Grafting poly(ethylene glycol) epoxide to amino-derivatized quartz: Effect of temperature and pH on grafting density

Microparticle capillary electrophoresis was used to characterize the surface of quartz capillaries grafted with the glycidyl ether of poly(ethylene glycol) (E-PEG). Site dissociation modeling of capillary electrokinetic behavior provided estimates of surface group pK and density, plus the distance (d) from the surface to the hydrodynamic plane of shear. Native quartz appeared to possess silanol groups of pK 3.6 and 6.9 whose surface densities varied with quartz treatment. Aminopropylsilane derivatization of quartz silanol groups in toluene yielded a coating which was stable (>6 h) at pH 10.3 and 60 degrees C. Aqueous grafting of E-PEG to this surface was relatively independent of pH (7.3-10.3) and reaction time (6-24 h) but was significantly influenced by reaction temperature (25-95 degrees C) and salt composition. PEG-grafted capillaries exhibited greatly reduced electroosmosis from pH 2 to 11. Significant grafting could be obtained under mild conditions (6 h, 35 degrees C, 0.4 M K2SO4, pH 6.9). These results suggest that PEG chains increasingly extend normal to a surface as their grafting density increases, and that PEG conformation influences grafting density. The methods described should aid the use of PEG-coated surfaces in a variety of applications.