A gas sensor

Abstract

The invention provides a gas sensor comprising an optically transparent substrate (100); an array of nanoantennae (200) being disposed on the optically transparent substrate (100); and a metal-organic framework hybridised with a polymer being integrated into the gas sensor; wherein the nanoantennae (200) are configured as dark-bright mode coupling antennae with one or more nanogaps being formed adjacent to one or more hotspots of the nanoantennae (200) in bright mode.

Description

A GAS SENSORRELATED APPLICATIONS

[0001] The present invention claims priority to Singapore patent application no. 10202400444Q filed on 19 February 2024, the disclosure of which is incorporated in its entirety.FIELD OF INVENTION

[0002] This invention relates to a gas sensor, hi more particular, the invention is a midinfrared nanoantenna gas sensor.BACKGROUND OF THE INVENTION

[0003] A mid-infrared (mid-IR) gas sensor detects and measures gases using light in the midinfrared wavelength range based on the principle that gas molecules absorb specific infrared wavelengths matching their inherent resonant frequencies, causing vibrations and rotations in their molecular bonds. These vibrations enable advanced applications in label-free and real-time gas sensing by measuring the absorbed light to identify the type and concentration of gases present. Such applications include environmental monitoring, healthcare, and clinical diagnosis, allowing for precise and efficient detection and analysis of various gases without the need for additional labelling or markers.

[0004] Commercial infrared (IR) gas sensors suffer from bulky volume, typically at the cubic centimeter scale, and exhibit poor detection sensitivity (tens of ppm) due to the inherently low cross-sections of molecular IR absorption. Additionally, the performance of nanoantennae is limited when it comes to ultra-low concentration gas detection. Bare nanoantennae can only realize several hundred ppm gas detection, which has no significant advantages over commercial infrared IR gas sensors.

[0005] Nanoantenna techniques can be introduced to provide a high-intensity electric field for improving the inherent low cross-section issue by utilizing the surface -enhanced infrared absorption (SEIRA) effect. An example is the plasmonic nanoantennae that induces plasmon-phonon interaction in the mid-infrared region which holds most of the molecular fingerprint absorption peaks. The mid-TR fingerprints provide specific identificationinformation of each type of chemical stretch in molecular structures. Compared to traditional optical sensors leveraging the mid-IR fingerprints, plasmonic sensors have the advantages of compact size, fast response time, and compatibility for nano integration, which bring the potential for the wearable Internet of Things (loT) sensor applications such as environmental monitoring, healthcare, and clinical diagnostics.

[0006] Different nanoantenna designs with its hotspots being indicated by tire circled parts are shown in FIGs. 1A -1C. FIG. 1A is a known nanorod design having hotspots at each end FIG. IB is a bright-bright coupling design having one hotspot at its center, and in more specific, at the center of its horizontal portion. FIG. 1C is a bright-dark coupling design having two hotspots, each at end of a horizontal portion that is adjacent to center of a vertical portion. These nanoantenna designs only allow one or two hotspots, not fully reaching the potential of nanoantennae that can have more hotspots.

[0007] The present invention is developed to overcome bottlenecks and limitations of existing gas sensors by leveraging the advancements in plasmonic sensors via functionalizing and modifying the nanoantennae used in the gas sensor with specific designs and materials.SUMMARY OF INVENTION

[0008] An object of the present invention is to provide a gas sensor for mid-infrared detection that enable label-free and real-time gas sensing, including CO2gas, volatile organic compounds (VOCs) gases, pollutant gases, industrial exhaust gases, etc for various applications such as environmental monitoring, healthcare, and clinical diagnosis.

[0009] Another object of the present invention is to provide a gas sensor that utilizes nanoantenna techniques with a multi -hotspot design which increases tire number of hotspots for achieving high detection sensitivity in gas sensing applications.

[0010] Further another object of the present invention is to provide a gas sensor integrated with a framework to achieve high detection selection and low detection limit.

[0011] Yet another object of the present invention is to provide an ultra-sensitive gas sensor with high detection resolution.

[0012] In addition, it is the object of the present invention to provide a gas sensor with compact size, fast response time, robustness, simple fabrication and high compatibility for nano integration which bring the potential for wearable Internet of Things (loT) sensor applications.

[0013] At least one of the preceding objects is met, in whole or in part, by the present invention, in which the present invention provides gas sensor comprising an optically transparent substrate; an array of nanoantennae being disposed on the optically transparent substrate; and a metal-organic framework hybridised with a polymer being integrated into the gas sensor; wherein the nanoantennae are configured as dark -bright mode coupling antennae with one or more nanogaps being formed adjacent to one or more hotspots of the nanoantennae in bright mode.

[0014] Preferably, the nanoantennae are each divided into a plurality of portions by the nanogaps at end -aligned positions of adjacent portions.

[0015] It is preferred that each nanoantcnnac is configured with more than two hotspots.

[0016] According to a preferred embodiment, each nanoantenna is in an H-shaped configuration.

[0017] Based on a preferred embodiment, the metal-organic framework is a zeolitic imidazolate framework.

[0018] The polymer is preferred to be polycthylcnciminc.

[0019] In a preferred embodiment, the polymer has an average molecular weight of 800 Da.

[0020] With reference to a preferred embodiment, the metal-organic framework that is hybridised with a polymer is in the form of a porous film that is incorporated into the gas sensor via spin-coating.

[0021] Preferably, the thickness of the film is in the range of substantially 180nm to 190nm .

[0022] Tt is preferred that the optically transparent substrate is made of calcium fluoride.

[0023] The gas sensor is preferred to further comprise a heating element to thermally reset the gas sensor for reuse.

[0024] One skilled in the art will readily appreciate that the invention is well adapted to carry" out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments described herein are not intended as limitations on the scope of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] To facilitate an understanding of the invention, there are illustrated in the accompanying drawings the preferred embodiments from an inspection of which when considered in connection with the following description, the invention, its construction and operation and many of its advantages would be readily" understood and appreciated

[0026] FIGs. 1A-1C illustrate different nanoantenna designs. FIG. 1 A illustrates a common, known nanorod design; FIG. IB illustrates a bright-bright mode coupling design; and FIG. 1C illustrates a dual-hotspot bright-dark mode coupling.

[0027] FIG. 2 illustrates a schematic illustration of the gas sensor having a metal-organic framework that is hybridized with a polymer film, and a plurality of nanoantennae exhibiting multiple hotspots.

[0028] FIG. 3 illustrates a nanoantenna of the gas sensor in a bright-dark coupling design having six hotspots.

[0029] FIGs. 4A-4H illustrate a plurality of graphs showing characterization of vibrational signal enhancement including SEM images showing the details of nanorods (in FIG. 4A), bright-bright mode coupled antennae (in FIG. 4B), bright-dark mode coupled antennae (in FIG. 4C), and multi-hotspot bright-dark mode coupled antennae (in FIG. 4D), all the above- mentioned antennae having nanogap size of substantially 30nm; FIG. 4E illustrates reflection spectra corresponding to the above antennae configurations; FIG 4F illustrates transmission spectra of the above-mentioned nanoantcnnac with 40 nm thick PMMA as analytes; whilst FIG. 4G illustrates corresponding enhanced molecular signal of the above-mentionednanoantennae extracted from reflection spectra, and FIG. 4H illustrates corresponding enhanced molecular signal of the above-mentioned nanoantennae extracted from transmission spectra.

[0030] FIGs. 5A-5I illustrate a plurality of graphs showing material characterization including: 1R absorbance spectra of PEI, ZIF-8, and Z1F-8-PE1 in FIG. 5A; XRD spectra of PEI, 10% ZIF-8-PEI, and 30% ZIF-8-PEI in FIG. 5B showing their phase details; N2adsorption-desorption isotherms for PEI, ZIF-8, and ZIF-8-PEI at 77 k in FIG. 5C showing their BET surface area; SEM micrograph showing the details of the multi-hotspot nanoantennae in FIG. 5D; SEM images showing the nanoantennae with PEI film (left panel) and ZIF-8-PEI hybrid film (nght panel) in FIG. 5E; EDX mapping analysis of the ZIF-8-PEI hybrid film in FIG. 5F; Profile ciir.e of enhanced molecular signals versus ZTF-8 mass fraction showing the influence of ZIF-8 mass fraction on enhanced signals in FIG. 5G; Hybrid film thickness versus spectrum wavelength map revealing the effect of film thickness on enhanced molecular signals of the multi-hotspot nanoantennae in FIG. 5H; and thickness measurement of ZIF-8-PEI hybrid film using a profilometer in FIG. 51.

[0031] FIGs. 6A-6I illustrate a plurality of graphs demonstrating ultrasensitive CO2gas detection using the MOF-functionalized multi-hotspot platform including: measured spectral response of the hybrid platform when CO2concentrations vary over a wide range (0-1512 ppm) in FIG. 6A; corresponding differential signal with the measured spectrum of O ppm as reference in FIG. 6B; total molecular signal versus CO2concentration profile showing the sensing behavior of tire platform over a wide concentration range in FIG. 6C; measured spectral response of the platform in a small CO2concentration range (0-52 ppm) in FIG. 6D; corresponding differential signal in FIG. 6E; total molecular signal versus CO2concentration profile in a small CO2concentration range in FIG. 6F; limit of detection (LOD) of the hybrid platform for CO2gas detection in FIG. 6G; thermal recovery and repeatability of the platform in FIG. 6H; and selectivity of the platform for CO2gas detection in FIG. 61.

[0032] FIGs. 7A-7E illustrate a plurality of graphs showing dynamic behavior of the sensing platforms of the gas sensor including: real-time 3D plots of the differential spectra of platforms using PEI-coated nanorods in FIG. 7A, PEI-coated multi-hotspot antennae in FIG. 7B, and MOF-PEI-functionalized multi-hotspot antennae in FIG. 7C, when CO2gases with various concentrations are loaded and thermally desorbed. FIG 7D illustrates temporalvariation of the total molecular signal corresponding to FIGs 7A-7C, revealing the dynamic behavior of various platforms: panel i: PEI-coated nanorod platform; panel ii: PEI-coated multi-hotspot platform; panel iii: MOF-PEI-functionalized multi-hotspot platform. FIG. 7E illustrates performance comparison corresponding to FIG. 7D, with panel i: spectral signal intensity of platforms after loading 10 ppm CO2gas; panel ii: spectral signal intensity after loading 100 ppm CO2gas; and panel iii: sensitivity comparison.DETAILED DESCRIPTION OF THE INVENTION

[0033] For a better understanding of the invention, preferred embodiments of the invention that arc illustrated in the accompanying drawings will be described in detail.

[0034] The present invention relates to a gas sensor for detecting gas. Specifically, the gas sensor enables label-free and real-time gas sensing. The gas sensor is suitable for applications involving mid-infrared (mid-IR) detection, including parts per billion (pbb)- level gas detection. Target gas for detection includes but not limited to CO2gas, volatile organic compounds (VOCs) gases, pollutant gases, and industrial exhaust gases, etc for various applications such as environmental monitoring, healthcare, and clinical diagnosis.

[0035] Preferably, the gas sensor is a plasmonic sensor with compact size, fast response time, and high compatibility for nano integration to be used in wearable devices with Internet of Things (loT) sensor applications. The plasmonic sensor detects specific molecules through plasmon-phonon interaction. The resonance of plasmonic nanoantennae exhibits plasmon behaviour, while the resonance of molecules demonstrates phonon behaviour. When plasmon and phonon resonances are close to each other, a coupling effect occurs, leading to different line shapes such as electromagnetic-induced transparency (E1T), electromagnetic- induced absorption (EIA), and Fano-like resonance, depending on the coupling conditions. By analysing the amplitude changes in these spectral line shapes, information about molecular concentrations and species can be obtained.

[0036] FIG. 2 shows a schematic illustration of the gas sensor that comprises an optically transparent substrate 100; an array of nanoantennae 200 being disposed on the optically transparent substrate 100; and a metal-organic framework hybridised with a polymer being integrated into the gas sensor. The optically transparent substrate 100 can be a calciumfluoride (CaFj) substrate that can offer high transparency across a wide range of wavelengths, from ultraviolet to infrared, with low refractive index that reduces surface reflection. Further, the CaF2 substrate has high thermal stability and low dielectric constant.

[0037] It should be noted that the term “nanoantenna” herein refers to a nanoscale device designed to transmit or receive electromagnetic waves, particularly in the optical or infrared spectrum, and it can be interchangeably referred to as “antenna” in this disclosure. The nanoantennae 200 are each configured in a multi-hotspot nanoantenna design, whereby the hotspot refers to a specific area on the antenna where the electromagnetic field is significantly intensified with enhanced interactions with its surrounding environment for improving sensitivity. In a preferred embodiment, the multi-hotspot nanoantenna design adopts the concept of coupling subradiant “dark” mode nanoantennae 200 with bright mode nanoantennae 200 having hotspots. The subtradiant “dark” mode nanoantennae 200 interact weakly with incident optical field through well-engineered nanogaps. These nanogaps are formed adjacent to the hotspots of the bright mode nanoantennae 200 to excite the high- intensity hotspots.

[0038] FIG. 3 shows an exemplary embodiment of the nanoantenna 200 employed in the present invention. The nanoantenna 200 can be configured with more than two hotspots, preferably at least six hotspots. To achieve this number of hotspots, the nanoantenna 200 can be made up of separate portions 210a, 210b, 210c, 210d, 220 with a nanogap formed between the separate portions 210a, 210b, 210c, 210d, 220. The nanoantenna 200 shown in FIG. 3 is formed in an H-shaped configuration that is made up of five portions, with one horizontal portion 220, two upper vertical portions 210a, 210b above each end of the horizontal portion 220, and tw o lower vertical portions 210c, 210d below each end of the horizontal portion 220. The nanogaps at located at end-aligned positions of the adjacent portions, including the end of each vertical portions 210a, 210b, 210c, 210d that are adjacent to the honzontal portion 220 and the two ends of the horizontal portions, resulting in six hotspots that are indicated by the circled part in FIG. 3.

[0039] The metal-organic framework (MOF) enables parts per billion (pbb)-level gas detection. In this invention, the MOF is the zeolitic imidazolate framework (ZIF-8), which is composed of Zn2+atoms linked to imidazolate anions through nitrogen and fonns tetrahedral coordination. The aperture between the connecting nets is substantially 3.4 A, which matches well with tire kinetic diameter of CO? molecules, making porous ZIF-8 agood CO2gas adsorption material. Since the adsorption is physical and causes small variation at low gas concentrations, in order to maximize the gas-induced changes in the MOF, the MOF is hybridized with a polymer.

[0040] Preferably, the polymer is polyethy leneimine (PEI), whereby the amino groups in the polycthylcnciminc arc introduced into the MOF through post-synthetic modification to expand the chemisorption function of the MOF while maintaining its structural integrity for physisorption. The basic amine groups of PEI polymers can chemically react with hard- acidic CO2molecules due to the zwitterion mechanism. The reaction process is reversible since the formed species caused by the CO2absorption can be recovered by heating with negligible PEI loss. Additionally, PEI-modified MOF can be integrated with the nanoantennae 200 through spin-coating process.

[0041] The MOF-polymer hybrid functions as a gas-selective-trapping material to concentrate and adsorb CO2gases. The variation in the MOF-polymer hybrid caused by the adsorption of CO2gases is detected by the nanoantennae 200 through near-field coupling. More specifically, the IR vibration of MOF-polymer hybrid is coupled with the resonance of nanoantennae 200 via near field. When CO2gases are adsorbed by the MOF-polymer hybrid, the IR vibration of MOF-polymer hybrid changes accordingly, which is reflected in the IR spectrum of the nanoantenna 200 via near-field coupling. Therefore, the gas information according to the change in the IR spectrum of nanoantcnnac 200 can be determined. Throughout the detection process, the nanoantenna 200 serves as signal transduction to convert the gas changes in MOF-polymer hybrid into easily detectable spectral changes.

[0042] In the present invention, the MOFs are modified by PEI polymers to achieve the dualmechanism of both physical and chemical adsorption and maximize the gas-induced changes in the MOF. Additionally, it is preferred that the polymer has an average molecular weight of substantially 800 Da, for example low-molecular-weight branched PET (average Mw-800) is used due to the smooth penetration of short-chain PEI into the relatively narrow pore window of the MOF (ZIF-8)

[0043] Referring back to FIG. 2, the metal-organic framework (MOF) that is hybridised with the polymer is in the form of a porous film 300 which is preferably incorporated to the gas sensor via spin-coating The film 300 can be applied in different ways depending on thedesign and requirements of the gas sensor, including any one or combination of being directly coated onto the nanoantennae 200 to enhance their interaction with the target molecules, being around the nanoantennae 200 for creating a protective or functional layer without covering the nanoantcnnac 200 directly, and being coated onto the optically transparent substrate 100 before or after the nanoantennae 200 are deposited, providing a base layer that interacts with the target molecules. By changing the MOF and polymer species, the target gas can be extended to detect various volatile organic compounds (VOCs) gases, pollutant gases, industrial exhaust gases, etc.

[0044] The MOF -polymer hybrid film functions as a gas-selective-trapping material to adsorb the target gas both physically and chemically, whereas the nanoantennae 200 provide a strongly enhanced near field for the detection of gas molecules captured in the hybrid film 300. It is preferred that the thickness of the film is in the range of substantially 180nm to 190nm, most preferably 185nm, for optimum performance with respect to signal enhancement profile.

[0045] In one embodiment, the gas sensor is incorporated with a heating element to reset the gas sensor for reuse as heating can facilitate removal of contaminants or residues that may affect the performance of the gas sensor, thus resetting the gas sensor to its original state.Examples:

[0046] Example 1Multiple nanoantennae 200 of various configurations are fabricated and then characterized by their signal enhancement performance by spin-coating a polymethyl methacrylate (PMMA) film as the analyte. FIGs. 4A-4D respectively show the SEM images of antenna in nanorod design, bright-bright mode coupled antenna, bright-dark mode coupled antenna, and multi-hotspot bright-dark mode coupled antennae. The nanogap sizes of these nanoantennae are set to be substantially 30nm, and their resonances are all engineered to match the vibration of PMMA by fine-tuning the bnght-modc antenna arm length. The measured reflection and transmission spectra of these nanoantennae that utilizes PMMA of substantially 40nm thick are shown in FIGs. 4E and 4F. Based on observation, the antenna resonances are well matched with the C=O stretching vibrations of PMMA film, demonstrating high accuracy of the fabrication process of the nanoantennae. To compare the signal enhancement achieved by these nanoantennae, the enhanced molecular signals areextracted from these spectra by using a least-squares fitting, as shown in FTGs. 4G and 4H. As obscn cd. the multi-hotspot nanoantennae in FIG. 4D possess the highest signal enhancement in both reflection and transmission modes when compared with dual-hotspot configurations like nanorods exhibited in FIG. 4A, and mode-coupled antennae shown in FIGs. 4B and 4C. It demonstrates the effectiveness of the loss-enhanced multi-hotspot strategy. Specifically, in the reflection mode, the enhanced molecular signal of the multihotspot nanoantenna 200 is substantially two times higher than that of the nanorods. In transmission mode, the enhanced signal of the former is substantially five times higher than that of the latter.

[0047] Example 2To investigate the composition of MOF-polymer hybrid film 300 being ZIF-8 and PEI hybrid film, and the interactions between PEI and ZIF-8, Fourier transform infrared spectroscopy (FT-IR) analysis. X-ray diffraction (XRD) test, and energy dispersive X-ray spectroscopy (EDX) mapping, scanning electron microscope (SEM), and N2adsorption / desorption analysis are performed. As shown in FIG. 5A, the representative IR peaks near 1150 cm'1and 3140 cm'1corresponding to v(CN) and v(CH) stretching vibrations of ZIF-8 are clearly observed in the ZIF-8-PEI hybrid film, indicating the successful loading of ZIF-8. Notably, with reference to red region of FIG. 5A (colour not shown in the figure), a slight blue-shift of IR peaks in amine groups between 1300-1700 cm-1is observed due to the interactions from the C-N vibration of ZIF-8. It indicates that the hybrid film is not a simple mixture of ZIF-8 and PEI. In fact, there arc some interactions of chemical bonds between them. From tire perspective of Bragg diffraction angles shown in FIG. 5B, with tire loading of ZIF-8 into PEI, sharp peaks representing the ZIF-8 crystalline structure appear in the measurement results. Additionally, the intensity of these peaks is largely increased as the ZIF-8 loading increases, indicating that the ZIF-8 is successfully loaded with its cry stalline structure being preserved. The N2adsorption / desorption isotherms of PEI at 77 K before and after ZIF-8 loading are characterized to investigate the change in the Brunauer-Emmett-Teller (BET) surface area. As shown in FIG. 5C, the original PEI exhibits quite low N2uptakes, and after the loading of ZIF-8, the N2uptake, BET surface area, and pore volume increase sharply. For instance, the saturated N2uptake of 80% ZIF-8-PEI hybrid film reaches 419 cm3 / g, and the BET surface area is calculated to be 406 m2 / g by applying the Brunauer-Emmett-Teller equation. The high surface area of the hybrid film ensures the contact area between gases and films, thereby^ optimizing the spatial overlapping of molecules to the near field.

[0048] For nanoantennae 200 fabricated by electron beam lithography (EBL) and evaporation, the morphology and outline are clear and well-defined as exhibited in FIG. 5D. Referring to FIG. 5E, after the loading of gas enrichment, the nanoantenna pattern is no longer observed in the SEM image. When the gas enrichment material is pure PEI film, the surface morphology of the device is smooth and dense as indicated by the left panel in FIG. 5E, which is not friendly to the adsorption of gas molecules. When the gas enrichment material is the PEl-modified ZIF-8 film, rough surfaces with ZIF-8 particles inside are observed as indicated by the right panel in FIG. 5E, which is conducive to the diffusion of gas into the film. The EDS results in FIG. 5F demonstrate the uniform distribution of ZIF in PEI, whereby the Zn element is the element that distinguishes ZIF-8 from PEI. In addition to the morphology distribution of ZIF-8, its mass fraction also has a critical effect on the enhanced molecular signal achieved by the antenna system. As observed in FIG. 5G, the content of 15% ZIF-8 in the hybrid film is suitable for the nanoantenna. FIG. 5H shows the map of enhanced molecular signal when film thickness and spectrum wavelength change. As observed, the IR peaks in ZIF-8-PEI are distinct in tire map, and with increasing film thickness, the enhanced molecular signal between 5.5-7.5 pm first increases and then decreases slightly. According to the signal enhancement profile, 180 nm is selected as the optimal value of the film thickness. By adjusting the rotation speed and time during the spincoating process, the film thickness can be controlled. Based on the measurement results in FIG. 51, the final thickness of the hybrid film is about 185 nm.

[0049] Example 3The gas detection of the multi-hotspot MOF-PEI hy brid platform of the present invention is investigated through steady demonstrations. FIG. 6A shows the measured spectral response of the hybrid platform after reaching steady state at various concentrations. As observed, significant changes occur in the 6-8 pm band of the IR spectrum, which is due to the group changes in the MOF-PEI hybrids caused by the adsorption of CO2as the target gas. In order to observe the changes clearly, the differential reflection spectra are extracted by taking the spectrum without CO2as a reference, as illustrated in FIG. 6B. FIG. 6C depicts the total molecular signal versus CO2concentration profile over a wide concentration range (0-1512 ppm). As observed, with increasing CO2gas concentration, the total molecular signal undergoes an evolution that first sharply increases and then gradually reaches saturation. When CO2varies within a small range of ultra-low concentrations (0-52 ppm), the spectral response of the platform is similar, as indicated in FIGs. 6D and 6E. However, therelationship between the total molecular signal and CO2gas concentration becomes linear(lineanty: 10%), as shown in FIG. 6F.

[0050] The limit of detection (LOD) is a crucial figure of merit for evaluating the sensing behaviour of the platform and is closely related to noise. Total noise is obtained by acquiring the spectra of the platform at pure nitrogen (N2) gas 20 times, followed by extracting and averaging the signal fluctuations of these spectra. By plotting the total noise and the output signal of the platform at low concentrations together, the LOD of the platform can be analysed, as shown in FIG. 6G. As observed, the signal strength of the present platform at a CO2gas concentration of 0 ppm is still higher than the total noise of the measurement system . The margin of the signal above the noise at 0 ppm indicates that the LOD of the platform can reach ppb levels. Apart from LOD, the reversibility of the platform is also investigated. When the platform undergoes consecutive cycling between CO2inflow and thermal desorption as illustrated in FIG. 6H, the measured spectral peak and total molecular signal of the platform change accordingly, as indicated by the arrow to the left-hand or right-hand axis. The difference between all adsorption and desorption cycles is within substantially 2%. The reflection intensity and total molecular signal change repeatedly when the platform undergoes consecutive cycling between thermal desorption and CO2loading. In addition to the reversibility, the selectivity of the platform, shown in FIG. 61, is also investigated. When measuring different gas mixtures with N2 as carrier gas, the signal response of the platform is significant in the presence of CO2gas.

[0051] Example 4The dynamic behaviour of the multi-hotspot MOF-PEI hybrid platform of the present invention is investigated by placing it in a gas cell with a heating stage inside, and the gas concentration of the cell is controlled in real time by mass flow controllers. To investigate the advantages of MOF and multi-hotspot design in gas detection, platform using PEL covered nanorods and PEI-covered multi-hotspot antennae are set as the control group, and the platform using MOF-PEI-functionalized multi-hotspot antennae is the experimental group, as illustrated in FIGs. 7A-7C. By calculating the differential spectra as total molecular signals, the response of different platforms to concentration changes is distinct, as shown in FIG. 7D. When the CO2gas concentration is 10 ppm, no response is observed in the PEI-functionalizcd platform, while the response of the MOF-PEI-functionalized platform is significant. It indicates that the integration of physisorption and chemisorption greatlyimproves the detection limit, as indicated in FIG. 7E, panel i According to FIG. 7E, panel ii, when the CO2gas concentration was increased to 100 ppm, all the platforms show a clear response. The signal intensity of the platform using multi-hotspot antennae is more than two times higher than that using common nanorods, which is attributed to the coordination of contradictions by the multi-hotspot strategy. In terms of sensitivity, with reference to FIG. 7E, panel iii, the MOF-PE1 platform is substantially 6.5 times higher than the nanorod platform.

[0052] The performance of the present invention defined by the above metal-organic framework -integrated multi-hotspot ultrasensitive mid-IR nanoantenna gas sensor has been demonstrated against the state-of-art miniaturized IR CO2sensors. The present invention possesses low LOD (ppb-level), high sensitivity (0.18% / ppm), excellent reversibility (variation within 2%), and high selectivity (against C2H5OH, CH3OH, N2).

[0053] Compared with traditional optical gas sensors like nondispersive infrared (NDIR) sensors, tire multi-hotspot MOF-PEI hybrid platform of the present invention provides an ultra-short optical interaction length which is substantially 37000 times improvement. Such compact size shrinking to nanometer scale allows the platform to be accessible for integration with smart nano-devices, enabling the on-chip solutions compatible with future smart devices such as smart phones and wearable devices. For other applications of nanosensors 200 like electrical gas sensors, the platforms also show better selectivity and reliability due to an extensive identification information of molecular fingerprints embedded within the mid-IR range.

[0001] The present disclosure encompasses the contents described in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred forms with a degree of particularity, it is understood that the present disclosure of the preferred forms have been made only by way of examples and that numerous changes in the details of construction and the combination and arrangements of parts may be resorted to without departing from the scope of the invention.

Claims

CLAIMS:

1. A gas sensor comprising: an optically transparent substrate (100): an array of nanoantcnnac (200) being disposed on the optically transparent substrate; and a metal-organic framework hybridised with a polymer being integrated into the gas sensor; wherein the nanoantennae (200) are configured as dark-bright mode coupling antennae with one or more nanogaps being formed adjacent to one or more hotspots of the nanoantennae (200) in bright mode.

2. The gas sensor according to claim 1 , wherein the nanoantennae (200) are each divided into a plurality of portions (210a, 210b, 210c, 210d, 220) by the nanogaps at end- aligned positions of adjacent portions.

3. The gas sensor according to claim 1 or 2, wherein each nanoantennae (200) is configured with more than two hotspots.

4. Tire gas sensor according to any one of the preceding claims, wherein each nanoantenna (200) is in an H-shaped configuration.

5. The gas sensor according to any one of the preceding claims, wherein the metalorganic framework is a zeolitic imidazolate framework (ZIF-8).

6. The gas sensor according to any one of the preceding claims, wherein the polymer is polyethyleneimine (PEI).

7. The gas sensor according to any one of the preceding claims, wherein the polymer has an average molecular weight of substantially 800 Da.

8. The gas sensor according to any one of the preceding claims, wherein the metalorganic framework that is hybridised with a polymer is in the form of a porous film (300) that is incorporated to the gas sensor via spin-coating.

9. The gas sensor according to claim 8, wherein the thickness of the porous film (300) is in the range of substantially 180nm to 190nm.

10. The gas sensor according to any one of the preceding claims, wherein the optically transparent substrate (100) is made of calcium fluoride.

11. The gas sensor according to any one of the preceding claims, further comprising a heating element to thermally reset the gas sensor for reuse.

Images