GB2518263A - Diamond based electrochemical sensors - Google Patents

Abstract

A diamond electrochemical sensor comprises a sensing electrode formed of boron-doped diamond material and a pH changing electrode also formed of boron-doped diamond material, wherein the sensing electrode and the pH changing electrode are configured such that when placed in an aqueous solution of pH 7 the pH changing electrode can generate H+ and OH- allowing a reversible pH change over the sensing electrode across a pH range of at least 3 to 9.

Description

DIAMOND BASED ELECTROCHEMICAL SENSORS

Field of Invention

Certain embodiments of the present invention relate to a diamond based electrochemical sensor a method of detecting a target chemical species in a solution using a diamond based electrochemical sensor.

Backzround of invention Electrochemical sensors are well known, It has also been proposed in the prior art to provide a diamond based electrochemical sensor, Diamond can be doped with boron to form semi-conductive or frilly metallic conductive material for use as an electrode.

Diamond is also hard, inert, and has a very wide potential window making it a very desirable material for use as a sensing electrode for an electrochemical cell, particularly in harsh chemical, physical, and/or thermal environments which would degrade standard metal based electrocheniical sensors, In addition, it is known that the surface of a boron doped diamond electrode may be functionalized to sense certain species in a solution adjacent the electrode.

One problem with using diamond in such applications is that diamond material is inherently difficult to manufacture and form into suitable geometries for sophisticated electrochemical analysis. To date, diamond electrodes utilized as sensing electrodes in an electrochemical cell have tended to be reasonably simple in construction and mostly comprise the use of a single piece of boron doped diamond configured to sense one physical parameter or chemical species at any one time, More complex arrangements have involved introducing one or more channels into a piece of boron doped diamond through which a solution can flow for performing electrochemical analysis, However, to date the present applicant is unaware of sophisticated diamond based electrochemical sensors which can perform multiple sensing functions at the same time, particularly configured for use in harsh environments. Due to the inherent difficulties involved in manufacturing and forming diamond into multi-structural components, even apparently relatively simple target structures can represent a significant technical challenge.

In terms of prior art arrangements, the present applicant already has a number of patent publications relating to diamond based electrochemical sensor structures and methodology including: W02005/0]2894; W02007/1 07844; W020]2/126802; W02005/012894 describes a microelectrode comprising a diamond layer formed from electrically non-conducting diamond and containing one or more pin-like projections of electrically conducting diamond extending at least partially through the layer of non-conducting diamond and presenting areas of electrically conducting diamond at a front sensing surface, W02007/107844 describes a microelectrode array comprising a body of diamond material including alternating layers of electrically conducting and electrically non-conducting diamond material and passages extending through the body of diamond material. In use, fluid flows through the passages and the electrically conducting layers present ring-shaped electrode surfaces within the passages in the body of diamond material, W020l2/l26802 describes a diamond based electrochemical band sensor comprising a plurality of boron doped diamond band electrodes disposed within a diamond body.

Each boron doped diamond electrode has a length / width ratio of at least 10 at a front sensing surface of the sensor. It is described that it is advantageous from a fhnctional perspective for certain sensing applications to provide band electrodes which have a high aspect ratio at the sensing surface such that a length of a band electrode across the sensing surface is very much larger than a width of the band electrode.

W020l2/l26802 also suggested that a larger number of electrodes can be provided within the diamond body to support a range of sensing capabilities, For example, a plurality of boron doped diamond band electrodes may be configured to sense one or more of the follong properties of a solution adjacent the sensing surface: p1-I; conductivity; temperature; individual or total heavy metal concentration; and 112S. In this regard, the plurality of boron doped diamond band electrodes may be grouped into sets, each set comprising one or more boron doped diamond band electrodes, wherein each set is configured to sense a different parameter. Further still, W02012/126802 suggests that one of the bands may be configured to generate a proton gradient which flows over one or more of the other bands. This may be used to promote reactions which require a certain pH. In this regard, it is taught that a diamond electrode material is advantageous in that a very high electrode potential can be applied to alter pH via proton or hydroxide generation. For metal ions which are complexed in solution, digests are normally performed to free them so they are available for subsequent reduction. One way to do this is to generate very strong acid (or base) conditions electrochemically. Furthermore, generating very strong acid (or base) conditions can also be useful for cleaning the electrode.

W02012/1 56203 also suggests that a diamond electrode material is advantageous in that a very high electrode potential can be applied to alter pH via proton or hydroxide generation. WO2012/t56203 specifically relates to an electrochemical sensor comprising a diamond reference electrode in addition to a diamond sensing electrode, i.e. a robust all-diamond sensor structure, Since the diamond reference electrode is not a standard reference electrode which is independent of s&ution type, an in-situ calibration system is provided for assigning peaks in voltammetry data to chemical species thereby allowing the type and concentration of chemical species in the solution to be detemrined, Several different calibration systems are described including a system in which a calibration electrode is configured to change pH conditions in the solution by generating protons or hydroxide ions to promote a reaction over the diamond reference electrode to tune the reference electrode towards a known potential.

W02012/1 56307 also suggests that a diamond electrode material is advantageous in that a very high electrode potential can be applied to alter pH via proton or hydroxide generation. WO2012/156307 specifically relates to a combined electrochemical and spectroscopic methodology where a diamond electrode is used to electro-deposit a target species and then a spectroscopic analysis technique is applied to the electro-deposited chemical species. It is indicated that an in-situ p1-I change can improve the electro-deposition step and thus improve the sensitivity of the combined electro-deposition and spectroscopy analysis technique. Furthermore, it is suggested that an in-situ pH change can be used to aid cleaning of the diamond electro-deposition electrode.

In light of the above, it will be evident that the present applicant's previous work has disclosed the use of a diamond electrode for electrochemically generating an in-situ pH change in electrochemical deposition and electrode cleaning applications. It is an aim of certain embodiments of the present invention to provide diamond electrode configurations and associated methodology which is optimized towards creating large and stable in situ pH changes over a diamond sensing electrode such that a target p1-I can be generated and maintained over the diamond sensing electrode during an electrochemical sensing measurement.

Summary of Invention

While the present applicant has previously disclosed the use of diamond electrodes for generating in situ pH changes, recent work on optimisation of diamond electrode configurations and associated methodology has resulted in surprisingly large, stable, and reproducible in situ pH changes over diamond sensing electrodes which can be used for improved sensitivity and reproducibility of electrochemical sensing measurement.

According to one aspect of the present invention there is provide a diamond electrochemical sensor comprising: a sensing electrode formed of boron-doped diamond material; and a pH changing electrode formed of boron-doped diamond material; wherein the sensing electrode and the pH changing electrode are configured such that when placed in an aqueous solution of pH 7, the pH changing electrode can generate H and Off allowing a reversible pH change over the sensing electrode across a pH range of at least 3 to 9, more preferably at least 2 to 10, and most preferably at least t to ii.

According to a second aspect of the present invention there is provided a method of detecting a target chemical species in a solution using a diamond electrochemical sensor as defined above, the method comprising: applying a first potential to the pH changing electrode to generating a local pH change over the sensing electrode; applying a second potential to the sensing electrode to detect the target chemical species; and optionally applying a third potential to the sensing electrode to condition the sensing electrode prior to application of the second potential, wherein the third potential has an opposite polarity to the second potential and is larger in magnitude than said second potential.

Brief Description of the Drawinzs

For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows stripping peaks from cyclic voltammograms (CVs) conducted on a boron doped diamond (BDD) ring disc electrode at 0.1 vs -1 in 1 m\'l Hg(N03) solutions at pH 2, pH 5 and pH 7 (inset shows the full CV for the pH 2 solution); Figure 2(a) shows an optical microscope image of the ring disc electrode surface; Figure 2(b) shows a typical Open Circuit Potential (OCP) vs. pH calibration curve for an iridium oxide coated BDD disc (inset shows the behaviour of the iridium oxide film on a polycrystalline boron doped diamond (pBDD) disc in 1 M H2 SO4); Figure 2(c) shows a radial slice of a model of the proton flux from the ring to the disc at ôOOs; Figure 2(d) shows long term, 600 s proton generation for a range of currents (10, 20, 40, 50 RA) applied to the ring, illustrating good stability over time after equilibration and good agreement to the model (solid = experimental data; dashed = simulated data); Figure 3 shows mercury stripping peaks at 0.1 Vs' in 1mM Hg(NO3) solutions; Figure 4 summarizes in situ mercury detection using a ring disc electrode configuration; Figure 5 shows characterisation of a ring disc electrode, CVs in Ru(NH3)o23 for the disc (left) and ring (right) at a range of scan rates; Figure 6(a) shows a schematic diagram of the simulation domain; an axisymmetric section of a water cylinder on top of a ring-disc electrode arangement defined by points w1.3 and lengths h and w'r; Figure 6(b) shows an example of a radial slice of the [W] at 60 seconds for a gen of 1iA (the electrode geometry is superimposed for clarity); Figure 7(a) shows an FE-SEM (Field Emission -Scanning Electron Microscopy) image of an all-diamond dual band electrochemical sensor; Figure 7(b) shows a schematic illustration of the all-diamond dual band electrochemical sensor; Figure 7(c) shows the potentials applied and timescale of the cleaning step (-4 iA pulse cleaning for 20 s) as well as the sensing step that follows at +1.2 V together with the basic pH generating electrode potential (can vary for -t.7 V to -2 V); Figure 8(a) shows the detection response of increasing concentrations of dissolved hydrogen sulphide (Edetect = +1,2 V) at pH 10 using a 90 p.m pBDD band; Figure 8(b) shows a calibration graph constructed using data by averaging peak heights vs. concentration of dissolved sulphide -the limit of detection (L.O.D.) obtained for pH tO was 6.5 ppb [R2 = 0.997, Slope = 7.84 x i09 A p.M', intercept = 1.8 x 10 Al for Ed0t = +1.2 V; Figure 9(a) shows the FIA-EC detection (Flow-Injection Analysis with Electrochemical Detection) of 100 pl\'I dissolved hydrogen sulphide (Edt0t = +1.2 V) at pH 6.1 using the 90 im pBDD band -the peaks show the effect of the sensor signal with the absence of a cleaning step prior to detection as well as the effect of the detector signal to the cathodic polarisation of the upstream Xg electrode; Figure 9(b) shows a plot summarising all the FIA-EC data as an average peak height versus electrode treatments and in-situ pH generation -the effect of a freshly cleaned electrodes can be clearly seen with a -10% increase in signal and a better signal reproducibility noted by the size of the error bars; a 25% increase in the signal was achieved at Xg applied potential of -1.7 V and a further 20% increase when the Xg applied apotentialof-1.8 V; Figure 9(c) illustrates the processes occurring at solution/electrode interfaces for solution containing dissolved hydrogen sulphide using the sensor technology as described herein; Figure 9(d) shows a plot of the ratio of dissolved hydrogen sulphide vs. solution pH showing the relative amounts of each of the three soluble forms of sulphide at a given pH; Figure 10(a) illustrates the application of increasing cathodic potentials applied at the upstream generator electrode for 60 s using a potentiostatic technique; Figure tO(b) illustrates the pH measured simultaneously on the downstream electrode; Figure 11(a) shows a series of 60 s constant current pulses applied to the upstream electrode using a galvanostatic technique; and Figure 11(b) shows the downstream pH electrode recording the local flux of Ht'OH downstream.

Detailed description of Certain Embodiments

As described in the summary of invention section, embodiments of this invention provide a diamond electrochemical sensor comprising: a sensing electrode formed of boron-doped diamond material; and a pH changing electrode formed of boron-doped diamond material; wherein the sensing electrode and the pH changing electrode are configured such that when placed in an aqueous solution of pH 7, the pH changing electrode can + generate H and OH allowing a reversible pH change over the sensing electrode across a pH range of at least 3 to 9, more preferably at least 2 to 10, and most preferably at least ito ii.

Such a broad and reversible pH change generated over a diamond sensing electrode in situ by an adjacent diamond electrode has been found to enable electrochemical sensing of important environmental target species such mercury and sulphur to extreme levels of sensitivity.

Preferably the sensor is configured to generate the reversible pH change without any substantial gas bubble formation at the surface of the pH changing electrode. Gas bubbles tend to form when electrodes are driven to the extreme potentials needed for large in situ pH changes and these gas bubbles interfere with electrochemical sensing measurements. However, the present inventors have found that by careful selection of the boron-doped diamond material used for the pH changing electrode then gas bubble formation can be effectively eliminated. For example, one or both of the sensing electrode and the pH changing electrode can be formed of a boron-doped diamond material which has an sp2 carbon content sufficiently low as to not exhibit non-diamond carbon peaks in a Raman spectrum of the boron-doped diamond material. Such high phase purity boron doped diamond material is capable of being driven to the extreme potentials required for generating large in situ p11 changes without any substantial gas bubble formation, Accordingly, a pH change can be generated and maintained during an electrochemical sensing measurement without gas bubble formation interfering with the electrochemical sensing measurement.

Further to the above, advantageously a target pH can be generated over the sensing electrode by the pH changing electrode and held during electrochemical sensing of a target species using the sensing electrode, For example, the reversible pH change can be generated over extended time periods (e.g. at least 10 seconds, 30 seconds, 1 minute, 5 minutes, or 10 minutes) and/or multiple cycles without surface passivation of the pH changing electrode. This allows a range of electrochemical sensing measurements to be performed while generating and maintaining a target pH value over the diamond sensing electrode and for such measurements to be repeatedly performed in a reliable and reproducible manner. Again, the use of high phase purity boron doped diamond electrodes is advantageous for achieving this functionality. In addition, it is also advantageous to provide a surface termination which is stable. For example, an oxygen-terminated synthetic diamond material which can be achieved by acid washing and then polishing the surface of the synthetic diamond material with alumina micro polish.

A number of different diamond electrode configurations have been identified as being suitable for generating the large and stable pH changes as described herein and the specific configuration which is selected will to some extent be dependent on the end application. Advantageously, the boron-doped diamond sensing electrode and the boron-doped diamond pH changing electrode are both mounted within an intrinsic diamond support matrix. This "all-diamond" solution is advantageous in terms of mechanical and chemical robusiness and is also advantageous in terms of ability to clean and re-use without causing damage and without contamination of subsequent electrochemical measurements.

In one configuration the sensing electrode is a disc electrode and the pH changing electrode is a ring electrode which surrounds the disc electrode. While such ring and disc electrode configurations are known, the large, stable, and reproducible pH changes as described herein can be achieved by combining this electrode configuration with the selection of phase-pure boron doped diamond electrode material, a suitable treatment to provide a stable surface termination, and a suitable electric contact to provide a good ohmic connection.

In another configuration the sensing electrode can be a band electrode and the pH changing electrode is another band electrode positioned adjacent the sensing electrode. Such a configuration is similar to that described in W020121126802. In addition, it has been found that in this band electrode configuration the pH changing electrode should ideally have a larger surface area than the sensing electrode to provide large and stable pH changes over the sensing electrode as described herein.

In fact, such a configuration in which the pH changing electrode has a larger surface area than the sensing electrode is advantageous for other electrode configurations, such as the ring and disc configuration described previously, in order to ensure that a sufficiently large and consistent pH change is generated over the sensing electrode.

As such, it is advantageous that an outer peripheral edge of the pH changing electrode defines a larger area than the outer peripheral edge of the sensing electrode.

Furthermore, as described above, suitable electrode configurations may be combined with the use of phase-pure boron doped diamond electrode material, a suitable treatment to provide a stable surface termination, and a suitable electric contact to provide a good ohmic connection in order to achieve the large, stable, and reproducible pH changes over a sensing electrode as described herein.

The electrode configurations as described above may be disposed in a static electrochemical cell. Alternatively, the sensing electrode and the pH changing electrode can be disposed in a flow cell with at least a portion of the pH changing electrode being disposed upstream of the sensing electrode whereby H and OFF generated by the pH changing electrode flow over the sensing electrode in use. The precise electrochemical cell configuration will depend on the end application.

Further to the above, it can be advantageous for the pH changing electrode to be configured to generate the reversible p1-I change using a galvanostatic generation methodology rather than a potentiostatic approach. It has been found that the galvanostatic generation methodology can result in more stable and reproducible pH responses when compared to the potentiostatic approach The diamond electrochemical sensor can also be configured to apply a cleaning pulse to the sensing electrode to condition the sensing electrode prior to detection of a target species. For example, the diamond electrochemical sensor may be configured to apply the following pulses to the sensing electrode and the pH changing electrode: a cleaning pulse applied to the sensing electrode; a pH generating pulse applied to the pH changing electrode; a sensing pulse applied to the sensing electrode, In light of the above, another aspect of the invention is a method of detecting a target chemical species in a solution using a diamond electrochemical sensor according to any preceding claim, the method comprising: applying a first potential to the pH changing electrode to generating a local pH change over the sensing electrode; and applying a second potential to the sensing electrode to detect the target chemical species.

Advantageously, a third potential is applied to the sensing electrode to condition the sensing electrode prior to application of the second potential, wherein the third potential has an opposite polarity to the second potential and is larger in magnitude than said second potential.

Using the electrochemical sensing configurations as described herein, it is possible to detect target species such as mercury and sulphur at extremely low concentration levels. For example, mercury detection is most sensitive at low pH and embodiments of the present invention are capable of generating and maintaining an in situ pH change down to a pH of 2 for detecting mercury. As will be described in more detail later in this specification, the diamond electrochemical sensor can be configured such that a mercury stripping peak of at least 10 1iA is generated at the sensing electrode using a scan rate of 0.1 Vs' in a 1 mM Hg22 solution which has an intrinsic pH of 7 (i.e. a pH of 7 prior to application of an in situ pH change via the pH changing electrode). In contrast, H2S detection is most sensitive at high pH and embodiments of the present invention are capable of generating and maintaining a pH of 11 for detecting H2S. As will be described in more detail later in this specification, the diamond electrochemical sensor can be configured such that a limit of detection for dissolved hydrogen sulphide is lower than 100 ppb, 50 ppb, 20 ppb, or 10 ppb in a solution having an intrinsic pH of between 6 and 7 (i.e. a pH between 6 and 7 prior to application of an in situ pH change via the pH changing electrode). Such functionality can be achieved through the selection of a suitable electrode geometry, diamond material quality, system configuration, and electrode driving methodology.

In order to generate a target pH locally over the sensing electrode, the sensor may be calibrated using bulk pH buffered solutions over the working range of the sensor and an electrochemical system with a known response to pH changes, e.g an iridium oxide film. Alternatively, the sensor may be configured in the same manner as a sensor calibrated in such a manner, In either case, local pH changes over the sensing electrode are calibrated, directly or indirectly, versus an electrochemical response of an electrochemical system in bulk pH buffered solutions over a working pH range of the sensor.

In addition, when using a sensor as described herein it has been noted that there is a time lag between driving the pH changing electrode and a steady state pH being achieved over the sensing electrode, In certain applications where fast measurement results are required this time lag is not desirable, Accordingly, to shorten the amount of time to achieve a steady state pH over the sensing electrode it can be advantageous to provide an initial drive pulse to the pH changing electrode which is larger than that required for achieve a steady state target pH in order to more quickly drive the local pH over the sensing electrode towards the target value, The initial "over-drive" pulse can then be ramped back to a drive pulse suitable to hold the local pH over the sensing electrode at the target value, Prior to looking at specific electrode geometries, pulse methodologies, and applications, a section is provided below on preferred boron-doped diamond material properties and synthesis methodology for achieving such material properties, Diamond Material and Synthesis Thereof Certain embodiments utilize boron-doped diamond electrodes fabricated from a boron doped synthetic diamond material which has the folIong characteristics: a solvent window meeting one or both of the following criteria as measured by sweeping a potential of the boron doped synthetic diamond material with respect to a saturated calomel reference electrode in a solution containing only deionised water and 0.1 M KNO3 as a supporting electrolyte at p1-I 6: the solvent window extends over a potential range of at least 4.1 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 38 mA cm2; and the solvent window extends over a potential range of at least 3.3 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 0.4 mA cm2; a peak-to-peak separation AE (for a macroelectrode) or a quartile potential AE3/4]/4 (for a microelectrode) of no more than 70 my as measured by sweeping a potential of the boron doped synthetic diamond material at a rate of 100 my s' with respect to a saturated calomel reference electrode in a solution containing only deionised water, 0.1 NI KNO3 supporting electrolyte, and 1 mM of FcTMK or Ru(N1-1s)6' at p1-I 6; arid a capacitance of no more than 10 j.tF cm2 as measured by sweeping a potential of the boron doped synthetic diamond material with respect to a saturated calomel reference electrode between 70 mY and -70 mY in a solution containing only deionised water and 0.1 NI KNO2 supporting electrolyte at pH 6, measuring resultant current, subtracting a current value at 0 V when sweeping towards negative potentials from a current value at 0 V when sweeping towards positive potentials, dividing the subtracted current value by 2, and then dividing the result by an area (cm2) of the boron doped synthetic diamond material and by a rate at which the potential is swept (Vs') to give a value for capacitance in F cm2, Boron-doped diamond materials meeting the aforementioned definition are described in PCT/EP2013/035170 (published as W02013/135783). Such synthetic diamond materials include both polycrvstalline (pBDD) and single crystal (5cBDD) boron doped synthetic diamond materials which have been optimized for their electrochemical sensing performance. Such materials are ideal for implementing examples of the present invention as the boron-doped diamond materials can be driven to extreme potentials to change the in situ pH of a solution without substantial gas bubble formation and without causing adverse electrochemical reactions which interfere with an electrochemical sensing measurement. in I-,

For boron doped diamond materials, a low boron dopant content can aid in providing a large solvent window, flat electrochemical response, and low capacitance as desired.

However, such material will not show metallic like properties resulting in non-reversible electrochemical characteristics for simple fast electron transfer outer sphere redox couples in the both positive and negative potential windows awl is thus not desirable for electrochemical sensing applications. Increasing the boron dopant content significantly will cause the solvent window to shrink and the capacitance to increase, which is undesirable for the aforementioned reasons. As such, it has been found that an optimum range of boron concentration exists which balances the requirement of reversible electrochemistry for simple fast electron transfer outer sphere redox couples versus the desirable characteristics of a large solvent window, a flat electrochemical response, and a low capacitance.

In addition to the above, it has been found that sp2 carbon content within the boron doped diamond material is undesirable as this also tends to shrink the solvent window, increase capacitance, and increase non-uniformities in the electrochemical response of the electrode material, If the boron dopant content becomes too high then it is more difficult to control the presence of non-diamond carbon, e.g. sp2 carbon, providing an additive detrimental effect on the performance of the electrode material in terms of providing a wide, flat baseline for species detection.

The present inventors have developed p6lycrvstalline CVD synthetic diamond materials and single crystal CVD diamond materials which have optimized boron concentrations and substantially no sp2 carbon (as detectable via Raman spectroscopy).

The boron doped synthetic diamond materials have been defined above in terms of their functional electrochemical properties as this is the most convenient, clear, and concise way to characterize the materials. In practice, the present inventors have fabricated a number of different types of boron doped synthetic diamond materials which ifilfill these functional electrochemical properties, The materials may be categorized into three main types: bulk boron doped single crystal synthetic diamond materials which comprise a suitable boron dopant content and crystallographic quality to achieve the previously described functional electrochemi cal properties throughout a majority volume of the single crystal synthetic diamond materials; capped boron doped single crystal synthetic diamond materials which comprise a capping layer having a suitable boron dopant content and crystallographic quality to achieve the previously described functional electrochemical properties and a support layer having a lower boron content; and boron doped polycrystalline synthetic diamond materials which comprise a plurality of boron doped synthetic diamond grains with a sufficient portion of the grains at an exposed surface of the material having a suitable boron dopant content, while maintaining phase purity (i.e. substantially no sp2 carbon content), to achieve the previously described functional electrochemical properties, Whichever of the aforementioned types of material is provided, it has been found to be advantageous to fabricate a boron doped synthetic diamond material in which at least a portion of an exposed surface layer comprises boron doped synthetic diamond material having a boron content in a range 1 x 1020 boron atoms cm'3 to 7 x 1021 boron atoms cm'3, Preferably at least 50%, 70%, 90?/, or 95% of the exposed surface layer comprises boron doped synthetic diamond material having a boron content in a range I x boron atoms cm'3 to 7 x 1021 boron atoms cm'3.

It has been found that boron doped synthetic diamond material can be fabricated with a boron content over I x 1022 boron atoms cm'3. However, while such material can provide a low AE it possesses a relatively high capacitance (e.g. greater than 10 jiF cm'2). Furthermore, as the boron content is increased the sp2 carbon content and crystallographic defects tend to increase which also detrimentally increases the capacitance of the material in addition to lowering the solvent window. Accordingly, it has been found that a suitable upper limit for boron concentration is 7 x 1021 boron atoms cm'3 when taking all these factors into account, Conversely, if the boron content is lowered below 1 x 1020 boron atoms cm'3 the material is found to have a large solvent window (up to 8 V if the boron content is sufficiently low that the material exhibits p-type semi-conductive behavior) and a low capacitance with a flat electrochemical response. However, such material possesses a large AE (over 70 mV and even up to several hundred mV). As such, it has been found to be advantageous to select a boron content falling within the range t x 1020 boron atoms cm3 to 7 x 1021 boron atoms cm* In addition to controlling boron dopant content, in order to achieve material having optimized electrochemical characteristics it has been found to be important to minimize the formation of sp2 carbon during growth of the boron doped synthetic diamond material. Raman spectroscopy has been found to be a particularly useful technique for measuring sp2 carbon content. Non-diamond carbon peaks include: 1580 cm' -graphite; 1350-1580 cm' -nanocrysallite graphite; and 1550-1500 cm' -amorphous carbon and graphitic phases. It has been found that if sp2 carbon is evident in a Raman spectmm of a material then the material will have a smaller solvent window, a higher capacitance, and surface oxidation/reduction features.

Accordingly, preferably the sp2 carbon content is sufficiently low as to not exhibit non-diamond carbon peaks in a Raman spectrum of the material. An sp2 carbon signature in Raman spectra has been correlated with higher capacitance, non-Faradaic surface processes, and a reduced solvent window, Micro-Raman spectroscopy can be performed at room temperature with a Renishaw inVia Raman microscope using an excitation wavelength of 514.5 nm, an Ar laser with a power of 10 mW, and a CCD detector. Magnification for Raman spectroscopy can be x5, xlO, x20, x50 and xlOO objectives at visible and near infrared (NIR) frequencies and x5 arid x20 at ultraviolet (liv) frequencies. With xSO magnification the xy-spot size (and hence resolution) is approximately 5x5 microns and with xtOO the xy-spot size is approximately 2x2 microns. Typical values for magnification obj ectives in air thus range from x5 to xlOO.

In addition to the boron and sp2 carbon compositional requirements discussed above, it has also been found to be desirable to fabricate material which comprises little or no crystallographic defects observable by DIC (Normaski) visible microscopy at a magnification of up to xlOO. For example, samples of boron doped single crystal synthetic diamond material have been found to exhibit small rod-like features, extended polycrystalline inclusions, which are visible using microscope imaging.

Such defects can result in variable electrochemical behavior and are thus considered undesirable. As such, it is considered desirable to minimize such defects either by controlled growth or by processing material by selecting areas of material which are substantially free of such defects to form electrochemical electrodes. As such, advantageously an exposed surface of the single crystal boron doped synthetic diamond material may comprise no more than 5%, 3%, 1%, 0.5%, or 0.3% by area of crystallographic defects observable by visible microscopy at a magnification of up to x]00.

Materials as described above have been fabricated using a microwave plasma activated chemical vapour deposition (CYD) synthesis process. A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave, typical frequencies used for this heating application include 2.45 GHz and approximately 900 MHz depending on the RF spectrum allocation of each country. In this work the example conditions are given for a system equipped with a 2.45 GHz microwave source. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur. If a source of boron such as diborane gas is introduced into the synthesis atmosphere then boron doped synthetic diamond material can be grown. Single crystal synthetic diamond materials are typically fabricated via homoepitaxial growth on single crystal diamond substrates. In contrast, polycrystalline synthetic diamond wafers can be grown on silicon or refractory metal substrates.

Important growth parameters include the microwave power density introduced into the plasma chamber (typically ranging from less than or equal to I kW to 5 kW or more for a substrate area < 20 cm2), the pressure within the plasma chamber (typically ranging from less than or equal to 50 Torr (i.e. 667 kPa) to 350 Torr (i.e. 4666 kPa) or more), the gas flow velocity flowing through the plasma chamber (typically ranging from a few tOs of sccm (standard cm3 per minute) up to hundreds or even thousands of seem), the temperature of the substrate (typically ranging from 700 to 1200°C), and the composition of the synthesis atmosphere (typically comprising ito 20% by volume of carbon containing gas (usually methane) with the remainder of the synthesis atmosphere been made up of hydrogen). For boron doping the synthesis atmosphere will typically comprise a boron containing gas such as diborane at a concentration from equal to or less than 0,01% up to several % by volume.

The problem to be solved is what growth parameters to select in order to fabricate synthetic boron doped diamond materials with optimized electrochemical sensing properties. Suitable growth parameters for both single crystal and polycrystalline diamond materials are discussed below.

Single Crystal Boron Doped Diamond Materials As previously described, a boron dopant concentration in a range i x 1020 boron atoms cm -to 7 x tO boron atoms cm -has been found to be desirable to achieve high performance synthetic diamond material for electrochemical sensing applications, However, it has also been found that electrochemical performance of single crystal boron doped diamond materials can be affected by the presence of crystallographic defect features which are observable by visible microscopy at a magnification of up to xlOO. It has been found that growing single crystal CVD synthetic diamond material at higher power and pressure (e.g. 250 Ton (i.e. 33.33 kPa); 5.0 kW at an operating frequency of 2.45 GHz with a 5 cm diameter carrier substrate area) produces better crystal quality material but at high powers and pressures the uptake of boron dopant is reduced such that the required levels cannot be achieved. Conversely, if the power and pressure are reduced (e.g. 100 Ton (i.e. 13.33 kPa); 2kW) then boron uptake is increased to the desired level but the crystal quality of the material is reduced such that the desired electrochemical parameters are not achieved. It has been found that there is a narrow operating window within which the power and pressure are sufficiently high to achieve the required crystal quality and sufficiently low to achieve the desired level of boron dopmt uptake, Preferably the pressure is controlled to lie in a range U0 Torr to 60 Torr (i.e. 16.00 kPa to 21.33 kPa), more preferably in a range 130 Ton to 150 Torr (i.e. 17.33 kPa to 20.00 kPa), and most preferably around 140 Torr (ie. 18.67 kPa). In addition, preferably the power is controlled to lie in a range 3.1 kW to 3.9kw, more preferably in a range 3.3 kW to 3.8kW, and most preferably around 3.6 kW. The temperature of the substrate may be controlled to lie in a range 750°C to 850°C, In addition to the above, boron incorporation has been found to be increased by manipulating the gas flow firstly by using a reactor configured with a co-axial gas injection system, for example comprising a nozzle positioned between 50 mm and mm above the substrate to direct gas towards the substrate, and secondly by increasing the gas velocity via a combination of the total gas flow rate and the chamber gas injection nozzle diameter, Using an axial gas injection nozzle with a diameter of 2 mm, positioned 75 mm above the substrate, the total gas flow rate may be at least 500 sccm, more preferably at least 600 sccm, and most preferably over 650 sccm. For example, a hydrogen gas flow of between 500 and 700 sccm may be utilized with a methane gas flow of between 25 and 40 sccm and a diborane gas flow between 15 and 30 sccm. Argon gas may also be introduced into the synthesis atmosphere, for example at a flow rate in a range 20 to 30 sccm, Only by providing these growth parameters in combination has it been found to be possible to achieve single crystal diamond growth which meets both the boron content and crystallographic quality requirements which result in a material having the electrochemical performance characteristics as described herein, It has also been found that providing a shallow miss-cut angle on the single crystal diamond substrates relative to a crystallographic plane can aid in promoting step-flow growth leading to higher crystallographic quality material for a given power and pressure.

Po/ycrycta/Jine Boron Doped Diamond kiaterials Similar comments as those set out above for single crystal boron doped diamond also apply for polycrystalline boron doped diamond material. Having regard to polycrystalline material, the problem is how to achieve the high levels of boron doping while avoiding incorporation of sp2 carbon during growth. This has been achieved by controlling substrate temperature in a range 1050 to 1120°C, using a synthesis atmosphere which has a relatively low concentration of carbon containing gas (e.g. in a range 1% to 3% of total gas flow), a high power density (e.g. 5 to 6 RW over a 50 mm diameter substrate) in combination with a relatively high reactor pressure (e.g. in the range 200 to 300 Torr (i.e. 26.66 kPa to 40.00 kPa)) using a high gas flow configuration and total flow rate as previously described for single crystal growth, The present invention will now be illustrated by way of two different examples: (i) in situ control of local pH using a boron doped diamond ring disc electrode configuration for detecting extremely low concentrations of mercury; and (ii) in situ control of local pH using boron doped diamond band electrodes in a channel flow system for detecting extremely low concentrations of hydrogen sulphide.

(i In situ control of local oil using a boron doped diamond ring disc electrode configuration for detecting extremely low concentrations of mercury In this study a novel approach to generating a localised in situ pH change was investigated for heavy metal stripping analysis. A boron doped diamond ring disc electrode was developed, and the electrochemical decomposition of water on the ring electrode used to generate protons and adjust the pH locally to the disc, The system was used to investigate mercury deposition/stripping measurements with cyclic voltammetry, which are improved under acidic pH conditions. An iridium oxide film electrodeposited on the disc electrode was used to characterise pH change, Our results showed that, through generating a local pH change, the stripping peak for mercury in a pH 7.0 solution was transformed to resemble that seen in a pH 2.0 solution. With the ability to create a localised pH change during experiments this system could provide a simple in situ method of heavy metal analysis, avoiding the need to remove samples from the source to acidify for later analysis.

pH plays a critical role in many important processes ranging from maintaining biological homeostasis [Cockerill, C, R,, S. Essential Eluicl Electrolyte and p1-f Horneostasis, Wiley-Blackwell 2W] to environmental pollution control [Gundersen, P.Steinnes, E. Water Research, 2003, 37, 307-3 18]. Often the system under investigation is investigated in vitro where the optimal p11 conditions are re-created in the laboratory environment. In the electroanalysis arena, H ions, and therefore pH, play a major role in promoting many different reactions, often by acting as a catalyst [Brett, A. M. OGhica, M.-E. Electroanalysis, 2003, 15, 1745-1750] or by simply retarding competing side reactions [Nematollahi, D.Golabi, S. M. Electroanalysis, 2001, 13, 1008-1015]. One particularly important area of research is the use of electroanalysis for heavy metal detection via electrodeposition and subsequent stripping [Zeng, A. ;Liu, E. ;Tan, S. N. ;Zhang, S.Gao, J. Electroanalysis, 2002, 14, 1294-1298]. In this process pH plays a significant role, with the stripping response found to be significantly improved at low pH (typically p1-1 <3). This is thought to be due to the prevention of metal hydroxide formation [Reeder, G. S.Heineman, W. R. Senc. Aclualors, B, 1998, 52, 58-64]. In the laboratory, analysis is undertaken oflen by removing and acidifying water from the source of interest in order to make the measurement. However, as there is significant interest in deployment of electrodes in the water source of interest, e.g. river, sea, where the pH is often near neutral, pH 6 -9 in rivers [UK L},vironn;entai Standards and Conditions (Phase 1), Water Framework Directive. 2008], and pH 7.5 -8.5 in sea water [Ocean acidification due to increasing atmospheric carbon dioxide, The Royal Society. 2005], this can significantly complicate the measurement. In this technical note we describe an approach which enables the local pH of the measurement environment to be precisely controlled electrochemically whilst simultaneously carrying out electroanalysis under optimal pH conditions, It is well known that the electrochemical decomposition of water at sufficiently high oxidising potentials leads to the formation of protons (H), as shown in reaction scheme I [Michaud, P. A. ;Panizza, M. ;Ouattara, L. ;Diaco, T. ;Foti, G.Comninellis, C. I. App!. Electrochen,., 2003, 33, tst-t54 and Palmas, S.; Polcaro, A. M.;Vacca, A. ;Mascia, M.Ferrara, F. J Appi Electrochein., 2007, 37, 1357-1365].

2H2O-O2+4W+46 (1) The local electrogeneration of H to influence an (electro)chemical process of interest is rarely described in the literature. Reports in the scanning probe field describe a probe electrode (ultramicroelectrode or metal coated atomic force microscopy tip), placed close to a mineral crystal in aqueous solution, was used to locally generate different fluxes of H from water oxidation, to initiate different rates of dissolution of the surface McGeouch, C.-A.; Edwards, M. A.; Mbogoro, M. M.; Parkinson, C.Unwin, P. R. Anal Chem., 2010, 82, 9322-9328 and Jones, C. B.; Unwin, P. R.; Macpherson, J. V. ChemPhysChern, 2003, 4, 139-146].

In order to locally generate an acidic environment electrochemically to promote efficient electroanalysis, a convenient approach is to use a dual electrode system, also referred to as generation/collection (GC). GC electrodes come in various geometries, including dual bands [Fosset, B.; Amatore, C.; Bartelt, J.; Wightman, R. M. Anal Chem., 1991, 63, t403-1408 and Fosset, B.; Amatore, C. A.; Bartelt, J. E.; Michael, A. C.; Wightman, IC M. Anal Chern., 1991, 63, 306-314], ring discs [Liljeroth, P.; Johans, C.; Slevin, C. J.; Quinn, B. M.; Kontturi, K. Electrochem. (ommwi,, 2002, 4, 67-71 and Barnes, E. 0.; Lewis, U. E. M.; Dale, S. E. C.; Marken, F.; Compton, R. U. Analyst, 2012, 13, 1068-1081] and dual microdisc [Cutress, I. J.; Wang, Y.; Limon-Petersen, J, U,; Dale, S. B. C.; Rassaei, L.; Marken, F.; Compton, R. U. J Electroanal (hem., 2011, 655, 147-153] electrodes and have been used widely in electroanalysis, for example to investigate analyte detection in the presence of interfering species [Dale, S. E. C.; Vuorema, A.; Ashmore, E. lvi, Y.; Kasprzyk-Horden, B.; Sillanpaa, M.; Denuault, G,Marken, F. The Chemical Recorcl 2012, 12, 143-1481 dissolution processes [Sasaki, H.; Miyake, M.; Maeda, M. .1 Electrochem.

Soc., 2010, 157, E82-E87], and the role of different species in electrochemical reactions Damjanovic, A.; Genshaw, lvi,; Bockris, 1. 0. M. I. Electrochem. Soc., 1967, 114, 466-472]. OC electrodes can also offer high sensitivity, enabling trace level detection [Aoki, K. ;Morita, TV!, ;Niwa, O,Tabei, H. I. Electroanal Cham, 1988, 256, 269-282].

Mercury (Hg) is a highly toxic heavy metal contaminant from both industrial and natural sources, and hence detection and quantification in aquatic environments is essential [Kudo, A.; Miyahara, S. Water Sc!. Technol., 1991, 23, 283-290]. In stripping analysis, due to legislation Hg can no longer be employed as the electrode material of choice and is anyway, unsuitable for Hg22(aq) detection. As an alternative, boron-doped diamond (BDD) has attracted much attention [Babyak, C., Smart, R. B. Electroanalysis, 2004, 16, 175-182; Dragoe, D., Spataru, N., Kawasaki, R., Manivannan, A., Spãtam, T., Tryk, D. A., Fujishima, A. .Eiectrochim. Ada, 2006, 51, 2437-2441; McGaw, E. A., Swain, G. M. Anal Chin,. Ada, 2006, 575, 180-189; Manivannan, A., Tryk, D. A., Fujishima, A. Electrochem. Solid-State Lett., 1999, 2, 455-456] due to its wide solvent window, low background currents and corrosion resistance [Luong, J. I-I. T.; Male, K. B.; Glennon, J. D. Analyst, 2009, 134, 965- 1979 and ICraft, A. Jut. .1 Electrochem. Xci, 2007, 2, 355-385]. Traditionally, Hg electroanalysis, as with most heavy metals, is carried out under acidic conditions [Kumar Jena, B., Retna Raj, C. Anal. Chem., 2008, 80, 4836-4844 and Manivannan, A., Seehra, M. S., Tryk, D. A., Fujishima, A. Anal. Lett., 2002, 35, 355-368], in order to improve the voltammetric stripping response. However, this prevents the use of electrodes directly at the source e.g. river and sea water, Herein we show that, by employing a ring disc BDD electrode, it is possible to optimise detection sensitivity for Hg22 in solutions where the bulk pH is close to neutral. This is achieved by generating acidic H fluxes at the ring (generator) electrode during electrochemical deposition and stripping at the disc (collector). The successful application of this approach to Hg measurement bodes extremely well for the in situ electroanalysis of Hg and other heavy metals at the source.

Experimental Solutions: All solutions were prepared using Milli-Q water (resistivity 18.2 MQ cm at °C) and all reagents were used as received unless otherwise stated. The supporting electrolyte employed was 0. 1 M KNO3 unless otherwise stated. Electrochemical characterisation was carried out using 1 mM Ru(NH3)63 (98% hexamine-ruthenium (III) chloride, Aldrich), 1 nilvI FcTMA (synthesised in house), 0.1 M H2S04 (>95%, Fisher Chemical) and KNO3 (99%, Aldrich). Known pH calibration solutions were made up from FIXANAL buffer concentrates (Sigma-Aldrich). Hg solutions were prepared from Hg(N03)vH2O (Merck), and pH adjusted where necessary with 1 M HNO2 (70%, Fisher Chemical) and 1 M KOH (Fisher Chemical). The iridium oxide solution used was prepared as described in the literature [Yamanaka, K. Jpn. J App. Phyc., 1989, 623 and Yamanaka, K. Jpn. J App. Phys, 1991, 12851; 4.45 mM iridium tetrachloride, 1 mL H202 (30% wlw) and 39 mM oxalic acid dihydrate were added sequentially to 100 mL water and stirred for 30 mm, 10 mm, and 10 mm fin Li intervals respectively. Anhydrous potassium carbonate was added until a pH of 10.5 was achieved resulting in a pale yellow-green solution. This was stirred for 48 h until the solution had stabilized and appeared blue in colour. The iridium oxide solution was refrigerated between uses.

BDD electrode fabrication: High quality (substantially sp2 free) conducting polycrystalline BDD [Patten, H. V.; Meadows, K. B.; Hutton, L. A.; lacobini, J. G.; Battistel, D.; McKelvey, K.; Colbum, A. W.; Newton, M. E.; Macpherson, J. V.; lJnwin, P. R. Angew Chern., hit Ed, 2012, 51, 7002-7006], thickness 600 jim, supplied by Element Six Ltd (Ascot: DIAFILM ES grade) was laser machined into columns (1 mm diameter) and rings (outer diameter (OD) = t680 pm, ± 2.5 jim; inner diameter (ID) = 1270 jim, ± 2,5 jim) and acid cleaned in concentrated H2S04 (>cS%, Fisher Chemical) saturated with KNO3 on a hot plate for 30 mm, The dimensions of the ring-disc electrode were measured using an optical microscope (Axio Imager, Zeiss). Ideally, the ring-disc separation would be as small as possible. However, in practice a separation was provided to ensure that the electrodes were electrically isolated despite any small errors in the manual assembly method and accounting for the resolution of the laser cut (= 30 jim). If this fabrication processing is optimized then a smaller electrode spacing should be achievable while maintaining electrical isolation. Ti/Au electrical contacts 10 nm and 200 nm thick respectively were sputtered onto the back of the disk and annealed at 500 °C for 4 h, whilst the backside of the ring was graphitized using the laser, which provides a convenient and sufficient ohmic contact [Geis, M.; Rothschild, M.; Kunz, R.; Aggarwal, R.; Wall, K,; Parker, C.; McIntosh, K.; Efremow, N.; Zayhowski, J.Ehrlich, D. Applied physics letters, 1989, 55, 2295-2297], The pBDD ring was placed as centrally as possible around the pBDD column, in a Teflon mould and sealed in place with a thin layer of epoxy resin (Robnor Resins), Copper wires were connected via conductive silver epoxy (Circuitworks, ITW Chemtronics) to the Ti/Au contact on the disc and the graphitised region of the ring. Once the epoxy had set the electrodes were removed from the mould and polished back with silicon carbide pads (Buehler) of decreasing roughness until the surface of the ring disc was exposed, Finally an alumina polish (0.05 pm micropolish Buehler) was applied to provide an optimum electrode surface. For comparison, macro electrodes were prepared from 1 mm diameter laser micromachined pBDD columns sealed in glass capillaries [Hutton, L. A.; Newton, M. E.; Unwin, P. R.; Macpherson, S. V. Anal (]hem., 2008, 81, 1023-1032].

In order to characterise in si/it pH generation at the ring electrode, a pH sensitive iridium oxide film was employed. The film was electrodeposited on the pBDD disc using a three electrode system with a non-leak AgAgCl reference electrode and a platinum counter. Deposition was carried out using a potentiostat (CFH73OA, CII Instruments Inc.) connected to a desktop computer; a potential of 0.7 V was applied for a total of 900 s. Cyclic voltammograms (CV) were run in iridium oxide solution and H2S04 before and after deposition to confirm film formation and elucidate deposition potential. The pH response of the film is reliant on hydration [Bitziou, E.; O'Hare, D,; Patel, B. A. Anal C/win., 2008, 80, 8733-8740], so electrodes were stored in pH 7 phosphate buffer solution and left to hydrate for two days before use. The film's response to pH was characterised via calibration in known pH buffer solutions using a two electrode system. The open circuit potential OCP) was measured for 30 s in each buffer in order of decreasing acidity; this was reversed then repeated to obtain at least 3 measurements at each pH.

pH get era/ion: H generation experiments on the ring electrode were conducted using an IVIUM bipotentiostat with a Peripheral Differential Amplifier (PDA) attachment which allows the simultaneous recording of the potential at the disc electrode, A constant current in the range, 0 -100 MA, was applied to the ring for 60 s in an unbuffered pH 6.4 KNO3 solution to generate different local W concentrations which were measured at the iridium oxide coated disc electrode, For simultaneous amperometric Hg detection and galvanostatic H generation experiments, the generating current was applied using a Keithley Source Meter.

Iviercuty Detect/n;;: Initial experiments were conducted using 1 mM Hg21 solutions in 0.1 M KNO3 supporting electrolyte with the solution adjusted to a defined pH, in the range 2-7 (measured using a pH probe (SevenEasy S20, Mettler Toledo)) by dropwise addition of] M HNO or KOH. For ring disc experiments, in order to allow time for HE diffusion from the ring to the disc, there was a wait time of 60 s between the start of HE generation at the ring and the commencement of CV at the disc. CVs were recorded between +1,5 V and -1.2 V at 0.1 Vs". An in situ cleaning step to remove any unstripped Hg from the surface of the electrode involved holding the disc electrode at a potential of +2 V versus AgIAgCI for 400 s between each CV. This was found to be sufficient to return the electrode response in a solution of supporting electrolyte (KNO3) to its pre-deposition state.

Comsol modelling: A model of proton generation and diffusion was created using COMSOL Multiphysics 4.3a (COMSOL, SE) finite element analysis (FEA) software, as described later.

Results and Disc ussion Effect of p11 on mercury s/ripping: Figure 1 shows a comparison of the Hg stripping peaks recorded at a 1 mm diameter BDD disc electrode with the solution adjusted to differing ph values of 2, 5 and 7. The solution contained 1 mM Hg(N03)2H20 and the data shows only the oxidative part of the CV. Full CVs were recorded at a scan rate of 0.1 V as shown in Figure 1 inset, for a pH 2 solution, where Hg can be seen to electrodeposit on the electrode at around -0 V versus AgAgCl on the cathodic scan. Going more negative, the wave at -0.4 V is likely to be due to the reduction of oxygen at the electrodeposited Hg, as sp2 free BDD does not electrocatalyse oxygen under these pH conditions, On the anodic scan there is a clear stripping peak at +0.5 V. As the pH is increased from 2 to 5 and then to 7 the stripping peak shifts significantly more negative to 0.2 V and becomes much broader, with the peak current decreasing significantly in magnitude, complicating quantitative electroanalysis.

These effects are attributed to the interaction of Off with Hg ions in less acidic solutions, as pH is increased the Hg (II) solution begins to undergo hydrolysis before forming a hydroxide precipitate in neutral and alkaline conditions [Zen, i-Ni.; Chung, M.-J. AnaL (hem., 1995, 67, 3571-3577 and Wei, Y.; Gao, C.; Meng, F.-L.; Li, H.-H.; Wang, L.; Liu, J.-H.; Huang, X.-J. I. Phys. (hem. C, 2011, 116, 1034-1041].

hi situ ApH generation: The generation of fl using the ring electrode in order to modify the local ph environment of the disc electrode was characterised using a ring -pH sensitive disc arrangement. An optical microscopy image of the BDD ring disc electrode is shown iu Figure 2(a). This electrode arrangement was used for all studies and contained a disc of diameter 922.5 pm, surrounded by a ring of outer and inner diameter of 1744.5 pm and 1444.5 pm respectively, sealed in non-conductive epoxy.

The electrodes were filly characterised in Ru(NH5)63'2 prior to use, as described later.

In order to confirm successful film deposition, a CV in H2S04 was conducted and is shown in the Figure 2(b) inset. The shape of this CV arises from the reversible Ir(IIJIIV) redox process [Johnson, C. S., Hupp, J, T. J.Eieclroana/. C/win., 1993, 3-15, 35h362], and is characteristic of an iridium oxide film on BDD. The pH response of the iridium oxide coated BDD disc electrode was characterised by placing the ring-disc electrode into pH buffered solutions, from pH 2 to pH ii. Figure 2(b) shows the calibration curve of pH versus OCP. The calibration plot shows a linear pH response with a super nernstian slope (-68 mV/pH ±0.82, F° = 0.64 V, R2 = 0.99928), which is in agreement with what has been seen in literature, and is due to the complex equilibrium reaction characteristic of iridium oxide films [Wipf D. 0.; Ge, F.; Spaine, T. W.; Baur, J. E. AnaL C/win., 2000, 72, 4921-4927]. E° describes the OCP at pH 0, which is dependent on film thickness [Terashima, C.; Rao, T. N.; Sarada, B. V.; Spataru, N.Fujishima, A. .1 Electroanat 0/jern., 2003, 544, 65-74 and Elsen, H. A.; Monson, C. F.; Majda, M, I. Electrochern. Soc., 2009, 156, F1-F6], The film calibration remained stable throughout experimentation over a period of approximately two months and this compares well to the film stabilities reported in literature, values of which are reported ranging from two weeks [Salimi, A.; Hyde, M. E.; Banks, C. E,; Compton, R. G, Ana4v,s], 2004, 129, 9-14 and Salimi, A.; Alizadeh, V.; Hallaj, R, JiIanta, 2006, 68, t610-1616] to fifty days.

The OCP, and hence PH, at the iridium oxide modified disc was measured for different applied ring currents in the range 0 -50 pA (galvanostatic) over 600 s, as shown in Figure 2(d) (solid lines), alongside the Finite Element Model (FEM) simulated data (dashed lines). This time period was employed to ensure local pH generation remained stable throughout typical electrodeposition times employed during heavy metal analysis. The data clearly shows that upon application of a positive current at the ring the measured pH on the disc decreases on a timescale of typically 120 -60 s (10 -50 pA), before reaching an approximate steady state plateau.

This reflects the time taken for protons to transit from the ring and flood the disc, and also the response time of the iridium oxide film, As the current is increased, the transition to steady state occurs faster and the pH decreases further, due to the higher flux of protons generated at more positive currents. After reaching steady-state the local pH recorded at the disc electrode appears to remain relatively stable over the entire 600 s, which bodes well for application over the time scale required for electroanalysis.

Importantly, there is also good agreement between the simulated pH-time profiles arid the experimentally recorded data. The discrepancy between the data and simulation may be due to: (i) the model assumes a symmetrical ring-disc electrode geometry, which as Figure 2 (a) shows, is not true; (ii) the model doesn't take into account natural convection, this would effectively absorb protons and thus raise the pH; arid/or (iii) the response time of the iridium oxide sensor is not instantaneous, as noted by others, who account for it as being caused by the intrinsic capacitance of the material.

Importantly, the data shows clearly that for a ring disc electrode of the dimensions reported herein, it is possible to locally modify the proton concentration in the vicinity of the disc by up to five orders of magnitude, from pH 6.4 to pH 2 (at the highest applied current). Under these conditions the electrode is being driven at a high potential for very long periods of time, which for commonly used noble metal electrodes (Pt, Au) would cause the formation of metal oxides and passivate the surface. Due to its intrinsically inert nature, BDD provides a reproducible flux of protons over long time scales without the need for electrode activation [Chaston, J. C. Plalinuni Met. Rev., 1964, 8, 14 1-142]. The lower pH value generated is sufficient to produce the efficient electrodeposition and stripping seen in acidic bulk solution.

Simultaneous ApH generation and mercuty detection: Figure 3 shows the resulting CV recorded for electrodeposition and stripping of Hg for the disc electrode, at a potential scan rate of 0.1 Vs' in a solution containing mM I-Tg(N03)2'H20 and 0.1 M KNO5, pH = 7.0, for a ring current of 50 j.tA, equivalent to generating p112 locally at the disc, Also shown is the deposition and stripping data recorded in a solution deliberately acidified to p1-I 2.0, and the native solution (pH 7.0). Figure 3 clearly shows that by in situ controlling the pH environment of the disc electrode it is possible to obtain deposition and stripping data which very closely resembles that of the bulk acidified solution, This data clearly demonstrates that it is possible to directly electrochemically detect Hg in non-acidic bulk solutions, simply by locally generating acidic conditions.

Conclusion

Figure 4 summarizes the work described above. Proton generation through the decomposition of water on the ring of a ring disc electrode allowed pH to be controlled locally at the electrode surface; this enables experiments to be conducted at the optimum pH for the analyte, without needing to manually acidify the solution. The generated pH was found to remain stable over time after an initial equilibration period and showed good agreement with simulated data. In situ Hg detection with no need for sample pre-treatment was made possible using a pBDD ring-disc electrode. It has been shown that using this method a localised pH can be generated in situ, eliminating the need to remove a sample from its source for analysis, the pH generated remains sufficiently stable long enough for analytical experiments to be conducted on the disc electrode.

A move to all diamond devices would improve the lifetime and durability of the electrodes, allowing them to be used in even the harshest environments without suffering corrosion or degradation. It would also improve the reproducibility of the fabrication procedure, ensuring co-planar electrodes and a centrally aligned disc, Use of this method in a flow system would both allow for detection of much lower analyte concentrations, and shorten the time required for electrodeposition and thus overall analysis.

Electrochemical character/sat/on of 111)1) ring disc electrode Before use, the ring-disc electrode was electrochemically characterised. Figure 5 shows characteristic CVs for a disc and ring electrode (left and right respectively) in 1 mINI Ru(NI13)62'3 (0. t IV1 KNO3) over the scan rate range 20 mVs" to 500 mVs".

The peak to peak separations of both CVs are approximately 70 my at 100 mVs", showing close to reversible diffusion controlled behaviour for both the ring and the disc BDD electrodes.

Simulation fproton generation at the ring disc electrode To quantif,' the pH changes across the detector electrode as the generator electrode produces protons, a finite element model (FEM) was employed.

The model was created using COMSOL Multiphysics 4,3a (COMSOL AB, Sweden) finite element analysis (FEA) software. The model consisted of a 2D axisymmetric section, with coordinates r and z corresponding to a water cylinder on top of a ring disc arrangement of electrodes in an inert surface. The water cylinder was defined by wr and h = 50 mm. Figure 6 shows a schematic of the simulated domain. The ring and disc were defined by subdividing this boundary into sub-boundaries Ia, lb and Ic. Ia was defined the part boundary i from r = 0 to r = 0.46 125 mm. Ic was defined as the part of boundary I between r = 0.72225 mm and r = 0.87225 mm. Boundary b was defined as the remaining part of boundary 1.

The size of the box is chosen such that the diffusion of the generated protons does not reach the edge in the time scale of the simulation. Ia corresponds to the Iridium Oxide coated detector disc electrode and is functionally inert (Ic no flux). lb corresponds to the material that the ring and disc are mounted in and is also functionally inert (ie no flux), ic corresponds to the generator ring electrode and an inward flux is defined across this electrode corresponding to the generation of protons by electrolysis of water. The flux is assumed to be proportional to the current passed through the electrode: = ii A.F where ii is the unit normal vector, N1, is the total inward flux across boundary Ic, Igen is the total current through the electrode, A is the area of the electrode and F is Faraday's constant. Boundaries 3 and 4 were defined as having a fixed proton concentration of M, while boundary 2 was defined by an axisymmetric constraint. The diffusion coefficient of the protons (I)!,) was set to 9.3 1x109 m2 and the initial concentration of protons c1, was set to 10.6.4 lvi, corresponding to the apparent starting p1-1 of the experimental medium.

Diffusion of protons from the generator electrode was modelled according to Fick's Second Law: Bc, =V.W13c$t k where R1, is the total inward flux of protons into the system. Migration due to electric

field and natural convection were not considered.

A mesh was generated where a density of 500 elements per mm were specified for sub-boundaries Ia, lb and Ic; coresponding to an element length of 2 m on these boundaries. The rest of the area was meshed using an advancing front to a maximum element size of 0.5 mm. The total mesh consisted of 461389 elements. The simulation was solved in a time-dependant manner for a total of 600 seconds using the PARDISO solver as implemented in COMSOL.

Table I. Summary of boundary conditions used far the simulation of the p1-1 during the ring generation experiment.

0«=rSw1 I a Detector disc = . n r < Epoxy mount and lb. material w S r«=. z0

Ic Generator ring S = 2 Axis of symmetry a = Vc.n o < 0 r 3 Bulk solution. c = = h 7 = 4 Bulk solution = 0<z<h In table SI, n represents the unit nomial vector and Ni represents the total inward flux across the boundary 1 c, c represents the concentration of the electroactive species in the bulk solution. In the case of this simulation, the set of points Wj, w2 and it's were chosen to reflect the size of the experimental ring-disc system and were set to 0.46125 mm, 0.72225 mm and 0.87225 mm respectively.

(ii) In situ control of local DH using boron doDed diamond band electrodes in a channel flow system for detecting extremely low concentrations of hydrogen sulphide Hydrogen sulphide is a colourless, flammable and toxic gas at room temperature (1 atm) with a distinct rotten egg odour. Upon exposure it is known to act upon the central nervous system causing narcosis with exposure limits of 300 ppm considered instantly hazardous (LD5O = 713 ppm). Detection of gaseous hydrogen sulphide and dissociated sulphide (as HS and 2 depending on the protolytic equilibria) is therefore very important for numerous industries. Oil and natural gas, pulp and paper production, sewage treatment and sulphur production plants suffer from potentially high levels of 1125 that is either occuning naturally or is a by-product of an indusliial process.

There are several analytical methods that can be used for the detection of H2S in water with electrochemical techniques providing an attractive platform for industrial applications where dissolved sulphide can be readily released as gaseous H2S due to its high partition coefficient between liquid and vapour. A commercially available amperometric sensor from Unisense [http://www.unisense.com/H2 S/] uses the electrocatalytic oxidation of ferrocyanide in the presence of sulphide using a platinum working electrode encapsulated in a tip with a semipermeable membrane. In this well established sensing method the non-ionic hydrogen sulphide diflisses through the membrane [Jeroschewski, P.; Steuckart, C.; KuhI, M., An amperometric microsensor thr the determination of H2S in aquatic environments. AnaL Chem. 1996, 68 (24), 4351-4357]. The method is not very sensitive (-1 pA pM4) and does not have a very low detection limit (-4oppm). In this work we investigate the direct oxidation of sulphide at a boron doped diamond (BDD) electrode to achieve a high sensitivity and a lower detection limit.

Polycrystalline boron doped diamond (pBDD) is a robust electrode material that is known to possess an ultra-wide solvent window in aqueous media, low capacitive currents, and high resistance to fouling and corrosion processes. Most of the above attributes depend on the quality of the diamond electrode (sp2-sp3 ratio) and its termination. In this invention we are using a high quality pBDD band structure overgrown on an intrinsic polycrystalline substrate with the aim to detect the dissociated sulphide in solution and optimise the electrode response.

Sulphide exists in water in three soluble forms (I-12S, HS, 52.) and its speciation is pH dependant according to the following protolytic equilibrium reactions: H2S + H20 HS + H30 pKa = 6.9 Eq. (1) HS + 1120 S2 + H30 PICa = 14 Eq. (2) According to equations 1 & 2, at neutral pH 7, the ratio of dissolved H2S and HS is 50/50 and the higher the pH (i.e. more basic) the larger the fraction of the singly protonated HS ion in solution, It has to be noted that sodium suiphide salt (Na2S) was used in these studies instead of H2S gas for reasons of convenience and health and safety. Na2S in water dissociates to 2 which in turn becomes almost entirely protonated to 11S due to equation 2 at pH values less than 12.

The HS ion can be oxidised at an electrode at relatively low overpotentials according to the following reaction, 11S -S(s)1 + 1-f + 2e Eq, (3) which produces elemental sulphur (S(s)). The latter species passivates the electrode surface and blocks further electron transfer unless an electrode cleaning step is applied after the oxidation process, pBDD is the ideal electrode material for achieving a reproducibly clean &ectrode surface due to its low level of catalytic active groups that is resistant to corrosion even at very high polarisation currents, In this ii study a pBDD array of two band electrodes has been used with one of the electrode's acting as the suiphide sensor and the other as an in-situ pH generator (Figure 7a).

This embodiment is based upon the fact that a second electrode can cause a shift in the local pH of the analysed solution in order to achieve an equilibrium shift towards a more favourable species to aid detection. Laminar flow transports the pH gradient over the sensing electrode in a continuous manner therefore maintaining a relatively constant regional pH. Constant potential (potentiostatic) and constant current (galvanostatic) approaches have been adopted and compared in order to recognize the best experimental approach.

Eq)enrneniaf Electrodes: In this study a dual electrode pBDD band structure was employed for the detection of sulphide at different pH values. The two BDD bands were fabricated by laser micromaching an intrinsic optically transparent diamond substrate to form two recessed band structures, Then pBDD was overgrown on top of the structures under controlled conditions using chemical vapour deposition (CVD) and the resulting composite was lapped flat to expose the conducting bands that formed the two electrode bands, The larger band had a width of xg = 460 jim and the second band was = 90 jim wide separated by 200 pm intrinsic substrate (Figure 7b). The latter electrode was used to oxidise the sulphide while the larger electrode was held a constant (-)ve potential or (-)ve current to induce the desirable pH shift, Channel flow set-up: The channel flow cell used herein consisted of a planar surface that contained the pBDD band structure and a one piece flow unit that was positioned on top (Fig. 7b). The flow unit was fabricated using microstereolithography (MSL) and was especially designed to use for flow injection analysis (FTA) [Sansuk, S.; Bitziou, E.; Joseph, lvi. B.; Covington, I. A.; Boutelle, lvi. G.; Unwin, P. R.; Macpherson, S. V., Ultrasensitive Detection of Dopamine Using a Carbon Nanotube Network Microfluidic Flow Electrode, Anal Chern. 2012, 85 (1), 163-169] with a channel width of 3 mm, a length of 6 mm, and a height (2h) of 50 jim. This set-up was used to study the oxidation of sulphide by injecting 50 pL volumes of degassed sulfide containing solutions in a continuous stream of mobile phase (0.1 M KNO3) at a flow rate of LO mL minT The pH and background electrolyte composition of the mobile phase and injected solution were the same.

To complete the electrochemical cell, a platinum counter electrode and a quasi-reference electrode (AgAgCl wire) were positioned downstream of the pBDD electrodes. Electrochemical measurements were cathed out with a portable Ivium potentiostat (CompactStat, Alvatek Ltd, UK) which was operated in an amperometric mode for both the detection of sulphide (Ecietect = + t.2 V vs. AgIAgC1) at x electrode and the in-situ pH generation at Xg.

In order to characterize in-situ pH generation at the x electrode, an iridium oxide film was employed. The film was electrodeposited on the pBDD band while confined in the MSL flow cell and under stationery conditions by applying a potential of 0.7 V (vs. AgAgCl) for a total of 120 s. Afier the film had stabilized for two days the IrOx-pBDD band electrode was calibrated using different pH solutions by recording the open circuit potential (OCP) under flow condition (1 mL min1) mimicking in that way the pH generating conditions, The calibration plot produced a slope of 80.3 + 2 mV p1-f1 and an intercept of 0.56 + 0.01 V (R2 = 0.999) which was later used to estimate the local pH generated when the upstream electrode (xg) operated at a constant potential or constant current mode.

Electrode cleaning: Prior to sulphide detection it was necessary to apply a cleaning step to the sensing electrode because of the sulphur deposition that occurs during the oxidation of sulphide (Eq. 3). Figure 7(c) shows the potentials applied and timescale of the cleaning step (-4 pA pulse cleaning for 20 s) as well as the sensing step that follows at +1,2 V together with the basic pH generating electrode potential (can vary for -1.7 V to -2 V). This cleaning process was previously optimised (not shown) and repeated at each injection of the sulphide solution.

Solutions': The mobile phase that was in continuous flow over the electrodes contained 0.1 [vi KNO1 at pH 3.8 or pH 6.1 or pH 10 (pH adjusted by 0.1 M HNO3 or 0.1 Ni NaOH) depending on the experiment. Stock solutions of 2 mM sodium sulphide (Na2S) were made in dc-aerated solutions of 0.1 M KNO3 at pH 3.8 or pH 6.1 or pH 10. The stock solution was used to make a diluted solution for the flow injection of sulphide. The diluted solutions had the same composition as the mobile phase and stock (degassed 0.1 M KNO3 with certain pH value) and contained 100 jiM suiphide added from the stock (for Figures 9 & 10) or various increasing concentrations (1 -50 pM H2S, Figure 8). The diluted solution was injected in the FIA system four times (n = 4) in order to derive the limits of detection (L,O,D.s; 3 x sb) for the sulphide oxidation using the described setup.

Results and di.stuswion Preliminary measurements for the oxidation of ionic hydrogen suiphide were carried out using a 1 mm diameter pBDD macroelectrode. Cyclic voltammetry identified a single oxidation wave at approximately +1.2 V in a pH 10 solution containing a buffer (Borax) and 0.1 M KNO3 electrolyte. This oxidation peak is attributed to the two electron oxidation of sulphide (HS) to sulphur as described in Eq. 3. When the pH of the sensing solution decreases to more acidic values the oxidation wave diminishes because of the higher fraction of non-ionic H2S according to Eq. 1.

Figure 7(a) shows an FE-SEM image of a 90 pm wide pBDD band which is part of the all-diamond device described in the experimental section. The polycrystalline nature of the BDD is visible and a thorough electrochemical characterisation process indicated that the band electrodes are ifilly insulated by the intrinsic substrate without the presence of holes or defects. The electrode microarray was characterised using microscopy, electrochemistry and Raman and showed that the electrode is completely insulated by intrinsic diamond and relates corresponds to high phase purity pBDD from Element Six Limited. For example the double layer capacitance of the 90 pm and 240 pm bands are 3 pIE cm2 and 5 jiF cm2, respectively, with very wide and featureless solvent window similar to results obtained for a metallic-like bulk material containing 3 x 1020 boron atoms cm.

This dual band all-diamond device was then introduced into a channel flow setup [Sansuk, S.; Bitziou, F,; Joseph, Ni. B.; Covington, J. A.; Boutelle, Ni. C.; Unwin, P. R.; Macpherson, J. V., Ultrasensitive Detection of Dopamine Using a Carbon Nanotube Network Microfluidic Flow Electrode. AnaL (hem. 2012, 85 (1), 163-169] as shown in the schematic of figure 7(b) to enhance mass transport to the diamond electrode and enhance detection for hydrogen sulphide. In this format the sulphide sensing electrode (x5) was located downstream to the pH generating electrode (xg) separated by 200 pm intrinsic diamond substrate. This distance is non-ideal [Bitziou, E.; Snowden, M. E.; Joseph, M. B.; Leigh, S. J.; Covington, J. A.; Macpherson, J. V.; Unwin, P. R., Dual electrode micro-channel flow cell for redox titrations: Kinetics and analysis of homogeneous ascorbic acid oxidation, J Electroanaf Chem. 2013, 692, 72-791 as the closer the two electrodes are the more pronounced the pH change effect will be at the downstream electrode.

Figure 7(c) shows the experimental settings for detection of sulphide for the two pBDD band electrodes. Sensor electrode x is cleaned prior to sulphide detection using a galvanostatic current pulse of -4 pA (50x 200 ms pulses, overall time 20s) followed by lOs at E = 0 V and then Ed0tu0n = +1.2 V for lOOs. At the same time the generator electrode xg is inactive during the x cleaning step and a constant potential of-l.7 V or -1.8 V or -2.0 V is applied to Xg during the sulphide sensing step. The negative potential applied at the Xg generates a flux of hydroxyl ions (H0) that increases the pH of the solution in the vicinity of xg generating a pH gradient. Due to diffusion and convection caused by solution flow the pH at x electrode is also affected causing the sulphide equilibrium to shift (eq. 1) and therefore increasing the signal we can obtain for the oxidation of the singly protonated HS' ion, The all-diamond device was used in a FIA electrochemical (FIA-EC) detection setup.

Figure 8(a) shows the detection response of increasing concentrations of dissolved hydrogen sulphide (Edt0 = +1.2 V) at pH 10 using the 90 pm pBDD band, The peaks represent the third injection of 50 pL of diluted sulphide solution and this was repeated four times (n = 4). The cleaning step prior to this raw data is not shown and the larger band electrode at this stage was inactive, Figure 8(b) shows the calibration graph constructed using the above data by averaging the peak heights vs. concentration of dissolved sulphide. The limit of detection (1L.O.D.) obtained for pH was 6.5 ppb [R2 = 0.997, Slope = 7.84 x io9 A pM", intercept = 1.8 x io A] for = + .2 V. Figure 9(a) shows the FIA-EC detection of 100 pM dissolved hydrogen sulphide (Edetect = +1.2 V) at pH 6.1 using the 90 pm pBDD band. The peaks represent the third injection (n = 4) of 50 RL of diluted sulphide solution at pH 6,] and shows the effect of the sensor signal with the absence of a cleaning step prior to detection as well as the effect of the detector signal to the cathodic polarisation of the upstream Xg electrode. A plot summarising all the FIA-EC data as an average peak height versus electrode treatments and in-situ pH generation is shown in Figure 9(b). The effect of a freshly cleaned electrode can be clearly seen with a =30% increase in signal and a better signal reproducibility noted by the size of the error bars. Upon water reduction on the xg electrode an increase in the local pH causes the recorded signal for the sulfide oxidation to increase, A 25% increase in the signal was achieved at xg applied potential of -1.7 V and a further =20% increase when the xg applied a potential of -1.8 V. The data shown in Figure 9 demonstrate a qualitative proof of principle study that pH generation by cathodic polarisation of an electrode can cause a sufficiently basic shift to the pH (ApH) so that an adjacent electrode experiences a signal enhancement.

The homogeneous and heterogeneous equations that govern the processes occurring at the solution/electrode interfaces are shown below and as a schematic in Figure 9 (c).

Upstream electrode rxns: H20 -e -, 1/2 H2 + HO Eq. (4a) H5O + & 1/2 H2 + H20 Eq. (4b) Downstream electrode rxn: HS -. 5(s) + H + 26 Eq. (5) Solution equilibrium with basic ApH: H2S(l) HS + H Eq. (6) Figure 9(d) shows a plot of the ratio of dissolved hydrogen sulphide vs. solution pH according to equations 1 & 2 showing the relative amounts of each of the three soluble forms of sulphide at a given pH. These equilibria can change substantially with salt content and temperature and in contrast to gaseous oxygen in solution, the relationship between salinity, solubility and temperature is not well known, In order to further compare the achieved enhancement produced by the pH change with that expected for a known solution pH the x electrode was modified with a pH sensing film of iridium oxide in order to quantitatively measure the local pH in the vicinity of the electrode. Increasing cathodic potentials were applied at the upstream generator electrode for 60 s as shown in figure 10(a) and the pH was measured simultaneously on the downstream electrode (Figure 10(b)). Before and after the constant potential pulse the upstream electrode was floating at an OCP therefore allowing the pH sensor downstream to equilibrate at the solution pH (pH 6, 0. 1 M KNO3) under continuous flow. Upon application of increasingly negative potentials the pH measured with the iridium oxide electrode shows a consistent and reproducible increase in pH signifying that the solution has become more basic (pH > 6).

According to these measurements the higher the negative applied potential the larger the change in solution pH (ApR) consistent with water reduction reaction from Eq.

(4a). When a potential of-1.7 V and -1.8 V is applied on Xg electrode (vs. Pt counter and AgAgCl reference) similarly to the experimental conditions described in Figure 9 (a & b) for the H2S oxidation, the ApH recorded by the pH sensor was 6 and 7, respectively. Therefore at these applied potentials the pH of the solution changes significantly enough to drive the equilibrium of a soluble species like H2S from one form to another according to equation 6. This very large shift of the pH to more basic values highlights that the hydroxide (Off) generated by the electrode held at constant potential is a good way to achieve detection of species which are electrodeposited at high pH.

Under the same experimental conditions (pH 6, 0.1 lvi KNO3) a different approach to Oft generation was adopted. The galvanostatic reduction of water (Eq. 4a) produces a constant flux of hydroxyl ions according to the equation 7 [Rudd, N. C.; Cannan, S.; Bitziou, E,; Ciani, I.; Whitworth, A, L.; Unwin, P. R., Fluorescence Confocal Laser Scanning Microscopy as a Probe of pH Gradients in Electrode Reactions and Surface Activity. AnaL (hem. 2005, 77(19), 6205-62 17], = .FA Eq. (7) wherej is the flux of species generated by the electrode, 1app is the applied curent, 1H is the current from proton reduction (Eq. 4b), and A is the electrode area, Figure 1] (a) shows a series of 60 s constant current pulses the upstream electrode was held with a the downstream pH electrode recording the local flux of W/Off downstream (Figure 1] (b)). It is clearly visible that the pH responses are more stable and reproducible than the previous potentiostatic approach shown in Figure 0. This constant galvanostatic generation methodology highlights that this would be a more desired technique for a consistent and quantitative pH generation.

While this invention has been particularly shown and described with reference to prefered embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appendarn claims.

Claims (18)

1. Claims 1. A diamond electrochemical sensor comprising: a sensing electrode formed of boron-doped diamond material; and a p1-I changing electrode formed of boron-doped diamond material; wherein the sensing electrode and the pH changing electrode are configured such that when placed in an aqueous solution of pH 7 the pH changing electrode can generate HE and Oft allowing a reversible pt-I change over the sensing electrode across a pH range of at least 3 to 9.
2. 2. A diamond electrochemical sensor according to claim 1, wherein the sensing electrode and the pH changing electrode are configured such that the reversible pH change over the sensing electrode is across a pH range of at least 2 to 10 or at least Ito 11.
3. 3. A diamond electrochemical sensor according to claim 1 or 2, wherein said reversible pH change can be generated without any substantial gas bubble formation at the surface of the pH changing electrode.
4. 4. A diamond electrochemical sensor according to any preceding claim, wherein a target pH can be generated over the sensing electrode by the pH changing electrode and held during electrochemical sensing of a target species using the sensing electrode.
5. 5. A diamond electrochemical sensor according to any preceding claim, wherein the reversible pH change can be generated over extended time periods and/or multiple cycles without surface passivation of the pH changing electrode.
6. 6. A diamond electrochemical sensor according to any preceding claim, wherein the diamond electrochemical sensor is configured such that a mercury stripping peak of at least 10 FIA is generated at the sensing electrode using a scan rate of 0.1 vs'1 in a I mM Hg22 solution which has an intrinsic pH of 7, where the intrinsic pH of 7 is a pH of 7 prior to application of an in situ pH change via the pH changing electrode.
7. 7. A diamond electrochemical sensor according to any preceding claim, wherein the diamond electrochemical sensor is configured such that a limit of detection for dissolved hydrogen sulphide is lower than 100 ppb, 50 ppb, 20 ppb, or ppb in a solution having an intrinsic pH of between 6 and 7.
8. 8. A diamond electrochemical sensor according to any preceding claim, wherein the sensing electrode is a disc electrode and the pH changing electrode is a ring electrode which surrounds the disc electrode.
9. 9. A diamond electrochemical sensor according to any one of claims I to 7, wherein the sensing electrode is a band electrode and the pH changing electrode is another band electrode positioned adjacent the sensing electrode, the pH changing electrode having a larger surface area than the sensing electrode.
10. 10. A diamond electrochemical sensor according to any preceding claim, wherein the sensing electrode and the pH changing electrode are disposed in a flow cell with at least a portion of the pH changing electrode being disposed upstream of the sensing electrode whereby fl and Oft generated by the p1-I changing electrode flow over the sensing electrode in use.
11. 11. A diamond electrochemical sensor according to any preceding claim, wherein the boron-doped diamond sensing electrode and the boron-doped diamond p11 changing electrode are both mounted within an intrinsic diamond support matrix,
12. 12. A diamond electrochemical sensor according to any preceding claim, wherein one or both of the sensing electrode and the pH changing electrode are fonned of a boron-doped diamond material which has an sp2 carbon content sufficiently low as to not exhibit non-diamond carbon peaks in a Raman spectrum of the boron-doped diamond material.
13. 13. A diamond electrochemical sensor according to any preceding claim, wherein the pH changing electrode is configured to generate the reversible pH change using a galvanostatic generation methodology.
14. 14. A diamond electrochemical sensor according to any preceding claim, wherein the diamond electrochemical sensor is configured to apply a cleaning pulse to the sensing electrode to condition the sensing electrode prior to detection of a target species.
15. 15. A diamond electrochemical sensor according to any preceding claim, wherein the diamond electrochemical sensor is configured to apply the following pulses to the sensing electrode and the pH changing electrode: a cleaning pulse applied to the sensing electrode; a pH generating pulse applied to the pH changing electrode; a sensing pulse applied to the sensing electrode.
16. 16. A method of detecting a target chemical species in a solution using a diamond electrochemical sensor according to any preceding claim, the method comprising: applying a first potential to the pH changing electrode to generating a local pH change over the sensing electrode; and applying a second potential to the sensing electrode to detect the target chemical species.
17. 17. A method according to claim 16, wherein the method further comprises: applying a third potential to the sensing electrode to condition the sensing electrode prior to application of the second potential, wherein the third potential has an opposite polarity to the second potential and is larger in magnitude than said second potential.
18. 18. A method according to claim 16 or 17, wherein the first potential applied to the p1-I changing electrode comprises an initial drive pulse which is larger in magnitude than that required to achieve a steady state target pt-I to drive the local pH over the sensing electrode towards the target value and then ramping back to a drive pulse suitable to hold the local pH over the sensing electrode at the target value. /lfl -I-,19. A method of fablicating a sensor according to any one of claims 1 to 15, wherein local pH changes over the sensing electrode are calibrated versus an electrochemical response of an electrochemical system in bulk pH buffered solutions over a working pH range of the sensor.