Synthesis of cadmium selenide quantum dots modified with thiourea type ligands as fluorescent probes for iodide ions

Article abstract of DOI:10.1039/b806485g

Highly luminescent and stable CdSe quantum dots (QDs) modified with 4-substituted pyridine type ligands containing thiourea groups: 1-(4-fluorobenzoyl)-3-(5-(pyridin-4-yl)-1,3,4-thiadiazol-2-yl)thiourea (PyTU); were prepared through a ligand exchange process, and were characterized by transmission electron microscopy (TEM), fluorescence spectroscopy, UV-vis spectroscopy and FT-IR spectroscopy. The synthesized nanocomposites show higher fluorescence (FL) intensity and are more stable in comparison with original QDs. The PyTU-CdSe QDs allow a highly sensitive determination of iodide via significant FL intensity quenching. Under optimal conditions, the relative FL intensity is decreased linearly with increasing iodide concentration in the range 0-50 μM with a detection limit of 1.5 × 10^-9 M, while the FL intensity of PyTU-CdSe QDs to other anions including F^-, Cl^-, Br^-, SCN^-, HCOO^-, CH ₃COO^-, HSO₃^-, and C ₂O₄^2- is negligible. It is found that the decreased luminescence intensity of the PyTU-CdSe QDs dependent on the concentration of iodide is best described by a Stern-Volmer equation. The possible mechanism is discussed. The Royal Society of Chemistry.

Full text of DOI:10.1039/b806485g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis of cadmium selenide quantum dots modified with thiourea type
ligands as fluorescent probes for iodide ions†
*
Haibing Li, Cuiping Han and Liang Zhang
Received 16th April 2008, Accepted 3rd July 2008
First published as an Advance Article on the web 19th August 2008
DOI: 10.1039/b806485g
Highly luminescent and stable CdSe quantum dots (QDs) modified with 4-substituted pyridine type
ligands containing thiourea groups: 1-(4-fluorobenzoyl)-3-(5-(pyridin-4-yl)-1,3,4-thiadiazol-2-yl)
thiourea (PyTU); were prepared through a ligand exchange process, and were characterized by
transmission electron microscopy (TEM), fluorescence spectroscopy, UV-vis spectroscopy and FT-IR
spectroscopy. The synthesized nanocomposites show higher fluorescence (FL) intensity and are more
stable in comparison with original QDs. The PyTU–CdSe QDs allow a highly sensitive determination
of iodide via significant FL intensity quenching. Under optimal conditions, the relative FL intensity is
decreased linearly with increasing iodide concentration in the range 0–50 mM with a detection limit of
1.5 ꢀ 10ꢁ9 M, while the FL intensity of PyTU–CdSe QDs to other anions including Fꢁ, Clꢁ, Brꢁ, SCNꢁ,
HCOOꢁ, CH3COOꢁ, HSO3ꢁ, and C2O42ꢁ is negligible. It is found that the decreased luminescence
intensity of the PyTU–CdSe QDs dependent on the concentration of iodide is best described by a
Stern–Volmer equation. The possible mechanism is discussed.
Semiconductor quantum dots (QDs), due to their novel
Introduction
optical, electrical, and catalytic properties, have gained
increasing attention during the past decade.11 Compared with
organic fluorophores, QDs have several important advantages
including size-selection, narrow emission spectra, resistance to
photobleaching, improved brightness, and broad excitation
spectra.11 The uses of QDs have been demonstrated in analytical
chemistry as high sensitivity fluorescent sensors recently.12 Since
Chen and Rosenzweig first demonstrated luminescent quantum
dot probes for Cu2+ and Zn2+ ions by utilizing functionalized CdS
QDs capped with different organic ligands in aqueous media,13
the use of QDs or nanoparticles as selective chemosensors for
metal ions has been an active research field in recent years. For
The selective detection of anions is of importance in environ-
mental monitoring, medicinal diagnostics, and the analysis of
biological samples.1 Among anions, iodide plays an important
role in several biological activities such as neurological activity
and thyroid function. Hence the iodide content of urine and milk
is often required for nutritional, metabolic, and epidemiological
studies of thyroid disorder.2 Some analytical methods have been
applied to determine the iodide content, such as ICP-MS,3
capillary electrophoresis,4 iodide-selective electrodes5 etc.
Nowadays, fluorescent sensors have been extensively developed
with high sensitivity and selectivity. Many investigations have
been conducted to make fluorescent sensors for anions.6
However, few reports have been explored for iodide7 and
discrimination between iodide and chemically close anions
presents a challenge. The relatively strong hydrogen bonding of
urea and thiourea groups has been used in the development of
neutral receptors, because the hydrogen bond is directional in
character, and correct orientation of the hydrogen bond donors
can provide selective anion recognition.8 It has also been repor-
ted that some of these urea or thiourea derivatives could be used
in anion sensing based on absorption and fluorescence spectra9
or ion-selective electrodes.10 However, there seems to be no
report, to our knowledge, of a urea or thiourea group as a part of
the framework, modified on the surface of quantum dots and
used as fluorescent sensors for the determination of anions.
example, Isarov and Chrysochoos,14 Gattas-Asfura and
´
Leblanc,15 and Konishi and Hiratani16 and coworkers reported
CdS QD-based sensors for the determination of Cu2+ in the
presence of other biological metallic ions. Also, CdSe QDs
modified with mercaptoacetic acid and bovine serum albumin
(BSA) were assayed for the analysis of Ag+.17 More recently, we
demonstrated carnitine modified CdSe/ZnS QDs as selective
chemosensors for Cd2+.18 Meanwhile, the employment of QDs or
nanocrystals for determination of anions was almost unexplored
until 2004, when Sanz-Medel and coworkers reported the
synthesis of red photoluminescent CdSe quantum dots, which
were modified with tert-butyl-N-(2-mercaptoethyl)-carbamate
(BMC), as a selective fluorescent probe for the determination of
free cyanide in methanol.19 To our knowledge, there are few
reports about quantum dots with a selective fluorescence
response for iodide ions.20
We are interested in constructing a novel quantum dots based
chemosensor for iodide ions. To address this goal, semi-
conductor nanocrystals have to be further modified by appro-
priate functional groups to allow significantly diverse properties
and potential sensor applications. To date, different synthetic
Key Laboratory of Pesticide and Chemical Biology (CCNU), Ministry of
Education, College of Chemistry, Central China Normal University,
Wuhan 430079, P. R. China. E-mail: lhbing@mail.ccnu.edu.cn
† Electronic supplementary information (ESI) available: Detailed
synthetic procedure and characterization of 1-modified QDs. See DOI:
10.1039/b806485g
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Scheme 1 Synthesis of (a) PyTU and (b) PyTU–CdSe QDs.
strategies have been investigated to modify QDs. One of the most
frequently used approaches is the ligand exchange method, which
uses organic ligands to replace the original surface ligands, such
as thiol (–SH) ligands,21,22 and other non-thiol organic ligands
(4-substituted pyridine, oligomeric phosphine, etc.).23 Skaff and
Emrick synthesized water-soluble CdSe QDs modified with
4-substituted pyridine with polyethylene glycol (PEG) chains.24
This new ligand architecture gives rapid access to modified
quantum dots, where neither ionization nor the use of thiols are
required.
electron micrographs were recorded by a JEOL-JEM 2010
transmission electron microscope operating at 200 kV. All
fluorescence measurements were made with a FluoroMax-P
fluorescence spectrophotometer equipped with a 1 cm quartz cell.
The IR spectra were detected by a Thermo Nicolet NEXUS IR
spectrometer with KBr disks. The ultrasonic bath was a SB120D
Supersonic instrument.
Synthesis of 1-(4-fluorobenzoyl)-3-(5-(pyridin-4-yl)-1,3,4-
thiadiazol-2-yl)thiourea (PyTU)
As part of our ongoing studies on preparing QDs based
sensors for iodide recognition, here we present the synthesis and
anion binding studies of CdSe QDs modified with 4-substituted
pyridine type ligand containing thiourea groups as recognition
sites for anions. The strategy for the design of the ligand is based
upon three ideas. First, N–H bonds in the thiourea type ligand
aligned in parallel may effectively make a complex with an anion.
Second, the flexibility of the ligand on the surface of QDs would
allow encapsulation of large anions such as iodide. Third, the end
pyridine group of ligand affords the binding site for QDs.
In this paper, we report the synthesis of QDs modified with
the pyridine derivative 1-(4-fluorobenzoyl)-3-(5-(pyridin-4-yl)-
1,3,4-thiadiazol-2-yl)thiourea (PyTU) having a thiourea group
(Scheme 1) as fluorescent sensors for iodide.
To a solution of 2-amino-5-(pyrid-4-yl)-1,3,4-thiadiazole 1
(9 mmol) in dry DMF (10 mL) was added 4-fluorobenzoyl
isothiocyanate 2 (9.9 mmol) at room temperature. After the
reaction mixture was stirred and heated at 50 ꢂC for 3 h under an
argon atmosphere, the mixture was concentrated and purified by
column chromatography on silica gel to give the 1-(4-fluoro-
benzoyl)-3-(5-(pyridin-4-yl)-1,3,4-thiadiazol-2-yl)thiourea
(PyTU) as a pale yellow powder with a yield of 85%. m.p.
ꢂ
1
>300 C; H NMR (400 MHz, DMSO-d6) d 7.437 (t, 2H, J ¼
8.8 Hz), 7.959 (d, 2H, J ¼ 4.8 Hz), 8.244 (t, 2H, J ¼ 7.6 Hz), 8.750
(d, 2H, J ¼ 4.8 Hz), 10.871 (s, 1H, NH), 13.391 (s, 1H, NH); IR
(KBr) v: 3266, 3133 (N–H), 1705, 1676 (C]O) cmꢁ1. ESI-MS:
m/z ¼ 359 (M+) Anal. calcd for C15H10FN5OS2: C 50.13, H 2.80,
N 19.49; found C 50.15, H 2.85, N 19.43.
Experimental
Preparation of CdSe QDs
Materials and apparatus
CdSe quantum dots were synthesized using CdO as a precursor
via the procedure described by Qu and Peng,25 although some
slight modifications were made here. Briefly, 0.0127 g of CdO
(0.1 mmol), 0.1140 g of stearic acid (0.4 mmol), 1.94 g of ODA
All chemicals used were of analytical grade or of the highest
purity available. Cadmium oxide (99.99%), trioctylphosphine
oxide (TOPO, 99%), trioctylphosphine (TOP, 90%), and
selenium (powder, 99.99%) were purchased from Aldrich
(Milwaukee, WI, USA). n-Octadecylamine (ODA) was obtained
from Alfa Aesar (Karlsruhe, Germany). All anion samples were
obtained from Shanghai Chemical Factory, China. All anion
samples were potassium salts and prepared with anhydrous
ethanol.
ꢂ
and 1.94 g of TOPO were mixed whilst heating at 300–320 C
under argon flow for 5 min, and CdO was completely dissolved in
ODA and TOPO. The solution of selenium–TOP [0.079 g
(2 mL)ꢁ1] was swiftly injected and a change of solution color to
red was observed. After injection, CdSe nanocrystals were left to
ꢂ
grow for about 4 min at 250 C. Then, the mixture was cooled
UV-vis absorption spectra were acquired on a UV-2501
UV-vis spectrometer (SHIMADZU CORPORATION). Fluo-
rescence spectra were taken on a FluoroMax-P luminescence
spectrometer (HORIBA JOBIN YVON INC.). Transmission
down to room temperature. The precipitate was collected by
centrifugation at 12 000 rpm for 5 min followed by washing with
methanol several times. Then, the precipitate was re-dispersed in
typical nonpolar solvents such as chloroform and toluene.
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Finally, the CdSe quantum dot solutions at a concentration of
0.5 mg mLꢁ1 were stored.
about 23%. The TEM images (Fig. 2) demonstrate that the sizes
of the modified QD particles and TOPO QDs are virtually
identical, indicating that the modified particles are monodisperse
and uniform.
Synthesis of PyTU–CdSe QDs
Fig. 3 depicts typical transmission IR spectra of parent PyTU,
the PyTU complex of CdCl2, PyTU-capped CdSe QDs, and
TOPO-capped CdSe QDs, respectively. Comparing Fig. 3a
with 3c, it is found that the gross resemblance in both spectral
PyTU exchanged TOPO to give PyTU–CdSe QDs. Briefly, 2 mL
of TOPO–CdSe QDs chloroform solution (0.5 mg mLꢁ1) was
added to 2 mL of PyTU ethanol solution (1 ꢀ 10ꢁ4 M), the
mixture was refluxed at 80 ꢂC for 4 h. The PyTU–CdSe QDs were
purified by precipitation and centrifugation in anhydrous
methanol. The resulting PyTU–CdSe QDs were stored in
chloroform and ethanol (v : v ¼ 4 : 1) mixture solution at room
temperature for further investigations.
Results and discussion
Synthesis and characterization of PyTU
The synthetic route is depicted in Scheme 1. Reacting 2-amino-5-
(pyrid-4-yl)-1,3,4-thiadiazole 1 with 4-fluorobenzoyl isothio-
ꢂ
cyanate 2 in DMF at 50 C for 3 h gives the desired compound
PyTU with a yield of 85%, which is confirmed by 1H NMR, IR,
Fig. 2 TEM images of (A) original CdSe QDs in chloroform, and (B)
PyTU–CdSe QDs in a chloroform and ethanol mixture solution (v : v ¼
4 : 1). Scale bars are all 50 nm.
ESI-MS and elemental analysis.
Characterization of PyTU–CdSe QDs
Fig. 1 shows the emission spectra of PyTU–CdSe QDs in
chloroform and ethanol (v : v ¼ 4 : 1) solution obtained by
excitation at 450 nm. In comparison with the original QDs in
chloroform, the fluorescence (FL) intensity of PyTU–CdSe QD is
increased with a blue-shift of 4 nm, which was detected under the
same conditions. The absorption spectra of PyTU–CdSe QDs in
chloroform and ethanol (v : v ¼ 4 : 1) solution and TOPO–CdSe
QDs in chloroform are shown in the inset. As can be seen from
the inset, no distinct difference in the positions and peak widths
was found, which suggests that the modification process retained
the optical properties of the CdSe QDs. The quantum yield (QY)
determined by using rhodamine B as a criterion (QY ¼ 89%) is
Fig. 1 The fluorescence spectra of (a) PyTU–CdSe QDs in a chloroform
and ethanol mixture solution (v : v ¼ 4 : 1) and (b) TOPO-capped CdSe
QDs in chloroform (Ex: 450 nm). The inset shows UV-vis spectra of (a)
PyTU–CdSe QDs in a chloroform and ethanol mixture solution (v : v ¼
4 : 1) and (b) TOPO-capped CdSe QDs in chloroform taken under the
same conditions.
Fig. 3 IR spectra of (a) pure PyTU, (b) the complex of PyTU and CdCl2,
(c) a PyTU–CdSe QDs sample, and (d) TOPO-capped CdSe QDs.
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features and peak positions for several vibrational modes
guarantees a successful attachment of PyTU onto CdSe QDs.
The characteristic peaks of pyridine located at 1535 and
1409 cmꢁ1 are attributed to the symmetric and asymmetric ring
breathing and the C–C and C–N stretching modes, respectively.
Pyridine is a good electron donor due to the presence of a pair of
isolated nitrogen electrons. CdSe quantum dots show a good
electron affinity, meaning that the nitrogen readily coordinates
with them to form a stable complex. As a consequence, the
characteristic bands of pyridine were shifted to 1564 and
1414 cmꢁ1 for the PyTU QD complexes. And the IR spectrum of
the PyTU–Cd2+ complex (Fig. 3b) further proved PyTU was
bound onto the surface of the QDs. The characteristic band
associated with P]O at 1083 cmꢁ1 for TOPO nearly disappears
in the PyTU–CdSe QD spectra, suggesting that PyTU-capped
particles have very few adsorbed TOPO ligands remaining.
31P NMR measurements of the QDs in CDCl3 provide more
information about the fate of the TOPO ligand on the particle
surface when the particles were exposed to the PyTU. According
to literature,26 high-resolution 31P NMR measurements of
TOPO-capped CdSe quantum dots in solution usually exhibit
several broad signals associated with the bound TOPO ligand.
The complexity of the NMR signal suggests that a variety of
phosphorus chemical environments are available to TOPO
ligands bound to the QD surface, which may include bound
dimers of TOPO.27 In the experiments on QDs in a CDCl3
solution (ca. 5 mg mLꢁ1) in the absence of PyTU, no 31P signals
was observed, presumably because of the low concentration of
the nanoparticles. However, after PyTU (40 mg) was added with
ultrasonic irradiation for 5 min, a sharp 31P signal appeared at
47.5 ppm, which corresponds to free TOPO ligand in CDCl3.26
This result emphasizes the fact that ligand replacement occurred.
Fig. 4 Fluorescence spectra change of PyTU QDs upon addition of
potassium salts (5 ꢀ 10ꢁ5 M) with excitation at 450 nm. The inset shows
the fluorescence image of PyTU QDs, and PyTU QDs with iodide, which
were taken under the illumination of a UV light (365 nm). Typically,
2 mL of various potassium salt–ethanol solutions were added to 0.5 mL
of PyTU QDs solution (chloroform and ethanol, v : v ¼ 4 : 1), and then
the solution was mixed well for 10 min and tested.
wave function in the dots and therefore an increase of the size
quantization (size-dependent increase in band gap) of the QDs
was reported.28 This is a possible mechanism to account for the
observed quenching of luminescence emission by iodide. It was
found that iodide quenches the fluorescence of PyTU–CdSe QDs
with a concentration dependence that is best described by
a Stern–Volmer type equation:
Detection of iodide
I0/I ¼ 1 + KSV [S]
The FL emission of PyTU–CdSe QDs turned out to be highly
sensitive to the presence of iodide ions. The changes in FL
intensity of PyTU–CdSe QDs upon addition of a particular
anion are shown in Fig. 4, and the fluorescence ratio (I0 ꢁ I)/I0 is
displayed in Fig. 5. As can be seen from Fig. 4 and 5, it is clear
that there was marked quenching upon addition of iodide, and
The dependence of I0/I on [S], where [S] is the iodide concen-
tration and I is the FL intensity of PyTU–CdSe QDs at given
iodide concentrations, is shown in Fig. 6, inset. A good linear
relationship (R ¼ 0.9982) was found between the relative FL
no significant effect was observed in presence of Fꢁ, Clꢁ, Brꢁ, Iꢁ,
2ꢁ
SCNꢁ, HCOOꢁ, CH3COOꢁ, HSO3ꢁ, and C2O4
.
Fig. 6 shows the effect of increasing concentration of iodide on
the FL emission of PyTU–CdSe QDs. As can be seen, the QDs
fluorescence emission was progressively quenched by addition of
iodide. Also, a slight blue-shift could be observed. Results found
here could also suggest some possible quenching mechanisms of
the luminescence of QDs by iodide: first, the interaction of the
iodide with the functional groups of the QDs modified surface
would bring about a disruption by iodide of the hydrogen bonds
between the PyTU groups of the surface-modified CdSe QDs.
This would result in a destabilisation of the QDs in solution, with
a modification of their luminescence. Also, as a result of the very
high surface-to-volume ratio of these nanoparticles, iodide
adsorption on this large surface could be expected. The similar
phenomena that the adsorption of negatively charged cyanide on
the surface of films of CdSe quantum dots resulted in an
increased electron localization due to compression of the electron
Fig. 5 Fluorescence ratio (I0 ꢁ I)/I0 of PyTU–CdSe QDs and 1-modified
CdSe QDs upon addition of 5 ꢀ 10ꢁ5 M relevant anions (from 1 to 9: Fꢁ,
Clꢁ, Brꢁ, Iꢁ, SCNꢁ, HCOOꢁ, CH3COOꢁ, HSO3ꢁ, C2O42ꢁ).
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characterization of 1-modified QDs is given in the ESI†). No
significant changes in the fluorescence intensity were observed in
the typical experiment (shown in Fig. 5), indicating that there is
little interaction between 1-modified CdSe QDs and iodide. The
result clearly indicates that the thiourea group and carbonyl
group of PyTU play important roles for iodide recognition.
Conclusions
Studies to obtain functionalized nanoparticles of CdSe semi-
conductor QDs have been carried out. The novel PyTU ligand
was designed and synthesized. PyTU and CdSe quantum dots
have been successfully combined to develop a novel and highly
sensitive luminescence nanoprobe for optical recognition of
iodide. Quenching of the luminescence emitted by the synthe-
sized nanoparticles allows the detection of iodide concentrations
as low as 1.5 ꢀ 10ꢁ9 M, thus affording a very sensitive detection
system for this chemical species. A disadvantage of the synthe-
sized QDs, however, is that they are insoluble in water, a clear
limitation for environmental applications. However, the possi-
bility of modifying the chemistry of the core–shell system in the
QDs, by adding different water-soluble organic ligands, is
already in progress in our laboratory. In any case, the synthe-
sized surface-functionalized nanoparticles have considerable
potential as optical nanoprobes and are opening new applica-
tions as colorimetric probes, which allows a rapid quantitative
assay of iodide. Such optosensing strategies are in progress in our
laboratory and could eventually allow the development of highly
sensitive and selective optosensors for other analyte determina-
tions based either on fluorescence intensity or on colorimetric
shift induced by the analyte on the surface-modified QDs.
Fig. 6 The effect of increasing concentrations of iodide on the FL
intensity of PyTU–CdSe QDs (from a to g: 0, 5 ꢀ 10ꢁ8, 5 ꢀ 10ꢁ7, 5 ꢀ 10ꢁ6
,
1 ꢀ 10ꢁ5, 2.5 ꢀ 10ꢁ5, 5 ꢀ 10ꢁ5 M). The inset shows a Stern–Volmer plot of
the iodide concentration dependence of the FL intensity of PyTU–CdSe
QDs with a 0.9982 correlation coefficient. Typically, 2 mL of various
concentrations of iodide–ethanol was added to 0.5 mL of PyTU–CdSe
QDs solution (chloroform and ethanol, v : v ¼ 4 : 1), and then the
solution was mixed well for 10 min and tested.
intensity and the concentration of iodide in the range 0–50 mM.
The quenching constant KSV is found to be 3.9 ꢀ 104 Mꢁ1. The
detection limit, calculated following the 3s IUPAC criteria, is
lowered to 1.5 ꢀ 10ꢁ9 M (0.19 mg Lꢁ1).
The preference for iodide suggests that the cavity formed by
C]O/H–N and C]S/H–N of PyTUs on the surface of QDs
is more complementary to the sizes of the iodide ion than to the
size of other anions. In general, halide anions tend to associate
with receptors according to their basicity29 (i.e., in the order Fꢁ >
Clꢁ > Brꢁ > Iꢁ). However, many examples of size discrimination
of halides due to the size of receptor binding site have been
reported.30 As anions have diverse geometries, complementarity
between the receptor and anion is crucial in determining selec-
tivity.31 The complementarity between the receptor and halides is
mostly achieved by the size of the receptor binding site due to the
spherical shape of halides. We propose that the cavity in
a PyTU–CdSe QD fits well with the size of an iodide ion, thus,
iodide has a greater effect on the fluorescence emission of PyTU–
CdSe QDs (Scheme 2).
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (20602015), (20772038), the
Program for Distinguish Young Scientist of Hubei Province
(2007ABB017) and the Program for Chenguang Young Scientist
for Wuhan (200750731283).
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