U.S. patent application number 10/798196 was filed with the patent office on 2004-09-16 for method of manufacture of colloidal rod particles as nanobarcodes.
Invention is credited to Dietz, Louis J., Natan, Michael J., Stonas, Walter J., Walton, Ian D., Winkler, James L..
Application Number | 20040178076 10/798196 |
Document ID | / |
Family ID | 32966916 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040178076 |
Kind Code |
A1 |
Stonas, Walter J. ; et
al. |
September 16, 2004 |
Method of manufacture of colloidal rod particles as
nanobarcodes
Abstract
A method is disclosed for the manufacture of colloidal rod
particles as nanobarcodes. Template membranes for the deposition of
materials are prepared using photolithographic techniques.
Inventors: |
Stonas, Walter J.;
(Campbell, CA) ; Dietz, Louis J.; (Mountain View,
CA) ; Walton, Ian D.; (Redwood City, CA) ;
Natan, Michael J.; (Los Altos, CA) ; Winkler, James
L.; (San Diego, CA) |
Correspondence
Address: |
SWANSON & BRATSCHUN L.L.C.
1745 SHEA CENTER DRIVE
SUITE 330
HIGHLANDS RANCH
CO
80129
US
|
Family ID: |
32966916 |
Appl. No.: |
10/798196 |
Filed: |
March 11, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10798196 |
Mar 11, 2004 |
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09969518 |
Oct 2, 2001 |
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09969518 |
Oct 2, 2001 |
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09677203 |
Oct 2, 2000 |
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09677203 |
Oct 2, 2000 |
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09598395 |
Jun 20, 2000 |
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60157326 |
Oct 1, 1999 |
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60189151 |
Mar 14, 2000 |
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60190247 |
Mar 17, 2000 |
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60194616 |
Apr 5, 2000 |
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60212167 |
Jun 16, 2000 |
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Current U.S.
Class: |
205/74 ;
205/118 |
Current CPC
Class: |
C25D 1/00 20130101; B01J
13/0047 20130101; B01J 2219/00513 20130101; B22F 2999/00 20130101;
B01J 2219/00502 20130101; H01F 1/0081 20130101; B82Y 25/00
20130101; B22F 1/062 20220101; B01J 2219/00722 20130101; G01N
33/54346 20130101; C25D 1/006 20130101; B01J 2219/00596 20130101;
B01J 2219/0072 20130101; B82Y 30/00 20130101; C40B 40/06 20130101;
B01L 3/545 20130101; B01J 13/0091 20130101; B01J 2219/00545
20130101; B01J 19/10 20130101; B01J 2219/00554 20130101; B01J
2219/00547 20130101; C25D 1/04 20130101; B01J 19/0046 20130101;
C40B 70/00 20130101; B01J 2219/005 20130101; B01J 2219/00657
20130101; B01J 2219/00585 20130101; B82B 1/00 20130101; B22F
2999/00 20130101; B22F 1/062 20220101; B22F 2207/01 20130101; B22F
2999/00 20130101; B22F 1/062 20220101; B22F 2207/01 20130101 |
Class at
Publication: |
205/074 ;
205/118 |
International
Class: |
C25D 005/02 |
Claims
We claim:
1. A method for the manufacture of a free standing nanoparticle,
the method comprising: a. providing a planer substrate; b. causing
deposition of a layer of material on said surface; c. placing a
mask over said material coated substrate, said mask comprising a
plurality of cylindrical posts and prepared by the method
comprising: i) providing a resist-coated substrate; ii) exposing a
pattern on said resist-coated substrate using a photolithographic
technique; iii) developing said pattern; and iv) etching said
pattern; d. releasing said nanoparticles from the surface of said
substrate.
Description
RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. Utility
application Ser. No. 09/969,518, entitled "Method Of Manufacture Of
Colloidal Rod Particles As Nanobarcodes," filed Oct. 2, 2001, which
is a Continuation-in-Part of U.S. Utility application Ser. No.
09/677,203, entitled "Method of Manufacture of Colloidal Rod
Particles as Nanobar Codes," filed Oct. 2, 2000, which is a
Continuation-in-Part of U.S. Utility application Ser. No.
09/598,395, filed Jun. 20, 2000, entitled "Colloidal Rod Particles
as Nanobar Codes." The Ser. No. 09/598,395 Application was filed
claiming the benefit of the filing date of U.S. Provisional
Application Serial No. 60/157,326, filed Oct. 1, 1999, entitled
"Self Bar-coded Colloidal Metal Nanoparticles;" U.S. Provisional
Application Serial No. 60/189,151, filed Mar. 14, 2000, entitled
"Nanoscale Barcodes;" U.S. Provisional Application Serial No.
60/190,247, filed Mar. 17, 2000, entitled "Colloidal Rod Particles
as Barcodes;" and U.S. Provisional Application Serial No.
60/194,616, filed Apr. 5, 2000, entitled "Nanobarcodes: Technology
Platform for Phenotyping." The Ser. No. 09/677,203 application was
filed claiming the benefit of the filing date of U.S. Provisional
Application Serial No. 60/212,167, filed Jun. 16, 2000, entitled
"Techniques for Multiple Parallel Nanobarcode Synthesis." This
application also claims the benefit of the filing date of U.S.
Provisional Application Serial No. 60/237,322, filed Oct. 2, 2000,
entitled "Methods for the Manufacture of Colloidal Rod Particles as
Nanobar Codes," and U.S. Provisional Application Serial No.
60/285,017, filed Apr. 19, 2001, entitled "Method of Manufacture of
Colloidal Rod Particles."
FIELD OF THE INVENTION
[0002] The present invention is directed to methods of manufacture
of nanoparticles and approaches for such manufacture. In certain
preferred embodiments of the invention, the nanoparticles may be
used to encode information and thereby serve as molecular (or
cellular) tags, labels and substrates.
[0003] The membranes of the present invention may be used as
templates for the synthesis of nanoparticles according to methods
provided herein. The membranes include anodized alumina membranes,
polycarbonate trach-etched membranes, and membranes made using
photolithographic methods. Throughout this application, said
membranes may be interchangeably referred to as "porous membranes,"
"porous templates" and "templates." Nanoparticles may be formed
within the pores by deposition methods including, electrochemical
deposition, sequential chemical reaction, and chemical vapor
deposition (CVD). Alternatively, the nanoparticles maybe directly
manufactured using photolithographic techniques.
BACKGROUND OF THE INVENTION
[0004] The present invention relates to methods of manufacture of
segmented particles and assemblies of differentiable particles
(which may or may not be segmented). Without a doubt, there has
been a paradigm change in what is traditionally defined as
bioanalytical chemistry. A major focus of these new technologies is
to generate what could be called "increased per volume information
content". This term encompasses several approaches, from reduction
in the volume of sample required to carry out an assay, to highly
parallel measurements ("multiplexing"), such as those involving
immobilized molecular arrays, to incorporation of second (or third)
information channels, such as in 2-D gel electrophoresis or
CE-electrospray MS/MS.
[0005] Unfortunately, many of these seemingly revolutionary
technologies are limited by a reliance on relatively pedestrian
materials, methods, and analyses. For example, development of DNA
microarrays ("gene chips") for analysis of gene expression and
genotyping by Affymetrix, Incyte and similar companies has
generated the wherewithal to immobilize up to 20,000 different
fragments or full-length pieces of DNA in a spatially-defined
1-cm.sup.2 array. At the same time, however, the use of these chips
in all cases requires hybridization of DNA in solution to DNA
immobilized on a planar surface, which is marked both by a decrease
in the efficiency of hybridization (especially for cDNA) and a far
greater degree of non-specific binding. It is unclear whether these
problems can be completely overcome. Moreover, there is a general
sense of disillusionment both about the cost of acquiring external
technology and the lead-time required to develop DNA arraying
internally.
[0006] A second example of how groundbreaking can be slowed by
inferior tools is in pharmaceutical discovery by combinatorial
chemistry. At the moment, solution phase, 5-10 .mu.m diameter latex
beads are used extensively as sites for molecular immobilization.
Exploiting the widely adopted "split and pool" strategy, libraries
of upwards of 100,000 compounds can be simply and rapidly
generated. As a result, the bottleneck in drug discovery has
shifted from synthesis to screening, and equally importantly, to
compound identification, (i.e., which compound is on which bead?).
Current approaches to the latter comprise "bead encoding", whereby
each synthetic step applied to a bead is recorded by parallel
addition of an organic "code" molecule; reading the code allows the
identity of the drug lead on the bead to be identified.
Unfortunately, the "code reading" protocols are far from optimal:
in most every strategy, the code molecule must be cleaved from the
bead and separately analyzed by HPLC, mass spectrometry or other
methods. In other words, there is at present no way to identify
potentially interesting drug candidates by direct, rapid
interrogation of the beads on which they reside, even though there
are numerous screening protocols in which such a capability would
be desirable.
[0007] Two alternative technologies with potential relevance both
to combinatorial chemistry and genetic analysis involve
"self-encoded beads", in which a spectrally identifiable bead
substitutes for a spatially defined position. In the approach
pioneered by Walt and co-workers, beads are chemically modified
with a ratio of fluorescent dyes intended to uniquely identify the
beads, which are then further modified with a unique chemistry
(e.g. a different antibody or enzyme). The beads are then randomly
dispersed on an etched fiber array so that one bead associates with
each fiber. The identity of the bead is ascertained by its
fluorescence readout, and the analyte is detected by fluorescence
readout at the same fiber in a different spectral region. The
seminal paper (Michael et al., Anal. Chem. 70, 1242-1248 (1998)) on
this topic points out that with 6 different dyes (15 combinations
of pairs) and with 10 different ratios of dyes, 150 "unique optical
signatures" could be generated, each representing a different bead
"flavor". A very similar strategy is described by workers at
Luminex, who combine flavored beads ready for chemical modification
(100 commercially available) with a flow cytometry-like analysis.
(See, e.g., McDade et al., Med. Rev. Diag. Indust. 19, 75-82
(1997)). Once again, the particle flavor is determined by
fluorescence, and once the biochemistry is put onto the bead, any
spectrally distinct fluorescence generated due to the presence of
analyte can be read out. Note that as currently configured, it is
necessary to use one color of laser to interrogate the particle
flavor, and another, separate laser to excite the bioassay
fluorophores.
[0008] A more significant concern with self-encoded latex beads is
the limitations imposed by the wide bandwidth associated with
molecular fluorescence. If the frequency space of molecular
fluorescence is used both for encoding and for bioassay analysis,
it is hard to imagine how, for example, up to 20,000 different
flavors could be generated. This problem might be alleviated
somewhat by the use of combinations of glass-coated quantum dots,
which exhibit narrower fluorescence bandwidths. (See, e.g. Bruchez
et al., Science, 281, 2013-2016 (1998)). However, these "designer"
nanoparticles are quite difficult to prepare, and at the moment,
there exist more types of fluorophores than (published) quantum
dots. If, however, it were possible to generate very large numbers
of intrinsically-differentiable particles by some means, then
particle-based bioanalysis would become exceptionally attractive,
insofar as a single technology platform could then be considered
for the multiple high-information content research areas; including
combinatorial chemistry, genomics, and proteomics (via multiplexed
immunoassays).
[0009] Previous work has originally taught how metal can be
deposited into the pores of a metallized membrane to make an array
of metal nanoparticles embedded in the host. Their focus was on the
optical and/or electrochemical properties of these materials. A
similar technique was used to make segmented cylindrical magnetic
nanoparticles in a host membrane, where the composition of the
particles was varied along the length. In no case, however, have
freestanding, rod-shaped nanoparticles with variable compositions
along their length been prepared. Indeed, "freestanding" rod-shaped
metal nanoparticles of a single composition, in which the length is
at least one micron, have never been reported. Likewise,
freestanding rod-shaped metal nanoparticles not embedded or
otherwise contained within such host materials have never been
reported. See, Martin et al., Adv. Materials 11:1021-25 (1999).
SUMMARY OF THE INVENTION
[0010] Rod-shaped nanoparticles have been prepared whose
composition is varied along the length of the rod. These particles
are referred to as nanoparticles or nanobar codes, though in
reality some or all dimensions may be in the micron size range. The
present invention is directed to methods of manufacture of such
nanoparticles.
[0011] The present invention includes methods of manufacture of
free-standing particles comprising a plurality of segments, wherein
the particle length is from 10 nm to 50 .mu.m and particle width is
from 5 nm to 50 .mu.m. The segments of the particles of the present
invention may be comprised of any material. Included among the
possible materials are a metal, any metal chalcogenide, a metal
oxide, a metal sulfide, a metal selenide, a metal telluride, a
metal alloy, a metal nitride, a metal phosphide, a metal
antimonide, a semiconductor, a semi-metal, any organic compound or
material, any inorganic compound or material, a particulate layer
of material or a composite material. The segments of the particles
of the present invention may be comprised of polymeric materials,
crystalline or non-crystalline materials, amorphous materials or
glasses. In certain preferred embodiments of the invention, the
particles are "functionalized" (e.g., have their surface coated
with IgG antibody). Commonly, such functionalization may be
attached on selected or all segments, on the body or one or both
tips of the particle. The functionalization may actually coat
segments or the entire particle. Such functionalization may include
organic compounds, such as an antibody, an antibody fragment, or an
oligonucleotide, inorganic compounds, and combinations thereof.
Such functionalization may also be a detectable tag or comprise a
species that will bind a detectable tag.
[0012] Also included within the present invention are methods of
manufacture of an assembly or collection of particles comprising a
plurality of types of particles, wherein each particle is from 20
nm to 50 .mu.m in length and is comprised of a plurality of
segments, and wherein the types of particles are differentiable. In
the preferred embodiments, the particle types are differentiable
based on differences in the length, width or shape of the particles
and/or the number, composition, length or pattern of said segments.
In other embodiments, the particles are differentiable based on the
nature of their functionalization or physical properties (e.g., as
measured by mass spectrometry or light scattering).
[0013] The present invention includes the manufacture of nanobar
codes by the electrochemical deposition of metals inside a template
wherein the process is improved, separately and collectively, by i)
electroplating in an ultrasonication bath; and ii) controlling the
temperature of the deposition environment, preferably by using a
recirculating temperature bath.
[0014] Also included within the scope of the invention are methods
for the simultaneous or parallel manufacture of a plurality of
different types of nanobar codes. According to one such method, a
plurality of templates are held in a common solution chamber and
electrochemical deposition is accomplished by controlling
deposition at each membrane by applying current selectively to
predetermined electrodes associated with each such membrane.
[0015] Also included within this invention is an apparatus for the
manufacture of nanobar codes comprising: a plating solution cell, a
defined-pore size template, means for applying a current to cause
electrochemical deposition of a metal into said template, means for
agitation of the plating solution, such as an ultrasonic
transducer, and temperature control means.
[0016] Also included within this invention is an apparatus for the
simultaneous manufacture of a plurality of different types of
nanobar codes. In one embodiment, such apparatus comprises: a
solution chamber, a plurality of templates, means for selectively
applying a current to each of said templates, and control means for
operating said apparatus.
[0017] Also within the scope of the invention are methods of making
segmented nanoparticles using a porous template manufactured by
standard photolithographic techniques, comprising exposing a
pattern on a resist-coated substrate or multi-layer stack and then
etching the exposed pattern to form pores.
[0018] Also included within the invention are methods for forming
nanoparticles by exposing a pattern on a resist-coated substrate
comprising one or more layers of metal, then etching the exposed
pattern to form free-standing nanoparticles.
BRIEF DESCRIPTION OF THE FIGURES
[0019] FIG. 1 is a perspective view of an apparatus for
manufacturing a plurality of different types of nanobar codes.
[0020] FIG. 2 is a cross-sectional elevation view of the apparatus
of FIG. 1.
[0021] FIG. 3 is a schematic illustration of a four-layer stack on
a silicon wafer substrate (A) before exposure; (B) following
exposure and development of the photoresist, and etching to etch
stop; (C) following further etching to conductive layer, (D) after
formation of the segmented nanoparticle, and (E) the liberated
segmented nanoparticles.
[0022] FIG. 4 is an SEM (top view) of a template prepared using
photolithographic techniques in which nanoparticles have been
formed by electrochemical deposition. The pore diameter is
approximately 2.5 to 3 .mu.m.
[0023] FIG. 5 is an SEM (side view) of a free-standing nanoparticle
made by electrodeposition in a template prepared using
photolithographic techniques.
[0024] FIG. 6 is an SEM (cross-sectional view) of a template
prepared using photolithographic techniques.
DETAILED WRITTEN DESCRIPTION OF THE INVENTION
[0025] The present application is directed to methods of
manufacture of nanoparticles. Such nanoparticles and their uses are
described in detail in U.S. Utility application Ser. No.
09/598,395, filed Jun. 20, 2000, entitled "Colloidal Rod Particles
as Nanobar Codes," incorporated hereby in its entirety by
reference. Also incorporated herein in their entirety by reference,
are two U.S. Utility applications entitled "Methods of Imaging
Colloidal Rod Particles as Nanobarcodes" and "Colloidal Rod
Particles as Nanobar Codes." The present application is filed as a
Continuation-in-Part of the Ser. No. 09/677,203 application.
[0026] The synthesis and characterization of multiple segmented
particles is described in Martin et al., Adv. Materials 11: 1021-25
(1999). The article is incorporated herein by reference in its
entirety. Also incorporated herein by reference in their entirety
are U.S. Provisional Application Serial No. 60/157,326, filed Oct.
1, 1999, entitled "Self Bar-coded Colloidal Metal Nanoparticles";
U.S. Provisional Application Serial No. 60/189,151, filed Mar. 14,
2000, entitled "Nanoscale Barcodes"; U.S. Provisional Application
Serial No. 60/190,247, filed Mar. 17, 2000, entitled "Colloidal Rod
Particles as Barcodes"; U.S. Provisional Application Serial No.
60/194,616, filed Apr. 5, 2000, entitled "Nanobarcodes: Technology
Platform for Phenotyping;" U.S. Provisional Application Serial No.
60/237,322, filed Oct. 2, 2000, entitled "Methods for the
Manufacture of Colloidal Rod Particles as Nanobar Codes;" and U.S.
Provisional Application Serial No. 60/285,017, filed Apr. 19, 2001,
entitled "Method of Manufacture of Colloidal Rod Particles."
[0027] Because bar coding is so widely-used in the macroscopic
world, the concept has been translated to the molecular world in a
variety of figurative manifestations. Thus, there are "bar codes"
based on analysis of open reading frames, bar codes based on
isotopic mass variations, bar codes based on strings of chemical or
physical reporter beads, bar codes based on electrophoretic
patterns of restriction-enzyme cleaved mRNA, bar-coded surfaces for
repeatable imaging of biological molecules using scanning probe
microscopies, and chromosomal bar codes (a.k.a. chromosome
painting) produced by multi-chromophore fluorescence in situ
hybridization. All these methods comprise ways to code biological
information, but none offer the range of advantages of the bonafide
bar codes of the present invention, transformed to the nanometer
scale.
[0028] The particles to be manufactured according to the present
invention are alternately referred to as nanoparticles, nanobar
codes, rods, nanorods, Nanobarcodes.TM. particles, and rod shaped
particles. To the extent that any of these descriptions may be
considered as limiting the scope of the invention, the label
applied should be ignored. For example, although in certain
embodiments of the invention, the particle's composition contains
informational content, this is not true for all embodiments of the
invention. Likewise, although nanometer-sized particles fall within
the scope of the invention, not all of the particles of the
invention fall within such size range.
[0029] In certain preferred embodiments of the present invention,
the nanobar code particles are manufactured by electrochemical
deposition in an alumina or polycarbonate template, followed by
template dissolution, and typically, they are prepared by
alternating electrochemical reduction of metal ions, though they
may easily be prepared by other means, both with or without a
template material. Typically, the nanobar codes have widths between
30 nm and 1,000 nanometers, though they can have widths of several
microns. Likewise, while the lengths (i.e. the long dimension) of
the materials are typically on the order of 1 to 15 microns, they
can easily be prepared in lengths as long as 50 microns, and in
lengths as short as 20 nanometers. In some embodiments, the nanobar
codes comprise two or more different materials alternated along the
length, although in principle as many as dozens of different
materials could be used. Likewise, the segments could consist of
non-metallic material, including but not limited to polymers,
oxides, sulfides, semiconductors, insulators, plastics, and even
thin (i.e., monolayer) films of organic or inorganic species.
[0030] When the particles of the present invention are made by
electrochemical deposition, the length of the segments (as well as
their density and porosity) can be adjusted by controlling the
amount of current (or electrochemical potential) passed in each
electroplating step; as a result, the rod resembles a "bar code" on
the nanometer scale, with each segment length (and identity)
programmable in advance. Other forms of deposition can also yield
the same results. For example, deposition can be accomplished via
electroless processes and in electrochemical deposition by
controlling the area of the electrode, the heterogenous rate
constant, the concentration of the plating material, and the
potential and combinations thereof (collectively referred to herein
as electrochemical deposition). The same result could be achieved
using another method of manufacture in which the length or other
attribute of the segments can be controlled. While the diameter of
the rods and the segment lengths are typically of nanometer
dimensions, the overall length is such that in preferred
embodiments it can be visualized directly in an optical microscope,
exploiting the differential reflectivity of the metal
components.
[0031] The particles of this embodiment of the present invention
are defined in part by their size and by the existence of at least
2 segments. The length of the particles can be from 10 nm up to 50
.mu.m. In preferred embodiments the particle is 500 nm-30 .mu.m in
length. In the most preferred embodiments, the length of the
particles of this invention is 1-15 .mu.m. The width, or diameter,
of the particles of the invention is within the range of 5 nm-50
.mu.m. In preferred embodiments the width is 10 nm-1 .mu.m, and in
the most preferred embodiments the width or cross-sectional
dimension is 30 nm-500 nm.
[0032] As discussed above, the particles of the present invention
are characterized by the presence of at least two segments. A
segment represents a region of the particle that is
distinguishable, by any means, from adjacent regions of the
particle. Segments of the particle bisect the length of the
particle to form regions that have the same cross-section
(generally) and width as the whole particle, while representing a
portion of the length of the whole particle. In preferred
embodiments of the invention, a segment is composed of different
materials from its adjacent segments. However, not every segment
needs to be distinguishable from all other segments of the
particle. For example, a particle could be composed of 2 types of
segments, e.g., gold and platinum, while having 10 or even 20
different segments, simply by alternating segments of gold and
platinum. A particle of the present invention contains at least two
segments, and as many as 50. The particles of the invention
preferably have from 2-30 segments and most preferably from 3-20
segments. The particles may have from 2-10 different types of
segments, preferably 2 to 5 different types of segments.
[0033] A segment of the particle of the present invention is
defined by its being distinguishable from adjacent segments of the
particle. The ability to distinguish between segments includes
distinguishing by any physical or chemical means of interrogation,
including but not limited to electromagnetic, magnetic, optical,
spectrometric, spectroscopic and mechanical. In certain preferred
embodiments of the invention, the method of interrogating between
segments is optical (reflectivity).
[0034] Adjacent segments may even be of the same material, as long
as they are distinguishable by some means. For example, different
phases of the same elemental material, or enantiomers of organic
polymer materials can make up adjacent segments. In addition, a rod
comprised of a single material could be considered to fall within
the scope of the-invention if segments could be distinguished from
others, for example, by functionalization on the surface, or having
varying diameters. Also particles comprising organic polymer
materials could have segments defined by the inclusion of dyes that
would change the relative optical properties of the segments.
[0035] The composition of the particles of the present invention is
best defined by describing the compositions of the segments that
make up the particles. A particle may contain segments with
extremely different compositions. For example, a single particle
could be comprised of one segment that is a metal, and a segment
that is an organic polymer material.
[0036] The segments of the present invention may be comprised of
any material. In preferred embodiments of the present invention,
the segments comprise a metal (e.g., silver, gold, copper, nickel,
palladium, platinum, cobalt, rhodium, iridium); any metal
chalcognide; a metal oxide (e.g., cupric oxide, titanium dioxide);
a metal sulfide; a metal selenide; a metal telluride; a metal
alloy; a metal nitride; a metal phosphide; a metal antimonide; a
semiconductor; a semi-metal. A segment may also be comprised of an
organic mono- or bilayer such as a molecular film. For example,
monolayers of organic molecules or self assembled, controlled
layers of molecules can be associated with a variety of metal
surfaces.
[0037] A segment may be comprised of any organic compound or
material, or inorganic compound or material or organic polymeric
materials, including the large body of mono and copolymers known to
those skilled in the art. Biological polymers, such as peptides,
oligonucleotides and polysaccharides may also be the major
components of a segment. Segments may be comprised of particulate
materials, e.g., metals, metal oxide or organic particulate
materials; or composite materials, e.g., metal in polyacrylamide,
dye in polymeric material, porous metals. The segments of the
particles of the present invention may be comprised of polymeric
materials, crystalline or non-crystalline materials, amorphous
materials or glasses.
[0038] Segments may be defined by notches on the surface of the
particle, or by the presence of dents, divits, holes, vesicles,
bubbles, pores or tunnels that may or may not contact the surface
of the particle. Segments may also be defined by a discemable
change in the angle, shape, or density of such physical attributes
or in the contour of the surface. In embodiments of the invention
where the particle is coated, for example with a polymer or glass,
the segment may consist of a void between other materials.
[0039] The length of each segment may be from 10 nm to 50 .mu.m. In
preferred embodiments the length of each segment is 50 nm to 20
.mu.m. The interface between segments, in certain embodiments, need
not be perpendicular to the length of the particle or a smooth line
of transition. In addition, in certain embodiments the composition
of one segment may be blended into the composition of the adjacent
segment. For example, between segments of gold and platinum, there
may be a 5 nm to 5 .mu.m region that is comprised of both gold and
platinum. This type of transition is acceptable so long as the
segments are distinguishable. For any given particle the segments
may be of any length relative to the length of the segments of the
rest of the particle.
[0040] As described above, the particles of the present invention
can have any cross-sectional shape. In preferred embodiments, the
particles are generally straight along the lengthwise axis.
However, in certain embodiments the particles may be curved or
helical. The ends of the particles of the present invention may be
flat, convex or concave. In addition, the ends may be spiked or
pencil tipped. Sharp-tipped embodiments of the invention may be
preferred when the particles are used in Raman spectroscopy
applications or others in which energy field effects are important.
The ends of any given particle may be the same or different.
Similarly, the contour of the particle may be advantageously
selected to contribute to the sensitivity or specificity of the
assays (e.g., an undulating contour will be expected to enhance
"quenching" of fluorophores located in the troughs).
[0041] In many embodiments of the invention, an assembly or
collection of particles is prepared. In certain embodiments, the
members of the assembly are identical, while in other embodiments,
the assembly is comprised of a plurality of different types of
particles. In embodiments of the invention comprising assemblies of
identical particles, the length of substantially all of the
particles for particles in the 1 .mu.m -15 .mu.m range may vary up
to 50%. Segments of 10 nm in length will vary .+-.5 nm while
segments in 1 .mu.m range may vary up to 50%. The width of
substantially all of the particles may vary between 10 and 100%
preferably less than 50% and most preferably less than 10%.
[0042] The present invention includes assemblies or collections of
nanobar codes made up of a plurality of particles that are
differentiable from each other. Assembly or collection, as used
herein, does not mean that the nanoparticles that make up such an
assembly or collection are ordered or organized in any particular
manner. Such an assembly is considered to be made up of a plurality
of different types or "flavors" of particles. In some such
assemblies, each of the nanobar codes of the assembly may be
functionalized in some manner. In many applications, the
functionalization is different and specific to the specific flavor
of nanoparticle. The assemblies of the present invention can
include from 2 to 10.sup.12 different and identifiable
nanoparticles. Preferred assemblies include more than 10, more than
100, more than 1,000 and, in some cases, more than 10,000 different
flavors of nanoparticles. The particles that make up the assemblies
or collections of the present invention are segmented in most
embodiments. However, in certain embodiments of the invention the
particles of an assembly of particles do not necessarily contain a
plurality of segments.
[0043] In certain embodiments of the invention, the particles of
the present invention may include mono-molecular layers. Such
mono-molecular layers may be found at the tips or ends of the
particle, or between segments. Examples of the use of
mono-molecular layers between segments are described in the section
entitled ELECTRONIC DEVICES in U.S. Utility application Ser. No.
09/598,395, filed Jun. 20, 2000.
[0044] The present invention is directed to the manufacture of
freestanding nanobar codes. By "freestanding" it is meant that
nanobar codes that are produced by some form of deposition or
growth within a template have been released from the template. Such
nanobar codes are typically freely dispensable in a liquid and not
permanently associated with a stationary phase. Nanobar codes that
are not produced by some form of deposition or growth within a
template (e.g., self-assembled nanobar codes) may be considered
freestanding even though they have not been released from a
template. The term "free standing" does not imply that such
nanoparticles must be in solution (although they may be) or that
the nanobar codes can not be bound to, incorporated in, or a part
of a macro structure. Indeed, certain embodiments of the invention,
the nanoparticles may be dispersed in a solution, e.g., paint, or
incorporated within a polymeric composition.
[0045] The particles of the present invention may be prepared by a
variety of processes. The preferred process for the manufacture of
a particular particle can often be a function of the nature of the
segments comprising the particle. In most embodiments of the
invention, a template or mold is utilized into which the materials
that constitute the various segments are introduced. Defined-pore
materials are the preferred templates for many of the preferred
particles of the present invention. Al.sub.2O.sub.3 membranes
containing consistently sized pores are among the preferred
templates, while photolithographically prepared templates, porous
polycarbonate membranes, zeolites and block co-polymers may also be
used. Methods for forming segments of particles include
electrodeposition, chemical deposition, evaporation, chemical self
assembly, solid phase manufacturing techniques and photolithography
techniques. Chemical self assembly is a method of forming particles
from preformed segments whereby the segments are derivatized and a
chemical reaction between species on different segments create a
juncture between segments. Chemically self-assembled nanoparticles
have the unique ability of being controllably separated between
segments by reversing the chemical bond formation process.
[0046] One of the preferred synthetic protocols used to prepare
metallic nanobar codes according to the embodiments of the present
invention is an extension of the work of Al-Mawlawi et al.
(Al-Mawlawi, D.; Liu, C. Z.; Moskovits, M. J. Mater. Res. 1994, 9,
1014; Martin, C. R. Chem. Mater. 1996, 8, 1739) on
template-directed electrochemical synthesis. See, Example 1, below.
In this approach, metals are deposited electrochemically inside a
porous membrane. The synthetic method of the present invention
differs from previous work in several respects including the
following. First, the electroplating is done with agitation, such
as in an ultrasonication bath. Second, the temperature is
controlled, for example, by using a recirculating temperature bath.
These first two modifications increase the reproducibility and
monodispersity of rod samples by facilitating the mass transport of
ions and gases through the pores of the membrane. Third, rods with
multiple stripes are prepared by sequential electrochemical
reduction of metal ions (e.g., Pt.sup.2+, Au.sup.+) within the
pores of the membranes. Because the length of the segments can be
adjusted by controlling the amount of current passed in each
electroplating step, the rod resembles a "bar code" on the
nanometer scale, with each segment length (and identity)
programmable in advance. While the width of the rods and the
segment lengths are generally of nanometer dimensions, the overall
length is generally such that it can be visualized directly in an
optical microscope, exploiting the differential reflectivity of the
metal components.
[0047] There are many parameters in the nanorod synthesis that are
tunable, such that it is theoretically possible to generate many
millions of different patterns, uniquely identifiable by using
conventional optical microscopy or other methods. The most
important characteristic that can be changed is the composition of
the striped rods. The simplest form of a nanoparticle is one with
only one segment. To this end, several different types of these
solid bar codes have been prepared. By simply using only one
plating solution during the preparation, a solid nanoparticle is
produced.
[0048] To generate two-segment nanobar codes, two metals (e.g., Au,
Ag, Pd, Cu, etc.) can be electroplated sequentially, or
simultaneously to form alloys. Nanobar codes can also be generated
using 3 different metals. Synthesis of a Au/Pt/Au rod may be
accomplished with 1 C of Au, 8 C Pt, and 1 C of Au. The nominal
dimensions of the segments are 1 .mu.m of Au, 3 .mu.m of Pt, 1
.mu.m of Au. The 5-segment nanobar codes, Ag/Au/Ag/Au/Ag, were
generated by sequentially plating the appropriate metal. In some
embodiments it is possible to include all metals in solution but
control deposition by varying the charge potential current. A
nine-segment nanobar code, Au/Ag/Au/Ag/Au/Ag/Au/Ag/Au has also been
prepared. The number of segments can be altered to desired
specifications.
[0049] The next controllable factor is diameter (sometimes referred
to herein as width) of the individual rods. Many of the nanobar
codes described were synthesized using membranes with a pore
diameter of 200 nm. By altering the pore diameter, rods of
differing diameter can be made. Au rods have been synthesized in a
membrane that has 10 nm diameter pores, 40 nm pores and pores in
the range of 200-300 nm.
[0050] The ends of the rods typically have rounded ends or flat
ends. A TEM image of an Au rod that was made by reversing the
current flow (from reduction at -0.55 mA/cm.sup.2 to oxidation at
+0.55 mA/cm.sup.2) and removing some of the gold from the tip of
the rod generated a spike extending from the tip of the rod.
Additionally, branched ends can be generated. This can be typically
controlled by controlling the amount of metal that is plated into
the membrane. The edges of the membrane pores have a tendency to be
branched which lead to this type of structure.
[0051] An additional way to alter the ends of the rods is to
control the rate of deposition. Gold rods (2 C total, 3 .mu.m) were
plated at a current density of 0.55 mA/cm.sup.2. Then the current
density was reduced to 0.055 mA/cm.sup.2 and 0.1 C of Au was
plated. The last segment of gold deposits is a hollow tube along
the walls of the membrane.
[0052] Example 1 describes the manufacture of single flavors of
nanoparticles according to one embodiment of the invention.
[0053] In order to produce many thousands of flavors of nanorods,
in practical quantities, and to attach molecules to most or all,
novel combinatorial or multiplexed synthesis techniques are
necessary. Several synthesis embodiments are included within the
scope of the invention. Each approach has advantages and
disadvantages depending on the specific application and the
required number of types and total number of nanorods needed for
the application.
[0054] The present invention includes methods of manufacture of
nanoparticles that allow for the simultaneous or parallel
manufacture of a plurality of different flavors of nanobar
codes.
[0055] Prior to the present invention, no system or apparatus has
been described whereby it was possible to prepare more than one
type of nanobar code simultaneously or in parallel. In the
preferred embodiments of this invention, such method for the
simultaneous manufacture of nanobar codes allows for the
manufacture of 2 or more, more than 5, more than the 10 and
preferably more than 25 different flavors of nanobar codes. By
simultaneous or parallel it is meant that common elements are
employed in the manufacture of the more than one nanobar code. For
example, in the apparatus depicted in FIGS. 1 and 2, there are 25
separate membranes, each with a separately controllable electrode
connection on the back side, but with common access to the plating
solution. In other embodiments, the separate membranes (or regions
on a single membrane) may have a common electrode, but separately
controllable solution access. In still other embodiments, the
simultaneous manufacture of different types of nanoparticles is
commonly controlled. Any system or apparatus whereby a plurality of
different flavors of nanoparticles (e.g, particles having a
plurality of segments, that are 10 nm to 50 .mu.m in length, and
have a width from 5 nm to 50 .mu.m that are differentiable from
each other) can be prepared in parallel is included within the
scope of this invention. Among the options that can be employed to
effect this parallel manufacture are the following:
[0056] 1. Multi-electrode and Microfluidic Synthesis: To synthesize
many flavors of nanorods on a single membrane, the membrane can be
divided into separate electrical zones, with each zone using a
different plating recipe. Of course, several smaller membranes
could be used, one for each separate zone, as opposed to a single
membrane with multiple zones. The electrical zone approach can be
achieved by patterning the Ag evaporation that initially seals one
side of the membrane into many separate islands. Each island would
have its own electrode, and control circuitry can activate each
island separately for plating. The microfluidic approach utilizes a
single evaporated Ag electrode, but would divide the opposite side
of the membrane into separate fluidic regions, and control the flow
of plating solutions to each region. Both of these techniques may
be automated, and result in the synthesis of hundreds of nanorod
flavors per membrane. Thousands to millions of flavors is probably
not practical with either of these approaches due to practical
limitations in the number of electrical or fluidic connections to
the membrane
[0057] 2. Patterned front-side insulation: This approach applies
insulating patterned coatings (e.g., photoresist) to the front-side
(electrodeposition side) of a membrane. Where the membrane is
coated, electroplating is inhibited. The coating can be removed and
reapplied with different pattern between electroplating steps to
achieve synthesis of many flavors of nanobarcode within one
membrane.
[0058] 3. Patterned back-side insulation: This approach applies
insulating patterned coatings (e.g., photoresist) to the back-side
(electrode side) of a membrane, which is divided into many separate
electrical contacts. Where the electrode is coated, electroplating
is inhibited. The coating can be removed and reapplied with
different patterns between electroplating steps to achieve
synthesis of many flavors of nanobarcode within one membrane.
[0059] 4. Lithography vertical or horizontal: This technique, that
offers increased design flexibility in the size and shape of
nanorods, utilizes lithographic processes to pattern the deposition
of multiple layers of metals on a silicon substrate. This approach
takes advantage of the tremendous capabilities developed in
microelectronics and MEMS, and promises very high quality nanorods
with greater design flexibility in the size and shape of nanorods
than membrane-based techniques. Each of these synthetic approaches
must be mated to complementary well arrays to allow nanobar release
into separate vessels.
[0060] 5. Light-addressable electroplating: A further technique
that could produce thousands of flavors in one synthesis step also
utilizes membrane-based synthesis, but includes light-directed
control of the electroplating process. In this technique, a
light-addressable semiconductor device is used to spatially modify
the electrical potentials in the vicinity of the membrane, and thus
spatially modulate electroplating currents. In this manner, the
membrane is optically subdivided into many different zones, each of
which produces a different flavor of nanorod.
[0061] 6. Electrical multiplexing to multiple separate template
membranes immersed in common plating solution: In this approach,
multiple template membranes are immersed in a common plating
solution, with a common anode electrode (platinum). Each membrane
has a separate electrical connection from a computer-controlled
current and/or voltage source to its silver-coated backside.
[0062] 7. Template Dicing: A template may be cut into a number of
smaller pieces. This may be accomplished, for example, using a
dicing saw or by a "scribe and break" procedure where the wafer is
cut part of the way through and then broken; the latter may be
preferred in some embodiments because it generates less dust and
debris.
[0063] Several of these embodiments are based on existing
procedures using defined-pore membranes. (i) One technique
generates hundreds to perhaps a few thousand types of nanorods, by
lithographically patterning the backside silver that is deposited
on the membrane into isolated islands, each island forming an
individually addressable electrical contact. By way of example,
each island would have enough surface area to contain between
10.sup.6 and 10.sup.8 individual rods, all of the same type. (Note
that since the membrane thickness, and therefore pore length, is
much greater than the nanorod length, multiple nanorods can be
synthesized in each pore. Each nanorod may be separated from others
in the same pore by a silver plug that would later be dissolved.
This could increase the total yield by 10.times..) The membrane is
then placed, with careful registration, onto a "bed-of-nails"
apparatus, with individual spring-loaded pins contacting each
electrode on the membrane. Computer-controlled circuitry attached
to the bed-of-nails is able to individually turn on or off each
electrode. During the electroplating process, each island would be
plated with unique combinations of metal types and thicknesses. In
this manner, each island would produce rods of different lengths,
different numbers of stripes, and different material combinations,
allowing ultimate design flexibility. (ii) The above approach will
be limited in the number of types of rods that can be synthesized
by the reliability and packing density of the bed-of-nails
apparatus. To avoid this limitation, the bed-of-nails apparatus can
be replaced by a liquid metal contact. To prevent the liquid bath
from simultaneously contacting every electrode, the backside of the
membrane may be patterned with a nonconductive coating. To
individually address electrodes during synthesis, the pattern would
be removed and replaced with a different pattern between
electroplating steps. This approach will enable a much higher
density of isolated islands, and therefore more types of rods to be
synthesized. With island spacing of 100 microns, which would be
trivial to achieve using lithographical patterning, up to 10.sup.5
types of rods could be synthesized. Since the total number of pores
in each membrane is a constant there will be proportionally fewer
rods of each type. (iii) The above two approaches use commercially
available aluminum oxide membrane filters, which have pore size and
density that are suitable for nanorod synthesis. However, the
membrane thickness is typically greater than that required, which
can cause variability in rod and stripe lengths due to non-uniform
mass transport into the pores during electroplating. Also, the
largest pores available in these membranes (and thus nanorod
widths) are 250 nm, and it would be desirable for some applications
to have rod widths of 1 micron or more (this could also be used for
embodiments with widths of less than 1 .mu.m).
[0064] To address these issues, pore matrices may be constructed
using photolithography techniques, which will give ultimate control
over the pore dimensions and lengths, and increase the design
flexibility and quality of the resulting nanorods. According to
this embodiment a positive photoresist-coated substrate is exposed
to an interference pattern of light, using a technique similar to
that used for interference-lithography generated diffraction
gratings. Typically, the substrate is a silicon wafer, with (a) a
thin coating of a conductive material, such as titanium nitride, or
gold, (b) a thick coating of polymer, such as
polymethylmethacrylate (PMMA) or polyimide, (c) an etch stop, such
as SiO.sub.2, aluminum, or nickel, and (d) a photoresist. Exposure
and subsequent development yields a two-dimensional array of pores
in the photoresist. Reactive ion etching may then be used to
transfer the pore pattern down through the polymer layer. The
photoresist layer is removed, and the conductive layer under the
polymer becomes the cathode for electroplating into the pores. The
shape and diameter of the nanorods can be controlled by the mask or
by adjusting the light source and the resultant standing wave
pattern. For most applications, a conventional mask is preferred.
However, interference lithography techniques may be preferred when
the desired pore diameter is lower than the resolution limit
available from state of the art projection lithography tools.
Achieving smaller pore size may also benefit from the use of x-ray
or e-beam etching.
[0065] An advantage to this technique is that the template
thickness, which may be the same as pore length, can be tailored to
the length of the rods, which may improve uniformity of
electroplating across the membrane. With this technique, 10.sup.10
to 10.sup.12 nanorods can be constructed on a single substrate. The
two approaches described above can be utilized to synthesize many
types of nanobar code from a single wafer. (iv) A further approach
uses the customized lithographically-defined pores from above, and
achieves the ultimate in design flexibility by using novel
light-directed electroplating. The template pores are constructed
just as in the third approach, but on top of a photosensitive
semiconductor wafer. The pore-side of the wafer is immersed in an
electroplating reagent, and the other side is illuminated with
patterns of light. Light exposure is used to generate photocurrent
in the wafer, and switch the plating current on or off for each
conductive zone within the wafer. A computer-controlled spatial
light modulator selectively illuminates different zones at
different times, so that each zone will be subjected to a different
computer-controlled plating recipe. Depending on the resolution of
the optical system that exposes the wafer, this could result in
10.sup.4 to 10.sup.6 separate flavors of nanorods synthesized on a
single wafer. With 10.sup.12 total pores per wafer, 10.sup.6 to
10.sup.8 nanorods of each flavor could be synthesized.
[0066] Membranes with extremely high densities of uniform pores can
be created by photolithographic techniques and the resulting
nanoparticles are of very uniform size and length. Using the
methods described herein, a 4-inch silicon wafer can serve as the
substrate for the formation of, for example, 50 billion pores of
diameter 200 nm and period 400 nm. Pores of smaller diameter and
lower period are readily achievable. Unlike the pores formed in
anodized alumina membranes or polycarbonate trach-etched membranes,
pores formed by photolithographic techniques have a very tight
diameter distribution, do not overlap or branch, and are straight
and parallel.
[0067] Interference Lithography and Achromatic Interference
Lithography-Based Methods for Nanoparticle Template Formation
[0068] In one series of embodiments, membrane templates are formed
from resist-coated substrates or multi-layer stacks by means of
interference lithography (also known as "holographic" or
"interferometric" lithography). Interference lithography (IL) is
well known in the semiconductor and microfabrication arts as a
technique capable of patterning grids and gratings over a large
area of resist (up to 10 cm diameter) without using a mask.
Briefly, IL involves forming an optical standing wave through the
intersection of two laser beams. The standing wave creates a line
of alternating exposed and unexposed regions on the resist. By
exposing a resist first in one orientation, and then at 90.degree.
to the first orientation, a "grid" is patterned on the resist. This
pattern can be transferred into material that lies underneath the
resist, thereby forming pores, by developing the resist and then
performing an etch. The resist and/or the underlying material form
the walls of the membrane pores within which nanoparticle can
subsequently be formed.
[0069] The period of the patterns that interference lithography can
expose is given by the equation p=.lambda./(2 sin .theta.), wherein
.lambda. is the wavelength, and .theta. is the half angle between
the intersecting beams. Interference lithography is best performed
with wavelengths longer than 248 nm; this corresponds to a period
of approximately 200 nm. To generate patterns of lower period,
shorter wavelength laser light is required. However, lasers that
provide shorter wavelength light generally do not produce
sufficiently monochromatic light to generate robust interference
patterns.
[0070] To form patterns with a period less than 200 nm, achromatic
interference lithography (AIL) is preferred. An optical system for
generating 100 nm period patterns with 193 nm light from a ArF
excimer laser, for example, is described in Savas et al., J. Vac.
Sci. Tech. 1996, 14, 4167, incorporated herein by reference in its
entirety. Briefly, achromatic interference lithography uses a first
phase grating to generate two first-order light beams from an
incident light source. Two further phase gratings recombine these
divergent light beams by second-order diffraction. In the case of
193 nm light from the ArF laser, each phase grating has a period of
200 nm and each is fabricated by interference lithography and
reactive ion etching. The resulting light beam takes the form of a
standing optical wave of 100 nm period which can extend over a 10
cm diameter area. Orthogonal exposure of a resist-coated substrate
to the standing optical wave can form an array of pores in the same
way as described above for interference lithography.
[0071] In a typical embodiment, EL or AIL is performed on
multi-layer stack, comprising a substrate on which has been
deposited a conductive material layer, a polymer layer, an etch
stop layer, and a photoresist. Following exposure by the orthogonal
standing optical waves, the resist can be developed by techniques
well known in the art. The grid pattern in the developed resist can
then be transferred down into the etch mask layer, and the polymer
layer by any of the etching techniques known in the art, including
wet etching, dry etching, reactive ion etching, electron beam and
laser writing; reactive ion etching is preferred. In this
configuration, each pore passes through the resist layer, the etch
mask layer, and the polymer layer. Optionally, the resist layer can
be completely removed following etching so that the pores are
formed solely within the polymer layer. The depth of the pores, and
hence the length of nanoparticles that can be formed in the pores,
is determined by the thickness of the relevant layers of the
stack.
[0072] The conductive material may be a metal, a metal-containing
compound, or a metal alloy, including without limitation titanium
nitride, nickel, copper, zinc, silver and gold; titanium nitride
and gold are preferred. The conductive layer may be deposited by
any suitable means, including sputtering. To promote adhesion
between the conductive layer and the substrate, a layer of adhesion
promoting material may be deposited on the substrate before
deposition of the conductive layer. Adhesion promoting materials
include titanium and cromium; titanium is preferred where titanium
nitride is the conductive material; cromium is preferred where gold
is the conductive material.
[0073] The polymer may be polyimide, polymethylmethacrylate (PMMA),
photoresist, or other suitable polymer known in the art. Although
referred to herein as the "polymer" layer, to the extent it may be
considered as limiting the scope of the invention, that label
should be ignored. Thus, the "polymer" layer includes materials
that can be etched according to the methods of the present
invention to form pores even if those materials are not polymeric
(e.g., polysilicon, SiO.sub.2). The etch stop layer may be
SiO.sub.2, aluminum, and nickel, or other etch stop know in the
art. Suitable resists include both positive and negative
photoresists known in the art. Positive photoresists are preferred
for features less than 3 .mu.m in size. It will be understood by
those skilled in the art that there are many suitable resists
(including positive and negative resists), etch stops and polymers
that can be patterned into grids using IL and AIL. Any stack that
can be patterned by IL or by AIL to yield pores suitable for the
formation of nanoparticles is contemplated by the present
invention. Similarly, although silicon wafers are the preferred
substrates of the invention, many other suitable substrates are
known in the art.
[0074] An antireflective coating (ARC) may be included in the
stack. Such a coating acts to prevent the reflection of light by
the underlying substrate, thereby assisting the integrity of the
standing optical wave.
[0075] In embodiments where high etch rate selectivity is not
desired, the etch stop layer may be omitted from the stack. The
conductive layer may also be omitted if the substrate is
sufficiently conductive (e.g., if it has been doped) or if the
nanoparticles are to be made by a process other than
electrodeposition (e.g., CVD), and therefore do not require an
electrode. Indeed, in the most basic embodiments of the invention,
even the polymer layer may be omitted, so that the photoresist will
form the walls of the pores.
[0076] Mask-Based Photolithographic Methods for Nanoparticle
Templates Formation
[0077] Membrane templates for the formation of nanoparticles also
may be formed by conventional mask-based photolithography
techniques well-known in the semiconductor and microfabrication
arts. In these embodiments, masks with a grid pattern generated by
standard methods known in the art (e.g., e-beam writing and laser
writing) are used to expose a resist-coated substrates and
multi-layer stacks. The substrates and multi-layer stacks, as well
as the techniques for development and etching thereof, are
preferably based on those described above (e.g., a substrate
overlaid first with a polymer layer, an etch stop material, and a
photoresist).
[0078] An advantage of using masks is that each mask can be used a
number of times, thereby obviating the need to use the AIL or IL
optical configuration every time a membrane template is required.
The use of any type of mask known in the art is contemplated by the
invention. In some embodiments of the mask-based approach, the mask
itself may be formed by IL or AIL.
[0079] If the mask is a grating, then two orthogonal exposures of
the resist are necessary to pattern the resist with a grid; if the
mask is a grid, then a single resist exposure can be used to
pattern a grid. Methods for forming free-standing grating and grid
masks by IL and AIL are described in, for example, Wolf and Tauber,
Silicon Processing for the VLSI Era, Vol. 1 Process Technology (2nd
Ed.) Lattice Press, California (2000), incorporated herein by
reference in its entirety. Once a mask has been formed, it can be
used to expose a resist by, for example, contact or proximity
exposure techniques known in the art.
[0080] Use of Membrane Templates
[0081] The membrane templates produced by the methods of the
present invention, whether through IL, AIL, or mask-based
photolithography, can be used to form nanoparticles via a number of
techniques. Most preferably, electrochemical deposition is used,
requiring that the membrane template have a conductive material in
communication with the pores. This may be achieved, for example, by
depositing a layer of conductive material overlying the substrate,
as described above, doping the substrate so that it can act as the
electrode, or both.
[0082] In other embodiments, material can be deposited within pores
by chemical vapor deposition (e.g., organic-metallic vapor
deposition) or evaporation. For example, electron-beam evaporation
has been used to deposit metal within pores formed by AIL. Savas et
al., J. Applied Physics 1999, 85, 6160, incorporated herein by
reference in its entirety. Methods for the formation of
nanoparticles in membrane template pores by evaporative techniques
are discussed further herein.
[0083] In the aforementioned embodiments in which the nanoparticles
are formed by electrodeposition into pores of membrane template,
there must be a conductive material in communication with the pore
so that current may be applied. In some embodiments, the substrate
may first be coated with one or more conductive layers before the
stack is built. The photoresist is then exposed and developed, as
described above. When the stack is etched down to the conductive
layer, the resulting pores comprise a layer of conductive material
at their base. By applying a current to the substrate, the
conductive layer acts as an electrode at the base of each pore,
thereby allowing charged material to be electrodeposited within the
pore. In this way, segmented nanoparticles can be built through
sequential electrochemical deposition according to the methods
provided herein. Other configurations allowing conductive material
to be in communication with the pore are possible (e.g., additional
conductive material may be deposited within the pore, the walls of
the pore may comprise conductive material, there may be conductive
pillars or posts within the pore, and so on.). As discussed above,
in preferred embodiments, the conductive layer comprises a layer of
Ti overlaid with a layer of TiN, or Cr overlaid with a layer of Au.
Preferably the Ti or Cr layer is about 5 nm thick and the TiN or Au
layer is about 20 nm thick.
[0084] When the electrochemical deposition procedure is complete,
the material that forms the walls of the pore (e.g., PMMA) can be
removed, exposing the nanoparticles. The nanoparticles can be
released from the silicon wafer by a number of methods. For
example, the nanoparticles can be liberated by physically breaking
the linkage between the wafer and the nanoparticles using, for
example, sonication or high-pressure water. The nanoparticles also
may be liberated by etching the silicon wafer, for example using HF
acid. All of the aforementioned techniques for liberating
nanoparticles are equally applicable in embodiments where
nanoparticles are formed in pores through techniques other than
electrochemical deposition.
[0085] Alternatively, the nanoparticles may be released by
dissolving the conductive layer. Indeed, areas of sacrificial
conductive material may be deposited on the conductive layer such
that they lie at the bottom of the pores. Because it is conductive,
the material can transmit the current to the growing nanoparticles.
Selected so that it is easily dissolved or otherwise removed, the
sacrificial conductive material may be useful in liberating the
nanoparticles formed in the pores. A number of materials may be
used as sacrificial conductive materials, including without
limitation Ag, Cu, and Zn. In embodiments in which Ag is used as
the sacrifical conductive material, nitric acid can be used to
liberate the nanoparticles; when Cu is the sacrifical conductive
material, sulfuric acid can be used; when Zn is the sacrifical
conductive material, a weak acid can be used. It should be noted
that the use of a particular metal as the sacrificial conductive
layer will preclude the use of that metal as one of the segments of
the nanoparticle.
[0086] FIG. 3 illustrates schematically an embodiment of the
invention using a four-layer stack on a silicon wafer substrate
101. In FIG. 3A, the stack is shown before exposure to radiation.
Substrate 101 is overlaid with conductive layer 102, then with
polymer layer 103, then with a etch stop layer 104, and finally
with photoresist layer 105. In FIG. 3B, the stack is shown
following exposure and development of the photoresist, followed by
etching down to etch stop 104, thereby forming pores 106. Exposure
may be performed using IL or AlL (with two orthogonal exposures),
or using conventional mask-based photolithography with a grid mask.
In FIG. 3C, further etching down through polymer layer 103 to
conductive layer 102 results in the formation of pores 107. In FIG.
1D, nanoparticles 108 are formed within pores 107 by
electrochemical deposition using the conductive layer 102 as the
plating electrode. Finally, in FIG. 1E, free-standing nanoparticles
109 are liberated by dissolving the conductive layer 102.
[0087] While a single synthesis process using one silicon wafer can
yield tens of billions of identical nanobarcodes, it may be
desirable to manufacture many different codes simultaneously on a
single wafer to provide for the diversity of nanobarcodes required
for many applications. This can be achieved with a straightforward
modification to the processes described above, by patterning the
conductive layer immediately below the resist stack into separate
zones. Each zone can then be connected to a separate electrical
control system.
[0088] Reuseable Membrane Templates
[0089] In still further embodiments, the membrane template created
using photolithographic techniques is re-usable. This can be
achieved in the following way. Step 1: a layer of polysilicon is
deposited on a silicon wafer using either CVD or PECVD deposition.
Step 2: A layer of photoresist is spun on the silicon wafer. Step
3: The resist is exposed using a mask, IL, or AIL, to pattern a
grid. Step 4: The resist is developed and the polysilicon is
etched, revealing an array of pores in the polysilicon. Step 5: A
sacrificial layer of conductive metal is deposited,
electrochemically or by CVD, inside the pores that were etched into
the polysilicon. For example, Zn can be used as it is easily
dissolved in a weak acid. Step 6: A layer of silicon dioxide is
then deposited on the surface of the stack, followed by a hydrogen
bake to remove metal oxide formed in the silicon deposition
process. Step 7: Nanoparticles may be formed inside the pores using
CVD or electrochemical deposition; in the latter case, the Zn may
participate as an electrode. Step 8: the silicon dioxide deposited
in step 7 is dissolved using HF. Step 9: The Zn deposited in step 6
is dissolved, thereby liberating the nanoparticles. The membrane
template can be re-used by returning to step 5.
[0090] Photolithographic Formation of Nanoparticles By Etching a
Pre-formed Film Stack
[0091] The aforementioned embodiments are all directed to the
formation nanoparticles in porous membrane templates made by
photolithographic techniques. In another series of embodiments,
photolithographic techniques are used to etch nanoparticles from a
pre-formed stack of material, wherein each layer of the stack
corresponds to a particular segment of the subsequent nanoparticle.
In one such embodiment of the invention, layers of material, such
as metal films, are deposited onto a silicon wafer to form a stack.
A layer of photoresist is then spun on the material stack. The
stack is then exposed to radiation (e.g., UV light) by conventional
mask-based photolithography (or by IL or by AIL) to pattern a grid
on the resist. Following development of the resist, the entire film
stack is then etched revealing many cylindrical film stacks. The
nanoparticles can be liberated by one of the methods described
above (e.g., physically, or by dissolving a sacrificial base
layer).
[0092] Alternatively, photolithography techniques may be used to
synthesize the nanoparticles one layer at a time. Thus, in a
further embodiment of the invention, a layer of material is
depositied on a silicon wafer, via electrochemical deposition or
CVD, to form Film 1 (corresponding to the first segment of the
nanoparticle). Photoresist is then spun on top of Film 1. The
resist is exposed, e.g., using a mask or by IL or AIL, to pattern
an array of cylindrical posts (i.e., the opposite resist pattern to
a pore pattern). Following resist development, the film is etched
with an appropriate wet or dry etch method to leave cyclindrical
posts of the Film 1 material. To form the subsequent segments of
the nanoparticle, the preceding steps are repeated n times,
depositing and etching Films 2 through n, to form the segments in
the nanoparticle. The nanoparticles can be liberated by one of the
methods described above.
[0093] The use of photolithographic techniques provides an
opportunity to make rods from many different materials beyond
simple metals. For example, glass can be spin-coated on a planar
substrate and readily etched later. Glass in each layer could be
doped with metals or phosphorescent materials so that each layer
would fluoresce a different amount. When the striped rods are
liberated, the codes would be read with a fluorescent imaging
system. Metals and metal oxides can be deposited on a planar
surface and later etched. Alternating layers of metals and oxides
can provide high contrast segments that can easily be read by a
reflectance microscopy. It should be noted that there are numerous
other materials that can be used to prepare membranes or templates
for nanorod synthesis. One example of many are bundles of optical
fibers in which the cores are etchable under conditions where the
claddings are not. Carrying out this etching, followed by slicing
across the bundle, yields a membrane with hole diameters the size
of the fiber cores. Note that fibers can be drawn out (using heat)
to submicron diameters. Note also that fiber bundles with
collections of greater than 1,000,000 fibers are commercially
available; this could easily be extended to 10 million. Another
group of materials that could be used, for example, are molecular
sieve materials with well-defined cavities such as zeolites.
[0094] Note also that other methods can be used to prepare
templates or membranes from a variety of different methods. Such
methods include but are not limited to: MEMS, electron beam
lithography, x-ray lithography, uv-lithography, deep lithography,
projection lithography, standing wave lithography, interference
lithography, and microcontact printing.
[0095] Chemical self-assembly/deassembly methods may also be used.
For example, formation of an infinite, close-packed, 2-dimensional
hexagonal layer of latex balls on a planar surface has been
demonstrated. Such particles could be shrunk by 10% in size, e.g.,
by cooling the temperature. Then a polymer may be grown in the
spaces between the infinite 2-D array (that is no longer close
packed). Then the balls are selectively dissolved, leaving behind a
polymeric material with well defined holes equal to the final
diameter of the latex balls.
[0096] The particles of the present invention may also be prepared
in large scale by automating the basic electroplating process that
is described in Example 1. For example, an apparatus containing a
series of membranes and separate electrodes can be used to make a
large number of different flavors of nanoparticles in an efficient
computer controlled manner. An example of this type of apparatus is
depicted in FIGS. 1 and 2.
[0097] The embodiment of the invention depicted in FIGS. 1 and 2
synthesizes 25 types of nanobar codes simultaneously in 25 separate
template membranes (e.g., Whatman Anodisc membranes, 25 mm
diameter, 60 micron thick, with 200 nm pores) mounted in a liquid
flow cell. Before mounting the membranes in the flow cell, each
membrane is silver-coated on one side (which is the branched-pore
side of the membrane) in a vacuum evaporator. Then each membrane is
immersed in a silver plating solution with electrodes on both
sides, and additional silver is electroplated onto the evaporated
silver coating and into the pores (at 4 mA for about 30 minutes),
to completely close all of the membrane pores. Each membrane is
then mounted with its silver-coated side in contact with an
electrode in the flow cell. The flow cell is about 1.5 mm thick,
containing about 30 ml of liquid. Opposite the membranes is a
platinum mesh electrode with surface area slightly larger than the
entire 5.times.5 array of membranes.
[0098] The flow cell can be filled (by computer control) with
water, nitrogen gas, gold plating solution (e.g., Technics), silver
plating solution (e.g.,Technics Silver Streak and/or additional
plating solutions). The flow cell is in thermal contact with a
coolant water tank, the temperature of which is controlled by
recirculation through a temperature-controlled bath. In the coolant
tank opposite the flow cell is an ultrasonic transducer (Crest, 250
Watt), which is turned on during electroplating operations to
facilitate mass transport of ions and gases through the membrane
pores. Control software is used to automatically flow the
appropriate solutions through the flow cell, and individually
control the electroplating currents or potentials at each separate
membrane. The software also measures temperature at various
locations in the apparatus, and controls the sonicator and
peristaltic pump. The software allows the user to define recipes
describing the desired stripe pattern for each nanobar code in the
5.times.5 array. The software reads the recipe, and then
automatically executes all fluidic and electrical steps to
synthesize different types of nanobar codes in each membrane.
[0099] After nanorod synthesis is complete, the membranes are
removed from the flow cell, and individually postprocessed to free
the nanobar codes from the template pores. First, each membrane is
immersed in approximately 2M HN (nitric acid) for about 30 minutes
to dissolve the backside silver coating. Then the membrane is
immersed in NaOH to dissolve the alumina membrane, and release the
rods into solution. The rods are then allowed to settle under
gravity, and the NaOH is washed out and replaced with H.sub.2O or
Ethanol for storage. In a further embodiment, rather than moving
the solution exposed to a stationary membrane or template, the
membranes or templates may be moved from one plating solution to
another.
[0100] An apparatus for performing such manufacture of 25 types or
flavors of nanobar codes is depicted in FIGS. 1 and 2. As described
above, 25 separate membrane templates are placed in a common
solution environment, and deposition is controlled by the
application of current to the individual membranes. For example,
membranes 1-10 may begin with the deposition of a layer of gold
that is 50 nm thick, membranes 11-20 may begin with the deposition
of gold that is 100 nm thick, while membranes 21-25 may not have an
initial layer of gold. This deposition step can be easily
accomplished in the apparatus of this embodiment by filling the
solution reservoir with a gold plating solution and applying
current to membranes 1-10 for the predetermined length of time,
membranes 11-20 for twice as long and not at all to membranes
21-25. The gold plating solution is then removed from the chamber
and the chamber rinsed before introducing the next plating
solution.
[0101] The apparatus of this embodiment has been designed to be
rotatable around a pivot point for ease of access to the solution
chamber and the electric and plumbing controls on the back of the
apparatus. Referring to FIG. 1, the apparatus rests upon a base
101. The pivoting mechanism is comprised of the pivoting support
103, the pivot locking pin handle 105, and the pivot pin 107. The
apparatus is equipped with a halogen light, contained in the box
108, and a sonicator, located at 109, in fluid communication with a
solution chamber.
[0102] The flow cell is defined by the rear cell assembly 111 and
the front cell assembly 113. The electrical connectors 115 are on
the tops of the rear and front assemblies. The assemblies are held
in place by clamping bolts 117 to maintain a sealed solution
chamber. The 25 templates 119 for nanoparticle growth are held
between front and rear assemblies, and the front assembly has an
electroforming cell front window 121.
[0103] FIG. 2 is a cross-sectional view of the apparatus shown in
FIG. 1. Many of the same elements can be seen in FIG. 2 that were
defined with respect to FIG. 1, and they have been numbered the
same. FIG. 2 also allows visualization of cell partitioning gaskets
123 between front and rear assemblies and gasket alignment pin 125.
FIG. 2 also shows rear assembly glass window 127. The water tank
129 for temperature control is found adjacent to the rear assembly,
and the halogen lamp 131 is shown. The ultrasonic apparatus is
comprised of the ultrasonic transducer 133 and the ultrasonic tank
135.
[0104] While the embodiment described above clearly illustrates how
twenty-five types of nanobar codes comprising cylindrical,
segmented metal nanoparticles can be prepared by parallel
synthesis, the concept has very broad applicability. It is
straightforward to extend this embodiment to hundreds or thousands
of parallel reaction chambers. Likewise, it is straightforward to
extend this method to the fabrication of nanorods with three or
more different materials. Likewise, it should be clear that,
through appropriate use of Ag spacers, that more than one flavor of
nanobar code can be prepared within a single reaction vessel. In
other words, one could prepare an Au--Pt rod, deposit Ag, and then
prepare an Au--Pt--Au rod. After rod release from the membrane, Ag
dissolution will lead to production of two types of rods. Of
course, the number of a single type of particles could be increased
by growing multiple copies of a single rod within the same reaction
vessel.
[0105] It should likewise be realized that, rather than
introduction of one plating solution to a collection of membranes,
it is straightforward to employ microfluidics to address templates
individually. In other words, a different plating solution could
simultaneously be delivered to two or more locations. Thus, in
principle, one could be making stripes of 5 or 10 or more
compositions, and with 5 or 10 or more segment widths, at the same
time, but in different, pre-programmed locations.
[0106] Importantly, the materials chosen for this synthesis (Au,
Ag, Pt) are meant to be illustrative, and in no way limiting. There
are numerous materials that can be electrodeposited in this
fashion, including metals, metal oxides, polymers, and so forth,
that are amenable to multiplexed synthesis.
[0107] More generally, multiplexed synthesis of nanoparticles need
not be confined to electrochemical deposition into a host. For
example, the materials described herein could likewise be prepared
by sequential evaporation, or by sequential chemical reaction. This
expands the possibilities for multiplexed nanoparticle synthesis to
include all oxides, semiconductors, and metals.
[0108] Independent of the synthetic approach used, when synthesis
is done in a membrane a final critical step is required to separate
each unique type of nanorod and release all the nanorods into
solution, for surface preparation or denaturation. In the preferred
embodiments of the invention this is done by chemical dissolution
of the membrane and electrode backing, using a series of solvents.
These solvents could be acids, bases, organic or aqueous solutions,
at one or more temperature or pressures, with one or more treatment
times. Two additional release techniques are: (i) Following
synthesis, whether on membrane or planar substrate, die separation
techniques from the semiconductor industry can be utilized. The
substrate will be mated to a flexible adhesive material. A dicing
saw cuts through the substrate, leaving the adhesive intact. The
adhesive is then uniformly stretched to provide physical separation
between each island, each of which is then picked up automatically
by robot and placed into a separate microwell. An automated
fluidics station is used to introduce the necessary etching
solutions to release each rod into solution. (ii) An alternative
embodiment is a matching microwell substrate that contains wells in
the same pattern as the individual islands in the membrane, and a
matching array of channels through which flow etching solutions.
The membrane or wafer can be sandwiched between the microwell
substrate and the channel array. Etching fluid is then introduced
into the channels which dissolves the Ag backing and carries the
nanorods into the corresponding well. Other means for removing the
particles from the membrane are also possible, including but not
limited to laser ablation, heating, cooling, and other physical
methods.
[0109] The membrane-based template-directed synthesis techniques
are preferred because they are capable of making a very large
number of very small nanorods. The electroplating conditions can be
adequately controlled to produce many types of nanorod bar codes.
For applications such as multiplexed immunoassays, where tens to
many hundreds of types are required, known techniques are adequate
and can simply be scaled up to provide the necessary number. For
applications such as proteomic signatures, where from dozens to
many thousands of types are required, higher throughput synthesis
techniques and the ability to uniquely identify each of thousands
of different bar codes are required.
EXAMPLES
[0110] The following examples are provided to allow those skilled
in the art access to information regarding various embodiments of
the present invention, and are not intended in any way to limit the
scope of the invention.
Example 1
[0111] One embodiment of the present invention is directed to the
template-directed synthesis of multiple flavors of nanobar codes
for the purpose of multiplexed assays. For this application it is
desirable to construct a variety of different flavors which are
easily distinguished by optical microscopy. For example, 10
different flavors of nanobar codes were individually synthesized
according to the table below, using gold and silver segments. Note
that the description field of the table indicates the composition
of each nanobar code by segment material and length (in microns) in
parentheses. For example, Flavor #1 is 4 microns long gold, and
Flavor #2 is 2 microns gold followed by 1 micron silver, followed
by 2 microns gold.
1 Flavor # Description # Segments Length 1 Au (4) 1 4 .mu.m 2 Au
(2), Ag(1), Au(2) 3 5 .mu.m 3 Au(1), Ag(1), Au(1), Ag(1), 5 5 .mu.m
Au(1) 4 Au(2), Ag(2) 2 4 .mu.m 5 Ag(1), Au(1), Ag(1), Au(1), 5 5
.mu.m Ag(1) 6 Ag(1), Au(4) 2 5 .mu.m 7 Ag(4) 1 4 .mu.m 8 Ag(1),
Au(2), Ag(1) 3 4 .mu.m 9 Ag(1), Au(1), Ag(1), Au(2) 4 5 .mu.m 10
Ag(2), Au(1), Ag(1), Au(1) 4 5 .mu.m
[0112] A detailed description of the synthesis of Flavor #4
follows. (All other flavors were synthesized by minor and obvious
changes to this protocol.)
[0113] 25 mm diameter Whatman Anopore disks with 200 nm diameter
pores were used for template directed nanobar code synthesis.
Electrochemical metal deposition was carried out using commercially
available gold (Technic Orotemp 24), and silver (Technic ACR 1025
SilverStreak Bath) plating solutions. All of the electroplating
steps described below were carried out in an electrochemical cell
immersed in a sonication bath, which was temperature controlled to
25.degree. C.
[0114] The synthesis of nanobar code Flavor #4 was carried out as
follows. The membrane was pretreated by evaporating .about.500 nm
of silver on its branched side. To completely fill the pores on
this side, approximately 1 C of silver was electroplated onto the
evaporated silver, using 1.7 mA of plating current for
approximately 15 minutes. Then an additional 1 C of silver was
electroplated into the pores of the membrane from the side opposite
the evaporated silver, using 1.7 mA of plating current for
approximately 15 minutes. This silver layer is used to fill up the
several micron thick "branched-pore" region of the membrane. The
silver plating solution was removed by serial dilutions with water,
and was replaced by the gold plating solution. The 2 micron long
gold segments were then deposited using 1.7 mA of plating current
for approximately 30 minutes. The gold plating solution was removed
by serial dilutions with water, and was replaced by the silver
plating solution. The final 2 micron long silver segment was then
deposited using 1.7 mA of plating current for approximately 30
minutes. The membrane was removed from the apparatus, and the
evaporated silver layer (and the electrodeposited silver in the
branched pores) was removed by dissolution in 6 M nitric acid,
being careful to expose only the branched-pore side of the membrane
to the acid. After this step, the nanobar codes were released from
the alumina membrane by dissolving the membrane in 0.5 M NaOH. The
resulting suspension of nanobar codes were then repeatedly
centrifuged and washed with water.
Example 2
[0115] It is an important goal to demonstrate the ability to use a
wide number of materials in the nanobar codes of the present
invention. To date, rod structures formed by electrochemical
deposition into a membrane template (alumina or track etch
polycarbonate) include Ag, Au, Pt, Pd, Cu, Ni, CdSe, and Co.
Primarily, the 200-nm pore diameter alumina membranes have been
used for convenience. Many of the materials are now also being used
in the smaller diameter polycarbonate membranes.
[0116] CdSe is currently plated via a potential sweep method from a
solution of CdSO.sub.4 and SeO.sub.2. Mechanical stability problems
have been encountered with the metal:CdSe interface; i.e. they
break when sonicated during the process of removing them from the
membrane. This has been remedied with the addition of a
1,6-hexanedithiol layer between each surface.
[0117] The Cu and Ni are plated using a commercially available
plating solution. By running under similar conditions as the Ag and
Au solutions, it was found that these metals plate at roughly the
same rate, .about.3 .mu.m/hr. The Co is plated from a
CoSO.sub.4/Citrate solution. These rods seems to grow fairly
monodispersely, however they grow comparatively slowly, .about.1.5
.mu.m/hr.
Example 3
[0118] One embodiment of the present invention is directed to the
template-directed synthesis of nanoscale electronic devices, in
particular diodes. One approach, combines the membrane replication
electrochemical plating of rod-shaped metal electrodes with the
electroless layer-by-layer self-assembly of nanoparticle
semiconductor/polymer films sandwiched between the electrodes.
Described below, is the wet layer-by-layer self-assembly of
multilayer TiO.sub.2/polyaniline film on the top of a metal nanorod
inside 200 nm pores of an alumina membrane.
[0119] 1. Materials
[0120] 200 nm pore diameter Whatman Anoporedisks
(Al.sub.2O.sub.3-membrane- s) were used for template directed diode
synthesis. Electrochemical metal deposition was carried out using
commercially available gold (Technic Orotemp 24), platinum (Technic
TP), and silver plating solutions. Titanium
tetraisopropoxide[Ti(ipro).sub.4], mercaptoethylamine
hydrochloride(MEA),ethyltriethoxy silane, chlorotrimethyl silane
were purchased from Aldrich. All the reagents were used without
further purification. All other chemicals were reagent grade and
obtained from commercial sources.
[0121] TiO.sub.2 colloid was prepared as follows. Ti(ipro).sub.4
was dissolved in 2-methoxyethanol under cooling and stirring. The
solution was kept under stirring until it became slightly yellow,
after which another portion of 2-methoxyethanol containing HCl was
added. The molar ratio of the components in the prepared solution
was Ti(ipro).sub.4:HCl:2-metoxyethanol=1:0.2:20. This solution was
diluted with water to adjust TiO.sub.2 concentration to 1% and
allowed to age during 3 weeks. The resulting opalescent sol was
subjected to the rotary evaporation at 60.degree. C. to give shiny
powder of xerogel containing 75% (w/w) titania. This xerogel was
used as a precursor for the preparation of stock aqueous TiO.sub.2
sol with TiO.sub.2 concentration of 2.3% wt (0.29 M) and pH=3,
which was stable during several weeks. XRD investigations of the
titania xerogel allowed estimating average size of the colloidal
anatase crystals at 6 nm, TEM image of the stock TiO.sub.2 sol
shows particles of 4-13 nm in diameter.
[0122] The emmeraldine base (EB) form of polyaniline (PAN) was also
prepared. A dark blue solution of PAN in dimethyl formamide (0.006%
wt) was used as a stock solution for the film synthesis.
[0123] 2. Synthesis of Rod-Shaped Diodes
[0124] The synthesis of rod-shaped diodes was carried out as
follows. Metal electrodes were grown electrochemically inside
porous membrane. Briefly, the membrane was pretreated by
evaporating .about.150 nm of silver on its branched side. To
completely fill the pores on this side 1 C of silver was
electroplated onto the evaporated silver. These Ag "plugs" were
used as foundations onto which a bottom electrode was
electrochemically grown. The bottom gold electrode of desired
length was electroplated sonicating. The plating solution was
removed by soaking the membrane in water and drying in Ar stream.
Priming the bottom electrode surface with MEA preceded depositing
multilayer TiO.sub.2/PAN film. This was achieved by 24 hour
adsorption from MEA(5%) ethanolic solution. The multilayer film was
grown by repeating successive immersing the membrane in the
TiO.sub.2 aqueous solution and PAN solution in DMF for 1 h. Each
adsorption step was followed by removing the excess of reagents by
soaking the membrane in several portions of an appropriate solvent
(0.01 M aqueous HCl or DMF) for 1 h, and drying in Ar stream.
Finally, a top electrode (Ag or Pt) of desired length was
electroplated at the top of TiO.sub.2/PAN multilayer without
sonicating. Then the evaporated silver, "plugs" and alumina
membrane were removed by dissolving in 6 M nitric acid and 0.5M
NaOH, respectively. (2-4 C of Au was always electroplated on the
top of Ag electrode to prevent dissolving the latter in the nitric
acid. Also preliminary experiments showed that multilayer
TiO.sub.2/PAN film self-assembled on plane Au(MEA) substrate was
not destroyed in the 0.5 M NaOH.) The resulting rod-shaped diodes
were repeatedly centrifuged and washed with water.
[0125] In most of the experiments, chemical passivation of
Al.sub.2O.sub.3-membrane pore walls was applied using treatments
with propionic acid or alkylsilane derivatives. In the latter case,
a membrane was successively soaked in absolute ethanol andanhydrous
toluene or dichlorethane for 1 h, after which it was immersed in a
ethyltriethoxy silane solution in anhydrous toluene (2.5% vol) or a
chlorotrimethyl silane solution in anhydrous dichlorethane (2.5%
vol) for 15 h. Then the membrane was successively soaked for 1 h in
the appropriate anhydrous solvent, a mixture (1:1) of the solvent
and absolute ethanol, the absolute ethanol, and finally was dried
in Ar stream. Wetting so treated membranes with water revealed
hydrophobic properties of their external surface. Transmission IR
spectra of the membrane treated with ethyltriethoxy silane or
propionic acid showed the appearance of weak bands at 2940, 2865,
2800 cm-1, which can be assigned to C--H stretching vibrations of
alkyl and alkoxy groups.
[0126] 3. Characterization
[0127] Transmission electron microscope (TEM) images were obtained
with a JEOL 1200 EXII at 120 kV of accelerating voltage and 80mA of
filament current.
[0128] Optical microscope (OM)images were recorded. Transmission IR
spectra were recorded using a Specord M-80 CareZeiss Jena
spectrometer. I-V characteristics for rod-shaped diodes were
measured in air at ambient temperature.
[0129] TEM images of some typical "striped" bimetallic Au/Pt/Au
nanorods, grown electrochemically inside the porous alumina
membrane, showed that the two rod ends differed in their
topography--one of the rod ends appeared to be bulging or rounded
while the other rod end had an apparent hollow in the middle. Such
differences in rod end appearance could be explained by adsorption
of some amount of metal ions on pore walls, promoting metal (e.g.
Ag) growth in the near-wall space and causing the hollow formation
in the pore middle space. During the electroplating of a second
metal "stripe" (e.g. Au), the growing metal follows the surface of
the bottom rod and fills the hollow thus forming the rounded end.
Further rod growth results in a cup-like end due to the metal
adsorption on the pore walls. Each sequential metal segment grows
in the same way in the end of the underlying segment.
[0130] It is unlikely that the relatively rough surface on the top
end of a rod may be completely covered with the ultrathin
TiO.sub.2/PAN film thus preventing immediate contacts between
bottom and top metal electrodes. From preliminary experiments on
plane Au-substrates, it was found that the multilayer TiO.sub.2/PAN
films grown on smoother surfaces demonstrated better
reproducibility in their rectifying behavior. Passivation
(hydrophobization) of Al.sub.2O.sub.3-terminated surface of pore
walls with propionic acid or alkylsilane derivatives, such as
ethyltriethoxy silaneor chlorotrimethyl silane, was tried to smooth
down the top rod end surface by reducing the metal adsorption on
the pore walls. The hydrophobization of pore walls may also be
expected to prevent TiO.sub.2 particles from adsorption on the wall
surface rather than on metal electrode surface situated in the
depth (.about.65 .mu.m) of the pore. It was shown that the
TiO.sub.2 particles readily formed a densely packed layer on a
plane Al/Al.sub.2O.sub.3 substrate. A typical higher resolution
image of rod's upper part confirmed that the cup-like ends are
situated at the top of the rods, and showed that the wall
passivation to some extent resulted in smoothing of the surface of
rod ends.
[0131] An optical micrograph of Au/(TiO.sub.2/PAN).sub.10/Ag/Au
rods, prepared using the membrane derivatized with ethyltriethoxy
silane, showed nanorods of uniform length, in which a silver
segment is clearly seen between two gold ends. TEM images of such a
rod, recorded in the first several seconds, revealed no visible
signs of a metal/film/metal heterojunction within the rod. However,
after focusing the electron beam on this rod for some time
(typically tens of seconds), a break appeared in the rod and metal
segments became separated, perhaps due to beam-induced metal
melting, in the neighborhood of the Au/film/Ag heterojunction. In
higher resolution TEM images of this break, particles of 5-10 nm in
diameter, which adhere to both metal ends, were observed.
Apparently, TiO.sub.2 nanoparticles are present between two
electroplated metals. The OM and TEM data suggest that the
self-assembly of multilayer TiO.sub.2/PAN film on the Au rod top
can be realized inside the membrane pores, and that the
self-assembled film does not prevent Ag rod electroplating on the
top of the film. It should be noted that TEM images in all
likelihood do not give a true picture of the multilayer
TiO.sub.2/PAN film inside the rod because of high probability of
the mechanical film destruction while separating partially melted
metal rod ends. Longer time exposure of the rod to the electron
beam causes complete destruction of the heterojunction and arising
two individual nanorods with nanoparticles stuck to their ends.
[0132] In order to investigate multilayer TiO.sub.2/PAN film
sandwiched between Au and Ag rods, Au/(TiO.sub.2/PAN)r/Ag nanorods
were prepared and their top Ag electrode was dissolved in nitric
acid: The remaining 2C Au rods with (TiO.sub.2/PAN).sub.6 film
deposited on their top were analyzed by TEM. Preliminary studies
showed that ellipsometric thickness of multilayer TiO.sub.2/PAN
film self-assembled on plane Au(MEA) substrate did not decrease
after immersion in 6 M HNO.sub.3 for 30 min suggesting stability of
the film in the acidic medium. Furthermore, similar to the
Au/(TiO.sub.2/PAN).sub.10/Ag/Au rods described above, TEM image of
the Au/(TiO.sub.2/PAN).sub.6 rod taken in the first several seconds
did not reveal any particles. However, during longer exposure to
the electron beam, gold melted revealing nanoparticle film on the
rod's top. It can be seen that the upper contour line of the film
is very close to that of Au rod before melting. This fact is
consistent with the cup-shaped top of the metal rods. The
multilayer film grows on the surface both of cup bottom and cup
walls and approximately retains cup shape after the thin walls have
melted. This explanation is consistent with observed film height of
.about.100 nm, which allows estimating rather gold cup depth than
(TiO.sub.2/PAN).sub.6 film thickness. Ellipsometric thickness of
TiO.sub.2/PAN).sub.6 film self-assembled on a plane Au(MEA)
substrate is estimated at about 10 nm.
[0133] I-V characteristic of the Pt/(TiO.sub.2/PAN).sub.3
TiO.sub.2/Au rod-shaped device reveals current rectifying behavior.
The forward and reverse bias turn-on potentials are .about.-0.2 and
.about.0.9 V, respectively.
Example 4
[0134] Segmented nanoparticles can be synthesized from membranes
produced using photolithographic techniques as follows: A silicon
wafer is spin coated with a the photoresist AZ.RTM.4620 (Clariant
Corp., Somerville, N.J.). The spin coating is conducted at 1000 rpm
for 40 seconds. The photoresist-coated substrate is then baked for
about 200 seconds on a hot plate coater at 110.degree. C.
Outgassing to remove volatile materials is conducted by allowing
the material to sit out at room temperature for at least 24 hours.
The photoresist-coated substrate is then exposed to radiation for
about 3100 msec using a mask to pattern the resist. The photoresist
is then developed using AZ.RTM.400K (potassium based buffered
developer) (Clariant Corp.) for about 15 minutes to reveal
cylindrical pores. The porous membrane is spin rinsed and dried.
The membrane is then evacuated for about an hour using a high
vacuum.
[0135] The pores of the membrane can be filled with alternating
bands of metals to form segmented nanoparticles as follows: copper
is plated into the pores using 1.0 mA for about 5 minutes. Gold is
plated into the pores using 1.0 mA for about 120 minutes. Silver is
plated into the pores using 1.0 mA for about 30 minutes. Gold is
again plated into the pores using 1.0 mA for about 30 minutes. The
photoresist is then dissolved using acetone. An SEM (top view) of
the template after plating is complete and the photoresist has been
dissolved is shown in FIG. 4. The tops of the nanoparticles, having
diameter approximately 2.5 to 3 .mu.m are visible. The segmented
nanoparticles may then be separated from the membrane using acetic
acid for about 60 minutes.
Example 5
[0136] In another embodiment, segmented nanoparticles can be
synthesized from membranes produced using photolithographic
techniques as follows: A silicon wafer is initially sputtered with
chromium and then gold, to form layers of about 200 .ANG. and 1000
.ANG. in thickness, respectively. A wafer singe is then performed
to remove water vapor and/or residual organics by heating at a
temperature of about 150.degree. C. for 30 minutes. The conductive
substrate is then spin coated first with 50% hexamethyldisilazane
(HMDS), and then with SPR.RTM.220 (Shipley Corp., Marlborough,
Mass.). The spin coating is conducted at 3500 rpm for about 40
seconds. The photoresist-coated stack is then baked for about 200
seconds at 90.degree. C. on an SVG coater (Silicon Valley Group,
San Jose, Calif.). This "soft bake" step is recognized in the art
to reduce solvent concentrations in the photoresist and improve
adhesion by relieving film stresses. Outgassing to remove volatile
materials is then conducted by allowing the material to sit out at
room temperature for at least 24 hours. The photoresist-coated
stack is exposed to radiation for about 1600 msec using an
appropriate mask to pattern the surface. The photoresist is
developed using LDD26W for about 100 seconds. The developing step
is repeated three times to reveal cylindrical pores. Oxygen plasma
is applied at 65 W for about 4 minutes, removing any photoresist
debris. The porous membrane is evacuated for about an hour using
high vacuum.
[0137] The pores of the membrane can be filled with alternating
bands of metals to form nanoparticles as follows: copper is plated
into the pores using 1.0 mA for about 5 minutes. Gold is plated
into the pores using 1.0 mA for about 115 minutes. After plating is
complete, the photoresist is dissolved using acetone. The segmented
particles are separated from the substrate using acetic acid for
about 60 minutes. An SEM (side view) of a nanoparticle made
according to the above procedure is shown in FIG. 5.
Example 6
[0138] In another embodiment, segmented nanoparticles can be
synthesized from membranes produced using photolithographic
techniques as follows: Cr and then Au, are sputtered onto a silicon
wafer to a thickness of about 200 .ANG. and about 1000 .ANG.,
respectively. The wafer is heated at a temperature of about
150.degree. C. for 30 minutes and then spin coated with 50% HMDS to
promote adhesion. The stack is then spin coated with polyimide, to
a thickness of about 10 .mu.m followed by a soft bake to remove
solvent. Aluminum is then sputtered on the stack to form an etch
stop layer of about 3000 .ANG. in thickness. After spin coating
again with 50% HMDS, the stack is spin coated with Shipley 3612
(5,500 rpm for 30 min.) resulting in a layer about 1 .mu.m in
thickness, and then heated at a temperature of about 90 .degree. C.
for about 60 seconds. Outgassing to remove volatile materials is
conducted by allowing the material to sit out at room temperature
for at least 24 hours.
[0139] The stack is exposed to radiation which has traveled through
a mask to transfer a pattern which will leave cylindrical holes in
the photoresist. The photoresist is developed using LDD26W for 60
seconds. Deep reactive ion etching is used to transfer the pattern
in the photoresist to the aluminum etch stop (450 W, 200 mTorr, 60
sec. BCl.sub.3 40 sccm, Cl.sub.2 30 sccm, N.sub.2 40 sccm). The
stack is then placed in water to remove any residual choloride, and
then spin rinsed and dried. The polyimide is then etched via deep
reactive ion etching using oxygen plasma (500 W. 250 mTorr, 300
sec., O.sub.2 50 sccm) to reveal cylindrical pores in the
polyimide. The resulting membrane is evacuated for one hour using
high vacuum. An SEM (cross-sectional view) of a template prepared
according to the above procedure is shown in FIG. 6.
[0140] The cylindrical pores can subsequently be filled with
alternating bands of metals as set forth described above.
* * * * *