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Ten-nanometer nanogold showed the strongest catalytic effect on the particle reaction between Ag(I) and hydroquinone to form nanosilver particles that exhibited the strongest resonance scattering (RS) peak at 350 nm. The enhanced RS intensity was linear to the nanogold concentration in the range of 30-5,700 nM Au. The nanogold was used to label goat antihuman immunoglobulin M (GIgM) to obtain an immunonanogold probe (AuGIgM) for immunoglobulin M (IgM). Based on the nanogold-labeled immunoreaction between IgM and AuGIgM, centrifugation, and AuGIgM-Ag(I)-hydroquinone nanocatalytic reaction, a highly sensitive and selective immunonanogold-catalytic Ag particle RS assay for 0.2-300 ng mL −1 IgM was proposed, with a detection limit of 0.1 ng mL −1 . This assay was simple and sensitive and was applied to assay IgM in serum samples, with satisfactory results.

A sensitive and selective resonance scattering spectral (RSS) assay was proposed for the determination of sudan I (SDI), using 10 nm nanogold to label the antibody against sudan I (anti-SDI Ab) to obtain a RSS probe for SDI. The immunonanogold reaction between nanogold-labelled anti-SDI Ab and SDI took place in pH 4.92 KH 2 PO 4 -Na 2 HPO 4 buffer solution and in the presence of polyethylene glycol (PEG)-6000, and the intensity of resonance scattering peak at 580 nm decreased greatly. The decreased intensity ÁI 580 nm was proportional to the concentration of SDI in the range of 0.23-45.0 ng mL À1 . The linear regression equation was calculated as ÁI 580nm ¼ 1.20c þ 2.01 (R ¼ 0.9975, n ¼ 6), with a detection limit (3) of 0.13 ng mL À1 . The SDI in egg samples was assayed, with satisfactory results.

Nanogold, 8 nm in size, was used to label goat antihuman immunoglobulin M (GIgM) to obtain a new immunonanogold resonance scattering (RS) probe (Au-GIgG) for quantitation of trace immunoglobulin M (IgM). The Au-GIgG combined with IgM to form nanogold-labeled immunocomplex causes the RS intensity at 580 nm to be enhanced, in pH 4.49 KH 2 PO 4 -Na 2 HPO 4 buffer and in the presence of polyethylene glycol 6000. The enhanced RS intensity at 580 nm (ΔI 580 nm ) is proportional to the IgM concentration in the range of 1.5 to 2000 ng/mL, with a lower detection limit of 0.98 ng/mL. The immunonanogold RS assay was used to assay IgM in serum samples, with sensitivity, selectivity, and simplicity. (Journal of Biomolecular Screening 2008:302-308)