Point-of-need determination of hypochlorite ion based on analyte-induced anti-aggregation of Au nanoparticle attenuating their Tyndall effect

This work initially describes a highly sensitive colorimetric assay with smartphone readout that enables instrument-free, determination of hypochlorite (ClO − ) ion. The assay exploited analyte-induced anti-aggregation of Au nanoparticle (AuNP), which attenuated their Tyndall effect (TE). In the absence of the analyte, dithiothreitol (DTT) molecules (each bearing two terminal thiol groups) could form strong Au S bonds with the AuNP to induce the aggregation of these nanoprobes. This not only produced a familiar surface-plasmon-resonance-based red-to-blue color change but also markedly intensified the TE signal of the reaction mixture. In the presence of the ClO − , on the other hand, the analyte rapidly oxidized the thiol groups of DTT, thereby preventing the AuNP aggregation. These nanoprobes thus remained their original dispersed state, giving a red color and a weak TE response. The TE signal was inversely related to the ClO − concentration. Under optimized conditions, this new method, which required only an inexpensive 635 nm laser pointer to generate a red or red-yellow TE, offered far better visual discrimination of analyte concentration than the conventional SPR-based approach, reaching a visual limit of detection (LOD) of ~0.25 μM (~0.02 mg·L −1 ), ten-fold lower than that of the latter. Coupling the new assay with a smartphone for portable TE readout further allowed for linear quantification of ClO − over the 0.05–90 μM (0.0036–6.4 mg·L −1 ) range, lowering the quantitative LOD to ~14.2 nM (~0.001 mg·L −1 ) (3σ), about 30 times lower than that achieved by the traditional UV–Vis spectroscopy. The recovery of ClO − in four types of tested water samples (i.e., tap water, mineral water, pond water and pool water) ranged from 97.64 to 108.38%, additionally confirming the analytical accuracy and practical utility of the TE-enhanced smartphone-assisted equipment-free quantitative method developed herein.