U.S. patent application number 12/559659 was filed with the patent office on 2010-01-14 for optical sensing of measurands.
This patent application is currently assigned to Invivosense ASA. Invention is credited to Arne Berg, Astrid Bjorkoy, Reinold Ellingsen, Berit Falch, Dag R. Hjelme, Dan Ostling.
Application Number | 20100007892 12/559659 |
Document ID | / |
Family ID | 46324066 |
Filed Date | 2010-01-14 |
United States Patent
Application |
20100007892 |
Kind Code |
A1 |
Hjelme; Dag R. ; et
al. |
January 14, 2010 |
OPTICAL SENSING OF MEASURANDS
Abstract
A dome-shaped chemical sensing probe comprises an optical fiber
or may be mounted on an optical fiber. The probe has a chemically
sensitive measuring material which exhibits a change in volume
and/or a change in refractive index in the presence of a given
chemical. The change in volume and/or refractive index gives a
change in an optical path length through the probe which can be
measured interferometrically.
Inventors: |
Hjelme; Dag R.; (Trondheim,
NO) ; Berg; Arne; (Kattern, NO) ; Ellingsen;
Reinold; (Trondheim, NO) ; Falch; Berit;
(Trondheim, NO) ; Bjorkoy; Astrid; (Spogndal,
NO) ; Ostling; Dan; (Trondheim, NO) |
Correspondence
Address: |
MCCORMICK, PAULDING & HUBER LLP
CITY PLACE II, 185 ASYLUM STREET
HARTFORD
CT
06103
US
|
Assignee: |
Invivosense ASA
Trondheim
NO
|
Family ID: |
46324066 |
Appl. No.: |
12/559659 |
Filed: |
September 15, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11374800 |
Mar 14, 2006 |
7602498 |
|
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12559659 |
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10273483 |
Oct 18, 2002 |
7440110 |
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11374800 |
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60346941 |
Oct 19, 2001 |
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Current U.S.
Class: |
356/477 |
Current CPC
Class: |
G01N 21/45 20130101;
G01N 2021/7779 20130101; G01N 2021/7776 20130101; G01N 21/7703
20130101; G01N 2021/7723 20130101; G01N 2021/772 20130101 |
Class at
Publication: |
356/477 |
International
Class: |
G01B 9/02 20060101
G01B009/02; G01N 33/48 20060101 G01N033/48 |
Claims
1. An interferometric sensing probe comprising an optical fiber and
a dome-shaped measuring material adapted to exhibit a change in
volume and/or refractive index in response to a given measurand so
as to produce an interferometrically measurable change in at least
one optical path length through the probe.
2. The probe as claimed in claim 1, wherein the measuring material
is chemically responsive and is adapted to exhibit a change in
volume and/or refractive index in the presence of a given
chemical.
3. The probe as claimed in claim 1, wherein said measuring material
comprises a hydrogel.
4. The probe as claimed in claim 1 comprising an optical fiber and
an intermediate substrate layer between the optical fiber and the
measuring material.
5. The probe as claimed in claim 4 comprising an intermediate lens
between said substrate and said fiber.
6. An interferometric sensing probe comprising an optical fiber and
a dome-shaped measuring material adapted to exhibit a change in
volume and/or refractive index in response to a given measurand so
as to produce an interferometrically measurable change in at least
one optical path length through the probe, wherein said measuring
material comprises a cross-linked polymer.
7. The probe as claimed in claim 6, wherein the measuring material
comprises a swollen cross-linked polymer.
8. The probe as claimed in claim 6 wherein said cross-linked
polymer comprises a hydrogel.
9. The probe as claimed in claim 8, wherein said hydrogel is an
ionic hydrogel.
10. The probe as claimed in claim 6, wherein the measuring material
comprises a biomolecular recognition component.
11. The probe as claimed in claim 6, wherein said polymer comprises
polymer chains having cross links and wherein additional
cross-links are formed between antibodies and antigens or peptide
sequences immobilised on the polymer chains.
12. The probe as claimed in claim 6, wherein said polymer comprises
polymer chains having cross links and wherein additional
cross-links are formed by peptides.
13. The apparatus for sensing a measurand comprising a probe as
claimed in claim 1 and read out instrumentation for reading out a
measurement of said measurand from said probe.
14. The apparatus as claimed in claim 13 comprising a scanning
filter.
15. An apparatus for sensing a chemical interferometrically
comprising a substrate bearing a dome-shaped chemically responsive
measuring material adapted to exhibit a change in volume and/or
refractive index in the presence of a given chemical so as to
produce an interferometrically measurable change in at least one
optical path length through the apparatus, and at least one lens
through which in use light traversing said optical path passes.
16. The apparatus as claimed in claim 15, wherein the measuring
material is a swollen cross-linked polymer.
17. The apparatus as claimed in claim 16, wherein said swollen
cross-linked polymer is an ionic hydrogel.
18. The apparatus as claimed in claim 16, wherein said polymer
comprises polymer chains having cross-links and wherein additional
cross-links are formed by peptides.
19. An interferometric chemical sensing probe comprising an optical
fiber and a chemically responsive measuring material adapted to
exhibit a change in volume and/or refractive index in the presence
of a given chemical so as to produce an interferometrically
measurable change in at least one optical path length through the
probe, wherein said chemically responsive measuring material
comprises a swollen cross-linked polymer network.
20. The probe as claimed in claim 19 wherein the measuring material
comprises a hydrogel.
21. An interferometric sensing probe comprising an optical fiber
and a measuring material comprising a swollen cross-linked polymer
network adapted to exhibit a change in volume and/or refractive
index in response to a given measurand so as to produce an
interferometrically measurable change in at least one optical path
length through the probe.
22. An optical sensing probe comprising: a measuring material said
measuring material undergoing a change in volume in response to a
given measurand; and a reflector coupled to the measuring material
so as to be moved by the measuring material as the measuring
material changes in volume, thereby changing the length of a light
path through the apparatus, wherein said measuring material
comprises a swollen cross-linked polymer network.
23. The probe as claimed in claim 22 wherein: said reflector has a
first side and a second side; said first side has a first region
and a second region; said first region is coupled to said measuring
material; and said probe is arranged such that in use a beam of
light for measuring said measurand is incident upon the reflector
on said second region of said first side.
24. The probe as claimed in claim 23 wherein the measuring material
is arranged in the form of a hollow cylinder having a central space
and wherein said light beam is arranged to pass through said
central space to said reflector.
25. The probe as claimed in claim 23 wherein: said reflector has a
first side and a second side; said first side is coupled to said
measuring material; and said probe is arranged such that in use a
beam of light for measuring said measurand is incident upon the
second side of said reflector.
26. The probe as claimed in claim 19 arranged such that said
optical path through the probe does not include the measuring
material.
27. The probe as claimed in claim 19 comprising compensation means
for compensating for environmental parameters.
28. The probe as claimed in claim 21 comprising compensation means
for compensating for environmental parameters.
29. The probe as claimed in claim 27 comprising an optical fiber
having a fiber Bragg grating written into the optical fiber.
30. The probe as claimed in claim 27 wherein said compensation
means comprises a Fabry-Perot interferometer.
31. The probe as claimed in claim 30 wherein said Fabry-Perot
interferometer is provided in parallel with an interferometer for
measuring a target analyte.
32. The probe as claimed in claim 31 comprising measurement and
compensation cavities adjacent one another on a common support.
33. The probe as claimed in claim 32 comprising separate optical
fibers for interrogating said cavities.
34. The probe as claimed in claim 27 wherein the swollen
cross-linked polymer network is a first cross-linked polymer and
wherein the compensation means comprises a second cross-linked
polymer with a different response to a target measurand than the
first.
35. The probe as claimed in claim 27 comprising an optical fiber
having an end and an intermediate substrate layer between the
measuring material and the end of the optical fiber.
36. The probe as claimed in claim 35 wherein said substrate layer
is provided with means, such as a suitable indent, to locate the
optical fiber.
37. An interferometric chemical sensing probe comprising an optical
fiber and a first chemically responsive measuring material adapted
to exhibit a change in volume and/or refractive index in the
presence of a given chemical so as to produce an
interferometrically measurable change in at least a first optical
path length through the probe; and a second chemically responsive
measuring material adapted to exhibit a change in volume and/or
refractive index in the presence of a given environmental parameter
so as to produce an interferometrically measurable change in a
second optical path length through the probe; wherein said first
and second measuring materials are substantially the same and
wherein said probe is arranged such that in use said second
measuring material does not come into contact with a target
analyte.
38. The probe as claimed in claim 37 wherein said first and second
measuring materials provided at a distal end of an optical fiber
separated by a layer which act to reflect a proportion of light
incident on said reflector.
39. The probe as claimed in claim 37 comprising a membrane for
blocking access of said target analyte to said second measuring
material.
40. The probe as claimed in claim 27 wherein said compensation
means is provided in series with a measurement part of the
probe.
41. The probe as claimed in claim 27 having the measuring material
and a compensation material provided coaxially at a distal end of
an optical fiber.
42. The probe as claimed in claim 41 wherein said measuring and
compensation materials are covered by a membrane sleeve or a
capillary tube.
43. The probe as claimed in claim 27 comprising an optical fiber
and an intermediate substrate layer between the optical fiber and
the measuring material.
44. The probe as claimed in claim 43 comprising an intermediate
lens between said substrate and said fiber.
45. The apparatus for sensing a measurand comprising a probe as
claimed in claim 19 and read out instrumentation for reading out a
measurement of said measurand from said probe.
46. The apparatus for sensing a measurand comprising a probe as
claimed in claim 21 and read out instrumentation for reading out a
measurement of said measurand from said probe.
47. The apparatus as claimed in claim 46 comprising a scanning
filter.
48. An apparatus for sensing a chemical interferometrically
comprising a substrate bearing a chemically responsive measuring
material adapted to exhibit a change in volume and/or refractive
index in the presence of a given chemical so as to produce an
interferometrically measurable change in at least one optical path
length through the apparatus, and at least one lens through which
in use light traversing said optical path passes.
49. The probe as claimed in claim 20 wherein said polymer comprises
polymer chains having cross links and wherein additional
cross-links are formed between antibodies and antigens or peptide
sequences immobilised on the polymer chains.
50. The probe as claimed in claim 20 wherein said hydrogel
comprises a biomolecular recognition component linked to the
polymer chain.
51. The method of measuring a measurand comprising using a probe as
a or part of a laser cavity.
52. An interferometric sensing probe comprising an optical fiber,
and a measuring material comprising a cross-linked polymer adapted
to exhibit a change in volume and/or refractive index in response
to a given measurand so as to produce an interferometrically
measurable change in at least one optical path length through the
probe, wherein said measuring material comprises a biomolecular
recognition component.
53. The probe as claimed in claim 52, wherein said cross-linked
polymer is a hydrogel.
54. An interferometric sensing probe for use with an optical fiber,
said probe comprising a dome-shaped measuring material adapted to
exhibit a change in volume and/or refractive index in response to a
given measurand so as to produce an interferometrically measurable
change in at least one optical path length through the probe.
55. An interferometric sensing probe for use with an optical fiber,
said probe comprising a dome-shaped measuring material adapted to
exhibit a change in volume and/or refractive index in response to a
given measurand so as to produce an interferometrically measurable
change in at least one optical path length through the probe,
wherein said measuring material comprises a cross-linked
polymer.
56. An interferometric sensing probe for use with an optical fiber,
said probe comprising a chemically responsive measuring material
adapted to exhibit a change in volume and/or refractive index in
the presence of a given chemical so as to produce an
interferometrically measurable change in at least one optical path
length through the probe, wherein said chemically responsive
measuring material comprises a swollen cross-linked polymer
network.
57. An interferometric sensing probe for use with an optical fiber,
said probe comprising a measuring material comprising a swollen
cross-linked polymer network adapted to exhibit a change in volume
and/or refractive index in response to a given measurand so as to
produce an interferometrically measurable change in at least one
optical path length through the probe.
58. An interferometric sensing probe for use with an optical fiber,
said probe comprising a first chemically responsive measuring
material adapted to exhibit a change in volume and/or refractive
index in the presence of a given chemical so as to produce an
interferometrically measurable change in at least a first optical
path length through the probe; and a second chemically responsive
measuring material adapted to exhibit a change in volume and/or
refractive index in the presence of a given environmental parameter
so as to produce an interferometrically measurable change in a
second optical path length through the probe; wherein said first
and second measuring materials are substantially the same and
wherein said probe is arranged such that in use said second
measuring material does not come into contact with a target
analyte.
59. An interferometric sensing probe for use with an optical fiber,
said probe comprising a measuring material comprising a
cross-linked polymer adapted to exhibit a change in volume and/or
refractive index in response to a given measurand so as to produce
an interferometrically measurable change in at least one optical
path length through the probe, wherein said measuring material
comprises a biomolecular recognition component.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of currently pending U.S.
patent application Ser. No. 11/374,800 filed on Mar. 14, 2006 and
titled OPTICAL SENSING OF MEASURANDS, which is a
continuation-in-part of U.S. patent application Ser. No.
10/273,483, filed Oct. 18, 2002, and claims the benefit of U.S.
Provisional Application No. 60/346,941 filed Oct. 19, 2001, all of
which are hereby incorporated herein by reference in their
entirety.
FIELD OF THE INVENTION
[0002] This invention relates to the fiber-optic sensing of
chemical and non-chemical measurands particularly, although not
exclusively, within the body. The invention relates to in-vivo
medical applications and also to ex-vivo or in-vitro medical uses
or non-medical uses.
BACKGROUND OF THE INVENTION
[0003] With the advance of technology relating to medical diagnosis
and therapy, there is an on-going need to improve methods and
systems for sensing chemical parameters, particularly in a medical
context both outside and within the body. There is also a need more
generally to improve the sensitivity and cost effectiveness with
which various chemical species can be detected.
[0004] There are many optical techniques known in the art for
sensing various chemical parameters. For example, U.S. Pat. No.
5,132,057 discloses an optical fiber probe based on the
immobilisation of fluorescent dye in a hydrogel at the distal end
of the optical fiber. The dye is excited by passing light to it
from the optical fiber and the intensity of the fluorescence is
monitored to give an indication of blood pH.
[0005] It is also known from U.S. Pat. No. 5,804,453 to immobilise
a reagent such as an antigen layer at the end of an optical fiber.
When placed in a solution containing the complementary antibodies,
an antibody layer binds to the antigen layer. This growth in the
immobilised layers is detected by a change in the phase difference
between light reflected from the fiber/reagent boundary and the
distal edge of the immobilised layers respectively.
[0006] Furthermore U.S. Pat. No. 5,898,004 discloses a device
composed of a crystalline colloidal array polymerized in a
hydrogel. Swelling of the hydrogel in response to stimuli is
measured as a shift in the Bragg diffraction wavelength.
[0007] There remains a need, however, for a highly sensitive probe
capable of sensing a wide range of chemicals. Accordingly, when
viewed from a first aspect the present invention provides an
interferometric chemical sensing probe comprising or for mounting
to an optical fiber, and a chemically responsive measuring material
adapted to exhibit a change in volume and/or refractive index in
the presence of a given chemical so as to produce a change in at
least one optical path length through the probe.
BRIEF SUMMARY OF THE INVENTION
[0008] Thus it will be seen by those skilled in the art that in
accordance with the present invention there is provided a sensor
probe which is operable to sense a chemical by its effect on a
measuring material which produces a change in optical path length
which may be measured interferometrically.
[0009] When viewed from a second aspect therefore the invention
provides a method of sensing a chemical comprising providing a
probe including a measuring material that changes in volume and/or
refractive index in the presence of said chemical, exposing said
measuring material to said chemical and measuring
interferometrically the resulting change in an optical path length
through the probe.
[0010] Preferably, the measuring material is dome-shaped. By
dome-shaped it is meant that the surface of the measuring material
opposite to the surface which is attached to, or to be mounted to,
the distal end of the optical fiber is curved outwardly so as to
form a convex surface. Preferably, the surface of the measuring
material remote from the optical fiber has a curvature
corresponding to part of the surface of a sphere. In some preferred
embodiments the measuring material is hemispherical.
[0011] Such a dome-shaped measuring material is advantageous in
that the curved surface enhances the quality of the optical signal
received from the fiber by improving the optical coupling between
the measuring material and the optical fiber. The curvature of the
dome-shaped measuring material is preferably selected so as to be
matched to the phase front of the optical beam that passes through
the fiber in use so that optimum optical coupling is achieved
between the measuring material and the optical fiber.
[0012] The dome-shaped measuring material is also advantageous in
that it provides a relatively large surface area per unit volume
for exposure to the measurand. For example, the curved surface of
the dome-shaped measuring material provides a relatively large
surface area for analyte to diffuse into the measuring material.
Therefore, the analyte is able to diffuse into and then throughout
the measuring material at a high rate. This enables the measuring
material to respond quickly and effectively to the presence of the
measurand.
[0013] Dome-shaped measuring materials are also easier to form than
other shapes because the dome-shape can be formed naturally by
physical forces, such as surface tension, in the measuring
material. As such, the provision of additional means to shape or
mould the measuring material is avoided.
[0014] The provision of a dome-shaped measuring material is novel
and inventive in its own right. Accordingly, from another aspect
the present invention provides an interferometric sensing probe
comprising an optical fiber or for mounting to an optical fiber,
the probe comprising a dome-shaped measuring material adapted to
exhibit a change in volume and/or refractive index in response to a
given measurand so as to produce a change in at least one optical
path length through the probe.
[0015] From another aspect the present invention provides an
apparatus for sensing a chemical interferometrically comprising a
substrate bearing a dome-shaped chemically responsive measuring
material adapted to exhibit a change in volume and/or refractive
index in the presence of a given chemical so as to produce a change
in at least one optical path length through the apparatus, and at
least one lens through which in use light traversing the optical
path passes.
[0016] Optical path length is dependent both on the physical length
of the path traversed by light and on the refractive index of that
path. Thus, in accordance with the invention in its broadest terms
either of these parameters or both of them may be affected by the
measuring material.
[0017] Any suitable measuring material may be used but preferably
the measuring material comprises a cross-linked polymer. The
cross-linked polymer may change in volume as a result of the target
analyte binding to it. This may be because cross-links in the
polymer network are broken or because the analyte changes the
affinity of the polymer chains to the solvent containing the
analyte. The refractive index of the polymer be changed simply as a
result of the volume change (i.e. if the polymer swells it will
become more rarefied and so its refractive index will fall) and/or
because analyte molecules bind to the polymer chains.
[0018] It will be appreciated that the use of a cross-linked
polymer in an interferometric sensor probe is novel and inventive
in its own right--for measuring any parameter not just chemical
sensing. Thus when viewed from a further aspect the invention
provides an interferometric sensing probe comprising or for
mounting to an optical fiber, and a cross-linked polymer adapted to
exhibit a change in volume and/or refractive index in response to a
given measurand so as to produce a change in at least one optical
path length through the probe.
[0019] From another aspect the present invention provides an
interferometric sensing probe comprising an optical fibre or for
mounting to an optical fibre, the probe comprising a dome-shaped
measuring material adapted to exhibit a change in volume and/or
refractive index in response to a given measurand so as to produce
a change in at least one optical path length through the probe,
wherein the measuring material comprises a cross-linked
polymer.
[0020] Except where otherwise specified, hereinafter references to
a measuring material are to be taken to be include a reference to
the cross-linked polymer of the foregoing aspect of the
invention.
[0021] Preferably the probe is arranged such that the optical path
in question is between the two reflectors of a Fabry-Perot
interferometer. As is well known to those skilled in the art, a
Fabry-Perot interferometer provides a sharp fringe pattern. Shifts
in the resulting interference pattern can then be used as a
sensitive measure of changes in the optical path length within the
measuring material.
[0022] In some preferred embodiments the distal reflector of the
measuring cavity is formed by the boundary between the measuring
material and the external environment of the probe--i.e. the
peripheral edge of the measuring material. In a particularly simple
preferred embodiment for example, the measuring material is
provided at the end of the optical fiber and its respective
boundaries with the fiber and its environment provide the two
reflectors of the interferometer.
[0023] Alternatively, as in some preferred embodiments, a separate
reflector is provided. By using a separate reflector rather than
relying on the reflection occurring at the boundary between the
polymer and its environment, the strength of the reflected light
signal may be significantly enhanced. As will be appreciated, a
reflector may reflect substantially all incident light whereas the
reflection coefficient at a boundary between two media of similar
refractive indices will be relatively low.
[0024] Where provided, the reflector may be fixed, i.e. the
measuring cavity comprises an etalon. This will be used when a pure
refractive index change in the measuring material is being
measured. In other preferred embodiments however, the reflector is
arranged so as to be moved by the measuring material as it changes
in volume.
[0025] Such an arrangement is novel and inventive in its own right
and thus when viewed from a further aspect the invention may be
seen to provide an optical sensing probe comprising a measuring
material adapted to undergo a change in volume in response to a
given measurand; and a reflector coupled to the measuring material
so as to be moved by it as it changes in volume, thereby changing
the length of a light path through the apparatus.
[0026] The measuring material preferably comprises a cross-linked
polymer, most preferably sensitive to a predetermined chemical.
[0027] If a separate reflector is provided it is not essential that
light passes through the measuring material. Instead the light may
traverse a path whose length is affected by the change in volume of
the measuring material (by virtue of including the movable
reflector) without passing through it. This could be beneficial in
a number of circumstances. For example it allows non-transparent
measuring materials to be used. It also obviates the disadvantages
of scattering or photo degradation which might be caused by passing
light through the measuring material.
[0028] In one set of examples of such an arrangement, the measuring
light is incident upon the reflector on the same side which is
coupled to the measuring side, but in a different region thereof.
In a particularly preferred embodiment, the measuring material is
arranged in the form of a hollow cylinder and the light is arranged
to pass through the middle of the cylinder to the reflector at one
end. This is beneficial in allowing the measuring material to come
into contact with the medium containing the measurand around the
entire periphery of the cylinder.
[0029] In another preferred embodiment, the measuring material is
arranged to act upon the reflector on the face opposite that which
reflects the measuring light.
[0030] In fact, even without a separate reflector the measuring
light need not pass through the measuring material in order for its
optical path to be influenced by a change in volume of the
measuring material. Thus in a broad set of preferred embodiments
the probe is arranged such that variable optical path through the
probe does not include the measuring material.
[0031] As mentioned hereinbefore, in its broadest terms, the
invention is applicable to measuring materials which exhibit a
change in volume, a change in refractive index or both in response
to the chemical or other measurand. The Applicants have appreciated
that it is desirable in some circumstances to restrict the change
in the optical path length to a change in the refractive index
only. This could, for example, be because only the refractive index
changes--i.e. there is no increase or reduction in the volume of
the polymer. Alternatively, one may choose to measure only the
refractive index component of the response to a given
measurand.
[0032] Accordingly, in some preferred embodiments, the apparatus is
arranged such that the physical length of the optical path
traversed by the measuring beam does not change in response to the
analyte being measured. In other words, the measuring material is
provided in a fixed-length etalon through which the light passes in
use
[0033] If the physical path length is fixed, the measuring light
will be required to pass through the measuring material. If the
measuring material is one which is suitable for passing light
through it, it will be appreciated that, a much greater effect will
be yielded than if the measuring material influences only the
evanescent field of the incident beam. Also, in accordance with the
invention a refractive index change may take place throughout the
volume of the material rather than just a surface layer thereof,
e.g. where a cross-linked polymer is used. This makes better use of
the optical energy.
[0034] In a preferred embodiment of this arrangement, a portion of
measuring material is provided in a cavity of fixed-length (in the
direction parallel to the propagation of the measuring light), but
is free to expand in a direction orthogonal to the path of the
measuring light. This allows the use of measuring materials which
incidentally expand as well as undergoing a change in refractive
index. It is even conceivable with such an arrangement that both
the refractive index change and the volume change may be measured
independently of one another by two separate measuring beams. By
making the cavity open to lateral expansion, contact between the
measuring material and the analyte is facilitated.
[0035] In an alternative embodiment, the measuring material may be
incorporated as part of the core of a wave guide. For example, part
of the core of a D-profile optical fiber may be removed and
replaced with the polymer. This would allow the change in effective
refractive index of the fiber to be measured and also allows
interaction between the analyte and the measuring material. This
arrangement should be contrasted with known D-fibers which have a
coating on the flat side whose refractive index change can only
directly influence the evanescent field of the light beam passing
through the fiber.
[0036] Any suitable interferometric measurement technique may be
employed to measure the change in optical path length produced in
accordance with the invention. Preferably the reflection spectrum
is measured using a scanning filter. This allows the free spectral
range and phase of the interferogram to be extracted thereby
allowing the optical path length to be deduced. Most preferably the
apparatus described in U.S. Pat. No. 6,097,487 is used. Using the
built-in reference grating and comb filter allows the absolute
wavelength axis to be accessed which ensures accuracy and long-term
stability.
[0037] Potential alternative read-out systems include white light
interferometry, which gives an absolute measurement of free
spectral range but with limited resolution; and dual or
multi-wavelength modulated laser interrogation, which should give
very accurate phase measurement. A technique based on Fourier
transform spectroscopy could also be used.
[0038] Probes in accordance with the invention may be used as
described to sense chemical analytes. In some circumstances however
it may be advantageous to compensate for environmental parameters
such as temperature, pressure pH, salt concentration, dissolved
proteins etc., in the measurements obtained. Preferably therefore,
compensation means is provided for compensating for environmental
parameters. Any suitable known means of measurement could be used
to provide a compensating measurement. For example a fiber Bragg
grating (FBG) could be written into the optical fiber.
[0039] Alternatively, the compensation means comprises a
Fabry-Perot interferometer--i.e. similar to that used to measure
the target analyte in preferred embodiments. In some preferred
embodiments the compensation means comprises a cross-linked polymer
with a different response to the target analyte--e.g. it may have a
lower density of recognition components bound to the polymer
chains.
[0040] Alternatively a different material may be used--e.g. one of
known response to the environmental parameters. In an exemplary
such embodiment, the probe comprises an intermediate substrate
layer between the measuring material and the end of the optical
fiber. This provides temperature compensation if the substrate has
a similar thermal expansion coefficient to the measuring material
or at least a known relationship to it.
[0041] Preferably, the substrate layer is provided with means, such
as a suitable indent, to locate the optical fiber. This provides
more reliable lateral positioning of the optical fiber with respect
to the measuring material.
[0042] In a yet further alternative, the compensation means may
comprise the same measuring material as is used to sense the target
analyte but simply not be exposed to the analyte is use. This is
particularly advantageous where, as is preferred, the measuring
material comprises a hydrogel as these have been found to have a
very large coefficient of thermal expansion and are thus able to
provide sensitive temperature compensation.
[0043] In exemplary embodiments, two portions of the measuring
material are provided at the distal end of an optical fiber,
separated by a layer which acts to reflect a proportion of the
light incident on it. This is required since there would otherwise
be no boundary to reflect the light. If only temperature and/or
pressure compensation is required, the partially reflective layer
may be impermeable. However if compensation for pH or dissolved
substances is required, a membrane is required to pass solution to
the compensation portion of measuring material, but to block the
analyte. The partially reflective layer may provide this membrane.
Alternatively, the membrane may be provided separately--e.g. on the
side wall of a cylinder of the device. If provided, the partially
reflective layer need have no membrane function in this
arrangement.
[0044] Where the compensation means comprises a Fabry-Perot
interferometer, it may be provided in parallel with the probe for
measuring the target analyte. For example in one preferred
embodiment measurement and compensation cavities are provided
adjacent one another on a common support. This is advantageous
since it ensures as far as possible that the compensation cavity
reacts to the environmental parameters in the same way as the
measurement cavity--e.g. because they may be manufactured together
and they will experience the same degree of thermal expansion,
pressure etc.
[0045] In such an arrangement the two cavities are preferably
interrogated by separate optical fibers. Separate optical analysers
may be provided but preferably an optical switch is provided to
enable a common analyser to measure both cavities. The switching
and measurement times are typically negligible in comparison to the
response time of the cavities to the analyte or changes in the
environmental parameters.
[0046] The use of optical fibers in such embodiments is beneficial
since the overall probe may be relatively small, making it suitable
for human in-vivo use. For example two optical fibers adjacent one
another will typically have a width of the order of only a quarter
of a millimetre. Furthermore, small quantities of the measuring and
compensation materials may be used giving fast response times and
small minimum sample volumes.
[0047] In other preferred embodiments the compensation means is
provided in series with the measurement part of the probe. Some
such embodiments are described above--for example where a FBG is
written into the optical fiber or where two portions of the same
measuring material are provided at the end of a fiber. A further
set of embodiments have a measuring material and a compensation
material provided coaxially at the distal end of an optical fiber,
optionally separated by a partially reflective layer. Preferably
the two materials are covered by a membrane sleeve or a capillary
tube. As before, it may not be necessary to expose the compensation
material to the analyte solution--e.g. if only temperature or
pressure compensation is required.
[0048] The possibility has been described above of using an
intermediate substrate to provide compensation in the measurements
e.g. for temperature fluctuations. More generally an intermediate
substrate layer can be beneficial, particularly in probes suitable
for ex-vivo use, since it allows the measuring material to be
separate from the optical fiber. This leads to a reduction in the
manufacturing cost. It also allows the measuring material to be
provided on a test strip separate from a measuring device. Thus
some embodiments preferably comprise an intermediate substrate
layer between the optical fiber and the measuring material.
[0049] Where such a substrate is provided it may be directly
optically coupled to the fiber, but preferably an intermediate lens
is provided--most preferably a graded index (GRIN) lens. This is
advantageous since it allows a thicker substrate to be used as
compared to an arrangement with no such lens since the measuring
beam is not constrained by the numerical aperture of the fibre.
[0050] In fact in some circumstances if a lens or lens system is
provided in the path of the measuring beam upstream of the optical
cavity influenced by the measuring material, it is not necessary to
provide an optical fiber coupling the probe to the associated
read-out instrumentation. Thus, when viewed from a further aspect,
the present invention provides an apparatus for sensing a chemical
interferometrically comprising a substrate bearing a chemically
responsive measuring material adapted to exhibit a change in volume
and/or refractive index in the presence of a given chemical so as
to produce a change in at least one optical path length through the
apparatus, and at least one lens through which in use light
traversing said optical path passes.
[0051] From another aspect the present invention provides an
apparatus for sensing a chemical interferometrically comprising a
substrate bearing a dome-shaped chemically responsive measuring
material adapted to exhibit a change in volume and/or refractive
index in the presence of a given chemical so as to produce a change
in at least one optical path length through the apparatus, and at
least one lens through which in use light traversing the optical
path passes.
[0052] This arrangement is advantageous in that it allows the
measuring material to be provided on a, preferably disposable, test
strip, separate from the lens and other read-out
instrumentation.
[0053] The measuring material is preferably arranged in accordance
with one or more of the preferred features set out hereinabove with
reference to arrangements including an optical fiber.
[0054] The invention also extends to the other aspects of the
invention set out hereinabove with the optical fiber replaced with
a lens and substrate.
[0055] The cross-linked polymer used in accordance with any of the
foregoing aspects of the invention will depend upon the analyte
being measured. Preferably a water-swollen cross-linked polymeric
network--i.e. a hydrogel is used. Preferably the hydrogel comprises
a gel monomer e.g. acrylamide and a cross-linking agent e.g.
bisacrylamide. As well as polyacrylamide, other suitable hydrogels
include polyvinyl alcohol and polyhydroxyethyl methacrylate.
[0056] Preferably, the measuring material permits fast
macromolecular diffusion into and through the measuring material so
as to enable the analyte to be detected effectively. The rate of
diffusion of macromolecules through a solution is determined by the
Stokes-Einstein relationship, i.e. it is determined by the
hydrodynamic radius of the macromolecules and the viscosity of the
solution.
[0057] As described above, the measuring material is preferably a
hydrogel. In a hydrogel a macromolecule will experience additional
friction due to the polymer network, causing diffusion of the
macromolecules to be slowed. For most hydrogels the reduction in
the rate of diffusion through the hydrogel is proportional to the
ratio of the hydrodynamic radius of the macromolecules to the
correlation length of the polymer network. The size (hydrodynamic
radius) of the molecules can be determined from the molecular
weight for globular macromolecules, and the correlation length of
the polymer network is determined by the concentrations of monomer
and crosslinker in the pre-gel solution.
[0058] The hydrogel composition is preferably chosen such that the
reduction in the rate of diffusion caused by the polymer network is
small enough so that the hydrogel has a fast enough response time
for a given application. In order for large molecules to diffuse at
a high rate through a hydrogel, the hydrogel is required to
comprise low concentrations of monomer and crosslinker. However,
the concentrations of monomer and crosslinker must be high enough
to provide the hydrogel with the required optical surface quality.
For example, for the transport of macromolecules up to 70 kD in a
polyacrylamide gel, a good trade off between diffusion rate and
optical quality is achieved by providing a hydrogel formed from a
pre-gel solution having a concentration of 10% (weight/volume)
monomer and 2 mol % (relative to monomer concentration)
crosslinker.
[0059] In one embodiment, additional cross-links are formed between
antibodies and antigens immobilised on the polymer chains. This
antigen could either be a native antigen (i.e. natural protein) or
a peptide sequence similar to a linear epitope on the native
antigen. Such a hydrogel will swell in the presence of a free
antigen because antigen-antibody cross-linked binding can be
disassociated by exchange of the immobilised antigen for the free
antigen. In the absence of the free antigen, the gel will
shrink.
[0060] In some embodiments, the gel comprises a biomolecular
recognition component linked to the polymer chain. Such linking may
be direct or indirect through one or more linking molecules.
Examples of such biomolecular recognition components include
enzymes, antibodies, antigens and aptamers. The hydrogel volume
changes when a chemical species binds to the recognition component
and changes the hydrophobicity of the gel. Alternatively, the
hydrogel refractive index may change when analyte molecules bind to
the recognition component. In preferred embodiments enzymatic
substrates are used as the biomolecular recognition component. The
enzymatic substrates are preferably formed from short peptides
which are selected such that they respond to at least one target
enzyme. When a target enzyme is present in the analyte it is able
to diffuse into the gel and cut the peptides forming the
cross-links, causing a change in volume or refractive index of the
gel.
[0061] In most cases there will be a combination of both volume
change and refractive index change. In a case with cross-links
formed between antibodies and antigens, the concentration of
antibodies might be so low that the dissociation by exchange of
immobilized antigen for free antigen results in an insignificant
refractive index change whereas the volume change will be large and
therefore dominate. On the other hand in a case where only the
recognition component, e.g. antibody, is immobilized (in
sufficiently high concentration), the binding of antigen with no
net charge to the antibody will result in insignificant swelling,
however the refractive index change will be large and therefore
dominate. With charged antigens the volume change can be
significant such that both effects contribute.
[0062] In one set of embodiments the measuring material is an ionic
hydrogel. Ionic hydrogels are formed by immobilizing ionizable
groups on the polymer chain. Preferably, the ionic hydrogel is
formed by adding an ionizable monomer to a pre-gel solution
comprising another monomer and a cross-linker. For example, ionic
monomers such as acrylic acid (anionic) or
dimethylaminopropylacrylamide (cationic) may be added to a pre-gel
solution containing acrylamide and the cross-linker bisacrylamide.
The degree of ionization of the hydrogel and the concentration of
counter ions can be varied by altering the pH and the ionic
strength of the solution respectively so as to change the
equilibrium swelling of the hydrogel. This ionic strength of the
solution may be varied by, for example, adding salts to the
solution.
[0063] In some preferred embodiments of the invention, a porous
membrane is provided through which analytes and solvent may
diffuse. The pore size is chosen such that the membrane molecular
weight cut-off is well above the molecular weight of the analyte
being measured. Such a membrane might also be used to block large
molecules, and/or blood cells from reaching the measuring material
by choosing a sufficiently small pore size. Examples of suitable
membrane materials are nitrocellulose, cellulose acetate,
polycarbonate, Teflon, nylon, and polyestersulfone. Preferably, the
membrane material is cast onto a mechanical support member to
ensure sufficient mechanical strength.
[0064] Examples of receptor ligand pairs include anti-BSA IgG and
BSA (Bovine Serum Albumin), anti-CRP IgG and CRP (C-Reactive
Protein), and anti-Troponin IgG and Troponin.
[0065] The measuring material, e.g. hydrogel or other cross-linked
polymer may be formed directly onto the optical fiber. In some
preferred such embodiments, a gel is formed by immobilising the
polymer material on the end of the fiber, covering with a lid of
sufficient optical quality, and performing the gelation. The lid
may thereafter be removed e.g. in solution. This method is novel
and inventive in its own right and thus when viewed from a further
aspect the present invention provides a method of forming a gel on
the end of an optical fiber comprising immobilising a pre-gel
material on said fiber end, covering said pre-gel material with a
lid, performing a gelation of the pre-gel material to form a gel
and removing said lid.
[0066] Preferably the pre-gel material is laterally constrained by
a suitable containment means. Preferred examples of such a
containment means are a capillary tube or a cylinder cast. The
cylinder cast preferably comprises part of a fiber optic
connector.
[0067] Alternatively, in some preferred embodiments, a separate
probe element comprising the measuring material is formed and
subsequently mounted to an optical fiber. This facilitates
manufacture as it is then no longer to manipulate the whole fiber
during fabrication.
[0068] Although so far only passive measurements of the optical
path length through a probe in accordance with the invention have
been discussed, this is not the only option. The various probes
described hereinabove may also be used as active sensors--i.e. the
sensor cavity could be used as part of a laser cavity. In such
arrangements the sensor cavity would be coupled to the main laser
cavity--i.e. that formed by the fiber itself, preferably with gain.
This may also be seen as the sensor cavity providing external
optical feedback to the laser.
[0069] In accordance with all of the foregoing embodiments, the
optical fiber could either be a multimode fiber (large core) or
single mode (small core) of standard dimensions. The measuring
material is preferably between 10 and 1000 microns, most preferably
between 100 and 200 micrometres. Where a capillary tube is provided
it preferably has an inner diameter of approximately 130
micrometres (assuming a fiber of nominal size 125 micrometres) and
an outer diameter of approximately 600 micrometres. Where a ferrule
is provided on the optical fiber, it is preferably of inner
diameter 128 micrometres and outer diameter 2.5 millimetres.
BRIEF DESCRIPTION OF THE DRAWINGS
[0070] Certain preferred embodiments of the present invention will
now be described by way of example only, with reference to the
accompanying drawings in which:
[0071] FIG. 1 is a cross sectional view of sensor probe embodying
the present invention;
[0072] FIG. 2 is a graph of wavelength versus amplitude showing the
reflection spectrum of the probe of FIG. 1.
[0073] FIG. 3a is a schematic depiction of a cross-linked
polymer;
[0074] FIG. 3b is a schematic depiction of a cross-linked polymer
with some cross-links disassociated;
[0075] FIG. 4 shows a second embodiment, similar to that of FIG. 1
but with the addition of a capillary tube;
[0076] FIG. 5 shows a third embodiment, similar to the first
embodiment but including a reflector moved by the polymer gel;
[0077] FIG. 6 shows a fourth embodiment including a reflector;
[0078] FIG. 7 shows a fifth embodiment including a reflector and a
membrane;
[0079] FIG. 8 shows a further embodiment in which the reflector is
fixed;
[0080] FIGS. 9a and 9b are respectively embodiments with and
without a capillary tube utilising a relatively small
reflector;
[0081] FIG. 10 shows a yet further embodiment of the invention
which does not require a membrane;
[0082] FIG. 11 shows a further embodiment of the invention in which
the polymer gel is provided in an optical waveguide;
[0083] FIGS. 12a, 12b show respectively further embodiments
utilising a substrate layer with and without a reflector;
[0084] FIG. 13 shows schematically a further embodiment including a
reference fiber Bragg grating;
[0085] FIG. 14 shows a further schematic embodiment comprising two
sensors;
[0086] FIG. 15 shows an embodiment similar to FIG. 14 but without a
partially reflective layer;
[0087] FIG. 16 shows an embodiment similar to FIG. 14 but with a
capillary tube;
[0088] FIG. 17 shows schematically two cavities on a common
support;
[0089] FIG. 18 shows a method of forming a hydrogel layer on the
end of an optical fiber;
[0090] FIGS. 19a and 19b show schematically further embodiments of
the invention utilising lenses; and
[0091] FIGS. 20a and 20b show schematically yet further embodiments
which do not use an optical fiber.
DETAILED DESCRIPTION OF THE INVENTION
[0092] Turning to FIG. 1, a first embodiment of the invention is
shown. An optical fiber 2 comprising a core 4 and a cladding 6 has
a dome-shaped measuring material 8 formed at one end. The measuring
material is a cross-linked polymer hydrogel 8. The gel 8 is
immobilised on the tip of the fiber 2 using a silanization
technique, well known per se in the art. To ensure uniform gelation
with an optical quality surface, the gelation is performed in an
oxygen-free humid atmosphere provided by delivering nitrogen passed
through a water column to the curing chamber.
[0093] In use, a beam of light is passed along the optical fiber
core 4. A portion of the light is reflected back from the boundary
4a between the fiber core 4 and the hydrogel 8. This forms the
reference beam for a Fabry-Perot interferometer. The rest of the
light is transmitted into the hydrogel 8 which forms a cavity
between the fiber core/hydrogel boundary 4a and the
hydrogel/external boundary 12a. The light beam reflected back from
this cavity causes an interference pattern which may be seen as
curve A on the graph of FIG. 2.
[0094] The free end of the sensor is then placed into a solution
containing the target analyte 12. This could, for example, be
in-vivo, in which case the probe will be suitably sterilized. When
the hydrogel 8 comes into contact with the analyte 12, the hydrogel
8 will change in volume and/or refractive index. In one example,
the hydrogel comprises an acrylamide gel monomer and a
bisacrylamide cross-linking agent. Additional cross-links are
formed between antibodies and antigens immobilised on the polymer
chains (FIG. 3a). In the presence of a free antigen in the solution
12, the hydrogel 8 will swell since the additional cross-link
binding is disassociated by exchange of the immobilised antigen for
the free antigen (FIG. 3b).
[0095] As the hydrogel 8 swells, the physical length of the path of
the light through it is increased. Correspondingly, the wavelengths
at which constructive and destructive interference with the
reference beam occur are shifted slightly. This is shown by curve B
in FIG. 2.
[0096] By measuring this shift in wavelength, the swelling of the
hydrogel may be calculated and hence the concentration of analyte
may be deduced. The wavelength shift is measured using a scanning
filter as described in U.S. Pat. No. 6,097,487.
[0097] FIG. 4 shows an embodiment identical to FIG. 1 except for
the addition of a capillary tube 10 which acts as a sleeve around
the optical fiber 2 and hydrogel 8. The capillary tube 10 makes the
overall probe more robust and prevents it from bending. The
capillary tube 10 also allows the hydrogel 8 to be formed by a
slightly different method. In this method gelation is performed by
closing the end of the capillary tube 10 with an optical quality
glass slide (not shown). The glass slide is removed in solution
after complete gelation, exposing an optical quality gel
surface.
[0098] A similar method of casting a hydrogel 8 onto the end of an
optical fiber 2 is may be seen with reference to FIG. 18. In this
arrangement, the fiber 2 is encased in a ferrule 80 to form one
half a standard fiber optic connector. A metal cylinder cast 82 is
fitted around the ferrule 80. The cylinder 82 has a tapered neck
82a to engage a tapered mating surface 80a of the fiber ferrule.
The tapered surfaces 80a, 82a ensure that the fiber 2 is not
inserted fully into the cylinder 82 and thus a space is formed
between the end of the ferrule 80 and inner side wall of the
cylinder cast 82. Pre-gel material 84 is placed in this space. An
optical quality glass slide 86 is then placed over the pre-gel
material 84 in order to perform gelation. Again, of the lid is
removed in solution after gelation to leave an optical quality gel
surface.
[0099] FIG. 5 shows an alternative embodiment in which a reflector
14 is provided at the distal edge 12a of the hydrogel 8. The
reflector 14 ensures higher reflection at of the hydrogel boundary
12a, thereby amplifying the interference spectrum to be measured.
Contact between of the hydrogel 8 and of the analyte 12 now takes
place on of the outer side face 8a of the hydrogel. As in of the
embodiment of FIGS. 1 and 4, of the optical path length through
this probe is affected by changes in both of the volume and of the
refractive index of the hydrogel 8.
[0100] FIG. 6 shows an embodiment similar to FIG. 5, but in which
of the hydrogel 8' is formed as a hollow cylinder. Of the hollow
centre 16 of the cylinder is aligned with of the end of the optical
fiber core 4. This means that light transmitted through of the
boundary 4a at of the end of the fiber core 4, passes through of
the centre 16 of the cylinder before being reflected by of the
reflector 14, rather than passing through of the hydrogel 8'.
[0101] Of the result is that this sensor arrangement is not
sensitive to any changes in of the refractive index of the hydrogel
8' brought about by its contact with of the dissolved analyte 12,
but rather will measure only a change in volume of the hydrogel 8'.
Such a change in volume causes vertical movement of the reflector
14 attached to it and thus modifies the physical length of the
cavity formed between the end 4a of the fiber core and the
reflector 14.
[0102] This arrangement is useful, for example, where there is no
change in the refractive index of the hydrogel or where it is
desired to separate the volume change from the refractive index
change.
[0103] In a potential modified embodiment (not shown) an
expandable/contactable cylinder may be provided around the inner
surface of the hydrogel cylinder 8' in order to prevent the ingress
of the analyte solution 12.
[0104] A further embodiment is shown in FIG. 7. In common with the
embodiment of FIG. 4, in this embodiment the optical fiber 2 is
encased in a capillary tube 10'. However, in this embodiment the
capillary tube 10' extends further beyond the end of the optical
fiber 2 and is closed at its distal end by a porous membrane 18.
The membrane 18 has a pore size sufficiently large to allow the
ingress of the target analyte, but sufficiently small to prevent
larger particles e.g. blood cells from passing through.
[0105] Examples of suitable membrane materials include
nitrocellulose, cellulose acetate, polycarbonate, Teflon, nylon and
polyestersulfone. The membrane 18, together with the optical fiber
2, defines a space 20 inside the capillary tube 10'. The space 20
is approximately half-filled at its upper end with hydrogel 8. A
reflector 14 is attached to the lower face of the gel with its
reflective surface facing towards the optical fiber 2. An optical
cavity is thus defined between the reflector 14 and the optical
fiber 2.
[0106] In use, the membrane 18 allows analyte to come into contact
with the hydrogel 8 thereby causing it to swell. This will move the
reflector downwards and therefore shorten the optical path of light
traversing the cavity 22. This is in contrast to the previous
embodiment in which swelling of the hydrogel causes an increase in
the path length. In common with the previous embodiment however,
the light does not pass through the hydrogel 8 and is not therefore
affected by any changes in its refractive index. Instead, only
volume changes of the hydrogel 8 will affect the optical path
length traveled by the measuring beam of light.
[0107] In an alternative embodiment (not shown) the reflector 14 of
FIG. 7 may be omitted and the edge of the hydrogel 8 is used to
reflect the incident light.
[0108] A yet further embodiment is shown in FIG. 8. In this
embodiment, the optical fiber 2 is encased in a porous membrane
sleeve 24. The membrane sleeve 24 extends beyond the end of the
optical fiber 2 and is closed at the far end by a reflector 14,
thus defining a cavity 26 at the end of the optical fiber 2 which
is completely filled with hydrogel 8. In this embodiment, the
cavity 26 has a fixed volume and the hydrogel 8 cannot therefore
expand in response to the target analyte.
[0109] Instead, the hydrogel 8 changes in refractive index in the
presence of the target analyte. This alters the optical path length
and hence shifts the interference pattern just as a change in the
physical path length does. The hydrogel in this embodiment
comprises a gel monomer, a cross-linking agent and a biomolecular
recognition component linked to the polymer chain, directly or
through one or more linking molecules. Examples of such
biomolecular components include enzymes, antibodies, antigens, and
aptamers. The refractive index change is brought about when analyte
molecules bind to the recognition component linked to the polymer
chain.
[0110] As may be seen from the embodiment of FIG. 9a, if a separate
reflector 14' is provided, it need not extend the full width of the
optical fiber 2 and hydrogel 8. The reason for this is that light
emerges from the optical fiber core 4 through a relatively small
range of angles. The reduced area reflector 14' is therefore
sufficient to reflect light transmitted from the fiber core 4 and
through the hydrogel 8 back into the core. It also means that it
takes less time for the analyte to diffuse throughout the measuring
material.
[0111] FIG. 9b shows a variant of the embodiment of FIG. 9a where
the hydrogel 8 is immobilised in an open cavity formed at the end
of a capillary tube 10 in which the optical fiber 2 is received. It
will be appreciated that in this embodiment, the reduced size of
the reflector 14' is particularly advantageous because it allows
the hydrogel 8 to come into contact with the analyte solution 12
without requiring a porous membrane. FIGS. 10a to 10c show a
variant on the embodiment in FIG. 8 in which instead of a porous
sleeve 24, a capillary tube 30 may be used, thereby giving better
rigidity. However, to allow contact between the hydrogel 8 and the
analyte 12, the capillary tube 30 has two diametrically opposed
cut-outs 32 at the upper end thereof. These form, together with the
reflector 14, side windows which allow contact between the hydrogel
8 and the analyte. They also allow expansion of the hydrogel 8 in a
lateral direction, although this will not have any effect on the
vertical optical path traversed by the measuring beam of light.
[0112] A yet further embodiment of the invention is shown in FIG.
11. In this embodiment, an optical waveguide 38 is defined in a
substrate layer 40. A longitudinal section 42 of the waveguide 38,
is, however, replaced by a cross-linked polymer gel 42. The two
boundaries between the gel 42 and the waveguide 38 form the two
reflectors of a Fabry-Perot cavity. This may be interrogated by a
beam of light as previously described and may thus sense changes in
the refractive index of the polymer gel 42 in response to a target
analyte. The cavity 42 is fixed in length and so volume changes are
not relevant.
[0113] In an alternative embodiment (not shown) the gel has a
smaller transverse dimension than the waveguide mode and thus
lateral expansion may affect the effective refractive index of the
waveguide.
[0114] FIG. 12a shows a yet further embodiment in which a substrate
layer 34 is interposed between the end of the optical fiber 2 and
the hydrogel 8. It will be seen that the substrate layer 34 has a
cavity 36 etched into its underside in order to locate it on the
end of the optical fiber 2. This serves to ensure proper lateral
location of the hydrogel 8 with respect to the optical fiber 2. It
also allows the probe assembly, comprising the substrate layer 34
and hydrogel 8, to be fabricated separately and then mounted to the
end of the optical fiber 2. This obviates the need to manipulate
the whole fiber during manufacture. The probe may even be assembled
onto the optical fiber 2 by an end user, with only the probe
assembly being supplied.
[0115] It will be appreciated that in this embodiment, there are
three boundaries at which the incident light will be reflected. The
first is between the optical fiber core 4 and the substrate layer
34; the second is between the substrate layer 34 and the hydrogel
8; and the third is at the upper external boundary 8a of the
hydrogel 8. The three boundaries described above yield three
separate interference spectra. One is between the light reflected
from the first and second boundaries. The second is generated
between light from the second and third boundaries. The second
interference pattern yields the previously described measurement of
the change in optical path through the hydrogel 8 in response to
the target analyte. A third interference spectrum is generated
between light from the first and third boundaries. This interface
pattern could be used with the first and/or second pattern to
improve the measurement accuracy by reducing inherent measurement
uncertainty.
[0116] The first interference pattern, between the light reflected
from the first and second boundaries, can be used to compensate for
changes in temperature, assuming that the co-efficient of thermal
expansion of the substrate layer 34 is known.
[0117] The variant in FIG. 12b is identical to that in FIG. 12a
except that a reflector 14 is attached to the upper face of the
hydrogel 8.
[0118] FIG. 13 shows schematically another embodiment of the
invention. In this embodiment, the probe comprises a Fabry-Perot
sensor 44, of the sort previously described, at the distal end of
an optical fiber 46. A fiber Bragg grating 48 is written into the
fiber 46 close to the distal end. In use the measured Bragg
reflection wavelength indicates the temperature at the end of the
fiber 46 which allows this to be taken into account when measuring
the response of the Fabry-Perot sensor 44 to the analyte. The fiber
Bragg grating gives a wavelength shift of approximately 10
picometres per Kelvin.
[0119] An alternative arrangement embodying compensation for
environmental parameters is shown in FIG. 14. In this embodiment
first and second Fabry-Perot cavity sensors 50, 52 are provided at
the distal end of a fiber 46. The first sensor 50 contains a
hydrogel which swells in response to a target analyte. However it
also changes volume depending on the temperature, pressure, pH,
salt concentrations and presence of other proteins in the analyte
solution. The second sensor 52 contains the same hydrogel but
without the receptor ligands for the analyte. Thus it does not
swell in response to presence of the analyte, but does exhibit the
same response to the environmental factors. A partially reflective
layer 54 is provided between the two sensors 50, 52 since they are
unlikely to differ sufficiently in refractive index to generate a
useful reflected beam.
[0120] By subtracting the responses of the two sensors, an accurate
measurement of the analyte concentration may be obtained.
[0121] FIG. 15 shows a similar embodiment to FIG. 14 except that
the second sensor 52 does not comprise hydrogel but is rather a
different polymer. Since the polymer has a known coefficient of
thermal expansion, the measurement from the second sensor 52 may be
used to account for temperature variations. Since the refractive
indices of the two materials used in the two sensors 50, 52 is
significantly different, a partially reflecting layer is not
essential, although may be desirable to ensure a good quality
optical reflection.
[0122] A further embodiment is shown in FIG. 16. In this embodiment
the fiber 46 is encased in a capillary tube 56. The first and
second sensors 50, 52 both comprise exactly the same hydrogel and
are separated by a partially reflecting layer 58. However in this
embodiment the partially reflecting layer 58 also acts as a
membrane allowing the analyte solution to pass through but not the
analyte molecules. This allows the second sensor to compensate for
all environmental parameters e.g. temperature, pH, salinity of the
solution.
[0123] FIG. 17 shows schematically a probe comprising a measuring
Fabry-Perot cavity 56 and a reference cavity 58 provided on a
common substrate 60. These are interrogated by a pair of optical
fibers 62. Since the optical coupling is from below, the support 60
and measuring material(s) must both be transparent. Alternatively
coupling from above could be used or a transmission mode may be
employed.
[0124] This embodiment provides good compensation since the two
cavities 56, 58 are fabricated together and are sufficiently near
one another to experience virtually identical environmental
conditions.
[0125] Of course it should be appreciated that in general, any pair
of the probes described hereinabove could be used in a compensation
scheme such as that shown in FIG. 17.
[0126] FIG. 19a is a schematic representation of a further
embodiment of the invention. This embodiment is similar to that
shown in FIG. 12b except that a lens 90 is provided between the
optical fiber 2 and the substrate 92. The lens is a graded index
(GRIN) type thus causing the light passing through it to follow a
curved path. The lens 92 collimates the beam emerging from the
fiber 2 into the Fabry-Perot cavity formed by the hydrogel 8. FIG.
19b is identical to FIG. 19a except that a different lens 90' is
used which focuses the beam inside the hydrogel layer 8.
[0127] The use of a lens 90, 90' allows a thicker substrate to be
used since it is no longer constrained by the numerical aperture of
the optical fiber.
[0128] FIGS. 20a and 20b are schematic representations of yet
further embodiments of the invention. These are similar to the
embodiments of FIGS. 19a and 19b but do not have an optical fiber
and utilise an ordinary lens 94, 94 rather than a GRIN lens. As
previously, the lens 94 in FIG. 20a collimates the beam into the
hydrogel layer 8 whereas the lens 94' of FIG. 20b brings the beam
to a focus.
[0129] The embodiments of FIGS. 20a and 20b are particularly suited
to use in a configuration where the substrate 92, hydrogel 8 and
reflector 14 together form a disposable test strip, whereas the
lens 94, 94' and associated read-out instrumentation (not shown) is
provided in permanent measurement apparatus.
[0130] It will be appreciated by those skilled in the art that
there are many possible variations on this embodiments herein
described. Furthermore, the principles of the invention may have
many widely varying applications and are not limited to the
examples described. For example, the invention may be used in
immunoassays, molecular assays, or in the detection of short DNA
and RNA sequences. One particularly preferred application is in the
sensing of specific markers produced from muscle hurt during an
infarction.
[0131] The target analyte may not necessarily be in solution and
may be any suitable form of chemical or biological molecule
depending upon the cross-linked polymer used.
[0132] It will also be appreciated that certain aspects of the
invention do not require a cross-linked polymer and thus another
suitable measuring material could be provided.
[0133] While the present invention has been illustrated and
described with respect to a particular embodiment thereof, it
should be appreciated by those of ordinary skill in the art that
various modifications to this invention may be made without
departing from the spirit and scope of the present invention.
* * * * *