Biomedical subjects

Michelle L Gee

Publications and source records attributed to Michelle L Gee.

3 recordsLinked to original sources

PubMed2005

Postsynthesis vapor-phase functionalization of MCM-48 with hexamethyldisilazane and 3-aminopropyldimethylethoxylsilane for bioseparation applications.

MCM-48 was surface modified via vapor-phase reactions with hexamethyldisilazane (CH(3)-MCM-48) and 3-aminopropyldimethylethoxysilane (NH(2)-MCM-48). (29)Si NMR confirmed that the resulting materials contained covalently attached trimethylsilane and 3-aminopropyldimethylsilane moieties, both important functionalities for bioseparation applications. The surface coverage was approximately 1.8 and 0.9 groups per nm(2), respectively. The X-ray diffraction patterns and the narrow pore size distributions obtained from the gas sorption isotherms showed that the modified materials retained the characteristic pore structure of the underlying MCM-48 material. CH(3)-MCM-48 exhibited significantly improved hydrolytic stability over the unmodified MCM-48 under the aqueous conditions tested, whereas NH(2)-MCM-48 appeared to be less stable than the unmodified MCM-48. The decrease in stability is most likely due to the nature of the attachment of the 3-aminopropyldimethylsilane moiety, where the conversion of surface silanol groups is limited by H bonding with the amino end, leading to a 50% lower surface concentration and resulting in an increased likelihood of nucleophilic attack on the silica surface, enhancing the rate of hydrolysis. Hexamethyldisilazane thus appears to be a superior functional group for modifying the MCM-48 surface.

Indicators and Reagents↗

PubMed2005

Atomic force microscopy study of the interaction between adsorbed poly(ethylene oxide) layers: effects of surface modification and approach velocity.

The interaction forces between layers of the triblock copolymer Pluronic F108 adsorbed onto hydrophobic radio frequency glow discharge (RFGD) thin film surfaces and hydrophilic silica, in polymer-free 0.15 M NaCl solution, have been measured using the atomic force microscope (AFM) colloid probe technique. Compression of Pluronic F108 layers adsorbed on the hydrophobic RFGD surfaces results in a purely repulsive force due to the steric overlap of the layers, the form of which suggests that the PEO chains adopt a brush conformation. Subsequent fitting of these data to the polymer brush models of Alexander-de Gennes and Milner, Witten, and Cates confirms that the adsorbed Pluronic F108 adsorbs onto hydrophobic surfaces as a polymer brush with a parabolic segment density profile. In comparison, the interaction between Pluronic F108 layers adsorbed on silica exhibits a long ranged shallow attractive force and a weaker steric repulsion. The attractive component is reasonably well described by van der Waals forces, but polymer bridging cannot be ruled out. The weaker steric component of the force suggests that the polymer is less densely packed on the surface and is less extended into solution, existing as polymeric isolated mushrooms. When the surfaces are driven together at high piezo ramp velocities, an additional repulsive force is measured, attributable to hydrodynamic drainage forces between the surfaces. In comparing theoretical predictions of the hydrodynamic force to the experimentally obtained data, agreement could only be obtained if the flow profile of the aqueous solution penetrated significantly into the polymer brush. This finding is in line with the theoretical predictions of Milner and provides further evidence that the segment density profile of the adsorbed polymer brush is parabolic. A velocity dependent additional stepped repulsive force, reminiscent of a solvation oscillatory force, is also observed when the adsorbed layers are compressed under high loads. This additional force is presumably a result of hindered drainage of water due to the presence of a high volume fraction of polymer chains between the surfaces.

Journal Article↗

PubMed2003

Time-resolved evanescent wave-induced fluorescence anisotropy for the determination of molecular conformational changes of proteins at an interface.

We have shown that the molecular conformation of a protein at an interface can be probed spatially using time-resolved evanescent wave-induced fluorescence spectroscopic (TREWIFS) techniques. Specifically, by varying the penetration depth of the evanescent field, variable-angle TREWIFS, coupled with variable-angle evanescent wave-induced time-resolved fluorescence anisotropy measurements, allow us to monitor how fluorescence intensity and fluorescence depolarization vary normal to an interface as a function of time after excitation. We have applied this technique to the study of bovine serum albumin (BSA) complexed noncovalently with the fluorophore 1-anilinonaphthalene-8-sulfonic acid. The fluorescence decay varies as a function of the penetration depth of the evanescent wave in a manner that indicates a gradient of hydrophobicity through the adsorbed protein, normal to the interface. Restriction of the fluorescent probe's motion also occurs as a function of distance normal to the interface. The results are consistent with a model of partial protein denaturation: at the surface, an adsorbed BSA molecule unfolds, thus optimizing protein-silica interactions and the number of points of attachment to the surface. Further away, normal to the surface, the protein molecule maintains its coiled structure.

Adsorption↗