Rapid Visual Detection of Calcium Ions Using Glutathione Functionalized Gold Nanoparticles

In this paper, glutathione (GSH)-capped gold nanoparticles (GSH-GNPs) were prepared in water media and used for the colorimetric assay of Ca²⁺. GSH could quickly bind onto the GNP surfaces through its -SH group and the cross-linking of GSH-capped GNPs could be quickly achieved by chelation by its functional groups (-NH₂ and-COOH) upon the addition of Ca²⁺. Color changes were observed from red to blue (or purple) within minutes with the increased concentration of Ca²⁺, in response to surface plasmon absorption of dispersed and aggregated nanoparticles. Using this colorimetric sensor, the concentration of Ca²⁺ was determined with a limit of detection low than 1×10^-7 M by the naked eye and could further be monitored by a UV-vis spectrometry easily. The possible mechanism of the aggregated GSH-GNPs was discussed by the addition of ethylene diaminetetraacetate (EDTA) and Ca²⁺ in different orders.