Eleni Bitziou

Examination of the Factors Affecting the Electrochemical Performance of Oxygen-Terminated Polycrystalline Boron-Doped Diamond Electrodes

In order to produce polycrystalline oxygen-terminated boron-doped diamond (BDD) electrodes suitable for electroanalysis (i.e., widest solvent window, lowest capacitive currents, stable and reproducible current responses, and capable of demonstrating fast electron transfer) for outer sphere redox couples, the following factors must be considered. The material must contain enough boron that the electrode shows metal-like conductivity; electrical measurements demonstrate that this is achieved at [B] > 10(20) B atoms cm(-3). Even though BDD contains a lower density of states than a metal, it is not necessary to use extreme doping levels to achieve fast heterogeneous electron transfer (HET). An average [B] ~ 3 × 10(20) B atoms cm(-3) was found to be optimal; increasing [B] results in higher capacitive values and increases the likelihood of nondiamond carbon (NDC) incorporation. Hydrogen-termination causes a semiconducting BDD electrode to behave metal-like due to the additional surface conductivity hydrogen termination brings. Thus, unless [B] of the material is known, the electrical properties of the electrode may be incorrectly interpreted. Note, this layer (formed on a lapped electrode) is electrochemically unstable, an effect which is exacerbated at increased potentials. It is essential during growth that NDC is minimized as it acts to increase capacitive currents and decrease the solvent window. We found complete removal of NDC after growth using aggressive acid cleans, acid cycling, and diamond polishing impossible. Although hydrogen termination can mask the NDC signature in the solvent window and lower capacitive currents, this is not a practical procedure for improving sensitivity in electroanalysis. Finally, alumina polishing of lapped, NDC free, freestanding, BDD electrodes was found to be an effective way to produce well-defined, stable, and reproducible surfaces, which support fast (reversible) HET for Fe(CN)6(4-) electrolysis, the first time this has been reported at an oxygen-terminated surface.

Ultrasensitive Detection of Dopamine Using a Carbon Nanotube Network Microfluidic Flow Electrode

The electrochemical measurement of dopamine (DA), in phosphate buffer solution (pH 7.4), with a limit of detection (LOD) of ∼5 pM in 50 μL (∼ 250 attomol) is achieved using a band electrode comprised of a sparse network of pristine single-walled carbon nanotubes (SWNTs), which covers <1% of the insulating substrate. The SWNT electrodes are deployed as amperometric (anodic) detectors in microfluidic cells, produced by microstereolithography, designed specifically for flow injection analysis (FIA). The flow cells, have a channel (duct) geometry, with cell height of 25 μm, and are shown to be hydrodynamically well-defined, with laminar Poiseuille flow. In the arrangement where solution continuously flows over the electrode but the electrode is only exposed to the analyte for short periods of time, the SWNT electrodes do not foul and can be used repeatedly for many months. The LOD for dopamine (DA), reported herein, is significantly lower than previous reports using FIA-electrochemical detection. Furthermore, the SWNT electrodes can be used as grown, i.e., they do not require chemical modification or cleanup. The extremely low background signals of the SWNT electrodes, as a consequence of the sparse surface coverage and the low intrinsic capacitance of the SWNTs, means that no signal processing is required to measure the low currents for DA oxidation at trace levels. DA detection in artificial cerebral fluid is also possible with a LOD of ∼50 pM in 50 μL (∼2.5 fmol).

Simultaneous Detection of pH Changes and Histamine Release from Oxyntic Glands in Isolated Stomach

Real-time simultaneous detection of changes in pH and levels of histamine over the oxyntic glands of guinea pig stomach have been investigated. An iridium oxide pH microelectrode was used in a potentiometric mode to record the pH decrease associated with acid secretion when the sensor approached the isolated tissue. A boron-doped diamond (BDD) microelectrode was used in an amperometric mode to detect histamine when the electrode was placed over the tissue. Both sensors provided stable and reproducible responses that were qualitatively consistent with the signaling mechanism for acid secretion at the stomach. Simultaneous measurements in the presence of pharmacological treatments produced significant variations in the signals obtained by both sensors. As the H2 receptor antagonist cimetidine was perfused to the tissue, histamine levels increased that produced an increase in the signal of the BDD electrode whereas the pH sensor recorded a decrease in acid secretion as expected. Addition of acetylcholine (ACh) stimulated additional acid secretion detected with the pH microelectrode whereas the BDD sensor recorded the histamine levels decreasing significantly. This result shows that the primary influence of ACh is directly on the parietal cell receptors rather then the ECL cell receptors of the oxyntic glands. These results highlight the power of this simultaneous detection technique in the monitoring and diagnosis of physiological significant signaling mechanisms and pathways.

In situ optimization of pH for parts-per-billion electrochemical detection of dissolved hydrogen sulfide using boron doped diamond flow electrodes.

A novel electrochemical approach to the direct detection of hydrogen sulfide (H2S), in aqueous solutions, covering a wide pH range (acid to alkali), is described. In brief, a dual band electrode device is employed, in a hydrodynamic flow cell, where the upstream electrode is used to controllably generate hydroxide ions (OH(-)), which flood the downstream detector electrode and provide the correct pH environment for complete conversion of H2S to the electrochemically detectable, sulfide (HS(-)) ion. All-diamond, coplanar conducting diamond band electrodes, insulated in diamond, were used due to their exceptional stability and robustness when applying extreme potentials, essential attributes for both local OH(-) generation via the reduction of water, and for in situ cleaning of the electrode, post oxidation of sulfide. Using a galvanostatic approach, it was demonstrated the pH locally could be modified by over five pH units, depending on the initial pH of the mobile phase and the applied current. Electrochemical detection limits of 13.6 ppb sulfide were achieved using flow injection amperometry. This approach which offers local control of the pH of the detector electrode in a solution, which is far from ideal for optimized detection of the analyte of interest, enhances the capabilities of online electrochemical detection systems.

Electrochemical flow injection analysis of hydrazine in an excess of an active pharmaceutical ingredient: achieving pharmaceutical detection limits electrochemically.

The quantification of genotoxic impurities (GIs) such as hydrazine (HZ) is of critical importance in the pharmaceutical industry in order to uphold drug safety. HZ is a particularly intractable GI and its detection represents a significant technical challenge. Here, we present, for the first time, the use of electrochemical analysis to achieve the required detection limits by the pharmaceutical industry for the detection of HZ in the presence of a large excess of a common active pharmaceutical ingredient (API), acetaminophen (ACM) which itself is redox active, typical of many APIs. A flow injection analysis approach with electrochemical detection (FIA-EC) is utilized, in conjunction with a coplanar boron doped diamond (BDD) microband electrode, insulated in an insulating diamond platform for durability and integrated into a two piece flow cell. In order to separate the electrochemical signature for HZ such that it is not obscured by that of the ACM (present in excess), the BDD electrode is functionalized with Pt nanoparticles (NPs) to significantly shift the half wave potential for HZ oxidation to less positive potentials. Microstereolithography was used to fabricate flow cells with defined hydrodynamics which minimize dispersion of the analyte and optimize detection sensitivity. Importantly, the Pt NPs were shown to be stable under flow, and a limit of detection of 64.5 nM or 0.274 ppm for HZ with respect to the ACM, present in excess, was achieved. This represents the first electrochemical approach which surpasses the required detection limits set by the pharmaceutical industry for HZ detection in the presence of an API and paves the wave for online analysis and application to other GI and API systems.

Fabrication Route for the Production of Coplanar, Diamond Insulated, Boron Doped Diamond Macro- and Microelectrodes of any Geometry

Highly doped, boron doped diamond (BDD) is an electrode material with great potential, but the fabrication of suitable electrodes in a variety of different geometries both at the macro- and microscale, with an insulating material that does not compromise the material properties of the BDD, presents technical challenges. In this Technical Note, a novel solution to this problem is presented, resulting in the fabrication of coplanar macro- and microscale BDD electrodes, insulated by insulating diamond, at the single and multiple, individually addressable level. Using a laser micromachining approach, the required electrode(s) geometry is machined into an insulating diamond substrate, followed by overgrowth of high quality polycrystalline BDD (pBDD) and polishing to reveal approximately nanometer roughness, coplanar all-diamond structures. Electrical contacting is possible using both top and bottom contacts, where the latter are defined using the laser to produce non-diamond-carbon (NDC) in the vicinity of the back side of the BDD. We present the fabrication of individually addressable ring, band, and disk electrodes with minimum, reproducible controlled dimensions of 50 μm (limited only by the laser system employed). The pBDD grown into the insulating diamond recesses is shown to be free from NDC and possesses excellent electrochemical properties, in terms of extended solvent windows, electrochemical reversibility, and capacitance.

In Situ Control of Local pH Using a Boron Doped Diamond Ring Disk Electrode: Optimizing Heavy Metal (Mercury) Detection

A novel electrochemical approach to modifying aqueous solution pH in the vicinity of a detector electrode in order to optimize the electrochemical measurement signal is described. A ring disk electrode was employed where electrochemical decomposition of water on the ring was used to generate a flux of protons which adjusts the local pH controllably and quantifiably at the disk. Boron doped diamond (BDD) functioned as the electrode material given the stability of this electrode surface especially when applying high potentials (to electrolyze water) for significant periods of time. A pH sensitive iridium oxide electrode electrodeposited on the disk electrode demonstrated that applied positive currents on the BDD ring, up to +50 μA, resulted in a local pH decrease of over 4 orders of magnitude, which remained stable over the measurement time of 600 s. pH generation experiments were found to be in close agreement with finite element simulations. The dual electrode arrangement was used to significantly improve the stripping peak signature for Hg in close to neutral conditions by the generation of pH = 2.0, locally. With the ability to create a localized pH change electrochemically in the vicinity of the detector electrode, this system could provide a simple method for optimized analysis at the source, e.g., river and sea waters.

Spatial changes in acid secretion from isolated stomach tissue using a pH-histamine sensing microarray

The acid secretion mechanism can be studied by measuring a series of metabolic markers and neurotransmitters from in vitro isolated tissue. A microelectrode array was used to monitor proton concentration and histamine levels from isolated guinea pig stomach tissue. The device was partially modified using iridium oxide to form a series of pH sensors, whereas unmodified gold microelectrodes were used to measure the level of histamine in the gut. Real-time measurements in the presence of the H2-receptor antagonist ranitidine produced significant decreases in the overall Delta pH response, as expected. Also, a significant variation in the Delta pH response in between pH sensors was observed in the presence of pharmacological treatment due to structural features of the tissue. No significant differences in Delta i(H) were detected in the presence of ranitidine as expected. More significantly, clear variations in Delta pH responses between animals in control conditions and those in the presence of ranitidine was observed highlighting possible variation in parietal cell density and/or variations in tissue activity. These results identify great possibilities in applying these multi-sensing devices as a long-term stable personalised diagnostic tool for pharmacological screening and disease status.

Simultaneous detection of gastric acid and histamine release to unravel the regulation of acid secretion from the guinea pig stomach

Gastric acid secretion is regulated by three primary components that activate the parietal cell: histamine, gastrin, and acetylcholine (ACh). Although much is known about these regulatory components individually, little is known on the interplay of these multiple activators and the degree of regulation they pose on the gastric acid secretion mechanism. We utilized a novel dual-sensing approach, where an iridium oxide sensor was used to monitor pH and a boron-doped diamond electrode was used for the detection of histamine from in vitro guinea pig stomach mucosal sections. Under basal conditions, gastrin was shown to be the main regulatory component of the total acid secretion and directly activated the parietal cell rather than by mediating gastric acid secretion through the release of histamine from the enterochromaffin-like cell, although both pathways were active. Under stimulated conditions with ACh, the gastrin and histamine components of the total acid secretion were not altered compared with levels observed under basal conditions, suggestive that ACh had no direct effect on the enterochromaffin-like cell and G cell. These data identify a new unique approach to investigate the regulation pathways active during acid secretion and the degree that they are utilized to drive total gastric acid secretion. The findings of this study will enhance our understanding on how these signaling mechanisms vary under pathophysiology or therapeutic management.

Approaches to self-powered biochemical sensors for in-vivo applications

The requirement for electrical power is a major limitation in the development of biosensors for in-body applications. This paper considers motion powered energy harvesting devices for in-body use, showing that power levels are unlikely to exceed a few microwatts for devices of acceptable size. As a low power sensor, we are developing pH detectors based on metal-metal oxide electrodes, such as iridium oxide. The power requirement of these devices is considered, including signal conditioning, storage and transmission. A hybrid powering scheme of energy harvesting and wireless power delivery for data transmission is proposed.