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The strongly implicit procedure is used to model the diffusion-only chronoamperometric responses of microband electrode geometries which deviate from the ideal. Specifically, attention is focused on four electrode shapes: elevated microband electrodes (with conducting supporting sides), recessed microband electrodes (with insulating pit walls), platform electrodes (with insulating supporting sides) and, for the purposes of comparison, a hypothetical line electrode without any support which permits diffusional mass transport to both sides of the infinitesimally thin electrode. In all cases the short time transient is found to be described by simple analytical expressions in which the contributions from linear diffusion over the electrode area(s) and non-linear diffusion to the electrode perimeters are clearly distinguishable. © 1995 Elsevier Science S.A. All rights reserved.

Analytical theory is presented which allows rigorous mass transport-corrected Tafel analysis for the reversible electrode processes (1) 2B ⇄ A2 + 2e− and (2) B2 ⇄ 2A + 2e− occurring at electrodes in flowing systems. Predictions are compared with the conclusions of numerical finite-difference modelling for both wall-jet and channel flow cell geometries. Exceillent agreement is found.

The hydroxide-driven dissolution of the polymer hydroxypropyl methylcellulose phthalate (HPMCP) in aqueous solution is studied using atomic force microscopy. The novel technique of employing the surface-averaged z-piezo voltage as a direct measure of the mean absolute surface height is described, so as, after appropriate calibration, to permit the ready inference of dissolution fluxes. In the case of interest the following dissolution rate law was established: j/g·cm-2·s-1 = 105.0[OH-] over the pH range 8.0−9.2, where j is the flux of the dissolving polymer and [OH-] is measured in mol cm-3.

A quantitative study of the hydrolytic dissolution of solid p-chloranil at alkaline pH using a combination of the channel flow cell and in situ atomic force microscopy measurements shows that the dissolution is driven by reaction of the substrate with hydroxide ions at, or very close to, the dissolving surface. The dissolution rate equation, deduced from the channel flow cell for single crystals of p-chloranil, is rate/mol cm-2 s-1 = 3.8 × 10-7[OH-]0 where [OH-]0/M is the hydroxide ion concentration adjacent to the solid surface. An analogous rate law is derived for pressed pellet substrates. The novel application of the atomic force microscope to make absolute measurements of surface averaged dissolution rates is described. Comparison with the independent channel flow cell data reveals good mechanistic agreement and consistency of rate constants provided it is recognised that a thick stagnant diffusion layer can be present under the usual conditions employed for AFM. © 1997 Elsevier Science S.A.

Using the soft lithographic technique, microcontact printing, we demonstrate an example of how a biocompatible polymer can be easily patterned upon another polymer substrate, creating a surface with two spatially different properties. A poly(dimethylsiloxane) mold allowed the patterning of a negative replica of an E-PROM microchip, forming a spatially defined pattern with a period of approximately 1 μm. The amphiphilic biocompatible polymer, poly(lactic acid)−poly(ethylene glycol) (PLA−PEG), was used as the “ink” in order to block off areas of a hydrophobic, polystyrene (PS) substrate. This created a surface with two properties: 2−3 μm strips of PLA−PEG polymer, which resists protein adsorption, divided by micrometer strips of PS. The ability of the patterning technique to provide a true heterogeneous surface was analyzed using atomic force microscopy, while fluorescence microscopy provided a high-contrast method by which to trace the position of specific molecules. We believe this technique to be an ele...

We have demonstrated the site-specific adhesion of photobiotin as a method of producing protein micropatterns. These patterns were created by the selective UV irradiation of a thin film of deposited photobiotin. The UV activated areas of photobiotin were then developed using fluorescently labelled avidin. The size of pattern produced is an order of magnitude smaller than those previously reported by this method. The patterns were characterised using atomic force microscopy (AFM) to determine their microstructure. It was found that the AFM could discriminate between the areas of protein immobilised to the surface through the activated photobiotin, and the bare substrate surface where the inactivated photobiotin had been removed during the washing process. The potential of these patterns as sensing surfaces is demonstrated through the creation of a spatially patterned immunosensing surface. In this case, a biotinylated antibody was bound to the surface and the pattern developed using a second antibody specific to the immobilised biotinylated antibody. This technique could thus provide a simple and efficient method of producing high density immunoassay systems.

We report the use of atomic force microscopy to observe the initial stages of beta-amyloid fibrillization in situ. The growth of individual beta-amyloid protofibrils on a mica substrate was followed over several hours. The first in situ visualization of protofibril formation from single aggregate units of beta-amyloid is reported. The growth of these protofibrils through the subsequent addition of these aggregate units is also observed. Growth of the protofibrils is bi-directional and the outgrowth of protofibrils from a common amyloid/heterogeneous core is also observed. Elongation also occurred by the addition of protofibrils from solution. This data provides an exciting insight into the early stages of beta-amyloid fibrillization and can be used to enhance the understanding of the mechanism(s) by which beta-amyloid fibrillizes and may consequently enable inhibition of one or more stages of fibrillization as a potential therapeutic strategy.

A scanning probe microscope employing a shear force feedback mechanism as used in most near-field scanning optical microscopes has been utilized to provide phase images of a patterned protein boundary. The microscope employed a dithering fiber optic tip and tuning fork method of tip-sample distance control. The effect of tip vibration amplitude upon the phase contrast is investigated, and possible contrast mechanisms are discussed.

We have used albumin-modified atomic force microscope (AFM) tips to probe interactions with a range of hydrophilic polymer brush surfaces and protein. Copolymers of poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) (Pluronics) adsorbed onto polymer interfaces have been shown in previous studies to modify adsorption properties of blood components [using surface plasmon resonance (SPR) and AFM]. Here we have employed protein-coated AFM probes to study a series of PEO−PPO−PEO-coated interfaces prepared with a range of PEO and PPO molecular weights. Subsequent force−distance experiments have shown a good correlation between the forces of adhesion of an albumin-functionalized AFM probe with the various PEO−PPO−PEO surfaces and the adsorption trends of albumin onto these polymeric surfaces observed with SPR. The data suggest that the size of the hydrophobic PPO segment of the Pluronic is a major determinant of the polymer protein resistance properties. In addition, as the PEO layer d...

The application of power ultrasound in electrochemistry may promote or usefully modify electrode reactions. In the work reported here the four-electron reduction of nitrobenzene in alkaline (pH 13) aqueous media was studied as a model system. The electrochemical reduction is known to follow a complex mechanism [E. Laviron, A. Vallat and R. Meunier-Prest, J. Electroanal. Chem., 379 (1994) 427], involving protonations as well as a dehydration step; both surface and solution pathways for this reduction may be observed [C. Nishihara and H. Shindo, J. Electroanal. Chem., 221 (1987) 245], depending on the nature and state of the electrode. Both under silent and ultrasonic conditions nitrobenzene is reduced on glassy carbon electrodes in a chemically reversible one-electron process followed by an irreversible three-electron reduction. At sufficiently negative potentials the reduction process remains overall four-electron, with phenylhydroxylamine as the major product even at the high current densities obtained with intense ultrasound. Glassy carbon electrodes are shown to be suitable for kinetic studies, although damage, as endorsed by an increase in roughness and capacitance and probably initiated by mechanical damage at very short electrode–horn distances, was detected by a.c. impedance, voltammetric and atomic force microscopy (AFM) methods. On gold electrodes a more complicated mechanism due to a surface reaction pathway arises. A comparison of sonovoltammetric and rotating disk voltammetric results gives evidence for the homogeneous pathway being dominant under applied ultrasound conditions. The transition between surface and solution pathways is mass flux as well as concentration dependent, and an estimate for the rate of the surface catalyzed reaction for a concentration of 1.44 mM nitrobenzene (k = 4±2 × 10−2 cms−1), attributed to protonation of the nitrobenzene radical anion, is obtained from combined rotating disk and sonovoltammetric data. Damage to the gold surface as monitored by various techniques is small, but manifests itself by an interesting decrease in double layer capacitance.

The effect of power ultrasound in the modification of voltammetric processes is investigated and the role of surface activation by means of electrode erosion/roughening characterized. The potential benefits of sonovoltammetry are illustrated with two examples. First, the Fladè potential for the passivation of nickel in air-saturated aqueous KOH solution is shown to be anodically shifted in the presence of ultrasound. Second, the voltammetric study of Cr(CO)6 in acetonitrile solution is shown to be feasible without the passivation observed under silent conditions where the electrolysis results in surface-active species the adsorption of which block further electron transfer. © 1994 American Chemical Society.

A novel technique is presented that addresses the issue of how to apply internal standard (IS) to dried matrix spot (DMS) samples that allows the IS to integrate with the sample prior to extraction. The TouchSpray, a piezo electric spray system, from The Technology Partnership (TTP), was used to apply methanol containing IS to dried blood spot (DBS) samples. It is demonstrated that this method of IS application has the potential to work in practice, for use in quantitative determination of circulating exposures of pharmaceuticals in toxicokinetic and pharmacokinetic studies. Three different methods of IS application were compared: addition of IS to control blood prior to DBS sample preparation (control 1), incorporation into extraction solvent (control 2), and the novel use of TouchSpray technology (test). It is demonstrated that there was no significant difference in accuracy and precision data using these three techniques obtained using both manual extraction and direct elution.

A novel liquid flow cell allows atomic force microscopy (AFM) images to be obtained under defined hydrodynamic flow conditions, enabling reaction fluxes calculated from proposed heterogeneous reaction mechanisms to be compared with those determined experimentally. The cell employs an inclined jet to direct a fluid flow at the sample surface to cover the area under investigation including the AFM scanning cantilever tip. The flow pattern and velocity were calculated by using the finite element fluid dynamics program FIDAP and confirmed by placing an electrode at the sample position and measuring the limiting current for the one-electron oxidation of potassium hexacyanoferrate(II) in water as a function of flow rate. The operation of the cell has been further confirmed by the direct measurement of the dissolution rate of calcite exposed to a flow of 0.98 mM aqueous HCl by deducing the rate of removal of the surface from the change of the z-piezo voltage. The rate constant k1 = 0.035 cm s-1 obtained for the dissolution step was in excellent agreement with the value (0.043 ± 0.015) cm s-1 found from independent channel flow cell experiments. The dissolution of the (110) and (110) faces of salicylic acid (SA) single crystals in water and in solutions of salicylic acid was studied as a function of flow rate and was found to be consistent with a model combining a constant rate of dissolution with a simultaneous reprecipitation having a first-order dependence on [SA]0, with the flux J = kd - kp[SA]0 where the parameters are kd110 = 3 × 10-9 mol cm-2 s-1 and kp110 = 1.74 × 10-4 cm s-1 for the (110) face and kd110 = 1.5 × 10-8 mol cm-2 s-1 and kp110 = 1.06 × 10-3 cm s-1 for the (110) face.

A novel liquid flow cell is developed which allows atomic force microscopy images to be obtained under defined and modelled hydrodynamic flow conditions, enabling measured reaction fluxes to be compared with those calculated from proposed heterogeneous reaction mechanisms.

This paper describes the formation of highly defined two-dimensional protein arrays onto a polymeric substrate expressing biotin functionalities at its surface. Micron-scale arrays of avidin were created by exploiting its interaction with biotin, thereby creating a versatile patterned surface onto which any biotinylated species can be subsequently immobilized. The patterning process utilized the technique of microfluidic networks (μFN) to spatially confine the flow of protein solution on a substrate. We describe methods developed to control protein deposition with nanometer-scale precision. Utilizing atomic force microscopy (AFM), these patterned surfaces were analyzed to the molecular level. In addition, the applicability of this technique to a wider range of substrates has been investigated. In particular, developments within the μFN technique have enabled the successful formation of protein micropatterns onto a highly hydrophilic surface, a substrate which is generally not suited to the technique of μFN.