Functionalized carbon electrodes for pH determination

Platinum Black/Gold Nanoparticles/Polyaniline Modified Electrochemical Microneedle Sensors for Continuous In Vivo Monitoring of pH Value

Continuous in vivo monitoring (CIVM) of pH value is essential for personalized medicine, as many diseases are closely related to acid-base imbalances. However, conventional pH meters are limited in their ability to perform CIVM due to excessive blood consumption, large device volume, frequent calibration, and inadequate real-time monitoring. There is thus an urgent need for a portable method for CIVM of pH value. To address this need, we propose a minimally invasive, continuous monitoring solution in the form of an implantable pH microneedle sensor (MNS) in this study. The MNS is based on the integration of an acupuncture needle (AN) and a Ag/AgCl reference electrode. We fabricate the sensor by electrochemically depositing platinum black and gold nanoparticles onto the AN and further modifying it with polyaniline to increase its sensitivity to hydrogen ions. The pH value is obtained by calculating the open circuit voltage between the modified AN and the reference electrode. The resulting MNS demonstrates excellent selectivity and a high nernstian response to pH (-57.4 mV per pH) over a broad range (pH = 4.0 to pH = 9.0). Both in vitro and in vivo experiments have verified the performance of the sensor, showcasing its potential for biomedical research and clinical practice. The MNS provides an alternative to conventional pH meters, offering a less invasive and more convenient way to perform CIVM of pH value. Moreover, this electrochemical implantable sensor based on AN and silver wires provides a simple and sensitive method for continuous in vivo detection of other biomarkers.

A simplified methodology: pH sensing using an in situ fabricated Ir electrode under neutral conditions

Herein, a simplified fabrication method for the producing of a pH-sensitive iridium electrode is developed. The in situ electrochemical fabrication of an iridium oxide film is optimized and shown to be achievable under neutral conditions rather than the acidic conditions hitherto employed. The formation of a pH sensitive Ir(III/IV) hydrous film is confirmed via XPS. The amperometric pH-sensing properties of this electrochemically generated material were investigated using square wave voltammetry. In the pH range 2–13, the iridium oxide redox signal has a pH dependency of 86.1 ± 1.1 mV per pH unit for midpoint potentials with uncertainties being ± 0.01–0.05 pH. Finally, the newly developed pH sensor was used to measure the pH of a natural water sample with excellent results as compared to a conventional glass pH probe.

A prototype device of microliter volume voltammetric pH sensor based on carbazole-quinone redox-probe tethered MWCNT modified three-in-one screen-printed electrode

As an alternate for the conventional glass-based pH sensor which is associated with problems like fragile nature, alkaline error, and potential drift, the development of a new redox-sensitive pH probe-modified electrode that could show potential, current-drift and surface-fouling free voltammetric pH sensing is a demanding research interest, recently. Herein, we report a substituted carbazole-quinone (Car-HQ) based new redox-active pH-sensitive probe that contains benzyl and bromo-substituents, immobilized multiwalled carbon nanotube modified glassy carbon (GCE/MWCNT@Car-HQ) and screen-printed three-in-one (SPE/MWCNT@Car-HQ) electrodes for selective, surface-fouling free pH sensor application. This new system showed a well-defined surface-confined redox peak at an apparent standard electrode potential, Eo' = - 0.160 V versus Ag/AgCl with surface-excess value, Γ = 47 n mol cm-2 in pH 7 phosphate buffer solution. When tested with various electroactive chemicals and biochemicals such as cysteine, hydrazine, NADH, uric acid, and ascorbic acid, MWCNT@Car-HQ showed an unaltered redox-peak potential and current values without mediated oxidation/reduction behavior unlike the conventional hydroquinone, anthraquinone and other redox mediators based voltammetry sensors with serious electrocatalytic effects and in turn potential and current drifts. A strong π-π interaction, nitrogen-atom assisted surface orientation and C-C bond formation on the graphitic structure of MWCNT are the plausible reasons for stable and selective voltammetric pH sensing application of MWCNT@Car-HQ system. Using a programed/in-built three-in-one screen printed compatible potentiostat system, voltammetric pH sensing of 3 μL sample of urine, saliva, and orange juice samples with pH values comparable to that of milliliter volume-based pH-glass electrode measurements has been demonstrated.

Voltammetric pH sensor based on electrochemically modified pseudo-graphite

A nanocrystalline graphite-like amorphous carbon (graphite from the University of Idaho thermolyzed asphalt reaction, GUITAR) shares morphological features with classical graphites, including basal and edge planes (BP, EP). However, unlike graphites and other sp2-hybridized carbons, GUITAR has fast heterogenous electron transfer (HET) across its basal planes, and resistance to corrosion similar to sp3-C and boron-doped diamond electrodes. In this contribution, quinoid modified BP-GUITAR (q-GUITAR) is examined as a sensor for pH determination. This modification is performed by applying 2.0 V (vs. Ag/AgCl) for 150 seconds followed by 15 cyclic voltammetric scans from -0.7 to 1.0 V at 50 mV s-1 in 1.0 M H2SO4. The quinoid surface coverage of q-GUITAR is 1.35 × 10-9 mol cm-2, as measured by cyclic voltammetry. X-ray photoelectron spectroscopy analysis also confirms the high surface coverage. The quinoid surface concentration ranks highest in literature when compared with other basal plane graphitic materials. This yields a sensor that responds through a square wave voltammetric reduction peak shift of 63.3 mV per pH over a pH range from 0 to 11. The response on q-GUITAR is stable for >20 measurements and no surface re-activation is required between the measurements. The common interferents, Na+, K+ and dissolved oxygen, have no effect on the response of the q-GUITAR-based pH sensor.

Ex Vivo Electrochemical pH Mapping of the Gastrointestinal Tract in the Absence and Presence of Pharmacological Agents

Ex vivo pH profiling of the upper gastrointestinal (GI) tract (of a mouse), using an electrochemical pH probe, in both the absence and presence of pharmacological agents aimed at altering acid/bicarbonate production, is reported. Three pH electrodes were first assessed for suitability using a GI tract biological mimic buffer solution containing 0.5% mucin. These include a traditional glass pH probe, an iridium oxide (IrOx)-coated electrode (both operated potentiometrically), and a quinone (Q) surface-integrated boron-doped diamond (BDD-Q) electrode (voltammetric). In mucin, the time scale for both IrOx and glass to provide a representative pH reading was in the ∼100's of s, most likely due to mucin adsorption, in contrast to 6 s with the BDD-Q electrode. Both the glass and IrOx pH electrodes were also compromised on robustness due to fragility and delamination (IrOx) issues; contact with the GI tissue was an experimental requirement. BDD-Q was deemed the most appropriate. Ten measurements were made along the GI tract, esophagus (1), stomach (5), and duodenum (4). Under buffer only conditions, the BDD-Q probe tracked the pH from neutral in the esophagus to acidic in the stomach and rising to more alkaline in the duodenum. In the presence of omeprazole, a proton pump inhibitor, the body regions of the stomach exhibited elevated pH levels. Under melatonin treatment (a bicarbonate agonist and acid inhibitor), both the body of the stomach and the duodenum showed elevated pH levels. This study demonstrates the versatility of the BDD-Q pH electrode for real-time ex vivo biological tissue measurements.

An sp2 Patterned Boron Doped Diamond Electrode for the Simultaneous Detection of Dissolved Oxygen and pH

A hybrid sp²-sp³ electrochemical sensor comprising patterned regions of nondiamond-carbon (sp²) in a boron doped diamond (sp³) matrix is described for the simultaneous voltammetric detection of dissolved oxygen (DO) and pH in buffered aqueous solutions. Using a laser micropatterning process it is possible to write mechanically robust regions of sp² carbon into a BDD electrode. These regions both promote the electrocatalytic reduction of oxygen and facilitate the proton coupled electron transfer of quinone groups, integrated into the surface of the sp² carbon. In this way, in one voltammetric sweep (time of measurement ∼4 s) it is possible to determine both the DO concentration and solution pH. By varying the sp² pattern the response can be optimized toward both analytes. Using a closely spaced sp² microspot array, a linear response toward DO, across the range 0.0 to 8.0 mg L^-1 (0.0 to 0.25 mM; sensitivity = -8.77 × 10^-8 A L mg^-1, R² = 0.9991) and pH range 4-10 (sensitivity = 59.7 mV pH^-1, R² = 0.9983) is demonstrated. The electrode is also capable of measuring both DO concentration and pH in the more complex buffered environment of blood. Finally, we show how the peak position for ORR is independent of pH, and thus via measurement of the difference in ORR and pH peak position, internal referencing is possible. Such electrodes show great promise for use in applications ranging from biomedical sensing to water analysis.

Amperometric micro pH measurements in oxygenated saliva

An amperometric micro pH sensor has been developed based on the chemical oxidation of carbon fibre surfaces (diameter of 9 μm and length of ca. 1 mm) to enhance the population of surface quinone groups for the measurement of salivary pH. The pH analysis utilises the electrochemically reversible two-electron, two-proton behaviour of surface quinone groups on the micro-wire electrodes. A Nernstian response is observed across the pH range 2-8 which is the pH range of many biological fluids. We highlight the measurement of pH in small volumes of biological fluids without the need for oxygen removal and specifically the micro pH electrode is examined by measuring the pH of commercial synthetic saliva and authentic human saliva samples. The results correspond well with those obtained by using commercial glass pH electrodes on large volume samples.

Voltammetric pH sensing using carbon electrodes: glassy carbon behaves similarly to EPPG

Developing and building on recent work based on a simple sensor for pH determination using unmodified edge plane pyrolytic graphite (EPPG) electrodes, we present a voltammetric method for pH determination using a bare unmodified glassy carbon (GC) electrode. By exploiting the pH sensitive nature of quinones present on carbon edge-plane like sites within the GC, we show how GC electrodes can be used to measure pH. The electro-reduction of surface quinone groups on the glassy carbon electrode was characterised using cyclic voltammetry (CV) and optimised with square-wave voltammetry (SWV) at 298 K and 310 K. At both temperatures, a linear correlation was observed, corresponding to a 2 electron, 2 proton Nernstian response over the aqueous pH range 1.0 to 13.1. As such, unmodified glassy carbon electrodes are seen to be pH dependent, and the Nernstian response suggests its facile use for pH sensing. Given the widespread use of glassy carbon electrodes in electroanalysis, the approach offers a method for the near-simultaneous measurement and monitoring of pH during such analyses.

Voltammetric pH sensor based on an edge plane pyrolytic graphite electrode

A simple sensor for pH determination is reported using unmodified edge plane pyrolytic graphite (EPPG) electrodes. The analysis is based on the electro-reduction of surface quinone groups on the EPPG which was characterised using cyclic voltammetry (CV) and optimised with square-wave voltammetry (SWV). Under optimised conditions, a linear response is observed between the peak potential and pH with a gradient of ∼59 mV per pH (at 25 °C), which corresponds well with Nernstian behaviour based on a 2 proton, 2 electron system over the aqueous pH range 1.0 to 13.0. As such, an EPPG is suggested as a reagent free and robust pH sensing material.

A novel sensor based on electropolymerized substituted-phenols for pH detection in unbuffered systems

Polymeric waves associated with the electrochemical oxidation of salicylaldehyde and its derivatives exhibit Nernstian responses to pH variations in both buffered and unbuffered media.

Electrochemical Reduction of Quinones in Different Media: A Review

The electron transfer reactions involving quinones, hydroquinones, and catechols are very important in many areas of chemistry, especially in biological systems. The therapeutic efficiency as well as toxicity of anthracycline anticancer drugs, a class of anthraquinones, is governed by their electrochemical properties. Other quinones serve as important functional moiety in various biological systems like electron-proton carriers in the respiratory chain and their involvement in photosynthetic electron flow systems. The present paper summarizes literatures on the reduction of quinones in different solvents under various conditions using different electrochemical methods. The influence of different reaction conditions including pH of the media, nature of supporting electrolytes, nature of other additives, intramolecular or intermolecular hydrogen bonding, ion pair formation, polarity of the solvents, stabilization of the semiquinone and quinone dianion, catalytic property, and adsorption at the electrode surface, are discussed and relationships between reaction conditions and products formed have been presented.

Development and characterization of a voltammetric carbon-fiber microelectrode pH sensor

This work describes the development and characterization of a modified carbon-fiber microelectrode sensor capable of measuring real-time physiological pH changes in biological microenvironments. The reagentless sensor was fabricated under ambient conditions from voltammetric reduction of the diazonium salt Fast Blue RR onto a carbon-fiber surface in aprotic media. Fast-scan cyclic voltammetry was used to probe redox activity of the p-quinone moiety of the surface-bound molecule as a function of pH. In vitro calibration of the sensor in solutions ranging from pH 6.5 to 8.0 resulted in a pH-dependent anodic peak potential response. Flow-injection analysis was used to characterize the modified microelectrode, revealing sensitivity to acidic and basic changes discernible to 0.005 pH units. Furthermore, the modified electrode was used to measure dynamic in vivo pH changes evoked during neurotransmitter release in the central nervous system of the microanalytical model organism Drosophila melanogaster.

Screen printed electrochemical platforms for pH sensing

We explore the possible use of screen printing technology for fabricating disposable electrochemical platforms for the sensing of pH. These screen printed pH sensors incorporate the pH sensitive phenanthraquinone moiety which undergoes a Nernstian potential shift with pH, and the pH insensitive dimethylferrocene which acts as an internal reference. This generic approach offers a calibration-less and reproducible approach for portable pH measurements with the possibility of miniaturisation allowing incorporation into existing sensing devices. The advantages, limitations and future prospects of this fabrication approach for producing electrochemical platforms for pH sensing are also discussed.