Colorimetric assay for sulfate using positively-charged gold nanoparticles and its application for real-time monitoring of redox process.

A flexible nanoparticle-based sulfate assay is demonstrated in which the positively-charged gold nanoparticles (cysteamine-AuNPs) act as indicator. The aggregation of cysteamine-AuNPs is selectively induced by sulfate, which allows the rapid colorimetric sensing of sulfate without any precipitant, sample preparation and specific instruments. In this work, the cysteamine-AuNPs probe has been successfully applied to the colorimetric detection of sulfate and demonstrates superior sensitivity with a detection limit of sulfate of ~50 ppb. A surprise finding is that the proposed probe can achieve the goal of real-time monitoring and translating a redox process into an appreciable color change via the aggregation of nanoparticles. This is a novel application of a positively-charged AuNPs-based nanoprobe for sulfate detection, kinetic study of the redox process, and opens up new opportunities for design of more novel colorimetric strategies and expansion of AuNPs-based application in different fields.