Polymer-based biocompatible fluorescent sensor for nano-molar detection of Zn2+ in aqueous medium and biological samples

Abstract

Chemosensors are prepared through condensation reactions between polymeric amines and three different aldehydes; the activities of the three products were evaluated in DMF–H₂O (7 : 3, v/v) solvent system. Chemosensor 2c demonstrated selectivity for Zn²⁺ enhancement of the fluorescence intensity at 420 nm. Upon binding of 2c with Zn²⁺, an approximately 9 fold enhancement in fluorescence intensity was observed due to the combined effect of ESIPT, CN isomerisation and the two-channel quenching of PET. The investigation of the photophysical properties of 2c revealed that the sensor at low pH led to a shift in the emission profile because of the presence of two types of nitrogen i.e. sp² and sp³ nitrogens and the pH dependent fluorescence profile is due to the protonation at sp³ hybridised nitrogen that influences the charge transfer transitions of 2c. However, the salt effect has not much influence on the photophysical properties of 2c; and 2c was found to form organic nano-particles (ONP) in a DMF–H₂O (1 : 99, v/v) solvent system and this material was found to detect a minimum of 40 nM concentration of Zn²⁺. The receptor 2c was found to be biocompatible through MTT assay and potent to detect Zn²⁺ in real biological conditions of microbial cells. The compounds 2a, 2b and 2c while immobilized in PVC based membrane, exhibited their selectivity for zinc ions as ion selective electrodes. These electrodes responded linearly in a wide concentration range with a lower detection limit of 10 μM. The electrodes have been successfully used as indicator electrodes in potentiometric titration of zinc ions with EDTA.