Development of a simple pyrazine-derived "turn on" Al3+ fluorescent sensor with high selectivity and sensitivity

Article abstract of DOI:10.1016/j.ica.2015.02.004

Abstract In this article, a novel pyrazine-derived hydrazone Schiff-base ligand bearing the quinoline unit (1) has been designed, synthesized and evaluated as a "turn on" fluorescent chemosensor for Al³⁺ based on the photoinduced electron-transfer (PET) mechanism and the chelation-enhanced fluorescence (CHEF) phenomenon. The sensor 1 showed remarkably enhanced fluorescence intensity at 488 nm in the presence of Al³⁺ ion and it also showed high selectivity and sensitivity towards Al³⁺ ion over a wide range of metal ions in ethanol, for the detection limit of 1 for Al³⁺ could reach at 10^-7 mol/L.

Full text of DOI:10.1016/j.ica.2015.02.004

Inorganica Chimica Acta 430 (2015) 91–95
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Development of a simple pyrazine-derived ‘‘turn on’’ Al3+ fluorescent
sensor with high selectivity and sensitivity
⇑
Chao-rui Li, Jing-can Qin, Guan-qun Wang, Bao-dui Wang, An-kun Fu, Zheng-yin Yang
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this article, a novel pyrazine-derived hydrazone Schiff-base ligand bearing the quinoline unit (1) has
been designed, synthesized and evaluated as a ‘‘turn on’’ fluorescent chemosensor for Al3+ based on
the photoinduced electron-transfer (PET) mechanism and the chelation-enhanced fluorescence (CHEF)
phenomenon. The sensor 1 showed remarkably enhanced fluorescence intensity at 488 nm in the pres-
ence of Al3+ ion and it also showed high selectivity and sensitivity towards Al3+ ion over a wide range
of metal ions in ethanol, for the detection limit of 1 for Al3+ could reach at 10ꢀ7 mol/L.
Ó 2015 Elsevier B.V. All rights reserved.
Received 24 December 2014
Received in revised form 4 February 2015
Accepted 5 February 2015
Available online 5 March 2015
Keywords:
Pyrazine
Fluorescent sensor
Aluminum ion
Selectivity
1. Introduction
and induce many health problems, such as Alzheimer’s disease
and Parkinson’s disease [20]. Nevertheless, there have been only
The development of methods for the detection and recognition
of biologically related metallic ions has attracted much interest
due to their importance in the human body [1,2]. In recent years,
many techniques, such as atomic absorption spectrometry (AAS)
[3], atomic emission spectrometry (AES) [4], and electron para-
magnetic resonance (EPR) [5], have been developed to detect metal
ions. However, these techniques suffer from complicated opera-
tional sequence, sophisticated synthetic procedure and expensive
operational cost in medicinal and environmental research [6].
Owing to the features of high sensitivity, facile operation, real time
detection and instantaneous response, the fluorescence technique
has received considerable attention in the field of detecting and
recognizing metallic ions [7]. Consequently, fluorescent sensors
have been recognized as powerful tools for monitoring biologically
related metal ions [8–13].
a few reports about the development of fluorescent chemosensors
for Al3+ because of its poor coordination ability [21]. Therefore, it is
a great demand to design and synthesize Al3+ selective and sensi-
tive fluorescent chemosensors [22–26].
For a sensor which is based on the photoinduced electron-trans-
fer (PET) mechanism and the chelation-enhanced fluorescence
(CHEF) phenomenon, it is evident that the PET from the amine
group to the excited singlet state of fluorophore makes the fluores-
cence of the sensor quenched. However, when a suitable metal ion
is added to the sensor solution, the inhibition of PET process will
occur with the complexation of the sensor and this specific metal
ion, which will cause a large chelation-enhanced fluorescence
(CHEF) effect [27–30]. Thus, the enhancement of the fluorescence
emission intensity of the sensor can be observed. However, some
of the fluorescent sensors reported are lack of selectivity and
sensitivity over other common biologically related metal ions,
which restrains these sensors for practical application [31].
Herein, a novel pyrazine-derived hydrazone Schiff-base ligand
called 2-Acetylpyrazine (80-hydroxyquinolineyl-20-acetyl) hydra-
zone (1) was designed and synthesized through a three-step reac-
tion (Scheme 1). For this compound had so simple structure, it
could be used as a fluorescent chemosensor to detect and recog-
nize Al3+ ion. From the experimental process, we can observe that
the ethanol solution of 1 was nearly nonfluorescent. However,
when Al3+ was added to the solution of 1, the fluorescence
emission intensity at 488 nm would enhance remarkably.
Furthermore, this fluorescent sensor 1 had high selectivity and
The detection of Al3+ is of great importance because Al3+ is an
essential metal ion in the biological and physical systems [14]. It
is well known that aluminum is the most abundant metallic ele-
ment in the earth’s crust and has widespread applications in mod-
ern life [15,16]. However, Al3+ can cause harm to human beings
when it is exposed to high concentration levels [17]. Aluminum
is regarded as a toxic element, because Al3+ can cause damage to
central nervous system and immune system in human bodies
[18], affecting the absorb and use of other trace elements [19],
⇑
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.ica.2015.02.004
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

92
C.-r. Li et al. / Inorganica Chimica Acta 430 (2015) 91–95
Scheme 1. The synthetic route of compound 1.
sensitivity over a wide range of other common biologically related
metallic ions tested, and could respond Al3+ in a reversible manner,
which developed 1 for practical application.
J = 7.2 Hz, 1.6 Hz, H6), 5.03 (s, 2H, –CH2–), 2.63 (s, 3H, –CH3). MS
(ESI) (Fig. S2): m/z 322.1219 [M+H+]+, 344.1037 [M+Na+]+,
665.2140 [2M+H+]+. FT-IR (KBr pellet, cmꢀ1) (Fig. S3): 3441 (N–
H), 1695 (C@O), 1622 (C@N), 1275 (C–O), 1119 (C–N). UV–Vis
(nm) (Fig. 1): 295, 350. Anal. Calcd. for C17H15N5O2 (%): C, 63.54;
H, 4.71; N, 21.79; O, 9.96. Found: C, 63.04; H, 4.57; N, 21.40; O,
10.99.
2. Experimental
2.1. Materials
2.4. Analysis
8-Hydroxyquinoline, ethyl chloroacetate, hydrazine hydrate,
acetyl pyrazine, absolute ethanol and salts of Al3+, Ba2+, Ca2+
,
Stock solutions (10 mM) of the salts of Al3+, Ba2+, Ca2+, Cd2+
,
Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+
and Zn2+ were obtained from commercial suppliers and used
without further purification.
Co2+, Cr3+, Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+ and
Zn2+ in ethanol were prepared. Stock solution of the compound 1
(10 mM) in dimethyl sulfoxide (DMSO) was also prepared. Test
solutions were prepared by placing 10 lL of the probe stock solu-
2.2. Methods
tion into cuvettes, adding an appropriate aliquot of each metal ion
stock, and diluting the solution to 2 mL with ethanol. For all mea-
surements, excitation wavelength was at 350 nm; the excitation
slit width was 5.0 nm, and the emission slit width was 3.0 nm.
The binding constant values were determined from the emis-
sion intensity data following the modified Benesi–Hildebrand Eq.
(1) [33]:
Distilled water was used throughout all experiments. 1H NMR
spectra were measured on the JNM-ECS 400 MHz instruments
using TMS as an internal standard in CDCl3. The ESI-MS data were
obtained in ethanol from a Bruke Esquire 6000 spectrometer. FT-IR
spectra were recorded with a VERTEX-70 spectrometer. Elemental
analyses were performed using a VarioEL Cube V1.2.1 analyzer.
UV–Vis absorption spectra were recorded on a Perkin Elmer
Lamda 35 UV–Vis spectrophotometer in ethanol medium at
298 K. Fluorescence spectra were generated on a Hitachi RF-4500
spectrophotometer equipped with quartz cuvettes of 1 cm
path length. Melting points were determined on a Beijing X-4
microscopic melting point apparatus.
1
1
1
¼
þ
:
ð1Þ
KðFmax ꢀ FminÞ½AL3þꢁ
F ꢀ Fmin
Fmax ꢀ Fmin
2.3. Synthesize
2.3.1. Synthesize of compound 1 (2-acetylpyrazine
(80-hydroxyquinolineyl-20-acetyl) hydrazone)
Ethyl 2-(quinolin-8-yloxy) acetate (2) and 2-(quinolin-8-yloxy)
acetohydrazide (3) were prepared by the reported method [32].
A solution of the 2-(quinolin-8-yloxy) acetohydrazide (0.199 g,
0.968 mmol) in ethanol was added to a solution containing acetyl
pyrazine (0.118 g, 0.968 mmol) in ethanol. The mixture was
refluxed for 26 h, and the obtained light yellow solution was then
cooled to room temperature. Then the light yellow product was fil-
tered, washing three times with distilled water and absolute etha-
nol, respectively. Finally, the solid was dried under drying oven
overnight to give the desired product 1 as a white powder
(0.24 g, 77.24%) (Scheme 1). m.p. 213–215 °C, 1H NMR (400 MHz,
CDCl3) (Fig. S1): 11.26 (s, 1H, –NH–), 9.49 (d, 1H, J = 1.6 Hz, H1),
8.94 (dd, 1H, J = 4.0 Hz, 1.6 Hz, H3), 8.55–8.50 (m, 2H, H2, 4), 8.28
Fig. 1. UV–Vis absorption of compound 1 (100 lM) measured in ethanol upon
addition of various concentration of Al3+ (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0,
(d, 1H, J = 8.0 Hz, H5), 7.59–7.51 (m, 3H, H7,
9), 7.29 (dd, 1H,
2.4, 2.8, 3.2, 3.6, 4.0 equiv., respectively).
8,

C.-r. Li et al. / Inorganica Chimica Acta 430 (2015) 91–95
93
where Fmin, F, and Fmax are the emission intensities of the organic
moiety considered in the absence of aluminum ion, at an inter-
mediate aluminum concentration, and at a concentration of com-
plete interaction, respectively, and where Ka is the binding
constant concentration.
3. Results and discussion
3.1. UV–Vis analysis of compound 1
Fig. 1 depicts the UV–Vis absorption spectra of compound 1
(100 l
M) in ethanol upon the gradual addition of Al3+. The com-
pound 1 in the absence of any metal ion displayed a weak absorp-
tion band at 295 nm with a shoulder at 350 nm, which was
possibly assigned to the absorption band of pyrazine. However,
when Al3+ was added to the solution of 1, the absorption band at
295 nm enhanced with increasing Al3+ amount. Simultaneously, a
new absorption band which was centered at 251 nm appeared
with increasing intensity (Fig. 1), which suggested that Al3+
coordinated with pyrazine, and this compound 1 could respond
for Al3+ in the UV–Vis absorption spectra. According to the
UV–Vis Absorption spectra of compound 1, we chose the shoulder
(350 nm) as excitation wavelength in the following fluorescence
emission measurements.
Fig. 3. Change in fluorescence spectra of 1 (50
l
M) measured in ethanol upon
addition of various concentration of Al3+ (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0,
1.2, 1.4, 1.6, 1.8, 2.0 equiv., respectively) with an excitation at 350 nm. Inset:
fluorescence color of 1 in the absence (left) and presence (right) of Al3+ in ethanol
under a UV lamp.
We hypothesized that the lone pair electrons from the Schiff-base
nitrogen atom to the pyrazine contributed to the photoinduced
electron-transfer (PET) phenomenon, which made the fluorescence
emission of compound 1 quenched. Upon addition of less than
0.2 equiv. of Al3+, there was almost no change in the fluorescence
emission spectrum. Nevertheless, when the amount of Al3+ was
more than 0.2 equiv., the fluorescence emission intensity at
488 nm increased with increasing amount of Al3+ (Fig. S4). The rea-
son was attributed to the inhibition of the PET phenomenon upon
complexation of 1 with Al3+, which made the quenched fluores-
cence recur by occurring highly efficient chelation-enhanced fluo-
rescence (CHEF) effect [34–36] (Scheme 2). As a result, the
fluorescence emission intensity at 488 nm enhanced remarkably
by about 230-fold with a large red-shift in the presence of 1 equiv.
of Al3+. Simultaneously, the fluorescence color of ethanol solution
of 1 turned light green upon addition of Al3+ under a UV lamp,
which could be easily detected by the naked eye (Fig. 3).
Furthermore, the binding constant of 1 with Al3+ was determined
to be 1.35 ꢂ 104 from the fluorescence titration profile using the
Benesi–Hildebrand Eq. (1) (Fig. S5), and the detection limit of this
sensor 1 for Al3+ was estimated as 10ꢀ7 mol/L level conducted by
using fluorescence spectra (Fig. S6), which was lower than some
reported Al3+ selective and sensitive fluorescent sensors [37–40].
It demonstrated that the detection limit was low enough for this
sensor to detect and control Al3+ in the environmental and biologi-
cal systems.
3.2. Selectivity of compound 1 for Al3+ over other metal ions
The fluorescence emission spectra of compound 1 upon addi-
tion of various metal ions (Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+
,
Fe2+, Fe3+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Zn2+) in ethanol was illus-
trated in Fig. 2. The addition of Al3+ into the solution of 1 caused
the remarkably enhanced fluorescence intensity at 488 nm.
However, upon addition of Fe3+ and Cr3+, it resulted in relatively
weak enhancement in fluorescence intensity and was significantly
lower than that of Al3+. Nevertheless, no significant change of the
fluorescence emission was observed in the presence of other metal
ions investigated (Fig. 2). From the results above, it was concluded
that compound 1 had high selectivity for Al3+ over other biologi-
cally related metal ions.
3.3. Fluorescence titration of compound 1 with Al3+
As shown in Fig. 3, the compound 1 without any metal ion
showed very weak fluorescence at 391 nm in ethanol upon excita-
tion at 350 nm, and there was no emission at longer wavelength.
3.4. Selectivity of compound 1 for Al3+ in the presence of other metal
ions
Competition experiments were conducted to further explore
the selectivity of this fluorescent sensor 1 for Al3+ in the presence
of other biologically related metal ions. The fluorescence responses
of compound 1 in the presence of Al3+ mixed with a variety of other
metal ions (Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, K+, Mg2+
,
Mn2+, Na+, Ni2+, Pb2+, Zn2+) in ethanol was shown in Fig. 4. It was
evident that two magnetic metal ions Cu2+ and Ni2+ made the
fluorescence intensity at 488 nm completely quenched. However,
Cd2+, Co2+, Fe2+ and Fe3+ induced relatively slight quenching at
488 nm. Nevertheless, in the presence of other metal ions
investigated, the fluorescence emission spectra displayed a similar
Fig. 2. Fluorescence spectra of 1 (50
(1 equiv.) in ethanol with an excitation at 350 nm.
lM) upon addition of various metal ions

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C.-r. Li et al. / Inorganica Chimica Acta 430 (2015) 91–95
Scheme 2. Proposed mechanism for detection of Al3+ by 1.
Fig. 4. Fluorescence responses of 1 (50 l
M) to Al3+ (1 equiv.) in the presence of
various metal ions (5 equiv.) under the same conditions at a wavelength of 488 nm.
Bars represent the fluorescence intensity at 488 nm. ((1) Al3+; (2) Al3+ + Ba2+
(3) Al3+ + Ca2+; (4) Al3+ + Cd2+; (5) Al3+ + Co2+; (6) Al3+ + Cr3+; (7) Al3+ + Cu2+; (8)
Al3+ + Fe2+ (9) Al3+ + Fe3+ (10) Al3+ + K+; (11) Al3+ + Mg2+ (12) Al3+ + Mn2+
(13) Al3+ + Na+; (14) Al3+ + Ni2+; (15) Al3+ + Pb2+; (16) Al3+ + Zn2+).
;
Fig. 5. Fluorescence response of ethanol solution of 1 (50 l
M) and Al3+ (1 equiv.)
upon addition of EDTANa2 (1 equiv.) with an excitation at 350 nm.
;
;
;
;
pattern to that with Al3+ only. (Fig. 4). Thus, most competing metal
ions did not interfere with the detection of Al3+. Therefore, this
compound 1 was shown to be a promising selective fluorescent
sensor for Al3+ in the presence of most biologically related
metal ions.
3.5. The reversibility and regeneration of the binding of Al3+ by 1
In order to develop this fluorescent sensor 1 for practical appli-
cation, the reversibility and regeneration of compound 1 were
investigated. For this purpose, the Al3+ complex was treated with
EDTANa2 that was a good chelating agent with Al3+. As illustrated
in Fig. 5, the addition of EDTANa2 to a mixture of compound 1 and
Al3+ made the fluorescence emission intensity quenched com-
pletely at 488 nm. It seemed that the decomplexation of 1-Al3+
and the formation of Al3+–EDTANa2 complex made compound 1
free from 1-Al3+ solution. As a result, the fluorescence emission
spectrum was nearly equal to free 1. However, when the excessive
Al3+ was added to the solution above, the fluorescence intensity at
488 nm enhanced again (Fig. 5). Therefore, we concluded that this
fluorescent sensor 1 could be developed for practical application,
for the reversibility and regeneration of 1 were perfect.
Fig. 6. Job’s plot for determining the stoichiometry between 1 and Al3+ in ethanol
(XAl = [Al3+]/([Al3+] + [1]), the total concentration of 1 and Al3+was 100
lM).
3.6. Binding stoichiometry between compound 1 and Al3+
The binding stoichiometry between compound 1 and Al3+ was
evaluated from a Job’s plot which was obtained from the fluores-
cence emission intensity at 488 nm. As can be seen from Fig. 6,
the maximum fluorescence emission intensity was observed when

C.-r. Li et al. / Inorganica Chimica Acta 430 (2015) 91–95
95
the amount of Al3+ and 1 was equimolar, which indicated that
compound formed
1:1 complex with Al3+ (Fig. 6).
Furthermore, from the electrospray ionization mass spectra
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In summary, a simple structure fluorescent sensor 1 based on
PET and CHEF mechanism has been successfully designed and
synthesized through a facile three-step reaction. This fluorescent
sensor 1 had high selectivity for Al3+ over other interfering metal
ions with large fluorescence enhancement, and it also had high
sensitivity towards Al3+ with the detection limit reaching at
10ꢀ7 mol/L level in ethanol. Furthermore, this sensor 1 could
respond Al3+ in a reversible manner, which developed 1 for practi-
cal application, and the 1:1 stoichiometry between 1 and Al3+ was
confirmed by job’s plot and electrospray ionization mass spec-
trometry (ESI-MS). These results indicated that compound 1 could
be used as a fluorescent chemosensor for detection and recognition
of Al3+ and might accelerate the development of novel simple
structure sensors. Moreover, the applications of novel sensors in
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2015.02.004.
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