Adsorption of bovine serum albumin on silicon dioxide nanoparticles: Impact of pH on nanoparticle-protein interactions.

Bovine serum albumin (BSA) adsorbed on amorphous silicon dioxide (SiO) nanoparticles was studied as a function of pH across the range of 2 to 8. Aggregation, surface charge, surface coverage, and protein structure were investigated over this entire pH range. SiO nanoparticle aggregation is found to depend upon pH and differs in the presence of adsorbed BSA. For SiO nanoparticles truncated with hydroxyl groups, the largest aggregates were observed at pH 3, close to the isoelectric point of SiO nanoparticles, whereas for SiO nanoparticles with adsorbed BSA, the aggregate size was the greatest at pH 3.7, close to the isoelectric point of the BSA-SiO complex. Surface coverage of BSA was also the greatest at the isoelectric point of the BSA-SiO complex with a value of ca. 3 ±1 × 10 molecules cm. Furthermore, the secondary protein structure was modified when compared to the solution phase at all pH values, but the most significant differences were seen at pH 7.4 and below. It is concluded that protein-nanoparticle interactions vary with solution pH, which may have implications for nanoparticles in different biological fluids (e.g., blood, stomach, and lungs).