Ratiometric ultrasensitive electrochemical immunosensor based on redox substrate and immunoprobe

In this work, we presented a ratiometric electrochemical immunosensor based on redox substrate and immunoprobe. Carboxymethyl cellulose-Au-Pb²⁺ (CMC-Au-Pb²⁺) and carbon-Au-Cu²⁺ (C-Au-Cu²⁺) nanocomposites were firstly synthesized and implemented as redox substrate and immunoprobe with strong current signals at -0.45 V and 0.15 V, respectively. Human immunoglobulin G (IgG) was used as a model analyte to examine the analytical performance of the proposed method. The current signals of CMC-Au-Pb²⁺ (I_substrate) and C-Au-Cu²⁺ (I_probe) were monitored. The effect of redox substrate and immunoprobe behaved as a better linear relationship between I_probe/I_substrate and Lg C_IgG (ng mL^-1). By measuring the signal ratio I_probe/I_substrate, the sandwich immunosensor for IgG exhibited a wide linear range from 1 fg mL^-1 to 100 ng mL^-1, which was two orders of magnitude higher than other previous works. The limit of detection reached 0.26 fg mL^-1. Furthermore, for human serum samples, the results from this method were consistent with those of the enzyme linked immunosorbent assay (ELISA), demonstrating that the proposed immunoassay was of great potential in clinical diagnosis.