Patent Description:
Sensors, such as moisture sensors, are used by farmers and gardeners to measure soil properties. In "<NPL> discusses uses of a probe for monitoring of soil nutrients. <CIT> discloses a soil nutrient measuring instrument.

In accordance with one aspect of the invention, there is provided an apparatus as defined by claim <NUM>. Optiona1 features are defined by the dependent c1aims.

Existing soil sensing architectures typically rely on dedicated sensors that are designed to detect individual soil properties, such as moisture. As such, measuring multiple soil properties can become cumbersome, due to the need to include multiple discrete sensors, with their associated overhead (e.g., cabling, electronics, etc.). Moreover, known sensors are often fabricated from materials that rapidly degrade in the presence of soil, and therefore can exhibit a relatively short lifetime and a lack of predictability and stability in their performance.

Embodiments of the present disclosure address the above drawbacks of existing soil sensor technologies. For example, sensor assemblies of the present disclosure include one or multiple sensor blocks, each sensor block including sensors (e.g., arranged in rows or arrays) for detecting a wide range of soil conditions and nutrients. Sensors of the present disclosure include ruggedized, ion-sensitive membranes and are configured to withstand soil environments for substantially longer than known sensors. In some embodiments, one or more sensors of a sensor assembly is PVC-free. Alternatively or in addition, in some embodiments, a sensor assembly includes a copper-free reference electrode.

A sensor assembly (also referred to herein as a "stake" or "probe") of the present disclosure can include multiple sensors (i.e., a "suite" of sensors), including, but not limited to, sensors for temperature, humidity, light, soil moisture, electrical conductivity, pH, and one or more soil nutrients. The nutrients that can be sensed by sensor embodiments of the present disclosure include, but are not limited to, ammonium (NH<NUM> +), calcium (Ca<NUM>+), carbon dioxide/carbonates (CO<NUM>, HCO<NUM>-, and CO<NUM><NUM>-, depending on pH), chloride (Cl-), nitrate (NO<NUM>-), phosphates (H<NUM>PO<NUM>, H<NUM>PO<NUM>-, HPO<NUM><NUM>-, and PO<NUM><NUM>-, depending on pH), potassium (K+), sodium (Na+), and sulfate (SO<NUM><NUM>-).

A wide range of nutrients can be detected using ion-sensitive field-effect transistor (ISFET) and/or chemically-sensitive field-effect transistor (ChemFET) sensors. The ISFETs described herein facilitate an extensible, versatile platform for the detection of a wide variety of soil nutrients. An ISFET, in its base configuration, can sense protons (H+), thereby enabling pH monitoring. Through the deposition of a membrane on the exposed gate of an ISFET, the ISFET can be transformed into a ChemFET with chemical sensitivity. The sensor assembly can be configured to perform real-time or quasi-real-time transmission of key soil health metrics to the end user, as part of a precision agriculture system. Historically, ISFETs or ChemFETs have not been considered suitable for use in soil sensing contexts, for example due to durability issues, as discussed above. In the present disclosure, however, several methods are presented for improving the durability and stability of sensors, rendering them suitable for agricultural use. Key improvements include improved membrane materials and solid-state reference electrodes designed for long lifetimes in soil.

In some embodiments, the form factor of a sensor assembly (or "platform") is that of an elongate "stake" (e.g., suitable for insertion into the ground), with multiple sensors positioned therein and/or thereon at defined levels, for detecting and reporting nutrient levels (and/or other conditions) at corresponding levels/depths in soil. The multiple sensors can include (but are not limited to): ammonium ISFET sensor(s), calcium ISFET sensor(s), carbonate ISFET sensor(s), chloride ISFET sensor(s), nitrate ISFET sensor(s), phosphate ISFET or electrode sensor(s), potassium ISFET sensor(s), sodium ISFET sensor(s), sulfate ISFET sensor(s) and pH ISFET sensors, electrical conductivity (EC) sensor(s), soil moisture sensor(s), and temperature sensor(s). In some embodiments, the platform can include <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or more ISFET sensors, inclusive of all ranges and values therebetween. The sensor assembly can include one or more "sensor blocks," each sensor block including one or more sensors, each sensor block (or an active/exposed region thereof) being positioned at a predetermined location in or on a housing of the sensor assembly (e.g., centered at or beginning at about <NUM>" (<NUM>), about <NUM>" (<NUM>), about <NUM>" (<NUM>), and/or about <NUM>" (<NUM>) below the ground level). Depending on the implementation, some or all sensors in the sensor assembly are configured to be at least partially in direct contact with the soil once installed. In view of their direct contact with a soil environment during use, sensors (e.g., including sensor membranes) and sensor assemblies (e.g., including sensor assembly housings) of the present disclosure are designed to be durable, and resistant to a wide variety of soil environments.

In some embodiments, the sensor assembly includes a suite of sensors in a sensor probe head (or "probe head"), which includes wireless communications hardware and/or wired communications hardware. Each probe head can also include sensors for one or more of: air temperature, humidity, and light. In some such implementations, one or more carbon dioxide (CO<NUM>) gas sensors can be located at specific locations on or in the sensor assembly, for example at approximately <NUM>" (<NUM>) below a surface of the soil. Alternatively, or in addition, in some such implementations, oxygen gas (O<NUM>) sensors can be located at specific locations on or in the sensor probe, for example at approximately <NUM>" (<NUM>) below a surface of the soil. Alternatively or in addition, ammonia (NH<NUM>), nitrous oxide (N<NUM>O), or methane (CH<NUM>) gas sensors can be located at specific locations throughout the probe. A single, common solid-state reference electrode can be located at the tip of the sensor stake, or individual reference electrodes may be located near each sensor array. The reference electrodes can be shared electrically, for example via multiplexing circuits.

<FIG> are block diagrams showing sensor assembly configurations, according to some embodiments. As shown in <FIG>, a sensor assembly 100A includes a housing <NUM>, a first sensor array segment <NUM> ("probe head"), one or more second sensor array segments <NUM> ("sensor block(s)"), and a reference electrode <NUM>. The housing <NUM> can have any of a variety of different geometries and/or shapes, including but not limited to: elongate, disc-shaped, circular, sheet-like, and/or plate-like. The first sensor array segment <NUM> is disposed within the housing <NUM>, and includes communications-related equipment <NUM> (e.g., antenna, transceiver, wired communication components, processor and associated memory, etc.), and optionally one or more of an air temperature sensor <NUM>, a humidity sensor <NUM>, and a light sensor <NUM>. The one or more second sensor array segments <NUM> are disposed within the housing <NUM>, and include one or more of: a soil temperature sensor <NUM>, an electrical conductivity (EC) sensor <NUM>, a moisture sensor <NUM>, an ion-sensitive field effect transistor (ISFET) nitrate sensor <NUM>, an ISFET phosphate sensor <NUM>, an ISFET potassium sensor <NUM>, and an ISFET pH sensor <NUM>. The nitrate sensor <NUM> is configured to detect, during use and substantially in real time, nitrates in an adjacent region of soil. The phosphate sensor <NUM> is configured to detect, during use and substantially in real time, phosphates in an adjacent region of soil. The potassium sensor <NUM> is configured to detect, during use and substantially in real time, potassium in an adjacent region of soil. The pH sensor <NUM> is configured to detect, during use and substantially in real time, pH in an adjacent region of soil. The reference electrode <NUM> is electrically coupled to each of the first sensor array segment <NUM> and the second sensor array segment(s) <NUM>. The first sensor array segment <NUM> is disposed on a first (left) side of the second sensor array segment(s) <NUM>, and the reference electrode <NUM> is disposed on a second side of the second sensor array segment(s) <NUM>, the second side of the second sensor array segment(s) <NUM> opposite the first side of the second sensor array segment(s) <NUM>. In some embodiments, one or more of the sensor array segment <NUM>, the one or more second sensor array segments <NUM>, or the reference electrode <NUM> is removably coupled to or disposed within the sensor assembly 100A, such that it can be replaced. The removable coupling can be accomplished via any suitable means, such as screw-thread engagement, mechanical attachment (e.g., snaps, clamps, etc.), adhesive attachment, press-fit attachment, etc..

<FIG> includes elements similar to those of <FIG>, but with the first sensor array segment <NUM> disposed outside (and configured to be attached to) the housing <NUM>. <FIG> includes elements similar to those of <FIG>, but with the reference electrode <NUM> disposed outside (and configured to be attached to) the housing <NUM>.

Sensors of the sensor assemblies (e.g., 100A, 100B, 100C) can be calibrated prior to use, as discussed further below with reference to <FIG>, and the associated calibration curves can be analyzed and/or stored in firmware or remotely. The firmware can reside in one or more of: the first sensor array segment (e.g., first sensor array segment <NUM>), the one or more second sensor array segments (e.g., one or more second sensor array segments <NUM>), or "the cloud" (i.e., a cloud computing network).

Diagrams depicting an individual sensor block <NUM> (e.g., similar to sensor blocks <NUM> of <FIG>) coupled to a large reference electrode <NUM> are shown in <FIG>. As shown in <FIG>, a sensor block <NUM> can include a <NUM>-sensor ISFET array with nitrate, phosphate, potassium, and pH ISFET sensors, along with electrical conductivity (EC), soil moisture, and temperature sensors, at each of a plurality of positions in/on each sensor block of the sensor assembly. An example of a sensor assembly having three sensor blocks (such as the sensor block shown in <FIG>), each coupled to a single common, large reference electrode <NUM>, is shown in <FIG>.

Diagrams depicting an individual sensor block <NUM> (e.g., similar to sensor blocks <NUM> of <FIG>) including a mini reference electrode <NUM> are shown in <FIG>. As shown in <FIG>, a sensor block <NUM> includes a <NUM>-sensor ISFET array with nitrate, ammonium, low-pH phosphate, high-pH phosphate, potassium, and pH ISFET sensors, along with EC, soil moisture, and temperature sensors, at each of a plurality of positions in/on each sensor block of the sensor assembly, where each position corresponds to a soil depth during use. Substitutions may be made, where one nutrient sensor is replaced with another, e.g. a sulfate or calcium ISFET sensor may replace the phosphate ISFET sensor(s). In some such configurations, small, or "mini," reference electrodes are disposed adjacent to (rather than incorporated within) each sensor block, and connected to an associated set of individual ISFET circuits of the sensor block. An example of a sensor assembly having three sensor blocks (such as the sensor block shown in <FIG>), each coupled to an associated mini reference electrode <NUM> (rather than to a common large reference electrode), is shown in <FIG>.

<FIG> are photographs of a constructed sensor assembly including three sensor blocks, according to an embodiment.

<FIG> are schematic drawings of an example sensor die, according to an embodiment. As shown in <FIG>, a sensor die can have a rectangular shape with a length of about <NUM>,<NUM> micrometers (µm) and a width of about <NUM>,<NUM>. Bond pads of the conductive traces (connected to one or more ISFETs of the sensor die) can be about <NUM> by about <NUM> square. An active region of the sensor die can have a rectangular shape with a size of about <NUM>,<NUM> by about <NUM>,<NUM>. Sensor dies can be fabricated on rigid substrates, such as silicon wafers, or on flexible substrates, such as polyethylene terephthalate (PET). Although example geometries for the sensor die are presented in <FIG>, deviations from the sizes and/or proportionalities of the various sensor die features can be made without departing from the scope of the present disclosure. In some embodiments, the sensor die can have a length of about <NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, or about <NUM>,<NUM>, inclusive of all ranges therebetween. In some embodiments, the sensor die can have a width of about <NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, or about <NUM>,<NUM>, inclusive of all ranges therebetween. In some embodiments, the sensor die can have a thickness of about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>, about <NUM>,<NUM> (<NUM>), about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM>, about <NUM>,<NUM> (<NUM>), inclusive of all ranges therebetween.

As described above, nutrient sensing, according to some embodiments, can be achieved via sensor dies using ISFETs or ChemFETs, which are amenable to fabrication via high-volume CMOS processing. A representative geometry of an ISFET sensor die is shown in the light microscope photograph of <FIG>. These devices evince common transistor-like structures, however, the metal gate normally associated with a transistor has been removed and left exposed. In some embodiments, it is the exposed region (see circled region in <FIG>) that is disposed adjacent to soil during use, and is responsible for ion or nutrient sensing (e.g., via one or multiple ion-sensitive membranes). The exposed region prior to membrane deposition can include one or more materials such as such as carbon, graphene, carbon nanotubes, or films including silicon nitride (Si<NUM>N<NUM>), aluminum oxide (Al<NUM>O<NUM>), or tantalum oxide (Ta<NUM>O<NUM>). The exposed region can be smaller than or larger than the active region discussed above with reference to <FIG>.

There is a design trade-off between allowing a portion of the sensor to contact the soil or medium of interest, while also protecting sensitive electronics. In some embodiments, the electronics are "encapsulated" using a material such as epoxy or resin. A portion of the active region of the sensor can be left exposed after the encapsulation process is completed. This can be accomplished, for example, by "masking" (or otherwise protecting) the desired region to be exposed, such that no encapsulant material is applied thereto during the encapsulation process (e.g., during an "additive" encapsulation process, in which encapsulant material is added to the surface of the die/PCB). Alternatively, or in addition, the active region of the sensor die is first coated with the encapsulant material, and then a portion of the active region (corresponding to the desired region to be exposed) is exposed through a subtractive process (e.g., dry etching, wet chemical etching, mechanical removal, etc.).

An example of an encapsulated ChemFET sensor die mounted on a printed circuit board (PCB) is shown in the photograph of <FIG>. Additional examples of encapsulation can be seen in <FIG> with a <NUM>-sensor ISFET/ChemFET array and <FIG> with a set of boards containing <NUM>-sensor ISFET/ChemFET arrays.

In some embodiments, membranes (formed from a "matrix material") are synthesized and disposed on an exposed gate of a field-effect transistor (FET) for the detection of analytes. The analytes that can be detected by a membraned FET include, but are not limited to, ammonium, calcium, carbonate, chloride, nitrate, phosphate, potassium, sodium, and sulfate. A matrix material, as defined herein, can include a fluorosilicone (FS) sealant/adhesive, or other polymeric materials having mechanical properties that achieve satisfactory ratings under ASTM standards, such as ASTM D3359 (Standard Test Methods for Rating Adhesion by Tape Test) and ASTM D6677 (Evaluating Adhesion by Knife). For example, membranes can receive a rating of 5A (no peeling or removal) under ASTM D3359 Test Method A and a rating of <NUM> or higher under ASTM D6677.

In some embodiments, various additives may be added to the matrix materials to tailor electrical properties, for example, the addition of carbon black. Prior to the inclusion of ion-selective ionophores and other ionic additives, the FS matrix material can be dissolved in a suitable solvent, such as tetrahydrofuran (THF) or cyclohexanone. Alternatively, matrix materials can include one or more ionophore-doped conducting polymers (CPs), such as polyaniline (PANI), polypyrrole (PPy), poly(<NUM>,<NUM>-ethylenedioxythiophene) (PEDOT), or poly(<NUM>-octylthiophene) (POT), as examples. Alternatively or in addition, a sensor array can include FS, CPs, or a mixture thereof.

In some embodiments, membrane solutions including FS or CPs are dispensed on the exposed gate of the field-effect transistor to yield an ion-selective membrane. The dispensing of the matrix material onto the exposed gate can include one or more of: screen printing, inkjet printing, syringe dispensing, etc. The membrane material(s) may be allowed to cure in air or other ambient environments, or via vacuum processing. Photocurable membranes may be cured via UV/visible light exposure.

In some embodiments, to achieve ion selectivity for ammonium (NH<NUM>+), the ionophore nonactin, monactin, or a mixture thereof is added to the dissolved matrix material. Optionally, the ionic additive potassium tetrakis(<NUM>-chlorophenyl)borate can also be added. The matrix material, in some embodiments, may be present in amounts ranging from about <NUM>% to about <NUM>% (w/w), with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios.

In some embodiments, to achieve ion selectivity for calcium (Ca<NUM>+), the ionophore diethyl N,N'-[(4R,5R)-<NUM>,<NUM>-dimethyl-<NUM>,<NUM>-dioxo-<NUM>,<NUM>-dioxaoctamethylene]bis(<NUM>-methylamino-dodecanoate) (ETH <NUM>), N,N,N',N'-tetracyclohexyl-<NUM>-oxapentanediamide (ETH <NUM>), calcimycin, N,N-dicyclohexyl-N',N'-dioctadecyl-<NUM>-oxapentanediamide (ETH <NUM>), <NUM>,<NUM>-bis[(octadecylcarbamoyl)methoxyacetyl]-<NUM>,<NUM>,<NUM>,<NUM>,<NUM>-pentaoxa-<NUM>,<NUM>-diazacycloheneicosane, α-furildioxime, or a mixture thereof is added to the dissolved matrix material. Optionally, the ionic additive sodium tetrakis[<NUM>,<NUM>-bis(trifluoromethyl)phenyl]borate (NaTFPB) can also be added. The matrix material, in some embodiments, may be present in amounts ranging from <NUM>% - <NUM>% (w/w) or thereabouts, with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios.

In some embodiments, to achieve ion selectivity for carbonate (CO<NUM><NUM>-), the ionophore heptyl <NUM>-trifluoroacetylbenzoate (ETH <NUM>), <NUM>-(dodecylsulfonyl)-<NUM>-trifluoroacetylbenzene (ETH <NUM>), N-dodecyl-N-(<NUM>-trifluoroacetylphenyl)acetamide (ETH <NUM>), <NUM>-butyl-α,α,α-trifluoroacetophenone, N,N-dioctyl-3α,12α-bis(<NUM>-trifluoroacetylbenzoyloxy)-5β-cholan-<NUM>-amide, or a mixture thereof is added to the dissolved matrix material. Optionally, the ionic additive tridodecylmethylammonium chloride (TDMAC) can also be added. The matrix material, in some embodiments, may be present in amounts ranging from <NUM>% - <NUM>% (w/w) or thereabouts, with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios.

In some embodiments, to achieve ion selectivity for chloride (Cl-), the ionophore meso-tetraphenylporphyrin manganese(III)-chloride complex, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-dioctyloxy-o-phenylene-bis(mercurytrifluoroacetate) (ETH <NUM>), <NUM>,<NUM>-didodecyloxy-<NUM>,<NUM>-dimethyl-o-phenylene-bis(mercury chloride) (ETH <NUM>), <NUM>,<NUM>-bis-[N'-(butyl)thioureido]-<NUM>,<NUM>-di-tert-butyl-<NUM>,<NUM>-dimethylxanthene, or a mixture thereof is added to the dissolved matrix material. Optionally, the ionic additive tridodecylmethylammonium chloride (TDMAC) can also be added. The matrix material, in some embodiments, may be present in amounts ranging from <NUM>% - <NUM>% (w/w) or thereabouts, with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios.

In some embodiments, to achieve ion selectivity for nitrate (NO<NUM>-), the ionophore α,α,α,α-<NUM>,<NUM>,<NUM>,<NUM>-tetrakis{<NUM>-[<NUM>-(<NUM>-methylphenyl)ureido]phenyllporphyrine, <NUM>,<NUM>,<NUM>,<NUM>-tetraoxa-<NUM>,<NUM>,<NUM>,<NUM>-tetraazacyclooctodecane, [<NUM>,<NUM>,<NUM>,<NUM>]tetraazacyclotetradecine-<NUM>,<NUM>-dithione, <NUM>-hexadecyl-<NUM>,<NUM>,<NUM>,<NUM>-tetraoxa-<NUM>,<NUM>,<NUM>,<NUM>-tetraazacycloeicosane, or a mixture thereof is added to the dissolved matrix material. Optionally, tridodecylmethylammonium nitrate, tetradodecylammonium nitrate, tri-n-octylmethylammonium nitrate, or other quarternary ammoniun salts may can also be added. The matrix material, in some embodiments, may be present in amounts ranging from <NUM>% - <NUM>% (w/w) or thereabouts, with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios.

In accordance with the present invention, to achieve ion selectivity for the phosphates in the pH regime of approximately <NUM> - <NUM>, tin organometallics, such as tributyltin chloride (TBTC), are added to the dissolved matrix material. Ionic additives such as sodium tetrakis [<NUM>,<NUM>-bis(trifluoromethyl)-phenyl]borate (NaTFPB) may be added in varying mol% relative to the tin organometallics. To achieve ion selectivity for the phosphates in the pH regime of approximately <NUM> - <NUM>, cyclic polyamines such as the N<NUM>-, N<NUM>-, N<NUM>-, and N<NUM>-cyclic amines are used as ionophores. NaTFPB may be added in varying mol% relative to the cyclic amines. The matrix material, in some embodiments, may be present in amounts ranging from <NUM>% - <NUM>% (w/w) or thereabouts, with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios. In some embodiments, multiple ISFET-based phosphate sensors are coupled to cover a wider pH range, and these sensors may also be correlated to electrode-based sensors, such as those fabricated from cobalt wires, enabling the sensor to detect a broad spectrum of phosphate species in soil.

In some embodiments, to achieve ion selectivity for potassium, the ionophore valinomycin is added to the dissolved matrix material. Optionally, the ionic additives potassium tetrakis(<NUM>-chlorophenyl)borate (KT4ClPB) and/or sodium tetrakis[<NUM>,<NUM>-bis(trifluoromethyl)phenyl]borate (NaTFPB) can also be added. The matrix material, in some embodiments, may be present in amounts ranging from <NUM>% - <NUM>% (w/w) or thereabouts, with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios.

In some embodiments, to achieve ion selectivity for sodium (Na+), the ionophore N,N',N"-triheptyl-N,N',N"-trimethyl-<NUM>,<NUM>',<NUM>"-propylidynetris(<NUM>-oxabutyramide) (ETH <NUM>), N,N'-dibenzyl-N,N'-diphenyl-<NUM>,<NUM>-phenylenedioxydiacetamide (ETH <NUM>), N,N,N',N'-tetracyclohexyl-<NUM>,<NUM>-phenylenedioxydiacetamide (ETH <NUM>), <NUM>,<NUM>:<NUM>,<NUM>-didecalino-<NUM>-crown-<NUM>, bis[(<NUM>-crown-<NUM>)methyl] dodecylmethylmalonate, bis[(<NUM>-crown-<NUM>)methyl] <NUM>,<NUM>-didodecylmalonate, <NUM>-tert-butylcalix[<NUM>]arene-tetraacetic acid tetraethyl ester, or a mixture thereof is added to the dissolved matrix material. Optionally, the ionic additive sodium tetrakis[<NUM>,<NUM>-bis(trifluoromethyl)phenyl]borate (NaTFPB) can also be added. The matrix material, in some embodiments, may be present in amounts ranging from <NUM>% - <NUM>% (w/w) or thereabouts, with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios.

In some embodiments, to achieve ion selectivity for sulfate (SO<NUM><NUM>-), the ionophore <NUM>,<NUM>-[bis(<NUM>-phenylthioureidomethyl)]benzene, <NUM>-(<NUM>-bromophenyl)-<NUM>,<NUM>-diphenylpyrilium perchlorate (BDPP), or a mixture thereof is added to the dissolved matrix material. The matrix material, in some embodiments, may be present in amounts ranging from <NUM>% - <NUM>% (w/w) or thereabouts, with the balance comprising ionophores, ionic additives, and plasticizers in varying ratios.

In some embodiments, where metal gates are removed from the transistor structures to yield ISFETs/ChemFETs, one or more reference electrodes are added to the sensor assembly, the reference electrode(s) being configured for placement in (and durability within) the same medium as the ISFETs/ChemFETs (e.g., soil). Traditional reference electrode designs often contain liquids that are prone to drying out, and are typically constructed using glass, further reducing their ruggedness. By contrast, reference electrodes of the present disclosure, using solid-state electrolytes, address these weaknesses. For example, in some embodiments, the reference electrode assembly contains a "frit" to enable ionic transport from the soil to the solid-state electrolyte; the frit materials may be comprised of porous ceramic, glass, or polymeric materials.

As shown in <FIG>, a large reference electrode assembly <NUM>, compatible, for example, with the sensor assemblies of <FIG> and <FIG>, includes a main body portion with a male connector <NUM> extending therefrom, the male connector <NUM> configured to connect with (i.e., be received by) a female connector <NUM>. <FIG> is a photograph of an assembled large reference electrode <NUM>, compatible, for example, with the sensor assemblies of <FIG>, <FIG>, or <FIG>, according to an embodiment. Assembled reference electrodes of varying electrolyte composition have been tested against reference Ag/AgCl elements to monitor their stability of the reference potential and to examine potential drift. <FIG> is a plot showing comparison data for large reference electrodes ("RE's") vs. a silver/silver chloride (Ag/AgCl) reference electrode, according to an embodiment. As demonstrated by the plot data in <FIG>, each of the tested reference electrodes (RE1 through RE7 and versions of RE1 through RE3 "conditioned" in an electrolyte storage solution) compared favorably with bare Ag/AgCl reference electrodes. The assembly process for large reference electrodes is provided in the Example Reference Electrode Assembly Process section below.

Additional form factors have been developed, including small/miniature ("mini") versions of the solid-state reference electrode. These are typically intended to be located near the sensor arrays, rather than at the tip of the sensor stake. Schematic drawings of a small reference electrode <NUM>, compatible, for example, with the sensor assemblies of <FIG> and <FIG>. As shown in <FIG>, a small (or "mini") reference electrode <NUM> includes a silver (Ag) electrode <NUM> positioned within a recess formed by sleeve <NUM> and at least partially supported by an internal cap <NUM> (which, in some embodiments, is integrally formed with the sleeve <NUM>). A lid <NUM> is positioned at the top of the recess formed by sleeve <NUM>. A sensor module PCB <NUM> (e.g., including conductive traces and/or electronics) is mechanically attached to the sleeve <NUM> (or other components of the reference electrode <NUM>) and is electrically connected to the Ag electrode <NUM>. A photograph of a fully assembled mini reference electrode is provided in <FIG>. An example of where the mini reference electrode of <FIG> may be located relative to the overall sensor assembly can be seen, for example, in <FIG> (reference electrode <NUM> - in each instance of a sensor block, multiple of which may be included in an overall sensor assembly) and in <FIG> (reference electrodes <NUM> - in each instance of a sensor block <NUM>). <FIG> is a plot showing stability testing data for a small reference electrode, according to an embodiment. As demonstrated by the plot data in <FIG>, there is a "settling" or "equilibration" period (over the first ~<NUM> days) during which the potential being measured by the reference electrode decreases (e.g., asymptotically) until it approaches a steady-state value (in this case, about -<NUM> V). This "equilibration" period may be taken into account when processing sensor signals of the sensor assembly, for example by excluding or disregarding data collected during the "equilibration" period, or making adjustments thereto (e.g., based on a time at which the particular data point was measured/detected).

In some embodiments, a reference electrode includes a housing, a ceramic frit, a Ag/AgCl element, and a solid electrolyte comprising "Plaster of Paris" (CaSO<NUM>•<NUM><NUM>O) and NaCl. To assemble the reference electrode, the ceramic frit (e.g., made of a metal oxide such as alumina (Al<NUM>O<NUM>)) is affixed to the reference electrode housing, for example using an epoxy (in which case the assembly, one epoxied, is allowed to cure overnight). The Ag/AgCl element is secured to the reference electrode cap using a small quantity of epoxy to the upper face (where the bare Ag wire originates) of the Ag/AgCl element. The bare Ag wire protrudes from the cap and is affixed to a copper (or similar metal) wire using a solder or crimped connection. The result of this assembly step is the Ag/AgCl element, which is epoxied to the reference electrode cap, which in turn is connected to copper or a similarly conductive wire.

After the ceramic frit is secured to the reference electrode body, a solid electrolyte mixture of deionized water, "Plaster of Paris" (CaSO<NUM>•<NUM><NUM>O), and NaCl is poured into the main cavity in the reference electrode housing. Within about one minute, the Ag/AgCl element from the cap-Ag/AgCl assembly is submerged into the solid electrolyte mixture prior to solidification. The solidification process of the solid electrolyte material is completed within about approximately <NUM> minutes. The solid electrolyte composition can be varied based on the targeted soil under analysis.

Upon installation of the Ag/AgCl element into the solid electrolyte, the cap is epoxied or otherwise secured to the main reference electrode body, thereby completing the assembly of the reference electrode.

<FIG> is a photograph of five encapsulated four-sensor arrays (i.e., four sensors arranged in a single row per array, with five arrays on a single board), according to an embodiment. <FIG> is a photograph of a set of three encapsulated six-sensor array boards, according to an embodiment.

In some embodiments, the ISFET device operates under a constant-voltage, constant-current bias scheme. A voltage source enforces a constant source-drain voltage Vds (Vd-Vs), for example approximately <NUM> V. A feedback loop senses the source-drain current through the ISFET (Ids) and adjusts the gate voltage Vg, until the Ids reaches a specified current, in some embodiments approximately <NUM> uA. The gate-source voltage Vgs (Vg-Vs) is then measured and represents the output of the sensor. An ISFET interface circuit schematic diagram and example implementation are shown in <FIG> and <FIG>, respectively. In <FIG> a voltage source (Vsupply) defines the ISFET Vds potential. An ammeter senses the current flowing through the voltage source and the ISFET. Based on the output of the ammeter, a current controlled voltage source adjusts the gate voltage, Vg, to reach the desired ISFET current. In <FIG>, U2 and U3 provide stable positive and negative reference voltages for biasing the ISFET. The resistors R5, R6, and R7 act as a voltage divider to generate the necessary Vds. The operational amplifier U4A buffers the Vd voltage and provides a low output impedance to drive the ISFET. The resistor R14 is the current sensing element, and together with operational amplifier U4D, creates the current controlled voltage source that drives the reference electrode (RE) potential. In stable sensing operation, Vs will be approximately <NUM> V and the output of the sensor can be read as Vg.

Several ISFET devices may be multiplexed into a single interface circuit through standard multiplexing techniques. Here, several low on-resistance (Ron) single-pole-single-throw solid state switches are used to switch rows connected to drains of ISFET devices and columns connected to sources of ISFET devices. Optionally, the reference electrode may also be selectively connected to the interface circuit through similar switches. These switches may be discrete devices or integrated into a single package. An example ISFET multiplexing circuit is shown in <FIG>.

Calibration data for pH ISFETs and nitrate, phosphate, and potassium ChemFETs can be seen in <FIG>, respectively. Testing has been conducted to assess the selectivity of the ChemFETs toward interfering ions. In the case of nitrate sensors, they have been tested against anions such as sulfates (<FIG>), carbonates (<FIG>), and chloride ions (<FIG>). The potassium sensors have been tested against common cations found in soil, such as sodium (<FIG>), calcium (<FIG>), magnesium (<FIG>), and ammonium (<FIG>). Both sensors have demonstrated strong selectivity toward the ion of interest (e.g. nitrate, potassium) and reject interfering ions. Tests have been performed to test the dynamic response of sensors to doses of solutions containing the ion of interest in soil. An example of this study conducted in sand is shown in <FIG>. Data collected from field studies with the sensor stakes has been compared to lab-tested samples. Most results are within <NUM> ppm of the lab-derived results.

<FIG> is a plot showing calibration data for a pH ISFET sensor, according to an embodiment. During calibration, three measurements of the sensor response (in millivolts, mV) were taken, in solution, at three different associated pH levels (in this case, pH values of about <NUM>, about <NUM>, and about <NUM>). The sensor response data was plotted, and a line of best fit was drawn (in this case, y = <NUM>. 5x + <NUM>), and a coefficient of determination (R<NUM>) was calculated (in this case, R<NUM> = <NUM>).

<FIG> is a plot showing calibration data for a nitrate ISFET sensor, according to an embodiment. During calibration, three measurements of the sensor response (in mV) were taken, in solution, at three different values of log([NO<NUM>-]) (in this case, log([NO<NUM>-]) values of about -<NUM>, about -<NUM>, and about -<NUM>). The sensor response data was plotted, and a line of best fit was drawn (in this case, y = -<NUM>x - <NUM>), and a coefficient of determination (R<NUM>) was calculated (in this case, R<NUM> = <NUM>).

<FIG> is a plot showing calibration data for a phosphate ISFET sensor, according to an embodiment. During calibration, three measurements of the sensor response (in mV) were taken, in solution, at three different values of log([H<NUM>PO<NUM>-]) (in this case, log([H<NUM>PO<NUM>-]) values of about -<NUM>, about -<NUM>, and about -<NUM>). The sensor response data was plotted, and a line of best fit was drawn (in this case, y = -<NUM>. 5x - <NUM>), and a coefficient of determination (R<NUM>) was calculated (in this case, R<NUM> = <NUM>).

<FIG> is a plot showing calibration data for a potassium ISFET sensor, according to an embodiment. During calibration, three measurements of the sensor response (in mV) were taken, in solution, at three different values of log([K+]) (in this case, log([K+]) values of about -<NUM>, about -<NUM>, and about -<NUM>). The sensor response data was plotted, and a line of best fit was drawn (in this case, y = <NUM>x - <NUM>), and a coefficient of determination (R<NUM>) was calculated (in this case, R<NUM> = <NUM>).

<FIG> is a plot showing the effect of sulfate, as a potential contaminant, on the performance of nitrate sensors (using a Ag/AgCl reference electrode), according to an embodiment. As shown in <FIG>, side-by-side comparisons of sequential sensor readings (using four distinct sensors, Sensors <NUM> through <NUM>) for nitrate (NO<NUM>-) alone (left portion of each grouping) and nitrate in the presence of sulfate (right portion of each grouping) are shown for three different ratios of nitrate to sulfate (i.e., Grouping <NUM> = <NUM> nitrate, <NUM> sulfate; Grouping <NUM> = <NUM> nitrate, <NUM> sulfate; Grouping <NUM> = <NUM> nitrate, <NUM> sulfate). The data in <FIG> shows that the nitrate sensor performance was stable even in the presence of sulfate.

<FIG> is a plot showing the effect of carbonate, as a potential contaminant, on the performance of nitrate sensors (using a Ag/AgCl reference electrode), according to an embodiment. As shown in <FIG>, sequential sensor readings (using four distinct sensors, Sensors <NUM> through <NUM>) for nitrate in the presence of carbonate (CO<NUM><NUM>-) are shown for three different ratios of nitrate to CO<NUM><NUM>- (i.e., Grouping <NUM> = <NUM> nitrate, <NUM> carbonate; Grouping <NUM> = <NUM> nitrate, <NUM> carbonate; Grouping <NUM> = <NUM> nitrate, <NUM> carbonate). The data in <FIG> shows that the nitrate sensor performance was stable (e.g., Grouping <NUM>, showing negligible/near-zero variation in voltage) or substantially stable (e.g., Groupings <NUM> and <NUM>, showing variation in voltage of about <NUM>-<NUM> V), even in the presence of carbonate.

<FIG> is a plot showing the effect of chloride, as a potential contaminant, on the performance of nitrate sensors (using a Ag/AgCl reference electrode), according to an embodiment. As shown in <FIG>, sequential sensor readings (using four distinct sensors, Sensors <NUM> through <NUM>) for nitrate in the presence of chloride are shown for three different ratios of nitrate to chloride (i.e., Grouping <NUM> = <NUM> nitrate, <NUM> chloride; Grouping <NUM> = <NUM> nitrate, <NUM> chloride; Grouping <NUM> = <NUM> nitrate, <NUM> chloride). The data in <FIG> shows that the nitrate sensor performance was stable (e.g., Groupings <NUM> and <NUM>, showing negligible/near-zero variation in voltage) or substantially stable (e.g., Grouping <NUM>, showing variation in voltage of about <NUM> V), even in the presence of chloride.

<FIG> is a plot showing the effect of sodium ions (Na+), as a potential contaminant, on the performance of potassium sensors, according to an embodiment. As shown in <FIG>, sequential sensor readings (using four distinct sensors, Sensors <NUM> through <NUM>) for potassium in the presence of sodium are shown for six different ratios of potassium to sodium (i.e., Grouping <NUM> = <NUM> potassium, <NUM> sodium; Grouping <NUM> = <NUM> potassium, <NUM> sodium; Grouping <NUM> = <NUM> potassium, <NUM> sodium; Grouping <NUM> = <NUM> potassium, <NUM> sodium; Grouping <NUM> = <NUM> potassium, <NUM> sodium; Grouping <NUM> = <NUM> potassium, <NUM> sodium). The data in <FIG> shows that the potassium sensor performance was stable or substantially stable, even in the presence of sodium.

<FIG> is a plot showing the effect of calcium ions (Ca<NUM>+), as a potential contaminant, on the performance of potassium sensors, according to an embodiment. As shown in <FIG>, sequential sensor readings (using four distinct sensors, Sensors <NUM> through <NUM>) for potassium in the presence of calcium are shown for six different ratios of potassium to calcium (i.e., Grouping <NUM> = <NUM> potassium, <NUM> calcium; Grouping <NUM> = <NUM> potassium, <NUM> calcium; Grouping <NUM> = <NUM> potassium, <NUM> calcium; Grouping <NUM> = <NUM> potassium, <NUM> calcium; Grouping <NUM> = <NUM> potassium, <NUM> calcium; Grouping <NUM> = <NUM> potassium, <NUM> calcium). The data in <FIG> shows that the potassium sensor performance was stable or substantially stable, even in the presence of calcium.

<FIG> is a plot showing the effect of magnesium ions (Mg<NUM>+), as a potential contaminant, on the performance of potassium sensors, according to an embodiment. As shown in <FIG>, sequential sensor readings (using four distinct sensors, Sensors <NUM> through <NUM>) for potassium in the presence of magnesium are shown for six different ratios of potassium to magnesium (i.e., Grouping <NUM> = <NUM> potassium, <NUM> magnesium; Grouping <NUM> = <NUM> potassium, <NUM> magnesium; Grouping <NUM> = <NUM> potassium, <NUM> magnesium; Grouping <NUM> = <NUM> potassium, <NUM> magnesium; Grouping <NUM> = <NUM> potassium, <NUM> magnesium; Grouping <NUM> = <NUM> potassium, <NUM> magnesium). The data in <FIG> shows that the potassium sensor performance was stable or substantially stable, even in the presence of magnesium.

<FIG> is a plot showing the effect of ammonium (NH<NUM>+), as a potential contaminant, on the performance of potassium sensors, according to an embodiment. As shown in <FIG>, sequential sensor readings (using four distinct sensors, Sensors <NUM> through <NUM>) for potassium in the presence of ammonium are shown for six different ratios of potassium to magnesium (i.e., Grouping <NUM> = <NUM> potassium, <NUM> ammonium; Grouping <NUM> = <NUM> potassium, <NUM> ammonium; Grouping <NUM> = <NUM> potassium, <NUM> ammonium; Grouping <NUM> = <NUM> potassium, <NUM> ammonium; Grouping <NUM> = <NUM> potassium, <NUM> ammonium; Grouping <NUM> = <NUM> potassium, <NUM> ammonium). The data in <FIG> shows that the potassium sensor performance was stable or substantially stable, even in the presence of ammonium.

<FIG> is a plot showing a dynamic response of a nitrate sensor to an applied dose of nitrate solution, in accordance with some embodiments. As can be seen in <FIG>, there is an initial time period (< <NUM>,<NUM> seconds) during which the sensor is initially "wetted" with the nitrate-containing solution and detects a rapidly increasing amount of nitrate. After this initial equilibration period, the sensor reaches and maintains a steady-state detected level of nitrate.

In some embodiments, one or more components of the sensor assembly (e.g., a first sensor array segment or a second sensor array segment) includes a processor and a memory storing instructions to cause the processor to one or more of: receive signals from one or more sensors of the sensor assembly, analyze signals from one or more sensors of the sensor assembly to detect or calculate a soil parameter, or send signals to one or more remote computing devices, in response to a sensor detection event. Instructions to analyze signals from one or more sensors of the sensor assembly can include calculating a soil parameter based at least in part on calibration data associated with the one or more sensors, and/or based at least in part on a known equilibration period associated with the one or more sensors.

Claim 1:
An apparatus, comprising:
a housing (<NUM>);
a first sensor array segment comprising wireless communication hardware, the wireless communication hardware including an antenna;
a second sensor array segment disposed within the housing, the second sensor array segment including:
a soil moisture sensor (<NUM>),
an electrical conductivity (EC) sensor (<NUM>), and
an ion-sensitive field-effect transistor (ISFET) pH sensor (<NUM>) configured to detect, during use and in an adjacent region of soil, a pH;
the apparatus being characterised in that it comprises:
a first ISFET phosphate sensor (<NUM>) configured to detect phosphates in a pH range of <NUM>-<NUM>, the first ISFET phosphate sensor having a first membrane comprising a tin organometallic,
a second ISFET phosphate sensor configured to detect phosphates in a pH range of <NUM>-<NUM>, the second ISFET phosphate sensor having a second membrane comprising a cyclic polyamine;
a soil temperature sensor (<NUM>),
an ISFET nitrate sensor (<NUM>) configured to detect, during use, nitrates in an adjacent region of soil,
an ISFET ammonium sensor configured to detect, during use, ammonium in an adjacent region of soil, and
an ISFET potassium sensor (<NUM>) configured to detect, during use, potassium in an adjacent region of soil, and
a reference electrode (<NUM>) electrically coupled to each of the first sensor array segment and the second sensor array segment,
the first sensor array segment disposed on a first side of the second sensor array segment, and
the reference electrode disposed on a second side of the second sensor array segment, the second side of the second sensor array segment opposite the first side of the second sensor array segment.