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Abstract
Flexible surface-enhanced Raman spectroscopy (SERS) substrate has attracted great attention due to its convenient sampling and on-site monitoring capability. However, it is still challenging to fabricate a versatile flexible SERS substrate, which can be used for in situ detection of analytes either in water or on irregular solid surfaces. Here, we report a flexible and transparent SERS substrate based on a wrinkled polydimethylsiloxane (PDMS) film obtained by transferring corrugated structures on the aluminium/polystyrene bilayer film, onto which silver nanoparticles (Ag NPs) are deposited by thermal evaporation. The as-fabricated SERS substrate exhibits a high enhancement factor (∼1.19×105), good signal uniformity (RSD of 6.27%), and excellent batch-to-batch reproducibility (RSD of 7.3%) for rhodamine 6 G. In addition, the Ag NPs@W-PDMS film can maintain high detection sensitivity even after mechanical deformations of bending or torsion for 100 cycles. More importantly, being flexible, transparent, and light, the Ag NPs@W-PDMS film can both float on the water surface and conformally contact with the curved surface for in situ detection. The malachite green in aqueous environment and on apple peel can be easily detected down to 10−6 M with a portable Raman spectrometer. Therefore, it is expected that such a versatile flexible SERS substrate has great potential in on-site, in situ contaminant monitoring for realistic applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Malachite green (MG) is a popular triphenylmethane dye, which has been widely used in the silk, textile and paper industry [1]. In addition, it is also used as a fungicide and parasiticide in aquaculture because of its strong effect against fungal and parasite infections [2]. Unfortunately, because of its high teratogenicity, mutagenicity and carcinogenicity, residual MG used in aquiculture and aquatic products at the concentrations of 0.1–10 mg/L poses a threat to human health [3,4]. Thus, the sensitive detection of MG residues has become important for food safety. So far, a variety of analytical methods, such as high-performance liquid chromatography [5], liquid chromatography–tandem mass spectrometry [6], and fluorescence spectroscopy [7], have been successfully used for the sensitive detection of MG. Although these techniques have features of high sensitivity and accuracy, the bulky and costly instruments along with the complicated pretreatment process make these methods difficult to conduct real-time in situ analysis. Therefore, it is of great significance to develop a simple, portable, cost-effective, and sensitive method for rapid monitoring of the MG residues.
Surface-enhanced Raman scattering (SERS) has become a powerful analytical technique with rapid advances in recent years [8], which can greatly amplify Raman scattering signal of analytes down to the single molecule [9,10]. Due to its unique merits, such as ultra-sensitivity, high specificity, fast response, together with non-destructive analysis, SERS technique has been widely used in food safety [11,12], environmental monitoring [13,14], biomedicine [15,16], and other fields [17,18]. Generally, SERS effect is attributed to electromagnetic enhancement (EM) and chemical enhancement (CM) [19], among which the former is the dominant factor [20]. In addition, the giant local electric field enhancement is usually located in nanogaps (so-called hot spots) between adjacent mental nanostructures [21,22]. Therefore, design and fabrication of SERS substrate with high-density and uniform hot spots is of great significance.
To date, various types of SERS substrates, including metal nanoparticle colloids [23], metal nanostructures on rigid substrates [24,25], and plasmonic nanostructures on flexible materials [26–28], have been reported. Among them, flexible SERS substrates are intriguing and favorable for real-word applications, which can avoid tedious sample pretreatment and allow for on-site monitoring of analytes on curved surfaces [29–32]. Typically, for flexible SERS substrates, preparing high-intensity and high-density hot spots on the flexible template is a prerequisite for ultrasensitive and reproducible detection [33,34]. Currently, the most widely used strategy is to deposit metal nanostructure on the flexible templates with three-dimensional (3D) structures [35–39]. The 3D structures can increase the number and contact area of hot spots, and thus endow the SERS substrate with excellent detection capability [40]. Besides, from a practical perspective, development of flexible SERS substrate versatile for different detection scenarios, such as contaminants in aqueous environments and pollutants on irregular surfaces, should bring the SERS technology closer to users. Thus, it is of great significance to develop 3D multifunctional flexible SERS substrates that can simultaneously satisfy different scenario test requirements.
In this paper, we present a facile method for fabricating a 3D flexible SERS substrate featuring versatile capability for detecting analytes in aqueous environment and on curved surface. First, the wrinkled polydimethylsiloxane (PDMS) film was prepared by transferring isotropic wrinkles on the aluminum/polystyrene bilayer film generated by buckling. Then, silver nanoparticles (Ag NPs) were deposited on the surface of the wrinkled PDMS film via thermal evaporation, and optimization of SERS performance was achieved by adjusting the silver thickness. The optimal Ag NPs@W-PDMS film exhibits excellent SERS performance for detection of rhodamine 6 G. The practicability of the as-prepared SERS substrate was evaluated through in situ detection of contaminants in different test environments. The Ag NPs@W-PDMS film is light enough to float on the water surface, and on-site SERS detection of malachite green (MG) in water at a concentration down to 10−6 M is feasible by a portable Raman spectrometer. In addition, the Ag NPs@W-PDMS film also demonstrates the feasibility for in situ detection and quantitative analysis of MG on apple peels.
2. Materials and methods
2.1. Materials
Aluminum wires (99.999%) were purchased from Beijing Zhongcheng New Material Co., Ltd. Silver particles (99.99%) were obtained from Fuzhou Infineon Photoelectric Technology Co., Ltd. Polystyrene (PS) and malachite green (MG) was purchased from Aladdin Chemical Reagent Co., Ltd. Polydimethylsiloxane (PDMS, Dow Corning Sylgard 184) was obtained from Hangzhou Jingbo Technology Co., Ltd. Rhodamine 6 G (R6G) was purchased from Sigma-Aldrich. Deionized (DI) water was used for preparation of analyte aqueous solutions.
2.2. Preparation process of the flexible SERS substrate
The overall fabrication process of the Ag NPs@W-PDMS film is schematically illustrated in Fig. 1(a). It involves several steps: Firstly, isotropic wrinkles on the Al/PS bilayer films were prepared by using thermally-driven buckling, then wrinkled PDMS films were prepared by using micro-transfer molding, and finally, the preparation of Ag NPs@W-PDMS film was achieved by a thermal evaporation method.
Fig. 1. (a) Schematic illustration of the fabrication process of the Ag NPs@W-PDMS film. (b) Schematic representation of the setup for in situ SERS detection of contaminants in water and fruits using the Ag NPs@W-PDMS film.
2.2.1 Preparation of wrinkles on the metal/polymer bilayer film
The glass substrate was initially cleaned using acetone, ethanol, and deionized water for 10 minutes to eliminate impurities. Then, a drop (60 µL) of PS chloroform solution with a concentration of 10 mg/ml was spin-coated onto the cleaned glass substrate at 500 rpm for 5 s followed by 2500 rpm for 30 s. After completion, the sample was annealed at 80 °C for 12 hours to prepare the PS film. Then, the PS film was coated with a 20 nm thick aluminum (Al) film by DZS-500 EB evaporation system (SKY Technology Development Co., Ltd.). The above parameters were selected based on the following two aspects [41]. On one hand, when the rotation speed is below 2000rpm, the thickness of the PS film obtained is uneven. One the other hand, for preparing surface wrinkled structures, the metal film on the PS film must be a continuous and dense layer film with small grain size. The evaporation process was carried out under the vacuum pressure of ∼5.0 × 10−4 Pa and the current of 50 A. Finally, the Al/PS bilayer film was subsequently annealed at 145 °C for 4 hours. After thermal annealing at above the glass transition temperature of the PS polymer, the isotropic wrinkles were generated on the surface of the Al/PS bilayer film, due to stress resulting from mismatch in thermal expansion of the individual layers [42–44].
2.2.2 Preparation of the wrinkled PDMS film
PDMS was chosen to transfer the morphology of the isotropic wrinkles on the Al/PS bilayer films and served as a template to support Ag NPs, due to its good flexibility, high transparency, chemical stability and a low Raman cross-section [45]. To prepare the PDMS solution, precursor and crosslinking agent (a mass ratio of 10:1) were mixed and stirred for 20 minutes. The mixture was placed in a vacuum desiccator for 30 minutes to remove bubbles. Then, the PDMS solution was spin-coated onto the surface of the Al/PS bilayer film with wrinkled structures at 600 rpm for 8 s, and annealed at 80 °C for 1 hour. Finally, the PDMS film with negative replica from wrinkled structures on the bilayer film was prepared by peeling the PDMS film off from the glass substrate.
2.2.3 Fabrication of Ag NPs@W-PDMS film
The wrinkled PDMS film was served as a structured template to load Ag NPs. For depositing the Ag NPs, a DZS-500 EB evaporation system was employed, with the silver particles as the evaporator source. The wrinkled PDMS film was placed on the center of the substrate in the thermal evaporator and rotated at 10 rpm during the evaporation process. The evaporation process was carried out under the vacuum pressure of ∼5.0 × 10−4 Pa, the current of 100 A, and the deposition rate of ∼1.6 Å/s. To study the impact of Ag layers, four samples were made on the wrinkled PDMS film with various Ag layer thicknesses of 30, 40, 50 and 60 nm, which were monitored by a quartz crystal microbalance. After the temperature inside the chamber cooled down for half an hour in vacuum condition, the as-prepared samples were taken out from the thermal evaporation system.
2.3. Characterization and SERS measurements
The surface morphologies of the samples were investigated by an atomic force microscope (AFM, Bruke Icon) in tapping mode using silicon nitride tips, and a field-emission scanning electron microscope (SEM, Apero S. HiVac) operating at a 2 kV accelerating voltage. For AFM measurements, imaging was performed at 1.00 Hz and 256 samples/line. The SERS spectra were measured using a Renishaw inVia Raman Microscope (785 nm laser) with a 50× objective. The laser power used for bench-top measurements of all samples was 3 mW and the size of the laser spot was 2.5 µm. The data acquisition time was 10 s, and the SERS signal was scanned over a spectral range of 400-1600 cm−1 at a spectral resolution of 0.3 cm−1. All spectra were presented without any spectral correction. A portable Raman spectrometer (ATR6500, Optosky photoelectric Co., Ltd., China) with a 785 nm laser (20 mW power) was used for on-site, in situ pollutant monitoring. To ensure the reliability of the data for SERS performance evaluation, some spectrum was averaged over five randomly measured spectra. In the bending and twisting tests, the flexible SERS substrate was repeatedly bended and twisted by hand, respectively, and 5 SERS signals were measured for each round of 10 deformations (bending or twisting). For in situ monitoring under a liquid environment, the Ag NPs@W-PDMS film floated on the water surface with its back side upward, and was irradiated by a laser beam from its back side into the water. For the in situ detection of contaminant on curved sample surfaces, MG solution was dropped on the apple surface and dried at room temperature. Then, the Ag NPs@W-PDMS film was pasted over the fruit peels with Ag NPs facing the polluted areas and subsequently the SERS signal was collected in back side illumination mode. The corresponding setup is schematically illustrated in Fig. 1(b).
3. Results and discussion
3.1 Characterization of the Ag NPs@W-PDMS film
Figure 2(a) shows a resulting PS film on a glass substrate placed on the paper printed with the letters of LCU. After coating the PS film with an Al film, the Al/PS bilayer film displays a grey color, as shown in Supplement 1, Fig. S1(a). The AFM analysis illustrates that the surface of the Al/PS bilayer film before thermal annealing is smooth with a small surface roughness (Ra) of 0.587 nm, as shown in Supplement 1, Figs. S1(b-d). After thermal annealing, the color of the Al/PS bilayer film becomes iridescent, as demonstrated in Fig. 2(b). The AFM image (Fig. 2(c)) along with a cross-sectional profile (Fig. 2(d)) clearly suggests that the isotropic wrinkles are formed on the surface of the Al/PS bilayer film and the Ra increases to 12.0 nm. It can be observed that the isotropic wrinkle structures show no directional order, which is consistent with previous work [46]. The calculated amplitude and peak-to-peak wavelength of the wrinkled structures are 35.54 ± 15.54 nm and 2.13 ± 0.18 µm, respectively, which were obtained by statistical analysis of the AFM image based on 50 measurements, as depicted in Supplement 1, Fig. S2.
Fig. 2. (a) Photograph of PS film on a glass substrate placed on the paper printed with the letters of LCU (Liaocheng University). (b) Photograph of the Al/PS bilayer film after thermal annealing supported on a glass substrate. (c) AFM image, and (d) cross-sectional profile of wrinkles on the bilayer film.
Figure 3(a) illustrates a photo image of a wrinkled PDMS film placed on the same background as in Fig. 2(a). The AFM results reveal that the surface morphology of the wrinkles on the Al/PS bilayer film was successfully transferred to PDMS integrally, as shown in Figs. 3(b,c). The statistical analysis of the AFM image depicts that the calculated amplitude and peak-to-peak wavelength of the wrinkled structures on the PDMS film are 23.48 ± 8.64 nm and 2.35 ± 0.24 µm, respectively, as demonstrated in Supplement 1, Fig. S3. After decorating with Ag film, the modified PDMS film shows lustrous iridescence compared with the pure PDMS film, as shown in Fig. 3(d), indicating the formation of Ag NPs layers on the PDMS film. Furthermore, we can see that the Ag NPs@W-PDMS film is transparent (from the visibility of the printed letter) and flexible (from the bending deformation). With such good flexibility and transparency, the Ag NPs@W-PDMS film is suitable for achieving rapid, in situ SERS detection.
Fig. 3. (a) Photograph of a wrinkled PDMS film obtained from the Al/PS bilayer film. (b) AFM image, and (c) cross-sectional profile of wrinkles on the PDMS film. (d) The transparency and flexibility of the Ag NPs@W-PDMS film.
Figure 4 displays SEM images of the wrinkled PDMS films decorated with Ag layers of different thicknesses (30, 40, 50 and 60 nm). It can be seen that the deposited Ag film changes from isolated nanoparticles to a relatively continuous layer as the film thickness increases. Typically, when the Ag layer thickness was 30 nm (Fig. 4(a)), the Ag NPs were distributed as isolated nanoparticles with large nanogaps. With the increase of the Ag layer thickness (40-50 nm), the diameter of Ag NPs became larger and the adjacent nanogaps became narrower, as shown in Figs. 4(b, c). Then, some aggregation of Ag NPs occurred for the 60 nm thick film (Fig. 4(d)). Thus, through tuning the Ag layer thickness, the morphology of plasmonic nanostructures on the wrinkled PDMS film can be changed conveniently, which is beneficial for SERS performance optimization. In addition, low-magnification SEM images (Supplement 1, Fig. S4) indicate that Ag NPs are uniformly distributed throughout the films over large areas.
Fig. 4. SEM images of Ag NPs@W-PDMS films with different Ag deposition thicknesses: (a) 30 nm; (b) 40 nm; (c) 50 nm; (d) 60 nm.
The Ag NPs were also characterized by AFM for its detail size and morphology, as shown in Supplement 1, Fig. S5. The histogram of the particle size distribution (Supplement 1, Fig. S6) obtained from the AFM images shows that the particle sizes of the Ag NPs corresponding to different thicknesses are 27.978 ± 5.548 nm, 34.015 ± 6.824 nm, 43.623 ± 13.093 nm and 63.647 ± 17.459 nm, respectively, which were based on the measurements of 100 nanoparticles. In addition, the size of the nanogaps between the Ag NPs are found to be 16.145 ± 4.830 nm, 12.242 ± 2.778 nm, 10.577 ± 2.671 nm and 10.285 ± 3.717 nm, respectively, as shown in Fig. S7. Moreover, the 3D AFM images of the Ag NPs@W-PDMS film reveal that the substrate has undulations, and Ag NPs are evenly distributed on the surface of wrinkles, as demonstrated in Fig. S8. Additionally, the energy dispersive spectrometer (EDS) of the Ag NPs@W-PDMS film with the thickness of 50 nm (Supplement 1, Fig. S9(a)) confirms the successful preparation of Ag NPs. The corresponding elemental mapping (Supplement 1, Fig. S9(b)) demonstrates that Ag NPs are uniformly distributed on the surface of the wrinkled PDMS film.
3.2 SERS performance of Ag NPs@W-PDMS films
To investigate the SERS activity of Ag NPs@W-PDMS films with various Ag thicknesses, probe molecule R6G (10−5 M) was utilized for comparative analysis, as show in Fig. 5(a). The SERS spectra of R6G showed that different substrates presented distinct enhanced performance, and the Ag NPs@W-PDMS film with 50 nm thick Ag film has the strongest SERS effects. Furthermore, the SERS intensity at peak 1510 cm−1 (drawn as a function of the Ag thickness in the inset of Fig. 5(a)) shows a trend of first rise and then fall. This trend could be related to the size and shape of Ag NPs on the PDMS films. For the thinner thickness (30 nm), the Ag NPs of small diameter with limited number of hot spots were unable to effectively enhance the SERS signal. With the increase of the film thickness up to 40 and 50 nm, the size of the Ag NPs became larger and the interparticle nanogaps narrowed. The narrowest nanogaps can be found in the 50 nm thick film, and thus displayed the optimal SERS intensity. However, Ag NPs amalgamated to form bigger ones (60 nm thick film), and thus reduced the number of hot spots.
Fig. 5. (a) SERS spectra of R6G (10−5 M) collected from Ag NPs@W-PDMS films with different Ag thicknesses, the inset showing the SERS intensity at 1510 cm−1 versus the Ag thicknesses. (b) SERS spectra of R6G with different concentrations on the optimal Ag NPs@W-PDMS film. (c) The calibration curve of SERS intensity at 1510 cm−1 against the logarithmic concentration of R6G. (d) The Raman spectra of R6G (10−3 M) on the pure PDMS film and SERS spectra of R6G (10−7 M) on the optimal Ag NPs@W-PDMS film.
The Ag NPs@W-PDMS film with 50 nm thickness exhibited the highest SERS activity, and thus was chosen as a flexible SERS substrate. To evaluate the SERS sensitivity, the SERS spectra of R6G with different concentrations (10−3−10−7 M) were collected from the Ag NPs@W-PDMS film and plotted in Fig. 5(b). The SERS spectra can be still easily distinguished even at the low concentration of 10−7 M. Furthermore, the SERS spectra of the Ag NPs@W-PDMS film indicated that no competitive signals from PDMS were obtained during the SERS measurement. As shown in Fig. 5(c), the fitting curve of the relation between SERS intensity and logarithm of concentration has a good correlation coefficient of R2 = 0.955. A large deviation occurs at high concentrations, which results from the SERS saturation effect [47]. Besides, the SERS performance of the Ag NPs@W-PDMS film was evaluated by calculating the analytical enhancement factor (AEF): [48]
where ISERS and IRaman are the Raman intensity of analyte in the SERS spectrum and norm Raman spectrum, and CSERS and CRaman are the analyte concentration on the SERS substrate and norm substrate, respectively. Here, the SERS peak intensity of R6G (10−7 M) at 1510 cm−1 on the Ag NPs@W-PDMS film is 3051, while the normal Raman intensity of R6G (10−3 M) at 1510 cm−1 on the pure PDMS film is 257, as depicted in in Fig. 5(d). Thus, the AEF value of the Ag NPs@W-PDMS film was calculated to be 1.19 × 105.The uniformity of SERS substrate is critical for practical application. Supplement 1, Fig. S10 shows the SERS spectra of R6G (10−5 M) collected from 10 randomly selected positions on the Ag NPs@W-PDMS film. It can be seen that the band shape and the Raman intensity of the obtained SERS signals are similar. The relative standard deviation (RSD) value of peak intensity at 1510 cm−1 is 6.27%, as shown in Fig. 6(a), indicating that the Ag NPs@W-PDMS film has a good uniformity. The excellent homogeneity of the Ag NPs@W-PDMS film was further demonstrated by SERS mapping at peak 1510 cm−1 with an RSD value of 6.13% (Fig. 6(b)). The batch-to-batch reproducibility of the Ag NPs@W-PDMS films was also studied by the average SERS spectra of R6G in different batches, as depicted in Fig. 6(c). The RSD value of peak intensity at 1510 cm−1 obtained from five different batches is 7.3%, which proves that the batch-to-batch reproducibility is also excellent. Hence, the Ag NPs@W-PDMS film can provide reliable and reproducible SERS measurement for quantitative detection. Besides, from a practical point of view, the mechanical stability of the flexible SERS substrate is a very important prerequisite for in situ detection of nonplanar objects. Here, the mechanical property of the Ag NPs@W-PDMS film was evaluated by bending and twisting test. For the bending and twisting test, the Ag NPs@W-PDMS films were bent in half and twisted to ∼180°. The SERS spectra of R6G collected from the Ag NPs@W-PDMS film show a good consistency after bending or twisting for 100 cycles, as shown in Figs. 7(a, c). The SERS peak intensity at 1510 cm−1 is only slightly degraded after multiple deformations, as shown in Fig. 7(b, d), indicating that the Ag NPs@W-PDMS film has satisfactory mechanical stability. Furthermore, the long-term stability of the AgNPs@W-PDMS film was studied. The AgNPs@W-PDMS film was stored over different periods of time (from 1 to 4 weeks). The corresponding SERS spectra of 10−5 M R6G solution using the AgNPs@W-PDMS film were recorded, and the result is shown in Supplement 1, Fig. S11(a). Compared with the SERS intensity from the fresh substrate, the SERS intensity variation is less than 20% after 4 weeks, as depicted in Supplement 1, Fig. S11(b).
Fig. 6. (a) The distribution of peak intensity at 1510 cm−1 collected from SERS spectra of R6G (10−5 M) at 10 random sites using the Ag NPs@W-PDMS film. (b) SERS mapping of the peak intensity at 1510 cm−1 corresponding to R6G (10−5 M) on the Ag NPs@W-PDMS film. (c) The average SERS spectra of R6G (10−5 M) on Ag NPs@W-PDMS films for five different batches. (d) The distribution of SERS intensity at 1510 cm−1 corresponding to (c).
Fig. 7. (a) SERS spectra and (b) corresponding peak intensity of R6G (10−6 M) at 1510 cm−1 over 100 bending cycles. (c) SERS spectra and (d) corresponding peak intensity of R6G (10−6 M) at 1510 cm−1 over 100 twisting cycles.
3.3 In situ detection of MG in water and fruits
For the detection of analytes in liquids, the tested solution was usually dropped on the surfaces of the rigid SERS substrates or added to colloid SERS substrates. Compared with these two methods, in situ SERS monitoring of analytes in water environments can overcome the difficulty in detecting large molecules on the rigid SERS substrate due to the small nanogaps, and low sensitivity in the colloid SERS substrates resulting from highly dispersed metal NPs in the colloids [14,49]. Thus, in this work, a portable Raman spectrometer was used to evaluate the feasibility of the as-prepared SERS substrate for rapid monitoring of MG residues in on-site application. Owing to its light weight, flexibility and transparency, the Ag NPs@W-PDMS film can serve as a SERS substrate for on-site monitoring of specific contaminants in water, as shown in Fig. 8(a). The developed Ag NPs@W-PDMS film was tested using a portable Raman spectrometer for on-site pollutant detection in aqueous environment (Fig. 8(b)). Figure 8(c) shows the SERS spectra obtained from the Ag NPs@W-PDMS film in contact with MG aqueous solutions at the concentration of 10−6 M. All the characteristic peaks of MG observed at 796, 913, 1169, 1393 and 1616 cm−1 can be clearly detected [50,51], validating its applicability in real conditions. Taking advantages of good flexibility and optical transparency, the Ag NPs@W-PDMS film has a good adaptability to curved surfaces, thus it has potentials to be used for in situ analysis of pesticide residues on fruits/vegetables. To verify this capability, the Ag NPs@W-PDMS film was placed on the apple peel with the Ag NP side facing the peel spiked with MG and SERS measurement was performed under the direct back illumination mode (Fig. 8(d)). The SERS spectra of MG residues with different concentrations were displayed in Fig. 8(e). The Raman signal peak of MG was still distinguishable at the concentration of 10−6 M. A satisfactory linear relation between SERS intensity at 1616 cm−1 and logarithm of MG concentration ranging from 1 × 10−2 to 1 × 10−6 M was obtained with an R2 of 0.998, as shown in the inset of Fig. 8(e). The obtained high R2 value is probably due to the larger spot size of the portable Raman spectrometer, which can average out the intrinsic “hot-spot” variance on the Ag NPs [52]. The above results indicated that the Ag NPs@W-PDMS film combined with a portable Raman spectrometer can be applied to monitor contaminants in different environments.
Fig. 8. (a) Schematic illustration of in situ detection of malachite green in water. (b) Photograph of in situ contaminant testing equipment. (c) SERS spectra collected from the Ag NPs@W-PDMS film floating on the surface of MG aqueous solution (10−6 M). (d) Photographs of in situ detection of MG residue on apple peel. (e) SERS spectra of MG residues with different concentrations on apple peel collected from the Ag NPs@W-PDMS film. The inset shows the calibration curve of SERS intensity at 1616 cm−1 against the logarithmic concentration of MG.
4. Conclusions
In summary, a flexible and transparent SERS substrate was fabricated by depositing Ag NPs on a wrinkled PDMS film obtained through the transfer of buckling structures on Al/PS bilayer film. The SERS performance of Ag NPs@W-PDMS film was optimized by adjusting the Ag thickness. The optimal SERS substrate exhibited a satisfactory testing performance for R6G, with a minimum detectable concentration of 10−7 M, AEF value of ∼1.19 × 105, as well as good signal reproducibility with an RSD value of 6.27%. Besides, the Ag NPs@W-PDMS film demonstrated excellent mechanical stability. Compared with traditional flexible SERS substrates, the as-prepared SERS substrate can achieve in situ detection of analyte not only by floating on water surface but also by pasting on curved surface due to its flexibility, transparency, and light weight. On-site, in situ detection of MG in water and on apple peel were performed using the Ag NPs@W-PDMS film and a portable Raman spectrometer, and the minimum detectable concentration was 10−6 M. Thus, the Ag NPs@W-PDMS film holds great potential in environmental monitoring and food safety.
Funding
Natural Science Foundation of Shandong Province (ZR2021MF097); China Postdoctoral Science Foundation (2018M642289); Liaocheng University Start-up Fund for Doctoral Scientific Research (318051542).
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may.
Supplemental document
See Supplement 1 for supporting content.
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