Aggregation of Gold Nanoparticles Caused in Two Different Ways Involved in 4-Mercaptophenylboronic Acidand Hydrogen...

The difference in gold nanoparticle (AuNPs) aggregation caused by different mixing orders of AuNPs, 4-mercaptophenylboronic acid (4-MPBA), and hydrogen peroxide (H₂O₂) has been scarcely reported. We have found that the color change of a ((4-MPBA + AuNPs) + H₂O₂) mixture caused by H₂O₂ is more sensitive than that of a ((4-MPBA + H₂O₂) + AuNPs) mixture. For the former mixture, the color changes obviously with H₂O₂ concentrations in the range of 0~0.025%. However, for the latter mixture, the corresponding H₂O₂ concentration is in the range of 0~1.93%. The mechanisms on the color change originating from the aggregation of AuNPs occurring in the two mixtures were investigated in detail. For the ((4-MPBA + H₂O₂) + AuNPs) mixture, free 4-MPBA is oxidized by H₂O₂ to form bis(4-hydroxyphenyl) disulfide (BHPD) and peroxoboric acid. However, for the ((4-MPBA+AuNPs) + H₂O₂) mixture, immobilized 4-MPBA is oxidized by H₂O₂ to form 4-hydroxythiophenol (4-HTP) and boric acid. The decrease in charge on the surface of AuNPs caused by BHPD, which has alarger steric hindrance, is poorer than that caused by -4-HTP, and this is mainly responsible for the difference in the aggregation of AuNPs in the two mixtures. The formation of boric acid and peroxoboric acid in the reaction between 4-MPBA and H₂O₂ can alter the pH of the medium, and the effect of the pH change on the aggregation of AuNPs should not be ignored. These findings not only offer a new strategy in colorimetric assays to expand the detection range of hydrogen peroxide concentrations but also assist in deepening the understanding of the aggregation of citrate-capped AuNPs involved in 4-MPBA and H₂O₂, as well as in developing other probes.