DE3930768A1 - Chemo- and bio-sensor system for ion conc. determn. without contact - by measuring interface potential with variable capacity - Google Patents

Abstract

Chemo- and bio-sensor system for determining ion concns. in liquids consists of a multisensor element (MSE) and measuring appts. and is used for the determn. of individual sensor potentials without contact. The MSE has an insulating substrate and a series of different ion-selective membranes, opt. coated with biologically active material. The potentials at the membrane-substrate interface and/or electrolyte-substrate interface are determined by applying one or more external capacity Cs, varying with time, with moving electrodes (BE) oscillating about a given distance from the sensor elements or rotating discs (RSC) are used to vary the effective area with time. To determine the individual membrane potentials, the electric potential at BE or the counterelectrode (GE) is regulated with a counter-potential UK, so that the pd. between each of the effective electrodes of capacity Cs and hence the induced a.c. current i, is minimised. USE/ADVANTAGE - The sensor, which is of the disposable type, is very small, free from solid electrolyte and noble metal and suitable for automation. It can be used to test small amts. of liquids down to under 0.1ml. Sensors of this type are useful in medicine, veterinary medicien, agriculture, and bio-food and environmental technology.

Description

The invention relates to the device of a chemo- and bio- Sensor system for the determination of ion concentrations in Liquids from a multi-sensor element and a measuring device and a method for the contactless determination of the individual sensor potentials.

The sensor elements should be designed so that compli decorated layer sequences with solid electrolytes and on Precious metal layers can be dispensed with, and at the same time thermodynamic non-equilibrium conditions avoided will.

Such sensor systems with extremely simple and very inexpensive chemical or bio sensor elements for "single use" and the associated measuring equipment can be used in the following areas, among others:
Medicine, veterinary medicine, biotechnology, agriculture, food technology, environmental technology.

State of the art

It is known that chemosensors from ion-selective membranes with an internal electrical contact and a reference electrode are constructed as film sensors ( 1, 2 ). In biosensors, the ion-selective membrane is additionally e.g. B. coated with an enzyme so that products of enzyme reactions mH the ion-selective membrane properties can be detected ( 3 ). Membranes can also be coated with antibodies and microorganisms.

The sensor principle of the ion-selective electrode is based on this on that on a membrane with an electrolyte (e.g. the liquid to be examined) an electrical potential difference is formed. These Electroplating voltage occurs at the phase boundary between the electrolyte and membrane and can be used in the case of thermodynami write equilibrium as Nernst equation:

ϕ = ϕ₀ + RT / (z F) ln (a _i1 / a _i2 )

ϕ₀ is the standard electrode potential, R the universal gas constant, F the Faraday constant, z the valence of the measurement ion, a _i1 the activity of the ion in phase 1 and a _i2 the activity of the ion in phase 2 .

The response of such membranes is different depending on the membrane material for different types of ions. So z. B. with Si ₃ N ₄ layers H ₃ O⁺ and with Na-Al silicate layers determine Na⁺ ion concentrations.

In addition to such solid-state membranes, membranes made of organic materials gain particular importance. So can e.g. B. PVC membranes selectively by adding ionophores be made for certain types of ions, their concentration NEN can be determined potentiometrically.

In Fig. 1, an ion-selective "coated-film" electrode is provided, which consists only of a plastic carrier (T), a silver layer (S) and an ion-selective membrane (M). The potential of the underside of the membrane can be tapped via the potential tap (PA). During the measurement, the top of the membrane is in contact with the measurement solution, and the galvanic voltage can be determined potentiometrically between the tap and a reference electrode.

With such simple sensor elements, thermodynamic non-equilibrium states can occur at the interface between the silver layer (S) and the ion-selective membrane (M) ( 1 ), which falsify the measurement of the ion concentration.

This problem is avoided in thin-film electrodes ( 2 ) ( FIG. 2) because a solid electrolyte (FE) and a silver chloride layer (SC) are introduced between the membrane (M) and the silver layer (S).

The measurement of the Galvani potential can of FIG. 3 between an ion selective electrode (SE) and a stable Refe rence electrode (RE) or Fig. 4 between two difference union ion-selective electrodes (SE 1, SE 2) under a pseudo-reference electrode relation ( PRE).

Should several ion-selective electrodes on one carrier a multi-sensor element, so each must Electrode individually on the back either according to the type of "coated-film" - or the thin-film electrode contacted and be connected to the external signal electronics.

literature

(1) P.Berveld
DEVELOPMENT AND APPLICATION OF CHEMICAL SENSORS IN LIQUIDS in: Sensors and Sensory Systems for Advanced Robots, NATO ASI Series, Vol. F43, Edited by P. Dario, Springer-Verlag Berlin Heidelberg 1988
(2) US Patent No 42 14 968, Jul.29,1980
( 3 ) K. Cammann
BIO-SENSORS BASED ON ION-SELECTIVE ELECTRODES Fresenius Z. anal.Chem. 287, 1-9 (1977)

Criticism of the state of the art

In the current state of the art, it is disadvantageous that for ion-selective electrodes either complicated layer fol conditions or thermodynamic non-equilibrium states in Purchase must be made.

Furthermore, the use of precious metal layers (e.g. Sil ber) especially in the production of "disposable sensors" unfavorable for economic reasons.

With multi-sensor elements, it is also necessary to a large number of external contacts are available deliver.

task

The invention is based, the multisensorele the task elements of chemical and bio sensors as a minimal configuration for single use without solid electrolyte and precious metal layers so that the sensor signals m.H. one  Non-contact measuring equipment d. H. without galvanic contact tion can be determined.

The embodiment of the invention is also intended to be a automatic handling of the multi-sensor elements as well as the Un testing from small amounts of liquid down to Allow 0.1 ml.

solution

This object is achieved in that the Multi-sensor element of a chemical or bio sensor from one electrically non-conductive carrier material and one or several different ion selective on it There is membranes, optionally with a biological active material are coated, and the potentials at the membrane-substrate interface or the electrolyte-substrate interface m. H. one of externally attached time-varying capacity be determined that one or more moveable capacities Electrodes by a predetermined distance in front of the sensor elements swinging, or one or more electrodes forming the capacitors e.g. B. m. H. a rotating disc on the sensor membranes be led past that the effective area of the Kapa change with time.

To determine the membrane potential, the electrical Po potential on the movable electrode or on the multi-sensor element ment m.H. adjusted a counter voltage so that the pots tial difference between the effective electrodes of the Capacity and thus that which occurs in the capacity supply lines alternating current is minimal.

Here is the potential of the movable capacitance electrode as large as the membrane potential or interface potential, that this is opposite. The adjustable Ge voltage can be used as a sensor output signal.

Is the time-varying capacity m. H. one on one realized rotating disc counter electrode, in this way several interface potentials can be created sampled at different times and the counter voltage with one keyed regulation.  

The signals from the individual sensor elements of the multisensor can for z. B. be compared in pairs.

So that the potential of the electrolyte during the measurement remains constant over time, becomes one that cannot change over time Capacity arranged opposite the electrolyte. Is the value this capacity is large compared to the values of the variable Capacities, the potential of the electrolyte remains approximately constant.

Alternatively, the electrolyte m. H. a pseudo reference electrode contacted in the form of an electrically conductive layer and thus its electrical potential during the Measurement can be kept constant.

Achievable advantages

The advantages achieved with the invention are in particular in that the multisensor element only consists of the carrier material as well as the ion-selective membranes. A such a minimal configuration results in minimal manufacturing costs, which is of paramount importance for disposable sensors of this type are.

Since the ion-selective membranes on the back are none have galvanic contacts, there are also no through thermodynamic non-equilibrium states caused instabilities.

In addition, because of the capacitive, d. H. non-contact determination of sensor potentials on a variety of electrical connection contacts to be dispensed with.

The design of the multi-sensor elements allows automatic Handling as well as examining even small ones Amounts of liquid down to 0.1 ml.

Embodiments

The Fig. 5a shows a multi-sensor element in the plan view and in cross-section (Fig. 5b). The multisensor element consists of an electrically non-conductive carrier material (T) on which several ion-selective membranes (N) are attached. The electrolyte or the measurement solution (E) is applied to the multi-sensor element from above. The depressions on the carrier (T) serve to realize larger capacities for these areas compared to a movable electrode on the back, which is attached later. This is particularly necessary when measuring with a rotating disk in order to bring the areas closer to the movable electrode whose interface potentials are to be determined.

On the other hand, it is also possible according to FIG. 6, this Be rich m. H. to delimit an electrical rear shield (AS). In this figure, the multisensor element again consists of a carrier (T) and several ion-selective membranes (M).

However, with several vibrating electrodes under the Membranes worked, the shield (AS) can also ent fall.

In the case of differential measurements, the potentials on the backs of different membranes can be compared. It is also possible to make differential measurements m. (B Fig 5, Fig. 6) perform H. a sensor element (SE) and a rudimentary sensor element (RSE) without ion-selective membrane.

To measure, ie to determine the sensor potentials, the multisensor element (MSE) is placed on the measuring apparatus (MA) ( FIG. 7). The measuring solution (E) covers the multisensor element. Under the multisensor element there are one or more counter electrodes in the measuring apparatus, which form the variable capacitances.

It is also possible to rotate the arrangement according to FIG. 7 about a horizontal axis by 180 °, so that the multisensor element is brought onto the measurement solution from above.

In FIG. 8, a multi-sensor element with a plurality ionenselek tive membranes (M), a rudimentary sensor element (RSE) as well as a simplified measuring arrangement is shown. Movable electrodes (BE 1 , BE 2 , BE 3 ...) Are arranged under the membranes, the distances to the multisensor element are changed periodically. These movable counter electrodes form the variable capacitances (C _s1 , C _s2 , C _s3... ) On the back of the membrane.

If the interfacial potential at the membrane trode back a different value than the potential of the movable Gegenelek so located on the capacitance C _s, an electric voltage U _s. When the distance d between the counter electrode and the sensor element changes, the capacitance C _{s changes} and a time-varying current i flows:

i = dQ / dt = (dC _s / dt) U _s

with C _s = ε _o ε _r A / d and d = d₁ ± Δd

Q = electrical charge
t = time
ε _o = dielectric constant of empty space
ε _r = relative dielectric constant
A = area of capacity
d = distance of the electrodes of the capacitance

With the help of a counter voltage Uk, the potential difference U _s and thus the alternating current i can be regulated to the value zero. The voltage Uk now represents the sensor signal, which can be compared with the signal of another sensor element in the form of a difference measurement: U _diff = U _s1 - U _s2 The difference measurement can also be m. H. a rudimentary sensor element (RSE) without an ion-selective membrane. The interface potential at the phase boundary between electrolytes (E) and carrier material (T) is detected.

When determining the individual sensor potentials it is of Meaning that the electrical potential of the electroly not changed significantly.

In the arrangement according to FIG. 8 this is achieved in that the electrical potential of the electrolyte m. H. the capacity C _{g is kept} almost constant. For large capacitance values (C _g »C _s1 + C _s2 + C _s3 ...) this is also guaranteed if changing potentials occur at points S ₁ , S ₂ , S _{3 ... of} the circuit. The capacitance C _g is formed between the electrolyte (E) and a fixed counter electrode (GE) in the area of the tub (W). However, it is also possible to use the electrical potential of the electrolyte m. H. to keep a pseudo-reference electrode (PRE) in the form of an electrically conductive layer constant ( Fig. 9).

For measurements on multi-sensor elements according to Fig. 8 and Fig. 9, it is necessary to arrange a vibrating electrode (BE) under each sensor membrane.

The vibrating electrodes can e.g. B. m. H. an electro magnetic or piezoelectric system driven will.

In addition, the multi-sensor element can also face a vibrating counter electrode are spatially shifted.

The individual sensor potentials are successively sums up.

In the exemplary embodiment according to FIG. 10, the variable capacitance is realized in that a movable electrode (BE) is moved laterally under the sensor membranes. The change in capacitance ΔC _s is achieved here by a change in area ΔA, where A indicates the size of the area with which the movable electrode (BE) and the respective membrane are spatially superimposed.

If the movable electrode (BE) is connected to the ver passed different membranes M, so the pots tiale recorded on the back of the membrane sequentially.

With the help of the counter electrode (GE), a time-constant capacitance C _g compared to the electrolyte is formed in the region of the trough (W) so that its electrical potential remains approximately constant.

FIG. 11 shows the time profiles of the capacitance C _s ( FIG. 11b) and the current i ( FIG. 11c) in the event that the movable electrode (BE) is guided past a membrane (M). The current amplitude depends on the size of the potential difference in the capacitance C _s . If the counter voltage has been adjusted, the following applies: i → O

The multi-sensor element (MSE) from FIG. 10 is shown in plan view in FIG. 12a. Below this, a rotating disc (RSC) with one or more movable electrodes (BE) is arranged so that the movable electrode is passed one after the other at the membranes M ₁ , M ₂ , M ₃ ..., the time offset Current signals of the individual sensor elements in the electrical lead of the movable electrode (BE) or the fixed counter electrode (GE) occur ( Fig. 12b).

The sensor membranes can also be arranged according to FIG. 13 in two or more rows and scanned with two or more movable electrodes (BE 1 , BE 2 ). In Fig. 13b, the corresponding current waveform i is over the time t is Darge.

The rotating disc (RSC) can be z. B. made of plastic or make glass and m. H. drive an electric motor. The movable electrodes (BE) can be made from a thin Me tallschicht exist, the z. B. m. H. the vapor deposition process be applied to the disc. It is also possible screwing solid metal electrodes into the disc pour in or stick on.

The immovable counter electrode (GE) can be firmly attached to the holder tion that the multi-sensor element (MSE) in the measuring apparatus (MA).

Fig. 14 shows the simplified block diagram for the signal electronics. The successive current pulses according to FIG. 12b or FIG. 13b are determined for each sensor element and the counter voltage is pulsed m. H. the keyed control set so that the output voltage pulses u _s reflect the height of the individual sensor potentials.

Occur on a disc according to Fig. 12a and 13a having a diameter of about 10 cm rotation speeds of 20, 000 revolutions per minute, so can at ionenselekti ven membranes with a surface area of 0.1 cm ² sensor potential in the size of 1 mV be resolved.

The sensor elements are calibrated according to the known ones Procedure m. H. of test solutions with known ion concentration possible.  

The evaluation of the output signal can then z. B. with With the help of a computer (PC).

Ion-selective multisensor elements can be different ways are made.

Multi-sensor elements according to Fig. 5 can thereby generate that in the corresponding grooves of the carrier material (T) for a liquid membrane solution. B. m. H. a dosing system is brought. After the solvent has evaporated, the sensor membranes form in the wells.

Plastic (Teflon, polycarbonate, PVC or others) but also glass or ceramics the.

The known liquid membrane ma can be used for the membranes use materials. Ion selective polyvinyl chloride (PVC) For example, membranes can have the following composition: 33% by weight PVC, 66% by weight plasticizer and 1% by weight Ionophore.

The membranes are used for the production of bio-sensors according to the known methods in addition z. B. with enzymes or Microorganisms documented.

If prefabricated membranes are to be used, then the membranes into the recesses of the carrier material (T) be glued in.

Referring to FIG. 15 of the carrier (T) can also consist of a lower part (TU) and an upper part (TO) are made.

Upper and lower parts can again be called from the above Manufacture materials.

On the lower beam (TU) z. B. prefabricated rivers sigmembranen are placed, which then m. H. of the glued upper part (TO) are attached to the membrane edge.

It is also possible to use solid-state membranes e.g. B. from SiO ₂ , Si ₃ N ₄ or Ti _x O _y on the lower support part. For this, e.g. B. the following known methods of solid state technology can be used:

Evaporation in a vacuum, CVD process, spin-on process, sieve print.  

The configuration of the multi-sensor elements according to Fig. 5 Fig. 9 Fig. 10 and Fig. 15 prevented in a particularly advantageous manner the contact between the electrolyte (E) and the Membranrän countries or membrane rear.

The multi-sensor elements (MSE) can according to Fig. 16 also has a rim (R) own which prevents the measuring liquid (E) at the lateral drainage. In addition, the multisensor elements can also be provided with a cover (AD) ( Fig. 17).

It is also shown in FIG. 18 possible to mount the counter electrode (GE) on the cover (AD).

Likewise, the entire arrangement around a horizontal axis can be rotated by 180 °.

Claims (19)

1. Device of a chemical and bio-sensor system for determining ion concentrations in liquids from a multi-sensor element (MSE) and a measuring apparatus and method for contactless determination of the individual sensor potentials, characterized in that the multi-sensor element (MSE) from an electrically non-conductive carrier (T ) and one or more different ion-selective membranes (M) located there, which are optionally coated with a biologically active material, and the potentials at the membrane-substrate interfaces or the electrolyte-substrate interface m. H. one or more externally attached, time-varying capacitances C _s are determined by one or more movable electrodes (BE) forming the capacitances C _s vibrating a predetermined distance in front of the sensor elements, or one or more movable capacitances C _s forming Electrodes (BE) z. B. m. H. a rotating disk is guided past the sensor membranes (M) in such a way that the effective area (A) of the respectively effective capacitance C _s changes over time, and the electrical potential at the movable electrode (BE) for determining the individual membrane potentials or on the counter electrode (GE) m. H. a counter voltage U _k is readjusted so that the potential difference between the respectively effective electrodes of the capacitance C _s and thus the alternating current i occurring in the capacitance leads becomes minimal. 2. Device and method according to claim 1, characterized in that m. H. one with or several electrodes (BE) provided rotating disc (RSC) multiple membrane potentials or Interface potentials are offset in time be scanned that the movable electrode (BE)  successively on the membranes or senso-elements is led past. 3. Device and method according to claim 1, characterized in that the potentials of each two sensor membranes (M) and one sensor membrane (M) and a rudimentary sensor element (RSE) for difference measurements are compared in pairs. 4. The device and method according to claim 1, characterized in that to ensure a constant electrical potential in the electrolyte (E) with the help of a counter electrode (GE) a time-constant capacitance C _g the electrolyte (E) is arranged opposite, the capacity of which is large in Comparison to the capacitance values C _{s of} the movable electrodes (BE), and the electrical potential of the counter electrode (GE) is defined by the electrical circuit. 5. The device and method according to claim 1, characterized in that to ensure a constant electrical potential the electrolyte (E) m. H. a pseudo reference electrode (PRE) in the form of a electrically conductive layer of platinum, silver graphite or other materials is contacted. 6. The device and method according to claim 1, characterized in that on the carrier (T) of the Multisensorelementes (MSE) wells, which the Record sensor membranes (M) and serve the Brin areas closer to the movable electrode (BE) conditions whose interface potentials are to be determined. 7. The device and method according to claim 1, characterized in that on the underside of the Carrier (T) from an electrically conductive layer (AS)  Aluminum, copper, graphite or other materials located by the electrical shielding Delimitation of the areas guaranteed Interface potentials are to be determined. 8. The device and method according to claim 1, characterized in that for the measurement the Multi sensor element (MSE) brought to the measuring apparatus (MA) will be that the mobile belonging to the measuring apparatus Electrodes (BE) for determining the individual sensor potentials can be used. 9. The device and method according to claim 1, characterized in that the distance between the movable electrode (BE) and the sensor membrane (M) m. H. an electromagnetic or piezoelectric Drive system is changed periodically, and the Vibration frequency is in the kHz range. 10. The device and method according to claim 1, characterized in that the membranes (M) of the Multi-sensor element (MSE) in one or more Rows are arranged, and the individual membrane potentials m. H. one or more rotating on one Washer (RSC) electrodes (BE) in time be sampled from each other. 11. The device and method according to claim 1, characterized in that the rotating disc (RSC) made of plastic, glass or ceramic and with the help an electric motor is driven, and the movable Electrodes (BE) made of evaporated metal layers or solid metal electrodes exist in the disc screwed, poured or glued.   12. The apparatus and method according to claim 1, characterized in that with the help of the rotating disc (RSC) and the movable electrodes (BE) on the sensor membranes caused current pulses in succession for each sensor element and the counter voltage u _k pulsewise m. H. a keyed control is set so that the output voltage u _s successively reproduces the individual sensor potentials. 13. The apparatus and method according to claim 1, characterized in that the rotating disc (RSC) has a diameter between 1 cm and 5 cm and with 1000 up to 50,000 revolutions per minute in front of sensor membranes (M) rotates, which have a diameter of 1 mm to 10 cm. 14. The apparatus and method according to claim 1, characterized in that provided in the Wells of the support (T) a liquid membrane solution e.g. B. m. H. a dosing system is introduced after Volatilization of the solvent the sensor membranes (M) forms, and that the sensor membranes (M) z. B. from one PVC material with the following composition: 33 % By weight PVC, 66% by weight plasticizer, 1% by weight Ionophore, and the membranes (M) with if necessary Enzymes or microorganisms are occupied. 15. The apparatus and method according to claim 1, characterized in that the carrier (T) made of PVC Material, polycarbonate, teflon, other plastics. Glass or ceramic is made in one piece or from an upper (TO) and a lower part (TU) becomes. 16. The apparatus and method according to claim 1, characterized in that prefabricated liquid membrane NEN applied to the support base (TU) and  then glued to the carrier top or be welded. 17. The apparatus and method according to claim 1, characterized in that solid membranes z. B. from SiO ₂ -, Si ₃ N ₄ - or silicate glass layers on the lower support part (TU) z. B. using the vapor deposition, CVD, spin-on or screen printing process and then the upper carrier part (TO) is glued or welded. 18. The apparatus and method according to claim 1, characterized in that the multisensor element (MSE) has an edge (R) on the measuring liquid (E) prevents lateral drainage. 19. The apparatus and method according to claim 1, characterized in that the multisensor element (MSE) has a cover (AD) and possibly the Cover also carries the counter electrode (GE).