Synthesis of pyridine-based 1,3,4-oxadiazole derivative as fluorescence turn-on sensor for high selectivity of Ag+

Article abstract of DOI:10.1007/s10895-013-1213-y

An oxadiazole derivative(OXD) containing symmetrical pyridine-2- formamidophenyl-binded moiety was synthesised as fluorescence turn-on sensor OA1. Its ultraviolet-visible(UV-vis) and fluorescent spectra(FS) gave prominent fluorescence enhancement only for monovalent silver ion(Ag⁺) in HEPES buffer solution (10 mM, pH = 7.0, DMF-H₂O, 9:1, v/v), which indicated the photo-induced electron transfer(PET) occurred from the donor of pyridine-2-formamidophenyl group to oxadiazole fluorophore. The present study demonstrated that OA1 was a viable candidate as fluorescent receptor for a new Ag⁺ sensor. And the results of fluorescent spectral titration showed this sensor formed 1:1 complex with Ag⁺.

Full text of DOI:10.1007/s10895-013-1213-y

J Fluoresc (2013) 23:785–791
DOI 10.1007/s10895-013-1213-y
ORIGINAL PAPER
Synthesis of Pyridine-Based 1,3,4-Oxadiazole Derivative
as Fluorescence Turn-On Sensor for High Selectivity of Ag+
Chunling Zheng & Ailin Yuan & Zhengyu Zhang &
Hong Shen & Shuyuan Bai & Haibo Wang
Received: 10 December 2012 /Accepted: 7 March 2013 /Published online: 16 March 2013
#
Springer Science+Business Media New York 2013
Abstract An oxadiazole derivative(OXD) containing sym-
metrical pyridine-2-formamidophenyl-binded moiety was
synthesised as fluorescence turn-on sensor OA1. Its ultravi-
olet–visible(UV–vis) and fluorescent spectra(FS) gave
prominent fluorescence enhancement only for monovalent
silver ion(Ag+) in HEPES buffer solution (10 mM, pH=7.0,
DMF-H2O, 9:1, v/v), which indicated the photo-induced elec-
tron transfer(PET) occurred from the donor of pyridine-2-
formamidophenyl group to oxadiazole fluorophore. The pres-
ent study demonstrated that OA1 was a viable candidate as
fluorescent receptor for a new Ag+ sensor. And the results of
fluorescent spectral titration showed this sensor formed 1:1
complex with Ag+.
selectivity out of interference from coexisting metal ions
has been challenging. Therefore, the rational design and
synthesis of efficient sensors to selectively recognize targets
is the hot topic in molecular recognition studies. Because of
high sensitivity, fluorescent reagents have been used satis-
factorily for the fluorometric detection of most transition or
heavy metal ions [18–27], and fluorescent spectroscopy is
becoming more and more important for chemical trace
detection [20, 28–30].
Among different fluorescent cation sensing probes, pho-
toinduced electron transfer(PET)-type receptors have been
proved highly successful [2, 31–36], which are devised to
covalently link fluorophores by means of non-conjugating
spacer groups and reversibly switch fluorescent intensity by
binding cations. Usually, highly selective probes for transi-
tion or heavy elements that give a positive response rather
than fluorescent quenching upon analyst binding are pre-
ferred to promote the sensitivity factor. Consequently, the
design of such turn-on silver ion(Ag+) sensors is an intrigu-
ing challenge since many transition elements often causes
fluorescent quenching [37].
In view of above requirement and as part of an ongoing
investigation focused on the preparation and utilization of
small-molecule electroluminescent organisms, we reported
an oxadiazole derivative(OXD) as Ag+ fluorescent sensor
OA1 herein, which gave a positive response to Ag+ in
HEPES buffer solution(10 mM, pH=7.0, DMF-H2O,
9:1, v/v). In the design of OA1, we employed 1,3,4-
oxadiazole as fluorophore because of its excellent
photophysical properties and chemical stabilities [38]. Two
pyridine-2-formamidophenyl were symmetrically bound on
C-2 and C-5 of 1,3,4-oxadiazole, in which the nitrogen atom
of pyridine ring was both the cation receptor and the
quencher via PET. Acceptance of the metal ion sequestered
the lone pairs in nitrogen atom and rebuilt rigid molecular
system, which stopped the PET quenching and produced a
.
.
.
Keywords Fluorescent sensor Silver ion Oxadiazole
Photo-induced electron transfer
Introduction
Selective sensors for detection and quantification of various
metal ions and organic structures have received considerable
attention in many fields such as environmental and security
monitoring, waste management, nutrition, and clinical tox-
icology [1–6]. An ideal sensory system should contain mul-
tiple members that respond to analytes with high sensitivity
and selectivity. Although previous work has achieved a
wide variety of chemical [7–12] and physical [13–17] sen-
sors for the detection of cations, improving the analysis
:
:
:
:
:
C. Zheng (*) A. Yuan Z. Zhang H. Shen S. Bai
H. Wang (*)
Department of Light-Chemical Engineering, Institute of Food
and Light-Chemical Engineering, Nanjing University of Technology,
Nanjing 211816, People’s Republic of China
e-mail: zcl323@139.com
e-mail: wanghaibo@njut.edu.cn

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J Fluoresc (2013) 23:785–791
fluorescent enhancement in the 1,3,4-oxadiazole emission.
Sensor OA1 shown in Scheme 1 provided high sensitivity
and large optics signaling changes for Ag+.
o-aminobenzoic Acid (1) o-nitrobenzoic acid (33.4 g,
0.2 mol) and distilled water (100 mL) were put into a
250 mL four-neck flask, stirred with sodium hydroxide
solution (15 %) dropping until the precipitates were
dissolved completely, and heated to 50 °C. Pd/C (3 %)
was used as catalyst, and hydrazine hydrate (24 mL,
0.4 mol) was added to above reaction mixture dropwise.
Then the reaction temperature was increased to 60 °C for
6 h. Cooled filtrate was adjusted to pH 3 ~ 4 with
hydrochloric acid (10 %). Finally, the filtered solid was
recrystallized from ethanol to obtain light yellow crystal
(21 g, 75 %). M. p., 195~196 °C; Found, 193~195 °C
Experimental
Reagents
All reagents were of analytical grade and used without
purification.
1
Characterization Methods
[39]. H NMR (300 MHz, DMSO-d6, TMS) δ: 8.57 (s,
2H), 7.68 (d, J=8.04 Hz, 1H), 7.21 (t, J=8.4 Hz, 1H),
6.73 (d, J=8.4 Hz, 1H), 6.49 (t, J=8.07 Hz, 1H).
Thin layer chromatography(TLC) plates were visualized
with UV light to monitor the completion of all preparation
reactions. Whatman silica gel-60 plates of 1 mm thickness
were used as the solid phase for TLC. Melting points(M. p.)
of prepared compounds were measured on an X4 Micro-
melting point apparatus. A Bruker DRX500 spectrometer
recorded 1H NMR spectra of objective products at
Isatoic Anhydride (2) o-aminobenzoic acid (21 g, 0.15 mol)
and acetonitrile (100 mL) were added into a 250 mL four-
neck flask, and heated to 50 °C. Meanwhile, methylene
chloride solution containing pyridine (25 mL, 0.3 mol) and
triphosgene (15.16 g, 0.05 mol) was dropped. Then the
reaction was kept 50~55 °C for 2 h. Finally, the solvent
was removed under reduced pressure and the residual solid
was recrystallized from ethanol to yield white product
(12.3 g, 50 %). M. p., 218 °C; Found, 220~230 °C [40].
1H NMR (300 MHz, DMSO-d6, TMS) δ: 11.71 (s, 1H),
7.92 (d, J=7.86 Hz, 1H), 7.74 (t, J=7.78 Hz, 1H), 7.25 (t, J=
7.5 Hz, 1H), 7.15 (d, J=8.22 Hz,1H).
1
300 MHz. H chemical shifts were reported in ppm down-
field from tetramethylsiane(TMS, δ scale with the solvent
resonances as internal standards). An HP 1100LC-MSD
high performance system was utilized to obtain atmospheric
pressure ionization mass spectra(API-MS) of target
compounds.
Synthesis
N,N’-bis(2-aminobenzoyl)hydrazide (3) Isatoic anhydride
Synthetic procedures of OA1 were shown in Scheme 2.
(11.2 g, 68.7 mmol) and absolute alcohol (60 mL) were
Scheme 1 Proposed complex formation between Ag+ and OA1

J Fluoresc (2013) 23:785–791
787
Scheme 2 Synthetic
procedures of OA1
put into a 100 mL four-neck flask, and hydrazine hydrate
(2 mL, 34.3 mmol) was added dropwise at room tempera-
ture. The refluxing reaction lasted 4 h until TLC indicated
the complete transformation, then was cooled down.
Recrystallization for filtered solid from DMF/H2O would
afford the target as brown crystal (6.7 g, 72.2 %). M. p., 208
needlelike crystal (3.2 g, 83.6 %). M. p., 227~230 °C;
Found, 228~230 °C [41]. H NMR (300 MHz, DMSO-d6,
TMS) δ: 7.87 (d, J=7.89 Hz, 2H), 7.28 (t, J=7.14 Hz, 2H),
6.92 (d, J=8.4 Hz, 2H), 6.76 (s, 4H), 6.71 (t, J=8.04 Hz, 2H).
API-MS [M + H]+ m/z: 253.2.
1
1
~211 °C; Found, 220~230 °C [41]. H NMR (300 MHz,
2,5-bis(pyridine-2-formamidophenyl)-1,3,4-oxadiazole
(OA1) 2,5-bis(2-aminophenyl)-1,3,4-oxadiazole (0.85 g,
3.37 mmol), pyridine-2-formic acid (1.28 g, 10.11 mmol),
triphenyl phosphite (TPPi, 1.9 mL, 6.74 mmol) and pyridine
(10 mL) reacted in a 100 mL four-neck flask at 50 °C for
16 h. Additional TPPi (0.9 mL, 3.37 mmol) joined in the
reaction and refluxing continued for another 3 h. The solvents
were evaporated and benzene (5 mL) was utilized to wash the
residual. Recrystallization for filtered solid from
chloroform/ether would produce light yellow crystal (0.96 g,
DMSO-d6, TMS) δ: 10.02 (s, 1H), 7.60 (d, J=7.86 Hz, 2H),
7.18 (t, J=7.11 Hz, 2H), 6.73 (d, J=8.25 Hz, 2H), 6.53 (t, J=
7.14 Hz, 2H), 6.41 (s, 4H).
2,5-bis(2-aminophenyl)-1,3,4-oxadiazole (4) In a 100 mL
four-neck flask, N,N’-bis(2-aminobenzoyl)-hydrazide
(4.1 g, 15.2 mmol) and polyphosphoric acid (PPA, 10 mL)
were heated quickly to 160 °C and reacted for 2 h.
Following, system temperature was decreased to 50 °C.
Ice water (100 mL) was added slowly after completed
reaction was detected by TLC. The pH was adjusted to 5
with sodium hydroxide solution (15 %). The filtered solid
was recrystallized from DMF/H2O to bring brown
1
61.7 %). M. p., 244~246 °C; Found, 249~251 °C [41]. H
NMR (300 MHz, DMSO-d6, TMS) δ: 12.64 (s, 2H), 8.87
(t, J=4.5 Hz, 4H), 8.28 (m, 4H), 8.14 (t, J=7.8 Hz, 2H), 7.76
(m, 4H), 7.40 (t, J=7.5 Hz, 2H). API-MS [M + H]+ m/z: 463.3.

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J Fluoresc (2013) 23:785–791
General Spectroscopic Procedures
Cd2+,, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+ and Zn2+ were
prepared from their chloride salts. The Pb2+ was prepared
from lead nitrate. A solution of Cr3+ was prepared from
chromium acetate and stored at pH1. Solution of Fe2+ was
prepared immediately before use with ferrous ammonium
sulfate and water that was thoroughly purged with Ar. All
stock solutions were approximate 10 mM. In a typical
experiment, the emission spectrum of free OA1 was
recorded. A 20μL aliquot of a 10 mM metal solution was
then added to a 5.0μM solution of OA1 (3 mL) and the
emission spectrum was recorded with excitation at 370 nm.
Subsequently, a 20μL portion of 10 mM AgNO3 was added
and the emission spectrum was obtained. For all experiments,
the spectra were normalized with respect to free OA1 and
manipulated to each condition at least for three times.
N-2-Hydroxyethylpiperazine-N'-2-ethanesulfonic
acid(HEPES) from Calbiochem and AgNO3 (99.998 %)
were purchased and used as received. Millipore filtered
water was used to prepare all aqueous solutions. All spec-
troscopic measurements were performed at neutral pH with
10 mM HEPES buffer solution adjusting. An Orion glass
electrode, calibrated prior to use, was employed to record
solution pH. Silver solutions were prepared from 10 mM
stock solutions of AgNO3. Stock solution of OA1 (10 mM
in different solvent such as DMF, cyclohexane, toluene,
THF, ethyl acetate, acetone, acetonitrile, pyridine, ethylene
glycol, monomethyl ether) was prepared. After addition of
this stock solution to aqueous buffers, the resulting solution
contained 0.1 % solvent for fluorescence and 1 % solvent
for absorption measurements. The KaleidaGraph software
package was used to manipulate all spectral data.
Ag+ Binding Studies by Fluorescence Spectroscopy
Metal-binding titration experiments were done and the
method of continuous variations(Job’s analyses) [42] was
applied to calculate the correlative stoichiometry of OA1-
Ag+ complex according to the following equation.
Optical Absorption Spectroscopy
UV-visible spectra were collected on an Agilent 8453 diode
array spectrophotometer which was controlled by a Pentium
PC running manufacturer supplied software package. A
circulating water bath was used during acquisition to main-
tain the temperature at 25.0 1.0 °C. Samples were
contained in 1 cm path length quartz cuvettes (3.5 mL
volume). All manipulations were performed at least three
times.
F ꢀ Fmin
log
¼ n log½Agþꢁ þ B
Fmax ꢀ F
In this equation, Fmin, Fmax and F referred to the fluores-
cent intensity of metal-free OA1, OA1 with excessive Ag+
and OA1 with Ag+ at any concentration between the former
two, respectively. n meant the stoichiometry of OA1-Ag+
complex. In a typical titration, 3μL aliquots of a 1 mM
AgNO3 solution in water were added to a 5.0μM OA1
solution in DMF and the fluorescence changes at 368 nm
were plotted against equivalents of Ag+ added. All processes
were operated at least with three replicates to obtain statistical
analysis.
Fluorescence Spectroscopy
Fluorescence spectra(FS) were obtained with a Perkin Elmer
LS-55 fluorescence spectrophotometer linked to a Pentium
PC running SpectraCalc software package. A rhodamine
quantum counter and manufacturer supplied photomultiplier
curves were used to normalize the excitation and emission
spectra, respectively. A circulating water bath was conducted
during all experiments to regulate the temperature at 25.0
1.0 °C. Spectra were acquired with 3 nm slit widths and a
600 nm/min scan speed. All measurements were carried out at
least in triplicate.
Selective Fluorescence Sensing of Molecular Sensor OA1
for Ag+
In order to ascertain the affect of metal ions on the fluores-
cence of molecular sensor, a solution of 5.0μM OA1 in the
buffer containing cation of interest was prepared and the
emission spectrum was recorded. The selectivity of OA1 for
Ag+ against a background of various alkali, alkaline earth,
transition metal ions, and Al3+ or Pb2+ was also investigated
by using FS. Aqueous metal ion solutions of Al3+, Ba2+,
Fig. 1 Absorption and fluorescence spectra of OA1 in HEPES buffer
solution (10 mM, pH=7.0, DMF-H2O, 9:1, v/v)

J Fluoresc (2013) 23:785–791
789
Table 1 Excitation and emission spectral data of OA1 in different
solvents at room temperature
excitation maximum by around 33 nm, and the magnitude
of spectral shift was 15 nm in fluorescence. The effect of
medium polarity on absorption maximum was more pro-
nounced than that on fluorescence maximum. This meant
the ground state of OA1 should have more significant
polarity than that of the emitting one.
Solvent
Absorption
Fluorescence
Stokes
maximum(nm)
maximum(nm)
(λmax=370 nm)
shift(nm)
Cyclohexane
Toluene
269
281
288
290
293
290
297
299
302
368
99
368.5
371.5
371.5
375
87.5
83.5
81.5
82
Fluorescence Sensing of Molecular Sensor OA1 for Ag+
THF
Ethyl acetate
Acetone
The fluorescence sensing abilities were primarily investigat-
ed by adding different cations (Ag+, Al3+, Ba2+, Cd2+, Cr3+,
Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, Pb2+ and Zn2+) to
the HEPES buffer solution (10 mM, pH=7.0, DMF-H2O,
9:1, v/v) containing molecular sensor OA1. When 10
equivalent(equiv.) free Ag+ was added into the buffer solu-
tion including OA1 (5.0μM), the sensor responded with
dramatic fluorescent enhancement contrast to the slight
fluorescent change even quenching caused by the other
cations (Fig. 2). Meanwhile, the addition of metal ions did
not shift the emitting wavelength of OA1 and all the fluo-
rescent responses were transient without time dependence.
Therefore, fluorescent sensor OA1 exhibited high selectivity
and sensitivity for Ag+.
The fluorescent decrease of OA1 caused by selected
cations except Ag+, Cd2+, Pb2+ and Mn2+ was consistent
with the behavior of numerous turn-off molecular sensors.
PET theory could explain this phenomenon. The nitrogen
atom of pyridine ring in OA1 was the cation receptor which
would provide lone pairs and the electron transfer was
switched off during binding process between OA1 and
cation. To Ag+, Cd2+, Pb2+ and Mn2+, the rebuilding of rigid
molecular system for sensor-metal complex might more
Acetonitrile
Pyridine
377.5
378
87.5
81
Ethylene glycol
Monomethyl ether
378.5
383
79.5
81
Results and Discussion
Spectroscopic Properties of Molecular Sensor OA1
The maximum excitation and emission wavelengths of sen-
sor OA1 in HEPES buffer solution (10 mM, pH=7.0, DMF-
H2O, 9:1, v/v) at room temperature were at 269 nm and
372 nm respectively (Fig. 1).
Meanwhile, the excitation and emission spectra of above
OA1 system have been studied in various solvents with
different polarity and the spectral data have been collected
(Table 1). As was typical of a charge-transfer transition, an
increase in the polarity of medium led to a Stokes shift of
excitation maximum. While the change of solvent from
cyclohexane to monomethyl ether resulted in a shift of
Fig. 2 Relative fluorescence
intensity changes of OA1
(5.0μM) in the presence of
excessive selected cations
(10equiv.) in HEPES buffer
solution (10 mM, pH=7.0,
DMF-H2O, 9:1, v/v) at
room temperature. Inset:
Fluorescence spectra of OA1
(5.0μM) in the presence of
excessive selected cations
(10equiv.) in HEPES buffer
solution (10 mM, pH=7.0,
DMF-H2O, 9:1, v/v) at room
temperature. The excitation was
at 370 nm

790
J Fluoresc (2013) 23:785–791
immediately which was comparable to that of sensor solu-
tion with Ag+ fed into directly. These results indicated that
the selectivity of OA1 for Ag+ was hardly affected by the
coexisted metal ions as shown in Fig. 3.
As OA1 shown specific selectivity for Ag+, a series of
experiments were planned to discuss the Ag+ recognition
capability and mechanism of OA1. The binding properties
of sensor OA1 with Ag+ were studied by fluorescence
spectral titration experiments (Fig. 4a) and the method of
continuous variations(Job’s analyses) was further used to
gain an insight into the stoichiometry of OA1-Ag+ complex
(Fig. 4b).
Figure 4a showed the typical fluorescence response spec-
tra of sensor OA1 in HEPES buffer solution (10 mM, pH=
7.0, DMF-H2O, 9:1, v/v) with increasing Ag+ concentra-
tions and the background signal of HEPES alone was
deducted to evaluate the fluorescent response of proposed
assay. The sensing system exhibited a significant change in
fluorescence in the presence of different concentrations of
Ag+. The bottom curve was measured in the absence of Ag+,
where the sensing system had a very weak emission. When
differernt concentrations of Ag+ were added into the solu-
tion of OA1, a drastic increase in the fluorescence emission
was observed.
The intensity of excimer emission increased correspond-
ingly with an increase in Ag+ concentrations as revealed in
Fig. 4b, indicating the formation of OA1-Ag+ complex
which stopped the transferring of lone pairs and rebuilt the
rigid molecular system as were illustrated in Scheme 1.
Once the concentration of Ag+ was over 5.0μM, the fluo-
rescence intensity reached a plateau which suggesting the
saturated recognition sites of Ag+ binding. According to
Fig. 3 Relative fluorescence intensity changes of OA1 (5.0μM) to
Ag+ (10equiv.) in the presence of selected cations (10equiv.) in HEPES
buffer solution (10 mM, pH=7.0, DMF-H2O, 9:1, v/v) at room tem-
perature. The excitation was at 370 nm
powerful than PET, and a fluorescent increase in the 1,3,4-
oxadiazole emission occurred.
An important feature of sensor was its high selectivity
toward analyte over other competitive species. In order to
investigate the silver recognition abilities of OA1, we car-
ried out interference experiments in the same HEPES buffer
solution to discuss the competition effects from above se-
lected metal ions. As has been argued, all other cations
excluded Ag+ slightly enhanced even weakened the fluores-
cence intensity of HEPES buffer solution containing 5.0μM
OA1. But when Ag+ followed to be added into the mixed
solution, the fluorescence intensity increased greatly and
Fig. 4 a Fluorescence enhancement of OA1 (5.0μM) with different
concentrations of Ag+ (3equiv.) added in HEPES buffer solution
(10 mM, pH=7.0, DMF-H2O, 9:1, v/v) at room temperature. b A Job
plot of OA1 and Ag+ at 368 nm which indicated that the stoichiometry
of OA1-Ag+ complex was 1:1. Inset: Linear fitting curve of the
changes in fluorescence intensity at 368 nm of OA1 versus increasing
Ag+ concentration. The excitation was at 370 nm

J Fluoresc (2013) 23:785–791
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Job’s analyses, the linear fitting curve in the inset further
elucidated the binding mode between sensor OA1 and Ag+
that a stable 1:1 OA1-Ag+ complex was obtained.
It was clear that OA1 could be a potential sensor candi-
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In conclusion, we have synthesized a fluorescence turn-on
probe OA1 for Ag+ detecting with 1,3,4-oxadiazole moiety
as signal group and pyridine-2-formamidophenyl moiety as
binding site. A remarkable enhancement (342 %) in fluo-
rescent spectra with Ag+ addition could be observed, and
assays demonstrated this probe was specifically selective
and sensitive for Ag+ over competing cations in HEPES
buffer solution (10 mM, pH=7.0, DMF-H2O, 9:1, v/v).
During recognization, a stable 1:1 OA1-Ag+ complex
formed. Due to its excellent properties, OA1 could be a
viable candidate as fluorescent receptor for Ag+ analysis.
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Acknowledgments Financial support from General Program of Na-
tional Natural Science Foundation of China (Grant No.51003047), Nat-
ural Science Foundation of Jiangsu Province (Grant No.55129003) and
Higher Education Institutions Natural Science Foundation of Jiangsu
Educational Commission (Grant No.09KJB540001, 10KJB540001) are
gratefully acknowledged.
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