A colorimetric and fluorescence turn-on probe for the detection of palladium in aqueous solution and its application in vitro and in vivo

Article abstract of DOI:10.1016/j.saa.2020.118547

Palladium has attracted a growing number of attention due to its widely application and environmental toxicity. Consequently, a novel colorimetric and fluorescent turn-on probe (NT-Pd) was designed for sensing of palladium. This probe was capable of detecting palladium in aqueous solution (DMSO was less than 1%, v/v). Under this mild condition, NT-Pd displayed high selectivity and sensitivity for sensing of palladium in both colorimetric and fluorescent strategy, such as low detection limit (5.30 nM) and rapid response time (within 10 min). In addition, NT-Pd was successfully applied for imaging of exogenous palladium in living cells and zebrafishes with good biocompatibility and low toxicity, indicating this probe has satisfactory application potential to track palladium in the complicated biological system.

Full text of DOI:10.1016/j.saa.2020.118547

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A colorimetric and fluorescence turn-on probe for the detection of
palladium in aqueous solution and its application in vitro and in
vivo
Qi Sun, Yuan Qiu, Jun Chen, Feng-Shou Wu, Xiao-Gang Luo,
Yan-Rong Guo, Xin-Ya Han, Da-Wei Wang
PII:
S1386-1425(20)30526-6
https://doi.org/10.1016/j.saa.2020.118547
SAA 118547
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 8 February 2020
Revised date:
26 May 2020
26 May 2020
Accepted
date:
Please cite this article as: Q. Sun, Y. Qiu, J. Chen, et al., A colorimetric and fluorescence
turn-on probe for the detection of palladium in aqueous solution and its application in
vitro and in vivo, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
(2020), https://doi.org/10.1016/j.saa.2020.118547
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A colorimetric and fluorescence turn-on probe for the detection
of palladium in aqueous solution and its application in vitro and
in vivo
Qi Sun,a, # Yuan Qiu,a, # Jun Chen,a Feng-Shou Wu,a Xiao-Gang Luoa, d Yan-Rong
Guo,c Xin-Ya Han c, * and Da-Wei Wang b, *
a Key Laboratory for Green Chemical Process of Ministry of Education, School of
Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan
430205, China.
b State Key Laboratory of Elemento-Organic Chemistry, and Department of Chemical
Biology, National Pesticide Engineering Research Center, Collaborative Innovation
Center of Chemical Science and Engineering, College of Chemistry, Nankai
University, Tianjin 300071, P. R. China.
c
School of Chemistry and Chemical Engineering, Anhui University of Technology,
Maanshan 243002, China.
d
School of Materials Science and Engineering, Zhengzhou University, No.100
Science Avenue, Zhengzhou City, 450001, Henan Province, PR China
# These authors contributed equally to this work.
Corresponding authors:
E-mail addresses: xinyahan@ahut.edu.cn (X. Y. Han); wangdw@nankai.edu.
ABSTRACT: Palladium has attracted a growing number of attention due to its
widely application and environmental toxicity. Consequently, a novel colorimetric
and fluorescent turn-on probe (NT-Pd) was designed for sensing of palladium. This
probe was capable of detecting palladium in aqueous solution (DMSO was less than
1 %, v/v). Under this mild condition, NT-Pd displayed high selectivity and sensitivity
for sensing of palladium in both colorimetric and fluorescent strategy, such as low
detection limit (5.30 nM) and rapid response time (within 10 min). In addition,

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NT-Pd was successfully applied for imaging of exogenous palladium in living cells
and zebrafishes with good biocompatibility and low toxicity, indicating this probe has
satisfactory application potential to track palladium in the complicated biological
system.
Keywords: Fluorescence probe; Palladium; Bioimaging; Zebrafish
Introduction
Palladium (Pd), a rare transition metal, was first discovered by William Hyde
Wollaston in 1803 [1-2]. To date, it is widely applied in chemistry, biology and
environmental science owing to its special physical and chemical properties [3-4]. For
instance, palladium-containing catalyst including Pd(PPh3)4, PdCl2, Pd(CH3COO)2,
(CH3CN)2PdCl2, etc. is widely used due to its excellent catalytic ability which lead to
a significant effect on the progress of organic chemistry [5-8]. In addition, various
palladium-containing reagent has been reported as potential drugs for cancer cells,
which exhibit high anti-cancer activity [9]. Furthermore, as we know, palladium is
also used for jewelry, electric equipment, dental restorations and so on [11-12].
Although palladium has extensive practical applications in various fields, it may lead
to palladium accumulation in the environment [1]. This high concentration of
palladium would cause potential toxicity to human health because palladium is
capable of binding closely to biomolecules including thiol-containing protein, amino
acids, DNA, RNA, etc. [13-15]. Consequently, to protect people's health, the
European Agency for the Evaluation of Medicinal Products (EMEA) reports the
concentration limits of palladium which the palladium-containing drugs is 5-10 ppm
and the recommended daily meal intake is less than 1.5-15 μM [16-17]. Hence,
efficient strategy for simple and rapid detection of palladium are very urgent and
important in biological systems and environment.
So far, several traditional testing methods, such as atomic absorption spectrometry
(AAS), inductively coupled plasma mass spectrometry (ICP-MS), high performance
liquid chromatography (HPLC) and X-ray fluorescence (XRF), have been

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successfully used for palladium detection [18-21]. Although these strategies are
prominent sensitive to palladium, it still has some shortcomings, including
complicated sample preparation, expensive professional instruments and well-trained
individuals [1, 22]. More importantly, these techniques are unsuited for real-time
monitoring and imaging of palladium in complicated biological samples, especially
living cells, zebrafish and mice. To overcome this problem, variety of small molecule
fluorescence probes are designed for detection of palladium. However, these reported
fluorescent probes for palladium have several drawbacks, such as low fluorescence
quantum yield, poor water solubility and long response time, which limited its
applications (Table S1) [23-27]. As far as we know, few fluorescence probes have
been reported for imaging palladium in vivo. Thus, development of a new
water-soluble fluorescence probe for rapid detection and imaging of palladium in
vitro and in vivo is of significance.
In this work, we present a novel colorimetric and fluorescent turn-on probe NT-Pd,
(Scheme 1) for the sensitive and rapid response to palladium. NT-Pd was designed by
the famous Tsuji-Trost reaction with naphthalimide group (NT) as the fluorophore
and an allyl carbonate as the recognition group for palladium, as illustrated in Scheme
1 [28-29]. Naphthalimide was chosen as the excellent fluorophore due to its highly
fluorescent quantum yield, good water-solubility and excellent biocompatibility,
which may solve the problems described above [30-31]. As we expect, NT-Pd could
successfully determine palladium in aqueous solution (10 mM PBS Buffer, pH=7.4).
Under this condition of aqueous solution, NT-Pd displayed excellent sensing features,
such as high selectivity and sensitivity in both colorimetric (colorless to yellow) and
fluorescent (70-fold enhancement) methods, low detection limit (5.30 nM) and rapid
response time (10 min). Most importantly, NT-Pd was successful applied for
palladium imaging of living A549 cells and zebrafishes with extremely low toxicity,
which confirmed NT-Pd has the potential capabilities for sensing of palladium in vivo
and in vitro.

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Scheme 1. Proposed mechanism of probe NT-Pd for detection of palladium.
Materials and methods
Reagents and Instruments. All the reagents for the experiments were purchased
from Sigam-Aldrich in analytical grade. The zebrafish and A549 cells were obtained
from China Zebrafish Resource Center (Wuhan, China). The pH buffers were
1
prepared through the METTLER TOLEDO Five Easy Plus pH meter. All the H
13
NMR and C NMR spectra were recorded through a Varian 600 MHz spectrometer.
The UV-vis spectra and fluorescence spectra were tested by SHIMADZU UV-1800
spectrophotometer and Agilent Cary Eclipse Fluorescence spectrophotometer
respectively. The fluorescence imaging of living cells and zebrafishes was obtained
with inverted fluorescence microscopy (Olympus IX71, Japan).
Synthesis of probe. The probe NT-Pd and fluorophore NT were synthesized as
described in Scheme S1. In addition, the concrete synthesis route was displayed in
Supplementary Information.

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Solution preparation and spectral measurement. The probe NT-Pd (10 mM) was
first dissolved in dimethyl sulfoxied. And the various testing analytes such as BaCl2,
CaCl2, CuCl2, FeCl3, Hg(ClO)2, KCl, LiCl, MgSO4, MnCl2, NaCl, NiSO4, ZnCl2,
Pd(PPh3)4 were dissolved in ultrapure water (10 mM each). The different pH buffers
(4.0, 5.0, 6.0, 7.0, 7.4, 8.0 and 9.0) were prepared for the detection of pH dependence.
The fluorescence spectrum was tested at excitation wavelength was 450 nm.
Calculation of the detection limit. Based on our previous methods, the detection
limit (DL) was calculated according to the following equation (1) [32-34]. The
fluorescence spectrum of fluorescence probe and the standard deviation of blank
measurement were measured. The linear relationship between fluorescence intensity
of probe at 560 nm and concentration of Pd(PPh3)4 was measured to get the slope.
DL = 3σ/k
(1)
Where σ and k are the standard deviation and the slope of the regression line,
respectively.
Determination of quantum yield. According to the reported literature, the
fluorescence quantum yields of NT-Pd and NT were tested by the following equation
(2) [35-37]. The subscripts c and s represent the reference compound and sample
respectively. Φc is the quantum yield of fluorescein (Φ=0.98, 0.1 M NaOH). F and A
represented the fluorescence integral area and the absorbance, respectively.
(2)
Cell Cultures and Imaging. A549 cells were cultured in Roswell Park Memorial
Institute (RPMI-1640) supplemented with 10 % (v/v) FBS (fetal bovine serum), 1 %
(v/v) penicillin/streptomycin on a cell culture flask in the atmosphere of 5 % CO2 at
37 °C. The A549 cells were first seed in a plate for 24 h. Then, the probe (10 M) was
added into the cells for incubation 30 min at 37 °C. After washing the cells three
times with PBS buffer to remove the residual probe, the A549 cells were further
incubated with Pd(PPh3)4 at different concentration (0, 25, 50 and 100 M) for 30 min.
All the cell imaging was acquired at green channel (510-550 nm).
Imaging of Zebrafish. Zebrafish embryos were purchased from China Zebrafish

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Resource Center. The zebrafishes embryos were cultured in 200 mL medium
containing 1-phenyl-2-thiourea (PTU) in a petrie dish for 48 h at 28 °C. After
hatching zebrafish embryos, zebrafishes were first incubated with probe for 1 h at
28 °C. All the zebrafish samples were carefully washed to remove the residual probe.
And the zebrafishes were further treated with different concentrations of Pd(PPh3)4 (0,
25, 50, 100 μM) for another 2 h. At last, all the samples were imaged at green channel
(510-550 nm).
Results and Discussion
Design and synthesis of probe NT-Pd. In this work, NT was selected as the
fluorophore due to its large Stokes' shift, long emission wavelength, high quantum
yield, good water solubility and excellent biocompatibility. In additional,
allylcarbonate moiety was considered to be a widely used recognition group in the
design of palladium fluorescent probes. For instance, feng's group recently reported a
selective and sensitive NIR fluorescent probe for palladium.[17] Based on this
strategy, a novel fluorescence probe NT-Pd for palladium was designed through
connect allylcarbonate moiety to NT via a well-known Tsuji-Trost reaction. Thus,
NT-Pd was expected to be a sensitive and selective fluorescent probe for palladium.
To verify this hypothesis, NT-Pd was synthesized following the detailed synthetic
route depicted in Scheme S1. All the synthesized compounds were characterized by
1H NMR, 13C NMR and HR-MS, as shown in Figure S1-S9.
Optical determination of NT-Pd to palladium. In order to confirm the detection
ability of probe NT-Pd to palladium, the absorbance spectra and fluorescence spectra
had been measured in PBS buffer solution (10 mM, pH 7.4) at 30 °C. As we know,
Pd(PPh3)4 has been widely reported as the source of Pd0. Under this condition, as
shown in Figure 1a, the absorption peak of NT-Pd (10 μM) at 450 nm has been
apparent enhancement after treatment with 50 μM pd0. And the color of the solution
was obvious change from colorless to yellow under ambient light (insert of Figure 1a),
confirming NT-Pd was an excellent probe for visual sensing of Pd0. Furthermore, as
depicted in Figure 1b, the fluorescent spectra of NT-Pd was almost no fluorescence

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with a low quantum yield (Φ=0.005), indicated that NT-Pd has extremely low
fluorescence background. However, after being exposed to Pd0, the fluorescence
spectra of NT-Pd increased sharply in 10 min and without obvious change in the next
10 min. In addition, the insert picture depicted the fluorescence change from
non-fluorescence to green fluorescence (insert of Figure 1b), which was in agreement
with the test result. In order to measure the optimum reaction time, the fluorescence
intensity of NT-Pd at 560 nm toward Pd0 was carried out (Figure S10). As depicted,
the fluorescence intensity reached the approximate saturation at 10 min, and the
fluorescence enhancement was calculated to be about 70-fold (Φ=0.043). It revealed
that the best response time of NT-Pd for Pd0 was 10 min. All the experiment results
clearly demonstrated that NT-Pd was a prospective probe for colorimetric and
fluorescence response to Pd0.
Figure 1. (a) Absroption spectra changes of probe NT-Pd (10 μM) upon addition of
Pd(PPh3)4 (50 μM) for different time. Inset: color changes. (b) Fluorescent spectra
changes of NT-Pd (10 μM) upon addition of Pd(PPh3)4 (50 μM). λex=450 nm. Inset
(a, b): photographs of NT-Pd without or with Pd(PPh3)4 under room light or UV light.
All experiments were performed in PBS buffer (10 mM, pH 7.4) at 30 °C.
Sensitivity of NT-Pd. To further inspect the sensitivity of NT-Pd, the titration
experiments of the NT-Pd sensing of different concentrations (0-300 μM) of Pd0 were
carried out in the assay. As displayed in Figure 2a, with the increase of concentration
of Pd0, the fluorescent intensity of NT-Pd at 560 nm was gradually increased.
Moreover, the fluorescent intensity increased linearly at low concentration of Pd0 and
reached saturation after incubation with 200 μM Pd0 (Figure 2b). This result indicated

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that NT-Pd was a turn-on fluorescence probe for quantitative detection of Pd0. Based
on this, the detection limit of NT-Pd for Pd0 was further investigated. As revealed in
Figure S11, a satisfactory linear relationship (Y=3646.32+10491.43[Pd0], R2=0.99501)
was established between the fluorescence intensity at 560 nm and concentrations of
Pd0. Hence, the detection limit (DL) was obtained as low as 5.38 nM by the method of
equation DL=3σ/k, indicating NT-Pd was highly sensitive for sensing of Pd0.
Considering the rapid response to Pd0 (10 min) and low detection limit, it can be
convinced that NT-Pd could sensitive detection of Pd0 at nanomolar level.
Figure 2. (a) Fluorescence change spectra of NT-Pd (10 μM) upon addition of
increasing concentrations of Pd0 (0–300 μM) in PBS buffer (10 mM, pH 7.4) for 10
min at 30 °C. (b) Fluorescence intensity change of the NT-Pd at 560 nm as a function
of the concentration of Pd0. λex=450 nm.
Reaction mechanism. To understand the fluorescent turn-on mechanism of NT-Pd
for sensing of Pd0, the mixture system of NT-Pd and Pd0 was studied by HRMS. As
illustrated in Figure S12a, the HRMS peak of pure probe NT-Pd [M+Na]+ (calcd. for
376.1160) was obvious observed. Besides, the mixture system of NT-Pd and Pd0 was
also monitored through the HRMS. As depicted in Figure S12b, the extremely strong
HRMS peak of fluorophore NT [M-H]- (calcd. for 268.0974) was found, which
illustrated NT was released through fracture allylcarbonate moiety of NT-Pd. Notably,
these results clearly confirmed that the sensing mechanism NT-Pd for Pd0 was based
on the Tsuji-Trost reaction, as we illustrated in Scheme 1.
Theoretical Calculations. In order to understand the quenching mechanism of

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NT-Pd, we used Gaussian 09 program to calculate the frontier orbital energy of NT
and NT-Pd at B3LYP/6-31G(d) level [38-39]. As shown in Figure 3, the highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) of NT mainly located at naphthalimide rings and hydroxyl groups, while for
NT-Pd, the orbitals showed a little delocalization compared with that of its
fluorophore NT. Indicating that there was intramolecular charge transfer (ICT) effect
between the fluorophore NT and allyl carbonate group of NT-Pd. The HOMO energy
level of fluorophore NT was -6.10 eV, when allyl carbonate group was introduced the
HOMO energy level of NT-Pd was decreased to -6.51 eV, suggested that the
fluorescence of NT was quenched by the ICT process. Upon reacting with Pd0, the
non-fluorescence NT-Pd can release the fluorescent NT, our experimental results also
confirmed the calculation results.
Figure 3. Optimized structures and frontier orbital energy of NT and NT-Pd. DFT
calculations were performed by Gaussian 09 with B3LYP/6-31G(d) level.
Selectivity of NT-Pd. To evaluate the selectivity of NT-Pd for Pd0, a number of
interfering cations and anions, including Ba2+, Ca2+, Cu2+, Fe3+, Hg2+, K+, Li+, Mg2+,
Mn2+, Na+, Ni2+, Zn2+, CN-, ClO-, Br-, I- and Pd2+ were detected. As shown in Figure
4a, after adding various analytes (100 μM), the fluorescence signal of NT-Pd did not
display significant changes. Compared to the interfering metal ions, the fluorescence

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spectra of NT-Pd showed remarkably enhancement when added only 20 μM Pd0. It
indicated that NT-Pd exhibited highly selective for Pd0 against these cations (Figure
4b). Besides, NT-Pd also exhibited a specific selectivity against Pd0, without any
noticeable interference (Figure S13). Meanwhile, as a colorimetric probe NT-Pd, the
selectivity of NT-Pd was also investigated in absorbance spectra. As depicted in
Figure S14, the absorbance spectra of NT-Pd also showed high selectivity to Pd0. In
addition, this highly selectivity for Pd0 could be visibly seen by the naked eyes,
because only Pd0 could change the color from colorless to yellow (insert of Figure
S14). All of these experimental results confirmed that NT-Pd showed high selectivity
and strongly anti-interference capability for sensing of Pd0 using both UV-vis and
fluorescence method.
Figure 4. (a) Fluorescence spectra of NT-Pd for various analytes. (b) Fluorescence
intensity responses of NT-Pd toward various analytes. (1. Blank, 2. Ba2+, 3. Ca2+, 4.
Cu2+, 5. Fe3+, 6. Hg2+, 7. K+, 8. Li+, 9. Mg2+, 10. Mn2+, 11. Na+, 12. Ni2+, 13. Zn2+, 14.
CN-, 15. ClO-, 16. Br-, 17. I-, 18. Pd2+, 19. Pd0.) The concentration of Pd0 was 20 μM,
and the other was 100 μM. All experiments were monitor 10 min in PBS buffer (10
mM, pH 7.4) at 30 °C. λex=450 nm.
pH effect. The pH value of PBS buffer has affected the stability of NT-Pd and the
response ability of NT-Pd for Pd0, so the pH effect of NT-Pd for Pd0 was studied. As
exposed in Figure S15, the fluorescent intensity of NT-Pd was almost unchanged
over a wide pH rang (4-9), which verified its strong stability. However, when NT-Pd
was treated with Pd0, the fluorescent intensity increased sharply with the pH value
from 4 to 7.4 and decreased gradually with the pH value from 7.4 to 9. Notably, the
pH value at 7.4 was the optimal condition for sensing of Pd0, indicating NT-Pd was
suitable for detection Pd0 in living systems.

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Detection of Pd0 in living cells. To investigate the excellent sensing properties of
probe NT-Pd, the bioimaging application of NT-Pd in A549 living cells was further
explored. Prior to cell image, the cytotoxicity of NT-Pd at different concentrations (0,
1, 5, 10 and 20 μM) was assessed by using MTT assays in A549 cells for 18 h. As
displayed in Figure S16, the highly cell viability of A549 cells clearly revealed that
NT-Pd exhibited extremely low cytotoxicity. It illustrated NT-Pd could be applied
for bioimaging in A549 cells. Consequently, the A549 cell imaging of NT-Pd for
different concentrations of Pd0 (0, 25, 50 and 100 μM) was investigated (Figure 5).
The A549 cells showed almost no fluorescence when only incubated with NT-Pd for
30 min at 37 °C, indicating NT-Pd had low fluorescence background. Conversely,
upon adding NT-Pd to the culture medium for another 30 min, the green fluorescence
was obviously observed and gradually enhanced with the increased concentration of
Pd0. It was also confirmed by the optical data, as depicted in Figure 5m. The cell
cytotoxicity and imaging results revealed that NT-Pd could be permeate into cells and
image Pd0 with highly sensitivity.

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Figure 5. Fluorescence imaging of A549 cells for NT-Pd (10 μM) incubated with
different concentrations of Pd0 (0, 25, 50 and 100 μM). (a, d, g, j) bright field imaging,
(b, e, h, k) green channel (550 - 580 nm), (c, f, i, l) merge imaging. (m) Relative
fluorescence intensity of representative images with different concentrations of Pd0 (0,
25, 50 and 100 μM). Data represent mean standard error (n=3), scale bar=40 μm.

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Zebrafish Imaging. Zebrafish, one of the four classical model organisms, was widely
applied for bioimaging [40-41]. Thus, due to the high selectivity and sensitivity of
NT-Pd for Pd0, the imaging ability of NT-Pd for zebrafish was further investigated,
as depicted in Figure 6. One-week-old zebrafishes were incubated with NT-Pd for 0.5
h to ensure that the probe NT-Pd could be permeated into zebrafish, and following
treatment with four groups of Pd0 (0, 25, 50 and 100 μM) for another 1 h. Each group
of zebrafish was imaging by using a fluorescence microscope. As we expected, in
absence of Pd0, the zebrafish in the control group was undoubtedly alive and
practically no fluorescent, indicating NT-Pd was an excellent probe for imaging in
zebrafish with extremely low toxicity and fluorescence background. However, upon
treating with Pd0, the fluorescence of the zebrafish was clearly observed, and
gradually enhanced with the increase concentrations of Pd0. As depicted in Figure 6m,
the relative fluorescence intensity of zebrafish revealed that NT-Pd could detect
exgenous Pd0 by fluorescent imaging. All of these resulted demonstrated that probe
NT-Pd was able to image of Pd0 in living body with good biocompatibility and high
sensitivity.

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Figure 6. Fluorescence imaging of zebrafish for NT-Pd (10 μM) incubated with
different concentrations of Pd0 (0, 25, 50 and 100 μM) for 30 min. (a, d, g, j) bright
field imaging, (b, e, h, k) green channel (550 - 580 nm), (c, f, i, l) merge imaging. (m)
Relative fluorescence intensity of representative images with different concentrations
of Pd0 (0, 25, 50 and 100 μM). Data represent mean standard error (n=3), scale
bar=200 μm.

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Conclusion
In summary, a novel probe NT-Pd was designed for colorimetric and fluorescent
turn-on detection of palladium in vitro and in vivo. This new probe had excellent
selectivity and sensitivity to palladium in phosphate buffer at physiological pH, which
exhibited a rapid fluorescent turn-on response (10 min) and distinct color change.
Furthermore, the NT-Pd could be used for the quantitative determination of palladium
with a detection limit of 5.30 nM. In addition, NT-Pd has good biocompatibility and
low toxicity, which could contribute to its successful use in tracking palladium in
living cells and zebrafish model. Overall, all these features illustrated that NT-Pd was
a very promising tool for sensing of palladium in complicated biological system.
Acknowledgment
This work was financially supported by the National Natural Science Foundation of
China (21804102), the Outstanding Young and Middle-aged Scientific Innovation
Team of Colleges and Universities of Hubei Province: "Biomass chemical
technologies and materials" (Grant No. T201908), the National Key R&D Program of
China (2017YFE0118200), the Anhui Key Research and Development Program (No.
201904b11020025), the Anhui Natural Science Fund Project (No. 1908085MB35)
and the Natural Science Research Project of Anhui Higher Education Institution (Key
Project, No. KJ2018A0040).
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Credit Author Statement
There is no conflict of interest between the authors and they abide by the
academic ethics.

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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:

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Graphical Abstract

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Highlights
 A novel colorimetric and fluorescent probe NT-Pd was constructed for detection
of palladium.
 The probe NT-Pd was able to detect palladium in aqueous solution.
 The probe NT-Pd exhibited high selectivity and sensitivity to palladium.
 The probe NT-Pd could be used for imaging of palladium in living A549 cells
and zebrafishes.