Antibody labelling. The affinity purified immunoglobulin
(IgG) was conjugated to colloidal gold (40 nm) 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 concentration
of 0.1 mg/ml in sodium phosphate buffer (1 mM, pH 7.0).
The pH of colloidal gold solution and the diluted IgG was adjusted
to pH 8.0 with 0.1 M KCO3. Ten aliquots of variable concentrations
(0.01-0.1 mg/ml) of the diluted IgG were prepared
in 0.2 ml sodium phosphate buffer. Then, a series of diluted IgG
solution was 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 in the curve drawn from the concentration
and the absorbance. Aliquot (approximately 10 ml) of purified
IgG (0.1 mg/ml) was added drop-wise to l00 ml of colloidal-gold
solution (pH 8.0) followed by the addition of 10 ml of filtered
10% bovine serum albumin (BSA), pH 8.0 with gentle stirring
for 20-25 min. The solution was incubated for 10 min at 25°C
and centrifuged at 15,000 g for 30 min at 4°C.