Two-terminal screen-printed PH sensors for monitoring growing media

Abstract

An example apparatus includes a housing; an electrode array located in the housing, the electrode array comprising two or more electrodes, each comprising a pH-sensitive coating or encapsulation to measure a pH of a growing media; and a controller configured to: generate an electrical current between the two or more electrodes; measure, by a current sensor, the electrical current flow between the two or more electrodes; and output an estimate of pH based on the electrical current flow.

Description

TWO-TERMINAL SCREEN-PRINTED PH SENSORS FOR MONITORING GROWING MEDIACROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional patent application No. 63 / 650,372, filed on May 21, 2024. and titled ‘TWO-TERMINAL SCREEN-PRINTED PH SENSORS FOR MONITORING GROWING MEDIA.” the disclosure of which is expressly incorporated herein by reference in its entirety.STATEMENT REGARDING GOVERNMENT SUPPORT

[0002] This invention was made with government support under grant number DE-arOO 162 awarded by the U.S. Department of Energy'. The government has certain rights in the inventionBACKGROUND

[0003] pH is a measure of how acidic or basic a solution is. The pH scale is logarithmic, and correlates to the activity of hydrogen atoms in solution. Sensors for pH can be implemented as electrode pairs. Soil particles can trap water around them, and the pH of the water surrounding soil particles can be an important parameter for farmers to manage.

[0004] There is a benefit to improving measurement of pH.SUMMARY

[0005] An exemplary distributed sensor system and method are disclosed for precision agriculture that monitors and / or actively controls growing conditions and parameters for crops. The exemplary system and method employ a bulk electrode functionalized with apH-sensitive dye compound (e.g., Alizarin) to provide a high-precision and high accuracy sensor for scientific and industrial monitoring employing low-cost, screen-printed, two-electrode sensor, e.g., for real time pH data with high spatiotemporal resolution. The sensor system can measure the pH of multiple growing media, including soil and hydroponic solutions. In some embodiments, the sensor employs a controller (e.g., off-the-shelf. Arduino-based device) with readout electronics. The exemplary sensor can be manufactured using roll-to-roll printing to provide high-precision and high accuracy sensing at low-cost when manufactured in high volume. The manufacturing is optimized low-cost production (e.g., less than $1 per sensor or $0.50 per sensor) while providing reliable measurement over time (e.g., over a growing season, 3-4 months).

[0006] In some embodiments, the exemplar}' system and method employ a stabilizing Nation membrane on the working electrode and a salt membrane on the reference electrode providing improved stability in buffered and unbuffered media. A study was conducted to develop and evaluate the exemplary system and method as a two-electrode sensor. The sensor developed and employed in the study demonstrated aNemstian sensitivity of -59.08 mV pH- 1. The results show that a straightforward printed, electronic pH sensor paired with low-cost electronics can be used to effectively to monitor the pH of different unbuffered media with precision and accuracy.

[0007] In some aspects, implementations of the present disclosure include an apparatus including: a housing; an electrode array located in the housing, the electrode array including two or more electrodes, each including a pH-sensitive coating or encapsulation to measure a pH of a growing media; and a controller configured to: generate an electrical current between the two or more electrodes; measure, by a current sensor, an electrical current flow between the two or more electrodes; and output an estimate of pH based on the electrical current flow.

[0008] In some aspects, implementations of the present disclosure include an apparatus, wherein the pH-sensitive coating or encapsulation includes Alizarin.

[0009] In some aspects, implementations of the present disclosure include an apparatus, wherein the two or more electrodes include a working electrode, a reference electrode, and a counter electrode.

[0010] In some aspects, implementations of the present disclosure include an apparatus, wherein the two or more electrodes include a working electrode and a counter electrode, wherein the counter electrode is configured as a combined counter and reference electrode.

[0011] In some aspects, implementations of the present disclosure include an apparatus, wherein the working electrode and counter electrode are arranged in a concentric configuration.

[0012] In some aspects, implementations of the present disclosure include an apparatus, wherein the working electrode and counter electrode are arranged in an interdigitated configuration.

[0013] In some aspects, implementations of the present disclosure include an apparatus, wherein the working electrode and counter electrode are different sizes.

[0014] In some aspects, implementations of the present disclosure include an apparatus, wherein the working electrode includes screen-printable carbon ink combined with Alizarin.

[0015] In some aspects, implementations of the present disclosure include an apparatus, wherein the growing media includes a hydroponic solution.

[0016] In some aspects, implementations of the present disclosure include an apparatus, wherein the growing media includes soil.

[0017] In some aspects, implementations of the present disclosure include an apparatus, wherein the controller is configured to transmit measured pH measurements to a remote computing device for analysis.

[0018] In some aspects, implementations of the present disclosure include an apparatus, wherein the controller is configured to transmit measured pH measurements to a remote computing device to control a release of growing media.

[0019] In some aspects, implementations of the present disclosure include an apparatus, wherein the electrode array is encapsulated in a multi-layer structure including: a first substrate (e.g., polyethylene naphthalate (PEN) substrate); conductive traces (e.g., carbon) formed on the substrate, the conductive traces terminating with an electrode (e.g., Ag / AgCl); and a second substrate having one or more chamber regions for the pH-sensitive coating or encapsulation.

[0020] In some aspects, implementations of the present disclosure include an apparatus further including: a salt reservoir formed in a chamber region of the one or more chamber regions.

[0021] In some aspects, implementations of the present disclosure include an apparatus, further including aNafion membrane.

[0022] In some aspects, implementations of the present disclosure include a distributed sensor system including: a plurality 1-15 in operable communication with a system controller, wherein the system controller is configured to receive pH measurements from each of the plurality of apparatuses.

[0023] In some aspects, implementations of the present disclosure include a distributed sensor system, wherein the plurality of apparatuses are coupled to the system controller by a wireless network.

[0024] In some aspects, implementations of the present disclosure include a distributed sensor system, wherein the electrode array includes of a working electrode and a counter electrode, the working electrode including screen-printable carbon ink combined with Alizarin, and wherein the counter electrode is configured as a combined reference and counter electrode.

[0025] In some aspects, implementations of the present disclosure include a method of pH sensing by a two-terminal apparatus, including: immersing a working electrode and a reference electrode in a sample; generating, by a controller, a time-varying excitation potential between the working electrode and the reference electrode; measuring by a current sensor, an electricalcurrent between the working electrode and the reference electrode; and determining, based on the electrical current, a pH value of the sample.

[0026] In some aspects, implementations of the present disclosure include a method, wherein the controller is configured to output a square-wave voltammetry signal.

[0027] Other systems, methods, features, and / or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed descriptions. It is intended that all such additional systems, methods, features, and / or advantages be included within this description and be protected by the accompanying claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The skilled person in the art will understand that the drawings described below are for illustration purposes only.

[0029] FIG. 1A illustrates the layers of an example three-electrode pH sensor, according to an example implementation of the present disclosure.

[0030] FIG. IB illustrates the layers of an example two-electrode pH sensor, according to an example implementation of the present disclosure.

[0031] FIG. 1C illustrates an example three-electrode pH sensor, according to an example implementation of the present disclosure.

[0032] FIG. ID illustrates an example two-electrode pH sensor, according to an example implementation of the present disclosure.

[0033] FIG. IE illustrates an example experimental setup, according to a study of an example implementation of the present disclosure.

[0034] FIG. IF illustrates electrochemical deprotonation of Alizarin, according to a study of an example implementation of the present disclosure.

[0035] FIG. 1G illustrates a stake for mounting a three-electrode sensor, according to a study of an example implementation of the present disclosure.

[0036] FIG. 1H illustrates a stake for mounting a three-electrode sensor, according to a study of an example implementation of the present disclosure.

[0037] FIG. II illustrates an example of different-proportioned electrodes, according to an example implementation of the present disclosure.

[0038] FIG. 1J illustrates an example of concentric electrodes, according to an example implementation of the present disclosure.

[0039] FIG. IK illustrates an example of interdigitated electrodes, according to an example implementation of the present disclosure.

[0040] FIG. 2A illustrates example calibration curves for three-electrode pH sensors with uncoated electrodes, according to a study of an example implementation of the present disclosure.

[0041] FIG. 2B illustrates square wave voltammograms of devices exposed to pH buffers, according to a study of an example implementation of the present disclosure.

[0042] FIG. 2C illustrates drift of the location of the peak potential as a function of interrogation number for devices in a pH 6 buffer solution, according to a study of an example implementation of the present disclosure.

[0043] FIG. 3A illustrates example calibration curves for two- and three-electrode pH sensors, according to a study of an example implementation of the present disclosure.

[0044] FIG. 3B illustrates square wave voltammograms of devices exposed to pH buffers, according to a study of an example implementation of the present disclosure.

[0045] FIG. 3C illustrates drift of the location of the peak potential as a function of interrogation number for devices in pH 6 buffer solution, according to a study of an example implementation of the present disclosure.

[0046] FIG. 4A illustrates a two-electrode pH sensor in hydroponic solution, according to a study of an example implementation of the present disclosure.

[0047] FIG. 4B illustrates drift of the location of the peak potential as a function of interrogation number for the two- and three-electrode devices in hydroponic solution, according to a study of an example implementation of the present disclosure.

[0048] FIG. 4C illustrates a two-electrode pH sensor in soil, according to a study of an example implementation of the present disclosure.

[0049] FIG. 4D illustrates the drift of the location of the peak potential as a function of interrogation number, according to a study of an example implementation of the present disclosure.

[0050] FIG. 5A illustrates an example circuit schematic for readout of pH sensors according to implementations of the present disclosure.

[0051] FIG. 5B illustrates pH dependence of the peak current flow due to Alizarin deprotonation, according to a study of an example implementation of the present disclosure.

[0052] FIG. 5C illustrates square wave voltammograms of devices exposed to pH buffers, according to a study of an example implementation of the present disclosure.

[0053] FIG. 5D illustrates normalized peak current as a function of interrogation number, according to a study of an example implementation of the present disclosure.

[0054] FIG. 5E illustrates the location of peak potential as a function of interrogation number, according to a study of an example implementation of the present disclosure.

[0055] FIG. 6 illustrates an example system for distributed pH sensing, according to implementations of the present disclosure.DETAILED DESCRIPTION

[0056] Some references, which may include various patents, patent applications, and publications, are cited in a reference list and discussed in the disclosure provided herein. The citation and / or discussion of such references is provided merely to clarify the description of the present disclosure and is not an admission that any such reference is ‘‘prior art'’ to any aspects of the present disclosure described herein. In terms of notation, “[n]” corresponds to the nthreference in the list. All references cited and discussed in this specification are incorporated herein by reference in their entirety and to the same extent as if each reference was individually incorporated by reference.

[0057] Disclosed are components that can be used to perform the disclosed methods and systems. These and other components are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these components are disclosed that while specific reference of each various individual and collective combinations and permutation of these may not be explicitly disclosed, each is specifically contemplated and described herein, for all methods and systems. This applies to all aspects of this application, including, but not limited to, steps in disclosed methods. Thus, if there are a variety of additional steps that can be performed, it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the disclosed methods.

[0058] Embodiments of the present disclosure include improvements to systems and methods for soil sensing. Precision agriculture can benefit from simple and low-cost systems for measuring pH throughout an agricultural facility (e.g., fields, hydroponic systems, etc.). Acidity / alkalinity (pH) is an important soil parameter that impacts nutrient availability and carbon cycling, and is therefore an important factor that impacts soil and plant health, as well as microbial processes including greenhouse gas production and sequestration [7],

[0010] ,

[0011] ,

[0059] However, many pH sensors are expensive and complicated. Field pH measurements commonly use two-electrode, potentiometric pH sensors, which can be relatively expensive (e.g., $200 — $1000 per unit), fragile, and typically require frequent manual calibration, preventing data collection with high spatial and temporal resolution

[0012] .

[0013] ,

[0060] Embodiments of the present disclosure overcome these challenges with conventional systems and methods. An example embodiment can include printed pH sensors using a working electrode which includes the pH -sensitive redox compound 1,2- dihydroxyanthraquinone (Alizarin).

[0014] -

[0018] , This approach provides a number of benefits, including the presence of the pH -sensitive and selective material throughout the bulk of the working electrode, rather than only at a thin surface layer, and a readout signal which is based on measuring the voltage position of the current peak, which significantly mitigates issues of signal drift over time. Additionally, embodiments of the present disclosure can replace conventional 3-electrode sensor designs with two-electrode sensors. Removing an electrode from the sensor reduces costs and complexity and enables larger-scale deployment of pH sensors to become practical for large-scale agricultural and environmental engineering use cases. Thus, embodiments of the present disclosure include only two electrodes. The first electrode of the two electrodes is a working electrode where the electrochemical reaction occurs. The second electrode of the two electrodes is a combined reference and counter electrode. The second electrode provides both a reference and completes the electrical circuit.

[0061] Conventional systems use three electrodes to avoid damage to the reference electrode. The streamlined sensor designs described herein are not limited by cost and / or complexity7in the same way as conventional systems. Embodiments of the present disclosure can be printed at very low cost, and therefore are not limited by the design requirements of three-electrode systems that feature more complicated circuitry.

[0062] Furthermore, embodiments of the present disclosure described herein can capture in-situ, continuous high-spatial density7information directly in growing media (including soil and hydroponic solution), enabling their readout with simplified electronics. Features of these sensors include a bulk Alizarin composite working electrode coated with a Nafion membrane to improve device stability, a salt-membrane coated reference electrode to enhance sensitivity, and a streamlined two-electrode architecture that enables readout using low -cost electronics, which allow for large numbers of these devices to be used to capture spatially dense information. The printed structure of the sensors described herein can also enable much larger scale deployment of the sensors when compared to conventional designs.

[0063] Example System

[0064] Fig. 1 A shows an example sensor for a distributed sensor system, e.g., for precision agriculture, that monitors and actively controls growing conditions and parameters for crops. The example sensor employs a bulk electrode functionalized with a pH-sensitive dye compound (e.g., Alizarin) to provide a low-cost, screen-printed, two-electrode sensor. Thesensor system can measure the pH of multiple growing media, including soil and hydroponic solutions. In some embodiments, the sensor employs a controller (e.g., a microcontroller or other computing device) with readout electronics.

[0065] Fig. 1 A shows an example three-electrode configuration for the pH sensor including the printed and drop-cast layers for the example sensor. Fig. IB shows an example two- electrode configuration for the pH sensor. As shown in FIGS. 1A and IB, the sensors can include a salt reservoir 102. a Nafion membrane 104, a carbon or Alizarin electrode 106, a dielectric encapsulant 108, an AG / AgCl electrode and contact 110, carbon contacts 1 12, and PEN substrate 114.

[0066] The exemplary configuration can be implemented with minimal electronic and hardware components while still able to interface with low-cost readout electronics, e.g., that are disposable, to be mass deployable in the field to provide real-time, high spatial, and temporal resolution data.

[0067] As shown in FIGS. 1A and IB, the pH sensors can be arranged in a multi-layered structure (e.g., five-layers) that includes a screen-printed electrode array, e.g., on a polyethylene naphthalate (PEN) substrate 114. The three-electrode device configuration shown in FIG. 1 A can include a working electrode (WE), a reference electrode (RE), and a counter electrode (CE). The tw o-electrode device configuration shown in FIG. IB includes a w orking electrode (WE), a shared counter electrode (CE), and reference electrode (RE). Fig. 1C show s an example of the electrodes of the three-electrode PH sensor. FIG. ID shows an example of the electrodes of the two-electrode pH sensor.

[0068] The working electrode (WE) can include a screen-printable carbon ink that is combined with a pH-sensitive dye (e.g., Alizarin) to form a pH-sensitive electrode as shown in FIGS. 1C and ID. As anon-limiting example, a 10 wt.% Alizarin concentration in commercial carbon ink (C / Az) can be employed

[0018] , FIG. IF illustrates the working principle of the electrodes, which can sense pH by evaluating the potential at which Alizarin undergoes an oxidation-reduction (redox) reaction.

[0019] Alizarin is selective to H+due to its internal hydrogen bonding and the presence of hydroxyl and carbonyl groups which facilitates proton- coupled electron transfer (PCET) while minimizing interference from other ions in solution

[0014] -

[0016] ,

[0020] , This hydrogen bonding also enables pH sensing in unbuffered and low ionic strength solutions by stabilizing proton exchange

[0015] ,

[0020] ,

[0069] FIG. IF shows the electrochemical deprotonation reaction of Alizarin. This reaction is dependent on pH and can be modeled using the Nemst equation per Equation 1.2.303 RTEP=EO- log(HF+)(Eq. 1)

[0070] In Equation 1 , EPis the peak potential, Eois the standard potential, R is the universal gas constant, T is the temperature in Kelvin, and F is Faraday’s constant. The expected Nemstian sensitivity of the C / Az working electrodes is 59.2 mV pH'1at 293 K. The redox potential may be evaluated using square wave voltammetry (SWV), as an electrochemical technique that uses a combination of a square wave and staircase potential to perform a ty pe of potential sweep voltammetry

[0021] ,

[0022] ,

[0071] For use in growing media high sensitivity can be required in order to differentiate small changes in pH . Furthermore, for this use case, temporal measurement density is low, as only occasional pH measurements are needed. As such, the example embodiment used SWV as the measurement scheme for these sensors since it is highly sensitive to faradaic processes compared with other electrochemical techniques. This high sensitivity is due to forward and reverse current sampling, which isolates the faradaic signal and leads to a higher peak cunent and improved signal resolution, allowing for an increased ability to detect shifts in peak potential

[0023] -

[0027] , Other techniques that were considered, such as differential pulse voltammetry (DPV), does not entirely eliminate non-faradaic effects, leading to lower pH sensitivity. The sensor's pH response is based on the peak potential rather than the peak current which makes the sensor output tolerant to current drift ensuring a reliable and stable pH output over time

[0014] ,

[0022] ,

[0072] It should be understood that SWV is a non-limiting example. Alternatively or additionally, the sensor described herein can be interrogated using linear sweep voltammetry, differential pulse voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy, and / or any other electrochemical interrogation technique. In some implementations, SWV can be more sensitive to faradaic processes than other electrochemical techniques due to the method of sampling which minimizes non-faradaic current when interrogating the printed pH sensors

[0021] ,

[0022] ,

[0073] Fig. 1G sho s an example stake assembly used to mount three-electrode and FIG. 1H shows an example stake assembly used to mount two-electrode pH sensors for soil-based measurements. The sensor structure can be configured to be attachable to a staking device to be tested in soil. Fig. 1H shows an example housing and stake for an example pH sensor. As shown, the stake may be designed to facilitate the insertion of the working electrode into the soil, e.g., up to a depth of 5 cm, without damaging the printed layers of the sensors. The stakecan be made longer for inserted deeper into the soil, e.g., 5 cm, 6 cm, 7 cm, 8 cm, 9, cm, 10 cm, 11 cm. 12 cm, 13 cm, 14 cm, 15 cm, 16 cm, 17 cm, 18 cm, 19 cm. 20 cm. 21 cm, 22 cm, 23 cm, 24 cm, 25 cm, 26 cm, 27 cm, 28 cm, 29 cm, 30 cm. In some embodiments, the length of the stake can be longer than 30 cm to accommodate depth insertion greater than 30 cm.

[0074] With reference to FIGS. 1I-1K, it should be understood that the electrode geometries described in FIGS. 1A-1B are only non-limiting examples. FIG. II illustrates an example electrode geometry where the reference electrode 116 is larger than the working electrode 1 18. This can reduce current density at the reference electrode 11 .

[0075] Alternatively or additionally, the reference electrode 116 and working electrode 118 can be interdigitated as shown in FIG. 1 J to minimize spacing between the two electrodes. This can reduce dependence of the observed signal on the conductivity of the sensing medium (soil, hydroponic solution, etc.) by reducing the ohmic drop within the sensing medium. Interdigitated designs such as the one shown in FIG. 1J overcome challenges that are unique to two-electrode configurations.

[0076] Additionally, interdigitated designs can leverage the benefits of precision achievable through printing to achieve better performance. FIG. IK illustrates example configurations where the reference electrode 116 and working electrode 118 can be concentric with one another. Concentric electrodes can also have the benefits unique to two-electrode configurations as described with reference to FIG. 1J. Additionally, concentric electrodes can be configured so that either the reference electrode 116 or working electrode 1 18 is the larger electrode in the configuration. Optionally, the concentric electrode configurations allow for both the inner and outer electrodes to have any surface area, as w ell as different ratios of surface area, by configuring which electrode is on the inside, and the proportions of the electrodes.

[0077] With reference to FIG. 6, implementations of the present disclosure include distributed sensing systems that can be used for agricultural systems (e.g., fields, greenhouses, hydroponics, etc.). The example system can include a system controller 600 including a memory 602 and processor 604. The system controller 600 can be in wired or wireless communication with any number of apparatuses 606a, 606b. 606c, 606d. Each apparatus 606a, 606b, 606c, 606d can include a sensor 608a, 608b, 608c. 608d as described with reference to FIGS. 1 A-1H. Optionally, each apparatus 606a, 606b, 606c, 606d can include a controller 610a, 610b, 610c, 610d configured to interrogate the respective sensor 608a, 608b, 608c, 608d of each apparatus 606a, 606b, 606c, 606d. The processor 604 and memory 602 of the system controller 600 can be configured to cause the system controller 600 to receive pHmeasurements from each of the apparatuses 606a, 606b, 606c, 606d, and / or cause the apparatuses 606a, 606b, 606c, 606d to initiate measurements.

[0078] Example computing device. An example computing device upon which the methods described herein may be implemented can include but is not limited to multiprocessor systems, microprocessor-based systems, minicomputers, embedded systems, and / or distributed computing environments, including a plurality of any of the above systems or devices. Distributed computing environments enable remote computing devices, which are connected to a communication network or other data transmission medium, to perform various tasks. In the distributed computing environment, the program modules, applications, and other data may be stored on local and / or remote computer storage media.

[0079] In an example configuration, the computing device includes at least one processing unit and system memory. Depending on the exact configuration and type of computing device, system memory may be volatile (such as random access memory (RAM)), non-volatile (such as read-only memory (ROM), flash memory, etc.), or some combination of the two. The processing unit may be a programmable processor that performs arithmetic and logic operations necessary for the operation of the computing device. The computing device may also include a bus or other communication mechanism for communicating information among various components of the computing device.

[0080] Computing devices may have additional features / functionality. For example, computing devices may include additional storage, such as removable storage and nonremovable storage, including, but not limited to, magnetic or optical disks or tapes. Computing devices may also contain network connection(s) that allow the device to communicate with other devices. Computing device may also have input device(s), such as a keyboard, mouse, touch screen, etc. Output device(s), such as a display, speakers, printer, etc., may also be included. The additional devices may be connected to the bus in order to facilitate the communication of data among the components of the computing device.

[0081] The processing unit may be configured to execute program code encoded in tangible, computer-readable media. Tangible, computer-readable media refers to any media that is capable of providing data that causes the computing device (i.e., a machine) to operate in a particular fashion. Various computer-readable media may be utilized to provide instructions to the processing unit 406 for execution. Examples of tangible, computer-readable media may include, but are not limited to, volatile media, non-volatile media, removable media, and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data.System memory, removable storage, and non-removable storage are all examples of tangible computer storage media. Examples of tangible, computer-readable recording media include but are not limited to an integrated circuit (e.g., field-programmable gate array or applicationspecific IC), a hard disk, a solid-state device, RAM, ROM, electrically erasable program readonly memory' (EEPROM), flash memory' or other memory technology, optical storage, or magnetic storage devices.

[0082] In an example implementation, the processing unit may execute program code stored in the system memory. For example, the bus may carry data to the system memory, from which the processing unit receives and executes instructions. The data received by the system memory may optionally be stored on the removable storage or the non-removable storage before or after execution by the processing unit.

[0083] Experimental Results and Additional Examples

[0084] A study was conducted to develop and evaluate the exemplary system and method as a two-electrode or three-electrode sensor as a stabilized screen-printed pH sensor that can measure the pH of unbuffered hydroponic growing media and soil in real-time using both two- and three-electrode configurations. By implementing a salt membrane on the RE and aNafion membrane on the working electrode (WE), the study demonstrated a Nemstian sensitivity to pH as well as an improvement in sensor performance through a reduction in the drift of the peak potential and the peak current during extended use in buffered solution and unbuffered media. The study’ showed that a low-cost printed sensor with both a two- and three-electrode configuration can be used to accurately track the pH of hydroponic solution and soil. Notably, the eloquent two-electrode configuration maintains comparable performance to the more traditional three-electrode sensor configuration, allowing for straightforward readout electronics. Utilizing the two-electrode configuration, the study demonstrated an example of a readout protocol utilizing a controller including an Arduino microcontroller with Nemstian pH sensitivity. In addition, the study7demonstrated that the readout system can achieve comparable cycling stability’ in hydroponic solution and soil over 50 cycles. The results from the study described herein demonstrate affordable, disposable, screen-printed sensors and simple electronics which can be utilized to engineer high-density environmental and agricultural monitoring systems. FIG. IE shows an image of the experimental setup for a three-electrode measurement.

[0085] Device Fabrication. The study produced the electrochemical sensor devices using a screen-printing process with stainless-steel mesh screens. Before device printing, the substrates were cleaned using acetone and air-dried. After the substrates were cleaned, fourlayers of the device were printed. The first layer, which served as the trace for the counter, working, and reference electrodes, was printed using conductive carbon ink and then cured in an oven at 110 °C for 30 minutes for a final dry thickness of ~5 pm. The second layer, which served as a trace for the reference electrode, was printed on top of the carbon reference electrode trace using the Ag / AgCl ink and cured at 110 °C for 30 minutes for a final combined dry thickness of ~20 pm. Once cured, the reference electrode, the third printed layer, was printed using the Ag / AgCl ink and cured at 110 °C for 30 minutes for a final dry layer thickness of ~12 pm. The fourth layer, which served as an encapsulating dielectric layer, was printed over the device using the dielectric ink and cured at 110 °C for 30 minutes, resulting in a dry thickness of ~5 pm and exposing 5 mm2of the carbon trace to serve as the counter electrode and 4. 16 mm2of the Ag / AgCl trace to serve as the reference electrode. For the pH sensors that incorporated salt reservoirs, the next processing step was to drop cast 2 pL of the PVB / NaCl solution onto the exposed area of the reference electrode. The salt reservoir was left to dry for at least one hour, then conditioned in a 3M NaCl solution for 12 hours. After the conditioning soak, the fifth layer, which served as the working electrode, was printed using the carbon / Alizarin ink and then cured at 110 °C for 30 minutes, resulting in an exposed area of 12.56 mm2and a thickness of ~12 pm. For the electrodes that included a Nafion membrane, the working and counter electrodes were masked off, and 1 wt.% Nafion 117 solution was spin- coated on top of the C / Az working electrode at 1000 RPM and then cured at 110 °C for 15 minutes. The devices were printed with a TF-100 Screen Printer from Micro Printing Systems (MPS) Inti. The screens were from Sefar, Inc., with a 325.09 stainless steel mesh filled with their E80 emulsion formulation.

[0086] Materials. The study employed flexible substrates (PEN, TEONEX Q51), e.g., acquired from TOYOBO Co., Ltd; carbon (DuPont 7102) and dielectric (DuPont 5036), e.g., from Insulectro; the Ag / AgCl ink (Cl-4001), e.g., from Nagase ChemteX. The Alizarin, NaCl, Butyvar B-98 (PVB), methanol, acetone, and Nafion 117 were from Sigma- Aldrich. Conductive silver epoxy adhesive (MG Chemicals 833 ID) was from DigiKey. Closed-cell ultra-conformable foam mounting tape was purchased from McMaster-Carr. The FlexSeal Leak Sealer was purchased from Home Depot.

[0087] The in-house hydroponic solution included Jack’s Part A and B nutrients and pH Up buffer from General Hydroponics. Apera Instruments supplied the commercial fluid pH monitoring device (PH60 pH Tester) and the commercial soil pH probe (AL102G GroStar series GS2). The soil used in this experiment was all-purpose for in-ground use soil from Miracle-Gro. For the construction of the readout electronics, an operational amplifier (TL081),BJTs, and passive electronic components were acquired from DigiKey. The Arduino Mega 2560 Rev 3 microcontroller was acquired from Arduino.

[0088] Ink Preparation. The carbon-Alizarin composite inks included carbon ink combined with 10 wt.% Alizarin and mixed in a centrifugal mixer for 10 min. The carbon, Ag / AgCl, and dielectric inks were dispersed by hand mixing prior to printing. All inks were left to settle in standard laboratory conditions for 10 minutes before device fabrication.

[0089] Membrane Preparation. The Nafion membrane solution was prepared by mixing Nafion 117 solution with 100% ethanol to produce a final concentration of 1 wt. %. The solution was vortex ed and then left to settle in standard laboratory conditions for 10 minutes prior to application. The solution was spin-coated onto the working electrode at 1000 RPM for 10 seconds, then dried in an oven at 110 °C for 10 minutes. The salt reservoir membrane was prepared by mixing 1.58 g of PVB, 1 g ofNaCl, and 20 mL of methanol. An ultrasonic ice bath was used for 30 minutes to achieve a well-mixed solution. The solution was stored at 4 °C to prevent solvent evaporation. Before application, the solution is re-agitated using a vortex mixer and then left to settle at standard laboratory conditions for 10 minutes. Afterwards, 2 pL of solution was pipetted onto the reference electrode and then covered and left to dry for at least 1 hour before conditioning the membrane in 3M NaCl for 12 hours.

[0090] Soil Stake Fabrication. All three components of the stake assembly - base, body, and lid - were fabricated using PLA on a Raise 3D Pro2 3D printer. The process employed the standard printing template with the exclusion of the platform and wipe tower settings. The infill density was also changed to 8% with a gyroid geometry to maximize strength and flexibility while maintaining a short printing time. The support types were customized to lines and were exclusively used during the lid printing process. Wires were connected to the contacts of each sensor using a silver conductive epoxy and then waterproofed using a closed-cell foam mounting tape and Flex Seal. Upon initial deployment, the stake maintains an approximate 1.5 mm distance between the sensor and the soil. This gap is eliminated by gently compressing the soil around the sensor, ensuring contact between the electrodes and the soil.

[0091] Soil and Hydroponic Solution Preparation. The soil employed for testing was commercial Miracle-Gro Garden Soil. To achieve a controlled gravimetric water content (GWC), the soil underwent thorough desiccation in beakers within an oven for a minimum of 24 hours at 110°C. Following complete drying, the soil mass was measured, and a specific percentage of DI water was introduced based on the desired GWC. The dry' soil and the DI water were rigorously mixed by hand to uniformly distribute the water throughout the soil sample.

[0092] The hydroponic nutrient solution was prepared by adding Jack's Part A and Part B Nutrients in equal proportions (~1 g / L) to 1 L of deionized (DI) water until the electrical conductivity (EC) of the solution reached 1.8 mS / cm.

[0093] Printed Electrode Optimization. A series of three-electrode voltammetric experiments were performed using Britton-Robinson buffer solution ranging from pH 4 to 8 to study the sensitivity of the printed sensors. The stabil i ty of each sensor was also evaluated by soaking the sensor in pH 6 buffer solution and interrogating the sensor every 10 minutes for a total of 50 measurements. The parameters used for each SWV experiment had a voltage range from -0.1 to 0.7 V with a step size of 0.005 V and amplitude of 0.1 V at a frequency of 35" " Hz.pH6 buffer solution was selected based on the optimal growing pH ranges for most plant growth in soil-based and hydroponic growing systems, as the ideal pH range for most plant growth in soil-based growing systems is 5.5-7.5

[0028] , Hydroponic-based growing systems have an optimal pH range from 5.5-6.5 which keeps most ions available in solution that can precipitate at higher pH values ( >6.5 ) and cause nutrient deprivation

[0029] ,

[0030] , The screen- printed C / Az traces served as the WE, the screenprinted Ag / AgCl electrode as the RE, and the exposed screenprinted carbon trace as the CE.

[0094] Figs. 2A and 2B show the calibration plot for uncoated electrodes, which illustrates a suboptimal relationship between pH and peak potential. A significant reduction in peak current (A / / / o> 99.5%) was also observed after 21 interrogations leading to difficulty in distinguishing the peaks. It should be noted that, for these devices and readout scheme, the drift in peak current is both dependent upon the starting state, as various conditioning or bum in steps could be applied, and importantly is not the source of signal drift, since pH is measured not by measuring the absolute current but rather by measuring the voltage of the position of the current peak, making these devices tolerant of current drift. The uncoated electrodes had a large drift in peak potential ( Ep / Ep 0) likely due to the uncoated WE and RE as shown in FIG. 2C.

[0014] ,

[0031] , This rapid degradation in peak current density and, much more critically, drift in the peak potential highlights the instability- of the uncoated electrodes in aqueous environments.

[0095] The stability of the RE can be improved by adding a salt membrane that functions similarly to the highly concentrated electrolyte solutions containing Cl“used in commercial Ag / AgCl reference electrodes

[0031] ,

[0032] , The salt membrane surrounds the Ag / AgCl electrode with a stable chloride reservoir, preserving the AgCl half-reaction (Equation 2) maintaining a stable reference potential in variable ionic strength environments

[0031] ,(Eq. 2)

[0097] In the case of printed sensors, this electrolyte solution can be substituted with a solid-state chloride salt membrane

[0031] ,

[0033] -

[0035] . Adding a salt membrane to the Ag / AgCl RE improved the reproducibility and accuracy of pH measurements by stabilizing the reference potential, resulting in an increased sensitivity to different buffer pH solutions, as can be seen in the calibration plots of FIGS. 2A and 2B. The pH sensors, with the addition of a salt membrane to the reference electrode, displayed a sensitivity of 67.5 lmV / pH(R2= 98.93%). Despite the addition of a salt membrane to preserve the reference electrode's AgCl half-reaction, there was a comparable drift in peak current to the fully uncoated pH sensor. Additionally, as shown in FIG. 2C, the pH sensor with a salt membrane covering the RE also experienced moderate drift in peak potential compared to that of the sensor with uncoated electrodes.

[0098] To further enhance the stability and lifetime of the pH sensors, we investigated the incorporation of a Nafion membrane onto the working electrode. Oxidized Alizarin has been shown to be more prone to dissolution in more alkaline environments, typically leading to a decay in current density

[0014] , The addition of a Nafion proton exchange membrane introduces a barrier between the surface of the working electrode and the solution, preventing hydrophilic attack while allowing proton transfer and enhancing the long-term stability’ of the working electrode

[0014] , The calibration plots shown in FIG. 2 A and FIG. 2B, with the addition of a Nafion membrane on the WE and a salt membrane on the RE, is comparable to pH sensors with an uncoated WE and RE with a salt membrane with a near-Nemstian sensitivity of 54.92mV / pH(R2= 98.71%). The average noise level, estimated as the standard deviation of peak potentials across multiple sweeps at each pH was found to be was 20.95 mV , resulting in a pH resolution of 0.32 pH. The pH sensor with the addition of a Nafion membrane on the WE and a salt membrane on the RE showed minor drift in peak potential compared to the other sensor configurations as show n in FIG. 2C. The fully coated sensor configuration also showed a decrease in the decay of peak current over interrogations with 10-minute intervals was observed, indicating enhanced current stability due to the addition of the Nafion membrane's proton-selective barrier, which prevents Alizarin leaching. Overall, this data shows that the sensitivity of the sensors and the stability (reduction in peak potential drift) is significantly improved with the addition of a salt membrane to the RE and a Nafion membrane to the WE.

[0099] Characterization Equipment. Square wave voltammetry (SWV) was conducted using the EmStat3 Blue from PalmSens. The pH of the soil was measured using an Apera Instruments (AL102G GroStar series GS2) pH probe. The pH of the hydroponic solution was measured using an Apera PH60 pH Tester. Optical photographs were taken with an iPhone 12 Pro.

[0100] pH Parameter Calibration and Measurement. pH calibration measurements were conducted by dipping the printed sensor into Britton-Robinson buffers that varied in pH from 4 to 8. Using the potentiostat, the study employed an SWV experiment with a voltage range from -0. 1 to 0.7 V with a step of 0.005 V and an amplitude of 0. 1 V at a frequency of 35 Hz. The potential at which the faradaic current was at a maximum (peak potential) was then extracted and plotted with respect to pH to obtain pH calibration plots. Prior to interrogating the sensors with a salt reservoir on their reference electrodes, the sensor was left to soak in each buffer solution for at least 30 minutes to recondition the salt membrane.

[0101] To determine the pH of the hydroponic fluid, a commercial pH probe was calibrated then immediately used to measure the pH of the solution. The pH measurements using the printed pH sensors were conducted by dipping the sensor into the fluid and performing SWV. The fluid and soil’s pH were verified by using a calibrated Apera Instruments PH60 pH Tester and Apera Instruments AL102G GroStar series GS2 pH probe, respectively. The stability of the sensors was evaluated by soaking sensors in buffer pH solution and performing SWV every 10 minutes. The soil measurements using the printed sensors were conducted by inserting the pH sensor into the soil in a protective stake, exposing the sensor to the soil with 60% GWC, and then performing an interrogation using SWV.

[0102] Comparison of Two- and Three-Electrode Sensors with the Optimized Devices. Condensing the three-electrode configuration to a twoelectrode configuration simplifies the printing process and the readout electronics, thereby enabling collection of more spatially dense in-field pH data. This approach involves merging the reference and counter electrodes into a single Ag / AgCl electrode while maintaining the C / M working electrode as in the three-electrode device described above. Accessible potentiostats for both three-electrode measurements and two-electrode voltammetry readout systems have been demonstrated for sensors operating based on voltammetric transduction

[0036] -

[0039] , The key advantage of a twoelectrode implementation for low-current linear sweep voltammetry is further reduction of the active circuit components and overall system complexity at the sensor node

[0036] ,

[0040] , While this simplification causes the printed Ag / AgCl reference electrodes to carry current, they are low-cost, disposable, and can be readily replaced if degradation occurs. This reduction incomplexity is meaningful in the scaling of printed sensor nodes constrained by cost, power, and size for continuous monitoring of critical environmental and agricultural parameters

[0041] ,

[0042] .

[0103] Two-electrode devices which employ the stabilizing membranes described above were compared to the three-electrode devices. The experiments were performed using BrittonRobinson buffer solution ranging from pH 4 to 8 to evaluate the differences in performance between the sensor configurations. The stability of each configuration was tested by soaking each sensor in pH 6 buffer solution with interrogations every 10 minutes for a total of 50 measurements. The three-electrode sensor performance was used as a baseline to compare with the performance of the two-electrode sensors. The calibration plots shown in FIGS. 3A and 3B for the three-electrode sensors with the addition of a Nafion membrane on the WE and a salt membrane on the RE produced a near-Nemstian sensitivity of 54.92mV / pH ( R2= 98.71% ). The calibration for the two-electrode sensor with the addition of the salt and Nafion membranes showed a Nemstian sensitivity of 56.38mV / pH(R2= 99.42% ) as shown in FIGS. 3A and 3B. The average noise level, estimated as the standard deviation of peak potentials across multiple sweeps at each pH was found to be was 15.37 mV , resulting in a pH resolution of 0.28 pH . The two-electrode and three-electrode sensors showed comparable drift in their peak potentials over the course of 50 measurements as shown in FIG. 3C, and the peak current drift for both the three- and two-electrode pH sensors followed similar profiles. The comparable performance for calibration and drift between the two- and three-electrode sensors supports the argument for simplified, two-electrode pH sensors.

[0104] Evaluating the Performance of Two- and Three-Electrode pH Sensors in Growing Media. The optimized sensors in both the two- and three-electrode configurations showed improved performance (increased stability and sensitivity) in buffered solution, creating a path towards a reliable way to directly monitor the pH levels of various growing media. The screen- printed pH sensors in both the two- and three-electrode configurations with Nafion and salt membranes were deployed in unbuffered hydroponic solution and directly in soil. The hydroponic experiments shown in FIG. 4A were performed in a hydroponic solution at pH 5.67 , which is within the optimal pH range for hydroponic systems ( pH 5.5-6.5)

[0029] ,

[0030] , The soil-based experiments shown in FIG. 4C were performed in soil with 60% gravimetric water content (GWC) moisture at a measured pH average of 6.1, which is within the ideal pH range for growing plants in soil ( pH5.5 — 7.5 )

[0028] , For the hydroponic solution, the drift in the peakpotential for both the two- and three-electrode pH sensors followed a similar trend as shown in FIG. 4B. The drift in peak current for the three-electrode hydroponic experiment was comparable to the drift of the peak position for the two-electrode sensor in hydroponic solution. The drift in peak current for the sensors tested directly in soil for both the two- and three- electrode pH sensors was similar; however, the three-electrode sensor experienced a more pronounced drift after 20 cycles, which could potentially be attributed to variations in the soil's physical properties. The drift in the peak potential for the two- and three-electrode sensor in soil was comparable. The peak potential drift in soil was lower for both the two- and three- electrode sensors than it was in hydroponic solution as shown in FIG. 4D. This could potentially be attributed to a slow er rate of nucleophilic attack or Alizarin dissolution resulting from a dryer soil environment and, therefore, loss of contact with aqueous solution compared to the experiments that were submerged in hydroponic solution. These results demonstrate that both the two-electrode and three-electrode sensors perform comparably well in various growing media (soil and hydroponics), indicating their potential for broader applications in agricultural and environmental monitoring.

[0105]

[0106] Measurement Using a Two-Terminal Sensor.

[0107] The measurements described previously are made with research-grade readout electronics that would not be appropriate for use at high device count for in-field measurements. The two-electrode devices enable simplified sensor readout circuitry as shown in FIG. 5A which can be implemented utilizing an integrator based on a single operational amplifier (TL081) to monitor the total charge flow between the electrodes during a linear sweep voltammetry cycle.

[0108] It is critical in two-electrode configuration for the current density to be low at the reference electrode to prevent measurement artifacts due to large ohmic potential drops. Because of this, an integrator configuration was chosen in favor of a resistive feedback transimpedance amplifier configuration as it allow s for measurement of nanoamp currents without the need for high valued resistors.

[0109] To execute a measurement cycle the readout circuit utilizes an Arduino Mega 2560 Rev 3 as a microcontroller. During a measurement, the potential of the WE relative to the RE is raised from 0 — 900mV at an average scan rate of 300 mV / s. This is achieved using a low- pass filtered, linearly increasing duty' cycle pulse-width modulation signal. The 980 Hz PWM signal is sourced from one of the Arduino Mega's 5 V PWM pins. The voltage at the output node of the integrator is sampled at 1 kHz by the Arduino's on-board 10-bit analog to digitalconverter (ADC), allowing for timesynchronized excitation and measurement. After the excitation cycle is complete, the integration capacitor is discharged to reset the integrator output voltage using a bipolar junction transistor (BJT) as an electrical switch as shown in FIG. 5A. A second BJT is used to electrically isolate the working electrode halfcell from the PWM input to reduce leakage current through the sensor between cycles. For the electrode described in this work, we utilized a 4 F feedback capacitor for the integrator in all measurements. This capacitor was selected to prevent the integrator from railing during an integration cycle while maximizing the dynamic range accessible to the ADC of the sampled signal.

[0110]

[0111] FIGS. 5B-5E show' calibration and stability data collected utilizing the simplified readout circuit. The obtained calibration plot of FIG. 5B wdth the addition of a Nafion membrane on the WE and a salt membrane on the RE is comparable to results achieved using SWV on a commercial potentiostat with a Nemstian sensitivity of 56.78mV / pH(R2= 98.31%). In addition, stability over 50 cycles in buffer solution (pH 6) and soil (pH 5.11) are comparable to results achieved using SWV.

[0112] After data collection, a signal post-processing protocol locates faradaic peaks in the voltammogram associated with Alizarin deprotonation for determination of the sample pH .

[0113] To perform a quantitative pH measurement, the integrator voltage time series data is first smoothed by convolution with a constant valued step function, after which the first differential is computed yielding time series data proportional to the current through the cell. The obtained current data is further smoothed and then decimated by a factor of 5 to improve signal to noise ratio before a calculation of the second differential to suppress slowly-varying, non-faradaic current. The excitation voltage is linear in time, therefore we can relate the time series data to the voltage difference betw een the w orking and counter electrodes to construct a voltammogram. Similar to the peak extraction method utilized for SWV data detailed above, a baseline subtraction is applied before peak extraction. Automatic peak detection is implemented in Python utilizing the open-source SciPy library. The near-Nemstian sensitivity for the pH sensors using the microcontroller-based readout electronics calibrated in buffer solution was found to be 53.99mV / pH ± 9.35mV / pH(n = 3) . FIG. 5C illustrates square wave voltammograms (baseline corrected) of the devices exposed to a series of pH buffers. FIG. 5D illustrates drift of the location of the normalized peak current. FIG. 5E illustrates driftof the location of the peak potential as a function of the interrogation number for the two- electrode.

[0114] Discussion

[0115] The two-electrode embodiment of the present disclosure comparable performance to the more traditional three-electrode sensor configuration, allowing for simplified readout electronics. For the two-electrode configuration, an example of a simplified readout circuit utilizing an Arduino microcontroller with Nemstian pH sensitivity was validated. This readout circuit can achieve comparable cycling stability to research grade readout electronics. These results demonstrate that affordable, sensitive and stable screen-printed pH sensors can be operated continuously and directly in growing media and can be measured using simple low- cost readout electronics, enabling high spatial density environmental and agricultural pH monitoring.

[0116] The world's population is expected to reach 9-10 billion by the year 2050, increasing our food production needs by at least 70% [1],[2], Current agricultural practices have to be modernized and advanced to support this increase in population and resulting crop requirements [3J. A potential solution to this issue is implementing precision agriculture by monitoring the growing environment using sensors to provide real-time feedback on different growing conditions and needs with high spatial and temporal resolution [4], [5], Current agricultural monitoring techniques, however, are bulky, costly, and fragile, preventing environmental data from being collected with high temporal and spatial resolution [6], A solution to this issue is implementing affordable, portable, and disposable sensing options such as printed electronic sensors. There has been considerable interest in developing printed environmental and soil sensors for agricultural applications since they are low-cost, portable, and disposable [5], [7], Integrating these sensors into existing farm practices allows for agricultural inputs to be adjusted with varying crop needs, enhancing input use efficiency and optimizing soil and plant health [8], Over the past few- decades, significant advancements have occurred in the field of printed environmental sensors; however, poor device stability is the crutch inhibiting their widespread use [9] . One growing parameter of particular interest is the potential of hydrogen (pH); the pH of growing media determines nutrient availability and plant health in soil and hydroponic growing systems [7],

[0010] ,[l 1], Currently, growers commonly use two-electrode, potentiometric pH sensors, which can be relatively expensive ($200-$ 1000 per unit), fragile, and typically require frequent calibration, preventing data collection with high spatial and temporal resolution^12,131

[0117] Previous studies working to quantify pH volammetrically using printed electronics have used inks modified with the pH-sensitive redox compound 1.2-dihydroxy anthraquinone (Alizarin) and demonstrated the efficacy of Alizarin-based, screen-printed pH sensors in monitoring unbuffered solutions and soil

[0014] -

[0018] , However, there have not been advances toward producing a sensor that can be interfaced with multiple growing media and readout with affordable, readily available electronics. Singh et al. have reported pH sensing using a disposable three-electrode Alizarin-based sensor to evaluate soil pH by assessing the pH of the solution in which soil samples have been soaked

[0015] , Eldeeb et al. interfaced a screen-printed carbon electrode with an Alizarin-modified Nafion membrane with soil and were able to read soil pH using a relatively expensive (>$1000) EmStat Pico module potentiostat

[0016] , The printed pH sensor presented in this work extends the capability of screen-printed Alizarinbased pH sensors to directly interface with soil and hydroponic growing environments with a simplified two-electrode configuration a readout system based on an off-the-shelf microcontroller. The pH sensor presented in this paper features membrane-stabilized working and reference electrodes for improved sensor stability' in a novel two-electrode configuration designed to be inexpensive, disposable, and capable of providing pH measurements using simplified readout electronics while directly interfaced with growing media.

[0118] Conclusion

[0119] Various sizes and dimensions provided herein are merely examples. Other dimensions may be employed.

[0120] Although example embodiments of the present disclosure are explained in some instances in detail herein, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the present disclosure be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The present disclosure is capable of other embodiments and of being practiced or carried out in various ways.

[0121] It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and ’the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “abouf’ or “ 5 approximately’7one particular value and / or to "‘about” or “approximately” another particular value. When such a range is expressed, other exemplary embodiments include from the one particular value and / or to the other particular value.

[0122] By “comprising” or “containing” or “including” is meant that at least the name compound, element, particle, or method step is present in the composition or article or method,but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named.

[0123] In describing example embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. It is also to be understood that the mention of one or more steps of a method does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Steps of a method may be performed in a different order than those described herein without departing from the scope of the present disclosure. Similarly, it is also to be understood that the mention of one or more components in a device or system does not preclude the presence of additional components or intervening components between those components expressly identified.

[0124] The term “about,” as used herein, means approximately, in the region of, roughly, or around. When the term “about” is used in conjunction with a numerical range, it modifies that range by extending the boundaries above and below the numerical values set forth. In general, the term “about” is used herein to modify a numerical value above and below the stated value by a variance of 10%. In one aspect, the term “about” means plus or minus 10% of the numerical value of the number with which it is being used. Therefore, about 50% means in the range of 45%-55%. Numerical ranges recited herein by endpoints include all numbers and fractions subsumed within that range (e.g., 1 to 5 includes 1 , 1.5, 2, 2.75, 3, 3.90, 4, 4.24, and 5).

[0125] “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0126] Throughout the description and claims of this specification, the word “comprise” and variations of the word, such as “comprising” and “comprises,” means “including but not limited to,” and is not intended to exclude, for example, other additives, components, integers or steps. “Exemplary” means “an example of’ and is not intended to convey an indication of a preferred or ideal embodiment. “Such as” is not used in a restrictive sense, but for explanatory purposes.

[0127] Similarly, numerical ranges recited herein by endpoints include subranges subsumed within that range (e.g., 1 to 5 includes 1-1.5, 1.5-2, 2-2.75, 2.75-3, 3-3.90, 3.90-4,4-4.24, 4.24-5, 2-5, 3-5, 1-4, and 2-4). It is also to be understood that all numbers and fractions thereof are presumed to be modified by the term '’about."

[0128] While the methods and systems have been described in connection with certain embodiments and specific examples, it is not intended that the scope be limited to the particular embodiments set forth, as the embodiments herein are intended in all respects to be illustrative rather than restrictive.

[0129] The following patents, applications and publications as listed below and throughout this document are hereby incorporated by reference in their entirety herein.

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Claims

What is claimed is:

1. An apparatus comprising: a housing; an electrode array located in the housing, the electrode array comprising two or more electrodes, each comprising a pH-sensitive coating or encapsulation to measure a pH of a growing media; and a controller configured to: generate an electrical current between the two or more electrodes; measure, by a current sensor, an electrical current flow between the two or more electrodes; and output an estimate of pH based on the electrical current flow.

2. The apparatus of claim 1, wherein the pH-sensitive coating or encapsulation comprises Alizarin.

3. The apparatus of claim 1, wherein the two or more electrodes include a working electrode, a reference electrode, and a counter electrode.

4. The apparatus of claim 1, wherein the two or more electrodes include a working electrode and a counter electrode, wherein the counter electrode is configured as a combined counter and reference electrode.

5. The apparatus of claim 4, wherein the working electrode and counter electrode are arranged in a concentric configuration.

6. The apparatus of claim 4, wherein the working electrode and counter electrode are arranged in an interdigitated configuration.

7. The apparatus of claim 4, wherein the working electrode and counter electrode are different sizes.

8. The apparatus of claim 4, wherein the working electrode comprises screen-printable carbon ink combined with Alizarin.

9. The apparatus of claim 1, wherein the growing media includes a hydroponic solution.

10. The apparatus of claim 1, wherein the growing media includes soil.

11. The apparatus of claim 1 , wherein the controller is configured to transmit measured pH measurements to a remote computing device for analysis.

12. The apparatus of claim 1, wherein the controller is configured to transmit measured pH measurements to a remote computing device to control a release of growing media.

13. The apparatus of claim 1, wherein the electrode array is encapsulated in a multilayer structure comprising: a first substrate (e g., polyethylene naphthalate (PEN) substrate); conductive traces formed on the substrate, the conductive traces terminating with an electrode; and a second substrate having one or more chamber regions for the pH-sensitive coating or encapsulation.

14. The apparatus of claim 13 further comprising: a salt reservoir formed in a chamber region of the one or more chamber regions.

15. The apparatus of claim 11, further comprising a Nafion membrane.

16. A distributed sensor system comprising: a plurality of apparatuses according to any one of claims 1-15 in operable communication with a system controller, wherein the system controller is configured to receive pH measurements from each of the plurality of apparatuses.

17. The distributed sensor system of claim 16. wherein the plurality of apparatuses are coupled to the system controller by a wireless network.

18. The distributed sensor system of claim 16. wherein the electrode array comprises a working electrode and a counter electrode, the working electrode comprising screen-printable carbon ink combined with Alizarin, and wherein the counter electrode is configured as a combined reference and counter electrode.

19. A method of pH sensing by a two-terminal apparatus, comprising: immersing a working electrode and a reference electrode in a sample; generating, by a controller, a time-vary ing excitation potential between the working electrode and the reference electrode; measuring by a current sensor, an electrical current between the working electrode and the reference electrode; and determining, based on the electrical current, a pH value of the sample.

20. The method of claim 19, wherein the controller is configured to output a squarewave voltammetry signal.

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