Imidazole and imine coated ZnO nanoparticles for nanomolar detection of Al(III) and Zn(II) in semi-aqueous media

Article abstract of DOI:10.1016/j.tetlet.2014.10.080

Imidazole and imine-linked dipodal receptors were decorated on the surface of ZnO nanoparticles. Cation binding assays of these chemosensors showed that receptors 5 and 6 had high selectivity for Al(III) and Zn(II) ions, respectively. The detection limit

Full text of DOI:10.1016/j.tetlet.2014.10.080

Accepted Manuscript
Imidazole and imine coated ZnO nanoparticles for nanomolar detection of
Al(III) and Zn(II) in semi-aqueous media
Hemant Sharma, Narinder Singh, Doo Ok Jang
PII:
S0040-4039(14)01774-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.10.080
TETL 45309
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
23 July 2014
4 October 2014
14 October 2014
Please cite this article as: Sharma, H., Singh, N., Jang, D.O., Imidazole and imine coated ZnO nanoparticles for
nanomolar detection of Al(III) and Zn(II) in semi-aqueous media, Tetrahedron Letters (2014), doi: http://dx.doi.org/
1
0.1016/j.tetlet.2014.10.080
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Imidazole and imine coated ZnO
nanoparticles for nanomolar detection of
Al(III) and Zn(II) in semi-aqueous media
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Hemant Sharma, Narinder Singh* and Doo Ok Jang*

1
Tetrahedron Letters
jo u rn al h ome p ag e : w ww .e lse vie r .c om
Imidazole and imine coated ZnO nanoparticles for nanomolar detection of
Al(III) and Zn(II) in semi-aqueous media
a
Hemant Sharma, Narinder Singh, * and Doo Ok Jang *
a,
b,
a
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab140001, India
b
Department of Chemistry, Yonsei University, Wonju 220-710, Korea
ARTICLE INFO
ABSTRACT
Article history:
Received
Imidazole and imine-linked dipodal receptors were decorated on the surface of ZnO
nanoparticles. Cation binding assays of these chemosensors showed that receptors 5 and 6 had
high selectivity for Al(III) and Zn(II) ions, respectively. The detection limit for Al(III) with
receptor 5 was 11 nM, and the detection limit for Zn(II) ion with receptor 6 was 23 nM..
Received in revised form
Accepted
Available online
2
009 Elsevier Ltd. All rights reserved.
Keywords:
ZnO
nanomolar
Al(III)
Zn(II)
chemosensor
1
. Introduction
two different organic ligands were attached to the surface of ZnO
nanoparticles (ZnO-NPs). The best results were obtained using 1-
methyl-3-carboxymethylimidazole (1) along with ligands 2 or 3
as shown in Figure 1. Receptors 5 and 6 exhibited selective
affinity towards Al(III) and Zn(II), respectively.
Recently, organic-inorganic hybrid chemosensors that
1
-6
recognize metal ions have gained considerable attention. Ease
of preparation, a low detection limit, and a broad detection range
are advantageous features of organic-inorganic hybrid
chemosensors. Aluminum is the third most abundant element in
the earth’s crust, and trace amounts of aluminum are required for
7
various normal human biological processes. However, excessive
intake of aluminum may cause Parkinson’s disease, Alzheimer’s
8
-10
disease, and amyotrophic lateral sclerosis.
1
In addition,
Aluminum hinders the growth of plants. Zinc is another
1
1
2,13
essential transition metal in human physiology,
playing roles
in multiple biological processes including as an essential element
of antioxidants and cofactors, gene transcription, neural signal
1
4-17
transmission, regulation of metallo-enzymes, and apoptosis.
A deficiency in zinc may lead to growth retardation, diarrhea,
18-20
impotence, and delayed sexual maturation.
Therefore, it is
essential to develop chemosensors for the detection of Al(III) and
Zn(II) ions.
Figure 1. Synthesis of receptor 1, 2, 3, 4, 5 and 6.
Fluorogenic chemosensors have enjoyed substantial interest
due to their widespread applications in medical and
environmental areas. Most of the reported chemosensors for
Al(III) and Zn(II) have complex structures that require multistep
2
. Results and discussion
Ligands 1, 2, and 3 were synthesized by literature methods,
30-32
21-28
synthetic routes.
Therefore, it is essential to develop sensors,
and the ZnO-NPs were prepared via a sol-gel method.
Decoration of the organic ligands on the surface of ZnO-NPs was
performed through an in situ method. Receptor 4 was obtained by
which can be easily prepared and have a broad detection range,
low detection limit, and a high binding constant.
1
reaction of ligand 1 with Zn(NO
3
)
2
and NaOH. The H NMR
We previously reported the preparation of dipodal imine-
linked ZnO-coated chemosensors, which when decorated with
spectrum of receptor 4 was similar to that of ligand 1 except for
signal broadening due to the presence of ZnO-NPs. Transmission
electron microscopy (TEM) analysis showed that the modified
2
9
compounds 2 and 3 showed high selectivity for Mg(II) and
3
0
Co(II), respectively. To extend the scope of this methodology,

2
Tetrahedron
ZnO-NPs were roughly spherical in shape with a size of
approximately 16.5 nm (Figure S1A). Dynamic light scattering
DLS) studies revealed that these particles had a size of
(
approximately 98 nm (Figure S2A). It is noteworthy that there
was a difference in size from TEM and DLS measurements
because DLS measures the hydrodynamic size of nanoparticles,
while TEM records the size of discrete particles. In case of
coated nanoparticles, frictional drag was occurred due to coated
material, which results in influence on the Brownian motion of
3
3
particles. Therefore, agglomeration of particles was occurred
and the size measured via DLS is the size of agglomerate which
is greater than size measured through TEM (size of discrete
particles). The decoration of organic ligand on the surface of
ZnO-NPs was also confirmed by energy-dispersive X-ray
spectroscopy (EDX). The EDX spectra advocated the presence of
C, O and Zn elements (Figure S3A). Receptor 5 was obtained via
in situ decoration with ligands 1 and 2 in a ratio of 1:1 on the
Figure 2. (A) Fluorescence emission spectra of receptor 1 (10 µM) upon
addition of various metal ions (700 nM) in DMSO/H O (7:3, v/v), excitation
at 350 nm; (B) Fluorescence emission spectra of receptor 4 (10 µM) upon
1
surface of ZnO-NPs. The light yellow compound gave a H NMR
2
spectrum with signals in the aromatic region. Total number of
protons indicated the ratio of 1:1 between ligands 1 and 2. TEM
images showed spherical particles of approximately 5.8 nm in
size (Figure S1B), while DLS measurements revealed a size of
2
addition of various metal ions (700 nM) in DMSO/H O (7:3, v/v), excitation
at 350 nm.
3
O and Zn elements (Figure S3B). Similarly, receptor 6 was
4 nm (Figure S2B). EDX analysis confirmed the presence of C,
To further extend the scope of this methodology, two different
receptors 1 and 2 or 3 were decorated on the surface of ZnO-NPs
in an equal amount. The resulting complexes 5 and 6 were
evaluated for cation binding study. The affinity of receptor 5
towards Na(I), K(I), Mg(II), Ca(II), Sr(II), Ba(II), Cr(III), Mn(II),
Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II), Hg(II), Pb(II),
1
prepared using ligands 1 and 3. Shifts of the H NMR signals
confirmed the attachment of ligands 1 and 3 on the surface of
ZnO-NPs, and total number of protons confirmed the 1:1 ratio
between ligand 1 and ligand 3. TEM images indicated that the
spherical nanoparticles had a size of approximately 3.8 nm as
shown in Figure S1C. The hydrodynamic size of receptor 6 was
about 20 nm (Figure S2C), and EDX spectra showed the
presence of C, O and Zn elements (Figure S3C).
2
and Al(III) was examined in a DMSO/H O (7:3, v/v) solvent
system. Upon addition of Al(III), the fluorescence intensity
decreased along with a slight blue shift, while other metal ions
did not showed such pronounced changes (Figure 3A). Titration
of receptor 5 was performed with Al(III) as depicted in Figure 3B.
Successive additions of Al(III) to a solution of receptor 5 (10
µM) produced concomitant changes in the emission profile of
receptor 5 along with a slight blue shift (∆ 14 nm). Binding of
Al(III) ions with the receptor leads to transfer electron density
from nitrogen atoms of imine groups to Al(III) ions since Al(III)
ions have a high charge density. Then, the dipole moment of the
excited molecule is changed causing a blue shift with a decrease
in emission intensity, which corresponds to an internal charge
transfer (ICT) phenomena. For standardization of the recognition
studies, pH titrations of receptor 5 were performed. The optimum
pH range for the recognition of Al(III) with receptor 5 was found
to be between pH 4 to 10 (Figure S4A).
The metal binding affinity of receptors 1, 2 and 3 was
2
examined in a DMSO/H O solvent system. It was noticed that
receptor 1 has non-selective behavior towards library of metal
ions like Na(I), K(I), Mg(II), Ca(II), Sr(II), Ba(II), Cr(III), Mn(II),
Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II), Hg(II), Pb(II),
2
and Al(III) in DMSO/H O (7:3, v/v) as shown in Figure 2A. As
reported earlier, receptors 2 and 3 did not show any particular
2
9, 30
responses towards any metal ions.
However, decoration of 2
and 3 on the surface of ZnO-NPs led to selectivity towards
Mg(II) and Co(II) ions, respectively. Due to oxygenated zinc
defects at grain boundaries, ZnO has high affinity for –OH, -
3
0, 34
COOH and –CH=N groups.
molecule upon decoration on the surface of ZnO-NPs is one of
The increase in rigidity of
2
9, 30
the possible causes for enhanced selectivity.
was decorated with receptor 1 on the surface of ZnO-NPs showed
Receptor 4 that
a non-selective binding profile in interacting with metal ions
(
Figure 2B).

3
produce significant changes in the emission spectra. To better
understand the mechanism and confirm the authenticity of the
phenomena, a titration was performed with receptor 6 (10 µM)
and Zn(II) ions. Addition of Zn(II) to a solution of receptor 6 led
to fluorescence enhancement (Figure 5B), indicating that
cancellation of PET is a possible mechanism for Zn(II)
recognition.
Figure 3. (A) Fluorescence emission spectra of receptor 5 (10 µM) upon
addition of various metal ions (700 nM) in DMSO/H O (7:3, v/v), excitation
at 350 nm; (B) Changes in the emission spectra of receptor 5 (15 µM) upon
2
continuous addition of Al(III) ions (0-700 nM) in DMSO/H
2
O (7:3, v/v),
excitation at 350 nm.
The effect of ionic strength on the emission profile of receptor
was also evaluated. A set of solutions of receptor 5 (10 µM)
5
were
prepared
using
different
concentrations
of
tetrabutylammonium perchlorate (0, 10, 20, 40, 80 and 100 mM).
After shaking, the solutions were kept under ambient conditions
to allow them to reach equilibrium. The emission spectra of each
of these solutions were recorded. It was observed that the
emission profile of receptor 5 changed slightly with increasing
ionic strength of the solution (Figure S4B). The binding constant
3
5
(
log β) was calculated to be approximately 4.7. The detection
limit was approximately 11 nM. In order to determine the linear
range of detection, the calibration plot was drawn between
fluorescence intensity at 439 nm and concentration of Al(III)
ions. The plot has linearity in the concentration range of 0 to 80
nM as shown in Figure S4C. To check whether the chemosensor
functioned properly with more typical analytical samples,
competitive binding assays were performed. Figure 4 illustrates
that receptor 5 recognized Al(III) selectively even in the presence
of other competing metal ions.
Figure 5. (A) Fluorescence emission spectra of receptor 6 (10 µM) upon
2
addition of various metal ions (700 nM) in DMSO/H O (7:3, v/v), excitation
at 350 nm; (B) Changes in the emission spectrum of receptor 6 upon
successive additions of Zn(II) ions (0-700 nM) in DMSO/H
2
O (7:3, v/v),
excitation at 350 nm.
The effect of pH on the emission profile of receptor 6 was
examined. The addition of acid led to enhancement of the
emission profile of receptor 6 (Figure S5A); however, base did
not produce any significant changes. The optimum pH range for
the use of receptor 6 was pH 7 to 11. The salt effect was
investigated with different concentrations of tetrabutylammonim
perchlorate solution. The emission profile of receptor 6 remained
the same with up to 100 mM tetrabutylammonium perchlorate
(
Figure S5B). The calibration plot between fluorescence intensity
Figure 4. Determination of Al(III) through receptor 5 (10 µM) in the
presence of other metal ions [Na(I), K(I), Mg(II), Ca(II), Sr(II), Ba(II), Cr(III),
Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II), Hg(II) and Pb(II)]
at 451 nm and concentration of Zn(II) ions represented the linear
range of detection from 0 to 80 nM as shown in Figure S5C. The
association constant (log β) of receptor 6 for Zn(II) wa3s5
approximately 3.1, and the detection limit was 23 nM.
Interference in the recognition of Zn(II) by the receptor from
other competing metal ions was evaluated using 18 different
metal ion. Each solution contained receptor 6 (10 µM) and Zn(II)
along with an equivalent amount of competing metal ions.
Receptor 6 had high selectivity for Zn(II) even in the presence of
competing metal ions, as shown in Figure 6.
2
in DMSO/H O (7:3, v/v).
Similarly, a metal binding study of receptor 6 (10 µM) was
performed with a library of metal ions [Na(I), K(I), Mg(II),
Ca(II), Sr(II), Ba(II), Cr(III), Mn(II), Fe(III), Co(II), Ni(II),
Cu(II), Zn(II), Ag(I), Cd(II), Hg(II), Pb(II) and Al(III)] in a
2
DMSO/H O (7:3, v/v) solvent system. Enhancement in the
emission profile of receptor 6 was observed upon addition of
Zn(II) ions as shown in Figure 5A, while other metal ions did not

4
Tetrahedron
2
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Figure 6. Determination of Zn(II) through 6 (10 µM) in the presence of
other metal ions [Na(I), K(I), Mg(II), Ca(II), Sr(II), Ba(II), Cr(III), Mn(II),
Fe(III), Co(II), Ni(II), Cu(II), Ag(I), Cd(II), Hg(II), Pb(II) and Al(III)] in
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DMSO/H
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. Conclusions
Members of a new class of chemosensor, 4, 5 and 6, have
5. Magri, D. C.; Callan, J. F.; De Silva, A. P.; Fox, D. B.;
McClenaghan, N. D.; Sandanayake, K. R. A. S. J. Fluoresc. 2005,
been prepared. Receptor 5 showed high selectivity for Al(III)
ions with a detection limit of 11 nM, whereas receptor 6
selectively recognized Zn(II) with a detection limit of 23 nM.
Receptors 5 and 6 displayed sensitive and selective recognition
of Al(III) and Zn(II), respectively, even in the presence of other
metal ions.
1
5, 769.
Supplementary Material
† Electronic Supplementary Information (ESI) available:
details of any supplementary information available should be
[
included here]. See DOI: 10.1039/b000000x/
Acknowledgments
This work was supported by the India-Korea Joint Program of
Cooperation in Science &Technology (NRF-2011-0027710) and
by a CSIR project grant (01(2417)/10 EMR-II).
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