Electrochemical detector

An electrochemical detector including at least one substance selection structure disposed adjacent or proximate to an electronic device structure, wherein the substance selection structure is arranged to interact with a target substance having a molecular structure so as to alter an electrical characteristic of the electronic device structure.

TECHNICAL FIELD

The present invention relates to an electrochemical detector, although not exclusively, to an electrochemical detector for detecting plasticizer.

BACKGROUND

Food poisoning is one of the most serious public health problems everywhere around the world. Various harmful additives or contaminants in food may affect the health of the general public. Therefore, the quality of food must be regularly examined to ensure that some poisonous or hazardous substances do not exceed safety levels.

The concentration of these harmful substances may be determined by using testing agents. The traditional chemical testing approaches are time consuming and may not respond promptly. Alternatively, material characterization techniques in laboratories may be used to analyse the concentration of a target substance and even the composition of a testing sample. Although the results may be very accurate and sensitive, these techniques used in laboratories may not be suitable for daily applications which require prompt and low-cost testing results.

SUMMARY OF THE INVENTION

In one example embodiment, the present invention is advantageous in that it provides for improvement over conventional devices by providing improved detection limit in the neighbourhood of ppb level, without increase in production complexity.

In accordance with a first aspect of the present invention, there is provided an electrochemical detector comprising a substance selection structure disposed adjacent or proximate to an electronic device structure, wherein the substance selection structure is arranged to interact with a target substance having a molecular structure so as to alter an electrical characteristic of the electronic device structure.

In an embodiment of the first aspect, the substance selection structure arranged to interact with at least one molecule of the target substance.

In an embodiment of the first aspect, the substance selection structure includes a molecule-selective electrode.

In an embodiment of the first aspect, the molecule-selective electrode comprises a polymer layer arranged to accommodate the at least one molecule, and wherein the target substance is in contact with the polymer layer.

In an embodiment of the first aspect, the polymer layer includes a plurality of voids each arranged to accommodate a molecule of the target substance.

In an embodiment of the first aspect, each of the plurality of voids includes a structure matching with a molecular structure of the at least one molecule.

In an embodiment of the first aspect, the plurality of voids are selective to the target substance.

In an embodiment of the first aspect, the polymer layer includes a molecular imprinted polymer.

In an embodiment of the first aspect, the molecular imprinted polymer includes a plurality of polymer particles each having a size of around 50 μm.

In an embodiment of the first aspect, the electrical characteristic of the electronic device structure is altered when the polymer layer accommodates the at least one molecule.

In an embodiment of the first aspect, the electrical characteristic of the electronic device structure is altered by a change in a differential capacitance of the substance selection structure.

In an embodiment of the first aspect, the electrical characteristic of the electronic device structure is altered by a change in a permittivity and/or a dielectric constant of the polymer layer accommodating the at least one molecule.

In an embodiment of the first aspect, the electrochemical detector further comprises an intermediate substrate arranged to connect the substance selection structure and the electronic device structure.

In an embodiment of the first aspect, the intermediate substrate includes a copper substrate.

In an embodiment of the first aspect, the intermediate substrate is further arranged to electrically connect the substance selection structure and the electronic device structure.

In an embodiment of the first aspect, the substance selection structure and the electronic device structure are fabricated on a plurality of separate substrates.

In an embodiment of the first aspect, the plurality of separate substrates includes a plurality of doped silicon substrates.

In an embodiment of the first aspect, the electronic device structure includes a transistor.

In an embodiment of the first aspect, the substance selection structure is arrange to operate as a gate electrode of the transistor.

In an embodiment of the first aspect, the electrical characteristic includes a threshold voltage of the transistor.

In an embodiment of the first aspect, the electrical characteristic includes a saturation drain current across the transistor under a predetermined gate bias and a predetermined source-drain bias.

In an embodiment of the first aspect, the target substance includes plasticizer.

In accordance with a second aspect of the present invention, there is provided a method for fabricating an electrochemical detector in accordance with the first aspect, the method comprises the steps of: fabricating the substance selection structure and the electronic device structure on one or more substrates; and electrically and physically connecting the substance selection structure and the electronic device structure.

In an embodiment of the second aspect, the step of fabricating the substance selection structure comprises the step of: fabricating a combination of a molecular-imprinted polymer and a molecular template of the target substance using a polymerization process; extracting and removing the molecular template from the molecular-imprinted polymer; and depositing the molecular-imprinted polymer on the substrate to form a molecule-selective electrode.

In an embodiment of the second aspect, the step of fabricating the substance selection structure further comprises the step of grounding and sieving the combination of the molecular-imprinted polymer and the molecular template to obtain a plurality of polymer particles each having a size of around 50 μm.

In accordance with a second aspect of the present invention, there is provided a method for detecting a target substance with the electrochemical detector in accordance with the first aspect, the method comprises the steps of: applying an analyte solution containing a predetermined amount of the target substance to the at least one substance selection structure; applying a gate voltage to the thin-film transistor via the analyte solution and the at least one substance selection structure; applying a source-drain bias to the thin-film transistor; and determining a concentration of the target substance in the analyte solution based on a characterization of the electrical characteristic of the transistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The inventors have, through their own research, trials and experiments, devised that plasticizers which are commonly found in plastic, especially polyvinyl chloride, may be used for increasing the fluidity of a material. However, being in contact with excess amount of plasticizers will affect our health. There are several ortho-phthalate plasticizers have been classified as potential endocrine disruptors.

Gas chromatography-mass spectrometry (GC-MS) may be applied to detect plasticizers, but the instrument is big in size and expensive, and it requires a trained person to operate. As such this equipment is accessible to the general public. Therefore, inexpensive, quick and user friendly devices as pre-screening tools may be preferable for detecting plasticizers in aqueous solutions.

Preferably, thin film transistor (TFT) based sensors may be used due to their low cost and simple fabrication process. On the other hand, modular design which may enable the combinations to different types of chemical receptors for different applications. In addition, these modular devices may be fabricated by printing in large and flexible substrates.

In one example embodiment of the present invention, electrochemical sensors based on selective bonding of plasticizers in aqueous phase. The sensor may be used to detect extremely low levels (ppb) of plasticizers with appropriate receptors or sensing electrodes, thus providing a cost-effective, accurate and reliable means for plasticizers detection in aqueous media, such as water or other liquids.

With reference toFIG. 1, there is shown an example embodiment of an electrochemical detector100comprising at least one substance selection structure102disposed adjacent or proximate to an electronic device structure104, wherein the substance selection structure102is arranged to interact with a target substance having a molecular structure so as to alter an electrical characteristic of the electronic device structure104.

In this embodiment, the electrochemical detector100comprises a substance selection structure102which includes chemical receptors arranged to interact with other chemical substance which is in contact with the substance selection structure102, and the interaction is transformed to an electrical signal which may be further captured or detected by an electronic device structure104. For example, a drop of analyte solution may be disposed on a surface of the substance selection structure102, chemical receptors in the substance selection structure102may react/interact with the target substance in the analyte solution, and the substance selection structure102may produce an electrical signal to be detected by the electronic device structure104upon the chemical reaction. Alternatively, the substance selection structure102may alter an electrical characteristic, such as a resistance/impedance value of the electronic device structure104in response to a change in an electrical/physical characteristic (such as capacitance, permittivity, dielectric constant, etc.) of the substance selection structure102.

Preferably, the substance selection structure102includes a molecule-selective electrode, and the substance selection structure may interact with at least one molecule of the target substance.

The molecule-selective electrode102may be specifically designed for facilitating the detection of plasticizer molecules in the target substance.

Alternatively, the molecule-selective electrode102may include different structures and/or components for the detection of different/multiple target substances.

With reference to the example embodiment as shownFIG. 2, the molecule-selective electrode102comprises a polymer layer106. Preferably, the polymer layer may include a molecular imprinted polymer, in which the polymer layer106is provided with a plurality of voids each arranged to accommodate a molecule of the target substance, therefore the polymer layer106may accommodate the molecules of the target substance when the target substance is in contact with the polymer layer106.

In this example embodiment, each of the plurality of voids108includes a structure (such as a molecular structure) matching with a molecular structure of the molecules of the target substance. Hence, the voids108are selective to the target substance and may accommodate only the matching molecules of the target substance. The polymer layer106will not interact with substance with any molecular structures which do not match with the molecular imprinted structure.

Referring toFIG. 2, when the target substance is in contact with the target substance122, the voids108accommodate one or more molecules of the target substance122. Upon the interaction between the target substance and the substance selection structure102, the permittivity and hence the dielectric constant and the electrical capacitance of the molecular imprinted polymer layer106is changed, therefore it may subsequently alter the electrical characteristic of the electronic device structure104which is in electrical communication with the substance selection structure102.

In one example embodiment, the electronic device structure104is a thin-film transistor (TFT). An electronic transistor or a TFT may be characterized by different electrical characteristics such as threshold voltage (or turn on voltage) and source-drain current under different predetermined (gate and source-drain) voltage bias conditions.

When the substance selection structure102and the TFT104are electrically connected, the substance selection structure102may operate as a gate electrode of the TFT104. In such configuration, the gate voltage signal may be applied to the TFT104via the molecular imprinted polymer106. For example, a fix positive or negative gate bias may be applied using an electric source as the gate voltage bias, and the threshold voltage of the TFT104may be changed due to an interaction between the target substance122and the molecular imprinted polymer106, therefore the source-drain current across the source and drain electrodes may be increased/decreased when compare to the drain current under a same external gate/source/drain voltage bias applied to the transistor without a target substance in contact with the molecular imprinted polymer layer106.

Preferably, the electronic device structure104may comprises an active layer112such as a TFT material including silicon, a compound semiconductor, metal oxide semiconductor (e.g. zinc oxide) or an organic material such as copper hexadecafluorophthalocyanine (F16CuPc). A layer of dielectric material114may be included to serve as the gate dielectric of the TFT, and may include oxides such as SiO2, HfO2and/or Al2O3. The source/drain electrodes116may include typical conductive metals with suitable work functions such as Au, Ag and or Al.

The electronic device structure104and the substance selection structure(s)102are electrically and/or physically connected, such that electrical signal may be transmitted between different individual structures. Preferably, the electrochemical detector100further comprises an intermediate substrate118arranged to connect one or more substance selection structure102sand/or the electronic device. For example, the intermediate substrate118may include a copper substrate, or it may include an electrically conductive substrate, such as but not limited to an aluminum foil and a glass substrate coated with indium-doped tin oxide (ITO). The intermediate substrate118may also provide mechanical support to different structures disposed thereon.

With reference toFIGS. 3A to 3C, there is shown an example embodiment of a fabrication of the electrochemical detector100. In this embodiment a substance selection structure102and an electronic device structure104are fabricated on a plurality of separate substrates120(such as heavily doped semiconductor substrates or metal substrates), and the two fabricated structures are physically and electrically connected to an intermediate copper substrate118.

To fabricate the substance selection structure102, at step310, a heavily n-doped (n+) silicon substrate120is cleaned with standard cleaning procedures. At step312, the molecular imprinted polymer106is deposited on the surface of the silicon substrate120.

The molecular imprinted polymer (MIP)106may be prepared by mixing 14.5 mmol monomer (acrylamide or acrylic acid), 5.3 mmol cross-linker (N,N-methylene-biacrylamide), 0.7 mmol ammonium persulfate and 2.5 mmol template (Bis(2-ethylhexyl) phthalate, DEHP) in 25 mL Dimethylformamide and 10 mL DI water. After the solution was well mixed, the solution may be degassed by nitrogen, then 0.5 ml tetramethylethylenediamine (TEMED) may be added to initiate the fabrication. The solution may be kept for 16 hours in order to complete the polymerization. By completing such polemerization process, a combination of a molecular-imprinted polymer and a molecular template of the target substance (DEHP) is fabricated.

The fabricated combined MIP and template may be filtered, washed and dried under nitrogen atmosphere, and may then be ground and sieved to obtain a plurality of polymer particles each having a size of around 50 μm. The molecular template (DEHP) may then be extracted and removed from the MIP by the Soxhelt extraction in 24 hour with methanol. Finally, the MIP powder may be dried under a nitrogen atmosphere.

Preferably, the voids may be created after the DEHP template is extracted and removed from the MIP. In this example, acrylamide (monomer) belongs to the amide group, and phthalates belong to ester group. The formation of template-monomer complexes is due to the hydrogen bond prior to the polymerization reaction. This allows the template molecules to be embedded in the molecule post-polymerization.

Alternatively, the MIP may be fabricated for the detection of other target substances. In some other example embodiments, other phthalates, provided that they are able to form complexes prior to polymerization, may be used as templates in the fabrication of MIP layer106.

The MIP may be mixed with trichlorosilane (1 mg/mL) and dropped cast on the surface of the silicon substrate120. Alternatively, any other solvent suitable for dissolving the MIP polymer particles may be used. The solvent may then be driven away in a following drying process (e.g. under a nitrogen atmosphere) so as to form the MIP layer106. Referring toFIG. 6, there is shown an SEM image of the surface of the MIP layer106.

Alternatively, the prepared MIP may be deposited on the clean silicon substrate120by any solution process such as spin coating, sol-gel, printing, spray coating and/or screen printing.

The thickness of the MIP layer106is preferably at approximately 50-100 μm. The performance of the electrochemical detector100may depend on the MIP particle size, in which the response time of the electrochemical detector100may be reduced by reducing the particle size. In addition, the response time may also be reduced by reducing the thickness of the MIP layer106on the substance selection structure102, without affecting the detection limit and/or the linear dynamic range of the electrochemical detector100.

To fabricate the TFT structure104, at step320, an n+ silicon substrate120is cleaned with standard cleaning procedures. At step322, the metal-oxide dielectric (e.g. HfO2, Al2O3) layer114is coated on top of the silicon substrate120using either solution process sol-gel method (spin coating, printing. etc.) or atomic layer deposition (ALD), and preferably the dielectric layer114has a thickness of 5 nm-10 nm. Preferably, such dielectric layer may operate as a gate dielectric of the TFT device.

At step324, a semiconductor layer112(n-type semiconductor) is deposited using thermal evaporation or formed through a solution process sol-gel method (spin coating, printing. etc.) or any other suitable fabrication methods as discussed earlier. In this process, 30 nm-50 nm of semiconductor layer112is formed on top of the gate dielectric layer114, which may operate as the active layer of the TFT device.

At step326, gold or other suitable metal pads116are deposited on the semiconductor layer which form the source and drain electrodes of the TFT device104. The metal pads116may be deposited with thermal evaporation through a shadow-mask with the desired patterns. Alternatively, a conformal layer of metal may be patterned using conventional photolithography approaches and other suitable deposition methods to form the metal pads116. Preferably, the metal pads116may include a thickness of 30 nm-50 nm.

The abovementioned ranges of thickness of different layers may be changed in according to requirements or applications of the electrochemical detector100.

The fabricated substance selection structure102and TFT structure104may be connected to the intermediate copper substrate118at step330. For example, conductive adhesive may be applied to connect the two structures, either on the same side or opposite sides, to the copper substrate118. In such configuration, the substance selection structure102may operate as an extended gate electrode of the TFT device104. Alternatively, the substance selection structure102and the TFT structure104may be connected without an intermediate substrate118, or the two different structures may be fabricated on the same substrate such as on a same side of a silicon substrate or on opposite sides on a double-side polished silicon substrate, in which the electrical signal between the two structures may be transmitted through the silicon substrate.

In yet another example embodiment, the MIP layer106may be fabricated with two or more molecule templates such that the detector100may detect the corresponding two or more target substances. Alternatively, multiple substance selection structures102each includes an MIP layer prepared with different molecular templates may be connected to the intermediate copper substrate118, such that the electrochemical detector100may detect different target substances122applied to each of the different substance selection structures102.

With reference toFIGS. 4A and 4B, there is shown an embodiment of a method of detecting a target substance with an electrochemical detector100. The method comprises the steps of: applying an analyte solution122containing a predetermined amount of the target substance to the at least one substance selection structure102; applying a gate voltage to the thin-film transistor104via the analyte solution122and the at least one substance selection structure102; applying a source-drain bias to the thin-film transistor104; and determining a concentration of the target substance in the analyte solution122based on a characterization of the electrical characteristic of the thin-film transistor104.

In this example, an analyte solution122which contains the target substance such as DEHP is applied on a surface of MIP layer106of the substance selection structure102. As discussed earlier in this disclosure, the when the MIP106accommodates one or more DEHP molecules122in the analyte solution which is in contact with the MIP layer106, which leads to a change of permittivity/dielectric constant of the MIP layer106, and may also lead to a change in a differential capacitance of the substance selection structure102, the electrical characteristic of the electronic device structure104is altered. As a result, the threshold voltage and/or source-drain current are altered. Such change in electrical characteristics of the TFT device104corresponds to the concentration of the detected target substance in the analyte solution122, and the change may be characterized by conventional electrical measurement unit for characterizing a transistor or other types of electronic devices.

In an example measurement setup, referring toFIG. 4, the analyte solution122is dropped on the extended gate sensor. A reference electrode is dipped into the sample solution122. Using an SMU unit124, a positive gate voltage is applied though the reference electrode and at the same time a positive drain-source voltage is applied to the TFT device104. The signal is read out as a function of the threshold voltage shift of the TFT device104or the change in saturation drain current at a fixed gate voltage and source-drain voltage.

With reference toFIGS. 5A and 5B, there is shown a measurement result of the setup for the detection of 0, 25, 50, 100 and 200 ppb DEHP in an analyte solution122with drain current range in the range of 2×10−7to 6×10−7A. it is shown that the electrochemical detector100has a detection range of 25 ppb to 50 ppb, a detection limit of around 25 ppb and a detection sensitivity of approximately 2×10−9A/ppb.

As the voids of the MIP fill up with increasing analyte concentration or the molecules of the target substance122, there is an increase in drain current until the all the voids within the MIP are filled (saturation point). Upon reaching saturation, there are no further empty voids in the MIP to be filled. Therefore, there is no change in drain current with increase in analyte concentration beyond saturation.

With reference toFIG. 5C, there is shown a selectivity performance of the phthalate sensor100with DEHP imprinted polymer. The dotted line shows the predicted value of the normalized current for DEHP at 3000 ppb. It is shown that the electrochemical detector100has an excellent selectivity of DEHP (diethylhexyl phthalate) against DOS (dioctyl sebacate) and DnHP (Di-n-hexyl phthalate).

These embodiments are advantageous in that the electrochemical detector may be used in various applications including detection of harmful or toxic substances such as plasticizers. It may be implemented in portable electronic devices and hand-held sensors which may be used in domestic applications. The low-voltage operation of the electrochemical detectors ensures chemical stability (no induced electrolysis) of analyte solutions.

Advantageously, the performance of the detection shows that the electrochemical detector has high selectivity and sensitivity for target substance with the molecular-imprinted polymer electrode. The solid-state molecular-imprinted polymer layer may be easily tuned for selectively sensing different molecules (reversible & non-reversible), such as but not limited to DOS, DnHP, DBP, BBP, DNOP, DIDP, DINP by fabricating different MIP with different molecule templates. In addition, solid-state molecule-selective electrode (the extended gate) requires no internal filling solution.

The fabrication process is also simple and the electrochemical detector may be readily fabricated using different simple fabrication processes including low-cost fabrication methods such as printing and solution processing with low-cost materials. In addition, the substance selection structure and the electronic device structure may be separately fabricated on different substrates which may further simply the fabrication process involved. The fabrication of the electrochemical detectors may be easily scaled up.