Label-free electrochemiluminescence assay for aqueous Hg 2+ through oligonucleotide mediated assembly of gold nanoparticles.

Development of ultrasensitive method for Hg2+ analysis is important for human health protection and environment monitoring. In this work, we present a highly sensitive and selective electrochemiluminescence (ECL) assay in a "turn-on" mode for the detection of Hg2+ through selective assembly of gold nanoparticles (AuNPs) on the surface of indium tin oxide (ITO) electrode. In the absence of Hg2+ , the nonthiolated ssDNA could protected AuNPs against its assembly on ITO surface, producing rather low ECL emission for Ru(bpy)3 2+ /TPA system. Conversely, binding of Hg2+ with the Hg2+ -specific oligonucleotide through thymine-Hg2+ -thymine coordination formed the double-stranded structure, which could not effectively adsorb to AuNPs in solution. The assembly of free-state AuNPs is achieved, which well preserves electronical conductivity. The presence of AuNPs can catalyze the electro-oxidation of TPA, producing significantly enhanced ECL signal. Through detecting the ECL signal mediated by assembly of AuNPs, the proposed method was able to ensure substantial signal amplification and a low background. It was demonstrated that the ECL intensity was correlated with the ssDNA-based recognition reaction, enabling quantitative analysis of Hg2+ over the range of 8pM to 2nM, with a detection limit of 2pM. ECL intensity of the system were extremely specific for Hg2+ even in the presence of 1000-fold higher concentrations of other metal ions. Analytical results of Hg2+ spiked into water samples by the proposed ECL method were in good agreement with that obtained by atomic fluorescent spectrometry or mass spectrometry data.