title: Characterization of protein adsorption onto silica nanoparticles: influence of pH and ionic strength.
abstract: The adsorption of lysozyme and ß-lactoglobulin onto silica nanoparticles (diameter 21 nm) was studied in the pH range 2-11 at three different ionic strengths. Since the two proteins have a widely different isoelectric point (pI), electrostatic interactions with the negative silica surface lead to a different dependence of adsorption on pH. For lysozyme (pI ≈ 11), the adsorption level increases with pH and reaches a value corresponding to about two close-packed monolayers at pH = pI. In the multilayer adsorption region near pI, added electrolyte causes a decrease in adsorption, which is attributed to the screening of attractive interactions between protein molecules in the first and second adsorbed layer. For ß-lactoglobulin (pI ≈ 5), a pronounced maximum of the adsorbed amount is found at pH 4 in the absence of salt. It is attributed to the adsorption of oligomers of the protein that exist in the solution at this pH. An inversion in the influence of salt on the adsorbed amount occurs at pH > pI, where the protein and the surface are both negatively charged. This inversion is attributed to the screening of the repulsive protein-surface and protein-protein interactions. The adsorption isotherms were analyzed with the Guggenheim-Anderson-De Boer (GAB) model, which allows for two adsorption states (strongly and weakly bound protein).
authors.affiliation: Stranski Laboratorium, Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Meissner
authors.affiliation: Stranski Laboratorium, Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany.
authors.forename: Albert
authors.initials: A
authors.lastname: Prause
authors.affiliation: Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695 USA.
authors.forename: Bhuvnesh
authors.initials: B
authors.lastname: Bharti
authors.affiliation: Stranski Laboratorium, Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany.
authors.forename: Gerhard H
authors.initials: GH
authors.lastname: Findenegg
keywords: Adsorption; Electrostatic interactions; Lysozyme; Nanoparticles; Protein; Silica; ß-lactoglobulin