In Vitro Monitoring of Nitric Oxide Release in the Mouse Colon Using a Boron-Doped Diamond Microelectrode Modified with Platinum Nanoparticles and Nafion.

This research reports on the preparation of a boron-doped diamond microelectrode modified with platinum nanoparticles and Nafion and its application for detecting nitric oxide (NO) in vitro in the mouse colon. Platinum nanoparticle deposition was performed potentiodynamically using a 2.0 mmol L-1 potassium hexachloroplatinate solution and cycling from -0.2 to 1.3 V vs Ag/AgCl at 0.01 V s-1 for 10 cycles. The Nafion overlayer was applied by immersion in a solution containing 2.5% (w/v) colloidal Nafion and drying overnight at 55 °C in a humid environment. The optimal microelectrode preparation conditions were chosen based on the electrode response for NO oxidation as well as rejection of nitrite (NO2-) oxidation, the main interferent in the electrochemical detection of NO in biological media. Detection figures of merit include a sensitivity of 16.7 ± 2.7 mA M-1 cm-2 (n = 3 electrodes), a detection limit of 0.5 µmol L-1 (S/N = 3), and an electrode response reproducibility of 2.5% (RSD). Electrical stimulation and continuous amperometry were used to measure NO release from myenteric ganglia in wild-type male and female mice in response to an increasing number of electrical stimuli to study nitrergic signaling in the colon. We also present preliminary data regarding the use of optogenetics to selectively stimulate nitrergic myenteric neurons using blue light stimulation with a goal of understanding how inhibitory neuromuscular signaling is involved in the myenteric plexus circuitry that controls intestinal motility.

In vitro electrochemical measurement of serotonin release in the human jejunum mucosa using a diamond microelectrode.

We report herein on the use of a boron-doped diamond microelectrode (DME) to record oxidation currents in vitro associated with the release of serotonin from enterochromaffin cells in the epithelium of the human intestinal mucosa. Continuous amperometric measurements were made as a function of distance (ln current vs. distance) from the tissue surface in human jejunum specimens. The results demonstrate the capabilities of the DME for the stable and reproducible detection of serotonin in the complex environment of the human tissue. Serotonin release was evoked by the shearing force of a continuously flowing Krebs buffer solution at 36 °C with the tissue pinned down in a flow bath. Reproducible currents with distance were recorded for serotonin oxidation. Increased oxidation currents were observed in the presence of the selective serotonin reuptake inhibitor, fluoxetine, indicating that a significant fraction of the amperometric current recorded is attributable to serotonin oxidation. The nominal reciprocal slope, |slope-1|, of the ln current vs. distance plots increased from 270 ± 25 µm-1 in Krebs buffer (N = 3) to 471 ± 65 µm-1 during fluoxetine addition (N = 3), reflective of a reduced rate of reuptake in the presence of the SERT antagonist. The paper reports on the characterization of the diamond microelectrodes and the in vitro electrochemical measurement data.

Local and Long-Range Organization in Room Temperature Ionic Liquids.

Room temperature ionic liquids (RTILs) have a wide range of current and potential applications, in areas ranging from supercapacitor energy storage to sequestration of toxic gas phase species and use as reusable solvents for selected organic reactions. All these applications stem from their unique physical and chemical properties, which remain understood to only a limited extent. Among the issues of greatest importance is the extent to which RTILs exist as dissociated ionic species and the length scales over which some types of organizations are seen to exist in them. In this Invited Feature Article, we review the current understanding of organization in this family of materials, where opportunities lie in terms of deepening our understanding, and what potential applications would benefit from gaining such knowledge.

Characterizing the Magnitude and Structure-Dependence of Free Charge Density Gradients in Room-Temperature Ionic Liquids.

We have reported previously on the existence of charge-induced long-range organization in the room-temperature ionic liquid (RTIL), BMIM+BF4-. The induced organization is in the form of a free charge density gradient (ρf) that exists over ca. 100 µm into the RTIL in contact with a charged surface. The fluorescence anisotropy decay of a trace-level charged chromophore in the RTIL is measured as a function of distance from the indium-doped tin oxide support surface to probe this free charge density gradient. We report here on the characterization of the free charge density gradient in five different imidazolium RTILs and use these data to evaluate the magnitude of the induced free charge density gradient. Both the extent and magnitude of this gradient depend on the chemical structures of the cationic and anionic constituents of the RTIL used. Control over the magnitude of ρf has implications for the utility of RTILs for a host of applications that remain to be explored fully.

Effect of Surface Oxygen on the Wettability and Electrochemical Properties of Boron-Doped Nanocrystalline Diamond Electrodes in Room-Temperature Ionic Liquids.

This paper reports on how the surface chemistry of boron-doped nanocrystalline diamond (BDD) thin-film electrodes (H vs O) affects the wettability and electrochemical properties in two room-temperature ionic liquids (RTILs): [BMIM][PF6] and [HMIM][PF6]. Comparative measurements were made in 0.5 mol L-1 H2SO4. The BDD electrodes were modified by microwave or radio-frequency (RF) plasma treatment in H2 (H-BDD), Ar (Ar-BDD), or O2 (O-BDD). These modifications produced low-, medium-, and high-oxygen surface coverages. Atomic O/C ratios, as determined by X-ray photoelectron spectroscopy (XPS), were 0.01 for H-BDD, 0.08 for Ar-BDD, and 0.17 for O-BDD. The static contact angle of ultrapure water on the modified electrodes decreased from 110° (H-BDD) to 41° (O-BDD) with increasing surface oxygen coverage, as expected as the surface becomes more hydrophilic. Interestingly, the opposite trend was seen for both RTILs as the contact angle increased from 20° (H-BDD) to 50° (O-BDD) with increasing surface oxygen coverage. The cyclic voltammetric background current and potential-dependent capacitance in both RTILs were largest for BDD electrodes with the lowest O/C ratio (H-BDD) and smallest contact angle. Slightly larger voltammetric background currents and capacitance were observed in [HMIM][PF6] than in [BMIM][PF6]. Capacitance values ranged from 8 to 16 µF cm-2 over the potential range for H-BDD and from 4 to 6 µF cm-2 for O-BDD. The opposite trend was observed in H2SO4 as the voltammetric background current and capacitance were largest for BDD electrodes with the highest O/C ratio (O-BDD) and smallest contact angle. In summary, reducing the surface oxygen on BDD electrodes increases the wettability to two RTILs and this increases the voltammetric background current and capacitance.

Conductive diamond: synthesis, properties, and electrochemical applications.

Conductive diamond possesses unique features as compared to other solid electrodes, such as a wide electrochemical potential window, a low and stable background current, relatively rapid rates of electron-transfer for soluble redox systems without conventional pretreatment, long-term responses, stability, biocompatibility, and a rich surface chemistry. Conductive diamond microcrystalline and nanocrystalline films, structures and particles have been prepared using a variety of approaches. Given these highly desirable attributes, conductive diamond has found extensive use as an enabling electrode across a variety of fields encompassing chemical and biochemical sensing, environmental degradation, electrosynthesis, electrocatalysis, and energy storage and conversion. This review provides an overview of the fundamental properties and highlights recent progress and achievements in the growth of boron-doped (metal-like) and nitrogen and phosphorus-doped (semi-conducting) diamond and hydrogen-terminated undoped diamond electrodes. Applications in electroanalysis, environmental degradation, electrosynthesis electrocatalysis, and electrochemical energy storage are also discussed. Diamond electrochemical devices utilizing micro-scale, ultramicro-scale, and nano-scale electrodes as well as their counterpart arrays are viewed. The challenges and future research directions of conductive diamond are discussed and outlined. This review will be important and informative for chemists, biochemists, physicists, materials scientists, and engineers engaged in the use of these novel forms of carbon.

Inkjet-Printed Carbon Nanotube Electrodes for Measuring Pyocyanin and Uric Acid in a Wound Fluid Simulant and Culture Media.

Polyacrylamide-coated, carbon nanotube (PA/CNT) electrodes were prepared by an inkjet printing process and used to measure pyocyanin and uric acid in a wound fluid simulant at 37 °C. These two molecules are potential indicators of infection, and therefore their detection could prove useful for monitoring wound healing. Pyocyanin is a marker for the common wound bacterium Pseudomonas aeruginosa. Our long-term goal is to use these inexpensive and disposable electrodes to measure biomarkers of wound healing directly. In this proof-of-concept work, studies were performed in a wound fluid simulant to evaluate the stability of the electrodes and their responsiveness for the two bioanalytes. The PA/CNT inkjet-printed electrodes and electrical contacts were stable with unchanging physical and electrochemical properties in the wound fluid simulant over a 7-8-day period at 37 °C. The detection figures of merit for pyocyanin in the simulant at 37 °C were as follows: linear over the physiologically relevant range = 0.10 to 100 µmol L-1 (R2 = 0.9992), limit of detection = 0.10 µmol L-1 (S/N = 3), sensitivity = 35.6 ± 0.8 mA-L mol-1 and response variability ≤4% RSD. The detection figures of merit for uric acid in the simulant at 37 °C were as follows: linear over the physiologically relevant range = 100 to 1000 µmol L-1 (R2 = 0.9997), sensitivity = 2.83 ± 0.01 mA-L mol-1, and response variability ≤4% RSD. The limit of detection was not determined. The PA/CNT electrodes were also used to quantify pyocyanin concentrations in cell-free culture media from different strains of P. aeruginosa. The detected concentrations ranged from 1.00 ± 0.02 to 118 ± 6 µM depending on the strain.

Analysis of Ag(I) Biocide in Water Samples Using Anodic Stripping Voltammetry with a Boron-Doped Diamond Disk Electrode.

The electroanalytical performance of a new commercial boron-doped diamond disk and a traditional nanocrystalline thin-film electrode were compared for the anodic stripping voltammetric determination of Ag(I). The diamond disk electrode is more flexible than the planar film as the former is compatible with most electrochemical cell designs including those incorporating magnetic stirring. Additionally, mechanical polishing and surface cleaning are simpler to execute. Differential pulse anodic stripping voltammetry (DPASV) was used to detect Ag(I) in standard solutions after optimization of the deposition potential, deposition time and scan rate. The optimized conditions were used to determine the concentration of Ag(I) in a NASA simulated potable water sample and a NIST standard reference solution. The electrochemical results were validated by ICP-OES measurements of the same solutions. The detection figures of merit for the disk electrode were as good or superior to those for the thin-film electrode. Detection limits were ≤5 µg L-1 (S/N = 3) for a 120 s deposition period, and response variabilities were <5% RSD. The polished disk electrode presented a more limited linear dynamic range presumably because of the reduced surface area available for metal phase formation. The concentrations of Ag(I) in the two water samples, as determined by DPASV, were in good agreement with the concentrations determined by ICP-OES.

Isatin Detection Using a Boron-Doped Diamond 3-in-1 Sensing Platform.

Boron-doped diamond (BDD) is a promising electrochemical tool that exhibits excellent chemical sensitivity and stability. These intrinsic advantages coupled with the material's vast microfabrication flexibility make BDD an attractive sensing device. In this study, two different 3-in-1 BDD electrode sensors were fabricated, characterized, and investigated for their capability to detect isatin, an anxiogenic indole that possesses anticonvulsant activity. Each device was comprised of a working, reference, and auxiliary electrode, all made of BDD. Two different working electrode geometries were studied, a 2 mm diameter macroelectrode (MAC) and a microelectrode array (MEA). The BDD quasi-reference electrode was studied by measuring its potential against a traditional Ag/AgCl reference electrode. While the potential shifted as a function of solution pH, a miniscule potential drift was observed when holding the solution pH constant. Specifically, the BDD quasi-reference electrode had a potential of -0.2 V (vs Ag/AgCl) in a pH 7 solution, and this remained stable for a 30-h time period. For the detection of isatin, solutions were analyzed using both sensors in pH 7.4 phosphate buffered saline (PBS). Using the MEA sensor, the limit of detection (LOD, (3σ)/m) for isatin was found to be 0.04 µM; an increase to 0.22 µM was observed with the MAC sensor. These results were compared to those obtained from UV-vis spectrophotometry, where a 0.57 µM LOD was observed. The feasibility for use in a complex sample matrix was also examined by completing measurements in urine simulant. The results presented herein indicate that both 3-in-1 BDD sensors are applicable at low limits of detection with potential application as an electrochemical detector for chromatographic methods.

Sex-related differences in small intestinal transit and serotonin dynamics in high-fat-diet-induced obesity in mice.

Obesity alters gastrointestinal (GI) motility and 5-HT signalling. Altered 5-HT signalling disrupts control of GI motility. Levels of extracellular 5-HT depend on enterochromaffin (EC) cell release and serotonin transporter (SERT) uptake. We assessed GI transit and 5-HT signalling in the jejunum of normal and obese mice. Male and female mice were fed a control diet (CD; 10% of kilocalories as fat) or a high-fat diet (HFD; 60% of kilocalories as fat). Gastrointestinal transit was increased in male HFD-fed and female CD-fed compared with male CD-fed mice. The 5-HT3 receptor blocker, alosetron, increased gastric emptying in male CD-fed mice, but decreased transit in female CD-fed mice. The 5-HT-induced jejunal longitudinal muscle contractions in vitro were similar in all mice. In contrast to male CD-fed mice, 5-HT uptake (measured using continuous amperometry in vitro) in male HFD-fed mice was fluoxetine insensitive, yet sensitive to cocaine and the dopamine transporter (DAT) blocker GBR 12909. Immunoreactivity for DAT was present in the mucosa, and protein levels were greater in male HFD-fed compared with CD-fed mice. Extracellular 5-HT and mucosal 5-hydroxyindolacetic acid (5-HT metabolite) were similar in male HFD-fed compared with CD-fed mice. 5-Hydroxytryptamine uptake was fluoxetine sensitive in all females. Greater SERT protein, decreased extracellular 5-HT and greater mucosal 5-hydroxyindolacetic acid were observed in female HFD-fed compared with CD-fed mice. Mucosal 5-HT and EC cell numbers were similar in CD-fed and HFD-fed mice of both sexes; female 5-HT and EC cell numbers were increased compared with males. The HFD did not alter plasma sex hormone levels in any mice. Overall, obesity alters GI transit and 5-HT signalling in a sex-dependent manner.

Charge-Induced Long-Range Order in a Room-Temperature Ionic Liquid.

We report direct evidence for charge-induced long-range (ca. 100 µm) order in the room-temperature ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM(+)BF4(-)), supported on a silica surface. We have measured the rotational diffusion dynamics of anionic, cationic, and neutral chromophores as a function of distance from a silica surface. The results reflect the excess charge density gradient induced in the IL by the (negative) charge present on the silica surface. Identical measurements in ethylene glycol reveal spatially invariant reorientation dynamics for all chromophores. Capping the silica support with Me2SiCl2 results in spatially invariant reorientation dynamics in the IL. We understand these data in the context of the IL exhibiting a spatially damped piezoelectric response mediated by IL fluidity and disorder.

Evaluation of a nitrogen-incorporated tetrahedral amorphous carbon thin film for the detection of tryptophan and tyrosine using flow injection analysis with amperometric detection.

We report on the analytical performance of a tetrahedral amorphous carbon (ta-C:N) thin-film electrode in flow injection analysis with amperometric detection. Two model redox analytes were used to evaluate the electrode response because of their positive detection potentials and propensity (i.e., reaction products) to adsorb and foul sp2 carbon electrodes: tyrosine and tryptophan. ta-C:N electrodes are attractive for electroanalytical applications because they possesses many of the excellent properties of boron-doped nanocrystalline diamond (BDD) and they can be deposited at or near room temperature. The results show that the ta-C:N electrode exhibits lower background current and noise than glassy carbon (GC). The electrode was stable microstructurally at the positive potentials used for detection, ∼1.1 V, of these two amino acids and it exhibited superior analytical detection figures of merit as compared to GC and as good or superior to BDD. The linear dynamic range for both analytes at ta-C:N was from 0.1 to 100 µmol L-1, the sensitivity was 8-12 mA L mol-1, the short-term response variability was 1-2%, and the minimum detectable concentration was 89.7 ± 0.9 nM (18.3 µg L-1 or 0.46 ng) for tryptophan and 120 ± 11 nM (21.7 µg L-1 or 0.54 ng) for tyrosine. The analytical detection figures of merit for these amino acids at GC and BDD are also presented for comparison as is characterization data for the chemical composition and microstructure of the ta-C:N film.

Correction: Evaluation of a nitrogen-incorporated tetrahedral amorphous carbon thin film for the detection of tryptophan and tyrosine using flow injection analysis with amperometric detection.

Correction for 'Evaluation of a nitrogen-incorporated tetrahedral amorphous carbon thin film for the detection of tryptophan and tyrosine using flow injection analysis with amperometric detection' by Romana Jarosová, et al., Analyst, 2016, DOI: .

Electrochemical detection of peroxynitrite using hemin-PEDOT functionalized boron-doped diamond microelectrode.

Peroxynitrite is a potent nitroxidation agent and highly reactive metabolite, clinically correlated with a rich pathophysiology. Its sensitive and selective detection is challenging due to its high reactivity and short sub-second lifetime. Boron-doped diamond (BDD) microelectrodes have attracted interest because of their outstanding electroanalytical properties that include a wide working potential window and enhanced signal-to-noise ratio. Herein, we report on the modification of a BDD microelectrode with an electro-polymerized film of hemin and polyethylenedioxythiophene (PEDOT) for the purpose of selectively quantifying peroxynitrite. The nanostructured modified polymer layer was characterized by Raman spectroscopy and scanning electron microscopy (SEM). The electrochemical response to peroxynitrite was studied by voltammetry and time-based amperometry. The measured detection limit was 10 ± 0.5 nM (S/N = 3), the sensitivity was 4.5 ± 0.5 nA nM(-1) and the response time was 3.5 ± 1 s. The hemin-PEDOT BDD sensors exhibited a response variability of 5% or less (RSD). The stability of the sensors after a 20-day storage in 0.1 M PB (pH 7.4) at 4 °C was excellent as at least 93% of the initial response to 50 nM PON was maintained. The presence of PEDOT was correlated with a sensitivity increase.

Effects of Film Morphology and Surface Chemistry on the Direct Electrochemistry of Cytochrome c at Boron-Doped Diamond Electrodes.

The effects of film morphology and surface termination on the direct electron transfer of horse heart cytochrome c on boron-doped ultrananocrystalline (B-UNCD) and microcrystalline (B-MCD) diamond thin-film electrodes were investigated. Quasi-reversible, diffusion-controlled cyclic voltammetric responses were observed on oxygen-terminated (atomic O/C ~0.015), but not hydrogen-terminated (atomic O/C ~0.02) diamond thin films. The effect of the surface termination was the same for both the nanostructured B-UNCD film with sp2-bonded carbon atoms in the grain boundaries and the well faceted B-MCD film with micron-sized grains and largely devoid of sp2 carbon. Stable cyclic voltammetric i-E curves were recorded with cycling for both oxygen-terminated films indicating the absence of protein denaturation and electrode fouling. The peak currents increased linearly with the square root of the scan rate and the protein concentration; both indicative of a reaction rate limited by semi-infinite linear diffusion of the protein. Similar heterogeneous electron-transfer rate constants were observed for oxygen-terminated B-UNCD (3.48 (± 1.25) × 10-3 cm/s) and B-MCD films (2.38 (± 0.72) × 10-3 cm/s). The results clearly reveal that the oxygen-terminated surface is more active for electron-transfer with this soluble redox protein than is the hydrogen-terminated surface. The film morphology does not influence the diffusion-controlled response of the redox protein.

Macrophage depletion lowers blood pressure and restores sympathetic nerve α2-adrenergic receptor function in mesenteric arteries of DOCA-salt hypertensive rats.

We tested the hypothesis that vascular macrophage infiltration and O2 (-) release impairs sympathetic nerve α2-adrenergic autoreceptor (α2AR) function in mesenteric arteries (MAs) of DOCA-salt hypertensive rats. Male rats were uninephrectomized or sham operated (sham). DOCA pellets were implanted subcutaneously in uninephrectomized rats who were provided high-salt drinking water or high-salt water with apocynin. Sham rats received tap water. Blood pressure was measured using radiotelemetry. Treatment of sham and DOCA-salt rats with liposome-encapsulated clodronate was used to deplete macrophages. After 3-5, 10-13, and 18-21 days of DOCA-salt treatment, MAs and peritoneal fluid were harvested from euthanized rats. Norepinephrine (NE) release from periarterial sympathetic nerves was measured in vitro using amperometry with microelectrodes. Macrophage infiltration into MAs as well as TNF-α and p22(phox) were measured using immunohistochemistry. Peritoneal macrophage activation was measured by flow cytometry. O2 (-) was measured using dihydroethidium staining. Hypertension developed over 28 days, and apocynin reduced blood pressure on days 18-21. O2 (-) and macrophage infiltration were greater in DOCA-salt MAs compared with sham MAs after day 10. Peritoneal macrophage activation occurred after day 10 in DOCA-salt rats. Macrophages expressing TNF-α and p22(phox) were localized near sympathetic nerves. Impaired α2AR function and increased NE release from sympathetic nerves occurred in MAs from DOCA-salt rats after day 18. Macrophage depletion reduced blood pressure and vascular O2 (-) while restoring α2AR function in DOCA-salt rats. Macrophage infiltration into the vascular adventitia contributes to increased blood pressure in DOCA-salt rats by releasing O2 (-), which disrupts α2AR function, causing enhanced NE release from sympathetic nerves.

Electrochemical activation of diamond microelectrodes: implications for the in vitro measurement of serotonin in the bowel.

The electrochemical pretreatment of diamond microelectrodes was investigated for the purpose of learning how an anodic, cathodic or a combined anodic + cathodic polarization affects the charge-transfer kinetics for two surface-sensitive redox systems: ferri/ferrocyanide and serotonin (5-hydroxytryptamine, 5-HT). The pretreatments were performed in 0.5 mol L(-1) H2SO4. The anodic pretreatment was performed galvanically for 30 s at 250 mA cm(-2). The cathodic pretreatment was performed for 180 s at -250 mA cm(-2). The combined pretreatment involved application of the anodic step first followed by the cathodic step. The results clearly demonstrate that the best performance for both redox systems is obtained after the cathodic polarization, which presumably activates the electrode by cleaning the surface and removing site-blocking surface carbon-oxygen functionalities. The cathodic pretreatment was found to be effective at activating a fouled microelectrode in situ. This observation has important implication for the measurement of 5-HT in the bowel.

Activation of colonic mucosal 5-HT(4) receptors accelerates propulsive motility and inhibits visceral hypersensitivity.

BACKGROUND & AIMS: 5-hydroxytryptamine receptor (5-HT(4)R) agonists promote gastrointestinal motility and attenuate visceral pain, but concerns about adverse reactions have restricted their availability. We tested the hypotheses that 5-HT(4) receptors are expressed in the colonic epithelium and that 5-HT(4)R agonists can act intraluminally to increase motility and reduce visceral hypersensitivity. METHODS: Mucosal expression of the 5-HT(4)R was evaluated by reverse-transcriptase polymerase chain reaction and immunohistochemical analysis of tissues from 5-HT(4)R(BAC)-enhanced green fluorescent protein mice. Amperometry, histology, and short-circuit current measurements were used to study 5-HT, mucus, and Cl(-) secretion, respectively. Propulsive motility was measured in guinea pig distal colon, and visceromotor responses were recorded in a rat model of colonic hypersensitivity. 5-HT(4)R compounds included cisapride, tegaserod, naronapride, SB204070, and GR113808. RESULTS: Mucosal 5-HT(4) receptors were present in the small and large intestines. In the distal colon, 5-HT(4) receptors were expressed by most epithelial cells, including enterochromaffin and goblet cells. Stimulation of 5-HT(4)Rs evoked mucosal 5-HT release, goblet cell degranulation, and Cl(-) secretion. Luminal administration of 5-HT(4)R agonists accelerated propulsive motility; a 5-HT(4)R antagonist blocked this effect. Bath application of 5-HT(4)R agonists did not affect motility. Oral or intracolonic administration of 5-HT(4)R agonists attenuated visceral hypersensitivity. Intracolonic administration was more potent than oral administration, and was inhibited by a 5-HT(4)R antagonist. CONCLUSIONS: Mucosal 5-HT(4) receptor activation can mediate the prokinetic and antinociceptive actions of 5-HT(4)R agonists. Colon-targeted, intraluminal delivery of 5-HT(4)R agonists might be used to promote motility and alleviate visceral pain, while restricting systemic bioavailability and resulting adverse side effects.

In situ pH measurement during the formation of conversion coatings on an aluminum alloy (AA2024).

The measurement of interfacial pH change is important for understanding the formation mechanism of conversion coatings that are used to protect metals from corrosion. In this work, we used a tungsten microelectrode to measure the interfacial pH change near the surface of an aluminium alloy (AA2024) during the formation of two conversion coatings: (i) a trivalent chromium pretreatment (TCP) and (ii) a Ti-based, non-chromium-containing coating. The tungsten microelectrode exhibited an open circuit potential (OCP) that changed linearly as a function of the solution pH with a slope of -64 mV per pH. The microelectrode was positioned near the AA2024-T3 surface and its potential was measured as a function of time along with the OCP of the alloy sample during formation of the two coatings. The microelectrode exhibited a negative shift in potential immediately upon initiation of the coating formation, consistent with a significant increase in the interfacial pH of 2-6 units depending on the coating system. The pH increase is attributed to proton-consuming cathodic reactions that occur on the alloy surface once the passivating oxide layer is dissolved: hydrogen evolution and oxygen reduction. The increased pH causes the hydrolysis of the soluble fluorometalate precursor species in the baths, which precipitate forming a hydrated metal oxide coating (e.g., ZrO2·nH2O).

Ex Vivo Electrochemical Monitoring of Cholinergic Signaling in the Mouse Colon Using an Enzyme-Based Biosensor.

Cholinergic signaling, i.e., neurotransmission mediated by acetylcholine, is involved in a host of physiological processes, including learning and memory. Cholinergic dysfunction is commonly associated with neurodegenerative diseases, including Alzheimer's disease. In the gut, acetylcholine acts as an excitatory neuromuscular signaler to mediate smooth muscle contraction, which facilitates peristaltic propulsion. Gastrointestinal dysfunction has also been associated with Alzheimer's disease. This research focuses on the preparation of an electrochemical enzyme-based biosensor to monitor cholinergic signaling in the gut and its application for measuring electrically stimulated acetylcholine release in the mouse colon ex vivo. The biosensors were prepared by platinizing Pt microelectrodes through potential cycling in a potassium hexachloroplatinate (IV) solution to roughen the electrode surface and improve adhesion of the multienzyme film. These electrodes were then modified with a permselective poly(m-phenylenediamine) polymer film, which blocks electroactive interferents from reaching the underlying substrate while remaining permeable to small molecules like H2O2. A multienzyme film containing choline oxidase and acetylcholinesterase was then drop-cast on these modified electrodes. The sensor responds to acetylcholine and choline through the enzymatic production of H2O2, which is electrochemically oxidized to produce an increase in current with increasing acetylcholine or choline concentration. Important figures of merit include a sensitivity of 190 ± 10 mA mol-1 L cm-2, a limit of detection of 0.8 µmol L-1, and a batch reproducibility of 6.1% relative standard deviation at room temperature. These sensors were used to detect electrically stimulated acetylcholine release from mouse myenteric ganglia in the presence and absence of tetrodotoxin and neostigmine, an acetylcholinesterase inhibitor.