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A facile low-cost paper-based SERS substrate for label-free molecular detection
Sensors & Actuators: B. Chemical 291 (2019) 369–377
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A facile low-cost paper-based SERS substrate for label-free molecular detection
T
Vo Thi Nhat Linha,b, Jungil Moonc,e, ChaeWon Muna, Vasanthan Devarajd, Jin-Woo Ohd, ⁎ ⁎ ⁎ Sung-Gyu Parka, Dong-Ho Kima, Jaebum Chooe, , Yong-Ill Leeb, , Ho Sang Junga, a
Advanced Nano-Surface Department, Korea Institute of Materials Science (KIMS), Changwon, Gyeongnam, 51508, Republic of Korea Department of Chemistry, Changwon National University, Changwon, 51140, Republic of Korea c Department of Bionano Engineering, Hanyang University, Ansan, 15588, Republic of Korea d Research Center for Energy Convergence and Technology Division, Pusan National University, Busan 46241, Republic of Korea e Department of Chemistry, Chung-Ang University, Seoul, 06974, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Paper-based sensor Surface enhanced Raman scattering Label-free detection Au nanoparticle
We introduce a facile and low-cost method for fabricating gold nanostructures on cellulose filter paper (CFP) to prepare a paper-based surface-enhanced Raman scattering (SERS) sensor for label-free molecular detection. Polymerized dopamine (PD) was used as an adhesive layer on the CFP and simultaneously functioned as a reducing agent for gold nanoparticle (AuNP) nucleation. The size of the AuNPs was dependent on the pH of the gold precursor solution, and nanoparticles with an average size of 102 nm were formed on the PD-coated CFP at a pH 3, exhibiting high SERS activity. Finite-difference time-domain (FDTD) simulations of the electromagnetic field enhancement of AuNPs with different sizes and interparticle distances were performed to identify the origin of the SERS effect. The developed paper-based SERS substrate showed uniform and excellent molecular sensitivity with a limit of detection (LOD) of 10−7 M for methylene blue, as measured by a portable Raman spectrometer. Furthermore, as a field application test, surfaces of apples were pretreated with diquat (DQ) and paraquat (PQ) pesticides, which were then detected down to a concentration of 1 ppm after simple attachment of the sensor on the apple peels and performing a SERS measurement. The developed paper-based SERS sensor is expected to be applicable as a label-free sensor for a variety of chemical and biological molecules.
1. Introduction Surface-enhanced Raman scattering (SERS) has been a powerful and attractive spectroscopic technique for molecular detection due to its label-free identification of chemicals from their specific Raman spectra and significant Raman signal enhancement of target molecules adsorbed onto novel plasmonic metal nanostructures [1–3]. Gold and silver nanoparticles have been widely used to create nanogaps between particles, which are known as ‘hotspots’, for electromagnetic field enhancement, but aggregation-induced hotspot generation has limited the production of uniform and reproducible Raman signals [4]. On the other hand, SERS substrates have been developed that generate uniform Raman signals over a large area, but creating a high density of hotspots requires expensive manufacturing processes such as E-beam lithography [5], focused ion beams [6], nanoimprinting [7], and multistacking of plasmonic nanostructures [8]. Recently, paper-based SERS substrates have been developed that exhibit many advantages such as
⁎
low-cost, flexibility, portability, and biodegradability [9–13]. Especially, paper-based sensors absorb solution samples with capillary action that enables rapid target molecule adhesion and enrichment on the sensor surface [14]. To decorate novel metal nanostructures onto paper substrates, various methods have been utilized, including inkjet printing [15], solution dipping [16], physical vapor deposition [17], and successive ion layer adsorption and reaction (SILAR) [9]. However, these methods require complicated nanoparticle synthetic and decoration processes to produce the paper-based SERS substrates, and also special instruments and multiple reaction steps are needed for their fabrication. Nevertheless, the paper-based SERS substrate can be applied for direct target absorbent on 3 dimensional objects such as agricultural and food products by swabbing the surface [18–20], biomarker collection by dipping in the biological fluids [21,22], and development of lateral/vertical flow system for on-site SERS immunoassay [23]. Herein, we propose a facile and low-cost paper-based SERS
Corresponding authors. E-mail addresses: [email protected] (J. Choo), [email protected] (Y.-I. Lee), [email protected] (H.S. Jung).
https://doi.org/10.1016/j.snb.2019.04.077 Received 12 February 2019; Received in revised form 14 April 2019; Accepted 15 April 2019 Available online 16 April 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 291 (2019) 369–377
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Scheme 1. Schematic illustration of the paper-based SERS substrate fabrication method.
Fig. 1. Optical property and surface morphology observations (a) Photographs of bare CFP, PD-coated CFP and SERS substrates fabricated at three different pH values. (b) UV–vis reflectance spectra of bare CFP, PD-coated CFP and paper-based SERS substrates fabricated at different pH values. (c) SEM images of AuNPs decorated on CFP at different pH values (scale bar: 200 nm, left: pH 3, middle: pH 5, right: pH 8) and (d) their corresponding size distributions.
sensitivity of the sensor. Furthermore, as a field application test, the surface of an apple was pretreated with pesticides, which were detected by simple attachment of the sensor substrate over the apple peels and performing a SERS measurement. The developed paper-based SERS substrate is expected to be applied for a variety of cost-effective labelfree chemical sensors.
substrate for label-free molecular detection with high sensitivity and signal uniformity (Scheme 1). Gold nanoparticles (AuNPs) were directly grown on polymerized dopamine (PD)-coated cellulose filter paper (CFP) by a simple solution dipping method. PD functioned as an adhesive interlayer between the CFP and Au ion seeds and reduced the Au ions into AuNPs directly on the CFP surface without the introduction of reducing agents. The paper-based SERS substrate prepared from a HAuCl4 precursor solution at pH 3 exhibited significant SERS activity with an average AuNP size of 102 nm. Finite-difference time-domain (FDTD) simulations demonstrated the SERS effect of the AuNPs decorated on the SERS substrate. Then, the sensitivity and signal uniformity of the developed sensor were characterized by measuring the limit of detection (LOD) and Raman mapping of 200 μm × 200 μm area, respectively. The LOD for methylene blue (MB) was 2 ng/cm2, and that for both bipyridylium pesticides such as diquat (DQ) and paraquat (PQ) was 0.03 μg/cm2 for each, which showed the excellent molecular
2. Experimental section 2.1. Materials Dopamine hydrochloride, tris(hydroxymethyl)aminomethane, gold chloride trihydrate (HAuCl4·3H2O), methylene blue (MB), methyl viologen dichloride monohydrate (PQ), and diquat dibromide monohydrate (DQ) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Cellulose filter paper (CFP) with a pore size of 0.45 μm was purchased 370
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Fig. 2. FDTD simulations of the electromagnetic field enhancement at various interparticle distances with particle sizes of (a) 102 nm, (b) 70 nm, and (c) 36 nm.
2.3. Characterization of the paper-based SERS substrates
from Hyundai Micro Co., LTD (Seoul, South Korea).
The surface morphologies of the paper-based SERS substrates were observed by field emission scanning electron microscopy (FE-SEM; Joel JSM-6700F). Additionally, elemental analysis was conducted using Energy dispersive X-ray spectroscopy (EDS) attached to the FE-SEM. The average particle sizes and their distributions of the AuNPs synthesized in three different pH solutions were obtained by measuring the sizes of 100 different nanoparticles in the SEM image. UV–vis reflectance spectra were measured using a UV–vis-NIR spectrophotometer (Cary 5000, Agilent Technology).
2.2. Preparation of the paper-based SERS substrates Paper-based SERS substrates were fabricated using a simple dipcoating method. CFP was immersed in 10 mL of a dopamine solution at a concentration of 1 mg/mL prepared in Tris-buffer (10 mM, pH 8.5) for 24 h under stirring at room temperature to polymerize dopamine on the CFP surface. Then, the samples were washed with deionized (DI) water and dried at 45 °C. The PD-coated CFP was then immersed in a 1 mg/mL HAuCl4 solution at pH 3, pH 5, or pH 8 for 24 h at room temperature. After AuNP nucleation, the prepared substrates were washed with DI water and dried under ambient conditions. Control substrates such as bare CFP immersed in HAuCl4 solution and PD-coated CFP that was immersed in a pre-synthesized-AuNP solution with an average particle size of 40 nm were prepared.
2.4. Electromagnetic field distribution simulations Optical analysis of the plasmonic nanostructures was performed using three-dimensional finite difference time domain (3D FDTD) simulations (Lumerical solutions). The diameters of the AuNPs were set at 36 nm, 70 nm, and 102 nm with a mesh size of 0.5 nm. The refractive 371
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Fig. 3. SERS activity test of the paper-based SERS substrate using MB. (a) LOD for MB (concentration unit was expressed in initial MB solution concentrations), (b) standard curve at various MB concentrations, (c) Raman mapping of MB at a concentration of 10 μM within an area of 200 μm × 200 μm, and (d) Raman signal uniformity test at 15 different positions.
indices for the paper substrate and air were selected as 1.557 and 1.0, respectively. For dimer AuNPs, the gap between the two AuNPs was set at 5 nm, 10 nm, 20 nm, 40 nm, 60 nm, and 100 nm. The direction of optical excitation (TE polarized, broadband plane-wave source) was normal to the plane of the paper substrate. To absorb the scattered waves, perfectly matched layer (PML) boundary conditions in the XYZ directions were applied around the AuNP structures. A power monitor was placed close to the nanostructures to record the near-field enhancement results. The near-field enhancement was obtained from an integral volume average of |E/E0|4 using the following formula where the maximum local electromagnetic field, incident amplitude of the light source, and volume were denoted E, E0 and V, respectively [24,25].
Near-field enhancement =
2.5. LOD and Raman signal uniformity tests The LOD of a paper-based SERS substrate prepared at pH 3 was tested after dropping 10 μL of MB, DQ and PQ solutions at various concentrations onto the surface. The size of paper-based SERS substrate was prepared in 0.16 cm2 in a square shape. MB solutions at concentrations of 5 μM, 1 μM, 500 nM, 200 nM and 100 nM were prepared. Both DQ and PQ solutions were prepared at concentrations of 50 ppm, 25 ppm, 10 ppm, 5 ppm, 1 ppm and 0.5 ppm. Ten microliters of each MB, DQ and PQ solution was dropped on the SERS substrate and dried before the Raman measurement. Additionally, the Raman signal uniformity was characterized by Raman mapping of a 200 μm × 200 μm area in 20 μm intervals. The Raman intensity of MB at a concentration of 200 ng/cm2 (10 μM of MB solution on 0.16 cm2 SERS substrate) and both DQ and PQ at a concentration of 3.125 μg/cm2 (50 ppm of each DQ and PQ solution on 0.16 cm2 SERS substrate) were compared for paper-based SERS substrates prepared at pH 3, pH 5 and pH 8. The Raman signals were measured using a portable Raman spectrometer Nanoscope system (NS220 for 633 nm, Inc., Daejeon, South Korea) for MB with a laser wavelength, a laser power, an exposure time and a laser spot diameter of 633 nm, 0.5 mW, 0.5 s, and 1.68 μm, respectively. For the pesticides detection, a portable Raman spectrometer Nanoscope System (NS200 for 785 nm Inc., Daejeon, South Korea) was used with a laser wavelength, a laser power, an exposure time and a laser spot diameter of 785 nm, 1 mW, 5 s, and 2.08 μm, respectively. All the spectra were plotted from the average of five different sample points.
∭ |E/E0 |4 dV/V
For the dielectric function model of the nanostructures, the LorentzDrude dispersion model was used as given below. The terms wp, f0, and Γ0 represent the plasma frequency, oscillator strength, and damping constant, respectively. In addition, the Lorentz modification terms of m, wj, fj, and Γj represent the number of oscillations, frequency, and damping constant, respectively [26].
∈ (w ) = 1 − (f0 wp2)/ w (w − iΓ0) +
m
∑j=1 (f j wp2)/((wj2 − w 2) + iwΓj)
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Fig. 4. SERS detection of pesticides. (a) LOD for DQ, (b) standard curve at various concentrations of DQ, (c) Raman mapping of DQ, (d) LOD of PQ, (e) standard curve at various concentrations of PQ, and (f) Raman mapping of PQ (concentration is expressed as the initial DQ and PQ solution concentrations).
2.6. Field application for residual pesticide detection
2.7. Bending test of paper-based SERS substrate
Twenty microliter of DQ and PQ solutions at concentrations of 50 ppm, 25 ppm, 10 ppm, 5 ppm and 1 ppm were dropped on the apple peels and dried for 24 h. The pesticide treated area was calculated as 0.13 cm2 from the 4 mm of 20 μL solution drop diameter. Ten microliters of DI water was dropped on a prepared paper-based SERS substrate with a size of 4 mm × 4 mm, which was then attached on pesticides contaminated apple peels for 10 min. The Raman signals were obtained from the detached pesticide-transferred SERS substrates using a portable Raman spectrometer with a laser wavelength, a laser power and an exposure time of 785 nm, 1 mW, and 5 s, respectively.
CFP filter paper with a diameter of 47 mm was prepared and decorated with AuNPs in Au precursor solution with pH 3. Then, the sample was cut in a rectangular shape with a size of 1 cm × 4 cm. The MB solution with a concentration of 10 μM and a volume of 10 μL was dropped on the center of the prepared SERS paper where the highest bending stress is applied. The bending condition was set as 7 mm bending radius with a speed of 90 rpm using home-made bending test machine. Raman spectrum of MB was measured at every 200 cycles of bending test with the measurement condition for MB mentioned above.
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Fig. 5. Pesticide detection on an apple surface. (a) Schematic depiction of pesticide detection with a paper-based SERS substrate after wiping on a contaminated apple surface, Raman spectra of (b) DQ and (c) PQ from an apple surface pretreated with pesticides at various concentrations.
3. Results and discussion
increased the light absorbance and affected color of the prepared SERS substrates which is well matched with UV–vis reflectance spectra in Fig. 1b. The advantages of the developed SERS substrate in this work is that the large-sized plasmonic AuNPs with the diameter of ˜100 nm were decorated from one time immersion of PD-coated CFP in Au precursor solution while other paper-based SERS substrates were prepared by multiple Au growth steps [9], utilization of reducing agent and galvanic exchange methods [12]. Therefore, in the viewpoint of simple production process, the developed paper-based SERS substrate is considered to be cost-effective and mass-productive.
3.1. Characterization of the paper-based SERS substrates AuNPs decorated on the CFP were prepared by simple immersion of the PD-coated CFP into a HAuCl4 solution. Polymerized dopamine is an adhesive polymer that adheres to a variety of material surfaces such as metal, ceramic, and polymer surfaces [27]. Catechol oxidation in PD is known to reduce the metal ions in solution, resulting in the nucleation and growth of AuNPs on the PD surface [28,29]. As shown in Fig. 1a, a color change was clearly observed for CFP during each fabrication step. PD-coated CFP showed a uniform gray color and then turned brown-red after AuNP growth in the HAuCl4 solution at pH 3, while Au reduction at pH 5 and pH 8 produced a dark-violet color after the reaction. The CFP decorated with AuNPs at pH 3 showed a reflectance SPR peak at 650 nm from the UV–vis reflectance spectra (Fig. 1b), which corresponds to AuNPs with a size around 100 nm. Interestingly, without the addition of a reducing agent, AuNPs with average particle sizes of 102 nm, 70 nm, and 36 nm were grown in HAuCl4 precursor solutions at pH 3, pH 5, and pH 8, respectively. In Fig. 1c and Fig. S1a–S1c, scanning electron microscopy (SEM) images of AuNPs formed under the three different pH conditions show particle size differences over a large area with high uniformity. The size distributions of AuNPs were 102 ± 24.3 nm, 70 ± 26.8 nm, and 36 ± 6.6 nm for pH 3, pH 5 and pH 8, respectively (Fig. 1d). Energy dispersive X-ray spectroscopy (EDS) analysis confirmed that the elemental compositions of the nanoparticles formed on the CFP were reduced Au (Fig. S1d). Bare CFP and PD-coated CFP showed the basal surface morphology of cellulose paper (Fig. S2a and b), and no AuNPs were grown on bare CFP immersed in a HAuCl4 solution (Fig. S2c). Additionally, locally aggregated particles attached to the PD layer were observed on PD-coated CFP immersed in a solution of pre-synthesized AuNPs with an average particle size of 40 nm (Fig. S2d). From these results, we could confirm the successful synthesis of AuNPs on paper-based substrates in the presence of an adhesive polymer layer without the addition of a reducing agents. On the other hand, as shown in Fig. 1a, the color of AuNPs were growing darken as the size of nanoparticle decreasing which showed an opposite phenomenon considering previous report [30]. However, in this work, the density of AuNPs is higher for substrate with small-sized AuNPs because more AuNPs can be nucleated from the same amount of Au precursor solution compared to the large-sized AuNPs that was prepared at pH 3. The number of AuNPs in each SEM image of Fig. 2c was counted as 348, 501, and 715 for 102 nm, 70 nm, and 36 nm, respectively, in the area of 5 μm2. It is reported that the density of AuNPs affect the color of gold containing substrate and shows color darkening as the density of AuNPs increases [31]. Therefore, the higher density of small-sized AuNPs
3.2. FDTD simulation of electromagnetic field enhancement The origin of the SERS effect from the decorated AuNPs was analyzed by FDTD simulation using AuNPs with particle sizes of 102 nm, 70 nm and 36 nm and various interparticle distances ranging from 5 nm to 100 nm. It was meaningful to calculate the electromagnetic field distribution of AuNPs with a size larger than 100 nm because many reports focused on field distribution of plasmonic nanoparticles with size smaller than 100 nm [32–34]. A strong electromagnetic field was localized and confined to the nanogap between the two particles, and the amplitude of the field enhancement exponentially decreased as the interparticle distance increased. As shown in Fig. 2a, for AuNPs with a size of 102 nm, the maximum electromagnetic field enhancement was 7.6 × 107 at a 5 nm interparticle distance and the AuNPs were regarded as individual nanoparticles at interparticle distances greater than 60 nm. Similarly, for the AuNPs with average sizes of 70 nm and 36 nm, the maximum electromagnetic field enhancements were 3.5 × 107 and 3.6 × 105, respectively, and they show a distance-dependent field intensity decrease (Fig. 2b and c). However, electromagnetic field enhancement around a single AuNP that was separated more than 60 nm was observed with minimum electromagnetic field enhancement amplitudes of 5 × 103, 1.2 × 103, and 3.6 × 102 for AuNP sizes of 102 nm, 70 nm and 36 nm, respectively. Furthermore, electromagnetic field distribution calculations of an individual AuNP for each particle size were carried out by assuming AuNP interfaces of paper and air with the direction of light illumination being normal to the AuNP/paper interface. From the FDTD simulation results, we could conclude that the SERS effect of the developed paper-based SERS substrate originated dominantly from particles that formed clusters and it was supported by single nanoparticle field enhancement with a large particle size (Fig. S3) [32,35,36]. On the other hands, Raman signal uniformity in SERS substrates is an important criterion for the practical utilization of sensors for molecular detection [4,37]. The density of hotspots in the laser spot-size area was calculated by counting the number of nanoparticles in the SEM images. Although the AuNPs were randomly placed within 374
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(10 ppm), 0.313 μg/cm2 (5 ppm), 0.06 μg/cm2 (1 ppm), and 0.03 μg/ cm2 (0.5 ppm). As shown in Fig. 4a, DQ with initial concentrations from 50 ppm to 0.5 ppm was detected by the typical DQ peak at 1574 cm−1, and the standard curve for the various concentrations showed a linear correlation to the Raman intensities (Fig. 4b). Therefore, the LOD for DQ was 0.03 μg/cm2 using prepared SERS substrate. Furthermore, the Raman mapping image in Fig. 4c, which was obtained for DQ at a concentration of 3.125 μg/cm2, was uniform over a 200 μm x 200 μm area with a CV value of 8.3%. Additionally, in Fig. 4d, PQ was similarly tested at the typical PQ peak at 1647 cm−1, and it was detected down to a concentration of 0.03 μg/cm2 (0.5 ppm). The Raman intensity of PQ was linearly correlated to the various PQ concentrations (Fig. 4e). Therefore, the LOD for PQ was 0.03 μg/cm2 on the SERS substrate. Raman mapping of PQ at a concentration of 3.125 μg/cm2 showed a uniform Raman signal over the entire mapping area with a CV value of 8.7% (Fig. 4f). However, the maximum residual limit (MRL) value for both DQ and PQ is 0.05 mg/kg in the USA, EU and China. Converting the MRL into the density units of μg/cm2 gave a value of 31.1 μg/cm2 for residual PQ on apple peels, as suggested in the literature [42]. Considering the sensitivity of the developed SERS substrate, DQ and PQ were detected at much lower concentrations than the converted MRL value. On the other hand, a comparison of the Raman spectra of MB, DQ, and PQ on a paper-based SERS substrate prepared in Au precursor solutions of pH 3, pH 5 and pH 8 was carried out (Fig. S6). The Raman intensities of MB, DQ and PQ were highest for the substrate prepared in the pH 3 solution, which is well correlated with the SERS activities and electromagnetic field enhancement properties of the FDTD simulation results. Therefore, we could confirm that the paper-based SERS substrate prepared from Au precursor solution at pH 3 formed AuNPs with an average size of 102 nm and exhibited the highest Raman signal enhancement activity compared to that prepared at pH 5 and pH 8.
the nanoscale area, more than 100 particles were contained within a single laser spot-size of 2.22 μm2 which was calculated from the laser spot diameter of 1.68 μm of 633 nm laser source with a lens numerical aperture value of 0.46. Because the Raman signal is generated from the ensemble average of signals produced from the nanogaps within the laser excitation area, we can expect a uniform Raman signal generation from the developed SERS substrate. The signal uniformity test is further verified in the following sections. 3.3. SERS activity of the prepared paper-based SERS substrates The Raman signal enhancement characteristics of the paper-based SERS substrate were studied, especially for the substrate prepared in a HAuCl4 solution at pH 3, which showed the highest Raman activity. Ten microliters of methylene blue (MB) solutions at various concentrations were dropped onto the SERS substrates and measured by a portable Raman spectrometer with a laser wavelength of 633 nm. As shown in Fig. 3a, the limit of detection (LOD) for MB was 100 nM and showed a linear correlation to the Raman intensity over the observed concentration range at the typical MB peak of 1640 cm−1 (Fig. 3b). However, it is reasonable to convert the concentration of MB on the SERS substrate into units of gram/cm2 because the number of molecules is dependent on the drop volume after sample drying [38]. Therefore, 10 μL drop of MB solution on the SERS substrate with a size of 0.16 cm2 at each concentration can be converted to 100 ng/cm2 (5 μM), 20 ng/ cm2 (1 μM), 10 ng/cm2 (500 nM), 4 ng/cm2 (200 nM), and 2 ng/cm2 (100 nM). The limit of detection for MB was down to 2 ng/cm2 according to Raman data. Then, the Raman signal uniformity of the developed SERS substrate was analyzed by a Raman mapping image that was obtained at 1640 cm−1 over a 200 μm x 200 μm area with 20 μm laser spot intervals (Fig. 3c). Additionally, the Raman spectra of MB at 15 different sampling positions are plotted in Fig. 3d. A Raman signal coefficient variation (CV) value of 6.8% was calculated from 100 different positions in the Raman mapping image. According to these results, the SERS activity of the developed SERS substrate is uniform when considering the reported CV values of 14.8%, which was obtained for uniform Ag nanorod arrays, and 4.3%, which was the lowest reported value for a SERS sensor [39,40]. Furthermore, background signals that can interfere with the target signal were measured from CFP, PD-coated CFP, and the paper-based SERS substrate using Raman laser wavelengths of 633 nm and 785 nm with a laser power of 0.5 mW and 1 mW, respectively. As shown in Fig. S4a, bare CFP showed no significant Raman signal under both laser wavelengths. After coating with PD, typical polymerized dopamine peaks at 1382−1 and 1563−1 were observed under the 633 nm laser, while no peaks were appeared under 785 nm excitation (Fig. S4b) [41]. The background signal of the AuNPsdecorated SERS substrate showed no significant interference signal with the target molecules (Fig. S4c). Considering the Raman intensity counts of the background measurements, the prepared SERS substrate can be utilized for label-free molecular detection with low background interference. To characterize the PD-coating uniformity, Raman mapping of PD-coated CFP was carried out using the peak at 1563−1 cm over a 200 μm × 200 μm area. As shown in Fig. S5, the PD coating on CFP was uniform over the measurement area and is well matched to the photographic image in Fig. 1a.
3.5. Pesticide detection on apple peels For the field application test of the prepared paper-based SERS substrate as a label-free molecular detection sensor, pesticide-contaminated apples were prepared by dropping of 20 μL of DQ and PQ solutions with concentrations of 50 ppm, 25 ppm, 10 ppm, 5 ppm, and 1 ppm onto the apple peels as reported in previous work [38]. After complete drying of the pesticides for 24 h, a paper-based SERS substrate was soaked with 10 μL of DI water and attached on apple peels for 10 min. Because the size of paper-based SERS substrate attached on apple peels was 0.16 cm2, which was larger than the pesticide contaminated area of 0.13 cm2, surface-adhered pesticides were transferred within the SERS substrate area. The Raman spectra of DQ and PQ that had transferred onto the SERS substrate were measured after detachment from the apple peels using a Raman laser wavelength of 785 nm at a power of 1 mW for 5 s A photograph of the SERS substrate applied to an apple surface is shown in Fig. 5a. Then, as shown in Fig. 5b and c, concentration-dependent DQ and PQ Raman spectra were observed from the tested SERS substrate down to a concentration of 1 ppm. Because DQ and PQ are water-soluble bipyridylium pesticides, they were easily transferred and absorbed from the apple peels onto the paperbased SERS substrates. For pesticides that are soluble in organic solvents, the SERS substrate can be utilized after soaking with an organic solvent such as methanol, ethanol, and acetonitrile. As a flexibility test of developed paper-based SERS substrate, bending test was performed using a sample prepared in a size of 1 cm × 4 cm with a bending radius of 7 mm and speed of 90 rpm (Fig. S7a and b). After dropping 10 μM of MB solution on the center of the sample, Raman signal was monitored at every 200 cycles to observe any interference during the bending test. As shown in Fig. S7c, the intensity of methylene blue showed no significant change until 1000 cycles that confirmed reliable paper-based SERS substrate even after the flexible handling and target extraction from curved surface. Considering the flexibility and absorbent properties of the paper-based SERS substrate, the developed sensor is expected
3.4. Pesticide detection The developed paper-based SERS substrates were evaluated for label-free detection of pesticides such as diquat (DQ) and paraquat (PQ) which are typical bipyridylium pesticides. Ten microliters of DQ and PQ solutions with concentrations of 50 ppm, 25 ppm, 10 ppm, 5 ppm, 1 ppm and 0.5 ppm were dropped onto 0.16 cm2 SERS substrates and dried. Then, as calculated for MB density, the amounts of both DQ and PQ additions were converted from their initial solution concentrations to 3.125 μg/cm2 (50 ppm), 1.56 μg/cm2 (25 ppm), 0.625 μg/cm2 375
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to be applicable as a label-free and field-specific pesticide prescreening SERS substrate for monitoring agricultural product distributions.
analchem.7b00300. [10] W. Zhang, B. Li, L. Chen, Y. Wang, D. Gao, X. Ma, A. Wu, Brushing, a simple way to fabricate SERS active paper substrates, Anal. Methods 6 (2014) 2066–2071, https:// doi.org/10.1039/C4AY00046C. [11] B. Li, W. Zhang, L. Chen, B. Lin, A fast and low-cost spray method for prototyping and depositing surface-enhanced Raman scattering arrays on microfluidic paper based device, Electrophoresis 34 (2013) 2162–2168, https://doi.org/10.1002/elps. 201300138. [12] W. Kim, S.H. Lee, Y.J. Ahn, S.H. Lee, J. Ryu, S.K. Choi, S. Choi, A label-free cellulose SERS biosensor chip with improvement of nanoparticle-enhanced LSPR effects for early diagnosis of subarachnoid hemorrhage-induced complications, Biosens. Bioelectron. 111 (2018) 59–65, https://doi.org/10.1016/j.bios.2018.04.003. [13] A.K. Yetisen, M.S. Akram, C.R. Lowe, Paper-based microfluidic point-of-care diagnostic devices, Lab Chip 13 (2013) 2210–2251, https://doi.org/10.1039/ C3LC50169H. [14] A.T. Singh, D. Lantigua, A. Meka, S. Taing, M. Pandher, G. Camci-Unal, Paper-based sensors: emerging themes and applications, Sensors (Basel) 18 (2018) 2838, https://doi.org/10.3390/s18092838. [15] W.W. Yu, I.M. White, Inkjet-printed paper-based SERS dipsticks and swabs for trace chemical detection, Analyst 138 (2013) 1020–1025, https://doi.org/10.1039/ c2an36116g. [16] R. Tantra, R.J. Brown, M.J. Milton, D. Gohil, A practical method to fabricate gold substrates for surface-enhanced Raman spectroscopy, Appl. Spectrosc. 9 (2008) 992–1000, https://doi.org/10.1366/000370208785793272. [17] R. Zhang, B.B. Xu, X.Q. Liu, Y.L. Zhang, Y. Xu, Q.D. Chen, H.B. Sun, Highly efficient SERS test strips, Chem. Commun. 48 (2012) 5913–59155, https://doi.org/10.1039/ C2CC31604H. [18] C.H. Lee, L. Tian, S. Singamaneni, Paper-based SERS swab for rapid trace detection on real-world surfaces, ACS Appl. Mater. Interfaces 2 (2010) 3429–3435, https:// doi.org/10.1021/am1009875. [19] F. Zeng, W. Duan, B. Zhu, T. Mu, L. Zhu, J. Guo, X. Ma, Paper-based versatile SERS chip with smartphone-based Raman analyzer for point of care application, Anal. Chem. 91 (2019) 1064–1070, https://doi.org/10.1021/acs.analchem.8b04441. [20] Y. Xu, P. Man, Y. Huo, T. Ning, C. Li, B. Man, C. Yang, Synthesis of the 3D AgNF/ AgNP arrays for the paper-based surface enhancement Raman scattering application, Sens. Actuators B Chem. 265 (2018) 302–309, https://doi.org/10.1016/j.snb. 2018.03.035. [21] C.K. Tanga, A. Vazea, J.F. Rusling, Paper-based electrochemical immunoassay for rapid, inexpensive cancer biomarker protein detection, Anal. Methods 6 (2014) 8878–8881, https://doi.org/10.1039/C4AY01962H. [22] X. Zou, Y. Wang, W. Liu, L. Chen, m-Cresol purple functionalized surface enhanced Raman scattering paper chips for highly sensitive detection of pH in the neutral pH range, Analyst 142 (2017) 2333–2337, https://doi.org/10.1039/C7AN00653E. [23] J. Hwang, S. Lee, J. Choo, Application of a SERS-based lateral flow immunoassay strip for the rapid and sensitive detection of staphylococcal enterotoxin B, Nanoscale 8 (2016) 11418–11425, https://doi.org/10.1039/C5NR07243C. [24] N. Jiang, X. Zhuo, J. Wang, Active plasmonics: principles, structures, and applications, Chem. Rev. 118 (2018) 3054–3099, https://doi.org/10.1021/acs.chemrev. 7b00252. [25] V. Devaraj, J.-M. Lee, J.-W. Oh, Distinguishable plasmonic nanoparticle and gap mode properties in a silver nanoparticle on a gold film system using three-dimensional FDTD simulations, Nanomaterials 8 (2018) 582, https://doi.org/10.3390/ nano8080582. [26] M.I. Markovic, A.D. Rakic, Determination of the reflection coefficients of laser light of wavelengths λε (0.22 μm, 200 μm) from the surface of aluminum using the Lorentz-Drude model, Appl. Opt. 29 (1990) 3479–3483, https://doi.org/10.1364/ AO.29.003479. [27] J.H. Ryu, P.B. Messersmith, H. Lee, Polydopamine surface chemistry: a decade of discovery, ACS Appl. Mater. Interfaces 10 (2018) 7523–7540, https://doi.org/10. 1021/acsami.7b19865. [28] K.C.L. Black, Z. Liu, P.B. Messersmith, Catechol redox induced formation of metal core-polymer shell nanoparticles, Chem. Mater. 23 (2011) 1130–1135, https://doi. org/10.1021/cm1024487. [29] H.Y. Son, K.R. Kim, J.B. Lee, T.H.L. Kim, J. Jang, S.J. Kim, M.S. Yoon, J.W. Kim, Y.S. Nam, Bioinspired synthesis of mesoporous gold-silica hybrid microspheres as recyclable colloidal SERS substrates, Sci. Rep. 7 (2017) 14728, https://doi.org/10. 1038/s41598-017-15225-8. [30] W. Haiss, N.T. Thanh, J. Aveyard, D.G. Fernig, Determination of size and concentration of gold nanoparticles from UV-vis spectra, Anal. Chem. 79 (2007) 4215–4221, https://doi.org/10.1021/ac0702084. [31] X. Han, Y. Liu, Y. Yin, Colorimetric stress memory sensor based on disassembly of gold nanoparticle chains, Nano Lett. 14 (2014) 2466–2470, https://doi.org/10. 1021/nl500144k. [32] V. Mondes, E. Antonsson, J. Plenge, C. Raschpichler, I. Halfpap, A. Menski, C. Graf, M.F. Kling, E. Rühl, Plasmonic electric near-field enhancement in self-organized gold nanoparticles in macroscopic arrays, Appl. Phys. B 122 (2016) 155, https:// doi.org/10.1007/s00340-016-6412-1. [33] R.X. He, R. Liang, P. Peng, Y.N. Zhou, Effect of the size of silver nanoparticles on SERS signal enhancement, J. Nanopart. Res. 19 (2017) 267, https://doi.org/10. 1007/s11051-017-3953-0. [34] X. Liu, M. Osada, K. Kitamura, T. Nagata, D. Si, Ferroelectric-assisted gold nanoparticles array for centimeter-scale highly reproducible SERS substrates, Sci. Rep. 7 (2017) 3630, https://doi.org/10.1038/s41598-017-03301-y. [35] L. Tong, T. Zhu, Z. Liu, Approaching the electromagnetic mechanism of surfaceenhanced Raman scattering: from self-assembled arrays to individual gold nanoparticles, Chem. Soc. Rev. 40 (2011) 1296–1304, https://doi.org/10.1039/
4. Conclusion In summary, we prepared AuNP-decorated paper-based SERS substrates with a facile and low-cost method without the addition of a reducing agent. The size of the synthesized AuNPs was controlled by changing the pH of the Au-precursor solution, and the particles formed under acidic conditions, which had an average particle size of 102 nm, showed the highest SERS activity. FDTD simulations suggested the origin of electromagnetic field enhancement originated from clustered AuNPs and individualized AuNPs. After characterization of the SERS activity and signal uniformity, the developed SERS substrate was applied to the label-free detection of bipyridylium pesticides such as DQ and PQ. Furthermore, the sensor was capable of being applied in the field, which was tested using pesticide-contaminated apple peels. Based on these results, flexible and water-absorbing paper-based substrates are expected to be applicable to a variety of label-free chemical SERS sensors. Notes The authors declare no competing financial interests. Acknowledgments This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry (IPET) through the Advanced Production Technology Development Program of the Ministry of Agriculture, Food and Rural Affairs (MAFRA) (31608004). This work was also supported by the Fundamental Research Program (PNK 6070) of the Korea Institute of Materials Science (KIMS). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.04.077. References [1] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Biosensing with plasmonic nanosensors, Nat. Mater. 7 (2008) 442–453, https://doi.org/10. 1038/nmat2162. [2] Y.W.C. Cao, R.C. Jin, C.A. Mirkin, Nanoparticles with Raman spectroscopic fingerprints for DNA and RNA detection, Science 297 (2002) 1536–1540, https://doi. org/10.1126/science.297.5586.1536. [3] P. Reokrungruang, I. Chatnuntawech, T. Dharakul, S. Bamrungsap, A simple paperbased surface enhanced Raman scattering (SERS) platform and magnetic separation for cancer screening, Sens. Actuators B Chem. 285 (2019) 462–469, https://doi. org/10.1016/j.snb.2019.01.090. [4] Y. Sun, Y. Zhang, Y. Shi, X. Xiao, H. Dai, J. Hu, P. Ni, Z. Li, Facile preparation of silver nanoparticle films as an efficient surface-enhanced Raman scattering substrate, Appl. Surf. Sci. 283 (2013) 52–57, https://doi.org/10.1016/j.apsusc.2013. 05.154. [5] N.A. Hatab, C.H. Hsueh, A.L. Gaddis, S.T. Retterer, J.H. Li, G. Eres, Z. Zhang, B. Gu, Free-standing optical gold bowtie nanoantenna with variable gap size for enhanced Raman spectroscopy, Nano Lett. 10 (2010) 4952–4955, https://doi.org/10.1021/ nl102963g. [6] Y.Y. Lin, J.D. Liao, Y.H. Ju, C.W. Chang, A.L. Shiau, Focused ion beam-fabricated Au micro/nanostructures used as a surface enhanced Raman scattering-active substrate for trace detection of molecules and influenza virus, Nanotechnology 22 (2011) 185308, https://doi.org/10.1088/0957-4484/22/18/185308. [7] V. Suresh, Ding Lu, A.B. Chew, F.L. Yap, Fabrication of large-area flexible SERS substrates by nanoimprint lithography, ACS Appl. Nano. Mater. 1 (2018) 886–893, https://doi.org/10.1021/acsanm.7b00295. [8] J.W. Jeong, M.M.P. Arnob, K.-M. Baek, S.Y. Lee, W.-C. Shih, Y.S. Jung, 3D crosspoint plasmonic nanoarchitectures containing dense and regular hot spots for surface-enhanced Raman spectroscopy analysis, Adv. Mater. 28 (2016) 8695–8704, https://doi.org/10.1002/adma.201602603. [9] W. Kim, J.C. Lee, G.J. Lee, H.K. Park, A. Lee, S. Choi, Low-cost label-free biosensing bimetallic cellulose strip with SILAR-synthesized silver core-gold shell nanoparticle structures, Anal. Chem. 89 (2017) 6448–6454, https://doi.org/10.1021/acs.
376
Sensors & Actuators: B. Chemical 291 (2019) 369–377
V.T.N. Linh, et al.
nanopillar arrays with vertically integrated nanogaps for SERS‐active substrates, Adv. Funct. Mater. 25 (2015) 4681–4688, https://doi.org/10.1002/adfm. 201501274. [40] M.K. Lee, T.Y. Jeon, C.W. Mun, J. Kwon, J. Yun, S.H. Kim, D.H. Kim, S.-C. Chang, S.G. Park, Multilayered plasmonic nanostructures with high areal density for SERS, RSC Adv. 7 (2017) 17898–17905, https://doi.org/10.1039/C6RA28150H. [41] M. Kaya, M. Volkan, New approach for the surface enhanced resonance Raman scattering (SERRS) detection of dopamine at picomolar (pM) levels in the presence of ascorbic acid, Anal. Chem. 84 (2012) 7729–7735, https://doi.org/10.1021/ ac3010428. [42] H. Fang, X. Zhang, S.J. Zhang, L. Liu, Y.M. Zhao, H.J. Xu, Ultrasensitive and quantitative detection of paraquat on fruits skins via surface-enhanced Raman spectroscopy, Sens. Actuators B 213 (2015) 452–456, https://doi.org/10.1016/j. snb.2015.02.121.
C001054P. [36] F. Benz, R. Chikkaraddy, A. Salmon, H. Ohadi, B. de Nijs, J. Mertens, C. Carnegie, R.W. Bowman, J.J. Baumberg, SERS of individual nanoparticles on a mirror: size does matter, but so does shape, J. Phys. Chem. Lett. 7 (2016) 2264–2269, https:// doi.org/10.1021/acs.jpclett.6b00986. [37] T. Lee, J.S. Wi, A. Oh, H.K. Na, J. Lee, K. Lee, T.G. Lee, S. Haam, Highly robust, uniform and ultra-sensitive surface-enhanced Raman scattering substrates for microRNA detection fabricated by using silver nanostructures grown in gold nanobowls, Nanoscale 10 (2018) 3680–3687, https://doi.org/10.1039/C7NR08066B. [38] E.H. Koh, C. Mun, C. Kim, S.G. Park, E.J. Choi, S.H. Kim, J. Dang, J. Choo, J.-W. Oh, D.H. Kim, H.S. Jung, M13 Bacteriophage/silver Nanowire surface-enhanced Raman scattering sensor for sensitive and selective pesticide detection, ACS Appl. Mater. Interfaces 10 (2018) 10388–10397, https://doi.org/10.1021/acsami.8b01470. [39] T.Y. Jeon, S.-G. Park, D.-H. Kim, S.-H. Kim, Standing‐wave‐assisted creation of
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A facile low-cost paper-based SERS substrate for label-free molecular detection
T
Vo Thi Nhat Linha,b, Jungil Moonc,e, ChaeWon Muna, Vasanthan Devarajd, Jin-Woo Ohd, ⁎ ⁎ ⁎ Sung-Gyu Parka, Dong-Ho Kima, Jaebum Chooe, , Yong-Ill Leeb, , Ho Sang Junga, a
Advanced Nano-Surface Department, Korea Institute of Materials Science (KIMS), Changwon, Gyeongnam, 51508, Republic of Korea Department of Chemistry, Changwon National University, Changwon, 51140, Republic of Korea c Department of Bionano Engineering, Hanyang University, Ansan, 15588, Republic of Korea d Research Center for Energy Convergence and Technology Division, Pusan National University, Busan 46241, Republic of Korea e Department of Chemistry, Chung-Ang University, Seoul, 06974, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Paper-based sensor Surface enhanced Raman scattering Label-free detection Au nanoparticle
We introduce a facile and low-cost method for fabricating gold nanostructures on cellulose filter paper (CFP) to prepare a paper-based surface-enhanced Raman scattering (SERS) sensor for label-free molecular detection. Polymerized dopamine (PD) was used as an adhesive layer on the CFP and simultaneously functioned as a reducing agent for gold nanoparticle (AuNP) nucleation. The size of the AuNPs was dependent on the pH of the gold precursor solution, and nanoparticles with an average size of 102 nm were formed on the PD-coated CFP at a pH 3, exhibiting high SERS activity. Finite-difference time-domain (FDTD) simulations of the electromagnetic field enhancement of AuNPs with different sizes and interparticle distances were performed to identify the origin of the SERS effect. The developed paper-based SERS substrate showed uniform and excellent molecular sensitivity with a limit of detection (LOD) of 10−7 M for methylene blue, as measured by a portable Raman spectrometer. Furthermore, as a field application test, surfaces of apples were pretreated with diquat (DQ) and paraquat (PQ) pesticides, which were then detected down to a concentration of 1 ppm after simple attachment of the sensor on the apple peels and performing a SERS measurement. The developed paper-based SERS sensor is expected to be applicable as a label-free sensor for a variety of chemical and biological molecules.
1. Introduction Surface-enhanced Raman scattering (SERS) has been a powerful and attractive spectroscopic technique for molecular detection due to its label-free identification of chemicals from their specific Raman spectra and significant Raman signal enhancement of target molecules adsorbed onto novel plasmonic metal nanostructures [1–3]. Gold and silver nanoparticles have been widely used to create nanogaps between particles, which are known as ‘hotspots’, for electromagnetic field enhancement, but aggregation-induced hotspot generation has limited the production of uniform and reproducible Raman signals [4]. On the other hand, SERS substrates have been developed that generate uniform Raman signals over a large area, but creating a high density of hotspots requires expensive manufacturing processes such as E-beam lithography [5], focused ion beams [6], nanoimprinting [7], and multistacking of plasmonic nanostructures [8]. Recently, paper-based SERS substrates have been developed that exhibit many advantages such as
⁎
low-cost, flexibility, portability, and biodegradability [9–13]. Especially, paper-based sensors absorb solution samples with capillary action that enables rapid target molecule adhesion and enrichment on the sensor surface [14]. To decorate novel metal nanostructures onto paper substrates, various methods have been utilized, including inkjet printing [15], solution dipping [16], physical vapor deposition [17], and successive ion layer adsorption and reaction (SILAR) [9]. However, these methods require complicated nanoparticle synthetic and decoration processes to produce the paper-based SERS substrates, and also special instruments and multiple reaction steps are needed for their fabrication. Nevertheless, the paper-based SERS substrate can be applied for direct target absorbent on 3 dimensional objects such as agricultural and food products by swabbing the surface [18–20], biomarker collection by dipping in the biological fluids [21,22], and development of lateral/vertical flow system for on-site SERS immunoassay [23]. Herein, we propose a facile and low-cost paper-based SERS
Corresponding authors. E-mail addresses: [email protected] (J. Choo), [email protected] (Y.-I. Lee), [email protected] (H.S. Jung).
https://doi.org/10.1016/j.snb.2019.04.077 Received 12 February 2019; Received in revised form 14 April 2019; Accepted 15 April 2019 Available online 16 April 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic illustration of the paper-based SERS substrate fabrication method.
Fig. 1. Optical property and surface morphology observations (a) Photographs of bare CFP, PD-coated CFP and SERS substrates fabricated at three different pH values. (b) UV–vis reflectance spectra of bare CFP, PD-coated CFP and paper-based SERS substrates fabricated at different pH values. (c) SEM images of AuNPs decorated on CFP at different pH values (scale bar: 200 nm, left: pH 3, middle: pH 5, right: pH 8) and (d) their corresponding size distributions.
sensitivity of the sensor. Furthermore, as a field application test, the surface of an apple was pretreated with pesticides, which were detected by simple attachment of the sensor substrate over the apple peels and performing a SERS measurement. The developed paper-based SERS substrate is expected to be applied for a variety of cost-effective labelfree chemical sensors.
substrate for label-free molecular detection with high sensitivity and signal uniformity (Scheme 1). Gold nanoparticles (AuNPs) were directly grown on polymerized dopamine (PD)-coated cellulose filter paper (CFP) by a simple solution dipping method. PD functioned as an adhesive interlayer between the CFP and Au ion seeds and reduced the Au ions into AuNPs directly on the CFP surface without the introduction of reducing agents. The paper-based SERS substrate prepared from a HAuCl4 precursor solution at pH 3 exhibited significant SERS activity with an average AuNP size of 102 nm. Finite-difference time-domain (FDTD) simulations demonstrated the SERS effect of the AuNPs decorated on the SERS substrate. Then, the sensitivity and signal uniformity of the developed sensor were characterized by measuring the limit of detection (LOD) and Raman mapping of 200 μm × 200 μm area, respectively. The LOD for methylene blue (MB) was 2 ng/cm2, and that for both bipyridylium pesticides such as diquat (DQ) and paraquat (PQ) was 0.03 μg/cm2 for each, which showed the excellent molecular
2. Experimental section 2.1. Materials Dopamine hydrochloride, tris(hydroxymethyl)aminomethane, gold chloride trihydrate (HAuCl4·3H2O), methylene blue (MB), methyl viologen dichloride monohydrate (PQ), and diquat dibromide monohydrate (DQ) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Cellulose filter paper (CFP) with a pore size of 0.45 μm was purchased 370
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Fig. 2. FDTD simulations of the electromagnetic field enhancement at various interparticle distances with particle sizes of (a) 102 nm, (b) 70 nm, and (c) 36 nm.
2.3. Characterization of the paper-based SERS substrates
from Hyundai Micro Co., LTD (Seoul, South Korea).
The surface morphologies of the paper-based SERS substrates were observed by field emission scanning electron microscopy (FE-SEM; Joel JSM-6700F). Additionally, elemental analysis was conducted using Energy dispersive X-ray spectroscopy (EDS) attached to the FE-SEM. The average particle sizes and their distributions of the AuNPs synthesized in three different pH solutions were obtained by measuring the sizes of 100 different nanoparticles in the SEM image. UV–vis reflectance spectra were measured using a UV–vis-NIR spectrophotometer (Cary 5000, Agilent Technology).
2.2. Preparation of the paper-based SERS substrates Paper-based SERS substrates were fabricated using a simple dipcoating method. CFP was immersed in 10 mL of a dopamine solution at a concentration of 1 mg/mL prepared in Tris-buffer (10 mM, pH 8.5) for 24 h under stirring at room temperature to polymerize dopamine on the CFP surface. Then, the samples were washed with deionized (DI) water and dried at 45 °C. The PD-coated CFP was then immersed in a 1 mg/mL HAuCl4 solution at pH 3, pH 5, or pH 8 for 24 h at room temperature. After AuNP nucleation, the prepared substrates were washed with DI water and dried under ambient conditions. Control substrates such as bare CFP immersed in HAuCl4 solution and PD-coated CFP that was immersed in a pre-synthesized-AuNP solution with an average particle size of 40 nm were prepared.
2.4. Electromagnetic field distribution simulations Optical analysis of the plasmonic nanostructures was performed using three-dimensional finite difference time domain (3D FDTD) simulations (Lumerical solutions). The diameters of the AuNPs were set at 36 nm, 70 nm, and 102 nm with a mesh size of 0.5 nm. The refractive 371
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Fig. 3. SERS activity test of the paper-based SERS substrate using MB. (a) LOD for MB (concentration unit was expressed in initial MB solution concentrations), (b) standard curve at various MB concentrations, (c) Raman mapping of MB at a concentration of 10 μM within an area of 200 μm × 200 μm, and (d) Raman signal uniformity test at 15 different positions.
indices for the paper substrate and air were selected as 1.557 and 1.0, respectively. For dimer AuNPs, the gap between the two AuNPs was set at 5 nm, 10 nm, 20 nm, 40 nm, 60 nm, and 100 nm. The direction of optical excitation (TE polarized, broadband plane-wave source) was normal to the plane of the paper substrate. To absorb the scattered waves, perfectly matched layer (PML) boundary conditions in the XYZ directions were applied around the AuNP structures. A power monitor was placed close to the nanostructures to record the near-field enhancement results. The near-field enhancement was obtained from an integral volume average of |E/E0|4 using the following formula where the maximum local electromagnetic field, incident amplitude of the light source, and volume were denoted E, E0 and V, respectively [24,25].
Near-field enhancement =
2.5. LOD and Raman signal uniformity tests The LOD of a paper-based SERS substrate prepared at pH 3 was tested after dropping 10 μL of MB, DQ and PQ solutions at various concentrations onto the surface. The size of paper-based SERS substrate was prepared in 0.16 cm2 in a square shape. MB solutions at concentrations of 5 μM, 1 μM, 500 nM, 200 nM and 100 nM were prepared. Both DQ and PQ solutions were prepared at concentrations of 50 ppm, 25 ppm, 10 ppm, 5 ppm, 1 ppm and 0.5 ppm. Ten microliters of each MB, DQ and PQ solution was dropped on the SERS substrate and dried before the Raman measurement. Additionally, the Raman signal uniformity was characterized by Raman mapping of a 200 μm × 200 μm area in 20 μm intervals. The Raman intensity of MB at a concentration of 200 ng/cm2 (10 μM of MB solution on 0.16 cm2 SERS substrate) and both DQ and PQ at a concentration of 3.125 μg/cm2 (50 ppm of each DQ and PQ solution on 0.16 cm2 SERS substrate) were compared for paper-based SERS substrates prepared at pH 3, pH 5 and pH 8. The Raman signals were measured using a portable Raman spectrometer Nanoscope system (NS220 for 633 nm, Inc., Daejeon, South Korea) for MB with a laser wavelength, a laser power, an exposure time and a laser spot diameter of 633 nm, 0.5 mW, 0.5 s, and 1.68 μm, respectively. For the pesticides detection, a portable Raman spectrometer Nanoscope System (NS200 for 785 nm Inc., Daejeon, South Korea) was used with a laser wavelength, a laser power, an exposure time and a laser spot diameter of 785 nm, 1 mW, 5 s, and 2.08 μm, respectively. All the spectra were plotted from the average of five different sample points.
∭ |E/E0 |4 dV/V
For the dielectric function model of the nanostructures, the LorentzDrude dispersion model was used as given below. The terms wp, f0, and Γ0 represent the plasma frequency, oscillator strength, and damping constant, respectively. In addition, the Lorentz modification terms of m, wj, fj, and Γj represent the number of oscillations, frequency, and damping constant, respectively [26].
∈ (w ) = 1 − (f0 wp2)/ w (w − iΓ0) +
m
∑j=1 (f j wp2)/((wj2 − w 2) + iwΓj)
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Fig. 4. SERS detection of pesticides. (a) LOD for DQ, (b) standard curve at various concentrations of DQ, (c) Raman mapping of DQ, (d) LOD of PQ, (e) standard curve at various concentrations of PQ, and (f) Raman mapping of PQ (concentration is expressed as the initial DQ and PQ solution concentrations).
2.6. Field application for residual pesticide detection
2.7. Bending test of paper-based SERS substrate
Twenty microliter of DQ and PQ solutions at concentrations of 50 ppm, 25 ppm, 10 ppm, 5 ppm and 1 ppm were dropped on the apple peels and dried for 24 h. The pesticide treated area was calculated as 0.13 cm2 from the 4 mm of 20 μL solution drop diameter. Ten microliters of DI water was dropped on a prepared paper-based SERS substrate with a size of 4 mm × 4 mm, which was then attached on pesticides contaminated apple peels for 10 min. The Raman signals were obtained from the detached pesticide-transferred SERS substrates using a portable Raman spectrometer with a laser wavelength, a laser power and an exposure time of 785 nm, 1 mW, and 5 s, respectively.
CFP filter paper with a diameter of 47 mm was prepared and decorated with AuNPs in Au precursor solution with pH 3. Then, the sample was cut in a rectangular shape with a size of 1 cm × 4 cm. The MB solution with a concentration of 10 μM and a volume of 10 μL was dropped on the center of the prepared SERS paper where the highest bending stress is applied. The bending condition was set as 7 mm bending radius with a speed of 90 rpm using home-made bending test machine. Raman spectrum of MB was measured at every 200 cycles of bending test with the measurement condition for MB mentioned above.
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Fig. 5. Pesticide detection on an apple surface. (a) Schematic depiction of pesticide detection with a paper-based SERS substrate after wiping on a contaminated apple surface, Raman spectra of (b) DQ and (c) PQ from an apple surface pretreated with pesticides at various concentrations.
3. Results and discussion
increased the light absorbance and affected color of the prepared SERS substrates which is well matched with UV–vis reflectance spectra in Fig. 1b. The advantages of the developed SERS substrate in this work is that the large-sized plasmonic AuNPs with the diameter of ˜100 nm were decorated from one time immersion of PD-coated CFP in Au precursor solution while other paper-based SERS substrates were prepared by multiple Au growth steps [9], utilization of reducing agent and galvanic exchange methods [12]. Therefore, in the viewpoint of simple production process, the developed paper-based SERS substrate is considered to be cost-effective and mass-productive.
3.1. Characterization of the paper-based SERS substrates AuNPs decorated on the CFP were prepared by simple immersion of the PD-coated CFP into a HAuCl4 solution. Polymerized dopamine is an adhesive polymer that adheres to a variety of material surfaces such as metal, ceramic, and polymer surfaces [27]. Catechol oxidation in PD is known to reduce the metal ions in solution, resulting in the nucleation and growth of AuNPs on the PD surface [28,29]. As shown in Fig. 1a, a color change was clearly observed for CFP during each fabrication step. PD-coated CFP showed a uniform gray color and then turned brown-red after AuNP growth in the HAuCl4 solution at pH 3, while Au reduction at pH 5 and pH 8 produced a dark-violet color after the reaction. The CFP decorated with AuNPs at pH 3 showed a reflectance SPR peak at 650 nm from the UV–vis reflectance spectra (Fig. 1b), which corresponds to AuNPs with a size around 100 nm. Interestingly, without the addition of a reducing agent, AuNPs with average particle sizes of 102 nm, 70 nm, and 36 nm were grown in HAuCl4 precursor solutions at pH 3, pH 5, and pH 8, respectively. In Fig. 1c and Fig. S1a–S1c, scanning electron microscopy (SEM) images of AuNPs formed under the three different pH conditions show particle size differences over a large area with high uniformity. The size distributions of AuNPs were 102 ± 24.3 nm, 70 ± 26.8 nm, and 36 ± 6.6 nm for pH 3, pH 5 and pH 8, respectively (Fig. 1d). Energy dispersive X-ray spectroscopy (EDS) analysis confirmed that the elemental compositions of the nanoparticles formed on the CFP were reduced Au (Fig. S1d). Bare CFP and PD-coated CFP showed the basal surface morphology of cellulose paper (Fig. S2a and b), and no AuNPs were grown on bare CFP immersed in a HAuCl4 solution (Fig. S2c). Additionally, locally aggregated particles attached to the PD layer were observed on PD-coated CFP immersed in a solution of pre-synthesized AuNPs with an average particle size of 40 nm (Fig. S2d). From these results, we could confirm the successful synthesis of AuNPs on paper-based substrates in the presence of an adhesive polymer layer without the addition of a reducing agents. On the other hand, as shown in Fig. 1a, the color of AuNPs were growing darken as the size of nanoparticle decreasing which showed an opposite phenomenon considering previous report [30]. However, in this work, the density of AuNPs is higher for substrate with small-sized AuNPs because more AuNPs can be nucleated from the same amount of Au precursor solution compared to the large-sized AuNPs that was prepared at pH 3. The number of AuNPs in each SEM image of Fig. 2c was counted as 348, 501, and 715 for 102 nm, 70 nm, and 36 nm, respectively, in the area of 5 μm2. It is reported that the density of AuNPs affect the color of gold containing substrate and shows color darkening as the density of AuNPs increases [31]. Therefore, the higher density of small-sized AuNPs
3.2. FDTD simulation of electromagnetic field enhancement The origin of the SERS effect from the decorated AuNPs was analyzed by FDTD simulation using AuNPs with particle sizes of 102 nm, 70 nm and 36 nm and various interparticle distances ranging from 5 nm to 100 nm. It was meaningful to calculate the electromagnetic field distribution of AuNPs with a size larger than 100 nm because many reports focused on field distribution of plasmonic nanoparticles with size smaller than 100 nm [32–34]. A strong electromagnetic field was localized and confined to the nanogap between the two particles, and the amplitude of the field enhancement exponentially decreased as the interparticle distance increased. As shown in Fig. 2a, for AuNPs with a size of 102 nm, the maximum electromagnetic field enhancement was 7.6 × 107 at a 5 nm interparticle distance and the AuNPs were regarded as individual nanoparticles at interparticle distances greater than 60 nm. Similarly, for the AuNPs with average sizes of 70 nm and 36 nm, the maximum electromagnetic field enhancements were 3.5 × 107 and 3.6 × 105, respectively, and they show a distance-dependent field intensity decrease (Fig. 2b and c). However, electromagnetic field enhancement around a single AuNP that was separated more than 60 nm was observed with minimum electromagnetic field enhancement amplitudes of 5 × 103, 1.2 × 103, and 3.6 × 102 for AuNP sizes of 102 nm, 70 nm and 36 nm, respectively. Furthermore, electromagnetic field distribution calculations of an individual AuNP for each particle size were carried out by assuming AuNP interfaces of paper and air with the direction of light illumination being normal to the AuNP/paper interface. From the FDTD simulation results, we could conclude that the SERS effect of the developed paper-based SERS substrate originated dominantly from particles that formed clusters and it was supported by single nanoparticle field enhancement with a large particle size (Fig. S3) [32,35,36]. On the other hands, Raman signal uniformity in SERS substrates is an important criterion for the practical utilization of sensors for molecular detection [4,37]. The density of hotspots in the laser spot-size area was calculated by counting the number of nanoparticles in the SEM images. Although the AuNPs were randomly placed within 374
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(10 ppm), 0.313 μg/cm2 (5 ppm), 0.06 μg/cm2 (1 ppm), and 0.03 μg/ cm2 (0.5 ppm). As shown in Fig. 4a, DQ with initial concentrations from 50 ppm to 0.5 ppm was detected by the typical DQ peak at 1574 cm−1, and the standard curve for the various concentrations showed a linear correlation to the Raman intensities (Fig. 4b). Therefore, the LOD for DQ was 0.03 μg/cm2 using prepared SERS substrate. Furthermore, the Raman mapping image in Fig. 4c, which was obtained for DQ at a concentration of 3.125 μg/cm2, was uniform over a 200 μm x 200 μm area with a CV value of 8.3%. Additionally, in Fig. 4d, PQ was similarly tested at the typical PQ peak at 1647 cm−1, and it was detected down to a concentration of 0.03 μg/cm2 (0.5 ppm). The Raman intensity of PQ was linearly correlated to the various PQ concentrations (Fig. 4e). Therefore, the LOD for PQ was 0.03 μg/cm2 on the SERS substrate. Raman mapping of PQ at a concentration of 3.125 μg/cm2 showed a uniform Raman signal over the entire mapping area with a CV value of 8.7% (Fig. 4f). However, the maximum residual limit (MRL) value for both DQ and PQ is 0.05 mg/kg in the USA, EU and China. Converting the MRL into the density units of μg/cm2 gave a value of 31.1 μg/cm2 for residual PQ on apple peels, as suggested in the literature [42]. Considering the sensitivity of the developed SERS substrate, DQ and PQ were detected at much lower concentrations than the converted MRL value. On the other hand, a comparison of the Raman spectra of MB, DQ, and PQ on a paper-based SERS substrate prepared in Au precursor solutions of pH 3, pH 5 and pH 8 was carried out (Fig. S6). The Raman intensities of MB, DQ and PQ were highest for the substrate prepared in the pH 3 solution, which is well correlated with the SERS activities and electromagnetic field enhancement properties of the FDTD simulation results. Therefore, we could confirm that the paper-based SERS substrate prepared from Au precursor solution at pH 3 formed AuNPs with an average size of 102 nm and exhibited the highest Raman signal enhancement activity compared to that prepared at pH 5 and pH 8.
the nanoscale area, more than 100 particles were contained within a single laser spot-size of 2.22 μm2 which was calculated from the laser spot diameter of 1.68 μm of 633 nm laser source with a lens numerical aperture value of 0.46. Because the Raman signal is generated from the ensemble average of signals produced from the nanogaps within the laser excitation area, we can expect a uniform Raman signal generation from the developed SERS substrate. The signal uniformity test is further verified in the following sections. 3.3. SERS activity of the prepared paper-based SERS substrates The Raman signal enhancement characteristics of the paper-based SERS substrate were studied, especially for the substrate prepared in a HAuCl4 solution at pH 3, which showed the highest Raman activity. Ten microliters of methylene blue (MB) solutions at various concentrations were dropped onto the SERS substrates and measured by a portable Raman spectrometer with a laser wavelength of 633 nm. As shown in Fig. 3a, the limit of detection (LOD) for MB was 100 nM and showed a linear correlation to the Raman intensity over the observed concentration range at the typical MB peak of 1640 cm−1 (Fig. 3b). However, it is reasonable to convert the concentration of MB on the SERS substrate into units of gram/cm2 because the number of molecules is dependent on the drop volume after sample drying [38]. Therefore, 10 μL drop of MB solution on the SERS substrate with a size of 0.16 cm2 at each concentration can be converted to 100 ng/cm2 (5 μM), 20 ng/ cm2 (1 μM), 10 ng/cm2 (500 nM), 4 ng/cm2 (200 nM), and 2 ng/cm2 (100 nM). The limit of detection for MB was down to 2 ng/cm2 according to Raman data. Then, the Raman signal uniformity of the developed SERS substrate was analyzed by a Raman mapping image that was obtained at 1640 cm−1 over a 200 μm x 200 μm area with 20 μm laser spot intervals (Fig. 3c). Additionally, the Raman spectra of MB at 15 different sampling positions are plotted in Fig. 3d. A Raman signal coefficient variation (CV) value of 6.8% was calculated from 100 different positions in the Raman mapping image. According to these results, the SERS activity of the developed SERS substrate is uniform when considering the reported CV values of 14.8%, which was obtained for uniform Ag nanorod arrays, and 4.3%, which was the lowest reported value for a SERS sensor [39,40]. Furthermore, background signals that can interfere with the target signal were measured from CFP, PD-coated CFP, and the paper-based SERS substrate using Raman laser wavelengths of 633 nm and 785 nm with a laser power of 0.5 mW and 1 mW, respectively. As shown in Fig. S4a, bare CFP showed no significant Raman signal under both laser wavelengths. After coating with PD, typical polymerized dopamine peaks at 1382−1 and 1563−1 were observed under the 633 nm laser, while no peaks were appeared under 785 nm excitation (Fig. S4b) [41]. The background signal of the AuNPsdecorated SERS substrate showed no significant interference signal with the target molecules (Fig. S4c). Considering the Raman intensity counts of the background measurements, the prepared SERS substrate can be utilized for label-free molecular detection with low background interference. To characterize the PD-coating uniformity, Raman mapping of PD-coated CFP was carried out using the peak at 1563−1 cm over a 200 μm × 200 μm area. As shown in Fig. S5, the PD coating on CFP was uniform over the measurement area and is well matched to the photographic image in Fig. 1a.
3.5. Pesticide detection on apple peels For the field application test of the prepared paper-based SERS substrate as a label-free molecular detection sensor, pesticide-contaminated apples were prepared by dropping of 20 μL of DQ and PQ solutions with concentrations of 50 ppm, 25 ppm, 10 ppm, 5 ppm, and 1 ppm onto the apple peels as reported in previous work [38]. After complete drying of the pesticides for 24 h, a paper-based SERS substrate was soaked with 10 μL of DI water and attached on apple peels for 10 min. Because the size of paper-based SERS substrate attached on apple peels was 0.16 cm2, which was larger than the pesticide contaminated area of 0.13 cm2, surface-adhered pesticides were transferred within the SERS substrate area. The Raman spectra of DQ and PQ that had transferred onto the SERS substrate were measured after detachment from the apple peels using a Raman laser wavelength of 785 nm at a power of 1 mW for 5 s A photograph of the SERS substrate applied to an apple surface is shown in Fig. 5a. Then, as shown in Fig. 5b and c, concentration-dependent DQ and PQ Raman spectra were observed from the tested SERS substrate down to a concentration of 1 ppm. Because DQ and PQ are water-soluble bipyridylium pesticides, they were easily transferred and absorbed from the apple peels onto the paperbased SERS substrates. For pesticides that are soluble in organic solvents, the SERS substrate can be utilized after soaking with an organic solvent such as methanol, ethanol, and acetonitrile. As a flexibility test of developed paper-based SERS substrate, bending test was performed using a sample prepared in a size of 1 cm × 4 cm with a bending radius of 7 mm and speed of 90 rpm (Fig. S7a and b). After dropping 10 μM of MB solution on the center of the sample, Raman signal was monitored at every 200 cycles to observe any interference during the bending test. As shown in Fig. S7c, the intensity of methylene blue showed no significant change until 1000 cycles that confirmed reliable paper-based SERS substrate even after the flexible handling and target extraction from curved surface. Considering the flexibility and absorbent properties of the paper-based SERS substrate, the developed sensor is expected
3.4. Pesticide detection The developed paper-based SERS substrates were evaluated for label-free detection of pesticides such as diquat (DQ) and paraquat (PQ) which are typical bipyridylium pesticides. Ten microliters of DQ and PQ solutions with concentrations of 50 ppm, 25 ppm, 10 ppm, 5 ppm, 1 ppm and 0.5 ppm were dropped onto 0.16 cm2 SERS substrates and dried. Then, as calculated for MB density, the amounts of both DQ and PQ additions were converted from their initial solution concentrations to 3.125 μg/cm2 (50 ppm), 1.56 μg/cm2 (25 ppm), 0.625 μg/cm2 375
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to be applicable as a label-free and field-specific pesticide prescreening SERS substrate for monitoring agricultural product distributions.
analchem.7b00300. [10] W. Zhang, B. Li, L. Chen, Y. Wang, D. Gao, X. Ma, A. Wu, Brushing, a simple way to fabricate SERS active paper substrates, Anal. Methods 6 (2014) 2066–2071, https:// doi.org/10.1039/C4AY00046C. [11] B. Li, W. Zhang, L. Chen, B. Lin, A fast and low-cost spray method for prototyping and depositing surface-enhanced Raman scattering arrays on microfluidic paper based device, Electrophoresis 34 (2013) 2162–2168, https://doi.org/10.1002/elps. 201300138. [12] W. Kim, S.H. Lee, Y.J. Ahn, S.H. Lee, J. Ryu, S.K. Choi, S. Choi, A label-free cellulose SERS biosensor chip with improvement of nanoparticle-enhanced LSPR effects for early diagnosis of subarachnoid hemorrhage-induced complications, Biosens. Bioelectron. 111 (2018) 59–65, https://doi.org/10.1016/j.bios.2018.04.003. [13] A.K. Yetisen, M.S. Akram, C.R. Lowe, Paper-based microfluidic point-of-care diagnostic devices, Lab Chip 13 (2013) 2210–2251, https://doi.org/10.1039/ C3LC50169H. [14] A.T. Singh, D. Lantigua, A. Meka, S. Taing, M. Pandher, G. Camci-Unal, Paper-based sensors: emerging themes and applications, Sensors (Basel) 18 (2018) 2838, https://doi.org/10.3390/s18092838. [15] W.W. Yu, I.M. White, Inkjet-printed paper-based SERS dipsticks and swabs for trace chemical detection, Analyst 138 (2013) 1020–1025, https://doi.org/10.1039/ c2an36116g. [16] R. Tantra, R.J. Brown, M.J. Milton, D. Gohil, A practical method to fabricate gold substrates for surface-enhanced Raman spectroscopy, Appl. Spectrosc. 9 (2008) 992–1000, https://doi.org/10.1366/000370208785793272. [17] R. Zhang, B.B. Xu, X.Q. Liu, Y.L. Zhang, Y. Xu, Q.D. Chen, H.B. Sun, Highly efficient SERS test strips, Chem. Commun. 48 (2012) 5913–59155, https://doi.org/10.1039/ C2CC31604H. [18] C.H. Lee, L. Tian, S. Singamaneni, Paper-based SERS swab for rapid trace detection on real-world surfaces, ACS Appl. Mater. Interfaces 2 (2010) 3429–3435, https:// doi.org/10.1021/am1009875. [19] F. Zeng, W. Duan, B. Zhu, T. Mu, L. Zhu, J. Guo, X. Ma, Paper-based versatile SERS chip with smartphone-based Raman analyzer for point of care application, Anal. Chem. 91 (2019) 1064–1070, https://doi.org/10.1021/acs.analchem.8b04441. [20] Y. Xu, P. Man, Y. Huo, T. Ning, C. Li, B. Man, C. Yang, Synthesis of the 3D AgNF/ AgNP arrays for the paper-based surface enhancement Raman scattering application, Sens. Actuators B Chem. 265 (2018) 302–309, https://doi.org/10.1016/j.snb. 2018.03.035. [21] C.K. Tanga, A. Vazea, J.F. Rusling, Paper-based electrochemical immunoassay for rapid, inexpensive cancer biomarker protein detection, Anal. Methods 6 (2014) 8878–8881, https://doi.org/10.1039/C4AY01962H. [22] X. Zou, Y. Wang, W. Liu, L. Chen, m-Cresol purple functionalized surface enhanced Raman scattering paper chips for highly sensitive detection of pH in the neutral pH range, Analyst 142 (2017) 2333–2337, https://doi.org/10.1039/C7AN00653E. [23] J. Hwang, S. Lee, J. Choo, Application of a SERS-based lateral flow immunoassay strip for the rapid and sensitive detection of staphylococcal enterotoxin B, Nanoscale 8 (2016) 11418–11425, https://doi.org/10.1039/C5NR07243C. [24] N. Jiang, X. Zhuo, J. Wang, Active plasmonics: principles, structures, and applications, Chem. Rev. 118 (2018) 3054–3099, https://doi.org/10.1021/acs.chemrev. 7b00252. [25] V. Devaraj, J.-M. Lee, J.-W. Oh, Distinguishable plasmonic nanoparticle and gap mode properties in a silver nanoparticle on a gold film system using three-dimensional FDTD simulations, Nanomaterials 8 (2018) 582, https://doi.org/10.3390/ nano8080582. [26] M.I. Markovic, A.D. Rakic, Determination of the reflection coefficients of laser light of wavelengths λε (0.22 μm, 200 μm) from the surface of aluminum using the Lorentz-Drude model, Appl. Opt. 29 (1990) 3479–3483, https://doi.org/10.1364/ AO.29.003479. [27] J.H. Ryu, P.B. Messersmith, H. Lee, Polydopamine surface chemistry: a decade of discovery, ACS Appl. Mater. Interfaces 10 (2018) 7523–7540, https://doi.org/10. 1021/acsami.7b19865. [28] K.C.L. Black, Z. Liu, P.B. Messersmith, Catechol redox induced formation of metal core-polymer shell nanoparticles, Chem. Mater. 23 (2011) 1130–1135, https://doi. org/10.1021/cm1024487. [29] H.Y. Son, K.R. Kim, J.B. Lee, T.H.L. Kim, J. Jang, S.J. Kim, M.S. Yoon, J.W. Kim, Y.S. Nam, Bioinspired synthesis of mesoporous gold-silica hybrid microspheres as recyclable colloidal SERS substrates, Sci. Rep. 7 (2017) 14728, https://doi.org/10. 1038/s41598-017-15225-8. [30] W. Haiss, N.T. Thanh, J. Aveyard, D.G. Fernig, Determination of size and concentration of gold nanoparticles from UV-vis spectra, Anal. Chem. 79 (2007) 4215–4221, https://doi.org/10.1021/ac0702084. [31] X. Han, Y. Liu, Y. Yin, Colorimetric stress memory sensor based on disassembly of gold nanoparticle chains, Nano Lett. 14 (2014) 2466–2470, https://doi.org/10. 1021/nl500144k. [32] V. Mondes, E. Antonsson, J. Plenge, C. Raschpichler, I. Halfpap, A. Menski, C. Graf, M.F. Kling, E. Rühl, Plasmonic electric near-field enhancement in self-organized gold nanoparticles in macroscopic arrays, Appl. Phys. B 122 (2016) 155, https:// doi.org/10.1007/s00340-016-6412-1. [33] R.X. He, R. Liang, P. Peng, Y.N. Zhou, Effect of the size of silver nanoparticles on SERS signal enhancement, J. Nanopart. Res. 19 (2017) 267, https://doi.org/10. 1007/s11051-017-3953-0. [34] X. Liu, M. Osada, K. Kitamura, T. Nagata, D. Si, Ferroelectric-assisted gold nanoparticles array for centimeter-scale highly reproducible SERS substrates, Sci. Rep. 7 (2017) 3630, https://doi.org/10.1038/s41598-017-03301-y. [35] L. Tong, T. Zhu, Z. Liu, Approaching the electromagnetic mechanism of surfaceenhanced Raman scattering: from self-assembled arrays to individual gold nanoparticles, Chem. Soc. Rev. 40 (2011) 1296–1304, https://doi.org/10.1039/
4. Conclusion In summary, we prepared AuNP-decorated paper-based SERS substrates with a facile and low-cost method without the addition of a reducing agent. The size of the synthesized AuNPs was controlled by changing the pH of the Au-precursor solution, and the particles formed under acidic conditions, which had an average particle size of 102 nm, showed the highest SERS activity. FDTD simulations suggested the origin of electromagnetic field enhancement originated from clustered AuNPs and individualized AuNPs. After characterization of the SERS activity and signal uniformity, the developed SERS substrate was applied to the label-free detection of bipyridylium pesticides such as DQ and PQ. Furthermore, the sensor was capable of being applied in the field, which was tested using pesticide-contaminated apple peels. Based on these results, flexible and water-absorbing paper-based substrates are expected to be applicable to a variety of label-free chemical SERS sensors. Notes The authors declare no competing financial interests. Acknowledgments This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry (IPET) through the Advanced Production Technology Development Program of the Ministry of Agriculture, Food and Rural Affairs (MAFRA) (31608004). This work was also supported by the Fundamental Research Program (PNK 6070) of the Korea Institute of Materials Science (KIMS). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.04.077. References [1] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Biosensing with plasmonic nanosensors, Nat. Mater. 7 (2008) 442–453, https://doi.org/10. 1038/nmat2162. [2] Y.W.C. Cao, R.C. Jin, C.A. Mirkin, Nanoparticles with Raman spectroscopic fingerprints for DNA and RNA detection, Science 297 (2002) 1536–1540, https://doi. org/10.1126/science.297.5586.1536. [3] P. Reokrungruang, I. Chatnuntawech, T. Dharakul, S. Bamrungsap, A simple paperbased surface enhanced Raman scattering (SERS) platform and magnetic separation for cancer screening, Sens. Actuators B Chem. 285 (2019) 462–469, https://doi. org/10.1016/j.snb.2019.01.090. [4] Y. Sun, Y. Zhang, Y. Shi, X. Xiao, H. Dai, J. Hu, P. Ni, Z. Li, Facile preparation of silver nanoparticle films as an efficient surface-enhanced Raman scattering substrate, Appl. Surf. Sci. 283 (2013) 52–57, https://doi.org/10.1016/j.apsusc.2013. 05.154. [5] N.A. Hatab, C.H. Hsueh, A.L. Gaddis, S.T. Retterer, J.H. Li, G. Eres, Z. Zhang, B. Gu, Free-standing optical gold bowtie nanoantenna with variable gap size for enhanced Raman spectroscopy, Nano Lett. 10 (2010) 4952–4955, https://doi.org/10.1021/ nl102963g. [6] Y.Y. Lin, J.D. Liao, Y.H. Ju, C.W. Chang, A.L. Shiau, Focused ion beam-fabricated Au micro/nanostructures used as a surface enhanced Raman scattering-active substrate for trace detection of molecules and influenza virus, Nanotechnology 22 (2011) 185308, https://doi.org/10.1088/0957-4484/22/18/185308. [7] V. Suresh, Ding Lu, A.B. Chew, F.L. Yap, Fabrication of large-area flexible SERS substrates by nanoimprint lithography, ACS Appl. Nano. Mater. 1 (2018) 886–893, https://doi.org/10.1021/acsanm.7b00295. [8] J.W. Jeong, M.M.P. Arnob, K.-M. Baek, S.Y. Lee, W.-C. Shih, Y.S. Jung, 3D crosspoint plasmonic nanoarchitectures containing dense and regular hot spots for surface-enhanced Raman spectroscopy analysis, Adv. Mater. 28 (2016) 8695–8704, https://doi.org/10.1002/adma.201602603. [9] W. Kim, J.C. Lee, G.J. Lee, H.K. Park, A. Lee, S. Choi, Low-cost label-free biosensing bimetallic cellulose strip with SILAR-synthesized silver core-gold shell nanoparticle structures, Anal. Chem. 89 (2017) 6448–6454, https://doi.org/10.1021/acs.
376
Sensors & Actuators: B. Chemical 291 (2019) 369–377
V.T.N. Linh, et al.
nanopillar arrays with vertically integrated nanogaps for SERS‐active substrates, Adv. Funct. Mater. 25 (2015) 4681–4688, https://doi.org/10.1002/adfm. 201501274. [40] M.K. Lee, T.Y. Jeon, C.W. Mun, J. Kwon, J. Yun, S.H. Kim, D.H. Kim, S.-C. Chang, S.G. Park, Multilayered plasmonic nanostructures with high areal density for SERS, RSC Adv. 7 (2017) 17898–17905, https://doi.org/10.1039/C6RA28150H. [41] M. Kaya, M. Volkan, New approach for the surface enhanced resonance Raman scattering (SERRS) detection of dopamine at picomolar (pM) levels in the presence of ascorbic acid, Anal. Chem. 84 (2012) 7729–7735, https://doi.org/10.1021/ ac3010428. [42] H. Fang, X. Zhang, S.J. Zhang, L. Liu, Y.M. Zhao, H.J. Xu, Ultrasensitive and quantitative detection of paraquat on fruits skins via surface-enhanced Raman spectroscopy, Sens. Actuators B 213 (2015) 452–456, https://doi.org/10.1016/j. snb.2015.02.121.
C001054P. [36] F. Benz, R. Chikkaraddy, A. Salmon, H. Ohadi, B. de Nijs, J. Mertens, C. Carnegie, R.W. Bowman, J.J. Baumberg, SERS of individual nanoparticles on a mirror: size does matter, but so does shape, J. Phys. Chem. Lett. 7 (2016) 2264–2269, https:// doi.org/10.1021/acs.jpclett.6b00986. [37] T. Lee, J.S. Wi, A. Oh, H.K. Na, J. Lee, K. Lee, T.G. Lee, S. Haam, Highly robust, uniform and ultra-sensitive surface-enhanced Raman scattering substrates for microRNA detection fabricated by using silver nanostructures grown in gold nanobowls, Nanoscale 10 (2018) 3680–3687, https://doi.org/10.1039/C7NR08066B. [38] E.H. Koh, C. Mun, C. Kim, S.G. Park, E.J. Choi, S.H. Kim, J. Dang, J. Choo, J.-W. Oh, D.H. Kim, H.S. Jung, M13 Bacteriophage/silver Nanowire surface-enhanced Raman scattering sensor for sensitive and selective pesticide detection, ACS Appl. Mater. Interfaces 10 (2018) 10388–10397, https://doi.org/10.1021/acsami.8b01470. [39] T.Y. Jeon, S.-G. Park, D.-H. Kim, S.-H. Kim, Standing‐wave‐assisted creation of
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