Plasmonic 3D SemiconductorâMetal Nanopore Arrays for Reliable

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Plasmonic 3D semiconductor-metal nanopore arrays for reliable SERS detection and in-site catalytic reaction monitoring Maofeng Zhang, Tun Chen, Yongkai Liu, Jiluan Zhang, Haoran Sun, Jian Yang, jiping zhu, Jia-Qin Liu, and Yu-Cheng Wu ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b01023 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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Plasmonic 3D semiconductor-metal nanopore arrays for reliable SERS detection and in-site catalytic reaction monitoring

Maofeng Zhang, *a Tun Chen, a Yongkai Liu, a Jiluan Zhang, a Haoran Sun, a Jian Yang, a Jiping Zhu, a Jiaqin Liu, *b and Yucheng Wu a a. School of Materials Science and Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, 230009, China b. Institute of Industry & Equipment Technology, Hefei University of Technology, 193 Tunxi Road, Hefei, 230009, China

Abstract It is urgent to develop a rapid, reliable and in-site determination method to detect or monitor trace amounts of toxic substances in the field. Here, we report an alternative surfaceenhanced Raman scattering (SERS) method coupled with a portable Raman device on a plasmonic three-dimension (3D) hot spots sensing surface. Plasmonic AgNPs were uniformly deposited on a 3D TiO2 nanopore arrays as sensitive SERS substrate, and further coated with graphene oxide (GO). We demonstrate the plasmon-induced SERS enhancement (5.8-fold) and the improvement of catalytic activity by incorporation of plasmonic AgNPs into the 3D TiO2 nanopore arrays. The modification of GO on TiO2-Ag nanopore array further increases 6.2-fold Raman enhancement compared to TiO2-Ag while maintaining good uniformity (RSD＜10 %). The optimized TiO2-Ag-GO substrate shows powerful quantitative detection potential for drug residues in fish scales via a simple

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scrubbing method and the limit of detection (LOD) for crystal violet (CV) was 10-8 M. The SERS substrate also showed detection practicability of pesticide residues in banana peel with LOD of 10-7 M. In addition, our TiO2-Ag-GO substrate exhibits excellent SERS selfmonitoring performance for catalytic reduction of multiple organics in NaBH4 solution and the substrate shows good recyclability of 6 cycles. Such 3D TiO2-Ag-GO substrate is a promising SERS substrate with good sensitivity, uniformity, and reusability, and may be utilized for further miniaturisation for point of analytical applications. Keywords：Surface-enhanced Raman scattering, portable SERS detection, TiO2-Ag-GO, crystal violet, thiram, catalytic reaction monitoring, 3D nanopore array

Surface-enhanced Raman spectroscopy (SERS) has inspired to widespread analytical fields such as biology, chemistry, catalysis, medicine and so on, due to its high sensitivity, molecular fingerprint and good convenience [1-3]. Generally, the local electromagnetic field enhancement of the nanostructure on the noble metals (e.g. Au and Ag) can produce significant SERS effect, and even the ultra-high sensitivity of single molecule detection can be realized [4,5]. SERS technique has the advantages of multicomponent analysis, rapid analysis, simple sample manipulation, in-situ analyte identification, etc, which make it as one of the most powerful techniques in molecular detection [6,7]. Moreover, the advent of commercially available portable Raman spectrometers makes rapid in-site analysis of real world samples possible. For SERS practical applications, the major challenge is the controllable fabrication of SERS-active substrates with highly reproducible


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SERS signals and large enhancement factor [8,9]. Up to date, not only noble metals and transition-metals nanostructures, but also some semiconductor materials (TiO2, ZnO, CdTe, Ge, graphene, etc) are explored and utilized as SERS substrates for various applications [10-14]. The semiconductor materials possess good chemical stability, low cost and wellcontrolled structure uniformity, however, their application in SERS detection is frustrated because of their unsatisfactory enhancement performance [15]. If one can be able to incorporate of plasmonic metal nanostructure into the ordered semiconductor substrate to simultaneously induce metal’s electromagnetic mechanism (EM) and the semiconductor’s chemical mechanism (CM) contributions, an ideal SERS substrate with significant SERS enhancement and good signal reproducibility would be achieved. On the other hand, building of three-dimensional (3D) SERS substrates have proved to be another effective way to realize more highly sensitive SERS substrates due to their large specific area and high porosity structure [16, 17], which enables 3D substrates easy to absorb and capture more target molecules in extremely low concentration detection of analytes. Recent studies have showed the fabrication and trace detection of Au (or Ag) decorated 3D SERS substrates with abundant “hot spots” [18-21]. For example, grafting a silver film on a melamine sponge as a 3D porous SERS substrate with excellent absorption performance and low detection limit for adenine and thiram [22]; wrapping Ag NPs around the foam graphene based on nickel foams as 3D foam substrates with high adsorptivity and sensitivity [23]; loading AuNPs on cellulose paper to from a 3D paper-based SERS substrate for ultra-trace detection of amino acids and melamine [24]. Great efforts have


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been made to fabricate various 3D plasmonic SERS substrates with high sensitivity, however, larger pore size and vase-like defects tend to lose incident laser, thus rational design of semiconductor-metal 3D nanostructure arrays including the pore size and arrangement of semiconductor, the particle size and position of metal nanoparticles is crucial to obtain highly sensitive SERS substrate with excellent uniformity. In addition to high SERS enhancement, the reproducibility and stability are two critical factors allowing for SERS application as a general analytical tool. Thus, great efforts have been devoted to synthesizing SERS substrates with better structure uniformity, among which the periodic ordered arrays of metallic and metal-based hybrid nanostructures are the most common types [25-28]. Unfortunately, it is still not easy to fabricate ideal SERS substrate with good structural periodicity and spatial signal reproducibility to perform practical detections. Ag-based SERS substrates usually have larger enhancement factor than Au, however, Ag is prone to oxidize to lose their activity. And many Ag-based SERS sensors are not recyclable due to its instability, thus it is desired to improve the stability of Ag-based SERS substrates. The existing studies have demonstrated that graphene oxide (GO) coated metal nanostructures succeed in not only further enhancing the SERS sensitivity but also improving the long-term stability owing to the synergistic effects of plasmonic metal and GO layer [29, 30]. GO can act as the molecule enricher to efficiently improve the absorption rate of the probed molecules. Further, GO coating can prevent AgNPs from oxidizing in air and solution environments. More importantly, GO are of low cost and easily obtained, thus greatly improving their feasibility for practical applications.


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In general, it remains a great challenge to address the above-mentioned problems together and integrate multiple functional components into one uniform unit to optimize the SERS substrate. In this work, uniform 3D TiO2 nanopore arrays were controllably prepared as semiconductor building blocks for the further construction of TiO2-Ag and TiO2-Ag-GO hybirds. The GO coated TiO2-Ag nanopore arrays showed 5.3 times stronger Raman intensity than that of TiO2-Ag substrate and 67 times than TiO2 nanopore arrays. And the TiO2-Ag-GO substrate showed good signal uniformity with RSD less than 10 %. For practical detections, the substrate exhibited a superior detection sensitivity for drug residues in fish scales with LOD of 10-8 M CV, and thiram residues on banana peel with LOD of 10-7 M. The TiO2-Ag-GO substrate can be reused for 6 cycles when SERS selfmonitoring the catalytic reaction, indicating their good recyclability and stability. Such a 3D TiO2-Ag-GO nanopore array substrate with good sensitivity, reproducibility and reusability has the potential for rapid, in-site detection applications with a portable Raman device.


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Figure 1. Schematic illustration of the preparation process of TiO2-Ag-GO nanopore arrays and the portable Raman detection of the drug residues in the fish scales.

Experimental Section

Synthesis of 3D ordered TiO2 nanopore arrays The overall fabrication schematic outline of the 3D TiO2-Ag-GO nanopore array structures was shown in Figure 1. Firstly, TiO2 nanopore arrays were prepared by a onestep anodization at room temperature by a direct current power source through a titanium (Ti) foil (99.8% purity, 0.3 mm thickness, 2 cm × 2 cm width). Before anodizing, titanium foil as anode and platinum as cathode electrode were washed using isopropyl alcohol, methanol, ethanol and distilled water, respectively. One-step anodization was carried out in an electrolyte containing 0.2826 g of NH4F, 2.7 mL of H2O and 47.3 mL of ethylene


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glycol (EG) solution at 60 V for 30 minutes. Then the substrate was ultrasonically cleaned in pure EG for 5 minutes to remove impurities and dried for use. Finally, the patterned TiO2 nanopore array on the titanium foil was placed in a crucible, and then annealed at 500 ℃ for 1 hour at a temperature rising and cooling rate of 3 ℃ to obtain a target titanium dioxide nanopore array.

Preparation of GO coated TiO2-Ag nanocomposites Then the prepared sample was soaked in a silver-ammonia solution (containing 0.2 M AgNO3) for 1 minute in a typical procedure and immersed in an excess of glucose solution (0.35 M) for 1 minute. This process was repeated for 3 times. The resulting TiO2-Ag nanopore array was washed with water and dried for further use. In this way, we have loaded the Ag nanoparticles on the TiO2 nanopore array by a simple silver mirror reaction. The obtained TiO2-Ag nanopore arrays were washed with water and dried for further use. Finally, the TiO2-Ag nanopore arrays were coated by graphene oxide (GO) solution via spin-coating method on a spin coater at a rotational speed of 2000 r/min for 1 min. The coating process was repeated for 4 times. Before GO coating, the commercially obtained GO solution was dispersed in water and sonicated for 4 h to obtain GO aqueous solution of 0.003 g/L. The obtained TiO2-Ag-GO hybrids were dried and tested for SERS performances.


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Characterization and SERS measurements

The morphology of the samples at different stages were characterized by a scanning electron microscopy (SEM, Quanta 200FEG). The prepared samples were cut into pieces (5 mm × 5 mm) as SERS active substrates before Raman measurement. Crystal violet (CV), malachite green (MG) and thiram molecules were used as Raman probes to initially assess SERS sensitivity and the signal uniformity. In the SERS measurement, the same volume of different analytes was dropped onto the substrate and the SERS spectra were collected by a portable Raman instrument (i-Raman plus, B&W Tek Inc., USA) equipped with a 785 nm laser source. During the Raman measurement, a 20× objective lens in the microscope was selected for focusing, and the total accumulation time was 2 seconds. For selfmonitoring the catalytic reaction, the probe molecules absorbed by the substrate were placed in a 0.1 M NaBH4 solution and the real-time catalytic degradation process was recorded by a portable Raman instrument. After catalytic degradation, a fixed drop amount of 50 μL of the analyte was again added to the substrate for catalysis. This process was repeated several times to test the recyclability of the substrate.

Results and discussion

Morphology characterization of the samples

The TiO2 nanopore array samples synthesized at different anodizing reaction intervals were characterized by SEM as shown in Figure 2. It was found that the diameter of the


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TiO2 nanopores could be finely tuned by changing the anodizing time. The longer of the anodizing time, the larger pore size of the TiO2 nanopore arrays. The pore size gradually increased from 85 nm at 10 min to 145 nm at 40 min (Figure 2a-d). However, further increasing of the anodizing time resulted in the formation of TiO2 nano-blocks and nanoparticles. TiO2 nanopore arrays prepared at the anodization time of 30 minutes reached the optimum requirement with a pore size of about 101 nm, and thus was selected as semiconductor substrate to be further combined with the plasmonic silver nanoparticles. Typical SEM images of ordered TiO2 nanopore arrays were shown in Figure 3a, b. The broken part in Figure 3b clearly showed its 3D structure feature. Figure 3c, d showed SEM images of TiO2-Ag nanopore arrays. It can be seen that Ag nanoparticles were uniformly deposited both on the surface and inside of the TiO2 nanopores. Moreover, the deposition amount of Ag nanoparticles can be well controlled by depositing times and the concentration of AgNO3. SEM images of graphene oxide (GO) modified TiO2-Ag nanopore array was presented in Figure 3e, f. The darker contrast indicating that GO layer was evenly spin-coated on the TiO2-Ag nanopore surface.


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Figure 2. SEM images of TiO2 nanopore arrays synthesized with different electrolysis time: (A) 10 min, (B) 20 min, (C) 30 min, (D) 40 min, (E) 60 min, (F) 120 min.

Figure 3. Typical SEM image of TiO2 nanopore arrays (a, b), TiO2-Ag hybird nanopore arrays (c, d), and TiO2-Ag-GO nanopore arrays (e, f).


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Plasmon-induced Raman enhancement and the optimization of SERS performance The SERS intensity of both TiO2 nanopore arrays and TiO2-Ag nanopore arrays with different pore sizes were measured by mean of a portable Raman instrument, as shown in Figure S1 and Figure 4a. It can be seen that the Raman signal increases with the pore size increasing to 101 nm, at which the substrate exhibits the highest Raman enhancement. Nevertheless, larger pore size of 145 nm results in the decreasing of the Raman intensity, probably due to the loss of incident laser. Thus, an appropriate TiO2 pore size of 101 nm was used as building blocks for further use. It has been recognized that both EM and CM mechanisms contribute to the SERS enhancement for metal-based substrates, while the semiconductor SERS substrates only has a CM mechanism, which results in inferior SERS enhancement performance than noble metal substrate. However, the semiconductor material is easy to fabricate ordered arrays with controllable size and shape, and usually has a higher SERS uniformity than a noble metal. Recently, the integration of a plasmonic metal and a semiconductor material into metal-semiconductor nanocomposites has been commonly used to achieve higher SERS performance through synergetic contribution [31,32]. Here, we first deposited plasmonic AgNPs into the 3D TiO2 nanopore array to construction of TiO2-Ag hybrids while keeping the original nanopore array structure. Raman spectra indicated that TiO2-Ag hybrids exhibited a much higher Raman intensity than TiO2 nanopore array (Figure 4b). The calculated enhancement factor (EF) of TiO2-Ag is 1.25×105, which is 5.8-fold enhancement compared to that of TiO2 substrate (EF=5×104). The detailed EF calculation is shown in the supporting information and Figure S2. The significant enhancement is due to the strong EM of Ag and the charge transfer of TiO2 substrate. To further optimize the SERS performances in terms of sensitivity, reproducibility and stability, the TiO2-Ag nanopore array substrate was subsequently coated with GO via a spin-coating method. The optimized coating times of four cycles were determined from different coating cycles as shown in Figure S3. The optimized TiO2Ag-GO substrate showed an improved EF of 7.75×105 that is 6.2-fold enhancement


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compared to TiO2-Ag substrate (Figure S2). The increased enhancement can be attributed to the GO’s chemical enhancement and the superior absorbing abilities to the probed CV molecules.

Figure 4. (a) SERS spectra of CV obtained on TiO2-Ag nanopore array substrates with different pore sizes, and (b) comparation of SERS performance from TiO2 nanopore array, TiO2-Ag nanopore array and TiO2-Ag-GO hybrids. In addition to sensitivity, the signal reproducibility is another critical criterion in evaluating an excellent SERS substrate. To measure the signal reproducibility of the TiO2Ag-GO nanopore array substrates, we randomly measured 20 spots by means of a portable Raman spectrometer, as shown in Figure 5. The relative standard deviation (RSD) values of the main vibrating peaks intensity at 912, 1174, and 1589 cm-1 were estimated as 8.47, 7.08, and 9.55 %, respectively, further demonstrating the excellent spatial homogeneity of the substrate. The SERS signal reproducibility from TiO2-Ag-GO nanopore array substrate has well met the requirements for quantitative analysis according to the standards suggested by Natan [33]. The excellent reproducibility of TiO2-Ag-GO substrate can be attributed to following factors: First, the successful construction of ordered 3D TiO2


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nanopore arrays. Second, the uniformly growth of Ag nanoparticles on the TiO2 nanopore arrays, thus producing homogeneous high density of “hot spots”. Third, the coating of GO effectively and homogenously capturing target molecules, reducing the signal blinking induced by the drift of analyte molecules on SERS-active surfaces under excitation laser [34].

Figure 5. SERS spectra of CV (10-6 M) molecules collected at 20 randomly selected spots, and the corresponding RSD values of characteristic peaks.

Practical SERS determination of drug residues As it is known that crystal violet (CV) and malachite green (MG) are widely used in fishery industry due to its splendid bactericidal effect. Nevertheless, this substance has serious


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side-effects of leading to carcinogenic, teratogenic, and mutagenic after entering into the human or animal body. In addition, thiram is a highly toxic chemical used as a fungicide and seed disinfectant in crops which the Environmental Protection Agency (EPA) examined and deemed to be dangerous. Recently, the abuse of fungicides and pesticides have become a serious public health risk in foods, therefor how to quickly detect fungicides and pesticides has received more and more attention. As a result, an effective method must be found to sensitively detect trace amounts of harmful substances. Among many methods, SERS has emerged as a powerful tool to realize simple, rapid, and in-site detection applications using a portable Raman device. The SERS sensitivity of the TiO2-Ag-GO substrates was initially evaluated by several typical target molecules, such as CV, MG, and thiram with different concentrations as shown in Figure 6. It is observed that a low concentration of 10-10 M for CV, 10-9 M for MG, and 10-8 M for thiram can be readily detected, indicating that this SERS substrate is highly sensitive and applicable to detect practical toxic organic molecules.

Figure 6. SERS spectra obtained from different concentrations of (a) CV, (b) MG, and (c) Thiram adsorbed on the TiO2-Ag-GO substrates. To further demonstrate the practical applications, TiO2-Ag-GO substrates was employed as portable SERS detection platform for the in-site determination of real samples. Herein, we selected fish as a sample model to carry out trace CV molecule detection. We used the SERS substrate to scrub CV molecules off the fish scales (Figure 1), which were pretreated with different spiked CV concentrations. From Figure 7a, it is found that a low CV


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concentration of 10-8 M can be readily detected via a portable Raman spectroscope. Besides, there is a broad linear relationship between the detected concentration and the Raman intensity with R2 as 0.980 (Figure 7b). The detection sensitivity is comparable to those detections by laboratory-based large Raman spectrometer for fish muscles and fish samples [35,36]. To verify the practicability of this method, we further tested the pesticide residues on banana peel surface with different concentrations of spiked thiram. As shown in Figure 7c, a low detection concentration of 10-7 M (0.024 mg/kg) can be achieved, which is much lower than the current maximum residue limits (MRLs) of 2 mg/kg in Europe (Reg. (EU) 2017/171). Figure 7d displays its good linear dose responds ranging from 10-4 M to 10-7 M with R2 as 0.989. The above results indicate this method is of great potential for convenient and speedy detection of real-world samples.


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Figure 7. (a) SERS spectra detected on fish scales with different CV concentrations using TiO2-Ag-GO substrates, and (b) relationship between CV concentration and SERS intensity; (c) SERS spectra detected on banana peel with different thiram concentrations using TiO2-Ag-GO substrates, and (d) relationship between thiram concentration and SERS intensity.

3.4 Plasmonic SERS self-monitoring of catalytic reaction and the recyclability

The combination of both SERS and catalysis-active plasmonic metal substrate can be selfmonitored by SERS spectra for the catalysis reaction [37-39]. Herein, TiO2-Ag-GO nanopore array substrate is also applied to monitor the catalytic degradation of diverse


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organics in NaBH4 solution. Target molecules were first adsorbed on the substrate and then immersed in NaBH4 solution for a period of time. And the catalytic degradation process was monitored by SERS spectra in a portable Raman device using a 785nm laser irradiation (Figure 8). Figure 8a showed the intensity of characteristic peaks of CV quickly reduced and disappeared in 16 minutes, indicating CV molecules absorbed on the substrate were completely catalytically decomposed. To assess the plasmon-induced catalysis by metal nanoparticles, two TiO2 nanopore array with and without AgNPs were also selected to monitor the catalytic reaction (Figure S4), and the results indicated that TiO2-Ag substrate has a much higher catalytic efficiency than TiO2. The larger enhancement in catalytic activity for TiO2-Ag substrate can be explained in two aspects: (1) excited electrons are generated in TiO2-Ag substrate on laser irradiation, these excited electrons can then transfer to the AgNPs and enhance the catalytic reaction, which is consistent with the earlier studies [40, 41]. and (2) much stronger local electric field at the vertex and inside sites of the 3D TiO2-Ag nanopore, in agreement with the reported results [42,43]. To better demonstrate the reliability and diversity of the self-monitoring performance, the TiO2-Ag-GO substrates are thus employed for the SERS monitoring of MG and thiram (Figure 8c,d), methylene blue (MB), rhodamine 6G (R6G), and 4-mercaptobenzoic acid (p-MBA) catalytic reduction (Figure S5) It is found that these target molecules adsorbed on the substrate can be rapidly catalytically degraded and monitored by SERS spectra, revealing the practical feasibility of plasmonic SERS self-monitoring reactions.


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Figure 8. (a) Time-dependent catalytic degradation of CV (10-6 M) absorbed on the TiO2Ag-GO substrate, and (b) recyclable SERS detection of CV for 6 cycles. SERS monitoring of MG (c) and thiram (d) catalytic reduction in NaBH4 solution using TiO2Ag-GO substrates. In addition, the recyclability of our TiO2-Ag-GO substrates were tested. Figure 8b shows the recyclable SERS detection of CV molecules. After detecting of CV, the SERS activity of the substrate can be refreshed simply by catalytic decomposing of CV in NaBH4 solution. It is observed that TiO2-Ag-GO nanopore array substrates can be used to detect CV for 6 cycles without significantly losing their SERS activity. However, without GO modification, the TiO2-Ag substrates can only be recycled for 4 times (Figure S6), indicating that GO


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helps to improve the stability and retain the SERS activity. The presence of GO can make the Ag nanoparticles more stable and not agglomerate to improve the stability of the active substrate. Previous studies also indicate that GO coating can improve the support’s stability by preventing oxidation from environment [44, 45].

Conclusions

In conclusion, plasmonic AgNPs uniformly distributed on a 3D ordered TiO2 nanopore arrays was fabricated and used for SERS detection of drug residues on fish scales and SERS self-monitored catalytic reaction for organics via a portable Raman device. The combination of metal AgNPs and semiconductor TiO2 significantly enhanced the SERS enhancement by 5.8-fold while maintaining good structure uniformity of TiO2 nanopore arrays. The SERS performance of the TiO2-Ag nanopore arrays was further optimized by coating a protective layer of GO, which has a 6.2-fold increasing of Raman enhancement compared to TiO2-Ag substrate. Besides, the signal reproducibility was effectively improved with RSD less than 10 %. The optimized TiO2-Ag-GO substrate exhibits a low LOD of 10-8 M for drug residues on fish scale and of 10-7 M for thiram residues on banana peel with good linear relationship between detection concentration and SERS intensity. In addition, the TiO2-Ag-GO substrate also demonstrates excellent SERS self-monitoring performance for a variety of organic molecules catalytic reduction. And the substrate can be reused for 6 cycles without significantly losing their SERS activity. This TiO2-Ag-GO substrate with high Raman enhancement, good uniformity and recyclability may have


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broad applications in rapid, in-site detection or monitoring of real-world samples in the field by mean of a portable Raman instrument. Associated content Supporting Information Available: The following files are available free of charge. Effects of TiO2 nanopore sizes on the SERS performance, the detailed calculation of the enhancement factor (EF) of the samples, influences of GO spin-coating cycles on the SERS performance, catalytic reaction comparison of TiO2 with and without AgNPs, catalytic reduction monitoring of MB, R6G, and p-MBA, recyclable SERS detection using TiO2-Ag substrates. Author information *E-mail: [email protected], Tel: +86 18019595355 *E-mail: [email protected], Tel: +86 13866186310 Author contributions The manuscript was written through contributions of all authors. Notes There are no conflicts to declare. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (no. 61205150), Project of Application Breeding Program of Hefei University of Technology (JZ2016YYPY0060), the Youth Academic Team Capacity Promotion


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Program (PA2017GDQT0023), and the 111 Project "New Materials and Technology for Clean Energy" (B18018).

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for TOC only 47x32mm (300 x 300 DPI)


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