U.S. patent application number 13/026946 was filed with the patent office on 2011-07-28 for surface enhanced raman spectroscopy using shaped gold nanoparticles.
This patent application is currently assigned to UNIVERSITY OF SOUTH CAROLINA. Invention is credited to ANAND M. GOLE, CATHERINE J. MURPHY, CHRISTOPHER J. ORENDORFF, TAPAN K. SAU.
Application Number | 20110184202 13/026946 |
Document ID | / |
Family ID | 36588448 |
Filed Date | 2011-07-28 |
United States Patent
Application |
20110184202 |
Kind Code |
A1 |
MURPHY; CATHERINE J. ; et
al. |
July 28, 2011 |
SURFACE ENHANCED RAMAN SPECTROSCOPY USING SHAPED GOLD
NANOPARTICLES
Abstract
Surface enhanced Raman scattering (SERS) spectra of
4-mercaptobenzoic acid (4-MBA) self-assembled monolayers (SAMs) on
gold substrates is presented for SAMs onto which gold nanoparticles
of various shapes have been electrostatically immobilized. SERS
spectra of 4-MBA SAMs are enhanced in the presence of immobilized
gold nanocrystals by a factor of 10.sup.7-10.sup.9 relative to
4-MBA in solution. Large enhancement factors are a likely result of
plasmon coupling between the nanoparticles (localized surface
plasmon) and the smooth gold substrate (surface plasmon polariton),
creating large localized electromagnetic fields at their interface,
where 4-MBA molecules reside in this sandwich architecture.
Moreover, enhancement factors depend on nanoparticle shape, and
vary by a factor of 10.sup.2.
Inventors: |
MURPHY; CATHERINE J.;
(Columbia, SC) ; SAU; TAPAN K.; (Chandigarh,
IN) ; ORENDORFF; CHRISTOPHER J.; (Albuquerque,
NM) ; GOLE; ANAND M.; (Columbia, SC) |
Assignee: |
UNIVERSITY OF SOUTH
CAROLINA
Columbia
SC
|
Family ID: |
36588448 |
Appl. No.: |
13/026946 |
Filed: |
February 14, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11721554 |
Jul 7, 2008 |
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PCT/US2005/044963 |
Dec 13, 2005 |
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13026946 |
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60635704 |
Dec 13, 2004 |
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60648920 |
Feb 1, 2005 |
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Current U.S.
Class: |
556/110 ;
977/773 |
Current CPC
Class: |
G01N 33/553 20130101;
G01N 2610/00 20130101; G01N 33/54373 20130101; Y10T 428/25
20150115; Y10T 428/2991 20150115; Y10S 436/805 20130101; G01N
21/658 20130101; B82Y 30/00 20130101 |
Class at
Publication: |
556/110 ;
977/773 |
International
Class: |
C07F 1/12 20060101
C07F001/12 |
Goverment Interests
ACKNOWLEDGEMENT
[0002] This invention was made with government support under Grant
CHE-0336350 awarded by the National Science Foundation. The
government has certain rights in the invention.
Foreign Application Data
Date |
Code |
Application Number |
Dec 13, 2005 |
US |
PCT/US2005/044963 |
Claims
1. A cetyltrimethylammonium bromide-capped gold nanoparticle.
2. The composition of claim 1, wherein the shape comprises a cube,
a block, a tetrapod, a rod with an aspect ratio of at least about
3.2, or a dogbone.
3. A composition comprising: nanoparticulate gold and a
cetyltrimethylammonium bromide residue, wherein the composition has
a shape comprising a cube, a block, a tetrapod, a sphere, a rod, a
star, or a dogbone.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a Divisional Application of U.S. Serial No.
11/721,554 filed Jul. 7, 2008, which is a U.S. National Phase of
PCT/US2005/044963 filed Dec. 13, 2005, which claims the benefit of
U.S. Application No. 60/635,704, filed Dec. 13, 2004, and U.S.
Application No. 60/648,920, filed Feb. 1, 2005, all of which are
hereby incorporated herein by reference in their entireties.
BACKGROUND
[0003] Bulk solution synthetic methods often produce nanocrystals
of multiple sizes and shapes, and hence there is relatively low
yield of the desired size and shape. Murphy, C. J. Science 2002,
298, 2139-2141. Although colloid chemists have achieved excellent
control over particle size for several metallic and semiconductor
systems, there has been limited success in gaining control over the
shape of the nanocrystals. Schmid, G.; Ed. Clusters and Colloids.
From Theory to Applications; VCH: New York, 1994. Watzky, M. A.;
Finke, R. G., J. Am. Chem. Soc. 1997, 119, 10382-10400. Jana, N.
R.; Peng, X., J. Am. Chem. Soc. 2003, 125, 14280-14281. Controlling
size, shape, and structural architecture of the nanocrystals
requires manipulation of the kinetic and thermodynamic parameters
of the systems via utilization of various additives, light and
thermal energies, and their various combinations. Ahmadi, T. S.;
Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A., Science
1996, 272, 1924-1925. Pileni, M. P.; Ninham, B. W.;
Gulik-Krzywicki, T.; Tanori, J.; Lisiecki, I.; Filankembo, A., Adv.
Mater. 1999, 11, 1358-1362. Li, M.; Schnablegger, H.; Mann, S,
Nature 1999, 402, 393-395. Jin, R.; Cao, Y. C.; Hao, E.; Metraux,
G. S.; Schatz, G. C.; Mirkin, C. A., Nature 2003, 425, 487-490.
Sun, Y.; Xia, Y. Science 2002, 298, 2176-2179. Sun, Y.; Xia, Y.,
Adv. Mater. 2002, 14, 833-837. Sun, Y.; Mayers, B.; Herricks, T.;
Xia, Y. Nano Lett. 2003, 3, 955-960.
[0004] Therefore, there remains a need for methods and compositions
that overcome these deficiencies and that effectively provide
shaped nanoparticles.
[0005] Surface enhanced Raman spectroscopy (SERS) is a powerful
analytical tool for determining chemical information for molecules
on metallic substrates. Moskovits, M. Rev. Mod. Phys. 1985, 57,
783-826. In general, there are two traditional operational
mechanism to describe the overall SERS effect, electromagnetic (EM)
and chemical (CHEM) enhancement mechanisms. EM enhancement is
enhancement of the local electromagnetic field incident on an
adsorbed molecule at a metallic surface. CHEM enhancement results
from electronic resonance/charge transfer between a molecule and a
metal surface, which leads to an increase the polarizability of the
molecule. Otto, A.; Mrozek, I.; Pettenkofer, C. Surf Sci. 1990,
238, 192. Schultz, S. G.; Janik-Czachor, M.; Van Duyne, R. P. Surf
Sci. 1984, 104, 419. Since the introduction of the SERS phenomenon
on roughened silver electrodes, much attention has turned to SERS
on spherical colloidal substrates of either gold or silver.
Jeanmaire, D. L.; Van Duyne, R. P., J. Electroanal. Chem. 1977, 84,
1-20. Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99,
5215-5217. Nie, S. M.; Emery, S. R. Science 1997, 275, 1102-1106.
Krug, J. T.; Wang, G. D.; Emory, S. R.; Nie, S. M., J. Am. Chem.
Soc., 1999, 121, 9208-9214. Freeman, R. G.; Bright, R. M.; Hommer,
M. B.; Natan, M. J., J. Raman Spectrosc. 1999, 30, 733-738. Jensen,
T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P., J. Phys.
Chem. B 2000, 104, 10549-10556. Kneipp, K.; Kneipp, H.; Deinum, G.;
Itzkan, I.; Dasari, R. R.; Feld, M. S. Appl. Spectrosc. 1998, 52,
175-178. Colloidal nanoparticles are of interest as SERS substrates
not only because they are strong light scatterers, but because of
their tunable optical properties which depend on nanoparticle size,
shape, and aggregation state. El-Sayed, M. A., Acc. Chem. Res.
2001, 34, 257-264. Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz,
G. C. J. Phzys. Chem. B 2003, 107, 668-677.
[0006] Spheroidal or rod-shaped nanoparticles are of significant
interest as SERS substrates because of their tunable longitudinal
plasmon bands and the "lightning rod" effect on surface
enhancement. Schatz, G. C., Acc. Chem. Res. 1984, 17, 370-376.
Gersten, J. I., J. Chem. Phys. 1980, 72, 5779-5780. While electric
field enhancement is observed for 10-200 nM metallic particles,
even greater local field enhancements are observed at sharp surface
features, for example, at the tips of needle-shaped nanorods where
the curvature radius is much smaller than the size of the
nanoparticle. Gersten, J. I. J. Chem. Phys. 1980, 72, 5779-5780.
This phenomenon is known as the lightning rod effect. Despite the
desirable characteristics of metallic nanorods and nanowires as
SERS substrates, only a few reports exist for SERS on rod- or
wire-shaped nanoparticles. Tao, A.; Kim, F.; Hess, C.; Goldberger,
J.; He, R.; Sun, Y.; Xia, Y, Yang, P. Nano Lett. 2003, 3,
1229-1323. Jeong, D. H.; Zhang, Y. X.; Moskovits, M., J. Phys.
Chem. B 2004, 108, 12724-12728. Yao, J. L.; Pan, G. P.; Xue, K. H.;
Wu, D. Y.; Ren, B.; Sun, D. M.; Tang, J.; Xu, X.; Tian, Z. Q. Pure
Appl. Chem. 2000, 72, 221-228. Nikoobakht et al. have examined the
use of unaggregated and aggregated gold nanorods as SERS substrates
using pyridine and 4-aminothiophenol analytes. Nikoobakht, B. Wang,
J. El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. Nikoobakht,
B., El-Sayed, M. A., J. Phys. Chem. A 2003, 107, 3372-3378. For
SERS on unaggregated nanorods, the excitation wavelength was 1064
nm, far removed from the nanorod absorption bands (.about.520 nm
and 700 nm) where the EM enhancement mechanism is thought to be
inoperative. Nikoobakht, B. Wang, J. El-Sayed, M. A. Chem. Phys.
Lett. 2002, 366, 17-23. Despite the off-resonance condition,
appreciable SERS intensity was observed with surface enhancement
factors (EF) of 104 for pyridine. The authors attributed the
enhancement to a chemical (CHEM) enhancement mechanism of strongly
adsorbed pyridine on the Au{ 110} surface of these nanorods.
However, no reports have been made for SERS on nanorods where the
Raman excitation occurs at a wavelength that overlaps with nanorod
plasmon resonance, a condition where the EM enhancement mechanism
should be operative.
[0007] Large enhancement factors and even single molecule SERS have
been reported for molecules at junctions between aggregated
nanoparticles. Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L., J.
Phys. Chem. B 2003, 107, 9964-9972. Xu, H. X.; Bjerneld, E. J.;
Kall, M.; Borjesson, L. Phys. Rev. Lett. 1999, 83, 4357-4360.
Michaels, A. M.; Jiang, J.; Brus, L., J. Phys. Chem. B 2000, 104,
11965-11971. This is a result of localized surface plasmon (LSP)
coupling between nanoparticles and enhanced electromagnetic field
intensity localized at nanoparticle junctions. Michaels, A. M.;
Jiang, J.; Brus, L., J. Phys. Chem. B 2000, 104, 11965-11971.
Vidal, F. J. G-.; Pendry, J. B. Phys. Rev. Lett. 1996, 77,
1163-1166. Wang, D.-S.; Kerker, M. Phys. Rev. B 1981, 24,
1777-1790. Markel, V. A.; Shalaev, V. M.; Zhang, P.; Huynh, W.;
Tay, L.; Haslett, T. L.; Moskovits, M. Phys. Rev. B 1999, 59,
10903-10909. Su, K.-H.; Wei, Q.-H.; Zhang, X.; Mock, J. J.; Smith,
D. R.; Schultz, S, Nano Lett. 2003, 3, 1087-1090. Atay, T.; Song,
J-. H.; Murmikko, A. V. Nano Lett. 2004, 4, 1627-1731. Fromm, D.
P.; Sundaramurthy, A.; Schuck, P. J.; Kino, G.; Moemer, W. E. Nano
Lett. 2004, 4, 957-961. This LSP coupling between aggregated gold
nanorods is believed to contribute to SERS enhancement observed by
El-Sayed and coworkers. Nikoobakht, B., El-Sayed, M. A., J. Phys.
Chem. A 2003, 107, 3372-3378. It is important to note, that
although it is difficult to estimate enhancement factors for
aggregated nanoparticles, the authors stated that SERS enhancements
were always greater for aggregated gold nanorods than for
aggregated spherical nanoparticles. Nikoobakht, B., El-Sayed, M.
A., J. Phys. Chem. A 2003, 107, 3372-3378. Similarly, LSP coupling
between colloidal nanoparticles and the surface of planar
substrates, referred to as surface plasmon polariton (SPP), has
also been well documented and has been reported for surface plasmon
resonance (SPR) spectroscopy measurements. Shchegrov, A. V.;
Novikov, I. V.; Maradudin, A. A. Phys. Rev. Lett. 1997, 78,
4269-4272. Holland, W. R.; Hall, D. G., Phys. Rev. B 1983, 27,
7765-7768. Kume, T.; Nakagawa, N.; Yamamoto, K., Solid State
Commun. 1995, 93, 171-175. Lyon, L. A.; Musick, M. D.; Natan, M.,
J. Anal Chem. 1998, 70, 5177-5183. Lyon, L. A.; Pena, D. J.; Natan,
M. J., J. Phys. Chem. B 1999, 103, 5826-5831. Hutter, E.; Cha, S.;
Liu, J-F.; Park, J.; Yi, J.; Fendler, J. H.; Roy, D., J. Phys.
Chem. B 2001, 105, 8-12. A 20-fold increase in signal is observed
for biological sandwich assays where analytes are between
nanoparticles and a planar surface, and LSP-SPP coupling occurs.
LSP-SPP coupling has also been observed qualitatively by Zheng et
al. between silver nanoparticles and surface plasmons of planar
silver substrates. Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu,
R Langmuir 2003, 19, 632-636. They observed greater SERS intensity
for 4-aminothiophenol (4-ATP) self-assembled monolayers (SAMs) on
silver when colloidal silver nanoparticles are adsorbed to the SAM
than for the 4-ATP SAM of polished and electrochemically roughened
silver. Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu, R Langmuir
2003, 19, 632-636.
[0008] A significant challenge in SERS on colloidal nanoparticle
substrates is determining the number of analyte molecules sampled
during the experiment. It is essential to calculate not only the
number of nanoparticles in solution, but also the surface coverage
of analyte molecules adsorbed to these nanoparticles. This is
especially difficult for nanoparticles that are synthesized using
strongly adsorbed capping agents including cetyltrimethylammonium
bromide (CTAB), which may or may not be displaced by the analyte of
interest. Nikoobakht, B. Wang, J. El-Sayed, M. A. Chem. Phys. Lett.
2002, 366, 17-23. Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001,
17, 6368-6374. In most reports, monolayer surface coverage on the
nanocrystals is assumed, but if incorrect could lead to errors in
calculations of EF values. For SERS on self-assembled monolayers
(SAMs) on planar substrates, this problem is avoided altogether
because there are no capping agents on these substrates and the
number of molecules sampled is well known. Ulman, A. Chem. Rev.
1996, 96, 1533-1554. However, the tunability of the optical
properties of planar SERS substrates is more difficult than
solution-prepared colloids.
[0009] Previous research has demonstrated the high yield synthesis
of gold nanorods and a plethora of other shapes of nanocrystals.
Jana, N. R.; Gearheart, L. Murphy, C., J. Adv. Mater. 2001, 137,
1389-1393. Jana, N. R.; Gearheart, L. Murphy, C. J., J. Phys. Chem.
B 2001, 105, 4065-4067. Sau, T. K.; Murphy, C. J. Langmuir 2004,
20, 6414-6420. Sau, T. K.; Murphy, C. J., J. Am. Chem. Soc. 2004,
126, 8648-8649. Additionally, the immobilization of CTAB-protected
gold nanorods on carboxylate-terminated SAMs has been studied.
Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20,
7117-7122. In the present invention, CTAB-capped nanoparticles of
various shapes are immobilized on 4-mercaptobenzoic acid (4-MBA)
monolayers. SERS spectra of 4-MBA are acquired to determine the
effect of immobilizing gold nanoparticles on SERS of 4-MBA SAMS on
gold and to determine whether the nanoparticle shapes, specifically
their optical properties and surface structure, influence SERS of
4-MBA SAMs.
[0010] Therefore, there remains a need for methods and compositions
that overcome these deficiencies and that effectively provide
surface enhanced Raman spectroscopy.
SUMMARY
[0011] In accordance with the purpose(s) of the invention, as
embodied and broadly described herein, the invention, in one
aspect, relates to methods, products, and compositions for
preparing and using shaped nanoparticles. In a further aspect, the
invention relates to methods, products, and compositions for
surface enhanced Raman spectroscopy.
[0012] In one aspect, the invention relates to a method for
enhancing a Raman signal comprising the steps of providing a sample
comprising a metal surface, an analyte adhered to the surface, and
a metallic nanoparticle coupled to the surface, wherein the
nanoparticle has a plasmon resonance band; exposing the sample to
incident energy of an excitation wavelength that overlaps with the
metallic nanoparticle plasmon resonance band; and detecting the
Raman signal of the analyte.
[0013] In a further aspect, the invention relates to a method for
enhancing a Raman signal comprising the steps of providing a sample
comprising a metal surface, a functionalized self-assembled
monolayer adhered to the surface, wherein the self-assembled
monolayer comprises an analyte, and a cetyltrimethylammonium
bromide-capped metallic nanoparticle coupled to the surface;
exposing the sample to incident energy of an excitation wavelength;
and detecting the Raman signal of the analyte.
[0014] In a further aspect, the invention relates to a method for
enhancing a Raman signal comprising the steps of providing a sample
comprising a gold surface, a functionalized self-assembled
monolayer adhered to the surface, wherein the self-assembled
monolayer comprises an analyte, and a cetyltrimethylammonium
bromide-capped metallic nanoparticle coupled to the surface;
exposing the sample to incident energy of an excitation wavelength
that overlaps with the metallic nanoparticle plasmon resonance
band; and detecting the Raman signal of the analyte, wherein the
Raman signal has an enhancement factor of from about 10.sup.7 to
about 10.sup.9 relative to the analyte in solution.
[0015] In a further aspect, the invention relates to a composition
comprising a metal surface, a functionalized self-assembled
monolayer adhered to the surface, wherein the self-assembled
monolayer comprises an analyte, and a cetyltrialkylammonium
halide-capped metallic nanoparticle coupled to the surface.
[0016] In a further aspect, the invention relates to a method for
preparing a cetyltrimethylammonium bromide-capped gold nanoparticle
comprising the steps of providing a seed solution comprising a gold
nanoparticle; providing an aqueous growth solution comprising
cetyltrimethylammonium bromide, hydrogen tetrachloroaurate, and
ascorbic acid; and adding a quantity of the seed solution to the
aqueous growth solution, thereby producing a cetyltrimethylammonium
bromide-capped gold nanoparticle having a shape comprising a cube,
a block, a tetrapod, a sphere, a rod, a star, or a dogbone.
[0017] In a further aspect, the invention relates to the products
produced by the methods of the invention.
[0018] In a further aspect, the invention relates to a
cetyltrimethylammonium bromide-capped gold nanoparticle.
[0019] In a further aspect, the invention relates to a composition
comprising nanoparticulate gold and a cetyltrimethylammonium
bromide residue, wherein the composition has a shape comprising a
cube, a block, a tetrapod, a sphere, a rod, a star, or a
dogbone.
[0020] Additional advantages of the invention will be set forth in
part in the description which follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The advantages of the invention will be realized and
attained by means of the elements and combinations particularly
pointed out in the appended claims. It is to be understood that
both the foregoing general description and the following detailed
description are exemplary and explanatory only and are not
restrictive of the invention, as claimed.
BRIEF DESCRIPTION OF THE FIGURES
[0021] The accompanying figures, which are incorporated in and
constitute a part of this specification, illustrate several
embodiments and together with the description serve to explain the
principles of the invention.
[0022] FIG. 1 shows TEM (inset SEM) images of Au nanoparticles
synthesized under different conditions. AA increases from A to C;
and seed concentration increases from C to D. Scale bar=100 nm. See
also Table 1.
[0023] FIG. 2 shows TEM images showing cubic to rod-shaped gold
particles produced with low AA concentrations in the presence of a
small quantity of silver nitrate. CTAB is increased from
1.6.times.10.sup.-2 M (A), to 9.5.times.10.sup.-2 M (B,C,D).
Au.sup.3+ decreases from (B) to (C), whereas seed concentration
increases from C to D. Scale bar=100 nm. See also Table 1.
[0024] FIG. 3 shows TEM images of branched Au nanoparticles,
varying in the dimension and number of branches, prepared under
various combinations of [seed]/[Au.sup.3+] ratio, and the
concentrations of CTAB and AA. Tetra-pods (A), star-shape (B),
larger tetra-pods (C), and multi-pods (D and E). See also Table
1.
[0025] FIG. 4 shows optical absorption spectra of the solutions
containing Au nanocrystals of various shapes. Solutions contain a:
multiple shapes; b & c: particles with hexagonal and cubic
profiles, respectively. Particle solutions correspond to the shapes
given in FIGS. 1A, 1B, and 1C, respectively. d: cubic particles
corresponding to that given in FIG. 2A. e: rectangular particles
corresponding to that given in FIG. 2C. f: tetrapods corresponding
to that given in FIG. 3A.
[0026] FIG. 5 shows UV-vis-NIR absorption spectra of (a) spheres
(--), aspect ratio 3.2 rods (-----), aspect ratio 4.4 rods (.....),
aspect ratio 16 rods (--), (b) dogbones (--), cubes (.....),
tetrapods (--), and blocks (-----).
[0027] FIG. 6 shows SEM and TEM (inset) images of (a) aspect ratio
16 rods, (b) aspect ratio 3.2 rods, (c) aspect ratio 4.4 rods, (d)
spheres, (e) tetrapods, (f) dogbones, (g) cubes, and (h) blocks
immobilized on 4-MBA SAMs.
[0028] FIG. 7 shows a scheme of the nanoparticle-SAM sandwich
geometry for SERS of 4-MBA.
[0029] FIG. 8 shows Raman spectra of (a) 0.01 M 4-MBA and (b) 4-MBA
SAM on gold, and SERS spectra of 4-MBA SAMs on gold with
immobilized (c) spheres, (d) aspect ratio 3.2 rods, (e) aspect
ratio 4.4 rods, (f) aspect ratio 16 rods, (g) cubes, (h) blocks,
(i) tetrapods, and (j) dogbones. Integration times are (a) 300 s,
(b) 480 s, (c) 300 s, (d) 120 s, (e) 120 s, (f) 30 s, (g) 30 s, (h)
30 s, (i) 30 s, and (j) 30 s.
DETAILED DESCRIPTION
[0030] The present invention may be understood more readily by
reference to the following detailed description of aspects of the
invention and the Examples included therein and to the Figures and
their previous and following description.
[0031] Before the present compounds, compositions, articles,
devices, and/or methods are disclosed and described, it is to be
understood that they are not limited to specific synthetic methods
unless otherwise specified, or to particular reagents unless
otherwise specified, as such may, of course, vary. It is also to be
understood that the terminology used herein is for the purpose of
describing particular embodiments only and is not intended to be
limiting.
A. Definitions
[0032] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
any methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, example methods and materials are now described.
[0033] All publications mentioned herein are incorporated herein by
reference to disclose and describe the methods and/or materials in
connection with which the publications are cited. The publications
discussed herein are provided solely for their disclosure prior to
the filing date of the present application. Nothing herein is to be
construed as an admission that the present invention is not
entitled to antedate such publication by virtue of prior invention.
Further, the dates of publication provided herein may be different
from the actual publication dates, which may need to be
independently confirmed.
[0034] As used in the specification and the appended claims, the
singular forms "a," "an" and "the" include plural referents unless
the context clearly dictates otherwise. Thus, for example,
reference to "a component," "a polymer," or "a particle" includes
mixtures of two or more such components, polymers, or particles,
and the like.
[0035] Ranges can be expressed herein as from "about" one
particular value, and/or to "about" another particular value. When
such a range is expressed, another embodiment includes from the one
particular value and/or to the other particular value. Similarly,
when values are expressed as approximations, by use of the
antecedent "about," it will be understood that the particular value
forms another embodiment. It will be further understood that the
endpoints of each of the ranges are significant both in relation to
the other endpoint, and independently of the other endpoint. It is
also understood that there are a number of values disclosed herein,
and that each value is also herein disclosed as "about" that
particular value in addition to the value itself. For example, if
the value "10" is disclosed, then "about 10" is also disclosed. It
is also understood that when a value is disclosed that "less than
or equal to" the value, "greater than or equal to the value" and
possible ranges between values are also disclosed, as appropriately
understood by the skilled artisan. For example, if the value "10"
is disclosed the "less than or equal to 10" as well as "greater
than or equal to 10" is also disclosed. It is also understood that
throughout the application, data is provided in a number of
different formats and that this data represents endpoints and
starting points, and ranges for any combination of the data points.
For example, if a particular data point "10" and a particular data
point 15 are disclosed, it is understood that greater than, greater
than or equal to, less than, less than or equal to, and equal to 10
and 15 are considered disclosed as well as between 10 and 15. It is
also understood that each unit between two particular units are
also disclosed. For example, if 10 and 15 are disclosed, then 11,
12, 13, and 14 are also disclosed.
[0036] A residue of a chemical species, as used in the
specification and concluding claims, refers to the moiety that is
the resulting product of the chemical species in a particular
reaction scheme or subsequent formulation or chemical product,
regardless of whether the moiety is actually obtained from the
chemical species. Thus, an ethylene glycol residue in a polyester
refers to one or more --OCH.sub.2CH.sub.2O-- units in the
polyester, regardless of whether ethylene glycol was used to
prepare the polyester. Similarly, a sebacic acid residue in a
polyester refers to one or more --CO(CH.sub.2).sub.8CO-- moieties
in the polyester, regardless of whether the residue is obtained by
reacting sebacic acid or an ester thereof to obtain the
polyester.
[0037] As used herein, the terms "optional" or "optionally" means
that the subsequently described event or circumstance may or may
not occur, and that the description includes instances where said
event or circumstance occurs and instances where it does not.
[0038] Disclosed are the components to be used to prepare the
compositions of the invention as well as the compositions
themselves to be used within the methods disclosed herein. These
and other materials are disclosed herein, and it is understood that
when combinations, subsets, interactions, groups, etc. of these
materials are disclosed that while specific reference of each
various individual and collective combinations and permutation of
these compounds may not be explicitly disclosed, each is
specifically contemplated and described herein. For example, if a
particular compound is disclosed and discussed and a number of
modifications that can be made to a number of molecules including
the compounds are discussed, specifically contemplated is each and
every combination and permutation of the compound and the
modifications that are possible unless specifically indicated to
the contrary. Thus, if a class of molecules A, B, and C are
disclosed as well as a class of molecules D, E, and F and an
example of a combination molecule, A-D is disclosed, then even if
each is not individually recited each is individually and
collectively contemplated meaning combinations, A-E, A-F, B-D, B-E,
B--F, C-D, C-E, and C--F are considered disclosed. Likewise, any
subset or combination of these is also disclosed. Thus, for
example, the sub-group of A-E, B--F, and C-E would be considered
disclosed. This concept applies to all aspects of this application
including, but not limited to, steps in methods of making and using
the compositions of the invention. Thus, if there are a variety of
additional steps that can be performed it is understood that each
of these additional steps can be performed with any specific
embodiment or combination of embodiments of the methods of the
invention.
[0039] It is understood that the compositions disclosed herein have
certain functions. Disclosed herein are certain structural
requirements for performing the disclosed functions, and it is
understood that there are a variety of structures that can perform
the same function that are related to the disclosed structures, and
that these structures will typically achieve the same result.
B. Shaped Nanoparticles
[0040] Various aspect ratio Ag and Au nanorods, Ag nanowires, and
cubic Cu.sub.2O particles in aqueous solution have been produced.
Jana, N. R.; Gearheart, L.; Murphy, C. J., J. Phys. Chem. B 2001,
105, 4065-4067. Jana, N. R.; Gearheart, L.; Murphy, C., J. Adv.
Mater. 2001, 13, 1389-1393. Jana, N. R.; Gearheart, L.; Murphy, C.,
J. Chem. Commun. 2001, 617-618. Gao, J.; Bender, C. M.; Murphy, C.
J. Langmuir 2003, 19, 9065-9070. Gou, L.; Murphy, C. J. Nano Lett.
2003, 3, 231-234. Based upon electron diffraction analysis and
high-resolution transmission electron microscopy studies, the
mechanism for the evolution of cylindrical rod shapes in aqueous
solution by the seeded growth method was investigated. Johnson, C.
J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater.
Chem. 2002, 12, 1765-1770. Alivisatos et al., Peng et al., and
Cheon et al. utilized high-temperature solution methods to obtain a
score of interesting shapes for semiconductor systems. Peng, X.;
Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.;
Alivisatos, A. P. Nature 2000, 404, 59-61. Manna, L.; Scher, E. C.;
Alivisatos, A. P., J. Am. Chem. Soc. 2000, 122, 12700-12706. Manna,
L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P.
Nat. Mater. 2003, 2, 382-385. Peng, Z. A.; Peng, X., J. Am. Chem.
Soc. 2002, 124, 3343-3353. Peng, X. Adv. Mater. 2003, 15, 459-463.
Lee, S.-M.; Jun, Y.-W.; Cho, S.-N.; Cheon, J. J. Am. Chem. Soc.
2002, 124, 11244-11245. Au particles with hexagonal (icosahedral)
and pentagonal (decahedral) profiles have been synthesized by vapor
deposition methods. Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.;
Gardea-Torresdey, J. J Vac. Sci. Technol. B 2001, 19, 1091-1103.
Yang, C. Y.; Heinemann, K.; Yacaman, M. J.; Poppa, H. Thin Solid
Films 1979, 58, 163-168. Renou, A.; Gillet, M. Surf Sci. 1981, 106,
27-34. Recently, Chen et al. and Hao et al. reported the synthesis
of a mixture of branched gold Au nanocrystals by using two
different colloid chemical synthetic protocols. Chen, S.; Wang, Z.
L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc.
2003, 125, 16186-16187. Hao, E.; Bailey, R. C.; Schatz, G. C.;
Hupp, J. T.; Li, S, Nano Lett. 2004, 4, 327-330.
[0041] 1. Preparation Methods
[0042] In one aspect, the methods of the invention involve the
preparation of Au seed particles and the subsequent addition of an
appropriate quantity of the Au seed solution to the aqueous growth
solutions containing desired quantities of cetyltrimethylammonium
bromide (CTAB), HAuCl.sub.4, ascorbic acid (AA), and, optionally, a
small quantity of AgNO.sub.3.
[0043] While not wishing to be bound by theory, it is believed that
the morphology and dimension of the Au nanoparticles depend on the
concentrations of the seed particles and CTAB, in addition to the
reactants (Au.sup.3+ and AA). All of the above factors have been
found to be interdependent, thus giving rise to interesting
combinations for various shapes (Table 1). For example, at
1.6.times.10.sup.-2 M CTAB and 2.0.times.10.sup.-4 M Au.sup.3+
ions, nanorods, and other particles with triangular and square
outlines are formed, for an AA concentration 1.6 times the
Au.sup.3+ ion concentration (FIG. 1A). On increasing the AA
concentration, rod length and yield decrease, and particles with
hexagonal shapes appear (FIG. 1B). Upon a further increase in AA,
cube-shaped particles are formed (FIG. 1C). Simultaneous adjustment
of all four reactant concentrations can produce monodisperse Au
nanoparticles with hexagonal and cubic profiles in very high yield
(.about.90%) at room temperature in aqueous solution. FIG. 1D shows
that smaller particles with triangular outlines are the major
product instead of cubic ones, when the seed concentration is
raised, keeping other parameters the same as for the cubic
shapes.
TABLE-US-00001 TABLE 1 [CTAB]/M [Au].sub.seed/M [Au.sub.3+]/M
[AA]/M Shape/Profile Dimension.sub..sctn. % Yield 1.6 .times.
10.sub.-2 1.25 .times. 10.sub.-8 2.0 .times. 10.sub.-4 6.0 .times.
10.sub.-3 Cube 66 nm ~85 1.6 .times. 10.sub.-2 1.25 .times.
10.sub.-8 2.0 .times. 10.sub.-4 3.0 .times. 10.sub.-3 Hexagon 70 nm
~80 1.6 .times. 10.sub.-2 1.25 .times. 10.sub.-7 2.0 .times.
10.sub.-4 6.0 .times. 10.sub.-3 Triangle 35 nm ~80 1.6 .times.
10.sub.-2 1.25 .times. 10.sub.-8 4.0 .times. 10.sub.-4 6.4 .times.
10.sub.-4 Cube.sub.a 90 nm ~70 9.5 .times. 10.sub.-2 1.25 .times.
10.sub.-7 4.0 .times. 10.sub.-4 6.0 .times. 10.sub.-3
Tetrapod.sub.a 30 nm ~70 1.6 .times. 10.sub.-2 1.25 .times.
10.sub.-8 4.0 .times. 10.sub.-4 1.2 .times. 10.sub.-2 Star 66 nm
~50 5.0 .times. 10.sub.-2 6.25 .times. 10.sub.-7b 5.0 .times.
10.sub.-4 3.0 .times. 10.sub.-3 Tetrapod 293 nm ~75 9.5 .times.
10.sub.-2 2.5 .times. 10.sub.-7 4.0 .times. 10.sub.-4 6.4 .times.
10.sub.-4 Branched.sub.a 174 nm ~95 .sub..sctn.For triangular
profile and cubes, this corresponds to edge lengths; for hexagonal
profile, this corresponds to the distance between opposite sides;
and for tetrapods and branched particles, this corresponds to
center-to-tip distances. For cubes, triangles and hexagons, the
dimensions are averaged over ~120 particles and are reproducible to
within 5% of the given value; for the other shapes, the dimensions
are averaged over ~120 particles and are within ~10% of the given
value. .sub.a6.0 .times. 10-5 M AgNO3 was used. .sub.bSeed(5) was
used here, otherwise results are reported for seed(1).
[0044] 2. Mechanism and Theory
[0045] While not wishing to be bound by theory, it is believed that
the formation of various shapes is likely the outcome of the
interplay between the faceting tendency of the stabilizing agent
and the growth kinetics (rate of supply of Au.sup.0 to the
crystallographic planes). Ahmadi, T. S.; Wang, Z. L.; Green, T. C.;
Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924-1925.
Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.; Gardea-Torresdey, J.
J. Vac. Sci. Technol. B 2001, 19, 1091-1103. Yang, C. Y.;
Heinemann, K.; Yacaman, M. J.; Poppa, H. Thin Solid Films 1979, 58,
163-168. Renou, A.; Gillet, M. Surf Sci. 1981, 106, 27-34.
Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J.
Phys. Chem. B 1998, 102, 3316-3320. For example, both fcc
cubooctahedral and icosahedral particles may show hexagonal
profiles under TEM. While not wishing to be bound by theory, it is
believed that, as in the case of Pt.sup.0-polymer systems reported
by El-Sayed et al., the shape of the fine Au seeds produced in the
presence of CTAB is faceted with the most stable {111} faces
solvent-accessible. Petroski, J. M.; Wang, Z. L.; Green, T. C.;
El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316-3320. CTAB
molecules appear to bind more strongly to the {100} than the {111}
faces. Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.;
Mann, S. J. Mater. Chem. 2002, 12, 1765-1770. Thus, lower CTAB and
higher AA concentration conditions favor the faster formation and
deposition of Au.sup.0onto the {111} faces, leading to their
disappearance and the formation of {100} faces, thereby producing
cubic shapes. Under similar (or slightly higher) CTAB
concentrations and slightly lower AA concentration conditions,
truncated octahedra with both {100} and {111} faces can be
produced. The formation of truncated fcc shapes has also been
previously observed in the presence of passivating agents for gold
nanocrystals. Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.;
Gardea-Torresdey, J. J. Vac. Sci. Technol. B 2001, 19, 1091-1103. A
good yield of cube-shaped particles can be obtained even at low
[AA] conditions, such as for a [AA]=1.6.times.[Au.sup.3+], if a
small quantity of AgNO.sub.3 is added to the system (FIG. 2A).
These particles appear to have rough surfaces. The edge length of
these particles is a function of both [Au.sup.3+] and [seed].
However, noncubic shapes form especially upon decreasing or
increasing the concentrations of Au.sup.3+ ions. If CTAB
concentration is increased from 1.6.times.10.sup.-2 to
9.5.times.10.sup.-2 M, a very high yield (.about.97%) of gold
particles with rectangular outline to cylindrical rod-shapes are
formed, depending on the concentration ratio of seed particles to
Au.sup.3+ ions (FIGS. 2B, C and D). Preliminary high-resolution TEM
data show that the rectangular blocks are single-crystalline in
structure. In electroless metal plating, reduced Ag.sup.+ ions act
as sacrificial seeds for the reduction of Au.sup.3+ ions to form Au
tubes/rods. Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67,
1920-1928. While not wishing to be bound by theory, it is believed
that this mechanism is not operative in this invention, since
substantially no particle formation is detected in the absence of
seeds in the present experimental time scale. El-Sayed et al. have
proposed that silver ions could assist in the template elongation
by pairing with Br.sup.- ions of CTAB. Nikoobakht, B.; El-Sayed, M.
A. Chem. Mater. 2003, 15, 1957-1965.
[0046] A lowering of the ratio of concentrations of seed to
Au.sup.3+ ions along with an increase in the concentration of AA
can result in the formation of branched Au particles, depending on
the concentrations of CTAB and silver nitrate (FIG. 3). However, in
one aspect, silver nitrate is not essential for the branching. The
yield of the branched particles produced is as high as .about.70%.
The four arms in larger tetrapods are clearly out of plane. The
release of stress/strain effects in the growth of Au nanoparticles
has been previously observed to give rise to anomalous shapes.
Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.; Gardea-Torresdey, J.
J. Vac. Sci. Technol. B 2001, 19, 1091-1103. Yang, C. Y.;
Heinemann, K.; Yacaman, M. J.; Poppa, H. Thin Solid Films 1979, 58,
163-168. Renou, A.; Gillet, M. Surf Sci. 1981, 106, 27-34. However,
in the case of semiconductor systems, the formation of branched
structures typically requires first a relatively high supply of
monomer growth units to the seed and then the evolution of branches
of various kinds is determined by a balancing act between the
concentration buildup and the competition of ligand/stabilizing
molecules for the particle surface. Peng, X.; Manna, L.; Yang, W.
D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P.
Nature 2000, 404, 59-61. Manna, L.; Scher, E. C.; Alivisatos, A. P.
J. Am. Chem. Soc. 2000, 122, 12700-12706. Peng, X. Adv. Mater.
2003, 15, 459-463. Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001,
123, 1389-1395. Very recently, Chen et al. also proposed that
forced reduction of gold ions by ascorbic acid through the addition
of NaOH is key for branching of particles. Chen, S.; Wang, Z. L.;
Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003,
125, 16186-16187. Vis-NIR absorption spectra of the samples
containing Au nanocrystals of various shapes show clear differences
in optical absorption properties (FIG. 4).
[0047] The present invention, in one aspect, uses a simple
solution-based seed-mediated growth method where one can
controllably vary the morphology and dimension of the Au
nanocrystals by the manipulation of the synthetic parameters.
Moreover, these various shapes can be produced in aqueous solution
at room temperature and, in one aspect, by utilizing only one
surfactant, CTAB.
[0048] In one aspect, the invention relates to a method for
preparing a cetyltrimethylammonium bromide-capped gold nanoparticle
comprising the steps of providing a seed solution comprising a gold
nanoparticle; providing an aqueous growth solution comprising
cetyltrimethylammonium bromide, hydrogen tetrachloroaurate, and
ascorbic acid; and adding a quantity of the seed solution to the
aqueous growth solution, thereby producing a cetyltrimethylammonium
bromide-capped gold nanoparticle having a shape comprising a cube,
a block, a tetrapod, a sphere, a rod, a star, or a dogbone.
[0049] In one aspect, the nanoparticle has a shape comprising a
cube, a block, a tetrapod, a rod with an aspect ratio of at least
about 3.2, or a dogbone.
[0050] In one aspect, the providing a seed solution step comprises
the step of reducing hydrogen tetrachloroaurate with ascorbic acid
in the presence of cetyltrimethylammonium bromide, thereby
producing seeds of less than about 4 nm. In a further aspect, the
providing a seed solution step comprises the step of reducing
hydrogen tetrachloroaurate with sodium borohydride in the presence
of trisodium citrate, thereby producing seeds of from about 20 nm
to about 30 nm.
[0051] In one aspect, the aqueous growth mixture further comprises
silver nitrate.
[0052] 3. Cetyltrialkylammonium Halide-Capped Nanoparticles
[0053] In one aspect, the invention relates to a
cetyltrialkylammonium halide-capped nanoparticle. That is, the gold
nanoparticle has at least one cetyltrialkylammonium halide residue
associated with the surface of the nanoparticle. In a further
aspect, the nanoparticle is a cetyltrialkylammonium halide-capped
gold nanoparticle. In a further aspect, the invention relates to a
cetyltrimethylammonium bromide-capped gold nanoparticle. In one
aspect, the invention relates to a composition comprising
nanoparticulate gold and a cetyltrimethylammonium bromide residue,
wherein the composition has a shape comprising a cube, a block, a
tetrapod, a sphere, a rod, a star, or a dogbone. In a further
aspect, the shape comprises a cube, a block, a tetrapod, a rod with
an aspect ratio of at least about 3.2, or a dogbone.
C. Enhancing Raman Signals
[0054] 1. Methods
[0055] The methods and compositions of the invention can be used to
enhance Raman signals. In one aspect, the method comprises the
steps of providing a sample comprising a metal surface, an analyte
adhered to the surface, and a metallic nanoparticle coupled to the
surface, wherein the nanoparticle has a plasmon resonance band;
exposing the sample to incident energy of an excitation wavelength
that overlaps with the metallic nanoparticle plasmon resonance
band; and detecting the Raman signal of the analyte.
[0056] By "coupled," it is meant that the nanoparticle is in
relatively close proximity to the surface and the resulting
combination can operate to enhance the vibrational spectral
intensity of an analyte adhered to the surface. In one aspect, the
nanoparticle and the surface can form a "sandwich" with the
analyte, resulting in enhanced vibrational spectral intensity of
the analyte.
[0057] By "adhered," it is meant that the analyte is associated
with the surface. In one aspect, the analyte is chemically bonded
to the surface by, for example, at least one covalent bond, ionic
bond, coordination bond, or hydrogen bond. In a further aspect, the
analyte is attracted to the surface by, for example, hydrophobic
interactions or hydrophilic interactions. In a further aspect, the
analyte is reversibly associated with the surface.
[0058] In a further aspect, the method comprises the steps of
providing a sample comprising a metal surface, a functionalized
self-assembled monolayer adhered to the surface, wherein the
self-assembled monolayer comprises an analyte, and a
cetyltrimethylammonium bromide-capped metallic nanoparticle coupled
to the surface; exposing the sample to incident energy of an
excitation wavelength; and detecting the Raman signal of the
analyte. In one aspect, the nanoparticle has a plasmon resonance
band and the excitation wavelength overlaps with the metallic
nanoparticle plasmon resonance band. In a further aspect, the
surface comprises at least one of gold, silver, copper, or silicon
or a mixture or an alloy thereof. In a further aspect, the analyte
comprises a thiol moiety. In a further aspect, the analyte
comprises a carboxylic acid moiety. In a further aspect, the
nanoparticle comprises at least one of gold, silver, or copper or a
mixture or an alloy thereof.
[0059] Typically, the nanoparticle produced by the methods of the
invention has a shape. In one aspect, the shape comprises a cube, a
block, a tetrapod, a sphere, a rod, a star, or a dogbone. In a
further aspect, the shape comprises a cube and the plasmon
resonance band comprises a wavelength maximum of about 540 nm. In a
further aspect, the shape comprises a sphere and the plasmon
resonance band comprises a wavelength maximum of about 520 nm. In a
further aspect, the shape comprises a rod with an aspect ratio of
from about 3.2 to about 16 and the plasmon resonance band comprises
a longitudinal wavelength maximum of from about 685 nm to about
1200 nm and a transverse maximum of about 520 nm. In a further
aspect, the shape comprises a rod with an aspect ratio of greater
than about 16 and the plasmon resonance band comprises a wavelength
maximum of greater than about 1200 nm. In a further aspect, the
shape comprises a tetrapod or a dogbone and the plasmon resonance
band comprises a wavelength of about 633 nm.
[0060] In one aspect, the invention relates to a method for
enhancing a Raman signal comprising the steps of providing a sample
comprising a gold surface, a functionalized self-assembled
monolayer adhered to the surface, wherein the self-assembled
monolayer comprises an analyte, and a cetyltrimethylammonium
bromide-capped metallic nanoparticle coupled to the surface;
exposing the sample to incident energy of an excitation wavelength
that overlaps with the metallic nanoparticle plasmon resonance
band; and detecting the Raman signal of the analyte, wherein the
Raman signal has an enhancement factor of from about 10.sup.7 to
about 10.sup.9 relative to the analyte in solution.
[0061] a. Analyte
[0062] Those skilled in the art will recognize that there is a
great deal of latitude in the composition of an analyte that yields
a distinct Raman spectrum. For example, in some aspects, the
analyte is a molecule. In other aspects, the analyte is not a
molecule: it can be a positively or negatively charged ion (e.g.,
Na.sup.+ or CN). If the analyte is a molecule, it can be neutral,
positively charged, negatively charged, or amphoteric. The analyte
can be a solid, liquid or gas. Non-molecular species such as
metals, oxides, sulfides, etc. can serve as the Raman-active
species. For example, a film of SiO.sub.2 on Au exhibits a unique
and identifiable Raman spectrum. Any species or collection of
species that gives rise to a unique Raman spectrum, whether solid,
liquid, gas, or a combination thereof, can serve as the analyte.
Examples easily number in the many millions and include but are not
limited to Hg, dimethylformamide, HCl, H.sub.2O, CN.sup.-,
polypyrrole, hemoglobin, oligonucleotides, charcoal, carbon,
sulfur, rust, polyacrylamide, citric acid, and diamond. In the case
of diamond, the unique phonon mode of the particle can be used. For
hemoglobin, only the porphyrin prosthetic group exhibits
significant Raman activity; thus, complex substances can be used as
the analyte if only part of the molecular or atomic complexity is
present in the Raman spectrum.
[0063] The analyte can also be a polymer to which multiple
Raman-active moieties are attached. In one aspect, the polymer can
have different attached moieties yielding different Raman spectra.
The polymer backbone does not necessarily itself contribute to the
acquired Raman spectrum. In one aspect, the polymer can be a linear
chain containing amine or ammonium groups to which Raman-active
entities are attached. In a further aspect, the polymer can be a
dendrimer, a branched polymer with a tightly controlled tree-like
structure, with each branch terminating in a Raman-active species.
A suitable dendrimer structure can have four generations of
branches terminating in approximately 45 Raman-active entities.
[0064] Typically, the analyte can be any analyte known to those of
skill in the art for analysis by Raman spectroscopy. In one aspect,
the analyte comprises a thiol moiety. In a further aspect, the
analyte comprises a carboxylic acid moiety. In a yet further
aspect, the analyte comprises 4-mercaptobenzoic acid (4-MBA) or a
derivative or salt thereof. In a still further aspect, the analyte
comprises a portion of a functionalized self-assembled monolayer.
That is, in one aspect, the analyte can be used to prepare a
self-assembled monolayer on the surface.
[0065] b. Surface
[0066] Typically, the surface can be any surface known to those of
skill in the art for use in Raman spectroscopy. In a further
aspect, the surface comprises at least one of gold, silver, copper,
or silicon or a mixture or an alloy thereof. In a yet further
aspect, the surface comprises gold.
[0067] While, in one aspect, the analyte can comprise a portion of
a functionalized self-assembled monolayer, in a further aspect, the
surface can be functionalized with, for example, a self-assembled
monolayer. In such an aspect, the self-assembled monolayer can be
selected to have an affinity for an analyte, thereby providing an
alternate mechanism for adhering an analyte to the surface.
[0068] c. Nanoparticle
[0069] Typically, the nanoparticle can be any nanoparticle known to
those of skill in the art. In one aspect, the nanoparticles are the
cetyltrialkylammonium halide-capped nanoparticles. That is, the
nanoparticle has at least one cetyltrialkylammonium halide residue
associated with the surface of the nanoparticle. In a further
aspect, the nanoparticle comprises a cetyltrialkylammonium
bromide-capped metallic nanoparticle. In a further aspect, the
invention relates to a cetyltrimethylammonium bromide-capped gold
nanoparticle.
[0070] In one aspect, the nanoparticle comprises at least one of
gold, silver, or copper or a mixture or an alloy thereof. In a
further aspect, the nanoparticle comprises gold.
[0071] In one aspect, nanoparticle has a shape comprising a cube, a
block, a tetrapod, a sphere, a rod, a star, or a dogbone. In a
further aspect, the shape comprises a cube and the plasmon
resonance band comprises a wavelength maximum of about 540 nm. In a
further aspect, the shape comprises a sphere and the plasmon
resonance band comprises a wavelength maximum of about 520 nm. In a
further aspect, the shape comprises a rod with an aspect ratio of
from about 3.2 to about 16 and the plasmon resonance band comprises
a longitudinal wavelength maximum of from about 685 nm to about
1200 nm and a transverse wavelength maximum of about 520 nm. In a
further aspect, the shape comprises a rod with an aspect ratio of
greater than about 16 and the plasmon resonance band comprises a
wavelength maximum of greater than about 1200 nm. In a further
aspect, the shape comprising a tetrapod or a dogbone and the
plasmon resonance band comprises a wavelength of about 633 nm.
[0072] In certain aspects of the invention, the nanoparticles can
be random aggregates of nanoparticles (colloidal nanoparticles). In
other embodiments of the invention, nanoparticles can be
cross-linked to produce particular aggregates of nanoparticles,
such as dimers, trimers, tetramers or other aggregates. Certain
alternative aspects of the invention can use heterogeneous mixtures
of aggregates of different size, while other alternative aspects
can use homogenous populations of nanoparticle aggregates. In
certain aspects of the invention, aggregates containing a selected
number of nanoparticles (dimers, trimers, etc.) can be enriched or
purified by known techniques, such as ultracentrifugation in
sucrose gradient solutions.
[0073] d. Excitation Energy
[0074] Typically, any source of excitation energy known to those of
skill in the art can be used in connection with the invention.
Suitable excitation sources include a 514.5 nm line argon-ion laser
370 from SpectraPhysics, Model 166, and a 647.1 nm line of a
krypton-ion laser 370 (Innova 70, Coherent), a nitrogen laser 370
(Laser Science Inc.) at 337 nm and a helium-cadmium laser 370
(Liconox) at 325 nm (U.S. Pat. No. 6,174,677), a light emitting
diode, an Nd:YLF laser 370, and/or various ions lasers 370 and/or
dye lasers 370. The excitation beam 390 can be spectrally purified
with a bandpass filter (Corion) and can be focused on the Raman
active substrate 240, 340 using a 6.times. objective lens (Newport,
Model L6X). The objective lens can be used to both excite the
analytes and to collect the Raman signal, by using a holographic
beam splitter (Kaiser Optical Systems, Inc., Model HB 647-26N18) to
produce a right-angle geometry for the excitation beam 390 and the
emitted Raman signal. A holographic notch filter (Kaiser Optical
Systems, Inc.) can be used to reduce Rayleigh scattered radiation.
Alternative Raman detectors 380 include an ISA HR-320 spectrograph
equipped with a red-enhanced intensified charge-coupled device
(RE-ICCD) detection system (Princeton Instruments). Other types of
detectors 380 may be used, such as Fourier-transform spectrographs
(based on Michaelson interferometers), charged injection devices,
photodiode arrays, InGaAs detectors, electron-multiplied CCD,
intensified CCD and/or phototransistor arrays.
[0075] Typically, the excitation energy is selected so as to
overlap with a plasmon band of a nanoparticle of the invention. In
one aspect, the excitation energy is provided as light of an
excitation wavelength incident upon the surface or upon the
composition or upon the analyte. In a further aspect, the incident
energy is provided by a visible light wavelength laser. In a yet
further aspect, the incident energy is provided by a HeNe
laser.
[0076] In one aspect, the excitation comprises visible light. In a
further aspect, the excitation comprises ultraviolet light. In a
further aspect, the excitation comprises infrared light. In one
aspect, the excitation comprises light of a wavelength of from
about 400 nm to about 500 nm, from about 500 nm to about 600 nm,
from about 700 nm to about 800 nm, from about 800 nm to about 900
nm, from about 900 nm to about 1000 nm, from about 1000 nm to about
1100 nm, from about 1100 nm to about 1200 nm, or of greater than
about 1200 nm. In one aspect, the excitation wavelength comprises a
wavelength of about 633 nm.
[0077] e. Enhancement Factor
[0078] In one aspect, use of the methods and compositions of the
invention results in enhancement of a Raman signal. The amount of
enhancement can be referred to as an enhancement factor (EF). The
EF can be expressed relative to an analyte in solution or an
analyte on a surface. In one aspect, the Raman signal has an
enhancement factor of from about 10.sup.7 to about 10.sup.9
relative to the analyte in solution.
[0079] In a further aspect, the nanoparticle has a shape comprising
a cube, a block, a tetrapod, a rod with an aspect ratio of at least
about 3.2, or a dogbone and the Raman signal has an enhancement
factor of from about 10.sup.1 to about 10.sup.2 relative to the
analyte in a sample comprising a spherical nanoparticle.
[0080] Those skilled in the art of Raman spectroscopy are aware
that the general concept of inelastic light scattering has many
alternative manifestations that can be used for detection. The
basic "normal" Raman scattering experiment involves
detection/measurement of Stokes-shifted photons, i.e., those with a
lower energy than the incident photons. Anti-Stokes photons-those
with energies greater than the incident photons-are also generated
in a Raman experiment. While the intensity of anti-Stokes Raman
bands is typically low compared to the Stokes bands, they offer one
very significant advantage: the lack of interference from
fluorescence, which by definition occurs at lower energies than
excitation. In embodiments in which the overall SERS intensity is
sufficiently high, this may be an attractive method for
detection.
[0081] For molecules whose absorption spectrum overlaps with the
laser excitation wavelength, Raman experiments can be said to be in
resonance; both the theory and practice of resonance Raman are well
understood. SERS experiments carried out under these circumstances
can also be referred to as SERRS (surface enhanced resonance Raman
scattering). SERRS spectra are typically more intense than normal
Raman spectra, and may provide an additional benefit. Organic
molecules that possess high extinctions in the visible region of
the spectrum also exhibit relatively complex molecular structures,
and as such might not be optimal choices for the intermediate
layer. On the other hand, coordination complexes can have
reasonably high absorptivity and still possess simple structures.
For example, simple homoleptic complexes of Cu(I) and Cu(II) are
often intensely colored (e.g., [Cu(NH.sub.3).sub.4].sup.+).
[0082] In addition to SERS and SERRS, there are a variety of other
detection mechanisms contemplated by the instant invention,
including but not limited to surface enhanced infrared absorption
spectroscopy (SEIRA), surface enhanced hyperRaman spectroscopy
(SEHRS), and its resonant analog, SEHRRS. In SEHRS and SEHRRS, two
photons of frequency A generate a scattering event at a frequency
of 2 A. The primary benefit of this method is the total lack of
interference by fluorescence or any other background process: one
can excite a particle with 800 nm light and observe photons
Raman-shifted from 400 nm. In general, for a given analyte with N
atoms, there are either 3N-5 or 3N-6 unique vibrations; all of
these vibrations can be found in either the Raman, hyperRaman, or
infrared spectrum. Indeed, in some aspects, identification can rest
on a combination of optical interrogation methods, including
methods that rely on inelastic scattering of photons (e.g., SERS,
SERRS, SEHRS, and SEHRRS, in both Stokes and anti-Stokes modes),
methods that rely on elastic scattering of photons (e.g., Raleigh
scattering and hyperRaleigh scattering for particles with
dimensions at least 1/10th of the excitation wavelength), and
methods that rely on adsorption, e.g., SEIRA.
[0083] 2. Compositions
[0084] Typically, various compositions of the invention can be used
in connection with the methods of the invention. In one aspect, the
invention relates to a composition comprising a metal surface, a
functionalized self-assembled monolayer adhered to the surface,
wherein the self-assembled monolayer comprises an analyte, and a
cetyltrialkylammonium halide-capped metallic nanoparticle coupled
to the surface. In one aspect, the surface comprises at least one
of gold, silver, copper, or silicon or a mixture or an alloy
thereof. In a further aspect, the nanoparticle comprises a
cetyltrialkylammonium bromide-capped metallic nanoparticle. In a
further aspect, the nanoparticle comprises a cetyltrimethylammonium
bromide-capped metallic nanoparticle.
D. Experimental
[0085] The following examples are put forth so as to provide those
of ordinary skill in the art with a complete disclosure and
description of how the compounds, compositions, articles, devices
and/or methods claimed herein are made and evaluated, and are
intended to be purely exemplary of the invention and are not
intended to limit the scope of what the inventors regard as their
invention. Efforts have been made to ensure accuracy with respect
to numbers (e.g., amounts, temperature, etc.), but some errors and
deviations should be accounted for. Unless indicated otherwise,
parts are parts by weight, temperature is in .degree. C. or is at
ambient temperature, and pressure is at or near atmospheric.
[0086] 1. Preparation of Shaped Nanoparticles
[0087] a. General Procedures
[0088] In a typical seed synthesis, a 10 mL solution of Au seeds is
prepared by the reduction of HAuCl.sub.4.3H2O (2.5.times.10-4 M) by
ice-cold NaBH.sub.4 (6.0.times.10-4 M) in the presence of
cetyltrimethylammonium bromide (CTAB, 7.5.times.10-2 M). The
NaBH.sub.4 solution is added at a time to the solution containing
CTAB and HAuCl.sub.4 and the reaction mixture is then shaken (or
magnetically stirred) for two minutes allowing the escape of the
gas formed during the reaction. These seeds are likely four nm or
smaller in diameter and are designated as seed(1).
[0089] Au seeds are produced at two other CTAB concentration
conditions also, namely 9.5.times.10.sup.-2M and
5.0.times.10.sup.-2 M. One can produce various shapes using these
two seeds as well. However, the amounts of seed particles and AA
required for a given Au.sup.3+ ion and CTAB concentration are
different for each of them.
[0090] The same is true for a seed sample produced from
1.0.times.10.sup.4 M HAuCl.sub.4 solution. Relatively large seeds
(20-30 nm in diameter) were also prepared by the reduction of the
same quantity of HAuCl.sub.4 (2.5.times.10.sup.-4M) by ascorbic
acid (4.0.times.10.sup.-4 M) in the presence of tri-sodium citrate
(2.5.times.10.sup.-4 M) as stabilizer and designated these seeds as
seed(5). Cylindrical rod-shaped and armed Au particles were
produced in high yield by using these larger seed particles. All
the seeds' are used between 2 and 24 hours after their
preparation.
[0091] In a typical growth reaction, 0.20 mL HAuCl.sub.4 solution
is added to 4.75 mL CTAB solution (0.10 M) followed by the addition
of 0.03 mL AgNO.sub.3 (0.01 M), 0.032 mL L-ascorbic acid (0.10 M),
and 0.01 mL Au seed solutions. The solution is gently mixed by
inversion of the test tube after the addition of every
component.
[0092] b. TEM Studies
[0093] TEM images are obtained by Hitachi H-8000 or JEOL
JEM-100CXII electron microscope. Typically 1.5 mL of the solution
is centrifuged for 10 min at a speed of 10000 rpm to precipitate
the solid. The supernatant is discarded. Then, the solid residue is
redispersed in 1.5 mL DI water and centrifuged again. Finally the
solid residue is redispersed in a suitable volume of DI water
depending on the quantity of the residue. 7 .mu.L of this solution
is dropcast on a TEM grid and allowed to dry in open
atmosphere.
[0094] c. Role of Silver Nitrate
[0095] Rods of cylindrical shape and penta-twinned structure and
higher length along with a substantial number of spherical
particles are formed when silver nitrate is not used in the system.
Preliminary studies show that single crystalline rods are formed in
the presence of silver nitrate. In the presence of silver nitrate,
transverse growth typically occurs more and the seeds hardly form
spherical particles. EDAX studies show that 3 to 7 wt % of silver
is associated with the particles. It can be present as alloyed
Ag.sup.0 in the particle or as adsorbed Ag.sup.+ on the particle
surface.
[0096] 2. Surface Enhanced Raman Spectroscopy
A. Nanoshape Synthesis
[0097] Gold nanoparticles including cubes (edge length=61.+-.3 nm),
blocks (aspect ratio 2.4.+-.0.4, length=81.+-.9 nm, width=34.+-.3
nm), tetrapods (center width=81.+-.18 nm, edge length=107.+-.18
nm), spheres (diameter=29.+-.6 nm), and rods, aspect ratio
3.2.+-.0.6 (length=55.+-.7 nm, width=17.+-.3 nm), 4.4.+-.0.9
(length=62.+-.6 nm, width=14.+-.3 nm), and 16.0.+-.5.3
(length=372.+-.119 nm, width=23.+-.4 nm), are prepared using the
above procedure. "Dogbone"-shaped gold nanoparticles (center
width=21.+-.2 nm, end width=30.+-.4 nm, length=68.+-.11 nm) are
prepared by adding 10 mL of as-prepared gold nanorods (aspect ratio
4.4) to a growth solution containing 8.5 mL of 0.1 M CTAB, 0.5 mL
of 0.01 M HAuCl.sub.4, and 1.0 mL of 0.1 M ascorbic acid. The
unstirred solution changes color from tan to dark blue in .about.2
minutes and is stored at room temperature for >2 hours prior to
use.
B. Nanoparticle Immobilization of Self-Assembled Monolayers
[0098] Gold substrates are prepared by sputtering 10 nm of
chromium, followed by 100 nm of gold, on piranha-cleaned glass
microscope slides. 4-MBA self-assembled monolayers (SAMs) are
formed by immersing the gold-coated glass slides, cut to 1
cm.sup.2, into a 1 mM ethanolic solution of 4-MBA for 24 hrs,
rinsed thoroughly with ethanol, and dried with nitrogen.
CTAB-protected nanoparticles are immobilized onto 4-MBA SAMs using
a similar protocol described previously for the immobilization of
gold nanorods on 16-mercaptohexadecanoic acid (16-MHA) SAMs. See
Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20,
7117-7122. Briefly, 10 mL of as-prepared nanoparticles are
centrifuged at various speeds depending on nanoparticle size
(7,000-14,000 RPM) and the pellet is redispersed in 1 mL of
deionized water to remove excess surfactant in the supernatant.
After two centrifugation steps, 1 mL of nanoparticle solution is
diluted to 3 mL with deionized water. The 4-MBA SAMs on gold-coated
glass substrates are immersed in the resulting aqueous nanoparticle
solutions for 3 hours. Substrates are rinsed with deionized water
and dried with nitrogen. Under these conditions, the 4-MBA SAM is
deprotonated, allowing for electrostatic binding of the cationic
CTAB-capped gold nanocrystals.
C. Instrumentation
[0099] Surface enhanced Raman spectra are collected using a
Detection Limit Solution 633 Raman system using a 633 nm helium
neon laser with 25 mW laser power at the sample. Integration times
are given in the figure captions for each spectrum. Spectra are
corrected for background using GRAMS 32 software (Galactic).
Absorption spectra are acquired using a CARY 500 Scan UV-vis-NIR
spectrometer. Scanning electron micrographs are acquired using an
FEI Quanta 200 environmental scanning electron microscope. Various
magnifications are used and are provided in the figure captions.
Raman spectra and SEM images are acquired on the same sample in
approximately the same region in order to minimize possible effects
from sample heterogeneity. Transmission electron microscopy is
performed on either a Hitachi H-8000 or a JEOL 100CXII
instrument.
D. Results
[0100] Nanoparticle plasmon resonance and immobilization. FIG. 5
shows absorption spectra of gold nanoparticles of various shapes,
including rods, cubes, dogbones, tetrapods, and blocks. Cubes and
spheres have only one plasmon band at .about.540 and .about.520 nm,
respectively. Rod-shaped nanoparticles have plasmon bands
corresponding to both transverse and longitudinal absorption, where
the wavelength of longitudinal absorption increases with aspect
ratio, from 685 nm for aspect ratio 3.2 rods to >1200 nm for
aspect ratio 16 rods. Dogbones, tetrapods, and blocks have multiple
plasmon bands from 525 to 775 nm corresponding to the variable
dimensions along multiple axes of these particles.
[0101] Aspect ratio 3.2 nanorods, tetrapods, and dogbones have
significant plasmon absorption overlap with the HeNe laser
excitation source used in our SERS experiments (632.8 nm).
Resonance between the incident radiation and the electronic
absorption maxima should contribute to greater SERS enhancement for
these nanoparticles than those without appreciable absorption at
632.8 nm. Other chemical effects of these nanoparticles can
contribute to large SERS enhancements, including the surface free
energy of the nanocrystals and radius of curvature of the
nanoparticle features (i.e., lightning rod effect).
[0102] The utility of both SEM and AEM has been demonstrated in
imaging aspect ratio 18 nanorods immobilized on 16-MHA monolayers.
Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20,
7117-7122. For nanoparticles <100 nm in size and in various of
shapes, AFM can be an ideal technique for imaging them on surfaces,
as its lateral resolution is superior to that of SEM. However,
imaging these surfaces in contact or in tapping mode AFM typically
perturbs the monolayer-nanoparticle architecture by moving
nanoparticles on the surface with the AFM tip. While not wishing to
be bound by theory, it is believed that this is likely a
consequence of the smaller particle sizes employed herein; there is
less surface area in contact with the SAM, leading to fewer
favorable electrostatic interactions between nanoparticles and the
underlying substrate. Therefore, scanning electron micrographs can
be acquired for immobilized nanoparticles in order to calculate
nanoparticle density.
[0103] FIG. 6 shows representative SEM images of nanorods (aspect
ratio 3.2, 4.4, and 16), blocks, tetrapods, dogbones, cubes and
spheres immobilized on 4-MBA SAMs. Since the size of nanoparticles
>100 nm, with the exception of aspect ratio 16 rods, and near
the practical resolution of the instrument (.about.10-20 nm), it
can be difficult to distinguish different shapes of nanoparticles.
Therefore, TEM images of these nanoparticles are provided as insets
in FIG. 6. In FIG. 6 a, aspect ratio 16 nanorods are uniformly
distributed on the surface and are generally isolated, with a
density of 17 rods/.mu.m.sup.2. This is a slightly higher density
of nanorods than reported previously for 16-hexadecanoic acid SAMs.
Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20,
7117-7122. The smaller nanoparticles are well dispersed on the
4-MBA surface and number densities are calculated to be .about.44
rods/.mu.m.sup.2 for aspect ratio 3.2 nanorods, .about.38
rods/m.sup.2 for aspect ratio 4.4 nanorods, .mu.9.5
blocks/.mu.m.sup.2, and 11 tetrapods/.mu.m.sup.2, 9
cubes/.mu.m.sup.2, .about.32 spheres/.mu.m.sup.2, and .about.24
dogbones/.mu.m.sup.2. The number density of nanoparticles is a
factor in determining not only surface enhancement factors (EF),
but also in comparing the SERS spectra of 4-MBA with different
immobilized shapes.
[0104] SERS of 4-MBA SAMs using different shaped gold
nanoparticles. A scheme of this sandwich geometry for acquiring
SERS spectra of SAMs is shown in FIG. 7. In this architecture, the
analyte molecules are SAM molecules, and are sandwiched between the
smooth gold substrate and the electrostatically-immobilized gold
nanocrystals. Raman spectra of 0.01 M 4-MBA alone, a 4-MBA SAM on a
gold substrate with no nanoparticles, and 4-MBA SAMs on gold in the
sandwich geometry with different immobilized nanocrystals are shown
in FIG. 8. The corresponding peak frequency assignments are
provided in Table 2. Raman spectra and SERS spectra of 4-MBA are
comparable to those reported previously for 4-MBA in aqueous
solution and adsorbed onto gold substrates. Michota, A.; Bukowska,
J. J. Raman Spectrosc. 2003, 34, 21-25. Park, H.; Lee, S. B.; Kim,
K.; Kim, M. S. J. Phys. Chem. 1990, 94, 7576-7580. Lin-Vien, J. G.;
Golthup, M. B.; Fateley, W. G. Grasselli, J. G. The Handbook of
Infrared and Raman Characteristic Frequencies of Organic Molecules;
Academic Press: New York, 1991. Characteristic vibrational modes
including .nu.(CC) ring-breathing modes (.about.1070 and 1575
cm.sup.-1) observed in the Raman spectra of aqueous 4-MBA and SERS
spectra of 4-MBA, while other less intense modes including .delta.
(CH) (1132 and 1173 cm.sup.-1) and .nu..sub.s(COO.sup.-) (1375
cm.sup.-1) are observed in SERS spectra with immobilized
nanoparticles, but are below the signal-to-noise for the Raman
spectra of 0.01 M aqueous 4-MBA.
TABLE-US-00002 TABLE 2 Peak Frequency (cm.sup.-1) 4-MBA SAM on gold
0.01M 4-MBA Assignment .sup.a, b 1070 1073 .nu.(CC).sub.ring 1132
.sup.c .delta.(CH) 1173 .sup.c .delta.(CH) 1357 .sup.c
.nu..sub.s(COO.sup.-) 1575 1575 .nu.(CC).sub.ring .sup.a
Assignments from references 42-44. .sup.b .delta. = bend or
deformation; .nu. = stretch; ring = ring breathing mode; a =
antisymmetric. .sup.c Not Observed
[0105] Raman spectra of aromatic thiol SAMs on smooth gold
substrates has been reported previously. Taylor, C. E.; Pemberton,
J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl. Spectrosc. 1999,
53, 1212-1221. However, the instrument used in these experiments
was limited to acquiring only single spectral integrations, opposed
to multiple accumulations to improve the signal-to-noise ratio. No
characteristic 4-MBA vibrational modes are observed in the spectra
of 4-MBA SAMs on gold substrates in the absence of immobilized
nanoparticles. These observations are in good agreement with those
obtained by Zheng et al. for SERS of 4-ATP SAMs on silver
substrates with adsorbed silver nanoparticles in the same sandwich
geometry. Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu, R
Langmuir 2003, 19, 632-636. In that work, characteristic
vibrational bands for 4-ATP SAMs on polished silver substrates were
readily observed after the immobilization of silver colloids, but
were not observed for the SAMs in the absence of the colloids.
Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu, R Langmuir 2003,
19, 632-636.
[0106] Surface enhancement factors (EF) are calculated for each of
the different nanoparticle shapes using the following
expression:
EF=[I.sub.SERs]/[I.sub.Raman].times.[M.sub.Bulk]/[M.sub.Ads] Eq.
1
where M.sub.Bulk is the number of molecules sampled in the bulk,
M.sub.Ads is the number of molecules adsorbed and sampled on the
SERS-active substrate, I.sub.SERs is the intensity of a vibrational
mode in the surface-enhanced spectrum, and I.sub.Raman is the
intensity of the same mode in the Raman spectrum. For all spectra,
the intensity of the .nu.(C--C) ring-breathing mode (.about.1070
cm.sup.-1) is used to calculate EF values. Ideally, the Raman
spectra of the 4-MBA SAM would be used to normalize SERS spectra in
determining EF values as described by Taylor et al. Taylor, C. E.;
Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl.
Spectrosc. 1999, 53, 1212-1221. However, as described above,
instrumental limitation prevent the acquisition of interpretable
Raman spectra of the 4-MBA SAM in the absence of immobilized
nanoparticles. Therefore, the spectrum of aqueous 0.01 M 4-MBA is
used to normalize the SERS data in the EF calculation. All spectra
are normalized for acquisition time. The number of molecules
sampled in the SERS experiments is determined by calculating the
total two-dimensional area or "SERS footprint" occupied by the
nanoparticles in the illuminated laser spot on the surface. This is
approximated by multiplying the number density of nanoparticles
(from the SEM images in FIG. 6), the illuminated spot size
(.about.0.2 mm dia. at the focal point), and the nanoparticle
footprint area (from the TEM images in FIG. 6) to give the total
SERS surface area sampled. This number is multiplied by the bonding
density of 4-MBA molecules in a SAM, .about.0.5 nmol/cm.sup.2 to
give the total number of molecules sampled in the SERS experiments.
Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H.
Appl. Spectrosc. 1999, 53, 1212-1221. It is noteworthy that there
are many more molecules in the SAM that are not sandwiched between
nanoparticles and the gold substrate. Taylor et al. determined the
enhancement factor of smooth, vapor deposited gold substrates to be
.about.2 at 514.5 nm and .about.64 at 720 nm. Since EF values
expected for these nanoparticle-SAM samples should be several
orders of magnitude greater than a factor of 64, SERS contributions
from SAM molecules not sandwiched between nanoparticles and the
gold substrate are assumed to be negligible.
[0107] Calculated EF values for 4-MBA SAMS with immobilized
nanocrystals are given in Table 3.
TABLE-US-00003 TABLE 3 Nanoparticle shape EF Spheres 1.62 .+-. 0.63
.times. 10.sup.7 Aspect ratio 3.2 rods 1.02 .+-. 0.40 .times.
10.sup.8 Aspect ratio 4.4 rods 1.04 .+-. 0.11 .times. 10.sup.8
Aspect ratio 16 rods 1.08 .+-. 0.08 .times. 10.sup.8 Tetrapods 7.16
.+-. 0.09 .times. 10.sup.8 Dogbones 1.61 .+-. 0.11 .times. 10.sup.9
Cubes 2.43 .+-. 0.21 .times. 10.sup.9 Blocks 2.65 .+-. 0.19 .times.
10.sup.9
[0108] All EF values are between 10.sup.7 and 10.sup.9, which is
significantly greater than those estimated for aromatic SAMs on
rough planar substrates (.about.10.sup.6). Taylor, C. E.;
Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl.
Spectrosc. 1999, 53, 1212-1221. VanDuyne, R. P.; Hulteen, J. C.;
Triechel, D. A. J. Chem. Phys. 1993, 99, 2101-2115. Likewise, these
EF values are also greater than those estimated for
2-aminothiophenol adsorbed to unaggregated gold nanorods
(.about.10.sup.5) and 2,4-dinitrotoluene adsorbed on silver
nanowires (.about.10.sup.5). Nikooballht, B. Wang, J. El-Sayed, M.
A. Chem. Phys. Lett. 2002, 366, 17-23. Tao, A.; Kim, F.; Hess, C.;
Goldberger, J.; He, R.; Sun, Y.; Xia, Y, Yang, P. Nano Lett. 2003,
3, 1229-1323. This indicates that plasmon coupling between the
nanocrystals (LSP) and the gold substrate surface (SPP) contributes
to significant localized field enhancement for 4-MBA molecules in
the gold nanoparticle-planar substrate sandwich, resulting in large
SERS intensities. LSP-SPP coupling is believed to contribute to
increase SERS intensities observed by Zhang et al. for 4-ATP SAMs
on silver with immobilized silver colloids. Lyon, L. A.; Pena, D.
J.; Natan, M. J. J. Phys. Chem. B 1999, 103, 5826-5831. However, no
EF values were estimated in that work for comparison. These SERS
results are comparable to those obtained by Natan and others for
SPR colloid-enhanced sandwich assays, where LSP-SPP interactions
contribute to SPR angle shift enhancement. Lyon, L. A.; Musick, M.
D.; Natan, M. J. Anal. Chem. 1998, 70, 5177-5183.
[0109] While not wishing to be bound by theory, it is believed
that, for these nanoparticles, enhancements are a combination of
plasmon absorption, or EM contributions, and chemical effects.
Cubes, blocks, and dogbones have the largest surface enhancement
factors, .about.10.sup.9, and spheres have the smallest
enhancement, 10.sup.7. However, cubes, blocks, and dogbones have
less absorption at 633 nm than tetrapods and aspect ratio 3.2
nanorods, but still give greater Raman enhancement by a factor of
10. This indicates that differences in enhancements observed for
each of these nanoparticle shapes is less dependent on resonance
with the incident radiation source (EM factors) than other chemical
effects.
[0110] One contributing chemical effect is the absorption strength
of 4-MBA of the gold nanoparticles. El-Sayed and coworkers
determined that greater Raman scattering enhancement is observed
for molecules on Au{110} than Au{111}, because the Au{110} has a
higher surface energy. Nikoobakht, B. Wang, J. El-Sayed, M. A.
Chem. Phys. Lett. 2002, 366, 17-23. However, in this case the SERS
analyte is immobilized in a self-assembled monolayer, and the gold
nanocrystals are capped with CTAB. Therefore, chemical enhancement
effects due to preferential binding of the SERS analyte to
different crystal faces of the gold colloidal particles is not
likely. While not wishing to be bound by theory, it is believed
that the differences in EF values between nanoparticle shapes is
due to the lightning rod effect. Schatz, G. C. Acc. Chem. Res.
1984, 17, 370-376. Gersten, J. I., J. Chem. Phys. 1980, 72,
5779-5780. In general, greater field enhancements are observed near
the sharpest surface features. Dogbones, tetrapods, cubes, and
blocks all have more well-defined edges, corners and have generally
sharper surface features than rods and spheres, shown in FIG. 6. As
observed here, the lightning rod effect results in greater
localized field enhancement for dogbones, cubes, tetrapods, and
blocks than rods, and rods having greater field enhancement than
spheres.
E. Conclusions
[0111] Gold nanoparticles of various shapes and sizes are
immobilized on 4-MBA SAMs on gold via electrostatic interactions,
as described previously. Gole, A.; Orendorff, C. J.; Murphy, C. J.
Langmuir 2004, 20, 7117-7122. These nanoparticle-planar substrate
sandwich structures are used as SERS substrates. No vibrational
bands of 4-MBA are observed in the Raman spectra of the 4-MBA SAM
on sputtered gold, but vibrational bands are readily observed when
gold nanoparticles of any shape are immobilized on the SAM. Results
suggest that SERS of the 4-MBA SAMs in this sandwich geometry
originates from plasmon coupling between localized surface plasmon
of the nanoparticles and surface plasmon of the gold substrate;
creating a large localized electromagnetic field enhancement, or
SERS "hot spot," for the 4-MBA molecules between the nanoparticles
and the planar substrate. Differences in surface enhancement are
also observed for nanoparticles of different shape. Results
indicate that resonance between the incident radiation and the LSP
of nanoparticles, EM enhancement, is not sufficient to adequately
describe differences between EF values of the various nanoparticle
shapes. Chemical contributions to SERS including the surface
structure and sharpness of structural features of the gold
nanocrystals contribute to enhanced EF values for different shaped
nanoparticles.
[0112] It will be apparent to those skilled in the art that various
modifications and variations can be made in the present invention
without departing from the scope or spirit of the invention. Other
embodiments of the invention will be apparent to those skilled in
the art from consideration of the specification and practice of the
invention disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with a true scope and
spirit of the invention being indicated by the following
claims.
* * * * *