On-off-on gold nanocluster-based near infrared fluorescent probe for recognition of Cu(II) and vitamin C

The authors described gold nanoclusters (AuNCs) for use on an on - off - on NIR fluorescent probe for the determination of citrate and Cu(II) ion. The AuNCs were prepared by a microwave-assisted method using BSA as both the stabilizing and reducing agent. The resulting BSA-capped AuNCs display NIR fluorescence peaking at 680 nm under 500 nm excitation, a quantum yield of similar to 6.0%, an average size of 2.8 +/- 0.5 nm, water-dispersibility, stability and biocompatibility. The on-off probe for Cu(II) is based on the interaction between Cu(II) and BSA which causes the fluorescence of the BSA-AuNCs to be quenched. The quenched fluorescence is recovered on addition of vitamin C (VC), obviously due to complexation of Cu(II) by citrate. The probe was employed to image Cu(II) and citrate in HeLa cells and in aqueous solutions. The method works in the 20 nM to 0.1 mM concentration range for Cu(II), and in the 8 nM to 120 mu M concentration range for VC.