A Ligand Exchange-based Fluorogenic Assay for Cartap Using Cu2+-calcein Blue Complex

Yoon Ji Shin, Min Sik Eom, Seungyoon Kang, Min Su Han
2014 Bulletin of the Korean Chemical Society (Print)  
Cartap, an ion channel blocker of the insect nicotinic acetylcholine receptor, has been used as an insecticide since it was commercialized in 1967. 1 It is commonly used worldwide and is one of the most frequently used insecticides against pests in rice, tea trees, fruit trees, etc. because it has a high insecticidal activity and a low toxicity in humans. 2 The overuse of cartap can lead to dangerous levels of residues in various foods, which result in a hazard for human health because cartap
more » ... uses significant neuromuscular toxicity, and eventually results in multisystem organ failure. 1,3 Owing to the toxicities of cartap, food administrations in various countries have defined the maximum residue limits for cartap. For example, the permissible limit for cartap in crops was set at 0.5 ppm by the WHO. 4 The toxicity of cartap and the restriction of residue limits have led to the development of various detection methods for cartap residues in food. A number of detection methods have been developed based on gas chromatography (GC), high performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), and liquid chromatography-mass spectrometry (LC-MS). 5 These methods are very useful for the quantification of cartap in various samples with high sensitivity, but require expensive instruments and a laborious setup process including the determination of optimum eluent conditions, column selection, and flow rate optimization. A chemosensor-based assay for cartap is one of the most desirable methods to address these problems because it has multiple advantages over other types of assays including low cost, ease of application, versatility, and high sensitivity. 6 Although various detection methods for cartap have been developed, only a limited number of chemosensor methods have been developed. 7 Recently, fluorescent chemosensors for cartap were developed based on nanocrystals and a combination of cucurbit[7]uril and palamatine, which can detect cartap in aqueous samples with high sensitivity and selectivity. 7a,b However, these chemosensors are turn-off type chemosensors and produce a decreased fluorescence that is undesirable for analytical purposes. Therefore, it is highly desirable to develop a turn-on type chemosensor for cartap with high sensitivity and selectivity. Metal-ion fluorogenic dye complex-based chemosensors have been developed for the detection of analytes that have high binding affinities to metal ion in the complexes. 8 These detection methods are based on ligand exchanges in which the metal ions are removed from the complexes by analytes, thereby inducing a fluorescence change in the metal complexes. For example, a Cu 2+ -gelatin complex and a Cu 2+zincon complex were designed for the detection of phytate and cyanide ions, respectively. 8a,b In this paper, we report a fluorogenic chemosensor for cartap with high selectivity and sensitivity, using a Cu 2+ -calcein blue complex. Cartap has a high binding affinity for transition-metal ions because the molecule has two thiocarbamate moieties and one amine moiety. The fluorescence of calcein blue is quenched by Cu 2+ due to mechanisms inherent for paramagnetic species. 9 Therefore, the Cu 2+ -calcein blue complex would be converted to free calcein blue when exposed to cartap, which results in the enhancement of fluorescence in calcein blue as shown in Scheme 1. A fluorescence titration of Cu 2+ was carried out using a 100 nM solution of calcein blue in borate buffer at pH 8.0 to determine the optimal concentration of Cu 2+ for the cartap assay. Fluorescence emission of calcein blue was measured after the addition of Cu 2+ and the emission curves are shown in Figure 1 . The addition of Cu 2+ induced a reduction of fluorescence in calcein blue (Figure 1 inset) , and the fluorescence was nearly proportional to the Cu 2+ concentration until saturation occurred at 1.5 equiv of Cu 2+ . The Cu 2+ -cal-Scheme 1. Schematic illustration of the cartap assay.
doi:10.5012/bkcs.2014.35.12.3642 fatcat:udmdyab4gzbltn3hqky4l2hufm