An electrochemical quartz crystal microbalance (EQCM) was used to measure the effects of in situ potential changes on the adsorption and desorption of ferritin on polycrystalline Au. As-received horse-spleen ferritin, and purified ferritin monomer, dimer and higher oligomer fractions separated by gel chromatography were compared. Experiments were carried out at pH 5 where the ferritin is expected to have a net negative charge. Despite electrostatic repulsion, significant adsorption was observed on Au even at the most negative potentials studied, though it could be suppressed by using a low ionic strength electrolyte. This adsorption was attributed to hydrophobic interactions caused by partial denaturation of the protein, and it was greater for as-received ferritin containing higher oligomers than for monomeric ferritin. The reversibility of the adsorption was also lower when higher oligomers were present. Potential dependent adsorption/desorption could be useful as a diagnostic method for protein solutions. (c) 2012 Elsevier Ltd. All rights reserved.