CHEMICAL ANALYTES

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

[0001] This invention was made using funds obtained from the US Government (US Army, Contract No. W91 l SR-1 l -C-0051 ; and US Army Contract No. W911SR-12-C-0052), and the US Government therefore has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to system and methods for detecting chemical analytes.

[0003] There is a need for improved detection elements and methods that are capable of detecting liquid, liquid droplet and aerosol forms of chemical analytes, where said detection elements can function in wet, rainy and other environmentally challenging conditions over extended periods of time without degradation to the detection element's ability to function. Other particular examples of environmental conditions which the detection elements must be resistant to are intense sun exposure, heat and dust contaminants. Furthermore, detection systems incorporating these robust detection elements are needed, wherein such detection systems contain transduction mechanisms capable of transforming the detection element response into an electronic signal for data transmission and remote communication purposes. Other desired attributes of these detection systems are that they function autonomously, are self- reporting, and maintain low power requirements.

[0004] Examples of such detection elements that are widely used by the military for detecting chemical warfare agents are M8 detection paper and M9 detection paper. Other forms of these detection papers (e.g., 3 Way Paper) also exist. M8 paper is used to detect and differentiate the three major classes of liquid chemical warfare agents, which are G- and V- nerve and H-blister agents. The paper is impregnated with two dyes and an acid-base indicator which produce a visible color change when in contact with these liquid chemical agents. Each agent class mobilizes a different dye, resulting in a different color change that is then used to identify the agent class. For example, sulfur mustard (or HD) mobilizes a red dye, producing a red spot; tabun (GA), sarin (GB), soman (GD), and cyclosarin (GF), collective referred to as G- series nerve agents, mobilize the yellow dye; and VX nerve agent mobilizes the yellow dye and additionally reacts with an acid-base indicator contained in the detection paper matrix which generates a blue color, thus creating an overall green-black color change.

[0005] When used for facilities and installations monitoring, for example, sheets of the M8 paper are placed in areas (buildings or common areas) where a potential contamination or attack could occur. Since existing manufactured forms of M8 detection paper are fragile and are often rendered inoperable once exposed to e.g., rain water, typical operating procedures entail frequent replacement of M8 paper when deployed in outdoor environments. Furthermore, DoD personnel are required to view and assess each individual sheet of paper deployed. Only after a visual inspection has occurred can a detection decision be made. This decision is subjective since it relies upon the ability of a human operator to interpret color changes, often under battlefield conditions. As a result of these limitations, advancements are needed in the state of the art which includes improved detection elements for detecting the presence of chemical agents which are more durable in harsh environmental conditions such as rain. Additional advancements in the state of the art are also needed for detection systems that incorporate improved detection elements, where such detection systems are capable of self-reporting detection events produced by the detection elements without requiring human interaction.

SUMMARY OF INVENTION

[0006] The invention relates to a detection element capable of detecting liquid, liquid droplet and aerosol forms of chemical analytes, where the detection element also possesses the characteristic that it is able to function reliably in challenging environmental conditions over extended periods of time without degrading in performance. The detection element may also be part of a larger detection system which contains transduction mechanisms capable of

transforming the detection element response into an electronic signal(s) for data transmission and remote signaling of detection events.

[0007] In one embodiment, the detection element may be a substrate that is composed of paper, plastic, polymer material, glass, metal, metal oxide, ceramic, or combinations thereof. The substrate may contain impregnated materials such as dyes, reactive chemicals,

chemisorptive chemicals, physisorptive chemicals, and/or electronically or optically reactive media. In another embodiment, the impregnated materials may produce a color change in the visible part of the electromagnetic spectrum when exposed to certain chemical species.

[0008] In another embodiment, the detection element substrate may be coated with a thin film of polymer, metal, metal oxide, metal halide, silica or other semi-conductor material. In one embodiment the thin film may be continuous. The thin film may also have the property that it is superhydrophobic and oleophilic. In another embodiment, the thin film may also be semi- continuous, or an electrode pattern or any inter-digitated circuit design. In another embodiment, the detection element substrate may be coated with two or more films of dissimilar material. More specifically, the first film may be an electrode pattern composed of a metal, and a second film may coat the first film and substrate wherein the second film is a polymer that is

superhydrophobic and oleophilic.

[0009] In another embodiment, the detection element may be incorporated into a detection system. Specifically, the detection system may contain transduction mechanisms which convert a visible color change in the detection element into an electronic signal when the detection element is exposed to a liquid, liquid droplet or aerosol forms of a chemical agent. The transduction mechanisms may be electronic, potentiometric, magnetic, optical, thermal conduction, or any physical or mechanical change in shape of the substrate.

[0010] In another embodiment, the detection element may be incorporated into a detection system, where the detection system contains an optical source beam and detector capable of probing the detection element and detecting the presence of a chemical agent on the detection element. The optical source beam and detector may be an infrared spectrometer, Raman spectrometer, filtometer, or UV-fluorescence spectrometer, or laser based system.

Specifically, the optical source beam and detector may function in reflection, transmission, scatter, diffuse-reflectance, attenuated total reflection, or surface plasmon resonance modes.

[0011] In another embodiment, the detection system may be capable of self-reporting detection events produced by the detection element(s) incorporated therein. In another embodiment, the detection system may be part of a wireless network consisting of multiple detection systems, all of which are capable of self-reporting detection events produced by their respective detection elements.

[0012] These and other advantages of the invention will become apparent upon review of the following detailed description, the accompanying drawings and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a diagram showing a comparison of the detection of GB, HD and VX gases using the coated M8 paper of the present invention and a conventional M8 paper.

[0014] FIG. 2 is a schematic diagram of a self-reporting version of the detection system of the present invention.

[0015] FIG. 3 is a chart of static water contact angles for coated and uncoated M8 paper.

[0016] FIG. 4A is a representation of water droplet behavior on coated and uncoated M8 paper immediately upon application. FIG. 4B is a representation of the water droplet behavior on the coated and uncoated M8 paper of FIG. 4A and after two hours.

[0017] FIGS. 5A-5C comprise a set of charts of time to color registration using coated M8 paper for three different simulants.

[0018] FIGS. 6A and 6B represent coated and uncoated M8 paper after water submersion. FIGS. 6C and 6D represent the coated and uncoated M8 paper of FIGS. 6A and 6B after simulant deposition.

[0019] FIG. 7A is a representation of water droplets on coated M8 paper after exposition to 10 days of intense sunlight. FIG. 7B is a representation of coated M8 paper after exposition to 10 days of intense sunlight and then exposure to simulant.

[0020] FIGS. 8A and 8B represent coated and uncoated M8 paper after mud exposure. FIGS. 8C and 8D represent the coated and uncoated M8 paper of FIGS. 8A and 8B after simulant deposition.

[0021] FIG. 9 A is a representation of coated and uncoated M8 paper after exposure to a rain event. FIG. 9B is a representation of coated and uncoated M8 paper after simulant deposition after seven days deployed outdoors including the rain event.

[0022] FIG. 10 is a circuit diagram for use in transmitting simulant information associated with use of the coated M8 paper of the present invention.

[0023] FIG. 11 is a schematic diagram showing color change outputs when coated M8 paper has contacted a simulant generating a green color when exposed to a white LED light.

[0024] FIGS. 12A-12C are time lapse representations of color formation on the coated M8 paper for three different exposure resolutions.

[0025] FIG. 13 is a graph of time to signal for the three different sensing resolutions of FIGS. 12A-12C. [0026] FIG. 14 is a block diagram of information transfer using the coated M8 paper and circuitry previously described.

[0027] FIG. 15 is a schematic diagram of a wireless communication system for transferring the detection information obtained using the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] A version of the invention provides a self reporting detection system that utilizes a ruggedized version of M8 chemical agent detection paper for detecting and reporting potential liquid chemical warfare attacks. In this version, a robust, superhydrophobic and oleophilic coating was successfully developed, applied and tested on M8 paper, demonstrating that it aggressively repels water and dirt, while still allowing agents to be detected to rapidly penetrate the coating and produce the expected color changes on the underlying paper. F-coated and G- coated samples of the improved M8 test strips containing these novel protective coating formulations were shown to successfully detect 1 μΐ, droplets of VX, HD, and GB (FIG. 1) during a round of live agent testing. Additional testing showed that the coated M8 paper samples also produced a color change in the presence of other agents.

[0029] The broader system of the invention takes advantage of the new capabilities that are now enabled by the ruggedized version of M8 described, involving an automated detection system network containing transduction hardware and software capable of reading and self reporting color changes on the coated M8 test strips due to agent interactions. The self reporting detection system network continuously monitors facilities for agent attacks and report positive alarms in real time. The self reporting detection system that is based on the ruggedized M8 paper provides increased confidence in the detection capabilities with a much faster response time. Furthermore, if a sensor node within the network is registering an alarm for agents, personnel can immediately query the other nodes in the network to determine the scope of the attack. In addition to providing added monitoring capabilities, the ruggedized M8 paper that does not degrade in outdoor environments will enable the self reporting system to monitor a facility for several months without human effort, thus eliminating the need for regular M8 paper inspections and frequent replacement. A representation of the invention using integrated transduction and wireless communication systems is shown in FIG. 2. [0030] In an analysis of the invention, 12 different coating technologies and formulations were applied to M8 paper with the goal of making the paper surface superhydrophobic while retaining the paper's inherent ability to detect CWAs and CWA simulants. A parallel strategy was used to identify the optimum coating technology. While various hydrophobic coatings were applied to M8 paper using a variety of techniques (spray coatings, surface silanization modification using supercritical C0 ₂ as a solvent), Chemical vapor deposition is also an option. All coatings were first tested to determine whether or not they had sufficient hydrophobicity to achieve the stated goal. Hydrophobicity was determined by measuring static water contact angles on the paper surface (where higher contact angles corresponds to greater

hydrophobi city/water repellency), and by visually determining whether or not water beaded up and rolled off the paper when tilted at a 15° angle. None of the silane modifications or spray coatings produced films that were sufficiently hydrophobic that water rolled off the paper surface, so their response to CWA simulants was not evaluated.

[0031] Coatings that produced sufficiently hydrophobic surfaces were nano-structured PTFE films. FIG. 3 shows the static water contact angle measurements for uncoated M8 paper, as well as for the various formulations of PTFE films that were applied to M8. Uncoated M8 has a static water contact angle around 103°, which is quite hydrophobic, but not nearly as hydrophobic as the nano-structured PTFE coatings, the water contact angles of which ranged from 130°-148°. More importantly, all of the PTFE coated M8 paper samples caused water to instantly bead up and roll off the paper when tilted slightly. Superhydrophobicity is clearly demonstrated when drops of water are carefully positioned on coated and uncoated M8 paper samples (FIG. 4A). Over the course of several hours, the drops placed on uncoated M8 soak into the paper, while the drops placed on PTFE coated M8 evaporate before they can soak into the paper (FIG. 4B).

[0032] Testing of PTFE coated M8 Paper using CWA simulants

[0033] Benign simulants were used to test the coated M8 paper samples. The simulants were selected based on their fluid properties, as well as their ability to mimic the CWA response on M8 paper. Methyl Salicylate (MS) was chosen to simulate H series blister agents because it causes a red response on M8; Dimethyl Sulfoxide (DMSO) was used to simulate V series nerve agents because it causes a green response on M8; and Dimethyl Methylphosphonate (DMMP) was selected as a simulant for G series nerve agents because it causes yellow response on M8. The ability of each paper sample to respond to simulants was quantified in two ways: 1) measuring static contact angle for each simulant on the paper surface over time; and 2) measuring the amount of time it takes for a definitive color response to appear after a drop of simulant has been placed on the paper.

[0034] While measuring contact angles over time provided information that was used to improve the coating formulation, it alone was not an accurate predictor of the time it took for a drop of simulant to penetrate the film and generate a color response. This is because a visible color change occurs long before the simulant completely soaks into the paper. Therefore, the required time for a 20 μΐ, drop of CWA simulant to generate a color response via visual inspection was measured for each PTFE coating formulation (FIGS. 5A-5C). While all PTFE coating treatments facilitated a visible color change in response to H and G series simulants less than 2 minutes (FIGS. 5 A and 5B), the only coating formulation that generated the characteristic deep green color response to V series simulants was coating formulation G (FIG. 5C). M8 paper with all other coating formulations turned yellow in response to the simulant for V series nerve agents. Based on these tests, a coating formulation downselect was performed. Because M8 paper with coating G was the only sample that generated the correct color response to all 3 CWA simulant classes, and was always among the fastest to detect all CWA simulants, it was selected to undergo preliminary environmental durability experiments.

[0035] Preliminary Environmental Durability Testing of Coated M8 Paper Samples

[0036] Following the coating formulation down select that occurred, tests were conducted that compared the durability of G coated M8 paper to uncoated M8. The

environmental conditions simulated during this initial phase were: testing of water repellency, resistance to UV light, the ability to withstand dirt/mud, and preliminary outdoor deployment.

[0037] Prior to the coating formulation down select, all testing of coated M8 paper was conducted using paper samples that only had one side coated. After recognizing that coating G was the only formulation that facilitated the correct color response for all CWA simulant classes, a number of M8 paper samples were coated on both sides so that more extensive water repellency testing could be conducted. The water repellency of coated and uncoated M8 was tested by submerging the paper samples in a beaker of water. While uncoated M8 paper was immediately soaked (FIG. 6A), the coated M8 came out of the water completely dry (FIG. 6B). A drop of each simulant then was deposited onto the paper surfaces, and the color response was photographed (represented by FIGS. 6C-6D). FIG. 6C shows that wet uncoated M8 does not allow for full color development for H and G series agents, and did not exhibit a green color response to the V series simulant, while the color response for the coated M8 paper was as if the paper was new (FIG. 6D). Additionally, FIG. 6C shows that uncoated M8 paper becomes wrinkled and otherwise degraded even after a single wetting cycle.

[0038] Photochemical degradation caused by exposure to sunlight is another potential concern for sensing systems that are deployed outdoors. To determine whether or not the water repellent coating is damaged by UV light, coated M8 paper samples were brought to a tanning salon and placed in high power tanning beds for 75 minutes over the course of several days, which is equivalent to 10 days of intense summer sunlight. Drops of water were placed on the paper surface to test for any indication that the hydrophobic coating had become degraded by UV light. FIG. 7A shows the behavior of water droplets on the coated M8 paper samples following UV exposure. As was the case before exposure to UV light, the water quickly beaded on the coated M8 paper surface, while forming a puddle and eventually soaking into uncoated M8. Additionally, the color response to CWA simulants was not compromised by exposing the coated M8 paper samples to UV light (FIG. 7B).

[0039] In addition to remaining functionally viable after being submerged in water and exposed to intense UV light, coated and uncoated M8 paper samples were tested to see whether or not they remained functional in the presence of mud, which is another known interference for M8. For these tests, 500 mg of clay was spread on each paper sample and sprayed with water to generate mud. On coated M8, the water and mud immediately beaded up and rolled off the paper, leaving a dry surface (FIG. 8A). When the experiment was conducted for the uncoated M8 paper sample, a puddle of mud formed on the paper (FIG. 8B). A 20 μΐ, drop of H series simulant and G series simulant was then deposited on each sample. While the coated M8 paper sample immediately generated the correct color responses to H and G series simulants (FIG. 8C), the uncoated M8 sample registered a barely noticeable red color response for the H series simulant (FIG. 8D), and did not generate any response to the G series simulant, even after the majority of the mud and water was physically wiped from the surface using a towel. [0040] The final preliminary environmental durability test conducted was to deploy a sample of coated and uncoated M8 paper on an outside wall of a building for 7 days. This particular wall was chosen because a vehicle fleet passes within a few feet of the door on a daily basis, so the samples were exposed to engine exhaust as well as splashes of water, mud, and deicing chemicals. On top of regular wear and tear, vehicles were allowed to idle within one foot of the deployed paper samples for approximately 3 hours during the deployment period. During the 7 day deployment, 10 inches of snow melted from the roof of the building, and dirty melt and rainwater were allowed to splash and drip on the paper surfaces. Figure 9A shows that while coated M8 remained hydrophobic throughout the rain event, uncoated M8 paper allowed rain to stick to the paper surface and soak in. After the outdoor deployment, the paper samples were tested for their ability to detect CWA simulants (FIG. 9B). It should be noted that because of the results observed in FIG. 8D, the uncoated M8 paper was dried prior to the exposure to CWA simulants (FIG. 9B), while coated M8 paper remained superhydrophobic throughout the outdoor deployment.

[0041 J Live Agent Testing Using Coated M8 Paper

[0042] Prior to the coating formulation down select, coated M8 paper samples were sent to a test facility where they were tested using live agents. FIG. 1 shows that both the F-coated and G-coated M8 paper samples generated an intense color response to 1 μΐ _^ drops of GB, VX, and HD, levels far below a specified 20 μΤ goal. In addition to the traditional CWAs, a number of other agents of interest were positively detected.

[0043] An aspect of the system is an optical transduction system that uses light sensors to mimic the human eye's response to coated M8 color change. The light sensors for the color recognition device are light emitting diodes (LEDs). While LEDs are commonly used for low power lighting applications, they are also passive light sensors when hooked up to an instrument that reads voltage. In addition to having minimal power requirements, LEDs are selectively sensitive to the same color light that they would emit when powered. Therefore, a red LED can be used to detect a color change in response to H series agents, a green LED can be used to detect a response to V series agents, and a yellow LED can be used to detect a response to G series agents. Because the signal generated from LEDs in response to light is weak, it may be enhanced using an operational amplifier and a feedback loop, with the circuit diagram shown in FIG. 10. The resistance value for Rl is tuned in order to keep the voltage output signal Vout within the allowable input limits of the data logger. The purpose of capacitor CI is to prevent oscillations, which commonly occur in this type of circuitry where the input signal is low and the gain is high.

[0044] Because the system must recognize three different colors, three circuits are combined into a single unit. In addition to the 3 LEDs used as light sensors, a white LED can be used to illuminate coated M8, so the system can operate at any time, including during nighttime hours, as represented in FIG. 11. In the example shown in FIG. 11, coated M8 paper has turned green in response to a V series nerve agent attack, causing a rise in the green voltage signal, while the signals for the other two light channels decrease. By monitoring the voltage output of 3 sensors in an array, characteristic signatures for each class of CWA simulant are expected.

[0045] The system includes a flexible electrical sensing circuit applied to standard M8 paper. Preliminary studies have shown that electrical properties of M8 paper change over time when wetted by CWAs. Each CWA-type (V, G, H) and corresponding indicator dye is mobilized through the M8 paper at different rates, which can give rise to potentially unique electrical signatures. The system takes advantage of these properties for novel electrical sensing capabilities on the papers. The sensors can be designed to sense a positive response to CWAs, as well as the dynamic electrical response produced by different agent/dye mobilities through the paper with time. Various sensor electrode designs (e.g. 2-point, 4-point probe) can be used for optimal detecting and identifying CWA responses in coated M8. FIG. 12 is an overview of the transduction technology concept, visually showing how a drop of agent can spread between the electrode connections as a function of time. FIGS. 12 and 13 also show that by incorporating different distances between the electrodes (high resolution, medium resolution, and low resolution), potentially unique conductivity signatures can be observed for different types and concentrations of CWAs.

[0046] To incorporate the electrode patterns into coated M8 chemical test strips, electrodes must be placed on the M8 paper prior to depositing the hydrophobic film. There are several electrode fabrication procedures available. One is physical vapor deposition (PVD) of metal onto M8 paper using a Denton BTT IV benchtop metal deposition system. PVD is a vacuum technique that produces homogenous metal films on a wide range of target substrates. PVD is a line of sight deposition technique, so a physical mask can be applied to the target substrate to generate a specified electrode pattern, and the film thickness is easily monitored using an in-situ quartz crystal microbalance. Other metal deposition options include effusion cell, e-beam evaporation and magnetron sputtering.

[0047] The metal, such as aluminum, copper or silver is evaporated to thicknesses of 1 , 5, and 10 microns on uncoated M8 paper substrates. To take advantage of the dynamic electronic signals produced by different agents on the M8 test strips, different electrode spacings and geometries are possible for optimal signal generation. Electrode spacings ranging from 0.5 mm to 1 cm may provide the necessary resolution to detect and measure the agent dependent mobilities, thus allowing for the possibility of enhanced agent recognition through signal processing.

[0048] Once the electrodes are produced, the electrical continuity of each conductor wis considered, and the coated M8/electrode substrates are flexed, twisted, and bent to determine whether or not continuity is maintained during rough handling. The electrode production method that produces the highest quality M8/electrode substrates and optimal electrical responses during testing is used to produce conductivity sensors to be coated with the superhydrophobic film.

[0049] The system including the transduction system and the networking hardware are configured in a single unit that is capable of being deployed outdoors for several months. The sensor nodes may be operated using IEEE 802.15.4/Zigbee wireless standard, which is ideal for embedded sensor applications that require low rate/low power wireless transmission. ¹ The IEEE 802.15.4/Zigbee wireless standard operates at 2.4 GHz but operates without interference from WiFi and other wireless systems that operate at the same frequency. Additionally, IEEE

805.15.4 incorporates frequency hopping routine, so many sensor nodes can operate

simultaneously without interference. In an outdoor environment, 802.15.4/Zigbee radios can transmit data or alarms to a repeater and / or a base station that is up to 300 feet away. A signal will then be transmitted from the base station to a central command and/or mobile command site to enable integrated base defense actions due to a detection event.

[0050] The wireless sensor networking hardware also includes a low power requirement microprocessor that controls the overall functionality of the system, and an example block diagram showing the system operation and data handling process is shown in FIG. 14. These

¹ IEEE Standard 802.15.4, "Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANS)", IEEE, 1 October 2003. microprocessors can be programmed to dictate sampling rates, wireless transmissions, system status checks, and switch the transduction subsystem power on and off in order to conserve battery life. In addition to controlling the overall system operation, the onboard CPU can be programmed to perform calculations on the sensor data stream.

[0051] FIG. 15 shows one wireless sensor example, including hardware for multiple node mounting possibilities. The design includes positioning the coated M8 surface by at least 15 degrees relative to the ground so rain can easily roll off the surface, keeping the sensing clean and dry. The ultimate goal of the unit is to be rugged enough for a 4-month outdoor deployment.

[0052] The detection algorithm of the system is relatively straightforward because the system simply needs to detect a color change. The coated M8 paper that has been developed consistently generated a clear color change in response to simulants, even in the presence of common interferences. In this case, the detection algorithms can be as simple as if the output voltage for a single signal channel reaches a certain threshold, or if the rate of signal increase reaches a certain level, an alarm is sounded. For example, if HD were to be deposited on the coated M8 paper of the present invention, we would expect a significant rise in the red voltage channel signal, even in the presence of rain, dirt, or sun. However, if the color change recognition is more complicated than expected, detection algorithms using sophisticated time series analysis statistics may be required.

[0053] It is also expected that the detection algorithms are fairly straightforward for the transduction system expected. By sandwiching electrodes between the M8 paper substrate and the superhydrophobic coating, water should not interfere with analysis, which is the most common interferent for electrical property based sensing systems. Additionally, we expect that the different CWA classes exhibit different responses because the sensitive dyes that are brought to the paper surface during color change could cause characteristic changes in conductivity change. Even if the conductivity based transduction system cannot decipher between different agent classes, it still provides actionable information that a potential CWA attack has occurred. If interference testing shows that the response signal is changed due to environmental

interferences, a reliable detection algorithm may require the use of sophisticated time series statistics.

[0054] A tiered detection algorithm can be implemented to maximize the amount of information available to personnel in the event of potential CWA alarm. For example, the conductivity based transduction system (which is completely passive when there is no alarm) could be used to trigger the optical color sensor if the conductivity of the coated M8 changes drastically. Once triggered, the optical color sensor would then notify personnel details regarding a color change (Specific CWA Alarm), or if there is no color change (No Alarm). Either scenario provides personnel valuable information that is not available with current technology.

[0055] Various other aspects of this invention will become apparent to those skilled in the art upon review of the present disclosure. The appended claims are not intended to limit reasonable equivalents.