CA2092042A1 - Biosensor - Google Patents

Abstract

ABSTRACT
A biosensor with a selective detection system consisting of a polymer and a biochemical substance, particularly an enzyme, is characterized by a detection system produced in the following manner:
- An olefinic-unsaturated, epoxyfunctional polyether is applied to a carrier material in the form of a layer, - the polyether is cross-linked to form a large-mesh epoxyfunctional polymer matrix by means of high-energy radiation, - the layer is treated with an aqueous solution of the biochemical substance, whereby the biochemical substance is immobilized in the polymer matrix by reaction with epoxy groups, - the layer is stabilized by reaction of non-converted epoxy groups with a compound containing amino and/or carboxyl groups.

Description

20920~2 BIOSENSOR
FIELD OF THE INVENTION
The inventlon relates to biosensors with a selective detection system consisting of a polymer and a biochemical substance, particularly an enzyme.
BACKGROUND OF THE INVENTION
Biosensors are chemosensors with a biological detection system. ~his detection system consists of biologically active substances, such as enzymes, antibodies, lectins, hormone receptors, etc., which are immobilized on the surface of the sensor or in a thin layer located on it. In the detection process, a change is produced on the surface or in this layer of the sensor, by interaction with the gaseous or liquid medium to be characterized, which can be evaluated using electrical, optical, mechanical, acoustical or calorimetric measurement methods. In the case of equipment with electronic data acquisition and evaluation, the active surface or layer is directly coupled, as a signal emitter, with a signal transformer, called a transducer, which is connected with the evaluation electronics for this purpose.
The reliability of the entire sensor depends on the assignability and reproducibility of the signals generated in the sensitive layer of the biosensor. This means that the layer must demonstrate not only high selectivity~and sensitivity, but also a function that is free of hysteresis and dri~t, as well as chsmical and biological stability and contamination resistance.
For technical use, in particular, ease of operation, easy integration and the lowest possible measurement/regeneration time requirement, but also great long-term stability are reguired, while the production of the layer - according to 2~92~
methods which are efficient in terms of production technology and can be automated ~ is supposed to be as simple, reproducible and inexpensive as possible, and such that it can be integrated into the production process for sensor production.
Until now, only such biosensors which are based on ! enzymatic reactions have achieved any practical importance. In is used these reactions, the circumst ~ t products which can easily - co ns um ed./
be detected, such as H~, 2, H202, CO2 and NH3, are fo ~ ~cdt With regard to selectivity and sensitivity, the enzvmatic reactions fully meet the requirements. But a difficulty exists in immobilizing the enzymes - without loss of activity - in as thin a detection layer as possible, in such a way that they are easily accessible for the substances to be detected, and are resistant to poisoning as well as biochemical pollutants, and remain functionally stable for as long as possible.
For the immobilization of enzymes, the following methods have been known:
- adsorption on carrier surfaces - ionic binding to carrier surfaces - covalent binding to carrier surfaces - absorption in polymer layers - inclusion in a polymer lattice (matrix sheathing, microencapsulation) - inclusion by sheathing with a membrane (~acroencapsulation) - cross-linking or copolymerization with difunctional or polyfunctional monomers.
However, as is evident from the extensive literature on the immobilization of enzymes, all of these methods have disadvantages, which make them appear unattractive for industrial sensor production (see, for example: W. Hartmeier, 20920~ ~

"Immobilisierte Biokatalysatoren" ["Immobilized Biocatalysts"], Springer-Verlag Berlin, Heidelberg 1986, pages 23 to 51, as well as J~ Woodward, "Immobilised cells and enzymes", IRL Press, Oxford, Washlngton DC, 1985, pages 3 to 54).
Thus, adsorption and ionic binding of enzymes at the surface results in relatively unstable systems with a limited range of use: Changes in the pH and the ion intensity of solutions in contact with it, or the presence of other substances, already result in displacement of the surface-bound enzyme and thus to activity losses of the detection system.
Also in the case of absorption in polymer layers, with plasticized polyvinyl chloride being used in the predominant number of cases (see, for example: "Sensors and Actuators", Vol. 18 (1989), pages 329 to 336, and "Ber. Bunsenges. Phys.
Chem." ["Reports of the Bunsen Society for Physical Chemistry"], Vol. 92 (1988), pages 1423 to 1426), relatively unstable systems are obtained: migration and extraction of the enzvmes result in a constant decrease in activity (drift) and limit the lifetime of the sensor.
Significantly more stable systems are achieved if the enzymes are covalently bound to a carrier surface, made insoluble via cross-linking or copolymerization, or are immobilized by microencapsulation or macroencapsulation. For the formation of covalent bonds and for cross-linking, free amino, carboxyl, hydroxyl and mercapto groups are available on the part of the enzymes. Both inorganic materials, such as glass, and natural and synthetic organic polymers can-be used as the carrier material. A prerequisite in this connection is that the carrier materials contain reactive groups, such as isocyanate, isothiocyanate, acid chloride and epoxy groups.

20~2~2 j Less reactive groups can be activated, for example carboxyl groups can be activated using the carbodiimide or azide method, hydroxyl groups can be activated using the bromine cyan method, and amino groups can be activated using the isothiocyanate or azo method. It was possible, particularly on the basis of acrylic acid and methacrylic acid derivatives, to produce ! copolymers I numerous reactivelcopoli~cri~atc3lwith dinitrofluorophenyl, isothiocyanate, p~lranlor acid anhydride groups.

Polyacrylamides with ~irunlgroups as well as modified copolymers ~copolymeri~tc~ on the basis of vinyl acetate and divinyl oxirane ethylene urea with~oxir~n-lgroups are commercially available, for example.
Immobilization by cross-linking or by copolymeriza~ion represent special forms of covalent binding. In these methods, the formation of covalent bonds takes place between the enzyme molecules and difunctional or polyfunctional monomers, such as glutardialdehyde, or, in the case of copolymerization, additionally between the enzyme molecules and a polymerizing I substance. In this manner, insoluble aggregates with a high molecular weight are formed. Cross-linking is generally used as an immobilization method in combination with one of the other methods, for example in combination with adsorption or absorption. Here, the enzyme molecules are first adsorbed on the surface of the carrier, or are absorbed in a layer located 1 on it, and subsequently cross-linked.
¦ A significant disadvantage of immobilization by covalent binding is the great s~ress on the biocatalysts connected with it. The immobilization procedures that are necessary, some of which are rough, in which a strong change in reaction the pH occurs, organic solvents have to be used orIconvcroionl \~ i 20~20~2 with reactive substances with a low molecular weight takes place, almost always lead to strong conformation changes and thus to activity losses of enzymes bound in such manner.
~ n immobilization by inclusion, i.e., micro-encapsulation or macroencapsulation, the enzymes themselves are not made insoluble, rather their reaction range is limited by semiDermeable ~cmlpermanen~ polymers or polymer layers. A prerequisite for the ability of enzymes sheathed in this manner to function is that substrates and products can pass through the sheathing substance, while the enzymes themselves have to be held back.
In addition to natural polvmers, such as alginate, carrageenan, gelatin pectin, agar andlg~atinEI, which are, however, too large-meshed for permanent immobilization of enzymes, synthetic polymers, such as polyacrylamide, are particularly used for matrix sheathing. Polyamides, polyurethanes, polyesters and polyureas, for example, are used for encapsulation. The inclusion method has the disadvantage that relatively thick layers with long sensor response times are formed.
In the methods described, immobilization of the enzymes i5 carried out by hand in most cases, which is relatively slow, expensive and not v~ry reproducible, and is contrary to integration into modern production processes. In view of the advantages which enzyme sensors on an FET basis (ENFETs) would be able to offer, attempts have been made in recent years to include enzyme immobilization ~ ar technology in the production of integrated circuits. Thus, for exa~ple, the production and direct photo-structuring of layers based on polyvinyl alcohol which contain enzymes and can be photo-cross-linked has been described ~"Proc. 3rd Int. Conf.
Solid State Sensors and Actuators (Transducers '85)", June !

2~2 11-14, 1985, pa~es 148 to 151). For the purpose stated, it is also known to use photosensitive polyvinyl pyrrolidone ("IEEE
Trans. Electron Devices", Vol. ED-36 (1989), pages 1303 to 1310). According to this method~ structures which exactly cover the gates of the FETs can be produced on wafers. However, this method has the great disadvantage that the enzymes are at least partially inactivated during W irradiation.
inactivation It is also known to utilize enzyme ~6tivatio~ by means of UV radiation, in that first a layer of acetyl cellulose an e nz yme containing ~ ~e~ is produced, the enzyme is cross-linked with glutardialdehyde in this layer, and subsequently it is irradiated through a mask in such a way that the gate coverings are shaded and therefore remain active, while the remaining areas are inactivated ("Chemical Economy & Engineering Review", Vol. 17 (1985), No. 7-8, pages 22 to 27). The inactivated layer remains on the sensor, which proves to be a disadvantage for further insulation and packaging of the sensor required for its use.
The lift-off technique has also been described ("Sensors and Actuators", Vol. 13 (1988), pages 165 to 172). In this method, a photoresist is structured in such a way that only the gate surfaces remain free. The enzyme is then applied to this, together with glutardialdehyde, and cross-linked; the photo varnish is removed with acetone and ultrasound, using the lift-off technique. Here again, it is impossible to avoid at least partial denaturing of the enzyme.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a biosensor with a selective detection system (composed of a polymer and a biochemical substance), which can be produced in 20920~.~
technically simple, efficient and low-cost manner, where the production method is such that it can be integrated into modern production systems, and yields detection systems with stable function, if necessary also miniaturized and integrated, with uniform quality and long life expectancy, in a reproducible manner.
This is accomplished, according to the invention, by applying an olefinic-unsaturated, epoxyfunctional polyether to a carrier material in the form of a layer. The polyether is cross-linked to form a large-mesh epoxyfunctional polymer matrix by means of high-energy radiation. The layer is treated with an aqueous solution of the biochemical substance, whereby the biochemical substance is immobilized in the polymer matrix by reaction with epoxy groups. The layer is then stabilized by reaction of non-converted epoxy groups with a compound containing amino and/or carboxyl groups.
DETAILED DESCRIPTION OF THE INVENTION
The invention utilizes a new type of immobilization of enzymes and other biochemical substances with selective detection properties, specifically in layers of radiation-cross-linked epoxyfunctional polyethers. It was surprisingly found that these substances are a~le to penetrate into large-mesh cross-linked epoxyfunctional polyethers - from queous solution - and can be anchored in the polymer matrix, network i.e., in the polymerl~a~e~, under very mild conditions, by t~L~ i reaction with epoxy groups in chain posltlon. This~new, and it allows for ccovcry opcnc u~ the possibility of carrying out the production, structuring and cross-linking of the layers before immobilization of the biochemical substances, and thus of 2~920~2 avoiding damage to the substances, most of which are very sensitive, by the processes mentioned.
ThP production of the cletection system of the biosensor according to the invention includes the following steps, in general:
1. Layer preparation An epoxyfunctional polyether which can be cross-linked by radiation, or a mixture of such polyethers is applied, in the desired layer thickness, to a carrier material, if necessary in combination with a cross-linking initiator, a cross-linking reinforcer and/or other additives. Depending on the application case and the carrier material, this can be done out of a solution or without solvent, by dipping, spin-coating, conventional roller-coating, curtain-coating or anotherjordinaryrprocess, where it might be necessary to pretreat the carrier surface with an adhesion agent. The layer thickness can be controlled by adjusting the viscosity and by adding a solvent or a reactive diluent. The layer produced in this manner must be freed of volatile components, in every case, which can be done by drying or degassing, for example.
2. Cross-linking of the layer Cross-linking of the layer, i.e., the polyether, takes place by means of high-energy radiation, particularly W, electron or ~ radiation. In this connection, only the olefinic-unsaturated groups that can be polymerized by radicals are converted, while the epoxy groups are quantitatively maintained. As a result of the cross-linking, a large-mesh network polymer ~ttic~ is formed. The layer can also be structured if projection exposure or irradiation through a mask and sub~equent dissolution of the non-cross-linked regions is carried out.

20920~

3. Immobilization of the biochemical substance Upon contacting of the cross-linked layer with an aqueous solution of the biochemical substance, this substance migrates into the polymer matrix and is covalently bound there by reaction with the epoxy groups. A prerequisite for this process, along with the necessary mesh width, is sufficient hydrophilicity network lhydrophilia~of the polymer ~ticq formed during cross-linking.
Immobilization can therefore be accelerated by prior hydrophilization of the polyether. This is done by conversion of part of the epoxy groups with hydrophilic compounds which contain reactive groups, such as NH, OH, SH or COOH groups, causing the hydrophilic character of the polymer layer to be increased. The immobilization process can also be significantly accelerated by means of additives, such as polyvinyl pyrrolidone, which result in increased water absorption of the polyethers, as well as by solvents which are miscible with water, such as dioxane, tetrahydrofuran, alcohols or polyethers.
Furthermore, several different biochemical substances can also be immobilized in a single layer, and this can be done either simultaneously or consecutively.

4. Stabilization of the layer includes ¦I This step dnvolvoEithe reaction of ~q epoxy groups ~¦ remaining after immobilization, with a compound containing amino I and/or carboxyl groups, particularly an amino acid. Depending ¦ on the compound used, stabilization can be utilized to achieve I closer cross-linking of the layer, and thus improved mechanical j strength, or for adaptation of the material properties and the material transport. Furthermore, a superficial covering of the sensor layer with one or more additional layers is possible, which might also be practical for adjusting defined diffusion conditions~

g 2~9~0~2 For the biosensor according to the invention, epoxyfunctional polyethers with the following structure are particularly suitable; these are the subject of copending U.S.
patent application Ser. No. ... ... - "Polyethers", which was filed on the same day as this application:

CH2~CH-Rl-CH-CH2-0-R2-0-CH2-CH-Rl-(`jH-CH2 where the following applies:

z = CH2=C-Co-o-R3-NH-Co-, CH2=C-CO-, ~ -CH=CH-CO- or ~ N-R3-Co-, where R3 = -(CH2)m-, with m = 1 to 10 R4 = H or CH3;

R1 = -(CH2)o~~ with o = O to 18, -CH2-o-R5 -0-CH2-, ` where R5 = -(CH2)p-, 'I~(CH2)q~0~(CH2)q~ ~ ~CH2-1CH-O~CH2_;CH_ ~¦CH3 CH3 _(CH2)q ~ ~(CH2)q ~ -Ar-O ~(cH2jq-o ~ (CH2)q~~

I-CH2- ,CH~EO-CH2- ~CH3~0-Ar-O~CH-CH2-O~ICH-CH2-, ~CH3 CH3 CH3 !¦ 10 28920l~
with p = 2 to 20~ q = 2 to 4, r = 1 to 50, s = 0 to 50, t = 0 to 25, Ar = ~ , ~CH2 ~3 or ~}CH~

l CH3 ~ C~ H3 (CH2)3 Si- o - Si-(CH2)3~~ with u = 0 to 150, CH3 u CH3 or the corresponding grouping from 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate, i.e., the compound:

~C ~

R2 = -(CH2-CH=CH-CH2)n- , -R6- , -R6-1)-CO-R7-CO-O-R6- o~-. I -CH3 CH3 -(CH2)3 -Si--o. Sl (CH2)3-CH3 u CH3 where n = 1 to 50, u = 0 to 150, R6 has the same meaning as R5, .1 2~2~2 except ~ ~nd ~ , and R7 has the following meaning:

(CH2)v-~ ~(cH2)q-l-o ~(cH2)q-o ~ (cH2)q 1-~(CH2)q ~ (CH2)q3~0-Ar-O~(CH2)q-O~(CH2)q 1-~
with q = 2 to 4, s = 0 to 50, t = o to 25, v = O to 20, and Ar has the meaning indicated above.

The invention offers the following advantages:
- Immobilization of all biochemical substances which have reactive NH, OH, SH or COOH groups at their periphery is made possible.
- The layers which have the immobilized biochemical substances can also be stored dry and under non-sterile conditions, without any damage to these substances.
- Immobilization of the biochemical substances takes place under very mild conditions, in aqueous solution and in the absence of reactive components with a low molecular weight;
in this way losses, for example as the result of enzyme denaturing, are avoided.
- A relatively small number of polymer materials with great chemical and thermal stability, which can be produced on a large technical scale and which are therefore accessible at large low cost, is used for immobilization of a¦largeEInumber of different types of biochemical substances and for different sensor types.
~ I
, - The production and cross-linking of the layers, as well as I their structuring, if necessary, can be carried out according to planar technolo~y, i.e in technically simple, ~2 2~2~

reproducible and low-cost manner, and so as to be integrated into the sensor production.
Immobilization of the biochemical substances can take place independent of the layer production, depending on the need and intended use, if necessary not until just before llse, to be carried out by the user.
Desorption, migration and extraction losses are avoided by chemical anchoring of the biochemical substances in the polymer matrix.
By the formation of covalent bonds between the peripheral NH, OH, SH and COO~ groups of the biochemical substances and the very soft and flexible sheathing polymer material, the substances, some of which are very sensitive, for example enzymes, are given great functional and long-term stability.
Because of the possibility of the production of very thin layers (<~ 1 ~m), very short sensor response times can be achieved.
Miniaturization and integration of the detection systems into microelectronic circuits, for example for the production of ISFETs and ENFETs, is without problems.
The selective detection systems are basically suitable for all sensor measurement arrangements.
The invention will now be explained in more detail in the following examples which should be regarded in an illustrative rather than a restrictive sense.

2~9~0~

Example 1 Production of Polyether/Enzyme Layers 100 parts by mass of an epoxyfunctional polyether with the structure CH2=C-C~-O- (CH2 )2-NH-C=O
O
CH-CH2-0-(CH2)2 0 2~CH-CH2 ~ -(CH2)4~7 5 0-CH2 2 glycerol are mixed with 7 parts by mass propoxylated ~lyccrinltriacrylate as the reactive diluent and 2 parts by mass 2-hydroxy-2-methyl-l-phenyl propan-l-one as the photoinitiator, and mixed with a corresponding amount of toluene to adjust the desired processing properties. This solution is then applied to the sensitive surface of a sensor, which has been pretreated with an adhesion agent, if necessary, by dipping, dripping or spreading.
Parallel to this, silicon wafers are coated with the same solution, using a varnish centrifuge; the centrifuge time is approximately 10 s.
The layers are dried in a laminar box and subsequently cross-linked under nitrogen, by W irradiation (System F 450 of the company Fusion W-Curing Systems) in a wavelength range of 200 to 450 nm, irradiation period: 4.6 s. To remove soluble components, the cross-linked layers are extracted with dioxane hydrophilicity for 24 h, at room temperature. To increase the ~ of 2~9~0ll2 reacted the layers, part of the epoxy groups is ~onvcrtc~ with compounds containing NH groups, in the form of amino acids. In this connection, storage of the layers in a 2~ solution of proline or glutaminic acid in a 2:1 mixture of dioxane and water at 40 to particularly, 60C ~ o belpartioular~-yleffective. Using silicon a correspondin~
wafers treated in ~ , the conversion can be followed by IR spectroscopy. A conversion of 50% is sufficient in most cases; if needed, howevar, higher values can also be adjusted.
Immobilization of the enzymes takes place by incubation of the layers in an approximately 1 to 2% solution of the enzyme in water at 20 to 30C. To accelerate this process, the solution can be mixed with 10 to 50% dioxane, depending on the sensitivity of the enzymeA Immobilization is complete after 1 to 8 h. Remaining epoxy groups can be eliminated by gentle conversion with amino acids. As the last step, the layers are freed from extractable components by being intensively washed with water.
Table 1 contains a summary of the enzymes immobilized according to the invention, in identically pretreated layers with a thickness of 10 ~m, on silicon wafers, immobilized at 30C within 8 h, as well as the enzyme activity at 25C.

Enzyme Activlty Determination Method Glucose oxidase from 0.8 U/cm2 Gluc-DH Method of the Aspergillus niger, Merck company lyophil.
240 U/mg 2~920~2 Catalase from cattle 350 U/cm2 See: B. Stellmach~
liver, suspension "Bestimmungsmethoden 65,000 U/mg Enzyme", Steinkopff-Verlag, Darmstadt lg88, pages 152 to 155 Urease from broad 0.7 U/cm See: B. Stellma~h, beans, lyophil. "Bestimmungsmethoden 100 U/mg Enzyme", Steinkopff-Verlag, Darmstadt 1988, pages 269 to 271 Alcohol dehydrogenase 2.0 U/cm2 See: B. Stellmach, from yeast, lyophil. "Bestimmungsmethoden 400 U/mg Enzyme", Steinkopff-Verlag, Darmstadt 1988, pages 11 and 12 L-asparaginase, 0.6 U/cm2 See: B. Stellmach, 50% solution in ~'Bestimmungsmethoden glycerQl ~lycer1~ Enzyme", Steinkopff-80 U/mg solution Verlag, Darmstadt 1988, pages 63 to 68 The publication "Bestimmungsmethoden Enzyme" means "Determination Methods for Enzymes".

209~0~2 Example 2 Evaluation of the Functional Stability of the Immobilized Enzymes To evaluate the functional stability of enzymes immobilized according to the invention (duration: 8 h), the activities of the layers with a thickness of 10 ~m, produced wafers according to Example 1 on siliconlla~er~, was measured at 25C
over a period of several weeks (see Table 1 in this regard).
The activity of glucose oxidase was followed for 70 days, without any reduction in the initial value being found.
Parallel to this, the activity decrease of an aqueous glucose oxidase solution was determined at 20C, according to the determination method indicated in Table 1. This showed an activity loss of approximately S0% within 10 days, which documents the greater stability of the glucose oxidase immobili7ed according to the invention. An evaluation of the other immobilized enzymes listed in Table 1 yields the result that the initial activity value measured was maintained for at least 8 weeks.

Example 3 Evaluation of the Functional Stability of Biosensors with Immobilized Enzymes According to the Invention Polyether/enzyme layers are produced on sensor measurement arrangements, according to the method described in Example 1, and their function and functional stability is followed by measurement of the resulting sensor signal. Table 2 contains the enzymes evaluated, as well as the measurement 2Q920~ ~

arrangement selected for the eva:Luation, and the useful lifetime.

TABI,E 2 Enzyme Sensor Measurement Arrangement Useful Lifetime Glucose oxidase oxygen sensor > 8 weeks (GOD) according to EP-OS O 470 473 GOD + catalase oxygen sensor > 8 weeks (1:1) according to EP-OS O 470 473 Urease NH~+-sensitive glass electrode > 8 weeks (company: Tecan AG) L-asparaginase NH4+-sensitive glass electrode > 8 weeks (company: Tecan AG)

Claims (5)

1. A biosensor with a selective detection system comprising a polymer and a biochemical substance wherein the detection system is produced in the following manner:
- applying an olefinic-unsaturated, epoxyfunctional polyether to a carrier material in the form of a layer, - cross-linking the polyether by means of high-energy radiation to form a wide-meshed epoxyfunctional polymer matrix, - treating the layer with an aqueous solution of the biochemical substance, whereby the biochemical substance is immobilized in the polymer matrix by reaction with epoxy groups, and - stabilizing the layer by reaction of non-converted epoxy groups with a compound containing an amino group, a carboxyl group or an amino group and a carboxyl group.

2. The biosensor according to claim 1 wherein the layer is structured.

3. The biosensor according to claim 1 wherein the cross-linked polyether is hydrophilized.

4. The biosensor according to claim 1 wherein the biochemical substance is an enzyme.

5. The biosensor according to claim 1 wherein the polyether has the following structure:

, where the following applies:

Z = ' , or , where R3 = -(CH2)m-, with m = 1 to 10 R4 = H or CH3;

R1 = -(CH2)o-, with o = 0 to 18, -CH2-O-R5-o-CH2-, where R5 = - (CH2)p-, , , [(CH2)q-o]r(CH2)q-, , , , with p = 2 to 20, q = 2 to 4, r = 1 to 50, s = 0 to 50, t = 0 to 25, , , or , , with u = 0 to 150, or the corresponding grouping from 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate;

R2 = -(CH2-CH=CH-CH2)n-, -R6-, -R6-O-CO-R7-CO-O-R6- o?

, where n = 1 to 50, u = 0 to 150, R6 has the same meaning as R5, except and , and R7 has the following meaning:

, , with q = 2 to 4, s = 0 to 50, t = 0 to 25, v = 0 to 20, and Ar has the meaning indicated above.