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Abstract
To achieve high sensitivity and uniformity simultaneously in a surface-enhanced Raman scattering (SERS) substrate, this paper presents the preparation of a flexible and transparent three-dimensional (3D) ordered hemispherical array polydimethylsiloxane (PDMS) film. This is achieved by self-assembling a single-layer polystyrene (PS) microsphere array on a silicon substrate. The liquid-liquid interface method is then used to transfer Ag nanoparticles onto the PDMS film, which includes open nanocavity arrays created by etching the PS microsphere array. An open nanocavity assistant soft SERS sample, “Ag@PDMS,” is then prepared. For electromagnetic simulation of our sample, we utilized Comsol software. It has been experimentally confirmed that the Ag@PDMS substrate with silver particles of 50 nm in size is capable of achieving the largest localized electromagnetic hot spots in space. The optimal sample, Ag@PDMS, exhibits ultra-high sensitivity towards Rhodamine 6 G (R6G) probe molecules, with a limit of detection (LOD) of 10−15 mol/L, and an enhancement factor (EF) of ∼1012. Additionally, the substrate exhibits a highly uniform signal intensity for probe molecules, with a relative standard deviation (RSD) of approximately 6.86%. Moreover, it is capable of detecting multiple molecules and can perform real detection on non-flat surfaces.
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1. Introduction
As a recently developed analytical technique, SERS demonstrates high sensitivity, excellent resolution, and non-destructive testing, making it a highly desirable detection technology in various fields, particularly in food inspection [1,2]. In recent years, researchers have focused on developing SERS substrates to enhance SERS efficiency and achieve better performance. However, extracting probe molecules from curved food surfaces quickly and efficiently is a challenging task. Currently, most SERS substrates are rigid, made of glass or silicon wafers, and require pretreatment of the object to be measured, including dissolution, concentration, and drying. This limitation hinders the widespread use of SERS substrates in practical applications. To overcome this issue, researchers are exploring flexible substrates as they can be easily attached to the surface of the measured object. Common flexible substrates include filter paper, cotton swabs, adhesive tape, and polymers. Several studies have reported the successful application of flexible SERS substrates in detecting pesticide residues on fruits and vegetables, demonstrating promising results [3,4]. For instance, Joshi et al. demonstrated the detection of thiram on apples with a concentration of 104 ng cm−2 using silver nanostructures manufactured in situ on paper substrates via a printing-exposure-development process [5]. Gong et al. developed a method to extract triazophos from the surface of apples and cherry tomatoes using adhesive tape, followed by the addition of silver nanoparticles for detection, resulting in a limit of detection of 25 ng cm−2 [6]. Kong et al. modified cotton swabs with silver sol to prepare SERS substrates and utilized a wiping sampling-detection method to achieve the mixed detection of Thiabendazole (TBZ) and thiram at a concentration of 1 ng cm−2 on balsam pear [7]. Alyami et al. conducted a study in which they self-assembled spherical silver nanoparticles on a SERS substrate made from PDMS. They then applied this substrate to contaminated fish skin and orange skin, and were able to successfully detect food contaminants such as crystal violet (CV) and thiram pesticide. The minimum detection concentration of CV was found to be 10−7 mol/L, while the limit of detection (LOD) for thiram was 10−5 mol/L [8].
Although the above-mentioned methods have addressed the issue of rigid substrates being difficult to make full contact with the objects being measured, they are, to some extent, limited by their two-dimensional structures. The arrangement of metal nanoparticles in these substrates is two-dimensional (2D), resulting in the appearance of “hot spots” with moderate strength and suboptimal SERS enhancement effects [9] To overcome this limitation, numerous methods for synthesizing three-dimensional (3D) substrates have been developed, including template-assisted methods [10–12], electrospinning [13,14], and microsphere template composite methods [15–17], all of which have demonstrated superior performance in three-dimensional structures. Therefore, the primary research focus at present is on combining these two methods to produce flexible, three-dimensional SERS substrates. For instance, Kumar et al. poured PDMS prepolymer onto the surface of taro leaves, resulting in a substrate with a blade surface structure. Subsequently, AgNPs were deposited onto the aforementioned structure, resulting in a linear detection range of 10−11 to 10−3 mol/L for Malachite Green (MG) molecules [18]. Wang et al. utilized a silk fibroin film as a carrier and anodic aluminum oxide (AAO) as a template, along with gold nanoparticles as plasma, to create a new type of ultra-thin layered, flexible, wearable SERS substrate. This substrate achieved a minimum detection limit of 5.7 ppm for thiram on fruit surfaces [19]. Li et al. prepared a flexible, three-dimensional, hybrid poly(bisphenol A carbonate) (PC)/Ag SERS substrate via electrospinning and in-situ chemical reduction. The linear response range for Trinitrotoluene (TNT) was found to be 10−8 to 10−12 mol/L, with a minimum detection concentration of 2.05 × 10−13 mol/L [20].
Compared to the random geometric structures mentioned above, ordered geometric structures offer greater reproducibility and uniformity, but the cost is relatively high. Fang et al. prepared a flexible, structured polytetrafluoroethylene (PTFE) SERS substrate using femtosecond laser technology and the thermal evaporation method, demonstrating excellent potential for developing wearable electronic devices [21]. Li et al. transformed 3D AgNPs/MoS2/pyramid Si samples into 3D flexible MoS2/AgNPs/polymethylmethacrylate SERS substrates via chemical etching. This approach facilitated the detection of multiple molecules and resulted in good stability and reusability [22].
Regardless of the preparation method for flexible 3D SERS substrates or the composite method for depositing precious metal nanoparticles onto these substrates, such as magnetron sputtering and vacuum evaporation [23,24], these processes require complicated procedures or expensive equipment, making it difficult to widely implement them on a large scale. Therefore, the development of an affordable, flexible SERS substrate with a three-dimensional structure is necessary to enable in-situ detection on curved surfaces.
In order to address the aforementioned issues, we propose a straightforward and cost-effective solution: the Ag@PDMS flexible SERS substrate with a 3D ordered nanostructure. This substrate demonstrates exceptional sensitivity and uniformity, achieved by transferring self-assembled silver nanoparticles onto a PDMS film with a 3D nano cavity array structure.
2. Experiments
2.1 Materials and characterization equipments
Single-sided polished silicon wafers were obtained from Zhejiang Lijing Electronics Co., Ltd (China). Monodisperse PS microspheres (600 nm) obtained from Beijing Zhongke Leiming Daojin Technology Co., Ltd (China) were used as a template to create the cavity structure. The substrate was made from Dow Corning silicone rubber DC184 (Shanghai Aladdin Co., Ltd, China). To etch the PS microspheres, dimethylformamide (Shanghai Aladdin Co., Ltd, China) was used. Mercaptopropyl trimethoxysilane (MPTMS, Shanghai Aladdin Co., Ltd, China) was used to form the interface. For SERS detection, Rhodamine 6 G, crystal violet, Malachite Green and Thiram, purchased from Shanghai Aladdin Co., Ltd (China), were used as probe molecules.
The morphological characteristics of the samples were obtained using the Quattro SEM (ThermoFisher, U.S.A.), which combines the imaging performance of a scanning electron microscope with an environmental model. The UV-3600 spectrophotometer (Shimadzu, Japan) was used to study the absorption characteristics of the samples. Raman spectra were collected using a laser confocal Raman spectrometer (HORIBA Jobin Yvon S.A.S., France) equipped with a green Nd: Yag 532 nm excitation laser. To avoid heat damage to the samples, a 50× objective lens and 5% laser power with a total power of 50 mW were used, and the laser spot size on the samples was about 1 µm. The wavenumber range selected for the Raman characteristic peak of the probe molecule used was 500−2000cm−1, and the integration time for each measurement was set to 5 seconds. After the measurement, Labspec 5 software was used to remove the baseline of the spectrum and eliminate the background related to Raman intensity. To ensure the credibility of the data, all data were the average results of several measurements.
2.2 Process of sample preparation
The preparation process of the composite SERS substrate mainly involves several steps: Firstly, the preparation of single-layer PS microspheres, followed by the preparation of PDMS films. Finally, the preparation of single-layer Ag@PDMS substrate is carried out, as illustrated in Fig. 1(a). For this three-dimensional substrate, we speculate that there are three factors contributing to Raman enhancement, namely localized electronic field enhancement near the surface of Ag, the gap between AgNPs, and the open nano cavity assistant effect, as shown in Fig. 1(b).
Fig. 1. (a) Preparation process of Ag@PDMS substrate, and optical image of this soft SERS sample. (b) Three types of enhancement effects induced by localized electronic filed enhancement ① near the surface of Ag, ② at the gap between AgNPs and ③ open nano cavity assistant effect.
2.2.1 A: Preparation of single-layer PS microspheres
To prepare the silicon wafer, it was initially cleaned using acetone, anhydrous ethanol, and deionized water for several minutes to eliminate impurities. Then, the silicon wafer was immersed in piranha solution (24 mL H2SO4 adding to 10 mL H2O2) and placed in a water bath at 90°C for 1 hour. After being washed and dried multiple times, the silicon wafer was placed on a spin coater, and the spin coater's working mode was set to two stages. In the first stage, the speed was set to 500 r/min for 15 s, followed by the second stage, which had a working time of 60 s at a speed of 1500 r/min. A small amounts of PS microspheres were absorbed with a pipette and dropped onto the silicon wafer for spin coating. After completion, the substrate was allowed to stand and dry to obtain a single-layer PS microsphere array.
2.2.2 B: Preparation of the PDMS thin film with open nanocavity
To prepare PDMS, Sylgard 184 prepolymer and crosslinking agent were fully mixed at a ratio of 10:1 (w/w), and the mixture was allowed to stand until all bubbles were completely removed. The mixed solution was then spread onto the surface of the single-layer PS microspheres, dried in an oven at 70 °C for 3.5 hours, and peeled off the PS@PDMS film from the silicon wafer slide after cooling. Subsequently, the PS@PDMS film was placed in dimethylformamide for 4 hours. The PS microspheres in the film were etched away using a wet method, resulting in a PDMS film with a 3D ordered hemispherical open nanocavity array.
2.2.3 C: Preparation of a single-layer Ag@PDMS
In our experiment, we employed the liquid-liquid interface method to obtain single-layer AgNPs. To form the interface, an equal volume of hexane was added to the silver sol due to incompatibility. It should be noted that Ag with diameters of 50, 100, 150, and 200 nm were used in the experiments [25]. Subsequently, 100 µL of 10−4 mol/L MPTMS was added to the liquid surface. Ethanol was then slowly added to the interface to enrich AgNPs on the interface. After allowing the hexane to evaporate for some time, a bright silver film appeared on the interface. Next, a PDMS film with a three-dimensional ordered hemispherical open nano-cavity array was inserted obliquely under the interface and lifted vertically. Upon drying, a single-layer Ag@PDMS substrate was formed.
3. Results and discussion
3.1 SEM characterization
Figure 2 displays SEM images of samples obtained during the preparation process. Fig. 2(a) demonstrates that PS microspheres are self-assembled uniformly over a large area and tightly stacked on the silicon substrate. The calculated gap between PS microspheres is ∼32 nm. Consequently, the obtained periodic ordered array constitutes a perfectly continuous nanopore over a large area, which confirms the high uniformity of the prepared substrate. Fig. 2(b) reveals that PS microspheres remain uniform even after being transferred to a PDMS substrate. Following the removal of the PS microspheres, a periodic ordered open nano cavity array is obtained, as depicted in Fig. 2(c). The calculated gap between the holes is ∼103 nm. The approximate size of the nano cavity was determined by counting, and the calculated average size of the nano cavity is 496 nm, as presented in Fig. 2(d). According to Li et al., using effective medium theory, cavities with an average particle size of around 450 nm have been shown to be beneficial in achieving better light utilization [26].
Fig. 2. SEM images of (a) PS microsphere array on silicon wafer, (b) PS microsphere array on PDMS, (c) flexible PDMS substrate with open nano cavity arrays. (d) The corresponding calculated size of nano cavity. SEM images of Ag@PDMS with AgNPs size of (e) 50, (f) 100, (g) 150 and (h) 200 nm.
Figures 2(e, f, g, h) illustrate the images of 3D flexible Ag@PDMS substrates following the transfer of four different sizes (50, 100, 150, and 200 nm) of Ag particles onto the surface of transparent PDMS with open nano cavities. It can be observed that when the particle size of silver nanoparticles is around 50 nm, it can be deposited effectively within the 3D open nano cavity structure. However, as the particle size increases to 100, 150, and 200 nm, silver nanoparticles are more likely to be spread out on PDMS without forming a three-dimensional structure. We suspect that this may be the result of surface tension, van der Waals force and Brownian motion of particles. Among them, surface tension is the force that prevents particles from entering the cavity. Van der Waals force is one of the main driving forces of adsorption [27]. When the nanoparticles are in contact with the pores, the silver nanoparticles with small particle size will generate greater van der Waals force because of their larger specific surface area, so they are easily absorbed [28]. In addition, the effect of Brownian diffusion decreases as the particle size increases, so the thermal motion of small-sized particles helps particles overcome resistance to enter the pores [29].
3.2 Absorption characterization
The surface morphology of plasma determines its optical response. Fig. 3(a) shows the simulation result of the normalized absorption of Ag@PDMS samples with different particle sizes. The maximum absorption resonance peak is red-shifted, ranging from 430 to 545 nm, and this red-shift is closely related to the radiation effect. Fig. 3(b) displays the corresponding normalized experimental absorption. It can be observed that as the size of silver particles increases, the absorption peak shows a redshift from 430 nm to 535 nm, which is consistent with the reported study [30]. There are slight discrepancies between the simulation and experimental results, mainly due to the fact that the single-layer AgNPs obtained experimentally are not completely evenly distributed in a single layer. Meanwhile, we know that the enhancement effect is optimal when the laser wavelength is the same as or close to the absorption resonance peak of SERS substrate [31]. Therefore, a 532 nm laser was used for Raman testing, resulting in better enhancement.
Fig. 3. Normalized absorption spectra of Ag@PDMS samples: (a) simulation, (b) experiment. Samples with larger Ag sizes exhibit a red-shift to a longer wavelength theoretically and experimentally.
3.3 Raman characterization
3.3.1 Comsol analysis
The Raman enhancement observed in Ag@PDMS samples primarily arises from the localized surface plasmon resonance of Ag nanoparticles (AgNPs) present in the gaps between the nanoparticles and the nano-cavities, which contribute to additional enhancement. The distribution of the electromagnetic field was simulated using Comsol, and the theoretical enhancement was calculated based on these simulations.
In Fig. 2, we compared the deposition of silver nanoparticles using SEM images. As an example, we took the Ag@PDMS sample with 50 nm Ag and determined the arrangement of silver nanoparticles to be hexagonal closest packing with a 5 nm gap between particles, consistent with the experimental parameters. We used a plane wave of 532 nm as the source and incident an electric field with an intensity of 1 V/m along the −Z direction, with the polarization direction set as the Y axis (Fig. 4(a)). We employed a perfect matching layer (PML) as the boundary condition and divided the model into two parts (A and B), which were simulated separately. Part A included the open nanocavity effect, as shown in Figs. 4(b, c, d), while Part B represented the area of AgNPs on the surface of flat PDMS, as shown in Figs. 4(e, f). According to Comsol calculation results, the larger E-field intensity was mainly distributed between the gaps of metal particles, which we referred to as “hot spots”. The calculated maximum electric field intensity between gaps in Part A was 800 V/m, while in Part B, it was 100 V/m.
Fig. 4. The simulation results. (a) Simulation model (Ag size of 50 nm, with the gap of 5 nm). Electronic field distribution of (b) plane xy, (c) plane yz, and (d) plane xz in Part A. Electronic field distribution of (e) plane xy and (f) plane xz in Part B.
So the theoretical EF can be obtained as follows:
3.3.2 Raman measurements of samples with different particle sizes
To explore the SERS performance of the Ag@PDMS substrate, Raman measurements were conducted using different concentrations of analyte. The Raman spectra of R6G at varying concentrations on our sample are presented in Figs. 5(a−d), where several characteristic peaks were easily distinguished. The LOD can reach 10−15 mol/L. Among the four samples tested, the sample with Ag particle size of 50 nm exhibited the highest sensitivity. In addition, the analytical enhancement factor (AEF) was used to evaluate the SERS effect of the substrate, calculated as follows:
Fig. 5. Raman measurements of Ag@PDMS with Ag size of (a) 50 nm, (b) 100 nm, (c) 150 nm, (d) 200 nm, using R6G as the analyte molecule with concentrations ranging from 10−11 mol/L to 10−15 mol/L. (e) Comparison of four samples at several peaks, all with a concentration of 10−11 mol/L. (f) Raman intensity of different concentrations on different samples, the calculated total EF and cavity-assisted EF is shown in the figure too.
In Fig. 5(e), we compared the Raman results of several samples for 10−11 mol/L R6G. We found that the substrate with silver nanoparticles of 50 nm had a better enhancement effect than other substrates, which is consistent with our previous analysis of the mechanism of substrate enhancement. This was due to the third enhancement effect induced by the “open nanocavity” mentioned earlier. Furthermore, we compared the Raman measurement results of the Ag@PDMS substrate with and without cavity. As shown in Fig. 5 (f), there was an order of magnitude difference between them for the LOD of R6G. We calculated that the EF value of the cavity could reach approximately 20.
3.3.3 Uniformity
To demonstrate the superiority of the substrate, we conducted a uniformity experiment of the substrate. Using 10−11 mol/L R6G as the analyte, we carried out Raman mapping measurements in a region of 20 µm × 15 µm with a step size of 5 µm. The results showed that all samples exhibited good uniformity (Fig. 6). Sample Ag@PDMS of Ag 50 nm had the best relative standard deviation (RSD) at 6.86%, as shown in Fig. 6(a). Additionally, SEM images revealed that AgNPs with smaller sizes, such as 50 nm, were uniformly distributed in a 3D nano-hole structure, while the vast majority of Ag nanoparticles with larger particle sizes, such as 100, 150, and 200 nm, were spread out on the plane. Although the Raman intensity was smaller due to the lack of open nanocavity assistant enhancement, their uniformity was still acceptable.
Fig. 6. Raman mapping measurements obtained when R6G was dripped on Ag@PDMS substrates with Ag size of (a) 50 nm, (b) 100 nm, (c) 150 nm and (d) 200 nm, at a concentration of 10−11 mol/L. The illustrations depict the results of RSD analysis.
3.3.4 Practical application
The practical application ability of Ag@PDMS was mainly tested by two experiments. The first experiment involved the detection of thiram, while the second experiment involved the detection of multiple molecules. For the thiram detection experiment, we sprayed thiram with different concentrations on the surface of an apple. We then placed the Ag@PDMS film on the surface and ensured full contact with the analysis surface. The measurement results are shown in Fig. 7(a), with a minimum detection concentration of 10−10 mol/L for thiram. Table 1 provides a summary of flexible SERS substrates used for pesticide detection in recent years. Our substrate's LOD was lower than that of previous studies. For the second experiment, we used R6G (10−9 mol/L), CV (10−7 mol/L), MG (10−5 mol/L) and Thiram (10−9 mol/L) in two or three mixtures. We were able to distinguish the different peaks of the analytes (R6G: 610, 770, 1650 cm−1, etc., CV: 722, 913, 1614 cm−1, etc., MG: 1222, 1299, 1444 cm−1, etc., Thiram: 565, 1380 cm−1, etc.) clearly, indicating that our substrate has great application potential, as shown in Fig. 7(b).
Fig. 7. Raman measurement of (a) thriam on the apple’s surface, (b) multi-molecule detection using Ag@PDMS (50 nm Ag) as SERS substrates.
Table 1. A summary of flexible SERS substrates used for pesticide detection
4. Conclusion
Here, we have produced 3D PDMS films by etching a self-assembled single-layer PS microsphere array. Through the transfer of Ag nanoparticles, we have developed a flexible and cost-effective SERS substrate, known as “Ag@PDMS,” which possesses a 3D ordered nanostructure and enables the detection of low concentration substances. Our findings reveal that the Ag@PDMS substrate of 50 nm silver nanoparticles, provides optimal SERS enhancement performance (EF= ∼1012) and good uniformity (with an RSD of 6.86% at 770 cm−1). Given its outstanding performance, this three-dimensional flexible SERS substrate holds great promise for food inspection applications.
Funding
Chongqing Outstanding Youth Fund (cstc2019jcyjjqX0018); National Natural Science Foundation of China (62175023).
Acknowledgments
We would like to thank Dr. Gong Xiangnan at Analytical and Testing Centre of Chongqing University for his help in Raman measurement.
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.
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