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

A Free-Standing Boron-Doped Diamond Grid Electrode for Fundamental Spectroelectrochemistry

Spectroelectrochemistry (SEC) is a powerful technique that enables a variety of redox properties to be studied, including formal potential (Eo), thermodynamic values (ΔG, ΔH, ΔS), diffusion coefficient (D), electron transfer stoichiometry (n), and others. SEC requires an electrode which light can pass through while maintaining sufficient electrical conductivity. This has been traditionally composed of metal or metal oxide films atop transparent substrates like glass, quartz, or metallic mesh. Robust electrode materials like boron-doped diamond (BDD) could help expand the environments in which SEC can be performed, but most designs are limited to thin films (∼100-200 nm) on transparent substrates less resilient than free-standing BDD. This work presents a free-standing BDD grid electrode (G-BDD) for fundamental SEC measurements, using the well-characterized Fe(CN)63-/4- redox couple as proof-of-concept. With a combination of cyclic voltammetry (CV), thin-layer SEC, and chronoabsorptometry, several of the redox properties mentioned above were calculated and compared. For Eo', n, and D, similar results were obtained when comparing the CV [Eo' = +0.279 (±0.002) V vs Ag/AgCl; n = 0.97; D = 4.1 × 10-6 cm2·s-1] and SEC [Eo' = +0.278 (±0.001) V vs Ag/AgCl; n = 0.91; D = 5.2 × 10-6 cm2·s-1] techniques. Both values align with what has been previously reported. To calculate D from the SEC data, modification of the classical equation used in chronoabsorptometry was required to accommodate the G-BDD electrode geometry. Overall, this work expands on the applicability of SEC techniques and BDD as a versatile electrode material.

Enzyme-Modified Microelectrode for Simultaneous Local Measurements of O2 and pH

The use of miniaturized probes opens a new dimension in the analysis of (bio)chemical processes, enabling the possibility to perform measurements with local resolution. In addition, multiparametric measurements are highly valuable for a holistic understanding of the investigated process. Therefore, different strategies have been suggested for simultaneous local measurements of various parameters. Electroanalytical methods are a powerful strategy in this direction. However, they have been mainly restricted to coupling concurrent independent measurements with different miniaturized probes. Here, we present an enzymatic microbiosensor for the simultaneous detection of O2 and pH. The sensing strategy is based on the pH-dependent bioelectrocatalytic process associated with O2 reduction at a gold microelectrode modified with a multicopper oxidase. After initial investigations of the bioelectrocatalytic reaction over gold macroelectrodes, the fabrication and characterization of micrometer-sized probes are presented. The microbioelectrode exhibits a linear current increase with O2 concentration extending to 17.2 mg L-1, with a sensitivity of (5.56 ± 0.13) nA L mg-1 and a limit of detection of (0.5 ± 0.3) mg L-1. Moreover, a linear response allowing pH detection is obtained between pH 5.2 and 7.5 with a slope of -(47 ± 8) mV per pH unit. In addition, two proof-of-concept analytical examples are shown, demonstrating the capability of the developed sensing system for simultaneous local measurements of O2 and pH. Compared with other miniaturized probes reported before for simultaneous detection, our strategy stands out as the two investigated parameters are acquired from the very same measurement. This strategy greatly simplifies the analytical setup and for the first time provides truly simultaneous local detection in the micrometer scale.

Electrochemical Diagnosis of Urinary Tract Infection Using Boron-Doped Diamond Electrodes

Efficient detection of sodium nitrite in human urine could be used to diagnose urinary tract infections rapidly. Here, we demonstrate a fast and novel method for the selective detection of sodium nitrite in different human urine samples using electrolysis with a bare boron-doped diamond electrode. The measurement is performed without adding any other species, such as enzymes, and uses a simple electrochemical approach with an oxidation step followed by reduction. In the present study, we pay attention to the reduction potential range for the measurement, which is substantially different from many previous literature reports that focus on the oxidation reaction. The determination of added sodium nitrite based on cyclic voltammetry or differential pulse voltammetry is employed for two pooled urine samples and three individual urine matrices. From this, the linear response ranges for sodium nitrite detection are 0.5-10 mg/L (7.2-140 μmol/L) and 10-400 mg/L (140-5800 μmol/L). The results from these urine samples convert well to the calibration curve, with a limit of detection established as 0.82 mg/L (R2 = 0.9914), which is clinically relevant.

Versatile DIY Route for Incorporation of a Wide Range of Electrode Materials into Rotating Ring Disk Electrodes

Rotating ring disk electrodes (RRDEs) are a powerful and versatile tool for mechanistically investigating electrochemical reactions at electrode surfaces, particularly in the area of electroanalysis and catalysis. Despite their importance, only limited electrode materials (typically glassy carbon, platinum, and gold) and combinations thereof are available commercially. In this work, we present a method employing three-dimensional (3D) printing in conjunction with machined brass components to produce housing, which can accommodate any electrode material in, e.g., pressed powdered pellet, wafer, rod, foil, or vapor deposited onto a conductive substrate form. In this way, the range and usability of RRDEs is extended. This custom do-it-yourself (DIY) approach to fabricating RRDEs also enables RRDEs to be produced at a significant fraction of the cost of commercial RRDEs. To illustrate the versatility of our approach, coplanar boron-doped diamond (BDD) RRDEs are fabricated for the first time using the approach described. Experimental collection efficiencies for the redox couple FcTMA⁺/FcTMA²⁺ are found to be very close to those predicted theoretically. BDD electrodes serve as an ideal electrocatalyst support due to their low background currents, wide solvent potential window in aqueous solution, and chemical and electrochemical stability in acid and alkali solutions. The BDD RRDE configuration is employed to investigate the importance of surface-incorporated nondiamond carbon in BDD on hydrogen peroxide generation via the oxygen reduction reaction in acid solutions.

Robust Dual Optical Sensor for pH and Dissolved Oxygen

As part of moving our optical pH and dissolved oxygen (DO) optical chemosensors toward industrial applications, we decided to explore a many-sensors-in-one principle. It was tested if physical segregation of the optical sensor components in a single sensor polymer could remove cross-talk and quenching. It was found that a design concept with an oxygen-responsive dye in polymer nanoparticles and a pH-responsive dye in an organically modified siloxane polymer resulted in a robust pH/O₂ dual optical sensor. Individually, the O₂-sensitive nanoparticles, a known component for optical DO sensing, and the pH sensor are operational. Thus, it was decided to test if nanoparticles enclosed within the pH-sensitive responsive sol-gel (i) could work together if segregated and (ii) could operate with a single intensity signal that is without a reference signal; developments within industrial optical sensor technology indicate that this should be feasible. The prototype optode produced in this work was shown to have a negligible drift over 60 h, bulk diffusion-limited DO response, and independent response to pH and O₂. On the individual optode, pH calibration was found to show the expected sigmoidal shape and pK_a, while the complexity of the calibration function for the DO signal was significant. While the engineering of the sensor device, optics, and hardware are not robust enough to attempt generic sensor calibration, it was decided to demonstrate the design concept in simple fermentation experiments. We conclude that the dual sensor design with the physical segregation of components is viable.

Blood Oxygen Sensor Using a Boron-Doped Diamond Electrode

The electrochemical behavior of oxygen (O₂) in blood was studied using boron-doped diamond (BDD) electrodes. Cyclic voltammogram of O₂ in a 0.1 M phosphate buffer solution solution containing 1 × 10^-6 M of bovine hemoglobin exhibits a reduction peak at -1.4 V (vs Ag/AgCl). Moreover, the scan rate dependence was investigated to study the reduction reaction mechanism, which was attributable to the reduction of O₂ to H₂O₂ via two electrons. A linear calibration curve was observed in the concentration range of 86.88-314.63 mg L^-1 (R² = 0.99) with a detection limit of 1.0 mg L^-1 (S/B = 3). The analytical performance was better than those with glassy carbon or platinum electrodes as the working electrode. In addition, an application to bovine blood was performed. The O₂ concentration in the blood measured on the BDD electrodes was compared to that measured using an OxyLite Pro fiber-optic oxygen sensor device. Both methods gave similar values of the O₂ concentration in the range of ∼40 to 150 mmHg. This result confirms that BDD electrodes could potentially be used to detect the O₂ concentration in blood.

Alizarin-graphene nanocomposite for calibration-free and online pH monitoring of microbial fuel cell

Microbial fuel cells (MFCs) are sensitive to acidity variations in both bioelectricity generation and biochemical digestion aspects, therefore online pH monitoring is of necessity to guarantee optimal function of MFCs. Present pH meters hardly fulfill this special need. In this work, we designed a novel voltammetric pH sensor based on electrochemically reduced graphene oxide (rGO) modified screen printed electrode. By surface doping of alizarin, good linearity of pH sensing over the range of 4.0-9.0 can be realized. Fast readout can be acquired within 15 s for each test. pH monitoring for artificial wastewater with inoculum of granular activated sludge in a MFC was successfully illustrated. Specially, it was verified that the performance was improved with alizarin doping due to the enhanced rGO surface proton diffusion. This approach provides an online, calibration-free and long stable pH monitoring method for the future MFC development.

A Dual Approach of an Oil-Membrane Composite and Boron-Doped Diamond Electrode to Mitigate Biofluid Interferences

Electrochemical biosensors promise a simple method to measure analytes for both point-of-care diagnostics and continuous, wearable biomarker monitors. In a liquid environment, detecting the analyte of interest must compete with other solutes that impact the background current, such as redox-active molecules, conductivity changes in the biofluid, water electrolysis, and electrode fouling. Multiple methods exist to overcome a few of these challenges, but not a comprehensive solution. Presented here is a combined boron-doped diamond electrode and oil–membrane protection approach that broadly mitigates the impact of biofluid interferents without a biorecognition element. The oil–membrane blocks the majority of interferents in biofluids that are hydrophilic while permitting passage of important hydrophobic analytes such as hormones and drugs. The boron-doped diamond then suppresses water electrolysis current and maintains peak electrochemical performance due to the foulant-mitigation benefits of the oil–membrane protection. Results show up to a 365-fold reduction in detection limits using the boron-doped diamond electrode material alone compared with traditional gold in the buffer. Combining the boron-doped diamond material with the oil–membrane protection scheme maintained these detection limits while exposed to human serum for 18 h.

Miniaturized probe on polymer SU-8 with array of individually addressable microelectrodes for electrochemical analysis in neural and other biological tissues

An SU-8 probe with an array of nine, individually addressable gold microband electrodes (100 μm long, 4 μm wide, separated by 4-μm gaps) was photolithographically fabricated and characterized for detection of low concentrations of chemicals in confined spaces and in vivo studies of biological tissues. The probe's shank (6 mm long, 100 μm wide, 100 μm thick) is flexible, but exhibits sufficient sharpness and rigidity to be inserted into soft tissue. Laser micromachining was used to define probe geometry by spatially revealing the underlying sacrificial aluminum layer, which was then etched to free the probes from a silicon wafer. Perfusion with fluorescent nanobeads showed that, like a carbon fiber electrode, the probe produced no noticeable damage when inserted into rat brain, in contrast to damage from an inserted microdialysis probe. The individual addressability of the electrodes allows single and multiple electrode activation. Redox cycling is possible, where adjacent electrodes serve as generators (that oxidize or reduce molecules) and collectors (that do the opposite) to amplify signals of small concentrations without background subtraction. Information about electrochemical mechanisms and kinetics may also be obtained. Detection limits for potassium ferricyanide in potassium chloride electrolyte of 2.19, 1.25, and 2.08 μM and for dopamine in artificial cerebral spinal fluid of 1.94, 1.08, and 5.66 μM for generators alone and for generators and collectors during redox cycling, respectively, were obtained.

Integration of Multiple Redox Centers into Porous Coordination Networks for Ratiometric Sensing of Dissolved Oxygen

The application of porphyrin metal-organic frameworks (MOFs) as a ratiometric electrochemical sensing platform is still unexplored. In this paper, we report a ratiometric electrochemical sensor by the integration of multiple redox centers into porphyrin MOFs for the detection of dissolved oxygen (DO). Specifically, the ferrocene (Fc) group was integrated into the nanosized PCN-222(Fe) (PCN = porous coordination networks) via acid-base reaction to synthesize the Fc@PCN-222(Fe) composite with two redox centers of the Fc group and Fe-porphyrin. The Fc group that is insensitive to DO serves as an internal reference, and the Fe-porphyrin in PCN-222(Fe) is a DO indicator. The ratios of the cathodic currents for the two redox centers exhibit a linear relationship with DO concentrations from 2.8 to 28.9 mg mL^-1 and a limit of detection of 0.3 mg mL^-1. In addition, the ratiometric electrochemical sensor has high selectivity and stability for DO sensing results from the Fc@PCN-222(Fe) composite. Because there are numerous redox centers, such as methylene blue and thionine, which can be integrated into MOFs, many MOF-based ratiometric electrochemical sensors can be simply developed for high-performance biosensing.

Modification of boron-doped diamond electrodes with gold-palladium nanoparticles for an oxygen sensor

Modification of boron-doped diamond (BDD) with gold-palladium nanoparticles (Au@PdNPs) was successfully performed. Prior to the modification, BDD was modified with allylamine to provide active sites for the attachment of nanoparticles, while the synthesis of Au@PdNPs was performed by chemical reduction of a palladium salt solution in a colloidal solution of gold nanoparticles. Characterization using TEM images showed that by controlling the palladium concentration, flower and core-shell shaped Au@PdNPs can be prepared. XPS studies confirmed that the nanoparticles with a flower shape could be attached better on the BDD surface. The Au@PdNPs-modified BDD (Au@PdNPs-BDD) electrodes were then examined for the oxygen reduction reaction in comparison with gold and palladium-based electrodes. One order higher current response was observed at Au@PdNPs-BDD compared to AuNPs-BDD, indicating the contribution of palladium in the oxygen reduction reaction. Good linearity with comparable limits of detection suggested that Au@PdNPs-BDD electrodes are promising for use as oxygen sensors. Furthermore, their application as BOD sensors was demonstrated.

Voltammetric pH Measurements in Unadulterated Foodstuffs, Urine, and Serum with 3D-Printed Graphene/Poly(Lactic Acid) Electrodes

The pH of a system is a critical descriptor of its chemistry-impacting reaction rates, solubility, chemical speciation, and homeostasis. As a result, pH is one of the most commonly measured parameters in food safety, clinical, and environmental laboratories. Glass pH probes are the gold standard for pH measurements but suffer drawbacks including frequent recalibration, wet storage of the glass membrane, difficulty in miniaturization, and interferences from alkali metals. In this work, we describe a voltammetric pH sensor that uses a three-dimensional (3D)-printed graphene/poly(lactic acid) filament electrode that is pretreated to introduce quinone functional groups to the graphene surface. After thoroughly characterizing the pretreatment parameters using outer-sphere and inner-sphere redox couples, we measured pH by reducing the surface-bound quinones, which undergo a pH-dependent 2e^-/2H⁺ reduction. The position of the redox peak was found to shift -60 ± 2 mV pH^-1 at 25 °C, which is in excellent agreement with the theoretical value predicted by the Nernst Equation (-59.2 mV pH^-1). Importantly, the sensors did not require the removal of dissolved oxygen prior to successful pH measurements. We investigated the impact of common interfering species (Pb²⁺ and Cu²⁺) and found that there was no impact on the measured pH. We subsequently challenged the sensors to measure the pH of unadulterated complex samples, including cola, vinegar, an antacid tablet slurry, serum, and urine, and obtained excellent agreement compared to a glass pH electrode. In addition to the positive analytical characteristics, the sensors are extremely cheap and easy to fabricate, making them highly accessible to a wide range of researchers. These results pave the way for customizable pH sensors that can be fabricated in (nearly) any geometry for targeted applications using 3D printing.

Polychlorinated Biphenyl Electrochemical Aptasensor Based on a Diamond-Gold Nanocomposite to Realize a Sub-Femtomolar Detection Limit

Polychlorinated biphenyls (PCBs) with high toxicity, low lethal dose, and bioaccumulation have been inhibited for application in wide fields, and a highly efficient trace detection is thus greatly desirable. In this study, we produce dense Au-nanoparticles by twice sputtering and twice annealing (T-Au-NPs) on boron-doped diamond (BDD). The successful formation of T-Au-NPs/BDD nanocomposites was confirmed by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy analysis. Based on T-Au-NPs/BDD, an electronic biosensor with aptamers is fabricated to detect trace polychlorinated biphenyl-77 (PCB-77) by electrochemical impedance. A good linear relationship in the range of femtomolar to micromolar and significantly low detection limit of sub-femtomolar level (0.32 fM) are realized based on the biosensor. The emphasis of this research lies in the key role of the diamond substrate in the biosensor. It is demonstrated that the biosensor has excellent sensitivity, specificity, stability, and recyclability, which are favorable for detecting the trace PCB-77 molecule. It is attributed to the important effect presented by the BDD substrate and the synergistic influence of T-Au-NPs combined with aptamers.

Investigation of sp2-Carbon Pattern Geometry in Boron-Doped Diamond Electrodes for the Electrochemical Quantification of Hypochlorite at High Concentrations

An electrochemical sensor that contains patterned regions of sp²-carbon in a boron-doped diamond (BDD) matrix is presented for the quantitative detection of hypochlorite (OCl^-) at high concentrations in the alkaline, chemically oxidizing environment associated with bleach. As BDD itself is unresponsive to OCl^- reduction within the solvent window, by using a laser micromachining process, it is possible to write robust electrochemically active regions of sp²-carbon into the electrochemically inert sp³-BDD electrode. In this work, four different laser patterned BDD electrodes are examined, and their response compared across a range of OCl^- concentrations (0.02-1.50 M). A single macrospot (0.37 mm diameter disk) electrode and a closely spaced microspot (46 μm diameter disk) hexagonal array electrode, containing the same surface area of sp²-carbon, are shown to provide the most linear response toward OCl^- reduction. Finite element modeling (FEM) is employed to better understand the electrochemical system, due to the complexity of the electrode geometry, as well as the need to include contributions from migration and Ohmic drop at these high concentrations. FEM data suggest that only a small percentage (∼1 × 10^-3%) of the total laser-machined sp² area is active toward the OCl^- reduction process and that this process is kinetically very sluggish (∼k_eff = 1 × 10^-12 cm s^-1). The sensitivity at the array electrode (-0.127 ± 0.004 mA M^-1; R² = 0.992) is higher than that at the single-spot electrode (-0.075 ± 0.002 mA M^-1; R² = 0.996) due to the enhanced effect of transport to the edges of the microspots, shown via simulation. The electrodes returned a relatively stable response over a greater than 3 month period of use in the OCl^- solutions, demonstrating these hybrid sp²-BDD electrodes show promise for long-term monitoring applications in the harsh environments associated with bleaching applications.

Unusual Electrochemical Properties of Low-Doped Boron-Doped Diamond Electrodes Containing sp2 Carbon

Unexpected phenomena displayed by low-boron-doped diamond (BDD) electrodes are disclosed in the present work. Generally, the presence of sp² nondiamond carbon impurities in BDD electrodes causes undesirable electrochemical properties, such as a reduced potential window and increased background current, etc. However, we found that the potential window and redox reaction in normally doped (1%) BDD and low-doped (0.1%) BDD exhibited opposite tendencies depending on the extent of sp² carbon. Moreover, we found that contrary to the usual expectations, low-doped BDD containing sp² carbon hinders electron transfer, whereas in line with expectations, normally doped BDD containing sp² exhibits enhanced electron transfer. Surface analyses by X-ray/ultraviolet photoelectron spectroscopy (XPS/UPS) and electrochemical methods are utilized to explain these unusual phenomena. This work indicates that the electrochemical properties of low-doped BDD containing sp² might be due partially to the high level of surface oxygen, the large work function, the low carrier density, and the existence of different types of sp² carbon.