Scholarly article on topic 'Near Infrared Emission of Ag2S Quantum Dots and their Fluorescence Quenched by Gold Nanoparticles'

Near Infrared Emission of Ag2S Quantum Dots and their Fluorescence Quenched by Gold Nanoparticles Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Xiulan Wu, Lei Liao, Weilin Du, Aimiao Qin

Abstract The near-infrared emitting Ag2S quantum dots (QDs) coated with L-cysteine were prepared by one-pot aqueous synthesis at room temperature. Several characterization methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), fluorescence spectrophotometer (PL) and ultraviolet -visible spectrophotometer (UV-vis) were used to characterize the structure, morphology and optical properties of the samples. The effects of gold nanoparticles on the fluorescence of Ag2S QDs were systematically investigated, it was found that gold nanoparticles could enhance or quench the fluorescence intensity of Ag2S QDs in some situation. Within a certain range, the fluorescence intensity of Ag2S QDs shows a linear quenching proportional to the colloidal gold concentration. Under the optimized experimental conditions, the linear range was obtained for colloidal gold concentration from 5.93 - 27.09 ×10-6mol/l (r2 = 0.9956). The determined detection limit (LOD) was about 5.08×10-6 mol/l. The quenching mechanism of Ag2S QDs is discussed. Gold nanoparticles also have similar effects on Ag2Te QDs. A new method for the quantitative detection of inorganic nanoparticles was established, and it provides a new way for the bio-analytical applications.

Academic research paper on topic "Near Infrared Emission of Ag2S Quantum Dots and their Fluorescence Quenched by Gold Nanoparticles"

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Procedía Engineering 102 (2015) 273 - 277

Procedía Engineering

www.elsevier.com/locate/procedia

The 7th World Congress on Particle Technology (WCPT7)

Near Infrared Emission of Ag2S Quantum Dots and their Fluorescence Quenched by Gold Nanoparticles

Xiulan Wu1, Lei Liao2, Weilin Du1, Aimiao Qin1*

1Key Lab New Processing Technology for Nonferrous Metals & Materials Ministry of Education,

College of Materials science & engineering, Guilin University of Technology, Guilin, China 2 College of Environmental science & engineering, Guilin University of Technology, Guilin, China

Abstract

The near-infrared emitting Ag2S quantum dots (QDs) coated with L-cysteine were prepared by one-pot aqueous synthesis at room temperature. Several characterization methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), fluorescence spectrophotometer (PL) and ultraviolet -visible spectrophotometer (UV-vis) were used to characterize the structure, morphology and optical properties of the samples. The effects of gold nanoparticles on the fluorescence of Ag2S QDs were systematically investigated, it was found that gold nanoparticles could enhance or quench the fluorescence intensity of Ag2S QDs in some situation. Within a certain range, the fluorescence intensity of Ag2S QDs shows a linear quenching proportional to the colloidal gold concentration. Under the optimized experimental conditions, the linear range was obtained for colloidal gold concentration from 5.93 - 27.09 xi0-6mol/l (r2 = 0.9956). The determined detection limit (LOD) was about 5.08x10-6 mol/l. The quenching mechanism of Ag2S QDs is discussed. Gold nanoparticles also have similar effects on Ag2Te QDs. A new method for the quantitative detection of inorganic nanoparticles was established, and it provides a new way for the bio-analytical applications.

© 2015TheAuthors.PublishedbyElsevierLtd.Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Selection andpeer-reviewunderresponsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Academy of Sciences (CAS)

Keywords: Ag2S; Quantum dots; fluorescence quenching

* Corresponding author. Tel.: +086-773-5893252; fax: +086-773-5896672. E-mail address: miaoaiqin@aliyun.com

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Selection and peer-review under responsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Academy of Sciences (CAS) doi:10.1016/j.proeng.2015.01.143

Introduction

In recent years, quantum dots (semiconductor nanocrystals) aroused widespread interest from researchers, and its research involves physics, chemistry, material science and electronics engineering and many other disciplines. Initially, the quantum dots were used in microelectronics and optoelectronic materials1-1, 2], then quantum dots were applied in biological and medical aspects-3, 4] due to their visible area unique and excellent fluorescence properties. With the deepening research of quantum dots, quantum dots have been extended to quantitative analysis of heavy metal ions, small organic molecules and drugs-5, 6]. Currently most research is mainly concentrated on CdX (X = S, Se, Te) or CdX as the core, the rest of the semiconductor material for the shell of the core-shell quantum dots. So far, quantitative detection based on Ag2S QDs fluorescence quenching effect has not been reported in the literature. Similar to the traditional fluorescent probe analysis, the proposed methodology was based on the enhancement or quenching effect induced by the physics process, chemical action between the analyte and quantum dots, and then the content determination of inorganic/organic molecules content could be obtained. Cai et al[7] synthesized a water-soluble CdS quantum dots coated with L-cysteine, and the CdS QDs were used as fluorescence probes for Hg2+ detection. Under optimal conditions, the method shows a high sensitivity, and the limit of detection was 2.4nmolL-1. Wang et al[8]used pH-sensitive CdTe quantum dots as proton probes to detect tiopronin. The determination was established based on fluorescence quenching method. The limit of detection wa^s 0.15mg L-1. The method was used to determine tiopronin in tablets successfully. Cao[9]established a sensitive, rapid, simple and economical turn-on fluorescent detection assay for melamine using the inner filter effect of AuNPs on L-Cys-capped CdS QDs, a detection limit was as low as 0.017 mgL-1.

In this work, an analytical approach involving the use of aqueous infrared Ag2S QDs was for the first time applied, to our knowledge, in the determination of colloidal gold. The proposed methodology was based on the quenching effect by colloidal gold on L-cysteine-capped Ag2S QDs fluorescence signal.

1. Experiment

All chemical reagents were analytical grade without further purification. Distilled water was used throughout this experimental study.

Water-soluble Ag2S QDs were prepared as follows. In a typical synthesis process, 0.1g L-Cysteine was dissolved in 100mL distilled water and then 4mL of 0.1mM AgNO3 was added under stirring. Subsequently, the solution was adjusted to pH 11 with 1M NaOH. Later, 1mL of 0.1mM freshly prepared Na2S solution was injected into the reaction solution while the mixture continued to stir constant. The solution rapidly changed to red brown.

For the spectrofluorometric detection of AuNPs, 1.8 mL of 1.0*10-3mol/L Ag2S QDs solution was transfered into the calibrated test tube and was diluted to 3ml with 0.05mol/L PBS buffer solution(pH = 8.04), then certain amounts of Au NPs solution was added into the mixture with a microsyringe. The fluorescence intensity was measured with the following settings of the spectrofluo-photometer (excitation wavelength (_ex) 520 nm; excitation slit 10.0 nm; emission slit 10.0 nm).

The X-ray powder diffraction (XRD) patterns of the products were measured on an X'Pert PRO X-ray powder diffractometer with Cu Ka radiation (X=1.5418A) for 20 ranging from 20° to 70° The morphology, size and chemical composition of the products were observed by using Hitachi S-4800 field emission scanning electron microscope (FE-SEM) and FE-2000 transmission electron microscope (TEM). The zeta potential was measured on a nano particle analyzer(Malvern Nano ZS).

2. Results and discussions

2.1. XRD and TEM of Ag2S QDs

The crystal phase of the as-obtained Ag2S QDs was confirmed by the XRD patterns, which are shown in Fig. 1. And the characteristic peaks of the samples are identified as monoclinic Ag2S (JCPDS no.14-0072). In addition, several peaks are broadened due to the smaller size of nanoparticles.

Fig. 2a shows a typical TEM image of Ag2S QDs. It presents a spherical morphology and well dispersion of Ag2S QDs with an average diameter of 4.993±0.732nm which obtained by measuring the diameter of several hundred particles in the TEM image. It is noted that some Ag2S QDs arrange in chains and the edges are not clear. It is speculated that the redundant coordinating groups in the capped agent of L-cysteine make the nanoparticles bind together. The morphology of Ag2S QDs in the present of colloidal gold is shown in Fig. 2b, it shows that the shape and the size of the nanoparticles of Ag2S are similar to that of Ag2S QDs in the absence of colloidal gold, but the chain-shaped particles disappear and the crystallization of the nanopartilces seems better. This maybe due to the competition effect of gold nanoparticles which are capped by the dissociative L-cysteine molecules.

2Theta [° ]

Fig. 1. XRD pattern of Ag2S QDs

Fig. 2. (a) TEM of Ag2S QDs (b)TEM of Ag2S QDs contained colloidal gold

2.2. Spectral characteristic

Fig. 3. 1 shows the UV-vis, fluorescence emission spectra of Ag2S QDs and Ag2S QDs in the present of colloidal gold. The UV-vis spectra (Fig3. 1a) shows that characteristic peaks are not found in the absorbance curves of Ag2S QDs, but the absorbance of Ag2S QDs declined when Au colloid was added, which maybe be due to the plasmonic effect of Au nanoparticles. The undetected of the characteristic absorbance peak of Au colloid is due to the minute amounts of Au colloid. The fluorescence emission spectra (Fig. 3. 1b) show that the L-cysteine-capped-Ag2S QDs have a strong fluorescence in the near infrared range with a maximum emission at 750 nm. When Au colloid was added into Ag2S QDs solution, the fluorescence intensity of L-cysteine-capped-Ag2S QDs was found to be enhanced

and then significantly quenched with the increase of Au colloid concentration. Fig. 3.2a displays the quenching of the fluorescence intensity of Ag2S QDs induced by Au colloid. Under the optimal conditions, a liner between the quenched fluorescence intensity of Ag2S QDs and concentration of Au colloid was observed in the range of 5.93-27.09x 10-6mol/l with a correlation coefficient (r2) of 0.9956, the linear regression equation is AF =470.88-3.81C (x10-6mol/l), and the direction limit is 5.08 x10-6 mol/l. Similarly, a liner between the quenched fluorescence intensity of Ag2Te QDs and the concentration of Au colloid was also observed in the range of 3.39-16.93x 10-6mol/l with a correlation coefficient (r2) of 0.9865, the linear regression equation is AF =469.86-3.19C (x 10-6mol/l), and the direction limit is 2.54x 10-6 mol/l, as shown in Fig. 3. 2b.

Fig. 3. (1) UV-vis (a) and fluorescence spectra (b) of Ag2S QDs and Ag2S QDs in the present of Au colloid; (2) the liner relation between the quenched fluorescence of Ag2S QDs with various concentration of Au colloid(a); the liner relation between the fluorescence quenched of Ag2Te

QDs and the various concentration of Au colloid(b).

The quenching mechanism of Ag2S QDs induced by Au colloid can be explained with the electrostatic theory. Zeta potential measurement results show that Au colloid carries positive charges while Ag2S QDs solution does negative charges, shown in Fig. 4. 1. The fluorescence intensity of Ag2S QDs was quenched for the electrostatic interaction between Ag2S QDs and Au colloid. This theory is also useful for the revealing the fluorescence quenched of Ag2Te QDs with negative charges induced by Au colloid with positive charges (Fig. 4. 2).

800000700000 » 600000

§ 500000-o

U 400000-3

■g 300000-H

200000 100000 0

-100 0 100 Zeta Potential(mV)

500000 400000 300000

(2) -Au

-Ag2Te

-Ag2Te+Au

Zeta Potential(mV)

200000

100000

Fig. 4. (1) Zeta potential of Ag2S QDs, Au colloid; (2) Zeta potential of Ag2Te QDs, Ag2Te QDs + Au colloid, Au colloid.

3. Conclusions

Based on the L-cysteine-capped Ag2S QDs fluorescence quenching effect induced by colloidal gold (Au), a simple, rapid, highly selective determination of colloidal gold was established. The way to determination of

colloidal gold is also appropriate for the similar type QDs, such as, Ag2Te, Ag2Se QDs, and etc. In our work, a new method for the quantitative detection of inorganic nanoparticles was established, and it provides a new way for the bio-analytical applications.

Acknowledgements

This work was supported by the National Science Foundation of China (under Grant Numbers: 21063005, 50968005, 21264005 and 51163003), and by the National Science Foundation of Guangxi Province(under Grant Number: 2010GXNSFC013007 and 2014GXNSFAA118331).

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