Various flexible SERS sensors have attracted widespread concern in performing the direct identification of the analytes adsorbed on arbitrary surfaces. Here, a sample method was proposed to integrate plasmonic nanoparticles into polydimethylsiloxane (PDMS) to fabricate flexible substrate for the decoration of silver nanoparticles (AgNPs). The flexible SERS sensor based on AgNPs/AgNPs-PDMS offers highly sensitive Raman detection with enhancement factor up to 8.3 × 109, which can be attributed to the integrative effects from both the increase of the light absorption of the embedded AgNPs in PDMS substrate and the EM enhancement from the adjacent top-top, bottom-bottom and top-bottom AgNPs. After undergoing the cyclic mechanical deformation, the SERS substrate still maintains high mechanical stability and stable SERS signals. However, upon stretching the flexible substrate, there was an amusing phenomenon that SERS signals can be highly increased, which results from that the reduction of lateral nanogaps between top and bottom of the PDMS boundary strengthens the trigger of the plasmon coupling as demonstrated by the simulated result. This result reveals that the tuning and the coupling of the electromagnetic fields can be effectively controlled by the macroscopic mechanical solicitation. That will have an important significance for practical applications in strain-dependent sensors and detectors.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Surface-enhanced Raman scattering (SERS) is emerging as an analytical technique for the trace level detection of chemical and biological analytes up to single-molecule sensitivity via their characteristic vibrational signatures, which facilitates novel applications in medical diagnostics, forensics, environmental conservation, food safety and homeland security [1–3]. What matters for SERS detection is various SERS-active substrates, providing the huge enhancement of Raman signals attributed to a combination of two mechanisms: chemical mechanism (CM) and electromagnetic mechanism (EM). CM is a mechanism that is mostly accepted to be ascribed to the charge transfer between the molecules and the substrates, while EM is based on the enhancement of the local electromagnetic fields due to the surface plasmons stimulated by the incident light [4–8]. Over the past years, researchers have been engaged in searching for top-down and bottom-up approaches such as nanoimprint lithography (NIL), e-beam lithography (EBL), nanoindentation (NI) and self-assembly of nanoparticles (NPs) to prepare SERS-active substrates [9–11]. It is universally known that nanogaps between two metal nanostructures that act as “hot spots” [12,13], can produce highly intense and localized electromagnetic fields under the excitation of the localized surface plasmon resonance (LSPR), leading to a high Raman enhancement factor (EF) to 1014 for single molecular detection [14–16]. Currently, noble metal nanostructures including gold and silver spaced by small dielectric gaps have been demonstrated as the SERS substrate materials [17,18]. While these preparation techniques are predominantly surface-based, in recent years, significant efforts have been devoted to controllably synthesize the solution-dispersible, hot spot-optimized nanostructures dispersed upon arbitrary substrate surfaces [19,20], which can be capable of tracing detection molecules when the surface analytes are located near excited plasmonic nanostructures. It has also been demonstrated recently that synthesis of metallic nanostructures has made ascendant progress in morphology controllability such as nanorods , nanowires , nanoshells , nanotriangles  and nanostars . However, accomplishment of practical applications of these metallic nanostructures is still a challenge. To address the challenge, some substrates, including silicon and glass wafers, have been utilized to deposit these nanostructures and served as the SERS substrates. In many cases, the traditional rigid substrates are not suitable for direct analysis of surface analytes, especially on uneven or not easily accessible surfaces.
To avoid the undesired influence, integration of plasmonic nanostructures onto flexible substrates with advantages of transparency, flexibility, deformability, anti-static properties, biocompatibility and nondestructive measuring [10,25–27] is a highly practical and efficient way to maximize the particulate collection from a nonplanar surface. These flexible substrates have greatly potential to trace target molecules, this is because that the flexible substrates can be easily cut into diverse shapes and sizes, and wrapped onto arbitrary surfaces. Moreover, the flexible substrates can lower the permittivity and reduce the dielectric loss of supporting media, which has a critical implication for detection of SERS sensors [25,28]. It has been reported that diverse methods have been proposed to realize the flexible supporting films soaked or coated nanoparticles, including free-standing polymer film as polydimethylsiloxane (PDMS), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET) and polyvinyl alcohol (PVA), the film of two-dimensional transition-metal dichalcogenide (2D TMDC) nanocomposites, natural 3D materials as filter paper, taro leaf and cicada wing, and polymer nanofiber mats [21,29–35]. For instance, Park et al. have demonstrated a transparent and flexible SERS substrate based on the polydimethylsiloxane (PDMS) film embedded with gold nanostar (GNS) . We have also reported the graphene oxide (GO)/silver nanoparticles (AgNPs)/pyramid polymethyl methacrylate (PMMA) three-dimension (3D) flexible structure . A highly efficient SERS substrate based on common filter paper filled with gold nanorods to trace hazardous material has also been adopted . Moreover, free-standing polymer nanofiber films can be easily prepared through electrospinning means and used to assemble nanoparticles for flexible SERS sensors . It has been evaluated that some hydrophobic materials such as rose petal and taro leaf can shape the beaded droplets on their surfaces [32,37]. Droplet evaporation can lead to localized spots including analytes or nanoparticles, and thus enhance the identifiable sensitivity. Yet, the above-mentioned materials as flexible SERS substrates are mainly based on the pure polymer, which only provides a platform for direct analysis of surface analytes.
In this study, we have developed a SERS sensor via using the silver colloid embedded into PDMS to fabricate flexible and reproducible substrate. Compared with the pure PDMS, the proposed PDMS embedded with AgNPs not only can act as a platform for direct analysis on arbitrary surfaces, but also can provide a SERS enhancement based on EM. To further enhance the SERS performance of the proposed flexible substrate, additional silver nanoparticles were deposited on the surface of the solidified AgNPs-PDMS substrate. The obtained AgNPs/AgNPs-PDMS substrate was demonstrated as a flexible and efficient tool for highly sensitive Raman detection, which can be attributed to the EM from both the top-top, bottom-bottom and top-bottom AgNPs as well as the ability to increase the light absorption of the AgNPs embedded PDMS substrate . Besides, the flexible SERS sensor can withstand a high tensile strain and bending strain without losing SERS performance. After the cyclic stretching test with tensile strain (ε) value of 40% of Raman experiments on the flexible substrates, the SERS signals still remain almost constant for more than 50 cycles, and the bending strain remains similar SERS performance. Interestingly, the SERS signals can be highly increased during the stretching process, especially when the tensile strain was stabilized at 80%, which can be attributed to the small change in the relative distance between nanoparticles at the top and bottom of the PDMS boundary that seriously affects the resonance coupling effect [39,40]. These results indicate that the electric interaction between the nanoparticles can be tuned by using a reversible mechanical strain.
2. Experimental section
2.1 Materials and instruments
Acetone (CH3COCH3, 99.5%), alcohol (C2H6O, 99.7%), ethylene glycol (C2H6O2, 99.0%), Rhodamine 6G (R6G), silver nitrate (AgNO3), methylbenzene (C7H8), and polydimethylsiloxane (PDMS) were purchased from local chemical plant. Polyvinylpyrrolidone (PVP, Mw = 55000) was purchased from Sigma-Alorich.
The morphologies and microstructures of the flexible SERS substrates were investigated by a field-emission scanning electron microscope (SEM) (ZEISS Gemini Sigma 500). The Raman spectra of the analytes were excited with a 532 nm laser using a Horiba HR Evolution 800 Raman microscope system and objective lens (50 × ). The integration time was 8 s, and the laser power was 0.48 mW.
2.2 Synthesis of Ag nanoparticles
Ag nanoparticles were synthesized through the method described by Zhang et al . 20 ml ethylene glycol was heated to 70 °C with continuous stirring by using magnetic stirring apparatus, and then, 0.25 g PVP was dissolved in the ethylene glycol solution. After that, 0.05 g AgNO3 was added into the heating solution when the mixture was heated to 135 °C with the reaction for one hour. With the prolongation of the reaction time, the color of solution gradually changed from a red wine color to a bright yellow color, and the mixed solution eventually became a sticky colloid. Then, substantial numbers of acetone were added into the cooled colloid to establish precipitation, and thus contribute to well separating the pure silver nanoparticles from the solution. Finally pure Ag nanoparticles were gathered by centrifugation with 12000 rpm for five minutes and dispersed in a spot of deionized water (DI water).
2.3 Fabrication of flexible SERS substrates
The flexible AgNPs/AgNPs-PDMS substrate was fabricated as illustrated in Fig. 1. The mixture of the Sylgard 184 elastomer and curing agent at 10:1 weight ratio was oscillated under ultrasound to dislodge the trapped bubbles. Then, adding equal volume of methylbenzene to dilute PDMS mixture. After that, five times volume of silver colloid mixed this mixture was oscillated under ultrasound to remove bubbles. And then pouring them onto the cleaned glass slide and heating at 100 °C for 6 h. After drying, the thin film of AgNPs-PDMS was easily peeled off from the glass slide. Next, the resulting film was immersed in silver colloid to deposit Ag nanoparticles by dip-coating method. Herein, the flexible SERS substrate based on AgNPs/AgNPs-PDMS was utilized for Raman experiments.
3. Results and discussion
Figure. 2(a) clearly exhibits the SEM image of the obtained AgNPs spread onto the silicon wafer, where we can observe the uniformity of size, the regulation of shape and the compactness of distribution of the AgNPs. These uniform AgNPs indicate that the obtained nanoparticles can enable the AgNPs/AgNPs-PDMS substrate to possess high-density and uniform hot spots for the SERS.
In order to identify the size of the well distribution of these nanoparticles, the diameter distribution of the synthesized AgNPs is calculated in the histogram of normal distribution as shown in the insert of Fig. 2(a), where the diameter distribution of the AgNPs is complied with a typical Gaussian curve. On the basis of the peak position, we can acquire the average diameter of our synthesized AgNPs is 77.83 nm. Figure. 2(b) shows the photograph of the AgNPs-PDMS substrate with bent appearance indicating the favorable flexibility. From the insert photograph of Fig. 2(b), obviously, R6G solution droplets on the substrate can hold sphere shape which indicates the well hydrophobicity of this substrate. As depicted in Fig. 2(c), it shows the photograph of the AgNPs/AgNPs-PDMS substrate and hemispherical solution-droplets covered on the surface of the proposed substrate. Owing to the deposition of a layered AgNPs, the surface of the proposed substrate looks glossy. The hydrophobic surface will assemble the target molecules to a certain extent to increase the Raman signals, even though the surface of the AgNPs-PDMS substrate was transferred with nanoparticles. The SEM image in Fig. 2(d) illustrates the surface of AgNPs-PDMS substrate where we can observe the surface is flat and smooth, which is much beneficial for depositing the AgNPs to enhance the SERS sensitivity. The thickness of the AgNPs-PDMS film is about 243 μm as shown in the insert of Fig. 2(d) that exhibits the cross section of the film. Furthermore, based on the EDS spectrum of the AgNPs-PDMS substrate in Fig. 2(e), we can conclude that the AgNPs has been embedded into PDMS successfully and continuously from EDS elemental mapping. And Fig. 2(f) confirms the AgNPs-PDMS substrate is covered by the high-density, continuous, and well-distributed AgNPs monolayer via dip-coating method. A high electromagnetic field near the noble metals can be achieved by tunable nanoscale separation since the enhancement of electromagnetic field is inversely proportional to the nanogap. This observation with nanoparticles being the compactness can be strongly propitious to enhance the electromagnetic couple of the AgNPs and motivate uniform hot spots for the SERS.
The demand of SERS detection of high sensitivity has important implications for sensing performance. To investigate the Raman sensitivity of flexible SERS-active films, we carried out a succession of experiments to detect R6G molecular signals for the analysis of Raman spectroscopy and all the Raman spectra were obtained on the same conditions. Figure. 3(a) shows the Raman spectra of R6G molecules adsorbed onto the surface of AgNPs-PDMS substrate with various concentrations from 10−9 to 10−12 M. The obvious PDMS peaks have not been seen on the pure PDMS substrate and it makes no contribution to enhance SERS intensity based on the insert of Fig. 3(a). The obtained SEM image reveals that no obvious nanoparticle structure can be observed in the AgNPs-PDMS substrate, which has only the texture structure of the PDMS film. Evidently, even down to such a low concentration at 10−12 M, strong SERS signals can be distinguished obviously, which can be attributed to the EM introduced by the bottom-bottom AgNPs and the increase of the light absorption by the embedded AgNPs in PDMS as demonstrated in Fig. 2(e), and indicates the high sensitivity of the flexible AgNPs-PDMS substrate for the SERS detection. This phenomenon can be supposed that the light can easily penetrate the exterior PDMS film to access the interior AgNPs to arouse the plasmonic resonance. Importantly, the thin film has superior hydrophobicity for R6G molecule enrichment, and it inherits abundant nanoparticles to provide rich SERS hot spots, which together result in high sensitivity. As exhibited in Fig. 3(b), the Raman spectra recorded that R6G molecules were adsorbed onto the surface of AgNPs/AgNPs-PDMS substrate with various concentrations from 10−9 to 10−13 M, where we can discover clearly that compared with the former substrate, not only the intensities of R6G peaks at 613, 774, 1185, 1315, 1365, 1508 and 1650 cm−1 can be observed but also the order of magnitude of sensitivity greatly increases. In this case, the high sensitivity can be attributed to the additional plasmonic coupling of the top-top and top-bottom AgNPs in the AgNPs/AgNPs-PDMS substrate, which significantly induces the strong electromagnetic fields in and near the nanogaps. Due to all of these SERS enhanced effects, the proposed SERS-active film can markedly enhance the Raman intensity of target analytes with a high enhancement factor (8.3 × 109), which can be satisfied for the acquisition of highly sensitive Raman-active sensors compared to the previous flexible SERS sensors (Table 1).
In addition to the performance of sensibility, the ability of the flexible SERS sensors to retain mechanical stability has also significant implications for practical applications. The mechanical durability of flexible substrates was measured by cyclic deformation including stretching and bending (Fig. 4(a) and Fig. 5(a)). With regard to the stretching deformation, the tensile strain (ε) is defined as the ratio of the increased film length (∆L) to its original film length (L). The SERS response of the AgNPs/AgNPs-PDMS substrates was investigated after each cycle of a specified tensile strain (ε = 40%, 80%, 120%, 160%). Each cycle is composed with producing specified stretching length on the flexible substrates and then rendering it to relax back to the original position. Figure. 4(b) illustrates the recorded Raman spectra of R6G molecules (10−7 M) adsorbed on the flexible films with producing 40% tensile strain with various stretching cycles. No remarkable change of the Raman fingerprint peaks at 613, 774, 1185, 1315, 1365, 1508 and 1650 cm−1 can be observed undergoing various stretching cycles, which almost still maintains the accordant intensity compared to the original film. To ensure the dependability of the data, the average intensities of the vibrations of R6G located at 613 cm−1 according to ten spectra randomly collected on the AgNPs/AgNPs-PDMS substrates after undergoing different stretching cycles are selected as presented in Fig. 4(c). It is thoroughly obvious that the Raman fingerprint peaks at 613 cm−1 remain mostly unchanged compared to that the original substrate without stretching. To further investigate this, we performed the persuasive analysis under the different tensile strains with different stretching cycles. As depicted in the scatter diagram in Fig. 4(d), it shows the variation of Raman intensity at 613 cm−1 (I613) as a function of the cycle number under the different tensile strains (ε), where it is very interesting to note that the distribution of the I613 with various experiment conditions is restricted in a certain intensity scope. All the data exhibit a slight fluctuation around the green dotted line which represents the average intensity I613 with the film being at the original state, and the variation ranges are confined in the region from −13.7% to + 9.1%. Hence one can see that our proposed SERS substrates can withstand a tensile strain value as high as 160% and possess excellent elasticity to maintain the stability of Raman signals.
Besides, with regard to the bending deformation, the flexible substrates were folded and then released back to the original position with cyclical operations. The SERS response of the flexible substrates was shown in Fig. 5(b), it illustrates the Raman spectra of R6G molecules (10−7 M) adsorbed on the film surfaces after undergoing various bending cycles, which also shows no detectable change in the Raman fingerprint peaks at 613, 774, 1185, 1315, 1365, 1508 and 1650 cm−1 compared to that on the original substrate without bending. Similarly, we also implement numerous analyses of the I613 as a function of the cycle number in Fig. 5(c), the average intensities of I613 based on ten spectra randomly collected on the AgNPs/AgNPs-PDMS substrates after undergoing different stretching cycles are chosen. All the data exhibit the almost similar intensity with only minor fluctuation near the red dotted line which represents the average intensity I613 of the original state, and the variation ranges are confined in the region from −7.4% to + 8.3%. The proposed flexible substrates were demonstrated that they can withstand mechanical deformation without losing SERS performance, which is superior to fulfil the requirements for practical SERS measurements on uneven surfaces. Consequently, these results demonstrate that our proposed SERS sensors possess excellent mechanical durability and great resilience to back, which may be ascribed to the superior elasticity of the PDMS film and the stability of the embedded AgNPs in the film.
In the previous report, it has been demonstrated that a strong polarization-dependent plasmon shift can be induced by stretching the flexible substrate, and a smaller gap between neighboring particles can emerge from a mechanical strain, which enables to control the coupling and the electromagnetic fields at the nanoscale [39,40]. The surface plasmon resonance mode is a natural localized electromagnetic mode, and the energy coupling endowed by external light field transmit to the mode of surface plasmon resonance, leading to the localization of the electromagnetic energy in space, which allows one to control and tune the electromagnetic fields. Hence, the detection of the SERS response was operated through stretching the film to reach the specified tensile strain (ε = 40%, 80%, 120%, 160%) and controlling it to maintain this length for Raman measuring. Figure. 6(a) clearly presents the recorded Raman spectra of R6G molecules (10−7 M) adsorbed on the flexible substrates with undergoing various tensile strains. Amazingly, it is distinct to note that compared to the original state, the SERS intensities of the vibrations of R6G located at 613, 774, 1185, 1315, 1365, 1508 and 1650 cm−1 all gradually raise with the increase of the tensile strain from 40% to 80%, but at 160%, the SERS intensities all appear decrease compared to the relaxant state. In order to more intuitively reflect the internal relationship between Raman activity and tensile strain, the average value of the I613 and the enhancement ratio of the I613 changes as a function of tensile strain as depicted in Fig. 6(b), where we can find that during the flexible substrate stretched, the SERS substrate realized extremely high activity, particularly at ε = 80%. What is more miraculous is that the average SERS intensity at 613 cm−1 is improved by 2.4 times compared to the original SERS substrate at relaxant state. While ε = 160%, the average SERS intensity at 613 cm−1 is down about 40% from black dashed line represented the original state judged from the Raman spectra. The random distribution of the spherical AgNPs manifests as isotropic materials, however when stretching the film in on direction, it leads to a compression in the orthogonal one that induces strong variations in the coupling between nanoparticles. In addition, the stress may induce the smaller lateral nanogaps of the two layer metal nanoparticles of the top and bottom of the PDMS boundary, which may contribute to the coupling of the two layer metal nanoparticles and introduce hot spots for local SERS enhancement. In fact, the decreasing of the inter-particle gap gradually strengthens the trigger of the plasmon coupling, whereas the application of a mechanical stretching allows its tuning. Therefore the coupling effect is ineluctably impacted by a slight change in their relative distance between nanoparticles, further the stretched substrate will show collective LSPR, and the SERS substrate provides the superior SERS enhancement ability. That will impose the development of real-word sensors and detectors related to mechanical strain.
Aside from the SERS activity of the flexible substrate, the homogeneity of the SERS signals is indispensable parameter for the practical measurement. To verify the uniformity of Raman signals in stretching process, the SERS spectra with tensile strain at 80% from 16 random spots on the three samples were recorded in Fig. 7(a), and the results show that the peak value of corresponding position are greatly well consistent, and the intensities for various peaks only fluctuate quite mildly. The well conformity of the SERS spectra of the proposed substrate presents the excellent homogeneity of the AgNPs/AgNPs-PDMS substrate, which can result from the well-ordered and well-distributed AgNPs monolayer. Despite in a stretching state, the well-ordered and well-distributed AgNPs still arouse uniform hot spots to achieve SERS enhancement. Moreover, we also compared the intensities of R6G peaks at 613 cm−1 in Fig. 7(b), where the red dashed line represents the average value of the intensities of R6G peaks at 613 cm−1, and all the data display a mild fluctuation near the average value. The fluctuation ranges are restricted in the region from −7.8% to + 9.6%, and the calculated relative standard deviation (RSD) is only 6.1%, which demonstrates an excellent uniformity of the Raman signals.
To better understand the enhancement mechanism of this AgNPs/AgNPs-PDMS SERS-active substrate, the FDTD simulations are introduced here to analyze the local electric field distributions in these structures. Figures. 8(a)-8(f) show the corresponding results, where the distributions are all located at x-z plane at z = 0. By reference to SEM images, all the AgNPs are considered as the spherical shape in this simulation, and the diameter of them is set as 78 nm with a gap of 5 nm between the adjacent ones and 2 nm between the superficial and inlaid ones, respectively. The incident light is set as 532 nm according to the actual experiments. Compared to the individual one (the insert of Fig. 8(a)), PDMS film embedded with AgNPs is more likely to collect the electric fields (Fig. 8(a)) owing to the LSPR effect of AgNPs. On account of the strong interactions between adjacent AgNPs, most electric fields in this structure tend to concentrate in their gaps rather than the surface of the PDMS film. Through the detailed information (Fig. 8(b), a low scale-bar image of Fig. 8(a)), it can be found that the electric fields distributed around the film surface are only 1.1- to 1.9- times to the incident ones, marked as the dashed box in Fig. 8(b). Considering that the probe molecules are all dispersed on the surface of the PDMS film, these hot spot distributions can’t bring so much SERS enhancement for this structure. However, after covering a layer of AgNPs on the surface of this film, the intense electric fields will transfer upward (highlighted in Figs. 8(c) and 8(d)). These hot spot distributions not only locate in the nanogaps between the adjacent top-top AgNPs, bus also appear at the areas between the adjacent top-bottom AgNPs, which are both the anchored position for probe molecules, and thus bring stronger SERS enhancement in contrast with those for the AgNPs-PDMS. Further stretching the film in the x direction, a compression in the orthogonal direction will lead to decreasing lateral nanogaps between top and bottom of the PDMS boundary as shown in Fig. 8(e). These compression results in more intense horizontal interaction between the top-bottom AgNPs as marked in Figs. 8(e) and 8(f), causing stronger local electric field intensities in their gaps compared to that in Fig. 8(c). Therefore, the application of a mechanical stretching allows the electromagnetic fields to be tuned.
In summary, we have developed a quite low-cost, highly sensitive, excellently elastic, and greatly repeatable SERS sensor by means of the integration the PDMS elastomer and the AgNPs colloid decorated with silver nanoparticles. The increase of the light absorption and the enhancement introduced by the EM from the top-top, bottom-bottom and top-bottom AgNPs endows the proposed flexible AgNPs/AgNPs-PDMS substrate with high SERS performance (EF up to 8.3 × 109). Meanwhile, the flexible SERS sensor retains superior mechanical stability without any deterioration or losing SERS performance after undergoing cyclic deformation including stretching and bending. During the stretching process, it is interesting to note that SERS signals can be highly increased, particularly the tensile strain stabilized at 80%, the SERS intensity at 613 cm−1 on our proposed substrate is enhanced by 2.4 times compared to the original state, which can be attributed to the small change in the relative distance between nanoparticles at the top and bottom of the PDMS boundary that seriously affects the resonance coupling effect. This result contributes to the trigger for the plasmon coupling, and furthermore promoting the SERS enhancement ability. The tuning and the coupling of the electromagnetic fields via adopting a macroscopic mechanical solicitation will open the neoteric door to be applied in sensing and strain detectors, plasmonic waveguides, and optical limiting.
National Natural Science Foundation of China (NSFC) (11774208, 11747072, 11474187, 11747076); Natural Science Foundation of Shandong Province (ZR2017BA004, ZR2017BA018, ZR2013AQ012); A Project of Shandong Province Higher Educational Science and Technology Program (J18KZ011); China Postdoctoral Science Foundation (2016M602716).
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