AU615928B2

AU615928B2 - Waveguide sensor

Info

Publication number: AU615928B2
Application number: AU19493/88A
Authority: AU
Inventors: Craig George Sawyers
Original assignee: Applied Research Systems ARS Holding NV
Current assignee: Applied Research Systems ARS Holding NV
Priority date: 1987-06-20
Filing date: 1988-06-17
Publication date: 1991-10-17
Anticipated expiration: 2008-06-17
Also published as: DE3876398D1; AU1949388A; DE3876398T2

Description

Waveguide Sensor

Field of the invention

This invention concerns assay techniques by which qualitative and/or quantitative detection of chemical, biochemical or biological species in a sample can be effected and also concerns sensors suitable for use in such techniques.

Background to the invention

The assay techniques with which the present application is concerned are based on the affinity between the species which is to be assayed (hereinafter called "ligand") and a specific binding material for the ligand (hereinafter called "specific binding partner") which is coated on a particular type of surface. International Patent Publications WO84/02578 and WO86/01901 describe assay techniques comprising (a) coating at least a part of a surface (being of preformed relief profile) on a substrate with a thin film of a specific binding partner, the said part of the surface being optically active with respect to radiation over a band of wavelengths, and (b) contacting the coated surface with a sample and observing the optical properties of the said part of the surface in order to determine a qualitative and/or quantitative change in optical properties as a result of the binding of the ligand onto the thin film of specific binding partner on the surface.

As described in the above publications, the preformed relief profile is typically in the form of an optical grating which may be a simple single grating or two or more crossed gratings, the ridges of which may, for example, have square, sinusoidal or triangular cross-sectional shape, and as employed herein references to a grating are intended to encompass all such gratings.

In publication WO84/02578, the change in optical properties of the grating as a result of the binding of the ligand to be assayed (such as a specific antigen in blood serum) is brought about essentially as a result of (1) the mass or bulk of the bound ligand and (2) its dielectric properties. In publication WO86/01901 the binding events are monitored by changes in the fluorescent properties of a dye-tagged binding partner attached to the sensor surface.

It is an object of the present invention to provide an alternative method of assay and a more sensitive type of sensor by which this assay technique can be performed.

Summary of the invention

Thus, in its broadest aspect, the invention provides a method of assaying for a ligand in a sample which comprises a) incubating the sample in contact with the surface of an optical structure, said optical structure comprising a substrate with a first refractive index N1 and a surface layer with a second refractive index N2, said surface layer being such that, in use, a guided wave mode of resonant radiation may be propagated therein, and said surface layer having adsorbed thereon or bound thereto, either directly or indirectly, a specific binding partner for the ligand it is desired to detect; b) irradiating the optical structure at an angle or range of angles to the normal; and c) analysing the reflected, transmitted and/or propagated radiation in order to determine whether, and if desired the extent to which and/or rate at which, the guided wave mode characteristics of the optical structure are altered by formation of a complex between the ligand and the specific binding partner. It will be appreciated that, in order to propagate a guided wave mode within the surface layer, N2 must be greater than N1. However, it is particularly preferred that the refractive indices N1 and N2 are similar (but not precisely equal) and that both are approximately 1.5.

Preferably the optical structure is a diffraction grating with a substrate of clear plastics, for example polycarbonate, or glass and a surface layer of any suitable refractive index. It is advantageous to utilise an optical structure in the method according to the invention wherein the surface layer is less than 700 nm thick.

The surface layer of refractive index N2 may comprise a thin layer of material. For example, where the substrate of the optical structure is of plastics the thin layer of material thereon may be of glass or silica and where the substrate is of glass, for example phosphate glass, the thin layer of material may be of silica or spun-coated polymer, for example polystyrene. However, it is particularly preferred for the substrate of the optical structure to be of glass and for the surface layer of refractive index N2 to comprise the surface of said substrate impregnated with metal ions, for example silver and/or lead ions.

A further aspect of the present invention therefore provides a method of forming a surface layer of refractive index N2 on an optical structure as defined herein which comprises exposing the surface of the optical structure to silver nitrate and/or lead nitrate.

The invention further provides a sensor for detecting a ligand in a sample which comprises an optical structure having a substrate with a first refractive index N1 and a surface layer with a second refractive index N2, said surface layer being such that, in use, a guided wave mode of resonant radiation may be propagated therein, and said surface layer having adsorbed thereon or bound thereto, either directly or indirectly, a specific binding partner for the ligand it is desired to detect.

Sensors produced in accordance with the invention are especially useful for the detection of antibodies or antigens [in which case the specific binding partner may be an antigen or an antibody (monoclonal or polyclonal) respectively] but other ligands may be detected by the use of other specific binding partners, as discussed hereinafter. However, the sensors produced in accordance with the invention are not limited to biosensors and may comprise chemical sensors such as, for example, gaseous chemical detectors wherein specific adsorption onto or absorption into the surface layer may occur.

The invention will be particularly described hereinafter with reference to an antibody or an antigen as the specific binding partner or ligand. However, the invention is not to be taken as being limited to methods of antibody or antigen assay but includes within its scope assays which can be used to detect any chemical, biochemical or biological species in a sample. Examples of suitable binding partners which may be immobilised on an optical structure according to the invention and of ligands which may be assayed by the method of the invention are given in Table I below.

The method of the invention has very broad applicability but in particular may be used to assay: hormones , including peptide hormones (e . g . thyroid stimulating hormone (TSH), luteinising hormone (LH), human chorionic gonadotrophin (hCG), follicle stimulating hormone (FSH), insulin and prolactin) and non-peptide hormones (e.g. steroid hormones such as cortisol, estradiol, progesterone and testosterone, or thyroid hormones such as thyroxine (T4) and triiodothyronine), proteins (e.g. carcinoembryonic antigen (CEA) and alphafetoprotein (AFP)), drugs (e.g. digoxin), sugars, toxins, vitamins, viruses, bacteria or microorganisms. It will be understood that the term "antibody" used herein includes within its scope

a) any of the various classes or sub-classes of immunoglobulin, e.g. IgG, IgA, igM or

IgE, derived from any of the animals conventionally used, e.g. sheep, rabbits, goats or mice, b) monoclonal antibodies, c) intact molecules or "fragments" of antibodies, monoclonal or polyclonal, the fragments being those which contain the binding region of the antibody, i.e. fragments devoid of the Fc portion (e.g., Fab, Fab', F(ab')₂) or the so-called "half-molecule" fragments obtained by reductive cleavage of the disulphide bonds connecting the heavy chain components in the intact antibody.

The method of preparation of fragments of antibodies is well known in the art and will not be described herein.

The term "antigen" as used herein will be understood to include both permanently antigenic species (for example, proteins, bacteria, bacteria fragments, cells, cell fragments and viruses) and haptens which may be rendered antigenic under suitable conditions.

The invention will be described in more detail with reference to a preferred embodiment wherein the optical structure is a diffraction grating. However, it is to be understood that other optical structures such as, for example, optical waveguides, optical fibres, and coated prisms may, in the correct configuration, be included within the scope of the invention. The invention relies for its effect, when observing the zero order diffraction efficiency of the system, on the sharp resonance anomaly due to excitation of guided waves in the triple layer dielectric arrangement formed between the optical structure as described herein, such as a coated diffraction grating, and the sample in contact with the surface layer of said optical structure. Whilst theoretically this resonance would not be expected .to be observed, in practice there will usually be a sufficient attenuation of the incident radiation to observe the phenomenon. However, in some circumstances it may be desirable to encourage the absorbtion of coupled radiation by suitable alteration of one or more layers of the arrangement. Such an arrangement may consist of a substrate with a refractive index Nl, coated with a conformal coating of refractive index N2 in contact with air or a liquid of a still further refractive index N3. By appropriate choice of grating depth, N1 and N2, and grating spacing (between ridges), a resonance with a very narrow spectral width of the order of 0.1 nm is possible.

Variation in the thickness of the coating, which acts as a waveguide at resonance, causes a change in the mode structure of the guide and a corresponding shift in angle of incidence or wavelength required to excite it.

It can be shown that for a shift corresponding to the angular half-width of the coupling resonance (approx. 0.1°) the coating (waveguide) of refractive index N2 must change in thickness by approximately 2nm. This is consistent with the thickness of material which will adhere as a result of a biochemical reaction such as the specific binding of antigens and antibodies in a bioassay.

Sensitivity may be increased by making N1 and N2 very close in value and by reducing the thickness of the coating-waveguide. This arises because reducing the thickness of the coating increases the rate of change of propagation velocity therein for a given change in thickness of the coating, and this increases the shift in the critical angle of incidence (for a given wavelength) relative to changes in thickness.

Thus, one embodiment of the invention provides a sensor comprising a diffraction grating with N1 similar to N2 and both approximately equal to 1.5, a grating depth of approximately 0.1 microns, grating spacing of approximately 0.3 microns and a coating-waveguide thickness of approximately 0.7 microns, with a similar sensitivity to that of a sensor such as is described in International Patent Publication WO84/02578. When the thickness of the coating-waveguide is reduced below 0.7 microns, this sensitivity can be increased four or five- fold.

Diffraction gratings may be prepared by any conventional means. Thus, for example a grating of plastics may be fabricated by injection moulding from a grating master produced via laser interferometry. A sensor according to the present invention may then be synthesised by depositing a thin layer of a dielectric material onto the grating, for example by the deposition of glass or SiO₂ by sputtering or vacuum evaporation respectively. Alternatively, a glass diffraction grating, may be produced by drawing lines in the surface of optically flat glass or by spinning a photoresist onto an optically flat glass substrate, exposing it to an interference pattern, for example as produced by a light interferometer, and then etching the glass surface by exposing the interference pattern in the resist to reactive ions. A glass grating with an ion-impregnated surface according to the invention may then be constructed. For example, silver and/or lead ions may be bound into the surface layer of the grating by exposing the surface to silver nitrate and/or lead nitrate. This permits migration of silver and/or lead ions into the glass surface and thus establishes a very thin surface region having a refractive index slightly different from the remainder of the glass. The thinness of the surface layer and the similarity of the refractive indices of the substrate and the surface layer are both requirements for giving good sensitivity.

The optical properties of an optical structure coated with a specific binding partner are altered by complex formation between the specific binding partner and the complementary ligand and, in one embodiment of the invention, by comparing the optical characteristics of a standard (untreated) region with those of a treated test region of the surface it is possible to determine qualitatively or quantitatively whether a binding reaction has occurred in the test region. In an alternative embodiment where, for example, the specific binding partner is an antibody, an antibody specific to the antigen to be tested for is adsorbed on or bound to at least one discrete test region on the surface of an optical structure and a protein which is not a specific binding partner for any ligand which may be present in the sample to be tested (denoted herein as a "non-specific protein") is adsorbed on or bound to at least one discrete reference region on said surface. The non-specific protein may be, for example, an inactivated antibody or an antibody raised against a ligand not present in the samples to be tested e.g. where the samples to be tested are human sera, the non-specific protein may be an anti-mouse antibody. Any differences between the non-specific binding of e.g. proteins present in the test sample to either the specific antibody or the non-specific protein can be determined by comparing the optical properties of the test region with the reference region after exposure to a sample which does not contain the antigen to be tested for, so that an appropriate correction can, if necessary, be made. Comparison of the optical characteristics of the test and standard regions of a similar biosensor during or after exposure to a sample to be tested can then provide a measurement of complex formation between the antigen to be tested for and its specific antibody. Each region may comprise a continuous layer of a specific binding partner or each binding partner may be present at discrete intervals within any given region to form a discontinuous layer.

In one embodiment of the invention the optical structure is irradiated at a fixed angle of incidence with polychromatic light and the reflected and/or transmitted light is analysed to determine any wavelengths of light which are absorbed due to guided wave resonance. The means for analysing the reflected or transmitted light may be a spectrum detector which determines any wavelengths missing from the reflected or transmitted light, compared to the irradiating light, and which generates a signal indicative of the missing wavelengths. The signal generating means may be calibrated to provide a direct readout.

If a significant wavelength shift is found between the missing part of the spectrum before and after bringing a sample into contact with the coated grating surface, it can be deduced that binding of a complementary material has occurred at the surface and thus that the sample contained a soecific ligand known to bind to the coating of its specific binding partner.

In an alternative embodiment the optical structure is irradiated at one or more angles of incidence with monochromatic or quasi-monochromatic light and the reflected, transmitted and/or propagated light is analysed to determine whether, at a particular angle of incident light, a guided wave mode of resonant light is propagated.

Claims

Claims :

1. A method of assaying for a ligand in a sample which comprises a) incubating the sample in contact with the surface of an optical structure, said optical structure comprising a substrate with a first refractive index N1 and a surface layer with a second refractive index N2, said surface layer being such that, in use, a guided wave mode of resonant radiation may be propagated therein, and said surface layer having adsorbed thereon or bound thereto, either directly or indirectly, a specific binding partner for the ligand it is desired to detect; b) irradiating the optical structure at an angle or range of angles to the normal; and c) analysing the reflected, transmitted and/or propagated radiation in order to determine whether, and if desired the extent to which and/or rate at which, the guided wave mode characteristics of the optical structure are altered by formation of a complex between the ligand and the specific binding partner.

2. A method as claimed in claim 1 wherein the optical structure is a diffraction grating.

3. A method as claimed in claim 1 or claim 2 wherein the specific binding partner is an antigen or an antibody.

4. A method as claimed in any one of the preceding claims wherein the refractive indices N1 and N2 are similar (but not precisely equal) and both are approximately 1.5.

5. A method as claimed in any one of the preceding claims wherein the surface layer of refractive index N2 comprises a thin layer of material.

6. A method as claimed in claim 5 wherein the substrate of the optical structure is of glass and the thin layer of material in silica.

7. A method as claimed in claim 5 wherein the substrate of the optical structure is of plastics and the thin layer of material is glass or silica.

8. A method as claimed in any one of claims 1 to 4 wherein the surface layer of refractive index N2 comprises the surface of the substrate impregnated with metal ions.

9. A method as claimed in claim 8 wherein the substrate of the optical structure is of glass and the surface layer comprises the surface of said substrate impregnated with silver and/or lead ions.

10. A method as claimed in any one of the preceding claims wherein the surface layer of refractive index N2 is less than 700 nm thick.

11. A method as claimed in any one of the preceding claims wherein the optical structure is irradiated at a fixed angle of incidence with polychromatic light and the reflected and/or transmitted light is analysed to determine any wavelengths of light which are absorbed due to guided wave resonance.

12. A method as claimed in any one of claims

1 to 10 wherein the optical structure is irradiated at one or more angles of incidence with monochromatic or quasi-monochromatic light and the reflected, transmitted and/or propagated light is analysed to determine whether, at a particular angle of incident light, a guided wave mode of resonant light is propagated.

13. A sensor for detecting a ligand in a sample which comprises an optical structure having a substrate with a first refractive index N1 and a surface layer with a second refractive index N2 , said surface layer being such that, in use, a guided wave mode of resonant radiation may be propagated therein, and said surface layer having adsorbed thereon or bound thereto, either directly or indirectly, a specific binding partner for the ligand it is desired to detect.

14. A sensor as -claimed in claim 13 wherein the optical structure is as defined in any one of claims 2 to 10.

15. A method of forming a surface layer of refractive index N2 on an optical structure as defined in claim 9 which comprises exposing the surface of the optical structure to silver nitrate and/or lead nitrate.

16. Use of a sensor as claimed in claim 13 or claim 14 in a method of assaying for a ligand in a sample.