Flexible paper-based SERS substrate strategy for rapid detection of methyl parathion on the surface of fruit

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 231 (2020) 118104

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Flexible paper-based SERS substrate strategy for rapid detection of methyl parathion on the surface of fruit Jie Xie, Liangyu Li, Imran Mahmood Khan, Zhouping Wang, Xiaoyuan Ma ⁎ State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi 214122, PR China School of Food Science and Technology, Jiangnan University, Wuxi 214122, PR China International Joint Laboratory on Food Safety, Jiangnan University, Wuxi 214122, PR China

a r t i c l e

i n f o

Article history: Received 7 November 2019 Received in revised form 15 January 2020 Accepted 22 January 2020 Available online 23 January 2020 Keywords: Au NPs The paper-based substrate SERS Methyl parathion

a b s t r a c t Herein, we reported a simple, flexible and sensitive surface-enhanced Raman scattering (SERS) substrate to detect methyl parathion residues in real life. The substrate was fabricated by filter paper and gold nanoparticles (Au NPs) with excellent reproducibility and stability. First, Au NPs were synthesized by the seed mediated growth method and assembled to the filter paper through immersion. The Raman probe molecule 4-MBA was used to evaluate performance of the substrate for an optimized signal using a portable Raman spectrometer coupled with 785 nm laser. Then, the paper-based substrate was applied to detect methyl parathion standard solution whose detection limit was down to 0.011 μg/cm2, and the linear range was between 0.018 μg/cm2 and 0.354 μg/cm2. Afterwards, actual sample (apple) spiked with methyl parathion was taken to verify the practicality of the substrate by a simple way of “press–peel off”. The recovery rate was ranged from 94.09% to 98.72%, indicating that this method is reliable in actual sample detection without complicated pretreatment steps. This work demonstrates that the flexible paper-based substrate combined with portable Raman instruments can be potentially applied to on-site detection of hazardous substances in the field of food safety. © 2020 Published by Elsevier B.V.

1. Introduction With the increase of social population and development of agricultural production, organophosphorus pesticides (OP) are widely used in the production process [1]. Parathion, endogenous phosphorus, malathion, dimethoate, trichlorfon are most common OPs and used as insecticides in plant pest control. Those pesticides are easy to remain on the soil and the surface of fruits and vegetable crops, which is harmful to human body by bringing about neurological impact [2]. Therefore, it is urgent to study the detection methods of these pesticides [3]. There are two main types of detection methods for organophosphorus pesticides at present. One of the methods is the traditional chromatographic detection method, including gas chromatography (GC), highperformance liquid chromatography (HPLC), and gas chromatography–mass spectrometry (GC–MS) [4–8]. Those methods are carried out by on-site sampling, and then analyzed in the laboratory. They always have accurate test results but rely on large instruments and the pretreatment is quite complicated. The other is enzymatic method, whose main disadvantage is the lack of selectivity [9]. In order to solve

⁎ Corresponding author at: School of Food Science and Technology, Jiangnan University, Wuxi 214122, PR China. E-mail address: [email protected] (X. Ma).

https://doi.org/10.1016/j.saa.2020.118104 1386-1425/© 2020 Published by Elsevier B.V.

the above problems and drawbacks, the exploration of new methods for detecting pesticides has great practical significance. SERS is an emerging analytical technique that provides characteristic vibrational fingerprints of molecules by enhancing the Raman signal in the near vicinity of plasma nanostructures [10,11]. It has been used in various fields such as biology, [12,13] chemistry, [14,15] and medicine [16,17]. In terms of detection technology, SERS is considered to be a fast and sensitive general-purpose technology, and its detection limit can be reached to a single-molecule level [18,19]. The signal uniformity and sensitivity are important factors to evaluate SERS method, which are related to SERS substrates. Therefore, a number of SERS substrates have been constructed. Hu et al. have fabricated an ordered array of hierarchically-structured core-nanosphere@space-layer@shell-nanoparticles to detect methyl parathion in water [20]. Li et al. have developed a multifunctional Au-coated TiO2 nanotube array for detecting 4chlorophenol (4-CP), dichlorophenoxyacetic acid (2,4-D), and methyl parathion [21]. Most of the nanomaterials are constructed on rigid substrates, such as silicon wafers, glass sheets, and electrodes [22–24]. These materials are not easy to bend but easily broken. They also can not be in full contact with the sample surface, which makes them difficult to efficiently collect the target molecule. In order to overcome these shortcomings, flexible substrates become emerging researching objects, such as flexible devices, polymers, and carbon materials [25–28]. They are able to fully contact with sample and easy to prepare, so there is a

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Fig. 1. Schematic of the paper-based SERS substrates for detecting on the fruit peels surface.

great research prospect for applying flexible SERS substrates to pesticide detection [29]. Filter paper is one of the most common substrates and it can combine with nanostructures for many applications [30–32]. Liu et al. used filter paper and gold nanorods (GNRs) to prepare threedimensional heterogeneous scaffold for cancer screening [33]. Zheng et al. have reported a robust and recyclable ‘dip-catalyst’ based on a gold nanoparticle (Au NP)-loaded filter paper composite to act as catalysts [34]. Therefore, filter paper has great potential as a tool in various fields. Here we reported a new paper-based SERS substrate which was composed of Au NPs and laboratory filter paper to detect pesticides rapidly. This kind of substrate was able to generate strong Raman signals for detection of target molecules with portable Raman spectrometer coupled with a 785 nm laser. The filter paper sheet was immersed in Au NPs solution for a certain time in order to absorb Au NPs onto its surface. Reproducibility, stability and sensitivity of the substrate were evaluated by using the probe molecule 4-MBA. A linear relationship between 4-MBA concentration and characteristic peak intensity was established. Practically, the paper-based substrate was employed to the detection of methyl parathion standard solution. In addition, methyl parathion spiked samples (apples) were tested using the paper-based substrate by means of “paste–peel off” approach. Hence, the innovative method can be well applied to detect the presence of hazards in foods owing to simplicity and sensitivity. Fabrication of the paper-based substrate and detection process were expressed in Fig. 1. 2. Materials and methods 2.1. Chemicals and instruments Gold (III) chloride trihydrate (HAuCl4·3H2O), sodium citrate (C6H5Na3O7·2H2O), absolute ethanol and the filter paper which is

composed of 98% α-cellulose were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 4-Mercaptobenzoic acid (4MBA) and methyl parathion (C8H10NO5PS) were purchased from Aladdin Chemical Reagent Co., Ltd. (China). Ultrapure water was obtained from a Milli-Q device (18.2 MΩ cm, Millipore, Molsheim, France) and used throughout the experiment. UV–vis spectrum of colloidal Au NPs was recorded by UV–vis spectrophotometer (Shimadzu, Japan). TEM images were acquired from Transmission Electron Microscope (JEOL JEM-2100, Japan) operating at an acceleration voltage of 200 kV. SEM images of filter paper were obtained from the Cold Field Emission Scanning Electron Microscope (SU8100, Japan). A portable Raman spectrometer (B&W Tek Inc., USA) was used to collect the Raman spectra coupled with a 785 nm laser. 2.2. Preparation of Au NPs All glassware should be cleaned by freshly prepared HCl/HNO3 (v:v, 3:1) before using and washed thoroughly by ultrapure water. The Au NPs were synthesized following a kinetically controlled seeded growth strategy according to the previous report with slight modification [35]. The Au seeds were synthesized by the following steps. Firstly, 150 mL sodium citrate solution (2.2 mM) was added to a three-neck round bottom flask and placed in an oil bath at 110 °C for 15 min with the condensing device. After the solution was boiled, 1 mL HAuCl4 (25 mM) was added. The color of solution gradually turned into pink within 10 min, which showed that Au seeds had been successfully synthesized. Start the growth of Au NPs after the seed solution cooled to 90 °C. 1 mL sodium citrate (60 mM) was added to the seed solution, and then 1 mL HAuCl4 (25 mM) was added about 2 min later. After stirring for 30 min, remove 2 mL aliquots from the reaction solution. By repeating that growth strategy, Au NPs with progressively larger size were obtained. The prepared Au NPs was cooled and stored at 4 °C for further use.

Fig. 2. (A) A typical UV–vis spectrum, (B) the TEM image of Au NPs.

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Fig. 3. (A) SERS spectra of different immersing time, (B) the relationship between the SERS intensity at 1068.18 cm−1 and immersing time.

2.3. Fabrication of the paper-based substrate By punching the filter paper, circular filter papers with a diameter of 6 mm were obtained. The paper-based substrate was acquired by following steps. The bare circular filter paper was immersed in 200 μL Au NPs solution for different time (5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60 min) and then washed with ultrapure water until the washing liquid was colorless. Leave the filter paper at room temperature for 1 h to dry and the paper-based substrate was obtained. The SERS enhancing effect of the filter paper substrate was examined using 4-MBA as the Raman probe molecule to get the optimized immersing time. 3 μL of 10−5 M 4-MBA was dropped on the filter paper substrate which was placed on

a glass slide wrapped with tin foil. A portable Raman spectrometer was used to detect. During the detection, the integration time was 5 s with accumulation and the laser power used was 280 mW. 2.4. Detection of SERS performance of the paper-based substrate 4-MBA was used to evaluate the reproducibility, stability and sensitivity of the paper-based substrate. The reproducibility was examined by selecting three different batches of prepared paper-based substrates and 10 points were taken from each batch for SERS detection. The stability of the paper-based substrate was conducted by storing substrates in the air and the Raman spectrum was collected every other day. 3 μL of 4-

Fig. 4. SEM images of filter paper with immersing time (A) 0 min, (B) 15 min, (C) 30 min, (D) 45 min.

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Fig. 5. (A) SERS spectra of 4-MBA (10−5 M) acquired from 30 sites of three batches of substrates, (B) intensity distribution at 1068.18 cm−1 with 5.28% RSD.

MBA aqueous solution with different concentration (10−9, 5 × 10−9, 10−8, 5 × 10−8, 10−7, 5 × 10−7, 10−6, 5 × 10−6 M) was dropped onto the substrates whose sensitivity was evaluated. The Raman spectrometer was set to be the same as the previous part. After all, 10 μL of methyl parathion ethanol solutions with different concentrations (0.018, 0.035, 0.071, 0.141, 0.212, 0.283, 0.354, 0.707, 1.415, 2.122, 2.829, 3.537, 7.074, 21.221, 35.368 μg/cm2) was dropped onto the prepared substrates and the integration time was increased to 20 s.

2.5. Detection of methyl parathion on the apple surface The pesticide was detected on the surface of apples (bought from the local supermarket) to verify the practicality of the paper-based substrate. The measurement was carried out according to the previous report with some modifications [36]. Firstly, clean and peel the actual sample. Then, the peel of the apples was cut into square pieces with 1 × 1 cm 2 . The contaminated samples were prepared by dropping 10 μL of methyl parathion solutions with different concentrations (0.1, 0.2, 0.35, 1.0, 1.5 μg/cm2) onto the surface. After drying, one drop of ethanol solution was dropped onto the peels to extract the pesticide and the paper-based substrate was pasted onto the peel immediately. The substrate was pressed for a few minutes and then peeled off. This process was what we mentioned as “paste–peel off”. Subsequently, the Raman spectrum was recorded. The integration time was 20 s.

3. Results and discussion 3.1. Characterization of Au NPs UV–vis spectrum was recorded to measure the surface plasma resonance of synthesized colloidal Au NPs. As shown in Fig. 2A, there is one distinct peak at 526 nm with narrow half-width and the color of Au NPs is red wine (inset of Fig. 2A shows the optical image). In order to observe the morphology of Au NPs, TEM images were recorded. Fig. 2B illustrates that the particles have a round shape with good dispersion and uniform size. The average size of Au NPs is about 37.91 ± 3.26 nm (Supplementary Material, S1). The UV–vis and TEM results proved that homogeneous Au NPs was successfully synthesized. The concentration of Au NP solutions was calculated to be 56.254 μg/mL. 3.2. SERS signal optimization of the paper-based substrate The bare filter paper was immersed in the Au NPs solution for different time and then 3 μL 4-MBA (10−5 M) was dropped onto the substrate to optimize SERS signal. Fig. 3A showed SERS spectra of 4-MBA with two distinct characteristic peaks at 1068.18 cm−1 and 1579.79 cm−1, which are assigned to the vC-C ring-breathing mode and stretching mode, respectively [37]. The strongest signal peak of 4-MBA at 1068.18 cm−1 was taken as the characteristic peak in the following study. As shown in Fig. 3B, when the immersing time was less than 30 min, the SERS intensity of 4-MBA at 1068.18 cm−1 increased gradually with the increasing time. This phenomenon was attributed to the increase of Au NPs

Fig. 6. (A) SERS spectra of 4-MBA (10−5 M) within 12 days, (B) the relationship between the SERS intensity at 1068.18 cm−1 and storage time.

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Fig. 7. (A) SERS spectra of 4-MBA with concentration ranging from 10−9 to 5 × 10−6 M using paper-based substrate, (B) Adsorption-saturation Langmuir plot of the SERS intensity at 1068.18 cm−1 peak and the concentration of 4-MBA, (C) Linear calibration plot of SERS intensity versus the low concentration of 4-MBA ranging from 10−9 to 10−7 M.

adsorbed on the filter paper. But when the immersing time was longer than 30 min, the SERS intensity began to decline due to a large amount of Au NPs attached to the surface of filter paper causing aggregation. This kind of nanoparticles accumulation would reduce the SERS intensity [38]. Therefore, 30 min was considered to be the best immersing time. The high absorption of Au NPs onto paper was realized by van der Waals binding without any retention aid [39]. Fig. 4A-D are SEM images of filter paper with different immersing time, which are consistent with SERS signal. And Fig. 4C of filter paper immersed for 30 min revealed that the surface of the filter paper was totally covered by uniform dispersed nanoparticles. This kind of close distribution would lead to more “hot spots” which enhanced the Raman intensity. 3.3. Reproducibility, stability and sensitivity of the paper-based substrate Reproducibility, stability and sensitivity are significant to characterize the properties of the substrate. The performance of the substrate plays a crucial role in detection. 4-MBA was still taken as a probe molecule. The 30 random points from three different batches of paper-based substrates were used to characterize the reproducibility of the paperbased substrate. As illustrated in Fig. 5A, the resulting SERS peak shapes of 4-MBA are basically the same, and the peak intensity are also very

close. The result of relative standard deviation (RSD) of Raman intensity at 1068.18 cm−1 as described in Fig. 5B was calculated to be 5.28%, which is better than the published work [36,40]. From the above result, it can be concluded that the paper-based substrate has excellent reproducibility because nanoparticles were evenly distributed on the surface of the filter paper. Stability was assessed by placing the paper-based substrates in the air and measured the intensity every other day. As shown in Fig. 6B, the Raman intensity declined rapidly in the first 6 days of storage. But after 6 days, the intensity was basically unchanged and remained at a relatively high level (about 30,000 counts). The possible explanation for this phenomenon was that a few gold nanoparticles initially adsorbed on the substrate fell from the filter paper due to weak physical binding, which resulted in the decrease of intensity. The remaining nanoparticles were firmly attached to the filter paper so that the substrate still had high Raman intensity, which was also used for detection [39]. Therefore, the paper-based substrate stored in air for a certain period still exhibited good stability. The sensitivity of the paper-based substrate was demonstrated by different concentrations of 4-MBA. Before quantitative detection, we made an investigation on the background signal interference of the filter paper. The SERS spectrum of bare filter paper and filter paper with

Fig. 8. (A) SERS spectra of methyl parathion with concentration ranging from 0.018 μg/cm2 to 35.368 μg/cm2 using paper-based substrate, (B) Adsorption-saturation Langmuir plot of the SERS intensity at 1336.79 cm−1 peak and the concentration of MP, (C) Linear calibration plot at low concentration (0.018 μg/cm2 to 0.354 μg/cm2) for quantitative analysis.

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Table 1 Comparison of different SERS substrates in methyl parathion detection. Methods

Raman model

Laser wavelength (nm)

Detection limits

References

AuNRs-based sensor AuNPs-pseudo-paper films (APPFs) Au@Ag NPs-based SERS

Ocean Optics portable Raman B&W Tek portable Raman Confocal Raman system (Horiba France) Confocal microprobe Raman system (Renishaw, India) Confocal microprobe Raman spectrometer (Renishaw, inVia) Raman spectrometer (Ntegra Spectra) Miniature Raman spectrometer (BWS415, B&W Tek Inc.) Via-Reflex micro-Raman spectrometer B&W Tek portable Raman

785 785 633

1 μM 0.0008 mg/kg 0.001 mg/kg

[49] [51] [52]

532 532 532 785 633 785

2.0 pM 10−8 M 1 μg/mL 10−3 ppm 2.60 ng/cm2 0.011 μg/cm2

[53] [54] [55] [56] [57] This work

Ag nanocubes/graphene oxide hybrid Ag-NPs@ZnO- nanorods/PAN-nanopillar arrays) Ag-NaCMC film MSiO2@Ag microspheres Tape-based SERS substrate Filter paper-based substrate

nanoparticles showed no obvious signal at the characteristic peak position, which proves that the filter paper will not interfere with the quantitative detection (Supplementary Material, S2). Fig. 7A showed two distinct characteristic peaks at 1068.18 cm−1 and 1579.79 cm−1 for 4MBA. We further found that there is a simple adsorption-saturation Langmuir relationship between the characteristic peak intensity at 1068.18 cm−1 and concentration of 4-MBA (Fig. 7B), which was similar to previous reports [41,42]. When the concentration of 4-MBA is in the lower range (10−9 M–10−7 M), a linear relationship can be observed in Fig. 7C. We speculated that the SERS intensity was proportional to the number of “hot spots” molecules [43]. The linear equation was expressed as:

parathion in ethanol solution was evaluated. As shown in Fig. 8A, there were three apparent peaks ranging from 700 cm−1 to 1500 cm−1 at 848.97 cm−1, 1104.10 cm−1 and 1336.79 cm−1 which can be attributed to the P\\O, C\\N stretch and C\\H bend stretch, respectively [44,45]. The SERS intensity at 1336.79 cm−1 was used as a quantitative characteristic peak to establish a standard curve between intensity and concentration of methyl parathion. Obviously, the adsorption-saturation Langmuir relationship also exists between them, which is the same as 4-MBA (Fig. 8B). When the concentration of methyl parathion reached to a certain value, the peak intensity was close to saturation. Fig. 8C shows a linear relationship between intensity and methyl parathion concentration which is ranging from 0.018 μg/cm2 to 0.354 μg/cm2. A linear equation was expressed as:

ISERS ¼ 3:457  1011 C4−MBA þ 171:126; R2 ¼ 0:9989

ISERS ¼ 2318:415CMP þ 272:481，R2 ¼ 0:9916

The nonlinear relationship at 10−7 - 5 × 10−6 M was expressed as: ISERS ¼

46442:162C4−MBA

; R2 ¼ 0:9998 C4−MBA þ 3:468  10−8

where ISERS is the intensity of the 1068.18 cm−1 peak, and C4-MBA is the concentration of 4-MBA. The above results indicated that the paperbased substrate possessed superior reproducibility, stability and sensitivity, which were significant for actual sample detection. 3.4. Methyl parathion measurements by the paper-based substrate After the characterization of the paper-based substrate, the concentration-dependence performance of the substrate for methyl

The nonlinear relationship at 0.354–35.368 μg/cm2 was expressed as: ISERS ¼

16359:656CMP 2 ; R ¼ 0:9953 CMPþ7:416

where ISERS is the intensity of the 1336.79 cm−1 peak, and CMP is the concentration of methyl parathion. The results illustrated that the paper-based substrate can be used for quantitative detection of methyl parathion. According to the formula (Supplementary Material, S3), [46] the limit of detection (LOD) was calculated to be 0.011 μg/cm2 (~0.0099 mg/kg, Supplementary Material, S4) [47], which is much lower than the maximum residue limits (MRL) for pesticides (0.2 mg/kg per weight) in China and European legislation (Council Directives 90/642/EEC, 1990) [48]. Moreover, the integration time for methyl parathion detection is 20 s, and the laser didn't damage the paper-based substrate. Furthermore, Table 1 also shows that our paper-based substrate is comparable with other substrates for the detection of methyl parathion. 3.5. Detection of methyl parathion in real samples In order to verify the practical application of the paper-based substrate, the contaminated apple was used as a real sample to detect methyl parathion and Raman spectra were collected. As shown in Fig. 9, we still can see that there are three distinct peaks at 848.97 cm−1, 1104.10 cm−1 and 1336.79 cm−1. The control experiment proved that molecules in fruits have no effect on the detection of Table 2 Recovery study of methyl parathion spiked onto apples.

Fig. 9. SERS spectra of methyl parathion with various surface concentration of apple using paper-based substrate.

Group

Added(μg/cm2)

Founded(μg/cm2)

Recovery (%)

1 2 3

0.1 0.2 0.35

0.094 0.190 0.346

94.09% ± 12.59% 94.91 ± 6.05% 98.72% ± 7.11%

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pesticide (Supplementary Material, S5). The Raman intensity at 1336.79 cm−1 was acted as a quantitative characteristic peak. Several concentrations in the linear range were selected for spike recovery experiments. Then, the linear equation of SERS intensity and methyl parathion concentration in the range of 0.018–0.354 μg/cm2 was used to calculate the recovery rate which ranged from 94.09% to 98.72% with acceptable relative standard deviation (RSDs) as mentioned in Table 2. In previous reports [49,50], the recovery rates were 96.45% to 106.02% and 97.3% to 114.2%, respectively. As compared to the previously reported results, this study performed an analogous recovery rate, which indicated that such a SERS substrate has a great prospect of methyl parathion detection in real life. 4. Conclusion In summary, we presented the fabrication of a flexible and effective paper-based substrate which was applied to detect the pesticide residue rapidly by the method of “paste–peel off”. The paper-based substrate was realized by immersing the filter paper into Au NPs colloid. Compared with traditional detection methods, the proposed method based on SERS substrate is of low cost and can be used for on-site inspection. More importantly, its preparation process is relatively simple and saves a lot of pre-processing steps. The test of methyl parathion in the actual sample verified the feasibility of this method in analytical fields, especially in food safety and environmental issues. CRediT authorship contribution statement Jie Xie: Conceptualization, Methodology, Visualization, Writing original draft, Writing - review & editing. Liangyu Li: Validation, Data curation. Imran Mahmood Khan: Writing - review & editing. Zhouping Wang: Methodology, Resources. Xiaoyuan Ma: Conceptualization, Supervision, Writing - review & editing, Funding acquisition. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. Acknowledgments This work was supported by Key Research and Development Program of Jiangsu Province (BE2018306, BE2017623), the National First-class Discipline Program of Food Science and Technology (JUFSTR20180303, JUSRP51714B), and Synergetic Innovation Center of Food Safety and Quality Control of Jiangsu Province. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2020.118104. References [1] S.O. Pehkonen, Q. Zhang, The degradation of organophosphorus pesticides in natural waters: a critical review, Crit. Rev. Environ. Sci. Technol. 32 (2010) 17–72, https:// doi.org/10.1080/10643380290813444. [2] P.S. Devon, C. Jonathan, C. Rosemary, D.A. Axelrad, T.J. Woodruff, Evaluating cumulative organophosphorus pesticide body burden of children: a national case study, Environmental Science & Technology 43 (2009) 7924–7930, https://doi.org/10. 1021/es900713s. [3] J.L. Martinez Vidal, P. Plaza-Bolanos, R. Romero-Gonzalez, A. Garrido Frenich, Determination of pesticide transformation products: a review of extraction and detection methods, J. Chromatogr. A 1216 (2009) 6767–6788, https://doi.org/10.1016/j. chroma.2009.08.013. [4] L. He, X. Luo, H. Xie, C. Wang, X. Jiang, K. Lu, Ionic liquid-based dispersive liquidliquid microextraction followed high-performance liquid chromatography for the determination of organophosphorus pesticides in water sample, Anal. Chim. Acta 655 (2009) 52–59, https://doi.org/10.1016/j.aca.2009.09.044.

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