Multilayer electrodes for pH sensing.

Abstract

The present invention provides a multilayer electrode for sensing pH, the electrode comprising: a sensing layer on a substrate, the sensing layer comprising at least one proton-sensitive metal oxide, such that the pH-dependent potential of the multilayer electrode is measurable via an electrically conductive connection to the sensing layer; a proton permeable layer covering at least a portion of the sensing layer, the proton permeable layer comprising at least one electrically insulating, proton-conducting metal oxide; and a carbonaceous layer on the proton permeable layer, the carbonaceous layer comprising amorphous carbon.

Description

[Technical Field]

[0001] The present invention relates to a multilayer electrode for detecting pH, and more particularly to an electrode including a metal oxide-based sensing layer on a substrate, a metal oxide-based proton-permeable layer covering the sensing layer, and a carbonaceous layer containing amorphous carbon on the proton-permeable layer. The present invention also relates to a method for manufacturing the multilayer electrode, a pH sensor including the multilayer electrode, and a pH measurement method using the multilayer electrode. [Background technology]

[0002] pH is one of the most commonly measured analytical parameters in laboratories and industry. Accurate measurement of pH is crucial in many chemical, mineral and biological processes, including water quality monitoring, chemical and biological analysis, blood monitoring, environmental monitoring and various clinical tests. All of these applications require reliable and accurate pH sensors.

[0003] Traditional glass pH probes are still commonly used today. However, these devices are relatively large, fragile, require recalibration, and are not suitable for high-temperature applications or wet storage, making them less than ideal for all applications. Consequently, much research has been devoted to developing solid-state pH sensing systems over the past few decades. Solid-state pH electrodes use an active sensing material, typically a metal oxide, such as iridium dioxide, ruthenium dioxide, osmium dioxide, or tin dioxide. Ion-selective field-effect transistor (ISFET)-type sensors use a pH-sensitive material as the gate electrode, and the resulting current through the transistor correlates with the pH of the analyte in contact with the gate. However, ISFET sensors are limited to pressures of around 2 bar, suffer from drift and hysteresis effects, and are sensitive to light. Therefore, electrodes suitable for direct potentiometric pH measurement are preferred for many applications.

[0004] In metal oxide-based potentiometric pH sensing systems, a metal oxide sensing layer on an electrode substrate is exposed to the analyte, and its pH-dependent potential is directly measured relative to the reference potential of a reference electrode using a potentiostat. However, proton-sensitive metal oxides are susceptible to redox interference from reduced and oxidized species in the analyte, and some oxides react with halogens to form soluble compounds. This has limited the deployment of these sensors in many applications. Recently, protective layers made of electrically insulating, proton-conducting metal oxides or polymers have been used to protect the sensing layer from redox interference. For example, Kuo et al. reported the use of a tantalum pentoxide outer layer to protect iridium-oxide-based electrodes (Sensors and Actuators B: 2014, 193, 687).

[0005] Although the use of such proton-permeable layers provides protection from redox interference, further improvements are needed. Furthermore, reported metal oxide-based pH electrodes are still unsuitable for many applications due to the relative fragility of the metal oxide and protective layers. Multilayer metal oxide electrodes are susceptible to physical damage in abrasive analytes, such as mineral processing streams containing fine particles. Furthermore, biofouling of the electrode surface due to the attachment and proliferation of microorganisms at the sensor-analyte interface can limit applications requiring long-term use, such as environmental monitoring.

[0006] Thus, there is a need for new metal oxide-based potentiometric electrodes for sensing pH that at least partially address one or more of the above-mentioned drawbacks or provide a useful alternative.

[0007] Any reference herein to a patent document or other matter offered as prior art shall not be construed as an admission that the document or matter was known at the priority date of any claim or that the information it contains was part of the common general knowledge. Summary of the Invention

[0008] The present inventors have discovered that a carbonaceous layer containing amorphous carbon can be advantageously provided as the outer layer of a multilayer metal oxide-based potentiometric electrode for pH sensing. The carbonaceous layer protects the electrode against mechanical damage and / or fouling during use and, in cooperation with the metal oxide-based proton-permeable layer, can protect the underlying sensing layer from redox interference. The carbonaceous layer allows acceptable proton permeability and therefore does not unacceptably reduce the sensitivity of the electrode, even when present as a continuous coating on the underlying metal oxide layer.

[0009] According to a first aspect, there is provided a multi-layer electrode for sensing pH, the electrode comprising: a sensing layer on the substrate, the sensing layer comprising at least one proton-sensitive metal oxide, wherein the pH-dependent potential of the multilayer electrode is measurable via a conductive connection to the sensing layer; a proton permeable layer covering at least a portion of the sensing layer, the proton permeable layer comprising at least one electrically insulating, proton-conducting metal oxide; and A carbonaceous layer on the proton-permeable layer is provided, the carbonaceous layer comprising amorphous carbon.

[0010] In some embodiments, the carbonaceous layer is a continuous coating that covers at least a portion of the proton-permeable layer.

[0011] In some embodiments, the carbonaceous layer has a thickness of 500 nm or less, or between 5 nm and 100 nm, for example between 15 nm and 60 nm.

[0012] In some embodiments, the amorphous carbon comprises diamond-like carbon. The amorphous carbon comprises 10% or more sp of total carbon. 3 - Hybridized carbon, e.g., 20% to 85% sp of total carbon 3 In some embodiments, the carbonaceous layer is formed by plasma-enhanced chemical vapor deposition.

[0013] In some embodiments, the proton-conducting metal oxide is selected from the group consisting of tantalum pentoxide, yttrium-stabilized zirconia, yttrium-doped barium cerate, barium cerium yttrium zirconate, and gadolinium-doped cerium oxide, hi some embodiments, the proton-conducting metal oxide comprises tantalum pentoxide or yttrium-stabilized zirconia.

[0014] In some embodiments, the proton-sensitive metal oxide is selected from the group consisting of ruthenium oxide (RuO), platinum oxide (PtO), iridium oxide (IrO), osmium oxide (OsO), tin oxide (SnO), titanium oxide (TiO), antimony oxide (SbO), rhodium oxide (RhO), and palladium oxide (PdO). In some embodiments, the proton-sensitive metal oxide comprises ruthenium oxide.

[0015] In some embodiments, the substrate comprises a conductive material such that the conductive connections pass through or along the substrate. The conductive material may comprise a semiconductor such as doped silicon. In some embodiments, the substrate comprises a semiconductor wafer having a first side, a second side, and a thickness of 300 μm to 3 mm, e.g., 400 μm to 700 μm, the sensing layer being disposed on the first side, and the pH-dependent potential of the multilayer electrode being measurable via the second side.

[0016] In some embodiments, the sensing layer has a thickness of 200 nm to 1000 nm, for example, 250 nm to 500 nm, hi some embodiments, the proton-permeable layer has a thickness of 10 nm to 100 nm, for example, 15 nm to 60 nm.

[0017] In some embodiments, the sensing layer and the proton conducting layer are formed by sputtering, such as RF magnetron sputtering.

[0018] According to a second aspect, the present invention provides a pH sensor comprising a multilayer electrode according to any embodiment of the first aspect, a reference electrode, and means for measuring the difference between the pH-dependent potential of the multilayer electrode and a reference potential of the reference electrode, which may be a potentiostat or a voltmeter.

[0019] According to a third aspect, the present invention provides a method of measuring pH, the method comprising contacting a multilayer electrode according to any embodiment of the first aspect and a reference electrode with an analyte; and measuring the difference between the pH-dependent potential of the multilayer electrode and a reference potential of the reference electrode.

[0020] In some embodiments, the analyte consists of abrasive particulate matter and / or has a pH of less than or equal to 4. In some embodiments, the analyte is a leachate or leach slurry from a mineral leaching process or an acid mine drainage stream.

[0021] According to a fourth aspect, the present invention provides a method for manufacturing a multi-layer electrode for sensing pH, the method comprising: forming a sensing layer on the substrate, the sensing layer comprising at least one proton-sensitive metal oxide; forming a proton permeable layer covering at least a portion of the proton sensitive layer, the proton permeable layer comprising at least one electrically insulating proton conducting metal oxide; forming a carbonaceous layer on the proton permeable layer, the carbonaceous layer comprising amorphous carbon; and Providing a conductive connection to the sensing layer to measure the pH-dependent potential of the multilayer electrode.

[0022] In some embodiments, forming the carbonaceous layer comprises depositing amorphous carbon on the proton permeable layer by plasma-enhanced chemical vapor deposition.

[0023] In some embodiments, forming the sensing layer comprises depositing a proton-sensitive metal oxide on the substrate by sputtering, such as magnetron sputtering, for example RF magnetron sputtering.

[0024] In some embodiments, forming the proton-permeable layer comprises depositing an electrically insulating proton-conducting metal oxide onto the sensing layer by sputtering, such as magnetron sputtering, e.g., RF magnetron sputtering.

[0025] In some embodiments, the substrate comprises a conductive material, such as a semiconductor, e.g., doped silicon, such that the conductive connections pass through or along the substrate. The substrate may comprise a semiconductor wafer having a first side, a second side, and a thickness of 300 μm to 3 mm, e.g., 400 μm to 700 μm, and the sensing layer is formed on the first side.

[0026] In some embodiments, the proton-conducting metal oxide is selected from the group consisting of tantalum pentoxide, tantalum pentoxide, yttrium stabilized zirconia, yttrium doped barium cerate, barium cerium yttrium zirconate, and gadolinium oxide doped cerium oxide.

[0027] In some embodiments, the proton-sensitive metal oxide is selected from the group consisting of ruthenium oxide (RuO), platinum oxide (PtO), iridium oxide (IrO), osmium oxide (OsO), tin oxide (SnO), titanium oxide (TiO), antimony oxide (SbO), rhodium oxide (RhO), and palladium oxide (PdO).

[0028] When the terms "comprise", "comprises" and "comprising" are used in this specification (including the claims), they shall be interpreted as specifying stated features, integers, steps or components but not excluding the presence of one or more other features, integers, steps or components or groups thereof.

[0029] As used herein, the terms "first," "second," "third," etc., in connection with various features of the disclosed devices are assigned arbitrarily and are intended merely to distinguish between two or more such features that the devices may incorporate in various embodiments. The terms do not, in themselves, indicate a particular orientation or order. Furthermore, it will be understood that the presence of a "first" feature does not imply the presence of a "second" feature, the presence of a "second" feature does not imply the presence of a "first" feature, etc.

[0030] Further aspects of the present invention appear below in the detailed description of the invention. [Brief explanation of the drawings]

[0031] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0032] FIG. 1 is a schematic depiction of a multilayer electrode for sensing pH, according to an embodiment of the present invention.

[0033] FIG. 2 is a schematic illustration of a multi-layer electrode for sensing pH according to another embodiment of the present invention.

[0034] FIG. 3 is a diagram showing a schematic diagram of a film-forming apparatus for forming a diamond-like carbon thin film by plasma-enhanced chemical vapor deposition.

[0035] FIG. 4 is the X-ray diffraction pattern of the crystalline RuO2 thin film fabricated on a silicon substrate via RF-magnetron sputtering at 220° C. in Example 1.

[0036] FIG. 5 is an XPS spectrum of the RuO2 thin film fabricated on a silicon substrate via RF-magnetron sputtering at 220° C. in Example 1.

[0037] FIG. 6 is an XPS spectrum of the Ta2O5 thin film fabricated on the RuO2-coated silicon substrate in Example 1.

[0038] FIG. 7 depicts a plot of the electrode potential as a function of pH for Si—RuO 2 —Ta 2 O 5 electrodes with various different RuO 2 film thicknesses measured in Example 3.

[0039] FIG. 8 shows plots of electrode potential as a function of pH for Si—RuO 2 —Ta 2 O 5 and Si—RuO 2 —YSZ electrodes measured in Example 4.

[0040] FIG. 9 depicts a comparative plot of the electrode potential as a function of pH for the Si—RuO 2 —Ta 2 O 5 -amorphous carbon and Si—RuO 2 —Ta 2 O 5 multilayer electrodes measured in Example 5.

[0041] FIG. 10 is a diagram showing the electrode potential response of the Si—RuO 2 —Ta 2 O 5 -amorphous carbon multilayer electrode measured in Example 5 in buffer solutions of pH 1 to pH 10.

[0042] FIG. 11 shows the results of the measurements in Example 6, where the Fe concentration was varied from 0.001M to 1M. 2+ 1 shows the electrode potential response of a Si-RuO2-Ta2O5-amorphous carbon multilayer electrode in a pH 2 buffer solution with various concentrations.

[0043] FIG. 12 depicts the electrode potential responses of the Si—RuO 2 —Ta 2 O 5 -amorphous carbon and Si—RuO 2 —Ta 2 O 5 multilayer electrodes in pH 2 buffer solutions with ascorbic acid concentrations varying from 0 M to 10 M, as measured in Example 6.

[0044] FIG. 13 shows a comparative plot of the electrode potential as a function of pH for Si—RuO 2 —Ta 2 O 5 -amorphous carbon and Si—RuO 2 -amorphous carbon multilayer electrodes measured in Example 7.

[0045] FIG. 14 is a comparative plot of the electrode potential as a function of pH for Si—RuO 2 -YSZ-amorphous carbon and Si—RuO 2 -YSZ multilayer electrodes, as measured in Example 8.

[0046] FIG. 15 depicts a comparative plot of electrode potential as a function of pH for Si—RuO 2 -YSZ multilayer electrodes with continuous and discontinuous RuO 2 layers, as measured in Example 9.

[0047] FIG. 16 depicts a comparative plot of electrode potential as a function of pH for Si—RuO 2 -YSZ multilayer electrodes with varying YSZ layer thicknesses, as measured in Example 10. DETAILED DESCRIPTION OF THE INVENTION

[0048] The present invention relates to a multilayer electrode for sensing pH. The electrode may include a sensing layer comprising at least one proton-sensitive metal oxide on a substrate, a proton-permeable layer comprising at least one electrically insulating proton-conducting metal oxide covering at least an analyte-contacting portion of the sensing layer, and a carbonaceous layer comprising amorphous carbon on the proton-permeable layer. When the electrode is exposed to an aqueous analyte, the proton-permeable layer and the carbonaceous layer allow sufficient conduction of protons to the underlying sensing layer, so that the pH-dependent potential of the electrode can be measured via a conductive connection to the sensing layer.

[0049] In at least some embodiments, the carbonaceous layer present on the electrode surface can provide a mechanically hard, fouling-resistant electrode-analyte interface and protect the electrolyte from physical damage, such as abrasion, during use and / or fouling, such as biofouling. Additionally, the carbonaceous layer and proton-permeable layer together can protect the sensing layer from reduced and oxidized species in the analyte that can confound the pH measurement. Detection Layer

[0050] The sensing layer is composed of at least one proton-sensitive metal oxide, and in some embodiments, includes a proton-sensitive metal oxide. As used herein, a proton-sensitive metal oxide is a metal oxide that, when exposed to an aqueous solution, exhibits an oxidation-reduction potential (electrode potential) that depends on the concentration of hydrogen ions (protons) in the solution. Furthermore, proton-sensitive metal oxides have sufficient electrical conductivity to transmit the electrode potential for measurement. A wide range of metal oxides have been reported to be suitable for solid-state pH sensing electrode applications, and it is contemplated that any of these metal oxides may be used. In some embodiments, the sensing layer includes at least one of ruthenium oxide (RuO), platinum oxide (PtO), iridium oxide (IrO), osmium oxide (OsO), tin oxide (SnO), titanium oxide (TiO), antimony oxide (SbO), rhodium oxide (RhO), and palladium oxide (PdO). RuO2, IrO2, and SnO2 are particularly preferred metal oxides for pH sensing applications due to their chemical stability and high conductivity. Combinations of metal oxides, such as nanocomposites of RuO2 and Ta2O5, are also contemplated for use in the sensing layer. The metal oxide sensing layer may be suitably crystalline or amorphous. For example, both highly crystalline RuO2 and substantially amorphous RuO2 have been found to provide good sensitivity to pH.

[0051] In some embodiments, the sensing layer is a thin film, e.g., having a thickness of 200 nm to 1000 nm, e.g., 250 nm to 500 nm. Thin film metal oxide layers may be fabricated by various methodologies, including sputter deposition techniques, as described in more detail below. The layer may be a continuous film of metal oxide deposited on a portion of a substrate; however, it is not excluded that the layer may be patterned or otherwise discontinuously formed on the substrate, provided that the pH-dependent potential can be measured. In this case, a conductive additive, such as particulate conductive carbon, may optionally be included with the metal oxide. Board and electrical connections

[0052] The substrate can in principle be any substrate capable of supporting the electrode multilayer structure. The substrate can be conductive or insulating, subject to the requirement that a conductive connection to the sensing layer be provided so that the pH-dependent potential of the multilayer electrode can be measured. Thus, the substrate can comprise a metal, a semiconductor, or an insulator such as a polymer or ceramic.

[0053] In some embodiments, the substrate includes a conductive material, such as a metal or semiconductor, at least in a surface layer of the substrate. Advantageously, this can provide an opportunity to measure the pH-dependent potential of the sensing layer of the electrode via a conductive path extending through the substrate or along the substrate surface. Suitable conductive substrates include those formed from semiconductive silicon, stainless steel, platinum, gold, glassy carbon, or pyrolytic graphite, or substrates including a thin conductive surface film, such as indium tin oxide, fluorine-doped tin oxide, or aluminum-doped zinc oxide.

[0054] In some embodiments, the substrate comprises a semiconductor, e.g., doped silicon, such as n-type silicon. Such substrates have been found to provide a good balance of properties for the multilayer electrodes of the present invention. These properties include chemical inertness in aqueous analytes, particularly inertness to redox reactions that can undesirably interfere with the pH measured in the sensing layer, compatibility with thin film deposition techniques for fabricating successive functional layers of the electrode, and adequate electrical conductivity to allow the potential of the sensing layer to be measured through the substrate material.

[0055] In some embodiments, the substrate comprises a semiconductor wafer, e.g., a doped silicon semiconductor wafer. The wafer may have a thickness of 300 μm to 3 mm, e.g., 400 μm to 700 μm. The wafer may have a resistivity in the range of 0.0008 to 0.02 ohm-cm. A sensing layer is typically disposed on only one surface of the wafer, and the pH-dependent potential of the sensing layer can then be measured via a conductive path extending through the thickness of the substrate and the backside of the wafer. Advantageously, this structure eliminates the need to fabricate a separate conductive path on the substrate surface and allows for physical isolation of the electrical connection of the electrodes to the external circuitry of the pH sensing system from contact with the analyte by the wafer.

[0056] The electrode comprises a conductive connection to the sensing layer, allowing the electrode's pH-dependent potential to be measured, typically with a potentiometer connected to the electrode. As noted above, it has been found that the potential can be advantageously measured through a suitable conductive substrate on which the sensing layer is formed. However, alternative arrangements are contemplated and within the scope of the present invention. For example, conductive metal tracks of a metal such as platinum can be patterned on an otherwise insulating substrate, and the sensing layer is then deposited on one portion of the conductive tracks, with the potential measured on another portion. Alternatively, the conductive tracks can be formed in the sensing layer itself, such as by laser reduction of a proton-sensitive metal oxide. In yet another embodiment, conductive wires can be physically attached to the sensing layer after it has been formed. Proton-permeable layer

[0057] The multilayer electrode includes a proton-permeable layer comprising at least one electrically insulating, proton-conducting metal oxide covering at least a portion of the sensing layer. In some embodiments, the proton-permeable layer comprises an electrically insulating, proton-conducting metal oxide. One role of the metal-oxide-based proton-permeable layer is to protect the underlying sensing layer, at least to some extent, from strong oxidizing or reducing species present in the analyte, which may interfere with the measured potential correlated to pH. It will be appreciated, therefore, that this layer should generally be present so as to continuously cover the entire sensing layer, or at least the portion expected to come into contact with the analyte. During use, the proton-permeable layer excludes large redox species from the sensing layer while allowing sufficiently facile ionic conduction of small hydrogen ions. The inventors have also found that when a continuous amorphous carbon layer is present on the electrode, such an intermediate layer is important for obtaining a linear pH response. Without wishing to be bound by any theory, it is believed that the electrically insulating proton-permeable layer is necessary to physically and / or electrically separate the sensing layer from the carbonaceous layer to prevent interference of the carbonaceous layer with the measured electrode potential.

[0058] Proton-conducting metal oxides are electrically insulating, meaning that they have low electrical conductivity, especially relative to the proton-sensitive metal oxide in the electrode's sensing layer. The role of the proton-permeable and carbonaceous layers is to act in combination to protect the underlying sensing layer from interference and damage, without otherwise directly contributing to the measured potential. Therefore, the electrically insulating nature of the proton-conducting metal oxide ensures that the pH-sensitive response of the device is driven by the sensing layer, not the proton-permeable layer, even though the proton-conducting metal oxide can inherently participate in pH-dependent redox chemistry.

[0059] The contrasting electrical conductivities of suitable proton-sensitive and proton-conducting metal oxides can be seen by comparing the reported resistivities of these materials. Reported resistivities for RuO thin films range from 20 to 90 μΩ cm, while those for yttrium-stabilized zirconia (YSZ) are in the range of 2.5 × 109 ~1×10 12 μΩ·cm, and that of Ta2O5 is in the range of 9×10 4 ~3×10 6 Thus, in some embodiments, the electrically insulating proton-conducting metal oxide in the proton permeable layer has a conductivity in the range of 1×10 3 μΩ.cm or greater, or 1×10 4 μΩ.cm or greater, or 1×10 5 In some embodiments, the resistivity of the proton-conducting metal oxide in the proton-permeable layer is 10 times greater than the resistivity of the proton-sensitive metal oxide in the sensing layer. 2 , or 10 3 , or 10 4 , or 10 5 It has a resistivity twice as high.

[0060] Proton-permeable layers made from a variety of electrically insulating, proton-conducting metal oxides have previously been disclosed for pH sensing applications, and it is contemplated that any of these metal oxides may be used. In some embodiments, the proton-permeable layer is made from tantalum pentoxide (TaO), yttrium-stabilized zirconia (YSZ), yttrium-doped barium cerate (BaCe 0.2 Y 0.1 O 3-δ ; also known as BCYO), barium cerium yttrium zirconate (BaCeO 0.2 Y 0.1 Zr 0.7Examples include cerium oxide doped with gadolinium oxide (CeO2-90%-Gd2O3-10%). Proton-conducting layers containing these metal oxide materials can be fabricated on the metal oxide sensing layer by thin film deposition techniques such as magnetron sputtering. Ta2O5 and YSZ have proven particularly suitable due to their protective properties and the ease with which these materials can be deposited by sputtering. Other proton-conducting metal oxides are described in Chapter 10 of Knauth et al., "Solid State Proton Conductors, Properties and Applications in Fuel Cells," John Wiley and Sons, 2012, the contents of which are incorporated herein by reference.

[0061] The metal oxide in the proton-permeable layer may be substantially amorphous. Amorphous metal oxide layers lack grain boundaries and may therefore provide better protection against interfering redox species.

[0062] In some embodiments, the proton-conducting layer is a thin film having a thickness of, for example, 10 nm to 100 nm, e.g., 15 nm to 60 nm. Such thin film metal oxide layers can be fabricated by a variety of methodologies, including sputter deposition techniques, as described in more detail below. carbonaceous layer

[0063] A multilayer electrode according to the present invention comprises a carbonaceous layer comprising amorphous carbon on a proton-permeable layer, which is typically the outermost layer of the electrode and is therefore directly exposed to the analyte during use.

[0064] Amorphous carbon is a carbon material that lacks crystalline structure, although some short-range order may be present. In some embodiments, amorphous carbon comprises diamond-like carbon. As used herein, diamond-like carbon refers to a carbon material that is sp 2 - and sp 3-refers to a metastable form of substantially amorphous carbon containing both hybridized and unhybridized carbon atoms, including hydrogen-free, hydrogenated, and heteroatom-doped forms.

[0065] Diamond-like carbon coatings typically contain a significant proportion of tetrahedral sp 3 The amorphous carbon in the carbonaceous layer is composed of sp-bonded carbon atoms, thus establishing a 3D-network of bonds in the material and imparting "diamond-like" properties, including high elastic modulus, mechanical hardness, and chemical inertness. Thus, in some embodiments, the amorphous carbon in the carbonaceous layer comprises 10% or less of the total carbon, e.g., 20-85%, or 25-50% sp 3 -Hybridized carbon. sp in diamond-like carbon 3 The proportion of sp-hybridized carbon can be determined by electron energy loss spectroscopy (EELS) or Raman spectroscopy. In some embodiments, the diamond-like carbon in the carbonaceous layer is 3 Due to its carbon content, it has a hardness of at least 14 GPa, or at least 15 GPa, or at least 16 GPa, such as about 17 GPa. Hardness can be determined by nanoindentation, also known as instrumented indentation testing. This technique is commonly applied to test the mechanical properties of thin film materials.

[0066] Properties of diamond-like carbon, e.g. sp 3 The percentage of carbon and hydrogen content depends on the fabrication method, as discussed in more detail below. In some embodiments, the carbonaceous layer is formed by plasma-enhanced chemical vapor deposition. This method typically produces, for example, 30-70% sp of total carbon. 3 - Producing a hydrogenated form of diamond-like carbon with hybridized carbon and 20-60 mol% hydrogen.

[0067] Diamond-like carbon has favorable mechanical properties, chemical inertness, low surface roughness, hydrophobicity, and low surface energy, making it an attractive material for various coating applications. Diamond-like carbon coatings have also been used as the proton-sensitive layer in ion-selective field-effect transistor (ISFET) sensors.

[0068] In contrast, the multilayer electrode of the present invention utilizes a metal oxide-based sensing layer with two overlying protective layers present on the sensing layer. Because the carbonaceous layer is used simultaneously with the proton-permeable metal oxide layer, it functions differently, operates on a different principle, and offers different advantages than its previous use in ISFET sensors. It has surprisingly been found that a carbonaceous layer comprising amorphous carbon, even when present as a thin, continuous coating on the underlying metal oxide layer, allows acceptable proton permeability and therefore does not unacceptably reduce the sensitivity of the electrode.

[0069] The carbonaceous layer can provide physical protection to the electrode structure by improving the surface's mechanical properties, including hardness. Thus, the carbonaceous layer resists abrasion or other damage to the more fragile metal oxide-based layer. This may advantageously extend the electrode's lifespan and / or application, for example, to the analysis of highly abrasive leachates or leach slurries in mineral processing applications. Furthermore, the carbonaceous layer is expected to protect the electrode from fouling, particularly biofouling, over extended use. Biofouling occurs when microorganisms adhere to a surface and can be suppressed by altering the surface properties. The low surface energy and hydrophobicity of the carbonaceous layer can suppress the adhesion and growth of microorganisms on the electrode surface.

[0070] In at least some embodiments, the carbonaceous layer is also believed to act in combination with the proton permeable layer to reduce redox interference in the sensing layer. Without wishing to be bound by theory, it is believed that the amorphous carbonaceous layer is selectively permeable to protons while excluding some of the larger oxidized or reduced species present in the analyte.

[0071] In some embodiments, the carbonaceous layer is a continuous coating that covers at least a portion of the proton-conducting layer. In particular, the continuous coating may cover the proton-conducting layer at a location on the electrode where contact with the analyte is expected. Such a continuous coating may have a thickness of 5 nm to 100 nm, e.g., 15 nm to 60 nm. The inventors have found that a 50 nm thick continuous diamond-like carbon coating does not appreciably reduce the sensitivity of an electrode having a RuO sensing layer and a TaO or YSZ proton-conducting layer.

[0072] In other embodiments, the carbonaceous layer is discontinuous, such that the underlying metal oxide layer may be partially exposed to the analyte during use. Patterned structures, for example, consisting of regular holes or of discrete lines or dots, may also be utilized. At least some of the advantages of the present invention, such as resistance to abrasion and / or fouling, may also be realized through the discontinuous placement of amorphous carbon. Indeed, a discontinuous placement may allow for the use of a thicker carbonaceous layer, thus providing enhanced mechanical properties of the electrode while still providing excellent proton sensitivity. In such embodiments, carbonaceous layers up to 500 nm thick, or even up to 1 micron thick, may be suitable.

[0073] In some embodiments, the electrode includes multiple carbonaceous layers on its surface, for example, a thin continuous coating formed first on a metal oxide layer, followed by a thicker discontinuous layer to provide additional mechanical protection.

[0074] The amorphous carbon layer can be formed by a variety of techniques, as discussed in more detail below: In some embodiments, the carbonaceous layer is formed by plasma-enhanced chemical vapor deposition.

[0075] A multilayer electrode 100 according to an embodiment of the present invention is depicted schematically in FIG. 1. The substrate 101 is a doped silicon semiconductor wafer having a first surface 103 and a second surface 105. The sensing layer 107, comprising a proton-sensitive metal oxide such as RuO2, is a thin film formed on the first surface 103. The proton-permeable layer 109, comprising an electrically insulating, proton-conducting metal oxide such as Ta2O5 or YSZ, is a continuous thin film uniformly covering the sensing layer 107. The carbonaceous layer 111, comprising amorphous carbon such as diamond-like carbon, is a thin, continuous film uniformly covering the proton-permeable layer 109. The outer surface 113 of the carbonaceous layer 111 thus provides the analyte-contacting surface of the electrode. In use, the carbonaceous layer 111 and the proton-permeable layer 109 cooperatively allow the conduction of protons from the analyte to the sensing layer 107 while excluding interfering oxidized and reduced species from the sensing layer. The insulating, proton-permeable metal oxide layer 109 also physically separates the sensing layer 107 from the carbonaceous layer 111, preventing interference with the measured electrode potential by the carbonaceous layer. The mechanically robust carbonaceous layer advantageously protects the underlying metal oxide layer from abrasion, and the low surface energy and hydrophobic surface 113 resists the attachment and growth of contaminants, including microorganisms. The pH-dependent potential of the multilayer electrode, resulting from the redox reaction of the proton-sensitive metal oxide of the sensing layer 107, is measurable via a conductive path that passes through the thickness of the substrate 101, as depicted by arrow 115. The electrode may therefore be connected to a potentiometer via an electrical connection on the second surface 105.

[0076] A multilayer electrode 200 according to another embodiment of the present invention is depicted schematically in FIG. 2. Like-numbered items of electrode 200 are as described herein for electrode 100. However, in electrode 200, a carbonaceous layer 211 comprising amorphous carbon, such as diamond-like carbon, is provided as a discontinuous, patterned layer on top of proton-conducting layer 109. For example, it may be a membrane having regular pores 217 as depicted. Because of its discontinuous configuration, layer 211 may be thicker than layer 111 of electrode 100. Although carbonaceous layer 211 resides on the analyte-contacting side of the electrode, its discontinuous configuration may also expose portions of proton-conducting layer 109 directly to the analyte. During use, proton conductivity from the analyte to sensing layer 107 is provided through the proton-conducting metal oxide of layer 109, at least via the pores in carbonaceous layer 211. The proton-conducting layer excludes interfering oxidized or reduced species from the proton-sensitive layer. The mechanically robust carbonaceous layer 211 protects the underlying metal oxide-based layer from abrasion, while the low surface energy and hydrophobic surface of the amorphous carbon can still resist fouling of the electrode surface. Method for manufacturing multilayer electrodes

[0077] The present invention also relates to a method for fabricating a multilayer electrode suitable for pH sensing, comprising forming a sensing layer comprising at least one proton-sensitive metal oxide on a substrate, forming a proton-permeable layer comprising at least one electrically insulating proton-conducting metal oxide covering at least an analyte-contacting portion of the proton-sensitive layer, and forming a carbonaceous layer comprising amorphous carbon on the proton-permeable layer. Conductive connections are provided to the sensing layer to enable measurement of the pH-dependent potential of the multilayer electrode.

[0078] The substrate and conductive connections are provided as previously described herein. The sensing layer, proton-conducting layer, and carbon layer are preferably thin films and may therefore be formed by a variety of thin film fabrication techniques. Formation of the sensing layer and proton-permeable layer

[0079] In some embodiments, the sensing layer and / or the proton conducting layer are formed by sputter deposition. Sputter deposition techniques, particularly magnetron sputtering, are suitable plasma-based processes for the deposition of a wide range of thin film materials, including metal oxide-based layers according to embodiments of the present invention.

[0080] In the sputter deposition process, the substrate to be coated and the target source material are placed separately in a low-pressure deposition chamber containing an inert gas (usually argon). A negative charge is applied to the target, generating a plasma within the chamber. Positively charged argon ions in the plasma are attracted by the negative charge and collide with the source material at high speed, "sputtering" away atomic-sized particles of the source material. These neutral particles then travel through the deposition chamber and are deposited as a thin film on the substrate. In magnetron deposition, magnets are used to keep free electrons in the plasma near the target, preventing unwanted electrons from striking the substrate during coating. Sputter deposition can use a variety of power sources, including direct current (DC) and radio frequency (RF). In RF sputtering, the charge of the power source is periodically cycled between positive and negative (RF), advantageously preventing the accumulation of positively charged ions on the target surface. RF sputtering is particularly well-suited for depositing metal oxide coatings.

[0081] In some embodiments, forming the sensing layer includes depositing a proton-sensitive metal oxide on the substrate by magnetron sputtering, such as RF magnetron sputtering. The proton-sensitive metal oxide may be any of those described herein as suitable for the electrodes of the present invention. The target source material may suitably be a metal or metal oxide, including the metal to be deposited as the proton-sensitive metal oxide. Thus, for example, a RuO layer can be deposited using either a ruthenium metal or RuO target. Oxygen is co-fed into the chamber with argon in a controllable ratio to induce the desired RuO phase in the thin film.

[0082] Sputtering deposition can be performed at high temperatures, such as above 150°C or between 150°C and 250°C. High-temperature sputtering deposition can advantageously produce crystalline metal oxide sensing layers. For example, crystalline RuO thin films with controllable thickness can be formed using either DC or RF power sources, or by using both Ru metal and RuO power sources. The thickness can be controlled by the deposition time. However, sputtering deposition can also be performed at lower temperatures, such as room temperature, which can result in the formation of amorphous metal oxide sensing layers. Amorphous sensing layers are also suitable for pH measurements, and low-temperature processes may be advantageous because electrodes can be fabricated on heat-sensitive substrates, such as polymers.

[0083] In some embodiments, forming the proton-permeable layer includes depositing an electrically insulating, proton-conducting metal oxide on the substrate by magnetron sputtering, such as RF magnetron sputtering. The proton-conducting metal oxide may be any of those described herein as suitable for the electrodes of the present invention. The target source material may suitably be a metal or metal oxide containing the metal(s) to be deposited as the proton-conducting metal oxide. Thus, for example, a Ta2O5 layer may be deposited using either a tantalum metal or Ta2O5 target, with oxygen again being co-introduced into the chamber as needed to deposit the Ta2O5 phase. The deposition temperature may be sufficiently low so that the proton-conducting metal oxide is amorphous. For example, an amorphous Ta2O5 layer may be deposited at room temperature.

[0084] Although sputter deposition techniques are particularly preferred, it is contemplated that the sensing layer and proton-conducting layer may be formed by other techniques, including plasma-enhanced chemical vapor deposition (PECVD), plasma-enhanced and thermal atomic layer deposition (ALD), filtered vacuum cathodic arc deposition (FVCAD), plasma-ion-assisted deposition, ion beam sputtering, chemical vapor deposition, and sol-gel deposition. Formation of carbonaceous layer

[0085] Thin film carbonaceous layers may be formed by a variety of thin film fabrication techniques. These include mass-selected ion beam (MSIB) deposition, filtered cathodic vacuum arc (FCVA), and laser ablation, which are used to produce low-hydrogen amorphous carbon layers, as well as plasma-enhanced chemical vapor deposition (PECVD) or reactive sputtering of graphite in Ar / H2, which can produce more hydrogenated forms of amorphous carbon. Other suitable techniques and variations include chemical vapor deposition (CVD), plasma-assisted chemical vapor deposition (PACVD), inductively coupled plasma (ICP) PECVD, pulsed DC chemical vapor deposition, hot filament-assisted PECVD, electron cyclotron wave resonance (ECWR) plasma beam deposition, microwave chemical vapor deposition, cathodic vacuum arc deposition, ion beam sputtering, RF and DC magnetron sputtering, pulsed laser deposition (PLD), electrophoretic deposition, sol-gel deposition, and others.

[0086] In some embodiments, the carbonaceous layer is formed by plasma-enhanced chemical vapor deposition. The substrate to be coated is placed on an electrode (substrate holder) in a vacuum chamber with a controllable pressure. An RF power supply is used to apply an oscillating RF voltage to the electrode, generating a plasma in the vacuum chamber. Inert and reactive gases are introduced into the chamber at controlled flow rates while maintaining a target pressure. During deposition, the plasma transfers energy to the gas mixture through electron bombardment, initiating ionization and dissociation of the gas molecules. These gas molecules are transported to the substrate by diffusion and adsorb to the surface, forming a conformal thin-film coating. Hydrocarbon gases, such as acetylene (C2H2), are preferably used as reactive gases.

[0087] In plasma-enhanced chemical vapor deposition, film thickness can be controlled by deposition time. This technique generally produces a continuous coating of amorphous (diamond-like) carbon on the substrate. In embodiments where a discontinuous carbonaceous layer is desired, this can be achieved using techniques such as lithography or by using a shadow mask (e.g., a mask with holes) during deposition of the carbonaceous layer. pH sensors and pH measurement methods

[0088] The present invention also relates to a pH sensor, particularly a potentiometric pH sensor, comprising a multilayer electrode as disclosed herein, a reference electrode, and means for measuring the difference between the pH-dependent potential of the multilayer electrode and a reference potential of the reference electrode.

[0089] The means for measuring the potential difference may be a potentiostat, a voltmeter, or the like. The potentiostat (or other device for measuring potential) is coupled to the electrode to measure the pH-dependent potential via a conductive connection to the sensing layer. In embodiments where the electrode substrate is conductive, the potentiostat may be electrically connected to the substrate, for example, on the backside of a semiconductor wafer substrate.

[0090] The reference electrode is not intended to be particularly limited and may include standard commercially available reference electrodes, such as a saturated calomel electrode (SCE) or an Ag / AgCl electrode containing an internal liquid electrolyte reservoir. However, in some embodiments, the reference electrode may be a solid reference electrode, including, for example, an Ag / AgCl reference element embedded in a polymer composite material loaded with an inorganic chloride salt. Examples of such reference electrodes are disclosed in the applicant's co-filed PCT application, published as WO 2018 / 201200, which is incorporated herein by reference. Such reference electrodes, such as embodiments of the multilayer pH sensing electrode of the present invention, are well suited for use with acidic and / or abrasive analytes.

[0091] The present invention also relates to a method for measuring pH, which method comprises contacting a multilayer electrode as disclosed herein and a reference electrode with an analyte and measuring the difference between the pH-dependent potential of the multilayer electrode and the reference potential of the reference electrode. The electrode can be contacted with the analyte by immersion in the analyte.

[0092] The method can be used to measure the pH of a wide variety of analytes produced, for example, in the food industry, chemical industry, mineral processing, biological processing, and environmental monitoring. As described herein, advantages of the multilayer electrodes of the present invention may include resistance to interference from reduced or oxidized species and compatibility with corrosive, acidic, mechanically abrasive, or biofouling analytes. In some embodiments, the analyte comprises abrasive particulate matter. In some embodiments, the analyte has a pH of 4 or less, or 2 or less, or 1 or less. In some embodiments, the analyte is a process stream from mineral processing operations, such as heap leaching, including leachate and leach slurry, or an acid mine drainage stream. In other embodiments, the analyte is wastewater with various ORP levels and potential for bacterial growth. In other embodiments, the analyte is groundwater or surface water with varying ORP levels that may foul or scale surfaces. Example

[0093] The present invention will now be described with reference to the following examples, which are understood to be illustrative but not limiting of the invention described herein. Materials and Techniques

[0094] N100 prime antimony-doped silicon wafers with thicknesses of 500–50 μm and resistivities of 0.05–0.20 Ω were obtained from Silicon Wafer Enterprises, LLC, CA, USA.

[0095] Metal oxide thin films were deposited by radio frequency (RF) or direct current (DC) magnetron sputtering using an axial turret magnetron head and power supply (AJA DCXS-750). The sputtering targets selected were Ru, RuO2, Ta2O5, and yttrium-stabilized zirconia (ZSZ). The nominal size of the sputtering targets was 2 inches and they were high purity (>99.0%). The turret head was mounted vertically at the bottom of the vacuum system. The thickness of the deposited thin films was determined using a Dektak 3030 surface profilometer.

[0096] Scanning electron microscopy analysis was performed using a ZEISS Auriga scanning electron microscope (SEM / FIB) with a focused ion beam (Focused Ion Beam). X-ray diffraction patterns were measured using a Philips PANalytical X'Pert PRO diffractometer operating at 40 kV and 45 mA with Cu Kα radiation. Raman spectroscopy was performed using a Renishaw In Via confocal Raman microscope system. Raman scattering was measured in air using a He-Ne laser (633 nm) as the excitation light, with an incident power of 1 MW and a spot diameter of 1 μm. The laser beam was focused onto the analysis surface using a microscope objective (20x magnification). X-ray photoelectron spectroscopy (XPS) analysis was performed using a hemispherical analyzer (SPECS 150) to measure the electron binding energy (BE) of core excitations generated by Mg Kα (1256.6 eV) radiation. When the analyzer pass energy was 20 eV, the energy resolution of the spectrometer was 0.8 eV. -7 The analysis was carried out in an ultra-high vacuum of 100 Pa. The analysis area was approximately 5.0 mm 2 The XPS system was operated with a Mg Kα X-ray source (10 keV, 10 mA). The spectra were analyzed with the CasaXPS 2.3.12 program. The peaks were calibrated against the main C 1s peak at 284.6 eV.

[0097] All electrochemical tests were performed in commercially available buffer solutions (Merck) using a Metrohm Autolab PGSTAT302N potentiostat or a Keysight U1231A multichannel system. All electrode potentials were recorded versus commercially available SCE or Ag / AgCl (saturated) reference electrodes. Example 1. Preparation of electrodes

[0098] A 4-inch silicon wafer was diced into 25 mm x 25 mm pieces. The wafer pieces were then prepared through the following consecutive steps: ultrasonic treatment in an acetone bath for 15 minutes, ultrasonic treatment in an ethanol bath for 15 minutes, drying under nitrogen, and irradiation with ultraviolet light for 15 minutes.

[0099] Next, the silicon wafer pieces were loaded into the magnetron deposition chamber, and the chamber was evacuated overnight to a base pressure of 2 × 10. -6 A thin layer of RuO2 was then deposited on the wafer by RF or DC magnetron sputtering using the conditions shown in Table 1. TIFF0007724213000001.tif44170

[0100] Method S5 was the same as S4, except that a mask was used to deposit a discontinuous layer of RuO. This layer consisted of square arrays of RuO, including 2 μm × 2 μm square blocks, covering approximately 45% of the underlying Si surface.

[0101] The RuO2 sputter-coated wafers were then coated with thin films of Ta2O5 or yttrium-stabilized zirconia (YSZ) by RF magnetron sputtering under the conditions shown in Table 2. JPEG0007724213000002.jpg32170

[0102] The metal oxide-coated wafer was further coated with a layer of diamond-like carbon by plasma-enhanced chemical vapor deposition (PECVD). As shown in Figure 3, the substrate 300 was placed on an electrode (substrate holder) 301 in the vacuum chamber 303 of the deposition system. The chamber was evacuated via a vacuum pump 305, and the pressure was controlled via a throttle valve 307. A radio frequency (RF) power supply 309 with a matching network 311 was used to apply an RF oscillating voltage to the electrode, thereby generating plasma 313 within the vacuum chamber. Gases containing argon 317 and acetylene 319 were introduced into the chamber through a gas inlet pipe 321 using a mass flow controller to provide a controlled flow rate. During deposition, the plasma transfers energy to the gas mixture through electron bombardment, initiating ionization and dissociation of the gas molecules. The reacted gases are transported to the substrate by diffusion and adsorb to the surface, forming a uniform thin film coating. In this manner, diamond-like carbon films were deposited under the conditions shown in Table 3. TIFF0007724213000003.tif21170

[0103] After coating with RuO2, Ta2O5 / YSZ, and / or diamond-like carbon layers, the coated substrates were cut into 6 mm x 6 mm dice. The resulting electrodes are identified below according to the layer configuration on the Si substrate, as shown in Tables 1-3. For example, Si-S1-P1-C1 refers to a multilayer electrode having all three layers according to the present invention. The samples were observed under an optical microscope and resistivity measurements were performed to confirm that the surface was insulating. The electrodes were then ready for characterization or assembly into pH sensing devices.

[0104] For electrochemical testing, wires were attached to the uncoated side of the silicon wafer using conductive silver epoxy. The electrodes were then placed in a casing with an opening area of 5 mm. The electrodes were glued with epoxy (Struers), and the electrical connections were potted with the same epoxy. The electrodes were connected to a potentiostat (Metrohm Autolab PGSTAT302N potentiostat) along with a reference electrode (Standard Laboratories SCE) for electrochemical testing. Example 2. Electrode characterization

[0105] The crystallinity of the RuO2 sputter-coated thin films was investigated by scanning electron microscopy and X-ray diffraction. SEM images of the thin film deposited at 250 °C (Si-S1) confirmed its crystalline nature, while the thin film deposited at room temperature (Si-S3) was observed to be amorphous. This result was also confirmed by X-ray diffraction. For example, the X-ray diffraction pattern depicted in Figure 4 confirmed that the RuO2 thin film deposited at 250 °C (Si-S1) was highly crystalline. Raman spectroscopy analysis of the RuO2 thin films was consistent with reported literature data.

[0106] The surface chemical composition and chemical state of the RuO2 and Ta2O5 thin films were analyzed using XPS. Representative spectra for the Si-S1 and Si-S1-P1 electrodes are shown in Figures 5 and 6, respectively. The spectra show that the surface composition of the RuO2 thin film is 2:1 (oxygen to Ru), indicating the RuO2 phase, with no detectable contaminants. The binding energy position of the Ru region also confirmed the RuO2 phase, consistent with published data. For the tantalum oxide film, the binding position of the peaks confirmed the Ta2O5 pentoxide phase. For the Si-S1-P1 electrode, no RuO2 peaks were observed, indicating a consistent Ta2O5 coating on the RuO2 layer. This was also confirmed by ion scattering spectroscopy, a highly sensitive surface analysis technique, which again failed to detect any underlying RuO2. Example 3. Electrodes with different RuO2 layers

[0107] The pH response of the thin-film-coated electrode was evaluated in pH buffer solutions with known pH values ranging from 1 to 10. Figure 7 shows plots of electrode potential as a function of pH for Si-S2-P1 electrodes with different RuO2 film thicknesses. The response is highly linear for film thicknesses of 200, 250, and 300 nm. Long-term testing in buffer solutions with pH values of 2 and 7 demonstrated that the electrode provided stable potential measurements for at least 4.5 months.

[0108] The pH response of the electrode with an amorphous RuO2 layer, i.e., Si-S3-P1, was also evaluated in the pH range from 2 to 10. This response also showed high linearity, with a sensitivity of -50.2 mV / pH (R 2 =0.99). Example 4. Selection of proton-conducting metal oxides

[0109] Different metal oxides were investigated for use as the proton-transporting layer. Electrodes Si-S2-P1 and Si-S2-P2 were fabricated with thin film layers (30 nm thick) of Ta2O5 and yttrium-stabilized zirconia, respectively, deposited on a RuO2 layer as described in Example 1. The pH response of the thin-film-coated electrodes was evaluated in pH buffer solutions with known pH values ranging from 1 to 10. Figure 8 reproduces plots of the electrode potential as a function of pH for both electrodes. Both electrodes exhibit highly linear, Nernstian-like responses and good sensitivity. Example 5. pH response of RuO2+ Ta2O5+ diamond-like carbon electrode

[0110] The effect of the diamond-like carbon layer on electrode performance was evaluated in known pH buffer solutions ranging from pH 1 to 10. Figure 9 shows a comparative plot of the electrode potential as a function of pH for a Si-S2-P1-C1 electrode coated with a 250 nm thick RuO2 layer, a 30 nm thick Ta2O5 layer, and a 50 nm thick diamond-like carbon coating, compared to an equivalent unprotected Si-S2-P1 electrode. It can be seen that the carbon-coated electrode maintains a linear pH response with similar sensitivity. Figure 10 shows the electrode potential response of the Si-S1-P1-C1 electrode as a function of time in buffer solutions ranging from pH 1 to pH 10. The results demonstrate good sensitivity and a rapid response to pH changes. Example 6. Sensitivity of RuO2 + Ta2O5 + diamond-like carbon electrode to interference

[0111] The sensitivity of the Si-S2-P1-C1 electrode (same layer thickness as in Example 4) to reducing species was investigated using Fe 2+ The potential response was measured and evaluated in a pH 2 buffer solution with concentrations varying from 0.001 M to 1 M. The results are shown in Figure 11. It can be seen that the potential is stable over a wide range of reducing agent concentrations.

[0112] The sensitivity of the Si-S2-P1-C1 electrode (with the same layer thickness as in Example 4) to different reducing species was evaluated by measuring the potential response in a pH 2 buffer solution containing varying concentrations of ascorbic acid from 0 to 10 M. In this experiment, the potential of an ORP electrode (Pt) was also measured. Figure 12 shows the results compared to a Si-S2-P1 electrode without a diamond-like carbon coating. Both electrodes exhibited good potential stability despite the presence of various concentrations of ascorbic acid, with the diamond-like carbon-coated Si-S2-P1-C1 electrode exhibiting superior results. Ta2O5, when present, was assisted by the diamond-like carbon layer, protecting the RuO2 layer from interference by reducing agents. In comparison, the unprotected RuO2 layer alone was highly sensitive to interference by reducing agents. Example 7. Role of the proton-permeable layer

[0113] We investigated the role of the Ta2O5 layer on electrode performance. Figure 13 shows a comparative plot of the electrode potential as a function of pH for a Si-S2-P1-C1 electrode with a 250-nm-thick RuO2 layer, a 30-nm-thick Ta2O5 layer, and a 50-nm-thick diamond-like carbon coating, compared with a comparable Si-S2-C1 electrode without the intermediate Ta2O5 layer. The Si-S2-P1-C1 electrode exhibited a linear pH response. However, the Si-S2-C1 electrode did not exhibit a clear correlation between pH and electrode potential. Reproduction experiments also yielded similar results. To prevent interference with the measured potential due to the diamond-like carbon layer, an intermediate, electrically insulating Ta2O5 layer, which physically and electrically separates the pH-sensitive RuO2 layer, is likely necessary. Example 8 pH response of RuO2 + yttrium-stabilized zirconia + diamond-like carbon electrode

[0114] The effect of a diamond-like carbon layer with an intermediate yttrium-stabilized zirconia (YSZ) layer on electrode performance was evaluated in pH buffer solutions with known pH values ranging from 2 to 10. Figure 14 shows a comparative plot of electrode potential as a function of pH for a Si-S4-P2-C1 electrode with a 300 nm thick RuO2 layer, a 50 nm thick YSZ layer, and a 50 nm thick diamond-like carbon coating, and a Si-S2-P2 electrode without a protective carbon layer. It can be seen that the carbon-coated electrode maintains a linear pH response with sensitivity similar to that of the unprotected electrode. Example 9. RuO2 sensing layer configuration

[0115] The effect of the sensing layer configuration was investigated by comparing electrodes with continuous and discontinuous RuO sensing layers. Figure 15 shows a comparative plot of electrode potential as a function of pH for a Si-S2-P2 electrode with a 250 nm thick continuous RuO layer covered by a 30 nm thick continuous YSZ layer, and a Si-S5-P2 electrode with a 250 nm thick discontinuous RuO layer (a square array of 2 μm x 2 μm square blocks) on a 50 nm thick continuous YSZ layer. Both electrodes exhibit a linear pH response with similar sensitivity, demonstrating that a continuous sensing layer is not required. Example 10. Thickness of proton-permeable yttrium-stabilized zirconia layer

[0116] We investigated the effect of the thickness of the proton-conducting layer when using YSZ as the proton-conducting metal oxide. Figure 16 shows a comparative plot of the electrode potential as a function of pH for Si-S4-P2 electrodes with 25, 50, and 100 nm YSZ layers layered on a 300 nm RuO2 layer. A linear pH response was obtained in all cases, with only a slight decrease in sensitivity for the thickest YSZ layer. Thus, increasing the YSZ layer thickness may improve protection of the underlying RuO2 layer, resulting in little loss of selectivity.

[0117] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described, and it is to be understood that the invention includes all such variations and modifications that come within the spirit and scope of the invention. A preferred embodiment of the present invention is as follows. [1] 1. A multilayer electrode for sensing pH, the electrode comprising: a sensing layer on the substrate, the sensing layer comprising at least one proton-sensitive metal oxide, wherein the pH-dependent potential of the multilayer electrode is measurable via a conductive connection to the sensing layer; a proton permeable layer covering at least a portion of the sensing layer, the proton permeable layer comprising at least one electrically insulating, proton-conducting metal oxide; and A carbonaceous layer on the proton permeable layer, the carbonaceous layer comprising amorphous carbon. [2] The multilayer electrode according to [1], wherein the carbonaceous layer is a continuous film that covers at least a portion of the proton permeable layer. [3] The multilayer electrode according to [1] or [2], wherein the carbonaceous layer has a thickness of 500 nm or less. [4] The multilayer electrode according to any one of [1] to [3], wherein the amorphous carbon includes diamond-like carbon. [5] Amorphous carbon is 10% or more of the total carbon sp 3 The multilayer electrode according to any one of [1] to [5], which contains hybridized carbon. [6] The multilayer electrode according to any one of [1] to [5], wherein the carbonaceous layer is formed by plasma enhanced chemical vapor deposition. [7] The multilayer electrode according to any one of [1] to [6], wherein the proton-conductive metal oxide is selected from the group consisting of tantalum pentoxide, yttrium-stabilized zirconia, yttrium-doped barium cerate, barium cerium yttrium zirconate, and gadolinium oxide-doped cerium oxide. [8] The proton-sensitive metal oxide is ruthenium oxide (RuO 2 ), platinum oxide (PtO 2 ), iridium oxide (IrO 2 ), osmium oxide (OsO 2 ), tin oxide (SnO 2 ), titanium oxide (TiO 2 ), antimony oxide (Sb 2 O 3 ), rhodium oxide (RhO 2 The multilayer electrode according to any one of [1] to [7], wherein the metal oxide is selected from the group consisting of palladium oxide (PdO). [9] The multilayer electrode according to any one of [1] to [8], wherein the substrate comprises a conductive material such that the conductive connections pass through or along the substrate.

[10] The multilayer electrode according to [9], wherein the conductive material comprises a semiconductor.

[11] The multilayer electrode according to any one of [1] to

[10] , wherein the substrate comprises a semiconductor wafer having a first surface, a second surface, and a thickness of 300 μm or more and 3 mm or less, a detection layer is disposed on the first surface, and the pH-dependent potential of the multilayer electrode can be measured via the second surface.

[12] The multilayer electrode according to any one of [1] to

[11] , wherein the detection layer has a thickness of 200 nm or more and 1000 nm or less.

[13] The multilayer electrode according to any one of [1] to

[12] , wherein the proton permeable layer has a thickness of 10 nm or more and 100 nm or less.

[14] The multilayer electrode according to any one of [1] to

[13] , wherein the detection layer and the proton permeable layer are formed by sputtering.

[15] The multilayer electrode according to any one of [1] to

[14] , wherein the proton-sensitive metal oxide comprises ruthenium oxide.

[16] The multilayer electrode according to any one of [1] to

[15] , wherein the proton-conductive metal oxide comprises tantalum pentoxide or yttrium-stabilized zirconia.

[17] A pH sensor comprising the multilayer electrode according to any one of [1] to

[16] , a reference electrode, and a means for measuring the difference between the pH-dependent potential of the multilayer electrode and the reference potential of the reference electrode.

[18] 1. A method for measuring pH, comprising: contacting the multilayer electrode and the reference electrode according to any one of [1] to

[16] with an analyte; and Measuring the difference between the pH-dependent potential of the multilayer electrode and the reference potential of the reference electrode.

[19] The method of

[18] , wherein the analyte comprises abrasive particulate matter and / or has a pH of 4 or less.

[20] The method of

[18] or

[19] , wherein the analyte is a leachate or leach slurry from a mineral leaching process or an acid mine drainage stream.

[21] 1. A method for manufacturing a multilayer electrode for sensing pH, comprising: forming a sensing layer on the substrate, the sensing layer comprising at least one proton-sensitive metal oxide; forming a proton permeable layer covering at least a portion of the proton sensitive layer, the proton permeable layer comprising at least one electrically insulating proton conducting metal oxide; forming a carbonaceous layer on the proton permeable layer, the carbonaceous layer comprising amorphous carbon; and Providing conductive connections to the sensing layer to measure the pH-dependent potential of the multilayer electrode.

[22]

[21] The method of

[21] , wherein forming the carbonaceous layer comprises depositing amorphous carbon on the proton-permeable layer by plasma-enhanced chemical vapor deposition.

[23] The method of

[21] or

[22] , wherein forming the sensing layer comprises depositing a proton-sensitive metal oxide on the substrate by sputtering.

[24] The method according to any one of

[21] to

[23] , wherein forming the proton-permeable layer comprises depositing a proton-conducting metal oxide on the sensing layer by sputtering.

[25] The method according to any one of

[21] to

[24] , wherein the substrate comprises a conductive material such that the conductive connection passes through or along the substrate.

[26] The method according to any one of

[21] to

[25] , wherein the substrate includes a semiconductor wafer having a first surface, a second surface, and a thickness of 300 μm or more and 3 mm or less, and the detection layer is formed on the first surface.

[27] The method according to any one of

[21] to

[26] , wherein the proton-conducting metal oxide is selected from the group consisting of tantalum pentoxide, tantalum pentoxide, yttrium-stabilized zirconia, yttrium-doped barium cerate, barium cerium yttrium zirconate, and gadolinium oxide-doped cerium oxide.

[28] The proton-sensitive metal oxide is ruthenium oxide (RuO 2 ), platinum oxide (PtO 2 ), iridium oxide (IrO 2 ), osmium oxide (OsO 2 ), tin oxide (SnO 2 ), titanium oxide (TiO 2 ), antimony oxide (Sb 2 O 3 ), rhodium oxide (RhO 2 The method according to any one of

[21] to

[27] , wherein the compound is selected from the group consisting of palladium oxide (PdO).

Claims

1. 1. A multi-layer electrode for sensing pH, the electrode comprising: a sensing layer on the substrate, the sensing layer comprising at least one proton-sensitive metal oxide, wherein the pH-dependent potential of the multilayer electrode is measurable via a conductive connection to the sensing layer; a proton permeable layer covering at least a portion of the sensing layer, the proton permeable layer comprising at least one electrically insulating proton-conducting metal oxide; and A carbonaceous layer on the proton-permeable layer, the carbonaceous layer comprising amorphous carbon.

2. 10. The multilayer electrode of claim 1, wherein the carbonaceous layer is a continuous coating that covers at least a portion of the proton-permeable layer.

3. 3. The multilayer electrode according to claim 1, wherein the carbonaceous layer has a thickness of 500 nm or less.

4. 4. The multilayer electrode according to claim 1, wherein the amorphous carbon comprises diamond-like carbon.

5. Amorphous carbon is 10% or more of the total carbon sp ３ A multilayer electrode according to any one of claims 1 to 4, comprising hybridized carbon.

6. A multilayer electrode described in any one of claims 1 to 5, wherein the electrically insulating proton-conducting metal oxide in the proton-permeable layer is selected from the group consisting of tantalum pentoxide, yttrium-stabilized zirconia, yttrium-doped barium cerate, barium cerium yttrium zirconate, and gadolinium oxide-doped cerium oxide.

7. The proton-sensitive metal oxide in the sensing layer is ruthenium oxide (RuO ２ ), platinum oxide (PtO ２ ), iridium oxide (IrO ２ ), osmium oxide (OsO ２ ), tin oxide (SnO ２ ), titanium oxide (TiO ２ ), antimony oxide (Sb ２ O ３ ), rhodium oxide (RhO ２ 7. The multilayer electrode according to claim 1, wherein the metal oxide is selected from the group consisting of palladium oxide (PdO), palladium monoxide (PdO), and palladium monoxide (PdO).

8. A multilayer electrode according to any one of claims 1 to 7, wherein the substrate comprises a conductive material such that the conductive connections pass through or along the substrate.

9. The multi-layer electrode of claim 8 , wherein the conductive material comprises a semiconductor.

10. 10. The multilayer electrode according to claim 1, wherein the substrate comprises a semiconductor wafer having a first surface, a second surface, and a thickness of 300 μm or more and 3 mm or less, a sensing layer is disposed on the first surface, and a pH-dependent potential of the multilayer electrode can be measured via the second surface.

11. The multilayer electrode according to any one of claims 1 to 10, wherein the sensing layer has a thickness of 200 nm to 1000 nm.

12. 12. The multilayer electrode according to claim 1, wherein the proton-permeable layer has a thickness of 10 nm or more and 100 nm or less.

13. The multilayer electrode according to any one of claims 1 to 12, wherein the proton-sensitive metal oxide comprises ruthenium oxide.

14. A multilayer electrode according to any one of claims 1 to 13, wherein the electrically insulating proton-conducting metal oxide comprises tantalum pentoxide or yttrium-stabilized zirconia.

15. A pH sensor comprising the multilayer electrode according to any one of claims 1 to 14, a reference electrode, and a means for measuring the difference between the pH-dependent potential of the multilayer electrode and the reference potential of the reference electrode.

16. 1. A method for measuring pH, comprising: contacting the multilayer electrode and the reference electrode according to any one of claims 1 to 14 with an analyte; and Measuring the difference between the pH-dependent potential of the multilayer electrode and the reference potential of the reference electrode.

17. 17. The method of claim 16, wherein the analyte comprises abrasive particulate matter and / or has a pH of 4 or less.

18. 18. A method according to claim 16 or claim 17, wherein the analyte is a leachate or leach slurry from a mineral leaching process or an acid mine drainage stream.

19. 1. A method for manufacturing a multi-layer electrode for sensing pH, comprising: forming a sensing layer on the substrate, the sensing layer comprising at least one proton-sensitive metal oxide; forming a proton permeable layer overlying at least a portion of the proton sensitive layer, the proton permeable layer comprising at least one electrically insulating proton conducting metal oxide; forming a carbonaceous layer on the proton permeable layer, the carbonaceous layer comprising amorphous carbon; and Providing conductive connections to the sensing layer to measure the pH-dependent potential of the multilayer electrode.

20. 20. The method of claim 19, wherein forming the carbonaceous layer comprises depositing amorphous carbon on the proton permeable layer by plasma-enhanced chemical vapor deposition.

21. 21. The method of claim 19 or claim 20, wherein forming the sensing layer comprises depositing a proton-sensitive metal oxide on the substrate by sputtering.

22. 22. The method of any one of claims 19 to 21, wherein forming the proton-permeable layer comprises depositing an electrically insulating, proton-conducting metal oxide on the sensing layer by sputtering.

23. A method according to any one of claims 19 to 22, wherein the substrate comprises a conductive material such that the conductive connection passes through or along the substrate.

24. 24. The method of any one of claims 19 to 23, wherein the substrate comprises a semiconductor wafer having a first side, a second side, and a thickness of 300 μm or more and 3 mm or less, and the sensing layer is formed on the first side.

25. The method of claim 19, wherein the electrically insulating proton-conducting metal oxide is selected from the group consisting of tantalum pentoxide, tantalum pentoxide, yttrium-stabilized zirconia, yttrium-doped barium cerate, barium cerium yttrium zirconate, and gadolinium oxide-doped cerium oxide.

26. The proton-sensitive metal oxide is ruthenium oxide (RuO ２ ), platinum oxide (PtO ２ ), iridium oxide (IrO ２ ), osmium oxide (OsO ２ ), tin oxide (SnO ２ ), titanium oxide (TiO ２ ), antimony oxide (Sb ２ O ３ ), rhodium oxide (RhO ２ 26. The method of any one of claims 19 to 25, wherein the metal oxide is selected from the group consisting of palladium oxide (PdO) and palladium oxide (PdO).

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