Molecular imprinted polymer-based volatile organic compound sensors

Abstract

The present invention is directed to a polymer bead comprising a crosslinked optionally substituted polyacrylate, wherein the polyacrylate is obtained by crosslinking an optionally substituted acrylic acid with a crosslinker at a ratio of the optionally substituted acrylic acid to crosslinker in the range of about 1:3 to about 1:1 by moles, and wherein the polymer bead has a porous structure having a plurality of complexing cavities therein, wherein the complexing cavities are spatially oriented for binding a target compound, wherein the target compound comprises an optionally substituted C3 to C6 carboxylic acid. The present invention also refers to a method of preparing a polymer bead, a sensor for detecting a target compound comprising an optionally substituted C3 to C6 carboxylic acid, a method of preparing the sensor, use of the sensor and a method of detecting a target compound comprising an optionally substituted C3 to C6 carboxylic acid.

Description

[0001] MOLECULAR IMPRINTED POLYMER-BASED VOLATILE ORGANIC COMPOUND SENSORS TECHNICAL FIELD

[0002] The present invention generally relates to the field of electrochemistry and more specifically relates to molecular imprinted polymer-based volatile organic compound sensors.

[0003] BACKGROUND

[0004] Volatile organic compounds (VOCs) are low boiling point, high vapour pressure organic compounds with distinct scent which are found in various applications and products such as paints, cosmetics, and healthcare products. Within the food industry, these VOCs play two major roles - imparting favourable fragrance, making them desirable for customers, and acting as biomarkers for food degradation. In the former, favourable VOCs are released during the processing of food products while in the latter, degradation-related VOCs are released when the produced food products undergo degradation through several stressors such as bacteria and oxidation during its farm to fork lifecycle, resulting in the food products having a rancid odour. With the rapid development in the food industry, it is crucial to ensure fresh food for consumers. This can potentially be achieved by analysing freshness of the food product throughout the product lifecycle.

[0005] Traditional analysis such as detecting change in pH, peroxide value, and plate count method for analysis of edible product degradation is time consuming, and requires direct probing or sampling of the food product which is destructive in nature, rendering the food product inedible.

[0006] Analytical tools for the profiling of VOCs from edible products include gas chromatography -mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS). However, the large size footprint and need of skilled operators limit their usage within a laboratory and limit their portability. Additionally, these methods do not provide continuous VOC measurement or readouts to explain the state of the food product over a period of time.Noting these drawbacks, several miniature, portable, and real-time VOC sensors have been developed. Several miniaturised electrical sensors based off of VOC responsive metal oxides (SnO2, ZnO, TiO2 etc) and two-dimensional transition metal dichalcogenides (2D TMDs) such as M0S2, WSe2, WS2 were developed for the detection of VOCs released from edible products. However, their high operating temperatures (>300°C) and low selectivity due to their indiscriminate electron transfer behaviour between the sensor and VOCs make them unsuitable for VOC profiling in each food product where there may be presence of flavour and degradation-related VOCs for freshness estimation.

[0007] Molecular imprinted polymers (MIPs) are shape-selective synthetic polymeric nanoparticles whose operation mechanism is analogous to biological antigen-antibody interaction. Within the food industry, these MIPs have been utilised for preconcentration, detection and extraction of target analytes from food particles Tn terms of VOC profiling from edible products for degradation quantification, these MIPs have been used in tandem with GC-MS where the MIPs are integrated with headspace solid-phase extraction microfiber for extraction of the target VOCs followed by GC-MS analysis. In terms of portable sensors, MIPs have been integrated with mass-based and electrochemical resistive sensors for VOC profiling. However, these sensors face issues with sensitivity due to their large environmental sensitivity (in mass-based sensors) or interference from resistive elements due to electrode fouling caused by faradic probing (in resistive sensors).

[0008] In mass-based sensors such as quartz crystal microbalance (QCM) sensors which have been modified with MIP beads, exposure of MIPs to VOCs lead to non-covalent interactions between the MIPs and the VOCs. This leads to trapping of the VOCs within the MIP matrix thus increasing the mass of the MIP beads. Since the QCM sensors are mass-based sensors with any change in mass leading to a change in resonance frequency of the sensor, the trapping of the VOCs within the MIP on the QCM sensor leads to an increase in mass leading to a drop in resonance frequency of the sensor. However, a drawback of this sensor-based system is that due to its high environmental sensitivity, a non-specific interaction of the QCM sensor with other interfering VOCs or water (in the form of humidity) in the environment might lead to a drift of the sensor, thus affecting the sensitivity of the sensor.In electrochemical sensing, since most known MIPs are non-conductive in nature, MIPs are usually incorporated with conductive elements such as graphene, CNT or ionic liquid for electrical communication between the MIPs and the substrate of the sensor. Upon exposure of the sensor to the VOCs, the VOCs binding to the MIPs lead to subsequent swelling, leading to compression of conductive materials between adjacent beads, decreasing electron transfer resistance of the sensor. Such methods are not robust and face their own myriad of problems such as electrochemical fouling, lack of sensitivity due to interfering interactions between the VOCs and conductive materials instead of the MIPs, slow response time and sufferance from motion artefacts.

[0009] Therefore, these conventional methods are unstable due to non-specific adsorption or motion artefact which causes signal instability.

[0010] In view of the limitations of current sensors for detecting VOCs, there is a need for development of a sensor to overcome or at least ameliorate, one or more of the disadvantages described above.

[0011] SUMMARY

[0012] In an aspect, there is provided a polymer bead comprising a crosslinked optionally substituted polyacrylate, wherein the polyacrylate is obtained by crosslinking an optionally substituted acrylic acid with a crosslinker at a ratio of the optionally substituted acrylic acid to crosslinker in the range of about 1:3 to about 1:1 by moles, and wherein the polymer bead has a porous structure having a plurality of complexing cavities therein, wherein the complexing cavities are spatially oriented for binding a target compound, wherein the target compound comprises an optionally substituted Ci to C6 carboxylic acid.

[0013] Advantageously, the polymer bead as defined above may bind with a target molecule of interest through a “lock and key” mechanism, making them highly selective and sensitive.

[0014] Further advantageously, the polymer bead as defined above may bind to a linear aliphatic carboxylic acids, which may be found in various types of edible oils.In another aspect, there is provided a method of preparing a polymer bead as defined above, comprising the steps of: mixing a template compound with an optionally substituted acrylic acid in a solvent to form a prearrangement complex, wherein the template compound comprises an optionally substituted C3 to Ce carboxylic acid; adding an initiator and a crosslinker to the prearrangement complex under reaction conditions to crosslink the optionally substituted acrylic acid to form a crosslinked optionally substituted polyacrylate mixture, wherein the optionally substituted acrylic acid and crosslinker are present at a ratio in the range of about 1 :3 to about 1 : 1 by moles; and removing the template compound from the polyacrylate mixture to form the polymer bead.

[0015] Advantageously, the method as defined above may facilitate preparation of a polymer bead for a variety of template compounds to detect any molecule of interest.

[0016] Further advantageously, the method as defined above may allow for fabrication of MIPs for the detection of linear aliphatic carboxylic acids. The method may advantageously be extended to the preparation of a wide range of linear aliphatic carboxylic acids which can be found in various types of edible oils.

[0017] In another aspect, there is provided a polymer bead prepared by the method as defined above.

[0018] Advantageously, the polymer bead as defined above may be prepared to selectively detect any molecule of interest.

[0019] In another aspect, there is provided a sensor for detecting a target compound comprising an optionally substituted C3 to Ce carboxylic acid, comprising: a substrate; an adhesion promoter immobilised onto the substrate, and a polymer bead as defined above attached to the adhesion promoter immobilised onto the substrate.

[0020] Advantageously, the sensor as defined above may detect capacitance and subsequently impedance change, and may therefore not be dependent on other conductive materials for communication and may be robust to environmental interferences. The sensor as defined above may advantageously not be prone to motion artefacts of non-specific absorption of target analytes such as water vapour adsorption in high humidity environments.Advantageously, the sensor may be specific and sensitive to target gases and show reduced response to other flavour related VOCs that are not of interest. For example, the sensor as defined above may advantageously be responsive to smaller analogous molecules of the target compound, but may have reduced response to larger analogous molecules.

[0021] Advantageously, the sensor may be portable and may be used outside the laboratory.

[0022] Advantageously, the sensor as defined above may be probed by non-faradaic electrochemical impedance spectroscopy (EIS), which may not experience interference from resistive elements.

[0023] In another aspect, there is provided a method of preparing the sensor as defined above, comprising the steps of: providing a substrate; immersing the substrate in a solution of an adhesion promoter to form a surface-modified substrate, and immersing the surface-modified substrate in a suspension of the polymer bead as defined above.

[0024] Conventionally known immobilisation chemistry methods such as drop casting may cause certain physical effects such as the coffee-ring effect which leads to increased accumulation of the polymer bead on the edges but sparse polymer bead distribution in the centre. Similarly, spin coating may leads to a thin layer of the polymer bead on a surface reducing the sensitivity and causing wastage of polymers.

[0025] Advantageously, the method as disclosed above may increase the uniformity of distribution of the polymer beads or any other materials on the substrate, which in turn may increase the sensitivity of the sensor.

[0026] Additionally, post immobilisation, the remaining polymer solution may be advantageously reused for subsequent modifications for other sensors as well.

[0027] Further advantageously, the method as defined above may also be utilised to assemble other similar polymer and nanoparticle on a surface of any hydroxylated substrate or oxide substrate.In another aspect, there is provided a use of the sensor as defined above for detecting a target compound, wherein the target compound comprises an optionally substituted C3 to C , carboxylic acid.

[0028] Conventionally, MIP sensors have been used in several applications such as disease detection, identification of hazardous materials in several scenarios (i.e., VOCs for bomb detection, environmental monitoring and even food contamination).

[0029] Advantageously, the present disclosure is directed to development of MIP sensors for use in food safety and security, especially for use in quantification of food freshness.

[0030] Advantageously, by using the sensor for detecting the concentration of degradation-related VOCs, it may be possible to quantify the freshness of the food product.

[0031] Further advantageously, the sensor may be used to analyse freshness of the food product throughout the product lifecycle.

[0032] In another aspect, there is provided a method of detecting a target compound comprising an optionally substituted C3 to C , carboxylic acid, the method comprising the steps of: providing the sensor as defined above; exposing the sensor to a gas comprising the optionally substituted C3 to Ci, carboxylic acid; and measuring the capacitance or impedance of the sensor.

[0033] Advantageously, the method of detecting may involve capacitance and impedance change which is not prone to motion artefacts of non-specific adsorption of target analytes, such as water vapour adsorption in high humidity environment.

[0034] Advantageously, the method may be a non-destructive method of freshness analysis involving profiling the VOCs released by a food product and which are available in the headspace of the food packaging.

[0035] Additionally, these methods may advantageously allow continuous VOC measurement or readouts to explain the state of the food product over a period of time.BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0037] Figure 1 is a schematic diagram showing a synthesis process of hexanoic acid-sensitive MIP (HA-MLP) beads. (1002) refers to 1 mmol hexanoic acid (template), (1004) refers to 4 mmol methacrylic acid (monomer), (1006) refers to 35 mL acetonitrile, (1008) refers to 500rpm stirring for 20 minutes, (1010) refers to the prearrangement complex, (1012) refers to 86 pL AIBN (initiator) and 6 mmol EGDMA (crosslinker), (1014) refers to heat and nitrogen, (1016) refers to the HA-MIP nanobeads, (1018) refers to the methanol (MeOH) wash and template removal, and (1020) refers to the MTP nanobeads.

[0038] Figure 2 is a set of images showing a fabrication process of a sensor. Figure 2(a) is a photo showing a structure of interdigitated electrode (IDE) and a magnified photo of gold digits patterned on glass and Figure 2(b) is a flowchart showing a fabrication process for a HA-MIP sensor.

[0039] Figure 3 is a schematic diagram showing the mechanism of VOC binding to MIP sensor.

[0040] Figure 4 is a set of diagrams showing water contact angle of water on IDE. Figure 4(a) shows the water contact angle of water on bare IDE, Figure 4(b) shows the water contact angle of water on bare IDE modified with [(3 -aminopropyl)tri ethoxy silane (APTES)] (IDE-APTES) and Figure 4(c) shows the water contact angle of water on bare IDE modified with APTES and pentanoic acid-sensitive MIP (PA-MIP) beads (IDE-APTES-PA-MIP).

[0041] Figure 5 is a set of images showing a set-up to measure capacitance of the bare IDE and IDE modified with APTES and VOC sensitive MIP (IDE-APTES-MIP). Figure 5(a) is a photo showing a gas syringe attached to a syringe pump and introduced to a flow cell and a drop cell, Figure 5(b) is a photo showing constituents of the drop cell and the flow cell unit, Figure 5(c) is a photo showing the set-up of Figure 5(a) with the drop cell and the flow cell unit placed ina Faraday shield, and Figure 5(d) is a schematic diagram showing the drop cell and the flow cell unit.

[0042] Figure 6 is a set of graphs showing response of MIP sensors to different VOCs. Figure 6(a) is a line graph showing the HA-MIP sensor response to lOOppm of hexanoic acid at 40% relative humidity and at 23°C, Figure 6(b) is a line graph showing PA-MIP sensor response to lOOppm of hexanoic acid at 40% relative humidity and at 23°C, Figure 6(c) is a bar graph showing the HA-MIP sensor response to lOOppm of hexanoic acid, pentanoic acid, 2-pentylfuran and phenyl acetaldehyde at 40% relative humidity and at 23°C and Figure 6(d) is a bar graph showing the PA-MIP sensor response to lOOppm of hexanoic acid, pentanoic acid, 2-pentylfuran and phenyl acetaldehyde at 40% relative humidity and at 23°C.

[0043] Figure 7 is a set of graphs showing the response of MIP sensors to different VOCs Figure 7(a) is a line graph showing response of the PA-MIP sensor to 50ppm pentanoic acid at 40% relative humidity (RH), Figure 7(b) is a line graph showing a calibration curve of the PA-MIP sensor with a tested range of 25-188ppm of pentanoic acid at 40% RH, Figure 7(c) is a bar graph showing specificity of the PA-MIP sensor to 50ppm of key VOCs found in peanut oil (PNO) at 40% RH, Figure 7(d) is a line graph showing a calibration curve of the HA-MIP sensor with tested range of 25-200ppm of hexanoic acid at 40% RH, and Figure 7(e) is a bar graph showing specificity of the HA-MIP sensor to 50ppm of key VOCs found in PNO at 40% RH.

[0044] Figure 8 is a graph showing the capacitance at ambient atmosphere of four different HA-MIP sensors that were prepared in the same way.

[0045] Figure 9 is a line graph showing the PA-MIP sensor response to the pentanoic acid at different RH conditions.

[0046] Figure 10 is a set of graphs showing reusability of the PA-MIP sensor. Figure 10(a) is a line graph showing cyclic testing of the PA-MIP sensor to 188ppm pentanoic acid, Figure 10(b) is a bar graph showing relative impedance change of the PA-MIP sensor across 6 cycles of exposure to the pentanoic acid.Figure 11 is a set of bar graphs showing response of the MLP sensors to different sized analogous carboxylic acid VOCs. Figure 11(a) is a bar graph showing response of the HA-MIP sensor to heptanoic acid, hexanoic acid and pentanoic acid, and Figure 11(b) is a bar graph showing response of the PA-MIP sensor to hexanoic acid, pentanoic acid and butyric acid.

[0047] DEFINITIONS

[0048] The following words and terms used herein shall have the meaning indicated:

[0049] The term “spatially oriented” forthe purposes of this disclosure refers to a shape and orientation of the complexing cavity within the MLP that corresponds to the shape and orientation of the target compound.

[0050] The term “template compound” for the purposes of this disclosure refers to the target compound for which the MLP polymer bead is tuned to be sensitive to.

[0051] The term “prearrangement complex” for the purposes of this disclosure refers to an arrangement of optionally substituted acrylic acid monomers before the monomers crosslink to form the polymer bead.

[0052] The term “optionally substituted” as used herein means the group to which this term refers may be unsubstituted, or may be substituted with one or more groups independently selected from alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, cycloalkenyl, cycloalkylalkenyl, heterocycloalkyl, cycloalkylheteroalkyl, cycloalkyloxy, cycloalkenyloxy, cycloamino, halo, carboxyl, haloalkyl, haloalkenyl, haloalkynyl, alkynyloxy, heteroalkyl, heteroalkyloxy, hydroxyl, hydroxyalkyl, alkoxy, alkenyloxy, nitro, amino, alkylamino, dialkylamino, alkenylamine, aminoalkyl, alkynylamino, acyl, alkyloxy, alkyloxyalkyl, alkyloxyaryl, alkyloxycarbonyl, alkyl oxy cycloalky I, alkyl oxy heteroaryl, alkyloxyheterocycloalkyl, acylamino, alkylsulfonyloxy, heterocyclic, heterocycloalkenyl, heterocycloalkyl, heterocycloalkylalkyl, heterocycloalkylalkenyl, heterocycloalkylheteroalkyl, heterocycloalkyloxy, heterocycloalkenyloxy, heterocycloamino, haloheterocycloalkyl, alkylsulfinyl, alkyl sulfonyl, aminosulfonyl, sulfinyl, sulfinylamino, sulfonyl, sulfonylamino, aryl, heteroaryl, heteroarylalkyl, heteroarylalkenyl, heteroarylheteroalkyl, heteroarylamino,heteroaryloxy, arylalkenyl, arylalkyl, aryloxy, arylsulfonyl, cyano, cyanate, isocyanate, -C(O)NH(alkyl), and -C(O)N(alkyl)2.

[0053] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

[0054] As used herein, the term "about", typically means + / - 5% of the stated value, more typically + / - 4% of the stated value, more typically + / - 3% of the stated value, more typically, + / - 2% of the stated value, even more typically + / - 1% of the stated value, and even more typically + / -0.5% of the stated value.

[0055] Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 3 should be considered to have specifically disclosed sub-ranges such as from 1 to 2, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0056] Certain embodiments may also be described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0057] DETAILED DESCRIPTION OF EMBODIMENTS

[0058] There is provided a polymer bead comprising a crosslinked optionally substituted polyacrylate, wherein the polyacrylate is obtained by crosslinking an optionally substituted acrylic acid with a crosslinker at a ratio of the optionally substituted acrylic acid to crosslinker in the range ofabout 1 :3 to about 1: 1 by moles, and wherein the polymer bead has a porous structure having a plurality of complexing cavities therein, wherein the complexing cavities are spatially oriented for binding a target compound, wherein the target compound comprises an optionally substituted C3 to Cc carboxylic acid.

[0059] The optionally substituted polyacrylate may be polyacrylate, polymethacrylate, poly(methyl methacrylate) or poly(2-hydroxyethyl methacrylate).

[0060] The optionally substituted acrylic acid may be acrylic acid (AA), methacrylic acid (MA), methyl methacrylate (MMA), or 2 -hydroxy ethyl methacrylate (HEMA).

[0061] The crosslinker may be selected from the group consisting of ethylene glycol dimethacrylate (EGDMA), N,O-bismethacryloyl ethanolamine, divinyl benzene, trimethylolpropane trimethacrylate, pentaerythritol tetraacrylate, pentaerythritol triacrylate, N,O-bisacryloyl L-phenylanalionol, N,N-l,4-phenylene diacrylate, N,N- methylenebisacrylamide, glycidilmethacrylate and any mixture thereof.

[0062] The ratio of the optionally substituted acrylic acid to crosslinker may be in the range of about 1:3 to about 1:2, about 1:3 to about 3:5, about 1:3 to about 7:10, about 1:3 to about 4:5, about 1:3 to about 9:10, about 1:2 to about 3:5, about 1:2 to about 7:10, about 1:2 to about 4:5, about 1:2 to about 9:10, about 1:2 to about 1:1, about 3:5 to about 7:10, about 3:5 to about 4:5, about 3:5 to about 9:10, about 3:5 to about 1:1, about 7:10 to about 4:5, about 7:10 to about 9:10, about 7:10 to about 1:1, about 4:5 to about 9:10, about 4:5 to about 1:1 or about 9:10 to about 1 : 1 by moles.

[0063] The optionally substituted C3 to G> carboxylic acid may be propanoic acid, butyric acid, isobutyric acid, 3-methylbutanoic acid (isovaleric acid), dimethyl propanoic acid (pivalic acid), pentanoic acid or hexanoic acid.

[0064] There is also provided a method of preparing a polymer bead as defined above, comprising the steps of:i) mixing a template compound with an optionally substituted acrylic acid in a solvent to form a prearrangement complex, wherein the template compound comprises an optionally substituted C3 to Cr, carboxylic acid;

[0065] ii) adding an initiator and a crosslinker to the prearrangement complex under reaction conditions to crosslink the optionally substituted acrylic acid to form a crosslinked optionally substituted polyacrylate mixture, wherein the optionally substituted acrylic acid and crosslinker are present at a ratio in the range of about 1:3 to about 1 : 1 by moles; and

[0066] iii) removing the template compound from the polyacrylate mixture to form the polymer bead.

[0067] The solvent may be acetonitrile, toluene, methanol, dimethylformamide, tetrahydrofuran, acetone, chloroform, xylene or any mixture thereof

[0068] The mixing step may further comprise a step of stirring the optionally substituted acrylic acid and the crosslinker for a duration in the range of about 15 minutes to about 30 minutes to form the prearrangement complex.

[0069] The step of stirring the optionally substituted acrylic acid and the crosslinker to form the prearrangement complex may be for a duration in the range of about 15 minutes to about 25 minutes, about 15 minutes to about 20 minutes, about 20 minutes to about 30 minutes, about 20 minutes to about 25 minutes, or about 25 minutes to about 30 minutes.

[0070] The prearrangement complex may comprise the optionally substituted acrylic acid attached to the template compound via hydrogen bonding.

[0071] The initiator may be selected from the group consisting of azobi si sobutyronitrile (AfBN), benzoyl peroxide, ammonium persulfate, ethyl-2 chloro propionate, 2,2'-azobis(2,4-dimethylvaleronitrile) and any mixture thereof.

[0072] The reaction conditions may comprise heating under inert atmosphere. The inert atmosphere may be nitrogen, or any other inert gas such as argon, helium or neon The reaction conditions may also comprise a vacuum.The heating may be at a temperature in the range of about 40 °C to about 80 °C for a duration in the range of about 18 hours to about 30 hours.

[0073] The heating may be at a temperature in the range of about 40 °C to about 50 °C, about 40 °C to about 60 °C, about 40 °C to about 70 °C, about 50 °C to about 60 °C, about 50 °C to about 70 °C, about 50 °C to about 80 °C, about 60 °C to about 70 °C, about 60 °C to about 80 °C, or about 70 °C to about 80 °C.

[0074] The heating may be for a duration in the range of about 18 hours to about 26 hours, about 18 hours to about 22 hours, about 22 hours to about 30 hours, or about 26 hours to about 0 hours.

[0075] The reaction condition may further comprise a step of stirring.

[0076] The removing step may comprise washing the crosslinked polyacrylate mixture with a washing solvent.

[0077] The washing solvent may comprise an alcohol and an acid.

[0078] The alcohol may be methanol, ethanol, propanol, isopropanol or any suitable alcohol.

[0079] The acid may be acetic acid, citric acid, phosphoric acid, formic acid, oxalic acid or any suitable acid.

[0080] The washing solvent may comprise methanol and acetic acid, preferably at a ratio in the range of about 10:1 to about 9:2 v / v, about 10:1 to about 5:1 v / v, about 10:1 to about 6:1 v / v, about 10:1 to about 7:1 v / v, about 10:1 to about 8:1 v / v, about 10:1 to about 9:1 v / v, about 9:1 to about 9:2, about 9:1 to about 5:1, about 9:1 to about 6:1, about 9:1 to about 7:1, about 9:1 to about 8:1, about 8:1 to about 9:2, about 8:1 to about 5:1, about 8:1 to about 6:1, about 8:1 to about 7:1, 7:1 to about 9:2, about 7:1 to about 5:1, about 7:1 to about 6:1, about 6:1 to about 9:2, about 6: 1 to about 5:1 or about 5: 1 to about 9:2.The removing step may further comprise centrifugation to separate the polymer bead from the solvent.

[0081] The centrifugation may be performed by benchtop centrifugation, ultracentrifugation or any other centrifugation method.

[0082] The centrifugation may be performed at a speed ranging of about 3,000 revolutions per minute (rpm) to about 150,000 rpm, about 3,000 rpm to about 100,000 rpm, about 3,000 rpm to about 50, 000 rpm, about 3,000 rpm to about 10, 000 rpm, about 3,000 rpm to about 5,000 rpm, about 5,000 rpm to about 150,000 rpm, about 5,000 rpm to about 100,000 rpm, about 5,000 rpm to about 50,000 rpm, about 5,000 rpm to about 10,000 rpm, about 10,000 rpm to about 150,000 rpm, about 10,000 rpm to about 100,000 rpm, about 10,000 rpm to about 50,000 rpm, about 50,000 rpm to about 150,000 rpm, about 50,000 rpm to about 100,000 rpm, about 100,000 rpm to about 150,000 rpm.

[0083] The removing step may further comprise washing the polymer bead with a second alcohol after the step of washing the crosslinked polyacrylate mixture with a washing solvent. The second alcohol may be methanol, ethanol, propanol, isopropanol or any suitable alcohol.

[0084] There is also provided a polymer bead prepared by the method as defined above.

[0085] There is also provided a sensor for detecting a target compound comprising an optionally substituted C3 to G, carboxylic acid, comprising:

[0086] a substrate;

[0087] an adhesion promoter immobilised onto the substrate; and

[0088] a polymer bead as defined above attached to the adhesion promoter immobilised onto the substrate.

[0089] The substrate may be an electrode comprising metal digits patterned on glass or any suitable substance for use as a substrate.

[0090] The metal may be a noble metal, preferably gold, titanium, platinum or palladium.

[0091] The digits may be flat, circular, angular or any other suitable digit type.The patterns may be interdigitated, circular, rectangular, hexagonal or any other suitable pattern.

[0092] The adhesion promoter may be in the form of a self-assembled monolayer.

[0093] The adhesion promoter may be (3-Aminopropyl)triethoxysilane (APTES), (3-Aminopropyl)trimethoxysilane (APTMS), 6-Amino-l -hexanethiol hydrochloride, N-(2-Aminoethyl)-3 -aminopropyltrimethoxy silane (AE APTMS), (3- Aminopropylmethyl)diethoxysilane (APMDES), or p-Aminophenyltrimethoxysilane.

[0094] The polymer bead may be electrostatically attached to the adhesion promoter immobilised onto the substrate.

[0095] There is also provided a method of preparing the sensor as defined above, comprising the steps of:

[0096] a) providing a substrate;

[0097] b) immersing the substrate in a solution of an adhesion promoter to form a surface- modified substrate; and

[0098] c) immersing the surface-modified substrate in a suspension of the polymer bead as defined above.

[0099] The method may further comprise the step of plasma treating the substrate after step a) but before step b).

[0100] The plasma treating step may involve treating the substrate using ionized gas, such as oxygen, or any other suitable ionized gas.

[0101] The adhesion promoter may be dissolved in acetonitrile, toluene, methanol, tetrahydrofuran, acetone, chloroform, xylene, ethanol, isopropanol or any mixture thereof, to provide the solution of the adhesion promoter.

[0102] The solution of the adhesion promoter may have a concentration in the range of about 1% to about 5% w / v, about 1% to about 4% w / v, about 1% to about 3% w / v, about 1% to about 2%w / v, about 2% to about 5% w / v, about 2% to about 4% w / v, about 2% to about 3% w / v, about 3% to about 5% w / v, about 3% to about 4% w / v, or about 4% to about 5% w / v of the adhesion promoter.

[0103] The polymer bead may be suspended in a medium at a range of about 1 mg / mL (w / v) to about 7 mg / mL (w / v), about 1 mg / mL (w / v) to about 6 mg / mL (w / v), about 1 mg / mL (w / v) to about 5 mg / mL (w / v), about 1 mg / mL (w / v) to about 4 mg / mL (w / v), about 1 mg / mL (w / v) to about 3 mg / mL (w / v), about 1 mg / mL (w / v) to about 2 mg / mL (w / v), 2 mg / mL (w / v) to about 7 mg / mL (w / v), about 2 mg / mL (w / v) to about 6 mg / mL (w / v), about 2 mg / mL (w / v) to about 5 mg / mL (w / v), about 2 mg / mL (w / v) to about 4 mg / mL (w / v), about 2 mg / mL (w / v) to about 3 mg / mL (w / v), about 3 mg / mL (w / v) to about 6 mg / mL (w / v), about 3 mg / mL (w / v) to about 5 mg / mL (w / v), about 3 mg / mL (w / v) to about 4 mg / mL (w / v), about 4 mg / mL (w / v) to about 6 mg / mL (w / v), about 4 mg / mL (w / v) to about 5 mg / mL (w / v) or about 5 mg / mL (w / v) to about 6 mg / mL (w / v).

[0104] The medium may be water.

[0105] There is also provided a use of the sensor as defined above for detecting a target compound, wherein the target compound comprises an optionally substituted Ci to Cr, carboxylic acid.

[0106] The detection may be performed in food products or in the environment.

[0107] The food products may be agricultural products, such as fish, chicken, beef, edible oils such as peanut oil, olive oil, sesame oil, milk, fruits, vegetables such as spinach, kale, bok choy or any other food products.

[0108] The environment may be materials obtained from land, air or water, in particular gases obtained from air.

[0109] There is also provided a method of detecting a target compound comprising an optionally substituted C3 to C7 carboxylic acid, the method comprising the steps of:

[0110] I) providing the sensor as defined above;Il) exposing the sensor to a gas comprising the optionally substituted C3 to G, carboxylic acid; and measuring the capacitance or impedance of the sensor.

[0111] The method as defined above may further comprise the step of measuring the capacitance or the impedance of a control substrate and comparing it with the capacitance or the impedance of the sensor, wherein the control substrate is the sensor as defined above but without the adhesion promoter immobilised onto the substrate or the polymer bead as defined above attached to the adhesion promoter immobilised onto the substrate.

[0112] EXAMPLES

[0113] Materials

[0114] The reagents were obtained from Sigma Aldrich, Singapore. They are methacrylic acid, EGDMA, A1BN, acetonitrile, hexanoic acid (analytical grade and 99%), pentanoic acid (analytical grade and 99%), 2-pentylfuran (food grade) and phenylacetaldehyde (food grade), APTES, absolute ethanol, anhydrous toluene, methanol, and acetic acid.

[0115] The IDE utilised for the sensor development were provided by MicruX, Gijon, Spain. The IDE consist of 180 pairs of gold digits, with each digit having a thickness of 5 pm and interdigit distance of 5 pm.

[0116] Keysight E4980A Inductance-Capacitance-Resistance meter (LCR meter) was obtained from Keysight (Santa Rosa, California, United States of America).

[0117] Femto Science VITA 8 plasma cleaner was obtained from Femto Science, Gyeonggi-do, Republic of Korea.

[0118] Example 1: Synthesis of MIP for target molecule

[0119] A scheme showing synthesis of MIP for hexanoic acid is shown in Figure 1. The same process applies for pentanoic acid where the only difference is in the template compound.To synthesize MIP for necessary target VOC molecules, a fixed amount of template compound (in this case, hexanoic acid, Immol) and functional monomer (methacrylic acid, 4mmol) were added to a glass vial containing 35mL of acetonitrile which served as a solvent. The glass vial was then sealed and stirred at 500 rpm for 20 minutes to form a prearrangement complex. The prearrangement complex structure is drawn in Figure 1 for schematic representation. Following this process, a fixed concentration of crosslinker (ethylene glycol dimethacrylate, 6mmol) and initiator (azobisisobutyronitrile, with a volume of 86 uL) was added to the glass vial. The glass vial was then sealed and purged with nitrogen gas for 30 minutes before immersing the glass vial in an oil bath at 60 °C for 24 hours for the polymerisation of the MIP beads to take place. Post polymerisation, the MIP beads were extracted through centrifugation and washed thrice with a solution of 9:1 v / v of methanol and acetic acid to remove the template compound and washed thrice with pure methanol to remove any excess acetic acid, after which the MIP beads were dried in an oven for 12 hours at 70°C.

[0120] Example 2: Fabrication of MIP sensor

[0121] This disclosure also involves fabrication of an IDE-APTES-MIP, as shown in Figure 2. Throughout this disclosure, the term “IDE-APTES-MIP”, “MIP sensor” are used interchangeably. Similarly, throughout this disclosure, the term “IDE-APTES-PA-MIP” and “PA-MIP sensor” are used interchangeably. Further, throughout this disclosure, the term “1DE-APTES-HA-MIP” and “HA-MIP sensor” are used interchangeably.

[0122] The first step to prepare the MIP sensor was to provide a bare IDE comprising gold digits patterned on glass, as shown in Figure 2(a). As shown in Figure 2(b), the next step involved plasma treating (2002) the bare IDE with oxygen gas using a Femto Science VITA 8 plasma cleaner at 100 standard cubic centimetres per minute (seem), 150 watt (W) power and 500 millitorr (mtorr) vacuum for 2.5 minutes so that the bare IDE became hydroxylated. Subsequently, the bare IDE was immersed in a 2% w / v APTES solution for 4 hours (2004) so that APTES self-assembled to form a monolayer on a surface of the bare IDE, to become 1DE-APTES. The solution of IDE- APTES then underwent sonication in ethanol for 10 seconds (2006) followed by sonication in deionised water for 10 seconds (2008). The IDE- APTES was then immersed in a solution of 1 mL of MIPs in deinoised water, having a concentration of 2 mg / mL (2010), in this case HA-MIPs, for 21 hours for assembly of the HA -MIPs on a surfaceof the LDE-APTES to form LDE-APTES-HA-MfP. The immersion process helped form a uniform distribution of the HA-MIPs on the surface of the IDE-APTES. Following a fixed incubation period, the ZDE-APTES-EIA-MIP was removed from the solution of HA-MIPs and dried over a hotplate for 2 hours at a temperature of 115°C (2012). Post drying, the LDE-APTES-HA-MIP specific to target hexanoic acid, i.e. the HA-MIP sensor, was ready for VOC sensing experiments. This method was a modification of the Langmuir Blodgett method and was used to ensure a high coverage of the MIPs on the surface of the bare IDE.

[0123] The fabrication process for the PA-MIP sensor followed the exact same process, except that PA-MIP was used for modification instead of the HA-MIP.

[0124] Example 3: Mechanism of the interaction between MIP and target VOC

[0125] Non-covalent hydrogen bonding interactions formed between the MIP beads and the target VOC molecules and such interactions were accompanied with swelling of the MLP beads. Therefore, utilising non-faradaic EIS which measured capacitance of a system, upon exposure of the MIP sensor to the VOCs, the hydrogen bonding interactions between the VOCs and the MIP beads led to the swelling of the MIP beads This in turn led to an increase in bulk dielectric of the MIP on a surface of the sensor and increased the capacitance of the sensor. Therefore, utilising non-faradaic EIS which measured capacitance of a system, a higher concentration of VOCs led to more binding, which in turn led to a greater increase in the capacitance of the sensor.

[0126] While other electrochemical sensors are probed by faradaic methods, the fabricated sensor is advantageously probed by non-faradaic EIS, measuring the capacitance change of the sensor caused due to the non-covalent hydrogen bonding interaction of the VOCs with the MIP beads influencing a change in the electrical permittivity of the system resulting in the change of the system’s capacitance.

[0127] Furthermore, the IDE-APTES-MIP consists of the IDE functionalized with the APTES, which is a hydrophilic self-assembled monolayer, on top of which the MLP (either the PA-MLP or the HA-MLP) is functionalized. As shown in Figure 3, when operated under ambient conditions with natural humidity, the APTES is capable of attracting a thin nanoscale layer of water onthe sensor surface. This water layer is trapped within the MLP layer of the sensor. When the sensor is exposed to the VOCs, the VOCs get trapped within the water layer. This leads to vapour pressure lowering in the nanoscale water layer within the MLP layer (3002). The lowering of water vapour pressure leads to additional adsorption of water forming a thicker water layer in order to maintain a water equilibrium (3004). The thicker water layer leads to formation of more conductive pathways and lowers impedance of the sensor, and consequently, leads to a higher capacitance and effectively a lower impedance of the sensor. When flow of the VOCs is stopped, the VOCs are desorbed, resulting in a thinner water layer (3006). This increases the impedance of the sensor and results in a lower capacitance of the sensor.

[0128] To support the adsorption of the water on the IDE, water contact angles of the bare IDE, the IDE-APTES and the IDE-APTES-PA-M1P are shown in Figure 4. The water contact angle of the bare IDE is measured at 81.8° (Figure 4(a)) - indicating a close to hydrophobic surface. Upon functionalization of the bare IDE with the APTES, hydrophilicity of the sensor is improved, reflected by a reduced water contact angle of 37.4° (Figure 4(b)). Further modification of the sensor with the PA-MIP beads improves the hydrophilicity of the sensor leading to an even further drop in water contact angle to 24.8° (Figure 4(c)). The improved hydrophilicity is indicative of water trapping on the sensor surface for the sensing to occur.

[0129] Example 4: Method of detection

[0130] This disclosure further involves measuring the capacitance of the sensor to detect the target VOCs. The capacitance of the bare IDE and the LDE-APTES-MIP were measured utilising the LCR meter which was connected to a laptop for data acquisition. The electrical properties of the sensor were measured through the LCR meter with the aid of the drop cell. The drop cell was connected to the LCR meter through a micro - Universal Serial Bus - Bayonet Neill-Concelman (micro-USB-BNC) connection wire. For non-faradic measurement, a root-meansquare alternative current (AC) voltage of 250mV with a 0V direct current (DC) bias was applied across the electrodes throughout a frequency spectrum of 20Hz - 2000Hz defined by 36 frequency points at 18points / decade. Capacitance measurements of the sensors were taken pre and post modifications (i.e., bare IDE and LDE-APTES-MIP). The measurements were shielded using a faraday shield connected to a ground of the LCR meter for protection against external noise, as shown in Figure 5(c).To test the sensor versus different VOCs, the sensor was placed in the drop cell and covered with the flow cell, a customised attachment to isolate the sensor from external interferences, as shown in Figure 5(b) The drop cell had an inlet through which the VOCs were injected and an outlet through which the VOCs were removed, as shown in Figure 5(d).

[0131] The VOC source was generated by weighing a fixed amount of VOC analytical sample and placing them in a glass vial, followed by heating the vial to evaporate the liquid VOC into gaseous form. The concentration of gaseous VOC in the vial was determined through the following equation (A)

[0132]

[0133] where C denotes the ppm concentration of the given VOC, d representing the density of the liquid (g / mL), v representing the volume of liquid added in the glass vial in pL, p representing the purity of the solvent, T the temperature of the vial (K), R being the universal gas constant (0.0821 J. atm / (mol. K)), M the molar mass of the solvent, Vjiask and Pflask representing the volume of the flask (L) and pressure inside the flask (atm) respectively.

[0134] In an experiment, 0.13 pL of liquid analytical hexanoic acid was added into a 500 mL glass bottle. Thereafter, the glass bottle was heated in an oven at 70°C for 30 minutes to ensure complete evaporation of the liquid into gas. The glass bottle was then cooled down at room temperature to generate 50 ppm of hexanoic acid as a volatile organic compound (VOC).

[0135] In another experiment, 0.11 pL of liquid analytical pentanoic acid was added into a 500 mL glass bottle. Thereafter, the glass bottle was heated in an oven at 70°C for 30 minutes to ensure complete evaporation of the liquid into gas. The glass bottle was then cooled down at room temperature to generate 50 ppm of hexanoic acid as a volatile organic compound (VOC).

[0136] Once the VOC source was generated, the gas was extracted through a gas syringe and introduced to the inlet of the flow cell The rate of injection (volume flow rate) was pre-set bya syringe pump which was attached to the gas syringe, as shown in Figure 5(a). In an experiment, the rate of injection of the syringe pump into the sensor was 2 mL / min.

[0137] Example 5: VOC response experiment 1

[0138] In an experiment, before injection of the VOCs, 1 capacitance scan was taken every minute for 5 minutes to identify a baseline of the sensor. The capacitance measured on minute 5 was considered as the baseline capacitance (Cbaseime) of the sensor. After 5 minutes (minutes 1-5), the VOC injection started and lOOppm of target gas was injected. 1 capacitance scan was taken every minute for 20 minutes (minutes 6-25), which was a period of VOC flow. The capacitance measured at minute 25 was considered as the VOC response of the sensor (Crespmse). On minute 25, the VOC flow was stopped, and 1 capacitance scan was taken every minute for 10 minutes (minutes 26-35). Capacitance at 20Hz for all 35 scans were extracted and plotted to generate a response curve. The relative response of the MIP sensor towards any VOC was given by a formula (B) as follows :-

[0139]

[0140] For the target VOCs such as hexanoic acid and pentanoic acid, it was observed that upon exposure of the VOCs, the respective MIP sensors (HA-MIP and PA-MIP) sensors showed a significant increase in capacitance. Figure 6(a) shows response of the HA-MIP sensor to 100 ppm of hexanoic acid and Figure 6(b) shows response of the PA-MIP sensor to 100 ppm of pentanoic acid. The significant increase in capacitance was due to binding of the VOCs with the target selective MIP beads which led to the swelling of the MIP beads and resultant increase in the capacitance of the MIP sensor.

[0141] Figure 6(c) shows response of the HA-MIP sensor to lOOppm of hexanoic acid, pentanoic acid and other flavoured VOCs, which are 2-pentylfuran and phenyl acetaldehyde The relative capacitance charges in Figure 6(c) are hexanoic acid: 15.83±2.66, pentanoic acid: 97.69±28.16, 2-pentylfuran: -2.06±4.06, and phenylacetaldehyde: 4.33 ±6.67. Figure 6(d) shows response of the PA-MIP sensor to lOOppm of hexanoic acid, pentanoic acid and other flavoured VOCs, which are 2-pentylfuran and phenyl acetaldehyde. The relative capacitance charges in Figure6(d) are hexanoic acid: 8.87±2.97, pentanoic acid:42.67±13.23, 2-pentylfuran: -1.78±1.35, and phenylacetaldehyde: 4.49±5.99.

[0142] As seen in Figure 6(c) and 6(d), other nonspecific VOCs such as 2-pentylfuran and phenyl acetaldehyde, which are related to flavour of the food product, failed to generate any significant response when exposed to the degradation-related HA-MIP sensor or PA-MIP sensor. However, the HA-MIP sensor and the PA-MIP sensor produced a significant response to hexanoic acid and pentanoic acid respectively. In these experiments, three sensors were used to test against each VOC, except for the PA-MIP sensor against phenyl acetaldehyde, in which two sensors were used.

[0143] Example 6: VOC response experiment 2

[0144] In a separate experiment, sensor testing was optimized and testing time was brought down from 35 minutes to 15 minutes. Baseline impedance of the sensor was measured every minute for 5 minutes (minutes 1-5), followed by VOC exposure for 5 minutes (minutes 6-10) and stop of VOC flow to observe desorption behavior for 5 minutes (minutes 11-15). The sensors were tested at an ambient humidity of 40% RH.

[0145] As shown in Figure 7(a), it was observed that the baseline of the sensor (minutes 1-5) was very stable without any significant variations. An average of 5 baseline measurements was represented as the baseline impedance

[0146]

[0147] Upon exposure of the sensor to 50ppm of pentanoic acid (minutes 6-10), a steady decrease in impedance of the sensor was observed with a maximum decrease observed at minute 10 represented as

[0148]

[0149] Upon stopping the VOC flow, steady desorption of the VOC led to an increase in impedance back to the baseline. The relative impedance response of the sensor was given by a formula (C) as follows :-

[0150]

[0151] A calibration curve for the PA-MIP sensor and the HA-MIP sensor is shown in Figure 7(b) and 7(d), respectively. Figure 7(b) shows relative impedance response of the PA-MIP sensor against 25-188ppm of pentanoic acid and that saturation occurred from 150ppm onwards.Figure 7(d) shows relative impedance response of the HA-MLP sensor against 25-200ppm of hexanoic acid and that saturation occurred from 100 ppm onwards. For both sensors, the relative impedance response of the sensor increased with increase in concentration of the VOC with saturation noticed in higher concentrations.

[0152] Similarly, specificity of the sensor was demonstrated where the PA-MIP sensor and the HA-MIP sensor are tested against key VOCs found in peanut oil (PNO). As shown in Figure 7(c), it was observed that the PA-MIP sensor showed maximum response to pentanoic acid - the VOC that the PA-MIP sensor was tuned to be selective to. The relative impedance response shown in Figure 7(c) are pentanoic acid: 46.93±7.31, hexanoic acid: 14.37±5.57, 2-pentylfuran: 1.31±0.84, and phenylacetaldehyde: -0.15±2.58. Similarly, as shown in Figure 7(e), the HA-MIP sensor showed maximum response to hexanoic acid - the VOC that the HA-MIP sensor was tuned to be selective to. The relative capacitance response shown in Figure 7(e) are hexanoic acid: 61.66±5.56, pentanoic acid 28.35±12.18, 2-pentylfuran: 0.23±2.13, and phenylacetaldehyde: 1.65±1.94.

[0153] Example 7: Humidity stability experiment

[0154] Upon fabrication of the HA-MIP sensors, four different HA-MIP sensors that were prepared in the same way were placed in ambient atmosphere (-40% RH) and 50 capacitance scans were obtained. After 50 scans, the HA-MIP sensors were left undisturbed for 2 hours after which an additional 50 capacitance scans were obtained. Capacitance values at a frequency of 20Hz were extracted from all the scans and plotted to study humidity stability of the sensor. As shown in Figure 8, the results of the humidity stability experiment show that the HA-MIP sensors were stable in ambient atmosphere for a minimum period of 2 hours

[0155] Example 8: Humidity range experiment

[0156] Since a certain level of humidity was required to operate the sensor, RH levels were varied to observe any variations in response of the sensor. The response of the PA-MIP sensor exposed to the pentanoic acid at different RH, i.e. 40% RH, 80% RH and 0% RH, were measured.As shown in Figure 9, it was observed that the relative impedance response of the PA-M1P sensor increased when humidity level increased from 40% to 80% RH. This was due to presence of higher water content which led to formation of the thicker water layer and more conductive paths within the sensor, which increased the response of the sensor. In contrast, when the RH is reduced to 0%, the sensor response was negligible due to lack of humidity for sensor operation to occur.

[0157] Example 9: Reusability of sensor experiment

[0158] Reusability of the PA-MIP sensor is shown in Figure 10. The reusability of the PA-MIP sensor was tested through repeated exposure of the PA-MIP sensor to 188ppm pentanoic acid with sensor recovery between cycles performed through heating of the PA-MIP sensor on a hotplate at 60°C for 2.5 minutes. The heating ensured complete desorption of the pentanoic acid from the PA-MIP sensor which resulted in baseline recovery of the sensor.

[0159] Figure 10(a) shows the reusability profile of the PA-MIP sensor to 188 ppm pentanoic acid. Individual cycles of testing are represented with notation Ci, C2, C3, C4, C5 and Ce. Figure 10(b) shows that the PA-MIP sensor was reusable for 6 cycles with a relative impedance change of 73.86%, 75.64%, 77.67%, 74.38%, 71.87% and 72.50% across 6 cycles respectively and exhibited minimal variation (<3%). This confirmed the capability of the sensor to produce consistent and reliable responses.

[0160] Example 10: Size specificity experiment

[0161] In addition to determining these MIPs’ specificity to degradation related VOCs, it was also identified that these MIPs have a size specific filter function with the ability to bind to analogous molecules with smaller size such as butyric acid and propionic acids.

[0162] Response of the sensor is shown in Figure 11. Figure 11(a) and 11(b) show the response of the HA-MIP sensor and the PA-MIP sensor respectively to analogous VOC molecules which are smaller and larger than the target VOC by one methyl group, i.e. the response of HA-MIP sensor to heptanoic acid, hexanoic acid and pentanoic acid, and the response of PA-MIP sensorto hexanoic acid, pentanoic acid and butyric acid. Three HA-MIP sensors and three PA-MIP sensors were used for this experiment.

[0163] As shown in Figure 11(a), for the HA-MIP sensor, the sensor response was higher when exposed to the smaller pentanoic acid VOC and a reduced response was obtained when the sensor was exposed to the larger heptanoic acid VOC. Therefore, the HA-MIP sensor was much more sensitive to pentanoic acid compared to that of hexanoic acid, and less sensitive to heptanoic acid.

[0164] A similar effect can be seen in the responses of the PA-MIP sensor to smaller butyric acid and larger hexanoic acid, as shown in Figure 11(b). Therefore, the PA-MIP sensor showed higher sensitivity to the smaller butyric acid and lower sensitivity to the larger hexanoic acid. This was because the smaller size molecules could enter pores of MIP targeted towards larger sized molecules, which led to an increased binding and more response.

[0165] The sensor response and specificity demonstrated feasibility of the MIP synthesis and the fabrication of the MIP sensor. It showed that the MIP synthesis process was successful and that the MIPs were sensitive to the target gases and showed reduced response to other flavour related VOCs.

[0166] The filter function with potential for detecting analogous molecules of different sizes can be used to develop an array of MIP sensors that comprise multiple MIPs, i.e. MIP-e-nose sensor system, to detect individual VOCs in a given mixture. The MIP-e-nose sensor system can be used in combination with machine learning frameworks for detecting single atoms and methyl group discrimination of VOCs in a given sample. The functionality of the MIP-e-nose sensor system may be envisioned to have a performance similar to GC-MS.INDUSTRIAL APPLICABILITY

[0167] The polymer bead as defined herein may be useful to selectively bind with a wide range of target molecules. The method for preparing the polymer bead as defined herein may be useful in preparing the polymer bead as defined above. The sensor as defined herein may be useful in detecting VOCs to analyse freshness of a food product throughout the product lifecycle. The sensor as defined herein may also be useful to screen different VOCs. The method for preparing the sensor as defined herein may be useful in preparing the sensor as defined above. The method of detecting a target compound as defined herein may be useful to detect VOCs without destroying the food product. The method of detecting a target compound as defined herein may be useful to accurately detect VOCs even in the presence of other interferents.

[0168] Extending this knowledge, MIP sensors as defined herein may also be useful for several extended applications, such as:

[0169] 1. To utilise machine learning framework to identify the time left for safe consumption.

[0170] 2. To extend the fabrication of MIPs to detect flavour related VOCs such as 2- pentylfuran and phenylacetaldehyde and combining flavour related MIP sensors with degradation related VOC sensors to develop an e-nose to quantify flavour and freshness of the food product

[0171] 3. The e-nose as a whole or individual MIP sensors can be used in other food safety aspect such as to monitor adulteration.

[0172] MIP sensors may also be utilised to check the safety of frying oils. Frying oils may be reused a few times before disposal and this sensor may be used to check its quality between frying periods to check its safety. Additionally, the MIP sensor utilises only 10 mL of sample volume to obtain a discernable response. Therefore, the MIP sensor may be used in any examples where the available sample volume is low for example monitoring plant volatiles and human breath analysis.

[0173] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departingfrom the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

CLAIMS1. A polymer bead comprising a crosslinked optionally substituted polyacrylate, wherein the polyacrylate is obtained by crosslinking an optionally substituted acrylic acid with a crosslinker at a ratio of the optionally substituted acrylic acid to crosslinker in the range of about 1:3 to about 1:1 by moles, and wherein the polymer bead has a porous structure having a plurality of complexing cavities therein, wherein the complexing cavities are spatially oriented for binding a target compound, wherein the target compound comprises an optionally substituted Cr to Cs carboxylic acid.

2. The polymer bead of claim 1, wherein the optionally substituted C3 to Cs carboxylic acid is propanoic acid, butyric acid, isobutyric acid, 3-methylbutanoic acid (isovaleric acid), dimethyl propanoic acid (pivalic acid), pentanoic acid or hexanoic acid.

3. A method of preparing a polymer bead of claim 1 or 2, comprising the steps of:i) mixing a template compound with an optionally substituted acrylic acid in a solvent to form a prearrangement complex, wherein the template compound comprises an optionally substituted C3 to Cs carboxylic acid;ii) adding an initiator and a crosslinker to the prearrangement complex under reaction conditions to crosslink the optionally substituted acrylic acid to form a crosslinked optionally substituted polyacrylate mixture, wherein the optionally substituted acrylic acid and crosslinker are present at a ratio in the range of about 1 :3 to about 1 : 1 by moles; andiii) removing the template compound from the polyacrylate mixture to form the polymer bead.

4. The method of claim 3, wherein the optionally substituted C3 to Cs carboxylic acid is propanoic acid, butyric acid, isobutyric acid, 3-methylbutanoic acid (isovaleric acid), dimethyl propanoic acid (pivalic acid), pentanoic acid or hexanoic acid.

5. The method of claim 3 or 4, wherein the optionally substituted acrylic acid is acrylic acid (AA), methacrylic acid (MA), methyl methacrylate (MMA), or 2-hydroxyethyl methacrylate (HEMA).

6. The method of any one of claims 3 to 5, wherein the solvent is acetonitrile, toluene, methanol, dimethylformamide, tetrahydrofuran, acetone, chloroform or xylene.

7. The method of any one of claims 3 to 6, wherein the mixing step further comprises a step of stirring the optionally substituted acrylic acid and the crosslinker for a duration in the range of about 15 minutes to about 30 minutes to form the prearrangement complex.

8. The method of any one of claims 3 to 7, wherein the prearrangement complex comprises the optionally substituted acrylic acid attached to the template compound via hydrogen bonding.

9. The method of any one of claims 3 to 8, wherein the initiator is selected from the group consisting of azobisisobutyronitrile (AIBN), benzoyl peroxide, ammonium persulfate, ethyl-2 chloro propionate, 2,2'-azobis(2,4-dimethylvaleronitrile) and any mixture thereof, or the crosslinker is selected from the group consisting of ethylene glycol dimethacrylate (EGDMA), N,O-bismethacryloyl ethanolamine, divinyl benzene, trimethylolpropane trimethacrylate, pentaerythritol tetraacrylate, pentaerythritol triacrylate, N,O-bisacryloyl L-phenylanalionol, N,N-l,4-phenylene diacrylate, N,N- methylenebisacrylamide, glycidilmethacrylate and any mixture thereof.

10. The method of any one of claims 3 to 9, wherein the reaction conditions comprise heating under inert atmosphere.

11. The method of claim 10, wherein the heating is at a temperature in the range of about 40 °C to about 80 °C for a duration in the range of about 18 hours to about 30 hours.

12. The method of claim 10 or 11, wherein the inert atmosphere is nitrogen.

13. The method of any one of claims 3 to 12, wherein the reaction condition further comprises a step of stirring.

14. The method of any one of claims 3 to 13, wherein the removing step comprises centrifugation to separate the polymer bead from the solvent.

15. The method of any one of claims 3 to 14, wherein the removing step comprises washing the crosslinked polyacrylate mixture with a washing solvent.

16. The method of claim 15, wherein the washing solvent comprises an alcohol and an acid17. The method of claim 15 or 16, wherein the washing solvent comprises methanol and acetic acid, preferably at a ratio in the range of about 10: 1 to about 9:2 v / v.

18. The method of any one of claims 15 to 17, wherein the removing step further comprises washing the polymer bead with a second alcohol after the step of washing the crosslinked polyacrylate mixture with a washing solvent.

19. A polymer bead prepared by the method of any one of claims 3 to 18.

20. A sensor for detecting a target compound comprising an optionally substituted C3 to C& carboxylic acid, comprising:a substrate;an adhesion promoter immobilised onto the substrate; anda polymer bead of claims 1 or 2 or 19 attached to the adhesion promoter immobilised onto the substrate.

21. The sensor of claim 20, wherein the substrate is an electrode comprising metal digits patterned on glass.

22. The sensor of claim 21, wherein the metal is a noble metal, preferably gold, titanium, platinum or palladium.

23. The sensor of any one of claims 20 to 22, wherein the adhesion promoter is in the form of a self-assembled monolayer.

24. The sensor of any one of claims 20 to 23, wherein the adhesion promoter is (3- Aminopropyl)triethoxysilane (APTES), (3-Aminopropyl)trimethoxysilane (APTMS), 6-Amino-l -hexanethiol hydrochloride, N-(2-Aminoethyl)-3- aminopropyltrimethoxy silane (AEAPTMS), (3 -Aminopropylmethyl)diethoxysilane (APMDES), or p-Aminophenyltrimethoxysilane.

25. The sensor of any one of claims 20 to 24, wherein the polymer bead is electrostatically attached to the adhesion promoter immobilised onto the substrate.

26. A method of preparing the sensor of any one of claims 20 to 25, comprising the steps of:a) providing a substrate;b) immersing the substrate in a solution of an adhesion promoter to form a surface- modified substrate; andc) immersing the surface-modified substrate in a suspension of the polymer bead of claims 1 or 2 or 19.

27. The method of claim 26, further comprising the step of plasma treating the substrate after step a) but before step b).

28. The method of claim 26 or 27, wherein the solution of the adhesion promoter has a concentration in the range of 1% to 5% w / v of the adhesion promoter.

29. The method of claim 26 to 28, wherein polymer bead is suspended in a medium at a range of about 1 mg / mL (w / v) to about 7 mg / mL (w / v).

30. The method of claim 29, wherein the medium is water.

31. Use of the sensor of any one of claims 20 to 25 for detecting a target compound, wherein the target compound comprises an optionally substituted C3 to G> carboxylic acid32. The use of claim 31, wherein the detection is performed in food products or in the environment.

33. A method of detecting a target compound comprising an optionally substituted C3 to G, carboxylic acid, the method comprising the steps of:I) providing the sensor of any one of claims 20 to 25;II) exposing the sensor to a gas comprising the optionally substituted C3 to G carboxylic acid; andIII) measuring the capacitance or impedance of the sensor.

34. The method of claim 33, further comprising the step of measuring the capacitance or the impedance of a control substrate and comparing it with the capacitance or the impedance of the sensor, wherein the control substrate is the sensor of any one of claims 20 to 25 but without the adhesion promoter immobilised onto the substrate or the polymer bead of claims 1 or 2 or 19 attached to the adhesion promoter immobilised onto the substrate.

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