Electrolysis-induced-bubble-based carbon dioxide gas sensors

Abstract

An example carbon dioxide sensor (100) can include a channel (110) capable of containing water having carbon dioxide dissolved therein. A pair of electrodes (130, 132) can be positioned to contact the water. A voltage source (140) can be connected to the pair of electrodes and operable to supply sufficient voltage to convert a portion of the water to hydrogen gas and oxygen gas by electrolysis to form at least one gas bubble (150, 152) in the channel. A bubble size sensor (160) can be operable to measure a size change over time of the at least one gas bubble in the channel.

Description

[0001] ELECTROLYSIS-INDUCED-BUBBLE-BASED CARBON DIOXIDE GAS SENSORS

[0002] CROSS REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Application No. 63 / 650,059, filed May 21, 2024, which is hereby incorporated herein by reference.

[0004] STATEMENT REGARDING FEDERALLY SPONSORED

[0005] RESEARCH OR DEVELOPMENT

[0006] This invention was made with government support under DE-AR0001836 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

[0007] NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

[0008] Not applicable.

[0009] INCORPORATION BY REFERENCE STATEMENT

[0010] Not applicable.

[0011] BACKGROUND

[0012] Carbon dioxide monitoring and sequestration have the potential to help mitigate effects of global climate change. In particular, carbon dioxide monitoring and sequestration in the Earth’s oceans can have a great impact on global climate change. The oceans, which cover 71% of Earth’s surface, can potentially be used to sequester up to 38,000 gigatons of carbon (GtC). However, less than 1% of this potential has been realized. Despite the ocean's significant potential for carbon sequestration, there is still a significant knowledge gap regarding the carbon transfer cycle at the ocean surface. This lack of understanding is at least partially due to the ocean's vast size, which makes it difficult for researchers to identify optimal carbon sink locations for improved carbon sequestration.

[0013] Monitoring carbon flux in the vast expanse of the ocean presents challenges in both temporal and spatial aspects. The lack of advanced carbon dioxide sensors capable of accurate, long-term monitoring has hindered understanding of carbon sequestration. For years, satellite-based imaging has been the go-to method for quantifying carbon dioxide flux, offering speed and broad ocean coverage. However, this approach sacrifices spatial resolution and accuracy. In addition, carbon flux can deviate greatly from day to day in any given location due to constant changes in the environment. Based on current technology, satellite imaging cannot reasonably provide real-time monitoring of a large number of locations in the ocean simultaneously. In-situ carbon dioxide monitoring may help address the knowledge gap in the carbon transfer cycle at the ocean surface. Yet, existing in-situ carbon dioxide sensors, such as optical-based probes and acoustic sensing, suffer from relatively high power consumption, making widespread deployment in the ocean impractical and cost-inefficient. Bubble-based sensors have also been used. However, previous bubblebased sensors have relied on an external flow of gas to form bubbles, which prevents in-situ usage and leads to high power consumption.

[0014] SUMMARY

[0015] An example carbon dioxide sensor can include a channel capable of containing water having carbon dioxide dissolved therein. A pair of electrodes can be positioned to contact the water. A voltage source can be connected to the pair of electrodes and operable to supply sufficient voltage to convert a portion of the water to hydrogen gas and oxygen gas by electrolysis to form at least one gas bubble in the channel. A bubble size sensor can be operable to measure a size change over time of the at least one gas bubble in the channel.

[0016] Another example carbon dioxide sensor can include a channel capable of containing water. A pair of electrodes can be positioned to contact the water. A voltage source can be connected to the pair of electrodes and operable to supply sufficient voltage to convert a portion of the water to hydrogen gas and oxygen gas to form at least one gas bubble in the channel. A capacitive sensor can be positioned to measure a change in capacitance cause by a size change over time of the at least one gas bubble in the channel.

[0017] An example method of measuring a carbon dioxide concentration in water can include contacting a pair of electrodes with water in a channel, where the water has carbon dioxide dissolved therein. A sufficient voltage can be applied to the pair of electrodes to convert a portion of the water to hydrogen gas and oxygen gas by electrolysis to form at least one gas bubble in the channel. The method can further include measuring a size change over time of the at least one gas bubble in the channel. The size change overtime can be correlated with a carbon dioxide concentration in the water.

[0018] There has thus been outlined, rather broadly, the more important features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Other features of the present invention will become clearer from the following detailed description of the invention, taken with the accompanying drawings and claims, or may be learned by the practice of the invention.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1A is a schematic top-down view of an example carbon dioxide sensor in accordance with the present disclosure.

[0021] FIG. IB is a perspective view of a portion of the example carbon dioxide sensor shown in FIG. 1A.

[0022] FIG. 2A is a schematic top-down view of another example carbon dioxide sensor in accordance with the present disclosure.

[0023] FIG. 2B is a perspective view of electrodes from the example carbon dioxide sensor shown in FIG. 2A.

[0024] FIG. 3 is a schematic top-down view of another example carbon dioxide sensor in accordance with the present disclosure.

[0025] FIG. 4 is a schematic top-down view of yet another example carbon dioxide sensor in accordance with the present disclosure.

[0026] FIG. 5 is a schematic top-down view of another example carbon dioxide sensor in accordance with the present disclosure.

[0027] FIGs. 6A-6I show a process for making an example carbon dioxide sensor in accordance with the present disclosure.

[0028] FIG. 7 is a graph of power consumption vs. electrodes separation in example carbon dioxides sensors in accordance with the present disclosure.

[0029] FIG. 8 is a graph of rate of change of bubble volume vs. CO2 concentration measured with a carbon dioxide sensor in accordance with the present disclosure. FIG. 9 is a graph of sensor response vs. various types of gas measured with a carbon dioxide sensor in accordance with the present disclosure.

[0030] FIG. 10 is a graph of bubble volume vs. temperature for bubbles made using a carbon dioxide sensor in accordance with the present disclosure.

[0031] FIG. 11 is a graph of capacitance vs. time measured with a carbon dioxide sensor in accordance with the present disclosure.

[0032] FIG. 12 is a graph of bubble transportation time and transportation speed vs. magnetic flux density in accordance with the present disclosure.

[0033] FIG. 13 is a graph of bubble diameter vs. concentration of CO2 in accordance with the present disclosure.

[0034] FIG. 14 is a graph of measured capacitance and predicted capacitance vs. concentration of CO2 in accordance with the present disclosure.

[0035] These drawings are provided to illustrate various aspects of the invention and are not intended to be limiting of the scope in terms of dimensions, materials, configurations, arrangements or proportions unless otherwise limited by the claims.

[0036] DETAILED DESCRIPTION

[0037] While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the spirit and scope of the present invention. Thus, the following more detailed description of the embodiments of the present invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics of the present invention, to set forth the best mode of operation of the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the present invention is to be defined solely by the appended claims.

[0038] Definitions

[0039] In describing and claiming the present invention, the following terminology will be used. The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “an electrode” includes reference to one or more of such features and reference to “measuring” refers to one or more of such steps.

[0040] As used herein with respect to an identified property or circumstance, “substantially” refers to a degree of deviation that is sufficiently small so as to not measurably detract from the identified property or circumstance. The exact degree of deviation allowable may in some cases depend on the specific context.

[0041] As used herein, “adjacent” refers to the proximity of two structures or elements. Particularly, elements that are identified as being “adjacent” may be either abutting or connected. Such elements may also be near or close to each other without necessarily contacting each other. The exact degree of proximity may in some cases depend on the specific context.

[0042] As used herein, the term “about” is used to provide flexibility and imprecision associated with a given term, metric or value. The degree of flexibility for a particular variable can be readily determined by one skilled in the art. However, unless otherwise enunciated, the term “about” generally connotes flexibility of less than 2%, and most often less than 1%, and in some cases less than 0.01%.

[0043] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.

[0044] As used herein, the term “at least one of’ is intended to be synonymous with “one or more of.” For example, “at least one of A, B and C” and “at least one of A, B or C” explicitly includes only A, only B, only C, or combinations of each.

[0045] Numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly recited limits of 1 to about 4.5, but also to include individual numerals such as 2, 3, 4, and subranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only one numerical value, such as “less than about 4.5,” which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described.

[0046] Any steps recited in any method or process claims may be executed in any order and are not limited to the order presented in the claims. Means-plus-function or step-plus- function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) “means for” or “step for” is expressly recited; and b) a corresponding function is expressly recited. The structure, material or acts that support the means-plus function are expressly recited in the description herein. Accordingly, the scope of the invention should be determined solely by the appended claims and their legal equivalents, rather than by the descriptions and examples given herein.

[0047] Electrolysis-Induced-Bubble-Based Carbon Dioxide Gas Sensors

[0048] As mentioned above, some previous gas sensors have used reference bubbles formed with an external gas supply. This can make such sensors difficult or impossible to use in-situ in the ocean. These sensors can also be bulky and consume high amounts of energy. The carbon dioxide sensors described herein are capable of forming reference bubbles without the use of an external flow of gas. This is accomplished by using electrolysis to form hydrogen and oxygen gas bubbles directly from water. These sensors can be smaller and more power-efficient compared to previous carbon dioxide sensors. The power consumption of the sensors described herein can be as low as a micro-Watt in some examples. The sensors can be used in-situ in the ocean for long periods of time because of the lower power consumption, enabling long-term monitoring of carbon dioxide levels in the ocean. This can enable researchers to better understand carbon dioxide flux in the ocean and identify optimal carbon sink locations for carbon sequestration.

[0049] The carbon dioxide sensors described herein can operate based on the principle of time-dependent diffusion of dissolved carbon dioxide from water into reference gas bubbles. Carbon dioxide has a higher solubility in water than other gases present in the atmosphere, such as oxygen and nitrogen. However, when a gas bubble is present in water that contains dissolved carbon dioxide, some of the dissolved carbon dioxide will diffuse from the water into the gas bubble over time. The rate at which the carbon dioxide diffuses into the gas bubble can correlate to the concentration of carbon dioxide in the water. If the concentration of dissolved carbon dioxide is high enough, the diffusion of carbon dioxide into the reference bubble can cause the reference bubble to grow in volume over time. The higher the concentration of dissolved carbon dioxide is, the faster the reference bubble will grow. Thus, the sensors described herein can measure the growth of the reference bubble over time and then estimate the concentration of carbon dioxide in the water based on a correlation between the growth rate and the dissolved carbon dioxide concentration. The reference bubbles can be formed using a pair of electrodes, which can convert water to hydrogen and oxygen gas through electrolysis. Thus, no external gas flow is used to form the reference bubbles.

[0050] FIG. 1 A shows a top-down schematic view of an example carbon dioxide sensor 100 according to the present technology. The carbon dioxide sensor includes a channel 110 containing water that has carbon dioxide dissolved therein. The channel in this example has side walls 112, 114 formed in a layer of solid material 102. In this example, the channel extends between an inlet 120 and an outlet 122. Although not shown in this figure, the channel can have a closed ceiling covering the channel, while the inlet and outlet can be open to allow water to flow in through the inlet and water and / or bubbles to flow out through the outlet, i.e. in fluid communication with surrounding environment. The carbon dioxide sensor also includes a first electrode 130 and a second electrode 132 that are positioned to be in contact with the water. The electrodes are separated one from the other so that electric current flowing between the electrodes passes through water in the channel. A voltage source 140 is connected to the pair of electrodes by electrical connections 142, 144. The voltage source can be operable to supply sufficient voltage to the electrodes to convert a portion of the water in the channel to hydrogen gas and oxygen gas by electrolysis. This figure shows a hydrogen gas bubble 150 and an oxygen gas bubble 152 formed at the electrodes. In this example, the electrodes are positioned on the floor of the channel. However, in other examples the electrodes can be positioned in a variety of locations in the sensor, as long as both electrodes are in electrical contact with the water to allow for electrolysis. The carbon dioxide sensor also includes a bubble size sensor 160. In this example, the bubble size sensor is an optical camera positioned to record images of the bubbles in the channel. In other examples, the bubble size sensor can include any type of sensor capable of measuring a change in the size of the bubbles over time.

[0051] FIG. IB shows a perspective view of a portion of the carbon dioxide sensor to clarify the three-dimensional structure. The layer of solid material 102 has voids formed for the channel 110, inlet 120, and outlet 122. Edges of the solid material that are beneath the surface in this view are shown as dashed lines. As shown in this figure, the channel is enclosed with a floor, ceiling, and side walls. The inlet and outlet have an opening to allow water and bubbles to flow in and out (i.e. from a top side in this view). The first electrode 130 and second electrode 132 are formed of a conductive material deposited onto a substrate 104 such as glass. The substrate is not shown in FIG. 1A. The solid material layer can be formed of a polymer such as polydimethylsiloxane (PDMS), although other materials can be used such as, but not limited to, other polymers such as poly(methyl methacrylate (PMMA) or metals such as titanium. In some examples, the solid material layer can be formed and then bonded to the substrate, which already has the electrodes deposited thereon. In one example, the solid material and / or the substrate can be transparent so that the optical camera can record images of the bubbles in the channel. Opaque materials can also be used if other bubble size measurement techniques are used.

[0052] For clarity, the positions, orientations, and dimensions of components of the carbon dioxide sensors are described herein with reference to the width, length, and thickness of the channel. As used herein, “channel width,” or the width of the channel, refers to the distance between side walls of the channel. The “channel width direction” refers to a directional axis that extends along the side-to-side direction of the channel width. As used herein, “channel length” refers to the dimension of the channel from end to end, or from inlet to outlet. The “channel length direction” refers to a directional axis that extends along the end to end direction of the channel length, orthogonal to the channel width. The “channel thickness” or “channel height” refers to the dimension of the channel from floor to ceiling. The “channel thickness direction” is a directional axis orthogonal to the channel width and the channel length, extending in the direction from floor to ceiling of the channel.

[0053] The channel of the carbon dioxide sensors can have a variety of shapes. In some examples, the channel can have a rectangular cross-section, with flat side walls, a flat floor, and a flat ceiling. In other examples, the side walls or the floor or ceiling can be curved. In certain examples, the channel can a uniform channel thickness along the length of the channel. In further examples, the channel thickness can be uniform along the length of the channel and across the width of the channel. In many examples, the channel thickness can be smaller than the channel width and the channel length. In further examples, the channel length can be greater than the channel width and the channel thickness. However, in other examples the channel width can be greater than the channel length, or the channel thickness can be greater than the channel width, or other relationships between the dimensions can be used. This variable dimension can allow for the integration of variable interdigital electrode designs to adjust the sensitivity in the detection of the bubble size. As bubbles can vary greatly under high pressure in the ocean versus in atmospheric air, specific interdigital electrodes can be designed to fit the needed sensitivity for a specific application. In some examples, a filter can be provided at one or both of the inlet and outlet. This can help to prevent solid debris from entering the channel and potentially clogging the sensor. Another implementation is the use of an anti -algae coating to prevent stiction of algae on to the device overtime, which may cause damage. This can be formed by incorporating a thin hydrophobic polymer layer, such as PDMS, around the sensor surface, preventing accumulation of algae.

[0054] In some examples, the channel can be a microfluidic channel. This can mean that at least one of the dimensions of the channel can have a size on the order of microns, or in other words, less than 1 millimeter. In certain examples, the channel thickness can be from 20 pm to 500 pm, or from 20 pm to 400 pm, or from 20 pm to 300 pm, or from 20 pm to 200 pm, or from 20 pm to 100 pm, or from 100 pm to 500 pm, or from 100 pm to 400 pm, or from 100 pm to 300 pm, or from 100 pm to 200 pm, or from 200 pm to 500 pm, or from 200 pm to 400 pm, or from 200 pm to 300 pm, or from 300 pm to 500 pm, or from 300 pm to 400 pm, or from 400 pm to 500 pm. The channel width can be from 200 pm to 2,000 pm, or from 200 pm to 1,500 pm, or from 200 pm to 1,000 pm, or from 200 pm to 800 pm, or from 200 pm to 600 pm, or from 200 pm to 400 pm, or from 400 pm to 2,000 pm, or from 400 pm to 1,500 pm, or from 400 pm to 1,000 pm, or from 400 pm to 800 pm, or from 400 pm to 600 pm, or from 600 pm to 2,000 pm, or from 600 pm to 1,500 pm, or from 600 pm to 1,000 pm, or from 600 pm to 800 pm, or from 800 pm to 2,000 pm, or from 800 pm to 1,500 pm, or from 800 pm to 1,000 pm, or from 1,000 pm to 2,000 pm, or from 1,000 pm to 1,500 ri, or from 1,500 pm to 2,000 pm. In some examples, the channel length can be from 200 pm to 20,000 pm, or from 200 pm to 10,000 pm, or from 200 pm to 2,000 pm.

[0055] The pair of electrodes in the carbon dioxide sensors, can have a variety of arrangements. In the example shown in FIG. 1 A-1B, the electrodes are layers of conductive material deposited on the floor of the channel. In other examples, the electrodes can be on the ceiling of the channel, or on side walls of the channel, or in any other locations where the electrodes are in contact with the water. The electrodes can be spaced apart by a separation distance. In some examples the electrodes can be spaced apart in the channel width direction. In certain examples, the separation distance can be from 300 pm to 800 pm, or from 300 pm to 700 pm, or from 300 pm to 600 pm, or from 300 pm to 500 pm, or from 300 pm to 400 pm, or from 400 pm to 800 pm, or from 400 pm to 700 pm, or from 400 pm to 600 pm, or from 400 pm to 500 pm, or from 500 pm to 800 pm, or from 500 pm to 700 pm, or from 500 pm to 600 pm, or from 600 pm to 800 pm, or from 600 pm to 700 pm, or from 700 pm to 800 pm, in a channel width direction. In other examples, the electrodes can be separated by a separation distance in the channel thickness direction. For example, the first electrode can be positioned at the floor of the channel and the second electrode can be positioned at the ceiling of the channel. In certain examples, the first and second electrode can be formed as layers of conductive material with a layer of dielectric material between the electrodes. However, at least one surface of each electrode can contact the water in some examples. If the electrodes are separated in the channel thickness direction, then in some examples the separation distance can be from 20 pm to 500 pm, or from 20 pm to 400 pm, or from 20 pm to 300 pm, or from 20 pm to 200 pm, or from 20 pm to 100 pm, or from 100 pm to 500 pm, or from 100 pm to 400 pm, or from 100 pm to 300 pm, or from 100 pm to 200 pm, or from 200 pm to 500 pm, or from 200 pm to 400 pm, or from 200 pm to 300 pm, or from 300 pm to 500 pm, or from 300 pm to 400 pm, or from 400 pm to 500 pm.

[0056] The electrodes can be formed of any suitable conductive material. In some examples, the electrodes can comprise platinum, iridium, gold, silver, copper, graphite, steel, or a combination thereof. Platinum can be particularly useful because of its resistance to corrosion in ocean water. The electrodes can be formed by a deposition process in some examples, such as sputtering, electroplating, atomic layer deposition, physical vapor deposition, chemical vapor deposition, or others. The voltage source of the carbon dioxide sensor can be configured to supply a sufficient voltage to generate hydrogen and oxygen gas bubbles from the water through electrolysis. Based on Gibbs free energy, the theoretical minimum voltage for electrolysis is 1.23 V. In practice, a voltage higher than the minimum can be used to ensure that bubbles are formed. In some examples, the voltage source can supply a voltage from 1.23 V to 5 V, or from 2 V to 5 V, or from 2 V to 4 V, or from 2 V to 3 V, or from 3 V to 5 V, or from 3 V to 4 V, or from 4 V to 5 V. In some examples, the voltage source can be activated for a short period of time to form one or more bubbles in the water in the channel, and then the voltage source can be shut off while the size of the bubbles is measured over time. In certain examples, the voltage source can be activated for a time from 1 second to 60 seconds, or from 1 second to 30 seconds, or from 1 second to 10 seconds, or from 10 seconds to 30 seconds, or from 30 seconds to 60 seconds.

[0057] After the gas bubbles have been formed, the size of the bubbles can be measured over time. In some examples, the size can be measured continuously, while in other examples the size can be measured periodically. Carbon dioxide can begin diffusing into the bubble immediately after the bubbles are formed. In some cases, the diffusion can slow down over time as the concentration of carbon dioxide in the bubbles increases, which can reduce the driving force for diffusion of additional carbon dioxide. The initial rate of carbon dioxide diffusion can be of particular interest, which can be measured by measuring the change in bubble size starting immediately after the bubbles have formed. In some examples, the bubble size can be measured starting at or near formation (i.e., when the voltage source is turned off) and proceeding for a time period of 1 to 30 minutes, or 1 to 20 minutes, or 1 to 15 minutes, or 1 to 10 minutes, or 1 to 5 minutes, or 5 to 30 minutes, or 5 to 20 minutes, or 5 to 15 minutes, or 5 to 10 minutes, or 10 to 30 minutes, or 10 to 20 minutes, or 10 to 15 minutes, or 15 to 30 minutes, or 15 to 20 minutes, or 20 to 30 minutes.

[0058] The channel can have an inlet and an outlet in some examples. The inlet and outlet can allow water to flow through the sensor, such as water from the ocean. In other examples, the channel can have a single opening that can operate as an inlet and / or outlet. In still other examples, the channel can be sealed without any inlet or outlet. In such examples, carbon dioxide can diffuse into water held in the channel through a solid wall, floor, or ceiling of the channel (e.g. a CO permeable membrane). An inlet and outlet can allow bubbles to be flushed out of the channel after the sensor has been used to complete a measurement of the carbon dioxide concentration. In some examples, the carbon dioxide sensor can include a pump to actively flush bubbles out through the outlet. In other examples, the bubbles can escape from the channel through the outlet after some amount of time without any active flushing. In certain examples, the channel can be oriented vertically or at an angle so that the buoyant force of the bubbles can cause them to move upward and out of the outlet. In still further examples, the bubbles can remain in the channel until the oxygen and hydrogen dissolves into the water and eventually the bubbles collapse. Another incorporation is the usage of an electromagnetic flow. By incorporating a magnetic field perpendicular to direction of the electrical current, an electromagnetic force can be applied to the dipole water molecules. This provides the ability to transport bubbles to the capacitive sensing mechanism when needed, while assisting with the flow from inlet to outlet. Additionally, this mechanism can be applied during the bubble generation period when an electrical power source is applied to the bubble nucleating electrode, thus producing a flow without any added power. In some examples, the sensor can include one or more magnets positioned to provide a magnetic field at the electrodes. In certain examples, a first magnet can be located at a ceiling of the channel (i.e., above the electrodes) and a second magnet can be located at a floor of the channel (i.e., below the electrodes).

[0059] FIG. 2A shows a top-down schematic view of another example carbon dioxide sensor 200 in accordance with the present technology. This sensor also includes a channel 210 formed in a layer of solid material 202. The channel extends from an inlet 220 to an outlet 222. Again, the channel can be covered by a closed ceiling over the channel, but the ceiling can have openings at the inlet and the outlet. This example includes a first electrode 230 and a second electrode 232, which are spaced apart in the channel thickness direction. When viewed along the channel thickness direction, as in FIG. 2A, the electrodes overlap one another to form an interdigitated electrode set. FIG. 2B shows a perspective view of the electrodes, which are separated by a dielectric layer 234. The electrodes have a shape with a concave angle 236. Gas bubbles 250 can form at or near this concave angle, along the edges of the electrodes that are in contact with the water. This creates a cavity, serving as a defect, which reduces the surface energy. By integrating the cavity and diminishing the surface energy, the energy required to initiate bubble formation at the cavity site decreases, enabling controlled nucleation of bubbles at that location. Furthermore, the size of the nucleated reference bubble can be managed using Fritz’s formula, a derived equation for determining the detachment radius of a bubble within an electrode's cavity. By modulating the cavity, the reference bubble detachment size can be controlled. As in the previous example, the electrodes are connected to a voltage source 240 through electrical connections 242, 244. This example includes another type of bubble size sensor 260. In this example, the bubble size sensor is a capacitive sensor that measures the size change of the bubbles through capacitance. In this example, the capacitive sensor includes interdigitated electrodes 262, 264. These electrodes can be located in the floor or ceiling of the channel, with a layer of dielectric material covering the electrodes to prevent direct contact with the water in the channel. The interdigitated electrodes can be connected to a capacitance meter 266 that measures the capacitance between the interdigitated electrodes. The measured capacitance can change depending on what portion of the interdigitated electrodes are covered by water and what portion are covered by gas bubbles. Thus, changes in capacitance can indicate changes in bubble volume. Such capacitive techniques for measuring bubble sizes can be used in lieu of optical techniques, or in addition.

[0060] The capacitive sensor in the above example is one example of a bubble size sensor that can be used in the carbon dioxide sensors described herein. This particular capacitive sensor utilizes two interdigitated electrodes positioned adjacent to the channel so that bubbles in the channel can affect the capacitance measured between the electrodes. In particular, whenever one or more bubbles are positioned over the interdigitated electrodes, size changes in the bubbles can be detected as variations in the capacitance measured between the electrodes. Other examples can include capacitive sensors having different designs. For example, instead of interdigitated electrodes, a capacitive sensor can have two straight electrodes extending in parallel along the length of the channel. In other examples, the capacitive sensor can include more than two electrodes. Higher resolution capacitive sensors can be used, such as a capacitive sensor array used in touch screens. Such sensors can provide higher resolution and potentially measure size changes in individual bubbles in the channel. However, simpler capacitive sensors can also be sufficient and can allow the carbon dioxide sensor to operate without differentiating between individual bubbles. This is because any overall change in the size of bubbles in the channel, as a whole, can be correlated to the concentration of carbon dioxide in the water in the channel. The capacitive sensor can be positioned anywhere that allows the capacitive sensor to measure changes in capacitance caused by changes in bubble size. The capacitive sensor can be positioned adjacent to the channel, such as on a floor of the channel, or on a ceiling of the channel, or on one or more side walls of the channel, or a combination thereof. In some examples, the capacitive sensor can include electrodes covered by a layer of dielectric material. The water in the channel and the gas bubbles can directly contact the layer of dielectric material but may not contact the electrodes. The capacitive sensing mechanism is an acceptable interdigital -based electrode design, but a parallel plate design can be similarly be implemented such that a bubble flow in between two conductive plate for a more compact high resolution sensing of the bubbles.

[0061] In further examples, the carbon dioxide sensor can include a correlation module configured to correlate the size change over time of the gas bubbles in the channel with a concentration of carbon dioxide in the water. For example, multiple images can be recorded over time such that the actual or apparent size of identified gas bubbles can be compared across the images. Changes in bubble size over time can then be correlated with carbon dioxide concentration as outlined. The correlation module can be in communication with the bubble size sensor to receive data from the bubble size sensor. In one example, the bubble size sensor can include a capacitive sensor, and the correlation module can receive capacitance measurements from the capacitive sensor. In another example, the bubble size sensor can include a camera and the module can receive image data from the camera. The correlation module can include any electronics, software, or combinations thereof suitable for correlating the size change over time of the gas bubbles with the concentration of carbon dioxide in the water. In some examples, the module can include a processor programmed to correlate the size change over time of the gas bubbles with the concentration of carbon dioxide in the water. In other examples, the correlation module can include software to be run on a separate processor, where the software includes instructions to the processor to correlate the size change over time of the gas bubbles to the carbon dioxide concentration. The correlation can include calculating a carbon dioxide concentration based on changes in bubble size measurements received from the bubble size sensor. In some examples, this calculation can be performed using an onboard processor that is part of an integrated device that includes the channel, electrodes for electrolysis, and the bubble size sensor. In other examples, the channel, electrodes for electrolysis, and the bubble size sensor can be part of a remote measurement device and the correlation module can be separate, such as in a base station. The remote measurement device can communicate bubble size data to the correlation module, and the correlation module can calculate the carbon dioxide concentration based on the bubble size data.

[0062] Other types of modules and / or electronic components can also be included in the carbon dioxide sensors. For example, the carbon dioxide sensor can include a processor, a microcontroller, a data storage module, a wireless communication module, a wired communication module, an image processing module, additional sensor such as a temperature sensor, a humidity sensor, or others.

[0063] As an example, FIG. 3 is a schematic view of another example carbon dioxide sensor 300 that includes several electronic components. This example is similar to FIG. 2A, with a channel 310, a first electrode 330 and a second electrode 332 for generating hydrogen and oxygen gas 350 bubbles through electrolysis, and a capacitive sensor 360 for measuring size changes of the bubbles. This example also includes a controller 370 that includes a processor 372. The processor can be programmed to correlate measurements of bubble size change over time with the carbon dioxide concentration of the water in the channel. Thus, the processor acts as the correlation module configured to correlate the bubble size change over time with carbon dioxide concentration in this example. The controller also includes a voltage source 340, which supplies power to the electronic components of the controller and also to the electrode pair used for electrolysis. A capacitance meter 366 is also in the controller, which together with two interdigitated electrodes 362, 364 make up the capacitive sensor. The controller also includes a data storage module 374, a wireless communication module 376, a temperature sensor 378, and a humidity sensor 380. These components can be electronically connected to the processor to allow the processor to send and receive information to and from these components.

[0064] In some cases, the effect of carbon dioxide concentration on bubble size can be influenced by other variables such as ambient temperature, humidity, or other factors. Therefore, it can be useful to include a temperature sensor, humidity sensor, and / or other sensors to measure these factors. In some examples, the correlation module can calculate the carbon dioxide concentration in the water using a correlation model that accounts for the change in bubble size over time, the temperature, the humidity, and any other variables that may impact the correlation. In some examples, the effects of these variables on the correlation may be unknown, but a machine learning algorithm can be used with the carbon dioxide sensor to calibrate the correlation module to provide more accurate calculations of the carbon dioxide concentration while accounting for these variables. In certain examples, the machine learning algorithm can be trained using one or more carbon dioxide sensors taking multiple measurements over time at various conditions.

[0065] In more detail regarding the correlation between bubble size change and carbon dioxide concentration, the carbon dioxide sensor can generate bubbles of hydrogen and oxygen, or bubbles containing a mixture of hydrogen and oxygen if bubbles of each gas combine. These bubbles can be formed quickly at the electrodes. Once the initial bubbles are produced, the bubbles will increase or decrease in size depending on the CO2 gas concentration dissolved within the water. The gas dissolution process of a microbubble in the liquid phase can be described with Eq. 1, where R is the bubble radius at time t, k is the diffusion coefficient of the gas into the liquid, p is the gas density, Ci is the initial gas concentration in liquid, and Csgas solubility. This equation suggests that the size of the bubble is dependent primarily on the gas diffusivity (k), the ratio between the gas solubility with density (C3-p_1), and the ratio between the initial dissolved gas concentration with the gas solubility (CrCs-1). The carbon dioxide sensor can also be affected by the diffusion of the atmospheric gas through the PDMS structure and its solubility with the liquid, which can be described using Eq. 2, where F is the permeation gas flux, P is the permeability, d is the wall thickness, Ap is the pressure difference, and A is the area of diffusion. Together, these equations provide insight into the factors that will affect the performance of the carbon dioxide sensor.

[0066] After the hydrogen and oxygen gas bubbles form at the electrodes, carbon dioxide can begin to diffuse from the water in the channel into the gas bubbles. At the same time, oxygen and hydrogen may diffuse from the gas bubbles into the water to dissolve in the water. The diffusion of carbon dioxide into the bubbles tends to make the bubbles increase in size, while the diffusion of hydrogen and oxygen into the water tends to make the bubbles decrease in size. In some examples, the bubbles can decrease in size if the concentration of carbon dioxide in the water is below a certain threshold. The threshold can be the concentration at while the rate of diffusion of carbon dioxide into the bubbles balances out the rate of diffusion of oxygen and hydrogen out of the bubbles. In certain examples, this threshold concentration can be between about 450 ppm and about 550 ppm, or between about 470 ppm and about 530 ppm, or between about 490 ppm and about 530 ppm. If the bubbles decrease in size over time, this can indicate that the concentration of carbon dioxide in the water is below the threshold value. If the bubbles increase in size over time, this can indicate that the concentration of carbon dioxide is above the threshold value.

[0067] At standard temperature and pressure, the saturation concentration of carbon dioxide in water can be about 750 ppm. Therefore, the carbon dioxide sensors described herein can have a useful detection range up to about 750 ppm in some examples. Notably, carbon dioxide has a gas concentration and gas diffusivity which results in carbon dioxide bubbles constituting about 93.52% of bubble growth rate compared to other gases (e.g. nitrogen at 3.91%, oxygen at 2.57%, and other gases as negligible) such that this approach of bubble growth rate measurement is highly correlated to make an effective sensor of carbon dioxide concentration. However, in other examples, the pressure of the water in the channel can be adjusted to change the saturation concentration of carbon dioxide in the water. This can allow the useful range to be extended to higher concentrations. In some examples, the carbon dioxide sensors can detect carbon dioxide concentrations in the range of 800 ppm or less, or 750 ppm or less. In further examples, the carbon dioxide sensors can detect carbon dioxide concentrations down to the atmospheric carbon dioxide concentration, which is currently about 420 ppm. In further examples, the carbon dioxide sensor can measure carbon dioxide concentrations in a range from about 400 ppm to about 800 ppm, or from about 420 ppm to about 800 ppm, or from about 420 ppm to about 750 ppm. This CO2 concentration range is that of the atmosphere. To calculate the CO2 concentration within a water-fdled microchannel, Henry’s Law can be utilized in which the CO2 concentration in water is equal to the Henry’s constant for CO2 times the pressure of CO2 being applied. While the gas does saturate at around 700 ppm, this dynamic sensing range can drastically increase with the applied gas pressure as outlined by Henry’s law.

[0068] Because the size of the gas bubbles is affected by diffusion of oxygen and hydrogen out of the bubbles and into the water, it can be useful to account for the diffusion of oxygen and hydrogen in the correlation model. In some examples, the diffusion of hydrogen and oxygen can be accounted for together, and in other examples the diffusion of hydrogen and oxygen can be accounted for separately. In the examples shown above, it may be difficult to distinguish between gas bubbles that contain hydrogen gas, gas bubbles that contain oxygen gas, and gas bubbles that contain a mixture of hydrogen and oxygen. However, in certain examples the carbon dioxide sensor can be designed to separate the hydrogen gas bubbles from the oxygen gas bubbles and separately measure the size changes of the hydrogen gas bubbles and oxygen gas bubbles. Further, the potential H2 bubbles formed at the cathode can be separated from the O2 bubbles formed at the anode because electrolysis can produce a reference bubble at different size for each respective bubble type causing variations in measurements. Through the separation, reference growth between the two respective bubbles can be compared separately for a more accurate measurement. Additionally, by separating the two bubbles, additional parameters can be measured, in which the H2 bubbles can be utilized to quantify pH (e.g. due to pH correlation with hydrogen ions) and O2 bubbles can be utilized to quantify the CO2 concentration as described herein.

[0069] FIG. 4 shows a schematic top-down cross-sectional view of an example carbon dioxide sensor 400 that includes a channel 410 formed in a solid material 402 with a semi- permeable membrane 480 dividing the channel into a first channel portion 416 and a second channel portion 418. A first electrode 430 is positioned in the first channel portion to form hydrogen gas bubbles 450. A second electrode 432 is positioned in the second channel portion to form oxygen gas bubbles 452. This carbon dioxide sensor can independently measure size changes in the hydrogen gas bubbles and the oxygen gas bubbles using a first capacitive sensor 460 and a second capacitive sensor 462. In some examples, different correlation models can be used for the hydrogen gas bubbles and the oxygen gas bubbles to account for differences in the diffusivity and solubility of hydrogen and oxygen. The sensor can also include a correlation module that can utilize the data provided by the first and second capacitive sensors to calculate the carbon dioxide concentration in the water using the correlation models.

[0070] The size change of the gas bubbles over time can be defined as a rate of change in volume of the bubbles, a rate of change of area of the bubbles, or a rate of change of diameter of the bubbles, in some examples. The rate of change of the bubble size can be related to the carbon dioxide concentration through a correlation model. The correlation model can be calibrated by exposing the carbon dioxide sensor to multiple known carbon dioxide concentrations and measuring the rate of change of bubble size, and fitting a model to the measured data. Various equations can be used for the correlation model. In some examples, the rate of change of bubble size can be related to the carbon dioxide concentration through a linear equation, a polynomial equation, an exponential equation, a logarithmic equation, or other types of equations that can fit the data. In certain examples, the correlation model can use Equation 3: y = T^^ ~D (3)where y is the rate of change of bubble volume in percent per minute, x is the concentration of carbon dioxide in the water in parts per million (ppm), and A, B, C, and D are coefficients. Calibration of the carbon dioxide sensor can include fitting measured data to Equation 3 to determine values for the coefficients. Other equations can also be used and calibrated in a similar way.

[0071] FIG. 5 shows a top-down schematic view of another example carbon dioxide sensor 500. This sensor also includes a channel 510 formed in a layer of solid material 502. An inlet and outlet can be formed in the ceiling of the channel, which is not shown in this figure. A first electrode 530 and a second electrode 532. A voltage can be applied across these electrodes to generate bubbles 550 through electrolysis. The electrodes have a cavity shape with a concave angle 536, which can help nucleate bubbles with uniform size. A passivating coating 538 is also formed over the base portion of the electrodes, so that this portion of the electrodes is not exposed to the water in the channel. This can also help ensure that bubbles form only at the desired location in the cavity at the tip of the electrodes. As in previous examples, the sensors also includes interdigitated electrodes 562, 564 that are used as a capacitive sensor for sensing bubble size. This sensor also includes magnets 570 that provide a magnetic field. A single magnet is shown in the figure, and this magnet can be positioned at the floor of the channel or beneath the floor of the channel. A second magnet (not shown) can be positioned at the ceiling of the channel or above the ceiling of the channel, and aligned with the bottom magnet so that a magnetic field is present in the into-the-page direction, between the two magnets. The magnetic field from these magnets and the electric field formed when voltage is applied across the electrodes can increase the speed at which bubbles are transported from the nucleation sites on the electrodes to the capacitive sensor.

[0072] FIGs. 6A-6I show a process that can be used to make an example carbon dioxide sensor 600. In FIG. 6A, a photoresist pattern 680 is deposited on a substrate 682. In FIG. 6B, electrodes 630, 632 are formed by depositing metal such as chromium or platinum and lifting off the photoresist. In FIG. 6C, another photoresist layer is deposited to pattern for a dielectric layer. In FIG. 6D, a dielectric material 684 is added over the electrodes. FIG. 6E shows a separate substrate with a layer of photoresist forming a mold for making a microfluidic channel. In FIG. 6F, the microfluidic channel 686 is cast on the mold. In FIG. 6G, the microfluidic channel is peeled off the mold and a liquid inlet and outlet is formed. FIG. 6H shows the microfluidic channel being bonded to the substrate with electrodes and dielectric layer, thus forming the channel of the carbo dioxide sensor. FIG. 61 shows magnets 670 being added to the top and bottom of the sensor to provide a magnetic field to increase transportation speed of bubbles as described above.

[0073] In further detail regarding the electrode shape, the electrode can include a microcavity at the electrode tip with a concave angle in the microcavity. In some examples, the concave angle can be about 75°, or from about 50° to about 100°, or from about 60° to about 90°. The microcavity can have a cavity width from about 10 pm to about 40 pm, or from about 15 pm to about 30 pm, or about 20 pm in some examples. In other examples, the microcavity can have a cavity width from about 100 pm to about 500 pm, or from about 200 pm to about 500 pm, or from about 300 pm to about 500 pm, or from about 400 pm to about 500 pm.

[0074] The magnets can provide a magnetic flux over the electrodes where bubble nucleation takes place. In some examples, the magnetic flux can be from about 20 mT to about 400 mT, or from about 50 mT to about 300 mT, or from about 100 mT to about 200 mT, or greater than about 150 pm, or greater than about 190 mT in some examples. In some cases, the magnetic flux can dramatically increase the speed of bubbles being transported to the capacitive sensor. For example, bubbles can take around 10 minutes to flow to the capacitive sensor after nucleation when there is no magnetic flow system, but bubbles can move to the capacitive sensor in one second or less when a magnetic flow system is used. The magnets described above can cause the bubbles to move because the bubbles can have a negative surface charge. Negative oxygen dipoles of water molecules tend to be oriented toward the bubble surface. OH' ions also tend to absorb on the bubble surface. Therefore, the bubble can experience forces caused by the electric field between the electrodes and the magnetic field provided by the magnets.

[0075] The interdigitated electrodes can have a width of the interdigitated portions, a separation distance between the interdigitated portions, and a length of the interdigitated portions that are suitable for measuring size changes in the bubbles. In certain examples, the bubbles can have diameters from about 100 pm to about 1,000 pm. The electrode width and separation distance (i.e., pitch) can be from about 5 pm to about 50 pm, or from about 5 pm to about 20 pm, or about 10 pm in some examples. The length of the interdigitated portions can be from about 500 pm to about 2,000 pm, or from about 500 pm to about 1,500 pm, or about 1010 pm in some examples.

[0076] In addition to the carbon dioxide sensors described herein, this disclosure also describes methods of measuring carbon dioxide concentrations in water. In some examples, a method of measuring carbon dioxide concentration in water can include contacting a pair of electrodes with water in a channel. The water can have carbon dioxide dissolved therein. A sufficient voltage can be applied to the pair of electrodes to convert a portion of the water to hydrogen gas and oxygen gas by electrolysis, forming at least one gas bubble in the channel. The method can also include measuring a size change over time of the gas bubble in the channel. The size change of the gas bubble over time can then be correlated with a carbon dioxide concentration in the water. This method can be performed using a carbon dioxide sensor as described herein.

[0077] The carbon dioxide sensors described herein can be capable of providing carbon dioxide measurements while consuming very little power. This can allow for the carbon dioxide sensors to work with a modest sized battery for an extended period of time. For example, the carbon dioxide sensors can include a battery (which acts as the voltage source to drive electrolysis, and also powers other electronic components of the carbon dioxide sensor) that can last for weeks, months, or years without being recharged or replaced. This can enable the use of a network of many such carbon dioxide sensors in remote locations, such as distributed in the oceans. The methods used to measure carbon can be designed to minimize power consumption. For example, voltage can be applied to the electrode pair for a short period of time to form gas bubbles by electrolysis. These electrodes can then be turned off, and the carbon dioxide sensor can consume a smaller amount of power while the size of the bubbles is measured over time. In some examples, the carbon dioxide sensor can be configured to perform measurements of carbon dioxide periodically, and the carbon dioxide sensor can be in a very low power or sleep mode between these measurements. The time between measurements can be set at any desired interval, such as 12 hours, 1 day, 2 days, 1 week, etc. Thus, the highest power consumption mode, which occurs when electrolysis is used to form bubbles, can be limited to short periods of time that occur periodically. In some examples, the power consumed while forming the bubbles by electrolysis can be from 100 pW to 300 pW, or from 100 pW to 200 pW, or from 200 pW to 300 pW. The carbon dioxide sensors can use less power while measuring the bubble size change over time and even less power in sleep mode between measurements.

[0078] Examples

[0079] Fabrication of a Carbon Dioxide Sensor

[0080] A carbon dioxide sensor was fabricated in two parts: the first part included a pair of electrodes deposited on a glass wafer. LORI 0B photoresist was spin coated onto a 4-inch Pyrex glass wafer at 1200 rpm for 60 seconds to produce a 1.65 pm layer for a future lift-off process. The wafer was then prebaked at 190 °C for 5 minutes. A layer of nLOF 2020 negative photoresist was spin coated at 3500 rpm for 60 s for a thickness of 1.85 pm. The wafer was soft -baked at 115 °C for 60 s. The electrode plate design was patterned using UV lithography at 350 W for 25 s, then the wafer was post-baked at 95 °C for 1 min and developed in MIF300 for 90 s. A 30 nm chromium layer was deposited with magnetron sputtering. A 100 nm platinum layer was deposited with magnetron sputtering. The wafer was then placed into an ultrasonic acetone bath for 20 minutes to start the lift-off process. The wafer was diced into 5 x 10 mm2components, each with a set of electrodes. The second part of the carbon dioxide sensor was a polydimethylsiloxane (PDMS) layer having the channel formed therein. To fabricate this second part, SU-82075 photoresist was spin coated at 1000 rpm for 30 s to produce a 200-pm thick layer on a 4-inch silicon wafer. The SU-8 was cured at 65 °C for 7 minutes and then at 95 °C for 40 minutes. UV lithography was used at 350 W for 25 s to form a patterned microchannel. The wafer was then post-baked at 65 °C for 5 minutes and then at 95 °C for 15 minutes. The wafer was developed with a SU-8 developer (MicroChem) for 5 minutes to produce a patterned mold. The wafer was then post-baked at 95 °C for 1 min. Liquid PDMS solution (Sylgard 186 Silicone Elastomer) was mixed with a curing agent solution in a 10: 1 ratio. The mixture was poured onto the SU-8 mold and cured at 100 °C for 60 minutes. The cured PDMS was cut to fit on each glass component with the electrodes.

[0081] The two parts were assembled by bonding the PDMS layer to the glass wafer with the electrode plates aligned inside the channel. To do this, the glass wafer and the PDMS layer were treated with O2 plasma at 120 V for 30 s. Pressure was applied to both layers until they were firmly attached. The glass wafer was then heated at 95 °C for 60 s. A sealant was applied to the outer boundary of the glass-PDMS interface. Then, 1.5 mm inlet and outlet channels were formed in the PDMS layer. The microchannel in this example had a volume of about 35 pL.

[0082] Power Consumption in Relation to Electrode Separation

[0083] Tests were performed to show that the power consumption of the electrodes used for electrolysis decreases as the electrode separation increases. The power consumption can be described by Equation 4, where P represents the average power and x indicates the electrode separation distance.

[0084] This decrease in power consumption is the direct result of increasing the amount of distance and water in between the two electrode plates, increasing the resistance, thus leading to less current drawn. With the electrode separation tested, the overall power consumption ranged between 130-216 pW, which is indicative of a potential low-power gas sensor. FIG. 7 shows a graph of the power consumption vs. electrode separation distance. In comparison, the bubble-based sensing component consumes about 1000 times less power than the average microbolometer in most NDIR systems, which usually falls within the hundreds of mW range. It can be noted that that the decrease in power consumption due to an increase in electrode separation also results in the tradeoff off in the reduction of electrochemical reaction, thus less bubble production and bubbles to analyze.

[0085] CO 2 Calibration and Bubble Analysis

[0086] CO2 calibration was done by quantifying the relative change in the bubble volume over a 45-minute period, to observe the diffusion rate of CO2 within the proposed device for the initial experiment. When comparing the relative rate of change in volume for CO2 concentrations ranging from 430 ppm to 1,000,000 ppm, the bubble size increased at both the anode and cathode in relation to the increase in CO2 concentration as displayed in FIG. 8. This relationship between the CO2 concentration and the average change in bubble volume for the two electrodes can be described by the function shown in Equation 5, where x represents the CO2 concentration and y represents the relative rate of change in bubble volume.

[0087] At atmospheric CO2 levels, the bubble area decreased as the gas within the bubbles diffuses outwards over time, eventually collapsing when the pressure outside of the bubble exceeds the pressure inside the bubble. However, at higher CO2 concentrations, the diffusion of CO2 gas is reversed, with CO2 diffusing from the outside into the bubble, causing an increase in bubble size until saturation. The relative rate of change slows as the CO2 concentration increases due to gradually reaching the gas saturation level within water. The relative rate of change plateaus at around 700 ppm at room temperature, which is consistent with other research conducted under similar conditions.

[0088] CO2 Selectivity

[0089] While various gases exist in the atmosphere, the carbon dioxide sensor can be configured to measure only CO2 concentration. Testing was conducted with N2, O2, and CO2 at 100% concentration for each respective gas to observe the device's response to varying atmospheric gases. Initial test results indicate that CO2 has a much higher response, with approximately 28 times the response of N2 and around 14 times the response of O2, as demonstrated in FIG. 9. These results seem to be associated with the solubility of each gas in water at room temperature, with CO2 having a solubility of about 1.5 g-kg'1of water, N2 with a solubility of about 0.02 g-kg'1of water, and O2 with a solubility of around 0.045 g-kg'1of water. Although the correlation is similar, the experimental results showed CO2 with a lower relative ratio compared to the other gases. This result is likely due to most CO2 molecules actively reacting with water to form carbonic acid rather than staying as a dissolved gas state within the water. This test proves that the liquid solution used for electrolysis can possibly be adjusted to target other gases in accordance with gas’ solubility. Temperature Effects

[0090] FIG. 10 shows a graph of volume versus temperature for bubbles at the anode and bubbles at the cathode. This figure was the result of an experiment which characterizes the volume of the bubble as a function of the temperature. Theoretically, as the temperature increases, the gas solubility decreases, thus the bubble size should decrease with temperature, which is experimentally confirmed in FIG. 10.

[0091] Capacitance Change over Time

[0092] FIG. 11 shows a graph of capacitance over time. This figure shows the capacitive measurements of bubbles at atmospheric CO2 concentration and 545 ppm of CO2 concentration. Theoretically, the capacitance will decrease overtime as the bubble grows, this is because the dielectric permittivity of the air bubble (~1) is much lower than the dielectric permittivity of the water (~80). As seen in the figure, the capacitance decreases as the bubble grows naturally overtime, however, as the CO2 concentration in the sensor normalizes and bubbles begin to shrink in size and disappear, the capacitance will begin increasing again as seen in the atmospheric CO2 concentration condition. The data on which this graph is based are given in Table 1. Two trials were performed with atmospheric CO2 concentration (ATM1 and ATM2) and one trial was performed with a CO2 concentration of 545 ppm. Capacitance values are in units of nF.

[0093] Table 1: Capacitance of Bubbles with Varying CO2 Concentration

[0094] Effect of Microcavity and Passivation Layer on Electrodes

[0095] A pair of electrodes was used to generate bubbles and the bubble size was measured to determine the uniformity of bubbles generated in this way. The pair of electrodes were designed to have a microcavity at the tip, with a concave angle of 75° and a cavity width of 20 pm at the cavity opening. A passivation layer was formed over the electrode with the exception of the tip where the microcavity was located. The passivation layer was made of TiCh. For comparison, a control electrode pair was also constructed with electrodes that terminate at a pointed tip instead of a microcavity, and no passivation layer was formed over the control electrodes.

[0096] Each pair of electrodes was used to generate bubbles in salt water having different concentrations of CO2. With a CO2 concentration of 247 ppm, the bubble size deviation with the control electrodes was about 21% while with the microcavity electrodes the deviation was reduced to about 13%. With a CO2 concentration of 400 ppm (about the same concentration as in normal seawater), the control electrodes had a bubble size deviation of 30%, and the microcavity electrodes had a deviation of 14%. With a CO2 concentration of 987 ppm, the control electrodes had a bubble size deviation of about 63%, and the microcavity electrodes had a deviation of about 17%. Thus, the microcavity and passivation layer significantly reduced the bubble size deviation, making bubbles with a more uniform size. There was also very little deviation between the different CO2 concentrations for the microcavity electrodes.

[0097] Magnetic Flow System

[0098] Magnets were added to an example sensor to increase the speed of transporting bubbles from the nucleation electrodes to the bubble size sensor. Different magnets providing different levels of magnetic flux density were added and the bubble transport speed was measured. FIG. 12 is a graph showing the transportation time and transportation speed of the bubbles with various levels of magnetic flux density. It was found that an optimal transportation speed and transportation time were achieved when the magnetic flux density was above about 195 mT.

[0099] Correlation of Bubble Diameter and CO 2 Concentration

[0100] To show that bubble diameter can be correlated to CO2 concentration, bubbles were generated in water with varying dissolved CO2 concentrations. The diameter of the bubbles was measured optically after 5 minutes to allow time for CO2 to diffuse from the water into the bubbles. FIG. 13 is a graph showing the bubble diameter vs. concentration of CO2. This graph shows a correlation between bubble diameter and CO2 concentration.

[0101] Correlation of Capacitance and CO 2 Concentration

[0102] The bubbles formed at varying CO2 concentrations in the above example were also measured using a capacitive sensor with interdigitated electrodes. The capacitive measurement utilizes the dielectric difference between water and air. Air has a significantly lower dielectric permittivity than water, so the capacitance decreases as the size of the bubble increases. The capacitive sensor was designed to detect a 1 pm change in bubble radius, resulting in a 0.1 nF change in capacitance. FIG. 14 is a graph of the capacitance measurements vs. the CO2 concentration in the water. As expected, the capacitance measurements decrease with increasing CO2 concentration, because the bubble size increases with increasing CO2 concentration. The optically measured bubble sizes shown in FIG. 13 were converted mathematically to predicted capacitance values. These predicted capacitance values are also shown on the graph (circles represent values measured with the capacitive sensor and squares represent predicted values based on the optical bubble diameter measurements). The predicted values agree closely with the measured values. The capacitive sensor was found to be capable of use to determine the CO2 concentration in the water with an accuracy of plus or minus 6.9% and a hypothetical LOD of 2 ppb.

[0103] Additional Enumerated Examples

[0104] The technology described herein can also include any of the following enumerated examples:

[0105] 1. A carbon dioxide sensor, comprising: a channel capable of containing water having carbon dioxide dissolved therein; a pair of electrodes positioned to contact the water; a voltage source connected to the pair of electrodes operable to supply sufficient voltage to convert a portion of the water to hydrogen gas and oxygen gas by electrolysis to form at least one gas bubble in the channel; and a bubble size sensor operable to measure a size change over time of the at least one gas bubble in the channel.

[0106] 2. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, further comprising water having carbon dioxide dissolved therein inside the channel.

[0107] 3. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, further comprising a correlation module configured to correlate the size change over time of the at least one gas bubble with a concentration of carbon dioxide in the water.

[0108] 4. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the bubble size sensor comprises a capacitive sensor configured to measure a capacitance of the water and the at least one bubble in the channel.

[0109] 5. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the bubble size sensor comprises an image processing module configured to record multiple images over time of the at least one gas bubble and compare an apparent size of the at least one gas bubble in the images.

[0110] 6. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the channel has a channel thickness from 20 pm to 500 pm.

[0111] 7. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the channel has a channel width from 200 pm to 2,000 pm.

[0112] 8. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the electrodes are spaced apart by a separation distance from 300 pm to 800 pm in a channel width direction. 9. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the electrodes are spaced apart by a separation distance from 20 pm to 500 pm in a channel thickness direction.

[0113] 10. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the electrodes comprise platinum, iridium, gold, silver, copper, graphite, steel, or a combination thereof.

[0114] 11. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the voltage source is operable to supply a voltage from 1.23 V to 5 V.

[0115] 12. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, further comprising an inlet into the channel and an outlet out of the channel.

[0116] 13. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, further comprising a semi -permeable membrane dividing the channel into a first channel portion and a second channel portion, wherein a first electrode of the pair of electrodes is operable to form a first gas bubble in the first channel portion and a second electrode of the pair of electrodes is operable to form a second gas bubble in the second channel portion, wherein the bubble size sensor is configured to independently measure a size change of the first gas bubble and a size change of the second gas bubble.

[0117] 14. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the bubble size sensor comprises a first capacitive sensor positioned at the first channel portion to measure a change in capacitance caused by a size change over time of the first gas bubble, and a second capacitive sensor positioned at the second channel portion to measure a change in capacitance caused by a size change over time of the second gas bubble. 15. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, further comprising one or more magnets positioned to provide a magnetic field at the pair of electrodes.

[0118] 16. A carbon dioxide sensor, comprising: a channel capable of containing water; a pair of electrodes positioned to contact the water; a voltage source connected to the pair of electrodes operable to supply sufficient voltage to convert a portion of the water to hydrogen gas and oxygen gas to form at least one gas bubble in the channel; and a capacitive sensor positioned to measure a change in capacitance caused by a size change over time of the at least one gas bubble in the channel.

[0119] 17. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, further comprising a correlation module configured to correlate the size change over time of the at least one gas bubble with a concentration of carbon dioxide in the water.

[0120] 18. A method of measuring a carbon dioxide concentration in water, comprising: contacting a pair of electrodes with water in a channel, wherein the water has carbon dioxide dissolved therein; applying a sufficient voltage to the pair of electrodes to convert a portion of the water to hydrogen gas and oxygen gas by electrolysis to form at least one gas bubble in the channel; measuring a size change over time of the at least one gas bubble in the channel; and correlating the size change over time with a carbon dioxide concentration in the water.

[0121] 19. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the channel does not contain gas bubbles other than the at least one gas bubble formed by the electrolysis. 20. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein applying the sufficient voltage is performed for a time from about 1 second to about 60 seconds, following which the voltage is turned off.

[0122] 21. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein measuring the size change over time is performed after the voltage is turned off, such that changes in size of the at least one gas bubble during the measuring are due to diffusion of gas into and out of the at least one gas bubble.

[0123] 22. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the voltage applied is from 1.23 V to 5 V.

[0124] 23. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the power consumed while applying the sufficient voltage to form the at least one gas bubble is from 100 pW to 300 pW.

[0125] 24. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the at least one gas bubble comprises a plurality of gas bubbles, and wherein measuring the size change over time of the at least one gas bubble comprises measuring a total size change over time of the plurality of gas bubbles.

[0126] 25. A carbon dioxide sensor as in any of examples 1-17 or a method as in any of examples 18-25, wherein the carbon dioxide concentration is from about 400 ppm to about 800 ppm.

[0127] While the flowcharts presented for this technology may imply a specific order of execution, the order of execution may differ from what is illustrated. For example, the order of two more blocks may be rearranged relative to the order shown. Further, two or more blocks shown in succession may be executed in parallel or with partial parallelization. In some configurations, one or more blocks shown in the flow chart may be omitted or skipped. Any number of counters, state variables, warning semaphores, or messages might be added to the logical flow for purposes of enhanced utility, accounting, performance, measurement, troubleshooting or for similar reasons. Reference was made to the examples illustrated in the drawings and specific language was used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the technology is thereby intended. Alterations and further modifications of the features illustrated herein and additional applications of the examples as illustrated herein are to be considered within the scope of the description.

[0128] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more examples. In the preceding description, numerous specific details were provided, such as examples of various configurations to provide a thorough understanding of examples of the described technology. It will be recognized, however, that the technology may be practiced without one or more of the specific details, or with other methods, components, devices, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the technology.

[0129] Although the subject matter has been described in language specific to structural features and / or operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features and operations described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. Numerous modifications and alternative arrangements may be devised without departing from the spirit and scope of the described technology.

Claims

CL IMSWhat is claimed is:

1. A carbon dioxide sensor, comprising: a channel capable of containing water having carbon dioxide dissolved therein; a pair of electrodes positioned to contact the water; a voltage source connected to the pair of electrodes operable to supply sufficient voltage to convert a portion of the water to hydrogen gas and oxygen gas by electrolysis to form at least one gas bubble in the channel; and a bubble size sensor operable to measure a size change over time of the at least one gas bubble in the channel.

2. The carbon dioxide sensor of claim 1, further comprising water having carbon dioxide dissolved therein inside the channel.

3. The carbon dioxide sensor of claim 1, further comprising a correlation module configured to correlate the size change over time of the at least one gas bubble with a concentration of carbon dioxide in the water.

4. The carbon dioxide sensor of claim 1, wherein the bubble size sensor comprises a capacitive sensor configured to measure a capacitance of the water and the at least one bubble in the channel.

5. The carbon dioxide sensor of claim 1, wherein the bubble size sensor comprises an image processing module configured to record multiple images over time of the at least one gas bubble and compare an apparent size of the at least one gas bubble in the images.

6. The carbon dioxide sensor of claim 1, wherein the channel has a channel thickness from 20 pm to 500 pm.

7. The carbon dioxide sensor of claim 1, wherein the channel has a channel width from 200 pm to 2,000 pm.

8. The carbon dioxide sensor of claim 1, wherein the electrodes are spaced apart by a separation distance from 300 pm to 800 pm in a channel width direction.

9. The carbon dioxide sensor of claim 1, wherein the electrodes are spaced apart by a separation distance from 20 pm to 500 pm in a channel thickness direction.

10. The carbon dioxide sensor of claim 1, wherein the electrodes comprise platinum, iridium, gold, silver, copper, graphite, steel, or a combination thereof.

11. The carbon dioxide sensor of claim 1, wherein the voltage source is operable to supply a voltage from 1.23 V to 5 V.

12. The carbon dioxide sensor of claim 1, further comprising an inlet into the channel and an outlet out of the channel.

13. The carbon dioxide sensor of claim 1, further comprising a semi -permeable membrane dividing the channel into a first channel portion and a second channel portion, wherein a first electrode of the pair of electrodes is operable to form a first gas bubble in the first channel portion and a second electrode of the pair of electrodes is operable to form a second gas bubble in the second channel portion, wherein the bubble size sensor is configured to independently measure a size change of the first gas bubble and a size change of the second gas bubble.

14. The carbon dioxide sensor of claim 13, wherein the bubble size sensor comprises a first capacitive sensor positioned at the first channel portion to measure a change in capacitance caused by a size change over time of the first gas bubble, and a second capacitive sensor positioned at the second channel portion to measure a change in capacitance caused by a size change over time of the second gas bubble.

15. The carbon dioxide sensor of claim 1, further comprising one or more magnets positioned to provide a magnetic field at the pair of electrodes.

16. A carbon dioxide sensor, comprising: a channel capable of containing water; a pair of electrodes positioned to contact the water; a voltage source connected to the pair of electrodes operable to supply sufficient voltage to convert a portion of the water to hydrogen gas and oxygen gas to form at least one gas bubble in the channel; and a capacitive sensor positioned to measure a change in capacitance caused by a size change over time of the at least one gas bubble in the channel.

17. The carbon dioxide sensor of claim 16, further comprising a correlation module configured to correlate the size change over time of the at least one gas bubble with a concentration of carbon dioxide in the water.

18. A method of measuring a carbon dioxide concentration in water, comprising: contacting a pair of electrodes with water in a channel, wherein the water has carbon dioxide dissolved therein; applying a sufficient voltage to the pair of electrodes to convert a portion of the water to hydrogen gas and oxygen gas by electrolysis to form at least one gas bubble in the channel; measuring a size change over time of the at least one gas bubble in the channel; and correlating the size change over time with a carbon dioxide concentration in the water.

19. The method of claim 18, wherein the channel does not contain gas bubbles other than the at least one gas bubble formed by the electrolysis.

20. The method of claim 18, wherein applying the sufficient voltage is performed for a time from about 1 second to about 60 seconds, following which the voltage is turned off.

21. The method of claim 20, wherein measuring the size change over time is performed after the voltage is turned off, such that changes in size of the at least one gas bubble during the measuring are due to diffusion of gas into and out of the at least one gas bubble.

22. The method of claim 18, wherein the voltage applied is from 1.23 V to 5 V.

23. The method of claim 18, wherein the power consumed while applying the sufficient voltage to form the at least one gas bubble is from 100 pW to 300 pW.

24. The method of claim 18, wherein the at least one gas bubble comprises a plurality of gas bubbles, and wherein measuring the size change over time of the at least one gas bubble comprises measuring a total size change over time of the plurality of gas bubbles.

25. The method of claim 18, wherein the carbon dioxide concentration is from about 400 ppm to about 800 ppm.

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