The adsorption of bovine serum albumin BSA on a silica surface is highly dependent on the pH of the solution. BSA is a globular protein with a molecular weight of approximately 66.5 kDa and has an isoelectric point pI of around 4.7. At its isoelectric point, the protein carries no net charge, and its solubility in water is at its minimum. The pH of the solution can affect the adsorption of BSA on a silica surface through the following ways:1. Charge of BSA: The pH of the solution determines the ionization state of the amino acid residues in BSA. When the pH is below the pI of BSA pH < 4.7 , the protein carries a net positive charge due to the protonation of carboxyl groups COO- and deprotonation of amino groups NH3+ . When the pH is above the pI pH > 4.7 , the protein carries a net negative charge due to the deprotonation of carboxyl groups and protonation of amino groups.2. Charge of silica surface: The silica surface has silanol groups Si-OH that can ionize depending on the pH of the solution. At low pH, the silanol groups are protonated Si-OH2+ , giving the surface a net positive charge. At high pH, the silanol groups are deprotonated Si-O- , giving the surface a net negative charge.3. Electrostatic interactions: The adsorption of BSA on a silica surface is mainly governed by electrostatic interactions between the protein and the surface. When the pH is below the pI of BSA and the silica surface is positively charged, the electrostatic repulsion between the protein and the surface reduces the adsorption of BSA. Conversely, when the pH is above the pI of BSA and the silica surface is negatively charged, the electrostatic attraction between the protein and the surface enhances the adsorption of BSA.4. Hydrophobic and hydrogen bonding interactions: Besides electrostatic interactions, hydrophobic and hydrogen bonding interactions also play a role in the adsorption of BSA on a silica surface. The pH of the solution can affect the conformation of BSA, exposing or hiding hydrophobic and hydrogen bonding sites, which can influence the adsorption behavior.In summary, the pH of the solution significantly affects the adsorption of BSA on a silica surface. The adsorption is generally higher at pH values above the pI of BSA due to the electrostatic attraction between the negatively charged protein and the negatively charged silica surface. However, other factors such as hydrophobic and hydrogen bonding interactions can also influence the adsorption behavior, making the relationship between pH and adsorption complex.