JP6076749B2 - Small electrochemical sensor - Google Patents

Description

  The present invention generally relates to a miniature electrochemical sensor for detecting a component in a gas, a device for measuring the content of nitric oxide in exhaled air, including such a miniature electrochemical sensor, and such a miniature electrical sensor. The present invention relates to a method of manufacturing a chemical sensor.

  In recent years, extensive research has been conducted on NO as a biomarker. One use of such biomarkers is in the detection of respiratory inflammation such as asthma. The concentration of nitric oxide (NO) in the breath serves as an indicator of inflammation in the airways of asthmatic patients. Thus, the use of exhaled NO (eNO) is considered a promising means in the diagnosis of asthma. However, the concentration of eNO in exhaled breath is very low. In healthy adults, the concentration of NO is about 10-35 ppb (parts per billion), while in children, the concentration is about 5-25 ppb. More than 70-100 ppb in patients with a significant amount of inflammation.

  Sensors for detecting NO using electrochemical methods have been shown in the past. However, the combination of rapid response, small size, and high sensitivity is not sufficient for measuring exhaled NO. Conventional electrochemical sensors can detect gas concentrations down to several ppb, but suffer from long response times, typically on the order of 60-100 seconds. They therefore require manipulation and buffering of the complex flow of the breath sample.

  The object of the present invention is to alleviate the drawbacks discussed above and to provide a sensor for detecting components in a gas with high sensitivity and short response time.

  It is also an object to provide a sensor that can be manufactured cost-effectively and is small enough to be incorporated into a hand-held analytical device.

  Thus, the present disclosure relates to a miniature electrochemical sensor for detecting components in a gas. The miniature electrochemical sensor includes a reference electrode, a counter electrode, and a structure that includes a plurality of passages delineated by walls extending along the passages. The working electrode covers the wall of the structure, and the ionomer layer covers at least a portion of the working electrode along the structure wall. The ionomer layer is in ion conductive contact with the electrodes, ie, the reference electrode, the counter electrode, and the working electrode.

  By arranging the working electrode and ionomer layer along the walls of the passage, a sensor with a large detection area is realized. In this way, the sensitivity of the sensor can be increased while still maintaining the small size of the sensor. A structure having a passage containing a layer of ionomer can also achieve a short response time of the sensor.

  The passageway may have an aspect ratio of at least 0.25, at least 1, at least 4, at least 10, at least 20, or at least 50. Thus, the detection area can be wider or much wider than the corresponding planar sensor, and the sensitivity is directly determined by the aspect ratio of the passage.

  The passageway may have a cross-sectional dimension in the range of 1 to 300 micrometers, or in the range of 10 to 150 micrometers. Thus, the passage can provide a large surface area of the structure while facilitating the formation of the ionomer layer.

The surface area of the ionomer layer, footprint (foot print area) per 2000～2Cm ² ranging from 1 cm ^2, or 1 cm ² range footprint per 1000～10Cm ^2, or 1 cm ² footprint per 200～20Cm ² It may be a range. The installation area is defined in a plane perpendicular to the direction in which the passage extends. Therefore, the sensitivity of the sensor can be high.

  The sensor may include first and second surfaces, where the first surface is exposed to the gas and the passage extends from the first surface to the second surface. Thus, the passage may be exposed to gas at the first surface and exposed to another portion of the sensor at the second surface.

  The structure may be a porous structure in which the passages are formed as pores. The pores may extend in parallel throughout the structure. The pores may be provided in a close packed arrangement, and the close packed arrangement may be at least one of a hexagonal arrangement, a rectangular arrangement, a square arrangement, and a triangular arrangement. Thus, a large surface area of the structure can be obtained.

  The ionomer may be a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, such as Nafion. The ionomer may be a proton conductor. The ionomer may be Nafion product SE-5112 available from DuPont. In this way, the sensor can be highly sensitive to components in the gas.

  The thickness of the ionomer layer covering the working electrode may be in the range of 10-2000 nm, or 100-1000 nm, or 300-700 nm. In this way a short response time of the sensor can be obtained.

  The ionomer may comprise a nanostructured solid material. In this way, the effective surface area of the layer at the microscopic level can be greatly increased, further increasing the sensitivity of the sensor.

  The sensor may further include a liquid electrolyte in contact with the ionomer layer and the reference and counter electrodes. Therefore, the sensitivity of the sensor to the change in humidity in the surrounding gas can be lowered, and the long-term stability of the sensor can be improved. The second surface of the structure may be in contact with the liquid electrolyte so as to allow the layer of ionomer in the wall of the passage to contact, wet with or wet the liquid electrolyte.

  The sensor may include a housing for the volume of liquid electrolyte. Therefore, the operability of the sensor can be improved by enclosing the liquid electrolyte in the sensor.

  The structure may be formed by micromachining. The structure may be formed by micromachining of a silicon material. Thus, the structure may have features in the micrometer range to obtain a large surface area. The structure may also be manufactured in batches and at a low cost.

  The working electrode may comprise a material selected from the group consisting of platinum, gold, palladium, carbon, and ruthenium.

The working electrode may be insulated from the structure by an insulating layer. The insulating layer may include a material selected from the group consisting of Al ₂ O ₃ , SiO ₂ , HfO ₂ , and LaO. Accordingly, the electrode area is limited to the passages and surfaces of the structure.

  The component in the gas may be NO. Thus, the sensor can be used to analyze the concentration of NO in exhaled breath, for example as an asthma biomarker.

  The present disclosure further relates to a device for measuring the content of NO in exhaled breath, comprising a miniature electrochemical sensor according to the disclosure herein.

The disclosure further includes
Providing a structure including a plurality of passages delineated by walls extending along the passages;
Coating the wall of the structure with a working electrode;
Disposing a layer of ionomer covering at least a portion of the working electrode along the wall of the structure, and a method of manufacturing a miniature electrochemical sensor disclosed herein.

  The structure can be provided by etching a silicon material, preferably by a reactive ion deep etching process of the silicon material. Thus, it is possible to produce structures with passages having very high aspect ratios AR up to and above 100.

  Small electrochemical sensors may be batch manufactured using silicon micromachining.

  The ionomer layer may be deposited by dip coating and dried under low pressure. Thus, the ionomer layer can be deposited inside the channel in a simple manner.

  The working electrode may be formed by atomic layer deposition. Thus, the electrode can be formed inside the passage even if the aspect ratio of the passage is very high, covering the walls of the passage.

This method further:
-Providing a liquid electrolyte in electrical contact with the working electrode;
Providing a reference electrode and a counter electrode in electrical contact with the liquid electrolyte.

The invention will now be described by way of example with reference to the accompanying drawings:
FIG. 1 shows a cross-sectional view of a small electrochemical sensor. FIG. 2 shows an enlarged view of a portion of the structure in a small electrochemical sensor. FIG. 3 shows various arrangements of passages in a miniature electrochemical sensor.

DESCRIPTION OF EMBODIMENTS In the following, a detailed description of a small electrochemical sensor for detecting components in a gas is disclosed.

FIG. 1 shows a small electrochemical sensor 1 for detecting a component in a gas. The component in the gas may be a gas component such as NO. The sensor is an electrochemical sensor by current measurement, and includes a reference electrode 2, a counter electrode 3, and a working electrode 7. The reference and counter electrodes are supported by a substrate 14 formed of a material such as plastic (such as polycarbonate), glass, ceramic, or silicon. The counter electrode is made of silver (Ag) and the reference electrode is made of oxidized silver ( Ag ₂ O ). As further disclosed in WO 2011073393 A2, the electrode may comprise electrical vias 15, 16, and 17 for penetrating the substrate to obtain electrical contact. The sensor further includes a structure 4, which has a first surface 9 and a second surface 10, and has a plurality of passages 5 extending across the structure from the first surface to the second surface. Form a grid that contains. The passage 5 is formed as a pore which is contoured by walls (6a, 6b) extending along the passage. The sensor forms a chamber 13 enclosed by the substrate 14 and the second surface of the structure 4, which chamber contains a liquid electrolyte 11 such as a weak acid solution (eg, a 10% H ₂ SO ₄ aqueous solution). Thus, the second surface of the structure faces the chamber 13 and is in contact with the liquid electrolyte. A spacer is used to maintain a spacing of about 500 micrometers between the second surface of the structure having the working electrode and the counter / reference electrode. The base material 14 may include a through hole 18 for supplying a liquid electrolyte into the chamber. This may be used to compensate for the evaporation of the liquid electrolyte through the passage of the structure. As further disclosed in WO 2011073393 A2, the through hole may be sealed with a plug 19.

  In FIG. 2, the structure 4 defining the passage 5 delimited by the walls 6a, 6b is shown in more detail. The passages may be linear pores arranged in parallel and distributed over the entire structure 4. The structure 4 supports a working electrode 7, which covers the walls 6a, 6b of the structure and thus extends along the passage 5 from the first surface 9 to the second surface 10. The working electrode further covers at least a portion of the first surface 9 and the second surface 10 to electrically connect the electrodes in the passage to form a working electrode having a large surface area. The structure 4 may be formed of single crystal silicon, and the working electrode may be formed of platinum (Pt). The working electrode is electrically insulated from the structure by an insulating layer 12 covering the structure forming the passage.

The portion of the structure with the passage may occupy an area of several mm ² (such as 6 × 6 mm ² ). The passage may have a cross-sectional dimension in the range of 1-300 micrometers, or in the range of 10-150 micrometers, typically about 120 micrometers. The width of the grid walls defining the passage may be in the range of 1-100 micrometers, typically about 20 micrometers. The length of the passage may be in the range of 10-2000 micrometers, or in the range of 50-850 micrometers, typically about 300 micrometers. The term aspect ratio (AR) is defined as the ratio of the height (h) to the width (w) of a structure or passage, ie AR = h / w. Thus, the aspect ratio AR of the passage may be at least 0.25, at least 1, at least 4, at least 10, at least 20, or at least 50. A high AR can result in a large surface area of the walls that define the passage of the structure.

A layer 8 of Nafion (SE-5112, DuPont (Dupont), CAS number 31175-20-9) is deposited on the working electrode of the passage 5 and on the surfaces 9 and 10. Nafion, a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, is an ionomer selected to be a proton (hydrogen ion, H ⁺ ) conductor. This material has the following structure;

The Nafion layer is about 100-1000 nm thick, or about 500 nm thick and extends across the walls of the passage. The layer of Nafion comprises particles with dimensions in the nanometer range, i.e. nanoparticles, which results in a very large material surface area. The layer of Nafion is in electrical contact with the counter electrode and the reference electrode via the liquid electrolyte.
In FIG. 3, four different arrangements of passages and walls are disclosed. FIG. 3 a shows the arrangement of the passages 5 having a triangular cross section in the structure 4. The passages are defined by walls arranged at an angle of 60 ° with respect to each other. Thus, all the passages 5 are defined by three walls extending along the passages. FIG. 3 b shows the arrangement of the passages 5 having a hexagonal cross section in the structure 4. The passage 5 is thus defined by six walls arranged at an angle of 120 ° with respect to each other. FIG. 3 c shows the square arrangement of the passages 5 in the structure 4. The passages are defined by walls arranged at right angles to each other. Thus, all the passages 5 are defined by four walls extending along the passages. FIG. 3 d shows the arrangement of the cylindrical passages 5 formed in the structure 4. In this case, all the passages 5 are defined by walls forming a section of a cylindrical passage extending over the whole structure. In each of these examples, the passages are closely arranged, forming a tightly packed arrangement of passages in the structure.

In the following, an example of manufacturing a small electrochemical sensor is disclosed. A structure supporting the working electrode is produced by providing a double-side polished silicon wafer having a diameter of 100 mm and a thickness of 300 micrometers. A silicon wafer is spin coated with a layer of photoresist (AZ 9260) having a thickness of 6 micrometers. The wafer with the photoresist layer is then soft baked on a hot plate for 2 minutes and then exposed for 15 seconds with 300 mW / cm ² intensity UV light through a lithography mask. The photoresist is then developed for 3 minutes using a developer 2401 to define the pattern. The pattern defines the walls and passages of the structure 4. The structure is then etched using a reactive ion deep etching process for 1.5 hours to obtain a grid structure having walls and passages.

Next, the etched silicon wafer is transferred to an atomic layer deposition chamber (Beneq TFS 200). Here, a 10 nm Al ₂ O ₃ layer is deposited on the structure, followed by a 10 nm thick platinum layer. The wafer is then cut into chips of about 10 × 10 mm ² dimensions.

  The chip with the structure is then immersed in a 5% Nafion solution (SE-5112, DuPont) and dried in a low pressure chamber. This low pressure treatment helps to remove air in the micropores, thus facilitating the deposition of Nafion in the passage. The Nafion coated chip is then removed from the low pressure chamber and dried in air for 2 hours. A grid structure coated with Nafion is used as the working electrode of the sensor.

The counter electrode and the reference electrode are manufactured on a 2 mm thick polycarbonate (PC) substrate. A 500 nm-thick silver film is formed on one side of the PC substrate using electron beam evaporation. Silver is then patterned to define the counter and reference electrodes. The reference electrode is oxidized to Ag ₂ O by applying a voltage of 1.0 V to the silver electrode serving as the anode by using the platinum electrode as the cathode.

  A chip with a working electrode is fixed, eg glued, on the counter electrode and the reference electrode. The assembly may then be submerged in a liquid electrolyte solution and placed in a vacuum desiccator to fill the chamber between the working electrode and the counter / reference electrode. Thus, the liquid electrolyte ionically and thus electrically connects the working electrode, counter electrode, and reference electrode.

During operation of the electrochemical sensor, the gas to be analyzed is supplied to the first surface of the sensor. The potential of the working electrode is maintained at +0.7 V with respect to the Ag / Ag ₂ O reference electrode. At this potential, NO is oxidized to cause the following reaction: NO + 2H ₂ O → NO ³⁻ + 4H ⁺ + 3e ⁻

  The counter electrode allows current to flow through the sensor cell. The potential of the working electrode, the electrolyte, and the electrode material are selected such that the gas being measured is oxidized at the working electrode. The Nafion layer acts as a diffusion layer that allows interaction between the gas, electrode, and liquid. When oxidation occurs at the working electrode, oxygen is usually reduced to water at the counter electrode. The resulting current flowing through the sensor is directly proportional to the gas concentration. Thus, the oxidation of the analyte (in this case NO) at the working electrode produces a current that is detected using a potentiostat including a transimpedance amplifier. This is also used to maintain a constant potential between the working electrode and the reference electrode.

The manufactured sensor was tested to determine the concentration of 0-100 ppb NO in N ₂ gas. The detection limit (S / N = 2) was estimated to be 0.3 ppb and the sensitivity was measured to be 4 μA / ppm / cm ² . Sensor response and recovery time (time to return to 90% of starting signal) was measured as 6 seconds.

DESCRIPTION OF SYMBOLS 1 Small electrochemical sensor 2 Reference electrode 3 Counter electrode 4 Structure 5 Passage 6a Wall 6b Wall 7 Working electrode 8 Nafion (ionomer) layer 9 First surface 10 Second surface 11 Liquid electrolyte 12 Insulating layer 13 Chamber 14 Base Material 15 Electrical via 16 Electrical via 17 Electrical via 18 Through hole 19 Plug

Claims (15)

An electrochemical sensor (1) for detecting a component in a gas,
The sensor
Reference electrode (2),
Counter electrode (3),
A structure (4) comprising a plurality of passages (5) delineated by walls (6, 6a, 6b) extending along the passages;
Working electrode covering the walls of the structure (7), and along the walls of the structure seen including at least a layer of ionomer covering the portion (8) of the working electrode,
The passage has an aspect ratio of at least 2;
The ionomer layer is in ionic conductive contact with the working electrode ;
The sensor further comprises a liquid electrolyte (11) in contact with the layer of ionomer and the reference and counter electrodes.
A sensor characterized by that . The sensor includes a first surface (9) and a second surface (10),
The first surface is exposed to a gas; and
The passage extends from the first surface to the second surface
The sensor according to claim 1 , wherein: The second surface is in contact with the liquid electrolyte
The sensor according to claim 2 , wherein: Structure, Ri porous structure der,
The passages are formed as pores extending in parallel throughout the structure.
The sensor according to any one of claims 1 to 3 , wherein Pores are provided in the packed arrangement,
The closely packed arrangement is at least one of a hexagonal arrangement, a rectangular arrangement, a square arrangement, and a triangular arrangement.
The sensor according to claim 4 , wherein: The ionomer is a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer
The sensor according to any one of claims 1 to 5 , wherein The thickness of the ionomer layer covering the working electrode is in the range of 10 to 2000 nm, or 100 to 1000 nm, or 300 to 700 nm.
The sensor according to any one of claims 1 to 6, characterized by that . Ionomers contain nanostructured solid materials
The sensor according to any one of claims 1 to 7, characterized in that . Includes a housing for the volume of liquid electrolyte
The sensor according to claim 8 , wherein The working electrode comprises a material selected from the group consisting of platinum, gold, palladium, carbon, and ruthenium
The sensor according to any one of claims 1 to 9, characterized in that . The working electrode, are insulated from the structure by an insulating layer (12),
The insulating layer includes a material selected from the group consisting of Al ₂ O ₃ , SiO ₂ , HfO ₂ , and LaO.
The sensor according to any one of claims 1 to 10, characterized by that . A device for measuring the content of NO in exhaled breath, comprising the electrochemical sensor according to claim 1 . A method for producing the electrochemical sensor according to claim 1,
The method
Providing a structure including a plurality of passages delineated by walls extending along the passages;
Coating the wall of the structure with a working electrode;
Placing an ionomer layer covering at least a portion of the working electrode along the wall of the structure;
Providing a liquid electrolyte in contact with the ionomer layer;
Providing a reference electrode and a counter electrode in contact with the liquid electrolyte
A method characterized by that . The structure is provided by reactive ion deep etching of silicon material
The method according to claim 13 , wherein: A layer of ionomer is deposited by dip coating and dried under low pressure
The method according to claim 13, wherein:

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