Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF MANUFACTURE OF COLLOIDAL ROD PARTICLES
AS NANOBAR CODES
FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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.
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-cm2 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.
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
~m diameter latex beads are used extensively as sites for molecular
immobilization.
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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.
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.
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
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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).
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
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.
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 ~m and
particle width is from 5 nm to 50 Vim. 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
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4
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.
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 ~.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).
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.
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.
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
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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.
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.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a perspective view of an apparatus for manufacturing a plurality
of
different types of nanobar codes.
Figure 2 is a cross-sectional elevation view of the apparatus of Figure 1.
DETAILED WRITTEN DESRIPTION OF THE INVENTION
The present application is directed to methods of manufacture of
nanoparticles. Such
nanoparticles and their uses are described in detail in United States Utility
Application Serial
No. 09/598,395, filed June 20, 2000, entitled "Colloidal Rod Particles as
Nanobar Codes,"
incorporated hereby in its entirety by reference. Filed concurrently with the
present
application, and also incorporated herein in their entirety by reference, are
two United States
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 09/598,395 application.
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 United
States Provisional Application Serial No. 60/157,326, filed October 1, 1999,
entitled "Self
Bar-coded Colloidal Metal Nanoparticles"; United States Provisional
Application Serial No.
60/189,151, filed March 14, 2000, entitled "Nanoscale Barcodes"; United States
Provisional
Application Serial No. 60/190,247, filed March 17, 2000, entitled "Colloidal
Rod Particles as
Barcodes"; and United States Provisional Application Serial No. 60/194,616,
filed April 5,
2000, entitled "Nanobarcodes: Technology Platform for Phenotyping."
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
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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
S fluorescence in situ hybridization. All these methods comprise ways to code
biological
information, but none offer the range of advantages of the bona fide bar codes
of the present
invention, transformed to the manometer scale.
The particles to be manufactured according to the present invention are
alternately
referred to as nanoparticles, nanobar codes, rods, nanorods, nanobar codes,
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 manometer-
sized
particles fall within the scope of the invention, not all of the particles of
the invention fall
1 S within such size range.
In 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
mm and 1,000 manometers, 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 manometers. In some embodiments, the nanobar codes comprise two or
more
25 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.
When the particles of the present invention are made by electrochemical
deposition
30 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 manometer scale, with
each segment
length (and identity) programmable in advance. Other forms of deposition can
also yield the
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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.
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 Vim. In preferred embodiments the particle is 500 nm - 30 ~m in
length. In the
most preferred embodiments, the length of the particles of this invention is 1-
1 S Vim. The
width, or diameter, of the particles of the invention is within the range of 5
nm - 50 Vim. In
preferred embodiments the width is 10 nm - 1 ~.m, and in the most preferred
embodiments the
width or cross-sectional dimension is 30 nm - 500 nm.
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 tykes of segments, preferably 2 to 5
different types of
segments.
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,
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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).
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.
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.
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.
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.
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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 discernable 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.
The length of each segment may be from 10 nm to 50 Vim. In preferred
embodiments
the length of each segment is 50 nm to 20 qm. 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 pm 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.
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).
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 qm - 15 pm range may
vary up to 50%.
Segments of 10 nm in length will vary ~ 5 nm while segments in 1 pm 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% .
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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
1012 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.
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
United
States Utility Application Serial No. 09/598,395, filed June 20, 2000.
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.
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
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introduced. Defined pore materials are the preferred templates for many of the
preferred
particles of the present invention. A1203 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
S 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.
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 l, 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., Pt2+, Au+) 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.
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
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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.
To generate two-segment nanobar codes, two metals (e.g., Au, Ag, Pd, Cu, etc.)
can
S 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
g.m of Au,
3 ~m of Pt, 1 pm 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.
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.
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/cm2 to
oxidation at +0.55 mA/cm2) 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.
An additional way to alter the ends of the rods is to control the rate of
deposition.
Gold rods (2 C total, 3 ~.m) were plated at a current density of 0.55 mA/cm2.
Then the
current density was reduced to 0.055 mA/cmz and 0.1 C of Au was plated. The
last segment
of gold deposits is a hollow tube along the walls of the membrane.
Example 1 describes the manufacture of single flavors of nanoparticles
according to
one embodiment of the invention.
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.
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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.
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.
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 S, 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 Figures 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 ~m in length, and have a width from 5 nm to 50 ~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 affect this parallel manufacture are the
following:
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
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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.
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.
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.
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.
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.
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
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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
106 and 10g 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 10x.) 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 105 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 embodiment with widths of less than 1 pm).
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
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embodiment a positive photoresist-coated wafer is exposed to an interference
pattern of light,
using a technique similar to that used for interference-lithography generated
diffraction
gratings. The wafer is typically silicon, with a thin coating of titanium and
gold, a thick
coating of polymethylmethacrylate (PMMA), and a photoresist. Two exposures at
right
angles and subsequent development yields a two-dimensional array of holes in
the
photoresist. Reactive ion etching is then used to transfer the hole pattern
down through the
PMMA layer, which becomes the template. The photoresist layer is removed, and
the gold
layer under the PMMA becomes the cathods for electroplating into the PMMA
pores. The
shape and diameter of the nanorods can be controlled by adjusting the light
source and the
resultant standing wave pattern.
An advantage to this technique is that the template thickness, which is the
same as
pore length, can be tailored to the length of the rods, which improves
uniformity of
electroplating across the membrane. With this technique, 101° to 1012
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 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
104 to 106 separate
flavors of nanorods synthesized on a single wafer. With 1012 total pores per
wafer, 106 to 10g
nanorods of each flavor could be synthesized.
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
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of materials that could be used, for example, are molecular sieve materials
with well-defined
cavities such as zeolites.
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
S beam lithography, x-ray lithography, uv lithography, deep lithography,
projection
lithography, standing wave lithography, interference lithography, and
microcontact printing.
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.
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 Figures 1 and 2.
The embodiment of the invention depicted in Figures 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
Sx5 array of
membranes.
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,
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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 5x5 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.
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~0 or Ethanol for
storage. In
a further embodiment, rather than moving the solution exposed to a stationary
membrane or
template, moving the membranes or templates may be moved from one plating
solution to
another.
An apparatus for performing such manufacture of 25 types or flavors of nanobar
codes is depicted in Figures 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.
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 Figure 1, the apparatus rests upon a base
101. The
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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.
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.
Figure 2 is a cross-sectional view of the apparatus shown in Figure 1. Many of
the
same elements can be seen in Figure 2 that were defined with respect to Figure
1, and they
have been numbered the same. Figure 2 also allows visualization of cell
partitioning gaskets
123 between front and rear assemblies and gasket alignment pin 125. Figure 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.
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.
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.
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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.
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.
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.
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
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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
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
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.
Flavor Descri tion # Se ments Len th
#
1 Au 4 1 4 m
2 Au 2,A 1 ,Au2 3 5 m
3 Au( 1 ), Ag( 1 ), Au( 1 S 5 ~m
), Ag( 1 ),
Au 1
4 Au2,A 2 2 4 m
5 Ag( 1 ), Au( 1 ), Ag( 1 5 5 ~m
), Au( 1 ),
A 1
6 A 1 ,Au4 2 5 m
7 A 4 1 4 m
8 A 1 ,Au2,A 1 3 4 m
9 A l,Aul,A l,Au2 4 5 m
10 A 2,Aul,A l,Au1 4 5 m
A detailed description of the synthesis of Flavor #4 follows. (All other
flavors were
synthesized by minor and obvious changes to this protocol.)
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25 mm diameter Whatman Anopore disks with 200 nm diameter pores were used for
template directed nanobar code synthesis. Electrochemical metal deposition was
carned 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°C.
The synthesis of nanobar code Flavor #4 was carned out as follows. The
membrane
was pretreated by evaporating 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
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.
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CdSe is currently plated via a potential sweep method from a solution of CdS04
and
Se02. 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.
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, ~3 ~m/hr. The Co is plated from a CoSOa/Citrate
solution. These
rods seems to grow fairly monodispersely, however they grow comparatively
slowly, ~1.5
~.m/hr.
EXAMPLE 3
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 Ti02/polyaniline film on the top of a metal nanorod inside 200
nm pores of an
alumina membrane.
1. Materials
200 nm pore diameter Whatman Anoporedisks (A1z03-membranes) 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)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.
Ti02 colloid was prepared as follows. Ti(ipro)4 was dissolved in 2-
methoxyethanol
under cooling and stirring. The solution was kept under stirnng 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)4: HCl :2-
metoxyethanol
= 1:0.2:20. This solution was diluted with water to adjust TiOz concentration
to 1% and
allowed to age during 3 weeks. The resulting opalescent sol was subjected to
the rotary
evaporation at 60° C to give shiny powder of xerogel containing 75%
(w/w) titania. This
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xerogel was used as a precursor for the preparation of stock aqueous Ti02 sol
with Ti02
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 Ti02 sol shows particles of 4-13 nm
in diameter.
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.
2. Synthesis of rod-shaped diodes
The synthesis of rod-shaped diodes was carned out as follows. Metal electrodes
were
grown electrochemically inside porous membrane. Briefly, the membrane was
pretreated by
evaporating 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 Ti02/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 Ti02 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 Ti02/PAN multilayer without
sonicating. Then
the evaporated silver, "plugs" and alumina membrane were removed by dissolving
in 6 M
nitric acid and O.SM 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 Ti02/PAN film self assembled on plane
Au(MEA)
substrate did not destroy in the 0.5 M NaOH.) The resulting rod-shaped diodes
were
repeatedly centrifuged and washed with water.
In most of the experiments, chemical passivation of A1203-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
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(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.
3. Characterization
Transmission electron microscope (TEM) images were obtained with a JEOL 1200
EXII at 120 kV of accelerating voltage and 80mA of filament current.
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.
TEM images of some typical "striped" bimetallic AulPt/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.
It is unlikely that the relatively rough surface on the top end of a rod may
be
completely covered with the ultrathin Ti02/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 Ti02/PAN films grown on smoother
surfaces
demonstrated better reproducibility in their rectifying behavior. Passivation
(hydrophobization) of A1203-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 Ti02
particles from
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adsorption on the wall surface rather than on metal electrode surface situated
in the depth (~
65 mm) of the pore. It was shown that the Ti02 particles readily formed a
densely packed
layer on a plane Al/A1203 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.
An optical micrograph of Au/(Ti02/PAN)lo/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,
Ti02
nanoparticles are present between two electroplated metals. The OM and TEM
data suggest
that the self assembly of multilayer TiOz/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 TiOz/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.
In order to investigate multilayer Ti02/PAN film sandwiched between Au and Ag
rods, Au/(Ti02/PAN)~/Ag nanorods were prepared and their top Ag electrode was
dissolved
in nitric acid. The remaining 2C Au rods with (Ti02/PAN)6 film deposited on
their top were
analyzed by TEM. Preliminary studies showed that ellipsometric thickness of
multilayer
Ti02/PAN film self assembled on plane Au(MEA) substrate did not decrease after
immersion
in 6 M HN03 for 30 min suggesting stability of the film in the acidic medium.
Furthermore,
similar to the Au/(Ti02/PAN)lo/Ag/Au rods described above, TEM image of the
Au/(Ti02/PAN)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
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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 100 nm, which allows estimating rather gold cup depth
than
(Ti02/PAN)6 film thickness. Ellipsometric thickness of TiOz/PAN)6 film self
assembled on a
plane Au(MEA) substrate is estimated at about l Onm.
I-V characteristic of the Pt/( Ti02/PAN)3 Ti02/Au rod-shaped device reveals
current
rectifying behavior. The forward and reverse bias turn-on potentials are -0.2
and ~0.9 V,
respectively.
