Figure 1: Silver reduction as a signal amplification method
Antibodies are first conjugated to 10 nm gold colloids, and then immersed in a solution containing silver nitrate and hydroquinone. The gold colloids catalyze the reduction of silver ions to silver atoms to generate a silver film whose opacity is a function of the concentration of the antigen. This film partially blocks the transmission of light through the transparent polystyrene plate. The silver product is visible to naked eye, and easily detected by a low-cost and custom-made optical detector to give a quantitative reading.