UC San Diego

Bovine serum albumin (BSA) adsorbed on amorphous silicon dioxide (SiO₂) 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. SiO₂ nanoparticle aggregation is found to depend upon pH and differs in the presence of adsorbed BSA. For SiO₂ nanoparticles truncated with hydroxyl groups, the largest aggregates were observed at pH 3, close to the isoelectric point of SiO₂ nanoparticles, whereas for SiO₂ nanoparticles with adsorbed BSA, the aggregate size was the greatest at pH 3.7, close to the isoelectric point of the BSA-SiO₂ complex. Surface coverage of BSA was also the greatest at the isoelectric point of the BSA-SiO₂ complex with a value of ca. 3 ±1 × 10¹¹ molecules cm^-2. 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).