A surface plasmon resonance interferometer based on spatial phase modulation for protein array detection

Abstract

Thousands of kinds of proteins exist in a single cell. Proteomics research aims to characterize these proteins and simultaneously analyse modifications and interactions on a large scale. Here we present a label-free surface plasmon resonance (SPR) imaging interferometer based on spatial phase modulation, which can be useful in this field. It consists of a light source, a SPR sensing unit, a special phase modulator, a photoelectric conversion unit and a computer. Collimated light is projected into a prism and reflected at the gold–glass interface. The p- and s-polarized components of the reflected light pass through a one-dimensional beam expander and a Wollaston prism, and form an interference pattern on a CCD. Interference images are acquired and transferred to the computer for data processing. Protein interaction on the gold surface leads to a local refractive index change and results in interference fringe phase shift. By calculating the phase shift, interaction information can be obtained. It is demonstrated that this technique can detect different concentrations of NaCl solutions, and the phase change generated by a 0.9% NaCl solution is about 10°. In protein–protein interaction experiments, a model system of rabbit IgG and goat–anti-rabbit IgG is tested. The maximum phase change is up to 12°. The phase resolution of the system is 0.2°, equivalent to the refractive index resolution of 3 × 10−5 RIU, and this value can be improved to 2 × 10−6 RIU just by increasing the gold thickness of the sensing chip. It is concluded that the sensitivity of the interferometer is enough to achieve larger capacity than that detected by the present protein micro-array products. These results suggest that the SPR interferometer based on spatial phase modulation provides a potential facility to meet the requirements in proteomics research.