Analysis of wavelength shift versus surface coverage of proteins on nano-porous silicon immunosensor

The Fabry-Perot fringe pattern is determined by wavelength shifts in interferometric reflectance spectroscopy, which is a function of the refractive index (n) and thickness (l) of the porous silicon (PSi). In this paper, we demonstrated the shifts of wavelength as systematic controlling of adsorbed concentration of a protein on the PSi substrate. In order to correlate the wavelength shift of the Fabry-Perot fringe pattern with the protein adsorption, adsorbed concentrations of avidin (66 kDa) on the PSi substrate were measured by three independent methods: first, by means of bromophenol blue, which is applied to surface-confined avidin; second, with fluorescence intensity measurement of FITC-tagged avidin; and finally by utilizing Time of Flight-Secondary Ion Mass Spectrometry (TOF-SIMS). It was found that the wavelength shift is directly proportional to the surface-adsorbed avidins over the wide range of surface concentration on the PSi substrate, with a rate of 0.1 nm red-shift at avidin concentration change of 1.0 fmol/cm2.