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Biomedical subjects

Henry S White

Publications and source records attributed to Henry S White.

At least 19 recordsLinked to original sources
PubMed2006

Ionic conductivity of the aqueous layer separating a lipid bilayer membrane and a glass support.

The in-plane ionic conductivity of the approximately 1-nm-thick aqueous layer separating a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer membrane and a glass support was investigated. The aqueous layer conductivity was measured by tip-dip deposition of a POPC bilayer onto the surface of a 20- to 75-microm-thick glass membrane containing a single conical-shaped nanopore and recording the current-voltage (i-V) behavior of the glass membrane nanopore/POPC bilayer structure. The steady-state current across the glass membrane passes through the nanopore (45-480 nm radius) and spreads radially outward within the aqueous layer between the glass support and bilayer. This aqueous layer corresponds to the dominant resistance of the glass membrane nanopore/POPC bilayer structure. Fluorescence recovery after photobleaching measurements using dye-labeled lipids verified that the POPC bilayer maintains a significant degree of fluidity on the glass membrane. The slopes of ohmic i-V curves yield an aqueous layer conductivity of (3 +/- 1) x 10(-3) Omega(-1) cm(-1) assuming a layer thickness of 1.0 nm. This conductivity is essentially independent of the concentration of KCl in the bulk solution (10-4 to 1 M) in contact with the membrane. The results indicate that the concentration and mobility of charge carriers in the aqueous layer between the glass support and bilayer are largely determined by the local structure of the glass/water/bilayer interface.

Electric Conductivity↗
PubMed2006

Photon gated transport at the glass nanopore electrode.

The interior surface of the glass nanopore electrode was modified with spiropyran moieties to impart photochemical control of molecular transport through the pore orifice (15-90 nm radius). In low ionic strength acetonitrile solutions, diffusion of a positively charged species (Fe(bpy)(3)(2+)) is electrostatically blocked with approximately 100% efficiency by UV light-induced conversion of the neutral surface-bound spiropyran to its protonated merocyanine form (MEH+). Transport through the pore orifice is restored by either irradiation of the electrode with visible light to convert MEH+ back to spiropyran or addition of a sufficient quantity of supporting electrolyte to screen the electrostatic field associated with MEH+. The transport of neutral redox species through spiropyran-modified glass nanopores is not affected by light, allowing photoselective transport of redox molecules to the electrode surface based on charge discrimination. The glass nanopore electrode can also be employed as a photochemical trap, by UV light conversion of surface-bound spiropyran to MEH+, preventing Fe(bpy)(3)(2+) initially in the pore from diffusing through the orifice.

2,2'-Dipyridyl↗
PubMed2006

Alternating current impedance imaging of high-resistance membrane pores using a scanning electrochemical microscope. Application of membrane electrical shunts to increase measurement sensitivity and image contrast.

Whether an individual pore in a porous membrane can be imaged using scanning electrochemical microscopy (SECM), operated in ac impedance mode, is determined by the magnitude of the change in the total impedance of the imaging system as the SECM tip is scanned over the pore. In instances when the SECM tip resistance is small relative to the internal pore resistance, the total impedance changes by a negligible amount, rendering the pore invisible during impedance imaging. A simple solution to this problem is to introduce a low-impedance electrical shunt (i.e., a salt bridge) across the membrane. This principle is demonstrated by imaging polycarbonate membranes (6-12-microm thickness) containing between 1 and 2000 conical-shaped pores (60-nm- and 2.5-microm-diameter openings) using an approximately 1-microm-radius Pt tip. Theory and experiments show that image contrast (the change in ac current measured as the probe is scanned over the pore) is inversely proportional to the total resistance of the membrane and can be increased by a factor of approximately 50x by introducing a low-resistance electrical shunt across the membrane. Remarkably, SECM images of membranes containing a single high-resistance (approximately 1 G Omega) pore can only be imaged by short-circuiting the membrane. Image contrast also becomes independent of membrane resistance when an electrical shunt is used, allowing for more quantitative comparisons of the features in ac impedance images of different membranes.

Journal Article↗
PubMed2006

Electrostatic-gated transport in chemically modified glass nanopore electrodes.

Electrostatic-gated transport in chemically modified glass nanopore electrodes with orifice radii as small as 15 nm is reported. A single conical-shaped nanopore in glass, with a approximately 1 microm radius Pt disk located at the pore base, is prepared by etching the exposed surface of a glass-sealed Pt nanodisk. The electrochemical response of the nanopore electrode corresponds to diffusion of redox-active species through the nanopore orifice to the Pt microdisk. Silanization of the exterior glass surface with Cl(Me)(2)Si(CH(2))(3)CN and the interior pore surface with EtO(Me)(2)Si(CH(2))(3)NH(2) introduces pH-dependent ion selectivity at the pore orifice, a consequence of the electrostatic interactions between the redox ions and protonated surface amines. Nanopore electrodes with very small pore orifice radii (< approximately 50 nm) display anion permselectively at pH < 4, as demonstrated by electrochemical measurement of transport through the pore orifice. Ion selective transport vanishes at pH > 6 or when the pore radius is significantly larger than the Debye screening length, consistent with the observed ion selectivity resulting from electrostatic interactions. The ability to introduce different surface functionalities to the interior and exterior surfaces of glass nanopores is demonstrated using fluorescence microscopy to monitor the localized covalent attachment of 5- (and 6)-carboxytetramethylrhodamine succinimidyl ester to interior pore surfaces previously silanized with EtO(Me)(2)Si(CH(2))(3)NH(2).

Biosensing Techniques↗
PubMed2006

pH- and ionic strength-controlled cation permselectivity in amine-modified nanoporous opal films.

The influence of pH and ionic strength on permselective transport in nanoporous opal films prepared from 440 nm silica spheres was investigated by cyclic voltammetry in aqueous and acetonitrile solutions. Three-layer opal films were deposited from a 1.5 wt % colloidal solution of silica spheres onto 25-microm-diameter Pt microdisk electrodes shrouded in glass. The films were chemically modified by immersing them in a dry acetonitrile solution of 3-aminopropyl triethoxysilane. When the surface amino groups of the modified opal films are protonated and there is little or no supporting electrolyte present in solution, the flux of cationic redox species through the opal membrane is blocked because of electrostatic repulsion. The permselectivity is pH-dependent and can be modulated by adjusting the Debye screening length within the nanopores of the opal by changing the ionic strength of the contacting solution.

Journal Article↗
PubMed2006

Electrochemistry of nanopore electrodes in low ionic strength solutions.

The steady-state voltammetric behavior of truncated conical nanopore electrodes (20-200 nm orifice radii) has been investigated in low ionic strength solutions. Voltammetric currents at the nanopore electrode reflect both diffusive and migrational fluxes of the redox molecule and, thus, are strongly dependent on the charge of the redox molecule and the relative concentrations of the supporting electrolyte and redox molecule. In acetonitrile solutions, the limiting current for the oxidation of the positively charged ferrocenylmethyltrimethylammonium ion is suppressed at low supporting electrolyte concentrations, while the limiting current for the oxidation of the neutral species ferrocene is unaffected by the ionic strength. The dependence of the limiting current on the relative concentrations of the supporting electrolyte and redox molecule is accurately predicted by theory previously developed for microdisk electrodes. Anomalous values of the voltammetric half-wave potential observed at very small nanopore electrodes (<50 nm radius orifice radii) are ascribed to a boundary potential between the pore interior and bulk solution (i.e., a Donnan-type potential).

Electrochemistry↗
PubMed2006

Steady-state voltammetric response of the nanopore electrode.

The steady-state voltammetric response of the truncated conical-shaped glass nanopore electrode is presented. Analytical theory, finite-element simulations, and experimental measurement of the diffusive flux of a redox molecule through the pore orifice demonstrate that the steady-state current decreases rapidly as the pore depth increases and then asymptotically approaches a constant value when the pore depth is approximately 50x larger than the pore orifice. The asymptotic limit of the steady-state current is only a function of the pore orifice radius and the cone angle of the pore and has a finite value for all cone angles greater than zero. Experimental confirmation of the predicted dependence on pore depth is obtained using nanopore electrodes with 100-1000 nm orifice radii, by measuring the steady-state voltammetric current corresponding to the oxidation of ferrocene in acetonitrile solutions containing an excess of supporting electrolyte.

Computer Security↗
PubMed2005

Alternating current impedance imaging of membrane pores using scanning electrochemical microscopy.

Alternating current impedance imaging of a 6-microm thick membrane containing conical-shaped pores (60-nm and 2.5-microm diameter openings) using scanning electrochemical microscopy (SECM) is described. Impedance images of the pore openings were obtained by rastering a glass-sealed conically shaped Pt tip (approximately 1-microm radius) above the membrane surface, while measuring the total impedance between the tip and a large area Pt electrode located on the opposite side of the membrane. Individual pore openings in the high pore density membrane (approximately 8 x 10(4) pores/cm2) are observed in the SECM impedance image. The image contrast is due to the decrease in tip and membrane resistance, in the vicinity of the pore opening. An equivalent circuit for the SECM cell and membrane is proposed and evaluated against the measured SECM imaging impedance. Criteria for employing SECM in impedance mode to image membranes are discussed.

Electric Impedance↗
PubMed2005

Chemically modified opals as thin permselective nanoporous membranes.

Thin-film opals comprising three layers of 440 nm diameter SiO2 spheres were assembled on Pt electrodes and modified with amino groups on the silica surface. Diffusion of anionic, cationic, and neutral redox species through the opals was studied by cyclic voltammetry. The chemically modified opal membranes demonstrate high molecular throughput and, at low pH, selectively block transport of a cationic redox species relative to that of anionic and neutral redox species. This permselective behavior is attributed to the electrostatic interactions that are enhanced by the tortuous pathway within the opal and by the high surface area of the chemically modified spheres.

Journal Article↗
PubMed2005

Influence of asymmetric donor-receiver ion concentration upon transscleral iontophoretic transport.

Recent in vitro and in vivo studies have suggested transscleral iontophoresis as a means for non-invasive drug delivery to the eye. However, there remains a lack of information of the iontophoretic transport behavior of the sclera. The objective of the present study was to investigate the effects of permeant concentration upon transscleral iontophoretic transport. Constant current direct current (DC) iontophoresis was conducted with rabbit sclera in vitro at permeant concentration ranging from 0.015 to 1.0 M in the donor chamber without background electrolyte at 0.4-4 mA (current density: 2-20 mA/cm2). PBS (0.15 M) was the receiver solution. Salicylate (SA) and tetraethylammonium (TEA) were the model ionic permeants, and mannitol was the neutral probe permeant. Conductivity experiments of SA and TEA solutions were performed to determine the effects of ion concentration upon SA and TEA electromobilities. Model simulations were carried out and compared with the experimental data. It was found that the fluxes of the ionic permeants increased linearly with the electric current but were relatively independent of their donor concentrations. Electric field-induced convective solvent flow (electroosmosis) in the sclera was observed to be from the anode to cathode, suggesting that the sclera is net negatively charge at neutral pH. For the studied permeants, electrophoresis was the main transport enhancing mechanism with electroosmosis as a secondary effect. No significant interaction between the permeants and sclera was observed that significantly altered electroosmosis in the membrane. Under the asymmetric donor and receiver conditions, the transference of the permeants could not be predicted by the concentrations of the ions in the donor and receiver chambers with the assumption of constant electric field in the membrane. The membrane ion concentrations were different from those in the chambers due to the requirement of charge neutrality in the membrane.

Administration, Cutaneous↗
PubMed2004

The nanopore electrode.

The fabrication and electrochemical characterization of truncated cone-shaped nanopore electrodes are reported. A nanopore electrode is a Pt disk electrode embedded at the bottom of a conical pore, the circular orifice of the pore having nanometer dimensions. The electrochemical properties of nanopore electrodes with orifice radii of 39 and 74 nm are presented. Both the steady-state and transient voltammetric behavior of the nanopore electrode are reported and compared to predictions obtained using finite-element simulations. The truncated cone-shaped pore electrode possesses a unique transport property-the steady-state flux of molecules into a deep pore is limited by the restriction near the pore orifice, and thus, the steady-state current is independent of the pore depth. This characteristic is potentially useful in studying transport through nanometer-scale orifices.

Journal Article↗
PubMed2004

Electrophoretic capture and detection of nanoparticles at the opening of a membrane pore using scanning electrochemical microscopy.

Stochastic electrophoretic capture of individual nanometer-scale particles at the small opening of a conically shaped nanopore in a synthetic membrane is described. Particle capture is sensed using a scanning electrochemical microscope (SECM) to measure the decrease in the transport rate of a redox-active molecule through the pore. The SECM tip is positioned at the larger backside opening of pore and used to amperometrically monitor the transport rate prior, during, and after particle capture. Following capture, the particle is released by electrophoretically driving it out of the pore opening and back into the solution. The capture and release method is demonstrated by detection of charged polystyrene spheres (43-150-nm diameter) using a polycarbonate membrane with conically shaped pores, the small opening of the pore having a diameter of 60 nm. The inverse of the time to capture polystyrene spheres increases with particle concentration over the range 10(8)-10(10) particles/mL. Selective detection based on nanoparticle charge and size is also demonstrated. A quantitative theoretical description of the rate of particle capture is presented, and the physical mechanism of particle capture, based on the balance of electrostatic and entropic forces, is considered.

Journal Article↗
PubMed2004

The role of the electrical double layer and ion pairing on the electrochemical oxidation of hexachloroiridate(III) at Pt electrodes of nanometer dimensions.

The steady-state voltammetric oxidation of hexachloroiridate(III), IrCl6(3-) (1-5 mM), in the presence and absence of an excess supporting electrolyte was investigated at disk- and hemispherical-shaped Pt electrodes with radii ranging from 48 nm to 12.5 microm. Thermodynamic, kinetic, and transport parameters that define the shape and magnitude of the voltammetric wave exhibit a complex dependence on whether a supporting electrolyte is present in the solution. First, the half-wave potential, E1/2, for oxidation of IrCl6(3-) shifts to more positive potentials in the presence of a supporting electrolyte, a consequence of the relative difference in the strength of ion pairing of IrCl6(3-) and IrCl6(2-) by the supporting electrolyte cation. E1/2 increases in the order no electrolyte < n-tetrabutylammonium < Na+ approximately K+ approximately Ca2+, but is independent of the supporting electrolyte anion (Cl-, NO3-, PF6-). Second, the heterogeneous electron-transfer rate constant for oxidation of IrCl6(3-) increases by approximately an order of magnitude in the presence of a supporting electrolyte. Third, in the absence of electrolyte, mass transport limited currents deviate significantly from predicted values based on the Nernst-Planck equation, but only when the electrode radius is smaller than ca. 1 microm. The latter two effects (Frumkin and dynamic diffuse layer effects) result from the dependence of interfacial electrical fields and, thus, the rates of electron-transfer and ion migration, on the supporting electrolyte concentration. We also demonstrate that the theoretical shape of the voltammetric response for oxidation or reduction of a highly charged redox species (e.g., IrCl6(3-)) is essentially independent of whether a supporting electrolyte is present in the solution. This finding can greatly simplify the analysis of heterogeneous electron-transfer rates using steady-state voltammetry in low ionic strength solutions.

Journal Article↗
PubMed2003

Zeptomole voltammetric detection and electron-transfer rate measurements using platinum electrodes of nanometer dimensions.

The characterization of quasi-hemispherical Pt electrodes of nanometer dimensions (radius 2-150 nm), prepared by electrophoretic coating of etched Pt wires with poly-(acrylic acid), is described. The goals of these experiments are to estimate the accuracy of using steady-state voltammetric limiting currents (i(lim)) in determining the true electrode area and to develop new electrochemical methods for rapidly screening individual electrodes for non-ideal geometries. Electrochemical active areas were determined by measuring the electrical charge (Q) associated with oxidation of adsorbed bis(2,2'-bipyridine)chloro(4,4'-trimethylenedipyridine)osmium(II) in fast-scan voltammetric measurements (scan rate 1000 V/s). Voltammetric peaks corresponding to oxidation of as few as approximately 7000 molecules (approximately 11 zmol) at individual electrodes are reported, allowing precise measurement of electrode areas as small as approximately 10(-10) cm2. A plot of i(lim) (for a soluble redox species) versus Q1/2 (for an adsorbed redox species), constructed from i(lim)-Q1/2 data pairs obtained as a function of the electrode radius, is shown to be linear if the electrode geometry is independent of electrode radius; departure of experimental values from the straight-line plot is a diagnostic indicator of a nonideal electrode geometry. The results indicate that approximately 50% of the electrodes prepared by the electrophoretic polymer-coating procedure are quasi-hemispherical, the remaining being recessed slightly below the polymer coating. The heterogeneous electron-transfer rate constant for the oxidation of the ferrocenylmethyltrimethylammonium cation in H2O/ 0.2 M KCl was also determined from steady-state voltammetry using the method of Mirkin and Bard and found to be 4.(8) +/- 3.(2) cm/s with alpha = 0.6(4) +/- 0.1(5).

Journal Article↗
PubMed2003

Finite-element analysis of magnetic field driven transport at inlaid platinum microdisk electrodes.

We describe a computer-assisted analysis of three-dimensional magnetohydrodynamic (MHD) fluid flow resulting from the passage of electrochemically generated charge through a uniform magnetic field. Magnetic field driven molecular transport in electrochemical systems offers a number of emerging opportunities in research and technology. For instance, electrochemical microfluidic transport and molecule trapping using magnetic fields and field gradients have been demonstrated in recent reports from this laboratory. A key limitation of these investigations is the difficulty in analyzing magnetic field driven flow and transport, due to the complexity of the governing equations of fluid mechanics, electrochemical molecular transport, and magnetic forces. In general, quantitative expressions describing the distribution and fluxes of electroactive species under the influence of a magnetic field cannot be obtained by a direct analytical solution.

Journal Article↗
PubMed2003

Electroosmotic pore transport in human skin.

PURPOSE: To determine the pathways and origin of electroosmotic flow in human skin. METHODS: Iontophoretic transport of acetaminophen in full thickness human cadaver skin was visualized and quantified by scanning electrochemical microscopy. Electroosmotic flow in the shunt pathways of full thickness skin was compared to flow in the pores of excised stratum corneum and a synthetic membrane pore. The penetration of rhodamine 6G into pore structures was investigated by laser scanning confocal microscopy. RESULTS: Electroosmotic transport is observed in shunt pathways in full thickness human skin (e.g., hair follicles and sweat glands), but not in pore openings of freestanding stratum corneum. Absolute values of the diffusive and iontophoretic pore fluxes of acetaminophen in full thickness human skin are also reported. Rhodamine 6G is observed to penetrate to significant depths (approximately 200 microm) along pore pathways. CONCLUSIONS: Iontophoresis in human cadaver skin induces localized electroosmotic flow along pore shunt paths. Electroosmotic forces arise from the passage of current through negatively charged mesoor nanoscale pores (e.g., gap functions) within cellular regions that define the pore structure beneath the stratum corneum.

Acetaminophen↗