Nitrogen in Diamond.

Nitrogen is ubiquitous in both natural and laboratory-grown diamond, but the number and nature of the nitrogen-containing defects can have a profound effect on the diamond material and its properties. An ever-growing fraction of the supply of diamond appearing on the world market is now lab-grown. Here, we survey recent progress in two complementary diamond synthesis methods-high pressure high temperature (HPHT) growth and chemical vapor deposition (CVD), how each is allowing ever more precise control of nitrogen incorporation in the resulting diamond, and how the diamond produced by either method can be further processed (e.g., by implantation or annealing) to achieve a particular outcome or property. The burgeoning availability of diamond samples grown under well-defined conditions has also enabled huge advances in the characterization and understanding of nitrogen-containing defects in diamond-alone and in association with vacancies, hydrogen, and transition metal atoms. Among these, the negatively charged nitrogen-vacancy (NV-) defect in diamond is attracting particular current interest in account of the many new and exciting opportunities it offers for, for example, quantum technologies, nanoscale magnetometry, and biosensing.

Deconvoluting Surface-Bound Quinone Proton Coupled Electron Transfer in Unbuffered Solutions: Toward a Universal Voltammetric pH Electrode.

While quinone proton coupled electron transfer (PCET) under buffered conditions is well understood, the situation is more complicated in unbuffered aqueous solutions. With a view to producing a quinone-based voltammetric pH electrode that can function universally in both buffered and unbuffered solutions by following a two-electron (2e-)/two-proton (2H+) Nernstian pathway over a wide pH range, the voltammetric response of strongly electronically coupled surface-bound quinones, directly integrated into a boron-doped diamond (BDD) electrode, is investigated. A laser ablation process enables integration of quinones into the BDD electrode surface with a high p Ka1 (first protonation state) and with controllable, very low surface coverages (as low as 2 orders of magnitude below monolayer coverage). Under buffered conditions, one wave results for all pH values, and the 2e-/2H+ pathway is followed across the entire pH range. The measured ET rate constant values, from Laviron analysis, are also high, indicative of fast ET pathways. Under unbuffered conditions, one wave is again observed for all pH values; however, deviations from the buffered 2e-/2H+ behavior are seen in the neutral region (pH 6-8). While 2e-/2H+ transfer is maintained at all times, we attribute the observed deviation to local pH changes caused by the consumption and generation of protons at the electrode surface during the redox electrochemistry of the quinone. The associated proton fluxes generated at such sparse surface coverages are thought to be sufficiently high enough to prevent ET from occurring exclusively via a proton-independent route. By reducing surface coverage (down to ∼4 × 10-12 mol cm-2; the limit of our laser ablation process) local pH changes can be reduced but are not eradicated completely. By moving to a pulsed voltammetric technique, where for each potential step protons consumed at the electrode are immediately replaced, it is possible, provided the surface coverage is low enough, to obtain a Nernstian 2e-/2H+ response across a wide pH range in unbuffered solution.

Controlled sp(2) Functionalization of Boron Doped Diamond as a Route for the Fabrication of Robust and Nernstian pH Electrodes.

The development of a voltammetric boron doped diamond (BDD) pH sensor is described. To obtain pH sensitivity, laser micromachining (ablation) is utilized to introduce controlled regions of sp(2) carbon into a high quality polycrystalline BDD electrode. The resulting sp(2) carbon is activated to produce electrochemically reducible quinone groups using a high temperature acid treatment, followed by anodic polarization. Once activated, no further treatment is required. The quinone groups show a linear (R(2) = 0.999) and Nernstian (59 mV/(pH unit)) pH-dependent reductive current-voltage response over a large analyzable pH range, from pH 2 to pH 12. Using the laser approach, it is possible to optimize sp(2) coverage on the BDD surface, such that a measurable pH response is recorded, while minimizing background currents arising from oxygen reduction reactions on sp(2) carbon in the potential region of interest. This enables the sensor to be used in aerated solutions, boding well for in situ analysis. The voltammetric response of the electrode is not compromised by the presence of excess metal ions such as Pb(2+), Cd(2+), Cu(2+), and Zn(2+). Furthermore, the pH sensor is stable over a 3 month period (the current time period of testing), can be stored in air between measurements, requires no reactivation of the surface between measurements, and can be reproducibly fabricated using the proposed approach. The efficacy of this pH sensor in a real-world sample is demonstrated with pH measurements in U.K. seawater.

Quantitative analysis of trace palladium contamination in solution using electrochemical X-ray fluorescence (EC-XRF).

The application of electrochemical X-ray fluorescence (EC-XRF), for the detection of palladium (Pd) contamination in a range of solutions containing electrochemically active compounds, present in excess and relevant to the pharmaceutical and food industries, is reported. In EC-XRF, EC is used to electrochemically pre-concentrate metal on an electrode under forced convection conditions, whilst XRF is employed to spectroscopically quantify the amount of metal deposited, which quantitatively correlates with the original metal concentration in solution. Boron doped diamond is used as the electrode due to its very wide cathodic window and the fact that B and C are non-interfering elements for XRF analysis. The effect of several parameters on the Pd XRF signal intensity are explored including: deposition potential (Edep), deposition time (tdep) and Pd(2+) concentration, [Pd(2+)]. Under high deposition potentials (Edep = -1.5 V), the Pd XRF peak intensity varies linearly with both tdep and [Pd(2+)]. Quantification of [Pd(2+)] is demonstrated in the presence of excess acetaminophen (ACM), l-ascorbic acid, caffeine and riboflavin. We show the same Pd XRF signal intensity (for [Pd(2+)] = 1.1 µM and tdep = 325 s) is observed, i.e. same amount of Pd is deposited on the electrode surface, irrespective of whether these redox active molecules are present or absent. For tdep = 900 s we report a limit of detection for [Pd(2+)] of 3.6 ppb (34 nM). Even lower LODs are possible by increasing tdep or by optimising the X-ray source specifically for Pd. The work presented for Pd detection in the presence of ACM, achieves the required detection sensitivity stipulated by international pharmacopeia guidelines.

Direct identification and analysis of heavy metals in solution (Hg, Cu, Pb, Zn, Ni) by use of in situ electrochemical X-ray fluorescence.

The development and application of a new methodology, in situ electrochemical X-ray fluorescence (EC-XRF), is described that enables direct identification and quantification of heavy metals in solution. A freestanding film of boron-doped diamond serves as both an X-ray window and the electrode material. The electrode is biased at a suitable driving potential to electroplate metals from solution onto the electrode surface. Simultaneously, X-rays that pass through the back side of the electrode interrogate the time-dependent electrodeposition process by virtue of the XRF signals, which are unique to each metal. In this way it is possible to unambiguously identify which metals are in solution and relate the XRF signal intensity to a concentration of metal species in solution. To increase detection sensitivity and reduce detection times, solution is flown over the electrode surface by use of a wall-jet configuration. Initial studies focused on the in situ detection of Pb(2+), where concentration detection limits of 99 nM were established in this proof-of-concept study (although significantly lower values are anticipated with system refinement). This is more than 3 orders of magnitude lower than that achievable by XRF alone in a flowing solution (0.68 mM). In situ EC-XRF measurements were also carried out on a multimetal solution containing Hg(2+), Pb(2+), Cu(2+), Ni(2+), Zn(2+), and Fe(3+) (all at 10 µM concentration). Identification of five of these metals was possible in one simple measurement. In contrast, while anodic stripping voltammetry (ASV) also revealed five peaks, peak identification was not straightforward, requiring further experiments and prior knowledge of the metals in solution. Time-dependent EC-XRF nucleation data for the five metals, recorded simultaneously, demonstrated similar deposition rates. Studies are now underway to lower detection limits and provide a quantitative understanding of EC-XRF responses in real, multimetal solutions. Finally, the production of custom-designed portable in situ EC-XRF instrumentation will make heavy metal analysis at the source a very realistic possibility.

Electrochemical electron paramagnetic resonance utilizing loop gap resonators and micro-electrochemical cells.

A miniaturised electrochemical cell design for Electron Paramagnetic Resonance (EPR) studies is reported. The cell incorporates a Loop Gap Resonator (LGR) for EPR investigation of electrochemically generated radicals in aqueous (and other large dielectric loss) samples and achieves accurate potential control for electrochemistry by using micro-wires as working electrodes. The electrochemical behaviour of the cell is analysed with COMSOL finite element models and the EPR sensitivity compared to a commercial TE011 cavity resonator using 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) as a reference. The electrochemical EPR performance is demonstrated using the reduction of methyl viologen as a redox probe in both water and acetonitrile. The data reported herein suggest that sub-micromolar concentrations of radical species can be detected in aqueous samples with accurate potential control, and that subtle solution processes coupled to electron transfer, such as comproportionation reactions, can be studied quantitatively using EPR.

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 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 X-ray fluorescence spectroscopy for trace heavy metal analysis: enhancing X-ray fluorescence detection capabilities by four orders of magnitude.

The development of a novel analytical technique, electrochemical X-ray fluorescence (EC-XRF), is described and applied to the quantitative detection of heavy metals in solution, achieving sub-ppb limits of detection (LOD). In EC-XRF, electrochemical preconcentration of a species of interest onto the target electrode is achieved here by cathodic electrodeposition. Unambiguous elemental identification and quantification of metal concentration is then made using XRF. This simple electrochemical preconcentration step improves the LOD of energy dispersive XRF by over 4 orders of magnitude (for similar sample preparation time scales). Large area free-standing boron doped diamond grown using microwave plasma chemical vapor deposition techniques is found to be ideal as the electrode material for both electrodeposition and XRF due to its wide solvent window, transparency to the XRF beam, and ability to be produced in mechanically robust freestanding thin film form. During electrodeposition it is possible to vary both the deposition potential (Edep) and deposition time (tdep). For the metals Cu(2+) and Pb(2+) the highest detection sensitivities were found for Edep = -1.75 V and tdep (=) 4000 s with LODs of 0.05 and 0.04 ppb achieved, respectively. In mixed Cu(2+)/Pb(2+) solutions, EC-XRF shows that Cu(2+) deposition is unimpeded by Pb(2+), across a broad concentration range, but this is only true for Pb(2+) when both metals are present at low concentrations (10 nM), boding well for trace level measurements. In a dual mixed metal solution, EC-XRF can also be employed to either selectively deposit the metal which has the most positive formal reduction potential, E(0), or exhaustively deplete it from solution, enabling uninhibited detection of the metal with the more negative E(0).

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.

Electron Paramagnetic Resonance for the Detection of Electrochemically Generated Hydroxyl Radicals: Issues Associated with Electrochemical Oxidation of the Spin Trap.

For the detection of electrochemically produced hydroxyl radicals (HO·) from the oxidation of water on a boron-doped diamond (BDD) electrode, electron paramagnetic resonance spectroscopy (EPR) in combination with spin trap labels is a popular technique. Here, we show that quantification of the concentration of HO· from water oxidation via spin trap electrochemical (EC)-EPR is problematic. This is primarily due to the spin trap oxidizing at potentials less positive than water, resulting in the same spin trap-OH· adduct as formed from the solution reaction of OH· with the spin trap. We illustrate this through consideration of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap for OH·. DMPO oxidation on a BDD electrode in an acidic aqueous solution occurs at a peak current potential of +1.90 V vs SCE; the current for water oxidation starts to rise rapidly at ca. +2.3 V vs SCE. EC-EPR spectra show signatures due to the spin trap adduct (DMPO-OH·) at potentials lower than that predicted thermodynamically (for water/HO·) and in the region for DMPO oxidation. Increasing the potential into the water oxidation region, surprisingly, shows a lower DMPO-OH· concentration than when the potential is in the DMPO oxidation region. This behavior is attributed to further oxidation of DMPO-OH·, production of fouling products on the electrode surface, and bubble formation. Radical scavengers (ethanol) and other spin traps, here N-tert-butyl-α-phenylnitrone, α-(4-pyridyl N-oxide)-N-tert-butylnitrone, and 2-methyl-2-nitrosopropane dimer, also show electrochemical oxidation signals less positive than that of water on a BDD electrode. Such behavior also complicates their use for the intended application.

Tryptophan switch for a photoactivated platinum anticancer complex.

The octahedral Pt(IV) complex trans,trans,trans-[Pt(N(3))(2)(OH)(2)(py)(2)] (1) is potently cytotoxic to cancer cells when irradiated with visible (blue) light. We show that the acute photocytotoxicity can be switched off by low doses (500 µM) of the amino acid l-tryptophan. EPR and NMR spectroscopic experiments using spin traps show that l-Trp quenches the formation of azidyl radicals, probably by acting as an electron donor. l-Trp is well-known as a mediator of electron transfer between distant electron acceptor/donor centers in proteins, and such properties may make the free amino acid clinically useful for controlling the activity of photochemotherapeutic azido Pt(IV) drugs. Since previous work has demonstrated the ability of photoactivated 1 to platinate DNA, this suggests that the high potency of such photoactive platinum complexes is related to their dual attack on cancer cells by radicals and Pt(II) photoproducts.

Electrochemical mapping reveals direct correlation between heterogeneous electron-transfer kinetics and local density of states in diamond electrodes.

Conducting carbon materials: a multi-microscopy approach shows that local heterogeneous electron-transfer rates at conducting diamond electrodes correlate with the local density of electronic states. This model of electroactivity is of considerable value for the rational design of conducting diamond electrochemical technologies, and also provides key general insights on electrode structure controls in electrochemical kinetics.

Factors controlling stripping voltammetry of lead at polycrystalline boron doped diamond electrodes: new insights from high-resolution microscopy.

We report wide-ranging studies to elucidate the factors and issues controlling stripping voltammetry of metal ions on solid electrodes using the well-known Pb/Pb(2+) couple on polycrystalline boron doped diamond (pBDD) as an exemplar system. Notably, high-resolution microscopy techniques have revealed new insights into the features observed in differential pulse anodic stripping voltammetry (DPV-ASV) which provide a deeper understanding of how best to utilize this technique. DPV-ASV was employed in an impinging wall-jet configuration to detect Pb(2+) in the nanomolar to micromolar concentration range at a pBDD macrodisk electrode. The deposition process was driven to produce a grain-independent homogeneous distribution of Pb nanoparticles (NPs) on the electrode surface; this resulted in the observation of narrow stripping peaks. Lower calibration gradients of current or charge versus concentration were found for the low concentrations, correlating with a lower than expected (from consideration of the simple convective-diffusive nature of the deposition process) amount of Pb deposited on the surface. This was attributed to the complex nature of nucleation and growth at solid surfaces in this concentration regime, complicating mass transport. Furthermore, a clear shift negative in the stripping peak potential with decreasing concentration was seen correlating with a change in the size of the deposited NP, suggesting an NP size-dependent redox potential for the Pb/Pb(2+) couple. At high concentrations a nonlinear response was observed, with less Pb detected than expected, in addition to the observation of a second stripping peak. Atomic force microscopy (AFM) and field emission scanning electron microscopy revealed the second peak to be due to a change in deposition morphology from isolated NPs to grain-independent heterogeneous structures comprising both thin films and NPs; the second peak is associated with stripping from the thin-film structures. AFM also revealed a substantial amount of Pb remaining on the surface after stripping at high concentration, explaining the nonlinear relationship between stripping peak current (or charge) and concentration. Finally, the use of an in situ cleaning procedure between each measurement was advocated to ensure a clean Pb-free surface (verified by AFM and X-ray photoelectron spectroscopy analysis) between each run. The studies herein highlight important and complex physicochemical processes involved in the electroanalysis of heavy metals at solid electrodes, such as pBDD, that need to be accounted for when using stripping voltammetry methods.

Fabrication and characterization of an all-diamond tubular flow microelectrode for electroanalysis.

The development of the first all-diamond hydrodynamic flow device for electroanalytical applications is described. Here alternate layers of intrinsic (insulating), conducting (heavily boron doped), and intrinsic polycrystalline diamond are grown to create a sandwich structure. By laser cutting a hole through the material, it is possible to produce a tubular flow ring electrode of a characteristic length defined by the thickness of the conducting layer (for these studies ∼90 µm). The inside of the tube can be polished to 17 ± 10 nm surface roughness using a diamond impregnanted wire resulting in a coplanar, smooth, all-diamond surface. The steady-state limiting current versus volume flow rate characteristics for the one electron oxidation of FcTMA(+) are in agreement with those expected for laminar flow in a tubular electrode geometry. For dopamine detection, it is shown that the combination of the reduced fouling properties of boron doped diamond, coupled with the flow geometry design where the products of electrolysis are washed away downstream of the electrode, completely eradicates fouling during electrolysis. This paves the way for incorporation of this flow design into online electroanalytical detection systems. Finally, the all diamond tubular flow electrode system described here provides a platform for future developments including the development of ultrathin ring electrodes, multiple apertures for increased current response, and multiple, individually addressable ring electrodes incorporated into the same flow tube.

Determination of the temperature dependence of the dynamic nuclear polarisation enhancement of water protons at 3.4 Tesla.

It is shown that the temperature dependence of the DNP enhancement of the NMR signal from water protons at 3.4 T using TEMPOL as a polarising agent can be obtained provided that the nuclear relaxation, T(1I), is sufficiently fast and the resolution sufficient to measure the (1)H NMR shift. For high radical concentrations (∼100 mM) the leakage factor is approximately 1 and, provided sufficient microwave power is available, the saturation factor is also approximately 1. In this situation the DNP enhancement is solely a product of the ratio of the electron and nuclear gyromagnetic ratios and the coupling factor enabling the latter to be directly determined. Although the use of high microwave power levels needed to ensure saturation causes rapid heating of the sample, this does not prevent maximum DNP enhancements, ε(0), being obtained since T(1I) is very much less than the characteristic heating time at these concentrations. It is necessary, however, to know the temperature variation of T(1I) to allow accurate modelling of the behaviour. The DNP enhancement is found to vary linearly with temperature with ε(0)(T) = -2 ± 2 - (1.35 ± 0.02)T for 6 °C ≤ T ≤ 100 °C. The value determined for the coupling factor, 0.055 ± 0.003 at 25 °C, agrees very well with the molecular dynamics simulations of Sezer et al. (Phys. Chem. Chem. Phys., 2009, 11, 6626) who calculated 0.0534, however the experimental values increase much more rapidly with increasing temperature than predicted by these simulations. Large DNP enhancements (|ε(0)| > 100) are reported at high temperatures but it is also shown that significant enhancements (e.g.∼40) can be achieved whilst maintaining the sample temperature at 40 °C by adjusting the microwave power and irradiation time. In addition, short polarisation times enable rapid data acquisition which permits further enhancement of the signal, such that useful liquid state DNP-NMR experiments could be carried out on very small samples.

DNP enhanced NMR using a high-power 94 GHz microwave source: a study of the TEMPOL radical in toluene.

DNP enhanced (1)H NMR at 143 MHz in toluene is investigated using an NMR spectrometer coupled with a modified EPR spectrometer operating at 94 GHz and TEMPOL as the polarisation agent. A 100 W microwave amplifier was incorporated into the output stage of the EPR instrument so that high microwave powers could be delivered to the probe in either CW or pulsed mode. The maximum enhancement for the ring protons increases from approximately -16 for a 5 mM TEMPOL solution to approximately -50 for a 20 mM solution at a microwave power of approximately 480 mW. The temperature dependence of the enhancement, the NMR relaxation rates and the ESR spectrum of TEMPOL were also studied in an effort to obtain information on the dynamics of the system.

An Ultrafast Shakedown Reveals the Energy Landscape, Relaxation Dynamics, and Concentration of the N3VH0 Defect in Diamond.

Atomic-scale defects can control the exploitable optoelectronic performance of crystalline materials, and several point defects in diamond are emerging functional components for a range of quantum technologies. Nitrogen and hydrogen are common impurities incorporated into diamond, and there is a family of defects that includes both. The N3VH0 defect is a lattice vacancy where three nearest neighbor carbon atoms are replaced with nitrogen atoms and a hydrogen is bonded to the remaining carbon. It is regularly observed in natural and high-temperature annealed synthetic diamond and gives rise to prominent absorption features in the mid-infrared. Here, we combine time- and spectrally resolved infrared absorption spectroscopy to yield unprecedented insight into the N3VH0 defect's vibrational dynamics following infrared excitation of the C-H stretch. In doing so, we gain fundamental information about the energies of quantized vibrational states and corroborate our results with theory. We map out, for the first time, energy relaxation pathways, which include multiphonon relaxation processes and anharmonic coupling to the C-H bend mode. These advances provide new routes to quantify and probe atomic-scale defects.

Amperometric oxygen sensor based on a platinum nanoparticle-modified polycrystalline boron doped diamond disk electrode.

Pt nanoparticle (NP)-modified polycrystalline boron-doped diamond (pBDD) disk electrodes have been fabricated and employed as amperometric sensors for the determination of dissolved oxygen concentration in aqueous solution. pBDD columns were cut using laser micromachining techniques and sealed in glass, in order to make disk electrodes which were then characterized electrochemically. Electrodeposition of Pt onto the diamond electrodes was optimized so as to give the maximum oxygen reduction peak current with the lowest background signal. Pt NPs, >0-10 nm diameter, were found to deposit randomly across the pBDD electrode, with no preference for grain boundaries. The more conductive grains were found to promote the formation of smaller nanoparticles at higher density. With the use of potential step chronoamperometry, in which the potential was stepped to a diffusion-limited value, a four electron oxygen reduction process was found to occur at the Pt NP-modified pBDD electrode. Furthermore the chronoamperometric response scaled linearly with dissolved oxygen concentration, varied by changing the oxygen/nitrogen ratio of gas flowed into solution. The sensor was used to detect dissolved oxygen concentrations with high precision over the pH range 4-10.