WO2013009251A1

WO2013009251A1 - Method to use a probe to monitor interfacial changes of capacitance and resistance

Info

Publication number: WO2013009251A1
Application number: PCT/SE2012/050806
Authority: WO
Inventors: Anatol Krozer; Andrea ASTALAN; Dimitar KOLEV; Christer Johansson
Original assignee: Imego Ab
Priority date: 2011-07-08
Filing date: 2012-07-06
Publication date: 2013-01-17
Also published as: EP2729820A4; EP2729820A1

Abstract

The present invention relates to method to use a probe (1) adapted to monitor interfacial changes of capacitance and resistance. It is specifically taught that the probe consist of one single electrode, which electrode is in the form of a planar coil (11).

Description

Method to use a probe to monitor interfacial changes of capacitance and resistance

TECHNICAL FIELD

The present invention relates to a method on how to use a probe adapted to monitor interfacial changes of capacitance and resistance in different measurement applications.

PRIOR ART

It is previously known to use electromechanical impedance (El) as a technique in sensing applications to monitor the status of the solid-liquid interface, or to monitor changes of a fluid. One example where El can be successfully applied is to measure the aging of motor oil in the cars or to differentiate between different oil suppliers or to measure quality and concentration of urea. Another application discussed below is to sense changes of the charge within thin layers deposited onto electrodes.

The measurement is usually performed with the sophisticated and dedicated equipment using either 3-electrode configuration, with an anode, a cathode and a reference electrode, that, nominally, does not draw any current, or two-electrode configuration, with an anode and a cathode.

The classical electrode configuration is the one with two planar parallel plates facing each other, and perhaps a reference electrode near-by. During the measurement a small time variable voltage is usually applied, an AC-voltage, between the anode and cathode, or anode versus reference and cathode versus reference, and the induced current and the phase delay between current and voltage at different frequencies is monitored. Sometimes the DC offset voltage is also applied. The data is usually displayed as absolute value of impedance versus frequency and phase angle versus frequency. Ideally the results can be modelled by the suitable combinations of electrical components, resistivities and

capacitances. The capacitances are related to the immobile charges while the resistances characterise charge motion, for example, the motion of free ions in a liquid.

In biochemical applications the metallic electrode(s) are often covered by biomolecules. The biomolecular layers are often charged. In many cases changes of the charge within these layers is monitored. Such changes may mirror pH but may also arise due to a chemical reaction that neutralizes the charged species, or for a number of other reasons. Usually the changes of the charge state of the layer lead to changes of other parameters that characterise the layer, for example, its thickness or its geometry. Each of these lead to variation of capacitance values and possibly also of resistances.

The liquid often penetrates the layer, the layer might even be solvated, and the charged groups from the liquid may be present within the bio-layer. The amount of charged species within the layer is then given by the concentration of ions in the liquid adjacent to the layer, the so called interface double layer and also by the concentration of charged species within the layer itself. The interface double layer of charge builds always up near the (charged) interfaces whereby the mobile ions in the liquid screen the electric field that arises due to applied voltage or due to the charge built-up at the interface from the interior of the liquid. The interior is thus field-free.

All these charges within the layer or adjacent to it are interdependent from each other. The change in one of them influences the changes of the others. It is therefore of utmost importance to be able to measure as many parameters of this complex system as possible.

With recent advances in miniaturization it is known to use other electrode configurations than planar parallel electrodes. A particularly common configuration is the so called "interdigitated finger" configuration. One advantage of such electrodes is the possibility to incorporate several of them onto a small area, and therefore save on analytes, which are often expensive, and detect several analytes at different electrodes. The disadvantage is the rather complex

capacitance modelling because the electric field that arises due to applied voltage no longer follows straight lines as in the case of planar electrodes but protrudes into liquid and attains complex shapes. However the rule of thumb is that the field depth, and therefore the sensing depth, is of the order of one inter electrode spacing (within 90%).

The results obtained are usually modelled by more or less complex electro-chemical circuitry, including capacitances, resistances and complex capacitances arising mainly due to "parasitic" effects, eg, diffusion, electrode surface heterogeneity, etc. It is known that irrespective from "traditional" electrode configurations, all modelling always includes an RC product. It never allows for only capacitance to be measured; although sometimes its value can be deduced using modelling. Yet, one often wants to know the changes of charges which is a purely capacitive effect. Modelling involves always an element of uncertainty in determination of capacitance changes.

In addition the resistance may arise from at least two different sources, it may be due to a "friction" during charge motion mentioned above, for example during voltage (field) driven motion of ions in liquid, but it also may be due to a fact that the occurring charge changes take time where the resistance mirrors this time, i.e., the time for charge rearrangement.

SUMMARY OF THE PRESENT INVENTION

Problems

Taking the above mentioned prior art into consideration it is a technical problem to measure the changes of the charge state of the (bio)-layer(s) deposited on the electrode(s) or the changes of thickness or geometry induced by charge changes independently from the changes of the charge in the adjacent liquid (the changes of the interface double layer capacitances).

It is also a technical problem to measure charge changes in these layer(s) independently from the motion which is involved in charge rearrangement, for example motion of ions in a liquid adjacent to the electrode.

Another problem with prior art is the inability to measure the resistance and the capacitance separately instead of at best deducing their values via complex modelling.

Another problem is that the voltage applied across the electrodes induces additional charge separation due to motion of ions in the liquid at the electrode - liquid interface. Solution

With the purpose of solving one or several of the above problems, and on the basis of prior art and the indicated technical field as it has been shown above, the present invention teaches that the probe consist of one single electrode, which electrode is in the form of a coil, such as a planar coil. This opens up for a possibility to use yet another parameter of the electrical circuitry; the inductance. Inductance is an inherent property characterised only by the construction of the coil and does not change when the surroundings of the coil change, as described above.

The coil can be covered by at least one complex polymer film aimed to monitor different parameters in a fluid.

It is also proposed that the coil can be covered by a first passive layer, which is adapted to provide a chemically homogenous surface to be covered by the at least one complex polymer film. One example of such passive layer can be a layer of mussel adhesive protein. It binds strongly to most substrates and gives adherence to many functional layers on top. The invention is not limited by this particular passive layer; other examples are alkenothiols with suitably tailored end- groups.

It should be understood that the present invention is not limited to the measurement of any specific type of layer or film and that different films can be used for different embodiments of the present invention. By way of example it is proposed that the film can consist of pH sensitive overlayers, charged overlayers adapted to enable detection of specific molecules in the fluid, overlayers adapted swell or shrink as a result of a chemical reaction with specific molecules in the fluid, overlayers where the weight, volume, geometry or viscosity of the overlayers are changed as a result of a chemical reaction with specific molecules in the fluid, overlayers where the complex electrical impedance, i.e., either capacitance or resistance, or both, of the overlayers is changed as a result of a chemical reaction with specific molecules in the fluid, or overlayers where the dielectric constant of the overlayers are changed as a result of a chemical reaction with specific molecules in the fluid.

Used overlayers can be adapted to function for different purposes, and according to one proposed embodiment the overlayers comprise in general proteins that interact with other proteins, such as antibodies that react with antigens specific for a given disease.

It is also proposed that the film can consist of a molecularly imprinted polymer formed to be a specific to a particular compound in said fluid.

It is also possible that the films are neutral adapted to bind charged nanoparticles, or charged adapted to bind neutral nanoparticles. Regardless of if the coil is covered, and if so, by what it is covered, it should be understood that the inventive probe is adapted to detect chemical reactions taking place within the probed detection depth of the coil.

The present invention relates to a method to use a probe as described above, and different parameters that can be monitored in the use of an inventive probe.

One possibility is that any interfacial changes are monitored by means of monitoring the voltage response amplitude and phase shift or resonance frequency of the coil as an AC-voltage is applied to the coil. This will enable a separate measurement of the changes in the capacitance and changes in the resistance of the coil, where the capacitive and resistive changes are measured separately by measuring the change of the resonance frequency and the changes of its full width at half maximum, respectively.

It is proposed that the coil can be used a part of a feedback circuitry in an oscillator. In that case the resonance frequency of an oscillator corresponds to the resonance frequency of the coil while the damping within the oscillator circuitry is a measure of the full width at half maximum of the coil resonance peak or, equivalently, to the Q value of the coil.

There are different ways of tuning a probe according to the present invention, and it is proposed that the resonance frequency of the coil is tuned by tuning the distance between the windings and the number of windings of the coil, and that the probed detection depth for the coil is tailored by the use of narrow conductor-conductor distance between the windings of the coil. In addition the voltage drop along the coil can be tailored by changes of the thickness of the windings independently from the total length of the coil, i.e., independently from the distance between the windings and the number of windings of the coil.

The probe can be used as a single probe, in a 2-electrode configuration where an additional electrode is used parallel to and facing the coil surface of the coil, or in a 3-electrode configuration where a reference electrode and an additional electrode, parallel to and facing the coil surface of the coil, is used. The coil of the probe can be used as a working electrode, a counter electrode, or a floating electrode.

If used as a floating electrode the coil may be biased by a DC voltage. It is also proposed that when using in a configuration with other

electrodes, the coil can be used as a grounded electrode, where the coil is grounded at one of its ends.

The voltage difference between the coil and any additional electrode can be used to change the field distribution and tailor the detection depth.

Examples have been given where a probe is used for different kinds of detection, but it should be understood that the probe can also be used to monitor a fluid level. Advantages

The main advantages that foremost may be associated with a method to use a probe according to the present invention are that monitoring the changes of the voltage response amplitude and the coil resonance line shape and position versus frequency allows one to separate the resistive effects from the capacitive ones, and measure each of the effects separately.

The voltage drop over a coil is very low which implies the electrode potential applied to a coil is almost uniform, and low. The voltage drop can be tailored by changing coil geometry but also by changing the thickness of the metal windings, independently from the coil geometry. This result in a very low

perturbation of the adjacent liquid which implies that the influence of changes within the interface double layer on changes induced by chemical reaction(s) taking place within the film/layer deposited on to the coil substrate are also small.

Detection of changes in resonance frequency becomes easy if the coil is used as a part of a feedback circuitry in an oscillator.

The resonance frequency can be easily tuned by tuning the distance between the winding and the number of winding.

It is possible to adapt coil shape to desired measurement conditions using, e.g., circular coil geometry, square geometry, etc.

The probed detection depth can be tailored by using narrow conductor- conductor distance between the windings.

It is easy to expand the circuitry to allow traditional impedance

measurement by simply adding an additional electrode parallel to and facing the coil surface (2-electrode configuration) or an electrode and a reference electrode (3-electrode configuration). The coil electrode can be used either as working electrode or as a counter electrode and it can be floating (biased by a DC - voltage) or grounded (at one of its ends).

If used with an additional electrode one advantage is that the voltage difference between the two electrodes changes the field distribution and therefore the detection depth can be varied.

Additional information can be derived from a configuration with more than one electrode. For example one may be able to measure the charge drift through a cell and how it influences the charge at the interface (time scale for a double layer formation) but at the same time measure the changes that take place at the coil only.

The planar coil configuration according to the present invention is similar to the known interdigitated finger electrode except that the field shape is even more complex. Despite this fact the rule of the thumb mentioned above applies and the field penetration, and therefore the sensing depth, will be of the same order of magnitude as the distance between the windings of the coil.

BRIEF DESCRIPTION OF THE DRAWINGS

A probe and a method to use a probe according to the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a schematic and simplified illustration of a probe according to the present invention,

Figure 2 shows two graphs indicating changes of the electrical

characteristics of an inventive probe in dry condition and partly immersed in water,

Figure 3 is a simplified illustration of a cross sectional view of a thread forming the windings of a coil,

Figure 4 is a schematic and simplified illustration of a measuring setup using an inventive probe,

Figure 5 is a schematic and simplified illustration of an inventive probe used in a 2 electrode configuration, and

Figure 6 is a schematic and simplified illustration of an inventive probe used in a 3 electrode configuration. It should be understood that the figures are simplified and are used only to illustrate the inventive concept; the figures do in no way represent the dimensions that are found and used in real applications of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, the present invention will be described and the

background will first be described in some more detail.

A coil is an example of practical implementation of inductors - one of the three basic components in an electrical circuit; the other two being resistors and capacitors. As a representation of an inductor, a coil has been known for years. However as any other electrical component the "coil" is not a pure inductor. In fact any coil behaves always as a combination of the three. For example, when a DC current flows through a coil it behaves as a resistor. When a time dependent current flows through a coil it behaves as a combination of inductors, capacitors and resistors. It is an easy task to check that the simplest combination of the three basic circuit components that is able to describe the response of the coil to time variable current is the so called RLC circuit in parallel with an additional (stray) capacitance Co.

It is also well known that for certain time periods, or rather frequencies, the effective impedance of such a circuit becomes very high - the circuit exhibits a resonant behaviour. It occurs at an angular frequency, a)_res, given by

ωτ_βε = I/Vic, where L is the coil inductance and C its capacitance.

The coil damps the current, it loses some energy. The damping can be described by the so called quality factor, Q, and is given by Q = <¾_res L/.R, where R is the resistance of the coil.

The present invention relates to a probe adapted to monitor interfacial changes of capacitance and resistance, which probe consists of one single electrode, and where that electrode is in the form of a coil.

A so called planar coil will be used to exemplify the invention, as can be seen in figure 1 .

In Figure 1 a possible planar inductor lay-out is illustrated showing a probe 1 according to the present invention with a coil 1 1 and a first and a second electrical lead 12, 13. In this case the shape is circular. However, it should be understood that other shapes, such as square ones, are also possible to use. The capacitance mentioned is due to charges in between each pair of windings. The coil can be built of a copper conductor electrodeposited on top of a polymer substrate. It is obvious that also other materials can be used to build the coil, such as silver, gold or any other electrically conductive material. Similar results apply to the conductor embedded into a polymer or any other dielectric. The figure 2 represents the resonance mentioned whereby figure 2a is the power spectrum of the impedance while figure 2b is the phase angle change at resonance. When the coil is partially immersed in water the position and the shape of both resonance and the phase change, and in the figure the curve 21 a shows the resonance and 22a shows the phase angle shift with a dry coil and the curve 21 b shows the resonance and 22b shows the phase angel shift with a partially immersed coil.

In general, one should use terms reactive part of impedance and resistive part of impedance of a coil since the formulas above are only approximations to a "true" model of a coil, in particular, of a planar coil.

The capacitance is a property of material to polarize the charge. The charge which is mobile, e.g., electrons in a metal or ions in a liquid, produces current. The flow resistance of such charges, energy loss during their motion, i.e., friction, is described by the resistance, R, and produces voltage drop, an electric field change, according to Ohms law R = V/I, where V is the voltage and I is the current.

However when the charges are stopped at two interfaces, or are immobile altogether, for example Ionic groups on a monomer in a polymer, the component behaves as a capacitor and is described by capacitance. The voltage relates the charge with capacitance similarly to Ohms law C = Q_c/V, where Q_c is the available charge.

Similarly to the resistance, also a capacitance is constant for a given geometry and a given material. Therefore when the number of charges changes they can be detected by the changed voltage.

For example two traditional electrodes in the form of plates parallel to and facing each other, separated by a distance d constitute a capacitor with

capacitance given by the expression C = ε₀ εΑ/d where A is a surface of a plate while ε is the so called dielectric constant of a medium between the plates, while ε₀ is the dielectric constant of vacuum. Of course, when the geometry is not as simple as the one above the formula for capacitance becomes much more complex.

This is also the case for a planar coil mentioned above. The capacitance of such coil arises due to the charges confined in-between the current leads. It is difficult to express this capacitance by the simple geometrical parameters of the coil in a way similar to the case with two planar parallel plates although

approximate complex expressions relating the capacitance with the coil geometry do exist. Nevertheless similarly to the case with planar electrodes the planar coil capacitance depends on the dielectric constant of the material in-between the turns, and on the intra-winding distance, and on the stored charge at a given voltage.

Thus the voltage over the turns will change given the charge between the leads changes, at a given capacitance. However, if one changes the dielectric constant of the material in-between the leads or changes the distance between the leads or other geometrical parameters, the capacitance itself changes. The latter may also induce the variation of voltage and may be falsely interpreted as being induced by the change of Q_c.

For example water, a polar liquid, has a high dielectric constant, where ε « 75, as compared to air, where ε « ε₀ « 1. Thus water should result in changes of resonance since the dielectric constant should change. This is also the case as can be seen in figure 2.

However, there will arise two additional effects upon introduction of water:

(i) The charges that exist in water will tend to separate and move towards metallic interfaces where the voltage is applied. The capacitance formed by these charges, the so called the double layer capacitance,

Cdi, will add to the effective turn-turn capacitance. At the ionic strength of 10mM this capacitance protrudes only ~ 100nm which should be put in relation to the distance d between the turns of the coil, as shown in figure 1 . This distance d is a design feature when building coils and it is possible to produce planar coils with an intra-winding spacing d of any desired distance down to the order of 100 nm.

(ii) Any water, also ultra pure, contains some charges, ions. The amount of each charge depends on the pH or pK of the water and on the temperature, T. For example at equilibrium and at pH=7 there exist equal amount of OH^" and H3O⁺ ions, where their concentration is proportional to the expression -ΔΗ/kT where AH is the heat of water molecule dissociation into each of the ions. Thus the water will behave as a resistor, too. The resistance of a pure water is very high, Λ-10 Ω, but decreases very fast due to dissolution of ambient impurities and drops down to < lOkQ.. At 10mM the resistivity becomes -lkQ.. This is still a much higher value than the resistance drop over the entire coil which, at DC is « 10Ω .

When the water is very pure the resistance will be very high and it introduces only small changes in the quality factor, but affects the capacitance. However when there are many ions, high ionic strength, then both the resistance and the capacitance will be affected. The effect on resistance arises since the resistance of a coil at resonance is different from the DC-resistance and may be comparable to the water resistance. For example, the water used in figure 1 was ultra pure and the effect on resonance width, on the Q-value, is small. By increasing ionic strength the influence becomes larger and larger.

The water level has therefore pronounced influence on the response of the planar coil. Conversely, the coil can be used to monitor how large of its part is immersed into water, i.e., it can be used to monitor the water level. This applies not only to water but is equally valid for any polar liquid, and also for non-polar liquids, albeit with worse resolution in the latter case.

It can thus be understood that a probe according to the present invention can be used to monitor a fluid level.

An electrode according to the present invention can also be used to monitor interfacial changes of capacitance, the reactive part of the impedance, and of resistance, the resistive, real part of the impedance, especially when the electrode is covered by a complex polymer film aimed to monitor different parameters in a fluid.

Figure 3 shows schematically a cross sectional view of a thread 3 forming the windings of a coil. The coil can be covered by a first passive layer 31 , which first passive layer is adapted to provide a chemically homogenous surface to be covered by the at least one complex polymer film 32.

The film 32 can consist of pH sensitive overlayers, charged overlayers adapted to enable detection of specific molecules in the fluid, overlayers adapted swell or shrink as a result of a chemical reaction with specific molecules in the fluid, or overlayers where the weight, volume, geometry or viscosity of the overlayers are changed as a result of a chemical reaction with specific molecules in the fluid.

It is also possible that the film 32 consist of overlayers where the complex electrical impedance of the overlayers is changed as a result of a chemical reaction with specific molecules in the fluid, or overlayers where the dielectric constant of the overlayers are changed as a result of a chemical reaction with specific molecules in the fluid.

The overlayers comprises in general proteins that interact with other proteins, such as antibodies that react with antigens specific for a given disease.

It is also proposed that the film 32 can consist of a molecularly imprinted polymer formed to be a specific to a particular compound in the fluid. These man- maid polymers are formed to be specific to a particular compound, e.g., a toxin, a drug candidate, an antibiotic, a narcotic, etc., which enables the detection of the occurrence of the compound in question when it binds to a tailored cavity in the imprinted polymer. The binding usually neutralizes a charge within the cavity, electrostatic binding, which leads to charge decrease in proportion to the number of binding events. It also leads to changed geometry of the imprinted polymer films and thus induces capacitance changes.

It is also proposed that the film 32 is neutral adapted to bind charged nanoparticles, or that the film 31 is charged adapted to bind neutral nanoparticles.

Thus it is understood that an inventive probe can be adapted to detect chemical reactions taking place within the probed detection depth 33 of the coil.

Figure 4 shows a schematic measuring setting using an inventive probe

41 with a coil 41 a. Here it can be seen that the probe 41 is connected to a device

42 whereby an AC voltage can be applied to the probe 41 . The device 42 can also be adapted to receive and monitor the signal from the probe 41 .

The present invention teaches that any interfacial changes can be monitored by means of monitoring the voltage response amplitude and the phase shift, or resonance frequency, of the coil as an AC-voltage is applied to the coil 41 a. The changes in the capacitance and changes in the resistance of the coil can be measured separately by measuring the change of the resonance frequency and the changes of its full width at half maximum.

It is also possible to monitor any interfacial changes by means of monitoring the changes of frequency at which the phase shift of the coil occurs as an AC-voltage is applied to the coil.

It is also possible to monitor any interfacial changes by means of monitoring the changes of the frequency window at which the phase shift of said coil occurs as an AC-voltage is applied to the coil.

A practical realisation of the present invention is that the coil 41 a is used a part of a feedback circuitry in an oscillator 42a.

The resonance frequency of the coil can be tuned by tuning the distance d between the windings and the number of windings of the coil 1 1 and the probed detection depth 33 for the coil can be tailored by the use of narrow conductor- conductor distance d between the windings of the coil 1 1 .

Figure 5 shows that it is proposed that an inventive probe 51 can be used in a 2-electrode configuration where an additional electrode 52 is used parallel to and facing the coil surface of the coil 51 a.

Figure 6 shows that it is also possible to use an inventive probe 61 in a 3- electrode configuration where a reference electrode 62 and an additional electrode 63, parallel to and facing the coil surface of the coil 61 a, is used.

In these configurations with other electrodes it is possible to use the coil 51 a, 61 a as a working electrode, as a counter electrode, or as a floating electrode.

If used as a floating electrode the coil 51 a, 61 a can be biased by a DC voltage.

Whenever the inventive probe is used in 2- or 3-electrode configuration it is possible to use the coil 51 a, 61 a as a grounded electrode, the coil being grounded at one of its ends.

The voltage difference between the coil 51 a, 61 a and any additional electrode 52, 62, 63 can be used to change the field distribution and tailor the detection depth 33 of the probe.

It will be understood that the invention is not restricted to the aforede- scribed and illustrated exemplifying embodiments thereof and that modifications can be made within the scope of the invention as defined by the accompanying Claims.

Claims

1 . Method to use a probe adapted to monitor interfacial changes of capacitance and resistance, where said probe consists of one single electrode, and where said electrode is in the form of a planar coil, characterised in, that any interfacial changes are monitored by means of monitoring the voltage response amplitude and the phase shift of said coil as an AC-voltage is applied to said coil.

2. Method according to claim 1 , characterised in, that any interfacial changes are monitored by means of monitoring the resonance frequency of said coil as an AC-voltage is applied to said coil, and that changes in the capacitance and changes in the resistance of said coil are measured separately.

3. Method according to claim 2, characterised in, that the capacitive and resistive changes are measured separately by measuring the change of the resonance frequency and the changes of its full width at half maximum.

4. Method according to claim 1 , characterised in, that any interfacial changes are monitored by means of monitoring the changes of frequency at which the phase shift of said coil occurs as an AC-voltage is applied to said coil.

5. Method according to claim 4, characterised in, that any interfacial changes are monitored by means of monitoring the changes of the frequency window at which the phase shift of said coil occurs as an AC-voltage is applied to said coil.

6. Method according to any preceding claim, characterised in, that said coil is used a part of a feedback circuitry in an oscillator.

7. Method according to any preceding claim, characterised in, that the resonance frequency of said coil is tuned by tuning the distance between the windings and the number of windings of said coil.

8. Method according to claim 2 or 3, characterised in, that the probed detection depth for said coil is tailored by the use of narrow conductor-conductor distance between the windings of said coil.

9. Method according to any preceding claim, characterised in, that said probe is used in a 2-electrode configuration where an additional electrode is used parallel to and facing the coil surface of said coil.

10. Method according to any one of claims 1 to 8, characterised in, that said probe is used in a 3-electrode configuration where a reference electrode and an additional electrode, parallel to and facing the coil surface of said coil, is used.

1 1 . Method according to claim 9 or 10, characterised in, that said coil is used as a working electrode.

12. Method according to claim 9 or 10, characterised in, that said coil is used as a counter electrode.

13. Method according to claim 9 or 10, characterised in, that said coil is used as a floating electrode.

14. Method according to claim 13, characterised in, that said coil is biased by a DC voltage.

15. Method according to claim 9 or 10, characterised in, that said coil is used as a grounded electrode, said coil being grounded at one of its ends.

16. Method according to any one of claims 9 to 15, characterised in, that the voltage difference between said coil and said additional electrode is used to change the field distribution and tailor the detection depth.

17. Method according to any preceding claim, characterised in, that said probe is used to monitor a fluid level.

18. Method according to any preceding claim, characterised in, that said coil is covered by at least one complex polymer film aimed to monitor different parameters in a fluid.

19. Method according to claim 18, characterised in, that said coil is covered by a first passive layer, and that said first passive layer provides a chemically homogenous surface to be covered by said at least one complex polymer film.

20. Method according to claim 18 or 19, characterised in, that said film consist of pH sensitive overlayers.

21 . Method according to claim 18 or 19, characterised in, that said film consist of charged overlayers enabling detection of specific molecules in said fluid.

22. Method according to claim 18 or 19, characterised in, that said film consist of overlayers that swell or shrink as a result of a chemical reaction with specific molecules in said fluid.

23. Method according to claim 18 or 19, characterised in, that said film consist of overlayers where the weight, volume, geometry or viscosity of said overlayers changes as a result of a chemical reaction with specific molecules in said fluid.

24. Method according to claim 18 or 19, characterised in, that said film consist of overlayers where the complex electrical impedance of said overlayers changes as a result of a chemical reaction with specific molecules in said fluid.

25. Method according to claim 18 or 19, characterised in, that said film consist of overlayers where the dielectric constant of said overlayers changes as a result of a chemical reaction with specific molecules in said fluid.

26. Method according to any one of claim 21 to 25, characterised in, that said overlayers comprises in general proteins that interact with other proteins.

27. Method according to claim 26, characterised in, that said overlayers comprises antibodies that react with antigens specific for a given disease.

28. Method according to claim 18 or 19, characterised in, that said film consist of a molecularly imprinted polymer formed to be a specific to a particular compound in said fluid.

29. Method according to claim 18 or 19, characterised in, that said films are neutral and binding charged nanoparticles.

30. Method according to claim 18 or 19, characterised in, that said films are charged and binding neutral nanoparticles.

31 . Method according to any one of claims 18 to 30, characterised in, that said probe detects chemical reactions taking place within the probed detection depth of said coil.

32. Method according to any one of claims 18 to 31 , characterised in, that said coil acts as an antenna, and that the signal read-out from said coil is made wireless and takes place from a distance from said coil.

33. Method according to claim 32, characterised in, that said distance is a function of coil geometry and of the resonance frequency of said coil, and that said distance from said coil is tailored by changing the distance between the windings and the number of windings of said coil.