A highly sensitive fluorescent chemosensor for selective detection of zinc (II) ion based on the oxadiazole derivative

Article abstract of DOI:10.1016/j.saa.2016.11.053

A novel fluorescent chemosensor based on the oxadiazole, 2-(2-hydroxyphenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole, was designed and synthesized. The interaction of the oxadiazole with different metal ions had been investigated through UV–vis absorption and fluorescence spectra in 9:1 (v/v) ethanol-water (pH?=?7.0) solution. The oxadiazole showed a pronounced fluorescence enhancement at 430?nm upon addition of Zn^2?+ in aqueous solution, whereas it had no apparent interference from other metal ions. The results indicated that the oxadiazole possessed high selectivity and sensitivity to Zn^2?+ ion. The stoichiometric ratio between the oxadiazole and Zn^2?+ ion was calculated to be 2:1 by Job plot experiment, meanwhile their binding modes was confirmed by ¹H NMR and mass spectrometry. Their association constant was determined to be 1.95?×?10⁵?M^??1 and the detection limit for Zn^2?+ ion was 6.14?×?10^??7?mol/L.

Full text of DOI:10.1016/j.saa.2016.11.053

Accepted Manuscript
A highly sensitive fluorescent chemosensor for selective detection
of zinc (II) ion based on the oxadiazole derivative
Lu Lin, Dan Wang, Si-Hong Chen, Dun-Jia Wang, Guo-Dong Yin
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DOI:
Reference:
S1386-1425(16)30715-6
doi: 10.1016/j.saa.2016.11.053
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Spectrochimica Acta Part A: Molecular and Biomolecular
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30 November 2016
Please cite this article as: Lu Lin, Dan Wang, Si-Hong Chen, Dun-Jia Wang, Guo-Dong
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ACCEPTED MANUSCRIPT
A highly sensitive fluorescent chemosensor for selective detection of
zinc (II) ion based on the oxadiazole derivative
Lu Lin, Dan Wang, Si-Hong Chen, Dun-Jia Wang*, Guo-Dong Yin
College of Chemistry and Chemical Engineering, Hubei Collaborative Innovation Center for Rare
Metal Chemistry, Hubei Key Laboratory of Pollutant Analysis and Reuse Technology, Hubei
Normal University, Huangshi 435002, China
Abstract: A novel fluorescent chemosensor based on the oxadiazole,
2-(2-hydroxyphenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole, was designed and
synthesized. The interaction of the oxadiazole with different metal ions had been
investigated through UV–vis absorption and fluorescence spectra in 9:1 (v/v)
ethanolwater (pH = 7.0) solution. The oxadiazole showed a pronounced
fluorescence enhancement at 430nm upon addition of Zn2+ in aqueous solution,
whereas it had no apparent interference from other metal ions. The results indicated
that the oxadiazole possessed high selectivity and sensitivity to Zn2+ ion. The
stoichiometric ratio between the oxadiazole and Zn2+ ion was calculated to be 2:1 by
Job plot experiment, meanwhile their binding modes was confirmed by 1H NMR and
mass spectrometry. Their association constant was determined to be 1.95  105 M−1
and the detection limit for Zn2+ ion was 6.14  107 mol L1.
Keywords: Fluorescent sensor, zinc (II) ion detection, selective, oxadiazole derivative.

*Corresponding author. Tel: +86-714-6515602; Fax: +86-714-6573832
E-mail: dunjiawang@163.com (D.J. Wang)
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1. Introduction
As is known, the zinc (II) ion is the second most abundant transition metal ion in
the human body [1] and present as essential element in biological systems [2,3]. It
plays an indispensable role in several vital biological processes such as oxygen
transport, gene expression, energy generation cellular metabolism and DNA binding
or recognition [4,5]. But the unregulated level of zinc (II) ion in human body may
cause some neurodegenerative disorders such as Parkinson’s disease, Alzheimer’s
disease, epilepsy and cerebral ischemia [68]. Therefore, it is necessary and
important to develop some analytical methods for detecting zinc (II) ion in both
environmental and biological samples. In recent years, many highly selective and
sensitive fluorescence chemosensors toward zinc (II) ion were reported [917].
Among these fluorescent chemosensors, many cannot distinguish cadmium ion from
zinc ion [18], because of their interfering chemical properties. Thus, it is interesting
and challenging to design and synthesize some new fluorescent chemosensors for
zinc (II) ion, which can avoid interferences from other metal ions, especially from
cadmium ion.
In this context, we designed and synthesized a novel fluorescent chemosensor
based on the oxadiazole, 2-(2-hydroxyphenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole
(HMOD). The fluorescence responses of the chemosensor HMOD toward various
metal ions, Na+, K+, Mg2+, Ca2+, Ba2+, Al3+, Pb2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+,
Cu2+, Zn2+, Cd2+ and Hg2+ were studied. Among these metal ions, only zinc (II) ion
caused a prominent fluorescence enhancement at 430 nm, indicating that the
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chemosensor HMOD hold the potential to act as Zn2+ ion fluorescence probe.
2. Experimental
2.1. Materials and methods
Propylphosphonic anhydride (50 wt. % in ethyl acetate), salicylic acid, methyl
methoxybenzoate, hydrazine hydrate, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic
acid (Hepes) and N,N-diisopropylethylamine were purchased from Sun Chemical
Technology (Shanghai) Co., Ltd. All other reagents and solvents were of analytical
grade and used without further purification.
Fluorescence emission spectra were conducted on a Varian Cary Eclipse
fluorescence spectrometer. UV-vis spectra were obtained on a Hitachi U-3010
spectrometer. 1H NMR spectra were measured on an Advance IIITM 300 MHz NB
Digital NMR spectrometer in DMSOd6 solution with TMS as internal standard. The
IR spectra were performed with a Nicolet FTIR 5700 spectrophotometer with KBr
pellets. Electrospray ionization mass spectra (ESIMS) were obtained on a Finnigan
LCQ Advantage Max spectrometer. Elemental analysis (C, H, N) was recorded on a
PerkinElmer 2400 elemental analyzer. Melting points were determined on an X-4
digital melting point apparatus. The pH measurements were carried out on a Model
PHS-3C pH meter (Shanghai, China).
2.2. Synthesis of 2-(2-hydroxyphenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole
Synthetic pathway for the target compound HMOD is illustrated in Scheme 1.
According to the method of references [1921], the compound HMOD was
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synthesized by treatment methyl methoxybenzoate and hydrazine hydrate in ethanol,
followed by cyclization with salicylic acid at organic alkaline solution in the presence
of propylphosphonic anhydride. Its structure was characterized by elemental analysis,
FTIR, 1H NMR, and mass spectroscopy.
<Scheme 1>
4-Methoxybenzohydrazide was synthesized according to the literature [22]. A
mixture of methyl methoxybenzoate (1.66 g, 0.01 mol) and hydrazine hydrate (3.6 ml,
0.06 mol) in ethanol (20 ml) was heated under reflux for 6h. The solvent was
removed and the precipitate was obtained. The solid product was recrystallized with
ethanol to give 4-methoxybenzohydrazide as colorless crystals. Yield 96%, mp
138139 oC.
4-Methoxybenzohydrazide (1.66 g, 0.01 mol), salicylic acid (1.38 g, 0.01 mol)
and N,N-diisopropylethylamine (5.2 ml, 0.03 mol) were dissolved in ethyl acetate (20
ml). To this solution, propylphosphonic anhydride (12 ml, 0.02 mol) was added
dropwise under stirring and heated to reflux for 8h. The resulting mixture was poured
into 50 ml ice-water and extracted with ethyl acetate. The organic layer was separated
and the solvent removed by evaporation. The residual solid was recrystallized from
absolute ethanol to get the compound HMOD as pale crystals. Yield 85%, mp
218219 oC; IR (KBr): ν 3424 (s), 3055 (m), 2981 (w), 2859 (w), 1612 (s), 1593 (s),
1543 (m), 1499 (s), 1401 (m), 1304 (s), 1260 (s), 1186 (m), 1020 (m), 835(m), 748(s),
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707(m) cm1; 1H NMR (300 MHz, DMSOd6): δ 10.32 (s, 1H, OH), 8.05 (d, J = 8.8
Hz, 2H, ArH), 7.93 (dd, J = 1.3, 7.8 Hz, 1H, ArH), 7.527.46 (m, 1H, ArH), 7.19
(d, J = 8.8 Hz, 2H, ArH), 7.11 (d, J = 8.3 Hz, 1H, ArH), 7.05 (t, J = 7.5 Hz, 1H,
ArH), 3.87 (s, 3H, OCH3) ppm; ESIMS mz: 268.92 [M+1]+; Anal. calcd. for
C15H12N2O3: C 67.16, H 4.51, N 10.44; found C 67.48; H 4.49; N 10.46.
2.3. Analytical procedure
Hepes buffer solution (1.0×102 mol/L, pH = 7.0) was prepared using Hepes and
sodium hydroxide. Stock solutions of the metal ion were prepared in H2O with a
concentration of 1.0×103 mol/L. The stock solution of HMOD (2.0×103 mol/L) was
prepared in ethanol and its test solution (2.0×105 mol/L) was prepared in 9:1 (v/v)
ethanolHepes buffer solution. In the experiments, 3.0 mL solution of HMOD
(2.0×105 mol/L) was placed in a quartz cell (1cm width) and the solution of metal
ions was added into the quartz cell gradually by micro-syringe to obtain the working
solution. The fluorescence emission spectra of HMOD were recorded in the λem of
380600 nm at λex = 378 nm with 5.0/5.0 nm slit width and 550 V PMT.
3. Results and discussion
3.1. UVvis absorption spectral studies
The UV–vis absorption spectra of the chemosensor HMOD (2.0×105 mol/L)
showed two peaks at 288 and 316nm in 9:1 (v/v) ethanolHepes buffer solution (pH
= 7.0). Upon addition of 1.0 equiv of various metal ions (Na+, K+, Mg2+, Ca2+, Ba2+,
Al3+, Pb2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+) in aqueous
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solution, the absorption spectra changed in different manner as presented in Fig. 1. It
was found that the absorption of HMOD appeared a new weak peak at region of
350380nm in the presence of Mg2+, Ca2+, Co2+, Pb2+, Mn2+, Fe3+, Fe2+, Ni2+ and Cu2+
ions. However, the addition of Zn2+ ion caused a large decrease of absorption
intensity at 316nm, accompanied by a new strong absorption band appeared at 370nm,
indicating the complexation between the chemosensor HMOD and Zn2+ ion.
Meanwhile, the addition of Na+, K+, Ba2+, Al3+, Cr3+, Cd2+, Hg2+ ions, the absorption
bands of HMOD had little change.
< Figure 1>
In order to elicit the binding property of HMOD toward Zn2+ ion, UV-vis
titration of HMOD (2.0×105 mol/L) with Zn2+ ion (00.75 equiv) was carried out
(Fig. 2). It can be found that successive additions of Zn2+ ion to HMOD in 9:1 (v/v)
ethanolHepes buffer solution caused a gradual decrease in absorbance at 316 nm
and appeared a new absorption band centered at 360 nm. Moreover, a clear isosbestic
point at 336 nm can be observed. In addition, the absorbance at 360 nm (inset) tended
to saturation when the amount of Zn2+ ion was beyond 0.5 equiv, indicating the
possible formation of a 2:1complex between chemosensor HMOD and Zn2+ ion.
< Figure 2>
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3.2. Fluorescence emission studies
To investigate the selectivity of chemosensor HMOD towards different metal
ions, the fluorescence emission studies were performed by adding 1.0 equiv of metal
ions (Na+, K+, Mg2+, Ca2+, Ba2+, Al3+, Pb2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+,
Zn2+, Cd2+ and Hg2+) into HMOD (2.0×105 mol/L) in 9:1 (v/v) ethanolHepes buffer
solution (Fig. 3). From Fig. 3, it was observed that the emission spectrum of HMOD
had hardly changed in the presence of Na+, Ba2+ and Pb2+ ions. Metal ions K+, Al3+,
Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+ and Hg2+ caused a obvious fluorescence
quenching at 430nm. Mg2+, Ca2+ and Cd2+ ions resulted in a certain degree of
fluorescence enhancement at 430 nm. However, Zn2+ ion exhibited a very
pronounced fluorescence enhancement at 430 nm over other metal ions, which
further confirmed the interaction between the chemosensor HMOD and Zn2+ ion.
This result was in agreement with its absorption spectrum. Consequently, the
chemosensor HMOD could be used as a highly selective chemosensor for Zn2+ ion.
< Figure 3>
The fluorescence titration was also performed by adding Zn2+ ion aqueous
solution (1.0×10−3 mol/L) into the solution of HMOD (2.0×105 mol/L) in 9:1 (v/v)
ethanolHepes buffer solution (pH = 7.0) at room temperature. As shown in Fig. 4,
upon gradually addition of Zn2+ ion from 0 to 0.75 equiv, the emission intensity of
HMOD gradually enhanced at 430 nm, which was due to the coordination between
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HMOD and Zn2+ ion to enhance the planarity and rigidity of the molecular structure.
The emission intensity of HMOD increased rapidly when the amount of Zn2+ ion
added were less than 0.5 equiv. Then the more Zn2+ ion was added, its emission
intensity changed much more slowly. The nonlinear curve fitting of the fluorescence
titration (inset) also gives a 2:1 stoichiometric ratio between HMOD and Zn2+ ion,
which was similar to the result of UV–vis titration. For determination of the
complexation ratio between HMOD and Zn2+ ion, the fluorescence Job’s plot
analysis [23] (Fig. 5) was used. The result showed that the maximum point appeared
at the molar fraction (Zn2+ ion) of about 0.33, which further confirmed that the
chemosensor HMOD and Zn2+ ion formed a 2:1 complex. This is also in accord with
the result of UV–vis titration.
< Figure 4>
< Figure 5>
3.3. The association constant and detection limit
According to the fluorescence titration curves of HMOD with Zn2+ ion at room
temperatures, the association constant Ka was calculated as 1.95 × 105 M−1 (R =
0.99625) for HMODZn2+ complex by Benesi–Hildebrand plot [24] (Fig. 6).
Moreover, the emission intensity of HMOD is linearly proportional to the Zn2+
concentration in the range of 0–10 M (Fig. 7). The detection limit was determined
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by the equation [25,26] LOD = 3SD / B, where SD is the standard deviation of blank
measurement; B is the slope between the emission intensity versus Zn2+ concentration.
The detection limit of Zn2+ ion was calculated to be 6.14  107 M (R = 0.99832),
which was adequately low to enable the detection of micromolar amount of Zn2+ ion.
< Figure 6>
< Figure 7>
3.4. 1H NMR and mass studies
The mechanism of interaction between HMOD and Zn2+ ion was further
investigated through 1H NMR and mass spectrometry upon the addition of Zn2+ ion.
The significant 1H NMR spectral changes were observed as shown in Fig. 8. The 1H
NMR spectrum of HMOD showed a singlet peak at  10.32 ppm corresponding to Hg
of the hydroxyl group, which was disappeared completely when one equiv of Zn2+
ion was added to HMOD in DMSOd6 solution. It suggested that the hydroxyl group
takes part in the coordination with Zn2+ ion. The aromatic proton signals of
4-methoxyphenyl moiety (Ha, Ha’, Hb and Hb’) were nearly unchanged. However, the
aromatic proton signals of 2-hydroxyphenyl moiety (Hc, Hd, He and Hf) were
obviously shifted to high-field on addition of one equiv of Zn2+ ion, especially Hf and
Hd ( the ortho- and para-protons of the hydroxyl group, respectively), due to the
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coordination effect of HMOD with Zn2+ ion. In addition, the formation of a 2:1
complex between HMOD and Zn2+ ion was further confirmed by the appearance of a
peak at m/z 599.06, which was assigned to [2([HMOD+1]+–1)+Zn2+–2H+] + in the
electrospray ionization mass spectrum (Fig. 9). According to these results, we can
deduce the possible binding mode between HMOD and Zn2+ ion (Scheme 2).
< Figure 8>
< Figure 9>
<Scheme 2>
3.5. The interference from other metal ions
In order to investigate the practical applicability of HMOD as a selective
fluorescence chemosensor for Zn2+, the competition experiments were also performed
by addition of one equivalent of other metal ions, such as Na+, K+, Mg2+, Ca2+, Ba2+,
Al3+, Pb2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+ and Hg2+, to HMOD
(2.0×105 mol/L) in 9:1 (v/v) ethanolHepes buffer solution (pH = 7.0) in the
presence of one equivalent of Zn2+ ion. From Fig. 10, it was found that the coexistent
metal ion had no apparent interference with the detection of Zn2+ ion. Especially, the
emission intensity of HMOD−Zn2+ hardly changed in the presence of Cd2+ ion
commonly having similar properties. The results indicated that HMOD would be an
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excellent zinc selective fluorescent chemosensor.
< Figure 10>
4. Conclusion
In summary, a novel fluorescent chemosensor based on the oxadiazole,
2-(2-hydroxyphenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole, was synthesized and
characterized by elemental analysis, FTIR, 1H NMR, and mass spectroscopy. The
UV-vis absorption and fluorescence properties of HMOD with Zn2+ ion were studied
in 9:1 (v/v) ethanolHepes buffer (pH = 7.0) solution. The fluorescence emission of
HMOD showed a pronounced enhancement at 430nm upon addition of Zn2+ in
aqueous solution. The results indicated that HMOD was a highly selective and
sensitive chemosensor in detection of Zn2+ in presence of other metal ions. The
complex formed between HMOD and Zn2+ ion is found to be 2:1 stoichiometry by
Job plot experiment, while their binding modes was confirmed by 1H NMR and mass
spectrometry. The association constant Ka was calculated to be 1.95  105 M−1 and the
detection limit of HMOD for Zn2+ was 6.14  107 mol L1. Consequently, this
fluorescence probe could be served as potential candidate in various biological
environments.
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Acknowledgement
We gratefully acknowledge financial support from National Natural Science
Foundation of China (No. 21542009) and Foundation for Hubei Key Laboratory of
Pollutant Analysis and Reuse Technology (No. PA160203).
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Schemes:
O
O
COOH
OH
C H
.
2
2
5
NH
NH NH H O
2
O
2
2
N
H
+
C H OH
2
5
O
O
HO
N
N
N
O
Propylphosphonic anhydride
O
HMOD
Scheme 1 Synthetic route of the oxadiazole HMOD.
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O
O
HO
N
N
N N
O
2+
O
Zn
Zn
Fluorescence enhancement
O
O
N
N
O
O
Scheme 2 Proposed binding mode between HMOD and Zn2+ ion.
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Figure Captions:
Fig. 1 UVvis spectra of HMOD (2.0×105 mol/L) in 9:1 (v/v) ethanolHepes buffer
solution (pH = 7.0) upon 1:1 mol ratio addition of various metal ions.
Fig. 2 Changes in UV-vis spectra of HMOD (2.0×105 mol/L) in 9:1 (v/v)
ethanolHepes buffer solution (pH = 7.0) upon addition of various amounts of Zn2+
ion (00.75 equiv). Inset: plots of absorbance at 360nm versus the value of [Zn2+] /
[HMOD].
Fig. 3 Normalized emission spectra of HMOD (2.0×105 mol/L) in 9:1 (v/v)
ethanolHepes buffer solution (pH = 7.0) upon 1:1 mol ratio addition of various
metal ions (λex = 378 nm).
Fig. 4 Changes in normalized emission spectra of HMOD (2.0×105 mol/L) in 9:1
(v/v) ethanolHepes buffer solution (pH = 7.0) upon addition of various amounts of
Zn2+ ion (00.75 equiv) (λex = 378 nm). Inset: plots of fluorescence at 430nm versus
the value of [Zn2+] / [HMOD].
Fig. 5 Job’s plot for the complexation proceeding of HMOD with Zn2+ ion in 9:1 (v/v)
ethanolHepes buffer solution (pH = 7.0) yield from fluorescence emission at 430nm
(λex = 378 nm).
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Fig. 6 Benesi–Hildebrand plot of HMOD with Zn2+ ion in 9:1 (v/v) ethanolHepes
buffer solution (pH = 7.0) (λem = 430 nm, λex = 378 nm).
Fig. 7 Plot of emission intensity versus the concentrations of Zn2+ ion (λem = 430 nm,
λex = 378 nm).
Fig. 8 1H NMR spectra of (A) HMOD in DMSOd6 and (B) addition of 1.0 equiv of
Zn2+ ion.
Fig. 9 Electrospray ionization mass spectrum of HMOD (2.0×105 mol/L) in
methanol upon addition of 1.0 equiv of Zn2+ ion.
Fig. 10 Competitive selectivity of HMOD (2.0×105 mol/L) in 9:1 (v/v)
ethanolHepes buffer solution (pH = 7.0) toward Zn2+ (1.0 equiv) in the presence of
other metal ions (1.0 equiv) (λem = 430 nm, λex = 378 nm).
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Fig.1
0.6
0.5
0.4
0.3
0.2
0.1
0.0
HMOD
Na+
K+
Mg2+
Ca2+
Ba2+
Al3+
Pb2+
Cr3+
Mn2+
Fe3+
Fe2+
Co2+
Ni2+
Zn2+
Cu2+
Zn2+
Cd2+
Hg2+
260 280 300 320 340 360 380 400 420 440
Wavelength (nm)
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Fig.2
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.20
0.18
0.16
0.14
0.12
0
0.10
0.08
Zn2+
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
[Zn2+] / [HMOD]
0.75eq
260
280
300
320
340
360
380
400
420
Wavlength (nm)
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Fig.3
HMOD
Na+
1.0
0.8
0.6
0.4
0.2
0.0
Zn2+
K+
Mg2+
Ca2+
Ba2+
Al3+
Pb2+
Cr3+
Mn2+
Fe3+
Fe2+
Co2+
Ni2+
Cu2+
Zn2+
Cd2+
Hg2+
380
400
420
440
460
480
500
520
540
Wavelength (nm)
21

ACCEPTED MANUSCRIPT
Fig.4
0.75eq
Zn2+
1.0
0.8
0.6
0.4
0.2
0.0
1100
1000
900
800
700
600
500
400
300
200
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
[Zn2+] / [HMOD]
380 400 420 440 460 480 500 520 540 560 580
Wavelength (nm)
22

ACCEPTED MANUSCRIPT
Fig.5
900
800
700
600
500
400
300
200
100
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
[Zn2+] / ([Zn2+]+[HMOD])
23

ACCEPTED MANUSCRIPT
Fig.6
Y = A + B * X
A 6.44464E-4
B 0.00533
R 0.99625
SD 3.84925E-5
0.0024
0.0022
0.0020
0.0018
0.0016
0.0014
0.0012
0.0010
0.05
0.10
0.15
0.20
0.25
0.30
0.35
1 / [Zn2+](106 M-1)
24

ACCEPTED MANUSCRIPT
Fig.7
1000
900
800
700
600
500
400
300
200
Y = A + B * X
A 261.3186
B 70.51861
R 0.99832
SD 14.44207
0
1
2
3
4
5
6
7
8
9
10 11
[Zn2+](10-6M)
25

ACCEPTED MANUSCRIPT
Fig.8
26

ACCEPTED MANUSCRIPT
Fig.9
1_160117144740 #198 RT: 2.94 AV: 1 NL: 6.04E5
T: + c NSIFull ms [ 400.00-1000.00]
599.06
100
90
80
70
60
506.54
50
440.53
615.35
40
425.56
30
725.94
20
462.61
933.09
774.85
711.00
683.06
521.74
474.68
10
0
586.44
632.70
753.47
750
810.90
800
909.58
886.28
850
967.01
950 1000
400
450
500
550
600
650
700
m/z
900
27

ACCEPTED MANUSCRIPT
Fig.10
HMOD
HMOD+ions
HMOD+ions+Zn2+
1200
1000
800
600
400
200
0
HMOD Na+ K+ Mg2+ Ca2+ Ba2+ Al3+ Pb2+ Cr3+ Mn2+ Fe3+ Fe2+ Co2+ Ni2+ Cu2+ Zn2+ Cd2+ Hg2+
28

ACCEPTED MANUSCRIPT
Graphical Abstract
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.75eq
Zn2+
0
1.0
0.8
0.6
0.4
0.2
0.0
0.20
0.18
0.16
0.14
0.12
0.10
0.08
1100
1000
900
800
700
600
500
400
300
200
0
Zn2+
0.0
0.1
0.2
0.3
[Zn2+
0.4
0.5
0.6
0.7
0.8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
]
/ [HMOD]
[Zn2+] / [HMOD]
0.75eq
380 400 420 440 460 480 500 520 540 560 580
Wavelength (nm)
260
280
300
320
340
360
380
400
420
Wavlength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
Zn2+
HMOD
Na+
K+
Mg2+
Ca2+
Ba2+
Al3+
O
Pb2+
Cr3+
Mn2+
Fe3+
Fe2+
Co2+
Ni2+
Cu2+
Zn2+
Cd2+
Hg2+
O
HO
N
N
N
N
Zn2+
O
Zn
O
Fluorescence enhancement
O
O
N
N
O
O
380
400
420
440
460
480
500
520
540
Wavelength (nm)
29