Flexible and robust SERS active substrates for conformal rapid detection of pesticide residues from fruits

Sensors and Actuators B 241 (2017) 577–583

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Research paper

Flexible and robust SERS active substrates for conformal rapid detection of pesticide residues from fruits Samir Kumar, Pratibha Goel 1 , Jitendra P. Singh ∗ Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi- 110016, India

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Article history: Received 3 July 2016 Received in revised form 27 September 2016 Accepted 21 October 2016 Available online 27 October 2016 Keywords: Silver nanorods Glancing angle deposition SERS Pesticide detection

a b s t r a c t Flexible surface enhanced Raman scattering (SERS) substrates have advantage over the conventional rigid substrates as they can conform to the underlying object for the efficient extraction of target molecules from complex surfaces. Here, we demonstrate a simple and facile method for fabricating large area SERS-active, flexible and robust substrate for conformal and rapid extraction and detection of trace molecules. The novel SERS substrate was fabricated by embedding Ag nanorods into the polydimethylsiloxane (PDMS) polymer. The AgNRs embedded SERS substrates exhibited a high sensitivity and excellent reproducibility for analytes employed, demonstrating a direct application in trace detection for on-field applications. The in situ SERS measurements on these flexible substrates under mechanical tensile strain conditions were performed. Our results show that flexible SERS substrates can withstand a tensile strain (ε) value as high as 30% without losing SERS performance. The functionality of AgNRs embedded PDMS SERS substrate was demonstrated by directly extracting trace amount (∼10−9 g/cm2 ) of thiram pesticide directly from fruit peels via simple “paste and peel off” method. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Surface enhanced Raman spectroscopy (SERS) has emerged as a promising technique for chemical and biosensing applications as it combines molecular fingerprint specificity with potential single-molecule sensitivity [1–5]. SERS shows an enormous (∼1010 ) enhancement of the Raman signal intensity for molecules of interests in close proximity to a plasmonic nanostructure [6–9]. SERS substrates have been fabricated ranging from rough metal surfaces to fractals, nanowires and well-ordered substrates [10–17]. Although SERS has emerged as a potential technique for chemical and biological sensing it has few limitations like all the other techniques. Silicon wafers and glass slides are two of the most common substrates used for the growth of SERS active layers. However, these substrates are rigid and brittle and hence, these static substrates severely limit the application of plasmonic nanostructures, such as packaging or tracking where a flexible SERS substrate would be more appropriate. There is another factor limiting the full potential of these SERS substrates i.e. adhesion of thin metal films to dielectric substrates. Many studies have been carried out

∗ Corresponding author. E-mail address: [email protected] (J.P. Singh). 1 Present address: Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing-100871, China. http://dx.doi.org/10.1016/j.snb.2016.10.106 0925-4005/© 2016 Elsevier B.V. All rights reserved.

to clarify the mechanism of adhesion between these two dissimilar materials. For example, Chou et al. have carried out insitu X-ray photoelectron spectroscopy (XPS) studies and found that reactive metals such as Cr undergo a chemical reaction with the cured polyimide substrate while non-reactive metals such as Au or Cu do not [18,19]. The metal nanostructures deposited by using conventional techniques such as physical vapor deposition suffers from a low adhesion strength of Ag or Au on to silicon or glass substrates and hence, these techniques do not ensure to get robust SERS substrates. It is difficult to prepare robust and reproducible metal-coated substrates of the correct surface morphology to provide maximum SERS enhancements. Flexible substrates have advantage over the conventional rigid substrates because of their ability to conform to the underlying object. They can be wrapped onto curved surfaces and can be easily cut into different shapes and sizes for applications which demand non-planar, flexible or conformal surfaces. There have been few attempts to achieve polydimethylsiloxane (PDMS) based flexible and large area SERS substrates, but no study have been done to develop both robust as well as flexible SERS substrate [15–17]. Zhan et al. have reported a transfer printing method to fabricate flexible nanostructured films with coating of Ag/Au nanoparticles on PDMS [20]. They have applied PDMS onto self-assembled polystyrene nanoparticles template to produce ordered nanostructures on PDMS films. These substrates were examined by the researchers

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Fig. 1. A schematic illustration of the fabrication of AgNRs embedded PDMS SERS substrates.

for flexibility and reproducibility without any inside information on the robustness and adhesion of the metal layer onto the PDMS films. In an another work, Lee and researchers have reported SERS substrates based on flexible filter paper [21,22]. These SERS substrates have advantages of being biodegradable and having the ability to easily absorb the fluids. But, the reproducibility of the SERS signal over these paper substrates is still a matter of research [22]. SERS substrates containing a (3-mercaptopropyl)triethoxysilane (MPTES) adhesive layer between the SiO2 and gold in contrast to traditional titanium adhesive layers has also been reported [23]. These SERS substrates were prepared by etching with e-beam lithography (EBL). The MPTES adhesion layer has shown to provide an improved adhesion strength between gold and SiO2 surface, but EBL systems are generally expensive, time consuming and highly complex machines require substantial maintenance. To our amazement, there are no reports on the performance of a flexible and robust SERS substrates under mechanical strain conditions, which is critically needed for realization of practical applications. Of late, we have shown that glancing angle deposition (GLAD) can be utilized to prepare aligned silver nanorods (AgNRs) arrays having a large enhancement factor onto the PDMS substrate. GLAD is based on a conventional physical vapor deposition principle and can be used to fabricate aligned and tilted AgNRs arrays on large substrate areas. Here, we report a facile method to fabricate highly adhesive AgNRs arrays embedded in the PDMS film as robust, flexible and extremely sensitive SERS active substrates. The AgNRs arrays are embedded in low index PDMS to achieve enhanced portability and mechanical stability. Interestingly, the embedded AgNRs layers show good adhesion onto the PDMS surface as shown by the scotch tape peeling test. These SERS substrates exhibit comparatively similar SERS enhancement to AgNRs arrays on the Si/glass substrate. The in situ SERS measurements over these flexible substrates under mechanical strain conditions show that the SERS signal intensity remains almost constant for an induced tensile strain value as high as 30%. Finally, the AgNRs embedded PDMS SERS substrates were used for the label-free detection of pesticides thiram and chlorpyrifos. Through strongly enhanced Raman signals on the AgNRs embedded SERS substrate, thiram was effectively detected on apple peels at concentrations as low as 2.4 × 10−9 g/cm2 . These robust and flexible SERS substrates clearly demonstrate its potential as an onsite, rapid, sensitive method for in routine SERS detection appli-

Fig. 2. (a)The schematic of the setup. The figure shows AgNRs embedded PDMS mounted on the optical rails. (b) A photograph of a flexible AgNRs embedded PDMS SERS substrate.

cations that include practical pathogen identification, packaging, chemical sensing and trace detection. 2. Material and methods 2.1. Fabrication of AgNRs embedded flexible SERS substrates A schematic showing the major steps followed for the fabrication of AgNRs embedded PDMS are shown in Fig. 1. The AgNRs arrays were grown over Si(100) substrates by thermal evaporation of silver powder (99.9%) using GLAD technique [24–28]. Before the deposition Si substrates were ultrasonically cleaned successively using acetone, isopropyl alcohol and, de-ionized (DI) water for 15 min each. For the growth of AgNRs films, the substrates were mounted on a sample holder such that the angle between the substrate normal and the incident vapor flux was 85◦ [29–31]. During the initial growth of the films, the impinging atoms form isolated nucleation centers which cast shadows for the arriving vapor flux. The nucleated islands act as shadowing centers and hence, the larger nucleation centers will receive more impinging atoms as compared to the smaller ones and only the larger islands will grow. The competition between limited adatom surface mobility and shadowing effect results in the evolution of the columnar structure with the growth of AgNRs in the direction of the incident vapor flux. The chamber pressure during the deposition was better than 2 × 10−6 Torr. The PDMS was prepared using Sylgard 184 silicone elastomer (Dow Corning Inc.) by mixing base with a curing agent in a 10:1 ratio at room temperature. The solution was kept in a desiccator attached with rotary pump to remove the trapped air bubbles. After the air bubbles were removed, the mixture was poured onto

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Fig. 3. The SEM micrographs of (a) as prepared AgNRs arrays on Si wafer and (b) AgNRs embedded in PDMS film.

2.4. Tensile and reproducibility tests

Fig. 4. SERS spectra of 10−4 M Rh6G solution on AgNRs on glass substrate, AgNRs embedded PDMS and conventional Ag thin film.

the AgNRs deposited Si substrate and cured for about 30 min at 80 ◦ C temperature. The PDMS film was then peeled off from the Si wafer and used as free standing and flexible AgNRs embedded PDMS film. 2.2. SERS characterization The SERS spectra from flexible substrates were acquired using a micro-Raman system (Horiba LabRAM HR Evolution). A 514 nm argon ion laser was used for excitation at 7 mW power at the sample. The spectral collection time was 2 s. SERS spectra over the range of 500 cm−1 –1800 cm−1 were collected from 5 randomly selected spots across the substrates. All measurements were done at room temperature and 520 cm−1 peak of silicon was used for the frequency calibration. The molecular probe used in this study was Rhodamine 6G (Rh6G, 99.9 + %, Sigma Aldrich). A 10−4 M Rh6G solution was prepared by sequential dilution in DI water. The AgNRs embedded PDMS sample was dipped in the Rh6G solution for 15 min and dried using gentle blow of nitrogen before the acquisition of data. Pesticide thiram (99.99%) and chlorpyrifos (99.3%) were obtained from Sigma Aldrich and were used as received. 2.3. Surface morphology The morphology and structural analysis of as-prepared samples were characterized by scanning electron microscope (SEM) (Zeiss, EVO 50). The surface morphology was obtained across 2 mm × 2 ␮m regions using Bruker’s Dimension ICON atomic force microscope (AFM) in tapping mode.

The SERS performance of the flexible AgNRs embedded PDMS substrates was analyzed in situ as a function of mechanical strain. For the tensile test, a custom-designed tensile tester was used which consists of two posts (stationary and movable) mounted on the optical rails. During tensile experiment, the sample was held in between the two posts and tensile tests were performed by moving the mount post with respect to the stationary post and measuring the increment in the film length. SERS spectra of AgNRs embedded PDMS substrates was initially measured in its relaxed position. Tensile experiments were performed by moving the posts further away from each other while monitoring the SERS spectra. The schematic of the setup is shown in Fig. 2. The figure shows AgNRs embedded PDMS mounted on the optical rails. Fig. 2(b) shows a photograph of the AgNRs embedded PDMS SERS substrate. To examine the reproducibility of the AgNRs embedded substrate spectra at 11 different spots was taken with the same conditions.

3. Results and discussion 3.1. Morphology characterization and scotch-tape adhesion test of AgNRs embedded PDMS substrates The SEM micrographs of as prepared AgNRs arrays on Si wafer and AgNRs embedded in PDMS film are shown in Fig. 3(a) and (b), respectively. The SEM micrographs of as prepared AgNRs arrays on Si wafer and AgNRs embedded in PDMS film are shown in Fig. 3(a) and (b), respectively. The SEM images clearly show the permeation of PDMS into the voids present in the porous columnar silver film. Since, one end of the AgNRs gets buried inside PDMS, this makes the adhesion between the silver and the polymeric PDMS film outstanding.From AFM the rms surface roughness of the AgNRs and AgNRs embedded in PDMS was found to be around 70 nm and 15 nm with maximum peak to valley height of around 280 nm and 90 nm respectively, (Fig. S1, ESI). The surface area of the AgNRs decreased to around 1/5 of its initial value after embedding into the PDMS. The adhesion between the embedded AgNRs and the PDMS substrate was tested by using a scotch-tape. The scotch-tape peel test is a popular adhesion test performed for the characterization of the thin film adhesion on a substrate [32]. Fig. 4 shows the adhesion test performed on the embedded AgNRs PDMS substrate (see ESI). A new scotch-tape with a size of 5 cm × 1 cm was applied on the AgNRs embedded PDMS surface with a size of 2 cm × 2 cm. Mechanical pressure is applied by hands to enhance the adhesion of the tape to the metal surface. Then the tape was quickly (<1 s) peeled off. No Ag layer or residue was seen on the tape surface, indicating an excellent adhesion between the AgNRs arrays and PDMS film, Fig. 4(f). The adhesive strength in coatings is defined in terms of the resistance of the coating against mechanical sep-

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Fig. 5. SERS spectra of Rh6G (a) with different concentrations on AgNRs embedded substrate, the concentration of Rh6G (10−12 M–10−4 M). (b) The calibration plot of SERS intensity of peak 613 cm−1 versus the logarithm of Rh6G concentration. The error bars represent the standard deviations of five identical measurements.

aration from the substrate. The mechanical interlocking theory of adhesion states that good adhesion occurs only when an adhesive penetrates into the pores, holes, crevices, and other irregularities of the adhered surface of a substrate, and locks mechanically to the substrate [33–35]. The reason for the increase in mechanical adhesion is that the liquid PDMS coating hardens within the hollows or pores of the AgNRs of the substrate, where it is mechanically anchored with nanorods [36]. Mechanical interlocking can produce strong adhesive bonds that are resistant to hydrolytic and thermal degradation. Therefore, even the adhesive tapes were found not to remove any silver from this composite AgNRs embedded PDMS surface. The peel tests were repeated 3 times on the same SERS substrate and no apparent impairment of the structures was

observed. The AgNRs arrays films were intact to the surface even after ultra-sonicating these SERS substrates continuously for 1 h. These results suggest that the SERS substrate fabricated by the proposed method would be suitable for realistic applications such as biomolecule sensing and environmental monitoring. 3.2. SERS measurements on flexible AgNRs embedded PDMS substrates The SERS spectra of Rh6G on AgNRs embedded PDMS substrates is shown in Fig. 5. Raman bands at 613, 775, 1190 cm−1 are associated with C C C ring in-plane, out-of-plane bending and C C stretching vibrations, respectively. The other Raman bands at 1363,

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1510 and 1651 cm−1 are usually assigned to aromatic C C stretching vibrations of Rh6G molecule [37]. Fig. 4 clearly shows a large enhancement in the SERS signal of Rh6G on AgNRs embedded PDMS substrate. The baseline-corrected peak height of the Rh6G peak located at about 613 cm−1 was used to quantify the overall SERS response, and is denoted as I613 . The SERS response of AgNRs arrays embedded PDMS substrate gets declined by about 33% as compared to that of AgNRs arrays on silicon substrates but still the SERS enhancement by AgNRs embedded PDMS substrates is high enough to use it as an efficient flexible SERS sensor. To further evaluate SERS performances of our SERS substrate, the enhancement factor (EF) was calculated as follows [38] EF =

I

I

SERS

⁄cSERS



Raman cRaman

⁄



(1)

where cRaman and IRaman are the concentration and peak intensity for the regular Raman measurements with 10−3 M Rh6G solution on conventional silver thin film respectively, whereas cSERS and ISERS are the concentration (1 × 10−12 M) and peak intensity for the SERS measurement, respectively. The EF of AgNRs embedded PDMS substrate was calculated as 108 . 3.3. Sensitivity and reproducibility of SERS substrates Sensitivity and uniformity are major concerns for any SERS substrates. Hence, SERS measurements of Rh6G with different concentrations ranging from 10−4 M to 10−12 M were performed to test the sensitivity of the AgNRs embedded PDMS substrates as shown in Fig. 5(a). The SERS peak at 613 cm−1 was used as a measure for quantitative evaluation of SERS sensitivity. The 613 cm−1 can be clearly identified for the concentration value of 10−12 M. Fig. 5(b) shows the linear correlation between the log of peak intensity at 613 cm−1 and the log Rh6G concentrations for concentration lower than 10−8 indicating a direct interaction of the Rh6G with the AgNRs surface and a negligible intermolecular interaction between molecules of the Rh6G. To assess the reproducibility of Rh6G SERS signals, the intensity of the 613 cm−1 peak for 10−5 M Rh6G solution from 11 spots on AgNRs embedded substrate were taken (Fig. S2, ESI). The variation of intensity from sample to sample was found to be within 20%. 3.4. Effect of cyclic tensile strains on the SERS performance of AgNRs embedded PDMS substrates The robustness of the SERS substrates against external forces is an important factor for its application. The flexible substrate must retain superior SERS performance during the mechanical deformation. The insitu SERS measurements were performed on AgNRs embedded flexible PDMS substrates under tensile strains. The SERS response of AgNRs embedded flexible PDMS substrates was measured after each cycle of a pre-specified tensile strain (ε) value of 10% up to the maximum tensile value of 30%. The ε is defined as the ratio of increase in the film length (L) to its original unstrained film length (L). The SERS response of AgNRs arrays on flexible PDMS substrate as a function of cyclic tensile strain is shown in Fig. 6. Here, SERS response is plotted as the intensity of 613 cm−1 (I613 ) Raman peak of Rh6G as a function of ε. Each cycle consists of producing 10% tensile strain in the film and then relaxing back it to the normal relaxed state. Surprisingly, an increase in the SERS intensity was observed with the increasing the strain value. For 10% strain, I613 was found to increase by about 12% from upstretched value. As the strain was increased further, the SERS intensity I613 increases by 25% for 20% strain value and above 20% strain I613 decreases again. Thus, completing the first cycle and returning to the unstretched configuration, the substrate retains 90% of its initial SERS inten-

Fig. 6. The SERS response of AgNRs arrays on flexible PDMS substrate as a function of tensile strain (␧). The error bars represent the standard deviations of five identical measurements.

sity. It is important to notice that after a complete cycle, the Raman intensity remains almost constant with a maximum decrease of about 10%. This cyclic test was repeated for another few cycles. A maximum decrease of 20% in Raman intensity was observed after 2 consecutive cycles. These results demonstrate that AgNRs embedded flexible PDMS SERS substrates are flexible and mechanically robust. 3.5. Detection of pesticide residues Due to high enhancement factor SERS is particularly suitable to probe the residual molecules at various surfaces and with an added advantage of flexibility our SERS substrate can be used for rapid extraction of trace molecules from the various surfaces. We propose a proof-of-concept surface SERS active substrate for direct and rapid extraction and detection of target molecules from complex surfaces. Pesticide residues is a major problem and caused serious pollution and becoming a serious threat to human health [39]. We perform a series of SERS experiments to detect thiram using this highly flexible and reproducible AgNRs embedded substrates and estimated the limit of detection (LOD) of thiram. Thiram is often used to prevent fungal diseases in seed and crops and as an animal repellent to protect fruit trees from damage by rabbits, rodents, and deer [40,41]. A 10 ␮L of thiram pesticide solution with different concentrations (10−2 M − 10−7 M) was evenly dropped onto the apple surface and dried at room temperature. Prior to the detection of pesticides, 10 ␮L of ethanol was first dropped onto the surface of spiked peels, and then evaporated naturally. The traces of the thiram were collected from apple surface by AgNRs embedded PDMS substrates via simple “paste and peel off” method. The SERS spectra obtained from the fruit peel is shown in Fig. 7. The most intense band of the thiram SERS can be seen at 1386 cm−1 , and corresponds to ␦s (CH3 ) vibrations. Other bands which are attributed to the methyl group appear at 1448 cm−1 with contribution from ␦as (CH3 ) and at 1148 cm−1 corresponding to both ␳(CH3 ) and (N CH3 ) vibrations, band appearing at 1514 cm−1 is assigned to pure (C N) vibration. The band at 561 cm−1 can be assigned to s (S-S) [41,42]. The Raman band at 1386 cm−1 can be clearly identified even when the concentration was down to 10−6 M which is one order better than that of conventional techniques like high performance liquid chromatography (HPLC) with the advantage of direct analysis of the sample. The LOD of thiram on the AgNRs embedded PDMS SERS substrate converted to mass-to-area ratio was 2.4 × 10−9 g/cm2 , which shows that the AgNRs embedded PDMS SERS substrates can be suc-

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[2]

[3] [4]

[5]

[6]

[7] [8] [9] Fig. 7. Raman spectra of thiram pesticide. The exposure time was 10 s, laser wavelength was 785 nm, and laser power was 0.7 mW on the sample.

cessfully used for rapid trace detection of thiram with LOD much lower than the permissible limit for apple peels (∼2 × 10−6 g/cm2 ) [43]. Another pesticide chlorpyrifos was also tested for the trace detection. Fig. S3 shows the SERS spectra of different concentrations of chlorpyrifos pesticide on AgNRs embedded PDMS substrates (ESI). Therefore, these flexible and adhesive SERS active substrates can be used for the extraction and rapid detection of trace molecules from complex surfaces in real samples. 4. Conclusions In conclusion, we demonstrated a simple and facile method to fabricate a highly sensitive, flexible and robust SERS active substrate. Ag nanorods arrays can be embedded within flexible PDMS substrates to produce a comparable SERS response to those deposited onto the PDMS. The AgNRs arrays on these flexible substrates retain their SERS activity after repeated cyclic tensile tests. The robustness of the AgNRs arrays based PDMS SERS substrates reveals possibilities for sensing with non-planar surfaces in terms of flexible and conformal labels capable of monitoring biological and chemical agents. As an example, thiram pesticide with concentrations value 1000-fold lower than the level currently permissible in farming has been detected on apple peels. These flexible substrates offer a number of significant benefits over other conventional SERS substrates in that they are low-cost, flexible, conformal, achieve enhanced portability and mechanically robust. Conflict of interest The authors declare no competing financial interest.

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Acknowledgment We kindly acknowledge the Nanoscale Research facility (NRF), IIT Delhi for providing characterization facilities.

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Appendix A. Supplementary data [26]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.10.106.

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Biographies Samir Kumar received his master’s degree in Physics at the University of Kalyani, India in 2009 and his master’s in technology from National Institute of Technology Hamirpur, India in 2012. He is currently a Ph.D. student at the Indian Institute of Technology Delhi India since 2012. His research interest focuses on the chemical and biosensing by SERS. Pratibha Goel received her Ph.D. in Physics from the Indian Institute of Technology Delhi, India in 2016. Since April 2016 she has been a post-doctoral fellow at the Peking University, China. Her research includes study of optical, electrical and wetting behavior of silver nanorods on patterned and flexible substrates. J.P. Singh received his PhD in the field of scanning probe microscopic studies of ion beam irradiated semiconducting surface in 2002 from Inter University Accelerator Centre, New Delhi (Jawaharlal Nehru University, New Delhi). He was at Rensselaer Polytechnic Institute from 2001 to 2005, NY working on the field of applications of glancing angle deposited sculptured thin films. In 2005, he joined the Indian Institute of Technology Delhi where he is currently an associate Professor of physics. In addition, he has been a visiting Scientist at, Max Planck Institute for Intelligent Systems, Stuttgart, Germany (2014), Department of Infectious Diseases, University of Georgia, Athens, USA (2011- 2012), Rensselaer Polytechnic Institute, Troy, NY, USA (Summer 2006, 2007, 2008, 2009), Department of Electrical Engineering, South Dakota State University, SD, USA (Summer 2010). His current research includes photomechanical properties of carbon nanotubes, glancing angle deposition, optical and mechanical properties of metal oxides, nanomechanical properties of nanorods and nanospring structures, and nanofluidics, fabrication and study of plasmonic nanostructures for chemical and biosensing using SERS.