β-Hydroxy-α-naphthaldehyde [2-(quinolin-8′-yloxy) acetyl] hydrazone as an efficient fluorescent chemosensor for Mg2+

Article abstract of DOI:10.1016/j.saa.2014.12.094

An efficient fluorescent sensor based on β-hydroxy-α-naphthaldehyde [2-(quinolin-8′-yloxy) acetyl] hydrazone (HL) for Mg²⁺ has been designed and synthesized. The receptor showed "off-on" fluorescent responses toward Mg²⁺ in acetonitrile. The reasons for this phenomenon is that the addition of Mg²⁺ to the solution of HL induce the formation of a 1:1 ligand-metal complex which inhibit excited-state intramolecular proton transfer (ESIPT).

Full text of DOI:10.1016/j.saa.2014.12.094

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 21–26
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
0
b-Hydroxy-
a
-naphthaldehyde [2-(quinolin-8 -yloxy) acetyl] hydrazone
2+
as an efficient fluorescent chemosensor for Mg
⇑
Jing-can Qin, Zheng-yin Yang , Long Fan, Bao-dui Wang
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
The sensor is applied to recognition of
2
+
Mg
.
It shows great fluorescence based on
excited-state intramolecular proton
transfer.
ꢀ
It has lower detection limit.
a r t i c l e i n f o
a b s t r a c t
0
Article history:
Received 9 October 2014
Received in revised form 29 November 2014
Accepted 26 December 2014
Available online 2 January 2015
An efficient fluorescent sensor based on b-hydroxy-a-naphthaldehyde [2-(quinolin-8 -yloxy) acetyl]
2+
hydrazone (HL) for Mg has been designed and synthesized. The receptor showed ‘‘off–on’’ fluorescent
2+
2+
responses toward Mg in acetonitrile. The reasons for this phenomenon is that the addition of Mg to
the solution of HL induce the formation of a 1:1 ligand–metal complex which inhibit excited-state
intramolecular proton transfer (ESIPT).
Ó 2014 Published by Elsevier B.V.
Keywords:
Fluorescence sensor
Naphthalene derivative
Magnesium ion
ESIPT
Introduction
conformation [5–9]. In addition, magnesium ion is also required
for the proper functioning of nerves and immune systems and for
Fluorescent chemosensors with high selectivity, sensitivity and
real-time detection have been developed to be efficient tools to rec-
ognition of metal ions in biological industry and environmental pro-
cesses [1–4]. Among various chemosensors, the reports about
fluorescent sensor for Mg2 are still few in number. Magnesium
ion is one of the most abundant divalent metal ions in biological sys-
tems. As an important trace element, it plays an active role in many
biological processes at the cellular level such as proliferation of cells,
enzyme-driven biochemical reactions and stabilization of DNA
muscle and bone health. Its concentrations in cells typically varies
between 0.1 mM and 6 mM: ꢁ0.3 mM in synaptosomes; 0.37 mM
in hepatocytes; 0.5–1.2 mM in cardiac cells, while 0.44–1.5 mM in
normal serum [10]. We can apply these concentrations to diagnose
whether the human body is in diseased states. Therefore, in order to
protect the environment and human health, it is crucial to develop
an effective fluorescent probe for detection of the concentration
levels of magnesium ion.
+
Schiff-bases are well known to be good ligands, which provide a
nitrogen–oxygen-rich coordination environment for metal ions
[
11–13]. The ligands with a fluorescence group will probably shows
⇑
Corresponding author. Tel.: +86 931 8913515; fax: +86 931 8912582.
E-mail address: yangzy@lzu.edu.cn (Z.-y. Yang).
characteristic fluorescence emission in the presence of metal ions,
http://dx.doi.org/10.1016/j.saa.2014.12.094
386-1425/Ó 2014 Published by Elsevier B.V.
1

2
2
J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 21–26
which make it feasible to develop a fluorescent probe [14–16], espe-
cially naphthalene derivative.
achi RF-5301 spectrophotometer equipped with quartz cuvettes of
1 cm path length. The melting point of the Schiff base was deter-
mined on a Beijing XT4-100ꢃ microscopic melting point apparatus.
Compared with others compound, 2-hydroxy-1-naphthalde-
hyde Schiff-bases are characterised by the presence of the –OHꢂꢂꢂN
intramolecular hydrogen bond involved in a six-membered chelate
ring. Nevertheless, the existence of –OHꢂꢂꢂꢂꢂN or OꢂꢂꢂH–N type
hydrogen bonds is a prerequisite for ESIPT process which is used
to design fluorescent probe [17–20]. So-called ESIPT process, and,
in short, commonly involves the transfer of a hydroxyl (or amino)
proton to an adjacent carbonyl oxygen (or imine nitrogen) through
a five- or six-membered ring of hydrogen-bonding configuration.
Once coordinated with metal ions, the interaction of the receptor
and metal ions will remove the proton of the receptor, which
results in inhibiting the ESIPT processes, as a result, a significant
emission enhancement can be observed [21–22]. So Far, there are
few fluorescent probes based on excited-state intramolecular pro-
ton transfer (ESIPT), thus, the design and synthesis of multi-ion
selective fluorescent probes based on ESIPT have attracted consid-
erable attention [23–24].
Synthesis
Synthesis of 2-hydroxy-1-naphthaldehyde [26]
A mixture of 2-naphthol (10.5 g, 73 mmol) and hexamethylene-
tetramine (12 g, 86 mmol) was dissolved in 20 mL of acetic acid and
stirred and heated for 1 h at 50–60 °C. Then rising the temperature
to 90 °C, H SO (98%, 9 mL) was added dropwise by dropping funnel
2 4
within 30 min. Upon completion, the reaction mixture was heated
to 96 °C and stirred for additional 4 h, and then cooled to room tem-
2
perature pouring into 100 mL H O. The crude product was filtered
and washed to pH 7 with cold water. The final product was recrys-
tallized from ethanol to afford HN as a yellow solid and allowed to
1
vacuum drying in low temperature. Yield: 55%, Mp: 79–80 °C,
NMR (Fig. S2): (400 MHz; CDCl
H
3
) (Fig. S1) d (ppm) = 13.1(s, 1H),
1
7
0.83(s, 1H), 8.36(d, 1H), 7.99(d, 1H), 7.81(d, 1H), 7.63(m, 1H,),
.45(m, 1H), 7.15(d, 1H).
With this in mind, our choice of 2-hydroxy-1-naphthaldehyde
as a second moiety of the Schiff-base develop an on–off fluorescent
responsive molecular sensors for Mg2 (Scheme 1). In fact, the sen-
sor showed ‘‘off–on’’ fluorescent responses toward Al3 which is
reported by related article [25]. But, more interestingly, the detec-
tion of the sensor could be switched for Mg2+ by swapping the
solvent.
+
Synthesis of 2-(quinolin-8-yloxy) acetohydrazide
+
To a solution of 8-hydroxy quinoline (0.5 mol) in dry acetone
(
150 mL), ethyl chloroacetate (0.60 mol) were added followed by
anhydrous K CO (1.0 mol), After 24 h, the solvent was removed
2
3
and the reaction mixture was treated with water, extracted with
benzene three times and the collected organic layer was dried on
anhydrous Na SO , filtered and evaporated to give liquid ester.
2 4
Experimental
The ester was directly used for next step without further purifica-
tion. Hydrazine hydrate (80%, 10 equiv.) was added dropwise to
an ethanol solution (40 mL) of ethyl (quinolin-8-yloxy) acetate.
The mixture was refluxed under stirring for 24 h. Then the solvent
was concentrated under reduced pressure and let stand overnight
in refrigerator. A white needle crystal was observed. The final prod-
Apparatus and reagents
Unless mentioned otherwise, all chemicals for synthesis were
purchased from commercial suppliers and used without further
1
purification. H NMR spectra were measured on the JNM-ECS400
instruments using TMS as an internal standard. ESI-MS were deter-
mined on a Bruker esquire 6000 spectrometer. UV–Vis absorption
spectra were recorded on a Perkin Elmer Lambda 35 UV–Vis
spectrophotometer. Fluorescence spectra were generated on a Hit-
uct was filtered, dried and recrystallized from ethanol. Yield: 55%,
1
Mp: 140–141 °C, H NMR (Fig. S2): (400 MHz; CDCl
3
) d = 3.98 (s,
), 7.44–7.52 (m, H ,H
), 8.17 (d, J = 8.3 Hz, H ), 9.65 (s, H ).
2
9
H ), 4.84 (s, 2H
7
), 7.15 (d, J = 7.2 Hz, H
1
2
3
,
H
5
), 8.91 (d, J = 4.2 Hz, H
6
4
8
Synthesis of HL
An ethanol solution (20 mL) of 2-(quinolin-8-yloxy) acetohyd-
razide (1 mmol) was added to another ethanol (20 mL) containing
2
-hydroxy-1-naphthaldehyde (1 mmol). Then the solution was
refluxing for 12 h under stirring and some white precipitant
appeared. Excess solvent was removed under reduced pressure.
After cooling to room temperature, the mixture was filtered and
1
dried. Recrystallization from DMF. Yield: 76%, Mp: 205–206 °C, H
NMR (Fig. S3): (400 MHz; DMSO-d
6
): the major: d = 4.98 (s, 2H
), 7.29 (d, J = 8.5 Hz, H15), 7.35 (m, H12), 7.47–
, H11), 7.84 (d, J = 8 Hz, H10), 7.88 (d, J = 9 Hz,
13), 8.24 (d, J = 8.5 Hz, H14), 8.35 (dd, J = 8.4 Hz, J = 1.7 Hz, H ),
.93 (dd, J = 4.1 Hz, J = 1.7 Hz, H ), 9.39 (s, H ), 12.17 (s, H ), 12,48
s, H16). The minor: d = 5.38 (s, 2H ), 7.12 (d, J = 9 Hz, H ), 7.31 (m,
15), 7.38 (m, H12), 7.40–7.67 (m, H , H , H , H11), 7.83 (d, J = 9 Hz,
13), 7.99 (d, J = 8 Hz, H10), 8.30 (dd, J = 8.3 Hz, J = 1.7 Hz, H ), 8.71
), 8.89 (s, H ),
). ESI-MS (Fig. S4): [M+1] : 372.092.
7
),
7
7
.18 (d, J = 9 Hz, H
.61 (m, H , H , H
5
1
2
3
H
4
8
6
9
8
(
7
1
H
H
2
3
5
4
(
d, J = 8.5 Hz, H14), 8.83 (dd, J = 4.1 Hz, J = 1.7 Hz, H
6
9
+
1
0.73(s, H16), 11,48 (s, H
8
+
[
M+Na] : 394.04.
Results and discussion
General information
Scheme 1. Reagents and conditions: (a) ethyl chloroacetate, acetone, anhydrous
Stock solutions of various cations (1 mM) were prepared using
nitrate salts. A stock solution of HL (1 mM) was prepared. The ions
K
2
CO
3
, reflux, 24 h; (b) EtOH, N
2
H
4 2
ꢂH O, 80 °C, 20 h; (c) HMTA, glacial acetic acid
reflux, 6 h (d) EtOH, reflux, 8–10 h.

J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 21–26
23
Fig. 1. Changes in the absorption spectra of HL (50
l
M) in acetonitrile at room
temperature as a function of added Mg (0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and
M).
2
+
50 l
Fig. 3. Fluorescence intensity of HL and its complexation with Mg2+ in the presence
stock solution was then diluted for 100 times with ethanol in a
quartz optical cell of 1 cm optical path length. The ions stock solu-
tion was added into the quartz optical cell gradually by using a pip-
ette. Spectral data were recorded at 3 min after addition of the
ions. Both the excitation and the emission slit widths were 3.0 nm.
The detection limit was calculated based on the fluorescence
titration. To determine the S/N ratio, the detection limit is then
of various metal ions in acetonitrile. Black bar: HL (50.0 lM) and HL with 1.0 equiv.
+
+
+
+
2+
2+
3+
2+
2+
2+
2+
3+
2+
2+
2+
2+
of Li , Na , K , Ag , Ca , Hg , Cr , Mn , Fe , Ni , Co , Fe , Cu , Ba , Cd , Zn
,
and Al3 stated. Red bar: 50.0
+
l
M of HL and 1 equiv. of Mg ; 50.0 lM of HL and
2+
1
.0 equiv. of Mg2 with 1.0 equiv. of metal ions stated in acetonitrile (kex = 398 nm).
+
(
For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
calculated with the following equation: detection limit 3
r
/K.
addition, two clear isosbestic points at 325 nm, 365 nm, are
observed while two new absorption bands appear at 398 nm and
420 nm with increasing Intensity. All of these might be attributed
where is the standard deviation of blank measurements, and K
r
is the slope between intensity versus sample concentration [27].
to form a 1:1 stoichiometry of the binding mode of HL with Mg2+
,
resulting in breaking hydrogen bonds and the deprotonation which
was further confirmed by ESI-MS, NMR titration.
UV–Vis titrations
The binding ability of HL and metal ions was initially investi-
2
+
gated by UV–Vis titration. As shown in Fig. 1a, the free ligand in
Fluorescence response of HL to Mg
⁄
solution exhibits characteristic absorption bands due to the
p–p
transitions of the two configurations of naphthaldimine chromo-
The fluorometric behavior of HL was investigated upon addition
+
+
+
+
2+
2+
2+
3+
phore at about 310 nm, 317 nm, 357 nm and 362 nm [28–30].
of several metal ions such as Li , Na , K , Ag , Mg , Ca , Hg , Cr
,
2
+
2+
2+
2+
2+
3+
2+
2+
2+
2+
3+
Upon addition of Mg to a solution of HL, the absorption band at
10 nm and 317 nm gradually merged to one, the absorption band
at 357 nm and 362 gradually merged to one, respectively. In
Mn , Fe , Ni , Co , Fe , Cu , Ba , Cd , Zn , and Al . As shown
in Fig. 2, the receptor HL show weak fluorescence signal at 445 nm
when it is excited at 398 nm in acetonitrile. Upon the addition of
3
1
.0 equiv. of various metal ions, the fluorescence intensity of HL
only shows significant fluorescence enhancement in the presence
Fig. 2. Fluorescence spectra of HL (50 lM) upon the addition of metal salts
2+
1.0 equiv.) of Li , Na , K , Ag , Al , Ca2+, Pb , Cr , Mn2+, Fe , Ni , Co , Fe3+, Cu
,
+
+
+
+
3+
3+
2+
2+
2+
2+
(
2
+
2+
2+
2+
2+
2+
Fig. 4. Job’s plot for determining the stoichiometry of HL and Mg in acetonitrile
Ba , Cd , Hg , Zn , and Mg in acetonitrile, (kex = 398 nm, slit widths: 3 nm/
nm).
(X lM).
Mg = [Mg2 ]/([Mg ] + [HL]), the total concentration of HL and Mg was 50
+
2+
2+
3

2
4
J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 21–26
of Mg2+, other ions have almost no influence on the fluorescence
emission. Hence, the receptor is highly selective and sensitive for
2
+
Mg detection, which makes it feasible for biological and environ-
mental applications.
To get further insight into the recognition abilities of HL toward
Mg2+ competition experiments (Fig. 3), the Job’s plot analysis
(Fig. 4) and the fluorescence titration (Fig. 5) were carried out in
acetonitrile. From competition experiments, the receptor HL was
2
+
treated with 1.0 equiv. of Mg in the presence of other metal ions.
As shown in Fig. 3, the fluorescence intensity is low but detectable
except in the case of Cu(II), Fe(III), Co(II), Zn(II), which be attributed
to their inherent to the magnetic property. It means that the recep-
tor can be used as a selective fluorescent sensor for Mg2+ in the
presence of most competing metal ions.
From the Job’s plot analysis. As shown in Fig. 4, the Job’s plot of
2
+
2+
HL with Mg (at 445 nm), the L–Mg complex concentration
3+
approaches a maximum when the molar fraction of Al is 0.5,
which means HL and Mg forms a 1:1 complex. The result was fur-
2
+
Fig. 5. Fluorescence spectra of HL (50
NO (0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, and 55
98 nm. Inset: color of HL and HL + Mg system under UV lamp. (For interpretation
l
M) in acetonitrile upon the addition of Mg
ther conformed by the appearance of a peak at m/z 435.13, assign-
(
3
)
3 2
l
M) with an excitation of
2
+
+
2
+
3
able to [HL + Mg + CH CN-H] in the ESI-MS spectra (Fig. S5). In
of the references to color in this figure legend, the reader is referred to the web
version of this article.)
the fluorescence titration profile, the binding constant of HL with
2+
4
Mg has been estimated as 3.45 ꢃ 10 (Fig. S6) based on the mod-
ified Benesi–Hildebrand equations [31] respectively. The detection
2
+
ꢄ7
limit for Mg was calculated to 6.8 ꢃ 10 M (Fig. S7) and was suf-
ficiently low to detect submicromolar concentration of Mg2+
.
1
H NMR titration
To better understand the binding behavior of receptor HL with
Mg2+, H NMR titration, Mg (as its nitrate salt) was added to the
1
2+
1
DMSO-d
6
solution of HL (Fig. S8). In the H NMR of the neutral HL,
the normal signal is located at about 10 ppm, its downfield location
at 12.48 ppm) is attributed to the intramolecular hydrogen bond-
(
ing of the naphtholic hydrogen with imine nitrogen which was fur-
ther conformed by related article [25]. Hence, the two groups of
peaks were correspond to the major (a) and minor (b) isomers in
1
H NMR spectra as shown in Fig. 6 and compared with the config-
uration b, the chemical shift of the configuration (a) is located in
the low field due to the formation of a strong intra-molecular
hydrogen bond (OHꢂꢂꢂꢂN) with the imine-nitrogen [32–35]. The
spectral differences are depicted in Fig. 7. Upon complexation with
2
+
Mg ion, the proton signal of the naphthol hydroxyl (H16a) at
12.48 ppm and the imine (H8a) at 12.17 ppm gradually weaken
Fig. 6. The major (a) and minor (b) isomers.
and disappeared, the proton peak of the naphthol hydroxyl (H8b
)
Fig. 7. 1H NMR spectra of HL with metal ions in DMSO-d : (I) HL; (II) HL with Mg2+
.
6

J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 21–26
25
Scheme 2. Proposed mechanism for detection of Mg2+ by HL.
at 10.73 ppm disappeared, the proton signal of the imine (H8b
)
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Appendix A. Supplementary data
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