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Patrick R Unwin

Publications and source records attributed to Patrick R Unwin.

6 recordsLinked to original sources
PubMed2003

Effect of surface pressure on the insulator to metal transition of a langmuir polyaniline monolayer.

A remarkable change in the conductivity of a polyaniline (PAN) Langmuir monolayer in the conducting state, as a function of surface pressure, has been observed using scanning electrochemical microscopy (SECM). The film conductivity, as expressed by the SECM current response of a redox mediator, was measured in-situ in a Langmuir film balance. The conductivity of the film increases significantly with surface pressure, above a threshold value of ca. 20 mN m-1.

Journal Article↗
PubMed2003

In situ observation of the surface processes involved in dissolution from the cleavage surface of calcite in aqueous solution using combined scanning electrochemical-atomic force microscopy (SECM-AFM).

The surface processes involved in the initial stages of the proton-assisted dissolution of the calcite single crystal cleavage plane (1014) have been identified using a combined scanning electro-chemical-atomic force microscope (SECM-AFM). This instrument employs a platinum-coated AFM probe, which functions as an electrode as well as a high-resolution topographical sensor. Dissolution in this arrangement is effected by the local electrogeneration of protons, produced by oxidation of water at the probe electrode. By careful control of the applied potential, it is possible to vary the magnitude of the electrogenerated flux of protons from the probe towards the calcite surface. Crucially, by generating a small proton flux for short time periods (0.5 s) it is possible to observe and monitor the initial sites in the dissolution process. Topographical images were recorded in the same area of the surface both prior to and after inducing dissolution, as a function of the proton flux. At low proton fluxes, of the order of 1 nmol cm-2s-1 or less, the surface was observed to dissolve by the nucleation of monolayer deep pits, with densities of about 10(8) cm-2. These pits are likely to be formed at point vacancies or atomic (impurity, for example) defects in the crystal lattice. As the proton flux was increased (over two orders of magnitude), these same etch pits were found to open into wider macro-pits, with an outline morphology that reflected the crystallographic orientation of the surface. At the highest proton fluxes, dissolution from macroscopic step edges became significant.

Journal Article↗
PubMed2003

A microelectrochemical actinometer for scanning electrochemical microscopy studies of photochemical processes.

A new in situ electrochemical actinometry method has been developed and used to determine the light flux through a quartz fibre, employed in a scanning electrochemical microscopy (SECM) system developed to study the kinetics of interfacial photochemical processes. In this system an ultramicroelectrode (UME) probe is positioned with high precision at a known distance close to the end of the fibre, through which light is guided, and used to detect reactants or products of the ongoing photochemical process. The microelectrochemical actinometer was developed using the well-known liquid phase potassium ferrioxalate actinometer. The approach involved recording the steady-state current for Fe(III) reduction at a 25 microm diameter disc-shaped ultramicroelectrode (UME) positioned close to the fibre. A step function in the light flux through the fibre (off-on) was then applied which led to a depletion in the local Fe(III) concentration. The resulting chronoamperometric behaviour at the UME, as a consequence of the solution photochemical process, was measured. A theoretical model has been developed to simulate experimental current-time profiles, which enabled measurements of the light flux initiating the photoprocess.

Journal Article↗
PubMed2002

Electrochemical imaging of diffusion through single nanoscale pores.

A combined scanning electrochemical-atomic force microscope (SECM-AFM) has been used to probe the diffusional transport of target electroactive solutes in isolated nanopores of a track-etched membrane. A polycarbonate membrane (100-nm-diam pore size) hydrated with an electrolyte solution, containing a redox-active probe molecule, such as IrCl6(3-) or Fe(phen)3(2+), functions as the model membrane system. The use of a mobile Pt-coated AFM probe enables individual solution-filled pores to be topographically identified. Analysis of the corresponding current images for the diffusion-limited oxidation of the redox mediator indicates that solution is largely confined to pores in the membrane. Moreover, the tip collector current response provides information on diffusion of the mediator through the pore. Force-distance tip approach and retract measurements allow the radius of contact between the electrochemical-AFM tip and solution confined within a pore at the point of pull-off to be estimated.

Journal Article↗
PubMed2002

Proton diffusion at phospholipid assemblies.

A new scanning electrochemical microscopy proton feedback method has been developed for investigating lateral proton diffusion at phospholipid assemblies: specifically Langmuir monolayers at the water/air interface. In this approach, a base is electrogenerated by the reduction of a weak acid (producing hydrogen) at a "submarine" ultramicroelectrode (UME) placed in the aqueous subphase of a Langmuir trough close to a monolayer. The electrogenerated base diffuses to and titrates monolayer-bound protons and is converted back to its initial form, so enhancing the current response at the UME. Local deprotonation of the monolayer creates a concentration gradient for lateral proton diffusion. A numerical model has been developed, taking into account the potential-dependent association/dissociation constant of the interfacial acid groups. A comparison is made of monolayers comprising either acidic DL-alpha-phosphatidyl-L-serine, dipalmitoyl (DPPS) or zwitterionic L-alpha-phosphatidylcholine, dipalmitoyl (DPPC) monolayers at a range of surface pressures. It is demonstrated that lateral proton fluxes at DPPS are significant, but the lateral proton diffusion coefficient is lower than in bulk solution. In contrast, lateral proton diffusion cannot be detected at DPPC, suggesting that the acid/base character of the phospholipid is important in determining the magnitude of interfacial proton fluxes.

Diffusion↗