Journal of Analysis and Testing

, Volume 1, Issue 4, pp 322–329 | Cite as

Fabrication of Non-woven Fabric-Based SERS Substrate for Direct Detection of Pesticide Residues in Fruits

Original Paper
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Abstract

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.

Keywords

SERS Non-woven fabric Flexible substrate Pesticide residues 

1 Introduction

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 [31]. 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 Experimental

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.

2.2 Instrumentation

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 [32]. 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 [33]. 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.

The SERS analysis process for real samples was shown in Fig. 1. 10 μL of as-prepared pesticide solution was drop onto an apple surface and dried at room temperature. 5 μL of acetone was spread onto the area drop with pesticide followed by wiping with the flexible substrate. The SERS measurements were performed four times randomly within the area, and the obtained SERS data were averaged.
Fig. 1

Schematic of the pesticide residues detection on an apple surface with flexible SERS substrates

3 Results and Discussion

3.1 Characterization of the Non-woven Fabric

The prepared flexible substrates using the above different way were characterized by SEM. As shown in Fig. 2a, the blank non-woven fabric was made of random chemical fibers, leading to a large number of staggered structures. Furthermore, there were few folds on the fiber surfaces from the magnification image (Fig. 2b). During the self-assembly process, APTMS was used as the activating agent to effectively increase the active groups on the surface of polyacrylonitrile fiber, resulting in the large amount absorption of Ag NPs. The white non-woven fabric turned to gray color after the self-assembly process (Fig. 2c). As shown in Fig. 2d, many Ag NPs aggregated on the fibers to generate hot spots for SERS measurements. In the procedure of in situ growing, Ag+ adsorbed on the surface of the fiber due to the electrostatic interaction, and then the adsorbed Ag+ was reduced by NaBH4 to Ag NPs. The synthesized substrate (Fig. 2e, f) was yellow and the distribution of Ag NPs was more sparse and inhomogeneous than that of self-assembly because of the weak electrostatic adsorption between the fiber and Ag+. Finally, in the self-assembly/in situ growing approach, the uniform and densely scattered Ag NPs could be obtained as displayed in Fig. 2g, h. This method combined the advantages of self-assembly and in situ growing, which improved the amount and distribution of Ag NPs on the fiber surface.
Fig. 2

SEM images of: a blank non-woven fabrics, c self-assembly Ag NPs, e in situ growing Ag NPs and g self-assembly/in situ combination growing Ag NPs. b, d, f, h are the magnification images of the yellow box in a, c, e and g, respectively

3.2 Evaluation of the Non-woven Fabric for SERS Applications

PATP was selected as a model probe for the SERS evaluation and comparison of the different non-woven fabric substrate modified Ag NPs. The substrates were soaked in 100 μg L−1 PATP solution for 30 min under ultrasonic agitation. As a result, PATP was absorbed on Ag NPs through the thiol group. The SERS spectra of the three substrates are shown in Fig. 3. It is clear that the substrate fabricated by the self-assembly/in situ growing approach presented the best SERS enhancement, which could be consistent with the SEM results. The higher density and more uniform distribution of Ag NPs on the surface contributed more hot spots, leading to the highest SERS sensitivity. Thus, the substrate was adopted in the next experiments.
Fig. 3

Comparison of SERS performance of the three different flexible SERS substrates. a self-assembly combined with in situ growing, b self-assembly, c in situ growing

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].

$${\text{EF}} = \, (I_{\text{SERS}} /I_{\text{Raman}} ) \cdot (N_{\text{bulk}} /N_{\text{ads}} ),$$
where I SERS and I Raman are the SERS and normal Raman integrated intensities at 1074 cm−1 for PATP, respectively. N ads and N bulk are the number of the molecules found in the laser (785 nm) excitation area on SERS non-woven fabric and in the bulk solution in the probe volume, respectively. The following approximation equations could be used to replace above equation for the difficulty in specifying N ads and N bulk.
$$N_{\text{ads}} = A_{\text{eff}} /A_{\text{sum}} \cdot V_{\text{ads}} \cdot C_{1} ,$$
$$N_{\text{bulk}} = \, A_{\text{eff}} \cdot H_{\text{eff}} \cdot C_{\text{sol}} ,$$
where A eff and A sum are the area of the laser spot and substrate, respectively. V ads and C 1 are the volume and concentration of the measurement solution, respectively. H eff is the effective length of the scattering volume. C sol represents the concentration of the solution in normal Raman detection. As a result, the EF of the proposed SERS non-woven fabric was calculated to be 3.5 × 106, illustrating the excellent SERS enhancement similar to or stronger than many previous SERS substrates [30, 36, 37, 38].

3.3 Reproducibility of the SERS Signal

To further access the SERS performance of non-woven fabric prepared by self-assembly/in situ growing combination approach, the reproducibility of the SERS signal was evaluated. Eight batches of the same size of the flexible substrates were immersed into 2 mL of 50 μg/L PATP solution for 30 min under ultrasonic agitation. SERS spectra were collected from six randomly selected positions within the flexible SERS substrate. As shown in Fig. 4, the SERS signals at 1074 cm−1 obtained from different regions of the same substrate were high consistency, whose relative standard deviations were from 6.6 to 14.2%. Furthermore, the reproducibility of the SERS signal from eight different batches was also good with the relative standard deviation of 6.3%. Because of the same morphology of the non-woven fabric and the uniform distribution of Ag NPs using self-assembly/in situ growing combination approach, the SERS signals showed high reproducibility and practicality.
Fig. 4

Reproducibility of the flexible SERS substrate. The red dot line represents the average SERS intensity of the eight flexible SERS substrates. The error bar indicates the signal derivation within single flexible SERS substrate (six spots from different area)

3.4 Evaluation of SERS Non-woven Fabric Detection Performance

The sensitivity of PATP using the flexible SERS substrate was further evaluated. Generally, the substrates were soaked into 2 mL solution containing different PATP concentration from 1 to 500 μg L−1 for 30 min under ultrasonic condition and the obtained SERS spectra were collected and shown in Fig. 5. The SERS signal decreased with the concentration decrease of PATP. The characteristic peak of PATP could still be observed at a concentration of 1 μg L−1, suggesting high sensitivity using the substrates. The linear correlation coefficient of simulation curve of the SERS signals at 1074 cm−1 was 0.988 and the detection limit was found to be 0.2 μg L−1. These results indicated the high sensitivity of the flexible substrate for molecules containing thiol groups. Thus, it was of great value in practical application.
Fig. 5

SERS spectra of PATP after extraction by flexible SERS substrates in different concentrations and the inset figure is the calibration curve of PATP plotted peak intensity of 1074 cm−1

3.5 Application of SERS Non-woven Fabric in Detecting Pesticide Residues

Four kinds of organochlorine pesticides, p,p′-DDT, isocarbophos, sumicidin and phosgene, were chosen as the target analytes to verify the practicability of the flexible SERS substrates. Pesticide solution with different concentration was sprayed onto an apple surface, and the target pesticide was collected and detected by a simple wipe sampling method. It can be seen from Fig. 6 that the flexible substrates presented good Raman enhancement effect for all four pesticides. The SERS spectra of the pesticides were still obvious even the concentration was as low as 5 ng/cm2. Furthermore, the total time for the sampling and the SERS detection was less 1 min. The high sensitivity, short detection time and non-destructive characteristic made the approach to be potential in rapid on-site test of pesticide residues.
Fig. 6

SERS spectra of pesticide residues in different concentrations obtained by wiping orange surface using the flexible SERS substrate, inset was the molecular formula of the pesticide. (a p,p′-DDT, b sumicidin, c isocarbophos, d phostene)

4 Conclusions

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.

Notes

Acknowledgements

This work was supported by Natural Science Foundation of Fujian Province (No. 2015J01058) and NFFTBS (No. J1310024) which are gratefully acknowledged.

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Copyright information

© The Nonferrous Metals Society of China and Springer Nature Singapore Pte Ltd. 2017

Authors and Affiliations

  1. 1.Department of Chemistry and the MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, College of Chemistry and Chemical EngineeringXiamen UniversityXiamenChina
  2. 2.State Key Laboratory of Marine Environmental ScienceXiamen UniversityXiamenChina

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