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Rapid and simple detections of two kinds of prohibited fish drugs, crystal violet (CV) and malachite green (MG), were accomplished by surface-enhanced Raman scattering (SERS). Based on the optimized Au/cicada wing, the detectable concentration of CV/MG can reach 10−7 M, and the linear logarithmic quantitative relationship curves between logI and logC allows for the determination of the unknown concentration of CV/MG solution. The detection of these two analytes in real environment was also achieved, demonstrating the application potential of SERS in the fast screening of the prohibited fish drugs, which is of great benefit for food safety and environmental monitoring.
Some drugs, including crystal violet (CV) and malachite green (MG), can be used to keep the health of fish, but they all have bad effects on people.[1, 2] CV, a kind of triphenylmethane dye, has been found to be quite effective as fungicide and biocide in the fishery industry owing to its splendid bactericidal effect. Though it can be used as anti-fungal additive in the production of feed to improve the health of living body, CV gives side-effects by biotransformation after entering into the human or animal body and is carcinogenic, teratogenic, mutagenic, and so on.[1] MG is a cationic triphenylmethane dye and can act as a drug to sterilize and kill parasites in aquaculture. Nevertheless, this substance is resistant to biodegradation and has serious carcinogenicity and genotoxicity.[2] Though being forbidden to be used in aquatic products,the abuse of both CV and MG cannot be stopped due to their low cost and high efficiency. In consequence, it is necessary to find an expeditious approach to realize the sensitive detection of trace amounts of CV and MG.
There are many methods to detect CV and MG, such as liquid chromatography–tandem mass spectrometer, gas chromatography–mass spectrometry, and high-performance liquid chromatography,[3, 4] but they are either time-consuming or complicated. Compared with these methods, surface-enhanced Raman scattering (SERS) can realize rapid, simple, and inexpensive detection. In the past decades, SERS has gained much attention all around the world due to its characteristics of non-destructive testing feature, rapid response, sensitive detection and trace analysis ability. As a convenient tool, SERS is widely used in many fields of science, including food safety, environmental evaluation, biosensing and analytical chemistry.[5–7] Benefiting from the distinct enhancement of signal intensity, SERS can be used to analyze a substance at the single molecule level.[8] The enhancement effect is generally owing to a combination of the electromagnetic (EM) mechanism and the chemical mechanism, and the EM mechanism is predominant. To highly enhance the signal of the analyte molecules, noble metals, such as Au, Ag, and Pt, are needed to prepare SERS substrates, and simultaneously microstructure having dense nanogaps is desired to facilitate the formation of enormous hot spots[9] on the substrates. Qi Jiwei et al. fabricated high-performance SERS substrate with deep controllable sub-10-nm gap structure by depositing Au film on the cicada wing, but the reproducibility is hard to control because of the method of ion beam sputtering.[9] Ichiro Tanahashi and Yoshiyuki Harada fabricated naturally inspired SERS substrate by photocatalytically depositing silver nanoparticles on cicada wings, but the substrate is not stable enough to store because Ag is easily oxidized in air.[10] Though great enhancement has been achieved by fabricating complicated substrates, it is difficult to reach the goal of the quantitative detection of some specific analytes.
In this study, using Au/cicada wing as SERS substrate, the quantitative detection of CV and MG was achieved. The detectable concentration of CV and MG can both attain as low as 10−7 M, and the logarithmic quantitative relationship is linear which can serve as a standard for the determination of the unknown concentration of CV/MG solution. Furthermore, CV and MG can also be quantitatively determined in a real environment, indicating the practicability of the SERS technique for the CV/MG detection.
Cryptotympana atrata fabricius were purchased from Jia-Ying Art Museum of Entomology. Rhodamine 6G (R6G), crystal violet (CV), and malachite green (MG) were procured from J & K Scientific Ltd. Deionized water (18 MΩ) acquired from Beijing Chemical Works was used for all experiments. The water for aquatic product was taken from a local aquaculture market located near Huixinxijie Nankou Station of Beijing Subway, named Tianlihong market. All chemicals, unless otherwise noted, were of analytical grade and were used as received.
The typical morphologies of cicada wing and Au/cicada wing were observed by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-7800F).
The aqueous solutions of 10−2-M R6G, CV and MG were prepared first, and then they were diluted to lower concentrations. The SERS signals of R6G, CV, and MG were obtained after the 10-
The cicada wing has homogeneous micropapillae structure on its surface, as shown in Fig.
The SERS spectra of R6G at different concentrations were checked to evaluate the SERS property of the Au/cicada wing, as shown in Fig.
In order to calculate the enhancement factor (EF) of the substrate, series of R6G solution dropped on the Au/cicada wing were checked to ascertain the saturation value, upon which the concentration of R6G used for calculating EF could be determined, as exhibited in Fig.
To testify the SERS performance of the Au/cicada wing theoretically, the finite-difference time-domain (FDTD) simulations of electric field distribution were conducted. Figure
Using the Au/cicada wing substrate, SERS spectra of CV solutions with different concentrations were measured, and the result was shown in Fig.
To achieve the quantitative detection of CV, the logarithmic relationship curve of the 1618-cm−1 peak intensity was plotted to serve as a standard. The linear relationship between logI and logC is obvious, which is important for determining the CV concentration in solution. In order to examine whether the method can be applied in real environment, two types of water, including for life (E1) and for aquatic product (E2), were employed. The water for life was obtained from tap water in our lab and that for aquatic product was from a local aquaculture market. These two samples showed no CV signal before adding it. After being spiked with CV of different concentrations (
Figure
In this study, the quantitative detection of two prohibited fish drugs was achieved using the sensitive SERS technology. The detectable concentration of both CV and MG can reach as low as 100 nM, and their logarithmic quantitative relationship curves show good linearity, allowing for the quantitative determination of unknown CV/MG concentrations in various samples. The detection of CV/MG in a real environment was also demonstrated by SERS, which is of great importance for food safety and environmental protection.
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