Nanogold Catalysis Based Immunoresonance-Scattering Spectral Assay for Trace Complement Component 3

Complement component 3 (C3) is an essential bridge linking innate immunity and adaptive immunity. We describe an immunonanogold catalytic resonance-scattering (RS) technique for assaying C3 in serum. We used nanogold to label goat antihuman C3 antibody to obtain an immunonanogold RS probe for C3. The immune reaction between nanogold-labeled antibodies and antigens was carried out in Na(2)HPO(4)-sodium citrate buffer, pH 5.6, containing polyethylene glycol. After centrifuging the particle suspension, we used RS to monitor the catalytic effect of nanogold-labeled anti-C3 in the supernatant on the chlorauric acid-hydroxylamine (HAuCl(4)-NH(2)OH) particle reaction and used electron microscopy to monitor particle shape. We assayed 36 human serum samples with the immunonanogold catalytic RS assay and immunoturbidimetry. Nanogold-labeled anti-C3 had a marked catalytic effect on the reaction of HAuCl(4) and NH(2)OH to form particles, which exhibit a maximum RS peak at 585 nm. The decrease in RS intensity, DeltaI(RS), of the nanocatalytic system was proportional to C3 concentration from 5.0 to 160.0 ng/L. The detection limit for the C3 assay was 1.52 ng/L. Results obtained with serum samples agreed with those obtained with an immunoturbidimetric method. A linear regression analysis of 28 nonpathologic serum samples revealed a correlation coefficient of 0.960, with mean (SD) slope and intercept values of 0.787 (0.0218) g/L and 0.28 (0.026) g/L C3, respectively. The immunonanogold catalytic RS assay showed high sensitivity and good selectivity for measuring C3 in human serum. This method may become useful for diagnosing certain diseases, such as hepatitis.