A simple and versatile route to stable quantum dot-dye hybrids in nonaqueous and aqueous solutions

Full text of DOI:10.1021/ja8073962

Published on Web 12/03/2008
A Simple and Versatile Route to Stable Quantum Dot-Dye Hybrids in
Nonaqueous and Aqueous Solutions
Ting Ren, Prasun K. Mandal, Wolfgang Erker, Zhihong Liu, Yuri Avlasevich, Larissa Puhl,
Klaus Mu¨llen, and Thomas Basche´*
Institut fu¨r Physikalische Chemie, Johannes Gutenberg-UniVersita¨t Mainz, Welderweg 11, 55099 Mainz, Germany,
and Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany
Received September 18, 2008; E-mail: thomas.basche@uni-mainz.de
Scheme 1. Rylene Dyes and Dye-QD Binding Model
Hybrid materials composed of inorganic semiconductor quantum
dots (QDs) and π-conjugated organic molecules (Dye) are being
increasingly considered for optoelectronic and sensing applications.1
Recently, a number of approaches have been developed to prepare
Dye/QD conjugates in solution. In early experiments the emission
of CdSe/ZnS QDs was completely quenched by unspecific complex
formation with a diazaperylene dye.2 Later on more reliable
protocols have been developed in which attachment of dye to QD
was mediated by an additional component such as proteins, peptides,
or polymers.3 Other approaches used pyridyl moieties for Dye/QD
complex formation or interdigitization of alkylamino chains into
the alkyl-capping layer of QDs.4 In most of these conjugates the
QD (Dye) acts as an energy donor (acceptor) and excitation energy
transfer (ET) signals complex formation. The modulation of ET
efficiency, e.g. by analyte binding, typically provides the basis of
the sensing capability. In several instances energy transfer from
QD to dye could be satisfactorily modeled in terms of the Fo¨rster
mechanism (Fluorescence resonance energy transfer: FRET),5,6
although deviations have also been reported.4b
Using dye 1 and going through exactly the same sequence of
steps as those described before did not result in any detectable
complex formation. These results strongly suggest that dyes 2 and
3 bind specifically via their dicarboxylate groups to QDs, while
nonspecific binding of 1 did not occur. Accordingly, we propose a
chelate type binding of the bidentate anchors in 2 and 3 to Zn2+
ions on the QD surface (Scheme 1). For a given Dye/QD ratio,
emission spectra remained almost constant for now up to 9 months
corroborating the high stability of the complexes.
Here, we report a versatile approach to extraordinarily stable
Dye/QD hybrids, which uses dicarboxylate anchors7 to bind rylene
dyes to QD. In the example shown in Scheme 1, the starting
perylene dye 18a was partially saponified under basic conditions to
give monoanhydride 2. Imidization of 2 with ꢀ-glutamic acid in
NMP affords 3. Under basic conditions both dyes (2, 3) expose
two carboxylate moieties for binding to the QD surface (see below).
Multishell QDs composed of a CdSe core and covered by various
layers of CdS and ZnS were synthesized according to existing
protocols.9 Exemplarily, complexes between CdSe/CdS/ZnS QDs
and dyes 2 or 3 were formed by sonicating a mixture of dye (in
methanol) and QD (in chloroform) after addition of a weak base
(K2CO3) for 4 h.7 Next, the Dye/QD complexes were precipitated,
washed, and redissolved in chloroform yielding clear solutions. The
(spectroscopic)observationthatafterrepeatedprecipitation-dissolution
cycles only minor amounts of dye detached from QD, with 3
showing qualitatively a higher binding affinity than 2, is a first
indication of the high stability of the complexes.
In Figure 1a, a series of emission spectra obtained for different
Dye 3/QD ratios is shown. (Similar results have been obtained for
2, see Supporting Information.) The given ratios are the nominal
valuesofthestartingmixtures,andforeachratiotheprecipitation-dissolution
step has been carried out. With increasing dye amounts the QD
emission is successively quenched and sensitized dye emission
increased, indicating efficient ET in the complexes, which will be
analyzed in full detail below. All spectra have been corrected for
the small contribution from direct excitation of the dye. Fluores-
cence decay curves of QDs in the complexes (see Supporting
Information) show an additional fast component also corroborating
efficient ET.
So far we have restricted the discussion to perylene derivatives
2 and 3. Our route for Dye/QD complex formation, however, can
be straightforwardly adapted to their terrylene and quaterrylene
analogues.8b Hence, in combination with QD size quantization,
broad color tunability of the complexes can be achieved. Indeed,
stable complex formation has been observed, e.g., between terrylene
dye 4 (scheme 1) and QDs using the same procedure as that
described before. An emission spectrum is presented in Figure 1b
clearly showing emission from QDs (λmax ∼600 nm) and dye 4
(λmax ∼670 nm) after selective excitation of the QDs. Here,
multishell QDs with red-shifted emission have been used.
The Dye/QD complexes can be transferred from the organic into
the aqueous phase by exchange of the original ligands (oleic acid,
oleyl amine) by mercapto-propionic acid (MPA) under basic
conditions. Dye 3/QD and MPA in an isopropanol/chloroform
mixture were refluxed under Ar for 5 min. As can be clearly seen
in Figure 1c, the aqueous solution contains Dye/QD complexes.
An appreciable fraction of 3, however, has been replaced by MPA,
which is known to bind strongly via its thiol group to the QD surface
Figure 1. Fluorescence spectra of (a) Dye 3/QD complexes in CHCl3 as
function of Dye/QD ratio; (b) Dye 4/QD complex (ratio 1:1) in CHCl3; (c)
Dye 3/QD complex in CHCl3 and after transfer into water. In all cases the
excitation wavelength was 390 nm.
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10.1021/ja8073962 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
will decrease the number of intact complexes. This result, obtained
at the single molecule level, is another clear example of the
extraordinary stability of our Dye/QD complexes.
In summary, we have established a versatile route for the
preparation of extraordinarily stable Dye/QD complexes by furnish-
ing rylene dyes with dicarboxylate anchors. By proper choice of
dye and QD components, a broad spectral range from the visible
up to the NIR can be covered and additionally the efficiency of ET
can be easily tuned. Future work will focus on the selection of
more appropriate ligands to optimize transfer of the complexes into
the aqueous phase. Furthermore, the time scale of ET will be directly
probed by transient absorption measurements. In combination with
the capability to vary the Dye-QD distance with, e.g., an increasing
number of ZnS layers while preserving the binding geometry, such
investigations will give unprecedented insights into the ET mech-
anism operating in such complexes.
Figure 2. (a) Stern-Volmer plot of QD fluorescence quenching in complex
with dye 3. Data were fitted with a model assuming a binomial distribution
of the number of dyes bound to QD and an FRET process (see text). (b)
Sequence of emission spectra of a single Dye 3/QD complex in PMMA
(λexc ) 488 nm, details below).
(ZnS).10 Consequently, the overall ET efficiency in the complex is
reduced after transfer into water.
In Figure 2a QD quenching expressed as F0/F - 1 from the
series in Figure 1a is plotted versus the Dye/QD ratio where F0
and F are the fluorescence intensity of QDs in the absence and
presence of the dye, respectively. As we observed reduced donor
emission and sensitized acceptor emission as well as strong spectral
overlap between QD emission and dye absorption (Figure S1), the
quenching of QD emission is described by the following FRET
expression with transfer efficiency E (for details see Supporting
Information):
Acknowledgment. This work was supported by the Volkswagen
Foundation and by the DFG (SFB 625).
Supporting Information Available: Synthetic procedures for 2-4,
QD, and Dye/QD complexes; experimental details of absorption,
emission, and single molecule spectroscopy; details of FRET approach.
This material is available free of charge via the Internet at http://
pubs.acs.org.
0 - 1 )
with E ) dΣmax a(d)
F
E
1 - E
d
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