Ligands dissociation induced gold nanoparticles aggregation for colorimetric Al³⁺ detection

Aluminum is a very important analyte, and developing biosensors for aluminum is an important analytical task. In this work, we report a novel mechanism to design colorimetric sensor based on gold nanoparticles (AuNPs). The AuNPs were prepared by reducing HAuCl₄ using catechols, and the resulting AuNPs can be directly adapted for Al³⁺ detection without any post-modifications, showing high sensitivity and selectivity against other metal ions. Interestingly, our mechanistic studies revealed that Al³⁺-induced AuNPs aggregation was not due to the formation of interparticle crosslinks that refers to the design principle of most AuNPs-based colorimetric sensors reported before. But rather, Al³⁺ competitively coordinated with the capping ligands on AuNPs surface through the formation of stable Al-O bond, which dissociated these ligands from AuNPs surface. As a result, the AuNPs aggregated due to the loss of surface stabilizers. Based on this mechanism, several catechols, including pyrocatechol (PC), 3-(3,4-dihydroxyphenyl) propionic acid (DHCA), levodopa (LDA) and dopamine (DA), were used as reductant to prepare AuNPs for Al³⁺ sensing, and the AuNPs prepared by DA (AuNPs/DA) displayed the highest sensitivity, with detection limit of 0.81 μM. The sensor was then tested for Al content analysis in river water and food samples, and the results supported its practical applications. Importantly, this work expands the design principles for colorimetric sensors by using AuNPs.