Recent Advances in Laser Utilization in the Chemical Modification of Graphene Oxide and Its Applications Academic research paper on "Nano-technology"

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Recent Advances in Laser Utilization in the Chemical Modification of Graphene Oxide and Its Applications

Romualdas Trusovas, Gediminas Raciukaitis, Gediminas Niaura, Jurgis Barkauskas, Gintaras Valusis, and Rasa Pauliukaite*

A dramatic rise in research interest in laser-induced graphene oxide (GO) reduction and modification requires an overview of the most recent works on this subject. Typical methods for the recognition and confirmation of modified graphene and its derivatives, such as Raman, Fourier-transform infra-red (FTIR), X-ray photoelectron (XP), and ultraviolet-visible (UV-vis) spectroscopies, are introduced briefly in this review. A major part of the survey is devoted to the main modification ways and the laser parameters used in the literature. A discussion of possible reduction and modification mechanisms is also presented. Recent applications, especially in the biomedical field such as cell therapy treatment, as well as significant results of GO modification, are discussed in detail. Finally, perspectives for the application of laser-induced GO modifications in passive THz photonics and biomedicine are briefly addressed.

1. Introduction

A great number of works has been published on graphene, its investigation, and applications since it was discovered by Novo-selov and Geim'1' in 2004. The Web of Science database shows over 100 000 publications of this kind. The properties of this carbonaceous nanomaterial, such as its thermal conductivity, '2' electronic structure,'3' topographic properties on metal surfaces,'3b'4' electrochemical behavior, '5' as well as its applications for energy storage, '2' sensing,'6' batteries,'63' optoelectronic devices,'7' and interaction with biological molecules'8' have been studied in detail over the past decade, and a great variety of applications have been proposed. Amongst a large number of research articles and reviews on graphene, papers with laser applications for

the manipulation of graphene and its derivatives occupy a relatively insignificant part, i.e., less than 1%. The first publications on graphene modification with laser irradiation appeared in 2008, however, since 2010, the number of publications has risen exponentially.

Graphene oxide (GO) has emerged as a versatile material for nanocarbon research. Several types of oxygen-containing functional groups present on the basal plane, and the sheet edge allows GO to interact with a broad range of organic and inorganic materials. '9' Furthermore, GO is an abundant and low-cost material synthesized from graphite or graphite oxide; it retains a layered structure but, at the same time, it can be characterized by the loss of electronic conjugation caused by the oxy groups at the edge or plane defects. GO has a potential use in energy, '10' electronics,'11' molecular sensing areas, '12' catalysis,'13' etc. Moreover, it can be reduced chemically or thermally in order to achieve graphene-like properties. '14' The material produced during such a procedure is usually referred to as "reduced GO" (rGO). However, residual defects such as remnant oxygen atoms, '15' Stone-Wales defects (pentagon-heptagon pairs),'16' and holes'17' appearing due to the loss of carbon (in the form of CO or CO2) from the basal plane '18' limit the electronic quality of rGO.

A laser is a powerful tool which is used for various applications in industry, medicine, science, and material processing. ' 19' It is usually employed in different processes such as ablation, etching, cutting of various materials, as well as direct

Dr. R. Trusovas, Dr. G. Raciukaitis Department of Laser Technology Center for Physical Sciences and Technology Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania Dr. G. Niaura

Department of Organic Chemistry

Center for Physical Sciences and Technology

A. Gostauto str. 9, LT-01180 Vilnius, Lithuania

Prof. J. Barkauskas

Department of Inorganic Chemistry

Faculty of Chemistry

Vilnius University

Naugarduko str. 24, LT-03225 Vilnius, Lithuania

DOI: 10.1002/adom.201500469

Prof. G. Valusis

Department of Optoelectronics Center for Physical Sciences and Technology A. Gostauto str. 11, LT-01180 Vilnius, Lithuania Dr. R. Pauliukaite Department of Nanoengineering Center for Physical Sciences and Technology Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania E-mail: pauliukaite@ftmc.lt

The copyright line of this paper was changed 6 November 2015 after initial publication.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

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\Ji*S>

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Figure 1. Manipulating graphene oxide employing various methods, including laser manipulation, single-layer, bilayer, three-layer, and multilayer graphene can be obtained (from directions (1) to (4)). The laser irradiation of GO in the presence of a dopant leads to graphene doping: direction (5) gives the example of doping with nitrogen, showing different possible doping degrees. Direction (6) shows the product of laser irradiation of a GO film, suspension, or powder in the presence of chemical compounds generally named "R", leading to the modification of rGO. Directions (1-3) adapted with permissionJ25a] Copyright 2011, Nature Publishing. Direction (4) adapted with permission.!25b] Copyright 2012, Royal Society of Chemistry. Directions (5) adapted with permission.!25c] Copyright 2012, IOP Science Publishing.

writing on surfaces and the polymerization of some organic monomers.!20! Lasers can also be a useful tool to synthesize and manipulate graphene.! 21 Graphene can be likewise used to improve the laser device properties.! 22] The number of reports where lasers are applied for graphene manipulation and modification is rising dramatically. However, this type of work has not been overviewed in detail, although the application of the laser technology to the synthesis of graphene and related material was reviewed by Kumar! 23 in 2013, and the photoreduction of graphene oxide (GO) and its applications were discussed by Zhang and co-authors! 24 in 2014. The aim of this review is to shed light on the importance of and possibilities for employing laser irradiation for graphene modification with different functional moieties, and further applications of such modified graphene.

Generally, laser irradiation leads to various changes in graphene and GO depending on the number of graphene layers—single, double layer, triple layer, and multilayer—as presented in Figure 1. Moreover, if laser irradiation is performed under controlled ambient conditions in the presence of chemical compounds, modifications such as doping or the functionalization of rGO occurs at the same time. Commonly, multilayer graphene is applied for laser-induced modifications, therefore, mainly multilayer GO and graphene are analyzed here.

This review surveys the application of lasers to GO modification, such as the main mechanisms of thermal and chemical functionalization, methods for the determination of the products obtained, etc. The influence of the laser parameters and other processing conditions is overviewed and discussed,

Romuldas Trusovas is

a young research fellow in the Laboratory of Laser Microfabrication Technologies, Department of Laser Technologies, at the Center for Physical Sciences and Technology. He graduated from Vilnius University in 2009 and got his PhD in 2015 in Material Engineering. His research interest is laser micromachining, including the formation and modification of graphene layers by applying laser irradiation. The latter was the subject of his PhD thesis.

Gediminas RaCiukaitis is a

chief research fellow and head of the Department of Laser Technologies at the Center for Physical Sciences and Technology. He graduated from Vilnius University in 1978 and got his PhD in 1985 in the field of laser spectroscopy. Since 1995, he has been with the laser company "Ekspla"; his current position is as a consultant on laser technologies. His scientific activities include the development of new laser sources and laser applications in material processing and photonics.

Rasa Pauliukaite received a PhD in the field of physical chemistry from the Institute of Chemistry Vilnius, Lithuania, in 1998. She did her postdoctoral studies in Karl-Franzens University, Graz, Austria, (2000), the National Institute of Chemistry, Ljubljana, Slovenia (2001), ETH Zurich, Switzerland (2002), and the University of Coimbra, Coimbra, Portugal (2003-2010). She is the head of the Laboratory of Functional Nanomaterials and a chief research associate in the Center for Physical Sciences and Technology. Her current interests are the development and application of functional nanomaterials, including graphene, in electroanalytical chemistry.

as well as the confirmation of graphene modifications with vibrational spectroscopy. Additionally, the introduction of gra-phene into laser technologies and devices is briefly considered in this review. Some recommendations and perspectives

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for the further practice of GO modification with lasers are provided.

2. Spectroscopic Characterization of Graphene Oxide and Its Derivatives

Laser-assisted GO modification requires the application of a complex technique capable of probing structures at the molecular level. Some vibrational spectroscopies such as Raman and Fourier-transform infrared (FTIR) are most often engaged in the characterization of graphene and its derivatives. An additional method for characterization is X-ray photoelectron spectroscopy (XPS). The application of these techniques has already been reviewed,! 21>22>251 so they are just briefly described in this review for a better understanding of the changes induced in graphene and its derivatives after laser irradiation and modification. Raman spectroscopy provides information on the structure of the carbon matrix, while infrared spectroscopy is more suitable for the identification of functional groups attached during the modification procedure. Besides this, a simple spectroscopic technique such as UV-visible (UV-vis) spectroscopy is also used to characterize graphene oxide and its derivatives.! 26

2.1. Raman Spectroscopy

Raman spectroscopy is one of the most powerful tools to characterize the structure, bonding, level of disorder, as well as the modification of carbon-based materials.' 27' Raman spectra of carbon materials in the visible excitation wavelength range are always resonantly enhanced, because the laser line falls within the electronic transition of material. Due to this resonant enhancement, the Raman spectrum yields information on the structure of carbon network. However, less information is available on the structure of functional groups attached to the basal graphene plane. The Raman spectrum of graphene and its derivatives consist of three prominent peaks assigned to the first order D (1333 cm"1) and G modes (1578 cm"1) , and the

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second-order 2D mode (2682 cm-1), as presented in Figure 2a. The D mode has A1g symmetry and can be described as an inplane breathing vibration of the six-membered aromatic carbon rings (Figure 2b). That is the Raman-forbidden band in perfect graphite, and it becomes active in the presence of any disorder from an ideal structure. Therefore, this feature can be considered a disorder-induced band (D stands for "disorder"). It should be noted that the frequency of this mode depends on the excitation wavelength (dispersion) with an experimentally observed slope 40-50 cm' 1 eV-1 due to the double-resonance scattering described by Thomsen and Reich.'28' Such a Raman resonance process takes place in the system when both intermediate and initial scattering states are actual electronic states.'28,29' The G mode (E2g symmetry) is due to the in-plane stretching vibration of carbon atom pairs (Figure 2b). This mode is always allowed and can be observed in the Raman spectrum for all carbon structures containing the sp2 network, and it is characteristic not only of the aromatic rings of carbon but also of other sp2 structures. In contrast to the D band, the 2D mode is always allowed (consequently, it is also called G') and appears as the most intense feature in perfect single-layer graphene.' 25d' One should keep in mind that this mode is also dispersive; without information on an excitation wavelength, the absolute wavenumber of this feature has negligible value. The second Raman-forbidden band, named D', appears as a shoulder at the high-frequency side of the G peak due to the presence of disorder (Figure 2a). Moreover, this band is dispersive in the frequency because of the double resonant scattering;'28,30' however, the relative intensity of this band is usually low.

Extensive Raman spectroscopic analysis led to the identification of certain Raman markers, indicated in Table 1, which are extremely useful in the structural characterization of carbon-based materials.' 25d,i,31 ' Historically, Raman spectroscopy was used as a straightforward method to identify single-layer gra-phene and to estimate the number of graphene layers.' 32 ' Nonetheless, the frequently used application of the I(2D)/I(G) (where I is for peak intensity) ratio is not sufficient for the reliable recognition of single-layer graphene' 33 ' because the effect of the substrate and the presence of charged impurities leading to considerable changes in the I(2D)/I(G) ratio. For correct

Figure 2. a) Raman spectrum of disordered graphene flakes; b) motions of carbon atoms in the D and G vibrational modes.

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Table 1. Raman marker bands for the structural characterization of carbon-based materials. Abbreviations: A, integrated intensity; I, peak intensity; FWHM, full width at half maximum.

Marker

Carbon material

Description

Advantages

Limitations

i(2D)/i(G)

>/a(g)/a(2d)

i(D)/i(G)

i(D)/i(G) i(D)/i(D')

D''-band (broad band near 1500 cm-1)

FWHM(G)

FWHM(G)

FWHM(2D)

Graphene

Graphene

Graphene

Covalently functionalized graphene

Graphene, GO

Graphite, GO

Graphite

Nanostructured

carbon (nanographite)

Graphene

Allows identification of single-layer, bilayer and multilayer grapheneP5d]

The average Fermi level can be obtained using the equation:P1d] >/a(g)/a(2d) = C (Yep + 0.071 Ef|) where C is constant (10 eV-1 for 633 nm excitation) and Yep is the energy of electron scattering due to phonon emission!31^ (33 meV for 633 nm excitation).

Quantifies the point-like defects (distance between defects LD and defect density nD) for any excitation wavelength using Equations ( 1) and ( 2) .

Quantifies the fraction of sp3 hybridized carbon, F(sp3):[31e]i(D)/i(G) = 0.65/F(sp3]

Probes the nature of defects. Intensity ratio is -13 for sp3 defects, ~7 vacancy-like defects, and -3.5 boundaries in graphiteP1g]

Appearance of single peak near 1500 cm-1 indicates amorphization of the sample[31a,b,h]

Rise with an increase in interlayer spacing d002^31c]

Linear increase with the inverse value of mean crystallite diameter La [FWHM(G) = 14 + 430/la][30]

FWHM(2D) - 24 cm-1 (514.5 nm excitation) for single-layer graphene (Lorentzian form), while four components are visible for the bilayer t 25d]

Graphene identification marker

Marker of average Fermi level

Substrate nature and charged impurities might significantly affect the ratiot33]

Equation is valid for Ef ^ 0.4 eV; The intensity ratio depends on the spectrometer sensitivity; it depends on excitation wavelength.

Point defect density marker Marker is valid for LD > 10 nm^25'

Marker of the fraction of sp3 content

Detailed characterization of the nature of defects in graphene

Carbon amorphization marker band

Marker of the graphitization of carbon material^

Marker of the mean crystallite diameteK30]

Valid for 532 nm excitation; only sp3 defects can be detected.

In many cases, the D' band is not well-resolved, not useful for highly disordered GO

Very broad band, decomposition of spectrum into the components is required

Depends on excitation wavelength

Correlation is valid for crystallite diameters larger than 15 nm^30]

Graphene identification marker Depends on excitation wavelength

and precise identification, an additional Raman marker, the 2D-linewidth FWHM(2D) (full width at half maximum of the band), should be applied simultaneously.'25^331 The 1(D)/1(G) ratio has been extensively used for the quantification of defects in carbon-based materials since the pioneering work of Tuin-stra and Koenig '341 was published in 1970. The detailed Raman analysis of graphene after Ar+ ion bombardment revealed equations for quantifying the distance between the defects Id (nm) and point defect density nD (cm-2) at any laser excitation wavelength 4 (nm):'25e1

Id =(1.8 ± 0.5) x 10-9 A ^ iGy ] (1)

_(1.8 ±0.5) x 1022 (I(D)]

nD _ A 11(G)J (2)

Maslova et al.'301 demonstrated that FWHM(G) exhibits a linear correlation with the inverse diameter of crystallite size for the analysis of sp2 - nanostructured carbon (nanographite),

moreover, the I(D)/I(G) ratio was found to be dependent on the preparation procedure of the nanographites.

2.2. Fourier-Transform Infrared Spectroscopy

Infrared spectroscopy provides direct information on the structure and the state of carbon surface oxides and has been extensively used to characterize carbon black,1351 activated carbon,136' GO, 1371 reduced GO,'381 and GO derivatives.'391 Because chemical modification of GO usually takes place through oxygen-containing functionalities, infrared spectroscopy allows probing of the derivatization process.'21b-371 Figure 3' shows the main oxygenated functional groups at the graphene plane edges and epoxide as well as hydroxyl moieties positioned axially to the basal plane. '401 The relative amount of functional groups depends on the treatment conditions. For example, in the case of GO prepared by the modified Hummer's version,'411 the XPS-determined amount of functional oxygen groups was distributed as follows: hydroxyl (20.3%), carboxyl (19.5%), epoxy groups

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Figure 3. a) Structure of functional groups at the edges of graphene oxide. Adapted with permission.'403' Copyright 2003, American Chemical Society. b) Structure of functional groups at basal graphene plane epoxide (C-O-C) and hydroxyl (C-OH) functional groups. Adapted with permission.'4015' Copyright 1998, Elsevier.

(13.8%). The successful application of FTIR spectroscopy for the analysis of oxygenated functional groups requires correct assignments of the observed bands. This is a non-trivial task, because of the astoichiometric structure of GO, overlapping

the bands. Moreover, the neighboring functional groups affect the parameters of the observed features.)40a'42] In addition, adsorbed and/or intercalated water is usually present in GO samples.I42al Nevertheless, based on extensive theoretical modeling of functional groups " 40a>42bl and isotopic OH/OD substitution experiments,"42d] a certain consensus concerning the interpretation of GO infrared spectra was reached, as seen from Table 2. The characteristic stretching C = O vibration of carboxylic acid groups attached to the aromatic ring appears near 1680-1700 cm-1, while in the case of the non-conjugated COOH groups the peak frequency increases by 30-40 cm_1.[36'40a'42b,d] The peak frequencies of the C = O stretch in non-conjugated ketones appear at higher wavenumbers. However, it should be noted that the infrared cross section for the C = O stretching mode in carboxyl groups is higher compared with that of ketones.! 42b] Theoretical modeling suggests that five-membered lactone rings have higher C = O stretching frequencies (1820 cm" 1 ) compared with that of the six-mem-bered ring lactone structure (1780 cm_1).l40al The basal plane epoxide group can be probed with FTIR spectroscopy by two

Table 2. Characteristic FTIR vibrational modes for GO. Abbreviations: v, stretching; vs, symmetric stretching; vas, asymmetric stretching; 8, deformation.

Molecular group (see Figure 3) Frequency[cm 1] Assignment Commentsandreferences

sp2 hybridized carbon bonds 1463/1594 v(C = C) aromatic ring in-plane stretching Low intensity bands'42b]

sp2 hybridized carbon bonds 1585-1600 v(C = C) aromatic ring in-plane stretching Band is associated with the presence of hydroxyl groups and pyrones l37,40al

Carboxyl 1080-1100 v(C-O) Overlapped bands I37,38!

1680-1700 v(C = O) Aromatic carboxyl groups on the edges of the layer planes; Frequency is affected by peripheral groups l36,40al

1712-1720 v(C = O) Non-conjugated carboxylic carbonyl groups l36,42b,dl

3400-3600 v(O-H) Broad, overlapped band'37,38,40a,42b-dl

Epoxide «850 8(COC) [38,42a]

1280-1320 VaS(C-O-C) [38,42a]

Lactone 1720-1740 v(C = O) [40a]

Phenol and hydroxyl 1070 v(C-O) [38,42a]

1360-1480 8(COH) +v(C = C) ring Characteristic band for nearby located several phenyl groups, isolated groups give rise to lower frequency band («1300 cm-1)[40a,42d]

3000-3700 v(O-H) Broad, overlapped bandl37 38,42b-e]

Etheric rings 900-1100 v(C-O) Overlapped bands[38,42a]

Ketone 1600-1660 v(C = O) Conjugated ketones'36,38,408!

1750-1850 v(C = O) Nonconjugated ketones '42a,b]

Adsorbed/intercalated water 1620-1640 8(HOH) [42c-e]

3000-3700 v(O-H) Broad, overlapped band[ 37,42b,d,e]

Oxygen atoms aggregated at the edges of defects (cyclic edge ethers) 800 VaS(C-O-C) Giant infrared absorption band in thermally reduced graphene oxide[ 38]

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characteristic modes located near 1280-1320 cm 1 (asymmetric C-O-C stretching) and 850 cm-1 (COC deformation) ;'38-42a-b1 the origin of the bands was confirmed by the presence of the modes at 1320 and 896 cm-1, respectively, in the theoretically modeled spectrum. '42b1 In contrast to the epoxide group, basal plane hydroxyls are difficult to follow with infrared spectros-copy; the O-H stretching vibration appears in a wide wave-number region (3000-3700 cm'1) and strongly overlaps with the water, COOH, and phenol vibrational modes. The bending vibration of the C-OH group at 1368 cm-1 in phenols was experimentally confirmed by a line-shift to 968 cm-1 in H2O/ D2O isotopic exchange studies by diffuse reflectance FTIR. '42d| Further, an isotopic substitution experiment confirmed the presence of the water deformation vibration band 5(HOH) near 1620-1640 cm-1 and a weak feature due to the sp2-hybridized C_C in-plane stretching band near 1574 cm-1.'42d1 The spectral region between 900 and 1500 cm'1 is complicated due to the presence of the C-O stretching vibrational bands of various lac-tone, ketone, pyrone, and ether functional groups at the edges of GO (Figure 3.).

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nanoclusters. The UV-vis spectrum of GO exhibits a characteristic absorption peak near 230-240 nm due to the n-n* transitions of the C _ C aromatic rings and a shoulder near 290-300 nm corresponding to the n-n* transitions of the C _ O bonds. '481 The 230-240 nm band can be employed to monitor the number of GO layers deposited at the transparent substrate. '491 Upon reduction, the 230-240 nm band redshifts to 260-290 nm due to an increase in restored carbon network conjugation length, while the 290-300 nm band disappears or decreases in intensity, because of the elimination of C _ O bonds. '48d-501 Thus, the GO reduction process can be followed by UV-vis spectros-copy. However, some reduction procedures, e.g., thermal treatment in water, do not result in an extension of conjugation length; consequently, in this case no redshift is observed.'511 UV-vis spectroscopy provides the possibility to determine the optical energy gap of GO materials. ' 521 The optical bandgaps can be determined from absorption spectra using Tauc's expression, developed originally for amorphous germanium: ' 531

co2e_(ha- Eg )2

2.3. X-ray Photoelectron Spectroscopy

XPS is a surface-sensitive technique which detects the elemental composition and bonds between ions in a sample. In the case of graphene characterization, this technique is applied to analyze graphene derivatives, i.e., GO, modified or function-alized graphene, to determine their chemical composition.' 43 1 Moreover, XPS was demonstrated as a brilliant method to evaluate the graphene doping with nitrogen' 44 1 or fallow graphene growth in situ. '451 Hence, XPS can easily identify oxygen-containing functional groups in GO from spectral deconvolution of the O(1s) or C(1s) XPS lines. Since different species are present in functionalized graphene, e.g., C-H, C-O, C _ O, COOH, it is obvious that spectral deconvolution is a complicated procedure due to the presence of species with similar binding energies. '461 Figure 4 shows the XPS spectra lines C(1s) and O(1s) of the HOPG-PP and G-PP samples (HOPG: higly oriented pyro-lytic graphite, PP: pink plasma). The spectra are fitted to separate lines of chemical components described by asymmetric Gaussian-Lorentzian functions. Srivastava and Piontek,'471 in their review, described the C(1s) XPS spectrum of graphite oxide, which clearly indicates a considerable degree of oxidation with four components. These components coincide with carbon atoms in different functional groups, i.e., the non-oxygenated ring C (284.8 eV), C in the C-O bond (286.2 eV), the carbonyl C (C _ O, 287.8 eV), and the carboxylate carbon (O-C _ O, 289.0 eV). Such an interpretation is used in most XPS spectra for graphene oxide.

2.4. UV-vis Spectroscopy

Electronic UV-vis spectroscopy provides the possibility to i) determine the number of attached GO layers, ii) follow the GO reduction process, iii) determine the optical bandgap, and iv) monitor the formation of the hybrid material from GO and molecules containing a chromophoric group or plasmonic metal

where e is the optical absorbance, a _ 2n/A is the angular frequency of the incident radiation, A is the wavelength, Eg _ hc/Ag is the optical bandgap, and Ag is the gap wavelength. To determine Ag, a straight line plot of e1/2/A against 1/A should be constructed and the intersection point with the abscissa axis gives the 1/Ag value. Using such an approach, Ajayan's group' 52a 1 have monitored the changes in the bandgap during the controlled hydrazine-vapor reduction of GO from the initially insulating structure (Eg _ 3.5 eV) towards a semiconductor-like material (Eg _ 1.6 eV). Finally, UV-vis spectroscopy is a very attractive technique to monitor the formation of hybrid materials composed of GO and molecules containing chromo-

phoric groups or plasmonic metal nanoclusters.' 26,48a,b,54 1 In this

case, not only the process of the chromophore attachment can be monitored spectroscopically, but also the electronic interaction between GO and the linkage molecule/metal nanocluster may be analyzed.

3. Laser-Induced GO Modification and Functionalization

Laser irradiation is a modern nontoxic tool for local and/or large-area GO modification. A number of papers are devoted to such a photothermal GO modification and functionalization, which provides the possibility to manipulate the physical and the chemical properties of graphene as well as opening up new applications for this material. Such a modification is possible due to the rise in local temperature caused by laser irradiation. Local heating introduces local changes in the GO structure: It breaks bonds with functional groups attached to the graphene flake or forms chemical bonds with the compounds present in the local ambient environment. In fact, such modifications can be performed on both inorganic and organic compounds as well as polymers, as seen from Figure 5 , Changes to the physical GO peculiarities are also possible in the absence of any specific compounds during laser irradiation by only changing the

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Figure 4. The XPS spectra of the HOPG-PP and G-PP samples fitted to chemical-group components, described by asymmetric Gaussian-Lorentzian functions. a) C 1s. b) O 1s. PP is for the synthesis method of the materials using oxidation in low-energy (cold) glow of air pink plasma. Reproduced with permission.'46' Copyright2014, Elsevier.

ambient environment and the laser parameters. In such a way, graphene is photothermally synthesized from GO, while the mechanism of the synthesis mostly depends on the ambient atmosphere. As mentioned in the introduction, such a synthesis was overviewed in detail by Kumar in 2013." 23] A brief discussion of the most recent works focused on GO reduction either directly in suspensions of GO or in supported or freestanding films is presented in Section 3.1. The advantage of film irradiation is the formation of local micropatterns of the reduced graphene in GO. GO reduction by laser treatment is

also much faster than conventional chemical reductions. The laser parameters are vital in both cases (Figure 5).

In such a manner, the main tendencies collected from the scientific papers^5 demonstrating the influence of the laser parameters on the efficiency of GO reduction and modification are given in Table 3, Table 4, and Figure 6 . A variety of lasers has been used for GO reduction, its modification, or the production and the modification of graphene (Table 3, Figure 6a). UV excimer lasers, femtosecond, picosecond, CO2 lasers, as well as various continuous-wave (CW) lasers have

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Figure 5. Schematic presentation of the most used GO modifications under laser irradiation: 1) photothermal reduction of graphene oxide in vacuum; 2) GO doping with nitrogen during laser irradiation in ambient nitrogen atmosphere; in the same way, GO can be doped with other inorganic molecules, adding corresponding compounds to the irradiation environment; 3) modification with organic molecules during laser irradiation, where R indicates the required organic compound; 4) modification with a polymer during laser irradiation; 5) laser irradiation of GO in the presence of nanoparticles.

been applied for treatment of GO and its derivatives. According to Smirnov et al., ' 55a1 GO photoreduction takes place when the photon energy exceeds 3.2 eV (387 nm); while, when the laser irradiation wavelength is lower than 390 nm (excimer lasers -248 nm), the photochemical effect dominates. However, if the laser wavelength is larger than 390 nm, GO reduction is caused mainly by the photothermal process.' 24 1

The pulse duration also plays a significant role in the mechanism of GO reduction and modification. In the case of ultrashort pulses, the reduction mechanism is induced by the electronic system excitation and thermal effect caused by electron-hole recombination. Due to two- or multi-photon absorption occurring when the femtosecond laser irradiation is tightly focused, GO reduction resulting from the laser treatment can be considered a combination of photochemical and photothermal effects.' 55b

The analysis of graphite ablation with femtosecond laser pulses performed by Lenner et al.' 55c1 revealed the generation of graphite nanoparticles during the ablation process. These generated nanoparticles tend to rearrange to form thin surface layers. The laser fluence is a crucial factor in this process,'2'21a'55d-v1 as the size and the orientation of such formations strongly depend on the fluence value (Table 3, Figure 6a). When the fluence is close to the damage threshold (100 mJ cm-2), the size of the particles reaches a few angstroms. According to Sokolov et al.,'55d1 GO reduction with nanosecond (532 nm and 355 nm) and CW (532 nm) lasers induced clustering reactions with large sp2 domains within the laser plume. Once deposited on the surface, these products serve as a seed material, leading to the formation of larger graphene sheets.

In the case of CW lasers, the analysis of the D and 2D band intensities in the Raman spectra revealed that the formation of graphene-like materials can be controlled by varying the laser power;' 48d 1 in the case of pulsed-laser irradiation, the reduction degree depends on the product of the laser fluence and

the irradiation dose, ! 21a] as seen from the generalized data presented in Table 4 and Figure 6b.

3.1. Laser-Caused Structural Modification ofthe Physical Properties of Graphene

Usually, the modification of GO under laser irradiation is performed either in suspension—wet photochemistry—or in films deposited on various substrates. The latter is a more popular mthod. To our best knowledge, GO powder has not been used for modification or reduction under laser irradiation so far. GO nanosheets and nanoflakes are commonly transformed into films before laser-induced reduction or/and chemical modification.!56!

3.1.1. Suspensions

As mentioned previously, the number of publications revealing GO reduction under laser irradiation has increased significantly since 2013, because it is a rather simple, fast, efficient, localized and nontoxic method. As seen from Figure 6, a pulsed laser is more frequently used for GO reduction in comparison with continuous wave lasers. A nanosecond pulsed laser of 532 nm with an average power of 0.3 W was employed by Ghadim et al.'571 for effective, sustainable, and environmentally friendly reduction of GO sheets in a suspension of ammonia solution (pH - 9) at room temperature. Such a suspension was chosen in order to perform a controlled reduction with hydrazine under similar conditions. The dependence of the reduction efficiency on irradiation time was investigated with UV-vis, FTIR, Raman, XPS spectroscopies and thermo-gravimetric analysis. Figure 7 reflects the results of the influence of irradiation time on GO reduction efficiency. The

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Table 3. Pulsed laser treatment parameters for GO reduction, patterning and modification as well as for graphene patterning and ablation.

Irradiation wavelength [nm]

Pulse duration

Fluence [mJ cm-2]

Pulse repetition rate [Hz]

Type of laser treatment and treated material

Reference

248 248 248 532 532 790 800 800 1030 1064 532 800 1064 1064 415 248 266 343 532 840 1064 248 248

532 1064

20 ns 25 ns 20 ns 5 ns

9 ns 120 fs 100 fs 120 fs 170 fs

10 ps 7 ns

120 fs 10 ps 220 ns 10 ns 20 ns 5 ns 550 fs 5 ns 150 fs 5 ns 25 ns 20 ns

60-190 150 80 320 25 3.5 0.057 3.2 50 0.16 100 4

3340 400 25 800 200-850 160 150 200 4.4

14 000 40 000

15 000

5000 60 000

1 Patterning/reduction (GO) [55e]

1 Patterning/reduction (GO) [55f]

1 Patterning/reduction (GO) [55g]

10 Patterning/reduction (GO) [55h]

20 Patterning/reduction (GO) [55d]

8 x 107 Patterning/reduction (GO) [55i]

n/a Patterning/reduction (GO) [55j]

8 x 107 Patterning/reduction (GO) [55b]

6 x 104 Patterning/reduction (GO) [55k]

105 Patterning/reduction (GO) [21a]

30 Modification/reduction (GO) [55l]

8 x 107 Modification/reduction (GO) [55m]

105 Modification/reduction (GO) [21b]

5 x 104 Modification (GO) [55n]

10 GO solution reduction [55o]

n/a Ablation/patterning (Graphene) [55p]

n/a Ablation/patterning (Graphene) [55q]

103 Ablation/patterning (Graphene) [55r]

n/a Ablation/patterning (Graphene) [55q]

7.6 x 107 Patterning (Graphene) [55s]

10 Patterning (Graphene) [55t]

5 Ablation (HOPG)/production (Graphene) [55u]

10 Ablation (HOPG)/production, [55v] modification (Graphene)

n/a Ablation (HOPG)/production (Graphene) [55w]

5 Ablation (HOPG)/production (Graphene) [55x]

Table 4. Continuous wave laser treatment parameters for the GO and graphene patterning, reduction and modification reported in scientific papers.

Irradiation wavelength [nml Average power [mW] Average power density [MW cm-2] Type of laser treatment and treated material Reference

532 80 2.5 Patterning/reduction (GO) [55y]

532 12 0.4 Patterning/reduction (GO) [55z]

532 20 n/a Patterning/reduction (GO) [55d]

663 24 0.086 Patterning/reduction (GO) [55aa]

780 200 n/a Patterning/reduction (GO) [55ab]

788 5 n/a Patterning/reduction (GO) [55ad]

788 5 n/a Patterning/reduction (GO) [55ac]

808 n/a 7 x 10-8 Reduction/modification (GO) [55ae]

488 0.3 0.04 Modification (Graphene) [55af]

3 0.38

532 8.7 0.3 Modification (Graphene) [55ag]

1064 6 x 105 0.006 Laser-assisted CVD [126]

10600 7.2 x 104 0.0004 Patterning/modification (Graphene) [55ai]

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Figure 6. Distribution of a) the pulsed laser treatment parameters for graphene ablation, patterning,[55p-x] including GO modification,I21b,55l-n] and its reduction and patterning'21a,55b,d-k over the broad wavelength range of 200-1200 nm; b) continuous-wave laser treatment parameters for graphene ablation patterning and modification, including GO reduction over the wavelength range of 400-900 nm. Data is collected from the literature and summarized in diagrams (a) and (b). Different colors in (a) indicate the laser treatment method: yellow - GO reduction, green - GO modification, and red - graphene ablation and patterning; other colors indicate overlapping parameters between different treatments. The colors in (b) reflect the wavelength.

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degree of GO deoxygenation was determined from the position of the absorption peak of the UV-vis spectra. The optical absorption of the samples increased with an increase in laser irradiation time from 3 to 10 min, which was confirmed by a change in suspension color as shown in the inset of Figure 7 (left), indicating progressive restoration of the electronic conjugation within the rGO sheets under laser irradiation. The FTIR spectra presented in Figure 7 (right) revealed that the absorption bands for GO sheets were located at 3340 cm-1 (O-H stretching band), 1730 cm"1 (C = O stretching band), 1630 cm-1 (skeletal vibrations of aromatic domains and HOH deformation), 1401 cm"1 (bending absorption of the carboxyl group O = C-O), 1226 cm" 1 (O-H bending vibrations), and 1044 cm"1 (C-O stretching vibrations)."7! Upon laser irradiation for 10 min, all the absorption peaks related to the oxygen-containing functional groups were significantly reduced and well comparable with the graphene obtained after reduction with hydrazine. The spectroscopic analysis evidenced the decrease in the number oxygenated groups from 49% to 21% after the maximal period of laser irradiation of 10 min, which was rather similar to chemical reduction decrease to 15%J 57]

Notably, the role of ammonia in the aqueous graphene suspensions was manifold: i) to increase the pH of the suspension (usually to pH 9); ii) to inhibit rGO aggregation after laser irradiation, and; iii) to stabilize incompletely removed, negatively charged, functional oxy groups, assuring an alkaline medium.! 57,58 In addition, GO with ammonia displays a small increase in absorption in the whole spectral region compared to that of as-prepared GO. This phenomenon is attributed to a partial reduction of the GO suspension immediately after the introduction of ammonia.! 58

3.1.2. Film Structuring and Patterning

Application areas of GO photoreduction in the form of supported or free-standing films are expanding every year, according to the publications on this topic. Such an interest in film modification is probably related to broader application possibilities. As has been found, GO reduction induced during laser irradiation is partially a chemical process which depends

Figure 7. Optical absorption spectra (top) of a) GO suspensions of 0.02 mg mL-1 in water, and GO suspensions reduced by the pulsed laser irradiation for the different irradiation periods of b) 3, c) 5, d) 7, and e) 10 min. FTIR spectra (bottom) of a) GO, of reduced GO for the different irradiation times of b) 3, c) 5, (d) 7, (e) 10 min, and f) of GO reduced with hydrazine. Reproduced with permission.'57! Copyright 2014, Elsevier.

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on GO hydration. I23-24-59! The earlier investigations of the GO reduction mechanism dependence on the laser parameters I55a] showed the photothermal nature of the excimer laser; moreover, recent studies by Gengler and co-workers I59b' confirmed that the photo-induced transformation of GO to rGO employing a femtosecond UV laser (400 nm, 70 fs and 266 nm, 90 fs) is an indirect process. The authors demonstrated that GO was reduced after the capture of solvated electrons produced by UV pho-toionization of the solvent, subsequently, the reduction had a nonthermal nature, but was driven by the chemical potential of the solvated electrons rather than simple heating effects on barrier crossings. So, the authors proposed the description of the chemical process after two-photon initialization with following Equations ((4)-(6)):[59bl

GO + H2O + 2hv ^ GO* + H2O*

GO+2H,O*

■■•aq 1 xaq 1

GO + OHaq+ H3O+ + e-q n > rGO + 2H2O,

where the excited GO and solvent molecules in 1 ps after UV irradiation generate solvated electrons (e-aq) , and this reaction is ultrafast. The further reaction of solvated electrons with GO and its reduction is a significantly slower process.

Moreover, defects are often formed during GO reduction, which depend on the reduction mechanism during the photogenerated process (Equations (1), (2), (3), (4), (5), (6), (7), (8), ( 9) , ( 10) ), including the reactions between functional groups and holes and/or electrons: I55l]

C-C-OH + 3H2O + 3h+ ^ C (defect carbon) + CO2 + 3H+ (7)

C-C-OH + H++ e- ^ C-C (defect carbon) + H2O,

or epoxy groups:

C-O-C + H2O + 2h+ ^ C (defect carbon) + CO2 + 2H+ (9)

C-O-C + 2H++ 2e- ^ C-C (defect carbon) + H2O

The laser-induced GO reduction in films is usually performed either applying ablation with exfoliation of the surface or using local photothermal initiation based on the heating effect of the material. Routinely, the changes in GO layers and films after laser treatment are confirmed utilizing spectroscopic methods, as described in Section 2. A combined process of the multiphoton-induced reduction together with the ablation of the GO film was demonstrated by Li et al. I 55af' An ultrafast laser with a maximum pulse energy of 400 ^J and pulse duration of 90 fs, with an adjustable wavelength from 750 nm to 830 nm and pulse repetition rate of 10 kHz was employed in the experiments. The authors controlled the ablation process by changing the laser irradiation dose, i.e., the laser pulse number as shown

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in Figure 8.'60' Figure 8a presents an example of laser irradiation with six different pulse numbers, specifically, 1000, 2800, 4600, 6400, 8200, and 10 000. Those were adapted to reduce the same GO film at different locations. In this case, the pulse energy was 1.4 ^J; the pattern size was 30 x 30 ^m. As can be seen, a bright-field image of the reduced square patterns turned into a darker one with an increase in pulse number, implying a stronger GO reduction. In some cases, even ablation of the surface (bright spots in dark squares) was detected. The differently irradiated areas were inspected with micro-Raman spectroscopy (Figure 8b). Clearly, the intensity of the Raman band diminished with an increase in the pulse number. The authors attributed this effect to the simultaneous laser ablation of the film and GO reduction: the higher the pulse number, the greater the number of ablated GO layers. Furthermore, the values of the ratio I(G)/I(D) varied with different numbers of pulses, as seen from Figure 8c. Hence, this led to the conclusion that the appropriate pulse number around 4600 at an energy of 1.4 ^J per pulse was optimal to gain a superior GO reduction based on temporal focusing and patterned excitation. However, if the pulse number exceeded 4600, the total laser dose was too strong and, therefore, it caused a decrease in the I(G)/I(D) value due to damage in the GO thin film. I61' A similar increase in defect concentration with laser irradiation energy during GO reduction was also obtained in our work.'21,61' These studies were carried out employing a picosecond laser (1064 nm, pulse duration of 10 ps); the optimization was evaluated measuring the electrical resistance and Raman spectra. The optimal laser parameters were found to be as follows: a mean laser power of 10-100 mW (laser fluence of 0.01-0.3 J cm-2), scanning speed of 10 to 50 mm s-1 (at a 100 kHz repetition rate), depending on the GO sample thickness and the method of its preparation. As such, the resistance of the GO films decreased by 4-5 orders of magnitude after laser irradiation. Moreover, the Raman spectra confirmed the formation of a graphene phase. I61' Furthermore, both the decrease in the width of the G, D, and 2D Raman bands and the decrease in the number of graphene sheets during the laser treatment with the optimal mean laser power of 50 mW (the laser fluence 0.04 J cm-2) indicated a more regular ordering in the GO film. Modeling of the temperature dynamics revealed that a single laser pulse under the optimized irradiation conditions increased the local temperature of GO up to 1700 K, which was sufficient for GO reduction by removing the functional groups, i.e., carboxyl, hydroxyl, and epoxide, and formation of their oxidation to CO and CO2 and creating structural defects in GO. I21a' In addition, a polymer— chitosan (partially acetylated polyglucoseamine)—was introduced into the GO film to assure its biological compatibility in the investigations. I21b' The laser treatment caused a partial evaporation of the polymer, providing better adhesion of the GO to the surface of the supporting material, and decreased the thickness of the GO layer by ablation as well as cutting graphene sheets into nanocrystalline structures. It was found that the effect of laser irradiation depended both on the laser dose and the GO load in the film. The increase of the sheet edge defects and the formation of nanocrystalline structures were confirmed by Raman spectroscopy after the laser treatment with a mean power of more than 100 mW (fluence of 2.23 J cm-2). After laser treatment at these fluences, the I(D)/I(G) ratio was found

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Figure 8. a) Bright-field images of reduced GO squares using different pulse numbers, as indicated on the images, and b) micro-Raman spectra corresponding to the reduced patterns in (a). c) The /(G)//(D) values corresponding to the pulse numbers in (b). Reproduced with permission.'60 Copyright 2014, The Optical Society (OSA).

to be close to 2, indicating the formation of highly disordered nanocrystalline graphite.! 21b]

As noted, the laser-induced photochemical reduction of GO films opens new broad choices for the local and fast modification of graphene for various applications. The efficiency of the reduction depends on GO hydration as well as laser parameters, and it is considerably more efficient than a simple thermal GO reduction.!55ad]

Several works are devoted to the kind of laser irradiation which induces the periodic structures in GO responsible for its physical properties, enabling a fast and simple method to manipulate this material and create new devices. !55s>ah>62] In this case, no additional compounds are used to modify GO. Such a periodically structured GO film features supe-rhydrophobicity, and brilliant structural color! 55s ah] can be fabricated using either the pulsed laser! 62] or laser holography technique (two-beam laser interference (TBLI)). Jiang et al.!55ah] created such features inspired by nature: butterfly wings. An irradiation wavelength of 355 nm and a 10 ns pulse duration laser were used in the TBLI setup. TBLI and dual TBLI treatments caused both laser-induced ablation and reduction effects, which led to the formation of periodic graphene nanostructures. In addition, the laser treatment of GO led to a drastic removal of hydrophilic oxygen groups from the GO sheets, which was confirmed by inspection with C1s XPS. The hydrophobicity can be controlled by

changing the laser power, as seen from Figure 9: the higher the laser power, the higher the hydrophobicity. By using direct laser writing (DLW), it is possible to induce nanostruc-tural changes in the chemical composition of the graphene surface, increasing the unique superhydrophobic properties that mimic butterfly wings.' 55ah1

According to the unique idea by Zijlstra et al., ' 631 laser irradiation could improve graphene properties by healing its defects. The work of these authors was based on simulations of how graphene defects, in particular, the Stone-Wales defect in basal plane graphene, can be healed with a femtosecond laser to improve its physical properties. However, the authors could not practically succeed in healing such defects because, in their opinion, the defect was not sufficiently small in the simulations.'63]

As mentioned above, the laser-induced local thermal changes lead to a rearrangement of the graphene structure and the improvement of its physical properties when GO is deposited onto a solid substrate. Kumar and Khare' 55ah 1 explained the changes occurring at different temperatures: at room temperature and low substrate temperatures, carbon atoms are deposited onto a fused silica substrate in a highly localized manner due to the lack of sufficient mobility to form any crystalline structure. With a rise in substrate temperature, the mobility of the carbon adatoms increases due to a boost in the diffusion coefficient, which is given by Equation (11):' 55w]

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Figure 9. a) Dependence of the CA (contact angle) on the laser power in both vertical and parallel directions of the grating structure. Photographic images of a water droplet on different substrates have been inserted; b) 3D-AFM (atomic force microscopy) image of the biomimetic graphene. Adapted with permissionJ62a] Copyright 2012, John Wiley & Sons.

D = Do e

where D is the diffusion coefficient, D0 is the maximum diffusion coefficient at infinite temperature, Ea is the activation energy for diffusion, T is the substrate temperature and R is the gas constant. At higher temperatures, the diffusion rate increases and determines the rearrangement of the adatoms to form more ordered graphene layers. That was proved by a lower intensity ratio I(D)/I(G) in the Raman spectra at higher substrate temperaturesJ55w] In this way, GO modification can be performed simply by controlling the physicochemical environment, i.e., the photoreduction source and the local heating energy.

The manipulation of physical properties of a multilayered graphene film can be performed by exploring thermal laser annealing, which decreases the resistance of the layer and enables the fast and simple formation of transparent conductive films. The transparency of such films was found to be 90%. The increase in the conductivity of such films after laser annealing is related to the reduction of the structural disorder as confirmed with the help of Raman spectroscopy.! 64] Simulations of temperature dynamics after laser irradiation were also studied in our works. The transient temperature distribution in GO films was simulated using pulsed laser irradiation (t = 10 ps) with a wavelength of 1064 nm and a Gaussian temporal beam profile applying the software COMSOL Multiphysics, and the results are presented in Figure 10. The mean laser power (pulse energy) was varied between simulations. A heat transfer equation was modeled for a GO layer with a thickness of 1.2 pm, coated on a polycarbonate film with a thickness of 10 pm. The boundary conditions were selected as thermally insulating in all directions because of the significant difference in the thermal conductivities of GO and air. I 21a,65l

The transient temperature distribution was found at various depths in the GO film from the surface point. The GO absorption coefficient at a radiation wavelength of 1064 nm is equal to a = 5.3 x 104 cm-1, which corresponds to a penetration depth of 189 nm for this laser wavelength. Thus, all the laser radiation was absorbed by linear absorption in the GO layer.! 66] Absorption of the laser energy induced a temperature increase in the GO film, and it decayed quickly with depth and time.

Our thermal simulation results showed that the surface of GO was heated up to 1000 °C for a few nanoseconds with a single laser pulse, when the pulse energy exceeded 0.3 pJ (30 mW). The heating was necessary for the effective thermal reduction of GO.) 67 Duration of"the high temperature period" was multiplied by the number of laser pulses per spot, depending on the scanning speed at a constant pulse repetition rate (Figure 10d). The highest effectiveness of the reduction was achieved at irradiation dose (pulse energy and the number of laser pulses per spot product) values of 900 and 2500, and the temperature of GO above 1300 K was kept for as long as 9 ns and 31 ns, respectively. At the 100 kHz pulse repetition rate, the time interval between two pulses was longer than the decay of graphene temperature. Therefore, the cooling time of the GO sheet was shorter than the interval between pulses.) 21a,65]

According to Wei et al., I 68] the thermal reduction of GO occurs even at moderate temperatures (400 to 550 K). However, the majority of the reports are devoted to GO reduction, which takes place in the temperature range from 400 to 1300 K.I 14,67b,68,69] Huh'70' found out that the vaporization of water molecules occurs at a temperature of 500 K, the removal of carboxyl groups takes place at -500-900 K, further follows the removal of residual carboxyl and hydroxyl groups at -1100 K and, finally, the removal of residual hydroxyl groups with a partial removal of the epoxy group and the removal of aromatic C = C bonds occurs at -1300 K, which corresponds to an irradiation power of 50 mW. Moreover, it should be noted that the phenol groups stay in GO even after treatment at 1300 K.I 70] The ablation of graphene flakes was induced at high pulse energies when the temperature exceeded the evaporation limit. A precise balance between the removal of oxygen-containing groups and the preservation of the conjugation structure can be reached by controlling the temperature conditions in the layer and by using appropriate ambient gases.

The physical properties of GO can also be altered using a programmable laser-mediated N-doping and simultaneous reduction of GO, which can be achieved by DLW with a femtosecond laser of GO in ammonia) 55m] or nitrogen)71' atmosphere, or by depositing GO on a GaN support and applying irradiation with an excimer laser.) 72] DLW is a unique method which

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Figure 10. Temporal temperature distribution after the first laser pulse at different depths inside the GO film: a) 0.2 mJ (20 mW), b) 0.3 mJ (30 mW), and c) 0.5 mJ (50 mW). 1 - GO surface, 2 - 300 nm under the surface, 3 - 500 nm under the surface, 4 - 700 nm under the surface, 5 - 900 nm under the surface, 6 - at the GO and PC substrate boundary (1200 nm). Part (d) presents the dependence of temperature on the accumulated time according to temporal temperature distribution simulations, which determines how long GO was heated with different laser irradiation parameters. Adapted with permissionJ21a] Copyright 2013, Elsevier.

permits precise control over the doping area, therefore, it opens up the possibility to fabricate complex micropatterns on both glass and polymer substrates. By tuning the laser power, both the N-doping concentration and the bond types of N species could be modulated. The highest nitrogen concentration in rGO (up to 10.3%) was achieved with a femtosecond laser (800 nm wavelength, 120 fs pulse duration and 80 MHz repetition rate) by Guo et al.)55m] Moreover, if GO reduction with a UV laser irradiation is performed in a Cl2 environment instead of N2, Cl-modified rGO is obtained. As XPS and Raman spectros-copy characterizations confirmed, Cl is linked to the edge and basal plane defects.) 73]

Applying GO modifications with laser-assisted fabrications, nanostructures can be created in graphene membranes on polymer films.) 74] As mentioned previously, the laser beam deposits heat locally and, in this manner, irradiated polymer instantaneously melts and vaporizes. In such a way, a single-layer graphene membrane can adhere to the polymer surface and consequently form -10 nm deep wells during such a laser drilling of the polymer. Li et al.) 74] found out that, due to the short timescale of laser irradiation, heat diffusion into the polymer was negligible, and the excitation energy was highly confined in the polymer. A 532 nm laser with a repetition rate of 10 Hz and a pulse duration tfwhm of 25±2 ps was used in experiments. Such a modification allows patterning of the graphene nanostructured wells of hundreds of nanometers in diameter with high fidelity. Interestingly, Raman spectra

indicated that no additional defects were introduced in graphene by laser irradiation under the given conditions, which means that such heating affects only the polymer and adhesion is thermal rather than chemical.

In summary, the GO and graphene properties can be manipulated with laser irradiation just by controlling laser parameters, since they determine the local temperature. The local heat causes different GO reduction mechanisms, thus changing the physical properties of graphene or rGO. Other critical factors are the ambient conditions (vacuum or gases) and the supporting substrate of GO. In such a way, GO can be reduced or doped with elements present in the ambient gas. The supporting substrate can influence the heating effects as well as release some atoms and lead to the doping of rGO.

3.2. Modification with Organic Molecules and Polymers

GO functionalization or, in other words, modification with organic molecules not only changes its chemical activity but also provides more feasibility to attach it to different surfaces and expands its application areas. Usually, chemical GO functionalization is common for various applications; however, only GO modification under laser irradiation is discussed in this section. The choice of functional compound for the modification usually depends on the aim of application of modified GO. The latter is considered below in Section 4. GO modification is

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usually conducted for the following purposes: i) to change its physical and/or chemical properties; ii) to link GO to surfaces or substrates; iii) to increase its biological compatibility. The majority of papers are devoted to the second point.

A picosecond laser irradiation of 1064 nm was applied to GO-Congo red nanocomposite coatings."75 The mean laser power and the scanning speed were varied to find the optimal irradiation conditions for graphene phase formation. In order to confirm GO modification, the laser-treated areas of the samples were investigated employing Raman spectroscopy. The absolute best results were obtained when the laser power was 40 mW and scanning speed was 25 mm s-1. The presence of an intense 2D feature indicates the formation of a graphene-like structure due to the laser treatment of the GO-Congo red film. The frequency of the symmetric 2D band (2660 cm"1) coincides well with the estimated value for a single graphene layer. A relatively low I(2D)/I(G) ratio and an intense D band indicate the presence of defects and a structure composed of several graphene layers. The ratio of the intensities of the Raman lines depends on the Congo red concentration, as seen from Figure 11. The variation in the intensity ratios is related to the quality of the

Figure 11. a) Raman spectrum of laser treated GO-Congo red nanocomposite. b) Dependence of the intensities ratio of Raman lines D, G, and 2D on the Congo red concentration in the laser-treated nanocomposite. Laser process parameters: wavelength 1064 nm; mean laser power 40 mW, scanning speed 25 mm s"1. Reproduced with permission.!.751 Copyright 2012, Springer.

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resulting graphene film, indicating the importance of the Congo red concentration on linking graphene sheets together and the formation of larger clusters with an increase in order.

Other dye compounds, in particular phenothiazinyl molecules, can be covalently attached to GO by nitrene insertion into the C-C bonds." 76 Laser flash photolysis of this material indicates that tethering of phenothiazinyl molecules to GO enhances its interaction significantly with respect to the mixtures of both components, leading to the observation of photo-induced electron transfer. The mechanism of e- release was explained by three events observed when phenothiazinyl molecules were bound to GO sheets:" 76] i) prompt h+/e- recombination, ii) delayed generation of additional e- by phenothiazinyl molecules donating electrons to the GO sheet, and iii) slow e- annihilation. Such a mechanism can be applied to GO modification with other similar electron-donor compounds. Krishna et al." 77] studied GO interaction with metal porphyrins of Cu, Sn, and Zn, employing an ultrashort pulsed laser with two different wavelengths of 532 nm and 800 nm for the investigation of their nonlinear optical transmission. An interesting discovery was that the GO-metal-porphyrin composites had different absorption behavior as compared to those of pure porphyrins or pure GO. The variation in intensity towards the focus was observed due to strong two-photon absorption as well as excited-state absorption in the picosecond regime.)77 This change in the absorption behavior was interpreted as the result of the long lifetime and saturation of the excited states as well as the formation of stable GO-porphyrin composites. According to the UV-vis spectra obtained for separate compounds and composites, only the Cu porphyrin showed enhanced absorption in the composite, while the other two metal porphyrins exhibited suppressed absorption at the wavelengths typical of porphyrins.

As previously mentioned, GO can be modified and function-alized with different compounds, including polymers, in order to cast GO to the required surface and to amplify its optical and/ or electronic properties. As is known, a firm GO attachment to the conductive metal or oxide surface is an important challenge. Trying to solve this problem, Viskadouros and co-workers "55k] deposited rGO on the rGO-polymer composite, where the polymer was poly(3-hexylthiophene), applying the DLW technique. Experiments were performed using a 170 fs laser operating at 1030 nm wavelength and a 60 kHz repetition rate. The mechanical stability of such a field-effect transistor electrode as well as its conductive properties were improved. Moreover, the same technique can be implemented to improve the properties of a field-effect transistor electrode by n- and p-doping graphene using spin-coated 6,13-bis-(triisopropylsilylethynyl) pentacene. The doping as well as electrical properties can be controlled by modifying the laser irradiation regime." 55ag

The fluoropolymer can be applied to form fluorinated gra-phene or fluorographene, which is a novel 2D material with excellent electronic properties, widely used for transistor appli-cations."78] It can be synthesized by various approaches, the fastest one being laser irradiation (Figure 12). Lee at al." 55af obtained fluorographene from the selective photoreaction of the fluoropolymer (commercial name CYTOP)-covered GO induced by blue laser irradiation (488 nm). Therefore, it is in the section of GO modification with organic compounds. This fluorogra-phene showed practically the same structural and electronic

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Figure 12. a) AFM image and height profile of the CYTOP-covered graphene (CYTOP is a commercial name for the fluoropolymer used in this work) after selective-area laser exposure. b) A mechanism for fluorination by using CYTOP and laser irradiation. c) (Top) Raman map (D-band: from 1300 to 1450 cm-1) of the fluorinated graphene film adhered to a SiNx membrane (with a hole diameter of 3 pm). The inset shows a single spectrum of a freestanding graphene film after fluorination. (Bottom) A typical electron diffraction pattern of the selected area of the fluorinated graphene membrane. Reproduced with permissionJ55af Copyright 2012, American Chemical Society.

properties as that of the single-side fluorographene synthesized from XeF2 as well as exfoliated graphite fluoride.! 55af] In a similar way, the electronic properties of GO can be changed by Si-H doping of graphene-based lamellae nanoplates/rib-bons. The modification was achieved by the pulse laser ablation of bulk graphite in tetraethylethoxisilane. Hence, a rather long pulse duration and low energy density caused the simultaneous formation of by-products: polyynes.!79 Raman spectroscopy characterization revealed a high fraction of sp3 bonds in the nanoribbons, which can be ascribed to capillarity force and Si-H solute trapping under the influence of particle size and lattice imperfections.

To assure a strong GO attachment to the substrate surface, laser treatment at various fluences was applied to a GO-chi-tosan composite deposited onto ITO-covered glass by our group.! 21b] The same procedure can also be used for GO-composite structuring. FTIR spectra confirmed the formation of the composite material, although only electrostatic interactions between GO and chitosan were expected under the given conditions. Furthermore, the pulsed laser irradiation caused not only a partial chitosan evaporation but also generated an increase in the number of side defects in graphene sheets. Simultaneously, nanocrystalline structures were formed, depending on the laser

irradiation dose,! 21bl which allowed control over both the defect number and surface structure, thus increasing the biocompat-ibility of such a surface.

GO modification with biomolecules or the synthesis of biocompatible GO composites were reviewed by Chung et al.!80 in 2013. Some newer works are briefly presented below in this section. Remarkably, GO modification can also be conducted in the opposite way, i.e., first modifying GO chemically with polyethylene glycol (PEG) and then applying laser irradiation to reduce GO-PEG. Such rGO-PEG is an excellent substrate for biological applications, for example, cell culture investigations !81] (see more in Section 4.2.1). Nano-GO-PEG can be handled as a multifunctional nanocarrier to load a photosensitizer Chlorin e6 (Ce6) for photothermally enhanced photodynamic therapy. The combination of NIR light-triggered, mild photothermal heating of graphene and photodynamic treatment using Ce6 delivered by GO-PEG remarkably enhances the efficiency of the photodynamic therapy. Unique physical properties such as strong NIR absorption and chemical (i.e., surface n-n stacking) properties of nano-GO-PEG were exploited for the efficient treatment of cancer cells ! 78] (see more in Section 4). Another way to compose biologically friendly graphene is the formation of a self-assembled hydrogel with GO nanosheets by

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Ag++GO(e- ) ^ GO + Ag

GO + 4e- + 4H+ ^ rGO + 2H2O

Figure 13. Schematic representation of thiofavin-S-modified GO (GO-ThS) with high NIR absorbance. GO-ThS can effectively dissolve amyloid deposits of A^ 1-40 upon near-infrared laser irradiation. Adapted with permission.^83! Copyright 2012, John Wiley & Sons.

physical crosslinking at low concentrations without any chemical modification of GO. This hydrogel undergoes a sol-gel transition upon exposure to various stimuli, such as temperature, NIR light, and pH.I82! Further, GO modification with biological molecules was performed by Li et aU83] to create a novel method for Alzheimer's disease treatment. The authors locally and remotely heated and dissociated amyloid aggregations, employing thioflavin-S (ThS)-modified GO with NIR laser irradiation, as shown in Figure 13. GO was covalently linked to ThS, which can selectively attach to amyloid-^ (A^) aggregates and form conjugated GO-ThS-A^. The A^ morphology can be changed under the NIR laser irradiation applying it to GO-ThS in real-time (see more in Section 4).

Summarizing, the simultaneous or cascade laser-induced GO reduction and modification with organic molecules is usually performed to change rGO physicochemical properties or attach rGO to solid substrates. As in the other cases, the modification and irradiation conditions, as well as the modification pathway, are chosen depending on the prospective applications of the product.

3.3. Laser-Induced Decoration ofGraphene with Inorganic Elements or Nano-objects

A considerable number of publications in scientific journals are dedicated to GO modification with inorganic nano-objects under laser irradiation. These nano-objects, such as nanoparti-cles (NPs) or nanowires (NWs), are usually applied for graphene decoration to enhance its electrical properties. A direct and facile method for the microdecoration of AgNPs on the reduced rGO can be achieved employing a focused laser beam to cause the local reduction of Ag+ ions to AgNPs (Equations (12), (13), ( 14), ( 15)) by the laser irradiation of a GO film submerged in an aqueous AgNO3 solution. Teoh and colleagues +41 suggested a mechanism for such a local reduction. Firstly, laser irradiation produces electrons and holes: the electrons are used for the reduction of Ag+ or GO (Equations (14),(15)), while the holes cause oxygen evolution from water (Equation (13)):+84

A similar procedure can be applied for AuNP deposition on rGO.!84,85! In such a way, the NPs are directly anchored on the rGO film, creating a microlandscape, as presented in Figure 14J84] In addition, by varying the intensity of the laser beam, the shape, size, and distribution of NPs can be controlled: a higher intensity leads to larger NPs at a lower concentration. Interestingly, the size strongly depends on the laser beam intensity, but it is independent of the precursor concentration in the range between 0.01 and 0.1 M. The resulting hybrid materials exhibit surface-enhanced Raman scattering up to 16 times higher than that of GO, depending on the size and density of AuNPsJ84] The elemental analysis of AuNP-rGO composites showed the presence of almost 2% Au. The Raman spectra confirmed changes after the composite formation: the peak intensities at 1580 and 2680 cm+x decreased significantly after AuNP decoration. Sid-dhardha and colleagues+51 used fixed laser parameters in their studies: a nanosecond laser of 1064 nm with a pulse energy of 50 mJ and repetition rate of 10 Hz. However, if the authors varied the parameters, different properties and a different composite structure might be obtained. In contrast, the peak intensity at 1300 cm-1 of AuNPs increased significantly when Au nanostars were deposited on the surface of GO, which was electrostatically attached to the aminooxysilane treated surfaces.!+61 Metal NPs can also be deposited from mixtures of aqueous and nonaqueous solutions, e.g., water:ethanol, water:methanol.I5ll

GO + hv ^ GO(h++ e- )

4h++ 2H2O ^ O2 + 4H+

Figure 14. SEM images of micropatterns comprising AgNP decoration on GO films. a) Ag micropatterns with different sizes on the same sample. On the left, the laser power used was low compared to that on the right, where a high laser power was used. The contrast in the size of the AgNPs in both left and right regions was created by changing the laser power in a single scan; b) a regular box with controlled features comprising AgNPs; c) a regular pattern of circular patches made of AgNPs, and; d) a simple grid pattern. Reproduced with permission.^84 Copyright 2014, the Royal Society of Chemistry.

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Again, the decoration with metal NPs occurs during partial GO reduction by laser irradiation, as presented in Equations ( 12), (13), (14), (15); however, Equations (13) and (15) are different, but again the holes react with the solvent in the same manner as shown in Equations (16), (17), (18), (19):'551'

4h++ C2H5OH ^ C2H4OH + H+

nC2H4OH + GO ^ rGO + n ■ CH3COH + n/2H2O (17)

Gases can be released in the case of methanol: 6h + + CH3OH + 6OH-^ CO2 + 5H2O (18)

6H++6e-^ 3H2 (19)

The quality of a GO reduction together with a PdNP deposition was investigated by XPS (C1s), and it was confirmed that the laser-induced GO reduction in the absence of a Pd precursor was independent of the medium, and the majority of C = O groups were removed. In the presence of Pd2+, some C = O groups were present and, therefore, GO was just partially reduced due to the competition between the photoreduction of GO and Pd2+.'551' Furthermore, the success of Pd-rGO formation in all solvents was similar and, according to XPS data, Pd0(3d5/2) was predominant, although Pd0(3d3/2( was also present in a lower amount. The intensities at 335 eV (Pd0(3d5/2)) were similar in all solvents. However, water:methanol resulted in a 2% higher peak intensity compared to those in the other two media. |5511

In addition, Jurewicz and co-workers'87' performed graphene modification in the opposite way, i.e., they chemically deposited AgNWs on the rGO surface and then applied 1064 nm laser ablation to form structures used in nanoelectrode construction. Since the authors did not provide any spectroscopic characterization to register the changes after the graphene modification, it is difficult to compare the quality of the material with that prepared by different methods. Furthermore, graphene can be modified by pulse laser deposition on the top of metal nanocones, protecting the nanocones and enhancing the emission properties of such nano-objects compared to those of unmodified nanostructures. '88'

Nonetheless, the simultaneous formation of inorganic gra-phene composites is another method to change the graphene properties. TiO2-GO nanocomposite thin films can be grown by applying UV matrix-assisted pulsed laser evaporation, usually abbreviated as MAPLE, a technique in controlled oxygen or nitrogen atmospheres. '80' Such a graphene modification allows the manipulation of the hydrophobic and hydrophilic properties by controlling the component ratio in the composite and UV exposure. Doping with nitrogen leads to a further decrease in the static contact angle of composite films.'71,89' In addition, a GO-Fe nanocomposite can be synthesized after laser sintering GO and Fe powder in a dispersion of polyvinyl alcohol on a stainless steel substrate. '55n' The most important factor, in this case, is the thermal treatment of the composite by fast heating and cooling. The laser sintering of the nanocomposite improved its physical properties, i.e., increased the strength and hardness, and extended the three-point bending fatigue life. Lin et al. '55n' developed a model to calculate the tensile strength and Young's modulus of GO-Fe nanocomposites. This model could be also

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applied for other GO nanocomposites, because it is based on the dimensions of the nanoparticles in the film. One more graphene-based composite was synthesized employing laser sintering; it was applied as a biocompatible scaffold for bones: CaSiO3-G. This ceramic composite scaffold was created via selective laser sintering, in which the sintering time was reduced to seconds or even microseconds through rapid heating and cooling. The physical properties of such a biocompatible composite depended mostly on the graphene load in the composite. !90'

The other important group of compounds is polyoxometa-lates (POMs). They are known as photocatalytically and electro-chemically active inorganic compounds and, therefore, modification with these compounds may grant broader possibilities for GO reduction and its applications. Li and colleagues'601 adopted the photocatalytic abilities of three Keggin-type poly-oxometalate clusters, H3PW12O40 (PW), H4SiW12O40 (SiW), and H3PMo12O40 (PMo), to reduce GO under UV irradiation in water. UV-vis absorption and XPS spectra demonstrated that PW and SiW can photoreduce GO effectively, in contrast to PMo. The authors concluded that the LUMO levels of POMs should be located energetically above the work function of GO to enable electron transfer from POM to GO. !74' Hence, this was not a disadvantage since the modification of non-reduced GO also improved the photocatalytic properties, allowing it to be employed in practical applications.

As seen, GO modification with the help of laser irradiation occurs in all cases together with GO reduction. The modification mechanism is usually thermochemical, depending on the environment and solvents. In most cases, produced holes react with the solvent, and the modifying compound is linked to rGO defect sites during the photothermally initiated GO reduction reaction. The modifications quite often increase the GO reduction rate. Laser irradiation enables a fast, simple and nontoxic way to modify rGO and, at the same time, it enables the manipulation of its physicochemical properties as well as a broad array of applications.

4. Applications of Laser-Irradiated Graphene and Graphene Oxide

The ability of lasers to reduce GO locally on a sample has been proposed for several fields of applications: i) the formation of electrical devices either using DLW'21b-55d-f-j-y-z-59a-69b-91' (including batteries),'82' or ablation and micropatterning;'60,921 ii) the usage of graphene in laser devices including mode-locking lasers' 93 ' and photonics, and; iii) applications in medicine for the treatment of some diseases.' 81,83,94 ' The application of DLW and micropatterning were already discussed in a previous review by Zhang et al.; '24' graphene application to terahertz (THz) optics were also overviewed by several researcher groups.'95' Therefore, only mode-locking devices and applications in medicine, which is the broadest area of applications of modified graphene, are discussed in detail in this section.

Nevertheless, laser-reduced, patterned, and nanostructured GO may also be applied for humidity sensing because such a GO is sensitive to gaseous water molecules. '62b-96' Detection can be either via measuring of the impedance' 62b ' or by using a quartz crystal microbalance.'96' Sensing properties can be

controlled by the GO reduction degree by changing the TBLI laser power.! 62bl Since this area has not been widely explored yet, it will be reviewed in more detailed in Section 5, as part of the future perspectives.

4.1. Graphene Applications in Optics

As mentioned above, reduced and/or modified GO can be used in various laser devices. For instance, saturable absorbers made from graphene can be applied for the formation of fiber lasers. The main advantage of the graphene-based saturable absorbers is their broadband operation. This feature is important for devices used in spectroscopy or chemical sensing applications.! 97

4.1.1. Devices

Graphene, together with carbon nanotubes, is a promising material for mode-locking devices.! 98] Mode locking is a technique where a laser can produce ultrashort picosecond or femtosecond pulses. Husaini et al.! 93 developed a graphene-based saturable absorber for the high-power semiconductor disk laser. The integration of an antiresonant graphene saturable absorber mirror led to the generation of mode-locked pulses with a duration of 353 fs and a pulse energy of 2.8 nJ. A novel graphene-based, Q-switched, S-band fiber laser with a Q-switching threshold of 65.29 mW was presented by Muhammad et al. [99] The minimum pulse duration of 1.218 ps was obtained at a pump power of 100.44 mW. The device can provide pulse repetition rate from 73.6 kHz to 331.1 kHz. Lasers of this kind can be used in data traffic applications.

One more application was proposed by Li et al.! 10°1 The authors constructed a graphene-clad microfiber all-optical modulator, which can achieve the modulation depth of 38% and a response time of -2.2 ps. A weak infrared signal wave coupled into the graphene-clad microfiber encounters a critical weakening because of absorption in graphene during the wave propagation. A coupled light excites carriers in the graphene and, through Pauli blocking of interband transitions, it moves the absorption threshold of graphene to a higher frequency, causing a decrease in the signal wave attenuation. The propagating light prompts modulation of the signal output from the fiber, and its response time is limited by the relaxation of the excited carriers. The induced refractive index change in graphene is too small to cause any changes in the waveguiding properties of the microfiber. A significant advantage is that the tapering of the standard fiber into a microfiber minimizes losses. That permits a graphene-clad microfiber modulator to be efficiently inte-gratable into the standard fiber optic frameworks for in-fiber operation.

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Del and co-workers ! 64] attempted to adjust graphene ink for electronic applications such as transparent electrostatic dissipation, by laser annealing the graphene films. They employed a CW laser (532 nm) to optimize the optical and electronic properties of such graphene ink thin films. The laser treatment of inkjet-printed graphene films caused a decrease in sheet resistance from -180 kQ cm-2 to -60 kQ cm"2 and an increase in transmittance from 52% to 54% at X = 550 nm. Peng and colleagues ! 101] aimed their research to create a flexible and stack-able supercapacitor from laser-induced graphene. Graphene was synthesized by laser induction of commercially available polyimide films. Irradiation with a CO2 laser of 10.6 pm (pulse duration of 14 ps and average power of 4.8 W) caused the formation of porous graphene structures. Such structures can be used for the production of stackable supercapacitors. The tested devices with a solid-state polymeric electrolyte achieved an aerial capacitance of 9 pF cm-2.

Our group[65] employed the laser-induced reduction of graphite oxide for the formation of heat-conductive channels for heat-sinking applications. COMSOL software was used for heat-transfer modeling in the graphene channels. A graphite oxide coating was modified with picosecond laser irradiation to form 1.2 mm wide and 44 mm long zigzag channels. One end of the channel was heated with a soldering iron tip while the other was connected to a massive copper radiator. A Fluke thermovisual camera was used to record the temperature field in the sample. Reduced to graphene and ablated, the channels in GO were clearly distinguished from the unmodified material background, indicating a significant difference between the thermal properties of those that were unreduced and the laser coating of the reduced GO locations (Figure 15).

Figure 15. Temperature distribution in laser scribed GO. a) A schematic view of the setup used for the thermal imaging of laser-scribed graphene channels on the GO substrate; b) temperature distribution in the zigzag structure of graphite oxide coatings formed by the localized laser reduction method measured with a thermovisual camera. The red dot is the contact point of the heater. The opposite end of the reduced conductor was connected to a massive copper radiator; c) temperature distribution in the graphene channel in the GO layer on polycarbonate, simulated by COMSOL.!65!

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4.1.2. THz Plasmonic Response

First of all, one of the hottest topics in photonics and electronics is the development of THz technology. The range of electromagnetic radiation spaced in between 300 GHz and 10 THz is useful for a large variety of applications,' 95e ' but still lacks compact photonics components for room-temperature operation. Graphene and graphene-related structures can serve as an important route to proceed in the design and fabrication of photonics components. '95a,f' It has been nicely shown that graphene can successfully be employed for sensitive room-temperature THz detection via the photo-thermoelectric effect. '95h' However, it is worth noting that probably the most attractive spotlight of scientific interest in THz application is plasmons—collective oscillations of electrons—in graphene. '102' Plasma wave-related effects are powerful tools in realizing highly sensitive and widely tunable THz detection in graphene field-effect transistors. '103' Moreover, plasmons can be used for the enhancement of THz emissions induced by ultrafast optical excitation from single-layer graphene.'104' It is important to mention that sinu-soidally corrugated graphene sheets can be an environment suited for the generation of THz radiation. '95g|

Omitting a detailed description of the physics behind these principles, but touching on a route in passive THz optics fabrication, the DLW technique can be employed very effectively. As a rule, THz optics utilizes parabolic or spherical mirrors, high-density polyethylene lenses, i.e., components which are rather massive but of centimeter-range in size. The reduction in size of THz imaging systems, which are of particular importance in modern security and material control applications, is one of the most important tasks in order to make their implementation convenient and alignment-free for optics. Once again, plasmons in graphene employing the metamaterials approach seem to be a very promising choice.'105'

Recently, a flat optics approach—Fresnel zone plates and zones plates with integrated bandpass filters—has been used to focus and filter THz radiation.'106' It was shown that terahertz zone plates with integrated resonant filters are a powerful tool to manipulate the THz laser beam profile, aiming to find an optimal design to reveal the effect of zone number and the influence of the plate thickness. '107' Further, it was demonstrated that the flat optics technology can serve as an effective instrument to integrate THz optics and sensing elements in one chip. '108' The DLW method was applied to produce the zone plate either in 30 ^m-thick steel'106' or in a 200 nm-thick gold layers deposited on one of the bottom surfaces of an InP wafer with integrated InGaAs bow-tie detectors. '108' It is reasonable to suppose that gra-phene can also be an option to squeeze flat optics into ultrathin dimensions. Quite recently it was demonstrated computationally that monolayers and multilayers of graphene fabricated into Fresnel zones can operate in the visible and terahertz regimes. '109'

4.2. Bioapplications of Nanographene Materials in Medicine Using Laser Therapy

Laser therapy was introduced to medicine decades ago, and it provides a range of efficient techniques. Graphene nano-materials (GNMs) are a relatively new subject to the field of

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nanomedicine; nevertheless, this is a rapidly developing and prospective area. A significant number of papers has been published in the literature, including a couple of reviews ' 110' on the design, synthesis, and characterization of graphene nanoparti-cles and hybrid materials for bioapplications. In comparison, the number of publications on the laser-assisted bioapplications of graphene is considerably low. These publications include two sets of topics: i) photoluminescence in cell imaging/bio-analysis, and ii) cell therapies. The former does not fully meet the topic of our survey, moreover, it has been discussed in detail

elsewhere.' 111 '

Laser-assisted therapy involves interactions between the cell and its environment. Novoselov and co-authors' 112 ' critically assessed whether this technology, which enables the use of gra-phene to be near the market before 2030 due to the high safety, clinical and regulatory hurdles, and long timescales associated with drug development, which are exacerbated when new materials are involved. Beginning in 2010, when the first publications appeared in this field, the number of articles has doubled annually; the number of citations reached 1000 in 2015.

The laser treatment of cells using GNMs generally includes four key elements: i) a choice of graphene nanomaterial, ii) conjugation of the graphene nanomaterials with functional moieties; iii) a choice of laser parameters, and iv) some sort of cell therapy, as presented in Figure 16. The treatment protocols, gra-phene conjugation moieties, and laser parameters are collected in Table 5 J6i-7c.73.86.113' The sequence of attachment of functional moieties to GO is marked by ">" in this table. A selection of the correct combination of these elements is significant for the efficient usage of the laser treatment. Although the interaction between nanomaterials and bioactive components is extremely complex,and the final result is still difficult to forecast, a few important conclusions can be made from the literature survey.

Most publications address laser-assisted cancer therapy using GNMs. Two main directions can be distinguished in this area: photothermal therapy (PTT) and photodynamic therapy (PDT).

4.2.1. Photothermal Anticancer Therapy

PTT refers to the use of IR radiation for the treatment of cancer cells. GO, rGO, and other GNMs absorb light across a broad spectrum of wavelengths from the UV to the IR. '113f-g! In this range, the NIR region is of utmost importance. The NIR window (also known as the optical window or therapeutic window) defines the range of wavelengths from 650 to 1350 nm where light has its maximum depth of penetration in tissue. Within the NIR window, scattering is the most dominant light-tissue interaction and, therefore, the propagating light diffuses rapidly. GO and especially rGO with a high NIR light absorbance have found extensive application in PTT.

Among the most common treatment strategies, the maximal difference obtained between the absorbance values of GNMs and tissue, causing minimal damage to normal cells, is the most important one. Yang's group'113b' examined the in-vivo behavior of graphene nanosheets with PEG coatings in one of their first studies. Later on, the same group studied how the size and surface chemistry of ultrasmall rGO with a noncovalent

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Figure 16. Cell treatment using a laser and GNMs. The references in the literature about the use of GNMs are: graphene, GO, rGO, graphene quantum dots. GMNs are modified with functional moieties using glutaraldehide, polymers, drugs (doxorubicin, thioflavin-S, folic acid), photoactive sensitizers (cyanine 7, chlorin e6, methylene blue, hypocrellin A, cell-targeting substances (antibodies, peptides, nucleotides, transferrin), composite structures (Au, Fe3O4, CdSe/ZnS, BaGdF5, SiO2). Cell treatment protocols include: photothermal treatment (PTT) of cancer, neural cells, gene delivery, prevention of amyloid aggregation and bacterial killing; photodynamic treatment (PDT) of cancer; pulsed laser treatment (PLT) of cancer, neural cells and cell transfection. Laser sources used are: NIR (808 nm), femtosecond pulsed laser (800 nm) and in a few cases, tunable wavelength lasers.

PEG coating affect the in-vivo action of graphene, and they succeeded in remarkably improving the performance of graphene-based in-vivo photothermal cancer treatment.! 113c] The authors suggested that the surface chemistry and the sizes of the nano-materials have undeniable effects on their attitudes in biological systems, especially in vivo. According to the authors, no obvious sign of toxicity was observed for NGS-PEG-injected mice from histology examination, blood chemistry, or whole blood panel analysis. Similar results using PEGylated graphene nano-mesh were obtained by other researchers.! 113cdl Vila et al.!113f investigated the laser dosage and irradiation exposure time to control the temperature rise and consequent damage in GOs containing cell culture media. These results showed that the cell culture temperature increases preferentially with the laser power rather than with the exposure time. The fact of the existence of a significant release of the interleukin-6 from the cells after low-power irradiation at a long exposure time means that, even with lower necrotic damage, immune activation could be induced for softer treatment parameters. A pH-dependent, NIR-sensitive, rGO hybrid nanocomposite was synthesized via electrostatic interaction with indocyanine green, which was

designed not only to destroy localized cancer cells but also to be minimally invasive to surrounding normal cells.! 114 The NIR-irradiated hybrid nanocomposites showed a pH dependent photothermal heat generation capability from pH 5.0 to 7.4 due to the pH response relief and quenching effects of poly(2-dimethyl amino ethyl methacrylate) with indocyanine green on a single rGO sheet. The image-guided synergistic photothermal antitumor effects of photoresponsive NIR imaging agent, indocyanine green, by loading onto hyaluronic acid-anchored rGO nanosheets, were reported by Miao et al.! 115] Loading of indocyanine green onto either rGO (ICG/rGO) or hyaluronic acid-anchored rGO substantially improved the photostability of photoresponsive indocyanine green upon NIR irradiation. After 1 min of irradiation, the NIR absorption peak of indocyanine green almost disappeared, whereas the peak of indocyanine green on rGO or hyaluronic acid-anchored rGO was retained even after 5 min of irradiation. In the study of Mauro et al.,! 1161 in-vitro rGO-induced hyperthermia was assessed and combined with the stimuli-sensitive anticancer effect of a biotinylated inulin-doxorubicin conjugate, hence, getting a nanosystem endowed with synergic anticancer effects and high specificity.

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Table 5. A summary of cell therapy protocols reported in the literature using GNMs and laser treatment.

Cell therapy protocol Functional moieties Graphene nanomaterial and its size [nm] Laser parameters: laser; power density [W cm 2]; irradiation time [min] Reference

PTT-C PEG GO; 10-50 NIR; 2; 1440 [113b]

PEG rGO; 27 NIR; 0.15, 5 [113c]

PEG > P rGO; 60 NIR; 0.1; 7 [113d]

PEG > P rGO; 20 NIR; 0.6; 5 [113e]

PEG GO; 100 NIR; 1.5-3.0; 1-15 [113f]

Au GO; 500 NIR; 0.75; 2 [86]

ICG rGO; 200 NIR; 2; 5 [114]

H > ICG rGO; 100 NIR; 1.2; 0.5 [115]

In > DOX rGO; 400 NIR; n/a [116]

PEG rGO; 50 NIR; 6; 10 [117]

FA > An > Nu > Au GO; 100 TL; 20; 10 [118]

PEG > BaGdF5 GO; 200 NIR; 0.5; 1 [113h]

Fe3O4 G NIR; 2.3; 10 [113i]

PTT-NC - rGO NIR; 0.1; 10 [113j]

PTT-GD PEI + PEG rGO; 400 NIR; 6; 20 [127]

PEG + PEI GO NIR, n/a; n/a [113l]

PTT-A TS > Au GO; 600 NIR; 1;8 [112n]

PTT-PDT PEG;PPI > Pc > P G; n/a TL (690 nm); 0.95; 20 [122]

PTT-B GA > Fe3O4 GO, rGO NIR; 1.5; 10 [6i]

PTT-B - GO; n/a NIR (1064 nm); n/a 123

PDT PEG > Ce 6 GO; 50 NIR; 0.1; 10 [94]

PEG GO; 50 TL (671 nm); 0.75; 20 [113o]

FA > Ce 6 GO; 1500 TL (632.8 nm); 30; 10 [113p]

GA GQDs; 5 TL (670 nm); 0.3; 30 [113q]

FA > DOX > P > Ag/SiO2 GO, n/a NIR; 2; 3 [113r]

FA > DOX rGO; 100 NIR;1.0; 5 [113s]

PEG > DVDMS GO; 50 TL (630 nm); n/a [120]

H > MT GO; n/a NIR; 2; 6 [121]

PLT-C PEG > T rGO; 30 FPL, n/a; n/a [113a]

PEG > T GO; 30 FPL, n/a; n/a [74]

PLT-NC GO, rGO, n/a FPL, n/a; n/a [113t]

PLT-CT - G, n/a FPL (1064 and 532 nm); n/a; n/a [113u]

In-vitro tests using cancer breast cells showed the ability of the bioconjugate to efficiently kill cancer cells both via a selective laser beam thermo-ablation and hyperthermia-triggered chemotherapy. The effects of lateral size and reduction level of PEG-modified GO nanosheets on the photothermal properties were investigated in the study by Turcheniuk et al. '117' PEG-modified GO and rGO-PEG matrices were synthesized through amide bond formation between the carboxyl groups of carboxylated GO and rGO and the amine groups of a PEG linker. The authors found a significant influence of the reaction temperature on the morphology and size of the PEGylated nanostruc-tures. When rGO-PEG is formed at 80 °C, it is of nanometer size; at room temperature it has a needle-like shape with micro-metric dimensions. Importantly, the rGO-PEG matrix was

found to be highly soluble under physiological conditions with no aggregation even after 6 months. The cytotoxicity of both matrices as well as their photothermal properties to ablate cervical HeLa cancer cells and MDA-MB-231 human breast carcinoma cells were studied. There was no sign of acute toxicity of rGO-PEG for HeLa and MDA-MB-31 cancer cells over a wide concentration range. A complete destruction of the tumor cells could be achieved with a laser power of 6 W cm-2 and a concentration of 60 ^g mL-1 of rGO-PEG.

In recent years, some researchers have used hybrid composite GNMs loaded with various inorganic phases for PTT. This research area is determined by the possibility of better cell imaging and manipulation. '106d-g' A combination of monoclonal P-glycoprotein antibodies, folic acid and miR-122-loaded gold

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nanoparticles on graphene nanocomposites promoted drug-resistant HepG2 cell apoptosis with drug targeting and controlled release properties. The authors found that the properties of graphene and gold nanoparticles result in low toxicity. !118' Zhang et al. !113h' used a GO/BaGdF5/PEG composite in dual-modality magnetic resonance/computer tomography imaging and PTT of cancers. The histological and biochemical analysis data revealed no perceptible toxicity of GO/BaGdF5/PEG in mice after treatment. Fe3 O4- decorated graphene particles also show an excellent PTT activity and, therefore, they have been used in the treatment of human adenocarcinoma A549 cells using NIR-triggered cell death.!113i| The result was =98% cell death within the optimized parameters (Table 5).

4.2.2. Photodynamic Anticancer Therapy

As already mentioned, the other technology in progress for anticancer therapy is photodynamic treatment. PDT, sometimes called photo-chemotherapy, uses light-sensitive compounds selectively exposed to light, after which they become toxic to targeted malignant cells. The use of GNMs for PDT is promoted by the fact that they are nontoxic for healthy cells. Moreover, the anti-metastatic effect of GO, which is crucial in cancer treatment, has been evaluated recently. !119' The anti-metastatic activity of GO is realized most likely by disrupting electron transfer between iron-sulfur centers, which is due to its stronger ability to accept electrons compared to iron-sulfur clusters (determined through theoretical calculations). The decreased electron transfer chain activity caused a reduction in the production of ATP and subsequent impairment of F-actin cytoskeleton assembly, which is crucial for the migration and invasion of metastatic cancer cells. The inhibition of cancer cell metastasis by graphene and GO might provide new insights into specific cancer treatments, including laser-assisted cancer treatment. In one of the first works on this, it was shown that the combination of NIR light-triggered mild photothermal heating of graphene and photodynamic treatment using Ce6 delivered by GO-PEG remarkably enhances the PDT efficacy.!94' In the meantime, Rong et al. !113o] used PEGylated GO loaded with a photosensitizer molecule, 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-alpha (HPPH), for the photodynamic therapy of tumors. GO-PEG-HPPH induced more oxygen consumption within the tumor compared with that of free HPPH. The authors uncovered that covalently PEGylated nano-GO (nGO-PEG) induced no appreciable toxicity at the tested dose (20 mg kg-1) in mice over a period of 3 months, and could be gradually excreted via both renal and fecal excretion. Earlier, it was also noticed that nanocarriers can significantly increase the accumulation of photosensitizers (e.g., Ce6) in tumor cells and lead to a remarkable photodynamic efficiency. ! 113p'

Nurunnabi and colleagues !113q' applied graphene quantum dots (GQDs) for PDT. The authors revealed that, upon laser irradiation by NIR sensitizer (wavelength 670 nm), electrons of the GQDs start to vibrate and form electron clouds, thereby generating sufficient heat (>50 °C) to kill the cancer cells by thermal ablation. The generation of singlet oxygen also occurs due to irradiation, thus acting similarly to pheophorbide-A, a well-known photodynamic therapeutic agent. The GQDs killed

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MDAMB231 cancer cells (more than 70%) through both pho-todynamic and photothermal effects. No signs or symptoms of severe toxicity were observed after the administration of the GQDs, and a similar profile with the control groups was obtained.

Shi et al.!113r' prepared composite nanoparticles composed of GO, Ag, desoxirubicin, and peptide, which served as a powerful tumor diagnostic X-ray contrast agent, and simultaneously as a strong agent to combine the local specific chemotherapy with external PTT. Compared with free desoxirubicin in an in-vivo murine tumor model, these nanoparticles afforded a much higher antitumor efficacy without obvious toxic effects to normal organs owing to an 8.4-fold higher desoxirubicin uptake of tumor and 1.7-fold higher desoxirubicin released in tumor with the NIR laser than for the other tissues.

A multifunctional nanomaterial has also been developed for chemo-photothermal therapeutics based on the silica and gra-phene core/shell structure (SiO2 @GN) because of the ability of GN to convert light energy into heat.!113s' The as-synthesized SiO2@GN-serum nanoparticles demonstrated a high photothermal conversion efficiency, excellent biocompatibility and stability, as well as high storage and release capacities for the anticancer drug doxorubicin.

A novel phototheranostic agent based on sinoporphyrin sodium (DVDMS)-loaded PEGylated GO with an improved fluorescence property for enhanced optical imaging-guided PDT was also reported recently.!120' The fluorescence ofloaded DVDMS is drastically enhanced via intramolecular charge transfer. The GO-PEG vehicles can significantly increase the tumor accumulation efficiency of DVDMS and lead to an improved PDT efficacy as compared to DVDMS alone. The cancer theranostic capability of the as-prepared GO-PEG-DVDMS was carefully investigated both in vitro and in vivo. Most intriguingly, 100% in-vivo tumor elimination was achieved by intravenous injection of GO-PEG-DVDMS (2 mg kg-1 of DVDMS, 50 J) without tumor recurrence, loss of body weight, or other noticeable toxicity.

Another approach is synthesized hyaluronic acid-functional-ized GO (H-GO), used for the investigation of the controlled loading of mitoxantrone onto H-GO via n-n stacking interactions.!113' The results revealed that drug-loaded nanosheets with a high loading efficiency of 45 wt% exhibited pH-sensitive responses to tumor environments. Owing to receptor-mediated endocytosis, cellular uptake analysis of H-GO showed enhanced internalization. An in-vivo optical imaging test demonstrated that H-GO nanosheets could enhance the targeting ability and residence time at the tumor site. The in-vitro cytotoxicity study revealed that H-GO could stand as a biocompatible nanocarrier.

4.2.3. Miscellaneous Methods and Techniques of Laser Treatment

Recently, several publications have appeared dealing with pulsed laser treatment (PLT) in cancer therapy, where GNMs are applied. Taking into account the processes that occur during such a procedure, PLT is a really prospective technology. An example of this technology is the ultrafast reduction of GO nanoparticles with a femtosecond laser beam, which creates extensive microbubbling using a repetition rate of 80 MHz and

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pulse width of 100 fs (Table 5) .'113n' When microbubbles were produced, the effective laser power was reduced to less than half of what is needed when microbubbling is absent. It was also demonstrated that a strong photoluminescence from two-photon excitation can be induced in GO by an ultrafast pulsed laser treatment.'113"' The intensive microbubbling was induced by the irradiation at the laser power as low as 4 mW in the presence of GO, which caused instant cell damage.

Furthermore, PLT can be used for neural cell proliferation.'1124' It was discovered that PLT resulted in the self-organization of a radial neuronal network on the surface of rGO sheets. Similarly, under NIR laser stimulation, the graphene layers exhibited significant cell differentiation, including more elongations of the cells and a higher differentiation of neurons than glia.' 113j' The effects of the femtosecond laser irradiation on a cell viability and a cytotoxicity at 1064 and 532 nm for cells plated and grown on graphene and pure glass were assessed by Mthunzi et al. '113u' The optical transfection of CHO-K1 and mES cells was performed on graphene-coated versus plain glass substrates. A more efficient optical transfec-tion in mES at 1064 (82%) and 532 (25%) nm was obtained due to the presence of a graphene support as compared to the pristine glass.

Taratula et al. '122' reported a cancer-targeted nanomedicine platform for the imaging and treatment of un-resected ovarian cancer tumors by intraoperative multimodal phototherapy. To develop the required theranostic system, low-oxygen graphene nanosheets were chemically modified with polypropylenimine (PPI) dendrimers loaded with phthalocyanine as a photosensi-tizer. The developed nanoplatform was conjugated with PEG and with the luteinizing hormone-releasing hormone pep-tide to improve biocompatibility and assure a tumor-targeted delivery, respectively. Notably, the low-power NIR irradiation of a single wavelength was used for both heat generation by the graphene nanosheets (PTT), and for reactive oxygen species production by phthalocyanine (PDT). The combinatorial phototherapy resulted in an enhanced destruction of ovarian cancer cells, with a killing efficacy of 90-95% at low phthalocyanine and low-oxygen graphene dosages, presumably conferring cyto-toxicity through the synergistic effects of generated reactive oxygen species and mild hyperthermia.

PTT has also been applied for a controlled gene-delivery procedure employing GO and rGO nanosheets. '113i,j' The authors claim that mild photothermal heating increased the cell membrane permeability without any significant damage to the cells. The PEG-PEI-rGO nanocomposite demonstrated enhanced gene transfection efficiency upon the NIR irradiation, which was attributed to accelerated endosomal escape of polyplexes augmented by the locally induced heat. Another approach to PTT application was the prevention of amyloid aggregation using ThS-modified GO with NIR laser irradiation. '84' As mentioned in Section 3.2, GO was covalently linked to ThS, which can selectively attach to Ap aggregates, forming the conjugated GO-ThS-Ap (Figure 13). The strong NIR optical absorption ability of nano-GO was then utilized to generate local heat, which causes dissociation of the Ap fibrils following low-power NIR laser irradiation. Similarly, nanocomposites combining GO with AuNPs have been reported and their application to modulate amyloid peptide aggregation was demonstrated by Li

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et al.'83' These results are promising for the photothermal treatment of Alzheimer's disease.

Magnetic r-GO functionalized with glutaraldehyde (MrGOGA) particles was presented as a rapid (within 10 min) and efficient (99% killing efficiency) photothermal agent toward both gram-positive S. aureus and gram-negative E. coli bacteria under NIR laser irradiation.'6i' The MrGOGA not only possesses a good superparamagnetic property (Ms = =26 emu g-1) and optical absorption from the UV to the NIR regions but also exhibits an efficient capturing capacity owing to the cross-linking ability of glutaraldehyde and a high killing efficiency of the photothermal ability of graphene toward both bacterial strains. The photothermal treatment of GO for antibacterial, antifungal, and wound infection treatments using an NIR laser (Nd-YAG (X = 1064 nm)) were reported.'123' Various pathogenic bacteria (Pseudomonas aeruginosa, Staphylococcus aureus' and fungi (Saccharomyces cer-evisiae, Candida utilis) were investigated. The cytotoxicity was measured using proteomic analysis. The laser-mediated surface activation of GO offers a high efficiency of antifungal and antibacterial activity.

The use of GNMs in laser-assisted therapy is based on previous experience in this field working with a number of different nanomaterials: nanoparticles of gold, silver, and silica; carbon nanotubes; etc. '124' Experience gained in this field has served as the basis for the successful application of GNMs. It is generally recognized that, compared to gold nanoparticles and carbon nanotubes, the shape and spatial parameters of GNMs are less controllable. Nevertheless, many authors agree'86'113b'e,m'125-127' that, due to better dispersivity and a smaller size, GNMs should rival other nanomaterials in the future. An extremely high photoresponsiveness of GNMs is

also emphasized.' 86,94,113b,e ' This, in addition to a large surface

area, a low toxicity, and an extremely low cost, makes GNMs promising candidates for laser-assisted therapy and treatment.

In summary, a biomedical use of GNMs in conjunction with laser therapy is gaining a wider scale; the number of publications in this field is increasing. The main focus of research is photothermal cancer therapy, but additional treatment protocols are also being developed. GNMs perform a multifunctional role: after entering the cell they absorb laser radiation and release attached functional moieties, thus killing the malignant tumor. Many authors point to an excellent biocompatibility and a low toxicity of GNMs to the healthy cells. Moreover, laser radiation can be dosed accurately, which allows healthy cells to remain intact. We can expect that, in the near future, research will concentrate on a search for the most suitable GNMs as well as the use of GNM/laser treatments for a wider range of bioapplications.

5. Conclusion and Perspectives

The laser-induced reduction and functionalization of GO have gained much attention over the last five years. The mechanism of GO modification is complex, and it is explained in various ways the literature. However, this overview lets us conclude that it mostly depends on the laser parameters. It is complicated to distinguish the best recipe for GO modification, but

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mainly the local temperature plays the most important role, thus, the modification mechanism depends on the local temperature, and the mechanism changes from the photochemical reduction to the photothermal one. Laser-induced GO modification can be performed in both suspensions and films in the presence of functional moieties. Two major ways are used by researchers: i) laser-induced reduction of GO and its modification with a required compound simultaneously or in two steps, and ii) chemical modification of GO followed by laser irradiation to reduce GO. The first way is prevalent in the literature. In some cases, the GO physical properties are modified without any chemical modification, just with laser irradiation.

A tentative classification of the laser parameter choices for different modifications gives an ambiguous view. It can be stated that the excimer laser is usually employed to change the GO physical properties and to dope GO with inorganic atoms, but for modification with organic molecules and nanoparti-cles, NIR lasers are employed more often. The laser fluence and average power density frequently depend on the chosen laser.

The main application areas of laser-irradiated and/or chemically modified GO are directed to the construction of electrical devices, supercapacitors, various laser devices including mode-locking, and the biomedical treatment of some diseases, namely cancer. However, some applications have not been widely explored yet and, therefore, it is a niche for the further investigation of laser irradiation and its application in one of the hottest topics in photonics and electronics: the development of THz technology. Flat optics is also one of the most promising applications in optics and the formation of prospective, ultracompact THz optics and active device integration.

Graphene nanomaterials have been extensively studied as highly effective components for laser-based therapy in a number of biomedical applications. The unique physicochemical properties of GNMs allow efficient loading via both the physical absorption and chemical conjugation of bioactive compounds. Various strategies have been developed up to date. Still, more studies are required to find out the correlations between the physicochemical characteristics or structural modifications of graphenes and their biological impact. Consequently, the next step would be the introduction of laser-based treatments from laboratories to clinical trials.

As previously mentioned (see Section 4), the laser modification of GO also opens up great potential for the development of humidity sensors.|62b'96' By choosing different sensor substrates, different humidity sensors may be obtained, e.g., a polymeric substrate can be employed in impedance sensors |62b| and quartz crystal substrates in microgravimetric sensors.11961 For instance, the femtosecond laser modification of GO fibers for humidity-driven actuators was reported by Cheng et al. 1 128] Very recently, Han et al. 1 129] reported the controllable photoreduction of GO papers for smart manipulators. Anisotropic GO/r-GO bilayer paper was directly prepared by controlling the photore-duction gradient. That is a good strategy for smart, humidity-driven graphene actuators, and those that mimic the cilia of a respiratory tract and the tendrils of a climber plant have been developed for object transport. |129b| Laser-reduced GO may be employed as a smart humidity sensor on such simple materials as textiles, which are cheap and easy to use. Along with water

molecules, there is a space for sensors and actuators for other gaseous molecules, e.g., hydrazine, !130' CO, SO2, and NO2.

Acknowledgements

This research was partially funded by the European Social Fund under the Global Grant measure, Project No. VP1-3.1-SMM-07-K-01-124.

Received: August 18, 2015 Revised: September 23, 2015 Published online: October, 26 2015

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