Biomedical subjects

J Hugh Horton

Publications and source records attributed to J Hugh Horton.

7 recordsLinked to original sources

PubMed2006

Chemical force titrations of antigen- and antibody-modified poly(methylmethacrylate).

Poly(methylmethacrylate) (PMMA) is a versatile polymer that displays desirable properties for development of cheap and disposable microfluidic devices for sensing biomolecular interactions. Atomic force microscopy (AFM) and chemical force titrations were used to determine the efficacy of surface modifications made to accommodate protein-substrate linkage. AFM images show the effects on surface morphology of carboxylated-, amine-, hCG antigen- and anti-hCG antibody-modified PMMA substrates. Confocal microscopy was used to determine the fluorescent intensity of labeled antibody species on the PMMA substrate, confirming the success of surface antigen/antibody immobilization. Surface pK(1/2) value for carboxylic acid and amine species grafted on PMMA were determined. When carboxylic acid or amine-terminated tips were titrated against PMMA samples terminated with the hCG antigen and anti-hCG antibody, peaks appeared in the force titration curve consistent with the pI range of the antigen or antibody species. Strong adhesive forces were present at pH values above 7.0 when the antigen was present on the PMMA substrate, and these were attributed to hydrophobic interactions between the antigen and the alkane "linker" chain attaching the amine or carboxylate group to the AFM tip. Such hydrophobic interactions were not observed with the carboxylic acid or amine/antibody combinations suggesting that the surface-linked antibody was more resistant to denaturation under higher pH. The results demonstrated the feasibility of using AFM approaches for interrogating protein grafting strategies in the fabrication of PMMA-based microsystems.

Antibodies↗

PubMed2006

Chemical force titrations of functionalized Si(111) surfaces.

Chemical force titrations-plots of the adhesive force between an atomic force microscope tip and sample as a function of pH-were acquired on alkyl monolayer-derivatized Si(111) surfaces. Gold-coated AFM tips modified with thioalkanoic acid self-assembled monolayers (SAM) were employed. Alkyl monolayer-derivatized Si(111) surfaces terminated with methyl, carboxyl, and amine groups were produced via hydrosilylation reactions between 1-alkene reagents and H-terminated silicon. The functionalized surfaces were characterized using standard surface science techniques (AFM, FTIR, and XPS). Titration of the methyl-terminated surface using the modified (carboxyl-terminated) atomic force microscope tip resulted in a small pH-independent hydrophobic interaction. Titration of the amine-terminated surface using the same tip resulted in the determination of a surface pKa of 5.8 for the amine from the pH value from the maximum in the force titration curve. A pK(1/2) of 4.3 was determined for the carboxyl-terminated Si(111) in a similar way. These results will be discussed in relation to the modified Si(111) surface chemistry and organic layer structure, as well as with respect to existing results on Au surfaces modified with SAMs bearing the same functional groups.

Journal Article↗

PubMed2006

Solvation of the N-(1-Phenylethyl)-N'-[3-(triethoxysilyl)propyl]-urea chiral stationary phase in mixed alcohol/water solvents.

A theoretical and experimental study of alcohol/water and alcohol/alcohol solvent mixtures near a surface of N-(1-phenylethyl)-N'-[3-(triethoxysilyl)propyl]-urea (PEPU), a Pirkle-type chiral stationary phase, is presented. Molecular dynamics simulations are performed at room temperature for water/methanol, water/1-propanol, water/2-propanol, and methanol/1-propanol solvent mixtures confined between two PEPU surfaces. The interface was also prepared experimentally by attaching the PEPU molecules to atomic force microscopy tips and oxidized Si(111) substrates. Chemical force spectrometric measurements between such PEPU-terminated tips and samples were taken in the solvent mixtures, and the results are compared to the molecular dynamics study. We find that the extent of hydrogen bonding at the surface is the dominant contributor to the measured forces.

Journal Article↗

PubMed2005

Fabrication and characterization of poly(methylmethacrylate) microfluidic devices bonded using surface modifications and solvents.

The fabrication of polymer microchips allows inexpensive, durable, high-throughput and disposable devices to be made. Poly(methylmethacrylate) (PMMA) microchips have been fabricated by hot embossing microstructures into the substrate followed by bonding a cover plate. Different surface modifications have been examined to enhance substrate and cover plate adhesion, including: air plasma treatment, and both acid catalyzed hydrolysis and aminolysis of the acrylate to yield carboxyl and amine-terminated PMMA surfaces. Unmodified PMMA surfaces were also studied. The substrate and cover plate adhesion strengths were found to increase with the hydrophilicity of the PMMA surface and reached a peak at 600 kN m(-2) for plasma treated PMMA. A solvent assisted system has also been designed to soften less than 50 nm of the surface of PMMA during bonding, while still maintaining microchannel integrity. The extent to which both surface modifications and solvent treatment affected the adhesion of the substrate to the cover plate was examined using nanoindentation methods. The solvent bonding system greatly increased the adhesion strengths for both unmodified and modified PMMA, with a maximum adhesion force of 5500 kN m(-2) achieved for unmodified PMMA substrates. The bond strength decreased with increasing surface hydrophilicity after solvent bonding, a trend that was opposite to what was observed for non-solvent thermal bonding.

Equipment Design↗

PubMed2005

Interactions of benzoic acid and phosphates with iron oxide colloids using chemical force titration.

Colloidal iron oxides are an important component in soil systems and in water treatment processes. Humic-based organic compounds, containing both phenol and benzoate functional groups, are often present in these systems and compete strongly with phosphate species for binding sites on the iron oxide surfaces. Here, we examine the interaction of benzoate and phenolic groups with various iron oxide colloids using atomic force microscopy (AFM) chemical force titration measurements. Self-assembled monolayers (SAMs) of 4-(12-mercaptododecyloxy)benzoic acid and 4-(12-mercaptododecyloxy)phenol were used to prepare chemically modified Au-coated AFM tips, and these were used to probe the surface chemistry of a series of iron oxide colloids. The SAMs formed were also characterized using scanning tunneling microscopy, reflection-absorption infrared spectroscopy, and X-ray photoelectron spectroscopy. The surface pK(a) of 4-(12- mercaptododecyloxy)benzoic acid has been determined to be 4.0 +/- 0.5, and the interaction between the tip and the sample coated with a SAM of this species is dominated by hydrogen bonding. The chemical force titraton profile for an AFM probe coated with 4-(12- mercaptododecyloxy)benzoic acid and a bare iron oxide colloid demonstrates that the benzoic acid function group interacts with all three types of iron oxide sites present on the colloid surface over a wide pH range. Similar experiments were carried out on colloids precipitated in the presence of phosphoric, gallic, and tannic acids. The results are discussed in the context of the competitive binding interactions of solution species present in soils or in water treatment processes.

Journal Article↗

PubMed2005

Chemical force titrations of amine- and sulfonic acid-modified poly(dimethylsiloxane).

Chemical force titrations-measurements of the adhesive interaction between a pair of suitably chemically modified atomic force microscopy (AFM) tip and sample surfaces as a function of pH-have been carried out for various combinations of silanol, amine, carboxylic acid, and sulfonic acid functional groups on both tip and sample. The primary surface material studied was poly(dimethylsiloxane) (PDMS). Surface modification was carried out using a plasma oxidation process to form silanol sites; further modification with amine or sulfonic acid sites was carried out by reaction of the silanol sites with the appropriate trialkoxysilane derivative. AFM tips were also modified using trialkoxysilane compounds. In the cases of tip/sample combinations with the same functional group on each, surface pK(1/2) values could be determined. In several "mixed" tip/sample combinations, a peak appeared in the titration curve midway between the surface pK(1/2) values of the tip and sample, consistent with an ionic H-bonding model for the interactions. The amine/sulfonic acid pair showed more complex behavior; the amine-terminated tip/sulfonic acid-terminated PDMS surface force titration curve consisted of two peaks centered at pH 4 and pH 8. Reversing the tip/sample pair resulted in the peak positions being shifted upward by 1.0 pH unit. The peak appearing at lower pH is assigned to electrostatic interactions between the two oppositely charged surfaces, whereas the higher pH peak is believed to arise due to ionic H-bonding interactions. AFM images show the effects on surface patterning of amine- and sulfonic acid-modified PDMS surfaces that have undergone two different oxidation methods (air plasma oxidation and Tesla coil oxidation). The surface morphologies of freshly prepared and 24 h aged air plasma oxidized PDMS are also discussed in this study.

Journal Article↗

PubMed2003

Surface characterization using chemical force microscopy and the flow performance of modified polydimethylsiloxane for microfluidic device applications.

The widespread interest in micro total analysis systems has resulted in efforts to develop devices in cheaper polymer materials such as polydimethylsiloxane (PDMS) as an alternative to expensive glass and silicon devices. We describe the oxidation of the PDMS surface to form ionizable groups using a discharge from a Tesla coil and subsequent chemical modification to augment electroosmotic flow (EOF) within the microfluidic devices. The flow performance of oxidized, amine-modified and unmodified PDMS materials has been determined and directly compared to conventional glass devices. Exact PDMS replicas of glass substrates were prepared using a novel two step micromolding protocol. Chemical force microscopy has been utilized to monitor and measure the efficacy of surface modification yielding information about the acid/base properties of the modified and unmodified surfaces. Results with different substrate materials correlates well with expected flow modifications as a result of surface modification. Oxidized PDMS devices were found to support faster EOF (twice that of native PDMS) similar to glass while those derivatized with 3-aminopropyl triethoxysilane (APTES) showed slower flow rates compared to native PDMS substrates as a result of masking surface charge. Results demonstrate that the surface of PDMS microdevices can be manipulated to control EOF characteristics using a facile surface derivatization methodology allowing surfaces to be tailored for specific microfluidic applications and characterized with chemical force microscopy.

Dimethylpolysiloxanes↗