Colorimetric determination of nine metal ions based on the de-aggregation of papain-functionalized gold nanoparticles and using three chelating agents

A colorimetric array for metal ions is described that is based on the de-aggregation of papain-capped gold nanoparticles (AuNPs) for discrimination of metal ions. The functionalized AuNPs were used as colorimetric probes, and three chelating agents (tripolyphosphate, citrate, and ethylenediamine tetraacetate were employed as the recognition receptors. In the absence of metal ions (Fe3+, Cr3+, Co2+, Mg2+, Pb2+, Ca2+, Zn2+, Ti4+, and Sn4+ were studied), the capped AuNPs are well dispersed in solution. In the presence of these ions, the metal ions bind papain and cause the AuNPs to aggregate. This causes a color change from red to purple. Different chelating agents induce different affinities between metal ions and papain. This results in distinct colorimetric response patterns. Linear discriminant analysis is used to discriminate the various metal ions on a three-dimensional spatial dispersion graph.