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Dec 28, 2017 - Pesticide Detection by Hydrophobicity Change of Paper Surface. Minwoo Lee,. †,#. Kyudeok Oh,. ‡,§,#. Han-Kyu Choi,. ⊥. Sung Gun ...
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Sub-nanomolar Sensitivity of Filter Paper–Based SERS Sensor for Pesticide Detection by Hydrophobicity Change of Paper Surface Minwoo Lee, Kyudeok Oh, Han-Kyu Choi, Sung Gun Lee, Hye Jung Youn, Hak Lae Lee, and Dae Hong Jeong ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00782 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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Sub-nanomolar Sensitivity of Filter Paper–Based SERS Sensor for Pesticide Detection by Hydrophobicity Change of Paper Surface Minwoo Leea+, Kyudeok Ohb,c+, Han-Kyu Choid, Sung Gun Leea, Hye Jung Younb,c, Hak Lae Leeb,c*, and Dae Hong Jeonga* a

Department of Chemistry Education, College of Education, Seoul National University, Seoul 08826, Korea

b

Department of Forest Sciences, College of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea c

Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea

d

Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis 55455, USA

KEYWORDS. Surface-enhanced Raman scattering, filter paper-based SERS sensor, trace analysis, silver nanoparticles (AgNPs), hydrophobic modification

ABSTRACT: As a cost-effective approach for detecting trace amounts of pesticides, filter paper–based SERS sensors have been the subject of intensive research. One of the hurdles to overcome is the difficulty of retaining nanoparticles on the surface of the paper because of the hydrophilic nature of the cellulose fibers in paper. This reduces the sensitivity and reproducibility of paper-based SERS sensors due to the low density of nanoparticles and short retention time of analytes on the paper surface. In this study, filter paper was treated with alkyl ketene dimer (AKD) to modify its property from hydrophilic to hydrophobic. AKD treatment increased the contact angle of the aqueous silver nanoparticle (AgNP) dispersion, which consequently increased the density of AgNPs. The retention time of the analyte was also increased by preventing its rapid absorption into the filter paper. The SERS signal was strongly enhanced by the increased number of SERS hot spots owing to the increased density of AgNPs on a small contact area of the filter surface. The reproducibility and sensitivity of the SERS signal were optimized by controlling the distribution of AgNPs on the surface of the filter paper by adjusting the concentration of the AgNP solution. Using this SERS sensor with a hydrophobicity-modified filter paper, the spot-to-spot variation of the SERS intensity of 25 spots of 4-aminothiophenol was 6.19%, and the limits of detection of thiram and ferbam as test pesticides were measured to be 0.46 nM and 0.49 nM, respectively. These proof-of-concept results indicate that this paper-based SERS sensor can serve for highly sensitive pesticide detection with low cost and easy fabrication.

When an analyte is adsorbed on novel metal surface, its Raman scattering is dramatically increased. This phenomenon, called as surface-enhanced Raman scattering (SERS), was discovered by Fleischmann in 19741. Since the discovery of SERS phenomena, many researchers have attempted to apply the phenomenon to the molecular detection by exploiting the advantages of SERS, including high sensitivity, unique molecular fingerprint and narrow spectral bandwidth for multiplexed detection. However, despite these advantages, suitable SERS substrates have been hard to find because of low spot-to-spot reproducibility and the large SERS signal variations affected by the conformation of novel metal nanoparticles.

To overcome these disadvantages of SERS, various methods for fabricating suitable nanostructures, such as electrochemical deposition2-3, vapor deposition4-5, electron beam lithography6-7 and colloidal lithography8, have been applied to the creation of sensitive and reproducible SERS substrates on conventional solid supports such as glass9-11, silicon12-13, anodic aluminum oxide (AAO)14-15, polydimethylsiloxane (PDMS)16-17. The substrates fabricated by these methods have shown high reproducibility and sensitivity. However, the overall processes for fabricating these substrates are complex, and conventional solid supports are difficult to functionalize, eco-unfriendly and expensive. For these reasons, the development of cheap, simple and easily producible SERS substrates is required.

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Figure 1. Schematic illustration of fabrication process of filter paper-based SERS sensor.

Recently, researchers have focused on fabricating paper-based SERS substrates as an alternative to conventional substrates. The paper–based SERS substrates have several advantages compared with conventional SERS substrates. At first, the price of paper is hundreds of times cheaper than that of conventional substrates such as poly(ethylene terephthalate) and glass18. Moreover, it is easy to functionalize the surface of paper owing to the hydroxyl group in cellulose while the surface functionalization of conventional substrates requires complicated and toxic processes such as piranha treatment. The paperbased SERS substrates are adequate in point of biodegradability and disposability because the paper substrates are mainly composed by cellulose fibrils, which is environmentally friendly substances. Finally, the paper substrates are easy to control shapes according to purposes, which is caused by low hardness and high flexibility of paper substrates. However, the some of the conventional substrates are hard to control the shapes of substrates due to high hardness and low flexibility of substrates18. Therefore, paper substrate is emerging as a potential replacement for conventional substrates. However, due to the hydrophilic nature of filter paper, it is difficult to retain nanoparticles and analyte solution on the filter paper surface because the solutions are quickly absorbed into the paper and become widely dispersed. For these reasons, the SERS signals of filter paper–based SERS substrates tend to be very weak, with a large standard deviation. To enhance the signal of filter paper–based SERS substrates, many methods have been studied, such as electrostatic adsorption19-22, aggregation or filtration of nanoparticles23-24, chemical growth25-28, light-induced deposition of nanoparticles29-30, wax printing for fabrication SERS active spot on filter paper31-32, printing of nanoparticles in ink form33 and vapor deposition of metals on the filter paper surface30, 34. However, these methods still face the drawbacks of non-uniform distribution of nanoparticles, large

spot-to-spot variation of SERS intensity, low sensitivity, requirement of complicated instruments, complex processes and high cost of substrate fabrication. In this study, we developed a simple, inexpensive and easily fabricable SERS sensor with high sensitivity and reproducibility by hydrophobic modification of filter paper. To prevent the absorption and spreading of silver nanoparticle (AgNP) and analyte solution s into the filter paper, the paper was treated with alkyl ketene dimer (AKD) to convert the hydroxyl groups of the cellulose fibers in the paper into hydrophobic alkyl groups35. The AKD-treated filter paper showed an increased contact angle, resulting in a longer retention time of aqueous solution on the paper. The increased contact angle reduced the contact area of aqueous AgNP solution on the filter paper, thereby concentrating the AgNPs in reduced contact area. The retention time of analyte solution on the contact area was increased, as was that of the aqueous solution on the filter paper surface. As a result, the concentrated AgNP solution within a reduced contact area created SERS hot-spots, with a highly intensified SERS signal. To optimize the SERS intensity and minimize the spot-to-spot variation of SERS signals, the distribution of AgNPs in the contact area was controlled by varying the concentrations of the AgNP solutions. A low spot-to-spot variation in SERS intensity was achieved, with a relative standard deviation (RSD) of about 6%. To demonstrate the feasibility of the developed SERS sensor as an SERS sensor for pesticide detection, it was applied for the detection of thiram and ferbam as representative pesticides and the limits of detections of pesticides was measured 0.46 nM and 0.49 nM, respectively. This proof-of-concept experiment confirmed the sensitive limit of detection and the low spot-to-spot variation of measurement.

Experimental section

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Figure 3. FE-SEM images of AgNP spots on (a) calendered filter paper and (b) AKD-treated filter paper. (c) SERS spectra of AgNP spots on filter paper and AKD-treated filter paper treated by 5 μL of 1 μM 4-ATP solution. Black line: SERS spectra of AgNP spots on filter paper. Red line: SERS spectra of AgNP spots on AKD-treated filter paper. Scale bar: 100 nm. Figure 2. (a) Surface roughness analysis of filter paper and calendered filter paper (Parker Print Surf instrument). (b) Contact angle of calendered filter paper and AKD-treated filter paper. Inset photographs are water droplets on calendered filter paper and AKD-treated filter paper, respectively.

Materials and reagents Silver nitrate (AgNO3, 99.999%), sodium citrate tribasic dihydrate (C6H5O7Na3·2H2O, 99%), 4-aminothiophenol (4-ATP, C6H7NS, 98%) and bis(dimethylthiocarbamoyl)disulfide (thiram, C6H12N2S4, 97%) were purchased from SigmaAldrich (St. Louis, USA), and iron(III) dimethyldithiocarbamate (ferbam, C9H18FeN3S6, 97%) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Filter paper was purchased from Advantec (grade 5C, Dublin, USA), and AKD was purchased from Solenis (Hercon-WI 155, Kimchun, Korea). All chemicals were used without further purification. Silver nanoparticle synthesis AgNPs were synthesized by the citrate-based reduction of silver nitrate36. Silver nitrate solution, which consisted of 70 mg of silver nitrate dissolved in 400 mL of distilled water (DW), was heated until boiling in a 3-neck round-bottom flask with vigorous stirring. After boiling the silver nitrate solution, 8 mL of 1 wt% aqueous sodium citrate solution was rapidly injected into the 3-neck round-bottom flask. After 30 min further boiling, it was cooled to room temperature. To remove excess citrate in AgNP solution, the AgNP solution was centrifuged with the condition 3000 rpm, 15 min. After centrifugation, the AgNPs was dispersed at DW. The synthesized AgNPs showed a plasmonic band at

422 nm and their size is 92 ± 21 nm in average, as shown in Figure S1 and S2. The concentration of the synthesized AgNPs was 0.15 nM, measured by a Nanosight (LM10, Malvern, UK). Fabrication of filter paper–based SERS sensor To fabricate a sensitive and uniform filter paper–based SERS sensor, the filter paper was subjected to calendering and AKD treatment. An overall schematic illustration of the fabrication of the filter paper–based SERS sensor is shown in Figure 1. At first, the filter paper was cut to a size of 80 mm by 50 mm, and the calendering process was applied to filter paper to reduce the roughness of the paper. For the calendering process, the paper was passed between rotating rolls. The conditions of rolling speed, calendering temperature, relative humidity and line pressure were set as 10 m/min, 23 °C, 50% and 130 kgf/cm2, respectively. The roughness of the filter paper was then evaluated using a Parker Print Surf (PPS, Lorentzen & Wettre, Sweden) instrument (ISO 8791-4). After calendering, the filter paper was treated with AKD as a hydrophobic agent to increase its hydrophobicity. The calendered filter paper was soaked in 0.1% AKD dispersion for 2 min. After AKD treatment, the filter paper was rinsed with DW to eliminate the remaining AKD. Excess water on AKD-treated filter paper was eliminated using another filter paper, and the treated filter paper was dried using a drum drier. The temperature of the drum was 120°C. The contact angles of 5 μL water droplets on the non-treated and AKD-treated filter papers were measured by a contact angle meter (DSA100, Krüss, Germany).

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Figure 4. FE-SEM images of AgNP spots on AKD-treated filter paper with different concentrations of AgNP solutions: (a) the reference AgNP solution of 0.15 nM, (b) 2.5 times concentrated, (c) 5 times concentrated, (d) 10 times concentrated, (e) -1 20 times concentrated and (f) 40 times concentrated AgNP solutions. (g) Graph of SERS intensity of 1073 cm band of 4ATP versus concentration ratio of AgNP solutions. Each AgNP spot was treated with 5 μL of 1 μM 4-ATP solution in deionized water. Concentrated ratio of AgNP solution of 1 was the reference AgNP solution of 0.15 nM. Scale bar: 1000 nm.

Fabrication of SERS-active AgNP Spots For fabrication of an SERS-active AgNP spots on the hydrophobic filter paper, 2 μL droplets of AgNP solution were dropped onto the paper and dried at room temperature for around 1 h. After drying of the AgNP solution, 5 μL droplets of each analyte solution were dropped and dried on the AgNP spots for SERS measurement. The Photographs and FE-SEM images of AgNP spots on hydrophobic filter paper were shown in Figure S3 and figure S5, respectively. The average size of AgNP spots are 1.3 mm. Characterization of AgNPs and filter paper A UV/Visible absorption spectrometer (Cary Bio200, Varian, USA) was used to measure the extinction spectrum of the AgNP solution. The morphology of filter paper and distribution of the AgNPs on the filter paper were measured by a field-emission scanning electron microscope (FE-SEM, AURIGA, Carl Zeiss, Germany), transmission electron microscope (TEM, LIBRA 120, Carl Zeiss, Germany) and high-resolution transmission electron microscope (HRTEM, JEM-3010, JEOL Ltd, Japan). The X-ray diffraction (XRD) analysis was measured to identify the loading of AgNP on hydrophobic filter paper using X-ray diffractometer (New D8 Advance, Bruker, USA). SERS measurements The SERS spectra were obtained by a custom-made Raman read-out system for large-area

scanning. A 643-nm laser line (110-81040-019, Ondax, US) was used for the excitation source. The output of laser power was set as 1.5 mW. The laser line was delivered through a 2-axis galvanometric mirror with an area of 300 μm × 300 μm at the center of an AgNPs spot for 10 seconds using a 20× objective lens (NA = 0.40, Olympus, Japan). The scattered light was read by a charge-coupled device (iDus 419, Andor, UK). Theoretical simulation of nanostructures for investigating SERS enhancement To investigate the SERS enhancement as a function of the AgNPs distribution, the electric field (E-field) of the AgNPs nanostructures was calculated using the discrete dipole approximation (DDA, DDSCAT 7.1)37. In these calculations, the diameter of the AgNPs was set as 80 nm, the interparticle distance of the AgNPs was set as 4 nm and the inter-dipole distance between dipoles was set as 4 nm. The dielectric constant was obtained from Palik38 and the surrounding medium of the nanostructures was set as vacuum with a refractive index of 1.00 + 0i. The incident wavelength of the laser was set as 643 nm, the same as for the SERS measurement.

Results and discussions

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Figure 5. (a) The calculated structure of AgNP clusters for calculation of local field enhancement with the DDA method. (b) Maximum E-field intensity versus number of AgNP layers.

Hydrophobic surface modification of filter paper In an effort to overcome the low sensitivity and reproducibility of conventional filter paper–based SERS sensors, the filter paper was treated with hydrophobic AKD to reduce the contact area between water droplet and paper surface39. The aim of the hydrophobic modification was to prevent the quick absorption of aqueous solution into the filter paper, allowing the analyte solution to be retained as an aqueous droplet on the filter paper surface. This would allow more AgNPs to be immobilized on the paper surface in a localized contact area, which would be advantageous for SERS hot-spot generation and allow more analyte molecules to adsorb on the SERS-active surface. The overall scheme for fabrication of the filter paper– based SERS sensor is shown in Figure. 1. Before surface modification of the filter paper, it was subjected to a calendering process to reduce surface roughness of the filter paper. To apply calendering process to filter paper, it was passed between rotating two rolls at high pressure40. To apply calendering process to filter paper, it was passed between rotating two rolls at high pressure. Through the calendering process, the PPS roughness of the filter paper was reduced from 9.4 μm to 4.0 μm, as shown in Figure 2a. After calendering, the filter paper was immersed into AKD solution to allow esterification reaction of AKD with the hydroxyl groups of the cellulose fibers in the filter paper. Via the AKD treatment, the cellulose fibers were functionalized with alkyl groups, which modified the nature of the filter paper from hydrophilic to hydrophobic. To verify the hydrophobic modification of the filter paper, the contact angles of filter paper before and after AKD treatment were measured, as shown in Figure 2b. The contact angle of a water droplet on bare filter paper was 15°, and the droplet was quickly absorbed into the paper within several minutes. However, the contact angle of a water droplet on AKD-treated filter paper was increased to 114°, and the droplet was retained on the surface of the paper for 1 h to dry. As a consequence, the aqueous solutions of AgNP and the analyte could both be retained within a small area of the filter paper surface for a longer time. Furthermore, the XRD spectrum was measured by X-ray diffractometer to identify the loading of AgNP on hydrophobic filter paper as shown in Figure

Figure 6. (a) Reproducibility of AgNP spots on AKDtreated filter paper. SERS spectra were measured from 25 AgNP spots treated with 5 μL of 1-μM 4-ATP solution. (b) -1 SERS intensity distribution of 1073 cm band of 4-ATP of 25 AgNP spots on AKD-treated filter paper.

S4. The spectral bands below the 35° were originated from cellulose fibril in filter paper, and the spectral bands more than 35° were originated from AgNP on filter paper41-42. From XRD analysis, it was confirmed that the AgNP were well loaded on hydrophobic filter paper. To investigate the effect of hydrophobic modification on the SERS activity of the filter paper, the SERS intensity of 4-ATP, as a test analyte, was compared for the AKDtreated filter paper–based SERS sensor and the untreated sensor. To evaluate the SERS intensity, AgNP solution was dropped and dried on the surface of each filter paper to incorporate the AgNPs on the paper surface as an SERS sensor. Then, 5 μL of 1 μM 4-ATP solution was dropped on each SERS sensor and dried before SERS measurement. The AgNPs distribution and SERS spectra of both SERS sensors are shown in Figure 3. For the untreated filter paper, the AgNPs density on the paper surface was low. However, the AKD-treated filter paper showed a high density of AgNPs, forming small AgNP clusters. The highly localized concentration of AgNPs in a small area, enabled by the hydrophobic modification of the filter paper, markedly enhanced the SERS signal of the AKD-treated filter paper. This enhancement resulted from SERS hot spots formed by AgNP clusters on the modified filter paper surface. This confirmed that hydrophobic modification of the paper surface successfully promoted the SERS intensity of the filter paper–based SERS sensor.

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tures. As the size of the AgNP clusters was increased, the Control of AgNPs distribution for SERS optimization Because SERS enhancement can be affected by the distribution and conformation of nanoparticles10, 30, to optimize the SERS enhancement of the hydrophobically modified filter paper–based SERS sensor, the AgNPs distributions and SERS enhancement were compared as a function of the concentration ratios of the AgNP solutions. To evaluate the SERS enhancement, AgNP solutions of various concentrations were dropped on hydrophobically modified filter papers and dried. After drying of the AgNP solutions, 5 μL droplets of 1 μM 4-ATP solution were dropped on the SERS sensors, and the SERS spectra of each SERS sensors were measured for comparison. Figures 4a–4f show the AgNP distributions on hydrophobically modified filter paper versus the concentration ratio of the AgNP solutions by FE-SEM images. The concentration of the unconcentrated AgNP solution was 0.15 nM, and the unconcentrated AgNP solution referred to the reference AgNP solution. In the relatively low AgNP concentration region, the AgNPs existed as monomers and small AgNP clusters with low density on the surface of hydrophobic filter paper. Furthermore, as the concentration of AgNP solution increased, the AgNP density increased and the AgNP cluster grew larger. However, in the relatively high AgNP concentration region, the AgNPs formed vertically stacked three-dimensional and multilayer AgNPs aggregated structure, with sizes larger than several micrometers, on the surface of filter paper. For comparison of the SERS enhancement with different concentrations of AgNP solutions, the SERS spectra of each SERS sensors were measured. The 1073 cm-1 band of 4ATP was used for the SERS enhancement comparison. The SERS spectra of SERS sensors and the SERS intensities of 4-ATP at 1073 cm-1 are shown in Figure S6 and Figure 4g, respectively. The SERS intensities of the 1073 cm-1 band increased gradually as the concentrations of the AgNP solutions were increased and reached a maximum SERS intensity when the concentration ratio of the AgNP solution was 10 times. However, the SERS intensity slightly decreased as the concentration ratio was increased to 20 times and above. To investigate the variation of SERS intensity as a function of AgNP concentrations, the E-field enhancement with different numbers of AgNP layers in the AgNP clusters was theoretically simulated by DDA37. The structures of theoretically calculated AgNP clusters, E-field distributions and maximum E-field intensities of those structures are shown in Figure 5 and Figure S7. To identify the effect of the size of the AgNP clusters on the E-field intensity, a series of AgNP clusters was considered, the calculated nanostructures of which are shown in Figure 5a. The increase in the AgNP cluster size resulted in an increase in the number of AgNP layers in the calculated nanostruc-

Figure 7. (a) SERS spectra of AgNP spots treated with different concentrations of 4-ATP from 1000 nM to 0.1 -1 nM. (b) SERS intensity of 1073 cm band of 4-ATP versus concentration of 4-ATP. Each SERS intensities were averaged from 7 AgNP spots. Error bars represent standard deviations.

maximum E-field intensity also increased initially. However, as the cluster size was increased beyond the optimal value, the maximum E-field intensity then steadily decreased as the number of AgNP layers in the clusters grew beyond two. The decrease in E-field intensity was caused by delocalization of the E-field as the AgNP clusters grew larger. With small AgNP clusters, the E-field was concentrated in smaller regions between AgNPs, which enhanced the maximum E-field intensity. However, as the number of AgNPs in the clusters increased, the E-field of nanostructures became delocalized throughout the larger structures and its maximum value decreased10, 43. This result is consistent with an earlier finding that aggregated and vertically piled structures of nanoparticles showed reduced SERS intensity due to the delocalization of optical field as well as hindrance of irradiation and the scattering of light30. Here, it was shown that the optimal concentration of AgNP solution for fabrication of AgNPs spots was 1.5 nM, that is, a concentration ratio of 10 times of AgNP solution.

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Figure 8. SERS spectra of AgNP spots treated with different concentrations of (a) thiram and (b) ferbam from 1000 nM to 0.1 -1 nM. SERS intensities of 1400 cm band of (c) thiram and (d) ferbam versus concentration of analyte. Each SERS intensities were averaged from 7 AgNP spots. Error bars represent standard deviations.

Reproducibility and sensitivity of SERS sensor High reproducibility and sensitivity are important criteria for a molecule detection sensor. First, to evaluate the reproducibility of the hydrophobically modified filter paper– based SERS sensor, we measured the SERS spectra of 25 AgNP spots treated with 5 μL of 1 μM 4-ATP solution. To measure the reproducibility, the spot-to-spot variation of SERS intensity was quantified by the 1073 cm-1 band of 4ATP. The SERS spectra of 4-ATP on AgNP spots and SERS intensities of the 1073 cm-1 band are shown in Figure 6. The RSD of the SERS intensities of the 1073 cm-1 band of 4-ATP among the 25 AgNP spots was calculated as 6.19%, slightly higher than or similar to those of previous studies20, 25, 44. This RSD value confirmed that the hydrophobically modified filter paper functioned as a highly reproducible SERS sensor. The high reproducibility evidently originated from the hydrophobic modification of the filter paper, which prevented the aqueous AgNP solution from absorption into the filter paper and allowed the AgNPs to be uniformly retained on the surface of the paper. Furthermore, the area scanning method combined with a micro Raman system also contributed to the reproducibility of paper-based SERS sensor because this measurement method integrated the SERS signal over the whole area rather than sampling45. As a result, the RSD of SERS intensity of the hydrophobic filter-paper based SERS sensor was increased up to 6 %. Furthermore, to verify the application of various nanoparticles to the hydrophobically modified filter paper, the

SERS spectra of hydrophobic filter paper-based SERS sensors treated by gold nanoparticle (AuNP) and gold nanorod (AuNR) were measured. Each SERS sensor was treated by 5 μL of 10 μM of 4-ATP solution. With respect of reproducibility of each sensor RSD was similar compared with that of hydrophobic filter paper-based SERS sensor treated by AgNP as shown in figure S8. The RSDs of the sensors treated by AuNP and AuNR were 7.14% and 5.53%, respectively. These results indicated that the hydrophobic filter paper-based SERS sensor could easily tune the plasmonic properties of paper-based SERS sensor. To evaluate the sensitivity of the hydrophobic filter paper–based SERS sensor, we collected the SERS spectra of AgNP spots treated with 5 μL droplets of 4-ATP with concentrations from 0.1 nM to 1000 nM, as shown in Figure 7a. The SERS spectrum of each concentration was obtained from 7 AgNP spots on hydrophobically modified filter paper. The averaged SERS intensities of the 1073 cm-1 band of 4-ATP with different 4-ATP concentrations are shown in Figure 7b. The SERS intensities of 4-ATP decreased as the concentration of 4-ATP decreased until 1 nM. The limit of detection (LOD) of 4-ATP using the hydrophobically modified filter paper–based SERS sensor was estimated by linear fitting of the SERS intensities versus concentrations of 4-ATP, as shown in Figure S8 and Table S1. The calculated LOD of 4-ATP using this sensor was 0.60 nM. Furthermore, to evaluate the stability of the hydrophobic filter paper-based SERS sensor, we collected

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the SERS spectra of AgNP spots treated with 5 uL of 1 uM 4-ATP solution during 15 days as shown in figure S9. The SERS intensity of 1073 cm-1 band of 4-ATP was decreased in a few days and reached a plateau there. After 5 days, the SERS intensity was maintained with the 15% decreasing of SERS intensity compared with freshly prepared SERS sensor. These reproducibility and sensitivity results indicated that the hydrophobically modified filter paper–based SERS sensor, fabricated by a simple method, was suitable for practical use. Application to pesticide detection To confirm its applicability to detection of actual pesticides, the hydrophobically modified SERS sensor was used to analyze thiram and ferbam. For this procedure, we collected the SERS spectra of AgNP spots treated with 5 μL droplets of solutions of each pesticide dissolved in DW with concentrations from 0.1 nM to 10000 nM as shown in Figure 8a, 8b. The averaged SERS intensities at 1400 cm-1 with different concentrations of pesticides are shown in Figure 8c, 8d. These SERS intensities were collected from 10 AgNP spots on hydrophobically modified filter paper. Analysis of the SERS spectra as a function of the concentration of each pesticide in Figure 8a, 8b confirmed that thiram and ferbam could be detected at the nanomolar level. Furthermore, the LODs of thiram and ferbam, estimated by linear fitting of the SERS intensities versus concentrations of each pesticide, were 0.46 nM and 0.49 nM, respectively, as shown in Figure S8 and Table S1. These results confirmed that the SERS sensor based on hydrophobically modified filter paper can be applied to the detection of trace amounts of pesticides at the sub-nanomolar level. Furthermore, the presented paper-based SERS sensor showed better detection limits compared to other SERS sensors as shown in Table S228, 46-51. Conclusion In this study, to increase the sensitivity and reproducibility of a paper-based SERS sensor, the filter paper was subjected to hydrophobic surface modification by treatment with AKD. Using the hydrophobically modified filter paper, the aqueous AgNP and analyte solutions were prevented from absorption into the filter paper and were instead retained on the paper surface during drying. The hydrophobic modification of the filter paper was confirmed by the change in the contact angle. Unlike the conventional filter paper–based SERS sensors, the hydrophobically modified filter paper–based SERS sensor was found to form many SERS hot-spots composed of AgNP clusters on the paper surface, without absorption of AgNPs into the paper. The resulting SERS sensor was shown to be highly reproducible and sensitive, with an RSD of 6.19% and LOD of 4-ATP of 0.60 nM, respectively. Furthermore, the presented SERS sensor proved able to detect pesticides at the sub-nanomolar level. The presented SERS sensor based on hydrophobically modified

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filter paper was confirmed to be capable of highly reproducible and sensitive molecular detection of SERS sensors without using complicated and expensive processes.

ASSOCIATED CONTENT Supporting Information. The characterization of AgNPs and filter paper-based SERS sensors such as UV/Vis absorption spectrum, FE-SEM, TEM and HR-TEM images, photographs, SERS spectra of each analytes and XRD spectrum was included in supporting information. In addition, the result of E-field distribution, calculation of LODs and the comparison of LOD of thiram with other sensors were included in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * D. H. Jeong, E-mail: [email protected]. Tel: +82 2 880 8012 * H. L. Lee, E-mail: [email protected]. Tel: +82 2 880 4786

Author Contributions Minwoo Lee and Kyudeok Oh contributed to main experiments and manuscript. Han-Kyu Choi and Sung Gun Lee helped to analysis SERS data and revision of manuscript. Hye Jung Youn helped fabrication of filter paper and Hak Lae Lee and Dae Hong Jeong proposed and instructed the project. + These authors contributed equally to presented work.

Notes The authors declare no competing financial interests

ACKNOWLEDGMENT This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF-2017R1D1A1B03029880 and NRF2016M2A2A4A03913619), BrainKorea 21 PLUS project through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (21B20151713505) and The Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (HI17C1264).

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