Low part-per-trillion, humidity resistant detection of nitric oxide using microtoroid optical resonators

Abstract

A nitric oxide detector includes a vessel defining a gas chamber therein; an optical resonator disposed in the gas chamber, the optical resonator having a whispering-gallery mode of resonance; an optical source arranged at least one of proximate the vessel or within the gas chamber to provide light to be coupled into the optical resonator to thereby excite the whispering-gallery mode of resonance; and an optical detector arranged at least one of proximate the vessel or within the gas chamber to receive light to be coupled out of the optical resonator to provide a detection signal.

Description

LOW PART-PER-TRILLION, HUMIDITY RESISTANT DETECTION OF NITRIC OXIDE USING MICROTOROID OPTICAL RESONATORSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present patent application claims priority benefit to U.S. Provisional Patent Application No. 63 / 546,132, filed on October 27, 2023, the entire content of which is incorporated herein by reference. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.FEDERAL FUNDING

[0002] This invention was made with government support under Grant No.HDTRA1- 18-1-0044 awarded by DOD / DTRA. Tire government has certain rights in the invention.BACKGROUND1. Technical Field

[0003] The field of currently claimed embodiments of this invention relates to nitric oxide detectors, and more particularly optical-resonator nitric oxide detectors, polymer- coated optical resonators, and the polymer for coating the resonators.2. Discussion of Related Art

[0004] Systems for the selective and rapid detection of gases are important tools used to monitor environmental impacts,1occupational safety,2and human biomarkers.3Nitric oxide is a common byproduct of vehicle exhaust and industrial processes involving combustion, making it a main contributor to air pollution.4In fact, nitric oxide is a major industrial emission contributing to ozone layer depletion.5In addition to its environmentalimpacts, nitric oxide also serves as an important biomarker of respiratory health. Higher concentrations of exhaled nitric oxide are associated with asthma and Chronic Obstructive Pulmonary Disease (COPD), and the exhaled nitric oxide (FENO) test is commonly administered in the diagnosis of these diseases.6Furthermore, nitric oxide is easily oxidized in air to nitrogen dioxide, a common air pollutant which is highly corrosive and toxic and has been implicated as a danger to human respiratory health.7To mitigate injury due to gas exposure and monitor the environmental impacts of industrial processes, selective sensors for NO detection must be developed and deployed.8Additionally, for sensors to produce reliable and reproducible results in the field, resistance to external environmental factors such as humidity must be demonstrated.

[0005] A variety of sensors have been established for nitric oxide monitoring, including chemical electrode sensors,9chemiresi stive gas sensors (CGS),10 14Rayleigh surface acoustic wave resonators (RS AW)15 16, and chemiluminescence-based sensors.17The lowest limits of NO detection (sub-ppt) have been achieved with single-wall carbon nanotube (SWNT) based sensors, albeit with a lack of analyte specificity and lab-to-lab perturbations in sensitivity.11Semiconductor metal oxide based chemiresistive gas sensors have also demonstrated merit in NO sensing, however their practicality is limited by their extreme operating temperatures.18Additionally, chemiresistive gas sensors often display poor humidity tolerance due to the competitive absorption of water onto metal oxide semiconductor surfaces.19Recently, the development of humidity resistant materials for gas sensing has received much attention.20 22Highly practical, wearable, flexible, and humidity resistant NO / NO2 gas sensors based on laser-induced graphene were recently reported by Yang and coworkers, displaying a limit of detection (LOD) of 8.3 ppb for NO.10Despite this advance, there have been no reports of part-per-trillion level nitric oxide sensors that are selective as well as humidity resistant, a gap which we aimed to address through our research efforts.

[0006] Recently, our lab developed a platform for selective, ultra-sensitive gas sensing using whispering-gallery mode (WGM) microtoroid sensors coated with polymer brushes.23WGM optical microresonators have attracted intense interest in the past decadesdue to their high quality factors (Q) and small mode volumes, which cause enhanced light matter interaction. In WGM microresonators, photons will take hundreds of thousands of round trips while interacting with analytes on the surface, significantly increasing the optical sensing signal response to analytes. Since 2002, sensing based on WGM mode shift has developed a wide application area, including proteins,24 27gases,23exosomes,28,29nanoparticles,24viruses,30and even single molecules.24,31Frequency locked optical whispering evanescent resonator (FLOWER) is a sensing system that provides a lock-in method to track WGM resonances in real time with high (sub-femtometer) resol uti on.24 273235This system allows us to further investigate the resonance shift of a WGM sensor at timescales as short as milliseconds when the sensor is placed in a continuously changing external environment. FLOWER provides advantages over plasmonic36or hybrid plasmonic- WGM sensors37in that it uses a bare microcavity, which enables a greater capture area and thus response time.38Therefore, there remains a need for improved nitric oxide detectors.SUMMARY

[0007] A nitric oxide detector according to some embodiments of the current invention includes a vessel defining a gas chamber therein; an optical resonator disposed in the gas chamber, the optical resonator having a whispering-gallery mode of resonance; an optical source arranged at least one of proximate the vessel or within the gas chamber to provide light to be coupled into the optical resonator to thereby excite the whispering-gallery mode of resonance; an optical detector arranged at least one of proximate the vessel or within the gas chamber to receive light to be coupled out of the optical resonator to provide a detection signal. The optical resonator is at least partially coated with a polymer that satisfies the formula:where n is an integer within the range of 1 to 50, and m is an integer within the range of 0 to 200, and Me is -CH3

[0008] An optical resonator for a nitric oxide detector according to some embodiments of the current invention includes at least a partial coating of a polymer that satisfies the formula:where n is an integer within the range of 1 to 50, m is an integer within the range of 0 to 200, and Me is -CH3.

[0009] A polymer produced from co-polymers 2 -(methacryloyloxy) ferrocenecarboxylate (FcMA) and methyl methacrylate (MMA) prepared via reversible addition fragmentation chaintransfer (RAFT) polymerization so as to satisfy the formulawhere n is an integer within the range of 1 to 50, m is an integer within the range of 0 to 200, and Me is -CH3.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present disclosure, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

[0011] FIG. 1 shows Scheme 1, synthesis of statistical copolymers of 2-(methacryloyloxy) ferrocenecarboxylate and methyl methacrylate via RAFT polymerization according to an embodiment of the present invention.

[0012] FIG. 2 shows Scheme 2, deposition of polymer brush onto microtoroid surface according to an embodiment of the present invention.

[0013] FIGS. 3A-3C show: (a) Nitric oxide gas sensing schematic. FLOWER consists of a tunable laser (Velocity TLB-6700), Digilock frequency locking feedback controller, polarization controller, fiber splitter, balanced photodetector (PD) and microtoroid, (b) Gas chamber with humidity sensor and coupled toroid. Inset. Schematic of an optical fiber coupled to a microtoroid. The small chamber is designed with a slit for the gas delivery tube. A TEC (thermoelectic cooler) is set at the bottom of the chamber. A temperature sensor is placed between the toroid and the inlet gas tube (not shown in the figure). A glass top is used to keep the gas flow stable and directed to the other side of chamber, (c) Schematic of tire coated microtoroid and nitric oxide binding process.

[0014] FIGS. 4A-4B show: (a) FLOWER response to nitric oxide using a Polymer A as the toroid coating, (b) FLOWER response to nitric oxide sensing using Polymer B as the toroid coating.

[0015] FIGS. 5A-5C show: (a) Plot of the sensor response to the tested concentrations of nitric oxide, (b) One-site specific binding curve fit of each response trace. The blank signal is represented by a dashed line, (c) Sensor coated with polymer A response to nitric oxide in different humidity environments.

[0016] FIGS. 6A-6C show selectivity demonstration of the FLOWER nitric oxide sensor. Tire sensor is tested in response to (a) DIMP, (b) hexane, (c) benzene. The grey and red line are linear fits to the blank (nitrogen) and the target gas response.

[0017] FIGS. 7A-7B show: (a) Raw resonance shift data and real-time temperature trace, (b) Resonance shift after calibration.

[0018] FIG. 8 is for a sensor recovery experiment. The arrow points to the time that heating was applied. Comparing the response from 40 min to 60 min and 80 min to 100 min, heating can recover the sensors performance.

[0019] FIGS. 9A-9C show: (a) Structures of ferrocenyl monolayer and PMMA attached to the microtoroid, (b) Sensor response to nitric oxide for microtoroid coated with a ferrocenyl monolayer (c) Sensor response to nitric oxide for microtoroid coated with PMMA.

[0020] FIGS. 10A-10E show: (a) Structures of Polymers C - E. (b) FLOWER response to nitric oxide using a coating of Polymer C. (c) FLOWER response to nitric oxide using a coating ofPolymer D. (d) FLOWER response to nitric oxide using a coating of Polymer E. (e) FLOWER response to nitric oxide using a coating of Polymer F.

[0021] FIGS. 11A-11B show: (a) FLOWER response to nitric oxide using Polymer B from 0.26 to 1.5 ppb of NO. (b) FLOWER response to nitric oxide using Polymer F from 0.5 to 2 ppb.

[0022] FIG. 12 shows a top view of the big chamber. Water containers inside the chamber generate a humid environment. The chamber is covered during experimentation.

[0023] FIG. 13 shows the scheme for examples of polymer synthesis and characterization according to some embodiments of the present invention.

[0024] FIGS. 14A-14F are NMR spectra for the examples corresponding to FIG. 13.

[0025] FIGS. 15A-15F are size-exclusion chromatography data for the examples corresponding to FIG. 13.

[0026] FIG. 16 demonstrates polymer stability over time according to an embodiment of the present invention.DETAILED DESCRIPTION

[0027] Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed and other methods developed without departing from the broad concepts of the current invention. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.

[0028] The term “light” is intended to have a broad definition that can include light in the visible as well as non-visible regions of the electromagnetic spectrum. For example, theterm “light” can include, but is not limited to, visible, infrared and ultraviolet light. Similarly, the term “optical” has a correspondingly broad definition as with the term “light”.

[0029] The abbreviations parts-per-billion (ppb), parts-per-trillion (ppt) and parts-per- quadrillion (ppq), etc. are used herein.

[0030] The nitric oxide radical plays pivotal roles in physiological as well as atmospheric contexts. Although the detection of dissolved nitric oxide in vivo has been widely explored, highly sensitive (i.e. low part-per-trillion level), selective, and humidity resistant detection of gaseous nitric oxide in air remains challenging. In the field, humidity can have dramatic effects on the accuracy and selectivity of gas sensors, confounding data and leading to overestimation of gas concentration. Highly selective and humidity resistant gaseous NO sensors based on laser-induced graphene (LIG) were recently reported, displaying a limit of detection (LOD) of 8.3 ppb. Although highly sensitive (LOD = 590 ppq) single-wall carbon nanotube NO sensors have been reported, these sensors lack selectivity and humidity resistance. In this report, we disclose a highly sensitive (LOD = 2.34 ppt), selective, and humidity resistant nitric oxide sensor based on a whispering-gallery mode (WGM) microtoroid optical resonator. Excellent analyte selectivity was enabled via novel ferrocene-containing polymeric coatings synthesized via RAFT polymerization. Utilizing a Frequency Locked Optical Whispering Evanescent Resonator (FLOWER) system, the microtoroid’s real-time resonance frequency shift response to nitric oxide, was tracked with sub -femtometer resolution. The lowest concentration experimentally detected was 6.4 ppt, which is the lowest reported to date. Additionally, the performance of the sensor remained consistent across different humidity environments. Lastly, the impact of the chemical composition and molecular weight of the novel ferrocene-containing polymeric coatings on sensing performance was evaluated. We anticipate that our results will have impact on a wide variety of fields where NO sensing is important such as medical diagnostics through exhaled breath, determination of planetary habitability, marine science, climate change, air quality monitoring, and treating cardiovascular and neurological disorders.

[0031] In some embodiments of the current invention, we create a WGM gas-sensing platform to enable selective nitric oxide sensing in the low part-per-trillion range (6.4 ppt -240 ppt). This is two orders of magnitude lower than what has previously been demonstrated with whispering gallery mode sensors for volatile organics such as DIMP, ammonia, and formaldehyde.39Co-polymers of 2-(methacryloyloxy) ferrocenecarboxylate (FcMA) and methyl methacrylate (MMA) were synthesized and deposited on the surface of the microtoroid in a polymer brush. (See, FIGS. 1 and 2.) We hypothesized that the binding of nitric oxide to the iron centers in the polymer would lead to swelling of the coating and a change in refractive index, causing a WGM resonance shift. While polymers of this type have been previously accessed via free-radical polymerization40and atom-transfer radical polymerization,41to the best of our knowledge, this is the first report of FcMA / MMA statistical co-polymers prepared via reversible addition fragmentation chain-transfer (RAFT) polymerization. Employing RAFT polymerization allowed us to access polymers with precise control over molecular weight while ensuring end-group fidelity. By testing polymeric coatings with various molecular weights and ferrocene content in our gas sensing experiments, we gauged the impacts of these factors on sensor performance. Additionally, our sensor displayed excellent specificity for NO, exhibiting no response greater than blank noise to volatile organics including benzene, hexanes, and diisopropyl methylphosphonate (DIMP). To evaluate the selectivity and practicality of our device, we tested it in different humidity environments, finding that the sensor’s response was consistent despite changes in humidity.

[0032] FIGS. 3A-3C illustrate a nitric oxide detector 100 according to an embodiment of the current invention. The nitric oxide detector 100 includes a vessel 102 defining a gas chamber therein and an optical resonator 104 disposed in the gas chamber defined by the vessel 102. The optical resonator 104 has a whispering-gallery mode of resonance. The nitric oxide detector 100 also includes an optical source 106 arranged at least one of proximate said vessel 102 or within said gas chamber to provide light to be coupled into said optical resonator 104 to thereby excite said whispering-gallery mode of resonance. The nitric oxide detector 100 also includes an optical detector 108 arranged at least one of proximate the vessel 102 or within the gas chamber to receive light to be coupled out of the optical resonator 104 to provide a detection signal.Experimental MethodsPolymer Synthesis and Microtoroid Coating

[0033] Statistical co-polymers of 2-(methacryloyloxy) ferrocenecarboxylate and methyl methacrylate were prepared via RAFT polymerization (FIG. 1). Chain transfer agent CTA-OSu and radical initiator Int-OSu were employed to reliably install the desired succinimide ester end group needed for toroid functionalization. Polymer A was prepared by employing a 1 : 10 ratio of FcMA to MMA, leading to a statistical co-polymer with a 1 : 8.8 ratio of FcMA: MMA units (determined by 'H NMR) and a Mn of 10.1 kDa, with excellent poly dispersity of Mn / Mw = 1.032. Polymer B was prepared in the same fashion, however a 1: 20 ratio of FcMA to MMA was employed, leading to a polymer with Mn~ 8.5 kDa, Mn / Mw~ 1.08 and a FcMA: MMA ratio of 1: 18.5.

[0034] Microdisks were fabricated as previously described24in a cleanroom via photolithography, patterning, and etching. Then, the silica microdisk was reflowed using a carbon dioxide laser to form a microtoroid structure with a major radius of 100 pm in diameter and a minor radius of 10 / / m. Following plasma treatment of the microtoroid surface, the chip was immersed in a glass vial containing a 1% solution of 3 -(aminopropyl) triethoxysilane (APTES) in 990 pL of chloroform, functionalizing the silica surface with free amine groups (FIG. 2). The vial was placed on a nutator for 10 min and the chip was then rinsed with chloroform and dried with nitrogen. Afterwards, the chip was immersed in a 5 mM solution of either Polymer A or Polymer B for 2 hours, depositing the polymer brush via an on-toroid amide coupling reaction. The chip was rinsed and blown-dry and then baked on a 100 °C hotplate for 30 min to evaporate most of the solvent and enhance the stability of the surface. Lastly, the chip was placed in a room-temperature vacuum overnight to thoroughly remove residual solvent.Gas Sensing

[0035] The chip was placed inside a small chamber (FIG. 3A) enclosed inside a larger 20 cm x 10 cm x 10 cm stainless steel box connected to a vacuum to remove residual gas. A TEC element (Thorlabs TECD2S) was placed at the bottom of the small chamber. The small chamber was designed to reduce the space for diffusion and to make gas flow reach the microtoroid more evenly. A FlexStream™ Gas Standard Generator was connected to a flowmeter to read the exact flow rate into the chamber and then connected to the chamber to blow a target gas at a specific flow rate and concentration. The target gas was generated by mixing high concentration gas from a permeation tube and a gas carrier (either nitrogen or argon). This brings the gas to a secondary dilution system for further dilution to low part-per-trillion values. The concentration was controlled by adjusting the primary dilution flow rate, FPa, the component gas flow rate, Fc, and diluted gas rate, Fsd, in the second dilution system. The final diluted gas concentration in ppm unitis given by:where J ? is molecule weight of target gas andis the emission rate of the permeation tube. The flowmeter sets the flow rate after the second dilution system.

[0036] The FLOWER system uses a tunable laser (Velocity TLB-6700). The laser light polarization is adjusted by a polarization controller and goes through a 50:50 fiber splitter. One beam goes directly to a balanced photodetector (PD) while another goes through a tapered optical fiber. The balanced PD minimizes the effects of laser power fluctuations. A 24-bit data acquisition (DAQ) card collects the real time wavelength signal from both the laser and the temperature sensor. The fundamental mode was selected for tracking, as it exhibits the highest quality factors and greatest field area overlap with the polymer layer where the gas absorption is changing the optical properties of the layer (FIG. 3C). Afterwards, the laser wavelength was locked-in to the selected fundamental mode andthe DAQ monitored both the laser wavelength shift and the temperature sensor to give the raw data of WGM resonance and temperature.

[0037] Before testing the response to nitric oxide, argon was blown into the sample chamber for 30 minutes until thermal equilibration occurred. During an experiment, we cycled between blowing in pure argon for 10 minutes as a blank comparison and blowing in the mixed target gas and gas carrier for another 10 minutes. Argon was chosen as the carrier gas for the nitric oxide sensing experiments, as it is more dense than nitric oxide and therefore can reliably displace it in between cycles. After completing one cycle, recording was stopped for several minutes until the residual waste gas was removed and the chamber re-filled with pure argon (FIGS. 4A-4B).Results and Discussion

[0038] The WGM resonance wavelength (A ) was measured in real time at concentrations from 6.4 ppt to 240 ppt (FIGS. 4A-4B). The chamber was purged with argon between each concentration tested in order to release any NO absorbed in the polymer coating. Linear fits were assigned to track the sensor’s response during the intervals of Ar purging (white intervals) and nitric oxide exposure (shaded intervals).

[0039] Nitric oxide detection real-time traces post temperature calibration are shown in FIGS. 5A-5C, where the sensor’s response is plotted against nitric oxide concentration. The wavelength shift value (AX) is the relative wavelength shift from the starting point. The slope of the wavelength shift over time is considered to be the target gas response. Langmuir’s theory of adsorption42is applied in our case to describe the molecule binding dynamic and fit the curve of response. The two groups of data were fit to:where B andare both fitting parameters (FIG. 5B). By calculating the intersection point between the curve and the blank signal (1.5 fm / min), we derived the LOD (limit of detection) of the sensor for Polymer A to be 2.43 ppt and Polymer B to be 2.91 ppt. The response slopewas relatively consistent across trials for concentrations from 0 to 100 ppt. However, it was noted that the sensor’s response began to decrease when concentrations above 100 ppt were tested, implying that the polymer coating on the toroid was quickly saturating with gas. Concentrations above 240 ppt could also be detected, although this causes the sensor to saturate (FIGS. 11A-11B). This capability can be valuable for issuing early warnings, as it allows for the detection of NO concentrations within the 6.4 to 240 ppt range, while also indicating when larger concentrations are present. The sensor can also be recovered using heat, thus making it reusable after saturation (FIG. 8).

[0040] The impact of the chemical composition and molecular weight of the polymeric coating on the toroid was also evaluated. Polymer A, which contains a higher ratio of FcMA to MMA, displayed a higher response slope than Polymer B for concentrations between 0 and 100 ppt. Polymer A contains twice as many Fe binding sites for nitric oxide relative to Polymer B, thereby increasing the swelling of the polymer coating and producing a higher response slope. Polymer coatings of lower (~ 6.9 kDa) and higher (~ 19.2 kDa) molecular weights were also tested, but these either provided lower response slopes or saturated more quickly (FIGS. 10A-10C). Additionally, a polymer coating with low molecular weight (~ 5.0 kDa) but high FcMA content (1 : 5.7 ratio of FcMA:MMA) produced an irreversible response to NO at ppb-level concentrations (FIG. 1 IB).

[0041] To test the practicality of our sensor in the field, we also measured its response to NO in different humidity environments (FIG. 5C). Due to the hydrophobic nature of the polymer coating, we anticipated the sensor’s performance would remain consistent.43Indeed, the sensor displayed a similar response at 47% humidity as compared to 20% humidity. Notably, this is the first report of a humidity resistant, part-per-trillion level nitric oxide sensor.

[0042] In addition to humidity resistance, the sensor reported herein also demonstrates excellent selectivity for nitric oxide when compared to other hazard gases including diisopropyl methylphosphonate (DIMP), hexane, and benzene. (FIGS. 6A-6C) None of these gases gave a response higher than the blank signal, even when tested at part- per-billion level concentrations.

[0043] In comparison to existing technologies for nitric oxide sensing, the FLOWER sensor reported herein displayed the lowest experimentally detected concentration to date of 6.4 ppt, with an LOD of 2.43 ppt (Table 1). Slightly lower limits of detection were reported for carbon nanotube sensors11, albeit with a lack of selectivity. Except for carbon nanotube sensors, our FLOWER sensor is the only other reported device that can measure nitric oxide in the low part-per-trillion range, and its humidity resistance and selectivity provide distinct advantages over existing nitric oxide sensors.

[0044] Table 1. Comparison of FLOWER-based NO sensor against existing technologies.Conclusion

[0045] In the above-described examples, we exhibit low part-per-trillion level selective nitric oxide detection using WGM microtoroids functionalized with novel polymeric coatings. We synthesized ferrocene-containing polymers via RAFT polymerization that enable the sensor’s selective responsive to nitric oxide, finding that more ferrocene-rich polymers with molecular weights around 10 kDa displayed the best sensing capabilities. With FLOWER real-time resonance tracking, we experimentally detected nitric oxide in concentrations as low as 6.4 ppt, the lowest experimentally demonstrated concentration reported in the literature, and we theoretically calculated an LOD of 2.43 ppt with a one-site binding model. Humidity resistance of the sensor was enabled by the hydrophobic nature of the polymer coatings employed. Control experiments with several hazardous gases demonstrated the selectivity of our sensor to nitric oxide.

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Water Absorption of Poly(Methyl Methacrylate) Measured by Vertical Interference Microscopy.Langmuir 2012, 28 (31), 11609-11614. https: / / doi.org / 10.1021 / la302260a.Supporting InformationTemperature calibration

[0047] Lock-in resonance shift raw data from FLOWER system and concurrent temperature sensor trace data. This data is subtracted to give the final trace. (See FIGS. 7A- 7B.)Sensor recovery

[0048] After saturation was observed for polymer C, a TEC heating element was set to 100°C to recover the signal. Sensor recovery was then observed. (See FIG. 8.)Control experiments

[0049] To demonstrate the necessity of the FcMA / MMA copolymer coating, we also tested microtoroids coated with either a ferrocenyl monolayer or poly(methyl methacrylate) (PMMA) in our nitric oxide sensing experiments (FIG. 9A). The ferrocenyl monolayer- coated microtoroid sensor displayed some response to nitric oxide at very low concentrations (6.4 ppt - 10 ppt) but quicky saturated (FIG. 9A). The PMMA coated microtoroid did not provide a response above that of the control for concentrations from 6.4 ppt - 480 ppt (FIG. 9B). Taken together, these results imply that the FcMA / MMA co-polymer coating is necessary to generate a sensitive and selective response of the sensor to nitric oxide.Effect of Polymer Molecular Weight on Sensing Capabilities

[0050] In addition to polymer A and B, we also evaluated the sensing capabilities of the polymeric coatings shown in FIG. 10A. Polymers C and D contain similar FcMA content to Polymer B, but they differ in polymer chain length. Polymer E is a homopolymer of 2- (methacryloyloxy) ferrocenecarboxylate (FcMA) with a molecular weight of ~ 5.9 kDa. FIG. 10B shows the response of a microtoroid coated with Polymer C to nitric oxide. Although a response was observed for concentrations from 6.4 ppt - 40 ppt, saturationbehavior was observed at a lower concentration of 40 ppt, whereas Polymer B saturated at a higher concentration of 160 ppt. Polymer D has a similar n: m ratio with polymer C while it has smaller molecule weight. However, the performance in FIG. 10C shows that polymer D has a higher response and saturation concentration. It indicates that the amount of binding sites on the microtoroid surface is not simply linear related to molecule weight of polymer chain and n: m ratio. This phenomenon can be explained that the length of polymer chain will affect the functionalization process, a too long chain may reduce the surface density of the polymer. Polymer E showed some response to nitric oxide from 10 ppt to 80 ppt, but the response slope was significantly lower at all concentrations measured. Interestingly, Polymer F, the most ferrocene-rich polymer tested, displayed a lower response slope to NO for the concentration range evaluated.Sensor performance at high concentration (ppb level)

[0051] The sensors using Polymer B and Polymer F are tested at a higher concentration range. Both polymers showed saturation after the first concentration. For high concentration nitric oxide detection, a cleaning process is necessary if the sensor is to be used multiple times. (See FIGS. 11A-11B.)Humidity sensing experiment

[0052] High humidity environments were generated by putting water containers into the sample chamber (FIG. 12). This approach was chosen to minimize noise generated by commercial humidifiers. A humidity sensor recorded the real-time humidity value in the chamber. This relatively sealed environment reached vapor-liquid equilibrium after over an hour. After reaching equilibrium, temperature recording began and resonance shift tracking began.Polymer synthesis and characterization

[0053] All synthetic procedures were carried out using dry and degassed acetonitrile. CTA-OSu1and int-OSu2were prepared according to the literature protocol. FcMA was prepared according to the literature protocol3. Methyl methacrylate (stabilized with 6-tert-Butyl-2,4-xylenol) was obtained from Tokyo Chemical Industry, and the inhibitor was removed immediately prior to polymerization via a short basic alumina plug.JH NMR spectra were collected with a 400 mHz Bruker spectrometer and reported relative to residual CHCI3 (5 = 7.26 ppm). Size Exclusion Chromatography data were collected using an Agilent 1260 series pump equipped with two Agilent PLgel MIXED-B columns (7.5 x 300 mm), an Agilent 1200 series diode array detector, a Wyatt 18-angle DAWN HELEOS light scattering detector, and an Optilab rEX differential refractive index detector. The mobile phase was THF at a flow rate of 1 mL / min. The dn / dc value of the FcMA / MMA co-polymer at 25 °C in THF was determined to be 0.094 mL / g via online calculation using injections of known concentration and mass. (See FIG. 13.)General Procedure for FcMA / MMA Co-polymer Synthesis

[0054] A 25 mL Schlenk tube was charged with a stir bar, CTA-OSu (0.2 equiv), int- OSu (0.01 equiv), and FcMA (1 equiv) as solids. Methyl methacrylate (10 or 20 equiv) was added via syringe, then dry MeCN (2.0 M in FcMA) was added via syringe. The flask was sealed and subjected to three freeze-pump-thaw cycles. After the final thaw, the flask was not backfilled with inert gas. The reaction was placed in a 70 °C oil bath, protected with a blast shield, and stirred for 17 h. The reaction was allowed to cool to 23 °C, opened to air, and diluted with ~ 5 mL CHCI3 and the resulting viscous orange polymer was dissolved by vortexing. The solution was added dropwise to a flask of Et2O (150 mL) at -78 °C with stirring to precipitate the FcMA / PMMA co-polymer. The solution was filtered through a medium porosity sintered glass frit, and the resulting sticky orange residue was re-dissolved in 2-5 mL of CHCI3 and added dropwise to a flask of hexanes (150 mL) at -78 °C with stirring to re-precipitate the polymer. The solution was filtered as described above and the filtrate was washed with ~ 60 mL of hexanes (3 x 20 mL portions). The resulting light orange powder was dried under high vacuum at 40 °C overnight to remove any residual solvent.Polymer A (n: m = 1 : 8.8)

[0055] Polymer A was prepared as described by the general procedure above using 10 equiv of MMA relative to 1 equiv of FcMA. Mn(GPC) - 10.1 kDa, PDI = 1.03. 'H NMR(400 MHz, CDCh, 298 K) 5 4.84 (bs), 4.44 (bs, overlapped), 4.41 (bs, overlapped), 4.23 (bs, overlapped), 3.59 (bs), 3.23 (t), 2.85 (bs, overlapped), 1.80 (bm), 1.25 (bs), 1.02 (bs), 0.85 (bm). Note: many of the aliphatic signals below 2 ppm are indiscernible due to substantial overlap. However, signals attributable to the succinimide ester end group and ferrocenyl protons are clearly marked.Polymer B (n: m = 1 : 18.5)

[0056] Polymer B was prepared as described by the general procedure above using 20 equiv of MMA relative to 1 equiv of FcMA. Mn(GPC) ~ 8.5 kDa, PDI = 1.06. 'HNMR (400 MHz, CDCh, 298 K) 5 4.94 (bs), 4.36 (bs, overlapped), 4.22 (bs, overlapped), 3.59 (bs), 2.85 (bs, overlapped), 1.80 (bm), 1.24 (bs), 1.01 (bs), 0.85 (bm). Note: many of the aliphatic signals below 2 ppm are indiscernible due to substantial overlap. However, signals attributable to the succinimide ester end group and ferrocenyl protons are clearly marked. Additionally, the use of different experimental parameters (number of scans and relaxation delay) in the collection of this spectra is responsible for the peak broadening observed.Polymer C (n: m = 1 : 22)

[0057] Polymer C was prepared as described by the general procedure above using 20 equiv of MMA relative to 1 equiv of FcMA, and 0.1 equiv of the CTA-OSu was used instead of 0.2 equiv. Mn (GPC) ~ 19.2 kDa, PDI = 1.05. 'H NMR (400 MHz, CDCh, 298 K) 5 4.83 (bs), 4.44 (bs, overlapped), 4.40 (bs, overlapped), 4.22 (bs, overlapped), 3.58 (bs), 2.84 (bs, overlapped), 1.80 (bm), 1.24 (bs), 1.00 (bs), 0.83 (bm).Polymer D (n: m = 1 : 21.5)

[0058] Polymer D was prepared as described by the general procedure above using 20 equiv of MMA relative to 1 equiv of FcMA, and 0.4 equiv of the CTA-OSu were used instead of 0.2 equiv. Mn(GPC) ~ 6.9 kDa, PDI = 1.22. 1H NMR (400 MHz, CDC13, 298 K) 6 5.01 (bs), 4.42 (bs, overlapped), 4.33 (bs, overlapped), 4.21 (bs, overlapped), 3.59 (bs), 2.85 (bs, overlapped), 1.80 (bm), 1.25 (bs), 1.01 (bs), 0.85 (bm).Polymer E

[0059] To a solid mixture of 2-(Methacryloyloxy)ethyl ferrocenecarboxylate (1.07 g, 3.12 mmol), CTA1 (100 mg, 0.2 mmol), and OSu initiator (4.3 mg, 0.010 mmol) was added anhydrous acetonitrile (2.5 mL). The resulting suspension was sealed in a thick walled reaction vessel fitted with a threaded PTFE high vacuum valve. The reaction mixture was degassed by three freeze-pump-thaw cycles. The reaction vessel was not backfilled with inert gas prior to heating. The reaction mixture was heated at 70 °C for 24 h. [Note: the reaction mixture was observed to become homogeneous within ca. 5 minutes of heating.] Upon cooling, an amorphous mass separated from the solution. Following exposure to air, the reaction mixture was dissolved into 10 mL of chloroform and precipitated into ca. 200 mL of methanol cooled to -78 °C. The resulting suspension was filtered through a medium porosity sintered glass frit, and the isolated solids were washed with room temperature methanol (3 x 20 mL). The resulting yellow solids were exposed to high vacuum at 40 °C overnight to remove residual volatiles. Mn(GPC) = 5.9 kDa, PDI = 1.04. 'H NMR (400 MHz, CDCI3, 298 K): 5 4.94 (bs), 4.74-3.99 (m), 3.21 (bs), 2.84 (s), 2.74 (bs), 2.17 (s, overlapped), 1.95 (bs), 1.80 (s), 1.62 (bs), 1.25 (s), 1.13 (s), 0.98 (bs), 0.92-0.84 (m).Polymer F (n: m = 1 : 5.7)

[0060] Polmer F was prepared as described by the general procedure above using 7.5 equiv of MMA relative to FcMA. M„ (GPC) = 5.0 kDa, PDI = 1.11. ’H NMR (400 MHz, CDCI3, 298 K): 5 4.95 (bs), 4.36 (bs, overlapped), 4.23 (bs, overlapped), 3.59 (bs), 3.23 (bs), 2.85 (bs, overlapped), 1.81(bm), 1.25 (bs), 1.02 (bs), 0.88 (bm).

[0061] NMR Spectra (FIGS. 14A- 14F)

[0062] Size-Exclusion Chromatography Data (FIGS. 15A-15F)Polymer Stability Over TimeNMR of Polymer B After One Year of Storage at 23 °C:

[0063] JH NMR (400 MHz, CDCI3, 298 K) 8 4.84 (bs), 4.44 (bs, overlapped), 4.40 (bs, overlapped), 4.23 (bs, overlapped), 3.59 (bs), 2.85 (bs, overlapped), 1.80 (bs), 1.24 (bs),1.01 (bs), 0.84 (bm). Comparing to the spectrum taken immediately after isolation, there is no evidence of polymer chain scission or decomposition over time. (See FIG. 16.)

[0064] The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use the invention. In describing embodiments of the invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.

Claims

We claim:1 . A nitric oxide detector, comprising: a vessel defining a gas chamber therein; an optical resonator disposed in said gas chamber, said optical resonator having a whispering-gallery mode of resonance; an optical source arranged at least one of proximate said vessel or within said gas chamber to provide light to be coupled into said optical resonator to thereby excite said whispering-gallery mode of resonance; an optical detector arranged at least one of proximate said vessel or within said gas chamber to receive light to be coupled out of said optical resonator to provide a detection signal, wherein said optical resonator is at least partially coated with a polymer that satisfies the formula:where n is an integer within the range of 1 to 50, and m is an integer within the range of 0 to 200, and Me is -CH3.

2. The nitric oxide detector according to claim 1, wherein n is in the range 2 to 12, and m is in the range 60 to 85.

3. The nitric oxide detector according to claim 2, wherein n is in the range 3 to 9, and m is in the range 65 to 85.

4. The nitric oxide detector according to any one of claims 1-3, wherein said optical resonator is a toroidal optical resonator.

5. An optical resonator for a nitric oxide detector comprising at least a partial coating of a polymer that satisfies the formula:where n is an integer within the range of 1 to 50, m is an integer within the range of 0 to 200, and Me is -CH?.

6. The optical resonator according to claim 5, wherein n is in the range 2 to 12, and m is in the range 60 to 85.

7. The optical resonator according to claim 6, wherein n is in the range 3 to 9, and m is in the range 65 to 85.

8. The optical resonator according to any one of claims 5-7, wherein said optical resonator is a toroidal optical resonator.

9. A polymer produced from co-polymers 2-(methacryloyloxy) ferrocenecarboxylate (FcMA) and methyl methacrylate (MMA) prepared via reversible addition fragmentation chain-transfer (RAFT) polymerization so as to satisfy the formulawhere n is an integer within the range of 1 to 50, m is an integer within the range of 0 to 200, and Me is -CH3.

10. The polymer according to claim 9, wherein n is in the range 2 to 12, and m is in the range 60 to 85.

11. The polymer according to claim 10, wherein n is in the range 3 to 9, and m is in the range 65 to 85.

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