A dual-mode probe for colorimetric and fluorometric detection of cysteine based on phosphorus/nitrogen co-doped CQDs and gold nanorods

Abstract

In this study a novel method was developed for the fluorescent and colorimetric determination of cysteine (Cys). The fluorescent determination was designed based on surface plasmon-enhanced energy transfer (SPEET), and phosphorus/nitrogen co-doped CQDs (PNCQDs) were employed as the donor, of which the fluorescence was quenched by gold nanorods (AuNRs) (acceptor). For the colorimetric assay, under the catalytic effect of Fe3+, AuNRs were effectively etched by OH· that was generated by the Fenton reaction. The etch of the AuNRs lead to the decrease in the absorbance intensity, the blue shift of longitudinal surface plasmon resonance (LSPR) absorption peak of AuNRs as well as the fluorescence of PNCQDs recovery. However, Cys can suppress this etching reaction by coordination effect between hydrosulfide group (SH) in Cys and Fe3+, leading to red shift of the LSPR absorption peak. Similarly, the fluorescence will change with the absorbance. The peak shift and the fluorescence change can be applied to monitor of Cys. Under the optimal operation conditions, highly sensitive determination of Cys was achieved, with the fluorescent detection limit of 1.2 nM and the colorimetric detection limit of 0.9 nM. The linearity of the system towards Cys was in the range of 0.005–25 μM. The highly sensitive determination of Cys in urine samples demonstrates that this method possesses high potential for on-site and visual detection of urine Cys.