Amplified immunosensing based on ionic liquid-doped chitosan film as a matrix and Au nanoparticle decorated graphene nanosheets as labels

This paper describes a new signal amplification strategy based on ionic liquid-doped chitosan film as a matrix and Au nanoparticle decorated graphene nanosheets (AuNP–graphene) as labels for the sensitivity improvement of an electrochemical immunosensor. At first, an ionic liquid was doped into ferrocene-branched chitosan film to obtain a novel redox composite, which was employed as an antibody immobilization matrix due to its better biocompatibility and higher electron transfer mobility. Then, the AuNP–graphene were prepared by a one-pot method in a aqueous-phase synthesis and were provided with a large surface area and multiple binding sites to allow high accessibility for the immobilization of secondary antibody (Ab2) and horseradish peroxidase (HRP). Based on the sandwich immunoassay format, the electrochemical signal could be amplified and adequately achieved, according to the catalytic reaction of the carried HRP towards the reduction of H₂O₂ with the aid of the IL and ferrocene synergistic effect. Using Immunoglobulin G (IgG) as a protein model, a good and repeatable linear relationship was found between the electrical signal outputs and human IgG concentration on a logarithm scale for a wide range of 2.0 × 10⁻¹⁰ to 5.0 × 10⁻⁷ g/mL, with a detection limit of 50 pg/mL. In conclusion, the use of ionic liquid-doped chitosan redox film and AuNP–graphene greatly enhances the electrochemical signal and shows high sensitivity.