Synthesis of cationic quantum dots via a two-step ligand exchange process

Article abstract of DOI:10.1039/c0cc04975a

A new class of quaternary ammonium derivatives has been used to synthesize cationic CdSe/ZnS quantum dots with exceptional stability in water as well as in biological media.

Full text of DOI:10.1039/c0cc04975a

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COMMUNICATION
Synthesis of cationic quantum dots via a two-step ligand exchange
processw
Yi-Cheun Yeh, Debabrata Patra, Bo Yan, Krishnendu Saha, Oscar R. Miranda,
Chae Kyu Kim and Vincent M. Rotello*
Received 15th November 2010, Accepted 24th January 2011
DOI: 10.1039/c0cc04975a
A new class of quaternary ammonium derivatives has been used
to synthesize cationic CdSe/ZnS quantum dots with exceptional
stability in water as well as in biological media.
of amine groups as QDs tend to aggregate and precipitate
upon deprotonation at higher pH.16 This pH dependence
complicates both synthesis and applications of these cationic
particles.
Despite the potential advantages of permanently cationic
QDs, to our knowledge there have been no examples of
particle of this sort reported to date. To provide QDs with a
permanent positive charge featuring extended pH and biofluid
stability profiles we have explored protocols for the synthesis
of quaternary ammonium functionalized QDs. The surface
properties, i.e. hydrophobicity and hydrophilicity, of these
cationic QDs are tunable through the choice of terminal
head group.
Semiconductor quantum dots (QDs) are important class of
nanomaterials due to their inherent physical properties such as
size/composition-tunable
fluorescence
emission,
high
fluorescence quantum yield, extended fluorescence lifetime
and photostability.1 QDs can provide better sensitivity and
stability for biological application than that of traditional
organic dyes and protein-based fluorophores.2 The unique
optical properties of QDs make them appealing for bio-
molecular interaction studies3 and biomedical applications
including sensing,4 labelling5 and intracellular imaging.6
The essential prerequisite for biological applications of QDs
is solubility and stability in water and biofluids. Common
synthetic routes for QDs, however, generate particles capped
with hydrophobic surface ligands such as trioctylphosphine
oxide (TOPO) and trioctylphosphine (TOP).7 Several
strategies have been developed to solubilize QDs in aqueous
medium, including encapsulation and ligand exchange.8
Hydrophilic ligands, featuring negative, positive and neutral
termini have also been used to displace the hydrophobic
ligands on the QDs and provide solubility.9
The cationic QDs were synthesized via a two-step ligand
exchange reaction featuring conversion of hydrophobic QDs
to amphiphilic QDs followed by creation of cationic QDs from
the amphiphilic QD intermediate. The subsequent change in
ligand polarity is the key factor in providing solubility of the
QDs. This process is highly efficient, providing an essentially
quantitative yield of the cationic particle from the hydro-
phobic precursor. These particles feature high purity, pH
stability, and dispersibility in biofluids including serum,
making them excellent candidates for biological applications.
Our ligand design features dihydrolipoic acid (DHLA) as
the bidentate anchor on the surface17 and a tetra(ethylene
glycol) (TEG) spacer to minimize non-specific protein and cell
interactions.3a Positive charge and enhanced water solubility
are then imparted by the quaternary ammonium terminus,
whose physicochemical properties can be tuned through head
group choice (Scheme 1a). The general synthetic scheme for
synthesizing dithiol cationic ligands is given in Scheme 1c, with
details available in the ESI.w
Our initial efforts to produce cationic QDs focused on direct
functionalization of TOPO/TOP-capped QDs (TOPO/TOP-QDs).
This process, however, provided a very low yield of QD
(o 10%). To address this problem, we developed a two-step
ligand exchange process. As shown in Scheme 1b, the first step
involves the phase transfer conversion of hydrophobic
TOPO/TOP-capped QDs to amphiphilic HS-C5-TEG capped
QDs using MeOH as solvent. The particles are then purified,
resuspended in MeOH and the corresponding dithiol ligand
added to provide hydrophilic DHLA–TEG–N+(CH3)2–R
capped QDs. These particles are quite clean; mass
Positively charged QDs are of particular interest for bio-
logical applications, providing a complementary surface
binding for negatively charged proteins10 and nuclei acids11
via electrostatic interactions. This supramolecular design
enables applications including intracellular delivery and
sensing.12 Additionally, positively charged QDs have higher
stability at low pH13 and possess higher cellular permeability
than uncharged and anionic analogs.14 Several groups have
generated cationic QDs by using the amine-containing ligands
or polymers, using either thiol capping or hydrophobic
encapsulation.15 However, amine-functionalized QDs have a
limited useful pH range due to the protonation/deprotonation
Department of Chemistry, University of Massachusetts Amherst,
710 North Pleasant Street, Amherst, MA 01003, USA.
E-mail: rotello@chem.umass.edu; Fax: +1 413-5452058
w Electronic supplementary information (ESI) available: Synthesis
and characterization of QDs and ligands. TEM data of cationic
QDs. See DOI: 10.1039/c0cc04975a
c
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Chem. Commun., 2011, 47, 3069–3071 3069

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Table 1 Physical properties of cationic QD 1–3
Absorption Emission Hydrodynamic Zeta
peak/nm peak/nm diameter/nm potential/mV
TOPO/TOP-QD 578
593
595
594
595
—
—
QD 1
QD 2
QD 3
578
578
578
11.7 Æ 3.9
11.7 Æ 2.7
14.6 Æ 2.9
19.7 Æ 5.7
19.4 Æ 6.3
24.4 Æ 7.8
ligand exchange reaction did not cause QD aggregation, with
the size of the CdSe/ZnS core–shell was about 5 nm diameter,
same as the precursor TOPO/TOP-QDs (Fig. S2, ESIw).
Fig. 1a and b presents the absorption and emission spectra
of QDs before and after the ligand exchange process. The
absorption and emission maxima of the hydrophobic and
hydrophilic QDs were unchanged (Table 1), indicating the
optical properties of cationic QDs exhibited no major changes
after transferring into water. However, the relative emission
intensity indicates that the cationic QDs in aqueous media had
photoluminescence quantum yield (QY) 40–60% of those of
the precursor particles in organic solvent, an observation for
QDs after their phase transfer into water.18 The hydrodynamic
diameter and the surface charge of the QD 1 were measured
using Malvern Zetasizer Nano ZS (Fig. 1c and d). The
hydrodynamic diameter of cationic QDs 1–3 ranged from 11
to 15 nm due to the different sized head group, and all QDs
showed a net positive surface charge (20–25 mV) with a
narrow charge distribution (Table 1). The narrow size distri-
bution further confirms that the cationic QDs were uniformly
dispersed in aqueous media.
Scheme 1 (a) Design of dithiol cationic ligand. (b) Ligand structure
for functionalization of QDs and synthetic route for a two-step ligand
exchange reaction: Step 1: conversion of hydrophobic QDs to amphi-
philic QDs into MeOH; Step 2: conversion of amphiphilic QDs to
hydrophilic QDs into water. (c) Synthetic route of the cationic ligands.
Reagents and conditions: (i) EDC, HOBt, DIPEA, DCM, rt, 24 h;
(ii) MsCl, NEt3, DCM, 0 1C to rt, 24 h; (iii) N(CH3)2–R/EtOH, 35 1C,
48 h; (iv) NaBH4, EtOH/H2O, rt, 2 h.
Colloidal stability in physiological media is a significant
challenge for biological applications of QDs. The stability of
the cationic QDs were monitored by photoluminescence intensity
(lmax at 590 nm) for 8 h. Fig. 2a–c show that cationic QDs are
exceptionally stable in Tris buffer, media, and serum. As shown
in Fig. 2d the photoluminescence intensity and hence stability of
QD 1 was maintained for 6 h in phosphate buffers ranging from
pH 4 to pH 11. In addition, the QDs were stable in the saturated
NaCl solution after 8 h incubation (Fig. S3, ESIw).
spectrometric analysis revealed complete removal of both the
initial capping ligand (TOPO/TOP) and the intermediate
amphiphilic ligands (HS-C5-TEG) (Fig. S1, ESIw).
Further characterization of the particles was obtained by
transmission electron microscopy (TEM). As expected, the
micrographs showed primarily single dots, indicating that the
The stability of the QDs enables their application in cell
imaging. QD 1 was incubated with HeLa cell for 3 h (Fig. 3).
Fig. 1 (a) Absorption and (b) emission spectra of cationic QDs after
a two-step ligand exchange reaction: TOPO/TOP-QDs in toluene
(black), QD 1 in water (red), QD 2 in water (blue) and QD 3 in water
(green). (c) Hydrodynamic diameter and (d) surface zeta potential of
QD 1.
Fig. 2 Stability of cationic QDs in (a) Tris buffer, (b) media and (c)
serum with different QDs: QD 1 (black), QD 2 (red) and QD 3 (green).
(d) QD 1 in different pH phosphate buffers after incubation for 6 h.
c
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In summary we have demonstrated a two-step method for
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cationic QDs exhibit uniform dispersibility in aqueous
solution and high stability under biological conditions. The
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This research was supported by the NSF: (CHE-1025889,
VR), MRSEC facilities, and the DOE DE-FG02-04ER46141.
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Chem. Commun., 2011, 47, 3069–3071 3071