Investigations of Chemical Modifications of Amino-Terminated Organic Films on Silicon Substrates and Controlled Protein Immobilization

Fourier transform infrared spectroscopy by grazing-angle attenuated total reflection (FTIR-GATR), ellipsometry, atomic force microscopy (AFM), UV-visible spectroscopy, and fluorescence microscopy were employed to investigate chemical modifications of amino-terminated organic thin films on silicon substrates, protein immobilization, and the biological activity and hydrolytic stability of immobilized proteins. Amino-terminated organic films were prepared on silicon wafers by self-assembling 3-aminopropyltriethoxysilane (APTES) in anhydrous toluene. Surface amino groups were derivatized into three different linkers: N-hydroxysuccinimide (NHS) ester, hydrazide, and maleimide ester groups. UV-visible absorption measurements and fluorescence microscopy revealed that more than 40% of surface amino groups were chemically modified. Protein immobilization was carried out on modified APTES films containing these linkers via coupling with primary amines (-NH(2)) in intact monoclonal rabbit immunoglobulin G (IgG), the aldehyde (-CHO) of an oxidized carbohydrate residue in IgG, or the sulfhydryl (-SH) of fragmented half-IgG, respectively. FTIR spectra contain vibrational signatures of these functional groups present in modified APTES films and immobilized IgGs. Changes in the APTES film thickness after chemical modifications and protein immobilization were also observed by ellipsometric measurements. The biological activity and long-term hydrolytic stability of immobilized IgGs on modified APTES films were estimated by fluorescence measurements of an adsorbed antigen, fluorescein isothiocyanate (FITC)-labeled goat anti-rabbit IgG (FITC-Ab). Our results indicate that the FITC-Ab binding capacity of half-IgG immobilized via maleimide groups is greater than that of the oxidized IgG and the intact IgG immobilized via hydrazide and NHS ester groups, respectively. In addition, IgGs immobilized using all coupling chemistries were hydrolytically stable in phosphate-buffered saline (PBS).