Patent Publication Number: US-11378541-B1

Title: Self-contained, automated, long-term sensor system for monitoring of soil and water nutrients in fields

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Provisional Application U.S. Ser. No. 62/750,663, filed on Oct. 25, 2018, all of which are herein incorporated by reference in their entirety. 
    
    
     GOVERNMENT RIGHTS CLAUSE 
     This invention was made with Government support under (1) Department of Energy Contract No. DE-AR0000824 and (2) USDA/NIFA Grant No. 2017-67013-26463. The government has certain rights in the invention. 
    
    
     I. BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The invention is about an automated, fieldable ion-selective sensor system for frequent detection of nutrients (nitrate, phosphate, sulfate, potassium, etc.) in soil and water. One aspect of the invention includes an all-solid-state ion-selective sensing unit using a printed working electrode of, e.g., poly(3-octyl-thiophene)-molybdenum disulfide (or POT-MoS 2 ) nanocomposite as an ion-to-electron transducing layer and a printed or coated sandwiched solid-state reference electrode, e.g., (S 3 RE) of Nafion-polyvinyl butyral/sodium chloride-silver/silver chloride (or Nafion-PVB/KCl-Ag/AgCl), for high sensitivity and selectivity measurements. 
     As will be further discussed herein, variations are possible. Non-limiting examples include that POT could be replaced by other polymers which are conducting, provide redox properties, and are lipophilic. MoS 2  is a very specific material used with POT as it provides high redox activity, high conductivity, and improved electron conducting property of POT. 
     B. Problem Statement 
     As is made clear in the publications cited in the Background References section below, the benefits of measuring soil nutrient levels can be very significant for agricultural crop producers. As such, a variety of attempts at obtaining meaningful soil nutrient measurements have been made over the years. More recently, recognition of benefits of not just soil nutrients for a whole field but rather both at different spatial locations around a field and different depths has become apparent. However, the challenges are substantial. 
     Currently, farmers take soil samples to a laboratory for assessing contents of important nutrients available in the soil. It is difficult for farmers to know the information on dynamic changes in nutrient availability over time. Existing technologies available today do not provide the capacity to automatically monitor soil nutrients in fields at multiple times. 
     Problem 1: Currently, existing nutrient sensors, when they operate in the field for frequent measurements of nutrient ions, are not able to automatically reset themselves to their original status after use. For example, after ion-selective membrane-based sensors detect particular ions for multiple times, the ion-selective membranes coated on the working electrodes cannot be automatically cleaned, reconditioned, and recalibrated in the field. The detection accuracy of the sensors for the following measurements are affected. Long-term sensor performance stability and accuracy should be improved. There is a critical need to realize in situ cleaning, reconditioning, and recalibration for ion-selective membrane based potentiometric sensors when they are deployed in the field. 
     Problem 2: Many types of soil sensors and sensing units of sensor systems have been developed to monitor nutrients in soil and water. Common measurement practices include ion chromatography, spectrophotometry, ion-selective membrane (ISM) based sensors, and electrochemical sensors. Among these, chromatography and spectrophotometry are limited to laboratory settings due to high cost, large size, complex sample preparation and high power consumption, while the goal is design of affordable sensors for site-specific and real-time measurements. Enzymatic electrochemical sensors, using an ion-specific enzyme for molecular recognition, have also been developed to realize detection of a specific ion. This type of sensor, however, has short lifetime, is limited by the availability of the ion-specific enzymes, and is only suitable for laboratory measurement. The ISM based sensors are simple and field deployable and can convert the activity of a specific ion in a solution into an electrical signal. They, however, require specific ion-selective mechanisms and careful design in material, structure and manufacturing to obtain both high performance and low cost. Unfortunately, almost no ISM based sensors are commercially available for long term, real time, continuous monitoring of nitrate in the field with good selectivity and high sensitivity. There is a significant need to address the issues of sensor lifetime, accuracy, and stability of the sensors or the sensing units of sensor systems for field measurements. This can be achieved by improving the quality of both working and reference electrodes of the ISM based sensors or the sensing units of sensor systems. 
     Problem 3: Conventional porous ceramic capillary based suction heads for soil water extraction have one end opened and the other end closed. For miniaturized capillary tubes, manual removal of the remaining chemicals from the interior of such ceramic tubes is challenging, due to limited space. The smaller the ceramic tubes, the more difficult the operations (rinsing, flushing, etc.) become. Integration of such ceramic tubes with soil water detection units will not lead to field-deployable sensors for long-term use. 
     It therefore is apparent there is room for improvement in this technological area. 
     Background References 
     The following provide background information and detail of the type indicated. Each is incorporated by reference herein in its entirety: 
     
       
         
           
               
               
               
             
               
                   
               
               
                   
                   
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                 1 
                 US 20070013908 A1: Portable Raman 
                 Raman based sensor for 
               
               
                   
                 Sensor for Soil Nutrient Detection. 
                 phosphorus detection 
               
               
                   
                   
                 Probe in situ 
               
               
                   
                   
                 Housing with 
               
               
                   
                   
                 integrated-computer, 
               
               
                   
                   
                 laser, batteries, fiber 
               
               
                   
                   
                 optic cable 
               
               
                 2 
                 U.S. Pat. No. 5,033,397: Soil Chemical  
                 Pull-through-ground  
               
               
                   
                 Sensor and Precision Agricultural  
                   
               
               
                   
                 Chemical Delivery 
                 real time soil  
               
               
                   
                 System and Method. 
                 sensing implement 
               
               
                 3 
                 U.S. Pat. No. 8,444,937 B2: In-Situ Soil  
                 In situ soil nitrate ISE 
               
               
                   
                 Nitrate Ion Concentration Sensor 
                 Porous sleeve in soil 
               
               
                   
                   
                 probe inserted 
               
               
                 4 
                 U.S. Pat. No. 5,985,117: Ion-Selective  
                 Ion-selective electrodes 
               
               
                   
                 Membrane Sensors With  
                 membranes 
               
               
                   
                 Mercuracarborand Ionophore 
                   
               
               
                 5 
                 US 20140345394 A1: Portable Soil 
                 Portable soil testing unit 
               
               
                   
                 Testing Apparatus and Method 
                 On-board mixing cup to 
               
               
                   
                   
                 mix soil and water and 
               
               
                   
                   
                 then insert detector into 
               
               
                   
                   
                 mixture in cup. 
               
               
                   
                   
                 Ion selective sensors 
               
               
                   
                   
                 [0026] 
               
               
                   
                   
                 Detect different 
               
               
                   
                   
                 nutrients [0027] 
               
               
                 6 
                 U.S. Pat. No. 6,398,931 B1: Combination  
                 Ion-Selective Electrodes 
               
               
                   
                 Ion-Selective Electrode With A  
                 Replacement sensing 
               
               
                   
                 Replaceable Sensing Membrane. 
                 membranes 
               
               
                 7 
                 US 20140165713 A1: Systems, Devices, 
                 Soil monitoring, 
               
               
                   
                 And Methods For Environmental 
                 including nitrates 
               
               
                   
                 Monitoring In Agriculture 
                 Ion selective electrodes. 
               
               
                   
                   
                 In ground water 
               
               
                   
                   
                 collection 
               
               
                   
                   
                 Micropumps 
               
               
                   
                   
                 ISE sensors 
               
               
                   
                   
                 Porous soil water 
               
               
                   
                   
                 collection tubes -water 
               
               
                   
                   
                 in during wetting events 
               
               
                   
                   
                 Battery 
               
               
                   
                   
                 Processor 
               
               
                   
                   
                 In situ 
               
               
                 8 
                 U.S. Pat. No. 5,526,705: Automated  
                 Automated soil sample 
               
               
                   
                 Work Station For Analyzing Soil Samples 
                 processing on bench top. 
               
               
                 9 
                 U.S. Pat. No. 8767194 B2: Automated  
                 Soil measurement 
               
               
                   
                 Soil Measurement 
                 Device Mixing chamber 
               
               
                   
                   
                 Optical detector 
               
               
                 10 
                 U.S. Ser. No. 62/411,315: 
                 Ion species detection in 
               
               
                   
                 Electrophoretic Soil Nutrient  
                 soil on the go 
               
               
                   
                 Sensor For Agriculture 
                 Variety of species 
               
               
                   
                   
                 Microchip 
               
               
                   
                   
                 Electrophoresis 
               
               
                   
                   
                 Vacuum sucks soil 
               
               
                   
                   
                 solution into 
               
               
                   
                   
                 microfluidic circuit. 
               
               
                   
                   
                 Microfluidics controls 
               
               
                   
               
            
           
         
       
     
     II. SUMMARY OF THE INVENTION 
     A. Objects, Features, and Advantages of the Invention 
     A main object, feature, and advantage of the present invention is to improve over or solve problems and deficiencies in the state-of-the-art related to systems, methods, and apparatus for measuring or estimating soil nutrient levels in agricultural fields. 
     Further objects, features, and advantages of the invention include one or more of:
         a. Ability for in situ monitoring over extended periods of time.   b. Robustness over a range of sometimes harsh and extreme environmental conditions.   c. Minimal disruption to planting, harvesting, and intermediate agricultural production and field tasks.   d. Self-contained and self-sufficient over a long useful measuring time span, as well as at least semi-automated or fully automated.   e. Adaptable for different applications and chemical sensing tasks, including ion-selective detection.   f. Integratable regarding a variety of different components (sensing elements, fluidic circuitry, etc.) and functionalities in a relatively small form factor.   g. Ability to auto reset, auto recondition and auto calibration for multiple new detection cycles.   h. Increased performance and accuracy of detection, including higher sensitivity, high selectivity working electrode configuration, and higher stability reference electrode configuration.   i. Versatile sample collection head.   j. Can provide sufficiently accurate and precise measurements with quite small sample volume sizes.   k. Can be integrated with a variety of sensing techniques   l. Can be used for monitoring of nutrient ions in both soil and water.       

     B. Aspects of the Invention 
     The disclosed sensor system can automatically reset, recondition, and recalibrate the sensing unit for long-term, high-precision detection of nutrient ions in the field. In contrast, almost no existing nutrient sensors are able to do so. The ability of the disclosed sensor system to reset, recondition, and recalibrate itself in the field is achieved using multiple simple fluid manipulation units (e.g., in one example each consists of two normally-closed mini peristaltic actuators and a mini vacuum pump) and a unique flow-through suction head with openings at two sides. 
     The disclosed sensor system uses a new ISM-based sensing unit with an improved sensitivity, selectivity, and reliability. In the ISM-based sensing element, the working electrode contains a novel ion-to-electron transducing layer sandwiched between an ISM and a metallic electrode. In one example, the ion-to-electron transfer layer is a nanocomposite of poly(3-octyl-thiophene) and molybdenum disulfide (POT-MoS 2 ). The use of POT-MoS 2  in the working electrode leads to increased redox activity and electron conduction, thus increasing sensitivity and selectivity. Also, because high lipophilicity of the POT-MoS 2  nanocomposite helps to minimize formation of an aqueous film (containing interference ions) commonly formed between the ISM and the metallic electrode, and restrict the accumulation of interfering ions in the interface, thus improving selectivity of the sensor to nitrate. In addition, the ISM-based sensing element can use a sandwiched solid-state reference electrode (S 3 RE) to minimize chloride leaching and signal drifting, thus improving stability of the sensing unit. 
     The disclosed sensor uses a new flow-through soil water suction head. Ceramic porous capillary tubes have been widely used to extract soil water for analysis. The suction head is unique in that it utilizes a porous ceramic tube with openings at both sides. This design allows not only in situ sampling of soil water, but also in situ cleaning of the interior of the ceramic tube by simply flowing cleaning liquids from one side through the ceramic tube into a waste reservoir connecting on the other side of the tube. Therefore, every time after the soil water is sampled and analyzed, the suction head can be easily washed in situ, without pulling it out of the soil. 
     In addition, a nutrient sensing unit and a water-level sensor are directly made inside or socketed into a soil solution collector-to-detection cell. This integrated design eliminates the need of transporting extracted soil water from a collector to the sensing unit, thus not only simplifying the cost and footprint of the sensor system, but also reducing the amount of soil water (no more than 200 microliters) required for analysis. 
     Non-limiting Examples of Applications 
     
         
         
           
             1. Soil and water nutrients (N, P, S, K, etc.) detection in field. No substantial changes are required for using the same sensor system to detect nutrient in either soil or water. 
             2. Pesticide detection in field (by changing the working electrode of the sensor). 
             3. Water pollution (N, P, S, K, etc.) detection in field. 
           
         
       
    
     In addition, the disclosed fluid manipulation unit, the flow-through water extraction unit, and the system integration approach can be applied to many other existing soil and water sensor technologies to move these sensors from the laboratory to the field. 
     A further aspect of the invention comprises an integrated self-contained system that can include any or all of the above aspects. In one example, the system includes a soil water sampling head comprising a porous tube that is permeable to relevant gas/fluids in the soil but allows fluid flow in either direction through it. A ruggedized housing includes a chemical detection subsystem. In the case of ion-selective membrane (ISM) based detection, a sensing unit includes a reference electrode, a working electrode, an ion-selective membrane at the working electrode and an ion-to-electron interface between the membrane and the working electrode. In one example, the interface comprises MoS 2 -POT. 
     Alternatives to POT include but are not limited to, other polymers, such as poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate, polyaniline, and polypyrrole, can also be integrated with MoS 2  to form the ion-to-electron transducing layer. 
     Alternatives to a Nafion coating on top the PVB layer include but are not limited to, polyurethane can be coated to block both interfering anions and cations for minimizing chloride leaching from the PVB layer to surrounding environment. 
     A fluid manipulation subsystem includes soil water extraction and delivery fluid circuit that can be controlled to draw soil water and air into the sampling head and convey it to the detection cell. A controller can supply electrical power to the electrodes and collect data correlated to detection of a chemical species. Additionally, a reset/reconditioning fluid circuit can be selectively actuated to push cleaning fluid through the detection cell and sampling head after a detection cycle to refresh or recondition the system for a next sample. 
     In one aspect of the invention, an improved sensitivity sensor comprises a printed, coated, or otherwise applied nanocomposite of POT-MoS 2  as the ion-to-electron transfer layer of a working electrode and a printed, coated, or otherwise applied sandwiched solid-state reference electrode (S 3 RE). 
    
    
     
       III. BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing or photograph executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       The appended drawings will be referred to in the detailed description of exemplary embodiments and are summarized as follows: 
         FIG. 1  is a diagrammatic view of certain features of an overall system according to one embodiment of the invention, a self-contained, ion-selective sensor system. 
         FIG. 2  are similar to  FIG. 1  but show a first operational state of fluid manipulation from a fluid collection sampling head to a detection cell for electrochemical sensing ( FIG. 2A ) and a second state of recalibration after a first measurement ( FIG. 2B ), and fluid manipulation of cleaning out/reconditioning the system to reset it for a next detection cycle ( FIG. 2C ). 
         FIG. 3  is an enlarged diagrammatic depiction of a modified electrode-based ion-selective detector including an ion-to-electron transfer layer between ion-selective membrane and metallic conducting electrode according to one aspect of the invention. 
         FIGS. 4A-E  are a collection of photographs showing different features of a complete, field-deployable sensor system such as could be used with the systems of  FIGS. 1-3 . Each of  FIGS. 4A-E  is reproduced in enlarged form on succeeding pages of the drawings. Note that the enlargement of  FIG. 4E  includes an added diagrammatic illustration of the porous ceramic tube of  FIG. 4E . 
         FIG. 5  is a graph illustrating efficacy of improved sensitivity of the sensor of  FIG. 3 .  FIG. 5  shows: (A) (B) Sensitivity test: voltage responses of the sensor to changing nitrate concentrations. The sensors used the POT-MoS 2  nanocomposite, POT, and MoS 2  in their working electrodes. (C) (D) Selectivity test: voltage responses of the sensor to a mixture of nitrate (50 ppm) and different interfering ions (500 ppm each). The sensor used the POT-MoS2 nanocomposite in its working electrode. 
         FIG. 6  is a photograph of an overall system such as  FIG. 4A  packed in a thermally insulated covering and a plurality of which are installed at spatially separated distance in a field for measuring soil nutrient levels at different points in the field.  FIG. 6  is illustrations of a field installation of the ion-selective sensor systems according to the invention. Fifty-four sensors have been installed in the field of an initial field testing 
         FIG. 7  is a graph showing measurements throughout several days by a sensor such as  FIG. 6  compared to a calibration measurement at the same location and times with a UV spectrophotometry based detector.  FIG. 7  are typical measurement data from field deployable sensors according to the invention. 
         FIG. 8  is a flow chart showing one exemplary methodology of using the sensor system of  FIGS. 1-7 . 
         FIG. 9  is a diagrammatic view of a variety of options regarding to sensor unit placement and communications capabilities. 
         FIG. 10  is a diagrammatic view of a system like  FIG. 1  but showing some alternatives.  FIG. 10  shows a non-limiting exemplary all in one detector unit including:
         Electrochem sensors or ISM-based sensors in detection unit.   Suction head allows permeable infiltration of soil water/air via vacuum and bi-directional flow between ends.   Suction head (single or plural) can be placed in situ at different heights for sensing different depths and allow differentiation detection by varying length of tubing.   Fluid manipulation allows movement of soil water/air from suction head to detection unit.   Water level sensor in detection unit—two electrode sensors—allows automated detection of enough sample loading.   Peristaltic actuation and check valve(s) with flush tubing to opposite ends of suction head allows flushing/cleaning/reset of system. Essentially “artificial rain” to clear out system for next measurement.       

         FIGS. 11 to 19  are illustrations and graphs relating to a specific embodiment Example 1 discussed herein of a sensor that can be used with the present invention, in particular, to a solid state sensor using a printed nanocomposite of POT-MoS 2  as the ion-to-electron transfer layer of a working electrode and a printed sandwiched solid-state reference electrode (S 3 RE) for measurement of soil or water nutrients. 
         FIG. 11  is a step-wise representation for the fabrication of all-solid-state ion selective sensor. (a) A photograph of wafer-scale PCB containing a series of WE and RE. (b) Stencil-based screen printing setup for printing Ag/AgCl paste on the circular shaped silver (Ag) PCB substrate. (c) A photograph during materials dispensing (POT-MoS 2  in THF solvent) on circular-shaped electrodes using high-precision Nordson EFD auto-dispenser at 2 psi pressure. A 100 cc syringe with 0.05 mm diameter of tip is loaded POT-MoS 2  material to deposit. (d) Schematic presentation of PCB substrate containing RE and WE with wire connections and (e) both electrodes were modified with electrode materials and green glue covering side-wall coating and connection pads. (f) A photograph of real device. 
         FIG. 12  is (a) Scanning electron microscopic (SEM) image for Ag/AgCl surface on Si needle, (b) schematic for different layers of the home made reference electrode on Si/SiO 2 . (c) SEM image the cross-sectional view of PVB and Nafion layers on top of the Ag/AgCl substrate. (d) Schematic shows the different layers for the formation of S 3 RE. 
         FIG. 13  is (a) Chloride sensitivity studies for different fabricated electrodes by varying Cl −  ions concentration. The measurements were conducted using a commercial RE with respect to fabricated electrodes. (b) Long-term studies for different electrodes at 0.000M Cl −  concentration. The commercial RE is obtained from eDAQ Pty. Ltd. (leak-less miniature Ag/AgCl RE; internal filling solution: 3.4 mol/L KCl; model: ET072). 
         FIG. 14  is SEM images for MoS 2  (a), (POT) (b) and POT-MoS 2 (c) . (d) Schematic shows the different layers for working electrode (e) cross-sectional view of the working electrode showing the different layers, (f) cyclic voltammetry of MoS 2 , POT and POT-MoS 2  electrodes in presence of buffer solution (containing 2 mM of ferro/ferricyanide redox species) and (g) voltage measurements of all three electrodes in presence of 1000 ppm NO 3 —N. In this measurements (g), all electrodes were coated with nitrate ions selective membrane and was carried out in nitrate solution. (h) A pictorial presentation for the formation of all relevant interfaces in the fabricated all-solid-state sensor. Anion (M) selective membranes contains an electrically neutral ionophore (L) and cationic sites (R + ). (i) Molecular structure of POT and MoS 2  for composite formation in this sensor. 
         FIG. 15  is the contact angle studies for the investigation of hydrophobicity of the working electrode materials. A syringe was used to drop 3 μL volume of DI water on the Au/PCB substrate coated with different working electrode materials including POT (a) and POT-MoS 2  (b). 
         FIG. 16  is (a)-(b) Sensitivity test: voltage responses of the sensor to changing nitrate concentrations. The sensors used the POT-MoS2 nanocomposite, POT, and MoS 2  in their working electrodes. (c)-(d) Selectivity test: voltage responses of the sensor to a mixture of nitrate (50 ppm) and different interfering ions (500 ppm each). The sensor used the POT-MoS2 nanocomposite in its working electrode. 
         FIG. 17  is (a) and (b) repeatability test for the fabricated sensor made of ISM/POT-MoS 2 /Au in presence of 1 ppm and 1300 ppm of nitrate-nitrogen solution. (c) Long-term stability test of the sensor in presence of 5 ppm of nitrate-nitrogen. 
         FIG. 18  is XPS analysis for the printed electrodes made by POT, MoS 2 , POT-MoS 2  and ISM/POT-MoS 2  materials. For this study, these materials were printed on the surface of silicon. The XPS spectra of the carbon is region of MoS 2  (a), POT-MoS 2  (b) and ISM/POT-MoS 2  (c). Sulphur (S 2p) peaks for the MoS 2  (d), POT-MoS 2  (e) and ISM/POT-MoS 2  (f) electrodes. XPS peaks for molybdenum (Mo) 3d found for the MoS 2  (g) film and POT-MoS 2  (h) film. XPS spectra for the nitrogen is peaks region of ISM/POT-MoS 2  film (i). 
         FIG. 19  is at graph (a) cyclic voltammetry (CV) studies of MoS 2 , POT and POT-MoS 2  electrodes. These experiments were conducted using phosphate buffered saline (PBS) solution mixed with a 2 mM concentration of ferro/ferricyanide ([Fe(CN) 6 ] 3−/4− . CV responses for the POT/Au (graph (b)), MoS 2 /Au (graph (c)) and POT-MoS 2  (graph (d)) by varying scan rate from 20 mV/s to 200 mV/s. Potential differences versus root square of scan rates for all fabricated electrodes are at graph (e). CV responses of POT/Au electrode with and without ion selective membrane are at graph (f). 
         FIGS. 20 to 29A -C relate to a second Specific Example 2 according to aspects of the present invention. 
         FIG. 20  is a diagrammatic depiction of concepts from the Example 2 according to aspects of the present invention. 
         FIG. 21  relates to the Example 2 and is a stepwise representation of the fabrication of all-solid-state soil nitrate sensor. (a) Photograph taken during printing Ag/AgCl paste on circular-shaped silver (Ag) electrodes using a stencil printer. (b) Photograph taken during materials dispensing (POT-MoS 2  in THF solvent) on circular-shaped Au electrodes using a programmable high-precision automated fluid-dispensing robot. (c) Photograph of the device. 
         FIG. 22  relates to the Example 2 and is a schematic of the working principle of the soil sensor. (a) Oxidation process for the WE (ISM/POT-MoS 2 /Au) in the presence of soil solution NO 3   −  ions. R +  and R −  represent the anion and cation exchangers at the organic membrane, and M +  and A −  are the hydrophilic ions in soil water. POT-MoS 2  and POT-MoS 2   +  indicate neutral and oxidized POT-MoS 2  units. Oxidation/reduction is shown for the Ag/AgCl RE. (b) Molecular structure of POT and MoS 2  for composite formation in this sensor. (c) Mechanism of the reduction process for the WE (ISM/POT-MoS 2 /Au). 
         FIG. 23  relates to the Example 2 and is a scanning electron micrographs for MoS 2  sheets (a), POT (b), and POT-MoS 2  materials (c) with schematic representation of various layers. (d) Cross-sectional view of the SEM image for the POT-MoS 2  composite on Au. Contact angle (CA or Θ) studies for the investigation of the hydrophobicity of the WE materials. A syringe was used to drop 3 μL volume of deionized water on the Au/PCB substrate coated with different WE materials, including MoS 2  (e), POT (f), and POT-MoS 2  (g). Images were analyzed using image J plugin software. 
         FIG. 24  relates to the Example 2 and is an XPS analysis for the WEs using MoS 2 , POT-MoS 2 , and ISM/POT-MoS 2  materials. XPS spectra of the carbon is region of MoS 2  (a), POT-MoS 2  (b), and ISM/POT-MoS 2  (c). Sulfur (S 2p) peaks for the MoS 2  (d), POT-MoS 2  (e), and ISM/POT-MoS 2  (f) electrodes. XPS peaks for molybdenum (Mo) 3d found for the MoS 2  (g) film and POT-MoS 2  (h) film. XPS spectra for the nitrogen 1 s peak region of the ISM/POT-MoS 2  (i) film. 
         FIG. 25  relates to the Example 2 and is a (a) CV for different electrodes: MoS 2 , POT, POT-MoS 2 , and ISM/POT-MoS 2 . These experiments were conducted using a phosphate buffered saline solution mixed with a ferro-/ferricyanide species ([Fe(CN) 6 ] 3−/4− ) of concentration 2 mM. Inset shows the zoomed CV curve of ISM/POT-MoS 2 . (b) Potential differences (ΔE) obtained from the CV curves plotted against the ratio of POT-MoS 2 -based electrodes. In the composite formation, the ratio of POT to MoS 2  was varied from 1:1 to 1:10 by weight percentage. (c) Oxidation current obtained from the CV curves vs the ratio of POT to MoS 2  (1:1 to 1:10). (d) CV graphs for the optimized electrode based on POT-MoS 2  (at a ratio of 1:4) in the presence of [Fe(CN) 6 ] 3−/4− . (e) Voltage measurements (OCP) for three electrodes, such as MoS 2   − , POT-, and POT-MoS 2 -based sensors, after coating with a nitrate ion-selective membrane in the presence of 1000 ppm NO 3   − —N. (f) Chronopotentiometry measurements for three nitrate sensors using POT, MoS 2 , and POT-MoS 2  as the ion-to-electron transducing layers. Constant 50 nA anodic and cathodic currents were applied uninterruptedly for 60 s each, and the respective potential responses over time were recorded. 
         FIG. 26  relates to the Example 2 and is a (a) Sensor responses in millivolts (mV) made by MoS 2 , POT, and POT-MoS 2  electrodes modified with ISM. A stock solution of 1500 ppm of nitrate-nitrogen was made in DI water and diluted from 1500 to 1 ppm. Sensor measurements were conducted for 2 min at each concentration. The corresponding average voltages for all the sensors (MoS 2 , POT, and POT-MoS 2 ) were plotted against the logarithm of nitrate-nitrogen in ppm. Error bars were calculated using three consecutive measurements for each concentration. (b) For the selectivity studies, the NO 3   − —N concentration was set to 100 ppm, and the hydrophilic interfering ions were set to 400 ppm. The selectivity coefficients were calculated for MoS 2 -, POT-, and POT-MoS 2 -based ISM sensors using SSM. (c) Stability of the fabricated RE (Ag/AgCl) with and without Nafion coating was tested separately by varying the concentration of KCl from 0.01 to 3 M. For the stability test, the OCP of the fabricated RE was measured with respect to a leakless miniature Ag/AgCl RE having an internal electrolyte of 3.4 M KCl (obtained from EDAQ, ET072-1). (d) Long-term stability measurement of the Ag/AgCl electrodes with and without Nafion coating: plot of the OCP of the electrodes vs time in the presence of 0.01 M KCl solution. (e) Interference studies of the POT-MoS 2 -based sensor in the presence of CO 2  and N 2  gases purging into a nitrate solution. After the nitrate measurement, the sensor was tested in a closed chamber where CO 2  and N 2  gases continuously flowed for 15 min before the measurement. 
         FIG. 27  relates to Example 2 and is: Graph (a) Sensor responses (commercial and fabricated) for real soil extracted solutions collected directly from Ames, Iowa, with a suction lysimeter. Graph (b) Schematic presentation of soil-column setup for nitrate-nitrogen measurement. Graph (c) Photographs of soil column beakers with soil slurries wherein the sensors were hung on the wall of the column. Graph (d) Short-term soil nitrate-nitrogen sensing in the soil column, where the baseline was set in the presence of DI water (baseline), and the column was flushed with DI water after the soil was treated with 100 and 50 ppm of NO 3 —N, and Graph (e) plot for corresponding sensor readings. 
         FIG. 28  relates to Example 2 and is a long-term measurements (approximately 4 weeks) using two different individual sensors (made with POT-MoS 2  material), wherein sensor 1 and sensor 2 were deployed in beakers containing soil slurries. Photographs of column beakers without soil slurries (a) and with soil slurries and sensors 1 and 2 (b). For sensor 1 (c), the soil beaker was filled with DI water and then left to dry, and the soil slurry was again treated with water multiple times and then parched. Finally, DI water mixed with nitrate-nitrogen (50 ppm) was poured into the soil slurry in the column beaker with sensor 1 and left to dry. The process was repeated multiple times (for approximately 4 weeks) for sensor 1. For sensor 2, the soil slurry was initially filled with DI water, parched, and flushed with 100 ppm nitrate-nitrogen (d). After drying, sensor 2 was kept in the parched condition for about 2 weeks (e). 
         FIG. 29A  relates to Example 2 and is a reproducibility studies for POT-MoS 2  based sensor with individual identical copies of the electrode. 
         FIG. 29B  relates to Example 2 and is the potential stability of the sensor (ISM/POT-MoS 2 ) for nitrate-nitrogen detection. In this measurement, the ISM/POT-MoS 2  electrode was conditioned for 3 d at high concentration of nitrate-nitrogen (1500 ppm) and inserted an open vial filled with 100 ppm of nitrate-nitrogen for long-term stability measurement. Results show the initial potential (starting potential ˜−184 mV) is almost matched after 10 days of potential measurement (end potential ˜−186 mV). However, it has been seen the there is a potential variation of 0.2 mV per day during continuous measurement. 
         FIG. 29C  relates to Example 2 and is an activity of high (1300 ppm) and low (1 ppm) concentration nitrate-nitrogen was performed to investigate the repeatability of the electrode. 
         FIGS. 30-35  relate to a Specific Example 3 according to aspects of the present invention. 
         FIG. 30  is a diagrammatic view of an Example 3 according to the present Invention, including a method of manufacturing small-scale solid sensors according to aspects of the present Invention.  FIG. 30  shows: (a) Coating POT-MoS 2  nanocomposite on the surface of substrate using a programmable robotic dispensing system. (b) Photo of a fabricated nitrate sensing unit. 
         FIG. 31  relates to Example 3 and shows: SEM for MoS 2  sheets (a), POT (b), and POT-MoS 2  materials (c) with schematic of various layers. (d) A cross-sectional view of POT-MoS 2 /Au. (e) Molecular structure of POT and MoS 2 . (f) The oxidation and reduction for printed WE (ISM/POT-MoS 2 /Au) in presence of soil NO 3   −  ions. R +  and R −  represent anion and cation exchangers at organic membrane, and A are hydrophilic ions. 
         FIG. 32  relates to Example 3 and shows: (a) Cyclic voltagramms for various electrodes in presence of standard PBS solution. (b) Ratio optimization for POT and MoS 2 . Inset shows the voltages of all three electrodes after coated with nitrate-selective membrane in presence of 1000 ppm NO 3   − —N. 
         FIG. 33  relates to Example 4 and shows: Responses to varying nitrate concentrations of standard nitrate solutions for MoS 2 -, POT- and POT-MoS 2 -based sensors (a) and corresponding calibration curves (b). (c) Effect of Cl −  ions on voltage output of the POT-MoS 2 -based sensor. (d) Selectivity studies. The nitrate concentration was set to 100 ppm NO 3   − —N and the interfering ions were set to 400 ppm. The selectivity coefficients were calculated using separate solution method. 
         FIG. 34  relates to Example 3 and shows: (a) Nitrate sensing for extracted soil solutions and validated using a commercial sensor. (b-c) Experimental setup. (d) Short-term soil nitrogen sensing in the soil column, where the baseline was set in the presence of water (baseline), and the column was flushed with DI water after the soil was treated with 100 and 50 ppm of NO 3   − —N, and (e) Plot for corresponding sensor readings. 
         FIG. 35  relates to Example 3 and shows. Long-term measurement with two sensors. Photographs of column beakers without soil slurries (a), and with soil slurries and sensor 1 and sensor 2 (b). For sensor 1 (c), the soil beaker was filled with DI water and then left to dry, and the soil slurry was again treated with water multiple times and then parched. Finally, DI water mixed with nitrate-nitrogen (50 ppm) was poured into the soil slurry in the column beaker with sensor 1 and left to dry. The process was repeated multiple times (for approximately 4 weeks) for sensor 1. For sensor 2, the soil slurry was initially filled with DI water, parched, and flushed with 100 ppm nitrate-nitrogen (d). After drying, sensor 2 was kept in the parched condition for about 2 w (e). 
     
    
    
     IV. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     A. Overview 
     As will be appreciated by those skilled in the art, the invention can take many forms and embodiments. For a better understanding of the invention, examples of non-limiting specific embodiments will now be set forth. It is to be understood these are neither inclusive nor exclusive of all forms the invention can take. 
     As will be appreciated, the examples focus on in situ, long term detection of nutrients in soil and water in an agricultural field. A particular example is soil nitrate. As indicated, however, the invention is not limited to those specifics. 
     B. Apparatus/System 
     With reference to  FIGS. 1-7 , a first exemplary embodiment according to the invention will now be described. As will be appreciated with reference to the brief description of the appendices, background information on such things as how ion-selective detection works for soil nutrient measurement, how porous sampling heads can be used to collect a soil water sample, and how onboard, integrated fluid, electrical, and electronic subsystems can be powered by a battery and controlled by a processor are set forth. The main features and aspects of the invention will now be discussed. 
     The main features of the disclosed soil or water nutrient sensor system are as follows: 
     #1. The disclosed sensor system  10  can automatically reset, recondition, and recalibrate for long-term, high-precision detection of nutrient in soil and water (e.g., tile drainage water) in crop fields ( FIG. 1 ): Currently, there are several soil nutrient sensor technologies, including ion selective electrodes, ion selective field-effect transistors, enzyme-based electrochemical sensors, and electrochemical sensors. However, none of these sensors are able to automatically reset themselves after long term use. Manual reconditioning and recalibration are required for high-accuracy measurement over a period of time (e.g., &gt;30 days). Therefore, existing nutrient sensors are usually used in laboratory. The disclosed sensor system is designed as a novel self-contained system to realize automated sampling, detection, reconditioning, and recalibration, without any manual interferences. A simple fluid manipulation unit (consisting of two normally-closed peristaltic actuators and a vacuum pump), in incorporation with a unique flow-through soil water suction head (see #3 below), allows sampling, conditioning and reconditioning, and calibration and recalibration, through manipulation of liquid fluids within the sensor system  10  ( FIGS. 2A-C ). The reconditioning liquids can be delivered to and from an embedded nutrient detection unit to wash the sensing surface and the interior of the sampling head. 
     #2. A new ion-to-electron interface for ion-selective membrane to improve both sensitivity and selectivity of ion-selective membrane-based sensing unit used in the system: (1) The working electrode  30  of the sensing unit  10  contains an ion-to-electron transfer layer  34  between the ion-selective membrane  32  and the metallic (e.g., gold, Ag/AgCl) conducting layer  36 . This ion-to-electron transfer layer  34  can be a nanocomposite of poly(3-octyl-thiophene) and molybdenum disulfide (or POT-MoS 2 ) that can improve sensitivity, selectivity and long-term stability of the sensor  14  ( FIG. 3 ). The POT-MoS 2  is a first-ever nanocomposite introduced to the ion-selective membrane-based sensor and provides high redox activity and electron conduction ability to improve sensitivity, where POT helps increase redox activity and MoS 2  nanoflakes help increase electron conductance. Also, the incorporation of the POT-MoS 2  into the working electrode can minimize formation of a thin aqueous film between the ion-selective membrane and the metallic conducting layer. This can help increase selectivity and reduce signal drifting because the POT-MoS 2  exhibits a high lipophilicity inhibiting the formation of aqueous film that contains other interfering ions. Also, the aqueous film is a main source for substantial potential instabilities because the electrolyte composition of this layer may slowly change leading to drifting potentials. Therefore, minimizing or eliminating this aqueous layer using the POT-MoS 2  can reduce potential drifting at the working electrode, thus increasing stability of the sensing unit. (2) The sensing unit used in the disclosed sensor system  10  uses a novel integrated, low-cost, high-performance reference electrode  40 , i.e., S 3 RE or sandwiched solid-state reference electrode. The S 3 RE  40  allows immobilizing a chloride reservoir on top of a base electrode  45  of Ag/AgCl to stabilize potential at the reference electrode  40 . Generally, conventional ion-selective membrane based sensors use Ag/AgCl reference electrodes alone surrounded by high concentration Cl −  ions with a porous glass/plastic frit tip. But, these simple reference electrodes undergo a redox reaction and leaching of chloride ions, which, in turn, alters the chloride equilibrium at the electrode surface, resulting in drifting of reference potential. On contrary, in the S 3 RE  40 , a polymeric membrane  44  of polyvinyl butyral (PVB) contains a saturated concentration of chloride and a proton exchange membrane (PEM)  42  such as Nafion serves as a protection coating to minimize or even prevent leaching of chloride, thus stabilizing the reference potential of the sensing unit  10 . See infra., and  FIGS. 11 to 19  for more details. An appropriate voltage signal  50  is established between electrodes  30  and  40  during operation via an appropriate processor/control circuit/power source  18 . Mounting surfaces or substrates  38  and  48 , and waterproof mountings  39  and  49  can be used to support the metallic conducting layers  36  and  46 , and other layers, respectively, on the substrates  38  and  48 . Other techniques are possible. 
     #3. A flow-through soil water suction head: Ceramic porous capillary tubes have been widely used to extract soil water for analysis, and test soil water potentials for irrigation management. See, e.g., Di Bonito, Marcello, et al. “Overview of selected soil pore water extraction methods for the determination of potentially toxic elements in contaminated soils: operational and technical aspects.” Environmental Geochemistry. 2008. 213-249, incorporated by reference herein. The disclosed water suction head  20  is unique in that it utilizes a porous ceramic tube with openings at both sides (see openings  21 A and B). This design allows not only in situ sampling of soil water; but also in situ cleaning of the interior of the ceramic tube  20  by flowing a cleaning solution  27  from one side through the ceramic tube  20  into a waste reservoir  28  connecting on the other side of the tube  20  ( FIG. 2  and  FIG. 4E ). This design also allows reconditioning and recalibration of the sensing unit  10  by flowing the calibration solutions to a detection cell  12 . Therefore, after the soil water is sampled and analyzed, the suction head  20  can be easily washed and recalibrated in situ, without pulling it out of the soil. In contrast, a traditional soil water suction head has a closed end, making it difficult to clean the interior of the ceramic tube and to recalibrate the sensors. 
     #4. Integration of a nutrient detection unit  14  and a water-level sensor  16  into a soil water collector or detection cell  12 : The nutrient detection unit  14  is made inside or socketed into the soil water collector (or detection cell  12 ). A conductivity measurement unit  16  is designed to serve as a water-level sensor to control ON/OFF states of a vacuum pump  22 . Soil water solutions cannot be extracted continuously; instead, they appear a mixture of tiny water droplets and air in the ceramic suction head  20 . Vacuum pumping via tubing or conduit  23  directs the water droplet-air mixture along a plastic channel  29 A into the detection cell  12  for analysis. The disclosed integration method reduces the amount of soil water required for analysis (e.g., less than 200 microliters). 
     #5. In the disclosed sensor system  10 , a single sensing unit  14  can be used for nutrient detection at least several dozens of times in the field. The sensing unit  14  is replaceable. Replacing is simple: simply sliding a new sensing unit  14  into the detection cell  12  (i.e., soil water collector). Without the disclosed fluid manipulation/control unit and the suction head with opening at two sides, the device  12  could work only for once in the field. The sensitivity, selectivity, and stability of the disclosed sensor  12  are increased ( FIG. 5 ) due to the use of the improved sensing unit  14 . See infra., and  FIGS. 11 to 19  for more details. 
     C. Operation 
     Device Structure, Working Principle, and Operation 
     The disclosed sensor system  10  consists of three main units, i.e., (1) a special flow-through porous ceramic suction head  20  with openings  21  at both sides, (2) a vertically placed nutrient detection cell  12  embedded with an improved ion-selective membrane-based sensing unit  14  and a water-level detection unit  16 , and (3) a novel fluid control unit with only two normally-closed peristaltic actuators (PeriA-3 and -4) and one vacuum pump (VacP). Also, please refer to the flow chart of  FIG. 8 . 
     Step 1: Initialization ( FIG. 2A ): PeriA_3 (ref. no.  24 - 3 ) and PeriA_4 (ref. no.  24 - 4 ) are turned ON. PeriA_1 (ref. no.  24 - 1 ), PeriA_2 (ref. no.  24 - 2 ) and VacP (ref. no.  22 ) are OFF. Liquids in the detection cell  12 , plastic tubing  29 A and  29 B, and suction head  20  are pumped into the waste reservoir  28 . 
     Step 2: Recalibration ( FIG. 2B ): PeriA_1 (ref no.  24 - 1 ) is turned ON. PeriA_2 (ref. no.  24 - 2 ), PeriA_3 (ref no.  24 - 3 ), PeriA_4 (ref no.  24 - 4 ) and VacP (ref. no.  22 ) are OFF. The standard calibration nitrate solution  26  is delivered through tubing  25  into the detection cell  12 . 
     Step 3: When the standard calibration nitrate solution  26  reaches a preset volume (e.g., 200 microliters) in the detection cell  12 , the water level sensor  16  is triggered to turn off the peristaltic actuators. Here, the water level sensor  16  is formed by two electrode elements. When the standard calibration nitrate solution (or extracted soil water solution) contacts the two electrode elements, the electrical impedance between the two electrode elements is changed, providing a signal to turn off the peristaltic actuators. 
     Step 4: The nutrient sensing element  14  in the detection cell  12  quantifies a specific ion species (e.g., nitrate). 
     Step 5: Cleaning ( FIG. 2C ): After the detection is done, PeriA_2 (ref. no.  24 - 2 ) and PeriA_4 (ref. no.  24 - 4 ) are turned ON to direct the standard calibration nitrate solution  26  from the detection cell  12  into the waste reservoir  28 , and then flow the cleaning water from the water reservoir  27  through both the detection cell  12  and the suction head  20  into the waste reservoir  28 . Flushing and cleaning by deionized water can reset and recondition the sensing element  14 . 
     Step 6: Empty tubing: PeriA_3 (ref no.  24 - 3 ) and PeriA_4 (ref. no.  24 - 4 ) are turned ON, other peristaltic actuators  24 - 1  and  24 - 2  and vacuum pump  22  are OFF. Liquids in the detection cell  12 , plastic tubing  29 A and  29 B, and suction head  20  are pumped into the waste reservoir  28 . Here, liquid in the tubing  25  between the detection cell  12  and peristaltic actuators  24  must be pumped into the waste reservoir  28 . Otherwise when the vacuum pump  22  is turned ON to extract the soil water, air dissolved in this liquid in the tubing  25  will expand on vacuum environment and push the liquid into the detection cell  12 , dilutes the soil water and affects the soil water analyzing. 
     Step 7: Analyzing: All peristaltic actuators  24 - 1  to  24 - 4  are turned OFF. VacP 22 is turned ON. A mixture of soil water and air is sucked into the ceramic tube  20  through the pores embedded in the wall of the ceramic tube  20 , and further, is driven into the vertically placed detection cell  12 . Here, the uniqueness is that the use of the normally closed peristaltic actuators  24 - 3  and  24 - 4  eliminates the need of any valves (e.g., solenoid valves), to avoid directly pumping the cleaning (water or buffer solutions), standard nitrate solution and waste liquids from three reservoirs  26 ,  27 , and  28  into the detection cell  12 . This saves space and cost of the system (note: solenoid valves with sufficient holding pressure are costly and have large dimensions). 
     Step 8: When the soil water reaches a preset volume (e.g., 200 microliters) in the detection cell  12 , the water level sensor  16  is triggered to turn off the vacuum pump  22  and start analyzing the soil water with the selective sensor  14  embedded in the detection cell  12 . 
     Step 9: Cleaning: After the detection is done, PeriA_2 (ref. no.  24 - 2 ) and PeriA_4 (ref. no.  24 - 4 ) are turned ON, directing the tested soil water sample solution from the detection cell  12  into the waste reservoir  28 , and flowing the washing water from the water reservoir  27  through both the detection cell  12  and the ceramic suction head  20  into the waste reservoir  28 . 
     To conduct new measurements, go and repeat Step 1-9.  FIG. 8  gives one non-limiting example of methodology  100  for these different stages/modes of operation possible with a unit  10 . 
     D. Non-limiting Options and Alternatives 
     As mentioned earlier, the foregoing examples are not limiting to the invention. Variations obvious to those skilled in the art will be included. 
     For example, some of the Appendices mention different ion-selective membranes or detectors that can be configured for different ionic species and in different applications. See infra., and  FIGS. 11 to 19  for a specific example. 
     A few other nonlimiting examples relevant to the foregoing are set forth below. 
     1. Integration 
     As indicated above, an overall system  10  can be created which includes or integrates one or more of the features and aspects of the invention described herein. It can be an all-in-one system, including a housing, power source, detection cell (e.g. electrodes and membrane), a sampling suction head (e.g. flow through) and built-in water level sensing; refresh/flushing operation, fluid manipulation components and reservoir, data storage, a readout, and a data interface (e.g. communication including wireless). This description includes other non-limiting options and alternatives. 
       FIGS. 1-8 and 10  illustrate an all-in-one unit  10  including a ruggedized (e.g. metal or plastic) housing  60  with internal onboard power supply  66 . Subsystems allow collection of soil water and then transport over a variable length fluid conduit to the housing  60  and a detector cell  12  in the housing. A miniaturized or micronized electrode based detection cell  12  can sense when enough soil water sample is present and then conduct the electrochemical sensing to measure a chemical species of interest related to the selected ion-selective membrane used. In this first measurement cycle state, the integrated system can derive from the electrodes electrical measurements that are converted to data that can be processed and stored as a measurement of interest. The onboard battery  66  can supply needed electrical power for these functions to both electrical and electronic systems as well as actuators  24  for fluid manipulation unit  64  and reservoirs  62 . 
     In a second state, electrical power managed by the processor  68  can control fluid manipulation components  64  for a flushing of the system to recondition it in preparation for a next sampling and detection cycle. The onboard system can include other components such as data storage  69  and a readout circuit associated with processor  68 . There can be data ports or interfaces for downloading data to another device. As indicated in certain Appendices, an option would be to include electrical communication whether wired or wireless. See, e.g.,  FIG. 9 . 
       FIG. 10  illustrates a variation on an all-in-one cell  12  with sensor unit  14  and water level sensor  16 . Multiple suction heads  10 - 2 ,  20 - 2 , to  20 - n  could be used with one cell  12 . By programming and control of actuation, all sampling heads could simultaneously by suction retrieve soil water samples for collective chemical sensing in cell  12 . A read-out based on the collective samplings could be used. Alternatively, by appropriate set-up and control, sampling could be done seriatim one sampling head  20  at a time and measurements made and stored/communicated. Still further, each cell  12  could have one sensor unit  14  for all heads  20 , or one unit  14  for each head  20 , or other arrangements. Still further, tubing to each head  20  can vary in length or be selected in length allowing each head  20  to be placed to different sub-soil depths or different locations relative to cell  12 . This can allow either a collective average measurement if all samples are collectively retrieved and one measurement taken, or individual measurements made for each head to gain information about variation of the measurement per head location. 
     2. Form Factor 
     The integrated example is at centimeter scale or less. One example as indicated at  FIG. 4B  is a housing 15×15×7.5 cm with an approximate 8 cm long by 2 cm outside diameter ceramic porous tube  20  as the sampling head. Tubing to the sampling head can be selected based on the ability to place the housing at ground level and extend the sampling head into the ground around it to various steps. References cited in the Background section above suggest sampling depths from 6 to 60 cm might be of interest. 
     That general form factor is not materially disruptive to planting and other field events. If desired, the housing could be buried to further protect it. Some sort of GPS waypoint or physical visible aboveground indication might be used to locate and remove it if buried. The system can be ruggedized and sealed. 
     3. Power 
     Electrical connections can be conventional. Some are indicated in the figures. 
     4. Fluid Manipulation 
       FIGS. 1-8 and 10  show a vacuum pump  22  as the primary fluid manipulation source for state one of the system  10 . Such a vacuum pump can be small and inside the housing  60  and provide sufficient pressure difference to effectively collect soil water at the sampling head  20  and move it to the detection cell  12  repeated times over a battery life for the system. An example of such a pump  22  is model number D2028 from vendor Karlsson Robotics, FL, USA. Alternatives are, of course, possible. 
     State two system flushing is shown using peristaltic type pumps  24  and fluid conduits between cleaning liquid reservoirs  27  and waste reservoirs  28 . Examples are model 1150 from vendor ZJchao Inc., China. They too could operate over repeated flushing cycles for the intended normal operating life of the battery and system. 
     As can be appreciated, water level sensors in the detection cell can stop fluid accumulation above the sensed maximum level. This can avoid having to use valves or fluid gates which could have actuators and use additional power and resources. 
     Similarly, the peristaltic actuators function both as a pump and as a check valve. 
     5. Suction Head 
     One suction head  20  is shown in most of the systems in the drawings. As indicated at  FIG. 10 , it can be possible to attach multiple heads  20 - 1 ,  20 - 2 ,  20 - n  to a single detection cell  12 . Other variations are indicated. 
     It is to be appreciated that the relatively small form factor ceramic tube  20  in the drawings could be placed at various depths in the soil for different data acquisitions based on depth. Different lengths can be used/selected to sample to different soil water depths in the Z-direction relative to the ground. 
     Measurements from different spatial locations, depths at or near the same spatial location, or otherwise from different detections can be acquired and compared. Measurements can be compared for differences. 
     One example of a sampling head is a ceramic tube having open ends. Permeability can be selected by the designer according to need or desire. Other porous tubular structures are possible. Non-limiting examples are plastic or metallic tubing with selected permeability. 
     Similarly, the fluid conduits connecting the head to the detection cell, the vacuum pump to the detection cell, and the reservoirs to other components can be plastic of a substantially robust grade for the environmental extremes of an agricultural field over various seasons of the year. Other non-limiting examples are possible. 
     6. Water Level 
       FIGS. 1 and 2  show a water level sensor  16  at each electrode of the electrode-based ion-selective detector  14 . In this non-limiting example, use of two sensors  16  can detect/determine water level by the following technique: impedance measurement. 
     7. Electrochem Detection 
     Both types of detector  14  and parameters related to the novel ion-to-electron transfer layer  34  at the working electrode  30  and the novel sandwiched solid-state reference electrode  40  are set forth above. Variations obvious to those skilled in the art are included. 
     Non-limiting examples of alternatives are: ion selective field-effect transistor-based sensors (e.g. extended gate), enzymatic electrochemical sensors, optical chemical sensors, microwave based chemical sensors. 
     Alternatives to POT and Nafion have been mentioned elsewhere. 
     8. Flushing Subsystem 
     The flushing subsystem not only can move out of the detection cell  12  any fluid as well as out of the sampling head  20 , variations as to the type of fluid that can be inserted into the system are possible. Additionally, the flushing system might be activated to try to unplug a blocked conduit or blocked suction head. The vacuum pump  22  could be operated for that purpose also. Both the vacuum and the peristaltic pumps could be operated in a toggling mode for such purpose. 
     Essentially the flushing can be artificial rain to rinse soil water for the next measurement. The unit can be cycled by vacuum if not working to try to unplug it. 
     9. Applications 
     A few non-limiting examples of applications and variations of the system are set forth above. Variations obvious to those skilled in the art will be included. Some additional examples are: monitoring of nutrient in water (e.g., agricultural subsurface water, drainage water, sewage pipe water). 
     A primary use is as in situ soil nutrient sensors (e.g. housed on the ground or at ground level with sampling head  20  buried to a desired depth or depth(s)). Other non-limiting examples are indicated herein. 
     10. Distributed System with Multiple Detectors in Field and Optional Communication Capability to Remote Sites. 
     With reference to  FIG. 9 , as will be appreciated by those skilled in this technical area, the following types of variations and options with the system of  FIGS. 1-8  are possible. 
     Multiple Spatially-distributed Sensors. A plurality of sensor systems  10 - 1 ,  10 - 2 ,  10 - 3 ,  10 - n  as in  FIGS. 1-8  (particularly  FIG. 6 ) could be placed at spaced apart positions, individually monitor and take measurements at its position over time, and store or communicate those measurements for further use. They could be in the same or separate crop fields. They could be relatively closely spaced or more widely spaced. They could be individually readable at selected times or collectively report each of their readings. The designer could select these and other parameters based on need or desire, as well as practicality. A benefit of this arrangement is that each sensor  10  is relatively inexpensive to make and operate, and could generate measurements over extended periods of time. This allows collection of a lot of data specific to each sensor location over an extended time in situ and without disruption of normal agricultural activities in the field. But it also allows collection of data from multiple sensors  10  for collective analysis. Such collective analysis can be used in a number of advantageous ways by agricultural producers, as is well-known. To have not only data resolved to individual spatial locations, but also over relatively long periods of time can lead to insights that can be very helpful to the producers. 
     As indicated in  FIG. 9 , one way to communicate results automatically or semi-automatically is via wireless communications from each sensor unit  10 . Of course, other ways of data collection for each is possible, including but not limited to a reader that can be moved across the field and wirelessly interrogate stored measurement data from each sensor. Still further, a hard-wire connection could be made to each sensor unit  10  to read the data. But with current Blue-tooth and other wireless relatively low power reading techniques, a worker could walk or move by in a vehicle each of the sensor units  10 - 1  to  10 - n  and get wireless data transfer of measurements. 
     One or More Sensor Cells at Each Spatial Location. As indicated in the enlarged diagram at the lower left hand of  FIG. 9 , another optional feature is that there could be either a single sensing head  20  or cell  12  embedded into soil or water at a pre-selected depth and in electrical communication (e.g. via hardwire or possible wireless) with a separated Readout Circuit/Timing Control/Data Storage/Power Supply (or collectively Control Circuit  18 ) as indicated diagrammatically in  FIG. 1 . This could allow the Control Circuit  18  to be placed above-ground level or above water level with the sensor cell  12  or sampling head  20  at a selected depth in the soil or water. This can allow for easy access for, inter alia, maintenance, repair, programming, battery replacement, and the like. It would also allow, if desired, recovery and reuse. The form factor of such Control Circuits  18  can be relatively small, robust, and low profile so as to not disrupt plant growth or vehicle travel by or even over them. However, it is to be understood that any housing and components of the Control Circuit  18  could be made to allow it also to be embedded into the soil or water and operate from that position, if desired or needed. As mentioned, wireless technologies like Bluetooth allow wireless reading of embedded Control Circuits at least if sufficiently close in distance. 
     But there could be multiple such heads  20  and/or cells  12  for each Control Circuit  18  (see ref. nos.  12 - 1 ,  12 - 2 , to  12 - n  and  20 - 1 ,  20 - 2 , to  20 - n  in  FIG. 9 , lower left hand enlargement). This could, for example, allow measurements at the same geospatial location from plural depths. Each sensor cell/sampling head combination  12 / 20  could report this information and allow further differentiated analysis of measurements from such different depths, for a single Control Circuit  18 . 
     Communication Options. As further indicated diagrammatically in  FIG. 9 , by techniques and components well-known to those of skill in this technical area, collection of data from each measurement cell can be individually, but also collectively. In one non-limiting example, all cells and Control Circuits in a field could wirelessly communicate to a wireless collection point within wireless communication range of each Control Circuit. In one example it could be within perhaps tens to hundreds of feet to each collection tower. The collection tower (labelled wireless hub in  FIG. 9 ) could then have components with sufficient power and capabilities to communication wireless or otherwise to a LAN, WAN, or the Internet. By appropriate software, the measurement data from all the sensor cells can be collected, with registration to the geospatial position of each cell, and communicated from further use. Examples are to cloud storage for later access and use by any of remote computers, servers, or databases, or access and use by any of a number of digital devices (e.g. smart phones, tablets, lap tops, desk tops, etc.) 
     This collection of big data can be from a single field for use by a landowner/producer. But it could be from multiple fields, multiple farms, multiple landowners/producers, multiple counties, states, and even countries. Having this big data can lead to understandings or insights that can be helpful to not only individual producers but researchers, scientists, governments, seed companies, and others. 
     E. Specific Example 1 of Solid State Nutrient Sensor 
     With particular reference to  FIGS. 10-19 , details about a specific solid-state sensor configuration useable according to aspects of the invention are as follows. 
     All-Solid-State Nutrient Sensors Using Printed Polymeric Composite of POT-MoS 2  as an Ion-to-Electron Transfer Layer and Sandwiched Solid-State Reference Electrode (S 3 RE) for Detection of Nitrate in Soil and Water 
     Unlike conventional ion-sensitive sensor, the main motivation was to construct a novel, deployable, miniature, mass production and stable all-solid state nitrate sensor. Selection of appropriate materials is crucial for the better performance of the sensor. The potential issues with conventional ion selective devices are signal drifting, instability, bulky and not miniaturize. The internal filling solutions (such as nitrate for nitrate selective sensor) is commonly used for the development of ion selective sensors. Our approach was to replace the internal filling solution by introducing an ion-to-electron transfer layer in between ion selective membrane (ISM) and conductive layer and reducing potential drift at output signal. The potential drift occurred not only in the working electrode but also happened in the reference electrode of the device. For working electrode issues, we introduced a composite material of poly(3-octyl-thiophene) and molybdenum disulfide (POT-MoS 2 ) as ion-to-electron transfer layer. POT-MoS 2  material can also replace the internal nitrate solution between the conductive electrode and ISM. The POT is an attractive material for the construction of all-solid-state ion selective sensors due to their high redox property and prevent the formation of water layer due to its high lipophilicity. However, the POT is found to be not significantly conductive material, thus we introduced MoS 2  into POT polymer to enhance the device performance. The incorporation of MoS 2  can increase the redox property, amplify the voltage signal, lowering the limit of detection and reduced the potential drift of the sensor. For the all-solid-state Ag/AgCl reference electrode (RE), there are several issues such as constant supply of Cl— ions to compensate Cl— equilibrium in the redox reaction of Ag/AgCl surface, sensitive to Cl— and other ions and potential drift over time. A three layered sandwiched type solid-state structure can be a potential solution by reducing the chloride leaching over time, holding the chloride ions in a polymeric membrane and blocking other ions. The polymeric membrane of polyvinyl butyral (PVB) can hold KCl crystal on Ag/AgCl layer to supply Cl— ions constantly due to its nanoporous feature. A proton exchange membrane such as Nafion can block the leaching of Cl— from the RE surface and block the other interfering ions. 
     Our work is to make portable and miniaturize nitrate sensor, we have constructed both electrodes by using silicon needle via microfabrication technique and coated these materials using liquid auto-dispensing unit for mass fabrication. Combining both solid-state electrodes containing all the materials on the surface of Si needle, the needles were placed in 3D printed case as silicon needles are fragile. Affordability, portability, all-solid-state electrode with ion-to-electron transducer and mass fabrication are the attractive features of this nitrate sensor. The fabrication process is shown in  FIG. 11 . 
     A main feature of the reference electrode is to make a sandwiched solid-state reference electrode (S 3 RE) to provide a stable voltage for both long and long term measurements ( FIG. 12 c   ). The Ag/AgCl RE electrode  40  surface undergoes redox reactions and leaching of chloride ions (Cl − ) which alters the chloride equilibrium resulting in voltage drifting chloride equilibrium. To compensate Cl −  ions for the redox reactions and to isolate the electrode  40  from other interfering ions, we introduced a low-cost and rugged polymeric material  44  (polyvinyl butyral or PVB) with pore size ranging from 50-200 nm. PVB can hold Cl −  ions in its membrane form and supply Cl −  ions when needed. Unlike the conventional RE which contains a reference internal solution of KCl (3M) separated with Ag/AgCl wire, the S 3 RE electrode  40  is free from internal reference solution by replacing solid-state membrane with solid KCl crystals in order miniaturize the sensor system. 
     The S 3 RE was made on a silicon with grown oxide ( FIG. 12 c   ). The S 3 RE has three layers such as Ag/AgCl, PVB and Nafion. The 1 μm thick of silver was deposited on Si/SiO 2  needle and further treated with FeCl 3  (0.1M) plus HCl (0.001 mM) solution for 30 s to form Ag/AgCl on Ag/SiO 2 /Si surface and then dried at 100° C. for 2 hr. The formation of Ag/AgCl is shows in  FIG. 12 a   . The formation of Ag/AgCl layer (first layer) provides a constant half-cell potential by enabling a reverse-redox reaction in the nitrate sensor device. Another polymeric layer (thickness: 1.7 μm) made of polyvinyl butyral (PVB) containing KCl was deposited on the surface of AgCl layer (second layer).  FIG. 12 b    shows a cross-sectional scanning electron microscopic image for the formation of S 3 RE. To prepare this PVB solution, 395 mg of PVB and 250 mg of KCl salt were dissolved in 5 mL in tetrahydrofuran solvent (THF) or methanol and sonicated for 24 hrs and then deposited using a liquid auto-dispenser. The PVB stock solution was stored at 4° C. to avoid the evaporation. The PVB is useful for dispense casting as it is fully dissolvable material in THF solvent resulting in a uniform coating on the substrate without using any surfactants, an important advantage to develop uniform films without the need for using surfactants which may deteriorate the performance of the electrode. Finally, a perfluorinated polymer, Nafion (thickness ˜10-20 nm) was deposited on PVB layer using auto-dispenser machine. The role for incorporating the third layer of this reference electrode is to prevent chloride ions leaving resulting from an enormous redox reactions of Ag/AgCl layer and to prevent the potential drift at RE. In addition, this proton exchange membrane also avoids other anions to enter into the membrane. 
     To investigate the chloride sensitivity, we have measured potentials for the fabricated electrodes such as Ag/AgCl wire, Ag/AgCl on Si needle, Ag/AgCl+PVC+Nafion and Ag/AgCl+PVB+Nafion with respect to commercial RE by varying Cl −  concentration from 1 mM to 1M in DI water. Without coating of PVC, PVB and Nafion, the Ag/AgCl electrodes are highly sensitive to Cl −  ions ( FIG. 13 a   ). Interestingly, with PVB and Nafion coating on Ag/AgCl electrode, S 3 RE (Nafion/PVB/Ag/AgCl) do not show a potential change for different Cl −  concentration (1 mM to 1M Cl − ). However, the S 3 RE with PVC is showed a minute change of potential with different Cl −  ionic solution. The higher polarity of PVB compared to PVC makes it less prone to fouling the membrane. This result suggests that the stable potential with PVC may be due to the formation of nanopores on the surface, which supply and control the flow of Cl—. To investigate the long-term stability, the fabricated electrodes were conducted to measure voltage for 30 days continuously. The voltage measurements were performed in a chamber filled with 0.1 mM Cl −  solution with respect to commercial RE. Within 30 days of voltage measurements, the S 3 RE with PVC incorporating does not change its potential ( FIG. 13 b   ). However, with PVC and without coating the Ag/AgCl showed a significant drift of potential. This new S 3 RE can not only simplify the sensor structure by eliminating Cl −  solutions, and but also reduce a potential drift of the sensor in response to different ionic strengths and for long-term measurement. With the long-term stability testing results, it can be concluded that the Nafion acted as a protection layer for S 3 RE to prevent the leakage of Cl −  ions. 
     In our design of working electrode or WE  30  ( FIG. 14 d   ), an 80 nm-thick Au was coated on Si/SiO 2  needle. The POT-MoS 2 , a composite material (1 μm-thick) was deposited on the Au surface of the needle via liquid auto-dispenser machine and dried at room temperature. The POT-MoS 2  composite material was used to dissolve into THF solvent for a uniform coating on Au substrate. The main rationale for this composite material is boost the electrochemical signal (voltage) for the detection of nitrate ions and to act as ion-to-electron transfer in between Au and ion selective membrane (ISM). Finally, a ˜4-5 μm-thick nitrate ion selective membrane was coated using auto-dispensing machine and the ISM is covered the entire surface of POT-MoS 2  material. SEM images ( FIG. 14 a -4 c   ) show the morphological structure of MoS 2  sheets, porous structures of POT and full coverage of MoS 2  sheets by POT layers.  FIG. 14 d    shows the pictorial representation of all layers with SEM images and  FIG. 14 e    shows the cross-sectional of POT-MoS 2  layer. 
     Cyclic voltammetry studies of the fabricated sensors by incorporating different materials such as POT, MoS 2  and POT-MoS 2  showed the redox behavior in buffer solution ( FIG. 14 f   ). Oxidation current for MoS 2  electrode is obtained as higher (85 μA) compared to POT electrode (−28 μA). As we are interested in the voltage monitoring for our sensor system, when incorporated the MoS 2  into POT, the change of current is found to be slight (−5 μA) compared to POT electrode, however, the voltage shifted to higher value (85 mV). The inherent conducting feature of MoS 2  material is responsible to boost the voltage signal. A similar observation also found in  FIG. 14 g    wherein the voltage between two electrodes during nitrate sensing wherein the magnitude of potential for POT-MoS 2  electrode exhibited a maximum value of 325 mV compared other electrodes such as POT (255 mV) and MoS 2  (66 mV). 
       FIG. 15  shows the contact angle studies to investigate the amphiphilic property of POT and POT-MoS 2  surface. The POT-MoS 2  has a high lipophilicity, indicating this has high capability to repel water molecules. Thus, this POT-MoS 2  acts as an ion-to-electron transfer layer and prevent the formation of water layer. 
       FIG. 16 a -6 c    shows the output voltage of the nitrate sensor using the combined S 3 RE and POT-MoS 2  based electrodes when the sensor responded to varying nitrate-nitrogen (NO 3   − —N) concentrations from 1 ppm to 1500 ppm (NO 3   − —N). As the concentration increases, the output voltage of the sensor becomes more negative. The sensor with the S 3 RE and POT-MoS 2  exhibited a higher sensitivity, compared to the other two counterpart devices in which the POT or MoS 2  alone was combined with the ISM to form the WE, and the S 3 RE was used to provide a reference potential. 
       FIG. 16 d    shows the selectivity of the nitrate sensor and its comparison with the other two counterpart devices. It is shown that by incorporating the POT-MoS 2  nanocomposite in the working electrode, the sensor provided the highest selectivity in presence of the main interfering ions (SO 4   2− , PO 4   3− , Cl − , and K + ) in the soil and water as evident by its low relative voltage change of ˜13% compared to MoS 2  (˜48%) and POT (˜25%) electrodes. 
       FIG. 17  shows the repeatability (a) and long-term stability (b) test results for the fabricated sensor. In the repeatability test, the sensor was tested in presence of high and low concentration of nitrate-nitrogen solution. At high concentration of nitrate-nitrogen is found to be not stable with repeated time. This may be due to the water layer formation for the side-wall for the electrode. For long-term stability test of the sensor, it has found that the sensor has a potential drift in its output signal again due to the water layer formation at interface between ISM and POT-MoS2 or due to the leaching of Cl— ion at reference electrode. 
     XPS analysis: To confirm the chemical structures of POT-MoS 2  and ISM, XPS spectra were investigated. The XPS spectra shows the carbon is of MoS 2 , POT-MoS 2  and ISM/POT-MoS 2  films ( FIG. 18 a -8 c   ). After deconvolution into characteristic peaks, the C is peaks found to be at 284.9 eV, 285.9 and 289.5 eV indicating in the presence of C—C, C—OH and O—C═O groups. The presence of carbon may be due to the treatment of THF solution during their exfoliation. With incorporation of MoS 2  into POT solution, the peak found at 284.9 eV is changed to 0.4 eV with lower a full-width half maximum of 2.5 eV ( FIG. 18 b   ). After coating with ion-selective membrane (ISM), the peak location due to C—C is found to be at 284.3 eV ( FIG. 18 c   ). Another peak is obtained on the surface of ISM at 286.1 eV due to the C—O groups present in the membrane.  FIG. 18 d    shows XPS two S 2p peaks of MoS 2  due to 2p 1/2  and 2p 3/2  at 165.5 eV and 164.9 eV, respectively. For the composite material of POT-MoS 2 , S 2p peaks are found at 262.7 eV and 263.9 eV due to S*—Mo groups and C—S*—C groups [Ref: Wang et al., Polymer Journal, 38, 484-489, 2006] and the peak found at 269.2 eV is due to S—O groups. In the XPS peaks for Mo 3d, it shows a peak at 227.2 eV due to S 2s, and other peaks at 229.9 eV, 233.1 eV and 236.4 eV are due to the Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2  and Mo 6+ 3d 3/2  ( FIG. 18 g   ). However, additional two found at 233.1 eV and 231 eV are because of Mo 6+ 3d 5/2  and Mo 5+ 3d after incorporating of POT ( FIG. 18 h   ). These studies can confirm that for the composite material between POT and MoS 2  due to their electrostatic interactions. N is peaks in the XPS spectra of ISM on POT-MoS 2  are obtained at 402.7 eV and 408.4 eV indicate —NH 2  and nitrooxy (—N—NO 2 ) groups due to presence of nitrocellulose in the membrane. 
     Electrochemical Characterization: Cyclic voltammetry studies of the fabricated sensors by incorporating different materials such as POT, MoS 2  and POT-MoS 2  showed the redox behavior in phosphate buffered saline (PBS) solution ( FIG. 19 a   ) containing ferro-ferri cyanide redox probe. Oxidation current for MoS 2  electrode is obtained as higher (91 μA) compared to POT electrode (32 μA). With composite, the oxidation current is increased to 47 μA due to high conductive nature of MoS 2  sheets. As we are interested in the voltage monitoring for our sensor system, when incorporated the MoS 2  into POT, the change of current is found to be slight (15 μA) compared to POT electrode, however, the voltage shifted to higher value (95 mV). Thus, inherent conducting feature of MoS 2  material is responsible to boost the voltage signal which may result higher sensing ability of the device. All the three electrode were conducted to investigate the redox properties by varying the scan rate from 20 to 200 mV/s ( FIG. 19 b -9 c   ). The electrodes showed the surface-controlled process, quasi-reversible and the potential differences due to oxidation and reduction of redox molecules are found to shift towards higher values with increase scan rates ( FIG. 19 d   ). The lower peak-to-peak potential difference of the POT-MoS 2  electrode is found to less compare to bare MoS 2  electrode due to fast electron transfer between the solution and electrode ( FIG. 19 e   ). With ion-selective membrane coating on POT surface, the electrodes do not show the redox reaction of ferro-ferry cyanide molecules due to non-conducting behavior of ion-selective membrane ( FIG. 19 f   ). 
     E. Specific Example 2 
     With particular reference to  FIGS. 20 to 29A -C, a second example of apparatus, methods, and systems according to the invention is set forth. This is taken from Md. Azahar Ali, Xinran Wang, Yuncong Chen, Yueyi Jiao, Navreet K. Mahal, Satyanarayana Mom, Michael J. Castellano, James C. Schnable, Patrick S. Schnable, and Liang Dong, ACS Appl. Mater. Interfaces 2019, 11, 29195-29206, which is incorporated by reference herein in its entirety, including Supporting Information available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07120.  FIG. 20  shows the basic concepts, as further described below. 
     Continuous Monitoring of Soil Nitrate Using a Miniature Sensor with Poly(3-Octyl-Thiophene) and Molybdenum Disulfide Nanocomposite 
     ABSTRACT: There is an unmet need for improved fertilizer management in agriculture. Continuous monitoring of soil nitrate would address this need. This paper reports an all-solid-state miniature potentiometric soil sensor that works in direct contact with soils to monitor nitrate-nitrogen (NO 3   − —N) in soil solution with parts-per-million (ppm) resolution. A working electrode is formed from a novel nanocomposite of poly(3-octyl-thiophene) and molybdenum disulfide (POT-MoS 2 ) coated on a patterned Au electrode and covered with a nitrate-selective membrane using a robotic dispenser. The POT-MoS 2  layer acts as an ion-to-electron transducing layer with high hydrophobicity and redox properties. The modification of the POT chain with MoS 2  increases both conductivity and anion exchange, while minimizing the formation of a thin water layer at the interface between the Au electrode and the ion-selective membrane, which is notorious for solid-state potentiometric ion sensors. Therefore, the use of POT-MoS 2  results in an improved sensitivity and selectivity of the working electrode. The reference electrode comprises a screen-printed silver/silver chloride (Ag/AgCl) electrode covered by a protonated Nafion layer to prevent chloride (Cl − ) leaching in long-term measurements. This sensor was calibrated using both standard and extracted soil solutions, exhibiting a dynamic range that includes all concentrations relevant for agricultural applications (1-1500 ppm NO 3   − —N). With the POT-MoS 2  nanocomposite, the sensor offers a sensitivity of 64 mV/decade for nitrate detection, compared to 48 mV/decade for POT and 38 mV/decade for MoS 2 . The sensor was embedded into soil slurries where it accurately monitored nitrate for a duration of 27 days. 
     Introduction 
     Low-cost, high-performance nutrient sensors that continuously monitor soil conditions for precision agriculture, 1,2  plantphenotyping, 3  and environmental quality 2  are in high demand. Soil is the primary source of nutrients for plant growth. 4-7  Biologically available soil nitrogen (N) is one of the key limiting factors in plant growth, and crop productivity relies heavily on the application of supplemental N in the form of fertilizers. Yet, the proper amount of N fertilizer input can vary within fields by &gt;100% per year because of the variation in the soil N supply that is mostly caused by interannual weather variability. Insufficient N fertilizer input reduces crop production and excessive N input harms the environment. Farmer income suffers from both. 
     Continuous monitoring of N dynamics in agricultural fields would help maximize control over fertilizer management. Several laboratory-based soil N measurement methods are widely used, such as gas chromatography-mass spectrometry (GC-MS), ultraviolet-visible (UV-vis) spectrophotometry, ion chromatography, and chemiluminescence. 8-12  Although these methods are highly sensitive and selective and exhibit superior performance, they are known to have instrumentation complexity and need laborious and time-consuming tasks. Colorimetric determination of nitrate relies on the reduction of nitrate by vanadium(III), combined with detection by the Griess reaction, and needs extraction of nitrate ions from soil samples using a high-concentration (e.g., 2 M) KCl solution, which limits its practical operation in fields. 7  With an increasing demand for on-site nitrate monitoring, mobile vehicle-based nitrate sensors 13  have been reported but still require significant labor and are relatively expensive. Satellite remote sensing 14  provides an indirect measure of plant N dynamics and does not currently provide high accuracy or spatial resolution. The development of field-deployable soil nitrate sensors is an attractive solution to better manage N fertilizers. Noteworthy in-field nitrate sensing methods include electrochemical sensors, 5,6,15  ion-selective electrodes (ISEs), 16,17  and microfluidic electrophoresis. 18  However, these miniature sensor methods need further development or remain challenging mainly because of the suboptimal sensitivity, relatively high signal drift, and material instability. 19    
     Ion-selective membrane (ISM)-based sensors are considered a promising approach to detecting soil nutrients. Many ISEs are manufactured by simply coating thin metal wires with ISMs. However, redox-active charged species are difficult to be transferred to metal wires, leading to a capacitive interface with the wire. 20  Conversely, nonmetal wire-based ISEs often require an inner filling solution between the ISM and a conductive metal layer substrate; 21-25  the main drawbacks, however, include easy contamination of the filling solution with interfering ions, gradual evaporation of the solution, variations in both osmolality and ionic strength, membrane delamination, poor adhesion, and difficulty in device miniaturization. 23,26    
     Although ISEs that do not use inner filling solutions are an attractive option, a thin water layer that often forms at the interface between the conducting metal layer and the ISM has created a major challenge to the development of these sensors. Usually, this thin water layer presents an interfacial barrier to fast electron transfer and negatively impacts the selectivity of the sensor to specific ions because different ions are trapped inside 4. 27,28  Therefore, significant attempts have been made to replace the inner filling solutions with solid-contact materials as ion-to-electron transducing layers, with the objective of realizing an all-solid-state miniature ionsensor. 29-33  Many solid-contact candidate materials have been investigated, including hydrogel, 34  carbon nanotubes (CNTs), 35,36  graphene, 32  polymer-carbon composites, 37  metallic nanostructures, 38  macroporous carbon, 39  and conjugated conducting polymers such as polyaniline, 19  poly(3,4-ethylenedioxythiophene) (PEDOT), 40  and poly(3-octylthiophene-2,5-diyl) (POT). 27,41,42  Of these, PEDOT has a strong ability to oxidize to PEDOT +  and thus has been extensively used as a solid-contact material  30  to attract lipophilic ions from the ISM to the conducting metal layer to establish a potential equilibrium. As another promising candidate, electropolymerized, 41,42  drop-casted, 43  and Langmuir-Blodgett 44  POT is redox-sensitive and can be oxidized reversibly in anion solutes with a low ohmic voltage drop; in addition, the high hydrophobicity of POT restricts the formation of a water layer between POT and the ISM. Recently, the incorporation of 7,7,8,8-tetracyanoquinodimethane (TCNQ) into a POT matrix contributed to reducing the potential drift by more than one order of magnitude because of the introduction of a TCNQ/TCNQ −  redox couple. 45  Despite its high redoxproperty,41 POT has a relatively low conductivity (approximately 10 −6  S/cm) 28  and is also sensitive to light, 36  which negatively impacts the efficiency of charge transport through POT to the conducting metal substrates. 
     Here, we report a miniature solid-state potentiometric sensor for the continuous monitoring of soil nitrate. The sensor uses a nanocomposite of POT and transition-metal dichalcogenides of molybdenum disulfide (MoS 2 ) nanosheets 46  as a solid-contact ion-to-electron transducing layer. MoS 2  nanosheets provide large surface area, high conductivity, 47  insensitivity to light and pH, and absence of any side reactions. The working electrode (WE) was built on top of a copper pad of a printed circuit board (PCB) covered by a thin, patterned gold (Au) layer, a MoS 2  (POT-MoS 2 ) nanocomposite-based solid-contact layer, and a nitrate-specific ISM. The incorporation of MoS 2  into POT not only increases the redox properties of POT 48  but also maintains high hydrophobicity to minimize the formation of a thin water layer between the ISM and the Au layers. The use of POT-MoS 2  remedies the issue of the trapped water layer, thus contributing to the increased charge transfer and ion selectivity of the WE. 49-51  The reference electrode (RE) of this nitrate sensor includes a silver/silver chloride (Ag/AgCl) electrode covered by a proton exchange membrane to reduce the redox reaction induced chloride leaching from the RE, thus minimizing the drift of the reference potential. The sensor features an all-solidstate design that incorporates the POT-MoS 2  nanocomposite for improved device performance. The sensor can also be directly embedded in soil slurries for continuous measurement of nitrate dynamics for approximately 4 weeks. Furthermore, all the sensor materials (except for the screen-printed Ag/AgCl and evaporated Au) are deposited and patterned using a high resolution dispensing robot with good control over the uniformity of material thickness. 
     Materials, Manufacturing, and Circuits 
     Materials. Methyltriphenylphosphonium bromide, polyvinylchloride, Nafion, nitrocellulose, 2-nitrophenyl octyl ether, tetrahydrofuran (THF), and tridodecylmethylammonium nitrate were purchased from Sigma-Aldrich, MO. Polyvinyl butyral, regioregular POT, and Ag/AgCl ink (composed of finely dispersed chloritized silver flakes) were obtained from Fisher Scientific, MA. Ultrafine powders of MoS 2  nanosheets were obtained from Graphene Supermarket, NY. Deionized water with a resistivity of 18.2 M Ω cm was obtained using a purification system from Millipore, Mass. Potassium nitrate (KNO 3 ), calcium sulfate (CaSO 4 ), sodium chloride (NaCl), sodium bicarbonate (NaHCO 3 ), and sodium phosphate monobasic (NaH 2 PO 4 ) were also obtained from Fisher Scientific, MA. The PCB was manufactured by OHS PARK, OR. 
     The NO 3   −  ISM cocktail contained methyltriphenylphosphoniumbromide (0.25 wt %), nitrocellulose (moistened with 2-propanol (35%); 1.93 wt %), 2-nitrophenyl octyl ether (16.25 wt %), polyvinylchloride (5.75 wt %), THF (74.3 wt %), and tridodecylmethylammonium nitrate (1.50 wt %). This solution was sealed and stored at −20° C. 52    
     Nanocomposites of POT-MoS 2 . The weight ratio of POT to MoS 2  was varied from 1:1 to 1:10 to study the influence of material composition on the redox properties of different POT-MoS 2  nanocomposites. In each case, the concentration of the POT solution was fixed at 2.6 mg/mL. For example, to prepare a POT-MoS 2  sample with a 1:4 weight ratio of POT to MoS 2 , 2.6 mg of POT powder was dissolved in 1 mL of THF solvent. A 10.4 mg MoS 2  was added to the POT solution and sonicated for 4 h. Because of the attraction between the opposite charges of MoS 2  and POT, a homogeneous solution of POT-MoS 2  nanocomposite was formed. 
     Electronic Circuitry. A homemade data logger with an embedded readout circuitry was used to detect and record potential variations between the WE and RE. The voltage potential provided by the sensor was first isolated from other parts of the readout circuit using two buffer amplifiers. Then, the output signal from the buffer amplifiers was fed to a differential amplifier to obtain a single output voltage, which could be further enhanced fivefold by using an inverting amplifier. Further, a voltage lifter circuit was introduced to obtain both negative and positive data from the sensor using a microcontroller. A two-order filter with 1 Hz cutoff frequency was then used to reduce the noise signal at the output. Finally, an Adafruit Feather 32u4 microcontroller was used to realize the analogue-to digital signal conversion. 
     Device Fabrication. The sensor had two 5 mm diameter, round shaped electrodes formed on the PCB that served as WE  30  and RE  40 . The rectangular pads on the PCB allowed for connecting the WE and RE to an external data logger. The base material of the WE and RE was copper. With the help of a shadow mask, a 5.2 mm diameter and 100 nm thick Au layer was deposited on top of one of the base electrodes using electron beam evaporation. The same approach was used to form a 5.2 mm diameter and 500 nm thick Ag layer on top of the other base electrode.  FIG. 21 a    shows a wafer-scale PCB containing arrays of RE  40  and WE  30 . To form the POT-MoS 2  nanocomposite and nitrate-selective ISM layers, a high-precision, automated fluid dispensing robot (Nordson EFD, RI) was used to dispense the prepared POT-MoS 2  and ISM solutions, respectively, on top of the Au surface ( FIG. 21 c   ). During this process, the POT-MoS 2  solution was first dispensed out of a syringe (size 10 cc) under an air pressure of 2 psi, followed by thermal treatment on a hot plate at 65° C. for 1 h. After the ISM solution was dispensed, the WE was dried at room temperature for 10 h. The same material coating technique was applied to make other WEs using POT or MoS 2  alone as the solid contact ion-to-electron transducing layer for comparison with the proposed WE with the POT-MoS 2  nanocomposite. To form the RE of the sensor, the round-shaped Ag electrode was further screen printed with Ag/AgCl paste using a stencil mask placed on top of the PCB. The 200 μm thick Ag/AgCl paste was dried at 110° C. for 2 h. To prevent the leaching of chloride ions as a result of the redox reaction of Ag/AgCl during long-term measurements, 53  a 15 nm thick perfluorinated polymer layer, or Nafion, was coated on the surface of Ag/AgCl using the above-mentioned fluid-dispensing robot and was then dried at 90° C. for 1 h. In addition, the Nafion layer could also block anions entering the RE from the surrounding environment. Finally, a 1.2 mm thick waterproof insulating epoxy (CircuitWorks, CW2500) was used to cover the PCB, except for the regions of the WE, RE, and contact pads. This insulation layer impedes water penetration from the sidewalls of the coated materials when the sensor is embedded in soil slurries. The sensor was preconditioned by dipping it into 1500 ppm NO −   3 —N solution for 24 h.  FIG. 21 c    shows the fabricated solid-state nitrate sensor. 
     Working Principle. As the ion-to-electron transducing layer, the POT-MoS 2  nanocomposite layer undergoes a redox reaction during sensing. The mechanism of anion (or cation) exchange through POT is demonstrated in the previously reported literature. 27  Si and Bakker demonstrated a cyclic voltammetric experiment for the anion (lipophilic) exchange process in a POT electrode-based ion-selective membrane. 27  Kim and Amemiya also explained the anion exchange in a POT film coated with ISM using ion-transfer stripping voltammetry. 42    FIG. 22  shows the oxidation and reduction associated with the sensing mechanism. The mechanism for extracting electrochemically mediated anions (NO 3   − ) into the ISM involves three phases, 27  including (1) oxidizing POT-MoS 2  (or P) to (POT-MoS) +  (or P + ); (2) triggering the extraction of NO 3   −  from the test sample; and (3) redistributing lipophilic anions (R − ) from the ISM to the POT-MoS 2  layer. The corresponding redox reaction accompanied by NO 3   −  transfer at the ISM is given by
 
P (p)   +   +e   − +R − ( p )+NO 3   − ( m ) P( p )+R (m)   − +NO 3   −   (aq)   (1)
 
where m, p, and aq represent the ISM phase, POT-MoS 2  nanocomposite phase, and aqueous phase, respectively, and P (p)  and P (p)   +  represent a few monomeric units of the POT chain in the neutral insulating state and the oxidized state with polaronic sites, respectively. Owing to the oxidation process ( FIG. 22 a   ), POT-MoS 2  extracts the sample anions NO 3   −   (aq)  into the ISM and forces their distribution of the lipophilic anions (R − ) into the POT-MoS 2  layer. In the reduction process ( FIG. 22 b   ), (POT-MoS 2 )+becomes neutral POT-MoS 2 , releasing the lipophilic anions R −  (p) into the ISM, which in turn leads to a release of NO 3   −   (aq)  from the outer membrane (NO 3   −   (m) ) into the test solution. Therefore, by combining the redox and ion-exchange processes at the WE, an equilibrium is established at the aqueous—nanocomposite—ISM interfaces, leading to charge separation at each interface, thus generating a phase boundary potential.12 This phase-boundary potential E 1  is given by E 1 =(RT/zF)×ln a I , where R, T, z, F, and a I  represent the gas constant, temperature, charge of the target ion, the Faraday constant, and the primary ion activity without interfering ions, respectively. On the other hand, the RE of the sensor also undergoes a redox reaction, providing a constant potential (E 0 ). 54  The Nafion layer coated on the surface of Ag/AgCl not only minimizes the leaching of the chloride ion from Ag/AgCl but also blocks the other anions in the external environment from entering the RE. As the anions or cations move from high to low regions of concentration, a potential difference is produced during the ion exchange. Therefore, the potential (E) is dependent on the logarithm of the ion activity and is described by the Nernst equation 55  
 
E=E 0 +E 1 =E 0 +(RT/zF)ln  a   I   (2)
 
     To determine the ion selectivity of the sensor, according to Nikolskii-Eisenman formalism, 56  the logarithm term in eq 2 can be replaced by a sum of selectivity-weighted activities given by
 
E=E 0 +(RT/nF)ln( a   I +K I   p   J   a   J   Z     I     /Z     J   )  (3)
 
     where K I   p   J  is the selectivity coefficient, a I  and a J  are the activities of I and J, respectively, in the test solution, and Z I  and Z J  are the charges of the primary and interfering ions, respectively. 
     Results and Discussion 
     Surface Morphology and Water-Repellent Properties.  FIG. 23  shows the scanning electron microscopy (SEM) images for different ion-to-electron transducing layers formed on the Au surface, including MoS 2 , POT, and POT-MoS 2  nanocomposites. The MoS 2  layer is seen as a mixture of MoS 2  sheets of different sizes ( FIG. 23 a   ). The POT film exhibits continuous distribution and microtexture ( FIG. 23 b   ). In the POT-MoS 2  nanocomposite, MoS 2  sheets are embedded with POT because of the electrostatic interactions between them ( FIG. 23 c,d   ). In addition,  FIG. 23 e - g    shows the measured water contact angles of the MoS 2  (Θ=68°), POT (Θ=86°), and POT-MoS 2  (Θ=107°) surfaces. With MoS 2 , the nanocomposite remains hydrophobic, which may contribute to minimizing the formation of a thin water layer between the ISM and Au layers. 
     XPS Analysis. X-ray photoelectron spectroscopy (XPS) was conducted to confirm the chemical structures of MoS 2 , POT-MoS 2 , and ISM/POT-MoS 2 .  FIG. 24 a - c    shows the carbon is spectra of the MoS 2 , POT-MoS 2 , and ISM/POT-MoS 2  layers coated on the Au surface. After deconvolution into characteristic peaks, the C is peaks of MoS 2  are found at 284.9, 285.9, and 289.5 eV, indicating the presence of C—C, C—OH, and O—C══O groups, respectively. 57  The presence of carbon may be because of the impurity of the MoS 2  sheets. The incorporation of MoS 2  into the POT matrix leads to a shift in the peak location from 284.9 to 285.3 eV with a full width half maximum of 2.5 eV ( FIG. 24 b   ), perhaps because of the POT hydrocarbons. A peak at 285.8 eV can be ascribed to the C—S bond, indicating the formation of a strong chemical bonding at the interface between MoS 2  and POT. After the ISM was coated on the POT-MoS 2  layer, the peak for the C—C bond was found to be at 284.3 eV ( FIG. 24 c   ). Another peak at 286.1 eV was obtained on the surface of ISM because of the C—O group present in the ISM. 
       FIG. 24 d    shows the MoS 2  layer with two S 2p core-level peaks of MoS 2  at the binding energies of 165.5 and 164.9 eV, corresponding to the S 2p 1/2  and S 2p 3/2  orbitals of divalent sulfide ions (S 2− ). In  FIG. 24 e   , two S 2p peaks appear at 162.7 and 163.9 eV because of the formation of S*—Mo and C—S*—C groups, respectively, 58  indicating the incorporation of POT into MoS 2 , and another peak found at 169.2 eV is associated with S in sulfone. Furthermore, the S peaks were observed to shift toward higher energies of 1.6 and 1.2 eV because of the ISM coating on the POT-MoS 2  film ( FIG. 24 f   ). 
     In the Mo 3d spectrum of MoS 2 , a peak at 227.2 eV corresponds to S 2s with a chemical state of S 2 , whereas other peaks at 229.9, 233.1, and 236.4 eV are ascribed to Mo 4+ 3d 5/2′ Mo 4+ 3d 3/2 , and Mo 6+ 3d 3/2 , respectively ( FIG. 24 g   ). For POT-MoS 2  ( FIG. 24 h   ), two additional peaks appear at 233.1 and 231 eV because of Mo 6+ 3d 5/2  and Mo 5+ 3d, respectively. In the N is spectrum of ISM/POT-MoS 2  ( FIG. 24 i   ), the peaks seen at 402.7 and 408.4 eV correspond to the —NH 2  and nitrooxy (—N—NO 2 ) groups because of the presence of nitrocellulose in the ISM. Therefore, the formation of a composite between POT and MoS 2  because of the appearance of the chemical C—S bond is confirmed. Further, the presence of the —NH 2  and nitrooxy (—N—NO 2 ) groups at ISM/POT-MoS 2  indicates the ISM coating on the surface of the POT-MoS 2  matrix. 
     Electrochemical Characterizations. Cyclic voltammetry (CV) was conducted at room temperate to investigate the redox properties of the MoS 2 , POT, POT-MoS 2 , and ISM/POT-MoS 2  layers coated on the Au electrodes ( FIG. 25 a   ), and the POT to MoS 2  ratio of the composite was set to 1:4. The cyclic voltammograms for the MoS 2 -, POT-, and POT-MoS 2 -based electrodes exhibited clear reversible oxidation and reduction reactions for the [Fe(CN) 6 ] 3−/4−  redox probes. The oxidation current for the POT-MoS 2 -based electrode was higher (115 μA) than that for the MoS 2 -based electrode (90 μA) and the POT-based electrode (65 μA) because the incorporation of high-conductivity MoS 2  facilitates an improved electron transfer from POT-MoS 2  to the Au current conductor. Also, the values of peak-to-peak potential difference (4E) for the POT- and POT-MoS 2 -based electrodes were found to be 0.127 and 0.38 V, respectively. After modification with the ISM, the POT-MoS 2 -based electrode exhibited reduced oxidation and reduction peaks for the [Fe(CN) 6 ] 3−/4−  redox probes, perhaps because of sluggish ion exchanges or a high selectivity of ISM that rejected[Fe(CN) 6 ] 3−/4−  ions (inset of  FIG. 25 a   ). 
     To optimize the weight ratio of POT to MoS 2  in composite formation, CV measurements were taken for composites at varying weight ratios with the objective of obtaining the composite that offered the largest value of ΔE.  FIG. 25 b    shows that as the weight ratio of POT to MoS 2  changes from 1:1 to 1:10, the obtained ΔE increases at lower weight ratios, reaches a maximum ΔE=0.345 V at a 1:4 weight ratio, and then deceases at higher weight ratios. Further,  FIG. 25 c    shows that the oxidation current decreases with increasing POT-to-MoS 2  weight ratios from 1:1 to 1:3 because of a reduction in the free POT in the POT-MoS 2  matrix. At a weight ratio between 1:4 and 1:6, the oxidation current is observed to be relatively stable at a low value because of the full bond formation. With a further increase in the MoS 2  component, the free MoS 2  in the matrix prompts the oxidation current because of the inherent electroactivity of MoS 2  ( FIG. 25 c   ). Therefore, for potentiometric measurements, the optimum POT-to-MoS 2  weight ratio was chosen to be 1:4. 
       FIG. 25 d    presents the redox activity studies of the POT-MoS 2 -based electrode (POT-to-MoS 2  weight ratio, 1:4). The POT-MoS 2 -based electrode shows a good redox behavior for the oxidation and reduction of ferro-/ferricyanide redox species. The difference between the oxidation and reduction potentials is found to increase with an increase in the scan rate. The peak current is proportional to the square root of the scan rate (inset of  FIG. 25 d   ), indicating a diffusion-controlled process on this redox-sensitive material. 
     For open-circuit potential (OCP) measurements, the MoS 2 , POT, and POT-MoS 2  layers were coated with nitrate-specific ISM.  FIG. 25 e    shows the output voltage signals of the fabricated sensors in response to 1000 ppm NO 3   − —N. The magnitude of the potential for the POT-MoS 2 -based electrode exhibits a maximum value of 325 mV, higher than the counterpart electrodes using POT (255 mV) and MoS 2  (66 mV). As is evident in the CV studies ( FIG. 25 a   ), compared to POT alone, the POT-MoS 2  nanocomposite offers a better redox property and functions as a good electroactive mediator to allow selective interaction with NO 3 — ions in the surrounding solutions ( FIG. 23 a,b   ), thus providing an increased OCP. 
     The potential stability of the electrodes and the electrical capacitance of the solid contact were evaluated using chronopotentiometry 19  ( FIG. 26 f   ). The characteristic chronopotentiometric curves present the change in potential overtime measured in a 500 ppm NO 3   − —N solution. The obtained results are shown in  FIG. 26 . The potential drift of electrode was calculated as ΔE/Δt. The ΔE/Δt values for the POT-, MoS 2 -, and POT-MoS 2 -based nitrate-selective electrodes were found to be 115.4, 213.3, and 95 μV s −1 , respectively. Similarly, the low-frequency capacitances C of the POT-, MoS 2 -, and POT-MoS 2 -based electrodes were estimated to be 433, 234, and 526 μF, respectively, according to the equation ΔE/Δt=I/C. These results indicate that the POT-MoS 2 -based electrode has a larger capacitance and a lower potential drift compared to the electrode using POT or MoS 2  (see, infra, Table S1, Supporting Information). 
     Quantification of Nitrate-Nitrogen. Nitrate detection by the sensors using MoS 2 , POT, and POT-MoS 2  as the solid contact ion-to-electron transfer layer materials was investigated.  FIG. 26 a    shows the calibration curves, that is, the OCP values of the sensors as a function of nitrate concentration ranging from 1 to 1500 ppm (NO 3   − —N). The slope of the voltage response versus logarithm concentration for the POT-MoS 2 -based sensor is 64 mV/decade (10-1500 ppm), which is higher than that of POT (approximately 48 mV/decade, 10-1500 ppm) and MoS 2  (approximately 38 mV/decade, 10-1500 ppm). The high electroactivity and redox property of the POT-MoS 2  layer is believed to contribute to improved sensitivity in nitrate detection. In addition, the high hydrophobicity of the POT-MoS 2  layer could minimize water accumulation between the ISM and the Au current collector, lowering the barrier of charge transfer to the Au layer and thus improving the sensor sensitivity. Although the MoS 2 -based sensor also provides a wide dynamic range up to 1000 ppm (NO 3   − —N), the output voltage was found to be unstable (particularly during the detection of high nitrate concentrations), possibly because of poor adhesion of the MoS 2  layer to the ISM layer, leading to membrane delamination. Following the method described by Buck and Lindner, 59  we calculated the limit of detection (LOD) as 0.84, 1.3, and 1.4 ppm for the three sensors using MoS 2 , POT, and POT-MoS 2 , respectively, according to the obtained calibration plots ( FIG. 26 a   ). Table 1 below compares nitrate monitoring using different nanostructured materials. The laboratory-based nitrate measurement methods based on Griess assay, UV-Vis spectrophotometry, GC-MS, and chemiluminescence for nitrate monitoring in different media showed higher performance in terms of their LOD compared to the POT-MoS 2 -based sensor. However, our sensor can perform long-term measurements, exhibit a wider detection range, and have considerable performances suitable for field applications. 8-12  In addition, our sensor uses an integrated solid-state RE, thus offering the possibility of miniaturization and mass production, whereas the above-mentioned counterpart sensors require commercial large-sized REs. The sensor is used in direct and long-term contact with soil particles across a range of wetness for nitrate quantification. 
     Selectivity, Repeatability, and Stability Studies.  FIG. 26 b    shows the selectivity of the sensors using MoS 2 , POT, and POT-MoS 2  as the ion-to-electron transducing layers in the presence of interfering anions such as chloride (Cl − ), phosphate (PO 4   3− ), bicarbonate (HCO 3   − ), sulfate (SO 4   2− ), and nitrite (NO 2   − ). The selectivity coefficient, K I   p   J , described in eq 4, is a numerical measure of how adequately the sensor is able to discriminate against the interfering ions.
 
K I   p   J   =a   I /( a   J   Z     I     /Z     J   )  (4)
 
where a I , a J , Z I , and Z J  are the activity of primary ions, activity of interfering ions, charge of the primary ions, and charge of the interfering ions, respectively. According to IUPAC recommendations, a matched potential method, including the separate solution method (SSM), is practical and unique for estimating K I   p   J , which does not depend on the Nikolskii-Eisenman equation. 70,71  In the SSM method, the potential of the sensor is adjusted by introducing two different concentration solutions separately, wherein one contains the ion I with activity a I  (no J) and the other one contains the ion J with the same activity a J  (no I) to attain the same measured potential. To calculate the value of K I   p   J , a I  was calculated from the extrapolated calibration graph where the potential of the interfering ion concentration (a J ) is equal. The result demonstrates that the POT-MoS 2 -based sensor shows less susceptibility to PO 4   3−  and SO 4   2−  than the sensor using POT or MoS 2  alone as the transducing layer, perhaps because of the improved hydrophobicity of the POT-MoS 2  layer, whereas the influence of HCO 3   −  and Cl −  on the output potential is comparable among all the sensors. For NO 2   − , the sensors based on POT or MoS 2  showed more negative selectivity coefficients compared to the POT-MoS 2 -based sensor.
 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of NO 3 —-N Monitoring Using Different  
               
               
                 Nanomaterials and Techniques 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 electrode materials 
                   
                 test range  
                 detection 
                 sensitivity 
                 test period and 
                   
               
               
                 or transducers 
                 methods 
                 (ppm) 
                 limit (ppm) 
                 (mV/dec) 
                 environment 
                 refs 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 CNTs 
                 OCP 
                 0.14 × 10 −3   
                 0.0014 
                 58.9 
                 NA 
                 60 
               
               
                   
                   
                 to 14.02 
                   
                   
                   
                   
               
               
                 polypyrrole 
                 OCP 
                  0.14-1400.6 
                 0.42 
                 53.9 
                 7 d in water 
                 61 
               
               
                 graphene 
                 OCP 
                  0.14-1400.6 
                 0.3 
                 54.8 ± 2.5 
                 not tested 
                 62 
               
               
                 polypyrrole 
                 OCP 
                  1.4-56.1 
                 1.68 
                 51.6 
                 not tested 
                 63 
               
               
                 POT 
                 OCP 
                  0.14-1400.6 
                 NA 
                 53 
                 ~90 d in water 
                 64 
               
               
                 poly(aniline) 
                 OCP 
                  0.14-1400.6 
                 NA 
                 51.5 
                 ~90 d in water 
                 64 
               
               
                 PEDOT 
                 OCP 
                 0.011-63.34 
                 0.25 
                 NA 
                 NA 
                 65 
               
               
                 ionic liquid 
                 OCP 
                 0.044-442.8 
                 0.012 
                 60.1 
                 NA 
                 66 
               
               
                 graphene- 
                 OCP 
                 0.004-442.8 
                 ~0.004 
                 59.14 
                 NA 
                 67 
               
               
                 tetrathiafulvalene 
                   
                   
                   
                   
                   
                   
               
               
                 carbon black 
                 OCP 
                 0.044-442.8 
                 0.1 
                 60 
                 NA 
                 68 
               
               
                 tetrathiafulvalene 
                 OCP 
                 0.044-442.8 
                 0.01 
                 58.8 
                 NA 
                 69 
               
               
                 spectroscopic 
                 VCl3/Griess 
                 0.02-5   
                 0.016 
                 NA 
                 NA 
                 8 
               
               
                 optical 
                 Greiss 
                   0-9.3 
                 0.027 
                 NA 
                 sea water 
                 9 
               
               
                 optical 
                 V 
                 0.3-3.1 
                 0.007 
                 NA 
                 waste water 
                 10 
               
               
                 optical 
                 chemi- 
                 0.001-0.9  
                 0.001 
                 NA 
                 atmospheric 
                 11 
               
               
                   
                 luminescence 
                   
                   
                   
                   
                   
               
               
                 gas chromatography 
                 nitration 
                 0.062-6.2  
                 0.1 
                 NA 
                 ~3 d in 
                 12 
               
               
                 POT —MoS2 
                 OCP 
                   1-1500 
                 1.3 
                 64 
                 25 d in soil 
                 this 
               
               
                   
                   
                   
                   
                   
                   
                 work 
               
               
                   
               
            
           
         
       
     
       FIG. 26 c    shows the stability of fabricated Nafion-modified Ag/AgCl RE with respect to commercial RE by varying the KCl concentration from 0.01 to 3 M KCl. Without the Nafion coating, the Ag/AgCl electrode shows a stable OCP with 0.01 and 0.05 M of KCl concentration; however, with a higher concentration of KCl, such as 1 and 3 M, the electrode shows a significant potential change. This change of potential is due to the considerable electrochemical reaction in the AgCl layer, which may leach Cl −  ions from the AgCl layer, resulting in an unstable OCP. With increasing KCl concentration, the Nafion modified Ag/AgCl electrode does not show a change in OCP. The pronated Nafion layer on the Ag/AgCl surface acts as a protective layer that does not allow Cl −  ions to leach out and rejects Cl −  from outside the Nafion.  FIG. 26 d    shows the long term stability (approximately 32 days) of the fabricated solidstate Ag/AgCl electrode with and without a Nafion layer in the presence of 0.01 M of KCl. With no Nafion coating on the Ag/AgCl surface, the OCP was not constant in long-term measurements because of Cl −  leaching. However, blocking the Ag/AgCl surface with Nafion resulted in an almost constant OCP for 32 days with a minimum drift. This indicates that Nafion-coated Ag/AgCl is not externally influenced by Cl −  ions and is more stable for long-term measurements. 
     To investigate the repeatability of the sensor, we repeatedly measured OCP as the sensor was transferred between 1 and 1300 ppm NO 3   − —N( FIG. 29C , Supporting Information). For 12 repeated measurements, the sensor was dipped in a high nitrate-nitrogen (1300 ppm) concentration for 2 min, and the OCP was recorded. Then, the sensor was immediately dipped in a low concentration of nitrate-nitrogen (1 ppm) and then washed with DI water for another 2 min, and the OCP was recorded. The sensor responded in less than 5 s when switching from a high to low concentration, or vice versa. With the high concentration of NO 3   − —N(1300 ppm), the percentage of relative standard deviation for the output voltage was calculated as ±3.0%, whereas with the low concentration, the sensor showed a deviation of ±5.0% over six repeated measurements. This result indicates high repeatability of the test. 
     For the interference study in the presence of CO 2 , the POT-MoS 2 -based sensor was tested in a closed chamber with a controlled CO 2  environment ( FIG. 26 e   ). Before the measurement, the CO 2  gas (saturated) was injected into a nitrate solution for 15 min to ensure satirized dissolution of CO 2  in the solution. The test result shows that the introduction of CO 2  into the solution led to a ±5% relative deviation from the initial signal of the sensor. This may be caused by a pH change induced by the dissolved CO 2  in the solution. Also, we found that the introduction of N 2  into the solution had almost no influence on the sensor readout. Nevertheless, the sensor exhibited a good potential stability in the CO 2  environment. 
     We studied the reproducibility of the POT-MoS 2 -based nitrate electrode ( FIG. 29A , Supporting Information). The concentration of NO 3   − —N was set to 100 ppm in DI water, and the measurement was performed for 2 min for each POT-MoS 2  electrode. The results show that the variation in potential among these electrodes is negligibly small, as evident by its low relative standard deviation (RSD=˜3.5%) because of the uniform coating of the electrode materials (i.e., POT-MoS 2  and ISM) using a high-resolution robotic dispensing machine. 
     We carried out potential stability measurements for the POT-MoS 2 -based nitrate sensor over ˜10 days ( FIG. 29B , Supporting Information). The electrode was preconditioned in a 1500 ppm NO 3   − —N solution for 3 days. The result of the continuous measurement shows that the potential at day 10 (˜−184 mV) remained almost unchanged from the initial potential (˜−186 mV). Therefore, the preconditioned electrode was found relatively stable. 
     Nitrate Measurement in Extracted Soil Solution. To demonstrate the nitrate measurement in extracted soil water, soil water was extracted from three locations at the Iowa State BioCentury Research Farm (Ames, Iowa) using a suction lysimeter. The suction head of the lysimeter was inserted to a depth of 25 cm from the soil surface. As the POT-MoS 2 -based sensor was dipped into different test solutions, the sensor responded by providing different voltage signals ( FIG. 27 a   ). The inset of  FIG. 27 a    shows the converted nitrate concentration using the calibration curve of the sensor ( FIG. 26 a   ). For comparison, a commercial sensor (LaQua Horiba nitrate sensor) was used to measure the same sample solutions. Our sensor and the commercial sensor showed comparable readings. 
     Short-Term Nitrate Measurement in Soil Column. To demonstrate the short-term nitrate measurement in a soil column, two identical POT-MoS 2  based sensors were fixed on the walls of two column beakers filled with soil slurries ( FIG. 27 c   ). The column beakers were 6 cm in diameter and 10 cm in height and loaded with soils to a height of 9 cm from the bottom of the beaker. Several 3 mm diameter holes were created at the bottom of the beaker to flush out the water. Each sensor was located 7 cm from the bottom, as shown in  FIG. 27 b   . The soil used here was collected from the soil surface at the research farm mentioned above. During the demonstration, the soil in one beaker was flushed with alternating solutions of 0 and 50 ppm NO 3   − —N at different time points, each time lasting 2 min, whereas the soil in the other beaker was flushed with 0 and 100 ppm NO 3   − —N.  FIG. 27 d    shows the voltage outputs of the two sensors installed in the two beakers. When the soil was flushed with DI water (0 ppm), the output voltage of the sensor reached a baseline voltage of approximately −110 mV. When the soil was treated with 50 or 100 ppm nitrate solution, the sensor 1 and sensor 2 outputs went down to approximately −123 mV or approximately −150 mV, respectively.  FIG. 27 e    shows the nitrate concentrations converted from the voltage outputs of the sensors. It should be noted that the converted concentrations are evidently lower than the known input nitrate concentrations. The nitrate solution was flushed out of the soil slurries immediately after introducing the solution, and the prewetted soil particles already had water content that may have diluted the external original concentration of nitrate in the testing soil slurries, resulting in reduced ppm levels compared with the original input concentration of nitrate. Alternatively, when we introduced the external nitrate concentration into the soil, as nitrate has a low charge density compared to other common pre-existing anions in soil solution and they always occupy the few positively charges sites, in turn, the nitrate ions may have failed to bind with soil particles within a short period of time or denitrification of nitrate ions, thus, both sensors showed reduced ppm nitrate levels. However, the sensor response returned to the baseline ppm level immediately as we flushed with DI water. 
     For long-term measurements, two identical sensors (sensor 1 and sensor 2) were deployed directly into the soil slurries in column beakers over approximately 4 weeks with different rates of nitrate concentration (50 and 100 ppm NO 3   − —N), and OCP was measured continuously ( FIG. 28 a,b   ). For this measurement, the beaker dimensions were the same, and the sensors were fixed at the same location as for the short-term measurement. However, unlike the previous design of the beakers for the short-term measurement, there were no holes at the bottom of the beakers to promote denitrification of nitrate ions in the soil slurries before evaporation. 
     The long-term monitoring of nitrates in soil slurries using sensor 1 and sensor 2 is shown in  FIG. 28 c - e   . For sensor 1, when the soil beaker was treated with DI water, the NO 3   − —N level was found to be approximately 14-23 ppm ( FIG. 28 c   , marked with box), which is similar to that observed in the short-term measurement. Because of the slow diffusion of preoccurring nitrate ions from the soil slurry into water, the nitrate level slowly increased after water was poured, until it reached a maximum concentration. Further, the sensor showed a slow decrease in NO 3   − —N concentration to the range of 2-5 ppm because of the denitrification at room temperature (25° C.). In this parched soil condition, the nitrate ppm was found to be almost constant. Upon further repeating the experiment two times, the sensor showed similar results. 
     Interestingly, when the 50 ppm NO 3   − —N was poured into the soil beaker, sensor 1 began to show a slow increase in NO 3   − —N and reached a maximum value of 53 ppm NO 3   − —N. With the addition of external nitrate into the soil, the sensor took approximately 3 h to reach a maximum nitrate level, indicating a slow diffusion of nitrate ions into the soil. This is because when the soil particles at the sensor interface are completely wet, nitrate ions may diffuse slowly from the external nitrate solution (as we filled the beaker) because of the concentration gradient. The NO 3   − —N concentration was further decreased to a low value of 2-5 ppm when the soil particles became parched because of water evaporation, which restricted the mobility of the nitrate ions. Sensor 1 showed an almost similar performance of NO 3   − —N, whereas the sensor was further flushed with 50 ppm NO 3   − —N concentration another three times. When more water containing NO 3   − —N(see the last two repeated measurements,  FIG. 28 c   ) was poured, the sensor showed a longer nitrate response at 50 ppm, as the evaporation of water from the soil takes time. 
     Similarly, for sensor 2, the sensor performance was investigated in the presence of DI water and 100 ppm of NO 3   − —N concentration for 2 weeks ( FIG. 28 d   ), and the sensor was kept in parched soil conditions for another 2 weeks ( FIG. 28 e   ). With DI water filling, the sensor exhibited a concentration of approximately 20-25 ppm of NO 3   − —N because of the pre-existing nitrate ions in the soil. Further, the soil water content dried slowly, and the soil became parched under this condition. The sensor showed a similar NO 3   − —N response as was observed in the case of sensor 1. When the soil slurry was flushed with 100 ppm NO 3   − —N solution, the output of the sensor reached a maximum value of NO 3   − —N(approximately 104 ppm), after which the sensor response began to decay to less than 10 ppm of NO 3   − —N because of water evaporation. Further, sensor 2 was kept in the same soil without the addition of water for approximately 2 weeks, and the concentration variability was investigated ( FIG. 28 e   ). The soil became parched without the addition of water and NO 3   − —N solution. Under this condition, however, the sensor still exhibited a low ppm of nitrate (approximately 10-2 ppm). Interestingly, the sensor response decreased from approximately 10 to 3 ppm over a long period of time (13 days), but the sensor response was found to be irregular, perhaps because of the changing room temperature or humidity level. The sensor deployed into soil slurries can monitor nitrate-nitrogen accurately for at least a duration of 27 days. 
     Conclusions 
     In this manuscript, a novel all-solid-state miniature sensor designed for long-term use in continuous monitoring of soil nitrate was presented. The sensor was fabricated on a PCB using patterned WE and RE. To characterize the sensor materials, solid-state components using MoS 2 , POT, and POT-MoS 2  were directly coated on the patterned PCB and functionalized with an ISM using a high-precision robotic-armed auto-dispenser machine. The electroactivity property of the POT-MoS 2  composite was found to be excellent, and the material was used as an ion-to-electron transducing layer for nitrate detection in the sensor. The POT-MoS 2  composite material produced superior sensor performance in terms of selectivity and sensitivity compared with MoS 2  and POT and the reported nitrate sensors shown in Table 1. This may be the result of the high hydrophobicity and high redox properties of the POT-MoS 2  layer. The solid-state sensor is selective to nitrate ions even when other anions are present at significant concentrations and offers long-term stability. This sensor can be deployed into the soil for long-term nitrate monitoring (about 4 weeks). In the future, by replacing the ion-selective membrane, the sensor could be adapted to detect other soil nutrients, including potassium, phosphate, and sulfate. These other nutrients are also essential to plant growth and agricultural productivity. Continuous measurements of these nutrients thus have significant potential applications in plant biology, plant breeding, environmental science, and production agriculture. 
     Associated Content 
     *S Supporting Information 
     The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07120. Reproducibility, repeatability studies, and potential stability of the sensor (PDF), and incorporated by reference herein. 
     With respect to Specific Example 3, supporting information referenced above can be found at  FIGS. 29A, 29B, and 29C  and Table S1 below. 
     
       
         
           
               
             
               
                 TABLE S1 
               
             
            
               
                   
               
               
                 Values of potential drifts per second (ΔE/Δt) and capacitate for  
               
               
                 different electrode materials (POT, MoS 2  and POT-MoS 2 ) 
               
            
           
           
               
               
               
            
               
                 Nitrate Selective Electrode 
                   
                   
               
               
                 materials 
                 ΔE/Δt (μV/s) 
                 C (μF) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 POT 
                 115.4 
                 433 
               
               
                 MoS 2   
                 213.3 
                 234 
               
               
                 POT-MoS 2   
                 95.0 
                 526 
               
               
                   
               
            
           
         
       
     
     REFERENCES REGARDING EXAMPLE 3 
     
         
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         (3) White, J. W.; Andrade-Sanchez, P.; Gore, M. A.; Bronson, K. F.; Coffelt, T. A.; Conley, M. M.; Feldmann, K. A.; French, A. N.; Heun, J. T.; Hunsaker, D. J.; Jenks, M. A.; Kimball, B. A.; Roth, R. L.; Strand, R. J.; Thorp, K. R.; Wall, G. W.; Wang, G. Field-based Phenomics for Plant Genetics Research. Field Crop. Res. 2012, 133, 101-112. 
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     F. Example 3 
     With particular reference to  FIGS. 30-35 , an Example 3 showing methods, apparatus and systems according to the invention is shown and described below. 
     The following is additional and supplementing description of aspects of the invention taken from Md. Azahar Ali, Xinran Wang, Yuncong Chen, Yueyi Jiao, Michael J. Castellano, James C. Schnable, Patrick S. Schnable, and Liang Dong. Transducers 2019-EUROSENSORS XXXIII, Berlin, Germany, 23-27 Jun. 2019, and incorporated by reference herein in its entirety. 
     Novel all-Solid-State Soil Nutrient Sensor Using Nanocomposite of Poly(3-Octyl-Thiophene) and Molybdenum Sulfate 
     Abstract 
     Nitrate is a major macronutrient for plant growth. There is a high demand to develop robust, reliable, and maintenance-free soil sensors for long-term monitoring of nitrate variations in field. An ion-selective membrane-based solid-state nitrate sensor is developed using poly(3-octyl-thiophene)-molybdenum disulfide nanocomposite as an ion-to-electron transducing layer. The sensor offers a high sensitivity of 64 mV/decade, a dynamic range of 1-1500 ppm NO 3   − —N, and an improved selectivity for nitrate detection in soils. The sensor has demonstrated the ability to continuously monitor soil nitrate concentrations over a period of four weeks. 
     Introduction 
     After water, soil nitrogen (N) is the most limiting factor for plant growth. Crop productivity relies heavily on N fertilization [1]. At present N fertilizer is not efficiently assimilated by crops. Excess N fertilizer leaks into the environment, leading to significant negative environmental impacts on water quality, biodiversity, and atmospheric pollution. Continuous monitoring of nitrate dynamics in crop fields provides us maximum control over fertilizer management. Laboratory measurement of soil nitrogen is time-consuming and labor intensive. Remote sensors and on-the-go vehicle-based sensors have been used for in-field measurement of soil nitrate [1][2]. Accurate, long-term and field deployable nitrate sensors are still challenging. 
     Ion-selective membrane (ISM)-based potentiometric sensors are widely used to detect nitrate in water [3]. But, due to the necessity of using inner-filling solutions, conventional ISM-based nitrate sensors lack portability and long-term deployability [4]. Efforts have been made to develop all-state-solid sensors by introducing solid-contact ion-to-electron transducing materials, such as carbon nanotube [5], polypyrrole [6] and graphene [7], between the ISM and an electron conducting layer of the sensor. A desired solid-contact ion-to-electron transducing material requires both high redox property and hydrophobicity to increase stability, selectivity, and sensitivity. 
     This paper reports an all-solid-state potentiometric sensor for continuous monitoring of soil nitrate levels. The sensor uses a novel nanocomposite of poly(3-octyl-thiophene) (POT) and two-dimensional transition metal dichalcogenides of molybdenum disulfide (MoS 2 ) sheets as a solid-contact ion-to-electron transducing layer. POT has an excellent redox property but its electrical conductivity is low. MoS 2  sheets can provide a larger surface area, high conductivity, insensitivity to light or pH, and absence of possible side-reactions. The working electrode (WE) of the sensor is built on top of a copper pad of printed circuit board (PCB) covered by a patterned thin Au layer, a POT-MoS 2  nanocomposite-based solid-contact layer, and a nitrate-specific ISM. The incorporation of MoS 2  into POT can not only increase redox properties of POT, but also provide high hydrophobicity to minimize the formation of water thin layer between the ISM and metallic electron conduction layer. Generally, a notorious thin waterlayer is often formed at the interface between the ISM and conducting metal layer, acting as an interfacial barrier to fast electron transfer and negatively impacting selectivity of the sensor because different ions may be trapped inside this water layer. In our case, due to high hydrophobilicity, the POT-MoS 2  can counter the issue with the trapped water layer, contributing to increasing charge transfer and ion selectivity of the sensor. The reference electrode (RE) of the sensor includes a silver/silver chloride (Ag/AgCl) electrode covered by a proton exchange membrane to reduce redox reaction-induced chloride leaching from the RE, contributing to minimizing drift of reference potential. 
     The presented sensor is featured with an all-solid-state design that incorporates the POT-MoS 2  nanocomposite for improved device performance. It should be noted that the sensor is embedded in soil for continuous monitoring of nitrate dynamics for about four weeks. 
     Device Fabrication 
     This sensor was made on a PCB with two electrodes wherein one served as a WE  30  and the other as a RE  40 . To functionalize the WE  30 , POT and MoS 2  with a weight ratio of 1:4 (concentration of POT: 2.6 mg/mL) were dispersed in tetrahydrofuran. This composite solution was coated on a 4 mm-diameter circular Au using a high-precision liquid dispensing machine ( FIG. 30 a   ), and then dried at room temperature. After that, a nitrate-specific ISM was coated on the POT-MoS 2  layer and then conditioned in a 1500 ppm NO 3   − —N solution for 24 hr. 
     For the RE  40 , Ag/AgCl paste was screen-printed on a silver layer and then dried at 110° C. for 2 hr. A perfluorinated polymer, Nafion, was then coated on the surface of Ag/AgCl. The Nafion layer was dried at 90° C. for 1 hr. The role of Nafion is to prevent redox reaction-induced chloride leaching from the Ag/AgCl material to increase potential stability at the RE. 
       FIG. 31  shows the scanning electron microscopic (SEM) images for MoS 2  sheets (a), POT (b), and POT-MoS 2  (c) with schematics of different structural layers. Image (a) shows MoS 2  sheets with a combination of large (&gt;1 μm) and small (&lt;1 μm) sheets. The observed porous morphology of POT is useful for forming a composite with MoS 2  (image b). In the POT-MoS 2  composite, MoS 2  is covered by POT maybe due to the oppositely charged interactions. A morphological transition of POT and MoS 2  indicate the formation of POT-MoS 2  composite (image c). The POT-MoS 2  is ˜1 μm thick (image d).  FIG. 31 e    shows a schematic presentation and molecular structure of both POT and MoS 2 . 
       FIG. 31 f    shows the ion exchange mechanism of the sensor for nitrate detection. The redox reaction is given by:
 P (p)   +   +e   − +R − ( p )+NO 3(m)   −   P (p) +R −   (m) +NO 3   −   (aq)   (1)
 
where m, p, and aq represent the ISM phase, POT-MoS 2  nanocomposite phase, and aqueous phase, respectively, and P (p)  and P +   (p)  represent a few monomeric units of the POT-MoS 2  in the neutral insulating state, and the oxidized state with polaronic sites, respectively. The movement of anions or cations, with the help of specific binding sites, produces a potential difference. E depends on the logarithm of the ion activity and described by Nernst equation,
 
E=E 0 +E 1 =E 0 +(RT/zF)ln  a   I (I)  (2)
 
where a I (I), R, T, z and F are the primary ion activity without any interfering, ions gas constant, temperature, charge in target ion, and Faraday constant, respectively. E 0  and E 1  are the constant potential at the RE and the potential devolved at the WE, respectively.
 
     Experimental Results 
     Electrochemical Studies 
     The cyclic voltammetry (CV) studies ( FIG. 32 a   ) show that the POT-MoS 2 -based sensor exhibited a higher redox current and peak-to-peak potential, compared to the POT-based electrode. The conductive nature of MoS 2  results in an increase in electron transfer. When incorporating the ISM, the redox current of the POT-MoS 2  decreased due to the insulating nature of ISM. The negatively charged ISM may repel ferro/ferricyanide molecules, thus reducing electron transfer. 
     The weight ratio of POT and MoS 2  was optimized ( FIG. 32 b   ) using the CV method wherein the peak-to-peak potential (ΔE) was calculated for the POT-MoS 2  electrode by varying MoS 2  concentration in the composite. At 1:4 weight ratio of POT and MoS 2 , the ΔE was found to reach maximum. Measurement results for open circuit potential (OCP) of three electrodes (coated with MoS 2 , POT or POT-MOS 2  and a nitrate-specific ISM) are shown in the inset of  FIG. 32 b   . The OCP for the POT-MOS 2 -based electrode (−327 mV) was shown higher than that for the POT- (−263 mV) and the MOS 2 -based (−63 mV) electrodes. As evident in the CV studies, the POT-MoS 2  electrode shows a high redox capacitance property and redox electroactivity, allowing selective uptaking or release of hydrophilic NO 3   −  ions. 
     Nitrate Detection in Standard Solution 
     Nitrate detection was investigated for all the sensors using MoS 2 , POT and POT-MoS 2  as different solid-contact materials. A home-made circuit was used to read out the OCP values as a function of concentration from 1 to 1500 ppm NO 3   − —N. As the concentration changed, OCP would change.  FIG. 33 a    shows the output voltages for different WEs when responding to different nitrate concentrations. The corresponding calibration curves for the sensors are shown in  FIG. 33 b   . The sensor using the POT-MoS 2  shows a higher sensitivity (64 mV/dec) with a wide dynamic range of 1 to 1500 ppm NO 3   − —N, compared to that using POT (48 mV/dec) or MoS 2  alone (38 mV/dec). 
       FIG. 33 c    shows the effect of environmental chloride ions on the ability of the POT-MoS 2 -based sensor to detect nitrate ions. The result shows that the sensor was little affected by low-concentration ions (e.g., 0.01 M and 0.1 M) in the nitrate solution. However, with extreme high Cl— concentrations such as 0.5 M and 1 M, the sensor output was deviated by ±2.2% and ±4.0%, respectively, from the output without any interfering ions. The potential drift may be caused by the non-selective interaction of the sensor with ions in the ISM. Additional selectivity studies ( FIG. 33 d   ) of the different sensors were performed in presence of interfering anions such as phosphate (PO 4   3 ), bicarbonate (HCO 3   − ), and sulphate (SO 4   2− ). The result shows that the POT-based or MoS 2 -based sensor shows low selectivity, compared to the POT-MoS 2 -based sensor. 
     Nitrate Detection in Soil 
     The POT-MoS 2 -based nitrate sensor was applied to measure nitrate concentrations in extracted soil solutions collected from a corn field in Ames, Iowa. The readings from this sensor were compared with those from a commercial nitrate sensor (Lague Horiba). Our sensor measurement results agreed well with those obtained using the commercial sensor ( FIG. 34 a   ). 
     Two POT-MoS 2 -based sensors were fixed on the walls of two column beakers filled with soil slurries ( FIG. 34 b - c   ). The beaker was 6 cm in diameter and 10 cm in height, and was loaded with mineral soils till 9 cm height from the bottom of the beaker. Small drain holes were created at the bottom. Each sensor was located at 7 cm from the bottom. During the measurement, the soil in one beaker was flushed with alternating solutions of 0 and 50 ppm NO 3   − —N at different times, each time lasting 2 minutes, while the soil in the other beaker was flushed with 0 and 100 ppm NO 3   − —N.  FIG. 34 d    shows the outputs of the two sensors. When the soil was flushed with water (0 ppm), the output voltage of the sensor reached the −110 mV voltage baseline. When the soil was treated with the 50 ppm and 100 ppm NO 3   − —N solutions, one sensor output went down to −123 mV, while the other to −150 mV, respectively.  FIG. 34 e    shows the nitrate levels converted from the output voltages of the sensors. 
     For long-term measurement, two sensors (sensor 1 and sensor 2) were deployed directly in soil slurries for ˜27 days with different rates of nitrate concentration (50 and 100 ppm NO 3 −N) ( FIG. 35 a - b   ). For the sensor 1, when the beaker (without holes at the bottom) was treated with water, NO 3   − —N was found to be ˜14-23 ppm ( FIG. 35 c   ). Due to the slow diffusion of pre-occurrence nitrate ions from the soil slurry into water, the nitrate concentration slowly increased until a maximum concentration was reached. Further, the sensor showed a gradual decrease in concentration in a range of 2-5 ppm NO 3   − —N due to slow evaporation. In this parched soil condition, the nitrate concentration was found to be almost constant. Upon further repeating the experiment two times, the sensor showed similar results. 
     Interestingly, when the 50 ppm of NO 3   − —N was poured into the soil beaker, sensor 1 began to show a slow increase in NO 3   − —N, and reached a maximum value of 53 ppm NO 3   − —N. With the addition of external nitrate into the soil, the sensor took approximately 3 h to reach a maximum nitrate level, indicating slow diffusion of nitrate ions into the soil. This is because when soil particles at the sensor interface are completely wet, nitrate ions may diffuse slowly from the external nitrate solution (as we filled the beaker) due to the concentration gradient. The NO 3   − —N concentration was further decreased to a low value of 2-5 ppm when the soil particles became parched due to water evaporation, which restricted the mobility of the nitrate ions. Sensor 1 showed an almost similar performance of NO 3   − —N, while the sensor was further flushed with 50 ppm NO 3   − —N concentration another three times. When more water containing NO 3   − —N(see the last two repeated measurements,  FIG. 35 c   ) was poured, the sensor showed a longer nitrate response at 50 ppm, as the evaporation of water from the soil takes time. 
     Similarly, for sensor 2, the performance was studied in the presence of 0 ppm (water) and 100 ppm of NO 3   − —N concentration for 2 weeks ( FIG. 35 d   ), and the sensor was kept in parched soil conditions for another 2 weeks ( FIG. 35 e   ). With water filling, the sensor exhibited a concentration of approximately 20-25 ppm of NO 3   − —N due to the pre-existing nitrate ions in the soil. Further, the soil water content dried slowly, and the soil became parched under this condition. The sensor showed a similar NO 3   − —N response as was observed in the case of sensor 1. When the soil slurry was flushed with 100 ppm NO 3   − —N solution, the output of the sensor reached a maximum value of NO 3   − —N(approximately 104 ppm), after which the sensor response began to decay to less than 10 ppm of NO 3   − —N due to water evaporation. Further, sensor 2 was kept in the same soil without the addition of water for approximately 2 weeks, and the concentration variability was investigated ( FIG. 35 e   ). The soil become parched without the addition of water and NO 3   − —N solution. Under this condition, however, the sensor still exhibited a low ppm of nitrate (approximately 10-2 ppm). Interestingly, the sensor response decreased from approximately 10 ppm to 3 ppm over a long period of time (13 days), but the sensor response was found to be irregular, perhaps because of the changing room temperature or humidity level. 
     Conclusions 
     An all-solid-state miniature sensor designed for long-term use in continuous monitoring of soil nitrate was presented. The electro-activity properties of POT-MoS 2  composite were found to be excellent, and the material was used as an ion-to-electron transducing layer for nitrate detection in the sensor. The POT-MoS 2  composite material produced excellent sensor performance in terms of selectivity and sensitivity compared with MoS 2  and POT, and the reported nitrate sensors. This may be the result of the high lipophilicity and high redox properties of the POT-MoS 2  layer. The sensor is highly selectable with other anions and offers long-term stability. This sensor can be deployed into the soil for long-term nitrate monitoring. In the future, by replacing the ion selective membrane, the sensor can work to detect other soil nutrients, including potassium, phosphate, and sulfate, which will also help the phenotypic activity and nutrient uptake of plants [8]. With this evidence of sensor performance, the nitrate sensor may detect the variation in nitrate concentration in an agricultural setting [9]. This solid-state sensor may help farmers manage nitrogen fertilizer levels in fields to enhance crop yield. 
     REFERENCES RELATING TO EXAMPLE 3 
     
         
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         [3] P. SjoEberg, et al. “All-solid-state chloride-selective electrode based on poly (3-octylthiophene) and tridodecylmethylammonium chloride.”  Electroanalysis,  11.10-11 (1999): 821-824. 
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         [5] M. Cuartero, et al. “Tandem electrochemical desalination-potentiometric nitrate sensing for seawater analysis.”  Analytical Chemistry,  87.16 (2015) 8084-8089. 
         [6] T. A. Bendikov, et al. “Development and environmental application of a nitrate selective microsensor based on doped polypyrrole films.”  Sensors and Actuators B. Chemical,  106.2 (2005) 512-517. 
         [7] N. T. Garland, et al. “Flexible laser-induced graphene for nitrogen sensing in soil.”  ACS Applied Materials Interfaces,  10.45 (2018) 39124-39133. 
         [8] M. A. Ali, et al. “Microfluidic impedimetric sensor for soil nitrate detection using graphene oxide and conductive nanofibers enabled sensing interface.”  Sensors and Actuators B. Chemical,  239 (2017): 1289-1299. 
         [9] M. A. Ali, et al. “Tunable bioelectrodes with wrinkled-ridged graphene oxide surfaces for electrochemical nitrate sensors.”  RSC Advances  6.71 (2016): 67184-67195.