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Here we demonstrate the use of microstereolithography (MSL), a 3D direct manufacturing technique, as a viable method to produce small-scale microfluidic components for electrochemical flow detection. The flow cell is assembled simply by resting the microfabricated component on the electrode of interest and securing with thread! This configuration allows the use of a wide range of electrode materials. Furthermore, our approach eliminates the need for additional sealing methods, such as adhesives, waxes, and screws, which have previously been deployed. In addition, it removes any issues associated with compression of the cell chamber. MSL allows a reduction of the dimensions of the channel geometry (and the resultant component) and, compared to most previously produced devices, it offers a high degree of flexibility in the design, reduced manufacture time, and high reliability. Importantly, the polymer utilized does not distort so that the cell maintains well-defined geometrical dimensions after assembly. For the studies herein the channel dimensions were 3 mm wide, 3.5 mm long, and 192 or 250 [micro]m high. The channel flow cell dimensions were chosen to ensure that the substrate electrodes experienced laminar flow conditions, even with volume flow rates of up to 64 mL [min.sup.-1] (the limit of our pumping system). The steady-state transport-limited current response, for the oxidation of ferrocenylmethyl trimethylammonium hexaflorophosphate ([FcTMA.sup.+]), at gold and polycrystalline boron doped diamond (pBDD) band electrodes was in agreement with the Levich equation and/or finite element simulations of mass transport. We believe that this method of creating and using channel flow electrodes offers a wide range of new applications from electroanalysis to electrocatalysis. 10.1021/ac100345v

A scanning micropipet contact method (SMCM) is described which promises wide-ranging application in imaging and quantifying electrode activity at high spatial resolution. In SMCM, a moveable micropipet probe (diameter 300 nm to 1 [micro]m) containing an electroactive species in electrolyte solution is brought to a sample electrode surface so that the liquid meniscus makes contact. The micropipet contains a reference-counter electrode, and the sample is connected as the working electrode to make a two-electrode voltammetric measurement. SMCM thus makes possible highly localized electrochemical experiments, and furthermore, heterogeneous electrode surfaces may be investigated without the substrate being completely immersed in solution. This opens up the possibility of making measurements on a wide range of electrode materials without having to encapsulate the electrode. Furthermore, the electrode/ solution contact can be made rapidly and briefly, which is useful for situations where the electrode would be unstable for longer periods (e.g., due to corrosion or surface adsorption). For heterogeneously active surfaces the technique is particularly powerful as it allows defined areas to be targeted and individual sites to be probed. To exemplify the approach, the electroactivity of basal plane highly oriented pyrolytic graphite (HOPG) and two types of aluminum alloy were investigated. SMCM measurements indicate that basal plane HOPG shows much greater activity than present consensus. Measurements of chemically heterogeneous aluminum alloy surfaces with SMCM allow variations in redox activity to be mapped with high spatial resolution.

Pt nanoparticle (NP)-modified polycrystalline boron-doped diamond (pBDD) disk electrodes have been fabricated and employed as amperometric sensors for the determination of dissolved oxygen concentration in aqueous solution, pBDD columns were cut using laser micro-machining techniques and sealed in glass, in order to make disk electrodes which were then characterized electrochemically. Electrodeposition of Pt onto the diamond electrodes was optimized so as to give the maximum oxygen reduction peak current with the lowest background signal. Pt NPs, >0-10 nm diameter, were found to deposit randomly across the pBDD electrode, with no preference for grain boundaries. The more conductive grains were found to promote the formation of smaller nanoparticles at higher density. With the use of potential step chronoamperometry, in which the potential was stepped to a diffusion-limited value, a four electron oxygen reduction process was found to occur at the Pt NP-modified pBDD electrode. Furthermore the chronoamperometric response scaled linearly with dissolved oxygen concentration, varied by changing the oxygen/nitrogen ratio of gas flowed into solution. The sensor was used to detect dissolved oxygen concentrations with high precision over the pH range 4-10.

Ultramicroelectrodes (UMEs) fabricated from networks of chemical vapor deposited single-walled carbon nanotubes (SWNTs) on insulating silicon oxide surfaces are shown to offer superior qualities over solid UMEs of the same size and dimensions. Disk shaped UMEs, comprising two-dimensional 'metallic' networks of SWNTs, have been fabricated lithographically, with a surface coverage of < 1% of the underlying insulating surface. The electrodes are long lasting and give highly reproducible responses (either for repeat runs with the same electrode or when comparing several electrodes with the same size). For redox concentrations [less than or equal to] 1 mM the steady-state behavior of SWNT network UMEs is as expected for conventional solid metal UMEs (e.g., Pt, Au), due to diffusional overlap between neighboring SWNTs. Importantly, the low intrinsic capacitance of the SWNTs and much reduced surface area lead to much faster response times and lower background currents. The well-defined geometry of the SWNT network electrode is also useful for studying electron transfer (ET) kinetics at SWNTs. Given the intrinsically high mass transport rates to SWNTs within a network electrode, the reversible nature of the CVs recorded for Ru[(N[H.sub.3]).sub.6.sup.3-/2-] and [FcTMA.sup.+/2+] suggests that ET driven solely by defects on the sidewalls of the SWNTs is highly unlikely.

Scanning electrochemical microscopy (SECM) has been employed in the feedback mode to assess the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes (SWNTs). It is shown that, even though the network comprises both metallic and semiconducting SWNTs, at high density (well above the percolation threshold for metallic SWNTs) and with approximately millimolar concentrations of redox species the network behaves as a thin metallic film, irrespective of the formal potential of the redox couple. This result is particularly striking since the fractional surface coverage of SWNTs is only ~1% and SECM delivers high mass transport rates to the network. Finite element simulations demonstrate that under these conditions diffusional overlap between neighboring SWNTs is significant so that planar diffusion prevails in the gap between the SECM tip and the underlying SWNT substrate. The SECM feedback response diminishes at higher concentrations of the redox species. However, wet gate measurements show that at the solution potentials of interest the conductivity is sufficiently high that lateral conductivity is not expected to be limiting. This suggests that reaction kinetics may be a limiting factor, especially since the low surface coverage of the SWNT network results in large fluxes to the SWNTs, which are characterized by a low density of electronic states. For electroanalytical purposes, significantly, two-dimensional SWNT networks can be considered as metallic films for typical millimolar concentrations employed in amperometry and voltammetry. Moreover, SWNT networks can be inexpensively and easily formed over large scales, opening up the possibility of further electroanalytical applications.

The application of evanescent wave cavity ring-down spectroscopy (EW-CRDS) in monitoring electrogenerated species within a thin-layer electrochemical cell is demonstrated. In the proof-of-concept experiments described, ferricyanide, Fe[(CN).sub.6.sup.3-], was produced by the transport-limited oxidation of ferrocyanide, Fe[(CN).sub.6.sup.4-], in a thin-layer solution cell (25-250 [micro]m) formed between an electrode and the hypotenuse of a fused-silica prism. The prism constituted one element of a high-finesse optical cavity arranged in a triangular ring geometry with light being totally internally reflected at the silica/solution interface. The cavity was pumped with the output (~417 nm) of a single-mode external cavity diode laser, which was continuously scanned across the cavity modes. The presence of electrogenerated ferricyanide within the resulting evanescent field, beyond the optical interface, was detected by the enhanced loss of light trapped within the cavity, as measured by the characteristic cavity ring down. In this way, the EW-CRDS technique is sensitive to absorption in only the first few hundred nanometers of solution above the silica surface. The cavity ring-down response accompanying both cyclic voltammetric and step potential chronoamperometry experiments at a variety of electrode-surface distances is presented, and the results are shown to be well reproduced in modeling by finite element methods. The studies herein thus provide a foundation for further applications of EW-CRDS combined with electrochemistry.

Spatial variations in the electrical and electrochemical activity of microarray electrodes, fabricated entirely from diamond, have been investigated. The arrays contain ~50[micro]m-diameter boron-doped diamond (BDD) disks spaced 250 [micro]m apart (center to center) in insulating intrinsic diamond supports, such that the BDD regions are coplanar with the intrinsic diamond. Atomic force microscopy (AFM) imaging of the surface reveals a roughness of no more than [+ or -]10 nm over the array. Each BDD microdisk within the array contains polycrystalline BDD with a variety of different grains exposed. Using conducting-AFM, the conductivity of the different grains was found to vary within a BDD microdisk. Electrochemical imaging of the electroactivity of the microdisk electrodes using scanning electrochemical microscopy operating in substrate generation-tip collection mode revealed that, under apparently diffusion-limited steady-state conditions, there was a small variation in the response between electrodes. However, the majority of electrodes in the array appeared to show predominantly metallic behavior. For the electrodes that showed a lower activity, all grains within the microdisk supported electron transfer, albeit at different rates, as evidenced by studies on the electrode-position of metallic silver, at potentials far negative of the flat band potential of oxygen-terminated polycrystalline diamond. The possibility of using these array electrodes for steady-state diffusion-limited measurements in electroanalytical applications is far-reaching. However, caution should be exercised in the kinetic analysis of voltammetric measurements, since wide variations in the electroactivity of individual grains are apparent when the potential is below the diffusion-limited value.

The first comparison of the structure and electrical conductivity properties of spin cast (SC) and Langmuir-Schaeffer (LS) films of regioregular poly(3-hexylthiophene) (P3HT) is presented. Results show that, while the inclusion of Au nanoparticles (NPs) caused the lateral conductivity to decrease in SC films, in LS films the lateral conductivity increased by a factor of 2.

A procedure for the batch microfabrication of scanning electrochemical-atomic force microscopy (SECM-AFM) probes is described. The process yields sharp AFM tips, incorporating a triangular-shaped electrode (base width 1 [micro]m, height 0.65 [micro]m) at the apex. Microfabrication was typically carried out on 1/4 3-in. wafers, yielding 60 probes in each run. The measured spring constant of the probes was in the range 1-1.5 N [m.sup.-1]. To date, processing has been carried out twice successfully, with an estimated success rate for the fabrication process in excess of 80%, based on field emission-scanning electron microscopy imaging of all probes and current voltage measurements on a random selection of ~30 probes. Steady-state voltammetric measurements for the reduction of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] in aqueous solution indicate that the electrode response is well-defined, reproducible, and quantitative, based on a comparison of the experimental diffusion-limited current with finite element simulations of the corresponding mass transport (diffusion) problem. Topographical imaging of a sputtered Au film with the SECM-AFM probes demonstrates lateral resolution comparable to that of conventional [Si.sub.3][N.sub.4] AFM probes. Combined electrochemical topographical imaging studies have been carried out on two model substrates: a 10-[micro]m-diameter disk ultramicroelectrode (UME) and an array of 1-[micro] m-diameter UMEs, spaced 12.5 [micro]m apart (center to center). In both cases, an SECM-AFM probe was first employed to image the topography of the substrates. The tip was then moved back a defined distance from the surface and use to detect [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] produced at the substrate, biased at a potential to reduce [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.], present in bulk solution, at a diffusion-controlled rate (substrate generation-tip collection mode). These studies establish the success of the batch process for the mass microfabrication of SECM-AFM tips.

Electrospray ionization (ESI) is extensively used in the analysis of biological compounds; yet some fundamental properties of this technique are not completely understood. It is widely recognized that care should be exercised when noncovalent complexes are being studied by ESI, since weak noncovalent binding can be broken or formed during the desolvation process. In the present work, spectra from the noncovalent complex, vancomycin/ diacetyl-L-iysyl-D-alanyl-D-alanine, obtained from ESI and from nanoelectrospray ionization (nanoESI), have been compared. The results indicated that the milder desolvation conditions arising as a result of the smaller sizes of droplets produced in the nanoESI source attenuated effects upon weak bonds in the desolvation process. The association constant values calculated from the relative peak intensities suggest that, when using ESI, the analyzed noncovalent complex dissociated in the condensed phase during the spraying process. The influences of experimental parameters such as tip diameter and coating for nanoESI needles were investigated. Principal component analysis, a multivariate analysis method, was applied to achieve a better evaluation of the spectra obtained using different needle diameters and coatings for the analysis of the noncovalent complex vancomycin/ diacetyl-L-lysyl-D-alanyl-D-alanine. It was found that 2-[micro]m tip diameter resulted in more reproducible spectra than the larger tip diameters tested (6-20 [micro]m).

The heterogeneous electronic conductivity of the native oxide film on polycrystalline Ti electrodes is studied by using the atomic force microscopy. It is revealed that there are large spatial variations in the oxide film conductivity on topographically featureless regions of grain faces.

Study was conducted to examine the mass transport to micrometer-sized electrodes in a microjet (wall-tube) electrode configuration experimentally and through finite element modeling. According to the electrochemical imaging experiments, local mass transport is highly sensitive to the lateral position of the nozzle with respect to the electrode.

Using an electrochemical potential pulse methodology in a mixed solvent system, electrochemical deposition of amorphous vanadium pentoxide (V2O5) nanobelts is possible. Crystallisation of the material is achieved using in air annealing with the temperature of crystallisation identified using in situ heating transmission electron microscopy (TEM). The resulting α-phase V2O5 nanobelts have typical thicknesses of 10-20 nm, widths and lengths in the range 5-37 nm (mean 9 nm) and 15-221 nm (mean 134 nm), respectively. One-cycle reversibility studies for lithium intercalation (discharge) and de-intercalation (charge) reveal a maximum specific capacity associated with three lithium ions incorporated per unit cell, indicative of ω-Li3V2O5 formation. Aberration corrected scanning TEM confirm the formation of ω-Li3V2O5 across the entirety of a nanobelt during discharge and also the reversible formation of the α-V2O5 phase upon full charge. Preliminary second cycle studies reveal reformation of the ω-Li3V2O5, accompanied with a morphological change in the nanobelt dimensions. Achieving α-V2O5 to ω-Li3V2O5 to α-V2O5 reversibility is extremely challenging given the large structural rearrangements required. This phenomenon has only been seen before in a very limited number of studies, mostly employing nanosized V2O5 materials and never before with electrodeposited material. [ABSTRACT FROM AUTHOR]

A method for producing and characterizing electrically connected single-wall carbon nanotube scanned-probe tips is discussed. This method provides a new geometry for investigating the transport properties of a nanotube as a continuous function of its conducting length.