U.S. patent application number 12/670419 was filed with the patent office on 2010-12-02 for plasmonic-driven synthesis of nanoprisms from isotropic and anisotropic gold cores.
This patent application is currently assigned to NORTHWESTERN UNIVERSITY. Invention is credited to Jill E. Millstone, Chad A. Mirkin, Can Xue.
Application Number | 20100304173 12/670419 |
Document ID | / |
Family ID | 39884158 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100304173 |
Kind Code |
A1 |
Mirkin; Chad A. ; et
al. |
December 2, 2010 |
Plasmonic-Driven Synthesis of Nanoprisms from Isotropic and
Anisotropic Gold Cores
Abstract
A nanoprism having a prismatic silver shell formed about a gold
core and a process of forming the same are disclosed. The process
includes irradiating a mixture of gold and silver nanoparticles
with a narrow band of wavelengths capable of exciting the surface
plasmon resonance of the gold.
Inventors: |
Mirkin; Chad A.; (Wilmette,
IL) ; Xue; Can; (Evanston, IL) ; Millstone;
Jill E.; (Jacksonville, FL) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 SOUTH WACKER DRIVE, 6300 WILLIS TOWER
CHICAGO
IL
60606-6357
US
|
Assignee: |
NORTHWESTERN UNIVERSITY
Evanston
IL
|
Family ID: |
39884158 |
Appl. No.: |
12/670419 |
Filed: |
July 28, 2008 |
PCT Filed: |
July 28, 2008 |
PCT NO: |
PCT/US2008/071296 |
371 Date: |
July 12, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60962061 |
Jul 26, 2007 |
|
|
|
Current U.S.
Class: |
428/570 |
Current CPC
Class: |
B82Y 30/00 20130101;
B22F 1/0018 20130101; B22F 9/24 20130101; B22F 2201/11 20130101;
B22F 1/025 20130101; B22F 2001/0037 20130101; B22F 2998/00
20130101; B22F 2999/00 20130101; B22F 2998/00 20130101; Y10T
428/12181 20150115; B22F 1/025 20130101; B22F 2999/00 20130101 |
Class at
Publication: |
428/570 |
International
Class: |
B22F 1/02 20060101
B22F001/02; B22F 9/02 20060101 B22F009/02 |
Goverment Interests
STATEMENT OF GOVERNMENTAL INTEREST
[0002] This invention was made with government support under grant
DMR-0520513 awarded by the National Science Foundation Materials
Research Science and Engineering Centers, and under grant
N00014-06-1-0079 awarded by the Office of Naval Research. The
government has certain rights in the invention.
Claims
1. A nanoprism comprising a gold core and a silver prismatic
shell.
2. A nanoprism having a gold core and a silver prismatic shell
produced by a process comprising: irradiating a mixture comprising
at least one gold nanoparticle and at least one silver nanoparticle
with a narrow band of wavelengths capable of exciting a surface
plasmon resonance of the gold nanoparticle.
3. The nanoprism of claim 1, wherein the nanoprism has a surface
plasmon excitation resonance hand of about 500 nm.
4. The nanoprism of claim 1, wherein the nanoprism has a
characteristic resonance at one or more of about 336 nm about 452
nm, and about 632 nm.
5. The nanoprism of claim 1, wherein the gold core is substantially
spherical.
6. The nanoprism of claim 1, wherein the gold core is a prism.
7. The nanoprism of claim 6, wherein the gold core is a triangular
prism.
8. The nanoprism of claim 1, wherein the silver prismatic shell is
a triangular prism.
9. The nanoprism of claim 1, wherein the nanoprism has a thickness
of about 2 to about 50 nm.
10. The nanoprism of claim 1, wherein the nanoprism has an average
edge length of about 50 to about 300 nm.
11. The nanoprism of claim 2, wherein the mixture is irradiated for
a time of about 1 to about 100 hours.
12. The nanoprism of claim 2, wherein the ratio of gold to silver
nanoparticles is about 1:5 to about 1:100.
13. The nanoprism of claim 2, wherein the gold nanoparticle has a
diameter of about 2 to about 50 nm.
14. The nanoprism of claim 2, wherein the gold nanoparticle has a
diameter of about 2 to about 8 nm, and the nanoprism has a
thickness of about 7 to about 10 nm.
15. The nanoprism of claim 2, wherein the gold nanoparticle has a
diameter of about 20 to about 30 nm, and the nanoprism has a
thickness of about 25 to about 45 nm.
16. The nanoprism of claim 2, wherein the silver nanoparticle has a
diameter of about 1 to about 10 nm.
17. The nanoprism of claim 2, comprising irradiating with a narrow
band of wavelengths of about 500 nm to about 700 nm.
18. The nanoprism of claim 2, wherein the narrow band of
wavelengths comprises wavelengths of about 600 to about 700 nm and
the nanoprism has an average edge length of about 75 to about 100
nm.
19. The nanoprism of claim 2, wherein the narrow band of
wavelengths comprises wavelengths of about 500 to about 525 nm and
the nanoprism has an average edge length of about 40 to about 60
nm.
20. The nanoprism of claim 2, wherein the narrow band of
wavelengths comprises wavelengths of about 530 to about 580 nm and
the nanoprism has an average edge length of about 65 to about 80
nm.
21. The nanoprism of claim 2, wherein the narrow band of
wavelengths comprises wavelengths of about 750 to about 1400 nm and
the gold core is a prism.
22. The nanoprism of claim 21, wherein the gold core is a
triangular prism.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/962,061, filed Jul. 26, 2007, which is
incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0003] The invention relates to methods of forming nanoprisms from
isotropic and anisotropic gold cores. In particular, the invention
relates to methods of forming nanoprisms having a silver nanoprism
shell formed about a gold core.
BACKGROUND
[0004] An interesting, unresolved issue in nanoscience involves the
use of surface plasmon resonance (SPR) to effect nanocluster
chemistry in a controllable manner. These electronic resonances
have been studied for decades in the context of physical phenomena,
such as plasmon coupling and wave-guiding (see Sanders et al., Nano
Lett. 6, 1822 (2006)); and Jain et al., J. Phys. Chem. B. 110,
18243 (2006)), surface-enhanced Raman scattering (SERS) (see
Hunyadi et al. J. Mater. Chem. 16, 3929 (2006); Wang et al., J. Am.
Chem. Soc. 127, 14992 (2005); Qin et al., Proc. Nat. Acad. Sci.
U.S.A. 103, 13300 (2006); and McLellan et al., Nano Lett. 7, 1013
(2007)), and electromagnetic field enhanced fluorescence (see Aslan
et al., J. Am. Chem. Soc. 129, 1524 (2007); and Tam et al., Nano
Lett. 7, 496 (2007)). In silver nanocluster synthesis (see Jin et
al., Science, 294, 1901 (2001); Jin et al., Nature 425, 287 (2003);
Bastys et al., Adv. Funct. Mater. 16, 766 (2006); and Xue et al.,
Ang. Chem. Int. Edit. 46, 2036 (2007)) observations have been made
regarding the ability for surface plasmon excitation to control the
growth of triangular prisms. It has been shown that silver
nanoprisms grow under irradiation until their dipole plasmon
resonance red-shifted to the excitation wavelength (see Jin et al.,
Nature 425, 487 (2003)). Thus, a need exists for core-shell
structured nanoprisms and a process of forming the same using
plasmonic driven synthesis.
SUMMARY OF THE INVENTION
[0005] Disclosed herein are nanoprisms having gold cores and silver
prism shells and a process for forming the same.
[0006] One aspect of the invention is directed to a nanoprism
having a gold core and a silver prismatic shell.
[0007] Another aspect of the invention is directed to a nanoprism
having a gold core and a silver prismatic shell formed by
irradiating a mixture of gold and silver nanoparticles with a
narrow band of wavelengths capable of exciting a surface plasmon
resonance of the gold nanoparticle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1A is a transmission electron microscopy image of a
nanoprism synthesized in accordance with an aspect of the
invention, by irradiation with 550 nm light.
[0009] FIG. 1B is an extinction spectrum of the nanoprisms formed
by a method in accordance with an aspect of the invention, after
centrifugation.
[0010] FIG. 1C is a high-resolution transmission electron
microscopy image of the {111} face of the nanoprisms formed by a
method in accordance with an aspect of the invention.
[0011] FIG. 2 is a scanning transmission electron microscopy
(STEM)--energy dispersive X-ray (EDS) analysis of a gold-silver
nanoprism in accordance with an aspect of the invention. (A) STEM
image of the nanoprism. (B) EDS spectrum of the silver nanoprism
matrix (square spot), showing only silver signal. (C) EDS spectrum
of the silver nanoprism matrix (circle spot), showing both a gold
and a silver signal.
[0012] FIG. 3A is time-resolved extinction spectra of a mixture
solution of 11 nm gold nanoparticles and 5 nm silver nanoparticles
when irradiated with 550 nm light.
[0013] FIG. 3B is a transmission electron microscopy image of the
product after irradiation of the mixture of FIG. 3A with 550 nm
light.
[0014] FIG. 4 is a high-resolution transmission electron microscopy
image of stacks of gold-silver nanoprisms in accordance with an
aspect of the invention.
[0015] FIG. 5 is a transmission electron microscopy image of
nanoprisms formed by a method in accordance with an aspect of the
invention by (A) irradiation with 514 nm light and (B) 600 nm
light.
[0016] FIG. 6 are transmission electron microscopy images of
nanoprisms formed from (A) 5 nm gold nanoparticles and (B) 25 nm
gold nanoparticles in accordance an aspect of the invention.
[0017] FIG. 7 is a schematic drawing showing the growth pathways
for a nanoprism formed by a method in accordance with an aspect of
the invention.
[0018] FIG. 8 are representative transmission electron microscopy
images of nanoprisms in accordance with an aspect of the
invention.
DETAILED DESCRIPTION
[0019] Disclosed herein are nanoprisms derived from gold
nanoparticles and silver nanoparticles. The nanoprisms have a
silver prismatic shell formed about a gold core (core-shell
structure). The nanoprisms of the present invention have
architectures that can be tuned by controlling excitation
wavelength and core diameter.
[0020] The term "nanoparticle" as used herein refers to a metal
composition that is, typically, less than about 1 .mu.m in any one
direction, but can be less than about 500 nm, less than about 200
nm, or less than about 100 nm. Alternatively, the nanoparticle can
be up to about 5 .mu.m. The nanoparticles can be, for example, 5,
10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90,
95, and 100 nm.
[0021] The term "nanoprism" as used herein refers to a metal
composition that exhibits prismatic properties. The nanoprism can
have a single metal, such as a silver or gold nanoprism, but can
also be a core-shell nanoprism, where a core metal has a prismatic
shell. Thus, referring to FIG. 1A, a nanoprism in accordance with
the invention has prismatic shell formed about a core (e.g., a
core-shell structure). The core comprises gold, such as an
isotropic gold nanoparticle, and the prismatic shell comprises
silver, such as silver nanoparticles. Thus, the nanoprism has a
gold core-silver prismatic shell structure. Referring to FIGS. 2B
and 2C, this core-shell structure of the nanoprism can be confirmed
using EDS. The gold core can be substantially spherical.
Alternatively, the gold core can have a triangular prism shape. The
silver prism shell can have, for example, a triangular or a
hexagonal shape. The nanoprism can have an edge length of about 25
nm to about 500 nm, about 50 nm to about 400 nm, and about 100 nm
to about 300 nm. The nanoprism can have an edge length, for
example, of 25, 30, 35, 40, 45, 50, 55, 60, 65, 60, 75, 80, 85, 90,
95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155,
160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220,
225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285,
290, 295, and 300.
[0022] Prismatic properties can be detected using known techniques.
Prismatic properties include, but are not limited to,
characteristic resonances. Referring to FIG. 1B, the nanoprism has
a characteristic resonance at about 336 nm (corresponding to an
out-of-plane quadrupole resonance), about 452 nm (corresponding to
an in-plane quadrupole resonance), and/or about 632 nm
(corresponding to an in-plane dipole resonance). The characteristic
resonance of the nanoprism indicates that these gold core-silver
shell nanoprisms can have optical features similar to optical
features of pure silver nanoprisms without gold cores.
[0023] A nanoprism in accordance with the invention exhibits
optical properties similar to pure silver nanoprism without a gold
core. Use of silver nanoprisms is disclosed in U.S. Pat. Nos.
7,135,054 and 7,033,415, each of which is incorporated by reference
in its entirety. The optical properties of the nanoprisms can be
used in many biodiagnostic applications. The scattering properties
of the nanoprism can be tailored by adjusting the size and shape of
the nanoprisms, making the nanoprism useful as multicolor
labels.
[0024] The nanoprism can be used as a diagnostic label, lighting up
when target DNA is present. Biodetectors incorporating nanoprisms
can be used to quickly, easily, and accurately detect biological
molecules, as well as a wide range of genetic and pathogenic
diseases, from genetic markers for cancer and neurodegenerative
disease to HIV and sexually transmitted diseases.
Formation of Gold and Silver Nanoparticles
[0025] Gold and silver nanoparticles can be formed according to
known methods. See Jin et al., Nature 425, 487 (2003); and Grabar
et al., Anal. Chem. 67, 735 (1995). For example, gold nanoparticles
can be formed by bringing an aqueous solution of a gold source,
such as HAuCl.sub.4, to reflux, and then adding trisodium citrate.
The solution preferably is refluxed for about 15 minutes, and then
allowed to cool to about room temperature. The gold colloid can be
centrifuged and resuspended in trisodium citrate solution.
[0026] Alternatively, the gold nanoparticles can be formed by
mixing an aqueous solution of a gold source, such as HAuCl.sub.4,
with trisodium citrate. A reducing agent, such as sodium
borohydride (NaBH.sub.4) then can be added to the mixture while
stiffing vigorously. The solution preferably is allowed to age for
about 3 hours, and then used as seed solution. The seed solution
can be mixed with an aqueous solution of a gold source, such as
HAuCl.sub.4, and a stabilizer, such as poly(vinylpyrrolidone)
(PVP). Ascorbic acid then can be added to the mixture and allowed
to react for about 20 minutes to form the gold nanoparticles.
[0027] The gold nanoparticles can also be formed, for example, by
bringing an aqueous solution of a gold source, such as HAuCl.sub.4,
to a boil, and then adding trisodium citrate quickly the boiling
solution. The solution can be refluxed for about 15 minutes, and
then allowed to cool to room temperature.
[0028] The gold nanoparticles can have a substantially spherical
shape, or any other suitable shape. The gold nanoparticles
typically have a diameter of about, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,
29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45,
46, 47, 48, 49, and 50 nm. Isotropic gold nanoparticles typically
have a surface plasmon resonance of about 500 nm to about 600 nm.
For example, the gold nanoparticles can have a surface plasmon
resonance of about 516 nm. The gold nanoparticles can also be
prismatic, for example, the gold nanoparticles can be triangular
nanoprisms. The surface plasmon resonance of triangular gold
nanoprism is in the near-infrared region of the spectrum.
[0029] Silver nanoparticles can be formed, for example, by bubbling
ice-cold deionized water with nitrogen gas in the dark and stiffing
vigorously for about 30 minutes. A silver source, such as silver
nitrate (AgNO.sub.3), and trisodium citrate then can be added. An
aqueous solution of a reducing agent, such as sodium borohydride
(NaBH.sub.4), can be rapidly injected into the solution. The
reducing agent can be further added dropwise about every 2 minutes
for about 15 minutes. The reducing agent and an aqueous stabilizer,
such as bis(p-sulfonatophenyl) phenylphosphine (BSPP), then can be
added dropwise. The resulting silver nanoparticle solution can be
stirred gently for about 5 hours in an ice bath, and then allowed
to stand overnight in the dark at about 4.degree. C.
[0030] The silver nanoparticles can have a substantially spherical
shape, or any other suitable shape. The silver nanoparticles
typically have a diameter of about 1, 2, 3, 4, 5, 6, 7, 8, 9, and
10 nm. The silver nanoparticles typically have a surface plasmon
resonance of about 395 nm.
Core-Shell Nanoprism Formation
[0031] Core-shell nanoprisms as disclosed herein can be formed by
irradiating a mixture of gold and silver nanoparticles with a
narrow band of wavelengths capable of exciting a surface plasmon
resonance of the gold nanoparticle. The phrase "narrow band of
wavelengths" as used herein refers to a specific wavelength plus or
minus about 20 nm. In some embodiments, the narrow band of
wavelengths has less than plus or minus 20 nm, e.g., plus or minus
15 nm, plus or minus 10 nm, or plus or minus 5 nm. Typically, a
specific wavelength is selected by placing an optical filter
between a light source and the object to be irradiated. This
optical filter can have a width of about 40 nm, about 30 nm, about
20 nm, or about 10 nm.
[0032] The gold and silver nanoparticle mixture can have a ratio of
gold to silver nanoparticles of about 1:5 to about 1:100; about
1:10 to about 1:50, and about 1:10 to about 1:20. The use of silver
nanoparticles as the silver source keeps the concentration of
silver ions (Ag.sup.+) low, but consistent throughout the
photochemically induced Ag.sup.+ reduction and deposition onto the
gold nanoparticle surfaces. Alternatively, AgNO.sub.3 can be used
as the silver source instead of silver nanoparticles, but the
reaction and quality of the resulting nanoprisms are more difficult
to control.
[0033] Irradiation can be performed for about 1 to about 100 hours,
about 5 to about 50 hours, and about 10 to about 25 hours. For
example, irradiation can be performed for about 1, 5, 10, 15, 20,
25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, and 100
hours. Irradiation is performed using a narrow band of wavelengths
that is capable of exciting the surface plasmon resonance of the
gold nanoparticle. For example, when a gold nanoparticle having a
surface plasmon resonance of about 516 nm, irradiation can be
performed using a narrow band of wavelengths of from about 450 to
about 700 nm; from about 500 to about 650 nm; and from about 550 to
about 600 nm.
[0034] Referring to FIG. 8, the nanoprism can have a gold prism
core and a silver prism shell. Formation of nanoprisms having a
prismatic gold core and a prismatic silver shell can be formed
using gold nanoparticles having a prism shape and exhibiting a
surface plasmon resonance in the near-infrared region. See
Millstone et al., Adv. Funct. Mater. 16, 1209 (2006). For example,
the gold nanoprisms can be triangular nanoprisms. A mixture of
silver nanoparticles and gold nanoprisms can be irradiated in the
near infrared region, for example, using a with a narrow band of
wavelengths from about 750 to about 1400 nm; from about 850 to
about 1200; and from about 900 to about 1100 nm.
[0035] Without intending to be bound by theory, it is believed that
the growth of the nanoprism is attributed to plasmon excitation,
and can be considered as a two-step pathway. Referring to FIG. 7,
in the first step, the incident light preferentially excites the
dipole plasmon resonance of the gold nanoparticles, which induces
deposition of the silver layers. The silver ions are contributed
from the dissolution of the silver nanoparticles. Preferential
excitation of the gold nanoparticles occurs because the surface
plasmon resonance of the gold nanoparticles is closer to the
excitation wavelength than the surface plasmon resonance of the
silver nanoparticles.
[0036] Without intending to be bound by theory, it is believed that
the gold is behaving as a photocatalyst. The redox chemistry of
Ag.sup.+ and citrate is thermodynamically downhill based upon their
redox potentials (E.sub.Ag+Ag=0.7996 V verses NHE; and E.sub.ADE,
CO2/citrate<-0.01 V at a pH above 8 (see Trettenhahn &
Koberi, Electrochim. Act. 52, 2716 (2007)), and photoexcitation of
a photocatalyst is not necessary to effect the redox reaction.
AgNO.sub.3 and citrate will react when boiling in the dark to form
silver particles, carbon dioxide (CO.sub.2), and
1,3-acetonediacarboxylate (ADE, the oxidation product of citrate).
Photoexcitation of the particles may result in a catalyst that
facilitates the reaction between Ag.sup.+ and citrate through
photoinduced ligand rearrangement. It is unlikely that the process
is through ligand dissociation because citrate and Ag.sup.+ are
indefinitely stable (at least about 1 month) in the presence of
gold nanoparticles. Furthermore, citrate is weakly bound to the
surface of the gold nanoparticle. Therefore, it is unlikely that
the particle is simply acting as a pre-catalyst activated by ligand
dissociation, as one would expect to observe some background
catalytic activity in the dark.
[0037] Referring to FIG. 3A, time-resolved UV-vis spectra
demonstrates that as the gold core is coated with the silver shell
during about the first 30 minutes of irradiation, the surface
plasmon resonance band of the gold-silver nanostructure blue shifts
from about 516 nm to about 500 nm. Referring to FIG. 3B,
transmission electron microscopy analysis of the particles after
the initial irradiation period demonstrates that the gold
nanoparticles are covered by a silver shell with irregular
shapes.
[0038] Referring to FIG. 3B and FIG. 7, during the early stage of
nanoprism growth, the silver shell begins to exhibit an anisotropic
morphology, but does not have the same morphology for each particle
in solution. The early dispersity in nanoparticle architecture may
be due in part to momentarily different electric field polarization
across the nanoparticle surface during excitation, which could play
a role in local redox chemistry. See Kelly, et al., J. Phys. Chem.
B. 107, 668 (2003). As the excitation bands of the gold-silver
particles approach the excitation wavelength, further excitation
leads to the reconstruction of surface silver atoms into prismatic
shells. The gold nanoparticles do not generally exhibit a change in
the morphology. The growth of the silver shell does not exhibit
dependence on the shape and symmetry of the gold cores. Without
intending to be bound by theory, it is believed that the plasmon
excitation leads to deposition of silver layers on the gold
nanoparticle surface, and creates silver {111} twin planes. With
continuous plasmon excitation, further silver shell growth
accelerates in the direction of the parallel twin planes, while
growth on the direction perpendicular to the {111} facet is much
slower. Such a growth pattern can lead to three-fold symmetry which
results in small triangular or hexagonal silver shells. Referring
again to FIG. 3A, the growth of the silver shell stops when the
dipole plasmon resonance band of the nanoprism red shifts with
respect to the excitation wavelength (i.e. until the final
nanoprism structure no longer absorbs the wavelength of
irradiation).
[0039] Referring to FIG. 4, the side planes of the nanoprisms
demonstrates that the number of twin planes varies, which could be
due to the non-uniformity of the gold nanoparticle seeds. Hexagonal
and triangular silver shells with different degrees of truncation
are observed. Without intending to be bound by theory, it is
believed that the varying degrees of truncation are structures that
have not been fully transformed due to a depletion of the silver
feedstock.
[0040] The nanoprisms have an average edge length of about 50 to
about 100 nm. Referring to FIG. 5A, for example, when irradiation
is performed using longer excitation wavelengths, such as about 600
nm, the nanoprisms have longer average edge lengths of about 75 to
about 100 nm. Referring to FIG. 5B, when irradiation is performed
using a wavelength that almost coincide with the plasmon band of
the gold nanoparticles (for example, about 514 nm), the nanoprisms
have average edge lengths of about 40 to about 60 nm. When
irradiation is performed using a wavelength of about 550 nm, the
nanoprisms have an average edge length of about 65 to about 80.
Irradiation at wavelengths shorter than about 514 nm, such as about
488 nm, can lead to an increase in irregular anisotropic particles
with a low yield of nanoprisms having the core-shell structure.
[0041] The nanoprisms have a thickness of about 2 to about 100 nm.
The thickness of the nanoprism is dependent on the size of the gold
nanoparticles used to form the nanoprism. Referring FIG. 6A, for
example, formation of nanoprisms using gold particles having a
diameter of about 2 to about 8 nm can lead to nanoprisms having a
thickness of about 7 to about 10 nm. Referring to FIG. 6B, for
example, formation of nanoprism using gold nanoparticles having a
diameter of about 20 to about 30 nm can result in nanoprisms having
a thickness of about 25 to about 45 nm.
[0042] Additional aspects of the invention will be apparent from
the following examples, which are intended to be illustrative
rather than limiting.
EXAMPLES
Formation of 5 nm Gold Nanoparticles
[0043] Gold nanoparticles having a diameter of about 5 nm were
prepared in accordance with known methods. An aqueous solution
(about 20 mL) containing 0.25 mM HAuCl.sub.4 and 0.25 mM trisodium
citrate was prepared in a flask. Ice-cold, freshly prepared 0.1M
NaBH.sub.4 solution (about 0.6 mL) was added to the aqueous
solution while stirring vigorously. The mixture was allowed to age
for about 3 hours, and then was used as a seed solution. While
stirring, about 2.5 mL of the seed solution was mixed with about
7.5 mL of an aqueous solution containing 0.25 mM HAuCl.sub.4 and
polyvinylpyrrolidone (0.1% PVP, MW about 58,000). Then, about 0.5
mL of 0.1 M ascorbic acid solution was added to the mixture and
allowed to react for about 20 minutes while stirring. Transmission
electron microscopy was used to determine the average diameter of
the gold nanoparticles, which was about 5.3.+-.0.8 nm.
Formation of 11 nm Nanoparticles
[0044] Gold nanoparticles having a diameter of about 11 nm were
prepared in accordance with known methods. An aqueous solution
(about 500 mL) of 1 mM HAuCl.sub.4 was brought to reflux while
stiffing. Fifty milliliters of 77.6 mM trisodium citrate solution
was added quickly to the boiling aqueous solution. The mixture was
refluxed for about an additional 15 minutes, and then allowed to
cool to room temperature. The gold colloid was centrifuged at about
13.2 krpm (Eppendorf, 5415D) for about 30 minutes, and then
resuspended in a 0.3 mM trisodium citrate solution. Transmission
electron microscopy was used to determine the average diameter of
the gold nanoparticles, which was about 11.2.+-.1.8 nm.
Formation of 25 nm Nanoparticles
[0045] Gold nanoparticles having a diameter of about 25 nm were
prepare by heating about 50 mL of an aqueous solution of
HAuCl.sub.4 (1% w/v) to a boil, while stiffing. Then, about 0.75 mL
of trisodium citrate (1% w/v) was added quickly to the boiling
mixture. The mixture was refluxed for about 15 minutes, and then
allowed to cool to room temperature. The gold colloid was
centrifuged at about 13.2 krpm for about 20 minutes, and
resuspended in 0.3 mM trisodium citrate solution. Transmission
electron microscopy was used to determine the average diameter of
the gold nanoparticles, which was about 25.1.+-.2.7 nm.
Formation of Silver Nanoparticles
[0046] Silver nanoparticles were formed by filling a three-neck
flask with about 95 mL of NANORPURE.TM. water and immersing the
flask in an ice bath. The water was bubbled with nitrogen gas in
the dark with vigorous stirring for about 30 minutes. Then, about
0.5 mL of 20 mM AgNO.sub.3 and 1 mL of 30 mM of trisodium citrate
were added to the ice-cold solution. One milliliter of 50 mM
NaBH.sub.4 (freshly prepared with ice-cold NANORPURE.TM. water) was
rapidly injected into the solution. Five to six drops of NaBH.sub.4
solution was added to the mixture every 2 minutes for about 15
minutes. One milliliter of NaBH.sub.4 solution and 1 mL of 5 mM
bis(p-sulfonatophenyl) phenylphosphine (BSPP) solution were added
to the mixture dropwise. The resulting silver nanoparticle solution
was gently stirred for about 5 hours in an ice bath, and then
allowed to stand overnight in the dark at about 4.degree. C.
Transmission electron microscopy was used to determine the average
diameter of the silver nanoparticles, which was about 5.0.+-.1.2
nm.
Formation of Gold-Silver Nanoprisms
[0047] About 9 mL of a solution of silver nanoparticles was mixed
with about 1 mL of a solution of gold nanoparticles that was
diluted to have an optical density of about 1 O.D./mL at extinction
maximum. The solution was irradiated with a 150 W halogen lamp
coupled with an optical bandpass filter (Intor, Inc.). When the
optical bandpass filter was centered at about 550.+-.20 nm,
triangular nanoprisms having an average edge length of 70.+-.6 nm
were formed. When the optical bandpass filter was centered at about
514.+-.20 nm, triangular nanoprisms having an average edge length
of about 48.+-.5 nm were formed. When the optical bandpass filter
was centered at about 600.+-.20 nm, triangular nanoprisms with an
average edge length of about 80.+-.7 nm were formed.
[0048] The foregoing describes and exemplifies the invention but is
not intended to limit the invention defined by the claims that
follow. All of the methods disclosed and claimed herein can be made
and executed without undue experimentation in light of the present
disclosure. While the materials and methods of this invention have
been described in terms of specific embodiments, it will be
apparent to those of skill in the art that variations may be
applied to the materials and/or methods and in the steps or in the
sequence of steps of the methods described herein, without
departing from the concept, spirit, and scope of the invention.
More specifically, it will be apparent that certain agents which
are both chemically and physiologically related may be substituted
for the agents described herein while the same or similar results
would be achieved. All such similar substitutes and modifications
apparent to those of ordinary skill in the art are deemed to be
within the spirit, scope, and concept of the invention as defined
in the appended claims
* * * * *