U.S. patent number RE37,473 [Application Number 09/498,758] was granted by the patent office on 2001-12-18 for diffraction anomaly sensor having grating coated with protective dielectric layer.
This patent grant is currently assigned to Imation Corp.. Invention is credited to William A. Challener.
United States Patent |
RE37,473 |
Challener |
December 18, 2001 |
Diffraction anomaly sensor having grating coated with protective
dielectric layer
Abstract
A method and apparatus for optically assaying a targeted
substance in a sample using a diffraction anomaly grating sensor.
The optical sensor has a diffraction grating coated with at least
one dielectric layer such that the sensor is sensitized to interact
with the targeted substance. Upon interaction, light incident upon
the sensor at a particular angle propagates through the dielectric,
thereby exhibiting a dip in zero-order reflectance. Advantages of
the present invention include facilitating increased sensitivity
while protecting the metal grating from tarnishing and degradation.
The present invention also allows for the construction of sensors
that are sensitized to a plurality of substances, thus eliminating
the need for an operator to reconfigure the sensing system in order
to assay different substances.
Inventors: |
Challener; William A. (Grant,
MN) |
Assignee: |
Imation Corp. (Oakdale,
MN)
|
Family
ID: |
25435616 |
Appl.
No.: |
09/498,758 |
Filed: |
February 3, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
915357 |
Aug 20, 1997 |
05925878 |
Jul 20, 1999 |
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Current U.S.
Class: |
250/225;
250/559.4; 422/68.1 |
Current CPC
Class: |
G01N
21/211 (20130101); G01N 21/4788 (20130101); G01N
21/553 (20130101); G01N 21/7743 (20130101) |
Current International
Class: |
G01N
21/47 (20060101); G01N 21/21 (20060101); G01N
21/55 (20060101); G02F 001/01 () |
Field of
Search: |
;250/225,559.22,559.4
;356/361,351,367 ;422/68.1,79,82.01,82.09 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 321 523 |
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Feb 1992 |
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EP |
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0 517 930 |
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Dec 1992 |
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EP |
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0677734 |
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Oct 1995 |
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EP |
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WO92/04617 |
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Mar 1992 |
|
WO |
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WO97/09608 |
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Mar 1997 |
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WO |
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1859-1869..
|
Primary Examiner: Le; Que T.
Attorney, Agent or Firm: Levinson; Eric D.
Claims
I claim:
1. A sensor for assaying a substance in a sample comprising:
a substrate having a plurality of parallel grooves in a
surface;
a metal layer formed outwardly from the surface of the substrate,
the metal layer substantially conforming to the grooved surface of
the substrate; and
a dielectric layer formed outwardly from the metal layer, .[.the
dielectric layer for suppressing reflection of incident light
having a polarization parallel to the grooves of the
substrate.].
.Iadd.wherein for light having a plane of incidence perpendicular
to the grooves of the substrate the dielectric layer suppresses a
component of the light having a polarization parallel to the
grooves of the substrate.Iaddend..
2. The sensor of claim 1, further comprising a sensitizing layer
formed outwardly from the dielectric layer, wherein the sensitizing
layer interacts with the substance in the sample.
3. The sensor of claim 1, wherein the dielectric layer interacts
with the substance in the sample.
4. The sensor of claim 1, wherein the dielectric layer has a
thickness of at least 50 nm.
5. The sensor of claim 1, wherein the dielectric layer has a
thickness of at least 130 nm.
6. The sensor of claim 1, wherein a cross-sectional shape of the
grooved surface of the substrate is substantially periodic.
7. The sensor of claim 6, wherein the cross-sectional shape of the
grooved surface is selected from the set of sinusoidal, trapezoidal
and triangular.
8. The sensor of claim 2, wherein the sensitizing layer comprises a
layer of antigens.
9. A sensor for assaying a plurality of substances in a sample
comprising:
a substrate having a plurality of parallel grooves in a
surface;
a metal layer formed outwardly from the surface of the substrate,
the metal layer substantially conforming to the grooved surface of
the substrate; and
a plurality of substantially non-overlapping dielectric layers
formed along the metal layer for suppressing reflectance of an
incident light beam having a polarization component parallel to the
grooves of the substrate .Iadd.and a plane of incidence
perpendicular to the grooves of the substrate.Iaddend..
10. The sensor of claim 9, wherein each dielectric layer interacts
with at least one of the plurality of substances in the sample.
11. The sensor of claim 9, further comprising a sensitizing layer
formed outwardly from the plurality of dielectric layers, the
sensitizing layer capable of interacting with the substance in the
sample.
12. The sensor of claim 9, wherein each of the dielectric layers
has a thickness of at least 50 nm.
13. The sensor of claim 9, wherein each of the dielectric layers
has a thickness of at least 130 nm.
14. The sensor of claim 9, wherein a cross-sectional shape of the
grooved surface of the substrate is substantially periodic.
15. The sensor of claim 14, wherein the cross-sectional shape of
the grooved surface is selected from the set of sinusoidal,
trapezoidal and triangular.
16. The sensor of claim 11, wherein the sensitizing layer comprises
a layer of antigens.
17. A method for assaying a substance in a sample comprising the
steps of:
providing a sensor having a metal diffraction grating coated with a
dielectric layer;
emitting a light beam having a polarization component parallel to
grooves in the metal diffraction grating .Iadd.and incident within
a plane perpendicular to the grooves in the metal diffraction
grating.Iaddend.;
exposing the sensor with the light beam over a plurality of
incident angles;
detecting a first diffraction anomaly angle during the first
exposing step at which zero-order reflectance of the incident light
changes;
interacting the sensor with the sample;
exposing the sensor a second time with a light beam over the
plurality of incident angles;
detecting a second diffraction anomaly angle during the second
exposing step; and
determining a measure of the substance in the sample as a function
of the first angle and the second angle.
18. The method of claim 17, further comprising the step of sounding
an alarm when the determined measure of the substance in the sample
exceeds a predetermined threshold.
19. A method for assaying a substance in a sample comprising the
steps of:
providing a sensor having a metal diffraction grating having a
plurality of grooves, wherein the metal diffraction is coated with
a dielectric layer;
exposing the sensor with a light beam having a .Iadd.polarization
.Iaddend.component parallel to the grooves of the grating .Iadd.and
incident within a plane perpendicular to the grooves in the
grating.Iaddend., and further wherein the component propagates
substantially within the dielectric layer when the sensor is
exposed with the light beam at a diffraction anomaly angle;
interacting the sensor with the sample; and
determining a measure of the substance in the sample as a function
of a shift in the diffraction anomaly angle.
20. The method of claim 19, wherein the exposing step comprises the
step of polarizing the light beam parallel to the grooves in the
metal diffraction grating.
21. The method of claim 19, further comprising the step of sounding
an alarm when the determined measure of the substance in the sample
exceeds a predetermined threshold.
22. The method of claim 19, wherein the determining step comprises
the steps of:
splitting a reflected light beam from the sensor into a first light
beam having a polarization vector parallel to grooves in the metal
diffraction grating and a second light beam having a polarization
vector perpendicular to grooves of the metal diffraction grating,
wherein the first light beam and the second light beam each have a
corresponding intensity; and
monitoring a ratio of the intensities of the first light beam and
the second light beam.
23. A method for assaying a plurality of substances in a sample
comprising the steps of:
providing a sensor having a metal diffraction grating coated with a
plurality of substantially non-overlapping dielectric layers, each
dielectric layer for interacting with at least one of the
substances in the sample, wherein the diffraction grating has a
plurality of parallel grooves;
selectively exposing at least one of the dielectric layers with a
light beam at a corresponding diffraction anomaly angle .Iadd.and
incident within a plane perpendicular to the grooves .Iaddend.such
that a component of the light beam having a polarization parallel
to the grooves in the sensor propagates substantially within the
exposed dielectric layer;
interacting the sensor with the sample; and
determining a measure for each substance in the sample as a
function of a shift in the diffraction anomaly angle of the
corresponding dielectric layer capable of interacting with the
substance.
24. The method of claim 23, further comprising the step of sounding
an alarm when the determined measure of each substance in the
sample exceeds a corresponding one of a plurality of predetermined
thresholds.
25. A system for assaying a substance in a sample, comprising:
a sensor sensitized for interacting with the substance in the
sample comprising:
a substrate having a .[.grooved.]. .Iadd.plurality of parallel
grooves in a .Iaddend.surface,
a metal layer formed outwardly from the surface of the substrate,
the metal layer substantially conforming to the grooved surface of
the substrate, and
a dielectric layer formed outwardly from the metal layer;
a light source exposing the sensor with a light beam .[.at an angle
of incidence.]. .Iadd.having a plane of incidence perpendicular to
the grooves in the surface of the substrate.Iaddend.;
a detector receiving light reflected from the sensor, the detector
responsive to light polarized parallel to the grooves in the
surface of the substrate; and
a controller coupled to the detector for calculating a measure of
the substance in the sample as a function of an anomaly angle at
which a change in zero-order reflectance of the light beam
.Iadd.occurs.Iaddend..
26. The system of claim 25, wherein the sensor further comprises a
sensitizing layer formed outwardly from the dielectric layer.
27. The system of claim 25, wherein the dielectric layer interacts
with the substance in the sample.
28. The system of claim 25, wherein the dielectric layer has a
thickness of at least 50 nm.
29. The system of claim 25, wherein the dielectric layer has a
thickness of at least 130 nm.
30. The system of claim 25, wherein a cross-sectional shape of the
grooved surface of the substrate is substantially periodic.
31. The system of claim 30, wherein the cross-sectional shape of
the grooved surface is selected from the set of sinusoidal,
trapezoidal and triangular.
32. The system of claim 26, wherein the sensitizing layer comprises
a layer of antigens.
33. The system of claim 25 wherein the detector further
comprises:
a polarizing beamsplitter for receiving the reflected light and
splitting the reflected light into a first component and a second
component, wherein the first component has a polarization vector
parallel to the grooves of the substrate and the second component
has a polarization vector perpendicular to the grooves of the
substrate;
a first detector for receiving the first component of the reflected
light, wherein the first detector has an output signal
representative of an intensity of the first component; and
a second detector for receiving the second component of the
reflected light, wherein the second detector has an output signal
representative of an intensity of the second component, wherein the
controller ratios the output signal of the first detector and the
output signal of the second detector, and further wherein the
controller determines the anomaly angle according the ratio of the
output signals.
34. The system of claim 25 wherein the light beam has a
polarization parallel to the grooves in the surface of the sensor.
Description
FIELD OF THE INVENTION
This invention relates generally to the field of optical sensing
and, more particularly, to a method and apparatus for assaying
chemical and biological materials.
BACKGROUND OF THE INVENTION
Recently, extremely sensitive optical sensors have been constructed
by exploiting an effect known as surface plasmon resonance (SPR).
These sensors are capable of detecting the presence of a wide
variety of materials in concentrations as low as picomoles per
liter. SPR sensors have been constructed to detect many
biomolecules including dinitrophenyl, keyhole limpet hemocyanin,
.alpha.-Feto protein, IgE, IgG, bovine and human serum albumin,
glucose, urea, avidin, lectin, DNA, RNA, hapten, HIV antibodies,
human transferrin, and chymotrypsinogen. Additionally, SPR sensors
have been built which detect chemicals such as polyazulene and
various gases including halothane, tricloroethane and carbon
tetrachloride.
An SPR sensor is constructed by sensitizing a surface of a
substrate to a specific substance. Typically, the surface of the
substrate is coated with a thin film of metal such as silver, gold
or aluminum. Next a sensitizing layer, such as a monomolecular
layer of complementary antigens, is covalently bonded to the
surface of the thin film. In this manner, the thin film is capable
of interacting with a predetermined chemical, biochemical or
biological substance. When an SPR sensor is exposed to a sample
that includes the targeted substance, the substance attaches to the
sensitizing layer and changes the effective index of refraction at
the surface of the sensor. Detection of the targeted substance is
accomplished by observing the optical properties of the surface of
the SPR sensor.
The most common SPR sensor involves exposing the surface of the
sensor to a light beam through a glass prism. At a specific angle
of incidence, known as the resonance angle, a component of the
light beam's wavevector in the plane of the sensor surface matches
a wavevector of a surface plasmon in the thin film, resulting in
very efficient energy transfer and excitation of the surface
plasmon in the thin film. As a result, at the resonance angle the
reflected light from the surface of the sensor exhibits a sharp dip
that is readily detected. When the targeted substance attaches to
the surface of the sensor, a shift in the resonance angle occurs
due to the change in the refractive index at the surface of the
sensor. A quantitative measure of the concentration of the targeted
substance can be calculated according to the magnitude of shift in
the resonance angle.
SPR sensors have also been constructed using metallized diffraction
gratings instead of prisms. For SPR grating sensors, resonance
occurs when a component of the incident light polarization is
perpendicular to the groove direction of the grating and the angle
of incidence is appropriate for energy transfer and excitation of
the thin metal film. As with prism-based sensors, a change in the
amount of light reflected is observed when the angle of incidence
equals the resonance angle. Previous SPR grating sensors have
incorporated square-wave or sinusoidal groove profiles.
SPR grating sensors offer many benefits over SPR sensors having
glass prisms including a thicker, more robust metal film.
Furthermore, the grating period for an SPR grating sensor can be
adjusted for any desired resonance angle. Despite these benefits,
both current SPR grating sensors and prism-based sensors are
susceptible to degradation due to oxidation of the metal film and
its continuous exposure to the sample. For this reason, the thin
metal film is usually constructed with a metal that tarnishes
slowly, such as gold, rather than a highly sensitive metal which
tarnishes more quickly, such as silver. Another disadvantage of
current SPR sensors is that the metal film causes many biological
substances to denature, thus leading to erroneous readings. For the
reasons stated above, and for other reasons stated below which will
become apparent to those skilled in the art upon understanding the
present invention, there is a need in the art for an optical sensor
having improved sensitivity and less susceptibility to
degradation.
SUMMARY OF THE INVENTION
In one aspect, the invention is a sensor for assaying a substance
in a sample. The sensor comprises a substrate having a plurality of
grooves in a surface. The grooves are formed in a substantially
periodic profile such as sinusoidal, trapezoidal or any other
suitable formation. A metal layer is formed outwardly from the
surface of the substrate. In this manner, the metal layer
substantially conforms to the grooved profile of the surface of the
substrate. A dielectric layer is formed outwardly from the metal
layer and is selected so as to suppress the zero-order reflectance
of light polarized parallel to the grooves of the surface for at
least one angle of incidence. It is preferable that the dielectric
layer has a thickness of at least 50 nm or, more preferably, at
least 130 nm.
The sensor may include a sensitizing layer formed outwardly from
the dielectric layer. The sensitizing layer is capable of
interacting with the substance in the sample and thereby changes
the angle of incidence at which the sensor suppresses the
zero-order reflectance of incident light. In one embodiment the
sensitizing layer comprises a layer of antigens. In another
embodiment, the dielectric layer is capable of interacting with the
substance in the sample.
In yet another embodiment, the invention is a sensor for assaying a
plurality of substances in a sample. The sensor includes a
substrate having a surface formed in a substantially periodic
grooved profile and a metal layer is formed outwardly from the
surface of the substrate. A plurality of substantially
non-overlapping dielectric layers are formed outwardly from the
metal layer. Each of the dielectric layers is capable of
suppressing the zero-order reflectance of incident light for at
least one corresponding angle of incidence and polarization. This
embodiment facilitates the sensing of a plurality of substances in
a sample or the reuse of a single sensor for multiple samples
having various substances.
In one aspect, the invention is a method for assaying a substance
in a sample using a sensor having a grooved diffraction grating
surface coated with a dielectric layer. The sensor is exposed over
a plurality of incident angles by a light beam polarized parallel
to the grooves in the diffraction grating surface. A controller
detects a first diffraction anomaly angle at which zero-order
reflectance of a component of the incident light having a
polarization parallel to the grooves of the sensor is a minimum.
After interacting the sensor with the sample, the sensor is exposed
a second time with a light beam over the plurality of incident
angles and a second diffraction anomaly angle is detected. A
measure of the substance in the sample is determined as a function
of the first diffraction anomaly angle and the second diffraction
anomaly angle.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic side view of one embodiment of a sensing
system that detects a substance by exposing a diffraction anomaly
sensor having a dielectric coated metal grating and detecting a
shift in the incidence angle that results in minimum
reflectivity;
FIG. 2 is a schematic side view of one embodiment of a diffraction
anomaly sensor having a dielectric-coated metal grating in
accordance with the present invention;
FIG. 3 is a graph that illustrates the calculated zero-order
reflectance over angles of incidence ranging from 0.degree. to
20.degree. for several diffraction anomaly sensors having
dielectric layers of differing thickness; and
FIG. 4 is a schematic side view of one embodiment of a diffraction
anomaly sensor having a plurality of dielectric layers, each
dielectric layer substantially sensitized for a different targeted
substance.
DETAILED DESCRIPTION
In the following detailed description, references are made to the
accompanying drawings that illustrate specific embodiments in which
the invention may be practiced. Electrical, mechanical and
structural changes may be made to the embodiments without departing
from the spirit and scope of the present invention. The following
detailed description is, therefore, not to be taken in a limiting
sense and the scope of the present invention is defined by the
appended claims and their equivalents.
FIG. 1 illustrates a sensing system 10 in accordance with the
present invention. Sensing system 10 includes light source 20,
diffraction anomaly sensor 50, polarizing beamsplitter 80, detector
60 and detector 65. Light source 20, such as a laser, produces a
light beam 25 incident upon sensor 50. Sensor 50 reflects light
beam 25 as light beam 70 onto polarizing beamsplitter 80.
Polarizing beamsplitter 80 splits light beam 70 into component 85
and component 90 which are incident upon detector array 60 and
detector array 65, respectively.
Sensor 50 is a diffraction anomaly sensor having a metal grating
that is coated with a dielectric layer as discussed in detail
below. Similar to SPR sensors, sensor 50 exhibits a change in
reflectivity, referred to as a diffraction anomaly, when exposed
with light beam 25 at a particular angle of incidence. Unlike an
SPR sensor, however, the change in reflectivity of sensor 50 occurs
for light polarized parallel to the grating grooves rather than
perpendicular to the grating grooves. In accordance with the
present invention, it is observed that the effective index of
refraction at the surface of sensor 50 changes in a manner similar
to an SPR sensor when sensor 50 is smeared with a sample containing
a targeted substance. It is further observed that the change in the
index of refraction in turn shifts the incidence angle at which the
diffraction anomaly occurs. Furthermore, for a fixed wavelength of
light beam 25, the diffraction anomaly angle is strongly dependent
upon the amount of targeted substance present in the sample. In
this manner, sensor 50 exhibits a shift in the anomaly angle that
is comparable to a SPR sensor, even though the metal grating of
sensor 50 is coated with a dielectric layer. Therefore, a
quantitative measure of the targeted substance can be calculated by
measuring the resulting shift in the anomaly angle.
After exposing sensor 50 to the sample, the new anomaly angle for
sensor 50 is determined by directing light beam 25 to sensor 50
over a range of incidence angles. In one embodiment, sensor 50 is
rotated so as to vary the angle of incidence of light beam 25. In
another embodiment, sensor 50 is fixed and light source 20 directs
light beam 25 to sensor 50 over a range of incidence angles.
Polarizing beamsplitter 80 splits light beam 70 such that component
85 has a wave vector parallel to the grooves of the surface of
sensor 50 and component 90 has a wave vector perpendicular to the
grooves of the surface of sensor 50. A controller (not shown)
monitors detectors 60 and 65 and continuously ratios the
intensities of light component 85 and light component 90 received
by detectors 60 and 65. In this manner, light fluctuations of the
light source, or other system variations such as ripples in the
sample, do not affect the calculation of the targeted species in
the sample. Based on the calculated ratio for each sensing element
for detector arrays 60 and 65, the controller determines a new
diffraction anomaly angle and calculates a measure of the targeted
substance in the sample based on the new diffraction anomaly angle.
In another embodiment, the controller monitors the diffraction
anomaly angle and sounds an alarm when the calculated measure of
the targeted substance exceeds a predetermined threshold. After
sensing is complete, SPR grating sensor 50 may be disposed or may
be washed and reused.
FIG. 2 illustrates in detail one embodiment of diffraction anomaly
sensor 50 in accordance with the present invention. Sensor 50
includes substrate 100 having a grooved surface 105. The grooves of
surface 105 are periodic and, in one embodiment, have a square
cross-sectional shape. Other cross-sectional shapes are
contemplated for the grooves of surface 105 including but not
limited to sinusoidal, trapezoidal and triangular. The period of
the grooves of surface 105 may range from less than 0.4 .mu.m to
over 2.0 .mu.m.
Thin metal layer 110 is formed outwardly from surface 105 of
substrate 100 and comprises any suitable metal such as aluminum,
gold or silver. Dielectric layer 120 is formed outwardly from metal
layer 110 and thereby protects metal layer 110 from oxidation and
general degradation. In this manner, metal layer 110 may comprise
any suitable metal and may be selected to optimize sensitivity. In
one embodiment, layer 110 comprises silver having a thickness of
approximately 100 nm. As is described in detail below, the
diffraction anomaly exhibited by sensor 50 is directly affected by
the thickness and dielectric constant of dielectric layer 120.
Dielectric layer 120 comprises any suitable dielectric material
such as silicon nitride, Si.sub.3 N.sub.4, and is preferably at
least 50 nm thick. Alternatively, dielectric layer 120 may have a
thickness of at least 100 nm or, more preferably, at least 130
nm.
In one embodiment, sensitizing layer 130 is formed outwardly from
dielectric layer 120. Because dielectric layer 120 is disposed
between sensitizing layer 130 and metal layer 110, dielectric layer
120 prevents chemical reaction between the metal layer 110 and
sensitizing layer 130. Sensitizing layer 130 is selected to
interact with a predetermined chemical, biochemical or biological
substance 140 contained in the sample. In this manner, sensitizing
layer 130 may comprises a layer of antigens capable of trapping a
complementary antibody. Recently, several techniques have been
developed for attaching antigens to dielectric layer 120 such as by
spin coating with a porous silica sol-gel or a hydrogel matrix.
Preferably, sensitizing layer 130 is less than 100 nm thick. In
another embodiment, dielectric layer 120 is selected so as to
interact directly with substance 140, thereby eliminating the need
for sensitizing layer 130.
Unlike conventional SPR sensors, diffraction anomaly sensor 50
exhibits a reflectance dip for light polarized parallel to the
grooves of surface 105. When light beam 25 has an angle of
incidence equal to the diffraction anomaly angle for sensor 50,
light beam 70 is not received by detector 60 but propagates within
dielectric layer 120. In this manner, dielectric layer 120 acts as
a waveguide and the dip in reflectivity is readily detected by the
controller.
The following equations can be used to select dielectric layer 120
such that a diffraction anomaly angle, .theta..sub.SP, occurs for
component 85 having a polarization parallel to the grooves of
surface 105. Using an iterative process, a wave-vector for the
diffraction anomaly resonance, k.sub.x, can be calculated from the
following equation: ##EQU1##
In this equation, .epsilon..sub.0 is the dielectric constant of the
medium above the substrate, such as air or water, etc.,
.epsilon..sub.1 is the dielectric constant of the dielectric layer,
and .epsilon..sub.2 is the dielectric constant of the metal film.
Furthermore, k.sub.0 is a wavevector of the incident light in
vacuum and equals 2.pi./.lambda.. Finally, d is the thickness of
the dielectric layer.
Once the wavevector for the diffraction anomaly resonance has been
found, the following equation can be used to solve for the
diffraction anomaly angle, .theta..sub.SP : ##EQU2##
In this equation, .phi..sub.SP is the azimuthal angle of incident
light beam 25 with respect to the grooves of surface 105, where
0.degree. corresponds to the plane of incidence perpendicular to
the groove direction, n.sub.0 is the index of refraction of the
sample, .lambda. is the wavelength of light beam 25, p is the
period of the grooves of surface 105, and m is an integer. Thus, a
dielectric layer having a suitable dielectric constant may be
readily selected so as to suppress component 85 which has a
polarization parallel to the grooves in surface 105 of sensor
50.
FIG. 3 plots the calculated reflectance of sensor 50 over a range
of incidence angles for light beam 25. More specifically, the
zero-order reflectance of sensor 50 is plotted for incidence angles
ranging from 0.degree. (normal) to 20.degree. and an azimuthal
angle of 0.degree.. Furthermore, reflectivity is plotted for
various thicknesses of dielectric layer 120 of sensor 50. In
modeling the reflectivity of sensor 50, a sinusoidal profile for
surface 105 was selected having a period of 0.66 .mu.m and a
peak-to-peak amplitude of 0.107 .mu.m. Metal layer 110 was modeled
by selecting optically thick aluminum having an index of refraction
of n=2+i8, assuming a wavelength of 780 nm for light beam 25.
Furthermore, dielectric layer 120 was modeled by a dielectric
having an index of refraction of n=2.
FIG. 3 illustrates that the diffraction anomaly angle shifts as the
thickness of dielectric layer 120 is increased. In addition, FIG. 3
is representative of the shifting of the diffraction anomaly angle
of sensor 50 due to interaction with the targeted species.
Increasing the thickness of dielectric layer 120 represents the
increase in thickness of the sensitizing layer as the targeted
substance attaches to it, resulting in a shift of the diffraction
anomaly angle by approximately 10 degrees.
As described above, dielectric layer 120 may be selected such that
it interacts with substance 140 directly, thereby eliminating the
need for sensitizing layer 130. This technique is advantageous in
that a sensor may be developed that is sensitized to a variety of
different substances. FIG. 4 illustrates one embodiment of a
diffraction anomaly sensor that is sensitized to interact with a
plurality of substances. More specifically, sensor 200 includes
substrate 200 having grooves with square cross-sectional profiles.
Other cross-sectional profiles are contemplated including
sinusoidal, trapezoidal and triangular and the square groove
profile is chosen for exemplary purposes only. The period of the
grooves in the surface of substrate 200 may range from less than
0.4 .mu.m to over 2.0 .mu.m.
Thin metal layer 210 is formed outwardly from substrate 200 and is
substantially similar to thin metal layer 110 of FIG. 2. A
plurality of dielectric layers 220.sub.1 through 220.sub.N are
formed along metal layer 210 and are substantially non-overlapping
as shown in FIG. 4. Dielectric layers 220 protect metal layer 210
from oxidation and degradation. Each dielectric layer 220 is
selected to interact with different chemical, biochemical or
biological substance 140 contained in the sample. In this
configuration, sensor 200 is sensitized to interact with a variety
of substances. One advantage of this configuration is that an
operator does not need to reconfigure sensing system 10 in order to
assay different substances. Another advantage is that a single
sample may be assayed for a plurality of substances simply be
interacting sensor 200 with the sample, selectively exposing
dielectric layers 220 with an incident light beam and detecting any
shift in the corresponding diffraction anomaly angle. The
diffraction anomaly exhibited by sensor 200 is directly affected by
the thickness and index of refraction of the exposed dielectric
layer 220. In one embodiment, dielectric layers 220 have
substantially equal thickness of at least 120 nm. In another
embodiment, the thickness of dielectric layers 220 varies in order
to optimize the sensitivity of sensor 200 to the corresponding
substance 140.
CONCLUSION
Various embodiments of a method and system for assaying a substance
in a sample using a diffraction anomaly sensor have been described.
In one embodiment, the present invention is a sensing system that
exposes a diffraction anomaly sensor with a light beam and
quantitatively measures the concentration of targeted substance by
determining the corresponding diffraction anomaly angle. In another
embodiment, the present invention is a diffraction anomaly sensor
having a metal diffraction grating coated with a protective
dielectric. For a particular angle of incidence, incident light
having a polarization component parallel to the grooves within the
metal grating propagates through the dielectric layer causing a dip
in zero-order reflectance. In another embodiment, the present
invention is a diffraction anomaly sensor that is sensitized to
interact with a plurality of substances.
Several advantages of the present invention have been illustrated
including eliminating the degradation and oxidation of the metal
grating of conventional SPR grating sensors. In this manner, the
present invention allows the metal grating to be selected so as to
optimize the sensitivity of the system. Furthermore, the present
invention allows for the construction of sensors that are
sensitized to a plurality of substances, thus eliminating the need
for an operator to reconfigure the sensing system in order to assay
different substances.
This application is intended to cover any adaptations or variations
of the present invention. It is manifestly intended that this
invention be limited only by the claims and equivalents
thereof.
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