*2.3.1 Detection of amino acids*

The fluorescent graphene quantum dots-gold nanoparticles as nanosensor showed a high selectivity and high sensitivity for the detection of cysteine [23]. The AuNPs@r-GQDs nanocomposite was prepared by the following processes. First, nitrogen-doped graphene quantum dots (N-GQDs) were reduced to r-GQDs by NaBH4 as reductant and subsequently the r-GQDs converted HAuCl4 to Au nanoparticles (AuNPs) by reduction reaction and coated onto AuNPs forming core-shell-structured AuNPs@r-GQDs. The AuNPs@r-GQDs showed good dispersion behavior with an intensive surface plasma band at 525 nm. The AuNPs@r-GQDs exhibited aggregation behavior and led to their color change by using cysteine as cross-linking agent through adsorption of Ag ions onto their surface. The detection limit was 5.6 nM. Furthermore, the AuNPs@r-GQDs showed higher selectivity for cysteine than that of glutathione (GSH) even at the interfere condition of 1000-fold concentrations of GSH.

The GQD-MnO2 complex as a convenient fluorescence nanosensor has been constructed and prepared for the detection of glutathione (GSH) with high selectivity and high sensitivity [24]. The fluorescence intensity of GQDs was quenched upon addition of MnO2 nanosheet based on the mechanism of fluorescence resonance energy transfer (FRET). The fluorescent signal recovered upon GSH reducing MnO2 nanosheets into Mn2+ ions and releasing GQDs. The GQD-MnO2 complex as nanoprobe showed a sensitive response to GSH between 0.5 and 10 μmol/L. The fluorescence intensity showed a good linear relationship with the concentrations of GSH. The detection limit was 150 nmol/L. The GQD-MnO2 complex exhibited higher selectivity for the GSH than that of other metal ions and biomolecules and successfully applied to detect GSH in living cells.
