Cysteine Modification of Glutathione-Stabilized Au Nanoclusters to Red-Shift and Enhance the Electrochemiluminescence for Sensitive Bioanalysis

Screening new electrochemiluminescence (ECL) emitters for the design of sensitive detection strategies with even long emission wavelength is intensively anticipated in ECL evolution. Herein, a promising modification strategy for improving the ECL performance of Au nanoclusters (AuNCs) as a water-soluble luminophore was proposed. Upon the introduction of l-cysteine (l-Cys) onto the surface of glutathione (GSH)-stabilized AuNCs (GSH-AuNCs), the dual-thiol bond between l-Cys and GSH was formed to limit the intramolecular motion and nonradiative relaxation of the excited state from the capping agents, which resulted in the enhancement of monochromatic ECL emission of GSH-AuNCs with a red-shifted wavelength. By utilizing triethylamine as a coreactant, the ECL of l-Cys/GSH-AuNCs was about 1.5-fold stronger than that of GSH-AuNCs, and the emission wavelength red-shifted from 660 to 780 nm at a relatively low potential, which could decrease the interference in bioassay and the photochemical damage in nondestructive detection. As a proof of application, a sandwich-type immunosensing method for CYFRA 21-1 was proposed with l-Cys/GSH-AuNCs as the signal tag, which displayed a wide linear ranging from 0.2 fg/mL to 2 ng/mL and a limit of detection down to 0.067 fg/mL at 3S/N. This work provides a wonderful strategy for promoting the performance of ECL emitters.