Photothermometric analysis of bismuth ions using aggregation-induced nanozyme system with a target-triggered surface cleaning effect.

The development of nanozyme-based photothermometric sensing for point-of-care testing (POCT) heavy metal ions is of great significance for disease diagnosis and health management. Considering the low catalytic activity of most nanozymes at physiological pH, we found bismuth ions (Bi3+) could effectively enhance the peroxidase (POX)-like activity of cetyltrimethylammonium bromide and citrate-capped octahedral gold nanoparticle (CTAB/Cit-AuNP) nanozymes. It is mainly based on Bi3+ ions being able to trigger the surface cleaning effect of CTAB/Cit-AuNPs. Because the more active Bi3+ ions could effectively bind with citrate on the gold surface and competitively destroy the electrostatic interaction between citrate and CTAB, resulting in the removal of CTAB ligands from the gold surface. Without the ligand protection, CTAB/Cit-AuNPs aggregated immediately, and further resulted in a significant activation of the POX-like activity of AuNP nanozymes. Based on this principle, we introduced the enzyme substrate 3,3',5,5'-tetramethylbenzidine (TMB) into this aggregation-induced nanozyme system, and rationally designed a photothermometric platform to quickly and sensitively detect Bi3+ ions by using the good photothermal effect of the oxidation product of TMB (oxTMB). The developed photothermometric method only using a common thermometer has a limit of detection (LOD) as low as 45.7 nM for POCT analysis of Bi3+ ions. This study not only provides a more accurate understanding of the aggregation-induced nanozymes based on the surface cleaning principle, but also shows the potential applications of aggregation-induced nanozymes in the POCT field.