Paper SS-TuP2
Size-Dependent Surface Chemistry of Alumina Nanoparticles

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

The particle size of a number of different oxide materials has been found to influence cell adhesion and growth; specifically, nanophase (rather than conventional micron-sized) particles enhance these processes. The increased adsorption of proteins to nanophase particles has been implicated in this size-dependent phenomenon. The current study focuses on the surface chemistry of alumina particles as a function of average particle size, specifically to determine whether or not there are size-dependent differences in surface chemical species that may affect protein (and other biomolecule) adsorption. Alumina nanoparticles of varying phase and diameter were spin-coated onto silicon substrates and their uniformity characterized via SEM and XPS. The distribution of surface hydroxyl (OH) species was analyzed via transmission infrared (IR) spectroscopy. The OH stretches observed for all alumina samples are signficantly redshifted (~200 cm^-1) from the frequencies observed for dried alumina powders (e.g. using diffuse reflectance IR). While a common cause of such redshifting in ν(OH) peaks is hydrogen bonding with surface water, the spin-coated samples do not exhibit the concomitant peak broadening associated with this kind of hydrogen bonding. We propose that the unique ν(OH) signatures of spin-coated alumina particles arise from discrete hydrogen bonding interactions between alumina hydroxyls and surface silanols on the underlying silicon substrate. Differences in the OH-stretching peaks for different phases (e.g. γ vs. α) of alumina provide evidence that these features do, in fact, arise from the alumina particles themselves. We do in fact see a size dependence in the distribution of surface hydroxyl species, with distinct populations of different OH species arising from conventional vs. nanophase alumina of all phases studied. These differences are likely to arise from different relative populations of edge vs. facet sites as a function of particle size.