On-line sensitive detection of aromatic vapor through PDMS/C₃H₇S-assisted SERS amplification

Abstract

Much attention has been paid to surface enhanced Raman spectroscopy (SERS) in recent years due to its capability to detect trace amounts of chemical and biological analytes with single molecule sensitivity and its rapid response. Unfortunately, sensitive SERS detection is often limited to molecules that can strongly adsorb onto the surface of plasmonic substrates. The detection of molecules without specific adsorption groups still remains a great challenge, especially for vapor molecules in a flowing system. Herein, we present a novel PDMS/C₃H₇S-assisted SERS amplification process for on-line sensitive detection of aromatic vapors. Specifically, the SERS amplification process relies on a PDMS-coated 1-propanethiol-modified Au@Ag nanoparticle monolayer film (PDMS/C₃H₇S–Au@Ag MLF) as the SERS substrate, in which the PDMS and 1-propanethiol layers are capable of capturing vapor molecules from flowing air and placing them in close proximity in plasmonic hot spots. By taking advantage of the high adsorption capability of PDMS and strong plasmonic properties of the nanoparticle film, our PDMS/C₃H₇S-assisted SERS amplification can result in about two orders of magnitude enhancement of the SERS signal in comparison with that of the naked Au@Ag MLF. We also demonstrate that our developed plasmonic nanoparticles film could be applicable for on-line SERS detection of a series of aromatic vapors in the flowing system. These results provide exciting opportunities for in situ monitoring of volatile organic pollutants in the atmospheric environment.

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Introduction

The increasing diffusion and accumulation of volatile aromatic compounds (e.g. benzene and its derivatives) in the environment has aroused wide public concern.^1–4 Intense research has been fueled by the need for a robust analytical technique to monitor trace amounts of these vapor molecules with high sensitivity and rapid response. Till now, various identification techniques have been explored for detection of pollutants diffused in the atmosphere. However, most of them suffer from disadvantages, such as, the use of high cost, complicated sample preparation processes, sophisticated operation, and time-consuming analytical procedures.⁵

Surface enhanced Raman spectroscopy (SERS) has emerged as a powerful analytical tool for identification of molecules.^6–10 In SERS, the Raman signal of a molecule in close proximity to a plasmonic surface can be enhanced by numerous orders of magnitude. As such, the detection limit of SERS could be down to the single molecule level. Unfortunately, such high sensitivity is generally restricted to the molecules with specific adsorption groups (e.g. thio, amino, nitrile, and nitro) that strongly chemisorb on the plasmonic metal surface.^11–13 The challenge of SERS detection for the molecules without specific groups (especially for volatile vapors), because of their low affinity toward the plasmonic metallic surface, has been met with limited success since the discovery of SERS phenomenon in 1974.¹⁴

To overcome this issue, various approaches have been developed to capture weakly adsorbed molecules on the plasmonic surface for the intense SERS signal.^15–28 An excellent strategy for SERS detection of aromatic vapor is to modify a hydrophobic thiol or alkylsilane monolayer on the metallic surface. The hydrophobic interaction between the substrate and aromatic molecules allowed the molecules to locate within the electromagnetic enhancement field, resulting in an enhanced SERS signal.^16–19 In a set of parallel efforts, metal–organic frameworks (MOFs) were employed to capture the weakly adsorbed molecules on the plasmonic substrates for improving SERS signals due to their inherent porous nature.^20–23 In our recent demonstration, we achieved an enhancement in SERS intensity by spin-coating PDMS onto Au nanoparticle monolayer film.²⁴ The special surface property and permeability of PDMS could trap the volatile aromatic compounds within the PDMS layer on the Au film surface, thus improving the SERS detection.^25–32 Despite advance in enhanced SERS detection of weakly adsorbed molecules, most of the molecules captured by PDMS are still located far away from the hot spots of plasmonic substrates. The SERS signal dominantly originated from a small amount of molecules within the plasmonic hot spots. This drawback has prompted us to develop a robust strategy to effectively immobilize aromatic vapor on the metal surface for strong SERS enhancement.

In this work, we describe a novel PDMS/C₃H₇S-assisted SERS amplification approach, by coupling a PDMS layer, alkanethiol linkers and a plasmonic metal nanostructure, for on-line SERS detection of aromatic vapor in the atmosphere with a high sensitivity and rapid response. By carefully controlling the experimental parameters, the thickness of PDMS layer can be optimized to obtain the maximum vapor adsorption efficiency and minimize the propagation loss of the excitation laser and SERS signals. The hydrophobic interaction offered by the alkanethiol ligands can effectively place the vapor molecules close to plasmonic hot spots and results in a plasmonic enhancement (Scheme 1). Our mechanistic investigation reveals the capture and immobilization process of aromatic vapor molecules in our PDMS/C₃H₇S-assisted SERS amplification. This strategy allows for rapid and sensitive SERS detection of weakly adsorbed vapor molecules at ambient condition.

Scheme 1 The schematic illustration for on-line detection of aromatic vapor in the flowing-system through PDMS/C₃H₇S-assisted SERS amplification. The detection is based on a coupled system comprising a flowing-system and a Raman microscope. The aromatic vapor was delivered from the analyte chamber to the substrate chamber (with PDMS/C₃H₇S–Au@Ag MLF) by a micropump. Subsequently, the vapor was captured by the PDMS layer, followed by immobilization on the enhancement source (Au@Ag NPs) via hydrophobic interaction with 1-propanethiol. The SERS signal on the PDMS/C₃H₇S–Au@Ag MLF was then recorded by the Raman microscope.

Experimental section

General

SYLGARD silicone elastomer base and SYLGARD 184 silicone elastomer curing agent were purchased from Dow Corning Company. Polyvinylpyrrolidone (PVP) was bought from Acros organics. Chloroauric acid tetrahydrate (HAuCl₄·4H₂O), silver nitrate (AgNO₃), hydroxylamine hydrochloride (NH₂OH·HCl), trisodium citrate, ascorbic acid, sulfuric acid (H₂SO₄, 95–98%), hydrogen peroxide (H₂O₂, 30%), toluene, benzene, m-xylene, o-xylene, p-xylene, n-hexane and ethanol were bought from Sinopharm Chemical reagent corporation with analytical reagent grade. The aqueous solutions were prepared by using Milli-Q water (≧18.2 MΩ cm).

Characterizations

Scanning electron microscopy (SEM) imaging was carried out using an S-4700 from Hitachi and transmission electron microscopy (TEM) images were recorded with a Tecnai G220 from FEI. UV-vis absorption measurements were carried out on a TU-1900 version spectrometer from Persee. SERS experiments were performed by using a LabRam HR800 from HORIBA with a 632.8 nm He–Ne laser. The objective lens was 50× with a working distance of 8 mm. The slit width was 100 μm while the confocal hole was 400 μm. The laser power on the substrates was about 5 mW.

Preparation of Au@Ag NPs and the formation of Au@Ag MLF

The Au@Ag NPs was prepared by a seeds growth method. Trisodium citrate was used for reducing HAuCl₄ as Au seeds for the Au cores of the Au@Ag NPs according to the Frens' method.³³ 100 mL aqueous solution of HAuCl₄ (0.01 wt%) was heated to boiling. Then 2 mL of trisodium citrate (1 wt%) solution was added immediately. The mixed solution was heated for another 15 min and was cooled down to room temperature. The obtained Au NPs with an average diameter of 15 nm served as Au seeds.

30 nm Au NPs were synthesized according to an epitaxial growth using 15 nm Au as the seeds. Typically, the 25 mL of 15 nm Au seed solution, 1 mL of PVP (1 wt%), 1 mL of trisodium citrate (1 wt%) solution, and 20 mL of NH₂OH·HCl (25 mmol dm⁻³) solution were mixed together under room temperature. Then 20 mL of HAuCl₄ (0.1 wt%) solution was added drop by drop under stirring for 1 h. The average diameter of the resulting Au NPs was about 30 nm.³⁴ 50 nm Au@Ag NPs were synthesized by using the 30 nm Au as the cores via an additional epitaxial growth. In a typical procedure, an aqueous solution of trisodium citrate (1 mL, 1 wt%), PVP (0.3 mL, 10 wt%) and ascorbic acid (4 mL, 34 mmol dm⁻³) was added to the 30 nm Au cores (10 mL) solution. Subsequently, 20 mL of AgNO₃ (0.03 wt%) solution was injected drop by drop with stirring at room temperature. After that, the Au@Ag NPs solution was transferred to lab-made evaporation device and then dried at 40 °C for 16 h. The resulting compact Au@Ag MLF was transferred onto a clean silicon wafer for further SERS study.³⁵

Preparation of C₃H₇S–Au@Ag MLF

The C₃H₇S–Au@Ag MLF was obtained by dipping the Au@Ag MLF into an ethanol solution of 1-propanethiol (1 mmol dm⁻³) for 24 h at room temperature. Then washed by ethanol and dried at room temperature for further study.

Fabrication of PDMS/C₃H₇S–Au@Ag MLF

Silicone elastomer base and its curing agent were completely mixed in a ratio of 10 : 1 (wt), and were kept oscillating for at least 1 h. The PDMS elastomer was diluted in n-hexane to obtain the concentration range from 2 wt% to 50 wt%. In our study, the thickness of the PDMS layer on the substrate was controlled by varying the concentration of the PDMS elastomer solution. In a typical experiment, 3 μL of PDMS elastomer solution was dropped and spin-coated onto a C₃H₇S–Au@Ag MLF for 300 s at a spin speed of 5000 rpm with an acceleration of 2500 rpm s⁻¹. The PDMS-coated C₃H₇S–Au@Ag MLF was finally cured in vacuum at 40 °C for 2 h.³⁶

SERS detection of aromatic vapor

The vapor flowing-system was used to simulate vapor detection in flowing air according to a modified Alison Chou's flow-through system.³⁷ The system was flushed with dry nitrogen before using. 20 μL of the aromatic compound was firstly transferred into the analyte chamber. When the carrier gas flow was adjacent to the liquid analyte surface, evaporation occurred. And the vapor was then transferred into the carrier gas stream. The micropump with a flowing rate of 200 mL min⁻¹ was used for delivering the vapor molecules from the analyte chamber to the substrate surface. The substrate chamber equipped with a glass window was placed directly under a 50× objective lens of a Raman microscope for SERS measurement. Note that the whole device was attached on a thick steel plate to fix the focus point during the vapor detection procedure.

Results and discussion

Synthesis and characterization of PDMS/C₃H₇S–Au@Ag MLF

In the present study, Au@Ag core–shell nanoparticles were synthesized as the SERS substrate according to the seeds growth method which was previously reported.³⁴ Typical TEM images of the as-synthesized Au and Au@Ag NPs are shown in Fig. 1a. The Au nanoparticles have an average diameter of 30 nm, while the corresponding Au@Ag NPs have an average size of 50 nm. UV-vis adsorption reveals that Ag coating results in a blue shift (from 524 nm to 492 nm) of the absorption band of Au nanoparticles. Furthermore, a new peak at ∼415 nm can be clearly observed, suggesting the successful Ag shell coating. Subsequently, Au@Ag MLF was fabricated by a self-assembly process and was then transferred to a silicon wafer. The SEM image shows that the as-prepared Au@Ag MLF has a compact monolayer structure (Fig. 1b).

Fig. 1 (a) UV-vis spectrum of 30 nm Au and 50 nm Au@Ag NPs. The insets show the corresponding TEM images of the Au and Au@Ag NPs. Scale bars: 40 nm. (b) SEM image of the Au@Ag MLF. Scale bars: 1 μm; (c) SERS spectra of TP from 60 randomly selected spots on the Au@Ag MLF. (d) The corresponding SERS intensities of TP at 1072 cm⁻¹. The average SERS intensity (blue line) was estimated to 3986.7 cps with the relative standard deviation of 1.45%.

We first evaluated the SERS activity of the as-prepared Au@Ag MLF by using thiophenol (TP) as a Raman reporter. As a result, we observed an intense SERS signal at 1024, 1072, and 1572 cm⁻¹ (Fig. 1c), which is attributed to the in-plane ring deformation mode, the in-plane phenyl ring stretching band, and the C–C stretching mode, respectively.^38,39 In order to further investigate the uniformity of SERS activity on the Au@Ag MLF, a set of SERS spectra were collected from 60 random spots on the Au@Ag MLF after dipping it into 1.0 × 10⁻⁴ mmol dm⁻³ TP solution for 30 min (Fig. 1c). As shown in Fig. 2d, it could be found that the intensity variation of TP signal at 1072 cm⁻¹ is within the relative standard deviation of 1.45%, indicating that the excellent uniformity of SERS activity of the as-fabricated Au@Ag MLF.

Fig. 2 (a) SERS spectra of C₃H₇S–Au@Ag (1, 2), C₈H₁₇S–Au@Ag (3, 4), HOOCC₂H₄S–Au@Ag (5, 6), and Au@Ag (7, 8) MLFs with (1, 3, 5, 7) and without (2, 4, 6, 8) treatment of toluene vapor. (b) SERS intensity of the toluene at 1003 cm⁻¹ on the corresponding MLFs. (c) SERS spectra of 1-propanethiol from 60 randomly selected spots on a C₃H₇S–Au@Ag MLF. (d) The corresponding SERS intensities of 1-propanethiol at 1025 cm⁻¹. The average SERS intensity (blue line) was estimated to 8882.2 cps with the relative standard deviation of 2.95%.

Influence of alkanethiols modification on the vapor detection

Self-assembled monolayers of alkanethiols on metal surface has proven effective for immobilization of the hydrophobic molecules on substrate through the hydrophobic interaction.¹⁶ This hydrophobic interaction can allow the Raman-active molecules located in the close proximity to the plasmonic substrate, thus resulting in a SERS enhancement due to a strong adsorption of the Raman-active molecule on the plasmonic metal surface. Compared to the strongly adsorbed molecules, the aromatic vapors are difficult to be immobilized in the metallic surface for an intense SERS signal. On the basis of above-mentioned principle, a self-assembled monolayer of alkanethiols on the plasmonic surface might offer an opportunity to capture aromatic vapor molecules via the hydrophobic interaction. To validate our hypothesis, we modified a set of alkanethiol molecules (1-propanethiol (C₃H₇SH), octanethiol (C₈H₁₇SH), and thiohydracrylic acid (HOOCC₂H₄SH)) on the surface of Au@Ag MLF and tested their SERS activities of the resulting substrates using toluene vapor as the Raman reporter. As shown in Fig. 2a and b, the alkanethiol-modified Au@Ag MLF gives rise to much higher SERS intensity of toluene at 1003 cm⁻¹ (corresponding to C–C stretching mode). Note that the Raman peaks at 1025 cm⁻¹ is ascribed to the vibrational band of 1-propanethiol. Notably, due to the strong hydrophobic behavior of their long carbon chain, 1-propanethiol and octanethiol on the metal surface both results in an obvious enhancement in SERS intensity of toluene molecules. However, the octanethiol modification shows a weaker enhancement effect. We reason that its long carbon chain makes toluene molecules to locate relatively far from the SERS hot spots. On the contrary, the thiohydracrylic acid modification barely has any positive effect on SERS enhancement because the weak hydrophobicity of thiohydracrylic acid is not favorable for capturing toluene vapor. Taken together, these results suggest the hydrophobic interaction offered by alkanethiol on the metallic surface plays the vital role in SERS enhancement of aromatic vapor molecules.

We further collected a set of SERS spectra from 60 random spots on the 1-propanethiol-modified Au@Ag MLF (Fig. 2c). Fig. 2d presents the SERS intensity distribution of the characteristic band at 1025 cm⁻¹. One can find that the SERS intensity is in the relative standard deviation of 2.95%, demonstrating the SERS activity uniformity of the C₃H₇S–Au@Ag MLF.

Effect of PDMS coating on the vapor detection

It is well known that PDMS has a high ability for the adsorption of trace aromatic molecules, and is used for the removal of benzene and toluene from polluted water.^30–32 To further improve the capability of the SERS substrate for immobilizing and detecting toluene vapor, a PDMS layer was employed to coat C₃H₇S–Au@Ag MLF. In principle, a thicker PDMS layer is beneficial to vapor capture, but it can also obstruct the propagation of excitation laser and SERS signal. For obtaining maximum SERS effect, controlling the thickness of the PDMS layer is of great importance. Our previous studies showed that the thickness of the PDMS layer could be controlled by varying the spin speed, spin duration, and the concentration of the PDMS precursor. Unfortunately, the precise control of PDMS coating thickness in the sub-5 μm region remains challenging.⁴⁰ To overcome this problem, we used n-hexane to dilute PDMS precursors for effective controlling the thickness of PDMS layer down to sub-100 nm. In our study, the thickness of PDMS layer was tuned by varying the concentration of PDMS precursor.

In order to investigate the effect of PDMS coating thickness on the SERS activity of plasmonic nanoparticle film, we prepared a series of PDMS-coated C₃H₇S–Au@Ag MLF by varying the concentration of the PDMS precursor. Fig. 3a shows a collective of SERS spectra of toluene vapor on the PDMS-coated C₃H₇S–Au@Ag MLF as a function of the concentration of PDMS precursor. It can be found that, with increasing the concentration of PDMS precursor, the SERS intensity of toluene first increased and then decreased. By evaluating the SERS intensity of toluene at 1003 cm⁻¹ and using the SERS signal of 1-propanethiol at 1025 cm⁻¹ as a reference, we found that the optimal concentration of PDMS precursor for maximum SERS intensity of vapor was estimated to 5% (Fig. 3b). The corresponding thickness of PDMS layer is 15–25 nm, as evidenced by the SEM images (Fig. 3c and d). On a separate note, when the concentration of PDMS precursor is higher than 10%, the decrease in SERS intensity of 1-propanethiol is caused by the thick PDMS (≥30 nm) layer which could result in a great loss of excitation laser and SERS signal.

Fig. 3 (a) SERS spectra of toluene vapor on the PDMS/C₃H₇S–Au@Ag MLF obtained by using PDMS precursors at different concentrations (100%, 50%, 20%, 10%, 5% and 2%). (b) The SERS intensity change of toluene vapor (1003 cm⁻¹) and 1-propanethiol (1025 cm⁻¹) with increasing PDMS concentrations. (c) SEM imaging shows the sectional view of the as-prepared C₃H₇S–Au@Ag MLF. Scale bars: 500 nm. (d) SEM imaging shows the sectional view of PDMS/C₃H₇S–Au@Ag MLF obtained by using 5% PDMS precursor. Scale bars: 500 nm. (e) SERS spectra of 1-propanethiol from 60 randomly selected spots on the as-prepared PDMS/C₃H₇S–Au@Ag MLF. (f) The corresponding SERS intensities of 1-propanethiol at 1025 cm⁻¹. The average SERS intensity (blue line) was estimated to 8810.4 cps with the relative standard deviation of 5.08%.

The SERS activity uniformity of the as-prepared optimal PDMS/C₃H₇S–Au@Ag MLF was also studied by using SERS signal of 1-propanethiol at 1025 cm⁻¹ as a reference. A collective of SERS spectra recorded from 60 random spots on the PDMS/C₃H₇S–Au@Ag MLF revealed that all the intensities are in the relative standard deviation of 5.08%, suggesting a highly uniform distribution of SERS activity of the as-prepared substrate (Fig. 3e and f).

To shed more light on the synergetic effect of PDMS and 1-propanethiol for vapor capture, we further compared the SERS intensity of toluene vapor using PDMS/C₃H₇S–Au@Ag, C₃H₇S–Au@Ag MLF, PDMS–Au@Ag and Au@Ag MLF as the plasmonic substrate, respectively. As shown in Fig. 4a and b, the PDMS coating resulted in a 6-fold enhancement in SERS intensity of toluene vapors compared with the bare Au@Ag MLF due to the strong adsorption capability of PDMS for vapor molecules. Relative to the unmodified control, the C₃H₇S–Au@Ag MLF showed a 10-fold increase of SERS intensity by hydrophobic interaction. Remarkably, we observed a two orders of magnitude enhancement (∼50 times) in SERS intensity from the PDMS/C₃H₇S–Au@Ag MLF. Such high enhancement is dominantly contributed by the synergetic effect of PDMS and 1-propanethiol layer for vapor capture. Despite high adsorption capability of PDMS layer for vapor molecules, most of the molecules captured by PDMS are located far from the metal surface and have a weak coupling with the surface plasmon. Therefore, the SERS signal only stems from a small number of molecules near the plasmonic hot spots. Alternatively, the hydrophobic interaction offered by alkanethiol chain place the toluene vapor molecules in close proximity to the plasmonic substrate. However, the improvement of SERS signal is often restricted by the deficiency in the vapor molecules due to the weak adsorption behavior of the alkanethiol ligands in the flowing system. As a result, the combination of PDMS coating and alkanethiol ligands can contribute to a dramatic improvement in vapor capture on the plasmonic surface, thus resulting in a significant increase of SERS intensity.

Fig. 4 (a) SERS spectra of toluene on the PDMS/C₃H₇S–Au@Ag (1), C₃H₇S–Au@Ag (2), PDMS–Au@Ag (4), and Au@Ag (5) MLFs. And the blank signal of C₃H₇S–Au@Ag (3) and Au@Ag MLFs (6). (b) The corresponding SERS intensity of toluene at 1003 cm⁻¹ on the four types of plasmonic substrates. (c) Adsorption isotherm curves of toluene vapor in the flowing-system using PDMS/C₃H₇S–Au@Ag (1), C₃H₇S–Au@Ag (2), PDMS–Au@Ag (4), and Au@Ag (5) MLFs, by evaluating SERS intensity at 1003 cm⁻¹.

To further study the capability of PDMS coating layer and 1-propanethiol molecules, we investigated the adsorption kinetics of different plasmonic substrates (PDMS/C₃H₇S–Au@Ag, C₃H₇S–Au@Ag MLF, PDMS–Au@Ag and Au@Ag MLF) for toluene vapor capture in the flowing-system by evaluating the SERS intensity at 1003 cm⁻¹. In our study, flow rate of toluene vapor in the flowing-system was set to 137.914 ppm min⁻¹. As shown in Fig. 5c, all SERS intensity of toluene vapor showed a dramatic increase at the initial stage and then reached a plateau. It could be found that PDMS and 1-propanethiol can both prolong the adsorption saturation time of the plasmonic substrate for toluene vapor. By comparison, the adsorption saturation time of PDMS (60 s) layer is much shorter than that of 1-propanethiol (170 s). We reasoned that the excellent permeability nature of PDMS offers a higher efficiency for vapor adsorption compared with the hydrophobic interaction provided by 1-propanethiol layer. Therefore, the synergetic effect of PDMS layer and hydrophobic 1-propanethiol molecules showed the optimal adsorption efficiency for vapor. Importantly, the hydrophobic interaction further put the vapor molecules and plasmonic hot spots within a close proximity, thus resulting in a significant SERS enhancement. Taken together, these results further demonstrate that the as-prepared PDMS/C₃H₇S–Au@Ag MLF holds great promise in practical SERS detection of vapor in the atmospheric environment.

Fig. 5 (a) SERS signal of benzene, p-xylene, m-xylene, o-xylene vapor on PDMS/C₃H₇S–Au@Ag (red), C₃H₇S–Au@Ag MLF (blue), and Au@Ag (black) MLFs. The blank shows the SERS signal of the 3 kinds of substrates without probe molecules. (b) The SERS intensity of benzene, p-xylene, m-xylene, o-xylene at 992 cm⁻¹, 1179 cm⁻¹, 999 cm⁻¹, and 1048 cm⁻¹ on the PDMS/C₃H₇S–Au@Ag and C₃H₇S–Au@Ag MLFs, respectively. (c) Plot of SERS enhancement factors of different aromatic molecule on the PDMS/C₃H₇S–Au@Ag relative to the C₃H₇S–Au@Ag MLFs.

The SERS detection of benzene and xylene vapor

To test the generality of our developed PDMS/C₃H₇S-assisted SERS amplification for aromatic vapor detection, we investigated the SERS signal of a series of vapor molecules (benzene, p-xylene, m-xylene, o-xylene) in the flowing system using PDMS/C₃H₇S–Au@Ag, C₃H₇S–Au@Ag and Au@Ag MLFs as the plasmonic substrates. As anticipated, we observed the best SERS signals on PDMS/C₃H₇S–Au@Ag MLF from benzene, p-xylene, m-xylene, o-xylene at 992, 1179, 999, and 1048 cm⁻¹, respectively. In stark contrast, a large decrease in the corresponding SERS signals could be detected on the C₃H₇S–Au@Ag MLF while naked Au@Ag MLF had no response to these vapors (Fig. 5a and b). In addition, PDMS shows a stronger adsorption of benzene than toluene and xylene molecules, as revealed by the comparison result of SERS enhancement factor for different aromatic molecules on the plasmonic substrate with and without the PDMS layer (Fig. 5c). We reasoned that the aromatic molecules with less number of substituent groups and better hydrophobicity, which had a weaker interaction with the PDMS, were easy to diffuse through the PDMS layer and tended to reach the interface between PDMS layer and the hydrophobic plasmonic metal nanostructure (Fig. 5c).⁴¹

Conclusions

We have developed a PDMS/C₃H₇S-assisted SERS amplification method for on-line detection of aromatic vapor. Our approach provides a rapid, efficient route to significantly improve the capture and immobilization of vapor molecules on the plasmonic surface. By carefully controlling the thickness of PDMS layer, our fabricated substrate can result in about two orders of magnitude enhancement of SERS signal. Our experimental investigations will allow for designing a new generation of SERS substrates for in situ detection of volatile organic pollutants in the atmospheric environment.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21173155, 21303115 and 21473118), the National Instrumentation Program (2011YQ031240402). The partial financial support is from a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We also thank Dr Sanyang Han for his helpful discussion.

Notes and references

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