Signal-off electrochemiluminescence immunosensors based on the quenching effect between curcumin-conjugated Au nanoparticles encapsulated in ZIF-8 and CdS-decorated TiO2 nanobelts for insulin detection

A new strategy for the highly sensitive electrochemiluminescence (ECL) detection of insulin was developed based on curcumin-conjugated Au nanoparticles wrapped in zeolitic Zn2+-imidazolate cross-linked framework nanoparticles (Au-Cur/ZIF-8) quenching the ECL of CdS-decorated TiO2 nanobelts (CdS@TiO2). Curcumin-conjugated Au nanoparticles were embedded in ZIF-8 through epitaxial growth or nucleus coalescence. Herein, dual coreactants H2O2 and K2S2O8 were employed to enhance the ECL emission of CdS@TiO2, and the enhancing mechanism may be due to the electrogenerated hydroxyl radical (OH˙) strengthening the amount of sulfate radical anion (SO4˙-), which was the main species of generating ECL signals. In order to sensitively detect insulin, Au-Cur/ZIF-8 labels were used to anchor secondary antibodies via amidation reaction and inhibit the ECL intensity of CdS@TiO2. Moreover, the possible mechanism of quenching between Au-Cur/ZIF-8 and CdS@TiO2 was ascribed to the poor conductivity of ZIF-8, the consumption of OH˙ and the ECL resonance energy transfer between CdS@TiO2 and Au-Cur. Under optimal conditions, a sandwich immunosensor was prepared to determine insulin with a good linear range from 0.3 pg mL-1 to 20 ng mL-1 and a low detection limit of 0.09 pg mL-1 (S/N = 3).