A highly selective and sensitive boronic acid-based sensor for detecting Pd2+ ion under mild conditions

Article abstract of DOI:10.1016/j.bmcl.2020.127397

Herein, a boronic acid-based sensor was reported selectively to recognize Pd²⁺ ion. The fluorescence intensity increased 36-fold after sensor binding with 2.47 × 10^?5 M of Pd²⁺ ion. It was carried out in the 99% aqueous so

Full text of DOI:10.1016/j.bmcl.2020.127397

Journal Pre-proofs
A highly selective and sensitive boronic acid-based sensor for detecting Pd2+
ion under mild conditions
Guiqian Fang, Dongxue Zhan, Ran Wang, Zhancun Bian, Guimin Zhang,
Zhongyu Wu, Qingqiang Yao
PII:
S0960-894X(20)30508-4
DOI:
Reference:
https://doi.org/10.1016/j.bmcl.2020.127397
BMCL 127397
To appear in:
Bioorganic & Medicinal Chemistry Letters
Received Date:
Revised Date:
Accepted Date:
26 February 2020
29 June 2020
6 July 2020
Please cite this article as: Fang, G., Zhan, D., Wang, R., Bian, Z., Zhang, G., Wu, Z., Yao, Q., A highly selective
and sensitive boronic acid-based sensor for detecting Pd2+ ion under mild conditions, Bioorganic & Medicinal
Chemistry Letters (2020), doi: https://doi.org/10.1016/j.bmcl.2020.127397
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A highly selective and sensitive boronic acid-based sensor for detecting
Pd2+ ion under mild conditions
Guiqian Fang,a, b, c, d Dongxue Zhan,a, b, c, d Ran Wang,a, b, c, d Zhancun Bian,a, b, c, d Guimin Zhang, e, f
Zhongyu Wu*b, c, d and Qingqiang Yao*b, c, d
a. School of Medicine and Life Sciences, University of Jinan-Shandong Academy of Medical Sciences, Jinan 250200, Shandong,China
b. Institute of Materia Medica, Shandong First Medical University & Shandong Academy of Medical Sciences, Jinan 250062, Shandong, China
c. Key Laboratory for Biotech-Drugs Ministry of Health, Jinan 250062, Shandong, China
d. Key Laboratory for Rare ＆ Uncommon Diseases of Shandong Province, Jinan 250062, Shandong, China
e. Laboratory, Generic Manufacture Technology of Chinese Traditional Medicine, Jinan, China
f. Center for New Drug Pharmacological Research of Lunan Pharmaceutical Group, Jinan, China
*Corresponding authors: E-Mail: wu_med@foxmail.com (Zhongyu Wu), yao_imm@163.com (Qingqiang Yao).
Herein, a boronic acid-based sensor was reported selectively to recognize Pd2+ ion. The fluorescence intensity increased 36-fold
after sensor binding with 2.47×10-5 M of Pd2+ ion. It was carried out in the 99% aqueous solution for binding tests, indicating
sensor having good water solubility. In addition, it is discernible that Pd2+ ion turned on the blue fluorescence of sensor under a
UV–lamp (365 nm), while other ions (Ag+, Al3+, Ba2+, Ca2+, Cr2+, Cd2+, Co2+, Cs2+, Cu2+, Fe2+, Fe3+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+
and Zn2+) did not show the similar change. Furthermore, sensor has a low limit of detection (38 nM) and high selectivity, which
exhibits the potential for the development of Pd2+ recognition in practical environments.
Keywords: Boronic acid; Fluorescence sensor; Pd2+ ion; Mild conditions
Considered to be one of the most important catalysts, palladium is used widely not only in numerous chemical
transformations employed for the production of key chemical intermediates, fine chemicals, therapeutic drugs and
so on,1, 2 but also in commercial materials including fuel cells, jewellery, and catalytic converters.3, 4 In this context,
it is difficult to avoid the residual palladium in pharmaceuticals and waste containing palladium accidentally leaking
into the environment, and eventually into the body, causing serious impact on the biological system. It is known that
palladium species bind to DNA, amino acids, protein, and other biomolecules, which disturb biological processes
and cause serious physiological disorders including dizziness, allergies, memory loss, facial paralysis and so on.
Therefore, it is significant to develop a simple and effective methodology for the detection of palladium species.
Compared to several typical analytical methods, including HPLC coupled with solid-phase microextraction,5 atomic
absorption spectroscopy (AAS)6 and inductively coupled plasma mass spectrometry (ICP-MS),7 a solution
fluorescence-based sensing method is receiving continued attention due to simple, rapid and sensitive detection of
palladium. Consequently, various fluorescent sensors have been designed and synthesized for the detection of Pd2+
ion (Table 1). However, these fluorescent sensors still have some drawbacks and limitations such as poor water-
soluble,8 slow response,9, 10 necessity of heating at high temperature,9-11 and complex synthetic process of sensor.12,
13 Therefore, the development of a new fluorescent sensor for detecting Pd2+ ion is still increasing interest.
It is well known that Pd can be used as an effective catalyst for the Suzuki-Miyaura coupling, which is the coupling
reaction between an arylboronic acid and an aryl halide.14-16 Currently, there are few boronic acid-based fluorescent
sensors for the detection of Pd2+ ion. Higashi reported the first boronic acid-based fluorescent sensor for Pd2+ ion
1

based on benzofuran-2-boronic acid.8 However, although there is a lower limit of detection (LOD) and a faster
response time, organic solvent acetonitrile is used as sensor buffer solvent, which reflects the poor water solubility
and is not conducive to further application of biological experiments. Furthermore, owing to shorter emission
wavelengths and the absence of functional groups that can be further modified, it brings much difficult to design
Pd2+ ion sensors with longer emission wavelengths on this basis. Therefore, it is quite meaningful to find a well
water-soluble sensor that can be further modified functional groups on this basis. However, compared to the reported
sensors (Table 1), the water-soluble sensor 1 has a low LOD to Pd2+ ion, as well as a handle for tethering other
functional groups. In addition, the colour of solution changed blue from colourless after Pd2+ ion combined with
sensor 1 but other ions caused almost unchanged, which was easily recognized by the naked eyes under a UV–lamp
(365 nm).
The UV‒vis absorption of the two sensors is shown in the Fig. S1. Sensor 1 has the largest absorption peak at
about 337 nm. When the excitation wavelength of sensor 1 was set at 337 nm (slit: 5 nm/5 nm), sensor 1 has the
largest emission fluorescence at about 400 nm. However, sensor 1 presents a red-shifted emission peak at 450 nm
after adding of Pd2+ ion (Fig. 3A). In addition, sensor 2 has strongest emission fluorescence when the excitation
wavelength was set at 340 nm for fluorescence tests (slit: 5 nm/5 nm). The quantum yields of sensor 1 and 2
2
( )
푛퐷
퐹퐷
퐹푆
퐴푆
fluorescence were calculated 0.06 and 0.05 respectively using the following equation:12
2
,
훷퐷
=
훷푆
×
×
×
퐴퐷
(
푛푆
)
where ΦS is the fluorescence quantum yield of the standard (quinine sulfate in ethanol, Φ =0.55, 25°C); FD and FS
are the integrated areas of fluorescence intensity at the same excitation wavelength; AD and AS are the absorbances
at the defined excitation wavelength; nD and nS are the refractive indices of the solvents (ethanol) at 25°C.
Fluorescence properties of sensor 1 were investigated toward different cations in DMSO/PBS (pH 8, 0.1M)
solution (1:99, v/v), at room temperature. Interestingly, upon addition of Pd2+ ion to the solution of sensor 1, a
remarkable fluorescence enhancement appeared at around 450 nm (Fig. 3A). In contrast, upon addition of other
metal ions (Ag+, Al3+, Ba2+, Ca2+, Cr2+, Cd2+, Co2+, Cs2+, Cu2+, Fe2+, Fe3+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+ and Zn2+)
to the solution of sensor 1, there were not displaying the similar phenomenon (Fig. 1). These results demonstrated
that sensor 1 had remarkably high selectivity toward Pd2+ ion.
Fig. 1 Fluorescence spectra of sensor 1 (1×10-5 M) in the absence and presence of 2.5×10-5 M of various metal ions in DMSO/PBS (pH 8,
0.1M) solution (1:99, v/v), at room temperature. Inset: Photograph of 1 (1×10-5 M) upon adding 2.5×10-5 M of various ions, which was
observed under a UV–lamp (365 nm).
As an important aspect of a chemical sensor used to practical applications, the sensitivity of sensor 1 toward Pd2+
ion was necessary to be evaluated. Therefore, the response time experiments of the reaction system were carried out.
Upon addition of 2.5×10-5 M of Pd2+ ion, the fluorescence of sensor 1 was enhanced at 450 nm and reached the
2

maximum fluorescence emission intensity after 10 min, followed by very weak fluorescence decay (Fig. 2). In
addition, sensor 1 recognized Pd2+ ion at room temperature (such as 25 °C). These results indicated that sensor 1 had
remarkably high sensitivity toward Pd2+ ion.
Fig. 2 Fluorescence spectra of sensor 1 (1×10-5 M) upon addition of 2.5×10-5 M of Pd2+ ion from 0 min to 30 min in DMSO/PBS (pH 8,
0.1M) solution (1:99, v/v), at room temperature. Inset: Plot of the fluorescence intensities at 450 nm over a period of 30 min.
In order to get more insight on the binding ability of sensor 1 toward Pd2+ ion, fluorescence spectrum titrations
were performed. Upon the addition of increasing concentrations of Pd2+ ion, the fluorescence intensity of sensor 1
increased 36-fold at 450 nm (Fig. 3A). In order to investigate the function of boronic acids groups recognizing Pd2+
ion, sensor 2 without boronic acids groups was also used in the fluorescence spectrum titration study.17 Interestingly,
as the concentration of Pd2+ ion increased, the fluorescence intensity of sensor 2 was almost unchanged (Fig. 3B).
This study indicates that boronic acids group plays a key role in the recognition of Pd2+ ion. Moreover, the formed
structure of sensor 1 and Pd2+ ion has been discussed according to mass spectrometry. It showed that the sensing
mechanism of sensor combined with 2 equivalents of Pd2+ ion (Fig. S10).18, 19 After sensor 1 combined with catechol,
the fluorescence of sensor 1-catechol is quenched owing to photoinduced electron transfer (PET) mechanism.17
Interestingly, after sensor 1 combined with 2 equivalents of Pd2+ ion, however, the fluorescence of sensor 1-2Pd2+
is enhanced. Therefore, we speculate that sensor 1 has a certain action of PET, the formed sensor 1-catechol enhanced
it while the formed sensor 1-2Pd2+ may inhibit it, thereby Pd2+ turning on the fluorescence sensor 1. In addition,
Tharmaraj reported similar compound structure which has inhibition of PET mechanism.20 Nevertheless, all these
results indicated that sensor 1 could be used as a fluorescent response sensor for rapid detection of Pd2+ ion in
aqueous solutions under mild conditions.
Fig. 3 A) Fluorescence spectra of 1 (1×10-5 M) in the presence of different concentrations of Pd2+ ion in DMSO/PBS (pH 8, 0.1M) solution
(1:99, v/v), at room temperature, measured after addition of Pd2+ ion for 30 min. Inset: Photograph of 1 (1×10-5 M) upon adding 2.47×10-
3

5
M of Pd2+ ion, which was observed under a UV–lamp (365 nm); B) Fluorescence spectra of 2 (1×10-5 M) in the presence of different
concentrations of Pd2+ ion in DMSO/PBS (pH 8, 0.1M) solution (1:99, v/v), at room temperature, measured after addition of Pd2+ ion for
30 min.
Moreover, a significant fluorescence increase at 450 nm was observed with 36-fold emission enhancements when
nearly 2 equiv. Pd2+ ion was added to the solution of sensor 1 (Fig.4). Pd2+ ion exhibited the dependence of the
intensity of emission at 450 nm, which indicated the formation of a sensor 1/ Pd2+ adduct of 1 : 2 stoichiometry.21, 22
LOD = 3훿/푆
Then the LOD was calculated to 38 nM with the following equation (Fig. S2):
, where δ is the standard
deviation of the 8 times blank signal of sensor, and S is the slope of the calibration curve. Sensor 1 has a low LOD
and good linear relationship in the range of 1.48 μM to 21.7 μM. As a “turn on” type sensor, sensor 1 is more
sensitive than “turn off” type sensors. In addition, the fluorescence tests of sensor 1 under the 99% aqueous solution
is more advantageous than the sensors with a larger proportion of organic solvent 19. Furthermore, the sensitivity of
sensor 1 can satisfy the requirement which the WHO specified threshold limit for palladium content in drug
chemicals (47.0 μM to 94.0 μM) 19. The enhancement efficiency was quantitatively explained by the Stern–Volmer
퐼0/퐼 = 1 + 퐾푆푉 × [푀],
(SV) equation:23
where I0 and I are the maximum fluorescence intensity of 1 before and after
the addition of Pd2+ ion, respectively, and [M] is the molar concentration of Pd2+ ion. Pd2+ ion has a significant effect
on the fluorescence enhancement of the sensor 1 with a large KSV of -3.9×10-4 M-1. The SV plot for Pd2+ was nearly
linear at low concentrations, as shown in Fig. S3.
Fig. 4 Photograph of sensor 1 linear relationship.
To further evaluate the selectivity of sensor 1 for Pd2+ ion, the interference experiments of sensor 1 for Pd2+ ion
were carried out in the presence of various ions under the same conditions, as shown in Fig. 5. It was found that
competitive ions had litter interference to sensor 1 responding to Pd2+ ion. Sensor 1 and 2 had neglectable
fluorescence change in the presence of 2.5×10-5 M of the ions Ag+, Al3+, Ba2+, Ca2+, Cr2+, Cd2+, Co2+, Cs2+, Cu2+,
Fe2+, Fe3+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+ and Zn2+. However, after addition of Pd2+ ion, sensor 1 had remarkable
fluorescence enhancement.
4

Fig. 5 Fluorescence intensities of 1 (1×10-5 M) in DMSO/PBS (pH 8, 0.1M) solution (1:99, v/v), at room temperature, measured after
addition various metal ions for 30 min. Black bars represent the fluorescence intensity (450 nm) of 1 in the presence of 2.5×10-5 M of the
ions Pd2+, Ag+, Al3+, Ba2+, Ca2+, Cr2+, Cd2+, Co2+, Cs2+, Cu2+, Fe2+, Fe3+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+ and Zn2+. Red bars represent the
fluorescence intensity (450 nm) of 1 in the presence of various metal ions after the addition of Pd2+ ion.
As Lewis acids, boronic acids can be affected by acidic and alkaline environments.24 Therefore, it is necessarily
to investigated a suitable pH condition for the interesting process of sensor 1 recognizing Pd2+ ion. As shown in the
Fig. 6, sensor 1 has a large fluorescence response in the range of pH 7 to 10 under the same condition. These
fluorescence intensity data come from strongest emission wavelength (450nm). Interestingly, boronic acid exists in
the form of trivalent plane sp2 (neutral) under acidic conditions and turns into tetravalent tetrahedral sp3 (anionic)
under basic conditions.25 Due to the electrostatic attraction, the anionic form of boronic acid can interact with the
cationic Pd2+ under slightly alkaline condition, inducing fluorescence enhancement. However, OH- can interact with
Pd2+ when the alkaline condition is too strong, weakening this induction effect. Therefore, the bell-shaped pH vs
fluorescence intensity plot is obtained.
Fig. 6 Fluorescence responses of sensor 1 (1×10-5 M) to Pd2+ ion in DMSO/PBS (1:99, v/v, 0.1 M) at different pH values, at room
temperature.
Many sensors were reported earlier selectively recognize Pd2+ ion, however, there still had some drawbacks.
Although some of the reported sensors have superior optical properties, there are some aspects that require further
improvement. For example, as an important parameter, the LOD of many reported sensors has reached the nM level,
which plays a significant role in the trace detection of real samples. However, these tests were performed under non-
ideal conditions, such as long binding time, high binding temperature and high proportion of organic solvent used
in the buffer.8-11 Interestingly, we found that the mother nucleus (sensor 1) designed previously into diboronic acid
sensors by us had a significant fluorescence enhancement with Pd2+ ion but did not show the similar phenomenon
5

with other ions. We have reported the recognition of Fe3+ ion by diboronic acid sensor mentioning sensor 1 having
a fluorescence quenching response to Fe3+, Cr+ and Al3+, but it does not affect the response of sensor 1 to Pd2+ ion.18
Owing to the difference of turning on and off in responding mode, it is easier to use sensor 1 to recognize the Pd2+
ion among various ions. The biggest advantage of sensor 1 over the reported sensors is water-soluble and mild
detection conditions (room temperature, 30 min response time, etc.). In addition, as a known and commercially
available compound, sensor 1 is more suitable to be further researched. The LOD of sensor 1 was measured at 38nM
indicated high sensitivity to Pd2+ ion. Moreover, it is discernible that Pd2+ ion turned on the blue fluorescence of
sensor 1 under a UV–lamp (365 nm), while other ions did not show the similar change.
Table 1 key information of the reported sensor for Pd2+ ion.
Time；
Sensor
Buffer
λex; λem
LOD
Temperature
CN
Chen et al.
PBS buffer/EtOH
(99:1, v/v)
CN
60 min; 37 °C
395 nm; 457 nm↑
53 nM
O
reported 11
315 nm; 360
Higashi et
OH
OH
B
acetonitrile
5 min; 25 °C
9.8 nM
260 nM
nm↑
al. reported 8
O
O
N
N
N
H
PBS buffer/DMF
(200:1, v/v)
Hou et al.
410 nm; 491
N
H
5 min; -
N
reported 26
nm↓
O
N
O
O
HO
N
O
Long et al.
EtOH/HEPES buffer
(4/6, v/v)
540 nm; 583
1 min; -
3490 nM
N
N
reported 27
nm↑
N
O
N
O
O
N
O
Ren et al.
MeOH/PBS buffer
(8:2, v:v)
550 nm; 586
30 min; 25 °C
50 nM
O
HN
reported 28
nm↑
S
NO2
N
O
N
S
Xu et al.
480 nm; 526
PEG400 solutions
120 min; 55°C
1.13 nM
reported 9
O
N
nm↑
O
Br
Qin et al.
240 nm; 359
EtOH/H2O (3:7, v:v)
120 min; 70°C
6 min; 25°C
50 nM
reported 10
nm↑
MeOH/PBS buffer
(1:1, v:v)
O
Wang et al.
405 nm; 511
O
690 nM
reported 29
nm↑
O
O
O
6

COOH
N
DMSO/PBS buffer
(1:99, v:v)
337 nm; 450
Our sensor
10 min; 25°C
38 nM
OH
nm↑
B
OH
↑：turn on; ↓：turn off
Sensor 1 is an interesting compound because the quinoline ring has a handle (carboxylic acid functional group)
for tethering other functional groups. A very striking example is the structural modification of sensor 1 by Chen et
al., which synthesized PBAQA-PGMA fluorescent composite nano-microsphere biosensor.30 It showed good
detection sensitivity and selectivity to 5-hydroxymethylcytosine in genomic DNA. Compared to the fluorescence
tests of none boronic acid-based sensor 2 to Pd2+ ion, it is determined that the boronic acid group of sensor 1 is a key
site for interaction with Pd2+ ion. There are some excellent features in sensor 1. The connection of phenylboronic
acid to the quinoline ring provides the possibility of electron transfer between the quinoline ring and the benzene
ring after the combination of boronic acid and the analyte. In addition, both the boronic acid group and N on the
quinoline are hydrophilic, which offer a water-soluble feature. Furthermore, the quinoline ring has a large conjugated
structure which provides potential for its derivatives to have long emission wavelengths. The long wavelength and
water solubility may be increased when the carboxylic acid on sensor 1 is further modified to a larger conjugated
compound or linked to new materials. Thus, these studies indicate that sensor 1 has the potential to be modified for
the detection of other meaningful analytes, including Pd2+ ion.
In summary, a highly selective and sensitive boronic acid-based sensor was reported selectively recognizing Pd2+
ion under mild conditions. Interestingly, Pd2+ ion turned on the fluorescence of sensor 1 while other ions did not
show the similar change. In addition, the conditions of tests are quite mild, which were carried out under the 99%
aqueous solution, at room temperature. Moreover, sensor 1 not only has high selectivity and sensitivity toward Pd2+
ion, but also provides further modified functional group, which bring great potential for designing higher binding
affinity and longer emission wavelength sensors on this basis. These studies indicate that sensor 1 has potential for
Pd2+ ion recognition in practical environments.
Acknowledgements
The authors would like to thank Shiyanjia Lab (www.shiyanjia.com) for analysis
molecular weight of thesensor 1-2Pd2+andthe Innovation Project of Shandong Academy of Medical
Sciences for financial support. This work was supported by National Natural Science Foundation of China (Grant
No. 21801158), Shandong Academy of Medical Sciences Foundation (Grant No. 2018-17), Graduate Instructor
Guidance Ability Improvement Project of University of Jinan (Grant No. JDYY1804), Academic Promotion
Programme of Shandong First Medical University (nos. 2019LJ003).
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Graphical Abstract
O
OH
O
OH
2Pd2+
N
N
O
Pd
OH
PET
Low fluorescence
PET
High fluorescence
B
B
O
OH
Pd
A boronic acid-based sensor was reported selectively to recognize Pd2+ ion, which has a low limit
of detection (38 nM) and exhibits the potential for the development of Pd2+ recognition in practical
environments.
9