Efficient quenching of TGA-capped CdTe quantum dot emission by a surface-coordinated europium(III) cyclen complex

Article abstract of DOI:10.1021/ic3027623

Extremely efficient quenching of the excited state of aqueous CdTe quantum dots (QDs) by photoinduced electron transfer to a europium cyclen complex is facilitated by surface coordination to the thioglycolic acid capping ligand. The quenching dynamics are elucidated using steady-state emission and picosecond transient absorption.

Full text of DOI:10.1021/ic3027623

Communication
pubs.acs.org/IC
Efficient Quenching of TGA-Capped CdTe Quantum Dot Emission by
a Surface-Coordinated Europium(III) Cyclen Complex
Shane A. Gallagher,† Steve Comby,‡ Michal Wojdyla,‡ Thorfinnur Gunnlaugsson,‡ John M. Kelly,‡
Yurii K. Gun’ko,† Ian P. Clark,§ Gregory M. Greetham,§ Michael Towrie,§ and Susan J. Quinn*,⊥
†School of Chemistry, Trinity College Dublin, Dublin 2, Ireland
‡School of Chemistry, Trinity Biomedical Sciences Institute, Trinity College Dublin, Dublin 2, Ireland
§Central Laser Facility, Research Complex at Harwell, Science & Technology Facilities Council, Rutherford Appleton Laboratory,
Didcot, Oxfordshire OX11 0QX, U.K.
⊥School of Chemistry and Chemical Biology, University College Dublin, Dublin 4, Ireland
S
* Supporting Information
Scheme 1. Formation of the Eu.1-CdTe Conjugate
ABSTRACT: Extremely efficient quenching of the excited
state of aqueous CdTe quantum dots (QDs) by
photoinduced electron transfer to a europium cyclen
complex is facilitated by surface coordination to the
thioglycolic acid capping ligand. The quenching dynamics
are elucidated using steady-state emission and picosecond
transient absorption.
The luminescent properties of cadmium chalcogenide quantum
dots (QDs), which include narrow emission band and high
quantum yields, render them attractive for a range of applications
including optoelectronics, light-harvesting, biological imaging
and biosensing.1−7 In general, excitation of QDs generates
exciton (electron−hole pair) excited states, which can
subsequently recombine through a number of processes. These
occur over various time scales with fast carrier trapping known to
happen on the picosecond time scale, while electron−hole
recombination occurs over longer times.8 Surface states play an
important role in these processes, and consequently QDs are
highly sensitive to surface modifications.9 In particular, the
binding of suitable molecules at the surface can facilitate efficient
charge10 or energy transfer11 (both are typically signaled by
changes in luminescence). Lanthanide complexes have been used
extensively for sensing and imaging due to their long-lived
excited states (ms) and well spaced, line-like emission in the
visible {Eu(III), Tb(III)} and near infrared {Yb(III), Nd(III)}
regions.15 Recently, the sensing capacity of a lanthanide complex
attached to gold nanoparticles for biologically relevant species
has been demonstrated.13 However, combining the properties of
Lanthanide and QD systems offers the potential to achieve
enhanced optical properties.12 In this work, we investigate the
properties of aqueous lanthanide-CdTe systems formed by the
coordination of a caged europium complex Eu.1 to the terminal
COOH groups of the thioglycolic (TGA) capped CdTe QDs as
shown in Scheme 1 and compare these results to the system
formed in the presence of the uncaged Eu(III) triflate salt.
Water-soluble TGA-CdTe QDs were prepared using the
method developed by Gaponik et al.15 The steady-state
absorption spectrum in water shows a band at 510 nm, which
is assigned to the first excitonic transition (1Se−1Sh) (Figure 1).
Figure 1. Absorption spectrum of 0.1 M Eu.1 and 1.29 × 10−5 M TGA-
CdTe QDs in water and emission spectrum (λex = 450 nm) of QDs.
Spectra have been normalized for the sake of comparison.
From this, the diameter of the particles was determined to be 2.3
nm, which was corroborated by high-resolution transmission
electron microscopy; see Figure S1 in the Supporting
Information (SI).16 The presence of negatively charged COO−
surface groups was confirmed by ζ-potential measurements (−48
mV); see Figure S2 in the SI. Excitation of the QDs at 450 nm
resulted in band-edge photoluminescence centered at 550 nm
with a QY of 22% measured against Rhodamine 6G. The Eu.1
complex was synthesized according to literature procedures.12
The strongest transition in the visible spectrum of Eu.1 occurs at
394 nm and has a molar extinction coefficient of just 2 M−1 cm−1,
see Figure 1, (the small value is due to a combination of strongly
forbidden f−f electronic transitions of EuIII and the nature of the
Received: December 16, 2012
Published: March 25, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
cyclen ligand). Therefore, accurate concentrations of Eu.1 were
determined using a displacement titration (see Figures S3−S5 in
the SI).12,13
The addition of Eu.1 to QDs resulted in no significant change
in the UV−visible absorption at 510 nm but had a dramatic effect
on the steady-state emission of the QDs (see Figure 2a).
quench a precursor to the exciton by subnanosecond processes
that are too fast to measure with the SPC instrument.
Picosecond transient absorption (ps-TA) spectroscopy was
performed using excitation energies below ca. 0.56 mJ cm−2 to
minimize multiphoton effects. The ps-TA spectrum of QDs
following excitation at 400 nm (50 fs) is shown in Figure 3a.20
Figure 3. Transient absorption spectra following 400 nm (50 fs)
excitation of 3.7 × 10−6 M TGA-CdTe QDs in the absence (a) and
presence (b) of Eu.1 (3 equiv). (c) Comparison of the kinetics for
recovery of bleach in TGA-CdTe QDs upon the addition of Eu.1.
The dominant feature is that of a strong bleached band that
corresponds to the depletion of the ground state. Multi-
component analysis was needed to determine the kinetics of
recovery of this band, and assuming a biexponential model,
lifetimes of 8 3 ps (22%) and 1.6 0.5 ns (34%) were found;
the latter state results in incomplete recovery (44%) of the bleach
on the time scale of the experiment (Figure 3c). These values are
in agreement with recent ps-TA observations for similar QDs.21
Figure 3b shows the transient absorption of TGA-CdTe
recorded in the presence of 3 equiv of Eu.1. In this case, we see
almost complete signal recovery of the bleached band, which is
more clearly visible in Figure 3c. Biexponential analysis of the
recovery kinetics, for the bleach, yielded lifetimes of 16 4 ps
(22%) and 475 42 ps (72%) and a small amount of longer-lived
species (ca. 6%). Thus, in the presence of Eu.1, a 475 ps
component dominates. This indicates that the principal recovery
pathway is due to surface interactions.22
Next we considered the mechanism of quenching. The
possibilities of energy and electron transfer were investigated
because both processes would affect the excited-state lifetimes.
Energy transfer was ruled out because of the absence of any EuIII-
based phosphorescence from the Eu.1-CdTe sample. This is not
unexpected because of the extremely small overlap between the
absorption of Eu.1 and emission of the QDs. However, the close
proximity of the surface-coordinated Eu.1 can facilitate photo-
induced electron transfer from the QDs to the complex.10 The
importance of the chemical headgroup through which the
electron acceptor adsorbs at the QD surface has recently been
highlighted in the case of electron transfer from the QD to a
ruthenium metal cluster.23 If electron transfer from the QDs to
Eu.1 is responsible to some extent for quenching of the QD
emission, then a similar behavior should be observed when
Figure 2. Changes in (a) the PL intensity (λex = 450 nm and (b) the ns
fluorescence decay profiles of TGA-CdTe QDs (3.7 × 10−6 M) in the
presence of Eu.1 [(0−3.7) × 10−5 M].
Strong (80%) quenching occurred upon titration of 1 equiv,
and almost complete quenching (∼96%) was reached with 2
equiv (Figure 2a). If quenching was treated as a dynamic process,
the Stern−Volmer analysis yielded a bimolecular quenching
constant of 1.9 × 1013 M−1 s−1. This is orders of magnitude
greater than the diffusion-limited value of 1.0 × 1010 M−1 s−1
predicted for aqueous solutions and suggests a dominant role of
static quenching.17 The role of static quenching is also supported
by a decrease in the initial amplitude of the decay profiles (Figure
2b). In the absence of Eu.1, biexponential behavior was observed
with τ values of 6 ns (25%) and 20 ns (75%), respectively.
Successive additions of Eu.1 result only in a small reduction in
the average emission lifetime, from 18 to 14 ns upon the addition
of 1 equiv of Eu.1.
Recently, it was reported that strong surface interactions can
result in a preferential decrease in one component of the QD
emission, which manifests as a reduction in the average lifetime.18
Furthermore, the same efficient quenching behavior was also
observed for a much weaker (100 nM) solution of QDs (see
Figure S6 in the SI). This contrasts with studies that employ hole
and electron trapping, where the reagents would have to be in
large excess for such low concentrations of QDs.10 The strong
quenching at low equivalents is attributed to the significant
driving force for Eu.1 to bonds to the surface TGA, which results
in the release of two water molecules.19 Taken together, these
results strongly indicate that the europium complex is able to
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Inorganic Chemistry
Communication
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quench the CdTe emission very effectively (see Figure S7 in the
SI), similar to what was reported recently for EuCl3 by Hong et
al.24 However, contrary to the latter study, the triflate salt caused
precipitation of the QDs from solution in the experimental
conditions used (see Figure S8 in the SI). In contrast, the QDs
were found to be stable to aggregation in the presence of Eu.1.
This is attributed to the entrapment of the EuIII ion within the
cyclen cage. Having observed efficient quenching for both
europium species (salt and complex), we considered the
quenching ability of a terbium triflate salt for which electron
transfer is prohibited (the standard redox potential values of
couple Ln3+/Ln2+ are E = −0.34 V and −3.7 V vs SHE for Ln =
EuIII and TbIII, respectively).25 The addition of Tb(CF3SO3)3 did
not result in efficient quenching (see Figure S9 in the SI) and also
induced some particle aggregation. This result was taken to
further confirm that quenching arises due to electron transfer
from the QDs to Eu.1.
In summary, a thorough photophysical investigation of a
hybrid lanthanide QD system has been undertaken using steady-
state emission and ps-TA spectroscopy. The europium triflate
salt was found to quench the QD emission, but this was
accompanied by precipitation of the sample. Extremely efficient
quenching was also observed for Eu.1 without compromising the
QD stability. The highly stable and robust QD system is
attributed to the caged nature of Eu.1. The quenching behavior
was also observed for low equivalents of Eu.1 at nanomolar
concentrations of QDs in water. The coordination of Eu.1 to a
carboxylate at the surface is seen as a key driver for highly efficient
quenching. Transient absorption indicated that surface binding
of the europium complex resulted in removal of the nanosecond
decay component, which dominates deactivation in the parent
TGA-CdTe particles, and this occurs principally through static
electron transfer. We believe that such lanthanide QD systems
have great potential as electron-transfer components for sensing
and signal applications.
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ASSOCIATED CONTENT
* Supporting Information
Details for experimental methods and additional titration data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(16) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15,
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(17) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
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■
S
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ChemPhysChem 2011, 12, 2735.
(19) On the basis of the surface area of 16.6 nm2 calculated for a 2.3 nm
spherical particle and the thiol footprint of 0.22 nm2, the maximum
number of surface TGA molecules was estimated to be approximately
75.
AUTHOR INFORMATION
Corresponding Author
*E-mail: susan.quinn@ucd.ie.
Notes
■
(20) The low absorption of Eu.1 at 394 nm does not influence the
transient spectrum obtained using fs laser excitation at 400 nm.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the EU for funded access to STFC (App1110017) and
the Science Foundation Ireland (Grants SFI SFI 07/IN.1/I1862,
07/RFP/MASF250, and 07/RFP/CHEF437) and Swiss Na-
tional Science Foundation (SC) for financial support.
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Seideman, T.; Kubiak, C. P.; Weiss, E. A. Phys. Chem. Chem. Phys. 2012,
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