Multimodal Ion Sensing by Structurally Simple Pyridine-End Oligo p-Phenylenevinylenes for Sustainable Detection of Toxic Industrial Waste

Environmental pollution induced by toxic metal ions and harmful chemicals mainly from industrial waste poses a significant threat which urges for their rapid detection before release into the ecosystem above the permissible level. Optical sensors are inexpensive, simple, yet efficient in sensing such toxicants. Herein, we show that structurally simple pi-conjugated pyridine-end p-phenylenevinylene oligomers can selectively sense toxic metal ions and anions in solution, supramolecular gels, as well as in solid support. Interaction of Hg2+ at nanomolar concentrations with the linear pyridine-ends via two-site coordination was clearly seen from ``naked eye'' color changes and fluorometric investigations. The sensitivity as well as selectivity of the oligomers toward Hg2+ was found to be greatly affected by the extent of aromatic conjugation and pK(a) of the end-pyridine functionalities. Interestingly, one of the oligomers (3) containing an isoniazid moiety renders visual color changes with both Hg2+ and CN- ions through two different binding sites involving two nonidentical sensing pathways which enable this probe for the detection of multiple analytes at the same time. Moreover, these ions (Hg2+/CM-) showed remarkable tuning of the supramolecular assembly (molecular gels) of 3, depicting reversible sol gel transformation on complementary addition of Hg2+/CN- in proper stoichiometry which could be useful in scavenging toxic ions from industrial wastes. In addition, the low-cost, reusable paper discs coated with the probe molecules show rapid, onsite detection of toxic ions even from the contaminated water samples. Therefore, this highly efficient multimodal sensing of toxic ions by easy-to-synthesize molecular probes could inspire the design of new sensors with varying chromophores for the color-tunable sensing of toxic ions