A novel long-wavelength fluorescent probe for discrimination of different palladium species based on Pd-catalyzed reactions

Article abstract of DOI:10.1039/c7ra01731f

We have synthesized a novel long-wavelength fluorescent probe MFC for detection of palladium (Pd⁰). This probe not only shows high selectivity for palladium, but also could quickly discriminate different palladium species (Pd⁰, Pd²⁺, Pd⁴⁺) in phosphate buffered solution. After the reaction with Pd(PPh₃)₄, the UV absorption of the probe MFC shifts from 334 nm to 594 nm within a few minutes. The probe MFC conjugated with alkyl carbamate, which destroys the principle of intramolecular charge transfer (ICT) and shows weak fluorescence. Based on the Pd⁰-catalyzed Tsuji-Trost deallylation reaction, the fluorogen was released and the fluorescence intensity at 612 nm was observed with 13-fold enhancement along with an obvious color change from yellow to purple with low detection limit.

Full text of DOI:10.1039/c7ra01731f

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A novel long-wavelength fluorescent probe for
discrimination of different palladium species based
on Pd-catalyzed reactions†
Cite this: RSC Adv., 2017, 7, 24822
*
*
Youyu Zhang
Ting Yu, Guoxing Yin, Peng Yin,
and Shouzhuo Yao
Ying Zeng, Haitao Li,
We have synthesized a novel long-wavelength fluorescent probe MFC for detection of palladium (Pd0). This
probe not only shows high selectivity for palladium, but also could quickly discriminate different palladium
species (Pd0, Pd2+, Pd4+) in phosphate buffered solution. After the reaction with Pd(PPh3)4, the UV
absorption of the probe MFC shifts from 334 nm to 594 nm within a few minutes. The probe MFC
conjugated with alkyl carbamate, which destroys the principle of intramolecular charge transfer (ICT) and
shows weak fluorescence. Based on the Pd0-catalyzed Tsuji–Trost deallylation reaction, the fluorogen
was released and the fluorescence intensity at 612 nm was observed with 13-fold enhancement along
with an obvious color change from yellow to purple with low detection limit.
Received 11th February 2017
Accepted 3rd May 2017
DOI: 10.1039/c7ra01731f
rsc.li/rsc-advances
have been reported for the detection of palladium, most of
which were based on the Pd0-catalyzed Tsuji–Trost deallylation
and depropargylation.20–24 Consequently, several uorescence
Introduction
Palladium, one of the most important noble transitional metal
catalysts, is widely used in organic reactions, medical instru-
ments, fuel crowns, electronics and jewellery.1–5 The concern of
palladium for human health and environmental contamination
is increasing and a lot of attention has been paid to it recently.6
In particular, palladium could bind to thiol-containing
proteins, DNA, and other biomolecules, causing potential
toxicity and carcinogenicity to humans, and it is strongly
limited by governmental regulations to 5–10 ppm in end
products.7–10 Therefore, it is urgently required to develop
methods for selective and sensitive monitoring of palladium
species in the environment, food and living organisms.
The traditional analytical methods for detecting palladium
species are atomic absorption spectroscopy (AAS), inductively
coupled plasma mass spectrometry (ICP-MS), inductively
coupled plasma optical emission spectrometry (ICP-OES), solid
phase micro extraction-high performance liquid chromatog-
raphy and X-ray uorescence, which require expensive instru-
ments and well-trained individuals.11–14 Recently, uorescent
probes have been widely applied in biological and environ-
mental studies owing to its simple operation, low-cost, high
sensitivity and high selectivity.15–19 Plenty of uorescent probes
probes based on intramolecular charge transfer (ICT) and the
excited stated intramolecular proton transfer (ESIPT) had been
reported. Liu24 et al. designed a novel turn-on ratiometric uo-
rescence probe by ICT, which conducted two-photon uores-
cent platform with a rigid oxygen-bridge conformation based on
the green uorescence protein. Zhang25 et al. designed
a coumarin-based uorescent probe for the detection of palla-
dium with the detection limit to be 0.34 nM. Also, this probe
showed nonuorescence when connecting with allky carbonate
ester. Wang26 et al. synthesized a colorimetric and turn-on
uorescent probe for the detection of palladium with high
selectivity and sensitivity, which also used the allyl carbamate as
the recognition unit for palladium likewise others. However,
some of them still have some limitations including long time
reaction times, emission in the short-wavelength region and
small Stroke shi, which limits their applications in vivo.
Therefore, developing some palladium uorescent probe that
could emit in the red or NIR region is of great importance, for
the longer wavelength uorescence could reduce the back-
ground interference from the indigenous biomolecules of the
living organisms.
2-Dicyano-methylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran
(TCF) and its derivatives have typical donor–p–acceptor (D–p–A)
structure, longer emission wavelength in the red (or NIR) region,
which contribute their application in dye-sensitized solar cells,
bio-imaging and organic nonlinear optical crystals.27–35 Herein,
a long-wavelength uorescent probe (MFC) was designed for
sensing palladium species, based on the properties of TCF and
the principle of intramolecular charge transfer (ICT) (Scheme 1).36
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research,
Ministry of Education, College of Chemistry and Chemical Engineering, Hunan
Normal University, Changsha 410081, PR China. E-mail: yinpeng@hunnu.edu.cn;
haitaoli@hunnu.edu.cn
† Electronic
supplementary
information
(ESI)
available:
Structural
characterization data, absorption and uorescence spectra, TLC, 1H NMR, 13C
NMR, and HRMS. See DOI: 10.1039/c7ra01731f
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compound TCF had been consumed, which was monitored by
TLC. Aer cooled to room temperature, the mixture was ltered
and washed with cooled ethanol three times. The residue was
dried under vacuum to give 0.80 g (88.88%) pure product of MFO
as red solid. 1H NMR (500 MHz, DMSO-d6) d: 10.62 (s, 1H), 7.91
(d, J ¼ 16.1 Hz, 1H), 7.81 (d, J ¼ 8.3 Hz, 2H), 7.02 (d, J ¼ 16.2 Hz,
1H), 6.91 (d, J ¼ 8.3 Hz, 2H), 1.78 (s, 6H). 13C NMR (126 MHz,
DMSO-d6) d 177.7, 176.3, 162.8, 148.8, 132.8, 126.2, 116.9, 113.4,
112.6, 112.2, 111.7, 99.5, 97.0, 53.7, 25.8 ppm.
Scheme 1 Structure of probe MFC and the proposed mechanism for
selective recognition of Pd0.
Synthesis of probe MFC
Compound MFO (0.60 g, 1.98 mmol) was dissolved in anhydrous
acetonitrile and cooled to 0 ꢁC by ice-water bath. Then allyl
chloroformate (0.24 mL, 2.19 mmol) and a few drops of redis-
tilled triethylamine were added to the solution under stirring.
The reaction mixture was stirred for 1 h and warmed to room
temperature. Aer quenched by adding 10 mL water, the mixture
was extracted with dichloromethane for three times. The
combined organic layers ware dried over anhydrous Na2SO4. The
solvent was ltered, and dried under vacuum to yield the brown
solid (704 mg, 91.87%). 1H NMR (500 MHz, chloroform-d) d 7.76–
7.69 (m, 2H), 7.67 (d, J ¼ 16.4 Hz, 1H), 7.41–7.34 (m, 2H), 7.02 (d, J
¼ 16.4 Hz, 1H), 6.03 (ddt, J ¼ 16.5, 10.4, 5.9 Hz, 1H), 5.48 (dq, J ¼
17.2, 1.4 Hz, 1H), 5.39 (dd, J ¼ 10.4, 1.3 Hz, 1H), 4.79 (dt, J ¼ 5.9,
1.4 Hz, 2H), 1.83 (s, 6H). 13C NMR (126 MHz, chloroform-d)
d 173.5, 154.1, 152.6, 145.9, 131.6, 130.8, 130.3, 122.2, 120.0,
Allyl carbamate group was usually used as the response unit for
Pd0. Aer reaction with palladium, the ally carbamate group
would be cleaved from the probe, and substrate with strong
uorescence would be released. The probe MFC showed high
sensitivity and selectivity toward palladium and met the require-
ment of both emitting in the red wavelength region and naked eye
detection.
Experimental
Materials and instrumentation
The chemicals and solvents were purchased from Aladdin
(Shanghai, China) and used without further purication.
Pd(PPh3)4, NaCl, KCl, AgNO3, ZnCl2, CdCl2, FeCl3, CrCl3, CuCl2,
Hg(NO3)2, MnCl2, PbCl2, Na2HPO4, NaH2PO4, Pd(PPh3)2Cl2,
(NH4)2PdCl6, (MeCN)2PdCl2, NaBH4 were of analytical grade.
TLC analysis was performed using precoated silica plates. UV-
vis absorption spectra were collected on a UV-2450 spectro-
photometer (Shimadzu Co., Japan). Fluorescence spectra were
recorded on Hitachi F-7000 spectrophotometer (Hitachi Ltd,
115.1, 111.5, 110.7, 110.1, 100.4, 97.8, 69.6, 26.4 ppm. IR (cmꢀ1
)
3070, 3046, 2994, 2965, 2748, 2417, 2177, 1765, 1580, 1621, 1378,
1309, 950, 858, 774. HRMS (FTMS ESI+): m/z calcd for C22H17N3O4
386.1141 [M–H]ꢀ; found 386.1122.
General procedure for uorescence and UV-visible
measurements
1
Japan). H NMR and 13C NMR spectra were obtained with tet-
ramethyl silane (TMS) as the internal standard on a BRUKER
AVANCE-500 spectrometer. IR spectrum was recorded on
NEXUS as KBr pellets and were reported in cmꢀ1. HRMS spec-
trum was recorded on AB TripleTOF 5600+ (U.S AB SCIEX).
UV-vis absorption and uorescence spectra studies were con-
ducted in a mixture solution of DMSO/PBS (10 mM, pH ¼ 7.4,
3 : 7, v/v). A volume of 2.0 mL of the solution containing probe
MFC (25 mM for UV-vis absorption and 10 mM for uorescence
experiments) was rst introduced to a quartz cell, following
additions of 10 mL of Pd(PPh3)4 stock solution (10 mM). The
kinetic investigations were carried out by measuring uorescence
intensities of the resulting mixture at different time intervals. The
uorescence intensities were recorded with an excitation wave-
length of 560 nm and emission wavelength ranging from 580 nm
to 700 nm. Both the excitation and emission slit was set as
5.0 nm. The responses of interferences were performed by adding
Synthesis of compound TCF
Compound TCF was prepared according to the literature
method.37 To a mixture of 3-hydroxy-3-methylbutan-2-one
(9.00 g, 88.12 mmol), malononitrile (11.92 g, 180.43 mmol) in
ethanol (30 mL) was added sodium ethanol (0.90 g, 13.22
mmol), then the mixture was heated to reux for 2 h until no
starting compound was indicated by thin layer chromatography
(TLC). Aer cooled to room temperature, the mixture was
ltered and washed with cooled ethanol three times. The
residue was dried under vacuum to give 15.00 g (85.71%) of TCF
4
equiv. of different anions and palladium-containing
compounds (20 mM) to each sample. PBS buffers with different
pH values were chosen for investigation the effect of pH. The
limit of detection (LOD) was determined by adding Pd(PPh3)4
over a series of concentrations into the probe MFC samples.
1
as gray solid. H NMR (500 MHz, chloroform-d) d: 2.39 (s, 3H),
1.65 (s, 6H). 13C NMR (126 MHz, chloroform-d) d: 182.7, 175.2,
111.0, 109.0, 104.8, 99.8, 58.4, 24.4, 14.2.
Results and discussion
Synthesis of MFO
Probe design and synthesis
A mixture of TCF (0.60 g, 3.01 mmol) and 4-hydroxybenzaldehyde The probe MFC was prepared conveniently according to the
(0.38 g, 3.10 mmol) in ethanol (20 mL) was heated to reux until synthetic routine in Scheme 2. Firstly, the compound TCF was
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The effect of the ratio of DMSO : PBS (pH ¼ 7.4) was also
investigated (Fig. S1 in ESI†). And the best ratio of DMSO/PBS
(pH ¼ 7.4) was found to be 3 : 7. Furthermore, normally used
buffer solutions (PBS, HEPES and Tris–HCl) were also tested for
the probe MFC and the mixture of probe MFC with Pd(PPh3)4.
Judging from the results, the buffer solution of PBS exhibited
the best results (Fig. S2 in ESI†). Therefore, the following
experiments were carried out in the solution of DMSO/PBS
(3 : 7, v/v). Encouraged by these observations, the time-
dependent uorescence intensity changes of probe MFC in
presence of Pd(PPh3)4 was studied (Fig. S3 in ESI†). Firstly, the
stability experiments of the probe MFC at pH 7.4 was investi-
gated. Encouragingly, the probe MFC has little hydrolysis under
pH 7.4 for 1 h (Fig. S3a in ESI†). While, upon addition of 2.0
equiv. of Pd(PPh3)4 to the probe solution, an initial fast, fol-
lowed by gradual increasement of the uorescent intensity was
obviously observed, and it came to a plateau in about 20 min.
Furthermore, the reaction of the probe MFC and Pd0 could be
easily detected by TLC plate and most of the probe had been
consumed aer reacted with Pd(PPh3)4 for 20 min (Fig. S4 in
ESI†).
Scheme 2 Preparation of probe MFC.
prepared by the 3-hydroxy-3-methylbutan-2-one and malononi-
trile in ethanol and sodium ethanol. Next, the compound TCF
and 4-hydroxybenzaldehyde was heated to reux in ethanol,
yielding the product MFO. Finally, the probe MFC was synthe-
sized from MFO and allyl chloroformate with a satisfactory
yield. The detailed characterizations of the intermediates and
probe MFC are shown in the ESI (Fig. S5–S11 in ESI†).
Effect of pH
With respect to the pH effect, as is well known, the carbonate
group is sensitive to pH change and easily hydrolyzed under
both acidic and basic media conditions. We examined the pH
effect on probe MFC in the absence and presence of Pd(PPh3)4,
respectively (Fig. 2). The probe was stable in a pH range of 2.0–
6.0 monitored at 612 nm. Fluorescence intensity changed little
when pH value range 6.0–7.5. Once the probe was incubated
with Pd(PPh3)4 within the pH value range 7.0–10.0, uorescence
intensity changed distinctly. These results indicated the
response of probe MFC to Pd0 were favorable at pH range from
7.0–8.0 including the physiological conditions and the probe
could be available for Pd0 detection in living organism.
The optical property and sensing potential of MFC
The spectral properties of probe MFC were initially investigated
in the absence and presence of Pd0 in a mixture solution of
DMSO and PBS (pH ¼ 7.4, v/v 3 : 7). Pd(PPh3)4 was used as the
resource of Pd0 in all of the experiments. The free probe MFC
exhibits a major UV absorption at 334 nm as shown in Fig. 1a.
However, aer treated with 2.0 equiv. of Pd(PPh3)4, a remark-
able change in the absorption spectrum was obtained and
a high absorption peak was observed at 594 nm (Fig. 1a).
Meanwhile, marked color changes of the solution were obvi-
ously noticed from yellow to purple in visible light, which could
enable the naked-eye visual detection of Pd0 (inset Fig. 1a).
Consistently, in the uorescence spectra, probe MFC exhibited
very weak uorescence at 612 nm as expected when excited at
560 nm, and a dramatic turn-on uorescence enhancement at
612 nm was observed aer triggered with Pd(PPh3)4 (Fig. 1b).
Correspondingly, a notable colour change from bright blue to
green was observed by naked eye when excited at 365 nm by
handheld UV lamp (Fig. 1b inset).
The sensitivity of probe MFC for palladium
The capability of probe MFC for recognizing Pd0 was investigated
by monitoring uorescent intensities aer adding different
concentrations of Pd(PPh3)4 under the optimal conditions. The
uorescence intensities at 612 nm increased gradually with the
Fig. 2 (a) pH-dependent normalized fluorescence intensity spectrum
of free probe MFC (10 mM) in DMSO-PBS buffer (10 mM). (b) pH-
Fig. 1 (a) The absorption spectra and (b) the fluorescence spectra of dependent normalized fluorescence intensity spectrum of probe MFC
probe MFC (25 mM) and probe MFC with 2 equiv. of Pd(PPh3)4 at 37 ꢁC (10 mM) with Pd(PPh3)4 (20 mM) in DMSO-PBS buffer (10 mM). lEx
¼
in DMSO/PBS buffer (10 mM, pH ¼ 7.4, 3 : 7, v/v). lEx ¼ 560 nm, lEm
¼
560 nm, lEm ¼ 612 nm. (c) pH effect on the normalized fluorescence
612 nm. Inset image: (a), left: visible-light of 10 mM probe, right: 10 mM intensity spectrum of free probe MFC (10 mM, black line) and probe
probe with 20 mM Pd(PPh3)4; (b), left: fluorescence image of 10 mM MFC (10 mM) with Pd(PPh3)4 (20 mM, red line) in DMSO-PBS buffer (10
probe, right: 10 mM probe with 20 mM Pd(PPh3)4.
mM). lEx ¼ 560 nm, lEm ¼ 612 nm.
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increase of the concentration of Pd(PPh3)4 (Fig. 3). A good line-
arity of the uorescence intensity at 612 nm versus the concen-
tration of Pd(PPh3)4 from 0 to 2.0 mM was obtained with the
detection limit as 1.4 nM (R2 ¼ 0.99356). And only 0.2 equiv. Pd0
could consume most of the probe MFC aer 20 min. Owing to the
specic properties of palladium, probe MFC displayed a high
sensitivity toward Pd0. The results indicated that a free product
MFO was generated by the palladium-triggered reaction, which
could indirectly be conformed by thin layer chromatography
(TLC) (Fig. S4 in ESI†).
The selectivity of probe MFC for palladium
To illustrate the good selectivity of this novel probe to Pd0,
a series of metal ions were evaluated. As shown in Fig. 4a, only
the addition of Pd0 induced remarkable uorescence increase.
In contrast, nearly no or little uorescence changes were
observed in the presence of Fe3+, Mg2+, Cd2+, Co2+, Ba2+, Ni+,
Cu2+, Ag+, Al3+, Ca2+, Li+, Zn2+, Mn2+, Hg2+ and Ti3+. Competitive
experiments involving the impacts of above mentioned metal
ions in the detection of Pd0 were performed, which indicated
the addition of various metal ions had a negligible effect on the
Pd0 detection (Fig. 4b). These results demonstrated that the
probe MFC showed an excellent selectivity to Pd0 over other
competitive metal ions, which should be attributed to the
specic Pd0-triggered reactions.
Fig. 4 (a) Normalized fluorescence intensity of probe MFC (10 mM)
and (b) probe MFC (10 mM) with Pd(PPh3)4 (20 mM) in the presence of
40 mM of different interferences in DMSO-PBS buffer (10 mM, pH ¼
7.4): (1) blank, (2) Pd(PPh3)4, (3) FeCl3, (4) MgCl2, (5) CdCl2, (6) CoCl2, (7)
BaCl2, (8) NiCl2, (9) CuCl2, (10) AgNO3, (11) AlCl3, (12) CaCl2, (13) LiCl,
(14) ZnCl2, (15) MnCl2, (16) Hg(NO3)2, (17) TiCl3. (c) Normalized fluo-
rescence intensity of probe MFC (10 mM) in the presence of 20 mM of
different palladium-containing compound (d) normalized fluores-
cence responses of probe MFC (10 mM) to palladium-containing
compound (20 mM) with NaBH4 at trace concentration [except
Pd(PPh3)4] in DMSO-PBS buffer (10 mM, pH ¼ 7.4), lEx ¼ 560 nm, lEm
¼
612 nm. (1) Pd(PPh3)4, (2) Pd(PPh3)2Cl2, (3) (NH4)2PdCl6, (4) (MeCN)2-
PdCl2, (5) PdCl2.
Mechanism
The selectivity of probe MFC for different palladium
According to previously reported probes with an allyoxy allyox-
ycarbonyl group for the sensing of Pd0, it is proposed that the
trigger moiety of the allyl chloroformate unit is initially conju-
gated with palladium and ionized to form MFC–Pd0 complex
and further dissociates p-allylpalladium(II) to produce (E)-4-(2-
(4-cyano-5-(dicyanomethylene)-2,2-dimethyl-2,5-dihydrofuran-
3-yl)vinyl)phenyl carbonate, then decarboxylates to produce the
compound MFO (Scheme 1).38 which was further veried by the
1H NMR spectra for the probe MFC and the mixture of the probe
MFC with Pd(PPh3)4 in deuterated reagents (Fig. 5). The
chemical shis of Ha (Hb), Ha0 (Hb0) in compound MFO and
MFC are 7.79 (6.89) ppm and 8.02 (7.42) ppm, respectively.
When probe MFC was treated with Pd(PPh3)4, it was clearly
observed that new peaks attributed to Ha and Hb was gener-
ated. Moreover, HRMS analyses also indicated that the free
MFO was produced during the detection process (Fig. S12 in
ESI†).
The reactivities of probe MFC towards other oxidation states
of palladium metal sources, such as PbCl2, (MeCN)2PdCl2,
Pd(PPh3)2Cl2, (NH4)2PdCl6, were also examined. The addition
of Pd2+ and Pd4+ to the solution of probe MFC had little
uorescence change even with long times (Fig. 4c). While,
aer the addition of micromolar NaBH4, similar responses to
probe MFC as Pd0 were obtained, which meant that Pd2+/Pd4+
species could be reduced to Pd0 (Fig. 4d). That meant probe
MFC could recognize the palladium species in all of the
typical oxidation states and discriminate Pd0 from Pd2+/Pd4+
under different test conditions. These results demonstrated
that probe MFC could be employed for specic recognition of
Pd0 species.
Cell uorescence imaging
Those above ndings were encouraging to explore the possi-
bility of using probe MFC for intracellular sensing of Pd0 in
living cells, which was conducted using HEK293 cells. The cells
were purchased from China Center for Type Culture Collection
(Wuhan, China). As shown in Fig. 6c, free probe MFC exhibited
almost no uorescence signal, which was consistent with the
above uorescence studies. Contrastingly, aer incubation with
Pd0 for 30 min, a strong uorescence response could be
observed (Fig. 6b). The results demonstrated that probe MFC is
Fig. 3 (a) Normalized fluorescence intensity spectra of probe MFC (10
mM) upon addition of increasing concentrations of Pd(PPh3)4 (0–10
mM) in DMSO-PBS buffer (10 mM, pH 7.4, 3 : 7, v/v). lEx ¼ 560 nm, lEm
¼ 612 nm. (b) The linear relationship between the fluorescence
intensity of probe MFC (10 mM) at 612 nm and the concentrations of
Pd(PPh3)4 (0–10 mM) in DMSO-PBS buffer (10 mM, pH 7.4, 3 : 7, v/v).
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competitive metal ions. The excellent selectivity is due to the
highly specic Pd0-triggered cleavage process and the long
wavelength emission uorescence should be attributed to the
substrate preference and the principle of intramolecular change
transfer. Probe MFC responds totally different to the different
oxidation states of palladium species, and have short response
times and low detection limit towards Pd0 species. This strategy
would provide a new opportunity for uorescent sensing of
palladium species with the emission in the red range and high
selectivity.
Acknowledgements
This work was supported by the funding from the National
Natural Science Foundation of China (Grant No. 21405043,
21675051, 21375037 and 21205038) and the Hunan Natural
Science Foundation (13JJ2022).
Fig. 5 1H NMR spectra of (a) compound MFO in DMSO-d6 and (b)
probe MFC in DMSO-d6 and (c) probe MFC upon addition of 2 equiv. of
Pd(PPh3)4 in DMSO-d6 and D2O.
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Conclusions
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