Ion sensitive field effect transistors with europium ion probes

Abstract

Devices, systems, and methods that utilize an ion probe bonded to a sensing layer to detect ions in solutions are provided herein. The ion probe can include one or more europium ions. For example, the devices provided herein can include: one or more source contacts disposed on a surface of a substrate; one or more drain contacts disposed on the surface of the substrate; a channel material disposed between at least one source contact and at least one drain contact; a sensing layer disposed on at least a portion of a surface of the channel material; and one or more europium ions bonded to the sensing layer.

Description

ION SENSITIVE FIELD EFFECT TRANSISTORS WITH EUROPIUM ION PROBESCLAIM OF PRIORITY

[0001] This application claims priority under 35 USC §119(e) to U.S. Patent Application Serial No. 63 / 544,057, filed on October 13, 2023, the entire contents of which are hereby incorporated by reference.STATEMENT OF GOVERNMENT INTEREST

[0002] This invention was made with government support under CBET2034385 and ITE2344373, awarded by the National Science Foundation. The government has certain rights in the invention.TECHNICAL FIELD

[0003] This document relates to field-effect-transistor based devices for sensing chemical compounds.BACKGROUND

[0004] Accurate and wide-ranging testing of phosphorous concentrations in fluids is crucial for evaluation of water pollution and for human health diagnostics.SUMMARY

[0005] The present document relates to methods and materials that can be used to detect phosphorous (e.g., compounds containing inorganic phosphorus, such as phosphate). For example, this document provides devices (e.g., sensors) and systems that can be used to detect phosphorus. In some cases, the methods and materials (e.g., devices and systems) provided herein can be used to determine a level or concentration of phosphorous in a sample. In some cases, the systems, devices, and methods provided herein also can be configured to detect and / or determine the level of other ions, such as lead and nitrate.

[0006] In a first aspect, the present document encompasses a device that includes, consists essentially of, or consists of: one or more source contacts disposed on a surface of a substrate; one or more drain contacts disposed on the surface of the substrate; a channel material disposed between at least one source contact and at least one drain contact; a sensing layer disposed on atleast a portion of a surface of the channel material; and one or more europium ions bonded to the sensing layer.

[0007] In another aspect, the present document encompasses a method that includes, or consists essentially of: depositing, onto a substrate, two or more discrete portions of a conductive material to form two or more contacts; depositing a layer of a channel material to cover an exposed portion of the substrate between at least two contacts; depositing a sensing layer on an exposed portion of the channel material; and introducing europium ions to the sensing layer.

[0008] In yet another aspect, the present document encompasses a method that includes, or consists essentially of: varying a gating voltage while measuring a first drain-source current in a device including a transistor; determining a first reference voltage where a first minimum in the first drain-source current in the device occurs; flushing the device with a test solution, thereby causing phosphorous ions, if present, in the test solution to bond with europium probes bonded to the sensing layer; varying the gating voltage in the device while measuring a second drain-source current in the device; determining a second reference voltage where a second minimum in the second drain-source current in the device occurs; determining a voltage difference between the second and first reference voltages; and determining, using the voltage difference, a concentration of the phosphorous ions in the test solution. The transistor can include a sensing layer disposed on at least a portion of a channel material.

[0009] In yet another aspect, the present document encompasses a method that includes, or consists essentially of: determining a Dirac point of a device including europium ions attached to a sensing layer; exposing the device to a test solution; measuring a shift in a gating potential in the device away from the Dirac point; and determining a presence or absence of phosphorous ions in the test solution based on the shift in the gating potential.

[0010] In some implementations, the device further can include a chamber above the sensing layer, coupled to an input port and an output port, and configured to receive and flush a solution; and a reference electrode disposed in solution.

[0011] In some implementations, the reference electrode can include at least one of silver and silver chloride.

[0012] In some implementations, the device is a transistor.

[0013] In some implementations, the device further can include an encapsulation layer (i) defining one or more sensing channels where the encapsulation layer has one or more holes froma top side to a bottom side and (ii) disposed to partially cover the channel material above the one or more source contacts and one or more drain contacts.

[0014] In some implementations, the encapsulation layer can include a negative photoresist.

[0015] In some implementations, the sensing layer can include self-assembled graphene oxide disposed on the encapsulation layer and contacting the channel material through the one or more sensing channels.

[0016] In some implementations, the sensing layer can include graphene, graphene oxide, carbon nanotubes, carbon nanofibers, or a combination thereof.

[0017] In some implementations, at least one of the one or more source contacts and / or one or more drain contacts include gold.

[0018] In some implementations, the one or more source contacts are interdigitated with the one or more drain contacts.

[0019] In some implementations, the one or more source contacts and one or more drain contacts are one of interdigitated transducers (IDTs), bar electrodes, or strip electrodes.

[0020] In some implementations, a system includes: the device, an electronics module configured to receive and / or transmit electrical signals to the device; and a fluidics module configured to provide one or more fluids to the sensing layer of the device.

[0021] In some implementations, the fluidics module can include a first chamber in fluidic communication with the sensing layer, and the first chamber is configured to deliver a test fluid and / or a regeneration fluid.

[0022] In some implementations, the fluidics module includes: a first chamber in fluidic communication with the sensing layer; a second chamber in fluidic communication with the sensing layer; and a movable component configured to transport the device between the first and second chambers. The first chamber can be configured to provide a test fluid, and the first chamber can be configured to provide a regeneration fluid.

[0023] In some implementations, the method further includes, before depositing the sensing layer, depositing an encapsulation material to cover one or more areas of the channel material above the one or more contacts.

[0024] In some implementations, the sensing layer is graphene oxide that was self-assembled using PDDA.

[0025] In some implementations, the sensing layer is self-assembled (e.g., layer-by-layer assembled) graphene oxide, graphene, carbon nanotubes, carbon nanofibers, or a combination thereof.

[0026] In some implementations, the europium ions are salts dissolved in a solvent.

[0027] In some implementations, a pH of the europium ions dissolved in the solvent is from about 4 to about 5.

[0028] In some implementations, the method further includes: before varying the gating voltage while measuring the first drain-source current, varying the gating voltage while measuring a third drain-source current; determining a third reference voltage where a third minimum in the third drain-source current occurs; flushing the device with a regeneration fluid containing europium ions, thereby causing europium ions in the regeneration fluid to bond with functional groups on the sensing layer; varying the gating voltage while measuring a fourth drain-source current; determining a fourth reference voltage where a fourth minimum in the fourth drain-source current occurs; and determining a second voltage difference between the fourth and third reference voltages.

[0029] In some implementations, the method further can include using the second voltage difference, determining to flush the device with the regeneration fluid.

[0030] Application of the methods and materials (e.g., systems and devices) provided herein can result in one or more of the following advantages. In some implementations, phosphorus can be accurately detected, even in solutions with low concentrations of phosphorus. Further, the europium ion probes that are periodically replaced are environmentally friendly.

[0031] Compared to sensors that use a membrane (e.g., a molecular imprinted polymer) and without wishing to be limited by mechanism or theory, components of the disclosed sensors can be more stable, more selective, and / or longer-lasting. For example, once an ion-sensitive sensor using membranes has reached a certain concentration measurement, the sensor typically cannot measure a concentration with a lower value, rendering the membranes as one-time-use materials. In some implementations, the sensors disclosed herein can be portable.

[0032] Moreover, the disclosed materials (e.g., systems and sensors) can feature structures with microfluidic channels and pumps that allow for automatic cycling through alternate stages of generating europium ion probes and depleting the europium ion probes by testing a solution. In some implementations, these devices can be disposed for long periods of time (e.g., three tonine months) in a test solution and can periodically measure phosphorous concentrations in the test solution.Definitions

[0033] As used herein, the term “about” means + / -10% of any recited value. As used herein, this term modifies any recited value, range of values, or endpoints of one or more ranges.

[0034] By “microfluidic” or “micro” is meant having at least one dimension that is less than 1 mm. For instance, a microfluidic structure (e.g., any structure described herein) can have a length, width, height, cross-sectional dimension, circumference, radius (e.g., external or internal radius), or diameter that is less than 1 mm.

[0035] By “nanofluidic” or “nano” is meant having at least one dimension that is less than 1 pm. For instance, a nanoparticle can have a length, width, height, cross-sectional dimension, circumference, radius (e.g., external or internal radius), or diameter that is less than 1 pm.

[0036] “Electrical connection,” as used herein, refers to any conductive or semi -conductive structure through which an electrical signal may pass. The electrical signal can be any useful change in electrical field, electric potential, current, or voltage. Non-limiting structures include lines, contact pads, busses, pins, connectors, bond lines, bond pads, etc., formed from any useful material (e.g., an ohmic material, a metal, etc.).

[0037] “Fluidic communication,” as used herein, refers to any duct, channel, tube, pipe, chamber, or pathway through which a substance, such as a liquid, gas, or solid may pass substantially unrestricted when the pathway is open. When the pathway is closed, the substance is substantially restricted from passing through. Typically, limited diffusion of a substance through the material of a plate, base, and / or a substrate, which may or may not occur depending on the compositions of the substance and materials, does not constitute fluidic communication.

[0038] As used herein, the terms “top,” “bottom,” “upper,” “lower,” “above,” and “below” are used to provide a relative relationship between structures. The use of these terms does not indicate or require that a particular structure must be located at a particular location in the apparatus.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although methods and materials similar or equivalent to those described herein can be used to practice the invention, suitable methods and materials are described below. Allpublications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

[0040] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1A is an exploded view of an example of a sensor.

[0042] FIG. IB is a schematic depicting an example of a system including the sensor of FIG.1A.

[0043] FIG. 2 is a schematic depicting europium ions bonding with a sensing layer of the sensor of FIG. 1A.

[0044] FIGS. 3A and 3B are plots depicting a Dirac point shift.

[0045] FIGS. 4A, 4B, 4C, 4D, 4E, and 4F are schematics depicting a non-limiting method for fabricating the sensor of FIG. 1A.

[0046] FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, and 5H are plots depicting experimental results for the sensor of FIG. 1A with different test fluids and regeneration fluids. Provided are drainsource current (Ids) and gate potential (Vg) curves (A) before and after europium immobilization and (B) showing shifts at different concentration of HPCh2'. Also provided are graphs showing (C) calibration curves for HPCh2’ detection and (D) device performance with and without Eu probe and after 10 regeneration-measurement cycles. Provided are (E) Ids - Vgcurve shift and (F) EU3+probe regeneration and removal with 10 cycles of continuous measurement at 10 ppm of HPO42' with uniform AVg. Also provided are (G) Ids - Vgcurves and (H) calibration curves for PO43' detection. A constant bias of 0.1 V was applied between source and drain contacts of the sensor.

[0047] FIGS. 6A, 6B, 6C, and 6D depict experimental results for characterizing layers of the sensor of FIG. 1A. Provided are scanning electron microscopy (SEM) images of (A) monolayer graphene and (C) graphene oxide and Raman spectra of (B) monolayer graphene and (D) graphene oxide.

[0048] FIGS. 7A, 7B, and 7C depict examples of devices that include the sensor of FIG. 1A.

[0049] FIG. 8 depicts an exploded view of a device including the sensor of FIG. 1 A.

[0050] FIG. 9 is a schematic depicting the stages of operating the device of FIG. 8.DETAILED DESCRIPTION

[0051] In some implementations, the present document provides devices (e.g., sensors) that can be used for universal detection of phosphorous, including hydrogen phosphate and phosphate ions in water or other solutions. The underlying mechanism of the devices provided herein is an ion-sensitive field-effect transistor (ISFET) using europium ion probes on a sensing layer. Generally, ISFETs can be used for measuring ion concentrations in solutions, since the concentration of a particular ion can cause the drain-source current flowing through the transistor to change. As described in the present disclosure, an ISFET can be configured to measure a Dirac point shift (e.g., the charge neutrality point) in the channel material of a sensor to determine a concentration of phosphorous. Non-limiting examples of phosphorous forms that can be measured using the methods and materials provided herein include phosphate ions (PO43) and hydrogenated forms thereof, such as hydrogen phosphate ions (HPO42) and dihydrogen phosphate ions (FEPOL).

[0052] FIG. 1A depicts an exploded view of a sensor 100 for detecting phosphorus. The sensor 100 includes a substrate 102, source and drain contacts 104 and 106, a channel material 108, a sensing layer 110, and a reference electrode 114. The source and drain contacts 104 and 106 are disposed on the substrate 102 and leave portions of the substrate 102 uncovered by electrodes. The channel material 108 is disposed on the uncovered portions of the substrate that is between the source and drain contacts 104 and 106, acting as the semiconductor channel in the ISFET. The reference electrode 114 can vary an applied voltage, which controls current flowing from the source to the drain contacts 104 and 106 and through the channel material 108. Different biases between the source and drain contacts 104 and 106 can affect the drain-source current in a field effect transistor. In some implementations, the bias between the source and drain contacts is between 0.05 and 0.1 V. In some implementations, the drain contact 104 is connected to ground.

[0053] In general, the channel material is a semiconductor material. In some implementations, the channel material 108 is graphene, e.g., a monolayer of graphene or multilayers of graphene. Compared to multilayer graphene, monolayer graphene has a highcarrier mobility, which can increase the lower detection limit of the sensor 100. The channel material, in turn, can be provided as a layer.

[0054] In some implementations, the sensing layer 110 is self-assembled graphene oxide, graphene oxide, carbon nanotubes, carbon nanofibers, or a combination thereof. The selfassembled graphene oxide can be layer-by-layer graphene oxide, e.g., assembly with a cationic compound and / or an ionic compound. In some implementations, a self-assembled material (e.g., self-assembled graphene oxide) can provide a multifaceted surface with a surface area greater than the surface area of the substrate 102. Without wishing to be limited by mechanism or theory, an increased surface area can allow more space for more europium ions to bind and form probes.

[0055] The thickness of the sensing layer 110 can impact the performance of the sensor 100. For example, if the sensing layer 110 is too thick, the ISFET will not be able to detect a Dirac point shift. Conversely, if the sensing layer 110 is too thin, the channel material 108 will be exposed to the solution, which can degrade the channel layer. In some implementations, the surface area of the sensing layer 110 is about 8.5 mm (channel width) x 11.5 mm (channel length).

[0056] In some implementations, the sensor includes encapsulation 112 that partially covers the channel material 108 where the channel material 108 is directly above source or drain contacts 104 or 106. The encapsulation 112 can include one or more through holes, e.g., a hole from a top side to a bottom side of the encapsulation 112, that define one or more sensing channels. When the sensor includes encapsulation 112, the sensing layer 110 contacts the channel material 108 in the sensing channel(s), e.g., extends through the holes of the encapsulation 112. In some implementations, the encapsulation 112 is a negative photoresist, e.g., an epoxy-based photoresist, such as KMPR® from Kayaku Advanced Materials, Inc., Westborough, MA).

[0057] The source and drain contacts 104 and 106 can have any useful configuration. The positions and configuration of the source and drain contacts 104 and 106 can be optimized. In some implementations, the source and drain contacts 104 and 106 include interdigitated transducers (IDT), in which comb electrodes are arranged to interdigitate the fingers of the electrodes. One of these electrodes can be an active electrode, and the other a grounded electrode. Each electrode can have any useful configuration and geometry. Other useful configurations and geometries for the electrodes can be employed. For instance, one, two, ormore pairs of transducers can be employed. The design of the IDT can be selected from single finger electrodes, double split finger electrodes, pruned double split finger electrodes, or unidirectional electrodes.

[0058] Yet other exemplary source and drain contacts 104 and 106 include a planar electrode, a grating electrode, a thin film electrode, and / or a floating electrode having any useful thickness, period, material, or geometric arrangement and formed by any useful process, such as sputtering, vacuum deposition, or ion-plating. Non-limiting materials for source and drain contacts 104 and 106 include aluminum (Al), titanium (Ti), gold (Au), copper (Cu), tungsten (W), molybdenum (Mo), platinum (Pt), ruthenium (Ru), tantalum (Ta), chromium (Cr), osmium (Os), rhenium (Re), iridium (Ir), as well as alloys, doped forms, and multilayers thereof (e.g., Ag / AgCl, TiW / AICu or TiW / Cu layers). Arrays of n electrodes or n pairs of electrodes (e.g., n is 2, 4, 5, 10, 15, 20, 24, etc.) can also be incorporated with the devices and systems herein.

[0059] Although not pictured in FIG. 1 A, the sensing layer 110 is functionalized, allowing europium ions (e.g., in a regeneration fluid) to bond to the sensing layer 110. The europium ions effectively form probes for detecting phosphorus ions, as the drain-source current in the ISFET changes when phosphorus ions selectively remove the europium ions from the functionalized sensing layer 110. Eventually, when enough of the europium has been removed from the sensing layer 110, the europium ions should be regenerated. The cycle of regenerating europium probes and using europium probes can be understood with reference to FIG. IB.

[0060] The sensor can include any useful combination of components described herein. In some implementations, the sensor includes a two-terminal device having a source contact, a drain contact, and a semiconductor channel disposed between these contacts. In turn, the semiconductor channel can include a channel material disposed on a surface of the substrate and disposed between the source and drain contacts. Optionally, encapsulation can be used to electrically isolate portions of the channel material that directly overlay the source and drain contacts.

[0061] In some implementations, the sensor can include a plurality of semiconductor channels. Each semiconductor channel, in turn, can form an electrical connection to a source contact and a drain contact. Any useful electrical connections can be implemented, e.g., by the use of pairs of contacts connected to each semiconductor channel, the use of an array of contacts configured to connect to each semiconductor channel or groups of semiconductor channels, or the use of a pair of multi-fingered contacts in which a particular finger connect to a particularsemiconductor channel. In some implementations, the sensor can include an array of individual sensors, in which each individual sensor is associated with an individual semiconductor channel. In turn, electronics modules can be configured to receive and / or transmit electrical signals to the plurality of semiconductor channels in any useful manner (e.g., to provide continuous detection with optional signal processing to filter, multiplex, amplify, or otherwise modify such electrical signals).Systems

[0062] FIG. IB depicts a system 1000 including the sensor 100. In a first configuration, a movable component 103 positions the sensor 100 in a test chamber 109a containing a test fluid. When the sensor 100 contacts the test fluid, the drain-source current in the sensor 100 will change, reflecting a change in the concentration of phosphorus of the test fluid. As described above, detection of the phosphorus is due to the removal of europium ion probes. Eventually, the movable component 103 will reposition the sensor 100 to regenerate the europium ion probes.

[0063] In a second configuration, the movable component 103 positions the sensor 100 into a regeneration chamber 109b containing a regeneration fluid, e.g., Eu(NO3)3 or other solution include a europium salt. When the sensor 100 contacts the regeneration fluid, europium ions flush the sensing layer 110 and bond with the functionalized surface. In some implementations, the concentration of europium ions in the regeneration solution is 500 to 2000 ppm, e.g., above 1000 ppm.

[0064] An electronics module 105 provides electrical signals to one or more sensors 100 in the system 1000. For example, the electronics module 105 can include any number of electrical components configured to deliver an electrical signal to the sensor. The electrical components can be configured to have an electrical connection (e.g., by way of wires, lines, and the like) with one or more electrodes, e.g., drain and source contacts 104 and 106 and the reference electrode 114, configured in any useful manner.

[0065] The dimensions of the system 1000 components can vary. For example, each sensor 100 can have a size of about 1 mm by about 10 mm (e.g., about 1 mm by about 4 mm, about 2 mm by about 5 mm, about 3 mm by about 6 mm, about 4 mm by about 7 mm, about 5 mm by about 8 mm, about 6 mm by about 9 mm, about 7 mm by about 10 mm, about 2 mm by about 4 mm, about 3 mm by about 5 mm, about 4 mm by about 6 mm, about 5 mm by about 7 mm, about 6 mm by about 8 mm, about 7 mm by about 9 mm, about 8 mm by about 10 mm, about 3 mm byabout 4 mm, about 4 mm by about 5 mm, about 5 mm by about 6 mm, about 6 mm by about 7 mm, about 7 mm by about 8 mm, about 8 mm by about 9 mm, about 9 mm by about 10 mm, about 3 mm by about 3 mm, about 4 mm by about 4 mm, about 5 mm by about 5 mm, about 6 mm by about 6 mm, about 7 mm by about 7 mm, about 8 mm by about 8 mm, about 9 mm by about 9 mm, or about 10 mm by about 10 mm), and the sensor 100 can be designed to fit inside a chamber having a size of about 10 mm by about 50 mm (e.g., about 10 mm by about 20 mm, about 20 mm by about 30 mm, about 30 mm by about 40 mm, about 40 mm by about 50 mm, about 15 mm by about 45 mm, about 20 mm by about 40 mm, or about 17 mm by about 35 mm). The wires or lines can have any appropriate diameter (e.g., from about 1 to about 6 mm (e.g., about 1, about 2, about 3, about 4, about 5, or about 6 mm).

[0066] In some implementation, the wires or lines can provide electrical signals to a device provided herein, and / or can be used to transfer a device provided herein from a test fluid to a regeneration fluid or vice versa (see, e.g., FIG. 7C). The electrodes can form a delay line. In addition, such lines can be unidirectional or bidirectional. The electrodes can be of any useful configuration (e.g., an interdigitated configuration, an arrayed configuration, a gate configuration, a one-port configuration, a two-port configuration, a delay line configuration, a unidirectional configuration, a bidirectional configuration, etc.), geometry (e.g., bar electrodes, single finger electrodes, double finger electrodes, split finger electrodes, pruned double split finger electrodes, etc.), orientation (e.g., compared to a crystal orientation of the substrate), or electrical connection (e.g., shorted, grounded, open, closed, arrayed, etc.).

[0067] The electronics module 105 can include any useful circuit components, such as an oscillator circuit (e.g., a Pierce circuit, a Colpitts circuit, or a Clapp circuit including an amplifier or a transistor, such as a bipolar junction transistor); one or more signal generators (e.g., configured to generate an electrical signal or a radiofrequency (RF) signal); one or more relay circuits (e.g., including one or more circuits configured to transmit the electrical signal or the RF signal the contacts); one or more attenuation networks (e.g., including one or more circuit components to reduce the amplitude of a signal, such as by use of one or more resistors); one or more filters (e.g., a voltage- or current- selective, a high pass filter, a low pass filter, or a phase shifting filter); one or more amplifiers (e.g., transistors); one or more impedance matching networks (e.g., including one or more circuit components configured to match the impedance of the sensor to another electronic component, in which exemplary impedance matching networks can include an inductor with an optional resistor in series with the non-grounded contact(s) of thesensor); and / or one or more coupling networks (e g., configured to provide an output, such as a measured current, voltage, or frequency shift).

[0068] The electronics module 105 is electronically coupled, e.g., to receive and / or transmit electrical signals, to the sensor 100 and the movable component 103. For example, the electronics module 105 can control the reference electrode 114 to vary the gate voltage in the ISFET. In another example, the electronics module 105 can apply a bias voltage between the source and drain contacts and / or measure a current between the source and drain contacts. In some embodiments, the electronic module can include an ammeter (e.g., configured to measure the drain source current). As another example, the electronics module 105 can control movable component 103 to position the sensor 100 and either of the test and regeneration chambers 109a and 109b, such that the sensor 100 is at least partially submerged in either the test or regeneration fluids. In some implementations, the sensor 100 is attached to the movable component 103 by a controllable magnet or other releasable attachment.

[0069] A fluidics module 107 is fluidically coupled to the test and regeneration chambers 109a and 109b. The fluidics module 107 provides test fluid to the test chamber 109a and regeneration fluid to the regeneration chamber 109b. The fluidics module 107 can include any useful port, via, chamber, and / or channel to provide or deliver a sample to the sensing layer of the sensor. For example and without limitation, the fluidics module can include one or more chambers having a fluid, and a movable component can dip the sensor into the fluid within the one or more chambers. In another example, the fluidics module can include one or more channel configured to be in fluidic communication with a sensing layer of a sensor, and a fluidic controller is configured to deliver fluids to the channel(s) of the sensor. The fluidics module 107 may be configured to be a disposable manifold that aligns with the sensor 100 and the electronics module 105. Alternatively, the fluidics module may be reusable for repeat use. The fluidics module 107 can optionally include any useful fluidic structure having a length, width, height, cross-sectional dimension, circumference, radius (e.g., external or internal radius), or diameter (e.g., that is less than 1 mm, such for a microfluidic structure, or that is greater than 1 mm in some instances).

[0070] In some implementations, a fluidics module 107 can include a fluidics layer configured to be in fluidic communication with at least a portion of the sensing layer for a sensor 100, in which the fluidics layer can include a channel designed to overlie at least a portion of the sensing layer. In some implementations, the fluidics layer can include a bulk substrate havingone or more channels disposed therein. The bulk substrate can include any useful material, including but not limited to glass, quartz, silicon, polymers, metals, or combinations thereof. A fluidics module can include one or more inlet ports and / or outlet ports configured to provide fluidic communication between a fluid (e.g., a test fluid or a regeneration fluid) and the sensor 100. Fluidic communication can be provided by way of a fluidic interconnect (e.g., a tubing, a manifold, etc.) attached to the inlet port(s) and outlet port(s).

[0071] In some implementations, a fluidics module 107 can include a plurality of chambers, and the electronics module 105 is programmed to periodically move the sensor 100 from one chamber to the other. Additionally or alternatively, the electronics module 105 is programmed to move the sensor 100 from one chamber to the other based on a drain-source current measurement. For example, a drain-source current measurement can indicate that the functional groups on the sensing layer 110 are fully saturated with europium ions or that the europium ion probes are completely depleted.

[0072] In some implementations, the fluidics module 107 is configured to periodically provide the test or regeneration fluid to the test or regeneration chambers 109a and 109b, respectively. Additionally or alternatively, the fluidics module 107 is configured to provide test or regeneration fluid to the test or regeneration chambers 109a and 109b, respectively, when the sensor 100 is within the test or regeneration chambers 109a and 109b, respectively. For example, the electronics module 105 and fluidics module 107 can be programmed such that the sensor 100 is dipped into the regeneration solution for five seconds or at least one, two, three, four, five, seven, ten, or more seconds.

[0073] One or more sensors 100 can be implemented with one or more modules, e.g., electronics and fluidic modules 105 and 107, within the system 1000. In some implementations, the system 1000 includes a plurality of devices, e.g., one or more sensors 100, in which each device can be same or different. In some implementations, at least two of the plurality of devices are fluidically connected in series or in parallel.Chemistry

[0074] FIG. 2 depicts a schematic of chemical reactions on the sensing layer 110. As an example, the sensing layer 110 can be self-assembled graphene oxide functionalized with binding groups 202, e.g., carboxyl groups, such as -COO or -COOH; hydroxyl groups, suchas -OH or -O ; other anionic groups or chelating groups; and other functional groups such as phenolic-OH (e.g., R-OH or R-0 , in which R includes a phenyl group), ketone (e.g., a carbonyl group of a ketone, such as in RC(O)R’, in which each of R and R’ is, independently, an optionally substituted aliphatic group), lactone (e.g., a carbonyl group or an ester group of a lactone, such as in RC(O)OR’, in which R and R’, taken together, form an optionally substituted heterocyclic group), quinone (e.g., a carbonyl group or a hydroxyl group of a quinone), and epoxy (e.g., an ether group of an epoxy), as well as other oxygen-containing groups (e.g., carbonyl-containing groups (-C(O)- containing groups), hydroxyl-containing groups (-OH containing groups), or oxy-containing groups (-0- containing groups), such as in alcohols, phenols, ethers, aldehydes, ketones, carboxylic acids, and esters). When the sensor 100 is flushed with regeneration fluid, europium ions 204 (e.g., Eu ) bond with the binding groups 202, forming a coordination complex, e.g., -[COO]mEu3+, wherein m > 1. Each europium ion may bind with one binding group or a plurality of binding groups (e.g., two or three binding groups). When the sensor 100 is flushed with test fluid, ions or compounds in the test fluid can break the bond between the europium ions 204 and the binding groups 202, depending on their electron affinity. For example, a phosphate ion 206 (e.g., PO43) has a higher electron affinity than that of a carboxyl group to a europium ion 204, making it more likely that phosphate ions 206 will remove the europium ions 204 from the sensing layer 110 and bond with the europium ions 204, thereby forming molecule 208, e.g., a coordination complex where the phosphate acts as a ligand and the europium acts as a central metal ion.

[0075] The addition and removal of europium ions 204 bonded to the sensing layer 110 affects electrical properties of the channel material 108. Specifically, the location of the Dirac point, e.g., the charge neutrality point, shifts depending on whether the europium ions 204 are bonded to the sensing layer 110. In some implementations, the channel material 108 is or comprises graphene. Graphene has two inequivalent symmetry points called Dirac points, e.g., K and K’, in the Brillouin zone that mark where the valence and conduction band touch. In graphene, the electron density of states is lowest at these Dirac points, hence the term “charge neutrality point.” The density of states in a channel material of the transistor depends on the gating voltage, so the gating voltage can be varied to locate the Dirac point.

[0076] Given that the Dirac points are locations of charge neutrality, whether electrons or holes are the majority carriers in the channel material 108 switches at the Dirac point, e.g., conduction of holes in the negative slope region and conduction of electrons in the positive sloperegion. The dispersion properties and thus location of the Dirac point depend on the level of doping on the sensing layer 110. When ions are added to or removed from the sensing layer 110 in contact with the channel material 108, the electrical properties, e.g., the carrier density, of the channel material 108 change. With reference to FIGS. 3 A and 3B, the transfer curve for graphene has a V-shape with the Dirac point being the voltage Voirac at which a minimum current occurs.

[0077] FIG. 3A depicts the shift of the transfer curve, e.g., drain- source current versus gate potential, and therefore shift of the minimum voltage Voirac when binding groups on the functionalized sensing layer 110 bond with europium ions. The transfer curve is generated by measuring the drain-source current while varying the gating voltage with the reference electrode 114. The solid line 302 represents the drain-source current before the sensor is flushed with regeneration fluid, and the dotted line 304 represents the drain-source current after the sensor is flushed with regeneration fluid. As indicated by the arrow 306, the minimum voltage Voirac negatively shifts by an amount -AVg.

[0078] FIG. 3B depicts the Dirac point shift when the sensor 100 is flushed with test fluid and ions, e.g., inorganic phosphorus and hydrated phosphate, within the test fluid remove europium ions from the sensing layer 110. The dashed line 308 represents the drain-source current before the sensor 100 is flushed with test fluid and is in the same location as dotted line 304 from FIG. 3A. The solid line 310 represents the drain-source current after the sensor 100 is flushed with test fluid. As indicated by the arrow 312, the minimum voltage Voirac positively shifts by an amount +AVg.

[0079] In some implementations, the solid line 310 from FIG. 3B is in the same location as the solid line 302 from FIG. 3A, and the minimum voltage Voirac is the same. When the minimum voltage Voirac returns to the same value after cycle of flushing both regeneration and test fluids, the sensor 100 has returned to a state of europium ion probe depletion. In other words, the chemical and physical properties of channel material 108 have returned to an initial state where no europium ions are bonded with the binding group on the sensing layer 110.

[0080] Without being limited to any particular theory, the concentration of anions, e.g., phosphate ions, in the test fluid affects the Dirac point shift +AVg. Specifically, the logarithmic concentration of phosphorus is proportional (e.g., linearly proportional) to the Dirac point shift, which in turn can be measured using any sensor described herein. Therefore, the electronics module 105 can be configured to determine the concentration of phosphorus in a test fluid usingthe Dirac point shift in the relationship AVg~ log(Cp), where Cp is the concentration of phosphorus.Methods

[0081] FIGS. 4A - 4F depict the stages of a representative method of fabricating a sensor 100. With reference to FIG. 4A, the method begins with providing a substrate 502. The description for substrate 102 generally applies to the substrate 502 and will therefore be omitted. In some implementations, the substrate 502 comprises silicon oxide (SiCh), silicon (Si), or other semiconductor material.

[0082] With reference to FIG. 4B, the method continues with depositing, onto the substrate 502, two or more discrete portions of a conductive material to form two or more contacts 512. The two or more contacts 512 can be source and drain contacts 104 and 106. In some implementations, electrodes comprise a conductive and ohmic material, such as gold or other materials described herein. Deposition can include any useful lithography or microfabrication process, such as, e.g., electroplating, electroless plating, electrodeposition, chemical vapor deposition (CVD, including laser assisted CVD or plasma enhanced CVD), atomic layer deposition (ALD, including laser assisted ALD or plasma enhanced ALD), physical vapor deposition (PVD), localized electrochemical deposition (LECD), electron-beam evaporation, or any described herein.

[0083] With reference to FIGS. 4B and 4C, the method continues with depositing a layer of a channel material 522 to cover an exposed region 514 of the substrate 502 between at least two contacts 512. The description for the channel material 108 generally applies to the channel material 522 and can include any description provided herein. The channel material 522 can partially cover the two or more contacts 512, e.g., electrodes. In some embodiments, the channel material 522 is deposited predominantly in the exposed region 514 of the substrate 502. In some implementations, the channel material includes graphene (e.g., monolayer graphene). Deposition can include any useful lithography or microfabrication process, such as, e.g., growth of channel material by way of CVD, ALD, PVD, or any described herein. Such material can be grown on the substrate or transferred to the substrate (e.g., by way of mechanical or chemical exfoliation, layer-by-layer (LbL) assembly with a cationic compound and / or an anionic compound, or a combination thereof, etc.).

[0084] With reference to FIG. 4D, optionally, the method can continue with depositing an encapsulation material 532 to cover areas of the channel material 522 above the one or more contacts 512. The description for the encapsulation 112 generally applies to the encapsulation material 532 and can include any description provided herein. Deposition can include any useful lithography or microfabrication process, such as any described herein.

[0085] With reference to FIGS. 4D and 4E, the method continues with depositing a sensing material (e.g., as a sensing layer 542) on an exposed portion 534 of the channel material 522. The description for the sensing layer 110 generally applies to the sensing layer 542 and can include any description provided herein. In some implementations, depositing the sensing layer 542 includes depositing positively charged poly(diallyldimethylammonium chloride) (PDDA) or another polymer such as PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) in alternative layers with graphene oxide through one or more deposition cycles (in which each cycle includes deposition of PDDA and deposition of graphene oxide). In some implementations, depositing the sensing layer 542 includes depositing positively charged poly(diallyldimethylammonium chloride) (PDDA) in alternative layers with a negatively charged material, such as poly(sodium styrene sulfonate) (PSS), poly(allylamine hydrochloride) (PAH), poly(4-vinylpyridine) (P4VP), and linear poly(ethylene imine) (LPEI), through one or more deposition cycles. The sensing layer 542 can include graphene, graphene oxide, carbon nanotube, and / or carbon nanofiber, e.g., layer-by-layer (LbL) assembly with a cationic compound and / or an anionic compound, or a combination thereof. More details regarding LbL assembly and device components can be found in U. S. Patent Application Nos. 2007 / 0081681 Al and 2017 / 0059507 Al, each of which is hereby incorporated by reference.

[0086] With reference to FIG. 4F, the method continues with introducing europium ions 554 to the sensing layer 542. The europium ions 554 bond with binding groups 552, e.g., carboxyl groups, on the surface of the sensing layer 542, thereby immobilizing the europium ions 554 and forming europium probes. The carboxyl groups are added by treating graphene oxide (e.g., before or after forming the sensing layer 110) with carboxylic acid. In some embodiments, treatment with carboxylic acid provides a uniform dispersion with minimal precipitation. In some implementations, the europium probes are immobilized via dip coating. In some implementations, the europium ions 554 are salts dissolved in a solvent. The pH of the solution of salt and the solvent can be in a range of 4-5 when the concentration is 1000 ppm, and the solvents can include deionized water or another aqueous solvent.

[0087] In FIG. 4E, the sensing layer 542 can include one or more binding groups. In some implementations, the deposited or transferred sensing material (e.g., including graphene oxide) is treated to provide hydroxyl groups, which in turn are functionalized with an acid (e.g., including one or more carboxyl groups) to provide acidic binding groups (e.g., carboxyl groups) on a surface of the sensing layer. In turn, the acidic binding groups can chelate or other react with europium ions to provide one or more europium probes. In some implementations, the sensing material itself includes binding groups (e.g., binding group-functionalized forms of any material described herein, such as carboxyl-functionalized and / or hydroxyl-functionalized forms of graphene, graphene oxide, carbon nanotube, and / or carbon nanofiber). It is to be noted that multiple sensors fabricated in this manner can be formed on a single substrate or wafer (e.g., substrate 502).

[0088] For any material or layer herein, deposition can include any useful lithography or microfabrication process, such as, e.g., electroplating, electroless plating, electrodeposition, chemical vapor deposition (CVD, including laser assisted CVD or plasma enhanced CVD), atomic layer deposition (ALD, including laser assisted ALD or plasma enhanced ALD), physical vapor deposition (PVD), localized electrochemical deposition (LECD), electron-beam evaporation, galvanization, casting, metal infiltration, laser-induced forward transfer (LIFT), additive manufacturing techniques, electron beam lithography (EBL), direct laser writing (DLW), direct laser lithography, photolithography, near-field optical lithography, nanoimprint lithography (NIL), deep UV lithography (DUV), extreme UV lithography (EUV), multiphoton polymerization (MPP) lithography, dip pen lithography (DPL), scanning tunneling microscopy lithography, atomic force microscopy lithography, microstereolithography, molecular beam epitaxy (MBE), inkjet printing, electrohydrodynamic (EHD) jet printing, focused-ion-beam (FIB) milling, and the like.Uses

[0089] The technology described herein can be applied for various uses, including testing water (or environmental samples) for levels of phosphorous or testing nutrient levels in blood samples (or other biological samples).EXAMPLESExample 1: Detecting HPO42'

[0090] FIG. 5A shows a transfer curve before and after europium ion mobilization. Line 402 represents the transfer curve before mobilization, line 404 represents the transfer curve after one europium cycle of immobilization, and line 406 represents the transfer curve after ten cycles of europium immobilizations. As depicted in FIG. 5A, the Dirac point shifted to a lower voltage after the first europium mobilization. However, the Dirac point remained relatively stable even after ten europium immobilizations, e.g., within 15%.

[0091] FIG. 5B shows the transfer curve using test solutions having different concentrations of HPO42, which bonded with the europium ions 204 to form coordination complexes, e.g., molecules 208. The sensing layer 110, e.g., graphene oxide, was saturated by europium ion probes via dip coating in 1000 ppm of europium nitrate solution in deionized water. The concentration of HPCL2’ ranged from zero, e.g., a control being deionized water, to 100 ppm. As visible from various curves, the higher the concentration, the greater the Dirac point shift in a positive direction.

[0092] The minimum voltages of each of the curves from FIG. 5B were plotted versus the log of the concentration of each of the curves (FIG. 5C). The calibration curve, depicting the relationship between the Dirac point shift and the logarithm of the concentration of HPCL2', included a region, indicated by square symbols 409, of nonlinear growth and a region of linear growth indicated by circular symbols 410. In the region of nonlinear growth, the concentration of phosphorus in the test fluid was too low, e.g., less than a threshold concentration of 87 ppb, to be detected let alone follow a consistent growth pattern. In some implementations, the sensor achieved super-Nernstein sensitivity (69.9 mV per decade change in ion concentration of HPO42). Once the concentration reached tenths of parts per million, the Dirac point shift began to follow a linear trend with respect to the logarithm of the concentration.

[0093] The europium probes increased the sensitivity and improved the accuracy of the sensor. With reference to FIG. 5D, the calibration curve was dependent on the presence of the europium ions. For example, square symbols 412 represent the calibration curve without europium probes, circular symbols 414 represent the calibration curve with europium probes after a single regeneration-test cycle, and triangular symbols 416 represent the calibration curve with europium probes after ten regeneration-test cycles. Similarly to FIG. 5C, the calibrationcurves plotted in FIG. 5D had a nonlinear region below a certain concentration, e.g., 10’1ppm, but then entered a linear growth region until reaching a concentration of 10 ppm. Above 10 ppm, the relationship between the logarithm of the concentration and Dirac point shift was no longer linear, e.g., the sensor 100 was saturated because there was too much phosphate relative to the europium ion probes. As seen in FIG. 5D, when HPO42- bound with the europium ion probes, a positive AVgwas observed with super-Nernstian sensitivity of 69.9 mV / dec surpassing the Nernstian limit of 59.2 mV / dec.

[0094] As depicted by the calibration curves using europium probes after a single and ten regeneration-test cycles, the accuracy and sensitivity did not significantly deteriorate after 10 regeneration-measurement cycles, as indicated by the similarity of the curves with circular symbols 414 and the curves with triangular symbols 416.

[0095] In the absence of europium probes for measurements and when the concentration was less than the threshold concentration, the Dirac point shift was actually negative due to interference from cations such as sodium, leading to inaccurate results (e.g., negative concentrations of receiving faulty). Europium immobilization led to saturation of the oxygen functional groups, suppressing the cation interference below 100 ppb.

[0096] The positive Dirac point shift +AVgdue to phosphate ions selectively removing the europium probes from the sensing layer 110 was used to determine concentration of phosphorus in a test fluid, and the negative Dirac point shift -AVgdue to europium ions bonding with the binding groups on the sensing layer 110 was used to determine the status of remaining europium probes. When the Dirac point returned to an initial value before flushing the sensor 100 with regeneration fluid and test fluid, there were no more bonds between europium ions in the binding groups on the functionalized sensing layer 110.

[0097] In some implementations, the Dirac point shifted slightly varies as the sensor 100 went through regeneration-test cycles. With reference to FIG. 5E, the transfers curve for ten regeneration-test cycles shows how the Dirac point slightly changed, e.g., increased or decreased, with each cycle. The transfer curves during detection are represented by solid gray lines, and the transfer curves during regeneration are represented by solid black lines.

[0098] FIG. 5F is a plot depicting the ranges and averages of the Dirac point shifts, e.g., shifts indicating potential differences between the dotted and solid lines from FIG. 5E. The positive Dirac point shifts due to europium ion removal had a range of 0.06 V and an average ofabout 0.03 V, and the negative Dirac point shifts due to europium probe generation had a range of about 0.04 V and an average of about -0.03 V.

[0099] In some implementations, rather than using the transfer curve to determine the Dirac point shift, the conductivity or resistance of the channel material 108, e.g., graphene, was used to infer the Dirac point shift.Example 2: Detecting P(3~

[0100] Similarly to FIG. 5B, FIG. 5G depicts a transfer curve using test solutions having different concentrations of PCh3’, instead of HPO42’. As in FIG. 5G, the Dirac point shift generally increased as the concentration of PO43' in the test fluid increased. Similarly to FIG. 5C, the Dirac point shift as a function of the logarithm of the concentration of PC>43’ is plotted in FIG. 5H. Below concentrations of tenths of parts per million, the growth was nonlinear, e.g., the concentration was too low to follow linear behavior. However, the sensor could still detect a Dirac point shift below this threshold for linear growth, e.g., can detect PC>43’ with concentrations as well as 44 ppb. Above the order of 1 ppm, the relationship between the Dirac point shift and the logarithm of the concentration of PO43' was linear, but with a different slope compared to the linear growth below the order of 1 ppm. As seen in FIG. 5H, the non-limiting sensor achieved a super-Nernstian sensitivity of 85.5 mV / dec and a limit of detection of 44 ppb for PO43'.Imaging Fabricated Devices

[0101] FIGS. 6A and 6C depict images taken by a scanning electron microscope (SEM) to evaluate the quality of the channel material 522, the sensing material, and the sensing layer 542. FIGS. 6B and 6D are graphs representing data collected using Raman spectroscopy to evaluate the quality of the channel material 522, the sensing material, and the sensing layer 542. Other analytical methodologies (e.g., atomic force microscopy, transmission emission microscopy, x- ray diffraction, x-ray photoelectron spectroscopy, nuclear magnetic spectroscopy, and the like) can also be used to evaluate the quality of the devices disclosed herein. In FIGS. 6A-6D, the channel material 522 was graphene, and the sensing layer 542 was graphene oxide. The SEM images of the graphene channel material 522 and graphene oxide sensing layer 542 show clean and residue free surfaces of graphene and uniformly distributed graphene oxide nanoparticles, respectively.

[0102] FIGS. 6B and 6D are plots of the intensity of light generated from interactions with vibrations of the atoms in monolayer graphene and self-assembled graphene oxide, respectively. Raman spectroscopy plots can be predicted by theory, so the experimental results can be compared with theoretical predictions to assess the quality of materials. The letters above each peak, e.g., D, G, and 2D, refer to the bands in the phonon spectrum associated with the peak frequency. FIG. 6B shows a sharp G band, a sharp 2D band with greater intensity compared to the G band, and a D band with lower intensity compared to the G band. These bands indicate high quality monolayer graphene with low density of defects and disorders in the graphene lattice. FIG. 6D shows signatures of graphene oxide, e.g., the D and G peak at 1350 and 1580 cm ’, respectively.

[0103] In some implementations, the characterization of the quality of the channel material 522 and the sensing layer 542 can be done during or after the operations in FIGS. 4C and 4E, respectively, before proceeding to future steps. This way, updates to these operations can be carried out midway if either the SEM images or Raman spectroscopy indicate an issue with the material making up the channel material 522 and the sensing layer 542, respectively.

[0104] FIGS. 7A-7C and 8 illustrate devices containing sensors 700a, 700b, 700c, and 800, respectively, that were fabricated according to the stages illustrated in FIGS. 4A-4F. As depicted in FIG. 7A, device 700a included a single wafer 701a (which was about 100 mm in diameter). It is to be noted that the devices provided herein each can include a plurality of sensors (e.g., ten, 20, 30, 40, or more than 40 individual sensors 100). Each individual sensor 100 had a single pair of source and drain contacts 104 and 106. It is to be noted that the number of “teeth” in each source and drain contacts 104 and 106 can vary for different sensitivity standards. For example, the source and drain contacts 104 and 106 can have a total 2, 7, 9, 11, 14, or 26 teeth, with the channel material between each pair of teeth. For example, a sensor with seven teeth would have six channels. Each individual sensor may include one or more (e.g., two, three, four, five, or more than five) active channels (e.g., active semiconductor channels) disposed between each pair of source and drain contacts. In this example, device 700a included 54 sensors, each sensor having 2 or more teeth.

[0105] Device 700b, shown in FIG. 7B included a plurality of sensors 100. Similarly to device 700a, each sensor 100 in device 700b includes source and drain contacts 104 and 106, integrated onto a wafer 701b. Depending on the implementation, there can be benefits to having more or fewer teeth in the source and drain contacts 104 and 106. For example, a higher numberof teeth, e.g., 25, leads to a lower detection threshold, such that phosphate can be detected at relatively low concentrations. On the other hand, a higher number of teeth also can reduce stability and device resistance. Thus, in some implementations, a lower number of teeth are included to increase stability for devices, e.g., the devices will experience less drift in accuracy over time.

[0106] Device 700c, shown in FIG. 7C, included a sensor that was encased in housing 704 and attached to wires 702 for holding the device 700c (e.g., as the device 700c is dipped into test and regeneration fluid).

[0107] FIG. 8 shows an exploded view of a device 800 that was generated to remain stationary within an analyte solution and automatically cycle between regenerating europium ion probes and testing the analyte solution. As depicted, the device 800 included multiple sensor modules 802 supported on a wafer 804. Wafer 804 was about four inches in diameter and included eight sensor modules 802, although it is to be noted that other dimensions and numbers of sensor modules are contemplated.

[0108] Each sensor module 802 included a sensor 806 supported by a substrate 808 (in this case, a glass substrate) that included microfluidic channels 810. The microfluidic channels 810 were fluidically coupled to a measurement chamber 812 in which the sensor 806 was disposed. The sensor 806 was substantially similar to the sensors described herein, such as sensor 100.

[0109] The measurement chamber 812 was fluidically coupled to an analyte solution inlet 814a and an analyte solution outlet 814b. During measurement, a test solution potentially containing phosphate, e.g., water from a natural body of water in which the device 800 was disposed, flowed into the measurement chamber 812 through the analyte solution inlet 814a. It is to be noted that in some cases, the electronic module 105 can control analyte solution inlet 814a with a mechanical switch and micropump. After the phosphate in the test solution interacted with the europium ion probes of sensor 806, the test solution flowed out of the measurement chamber 812 via the analyte solution outlet 814b. It is to be noted that in some cases, a micropump positioned near the analyte solution inlet 814a can cause flow of the testing solution toward the analyte solution outlet 814b, and surface tension can prevent the tested testing solution from reentering the measurement chamber 812.

[0110] The measurement chamber 812 was fluidically coupled to the microfluidic channels 810 at a regeneration fluid inlet 814c and a regeneration fluid outlet 814d. The device 800 also included a micropump 816, which created a pressure differential to cause regeneration fluid in aregeneration fluid chamber 818 to circulate through the microfluidic channels 810. In this example, regeneration fluid flowed in a clockwise direction. In some cases, the pump 816 was a diffuser / nozzle pump. A piezoelectric disc 820, e.g., composed of lead zirconate titanate (PZT), was used to activate the pump 816. As shown, the pump 816 was located within one of the microfluidic channels 810, and the piezoelectric disc 820 was supported by the substrate 808. Device 800 also included an electronics module 105 to control the pump 816 and the piezoelectric disc 820.

[0111] The device 800 was configured to prevent the analyte solution in the measurement chamber 812 from entering the microfluidic channels 810. For example, since the inlets 814a and 814c for the analyte solution and regeneration fluid were separate, and the outlets 814b and 814d for the analyte solution and regeneration fluid were separate, an analyte solution will not have to enter the microfluidic channels 810 in order to be detected by the sensor 806. Further, the microfluidic channels 810 were designed to have a sufficiently small cross-sectional area to prevent the analyte solution from entering the microfluidic channels 810. Due to both the surface tension of water and the pressure differential between the measurement chamber 812 and the microfluidic channels 810, fluid from the measurement chamber 812 cannot easily flow into the microfluidic channels 810.

[0112] FIG. 9 depicts various stages of operating the device 800. During operation, the device 800 began in stage 910a, in which the device 800 is turned off, e.g., the pump 816 was deactivated. Then, the device 800 was powered on and entered stage 910b, in which probe generation occurred. When the pump 816 was activated, regeneration fluid from the regeneration fluid chamber 818 circulated through the microfluidic channels 810, through the measurement chamber 812, and back to the regeneration fluid chamber 818. The regeneration fluid included europium ions, which bonded to the functionalized surface of the sensor 806 to create europium ion probes for detecting phosphate. Stage 910b lasted about five minutes, which was sufficient to allow the europium ion probes to fully saturate.

[0113] After stage 910b, the device entered stage 910c, in which testing occurs. In stage 910c, the pump 916 was turned off, and a test solution flowed from inlet 814a to outlet 814b to expose the sensor 806 to a test solution. Since phosphorus (e.g., in the form of phosphate) was present in the test solution, the phosphorus caused the bond between the europium ions and the functionalized surface to break, thereby depleting the europium ion probes.

[0114] As the device 800 alternated being stages 910b and 910c, the device 800 flushed the measurement chamber 812 with fresh regeneration fluid containing fresh europium ions and fresh test fluid containing fresh phosphorous compounds at the beginning of each stage. During each instance of stage 910c, the sensor 806 determined a fresh concentration of phosphorous ions in the test solution. It is to be noted that in some implementations, the measurement of the Dirac point after europium probe immobilization, e.g., stage 910b, can be used to determine when to either end stage 910c and either return to 910b or power off the device 800. For example, a Dirac point below a designated threshold value can indicate that stage 910b was unsuccessful, e g., due to sensor failure or the need to replace the regeneration fluid.

[0115] After completing a first regeneration-test cycle, e.g., one instance of each of stages 910b and 910c, the device repeated the cycle multiple times, e.g., once or twice a day for three to six months (several hundred cycles). For example, each regeneration-test cycle can last about 10 to 30 minutes, with stage 910b lasting between 5-15 minutes (e.g., about 5, about 10, or about 15 minutes) and stage 910c lasting between 5-15 minutes (e.g., about 5, about 10, or about 15 minutes). The device 800 can be configured to automatically alternate between stages 910b and 910c. After completing a certain number of cycles, the device 800 reentered stage 910a, in which the device 800 was powered off.

[0116] This configuration means that the device does not have to be transferred from a test solution chamber to a regeneration fluid chamber to complete a testing-regeneration cycle. Rather, for example, the device 800 can be suspended in in a test solution for long periods of time, e.g., about three to twelve months, about four to eight months, or about six months. Thus, the device 800 can measure the concentration of phosphorus over time in a natural body of water (e.g., a river, lake, or stream) in real time. The regeneration fluid can be replaced regularly, e.g., every n months, where n is a number from 1 to 12, to account for reduced europium concentration in the regeneration fluid over time. In such cases, during stage 910c, the device 800 allows the test solution to flow into and out of the measurement chamber 812. The test solution that flows out of outlet 814b includes bonded europium phosphate, which is not harmful to the environment and can safely reenter the natural body of water.

[0117] It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

Claims

CLAIMS1. A device comprising: one or more source contacts disposed on a surface of a substrate; one or more drain contacts disposed on the surface of the substrate; a channel material disposed between at least one source contact and at least one drain contact; a sensing layer disposed on at least a portion of a surface of the channel material; and one or more europium ions bonded to the sensing layer.

2. The device of claim 1, further comprising: a chamber above the sensing layer, coupled to an input port and an output port, and configured to receive and flush a solution; and a reference electrode disposed in solution.

3. The device of claim 2, wherein the reference electrode includes at least one of silver and silver chloride.

4. The device of claim 1, wherein the device is a transistor.

5. The device of claim 1, further comprising an encapsulation layer (i) defining one or more sensing channels where the encapsulation layer has one or more holes from a top side to a bottom side and (ii) disposed to partially cover the channel material above the one or more source contacts and one or more drain contacts.

6. The device of claim 5, wherein the encapsulation layer comprises a negative photoresist.

7. The device of claim 5, wherein the sensing layer comprises self-assembled graphene oxide disposed on the encapsulation layer and contacting the channel material through the one or more sensing channels.

8. The device of claim 1 , wherein the sensing layer comprises graphene, graphene oxide, carbon nanotubes, carbon nanofibers, or a combination thereof.

9. The device of claim 1, wherein at least one of the one or more source contacts and / or one or more drain contacts comprise gold.

10. The device of claim 1, wherein the one or more source contacts are interdigitated with the one or more drain contacts.

11. The device of claim 10, wherein the one or more source contacts and one or more drain contacts are one of interdigitated transducers (IDTs), bar electrodes, or strip electrodes.

12. A system comprising: a device of claim 1; an electronics module configured to receive electrical signals from the device, transmit electrical signals to the device, or transmit and receive electrical signals to and from the device; and a fluidics module configured to provide one or more fluids to the sensing layer of the device.

13. The system of claim 12, wherein the fluidics module comprises: a first chamber in fluidic communication with the sensing layer, wherein the first chamber is configured to provide a test fluid; a second chamber in fluidic communication with the sensing layer, wherein the first chamber is configured to provide a regeneration fluid; and a movable component configured to transport the device between the first and second chambers.

14. The system of claim 12, further comprising: a pump in electronic communication with the electronics module; and a fluid channel, wherein the pump is configured to cause circulation, in the fluid channel,of a regeneration fluid containing europium ions, wherein the circulation of the regeneration fluid comprises flowing the regeneration fluid from a regeneration fluid chamber, to a first inlet of the device, to a first outlet of the device, and back to the regeneration fluid chamber.

15. The system of claim 14, further comprising: a second inlet of the device; a second outlet of the device; and an analyte solution chamber in fluidic communication with the second inlet.

16. The system of claim 15, wherein the first inlet is a separate inlet from the second inlet, and the first outlet is a separate outlet from the second outlet.

17. A method of making a device comprising: depositing, onto a substrate, two or more discrete portions of a conductive material to form two or more contacts; depositing a layer of a channel material to cover an exposed portion of the substrate between at least two contacts; depositing a sensing layer on an exposed portion of the channel material; and introducing europium ions to the sensing layer.

18. The method of claim 17, further comprising: before depositing the sensing layer, depositing an encapsulation material to cover one or more areas of the channel material above the one or more contacts.

19. The method of claim 17, wherein the sensing layer is graphene oxide that was selfassembled using PDDA.

20. The method of claim 17, wherein the sensing layer is self-assembled graphene oxide, graphene, carbon nanotubes, carbon nanofibers, or a combination thereof.

21. The method of claim 17, wherein the europium ions are salts dissolved in a solvent.

22. The method of claim 21, wherein a pH of the europium ions dissolved in the solvent is from about 4 to about 5.

23. A method comprising: varying a gating voltage while measuring a first drain-source current in a device comprising a transistor, wherein the transistor comprises a sensing layer disposed on at least a portion of a channel material; determining a first reference voltage where a first minimum in the first drain-source current in the device occurs; flushing the device with a test solution, thereby causing phosphorous ions, if present in the test solution, to bond with europium probes bonded to the sensing layer; varying the gating voltage in the device while measuring a second drain-source current in the device; determining a second reference voltage where a second minimum in the second drainsource current in the device occurs; determining a voltage difference between the second and first reference voltages; and determining, using the voltage difference, a concentration of the phosphorous ions in the test solution.

24. The method of claim 23, further comprising: before varying the gating voltage while measuring the first drain-source current, varying the gating voltage while measuring a third drain-source current; determining a third reference voltage where a third minimum in the third drain-source current occurs; flushing the device with a regeneration fluid thereby causing europium ions, if present, to bond with functional groups on the sensing layer; varying the gating voltage while measuring a fourth drain-source current; determining a fourth reference voltage where a fourth minimum in the fourth drainsource current occurs; and determining a second voltage difference between the fourth and third reference voltages.

25. The method of claim 24, wherein flushing the device with the regeneration fluid comprises activating a pump to circulate the regeneration fluid through a fluidic channel in communication with a regeneration fluid source within the device.

26. The method of claim 25, wherein flushing the device with the test solution comprises flowing test solution from an inlet of the device to an outlet of the device when the pump is deactivated.

27. The method of claim 24, comprising: using the second voltage difference, determining to flush the device with the regeneration fluid.

28. The method of claim 24, wherein flushing the device with the test solution comprises flushing the device with the test solution while the device is suspended in a fluid comprising the test solution, and wherein flushing the device with the regeneration fluid comprises flushing the device with the regeneration fluid while the device is suspended in the fluid comprising the test solution.

29. The method of claim 24, further comprising flushing the device with fresh regeneration fluid comprising europium ions, thereby causing fresh europium ions in the regeneration fluid to bond with the functional groups on the sensing layer, thereby forming fresh europium probes.

30. The method of claim 29, further comprising flushing the device with fresh test solution, thereby causing fresh phosphorous ions, if present in the fresh test solution, to bond with the fresh europium probes bonded to the sensing layer.

31. The method of claim 30, further comprising determining a concentration of the fresh phosphorous ions in the fresh test solution.

32. The method of claim 30, wherein flushing the device with the fresh test solution automatically alternates with flushing the device the fresh regeneration fluid.

33. A method comprising: determining a Dirac point of a device comprising europium ions attached to a sensing layer; exposing the device to a test solution; measuring a shift in a gating potential in the device away from the Dirac point; and determining a presence or absence of phosphorous ions in the test solution based on the shift in the gating potential.

Images