Selective luminescence determination of cysteine by using terbium-modified silver nanoparticles or terbium-modified graphene quantum dots

Two methods for the luminescence determination of cysteine (Cys) are presented. They make use of either silver nanoparticles (Ag NPs) or graphene quantum dots (GQDs), both doped with terbium(III). The methods are based on the finding that Cys quenches the green luminescence of Tb(III)-Ag NPs and Tb(III)-GQDs. The excitation/emission maxima are at 306/545 and 257/545 nm, for both nanoprobes, respectively. Response is linear in the 0.28-5.0 μg mL^-1 Cys concentration range for the Tb(III)-Ag NP system, and from 0.05-3.0 μg mL^-1 for the Tb(III)-GQD system. The respective limits of detection are 0.09 and 0.015 μg mL^-1. The probes were applied to the time-resolved luminometric determination of Cys in (spiked) food supplements and gave satisfactory results. Graphical abstractSchematic representation of the quenching by cysteine (Cys) of the time-resolved luminescence (TSL) of terbium-graphene quantum dots [Tb(III)-GQD] and of terbium-silver nanoparticles [Tb(III)-Ag NP].