1. Introduction

Raman spectrum is a kind of scattering spectrum which distinguishes the molecular structure of matter by analyzing the scattered light which is different from the frequency of incident light [1]. However, the scattered light intensity of the traditional Raman spectrum is very weak, which limits the development of the traditional Raman spectrum [2]. Surface-enhanced Raman scattering (SERS) is a technique that significantly enhances the Raman signals of molecules adsorbed on special substrates to identify chemical and biological molecules [3]. As a powerful and effective analytical technology, SERS has been widely used in food safety, medical diagnosis, environmental monitoring, chemical analysis and other fields [4–11]. At present, the generally accepted mechanisms of SERS enhancement include electromagnetic enhancement mechanism (EM) and chemical enhancement mechanism (CM), and EM plays a major role in the SERS effect [12]. For precious metal materials, when incident light of a certain frequency irradiates its surface, due to the coupling of free electrons on its surface and incident light, precious metal nanoparticles will produce a strong local surface plasmon resonance (LSPR), resulting in SERS based on EM [13,14]. The precious metals (such as Au and Ag) SERS substrate based on EM have significant SERS effect, and the enhancement factor is as high as 10¹⁴, which can realize single molecule detection [15]. For non-precious metal materials, there are three main situations: (1) The CM of most semiconductor SERS substrates mainly comes from the enhancement of charge transfer, which is closely related to the inherent properties of the adsorbed molecules and the surface properties of non-noble metal substrates. When the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) match with the valence band and conduction band energy levels of the substrate, there is an effective charge shift, which changes the polarizability tensor and electron density distribution of the molecule, thus the SERS effect is observed [16]. (2) For some metal-like materials such as MoO₂, transition metal Cu, etc., the free electron density of the surface can be adjusted by changing its microstructure, thus generating LSPR in the visible region to improve the SERS performance of the substrate [17]. (3) Mie scattering occurs when the size of the nanostructure or cavity structure on the substrate is equal to the wavelength of the incident light, which can also improve the SERS performance of the substrate [18]. Common non-precious metal materials such as CdTe, TiO₂, ZnO, Cu₂O, Ta₂O₅ have been developed into SERS active substrate materials [19–22]. They enhance the performance of SERS by changing the shape and structure or by doping with self-doping and hetero-doping. Among them, Ta₂O₅ nanoparticles have the advantages of high biocompatibility, large specific surface area and high chemical stability, so they are one of the semiconductor substrates with the highest enhancement factor [23–26]. In addition, graphene, as an outstanding representative of two-dimensional materials, has been widely used in the study of SERS substrates because of its strong oxidation resistance, strong light transmittance, high electron mobility, excellent heat conduction performance, high toughness and strong chemical stability [27–31].

However, with the continuous development of SERS technology in practical application, the shortcomings of traditional substrates are increasingly exposed because of the single preparation material, such as poor spectral stability and weak repeatability of precious metal substrates [32], low sensitivity and weak enhancement effect of non-precious metal substrates [33]. Therefore, to solve the limitations of single precious metal substrate and non-precious metal substrate and improve the substrate performance to meet the practical application is one of the important topics in the future SERS field [34]. Compared with the substrate of single nanomaterial, the composite substrate composed of two or more materials not only has high sensitivity, but also has uniform Raman signal, stable chemical properties and high biocompatibility [35,36]. In the metal-semiconductor structure, the precious metal shows a strong surface plasmon resonance effect in the visible region, which greatly enhances the SERS effect. Among the common precious metal nanostructures, silver nanoparticles (AgNPs) has excellent SERS properties and controllable morphology, so it has been widely used in the preparation of metal-semiconductor composite substrates [37,38]. At the same time, semiconductor materials, as the bottom or covering layer of metal nanostructures, can increase the specific surface area, which is conducive to the adsorption of the measured molecules, thus enhancing the Raman signal of the tested molecules. In addition, it can avoid the oxidation of precious metal nanostructures directly exposed to air, thus enhancing the stability of the substrate. Based on this theory, the composite substrates of graphene and precious metal materials show excellent SERS properties [39–40]. Compared with graphene, graphene oxide (GO) contains a large number of oxygen-containing functional groups and has higher surface activity, and can combine well with precious metals and semiconductors [41–44]. The multilayer GO not only has the same charge transfer mechanism of π-π stacking as graphene, but also the oxygen-containing groups can adsorb dye molecules, thus enhancing the SERS effect. Therefore, the SERS performance of GO-based composite substrate is better than that of graphene [45–47]. To sum up, GO composite substrate has become a hot spot in the research and preparation of SERS substrate because of its excellent performance and potential application value.

In addition, it has been reported that the sensitivity and quantitative capability of SERS are mutually exclusive [48], because the contribution of analyte molecules at different positions to SERS is very different, which inevitably leads to poor quantitative capability of [49]. In order to solve this problem, we improve the quantitative capability of the Ag/Ta₂O₅ composite substrate by reducing the size of AgNPs and increasing the number of hot spots per unit volume. Ta₂O₅ and AgNPs with smaller size were prepared by hydrothermal reaction. Combined with the advantages of precious metal materials and semiconductor materials, Ag/Ta₂O₅ composite substrate with sandwich structure were designed. The Ta₂O₅ placed at the bottom can attach uniform AgNPs to establish high density active hot spots. In addition, the hydrothermal reaction makes Ta₂O₅ surface bound with a large number of hydroxyl groups, which makes it have stronger electrostatic attraction to basic dyes molecule, thus improving the Raman signal of the measured molecules. We use different concentrations of rhodamine 6G (R6G) to detect the sensitivity and quantification capability of different batches of Ag/Ta₂O₅ composite substrates, and the mapping diagram is used to reflect the consistency of the substrate performance. Through experiments, we believe that the SERS enhancement of the composite substrate is the synergistic effect of electromagnetic enhancement and chemical enhancement, in which the chemical enhancement is the cooperative charge transfer in the system composed of probe molecules and Ag/Ta₂O₅ substrate, and the electromagnetic enhancement comes from the strong local surface plasmon resonance between the adjacent AgNPs. We also discuss the stability of the composite substrate with or without GO. The results show that GO can be used as an inert film to prevent AgNPs from being oxidized, which is beneficial to improve the stability of the composite substrate. In practical application, the composite substrate is still sensitive to malachite green (MG) solution prepared with lake water. Finally, we use the finite-difference time-domain method (FDTD) to simulate the experiment, and the results are consistent with our experiments. Our work shows that GO/Ag/Ta₂O₅ composite SERS substrate has a broad application prospect in the field of food safety and chemical analysis.

2. Experimental details

2.1 Materials

Tantalum oxide (Ta₂O₅, 99.9% metals basis), hydrogen peroxide solution (H₂O₂, AR, 30 wt. % in H₂O), silver nitrate (AgNO₃, AR, 99.8%), hydrofluoric acid (HF, AR, ≥40%), polyvinylpyrrolidone (PVP, average Mw 10000) were purchased from Aladdin Co., Ltd. (Shanghai, China). Ethanol absolute (C₂H₆O, AR, ≥99.7%), acetone (C₃H₆O, AR, ≥99.5%), ethylene glycol (C₂H₆O₂, AR, ≥99.5%) and ammonium hydroxide aqueous solution (NH₃·H₂O, AR, 25∼28%) were bought from Sinopharm Chemical Reagent Co., Ltd. GO dispersion was prepared by Hummers method [50]. All the water used in the experiment is deionized water.

2.2 Synthesis of Ta₂O₅ nanoparticles and AgNPs

Ta₂O₅ nanoparticles were prepared by hydrothermal synthesis in two steps [23]. The first step is to dissolve 2.0 g Ta₂O₅ powder in 20 ml HF and then keep it at 110 °C for 12 hours. Subsequently, NH₃·H₂O is dropped into the mixed solution to form a large number of white flocculent precipitates. Then the precipitates were cleaned and dried to obtain Ta₂O₅ precursors. The Ta₂O₅ precursor is the Ta₂O₅ powder purified from the purchased Ta₂O₅ powder. In the second step, the 0.04 g Ta₂O₅ precursor was fully dissolved in the mixed solution of 50 ml H₂O₂ and 10 ml NH₃·H₂O, then the mixed solution was transferred to a hydrothermal reactor and a silicon wafer was added at the same time and kept at 240 °C for 36 hours. Finally, the white powdered Ta₂O₅ nanoparticles were obtained by cleaning and drying.

AgNPs were prepared by hydrothermal synthesis method [51]. First, 0.5 g PVP and 0.05 g AgNO₃ were dissolved in 20 ml ethylene glycol solution. Then heat the mixed solution to 130 °C and hold for 1.5 hours. When the mixed solution turns bright yellow, stop heating and add 45 ml of acetone to it. Finally, AgNPs were obtained by centrifugation and cleaning.

2.3 Preparation of the composite substrate

The preparation process of GO/Ag/Ta₂O₅ composite SERS substrate is shown in the Fig. 1. Firstly, 0.002 g Ta₂O₅ nanoparticles were dissolved in 20 ml deionized water to form Ta₂O₅ solution. 30 μl Ta₂O₅ solution was dripped on the hydrophilic quartz sheet, then heated and solidified to form the bottom layer of Ta₂O₅. A layer of AgNPs was deposited on the Ta₂O₅ substrate by impregnation method. Finally, the GO/Ag/Ta₂O₅ composite substrate was obtained by spin-coating GO dispersion on the Ag/Ta₂O₅ composite substrate.

 

Fig. 1. Preparation process of GO/Ag/Ta₂O₅ composite SERS substrate.

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2.4 Experimental instrument

Field emission scanning electron microscopy (SEM, ZEISS Gemini Sigma 500) is used to characterize the morphology and structure of composite substrates. Ultraviolet and visible spectrophotometer (UV-Vis, Alpha-1500) is used to measure the absorption spectra of AgNPs solution. Raman detection were performed by using a Raman spectrometer (Horiba HR Evolution 800) at a laser wavelength of 532 nm. The laser power was set as 0.48 mW with an integration time of 4 s and the diffraction grating is 600 gr/mm. When the laser was irradiated on the substrate, the laser power was 0.48 mW and diameter of the spot was about 1µm. Raman spectra were randomly collected on 15 positions, the average values were calculated. Throughout the SERS experiment, the sample was observed with a microscope equipped with a 50-fold objective.

3. Results and discussion

3.1 Characterization of the composite substrate

The morphology and structure of GO/Ag/Ta₂O₅ composite substrate were characterized by scanning electron microscope (SEM). As shown in the Fig. 2(a), it can be observed that the prepared Ta₂O₅ nanoparticles are uniform in size and spherical in shape. We use image J software to analyze the particle diameter of SEM photos. We calculate the particle diameter in the range of 1.0 × 0.8 μm, and there are 204 Ta₂O₅ nanoparticles in this area. The results (Fig. 2(h)) show that the average diameter of Ta₂O₅ nanoparticles is 28.4 nm, and the Ta₂O₅ nanoparticles with particle diameter distribution between 25-30 nm account for 41.2% of the total. These uniform Ta₂O₅ nanoparticles can make Ag/Ta₂O₅ composite substrate have high density and uniform SERS hot spots. As shown in Fig. 2(b), the morphology and structure of AgNPs were characterized by SEM. The prepared AgNPs are uniform in size and spherical in shape, which is beneficial to improve the plasma coupling efficiency in the later stage [52]. Similarly, we calculate the particle diameter in the range of 3.0 × 3.0 μm, and there are 600 AgNPs in this area. The results (Fig. 2(i)) show that the average particle size of AgNPs is 72.9 nm. As shown in Fig. 2(c), we can see that the AgNPs in the Ag/Ta₂O₅ composite substrate is closely arranged, and the Ta₂O₅ nanoparticles at the bottom can be seen from the gap of the Ag. In the white circle marked (Fig. 2(d)), we can clearly see the existence of gauze-like GO film. In Fig. 2(e), the GO/Ag/Ta₂O₅ composite substrate covered with GO film and the Ag/Ta₂O₅ composite substrate without GO film can be clearly distinguished. The element distribution of GO/Ag/Ta₂O₅ composite substrate was analyzed by energy dispersion spectrometer (EDS). As shown in Fig. 2(f), the standard diffraction peaks of Ta, Ag, C, Si and O further confirm the composite structure of Ta₂O₅, Ag and GO. As shown in Fig. 2(g), when the local composition of the sample is measured by EDS element mapping, we can clearly see the uniform distribution of Ta, Ag, C and O on the substrate, which proves that GO, Ag and Ta₂O₅ are uniformly distributed on the quartz substrate. Figure 2(j) shows the absorption peak of the visible part is about 455 nm, so the SERS signal with high signal-to-noise ratio can be produced under the incident laser with 532 nm wavelength. The full width at half maxima (FWHM) is only 100 nm, which indicates that the prepared nanoparticles are uniform. Figure 2(k) shows the Raman spectrum of GO on the GO/Ag/Ta₂O₅ composite substrate. The D peak at 1353 cm^-1 is half of the G peak at 1583 cm^-1 intensity and has a large bandwidth, indicating that GO has obvious structural disorder. There is no obvious 2D peak at 2800 cm^-1, on the contrary, a defect activation peak called Ding can be seen at 2940 cm^-1, which also indicates that the structure of GO is disordered [53].

 

Fig. 2. (a-d) SEM image of Ta₂O₅, AgNPs, Ag/Ta₂O₅ and GO/Ag/Ta₂O₅ composite substrates. (e) SEM image of Ag/Ta₂O₅ and GO/Ag/Ta₂O₅ composite substrates. (f) The energy dispersion spectrometer of the GO/Ag/Ta₂O₅ composite substrates. (g) EDS elemental maps from the GO/Ag/Ta₂O₅ composite substrates. (h,i) The size distribution of Ta₂O₅ and AgNPs. (j) The UV-Vis absorption spectrum of AgNPs. (k) The Raman spectrum of GO.

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The chemical states of Ag, Ta and O in purchased Ta₂O₅ and Ag/Ta₂O₅ composite substrate were analyzed by X-ray photoelectron spectroscopy (XPS). The standard value 284.8 eV of C_1s is used to correct the charge of XPS energy spectrum. Figure (a,b), as a control group, shows the XPS energy spectrum of the purchased Ta₂O₅. As shown above, the Ta_4f spectra of purchased Ta₂O₅ (Fig. 3(a)) and Ag/Ta₂O₅ (Fig. 3(d)) composite substrates can be divided into two peaks, belonging to the characteristic peaks Ta⁵⁺ 4f_7/2 and Ta⁵⁺ 4f_5/2. Compared with purchased Ta₂O₅, the Ta_4f peak of Ag/Ta₂O₅ composite substrate moves slightly to higher binding energy, from 27.8 eV and 26 eV to 28 eV and 26.2 eV, this may be the charge transfer between Ta₂O₅ and AgNPs. In addition, compared with purchased Ta₂O₅ (Fig. 3(b)), the proportion of Ta-OH in Ag/Ta₂O₅ (Fig. 3(e)) composite substrate is much larger. This shows that a large number of hydroxyl groups (-OH) are bound to the surface of Ta₂O₅ nanostructures after hydrothermal reaction. The -OH groups (function as Bronsted acid sites) on the surface of Ta₂O₅ nanostructures make them more electrostatically attractive to the basic dye R6G, CV and MG molecules [24]. Therefore, it directly affects the direction and efficiency of charge transfer between the composite substrate and dye molecules, thus improving the enhancement effect of SERS. As shown in Fig. 3(c,f), the signals clearly observed at 367.8 eV and 373.8 eV are Ag 3d_5/2 and Ag 3d_3/2, indicating the presence of AgNPs.

 

Fig. 3. (a,b) Ta_4f and O_1s detailed XPS spectra of purchased Ta₂O₅ (c) Ag_3d detailed XPS spectra of Ag/Ta₂O₅ composite substrate. (d,e) Ta_4f and O_1s detailed XPS spectra of Ag/Ta₂O₅ composite substrate.(f) Survey XPS spectra of Ag/Ta₂O₅ composite substrate.

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3.2 SERS performance of the composite substrates

We select R6G dye molecules for limit testing on SiO₂, Ta₂O₅, AgNPs and Ag/Ta₂O₅ composite substrates. As shown in Fig. 4(a), the characteristic peak of R6G on AgNPs and Ag/Ta₂O₅ composite substrates is much stronger than that of Ta₂O₅, which can be attributed to the strong LSPR produced by dense and uniform AgNPs. The comparison between Ta₂O₅ and SiO₂ shows that Ta₂O₅ has better SERS activity. In addition, the characteristic peak of R6G on Ag/Ta₂O₅ composite substrate is slightly stronger than that of AgNPs. This is attributed to the charge transfer between R6G and Ag/Ta₂O₅, which increases the SERS effect. Figure 4(b) shows the Raman spectra of different concentrations of R6G on Ag/Ta₂O₅ composite substrates prepared under the same experimental conditions. When the concentration of R6G is 10⁻¹² M, we can still clearly distinguish the main characteristic peaks of R6G molecules, such as 613, 1363 and 1511 cm^-1. As shown in Fig. 4(c), we analyze the relationship between the intensity of the R6G peak and the molecular concentration, and the error strip represents the standard deviation of 15 spectra. The linear fitting was carried out with the concentration as the variable and the peak intensity as the dependent variable. The results showed that the coefficient of determination (R²) at 613 cm^-1 and 1364 cm^-1 were 0.989 and 0.969, respectively. Based on the fitting line, we can quantify the R6G dye molecules to a certain extent based on Raman spectra. Figure 4(d) shows the intensity distribution of the characteristic peaks of R6G molecules obtained on Ag/Ta₂O₅ composite substrates at 613, 1364 and 1511 cm^-1. According to the formula of relative standard deviation (RSD):

In the formula, S is the standard deviation, $\bar{x}$ represents the corresponding average value, i represents each measured value, and n represents the number of tests. Through the calculation, the RSD is 4.47%, 4.67% and 7.23% respectively, demonstrating consistent performance between composite substrates. Figure 4(e) shows the Raman mapping of the characteristic peak at 613 cm^-1 obtained in the range of 20 × 20 μm on the Ag/Ta₂O₅ composite substrate. The relatively uniform color of the spectrum shows that the intensity of the Raman characteristic peak is uniform in most ranges, which is attributed to the fact that the bottom Ta₂O₅ can attach uniform AgNPs, so that the composite substrate has a high uniformity. Figure 5(f) shows the charge transfer between R6G, Ta₂O₅ and AgNPs. There is a charge transfer effect between R6G and Ta₂O₅. The HOMO and the LUMO energy levels of R6G are -5.70 eV and -3.40 eV [54], respectively. The valence band (VB) and conduction band (CB) of Ta₂O₅ nanoparticles are -7.91 eV and -3.78 eV, respectively. Because the band gap between the HOMO level of R6G molecule and the conduction band of Ta₂O₅ nanoparticles is about 1.92 eV, which is obviously smaller than the incident laser energy (2.33 eV). Therefore, electrons may be transferred from the HOMO level of R6G to the Ta₂O₅ conduction band. Secondly, there is charge transfer between R6G and Ta₂O₅ and AgNPs. This is due to the fact that the energy level difference between the Fermi level (-4.84 eV) of AgNPs and the LUMO level of R6G molecules is about 0.86 eV [38], and the energy level difference between AgNPs and the conduction band of Ta₂O₅ is 0.38 eV, which is obviously smaller than the incident laser energy (2.33eV). This series of charge transfer effectively increases the polarizability of the molecule, which in turn enhances the SERS effect of the substrate. Therefore, there is charge transfer in the system formed by R6G molecules and Ag/Ta₂O₅ composites, which plays an important role in the enhancement of SERS signals of R6G molecules.

 

Fig. 4. (a) The Raman spectra of different concentrations of R6G on SiO₂, Ta₂O₅, AgNPs and Ag/Ta₂O₅ composite substrates. (b) Raman spectra of R6G on the Ag/Ta₂O₅ composite substrates from 10⁻⁷ M to 10⁻¹² M. (c) Log−log plot of average intensity of SERS signals at 613 and 1363 cm⁻¹ versus the concentration of R6G. (d) The intensity distributions of the 613, 1364 and 1511 cm⁻¹ peaks for R6G. (e) Corresponding Raman mapping of the 613 cm⁻¹ characteristic peak of R6G (10⁻⁷ M). (f) The charge transfer between R6G, Ta₂O₅ and AgNPs.

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Fig. 5. (a) The Raman spectra of different concentrations of CV on SiO₂, Ag/Ta₂O₅ and GO/Ag/Ta₂O₅ composite substrates. (b) Raman spectra of CV on the GO/Ag/Ta₂O₅ composite substrates from 10⁻⁶ M to 10⁻¹¹ M. (c) Log−log plot of average intensity of SERS signals at 915 and 1621 cm⁻¹ versus the concentration of CV. (d) Raman spectra of CV (10⁻⁷ M) on GO/Ag/Ta₂O₅ composite substrates at different time. (e) Raman spectra of CV (915 cm^-1) on GO/Ag/Ta₂O₅ composite substrate and Ag/Ta₂O₅ composite substrate. (f) Corresponding Raman mapping of the 915 cm⁻¹ characteristic peak of CV (10⁻⁶ M).

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As a triarylmethane dye, Crystal Violet (CV) can be used as a disinfectant for human skin. However, this dye has carcinogenicity and is classified as a stubborn molecule because it is not easy to be metabolized by microorganisms and can persist in a variety of environments [55]. Therefore, it is necessary to detect CV accurately and quickly. As shown in Fig. 5(a), we used CV to test the limit of SiO₂, Ag/Ta₂O₅ and GO/Ag/Ta₂O₅ composite substrate. When the concentration of CV is 10⁻¹¹ M, the main characteristic peaks of CV can still be clearly distinguished. The intensity of the characteristic peak of CV on GO/Ag/Ta₂O₅ composite substrate is slightly stronger than that on Ag/Ta₂O₅ composite substrate, which can be attributed to the chemical enhancement caused by π-π stacking of GO films and charge transfer between oxygen-containing functional groups and CV molecules. Figure 5(b) shows the Raman spectra of different concentrations of CV on GO/Ag/Ta₂O₅ composite substrates prepared under the same experimental conditions. As shown in Fig. 4(c), we have analyzed the relationship between the intensity of the characteristic peak and the molecular concentration. The linear fitting was carried out with the concentration as the variable and the peak intensity as the dependent variable, and the error strip represents the standard deviation of 15 spectra. The results showed that the R² at 915 and 1621 cm^-1 were 0.995 and 0.990, respectively. Based on the fitting line, we can quantify the CV dye molecules to a certain extent based on Raman spectra. In order to verify the effect of GO on the stability of composite substrates, Ag/Ta₂O₅ and GO/Ag/Ta₂O₅ composite substrates were exposed to air and tested every 7 days. As shown in Fig. 5(d), it can be clearly observed that the spectral peak intensity of CV on the GO/Ag/Ta₂O₅ composite substrate attenuates slowly with the passage of time. As shown in Fig. 5(e), we analyze the relationship between the intensity of the characteristic peak of Ag/Ta₂O₅ and the GO/Ag/Ta₂O₅ composite substrate at 915 cm^-1 and the time, and the error strip represents the standard deviation of 15 spectra. The signal on the Ag/Ta₂O₅ substrate decreased by 36.26%, while the signal on the GO/Ag/Ta₂O₅ composite substrate decreased by 20.49%. This phenomenon can be attributed to the protective effect of GO. GO film can be used as a barrier for atomic diffusion, which can effectively prevent the penetration of oxygen and protect AgNPs from oxidation, so that the GO/Ag/Ta₂O₅ composite substrate has good stability. Figure 5(f) shows the Raman mapping of the 915 cm^-1 characteristic peak obtained on the GO/Ag/Ta₂O₅ composite substrate. The relatively uniform color of the spectrum shows that the intensity of the Raman characteristic peak is uniform in most ranges, indicating that the introduction of GO will not affect the uniformity of the composite substrate.

The SERS performance of the composite substrate can be evaluated intuitively by calculating the enhancement factor (EF). The EF for CV molecules is calculated by the standard equation [56]:

In the formula, I_SERS and I_RS are the intensity of the SERS peak and the normal Raman intensity respectively, and N_RS and N_SERS are the number of CV molecules per unit volume of SERS and normal Raman scattering. The peak strength of 10⁻² M CV at 915 cm^-1 on SiO₂ is 202.15 and the peak intensity of 10⁻¹¹ M CV on GO/Ag/Ta₂O₅ composite substrate is 304.61. As a result, the average EF of GO/Ag/Ta₂O₅ composite substrate is 1.5 × 10⁹, which is higher than that of several Ta₂O₅-based or GO/Ag-based composite SERS substrates that have been reported, as shown in Table 1.

Table 1. Sensitivity Comparison of Several Ta₂O₅-based or GO/Ag-based Composite SERS Substrates

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Malachite green (MG) is a common dye and fungicide, which is widely used in aquaculture because of its low cost. However, it is also a carcinogen, which can stay in fish for a long time and may cause human teratogenicity, carcinogenicity and mutagenicity through the food chain [57]. Therefore, it is necessary to detect MG accurately and quickly in aquatic products. In order to study the possibility of the application of GO/Ag/Ta₂O₅ composite substrate in practical samples, we carried out SERS detection of MG in deionized water and lake water. We dripped 0.2 μL of lake water on the quartz substrate and tested it after natural drying. As shown in Fig. 6(a), it can be seen that there is only one Raman characteristic peak (482 cm^-1) from the quartz substrate in the Raman spectrum of the lake water. Similarly, the Raman spectra of 10⁻² M concentration MG prepared with lake water also have Raman characteristic peaks from quartz substrate. Therefore, it can be proved that there is no obvious Raman characteristic peak in the lake water. We prepared MG solutions with different concentrations from lake water to test the practical application effect of GO/Ag/Ta₂O₅ composite substrates. In Fig. 6(b), we can easily detect the characteristic Raman peaks of MG prepared by lake water in the concentration range of 10⁻⁵ M to 10⁻¹⁰ M. In Fig. 6(c), we linearly fit the intensity of the MG peak at 1618 and 1368 cm^-1 with the molecular concentration, and the error strip represents the standard deviation of 15 spectra. The results show that the R² at 1618 and 1368 cm^-1 is about 0.995 and 0.985. On the basis of the fitting line, the MG dye molecules in lake water can be quantified to a certain extent according to Raman spectra. Therefore, the above results show that the proposed GO/Ag/Ta₂O₅ composite substrate can meet the practical application, and show the great application potential of the composite substrate in the field of food safety and chemical detection.

 

Fig. 6. (a) The Raman spectra of the measured lake water without MG on SiO₂, the Raman spectra of 10⁻² M MG prepared with lake water on SiO₂, and the Raman spectra of 10⁻¹⁰ M MG droplets prepared with lake water on the composite substrate. (b) Raman spectra of MG on the GO/Ag/Ta₂O₅ composite substrates from 10⁻⁵ M to 10⁻¹⁰ M. (c) Log−log plot of average intensity of SERS signals at 1618 and 1368 cm⁻¹ versus the concentration of MG.

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In order to better understand the electric field enhancement mechanism of GO/Ag/Ta₂O₅ composite substrates, we use the finite-difference time-domain method (FDTD) software to analyze the local electric field distribution of these structures. According to the SEM image and the actual experimental conditions, we set the wavelength of incident light to 532 nm, the diameter of AgNPs to 72.9 nm the diameter of Ta₂O₅ particles to 28.4nm, and the thickness of GO to 1 nm to simulate. By comparing the Fig. 7(a,b), it is obvious that due to the strong LSPR between AgNPs, the local electric field between AgNPs is very strong. It can be seen from Fig. 7(c) that the local electric field strength between AgNPs and Ta₂O₅ is much higher than that in Fig. 7(a), which can be attributed to the chemical enhancement caused by the charge transfer between AgNPs and Ta₂O₅. This is due to the fact that the energy difference between the Fermi level of AgNPs and the conduction band of Ta₂O₅ is 0.38eV, which is obviously smaller than the incident laser energy (2.33eV), so the charge transfer occurs. As shown in Fig. 7(d), when GO film is introduced, there is no new hot spot between GO film and AgNPs. This is due to the fact that the GO monolayer we set up does not have π-π stacking and there is no charge transfer between the oxygen-containing functional groups and the probe molecules, so it does not cause any SERS enhancement. Through FDTD, we verified that AgNPs and Ta₂O₅ can also have charge transfer in the absence of probe molecules, which is consistent with our experimental results.

 

Fig. 7. (a)-(d) Simulated electric field distribution of Ta₂O₅, AgNPs, Ag/Ta₂O₅ and GO/Ag/Ta₂O₅ composite substrates.

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4. Conclusions

In this paper, we have prepared a GO/Ag/Ta₂O₅ composite substrate with high sensitivity, high quantification capability and stable properties. We use R6G and CV molecules to carry out Raman detection of the composite substrate under different conditions. It is proved that the reason for the high sensitivity is the synergistic effect of chemical enhancement and electromagnetic enhancement, and the reason for the stability comes from the protection of GO. At the same time, a large number of hydroxyl groups on the surface of Ta₂O₅ nanoparticles and oxygen-containing functional groups on the surface of GO enhance the molecular selectivity of the GO/Ag/Ta₂O₅ composite substrate. We also prove that the introduction of GO not only improves the stability of the composite substrate, but also keeps the performance of the substrate consistent. Based on the practical application, we verify the feasibility of detecting MG content in lake water with GO/Ag/Ta₂O₅ composite substrate, which provides a new choice for water quality and food detection.