A coumarin-based fluorescent probe for the fast detection of Pd0 with low detection limit

Article abstract of DOI:10.1016/j.tetlet.2016.02.062

A off-on coumarin-based fluorescent probe for the fast detection of Pd⁰ is designed and synthesized. The probe shows not only good selectivity but also fast response to Pd⁰ in phosphate buffered saline (PBS) solution. The absorption band at 412 nm rises sharply as well as the fluorescence at 450 nm after the addition of Pd⁰ within few minutes. The detection limit is as low as 0.34 nM. The probe shows nonfluorescence when it conjuncted with allyl carbonate ester which destroys the Intramolecular Charge Transfer (ICT) system. The effect of Pd⁰ to the probe can change the structure and make the departure of the conjuncted allyl carbonate ester. As a result, the fluorogen is exposed and it shows fluorescence.

Full text of DOI:10.1016/j.tetlet.2016.02.062

Accepted Manuscript
A coumarin-based fluorescent probe for the fast detection of Pd0 with low de-
tection limit
Yunchang Liu, Kaiqiang Xiang, Min Guo, Baozhu Tian, Jinlong Zhang
PII:
S0040-4039(16)30171-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.02.062
TETL 47337
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
12 December 2015
12 February 2016
16 February 2016
Please cite this article as: Liu, Y., Xiang, K., Guo, M., Tian, B., Zhang, J., A coumarin-based fluorescent probe for
the fast detection of Pd0 with low detection limit, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.
2016.02.062
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A coumarin-based fluorescent probe for the fast detection of
Pd0 with low detection limit
Yunchang Liu, Kaiqiang Xiang, Min Guo, Baozhu Tian and Jinlong Zhang*
Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, Shanghai, 200237, China
Corresponding author:
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of
Science and Technology, Shanghai 200237, P.R. China
Tel & Fax: +86-21-64252062.
E-mail: jlzhang@ecust.edu.cn
1

Abstract
A off-on coumarin-based fluorescent probe for the fast detection of Pd0 is designed and synthesized.
The probe shows not only good selectivity but also fast response to Pd0 in phosphate buffered saline
(PBS) solution. The absorption band at 412 nm rises sharply as well as the fluorescence at 450 nm after
the addition of Pd0 within few minutes. The detection limit is as low as 0.34 nM. The probe shows
nonfluorescence when it conjuncted with allyl carbonate ester which destroys the Intramolecular
Charge Transfer (ICT) system. The effect of Pd0 to the probe can change the structure and make the
departure of the conjuncted allyl carbonate ester. As a result, the fluorogen is exposed and it shows
fluorescence.
Key words: Pd0; fluorescent probe; coumarin; low detection limit
2

1. Introduction
With the development of modern industry, heavy and transition-metal(HTM) has been widely used in
many fields such as chemistry, biology and environmental science.1-6 Among all these heavy and
transition-metal, palladium plays crucial roles in various fields because it is a perfect catalyst which has
excellent ability of forming stable bonds. Recently, it is widely used in the synthesis of drugs7 and
disposing automobile exhaust. What’s more, palladium is also widely used in the preparation of dental
materials and fine jewellery8. However, even after purification of the final products, 300-2000 ppm of
residual palladium is still left which is much higher than the specific threshold in drugs(5-10 ppm)9. It
is well known that palladium can cause serious health hazard by forming palladium complex with
thiol-containing proteins( such as casein, silk fibroin), DNA and RNA10,11 and other
macromolecules(Vitamin B6)12. Therefore, governmental restriction for the threshold level of palladium
in drugs is as low as 5-10 ppm9. Therefore, it seems to be urgent to develop sensors for the fast and
selectively detection of palladium. Some methods such as atomic absorption spectroscopy(AAS), solid
phase microextraction-high performance liquid chromatography, X-ray fluorescence, and plasma
emission methods have been developed to test palladium, however, they all suffer from high cost of
instruments and require good training individuals.
Fluorescent probe, on the other hand, has received much attention for the detection of palladium13 and
different metal ions14,15 because of its obvious advantages. Their high sensitivity, relatively simple
analysis protocols16-20 and far less expensive makes it more adaptable in bioimaging analysis in vivo or
even in living cells21-24. Consequently, many fluorescent probes for the detection of palladium have
been designed. Zhu25 etc. designed a naked eye and ratiometric probe which can detect palladium in the
near-infrared region with the detection limit to be 0.3 ppb. Also, the modulation of the p-conjugated
3

electrons in cyanine dyes can result in a ratiometric fluorescence change with a large Stokes shift (270
nm). Liu26 etc presented a reactive fluorescent probe for the colorimetric and ratiometric detection of
palladium. The probe displays highly sensitive and selective response to palladium with the detection
limit to be 6.1 nM. Likewise, another probe by Liu27 was also reported. The probe has the similar
structure but has good solubility in water which can be tested in phosphate buffer saline(PBS)
containing less than 1% organic cosolvent without adding any additional reagents with the low
detection limit of 25 nM. Ahn28 etc synthesized a fluorescent probe for the detection of palladium
species based on fluorescein. It is the first fluorescent probe which can be used in living organisms.
Lin29 etc designed a NIR fluorescent probe for the detection of palladium species based on a HD NIR
fluorophore with high sensitivity and selectivity. Meanwhile, the probe can be introduced in cells for
the detection of palladium species. Qian30 etc designed a ratiometric probe for the detection of
palladium species with large stoke shift while Yin31 etc synthesized a ratiometric probe for the
detection of palladium with the detection limit of 12 nM. Peng32 etc described a turn-on fluoresent
probe for the detection of palladium species by forming clathrate.
Herein, we design a fluorescent probe for the detection of Pd0 with the high sensitivity and
selectivity. What’s more, the detection limit can reach to 0.34 nM. Like many papers33-36, our probe is
also based on the ICT system. The probe showed nonfluorescence when Connecting with allyl
carbonate ester while the fluorescence appears when it reacts with Pd0. The process makes the
absorption at 412 nm increase sharply as well as the fluorescence at 450 nm. Palladium is the only
metal which can react with the probe while the other metal ions can not.
2. Experiment
2.1 Chemicals and instrumentals
4

2, 4-dihydroxybenzaldehyde was purchased from Energy Chemical and used directly without any
purification. The other chemicals were of the highest grade available and were used without further
purification. All employed solvents were analytical pure and were employed without any further drying
or purification.
1
Reactions were monitored by TLC. H NMR and 13C NMR spectra were recorded on Brucker
AM-400 MHz instruments with tetramethylsilane as internal standard. ESI was performed using a
Waters LCT Premier XE spectrometer. Absorption spectra were carried out on a SHMADZU UV-Vis
spectrophotometer. Fluorescence spectra were measured on a SHMADZU RF-5301PC Fluorescence
spectrophotometer.
2.2 Preparation and characterization of 1
Compound 1 was synthesized according to the reported papers37-39. 2-4-dihydroxybenzaldehyde 2.76
g(20 mmol) and diethylmalonate 6.4 mL(40 mmol) were mixed together in 30 mL anhydrous ethanol.
1.5 mL piperidine was added to the above solution dropwise after that. The whole mixture was refluxed
for 12 h after which the solvent was evaporated. The product was extracted with CH2Cl2, washed with
brine and purified by column chromatography to give compound 1 as a white yellow solids(4.1 g 12.89
1
mmol). HNMR(400 MHz, DMSO) δ 8.677(s,1H), 7.768(d, J=8, 1H) 6.84(dd, J=4, 1H), 6.73(d,
J=1.2,1H), 4.26(q, 2 H), 1.29(t, J=8, 3H) (Fig.S1). 13C NMR δ 164.03, 162.91, 157.07, 156.37, 149.41,
132.08, 113.97, 112.04, 110.39, 101.75, 60.77, 39.47, 14.10 (Fig.S2). HRMS: 257.0422 , calcd for:
257.0426 (Fig.S5). Yield: 64.5%.
2.3 Preparation and characterization of probe PC
Using the method of the reported literature40, probe PC was synthesized. A mixture of 0.234 g(1
mmol) compound 1 and triethylamine 0.27 mL (2 mmol) in 10 mL CH2Cl2 was stirred under an ice
5

bath. 65 μL(1.08 mmol) allyl chloroformate in 5 mL CH2Cl2 was added dropwise to the above solution.
The whole mixture was stirred in an ice bath for 4 h and the solvents were evaporated after that. The
solid was purified by column chromatography to give probe PC as a white solid(0.18 g, 0.56 mmol).
1HNMR(400MHz, DMSO) δ 8.79(s, 1H), 8.00(d, J=8, 1H), 7.48( d, J=4, 1H), 7.45(dd, J=8, 1H), 6.0(m,
1H), 5.38(dd, J=4, 2H), 4.77( d, J=8, 2H), 4.3(q, 2H), 3.09(t, J=4, 3H) (Fig.S3). 13C NMR 166.44,
155.65, 155.19, 154,65, 151.86, 148.18, 131.45, 119.15, 118.39, 116.99, 115.95, 109.38, 69.19, 61.23,
39.88, 14.01 (Fig.S4). HRMS: 342.0771, calcd for : 342.0775(Fig.S6). Yield: 56%.
3. Results and discussion
3.1 Synthesis
The synthetic routine of probe PC is outlined in Scheme 1. By two simple steps, we can get probe PC
with high yields. Coumarin derivate 1 is in high yields and it displays excellent fluorescence. The
esterification reaction by connecting an allyl carbonate ester makes the disappearance of the strong
fluorescence. The above reaction destroyed the ICT structure. When the probe encounters palladium,
the interaction of palladium with double bond makes the separation of the allyl carbonate ester with
coumarin. As a result, the fluorogen is exposed. The spectroscopic properties of probe PC were all
tested in aqueous phosphate buffered saline (PBS) solutions(10mM, pH=7.4) containing 50% DMSO
in the concentration of 10-5 M.
6

Scheme1. The synthesis procedure of the probe PC
3.2 The response time towards Pd0
The time-dependent absorption and fluorescence intensity changes of probe PC after adding 5 equiv
of Pd0 is tested To confirm when will the probe get saturated after adding 5 equiv of palladium
( Pd(PPh3)4 is the source of palladium). The probe PC is in the concentration of 10-5 M which is in the
solution of phosphate buffered saline (PBS) containing 50% DMSO. 5 equiv of Palladium(Pd0) is
added to the above solution, and absorption spectra and fluorescence spectra are collected as shown in
Fig.1. As it can be seen in Fig.1(a) that probe PC has nearly none absorption at 412 nm without Pd0.
However, the adding of Pd0 makes the absorption at 412 nm increase a lot in one minute and grow
gradually with time going on. Then the relationship between time and the changes of absorption at 412
nm is showed in Fig.1(b). The whole reaction can get saturation within 30 min as can be seen in
Fig.1(b). The reaction rate is slowly decreased with the time going on corresponding to the smoothness
of the curve in the figure. Likewise, the fluorescence is also collected in the same way with the
excitation at 400 nm. The fluorescence shows the same tendency with the absorption as shown in
Fig.1(c). The fluorescence at 452 nm increases gradually. Similarly, the relationship between time and
the changes of fluorescence is collected as Fig.1(d). The difference is that the fluorescence intensity
7

changes more quickly. The fluorescence intensity increases obviously within 30 seconds and get
saturation in 15 min which can be seen in Fig.1(d). All of this shows a rapid detection of Pd0 which can
be used in the practical application.
Fig.1. (a) Time-dependent absorption changes of probe PC after adding 5 equiv. of Palladium(Pd0). (b)
The changes of absorption intensity at 412 nm in the time range of 0 min-40 min. (c) Time-dependent
changes of fluorescence intensity of probe PC after adding 5 equiv. Palladium(Pd0) when excited with
400 nm. (d) The changes of fluorescence intensity at 452 nm in the time range of 0 -30 min. The
spectroscopic properties of probe PC were tested in aqueous phosphate buffered saline (PBS)
solutions(10mM, pH=7.4) containing 50% DMSO in the concentration of 10-5 M.
3.3 The selectivity towards different metal ions
As we all know, the selectivity towards only one ion is of vital importance for a good probe. Then
the selectivity towards different ions is tested. The selectivity towards different ions is shown in Fig.2.
CaCl2, CuCl2, FeCl3, HgCl2, CoCl2, NiCl2, ZnCl2, PbCl3, NaCl, KCl, AgNO3, MnCl2, AlCl3, BaCl2,
8

Cr(NO3)3 are used as the source of other metal ions and Pd(PPh3)4 is the source of palladium which is
dissolved in water and THF, respectively. The probe is dissolved in the concentration of 10-5 M in the
solution of phosphate buffered saline (PBS) containing 50% DMSO. 100 equiv of other ions and 5
equiv of palladium(Pd0) is added to the above solutions respectively. The spectra of absorption and
fluorescence are tested 30 min later after the adding of different ions. It can be found from the
absorption spectra that only Pd0 can cause the increase in 412 nm(Fig.2(a)) while the other ions can not.
The fluorescence changes is also tested with the same conditions(Fig.2(b)). It can be seen that nearly
no fluorescence can be found with other metal ions while Pd0 can cause the fluorescence increase
obviously. All these data demonstrate that the probe PC has a good selectivity towards Pd0 over other
metal ions. Thus, the probe has metal ion specificity. However, only metal ion specificity for a good
probe is not enough. Many metal ions are mixed together whether they are in river or in other places.
Therefore, it is very important for the detection of a specific ion without the interference of other metal
ions, especially when the other metal ions are in good quantity. Then, the effect of other metal ions is
tested. First, the probe PC is dissolved in phosphate buffered saline (PBS) containing 50% DMSO in
the concentration of 10-5 M. 100 equiv of different metal ions are added to the above solution
respectively. The fluorescence is tested and collected after 30 min, which is shown in Fig.3. The
fluorescence intensity after adding 100 equi of other metal ions is corresponding to the left bar in the
figure. All the solutions show nearly nonfluorescence. 5 equiv of palladium(Pd0) is added to the above
solutions respectively. The fluorescence is investigated and collected after 30 min as shown in Fig.3.
The fluorescence intensity after adding 5 equiv of palladium(Pd0) is corresponding to the right bar in
the figure. It can be seen that the probe PC can still detect Pd0 even with adding quantity of metal ions.
All this proved that this sensing for Pd0 is hardly interfered by other metal ions as can be seen in Fig.3.
9

There’s little influence to the detection of Pd0 even with quantity of other metal ions.
Fig.2. (a) Absorption of probe PC and probe PC in the presence of different ions including Ag+, Al3+,
Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+, Mn2+, Na+, Ni2+, Pb2+ and Zn2+; (b) Fluorescence intensity
of probe PC and probe PC in the presence of different ions including Ag+, Al3+, Ba2+, Ca2+, Co2+, Cr3+,
Cu2+, Fe3+, Hg2+, K+, Mn2+, Na+, Ni2+, Pb2+ and Zn2+ with excitation at 400 nm. Other metal ions are in
the quantity of 100 equiv and Pd0 is in the quantity of 5 equiv. The spectroscopic properties of probe
PC were measured in aqueous phosphate buffered saline (PBS) solutions(10mM, pH=7.4) containing
50% DMSO in the concentration of 10-5 M.
Fig.3. Effect of metal ions. The fluorescence intensity changes of probe PC in the presence of 100
equiv of various of cations. The short bar in the left is the fluorescence intensity of probe PC with other
ions in the concentration of 1μM. The ions including Ag+, Al3+, Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+,
10

K+, Mn2+, Na+, Ni2+, Pb2+ and Zn2+ . The high bar in the right is the fluorescence intensity of probe PC
with the adding of 5 equiv Pd0 to the above solutions. The spectroscopic properties of probe PC were
measured in aqueous phosphate buffered saline (PBS) solutions(10 mM, pH=7.4) containing 50%
DMSO in the concentration of 10-5 M.
3.4 pH effect to the probe and probe PC
In our surrounding atmosphere, the pH is varied in a wide range and it also has slight variation in
human bodies. Therefore, it is very important for a fluorescent probe to be stable in a wide range of pH.
At least, it must be stable in physiological environment. Therefore, pH effect on the fluorescence
behavior of Probe(PC) and Probe PC with Pd0 is investigated. A wide pH range from 2 to 12 is adjusted
using phosphate buffered saline (PBS) solutions(10 mM, pH=7.4). Then, probe PC is dissolved in the
prepared phosphate buffered saline (PBS) solutions(10 mM, pH=7.4) using 50% DMSO as good
solvent and then 5 equiv of Pd0 is added to the above solutions. The spectra of probe PC and probe PC
with Pd0 are measured after 30 min and the fluorescence spectra are collected as shown in Fig.4. In a
wide range of pH from 2 to 12, it can be seen that Probe PC is stable in a wide pH range from 2 to 10.
When the pH is higher than 10, hydrolysis is occurred which results in the increase of fluorescence.
This prove that probe PC can be used in physiological environment. What’s more, in a pH range of 6 to
12, probe PC can detect Pd0 with slight interference of the pH changes which can be used in
physiological environment.
11

Fig. 4. pH effect on the fluorescence behavior of probe PC(red line) and probe PC with Pd0(black line).
The fluorescence spectra were collected with the excitation at 400 nm. The spectroscopic properties of
probe PC were tested in aqueous phosphate buffered saline (PBS) solutions(10 mM, pH=7.4)
containing 50% DMSO in the concentration of 10-5 M.
3.5 Determination of the detection limit
To calculate the detection limit, titration of of Pd0 (0-0.18 μmol) to the probe PC solution is
investigated. Probe PC is dissolved in phosphate buffered saline (PBS) solutions(10 mM, pH=7.4)
containing 50% DMSO in the concentration of 10-5 M. 0.2 μmol, 0.4 μmol, 0.6 μmol, 0.8 μmol, 1 μmol,
1.4 μmol, 1.8 μmol and 2 μmol Pd0 were added to the solution respectively. Upon the titration of
Pd0( Fig.5(a) ), the fluorescence at 450 nm increases gradually. The detection limit is calculated based
on the fluorescence titration and is calculated by the equation: detection limit=3σ/k. Where σ is the
standard deviation of blank measurement, k is the slope between the fluorescence intensity versus Pd0
concentration. The fluorescence intensity of probe PC is measured by 15 times and the standard
deviation(σ) of blank measurement is obtained. To get the slope, the fluorescence intensity at 450 nm is
12

plotted as a concentration of Pd0 as shown in Fig.5(b). And the detection limit is calculated to be 0.34
nM which is much lower than the Pd0 concentration of threshold in drugs.
Fig.5. (a) Fluorescence response upon titration of Pd0 (0-0.18 μmol) with excitation at 400 nm taken
after 30 minutes. It is from 0.2 equiv to 2 equiv from the bottom to the top. (b) Fluorescence intensity
versus concentration of Pd0( pd: 0-0.18 μmol) at 450 nm with excitation at 400 nm taken after 30
minutes. The spectroscopic properties of probe PC were measured in aqueous phosphate buffered
saline (PBS) solutions(10 mM, pH=7.4) containing 50% DMSO in the concentration of 10-5 M.
3.6 The proposed mechanism for the detection of Pd0
Pd0 is capable of catalyzing the allylic oxidative insertion to cleave the allylic C-O bond of allylic
ethers to complex. These complexes then react with various nucleophiles which is know as Tsuji-Trost
reaction41,42. The excellent selectivity towards Pd0 should be attributed to the highly specific
Pd0-triggered cleavage process. As shown in Fig.6, the probe shows no fluorescence when conjugated
with an allyl carbamate group. However, when Pd0 is added, they will react with the allyl carbamate
group of the probe PC and ionize to form π-allylpalladium(II) complex 2 and further transferring the
allyl unit to morpholine as the nucleophile, and then producing carbamate 3. Finally, decarboxylation
of the compound delivers the compound 1 with strong blue fluorescence. To further prove the
mechanism of the reaction, the reaction of probe PC with Pd(PPh3)4 was conducted under the same
conditions and the products were subjected to mass spectral analysis. The peak at m/z 257.0442
13

corresponding to compound 1 was observed, which proved the proposed mechanism(Fig.S7).
Fig. 6. Mechanism for the detection of palladium
4. Conclusions
In summary, we have designed and synthesized a fluorescent probe for the fast detection of Pd0 based
on coumarin. The synthesized probe shows not only good selectivity but also fast response to Pd0 in
phosphate buffered saline (PBS). The absorption at 412 nm rises sharply as well as the fluorescence
intensity at 450 nm after the addition of Pd0 within few minutes. The detection limit can be as low as
0.34 nM. The probe shows nonfluorescence when connected with allyl carbonate ester which destroys
the ICT system. The effect of Pd0 to the probe can change the structure. As a result, the fluorophore is
exposed and it shows fluorescence.
Acknowledgment
This work has been supported by the National Natural Science Foundation of China (21277046,
21173077, 21377038), the Shanghai Committee of Science and Technology (13NM1401000), the
14

Research Fund of Education of the People’s Republic of China (JPPT-125-4-076), and the National
Basic Research Program of China (973 Program, 2013CB632403).
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Graphical abstract
We design a fluorescent probe for the detection of palladium with the high sensitivity and selectivity.
The probe shows nonfluorescence when connected with allyl carbonate ester which destroys the ICT
system. The effect of palladium to the probe can change the structure. As a result, the fluorophore is
exposed and it shows fluorescence.
18

Highlights:
1. A coumarin-based fluorescent probe for the detection of Pd0 is
designed and synthesized.
2. The probe shows good selectivity towards Pd0 compared with many
other ions
3. The probe can detect Pd0 even with the disturbance of quantity of other
metal ions.
4. The detection limit is as low as 0.34 nM which is lower than the
specific threshold in drugs.
19