Rapid detection of malachite green in fish based on CdTe quantum dots coated with molecularly imprinted silica
Introduction
Malachite green (MG) is a cationic triphenylmethane dye that has been widely used as a strong antifungal, antibacterial and antiparasitical agent (Plakas, El Said, Stehly, Gingerich, & Allen, 1996). Due to its broad antimicrobial spectrum and effectiveness at extremely low concentrations, MG has been used as the most efficacious antifungal agent in fish farming industries (Alderman & Clifton Hadley, 1993). However, many studies have indicated that MG is environmentally persistent and acutely toxic to aquatic and terrestrial animals, which may result in teratogenic, carcinogenic and mutagenic effects on human body (Srivastava et al., 2004, Zhang et al., 2012). Thus, the use of MG in aquaculture has been banned or restricted in many countries (Bilandžić, Varenina, Kolanović, Oraić, & Zrnčić, 2012). Despite this, the illegal use of MG in fish farming has still been found because of its availability and low cost (Bañuelos et al., 2016). Therefore, it is still important to develop detection methods for MG. To date, some detection methods have been developed, including high-performance liquid chromatography (HPLC) (Sagar, Smyth, Wilson, & McLaughlin, 1994), HPLC-mass spectrometry (HPLC-MS) (Halme, Lindfors, & Peltonen, 2004), and gas chromatography-mass spectrometry (GC–MS) (Aprea, Colosio, Mammone, Minoia, & Maroni, 2002), etc. Although these methods exhibit excellent selectivity and sensitivity, sophisticated and time-consuming sample pre-treatment is required (Rodríguez, Navarro-Villoslada, Benito-Peña, Marazuela, & Moreno-Bondi, 2011). Hence, the aim of the present study is to develop a rapid, simple and accurate method for the determination of MG in fish samples.
Semiconductor nanocrystals, also named quantum dots (QDs), have attracted a lot of attentions in recent years as a new fluorescent material. The remarkable unique properties of QDs, such as narrow emission spectra, broad excitation spectra, excellent photostability and size tenability (Dabbousi et al., 1997, Valizadeh et al., 2012), allow their wide applications in sensor probes, biological labeling and cellular effectors (Chan & Nie, 1998). Fluorescence resonance energy transfer (FRET) refers to a near-field energy transfer from an excited state donor (usually a fluorophore) to a ground state acceptor within close proximity through long-range dipole-dipole interactions (Sapsford, Berti, & Medintz, 2006). In recent years, FRET systems, especially QDs-based FRET systems, have attracted great attention due to the advantages of simple equipment and high sensitivity and speed compared with those systems using traditional organic fluorescent molecules (Selvin, 2000). For example, CdTe QDs and rhodamine B were combined as a FRET system to detect melamine in milk samples based on the changes of fluorescence intensities between rhodamine B and CdTe QDs (Tang, Du, & Su, 2013). Tan et al. applied thioglycolic acid-capped CdTe QDs as a fluorescence probe to quantitatively detect kaempferol in pharmaceutical preparations by a FRET system (Tan, Liu, Shen, He, & Yang, 2014).
Molecular imprinting is used to design tailor-made binding sites potentially complementary to a target molecule in size, shape and functionality (Lin et al., 2016). Typically, in imprinting process, the functional monomers and cross-linkers are copolymerized to form molecularly imprinted polymers (MIPs) in the presence of the target analyte which acts as the template. After the removal of the template, complementary binding sites are formed within the polymer network that allows specific rebinding of the template (Liu et al., 2013). The application of MIPs as a capping agent can clearly enhance the detection selectivity of QDs (Zhang, He, Chen, Li, & Zhang, 2012). Recently, MIPs-coated QDs have attracted much attention in various fields. Xiao et al. appended molecularly imprinted silica layers to QDs and established an enzyme-linked immunosorbent assay (ELISA)-like method which has been successfully employed to detect the residue of cypermethrin in fish samples (Xiao et al., 2016). Chantada-Vázquez et al. prepared Mn-doped ZnS QDs coated with MIPs for the detection and assessment of cocaine in human urine samples (Chantada-Vázquez et al., 2016).
In the present work, a fluorescent probe was synthesized with the reverse microemulsion method. The prepared sensor was applied to the rapid detection of MG in fish samples with high sensitivity and selectivity.
Section snippets
Reagents and materials
Malachite green (MG) was obtained from Fuchen Chemical Reagent Factory (Tianjin, China). NaBH4, tellurium powder, 3-mercaptopropionic acid (MPA), n-hexanol, triton X-100, (3-aminopropyl)triethoxysilane (APTES), poly dimethyl diallyl ammonium chloride solution (PDDA, 35%) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Tetraethyl orthosilicate (TEOS) were purchased from Sigma-Aldrich Corporation (Missouri, USA). CdCl2, NaOH, hydroxylamine hydrochloride, acetic acid/ammonium
Characterization of MIP-coated QDs
The pH dependent photoluminescence behavior of CdTe QDs in aqueous solution was provided in Fig. S1a. The buffer pH presented a great effect on the fluorescence intensities of CdTe QDs. The fluorescence intensities first increased then decreased with pH value. The highest fluorescence intensity was observed at pH 8.0. The characteristic absorption and emission spectra of CdTe QDs in water were shown in Fig. S1b. The maximum absorption peak of QDs was about 575 nm and the maximum emission was
Conclusions
A modest silica imprinting layer was successfully anchored to the surface of CdTe QDs with a reverse microemulsion method using MG as the template. The spherical MIP-coated QDs showed core-shell structure with CdTe QDs located in the center. The MIP-coated QDs presented fluorescence quenching efficiency for MG in acetonitrile solution with two linear relationships of MG ranging from 0.08 to 20 μmol·L−1, the detection limit was 12 μg·kg−1 (3σ, n = 9). The excellent recovery and RSDs for the
Acknowledgements
This work was supported by the Science and Technology Planning Project of Fujian Province, China (2016Y0064, 2014Y0045); EUR-cooperative Foundation of Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources (FJMBIO1605); the Science and Technology Planning Project of Xiamen, China (3502Z20143018); Fujian Provincial Key Laboratory of Food Microbiology and Enzyme Engineering (M20140902); the Foundation for Innovative Research Team of Jimei
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