Description

The present invention relates to a microelectrode, in particular a microelectrode for Scanning Electrochemical Microscopy and electroanalysis or for microchemical reactors and for so-called electrochemical micromachining (EMM), and to a manufacturing method thereof.

The use of microelectrodes, introduced in the 1980s, offers the typical advantages of electrochemical analysis devices, such as high sensitivity, precision and accuracy and the possibility to record an electrical signal (potential or current difference) correlated to the activity (concentration) of the analyte in real time. Microelectrodes allow particularly fast response times (in the order of milliseconds), mainly due to the peculiar spatial distribution of the material transport profiles which allow very rapid diffusion of the reacting species on the surface.

Microelectrodes can have various geometries with at least one of the dimensions comprised between 1 and 100 microns. They are typically metallic devices and can be functionalized on the surface, e.g. with enzymes. In particular cases such as microcavities, a metal microsupport can allow the study of small quantities of low/medium conductivity materials.

The most common geometry is the metal microdisc, based on a microwire (commercially available) coated with an insulating material, a section of which is exposed.

The main disadvantage in the use of microelectrodes and, consequently, the major limitation of their commercialization in the civil and health sector (they are devices widely used in the academic field but not very widespread in the industrial field) can be identified in (i) the difficulty of preparation, which requires know-how often handed down orally and (ii) the fragility, being based on a glass capillary. The customization procedures reported in the literature are often complex and difficult to reproduce.

Therefore, the problem at the basis of the present invention is to make available a microelectrode which can be made in a short time in an automated (and therefore scalable) manner and of high robustness.

Such a problem is solved by a microelectrode and by a method for manufacturing as defined in the accompanying claims, the definitions of which form an integral part of the present description.

The object of the present invention is a microelectrode comprising at least one electrode element comprising an electrode wire and a contact wire, and an electrode body made of a polymer material, wherein the electrode wire and the contact wire are joined by a junction made of an at least partially conductive material and are embedded in said electrode body, and wherein the electrode wire ends at the level of a distal end of the electrode body so that only a discoid surface of the electrode wire remains exposed, and wherein the contact wire ends outside the proximal end of the electrode body. In given embodiments, the microelectrode of the invention may comprise two electrode elements in the same electrode body.

In given embodiments, the microelectrode of the invention may comprise a longitudinal channel in the electrode body, into which an optical fiber can be inserted.

In given embodiments, the microelectrode of the invention may comprise, on at least part of the outer surface of the electrode body, a coating of silver or other metals with a similar function. A further object of the invention is a method for manufacturing the microelectrode of the invention which comprises a liquid injection phase of a polymeric hardening material into a tube mold into which said electrode element is inserted.

Further features and advantages of the present invention will be apparent from the description of some examples of embodiment, given here by way of non-limiting example with reference to the following figures:

Figure 1 is an enlarged side view of a microelectrode according to the invention;

Figure 2 is an enlarged side view of a second embodiment of the microelectrode according to the invention;

Figure 3A is an enlarged side view of a third embodiment of the microelectrode according to the invention;

Figure 3B is an enlarged front view of the microelectrode in figure 3A;

Figure 4A is an enlarged side view of a fourth embodiment of the microelectrode of the invention;

Figure 4B is an enlarged front view of the microelectrode in figure 4A; Figure 5 is an enlarged side view of the step of manufacturing of the microelectrode according to the invention.

The microelectrode according to the invention, indicated by reference numeral 1 as a whole, comprises an electrode element 10, comprising an electrode wire 2 and a contact wire 3, and an electrode body 4 made of plastic material, wherein the electrode wire 2 and the contact wire 3 are joined by a junction 5 made of an at least partially conductive material and are embedded in said electrode body 4 and wherein the electrode wire ends at the level of a distal end 4a of the electrode body 4 and the contact wire 3 ends outside the proximal end 4b of the electrode body 4.

It is important for the electrode wire 2 ends to end leveled with the distal end 4a of the electrode body 4 because in this manner only a discoidal surface 2a, which is essential for given applications such as Scanning Electrochemical Microscopy (SECM), remains exposed.

It is also important for the contact wire 3 to protrude from the electrode body 4 so that it can be connected to a measuring instrument.

In preferred embodiments, the electrode wire 2 is centered in the electrode body 4 i.e. the distance of the X-X axis of the electrode wire 2 from the outer surface 4c of the electrode body 4 is substantially uniform.

The electrode body 4 can have various shapes, according to the needs and applications to which it will be dedicated. Figures 1 and 2 show two possible shapes of the electrode body 4, 104 by way of example. The electrode body 4 in figure 1 has a substantially cylindrical shape, while the electrode body 104 in figure 2 has a cylindrically shaped portion 104c, adjacent to the proximal end 104b and a substantially conical or conical-frustum portion 104d at the distal end 104a.

A third embodiment is shown in figures 3A and 3B. The microelectrode 201 has a double wire and comprises a first electrode element 10 and a second electrode element 10', separated by an insulating septum 6, e.g. a septum made of polymeric material, such as polyethylene terephthalate (PET). Again in this case, the electrode elements 10, 10' are formed by an electrode wire 2, a contact wire 3 and a junction 5 and are embedded in an electrode body 204.

The shape shown, which has a tip at the distal end 204a, is only indicative because other shapes of the microelectrode 201 can be prepared according to needs and applications. The double wire microelectrode 201 can be used for example, for the simultaneous determination of two parameters.

Figure 4 shows a fourth embodiment of the microelectrode of the invention, wherein an electrode element 10 is embedded in an electrode body 304 and wherein the electrode body 304 comprises a longitudinal channel 7 which connects the distal end 304a with the proximal end 304b of the electrode body 304. The longitudinal channel 7 may comprise an optical fiber. Again, the shape of the microelectrode 301 is by way of example only.

According to a different embodiment, the microelectrode 1 comprises a coating made of silver or other metals, such as gold or platinum, on at least part of the electrode 4 body, which acts as a reference electrode or counter electrode. The result is a self-standing microelectrode which can be immersed in a solution for measurement without the need for other electrodes.

According to a different embodiment, the microelectrode 1 can be a microcavity microelectrode, i.e. having a discoid surface 2a on which a microcavity is made, as described below. Such microelectrodes can be used to house a finely subdivided material in the form of micro or nanometric particles to study their electrochemical properties or use them as an active phase in sensor technology .

In given embodiments, by way of example, the electrode body 4, 104, 204, 304 has a diameter comprised between 0.5 mm and 1 mm, or a diameter between 0.3 mm and 0.5 mm, or may comprise a proximal portion at the distal end 4a, 104a, 204a, 304a having a diameter between 0.3 mm and 0.5 mm and a proximal portion at the proximal end 4b, 104b, 204b, 304b having a diameter between 0.5 mm and 1 mm.

In given embodiments, the electrode body 4, 104,

204, 304 is made of an epoxy resin, in particular a two-component epoxy resin which can be polymerized at room temperature, or it can be made of a thermosetting or UV-crosslinking polymer or of a thermo-shrinkable polymer. It is also possible to make the electrode body 4, 104, 204, 304 of a silicone polymer or silicone rubber, preferably of the two-component type that can be polymerized at room temperature. In the case of polymer which can be polymerized at room temperature, however, the curing time must be long enough to allow the microelectrode

1 101 201 301 to be made using the method described below. In given embodiments, the electrode wire 2 has a diameter between 10 and 100 microns and a length comprised between 1 and 2 cm, but embodiments in which the diameter and length of electrode wire 2 are outside the indicated measurements cannot be excluded, as needed.

The electrode wire 2 can be a metal wire, in particular a wire of a metal normally used for electrodes, such as, for example, gold or platinum, or it can be a carbon fiber wire.

In given embodiments, the contact wire 3 has a diameter comprised between 0.1 and 0.2 millimeters and a length comprised between 9 and 10 cm, but also in this case embodiments in which the diameter and length of contact wire 3 are outside the indicated measurements cannot be excluded, as needed.

The contact wire 3 is a wire made of a conductive material chosen from those normally used for electrical contacts, e.g. a copper wire. The junction 5 is preferably made with an electrically conductive adhesive, i.e. an adhesive which comprises a conductive component in a percentage by weight of 20% or more. The conductive component is preferably chosen between silver, nickel, copper or graphite. An example of an electrically conductive adhesive is the silver paste (mono- or bi-component), which can contain a percentage by weight of silver varying between 30% and 60%, the remaining part being glycols and solvents.

The microelectrode 1 according to the invention can be made, as shown in figure 5, by means of the procedure described below comprising the following steps: a) providing an electrode wire 2, a contact wire 3, a material for the junction 5 and a tube 11 made of a plastic material having a first end 11a and a second end lib and having an inner diameter substantially corresponding to the outer diameter of the electrode body 4 to be manufactured; b) joining one end of the electrode wire 2 with one end of the contact wire 3 by means of the junction 5, to form an electrode element 10; c) inserting the electrode element 10 into said tube 11 so that the contact wire 3 projects from said first end 11a of the tube 11 for a length m, and that a section of length n of the tube 11 between the discoid surface 2a of the electrode wire 2 and the second end lib of the tube 11 remains empty; d) injecting, through said first end 11a of the tube 11, a hardening polymer material to form the electrode body 4, at least up to the level of the discoid surface 2a of the electrode wire 2 and at a distance of about 1 cm or more from the second end lib of the tube 11; e) hardening the polymer material and then removing, e.g. by peeling, the tube (11) from the electrode body (4) starting from the second end (lib); f) if the electrode body 4 extends beyond the discoid surface 2a of the electrode wire 2, cutting the excess portion so that the discoid surface 2a of the electrode wire 2 is at the same level as the distal end 4a of the electrode body 4; g) optionally, lapping and polishing the discoid surface 2a of the electrode wire 2 and the distal end 4a of the electrode body 4, to manufacture the microelectrode 1 of the invention.

If one were to make a microelectrode 101 like the one shown in figure 2, the method of the invention comprises the following steps: al) providing an electrode wire 2, a contact wire 3, a material for the junction 5 and a tube 11 made of a plastic material having a first end 11a and a second end lib and comprising a first portion having an inner diameter substantially corresponding to the outer diameter of the cylindrical portion 104c and a portion of a smaller diameter at least at the conical or conical-frustum portion 104d of the electrode body 104 to be manufactured; bl) joining one end of the electrode wire 2 with one end of the contact wire 3 by means of the junction 5, to form an electrode element 10; cl) inserting the electrode element 10 into said tube 11 so that the contact wire 3 projects from said first end 11a of the tube 11 for a length m and that a section of length n of the tube 11 between the discoid surface 2a of the electrode wire 2 and the second end lib of the tube 11 remains empty; dl) injecting, through said first end 11a of the tube 11, a hardening polymer material to form the electrode body 104, at least up to the level of the discoid surface 2a of the electrode wire 2 and at a distance of about 1 cm or more from the second end lib of the tube 11; el) hardening the polymer material and then remove, e.g. by peeling, the tube 11 from the electrode body 104 starting from the second end lib; fl) if the electrode body 104 extends beyond the discoid surface 2a of the electrode wire 2, cutting the excess portion so that the discoid surface 2a of the electrode wire 2 is at the same level as the distal end 104a of the electrode body 104; gl) optionally, lapping and polishing the discoid surface 2a of the electrode wire 2 and the distal end 104a of the electrode body 104; h) forming the conical or conical-frustum portion 104d by chemical etching;

1) optionally, soaking the microelectrode 104 or a part thereof into a bleaching solution.

The tube 11 is preferably made of a non-stick material, more preferably PTFE (polytetrafluoroethylene ) and has an inner diameter preferably comprised between 0.4 and 1 mm, more preferably between 0.5 and 0.7 mm, and a length preferably of about 8-12 cm.

The length of the tube 11 depends on the length of the electrode element 10 and in any event, it is such that, once the electrode element 11 has been inserted, a portion of length n from the second end lib is left empty and the contact wire 3 is made to protrude from the first end 11a by a sufficient length for its electrical connection to an instrument .

The tube 11 used in step al) for making the microelectrode 101 preferably consists of two pieces of tube, a first piece with a diameter not exceeding 0.5 mm and a length preferably of 3-5 cm and a second piece with an inner diameter preferably comprised between 0.7 and 1 mm and a length preferably of 6-8 cm, in which the first piece can be inserted in the second piece, e.g. for about 1-2 mm in length, to make the tube 11. The tube 11 can have a section of any shape, e.g. circular (as shown in the figures), elliptical, square, rectangular, polygonal, etc. In given embodiments, the tube 11 may also be conical, spherical, ogival or have any other shape that is adaptable to various needs.

Steps b) and bl) are preferably carried out by using an electrically conductive adhesive as defined above, more preferably silver paste, for the junction 5. Steps c) and cl) are carried out by threading the free end of the electrode wire 2 into the first end 11a of the tube 11 and making it advance up to a distance n from the second end lib of the tube 11, wherein such a distance n is preferably at least 1 cm. For step cl), it is preferable to place the junction 5 at about the level of the junction between the first piece and the second piece of the tube 11.

The injection of the polymeric hardener material according to steps d) and dl) may be by any means suited for such injection, e.g. a syringe. Steps g) and gl) can be carried out, e.g. by means of a lapping machine using abrasive paper and abrasive suspensions in the order of 1000 mesh, 2400 mesh, 4000 mesh and 0.3 micron alumina, for a time which ensures the disappearance of traces of polymeric material left by the previous step.

The step h) of chemical etching preferably provides soaking the portion of the electrode body

104 adjacent to the distal end 104a in an acid solution adapted to the erosion of the polymeric material of the electrode body 104. For example, a sulfuric acid solution may be used, preferably in the presence of hydrogen peroxide. An example is the so- called "piranha solution" consisting of a mixture of concentrated sulfuric acid (95-97% by weight) and hydrogen peroxide in a 2:1 ratio.

Step h) provides soaking of the distal end 104a of the electrode body 104 perpendicularly to the surface of the solution and for a length substantially corresponding to the conical or conical-frustum portion 104d to be achieved.

Preferably, the electrode body 104 is moved vertically with soaking/removal cycles lasting 2 seconds each and for a total time preferably comprised between 3 and 5 minutes. Step 1), which is necessary if the preceding step blackened the treated electrode body 204, can be carried out using a bleaching agent, e.g. chosen between hydrogen peroxide and sodium hypochlorite. In given embodiments, the electrode element 10 must be centered in the electrode body 4, 104. To obtain such a centering, the method of the invention comprises, between step b), bl) and step c), cl), the following steps: i) soaking a portion of the electrode element 10 from the end of the electrode wire 2 into melted paraffin; ii) soaking for a fraction of time the portion of step i) into the hardening polymer material before hardening; iii) hardening the polymer material adhered to said portion of the electrode element 10.

In this manner, drops of homogeneously thick paraffin are first formed on the soaked portion of the electrode element 10, which act as an adhesion promoter for the liquid hardening polymeric material, which in turn forms drops of homogeneous thickness. This produces centering elements which will keep the electrode element 10 at a distance from the walls of the tube 11. When steps i), ii) and iii) are operated, the introduction of electrode element 10 into tube 11 can be carried out by inserting the contact wire 3 through the second end lib of the tube 11. The double wire microelectrode 201 in figures 3A and 3B is obtained by a procedure similar to the one described above for the microelectrodes 1 and 101, with the difference that two electrode elements 10,

10' - obtained as described above - are bonded to two opposite faces of the insulating septum 6 by the interposition of the polymeric hardening material used for the electrode body 204, i.e. for example an epoxy resin or a thermosetting polymer as described above. The assembly thus formed is placed in a tube 11 as defined above, which may have a diameter of up to 2 mm, and will proceed as described in steps c), d) e), f) and optionally g), h) and 1).

The microelectrode in the embodiment in figures 4A and 4B, which comprises the longitudinal channel 7, may be obtained by the procedure used for the microelectrode 1, but by inserting a wire or tube made of a non-stick polymer, e.g. PTFE, into the tube 11, in parallel to the electrode element 10. After the hardening of the polymeric hardening material, such a wire or tube can be pulled out, so that the longitudinal channel 7 can be created inside the electrode body 204.

The microelectrode with a silver outer coating can be obtained as follows. This procedure, applicable to all the cases described above, aims to obtain a homogeneous layer of Ag on the external walls of the microelectrode, to be used as a reference electrode and/or counter electrode. In this manner, the microelectrode becomes self-standing and can be soaked in the desired solution without the need to introduce other electrodes .

Other known and conventional techniques may be used for the deposition of other metals, such as gold or platinum.

The procedure comprises the following steps:

- soaking the electrode in hydrochloric acid HC1 (17%) for about 15 minutes to prepare the surface for metal deposition; - preparing an aqueous solution of AgNC 1.5 M and an aqueous solution of NaOH 2.5 M;

- transferring an aliquot (e.g. 3 ml) of AgNC solution into a container (e.g. a 10 ml flask or a 10-15 ml volumetric jacket reactor) under magnetic stirring and thermostatic bath heating; - under magnetic stirring, adding an aliquot

(e.g., 5 ml) of NaOH solution drop by drop to form a black precipitate;

- adding a 35% ammonia solution drop by drop until the black precipitate has completely disappeared (approximately 2-3 ml is needed);

- soaking the electrode and add drop by drop an aliquot (e.g. 2 ml) of a 2 M glucose solution;

- adding water and heat to about 90°C, then stop stirring, forming a black solution;

- after about 5 minutes, removing the electrode and rinsing it with water.

The formation of an Ag mirror on the container walls and a white Ag layer on the electrode will be observed.

As a result of the deposition, an electrical contact can be generated using the aforementioned conductive adhesive to connect a copper wire or other metal. It is also possible to protect the deposit and contact with a protective layer, e.g. epoxy resin.

The microelectrode with discoid surface 2a with microcavity can be obtained as follows.

It is first necessary to determine the radius of the previously prepared 2a discoid surface. This is done by recording the steady-state limit current in the presence of a reversible redox species (e.g. an aqueous solution containing 1 mM Ru(NH3)6Cl3and 0.5 M KC1).

The radius is determined by solving the following equation: Iss = 4nFCDr

Where r (in m) is the radius of the discoid surface 2a, Iss (in A) the steady-state current limit, C (in mol m ^-3 ) and D (in m ² s ^-1 ) are respectively the concentration and the diffusion coefficient of the redox species, n are the electron moles/reagent moles and F the Faraday constant (96485 C/electron moles).

The etching is carried out by applying a pulsed current between 1.5 mA cm ^-2 (1000 s) and -0.075 mA cm- ² (1000 s). Among these, it is possible to add a current-free step. The current density values and their duration can be varied at will. It is also possible to apply a single anodic galvanostatic pulse. The number of cycles or the etching time depends on the desired depth.

After etching, it is possible to make the bottom of the cavity homogeneous by cycling the electrode potential between the evolution regions of ¾ and O2 at 0.5-1 V s ¹ 500 times. Sonic the electrode tip to remove any residual Au wire etching.

Finally, it is possible to evaluate the actual cavity depth by optical microscopy or by solving the following equation for L:

4n.F Dr ²

1ss 4 L 4- r where L is the cavity depth.

The cavity can be filled with any finely divided solid substance by pressing the electrode tip repeatedly (5-10 times) on a small amount of the powder. The powders (materials finely divided into the form of micro or nanometric particles) can consist of any conductive or semiconductor material, to study its electrochemical or photoelectrochemical properties or to be used as an active material in sensor technology. It is advisable to check the correct filling under a microscope. To empty the cavity, simply soak the tip in an ultrasonic bath for a few seconds and check again under the microscope.

The microelectrodes according to the invention can be used for multiple applications.

The microelectrode 101 with conical or conical- frustum portion 104d can be used as a tip for Scanning Electrochemical Microscopy. In this case, it is important for the electrode element 10 or at least electrode wire 2 to be centered in the electrode body 104, because a small and constant ratio between the radius of the discoid surface 2a and the total radius of the distal end 104a of the electrode body 104 must be determined.

The double wire microelectrode 201 can be used for the simultaneous determination of two parameters in a solution or as conductometric cells.

The microelectrode 301 equipped with longitudinal channel 7, into which an optical fiber can be inserted, can be used in photochemical or photoelectrochemical reactions or for reading an optical signal.

The silver-coated microelectrode can be used as a self-standing microelectrode.

Specific fields of application of the microelectrodes of the invention can be in water analysis, as sensors for pollutants, in the food industry or other uses both in research and industrial applications, in the field of microreactors and micromachining.

The microelectrodes of the invention are low cost, as they can be prepared with an easily scalable process, can have the desired shape according to the needs and have a high resistance by virtue of the mechanical properties of the polymeric material used. In this specification a method for the injection of the polymeric hardening material has been described but an extrusion and 3D printing procedure can also be provided. It is apparent that only some particular embodiments of the present invention have been described, to which a person skilled in the art will be able to make all the changes necessary to adapt it to particular applications, without because of this departing from the scope of protection of the present invention.