Fabrication of Non-woven Fabric-Based SERS Substrate for Direct Detection of Pesticide Residues in Fruits
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In surface-enhanced Raman scattering (SERS), flexible substrate plays an important role in target molecular collection from various shape surfaces and increases the analytical sensitivity. In this study, silver nanoparticles (Ag NPs) were deposited on a non-woven fabric used as an SERS substrate by self-assembly, in situ growing or the self-assembly/in situ growing combination method. 4-Aminothiophenol was selected as a model molecular for the evaluation of the SERS performance using the substrates. The Ag NPs substrate prepared by self-assembly/in situ growing method presented the best Raman enhancement effect and its enhancement factor was estimated as high as 3.5 × 106. The substrate was applied to the determination of four pesticide residues on the surfaces of fruit samples through wipe sampling, and the results revealed the good reproducibility of SERS responses and high detection sensitivity. The prepared flexible substrate was simple to fabricate and environmentally friendly. It could be expected to be a useful tool in rapid on-site test of pesticide residues on fruit surfaces because of its high sensitivity, convenience and non-destructive characteristics.
KeywordsSERS Non-woven fabric Flexible substrate Pesticide residues
In recent years, a series of endangering issues in environment and health issues have been caused by excessive pesticide residues [1, 2]. The problems of pesticide residues on fruits and vegetables have also been paid more attention due to the wide use of pesticide. Various conventional methods, such as mass spectrometry, capillary electrophoresis and chromatography, have been applied for the detection of trace level of pesticide residues [3, 4, 5, 6, 7, 8]. However, these methods are usually time-consuming requiring for complicated pre-treatments and must be carried out in laboratory with the expensive instruments. Consequently, it is of great practical value to develop a fast, convenient, and on-site detection method for pesticide residues.
Surface enhance Raman scattering (SERS) has been applied in many fields, such as biotechnology, homeland security, and food safety due to its non-destructive and highly sensitive characteristic features [9, 10, 11, 12, 13]. Other than optimizing the sizes, shapes and components of noble metallic nanoparticles (typically, silver or gold) [13, 14, 15, 16, 17, 18], the design of ideal SERS substrate with the capability to extract target from complex matrix more efficient has become a commonly used way to improve the Raman signals [12, 19, 20, 21, 22, 23, 24]. Conventional types of the solid SERS substrate, such as glass slide or silicon wafers [25, 26], possess a rigid architecture resulting in low conformal contact with the sample surface and thus poor extraction ability. Recently, the emerging flexible substrate materials have provided an effective way for the target analyte collection. Up to now, polymer, paper and cotton have been used as flexible SERS substrates [20, 27, 28, 29, 30]. These substrates could be applied to swab the complex surface of diverse actual analytes (swab sampling) for convenient and effective SERS detection. The choice of flexible substrate materials is critical to realize a convenient, sensitive and efficient target collection and SERS measurement.
Fiber-based material possesses excellent porous properties and large specific surface area due to its staggered structure, which makes it become an ideal flexible extraction material to closely touch the sample surface . Furthermore, since the chemical fiber or plant fiber has a micron-sized roughened fibrous structure, it is easy to modify the sol-metal nanoparticles on its surface, hence, to construct an SERS active substrate. In this study, silver nanoparticles (Ag NPs) were modified on the surface of polyacrylonitrile fiber (ultrafine non-woven fabric) by self-assembly synthesis, in situ growing and self-assembly/in situ growing combination methods. Their morphologies and performance were evaluated to optimize the preparation conditions. Finally, the obtained flexible SERS substrate was used for the determination of pesticide residues on the surfaces of fruit samples by wipe sampling, and the results illustrated that the proposed approach is of excellent application prospect in the practical fields. The approach using the wipe sampling-SERS determination provides a strategy for the rapid on-site test of pesticide residues, which is fast, non-destructive, sensitive and efficient.
2.1 Chemicals and Materials
Analytical grade silver nitrate (AgNO3), sodium citrate (Na3C6H5O7·2H2O) and sodium borohydride (NaBH4) were purchased from J & K Scientific Company (Beijing, China). (3-Aminopropyl) trimethoxysilane (APTMS) and 4-aminothiophenol (PATP) were from Sigma-Aldrich (Shanghai, China). Analytical grade acetone and ethanol were from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Acetone (pesticide analysis grade) was from TEDIA company, Inc. (Ohio, America). p,p′-DDT, isocarbophos, sumicidin and phosgene standard solution were from Agro-Environmental Protection Institute (AEPI) of Ministry of Agriculture (MOA). Their concentration was all 100 μg mL−1. Pure water in the experiments was obtained from a Simplicity Water Purification Systems (Millipore, Molsheim, France). The microfiber non-woven fabric (polyacrylonitrile fiber) was purchased from SANWA SUPPLY Co., Ltd. (Okayama, Japan). It was cut into the same size (1.5 cm × 3.0 cm) and then washed using acetone, ethanol, and pure water, respectively. They were dried under 60 °C before used.
Scanning electron microscope (SEM) images were acquired by an S4800 field emission SEM (Hitachi, Tokyo, Japan). Electronic balance (Saiduolisi balance Co. Ltd., Sartorius BS 110, Beijing, China), high-speed dispersion machine (ULTRA-TURRAX, IKA T18, Germany), Ultrasonic instrument (Kunshan City ultrasound instrument Co., Ltd., KQ-200KDE, Jiangsu, China) and electrothermal constant temperature blast oven (Tak Instrument Equipment Co., Ltd, DYG-9070A, Xiamen, China) were used in the synthesis of the flexible substrate.
The SERS spectra were obtained on a commercial portable spectrometer (DeltaNu Inspector Raman, USA) equipped with 785 nm laser. The laser power was 120 mW and the system resolution was 8 cm−1. The SERS signal acquisition integration time was set as 1 s. All measurements were performed at room temperature.
2.3 Preparation of Ag NPs
The Ag NPs were prepared according to the previous report . Briefly, 50 mg of silver nitrate (AgNO3) and 120 mL of pure water were added into a 250 mL three-necked flask to stir until the AgNO3 was dissolved thoroughly. After that, it was heated to slightly boiling at 120 °C under 1100 r/min in reflux conditions. Then, 5 mL of 1% (w/w) sodium citrate solution was quickly added. The solution was kept heating for 30 min and became pale yellow gradually. The synthesized Ag NPs were stored in a refrigerator at 4 °C after the solution was cooled to room temperature (around 20 °C).
2.4 Decoration of Non-woven Fabric
Three different methods for the decoration of non-woven fabric were applied according to the reference with minor modification . In the Ag NPs self-assembly approach, a prepared clean non-woven fabric was immersed in 2% (v/v) APTMS ethanol solution for 30 min under ultrasonic agitation, and then taken out to be rinsed with ethanol thoroughly. Next, the non-woven fabric was incubated in an oven at 100 °C for 30 min to activate the fibers. The non-woven fabric was then cooled to room temperature followed by being washed with ethanol and pure water for three times, respectively. After that, the non-woven fabric was soaked in a prepared Ag NPs solution for 30 min under sonication. Finally, the flexible substrates were taken out from the solution, and then washed using pure water to remove the excess Ag NPs. In situ growing approach, a prepared clean non-woven fabric was immersed in a beaker containing 10 mL of 50 mmol L−1 AgNO3 solution for 30 min under ultrasonic agitation, and then removed from the solution to be rinsed with ethanol twice. After that, the non-woven fabric was soaked in 10 mL of 50 mmol L−1 NaBH4 for 10 min followed by rinsed with pure water three times. In the self-assembly/in situ growing combination approach, the prepared clean non-woven fabric was first treated by the approach described in self-assembly approach, and then modified according to the method in situ growing approach. Finally, the all prepared substrates were washed and stored in pure water.
2.5 SERS Measurements
PATP was used as the Raman model probe to evaluate the performance of previous synthesized flexible substrates. A series of different concentrations PATP solution (1, 10, 50, 100, 200, 500 μg L−1) were prepared. The flexible substrates were soaked in 2 mL of PATP solution for 30 min under ultrasonic condition at room temperature. Then, the substrates were taken out and placed on a surface of glass slide for SERS detection.
3 Results and Discussion
3.1 Characterization of the Non-woven Fabric
3.2 Evaluation of the Non-woven Fabric for SERS Applications
To further evaluate the SERS performances of the selected substrate fabricated by the self-assembly/in situ growing combination approach, the enhancement factor (EF) of the substrate was calculated as follows [34, 35].
3.3 Reproducibility of the SERS Signal
3.4 Evaluation of SERS Non-woven Fabric Detection Performance
3.5 Application of SERS Non-woven Fabric in Detecting Pesticide Residues
In this study, a non-woven fabric deposited Ag NPs by self-assembly/in situ growing approach was developed for the sensitive and cost-effective SERS applications, by which several pesticide residues on a fruit surface could be detected by simple wipe sampling. The uniform and dense Ag NPs on the flexible SERS substrate resulted in the high collection efficiency and sensitivity SERS signals. Combined with a portable Raman spectrometer, we believed that the approach would be of great potential in homeland security, as well as crime scene investigation.
This work was supported by Natural Science Foundation of Fujian Province (No. 2015J01058) and NFFTBS (No. J1310024) which are gratefully acknowledged.
- 2.Pang S, Yang T, He L. Review of surface enhanced Raman spectroscopic (SERS) 225 detection of synthetic chemical pesticides TrAC Trends. Anal Chem. 2016;85:73–82.Google Scholar
- 3.Seebunrueng K, Santaladchaiyakit Y, Soisungnoen P, Srijaranai S. Catanionic surfactant ambient cloud point extraction and high-performance liquid chromatography for simultaneous analysis of organophosphorus pesticide residues in water and fruit juice samples. Anal Bioanal Chem. 2011;401:1703–12.CrossRefGoogle Scholar
- 7.Menezes Filho A, dos Santos FN, de Paula Pereira PA. Development, validation and application of a methodology based on solid-phase micro extraction followed by gas chromatography coupled to mass spectrometry (SPME/GC–MS) for the determination of pesticide residues in mangoes. Talanta. 2010;81:346–54.CrossRefGoogle Scholar
- 8.Brito N, Navickiene S, Polese L, Jardim E, Abakerli R, Ribeiro M. Determination of pesticide residues in coconut water by liquid-liquid extraction and gas chromatography with electron-capture plus thermionic specific detection and solid-phase extraction and high-performance liquid chromatography with ultraviolet detection. J Chromatogr A. 2002;957:201–9.CrossRefGoogle Scholar
- 26.Fan M, Brolo AG. Silver nanoparticles self assembly as SERS substrates with near single molecule detection limit. Phys Chem Chem Phys. 2009;11:7981.Google Scholar
- 31.Hubbe MA, Ayoub A, Daystar JS, Venditti RA, Pawlak JJ. Enhanced absorbent products incorporating cellulose and its derivatives. BioResources. 2013;8:6556–629.Google Scholar
- 37.Qu LL, Li DW, Xue JQ, Zhai WL, Fossey JS, Long YT. Batch fabrication of disposable screen printed SERS arrays. Lab Chip. 2012;12:879–81.Google Scholar