This application is a continuation-in-part of U.S. patent application Ser. No. 16/794,226, filed Feb. 19, 2020, which claims priority to U.S.
The invention relates to electrochemical gas sensors.
Electrochemical gas sensors measure the concentration of a target gas by recording the oxidizing or reducing Faradic current of the gas molecules at an electrode surface. Conventional electrochemical gas sensors are normally constructed by soaking a stack of working electrodes, a reference electrode, and a counter electrode in a liquid electrolyte and sealing them in a container with a cavity. A gas-permeable membrane next to the cavity allows gas to pass through and retains the electrolyte within the container. Electrochemical gas sensors outperform other types of sensors, such as microfabricated metal oxide sensors, in their sensitivity, selectivity, response time, and power consumption but are significantly larger in size. Although electrodes can be miniaturized through microfabrication, incorporating electrolytes and membranes into the microfabrication process is challenging. Low production rates and the use of a significant amount of noble metals keep the cost high. These disadvantages limit the application of electrochemical sensors in smartphones and wearable devices.
To address the challenges in the aforementioned art, the present disclosure involves microfabricated electrochemical gas sensor structure without utilizing a gas permeable membrane. In accordance with an embodiment of the present disclosure, the sensor structure consists of microcavities which penetrate through a stack of microfabricated electrodes, a cavity connecting the microcavities, and a liquid electrolyte in contact with the electrodes. The microcavities allow gases to pass through and, at the same time, prevent leakage of the electrolyte through surface tension.
A first exemplary embodiment of the microfabricated electrochemical gas sensor is depicted in
A second exemplary embodiment of the microfabricated electrochemical gas sensor consists of two conductor layers and three dielectric layers. A cross-section view of the second exemplary embodiment is depicted in
A third exemplary embodiment of the microfabricated electrochemical gas sensor consists of only one conductor layer that is sandwiched between two dielectric layers. A cross-section view of the third exemplary embodiment is depicted in
A fourth exemplary embodiment of the microfabricated electrochemical gas sensor consists of multiple sensing units.
A fifth exemplary embodiment of the microfabricated electrochemical gas sensor is shown in
Referring to the substrate materials, a flat and smooth substrate surface is required by photolithography in order to accurately produce the electrodes and the microcavities. The substrate material can be a conductor, a semiconductor, an insulator, or a composite of different materials. A substrate either is a dielectric in nature, or can be processed with a dielectric coating, so that the electrodes produced with the bottom conductor layer are not short-circuited. For example, oxidized silicon wafer, polyimide sheets, quartz plates, and complementary metal oxide semiconductor (CMOS) integrated circuits (IC) are commonly used substrates. The substrate can also be an assembly of two or more substrate materials, which can be raw or processed. For example, a polyimide sheet is fabricated with electrodes, and then is aligned and bonded onto a rigid printed circuit board with pre-drilled holes acting as the backside cavities.
Referring to the conductor layers and the electrode materials that compose the microfabricated electrochemical gas sensor, an electrode layer is fabricated with conductor materials with their necessary electrochemical functions. The conductor layers 102, 104, and 106 comprise conductor or semiconductor materials including, but not limited to, metals, such as gold (Au), silver (Ag), platinum (Pt), palladium, rhodium, and nickel, metal oxides or ceramics, such as aluminum oxide, indium tin oxide (ITO), and titanium nitride (TiN), conductive polymers, such as polypyrrole and polyaniline, and carbon of certain forms, such as boron doped diamond, grapheme, and carbon nanotubes (CNTs). A conductor layer may require a sublayer of adhesion material, such as 5 nm of titanium. The surface of a conductor layer can be partially modified with a different material, for example, carbon nanotubes modified with platinum nanoparticles. A conductor layer or a portion of it can be transformed into a compound. For example, Ag can be partially transformed into AgCl to form Ag/AgCl. The reference electrode material is selected to be compatible with the electrolyte and stable over time.
Referring to the dielectric layer materials that compose the microfabricated electrochemical gas sensor, the dielectric layers 101, 103, and 105 provide electrical insulation and spacing between electrodes and assist in retaining the electrolyte. Dielectric materials, such as silicon oxide, silicon nitride, polyimide, polytetrafluoroethylene (PTFE), or enzocyclobutene (BCB) are commonly used. A dielectric layer may consist of multiple sublayers, for example, a stack of silicon oxide, silicon nitride, and silicon oxide layers. A dummy conductor sublayer can be sandwiched between two dielectric sublayers. The surface of a capping layer can be modified hydrophobic to have a large contact angle with the electrolyte.
An exemplary SO2 sensor, for instance, can be produced with a gold working electrode layer, a silver/silver chloride (Ag/AgCl) pseudo reference electrode layer, and a platinum (Pt) counter electrode layer on top of an oxidized silicon wafer. The Ag/AgCl interface can be made by chlorinating the Ag electrode. Cylindrical microcavities of tens of nanometers to tens of micrometers in diameter penetrate through the electrodes into the substrate. A backside about the size of the overall electrode area penetrates the substrate and connects to the microcavities. The microcavities are filled with electrolyte, for example, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM NTF2) ionic liquid. To detect sulfur dioxide (SO2), the sensor is operated at a bias potential of 1.4 V (vs Ag/AgCl). With a reduction bias potential, the same SO2 sensor can be converted into a hydrogen sulfide (H2S) sensor. The sensor can be modified to detect other gases such as: carbon monoxide, carbon dioxide, alcohol vapor, propane, formaldehyde, benzene, methane, hydrogen cyanide, nitric oxide, nitrogen dioxide, oxygen, and ozone.
Referring to the fabrication process for producing the present invention, an exemplary microfabrication process flow for the second embodiment is illustrated in
In broad embodiment, the present invention is a microfabricated membraneless electrochemical gas sensor. The sensor consists of a layer stack on the top surface of a substrate. The layer stack consists of two or more thin-film electrodes produced from one or more conductor layers, and a capping layer over the top conductor layer. The sensor also consists of one or more microcavities that penetrate the layer stack, and a cavity from the bottom surface of the substrate connecting to the microcavities in the substrate. An electrolyte is housed in the internal space created by the microcavities and the cavity, allowing electrolytic communication among all electrodes exposed in the microcavity or the cavity. The electrolyte is kept within the microcavities by its surface tension with the capping layer. Multiple sensor units can be produced in the same substrate. The electrode material, the electrolyte, and the bias potential can be the same or different from one sensor unit to another.
While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention.
This invention was made with Government support under Grant No. 1913640, awarded by the National Science Foundation. The Government has certain rights in this invention.
Number | Name | Date | Kind |
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11346804 | Huang | May 2022 | B2 |
Number | Date | Country |
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202020106890 | Mar 2021 | DE |
2019095329 | Jun 2019 | JP |
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Number | Date | Country | |
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20220244215 A1 | Aug 2022 | US |