A new ratiometric ESIPT sensor for detection of palladium species in aqueous solution

Article abstract of DOI:10.1039/c2cc17677g

An aqueous ratiometric ESIPT sensor with a 87 nM (15.4 ppb) detection limit was successfully synthesized and applied for detection of all oxidation states of palladium species. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17677g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 2867–2869
www.rsc.org/chemcomm
COMMUNICATION
A new ratiometric ESIPT sensor for detection of palladium species in
aqueous solutionw
Bin Liu, Hu Wang, Taisheng Wang, Yinyin Bao, Fanfan Du, Jiao Tian, Qianbiao Li and
Ruke Bai*
Received 8th December 2011, Accepted 12th January 2012
DOI: 10.1039/c2cc17677g
An aqueous ratiometric ESIPT sensor with a 87 nM (15.4 ppb)
detection limit was successfully synthesized and applied for
detection of all oxidation states of palladium species.
sensors are highly desired for detection of palladium species
with a large Stokes’ shift.
Here we present a new aqueous ratiometric sensor 3-(prop-
2-ynyloxy)hydroxyflavone (1) based on excited-state intra-
molecular photon-transfer (ESIPT) as a probe for Pd species.
The experimental results show that this new sensor has
excellent properties for detection of Pd species. The proposed
sensing mechanism is shown in Scheme 1 via depropargylation
of 1 catalyzed by Pd species. In order to design a new sensor,
we chose 3-hydroxylflavone (2) as the fluorophore which has a
large Stokes’ shift (>150 nm) because 3-hydroxylflavone can
undergo an excited-state intramolecular photon-transfer
(ESIPT) process upon photoexcitation whereby rapid photo-
induced proton transfer results in tautomerization, as shown
in Scheme 2. Excitation of 2, which exists as a normal isomer
(N) in the ground state, leads to an adiabatic ESIPT reaction
and results in the formation of the tautomer (T*). Blue
fluorescence occurs from N*, whereas green fluorescence
In view of extensive applications of palladium species in fuel cells
and catalyst industry,1 it is possible that food, medicine and
water may be contaminated with palladium species. Palladium
may cause a serious health problem owing to the formation of
complexes between Pd and some biomacromolecules such as
proteins, DNA and RNA.2 Thus, government restrictions on the
levels of residual heavy metals in end products are very strict,
and its threshold for palladium is 5–10 ppm.3 Therefore, it is
necessary and important to urgently develop an effective and
convenient method for detecting palladium species. Conventional
analytical methods can be used for the detection of palladium
in biological and environmental samples, including atomic
absorption spectrometry, plasma emission spectroscopy, solid-
phase microextraction-high performance liquid chromatography,
X-ray fluorescence and inductively coupled plasma mass spectro-
metry (ICP-MS).4 However, these methods often require sophisti-
cated and time-consuming sample preparation procedures or
expensive equipments. Thus, recently significant effort has gone
into developing fluorescent methods, because of their low cost,
simplicity and sensitivity for palladium species.5
Very recently, we have also reported some conjugated
fluorescent sensors based on aggregation-induced fluorescent
quenching mechanism for palladium detection.6 However,
we noticed most of the sensors based on a coordination
mechanism suffered varying degrees of interference from other
transitional metal ions, in contrast, sensors based on a
catalytic mechanism usually show excellent selectivity. Also,
most of the fluorescent sensors for palladium possess a small
Stokes’ shift, which is an extremely serious limitation in
quantitative determination and bioimaging because of the
excitation interference.7 Thus, catalytic mechanism-based
Scheme 1 Proposed depropargylation process of 1 via Pd species
catalytic reaction.
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer
Science and Engineering, University of Science and Technology of China,
Hefei, P. R. China 230026. E-mail: bairk@ustc.edu.cn;
Fax: 0086-551-3631760; Tel: 0086-551-3600722
w Electronic supplementary information (ESI) available: Synthetic
procedures, experimental details, additional spectroscopic data. See
DOI: 10.1039/c2cc17677g
Scheme 2 General mechanism of ESIPT process and chemical structures
of normal isomer (N) and tautomer (T) in the ESIPT process of 2.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2867–2869 2867

View Article Online
originates from T*.8 Modification of the hydroxyl group of 2
can block the ESIPT process and result in quenching of the
T emission. Conversely, regeneration of the free hydroxyl group
can restore the dual emission associated with N–T tautomerism.
In view of their large Stokes’ shift and dual emission, ESIPT
compounds have been successfully applied to sensors9 for fluoride
and cysteine. However, to the best of our knowledge, no report
has been published on sensors for palladium detection based on
ESIPT signal transduction mechanism.
Fig. 2 Fluorescence spectra changes (a) and fluorescence intensities
ratio (I517/I412) (b) of 1 (10 mM) in the presence of increasing concentra-
tions of PdCl2 (concentrations: 0, 0.001, 0.003, 0.005, 0.007, 0.01, 0.013,
0.016, 0.019, 0.022, 0.025, 0.03, 0.04, 0.05, 0.075, 0.1 mM) in
CH3CN–H2O (1 : 4, v/v) solution with HEPES buffer (10 mM).
The synthetic approach for sensor 1 is shown in ESIw. First,
the reaction between benzaldehyde and 20-hydroxyacetophenone
in MeOH afforded 3 in a yield of 67% and then 3 was oxidized
with H2O2 to obtain 2 in 43% yield. Finally, propargylation of
the hydroxyl group of 2 was achieved with 3-bromo-1-propyne
to give sensor 1 in a yield of 53%. The chemical structure of
Fig. 2a shows the fluorescence changes with various amounts
of PdCl2 in CH3CN–H2O (1 : 4, v/v) solution with HEPES buffer
(10 mM) after 3 h. When the concentration of PdCl2 is gradually
increased, the fluorescence peak at l = 412 nm shows a
10–15 nm red shift with a slow decrease. At the same time, a
new fluorescence peak occurred at l = 517 nm and then becomes
the maximum peak. It is observed that the ratios of fluorescence
intensities (I517/I412) change from 0.13 to 2.43 (R = 18.7-fold)
(Fig. 2b). More importantly, the ratios of the fluorescent
intensities are linearly proportional to the amount of PdCl2 from
0 to 30 mM. The ratiometric fluorescent measurement which is
based on the ratio of two fluorescent bands instead of the
absolute intensity of one band, makes it possible to measure
the analyte more accurately and sensitively with minimization of
the background signal.12 If we arbitrarily define the detection
limit as the Pd concentration at which a 10% increase in
fluorescence emission intensity ratio (I517/I412) can be measured
by employing 10 mM sensor 1 in CH3CN–H2O (1 : 4, v/v) solution
with HEPES buffer (10 mM), the detection limit of PdCl2 is
calculated to be 87 nM (15.4 ppb, Pd content = 9.2 ppb), which
is far below the allowable level of palladium ions (5–10 ppm)
from the European Agency for the Evaluation of Medicinal
Products (EMEA).13 The sensitivity of sensor 1 was also measured
in pure water which is particularly important for detection in
drinking water (Fig. S4 and Fig. S5 ESIw). Such sensitivity
readily satisfies the requirement of medical and environmental
monitoring.
1
compound 1 was verified by HRMS, H NMR and 13C NMR
spectroscopy (see ESIw). UV-Vis absorption and fluorescence
spectra of 1 and 2 were measured in CH3CN–H2O (1 : 4, v/v)
solution buffered at pH 7.0 (HEPES, 10 mM) at room temperature
(Fig. S1 and S2, ESIw). Fluorescence spectra shows that compound
2 can emit dual emission maximum at 412 nm (N form) and
517 nm (T form), while compound 1 only emits the N form
emission at 412 nm, as expected.
The fluorescence sensing behavior of 1 toward palladium
was investigated using a 10 mM solution of 1 in CH3CN–H2O
(1 : 4, v/v) buffered at pH 7.0 (HEPES, 10 mM). PdCl2 was
selected as the representative palladium species in the following
titration experiments since it is among the most toxic of palladium
compounds. As shown in Fig. 1a, upon addition of 0.1 mM
PdCl2, the emission at l = 412 nm initially decreases. This may be
attributed to that a Pd2+ addition-induced intermediate product
with a carbon–carbon double bond (Scheme 2) leads to fluores-
cence quenching through a photoinduced electron-transfer (PET)
process.10,11 After 30 min, the emission band slowly increases and
gradually levels off. This is because the PET-induced fluorescence
quenching terminates along with the completion of depropargyl-
ation. Meanwhile deprotection of the hydroxyl group of 1 results
in the formation of 2, which exhibits ESIPT process, and the
T-form emission band at l = 517 nm gradually grows. It takes
about 3 h to reach the relative saturation point, as can be seen
from the plot of the time-dependent intensity change (Fig. 1b).
The mechanism of the palladium catalyzed depropargylation was
verified by LC-MS. The peak at m/z 239.0700 [M ꢀ H]+
corresponding to compound 2 was observed in the LC-MS spectra
of the product mixture (Fig. S3 ESIw). Therefore, the fluorescence
change is due to the formation of compound 2 by the palladium-
catalyzed depropargylation reaction.
To demonstrate the potential application of this palladium-
specific method, we tested whether it could detect all the oxidation
states of palladium [Pd(0): Pd(PPh3)4; Pd(II): PdCl2,
PdCl2(PPh3)2, PdCl2(dppf)2; Pd(IV): K2PdCl6]. According to the
intensity ratios (I517/I412), sensitivities of sensor 1 to palladium
species was in the order: PdCl2 > Pd(PPh3)2Cl2 > K2PdCl6 >
Pd(PPh3)4 > PdCl2(dppf)2, as shown in Fig. 3. The results reveal
that chemosensor 1 remarkably responds to all the oxidation
states of palladium with significant fluorescence changes. These
results prove once again that the depropargylation reaction
can occur not only via Pd(IV)/Pd(II)-catalyzed hydration inter-
mediates, but also via allenylpalladium resulting from the
oxidative addition of Pd(0) without additional reagents, as shown
in Scheme 1.14 On the other hand, PdCl2 and K2PdCl6 induce the
largest enhancement of the fluorescent intensity at l = 517 nm.
Due to the wide use of the aqueous solutions of these two
palladium ions in plating baths, chemosensor 1 is particularly
important for detection in environmental water.
Fig. 1 Time-dependent fluorescence spectral changes (a) and intensity
changes (b) of 1 (10 mM) in the presence of 0.1 mM PdCl2 in
CH3CN–H2O (1 : 4, v/v) solution with HEPES buffer (10 mM).
c
2868 Chem. Commun., 2012, 48, 2867–2869
This journal is The Royal Society of Chemistry 2012

View Article Online
and Y. Tsuji, J. Am. Chem. Soc., 2004, 126, 6554; (c) M. Lafrance
and K. Fagnou, J. Am. Chem. Soc., 2006, 128, 16496.
2 (a) J. C. Wataha and C. T. Hanks, J. Oral Rehabil., 1996, 23, 309;
(b) Palladium, Environmental Health Criteria Series 226, International
Programme on Chemical Safety, World Health Organization, Geneva,
2002; (c) J. Kielhorn, C. Melber, D. Keller and I. Mangelsdorf, Int. J.
Hyg. Environ. Health, 2002, 205, 417.
3 C. E. Garrett and K. Prasad, Adv. Synth. Catal., 2004, 346, 889.
4 (a) B. Dimitrova, K. Benkhedda, E. Ivanova and F. Adams,
J. Anal. At. Spectrom., 2004, 19, 1394; (b) C. Locatelli,
D. Melucci and G. Torsi, Anal. Bioanal. Chem., 2005, 382, 1567;
(c) K. Van Meel, A. Smekens, M. Behets, P. Kazandjian and
R. Van Grieken, Anal. Chem., 2007, 79, 6383.
5 (a) E. Unterreitmaier and M. Schuster, Anal. Chim. Acta, 1995,
309, 339; (b) K. Kubo, Y. Miyazaki, K. Akutso and T. Sakurai,
Heterocycles, 1999, 51, 965; (c) B. K. Pal and M. S. Rahman,
Microchim. Acta, 1999, 131, 139; (d) Y. J. Fang, H. Chen,
Z. X. Gao and X. Y. Jin, Indian J. Chem., Sect. A., 2002,
41, 521; (e) B. Tang, H. Zhang and Y. Wang, Anal. Lett., 2004,
37, 1219; (f) H. Huang, K. Wang, W. Tan, D. An, X. Yang,
S. Huang, Q. Zhai, L. Zhou and Y. Jin, Angew. Chem., Int. Ed.,
Fig. 3 Comparison of the fluorescence spectra changes (a) and
intensity ratio (I517/I412) changes on different palladium complexes,
measured for a mixture of sensor 1 (10 mM) and the palladium species
(100 mM) in CH3CN–H2O (1 : 4, v/v) solution with HEPES buffer
(10 mM): 1, sensor 1 only; 2, Pd(PPh3)4; 3, PdCl2; 4, Pd(PPh3)2Cl2; 5,
PdCl2(dppf)2; 6, K2PdCl6.
2004, 43, 5635; (g) T. Schwarze, H. Muller, C. Dosche,
¨
¨
T. Klamroth, W. Mickler, A. Kelling, H. G. Lohmannsroben,
¨
P. Saalfrank and H. J. Holdt, Angew. Chem., Int. Ed., 2007,
46, 1671; (h) F. Song, A. L. Garner and K. Koide, J. Am. Chem.
Soc., 2007, 129, 12354; (i) L. P. Duan, Y. F. Xu and X. H. Qian,
Chem. Commun., 2008, 6339; (j) R. J. T. Houk, K. J. Wallace,
H. S. Hewage and E. V. Anslyn, Tetrahedron, 2008, 64, 8271;
(k) A. L. Garner and K. Koide, Chem. Commun., 2009, 86;
(l) A. L. Garner, F. Song and K. Koide, J. Am. Chem. Soc.,
2009, 131, 5163; (m) H. Li, J. Fan, J. Du, K. Guo, S. Sun, X. Liu
and X. Peng, Chem. Commun., 2010, 46, 1079; (n) F. L. Song,
E. J. Carder, C. C. Kohler and K. Koide, Chem.–Eur. J., 2010,
16, 13500; (o) H. Li, J. Fan, F. Song, H. Zhu, J. Du, S. Sun and
X. Peng, Chem.–Eur. J., 2010, 16, 12349; (p) M. Santra, S.-K. Ko,
I. Shin and K. H. Ahn, Chem. Commun., 2010, 46, 3964;
(q) M. E. Jun and K. H. Ahn, Org. Lett., 2010, 12, 2790;
(r) J. Jiang, H. Jiang, W. Liu, X. Tang, X. Zhou, W. Liu and
R. Liu, Org. Lett., 2011, 13, 4922; (s) S. Goswami, D. Sen,
N. K. Das, H. K. Fun and C. K. Quahb, Chem. Commun., 2011,
47, 9101; (t) B. Zhu, C. Gao, Y. Zhao, C. Liu, Y. Li, Q. Wei,
Z. Ma, B. Du and X. Zhang, Chem. Commun., 2011, 47, 8656.
6 (a) B. Liu, Y. Bao, F. Du, H. Wang, J. Tian and R. Bai, Chem.
Commun., 2011, 47, 1731; (b) B. Liu, H. Dai, Y. Bao, F. n. Du,
J. Tian and R. Bai, Polym. Chem., 2011, 2, 1699.
7 (a) K. Hanaoka, K. Kikuchi, H. Kojima, Y. Urano and
T. Nagano, J. Am. Chem. Soc., 2004, 126, 12470; (b) F. Qian,
C. Zhang, Y. Zhang, W. He, X. Gao, P. Hu and Z. Guo, J. Am.
Chem. Soc., 2009, 131, 1460.
8 (a) P. K. Sengupta and M. Kasha, Chem. Phys. Lett., 1979, 68, 382;
(b) K. M. Prasun and S. Anunay, J. Phys. Chem. A, 2003,
107, 6334.
9 For selected examples, see: (a) T. I. Kim, H. J. Kang, G. Han,
S. J. Chung and Y. Kim, Chem. Commun., 2009, 5895;
(b) F. Wang, J. Wu, X. Zhuang, W. Zhang, W. Liu, P. Wang
and S. Wu, Sens. Actuators, B, 2010, 146, 260; (c) R. Hu, J. Feng,
D. H. Hu, S. Q. Wang, S. Y. Li and G. Q. Yang, Angew. Chem.,
Int. Ed., 2010, 49, 4915; (d) X. Yang, Y. Guo and R. M. Strongin,
Angew. Chem., Int. Ed., 2011, 50, 1; (e) M. Santra, B. Roy and
K. H. Ahn, Org. Lett., 2011, 13, 3422.
Fig. 4 Fluorescence spectral changes (a) and fluorescence intensities ratio
(I517/I412) (b) of 1 upon addition of different metal cations in CH3CN–H2O
(1 : 4, v/v) solution with 10 mM of HEPES buffer. (1, sensor 1 only; 2,
Ag+; 3, Ca2+; 4, Cd2+; 5, Co2+; 6, Cu2+; 7, Fe3+; 8, Hg2+; 9, Mg2+; 10,
Ni2+; 11, Zn2+; 12, Pd2+): [1] 10 mM, [metal ion] = 0.5 mM.
An exceptional feature of this sensor is its very high selectivity
towards the target analyte over other competitive species. The
selectivity of chemosensor 1 toward different metal ions was
investigated and results are shown in Fig. 4. The results indicate
that the sensor 1 displays a selective fluorophores behavior
toward palladium ion with a fluorescence change from blue to
green, which can be easily seen by the naked eye (Fig. 4a inset).
Under the same conditions, nearly no ratiometric response
changes are observed in the presence of 0.5 mM of Ag+,
Ca2+, Cd2+, Co2+, Cu2+, Fe3+, Hg2+, Mg2+, Ni2+, Zn2+ in
CH3CN–H2O (1 : 4, v/v) solution with HEPES buffer (10 mM).
These results demonstrate that chemosensor 1 possesses excellent
selectivity towards palladium species.
In conclusion, we have developed a chemical reaction-based
ratiometric ESIPT fluorescent sensor 1 specific for palladium
species with a large Stokes’ shift of more than 150 nm. The
results demonstrate that sensor 1 exhibits a high sensitivity and
an excellent selectivity for Pd2+ detection in aqueous solution,
with detection limit down to B87 nM (15.4 ppb). Moreover,
obvious ratiometric responses are observed towards all the
oxidation states of palladium among other transition metal ions
without additional reagents. This work provides a new mild and
promising strategy for the detection of Pd species in biological
and environmental systems.
10 L. Yi, H. Y. Li, L. Sun, L. L. Liu, C. H. Zhang and Z. Xi, Angew.
Chem., Int. Ed., 2009, 48, 4034.
11 T. Matsumoto, Y. Urano, T. Shoda, H. Kojima and T. Nagano,
Org. Lett., 2007, 9, 3375.
12 (a) B. Zhu, C. Gao, Y. Zhao, C. Liu, Y. Li, Q. Wei, Z. Ma, B. Du
and X. Zhang, Chem. Commun., 2011, 47, 8656; (b) D. Domaille,
W. L. Zeng and C. J. Chang, J. Am. Chem. Soc., 2010, 132, 1194;
(c) Z. Xu, K. H. Baek, H. Kim, J. Cui, X. Qian, D. Spring, I. Shin
and J. Yoon, J. Am. Chem. Soc., 2010, 132, 601.
13 Note for Guidance on Specification Limits for Residues of Metal
Catalysts, The European Agency for the Evaluation of Medicinal
Products, Evaluation of Medicines for Human Use, London,
17th December 2002; http://www.emea.eu.int.
The authors acknowledge financial support from the Ministry
of Science and Technology of China (NO. 2007CB936401).
Notes and references
14 M. Pal, K. Parasuraman and K. R. Yeleswarapu, Org. Lett., 2003,
5, 349.
1 (a) J. Le Bars, U. Specht, J. S. Bradley and D. G. Blackmond,
Langmuir, 1999, 15, 7621; (b) T. Iwasawa, M. Tokunaga, Y. Obora
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2867–2869 2869