Coreactant-free and Near-Infrared Electrochemiluminescence Immunoassay with n-Type Au Nanocrystals as Luminophores.

The electrochemiluminescence (ECL) bioassay is prominently carried out with the involvement of the coreactant. To remove the detrimental effects of the coreactant on the ECL of luminophores, herein, a promising ECL immunoassay strategy with biocompatible nanoparticles as the luminophore is proposed, which involves directly and electrochemically oxidizing the luminophore methionine-capped Au (Met@Au) nanocrystals (NCs) without the participation of any coreactant. Met@Au NCs are a kind of n-type nanoparticles, and they can be electrochemically injected with valence band (VB) holes around +0.80 and +1.10 V (vs Ag/AgCl). The electrochemically injected exogenous VB hole can recombine with the endogenous conduction band electron of Met@Au NCs and eventually bring out two coreactant-free and near-infrared ECL processes around 0.80 V (ECL-1) and 1.10 V (ECL-2). The intensity of coreactant-free ECL is primarily determined by the electrochemical oxidation-induced hole-injection process. ECL-2 is considerably stronger than ECL-1 and can be exploited for determining the carcinoembryonic antigen (CEA) in a sandwich immunoassay procedure with a linear range from 0.1 to 50 pg/mL as well as a limit of detection of 0.03 pg/mL (S/N = 3).