Detection of Dithiocarbamate Pesticides with a Spongelike Surface

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Detection of Dithiocarbamate Pesticides with A SpongeLike Surface-Enhanced Raman Scattering Substrate Made of Reduced Graphene Oxide-Wrapped Silver Nanocubes Chuhong Zhu, Xiujuan Wang, Xiaofei Shi, Feng Yang, Guowen Meng, Qizhong Xiong, Yan Ke, Hua Wang, Yilin Lu, and Nianqiang Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13479 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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ACS Applied Materials & Interfaces

Detection of Dithiocarbamate Pesticides with A Sponge-Like Surface-Enhanced Raman Scattering Substrate Made of Reduced Graphene Oxide-Wrapped Silver Nanocubes Chuhong Zhu,a Xiujuan Wang,a Xiaofei Shi,c Feng Yang,c Guowen Meng,*, a Qizhong Xiong,a Yan Ke,a Hua Wang,a Yilin Lu,d and Nianqiang Wu*,b a

Key Laboratory of Materials Physics, CAS Center for Excellence in Nanoscience, and Anhui

Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, China. b

Department of Mechanical and Aerospace Engineering, West Virginia University, P.O. Box

6106, Morgantown, WV 26506, USA. c

Industrial and Management Systems Engineering Department, West Virginia University,

Morgantown, WV 26506, USA. d

Institute of Technical Biology and Agriculture Engineering, Hefei Institutes of Physical

Science, Chinese Academy of Sciences, Hefei, 230031, China.

KEYWORDS: surface-enhanced Raman scattering, dithiocarbamate, silver nanocube, reduced graphene oxide, surface plasmon resonance

ACS Paragon Plus Environment

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ABSTRACT

Dithiocarbamate (DTC) pesticides are widely used for fruits, vegetables and mature crops to control fungal diseases. Their residues in food could pose a threat to human health. Therefore surface-enhanced Raman scattering (SERS) based sensor is developed to detect DTC pesticides because SERS can provide the characteristic spectrum of pesticides and avoid the use of a molecular recognition probe in the sensor. To achieve high sensitivity, good anti-interference ability and robustness of the SERS sensor, a silver nanocube-reduced graphene oxide (AgNCrGO) sponge is devised. In the AgNC-rGO sponge, the rGO sheets form a porous scaffold that physically holds the AgNCs, which create narrow gaps between the neighboring AgNCs, leading to the formation of “hot spots” for SERS signal amplification. When DTC pesticides coexist with aromatic pesticides in a sample matrix, the AgNC-rGO sponge can selectively detect DTC pesticides due to the preferential adsorption of DTC pesticides on the Ag surface and aromatic pesticides on the rGO surface, respectively, which can effectively eliminate the interference of the SERS signals of aromatic pesticides, and facilitate the qualitative and quantitative analysis of DTC pesticides. The AgNC-rGO sponge shows a great potential as a SERS substrate for selective detection of DTC pesticides.


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INTRODUCTION Dithiocarbamate (DTC) pesticides such as thiram and ferbam, are widely used during growth of fruits and vegetables to control fungal diseases, and to preserve mature fruits and vegetables during storage and shipment. Extensive use of DTC pesticides poses a risk on human beings. Uptake of DTC pesticides could induce lethargy, hyperactivity, loss of muscle tone, dyspnea, ataxia, convulsions and even severe fetal malformations.1,2 Therefore, tolerances have been set up for DTC pesticides by the U.S. Environmental Protection Agency (EPA).3 For example, the tolerances for thiram residues in banana, apple, peach, strawberry and avocado range from 2 ppm to 15 ppm. The tolerances for ferbam residues in pear, apple, peach, grape, mango, cabbage, bean and lettuce, range from 4 ppm to 7 ppm. Typically DTC pesticides are measured by largescale analytical instruments such as high-performance liquid chromatography (HPLC) or HPLCmass spectrometry (HPLC-MS).4-7 These analytical methods cannot be used for on-site monitoring of DTC residues. Hence electrochemical, colorimetric and fluorescent sensors have been developed as portable tools for detection of pesticides.8-11 Generally molecular recognition probes such as antibody and enzyme are needed in the above sensors for selective recognition of the targeted pesticide. Molecular recognition probe-modified surface-enhanced Raman scattering (SERS) platforms can realize the selective detection of organic molecules,14,15 biomolecules,16-18 small inorganic molecules19 and ions19,20. And SERS has strong ability to resist the interference from other substances in the sample matrix21-29 On the other hand, SERS can provide the fingerprint spectrum of the pesticide analyte.12,13 This makes it possible for SERS to selectively detect DTC without the use of molecular recognition probe, simplifying the design of the sensor and saving the cost.


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In the present work, we have developed a sponge-like SERS substrate in which the Ag nanocubes (abbreviated as AgNCs) are wrapped in the reduced graphene-oxide (abbreviated as rGO) sheets. Individual plasmonic AgNCs, which have a higher SERS activity than spherical silver nanoparticle as a result of their sharp features on surfaces,30,31 are able to enhance the electromagnetic field (EM) remarkably. The rGO sheets serve as a scaffold to hold the AgNCs to create a porous sponge and to make the AgNCs in proximity to each other, creating “hot spots” between the adjacent AgNCs. The greatly compressed regularly-shaped button-like AgNC-rGO sponge has a flat surface and high-density of AgNCs, which can ensure good SERS signal uniformity, substrate-to-substrate reproducibility and high SERS activity.32 Therefore the regularly-shaped AgNC-rGO sponge fabricated in a cost-effective way shows a great potential as a commercialized SERS substrate.33-37 In addition, the rGO sheets can selectively capture the aromatic pesticide molecules due to the π-π interaction between the rGO and the aromatic ring structures that are widely present in pesticide molecules. In contrast, the analyte (thiram and ferbam) contains no any aromatic unit and thus have weak affinity to rGO; but they can strongly bind to silver through the thiol moieties. In the AgNC-rGO sponge, the AgNCs are mainly distributed on the surface and subsurface of the sponge; and the center of the sponge is occupied by the rGO. During evaporation of the analyte solution, the aromatic pesticide molecules are brought into the center of the sponge where rGO is present but AgNCs are almost absent. In this case, when the AgNC-rGO sponge is used for measurement of a mixture of non-aromatic DTC analyte and aromatic pesticide molecules, the aromatic pesticide molecules will not induce a SERS band because they are held by the rGO sheets, keeping away from the plasmonic AgNCs. This reduces the interference of aromatic pesticide molecules on the SERS signal of DTC pesticides.


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EXPERIMENTAL SECTION Chemicals. NaCl, glycol, AgNO3, pentanediol, thiram, NaHSO3, ferbam, thiophanate-methyl, polyvinylpyrrolidone (PVP)-29 and N,N-dimethylformamide were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). An aqueous solution of rGO was purchased from Institute of Coal Chemistry, Chinses Academy of Sciences. Dichlorodiphenyl-trichloroethane (DDT), carbendazim, isazofos and methyl parathion were purchased from J&K Chemicals Ltd. Synthesis of AgNCs. The AgNCs with edge length ~ 70 nm were synthesized according to a previously reported approach.38 NaCl (10 mg) was dissolved in glycol (10 mL) under stirring. Two precursor solutions were prepared by stirring AgNO3 (55 mg) in pentanediol (3 mL), and PVP-29 (55 mg) in pentanediol (3 mL) that contains Cl– ions, respectively. The reaction solution was prepared by heating pentanediol (5 mL) in a flask in an oil bath at 155 °C under continuous stirring for 1 h. Both the precursor solutions were simultaneously injected into the hot pentanediol at a rate 600 µL/min by a double syringe pump. After completion of the reaction (5 h later), the flask was taken from the oil bath immediately and cooled to room temperature under stirring. The product was centrifuged and washed repeatedly, and then dispersed in N,Ndimethylformamide. AgNCs with edge-length ~ 50 nm, ~ 90 nm and ~ 100 nm were provided by Hua Wang, whose fabrication procedure will be reported later. Synthesis of AgNC-rGO sponge. 10 mg of NaHSO3 was added into a light yellow aqueous solution (2.5 mL) containing 20 µg of graphene oxide and ~ 5 µg of AgNCs under mild ultrasound. Then, the reaction was carried out at 98 °C for 3 h, a gel of sponge-like rGO that embedded AgNCs was formed and the solution became transparent. After the product was rinsed with ultrapure water for several times and freeze-dried, the sponge-like hybrid was immersed


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into an ethanol solution (100 mL) containing high-density of AgNCs (~ 100 µg) for 1 hour. Then, the sponge-like rGO adsorbed high-density AgNCs was dried in an oven at 40 °C for 1 hour. It is worth noting that high-density of AgNCs in the aqueous mixture (~ 80 µg) would lead to the formation of disperse hybrid of AgNCs and rGO (Figure S1) rather than a sponge-like hybrid, therefore low concentration AgNCs should be used in the first step. Finally, the spongelike hybrid was compressed into a sub-mm “button” with a dimeter of ~ 0.50 mm and a height of ~ 0.18 mm±0.02. The compression was achieved by pressing the sponge hybrid into a pipette tip (10 µL, 0.5 mm in a diameter) with a steel rod (with a diameter of 0.5 mm). Characterization. The as-prepared AgNC-rGO sponge was characterized by field-emission scanning electron microscope (SEM, SU8020) and transmission electron microscope (TEM, JEOL 2010). The specific surface area of the sponge was measured by a Coulter Omnisorp 100CX Brunauer-Emmett-Teller (BET) instrument. Adsorption spectrum was recorded on a UVVis spectrophotometer (Cary 5E). SERS measurements were conducted with a confocal microprobe Raman system (Renishaw, inVia) with an excitation wavelength of 785 nm, 633 nm or 532 nm. During SERS measurement, the laser light was vertically projected onto the samples. For checking SERS performance of DTC pesticide detection, an acetone solution (20 µL) containing a certain amount of analyte molecules was dropped into the compressed AgNC-rGO sponge, and then was dried for several minutes.

RESULTS AND DISCUSSION Structural Characterization. Figure 1 schematically shows the procedure for fabrication of the AgNC-rGO sponge. First, an aqueous mixture of graphene-oxide (GO), AgNCs and NaHSO3 (Figure S2a) was heated to ~98 oC, the sponge-like AgNC-rGO hybrid was gradually formed


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(Figure S2b, and Step I in Figure 1). Second, the AgNC-rGO hybrid was subject to freeze-drying to obtain the AgNC-rGO assembly (Figure S2c, and Step II in Figure 1). Third, the AgNC-rGO assembly was immersed into an ethanol solution containing high concentration of AgNCs for 1 hour for loading more AgNCs (Figure S2d),39 and then dried in an oven at 40 oC for 1 hour (Figure S2e, and Step III in Figure 1). The AgNCs were mainly distributed on the surface and subsurface of the sponge. Fourth, the AgNC-rGO assembly was compressed into a button-like AgNC-rGO sponge (Step IV in Figure 1) in a dimeter of ~ 0.50 mm and a height of ~ 0.18 ±0.2 mm (Figure S3). Finally, a droplet containing DTC pesticide and aromatic pesticides was dropped on the compressed AgNC-rGO hybrid and naturally dried for SERS detection (Step V in Figure 1). The DTC pesticide molecules would strongly adsorb on the AgNCs by formation of a bidentate complex with the Ag atoms via their S-C-S group. While aromatic pesticides would be adsorbed on rGO via the π-π interaction between the rGO and the aromatic rings. During analyte solution evaporating, most of the aromatic pesticide molecules were brought into the center of the sponge where only sparse AgNCs existed. Therefore, only the SERS signal of DTC pesticide could be obtained (Step V in Figure 1). The AgNC-rGO assembly obtained after freeze-drying was highly porous (Figure 2a) with a specific surface area of 219 m2/g. Such a large specific surface area allows loading of highdensity of AgNCs.39 It can be seen that the AgNCs were decorated on the rGO sheets that located on the surface and subsurface of the sponge (Figure S4). Only sparse AgNCs distributed in the center of the sponge (Figure S5). After compression (Figure S3), the surface of the AgNC-rGO hybrid was flattened, and the pore size was reduced to be in the range of tens of nanometers to hundreds of nanometers (Figure 2b and 2c). The 70 nm sized AgNCs were wrapped by the rGO sheets (Figure 2c). Transmission electron microscopy (TEM) observation confirmed that there


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were high-density of AgNCs in the sponge, and small gaps even less than 10 nm were formed between the neighboring AgNCs (Figure 2d). It is expected that “hot spots” will be generated at these narrow gaps, leading to significant enhancement of the electromagnetic field and correspondingly the SERS sensitivity.29,40-42

Figure 1. Schematic of the fabrication procedure for a button-like SERS platform made of AgNC-rGO sponge. (I) Simultaneous reduction of GO and assembly of rGO with AgNCs in water. (II) Freeze-drying of the AgNC-rGO sponge. (III) Modification of more AgNCs on the rGO sponge. (IV) Compression of the AgNC-rGO sponge into a sub-mm “button”. (V) Addition of DTC pesticide and aromatic pesticides into the compressed AgNC-rGO hybrid for selective detection of the DTC pesticide.


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Figure 2. Morphology of the sponge-like rGO-wrapped AgNCs. (a) SEM observation of the freeze-dried uncompressed AgNC-rGO sponge. (b, c) SEM views of the compressed AgNC-rGO sponge. (d) TEM view of the AgNCs-decorated rGO sheets that were peeled off the AgNC-rGO sponge and the close-up view shown in the inset. SERS Sensitivity to Thiram. The optical absorption spectrum of the AgNCs showed strong localized surface plasmon resonance (LSPR) band in a wavelength range from 400 nm to 800 nm (Figure S6). SERS spectra of thiram were obtained when the button-like AgNC-rGO sponge was excited by the lasers with wavelengths of 532 nm, 633 nm or 785 nm, respectively (Figure S7). Among the lasers with three different wavelengths, the 785 nm laser excited the strongest SERS spectrum of thiram (Figure S7). This may be due to the fact that LSPR peak has a spectral overlap with the 785 nm laser after a high-density of AgNCs are physically close to each other (Figure S8).43 Therefore, the laser with 785 nm wavelength was employed for analyte analysis in this work. It was found that the SERS activity improved with an increase in the AgNC size in the range of 50 ~ 100 nm (Figure S9).44 Because of the size uniformity and simplicity in


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fabrication, AgNCs with an average size ~ 70 nm were used for SERS detection in this study. Compressing the AgNC-rGO assembly was important to improvement of SERS sensitivity and signal reproducibility. After the hybrid was compressed into a sub-mm “button” (Step IV in Figure 1 and Figure S3), the volume of the AgNC-rGO hybrid was reduced by about 99%, and the density of AgNCs increased by about 100 times accordingly, which should increase the SERS sensitivity significantly. For example, the SERS spectrum of 2 µM thiram obtained from the compressed sub-mm “button” (Curve I in Figure S10) was much stronger than that of 1 mM thiram from the AgNC-rGO assembly before compression (Curve II in Figure S10). The enhancement factor of thiram was estimated to be 1×106 for the 1382 cm–1 band (detailed calculation can be found in Supporting Information).41 The SERS-signal uniformity, as a main index of a SERS substrate, was also improved by compression because of the flatter surface of the compressed AgNC-rGO hybrid. SERS-signal uniformity was evaluated by SERS mapping of 0.1 mM thiram (Figure 3b), which was recorded on a randomly selected 60 µm×60 µm region (Figure 3a) with 121 measurement points (i.e., 5 µm×5 µm resolution per point). The SERS spectra taken from 20 randomly chosen points (Figure 3c) were highly similar. The relative intensities of the strongest peak at 1382 cm–1 from all of the 121 spectra are shown in Figure 3d. The relative standard deviation (RSD) of the peak intensities was calculated to be 9.9%, showing the good SERS-signal uniformity of the button-like AgNC-rGO sponge.45-47 By employing the uncompressed AgNC-rGO as SERS substrates, the RSD of the 1382 cm–1 peak intensity from 1 mM thiram was more than 50%. This indicated that compression treatment was very important to improve the SERS-signal uniformity. The RSD of the average peak intensities of the button-like AgNC-rGO sponge from different batches was calculated to be 15%, showing the good substrate-to-substrate SERS-signal reproducibility.46 The stability of the SERS activity is very


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important for practical application. Therefore the SERS activity of the AgNC-rGO sponge was stored under a dry condition for different durations (Figure S12). After the AgNC-rGO sponge hybrid was stored for 100 days, the SERS spectrum peak intensity of thiram decreased less than 5% (Figure S12), showing high stability of SERS activity. The high stability may be attributed to the fact that the rGO sheets prevented the AgNCs from oxidation.48

Figure 3. SERS-signal uniformity of the as-prepared AgNC-rGO sponge. (a) Optical image of the AgNC-rGO sponge. (b) SERS map of 0.1 mM thiram on the area shown in (a). (c) 20 randomly selected SERS spectra from the SERS map shown in (b). (d) The intensity deviation of 1382 cm–1 band calculated using the SERS spectra from the SERS map as shown in (b). Selectivity to DTC Pesticides. To prove the selectivity of the AgNC-rGO sponge toward DTC pesticides, the SERS spectra of a mixture of thiram and some other pesticide molecules form


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both the AgNC-rGO sponge and the bare AgNCs on a glass slide were investigated. When the bare AgNCs were employed to enhance the SERS signal of a mixture of methyl parathion and thiram, the characteristic peaks of both the analytes were observed in the SERS spectrum (Curve I in Figure 4a). In contrast, only the SERS signal of thiram was observed when the AgNC-rGO sponge was used to detect such analyte mixture (Curve III in Figure 4a). Similarly, the SERS signals of both thiram and DDT appeared in the SERS spectrum (Curve I in Figure 4b) measured using the bare AgNCs, while all the peaks of SERS spectrum came only from thiram when the AgNC-rGO sponge was employed as the SERS substrate (Curve III in Figure 4b). Additionally, it was also found that the AgNC-rGO sponge could selectively detect DTC pesticides (such as thiram or ferbam) mixed with isazofos, thiophanate-methyl, carbendazim and so on. The high selectivity may be attributed to the preferential adsorption of pesticides on AgNCs and rGO, respectively. It is well-known that rGO can serve as a highly efficient sorbent for organics,49,50 especially for the aromatic ring structures that are widely present in pesticide molecules.51-53 Therefore, most of aromatic pesticides are adsorbed on rGO via the π-π interaction between the rGO and the aromatic rings, which keep aromatic pesticides away from the plasmonic AgNC surfaces, resulting in the absence of the SERS signals of such pesticides in the measured SERS spectra. In contrast, DTC pesticides can be strongly adsorbed on the AgNCs because their S-C-S group can form a bidentate complex with the Ag atoms.54,55 DTC pesticides thus possess stronger affinity to the AgNCs than to rGO. As a result, the SERS spectra of DTC pesticides are highly enhanced because of the adsorption of the DTC pesticide molecules on the plasmonic AgNCs. The selectivity is also dependent on the structure of sponge. When our previously reported graphene sheet-Ag nanoparticle hybrid56 was used to detect an aromatic pesticide (e.g., methyl parathion), strong SERS signal of methyl parathion was obtained. But it did not have selectivity


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toward DTC pesticides. In this paper, only

Detection of Dithiocarbamate Pesticides with a Spongelike Surface

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