Patent Application
20180231514
None.
At present, chemical sensors and electronic nose technology lack the capability of operating at sub-10 nW or nearly-zero power, which limits their distribution over a large area due to limited lifetime during battery operation. Existing chemical sensors and electronic nose technology can be categorized into four main groups, depending on their working principles: conductivity, piezoelectric, optical and field-effect-transistor (FET) sensors. FET sensors operate based on threshold voltage changes that are caused by the interaction of a gate material with certain gases, resulting in changes in work functions. Such work function changes occur due to polarization of the gate material surface and interface with catalysts (e.g. metal oxides) by target gases. To enhance such interaction and thus sensitivity, these sensors preferably operate at an elevated temperature between 50° C. and 170° C., which is not appropriate for some low-power applications. Optical sensors utilize a coating of fluorescence dyes around an optical fiber and measure the optical property changes, such as wavelength shifts. However, optical sensors typically require a continuously-power-consuming scheme of light sources and detectors, making the system too complex and inappropriate for many low-power applications. Piezoelectric sensors, such as surface acoustic wave (SAW) and quartz crystal microbalance (QCM) sensors, measure the shifts in frequency of acoustic waves caused by interaction with or mass of gas molecules that are captured in a gas sensitive membrane. To produce high-frequency (>1 MHz) vibration of the device, piezoelectric sensors inherently require high power consumption of greater than 100 μW. Conductivity sensors produce changes in conductance by interaction with a gas and a gas sensitive membrane and are further categorized into three groups, depending on the membrane material types: polymer composites (non-conductive), conducting polymers and metal oxides. Among these materials, metal oxides require high temperature to operate as gas sensors, typically 200° C. to 500° C., thus requiring high-power consumption. Both non-conductive and conductive polymers operate at room temperature and do not need an integrated heater or high power consumption. However, their ‘off-current’ is non-trivial, typically above 1 μA considering their resistance values between 1 kΩ and 1000 kΩ at an operation voltage of 1.0 V, which results in power consumption of greater than 1 μW. Most recent percolation-based chemical sensors operate in liquid with non-trivial off-power consumption of greater than 1 μW. Additionally these sensors rely on pattern recognition electronics to achieve target selectivity, which further precludes nearly zero-power operation, which is not appropriate for extended lifetime from a battery. In short, existing chemical sensors and electronic nose technology have not simultaneously achieved chemical selectivity and ultra-low power consumption with long battery lifetime.
The present invention provides chemically-selective percolation switches and sensors incorporating such switches that can operate with zero or near-zero power consumption when a target chemical is present below a certain threshold concentration. In some examples, a chemically-selective percolation switch can include a positive electrode and a negative electrode separated from the positive electrode by a switch gap. A binding agent can be located at a plurality of binding sites in the switch gap. The binding agent can be selective for binding a target chemical compound. The binding sites can be distributed in the switch gap such that the binding sites are capable of binding molecules of the target chemical compound to form an electrically conductive pathway via percolation between the positive electrode and the negative electrode when the chemically-selective percolation switch is exposed to a threshold concentration of the target chemical compound.
In further examples of the present invention, a zero-power digital chemical analyzer can include a power supply, a detection circuit, and a chemically-selective percolation switch. The chemically-selective percolation switch can be electrically connected between the power supply and the detection circuit to switch the detection circuit to an on-state when the chemically-selective percolation switch is exposed to a threshold concentration of a target chemical compound. The chemically-selective percolation switch can include a positive electrode and a negative electrode separated from the positive electrode by a switch gap. A binding agent can be located at a plurality of binding sites in the switch gap. The binding agent can be selective for binding to the target chemical compound. The binding sites can be distributed in the switch gap such that the binding sites are capable of binding molecules of the target chemical compound to form an electrically conductive pathway via percolation between the positive electrode and the negative electrode when the chemically-selective percolation switch is exposed to the threshold concentration of the target chemical compound.
In still further examples of the present invention, a digital chemical analyzer can include a power supply, a first chemically-selective percolation switch, and a second chemically-selective percolation switch. The first chemically-selective percolation switch can be tuned to conduct electric current from the power supply when exposed to a first threshold concentration of a target chemical compound, and the second chemically-selective percolation switch can be tune to conduct electric current when exposed to a second threshold concentration. The second threshold concentration can be greater than the first threshold concentration or it can react to other target gases having its own threshold concentration. The first and second chemically-selective percolation switches can each include a positive electrode and a negative electrode separated from the positive electrode by a switch gap. A binding agent can be located at a plurality of binding sites in the switch gap. The binding agent can be selective for binding to the target chemical compound. The binding sites can be distributed in the switch gap such that the binding sites are capable of binding molecules of the target chemical compound to form an electrically conductive pathway via percolation between the positive electrode and the negative electrode when the chemically-selective percolation switch is exposed to the threshold concentration of the target chemical compound. These switches can be connected in parallel or in series. More than two switches can be connected in various combinations of parallel or series connections.
These drawings are provided to illustrate various aspects of the invention and are not intended to be limiting of the scope in terms of dimensions, materials, configurations, arrangements or proportions unless otherwise limited by the claims.
While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the spirit and scope of the present invention. Thus, the following more detailed description of the embodiments of the present invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics of the present invention, to set forth the best mode of operation of the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the present invention is to be defined solely by the appended claims.
In describing and claiming the present invention, the following terminology will be used.
It is noted that, as used in this specification and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “an electrode” includes one or more of such materials and reference to “detecting” includes reference to one or more of such steps.
As used herein, the terms “about” and “approximately” are used to provide flexibility, such as to indicate, for example, that a given value in a numerical range endpoint may be “a little above” or “a little below” the endpoint. The degree of flexibility for a particular variable can be readily determined by one skilled in the art based on the context.
As used herein, the term “substantially” refers to the complete or nearly complete extent or degree of an action, characteristic, property, state, structure, item, or result. The exact allowable degree of deviation from absolute completeness may in some cases depend on the specific context. However, the nearness of completion will generally be so as to have the same overall result as if absolute and total completion were obtained. The use of “substantially” is equally applicable when used in a negative connotation to refer to the complete or near complete lack of an action, characteristic, property, state, structure, item, or result.
As used herein with respect to an identified property or circumstance, “substantially” refers to a degree of deviation that is sufficiently small so as to not measurably detract from the identified property or circumstance. The exact degree of deviation allowable may in some cases depend on the specific context.
As used herein, “percolation” and “chemical percolation” refer to the natural phenomenon of molecules of a target chemical compound forming an electrical connection between two electrodes when the target chemical compound is present above a certain threshold concentration. This phenomenon is described in more detail below. As used herein, “structure-assisted percolation” refers to the phenomenon of target chemical compound molecules forming an electrical connection in a switch gap where electrically conductive structures are present to assist in forming the electrical connection across the switch gap. As described in more detail below, electrically conductive structures can be placed in the switch gap to provide additional control over binding sites for the target chemical compound.
As used herein, “switch gap” refers to a gap between a positive electrode and a negative electrode of a chemically-selective percolation switch. In some examples, the switch gap can be substantially empty space. In other examples, the switch gap can contain electrically conductive structures.
As used herein, “structure gap” refers to a gap between adjacent electrically conductive structures within a switch gap. Thus, when multiple electrically conductive structures are present in a switch gap, the structures can be separated one from another by a structure gap. The structure gap distance is typically smaller than the switch gap distance. Furthermore, although in most cases structure gaps can be a uniform gap distance within a particular switch gap, the structure gaps and even shapes can be varied in some embodiments.
As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.
Concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly recited limits of 1 to about 4.5, but also to include individual numerals such as 2, 3, 4, and sub-ranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only one numerical value, such as “less than about 4.5,” which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described.
Any steps recited in any method or process claims may be executed in any order and are not limited to the order presented in the claims. Means-plus-function or step-plus-function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) “means for” or “step for” is expressly recited; and b) a corresponding function is expressly recited. The structure, material or acts that support the means-plus function are expressly recited in the description herein. Accordingly, the scope of the invention should be determined solely by the appended claims and their legal equivalents, rather than by the descriptions and examples given herein.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the technology is thereby intended. Additional features and advantages of the technology will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the technology.
With the general examples set forth in the Summary above, it is noted in the present disclosure that when describing the system, or the related devices or methods, individual or separate descriptions are considered applicable to one other, whether or not explicitly discussed in the context of a particular example or embodiment. For example, in discussing a device per se, other device, system, and/or method embodiments are also included in such discussions, and vice versa.
Furthermore, various modifications and combinations can be derived from the present disclosure and illustrations, and as such, the following figures should not be considered limiting.
The present invention provides chemically-selective percolation switches and sensors incorporating these switches. Such chemical sensors can ‘sleep’ when the ambient concentration of the target chemical compound is below a certain threshold, thus normally not consuming any power. The sensor can ‘wake up’ only in the event of introduction of the chemical target above a certain threshold concentration. In this way, the sensor can minimize the power consumption by substantially eliminating any static (or always-on) power consumption. This can dramatically extend the lifetime of the sensor by multiple orders of magnitude, thus greatly reducing the need for periodic battery replacement. Accordingly, the present disclosure is directed at chemically-selective percolation switches and digital chemical analyzers that incorporate the chemically-selective percolation switches. These devices introduce a new operation principle of a sensor that normally sleeps but can sense the introduction of chemical targets above a certain threshold concentration. This device uses the natural phenomenon of percolation. Percolation is a result of the natural process of multiple particles or molecules randomly forming an electrical connection within a switch gap. The switch gap is a space between a positive electrode and negative electrode in a switch according to the present invention. It has been found that a switch gap can be filled in various ways to form a connection between the electrodes. It has also been mathematically shown that a certain threshold concentration of particles is needed to result in percolation. This process does not require any power consumption because it is based on natural phenomenon. This invention provides sensors that can sense airborne target chemical compounds by utilizing the percolation phenomenon and a switch to wake up the sensor system above a certain target chemical concentration, thus consuming nearly-zero power normally when no or insufficient target chemical compounds are present.
Because the target chemical molecules are constantly in motion, or only temporarily resides on the binding sites, the electrical connections formed by the molecules in the switch gap can be intermittent. In other words, an electrical connection formed by the random motion of molecules can be formed and last for a short period of time before one or more of the molecules moves away and breaks the connection. However, it has been found that when the surrounding concentration of the target chemical reaches a certain threshold, the molecules can tend to form electrical connections more often than not, so that the switch conducts electricity substantially constantly. Indeed, the current flowing through the switch can increase dramatically when the target chemical concentration rises above the threshold concentration.
When target chemical molecules become sufficiently available, the molecules will statistically adhere to the binding sites between the electrodes and form continuous paths for electron tunneling. However, this occurs only when the concentration is above a certain threshold. This phenomenon has been heavily modeled in mathematics, known as percolation theory. Various percolation models are available in 2D and 3D coordination. The simulations described herein use a site-percolation on a 2D Bethe lattice as the percolation model due to the similarities of molecular behaviors in the structure of the zero-power digital chemical analyzer. However, other 2D and 3D simulation models can be used such as, but not limited to, site percolation, bond percolation, Bernoulli percolation, Fortuin-Kasteleyn random cluster model, Potts models, directed percolation, first passage percolation, and the like. Corresponding lattice forms can include, but are not limited to, regular square grid, hexagonal grid, triangular grid, pentagonal grid, and the like. Note that the invention includes the possibility of including both 2D and 3D structure-assisted percolation by forming, i.e. hexagonal 2D gaps among conductive planar electrodes, or hexagonal 3D gaps among conductive pillar electrodes. Simulation results predict a sharp transition between off and on states as shown in
Output digitization can be achieved in the chemically-selective percolation switch due to the characteristics of switching operation across a threshold, and the digitization threshold can be tuned by properly selecting the design parameters such as material properties and geometry. To be able to ‘design’ the digitization threshold values, a percolation prediction model can be devised by combining a percolation model algorithm with experimentally-obtained coefficients. Specifically, the established percolation model can consider the combinatory effects among adsorption periods of a molecule, binding site densities, a gap distance (i.e. both switch gap and structure gap), surface areas and molecular transportation in both 2D and 3D coordination. Particularly, gap distance control between the two electrodes is one of the design factors that can be used to manipulate threshold concentration.
An algorithm was developed by utilizing the Leath-Alexanderowicz method, to predict threshold and conduction probability. The probability distribution of growing cluster is described as P(n, b)=m(n, α)Kn−x(c/cα)n-1[(1−c)/(1−cα)]αn, where n is the number of sites, C is the target concentration, Ca is the threshold concentration, and K, x are experimentally-adjusted constants, resulting in the percolation probability depending on target concentrations. The simulation results showed that a percolation threshold pc can be predicted in a normalized form to the total particle numbers required to completely fill the gap. Since the total particle number depends on electrode gap and width, binding site densities and adsorption (residence) period of the particle, the threshold concentration can be manipulated by controlling those parameters. For example, threshold concentration increases when the particle adsorption period decreases, the binding site densities decrease, and the electrode distance increases, thus allowing for threshold programming. Note that adsorption periods can also be adjusted by modifying binder properties, while binding site densities and electrode dimensions can be designed. Based on this algorithm, a percolation prediction model can be provided that is capable of providing design rules for a wide-range of percolation sensors, including electrode gap distances, structure gap distances, electrode widths, binding site densities and adsorption periods, by comprehensively incorporating the coefficients from the experimental data.
In various examples of the present invention, chemically-selective percolation switches can have a form as shown in
The example shown in
In further examples of the present invention, a chemically-selective percolation switch can include electrically conductive structures in the form of overlapping horizontal parallel plates formed in the switch gap. Thus, the structure gaps can be oriented perpendicular to the switch gaps. The parallel plates can be formed so as to create a network of fluid flow channels which allow fluid to enter and exit the switch gap and corresponding structure gaps. In one example,
To reliably fabricate nano-size gaps, standard lithography with vertical pillar deposition or sacrificial layer deposition-and-removal can be utilized as a simple and robust technique, with an e-beam technique as an alternative. In one example,
In some examples, the chemically-selective percolation switch sensors can be fabricated by a combined microfabrication and chemistry procedure. In one example, a fabrication method can include precisely defining a switch gap in the range of 10 nm˜50 μm. Due to lithographic limitations, a 10 nm gap can be defined by the e-beam nanolithography or the thickness of a deposited layer.
As mentioned above, in some examples the chemically-selective percolation switch can include vertical pillars within the switch gap.
The chemically-selective percolation switches according to the present invention can be designed to have any of a wide range of threshold target chemical concentrations, depending on the desired sensing threshold. In some examples, the threshold concentration can depend in part on residence time of target chemical molecules or particles in the binding sites of the switch. In certain examples, particle residence time in a binding site can be from 10 milliseconds to 100 seconds. Notably both threshold concentrations and adsorption period (switch-on-period) can be designed by selecting gap distance, binder types and densities, which enables programmability of detection levels.
In further examples, particle adsorption to the binding sites can be specific through ultra-selective host-guest recognition. This host-guest recognition is the process of holding molecules without covalent (permanent) bonding. A target molecule is adsorbed by a host molecule, such as a crown ether, only when the size, shape and charge-distribution of the target and the host match with each other, leading to ultra-specific binding. Since this does not form covalent bonding, the binding can be breakable, and the host thermo-dynamically desorbs the target molecules to reach a lower Gibbs energy equilibrium, enabling reversibility of adsorption. The reversibility depends on adsorption process (instead of absorption) where particles only temporarily attach onto the binding sites. As the adsorption period becomes longer, then the reversibility time becomes longer. The length, charges, etc. of the chemical tether can be selected to tune the half-adsorption-lifetime of the receptor complex. Note that the reversibility of the chemically-selective percolation switch is related to the ‘group’ reversibility or the percolation period based on these individual adsorption periods, upon the removal of target concentrations.
The chemically-selective percolation switches as described herein can be designed to detect a variety of target chemicals. In certain examples, the target chemicals can be chemical warfare agents, including paralytic shellfish toxins (PST), such as saxitoxin, tetrodotoxin, zetekitoxin, chiriquitoxin, or sarin. In other examples, the target chemical compound can be a fuel, an air pollutant, an airborne compound, an explosive, an airborne biological agent, or combinations thereof. Ultra-selectivity for these target chemicals, without the assistance of electronic pattern recognition, can be achieved by synthesizing and employing sophisticated chemistry binding that adsorbs only particular targets. Thus, bonding with the target molecule can be via hydrogen bonding, covalent bonding, van der Waals attraction, or any other association which allows electron transfer through the target molecule.
In one specific example, a chemically-selective percolation switch can include nano-gaps treated with amine-PEG-amine and crown-tetracarboxylic-acid layers, forming binding sites for target molecules. The configuration of the chemically-selective percolation switch can include electrically conductive structures with an interdigitated shape to accommodate multiple gaps in series.
In some examples, the threshold concentration of a switch can be adjusted by adjusting the surface concentration of binding agent linked to the surfaces of electrodes and/or electrically conductive structures in the switch. For example, a higher surface concentration of binding agent can make it more likely for bridges to form between the electrodes, thus reducing the threshold concentration of target chemical required to close the switch. Similarly, a lower surface concentration of binding agent can result in a higher threshold concentration for the switch. Surface concentration can be controlled by treating the electrodes with a solution having a known concentration of the binding agent. In some cases, a treatment solution can include a binding agent that is active for binding the target chemical as well as a non-binding agent that can link to the electrode surfaces, but which will not bind the target chemical. The proportion of the binding agent and non-binding agent can be adjusted to control the surface concentration of the binding agent on the electrodes. In one example, the binding agent and non-binding agent can link to the electrode surfaces through a linking group such as a thiol group, an amine group, a siloxy group, and others.
In a certain example, the binding agent can be applied to the electrodes using a compound comprising two binding agent groups attached to a central photocleavable group. This compound can be applied to electrodes with a nano-gap with a gap distance that is approximately the same as the length of the compound. After the binding agent groups link to the electrode surfaces on each side of the gap, the photocleavable group can be removed to leave two binding agents attached to the electrodes on opposite sides of the gap. This can ensure that the binding agents are aligned so that a target chemical molecule can be captured between the binding agents.
The selection of binding agents can also affect the threshold concentration of the chemically-selective percolation switch. Binding agents can be selected to have a high degree of conjugation to allow for more electrical conductivity. The length of the binding agent molecules can also be selected to make an appropriately sized space for a single target chemical molecule to be capture between two binding agent molecules. The type of capture group on the binding agent molecule can be selected to match with the target chemical. For example, the capture group can include hydrogen bond donors spaced apart at a distance that matches with hydrogen bond acceptors on the target molecule. Conversely, if the target chemical has hydrogen bond donors then the capture group on the binding agent can have matching hydrogen bond acceptors. In one particular example, saxitoxin can be the target chemical. Saxitoxin has 2 hydrogen bond donors spaces about 7 angstroms apart. Therefore, an effective capture group for saxitoxin can include 2 hydrogen bond acceptors spaced 7 angstroms apart. In another example, sarin has 2 hydrogen bond acceptors spaced about 3 angstroms apart. Therefore, an effective capture group for sarin can have 2 hydrogen bond donors spaced 3 angstroms apart.
The strength of interaction between the target chemical and the binding agent can affect the on-rate and off-rate of target chemical molecules bound to the binding agent. The off-rate can also be affected by the geometry of the switch. For example, a switch can include vertical pillars in the switch gap. The vertical pillars can be treated with a binding agent. The structure gap distance between the pillars can be sufficient to allow a single molecule of the target chemical to be bound between binding agent molecules attached to opposite pillars. In this example, the height of the pillars is one parameter that can be adjusted to change the off rate of the target chemical molecules bound in the structure gaps between the pillars. When the pillars are taller, a target chemical molecule can have a longer distance over which to diffuse in order to exit the structure gap between the pillars. While the target chemical molecule is moving through the structure, the target chemical molecule can become bound to other binding agent molecules. Thus, a target chemical molecule can take a longer time to become completely unbound from the binding agent molecules on the pillars. A slower off-rate can result in a lower threshold concentration for the switch because bound target chemical molecules tend to maintain an electrically conductive bridge for a longer period of time.
The critical or threshold concentration of the target chemical can also be modeled mathematically using the following equations:
In Equations 1-5, CJS is the surface concentration of junctions made up of a target molecule bound between two binding agent molecules; λ is a constant that can be determined experimentally; CPS is the surface concentration of binding agents (i.e., probes); CTS is the surface concentration of target molecules; a is the volume-to-surface conversion coefficient; CTA is the volumetric concentration of target molecules present; NJ is the number of junctions; Ael is the surface area of the electrode; and CTAC is the critical or threshold concentration of target chemical for the switch to close (N=1 in this case because a single junction can form a bridge between the electrodes). Thus, the threshold concentration is inversely proportional to the area of the electrode and the surface concentration of binding agent on the electrode surface.
In certain examples, the threshold concentration can be adjusted within a wide range, e.g. from 1 part per billion (ppb) to 100 part per million (ppm), and in some cases to 1000 ppm, depending on the switch geometry, binding agents and target molecules.
The chemically-selective percolation switches described herein can be used in zero-power digital chemical analyzers. In some examples, a zero-power digital chemical analyzer can include a power supply, a detection circuit, and a chemically-selective percolation switch electrically connected between the power supply and the detection circuit. The chemically-selective percolation switch can be configured to switch the detection circuit to an on state when the switch is exposed to a threshold concentration of a target chemical compound.
In certain examples, the chemically-selective percolation switch can conduct a trivial current of less than 1 pA (and in some cases less than 1000 pA at 1V) when the chemically-selective percolation switch is exposed to a concentration of the target chemical compound below the threshold concentration. The switch can conduct a significant amount of current of at least 1 nA when the chemically-selective percolation switch is exposed to the threshold concentration of the target chemical compound, forming an electrically-conductive path via the natural percolation phenomenon.
In some examples of the present technology, a zero-power digital chemical analyzer can be used to identify the presence of specific aerosol and vapor-phase chemical signatures, thus producing a digital output code identifying the target species when the concentration of the target species exceed as specified threshold. The zero-power chemical analyzer can include one or more chemically-selective percolation switches, which dramatically change resistance when exposed to a specific compound. None of these mechanisms require external power consumption other than for the provision of bias and the switching output voltage, and yet the system has the potential to digitally sense vapor concentrations in the ppm and ppb range, depending on the particular materials. This system can be used to detect many different chemical targets, several non-limiting examples of which include chemical-warfare-agent (CWA) aerosols and vapors, fuel, and explosive vapors.
In order to meet the ‘zero-power’ requirements while maintaining selectivity, sensitivity, and digitization capability of target chemical inputs, the system can take advantage of the mechanism of percolation of the target chemical species. Specifically, the chemically-selective percolation switches can use a chemically-selective percolation phenomenon to control the electrical conductivity between two electrodes. Random motions of particles can cause the particles to adsorb onto a binder-filled surface and form a particle bridge across a gap, if the concentration of the particles exceeds a certain threshold. Due to the particle bridge, a conductive path can be formed between the two electrodes for a limited adsorption period, transferring the electric potential from the input (battery) to the output and electrons conduct through the gap via percolated particles. When the concentration of target gas compounds is below the threshold, the gap remains open without a conduction path, enabling nearly ‘zero’-power operation. Notably both threshold concentrations and adsorption period (switch-on-period) can be designed by selecting gap distance, binder types and densities, which enables programmability of detection levels. The binders can be chemically-designed through ultra-selective host-guest recognition between the binder and the chemical target.
The chemically-selective percolation switches can be used both as a sensing mechanism for detecting a certain level of the chemical target, and also as an electrical switch for activating the zero-power digital chemical analyzer from a zero-power “off state” to an “on state” when electrical current flows through the chemically-selective percolation switch. According to the chemically-selective percolation mechanism, when aerosol particles of the chemical target selectively adsorb onto a binder-filled surface the particles can form a particle bridge across a gap if their concentrations exceed a critical concentration or percolation threshold. The resulting particle bridge can establish an electrically conductive path between two electrodes spaced by a narrow gap transferring the electric potential from the input (battery) to the output. Since electrons jump through the gap via tunneling, the electrical current increases exponentially due to the reduced effective gap distance. The switching device is in the ‘off-state’ when the gap is e.g. >10 nm, blocking current flows and resulting in off-current near zero (e.g. <1 pA), enabling near ‘zero’-power operation. In the ‘on’ state the electrical current through the particle path is sufficient, e.g. in the nA˜μA range. The percolation sensor is also reversible as the adsorbed particles can also desorb.
Preventing false signals is another aspect of realizing a distributed zero-power sensor network. False signals are triggered by either rare statistical distribution of target binding or random landing of non-specific molecules over the gap between the two electrodes, respectively, forming a conductive bridge for unwanted current flow. Both the false alarm and detection probabilities, caused by rare statistical distribution, can be computed by integrating the areas under the percolation curve (
Considering on-current of 1 nA, the chemically-selective percolation switch can require 1 ms to produce output voltage of 1 V by charging an output capacitor of 1 pF. Considering the saxitoxin molecular diameter of 3 nm and adsorption time of 30 ms, current percolation simulation model indicates that a chemically-selective percolation switch with an inter-electrode area of 1×1 μm2 would require 300 ms for a saxitoxin density of 1021/m3 (corresponding to 1.0 ppm) to percolate and form a conductive path. Decreasing the area to 100×100 nm2 would increase the percolation probability and decrease the detection time by ˜10 folds. By increasing the adsorption time from 30 ms to 300 ms, the simulation results predicted a decreased threshold value of 74 ppb. In one example, the detection sensitivity can reach to the lethal dose of 3.0 ppb by reducing the electrode gap to 10×10 nm2 and increasing the adsorption time up to 10 s.
The present invention also extends to digital chemical analyzers that can include multiple chemically-sensitive percolation sensors that are tuned to different threshold concentrations. The analyzer can determine the ambient concentration of the target chemical based on the switching threshold concentrations of the switches that are closed.
In further examples, a digital chemical analyzer can include multiple chemically-selective percolation switches that are configured to detect different target chemical compounds. For example, a single analyzer can detect zetekitoxin, tetrodotoxin and chiriquitoxin by incorporating parallel switches that are configured to selectively detect these target compounds.
The described features, structures, or characteristics may be combined in any suitable manner in one or more examples. In the preceding description numerous specific details were provided, such as examples of various configurations to provide a thorough understanding of examples of the described technology. One skilled in the relevant art will recognize, however, that the technology may be practiced without one or more of the specific details, or with other methods, components, devices, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the technology.
The foregoing detailed description describes the invention with reference to specific exemplary embodiments. However, it will be appreciated that various modifications and changes can be made without departing from the scope of the present invention as set forth in the appended claims. The detailed description and accompanying drawings are to be regarded as merely illustrative, rather than as restrictive, and all such modifications or changes, if any, are intended to fall within the scope of the present invention as described and set forth herein.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/284,929 filed on Oct. 13, 2015, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62284929 | Oct 2015 | US |
