Adsorption of proteins to fused-silica capillaries as probed by atomic force microscopy

In order to prove binding of proteins to the capillary wall, the inner surface of naked silica has been probed with the aid of atomic force microscopy. A large protein (ferritin, a particle of 12 nm diameter) has been left in contact with the capillary dissolved in buffers both below (pH 4.6) and above (pH 7.0) its pI (5.0-5.2) value. The capillary was then sliced lengthwise and its surface explored with the atomic force microscopy tip. Massive protein adsorption onto the naked fused-silica wall was observed, both below and above the protein pI, the thickness and extent of such deposition being proportional to the initial concentration of the protein bathing the wall. Such proteinaceous material could be largely desorbed by washing the capillary in 1 M NaOH, this process restoring the original topography of naked fused-silica. Additionally, such binding was also demonstrated electrophoretically by a displacement process which consisted of desorbing the bound ferritin by driving anionic detergent micelles (sodium dodecyl sulphate) from the cathodic compartment. Atomic force microscopy could thus become a powerful tool for probing surface adsorption also to coated capillaries, thus helping in designing better, more hydrophilic coatings.