Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
  • G01N27/301—Reference electrodes

Definitions

• the present invention generally relates to the field of solid state reference electrodes, and more particularly, to solid state electrochemical reference electrodes.
• a chemical "reference potential" is often used in conjunction with an electrochemical sensor such as, for example, a pHT. sensor.
• the reference potential is analogous to the ground potential in an electrical circuit.
• the reference potential is derived from a reference electrode that is immersed in a separate reference cell, where the reference cell is in ionic communication with the test solution.
• the reference electrode develops a reference potential through insoluble silver or mercury salts that are in contact with their metals.
• the salts are typically contained in a conducting, ionic solution that is in ionic contact with the test solution.
• the ionic contact between the ionic solution of the reference cell and the test solution is typically provided via a porous layer, which allows ions to flow from the reference cell to the test solution and vice versa.
• a porous layer which allows ions to flow from the reference cell to the test solution and vice versa.
• One problem with many of these systems is that if the ionic flow rate through the porous layer is too low, drift can be introduced into the measurement, and if the ionic flow rate is too high, the ionic solution can be a source of contamination to the test solution and visa versa.
• Summary of the Invention generally relates to solid state electrochemical reference electrodes. Solid state electrochemical reference electrodes in accordance with the present invention may reduce measurement drift, as well contamination of " the test solution.
• the solid state electrochemical reference electrodes of the present nrvention . y reliable than many previous reference electrode configurations.
• a reference electrode in one illustrative embodiment of the present invention, includes a conductive substrate that has a first surface and an ionically insulating, hydrophobic layer positioned adjacent to the first surface.
• the ionically insulating, hydrophobic layer has a plurality of non-selective ion exchange sites on its exposed surface.
• the ionically insulating, hydrophobic layer is exposed to the test solution, and because of the ion exchange sites, generates an ionic charge on the surface of the ionically insulating, hydrophobic layer.
• a non-selective ion exchange layer is also provided on or in the ionically insulating hydrophobic layer to increase the number of exposed ion exchange sites, and thus the sensitivity of the reference electrode.
• the non-selective ion exchange layer is a non-selective ion exchange zeolyte layer.
• any suitable non-selective ion exchange layer may be used, depending on the application.
• the reference potential generated on the conductive substrate is provided to a high impedance input of an electric circuit such as an amplifier or the like.
• the reference potential may be provided to the gate of a Field Effect Transistor (FET), or any other suitable high impedance circuit.
• the reference potential may be provided to the substrate of a FET.
• the gate of the FET may be connected to the output of an electrochemical sensor within the test solution. The reference potential at the gate voltage provided by the electrochemical sensor.
• FET configuration may help provide a good chemical to electrical transducer.
• Figure 1 is a cross-sectional view of a reference electrode according to an embodiment of the invention
• Figure 2 is a cross-sectional view of a reference electrode according to an embodiment of the invention
• Figure 3 is a cross-sectional view of a reference electrode electrically coupled to a field effect transistor
• Figure 4 is a cross-sectional view of a reference electrode according to an embodiment of the invention
• Figure 5 is a cross-sectional view of a reference electrode according to an embodiment of the invention
• Figure 6 is a graph of pH meter response (mV) using the inventive reference electrode over a pH range of 4 to 10 with Parylene "C” and Parylene "H” as an ionically insulating, hydrophobic layer
• Figure 7 is a graph of pH meter response (mV) using the inventive reference electrode over a pH range of 4 to 10 with Teflon AF as an ionically
• FIG. 1 is a cross-sectional view of a reference electrode 100 according to an illustrative embodiment of the invention.
• the reference electrode 100 includes a conducting substrate 110 having a first surface 115.
• An ionically insulating, hydrophobic layer 120 is provided adjacent to the conducting substrate 110 first surface 115.
• the ionically insulating, hydrophobic layer 120 has a plurality of non- selective ion exchange sites 130 that may be on or near an outer surface of the ionically insulating, hydrophobic layer 120.
• the conducting substrate 110 can be any electrically conductive material capable of forming an image charge 140.
• the conducting substrate 110 can be, for example, any metal or semiconductor material, as desired, and can be any suitable size.
• the conducting substrate can be any electrically conductive material capable of forming an image charge 140.
• the conducting substrate 110 can be, for example, any metal or semiconductor material, as desired, and can be any suitable size.
• the conducting substrate can be, for example, any metal or semiconductor material, as
• the 110 can include a non-conducting substrate with a conducting layer provided thereon.
• the ionically insulating, hydrophobic layer 120 can be any ionically insulating, hydrophobic material.
• the ionically insulating, hydrophobic layer 120 is non-porous, non-reactive and non-hydrating.
• c ⁇ O o e us rauv em o me an also be chemically inert, thermally stable, mechanically stable, readily processable using standard IC processing techniques (spin coating, ion milling, etc.,) provide a stable reference potential over a pH range of 1-14, provide low interference 5 background ionic strength, and/or demonstrate low potential drift over time.
• the ionically insulating, hydrophobic layer 120 may be a polymer or an amorphous polymer.
• the ionically insulating, hydrophobic layer 120 can be polytetrafluoroethylene, amorphous polytetrafluoroethylene, polystyrene, polyethylene, polypropylene, polycarbonate, 10 polymethyl methacrylate, parylene, or mixtures thereof.
• Illustrative examples of useful ionically insulating, hydrophobic layer 120 material includes Teflon AFTM from DuPont, FluoroPelTM from Cytronics, or any other suitable ionically insulating hydrophobic layer material or material composition.
• the ionically insulating, hydrophobic layer 120 can have any thickness, 15 however, it is preferable that the ionically insulating, hydrophobic layer 120 be as thin as possible, preferably less than or equal to 1 micron, but this is not required in all embodiments. In some embodiments, the ionically insulating, hydrophobic layer 120 has a thickness of 0.1 micron to 10 micron, 0.1 micron to 5 micron, or 0.5 micron to 1 micron. 20
• the plurality of non-selective ion exchange sites 130 can be formed from simple impurities on the ionically insulating, hydrophobic layer 120 surface.
• the plurality of non-selective ion exchange sites 130 may include zeolyte particles.
• the non-selective ion exchange sites 130 allow the intercalation of background ions (cations or anions) from a sample fluid.
• the non- 25 selective ion exchange sites 130 strips the ions (such as cations) from their hydrated i ⁇ uiiuings an e s a is es a sur ace c arge , an us an accompanying image charge 140 on the underlying conductive substrate 110.
• the image charge represents the reference potential on the conductive substrate 210.
• background ion concentrations can be many 5 orders of magnitude greater than analyte concentrations.
• a useful non-selective ion exchange site 130 particle is a zeolyte. Zeolyte particles can be disposed on the hydrophobic layer 120 in any useful amount to create a desired ion exchange site density. Useful zeolyte particles have a
• An adhesion promoter 116 can be disposed between the conductive substrate 110 and the ionically insulating, hydrophobic layer 120, but this is not required in all
• the adhesion promoter 116 can be any material that helps join the ionically insulating, hydrophobic layer 120 to the remaining reference electrode 100.
• the adhesion promoter 116 can be a siloxane such as, for example, hexamethyl disiloxane, and the like.
• Figure 2 is a cross-sectional view of a reference electrode 200 according to 0 another illustrative embodiment of the present invention.
• the reference electrode 200 includes a conducting substrate 210 having a first surface 215.
• An ionically insulating, hydrophobic layer 220 is provided adjacent to the conducting substrate 210 first surface 215.
• a non-selective ion exchange layer 225 is disposed on the ionically insulating, hydrophobic layer 220.
• TMin n c s ra e an e y rop o ic ayer ZZ ⁇ can e sim ar to that described above.
• the non-selective ion exchange layer 225 includes non-selective ion exchange sites sufficient in quantity to achieve a desired ion exchange site density.
• the non-selective ion exchange sites, shown at 230 are provided by non-selective ion exchange particles that are provided in sufficient density to constitute a layer 225, and may be formed with zeolyte particles.
• an adhesion promoter 216 can be disposed between the conductive substrate 210 and the ionically insulating, hydrophobic layer 220 as described above.
• the non-selective ion exchange sites 330 allow the intercalation of background ions (cations or anions) from the sample fluid.
• the non-selective ion exchange sites 230 strip the ions (such as cations) from their hydrated surroundings and establishes a surface charge 235 on the non-selective ion exchange layer 225, when then produces an accompanying image charge 240 on or in the underlying conductive substrate 210.
• the image charge produces the reference potential on or in the conductive substrate 210.
• Figure 3 is a cross-sectional view of a reference electrode 300 that is coupled to the gate of a Field Effect Transistor (FET).
• the reference electrode 300 of Figure 3 may be similar to the reference electrode 200 of Figure 2.
• the conductive substrate 310 of the reference electrode 300 is electrically coupled to the gate of a FET device 350, as shown. While a FET device
• the conductive substrate 310 may be electrically coupled to any suitable electrical device or circuit, as desired.
• the conductive substrate 310 is preferably electrically coupled to a relatively high impendence input of an electrical device or circuit. " v ew o a re erence e ec ro e t ⁇ accor ⁇ mg to yet another illustrative embodiment of the present invention.
• the reference electrode 400 includes a conducting substrate 410, with an ionically insulating, hydrophobic layer 420 positioned adjacent to the conducting substrate 410.
• a non-selective ion 5 exchange layer 425 is shown disposed on or adjacent to the ionically insulating, hydrophobic layer 420.
• the conducting substrate 410 is disposed on a dielectric layer 460.
• the dielectric layer 460 is shown disposed on an integrated circuit substrate 480 that includes electronics 450 previously formed therein.
• electronics 450 may include one or more conductive intercomiect pads that are adapted to electrically interconnect to the conducting substrate 410 of the reference electrode 400.
• a VIA 470 is formed through the dielectric layer 460 to electrically connect the conductive substrate 410 and the one or more conductive interconnect pads of electronics 450.
• the electronics may be
• FIG. 15 used to process the reference potential provided by the conducting substrate 410, preferably in conjunction with one or more electrical signals provided by one or more electrochemical sensors disposed in the test solution.
• Figure 5 is a cross-sectional view of a reference electrode 500 according to yet another illustrative embodiment of the present invention.
• the reference electrode 500
• the 20 includes a conducting substrate 510.
• the conducting substrate 510 is a semiconductor wafer.
• An ionically insulating, hydrophobic layer 520 is added adjacent to the conducting substrate 510 first surface 515.
• a non-selective ion exchange layer 525 may be disposed on the ionically insulating, hydrophobic layer 520, as described above.
• the ionically insulating, hydrophobic - e e io xc nge yer are provi ⁇ e ⁇ on tne b ack s e of the semiconductor wafer.
• Electronics 550 may be fabricated into the front side of the semiconductor wafer, as shown.
• electronics 550 include a FET device.
• the substrate of the FET device 550 corresponds to the conducting substrate 510 of the reference electrode 500.
• the reference potential on the conducting substrate 510 is provided to the substrate of the FET device 550.
• the gate of the FET device 550 may be connected to the output of an electrochemical sensor that is in the test solution.
• the reference potential then may help offset or regulate the current supplied by the FET device 550 for a given gate voltage provided by the electrochemical sensor.
• Such a configuration may help provide a good chemical to electrical transducer.
• test fixture was a pH sensor and was designed and fabricated as described below.
• a polycarbonate tube fitted with an O-ring seal on the bottom was clamped against a standard 3" silicon wafer forming a liquid-tight reservoir.
• Three-inch silicon wafers were chosen as the substrate because they are inexpensive, readily available, easily processed using available equipment and have well-controlled electrical and surface characteristics.
• the wafer was supposed against an aluminum base, which also provided a convenient means for making electrical contact to it.
• the hydrophobic, ionically insulating material under investigation was coated onto the top surface of the wafer and the potential of the coated surface was monitored against a standard pH probe and double junction Ag/AgCI reference probe through an Orion pH meter. The output was also recorded on a strip chart recorder to monitor stability and drift using buffers of pH 4, 6, 8 and 10.
• Parylene poly-p-xylene
• Parylene samples (“C” and “H”) were obtained from Specialty Coating Systems, Inc., (Clear Lake, WI), who specialize in vacuum deposition of parylene and other conformal coatings.
• Samples of Parylene "C”, a standard commercial grade material and Parylene "H”, a higher density material with lower water absorption were obtained as 0.5 ⁇ m pmhole free films on 3" silicone wafers (HTC supplied wafers). The films were tested in the test apparatus described in Part I, and the results are seen in Figure 6. s n re ⁇ oo ⁇ very goo , w
• Parylene is intended to be used as a thick film enviromnental coating with low (total) water absorption. In this application, water penetration in the first few microns of the material is inconsequential. In our case, however, the total film thickness is ⁇ 1 ⁇ m and even minor water absorption into the film has an effect over time. From the graph, we can also see that the effect is cumulative implying a gradual increase in ion conductivity rather than a sudden failure such as a loss of adhesion and lift off of the surface.
• Teflon AF poly-tetrafluroethylene
• Teflon AF 1601S
• FC-75 a perfluronated hydrocarbon solvent from 3M
• Wafers with native oxide were then spin-coated at 3000 RPMs for 30-40 seconds to get a 0.5 to 1 micron coating.
• the wafers were baked at 160°C for at least 10 minutes to boil off excess solvent and cure the polymer film.
• Elipsometry of the films indicated an average thickness of approximately 0.6 ⁇ m with an index of refraction of 12.3, which is consistent with the manufacturer's specifications. s ooin a inger across them, and completely lifted off immediately on contact with our test buffers.
• HMDS hexamehtyl disiloxane
• CBV5524G (lot number 1822-18) and CP814E (lot number 1822-35) are size exclusion zeolytes with an internal cavity to accommodate large ammonium ions. Smaller ions all freely exchange with the cavity with no preference toward chemical nature. This is almost the ideal case for a reference electrode where "ion non- specificity" may be important.
• Figure 10 shows the typical result for CP814E (lot , although slightly super-Nernstian. Zeolyte CBV5524G (lot number 1822-18) has somewhat smaller pore size than CP814E (lot number 1822-35).

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