The affinity purified immunoglobulin (IgG) was conjugated to colloidal gold (40nm) according to the methods,
as described previously (Horisberger, 1989). The optimal concentration of antibody for conjugation with colloidal gold was determined by titrating aliquots of diluted IgG with colloidal gold. The purified IgG was diluted to a con-
centration of 0.1mg/ml in sodium phosphate buffer (0.001M, pH7.0). The pH of colloidal gold solution and
the diluted IgG was adjusted to pH8.0 with 0.1M Na2CO3.Ten aliquots of variable concentrations (0.01 - 0.1mg/ml)
of the diluted IgG were prepared in 0.2 ml sodium phosphate buffer, and added separately to 1 ml of the colloidal-
gold solution, as described previously (Li et al., 2013).After incubating the mixture for 10 min, 0.1 ml of 10%
NaCl was added to the tubes and the absorbance was measured at 520 nm. The least amount of protein required to
stabilize the colloidal gold was identified from the abscissa