Monocationic versus dicationic-based monomethine cyanine dyes for ultrasensitive colorimetric detection of hypochlorite ion in water

Detecting residual chlorine as a hypochlorite ion (ClO^-) in drinking water is crucial for ensuring disinfection effectiveness and safety. In the present study, we report two novel Quinolium Benzothiazole-Based Cyanine (3ethylbenzothiazol-2(3 H)-ylidene)methyl)-1-(4-iodobutyl)quinolin-1-ium tetrafluoroborate (IBTQ) and 1-(3-(4-(dimethylamino)pyridin-1-ium-1-yl)propyl)-4-((3-methylbenzothiazol-2(3 H)-ylidene)methyl)quinolin-1-ium diiodide (DMP-BTQ) hypochlorite (ClO^-) sensors using UV- visible, colorimetric, and quartz crystal microbalance (QCM) techniques. The two sensors generate distinct absorption spectra, frequency shifts, and color changes that are visible to the naked eye. They exhibit high sensitivity and selectivity towards ClO^-. The sensors have limits of detection (LOD) values in the range of 13.92 ppm and 0.127 ppm for IBTQ and DMP-BTQ, respectively, based on absorption performance with no interference of potential ions in drinking water. The method yields good recovery results, ranging from 97.4 to 103.0%, for ClO^- detection in the studied water samples. In addition, the LOD for the QCM technique is 0.06 ppm for IBTQ and 0.045 ppm for DMP-BTQ with low quantification. The sensors can be loaded on paper strips for naked-eye detection of ClO^- in domestic tap water and water treatment facilities. The sensors also provide low-cost, low cytotoxicity, high sensitivity, selectivity, and reusability of ClO^- in water. The sensing mechanism was rationalized in terms of radical cation generation upon ClO^- oxidizing action. The ease of cyanine oxidation was substantiated by quantum chemical studies including density functional theory (DFT) calculations, natural bond orbital (NBO) analysis, molecular electrostatic potential (MESP), and time-dependent density functional theory to support the experimental results.