Flexible, Transparent, and Free-Standing Silicon Nanowire SERS

Feb 21, 2017 - ... as a surface enhanced Raman scattering (SERS) platform for in situ ... the three-dimension interconnected nanowire network structur...
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Flexible, Transparent, and Free-Standing Silicon Nanowire SERS Platform for in Situ Food Inspection Hao Cui,†,‡,§ Shuoyu Li,†,‡,§ Shaozhi Deng,†,∥,⊥ Huanjun Chen,*,†,∥,⊥ and Chengxin Wang*,†,‡,§ †

State Key Laboratory of Optoelectronic Materials and Technologies, ‡School of Materials Science and Engineering, §The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, ∥Guangdong Province Key Laboratory of Display Material and Technology, and ⊥School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: We demonstrated a flexible transparent and free-standing Si nanowire paper (SiNWP) as a surface enhanced Raman scattering (SERS) platform for in situ chemical sensing on warping surfaces with high sensitivity. The SERS activity has originated from the three-dimension interconnected nanowire network structure and electromagnetic coupling between closely separated nanowires in the SiNWP. In addition, the SERS activity can be highly improved by functionalizing the SiNWP with plasmonic Au nanoparticles. The hybrid substrate not only showed excellent reproducibility and stability of the SERS signal, but also maintained the flexibility and transparency of the pristine SiNWP. To demonstrate its potential application in food inspection, the Au nanoparticlesmodified SiNWP was directly wrapped onto the lemon surface for in situ identification and detection of the pesticide residues. The results showed that the excellent SERS activity and transparency of the hybrid substrate enabled the detection of the pesticides down to 72 ng/cm2, which was much lower than the permitted residue dose in food safety. KEYWORDS: flexible, free-standing, silicon nanowires, surface enhanced Raman scattering, pesticide residue monitoring

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nanowires, nanopillars, and nanoparticles) with sizes smaller than the incidence wavelength can sustain low-loss optical leaky modes or Mie-type internal resonances, giving rise to strong light scattering and electromagnetic near-field enhancement. Such properties make the silicon nanostructures promising candidates for SERS substrates.21−24 In addition, the silicon nanostructures can be further functionalized with metal nanoparticles to form hybrid structures, which exhibit outstanding optical response originating from the synergistic interactions between the dielectric and metal components.25 It was reported that hybrid structure of silicon nanowires functionalized with metal nanoparticles could serve as highly efficient SERS substrates for ultrasensitive chemical and biomolecule detection.26−30 Unfortunately, most of these conventional silicon-based SERS substrates are either fabricated on solid and brittle silicon wafers or rely on dispersive colloidal nanostructures in aqueous solution, which failed to meet the requirements of flexible SERS substrates. Recently, combining the multilayer-etching and roll-to-roll techniques, the Agdecorated multilayer silicon nanowire arrays were successfully transferred to flexible thermal release tape. The obtained flexible hybrid structure showed great potential as an efficient SERS substrate.31 However, the fabrication process was rather time-consuming. Furthermore, the introduction of extra supporting frameworks, i.e., the polymer release tape, strongly

urface enhanced Raman scattering (SERS) has been considered one of the most promising sensing techniques for low-dose sensing applications due to its noninvasive, nonlabeling, fingerprint-type way of sensing, as well as ultrahigh sensitivity.1−4 Due to their intriguing behavior of focusing light at the nanoscale derived from their plasmonic resonances, noble metal nanostructures, such as Au, Ag, and Cu, have been employed as prevailing SERS substrates.5−7 On the other hand, both the scientific and industrial communities show growing interest in flexible sensors capable of monitoring physiological and biomechanical signals from the human body and epidermis, which can pave the way for the design of body sensor networks (BSN) for personalized healthcare.8−10 Besides, transparent free-standing flexible sensors that are convenient to carry and can conform to surfaces with random curvature can also benefit food safety inspection by in situ detection of toxicants from food surfaces.11 In these regards, a few flexible SERS substrates were proposed by decorating noble metal nanoparticles onto flexible frameworks such as cellulose paper,12 polymer membranes,13,14 and electrospun fibers.15,16 These flexible SERS substrates exhibit distinct advantages in comparison with the conventional rigid SERS substrates in terms of conformal contact with the inspecting surfaces, high surface area for adsorption of analytes, and abundant electromagnetic hotspots. Dielectric nanostructures, especially the silicon nanostructures, with high refractive index have been shown to exhibit intriguing optical properties that are distinct from their bulk counterparts.17−20 Specifically, silicon nanostructures (such as © XXXX American Chemical Society

Received: November 7, 2016 Accepted: February 21, 2017 Published: February 21, 2017 A

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ACS Sensors hinders the SERS substrate from in situ sensing and applications in harsh environments. Recently, we have successfully synthesized flexible, transparent, and self-standing SiNWP consisting of ultrathin threedimensional silicon nanowire networks.32 The excellent flexibility and unique architecture of the SiNWP have great potential for various flexible functional devices applications. In our current study, we continue to explore the potential application of the SiNWP as flexible and self-standing SERS substrates that can be universally adapted onto warping surfaces for in situ and nondestructive identification and detection. We first showed that, due to the high nanowire density and electromagnetic coupling between closely separated nanowires, the SiNWP possessed strong optical field enhancements and exhibited good SERS activity. Moreover, after modification with Au nanoparticles onto the nanowire surfaces, the SERS performance of the hybrid SiNWP can be further improved, with a Raman enhancement factor as high as 105. Most strikingly, the Au nanoparticle-modified SiNWP (AuNP− SiNWP) held the flexibility and transparency of the pristine SiNWP, making it an excellent SERS substrate for in situ and nondestructive identification and detection of the molecules on a curved surface. As a simple demonstration, we showed that by directly wrapping the AuNP−SiNWP onto the lemon surface, the lowest concentration of the pesticide residues detected can be down to 72 ng/cm2, which was far less than the permitted dose in food safety. We want to emphasize that although a few studies have revealed the excellent SERS activity of various silicon nanostructures as well as their metallic hybrids, our current study is the first attempt to demonstrate the selfstanding silicon nanostructures as flexible SERS substrates.

Figure 1. Structure characterizations and electric field localizations of the SiNWP. (a) Scanning electron microscope (SEM) image of the SiNWP. Inset: digital photograph showing the transparency of the nanowire paper. (b) TEM image of the Si nanowires extracted from a typical SiNWP sample. Inset: HRTEM image of a typical Si nanowire. (c) Simulated electric field enhancement contour of an individual Si nanowire. (d) Simulated electric field enhancement contour of SiNWP with 8 Si nanowires. (e) Simulated electric field enhancement contour of SiNWP with 16 Si nanowires. (f) Line profiles of the electric field enhancements along the dashed white lines shown in (c−e). The diameters of the nanowires are 10 nm. The nanowire and SiNWP were excited at 633 nm. The light was incident perpendicular to the plane of the nanowire and SiNWP. The excitation polarization was perpendicular to the longitudinal axis of the nanowire shown in (c). The electric field enhancement contours were obtained on the central cross sections of the nanowire and SiNWP.

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RESULTS AND DISCUSSION Structure Characterizations and Electric Field Localizations of the SiNWP. The as-prepared SiNWP was woven by abundant nanowires, forming a three-dimensional highly porous network structure (Figure 1a). Due to such a unique architecture and its ultrathin nature, the SiNWP was shown to exhibit excellent transparency and flexibility (Figure 1a, inset). Each nanowire has a diameter of ∼10 nm (Figure 1b). Highresolution transmission electron microscope (HRTEM) characterizations further indicated that the nanowires were wrapped by a thin amorphous oxide layer (∼1 nm) around its surface (Figure 1b, inset). As previously reported, the novel three-dimensional porous nanowire network structure can help to release the deformation strain, making SiNWP easily stuck to the surface of random curvature without cracking (Figure S1, Supporting Information). The SiNWP is thus expected to be a promising flexible and adaptable substrate on complex topographies. Due to their high refractive index, the silicon nanowires can be strongly polarized under light excitation and thereafter serve as efficient subwavelength dielectric antennas capable of trapping light into their vicinities.21−24 Furthermore, the localized field can be enhanced by taking advantage of the electromagnetic coupling between closely spaced nanowires. To demonstrate the light focusing ability of the SiNWP, the finite element method (FEM) was utilized to calculate the near-field distributions around the silicon nanowires. A plane wave with wavelength of 633 nm and incidence direction perpendicular to the plane of the nanowires was utilized as the excitation. The electric field distributions were evaluated via normalizing the electric field magnitude by that of the incidence light. As shown in Figure 1c,

for the individual silicon nanowire with diameter of 10 nm, the electric field in the vicinity of its surface was enhanced. These regions were termed “hot-spots”. The hot-spots decayed rapidly away from the nanowire surface. The appearance of the hot-spots can be understood from a simple model describing light scattering by nanospheres of size much smaller than the incidence wavelength. According to the Mie scattering theory, the electric field distribution around a nanosphere excited by polarized plane wave can be stated as33 ⎞ ⎛ 2a3 ε − εm E = E0cos θ ⎜ 3 1 + 1⎟er̂ ⎠ ⎝ r ε1 + 2εm

⎯⇀ ⎯

⎞ ⎛ a3 ε − εm ε − εm 1 + E0 sin θ ⎜ 3 1 − 1⎟eθ̂ ∝ 1 ε1 + 2εm r 3 ⎠ ⎝ r ε1 + 2εm

(1)

where E0, a, r, and θ are the incidence electric field magnitude, radius of the nanosphere, distance from the center of the nanosphere, and polar angle measured relative to the incidence direction. ε1 and εm are the dielectric function of the nanosphere and the surrounding medium, respectively. As the silicon exhibit a large ε1 in the visible region, a strong electric field can therefore be induced around the silicon nanowire. Physically such an effect can be understood that more polarized charges will be generated at the surface of the silicon nanowire due to its large ε1, giving rise to an enhanced electric field. The hot-spots surrounding the silicon nanowires can be increased by raising the number of the nanowires (Figure 1d,e). In addition, at the regions where two or more nanowires were intercepted, the electric field can be further enhanced due to B

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profiles across typical regions of the SiNWP (Figure 2c). The brighter contrast suggests stronger optical near-field intensity. Therefore, these regions are associated with the hot-spots in the nanowire paper. The s-SNOM results are generally conformed to the previous numerical analysis. Normally, hot-spots with bright contrast should exist on the silicon nanowires. However, due to their finite absorption at visible region, the optical signals from the silicon nanowires are relatively weak, which give a darker appearance. Existence of the hot-spots in the SiNWP can be further corroborated by the far-field optical characterizations. Due to the localized enhanced electric field, the nanowire nets can be strongly polarized in comparison to their individual counterparts. The oscillating polarized charges in the nanowire can therefore induce strong light scattering into the far-field zone. Figure 2d shows the dark-field scattering spectrum of a specific SiNWP sample, which covers a very broad spectral range from 400 to 900 nm. In addition, the far-field scattering intensity tends to increase into the near-infrared region, suggesting the potential of the SiNWP in near-infrared photonics and optoelectronic applications. The strong light scattering behavior of the nanowire paper can be further manifested from its scattering image, which shows vivid color appearance under white light excitation (Figure 2d, inset). SERS Performance of the Pristine SiNWP and AuNP− SiNWP. The various hot-spots sustained by the SiNWP can make it a potential SERS substrate. We then conducted Raman measurements on the nanowire paper using R6G as probe molecules. Figure 3a gives a series of SERS results, which clearly indicate the SERS activity of the nanowire paper. For R6G molecules adsorbed onto the nanowire paper, their corresponding Raman fingerprints (Table S1, Supporting Information) still existed for molecule concentration down to 10−5 M. Such a SERS performance is competitive with most of the semiconductor-type SERS substrates.35−37 To further promote its SERS performance, the SiNWP was modified with Au nanoparticles to form hybrid substrate. The resulting structure still retained the porous three-dimensional network structure (Figure 3b), with the silicon nanowire densely covered with elongated Au nanoparticles of sizes around 5− 30 nm (Figure 3c). The Au nanoparticles are single crystalline, with interplanar spacing of 0.23 nm corresponding to the (111) plane (Figure 3c, inset). We then evaluated the flexibility and transparency of the AuNP−SiNWP. As shown in Figure 3d, the AuNP−SiNWP attached to the quartz substrate still exhibited a comparable transmittance spectrum to that of the pristine nanowire paper, with only ∼10% reduction of the transmittance intensity. Such an excellent transparency can be further manifested from the corresponding digital photographs (Figure 3d, insets). Additionally, the AuNP−SiNWP still possessed excellent flexibility and mechanical stability, which was able to be repeatedly bent to large angles (∼160°) without structural breakdown (Figure S3, Supporting Information). The SERS performance of Au NP−SiNWP was also evaluated using the R6G as probe molecules. As expected, the hybrid SiNWP showed improved SERS activity in comparison with the pristine one (Figure 3e). The Raman signals were still detectable for molecule concentration down to 10−8 M. To quantitatively characterize the SERS activity, we calculated the Raman enhancement factor (EF) according to a standard procedure

the electromagnetic coupling between them. Such effects can be seen more clearly by comparing the line profiles of the electric field distributions across the individual silicon nanowire and SiNWP with a different number of nanowires (Figure 1f). The abundant hot-spots can greatly benefit the SERS activity of the SiNWP, which will be discussed in later sections. On the other hand, one should note that for silicon nanowires usually thin oxide layer with thickness of 1−2 nm can be formed on their surfaces. In order to reveal the influence of the oxide layer on the electric field enhancement, we compare the hot-spot distribution of an individual silicon nanowire with that wrapped by an oxide layer of 2 nm (Figure S2, Supporting Information). The result indicates that incorporation of the oxide layer will not strongly deteriorate the magnitude as well as distribution of the hot-spots around the silicon nanowire. This is reasonable because the dielectric constant of the SiO2 layer is relatively small, and will not severely screen the polarized charges on the silicon nanowire surface. Near-Field Optical Characterizations of the SiNWP. The above calculation results can be experimentally verified using scanning near-field optical microscope (SNOM) technique. To that end, apertureless scattering-type SNOM (s-SNOM) was utilized to measure the hot-spot distributions in the SiNWP.34 Specifically, a linearly polarized laser of 633 nm was employed as the excitation, which illuminated an atomic force microscope (AFM) tip above the SiNWP. The backscattered light was recorded during scanning of the sample under the tip. A typical topography image of the nanowire paper is presented in Figure 2a, verifying the porous network composed of uniform nanowires. As indicated by the near-field optical images of the SiNWP (Figure 2b), regions between adjacent nanowires are manifested by their distinctly brighter contrast in comparison with those on the nanowires. Such behavior can be seen more clearly by investigating the line

Figure 2. (a) AFM topography of a typical SiNWP sample. (b) Nearfield optical amplitude image at 633 nm recorded in the same area as that in (a). (c) Line profile of the near-field optical amplitude along the dashed white lines shown in (b). Representative regions corresponding to the hot-spots have been indicated by the orange doubled dashed lines. (d) Dark-field scattering spectrum of the SiNWP. Inset: corresponding dark-field scattering image of the SiNWP. C

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pristine SiNWP, which is far less than the EF of the Au NP− SiNWP. Simulated Electric Field Enhancement Contour of an Individual Hybrid Nanowire. The superior Raman enhancement from the hybrid SiNWP can be ascribed to two aspects. First, in the hybrid substrate, the Au nanoparticles were closely attached to the silicon nanowire surfaces. Due to their small separations, under excitation polarized along the transverse direction of the nanowire, the induced polarized charges in the Au nanoparticle and nanowire are in phase, which can enhance the oscillation strengths of each other. As a result, the electric fields from both the Au nanoparticle and silicon nanowire near their contact regions will be enhanced (Figure 4a).38 Such an

Figure 4. (a) Individual Si nanowire modified with Au nanoparticles. (b) Individual SiO2 nanowire modified with Au nanoparticles. (c) Line profiles of the electric field enhancements along the dashed white lines shown in (a) and (b), respectively. The diameters of the nanowires are 10 nm. The diameters of the Au nanoparticles are 5 nm. The hybrid nanowires are excited at 633 nm, with incidence direction perpendicular to the plane of the nanowires. The excitation polarization was perpendicular to the longitudinal axis of the nanowire. The electric field enhancement contours were obtained on the central cross sections of the nanowires.

enhancement is much stronger than that induced by a freestanding silicon nanowire or Au nanoparticle, which therefore gives rise to the enhanced SERS activity. We have also prepared a reference sample with Au nanoparticles sputtered directly onto the smooth Si wafer. Under the same measurement conditions, no SERS signal was collected with a molecule concentration even up to 10−6 M (Figure S4, Supporting Information). This result further corroborates that the SERS activity is mainly originated from the electromagnetic coupling between the Au nanoparticles and silicon nanowire. One should note that such additional enhancements can only occur for Au nanoparticles adhered to nanowire of high refractive index. If the nanowire was composed of lowrefractive-index dielectrics, such as SiO2, the induced polarized charges in the nanowire will be very small. The coupling strength between the charges in the Au nanoparticle and nanowire will be weak, and therefore the electric field enhancements near the contact regions will be much lower (Figure 4b,c). In order to validate the calculation results, the SERS performance of Au nanoparticle-modified SiO2 nanowire paper (SiONWP) was also evaluated using R6G of 10−6 M (Figure S5, Supporting Information). The SiONWP was prepared by direct thermal oxidation of the SiNWP sample, whereby the porosity and morphology were the same as those of the pristine SiNWP. The results showed that no observable Raman bands were obtained from the SiONWP substrate modified with the Au nanoparticles, which was consistent with the numerical results. The second origin for the additional Raman enhancement from the hybrid structure was associated with the novel three-dimensional net structure of the Au NP− SiNWP. This unique structure can preclude the aggregation of

Figure 3. (a) SERS spectra of the R6G molecules adsorbed onto the pristine SiNWP. The molecule concentrations are 10−3 M (orange), 10−4 M (green), and 10−5 M (blue), respectively. (b) Typical SEM image of the AuNP−SiNWP. (c) Typical TEM image of the Au nanoparticle-modified Si nanowires. Inset: HRTEM image of a typical Au nanoparticle conjugated onto the Si nanowire. (d) Transmittance spectra of the SiNWP and AuNP−SiNWP samples, respectively. Insets: digital photographs of the SiNWP and AuNP−SiNWP. (e) SERS spectra of the AuNP−SiNWP adsorbed with 10−6 M (purple), 10−7 M (green), and 10−8 M (red) R6G molecules, respectively. For all of the Raman measurements, the excitation wavelength was 633 nm. The laser power was 100 μW, with an acquisition time of 10 s.

EF =

ISERSNref Iref NSERS

(2)

where ISERS and Iref were SERS and ordinary Raman intensities of the same Raman bands of the R6G molecules. For the calculation, ISERS was measured when 10−8 M R6G was adsorbed onto the Au NP−SiNWP, while the Iref was obtained for 10−4 M R6G adsorbed onto the silicon wafer, respectively. NSERS and Nref were the number of molecules on the SERS substrate and silicon wafer within the laser spot, respectively. Average integrated peak intensities at 1363 cm−1 from five randomly selected positions on the two substrates were chosen for the calculation. In addition, for simplification the value of Nref/NSERS was estimated from the ratio of the respective molecule concentrations. Consequently an EF value of ∼105 was deduced for the hybrid AuNP−SiNWP substrate. By using the same procedure, we also attained the EF of ∼80 for the D

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ACS Sensors Au nanoparticles onto the nanowires. If the Au was sputtered onto smooth silicon wafer under the same sputtering condition, a dense Au film will be formed on the silicon wafer. This is due to the aggregation of the Au nanoparticles during the sputtering. The Au film can deteriorate the corresponding SERS performance (Figure S4, Supporting Information). On the other hand, the Au NP−SiNWP networks can also provide more hot-spots and adsorption sites for the R6G molecules, whereby the Raman signal can be enhanced accordingly. SERS Activity of the AuNP−SiNWP Adsorbed with Thiram Molecules. On the other hand, pesticide residue detection in crops is very important for human health. Nowadays, detection techniques usually rely on complicated sample pretreatments. Besides, an effective manner is still lacking for fast, low-cost, high-precision, and in situ detection of pesticide residues on fruit surfaces with random curvature.11 Considering its distinct SERS activity and exceptional flexibility, the AuNP−SiNWP is particularly suitable for application in identification and detection of pesticide residues on fruit surfaces. To demonstrate such an application potential, thiram, a broadly used sulfur-containing pesticide molecule, was chosen as the model analyte. The thiram molecule has a disulfide residue with a high affinity to a metal surface,39 which can be easily attached to the Au nanoparticles through chemisorption (Figure S6a, Supporting Information). Before the measurements the AuNP−SiNWP was soaked into thiram solutions with different concentrations overnight. As shown in Figure 5a, the main characteristic Raman bands of the thiram were clearly observed from all of the samples inspected (Table S2, Supporting Information), even for concentrate down to 2.5 × 10−7 M. It should be noted that some of the Raman bands are slightly different from the standard ones of the thiram powder (Figure S6b, Supporting Information). Such differences are likely to be due to molecular structure changes induced by the formation of Au−S bonds and laser irradiation.11,39 Reproducibility of the Raman signal is one of the crucial parameters for characterizing the performance of a specific SERS substrate. We then evaluated the SERS reproducibility of the AuNP−SiNWP from two aspects, i.e., the spatial reproducibility of a specific sample and sample-to-sample reproducibility. To that end we randomly selected 15 positions on the sample and measured the Raman signal from the adsorbed thiram molecules. The results clearly indicated that the Raman spectra collected from different regions were comparable with each other (Figure 5b). The relative standard deviation (RSD) values of the Raman signal, a commonly used parameter for assessing the SERS reproducibility, can thereafter be calculated on the basis of these measurements (the detail calculation was given in the Supporting Information).15 For the entire spectral range, the RSD values are below 0.15 with little fluctuations, suggesting the excellent SERS reproducibility of the substrate. Besides, we have also conducted intensity mapping of the Raman signal from the thiram molecules adsorbed onto the Au NP−SiNWP, which showed excellent spatial reproducibility (Figure S7, Supporting Information). For evaluation of the sample-to-sample reproducibility, the SERS spectra of 10 different samples were collected to evaluate the sample-to-sample reproducibility. The results from different samples were comparable with each other, and the RSD values calculated from these samples were below 0.3 (Figure S8, Supporting Information). These results clearly demonstrate the excellent SERS reproducibility of the Au NP−SiNWP substrate.

Figure 5. (a) SERS spectra of the AuNP−SiNWP adsorbed with thiram molecules of different concentrations. (b) Spatial reproducibility of the SERS spectra. The AuNP−SiNWP was adsorbed with 2.5 × 10−6 M thiram molecules. The RSD curve was obtained by collecting Raman spectra from 15 positions randomly selected on the AuNP−SiNWP. (c) Sample stability evaluated by comparing the SERS spectra of the thiram molecules adsorbed onto the as-prepared AuNP−SiNWP substrate and substrate that had been stored in air for 30 days (green). The thiram molecule concentration used was 2.5 × 10−6 M. (d) SERS signal stability evaluated by comparing the SERS spectra between the as-prepared AuNP−SiNWP adsorbed with thiram molecules (magenta) and that had been stored in air for 30 days (green). The thiram molecule concentration used was 2.5 × 10−6 M. For all of the Raman measurements, the excitation wavelength was 633 nm. The laser power was 100 μW, with an acquisition time of 10 s.

Temporal signal stability of SERS platform is also important for practical application in pesticide detection. In our study we evaluated the temporal stability of the Au NP−SiNWP SERS substrate from two aspects. The first one referred to the stability of sample, i.e., whether the substrate could still be used as the SERS platform with comparable SERS activity after storing for a specific period of time. We compared the SERS spectra of the thiram molecules adsorbed onto as-prepared hybrid SiNWP and sample that was stored in air for 30 days. As shown in Figure 5c, neither a shift in the major Raman bands nor a significant change in the signal-to-noise-ratio (SNR) of the Raman spectrum was observed between these two substrates, indicating that the nanowire paper substrate is stable in air. Another aspect for characterizing the temporal stability was the signal stability, i.e., whether the SERS signal from the nanowire paper can be preserved for a long time. We accessed this by monitoring the SERS signal of the thiram molecules adsorbed onto a typical nanowire paper for 30 days (Figure 5d). One can clearly see that the spectral features of the SERS spectra were maintained, with only a little decrease in the Raman intensity. The above results unambiguously reveal the outstanding stability of the Au NP−SiNWP as a SERS substrate, which will not only facilitate the handling and storing of the substrate in practical detection, but also benefit the long-term tracking of the SERS signal. E

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ACS Sensors Detection of Pesticide Residues of the Au NP−SiNWP. The above results clearly indicate the potential of the hybrid SiNWP in pesticide detection. As a demonstration of its real-life applications as well as universal adaptability on the curve surface, the Au NP−SiNWP was introduced onto a lemon surface for in situ and nondestructive identification and detection of pesticide residues. Two approaches were employed for such characterizations. Specifically, for the first approach a small piece of Au NP−SiNWP was adhered onto the lemon surface sprayed with pesticide residues (Figure 6a). Due to the

Raman signal of the thiram. The detectable concentration was as low as ∼72 ng/cm2, which was much lower than the maximum permitted residue of thiram on fruits (∼2 μg/cm2 for apples).11 Such a good performance is associated with the transparency of Au NP−SiNWP. When the AuNP−SiNWP was directly adhered onto the lemon surface, the incidence laser and Raman scattering signal could easily penetrate through the Au NP−SiNWP substrate due to its excellent transparency. As a result, the SERS signal can be strong enough for low-dose detection. On the other hand, the Au NP−SiNWP also shows excellent SERS reproducibility for the in situ pesticide inspection, with a RSD value below 0.3 (Figure 6c). For the second approach (Figure 6a), the Au NP−SiNWP was torn off from the lemon surface before the ethanol completely evaporated (Figure 6d, inset). Subsequently the wet nanowire paper was transferred onto a flat silicon substrate or glass for Raman characterizations. As shown in Figure 6d, the characteristic Raman bands of thiram molecules can be easily recorded from the nanowire paper, with a similar detectable concentration down to 72 ng/cm2. Moreover, the RSD value of the SERS signal was below 0.15 (Figure 6e), indicating the exceptional reliability of the Au NP−SiNWP as a nondestructive SERS platform for pesticide detection from fruit surfaces.

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CONCLUSIONS In conclusion, we have demonstrated that the flexible and freestanding SiNWP have great potential and advantage in serving as a SERS substrate for the in situ and nondestructive sensing. In the SiNWP consisting of three-dimensional interconnected silicon nanowires, abundant hot-spots can be formed due to the electric field enhancement contributions from the individual nanowires and electromagnetic coupling between the closely separated nanowires. The SERS performance of flexible SiNWP was observed by using the R6G as probe molecules. Moreover, the SiNWP was further modified with Au nanoparticles to further improve its SERS activity, which could be universally adapted to complex surfaces. Specifically, the Au NP−SiNWP can be directly adhered onto the fruit surface for in situ and nondestructive detection of pesticide residues, with the detectable concentration down to 72 ng/cm2. We strongly believe that the results obtained in this study can pave the way for development of novel flexible SERS substrates that can be widely utilized for ultrasensitive life-science and environmental sensing.

Figure 6. (a) Schematic showing the in situ detection of pesticide residues on lemon peels using Au NP−SiNWP via two schemes. (b) SERS spectra of the thiram residues from the lemon peel surface (scheme 1). Inset: digital image of the Au NP−SiNWP adhered to the lemon surface. (c) Reproducibility of the SERS spectra for scheme 1. (d) SERS spectra of the thiram residues on the Au NP−SiNWP torn off from the lemon surface (Scheme 2). Inset: digital image showing tearing off the Au NP−SiNWP from the lemon surface. (e) Reproducibility of the SERS spectra for scheme 2. For all of the Raman measurements, the excitation wavelength was 633 nm. The laser power was 100 μW, with an acquisition time of 10 s. The RSDs were calculated using a molecule concentration of 2.5 × 10−6 M.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.6b00712. Experimental section, the digital images of the SiNWP, hot-spot distribution of an individual silicon nanowire with an oxide layer of 2 nm, demonstration of the flexibility of the AuNP−SiNWP, molecular structure and Raman spectrum of the thiram molecule (PDF)

its excellent flexibility and huge surface area, the hybrid SiNWP can easily conform to the rough lemon surface (inset of Figure 6b). Prior to the Raman measurements, ∼10 μL ethanol was dropped onto the nanowire paper to improve its contact with the lemon surface as well as adsorption of the pesticide molecules. The SERS spectra were collected directly from the adhered nanowire paper right after the evaporation of the ethanol (Figure 6a). As shown in Figure 6b, for thiram molecules adsorbed onto the pristine lemon surface, the Raman spectrum is almost the same as that of the lemon. In contrast, the introduction of the Au NP−SiNWP can greatly enhance the

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. F

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Huanjun Chen: 0000-0003-4699-009X Chengxin Wang: 0000-0001-8355-6431 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51472276, U1401241, and 51290271). Guangdong Natural Science Funds for Distinguished Young Scholars (Grant No. 2014A030306017). Pearl River S&T Nova Program of Guangzhou (Grant Nos. 201610010084 and 201610010085). Guangdong special support program (Grant No. 2014TQ01C483).

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DOI: 10.1021/acssensors.6b00712 ACS Sens. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssensors.6b00712 ACS Sens. XXXX, XXX, XXX−XXX