In situ synthesis of gold nanoclusters in hydrogels for the capillary based portable fluorescence analysis of hypochlorite in environmental samples

Abstract

Bovine serum albumin (BSA) reduced HAuCl₄ under alkaline conditions to achieve the rapid in situ growth of gold nanoclusters (Au NCs) in hydrogels. The hydrogels doped with BSA and HAuCl₄ could not only provide reaction sites for the growth of Au NCs but could also precisely regulate the in situ growth process of the Au NCs. The hydrogel-based synthesis route displayed high repeatability, easy operation, and intriguing anti-interference performance, contributing to the establishment of comparable sensing applications. Hydroxide ions could diffuse from aqueous solutions to the inter-sides of the hydrogels, facilitating the reducibility of BSA and formation of BSA-protected Au NCs. Consequently, strong red fluorescence was detected in the hydrogels, including fluorescence changes in the hydrogels in the presence of hypochlorite (ClO⁻). It was concluded that a small amount of strong oxidizing agent ClO⁻ could promote the free sulfhydryl groups of BSA to transform into disulfide bonds, and then the proteins aggregated together. The aggregation of BSA in hydrogels contributed to the high-efficiency formation of Au NCs and consequently the fluorescence of the Au NCs was enhanced. Nevertheless, in the presence of high-concentration ClO⁻, the reducibility capacity of BSA was consumed, leading to a weakened fluorescence intensity at 650 nm; thus, the purple–red fluorescence signal of Au NCs correspondingly decreased and gradually turned to bluish fluorescence. The limit of detection was estimated to be 5.84 × 10⁻⁵ M. This phenomenon was used in glass capillary tubes to construct a portable, rapid, accurate device, which could monitor ClO⁻ by observation with the naked eye under UV irradiation.