Fluorescent methionine-capped gold nanoclusters for ultra-sensitive determination of copper(II) and cobalt(II), and their use in a test strip.

A fluorometric assay was constructed for supersensitive determination of Cu2+ and Co2+ based on their quenching effect on the orange fluorescence of methionine-capped gold nanoclusters (Met-AuNCs). A simple one-step method was developed for the preparation of the Met-AuNCs, employing L-methionine as both a reducing and protecting reagent. Within 10 min, water soluble Met-AuNCs were obtained with an average size of 2.4 nm. Under photoexcitation at 370 nm, the Met-AuNCs possess a maximum emission at 580 nm and a quantum yield of 2.3%. The response is fast (1 min), and the selectivity for Cu2+ and Co2+ is high over other metal ions. The detection limits for Cu2+ and Co2+ are around 47 and 420 pM, respectively. The effects were used to design a test paper for visual detection of Cu2+ and Co2+. Using this test paper, 1 μM of Cu2+/Co2+ can be detect under the UV lamp (365 nm excitation). It is perceived to be a promising tool for the rapid on-site determination of Cu2+ and Co2+ in real water samples. Graphical abstract Schematic presentation of gold nanoclusters (AuNCs) synthesis with methionine as both reducing and protecting reagent, Cu2+/Co2+ detection based on the quenching of the fluorescence of AuNCs, and visual detection of Cu2+/Co2+ based on a respective test strip.