Sensitivity-enhancement of wavelength-modulation surface plasmon resonance biosensor for human complement factor 4.

Sandwich and colloidal Au techniques for enhancing the sensitivity of a wavelength-modulation surface plasmon resonance (SPR) immunosensor are demonstrated by the detection of human complement factor 4 (C4). The design of the wavelength-modulation SPR biosensor is based on fixing the incident angle of light and measuring the reflected intensity of light in the wavelength range spanning 500-900 nm simultaneously. The human C4 had good response in the concentration range 2-20 microg/mL in the direct assay. However, in the sandwich assay, the human C4 had good response in the concentration range 0.2-20 microg/mL and the lowest concentration is 10-fold lower than that obtained by the direct assay. With human C4-Au colloidal conjugate, the human C4 had good response in the concentration range 0.1-20 microg/mL and the lowest concentration is 20-fold lower than that obtained by the direct assay. In the colloidal-Au-enhanced sandwich assay, the human C4 had good response in the concentration range 0.05-5 microg/mL and the lowest concentration is 40-fold lower than that obtained by the direct assay. Under selected experimental conditions, the reproducibility, sensitivity, and reversibility of the enhanced SPR immunoassay are very satisfactory. The results represent potentially significant advantages in the sensitivity of SPR biosensors.