Elsevier

Vibrational Spectroscopy

Volume 68, September 2013, Pages 246-250
Vibrational Spectroscopy

Surface-enhanced Raman scattering of 4-aminobenzenethiol on silver nanoparticles substrate

https://doi.org/10.1016/j.vibspec.2013.08.003Get rights and content

Abstract

Active surface-enhanced Raman scattering (SERS) silver nanoparticles substrate was prepared by multiple depositions of Ag nanoparticles on glass slides. The substrate is based on five depositions of Ag nanoparticles on 3-aminopropyl-trimetoxisilane (APTMS) modified glass slides, using APTMS sol–gel as linker molecules between silver layers. The SERS performance of the substrate was investigated using 4-aminobenzenethiol (4-ABT) as Raman probe molecule. The spectral analyses reveal a 4-ABT Raman signal enhancement of band intensities, which allow the detection of this compound in different solutions. The average SERS intensity decreases significantly in 4-ABT diluted solutions (from 10−4 to 10−6 mol L−1), but the compound may still be detected with high signal/noise ratio. The obtained results demonstrate that the Ag nanoparticles sensor has a great potential as SERS substrate.

Introduction

Surface-enhanced Raman spectroscopy (SERS) is an analytical technique extensively used in chemistry, biology, forensics, and materials science, due to its excellent molecular vibrations sensitivity [1], [2], [3], [4]. The surface-enhanced effect in metals surface adsorbed molecules can provide results that are several orders of magnitude higher than normal Raman scattering signal [5], [6]. The signals amplification in SERS comes, mainly, through the electromagnetic interaction between the radiation and the localized surface plasmon (LSP) in noble metal surface structures. Another contribution arises from the charge transfer energy of the molecules adsorbed and/or very close to the metal surface [5], [6]. High electromagnetic fields can be generated when nanostructured metals (e.g. Ag, Au, and Cu) substrates are excited by light, generally in the visible range, causing LSP generation. This fact has significantly contributed to SERS to be considered an ultra-sensitive trace detection technique [7] or even as a single-molecule probing tool [8], [9]. Since the discovery of the enhanced Raman effect by Fleischmann [10], a lot of research has been directed to the development of efficient SERS substrates such as metal electrodes, Ag, Au, and Cu colloidal nanoparticles, and lithographically made substrates [11], [12], [13], [14]. However, many of those substrates do not offer Raman signal reproducibility, which is a well-known difficulty when SERS is used as an analytical spectroscopy technique.

In fact, there are large variety of SERS substrates prepared through many different methodologies such as colloidal suspensions synthesis, metal vapor depositions, and lithography [12], [14], [15]. Lithographically made substrates can provide an excellent control on density and position of the hot spots. However, those substrates have some limitations such as high cost and long preparation time [15], [16]. Some articles reporting the use of 3-mercaptopropyl-trimethoxysilane (MPTMS) and 3-aminopropyl-trimethoxysilane (APTMS) to functionalize SERS substrates were published in recent years [17], [18], [19]. These compounds have chemical groups, such as single bondSH and single bondNH2 that can chemically interact with Au and Ag nanoparticles. For example, after modifying a glass slide with MPTMS or APTMS a layer of gold or silver nanoparticles can be attached on the modified surface. Using this strategy, one moiety of bifunctional molecule could anchor to the first nanoparticles layer through a surface polymerization procedure, leaving another moiety “available” to immobilize nanoparticles from a colloidal solution via chemical bonding. A hierarchical structure can be built through repeating this procedure [19]. Generally this procedure achieves multi-layers and/or aggregates structures, creating hot spots on the substrate surface. This kind of material has typically been applied as SERS substrate with promise results [18], [19]. However, in the scientific literature still have few investigations about the influence of experimental parameters on Raman signal such as nanoparticle size and analyte concentration.

In this work, SERS active substrates based on multiple depositions of Ag nanoparticles onto APTMS modified glass slides were prepared. Samples with five depositions of silver nanoparticles were used as a SERS substrate with 4-aminobenzethiol as the Raman probe molecule. The average Raman signal variation on 10−4 mol L−1 4-ABT concentration was investigated for two substrates. At the same time, the variation of the 4-ABT molecule Raman signal present on Ag nanoparticles substrate was analyzed for three different concentrations of this compound.

Section snippets

Chemicals

All the chemicals were used without further purification. Silver nitrate (ACS grade), sodium citrate dihydrate, 3-aminopropyl-trimetoxisilane (APTMS), 4-aminobenzethiol (4-ABT) were purchased from Sigma–Aldrich. Deionized water was used in all aqueous solution preparation. All glassware were cleaned with piranha solution (4:1 sulphuric acid: hydrogen peroxide) and washed with deionized water. The ethylic alcohol (ACS grade), hydrochloric and sulphuric acid were purchased from Synth.

Ag nanoparticles and APTMS sol–gel synthesis

Ag

Results and discussion

Previous reports suggested that the SERS performance on metallic nanoparticles modified-substrates is strongly dependent on the number of nanoparticles depositions [19], [21], [22]. Fan and Brolo [22] reported the preparation of this sort of substrate and investigated SERS activity for some dyes. They observed dependence between SERS intensities and the number of Ag nanoparticles depositions. This can happen because few depositions could be not enough to produce aggregates that support large

Conclusions

The results presented in this work show that the Ag nanoparticles substrates with five depositions are sensitivity to detect 4-aminobenzenethiol in different concentrations. The preparation procedure led to formation of silver nanoparticles aggregates on the entire substrates surface. Taking advantage of the large SERS enhancement promoted in the Ag nanoparticles aggregates was possible to evaluate the SRES activity of the substrates. The average Raman signal variations at 1070 cm−1 were

Acknowledgments

EBS thanks FAPESP for a post-doc fellowship. The authors would like to thank the FAPESP, CAPES and CNPq for financial supports. Contributions from Multiuser Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ/UNICAMP) for Raman analysis and Brazilian Nanotechnology National Laboratory (LNNano, Campinas-SP, Brazil) for SEM-FEG analysis are also gratefully acknowledgment. This is a contribution of the National Institute of Science and Technology in Complex Functional Materials (CNPq-MCT/FAPESP).

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