Chronopotentiometric synthesis of quantum dots with efficient surface-derived near-infrared electrochemiluminescence for ultrasensitive microchip-based ion-selective sensing

Abstract

A novel QD with near-infrared (NIR) electrochemiluminescence (ECL) emission was prepared electrolytically by hydrodynamic chronopotentiometry, using Unithiol, a clinically-known metalantidote, as the capping agent for the fabrication of an ultrasensitive ion-selective microchip. The proposed synthetic route as well as the optical properties of QD was clarified with both morphological and spectroscopic characterization. In air-saturated pH 8.0 phosphate buffer with dissolved oxygen as the endogenous coreactant, an intensive NIR-ECL emission at 692 nm arose which was ascribed to the unique surface states of multidentate-chelated QDs. By tuning electrolytes, a low-potential ECL peaking at −0.79 V (vs. Ag/AgCl) could further be achieved. Based on the validated competition of heavy metallic cations with the stabilizer Unithiol as it stabilizes the aqueous dispersion of QDs, the ECL emission could be significantly quenched and a home-made ECL ion-selective chip was manufactured. It was further found that capping with a cation exchange membrane enabled accelerated adsorption of metal ions and sensitized the ECL signal. Using cupric cations as a model analyte, the devised sensor showed a linear range from 10.0 pM to 1.0 mM with a detection limit down to 6.7 pM, and was successfully used in the direct detection of a Bordeaux mixture with an inorganic pesticide residue on the grape skin with high accuracy and selectivity. The proposed strategy could also be extended to quantify Hg²⁺ and Pb²⁺ with stronger thiol-bonding capability than Cd²⁺. The developed microsensing system with replicable one-step synthesis should facilitate portable and integrated QD-based NIR-ECL applications for food hygiene inspection and environmental monitoring.