A ternary functional Ag@GO@Au sandwiched hybrid as an ultrasensitive and stable surface enhanced Raman scattering platform

Accepted Manuscript Title: A ternary functional Ag@GO@Au sandwiched hybrid as an ultrasensitive and stable surface enhanced Raman scattering platform Author: Cong-yun Zhang Rui Hao Bin Zhao Yao-wu Hao Ya-qing Liu PII: DOI: Reference:

S0169-4332(17)30674-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.023 APSUSC 35399

To appear in:

APSUSC

Received date: Revised date: Accepted date:

30-1-2017 1-3-2017 2-3-2017

Please cite this article as: C.-y. Zhang, R. Hao, B. Zhao, Y.-w. Hao, Y.-q. Liu, A ternary functional Ag@GO@Au sandwiched hybrid as an ultrasensitive and stable surface enhanced Raman scattering platform, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.03.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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The GO embedded sandwich nanoparticles are capable of serving as an

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ultrasensitive, highly reproducible and stable SERS platform.

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Highlights 1. A novel sandwiched hybrid (Ag@GO@Au) with graphene oxide (GO) films embedded between hierarchically flower-like Ag particles and Au nanoparticles was

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successfully fabricated. 2. The Ag@GO@Au sandwiched structures exhibited ultrasensitive SERS response by

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multi-dimensional coupling.

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3. The GO interlayer as an isolating shell endowed the sandwiched hybrids good signal

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reproducibility and prolonged stability.

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A ternary functional Ag@GO@Au sandwiched hybrid as an ultrasensitive and stable surface enhanced Raman scattering platform Cong-yun Zhang a, Rui Hao a, Bin Zhao a, Yao-wu Hao b,*, Ya-qing Liu a,*

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1. Shanxi Province Key Laboratory of Functional Nanocomposites, School of Materials

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Science and Engineering, North University of China, Taiyuan 030051, China,

2. The Department of Materials Science and Engineering, University of Texas at Arlington,

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Arlington, Texas 76019, USA

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*corresponding author: E-mail: [email protected]；[email protected]

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Abstract The graphene-mediated surface enhanced Raman scattering (SERS) substrates by virtues of plasmonic metal nanostructures and graphene or its derivatives have attracted

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tremendous interests which are expected to make up the deficiency of traditional

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plasmonic metal substrates. Herein, we designed and fabricated a novel ternary Ag@GO@Au sandwich hybrid wherein the ultrathin graphene oxide (GO) films were

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seamlessly wrapped around the hierarchical flower-like Ag particle core and meanwhile

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provided two-dimensional anchoring scaffold for the coating of Au nanoparticles (NPs). The surface coverage density of loading Au NPs could be readily controlled by tuning

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the dosage amount of Au particle solutions. These features endowed the sandwiched

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structures high enrichment capability for analytes such as aromatic molecules and

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astonishing SERS performance. The Raman signals were enormously enhanced with an ultrasensitive detection limit of rhodamine-6G (R6G) as low as 10-13 M based on the chemical enhancement from GO and multi-dimensional plasmonic coupling between the metal nanoparticles. In addition, the GO interlayer as an isolating shell could effectively prevent the metal-molecule direct interaction and suppress the oxidation of Ag after exposure at ambient condition which enabled the substrates excellent reproducibility with less than 6% signal variations and prolonged life-time. To evaluate the feasibility and the practical application for SERS detection in real-world samples based on GO sandwiched hybrid as SERS-active substrate, three different prohibited colorants with a series of concentrations were measured with a minimum detected

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concentration down to 10-9 M. Furthermore, the prepared GO sandwiched nanostructures can be used to identify different types of colorants existing in red wine, implying the great potential applications for single-particle SERS sensing of

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biotechnology and on-site monitoring in food security. Keywords: graphene oxide; sandwich nanohybrid; multi-dimensional coupling; SERS;

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high stability; selective detection

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1. Introduction

Surface-enhanced Raman scattering (SERS), as a powerful analysis technique, has

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generated widespread interest owing to its wide applications in the fields of analytical

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chemistry, life science, biochemistry and trace detection of chemical species [1-5]. Up

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to now, most of the SERS platforms were constructed by noble metal nanostructures (Au, Ag, Cu) [6-7], where dramatic Raman enhancement could be acquired in the areas of “hot spot” from the highly roughened surface or the gaps of coupled plasmonic nanoparticles (NPs) [8-10]. Significant progress has been made to design and fabricate active metal nanostructures with various shapes and tunable size as SERS platforms [11-14]. Among them, Ag has been demonstrated to have much more outstanding SERS performance than Au. Especially hierarchical Ag structures with high surface roughness assembled by nanoscale blocks possess astonishing and collective physicochemical properties relative to individual NPs of the same metals [15-17]. Moreover, the single-particle of constructed complex architectures could serve as sensitive SERS

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platforms which maintain intense electromagnetic fields at roughened surface, folds, apexes that induce enormous surface-enhanced Raman scattering (SERS). However, instability under ambient condition and weak biocompatibility of Ag-based substrates

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hinder the long-term practical applications. Although many strategies have been developed to fabricate shell-isolated SERS substrates to suppress the oxidation and

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avoid the direct metal-molecule contact by coating Ag NPs with Au, SiO2 and TiO2

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shells [18-20], the accurate control of shell thickness and the low affinity with

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nonthiolated molecules are desired to resolve.

Graphene, a single layer of carbon atoms arranged in perfect honeycomb lattice has

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attracted considerable attention due to its exceptional properties. Moreover, its

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derivatives such as graphene oxide (GO) and reduced graphene oxide (RGO) also

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exhibit additional distinct physical properties owing to the existence of various oxygen containing functional groups. Recently, many researches focused on the SERS performance of plasmonic metal-graphene hybrid nanostructures with controlled shape and size, in which graphene itself could be employed as efficient SERS substrate and generate additional Raman enhancement through chemical mechanism [21-22]. Moreover, the combination of plasmonic metals with graphene or derivatives improve substrates the capability of affinity and enrichment of target analytes, especially for the aromatic molecules [23]. Wang reported that GO/Ag NP hybrids exhibited strong enrichment for folic acid and extremely high sensitivity with detection limit of folic acid down to 9 nM [24]. Similarly, Liz-Marzán synthesized RGO-Au nanostar composites

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and explored them as SRES substrates to monitor the loading and release of anticancer drug-doxorubicin [25]. Jin’s group developed GO-supported Ag nanoplate hybrids, wherein the SERS enhancement factor was 4 orders of magnitude relative to the normal

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Ag NP SERS substrate [26]. Nevertheless, this class of hybrids where the NPs were decorated or resided on graphene or its derivatives still suffered from poor stability and

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the interference from strong chemical interaction between metal surface and analytes,

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which severely induce signal fluctuations and weak reproducibility. Based on the

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aforementioned issues, Zhang prepared graphene-veiled gold hybrid, where the nanogaps between gold nanoislands could produce huge electromagnetic enhancement

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after annealing [27]. The metallic nanostructures encapsulated by ultrathin and seamless

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graphene layers have been demonstrated to enable the substrates with prolonged

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lifetime and effectively improved signal cleanliness [28-29]. Recently, more complicated graphene-metal composites have been designed and explored as SERS substrates to achieve the extremely high sensitivity, reproducibility and selective detection [30-32]. Some sandwich structures where graphene or its derivatives were embedded between two layers of plasmonic metals as a tailored dielectric nanogap exhibited enormous Raman enhancement via the multi-dimensional plasmonic coupling [33-35]. However, triplex core-shell sandwich architectures with GO embedded between two layers of plasmonic metal NPs as single-particle SERS sensing have not been reported yet.

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Herein, a novel ternary core-shell particles consisting of a flower-like Ag microparticle core, an ultrathin GO interlayer and Au nanosphere shell were successfully fabricated in high yield. The ultrathin GO interlayers were seamlessly

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wrapped around the hierarchically flower-like Ag particles, which effectively prevented the direct metal-molecule interactions and suppressed the oxidation of Ag particles

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under ambient conditions, enabling the improved reproducibility and stability of Raman

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signals. Due to high enrichment capacity of aromatic molecules, chemical enhancement

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from GO and the huge electromagnetic enhancement from multi-dimensional plasmonic coupling between same metal NPs and that between vertical Ag/Au NPs, the prepared

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Ag@GO@Au sandwiched structures exhibited dramatic SERS enhancement, especially

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for the ultrasensitive and selective detection of prohibited colorants in real-world

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samples. Furthermore, the single particle of Ag@GO@Au sandwiched structures can be easily observed under ordinary optical microscope, which was advantageous for single-particle SERS sensing.

2. Experimental section 2.1 Reagents and Materials

Silver nitrate (AgNO3, 98%), ascorbic acid (AA, 99.8%), polyvinylpyrrolidone (PVP, Mr = 10 000), sodium citrate (Na3C6H5O7, 99.8%) chloroauric acid (HAuCl4·4H2O, 99.9%) amaranth (96%), allura red (96%) and erythrosin B (96%) were purchased from

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Macklin. All reagents were used without further purification. Ultrapure water (18MΩ) was used in all of the experiments. 2.2 Synthesis of hierarchical silver microspheres

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In a typical synthesis, 0.3 mL of 1 M AgNO3 aqueous solution and 2 mL of 1M PVP aqueous solution were added to 10 mL deionized water under magnetic stirring in an

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ice-water bath. Then, 0.2 mL of 1 M AA was quickly injected into the mixed solution

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and continually stirred for 15 min until the hierarchical silver microspheres were

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synthesized. Finally, the as-synthesized Ag particle suspensions were centrifuged (2000 rpm for 5 min) and washed with ultrapure water three times.

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2.3 Synthesis of Ag@GO composites

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Graphene oxide was fabricated by the modified Hummers method as described

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previously [42]. Then the synthesized graphene oxide was dispersed in water with ultra-sonication for 1h to obtain 0.01 mg/ mL GO aqueous solution. The Ag@GO composites were prepared using cysteamine as positively charged capping agents. Briefly, 1 mL as-prepared Ag particle dispersion was added into 9 mL ultrapure water under gently stirring, followed by the addition of cysteamine (50 µL, 0.01 mg/mL). After stirring for 1 min, 15 mL of 0.01 mg/mL GO aqueous solution was injected into the mixture and mildly stirred for another 1 hour at room temperature. The final products were centrifuged three times (3000 rpm for 15min) and re-suspended in 1 mL ultrapure water for subsequent experiments. 2.4 Synthesis of Ag@GO@Au sandwich nanostructure

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Gold particles were synthesized based on a previous method. Briefly, 0.8 mL citrate sodium (1 wt %) was added into 100 mL of boiling HAuCl4 solution (1 mM) under vigorous stirring. Then the solution was boiled for 30 min and cooled in ambient

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conditions. The Ag@GO@Au sandwich nanostructure was prepared using 2-MPy as linking agents. Firstly, 1 mL 2-MPy (0.01 mg/mL) and 1 mL Ag@GO hybrids were

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added to 10 mL ultrapure water under stirring for 6 h. Then the products were

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centrifuged three times (3000 rpm for 15min) and re-suspended in 1 mL ultrapure water.

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Finally, 0.5 mL 2-MPy modified Ag@GO and different volumes of gold particles (3 mL, 5 mL, 10mL) were added to ultrapure water and the final volume of the solution was

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2.5 Characterization

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re-dispersed in 5 mL ultrapure water.

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made to be 10 mL. The resulting products were centrifuged (3000 rpm for 15min) and

The structures of the samples were characterized using scanning electron microscopy (SEM, SU-8010) and transmission electron microscopy (TEM, JEOL- 2100F). X-ray photoelectron spectroscopy (XPS) measurement was carried with spectrometer with monochromatic Al Kα (Thermo ESCALAB 250Xi). The Raman spectra were recorded using a 785 nm laser with 1mW power and a 100× objective (Renishaw inVia). The integral time was 10 s and the laser power was 1.5 mW. For the detection of R6G, 20 µL of the Ag@GO@Au aqueous suspension was mixed with 20 µL of different concentrations of R6G, and sonicated for 30 min to reach the adsorption equilibrium.

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Then, 20 µL of the solution was dropped on a Si wafer and dried for SERS detection. Colorants were also detected using the same method.

3.1 Characterization of triplex Ag@GO@Au particles

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3. Results and discussion

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Fig. 1a schematically illustrates the detailed fabrication process of triplex core-shell

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Ag@GO@Au composite nanostructures. Flower-like Ag microparticles with high

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surface roughness and average diameter of 0.8-1.2 µm(Fig.1b) were first fabricated and then modified with cysteamine to endow positively charged groups on the surface,

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allowing for the self-assembly of GO layer around them. Fig. 1c shows that a wrinkled and veil-like GO film was seamlessly encapsulated around the Ag particles, wherein the

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high coverage area of GO ensures the further attachment of Au particles and molecule enrichment. Then, as a linkage agent, 2-mercaptopyridine (2-MPy) was used to further modify the Ag@GO composite particles through π-π cooperative interactions, and the Au nanospheres were successfully decorated on the surface of Ag@GO particles through strong binding with the thiol group on the aromatic ring of 2-MPy (Fig.1d). The UV-vis-NIR absorption spectra were used to estimate the surface plasmon resonance (SPR) properties of flower-like Ag, Ag@GO and Ag@GO@Au structures, as shown in Fig.S1.

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Figure 1. (a) Schematic illustration of the synthesis steps of Ag@GO@Au sandwich hybrids. SEM images of pure flower-like Ag particles (b), GO wrapped Ag hybrids -Ag@GO (c) and ternary

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Ag@GO@Au sandwich hybrids (d). The scale bar was 500 nm.

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Furthermore, the densities of Au NPs coating on the surface of GO layer were easily controlled by tuning the dosage amount of gold particle solution. Fig.2 shows SEM

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images of Ag@GO@Au hybrids prepared with different volumes of gold particle solutions at low and high magnifications, displaying high yield formation of the triplex core-shell Ag@GO@Au hybrids. As expected, with the increasing amount of gold particle solution from 3 mL (Fig.2A-a), 5 mL (Fig.2B-b) to 10 mL (Fig.2C-c), Au NPs adsorbed outside the Ag@GO gradually attached much more densely and homogeneously, which could further amplify the Raman signal. The intensities of Raman signals of 10-7 M R6G increase with the increasing extent of Au particle coverage on GO surface (Fig.S2). The Ag@GO@Au sandwiched hybrids prepared from 10 mL gold NPs solutions were chosen for the further investigation in the following sections.

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Figure 2.SEM images of Ag@GO@Au at low (The scale bars were 1 µm) and high magnifications (The scale bars were 500 nm) prepared from different volumes of Au particle solutions. (A) and (a) 3 mL; (B) and (b) 5 mL; (C) and (c) 10 mL Au particle solution.

The morphologies of prepared Ag@GO@Au sandwiched hybrids were also characterized by TEM and HRTEM. Fig.3a presents TEM micrograph of an individual Ag@GO@Au hybrid with roughened surface. It is distinct that many Au nanospheres were deposited around Ag@GO composite particle. The HRTEM image (Fig. 3b)

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clearly shows the uniform GO interlayer with thickness of approximately 2.7 nm was embedded between two types of plasmonic metal NPs wherein the average diameters of Au nanospheres were about 35 nm, demonstrating the successful fabrication of

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Ag@GO@Au sandwiched structures. Fig.3c depicts the Raman spectra of flower-like Ag, Ag@GO and Ag@GO@Au substrates, respectively. It is noted that two new

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prominent peaks of D (1336 cm-1) and G (1598 cm-1) bands are observed on GO

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wrapped Ag particles (black line) relative to pure Ag substrate (blue line), indicating the

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existence of GO layer. After the high loading of Au NPs, the intensities of D and G bands of GO were significantly enhanced, which could be attributed to the strong

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multi-dimensional plasmonic coupling including the same metal NPs-NPs coupling on

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nanospacer.

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the horizontal direction and Ag/Au coupling at the nanoscale gap created by GO

Figure 3. (a) TEM and (b) HRTEM images of Ag@GO@Au sandwich hybrids. (c) Raman spectra of flower-like Ag particles, Ag@GO, Ag@GO@Au sandwiched structures.

X-ray photoelectron spectroscopy (XPS) measurements were then performed to confirm the surface functionalization and chemical composition evolution upon the formation of GO sandwiched hybrids. Fig. 4a displays the XPS spectra of pure

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flower-like Ag particles, binary GO wrapped Ag particles (Ag@GO) and ternary Ag@GO@Au sandwich hybrids. After the functionalization with cysteamine and GO wrapping, the atomic percentages of carbon (C) and oxygen (O) XPS spectra were

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significantly improved while that of Ag decreased, revealing efficient wrapping of GO around Ag particles (Table S1 in supporting information). Compared with that of pure

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Ag particles, the presence of sulfur (S) in Ag@GO and Ag@GO@Au sandwich hybrids

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suggested the successful introduction of thiol groups. The distinct difference of XPS

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spectra between Ag@GO and Ag@GO@Au hybrids was the appearance of Au, which further demonstrated the loading of Au NPs on the surface of GO (Fig. 4a and Table S1).

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In deconvolved C 1s spectra of pure Ag, four peaks appeared at 284.5eV, 285.3eV

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286.1eV and 287.9eV, assigning to the C-C, C-N and C=O bonds of PVP. For Ag@GO

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and Ag@GO@Au, three characteristic types of carbon bonds originating from GO: C-C (284.5 eV and 285.1 eV in Ag@GO, 284.7 eV and 285.2 eV in Ag@GO@Au), C-O (286.9 eV in Ag@GO, 287.1 eV in Ag@GO@Au) and C=O (288.9 eV in Ag@GO, 288.5 eV in Ag@GO@Au) were distinctly observed (Fig.4b), confirming the presence of GO layer [36-38]. The weaker peak (286.1 eV in Ag, 285.9 eV in Ag@GO) corresponding to C-N bond from PVP became a dominant peak centered at 286.2 eV after 2-MPy modification, suggesting existence of C=N in pyridine ring, which was further validated by N 1s spectra (line iii shown in Fig.4c) [39]. In S 2p spectra, it is distinct that two main peaks appear at 161.9 eV(S 2p3/2) and 163.0 eV (S 2p1/2) on Ag@GO hybrids while no signal of S 2p core level spectrum is observed on pure Ag

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particles, implying the formation of Ag-SH bond and successful functionalization with cysteamine [28]. Compared with binary Ag@GO hybrids, the S 2p spectrum of Ag@GO@Au exhibits a new doublet appearing at162.7 eV, 163.9 eV, which are related

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to Au-SH bonds [39]. These results demonstrate the successful functionalization with both cysteamine and 2-MPy, which also confirm the formation of ternary of

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Ag@GO@Au sandwiched hybrids.

Figure 4.(a) XPS spectra of flower-like Ag particles, Ag@GO hybrids and Ag@GO@Au sandwich hybrids. Decovolved (b) C 1s, (c) N 1s and (d) S 2p spectra collected on Ag particles(i), Ag@GO hybrids (ii) and Ag@GO@Au hybrids (iii).

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3.2 SERS performance of triplex Ag@GO@Au The SERS activity of ternary particles prepared under optimal conditions was evaluated using R6G as a target molecule. Fig.5a shows the SERS spectra of 10-7 M

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R6G on pure Ag, binary Ag@GO, and ternary Ag@GO@Au particle substrates. The

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Raman intensity of R6G on GO coated Ag particles (red line) was stronger than that on

pure Ag substrate (blue line), demonstrating the extra chemical enhancement and the

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improved enrichment capability for analytes from the GO film. Particularly, with the

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anchoring of Au NPs on the surface of GO, the SERS enhancement of ternary hybrid (black line) becomes much more prominent relative to those of Ag and Ag@GO

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particles. An ultrasensitive detection limit of ternary Ag@GO@Au substrate down to

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10-13 M was achieved (Fig. 5b) and the enhancement factor was up to 5.97×108 which

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was much higher than that of either pure Ag or binary Ag@GO (The calculation details were shown in Fig. S3)[27].

The GO films serve as a nanospacer to create a

sub-nanometer gap between plasmonic Ag/Au, which significantly enhances the coupling

on

vertical

direction

in

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structures.

Therefore,

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multi-dimensional plasmonic coupling originating from the same metallic particles and that between vertical Ag and Au particles, in conjugation with the molecular enrichment capability and chemical enhancement of GO nanospacer, collectively give rise to the superior SERS activity of Ag@GO@Au substrate. The linear response between intensities at representative peak of 1514 cm-1 and the logarithmic concentrations of R6G were found (see Fig.S4). In most cases, inhomogeneous distribution of hot spots

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could adversely affect the SERS performance of substrate. Raman spectra of 10-7 M R6G were recorded from 20 randomly selected sites to validate the signal reproducibility of ternary substrate (Fig.5c and d). The variations of relative Raman

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intensity at 1514 cm-1 was less than 6%, suggesting the extremely high uniformity of “hot spots” on Ag@GO@Au hybrid substrates. The spot-to-spot reproducibility on the

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single Ag@GO@Au particle and the reusability of substrates were shown in Fig.S5 and

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Fig.S6, respectively.

Figure 5.(a) SERS spectra of 10-7 M R6G on pure Ag, Ag@GO, Ag@GO@Au substrates. (b) SERS spectra of R6G obtained from Ag@GO@Au substrate at different concentrations, from top to bottom: 10-7, 10-8, 10-9, 10-10, 10-11, 10-12,and 10-13 M R6G. (c) SERS spectra of 10-7 M R6G on

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Ag@GO@Au sandwich substrate collected from 20 random sites. (d) Intensity variations of characteristic peak at 1514 cm-1 from 20 SERS spectra in Fig.5c.

Moreover, the stability is another crucial parameter for the practical application of

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excellent SERS substrates. The time-dependent measurements were then conducted to verify the stability of GO sandwiched structures. Fig. 6a and 6b display SERS spectra of

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R6G recorded from bare Ag and triplex Ag@GO@Au core-shell microspheres. It

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should be noted that the Raman signal intensity on Ag@GO@Au hybrid structure decay

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slightly whereas those from Ag substrate decreased remarkably after exposing under ambient condition for different periods. Fig. 6c depicts the plot of relative Raman

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intensities of as-prepared samples exposed in air versus different exposure times. After exposure to ambient environment for 5 days, the signal intensities on Ag@GO@Au

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hybrid nanostructures presented no distinct changes, whereas that on pure Ag substrate decreased significantly by 18±8%. When as-synthesized samples were further exposed for 30 days, the Raman intensities on pure Ag and Ag@GO@Au hybrid substrate decreased by about 53±8% and 12±4%, respectively, confirming that GO layer as a protective shell could effectively suppress the oxidation of Ag particles.

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Figure 6.SERS spectra of 10-7 M R6G obtained from pure Ag particles (a) and Ag@GO@Au sandwich hybrids (b) after exposure under ambient air for different times. (c) Plot of SERS signal intensities of R6G at 1514 cm-1 on pure Ag and Ag@GO@Au hybrids after exposing at ambient air for different periods versus freshly synthesized samples. The error bars represent the deviation of

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3.3 Practical application for detection of prohibited colorant

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three measurements at different positions of single particles.

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So far the as-prepared GO sandwiched hybrid systems have been demonstrated to exhibit excellent SERS performance and good capability of analyte enrichment, which

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are particularly suitable for the practical detections of aromatic molecules in real-world complex systems. In our study, three kinds of prohibited colorants commonly existing

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in drinks were detected. Fig. 7a, b and c show the SERS spectra of amaranth, allura red,

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erythrosin B in a series of concentrations, respectively. The characteristic peaks from

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three colorants are all observed clearly according to the previous report [41]. The detected limit of three colorants on prepared GO embedded sandwich substrate could be achieved as low as 1×10-9 M due to the multi-dimensional plasmonic coupling and excellent capability of enrichment, which were much lower than values recently reported [41]. The SERS signals of solid colorants on bare silicon wafer were shown in Fig.S7 for comparison. To investigate the feasibility and recognition selectivity of detection in real-world samples, the fixed amount of three types of colorants were added to the diluted red wine brought from a local market. All the fingerprint peaks of three types of colorants could be easily distinguished, shown in Fig. 7d marked with different labels. These results demonstrate that the as-synthesized triplex Ag@GO@Au sandwich

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hybrids as SERS sensing platforms exhibited extremely high sensitivity and excellent spectroscopic identification for trace prohibited food additives, suggesting the great

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application potential for the on-site monitoring in food security and biotechnology.

Figure7.SERS spectra of (a) amaranth, (b) allura red, (c) erythrosin B at different concentrations on Ag@GO@Au substrates. (d) SERS spectra of mixed systems of red prohibited colorants (including 10-5 M amaranth, 10-5 M allura red, 10-5 M erythrosin B)

4. Conclusions

In summary, a novel Ag@GO@Au sandwiched hybrids wherein GO embedded between two types of plasmonic metal NPs were prepared successfully. The combination of bimetallic metal NPs and GO could act synergistically to further enormously enhance Raman signals relative to either GO/metallic nanostructures alone

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or even the binary GO-metal nanostructures based on the extra chemical enhancement from GO and the multi-dimensional coupling between same metallic NPs and those between vertical Au/Ag NPs. The ultrasensitive detected concentration of R6G as low

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as 10-13 M was achieved. Ultrathin GO films were seamlessly wrapped around Ag core, which can effectively isolate the direct metal-molecule interaction and prevent the

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oxidation of Ag particles under ambient air, endowing the as-prepared Ag@GO@Au

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sandwich hybrids good signal reproducibility and prolonged stability. The Raman

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intensities of R6G on Ag@GO@Au sandwich hybrids were only decreased by 12%± 4% after exposure for 30 days under ambient atmosphere relative to a 53±8% signal

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decay for pure Ag substrates.Furthermore, the as-prepared Ag@GO@Au sandwich

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hybrids could be explored for the sensitive and distinguishing SERS detection of

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common prohibited colorants in complex real-world systems, revealing the great practical potential for the rapid, sensitive and selective detection of single-particle SERS-based sensing and on-site monitoring in food and environmental safety.

ASSOCIATED CONTENT

Supporting Information. UV-vis-NIR absorption spectra, SERS spectra of 10-7 M R6G on Ag@GO@Au sandwich hybrids prepared with different volumes of Au particle solutions; Atom percentages of different elements obtained from XPS spectra of pure Ag, Ag@GO and Ag@GO@Au nanostructures; Raman spectrum of 10-7 M R6G recorded from Ag@GO@Au sandwich hybrids and that of solid R6G on silicon wafer; The linear relation of SERS intensity of peaks at 1514 cm-1 versus logarithmic

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concentrations of R6G; Spot-to-spot reproducibility and reusability data of Ag@GO@Au substrates ; Raman spectra of solid amaranth, allura red and erythrosine

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B on silicon wafer.

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AUTHOR INFORMATION Corresponding Author

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*Ya-qing Liu: [email protected]

Acknowledgements

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*Yao-wu Hao: [email protected]

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This work was supported by the Natural Science Foundation for Young Scientists of

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Shanxi Province of China (No.2015021078) and International Cooperation of Science and Technology Project in Shanxi Province of China (No. 2014081006-2).

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