Protein adsorption and covalent bonding to silicon nitride surfaces modified with organo-silanes: Comparison using AFM, angle-resolved XPS and multivariate ToF-SIMS analysis

Organo-silanes provide a suitable interface between the silicon-based transducers of various biosensing devices and the sensing proteins, immobilized through physical adsorption, as for (3-aminopropyl)triethoxysilane (APTES), or covalent binding, e.g. via protein amine groups to (3-glycidoxypropyl)trimethoxysilane (GOPS) modified surface. Immobilization of rabbit gamma globulins (RgG) to silicon nitride surfaces, modified either with APTES or GOPS, was examined as a function of incubation time using atomic force microscopy (AFM), angle-resolved X-ray photoelectron spectroscopy (ARXPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). Multivariate technique of principal component analysis was applied to ToF-SIMS spectra in order to enhance sensitivity of immobilized RgG detection. Principal component regression shows a linear relationship with surface density determined rigorously from ARXPS following an organic bilayer approach, allowing for protein coverage quantification by ToF-SIMS. Taking it overall the surface immobilized amount of RgG is higher and develops faster on the surfaces silanized with APTES rather than with GOPS. Similar, although less distinct, difference is observed between the two surface types concerning the temporal evolution of average AFM height. The average height of protein overlayer correlates well with ARXPS and ToF-SIMS data expressed in terms of protein surface density. However, determined linear regression coefficients are distinctively higher for the surfaces modified with epoxy- rather than amino-silane, suggesting different surface density and conformation of the proteins immobilized through to covalent binding and physical adsorption.