Open Access
Add to BibSonomy
Abstract
Graphene(G)-noble metal-ZnO hybrid systems were developed as highly sensitive and recyclable surface enhanced Raman scattering (SERS) platforms, in which ultrathin graphene of varying thickness was embedded between two metallic layers on top of a ZnO layer. Due to the multi-dimensional plasmonic coupling effect, the Au/G/Ag@ZnO multilayer structure possessed ultrahigh sensitivity with the detection limit of Rhodamine 6 G (R6G) as low as 1.0×10−13 mol/L and a high enhancement factor of 5.68×107. Both experimental and simulation results showed that graphene films could significantly regulate the interlayer plasmon resonance coupling strength, and single-layer graphene had the best interlayer regulation effect. Additionally, the SERS substrate structure prepared through physical methods exhibited high uniformity, the graphene component of the substrate possessed excellent molecular enrichment ability and silver oxidation inhibition characteristics, resulting in a substrate with high stability and exceptional reproducibility. The signal change was less than 15%. Simultaneously, due to the excellent photocatalytic performance of the low-cost and wide-band-gap semiconductor material ZnO, the SERS substrate exhibited exceptional reusability. Even after five cycles of adsorption-desorption, the SERS performance remained stable and maintained a reliable detection limit. The study introduced a novel approach to creating multilayer composite SERS substrates that exhibited exceptional performance, offering a new analytical tool with high sensitivity, stability, and reusability.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) technology has great potential application in the field of rapid detection of trace organic pollutants due to its advantages such as ultra-sensitivity, fingerprinting, and non-destructive detection [1–5]. This technology has been reported in many literatures for the detection of various trace pollutants and biomolecules in the environment, including the food additive Rhodamine 6 G (R6G) [6], organic dye Methylene Blue (MB) [7], antibacterial agent Ciproflxacin (CIP) [8], Paracetamol [9,10], deoxyribonucleic acid (DNA) and β-amyloid protein (Aβ), et al. [11]. Au and Ag nanomaterials exhibit excellent localized surface plasmon resonance effects in the visible/near-infrared region, which widely used to construct highly efficient SERS active substrates [12,13]. Numerous studies have been conducted on the synthesis of gold and silver nanostructures with highly anisotropic performance, including flower-like, dendritic, urchin-like and star-like, whose surface multilevel nano-gaps and bulges can provide sufficient surface-enhanced Raman spectroscopy (SERS) activity hotspots and improve the sensitivity of Raman signal detection [14–18]. However, the above methods exist defects of complex preparation process and poor uniformity of hot spots.
As a new stable carbonaceous material, graphene makes graphene-noble metal nanocomposites an important development direction for new efficient SERS active substrates due to its chemically enhanced, fluorescence quenching, passivation protection and molecular enrichment performance [19–23]. Researchers have shown a preference for sandwich SERS substrates with vertically oriented interlayer particle gaps constructed from graphene or graphene oxide as nanospatial layers among the many graphene-noble metal composites. This type of substrate allows for the creation of precisely designed localized surface plasmon resonances between layers of precious metals, resulting in significantly enhanced electromagnetic fields [24,25]. Li et al. proposed a novel nanoparticle-film gap (NFG) system by introducing an ultrathin monolayer graphene between Ag NPs and Ag films as a sub-nanometer spacer. They successfully constructed a G-NFG heterostructure with huge near-field enhancement (up to 1700 ratio), which has one of the highest enhancement ratios reported for a graphene-metal plasma combination system so far [26]; Han et al. developed a three-dimensional AuAg ANPs/graphene(G)/AuAg ANPs sandwiched hybrid nanostructured substrate by a reproducible interfacial self-assembly technique. The structure consisted of two layers of AuAg ANPs film arrays using monolayer graphene as a nanospacer. The substrate exhibited much higher SERS sensitivity and uniformity for the detection of R6G molecules [27]; Zhao et al. assembled graphene oxide (GO) and anisotropic metal nanostructures to construct a novel multi-coupling system, in which ultrathin graphene oxide was sandwiched between two layers of closely packed gold nanostars (AuNSts) as a nanospacer layer. The graphene oxide not only provided vertically oriented electric field coupling, but its additional chemical enhancement as well as molecular adsorption ability with detection limits as low as 10−13 mol/L for R6G and enhancement factors as high as 6.64 × 107 [28]. However, these multiple coupling systems of graphene/graphene oxide interlayer nanostructures are built on chemically prepared noble metal nanoparticles with limited signal homogeneity. Meanwhile, little research has been reported on regulating the number of different graphene layers to find the optimal nanogap for composite structures.
It is also worth noting that most SERS substrates are not reusable. Both from economic point and quantitative analysis point of view, it is unfavorable. Fortunately, the enhancement of multiple electric field and photocatalytic degradation ability in non-noble metal and noble metal/semiconductor composite systems has enabled the substrate to have both SERS ultra-sensitive detection performance and self-cleaning efficacy, which has attracted considerable attention [29–31]. The combination of ZnO, a representative photocatalytic material, with noble metals promote the plasma effect between noble metals and charge transfer effect at the noble metal-semiconductor interface, which significantly improve the effectiveness of SERS detection. At the same time, the Schottky barrier formed by the noble metal-ZnO heterostructure can induce electrons in the ZnO conduction band and transfer them to the noble metal, effectively inhibiting the recombination of electrons and holes, which is conducive to improve the photocatalytic degradation efficiency [32].
Based on this, we innovatively prepared a SERS active platform, using the zinc oxide layer as the self-cleaning layer, and introducing the graphene film as the ideal nano-gap between the magnetron sputtered silver particle layer and the thermally evaporated gold particle layer. The regulation mechanism of intra-layer and inter-layer multiple plasmon resonance coupling of composite substrates was simulated using FDTD simulation with varying thicknesses of graphene layers. The result was that monolayer graphene has the most excellent Controllable performance. The substrates exhibit significantly enhanced Raman sensitivity, high reproducibility and reusability on account of the molecular enrichment and passivation of graphene, the multi-dimensional plasma coupling (inter- and intra-layer), the high structural homogeneity of the substrates prepared by physical methods, and the photocatalysis of zinc oxide. The resulting Au/G/Ag@ZnO composite substrate had a detection limit of R6G up to 10−13 mol/L, a signal changes of <15% and good SERS performance even after 5 cycles.
2. Experimental
2.1 Materials
Zinc oxide targets (99.99%), silver targets (99.99%), and gold targets (99.99%) were purchased from Zhongnuo New Material Technology Co., Copper-based graphene graphene (10 × 5 cm) was purchased from Nanjing Xianfeng Nanomaterials Technology Co., Absolute ethanol (99.8%), polymethyl methacrylate (PMMA) (ethyl acrylate < 5 wt.%), and acetone (C3H6O, 98%) were purchased from Macklin (shanghai, China). Rhodamine 6 G (R6G) and methylene blue (MB) were purchased from Alfa Aesar (Ward Hill, MA, USA), and were used as received. Water purified with a Millipore system was used throughout all the experiments.
2.2 Preparation of Au/G/Ag@ZnO composite SERS substrates
The specific preparation processes were divided into the following four steps: (1) zinc oxide film was prepared on silicon wafers using magnetron sputtering apparatus (UHV-Sputter-350 L) with Zinc oxide as the sputtering material. The process involved using Ar gas with a flow rate of 30 sccm, sputtering pressure of 5.28 × 10−3 Torr, sputtering rate of 0.23 Å/s, and sputtering time of 144 min. (2) Ag@ZnO heterostructures were obtained by sputtering a silver particle layer (flow rate: 21 sccm, sputtering pressure: 4.36 × 10−3 Torr, sputtering time: 325 s). (3) The CVD graphene (G) is transferred onto the Ag particle layer using a wet chemical etching technique to obtain the G/Ag@ZnO multilayer structure, as described detailly in the group's previously published work [33]. (4) Gold nanoparticle layers were prepared on the G/Ag@ZnO multilayer structure through electron beam evaporation technique to obtain the Au/G/Ag@ZnO multilayer structure (vacuum: 9.1 × 10−9 Torr, evaporation rate: 0.25 Å/s, evaporation time: 400 s). Specifically, the composite structures with no graphene, monolayer graphene, few-layer graphene, and multilayer graphene as spacer layers were noted as Au/Ag@ZnO, Au/G1/Ag@ZnO, Au/G2/Ag@ZnO, and Au/G3/Ag@ZnO. The specific preparation processes are shown in Fig. 1.
2.3 Characterization
The morphology of the substrate was characterized by field emission scanning electron microscopy (FE-SEM, Zeiss Supra 55 VP). The surface morphology of the sample was measured using an atomic force microscopy (AFM, MFP3D Origin Plus, Asylum Research, Oxford, UK). The Crystal structure of zinc oxide was characterized by X-ray diffraction (XRD, Siemens D800) (Cu Kα source, λ= 0.154 nm, scanning rate: 2 (°) min−1, operating voltage: 40 kV, operating current: 40 mA). The chemical composition of the substrate was analyzed by X-ray photoelectron spectrometry (XPS, Thermo ESCALAB 250Xi). SERS spectra were recorded on a model confocal microscope Raman system (Renishaw in Via). With excitation wavelength of 532 nm, the integration time of 10 s, the laser power of 5 mW and a 100× objective lens of a diameter of 1 µm. The photocatalytic and self-cleaning measurements was conducted with a xenon lamp light source (PLS-SXE300) (Current power: 20A, distance between measurements solution and light source: 2-5 cm).
2.4 SERS measurements
Reproducibility measurements: 2 mL R6G solution with a concentration of 10−11 mol/L was dropped on the prepared substrate surface. After drying, 20 different sites were randomly selected on the substrate surface for Raman spectroscopy detection.
Photocatalytic degradation measurements: The Au/G1/Ag@ZnO composite substrate was immersed in R6G solution with a concentration of 10−6 mol/L for 20 min to ensure the adsorption equilibrium achieved, and then Raman spectroscopy was performed after the substrate was taken out and dried. Subsequently, the measured substrate was immersed in deionized water and conducted photocatalytic degradation with xenon lamp placed above it. During this period, the irradiated substrate was taken out every 30 min, rinsed with deionized water, and dried at room temperature before Raman measurements.
Self-cleaning performance measurements: The Au/G1/Ag@ZnO composite substrate was immersed in R6G solution with a concentration of 10−6 mol/L for 20 min to ensure the adsorption equilibrium achieved, and then Raman spectroscopy was performed after the substrate was taken out and dried. Subsequently, the measured substrate was immersed in deionized water and conducted photocatalytic degradation with xenon lamp placed above it for self-cleaning for 120 min. Then the substrate was thoroughly rinsed with deionized water, and repeated for Raman measurements after dried at room temperature. The above steps were a self-cleaning SERS detection cycle.
2.5 Finite difference time-domain (FDTD) simulation
Finite-difference time-domain (FDTD) simulations were used to investigate the electromagnetic field intensity distribution of Ag NPs and Au NPs in Au/G1/Ag@ZnO composite substrates and the effect of different graphene layers in the composite structure on the electric field intensity. The incident light was a plane wave, the direction was vertically downward along the z-axis, and the excitation wavelength was 532 nm. Periodic boundary condition was set in the X, Y planes and perfectly matched layer (PML) was chosen in the Z direction. As the geometric model was close to the sub-nanometre scale, the minimum unit of mesh set in the simulation was 10−4 nm for accurate calculations. Different position monitors were set for different regions to probe the electromagnetic enhancement effect of composite substrate.
3. Results and discussion
3.1 Structure and morphology analysis
The morphologies of the substrates prepared in the above experiments were analyzed by scanning electron microscopy. Figure 2(a) shows the SEM image of ZnO films, and the inset shows the image taken under high magnification microscope, from which it is clearly seen that the surface of ZnO films is dense and the size of nanoparticles is uniform. From the particle size distribution statistics in Fig. 2(b), the particle size is about 70 ± 20 nm. Figure 2(h) shows the XRD pattern of ZnO films, and the characteristic peak at 2θ=34.27° is the diffraction peak of a typical ZnO film [34]. Figure 2(c) shows the SEM image of Ag@ZnO substrate. compared with the high-magnification microscope image of ZnO thin films, it is obvious that a large number of small particles of Ag NPs are deposited on the surface of ZnO, and the particle size is about 23 ± 8 nm as shown in Fig. 2(d). Figure 2(e)-(g) shows SEM images of monolayer, few-layer and multilayer graphene after transferring respectively. From the three images, obvious graphene sheet structure can be seen, which indicate that graphene is successfully transferred to the surface of Ag@ZnO substrate. Moreover, with the increase of graphene thickness, the color of SEM image becomes darker, and the morphology of Ag@ZnO substrate becomes blurred.
Fig. 2. SEM images of (a) ZnO substrate, (c) Ag@ZnO substrate, (e)-(g) monolayer, few-layer, and multilayer graphene/Ag@ZnO substrate. Particle size distribution histogram of (b) ZnO substrate, (d) Ag@ZnO substrate. (h) XRD pattern of ZnO.
To characterize the number of graphene layers transferred clearly, the monolayer, few-layer and multilayer graphene are respectively transferred to the blank silicon wafer surface for Raman measurements. Figure 3(a)-(c) shows the Raman spectra of monolayer, few-layer and multilayer graphene respectively. In the Raman spectra of all samples, there are three representative characteristic peaks: The D peak is at 1350 cm−1, the G peak is at 1582 cm−1, and the 2D peak is at 2700 cm−1.The ratio of 2D peak to G peak can represent the number of graphene layers, and D peak is used to represent the degree of lattice defects of graphene, and the number of graphene layers is inversely proportional to the ratio of 2D peak to G peak (I2D/IG) [35–37]. Figure 3(a) shows that the ratio of 2D peak to G peak is much larger than 2, and the intensity of D peak is very small compared with G peak, which indicates that the graphene we transferred is high-quality monolayer graphene. In Fig. 3(b), obvious D peak appears, and the ratio of 2D peak to G peak is obviously reduced compared with Fig. 3(a), indicating that the number of transferred graphene layers is increased, which is presumed to be few-layer graphene. However, in Fig. 3(c), not only the obvious D peak appears, but also the ratio of the 2D peak to the G peak is less than 1, which indicates that the transferred graphene is multilayer.
Figure 4(a)-(d) shows the SEM images of Au nanoparticles after thermal evaporation, Fig. 4(a) shows the Au/Ag@ZnO composite structure, due to the absence of graphene as a spacer layer, Au nanoparticles are directly deposited on the surface of Ag nanoparticles, and most Au nanoparticles are aggregated into clusters with local areas showing membrane-like structure. Figure 4(b)-(d) shows the SEM images of Au/G1/Ag@ZnO, Au/G2/Ag@ZnO, and Au/G3/Ag@ZnO, respectively. As the graphene serves as a nano-spacer layer, the combination of Au nanoparticles and Ag nanoparticles is well separated, and it can be clearly observed that small Au nanoparticles are attached to the graphene surface. In addition, with the increase of graphene layers, the color of the image becomes darker, and it is difficult to clearly see the Au nanoparticles deposited on the surface.
To characterize the successful assembly of the Au nanolayer further clearly, the surface morphology of the Au nanolayer in the Au/G1/Ag@ZnO composite structure was characterized by AFM measurements. Figure 5(a) shows the AFM image of the Au/G1/Ag@ZnO. From the figure, the surface structure of the Au nanolayer after thermal evaporation is uniform and flat, and its surface roughness is only less than 3 nm, which indicates that the Au nanolayer is successfully constructed on the graphene surface. The composition of the constructed composite substrate was further determined and the distribution state of various elements in the composite substrate was explored. The SEM image of the Au/G1/Ag@ZnO composite substrate was analyzed by energy spectrum. The composition of the Au/G1/Ag@ZnO composite structure, as shown in Fig. 5(b), mainly consists of Zn, O, and C elements, with trace amounts of Ag and Au elements. The proportions of these elements are 32.45%, 35.76%, 15.70%, 13.54%, and 2.55%, respectively. These results confirm the successful preparation of the SERS substrate.
XPS analysis shows the charge correction is performed on the Au/G1/Ag@ZnO composite substrate (taking the C 1s standard value of 284.8 eV) in Fig. 6(a). The coexistence of C, O, Ag, Au and Zn can be observed in the XPS full spectrum curve. Since zinc oxide is at the bottom layer and silver, graphene and gold components are assembled on its surface, the spectrum peak of Zn is not obvious. Figure 6(b) shows the high-resolution energy spectrum of C 1s in Au/G1/Ag@ZnO composite structure, and three obvious spectral peaks can be found, 284.8 eV corresponding to C-C bond, 286.1 eV corresponding to C-O bond, and 288.6 eV corresponding to C = O bond [38]. Figure 6(c) shows the high-resolution spectrum of O 1s, which can be fitted to the synergistic effect of Zn-O (530.4 eV), Zn-OH (531.5 eV) and C-O (532.7 eV) in the lattice [39]. Figure 6(d) shows the high-resolution energy spectrum of Zn 2p. The fitted peaks of Zn 2p3/2 and Zn 2p1/2 at 1021.8 eV and 1044.7 eV are consistent with the experimental spectral lines, which proves that Zn in the heterostructure exists only in the ZnO lattice in the form of Zn2+ [40]. Figure 6(e) is the high-resolution energy spectrum of Ag 3d. The spectrum peak presented is a significant double-peak structure, which is attributed to the fitting peak of Ag 3d5/2 (368.1 eV) and Ag 3d3/2 (374.1 eV), proving that Ag in the structure exists in the form of Ag° [41]. Figure 6(f) is the high-resolution energy spectrum of Au 4f. After thermal evaporation of Au nanoparticles on the graphene surface, the obtained spectral line is also a double-peak structure, which is attributed to the fitting peaks of Au 4f7/2 (84.0 eV) and Au 4f5/2 (87.7 eV), and only exists on the graphene surface in the form of Au°, without doping and heterostructure [42].
Fig. 6. XPS spectra of (a) Au/G1/Ag@ZnO composite substrate. High-resolution spectra of (b) C1s, (c) O1s, (d) Zn 2p, (e) Ag 3d, (f) Au 4f.
3.2 FDTD calculations
To investigate the electromagnetic enhancement effect of different noble metal materials in the composite structure, the corresponding geometric model was established according to its SEM images, as shown in Fig. 7(a). The bottom layer was set with silver nano-layer with particle size of 24 nm and particle spacing of 5 nm, and graphene with different thickness was laid on the top layer as nano-spacer layer. Finally, the top layer was set with gold nano-layer with particle size of 5 nm and particle spacing of 2 nm. Figure 7(b) shows the electric field distribution of Ag nanoparticles. From the figure, the electric field is only stimulated around the Ag nanoparticles and at the particle gap, and the electric field stimulated at the particle gap is the strongest. Figure 7(c) shows the electric field distribution of Au nanoparticles. Similarly, the electric field is greatly enhanced around the gold nanoparticles and the gap. The FDTD theoretical simulation proves that the electromagnetic enhancement mainly occurs near the noble metal nanoparticles and the nanoparticle gap. Furthermore, the influence of different graphene thickness on electromagnetic enhancement in the composite structure is simulated by FDTD simulation. Specifically, the graphene thickness is set to 0, 0.5, 1.5, 3 nm respectively without changing size and spacing of noble metal Ag and Au nanoparticles to investigate the multiple electric field coupling effect of graphene as a nano-spacer layer on the composite structure. Figure 7(d)-(g) are the electric field distribution diagrams of different graphene thicknesses, from which it can be seen that there is obvious longitudinal electric field coupling between the gold nanolayer and the silver nanolayer in the composite structure with graphene thickness of 0 and 0.5 nm, and the graphene with graphene thickness of 0.5 nm (Monolayer graphene) coupling is stronger, and there are relatively more hot spots. With the thickening of graphene, multiple electric field coupling between the two metal layers cannot be observed in Figs. 7(f) and (g). The simulation results further confirm that the graphene component, as a noble metal nano-spacer layer, providing a good noble metal resonance coupling effect for the substrate, and the nanometer thickness unique to the monolayer graphene is precisely the most favorable space required for noble metal resonance coupling.
Fig. 7. (a) FDTD simulation model structure diagram and electric field distribution of (b) Ag, (c) Au. The electric field distribution of different thickness graphene (d) 0 nm, (e) 0.5 nm, (f) 1.5 nm, (g) 3 nm as the precious metal nano-spacer layer.
3.3 SERS performance
In order to verify the simulation results and investigate the effects of noble metal nano-components and graphene layers on the SERS performance of the substrates, the Raman spectra of Ag@ZnO, Au/Ag@ZnO, G1/Ag@ ZnO, G2/Ag@ZnO, G3/Ag@ZnO, Au/G1/Ag@ZnO, Au/G2/Ag@ZnO and Au/G3/Ag@ZnO composite SERS substrates were measured using 10−9 mol/L R6G solution. From Fig. 8, Raman characteristic peaks of R6G can be detected for all kinds of substrates, which are located at 611, 771, 1184, 1310, 1360, 1507, 1574 and 1648 cm−1 respectively. It is noteworthy that the Raman characteristic peak intensity of R6G on Au/G1/Ag@ZnO substrate is the highest, the monolayer graphene has a significant enhancement of the Raman signal on the substrate compared with Ag@ZnO, G1/Ag@ZnO, G2/Ag@ZnO, and G3/Ag@ZnO SERS substrates. However, with the increase of graphene layers, the SERS signal gradually attenuates, especially the Raman data measured on G3/Ag@ZnO substrate is weaker than measured on Ag@ZnO substrate. The reason for the above phenomenon may be that the graphene sheet with the function of enriching molecules plays a role, which improves the adsorption efficiency of the composite SERS substrate to the probe molecule R6G, and further improves the SERS performance of the composite structure. When the graphene sheet increases from a monolayer to a plurality of layers, the molecular enrichment ability of the sheet material gradually cannot offset the shielding of the Ag@ZnO substrate caused by the increase of the thickness of the graphene sheet, which weakens the light response ability of the composite structure. Therefore, the SERS performance of different composite structures is enhanced at first and then attenuated.
By further comparing the Raman characteristic peaks of probe molecule R6G on the surface of G1/Ag@ZnO, G2/Ag@ZnO, G3/Ag@ZnO, Au/Ag@ZnO, Au/G1/Ag@ZnO, Au/G2/Ag@ZnO and Au/G3/Ag@ZnO composite SERS substrates, it could be observed that the SERS performance of the composite structure was significantly enhanced after loading Au nano-components on the surface of the composite structure, and the SERS substrate signal of the monolayer graphene component was the strongest. The reasons for this phenomenon are analyzed. Firstly, it is attributed to two factors. The local surface plasmon resonance effect of gold nanoparticles enhances the SERS performance of the substrate. Secondly, the graphene component serves as a noble metal nano-spacer layer to provide a good noble metal resonance coupling effect for the substrate, and the unique nanometer thickness of the monolayer graphene is just the most favorable space required for the noble metal resonance coupling, so that the SERS performance of the substrate is further enhanced. The experimental results are consistent with the FDTD simulation results, so the monolayer graphene as the noble metal nano-spacer layer has the best SERS enhancement performance. Therefore, the Au/G1/Ag@ZnO composite structure with monolayer graphene component is selected for SERS spectrum analysis in the subsequent experiments.
To quantify the SERS performance of the as-prepared Au/G1/Ag@ZnO composite substrate, the enhancement factor (EF) of the composite structure was calculated according to the following formula:
In Eq. (1), IRaman and ISERS are that Raman intensity of the ordinary R6G molecule and the SERS intensity of the enhance R6G molecule, respectively. Nbulk is the number of all probe molecules under the laser for Raman measurements and NSERS is the number of probes excited by the Raman laser for SERS detection. In Eq. (2), the average molecular weight of R6G detected by Raman in its scattering region is calculated using the molecular weight of R6G (M = 479.01 g/mol) and the density (ρ=1.26 g/cm3).NA is Avogadro's constant, and Aspot is the area of the irradiation area of the laser beam having a diameter of 1 µm and a focal depth h of 20 µm. In formula (3), an aqueous solution of R6G (V = 2 mL, C = 10−9 mol/L) is rapidly dispersed into a circular region with a diameter of 0.8 cm and an area of Asubstrate. We chose the characteristic peak at 611 cm−1 for calculation. The enhancement factor of Au/G1/Ag@ZnO composite SERS substrate was calculated as EF = 5.68 × 107.
In addition, we aimed to measurements the sensitivity of the Au/G1/Ag@ZnO composite substrate, which exhibited the best SERS performance. Specifically, the Raman spectra of Au/G1/Ag@ZnO composite SERS substrates were measured by using R6G probe molecules with concentration gradients of 10−8, 10−9, 10−10, 10−11, 10−12, 10−13 and 10−14 mol/L. From Fig. 9, the Raman intensity of the characteristic peak of R6G decreases with the decrease of the solution concentration. When the concentration of R6G decreases to 10−14 mol/L, the Raman characteristic peak of R6G can no longer be detected, and the weak characteristic peak can be seen in the R6G solution with the concentration of 10−13 mol/L, which proves that the composite substrate has an ultra-sensitive detection limit as low as 10−13 mol/L.
Furthermore, the SERS reproducibility of Au/G1/Ag@ZnO nanostructures was investigated. 20 different positions were randomly selected from the substrate surface for Raman detection of 10−11 mol/L R6G, and recorded. As shown in Fig. 10, the SERS spectra of the 20 measurements sites are basically consistent. The relative standard deviations (RSD) of Raman intensities at 611,771 and 1648 cm−1 are calculated. The RSD values of the three characteristic peaks are 13.7%, 11.4% and 12.1%, respectively. The RSD values of the three characteristic peaks are all less than 15%, indicating that the composite substrate has good spectral reproducibility for SERS detection.
Fig. 10. (a) SERS spectrum of 20 different sites. RSD values of (b) 611 cm−1, (c) 771 cm−1, (d) 1648 cm−1.
3.4 Self-cleaning performance
In this section, we will make full use of the unique photoresponse characteristics of zinc oxide under visible light to achieve self-cleaning and renewable recycling purposes under UV-visible light. To investigate the self-cleaning performance of the composite structure, we utilized a R6G probe molecule with a concentration of 10−6 mol/L as our research subject. Figure 11(a) shows the corresponding Raman spectra under different irradiation time. The Raman signal of R6G is significantly reduced after irradiation for 30 min. As the irradiation time was extended, the Raman signal of R6G gradually diminished and was nearly undetectable after 120 minutes, which meant that it was completely degraded under UV-vis irradiation. This was due to the visible light excitation of electrons from the ZnO valence band (VB) to the conduction band (CB), the electrons on the conduction band would react with the adsorption of oxygen molecules on the surface or dissolved oxygen in water, and the active free radicals generated would react with the surrounding organic pollutants R6G to achieve self-cleaning process [43–45]. Figure 11(b) is the Raman spectrum of R6G of Au/G1/Ag@ZnO SERS substrate after 5 times of visible light catalytic self-cleaning SERS cycles. The SERS substrate exhibited excellent SERS performance even after five consecutive self-cleaning experiments, the Raman signal of R6G showed no significant attenuation. The results show that the SERS substrate has good self-cleaning performance.
Fig. 11. (a) Raman spectra of R6G adsorbed on Au/G1/Ag@ZnO composite substrate after ultraviolet irradiation for different time. (b) Raman spectra of R6G adsorbed on Au/G1/Ag@ZnO composite substrate (5 cleaning cycles).
4. Conclusion
In summary, we successfully synthesized Au/G/Ag@ZnO composite substrates in this work by magnetron sputtering, wet chemical transfer of graphene, and electron beam evaporation. These substrates exhibit both SERS and photocatalytic degradation capabilities. The effect mechanism of various graphene thicknesses as noble metal nano-spacers on the electric field enhancement was simulated using the finite-difference time domain (FDTD) method, and the ideal structure for SERS detection was discovered. The findings demonstrated that the composite substrate has high trace detection capabilities, good repeatability, and recyclability, with trace detection capability (LOD of 10−13 mol/L for R6G) and enhancement factor of 5.68 × 107. The prepared SERS substrate has high signal reproducibility with relative standard deviation (RSD) value of less than 15%. The SERS substrate has excellent recyclability (reusable five times) and potential applications in the fabrication of reusable and highly sensitive SERS sensors based on the photocatalytic degradation capabilities of ZnO.
Funding
Key Research and Development Project Key Program of Shanxi Province, China (202102040201007); Fundamental Research Program of Shanxi Province (20210302123073); International Cooperation on Key R&D program of Shanxi Province (201903D421078); National Natural Science Foundation of China (52275577).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. W. Y. Hu, L. Xia, Y. F. Hu, and G. K. Li, “Recent progress on three-dimensional substrates for surface-enhanced Raman spectroscopic analysis,” Microchem. J. 172, 106908 (2022). [CrossRef]
2. C. Ji, J. X. Lu, B. J. Shan, F. R. Li, X. F. Zhao, J. Yu, S. C. Xu, B. Y. Man, C. Zhang, and Z. Li, “The Origin of Mo2C Films for Surface-Enhanced Raman Scattering Analysis: Electromagnetic or Chemical Enhancement?” J. Phys. Chem. Lett. 13(38), 8864–8871 (2022). [CrossRef]
3. L. Meng, L. Shang, S. Feng, Z. Tang, C. Bi, H. Zhao, and G. Liu, “Fabrication of a three-dimensional (3D) SERS fiber probe and application of in situ detection,” Opt. Express 30(2), 2353–2363 (2022). [CrossRef]
4. Y. Sun, D. Lou, W. Liu, Z. Zheng, and X. Chen, “SERS Labels for Optical Anticounterfeiting: Structure, Fabrication, and Performance,” Adv. Opt. Mater. 11(6), 2201549 (2023). [CrossRef]
5. C. Lin, Y. Li, Y. Peng, S. Zhao, M. Xu, L. Zhang, Z. Huang, J. Shi, and Y. Yang, “Recent development of surface-enhanced Raman scattering for biosensing,” J. Nanobiotechnol. 21(1), 149 (2023). [CrossRef]
6. W. H. Liu, Y. R. Li, Z. Li, X. J. Du, S. Q. Xie, C. Liu, S. Z. Jiang, and Z. Li, “3D flexible compositing resonant cavity system for high-performance SERS sensing,” Opt. Express 31(4), 6925–6937 (2023). [CrossRef]
7. N. Li, G. Xu, M. Yan, B. Chen, Y. Yuan, and C. Zhu, “Fabrication of Vertically Aligned ZnO Nanorods Modified with Dense Silver Nanoparticles as Effective SERS Substrates,” Chemosensors 11(4), 210 (2023). [CrossRef]
8. J. Xu, Y. Huang, S. Zhang, Z. Liu, and S. Jiang, “Plasmon-induced hot carrier separation across multicomponent heterostructure in Ag@AgCl@g-C3N4 composites for recyclable detection-removal of organic pollutions via SERS sensing,” Appl. Surf. Sci. 610, 155604 (2023). [CrossRef]
9. V. T. Thu, N. M. Cuong, D. T. Cao, L. T. Hung, and L. T.-Q. Ngan, “Trace detection of ciprofloxacin antibiotic using surface-enhanced Raman scattering coupled with silver nanostars,” Optik 260, 169043 (2022). [CrossRef]
10. N. AlMasoud, T. S. Alomar, Y. Xu, C. Lima, and R. Goodacre, “Rapid detection and quantification of paracetamol and its major metabolites using surface enhanced Raman scattering,” Analyst 148(8), 1805–1814 (2023). [CrossRef]
11. S. Bai, X. L. Ren, K. Obata, Y. Ito, and K. Sugioka, “Label-free trace detection of bio-molecules by liquid-interface assisted surface-enhanced Raman scattering using a microfluidic chip,” Opto-Electron. Adv. 5(10), 210121 (2022). [CrossRef]
12. M. S. S. Bharati and V. R. Soma, “Flexible SERS substrates for hazardous materials detection: recent advances,” Opto-Electron. Adv. 4(11), 210048 (2021). [CrossRef]
13. Z. Y. Pei, J. Li, C. Ji, J. B. Tan, Z. D Shao, X. F. Zhao, Z. Li, B. Y Man, J. Yu, and C. Zhang, “Flexible Cascaded Wire-in-Cavity-in-Bowl Structure for HighPerformance and Polydirectional Sensing of Contaminants in Microdroplets,” J. Phys. Chem. Lett. 14(25), 5932–5939 (2023). [CrossRef]
14. F. Wan, Y. Lei, C. Wang, X. Zhang, H. He, L. Jia, T. Wang, and W. Chen, “Highly sensitive and reproducible CNTs@Ag modified Flower-Like silver nanoparticles for SERS situ detection of transformer Oil-dissolved furfural,” Spectrochim. Acta, Part A 273, 121067 (2022). [CrossRef]
15. W. Yang, Q. Ou, C. Li, M. Cheng, W. Li, and Y. Liu, “Ultrasensitive flower-like TiO2/Ag substrate for SERS detection of pigments and melamine,” RSC Adv. 12(12), 6958–6965 (2022). [CrossRef]
16. H. Ma, Q. Cui, L. Xu, Y. Tian, A. Jiao, C. Wang, M. Zhang, S. Li, and M. Chen, “Silk fibroin fibers decorated with urchin-like Au/Ag nanoalloys: a flexible hygroscopic SERS sensor for monitoring of folic acid in human sweat,” Opt. Express 29(19), 30892–30904 (2021). [CrossRef]
17. Y. Shao, S. Li, Y. Niu, Z. Wang, K. Zhang, L. Mei, and Y. Hao, “Three-Dimensional Dendritic Au-Ag Substrate for On-Site SERS Detection of Trace Molecules in Liquid Phase,” Nanomaterials 12(12), 2002 (2022). [CrossRef]
18. D. Su, X. Y. Zhang, X. Y. Chen, S. J. Wang, Q. D. Wan, and T. Zhang, “Centrifugation-induced assembly of dense hotspots based SERS substrate for enhanced Raman scattering and quenched fluorescence,” Nanotechnology 33(23), 235304 (2022). [CrossRef]
19. Z. Zhang, Z. Li, L. Wang, J. Li, J. Pan, S. Wang, C. Zhang, Z. Li, Q. Peng, and X. Xiu, “Preparation and surface-enhanced Raman scattering properties of GO/Ag/Ta2O5 composite substrates,” Opt. Express 29(21), 34552–34564 (2021). [CrossRef]
20. L. Liu, S. Hou, X. Zhao, C. Liu, Z. Li, C. Li, S. Xu, G. Wang, J. Yu, C. Zhang, and B. Man, “Role of Graphene in Constructing Multilayer Plasmonic SERS Substrate with Graphene/AgNPs as Chemical Mechanism-Electromagnetic Mechanism Unit,” Nanomaterials 10(12), 2371 (2020). [CrossRef]
21. N. Mhlanga, T. A. Ntho, H. Chauke, and L. Sikhwivhilu, “Surface-Enhanced Raman Spectroscopy Substrates: Plasmonic Metals to Graphene,” Front. Chem. 10, 832282 (2022). [CrossRef]
22. V. Kaushik, H. L. Kagdada, D. K. Singh, and S. Pathak, “Enhancement of SERS effect in Graphene-Silver hybrids,” Appl. Surf. Sci. 574, 151724 (2022). [CrossRef]
23. S. Nalini, S. Thomas, K. S. Anju, M. K. Jayaraj, and K. Rajeev Kumar, “Chemical vapour deposited graphene-mediated enhanced SERS performance in silver nanostructures,” Mater. Sci. Technol. 39(8), 933–940 (2023). [CrossRef]
24. J. M. Quan, J. Zhang, J. Y. Li, X. L. Zhang, M. Wang, N. Wang, and Y. Zhu, “Three-dimensional AgNPs-graphene-AgNPs sandwiched hybrid nanostructures with sub-nanometer gaps for ultrasensitive surfaceenhanced Raman spectroscopy,” Carbon 147, 105–111 (2019). [CrossRef]
25. J. Zhang, Z. H. Yin, T. C. Gong, Y. F. Luo, D. P. Wei, and Y. Zhu, “Graphene/Ag nanoholes composites for quantitative surface-enhanced Raman scattering,” Opt. Express 26(17), 22432–22439 (2018). [CrossRef]
26. X. Li, W. C. H. Choy, X. Ren, D. Zhang, and H. Lu, “Highly Intensified Surface Enhanced Raman Scattering by Using Monolayer Graphene as the Nanospacer of Metal Film-Metal Nanoparticle Coupling System,” Adv. Funct. Mater. 24(21), 3114–3122 (2014). [CrossRef]
27. Q. Han, Z. Lu, W. Gao, M. Wu, Y. Wang, Z. Wang, J. Qi, and J. Dong, “Three-dimensional AuAg alloy NPs/graphene/AuAg alloy NP sandwiched hybrid nanostructure for surface enhanced Raman scattering properties,” J. Mater. Chem. C 8(36), 12599–12606 (2020). [CrossRef]
28. B. Zhao, R. Hao, Z. Wang, H. Zhang, Y. Hao, C. Zhang, and Y. Liu, “Green synthesis of multi-dimensional plasmonic coupling structures: Graphene oxide gapped gold nanostars for highly intensified surface enhanced Raman scattering,” Chem. Eng. J. 349, 581–587 (2018). [CrossRef]
29. H. Y. Mou, C. X. Song, Y. H Zhou, B. Zhang, and D. B. Wang, “Design and synthesis of porous Ag/ZnO nanosheets assemblies as super photocatalysts for enhanced visible-light degradation of 4-nitrophenol and hydrogen evolution,” Appl. Catal., B 221, 565–573 (2018). [CrossRef]
30. Y. Z. Sun, M. M. Jiang, B. H. Li, X. H. Xie, C. X Shan, and D. Z. Shen, “Electron-hole plasma Fabry-Perot lasing in a Ga-incorporated ZnO microbelt via Ag nanoparticle deposition,” Opt. Express 30(2), 740–753 (2022). [CrossRef]
31. J. L. Liu, Y. H. Wang, J. Z. Ma, Y. Peng, and A. Q. Wang, “A review on bidirectional analogies between the photocatalysis and antibacterial properties of ZnO,” J. Alloys Compd. 783, 898–918 (2019). [CrossRef]
32. F. R. Li, B. J. Shan, X. F. Zhao, C. Ji, Z. Li, J. Yu, S. C. Xu, Y. Jiao, C. Zhang, and B. Y. Man, “Plasmonic enhanced piezoelectric photoresponse with flexible PVDF@Ag-ZnO/Au composite nanofiber membranes,” Opt. Express 30(18), 32509–32527 (2022). [CrossRef]
33. Z. Wang, S. Li, J. Wang, Y. Shao, and L. Mei, “A recyclable graphene/Ag/TiO2 SERS substrate with high stability and reproducibility for detection of dye molecules,” New J. Chem. 46(39), 18787–18795 (2022). [CrossRef]
34. O. Bayram, E. Sener, E. İgman, and O. Simsek, “Investigation of structural, morphological and optical properties of Nickel-doped Zinc oxide thin films fabricated by co-sputtering,” J. Mater. Sci.: Mater. Electron. 30(4), 3452–3458 (2019). [CrossRef]
35. A. K. Singh, A. K. Singh, and S. R. P. Sinha, “Fermi-Level Modulation of Chemical Vapor Deposition-Grown Monolayer Graphene via Nanoparticles to Macromolecular Dopants,” ACS Omega 7(1), 744–751 (2022). [CrossRef]
36. A. Virga, C. Ferrante, G. Batignani, D. De Fazio, A. D. G. Nunn, A. C. Ferrari, G. Cerullo, and T. Scopigno, “Coherent anti-Stokes Raman spectroscopy of single and multi-layer graphene,” Nat. Commun. 10(1), 3658 (2019). [CrossRef]
37. Y. Niu, K. Bi, Q. Li, X. Bi, S. Zhou, W. Fu, S. Zhang, S. Han, J. Mu, W. Geng, L. Mei, and X. Chou, “Multilayer graphene-enabled structure based on Salisbury shielding effect for high-performance terahertz absorption,” Opt. Express 31(7), 11547–11556 (2023). [CrossRef]
38. Y.-K. Niu, K.-X. Bi, Q.-N. Li, S.-Y. Zhou, J.-L. Mu, L.-G. Tan, and L.-Y. Mei, “Optical Study of Few-Layer Graphene Treated by Oxygen Plasma,” Phys. Status Solidi B 259(11), 2200197 (2022). [CrossRef]
39. S. B. Amor, M. Jacquet, P. Fioux, and M. Nardin, “XPS characterisation of plasma treated and zinc oxide coated PET,” Appl. Surf. Sci. 255(9), 5052–5061 (2009). [CrossRef]
40. Y. Quan, J. Yao, S. Yang, L. Chen, J. Li, Y. Liu, J. Lang, H. Shen, Y. Wang, Y. Wang, J. Yang, and M. Gao, “ZnO nanoparticles on MoS(2) microflowers for ultrasensitive SERS detection of bisphenol A,” Microchim. Acta 186(8), 593 (2019). [CrossRef]
41. I. Shaikh, M. A. Haque, H. Pathan, and S. Sartale, “Spin-Coated Ag NPs SERS Substrate: Role of Electromagnetic and Chemical Enhancement in Trace Detection of Methylene Blue and Congo Red,” Plasmonics 17(5), 1889–1900 (2022). [CrossRef]
42. Q. Li, K. Bi, Y. Niu, S. Zhou, L. Tan, J. Mu, S. Han, S. Zhang, W. Geng, L. Mei, and X. Chou, “Modulation of graphene THz absorption based on HAuCl4 doping method,” Opt. Express 30(22), 40482–40490 (2022). [CrossRef]
43. T. Mahardika, N. A. Putri, A. E. Putri, V. Fauzia, L. Roza, I. Sugihartono, and Y. Herbani, “Rapid and low temperature synthesis of Ag nanoparticles on the ZnO nanorods for photocatalytic activity improvement,” Results Phys. 13, 102209 (2019). [CrossRef]
44. D. T. Tieu, T. N. Q. Trang, L. V. T. Hung, and V. T. H. Thu, “Assembly engineering of Ag@ZnO hierarchical nanorod arrays as a pathway for highly reproducible surface-enhanced Raman spectroscopy applications,” J. Alloys Compd. 808, 151735 (2019). [CrossRef]
45. T. V. Nguyen, V. D. Truc, T. A. Nguyen, T. L. and Pham, and D. L. Tran, “A Self Cleaning UV Cured Organic Coating with ZnO–Ag Hybrid Nanoparticles,” J. Cluster Sci. 10876, 023–02448-1 (2023). [CrossRef]
