Effect of pH on the adsorption and interactions of Bovine Serum Albumin with functionalized silicon nitride surface

Force-distance curves between atomic force microscopy (AFM) tip (S_i3N₄ non-functionalized) and bovine serum Albumin (BSA) immobilized on S_i3N₄ substrates have been performed with the purpose to understand how multiple interactions between the protein and the tip were favored in different pHs (4, 6 and 10). In this work, 100 silicon wafer samples were used to deposit a layer of Si₃N₄. Protein immobilization consisted of the silanization of the substrates with 3-aminopropyltriethoxysilane (APTES) and crosslinking with glutaraldehyde (GA). All functionalization steps were evaluated by contact angle, X-Ray electron spectroscopy (XPS) and AFM. AFM images showed increase of roughness following functionalization. At pH 4, it was possible to note that small forces (49.1 ± 2.4 pN) were needed to stretch BSA, with a contour length of CL = (30.0 ± 1.1 nm). At pH 6, the force applied was higher (101.5 ± 5.0 pN) with a higher molecule stretch CL = (75.6 ± 3.8 nm) because the pH is close to the BSA isoelectric point where the folding of the protein is favored as surfaces charges are minimized leading to lower attractive intramolecular forces. Young's Modulus were also calculated and the lowest value (265 kPa) was observed at pH 10.