Design and Synthesis of Nanosensor Based on CdSe Quantum Dots Functionalized with 8-Hydroxyquinoline: a Fluorescent Sensor for Detection of Al3+ in Aqueous Solution

Article abstract of DOI:10.1007/s10895-018-2238-z

A novel nanosensor based on CdSe quantum dots (QDs) capped with 8-hydroxyqunoline (HQ) was developed for Al³⁺ ions determination in aqueous solutions. The method is based on the fluorescence enhancement of the HQ functionalized QDs in the presence of Al³⁺ ions, due to the strong interaction between Al³⁺ and HQ. Prepared nanosensor exhibited an acceptable selectivity and sensitivity for Al³⁺ ions in the presence of other metal ions. Plot of Log(I/I₀) against Log[Al³⁺] shows a good linearity in the range of 0.02–3.0?mM, and the method could be used for detection of Al³⁺ ions concentration in aqueous solutions.

Full text of DOI:10.1007/s10895-018-2238-z

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2238-z
ORIGINAL ARTICLE
Design and Synthesis of Nanosensor Based on CdSe Quantum Dots
Functionalized with 8-Hydroxyquinoline: a Fluorescent Sensor
3+
for Detection of Al in Aqueous Solution
1
1
1
Bagher Eftekhari-Sis & Khadijeh Samadneshan & Saleh Vahdati-Khajeh
Received: 15 February 2018 /Accepted: 11 May 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
3
+
A novel nanosensor based on CdSe quantum dots (QDs) capped with 8-hydroxyqunoline (HQ) was developed for Al ions
determination in aqueous solutions. The method is based on the fluorescence enhancement of the HQ functionalized QDs in the
presence of Al ions, due to the strong interaction between Al and HQ. Prepared nanosensor exhibited an acceptable
selectivity and sensitivity for Al ions in the presence of other metal ions. Plot of Log(I/I ) against Log[Al ] shows a good
linearity in the range of 0.02–3.0 mM, and the method could be used for detection of Al ions concentration in aqueous
3
+
3+
3
+
3+
0
3
+
solutions.
3
+
.
.
.
.
Keywords Nanosensor CdSe quantum dots 8-Hydroxyquinoline Fluorescent Al
Introduction
abdominal pain, learning disabilities and exhaustion [4, 5]. At
the moment, in many industrial activities many various sensi-
tive determination methods such as atomic absorption spec-
troscopy (AAS) [6], inductively coupled plasma atomic emis-
sion spectroscopy (ICP-OES) [7] and inductively coupled
plasma mass spectrometry (ICP-Ms) [8] have been used for
recognition of heavy metal ions in aqueous solution, but due
to the complicated and expensive methods and instruments,
these are not affordable. Therefore, the development of new
strategies with operational simplicity, rapid response and non-
destructive character are attractive in the material science
[9–13]. In this regard, design and synthesis of nanosensors
have received extensive attention due to the specific electronic
and optical properties.
Various pollutants have been released into the environment as
a result of industrialization of society. Heavy metal ions are
one of the most important pollutants, and their detection is
essential in view point of biology and ecology. Living organ-
isms need varying amounts of heavy metal ions such as cobalt,
iron, zinc, manganese, and molybdenum. Aluminum is the
third most prevalent and important metallic element in the
earth’s crust, which widely used in the industrial such as light
alloys, pharmaceuticals, and water purification [1]. However,
aluminum is abundantly around our life and acidification of
3
+
environment causing to increase the amount of Al ions,
which is toxic metals for the environment and human beings
[
2]. The World Health Organization (WHO) recommends that
Recently, quantum dots (QDs) have been developed in
sensors due to having a property known as quantum confine-
ment, which was first reported by Ekimov [14]. On the other
hand, due to the photobleaching and photooxidation of QDs
surface atom, modification of QDs surface with suitable or-
ganic ligand prevent of this phenomena and it is a very deli-
cate subject for the make them appropriate for detection of
various heavy metals [15–17]. In 2002, Chen and coworkers
for the first time used CdSe QDs functionalized with L-cyste-
ine and thioglycerol for detection of zinc and copper [18].
After this report, the impact of different ligands has been ex-
amined on the selectivity of QDs in identifying of metal ions
the bearable weekly dietary human intake of aluminum is
7
in the human body can lead to serious afflictions like
Alzheimer’s, liver disease, skin problems, digestive problems,
mg kg− body weight [3]. Excessive amounts of aluminum
1
*
Bagher Eftekhari-Sis
eftekharisis@maragheh.ac.ir; eftekhari.sis@gmail.com
1
Department of Chemistry, University of Maragheh,
Maragheh 55181-83111, Iran

J Fluoresc
and bioorganic species [19, 20]. Also, various sensitive
chemosensor base on 8-hydroxyquinoline derivatives as a de-
sirable fluorophore and binding moiety was developed for
After completion of the reaction, 10 mL of saturated aqueous
solution of NaCl was added to the mixture of reaction to
precipitated of product. Obtained brown solid was filtered
and washed several times with distilled water and dried under
vacuum. FTIR (KBr): ν 3409 (N―H and O―H), 3059
(CH ), 2821–2932 (CH ), 1657 (N―H bending), 1566
2
+
2+
2+
3+
3+
selective detection of Hg , Cd , Zn , Cr and Al
21–25]. Therefore, the combination of 8-hydroxyquinoline
[
with QDs nanoparticles can lead to development of a novel
nanosensor by unique properties for investigation of detection
metal ions.
Ar
2
(C=N), 1507 (C=C), 1464 (CH bending) 1325 (O―H bend-
2
ing), 1238 (C―O), 1163, 1080 (C―N), 833 (N―H wagging
−1
In continuing our works on the nanosensors [17, 20, 26,
vibrations), 710 (O―H)OOP cm .
2
7], in this paper, we developed a nanosensor based on CdSe
QDs functionalized with 8-hydroxyquinoline for detection of
Al ions in aqueous solution.
Synthesis of Tioglycolic Acid (TGA) Capped CdSe QDs
CdSe QDs was synthesized according to the literature.
3
+
[29–32] In summary, selenium precursor was prepared in a
Experimental
three-neck round-bottom flask by dissolving 0.3 mmol of Se
powder and 1 mmol of NaBH in 10 mL of deionized water,
4
Materials and Apparatus
under nitrogen atmosphere and constant stirring at 40 °C un-
der N atmosphere. In a second reaction flask, 0.4 mmol
2
All chemicals were purchased from Merck and Sigma-
Aldrich and used without any further purification.
Solvents were used as received from commercial sup-
pliers. FT-IR spectra were obtained using a Perkin
Elmer 781 spectrometer (USA). UV-Vis spectra were
recorded using Shimadzu UV-1800 spectrophotometer
(0.08 g) of CdCl and 1.2 mmol (83.8 μl) of tioglycolic acid
(TGA) was added to 20 mL of distilled water; the pH was
adjusted to 10.5 by addition of NaOH (0.1 M) solution and
2
stirred under N atmosphere. Next, the above selenium pre-
2
cursor solution was added to the latter reaction flask, and the
resulting solution was stirred and refluxed for 1 h. After com-
pletion of the reaction, EtOH was added into the mixture to
precipitate a red solid. Finally, solid was separated by centri-
fuge and washed with EtOH three times. The solid was soni-
cated and dispersed in 10 mL deionized water and kept at
below 4 °C for future uses. FTIR (KBr): ν 3424 (O―H),
2854–2929 (CH2), 1634, 1559 (CO ), 1463 (CH bending),
(
Japan) and fluorescence spectra were measured using
JASCO-FP-750, Rev. 1.00 fluorometer (United
Kingdom). TEM images were recorded using a Philips
CM120 microscope (United States). SEM images were
recorded using a MIRA3 TESCAN microscope
(
Czech Republic).
2
2
−1
5
88 (Cd―Se) cm .
Synthesis of 5-(Chloromethyl)quinolin-8-ol
Hydrochloride (CHQ)
Synthesis of Nanosensors Based on CdSe QD
Functionalized with AHQ
CHQ was synthesized according to the literature [28].
Briefly, a stream of HCl gas was bubbled into the solution
of 8-hydroxyquinoline (1.45 g, 10 mmol) in concentrated
HCl (5 mL) and aqueous solution of formaldehyde 37%
In a 10 mL flask, 2 mL of the dispersion of TGA-capped
CdSe QDs in deionized water, 10 mg EDC and 5 mg N-
hydroxysuccinimide (NHS) were added to activation of
carboxylic acid groups and stirred at room temperature
for 30 min. Then 30 mg of AHQ was added to reaction
mixture and stirred at room temperature for 24 h. Then
solid was separated by centrifugation and washed with
EtOH for several times. The solid was dispersed in
10 mL deionized water by sonication and kept at below
4 °C for future uses.
(
2 mL) under stirring at 50 °C for 8 h. The reaction mix-
ture was then filtered and the residual solid was washed
with concentrated HCl (4 × 5 mL) and dried under vacu-
um. Compound was obtained as a yellow solid (1.3 g,
6
1
7
9%). FTIR (KBr): ν 2100–3100 (O―H), 2835 (CH2),
547, 1597 (C=C), 1497 (CH )OOP, 1200–1300 (C―O),
2
−
1
85, 725 (C―Cl) cm .
Synthesis of 5-((2-aminoethylamino)methyl)
quinolin-8-ol (AHQ)
Fluorescence Spectrometry
Selectivity of nanosensor towards different metal ions: Stock
+
+
2+
2+
In a 20 mL flask, 5-(chloromethyl)quinolin-8-ol hydrochlo-
ride (0.194 g, 1 mmol) was dissolved in DMF (2 mL) and
then ethylenediamine (EDA, 0.6 g, 10 mmol) was added to
mixture of reaction and stirred for 24 h at room temperature.
solution of different cations including Li , Na , Mg , Ca ,
2
+
2+
2+
2+
2+
2+
2+
3+
3+
3+
Pb , Cd , Hg , Fe , Co , Zn , Cu , Al , Fe , Cr ,
2
+
2+
Zn and Ni in water was prepared with 10 mM concentra-
tion. Volume of a 100 μL of nanosensor solution (2 mg/mL)

J Fluoresc
Scheme 1 Synthesis of TGA
capped CdSe QDs nanoparticles
and functionalized with 8-
hydroxyquinoline derivative
AHQ
Fig. 1 FT-IR spectrum of AHQ,
TGA-capped CdSe QDs and
AHQ-capped CdSe QDs
nanosensor

J Fluoresc
Result and Discussion
To realize this hypothesis, in the first step, 8-hydroxyquinoline
was converted to compound with amine group (AHQ) in two
steps by chloromethylation with formaldehyde in acidic me-
dia, followed by nucleophilic substitution with EDA. Then
TGA capped CdSe QDs was initially prepared as a previous
report in literature [29], and functionalized by AHQ via EDC/
NHS assisted formation of amide groups, to generate final
nanosensors based on CdSe QDs (Scheme 1). Nanosensor
based on CdSe QD coated with AHQ ligand was characterized
using FT-IR, TEM, SEM, EDX and UV-Vis techniques.
The chemical structure and functional groups of the syn-
thesized nanosensor was studied by FT-IR spectroscopy. FT-
IR spectrum of AHQ ligand, CdSe QDs and nanosensor,
AHQ-capped CdSe QDs are shown in Fig. 1.
−1
FT-IR spectrum of AHQ shows a broad peak at 3409 cm
for O―H and N―H bonds. Peaks at 3059 and 2821–
Fig. 2 UV-Vis spectrum of AHQ, TGA-capped CdSe QDs and AHQ-
capped CdSe QDs nanosensor
−1
2932 cm are attributed to the stretching vibrations of aro-
matic and aliphatic C―H bonds, respectively. The presence of
and 100 μL of stock solution of each metal ion was reached to
primary amine (NH ) and secondary amine (NH) can be con-
2
−1
2
.0 mL with deionized water and left for 15 min before
cluded from bands at 1657 and 833 cm for N―H bending
and wagging vibrations, respectively. Also, the existence of
phenolic O―H is approved by bands at 1325 for bending
vibration and 710 for OOP vibrations. Other distinct peaks
collecting the fluorescence spectra. Fluorescence spectra were
measured in the range of 450–700 nm with excitation at
4
00 nm, without any correction.
−
1
−1
In order to investigate response of nanosensor to different
were shown at 1566 cm for C=N, 1507 cm for C=C bonds
3
+
−1
concentration of Al , to a 100 μL solution of nanosensor
2 mg/mL), different volume (10, 25, 100, 150, 200, 250
of aromatic moiety, 1464 cm for bending vibration of CH ,
2
1238 cm for C―O bond and 1163 and 1080 cm−1 for C―N
−1
(
3
+
and 300 μL) of Al solution (10 mM) was added and diluted
to 2.0 mL with deionized water and subjected to fluorescence
measurements after 15 min, at the range of 420–620 nm, un-
der excitation at 400 nm.
bonds. In the IR spectrum of TGA-capped CdSe QDs, a broad
−1
peak is shown at 3424 cm , relating to the stretching vibration
of O―H of carboxylic acid and adsorbed water on the surface.
−1
Three weak bands at 2929, 2854 and 1463 cm are attributed
Fig. 3 a EDX analysis and (b) SEM and (c) TEM images of AHQ-capped CdSe QDs nanosensor

J Fluoresc
Fig. 4 a Fluorescence spectra and (b) emission intensity at 527 nm of nanosensor in the presence of various metal ions
to the asymmetric and symmetric stretching and bending vi-
the change of the CdSe QDs electronic ground state induced
brations of CH of TGA on the surface of CdSe QDs, respec-
by AHQ conjugation, which is a clear evidence for modifica-
tion of the surface of QDs with organic residue, and therefore
successful functionalization of CdSe QDs nanoparticles by
AHQ ligand [37].
2
tively. Asymmetric and symmetric vibrations of carboxylate
−
anions (CO ) of TGA moiety are appeared at 1634 and
2
−
1
1
559 cm , respectively. Also, C―O bonds vibration is shown
−1
−1
at 1152 cm . Band at 588 cm is related to stretching vibra-
tion of Cd―Se (Fig. 1a). However, in the FT-IR spectrum of
AHQ-capped CdSe QDs, similar bands were shown. The co-
valently linkage of AHQ to TGA-capped CdSe QDs could be
A scanning electron microscopy was used for structural
investigation of the morphology. SEM image of nanosensor
shows nanocrystals of CdSe QDs were aggregated to form a
smooth surface (Fig. 3b). EDX analysis allowed ascertaining
the composition of AHQ-capped CdSe QDs nanosensor. The
automatic assignments of C (22.39 w%), O (8.33 w%), N
(4.07 w%), S (1.68 w%), Se (14.89 w%), and Cd (48.63
w%) elements suggests that AHQ is successfully anchored
to CdSe QDs surface (Fig. 3a). Based on nitrogen content,
−1
concluded by appearance of a new peak at 1644 cm , relating
to C=O of amide moiety, with disappearance of primary N―H
−1
bending at 1657 cm (Fig. 1b) [33–35].
The additional structure information about nanosensor was
obtained by UV-Vis spectroscopy. As shown in Fig. 2, UV-Vis
spectrum of AHQ shows two absorption at 315 and 362 nm,
corresponding to π → π* and n → π* transitions, respective-
ly. Due to the ion charge transfer, TGA-capped CdSe QDs
shows an absorption at 466 nm, form which the size of
CdSe QDs was determined to be ~2.1 nm, using an empirical
formula reported by Yu et al. [36]. The results showed the
absorption of π → π* transition of nanosensor moved to the
smallest wavelength (blue shift) relative to AHQ, attributed to
−1
the results showed that about 0.97 mmolg of AHQ was
supported on CdSe QDs nanoparticles. Figure 3c shows
the typical TEM image of nanosensor. The TEM images
indicated that the size distribution of AHQ-capped CdSe
QDs nanosensor was nearly mono-dispersed with the
average size of 6 nm.
After characterization of nanosensor, we tested the selec-
tivity of AHQ-capped CdSe QDs nanosensor to various metal
Fig. 5 Schematic representation
of chelation-enhanced
fluorescence effect (CHEF) of
3+
Al and AHQ ligand on the
surface of nanosensor

J Fluoresc
3
+
The large fluorescence enhancement for the Al might be
explained by CHEF effect (Chelating Enhancement
Fluorescence Effect) of aluminum and immobilized AHQ on
the CdSe QDs nanoparticles. The lone pair electrons of nitro-
gen atom of HQ contributed to the photo-induced electron-
transfer (PET) phenomenon (or excited-state intramolecular
proton transfer (ESIPT) process from oxygen to nitrogen),
which quenched fluorescence emission of AHQ-capped
CdSe QDs nanosensor. Upon chelating metal ion with 8-HQ
moiety, the fluorescence could be increased dramatically by
occurring highly efficient chelation-enhanced fluorescence ef-
3
+
fect (CHEF). [21, 38] This type of chelating effect for Al
ions is illustrated in Fig. 5.
3
+
The effect of pH on the fluorescence spectra of AHQ-
capped CdSe QDs nanosensor were investigated. As shown
in Fig. 6, the fluorescence of nanosensor at neutral pH exhib-
ited high emission intensity rather than acidic and basic solu-
tions. In acidic (pH = 2.4) and alkaline (pH = 12) medium the
emission of nanosensor was quenched, which is attributed to
the protonation of nitrogen of quinoline and deprotonation of
phenolic OH of HQ in acidic and basic solutions, respectively.
Also, the effect of pH on the response of nanosensor for
Fig. 6 Fluorescence spectra of nanosensor and nanosensor/Al at
different pH values
ions in aqueous solution. Nanosensor shows a weak emission
at 527 nm in the absence of any metal ion, when excited at
00 nm. But as shown in Fig. 4, after addition of each metal
4
+
+
2+
2+
3+
2+
3+
2+
ions including, Li , Na , Mg , Co , Fe , Fe , Al , Cu ,
2
+
2+
2+
3+
2+
2+
Pb , Ni , Zn , Cr , Hg and Cd , to a solution of
nanosensor (the final concentrations of metal ions and
nanosensor are 2 mM and 0.1 mg/mL, respectively), only
Al exhibited large fluorescence enhancement. Other metal
ions could not enhance the fluorescence intensity efficiently.
Therefore, it is obvious that nanosensor exhibited a high se-
3
+
Al ions was investigated, in which there is no binding
+3
3+
between AHQ moieties with Al ions in acidic and basic
mediums, with no change in fluorescent spectra of
nanosensor. While, the emission intensity of nanosensor
was increased at neutral pH.
3
+
2+
lectivity towards Al ion. The results exhibited the Zn and
Cd created a little limitation of this nanosensor for detection
2
+
3+
To validate of nanosensor selectivity for Al , competitive
3
+
of Al in aqueous samples.
metal ions titrations was examined through fluorescent
Fig. 7 The emission intensity of
3+
nanosensor/Al solution in the
presence of 2 equiv. of each metal
ion

J Fluoresc
3+
3+
Fig. 8 a Fluorescent spectra of nanosensor in the presence of various amount of Al , b the emission intensity diagram against amount of Al , and (c)
the plot of Log (I/I ) against Log[Al ]
0
3+
3
+
emission measurement of a solution of nanosensor/Al upon
addition of an excess amount (2 equiv.) of each metal ion. As
metal ions. FT-IR and EDX analysis confirmed the AHQ li-
gand was successfully immobilized on the surface of QDs
nanoparticles. TEM image analysis has revealed the nanoscale
particle size of nanosensor. The designed nanosensor exhibit-
ed high selectivity and sensitivity for Al ions determination
in aqueous solution.
2
+
2+
3+
shown in Fig. 7, some metal ions such as Hg , Fe , Fe and
Cu via energy or electron transfer phenomena quenched the
2
+
3
+
3+
emission of the solution of nanosensor/Al , but other relevant
ions exhibited little change in emission spectra of nanosensor/
Al solution (Fig. 6). Also Cd , Zn and Mg afforded
about 2 fold increase in emission intensity, which could be
mentioned as limitation of the method for the quantitative
detection of Al in a mixture possessing this metal ions.
The sensitivity of AHQ-capped CdSe QDs nanosensor in
recognizing Al was also performed by titration of solution of
nanosensor with various amount of Al (Fig. 8a). As shown
in Fig. 8b, the emissions at λ was enhanced by increasing
the concentration of Al until 3 mM. The plot of Log(I/I0)
against Log[Al ], exhibited a good linearity in the range of
3
+
2+
2+
2+
Acknowledgments The work was supported by research council of the
University of Maragheh. Iran Science Elites Federation (ISEF) was also
acknowledged.
3
+
3
+
3
+
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