On the use of Au@Ag core-shell nanorods for SERS detection of Thiram diluted solutions

Highlights

• AuNR@Ag presented structures with symmetrical and asymmetrical Ag layer deposition.

• BEM simulations indicated a synergism between core-shell structures that is beyond the plasmon band shift.

• 4-ABT was used as a probe molecule achieving a detection limit of 1.10⁻¹⁵ mol L⁻¹.

• Thiram was detected at concentrations is far below (0.02 ppm) the established by the US EPA.

• Results indicate the possibility to produce SERS substrates with ultra-low detection limit.

Abstract

Rod-shaped gold-silver core-shells (AuNR@Ag) were synthesized for an analysis of the amplification of Raman scattering (surface-enhanced Raman scattering, SERS). The microscopy characterization confirmed a hierarchically structured nanoparticle with well-defined size and morphology, however, with a degree of dispersion in terms of shell thickness and symmetry of Ag deposition. In this paper, we analyze the possible effects of such structural dispersion in the SERS spectra of 4-aminobenzothiol (4-ABT) and in its detection at low concentrations in solutions. The interpretation of experimental results was supported by classical electrodynamics simulations based on the boundary element method (BEM). We verified that even in the case of asymmetrical Ag deposition onto AuNRs, a large SERS normal may be observed, which leads to the possibility of using such nanostructures for SERS applications aiming at low analyte concentrations detections. We show that the SERS substrates based on such AuNR@Ag present very high sensitivity for the detection of ultra-low concentrations of 4-ABT reaching a detection limit of 1.10⁻¹⁵ mol L⁻¹, which indicates the possibility of analytical applications in the detection of analytes such as pesticides.

Introduction

Thousands of frequently used synthetic chemicals, such as drugs and pesticides, are potential water contaminants and are being released into the environment on a daily basis. Yet, not much information is available concerning the ecological and health impact of this contamination, in a sense that the development of powerful and versatile analytical tools is needed. Many efficient techniques are available for environmental analysis such as chromatography, mass spectrometry, and electrophoresis, but they have limitations such as complex sample preparation, matrix interference, and most of them require time and expensive instrumentation. In the context of developing sensitive, selective and low-cost techniques as an alternative for environmental analysis emerges the Surface-Enhanced Raman Spectroscopy (SERS) [1].

SERS effect involves the enhancement of the Raman scattering of an analyte in several orders of magnitude when the molecule is adsorbed on (or close to) a nanostructured metal surface [[2], [3], [4]]. This phenomenon has been recurrently attributed in the literature to two main mechanisms: electromagnetic (EM), in which enhancement is a result of the interaction between electromagnetic radiation and the Localized Surface Plasmon (LSP) modes of certain metal structures that leads to strong electric field amplifications close to the metal surface, and of lower contribution, the charge transfer (CT) mechanism, that involves chemical interaction between molecule and metal substrate, which leads to modifications of the molecule Raman tensor [2, 3, [5], [6], [7], [8]].

Metal nanoparticles have been studied as efficient SERS substrates not only for being good radiation scatterers but also because their optical properties can be easily modulated [9]. Gold and silver are the most commonly used noble metals to prepare nanoparticles for SERS applications due to their physicochemical properties [[10], [11], [12], [13]]. For instance, their LSP bands are often located in the visible-NIR range, where most of the lasers used as excitation sources for Raman experiments are found.

One crucial aspect regarding the production of highly efficient SERS substrates (i.e. platforms for the acquisition of SERS data) using deposited nanoparticles is the generation of hot spots (HSs), in which the short distance between two or more plasmonic particles allows their interaction in order to create a region of highly concentrated electromagnetic fields, giving rise to very large SERS signals of nearby molecules [4, 14]. This comes to an extent where a single-molecule detection regime could be achieved, despite a possible sparse distribution of HS over the substrate, which makes it difficult to obtain quantitative measurements and to reach good reproducibility [[15], [16], [17]]. HS generation might be achieved, among several strategies, by the self-assembly and immobilization of the nanoparticles on a substrate. In previous works from our research group, glass slides were functionalized with an amino-terminated molecule for the immobilization and aggregation of Ag nanoparticles [11, 18].

A diversity of nanoparticle shapes can be used in the construction of SERS substrates. Several studies have reported the use of anisotropic particles such as nanorods [19], nanotriangles [20] and nanostars [21] as efficient SERS sensors for a variety of practical applications. Among these structures, we highlight the nanorod shape: its geometrical anisotropy leads to the observation of two LSP modes that represent electron oscillations along directions that are parallel (longitudinal mode) and perpendicular (transversal mode) to the main rod axis [22]. In particular, the longitudinal mode is very sensitive to the nanoparticle aspect ratio and can be used to fine tune the LSP resonance wavelength for SERS applications. Another very efficient way to observe LSP shifts is by the nanoparticle assembly. In this case, the resonance condition depends not only on the coupling distance [23], but also on the specific AuNR relative orientations [24, 25]. For the observation of large SERS enhancements for analytes in aqueous solutions, the end-to-end AuNR aggregation should preferred, which leads to plasmon resonances close to the NIR region of the electromagnetic spectrum [24]. If the experiment is to be realized with red light source (for instance 633 nm from a He-Ne laser), a blueshift should be observed for efficient SERS enhancements. The side-by-side procedure also leads to spectral shifts, however, with a decrease in the SERS performance [25]. A route to extract very large field enhancements in the end-to-end configuration could be the use Ag nanorods. However, controlling the particle shape is a hard task due to the Ag chemistry [26], and an alternative is the use of AuNR as templates for Ag deposition [27, 28].

We herein report the synthesis and characterization of core-shell nanorods consisting of an Au core covered by an Ag shell. The Ag layer deposition process may occur by creating both symmetric or asymmetric arrangement of the Au core and Ag shell, the latter with a core that is shifted from nanoparticle geometrical center. In this manuscript, we consistently evaluate, by electron microscopy, the distribution of nanoparticle morphologies, whose dispersion could be a concern for analytical applications of SERS due to possible large variability of enhancements from HSs formed by such nanoparticles. In this manuscript, we investigate the SERS spectra of 4-aminobenzenethiol (4-ABT) from such nanoparticles, which reveal very low detection limits, indicating the presence of very strong HSs. This near-field behavior was analyzed by classical electrodynamics simulations of simple model systems, indicating strong SERS normals even in the presence of asymmetric Ag shell coatings. Finally, we evaluate the substrates as probes for environmental applications, by investigating the detection capabilities for the pesticide Thiram.