Molecular sensor for detecting methane and / or carbon dioxide dissolved in a liquid

Abstract

Disclosed is a molecular sensor for detecting molecules dissolved in liquid, the molecular sensor comprising: a sensor housing, a measurement chamber comprised in the sensor housing and being fluidly coupled to an exterior of the sensor housing, and a quartz crystal microbalance (QCM). A quartz crystal resonator comprised in the QCM is functionalised with a metal-organic framework (MOF) which can reversibly adsorb methane and / or reversibly adsorb carbon dioxide, and the molecular sensor is configured such that liquid which enters the measurement chamber can come in direct contact with a surface of the MOF. Also disclosed is a method of detecting and / or quantifying molecules dissolved in a liquid, the method comprising the steps of: providing a molecular sensor comprising a quartz crystal microbalance (QCM), the QCM comprising a quartz crystal resonator, the QCM being functionalised with a metal-organic framework (MOF) which can reversibly adsorb methane and / or reversibly adsorb carbon dioxide; providing the liquid to a functionalised surface of the QCM such that the liquid directly contacts the functionalised surface.

Description

[0001] MOLECULAR SENSOR FOR DETECTING METHANE AND / OR CARBON DIOXIDE

[0002] DISSOLVED IN A LIQUID

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a molecular sensor for detecting methane dissolved in liquid and / or carbon dioxide dissolved in liquid, specifically it relates to a molecular sensor comprising a Quartz Crystal Microbalance (QCM), a method of detecting and / or quantifying molecules dissolved in a liquid, and use of the molecular sensor to perform the method.

[0005] TECHNICAL BACKGROUND

[0006] A Quartz Crystal Microbalance (QCM) is a well-known and sensitive technique that can determine various characteristics of selected molecules, when the molecules interact with a sensor surface of the QCM. The determinations are based on the change in resonant frequency of a quartz crystal which is coupled to the sensor surface. By alternating the voltage applied to the quartz crystal, a resonant oscillation can be stimulated in the crystal and the wavelength of the shear displacement in the quartz is dependent on material properties of quartz, such as density, shear modulus etc., and certain interactions between the sensor surface and selected molecules. The resonance at multiple frequencies may be determined to measure both the fundamental and overtone resonances. A change in frequency of a resonance can be used to determine with a high sensitivity the presence and relative amount of molecules interacting with the sensor surface, or if functionalized, e.g., with a porous material, the materials surface, internal surface area and / or pores.

[0007] QCMs have been used to detect gases, such as methane and carbon dioxide, see e.g. “Room-Temperature Monitoring of CH 4 and CO2 Using a Metal-Organic Framework-Based QCM Sensor Showing Inherent Analyte Discrimination’’, ACS Sensors, Volume 8, Issue 9, p. 3478.

[0008] It is generally desirable to accurately detect methane and / or carbon dioxide.

[0009] It is generally desirable to provide an improved molecular sensor.

[0010] Further, it is desirable to provide an improved molecular sensor, which can be used for sensing of dissolved molecules.

[0011] It is further desirable to provide a molecular sensor for detection, which has one or more of the following properties: low complexity, low power consumption, robustness, low cost. In particular, an improved molecular sensor, which has increased flexibility, while maintaining high sensitivity would be advantageous.

[0012] Various embodiments relating to a molecular sensor disclosed herein address one or more of the above needs and / or other needs that exist in the field of molecular sensing.

[0013] SUMMARY

[0014] It has been found by the inventors that a molecular sensor comprising a Quartz Crystal Microbalance (QCM) as described herein may, surprisingly, be used to detect methane dissolved in liquid and / or carbon dioxide dissolved in liquid by contacting the liquid to a sensor surface of the QCM that is functionalized with a metal-organic framework. Theoretically, the adsorption of molecules by a metal-organic framework (MOF) is generally hindered by the presence of water molecules. However, the inventors have experimentally shown that a suitably configured MOF can adsorb dissolved molecules directly from the liquid that is without a gas permeable membrane to separate the dissolved molecules from the liquid as is otherwise used.

[0015] Thus, disclosed is a molecular sensor for detecting molecules dissolved in a liquid. The molecular sensor may be adapted to detect methane and / or carbon dioxide dissolved in an aqueous liquid such as water, e.g. freshwater, saline water, e.g. seawater, groundwater, or in a non-aqueous liquid such as alcohol or oil.

[0016] The molecular sensor comprises:

[0017] - a sensor housing,

[0018] - a measurement chamber comprised in the sensor housing, the measurement chamber being fluidly coupled to an exterior of the sensor housing, and

[0019] - a quartz crystal microbalance (QCM). A quartz crystal resonator comprised in the QCM is functionalised with a metal-organic framework (MOF) which can reversibly adsorb dissolved methane and / or reversibly adsorb dissolved carbon dioxide directly from the liquid.

[0020] Accordingly, the quartz crystal resonator has a functionalised surface. The molecular sensor is configured such that liquid which enters the measurement chamber can come in direct contact with the functionalised surface.

[0021] In some embodiments, the molecular sensor comprises a liquid inlet and a liquid outlet each fluidly coupling the measurement chamber to an exterior of the sensor housing and providing a flow path from the liquid inlet to the liquid outlet via the measurement chamber. Surprisingly to the inventors, the molecular sensor based on the described functionalized QCM can be used to reliably and reproducibly determine the presence and / or concentration of methane and / or carbon dioxide molecules in a liquid, when the functional surface of the QCM is exposed directly to the liquid, that is, when the liquid directly contacts the functional surface.

[0022] The QCM further comprises the common, known components such as an oscillation source that is configured to oscillate the quartz crystal resonator and a frequency sensor that is configured to sense a resonance frequency of the quartz crystal resonator.

[0023] The molecular sensor can be deployed at a site where monitoring of possible methane and / or carbon dioxide is desirable. In the case of dissolved methane / carbon dioxide, the standard testing method involves collections of liquid samples, which are then transported to a laboratory for analysis, which is costly and time-consuming and also associated with risk of sampling losses during storage and transport. The molecular sensor may be integrated into automated underwater vehicles (ALIVs) and may for example be used as part of the ship-of- opportunity program. In some embodiments, the molecular sensor is configured for being mounted on, or constructed into, an automated underwater vehicle (AUV).

[0024] In some embodiments, an electrode of the quartz crystal resonator comprises one or more conductive materials, such as one or more of: Gold, Indium Tin Oxide (ITO), Silver, and / or Silicon oxide, and the MOF is positioned on, such as attached to, adhered to, or bonded to, a surface of the electrode. The MOF functionalises the quartz crystal resonator and thereby the QCM for detection of particular molecules.

[0025] The MOF may comprise [Cu(hfipbb)(H2O)] in which Cu paddlewheel structures are connected by 2,2-bis(4-carboxyphenyl)hexafluoropropane. [Cu(hfipbb)(H2O)] comprises the functional group CF3 on the surface to which methane and carbon dioxide may bind, and internal channels with aromatic regions where methane and carbon dioxide may also bind. Other MOFs may be used. In some embodiments, the MOF comprises at least one functional group comprising at least one halogen selected from fluorine, chlorine, bromine or iodine, preferably fluorine. For example, in some embodiments, the MOF comprises at least one functional group comprising CF3 (trifluoromethyl), CCI3 (trichloromethyl), CBrs (trifbromomethyl), or CI3 (triiodomethyl).

[0026] In some embodiments, the MOF comprises a hydrophobic surface and / or a microporous structure comprising pores having a diameter of between 1.5 A and 5 A (Angstrom), such as between 1.75 A and 4.5 A, such as between 2.0 A and 4 A. The pore size diameter may be determined by high resolution gas adsorption-desorption isotherm method (i.e. by physical adsorption, and using either nitrogen or argon), or by computational methods based on the crystal structure of the MOF (e.g. using CSD Mercury, PoreBlazer, Zeo++, or Monte Carlo simulations). Preferably, the pore size (diameter) of the MOF is approximately the same size as a diameter of methane and / or carbon dioxide. This may optimize interaction between the MOF and molecules of methane and / or carbon dioxide. Furthermore, molecules larger than methane and / or carbon dioxide may thus be prevented / inhibited from going into the MOF.

[0027] In some embodiments, the molecular sensor further comprises a pump system arranged in the flow path between the liquid inlet and the liquid outlet, where the pump system is configured to pump liquid from an exterior of the sensor housing along the flow path. In other embodiments, the molecular sensor is configured to be a static sensor, i.e. a sensor without a pump system. The molecular sensor may be a microfluidic device. In other embodiments, the molecular sensor is configured as an immersion probe. An immersion probe is configured such that the sensor comes into direct contact with the liquid without use of a pumping system. An immersion probe may have a liquid inlet and a liquid outlet comprised in a single combined liquid inlet and outlet, i.e. a single opening providing access for the liquid to the sensor.

[0028] In some embodiments, the molecular sensor further comprises a reference quartz crystal resonator disposed within the sensor housing, the reference quartz crystal resonator being configured to provide at least one reference measurement, wherein the reference quartz crystal resonator is a non-functionalised quartz crystal resonator. The molecular sensor is preferably configured such that the reference quartz crystal resonator is exposed to similar environmental effects as the functionalised quartz crystal. This allows for correction of environmental effects, such as temperature and pressure effects, on the quartz crystal. For example, the quartz crystal will have a given fundamental frequency at one pressure, but the fundamental frequency may be different at a different pressure. Thus, if the molecular sensor is put, for example, on an underwater vehicle, a reference quartz crystal resonator may be used to correct for the difference in frequency measurements obtained underwater and at atmospheric pressure. Alternatively, or additionally, the QCM may be pre-calibrated, for example based on frequency measurements done at different pressures. The QCM may alternatively, or additionally, be pre-calibrated to correct for other environmental effects such as temperature, pH, salinity, etc. A molecular sensor may comprise multiple reference quartz crystal resonators. In some embodiments, the calculation of the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide is based at least in part on the at least one reference measurement provided by the reference quartz crystal resonator, or by one or more of a multiple of reference quartz crystal resonators. In some embodiments, the molecular sensor further comprises one or more communication units configured for wired and / or wireless transmission of data from the molecular sensor to an external system, such as an external processing unit, i.e. to a data processing unit external to the sensor housing.

[0029] The molecular sensor may be configured to obtain continuous or intermittent measurements of one or more oscillation frequencies of the quartz crystal resonator and, optionally, of the reference quartz crystal resonator. In some embodiments, the molecular sensor comprises a data processing unit within the sensor housing and / or the molecular sensor is communicatively coupled to an external data processing unit. The data processing unit, and / or external data processing unit, may be configured to calculate the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based at least in part on the obtained measurements.

[0030] In some embodiments, the calculation of the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide is based at least in part on sensor data received from one or more auxiliary sensors disposed within the sensor housing. The one or more auxiliary sensors may be one or more of: a temperature sensor, a pressure sensor, an oxygen sensor, a nutrient sensor (such as a nitrate, and / or phosphate sensor), a pH sensor, or an ORP sensor.

[0031] The interrogation depth within the MOF film increases with harmonics providing a way to measure the adsorption of methane and carbon dioxide at different depth of the MOF. A reason for this is the differences in kinetic diameter / van der Waals volume of methane and carbon dioxide, where carbon dioxide can penetrate deeper within the film compared to methane. This provides the possibility to use the sensor as a ‘virtual array’, where the harmonics / overtones of the QCM measurement is used to discriminate between methane and carbon dioxide using multivariate statistical data processing (e.g., principal component analysis, non-negative matrix factorization). Thus, in some embodiments, the data processing unit is configured to discriminate between adsorbed methane and adsorbed carbon dioxide based on analysis of measurements of a multiple harmonic response of the QCM. That is, a plurality of harmonics, e.g. fundamental frequency, first harmonic, and second harmonic, are measured and analysed. Another method to distinguish the adsorption of methane and carbon dioxide is to use a physical sensor array, i.e. multiple MOFs, to obtain information on the difference in adsorption at different depth of a MOF. Thus, alternatively or additionally, the molecular sensor may comprise a plurality of MOFs of varying thickness, i.e. where each MOF has a thickness that is different to the thickness of the other MOF(s), and where the QCM response from each of the plurality of MOFs is measured substantially simultaneously.

[0032] A data processing unit, such as a data processing unit disposed in the sensor housing or an external data processing unit, may comprise one or more data processors. Such one or more data processors may be configured to receive the measurements of one or more oscillation frequencies of the quartz crystal resonator and to process the measurements to extract information therefrom. The one or more data processors may be further configured to calculate the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based at least in part on the obtained measurements. Some or all of the processing steps may be performed by a local data processing unit, i.e. one disposed in the sensor housing, or some or all of the processing steps may be performed by an external data processing unit, or the processing steps may be distributed between a local processing unit and an external processing unit.

[0033] Here and in the following, the term data processor is intended to comprise any circuit and / or device suitably adapted to perform the functions described herein. In particular, the term data processor comprises a general- or special-purpose programmable microprocessor unit, such as a central processing unit (CPU) of a computer or of another data processing system, a digital signal processing unit (DSP), an application specific integrated circuits (ASIC), a programmable logic arrays (PLA), a field programmable gate array (FPGA), a special purpose electronic circuit, etc., or a combination thereof.

[0034] Further disclosed is a method of detecting and / or quantifying methane and / or carbon dioxide dissolved in a liquid. The liquid may be aqueous such as water, e.g. freshwater, saline water, e.g. seawater, groundwater, or non-aqueous such as alcohol or oil. The method comprising the steps of: providing a molecular sensor comprising a quartz crystal microbalance (QCM), the QCM comprising a quartz crystal resonator, the QCM being functionalised with a metal-organic framework (MOF) which can reversibly adsorb dissolved methane and / or reversibly adsorb dissolved carbon dioxide directly from the liquid; providing the liquid to a functionalised surface of the QCM such that the liquid directly contacts the functionalised surface; and obtaining a plurality of measurements of one or more oscillation frequencies of the quartz crystal resonator. In some embodiments, the method comprises the step of: calculating the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based on the obtained measurements.

[0035] In some embodiments, the molecular sensor is a molecular sensor as described herein.

[0036] In some embodiments, the method further comprises collecting oscillation frequency data continuously or intermittently.

[0037] In some embodiments, the method further comprises collecting oscillation frequency data continuously or intermittently from a non-functionalised reference quartz crystal resonator, and calculating the concentration is based at least in part on the collected data from the reference quartz crystal resonator.

[0038] In some embodiments, the method further comprises collecting sensor data from one or more auxiliary sensors disposed within the sensor housing, and calculating the concentration is based at least in part on the collected sensor data. The one or more auxiliary sensors may be one or more of: a temperature sensor, a pressure sensor, a pH sensor, or an ORP sensor.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 is a diagram of a quartz crystal microbalance (QCM) according to some embodiments;

[0041] FIGS. 2 and 3 show a diagram of a molecular sensor comprising a QCM according to some embodiments;

[0042] FIG. 4 shows an example of a molecular sensor according to some embodiments being deployed in a liquid environment;

[0043] FIG. 5 is a schematic of a deposited metal-organic framework (MOF) according to some embodiments;

[0044] FIGS. 6 and 7 shows graphs of experimental data of the quantitative response to dissolved methane or dissolved carbon dioxide, respectively, by a molecular sensor according to some embodiments;

[0045] FIG. 8 shows a flow chart of a method of detecting and / or quantifying molecules dissolved in a liquid; and FIG. 9 shows a diagram of a molecular sensor comprising a QCM according to some embodiments.

[0046] DETAILED DESCRIPTION

[0047] Various aspects and embodiments of a molecular sensor and a method of detecting and / or quantifying molecules dissolved in a liquid as disclosed herein will now be described with reference to the drawings.

[0048] FIG. 1 shows a diagram of a quartz crystal microbalance (QCM) according to some embodiments. Part (a) of FIG. 1 shows a plan view of the QCM 100, while part (b) shows a cross-sectional view taken along the line B-B' of part (a) of FIG. 1.

[0049] The QCM 100 comprises a quartz crystal resonator 101 and a metal-organic Framework (MOF) 108 provided on a surface of the quartz crystal resonator 101. The quartz crystal resonator 101 comprises a disc-shaped quartz substrate 102, with two thin film electrodes 104, 106 arranged on opposite sides of the quartz substrate 102. The quartz substrate 102 does not need to be disc-shaped, and may instead have e.g. a polygonal shape or the like. The MOF 108 is provided on a surface of one of the electrodes, for example the surface of the upper electrode 106.

[0050] A quartz crystal microbalance (QCM) measures changes in the frequency of a quartz resonator as a function of mass. The resonator has a functionalised surface, which can adsorb molecules and when the mass of the resonator changes due to adsorption, the measured frequency changes. MOFs are a class of porous coordination polymers prepared by connecting inorganic nodes with organic linkers yielding extended, periodic networks with high internal surface area, which makes them suitable for adsorption-based applications. The QCM 100 is functionalised with a MOF which can reversibly adsorb methane and / or reversibly adsorb carbon dioxide such that the QCM is suitable for use in sensing methane and / or carbon dioxide. The MOF may be a MOF as described in connection with FIG. 5.

[0051] Advantageously, the MOF may be configured to have a hydrophobic surface and / or a microporous structure. The hydrophobicity and microporosity may separately or in combination facilitate a favourable interaction with methane and / or carbon dioxide relative to that of the liquid in which the methane and / or carbon dioxide is dissolved.

[0052] The QCM further comprises an AC power source 110 for applying a voltage between the upper electrode 106 and the lower electrode 104 by a lead wire or the like, thus acting as an oscillation source for oscillating the quartz crystal resonator 101. FIGS. 2 and 3 show a diagram of a molecular sensor comprising a QCM according to some embodiments.

[0053] The molecular sensor comprises a sensor housing 201 , a measurement chamber 208 within the sensor housing, a liquid inlet 202 and a liquid outlet 214. In part (a) of FIG. 2 is shown the molecular sensor in a partially transparent view to show the components within the sensor housing 201 , while part (b) shows the same molecular sensor in a non-transparent view. The liquid inlet and outlet each fluidly couple the measurement chamber to an exterior of the sensor housing 201 and are connected via flow channels 206. The flow channels 206 provide a flow path for a liquid from the liquid inlet to the liquid outlet via the measurement chamber 208 entering the measurement chamber via a chamber inlet 210 and exiting via a chamber outlet 212. Disposed in the measurement chamber is a quartz crystal microbalance (QCM) 100 and the flow path of the molecular sensor is configured such that liquid which enters the measurement chamber can come in direct contact with a surface of the MOF. The QCM may be a QCM as described in connection with FIG. 1.

[0054] The molecular sensor may be a microfluidic device, and may comprise a pump system 204 arranged in the flow path between the liquid inlet and the liquid outlet, the pump system being configured to pump liquid from the exterior of the sensor housing along the flow path.

[0055] The molecular sensor further comprises QCM electronics 216, which comprises a frequency sensor configured to sense a resonance frequency of the quartz crystal resonator. The QCM electronics may comprise, or be communicatively connected to, a processing unit configured to calculate the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in a liquid based at least in part on the sensed resonance frequency of the quartz crystal resonator. Some or all of the data processing may be performed by a local data processing unit, i.e. a data processing unit disposed in the sensor housing 201 , or some or all of the processing steps may be performed by an external data processing unit, or the processing steps may be distributed between a local processing unit and an external processing unit. In FIG. 3 is shown an external processing unit 320 which is communicatively connected to the molecular sensor 200 via a wireless connection.

[0056] In the embodiment shown in FIG. 3 the molecular sensor further comprises a reference quartz crystal resonator 300 disposed within the sensor housing 201. The reference quartz crystal resonator is a non-functionalised quartz crystal resonator and is configured to provide reference measurements which may be used to correct for the effect of e.g. temperature and pressure on the resonance frequencies of the quartz crystal. The reference quartz crystal resonator has substantially the same physical characteristics as the functionalised quartz crystal resonator except that it does not comprise a functionalised surface, i.e. does not comprise a MOF. The calculation of the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide may be based at least in part on the at least one reference measurement.

[0057] The molecular sensor may further comprise a wireless communication unit 222 and one or more auxiliary sensors 220, where each may be electronically connected to the QCM electronics 216. Alternatively, or additionally, the molecular sensor may comprise a communication unit 218 configured for wired transmission of data from the molecular sensor to a system external to the sensor housing. The auxiliary sensors 220 may be one or more of: a temperature sensor, a pressure sensor, a pH sensor, or an ORP sensor. The calculation of the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide may be based at least in part on sensor data received from the one or more auxiliary sensors 220.

[0058] FIG. 4 shows an example of a molecular sensor according to some embodiments being deployed in a liquid environment.

[0059] A molecular sensor 200, 300 has been deployed in a cylindrical pipe 400, which is partially filled with liquid 402 and partially filled with air 403. The pipe may be open to atmosphere or sealed off. The molecular sensor is shown submerged in the liquid 402, but may have been installed while the installation site was dry following which the pipe filled with liquid to above the sensor installation site. The molecular sensor 200, 300 may be a molecular sensor as described in FIGS. 2 or 3.

[0060] While deployed the molecular sensor 200, 300 may continuously or intermittently monitor the concentration of methane and / or carbon dioxide dissolved in the liquid 402.

[0061] FIG. 5 shows a schematic of a deposited metal-organic framework (MOF) according to some embodiments. Shown is a 2D-structured MOF comprising an inorganic metal cluster 506 coordinated with an organic ligand 508 to form a layered structure on a self-assembled monolayer (SAM) 504, which couples the MOF to a surface 502, such as a surface of a quartz crystal resonator. While a 2D-structured MOF is shown, a MOF having a different type of structure may be used.

[0062] For use as a sensing surface for the sensing of methane and / or carbon dioxide, the MOF must display selectivity towards methane and carbon dioxide. To this end, the MOF may have at least one functional group comprising at least one halogen selected from fluorine, chlorine, bromine or iodine, preferably fluorine. For example, the MOF may have at least one functional group comprising CF3 (trifluoromethyl), CCI3 (trichloromethyl), CBrs (trifbromomethyl), or CI3 (triiodomethyl).

[0063] A 2D-structured MOF may be [Cu(hfipbb)(H2O)] which arranges in layers with narrow 1 D hydrophobic channels connected via smaller gates, and which shows a selectivity towards methane and carbon dioxide relative to other atmospheric gases.

[0064] A functionalised quartz crystal resonator constitutes a sensor thin-film. The sensor thin-films may be prepared via a layer-by-layer chemical deposition method. In the following, a synthesis method to produce a quartz crystal resonator functionalised with [Cu(hfipbb)(H2O)] is described.

[0065] First, the conductive surface of a QCM electrode is functionalised with a self-assembly monolayer (SAM) of a carboxylic-acid terminated thiol, for example 16- mercaptohexadecanoic acid. The carboxylate-head of the SAM layer is then saturated with Cu2+ions by treatment with ethanolic solutions of a copper salt such as copper acetate or nitrate. The layer of Cu2+metal ions is used as a basis for continued sequential growth of the metal-organic framework by alternate injection of alcoholic solutions of metal salt and organic linker. Between each precursor, a washing step is initiated by flow of pure alcohol.

[0066] The concentration of the solutions of metal salt and organic linker can be used to control the size and morphology of the thin-film crystalline layer and the number of deposition cycles will determine the thickness of the thin-film. The more deposition cycles, the thicker the film. A thicker film may provide a higher mass uptake capacity, i.e. a higher capacity for attracting methane and carbon dioxide, and thus a higher sensor sensitivity.

[0067] FIG. 6 and 7 shows graphs of experimental data of the quantitative response to methane or carbon dioxide, respectively, by a molecular sensor according to some embodiments.

[0068] A quartz crystal resonator was functionalised with the 2D-structured metal-organic framework (MOF) [Cu(hfipbb)(H2O)]. The functionalized quartz crystal resonator was then placed in a flow cell with a relatively small internal volume (20 to 100 pL) and connected to electronics forming a Quartz Crystal Microbalance (QCM) sensor capable of measuring the frequency and dissipation of the quartz resonator. A small, microfluidic pump system was used to provide a continuous flow of a liquid sample to the internal volume and thereby to the functionalised surface of the quartz crystal resonator. The pump system allowed for a flow rate between 100 to 1500 pL / min, and an increase in flow rate was seen to yield a higher response rate from the QCM at the cost of power consumption.

[0069] Prior to experimentally testing the QCM sensor with dissolved gas, a flow of degassed water was injected into the sensor, passing the functionalised surface, so as to establish the baseline value of the fundamental frequency.

[0070] The QCM sensor was then experimentally tested using stock solutions of methane or carbon dioxide dissolved in pure water (simulating groundwater monitoring) or saline water (simulating seawater monitoring, 3.5% salinity). The experimental data shown in FIGS. 6 and 7 were obtained using stock solutions dissolved in pure water. The sensor response was measured at increasing levels of dissolved gas in water, e.g., 1, 2.5, 5, 7.5, 10, 15, 20 ppm, at atmospheric pressure and room temperature to create a response curve, such as might be used for calibration of a QCM. At each concentration of dissolved methane or carbon dioxide the drop in fundamental frequency due to adsorption of methane or carbon dioxide is recorded and plotted in part (a) of FIGS. 6 and 7, respectively. From the data shown in part (a) of FIGS. 6 and 7, a graph of the determined frequency drop versus known concentration of dissolved methane or carbon dioxide can be obtained and is shown in part (b) of FIGS. 6 and 7, respectively. The thus demonstrated quantitative response to dissolved methane / carbon dioxide provides experimental basis for the function of a molecular sensor as described herein.

[0071] In part (a) of FIG. 6 is shown measurements of the fundamental frequency of the quartz crystal resonator in the QCM as a function of time. The measurements were obtained using six different concentrations of methane dissolved in pure water: 1 , 2, 5, 10, 15, 20 ppm as written in the upper right corner of each graph.

[0072] In part (b) of FIG. 6 is shown the determined frequency drop versus known concentration of dissolved methane. The data points have been fitted with a straight line. The linear relationship between the determined frequency drop and the known concentration of dissolved methane demonstrates the functionality of the QCM sensor for sensing of dissolved methane.

[0073] In part (a) of FIG. 7 is shown measurements of the fundamental frequency of the quartz crystal resonator in the QCM as a function of time. The measurements were obtained using three different concentrations of carbon dioxide dissolved in pure water: 10, 20, 30 ppm as written in the upper right corner of each graph. In part (b) of FIG. 7 is shown the determined frequency drop versus known concentration of dissolved carbon dioxide. The data points have been fitted with a straight line. The linear relationship between the determined frequency drop and the known concentration of dissolved carbon dioxide demonstrates the functionality of the QCM sensor for sensing of dissolved carbon dioxide.

[0074] FIG. 8 shows a flow chart of a method of detecting and / or quantifying molecules dissolved in a liquid.

[0075] In step S82, a molecular sensor comprising a quartz crystal microbalance (QCM) having a quartz crystal resonator is provided. The QCM is functionalised with a metal-organic framework (MOF) which can reversibly adsorb methane and / or reversibly adsorb carbon dioxide.

[0076] In step S84, a liquid is provided to a functionalised surface of the QCM such that the liquid directly contacts the functionalised surface. This allows for any methane and / or carbon dioxide dissolved in the liquid to adsorb on a surface of the MOF.

[0077] In step S86, a plurality of measurements of one or more oscillation frequencies of the quartz crystal resonator is obtained.

[0078] In step S88, the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based on the obtained measurements is calculated.

[0079] The process may then return to step S86 to obtain further measurements of one or more oscillation frequencies of the quartz crystal resonator or the process may end.

[0080] Thus, the detection method could also be defined by the following overall procedure steps: (S1) a contact step, wherein the liquid is brought into contact with the functionalised surface of the QCM, (S2) a measurement step, wherein a change in the physical quantity of the quartz crystal resonator due to the adsorption of methane or carbon dioxide is measured, and (S3) a determination step, wherein the type or amount of methane or carbon dioxide from the result of the measurement is determined.

[0081] FIG. 9 shows a diagram of a molecular sensor comprising a QCM according to some embodiments.

[0082] The molecular sensor comprises a sensor housing 201, a measurement chamber within the sensor housing, and a combined liquid inlet and outlet 224. In part (a) of FIG. 9 is shown the molecular sensor in a partially transparent view to show the components within the sensor housing, while part (b) shows the same molecular sensor in a non-transparent view.

[0083] The combined liquid inlet and outlet 224 fluidly couples the measurement chamber to an exterior of the sensor housing 201. The molecular sensor does not comprise a pump system and is configured for immersion probing, where the liquid in which the molecular sensor is immersed enters the measurement chamber via the combined inlet and outlet 224. Disposed in the measurement chamber is a quartz crystal microbalance (QCM) 100 and a flow path between the exterior of the sensor and the measurement chamber, via the combined liquid inlet and outlet, is configured such that liquid which enters the measurement chamber can come in direct contact with a surface of the MOF. The QCM may be a QCM as described in connection with FIG. 1.

[0084] The invention may be defined according to one, more, or all the following items. Each item represents a potential embodiment of the invention. Any embodiment may be claimed individually or in combination.

[0085] 1. A molecular sensor for detecting molecules dissolved in liquid, the molecular sensor comprising:

[0086] - a sensor housing,

[0087] - a measurement chamber comprised in the sensor housing,

[0088] - a liquid inlet and a liquid outlet each fluidly coupling the measurement chamber to an exterior of the sensor housing and providing a flow path from the liquid inlet to the liquid outlet via the measurement chamber, and

[0089] - a quartz crystal microbalance (QCM) disposed in the measurement chamber, wherein a quartz crystal resonator comprised in the QCM is functionalised with a metalorganic framework (MOF) which can reversibly adsorb methane and / or reversibly adsorb carbon dioxide, and wherein the molecular sensor is configured such that liquid which enters the measurement chamber can come in direct contact with a surface of the MOF. 2. The molecular sensor according to item 1, wherein the molecular sensor is a microfluidic device.

[0090] 3. The molecular sensor according to any one of the preceding items, wherein the MOF comprises at least one functional group comprising at least one halogen selected from fluorine, chlorine, bromine or iodine, preferably fluorine.

[0091] 4. The molecular sensor according to any one of the preceding items, wherein the MOF comprises at least one functional group comprising CF3 (trifluoromethyl), CCI3 (trichloromethyl), CBr3 (trifbromomethyl), or Ch (triiodomethyl).

[0092] 5. The molecular sensor according to any one of the preceding items, wherein the MOF comprises a hydrophobic surface and / or a microporous structure comprising pores having a diameter of between 1.5 A and 5 A (Angstrom), such as between 1.75 A and 4.5 A, such as between 2.0 A and 4 A, where the pore size diameter is determined by high resolution gas adsorption-desorption isotherm method or by computational methods based on the crystal structure of the MOF.

[0093] 6. The molecular sensor according to any one of the preceding items, wherein the molecular sensor is configured to obtain continuous or intermittent measurements of one or more oscillation frequencies of the quartz crystal resonator and, optionally, of the reference quartz crystal resonator.

[0094] 7. The molecular sensor according to item 6, wherein the molecular sensor comprises a processing unit within the sensor housing and / or the molecular sensor is communicatively coupled to an external processing unit, and wherein the processing unit, and / or external processing unit, is configured to calculate the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based at least in part on the obtained measurements.

[0095] 8. The molecular sensor according to any one of the preceding items, wherein the molecular sensor further comprises a reference quartz crystal resonator disposed within the sensor housing, the reference quartz crystal resonator being configured to provide at least one reference measurement, wherein the reference quartz crystal resonator is a nonfunctionalised quartz crystal resonator, and wherein the processing unit, and / or external processing unit, is configured to calculate the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based at least in part on the at least one reference measurement.

[0096] 9. The molecular sensor according to any of items 7 or 8, wherein the calculation of the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide is based at least in part on sensor data received from one or more auxiliary sensors disposed within the sensor housing, and wherein the one or more auxiliary sensors is one or more of: a temperature sensor, a pressure sensor, an oxygen sensor, a nutrient sensor, such as a nitrate and / or phosphate sensor, a pH sensor, or an ORP sensor.

[0097] 10. The molecular sensor according to any of the preceding items, wherein the processing unit is configured to discriminate between adsorbed methane and adsorbed carbon dioxide based on analysis of measurements of a multiple harmonic response of the QCM.

[0098] 11. The molecular sensor according to any one of the preceding items, wherein the molecular sensor further comprises one or more communication units configured for wired and / or wireless transmission of data from the molecular sensor to an external system, such as an external processing unit.

[0099] 12. The molecular sensor according to any one of the preceding items, wherein the molecular sensor is adapted to detect methane and / or carbon dioxide dissolved in an aqueous liquid such as water, e.g. freshwater, saline water, e.g. seawater, groundwater, or in a non-aqueous liquid such as alcohol or oil.

[0100] 13. A method of detecting and / or quantifying molecules dissolved in a liquid, the method comprising the steps of: providing a molecular sensor comprising a quartz crystal microbalance (QCM), the QCM comprising a quartz crystal resonator, the QCM being functionalised with a metal-organic framework (MOF) which can reversibly adsorb methane and / or reversibly adsorb carbon dioxide; providing the liquid to a functionalised surface of the QCM such that the liquid directly contacts the functionalised surface; obtaining a plurality of measurements of one or more oscillation frequencies of the quartz crystal resonator; and calculating the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based on the obtained measurements.

[0101] 14. The method according to item 12, wherein the molecular sensor is a molecular sensor according to any of items 1 - 11.

[0102] 15. Use of a molecular sensor according to any one of items 1-12 in a method according to any one of items 13-14 for detecting and / or quantifying methane dissolved in a liquid and / or carbon dioxide dissolved in a liquid.

[0103] LIST OF REFERENCES

[0104] 100 Quartz crystal Microbalance

[0105] 101 Quartz crystal resonator

[0106] 102 Quartz disc

[0107] 104, 106 Contact electrode

[0108] 108 Metal-organic Framework (MOF)

[0109] 110 AC Voltage source

[0110] 200 Microfluidic sensor device

[0111] 201 Sensor housing

[0112] 202 Liquid inlet

[0113] 204 Microfluidic pump system

[0114] 206 Flow channel

[0115] 208 Measurement chamber

[0116] 210 Measurement chamber inlet 212 Measurement chamber outlet

[0117] 214 Liquid outlet

[0118] 216 QCM (Quartz Crystal Microbalance) electronics

[0119] 218 Communication unit configured for wired communication 220 Auxiliary sensor(s)

[0120] 222 Wireless communications unit

[0121] 224 Combined liquid inlet and outlet

[0122] 300 Non-functionalized quartz crystal resonator

[0123] 308 Reference measurement chamber 310 Reference measurement chamber inlet

[0124] 312 Reference measurement chamber outlet

[0125] 320 External data processing unit

[0126] 400 Pipe

[0127] 402 Liquid 403 Air

[0128] 502 Quartz crystal resonator surface

[0129] 504 Self-assembled monolayer

[0130] 506 Inorganic metal cluster

[0131] 508 Organic ligand

Claims

CLAIMS1. A molecular sensor for detecting methane and / or carbon dioxide dissolved in liquid, the molecular sensor comprising:- a sensor housing,- a measurement chamber comprised in the sensor housing, the measurement chamber being fluidly coupled to an exterior of the sensor housing, and- a quartz crystal microbalance, i.e. QCM, wherein a quartz crystal resonator comprised in the QCM has a functionalised surface that is functionalised with a metal-organic framework, i.e. MOF, which can reversibly adsorb dissolved methane and / or reversibly adsorb dissolved carbon dioxide directly from the liquid, the functionalised surface being disposed in the measurement chamber, and wherein the molecular sensor is configured such that liquid which enters the measurement chamber can come in direct contact with the functionalised surface.

2. The molecular sensor according to claim 1 , wherein the molecular sensor comprises a liquid inlet and a liquid outlet each fluidly coupling the measurement chamber to the exterior of the sensor housing and providing a flow path from the liquid inlet to the liquid outlet via the measurement chamber.

3. The molecular sensor according to claim 2, wherein the molecular sensor is a microfluidic device.

4. The molecular sensor according to claim 1 , wherein the molecular sensor is configured for immersion probing, the molecular sensor comprising a combined liquid inlet and outlet fluidly coupling the measurement chamber to the exterior of the sensor housing.

5. The molecular sensor according to any of the preceding claims, wherein the MOF comprises at least one functional group comprising at least one halogen selected from fluorine, chlorine, bromine or iodine, preferably fluorine.

6. The molecular sensor according to any of the preceding claims, wherein the MOF comprises at least one functional group comprising CF3, i.e. trifluoromethyl, CCI3, i.e. trichloromethyl, CBrs, i.e. trifbromomethyl, or Ch, i.e. triiodomethyl.

7. The molecular sensor according to any of the preceding claims, wherein the MOF comprises a hydrophobic surface and / or a microporous structure comprising pores having a diameter of between 1.5 A and 5 A, i.e. Angstrom, such as between 1.75 A and 4.5 A, such as between 2.0 A and 4 A, where the pore size diameter may be determined by high resolution gas adsorption-desorption isotherm method or by computational methods based on the crystal structure of the MOF.

8. The molecular sensor according to any of the preceding claims, wherein the molecular sensor is configured to obtain continuous or intermittent measurements of one or more oscillation frequencies of the quartz crystal resonator.

9. The molecular sensor according to claim 8, wherein the molecular sensor comprises a processing unit within the sensor housing and / or the molecular sensor is communicatively coupled to an external processing unit, and wherein the processing unit, and / or external processing unit, is configured to calculate the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based at least in part on the obtained measurements.

10. The molecular sensor according to any of the preceding claims, wherein the molecular sensor further comprises a reference quartz crystal resonator disposed within the sensor housing, the molecular sensor being configured to provide at least one reference measurement from the reference quartz crystal resonator, wherein the reference quartz crystal resonator is a non-functionalised quartz crystal resonator.

11. The molecular sensor according to claim 10, wherein the molecular sensor is configured to obtain continuous or intermittent measurements of one or more oscillation frequencies of the reference quartz crystal resonator.

12. The molecular sensor according to any of the preceding claims, dependent on claims 9 and 10, wherein the processing unit, and / or external processing unit, is configured to calculate the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based at least in part on the at least one reference measurement.

13. The molecular sensor according to any of the preceding claims, dependent on claim 9, wherein the calculation of the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide is based at least in part on sensor data received from one or more auxiliary sensors disposed within the sensor housing, and wherein the one or more auxiliary sensors is one or more of: a temperature sensor, a pressure sensor, an oxygen sensor, a nutrient sensor, such as a nitrate and / or phosphate sensor, a pH sensor, or an ORP sensor.

14. The molecular sensor according to any of the preceding claims, dependent on claim 9, wherein the processing unit is configured to discriminate between adsorbed methane and adsorbed carbon dioxide based on analysis of measurements of a multiple harmonic response of the QCM.

15. The molecular sensor according to any of the preceding claims, wherein the molecular sensor further comprises one or more communication units configured for wired and / or wireless transmission of data from the molecular sensor to an external system, such as an external processing unit, such as the external processing unit when dependent on claim 9.

16. The molecular sensor according to any of the preceding claims, wherein the molecular sensor is adapted to detect methane and / or carbon dioxide dissolved in an aqueous liquid such as water, e.g. freshwater, saline water, e.g. seawater, groundwater, or in a nonaqueous liquid such as alcohol or oil.

17. A method of detecting and / or quantifying methane and / or carbon dioxide dissolved in a liquid, the method comprising the steps of:providing a molecular sensor comprising a quartz crystal microbalance, i.e. QCM, the QCM comprising a quartz crystal resonator having a functionalised surface being functionalised with a metal-organic framework, i.e. MOF, which can reversibly adsorb dissolved methane and / or reversibly adsorb dissolved carbon dioxide directly from the liquid; providing the liquid to the functionalised surface such that the liquid directly contacts the functionalised surface; and obtaining a plurality of measurements of one or more oscillation frequencies of the quartz crystal resonator.

18. The method according to claim 17, the method comprising calculating the concentration of dissolved methane and / or the concentration of dissolved carbon dioxide in the liquid based on the obtained measurements.

19. The method according to claim 17 or 18, wherein the molecular sensor is a molecular sensor according to any of claims 1 - 16.

20. Use of a molecular sensor according to any of claims 1-16 in a method according to any of claims 17-19 for detecting and / or quantifying methane dissolved in a liquid and / or carbon dioxide dissolved in a liquid.

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