A ready-to-use fluorescence probe of Pd2+ in water: novel tricyclic heterocyclic base on 1,3,4-oxadiazole

Article abstract of DOI:10.1002/bio.4110

A ready-to-use hetero-tricyclic compound, 5,5′-(furan-2,5-diyl) bis (1,3,4- oxadiazol-2-amine) (5), was synthesized with a good yield; it has an suitable fluorescence characteristic and research founded that it can respond to trace Pd²⁺ in water at a normal pH range. A fluorescence titration revealed the detection limit for Pd²⁺ was as low as 3.97 × 10^?9 M. Density-functional theory calculation using Guassian09 implied that the breakage of conjugation and coplanarity of compound 5 led to fluorescence quenching. Compound 5 could be applied as a chemical probe to detect trace amounts of Pd²⁺ with good accuracy, fast response time, excellent selectivity, and high sensitivity. FT-IR, NMR, and MS were used to characterize the chemical structure of compound 5.

Full text of DOI:10.1002/bio.4110

Received: 9 April 2021
DOI: 10.1002/bio.4110
Revised: 2 June 2021
Accepted: 12 June 2021
R E S E A R C H A R T I C L E
2
+
A ready-to-use fluorescence probe of Pd in water: novel
tricyclic heterocyclic base on 1,3,4-oxadiazole
1
,2
1
2
1
3
Ke Su
| Xiaomei Huang | Wei Wei | Xiaotong Zeng | Siyu Xiang
|
3
Haijun Yang
1
Department of Chemistry and Chemical
Engineering, Sichuan University of Arts and
Science, Dazhou, Sichuan, China
Abstract
0
A ready-to-use hetero-tricyclic compound, 5,5 -(furan-2,5-diyl) bis (1,3,4- oxadiazol-
2
Key Laboratory of Exploitation and Study of
2-amine) (5), was synthesized with a good yield; it has an suitable fluorescence char-
Distinctive Plants in Education Department of
Sichuan Province, Dazhou, Sichuan, China
2+
acteristic and research founded that it can respond to trace Pd in water at a normal
2+
3
School of Materials Science and Engineering,
pH range. A fluorescence titration revealed the detection limit for Pd was as low as
Southwest University of Science and
Technology, Mianyang, Sichuan, China
.97 ꢀ 10ꢁ M. Density-functional theory calculation using Guassian09 implied that
9
3
the breakage of conjugation and coplanarity of compound 5 led to fluorescence
Correspondence
quenching. Compound 5 could be applied as a chemical probe to detect trace
Haijun Yang, School of Materials Science and
Engineering, Southwest University of Science
and Technology, Mianyang 621010, Sichuan,
China.
2
+
amounts of Pd with good accuracy, fast response time, excellent selectivity, and
high sensitivity. FT-IR, NMR, and MS were used to characterize the chemical struc-
ture of compound 5.
Email: 65818933@qq.com
Funding information
K E Y W O R D S
the Key Laboratory of Exploitation and Study
of Distinctive Plants in Education Department
of Sichuan Province, Grant/Award Number:
TSZW2006; Sichuan Science and Technolgy
Program, Grant/Award Numbers:
2
+
DFT, fluorescent probe, Pd , UV–Vis
2
019YJ0324, 2019YJ0307; National
Engineering Technology Center for Insulation
Materials, Southwest University of Science and
Technology, Grant/Award Number: kfjc03;
Sichuan University of Arts and Science,
Grant/Award Number: 2016KZ003Z; Dazhou
municipal science and technology bureau
application foundation, Grant/Award Number:
1
8YYJC0002
1
|
INTRODUCTION
Traditional methods to detect heavy metal ions such as ICP-
[
5]
[6]
MS, ICP-OES, X-ray fluorescence (XRF), and flame atomic absorp-
[
7]
Along with unceasing economic development, people's living stan-
dards are improving, and environmental pollution problems are now
increasing producing concerns especially for the pollution from heavy
tion spectrometry (FAAS) can detect the target ions with good
sensitivity and speed, but most require well trained operators,
[
8]
expensive facilities, and a specific experimental environment. For
[
1]
metals. Palladium is a very important metal due to its wide applica-
the reason of low cost, easy operation, speed and sensitivity, a series
[9]
[2]
tions in laboratory and industry. Large amounts of residual palladium
of colorimetric and fluorimetric methods has been established for
[
3]
exist in the environment. Palladium is nonbiodegradable with the
determination of trace heavy metal ions. To date, some fluorescent
chemical probes for detecting metal ions have been reported involving
characteristics of bioaccumulation and biological concentration along
[4]
3+ [10]
2+ [11]
3+[[12]]
2+ [13]
the food chain. It is a serious hazard to animals and plants. There-
Fe
,
Cu
,
Cr
and Zn
.
fore, there is an urgent need to explore a quick, selective, and sensi-
Various fluorescent chemical probes have been designed to
2+ [14]
2
+
tive method to detect Pd
.
detect Pd
,
unfortunately most of these are derived from
Luminescence. 2021;1–7.
wileyonlinelibrary.com/journal/bio
© 2021 John Wiley & Sons Ltd.
1

2
SU ET AL.
ꢂ
(compound 5, 59.8% yield) were obtained; m.p. > 300 C, IR (KBr pel-
traditional fluorescent molecular structures such as coumarin, rhoda-
mine and fluorescein. Moreover, the synthesis of such probes is com-
plex and it is more difficult to further derivatize from those structures.
As an important class of heterocyclic compound, there have been
many studies on 1,3,4-oxadiazole in virtue of its excellent biological
ꢁ
1
1
let, cm ): 3440, 2988, 2920, 1649, 1089, and 594. H-NMR
13
(600 MHz, DMSO-d
6
, ppm): 7.65 (s, 4H), 7.07 (s, 2H). C-NMR
(125 MHz, DMSO-d
6
, ppm): 112.62, 140.42, 149.92, 163.52. MS
+
ꢂ
(ESI): m/z = 235 [M + H] . m.p. > 300 C.
[
15]
and optical properties,
been rarely studied.
but its strong fluorescence properties have
By directly dissolving compound 5 in ethanol, the stock solutions
2
of compound
5
ꢁ2
(1 ꢀ 10ꢁ M) were prepared. Different stock
0
2+
2+
2+
3+
2+
+
3+
Here,
a
hetero-tricyclic compound 5,5 -(furan-2,5-diyl)bis
solutions (1 ꢀ 10 M) of Cu , Zn , Mg , Y , Ca , Rb , Ce
,
2
+
2+
3+
2+
(
1,3,4-oxadiazole-2-amine) (5) was synthesized using a one-step-
Ni , Pb , La , and Mn
were prepared by reverse osmosis
(RO) water, in addition, Pd (1 ꢀ 10ꢁ2 M) was placed in dimethyl
formamide (DMF). Then to obtain the corresponding concentration
value, all the stock solutions were diluted with RO water.
2+
process from the condensation of 2,5-furandicarboxyl hydrazide with
bromide cyanogen followed by alkalization. Research shows that com-
pound 5 has very strong fluorescence intensity and that it could react
2
+
with Pd ions in water solvent rapidly, therefore it was designed as a
selective chemical probe with speed, sensitivity, and convenient
properties.
3
3
|
RESULTS AND DISCUSSION
| Synthesis and characterization
.1
2
|
EXPERIMENTAL
Routinely, it is easy to form an oxadiazole ring using the condensation
of cyanogen bromide with amine group compounds promoted using
sulfuric acid, hydrochloric acid, phosphoric acid, sodium bicarbonate,
or sodium hydroxide at different temperatures under alkalescent
All reagents and raw materials were acquired by commercial
procuration.
Compound 5 was synthesized following the procedures described
in Scheme 1. A solution of 0.920 g (5 mmol) dimethyl furan-2,-
0
conditions. The tricyclic compound, 5,5 -(furan-2,5-diyl)bis(1,3,4-
5
-dicarboxylate (1) in 30 ml ethanol was added into a flask (50 ml)
oxadiazole-2-amine) (5), has a pair of oxadiazole rings, consequently
it was synthesized using one-step-process by condensation of
2,5-furandicarboxyl hydrazide with cyanogen bromide in sodium
bicarbonate solution. Caution: cyanogen bromide is toxic, highly irri-
tating, and can cause death; laboratories and personnel must be prop-
erly trained. Note: the target compound was formed under acid or
base conditions, but the reaction mechanism is not clear.
containing a thermometer and magnetic stirrer. The mixture was
ꢂ
heated at 70 C. Then the mixture was added dropwise to 2 ml iso-
thermal hydrazine monohydrate, the reaction mixture was kept at
ꢂ
7
0 C for 30 min. A large amount of flocculent solids separated out,
and the solution was cooled to room temperature. The precipitate
was collected using filtration and gave 0.87 g of compound 1 as a
white flocculent crystal (compound 2, furan-2,5 -dicarboxylic acid
Compound 5 was characterized (Figure S1–S4). Detailed data
analysis is provided as follows.
ꢁ
1
dihydrazide). Yield: 94.6%; FT-IR spectrum (KBr pellet, cm ): 3262,
ꢂ
2
981, 1669, 1638, 1602, 1554, 1489 and 1298. m.p.178–179 C.
Compound 3 (0.552 g, 3 mmol) and potassium bicarbonate (5.544 g,
ꢂ
6
.6 mmol) were suspended in methanol (30 ml) at 0 C. Bromine cya-
3.1.1
|
Spectra date
nide (0.63 g, 6 mmol) was directly added into the mixture above. Then
ꢂ
stirred overnight at 0 C, heated up to room temperature and stirred
To evaluate the effects of different ions on the optical response of
compound 5, UV–Vis spectra of compound 5 with or without differ-
ent metal ions in water were recorded (Figure 1). According to the
UV–Vis absorbance data (Figure 1), the λmax of compound 5 was
for another 24 h. TLC under 365 nm ultraviolet irradiation indicated
that the target compound was mainly produced, the precipitate was
collected and recrystallized from methanol, 0.42 g brown crystals
0
SCHEME 1 Synthesis of 5,5 -(furan-2,5-diyl)bis(1,3,4-oxadiazole-2-amine) (5)

SU ET AL.
3
ꢁ
3
FIGURE 1 (a) UV–vis absorptions of compound 5 (1 ꢀ 10 M) with or without different metal ions (1 eq). (b) Maximum fluorescence
ꢁ
6
emission intensity of compound 5 (1 ꢀ 10 M) with or without different metal ions
3
05 nm, and metal ions were affected little by both wavelength and
intensity is shown in a histogram (Figure 1b). Interestingly, compared
2+
2+
3+
2+
2+
intensity of compound 5 except for Pd . This was because supramo-
with Pd , other metal ions such as Fe , Cu , and Mn , hardly
2+
lecular interaction only took place between reaction sites of com-
affected the fluorescence of compound 5, one equivalent Pd
2
+
pound 5 and metal ions with a complementary structure. For Pd
,
could almost totally quench the fluorescence. Probably, this is also
because the coordination reaction could form a nonfluorescent or
low-fluorescent complex. This unique feature indicated that com-
pound 5 could be further developed as a selective photochemical
the absorption spectrum of compound 5 showed a red shift accompa-
nied by an absorbance increase, meaning that the coordination reac-
2
+
tion between compound 5 and Pd weakened the conjugation and
resulted in a red shift. Correspondingly, the fluorescence emission
spectra of compound 5 determined with or without different metal
ions in water were recorded, and the maximum fluorescence emission
2+
probe for Pd
For further exploration of the effect of Pd
absorption spectrum of compound 5, UV–Vis titrations of compound
.
2+
on the UV–Vis

4
SU ET AL.
2
+
5
upon addition of Pd were carried out. As displayed in Figure 2,
2+
after the addition of Pd , λmax of compound 5 experienced a slight
red shift, at the same time a small absorbance enhancement occurred.
In addition, a Job's plot (inset of Figure 2) showed that the molar ratio
2+
of coordination between compound 5 and Pd was 1:1. What is even
more interesting is that the quantum yield of compound 5 was
0
.5149, which was calculated using the equation Y
ꢀ A ), where Y is the quantum yield of compound 5, Y
erence compound (quinine sulfate, 0.55), F is the fluorescence inten-
sity integral of compound 5, F refers to the fluorescence intensity
integral of quinine sulfate, Au indicates the absorbance of compound
, and A is the absorbance of quinine sulfate, indicating that com-
u
= (Y
s
ꢀ F
s
u
ꢀ A
s
)/
(
F
s
u
u
is the ref-
u
s
5
s
pound 5 had good luminescence efficiency.
Apart from metal ions, cations in solution could influence the
fluorescence properties of a fluorophore and the pH of the solution
could interfere with the normal proper working of fluorescent probes.
To understand how pH affected the detection of palladium ions using
compound 5, a further study in water was investigated. The fluores-
cence intensities of compound 5, in addition at pH range 7.0–5.0,
2
+
FIGURE 3 pH effects on the Pd recognition of compound 5
2
+
greatly changed. Moreover, the adjunction of one eq Pd
could
almost quench the fluorescence intensity of compound 5 over a wide
pH range (shown in Figure 3), suggesting that compound 5 could only
2
+
be used to detect Pd at the pH range from 5 to 7. To meet potential
applications in an internal environment for animals and plants, here
further fluorescence research was carried out using phosphate-
buffered saline (PBS) at pH 7.4.
To further investigate the fluorescence properties of compound
2
+
2+
5
and Pd , compound 5 was fluorescently titrated with Pd . As
shown in Figure 4, the maximum emission wavelength for compound
5
was 425 nm, corresponding to visible blue fluorescence.
During the fluorescence titration, the fluorescence intensity of
2
compound 5 decreased gradually. When the molar ratio between Pd
+
FIGURE 4 Fluorescence emission titration spectra of compound
ꢁ6
2+
and compound 5 reached 1:1, the fluorescence emission intensity
5 (10 M) with Pd at room temperature (RT) in PBS buffer,
2+
embedded as plot of F/F0 versus [Pd
]
was eventually reduced to 91.2%, confirming that the coordination
2
+
ratio of the complex formed between Pd and compound 5 was 1:1,
consistent with the results of the Job’s equation. Exploiting the data
from the fluorescence titration (11 groups in parallel experiments), the
2+
lowest detection limit (3S/slope) of compound 5 for Pd (where S is
standard deviation) was estimated to be 3.97 ꢀ 10ꢁ M.
9
For the reasons of simplicity, speed and convenience in the pro-
cess of practical application, and requiring no complex and expensive
equipment, a test paper plays an important role in practical applica-
tion. By impregnating a filter paper with compound 5, a portable
2+
test strip can be easily prepared to detect Pd . The test strips
(
s5) prepared above had bright blue fluorescence under ultraviolet
2+
light and completely disappeared when Pd was added. This visible
2+
phenomenon allowed quick and easy detection of Pd in the natural
environment. In addition, other ions could not affect the fluorescence
of test strips under ultraviolet light. For the effect on the
fluorescence of the phosphor, apart from the interference of cations,
anions should also be given attention. The influence of anions on the
ꢁ
3
FIGURE 2 UV–vis titration of compound 5 (5 ꢀ 10 M) with
2
+
2+
Pd . Inset: Job's plot of compound 5 and Pd ; total concentration
of compound 5 and Pd was kept constant at 5 ꢀ 10ꢁ3
2+
M

SU ET AL.
5
2
+
ꢁ1
method for compound 5 to detect Pd was studied (Figure S6), one
bands c. 1652 cm of complex 6 were slightly increased compared
with compound 5 accompanied by the disappearance of a slightly
structured vibration peak, showing that, after introducing acetate ions,
the skeleton structure of the heterocyclic ring was certain influenced.
2
+
ꢁ
ꢁ
ꢁ
equivalent of Pd salt with Br , Cl or AcO ions for fluorescence of
compound 5 was identically quenched after it was added and could
quench the fluorescence of compound 5.
ꢁ
1
ꢁ1
The practical application of measurement is of vital importance,
The bands at c. 1381 cm and 1090 cm were the skeleton vibra-
tions of heterocyclic rings, after complexation the intensity decreased
considerably, and revealed that the N and O atoms in the furan and
oxadiazole rings are probably sites where compound 5 participates in
the coordination.
2+
therefore a standard curve of compound 5 to detect known [Pd
]
in five samples was drawn, the samples were prepared with DMF
and diluted to corresponding concentrations with water (Figure 5).
Another five samples were diluted with filtered water from the
Fujiang river. The analysis results showed that, after the addition
Therefore, the optimized structures of compound 5 with or with-
out palladium metal ions are given in Figure 7 using DFT calculations
and the Guassian09 software (attention: basis set of C, H, O, N and S
2+
of Pd
in the samples, considerable accuracy (relative error
approximately is 6.64%) could be obtained, and testified that com-
2+
0
pound 5 was able to be used to detect Pd in real water samples
system.
is 6–31+G, LANL2DZ for Pd). As we can see in the Figure 7, 3 -N
atoms in the oxazole ring and O atoms in the furan ring of compound
2
+
The results of the experiment above suggested that the coordina-
5 played the role of donors, while Pd ion is a recipient, then the
2+
2
+
tion molar ratio between Pd and compound 5 was 1:1. A mixture of
compound 5 and Pd(CH CHO) solution in methanol was prepared
uniformly, after a full reaction it was then evaporated in a vacuum to
coordination bonds were formed. The addition of Pd
broke
3
2
the coplanarity of compound 5, and conjugation was also weakened,
therefore fluorescence was quenched totally.
2
+
prepare complex 6. The possible process that Pd and compound
experienced is shown (Scheme 2). Interestingly, the mass-to-charge
To search for the detailed mechanism of optical changes before
and after complexation, the molecular orbitals of compound 5 with or
without palladium metal were also studied. As shown in Figure 8, the
clouds of the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) implied that the electrons in
compound 5 were well distributed, but that those in complex 4 locate
were disordered.
5
ratio of complex 6 was found through the measurement of mass spec-
trometry (Figure S7) at m/z 459.56.
The FT-IR spectra of complex 6 and compound 5 are given in
ꢁ
1
ꢁ1
Figure 6. As the IR data shows, between 1800 cm and 900 cm
,
compound 5 and complex 6 were quite different. The adsorption
FIGURE 5 Standard curve of compound 5, red dots means the
2
+
known [Pd ] in Fujiang river
FIGURE 6 IR absorption of compound 5 and complex 6
SCHEME 2 Synthesis of
complex 6

6
SU ET AL.
FIGURE 7 Optimized conformations
of compounds 5(a) and complex 6(b)
FIGURE 8 HOMO–LUMO energy and the interfacial gap of the orbitals for compound 5 and complex 6
This indicates that the fluorescence quenching of compound
ACKNOWLEDGMENTS
2
+
5
was due to the introduction of Pd , which inhibits intramolecular
Thanks to Dr Liangchun Li for his guidance and help on Gaussian
calculations. Thanks to the analysis and measurement centre of
Southwest University of Science and Technology (SWUST) for
supporting this experiment. We appreciate the financial support from
the Key Laboratory of Exploitation and Study of Distinctive Plants in
Education Department of Sichuan Province No: TSZW2006; Sichuan
charge transfer. What is more interesting, after coordination, the
orbital energy gap between the HOMO and LUMO of complex 6 was
slightly lower compared with compound 5, which coordinates with
the tiny red shift existing in the UV–vis spectra and fluorescence
2+
emission compound 5 with Pd
.

SU ET AL.
7
Science and Technolgy Program (2019YJ0324; 2019YJ0307); and the
program of National Engineering Technology Center for Insulation
Materials, Southwest University of Science and Technology (kfjc03);
Sichuan University of Arts and Science (2016KZ003Z) project of
Dazhou municipal science and technology bureau application founda-
tion (18YYJC0002).
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https://orcid.org/0000-0002-0063-2919
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