Luminescent 'On-Off' CdSe/ZnS quantum dot chemodosimeter for hydroxide based on photoinduced electron transfer from a carboxylate moiety

Article abstract of DOI:10.1007/s10895-013-1212-z

A CdSe-ZnS quantum dot (QD) has been surface functionalised by a place exchange reaction with p-mercaptomethyl benzoate synthesized by a three-step procedure. The resulting lumophore-spacer-receptor QD-conjugate was characterized by IR, UV-visible and fluorescence spectroscopy. The emission profile of the QD reveals a narrow emission peak centred at 542 nm. Addition of hydroxide to the solution containing the QD-conjugate results in quenching of the original fluorescence, which is attributed to a photoinduced electron transfer reaction from the electron-rich benzoate moiety to the QD valence band. This is the first reported example of fluorescent quenching of a CdSe-ZnS QD luminescence by an aryl carboxylate moiety.

Full text of DOI:10.1007/s10895-013-1212-z

J Fluoresc
DOI 10.1007/s10895-013-1212-z
ORIGINAL PAPER
Luminescent ‘On-Off’ CdSe/ZnS Quantum Dot
Chemodosimeter for Hydroxide Based on Photoinduced
Electron Transfer from a Carboxylate Moiety
Lara A. Gauci & Lindsay G. Kelland &
David C. Magri
Received: 25 December 2012 /Accepted: 4 March 2013
#
Springer Science+Business Media New York 2013
Abstract A CdSe-ZnS quantum dot (QD) has been surface
functionalised by a place exchange reaction with p-
mercaptomethyl benzoate synthesized by a three-step proce-
dure. The resulting lumophore-spacer-receptor QD-conjugate
was characterized by IR, UV-visible and fluorescence spec-
troscopy. The emission profile of the QD reveals a narrow
emission peak centred at 542 nm. Addition of hydroxide to the
solution containing the QD-conjugate results in quenching of
the original fluorescence, which is attributed to a photoin-
duced electron transfer reaction from the electron-rich benzo-
ate moiety to the QD valence band. This is the first reported
example of fluorescent quenching of a CdSe-ZnS QD lumi-
nescence by an aryl carboxylate moiety.
properties across the visible electromagnetic spectrum simply
by tuning the CdSe core size, which typically ranges between
1.5 nm to 5.0 nm in diameter [7]. Other notable properties of
QDs include the ability to excite at variable wavelengths due to
a continuous absorption spectrum, a high resistance to
photobleaching and a narrow, symmetrical emission band
[8–10].
Perhaps the most common luminescent sensing mecha-
nism utilized to date with QD systems is based on Forster
resonance energy transfer [1, 11–15]. An alternative mech-
anism growing in popularity is that based on photoin-
duced electron transfer (PET) [16–20]. The rational
design of chemosensors based on PET requires knowledge
of the redox potentials and the photophysical parameters
of the lumophore [21]. Furthermore, another requirement
is the strategic engineering of the sensing system based on
a lumophore-spacer-receptor design [22, 23]. Although the
scientific literature is rich with a plethora of examples
based on all organic-based fluorophore-spacer-receptor
PET models [24, 25], the same cannot be said of
lumophore-spacer-receptor systems based on semiconduc-
tor quantum dots as only recently have systematic studies
on the redox properties of QDs been reported [26–28].
These studies have provided insight into their physico-
chemical properties and demonstrated that the redox po-
tentials of QDs are dependent on the luminescent core
diameter, the protective shell thickness and the nature of
the passivating ligands. The determination of the thermo-
dynamic feasibility for PET reaction is rather simple once
accurate standard potentials are known. Common PET
receptor units that have thus far been demonstrated with
QDs include receptors for protons, such as amines [16]
and phenolates [13], crown ethers [18] for sodium and
potassium ions, and thioureas [19, 20] for anions. A
classic pH-sensitive receptor that is yet to be demonstrated
with QDs is the carboxylate moiety [29].
.
.
Keywords Quantum dot Luminescence Photoinduced
.
electron transfer Chemodosimeter
Introduction
Colloidal quantum dot (QD) nanocrystals have been demon-
strated to be a complementary alternative to organic dyes for
luminescent sensing applications [1, 2]. The most popular QDs
used for biological imaging and sensing purposes are those
composed of a CdSe core surrounded by a coating of ZnS
[3–6]. A distinct advantageous property of these inorganic-
organic hybrid particles over organic fluorophores, from a
synthetic point of view, is the ability to tailor the emission
:
L. A. Gauci D. C. Magri (*)
Department of Chemistry, University of Malta,
Msida MSD 2080, Malta
e-mail: david.magri@um.edu.mt
:
L. G. Kelland D. C. Magri
Department of Chemistry, Acadia University, 15 University Ave,
Wolfville NS B4P 2R6, Canada

J Fluoresc
In this work, we report the synthesis of a thiol pendant
organic ligand, its attachment on the quantum dot and char-
acterization of the photophysical properties of the resulting
QD-spacer-receptor conjugate by UV-visible and steady-state
fluorescence spectroscopy. The ligand consists of an aromatic
carboxylate moiety with a short methylene spacer anchored to
the QD surface via a pendant thiol ligand according to a
lumophore-spacer-receptor architecture. A single carbon-
atom spacer is purposely incorporated into the ligand design
in order to minimize the distance between the ZnS layer and
the organic receptor, thus maximizing the rate of PET and the
fluorescence switching factor. Based on prior knowledge of
the redox potentials of both the QD and the carboxylate donor,
and the fluorescence properties of a QD, we successfully
demonstrate for the first time, a CdSe/ZnS QD nano-sized
sensor as an ‘on-off’ chemodosimeter for hydroxide based on
a PET quenching mechanism.
Finnigan Trace DSQ spectrometer using a BP1 bonded phase
(polysiloxane) column. Samples were injected as solutions in
1:1 diethyl ether/acetone. Melting points were recorded on a
Griffin melting point apparatus and are uncorrected. UV-
visible absorption and fluorescence spectra were recorded on
a Jasco V-650 spectrophotometer and Jasco FP-8300 fluores-
cence spectrometer, respectively. Fluorescence experiments
were performed in emission mode with excitation slit width
at 2.5 nm and the emission slit width at 5.0 nm, unless stated
otherwise. All fluorescent measurements performed in chlo-
roform. The excitation wavelength was set at 370 nm.
Synthesis
4-Bromomethylbenzoic Acid (2)
A solution of p-toluic acid (1) (3.0 g, 22 mmol), N-
bromosuccinimide (4.1 g, 23 mmol), benzoyl peroxide
(0.25 g, 1.0 mmol) and chlorobenzene (30 ml) were refluxed
for 1 h. After cooling to ambient temperature, the product was
filtered by suction filtration and the resulting mixture was
extracted with hexanes (3×10 ml), collected by suction filtra-
tion, washed with deionised water (2×15 ml) and hexanes (2×
15 ml). The solid was recrystallized from 10 ml of ethyl
acetate to yield white-beige crystals (1.7 g, 40 %). m.p. 214–
216 °C; υmax (cm−1) 2812, 1672, 1608, 1421, 1284, 1224; 1H
NMR (250 MHz, CDCl3:DMSO-d6): δ 4.70 (2H, s), 7.55(2H,
d, J=8.3 Hz), 7.95(2H, d, J=8.3 Hz); m/z(EI): 216(M.+,6),
214(M.+, 7), 135(13), 134(100).
Experimental
Chemicals
All chemicals were purchased from commercial sources and
used as received: p-toluic acid (Acros Organics), N-
bromosuccinimide (GPR), chlorobenzene (BDH), benzyl per-
oxide (FLUKA), hexanes (Sigma Aldrich), ethyl acetate (Sig-
ma Aldrich), potassium thioacetate (Acros Organics),
methanol (Sigma Aldrich), hydrochloric acid (Sigma Al-
drich), potassium carbonate (Analar), sodium sulfate (Sigma
Aldrich), acetone (Sigma Aldrich), DMSO-d6 (Sigma Al-
drich), CDCl3 (Sigma Aldrich), CdSe/ZnS core quantum dot
with 2.4 nm core diameter and 8.0 nm total diameter (Evident
Technologies), chloroform (FLUKA), tetrabutylammonium
hydroxide (0.1 M in 2-propanol/methanol 10:1 (v/v))
(FLUKA), chloroform (Fischer Chemical).
Thioacetic Acid S-(4-Carboxybenzyl) Ester (3)
p-Bromomethylbenzoic acid (1.5 g, 7.0 mmol) and potassium
thioacetate (0.81 g, 7.1 mmol) were dissolved in 100 ml of
methanol and stirred for 48 h. The product was identified by
TLC (Rf of 0.47) in 1:1 ethyl acetate/hexanes solution. The
solvent was removed under reduced pressure and the product
collected by suction filtration in a sintered funnel. The white
crystals were washed with water to remove the excess potas-
sium thioacetate (0.69 g, 50 %). m.p. 153–156 °C (lit. 154–
155 °C) [30]; υmax (cm−1) 2630, 2364, 1676, 1608, 1419,
Instrumentation
1H and 13C NMR spectra were recorded at room temperature
on a Bruker AM 250 NMR spectrometer equipped with a
1H/13C 5 mm dual probe at 250.1 and 62.9 MHz, respectively.
Spectra are reported in ppm versus tetramethylsilane (δ=0.00)
1
1282, 1126; H NMR (250 MHz, CDCl3): δ 2.37 (3H, s),
4.16 (2H, s), 7.40(2H, d, J=8.1 Hz), 8.04(2H, d, J=8.1 Hz);
13C NMR (250 MHz, DMSO-d6 ) δ 30.5, 32.5, 129.0, 129.7,
143.0, 167.3, 194.7; m/z (EI): 210 (M.+ ,8), 150(100), 135(4),
134(28).
1
for H NMR and CDCl3 (δ=77.00) for 13C NMR. 1H NMR
spectra were recorded in 0.8 ml of a 1:1 solution of
CDCl3/DMSO-d6. 13C NMR samples of thioacetic acid
S-(4-carboxybenzyl) ester were recorded in DMSO-d6. Infra-
red (IR) spectra were recorded on a Shimadzu IR-Affinity 1
spectrometer. Bromomethylbenzoic acid and thioacetic acid
S-(4-carboxybenzyl) ester were recorded by attenuated total
reflectance (ATR) as solid samples. The IR spectra of the QD
and QD-spacer-receptor were recorded as a thin film on sodi-
um chloride windows. GC-MS was performed on a Thermo
p-Mercaptomethylbenzoate (4)
Thioacetic acid S-(4-carboxybenzyl) ester (100 mg, 0.48 mmol)
was dissolved in 8 mL of methanol and drops of 6 M HCl were
added. The solution was refluxed and stirred under a nitrogen
atmosphere for 4 h. The reaction was monitored by TLC in 1:9

J Fluoresc
ethyl acetate/ hexanes (RF of 0.28). The reaction was worked-
up with potassium carbonate solution, extracted with
dichloromethane (3×10 ml), washed with 10 ml deionised
water and dried with sodium sulphate. The solvent was re-
moved under reduced pressure by rotary evaporator to yield a
pale yellow oil (67 mg, 77 %). 1H NMR (250 MHz, CDCl3): δ
1.79 (1H, br t), 3.73(2H, s), 3.89 (3H, s), 7.37 (2H, d, J=
8.0 Hz), 7.97 (2H, d, J=8.0 Hz). The thiol peak was confirmed
by deuterium exchange with D2O.
refluxing the mixture in chloroform for 40 h. The resulting
QD-spacer-receptor conjugate was isolated after reiterative se-
quences of precipitation from acetone and centrifugation to
yield an orange solid. The conversion of the carboxylic acid
to the ester was purposely incorporated into the synthetic
1
scheme as esters are easier to characterize by H NMR and
IR than carboxylic acids (since they are not as hydroscopic).
Furthermore, the ester functionality can serve as a platform for
future covalent modification with other functional molecules.
1
The H NMR of the original QD has broad peaks at 0.8
and 1.2 ppm due to the methyl and methylene protons of the
TOPO ligands, which is a strongly co-ordinating ligand
used to passivate the QD surface. Trace resonances were
observed at 2.00, 2.80, 5.00, and 5.80 ppm, which are
attributed to octadecene, a non-coordinating high boiling
solvent used in the synthesis of the QD. Attempts to confirm
the place exchange by comparison of the 1H NMR spectrum
of the QD with the QD-conjugate were not successful. The
QD-conjugate was found to have low solubility in deuterat-
ed chloroform and in a 1:1 ratio of CDCl3/DMSO-d6 even
when heated.
We also compared the infra-red spectra of the QD and the
QD-conjugate. The CdSe/ZnS QD exhibit intense bands
about 2,900 cm−1 from the alkyl chains of the TOPO
ligands. These peaks were barely visible in the IR spectrum
of the QD-conjugate suggesting most of the TOPO ligands
were replaced with p-mercaptomethyl benzoate. The spec-
trum of the QD-conjugate exhibits broad, intense peaks at
1,100 cm−1 and 1,650 cm−1, which are attributed to the C–O
and C=O stretching, respectively. No such peaks were ob-
served in the IR of the QD, thus providing supporting
evidence that the ligand place exchange reaction was
successful.
The absorption spectra of the CdSe/ZnS core-shell QD in
chloroform is shown before and after treatment with the p-
mercaptomethylbenzoate Fig. 1. The QD-spacer-receptor con-
jugate displays a noticable different UV-visible spectrum with
a peak at 245 nm, a shoulder emerging at 275 nm and a tailing
of the absorbance past 370 nm. This absorption profile re-
sembles that of the model ligand and corresponds to the π to
π* transition due to the aromatic chromophore and the tailing
due to the n to π* transition of the carbonyl. As reported
elsewhere, the exciton peak at 521 nm is relatively unchanged
[13, 14]. Titration with 15 mM tetrabutylammonium hydrox-
ide (Bu4NOH) solution results in no change at the excitation
Ligand Exchange Reaction: QD with p-Mercaptomethylbenzoate
A solution of CdSe/ZnS core shell quantum dot (0.6 ml,
6 mg) in toluene and p-mercaptomethylbenzoate in chloro-
form (10 ml) were refluxed for 40 h. After cooling to
ambient temperature, the solvent was removed under re-
duced pressure using a rotary evaporator. The residue was
suspended in 5 ml of acetone and centrifuged for 5 min. The
supernatant was decanted and the procedure repeated five
more times. The residue was washed by suspending in 3 ml
acetone and centrifuged for 3 min. The supernatant solution
was decanted and the solid washed twice with acetone. The
product was vacuum dried yielding a bright orange powder
(18 mg).
Results
The multi-step synthesis of the designed ligand is shown in
Scheme 1. The ligand was synthesized in three steps beginning
with the bromination of p-toluic acid (1) with
bromosuccinimide (NBS) in 37 % yield. Treatment of 4-
bromomethylbenzoic acid (2) with potassium thioacetate in
methanol resulted in the formation of thioacetic acid S-(4-
carboxybenzyl ester (3) in 50 % yield. The carboxylic acid
was then reacted with dilute hydrochloric acid in methanol
resulting in the simultaneous deprotection of the thioester to
the thiol, and conversion of the carboxylic acid moiety to the
ester (4). Formation of the thiol was confirmed by a deuterium
exchange with deuterium oxide (D2O), which results in the
disappearance of the broad resonance at 1.79 ppm.
Deprotection of the thiol was subsequently done in situ (to
prevent oxidation of the thiol to the disulfide). The place
exchange reaction of the trioctylphosphine oxide (TOPO) li-
gands on the CdSe/ZnS QD surface was accomplished by
Scheme 1 The synthesis of the
p-mercaptomethylbenzoate
ligand

J Fluoresc
Fig. 3 Irradiation of the QD-conjugate with 365 nm UV light in
chloroform in the absence (left) and presence (right) of excess
tetrabutylammonium hydroxide solution
Fig. 1 Normalized absorption spectra of the CdSe/ZnS core-shell QDs
in chloroform before (red line) and after (blue line) the place exchange
reaction with p-mercaptomethylbenzoate refluxed for 40 h
of the same amount of Bu4NOH to an identical concen-
tration of QD solution does not result in switching ‘off’
the green fluorescence.
wavelength of 370 nm. The exciton peak was observed to shift
ever slightly to 523 nm while the absorbance remained
constant.
Scheme 2 illustrates the postulated mechanism responsi-
ble for quenching the fluorescence. Addition of hydroxide
results in hydrolysis of the ester to the carboxylic acid,
which subsequently reacts with another equivalent of hy-
droxide by an acid-base reaction. The resulting electron-rich
carboxylate facilitiates an efficient PET reaction on irradia-
tion of the QD core that readily quenches the fluorescence.
The carboxylate reacts as an electron donor that is less
positive than the valence band of the QD. In a separate
experiment, addition of Bu4NOH to the QD with TOPO
ligands resulted in essentially no loss of fluorescence.
Hence, this is convincing evidence that the source of fluo-
rescence quenching is the carboxylate ligand and not the
hydroxide ions. Attempts to revive the fluorescence of the
QD-conjugate with the addition of trifluoroacetic acid
proved unsuccessful. It is quite conceivable that at low pH
the thiol ligand is unstable and deabsorbs from the QD
surface [18].
The emission spectrum of the QD shows a narrow
and symmetrical band centred at 545 nm. Little change
is observed between the initial QD and QD-spacer re-
ceptor conjugate when excited at 370 nm [19]. Addition
of excess Bu4NOH results in the quenching of the
fluorescence and a concomitant bathochromic shift of
5 nm without the formation of any new bands, as
shown in Fig. 2, which is attributed to a change in
solvent polarity on addition of the 2-propanol/methanol
Bu4NOH solution. The fluorescence response upon ad-
dition of Bu4NOH is discernible to the naked eye.
Figure 3 highlights the dramatic change in the fluores-
cence before and after the addition of Bu4NOH as the
QD is switched from ‘on’ to ‘off’ from emitting a green
glow to becoming a clear, colourless solution. Addition
3
hv
Fig. 2 Fluorescence spectra of QD-conjugate in chloroform upon
addition of increasing amounts of tetrabutylammonium hydroxide in
10:1 (v/v) 2-propanol/methanol solution
Scheme 2 The reactivity of the QD-conjugate with hydroxide resulting in
quenching of the fluorescence emission by photoinduced electron transfer

J Fluoresc
The driving force for PET reactions (in kcalmol−1) was
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À
Á
ΔGPET ¼ 23:06 Eox ꢀ Ered ꢀ ΔEs ꢀ q2=εr
ð1Þ
which requires knowledge of the redox potentials of the
donor Eox and the acceptor (quantum dot) Ered, the
optical band gap of the quantum dot ΔEs and the Cou-
lombic term q2/εr (where q is the elementary charge, ε is
the dielectric constant and r is the distance between the
donor and acceptor. The oxidation potential of the car-
boxylate is 1.22 V [31] versus SCE and the reduction
potential of the QD is −0.96 V [27] versus SCE and the
band gap is ca. 2.4 V [27]. The total radius of the QD is
4.0 nm. Since the core radius is 1.2 nm, the ZnS layer is
estimated to be 2.8 nm in thickness. Given the dielectric
constant of chloroform is 4.8, the Coulombic term pro-
vides a negligible contribution to the overall free energy
on the order of less than 0.1 V. Hence, the estimated
driving force for PET is~−0.2 V or −5 kcalmol−1.
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Conclusions
In this study, a CdSe/ZnS QD was surface functionalized
with p-mercaptomethylbenzoate according to a lumophore-
spacer-receptor format. The QD-conjugate exhibits modu-
larity as the absorbed ligands retain their chemical proper-
ties. Addition of increasing amount of tetrabutylammonium
hydroxide results in efficient quenching of the fluorescence
due to hydrolysis of the ester via a carboxylic acid interme-
diate to the aryl carboxylate, and subsequently to photoin-
duced electron transfer from the electron-rich carboxylate to
the QD lumophore core. Attempts to restore the fluores-
cence upon addition of acid were unsuccessful. A remedy
in the future might be to use bidentate dithiolane [32] or
carbodithioate ligands [33], which have greater stability in
acidic media.
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Acknowledgements We would like to acknowledge financial sup-
port from the University of Malta, Acadia University, and European
Cooperation in Science and Technology (COST CM1005). Prof.
Robert M. Borg is thanked for assistance with the acquisition of the
NMR spectra.
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