Electrochemical immunosensor based on Ag⁺-dependent CTAB-AuNPs for ultrasensitive detection of sulfamethazine

An electrochemical biosensor was proposed utilizing an improved amplification strategy for the rapid detection of sulfamethazine (as a model target) in aquatic environments. In this competitive immunoassay, cetyltrimethylammonium bromide-capped gold nanoparticles (CTAB-AuNPs) were used as a signal amplifier and electrode matrix and coated with an antigen-antibody (Cag-Ab₁) specific binding system as a recognition unit for the target compound. In addition, silver nanoparticle labels were functionalized with dendritic fibrous nanosilica (DFNS@AgNPs) and decorated onto chitosan/single walled carbon nanohorn (CS/SWCNH)-modified glass carbon electrodes (GCEs), which improved the electron transfer rate and increased the surface area, enabling more coating antigens to be captured. Under acidic conditions, massive amounts of the Ag⁺ bound to the surface of the AuNPs dissolved, and consequently, formed Ag⁺@CTAB-AuNP complexes, which resulted in a distinctly improved peroxidase-like activity and enhanced current response. Furthermore, the destroyed Ab₁-Ab₂-DFNS conjugation greatly decreased the impedance, bringing about the amplification of the electrochemical signals. After optimization of the parameters, the proposed approach exhibited excellent performance, including good sensitivity (LOD, 0.0655 ng/mL) and satisfactory accuracy (recoveries, 79.02%-118.39%; CV, 3.18%-9.82%), which indicates the great potential of this strategy for the rapid detection of trace pollutants in the environments.