A novel reverse fluorescent immunoassay approach for sensing human chorionic gonadotropin based on silver-gold nano-alloy and magnetic nanoparticles.

A novel and environmentally friendly reverse fluorescent immunoassay approach was proposed and utilized for sensing human chorionic gonadotropin (HCG) in human serum by coupling a newly prepared and highly fluorescent glutathione-stabilized silver-gold nano-alloy (GSH-AgAuNAs) with magnetic nanoparticles (MNPs). To construct such a reverse system, fluorescent GSH-AgAuNAs and MNPs were first prepared and bio-functionalized with monoclonal antibodies (Mab-I and Mab-II) toward HCG antigen, respectively. Then, the GSH-AgAuNAs functionalized with Mab-I were incubated with HCG, followed by the addition of MNPs attached to Mab-II. Thereafter, a sandwich-type immunoassay could be constructed for determination of HCG owing to the antibody-antigen recognition between the functionalized GSH-AgAuNAs and MNPs. Afterwards, a magnetic collection was employed. Hence, the amount of GSH-AgAuNAs would be reduced through an immuno-magnetic separation, thus weakening the fluorescent intensity. Different from conventional immunoassay, our work determined the quantitative signal by measuring the decreasing gradient fluorescent intensity. Under optimal conditions, the developed reverse method exhibited a wide linear range of 0.5-600 ng mL(-1) toward HCG with a detection limit of 0.25 ng mL(-1). Additionally, the proposed immunoassay was validated using spiked samples, illustrating a satisfactory result in practical application.