A Turn-On Fluorescent Probe for Highly Selective and Sensitive Detection of Palladium

Article abstract of DOI:10.1002/cjoc.201600129

A novel turn-on fluorescent probe for the detection of palladium has been designed. The probe can selectively and sensitively detect palladium in solution, and the limit of detection was calculated to be 11.4 nmol·L^?1. Furthermore, the probe was successfully used for fluorescence imaging of palladium in living cells.

Full text of DOI:10.1002/cjoc.201600129

FULL PAPER
DOI: 10.1002/cjoc.201600129
A Turn-On Fluorescent Probe for Highly Selective and
Sensitive Detection of Palladium
a,c
b
b
b
,a,c
,b
Junliang Zhou, Jian Zhang, Hang Ren, Xiaochun Dong, Xing Zheng,* and Weili Zhao*
a
Institute of Pharmacy and Pharmacology, University of South China, Hengyang, Hunan 421001, China
b
School of Pharmacy, Fudan University, Shanghai 201203, China
c
Hunan Province Cooperative Innovation Center for Molecular Target New Drug Study, Hengyang,
Hunan 421001, China
A novel turn-on fluorescent probe for the detection of palladium has been designed. The probe can selectively

1
and sensitively detect palladium in solution, and the limit of detection was calculated to be 11.4 nmol•L . Further-
more, the probe was successfully used for fluorescence imaging of palladium in living cells.
Keywords fluorescent probe, palladium, living cell
[
26-51]
Introduction
cent probes to detect palladium.
The fluorescent
probes for palladium are mostly based on two mecha-
nistic pathways: chemical reaction catalyzed by palla-
Palladium is the widely used precious metal in the
fields of scientific research and material science such as
drug discoveries and developments, organic catalytic
[30-41]
dium ions
or the coordination between the palla-
[42,45]
dium ions and the probes.
However, the coordina-
[
1-6]
reactions, and fuel cells etc.
The frequent use of pal-
tion-based probes may encounter selectivity issue be-
cause of interference from other transitional metal ions.
In contrast, nearly all published fluorescent probes for
palladium are based on Pd-catalyzed cleavage reactions
which can recognize palladium with high sensitivity and
selectivity. However, many reported sensing systems
have been suffering from long response time (need sev-
ladium in the pharmaceutical industry may lead to its
significant accumulation in the final organic waste and
even in the final medicinal products. Palladium ions can
bind to thiol-containing amino acids, proteins (casein,
silk fibroin and many enzymes), DNA or other macro-
molecules (e.g. vitamin B
variety of cellular processes.
6
) and thereby may disturb a
[
7,8]
[
31,41]
The proposed maxi-
eral hours to achieve saturation point),[
rigorous test
which great-
30,36]
mum dietary intake of palladium is less than 1.5－15 μg
per day per person, and its threshold in drugs is 5－10
conditions (additional additives, etc.),
ly restricted their practical applications. Koide and
[
9]
[
30]
ppm. Consequently, it is necessary to develop con-
venient and effective methods to detect palladium spe-
cies in various samples.
coworkers developed an innovative fluorescent sens-
ing system for palladium species based on the Pd(0)-
catalyzed Tsuji-Trost allylic oxidative insertion reaction.
This probe has been shown to be useful and selective for
measuring low concentrations of palladium. Neverthe-
less, the sensing system requires initial conversion of
Typical analytical methods used for quantification of
palladium species include inductively coupled plasma
mass spectrometry (ICP-MS), atomic absorption spec-
trometry (AAS), plasma emission spectroscopy (ICP-
AES), solid phase micro extraction-high performance
liquid chromatography (SPME-HPLC), and X-ray fluo-
Pd(II) to Pd(0) using a reducing agent such as PPh
tri-2-furylphosphine (TFP) or TFP-NaBH . More re-
3
,
4
[31]
cently, Ahn and coworkers developed a probe carried
propargyl ether moiety sensed palladium species in all
the typical oxidation states without adding any addi-
tional reagents. However, this assay requires long reac-
tion times (3 h).
Since recent efforts suggested that terminal pro-
pargyl ether moiety-functionalized probes are most se-
lective for palladium recognization,
[
10-12]
rescence (XRF).
However, these methods often
require complicated sample preparation steps, rigorous
experimental conditions, sophisticated instrumentation
and well-trained individuals. Thus, recent research ac-
tivities on palladium detection have been focused on
fluorescent methods due to the low cost, simplicity,
sensitivity, selectivity, rapid response and high spatial
[13]
[
31-33]
we expected
[
14-25]
[52]
resolution of fluorescence detection.
many efforts have been devoted to developing fluores-
Recently,
that a long excitation and emission resorufamine dye
carrying a propargyl moiety may become a new palla-
*
E-mail: zhengxing5018@yahoo.com, zhaoweili@fudan.edu.cn; Tel./Fax: 0086-021-51980111
Received March 1, 2016; accepted April 18, 2016; published online XXXX, 2016.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600129 or from the author.
Chin. J. Chem. 2016, XX, 1—5
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Zhou et al.
FULL PAPER
dium-selective probe. The masked long wavelength re-
sorufamine dye may become a turn-on fluorescent probe
for palladium with reduced background interference. To
facilitate fast response to palladium, the propargyl moi-
ety was connected to resorufamine through carbamate
linkage to furnish a novel probe (probe 1) as shown be-
low for palladium detection. As reported in this paper,
such a probe shows excellent selectivity and high sensi-
tivity for Pd over other metal ions under physiologi-
cal condition (at 37 ℃ in PBS-DMF, pH 7.4), and was
successfully applied for palladium imaging in living
cells.
37 ℃ in 5% CO
2
. HepG2 cells were seeded at a den-
4
sity of 5×10 cells per well (200 μL) in a 6-well plate.
The cells were imaged by using a confocal laser scan-
ning microscope (λex＝565 nm, OLYMPUS FV1000).
HepG2 cells were incubated in DMEM with 10% (V/V)
fetal bovine serum, 1% penicillin/streptomycin at 37 ℃
in 5% CO
2
. HepG2 cells were seeded at a density of 5×
4
10 cells per well (200 μL) in a 96-well plate. Probe 1
2
＋
1
1
(20 µmol•L , 2 mL) in PBS buffer (10 mmol•L , pH
7.4, 1% DMSO) was added to HepG2 cells in a
six-compartment cell culture plate that contained 2.0
mL culture medium, and was incubated at 37 ℃ for 30
min. After removing the culture medium and washing
with PBS twice, the fluorescence images of cells were
2
＋
1
taken. Pd (50 µmol•L , 2.0 mL) in PBS buffer (10
1
mmol•L , pH 7.4) was then added to the HepG2 cells,
which were further incubated at 37 ℃ for 30 min. Af-
ter washing with PBS twice, the fluorescence images of
cells were taken.
Experimental
Synthesis of probe 1
Materials and measurements
The synthetic approach for probe 1 is shown in
All reagents were purchased from commercial sup-
Supporting Information (Scheme S1). The chemical
pliers and used without further purification. Solvents
1
1
structure of probe 1 was characterized by HRMS, H
used were purified by standard methods prior to use. H
1
3
NMR and C NMR spectroscopy.
NMR spectra were recorded on a Varian Model Mer-
1
cury 400 MHz spectrometer. H NMR chemical shifts (δ)
Results and Discussion
are given (s＝singlet, d＝doublet, t＝triplet, q＝quartet,
4
m＝multiplet) downfield from Me Si, determined by
With probe 1 in hand, the experiments for the detec-
13
chloroform (δ＝7.26). C NMR spectra were recorded
on a Varian Model Mercury 150 MHz spectrometer.
Spectrometer UV-vis spectra were acquired on a Hitachi
U-2900 double beam UV-visible spectrophotometer.
Fluorescence spectra were measured by a Hitachi
F-2500 fluorescence spectrophotometers. Electrospray
ionization (ESI) mass spectra were acquired with Ag-
ilent 1100 Series LC/MSD and AB SCIEX Triple
TOFTM 5600＋ mass spectrometer. All spectra were
recorded at room temperature, except for the confocal
laser scanning microscopic images.
tion of palladium were carried out following the proce-
dures reported in the literatures.
[30-41]
Spectroscopic property of probe 1 and its response to
Pd(II)
Initial spectroscopic properties of the probe 1 were
measured in DMF/PBS solution (1∶3, V∶V, pH＝7.4).
PdCl was selected as the representative palladium spe-
2
cies in the following titration experiments since it is
among the most toxic palladium compounds. As masked
1
resorufamine, probe 1 (10 µmol•L ) is observed to be
non-fluorescent with a maximum absorption at 480 nm
Detection limits
(
2
Figure 1). However, upon the addition of PdCl the
The detection limit was calculated based on the flu-
solution of probe 1 exhibited a strong red-shifted ab-
sorption peak centered at 565 nm, and a dramatic in-
crease in fluorescent intensity at the peak of 593 nm was
noticed. It took about 1 h to reach the relative saturation
point judged from the plot of time-dependent intensity
change of fluorescence (Figure 2). Thus carbamate
linkage allows amine-type fluorescent dye to be used to
detect palladium and may facilitate faster response than
2
＋
orescence titration. In the absence of Pd , the fluo-
rescence emission spectrum of probe 1 was measured
five times and the standard deviation of blank meas-
urement was achieved. To gain the slop, the fluores-
cence intensity at 593 nm was plotted to the concentra-
2
＋
tion of Pd . So the detection limit was calculated with
the following equation:
[31-34]
most propargyl ether type probes reported.
LC-MS
Detection limit＝3σ/k
spectroscopic evidences confirmed the formation of
resorufamine, which led to the increase in fluorescence
emission (Figures S1, S2). Therefore, the change of flu-
orescence is due to the palladium catalyzed de-
propargylation reaction including Pd(II)-promoted hy-
drolysis of the propargyl ether or Pd(0)-participated de-
protection via allenylpalladium intermediate, followed
by subsequent decarboxylation process (Figure S3).
where σ is the standard deviation of blank measurement,
k is the slop between the fluorescence intensity versus
2
＋
Pd concentration.
Cell imaging experiments
HepG2 cells were incubated in DMEM with 10%
(V/V) fetal bovine serum, 1% penicillin/streptomycin at
2
www.cjc.wiley-vch.de
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2016, XX, 1—5

A Turn-On Fluorescent Probe for Highly Selective and Sensitive Detection of Palladium
sence of any interference ions, the presence of other
metal ions caused only tiny variations on the fluores-
3
＋
2＋
cence intensity except Au and Hg which resulted
in low to moderate quenching of the fluorescence. Thus,
these free metal ions would have little influence towards
probe 1 and do not hamper the fluorogenic detection of
2
＋
Pd .

1
Figure 1 Absorption spectral of probe 1 (10 µmol•L ) in the
presence of PdCl (5 equiv.) in DMF-PBS (1∶3, V∶V) solution
10 mmol•L , pH＝7.4) at 37 ℃.
2

1
(
Figure 3 Fluorescence intensity changes of probe 1 (10

1
2＋
µmol•L ) for Pd in the presence of various metal ions in

1
DMF-PBS (1∶3, V∶V) solution (10 mmol•L , pH＝7.4) at
7 ℃. Black bars represent the addition of a single analyte in-
3
＋
2＋
2＋
3＋
2＋
cluding: (0) None, (1) Ag , (2) Co , (3) Cu , (4) Fe , (5) Hg ,
2
＋
2＋
2＋
2＋
2＋
2＋
(
(
6) Mg , (7) Ni , (8) Zn , (9) Ca , (10) Mn , (11) Pb ,
＋ ＋ ＋
2＋
3＋
3＋
12) Li , (13) Na , (14) K , (15) Ba , (16) Al , (17) Au , (18)
Pd . c(Au )＝50 µmol•L , c(Pd )＝50 µmol•L , c(other
metal ion)＝100 µmol•L . Red bars represent the subsequent
addition of Pd (50 µmol•L ) to the mixture (E /E ＝565/593
2
＋
3＋
1
2＋
1

1
2
＋
1
x
m
Figure 2 Time-dependent fluorescence spectra of probe 1 (10
μmol•L ) and the corresponding responses (inset) in the presence
nm).

1
of 5 equiv. of PdCl in DMF-PBS (1∶3, V∶V) solution (10
2

1
Detection limit
mmol•L , pH＝7.4) at 37 ℃ (E /E ＝565/593 nm).
x
m
Figure 4 shows the fluorescence spectra of probe 1
Selectivity of probe 1
upon titration with PdCl . The fluorescence spectrum of
2
1
probe 1 (10 µmol•L ) in the absence of palladium in
DMF/PBS solution (1∶3, V∶V, pH＝7.4) exhibits
very weak fluorescence emission at 593 nm. With con-
An exceptional feature of probe 1 is its high selec-
tivity towards the target analyte over other potential
competitive species. The selectivity of probe 1 toward
various metal ions was investigated and the results are
shown in Figure 3 (black bars). The results indicate that
the probe 1 displays selective response toward palla-
dium ion. Under the same conditions, nearly no respon-
sive changes are observed in the presence of 100
tinuous addition of PdCl , the fluorescence intensity at
2
5
93 nm increased in accordance with the increasing
concentration of PdCl . About 110-fold fluorescence
enhancement was observed after 60 min when the con-
2
2
＋
1
centration of Pd ions was 50 µmol•L . The observed
fluorescence intensity is nearly proportional to the Pd
concentration up to 8 μmol•L (Figure 4). The detec-
tion limit of PdCl was calculated to be 11.4 nmol•L
1
＋
2＋
2＋
3＋
2＋
2＋
µmol•L (10-fold) of Ag , Co , Cu , Fe , Hg ,
2
＋
2＋
2＋
2＋
2＋
2＋
＋
＋
＋
1
Mg , Ni , Zn , Ca , Mn , Pb , Li , Na , K ,
2
＋
3＋
1
Ba and Al in DMF/PBS solution (1∶3, V∶V, pH
2
＝
7.4). A small enhancement in the fluorescence was
(2.1 ppb), which is far below the allowable level of pal-
3
＋
1
observed only in the case of Au (50 µmol•L , 5
equiv.) that has high p-electrophilicity.
cording to the obvious spectroscopy changes, the de-
ladium ions (5－10 ppm) from the European Agency for
[
31]
[9]
Thus, ac-
the Evaluation of Medicinal Products (EMEA).
2
＋
Probe 1 response to different palladium complexes
signed probe 1 can detect Pd with high selectivity.
2
＋
To further verify the selectivity of probe 1 for Pd ,
interference experiments involving the effects of other
metal ions (Figure 3, red bars) in the identification of
Pd ions were also performed. Compared with the in-
tensity of probe 1 responded toward Pd in the ab-
To demonstrate the potential application of the probe
to detect various sources of palladium species, we tested
whether it could respond to different initial oxidation
states of palladium with various kinds of ligand. Thus
typical commercially available palladium species such
2
＋
2
＋
Chin. J. Chem. 2016, XX, 1—5
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
3

Zhou et al.
FULL PAPER
Effect of pH
The potential pH effects on probe 1 in the absence or
presence of PdCl were subsequently evaluated (Figure
). The fact that fluorescence remained almost un-
2
6
changed for the free probe over pH range of 6.0－8.0
indicated that the probe 1 is very stable in such pH
range. In stark contrast, remarkable fluorescent signal
enhancement was observed within the pH range 6.0－
8
2
.0 when the probe 1 was treated with PdCl , suggesting
that the novel probe was promising for biological appli-
cations.
Figure 4 Fluorescence spectra changes and fluorescence inten-

1
sities of probe 1 (10 µmol•L ) in the presence of increasing con-
2
＋
centrations of Pd (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0,
.5, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0 equiv.) in DMF-PBS (1∶3, V∶V)
1

1
solution (10 mmol•L , pH＝7.4) at 37 ℃. Inset: A linear cali-
bration curve between the fluorescent intensity changes of probe

1
1
and the concentration of PdCl in the range of 1 to 8 μmol•L .
2
Each spectrum was recorded 60 min after addition of PdCl2
E /E ＝565/593 nm).
(
x
m
as Pd(PPh
)
3 4
,
PdCl
2
,
Pd(OAc)
2
,
2 2
PdCl (dppf) ,
Pd(PPh ) Cl , and Pd (dba) were selected and the re-
3
2
2
2
3
sults are shown in Figure 5. According to the fluores-
cence intensity increment, sensitivities of probe 1 to
palladium species were in the following order:
Figure 6 Effect of pH on the fluorescence intensity (λ ＝593 nm)
em
1
of probe 1 (10 μmol•L ) in DMF/phosphate buffer (1∶3, V∶V)
1
2＋
upon addition of 50 μmol•L Pd after incubation at 37 ℃.
Pd
Pd(PPh
2
(dba)
3
＞ Pd(OAc)
＞PdCl (dppf)
2
＞ PdCl
2
＞ Pd(PPh
3 2 2
) Cl ＞
[33]
)
4
2
2
.
The results revealed that
2＋
3
Imaging Pd in living cells
probe 1 remarkably responds to the different oxidation
states of palladium. These results indicated that the de-
propargylation reaction can be catalyzed via Pd(II)- cat-
alyzed hydration intermediates, but also via allenyl-
palladium resulting from the oxidative addition of
To demonstrate the practical applicability of the
probe in biological systems, fluorescence imaging ex-
periments in living cells (HepG2 cells) were carried out.
1
HepG2 cells were incubated with 10.0 µmol•L of
1
[
33,53,54]
probe 1 for 30 min, and then treated with 50.0 µmol•L
of PdCl . The fluorescence images of HepG2 cells were
recorded before and after addition of Pd . As shown in
Pd(0).
2
2
＋
2
＋
Figure 7, in the absence of Pd , free probe 1 provided
no detectable fluorescence signals in living cells (Figure
2
＋
7
A). However, after incubation with Pd , red fluores-
cence was observed in living cells (Figure 7B). Thus,
these experimental images suggested that the probe 1 is
cell membrane permeable and capable of sensing palla-
dium in living cells.
Conclusions
In conclusion, we have developed a chemical reac-
tion-based fluorescent probe 1 selective for recognition
of palladium species. The results demonstrate that probe
2
＋
Figure 5 Comparison of the fluorescence intensity changes on
different palladium complexes, measured for a mixture of probe 1
1 exhibits a high sensitivity and selectivity for Pd
1
detection, with detection limit of 11.4 nmol•L . More-
over, such a probe is able to respond to different oxida-
tion states of palladium among other transition metal
ions without additional reagents. The current work pro-
vides a new mild and promising strategy for the detec-
tion of palladium species in biological and environ

1
1
(
10 µmol•L ) and the palladium species (50 µmol•L ) in

1
DMF-PBS (1∶3, V∶V) solution (10 mmol•L , pH＝7.4) at
7 ℃. (0) probe only; (1) PdCl (dppf) ; (2) Pd(PPh ) ; (3)
3
2
2
3 4
Pd(PPh ) Cl ; (4) PdCl ; (5) Pd(OAc) ; (6) Pd (dba) (E /E ＝
3
2
2
2
2
2
3
x
m
565/593 nm).
4
www.cjc.wiley-vch.de
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2016, XX, 1—5

A Turn-On Fluorescent Probe for Highly Selective and Sensitive Detection of Palladium
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