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Karbala International Journal of Modern Science xx (2015) 1 — 11
http://www.journals.elsevier.com/karbala-international-journal-of-modern-science/
Studies on interaction of ribonucleotides with zinc ferrite nanoparticles using spectroscopic and microscopic techniques
Md. Asif Iqubal, Rachana Sharma, Kamaluddin*
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, UK, India Received 30 May 2015; revised 22 June 2015; accepted 30 June 2015
Abstract
Interaction of ribonucleotides with zinc ferrite nanoparticles (~15 nm) prepared by the sol—gel method was studied at physiological pH (~7.0). Ultra-violet (UV—Vis), Fourier transform infrared (FT-IR), Raman spectroscopy and field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) were employed to investigate the interaction of ribonucleotides with zinc ferrite surface. Langmuir and Freundlich adsorption models were used to describe the equilibrium isotherms in the concentration range of 1.0 x 10~4M—4.0 x 10 4 M of ribonucleotides. The maximum adsorption capacity determined for 5'-GMP, 5'-AMP, 5'-UMP, 5'-CMP was 22.37 mg/g, 17.42 mg/g, 16.03 mg/g and 14.03 mg/g, respectively. Langmuir model was found to show the best fit for experimental data. Adsorption kinetics were studied by pseudo-first order and pseudo-second order kinetic models, and the adsorption process was best described by the pseudo-second order kinetic model. FE-SEM images clearly showed that ribonucleotide adheres onto the zinc ferrite nanoparticles surface. AFM analysis demonstrated that the root mean square roughness (Rms, Sq) and average roughness (Sa) increased from 0.98 nm to 1.80 and 0.67 nm—1.38 nm, respectively following exposure to ribonucleotide. FT-IR spectroscopy revealed that the zinc ferrite nanoparticles interact strongly with the phosphate, carbonyl and amino groups of ribonucleotides. Raman spectra of 5'-AMP-zinc ferrite adduct showed the participation of amino and a phosphate group with zinc ferrite nanoparticles surface.
© 2015 The Authors. Published and hosting by Elsevier B.V. on behalf of University of Kerbala. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Ribonucleotides; Adsorption; AFM; FT-IR; FE-SEM
1. Introduction
Adsorption of nucleic acids and their components on mineral matrices are especially interesting in respect of several scientific issues. After the death of organism and subsequent cell lysis a significant
* Corresponding author. Tel.: +91 1332 285796; fax: +91 1332 285146.
E-mail addresses: kamalfcy@gmail.com, kamalfcy@iitr.ac.in ( Kamaluddin).
Peer review under responsibility of University of Kerbala.
amount of nucleic acids is released in natural environment, mainly in soil [1—3]. The released nucleic acids are adsorbed onto mineral surfaces and preserved in the natural environment. Mineral surface protects them from UV-mediated or enzymatic degradation [4—8]. This preserved nucleic acid further can be taken up by the microorganisms for horizontal gene transfer or transfection [9—11]. Several important biomedical experiments involving nucleic acid—mineral interaction have been carried out in recent past [12—17]. Apart from this the adsorption of nucleic acids and
http://dx.doi.org/10.1016/j.kijoms.2015.06.001
2405-609X/© 2015 The Authors. Published and hosting by Elsevier B.V. on behalf of University of Kerbala. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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their components on mineral surfaces may have important rule on the origin of life issue, where mineral surfaces concentrate the nucleic acid components from dilute oceans [18—21]. Several minerals such as olivine, pyrite, calcite, hematite and rutile [22], zinc oxide [23], alumina [24,25], manganese oxides [26], metal octacyanomolybdates [27], natural zeolite [28], clays [29—34] have been employed to study the adsorption of nucleic acid and their components.
Spinel ferrites a group of minerals having the general formula MnFe2nO4 where M may be Zn, Ni, Co, Cu, etc., are a special class of compounds having high magnetic properties. Ferrite nanoparticles are particularly useful in biomedicine for nucleic acid separation, gene detection, as contrast agent and biosensors [35,36]. Zinc ferrite mineral (ZnFe2O4) has attracted an increasing attention due to possessing excellent electrical, optical and magnetic properties. This important material can be used in areas devoted to hyperthermia [37], MRI contrast agent [38], gas sensors [39], catalyst [40], etc. Although several minerals have been tested for the adsorption of nucleic acid, only a few literature are available involving metal ferrite—nucleic acid interactions. Magnetite, cobalt ferrite and silica-magnetite composite have been used as an effective adsorbent for the isolation of genomic DNA molecules [41,42]. Pershina et al., (2009) studied the interaction between DNA and cobalt nano ferrite particles using FTIR spectroscopy [43]. Recent work involving adsorption of DNA on Fe3O4 nanoparticles has been carried out by Ghaemi and Absalan (2014) [44]. To the best of our knowledge spinel metal ferrites has not been tested to study the adsorption of nucleo-tides (monomeric unit of nucleic acid) that prompted us to investigate the interaction between nucleotides and zinc ferrite nanoparticles.
In the present work we report the interaction between ribonucleotides (5'-AMP, 5'-GMP, 5'-UMP and 5'-CMP) and zinc ferrite nanoparticles under physiological pH (~7.0) using spectroscopic and microscopic techniques. This study involving magnetic nano-particle—nucleic acid interaction may be of high importance because of the influence of nano materials on biosystems and further the mechanism of interaction can be used to develop bio-nano composites.
2. Experimental
2.1. Materials
Zinc(II) nitrate (Zn(NO3)2.6H2O) was purchased from E. Merck, Citric acid (C6H8O7.H2O) from
RANKEM, Ethylene glycol (C2H6O2) from Sisco Research Laboratory, India, Iron(III) nitrate (Fe(NO3)3.9H2O) and disodium salts of ribonucleo-tides were purchased from Sigma—Aldrich. The reagents were used without further purification. Millipore water was used throughout the studies. Graphs are plotted and smoothened in Origin Pro 8.1 software.
2.2. Apparatus and measurements
X-ray powder diffraction analysis of samples was carried out using a Brucker AXS D8 advanced X-ray diffractometer (Cu-Ka, 1 = 0.1540 nm). Ultraviolet—visible (UV—Vis) absorption spectra of the samples were recorded in the wavelength range of 200—800 nm with a resolution of 0.1 nm in a quartz cuvette by using a Shimadzu UV-16001 spectropho-tometer. The infrared spectra of samples were recorded using a KBr pressed disk technique by a Fourier transform infrared spectrometer (Perkin—Elmer), operating in the range of 400—4000 cm-1. About 10 mg of sample with 200 mg of KBr were ground in an agate mortar until a homogenous mixture was obtained. Then this mixture was hydraulic pressed to form disc pellets. Spectra were recorded with a scanning speed of 2 mm/s with resolution of 2 cm-1. Raman spectra of the samples were recorded using a RENISHAW in Via Raman spectrophotometer with 4 cm-1 resolution. An argon ion laser operating at 514.5 nm was used as the excitation source having laser power of 1 mW an and acquisition time of 30 s. The zeta potential of the zinc ferrites nanoparticles was measured at different pH (i.e., pH = 3, 5, 7, 9, 11) using Malvern Zeta Sizer Nano ZS90. The pH of the solutions was adjusted by adding dilute hydrochloric acid (0.01 M) or sodium hydroxide (0.01 M). The experiments were carried out by dispersing 5 mg of zinc ferrite in 50 mL of 10 mM NaCl solution and sonicated for 10 min. Surface morphological images of the samples were carried out using a FEI Quanta 200F scanning electron microscope operating at 20 kV. The sample is first made conductive for current by smearing the sample on a conducting carbon tape and then gold sputtered for 100 s in a high vacuum chamber. Elemental compositions of the samples were also analyzed using an energy dispersive X-ray analysis (EDX) facility attached with the FE-SEM. Surface topography of the samples was analyzed by recording 2D images on a NTEGRE (NT-MDT) atomic force microscope (AFM) equipped with NOVA software. The textural characteristics of the samples were
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determined from N2 adsorption isotherms at 77 K using a Nova 2200e (Quantachrome) instrument. Prior to analysis, samples were kept at 150 °C for 2 h. Helium gas was used for dead measurements. The BET (Brunauer, Emmet and Teller) equation was used to calculate the specific surface area of porous solids from pressure measurements [45].
P/V(Po - P) = 1/VmC + C - 1/Vm (P/Po)
where P/Po is relative pressure, V is the adsorbed gas quantity, Vm is the monolayer adsorbed gas quantity and C is BET constant. A linear plot of P/V(Po—P) vs. P/Po was used to find out the value of surface area of samples from the slope and intercept. The classical pore size model developed by Barret, Joyner and Halenda (BJH) in 1951, which is based on the Kelvin equation and corrected for multilayer adsorption, is most widely used for calculations of the pore size distribution over the mesopore and part of the macropore range [46]. The Nova 2200e (quantachrome) instrument allows to measure the BJH pore size distribution data automatically.
The magnetic hysteresis loop of zinc ferrite was obtained by using a vibrating-sample magnetometer (PAR 155). Approximately 50 mg powder samples were put in a diamagnetic plastic straw and packed into a minimal volume for magnetic measurements.
2.3. Synthesis of zinc ferrite nanoparticles
The nanosized zinc ferrite was prepared according to a modified version of a previously reported procedure [47,48]. In a typical synthesis of zinc ferrite stoichiometric amounts of Zn(NO3)2.6H2O (0.005 mol, 1.48 g) and Fe(NO3)3.9H2O (0.01 mol, 4.04 g) were dissolved in a minimum amount of Millipore water with constant stirring at 80—90 °C. After complete dissolution of the metal salts, citric acid (0.015 mol, 3.15 g) was added, followed by 10 mL of ethylene glycol to the solution. The solution was stirred until gel formation. The obtained gel was subjected to thermal treatment at 400 °C for 2 h in a muffle furnace.
2.4. Adsorption studies
was measured before and after the adsorption reaction and it was found to be stable within 0.1 units over the time of adsorption. In the adsorption experiment 25 mg of zinc ferrite was added in 5 mL of ribonucleotide solution (ranging from 1.0 x 10-4 M-4.0 x 10-4 M) in a 25 mL glass tube followed by shaking at room temperature for 2 min on a Vortex shaker (Spinix). After 24 h the samples were centrifuged at 4000 rpm for 10 min and the supernatant was analyzed for ribonucleotide concentration by UV—Visible spectrophotometry at their characteristic values of 1max which was 259 nm for 5'-AMP, 252 nm for 5'-GMP, 272 nm for 5'-CMP and 262 nm for 5'-UMP. Each adsorption experiment was performed in triplicate. The adsorbed amount of ribonucleotides at equilibrium, Xe (mg/g) was calculated by the following expression:
Xe = (Ci - Ceq) x M x V x 1000 / m mg where,
Xe = amount (mg) of adsorbate adsorbed on 1 g of adsorbent
Ci and Ceq = initial and equilibrium concentration of ribonucleotide solution, respectively. V = volume of the adsorbate solution M = molecular weight of adsorbate m = amount (mg) of adsorbent used
The equilibrium concentration of ribose nucleotide and the amount adsorbed were used to obtain the adsorption isotherms. The solid residue was dried for 24 h at 30 °C and analyzed with FT-IR, FE-SEM, AFM, XRD, and Raman spectroscopy to know the nature of adsorption.
2.5. Kinetic studies
Kinetic studies were performed by placing 25 mg of adsorbent in a glass tube containing a fixed concentration (3.0 x 10-4 M) of ribonucleotides at pH 7 for 33 h. At different time intervals concentration of ri-bonucleotides was measured by UV—Visible spectro-photometry. Each kinetic study was performed for three times.
All glasswares were cleaned in an aqueous HCl bath and rinsed with Millipore water prior to use. Stock solutions of the ribonucleotides were prepared from crystalline powder as supplied and the pH was adjusted to the desired value (pH = 7.0) by adding 0.01 mol dm-3 HCl or 0.01 mol dm-3 NaOH. The pH
3. Results and discussions
3.1. Characterization of zinc ferrite nanoparticles
Figure S1 (in Supplementary Section) shows the XRD pattern of the zinc ferrite after calcination at
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temperature 400 °C. The experimental XRD data agree closely with the standard values given in the JCPDS data card (01-1108) confirming the single cubic phase spinel formation of zinc ferrite. The average crystallite size of zinc ferrite was found to be 14.89 nm which has been calculated using the Debye—Scherrer equation as given below:
D =(0.89 x A)/(b x cosd)
where D is the crystallite size, l is the wavelength of X-ray radiation (0.1540 nm for Cu-Ka radiation), b is the full width at half maximum and 0 is the diffraction angle. The values of the lattice parameter 'a' was calculated from the most prominent peak in the XRD of zinc ferrite by using the formula a = d^Jh2 + k2 + l2, where h, k and l are the Miller indices of the crystal plane (311) and 'd' is the interplanar spacing (0.2544 nm) obtained by the XRD pattern. The X-ray density of zinc ferrite was calculated using the equation,
px = 8M/Na3
where M = the molecular weight of zinc ferrite (241.07 g/mol); Na is the Avogadro constant; 'a' is the lattice parameter. The obtained lattice parameter (8.437 A) and X-ray density (5.345 g/cm3) are in good agreement with the reported value, i.e. 8.43 A, and 5.346 g/cm3, respectively [49]. TEM was used to find out the morphology and particle size of synthesized zinc ferrite particles. TEM (Fig. 1A) confirms particle size and was used to find out to the morphology of the particles and the size distribution.
FTIR spectra (Figure S2) of zinc ferrite showed two fundamental absorption peaks at 546 cm-1 and 434 cm-1. The existence of these characteristic peaks confirmed the presence of spinel type structure. The stronger absorption peak (546 cm-1) was due to vibration mode between tetrahedral metal ion and oxygen complex while the weaker absorption peak (434 cm-1) was attributed to stretching vibration between octahedral metal ion and oxygen complex [50]. In the Raman spectrum (Figure S3) of zinc ferrite peaks at 217 cm-1, 260 cm-1, 360 cm-1, 469 cm-1 were observed as characteristic frequency modes of the octahedral sites (FeO6) whereas peak at 658 cm-1 is attributed to the motion of oxygen atoms in tetrahedral (ZnO4) environment.
Magnetic characteristics of a material are a crucial property which is determined by recording magnetization curve at room temperature using a Vibrating Sample Magnetometer (PAR 155 VSM). The hysteresis loops of zinc ferrite illustrated the magnetic
Fig. 1. (A) TEM image and corresponding particle size distribution curve in inset; (B) Zeta potential curve of zinc ferrite nanoparticles at different pH (i.e., pH = 3, 5, 7, 9, 11) using 10 mM NaCl as electrolyte.
behavior. The parameters such as saturation magnetization (Ms) and coercivity (Hc) were deduced from the magnetic curve (Figure S4). The saturation magnetization and coercivity values were observed to be 2.1 emu/g and 8.0 Oe, respectively for zinc ferrite which is a typical behavior of soft ferrites.
3.2. Surface properties of zinc ferrite
The Zeta potential indicates the global charge of the particles, and is calculated from the electrophoretic mobility. Fig. 1B shows the effect of pH on the zeta potential of zinc ferrite nanoparticles. The point of zero charge (pzc) describes the definite pH value at which the electrical charge density on a particular surface is zero. Point of zero charge value of zinc ferrite was found to be 9.3 as can be seen from Fig. 1(B). So the surface charge of zinc ferrite is positive at pH values lower than pzc, neutral at pzc, and negative at pH values higher than it. As indicated by the pzc of zinc ferrite, the net surface charge at pH 7 is positive, which
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Equilibrium Concentration (MX10*) /A/iv-in4\
Fig. 2. (A) Adsorption isotherms and (B) Langmuir plots of ribonucleotides at pH~7.0 on zinc ferrite; each point is the mean ± S.D., n = 3.
is beneficial for adsorbing the negatively charged ribonucleotides.
3.3. Adsorption data modeling
The adsorption isotherm models are usually used to investigate the interaction between the absorbent and the adsorbate when the adsorption process reaches equilibrium. Adsorption isotherms describe how the adsorbates interact with the adsorbent at constant temperature. In this work, the adsorption isotherm studies were conducted by varying the initial concentration of ribonucleotides from 1.0 x 10-4 M-4.0 x 10-4 M while the adsorbent mass (25 mg) was kept constant at pH 7.0 for 24 h. The equilibrium isotherms for the adsorption of ribonu-cleotides by zinc ferrite are shown in Fig. 2A. With the increasing initial concentration of ribonucleotides, the equilibrium adsorption capacity for ribonucleotides increases gradually until the equilibrium condition is reached. The percent binding for all the four ribonu-cleotides corresponding to the saturation point of adsorption isotherm was calculated and listed in Table 1.
The percent binding of purine nucleotides (50-AMP, 5'-GMP) was found higher than the pyrimidine nu-cleotides (50-CMP, 50-UMP) probably due to the presence of larger amounts of p electrons in the doublering purines as compared to the single-ring pyrimi-dines. The Langmuir and Freundlich isotherm models are often used to describe the equilibrium adsorption isotherms. The Langmuir model suggests that adsorption occurs on a homogeneous surface with no interaction between adsorbed molecules [51]. The linearized form of Langmuir model is given as:
CeqjXe Ceq f Xm + 1/XmkL
Xe = equilibrium adsorption capacity (mg/g) Xm = maximum adsorption capacity (mg/g) KL = Langmuir adsorption constant (L/mol)
A straight line plot (Fig. 2B) was obtained upon plotting Ceq/Xe vs. Ceq and thus the Langmuir adsorption isotherm was found to be obeyed. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant
Table 1
Percent binding and adsorption isotherms constants of ribonucleotides on the surface of zinc ferrite.
Ribonucleotide % binding' Langmuir Freundlich
Xm (mg/g) KL x 104 (L/mol) Rl R2 Kf (mg/g) n R2
5'-AMP 64.71 ± 2.37 17.42 6.52 0.048 0.99 191.52 3.55 0.95
5'-GMP 74.43 ± 3.38 22.37 5.32 0.058 0.99 547.30 2.75 0.95
5'-UMP 58.65 ± 3.51 16.03 3.63 0.084 0.99 289.45 2.87 0.94
5'-CMP 58.58 ± 3.31 14.30 4.01 0.076 0.99 266.93 2.88 0.98
a The value of percent binding represents the mean ± S.D. (n = 3).
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separation factor RL which is given by the following equation
Rl = 1/(1 + KLCo)
where Co is the initial concentration of ribonucleotides solution. The values of RL indicate the type of isotherm to be reversible (RL = 0), favorable (0 ' RL ' 1), linear (Rl = 1) or unfavorable (RL ' 1). In the present case for all the ribonucleotide studied the RL values were in the range of 0—1 suggested the favorable adsorption (Table 1). Freundlich equilibrium isotherm is an empirical equation which is used for the description of multilayer adsorption with interaction between adsorbed molecules [52].
lnXe = 1 /n(lnCeq} + lnKF
where KF and n are the Freundlich constants related to the adsorption capacity and intensity, respectively. The values of n are 3.55, 2.75, 2.87 and 2.88 from Freundlich plots (in Supplementary Section Figure S5) for 5'-AMP, 5'-GMP, 5'-UMP and 5'-CMP, respectively (Table 1). It is generally stated that values of n in the range 2—10 represent better, 1—2 moderately difficult, and less than 1 poor adsorption, respectively [53]. In our case values of n were found to lie in the range of 2—4 suggested the favorable adsorption of ribonucleotides on zinc ferrite surface. Both models describe the favorable adsorption of ribonucleotides but according to the correlation coefficient (R2) as given in Table 1, Langmuir model was found better than that of Freundlich model. The Langmuir and Freundlich adsorption constants and correlation coefficient (R2) values are summarized in Table 1.
3.4. Kinetics models
The adsorption behavior of the ribonucleotides toward zinc ferrite at different time intervals is shown in Fig. 3A. The results indicate that a fast adsorption process occurs during the first 12 h and reaches equilibrium within 24 h. The pseudo-first and pseudosecond order models were used to examine the kinetics of the adsorption process of the ribonucleotides. The pseudo-first-order kinetic model [54] is expressed by Eq.
ln(Xe - Xt) = lnXe - k11
where Xe and Xt are the amount adsorbed (mg/g) at equilibrium and at time t(h), respectively. k1 is the rate constant (h-1) of pseudo-first-order equation. The
linear plot of ln(Xe—Xt) vs. t is given in the supplementary section (Figure S6).
The linear form of the pseudo-second order rate equation [55] can be represented as,
t/Xt = 1/(k*X?) + t/Xe
where k2 denotes the rate constant in g/(mg h) for pseudo-second order kinetic model. The values of k2 and Xe were determined from the slope and intercept of the pseudo-second order kinetics plot (Fig. 3B). The rate constant and the correlation coefficient of each model are listed in Table 2. To evaluate the suitability of different models, the correlation coefficient R2 was introduced. The comparison of the two models based on R2, the pseudo-second-order kinetic model fits the experimental data more accurately than pseudo first order. This was mainly supposed to be due to the actual heterogeneous distribution of ribonucleotides on nano-particles surface. Pseudo second order model implies that chemisorption took place during the adsorption process [54].
3.5. Spectroscopic investigation of ribonucleotides upon adsorption
3.5.1. Fourier transform infrared spectroscopy (FT-IR)
A 25 mg of zinc ferrite was placed in 25 mL glass tube containing 4.0 x 10~4 M of ribonucleotides solution. The pH of the solution was adjusted to ~7 with 0.01 M NaOH. The tube contents were left for 24 h at room temperature. The supernatant was decanted and residue obtained was dried at 30 °C. In order to analyze the interaction of ribonucleotides with zinc ferrite nanoparticles FT-IR spectra of ribonucleotides and ri-bonucleotides adduct adsorbed on zinc ferrite were recorded under identical experimental conditions. Under the experimental condition (pH = 7.0) the surface of zinc ferrite nanoparticles is positively charged as indicated by its determined pzc value (9.3). At the experimental pH ribonucleotides are likely to be present in its dianionic form as suggested by the pka2 values of 5'-AMP (6.1), 5'-GMP (6.1), 5'-UMP (6.4) and 5'-CMP (6.3). The nature of the interaction between ribonucleotides and zinc ferrite is mainly electrostatic involving positively charged surface of zinc ferrite and negatively charged atoms in the phosphate groups, via the p electrons of the aromatic ring, and/or via the lone pair of electrons on the N and O atoms. Such type of electrostatic interaction was further confirmed by infrared spectroscopic results as discussed below.
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Fig. 3. (A) Time-dependent adsorption of ribonucleotides on zinc ferrite cleotides on zinc ferrite; each point is the mean ± S.D., n = 3.
The binding of the ribonucleotides on zinc ferrite was observed through 'C=O, NH2 and PO2- groups as indicated by a shift in frequencies due to these groups along with a change in the nature of their bands. Small shifting of pyrimidine and imidazole ring vibrational frequencies in ribonucleotides was also observed upon interaction with zinc ferrite. The characteristic peaks of ribonucleotides before and after adsorption are summarized in Table S1 (see Supplementary Section). Fig. 4A shows the FT-IR spectra of 5'-AMP and 5'-AMP adsorbed on zinc ferrite at pH 7. The infrared spectrum of 5'-AMP shows all of its characteristic peaks as reported in literature [56]. Upon interaction with zinc ferrite, the uNH2 peak (1645 cm-1) is shifted to 1656 cm-1, which showed the involvement of the NH2 groups in adsorption process. The strong peaks of U(PO2-) antisymmetric (1032 cm-1) and u(PO|-) symmetric (979 cm-1) shifted to the frequency 1078 cm-1 and 991 cm-1, strongly suggest that interaction is taking place through the phosphate moiety of 5'-AMP with zinc ferrite. The fact that there was no significant change in the infrared frequencies (546 cm-1, 434 cm-1) of zinc ferrite upon adsorption suggested that the ribonucleotides do not affect the
Table 2
Comparison of rate constants calculated based on first-order and second-order kinetic models.
Kinetic model 5'-AMP 5'-GMP 5'-UMP 5'-CMP
Pseudo first order
R2 0.97 0.99 0.98 0.98
k (h"1) 0.1786 0.1468 0.9408 0.2202
Xe (mg/g) 15.46 18.52 12.96 13.20
Pseudo second order
R2 0.99 0.99 0.99 0.99
k2 g/(mg h) 0.0264 0.03026 0.0444 0.0378
Xe (mg/g) 17.09 19.90 14.47 15.50
at pH 7 for 33 h; (B) Pseudo-second order kinetic model of ribonu-
spinel lattice of the zinc ferrite. Beside phosphate group considerable shifting in 'C=O frequencies from 1680 cm-1, 1681 cm-1 to 1694 cm-1, 1692 cm-1 was observed in case of both 5'-GMP and 5'-UMP, respectively, while significant change in frequency of NH2 group was noted in 5'-CMP from 1688 cm-1 to 1674 cm-1. The FT-IR spectra of the ribonucleotides (5'-GMP, 5'-UMP and 5'-CMP) upon adsorption were given in the supplementary section (Figure S7).
3.6. Raman spectroscopy
Raman spectra of 5 -AMP and interacting sample are given in Fig. 5(B). The band at 1635 cm-1 in the Raman spectrum of 5'-AMP is assigned as a scissoring mode of NH2 [57], shifted towards higher frequencies (1643 cm-1) on the adsorption that indicates interaction of amino group with the nanoparticle surface. The shifts of the band corresponding to symmetric [Us(PO2-)] and asymmetric [Uas(PO2-)] oscillation of phosphate group from 988, 1065 cm-1 to 1017, 1072 cm-1, respectively suggest the participation of PO23- group in interaction with the nanoparticle surface. The change in intensity ratios of the symmetric and asymmetric oscillation (us/uas) of PO2- group from 4.35 to 1.17 for 5'-AMP and interacting sample respectively, confirms the involvement of phosphate group in the formation of bionanocomposite [58]. A small shifting in Raman modes involving vibrations of both pyrimidine and imidazole ring is also observed.
3.7. Field emission scanning electron microscopy (FE-SEM)
FE-SEM was employed to observe the attachment of ribonucleotides on zinc ferrite surface. The SEM
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Wavenumber (cm"') Raman shift (cm' )
Fig. 4. (A) FT-IR; (B) Raman spectra of 5'-AMP and 5'-AMP adsorbed on zinc ferrite.
images of 50-AMP, zinc ferrite nanoparticles and adsorption adduct were given in Fig. 5. The surface in 5'-AMP consists of some sort of curl-slice morphology whereas zinc ferrite has spherical like morphology. After the adsorption of 5'-AMP surface of adduct becomes blunt. It is obvious from the SEM images that 5'-AMP molecule appeared to adhere on the surface of zinc ferrite. In the EDX spectrum
(Fig. 5D) presence of sodium, phosphorous, carbon and nitrogen in adsorption adduct in addition to the elements present in zinc ferrite also confirm the adsorption of 5'-AMP on the surface of zinc ferrite. Further the SEM EDX mapping image as shown in the supporting information clearly demonstrated the presence of C, N, Na and P in adsorption adduct (Figure S8).
Fig. 5. FE-SEM images of (A) 5'-AMP; (B) zinc ferrite; (C) Adsorption adduct and (D) EDX spectrum of adsorption adduct.
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3.8. Atomic force microscopy (AFM)
Atomic force microscopy was employed to investigate and visualize the change in the surface topography resulting from adsorption of ribonucleotides on zinc ferrite nanoparticle surface. AFM images of zinc ferrite before and after adsorption show significant morphological changes (Fig. 6). The roughness of the film surface is generally characterized by the root mean square roughness (Rms, Sq) and average roughness (Sa) values. The Rms roughness (Sq) and average roughness (Sa) as measured by AFM are 0.98 nm, 0.67 nm, respectively, for zinc ferrite, and values were increased to 1.80 and 1.38 nm after adsorption of 5'-AMP on zinc ferrite. The increased values of Sq and Sa indicate the adsorption of 5'-AMP on zinc ferrite nanoparticle surface.
3.9. Textural characteristics
Figure (S9) shows BET adsorption/desorption isotherms of zinc ferrite before and after adsorption of 5'-AMP. According to the IUPAC classification both the isotherms are mainly related to the type IV isotherm, which is a characteristic of mesoporous material. The BET surface areas of zinc ferrite before and after
adsorption are 28.547 m2 g-1 and 21.457 m2 g-1 and, respectively. The considerable decrease in the BET surface area of zinc ferrite suggests that a considerable amount of 5'-AMP has been adsorbed to the zinc ferrite nanoparticle surface. The values of the pore volume for zinc ferrite before and after adsorption are
0.0169 cc g~ and 0.0147 cc g~ , respectively, as determined using BJH pore size distribution method. The significant decrease in the pore volume of zinc ferrite further indicates the adsorption of ribonucleo-tide onto the zinc ferrite nanoparticle surface.
4. Conclusion
In this study, we have explored the interactions of ribonucleotides with zinc ferrite nanoparticles surface using spectroscopic and microscopic techniques. The percent binding of ribonucleotides on zinc ferrite surface was found in the range of 58%—74%. On the basis of FT-IR studies of ribonucleotides-zinc ferrite adduct, we proposed that amino, carbonyl and the phosphate group of ribonucleotides interact with zinc ferrite surface. The attachment of ribonucleotides on zinc ferrite surface was obvious from FE-SEM images of zinc ferrite before and after adsorption. AFM analysis revealed that surface roughness was increased after
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adsorption of ribonucleotide on nanoparticle surface. Our results of this study would provide valuable information in a wide range of topics including origin of life, formulation of new material for industrial, biomedical, agricultural and environmental applications.
Acknowledgment
This work was financially supported by the Indian Space Research Organization (ISRO), Bangalore, grant no: ISRO/RES/2/373/11-12 dated Dec. 5, 2011.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.kijoms.2015.06.001.
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