GB2278447A - Dielectric porosity change immunoassay - Google Patents

Abstract

A homogeneous immunoassay in which the detectable signal is produced by the effect of a reactive or catalytic species which, during the course of the reaction of the ingredients of the immunoassay, becomes immobilised at or very near to a dielectric layer covering an electrode. Said reactive or catalytic species directly or indirectly effects a reaction with the said dielectric layer wherein the dielectric layer becomes porous causing a measurable change in electrical properties at the electrode surface. The electrical property may be resistance or capacitance. The dielectric layer may be of a pH-sensitive polymer, panticularly cellulose acetate hydrogen phthalate and the species preferably causes a change in pH. The species may be urease.

Description

Dielectric Breakdown Immunoassay Background Of the Invention This invention relates to a novel homogeneous immunoassay format. Immunoassays may be divided into 'homogeneous' and 'heterogeneous' categories. In a heterogeneous immunoassay the analyte in the sample is used to bind a certain amount of a labelled species to a solid phase via antibody-antigen interaction. A wash step then separates the bound label from the free label. One or another of these label populations may then be quantitated, the amount of label measured bearing a known relationship to the concentration of analyte in the sample. In a homogeneous immunoassay no wash step is used: rather binding of analyte in the sample to its specific binding partner (antigen or antibody) causes a change in the measurable activity of the label used.The advantage of the homogeneous assay is that it is easier to perform, requiring less operations than its heterogeneous counterpart.

In operation, however, those few homogeneous formats that have been demonstrated lack sensitivity, often because the change in activity of the label is not great. For example, one assay format known commercially as EMIT form Syva relies on binding of enzyme-labelled antigen to an antibody, in competition with the analyte: binding of the enzyme changes its specific activity, but only by a factor of approximately 50 %. Thus there is a need of simple homogeneous immunoassays in which the activity of the label undergoes a quantitative change.

Background Of the Invention In the present invention, a novel homogeneous immunoassay format is described based on the impedance analysis of dielectric coatings of electrodes. The impedance of an electrode is sensitive to a number of factors. Changes in electrode impedance can usefully be measured by changes in double layer capacity. The double layer capacity (Cdl) arises from the separation at the surface of an electrode between the electronic charges in the metal and the (mobile) ionic charges in the solution in contact with the metal. For a metal (e.g. gold) in contact with a solution of aqueous potassium chloride, this separation is due to the presence of a layer of water molecules on the metal surface.This double layer capacity may be calculated from the standard formula for a parallel plate condenser, Cdl =0ear A/d where Er is the effective dielectric constant of the water layer, A is the electrode area and d is the diameter of a water molecule.

It has now been found that minute imperfections in a dielectric layer give rise to a dramatic increase in double layer capacity which can easily be measured.

Summary Of Invention The invention relies on the immunochemically modulated binding of an effector species to a dielectric coated on an electrode where the effector species is capable of affecting the dielectric properties of the electrode coating. Preferably but not exclusively, the effector species would cause the localised breakdown of the dielectric layer and this breakdown would be measured by a change in the double layer capacity at the electrode surface. However, other measures of dielectric breakdown could be envisaged, such as increasing current at a polarised electrode. The effector species is preferably an enzyme or catalyst that produces a product capable of reacting with the dielectric, or an enzyme that hydrolyses the dielectric directly.

For example, it is possible to use pH changing enzymes in connection with a dielectric that becomes soluble when the pH is changed.

One of many appropriate combinations of an enzyme with a dielectric is the combination of urease with materials such as those used in enteric coatings for tablets. Enteric coatings work by being insoluble at the low pH of the stomach, but are soluble at the pH in the intestine (pH 6 and above). One example of such a coating material is cellulose acetate hydrogen phthalate. Further examples are methyl vinyl ether-maleic anhydride copolymer esters, anionic polymerisates of methacrylic acid and esters of methacrylic acid (EudragitB of Röhm-Pharma, Darmstadt, Germany).

Urease catalyses the reaction H20 + urea --- > 2NH3 + CO2.

The resulting increase of the pH value leads to a solubilisation of said materials.

If, for example, an antibody immobilised onto said pH-sensitive materials captures an urease-labelled conjugate in an immunoassay, the urease produces a local pH change that leads to the solubilisation of the dielectric layer at the point of conjugate capture. Most importantly, it turned out that the local solubilisation even occurs in solutions where because of the buffering capacity of the bulk solution a significant pH change in the solution does not take place. Since the assays according to this invention do not require a wash step they can be characterized as homogeneous. However, as presumably the reactions do not take place entirely in solution it appears to be more correct to call them pseudo-homogeneous.

Additionally, this method can be enhanced by polarising the working electrode at a potential where reactive species can be produced when the electrode comes into contact with the electrolyte. For example, it is advantageous to polarise the electrode at 1V vs Ag/AgCl. No reaction takes place until some small amount of electrolyte comes into contact with the electrode, as the dielectric layer is just starting to break down. Oxygen reduction at the electrode surface then produces OH which increases the local pH further and accelerates the breakdown of the dielectric. This amplifies the original signal considerably.

The invention further can be reduced to practice by the use of an H202-producing enzyme such as glucose oxidase in conjunction with Fenton's reaction. This reaction is commonly used in synthetic organic chemistry to hydroxylate aromatics, and works by producing the extremely reactive radical OH.; e.g.

H2O2 + Fe2+ ---- > Fe3+ + OH + HO The hydroxyl radical is so reactive that it survives only until it encounters an organic species. In this case a dielectric coating is chosen which contains structure elements reacting with the HO radicals. The introduction of hydroxyl groups enhances the solubility of the coating giving rise to a significant increase of the double layer capacity.

Examples General mechanism Planar carbon electrodes (1 mm diameter) formed by screen printing were coated with cellulose acetate phthalate (CAP) (Kodak) as follows: 70 mg CAP was mixed with 30 mg diethyl phthalate plasticiser and added to 400 mg of acetone to form a viscous solution. 5 Fl of this solution was then placed onto the working electrode and allowed to dry in air at room temperature.

The capacitance of the coated electrodes in 150 mmol L-1 sodium chloride, pH 4, was measured using a frequency response analyser (Schlumberger) and compared with that of bare carbon electrodes.

Coated electrodes were then exposed to 150 mmol L-1 sodium chloride, pH 6.5, for 15 minutes and the capacitance remeasured.

Electrode Conditions: Capacitance: Bare electrode capacitance 24.6 pF cam~2 Coated electrode capacitance, pH 4 0.002 pF cam~2 Coated electrode after 15 minutes at pH 6.5 0.331 AF cm'2 Exposure of the coated electrode to a solution at pH 6.5 causes a 160 fold increase in the capacitance of the electrode.

Gold rod electrodes (4 mm diameter) were coated with cellulose acetate phthalate (CAP) (Kodak) as follows: The electrode was dip coated once using a 1:8 w/w CAP / 30 % diethyl phthalate plasticiser to acetone mixture and allowed to dry for 30 minutes at room temperature.

The capacitance of the coated electrodes in 140 mmol L-1 sodium chloride, pH 4, was measured using a frequency response analyser (Schlumberger) in a three electrode configuration using a silver/silver chloride reference and gold counter electrode.

The capacitance of the coated electrode was measured after 5 minutes and 1 hour. The pH 4 solution was removed from the cell and replaced with pH 6.5, 140 mmol L-1 sodium chloride solution and the capacitance measured at 30 minute intervals.

Electrode conditions: Capacitance (EF cm-2): Uncoated electrode 12.2 Coated electrode, pH 4, 5 minutes 0.002 Coated electrode, pH 4, 1 hour 0.002 Coated electrode, pH 6.5, 30 minutes 1.176 Coated electrode, pH 6,5, 1 hour 1.584 Coated electrode, pH 6.5, 1.5 hours 1.944 Coated electrode, pH 6.5, 2 hours 2.224 These results are represented graphically in Figure 1.

The pKa values of enteric coatings, CAP and Eudragit S100 polymers, have been calculated by titration of the solid in aqueous solution against dilute base. As both polymers' pH sensitivity is a direct result of the deprotonation of acid ester groups, the pKa value corresponds to the breakdown pH of the polymer.

The pKa of CAP is approximately 6.0, and the pKa of Eudragit S100 approximately 7.0.

Breakdown of polymer films using urease CAP polymer breakdown A gold rod electrode (4 mm diameter) was coated with CAP as previously described and inserted into a glass cell. The buffer solution (1 mL) was EDTA (0.5 mM), sodium chloride (140 mM), urea (16 mM) adjusted to pH 4.8 using hydrochloric acid (0.1 M). Capacitance values were obtained at pH 4.8 after 30 and 60 minutes and then urease (20 L of a 1 mg mL-l solution) was added and the capacitance measured at 90, 120 and 150 minutes.

Time (minx Capacitance (F cm'21; 30 0.0004 60 (urease added) 0.0004 90 0.0133 120 5.048 150 6.544 These results are represented graphically in Figure 2 and confirm that it is possible to breakdown the CAP polymer film by the use of the enzyme urease causing a change in pH and resulting a a 4 orders of magnitude change in electrode capacitance.

Eudragit S100 polymer breakdown The gold rod electrode (4 mm diameter) was dip coated twice using 20 % w/w Eudragit S100 (Röhm-Pharma) in acetone containing 20 % w/w dibutyl phthalate plasticiser and 5 % w/w Tween 80 and then placed in an atmosphere saturated with acetone for 15 minutes. The electrode was then removed and allowed to dry for 30 minutes at room temperature.

The glass cell was used containing 1 mL of buffer solution (described above) and capacitance values were obtained after 30 and 60 minutes. Urease (20 L of a 1 mg mL'1 solution) was added and the capacitance measured at 75 and 90 minutes.

Time (mien) : Capacitance (F cm~2): 30 0.0005 60 (urease added) 0.0006 75 1.488 90 9.760 The results are represented in Figure 3 and show that the film is stable in the buffer solution after 1 hour. 45 minutes after the addition of urease the electrode capacitance had returned to its uncoated value due to total breakdown of the polymer film.

Immunoassay protocols resulting in binding of an enzyme such as urease to the dielectric layer turn out to represent a highly sensitive method when used in conjunction with capacitance measurements.

The electrochemical assay according to this invention can be used in many different formats known to the man skilled in the art.

So it is possible to immobilise in any manner known to the man skilled in the art an antibody at the electrode surface for which the analyte to be measured and an analyte enzyme conjugate or an analyte analogue enzyme conjugate compete.

For a sandwich format a first antibody against the analyte to be measured is immobilised at the electrode surface and a second antibody labelled with an enzyme is present in the solution.

In a competition format the solution contains a biotin labelled analyte or a biotin labelled analogue of the analyte. An enzyme conjugate with avidin is also present in the solution or may be added after the capture reaction of the biotin labelled analyte or analyte analogue and the analyte to be measured with the immobilised antibody. Other binding pairs in place of avidin/biotin, e.g. IgG-aIgG may be used equally.

It is also possible to use a competitive assay where an analyte or an analogue of the analyte is conjugated with an anti-enzyme antibody. Also present in solution is the analyte to be measured and free enzyme, where the signal generated is inversely proportional to the analyte concentration being measured.

In a further competitive format, the competition between the analyte and an enzyme conjugate of the analyte or an analogue of the analyte with an enzyme can be performed in a wick (bibulous layer) or a capillary channel in which an antibody against the analyte is immobilised on the surface. After having passed the wick or capillary channel, the unbound enzyme conjugate of the analyte comes into contact with the electrode where an antienzyme antibody is immobilised. The signal generated is proportional to the concentration of analyte present.

Claims (12)

Claims

1. A method for determining the presence of an analyte in a sample suspected of containing said analyte, said analyte being a member of a specific binding pair consisting of ligand and anti-ligand, wherein the amount of a detectable signal is a function of the amount of analyte in the assay medium, said detectable signal being detected at an electrode, where a) the electrode is covered with a thin layer of an electrically resistive material that separates the electrode from an electrolyte; b) the detectable signal is produced by the effect of a reactive or catalytic species that during the course of the reaction becomes immobilised at or very near to the dielectric electrode covering layer;; c) the said reactive or catalytic species directly or indirectly effects a reaction with the said dielectric layer wherein the dielectric layer becomes porous or more porous to the electrolyte, causing a measurable change in electrical properties at the electrode surface.

2. A method according to claim 1 where the electrically resistive material is a pH-sensitive polymer and where the reactive or catalytic species causes a change in pH sufficient to increase the permeability of the electrode covering layer.

3. A method to claim 1 in which the polymer is cellulose acetate hydrogen phthalate.

4. A method according to claim 2 in which the reactive or catalytic species is urease.

5. An immunoassay method in which a urease-antibody or urease-antigen conjugate becomes bound at or very near a layer of cellulose acetate phthalate via a specific immunochemical reaction where the layer of cellulose acetate phthalate covers an electrode and where the urease activity of the conjugate leads to an increased permeability of the cellulose acetate phthalate layer to electrolyte, measured by a change in capacitance or current passed by the underlying electrode.

6. A method according to claim 1 in which the electrode is polarised at a potential at which a redox reaction can occur with a constituent of the electrolyte such that breakdown of the dielectric layer becomes accelerated once the electrode first becomes exposed to the electrolyte.

7. A method according to claim 1 where the activity of the catalytic species results in the formation of a hydroxyl radical, which is able to react with the dielectric layer resulting in an increase in the permeability to electrolyte of said layer.

8. A method according to claim 7 where the catalytic species is a hydrogen peroxide-producing oxidase and the electrolyte contains species able to promote Fenton's reaction.

9. A method according to claim 1 in which the catalytic species is a polymer-degrading enzyme which acts directly on the dielectric layer to increase the permeability of the layer.

10. A method according to claim 9 in which the catalytic species is amylase or amyloglucosidase.

11. A method according to claim 9 in which the catalytic species is an endonuclease and the dielectric comprises or contains a nucleic acid.

12. A method according to claim 9 in which the catalytic species is a lipase and the dielectric comprises or contains lipid.