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.

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