A urea-based polyphenol receptor demonstrates colorimetric and fluorometric detection of cyanide in real-life applications

Cyanide (CN^-) anions are highly toxic and pose a serious threat to biological systems. Therefore, a strategy for the rational design of a new optical sensor for the selective and sensitive recognition of CN^- in aqueous media is presented. The approach relies on the blend of structure-property relationships to design the optimal sensor unit and tune its sensing characteristics. Two urea-based receptors, NPH and NPN, have been designed to investigate their ability to recognize anions in the solution phase. The sensor shows the advantageous features of both absorption and emission spectrometry in the visible spectral range, a red shift in fluorescence, and enhancement upon analyte coordination. In addition, the limit of detection (LOD) determined using UV-visible and fluorescence titration experiments was found to be 0.28 μM and 1.36 μM, respectively, which were much lower than the maximum concentration set by the World Health Organization (WHO) guidelines for cyanide (1.9 μmol L^-1) in drinking water. Despite having the same anion-binding site, <i>i.e.</i>, urea unit, the sensitivity and selectivity of both the probes were found to be different. A plausible mechanistic study reveals hydrogen bonding interactions followed by deprotonation, which further facilitated the extent of intramolecular charge transfer (ICT). This paper explores the potential practical use of the system for screening paper strips, real-life water samples, and food samples, highlighting its potential as a cyanide detection chemo-sensor. Moreover, the DPPH free radical scavenging study revealed that NPH had good antioxidant properties.