Probing soft polymeric coatings of a capillary by atomic force microscopy.

Abstract

Atomic force microscopy (AFM) has been used to probe the surface of a capillary after coating with "soft" polymers, notably polyacrylamides. The aim was the investigation of the efficiency of coverage of the silica surface, so as to reduce or eliminate the electroosmotic flow (EOF), particularly noxious in the separation of macromolecules. The quality of such coating is strongly dependent on two variables: temperature and pH. In the first case, progressively higher temperatures produce open silica patches, where no polymer seems to be bound. The transition from coated to largely uncoated surfaces occurs at 50 degrees C. Also the pH of the polymerizing solution strongly affects the coating efficiency. Since in all coating procedures the monomer solution is not buffered, addition of accelerator (TEMED, N,N,N'N'-tetramethylethylendiamine) induces polymer growth at pH 10-11. These pH values generate hydrolysis of the siloxane bridge anchoring the bifunctional agent (Bind Silane, onto which the polymer chain should grow) to the wall. Thus, coating and de-coating occur simultaneously. Low temperatures during polymer growth (typically 10 degrees C) and buffered solutions (pH 7, titrated after TEMED addition) ensure a most efficient and thorough coating, with virtual elimination of EOF: well coated capillaries exhibit residual EOF values, at pH 10, of the order of 10(-7) cm2 V-1 s-1 vs. a standard value for uncoated capillaries of the order of 10(-4) cm2 V-1 s-1. The AFM data have been fully confirmed by direct measurement of EOF in coated and uncoated capillaries under an electric field.