A simple benzothiazolium-based sensor for cyanide detection: Applications in environmental analysis and bioimaging

A new sensor based on Ethylbenzothiazolium-2-hydroxynaphthaldehyde conjugate-based fluorescent sensor, (E)-3-ethyl-2-(2-(2-hydroxynaphthalen-1-yl) vinyl) benzo[d]thiazol-3-ium iodide (SU-1) was designed and synthesized. The structure of SU-1 was confirmed by ¹H NMR, ¹³C NMR, HRMS, and single crystal XRD spectral analysis. SU-1 displayed a colorimetric and fluorometric response in a DMSO:H₂O (1:1,v/v) matrix, changing color from pale yellow to colorless visible to the naked eye, accompanied by a ∼ 120 nm red-shift in the absorption spectra upon CN^- addition. This shift, due to formation of deprotonation followed by the nucleophilic attack on the benzothiazolium ring's double bond, disrupts π-conjugation, blocking intramolecular charge transfer within SU-1. However, competitive anions showed negligible interference while detecting CN^-. The Limit of detection for CN^- was determined to be 0.27 nM, significantly below the WHO's permissible CN^- concentration in drinking water (1.9 μM). Job's plot analysis shows that the binding stoichiometry of SU-1 to CN^- is a 1:1, with a stability constant (Ka) of 1.58 x 10⁴ M^-1. The sensor demonstrated practical applications in environmental water samples and fluorescence imaging of intracellular CN^- in CAD cell line.