U.S. patent application number 11/313971 was filed with the patent office on 2007-02-15 for zero-mode waveguides.
Invention is credited to Harold G. Craighead, Jonas Korlach, Michael J. Levene, Stephen W. Turner, Watt W. Webb.
Application Number | 20070036502 11/313971 |
Document ID | / |
Family ID | 28044620 |
Filed Date | 2007-02-15 |
United States Patent
Application |
20070036502 |
Kind Code |
A1 |
Levene; Michael J. ; et
al. |
February 15, 2007 |
ZERO-MODE WAVEGUIDES
Abstract
The present invention is directed to a method and an apparatus
for analysis of an analyte. The method involves providing a
zero-mode waveguide which includes a cladding surrounding a core
where the cladding is configured to preclude propagation of
electromagnetic energy of a frequency less than a cutoff frequency
longitudinally through the core of the zero-mode waveguide. The
analyte is positioned in the core of the zero-mode waveguide and is
then subjected, in the core of the zero-mode waveguide, to
activating electromagnetic radiation of a frequency less than the
cut-off frequency under conditions effective to permit analysis of
the analyte in an effective observation volume which is more
compact than if the analysis were carried out in the absence of the
zero-mode waveguide.
Inventors: |
Levene; Michael J.; (Ithaca,
NY) ; Korlach; Jonas; (Ithaca, NY) ; Turner;
Stephen W.; (Ithaca, NY) ; Craighead; Harold G.;
(Ithaca, NY) ; Webb; Watt W.; (Ithaca,
NY) |
Correspondence
Address: |
WILSON SONSINI GOODRICH & ROSATI
650 PAGE MILL ROAD
PALO ALTO
CA
94304-1050
US
|
Family ID: |
28044620 |
Appl. No.: |
11/313971 |
Filed: |
December 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11151807 |
Jun 13, 2005 |
7013054 |
|
|
11313971 |
Dec 20, 2005 |
|
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10259268 |
Sep 27, 2002 |
6917726 |
|
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11151807 |
Jun 13, 2005 |
|
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60325280 |
Sep 27, 2001 |
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Current U.S.
Class: |
385/132 ;
385/129; 385/130 |
Current CPC
Class: |
C12Q 1/25 20130101; G01Q
60/22 20130101; G02B 6/241 20130101; G01N 21/6452 20130101; G01N
21/65 20130101; G02B 6/10 20130101; G01N 21/47 20130101; C12Q
1/6825 20130101; G01N 2333/9015 20130101; G01N 33/54373 20130101;
B82Y 20/00 20130101; B82Y 35/00 20130101; G01N 21/6428 20130101;
G01N 21/645 20130101; G01N 21/6408 20130101 |
Class at
Publication: |
385/132 ;
385/129; 385/130 |
International
Class: |
G02B 6/10 20060101
G02B006/10 |
Goverment Interests
[0002] This invention was made with funds provided by the U.S.
Government under National Science Foundation Grant No. BIR8800278,
and National Institutes of Health Grant No. P412RR04224-11, and
Department of Energy Grant No. 066898-0003891. The U.S. Government
may have certain rights in this invention.
Claims
1-33. (canceled)
34. A zero mode waveguide, comprising: a substrate layer; a
cladding layer disposed upon the substrate layer; and a core
comprising a hole disposed through the cladding layer, wherein the
hole is configured to substantially preclude electromagnetic energy
of a frequency less than a cutoff frequency entering the core from
propagating longitudinally through said zero mode waveguide.
35. The zero mode waveguide of claim 34, wherein the cladding layer
comprises a metal film.
36. The zero mode waveguide of claim 35, wherein the metal film
comprises a metal selected from the group consisting of aluminum,
gold and chromium.
37. The zero mode waveguide of claim 34, wherein the hole comprises
a diameter of less than about 100 nm.
38. The zero mode waveguide of claim 34, wherein the hole comprises
a diameter that is between 1 nm and 100 nm.
39. The zero mode waveguide of claim 34, wherein the hoe comprises
a diameter that is between 50 nm and 100 nm.
40. The zero mode waveguide of claim 34, wherein the hole comprises
a diameter of between 30 nm and 100 nm.
41. The zero mode waveguide of claim 34, wherein the substrate is
transparent.
42. The zero mode waveguide of claim 41, wherein the substrate is
selected from glass and fused silica.
43. The zero mode waveguide of claim 34, further comprising a
superstructure defining a well, wherein the substrate, cladding and
core are disposed within the well.
44. The zero mode waveguide of claim 43, wherein the substrate,
cladding and core are disposed at a bottom surface of the well.
45. A zero mode waveguide, comprising: a transparent substrate
layer; a cladding layer disposed upon the substrate layer; and a
core comprising a hole disposed in the cladding layer, wherein the
hole is dimensioned such that electromagnetic radiation entering
the core provides an effective illumination volume that is
substantially internal to the core of said zero mode waveguide.
46. The zero mode waveguide of claim 45, wherein electromagnetic
radiation can enter the core through the transparent layer.
47. The zero mode waveguide of claim 45, wherein the cladding layer
comprises a metal film.
48. The zero mode waveguide of claim 46, wherein the metal film
comprises a metal selected from the group consisting of aluminum,
gold and chromium.
49. The zero mode waveguide of claim 45, wherein the hole comprises
a diameter of less than about 100 nm.
50. The zero mode waveguide of claim 45, wherein the hole comprises
a diameter that is between 1 nm and 100 nm.
51. The zero mode waveguide of claim 45, wherein the hole comprises
a diameter that is between 50 nm and 100 nm.
52. The zero mode waveguide of claim 45, wherein the hole comprises
a diameter of between 30 nm and 100 nm.
53. The zero mode waveguide of claim 45, further comprising a
superstructure defining a well, wherein the substrate, cladding and
core are disposed within the well.
54. The zero mode waveguide of claim 45, wherein the hole is
disposed completely through the cladding layer.
55. An array of zero mode waveguides, comprising a transparent
substrate layer; a cladding layer disposed upon the substrate
layer; and a plurality of cores, wherein each of the plurality of
cores comprises a hole disposed in the cladding layer and wherein
the plurality of cores are configured to substantially preclude
electromagnetic energy of a frequency less than a cutoff frequency
entering the core from propagating longitudinally through said zero
mode waveguide.
56. The array of claim 55, wherein at least one of the plurality of
cores is less than about 100 nm in diameter.
57. The array of claim 55, wherein each of the plurality of cores
is less than about 100 nm in diameter.
58. The array of claim 55, wherein each of the plurality of cores
comprises a diameter between 1 nm and 100 nm.
59. The array of claim 55, wherein each of the plurality of cores
comprises a diameter between 50 nm and 100 nm.
60. The array of claim 55, wherein each of the plurality of cores
comprises a diameter between 30 nm and 100 nm.
61. The array of claim 55, further comprising a superstructure that
defines a well, wherein the transparent substrate, cladding and
plurality of cores are disposed at a bottom surface of the
well.
62. The array of claim 61, wherein the superstructure comprises a
plurality of additional wells, each of the plurality of additional
wells having a separate array disposed therein, each array
comprising a transparent substrate layer, a cladding layer disposed
upon the substrate layer, and a plurality of cores disposed through
the cladding layer, wherein each of the plurality of cores
comprises a hole disposed through the cladding layer and wherein
the plurality of cores are configured to substantially preclude
electromagnetic energy of a frequency less than a cutoff frequency
entering the core from propagating longitudinally therethrough.
63. The array of claim 55, wherein a diameter of each of the
plurality of cores varies by less than 4 nm in diameter from each
other of the plurality of cores.
64. The array of claim 55, wherein each of the plurality of cores
is disposed completely through the cladding layer.
Description
CROSS-REFERENCES
[0001] This application is a continuation application of Ser. No.
11/151,807, filed Jun. 13, 2005, which is a continuation
application of Ser. No. 10/259,268, now U.S. Pat. No. 6,917,726,
filed Sep. 27, 2002, which claims priority to U.S. Provisional
Patent Application No. 60/325,280, filed Sep. 27, 2001, which is
related to U.S. patent application Ser. No. 09/572,530, all of
which are hereby incorporated by reference in their entirety.
FIELD OF THE INVENTION
[0003] The present invention is directed to zero-mode metal clad
waveguides for performing spectroscopy with confined effective
observation volumes.
BACKGROUND OF THE INVENTION
[0004] Observing and understanding the activity of a single
molecule, such as an enzyme, is critical to understanding the
dynamics of many important biochemical processes, such as
catalysis, signal transduction, and gene regulation. Many
biochemical reactions require micromolar ligand concentrations. In
order to perform spectroscopy on one or a few molecules at such
high concentrations, it is necessary to limit the size of the
effective observation volume.
[0005] Previous attempts at sub-diffraction limited spectroscopy
have included the utilization of near-field apertures. These
implementations typically involve an optical fiber that has been
tapered to a sub-wavelength point and coated with a metal such as
aluminum. A subwavelength aperture is formed in the metal at the
end of the fiber. Excitation light is sent down the fiber towards
the aperture, and the elements to be studied are present outside
the fiber and in close proximity to the aperture. The subwavelength
nature of the aperture results in a light diffraction pattern that
includes evanescent modes. These modes rapidly decay with distance
from the aperture, thus effectively confining the volume of
illumination. Only a very small percentage of light sent down the
fiber makes it through the near-field aperture to the illumination
region, making the prior art very inefficient.
[0006] Additionally, the spectroscopic signal from the analyte is
best collected externally by an additional apparatus, such as a
microscope objective, since the efficiency of collection by the
near-field fiber is very low.
[0007] The present invention is directed to overcoming these
deficiencies in the prior art.
SUMMARY OF THE INVENTION
[0008] The present invention is directed to a method for analysis
of an analyte. The method involves providing a zero-mode waveguide
which includes a cladding surrounding a core, where the cladding is
configured to preclude propagation of electromagnetic energy of a
frequency less than a cutoff frequency longitudinally through the
core of the zero-mode waveguide. The analyte is positioned in the
core of the zero-mode waveguide and is subjected, in the core of
the zero-mode waveguide, to activating electromagnetic radiation of
a frequency less than the cutoff frequency under conditions
effective to permit analysis of the analyte in a volume that is
more compact than if the analysis were carried out in the absence
of the zero-mode waveguide.
[0009] The apparatus of the present invention is used for analysis
of an analyte. This includes a zero-mode waveguide having a
cladding surrounding a core, where the cladding is configured to
preclude propagation of electromagnetic energy of a frequency less
than a cutoff frequency longitudinally through the core of the
zero-mode waveguide. In addition, this apparatus has a source of
electromagnetic radiation positioned relative to the zero-mode
waveguide to direct electromagnetic radiation of a frequency less
than the cutoff frequency into the core under conditions effective
to permit analysis of an analyte in an effective observation volume
which is more compact than if the analysis were carried out in the
absence of the zero-mode waveguide.
[0010] In one embodiment of the present invention, the apparatus
includes a superstructure contacted with the opaque film that
serves to facilitate the use of a single chip with several
different samples to be analyzed. In this embodiment, the zero-mode
waveguides are positioned in an array such that several identical
devices are spaced at equal intervals across the surface of the
chip. The superstructure serves to isolate each device on the chip
from all of the rest, allowing an individual device on a chip to be
used with a particular sample without contaminating the rest of the
devices on the chip.
[0011] In another embodiment of the present invention, a different
superstructure is applied to a chip containing an array of
zero-mode waveguide devices to facilitate the delivery of a small,
accurately metered quantity of sample to each device on the chip.
In this embodiment the superstructure contains microfluidic
channels positioned to allow sample introduced at one or more input
ports to be delivered to fluid cavities positioned over each of the
zero-mode waveguide devices. The microfluidic portions of the
superstructure can be used simply to convey and measure the sample,
or more sophisticated operations such as capillary electrophoresis
can be performed on the sample before it reaches the zero-mode
waveguide device for optical analysis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a perspective view of a metal-clad zero-mode
waveguide with lateral dimensions, d, less than half the wavelength
of illuminating light, in accordance with the present
invention.
[0013] FIG. 2 is a cross-sectional view of one embodiment of a
zero-mode waveguide in accordance with the present invention.
[0014] FIGS. 3A, 3B, 3C and 3D are a series of cross-sectional
views, depicting one embodiment for the preparation of a zero-mode
waveguide in accordance with the present invention.
[0015] FIGS. 4A, 4B, and 4C are a series of cross-sectional views,
depicting another embodiment for the preparation of a zero-mode
waveguide in accordance with the present invention.
[0016] FIGS. 5A, 5B, 5C, 5D and 5E are a series of cross-sectional
views, depicting a further embodiment for the preparation of a
zero-mode waveguide in accordance with the present invention.
[0017] FIGS. 6A, 6B, 6C and 6D are a series of cross-sectional
views, depicting another embodiment for the preparation of a
zero-mode waveguide in accordance with the present invention.
[0018] FIGS. 7A, 7B, 7C and 7D are a series of cross-sectional
views, depicting a further embodiment for the preparation of a
zero-mode waveguide in accordance with the present invention.
[0019] FIGS. 8A and 8B show an alternative embodiment of a
zero-mode waveguide in accordance with the present invention where
the waveguide is formed from an optical fiber and an enlargement of
the tip of the zero-mode waveguide.
[0020] FIGS. 9A and 9B show a system utilizing a zero-mode
waveguide in accordance with the present invention.
[0021] FIG. 10 is a scanning electron micrograph showing a top view
of a zero-mode waveguide prepared in accordance with the present
invention.
[0022] FIG. 11 is a contour plot of the logarithm of the intensity
distribution in a 50 nm diameter cylindrical waveguide viewed from
the side at the diameter. The heavy lines mark the borders of the
waveguide, and the calculation is for 500 nm light in a
water-filled, aluminum-clad waveguide.
[0023] FIG. 12 is a graph of the intensity squared as a function of
depth into cylindrical waveguides of various diameters for 500 nm
light.
[0024] FIG. 13 is a graph of the effective observation volume in
cylindrical waveguides as a function of waveguide diameter for 500
nm light.
[0025] FIG. 14 is a drawing of the chemical structure of the dye
BODIPY 515 pyrophosphate (Molecular Probes, Eugene, OR).
[0026] FIG. 15 is a graph fitting fluctuation correlation
spectroscopy model to data for waveguides of various diameters.
[0027] FIG. 16 is a cross-section of a zero-mode waveguide in
accordance with the present invention used for observation of
enzymatic activity.
[0028] FIG. 17 shows fluorescence correlation spectroscopy ("FCS")
curves from a waveguide before, during, and after use of a
polymerase.
[0029] FIG. 18 shows a time trace of fluorescence during use of a
polymerase.
[0030] FIG. 19 is a cross-sectional view of a chip containing
zero-mode waveguide devices and superstructure for isolating
individual zero-mode waveguide devices in an array from one another
to facilitate independent analysis of several samples on a single
chip.
[0031] FIG. 20 is a perspective view of the device shown in FIG.
19.
[0032] FIG. 21 is a cross-sectional view of a chip containing
zero-mode waveguide devices and superstructure for delivering
aliquots of a single sample to several zero-mode waveguide devices
on a single chip by way of the same sample inlet port.
[0033] FIG. 22 is a plan view of the device depicted in FIG.
21.
[0034] FIG. 23 is a perspective view of a zero-mode waveguide
constructed in conjunction with a fluid channel to allow analysis
of analytes in the channel.
DETAILED DESCRIPTION OF THE INVENTION
[0035] The present invention is directed to a method for analysis
of an analyte. The method involves providing a zero-mode waveguide
which includes a cladding surrounding (i.e. partially or fully) a
core, where the core is configured to preclude propagation of
electromagnetic energy of a frequency less than a cutoff frequency
longitudinally through the zero-mode waveguide. The cutoff
frequency is defined as the frequency below which the waveguide is
incapable of propagating electromagnetic energy along the waveguide
under the illumination geometry used. In one embodiment, the core
is sufficiently small to preclude the propagation of
electromagnetic energy of frequency below the cutoff frequency
under any illumination geometry. A further embodiment uses a
waveguide that is capable of supporting one or more longitudinally
propagation modes of electromagnetic energy. In this embodiment, a
special illumination geometry is used such that minimal or no
energy is coupled into the propagating modes of the waveguide. The
analyte is positioned in the core of the zero-mode waveguide and is
subjected, in the core of the zero-mode waveguide, to activating
electromagnetic radiation of a frequency less than the cutoff
frequency under conditions effective to permit analysis of the
analyte in an effective volume which is more compact than if the
analysis were carried out in the absence of the zero-mode
waveguide.
[0036] The apparatus of the present invention is used for analysis
of an analyte. This includes a zero-mode waveguide having a
cladding surrounding a core, where the core is configured to
preclude propagation of electromagnetic energy of a frequency less
than a cutoff frequency longitudinally through the zero-mode
waveguide. In addition, this apparatus has a source of
electromagnetic radiation positioned relative to the zero-mode
waveguide to direct electromagnetic radiation of a frequency less
than the cutoff frequency into the core under conditions effective
to permit analysis of an analyte in an effective observation volume
which is more compact than if the analysis were carried out in the
absence of the zero-mode waveguide.
[0037] The present invention relates to zero-mode waveguides and
their use to confine the effective observation volumes that are
smaller than the normal diffraction limit. As shown in FIG. 1,
excitation radiation (e.g., incident light) L enters waveguide 2,
having core 3 and cladding 5, at entrance pupil 4. The waveguide
comprises an internal volume with lateral dimensions (d) smaller
than half the wavelength (.lamda.) of the excitation light. This
internal volume (i.e. the core) is comprised of a material that
includes, or is capable of including, one or more elements of the
material on which one or more forms of spectroscopy are to be
performed. The volume external to the internal volume in the
lateral directions is the cladding which is composed of a metal or
metal-like material. The end of the waveguide opposite entrance
pupil 4 is the exit pupil 6. The excitation power within the
waveguide decays along the length of the guide. The waveguide is
long enough so that the observation volume is predominantly
confined to the region internal to the waveguide. As a result,
signal collected from extraneous elements is greatly reduced.
[0038] FIG. 2 is a cross-sectional view of one embodiment of a
zero-mode waveguide in accordance with the present invention. This
waveguide 2 is comprised of holes 8, which function as the core, in
metal film 10, which functions as the cladding surrounding the
core, on glass substrate 12. A solution of analyte is placed above
and inside the waveguides. Illumination L is from below the
zero-mode waveguide, and the spectroscopic signal from the analyte
is detected through the glass substrate. The waveguide of FIG. 2 is
preferably prepared by providing a glass or fused silica cover slip
with an aluminum film on it. Chromium and other metals with small
skin depth at the frequency of illumination used are also suitable
for use in the zero-mode waveguide of the present invention. Holes
8 can be formed in the film by electron beam lithography. See
Example 1, infra.
[0039] Other embodiments of the zero-mode waveguide of the present
invention can be made by a number of techniques.
[0040] In another embodiment, as shown in FIGS. 3A, B, C and D,
electroplating is used to progressively restrict larger holes in
metal patterned using a low resolution patterning technique, such
as photolithography. Electroplating will plate new metal only on
those areas already metalized, so it will not deposit metal on the
bare floors of the holes. However, it will deposit on the inside
edges of the holes, so the holes will grow smaller in diameter as
the film grows thicker. By choosing the appropriate time to
discontinue deposition, the holes can be made sufficiently small to
act as zero-mode waveguides. To fabricate these structures, a film
of an appropriate priming metal 20, such as gold, is deposited on
transparent (e.g., glass) substrate 22 (FIG. 3A). A layer of
photoresist is then applied over the layer of priming metal (FIG.
3B). A cost-effective lower-resolution lithography system, such as
248 nm optical lithography, commonly known to practitioners in the
art of optical lithography, produces holes in a photoresist mask
layer 24 as small as 150 nm in diameter (FIG. 3B). This pattern of
holes 26 is then transferred from layer 24 to the metal priming
layer 20 (FIG. 3C) by wet etching, reactive ion etching, or ion
milling--all techniques commonly known in the art. After removal of
excess resist, a cladding material 28, such as chromium, is then
electroplated onto priming layer 20 (FIG. 3D). The electroplating
process will deposit cladding on the metal primed surfaces 20 but
not the bare glass surfaces 22 located at holes 26. Because of
this, the radius of the holes will shrink a distance equal to the
thickness of the plated film. By appropriate selection of the
plating time, or by optical feedback control, the holes can be
constricted to much smaller dimensions. Holes as small as 1 nm can
be consistently fabricated using related techniques. See Kang et
al., "Investigations of Potential-Dependent Fluxes of Ionic
Permeates in Gold Nanotubule Membranes Prepared Via the Template
Method," Langmuir 17(9):2753-59 (2001) ("Kang"), which is hereby
incorporated by reference. After electroplating, the device is
ready for use.
[0041] In another embodiment for preparation of zero-mode
waveguides in accordance with the present invention, as shown in
FIGS. 4A, B and C, a membrane filter (commercially available under
trade names, such as NUCLEOPORE membranes (Whatman, Clifton, N.J.),
and TRACK ETCH membranes (Whatman, Clifton, N.J.)) is used in place
of an electron beam resist. as the etch mask. In techniques
commonly known in the art of filter fabrication, free-standing
films of polymer (usually polycarbonate) are subjected to
bombardment by energetic ionized nuclei, generated by either a Van
De Graff generator or a nuclear reactor. These fragments penetrate
the entire thickness of the membrane and cause chemical changes to
the polymer in the vicinity of the trajectory of the energetic
nucleus. These chemical changes induce a difference in solubility
of the polymer in an etchant, commonly potassium hydroxide. The
films are bathed for some duration in the etchant, causing the film
to dissolve in the vicinity of the nuclear trajectories in the
membrane. These holes can be quite small (as small as 15 nm) and
quite consistent in size (i.e., the variability is less than 4 nm).
Film 30 fabricated in this manner is directly applied to thin metal
film 34 which is deposited on transparent substrate 32 (FIG. 4A).
The pattern of pores 36 in polymer film 30 is then transferred to
the metal film by any of a number of techniques (FIG. 4B),
including reactive ion etching, wet chemical etching, and ion
milling. The remaining polymer film 30 is then removed (FIG. 4C)
with a solvent suitable to dissolve the polymer (such as toluene or
xylenes in the case of polycarbonate) or an oxygen plasma clean
process (known to those in the art of plasma processing).
[0042] In another embodiment for preparation of zero-mode
waveguides in accordance with the present invention, as shown in
FIG. 5A, B, C, D and E, a thin film of a suitable polymer material
is deposited on a metal film and exposed to bombardment by
high-energy ions and etched in a manner very similar to the etching
techniques discussed above. In this embodiment, cladding material
(e.g., chromium, aluminum, or gold) 40 is evaporated onto
transparent (e.g. glass) substrate 42 (FIG. 5A) and thin polymer
film 44 is spin-cast (FIG. 5B) directly onto the metal surface (as
opposed to a free-standing membrane as in the embodiment of FIGS.
4A-C). The entire substrate with the metal and polymer film is then
subjected to ion bombardment, as in the embodiment of FIGS. 4A-C,
and the polymer film is developed with a solvent which will
dissolve zones near the trajectory of an energetic nucleus (FIG.
5C), but not the unaltered film. The pattern of holes 46 which is
thus created in the thin film of polymer 44 is then transferred
into metal layer 42 using reactive ion etching, wet chemical
etching, or ion milling (FIG. 5D). After pattern transfer, the
remains of the polymer film are removed (as above) with a suitable
stripping solvent or an oxygen plasma (FIG. 5E).
[0043] In another embodiment for preparation of zero-mode
waveguides in accordance with the present invention, as shown in
FIGS. 6A, B, C and D, the zero-mode waveguides are fabricated
directly in etched membrane 50 (FIG. 6A).
[0044] Following the methods of Kang, a layer of gold 54 is
deposited on all surfaces of a polycarbonate film, including the
cylindrical interiors of pores 52 (FIG. 6B). This gold film is used
as a priming layer to electroplate a cladding layer material 56,
such as chromium (FIG. 6C). The membrane filters are commercially
available with pores in a wide variety of dimensions. In one
embodiment, 100 nm pores are primed with 5 nm of gold and then
plated with 30 nm of chromium, leaving a 30 nm interior waveguide
core with a 30 nm cladding layer of chromium. These structures can
be optionally immobilized on a transparent (e.g. glass) substrate
58 and optionally thinned by any of the material removal techniques
well-known to those in the art of thin film processing (FIG.
6D).
[0045] In another embodiment for the preparation of zero-mode
waveguides in accordance with the present invention, as shown in
FIGS. 7A, B, C and D, cladding material lift-off is carried out
using an opposite tone film or pattern. This embodiment involves
first applying polysilicon layer 72 over light transmitting (e.g.,
fused silica) substrate 70, as shown in FIG. 7A. Polysilicon
pillars 72A are then formed from polysilicon layer 72 using
conventional photolithography, as shown in FIG. 7B. Pillars 72A are
reduced in size by baking in an oxygen atmosphere followed by
treatment with an etchant, such as hydrofluoric acid. As shown in
FIG. 7C, substrate 70 and pillars 72A are then electroplated with
metal layer 74 and 74A, respectively. Gold is a suitable metal
layer for such purposes. Pillars 72A and metal 5 layer 74A are then
removed by conventional techniques, leaving behind metal layer 74
with holes 76 over substrate 70, as shown in FIG. 7D.
[0046] In another embodiment for preparation of zero-mode
waveguides in accordance with the present invention, as shown in
FIGS. 8A and B, a fiber tip 80, which is similar to a near-field
scanning optical microscope tip is constructed so that it
terminates with a zero-mode waveguide where the tip allows entrance
of analyte material into the interior of the zero-mode waveguide.
This embodiment of the present invention is prepared by heating the
end of an optical waveguide fiber 82 and drawing it so that its
diameter narrows at the heated end. The resulting tapered tip is
coated with zero-mode cladding material 84. The silica of fiber 82
is then etched (e.g., with hydrofluoric acid) a small distance to
form hole 86 which serves as the core for the zero-mode
waveguide.
[0047] The use of zero-mode waveguides, in accordance with the
present invention, to analyze analytes is shown in FIG. 9B.
[0048] Surface 102 with metallic layer 116 applied over surface 102
with small holes 118 etched into opaque layer 116 represents one
embodiment of the present invention. When illuminated from below,
the light cannot penetrate fully through the holes into reagent
solution R, because the diameter of holes 118 is smaller than one
half of the light's wavelength. As shown in FIG. 9B, the material
undergoing spectroscopic analysis is positioned in hole 118 and is
illuminated from below. Because the effective volume of observation
is very small, signals due to extraneous material in the vicinity
will be reduced.
[0049] The system for carrying out analysis of analytes in
accordance with the present invention is shown in FIG. 9A.
Illumination source 110 (e.g., a laser) directs excitation
radiation by way of dichroic beam splitter 108 through lens 106 and
surface 102 to the target material. This excites the material with
the resulting emitted radiation passing back through surface 102
and lens 106. Dichroic beam splitter 108 allows passage of the
emitted radiation to detector 112 which identifies the type of
emission. The detected emission information is then directed to
computer 114 where the material corresponding to the emission is
identified and its identity stored. The radiation being emitted can
be in the form of fluorescence, Raman scattered light,
Hyper-Rayleigh scattered light, luminescence, Hyper-Raman scattered
light, or phosphorescent light.
[0050] The zero-mode waveguide of the present invention can be used
to analyze a variety of analytes. Amongst these are biomolecules,
such as proteins and nucleic acids.
[0051] The zero-mode waveguide of the present invention can be used
for sequencing nucleic acid molecules, as fully described in U.S.
patent application Ser. No. 09/572,530, which is hereby
incorporated by reference. This method involves providing a complex
of a nucleic acid polymerizing enzyme and a target nucleic acid
molecule oriented with respect to each other in a position suitable
to add a nucleotide analog at an active site complementary to the
target nucleic acid. A plurality of types of nucleotide analogs are
provided proximate to the active site, where each type of
nucleotide analog is complementary to a different nucleotide in the
target nucleic acid, leaving the added nucleotide analog ready for
subsequent addition of nucleotide analogs. The nucleotide analog
added at the active site as a result of the polymerizing step is
identified. The steps of providing a plurality of nucleotide
analogs, polymerizing, and identifying are repeated so that the
sequence of the target nucleic acid is determined. The zero-mode
waveguide of the present invention is used to carry out the step of
identifying the nucleotide analog added to the target nucleic
acid.
[0052] The zero-mode waveguide of the present invention can also be
used to analyze other enzymatic reactions, including haplotyping
with DNA polymerase, enzymatic reactions with RNA polymerase or
helicase/primase, and analysis of ribosomes, spliceosomes,
transcription complexes, chaperon proteins, protein folding, virus
particle assembly, catalytic or non-catalytic antibodies,
ribozymes, proteins involved in nucleic acid recombination,
exonucleases, endonucleases, inorganic catalysts, and detection of
viruses or other small pathogens.
[0053] The zero-mode waveguide of the present invention can be used
to analyze one or more analytes, either sequentially or
simultaneously within the waveguide.
EXAMPLES
Example 1
Fabrication of Zero-Mode Waveguide Array
[0054] Arrays of zero-mode waveguides were manufactured as small
holes in a 50 nm thick film of aluminum on a glass or fused silica
coverslip. The steps for the fabrication of the devices are as
follows. First, the cover glasses were cleaned with a solution of
one part ammonium hydroxide, one part hydrogen peroxide, and six
parts water at 70.degree. C. The coverglasses were immersed in this
solution for minutes, and then rinsed in a overflowing bath of
deionized water for 10 minutes. The samples were dried with
compressed dry nitrogen and then subjected to oxygen plasma for 3
minutes. The cleaned cover glasses were then coated with 50 nm of
aluminum by thermal evaporation. An electron beam lithography
resist, ZEP-7000A, was spun onto the cover glasses for 60 seconds
at 3000 RPM. Excess solvent was driven from the films by baking on
a temperature-controlled hotplate for 30 minutes at 170.degree. C.
This process yields a film approximately 300 nm thick. The films
were then mounted for exposure in an electron-beam lithography
system. Electron beam exposure was performed in a pattern of dots
separated by micrometers (for optical isolation during use). A
range of doses can be used to generate a gradation of hole sizes on
a single coverglass for studies where variable hole size is useful.
The latent pattern was then developed using a solution of xylenes
at room temperature for 3 minutes. The development was stopped with
a rinse of isopropanol, followed immediately by drying with
compressed dry nitrogen. The developed pattern was then transferred
to the aluminum layer by reactive ion etching using an aluminum
etch recipe: 20 sccm Cl.sub.2, 40 sccm BCl.sub.3, and 2 sccm
H.sub.2. The pressure was maintained at 20 mT, and the
radio-frequency power was controlled by feedback to hold the sample
bias potential at 400 V. The etch proceeded for approximately 1
minute and 20 seconds. Immediately after removal from the etch
chamber, the samples were rinsed in deionized water to remove
residual chlorine radicals which can degrade the structures on
exposure to moisture in the air. The remaining resist was exposed
to short-wavelength ultraviolet radiation to expose it, and the
exposed resist was removed with another developer for this film:
methyl isobutyl ketone ("MIBK"). Again, the samples were rinsed in
isopropanol and blown dry with compressed dry nitrogen. The final
step before use was to subject them to an oxygen plasma to harden
the aluminum native oxide and remove any organic residue from
processing. A total of 3 minutes of exposure at 1000 watts was
used, but the radio-frequency power was turned off and on to keep
the substrate temperature below 120.degree. C to prevent damage to
the aluminum film.
[0055] FIG. 10 is a scanning electron micrograph showing a top view
of the zero-mode waveguide made by the process of this example.
Example 2
Intensity Evaluation with Zero-Mode Waveguide Array
[0056] Simulations of the electric field inside zero-mode
waveguides were performed using a commercial finite-element
time-domain Maxwell's equation solver (EMFlex, from Weidlinger
Associates, New York, N.Y.). Models were run for right, circular
cylindrical waveguides of water in 50 mm thick aluminum films on a
glass substrate (index of refraction 1.5). The entire region above
the aluminum and inside the waveguides was assumed to be filled
with water, and the illumination was, in all cases, normally
incident circularly polarized plane waves of light at
6.times.10.sup.14 Hz (corresponding to a vacuum wavelength of 500
mm). The entire region of the model was 1 .mu.m.sup.3, and the grid
spacing inside and in the vicinity of the waveguides was 1 nm.
Although actual experiments would in most cases use tightly focused
light, rather than the plane waves modeled here, the dimensions of
the waveguides modeled are small enough compared with the
wavelength of light to make the plane wave approximation accurate
enough to estimate the intensity distribution within the
waveguides. Models were run for waveguide diameters between 30 and
100 nm.
[0057] FIG. 11 shows the calculated intensity distribution for a 50
nm diameter waveguide. The intensity falls off quickly with
increasing propagation into the waveguide, as expected for light
that is well below the cut-off frequency for the guide. The
intensity distribution is relatively constant.across the waveguide,
as demonstrated in FIG. 11, and, therefore, one can estimate the
effective illumination volume by plotting the intensity as a
function of propagation distance. The ability of fluorescent
photons to couple out of the waveguide will also be a strong
function of distance from the entrance/exit pupil. This effect has
been assumed to follow a behavior that is similar to that of the
excitation light propagation. However, this assumption fails to
take into account the orientation of the emitting dipole, its exact
lateral location within the guide, or the different frequency of
the emitted light. The effective observation volume is described by
the illumination profile multiplied by the collection efficiency
profile. The effective observation volume within a guide can,
therefore, be approximated by the square of the intensity
distribution. FIG. 12 shows the intensity squared as a function of
propagation distance for guides of various diameters decreases. As
expected, the intensity decays faster as the waveguide diameter
decreases. Therefore, small diameter waveguides serve to decrease
the effective observation volume both by physically constraining
the experiment in the lateral dimensions and by decreasing the
propagation distance of light into and out of the guide.
Example 3
Spectroscopy with Zero-Mode Waveguides
[0058] For fluorescence applications, one can define the size of
effective observation volume, V, as V = .intg. S .function. ( r )
.times. d 3 .times. r .times. .intg. S .function. ( r ) .times. d 3
.times. r .intg. S 2 .function. ( r ) .times. d 3 .times. r
##EQU1## where S(r) is the observation efficiency at the point r,
and for this system S(r)=I.sup.2(r). FIG. 13 shows the effective
volume of zero order waveguides in aluminum illuminated with
6.times.10.sup.14 Hz light as a function of waveguide diameter.
Volumes are on the order of 50 zeptoliters (10.sup.-21 liters).
This compares to 0.5 femtoliters (10.sup.-15) for a typical
diffraction-limited focal volume.
[0059] The effectiveness of the waveguides in confining the volume
of illumination was evaluated using fluorescence correlation
spectroscopy ("FCS"). FCS involves illumination of a sample volume
containing dye molecules in solution. The diffusion of molecules
into and out of the effective observation volume leads to
fluctuations in the number of observed molecules and hence to
fluctuations in the fluorescence signal. These fluctuations occur
on a time scale that is characterized by an average residence time
of a molecule within the volume, .tau..sub.D. The autocorrelation
of the fluorescence signal, G(.tau.), is given by G .function. (
.tau. ) = .delta. .times. .times. F .function. ( t ) .times.
.delta. .times. .times. F .function. ( t + r ) F .function. ( t ) 2
, ##EQU2## where F(t) is the fluorescence signal at time t and
.delta.F(t) is the deviation in fluorescence from the mean. G(O) is
inversely proportional to the average number of molecules in the
volume and the half-max of G(.tau.) occurs at the typical residence
time of molecules diffusing in the volume. For a known
concentration of dye in solution, the average number of molecules
observed gives a useful estimate of the effective observation
volume and is critical for determining the expected background from
freely diffusing species in studies of single enzyme molecules in
the presence of fluorescent ligands. The average residence time of
diffusing dye molecules, and the overall shape of G(r), can be
combined with theoretical calculations to give an understanding of
the shape of the effective observation volume. The residence time
is also relevant to the temporal resolution of studies of enzymatic
dynamics. Reactions that produce fluorescent product or
intermediates may be distinguishable from background fluorescence
fluctuations if the relevant reaction rates are longer than the
typical diffusion time.
[0060] Arrays of waveguides were illuminated from the glass side
with 488 nm circularly polarized light from an argon ion laser
using a 60.times. water immersion microscope objective (UPlanApo
Olympus, Melville, N.Y., NA=1.2). Fluorescence was collected by the
same objective, passed through a dichroic mirror (dichroic
long-pass 500, Chroma Technology Corp., Brattleboro, VT) and two
emission filters (575/150 and 580/150) to block reflected laser
light and coupled into a 50 .mu.m optical fiber (OZ Optics Ltd.,
Corp., Ontario, Canada). A 50/50 fiber splitter was used to send
the signal to two avalanche photodiodes ("APD"s) (Perkin Elmer
Optoelectronics, Fremont, Calif.) for cross-correlation.
Cross-correlation was necessary to remove artifacts from
after-pulsing at short times in individual APDs. Cross-correlation
is similar to auto-correlation and yields the same information,
except that G(.tau.) is now given by G .function. ( .tau. ) =
.delta. .times. .times. F 1 .function. ( t ) .times. .delta.
.times. .times. F 2 .function. ( t + .tau. ) F 1 .function. ( t )
.times. F 2 .function. ( t ) ##EQU3## where subscripts indicate
signals measured at different APDs.
[0061] Waveguide arrays were exposed to oxygen plasma for one
minute and were pre-treated with a solution of heparin (50
.mu.g/ml) in HPLC grade water on the aluminum side to prevent
sticking of dye to the glass and metal surfaces. FCS was then
performed on arrays with a 1 .mu.M solution of the dye Bodipy 515
pyrophosphate (see FIG. 14) in HPLC grade water with 50 .mu.g/ml of
heparin. Molecules can diffuse into and out of the waveguide only
through the top of the waveguide and the gradient of light
intensity inside the waveguide is essentially one dimensional, as
discussed above. Therefore, one would expect G(.tau.) to be
dominated by the decay in 5 illumination intensity and fluorescence
output coupling efficiency with increasing distance from the
entrance pupil, both of which decay faster in smaller diameter
waveguides.
[0062] Using the output from the finite-element simulations,
theoretical FCS curves were generated for waveguides of various
diameters and fit to the normalized data curves. Despite the use of
heparin and oxygen-plasma treatment, the FCS curves displayed
long-time tails attributable to some sticking of dye. This was
accommodated for in the fitting function using an additive
exponential term such that the fitting function was of the form G
.function. ( .tau. ) = 1 N .times. G diff .function. ( .tau. ) + S
.times. .times. e - t / .tau. s + offset ##EQU4## where G.sub.diff
is the numerically-derived diffusion component, S and .tau..sub.S
are the sticking component amplitude and lifetime, and offset is a
small constant. FIG. 15 shows fits to waveguides of various
diameters, with good agreement between the theoretical curves and
data confirming the accuracy of the effective observation volume
model. These experiments, therefore, verify that zero-mode
waveguides have been constructed and that they effectively confine
the effective observation volume to zeptoliter dimensions.
[0063] The confinement mechanism of the zero-mode waveguide can add
usefulness to many of the single-molecule spectroscopic techniques
known in the art. In addition to FCS, and cross-correlation
spectroscopy, the technique of dual-color cross-correlation
spectroscopy is described in "Dual-color fluorescence
cross-correlation spectroscopy for multicomponent diffusional
analysis in solution" Schwille, P., Meyer-Almes, F. J., Rigler R.
Biophys. J. Vol. 72 No. 4: pp. 1878-1886 April 1997, which is
hereby incorporated by reference. This technique can be extended to
function with two-photon excitation as described in "Two-photon
fluorescence coincidence analysis: rapid measurements of enzyme
kinetics", Heinz, K. G., Rarbach, M., Jahnz, M. and Schwille, P.,
Biophys J, September 2002 pp. 1671-1681 Vol. 83, No. 3 which is
hereby incorporated by reference. Both of these techniques can be
enhanced by the inclusion of the zero-mode waveguide as a volume
limiting technique.
[0064] It will be seen by one skilled in the art of single-molecule
spectroscopy and analysis that several different types of
presentations of the analyte will be useful. Analytes can diffuse
freely in solution or be immobilized in the illuminated region of
the zero-mode waveguide. In cases where there is more than one
analyte and these various analytes are expected to interact with
one another, all of the permutations of bound and diffusing
analytes are possible. For two analytes, both can be freely
diffusing, one or the other of them can be immobilized, or both can
be immobilized (such is the case where more than one fluorescent
label is attached to different residues of a single amino acid
chain to observe the folding kinetics of the protein). In the case
of observation of enzymatic activity (see example 4 herein), it is
useful to immobilize the polymerase molecule to the device, while
allowing the dNTP analogs bearing the fluorescent labels to diffuse
freely. In a study of DNA-DNA renaturation kinetics from P.
Schwille et al 1997 above it was advantageous to have both
components of the analyte diffusing freely. Many useful
configurations of labels and biological molecules are outlined in
"Fluorescence spectroscopy of single biomolecules", S. Weiss,
Science, Vol. 283, pp. 1676-1683, which is hereby incorporated by
reference. In the present invention, the zero-mode waveguide is
contemplated in conjunction with all of these configurations to
improve the signal-to-noise, temporal resolution and tolerance to
high ligand concentration.
Example 4
Observation of Enzymatic Activity in Zero-mode Waveguides
[0065] SEQUENASE, a commercially available exonuclease-deficient
mutant of T7 DNA polymerase (USB Corporation, Cleveland, OH), was
immobilized on the bottom of a zero-mode waveguide by 15 minute
incubation of the zero-mode waveguide structure with a 1:10
dilution of the commercial stock solution (13 U/.mu.l in 20 mM
KPO.sub.4, pH 7.4, 1 mM DTT, 0.1 mM EDTA, 50% glycerol) in glycerol
enzyme dilution buffer (provided with the SEQUENASE enzyme, this
buffer contains 20 mM Tris-HCl, pH 7.5, 2 mM DTT, 0.1 mM EDTA, 50%
glycerol). The waveguide was made of a 50 nm thick aluminum film on
a clean fused silica coverslip (25.times.25 mm square (from Esco
Products, Oak Ridge, N.J.)) with an array of waveguides of
different sizes. After immobilization, excess unbound enzyme was
washed away by extensive flushing with 1.times.pH 7.5 buffer (40 mM
Tris-HCl, pH 7.5, 10 mM MgCl.sub.2, 10 mm NaCl). The reaction of
DNA polymerization using the fluorophore coumarin-5-dCTP instead of
dCTP was initiated by incubating the waveguide with a reaction
mixture containing 3 ng/.mu.l primed M13 DNA, 5 mM dithiothreitol,
7.5 .mu.M dATP, coumarin-5-dCTP, dGTP, dTTP, 6 ng/.mu.l
single-stranded DNA binding protein in 1.times.pH 7.5 buffer.
Primed M13 DNA was provided by annealing 2 .mu.g of M13mpl8 DNA to
(-40) M13 primer (2 pmol) in a 20 .mu.l volume of 40 mM Tris-HCl,
pH 8.0, 20 mM MgCl.sub.2, and 50 mM NaCl by heating for 2 minutes
at 65.degree. C. and subsequent slow cooling to <35.degree. C.
over 30 minutes.
[0066] As shown in FIG. 16, the zero-mode waveguide with
immobilized polymerase was illuminated from the glass side with 488
nm circularly polarized light from an argon ion laser using a
60.times. water immersion microscope objective (UPlanApo, Olympus,
Melville, N.Y., NA=1.2). Fluorescence was collected by the same
objective, passed through a dichroic mirror (dichroic long-pass
500, ChromaTechnology Corp., Brattleboro, VT) and two emission
filters (575/150 and 580/150, Chroma Technology Corp., Brattleboro,
VT) to block reflected laser light and coupled into a 100 .mu.m
optical fiber (OZ Optics Ltd.). This fiber was used to send the
signal to an avalanche photodiode (APD, Perkin Elmer
Optoelectronics, Fremont, Calif.) for detection of the fluorescence
photons. The signal from the APD was sent to a correlator card
(Correlator.com, Bridgewater, N.J.) in a computer, where the time
trace of fluorescence was recorded and stored, and the
autocorrelation function of the signal was calculated.
[0067] FIG. 17 shows the FCS curves from a waveguide before,
during, and after the polymerization reaction. Before initiation of
the polymerization reaction (gray solid curve; in the presence of
7.5 .mu.M coumarin-5-dCTP, but in the absence of DNA), the FCS
curve only shows a decay originating from the fast diffusion of the
fluorescent coumarin-5-dCTP, analogous to FIG. 15 for the diffusion
of Bodipy-515-PP inside zero-mode waveguides. After initiation of
the polymerization reaction by addition of all ingredients
necessary to support efficient DNA synthesis, FCS curves are
dominated by fluctuations originating from incorporation of
coumarin-5-dCTP into DNA by the enzymatic activity of DNA
polymerase, with a much longer time constant of .about.2 ms. This
is because the coumarin-5-dCTP is incorporated into DNA which in
turn is bound to the molecule of DNA polymerase, and therefore the
fluorophore spends a long time in the confined volume of the
zero-mode waveguide, continuously emitting fluorescence until it is
photobleached. After completion of DNA polymerization (all of the
single-stranded DNA has been extended into double-stranded DNA),
the FCS curve returns to the shape that was obtained before
initiation of the polymerization reaction (gray dashed curve).
[0068] FIG. 18 shows a time trace of fluorescence during the period
of polymerization of coumarin-5-dCTP into M13 DNA. Distinct bursts
of fluorescence are visible, corresponding to incorporation of a
coumarin-5-dCTP molecule into DNA, and subsequent photobleaching of
the fluorophore. Traces, such as shown in FIGS. 17 and 18, can be
used for characterization of the DNA polymerization process on a
single molecule level.
[0069] For the analysis of this example, the concentration of
fluorophore (coumarin-5-dCTP) is fairly high, at 7.5 .mu.M. The
fact.that single molecule enzymatic activity can be observed inside
the waveguide demonstrates that the zero-mode waveguide of the
present invention provides a confined volume to enable such
analysis. In unconfined volumes, the number of fluorophores would
be far too high to permit the observation of enzymatic turnovers of
DNA polymerase. For example, in a diffraction-limited excitation
volume of 0.2 fl, such as provided by focusing laser light with a
high numerical aperture objective lens, a concentration of 7.5
.mu.M corresponds to an average of ca. 900 fluorophores
simultaneously present inside the volume.
Example 5
Use of a Superstructure to Enable Use of Many Samples on a Single
Chip
[0070] The zero mode waveguide devices are very small and use a
minute fraction of the available surface area of a moderate-sized
chip. As seen in FIG. 19, it is possible to replicate the pattern
of holes indicated generally at 1910 in the aluminum film 1915 many
times over on a 25 mm square fused silica chip. It is desirable to
be able to apply each of these devices with a separate sample to
increase the number of experiments that can be conducted with a
single chip. A silicone (polydimethylsiloxane) superstructure such
as silicone rubber gaskets 1920 is applied to the aluminized
surface after the fabrication of the zero-mode waveguides in the
metal film. The silicone structure 1920 contains an array of holes
as seen in a perspective view of FIG. 19 at 2010 coincident with
the centers of the zero-mode waveguide devices so that the union of
these two forms wells into which a fluid sample can be placed while
preventing contamination of the neighboring zero-mode waveguide
devices. Such devices are commercially available from Grace
Biolabs, Inc. in Bend, Oregon. Through the use of this method a
single chip can be employed with dozens of separate samples. While
silicone has been used for this purpose, the present invention
contemplates any suitable material and means of attachment for the
use of a superstructure to isolate neighboring zero-mode waveguide
devices.
Example 6
Use of a Superstructure to Enable Multiple Analyses of a Single
Sample
[0071] As with the previous example, a silicone superstructure 2110
is applied to the aluminized surface 2120. The silicone
superstructure 2110 is patterned with grooves 2130 and rectangular
recesses 2140 that when mated with the aluminized surface create
fluid channels and cavities respectively. Techniques for creating
microfluidic structures for application in this way are known in
the art and are discussed in technical papers such as "fabrication
of a configurable, single-use microfluidic device", McDonald, J.
C., Metally, S. J. and Whitesides, G. M., Analytical Chemistry 73
(23):5645-5650 Dec. 1 (2001), which is hereby incorporated by
reference.
[0072] FIG. 22 is a plan view of the structures. The placement of
the rectangular recesses 2140 in the silicone superstructure is
chosen to coincide with the locations of zero-mode waveguide
devices 2150 on the fused silica chip. The fluid channels 2130 can
be used simply to convey liquid to the several cavities where
different analyses will take place. For example, each cavity can be
pre-loaded with different reagents prior to mating the
superstructure with the chip so that each cavity enables a distinct
experiment on the sample. Alternatively the fluid channels can be
employed to perform pre-processing of the sample before it arrives
in the cavity. For example, capillary electophoresis can be
performed to fractionate the sample into components before
delivering these components to separate cavities containing
zero-mode waveguide devices for further analysis of the sample.
Zero-mode waveguides such as waveguide 2310 can also be placed to
coincide with a channel 2320 as seen in FIG. 23, so as to allow
analysis of material as it passes through the channel. Air vents
2230 can optionally be used in cases where trapped gas prevents the
entry of fluid into the recesses or fluid channels. These air vents
can also serve as conduits to allow the introduction of other
materials to the cavities after the superstructure has been mated
to the fused silica chip. These microfluidic channels and fluid
cavities can also facilitate the use of much smaller quantities of
sample than can be conveniently managed using hand-pipetting
methods. While silicone has been used for this purpose, the present
invention contemplates any suitable material and means of
attachment for the use of a superstructure to provide microfluidic
channels and fluid cavities in conjunction with zero-mode
waveguides to allow multiple analyses in parallel on a single
sample.
COMCLUSION
[0073] In the present invention, the region of observation is
internal to the waveguide, as opposed to prior methods in which the
region of observation is external to the sub-wavelength aperture.
By making use of the light internal to the waveguide, the present
invention achieves good light efficiency.
[0074] The extreme confinement of the effective observation volume
by the zero-mode waveguide of the present invention enables a
proportional increase in the limiting concentration of analytes up
to which single molecule detection is still possible. This is
important for many applications involving the study of single
molecules. Dorre, et al., "Highly Efficient Single Molecule
Detection in Microstructures," J. Biotech. 86:225-36 (2001), which
is hereby incorporated by reference. As many processes,
particularly many biochemical reactions, occur efficiently only at
concentrations much higher than the pico- or nanomolar
concentration regime typically required for single molecule
analysis in solution, zero-mode waveguides offer the potential to
study these processes at more appropriate concentrations, for
example, under physiologically relevant conditions. With the use of
zero-mode waveguides; single molecule characterization is possible
at much higher concentrations, ranging into the micromolar regime,
thus extending the range of biochemical reactions that can
successfully be studied on a single molecule level. Zero-mode
waveguides, therefore, provide the field of single molecule
research with novel instrumentation so that higher concentrations
of analytes can be studied and higher background can be
tolerated.
[0075] In addition to permitting the use of higher concentrations,
the zero-mode waveguides of the present invention permit analysis
of small volumes with the feature that signal fluctuations from
diffusion of analytes occurs about 100 times more rapidly than by
using a volume created by high numerical aperture objective lenses.
As a result, enzymatic turnovers or chemical reactions can be more
confidently distinguished from diffusion. The diffusional residence
time of a molecule inside the waveguide is relevant to the temporal
resolution capabilities of studies of enzymatic and chemical
dynamics by setting a lower limit of what time regime can be
measured. Reactions that produce fluorescent products or
intermediates are distinguishable from diffusional background
fluorescence fluctuations if the relevant reaction rates are longer
than the typical diffusion time. Thus, one can analyze faster
processes than would be possible without using zero-mode
waveguides.
[0076] Although the invention has been described in detail for the
purposes of illustration, it is understood that such detail is
solely for that purpose, and variations can be made therein by
those skilled in the art without departing. from the spirit and
scope of the invention which is defined by the following
claims.
* * * * *