Functionalized gold nanoparticles as phosphorescent nanomaterials and sensors

Article abstract of DOI:10.1021/ja054347j

Ligand-capped gold nanoparticles were synthesized by capping monothiol derivatives of 2,2′-dipyridyl onto the surface of Au nanoparticles (Au-BT). The average size of the metal core is around 4 nm, with a shell of ～340 bipyridine ligands around the Au nanoparticle. The high local concentration of the chelating ligands (～5 M) around the Au nanoparticle makes these particles excellent ion sponges, and their complexation with Eu ^III/Tb^III ions yields phosphorescent nanomaterials. Absorption spectral studies confirm a 1:3 complexation between Eu ^III/Tb^III ions and bipyridines, functionalized on the surface of Au nanoparticles. The red-emitting Au-BT:Eu^III complex exhibits a long lifetime of 0.36 ms with six line-like emission peaks, whereas the green-emitting Au-BT:Tb^III complex exhibits a lifetime of 0.7 ms with four line-like emission peaks. These phosphorescent nanomaterials, designed by linking BT:Eu^III complexes to Au nanoparticles, were further utilized as sensors for metal cations. A dramatic decrease in the luminescence was observed upon addition of alkaline earth metal ions (Ca²⁺, Mg²⁺) and transition metal ions (Cu²⁺, Zn²⁺, Ni²⁺), resulting from an isomorphous substitution of Eu^III ions, whereas the luminescence intensity was not influenced by the addition of Na⁺ and K⁺ ions. Direct interaction of bipyridine-capped Au nanoparticles with Cu²⁺ ions brings the nanohybrid systems closer, leading to the formation of three-dimensional superstructures. Strong interparticle plasmon interactions were observed in these closely spaced Au nanoparticles.

Full text of DOI:10.1021/ja054347j

Published on Web 01/24/2006
Functionalized Gold Nanoparticles as Phosphorescent
Nanomaterials and Sensors
Binil Itty Ipe, Karuvath Yoosaf, and Kakkudiyil George Thomas*
Contribution from Photosciences and Photonics, Regional Research Laboratory (CSIR),
TriVandrum 695 019, India
Received June 30, 2005; E-mail: georgetk@md3.vsnl.net.in
Abstract: Ligand-capped gold nanoparticles were synthesized by capping monothiol derivatives of 2,2′-
dipyridyl onto the surface of Au nanoparticles (Au-BT). The average size of the metal core is around 4
nm, with a shell of ∼340 bipyridine ligands around the Au nanoparticle. The high local concentration of the
chelating ligands (∼5 M) around the Au nanoparticle makes these particles excellent ion sponges, and
their complexation with EuIII/TbIII ions yields phosphorescent nanomaterials. Absorption spectral studies
confirm a 1:3 complexation between EuIII/TbIII ions and bipyridines, functionalized on the surface of Au
nanoparticles. The red-emitting Au-BT:EuIII complex exhibits a long lifetime of 0.36 ms with six line-like
emission peaks, whereas the green-emitting Au-BT:TbIII complex exhibits a lifetime of 0.7 ms with four
line-like emission peaks. These phosphorescent nanomaterials, designed by linking BT:EuIII complexes to
Au nanoparticles, were further utilized as sensors for metal cations. A dramatic decrease in the luminescence
was observed upon addition of alkaline earth metal ions (Ca2+, Mg2+) and transition metal ions (Cu2+
,
Zn2+, Ni2+), resulting from an isomorphous substitution of EuIII ions, whereas the luminescence intensity
was not influenced by the addition of Na+ and K+ ions. Direct interaction of bipyridine-capped Au
nanoparticles with Cu2+ ions brings the nanohybrid systems closer, leading to the formation of three-
dimensional superstructures. Strong interparticle plasmon interactions were observed in these closely spaced
Au nanoparticles.
sensitizers for light-harvesting systems1h) has been demonstrated.
Significant progress has been made in recent years in the
Introduction
Rational design of functional nanomaterials is of current
interest because of a variety of potential applications ranging
from chemistry to biological sciences.1 Metal and semiconductor
nanoparticles exhibit interesting size- and shape-dependent
properties.1a,b Combining these properties with the intrinsic
functionalities of organic and biomolecular systems, by incor-
porating them on the surface of nanoparticles, will yield hybrid
materials with novel properties and functions.1c,d Using this
strategy, the role of gold nanoparticles in modulating the
properties of surface-bound fluorophores1c,2 and their possible
applications (for, e.g., electrochemical modulation of fluores-
cence,3 light-mediated binding and release of amino acids,4 and
development of functional nanomaterials by designing mono-
layer-protected metal clusters1k,5-7 and exploiting them as
building blocks for supramolecular structures and sensory
applications.1d,8
Another approach to the design of functional nanomaterials
is based on the assembly of biomolecules such as lipids, proteins,
and nucleic acids on nanoparticles.1e,9 Such biomolecular
conjugates of nanoparticles can perform specific biomolecular
functions and also can be used for the fast and sensitive detection
of various biomolecular analytes.8,10 In most such cases,
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7175.
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J. AM. CHEM. SOC. 2006, 128, 1907-1913
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A R T I C L E S
Ipe et al.
Scheme 1: Design Strategy for Luminescent Nanomaterials
detection is based on the changes in the plasmon absorption
band of the Au nanoparticles. Although fluorescence spectros-
copy is more versatile and sensitive, it is not often used as a
tool for following the properties of metal nanoparticles. This is
based on the general belief that the metal nanoparticles strongly
quench the singlet excited state of the chromophore when it is
attached to their surface. In contrast, recent studies by our
group1c,11a and others2a,11b have shown a dramatic suppression
of the quenching of fluorescence when these chromophores are
densely packed on the Au nanoparticle surface. This allows the
effective utilization of luminescent nanohybrid systems for
sensory applications.
Recent studies have shown that Au nanoparticles are rapidly
taken up into human cells and that these nanoparticles, with
suitable surface modifiers, are nontoxic to human cells.12a The
biocompatibility of Au nanoparticle conjugates thus allows their
potential use for site-specific cellular targeting and nuclear
delivery of diagnostic probes and therapeutics.12b-e Motivated
by these observations, we report here a novel strategy for the
design of phosphorescent nanomaterials (Scheme 1) through a
bottom-up approach, first by functionalizing a monolayer of 2,2′-
bipyridine units on Au nanoparticles (Au-BT) and then further
complexing them with lanthanide metal ions such as europium
and terbium. Lanthanide complexes with coordinating ligands
exhibit long-lived line-like emission13-17 and are widely used
for the detection of biologically important cations and anions,18-22
as well as organic molecules (for example, phosphoglycerate23).
More recently, luminescent rare-earth-doped lanthanum phos-
phate nanoparticles were biofunctionalized with a model protein,
namely, avidin, and demonstrated its ability to bind biotin.24
Use of the newly designed phosphorescent nanomaterial
(Au-BT:EuIII) as sensors for biologically important cations is
also demonstrated here.
Results and Discussion
Bipyridine-Capped Au Nanoparticles. Bipyridine-capped
Au nanoparticles (Au-BT) were synthesized through the
sequence of reactions shown in Scheme 2. 5,5′-Dimethyl-2,2′-
dipyridyl (1) was converted to its monobromo derivative (2)
using N-bromosuccinimide25 and subsequently to the thiol
derivative (BT) using a procedure reported by Fox and co-
workers2b that involves the reaction of 2 with a mixture of
tetrabutylammonium fluoride and hexamethyldisilathiane. The
thiol derivative, BT was functionalized onto gold nanoparticles
along with 1-dodecanethiol (Au-BT) by following two-phase
(water-toluene) reduction of HAuCl4.26 The presence of
alkanethiols was found to be essential for imparting stability to
the nanoparticles. The synthesis and characterization of BT and
Au-BT are described in the Experimental Section, and the
details of the intermediate compounds are available in the
Supporting Information.
Absorption spectra of BT and Au-BT in acetonitrile are
presented in Figure 1, and TEM images, obtained by drop
casting a dilute solution of Au-BT onto a Formvar-coated
copper grid, are included in the inset. A stock solution of
Au-BT was prepared in toluene from which microliter quanti-
ties were injected into acetonitrile (the toluene content in the
resultant solution is negligible, <3%). The absorption spectrum
of Au-BT (Figure 1B) consists of two bands: the additive
(11) (a) Ipe, B. I.; Thomas, K. G. J. Phys. Chem. B 2004, 108, 13265-13272.
(b) Imahori, H.; Arimura, M.; Hanada, T.; Nishimura, Y.; Yamazaki, I.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 335-336.
(12) (a) Connor, E. E.; Mwamuka, J.; Gole, A.; Murphy, C. J.; Wyatt, M. D.
Small 2005, 1, 325-327. (b) Tsai, C.-Y.; Shiau, A.-L.; Cheng, P.-C.; Sheih,
D.-B.; Chen, D.-H.; Chou, C.-H.; Yeh, C.-S.; Wu, C.-L. Nano Lett. 2004,
4, 1209-1212. (c) Thomas, M.; Klibanov, A. M. Proc. Natl. Acad. Sci.
U.S.A. 2003, 100, 9138-9143. (d) Tkachenko, A. G.; Xie, H.; Liu, Y.;
Coleman, D.; Ryan, J.; Glomm, W. R.; Shipton, M. K.; Franzen, S.;
Feldheim, D. L. Bioconjugate Chem. 2004, 15, 482-490. (e) Hirsch, L.
R.; Jackson, J. B.; Lee, A.; Halas, N. J.; West, J. L. Anal. Chem. 2003, 75,
2377-2381.
(13) Quici, S.; Cavazzini, M.; Marzanni, G.; Accorsi, G.; Armaroli, N.; Ventura,
B.; Barigelletti, F. Inorg. Chem. 2005, 44, 529-537.
(14) Quici, S.; Marzanni, G.; Cavazzini, M.; Anelli, P. L.; Botta, M.; Gianolio,
E.; Accorsi, G.; Armaroli, N.; Barigelletti, F. Inorg. Chem. 2002, 41, 2777-
2784.
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Boehme, C.; Wipff, G. J. Am. Chem. Soc. 2002, 124, 7779-7788.
(19) Heck, R.; Dumarcay, F.; Marsura, A. Chem.-Eur. J. 2002, 8, 2438-2445.
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ReV. 2000, 205, 41-57.
(21) Elbanowski, M.; Makowska, B. J. Photochem. Photobiol. A: Chem. 1996,
99, 85-92.
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2nd ed.: Ward, M. D., Ed.; Elsevier: Oxford, U.K., 2004; Vol. 9, p 913.
(23) Best, M. D.; Anslyn, E. V. Chem.-Eur. J. 2003, 9, 51-57.
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5957.
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Soc., Chem. Commun. 1994, 801-802.
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Au Nanoparticles as Phosphorescent Sensors
A R T I C L E S
Scheme 2. Reaction Scheme for the Preparation of BT and Au-BTa
a (a) NBS, CHCl3; (b) HMDST, TBAF; (c) 1-dodecanethiol, HAuCl4, TOAB, NaBH4.
absorption coefficient of ions resulting from the forbidden f-f
transition.16 It is well established that the phosphorescence of
lanthanide ions can be enhanced by means of an absorption-
energy transfer-emission (A-ET-E) process by encapsulating
them in a molecular cavity of coordinating ligands (for example,
bipyridine cryptates,16 phenanthroline derivatives,13,14 bipyri-
midine derivatives29). Several factors contribute to the efficiency
of luminescence from EuIII/TbIII-ion-based molecular systems:
(i) ligand absorption and its ability to shield the ions from their
surroundings, (ii) ligand-to-metal ion energy transfer, and (iii)
the decay rate constant of metal ion emitting states.16 The latter
aspect is mainly influenced by deactivation due to the pres-
ence of solvent molecules that coordinate to the metal ions.
EuIII/TbIII complexes containing coordinating solvent molecules
such as water exhibit low luminescence yields. This is due to
enhanced nonradiative deactivation resulting from the vibronic
Figure 1. Absorption spectra in acetonitrile of (A) BT and (B) Au-BT.
The inset shows the TEM image of Au-BT placed on a Formvar-coated
copper grid.
absorption spectrum of BT and gold nanoparticles. The band
at 295 nm corresponds to the characteristic π-π* transition of
bipyridine, which is observed as a shoulder,16 and the broad
band at around 520 nm corresponds to the plasmon absorption
of gold nanoparticles. For Au-BT, the short-wavelength band
of BT is merged with the absorption of gold nanoparticles.
The TEM image shown in the inset of Figure 1B indicates
an average particle size of 4 nm. The functionalization of BT
on nanoparticles is not quantitative, and hence, the concentration
of unreacted ligands was estimated from the absorbance of BT
in the filtrate. Assuming a uniform distribution of BT and
dodecanethiol on nanoparticles, the number of BT moieties per
nanoparticle is calculated as ∼340, judging from the amount
of BT reacted (absorption spectral studies) and the size of the
metal core (TEM studies). The above calculation is based on
the tight-packed spherical model suggested by Murray and co-
workers,27 according to which gold nanoparticles have a densely
packed core of density 58.01 atoms/nm3 covered with a skin of
hexagonally close-packed gold atoms. Thus, each hybrid nano-
particle system consists of a core of ∼820 Au atoms having
∼340 bipyridine ligands, from which the local concentration
of BT per nanoparticle is estimated as ∼5 M.
5
coupling of the D0/5D4 excited states with the high-frequency
OH oscillators. It is possible to overcome these factors by
shielding the lanthanide ions with coordinating ligands that
absorb in the UV-vis region and transfer their energy to the
lanthanide ions through an energy-transfer process (antenna
effect).13,14,16,29
One of the significant features of the present nanohybrid
system is its high absorption coefficient due to the presence of
∼340 molecules of coordinating bipyridine ligand per Au
nanoparticle. In an earlier attempt to increase the absorption
efficiency, calix[4]arane16 and cyclodextrins19 with varying
bipyridine units were synthesized, and it was found that the
molar extinction coefficient (ꢀ) increases with the number of
bipyridine units (the average ꢀ value per bipyridine unit for the
latter system is estimated as 12500 M-1cm-1).19 The complex-
ation ability of BT/Au-BT with EuIII and TbIII ions was further
investigated by following the changes in the absorption spectrum
(Figures 2 and 3). The thiol derivative of bipyridine (BT) in
acetonitrile exhibits two absorption maxima (Figure 1A): one
Design and Fabrication of Nanophosphors. Lanthanide ions
exhibit long-lived, line-like emission that originates from the
5
radiative transitions of the Di excited state (5D0 for the EuIII
(28) (a) Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W. Phys. Chem. Chem.
Phys. 2002, 4, 1542-1548. (b) Haas, Y.; Stein, G. J. Phys. Chem. 1971,
75, 3668-3677.
(29) Shavaleev, N. M.; Accorsi, G.; Virgili, D.; Bell, Z. R.; Lazarides, T.;
Calogero, G.; Armaroli, N.; Ward, M. D. Inorg. Chem. 2005, 44, 61-
72.
5
7
ion and D4 for the TbIII ion) to the various Fj states.21,28
However, they exhibit low emission yields because of the poor
(27) Hostetler, M. J., et al. Langmuir 1998, 14, 17-30.
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A R T I C L E S
Ipe et al.
Chart 1. Two Possible Modes of Complexation between EuIII/TbIII
Ions and Bipyridines Bound on Au Nanoparticles
of complexed and uncomplexed bipyridines on gold nano-
particles. The corresponding Job’s plot showed a maximum
value for the absorbance at 328 nm when the mole fraction of
EuIII/TbIII ions reached 0.25, indicating a 1:3 binding between
metal ions and bipyridines bound on Au nanoparticles (Figure
3B and Supporting Information).
Figure 2. Absorption spectral changes in acetonitrile of (A) BT upon
addition of trifluoroacetic acid at (a) 0, (b) 37, (c) 74, (d) 111, (e) 148, (f)
185, and (g) 223 µM; (B) BT upon addition of EuIII ions at (a) 0, (b) 12,
(c) 24, (d) 36, (e) 48, and (f) 60 µM; (C) Au-BT upon addition of EuIII
ions at (a) 0, (b) 1.1, (c) 2.2, (d) 3.3, (e) 4.4, (f) 5.5, (g) 6.6, (h) and 7.6
µM; and (D) Au-BT upon addition of TbIII ions at (a) 0, (b) 0.6, (c) 1.9,
(d) 3.2 (e) 4.5, (f) 5.8, (g) 7.2, and (h) 8.4 µM.
A 1:3 complexation stoichiometry between metal ions and
BT bound on Au nanoparticles allows two major modes of
interactions (Chart 1): (a) bipyridines bound on the same Au
nanoparticle coordinating with EuIII/TbIII ions and (b) bipyridines
from different nanoparticles coordinating to a given lanthanide
ion. To investigate these aspects, the TEM images of the
Au-BT complexes were recorded in the absence (inset of Figure
1B) and the presence of EuIII ions (Figure 4), under otherwise
identical conditions (by drop casting the solution used for
spectroscopic investigation onto a Formvar-coated copper grid).
Bipyridine-capped Au nanoparticles are randomly distributed
through out the grid (Figure 1B), whereas self-assembly is
observed in the presence of EuIII ions (Figure 4). The tendency
toward aggregation observed in absorption spectral studies and
self-assembled structures in the TEM images support the later
possibility (Chart 1b).
Luminescence Efficiency of Nanophosphors. The lumines-
cence properties of Au-BT complexes containing varying
concentrations of EuIII/TbIII ions were investigated by using
steady-state and time-resolved techniques. In the case of
Au-BT, a weak unstructured emission band was observed
between 320 and 450 nm that is characteristic of bipyridine
fluorescence (Supporting Information). A dramatic enhancement
of the emission intensity was observed upon addition of varying
concentrations of EuIII/TbIII ions (excitation at 320 nm), and
the luminescence intensity leveled off at higher concentrations
(Figure 5). The Au-BT:EuIII complex features six line-like
emission peaks at 578, 594, 619, 649, 693, and 702 nm, whereas
the Au-BT:TbIII complex exhibits four major line-like emission
peaks at 487, 546, 584, and 640 nm. The enhancement in line-
like emission is attributed to the complex’s ability to effectively
shield the encapsulated metal ion and efficient energy-transfer
process from the BT molecules to the coordinated EuIII/TbIII
Figure 3. (A) Plot of the absorbance at 322 nm as a function of EuIII/BT
mole ratio (BT bound on Au nanoparticles). (B) Job’s plot obtained for a
1:3 complexation between EuIII ions and BT bound on Au nanoparticles.
at 249 nm and the other at 295 nm. Upon addition of increasing
concentrations of EuIII ions, a decrease in the intensity of the
π-π* absorption band (295 nm) of BT along with the
concomitant emergence of a new band around 325 nm (Figure
2B) was observed. Similar spectral changes were observed for
BT upon addition of acid (Figure 2A), and the monoprotonation
results in a conformational change to the cis form. The
emergence of a new band for BT/Au-BT at 325 nm upon
addition of EuIII/TbIII ions is attributed to the polarization
resulting from the metal ion coordination and the conformation
change of the ligand to the cis form (Figure 2C,D).
The stoichiometry for the complexation of EuIII/TbIII ions with
Au-BT was investigated by means of the absorption isotherm30a
and Job’s continuous variation method30b (Figure 3). After
successive additions of EuIII ions, solutions were kept for
equilibration (∼30 min), and the changes in absorbance were
monitored. The increase in absorbance at 322 nm, when plotted
against the mole ratios of metal ions to BT (bound to Au
nanoparticles), reached a saturation point at 0.33, indicating a
1:3 metal ion-to-ligand stoichiometry (Figure 3A and Supporting
Information). To further confirm the stoichiometry, difference
absorption spectra were recorded by varying the mole ratio of
BT bound on Au nanoparticles to EuIII/TbIII ions (Supporting
Information). The difference absorption spectra highlight the
spectral differences resulting from the varying concentrations
(30) (a) Folmer-Andersen, J. F.; Lynch, V. M.; Anslyn, E. V. Chem. Eur. J.
2005, 11, 5319-5326. (b) Connors, K. A. Binding Constants: The
Measurements of Molecular Complex Stability; Wiley: New York, 1987.
Figure 4. TEM image of Au-BT in the presence of EuIII at two different
magnifications.
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Au Nanoparticles as Phosphorescent Sensors
A R T I C L E S
Figure 5. Luminescence spectrum of Au-BT in acetonitrile (A) upon addition of EuIII at (a) 2.4, (b) 4.8, (c) 7.2, (d) 9.7, (e) 12, and (f) 14.5 µM and (B)
upon addition of TbIII at (a) 5.5, (b) 11, (c) 16.6, (d) 22, and (e) 27.7 µM after excitation at 320 nm. The dotted line shows the excitation spectrum of the
complex in acetonitrile (A) at 14.5 µM EuIII (emission followed at 616 nm) and (B) at 27.7 µM TbIII (emission followed at 545 nm). The inset shows a
photograph of a cuvette containing acetonitrile solution of Au-BT:EuIII and Au-BT:TbIII complexes illuminated using a 320-nm UV light source.
ions. The insets of Figure 5A,B show photographs of cuvettes
containing acetonitrile solutions of red-emitting Au-BT:EuIII
and green-emitting Au-BT:TbIII complexes illuminated using
a 320-nm UV light source.
The luminescence quantum yields (φ) were found to be
9.6 × 10-3 and 38.0 × 10-3 for the Au-BT:EuIII and
Au-BT:TbIII complexes, respectively (measured for the 1:3
metal ion-to-ligand complex). The quantum yields obtained
by the method described by Haas and Stein28b using
Ru[(bpy)3]2+ (φ ) 0.028 in aerated water)31 as a standard for
the Au-BT:EuIII complex and quinine sulfate (φ ) 0.546 in
1 N H2SO4)33 for the Au-BT:TbIII complex. The luminescence
quantum yields of Au-BT:EuIII/TbIII complexes are higher than
those of EuIII/TbIII complexes of cryptands and calixarenes
containing bipyridine units.16 The origin of the luminescence
was further investigated by recording the excitation spectra
(dotted lines in Figure 5) of Au-BT:EuIII and Au-BT:TbIII
complexes, following the emission at 616 and 545 nm,
respectively. In both cases, the excitation spectrum peaks at 320
nm, indicating that the ligand-centered excited state populates
the emitting levels of EuIII/TbIII ions through an energy-transfer
Figure 6. Relative decrease in the luminescence intensity of Au-BT-EuIII
in the presence of various concentrations of different metal ions.
Au-BT:EuIII and Au-BT:TbIII complexes are presented in the
Supporting Information.
5
process, resulting in emission from the Di-7Fj states.
Sensor Performance. One of the significant features of the
Au-BT:EuIII/TbIII system is its ability to hold a large number
of lanthanide complexes around the Au nanoparticle without
affecting their luminescence behavior (long-lived line-like
emission). Such nanophosphors are ideal as high-affinity sensors
for the site-specific detection of biologically important cations.
It is well established that a proper balance in the concentrations
of various metal ions such as Ca2+ and Mg2+ is vital in
performing various biological functions. For example, intracel-
lular Ca2+ regulates the contraction of cardiac and smooth
muscles.35 However, their determination in complex biological
systems is rather difficult because they are present in specific
locations with a concentration gradient (calcium channels).
Lanthanide complexes are widely used as fluoroimmunoassay
labels by taking advantage of their long-lived emission (lifetimes
are in the millisecond range).16,21,32 The autofluorescence of
biological materials usually decays on the nanosecond time
scale, which overlaps with the lifetime of organic fluorophores,
limiting their application as fluorescent labels in biological
systems. In contrast, luminescent lanthanide complexes have
the obvious advantage of eliminating the background emission
from a biological matrix. Interestingly, the luminescence
lifetimes of lanthanide complexes are not influenced by linking
them onto gold nanoparticles. The room-temperature phospho-
rescence lifetimes of the Au-BT:EuIII and Au-BT:TbIII
complexes (τ measured for 1:3 metal ion-to-ligand com-
plex) were estimated as 0.36 and 0.7 ms, respectively.34 The
solutions were thoroughly degassed before the lifetime studies
were conducted, and the plots of the luminescence decay for
(34) Phosphorescence lifetimes were measured in a SPEX model 1934D
phosphorimeter equipped with a flash lamp. The signals from the sample
falling on the photomultiplier were collected by a control module for a
preset length of time. The luminescence intensities decay exponentially
with first-order kinetics over time t, obeying the equation It ) I0 exp(-t/τ),
where I0 and It are the luminescence intensities at time zero and time t,
respectively. The emission lifetime was obtained by fitting the decay curve
to the above equation.
(31) Nakamura, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697.
(32) Werts, M. H. V.; Woudenberg, R. H.; Emmerink, P. G.; van Gassel, R.;
Hofatraat, J. W.; Verhoeven, J. W. Angew. Chem., Int. Ed. 2000, 39, 4542-
4544.
(35) Voet, D.; Voet, J. G. Biochemistry, 2nd ed.; John Wiley & Sons: New
York, 1995.
(33) Meech, S. R.; Philips, D. J. Photochem. 1983, 23, 193.
9
J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006 1911

A R T I C L E S
Ipe et al.
Figure 7. (A) TEM image and (B) absorption spectra of Au-BT upon addition of [Cu2+] at (a) 0, (b) 32, (c) 63, (d) 95, (e) 127, (f) 158, and (g) 190 µM.
(C) TEM image of Au-BT in the presence of Cu2+
.
Isomorphous replacement of metal cations (for example,
Ca2+) by EuIII/TbIII ions has been widely used for sensing
applications.21 Substitution of EuIII/TbIII ions by various metal
cations mainly depends on the similarity in size of the cat-
ion and its binding ability.21 The ability of the luminescent
Au-BT:EuIII complex to sense alkali metal ions (Na+, K+),
alkaline earth metal ions (Ca2+, Mg2+), and transition metal ions
(Cu2+, Ni2+, Zn2+) was investigated. The relative decreases in
the luminescence intensities of an acetonitrile solution of the
Au-BT:EuIII complex in the presence of various metal ions
are represented in Figure 6. It can be noted that, upon addition
of various metal cations, no noticeable absorption spectral
changes were observed (Supporting Information).
A dramatic decrease in the luminescence was observed upon
addition of alkaline earth metal ions and transition metal ions,
whereas the luminescence intensity was not influenced by the
addition of Na+ and K+ ions (after successive additions, the
solution was allowed to stand for 5 min, for attaining equilib-
rium, in each case). The relative decrease in intensity is much
larger for transition metal ions as compared to alkaline earth
metals. This might be due to the better binding ability of the
transition metal ions over EuIII ions.
interaction of terpyridine-functionalized Au nanoparticles and
metal salts has been reported recently.8g The direct interaction
of Au-BT with Cu2+ ions was investigated by following the
absorption spectral changes (Figure 7). Upon the addition of
an acetonitrile solution of Cu2+ to Au-BT, we observe (i) the
formation of a new band at 310 nm that is merged with the Au
nanoparticle absorption and (ii) the emergence of red-shifted
band at 780 nm, along with a concomitant decrease in the
plasmon absorption band of Au-BT at 520 nm (inset of Figure
7B).
The band at 780 nm is not observed in the case of BT upon
addition of Cu2+ ions. The absorption band in the short-
wavelength region (310 nm) results from the conformation
change of the ligand to the cis form and the complexation of
Cu2+ ions with bipyridines. Attempts to determine the stoichi-
ometry of complexation between BT on Au nanoparticles and
Cu2+ ions using Job’s continuous variation method were not
successful because of the strong aggregation tendency of the
nanoparticles in the presence of copper ions. Analysis of TEM
images of Au-BT in the absence and presence of Cu2+ clearly
indicates the formation of superstructures upon metal ion
complexation. The strong coordinating ability of Cu2+ ions can
bring the Au-BT nanohybrids together, leading to the formation
of superstructures. The images presented in the highlighted
Superstructure Formation of Au-BT. Transition metal ions
form stable complexes with various coordinating ligands such
as bipyridine36 and phenanthroline.37 The formation of super-
structures and cross-linked nanoparticle stripes through the
(36) Kaes, C.; Katz, A.; Hosseini, M. W. Chem. ReV. 2000, 100, 3553-3590.
(37) Armaroli, N. Chem. Soc. ReV. 2001, 30, 113-124.
9
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Au Nanoparticles as Phosphorescent Sensors
A R T I C L E S
terbium(III) chloride in methanol, and microliter quantities were added
to the Au-BT solution. The monobromo derivative of 5,5′-dimethyl-
2,2′-dipyridyl (2) was prepared as per a reported procedure25 (Supporting
Information). Conversion of 2 to the thiol derivative (BT) and its
functionalization on gold nanoparticles are described below. Freshly
distilled solvents were used throughout.
portion of Figure 7C show ring structures with an external
diameter of about 10 nm.
The absorption band at 780 nm originates from interparticle
plasmon coupling because of the close interaction of metal
nanoparticles in these superstructures. When electromagnetic
radiation interacts with Au nanoparticles, its electric vector
displaces the electron cloud, which is restored by the Coulombic
attraction of the nucleus, and this establishes the oscillation of
plasmon electrons. Surface plasmon absorption originates when
this frequency of oscillation is in resonance with that of incident
radiation. When two or more metal nanoparticles are placed in
close proximity, an additional force of attraction between the
electron cloud and nucleus of adjacent nanoparticles becomes
significant. A decrease in the frequency of plasmon oscillation
was observed because of the perturbed charge distribution of
assembled Au nanoparticles. This results in the formation of
the red-shifted band at 780 nm and a concomitant decrease in
the plasmon absorption band of Au-BT at 520 nm. Such
interparticle plasmon interactions were observed in Au super-
structures,1d,i,k closed-loop structures of Au nanoparticles (nano-
necklaces),38 closely spaced Au nanoparticles deposited on ITO
glass plates,39 and longitudinally organized Au nanorods.40
Synthesis of Compound BT. To an ice-cooled solution of compound
2 (200 mg, 0.76 mmol) in THF (3 mL) was added a mixture of
tetrabutylammonium fluoride (221 mg, 0.84 mmol) and hexamethyl-
disilathiane (166.8 mg, 0.92 mmol) in THF (2 mL) under stirring. The
reaction mixture was stirred for a further period of 12 h. It was then
diluted with dichloromethane and washed with saturated ammonium
chloride (10 mL) and then with distilled water (20 mL × 2). The organic
layer was separated and dried over anhydrous sodium sulfate, and the
solvent was removed under reduced pressure to give a residue that was
recrystallized from a mixture (9:1) of hexane and ethyl acetate to yield
white crystals of BT (40 mg, 40%), mp 106 °C. IR (KBr) νmax: 2369,
1
1593, 1560, 1465, 1371, 1236, 1128, 1027, 838, 737, 663 cm-1. H
NMR (CDCl3, 300 MHz) δ (ppm): 2.39 (s, 3H, -CH3), 3.66 (s, 2H,
-CH2SH), 8.51 (s, 1H, ar), 8.49 (s, 1H, ar), 8.34-8.31 (d, 1H, J )
8.1 Hz, ar), 8.28-8.26 (d, 1H, J ) 8.1 Hz, ar), 7.70-7.68 (d, 1H, J )
8.1 Hz, ar), 7.69-7.67 (d, 1H, J ) 8.1 Hz, ar). 13C NMR (CDCl3, 75
MHz) δ (ppm): 32.57, 40.06, 120.6, 132.65, 137.5, 149.6, 153.16,
155.46. Exact mass calcd. for C12H12N2S [MH+]: 217.0799, found
217.0805 (FAB, high-resolution mass spectroscopy).
Conclusions
Synthesis of Gold Nanoparticles Capped with BT. To a stirred
solution of tetraoctylammonium bromide (0.068 g,) in toluene (5 mL)
was added dropwise an aqueous solution of hydrogentetrachloroaurate-
(III) hydrate (11 mg, 0.028 mmol), and the mixture was stirred for 20
min. The reaction mixture was washed with distilled water (10 mL)
several times, and the organic layer was separated. A mixture of BT
(4.2 mg, 0.0196 mmol) and dodecanethiol (1.7 mg, 0.0084 mmol) in
toluene (2 mL) was added to the above solution, and the resulting
mixture was stirred for 30 min. An aqueous solution of sodium
borohydride (11 mg, 0.28 mmol) was added dropwise, and the mixture
was stirred for a further period of 3 h. The organic layer was washed
with water and diluted with methanol (250 mL). It was kept in a
refrigerator, and the precipitate obtained was further purified by being
resuspended in toluene and then centrifuged after the addition of
methanol (10 mL). This process was repeated twice to remove any
unbound thiol. The residue obtained was redispersed in 5 mL of toluene
and used as stock solution.
The chelating agent 2,2′-bipyridine was anchored to Au
nanoparticles, and their complexation with EuIII/TbIII ions
yielded red-emitting Au-BT:EuIII and green-emitting Au-BT:
TbIII phosphorescent nanomaterials (nanophosphors). The ligand-
centered excited state of the complex populates the emitting
levels of EuIII/TbIII ions through an energy-transfer process,
5
resulting in the emission from the D0/5D4-7Fj levels. A 1:3
complexation stoichiometry was observed between EuIII/TbIII
ions and BT bound on Au nanoparticles, and the self-assembled
structures observed in TEM images indicate the possibility of
a given lanthanide ion coordinating with bipyridines from
different nanoparticles. An attractive feature of the present
systems is their ability to hold a large number of luminescent
lanthanide complexes around Au nanoparticles without affecting
the long-lived line-like emission properties. Such phosphorescent
nanohybrid systems might have wide range of applications in
optoelectronic devices and biomolecular systems. In the present
case, we have used them as high-affinity sensors for the selective
detection of cations through isomorphous substitution of lan-
thanide ions. Because gold nanoparticles can be easily func-
tionalized with molecules that can selectively bind at specific
sites, such phosphorescent nanomaterials can be used in
understanding the properties and functions of various biological
processes and in determining biologically active substances.
Acknowledgment. The authors thank the Council of Scientific
and Industrial Research (CMM 220239), and the Department
of Science and Technology (SP/S5/NM-75/2002), Government
of India, for financial support. We thank Prof. C. N. R. Rao for
use of HRTEM facilities at the Jawaharlal Nehru Centre for
Advanced Scientific Research, Bangalore, India. This is con-
tribution no. RRLT-PPD-200 from the Regional Research
Laboratory, Trivandrum, India.
Supporting Information Available: Details on the prepara-
tion, purification, and characterization of 2; difference absorption
spectra of Au-BT in the presence of EuIII/TbIII ions; absorption
isotherm and Job’s plot for a 1:3 complexation between TbIII
ions and BT bound on Au nanoparticles; lifetime studies of
Au-BT:EuIII and Au-BT:TbIII complexes; and complete refs
1g and 27. This material is available free of charge via the
Internet at http://pubs.acs.org.
Experimental Section
Details of instrumental methods for spectroscopic characterization
are presented in the Supporting Information. Stock solutions of the
lanthanides were prepared by dissolving europium(III) chloride and
(38) Dai, Q.; Worden, J. G.; Trullinger, J.; Huo, Q. J. Am. Chem. Soc. 2005,
127, 8008-8009.
(39) Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht, B.;
Aussenegg, F. R. Opt. Commun. 2003, 220, 137-141.
(40) (a) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P.
V. J. Phys. Chem. B 2004, 108, 13066-13068. (b) Sudeep, P. K.; Joseph,
S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516-6517.
JA054347J
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