Glutathione-protected fluorescent gold nanoclusters for sensitive and selective detection of Cu²⁺

We report a novel and environmentally friendly fluorescent probe for detecting copper ion (Cu²⁺) using glutathione (GS)-protected Au nanoclusters (GS-Au NCs). The fluorescence-based sensing behavior by GS-Au NCs toward metal ions was investigated in buffered aqueous media. Among the metal ions studied, Cu²⁺, Hg²⁺ and Pb²⁺ were found to efficiently quench the fluorescence emission of GS-Au NCs. In differentiation of Cu²⁺ and Hg²⁺ (or Pb²⁺), we found that (1) in the presence of Pb²⁺, the GS-Au NCs solution first appeared turbidity, but with ethylenediaminetetraacetate (EDTA) added to the solution, the fluorescence recovered and the turbidity disappeared; (2) in the presence of Hg²⁺, EDTA could not recover the quenched fluorescence of GS-Au CNs by Hg²⁺, which is in contrast with Pb²⁺ and Cu ²⁺. The Cu²⁺ detection limit was determined to be approximately 86 nM based on the fluorescence quenching and linear fluorescence response in the Cu²⁺ concentration range 1.00 × 10^-7 to 6.25 × 10^-6 mol/L. The high sensitivity of 86 nM Cu ²⁺ and selectivity to Cu²⁺ among 17 types of metal cations make GS-Au NCs a good candidate for fluorescent sensors for Cu²⁺.