Flexible and transparent Au nanoparticle/graphene/Au nanoparticle ‘sandwich’ substrate for surface-enhanced Raman scattering

Abstract

In this work, we designed and fabricated a simple and convenient Au nanoparticle/graphene/Au nanoparticle (AuNP/G/AuNP) flexible ‘sandwich’ substrate on the surface of the polyethylene (PE) film through self-assembly. The coupling effect between the AuNPs enables this substrate to strongly enhance the local electromagnetic field, and graphene can also provide additional chemical improvement. Experimental results show that the low Rh6G concentration (10⁻⁹ M) located on the surface of the AuNP/G/AuNP substrate can be detected. Furthermore, the flexible and transparent AuNP/G/AuNP substrate can detect low concentrations of thiram (0.24 ppm) on the surface of orange peels. This low concentration is below the maximum residue limit (5 ppm) regulated by the National Standard of China in fruits and vegetables. Therefore, this flexible substrate can be used as an effective surface-enhanced Raman scattering sensor for in situ detection in food safety and environmental monitoring.

Introduction

Metallic nanostructures with surface plasmon, such as gold, silver, and copper or aluminum, exhibit novel optical properties at specific excitation wavelengths. When the frequency of the incident light corresponds to the oscillation frequency of the free electrons on the surface of the metallic nanoparticles, localized surface plasmon resonance (LSPR) occurs and generate strong local electromagnetic field enhancement [[1], [2], [3]]. In the last century, researchers found that the Raman signal of probe molecules located near the metal nanostructure substrate is amplified, and this phenomenon is referred to as surface-enhanced Raman scattering (SERS) [[4], [5], [6], [7]]. Given its ultrasensitive, non-destructive, and ‘fingerprint’ characteristics, SERS can be used as a powerful tool for sensing, bioimaging, and food safety [[8], [9], [10], [11]]. The unique and novel optical properties of these self-assembled metal-nanostructured substrates have attracted researchers’ attention for the development of micro-nanoprocessing technology. Given the advantages of self-assembled metal-nanostructured substrate, such as low cost, easy acquisition, and most importantly their unusual surface plasmon optical properties, many reports on SERS have been published [[12], [13], [14], [15]]. However, conventional rigid SERS substrates are not easily bent, lack flexibility, and have difficulty in achieving actual SERS detection. Therefore, a flexible SERS substrate that can meet actual engineering needs is urgently needed. Li et al. [8] obtained Ag₂O@Ag core-shell substrate on the surface of polymethyl methacrylate (PMMA) through deposition, annealing, and transfer. The flexible substrate can detect Rh6G molecules with concentrations of as low as 10⁻¹¹ M and chlorpyrifos molecules on the surface of cucumber, thus exhibiting potential use in food safety field and analyte detection on irregular objects in the future. Zhong et al. [16] obtained gold nanoparticle–flexible SERS substrate through self-assembly on the surface of polyethylene (PE) films. The experimental results show that the flexible substrate can detect low-concentration malachite green on fish surfaces. This finding provides experimental methods and theoretical basis for the preparation of flexible ultrasensitive sensor chips. Although flexible SERS substrates have been reported, in view of the universality of the spectral mechanism and reproducibility, related work is still necessary to provide reliable sample support for flexible SERS sensing.

Since the discovery of 2D material graphene, it has been widely used in the fields of SERS, surface photocatalysis, and nanodevices because of its excellent physical and chemical properties [[17], [18], [19], [20]]. For instance, Xiao et al. [18] reported that graphene can be used as ultrasensitive SERS substrate for Raman enhancement. The Raman signal of the probe molecule on the graphene surface can be effectively amplified compared with that of the control group. Researchers believe that the enhancement of the molecular Raman signal is mainly because of the chemical enhancement caused by the charge transfer between the molecule and graphene. Cao et al. [19] used applied bias voltage and combined silver nanoparticles with monolayer graphene to prepare an electrooptical coordinated controlled substrate. The experimental results show that the photocatalytic reaction on the surface of the substrate is affected by the surface plasmon-exciton coupling between the metal and graphene, and the bias voltage can control the photocatalytic reaction rate. Zhao et al. [21] transferred graphene on the surface of silver nanodisk arrays and assembled dense gold nanoparticles to prepare a 3D hybrid structure. The substrate can detect crystal violet molecules at concentrations as low as 10⁻¹³ M and exhibits extraordinary reproducibility, which is attributed to the local electromagnetic field enhanced by the coupling between nanostructures and chemical enhancement provided by graphene. In addition to the above research reports, the π-π stacking of graphene can effectively adsorb probe molecules, and charge transfer easily occurs between molecules and graphene. Thus, this material can be used as a good substrate for surface enhancement spectroscopy.

Herein, we designed and fabricated a self-assembled AuNP/G/AuNP ‘sandwich’ structure flexible SERS substrate inspired by the above reports. The uniform AuNPs in the ‘sandwich’ structure can provide dense and large-area ‘hot spots’ distribution from different dimensions under the excitation of the applied light field. Furthermore, the charge transfer between graphene and molecules can cause additional chemical enhancement. The experimental results show that the as-fabricated AuNP/G/AuNP ‘sandwich’ structure substrate has good sensitivity and reliability. In addition, this substrate is transparent and flexible, thus enabling in situ detection on irregular surfaces that is usually conducted in the fields of food safety and environmental pollution detection.