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A novel immunonanogold-catalytic Cu2O-enhanced resonance scattering (RS) spectral assay is reported, based on nanogold modified rabbit anti-penicillin G (RAPG) antibody, its catalytic enhanced effect on the slow particle reaction between Cu(II) and glucose, and the RS effect of (Au)nucleus(Cu2O)shell complex particle at 608 nm. As a model, we used nanogold in size of 9 nm to label RAPG to obtain an immunonanogold probe (AuRAPG) for penicillin G (PG). The immunoreactions between the probe and PG took place in phosphate-citric acid buffer solutions. After centrifugation, the excess AuRAPG in the supernatant was used to catalyze the particle reaction to amplify the RS signal. With the addition of PG, the concentration of AuRAPG in the supernatant reduced linearly, which leaded the I608 nm to decrease accordingly. The decreased RS intensity was proportional to the PG concentration in the range of 0.09-21.6 ngldrmL-1, with a detection limit (DL) of 0.01 ngldrmL-1 PG. The assay was used to determine trace PG in raw milk sample, with the recovery between 100.6% and 109.2%.