A conductive sheet for an electrode of a biosensor and / or an alcohol sensor

Abstract

A conductive sheet (A) for an electrode of a biosensor and / or an alcohol sensor comprising fibers (1) being nanofibers and / or microfibers, wherein the fibers (1) form a network structure having interspaces (4) between the fibers (1). The conductive sheet (A) further comprises conductive nanoparticles (2) and ionic liquid (3), wherein the conductive nanoparticles (2) and ionic liquid (3) are confined in the interspaces (4) of the network structure.

Description

DESCRIPTIONTITLEA conductive sheet for an electrode of a biosensor and / or an alcohol sensorTECHNICAL FIELD[OOOl]The present disclosure relates to a conductive sheet for an electrode of a biosensor, in particular a non- invasive electrochemical biosensor, such as a skin-patchable sensor and / or an alcohol sensor, such as iso propyl alcohol (IPA) and / or ethanol.BACKGROUND

[0002] Measurement of biological signals such as electrocardiogram is generally performed by electrodes attached to the skin, which are so-called “on-skin electrodes” and / or “sheet-type electrodes” and are used in the medical and health monitoring fields. Such a sheet-type electrode is disclosed by JP 2014-226172 A. The conductive sheet disclosed in JP 2014-226172 A is formed by coating a surface of a cloth with conductive materials, such as metals. For accurate measurement, it would be important to attach the “on- skin electrode” and / or “sheet-type electrode” in close contact to the biological surface, and this would require that the electrode be highly flexible and thin. According to the conventional conductive sheets as disclosed in JP 2014-226172 A, the amount of conductive material applied on the surface of the cloth would have to be increased to increase conductivity, and therefore it has been challenging to reduce the thickness of the sheet, while maintaining conductivities and flexibilities.SUMMARY OF INVENTION

[0003] The technical problem to be solved may be formulated to provide a thin and flexible conductive sheet for an electrode of a biosensor and / or an alcohol sensor substrate having improved electrical conductivities.

[0004] According to the first aspect of the present disclosure, a conductive sheet for an electrode of a biosensor and / or an alcohol sensor comprising fibers (1) being nanofibers and / or microfibers. The (electrically) conductive sheet preferably has a thickness in micrometer ranges to millimeter ranges, for example 10 pm to too pm, which may be suitable for an electrode of a biosensing application, in particular for ab electrode of a non-invasive electrochemical biosensor, such as a skin-patchable sensor. An electrode for a skin- patchable sensor may be used, for example, for recording electroencephalogram (EEG) and / or electrocardiogram (ECG) and / or electromyography (EMG), which is / are typically measured in the mV range or V range. Additionally or alternatively, such an electrode for a skin-patchable sensor may be used for stimulating of nerves, such as central nervous system (CNS) and / or peripheral nervous system (PNS) with current pulses in the pA to mA range or with voltage pulses in the mV or V range. The conductive sheet preferably comprises only nanofibers or preferably only microfibers. The fibers may be electrically conductive and / or electrically non-conductive and / or insulative material. The nanofibers have preferablya diameter between t nm to t inn, more preferably between 5 to 30 nm. The microfibers have preferably a length between 1 pm to 1 mm. The nanofibers and / microfibers may comprise or consist of fiber segments and / or fiber fragments. Each of the nanofibers and / or microfibers (and / or fiber segments and / or fiber fragments thereof) preferably have a length of 1 pm to 1 mm, more preferably between 1.5 pm to 950 pm. The fibers (1) form a network structure and / or a three-dimensional network structure and / or a mesh structure having interspaces (4) between the fibers (1) and / or interspaces (4) between the fiber segments and / or the fiber fragments. The fibers and / or the network of the fibers may form a substrate and / or base material of the conductive sheet, wherein the substrate may have a porous structure in which the interspaces (4) may correspond to pores of the substrate.

[0005] The conductive sheet (A) further comprises conductive nanoparticles (2). The conductive nanoparticles (2) are electrically conductive. The term “nanoparticle” means a particle and / or an object having a (average) dimension and / or a diameter and / or a hydrodynamic diameter and / or a length and / or a width between 1 and 700 nm, more preferably between 20 nm to 650 nm, wherein preferably the average peak is at too ± 50 nm. A nanoparticle(s) may take any shape such as a nanosphere and / or a nanorod and / or a nanochain and / or a nanofiber. For example, a sphere nanoparticle may have a diameter (or hydrodynamic diameter) between 1 and 500 nm, more preferably between 1 and 700 nm, more preferably between 20 nm to 650 nm, wherein preferably the average peak is at too ± 50 nm. The conductive nanoparticles may be, for example, conductive polymers. Conductive polymers may be a class of organic materials that are electrically conductive. Conductive polymers may be composed of chains of repeating units, which are typically based on aromatic rings, such as thiophene, pyrrole, and aniline. These aromatic rings can be doped with various dopants, such as oxidizing or reducing agents, to increase their electrical conductivity. The doping process introduces charge carriers, either positive or negative, into the polymer chain, allowing it to conduct electricity. The conductive polymers may be poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Additionally or alternatively, the nanoparticles may be metal, such as gold and / or silver, and / or carbon, such as carbon nanoparticles and / or carbon nanotubes and / or carbon rods, and / or supramolecule.

[0006] The conductive sheet (A) further comprises ionic liquid. An ionic liquid is a salt in the liquid state at and / or below room temperature (25 °C). The ionic liquid comprises or consists of cations and anions. Examples for the ionic liquid are compounds based on the i-ethyl-3-methylimidazolium(EMIM) cation, such as EMIM-ethyl sulfate (ES) and / or EMIM-acetate (A) and / or EMIM-C1 and / or EMIM-Ac (acetate anion), EMIM-dicyanamide (C2H5)(CH3)C3H3N+2-N(CN)-2.

[0007] According to the present disclosure, the conductive nanoparticles (2) and ionic liquid (3) are confined and / or incorporated and / or embedded and / or captured and / or trapped and / or encapsulated and / or integrated in and / or inside and / or within the interspaces (4) of the network structure. For example, the (average) dimension(s) of the nanoparticles (2) may be equal to or larger than the (average) dimension(s) of the interspaces, so that the nanoparticles (2) can be mechanically confined in the network structure and can be substantially homogeneously or homogeneously distributed in / throughout the network structure. The (average) dimension of the interspaces may be defined by a (average) distance between the fibersand / or the fiber segments and / or the fiber fragments. A density and / or number of the nanoparticles in a unit volume in the conductive sheet may be therefore substantially uniform / homogeneous or uniform / homogeneous along the thickness of the conductive sheet. The cations and / or anions of the ionic liquid may be attracted and / or attached and / or adhered to the nanoparticles by charges, thereby being confined in the interspaces of the network structure together with the nanoparticles. For example, each of the nanoparticles may comprise a negatively and / or positively charged part. Additionally or alternatively, the nanoparticles may comprise negatively charged nanoparticles and / or positively charged nanoparticles. Additionally or alternatively, the cations and / or anions of the ionic liquid may be attracted and / or attached and / or adhered to the fibers by charges. For example, the fibers may be negatively or positively charged and / or the network structure of the fibers may comprise a negatively charged part and / or a positively charged part. Additionally or alternatively, the fibers may comprise negatively charged fibers and / or positively charged fibers.

[0008] The conductive sheet according to the present disclosure may have one or more of the following technical advantages: The fibers forming the base sheet structure would confine the conductive nanoparticles and ionic liquid in the interspaces, namely meshes of the network structure, and wherein the conductive nanoparticles and ionic liquid would add conductivities to the sheet. Inventors have found that the combination of the conductive nanoparticles and ionic liquid would provide a low resistivity (i.e. a high conductivity) in a range of 10'5fl-m, which can be used as an electrode for a biosensor, in particular non- invasive electrochemical biosensor. The nanofibers and / or microfibers (and / or fiber segments and / or fiber fragments thereof) in a length of 1 pm to 1 mm, more preferably between 1.5 pm to 950 pm, may be especially advantageous to more efficiently capture the nanoparticles and the ionic liquid in the interspaces of the fiber network structure and may further improve conductivities of the conductive sheet. It has been further found by the inventors that the ionic liquid would contribute to add high flexibility and / or resilience to the conductive sheet. The conductive sheet according to the present disclosure with these physical and electrical characteristics would have advantages over conventional metal electrodes and / or conventional textile-based electrode, and the conductive sheet would be especially suitable for skin-patchable electrode applications with a thickness in micrometer orders. The conductive sheet would be further suitable for an electrode of an alcohol sensor, as isopropanol and / or ethanol.

[0009] Preferably, the fibers (1) comprise a thermoset polymer, a thermoplastic polymer, an elastomer, a polysaccharide (such as cellulose), chitosan, chiton and / or combinations thereof. More particularly, the fibers (1) comprise a thermoset polymer and / or a thermoplastic polymer and / or an elastomer and / or polysaccharide such as cellulose and / or chitosan and / or chiton, wherein the fibers (1) are preferably obtained by an electrospinning method. Specifically, the fibers may be obtained or produced by applying the electrospinning method to a liquid comprising a thermoset polymer and / or a thermoplastic polymer and / or an elastomer and / or polysaccharide such as cellulose and / or chitosan and / or chiton. It may be further advantageous in that mechanical and / or chemical properties of the fibers would be controlled by selecting materials of the fibers. The electrospinning method may be further advantageous in that it would be possible to control the diameter and / or length of the fibers, which may influence the conductive sheet properties, such as a size of the interspaces or flexibility of the conductive sheet.[ooio]Preferably, the conductive nanoparticles (2) comprise metal nanoparticles and / or one or more carbon nanoparticles such as carbon nanotubes and / or carbon rods, and / or one or more conductive polymers, the one or more conductive polymers preferably comprise a conductive polymer mixture of at least two ionomers, such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Preferably, the ionic liquid (3) comprise compounds based on i-ethyl-3-methylidmidazolium (EMIM) cations, such as EMIM-ethyl sulfate (ES) and / or EMIM-acetate (A). Preferably, the fibers (1) are negatively charged and / or positively charged. The conductive sheet comprising the EMIM based compound as being the ionic liquid may be especially advantageous in that EMIM would provide flexibility to the conductive sheet and while the anions such as ES would enhance the conductivity.

[0011] Preferably, the conductive nanoparticles (2) comprise or consist of a conductive polymer mixture, the conductive polymer mixture comprises positively charged polymers and a negatively charged polymers.

[0012] Preferably, the conductive nanoparticles (2) have dimensions or an average dimension being equal to or larger than the interspaces (4), such that the conductive nanoparticles (2) are mechanically confined in the interspaces (4). Preferably, the conductive nanoparticles (2) have an average diameter and / or an average length and / or an average width between 1 nm and 700 nm, more preferably between 20 nm to 650 nm, wherein preferably the average peak is at too ± 50 nm. The conductive sheet may be further advantageous in that the conductive nanoparticles may be more stably confined in the interspaces of the fiber network structure.

[0013] Preferably, the fibers (1) (and / or fiber segments and / or fiber fragments thereof) have an average diameter between 1 nm to 1 pm, more preferably between 5 to 30 nm. Preferably, the fibers (and / or fiber segments and / or fiber fragments thereof) have in an average length and / or lengths between 1 pm to 1 mm, more preferably between 1.5 pm to 950 pm. The conductive sheet may be further advantageous in that the fibers would contribute to flexibility of the conductive sheet and / or may further confine more efficiently the nanoparticles and / or the ionic liquid in the interspaces.

[0014] According to the second aspect of the present disclosure, a mixture for manufacturing a conductive sheet according to the present disclosure is a liquid suspension. The mixture comprises a dispersion medium comprising deionized water and / or organic solvent, and the fibers (1) being nanofibers and / or microfibers, and the conductive nanoparticles (2), and the ionic liquid (3). The nanofibers (1) and the conductive nanoparticles (2) and the ionic liquid (3) are dispersed throughout the dispersion medium. The mixture may have one of more of following technical advantages: The mixture, which is the liquid suspension, would be especially advantageous to manufacture the conductive sheet having a homogeneous or substantially homogeneous conductivity and / or mechanical properties through the conductive sheet. For example, the mixture may be filtered by a filter such as a paper filter and / or membrane filter having a pore size in a range of 0.1 pm to 0.5 pm, thereby obtaining the conductive sheet after having removed the dispersion medium.

[0015] Preferably, the fibers (1) comprise cellulose, and the conductive nanoparticles (2) comprise poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and the ionic liquid (3) comprises 1-ethyl- 3methylidmidazolium-ethyl sulfate (EMIM-ES). The PEDOT:PSS as being the conductive nanoparticles and the EMIM based compound as being the ionic liquid may be especially advantageous in that EMIM would provide flexibility to the conductive sheet, while the anions such as ES would improve the conductivity of the conductive sheet including the PEDOT:PSS.

[0016] Preferably, the conductive sheet (1) according to the first aspect of the present disclosure is obtained by filtering the mixture according to the second aspect, the mixture being a liquid suspension and / or wherein the conductive sheet is obtained by evaporating the dispersion medium from the mixture.

[0017] Preferably, a microelectrode element comprises the conductive sheet according to the first aspect of the present disclosure.

[0018] According to one of aspects of the present disclosure, an alcohol sensor comprises an electrode for attaching to a skin. The electrode comprises a conductive sheet (A) according to the first aspect, and / or a microelectrode comprising a conductive sheet (A) according to the first aspect.

[0019] According to one of aspects of the present disclosure, a non-invasive electrochemical biosensor comprises an electrode for attaching to a skin. The electrode comprises a conductive sheet (A) according to the first aspect of the present disclosure and / or a microelectrode element comprising a conductive sheet (A) according to the first aspect of the present disclosure.

[0020] According to the third aspect of the present disclosure, a method for manufacturing a conductive sheet (A) comprises a step of preparing a mixture including a step of preparing a first liquid suspension comprising the fibers (1) and a first dispersion medium, the first dispersion medium being deionized water and / or organic solvent, and a step of preparing a second liquid suspension comprising conductive nanoparticles (2) and a second dispersion medium, the second dispersion medium being deionized water and / or organic solvent. The method further comprises a step of mixing the first liquid suspension and the second liquid suspension and ionic liquid (3). The first dispersion medium and / or the second dispersion medium may be deionized water and / or organic solvents. Preferably, sonication is applied to a mixture of the first liquid suspension and the second liquid suspension in an ice bath. The method further comprises a step of mixing the ionic liquid (3) to a mixture of the first liquid suspension and the second liquid suspension. Preferably, the steps are performed in this order. The method further comprises a step of removing the first dispersion medium and the second dispersion medium from the mixture.

[0021] Preferably, the step of removing the first dispersion medium and the second dispersion medium includes a step of filtering the mixture by a filter, wherein the filter has a pore size between 0.1 pm and 0.5 pm.

[0022] Preferably, the step of removing the first dispersion medium and the second dispersion medium includes a step of evaporating the first dispersion medium and the second dispersion medium from the mixture. Preferably, prior to the step of removing, the mixture is deposited on a substrate by spray coating or by printing, and the method further includes a step of removing the dried mixture being the conductive sheet from the substrate.

[0023] The fourth aspect of the present disclosure is a use of a conductive sheet (A) according to the first aspect of the present disclosure or a microelectrode element comprising a conductive sheet (A) according to the first aspect of the present disclosure, in integrated circuits, and / or in a PCB board and / or a conductive board, and / or in cable and / or wire formation(s) including patterned feedline(s), and / or for on-skin biosensing, and / or for temperature sensing, and / or as alcohol sensor and / or as humidity sensor.BRIEF DESCRIPTION OF THE DRAWINGFig. 1: schematic perspective views of a conductive sheet according to the first embodimentFigs. 2(a) to 2(d): schematic explanations of manufacturing processes of the conductive sheet according to the first embodimentFigs. 3(a) to 3(f): photos of conductive sheet samples, and photos of comparative samples Fig. 4: schematic plan view of an electrode sampleDETAILED DESCRIPTION OF EMBODIMENTS

[0024] 1. First embodimentFig. 1 shows a schematic illustration of a conductive sheet (A) according to the first embodiment. The conductive sheet (A) has a thickness (d) in the z-direction and comprises a first surface and a second surface, each of the first surface and the second surface extending in the x-direction and the y-direction (and / or in the xy-plane. The second surface is opposite to the first surface in the z-direction. The x-, y-, z- axes and / or directions are of the three-dimensional rectangular coordinate system, which are orthogonal to each other.

[0025] Fig. 1 further shows an illustrative explanation of the internal structure of the conductive sheet. This illustration is only for better understanding of the microscopic structure of the conductive sheet and may not reflect the real structure of the conductive sheet.

[0026] The conductive sheet (A) according to the first embodiment includes fibers (1), conductive nanoparticles (2) and ionic liquid (3). The fibers (1) maybe nanofibers and / or microfibers. An example for such fibers is cellulose. The fibers form a network structure and / or a mesh structure having interspaces (4) between the fibers (1). The interspace(s) is / are thus defined as internal spaces between the fibers. The characteristics of the network structure, such as a size of the interspace(s), which may correspond to an average distance between the fibers in the network, may be influenced by a diameter and length of the fibers forming the network structure.

[0027] As illustrated in Fig. 1, the conductive nanoparticles (2) and the ionic liquid (3) are confined in the interspaces of the network structure. Namely, the conductive nanoparticles (2) and the ionic liquid (3) are incorporated into the network structure, and the network structure traps and / or captures the nanoparticles(2) and the ionic liquid (3) in / inside the interspaces (4) of the network structure. The nanoparticles (2) and the ionic liquid (3) may therefore be homogeneously distributed throughout the conductive sheet (A). For example, the conductive sheet (A) may have a homogeneous density of the nanoparticles and / or the ionic liquid along the thickness of the conductive sheet. In other words, the nanoparticles (2) and the ionic liquid(3) are embedded or filled in the interspaces of the network structure.

[0028] In Fig. 1, the nanoparticles (3) are represented as spheres, however, the nanoparticles are not limited thereto and may take any structures such as a rod. The nanoparticles (2) may be conductive polymers. An example for such conductive polymers is FEDOT :PSS. Anions and cations of the ionic liquid are represented with a sign of - and + in Fig. 1.

[0029] According to the first embodiment, the nanoparticles (2) are surrounded by the fibers and mechanically or physically confined in the interspaces (4). Additionally, the fibers (1) may be negatively charged or positively charged. Alternatively, the fibers may be a mixture of negatively charged fibers and positively charged fibers. For example, negatively charged fibers, such as cellulose, would more stably confine the positively-charged part of the nanoparticles (2) in the interspaces. For example, in case the conductive nanoparticles (2) are PEDOT:PSS, a positively charged FEDOT part would be attracted by the negatively charged fibers (e.g. cellulose). The charges may play a role for the confinement of the anions and cations of the ionic liquid (3) in the interspaces. The anions of the ionic liquid may be attracted by the positively- charged part of the nanoparticles (2), and the cations of the ionic liquid may be attracted by the negatively- charged fibers and / or the negatively-charged part of the nanoparticles.

[0030] As the ionic liquid (3) is in the liquid state in the room temperature, the combination of the conductive nanoparticles (2) and the ionic liquid (3) confined in the interspaces would be especially advantageous to improve electrical conductivities of the conductive sheet (A) and further to obtain flexible mechanical properties of the conductive sheet (A).

[0031] 2. Manufacturing methodFigs. 2(a) to 2(d) schematically illustrate steps of the manufacturing method for the conductive sheet according to the first embodiment. The first liquid suspension (1S) comprises the fibers (1) and the first dispersion medium (1S). The second liquid suspension (2S) comprises the conductive nanoparticles (2) and a second dispersion medium. The first liquid suspension is mixed with the second liquid suspension (Fig. 1(a)). The first dispersion medium may be deionized water and / or organic solvent(s), in which the fibers (1) can be dispersed throughout the dispersion medium. For example, cellulose nanofibers, which is an example of the fibers, may be dispersed in deionized water, which is an example of the first dispersion medium. Similarly, the second dispersion medium may be deionized water and / or organic solvent(s), in which the nanoparticles (2) can be dispersed throughout the dispersion medium. For example,PEDOT:PSS, which is an example of the nanoparticles, may be dispersed in deionized water, which is an example of the second dispersion medium. The mixture of the first liquid suspension (1S) and the second liquid suspension (2S) is preferably sonicated (for example in an iced bath (I)) to obtain homogeneous or substantially homogeneous mixture (Fig. 2(b)).

[0032] The ionic liquid (3) is then added to the mixture of the first liquid suspension (1S) and the second liquid suspension (2S), and is mixed (Fig. 2(c)). Subsequently, the mixture of the first liquid suspension, the second liquid suspension and the ionic liquid (3) is filtered by a filter (F), for example by applying vacuum filtration (Fig. 2(d)), so that a solid conductive sheet would be formed on the filter (F). For example, polyvinylidene difluoride (PVDF) membrane filters maybe used in case the dispersion medium of the liquid suspension is water. Alternatively, other porous filters of e.g. PTFE (polytetrafluoroethylene) and / or glass fiber and / or cellulose may be used. The pore size of the filter (F) is preferably equal to or smaller than dimensions the fibers (1) (e.g. (average) lengths of the fibers) and / or the nanoparticles (2) (e.g. (average) diameter of the nanoparticles), preferably in a range of 0.1 pm to 1 pm and more preferably, 0.1 pm to 0.5 pm, so that the fibers capturing the nanoparticles and the ionic liquid would form a network structure and remain on the filter and the dispersion medium would be filtered out.

[0033] 3. Examples of a conductive sheetThe first embodiment will be further explained by conductive sheet examples shown in Figs. 3(a) to 3(f). The components and conditions used for the conductive sheet examples are only specific example and do not restrict the embodiment.

[0034] 3-1 Thickness characterizationCellulose nanofibers (CNF), which is an example of the nanofibers (1), was mixed with deionized water (2 w / w % cellulose) by sonicating for 40 min. in an ice bath to obtain the first liquid suspension. A PEDOT:PSS (1.3 wt%), which is an example the nanoparticles (2), was prepared by mixing PSS with EDOT by stirring and sonicating for 3 min. to obtain the second liquid suspension. The first liquid suspension (10 w / w %) and the second liquid suspension (90 w / w %) were then mixed by sonicating for 20 min. in an ice bath. The EMIM-ES (more than 98 % concentration), which is an example of the ionic liquid, was then added to the mixture of the first liquid suspension and the second liquid suspension (the volume ratio of the EMIM-ES solution to the mixture was 1:10), and mixed until gelatinization occurred.

[0035] The prepared mixture was then poured into a vacuum filtering station with 0.22 pm pore PVDF (Polyvinylidenfluorid) filter for 2 hours at 1 bar vacuum pressure, so that the dispersion medium of the first and second liquid suspension (here deionized water) was removed and a solid conductive sheet with a diameter of 3.6 cm was formed on the filter. The filter was removed from the funnel, and the conductive sheet was manually detached from the filter.

[0036] Five samples (1) to (5) were prepared by changing amounts of the mixture, which was loaded on the filter. A thickness of each sample was measured by laser microscopy and is summarized in Table 1.Table 1:

[0037] The results indicates that the thickness of the conductive sheet examples increases substantially proportion to the loaded amount of the mixture, and the thickness can be thus controlled by the mixture amount to be filtered.

[0038] 3-2 Resistivity characterizationFurther three samples (6) to (8) were prepared by changing the weight ratio of the first liquid suspension to the second liquid suspension in the mixture. Other steps of the manufacturing are identical to that of the sample (1) to (5). Furthermore, the comparative samples (i) to (iv) were prepared: The comparative sample (i) was prepared by filtering only the first liquid suspension (CNF); the comparative sample (ii) was prepared by filtering a mixture of the first liquid suspension and the EMIM-ES (the volume ratio of the EMIM-ES solution to the first liquid suspension was 1:10, i.e. without the second liquid suspension (PEDOT:PSS)); the comparative sample (iii) was prepared by filtering a mixture of the first liquid suspension (10 w / w %) and the second liquid solution (90 w / w %) (i.e. without the EMIM-ES); the comparative sample (iv) was prepared by filtering a mixture of the second liquid suspension (PEDOT:PSS) and the EMIM-ES (the volume ratio of the EMIM-ES solution to the PEDOT:PSS solution was 1:10, i.e. without the first liquid suspension (CNF)). Photos of the samples (6) to (8) are shown in Figs. 3(a) to 3 (c), respectively, and photos of the comparative samples (i), (iii) and (iv) are shown in Figs. 3 (d) to 3(f), respectively.

[0039] Each of the samples (6) to (8) and the comparative samples (i) to (iv) was then cut into an electrode shape (electrode sample) having a linear portion (2 cm length (1) and 0.3 cm width (w)) and contact pads (0.9 x 0.9 cm2square) at two ends of the linear portion as shown in Fig. 4. The resistivity was measured by chronoamperometiy. As a reference sample, a resistivity of a gold electrode, which is a 100 nm thickness gold electrode being sputtered of a 125 pm polyimide sheet, was measured. The results are summarized in Table 2.Table 2:

[0040] The comparative sample (i) consists of cellulose nanofibers (CNF) and thus exhibits a high resistivity, which is considered as being non-conductive. The resistivity decreases for the comparative sample (ii) formed by a combination of cellulose nanofibers (CNF) and EMIM-ES without PEDOT:PSS by three orders of magnitude in comparison to the comparative sample (i). The comparative sample (iii) formed by a combination of cellulose nanofibers (CNF) and PEDOT:PSS without EMIM-ES further shows a resistivity lower than the sample (i) by six orders of magnitude. The comparative example (iv) formed by PEDOT:PSS and EMIM-ES without cellulose nanofibers (CNF) shows a resistivity in a similar range of the comparative example (iii). The conductive sheet examples (6) to (8) show low resistivities (i.e. high conductivities). It can be also seen from the conductive sheet example (8) and the comparative example (iii), that the ionic liquid, here EMIM-ES, would largely contribute to improve conductivities of the conductive sheet. Furthermore, when comparing the conductive sheet sample (8) and the comparative example (iv), it would be evident that the internal structure of the conductive sheet, in which the fiber network incorporates the nanoparticles (PEDOT:PSS) and the ionic liquid (EMIM-ES), would be especially advantageous for providing higher electrical conductivities.

[0041] List of reference signsA: Conductive sheet, 1: Fibers, 2: Nanoparticles, 3: Ionic liquid, 4: Interspaces, 1S: First dispersion medium, 2S: Second dispersion medium, F: Filter, I: Iced bath

Claims

Claims1. A conductive sheet (A) for an electrode of a biosensor and / or an alcohol sensor comprising fibers (i) being nanofibers and / or microfibers, wherein the fibers (i) form a network structure having interspaces (4) between the fibers (1), wherein the conductive sheet (A) further comprises conductive nanoparticles (2), and ionic liquid (3), wherein the conductive nanoparticles (2) and the ionic liquid (3) are confined in the interspaces (4) of the network structure.

2. The conductive sheet (A) according to claim 1, the fibers (1) comprise a thermoset polymer, a thermoplastic polymer, an elastomer, a polysaccharide (such as cellulose), chitosan, chiton and / or combinations thereof, and / or wherein the fibers (1) are obtained by an electrospinning method.

3. The conductive sheet (A) according to any one of the preceding claims, wherein the conductive nanoparticles (2) comprise metal nanoparticles, and / or carbon nanoparticles such as carbon nanotubes and / or carbon rods, and / or conductive polymers, wherein the conductive polymers preferably comprise a conductive polymer mixture of at least two ionomers, such as poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and / or wherein the ionic liquid (3) comprise compounds based on i-ethyl-3- methylidmidazolium (EMIM) cations, such as EMIM-ethyl sulfate (ES) and / or EMIM-acetate (A).

4. The conductive sheet (A) according to any one of the preceding claims, wherein the fibers (1) are negatively charged and / or positively charged.

5. The conductive sheet (A) according to any one of the preceding claims, wherein the conductive nanoparticles (2) comprise a conductive polymer mixture, wherein the conductive polymer mixture comprises positively charged polymers and negatively charged polymers.

6. The conductive sheet (A) according to any one of the preceding claims, wherein the conductive nanoparticles (2) have dimensions being equal to or larger than the interspaces (4), such that the conductive nanoparticles (2) are mechanically confined in the interspaces (4).

7. The conductive sheet (A) according to any one of the preceding claims, wherein the fibers (1) have diameters between 1 nm and 1 pm, more preferably between 5 to 30 nm, and / or wherein the fibers (1) have lengths between 1 pm to 1 mm, more preferably between 1.5 pm to 950 pm.

8. A mixture for manufacturing a conductive sheet according to any one of claims 1 to 7, the mixture being a liquid suspension, which comprises:- a dispersion medium comprising deionized water and / or organic solvent,- fibers (1) being nanofibers and / or microfibers,- conductive nanoparticles (2), and- ionic liquid (3), wherein the nanofibers (1) and the conductive nanoparticles (2) and the ionic liquid (3) are dispersed throughout the dispersion medium.

9. The mixture according to claim 8, wherein the fibers (1) comprise cellulose, and the conductive nanoparticles (2) comprise poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and the ionic liquid (3) comprises i-ethyl-3methylidmidazolium-ethyl sulfate (EMIMES).

10. The conductive sheet (A) according to any one of claims 1 to 7, wherein the conductive sheet is obtained by filtering a mixture according to claim 8 or 9, and / or wherein the conductive sheet is obtained by evaporating the dispersion medium from the mixture according to claim 8 or 9.

11. A microelectrode element comprising the conductive sheet (A) according to any one of claims 1 to 7, 10.

12. An alcohol sensor comprising an electrode for attaching to a skin, wherein the electrode comprises a conductive sheet (A) according to any one of claims 1 to 7, 10, and / or a microelectrode element according to claim 11.

13. A non-invasive electrochemical biosensor comprising an electrode for attaching to a skin, wherein the electrode comprises a conductive sheet (A) according to any one of claims 1 to 7, 10 and / or a microelectrode element according to claim 11.

14. A method for manufacturing a conductive sheet (A) according to any one of claims 1 to 7, 10, the method comprising a step of preparing a mixture according to claim 8 including a step of preparing a first liquid suspension comprising the fibers (1) and a first dispersion medium, the first dispersion medium being deionized water and / or organic solvent, and a step of preparing a second liquid suspension comprising the conductive nanoparticles (2) and a second dispersion medium, the second dispersion medium being deionized water and / or organic solvent, wherein the method further comprises a step of mixing the first liquid suspension and the second liquid suspension, a step of mixing the ionic liquid (3) and a mixture of the first liquid suspension and the second liquid suspension,and a step of removing the first dispersion medium and the second dispersion medium from the mixture.

15. The method according to claim 14, wherein the step of removing the first dispersion medium and the second dispersion medium includes a step of filtering the mixture by a filter, wherein the filter has a pore size between 0.1 pm to 0.5 pm.

16. The method according to any one of claims 14 or 15, wherein the step of removing the first dispersion medium and the second dispersion medium includes a step of evaporating the first dispersion medium and the second dispersion medium from the mixture.

17. The method according to any one of claims 14 to 16, wherein prior to the step of removing, the mixture is deposited on a substrate by spray coating or by printing, and the method further includes a step of removing the dried mixture being the conductive sheet from the substrate.

18. Use of a conductive sheet (A) according to any one of claims 1 to 7, 10 or a microelectrode element according to claim 11, in integrated circuits, in a PCB board and / or a conductive board, in cable and / or wire formation(s) including patterned feedline(s), for on-skin biosensing, for temperature sensing, as alcohol sensor and / or as humidity sensor.

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