A colorimetric immunoassay chip has been developed based on gold nanoparticles for indicating the antibody-antigen binding activity and gold enhancement for amplifying the specific binding signal. Our investigations showed that the results of immunoassay can be represented by the level of color intensity. They were easily observed by a regular camera or naked eye, which is not needed of sophisticated laboratory equipment. Optimization of experimental conditions was carried out and the colorimetric detection had been compared to the standard chemifluorescent detection. Under the optimized conditions, colorimetric immunoassay chip had been demonstrated to detect different amount of immobilized antigens, i.e., human IgG. The results, i.e., color intensity, were mapped to the concentration of immobilized antigens in a dynamic range of 1-5,000 ng/ml. The proposed detection method does not require any sophisticated optical systems; therefore, it is possible to be miniaturized and integrated into a microfluidic system for developing a portable immunoassay device.

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