Triarylamine Rhodanine Derivatives as Red Emissive Sensor for Discriminative Detection of Ag+ and Hg2+ ions in Buffer-Free Aqueous Solutions

A completely water-soluble, red emitting, multibranched sensor based on the electron-rich triphenylamine and electron-deficient rhodanine-3-acetic acid has been developed. The sensors mRA, dRA, and tRA, respectively, have one, two, and three rhodanine-3-acetic acid groups, responsible for the interaction with the metal ions as well as the solubility of the probe in water. mRA, dRA, and tRA senses Ag+ and Hg2+ ions in a buffer-free aqueous solution with the lowest detection level of 0.06 and 0.02 ppm, respectively. The yellow color of free sensor turns into purple and colorless in the presence of Ag+ and Hg2+ ions, respectively, which can be witnessed even by the naked eye. The metal ion preferentially binds with electron-deficient rhodanine-3-acetic acid owing to the presence of an ideal coordination environment. The distinctly different signals for Ag+ and Hg2+ ions occur because of the dissimilar binding modes, wherein the former extends and the latter breaks the pi-electron conjugation that results in a different signaling mechanism. Nevertheless, the additional binding sites of dRA and tRA influences the binding constant and sensitivity; however, additional metal ion binding does not occur because of the changes in the electronic properties after initial binding. The applicability of these sensors in the biological medium was also tested using HaCaT cells using Ag+ and Hg2+ ions, which demonstrated the quenching of intense red fluorescence of the probe, and thus, these probes can be also be used as a potential biosensor.