Immunonanogold-catalytic resonance scattering spectral assay of trace human chorionic gonadotrophin

Gold nanoparticles in diameter of 10 nm were used to label rabbit anti-human chorionic gonadotrophin (RhCG) antiserum to obtain a resonance scattering spectral probe (AuRhCG) for human chorionic gonadotrophin (hCG). The immunoreaction between AuRhCG and hCG take place to form hCG–AuRhCG immunocomplex in pH 5.0 citric acid–Na₂HPO₄ buffer solution. The immunocomplex solutions were centrifuged to obtain the supernatant solution. The AuRhCG in the supernatant solution exhibited strong catalytic effect on the particle reaction between Ag⁺ and hydroquinone to produce gold–silver composite particles in pH 3.4 citric acid–trisodium citrate buffer solution. There is a stronger resonance scattering (RS) peak at 423 nm for the particles. With the addition of hCG, the AuRhCG in the supernatant solution decreased, and the RS intensity at 423 nm decreased. The decreased RS intensity ΔI_423 nm was proportional to the concentration of hCG in the range of 2.5–208.3 mIU/mL with a detection limit of 0.83 mIU/mL. This method has been applied to the determination of hCG in urine samples, with satisfactory results.