Lysine adsorption on the silanized SiO2-surface for immobilization of the estrogen receptor hERα

We investigated the adsorption of the l-lysine (200 mmol) molecule to a silanized SiO2 surface as a function of the pH value. The SSC (Spraying Spin Coating) method [Cherkouk et al., J. Colloid Interf. Sci. 337 (2009) 375–380] was applied to functionalize the SiO2 surface by using the (3-aminopropyl)trimethoxysilane (APMS) as coupling agent with a NH2 functional group. We adsorbed lysine molecules to the silane film for pH-values of 2.5, 7.5, 8.7, 9.5 and 13, which correspond to the di-cationic, cationic, zwitterinonic (pH 8.7 and 9.5) and the anionic charge state of lysine, respectively. The infrared spectroscopy is not suitable to investigate the system because the NH3+ signal at 1600 cm−1 originating from the silane film overlaps with the infrared signal of the deprotonated carboxyl group of the lysine molecule. X-ray photoelectron spectroscopy (XPS) was used to measure the binding energies C 1s and N 1s as function of the pH value. This pH change affects the charge state which was fitted in the XPS spectra to obtain the optimal adsorption conditions at pH 7.5 of the lysine to the functionalized SiO2 surface.