Surface plasmon resonance was used to measure binding of proteins from solution to poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes end-grafted from gold surfaces by atom transfer radical polymerization (ATRP). PDMAEMA brushes were prepared with a variety of grafting densities and degrees of polymerization. These brushes displayed charge selective protein uptake. The extent of uptake for net negatively charged bovine serum albumin (BSA) scaled linearly with the surface mass concentration of grafted PDMAEMA, regardless of grafting density. BSA was bound at a constant ratio of 120 DMAEMA monomer units per protein molecule for all brushes examined. The equivalent three-dimensional concentration of BSA bound in the brush (i.e., the bound BSA surface excess concentration divided by the brush thickness) decreased monotonically with decreasing grafting density. The concentration of BSA bound within brushes prepared at higher grafting densities was comparable with the aqueous protein solubility limit. BSA desorption from the brush required changes in solution pH and/or ionic strength to eliminate its net electrostatic attraction to PDMAEMA. Net positively charged lysozyme was completely rejected by the PDMAEMA brushes.
Adsorption of a water-soluble diblock copolymer, poly(fert-butylstyrene)-sodium poly-(styrenesulfonate) (PtBS-NaPSS), on silica surfaces in aqueous solutions was studied using ellipsometry and atomic-force microscopy (AFM). The block copolymers used were compositionally asymmetric, with large, hydrophilic, PSS blocks and small, hydrophobic, PtBS blocks. Materials with molecular weights of 87 000 and 160 000 were used. Adsorption could not be observed in pure water without added salt (NaCl). When the NaCl concentration was increased to 1 M, adsorption could be readily observed. The measured adsorbed amount at long times was significantly larger for the 87 000 diblock compared with that for a polyelectrolyte homopolymer of comparable molecular size, demonstrating the role played by the uncharged block in anchoring the diblock at the solid surface. The kinetics of adsorption showed a two-stage process: an initial diffusion-limited stage, followed by a slower buildup of surface coverage in a brush-limited stage. The number density of molecules at the surface was smaller for the higher molecular weight species, in agreement with simple scaling arguments.f This work forms part of the Ph.D. dissertation of Mohan Sikka.
This work presents a methodology to measure and quantitatively interpret force curves on supported lipid bilayers in water. We then use this method to correlate topographic imaging contrast in atomic force microscopy (AFM) images of phase-separated Langmuir-Blodgett bilayers with imaging load. Force curves collected on pure monolayers of both distearoylphosphatidylethanolamine (DSPE) and monogalactosylethanolamine (MGDG) and dioleoylethanolamine (DOPE) deposited at similar surface pressures onto a monolayer of DSPE show an abrupt breakthrough event at a repeatable, material-dependent force. The breakthrough force for DSPE and MGDG is sizable, whereas the breakthrough force for DOPE is too small to measure accurately. Contact-mode AFM images on 1:1 mixed monolayers of DSPE/DOPE and MGDG/DOPE have a high topographic contrast at loads between the breakthrough force of each phase, and a low topographic contrast at loads above the breakthrough force of both phases. Frictional contrast is inverted and magnified at loads above the breakthrough force of both phases. These results emphasize the important role that surface forces and mechanics can play in imaging multicomponent biomembranes with AFM.
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