Sol particle immunoassays using colloidal gold and neutron activation

The feasibility of performing immunoassays with colloidal gold labels and detection of¹⁹⁸Au by neutron activation has been demonstrated with measurements of human immunoglobulin and of serum antibodies to human immunodeficiency virus type 1. The detection sensitivity achieved after activation in a high flux reactor or with a water moderated²⁵²Cf source, by gamma-counting or by autoradiography, is similar to the sensitivity obtained with absorbance measurements in the more common enzyme immunoassays. The reactor based neutron activation assay allows detection of 10^-16 mol of analyte in routine operation with possible extension to 10^-20 mol. The sensitivity with the 1.3 Ci²⁵²Cf source is limited to about 10^-15 mol. The practical limitations of the assay's sensitivity at this point are due to background signals from reagents and/or nonspecific binding of the gold labeled reagent.