J. Hugh Horton

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.

A combined QCM and XPS investigation of asphaltene adsorption on metal surfaces

To investigate asphaltene-metal interactions, a combined quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS) study of asphaltene adsorption on a gold surface was conducted. Adsorption experiments were conducted at 25 degrees C with solutions of asphaltenes in toluene at concentrations ranging from 50 to 1500 ppm. QCM measurements yielded information on the kinetics of adsorption and further assessment of the data allowed the estimation of equilibrium adsorption levels. XPS analysis of adsorbed and bulk asphaltene demonstrated the presence of carboxylic, thiophenic, sulfide, pyridinic and pyrrolic type functional groups. The intensity of the main carbon (C-H) peak was related to surface coverage of adsorbed asphaltene as a function of asphaltene concentration by a simple mathematical model. The mass adsorption data from the QCM experiments also allowed estimation of the surface coverage, which was compared to those from XPS analyses. Surface coverage estimates as a function of asphaltene concentration could be described by a Langmuir (type-I) isotherm. The free energy of asphaltene adsorption was estimated to be -26.8+/-0.1 and -27.3+/-0.1 kJ/mol from QCM and XPS data, respectively assuming asphaltene molar mass of 750 g/gmol. QCM and XPS data was also analyzed to estimate adsorbed layer thickness after accounting for surface coverage. The thickness of the adsorbed asphaltene estimated from both XPS and QCM data analyses ranged from 6-8 nm over the entire range of adsorption concentrations investigated.

Competitive adsorption of phosphate and carboxylate with natural organic matter on hydrous iron oxides as investigated by chemical force microscopy

Abstract The behaviour of phosphate and natural organic matter (NOM) in soils and aqueous media is strongly influenced by their association with surfaces of colloidal mineral oxide and oxyhydroxide particles of iron. Here we investigate the interaction of atomic force microscope (AFM) tips terminated with bis(11-thioundecanoic)phosphate and 16-thiohexadecanoic acid as a function of pH in the context of competitive phosphate–organic matter adsorption interactions at the surface of hydrous iron oxide colloids. Experiments were carried out on unmodified colloids as well as colloids coated with gallic acid, tannic acid or peat-derived humic material. The colloids were also examined by infrared spectroscopy (IR) and zeta potentiometry. Force titrations on a gallic acid control surface revealed that the main mode of interaction of this compound with the tips was hydrogen bonding. A strong interaction in the pH range 4–8 observed for the PO2H probe on the unmodified iron (hydroxy)oxide was attributed to a specific adsorption reaction with an A-type FeOH surface site. The CO2H tip displayed a strong but less specific interaction that extended over the pH range 4–10. Force titrations of probes against colloids post-precipitated with gallic acid, tannic acid or peat-derived humic material were significantly different than for the unmodified surfaces. Surface-bound organic molecules reduced the specific mid pH range interactions and gave rise to two new force titration features with maxima about pH 4 and 8, which were assigned to H-bonding between the probes and benzoic and phenolic groups of surface-bound organic acids, respectively.

Surface modification of poly(dimethylsiloxane) with a perfluorinated alkoxysilane for selectivity toward fluorous tagged peptides.

Poly(dimethylsiloxane) (PDMS) and similar polymers have proved to be of widespread interest for use in microfluidic and similar microanalytical devices. Surface modification of PDMS is required to extend the range of applications for devices made of this polymer, however. Here we report on the grafting of perfluorooctyltriethoxysilane via hydrolysis onto an oxidized PDMS substrate in order to form a fluorinated microchannel. Such a fluorinated device could be used for separating fluorous tagged proteins or peptides, similar to that which has been recently demonstrated in a capillary electrophoresis system or in an open tubular capillary column. The modified polymer is characterized using chemical force titrations, contact angle measurements, and X-ray photoelectron spectroscopy (XPS). We also report on a novel means of performing electroosmotic measurements on this material to determine the surface zeta potential. As might be expected, contact angle and chemical force titration measurements indicate the fluorinated surface to be highly hydrophobic. XPS indicates that fluorocarbon groups segregate to the surface of the polymer over a period of days following the initial surface modification, presumably driven by a lower surface free energy. One of the most interesting results is the zeta potential measurements, which show that significant surface charge can be maintained across a wide range of pH on this modified polymer, sufficient to promote electroosmotic flow in a microfluidic chip. Matrix-assisted time-of-flight mass spectrometry (MALDI-TOF MS) measurements show that a fluorous-tagged peptide will selectively adsorb on the fluorinated PDMS in aqueous solution, demonstrating that the fluorinated polymer could be used in devices designed for the enrichment or enhanced detection of fluorous-labeled proteins and peptides.

Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing

The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol−1) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.

The adsorption of globular proteins onto a fluorinated PDMS surface.

Poly(dimethylsiloxane) (PDMS) has shown considerable promise in the fabrication of microfluidic devices. Surface modification of PDMS by the grafting of perfluorinated alkanes allows the selective adsorption of fluorous-tagged peptides, demonstrating that this material may be used in fluorous affinity tag technology to enrich and separate specific proteins or peptides from complex mixtures. Here, we explore the non-specific adsorption of proteins which may interfere with this process. The desorption of cytochrome c, carbonic anhydrase, insulin and ubiquitin onto the surfaces of unmodified, oxidized and fluorinated PDMS in solutions of varying water/methanol concentration has been studied using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). The interaction forces involving perfluorinated surfaces are probed using chemical force spectrometry. The denaturation of the proteins in solutions of high methanol concentration is followed using electrospray ionization mass spectrometry (ESI-MS) and the adsorption profiles discussed in the context of the surface hydrophobicity of each protein.

Characterization of covalently bonded proteins on poly(methyl methacrylate) by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) has been used to characterize a poly(methyl methacrylate) (PMMA) surface with covalently attached proteins. The PMMA surfaces were first aminated using hexamethyldiamine; the resulting -NH(2) sites were reacted with the hetero-bifunctional cross-linker Sulfo-EMCS to form a maleimide-terminated surface. The N-hydroxysuccinimide ester terminal and maleimide terminal groups of Sulfo-EMCS reacts with amine and sulfhydryl groups, respectively, exposed on the surface of the proteins. This study characterizes Thermotoga maritima beta-glucosidase 1 (TmGH1), which belongs to a family of proteins that facilitate hydrolysis of glucose-related monomers with retention of conformation. The surfaces were characterized by XPS to monitor surface composition, and to elucidate protein orientation on the surface. Results suggest that a covalently bonded surface of TmGH1 on PMMA has been obtained. These results demonstrate the feasibility of using XPS to study protein surface chemistry and demonstrate a useful method to anchor cysteine-terminated proteins for the purposes of creating biosensors or platforms for mechanical force experiments to investigate protein structure.

N-heterocyclic carbene self-assembled monolayers on copper and gold : dramatic effect of wingtip groups on binding, orientation and assembly

Abstract Self‐assembled monolayers of N‐heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N‐substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl‐ or ethyl‐substituted NHCs lie flat. Temperature‐programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of E des=152±10 kJ mol−1. NHCs that bind upright desorb cleanly, while flat‐lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing.

Anion and cation chemistry of phosphoryl chloride as an electron scavenger in a fuel-rich, methane—oxygen flame

Abstract A premixed, fuel-rich, methane—oxygen flame at atmospheric pressure was doped with 0.05 mol. % of phosphoryl chloride, POCl3. The phosphorus anions and cations produced by chemical ionization were observed by sampling the flame through a nozzle into a mass spectrometer. The POCl3 additive was a very effective scavenger of free electrons by negative ion formation in the burnt gas of the hydrocarbon flame, even more so than trimethylphosphate, TMP, studied previously under similar conditions. The kinetic breakdown of POCl3 is faster, yielding the major anions PO−2 and PO−2; the role of electronegative chlorine is relatively minor. To a remarkable degree, the ion chemistry observed with both additives is very similar, stemming from a common series of neutral phosphorus intermediates. In addition to PO−2 and PO−3, common anions include PO−, PO2(OH)−2 and PO2(CH2)−; common cations have the general form PO+n · xH2O(n=1−3, x=0−3). New phosphorus anions observed with POCl3 include: PO2(OH)(CH3)−; the disphosphorus species P2O5H−, P2O5CH−3, P2O5 · OH− and P2O5H− · H2O; and the chlorinated anions POx(OH)yCl−z formed by nucleophilic substitution of POCl3, as well as Cl−. New cations include POCl3 · H+ and the series PO+4, PO+5 · H2O, showing the extraordinary affinity of phosphorus for oxygen. The ion chemistry is discussed in detail, mainly involving proton transfer, nucleophilic substitution (SN2) and three-body association (e.g. hydration).

Characterization of hydroxyphenol-terminated alkanethiol self-assembled monolayers: interactions with phosphates by chemical force spectrometry.

Tannins and humic substances, commonly referred to as natural organic matter (NOM), constitute an important component of natural water and soil systems. These species contain numerous hydroxyl and carboxyl functional groups whose reactivity is strongly dependent on both the quantity and location of these moieties on the aromatic ring. In the present study, self-assembled monolayers (SAMs) of 4-(12-mercaptododecyl)benzene-1,2-diol (o-hydroxyphenol-terminated); 5-(12-mercaptododecyl)benzene-1,3-diol (m-hydroxyphenol-terminated); bis(11-thioundecyl) hydrogen phosphate (monoprotic phosphate); and 11-thioundecyl dihydrogen phosphate (diprotic phosphate) were prepared and characterized using X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), and water contact angle measurements. The interactions between phenolic groups with phosphates were examined as a function of pH using the chemical force spectrometry (CFS) technique. The observations are discussed in the context of hydrogen bonding and electrostatic repulsion interaction between corresponding species. Adhesion force profiles of hydroxyphenol isomers interacting with monoprotic phosphate are dominated by ionic H-bonding; however the strength of o-hydroxyphenol interactions is significantly higher. The difference in location of hydroxyl groups on the interface also results in significantly different force-distance profiles for the isomeric hydroxyphenols when interacting with diprotic phosphate.