Fluorescent detection of palladium species with an O-propargylated fluorescein

Article abstract of DOI:10.1039/c001922d

A fluorescein-based fluorescent probe displays fluorescence enhancement for palladium species in the typical oxidation states of 0, +2 and +4 and is applied to monitor accumulated palladium in living organisms.

Full text of DOI:10.1039/c001922d

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Fluorescent detection of palladium species with an O-propargylated
fluoresceinw
Mithun Santra,a Sung-Kyun Ko,b Injae Shin*b and Kyo Han Ahnz*a
Received 28th January 2010, Accepted 12th April 2010
First published as an Advance Article on the web 7th May 2010
DOI: 10.1039/c001922d
A fluorescein-based fluorescent probe displays fluorescence
enhancement for palladium species in the typical oxidation states
of 0, +2 and +4 and is applied to monitor accumulated
palladium in living organisms.
However, the sensing system requires initial conversion of
Pd(II) to Pd(0) using a reducing agent such as PPh3, TFP
(tri-2-furylphosphine) or TFP–NaBH4.7a The system can be
also used to detect Pd(II) or Pt(IV) directly through a catalytic
aromatic Claisen rearrangement at 50 1C in 1 : 4 DMSO–pH
10 buffer.7b At an optimized pH of 10, the system can also
discriminate Pd from competing Pt species.7c,d
Palladium plays a key role in chemical transformations and is
a catalyst in the synthesis of various molecules including
drugs.1 It is also widely used to prepare dental materials,
electric equipment, automobile exhaust catalysts, and fine
jewellery.2 The metal complexes, however, influence our health
and environment in an adverse way because palladium ions
can bind to thiol-containing amino acids, proteins (casein, silk
fibroin and many enzymes), DNA or other macromolecules
(vitamin B6) and thereby may disturb a variety of cellular
processes.2 As a consequence it is desirable to develop efficient
methods for detecting the presence of palladium species.
Typical analytical methods used for quantification of palladium
species are atomic absorption spectroscopy, inductively
coupled plasma mass spectrometry and X-ray fluorescence.3
Although these methods provide a rapid and extremely sensitive
analysis, they requires complicated sample preparation steps,
rigorous experimental conditions, sophisticated instrumenta-
tion, and well-trained individuals.4 A fluorescence sensing
method can provide an operationally simple and cost-effective
detection method together with high sensitivity and selectivity.
Herein, we describe a novel fluorescent sensing system,
which can selectively detect palladium species from other
metal species through a palladium-catalyzed chemical con-
version. A fluorescein-based propargyl ether derivative 1 is
found to detect palladium species selectively at room temperature
and shows a turn-on type fluorescent response. Significantly,
the probe senses palladium species in all the typical oxidation
states (0, +2 and +4) without adding any additional reagents.
Furthermore, such unique features of the probe enabled us to
apply it for the bioimaging of palladium in living organisms
for the first time.
Propargyl ether 1 was synthesized from commercially
available 2,7-dichlorofluorescein in two steps [(i) propargyl
bromide, K2CO3, DMF, 60 1C, 87%; (ii) DIBAL-H, dichloro-
methane, ꢀ78 - 25 1C; DDQ, 65%].w
Propargyl ether 1 shows very weak fluorescence as the
hydroxyl group is alkylated, but it converts to a highly
fluorescent compound 2 (FF = 0.89)8 in the presence of a
palladium species such as PdCl2 (Scheme 1). Thus, when probe
1 (10 mM) was treated with PdCl2 in water containing 10%
CH3CN at room temperature, the nonfluorescent solution
emitted green fluorescence (lmax = 520 nm) when excited at
l = 480 nm (Fig. 1). It takes about 3 h to reach the saturation
point under dilute conditions, as can be seen from the time-
dependent intensity change plot (Fig. 1, inset). After the
titration, compound 2 was isolated and characterized by
NMR and MS. Therefore, the fluorescence enhancement is
due to the formation of compound 2 formed by a palladium
catalyzed depropargylation reaction.
Palladium complexes exhibit oxidation states of 0, +2 and
2ꢀ
+4 as in the typical cases of Pd(PPh3)4, PdCl2 and PdCl6
,
respectively. Among the palladium species, PdCl2 is the most
toxic. A few fluorescent sensing systems for Pd(II) species are
known, which are composed of sulfur or alkyne groups as
the coordinating ligands for palladium.5 The selectivity
may be a major issue with such coordination based sensing
systems because the ligands for palladium can also bind to
other metal species. This selectivity issue may be alleviated
by fluorescent sensing systems based on metal ion-specific
chemical reactions.6
We then examined the reactivity of the probe toward other
metal species. The fluorescence titration experiment was carried
out under the same conditions by changing metal species.
Recently, Koide and co-workers devised an efficient fluores-
cent sensing system for palladium species based on the Pd(0)-
catalyzed Tsuji–Trost allylic oxidative insertion reaction.7a
a Department of Chemistry and Center for Electro-Photo Behaviors in
Advance Molecular Systems, POSTECH, San 31 Hyoja-dong,
Pohang, 790-784, Republic of Korea. E-mail: ahn@postech.ac.kr
b Center for Biofunctional Molecules, Department of Chemistry,
Yonsei University, Seoul 120-749, Republic of Korea.
E-mail: injae@yonsei.ac.kr
w Electronic supplementary information (ESI) available: Experimental
details and NMR spectra of compounds (1 and 2). See DOI: 10.1039/
c001922d
Scheme
1
Turn-on sensing of palladium species by probe 1
(photos show fluorescence for the solutions in water containing 10%
acetonitrile).
z Ahn K. H. dedicates this paper to the late Professor Chi Sun Hahn in
admiration of his contributions to organic chemistry of Korea.
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experiment no additional reagents were added to change the
oxidation state of the palladium species.
Interestingly, probe 1 responds to all the oxidation states of
palladium [Pd(0): Pd(PPh3)4; Pd(II): PdCl2, PdCl2(CH3CN)2,
Pd(OAc)2, Na2PdCl4; Pd(IV): (NH4)2PdCl6] and exhibits
significant fluorescent enhancement (Fig. 3). This is an impor-
tant feature of the present system over the known systems.7
Terminal propargyl ethers are cleaved by palladium catalysts
such as (PPh3)2PdCl2 or (Ph3P)4Pd (in DMF–H2O at 80 1C for
2–4 h) in the presence of Et3N, which is known to reduce Pd(II)
into Pd(0) in the catalytic reaction.9 They suggested the
formation of an allenylpalladium intermediate, which will
form through an oxidative addition of Pd(0) into the alkyne
C–H bond. Also, propargyl ethers are catalytically cleaved
with several Pd(II) species in the absence of any additional
reagents (in benzene at 60–65 1C for 7–15 h).10 A different
possibility is the hydration of a propargyl ether catalyzed by
Pd(II)/Pd(IV) species to form an internal and a terminal carbonyl
compound, the latter of which can undergo b-elimination
(depropargylation). We followed the depropargylation reac-
tion by in situ NMR studies and were able to observe only the
depropargylated product 2 but not the other keto compound,
suggesting that if the hydration process occurs, it is highly
regioselective. Also, we synthesized a methylpropargyl ether
(3) and examined the depropargylation reaction with Pd(0),
Pd(II) and Pd(IV) species [(Ph3P)4Pd, PdCl2, Na2PdCl4].
Methylpropargyl ether 3 underwent the depropargylation with
the Pd(II) and Pd(IV) species, albeit rather slowly compared to
the case of probe 1; however, the deparpargylation reaction
did not proceed with Pd(0) species. These results together with
literature data suggest that the depropargylation can proceed
either via allenylpalladium in the case of terminal propargyl
ethers, through the oxidative addition of Pd(0) species added
(or generated in situ, although less probable in the present
case) or via Pd(II)/Pd(IV)-catalyzed hydration intermediates in
the cases of terminal and internal propargyl ethers.
Fig. 1 Time-dependent fluorescence change for an equimolar mixture
of probe 1 (10 mM) and PdCl2 in water containing 10% CH3CN at
25 1C. Inset: time-dependent intensity change.
Fluorescence spectral data reveal that probe 1 selectively
responds to Pd(II) among various metal sources examined
(Fig. 2). Other metal sources such as Cr(II), Ca(II), Co(II),
Cu(II), Ni(II), Mg(II), Hg(II), Mn(II), Cd(II), Ag(I), Ba(II), Zn(II),
Pb(II), Pt(II), Fe(III), Cr(III), Fe(II), Pt(IV), Rh(III), Ru(III) and
Rh(I) show little fluorescence or sometimes quench the fluores-
cence. A small enhancement in the fluorescence was observed
only in the case of Au(III) that has high p-electrophilicity.
The palladium content in persons with amalgam fillings and
metallic dental appliances was determined to be 10.6 ꢂ 7.4 mg Lꢀ1
from samples of morning saliva.2 The detection limit (based
on the criteria of fluorescence enhancement in the low con-
centration range and a signal-to-noise ratio of 43) of probe
1 for PdCl2 is determined to be 0.03 mM (5.3 mg Lꢀ1; Pd
content = 3.2 mg Lꢀ1) (see ESIw).
Next, we examined the fluorescence response of probe 1
toward several typical palladium species in the oxidation states
of 0, +2 and +4. As mentioned above, in this sensing
As probe 1 senses Pd(II) species selectively among various
metal species with no additional reagents, it may be useful in
monitoring of Pd(II) in living organisms.
Fig. 2 Comparison of the fluorescence enhancement observed for a
1 : 1 mixture of probe 1 (10 mM) and each of metal species [P1, probe 1
only; A, Cr(II); B, Ca(II); C, Co(II); D, Cu(II); E, Ni(II); F, Mg(II); G,
Hg(II); H, Mn(II); I, Cd(II); J, Ag(I); K, Ba(II); L, Zn(II); M, Pb(II); N,
Pt(II); O, Fe(III); P, Au(III); Q, Cr(III); R, Fe(II); S, Pt(IV); T, Rh(III); U,
Ru(II); V, Rh(I); W, Pd(II)], measured after 1 h in water containing
10% CH3CN at 25 1C. Metal sources used are chloride complexes
except for Pt(IV) and Rh(I), for which H2PtCl6 and RhCl(PPh3)2 were
used, respectively (see ESIw).
Fig. 3 Comparison of the fluorescence change depending on different
palladium complexes, measured for an equimolar mixture of probe 1
(10 mM) and the palladium species in water containing 10% CH3CN at
25 1C after 1 h: P1, probe 1; A, Pd(PPh3)4; B, PdCl2; C,
PdCl2(CH3CN)2; D, Pd(OAc)2; E, (NH4)2PdCl6; F, Na2PdCl4.
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demonstrate for the first time that probe 1 is potentially useful
for monitoring palladium species in living organisms (Fig. 5)
(see ESIw).
In conclusion, we have developed a reaction-based turn-on
fluorescent probe for palladium species. An O-propargylated
fluorescein senses several palladium species in the oxidation
states of 0, +2 and +4 with no additional reagents through a
depropargylation reaction, which gives a turn-on type fluores-
cence change. The reaction is specific toward the palladium
species and little or slight fluorescent changes result in the
cases of various other metal species. The probe is used for the
fluorescent imaging of palladium chloride in a living species,
which demonstrates its potential usefulness as a molecular
probe in biological systems.
Fig. 4 Five-day-old zebrafish incubated with probe 1 (20 mM) and
various concentrations of PdCl2 (from top to bottom; 0, 5, 10, 20 mM)
(left; phase contrast image, right; fluorescence microscopic image).
This work is supported by the EPB center (R11-2008-052-
01001) and the National Creative Research Initiative Program.
Five-day-old zebrafish was incubated with probe 1 (20 mM)
and various concentrations (0, 5, 10, 20 mM) of PdCl2 in
incubation media for 30 min at 28 1C. The results of fluores-
cence microscope analysis of these specimens show that
palladium ions in zebrafish are fluorescently detectable by
the probe (Fig. 4).w
We then made an attempt to monitor the accumulated
palladium species in living organisms using the probe. For these
studies, adult zebrafish (three months old with identifiable
organs) were pre-incubated with 500 nM PdCl2 in media for
24 h. Then, the treated zebrafish was further exposed to probe
1 (20 mM) for 30 min. Zebrafish was dissected to isolate tissues
and organs that were examined using fluorescence microscopy.
Palladium species were detected in the brain, eye and fin,
and weakly seen in the heart and liver. These in vivo studies
showed that probe 1 can enter fish and then reacts with
palladium species to form the fluorescent product. The results
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3966 | Chem. Commun., 2010, 46, 3964–3966