Platinum and palladium nanoparticles on boron-doped diamond for the electrochemical detection of hydrogen peroxide: a comparison study.

Hydrogen peroxide (H2O2) plays a role in many facets - a household item, an important industrial chemical, a biomarker in vivo, and several others. For this reason, its measurement and quantification in a variety of media are important. While spectroscopic detection is primarily used for H2O2, electrochemical methods offer advantages in versatility, cost, and sensitivity. In this work, we investigate a 2-step surface metal nanoparticle (NP) modification for platinum (Pt) and palladium (Pd) on boron-doped diamond (BDD) electrodes for the detection of H2O2. Several parameters such as the metal salt concentration and electrodeposition charge in the 2-step modification were varied to find an optimum. Using cyclic voltammetry (CV), the BDD-PdNP electrode types were found to yield a sharper, more well-resolved H2O2 oxidation peak compared to the BDD-PtNP electrodes. Both metal NP electrode types significantly improved the response compared to the bare BDD electrode; a 150-200× improvement in sensitivity was observed across all modified electrode types. Calibration experiments were completed at both low and high concentration ranges in stagnant and flow-based solutions. The lowest limit of detection (LOD) obtained was 50 nM (5E-08 M) on a BDD-PdNP electrode modified with 1.0 mM PdCl2 to 5.0 mC in the wet chemical seeding and electrodeposition steps. 0.25 mM PdCl2 to 3.23 mC and 0.25 mM HPtCl6- to 3.23 mC also yielded a sufficient response for low-level H2O2, with LODs around 100 nM (1E-07 M). Overall, this work exemplifies the wide applicability of BDD and achieves sub-µM H2O2 LODs with a non-enzymatic electrode material.

Next-Generation Diamond Electrodes for Neurochemical Sensing: Challenges and Opportunities.

Carbon-based electrodes combined with fast-scan cyclic voltammetry (FSCV) enable neurochemical sensing with high spatiotemporal resolution and sensitivity. While their attractive electrochemical and conductive properties have established a long history of use in the detection of neurotransmitters both in vitro and in vivo, carbon fiber microelectrodes (CFMEs) also have limitations in their fabrication, flexibility, and chronic stability. Diamond is a form of carbon with a more rigid bonding structure (sp3-hybridized) which can become conductive when boron-doped. Boron-doped diamond (BDD) is characterized by an extremely wide potential window, low background current, and good biocompatibility. Additionally, methods for processing and patterning diamond allow for high-throughput batch fabrication and customization of electrode arrays with unique architectures. While tradeoffs in sensitivity can undermine the advantages of BDD as a neurochemical sensor, there are numerous untapped opportunities to further improve performance, including anodic pretreatment, or optimization of the FSCV waveform, instrumentation, sp2/sp3 character, doping, surface characteristics, and signal processing. Here, we review the state-of-the-art in diamond electrodes for neurochemical sensing and discuss potential opportunities for future advancements of the technology. We highlight our team's progress with the development of an all-diamond fiber ultramicroelectrode as a novel approach to advance the performance and applications of diamond-based neurochemical sensors.

Flexible, diamond-based microelectrodes fabricated using the diamond growth side for neural sensing.

Diamond possesses many favorable properties for biochemical sensors, including biocompatibility, chemical inertness, resistance to biofouling, an extremely wide potential window, and low double-layer capacitance. The hardness of diamond, however, has hindered its applications in neural implants due to the mechanical property mismatch between diamond and soft nervous tissues. Here, we present a flexible, diamond-based microelectrode probe consisting of multichannel boron-doped polycrystalline diamond (BDD) microelectrodes on a soft Parylene C substrate. We developed and optimized a wafer-scale fabrication approach that allows the use of the growth side of the BDD thin film as the sensing surface. Compared to the nucleation surface, the BDD growth side exhibited a rougher morphology, a higher sp 3 content, a wider water potential window, and a lower background current. The dopamine (DA) sensing capability of the BDD growth surface electrodes was validated in a 1.0 mM DA solution, which shows better sensitivity and stability than the BDD nucleation surface electrodes. The results of these comparative studies suggest that using the BDD growth surface for making implantable microelectrodes has significant advantages in terms of the sensitivity, selectivity, and stability of a neural implant. Furthermore, we validated the functionality of the BDD growth side electrodes for neural recordings both in vitro and in vivo. The biocompatibility of the microcrystalline diamond film was also assessed in vitro using rat cortical neuron cultures.

Indium Tin Oxide Film Characteristics for Cathodic Stripping Voltammetry.

The combination of conductivity, optical transparency, and wide anodic potential window has driven significant interest in indium tin oxide (ITO) as an electrode material for electrochemical measurements. More recently, ITO has been applied to the detection of trace metals using cathodic stripping voltammetry (CSV), specifically manganese (Mn). However, the optimization of ITO fabrication for a voltammetric method such as CSV is yet to be reported, nor have the microstructural properties of ITO been investigated for CSV. Furthermore, CSV does not require optical transparency, thereby allowing nontransparent substrates to be used for deposition. This enables microfabrication procedures to be expanded and simplified compared to glass or quartz. Combining this with the profound importance of sensitive, selective detection of toxic metal ions in environmentally and biologically relevant samples makes ITO especially attractive. In this work, we report a thorough investigation of ITO deposition and processing on silicon (Si) substrates for CSV analysis using Mn as the model analyte. Several ITO process parameters were examined such as heated deposition and post-process annealing. Each ITO film was characterized using a variety of surface, bulk (X-ray diffraction), and electrochemical measurements. Although each ITO film type showed electrochemical activity, the heated and annealed (HA) ITO fabrication process yielded superior results for Mn CSV; a limit of detection (LOD) of 0.1 ppb (1.8 nM) was obtained. This work exemplifies new applications of ITO as an electrode material while providing a baseline for trace detection of toxic metals and other contaminants amenable to detection by CSV.

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.

Large-scale, all polycrystalline diamond structures transferred onto flexible Parylene-C films for neurotransmitter sensing.

Boron-doped diamond (BDD) has superior electrochemical properties for bioelectronic systems. However, due to its high synthesis temperature, traditional microfabrication methods have limits to integrating BDD with emerging classes of flexible, polymer-based bioelectronic systems. This paper introduces a novel fabrication solution to this challenge, which features (i) a wafer-scale substrate transfer process with all diamond structures transferred onto a flexible Parylene-C substrate and (ii) Parylene anchors introduced to strengthen the bonding between BDD and Parylene substrates, as demonstrated by a peeling test. The electrochemical properties of the transferred BDD-polymer electrodes are evaluated using (i) an outer sphere redox couple Ru(NH3)62+/3+ to study the electron transfer process and (ii) quantitative and qualitative studies of the neurotransmitter redox couple dopamine/dopamine-o-quinone. A linear response of the BDD sensor to dopamine concentrations of 0.5 µM to 100 µM is observed (R2 = 0.999) with a sensitivity of 0.21 µA cm-2 µM-1. These examples of fabricated diamond-polymer devices suggest a broad application in advanced bioelectronics and optoelectronics.

Determination of Lead with a Copper-Based Electrochemical Sensor.

This work demonstrates determination of lead (Pb) in surface water samples using a low-cost copper (Cu)-based electrochemical sensor. Heavy metals require careful monitoring due to their toxicity, yet current methods are too complex or bulky for point-of-care (POC) use. Electrochemistry offers a convenient alternative for metal determination, but the traditional electrodes, such as carbon or gold/platinum, are costly and difficult to microfabricate. Our copper-based sensor features a low-cost electrode material-copper-that offers simple fabrication and competitive performance in electrochemical detection. For anodic stripping voltammetry (ASV) of Pb, our sensor shows 21 nM (4.4 ppb) limit of detection, resistance to interfering metals such as cadmium (Cd) and zinc (Zn), and stable response in natural water samples with minimum sample pretreatment. These results suggest this electrochemical sensor is suitable for environmental and potentially biological applications, where accurate and rapid, yet inexpensive, on-site monitoring is necessary.

Bare and Polymer-Coated Indium Tin Oxide as Working Electrodes for Manganese Cathodic Stripping Voltammetry.

Though an essential metal in the body, manganese (Mn) has a number of health implications when found in excess that are magnified by chronic exposure. These health complications include neurotoxicity, memory loss, infertility in males, and development of a neurologic psychiatric disorder, manganism. Thus, trace detection in environmental samples is increasingly important. Few electrode materials are able to reach the negative reductive potential of Mn required for anodic stripping voltammetry (ASV), so cathodic stripping voltammetry (CSV) has been shown to be a viable alternative. We demonstrate Mn CSV using an indium tin oxide (ITO) working electrode both bare and coated with a sulfonated charge selective polymer film, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene-sulfonate (SSEBS). ITO itself proved to be an excellent electrode material for Mn CSV, achieving a calculated detection limit of 5 nM (0.3 ppb) with a deposition time of 3 min. Coating the ITO with the SSEBS polymer was found to increase the sensitivity and lower the detection limit to 1 nM (0.06 ppb). This polymer modified electrode offers excellent selectivity for Mn as no interferences were observed from other metal ions tested (Zn(2+), Cd(2+), Pb(2+), In(3+), Sb(3+), Al(3+), Ba(2+), Co(2+), Cu(2+), Ni(3+), Bi(3+), and Sn(2+)) except Fe(2+), which was found to interfere with the analytical signal for Mn(2+) at a ratio 20:1 (Fe(2+)/Mn(2+)). The applicability of this procedure to the analysis of tap, river, and pond water samples was demonstrated. This simple, sensitive analytical method using ITO and SSEBS-ITO could be applied to a number of electroactive transition metals detectable by CSV.

Cloud Point Extraction for Electroanalysis: Anodic Stripping Voltammetry of Cadmium.

Cloud point extraction (CPE) is a well-established technique for the preconcentration of hydrophobic species from water without the use of organic solvents. Subsequent analysis is then typically performed via atomic absorption spectroscopy (AAS), UV-vis spectroscopy, or high performance liquid chromatography (HPLC). However, the suitability of CPE for electroanalytical methods such as stripping voltammetry has not been reported. We demonstrate the use of CPE for electroanalysis using the determination of cadmium (Cd(2+)) by anodic stripping voltammetry (ASV). Rather than using the chelating agents which are commonly used in CPE to form a hydrophobic, extractable metal complex, we used iodide and sulfuric acid to neutralize the charge on Cd(2+) to form an extractable ion pair. This offers good selectivity for Cd(2+) as no interferences were observed from other heavy metal ions. Triton X-114 was chosen as the surfactant for the extraction because its cloud point temperature is near room temperature (22-25 °C). Bare glassy carbon (GC), bismuth-coated glassy carbon (Bi-GC), and mercury-coated glassy carbon (Hg-GC) electrodes were compared for the CPE-ASV. A detection limit for Cd(2+) of 1.7 nM (0.2 ppb) was obtained with the Hg-GC electrode. ASV with CPE gave a 20x decrease (4.0 ppb) in the detection limit compared to ASV without CPE. The suitability of this procedure for the analysis of tap and river water samples was demonstrated. This simple, versatile, environmentally friendly, and cost-effective extraction method is potentially applicable to a wide variety of transition metals and organic compounds that are amenable to detection by electroanalytical methods.