Stable, Flexible, and High-Performance SERS Chip Enabled by a

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A Stable, Flexible and High-Performance SERS Chip Enabled by Ternary Films-Packaged Plasmonic Nanoparticles Array Kaiqiang Wang, Da-Wen Sun, Hongbin Pu, Qing-yi Wei, and Lunjie Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09746 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Materials & Interfaces

A Stable, Flexible and High-Performance SERS Chip Enabled by Ternary Films-Packaged Plasmonic Nanoparticles Array

Kaiqiang Wang1,2,3, Da-Wen Sun1,2,3,4 , Hongbin Pu1,2,3, Qingyi Wei1,2,3, Lunjie Huang1,2,3

School of Food Science and Engineering, South China University of Technology, Guangzhou 510641, China

1

2

Academy of Contemporary Food Engineering, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou 510006, China

3

Engineering and Technological Research Centre of Guangdong Province on Intelligent Sensing and Process Control of Cold Chain Foods, Guangzhou Higher Education Mega Center, Guangzhou 510006, China 4

Food Refrigeration and Computerized Food Technology (FRCFT), Agriculture and Food Science Centre, University College Dublin, National University of Ireland, Belfield, Dublin 4, Ireland

ABSTRACT The high sensitivity and long-term storage stability of a plasmonic substrate are vital for practical applications of the surface-enhanced Raman scattering (SERS) technique in real world analysis. In this study, a rationally designed ternary films-packaged silver-coated-gold nanoparticles (Au@Ag NPs) plasmonic array was fabricated and applied as a stable and high-performance SERS chip for highly sensitive sensing of thiabendazole (TBZ) residues in fruit juices. The ternary films played different roles in the plasmonic chip: a newborn poly(methyl methacrylate) (PMMA) film serving as a template for fixing the self-assembly closely-packed monolayer Au@Ag NPs array that provided intensive hotspot; a fluorescent quantitative polymerase chain reaction adhesive film (qPCR film) acting as a carrier to retrieve Au@Ag/PMMA film that was used to improve the robustness of the plasmonic array;



Corresponding author. School of Food Science and Engineering, South China University of Technology, Guangzhou

510641, China. Email: [email protected], URLs: http://www.ucd.ie/refrig; http://www.ucd.ie/sun 1

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and a polyethylene terephthalate (PET) film covered over the Au@Ag/PMMA/qPCR film performing as a barrier to improve the stability of the chip. The Au@Ag/PMMA/qPCR-PET film chip showed high sensitivity with an enhancement factor of 3.14x106, long-term storage stability without changing SERS signals for more than 2 months at room temperatures, and low limit of detection for sensing TBZ in pear juice (21 ppb), orange juice (43 ppb) and grape juice (69 ppb). In addition, the procedure for fabricating the Au@Ag/PMMA/qPCR-PET film SERS chip was easy to handle, offering a new strategy to develop flexible and wearable sensors for on-site monitoring chemical contaminants with a portable Raman spectrometer in the future. KEYWORDS: surface-enhanced Raman scattering, Au@Ag nanoparticles, plasmonic chip, PET film, storage stability

1. INTRODUCTION Surface-enhanced Raman scattering (SERS) as an effective and sensitive vibrational spectroscopic technique shows increasingly attraction since its discovery. The basic concept of SERS effect is that the Raman scattering cross sections of molecules located in the vicinity of plasmonic nanostructures can be tremendously enhanced by the amplification of electromagnetic fields resulted from the excited localized surface plasmon resonances.1 Due to their sensitive, rapid, nondestructive and fingerprinting characteristics, SERS-based techniques have provided unprecedented opportunities for detection of chemicals and microorganisms in ultra-low levels,2−5 and they are anticipated to make significant impacts on nanotechnology, materials science, life science and environmental engineering.6−11 Generally, gold, silver and bimetallic silver-coated gold nanostructures based substrates have been widely used for SERS analysis, and the enhancement performance of SERS-active substrates is closely related to the geometric shape, size, orientation and the gap of adjacent metallic nanostructures.12−14 In the past decades, with great advances in nanotechnology, a variety of high-performance SERSactive substrates have been developed, including nanoflower,15 nanocompass,16 nanocube,17 ordered 2


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nanoparticle arrays,18 flexible substrates,19−21 etc. Although silver-based nanostructures could achieve excellent SERS activity, silver metal is susceptible to oxidation in air, which inevitably affects Raman scattering enhancement during prolonged storage. In general, substrates stored under low temperature, vacuum dry condition or with inert gas protection could maintain the SERS activity to a certain extent.22−23 In some studies, to further extend the storage stability of substrates, researchers have attempted to coat the plasmonic nanostructures with other chemicals or materials. For example, Potara et al. demonstrated that chitosan-coated anisotropic silver nanoparticles were particularly stable, which could serve as a versatile plasmonic sensor.24 Jiang et al. reported that mesoporous silica microspherescoated Ag NPs could store for a long time without reducing the SERS properties.25 The deposition of silver surface with a thin layer of alumina or gold were also found in improving the stability of substrates.17,26 In addition, graphene was shown to be highly impermeable to gas and liquid, thus the graphene-silver nanocomposites showed enhanced durability compared with bare silver plasmonic nanoparticles.27,28 Nevertheless, most of the above-described available methods are complicated and drawbacks still exist in these substrates during long-time storage at room temperature and routine analysis. As a result, developments of novel and simple strategies to fabricate stable and highperformance SERS sensors with ordered plasmonic nanoparticles for achieving sensitive and on-site SERS analysis are required. Polymer films, such as polyethylene terephthalate (PET) film, polymethylmethacrylate (PMMA) film, polyethylene (PE) film and polyvinylchloride (PVC) film, have been widely used in various fields for wrapping goods, in particular, plastic wraps, initially created from PVC, play an important role in extending the shelf-life and maintaining the quality of food. PET films are now commonly used as a screen protector to protect electronic devices against damage. To the best our knowledge, no works so far have combined the polymer films for maintaining the stability of plasmonic chip. Inspired by the above, in the current study, a novel strategy was proposed to fabricate a stable and flexible platform with high-performance SERS activity, which was defined as Au@Ag/PMMA/qPCR3



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PET film chip. The procedure for fabricating this chip is shown in Scheme 1. Firstly, the core-shell Au@Ag nanoparticles array was formed at oil/water interface, and it was fixed by a newborn PMMA template after the spontaneous evaporation of organic phase. The highly-packed ordered nanoparticles array could generate intensive hot spots and uniform SERS signals. Afterwards, to improve the mechanical strength of the metal-polymer, a highly transparent adhesive film for fluorescent quantitative polymerase chain reaction (qPCR film) was used as a carrier to retrieve the prepared Au@Ag/PMMA array. In addition, a PET film was covered over the plasmonic nanoparticles to protect them against external environments. To our knowledge, this was the first time to application of PET film to extend the shelf-life of the SERS chip stored at ambient temperatures. The developed flexible ternary films-packaged plasmonic nanoparticles array chip was expected showing not only excellent SERS activities but also good mechanical property and storage stability, and it was thus anticipated to provide a basis for developing stable, wearable and portable SERS sensors in the future.

2. EXPERIMENTAL SECTION 2.1. Chemicals Chloroauric acid (HAuCl4·4H2O), toluene and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Trisodium citrate, ascorbic acid, silver nitrate (AgNO3), Rhodamine 6G (R6G), 4-Mercaptobenzoic acid (4-MBA), thiabendazole (TBZ), thiram, crystal violet, malachite green and orange ΙΙ were obtained from Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China). Adhesive film for fluorescent quantitative PCR (qPCR film) was brought from Shanghai Huake (SHHK) Co., Ltd. (Shanghai, China), and polyethylene terephthalate (PET) film was supplied by Suzhou BoYan Jingjin Photoelectric Co., Ltd. (Suzhou, China). Ultrapure water prepared by a Milli-Q system (Millipore Corp., Bedford, USA) was used throughout the study to prepare solutions. The glassware and magnetic stirring bars used for synthesizing gold nanoparticles (Au NPs) and silvercoated gold nanoparticles (Au@Ag NPs) were cleaned with aqua regia (HCl:HNO3 = 3:1, v/v) and 4


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rinsed with ultrapure water.

2.2. Fabrication of Au NPs and core-shell Au@Ag NPs The procedure for synthesizing Au NPs was performed according to the classic citrate reduction route reported by Frens (1973).29 The fresh prepared Au NPs colloid was centrifuged for 15 min at 6000 rpm and washed with ultrapure water for three times for further use. The core-shell Au@Ag NPs were fabricated by a seed-mediate growth method. In brief, 3 mL of as-prepared Au NPs was injected into a 10 mL centrifuge tube (Sangong Biotech Co., Ltd., Shanghai, China). After that, 60 μL of 1% trisodium citrate and 120 μL of 10 mM ascorbic acid were added into the solution. To deposit Ag shell (with the thickness of 5.2 nm) over the Au NPs, 120 μL of 10 mM AgNO3 solution was dropwise added to the mixture at a rate of one drop per 40 s under vigorous shaking by a IKA MS 3 digital shaker (IKA Inc., Staufen im Breisgau, Germany). The solution was continuously shaking in the dark for 30 min at 25 °C.

2.3. Protocol for interfacial self-assembly Au@Ag/PMMA plasmonic array The beaker glassware used for preparing the interfacial self-assembly Au@Ag/PMMA plasmonic array was cleaned by immersing in acetone for 2 h, followed by rinsing with ultrapure water and drying under 60 °C. The interfacial self-assembly Au@Ag/PMMA array was formed at the organic/aqueous interfacial system. In details, a portion of 3 mL fresh prepared Au@Ag NPs colloid was injected into the above treated beaker glassware, followed by adding 1 mL of PMMA toluene solution. Afterwards, in order to entrap Au@Ag NPs to organic/aqueous interface, 1.5 mL of ethanol was rapidly injected into the solution by a syringe. After the toluene evaporated spontaneously at room temperature, a thin newborn PMMA template was formed, and the entrapped Au@Ag NPs were self-assembled into an orderly Au@Ag NPs layer fixed by the PMMA template.

5



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2.4. Preparation of the Au@Ag/PMMA/qPCR-PET film platform After the formation of Au@Ag/PMMA array, a qPCR film was used as a carrier to fix the Au@Ag/PMMA array. After that, a PET film was used to cover over the substrate, which was defined as an Au@Ag/PMMA/qPCR-PET film chip. The qPCR film was found to be fastened tightly to the Au@Ag/PMMA array, and the PET film served as a barrier to protect Au@Ag NPs from oxidation during storage. During SERS experiments, the PET film should be removed from the platform. The whole procedure for fabricating the platform did not require complicated and expensive equipment, and could be completed within a few hours.

2.5. Preparation of SERS samples To evaluate the SERS enhancement factor (EF), uniformity and reproducibility of the Au@Ag/PMMA/qPCR-PET film chip, 5 μL of R6G solution was dropped onto the film. To study the storage stability of the platform, Au NPs were modified with 30 μL 4-MBA probe molecules (10−5 M) before fabrication of the Au@Ag/PMMA/qPCR-PET film chip. After that, this chip was stored at room temperature for two months, and the SERS intensities of 4-MBA molecules at different storage times of 0, 5, 10, 20, 30 and 60 days were detected. To investigate the SERS sensitivity for chemical contaminants, a series of concentrations of analytes were dropped on the platform for SERS measurements. For the detection of TBZ in fruit juices (pear juice, orange juice and grape juice), the juices spiked with different levels of TBZ were prepared, which were then centrifuged at 6000 rpm for 10 min. The suspension was used for SERS detection directly with a laser confocal Raman microscope system (LabRAM HR, Horiba France SAS, Villeneuve d'Ascq, France), equipped with a 50× objective lens and a 633 nm He-Ne laser as the excitation source (laser power: 4.25 mW). The acquisition time was 15 s with 2 accumulations.

2.6. Characterization 6


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The UV-Vis absorbance spectra of Au NPs, Au@Ag NPs colloid and Au@Ag/PMMA film were obtained from a spectrophotometer (UV-1800, Shimadzu Co., Kyoto, Japan). The spectra were recorded between 300 nm and 900 nm. The X-ray diffraction (XRD) patterns of nanoparticles were acquired using an X-ray diffractometer (Empyrean, PANalytical B.V., Almelo, Netherlands). The scanning electron microscopy (SEM) images were collected from a Zeiss Merlin field emission scanning electron microscope (Carl Zeiss NTS GmbH, Oberkochen, Germany). Transmission electron microscopy (TEM) images were taken from a transmission electron microscope (JEM-1400 Plus, JEOL Ltd., Tokyo, Japan). High-resolution TEM images and scanning transmission electron microscopy energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping images were taken from a JEM-2100F Plus high-resolution transmission electron microscope (JEOL Ltd., Tokyo, Japan) operating at an accelerating voltage of 200 kV. The surface morphology of the Au@Ag/PMMA/qPCR-PET film chip was recorded by an atomic force microscopy (AFM) (CSPM 5500, Benyuan Nano-Instrument, Guangzhou, China) equipped with a silicon AFM probe (force constant = 40 N/m) under tapping mode (resonance frequency = 75 kHz).

3. RESULTS AND DISCUSSION 3.1. Fabrication and characterization of the film chip Figure 1 illustrates the procedure for fabricating the ternary films-packaged plasmonic nanoparticles array chip, and the plasmonic nanoparticles and film chip were characterized by XRD, UV-Vis spectra, TEM, SEM, and AFM techniques. As shown in Figure 1A, the synthesized Au NPs showed uniform morphology with the average diameter of about 32 nm. The XRD patterns of Au NPs indicated that five diffraction peaks were observed at 38.1°, 44.2°, 64.5°, 77.4° and 81.6°, which could be assigned to the (111), (200), (220), (311) and (222) reflections of face-centered cubic (fcc) gold, respectively (JCPDS No. 04-0784) (Figure S1). The result revealed high crystallinity of the synthesized Au NPs. Due to similar lattice constants of Au and Ag, the core-shell Au@Ag NPs can be synthesized by 7



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depositing Ag shell over the as-prepared Au NPs.30 In comparison with the Au NPs and Ag NPs, the core-shell Au@Ag NPs showed excellence in both size-uniformity and SERS sensitivity, which were critical to fabricate the plasmonic array with highly sensitivity and well reproducibility at oil/water interface. The TEM observation indicated that Au core was uniformly covered over by Ag shell with an average thickness of 5.2 nm (Figure 1B). The inset in Figure 1B illustrates high-resolution TEM images in the Ag shell region, suggesting that the lattice fringes was 0.234 nm. This value matched well with the lattice constant of (111) planes of fcc Ag (0.2358nm), revealing that the Ag nanocrystal predominantly grew along the (111) direction. In addition, the STEM-EDS elemental mapping in Figure 1C shows that the Ag element was distributed on the surface of Au core, further evidencing the successful fabrication of core-shell Au@Ag architecture. In addition, the core-shell Au@Ag NPs with different Ag shell thickness (1.1−7.3 nm) can be found in Figure S2 and Figure S3. The UV-Vis spectra of Au NPs showed a strong absorption characteristic band centered at 525 nm due to the surface plasmon resonance (SPR) (Figure 2A). After the deposition of Ag shell, the color of the colloid changed from purple to orange, and the UV-Vis spectra of synthesized Au@Ag NPs had two SPR bands at 488 nm and 395 nm, corresponding to the plasmon resonances of Au core and Ag shell, respectively. Due to the deposition of Ag shell, the SPR band of the Au core was gradually attenuated and even completely disappeared with increasing the thickness of Ag shell (Figure S4). The synthesized Au@Ag NPs were used to prepare monolayer plasmonic array by the liquid-liquid interfacial self-assembly method. In this study, toluene containing PMMA was used as organic phase and was poured over Au@Ag NPs colloid. Ethanol as inducer was rapidly injected into the system to decrease Au@Ag NPs surface charge, increasing the particle/water surface tension and driving the Au@Ag NPs to the organic/aqueous interface (Scheme 1).31 This allowed Au@Ag NPs to rapidly move to the organic/aqueous interface and eventually coalesced into a monolayer array, as demonstrated by UV-Vis spectra, TEM and AFM observations. After the injection of ethanol, the color of Au@Ag NPs colloid turned into pale yellow, and the UV-Vis spectrum absorbance intensity of the 8


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residual Au@Ag NPs colloid decreased remarkably (Figure 2A). With the spontaneous evaporation of toluene, the monolayer Au@Ag NPs array was fixed with a thin newborn PMMA template. In comparison with the UV-Vis spectrum of Au@Ag NPs colloid, the absorption band of Au@Ag/PMMA film showed an obvious red shift from 488 nm to 521 nm. In addition, a new absorption band at 789 nm was observed. These results could be attributed to the strong plasmonic coupling between tightly adjacent Au@Ag NPs, demonstrating the formation of self-assembly plasmonic array on PMMA film.32 The SEM images in Figure 1E−G and Figure S5 further verified that Au@Ag NPs were closely packed into a two-dimensional array on the PMMA template surface. In this study, we chose the plasmonic array with Ag shell of 5.2 nm for further study (Figure S6). For this plasmonic array, the average gap between the adjacent Au@Ag NPs was less than 5 nm, which would offer numerous “hot spots” for amplifying the Raman scattering signals of molecules absorbed on the array. In addition, AFM was carried out to validate the SEM analysis of the array and the dimension of Au@Ag/PMMA film. The two-dimensional AFM reconstruction of Au@Ag NPs indicated the formation of closely packed nanoparticles array on PMMA film. According to the height profile in Figure 2B, the height of nanoparticles on PMMA film was less than 50 nm, which again confirmed the diameter of Au@Ag NPs as observed from TEM analysis.

3.2. Effects of PMMA content on SERS performance of film chip PMMA was added into toluene to form a template for fixing the self-assembly Au@Ag NPs array. Previous studies have demonstrated that the dosage of polymer would affect the structure and SERS performance of the plasmonic array.31,32 Herein, the thickness of the PMMA layer was controlled by tuning the PMMA dosage from 0.65 to 13 mg/cm2. As shown in Figure 3A, R6G was used as the probe molecule to characterize the SERS activity of Au@Ag/PMMA. For adding PMMA at 0.65 mg/cm2, the SERS intensity of R6G at 612 cm−1 was 7787±652. With increasing the PMMA dosage 9



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to 2.6 mg/cm2, the SERS intensity of R6G was increased more than fourfold. As indicated in the morphologies in Figure 3B, the Au@Ag NPs were not closely packed and there were many voids in the array for the PMMA at low dosages. However, with increasing the dosage of PMMA from 0.65 to 2.6 mg/cm2, no obvious voids were observed and the Au@Ag NPs assembled together, becoming closer. Previous studies also found that more layers of nanoparticles could be trapped in newborn template with increasing the polymer content.31,32 The UV-Vis spectra of Au@Ag/PMMA in Figure 3C showed that a new absorption band at 820 nm was shifted to 789 nm as the dosage of PMMA increased from 0.65 to 2.6 mg/cm2. This might be due to the strong plasmonic coupling between closer adjacent Au@Ag NPs. Therefore, the ordered and tightly packed array on 2.6 mg/cm2 PMMA film could offer abundant hot spots and strong electromagnetic field enhancement. Besides, Figure 3 revealed that although it was possible to entrap more Au@Ag NPs at the organic/aqueous interface and formed multi-layers array with further increasing PMMA dosages from 2.6 to 13 mg/cm2, however, the SERS intensity of R6G and the UV-Vis absorption band at around 750 nm were decreased accordingly, especially for 13 mg/cm2 PMMA. These could be resulted from that nanoparticles were completely covered inside the polymer component when polymer dosage exceeded a certain limit (Figure 3B5), which might affect the plasmonic coupling between adjacent Au@Ag NPs. Therefore, the dosage of 2.6 mg/cm2 PMMA was used to prepare Au@Ag/PMMA/qPCR-PET film chip in the subsequent studies.

3.3. The stability of Au@Ag/PMMA/qPCR-PET film chip The excellent mechanical stability of substrate is a crucial requirement for achieving routine SERS detection. In the case of the Au@Ag/PMMA film substrate, it was fragile and thus not suitable for long-distance carry. In previous studies, PE film was used to support the Au-NPs/PMMA substrate, while the substrate may be broken away from PE film during a long-term detection.17,32 In order to improve the stability of the substrate for practical analysis, the Au@Ag/PMMA substrate was 10


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transferred to a highly transparent qPCR film. Generally, PCR reaction depends on thermal cycling for facilitating DNA melting and replication. Except the highly transparent and adhesive properties, the qPCR film for sealing PCR well plate is also heat-resistant and water-proof.33 As can be seen from Figure 1D, the Au@Ag/PMMA/qPCR film was flexible, which was able to be folded by hand. The cross-sectional SEM image in Figure 1F showed that the thickness of the Au@Ag/PMMA film was 42 μm, and the qPCR film was about 160 μm in thickness. As a result, the observation demonstrated that the as-prepared Au@Ag/PMMA/qPCR film was thin, which is critical in developing flexible and wearable sensors for on-site analysis. The storage stability also needs be considered for long-distance transportation and routine analysis, as Ag nanoparticles are easily oxidized and their enhancement performances are reduced during prolonged storage. Previous methods for protecting plasmonic nanoparticles were complicated. PET film is one of the flexible packaging materials. Due to it lightweight, low-cost, high transparency, high stability, as well as good gas and moisture barrier properties, it has been widely used for prepackaged foodstuffs and electronic devices.34,35 In the current study, as inspired by the excellent packaging properties of PET film, for the first time, an attempt was made in the current study to cover the surface of Au@Ag/PMMA/qPCR film with PET film to improve the storage stability of SERS substrate. The thickness of the high transparency PET film used in this study was 30 μm (Figure S7). Due to the highly self-adhesive properties of qPCR film, the PET film can tightly combine with qPCR film. As a result, the oxygen between PET film and Au@Ag/PMMA/qPCR film could be evacuated, and the PET film as a barrier would protect the nanoparticles against external environment damage. To evaluate the storage stability, the Au@Ag/PMMA/qPCR film with and without PET film protection stored in room temperature for different periods were investigated in details. Figure 4A shows the SERS intensity of Raman tags (4-MBA), indicating that the SERS signals from Au@Ag/PMMA/qPCR film without PET film protection was unstable with increasing the storage periods. Notably, the Raman intensity of 4MBA at 1075 cm−1 was reduced by more than 70% after storage for 60 days. In contrast, after PET 11



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film protection, the obtained Au@Ag/PMMA/qPCR-PET film plasmonic chip still kept its good enhancement performance after storage for 60 days. The XRD patterns and SEM images of the Au@Ag/PMMA/qPCR film with and without PET film protection are shown in Figure S8. In comparison, the intensity of (111) diffraction peak was decreased, and the morphology of Au@Ag NPs seemed to be coarsened without the protection of PET film. These changes would be resulted from the oxidation of the surface of Au@Ag NPs during storage.36,37 The results demonstrated that the thin PET film should provide a protective barrier to the oxidation of the silver nanostructures during storage, thus maintaining the stable SERS signals from the plasmonic chip. In addition, the SERS spectra of the probe molecule collected from Au@Ag/PMMA film, Au@Ag/PMMA/qPCR film and Au@Ag/PMMA/qPCR-PET film chip are compared in Figure 4B. It was found that the SERS intensity of 4-MBA at 1075 cm−1 was similar, indicating that the highly transparent qPCR film and the process of “covering and peeling of” PET film did not affect the Raman enhancement effects of the Au@Ag NPs array fixed on PMMA film. In order to further evidence the stability of Au@Ag/PMMA/qPCR-PET film plasmonic chip, the robustness characteristics of the chip exposed to strong external forces (hydrothermal and ultrasonic treatments) were investigated. As shown in Figure 4B, the SERS spectra of 4-MBA from the Au@Ag/PMMA/qPCR-PET film chip after hydrothermal (80 °C for 30 min) and ultrasonic (15 W for 15 min) treatments were plotted. It was clearly indicated that the Au@Ag/PMMA/qPCR-PET film chip could still hold the original SERS enhancement performance even under strong external forces. Therefore, the developed Au@Ag/PMMA/qPCR-PET film chip showed excellent stability for SERS detection.

3.4. SERS activity, uniformity and reproducibility of film chip The high SERS activity of a substrate is important to enable the platform to identify analytes even at a low concentration. As the SERS activity of a substrate is closely related to the enhancement of the local electromagnetic field of nanostructures, the electromagnetic field distribution around the 12


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Au@Ag/PMMA/qPCR-PET film chip was simulated and shown in Figure 5A, clearly indicating that hot spots with strong electromagnetic field were formed in the gap regions between Au@Ag NPs, which confirmed that the Au@Ag/PMMA/qPCR-PET film plasmonic chip possessed strong SERS enhancement effects. To further evaluate the SERS activity of the chip, R6G was selected as a probe molecule and its Raman enhancement factor (EF) can be calculated according to the following equation: EF =

𝐼𝑆𝐸𝑅𝑆 𝐼𝑁𝑅

𝑁𝑁𝑅

(1)

× 𝑁𝑆𝐸𝑅𝑆

where ISERS and INR represent the SERS intensity of R6G absorbed on Au@Ag/PMMA/qPCR-PET film chip and normal Raman intensity of R6G without substrate, and NSERS and NNR are the corresponding number of molecules detected in the laser excitation area in SERS measurements and normal Raman measurements, respectively.38 The highest peak intensity of SERS spectrum from R6G molecule located at 612 cm−1 was selected to calculate EF. Based on Equation (1), the intensity of the peak at 612 cm−1 was amplified with an EF value of 3.14x106 under 633 nm laser (the calculation can be seen in Supporting Information). To verify the sensitivity of our developed SERS chip, we detected different kinds of chemicals, including pesticide (thiram), disinfectants (crystal violet and malachite green) and pigment (orange ΙΙ), using this plasmonic chip. As shown in Figure S9, the characteristic peaks of these four chemicals even in low concentrations could be discerned and enhanced by the SERS chip. Therefore, these results demonstrated that the plasmonic Au@Ag/PMMA/qPCR-PET film chip showed excellent enhancement performance, which enabled this platform to sense analytes in low-concentrations. Besides the SERS activity, uniformity and reproducibility are another two properties that should be considered for realizing reliable SERS quantitative analysis. In order to demonstrate the uniformity of this new substrate, SERS mapping was carried out across an area of 400 μm2 in the Au@Ag/PMMA/qPCR film. The collected 441 Raman spectra of R6G and their Raman mapping images at 612, 1180 and 1362 cm−1 are shown in Figure 5B and 5C. Each pixel of the mapping images 13



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indicated the SERS intensity of R6G at a spatial position on the chip. With regards to these three peaks, the color distributions on their Raman mapping images were uniform and no obvious fluctuations were observed. The relative standard deviation (RSD) of SERS signal intensities at 612, 1180 and 1362 cm−1 were calculated to be 10.54%, 9.42% and 9.17%, respectively. In addition, in order to examine the batch-to-batch reproducibility of the substrate, the SERS spectra of R6G at 50 randomly sites from 5 batches of Au@Ag/PMMA/qPCR-PET film chip were determined and their SERS intensity variations at 612 cm−1 are depicted at Figure 5D. The RSD value was calculated to be 11.58%. In previous studies, it was reported that the spot-to-spot or substrate-to-substrate RSD values of a novel SERS-active substrate for quantification analysis should be less than 20%.39 Therefore, the current excellent SERS performance with well uniformity and reasonable reproducibility ensured that the Au@Ag/PMMA/qPCR film chip could serve as a SERS platform for realistic applications.

3.5. Practical detection of TBZ in water and fruit juices using the proposed chip In recent years, unreasonable uses of pesticides in agricultural production has led to great concern on environment and public health. Developments of rapid and accurate techniques to detect pesticides contamination are thus important for identifying unsafe foods and controlling the abuse of pesticides.40 TBZ is a kind of fungicides commonly used to control fungi diseases both in fruit development and postharvest

storage.

To

demonstrate

the

practical

applications

of

the

developed

Au@Ag/PMMA/qPCR-PET film plasmonic chip substrate, TBZ in water and fruit juices were quantitatively detected using SERS method. Figure 6A1 shows the SERS spectra of TBZ in water with concentrations from 0 to10 ppm, and the Au@Ag/PMMA/qPCR-PET film showed weak background during SERS detection, which is crucial for practical quantitative analysis (Figure S10). As the concentrations of TBZ increase, several characteristic peaks of TBZ at 782, 880, 1006, 1194, 1277, 1321, 1402, 1576 and 1624 cm−1 were clearly observed. The assignments of these peaks are listed in Table S1. The peaks at 782 cm−1 attributed to C-S stretching and in-plane C=N bending, and 14


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that at 1006 cm−1 assigned to C-N stretching and C-C stretching were the two most intense peaks.41 Notably, these two peaks could still be obviously discerned even when the concentration of TBZ in water was decreased to 0.05 ppm. Therefore, these two peaks were selected to build the calibration curves of SERS intensities as a function of TBZ concentrations. Figure 6B1 and Figure 6C1 suggested that the logarithmic SERS intensities at 782 and 1006 cm−1 showed linear correlations with the logarithmic concentrations of TBZ from 0.05 to 10 ppm. The calibration curves were y=0.768x+3.585 and y=0.776x+3.522 with coefficient of determination (R2) of 0.989 and 0.983, respectively. In addition, the limit of detection (LOD) of this method was calculated by the follwing equation: LOD = 3σ/k

(2)

where σ represents the standard deviation of the SERS intensity at 782 or 1006 cm−1 from blank samples, and k is the slope of above established calibration curves. The LOD values of TBZ in water were calculated to be 20 and 26 ppb by targeting the Raman intensities at 782 and 1006 cm−1, respectively. Furthermore, the residue of TBZ in three kinds of fruit (pear, orange and grape) juices were also detected using the proposed chip. As illustrated in Figure 6, the characteristic peaks of TBZ at 782 and 1006 cm−1 could also be clearly identified. The LODs for pear juice, orange juice and grape juice were found to be as low as 21, 43 and 69 ppb, respectively (Table S2). The performance of this method was compared with other available methods reported in literatures (Table S3). It was indicated that the traditional HPLC and fluorescence methods, as well as the surface plasmon resonance biosensor showed lower LODs for detecting TBZ in food matrices. However, these approaches required tedious sample extraction or complicated clean up procedures. In comparison, SERS method is an emerging rapid analytical method. As shown in Table S3, many plasmonic nanoparticles-based SERS methods have been attempted to analyze TBZ in food matrices with LODs in the range of 0.06 – 4 ppm. Notably, Kim et al. reported that the gold nanofinger SERS sensor could be used for detection of TBZ on apple skin with LOD of 7 ppb, but the fabrication of the gold nanofinger structures included the nanoimprint 15



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lithography and e-beam evaporation techniques, which was complex and high production cost.42 In comparison, our developed ternary films-packaged Au@Ag NPs array SERS sensor was fabricated through the liquid-liquid interfacial self-assembly method. The fabrication process was easy to handle and no need for complex engineering, and more cost-effective. Most importantly, this SERS-active substrate showed unparalleled superiority in the long-term storage stability under room temperature. In the case of detecting TBZ in fruit juices, the LOD of our developed Au@Ag/PMMA/qPCR-PET film SERS chip was better than those obtained using standing gold nanorod arrays and silver colloid for detecting TBZ in fruit juices.43,44 In addition, the recovery experiments were carried out and listed in Table 1 and it was found that satisfactory recoveries (85%-139%) were accomplished for detecting TBZ using the current method, indicating that our developed Au@Ag/PMMA/qPCR-PET film plasmonic chip could be used as a SERS sensor for practically sensing contaminants in complex matrixes. On the other hand, multiple-analytes detection is a superiority of SERS technique due to its unique fingerprint effects.45,46 Therefore, the multiplex detection capability of the Au@Ag/PMMA/qPCRPET film chip was also investigated in the current study. Two widely used fungicides for fruit, TBZ and thiram, were added into orange juice for simultaneous measurements. The SERS spectra of orange juice containing TBZ, thiram and their mixture are displayed in Figure S11. Compared with these three spectra, the vibrational bands of TBZ (782, 1006, 1194 and 1576 cm−1) and thiram (562, 929, 1146, 1384 and 1514 cm−1) could still be discerned in the dual-analytes SERS spectra, revealing a good capability of the ternary films-packaged plasmonic chip for distinguishing dual-analytes fingerprints in real samples.

4. CONCLUSIONS In the current study, summary, a simple and high-throughput method for fabricating a stable ternary films-packaged bimetallic Au@Ag plasmonic chip as a robust SERS sensor was developed. The 16


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detection of food contaminant (thiabendazole) both in water and fruit juices over a wide concentration range using the proposed plasmonic chip were demonstrated, showing that the LOD could be as low as 20 ppb. In regard to the ternary films-packaged plasmonic chip, the PMMA and qPCR films for fixing and supporting the Au@Ag NPs could maintain the closely packed plasmonic array structure even under strong external forces. In addition, the PET film covered over the plasmonic chip showed unparalleled superiority for improving the long-term storage stability of the SERS sensor under room temperature, and the entire fabrication process were low-cost, easy to handle and no need for complex engineering. As a result, the proposed chip with high-density hotspots not only had high sensitivity, good uniformity and reproducibility for quantitative SERS analysis, but also showed excellent stability for real world analysis. It is therefore anticipated that the inexpensive, flexible and stable Au@Ag/PMM/qPCR-PET film chip could sever as a promising candidate for on-site monitoring of a variety of chemicals in the environmental and food industry. More importantly, the proposed simple strategy of protecting SERS substrate with polymer films provided a new insight for developing more stable and wearable sensors for on-site detection in the future.

ASSOCIATED CONTENT Supporting Information XRD patterns of Au NPs and core-shell Au@Ag NPs; TEM images, size distributions and UV-Vis spectra of core-shell Au@Ag NPs with different Ag shell sizes; SEM images and SERS activities of Au@Ag/PMMA/qPCR film chips with different Ag shell sizes; Transmittance spectra and SEM image of PET film; XRD patterns and SEM images of Au@Ag/PMMA/qPCR film stored for 60 days; SERS spectra of thiram, crystal violet, malachite green and orange ΙΙ; Raman spectra of Au@Ag/PMMA/qPCR-PET film chip, thiabendazole solid, and the SERS spectrum of 10 ppm thiabendazole in orange juice; The SERS spectra of thiabendazole, thiram and thiabendazole + thiram in orange juice; The assignment of Raman peaks from thiabendazole and thiram; Fitting equations, R2 17



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and LOD for the quantification of thiabendazole in different samples; Comparison of the proposed SERS method with other analytical methods for measuring thiabendazole in food; Calculation of enhancement factor. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], URLs: http://www.ucd.ie/refrig; http://www.ucd.ie/sun Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to the National Key R&D Program of China (2018YFC1603404) for its support. This research was also supported by the Fundamental Research Funds for the Central Universities (2018MS056, 2017MS075), the International and Hong Kong - Macau - Taiwan Collaborative Innovation Platform of Guangdong Province on Intelligent Food Quality Control and Process Technology & Equipment (2015KGJHZ001), the Guangdong Provincial R & D Centre for the Modern Agricultural Industry on Non-destructive Detection and Intensive Processing of Agricultural Products, the Common Technical Innovation Team of Guangdong Province on Preservation and Logistics of Agricultural Products (2016LM2154) and the Innovation Centre of Guangdong Province for Modern Agricultural Science and Technology on Intelligent Sensing and Precision Control of Agricultural Product Qualities.

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Table 1. Results of recovery experiments for detecting thiabendazole using Au@Ag/PMMA/qPCR-PET film plasmonic chip coupled with SERS technique. Samples

Spiked (ppm)

Detected at 782 cm−1 (ppm)

Recovery at 782 cm−1 (%)

Detected at 1006 cm−1 (ppm)

Recovery at 1006 cm−1 (%)

Water

5

4.35±0.54

87

4.94±0.99

99

1

0.85±0.16

85

0.86±0.11

86

0.5

0.51±0.09

102

0.45±0.05

90

5

4.41±0.06

89

4.23±0.11

85

1

1.39±0.13

139

1.16±0.09

116

0.5

0.48±0.09

96

0.47±0.16

94

5

4.53±0.72

91

4.57±0.88

92

1

1.39±0.20

139

1.10±0.18

110

0.5

0.51±0.03

102

0.46±0.04

92

5

4.44±0.81

89

4.43±0.92

89

1

1.23±0.12

123

1.04±0.13

104

0.5

0.59±0.22

118

0.56±±19

112

Pear juice

Orange juice

Grape juice

24


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Figure capations Scheme 1. The fabrication of ternary films-packaged core-shell nanoparticles array chip for SERS sensing. Figure 1. TEM images of (A) Au NPs and (B) Au@Ag NPs, the inset was the high-resolution TEM image acquired from the Ag shell region. (C) The STEM-EDS elemental mapping of Au@Ag NPs. (D) The flow-process diagram of the procedure for fabricating the ternary films-packaged plasmonic nanoparticles array chip, (a) and (b) the diagram of Au NPs and Au@Ag NPs colloids, (c) the toluene/Au@Ag NPs colloid biphase system of interfacial self-assembly by adding ethanol, (d) the spontaneous evaporation of toluene at room temperature, (e) the formation of Au@Ag/PMMA film after the completely volatilization of toluene, (f) the diagram of qPCR film and PET film, (g) the flexible Au@Ag/PMMA/qPCR-PET film chip, (h) when used for SERS detection, the PET film could remove from the chip conveniently. (E) and (F) The cross-sectional SEM image of Au@Ag/PMMA/qPCR film, (E) was the magnified image of the green region in (F). (G) The vertical view SEM image of the Au@Ag/PMMA/qPCR film. Figure 2. (A) UV-Vis absorption spectra of (a) Au NPs, (b) Au@Ag NPs, (c) Au@Ag NPs colloid after interfacial self-assembly and (d) the Au@Ag/PMMA film. (B) AFM image of Au@Ag/PMMA/qPCR-PET film plasmonic array chip, and the height profile of Au@Ag NPs along the white line in (B). Figure 3. (A) SERS spectra of R6G (10−6 M) collected on Au@Ag/PMMA film plasmonic array chip with increase of PMMA concentration (0.65−13 mg/cm2). (B) Corresponding SEM images and (C) UV-Vis absorbance spectra. Figure 4. (A) The SERS intensity of 4-MBA at 1075 cm−1 detected by the Au@Ag/PMMA/qPCR film (red line) and Au@Ag/PMMA/qPCR-PET film chip (blue line) at different storage times of 0, 5, 10, 20, 30 and 60 days. (B) SERS spectra of 4-MBA collected from (a) Au@Ag/PMMA film, (b) Au@Ag/PMMA/qPCR film, (c) Au@Ag/PMMA/qPCR-PET film chip, (d) and (e) the Au@Ag/PMMA/qPCR-PET film chip after hydrothermal and ultrasonic treatments, the inset illustrated the SERS intensity of 4-MBA at 1075 cm−1. Figure 5. (A) The COMSOL multiphysics computer simulation showing the electromagnetic field distribution around the Au@Ag/PMMA/qPCR-PET film chip under 633 nm laser. The 25



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simulation result of the chip was obtained based on Au@Ag NPs with Au core of 32 nm, Ag shell of 5.2 nm, and the interparticle gap between Au@Ag NPs of 3 nm. (B) The SERS spectra of 10−6 M R6G collected on the chip for Raman mapping. (C) (a) Optical image of the region for Raman mapping in (B), (b)-(d) Raman maps targeting the R6G signal at 612, 1180 and 1362 cm−1, respectively. (D) The SERS intensity distribution of R6G at 612 cm−1 collected from 50 randomly selected spots in 5 batches Au@Ag/PMMA/qPCR-PET film plasmonic array chip. Figure 6. The SERS spectra of thiabendazole (TBZ) in (A1) water, (A2) pear juice, (A3) orange juice and (A4) grape juice with its concentration ranging from 0 to 10 ppm collected from the Au@Ag/PMMA/qPCR-PET film chip. The corresponding logarithmic SERS intensity of TBZ at (B) 782 and (C) 1006 cm−1 as a function of the logarithmic TBZ concentration.

26


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Scheme 1. The fabrication of ternary films-packaged core-shell nanoparticles array chip for SERS sensing.

27



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Figure 1. TEM images of (A) Au NPs and (B) Au@Ag NPs, the inset was the high-resolution TEM image acquired from the Ag shell region. (C) The STEM-EDS elemental mapping of Au@Ag NPs. (D) The flow-process diagram of the procedure for fabricating the ternary films-packaged plasmonic nanoparticles array chip, (a) and (b) the diagram of Au NPs and Au@Ag NPs colloids, (c) the toluene/Au@Ag NPs colloid biphase system of interfacial self-assembly by adding ethanol, (d) the spontaneous evaporation of toluene at room temperature, (e) the formation of Au@Ag/PMMA film after the completely volatilization of toluene, (f) the diagram of qPCR film and PET film, (g) the flexible Au@Ag/PMMA/qPCR-PET film chip, (h) when used for SERS detection, the PET film could remove from the chip conveniently. (E) and (F) The cross-sectional SEM image of Au@Ag/PMMA/qPCR film, (E) was the magnified image of the green region in (F). (G) The vertical view SEM image of the Au@Ag/PMMA/qPCR film.

28


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Figure 2. (A) UV-Vis absorption spectra of (a) Au NPs, (b) Au@Ag NPs, (c) Au@Ag NPs colloid after interfacial self-assembly and (d) the Au@Ag/PMMA film. (B) AFM image of Au@Ag/PMMA/qPCR-PET film plasmonic array chip, and the height profile of Au@Ag NPs along the white line in (B).

29



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Figure 3. (A) SERS spectra of R6G (10−6 M) collected on Au@Ag/PMMA film plasmonic array chip with increase of PMMA concentration (0.65−13 mg/cm2). (B) Corresponding SEM images and (C) UV-Vis absorbance spectra.

30


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Figure 4. (A) The SERS intensity of 4-MBA at 1075 cm−1 detected by the Au@Ag/PMMA/qPCR film (red line) and Au@Ag/PMMA/qPCR-PET film chip (blue line) at different storage times of 0, 5, 10, 20, 30 and 60 days. (B) SERS spectra of 4-MBA collected from (a) Au@Ag/PMMA film, (b) Au@Ag/PMMA/qPCR film, (c) Au@Ag/PMMA/qPCR-PET film chip, (d) and (e) the Au@Ag/PMMA/qPCR-PET film chip after hydrothermal and ultrasonic treatments, the inset illustrated the SERS intensity of 4-MBA at 1075 cm−1.

31



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Figure 5. (A) The COMSOL multiphysics computer simulation showing the electromagnetic field distribution around the Au@Ag/PMMA/qPCR-PET film chip under 633 nm laser. The simulation result of the chip was obtained based on Au@Ag NPs with Au core of 32 nm, Ag shell of 5.2 nm, and the interparticle gap between Au@Ag NPs of 3 nm. (B) The SERS spectra of 10−6 M R6G collected on the chip for Raman mapping. (C) (a) Optical image of the region for Raman mapping in (B), (b)(d) Raman maps targeting the R6G signal at 612, 1180 and 1362 cm−1, respectively. (D) The SERS intensity distribution of R6G at 612 cm−1 collected from 50 randomly selected spots in 5 batches Au@Ag/PMMA/qPCR-PET film plasmonic array chip.

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Figure 6. The SERS spectra of thiabendazole (TBZ) in (A1) water, (A2) pear juice, (A3) orange juice and (A4) grape juice with its concentration ranging from 0 to 10 ppm collected from the Au@Ag/PMMA/qPCR-PET film chip. The corresponding logarithmic SERS intensity of TBZ at (B) 782 and (C) 1006 cm−1 as a function of the logarithmic TBZ concentration.

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Stable, Flexible, and High-Performance SERS Chip Enabled by a

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