Ultrabright supramolecular beacons based on the self-assembly of two-photon chromophores on metal nanoparticles

Article abstract of DOI:10.1021/ja0281277

Silver nanoparticles coated with a self-assembled layer of 2500 chromophoric alkylthiol ligands, that exhibit a huge per particle two-photon absorption cross section (2.7 × 10^-45 cm⁴ s photon^-1) and a high fluorescence qua

Full text of DOI:10.1021/ja0281277

Published on Web 12/13/2002
Ultrabright Supramolecular Beacons Based on the Self-Assembly of
Two-Photon Chromophores on Metal Nanoparticles
Francesco Stellacci,† Christina A. Bauer, Timo Meyer-Friedrichsen, Wim Wenseleers,
Seth R. Marder, and Joseph W. Perry*
Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721-0041
Received August 13, 2002 ; E-mail: jwperry@u.arizona.edu
There is currently great interest in nanoscience due to the many
potential applications in the fields of nanoelectronics1 and ultra-
sensitive biomolecular detection.2 Ligand-coated metal nanopar-
ticles are very promising building blocks for applications in
nanotechnology.3 They are readily synthesized,4 they show remark-
able stability,5 and they can be polyfunctionalized,6 allowing for
significant flexibility in the tailoring of their optical, electronic,
and chemical properties. Two-photon absorption is a nonlinear
optical property that is currently under extensive investigation for
applications such as biological imaging,7 3-D microfabrication,8 and
optical limiting.9 In biological imaging, there is a need for
fluorescent labels with a large two-photon cross-section (δ) and
high quantum efficiency (η) to enable detection at the single label
level. One approach to the development of such high sensitivity
labels is to concentrate a number of fluorophores into a supramo-
lecular assembly.10
Figure 1. Left: Structure of dye 1. Right: Schematic representation of
silver nanoparticles coated with densely packed chromophores.
to ∼2500). To impart a specific chemical functionality to the
particle, we performed a place exchange reaction of the pure 1-Np
with 11-mercaptoundecanoic acid16 and obtained fluorescent par-
ticles that were soluble in water/ethanol (95:5 vol/vol).
1-Np was found to be relatively insoluble in common organic
solvents and showed appreciable solubility (∼10-5 M) only in 1,2-
dichlorobenzene under reflux conditions (180 °C). In contrast,
mixed-ligand nanoparticles with a 1.53:1 ratio of octylthiol and
dodecylthiol to 1 (O-D/1-Np) were soluble in dichloromethane and
toluene (∼10-3 M). It is known that the ligands of alkylthiol-coated
nanoparticles can strongly interdigitate.17 We have found that the
solubility of ligand-coated nanoparticles can be correlated with the
free energy of interdigitation and that the mixed-ligand nanoparticles
are typically more soluble than their monoligand counterparts.18
An endothermic transition was observed by differential scanning
calorimetry for 1-Np at higher temperatures (200 °C) than that
which is typical for the de-interdigitation of alkylthiol-coated
nanoparticles (130 °C), but with a lower enthalpy change (2.2 kJ/
mol ligand vs ∼20 kJ/mol ligand for 1-octylthiol-coated particles).
1,2-Dichlorobenzene was chosen because its boiling point is near
the onset of the endothermic peak at 200 °C for 1-Np. Thus, if the
transition involves breakup of interparticle interactions (such as
π-π stacking of the chromophores), it would facilitate dissolution
of the particles at a temperature that is lower than the decomposition
temperature (240 °C). The particles did not precipitate even after
dilution with other solvents, such as dichloromethane and toluene.
The diameter distribution of the particles was obtained using
transmission electron microscopy (TEM). The molar ratio of silver
atoms to ligand molecules was determined by elemental analysis.
Using the mean diameter and the molar ratio, we estimated the
number of ligands per particle. For example, 1-Np was found to
have an average diameter of 6.5 ( 2 nm (1σ), see Figure 3. The
average ratio of silver atoms to ligands is 3.4; thus the average
particle has ∼2500 ligands on the surface. These data indicate a
coverage of ∼90%, showing a very high density of dye in the
outside shell of the particles.
Here we report on the preparation of the first supramolecular
assembly consisting completely of a self-assembled layer of
chromophores on a metal nanoparticle core, which allows for
packing of ∼2500 chromophores within a sphere of <10 nm
diameter. The resulting nanoparticles act as ultrabright two-photon
fluorescent nanobeacons, with an effective δ of ∼2500 times that
of the individual chromophores, a large η, and a decreased
sensitivity to photobleaching.
The chromophore-1-coated nanoparticles (1-Np) were synthe-
sized in 71% yield using a modification of the method of Kim et
al.,11 involving reduction of AgNO3 with NaBH4 in the presence
of chromophore-functionalized alkylthiol ligands (1, see Figure 1)
in ethanol at 0 °C. The acceptor-π-acceptor (A-π-A) design
of the chromophore was chosen to impart a high δ12 and a moderate
electron deficiency, such that it would neither reduce silver ions
nor react with sodium borohydride. Ligand 1 possesses these
properties and has a large ηδ (see Supporting Information for the
ligand synthesis).
Synthesis of the chromophore-coated silver nanoparticles using
this approach allows for a large chromophore density13 in the ligand
shell (Figure 1) as compared to that typically achieved by place
exchange14 or sequential reactions.15 For many purposes, nanopar-
ticles that are polyfunctionalized with ligands that provide different
functions, for example, specific binding or solubility properties,
would be very useful.6 To demonstrate the feasibility of the
polyfunctionalization for these particles, we synthesized mixed-
ligand-coated silver nanoparticles using 1, 1-dodecylthiol, and
1-octylthiol. Such nanoparticles were prepared both by reduction
of AgNO3 in the presence of a mixture of the ligands and by a
place exchange reaction starting with either octylthiol/dodecylthiol-
coated nanoparticles or 1-Np. We were able to prepare particles
with varying numbers of chromophores in their ligand shell (∼10
Infrared spectroscopy showed that the benzene ring stretching
modes at ∼1595 and 1520 cm-1, as well as a mode (nitro group
stretch) at 1345 cm-1, shifted to lower frequency (e.g., 1345 to
† Current address: Department of Materials Science and Engineering, MIT,
Cambridge, MA.
9
328
J. AM. CHEM. SOC. 2003, 125, 328-329
10.1021/ja0281277 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Two-photon fluorescence excitation spectra of 1 and 1-Np have
the same shape as shown in Figure 3, in accord with the one-photon
spectra. 1 showed a peak δ value of 165 GM (1 GM ) 10-50 cm4
s photon-1). The nanoparticles showed a huge per-particle δ value
of 2.7 × 105 GM and an ηδ of 0.9 × 105 GM. The value for the
1-Np is consistent with a linearly additive contribution from the
roughly 2500 chromophores and no significant enhancement
induced by the nanoparticle.21
The two-photon action cross section (ηδ) of the 1-coated
nanoparticle is one of the highest ever reported10b,12,22 and dem-
onstrates the utility of using the particles as a tool for concentrating
chromophores. The optical properties coupled with polyfunction-
alization to provide water solubility may allow for applications in
biological imaging. We believe that the use of nanoparticles as a
carrier for large numbers of chromophores is promising and may
have an impact on ultrasensitive detection.
Figure 2. Absorption (solid line) and fluorescence (dashed line) spectra
of 1 (red) and of 1-Np (black) in 1,2-dichlorobenzene.
Acknowledgment. Support of this work by NSF (BES-0086944
and CHE-0107105), AFOSR (F49620-02-1-0358), and NIH
(5R01RR14312-02) is gratefully acknowledged. We thank Stephanie
J. K. Pond for technical assistance. T.M.-F. thanks DFG for financial
support.
Supporting Information Available: Synthesis, infrared, and optical
spectra of 1 and of 1-Np (mixed- and mono-ligand) (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L.
Nature 1997, 389, 699-701.
Figure 3. Two-photon absorption spectra of 1 (red, abscissa on the left)
and of 1-Np (black, abscissa on the right) in 1,2-dichlorobenzene. Inset:
TEM image and electron diffraction pattern of 1-Np. Scale bar 66 nm.
(2) Taton, T. A.; Lu, G.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 5164-
5165.
(3) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18-52.
(4) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem.
Soc., Chem. Commun. 1994, 801-802.
1330 cm-1) on going from the free ligand 1 in chloroform solution
to crystallites of 1 in KBr pellets. The band frequencies for the
O-D/1-Np and the 1-Np (both nanoparticle samples in KBr pellets)
were intermediate to those for the ligand in the solution and solid
state, see Supporting Information. These results suggest that the
environment of the chromophores on the nanoparticles is intermedi-
ate to the solution and solid state of the unbound chromophore.
Electrochemical measurements indicated that there was no change
in reduction potential on going from the free dye to the dye bound
to the nanoparticle (both in solution), ruling out a significant
electronic coupling of the ligand molecular orbitals and the metal
electron levels.
(5) Terrill, R. H.; et al. J. Am. Chem. Soc. 1995, 117, 12537-12548.
(6) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000,
33, 27-36.
(7) Williams, R. M.; Zipfel, W. R.; Webb, W. W. Curr. Opin. Chem. Biol.
2001, 5, 603-608.
(8) (a) Cumpston, B. H.; et al. Nature 1999, 398, 51-54. (b) Stellacci, F.; et
al. AdV. Mater. 2002, 14, 194-198.
(9) Bhawalkar, J. D.; He, G. S.; Prasad, P. N. Rep. Prog. Phys. 1996, 59,
1041-1070.
(10) (a) Grayson, S. K.; Frechet, J. M. J. Chem. ReV. 2001, 101, 3819-3867.
(b) Drobizhev, M.; Karotki, A.; Rebane, A.; Spangler, C. W. Opt. Lett.
2001, 26, 1081-1083.
(11) Kang, S. Y.; Kim, K. Langmuir 1998, 14, 226-230.
(12) Albota, M.; et al. Science 1998, 281, 1653-1656.
(13) Imahori, H.; Fukuzumi, S. AdV. Mater. 2001, 13, 1197-1199.
(14) (a) Hu, J. J. Am. Chem. Soc. 2001, 123, 1464-1470. (b) Ingram, R. S.;
Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175-
9178.
The absorption and fluorescence spectra of the 1-Np in solution
are nearly identical to those of free 1 (Figure 2).19 Little difference
was observed in the fluorescence quantum yield of the free dye (η
) 0.47) and the dye-coated mono- or mixed-ligand nanoparticles
(η ) 0.33) in solution, consistent with the small spectral overlap
of the dye fluorescence and the nanoparticle absorption. The same
trend was observed for the fluorescence lifetime (2.13 ns vs 1.85
ns).20 The optical spectroscopy indicates that there is little interaction
between the bound chromophores for the nanoparticles in solution,
in contrast to the IR evidence for the nanoparticles dispersed in
the solid. These results suggest that a large degree of disorder in
the arrangement of the chromophores is induced by the high
temperature used to solvate the nanoparticles.
(15) Aguila, A.; Murray, R. W. Langmuir 2000, 16, 5949-5954.
(16) Gittins, D. I.; Caruso, F. ChemPhysChem 2002, 3, 110-113.
(17) (a) Badia, A. et al. Chem.-Eur. J. 1996, 2, 359-363. (b) Sandhyarani,
N.; et al. Phys. ReV. B 2000, 62, R739-R742.
(18) Bauer, C. A.; Stellacci, F.; Marder, S. R.; Perry, J. W., manuscript in
preparation.
(19) This is due to the large number of chromophores on the nanoparticle,
which dominate the absorption relative to the core. For O-D/1-Np, the
absorption of both the chromophore and the metal core was observed.
(20) Lakowicz, J. R. Anal. Biochem. 2001, 298, 1-24. These small changes
are associated with changes in both the radiative and the nonradiative
rates when the dye is bound to the nanoparticle [see also refs 14, 15a].
(21) Wenseleers, W.; et al. J. Phys. Chem. B 2002, 106, 6853-6863.
(22) (a) Najechalski, P.; Morel, Y.; Stephan, O.; Baldeck, P. Chem. Phys. Lett.
2001, 343, 44-48. (b) van Oijen, A. M.; et al. Appl. Phys. Lett. 2001, 79,
830-832.
JA0281277
9
J. AM. CHEM. SOC. VOL. 125, NO. 2, 2003 329