Aggregation of human serum albumin on graphite and single-walled carbon nanotubes as studied by scanning probe microscopies.

ABSTRACT

Human serum albumin (HSA) is the most abundant protein in blood plasma showing a remarkable ability to bind a broad range of hydrophobic substrates. We employed scanning tunneling microscopy and atomic force microscopy to characterize the morphology of HSA aggregates on highly-ordered pyrolytic graphite (HOPG) and single-walled carbon nanotubes (SWNTs). The morphologies found for albumin aggregates on HOPG are quite different from the ones observed on SWNTs. On HOPG, HSA forms aggregates of roughly 10-20 molecules; single protein molecules were observed as well. In the case of SWNTs, nanotubes were partially or totally covered with HSA, exhibiting four general types of aggregation (i) SWNT sidewalls contain single molecules of albumin which are away from each other at distances longer than the HSA molecular size; (ii) SWNTs are completely covered with HSA, which forms a thin and relatively homogeneous layer; (iii) SWNTs have a complete layer of HSA with additional accumulation of protein at separate sites; and (iv) several SWNTs totally covered with albumin assemble into a bundle-like structure common for bare nanotubes. These observations are interpreted in terms of stronger interactions of HSA with nanotube sidewalls than with flat graphite surface.