A novel dual-mode turn-on optional chemodosimeter for the visualization of Pd0 with a low detection limit

Article abstract of DOI:10.1016/j.jphotochem.2017.01.007

Rhodol is an ideal platform for fluorescent probes owing to its spirolactone framework and excellent photochemical properties. Herein, a novel rhodol-based colorimetric and fluorescent turn-on probe, DER-1, for the detection of Pd⁰, was rationally developed with an allyl carbamate group as the response unit. Base on the Pd⁰-triggered cleavage reaction and rhodol spiroring-opening mechanism, the proposed probe exhibited a high selectivity and sensitivity towards Pd⁰. Upon addition of Pd(PPh₃)₄, a significant fluorescence enhancement at 547?nm was observed with an obvious color change from colorless to pink, which can be easily identified by naked-eye. In addition, the fluorescence intensity at 547?nm was linearly proportional to the concentration of Pd⁰ in the range of 0–1.5?μM, and the detection limit was calculated to be 1.14?nM. That is, probe DER-1 can be quite a sensitive fluorescent turn-on probe for the quantitative detection of Pd⁰ in pretty low dose.

Full text of DOI:10.1016/j.jphotochem.2017.01.007

Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 25–32
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A novel dual-mode turn-on optional chemodosimeter for the
visualization of Pd0 with a low detection limit
Mengmeng Liua, Taohua Lengb, Kai Wanga, Yongjia Shena, Chengyun Wanga,
*
a
Key Lab for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and
Technology, 130 Meilong Road, Shanghai 200237, China
b
National Food Quality Supervision and Inspection Center (Shanghai)/Shanghai Institute of Quality Inspection and Technical Research, Shanghai 200233,
China
A R T I C L E I N F O
A B S T R A C T
Article history:
Rhodol is an ideal platform for fluorescent probes owing to its spirolactone framework and excellent
photochemical properties. Herein, a novel rhodol-based colorimetric and fluorescent turn-on probe,
DER-1, for the detection of Pd0, was rationally developed with an allyl carbamate group as the response
unit. Base on the Pd0-triggered cleavage reaction and rhodol spiroring-opening mechanism, the proposed
probe exhibited a high selectivity and sensitivity towards Pd0. Upon addition of Pd(PPh3)4, a significant
fluorescence enhancement at 547 nm was observed with an obvious color change from colorless to pink,
which can be easily identified by naked-eye. In addition, the fluorescence intensity at 547 nm was linearly
Received 18 November 2016
Received in revised form 5 January 2017
Accepted 7 January 2017
Available online 9 January 2017
Keywords:
Fluorescent probe
Palladium
Rhodol dye
Reaction-based
proportional to the concentration of Pd0 in the range of 0–1.5
mM, and the detection limit was calculated
to be 1.14 nM. That is, probe DER-1 can be quite a sensitive fluorescent turn-on probe for the quantitative
detection of Pd0 in pretty low dose.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
capabilities have been developed, such as atomic absorption
spectrometry (AAS), inductively coupled plasma atomic emission
Palladium, which is widely distributed in the environment due
to its use in alloys, fuel cells, chemical catalysts, and especially in
automobile catalytic converters, is one of the most ubiquitous and
poisonous heavy metals [1,2]. Available evidences indicated that
palladium compounds originating from environmental matrices
could be easily migrated to biological materials, thereby accumu-
lating in the food chain [3]. Moreover, palladium, as a thiophilic
element, can bind to thiol-containing amino acids, proteins, DNA
and RNA, and other macromolecular, possibly impair various cellar
processes, which may result in potential health hazards [4–6]. Even
low doses of palladium are sufficient to cause allergic in
susceptible individuals [7]. In view of this, effective assay methods
for monitoring low levels of the palladium species are urgently
required.
spectrometry (ICP-AES), inductively coupled plasma mass spec-
trometry (ICP-MS), solid phase microextraction high-performance
liquid chromatography (SPME–HPLC), X-ray fluorescence (XRF),
time-of-flight resonance ionization mass spectrometry (TOF-MS),
capillary zone electrophoresis (CZE) [8–12]. These methods do
provide a rapidly and extremely accurate analysis of palladium;
however, they need sophisticated sample-pretreatment proce-
dures, complicated apparatus, and rigorous experimental con-
ditions. Therefore, development of optical (colorimetric and
fluorescent) methods is highly beneficial for the detection of
palladium due to apparent advantages such as high selectivity and
sensitivity, operational simplicity, rapidity, and low cost, over other
methods [13–16].
Until now, various colorimetric and fluorescent chemosensors
have been rationally reported for palladium sensing, and the
detection of palladium is usually based on the complexation of
palladium with the detector or the reaction between palladium
and probe such as ring opening of rhodamine derivatives and
palladium-catalyzed reactions [17–22]. Palladium detection based
on coordination can always provide a reversible detection process
To determine the palladium concentration, various conven-
tional techniques with low detection limits and multi-element
* Corresponding author.
E-mail address: cywang@ecust.edu.cn (C. Wang).
http://dx.doi.org/10.1016/j.jphotochem.2017.01.007
1010-6030/© 2017 Elsevier B.V. All rights reserved.

26
M. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 25–32
and signal change [23]. These chemosensors can thus be used to
monitor the fluctuation of palladium content in various samples.
However, their selectivity and sensitivity cannot always be
guaranteed. Considering the low residue threshold of palladium
in samples of drugs and biomaterials [24], a new strategy with high
sensitivity and selectivity should be selected for low-dose
palladium sensing. Investigations have indicated that a catalyzed
mechanism based fluorescent probe usually exhibits high selec-
tivity and sensitivity towards palladium species [25].
As we all know, rhodol fluorophore (also named rhodafluor) is
the hybrid of fluorescein and rhodamine, and it is an interesting
candidate for fluorescent probe since it inherit all the excellent
photophysical properties, such as high extinction coefficient,
quantum yield, photostability, and solubility in a variety of
solvents, and low pH-dependence [26]. Owning to the spirolactone
scaffold in rhodol fluorophore, which undergoes a conformational
transformation from spirolactone (colorless and nonfluorescent) to
an open-ring structure (colored and fluorescent), it is well
considered to offer promise as fluorescent group to construct
off-on optional probe [27].
for about 10 min. Then a solution of allyl chloroformate (60 mg,
0.5 mmol) in CH2Cl2 (5 mL) was added dropwise to the above
mixture. After being stirred for 30 min at 0 ꢀC, the mixture was
heated to room temperature and stirred for another 1.5 h.
Eventually the solvent was evaporated under reduced pressure,
and the pale pink crude product was purified by column
chromatography (pure CH2Cl2) on silica gel, affording the desired
DER-1 as a white solid (65 mg, yield 51%). 1H NMR (400 MHz,
DMSO): 1.09 (t, J = 6.9 Hz, 6H), 3.36 (q, J = 6.9 Hz, 4H), 4.75 (d,
J = 5.6 Hz, 2H), 5.32 (dd, J = 10.5, 1.0 Hz, 1H), 5.42 (dd, J = 17.2, 1.4 Hz,
1H), 6.01 (ddd, J = 22.7, 10.8, 5.6 Hz, 1H), 6.54–6.45 (m, 3H), 6.83 (d,
J = 8.7 Hz,1H), 7.01 (dd, J = 8.7, 2.3 Hz,1H), 7.37–7.29 (m, 2 h), 7.74 (t,
J = 7.3 Hz, 1H), 8.03 (d, J = 7.5 Hz, 1H), 7.81 (t, J = 7.1 Hz, 1H). 13C NMR
(100 MHz, DMSO): d(ppm) = 168.58, 152.27, 152.14, 152.01, 151.77,
151.48, 149.33, 135.63, 131.57, 130.19, 129.14, 128.64, 126.14, 124.68,
124.06, 119.03, 117.25, 117.16, 109.77, 109.81, 104.07, 96.82, 82.69,
68.99, 43.76, 12.25. HRMS (ESI, m/z) calcd for [C28H25NO6 + H]+
472.1760; found 472.1754.
2.3. Preparation of stock solutions of probe and metal ions
In order to detect Pd0 in low dose, we have developed a novel
optional Pd0-selective probe with a low detection limit. Herein, we
present the design, synthesis and spectral properties of the
fluorescent probe DER-1 with a terminal allyl carbamate as the
recognition unit (see ESI, Scheme S1). The designed probe exhibits
prominent turn-on fluorescence response towards Pd0 in PBS
buffer containing 50% THF, corresponding to the obvious color
change from colorless to pink. Furthermore, the proposed probe
shows high selectivity and sensitivity towards Pd0, especially with
quite a low detection limit. Compared to our previous work, the
probe of this work exhibited a more sensitive response with a
lower detection limit.
Stock solutions of DER-1 and Pd(PPh3)4 were prepared in THF
with a concentration of 0.5 mM. The PdCl2 solution was also
prepared in THF, but with a concentration of 1.0 mM. The
solutions of LiClO4, CuCl, Ce(NO3)3, SnCl2, ZnCl2, CrCl3, MnCl2,
AlCl3, CoCl2, NaCl, NiCl2, CaCl2, Pb(NO3)2, CuCl2, MgCl2, FeCl2,
FeCl3, Bi(NO3)3, KCl, AgNO3, BaCl2, Hg(OAc)2, PtCl2 were prepared
in with a concentration of 1.0 mM. The solution of NaBH4 was
also prepared in twice-distilled water, but with a concentration
of 10À2 M.
2.4. General spectrophotometric experiments
2. Experimental section
Both the fluorescence and UV–vis absorption experiments were
conducted in PBS buffer solution (20 mM, pH = 7.4, 50% THF, v/v).
2.1. Materials and instruments
Test solutions were prepared by placing 30.0
mL of DER-1 solution
(0.5 mM), 1470.0 L of DMSO, and an appropriate aliquot of each
m
N,N-Diethylaminophenol and allyl chloroformate (AllocCl)
were purchased from Energy Chemical and used directly without
any purification. All other reagents were of the highest grade that
available and used as received unless otherwise noted. All solvents
were analytical pure and were without any dryness and purifica-
tion prior to use. Twice-distilled water was used throughout all the
experiments.
All reactions were monitored by TLC. The TLC analysis was
performed on silica gel plates and column chromatography was
conducted over silica gel (mesh 200–300), both of which were
purchased from the Qingdao Ocean Chemicals. NMR spectra were
recorded on a Bruker AV-400 spectrometer, while using TMS as an
internal standard. All pH measurements were performed with a
PHS-3C digital pH meter. High Resolution Mass Spectra (HRMS)
were obtained by a Waters LCT Premier XE spectrometer. The UV–
vis absorption spectra were carried out on a Varian CARY 100
spectrophotometer at 37 ꢀC. Photoluminescent spectra were
recorded with Varian Cary Eclipse spectrophotometer equipped
with quartz cell of 1 cm path length at 37 ꢀC. The fluorescence
quantum yields were determined on a Horiba Fluoromax-4
fluorescence spectrophotometer.
analyte stock solution into a 3.0 mL test tube, and diluting the
resulting solution to 3.0 mL with PBS buffer solution (20 mM,
pH = 7.4). The slight pH variations of the solutions were achieved by
adding the minimum volumes of NaOH (0.1 M) or HCl (0.2 M). The
resulting solutions were well-mixed and kept at 37 ꢀC for 60 min,
and then the fluorescence spectra or UV–vis absorption spectra
were recorded. For all the measurements of fluorescence spectra,
excitation was performed at 500 nm with slit widths for excitation
and emission of 5.0 and 5.0 nm, respectively.
2.5. Determination of detection limit
The detection limit was calculated by 3
d/k method [30,31],
where is the standard deviation of blank measurement and k is
d
the slope between the fluorescence intensity versus DER-1
concentration. In the absence of Pd0, the fluorescence emission
spectrum of DER-1 was measured 5 times and the standard
deviation (d) was achieved. To get the slope, the fluorescence
intensity at 547 nm was plotted as a concentration of Pd(PPh3)4.
3. Results and discussion
2.2. Synthesis preparation of DER-1
3.1. Probe design and synthesis
N,N-Diethylrhodol was synthesized according to the proce-
dures reported in the literature [28,29].
Synthesis of DER-1. To a stirred solution of N,N-diethylrhodol
(97 mg, 0.25 mmol) in dry CH2Cl2 (10 mL) was added Et3N (0.1 mL,
0.75 mmol) under ice bath, and the resulting mixture was stirred
It is well-known that the protection of the hydroxyl group of
fluorophores can quench their fluorescence and the removal of
functionalized reactive site can recover the fluorescent signals of
the fluorophores [32]. Such phenomenon can be attributed to the
remarked changes in electronic properties that induced by the

M. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 25–32
27
protection-deprotection of functional groups. This strategy has
been widely employed to develop reaction-based turn-on fluores-
cent probes.
A suitable protectable group is of quite importance. As is known
to all that Pd0-catalyzed Tsuji-Trost allylic reaction was often
adopted as an admirable mechanism to develop fluorescent probes
for sensing of Pd0. To employ the mechanism for the selective and
quantitative determination of Pd0, a terminal allyl carbamate was
usually incorporated to a fluorescent group as the recognition unit.
In our previous work, allyl carbamate moiety was incorporated to a
coumarin derivative and the fluorescent molecule perform
excellent selectivity and sensitivity towards Pd0 [33].
In order to further develop and improve the sensitivity and
detection limit, we constructed DER-1 for Pd0 sensing in mixed
aqueous solution. In our scaffold, allyl carbamate was selected as
the trigger moiety and N,N-diethylrhodol (DER) was the fluores-
cent emission group. DER was selected to be the fluorophore
because they inherit all the excellent photophysical properties
from fluorescein and rhodamine, such as high extinction coef-
ficients, quantum yields, photostability, and good solubility, yet
low pH-dependence. Moreover, convention of the O-substituted
rhodol to O-free rhodol would induce an increase in fluorescence
intensity, which is an ideal scaffold for developing turn-on
fluorescent probes [34,35].
Fig 1. Absorption spectra of DER-1 (5
m
M) in PBS buffer solution (20 mM, pH = 7.4)
containing 50% THF in the absence (black) or presence (red) of Pd(PPh3)4 (2.0 equiv).
Insert: color change of DER-1 upon addition of Pd(PPh3)4. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
3.2. Spectroscopic properties of DER-1 towards Pd0
The desired probe DER-1 was conveniently prepared according
to the synthetic routine that outlined in Scheme 1. Firstly, N,N-
diethylrhodol (DER) was prepared by the reaction of 2-(4-
diethylamino-2-hydroxybenzoyl) benzoic acid with resorcinol
following the reported procedure. Then, probe DER-1 could be
easily synthesized in a satisfactory yield by treating DER with allyl
chloroformate in dry dichloromethane under basic conditions at
The spectral properties of probe DER-1 were firstly examined in
the absence and presence of Pd0 in PBS buffer (20 mM, pH = 7.4)
containing 50% THF at 37 ꢀC. Pd(PPh3)4 was selected as the resource
of Pd0 in all experiments. The absorption spectrum of free probe
DER-1 shows no absorption bands beyond 400 nm, as shown in
room temperature, and its structure was confirmed by 1H NMR, 13
NMR, and HRMS spectra(see ESI, Figs. S1–S3).
C
Fig. 1. However, treatment of DER-1 (5
mM) with 2.0 equiv. of Pd
(PPh3)4 resulted in remarkable change in the absorption
a
spectrum. A new, high absorption peak at 520 nm with a shoulder
around 491 nm appeared. Meanwhile, the solution color changed
Scheme 1. Preparation of probe DER-1.

28
M. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 25–32
potentiality of sensing both Pd0 and Pd2+ under reducing
conditions.
For better understanding of the sensing ability of the designed
probe towards Pd0, the time-dependent fluorescence intensity
changes of probe DER-1 in the absence and presence of Pd(PPh3)4
were studied. The fluorescence intensity of free probe DER-1
displayed no noticeable changes with time, indicating that the
proposed probe is quite stable under sensing environment.
However, upon addition of 2.0 equiv. Pd(PPh3)4 to probe DER-1
(5 mM), it was obviously observed that the solution of DER-1
afforded an initial fast, followed by gradual increase in emission
intensity and it reached a plateau in about 75 min (Figs. 3 and S5).
However, it could reach about 90% of the saturation in 60 min at
established condition. In this work, 60 min was selected as the
assay time for the evaluation of the sensitivity and selectivity of
DER-1 towards Pd0.
3.3. Ion selectivity and competitiveness
Fig. 2. Fluorescence spectra of DER-1 (5 mM) in PBS buffer solution (20 mM,
It is well known that the most important property of a probe is
high selectivity towards the targeted analyte over other competi-
tive species. To investigate the selectivity of probe DER-1 towards
Pd0 and other metal ions, the UV–vis absorption and emission
spectra after addition of different metal ions such as Na+, K+, Cu+,
Li+, Ag+, Mn2+, Co2+,Ba2+, Mg2+, Ca2+, Hg2+, Zn2+, Cu2+, Ni2+, Pb2+, Pd2
+, Fe2+, Sn2+, Ce3+, Fe3+, Cr3+, Al3+, Bi3+, and Pd0 were recorded in PBS
buffer. As shown in Fig. 4, only the addition of Pd0 induced
remarkable signal changes in the UV–vis absorption and emission
spectra. However, nearly no or little absorption and emission
fluorescence changes were observed even in the presence of 10.0
equiv of above-mentioned metal ions. Specifically, a 330-fold
fluorescence intensity enhancement at 547 nm was observed in the
presence of Pd0, corresponding with the color change from
colorless to pink that could be easily distinguished by naked eye
(see ESI, Fig. S6). To further examine the selectivity of probe DER-1
towards Pd0, competitive experiments involving the effects of
other mentioned metal ions in the determination of Pd0 were
carried out, which indicated that the addition of various above-
mentioned metal ions promoted a negligible effect on Pd0 sensing
(Fig. 5).
Nickel and Platinum exist in the same group in the periodic
table as palladium, so they may possess similar chemical and
physical properties. In other words, Ni (0) and Pt (0) have the
highest possibilities to interfere the Pd0 detection process, if any.
Considering all these above, the selectivity of DER-1 to Ni2+, Pt2+, Ni
(0), Pt (0) were also investigated. As shown in Fig. S7, nearly no or
little changes were observed in their absorption and emission
spectra even in the presence of 10.0 equiv. of these four species.
These results did demonstrate that the proposed probe show an
excellent selectivity towards Pd0 over other competitive metal
ions. Clearly, the outstanding selectivity should be attributed to
Pd0-triggered cleavage progress.
pH = 7.4) containing 50% THF in the absence (black) or presence (red) of Pd(PPh3)4
(2.0 equiv). Insert: fluorescent emission change (under UV light) upon addition of
Pd(PPh3)4. lex = 500 nm; slits: 5 nm/5 nm. (For interpretation of the references to
colur in this figure legend, the reader is referred to the web version of this article.)
obviously from colorless to pink, which could facilitate naked-eye
visual detection of Pd0 (inset Fig. 1).
Probe DER-1 also exhibited excellent fluorescent signaling for
Pd0. In the emission spectrum, DER-1 (fluorescence quantum yield
K
< 0.001) displays no fluorescence at 547 nm as expected when
excited at 500 nm. The low fluorescent background was reasonably
attributed to that DER-1 existed predominantly in spirolactone
form, and the spirocyclic form of rhodol was always colorless and
nonfluorescent. However, treatment of probe DER-1 with Pd
(PPh3)4 triggered a dramatic turn-on fluorescence enhancement
(>330-fold,
K= 0.4182) at 547 nm and correspondingly an obvious
bright green-yellow fluorescence was clearly observed (Fig. 2).
Furthermore, Pd2+ species containing PdCl2, Pd(PPh3)2Cl2, Pd
(OAc)2 and Pd(CH3CN)2Cl2 gave the similar responses under
reducing conditions (NaBH4-PPh3) (see ESI, Fig. S4). Evidences
available above meant that the designed probe possessed the
3.4. Effect of pH on the detection of Pd0
It is generally known that ester group is susceptible to pH
change and easily hydrolyzed under both acid and basic
conditions. Therefore, studies of the pH effect on the fluorescent
responses of probe DER-1 were examined in the absence and
presence of Pd(PPh3)4, respectively. As shown in Fig. 6, the
designed probe was stable for quite a long time (over 60 min) in a
wide pH range of 2.0–10.0 monitored at 547 nm. However, an
obvious fluorescence intensity enhancement was observed above
pH of 10.0, which should be attributed to hydrolysis of the probe
Fig. 3. Time-dependent fluorescence spectra of DER-1 (5 mM) upon addition of Pd
(PPh3)4 (2.0 equiv.). The spectra were recorded in PBS buffer solution (20 mM,
pH = 7.4) containing 50% THF at 37 ꢀC. lex = 500 nm; slits: 5 nm/5 nm.
under strong basic conditions. Once the probe DER-1 (5
mM) was
incubated with 2.0 equiv Pd(PPh3)4, negligible fluorescence were

M. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 25–32
29
Fig. 4. UV–vis absorption (a) and fluorescence (b) spectra of DER-1 (5 m
M) only and in the presence of Pd0 (2 equiv.), Na+, K+, Cu+, Li+, Ag+, Mn2+, Co2+,Ba2+, Mg2+, Ca2+, Hg2+, Zn2
+, Cu2+ Ni2+, Pb2+, Pd2+, Fe2+, Sn2+, Ce3+, Fe3+, Cr3+, Al3+, and Bi3+ (10 equiv., respectively). The spectra were recorded at 1 h after addition of Pd0 in PBS buffer solutions (20 mM,
pH = 7.4) containing 50% THF at 37 ꢀC. lex = 500 nm; slits: 5 nm/5 nm.
observed within the pH range 2.0–5.0, and weak fluorescence
signal was obtained at pH 6.0. It suggested that Pd(PPh3)4 couldn’t
convert probe DER-1 to ring-opening form DER in strong acid
solutions. However, the fluorescence intensity at 547 nm showed
distinct changes over a wide pH range of 7.0–11.0. The results
indicated that the response of probe DER-1 towards Pd0 were
THF at 37 ꢀC. As proposed, the probe DER-1 (5
with different concentrations of Pd(PPh3)4 (0–3
m
m
M) was incubated
M) under above-
mentioned condition and the absorption and fluorescence spectra
were recorded after an interval of 60 min, respectively. The
absorption band at 520 nm enhanced gradually with the increase
of the concentration of Pd(PPh3)4, accompanied with an obvious
color change from colorless to pink (Fig. 7a and Fig. S8a). Also, there
was a good linearity of the absorption at 520 nm versus the Pd
favorable at
a wide pH range of 7.0–11.0 containing the
physiological conditions, and that the probe DER-1 could be
selected as a candidate for Pd0 detection.
(PPh3)4 concentration (0–1.5 m
M) with R2 = 0.98815 (Fig. 7b).
Similarly, the addition of Pd(PPh3)4 to probe DER-1 also led to
significant fluorescent emission spectra changes; the fluorescence
intensity recorded at 547 nm greatly enhanced with the increase of
Pd(PPh3)4 concentration (Figs. 7 c and S8b). Furthermore, the plots
of the fluorescence intensity at 547 nm fit linearly with the Pd
3.5. Sensitivity study
To further explore the recognition ability of probe DER-1
towards Pd0, the absorption and emission titration experiments
were carried out in PBS buffer (20 mM, pH = 7.4) containing 50%
(PPh3)4 concentration range of 0–1.5 mM with a correlation
Fig. 5. Fluorescence intensity of DER-1 at 547 nm in the presence of single metal ion (black) and a mixture of Pd0 (2 equiv.) and various metal ions (10 equiv.) (red). Data were
acquired at 1 h after addition of various metal ions in PBS buffer solutions (20 mM, pH = 7.4) containing 50% THF at 37 ꢀC. lex = 500 nm; slits: 5 nm/5 nm. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version of this article.)

30
M. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 25–32
coefficient of 0.9969 (Fig. 7d). The detection limit of DER-1 (5
for the sensing of Pd0 was calculated to be as low as 1.14 nM
according to a 3
method. This is sufficient sensitive for Pd0
mM)
d
detection in common chemical and industrial samples. Taking all
these results together, DER-1 displayed promising application
prospects for the selective and sensitive detection of Pd0 with
colorimetric and emission outputs.
3.6. Proposed sensing mechanism for the Pd0 detection
The excellent selectivity can be ascribed to the highly
specific Pd0-triggered cleavage process. According to previously
well-received sensing mechanism for Pd0 probes with an
allyoxy allyoxycarbonyl group, it is expected that the trigger
moiety of the allyl chloroformate unit is initially conjugated
with palladium and ionized to form
p-allylpalladium(II)
complex 1, and then dissociating and decarboxylating to release
the ring-opening DER under basic condition (Scheme 2) [36].
The presence or absence of Pd0 can be easily identified according
to the absorption and fluorescence spectrum of testing samples,
because DER-1 and DER have different spectroscopic properties.
Moreover, the fluorescence intensity at 547 nm is positively
Fig. 6. The effect of pH on the fluorescence intensity changes of DER-1 (5 mM) at
547 nm in absence (black) and presence (red) of Pd(PPh3)4 (2 equiv.) in PBS buffer
solution (20 mM, pH = 7.4, 1:1, v/v) at 37 ꢀC. lex = 500 nm; slits: 5 nm/5 nm. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
Fig. 7. (a) UV–vis absorption spectra evolution of DER-1 (5
m
M) in terms of palladium concentrations (0–3
m
M). (b) Absorption intensity at 520 nm of probe DER-1 versus
M). (d) Fluorescence intensity at
increasing concentrations of Pd0. (c) Fluorescence spectra evolution of probe DER-1 (5
mM) in terms of palladium concentrations (0–3 m
547 nm of probe DER-1 versus increasing concentrations of Pd0. All spectra were acquired at 1 h after Pd0 addition at 37 ꢀC in PBS buffer solutions (20 mM, pH = 7.4) containing
50% THF. lex = 500 nm; slits: 5 nm/5 nm.

M. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 25–32
31
Scheme 2. A proposed mechanism for Pd0 sensing by DER-1.
relative to Pd0 concentration, and then the Pd0 content can be
Acknowledgements
determined. To confirm the proposed sensing mechanism, the
reaction product of DER-1 with Pd(PPh3)4 was isolated by
column chromatography (CH2Cl2:EtOH = 20:1). The 1H NMR and
HRMS analysis indicated the free DER was produced during the
detection process. (see ESI, Figs. S9 and S10). Moreover, the fact
that both the absorption and fluorescence spectra of the test
system after addition of Pd(PPh3)4 resembled those of free
fluorophore DER further confirmed the above-mentioned sens-
ing mechanism (see ESI, Fig. S11).
This work is sponsored by Natural Science Foundation of
Shanghai (16ZR1408000), and the National Natural Science
Foundation of China (No. 21576087).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
jphotochem.2017.01.007.
4. Conclusion
References
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carbamate as the recognition unit was rationally designed and
synthesized for Pd0 sensing. The Pd0-triggered cleavage of allyl N,
N-diethylrhodol carbonate to the parent rhodol was used as the
basis of DER-1 for colorimetric and fluorescent signaling of Pd0.
Based on the Pd0-triggered cleavage reaction, the designed probe
shows high sensitivity and selectivity towards Pd0 in PBS buffer
solution under mild condition at 37 ꢀC. Upon addition of 2.0 equiv.
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