A highly selective fluorescent probe for Al3+ based on quinoline derivative

Article abstract of DOI:10.1016/j.saa.2015.05.041

Abstract A novel Schiff base fluorescent probe, 1-phenyl-3-methyl-5-hydroxypyrazole-4-carbaldehyde (2′-methylquinoline-4′-formyl) hydrazone (PMHCH), for selective detection of Al³⁺ has been designed and synthesized. Upon addition of various met

Full text of DOI:10.1016/j.saa.2015.05.041

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 21–25
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A highly selective fluorescent probe for Al3+ based on quinoline
derivative
⇑
Guan-qun Wang, Jin-can Qin, Chao-Rui Li, Zheng-yin Yang
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
Possible PET process of HL.
ꢀ
The receptor HL shows 286-fold
enhancement of fluorescence
intensity in the present of Al3+
.
ꢀ
ꢀ
The detection limit of HL for Al3+
could reach at 10ꢁ7 M level.
A new chemosensor for Al3+ via the
photo-induced electron transfer
effect have been reported.
a r t i c l e i n f o
a b s t r a c t
A novel Schiff base fluorescent probe, 1-phenyl-3-methyl-5-hydroxypyrazole-4-carbaldehyde (20-methyl
quinoline-40-formyl) hydrazone (PMHCH), for selective detection of Al3+ has been designed and synthe-
sized. Upon addition of various metal ions, the receptor only shows 286-fold enhancement of fluores-
cence intensity which might be attributed to a 1:1 stoichiometry between PMHCH and Al3+ and the
photo-induced electron transfer progress in the present of Al3+ at 505 nm. More importantly, the detec-
tion limit of PMHCH for Al3+ could reach at 10ꢁ7 M level.
Article history:
Received 1 November 2014
Received in revised form 23 April 2015
Accepted 3 May 2015
Available online 20 May 2015
Keywords:
Fluorescent probe
Schiff-base
Ó 2015 Published by Elsevier B.V.
Aluminum
Detection limit
Introduction
Parkinson’s disease and Alzheimer’s disease [4–8]. According to
The World Health Organization (WHO), the average daily human
Aluminum is the third most abundant element in the Earth’s
crust (approximately 8% of total mineral components) [1]. Its com-
pounds are widely used in food additives, cookware, drinking
water supplies, antiperspirants, deodorants, bleached flour, and
antacids [2,3]. Nevertheless, as a non-essential element for living
system, the extensive use of aluminum not only are deadly to
growing plants but could induce many diseases, such as
intake of Al3+ of around 3–10 mg and the weekly tolerable dietary
intake of 7 mg kgꢁ1 body weight [9]. The determination of very low
levels of aluminum has become increasingly very important in
environmental and clinical chemistry since its negative role in
the human life. Therefore, it is desirable to develop some new ana-
lytical methods for detecting Al3+
.
Among all detection techniques [10–13], fluorescent chemosen-
sors have attracted considerable attention due to its easily detect-
able signals upon recognition of metal ions with high sensitivity
and selectivity [14–17]. But the poor coordination ability, strong
⇑
Corresponding author. Tel.: +86 931 8913515; fax: +86 931 8912582.
E-mail address: yangzy@lzu.edu.cn (Z.-y. Yang).
http://dx.doi.org/10.1016/j.saa.2015.05.041
1386-1425/Ó 2015 Published by Elsevier B.V.

22
G.-q. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 21–25
hydration ability, and the lack of spectroscopic characteristics of Al3+
have hindered development of a suitable fluorescence sensor com-
pared to other metal ions [18]. Previous reports indicate that struc-
tures of most fluorescent probes for Al3+ contain nitrogen and
oxygen-rich coordination environments providing a hard base envi-
ronment for the hard acid Al3+ [19]. Therefore, design of selective
and sensitive chemo-sensors for Al3+ remains desirable. Schiff bases
are known to form stable complexes with metal ions and act as ion
carriers. The structures of Schiff bases contain nitrogen–
oxygen-rich coordination environments which provide
a
hard-base environment for the hard-acid Al3+. The resulting Schiff
base complexes have attracted increasing attention in the area of
ionic binding due to their unique properties and reactivity [20,21].
Here, we have reported a Schiff base ligand 1-phenyl-3-me
thyl-5-hydroxypyrazole-4-carbaldehyde (20-methylquinoline-40- f
ormyl) hydrazone (PMHCH) as a chemosensor for Al3+ via the
photo-induced electron transfer (PET) effect. The free receptor
PMHCH almost did not show fluorescence emission at 515 nm
when it was excited at 375 nm. The intensity dramatically
enhanced (286-fold) in the present of Al3+, other ions such as K+,
Na+, Mg2+, Ca2+, Cr2+, Ba2+, Mn2+, Cu2+, Co2+, Pb2+, Zn2+, Hg2+, Fe2+
and Fe3+ did not have much influence. Moreover, the detection
limit of PMHCH for Al3+ could be as low as 2.2 ꢂ 10ꢁ7 M in ethanol.
Scheme 1. Synthetic route of PMHCH.
formed immediately. Following reaction, the mixture was allowed
to cool to room temperature. The precipitated was filtered, washed
with cold absolute ethanol. The crude product was purified by
recrystallization from ethanol to give 0.484 g of PMHCH as a
light-green solid. Yield: 62%; m.p.: 253–255 °C. IR (KBr, cmꢁ1):
3118, 1583, 1387, 1281 (Fig. S2). 1H NMR (Fig. S3): (400 MHz;
DMSO-d6): d = 8.26 (m, 1H), 8.14 (s, 1H), 8.00 (d, J = 7.4 Hz, 1H),
7.96 (d, J = 7.4 Hz, 1H), 7.75 (m, 2H), 7.61 (m, 2H), 7.370 (m, 2H),
7.305 (m, 1H), 7.09 (t, J = 7.4 Hz, 1H), 4.44 (s, 1H), 2.69 (s, 3H),
2.20 (d, J = 7.4 Hz, 3H). 13C NMR (Fig. S4): (400 MHz; DMSO-d6):
d = 163.23, 161.17, 159.12, 158.78, 151.17, 140.00, 139.95,
139.09, 138.43, 130.60, 129.28, 127.11, 122.86, 121.03, 119.60,
118.11, 100.00, 98.55, 25.21, 13.23. ESI-MS (Fig. S5): m/z calcd
for C22H19N5O2 + H+: 386.15 [M+H+]: found, 386.11. Anal. Calcd
for C22H19N5O2: C, 68.56; H, 4.97; N, 18.17. Found: C, 68.14; H,
4.24; N, 18.13.
Experimental
Materials and instrumentation
All chemicals were of reagent grade quality, obtained from com-
mercial sources, and were used as received without further purifi-
cation. Melting points were determined using a Beijing XT4-100x
microscopic melting point apparatus. 1H-NMR spectra were
recorded on JNM-ECS 400 MHz instruments spectrometers with
TMS (tetramethylsilane) as internal standard and DMSO-d6 as sol-
vent. Mass spectra were recorded in methanol solvent on a Bruker
Esquire 6000 spectrometer. UV–vis absorption spectra were
obtained with
a Shimadzu UV-240 spectrophotometer and
Results and discussion
recorded in quartz cells with 1 cm optical path length. Emission
spectra were measured using a Hitachi RF-5301 fluorimeter.
General information
Synthesis of the probe PMHCH
Solutions of metal ions were prepared from the corresponding
metal nitrate salts. Stock solutions of metal ions were prepared
by dissolving the desired amount of material in ethanol. Test solu-
1-Phenyl-3-methyl-4-formyl-pyrazolone-5 (PMFP) was synthe-
sized as reported procedure [22]. Ethyl 2-methyl quinoline
-4-carboxylate was synthesized according to the method reported
[23]. The synthetic route of PMHCH is shown in Scheme 1.
tions were prepared by placing 20 lL of the probe stock solution
into cuvettes, adding an appropriate aliquot of each ions stock,
and diluting the solution to 2 mL with ethanol solutions. For fluo-
rescence measurements, the excitation and emission slit widths
were 5 nm and 3 nm, respectively.
Synthesis of 2-methyl quinoline-4-carboxylic hydrazide [24]
The association constant for PMHCH–Al3+ complex is further
estimated on the basis of the nonlinear filtting of the fluorescence
titration curve assuming a 1:1 stoichiometry by the Benesi–
Hildebrand method [25,26].
To
a solution of ethyl 2-methyl quinoline-4-carboxylate
(5 mmol, 1.015 g) in ethanol (40 mL) was added hydrazine hydrate
(80%, 4 mL) dropwise. Then the reaction mixture was stirred at
80 °C for 12 h under reflux. After evaporating the solvent in a vac-
uum, the precipitate was filtered and stand overnight in refrigera-
tor. A white needle crystalline solid was obtained and then
recrystallized with ethanol to get the final product. Yield: 53%;
m.p.: 178–179 °C. 1H NMR (Fig. S1): (400 MHz; CDCl3): d = 2.71
(s, 3He), 4.25 (s, 2Hh), 7.29 (s, 1Hf), 7.54 (m, 1Hb), 7.59 (s, 1Hg),
7.73 (m, 1Hc), 8.05 (d, J = 8.25 Hz, 1Hd), 8.14 (d, J = 8.25 Hz, 1Ha).
1
1
1
h
i
¼
þ
KðFmax ꢁ FminÞ Al3þ
F ꢁ Fmin
Fmax ꢁ Fmin
Fmin, F, and Fmax are the emission intensities of the organic moiety
considered in the absence of Al (III) ions, at an intermediate alu-
minum concentration, and at a concentration of complete interac-
tion, respectively. And K is the binding constant concentration.
The detection limit is estimated from the fluorescence titration.
The emission intensity of the complex (PMHCH–Al3+) without any
anion is measured to determine the S/N ratio [27,28]. And the stan-
dard deviation of blank measurements is calculated. The detection
limit is calculated based on 3 ꢂ dblank/k. Where dblank is the
Synthesis of PMHCH
An ethanol solution (30 mL) of 2-methyl quinoline-4-carboxylic
hydrazide (0.402 g, 2 mmol) was added dropwise to a solution
(30 mL)
of
1-phenyl-3-methyl-4-formyl-pyrazolone-5(PMFP)
(0.404 g, 2 mmol) in ethanol. The reaction mixture was heated to
reflux for 8 h with stirring, during which time a green precipitated

G.-q. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 21–25
23
standard deviation of the blank solution and k is the slope of the
calibration plot.
UV–vis analysis
We have carried out UV–vis titration experiments in ethanol at
room temperature to understand the nature of binding of PMHCH
to Al3+. As shown in Fig. 1, upon gradual addition of Al3+, a con-
comitant 16 nm red shift in the spectral position at 375 nm was
observed along with an increase in the absorption intensity. A
well-defined isosbestic point could be clearly observed at
368 nm, indicating a balance between PMHCH and PMHCH–Al3+
.
These phenomenons indicated the formation of a new complex
between ligand PMHCH and Al3+ [29].
Fluorescence study
The effect of reaction time on the fluorescence spectra was
studied and the results were shown in Fig. S6. Adding 1 equiv. of
Al3+ to ethanol solution of PMHCH, 1 min later the emission at
505 nm of the system got to maximum and did not increase with
Fig. 2. Fluorescence spectra of PMHCH (10
of Al3+ (0–19
l
M) with addition of increasing amount
l
M) in ethanol. Excitation wavelength was 375 nm, and emission was
at 505 nm. Inset: color of PMHCH (left) and PMHCH + Al3+ (right) under UV lamp.
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
more reaction time. It means PMHCH could be used as
a
real-time Al3+ chemosensor.
Fluorescence titration experiments were also performed in
ethanol. As shown in Fig. 2, the free receptor PMHCH almost not
show fluorescence emission at 515 nm when it was excited at
375 nm (U0 = 1.82). Upon successive addition of Al3+ ion to the
solution of receptor PMHCH, a new emission band with a maxi-
mum at 505 nm appeared and the intensity of this band gradually
increased with an increasing concentration of Al3+
(UAl = 4.62). The
response of the probe toward the addition of Al3+ was immediate
and the intensity dramatically enhanced (286-fold) which can be
attributed to the decrease of PET effect and the
chelation-enhanced fluorescence (CHEF) effect [30,31]. More
specifically, the free receptor displayed very weak fluorescence
band at 515 nm which was due to quenching by the carbonyl
group through a photo-induced electron transfer (PET), induced
by a lone pair electron from the Schiff base. Upon the addition of
Al3+, the chelation of the oxygen and nitrogen atom with Al3+
resulted in the efficient inhibition for the PET process of the
AC@N group as shown in Scheme 2.
Scheme 2. Possible PET process of PMHCH.
The Job’s plot obtained for PMHCH–Al3+ system in ethanol
clearly suggests a formation of 1:1 metal–ligand stoichiometry
[32] (Fig. 3). To confirm further the stoichiometry between
PMHCH and the Al3+
, mass spectra analysis was conducted
Fig. 3. Job’s plot of PMHCH with Al3+ in ethanol solution. Total concentration of
PMHCH + Al3+ was kept constant at 10
lM, the emission at 505 nm was used.
(Fig. S7).
A mass peak at m/z 473.07 corresponding to
[PMHCH + Al3+ + NOꢁ3 ꢁ H+]+ is indicative of the formation of a
Fig. 1. Changes in the absorption spectra of PMHCH (10 lM) in ethanol after
1:1 complex. According to linear Benesi–Hildebrand expression,
addition of Al3+ (1–9 equiv.).

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G.-q. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 21–25
ions have been carried out (Fig. 5). PMHCH responding for Al3+ in
the presence of Mg2+, Cd2+ and Zn2+ are relatively low but clearly
detectable. As for as Fe2+, Cu2+, those ions are typical ions that have
a pronounced quenching effect on fluorophores by mechanisms
inherent to the paramagnetic species [35,36]. This result indicated
that PMHCH is a selective fluorescent sensor toward Al3+ over most
competitive cations.
Furthermore, the reversibility nature of chemosensor PMHCH
was measured by the titration of EDTA with fluorescent probe
(PMHCH + Al3+) (Fig. S10). The addition of 1 equiv. of EDTA to a
solution of PMHCH–Al3+ complex resulted in its emission intensity
decreased dramatically, which indicates the regeneration of the
free PMHCH. The result indicated that sensor PMHCH could be a
reversible chemosensor for Al3+
To understand better the complexation behavior of PMHCH
with Al3+ 1H NMR experiments were carried out in d6-DMSO
.
,
(Fig. S11). Upon addition of Al3+, the NH proton of PMHCH at
around 7.75 ppm was shifted downfield toward 7.87 ppm. At the
same time, the signal for ACH@NA was shifted downfield by
0.08 and the hydrogens of quinoline were shifted downfield
slightly followed the addition of Al3+. The results suggested that
the binding of probe PMHCH to Al3+ forms a rigid system by a
chelation with the nitrogen of imine-nitrogen and the oxygen atom
of carbonyl group.
Fig. 4. Emission spectra of PMHCH (10 lM) upon addition of different mental ions
(1 equiv. K+, Na+, Mg2+, Ca2+, Cr2+, Ba2+, Mn2+, Cu2+, Co2+, Pb2+, Zn2+, Hg2+, Fe2+
Fe3+and Al3+) in ethanol.
,
Conclusion
In conclusion, we have presented the synthesis, characteriza-
tion, and spectral properties of PMHCH, a new fluorescent sensor
for Al3+ with high sensitivity. This phenomenon was attributed to
forms 1:1 complex which inhibit photo-induced electron transfer
process and chelation-enhanced fluorescence precess. The detec-
tion limit for Al3+ can reach at 10ꢁ7 M level. Further study will
focus on enhancing the water solubility of the receptor and its
potential applications in biological chemistry. The design strategy
and remarkable photo physical properties of the sensor would help
to extend the development of fluorescent sensors for other ions.
Acknowledgments
Fig. 5. Changes in fluorescence emission intensity at 505 nm of PMHCH (10.0
in the presence of various metal ions in ethanol. Black bars represent the addition of
metal ions written below the bars to a 10 M solution of PMHCH. Red bars
represent emission intensity of a mixture of PMHCH (10 M) with the metal ions
written below the bars (10 M) following addition of 10
M Al3+ to the solutions.
lM)
This work is supported by the National Natural Science
Foundation of China (81171337) and Gansu NSF (1308RJZA115).
l
l
l
l
Appendix A. Supplementary data
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2015.05.041.
the association constant between PMHCH and Al3+ was calculated
from the fluorescence titration result and was found to be
1.89 ꢂ 104 Mꢁ1 [33,34] (Fig. S8). For practical applications, the
detection limit of PMHCH was also estimated from the titration
experiment (Fig. S9). The detection limit for Al3+ was calculated
to 2.2 ꢂ 10ꢁ7 M, which is far below the WHO acceptable limit
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