An electrochemiluminescence biosensor for mercury ion detection based on gamma-polyglutamic acid-graphene-luminol composite and oligonucleotides

An ultrasensitive electrochemiluminescence (ECL) biosensor based on gamma-polyglutamic acid-graphene-luminol (γ-PGA-G-luminol) composite and oligonucleotides for the detection of mercury ion (Hg2+) was developed. γ-PGA-G-luminol composite coated on the glassy carbon electrode could produce a strong and stable ECL signal by luminol, and provide carboxyl groups to cross-link with amino-modified DNA1 enriching in thymine (T) by γ-PGA. Upon addition of Hg2+ and biotin-modified DNA2 enriching in T, T-Hg2+-T interaction occurred and mediated the coordination between DNA1 and DNA2. Afterwards, streptavidin was connected to the end of DNA2 through specific binding of streptavidin to biotin, which obviously reduced the ECL intensity due to the inert protein layer with poor electron transfer ability. Based on the specific T-Hg2+-T coordination chemistry and the specific binding of streptavidin-biotin system, ultrahigh sensitivity and selectivity for Hg2+ detection were obtained. Results revealed that the ECL intensity was logarithmically linear with the concentration of Hg2+ in a wide range from 0.01 to 100 nmol/L (about 0.002-20 ppb). Moreover, the biosensor also exhibited excellent selectivity for Hg2+ ions without significant interference from commonly co-existing metal ions in the sample matrix. Excellent sensitivity and selectivity make the developed biosensor a potential and simple tool for the detection of Hg2+.