In the presence of sodium citrate as stabilizer, a gold-platinum nanoalloy (GP) in the mole ratio of 49∶1 was prepared by using NaBH₄ as a reducing agent. The rabbit anti-human chorionic gonadotrophin antibody (RhCG) was labeled by the GP to obtain a stable immune GP probe (GP-RhCG) for the human chorionic gonadotrophin (hCG). In a pH 5.8 NaH₂PO₄-citric acid buffer solution and in the presence of KCl, the GP-RhCG probe was aggregated un-specifically to big particles that exhibited a resonance scattering (RS) peak at 590 nm. When the hCG was added, the GP-RhCG probe combined specifically with the hCG to form a dispersed GP-RhCG-hCG immunocomplex that resulted in the RS intensity decreasing at 590 nm. The decreased RS intensity (ΔI_{590 nm}) is linear with hCG concentration in the range of 6.67—86.7 ng/mL, with a detection limit of 3.7 ng/mL hCG. The immunocomplex exhibited a strong catalytic effect on the slow Cu₂O particle reaction of glucose-Cu(II), and the particles appeared a RS peak at 610 nm. When the hCG concentration increased in the immune reaction system, the immunocomplex concentration increased, and the catalytic product of Cu₂O particles increased, which caused the enhancement of RS intensity at 610 nm. The increased intensity (ΔI_{610 nm}) is linear with the hCG concentration in the range of 3.33—133 ng/mL, with a detection limit of 1.7 ng/mL hCG. In addition, the influence of foreign substances on the determination of hCG was examined. The results show that 150 times of L-aspartic acid, urea, BSA, glycine, Ca(II) and Zn(II), 50 times of L-lysine and L-cysteine, 30 times of IgG and IgA, 40 times of IgM did not interfered with the determination of 80 ng/mL hCG. This indicates that this method has good selectivity. The hCG content in the serum sample was analyzed by the GP RS method and GP-catalytic RS method, the results are agreement with those of the chemiluminescence experiments. Both RS methods are of some advantages including low-cost, simplicity, rapidity, high sensitivity and selectivity.

Key words: human chorionic gonadotrophin, gold-platinum nanoalloy, immunonanocatalysis, resonance scattering assay