Two similar Schiff-base receptor based quinoline derivate: Highly selective fluorescent probe for Zn(II)

Article abstract of DOI:10.1016/j.saa.2020.118347

As is known, Zn²⁺ plays a vital role in a variety of biological processes but excessive exposure of Zn²⁺ to human beings can cause toxicity, inducing a series of overt poisoning symptoms and neurodegenerative disorders. Thus, we designed and synthesized two quinoline-derived Schiff-bases HL₁ and HL₂, and investigated the fluorescence emission responses of these two Schiff-bases to various metal ions. A significant enhancement in fluorescence emission band centered at 450 nm was observed in the ethanolic solution of HL₁ with addition of Zn²⁺, while remarkably lower fluorescence emission enhancement was obtained in the case of HL₂ in which one methyl group was introduced to the azomethine carbon. In addition, HL₁ showed good selectivity and high sensitivity towards Zn²⁺ in the existence of other various interfering metal ions, and the reversibility and regeneration of HL₁ were also perfect for extending its applications in environmental and biological systems. Therefore, HL₁ could be identified as a fluorescent probe for sensing Zn²⁺ environmentally and biologically.

Full text of DOI:10.1016/j.saa.2020.118347

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Two similar Schiff-base receptor based quinoline derivate: Highly
selective fluorescent probe for Zn(II)
Long Fan, Jing-can Qin, Chao-rui Li, Zheng-yin Yang
PII:
S1386-1425(20)30325-5
https://doi.org/10.1016/j.saa.2020.118347
SAA 118347
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 29 January 2020
Revised date:
31 March 2020
6 April 2020
Accepted
date:
Please cite this article as: L. Fan, J.-c. Qin, C.-r. Li, et al., Two similar Schiff-base receptor
based quinoline derivate: Highly selective fluorescent probe for Zn(II), Spectrochimica
Acta Part A: Molecular and Biomolecular Spectroscopy (2018), https://doi.org/10.1016/
j.saa.2020.118347
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Two similar Schiff-base receptor based quinoline derivate :
Highly selective fluorescent probe for Zn(II)
Long Fana,b, Jing-can Qina, Chao-rui Lia, Zheng-yin Yanga,*
aCollege of Chemistry and Chemical Engineering, State Key Laboratory of Applied
Organic Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
bQuality and Technical Supervision and Inspection of Jin Chang, Jin Chang 737100,
People’s Republic of China
**Corresponding author. Tel.: +86 931 8913515; Fax: +86 931 8912582; e-mail:
yangzy@lzu.edu.cn (Z.Y. Yang)
ABSTRACT:
As is known, Zn2+ plays a vital role in a variety of biological processes but
excessive exposure of Zn2+ to human beings can cause toxicity, inducing a series of
overt poisoning symptoms and neurodegenerative disorders. Thus, we designed and
synthesized two quinoline-derived Schiff-bases HL1 and HL2, and investigated the
fluorescence emission responses of these two Schiff-bases to various metal ions. A
significant enhancement in fluorescence emission band centered at 450 nm was
observed in the ethanolic solution of HL1 with addition of Zn2+, while remarkably
lower fluorescence emission enhancement was obtained in the case of HL2 in which
one methyl group was introduced to the azomethine carbon. In addition, HL1 showed
good selectivity and high sensitivity towards Zn2+ in the existence of other various
interfering metal ions, and the reversibility and regeneration of HL1 were also perfect
for extending its applications in environmental and biological systems. Therefore,
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HL1 could be identified as a fluorescent probe for sensing Zn2+ environmentally and
biologically.
Keywords: fluorescence probe; Schiff-base; Zinc ion; Selectivity; Sensitivity
1. Introduction
Following iron, zinc is the second most abundant transition metal element in the
human bodies, and the ionic state of zinc (Zn2+) is involved in a variety of biological
processes such as gene transcription, DNA binding, neural signal transmission and
metalloenzyme functioning [1-6]. In general, a normal quantity of Zn2+ is essential
and beneficial for human health in our everyday life [7]. However, toxicity is
exhibited with overloading amount of Zn2+, causing some overt poisoning symptoms
and neurodegenerative disorders like Alzheimer’s disease, Parkinson’s disease,
Menkes disease and familial amyotropic lateral sclerosis [8-9]. On a basis of these
considerations, it is important to develop an efficient method for monitoring the
concentration level of Zn2+ in environmental assays and biological systems.
In the past few decades, there have been many methods reported for detecting
various transition metal ions, especially Zn2+ ion [10-14]. Among these methods,
fluorescence detection has been identified as the most convenient tool for sensing
Zn2+ in environmental research and analytical fields due to its good selectivity, high
sensitivity, rapid response and facile operational procedure [15-17]. To date, a number
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of fluorescent probes have been designed and synthesized for the detection of Zn2+
[18-26], but the practical applicability of these Zn2+ fluorescent probes is limited
because of the same group of zinc and cadmium in the periodic table, leading to
similar electron configurations and binding properties between Zn2+ and Cd2+ [27-30].
Additionally, to the best of our knowledge, no studies about the comparison of two
fluorescent probes with similar structures for sensing Zn2+ have been reported.
Therefore, it is still challenging to design two similar fluorescent probes with good
selectivity and high sensitivity in the existence of other various interfering metal ions
which exhibit different responses to Zn2+.
Out of consideration of these circumstances, we have designed two similar
schiff-base derived receptors which were synthesized by condensing 8-hydroxy
quinoline with 2, 4-dihydroxy-benzaldehyde (HL1) and 2, 4-dihydroxy-acetophenone
(HL2) respectively. Upon binding of Zn2+, the former receptor HL1 showed
remarkable enhancement in fluorescence emission intensity at 450 nm, while
significantly lower fluorescence emission enhancement was observed in the case of
the latter receptor HL2, which was not enough to detect the concentration level of
Zn2+ with high sensitivity. In addition, the selectivity of HL2 was too poor to
recognize Zn2+ environmentally and biologically. Therefore, HL1 could be utilized as
a selective fluorescent probe for Zn2+ in the existence of most competing metal ions in
environmental and biological systems.
2. Experimental
2.1. Materials and instruments
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All reagents for synthesis were obtained commercially and used with no further
depuration. All the used metal ions were originated form nitrate salt. 1H NMR spectra
were measured on a Bruker 400 MHz instruments using TMS as an internal standard
and DMSO-d6 as a solvent. UV-vis absorption spectra were determined on a
Shimadzu UV-240 spectrophotometer. ESI-MS analyses were performed using a
Bruker esquire 6000 spectrometer. Fluorescence spectra were recorded on a Hitachi
RF-4500 spectrophotometer equipped with quartz cuvettes of 1 cm path length. FT-IR
spectra were obtained with KBr discs on a Therrno Mattson FT-IR spectrometer in the
4000-400cm-1 region. The melting points of the compounds were determined on a
Beijing XT4-100x microscopic melting point apparatus with no correction.
2.2. Synthesis
2.2.1. Synthesis of 2-(quinolin-8-yloxy) acetohydrazide (2)[31]
To a solution of 8-hydroxyquinoline (0.5 mol) in dry acetone (150 mL), ethyl
chloroacetate (0.60 mol) followed by anhydrous K2CO3 (1.0 mol) was added. After
agitating the reaction mixture at ambient temperature for 24 hours, the solvent was
removed under reduced pressure and the residual was treated with water, extracted
with benzene for three times and the collected organic layer was dried by anhydrous
Na2SO4. Subsequently, the mixture was filtered and the filtrate was evaporated under
reduced pressure to give 1 as a liquid ester. 1H NMR (400 MHz, DMSO-d6) (Fig. S1):
δ 8.93 (d, 1H, J = 4.0 Hz, H1), 8.12 (d, 1H, J = 4.0 Hz, H3), 7.44-7.25 (m, 3H, H2, 4, 5),
6.95 (d, 1H, J = 4.0 Hz, H6), 4.93 (s, 1H, H7), 4.26 (q, 2H, J = 8.0 Hz, H8), 1.26 (t, 3H,
J = 8.0 Hz, H9). The ester was directly used for the next step without further
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purification. Hydrazine hydrate (80%, 10 equiv.) was added dropwise to an ethanolic
solution (40 mL) of ethyl (quinolin-8-yloxy) acetate. After refluxing the reaction
mixture under agitating for 24 h, half of the solvent was evaporated under reduced
pressure and the residual was standing overnight in the refrigerator. Subsequently, the
obtained precipitate was filtered under reduced pressure, and the crude product was
recrystallized from ethanol to furnish 2 as a white needle crystal. Yield: 55%, Mp:
140-141 °C, 1H NMR (400 MHz, CDCl3-d6) (Fig. S2): δ 9.65 (s, 1H, H8), 8.91 (d, 1H,
J = 4.2 Hz, H6), 8.17 (d, 1H, J = 8.3 Hz, H4), 7.52-7.44 (m, 3H, H2, 3, 5), 7.15 (d, 1H, J
= 7.2 Hz, H1), 4.84 (s, 2H, H7), 3.98 (s, 2H, H9).
2.2.2. Synthesis of HL1
An ethanolic solution (20 mL) of 2-(quinolin-8-yloxy) acetohydrazide (1 mmol)
was added to another ethanolic solution (20 mL) containing 2,
4-dihydroxy-benzaldehyde (1 mmol). After refluxing the reaction mixture under
agitating for 10 h, half of the solvent was evaporated under reduced pressure. Then
the reaction mixture was cooled to ambient temperature, leading to an appearance of
the white precipitate, which was filtered under reduced pressure. Subsequently, the
crude product was recrystallized from dimethyl formamide (DMF) to afford HL1 as a
1
white needle crystal. Yield: 66%, M.P.: 242–244oC. H NMR (400 MHz, DMSO-d6)
(Fig. S3): δ 11.18 (s, 1H, H16), 9.94 (s, 1H, H15), 9.78 (s, 1H, H8), 8.92 (d, 1H, H6),
8.38-8.33 (m, 3H, H1, 4, 9), 7.58-7.49 (m, 4H, H2, 3, 5, 14), 7.35 (m, 1H, H12), 6.33-6.29
13
(m, 2H, H11, 13), 4.88 (s, 2H, H7). C NMR (DMSO-d6, 100 MHz): 173.04, 161.07,
159.22, 156.34, 154.79, 149.10, 140.01, 135.63, 131.84, 128.85, 126.27, 121.31,
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119.76, 111.01, 108.42, 106.97. 103.03, 78.66. MS (ESI) m/z 338.1 [M+H]+ FT-IR
.
(KBr pellet, cm-1) (Fig. S9): 3438, 3061, 1678, 1622.
2.2.3. Synthesis of HL2
An ethanolic solution (20 mL) of 2-(quinolin-8-yloxy) acetohydrazide (1 mmol)
was added to another ethanolic solution (20 mL) containing 2,
4-dihydroxy-acetophenone (1 mmol). After refluxing the reaction mixture under
agitating for 12 h, half of the solvent was evaporated under reduced pressure. Then
the reaction mixture was cooled to ambient temperature, leading to an appearance of
the white precipitate, which was filtered under reduced pressure. Subsequently, the
crude product was recrystallized from dimethyl formamide (DMF) to afford HL2 as a
white needle crystal. Yield: 52%. M.P.: 256–258oC.1H NMR (400 MHz, DMSO-d6)
(Fig. S4): δ 13.19 (s, 1H, H16), 11.31 (s, 1H, H15), 9.85 (s, 1H, H8), 8.88 (d, 1H, H6),
8.35 (d, 1H, H1), 7.59-7.49 (m, 3H, H2, 5, 14), 7.41 (d, 1H, H3), 7.27 (d, 1H, H4),
13
6.31-6.21 (m, 2H, H11, 13), 4.99 (s, 2H, H7), 2.38 (s, 3H, H9). C NMR (DMSO-d6,
100 MHz): 172.21, 160.93, 159.03, 154.37, 154.30, 148.71, 140.00, 134.77, 130.08,
128.28, 126.82, 120.93, 116.27, 109.09, 106.90, 103.29. 104.07, 78.66. 12.63. MS
(ESI) m/z 352.3 [M+H]+ FT-IR (KBr pellet, cm-1) (Fig. S10): 3438, 3053, 1679, 1610.
.
2.3. Analysis
Stock solutions of various cations (5 mM) were prepared using nitrate salts in
ethanol. Stock solutions of HL1 and HL2 (5 mM) were also prepared in ethanol. Test
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solutions were prepared by placing 20 μL of the probe stock solution into cuvettes,
adding an appropriate aliquot of each ions stock, and diluting the solution to 2 mL
with ethanol. The excitation wavelength was set at 375 nm and the emission
wavelength was recorded at 450 nm. Both the excitation and emission slit widths were
5.0 nm. After introduction of the ions for 2 min, fluorescence emission spectra were
recorded at ambient temperature.
3. Results and discussion
3.1. Fluorescence emission response of HL1 to Zn2+ in various solvents
First of all, fluorescence emission response of HL1 to Zn2+ was investigated in
various solvents to determine which solvent was the best choice to detect Zn2+ by HL1.
For this purpose, we conducted fluorescence emission spectra of HL1 with addition of
Zn2+ in ethanol, methanol, dimethyl formamide (DMF), tetrahydrofuran (THF),
acetonitrile (CH3CN), trichloromethane (CHCl3) and a mixture of ethanol and water
(1 : 1). As shown in Fig. S5, the most overt fluorescence emission band at 450 nm
was observed in the ethanolic solution of HL1 with Zn2+, while inappreciable bands
were obtained under other solvent conditions. Moreover, it was interesting to note that
the fluorescence emission intensity at 450 nm also increased significantly in a mixture
of ethanol and water (1 : 1) but its value was lower than that in pure ethanol (Fig. S5).
As a result, ethanol was chosen to investigated the UV-vis absorption and
fluorescence emission responses of HL1 to Zn2+ in the following experiments.
3.2. UV-vis absorption response of HL1 to Zn2+
Under the optimal solvent condition of ethanol, UV-vis absorption response of
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HL1 to Zn2+ was tested by introducing increasing amounts of Zn2+ into an ethanolic
solution of HL1, and the corresponding spectra were illustrated in Fig. 1. HL1 alone
displayed two absorption bands centered at 321nm and 295nm, and these two bands
decreased gradually accompanied by an appearance of a new absorption band
centered at 306nm and a shoulder peak at 367nm with incremental addition of Zn2+,
emerging two isosbestic points at 341nm and 267nm (Fig. 1), which indicated that a
stable complex was formed between HL1 and Zn2+ in ethanol [32-33].
3.3. Fluorescence emission response of HL1 to Zn2+
Selectivity is a vital factor that must be considered for the construction of
fluorescent probes [34], There are many probe for Zn2+ reported and they showed
good selectivity,We have gave a table in the manuscript to show what merit they
achieved (table 1)[35-39]. So as to ascertain if HL1 exhibited good selectivity towards
Zn2+ in the existence of other various interfering metal ions, these corresponding
metal ions (Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, In3+, Li+, Mg2+, Mn2+,
Na+, Ni2+, Pb2+, Al3+, K+ and Ag+) was separately added to the ethanolic solution of
HL1 in the absence and existence of Zn2+. From Fig. 2 (a) depicting the fluorescence
emission spectra of HL1 with introduction of various metal ions in ethanol with
excited at 375 nm, we could clearly observe that only the introduction of Zn2+ induced
remarkable enhancement in fluorescence emission band of HL1 centered at 450 nm,
and the fluorescence emission intensity at 450 nm (F450 nm) of HL1 only increased
sharply with addition of Zn2+. Additionally, upon addition of other various interfering
metal ions to the ethanolic solution of HL1 with Zn2+, the values of F450 nm hardly
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changed except for the cases of Cr3+ and Hg2+, which quenched the fluorescence
emission to a certain degree due to the photoinduced electron and energy transfer
process from these two metal ions to HL1 (Fig. 2 (b)). These results suggested that
HL1 exhibited good selectivity towards Zn2+ in the existence of most of other various
interfering metal ions, for that these metal ions did not interfere with the detection of
Zn2+ by HL1.
Subsequently, fluorescence emission spectra of HL1 in the absence and existence
of increasing amounts of Zn2+ were carried out in ethanol to explore the binding
property of HL1 with Zn2+. As can be seen from Fig. 3 (a), upon excitation at 375 nm,
negligible fluorescence emission band was observed in free HL1 due to the
photoinduced electron-transfer (PET) phenomenon from the lone pair electrons of
Schiff-base nitrogen atom to the benzene moiety in HL1 [39-40], but the emission
band centered at 450 nm emerged with increasing intensity upon addition of
incremental addition of Zn2+, which was accompanied by an apparent color change
from colorless to blue upon illumination of a UV lamp at 375 nm (Fig. 3 (a)). The
reason was caused by the inhibition of the PET phenomenon upon complexation of
HL1 with Zn2+ (Scheme 1). Nevertheless, in the case of HL2 in which one methyl
group was introduced to the azomethine carbon, the addition of the identical amount
of Zn2+ caused significantly lower fluorescence emission enhancement than that of
HL1 (Fig. 3 (b)), which was probably attributed that the introduction of methyl group
made the steric hindrance of HL2 larger, thus the inhibition of the PET phenomenon
was not sufficient enough to restore the fluorescence emission. Accordingly, based on
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the good linearship between the fluorescence emission intensity at 450 nm (F450 nm
)
and Zn2+ concentration from fluorescence titration experiments, the limit of detection
(LOD) was calculated to be 3.5 × 10-7 M (Fig. S6), suggesting high sensitivity. These
results indicated that HL1 could be identified as a fluorescent probe for detecting and
monitoring the concentration level of Zn2+ in environmental and biological systems.
On a basis of the fluorescence emission enhancement of HL1 caused by Zn2+, we
deduced that HL1 could bind with Zn2+ through the coordination process to form a
stable complex HL1-Zn2+. Therefore, we conducted a Job’s plot experiment to
determine the binding stoichiometry between HL1 and Zn2+ by recording the
fluorescence emission intensity at 450 nm (F450 nm) of a mixture of HL1 and Zn2+ in
ethanol. In this regard, the molar ratio of HL1 in this mixture ([HL1]/[HL1] + [Zn2+])
varied from 0.1 to 0.9 with the total concentration of HL1 and Zn2+ keeping constant
at 100 μM. As depicted in Fig. 4, the value of F450 nm reached a maximum at the molar
ratio of 0.5, suggesting a 1 : 1 binding stoichiometry between HL1 and Zn2+.
According to the 1 : 1 binding stoichiometry, the binding constant (K) of HL1 with
Zn2+ was further calculated based on nonlinear Benesi-Hildebrand equation from
fluorescence titration experiments [41], and the result was found to be 2.3 × 104 M-1
(Fig. S7), proving strong binding property of HL1 with Zn2+.
In addition, reversibility and regeneration were also a crucial trait for
extending the practical applications of fluorescent probes for sensing a specific
analyte. Based on this consideration, we successively introduced Zn2+ and Na2EDTA
into the ethanolic solution of HL1, and recorded the fluorescence emission spectra and
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the corresponding intensity at 450 nm (F450 nm). As can be seen from Fig. 5(a), the
introduction of Na2EDTA to a mixture of HL1 and Zn2+ induced an obvious decrease
in fluorescence emission band centered at 450 nm, which was almost recovered to the
original state by the successive addition of the same amount of Zn2+, confirming the
good reversibility and generation of HL1. Furthermore, the reversibility and
generation could be repeated for 4 times with negligible changes in the value of F450
nm. From the results above, it was concluded that the reversibility and generation of
HL1 were sufficiently perfect for extending the practical applications of HL1 for
sensing Zn2+. In the presence of metal ions, the fluorescence intensity of HL1 was
specified depending on the response times for 10, 20, 30, 40, 50 and 60s. Fig. 6
indicates that the HL1 responses to Zn2+ in the shortest duration as 0-30 s. The
experimental data for response time show that HL1 has a more positive results for
Zn2+.
3.4. 1H NMR titration
1
To better understand the binding behavior of receptor HL with Zn2+, H NMR
titration, Zn2+ (as its nitrate salt) was added to the DMSO-d6 solution of HL1 (Fig.
S8). In the 1H NMR of the neutral HL1, the normal signal is located at about 10 ppm,
its downfield location (at 11.97 ppm) is attributed to the intramolecular hydrogen
bonding of the Salicylaldehyde hydrogen with imine nitrogen which was further
conformed by related article [42-43]. Hence, the two groups of peaks were correspond
1
to the major (a) and minor (b) isomers in H NMR spectra as shown in Fig. 7 and
compared with the configuration b, the chemical shift of the configuration (a) is
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located in the low field due to the formation of a strong intra-molecular hydrogen
bond (OH-N) with the imine-nitrogen [44-45]. The spectral differences are depicted in
Fig. 8. Upon complexation with Zn2+ ion, the proton signal of the Salicylaldehyde
hydroxyl (H16a) at 11.97 ppm and the imine (H8a) at 11.37 ppm gradually weaken and
disappeared, the proton peak of the naphthol hydroxyl (H8b)at 10.73 ppm disappeared.
These might be attributed to the binding mode of HL1with Zn2+, resulting in breaking
hydrogen bonds and the deprotonation. In addition, the proton peak (H9) was shifted
0.4ppm to the low field. Meanwhile, The other aromatic protons showed a slight
change for the system. The results suggest that Zn2+ is coordinated with the imine
nitrogen atom, the hydroxyl oxygen atom of HL1 [46-48].
4. Conclusion
In conclusion, we designed two similar schiff-base derived receptors which were
synthesized by condensing 8-hydroxy quinoline with 2, 4-dihydroxy-benzaldehyde
(HL1) and 2, 4-dihydroxy-acetophenone (HL2) respectively. Between them, HL1
displayed significant enhancement in fluorescence emission band centered at 450 nm
due to the inhibition of the photoinduced electron-transfer (PET) phenomenon, while
in the case of HL2 in which one methyl group was introduced to the azomethine
carbon, the addition of the identical amount of Zn2+ caused significantly lower
fluorescence emission enhancement than that of HL1, which was because that the
inhibition of the PET phenomenon was not sufficient enough to restore the
fluorescence emission. Additionally, HL1 exhibited good selectivity and high
sensitivity towards Zn2+ in the existence of most of other various interfering metal
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ions, for that these metal ions did not interfere with the detection of Zn2+ by HL1, and
the limit of detection (LOD) of HL1 for sensing Zn2+ reached 3.5 × 10-7 M. Moreover,
a 1 : 1 binding stoichiometry between HL1 and Zn2+ was determined by a Job’s plot
experiment, and the reversibility and generation of HL1 were sufficiently perfect for
extending the practical applications of HL1 for sensing Zn2+. As a result, the
experimental results might accelerate the development of novel fluorescent probes
which have similar structures but distinct fluorescence emission responses to a
specific analyte.
Acknowledgments
This work was supported by the National Natural Science Foundation of China
(21761019) and the Young Foundation of Jinchang (jrc201811).
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Figure captions
Fig. 1. Change in UV-vis absorption spectra of HL1 (50.0 μM) in ethanol after
addition of various amounts of Zn2+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
equiv.).
Fig. 2. (a) Changes in fluorescence emission spectra of HL1 (50.0 μM) with addition
of Zn2+ (1.0 equiv.) and other various interfering metal ions (5.0 equiv.) in ethanol. (b)
Histogram indicating fluorescence emission intensity at 450 nm (F450 nm) of HL1 (50.0
μM) with addition of various interfering metal ions (5.0 equiv.) in the absence and
existence of Zn2+ (1.0 equiv.) in ethanol. Black bar: HL1 and HL1 + metal ions, red
bar: HL1 + Zn2+ and HL1 + Zn2+ + metal ions. (λex = 375 nm, slit: 5.0/5.0 nm).
Fig. 3. (a) Change in fluorescence emission spectra of HL1 (50.0 μM) with addition
of various amounts of Zn2+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 equiv.) in
ethanol. (b) Fluorescence emission spectra of HL1 (50.0 μM) and HL2 (50.0 μM) in
the absence and existence of Zn2+ (1.0 equiv.) in ethanol. (λex = 375 nm, slit: 5.0/5.0
nm).
Fig. 4. A Job’s plot for determining the binding stoichiometry between HL1 and Zn2+
in ethanol (XHL1 = [HL1]/([HL1] + [Zn2+], the total concentration of HL1 and Zn2+
was 100 μM).
Fig. 5. (a) Change in fluorescence emission spectra of HL1 (50.0 μM) with Zn2+ (1.0
equiv.) upon successive additions of Na2EDTA (1.0 equiv.) and Zn2+ (1.0 equiv.) in
ethanol. (b) Change in fluorescence emission intensity at 450 nm (F450 nm) of HL1
(50.0 μM) upon alternate additions of Zn2+ (1.0 equiv.) and Na2EDTA (1.0 equiv.) in
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ethanol after 4 cycles. (λex = 375 nm, slit: 5.0/5.0 nm).
Fig. 6. Changes in maximum fluorescence intensity of HL1 for different contact time.
Fig. 7. The major (a) and minor (b) isomers.
1
Fig. 8. H NMR spectra of HL1 with metal ions in DMSO-d6: (I) HL1; (II) HL1 with
Zn2+.
Scheme 1. Proposed mechanism for the detection of Zn2+ by HL1.
Table 1. Comparison of various probes of Zn2+.
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Fig. 1. Change in UV-vis absorption spectra of HL1 (50.0 μM) in ethanol after
addition of various amounts of Zn2+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
equiv.).
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Fig. 2. (a) Changes in fluorescence emission spectra of HL1 (50.0 μM) with addition
of Zn2+ (1.0 equiv.) and other various interfering metal ions (5.0 equiv.) in ethanol. (b)
Histogram indicating fluorescence emission intensity at 450 nm (F450 nm) of HL1 (50.0
μM) with addition of various interfering metal ions (5.0 equiv.) in the absence and
existence of Zn2+ (1.0 equiv.) in ethanol. Black bar: HL1 and HL1 + metal ions, red
bar: HL1 + Zn2+ and HL1 + Zn2+ + metal ions. (λex = 375 nm, slit: 5.0/5.0 nm).
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Fig. 3. (a) Change in fluorescence emission spectra of HL1 (50.0 μM) with addition
of various amounts of Zn2+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 equiv.) in
ethanol. (b) Fluorescence emission spectra of HL1 (50.0 μM) and HL2 (50.0 μM) in
the absence and existence of Zn2+ (1.0 equiv.) in ethanol. (λex = 375 nm, slit: 5.0/5.0
nm).
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Fig. 4. A Job’s plot for determining the binding stoichiometry between HL1 and Zn2+
in ethanol (XHL1 = [HL1]/([HL1] + [Zn2+], the total concentration of HL1 and Zn2+
was 100 μM).
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Fig. 5. (a) Change in fluorescence emission spectra of HL1 (50.0 μM) with Zn2+ (1.0
equiv.) upon successive additions of Na2EDTA (1.0 equiv.) and Zn2+ (1.0 equiv.) in
ethanol. (b) Change in fluorescence emission intensity at 450 nm (F450 nm) of HL1
(50.0 μM) upon alternate additions of Zn2+ (1.0 equiv.) and Na2EDTA (1.0 equiv.) in
ethanol after 4 cycles. (λex = 375 nm, slit: 5.0/5.0 nm).
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Fig. 6. Changes in maximum fluorescence intensity of HL1 for different contact time.
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Fig. 7. The major (a) and minor (b) isomers.
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1
Fig. 8. H NMR spectra of HL1 with metal ions in DMSO-d6: (I) HL1; (II) HL1 with
Zn2+.
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Scheme 1. Proposed mechanism for the detection of Zn2+ by HL1.
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Table1. Comparison of various probes of Zn2+.
Selectivity
Zn2+ Al3+
Fluorescence or
colorimetric
reversibility mechanism detection
limits
solvent
Fluorescence and
colorimetric
/
ICT
3.75ꢀnM
MeOH-H2O
Zn2+
Zn2+ Cu2+
Zn2+
Fluorescence
Fluorescence
Fluorescence
Fluorescence and
colorimetric
/
ESIPT
PET
67.2ꢀnM
17ꢀnM
EtOH/PBS buffer
CH3CN/H2O
reversibility
/
CHEF
PET
37.13ꢀnM
10.3ꢀnM
aqueous medium
DMF/HEPES buffer
medium
Zn2+
reversibility
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Credit Author Statement
Long Fan ： Visualization, Writing - Original Draft, Investigation, Resources,
Development or design of methodology
Jing-can Qin：Resources,, creation of models
Chao-rui Li：Resources
Zheng-yin Yang：Project administration
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