The emission enhancement of the NIR distyryl Bodipy dyes by the indirect S0 → S2 excitation and their application towards a Hg2+ probe

Article abstract of DOI:10.1039/c2jm30963g

A new strategy for enhancing the brightness of the NIR distyryl Bodipy derivatives was proposed based on the indirect S₀ → S ₂ excitation, and applied to fluorescence turn-on detection of Hg²⁺, by which the marked fluorescence enhancement (2.5-fold) and the high sensitivity for Hg²⁺ (up to 3 ppb) could be realized.

Full text of DOI:10.1039/c2jm30963g

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The emission enhancement of the NIR distyryl Bodipy dyes by the indirect
S0 / S2 excitation and their application towards a Hg2+ probe†
*
Yun Zhao, Xin Lv, Yunlong Liu, Jing Liu, Yan Zhang, Heping Shi and Wei Guo
Received 16th February 2012, Accepted 23rd April 2012
DOI: 10.1039/c2jm30963g
Knoevenagel reaction of the 3- and 5-methyls with aromatic aldehyde
is a simple and efficient method, and the obtained distyryl Bodipys
have the desirable characteristics including moderate to strong
emission in the NIR region, good photostability and solubility in
many solvent systems. As a result, they have been widely studied as
fluorescent probes for metal ions,6 building blocks for artificial
photosynthetic models,7 and photosensitisers for photodynamic
therapy8 and dye-sensitised solar cells.9 However, there is still a need
to improve the brightness of this type of dyes for their wider
applications.
A new strategy for enhancing the brightness of the NIR distyryl
Bodipy derivatives was proposed based on the indirect S0 / S2
excitation, and applied to fluorescence turn-on detection of Hg2+, by
which the marked fluorescence enhancement (2.5-fold) and the high
sensitivity for Hg2+ (up to 3 ppb) could be realized.
The development of functional dyes that emit in the near-infrared
(NIR) region of the optical spectrum has received increasing attention
in the last decade.1–4 Besides their high significance for photophysical
technologies, NIR fluorophores are of enormous importance for
biological and medicinal applications.2 The NIR spectral region is
often referred to as the optical window in biological tissues because
autoabsorption and autofluorescence of biomolecules are least
between 650 and 1200 nm combined with low light scattering and
deep tissue penetration of NIR radiation.3 Thus, NIR fluorophores
provide an excellent tool for bioimaging with substantially increased
sensitivity compared to ultraviolet (UV) or visible (Vis) fluorophores.
From the sensitivity point of view, an important criterion that eval-
uates the quality of a fluorophore is its brightness. An ideal fluo-
rophore should feature sharp and intense absorption spectra
combined with a high fluorescence quantum yield. However, because
of their inherent low band gaps, NIR dyes often suffer from
a dominant vibronic coupling between ground and excited states
which promotes nonradiative deactivation pathways. As a result,
most NIR dyes show low fluorescence efficiency.
A prevalent method for enhancing the brightness of fluorophores
is the exploration of multifluorophores with energy-donor–acceptor
architectures, by which the amplified emission of acceptors could be
observed when these systems are excited at the donor lmax due to the
efficient fluorescence resonance energy transfer (FRET), the so-called
antenna effect.10 Herein, we hope to present another strategy to
enhance the brightness of certain single conjugated organic dyes by
the indirect S0 / S2 excitation by reference to a NIR di(methoxy-
styryl)-Bodipy fluorophore (B1) (Fig. 1). The obtained results
demonstrated that, by this strategy, the molecule shows a 2.5-fold
emission enhancement compared with the direct S0 / S1 excitation.
Furthermore, with B1 as a platform, we also developed a PET
(photo-induced electron transfer)-based fluorescence turn-on che-
mosensor, B2, by introducing a known Hg2+ binding group11 to B1,
and its highly sensitive fluorescence turn-on responses towards Hg2+
by the indirect excitation justified our idea.
Traditionally, cyanine dyes are the most widely used NIR chro-
mophores in various applications.4 However, whereas their absorp-
tion characteristics are very favorable, their fluorescence quantum
yields often do not exceed moderate values (5–15%). As a promising
alternative, borondipyrromethene (Bodipy) derivatives have gained
huge popularity in many fields.5 Bodipy dyes can be seen as rigid,
cross-conjugated cyanines. The simple Bodipy core absorbs and emits
in the 480–540 nm region. Long wavelength absorbing and emitting
Bodipys are often obtained by extending the conjugation of the
Bodipy chromophore via functionalization of the pyrrolic position.
Among these, the distyryl modification of the Bodipy core by
In fact, the indirect S0 / S2 excitation followed by the S2 / S1
internal conversion is a way of dissipating excess electronic energy,
and is important in the protection of biological molecules from
School of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan 030006, China. E-mail: guow@sxu.edu.cn; Fax: +86-351-
7011600; Tel: +86-351-7011600
† Electronic supplementary information (ESI) available: Experimental
procedures, supplemental data, and the 1H-, 13C-NMR spectra. See
DOI: 10.1039/c2jm30963g
Fig. 1 Chemical structures of B1 and B2.
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harmful UV radiation.12 However, the known photophysical process
has not previously been specifically exploited and applied in the
sensing field, presumably because many excellent dyes, such as
rhodamine, fluorescein, simple Bodipys, and cyanine derivatives, only
display the weak S0 / S2 transition, thus being less effective for
absorbing high-energy photons. It was interesting to note that, in the
absorption spectra of B1, despite the typical S0 / S1 transition
located at 640 nm, there exists an intense high energy absorption
band located at 370 nm, which can be assigned to the S0 / S2
transition (Fig. 2).13 The assignment could be rationalized by density
function theory (DFT) calculations at the B3LYP/6-31G(d) level of
the Gaussian 03 program (Fig. S3, Table S1 and S2 in the ESI†),
where two main electronic transitions (S0 / S1, oscillator strength fab
¼ 1.12; and S0 / S2, oscillator strength fab ¼ 1.54) were obtained.
The calculated absorption bands were 613 nm and 360 nm, respec-
tively, corresponding to the two absorption peaks at 640 nm and
370 nm obtained experimentally. Noteworthy is that the S0 / S2
transition has a big oscillator strength f (fab 1.54), corresponding to
the intense high energy absorption band located at 370 nm in
absorption spectra of B1. Inspection of the molecular orbitals
involved in the electronic transitions shows that in the S1 state, the
electronic delocalization is mainly on the Bodipy core, and less on
methoxystyryl moieties; however, in the S2 state, the electronic
delocalization is mainly on the methoxystyryl moiety, and less on the
Bodipy core (Fig. 3). Thus, the strong S0 / S2 transition appears to
be linked to the 3,5-substituted methoxystyryl moieties. In fact, the
3,5-dimethoxystyryl substituted Bodipy B1 is a typical D–A–D
(donor–acceptor–donor) system that usually has the big S0 / S2
transition-related two-photo absorption action cross-section.13,14
Thus, we speculated that the observed strong S0 / S2 transition in
B1 is probably due to its D–A–D structural feature that leads to the
obvious symmetric charge transfer upon excitation, and the
enhancement of the corresponding transition dipole moment.14
Similarly, the obvious S0 / S2 transition could also be found in other
3,5-distyryl-Bodipys,9,15 3,5-di(4-methoxyphenyl)-Bodipy,16 and 3,5-
difuranylvinyl-Bodipys,17 all of which possess the D–A–D motif.
Turning our attention now to the excitation spectrum of B1, the
corrected fluorescence excitation spectrum recorded by monitoring
fluorescence at 655 nm was nearly in agreement with the ground-state
absorption spectrum (Fig. 2), suggesting that both the S0 / S2 and
S0 / S1 transitions participate in the emission process. Importantly,
the shorter wavelength excitation peak is more dominant in the
Fig. 3 The energy levels and interfacial plots of the orbitals for B1.
excitation spectrum; thus, the fluorescence enhancement by the
indirect excitation can be expected. In fact, when exciting B1 at the
S0 / S2 transition (lex ¼ 370 nm), fluorescence was only observed
from the S1 state (lem ¼ 655 nm), indicative of the rapid and almost
quantitative S2 / S1 internal conversion.18 Moreover, compared
with the direct S0 / S1 excitation (lex ¼ 640 nm), the molecule shows
a 2.5-fold emission enhancement, an important feature for practical
applications.
Because the photon number absorbed is dependent on the wave-
length and it increases with a decrease in the wavelength, the observed
emission enhancement of B1 is probably a result of the fact that more
numbers of the high-energy photon are absorbed by the indirect
S0 / S2 excitation than by the direct S0 / S1 excitation, as well as
the efficient S2 / S1 internal conversion process, though the S0 / S2
transition is less intense than the S0 / S1 transition. In fact, the
significant increase in core emission in light-harvesting molecules is
mainly attributed to the large light-harvesting capability of the donor
fluorophores located at the periphery of these molecules,10 though the
efficient energy transfer from the donor to the core is another
important factor.
Because mercury is a deadly toxin to humans by causing cell
dysfunction,19 development of the highly sensitive and specific
mercury probes, which are applicable to the common biological
milieux, is an important goal.20 As a preliminary illustration of the
utility of the above-mentioned strategy, with B1 as a platform we
created the fluorescent probe B2 as a Hg2+ probe by introducing
a known Hg2+ binding group11 to B1 (Fig. 4). The detailed synthetic
Fig. 2 The absorption, excitation and emission spectra of B1 (5 mM) in
EtOH.
Fig. 4 The proposed sensing mechanism of B2 to Hg2+
.
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procedures are provided in the ESI†. In subsequent experiments,
CH3CN–HEPES buffer solution (10 mM, pH ¼ 7.4, 1 : 1, v/v)
was selected as a testing system to investigate the spectra response of
B2 to Hg2+.
In the absorption spectra of B2 (5 mM), despite the typical S0 / S1
transition located at 640 nm, a strong S0 / S2 transition located at
370 nm was also observed (Fig. S1, ESI†). The Hg2+ addition results
in a slightly red shift of the long-wavelength band and an isosbestic
point at 645 nm (Fig. S1, ESI†), suggesting that the binding of B2
with Hg2+ produces a single component. However, unlike B1, B2
displayed a considerably weak fluorescence at 655 nm (Fig. 5) due to
the efficient PET quenching from the aniline-containing binding
group to the Bodipy fluorophore.11 The PET quenching process in B2
could be rationalized by density function theory (DFT) calculations
(Fig. S2, ESI†). As expected, upon addition of Hg2+, the fluorescence
intensity increased remarkably with the virtually unchanged emission
shift, suggesting that the PET quenching is inhibited due to the
binding of B2 with Hg2+. Importantly, when the high energy part
of the absorption spectra of B2 was excited (lex ¼ 370 nm), the
fluorescence intensity of Hg2+-bound B2 was significantly higher
Fig. 6 Fluorescence spectra of B2 (5 mM) in the presence of different
amounts of Hg2+. Condition: CH3CN–HEPES buffer solution (10 mM,
pH ¼ 7.4, 1 : 1, v/v). slit: 1 nm/1 nm. lex/lem ¼ 370/655 nm.
Further, the response of B2 to other metal ions was also studied.
As shown in Fig. 7, the addition of 30 equiv. of Na+, K+, Ca2+, and
Mg2+ as well as 3 equiv. of Cd2+, Ag+, Pb2+, Mn2+, Cu2+, Co2+, Ni2+,
Zn2+, Fe2+, Cr3+, and Fe3+ has no obvious effect on the fluorescence
emission of B2, whereas Ag+, Ni2+, and Cd2+ responded with a slight
increase in the fluorescent intensity at 655 nm. In sharp contrast, the
addition of Hg2+ resulted in a significant enhancement of the emission
intensity. Thus, B2 was shown to be a promising fluorescent sensor
for Hg2+.
In addition, B2 was also used for in vitro Hg2+ detection in HeLa
cells. After HeLa cells were incubated with 10 mM of B2 in phos-
phate-buffered saline (PBS) for 30 min at 37 ꢂC, and then incubated
with 20 mM of Hg2+, their fluorescence images were taken using
a confocal fluorescence microscope (Fig. 8). The HeLa cells incubated
with only sensor B2 (5 mM) exhibited almost no fluorescence due to
the efficient PET quenching. By contrast, the cells stained with both
the sensor and Hg2+ showed strong red fluorescence. The results
establish that sensor B2 was efficiently internalized within the HeLa
cells and capable of sensing Hg2+ in cells. In addition, no change in the
cell morphology is induced by incubating the dyes and Hg2+ for at
(2.5-fold) than that obtained by direct excitation of Bodipy dye (lex
¼
640 nm), thus confirming that the indirect exciton induced fluores-
cence enhancement still works well in the system. In addition, the
binding process of Hg2+ to B2 was found to be very fast at room
temperature, and could be completed within several seconds.
The fluorescence titrations were conducted using a 5 mM solution
of B2 in CH3CN–HEPES buffer solution (10 mM, pH ¼ 7.4, 1 : 1,
v/v) with the excited wavelength of 370 nm (Fig. 6). With the increase
of the Hg2+ concentration, the titration reaction curve showed
a steady and smooth increase, and the changes of the emission
intensities became constant when the amount of Hg2+ added reached
2 equiv., and an approximately 30-fold fluorescence enhancement
could be observed. The association constant for Hg2+ was estimated
to be 2.1 ꢀ 105 Mꢁ1 (R2 ¼ 0.9996) on the basis of nonlinear fitting of
the titration curve assuming a 1 : 1 stoichiometry (Fig. 6, inset). This
1 : 1 binding mode was supported by a Job plot (Fig. S3, ESI†). In
addition, a typical calibration graph of the response to Hg2+ under the
optimum experimental conditions could be obtained with a good
linear relationship ranging from 0 to 5.0 ꢀ 10ꢁ6 M (Fig. S4, ESI†),
and the detection limit (S/N ¼ 3) for B2 was calculated to be
0.015 mM (3 ppb).
Fig. 7 The fluorescence spectra of B2 (5 mM) upon addition of 3 equiv.
of Hg2+ and various other metal ions, including of Na+, K+, Ca2+, and
Fig. 5 Fluorescence spectra of B2 (5 mM) in the absence and presence of
2 equiv. of Hg2+ with varied excitation. Condition: CH3CN–HEPES
buffer solution (10 mM, pH ¼ 7.4, 1 : 1, v/v), slit: 1 nm/1 nm.
Mg2+ (30 equiv.); Cd2+, Ag+, Pb2+, Mn2+, Cu2+, Co2+, Ni2+, Zn2+, Fe2+
,
Cr3+, Fe3+, and Hg2+ (3 equiv.), in CH3CN–HEPES buffer solution
(10 mM, pH ¼ 7.4, 1 : 1, v/v). Slit: 1 nm/1 nm. lex/lem ¼ 370/655 nm.
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D. Feith, J. H. Lee, S. H. Kim, Y. Ashitate, H. Hyun, G. Patonay,
L. Strekowski, M. Henary and J. V. Frangioni, Angew. Chem., Int.
Ed., 2011, 50, 6258; (g) X. Peng, Z. Yang, J. Wang, J. Fan, Y. He,
F. Song, B. Wang, S. Sun, J. Qu and M. Yan, J. Am. Chem. Soc.,
2011, 133, 6626; (h) X. Cao, W. Lin and L. He, Org. Lett., 2011, 13,
4716; (i) Z. Guo, W. Zhu, M. Zhu, X. Wu and H. Tian, Chem.–
Eur. J., 2010, 16, 14424; (j) K. Kiyose, H. Kojima, Y. Urano and
T. Nagano, J. Am. Chem. Soc., 2006, 128, 6548; (k) Y. Ueno,
Fig. 8 Confocal fluorescence images of HeLa cells incubated with B2
(10 mM), and then further incubated with Hg2+ (20 mM). (a) Brightfield
and (b) fluorescence images; (c) the overlay of (a) and (b); (d) fluorescence
image of cells treated with only probe B2. Emission was collected at
620–730 nm upon excitation using a 633 nm He–Ne laser.
ꢀ
ꢀ
J. Jose, A. Loudet, C. Peret-Bolıvar, P. Anzenbacher, Jr and
K. Burgess, J. Am. Chem. Soc., 2011, 133, 51; (l) G. Chen, F. Song,
J. Wang, Z. Yang, S. Sun, J. Fan, X. Qiang, X. Wang, B. Dou and
X. Peng, Chem. Commun., 2012, 48, 2949.
5 (a) G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int. Ed.,
2008, 47, 1184; (b) A. Loudet and K. Burgess, Chem. Rev., 2007,
107, 4891; (c) N. Boens, V. Leen and W. Dehaen, Chem. Soc. Rev.,
2012, 41, 1130.
6 (a) S. Atilgan, T. Ozdemir and E. U. Akkaya, Org. Lett., 2008, 10,
4065; (b) R. Guliyev, A. Coskun and E. U. Akkaya, J. Am. Chem.
Soc., 2009, 131, 9007.
7 (a) X. Zhang, Y. Xiao and X. Qian, Org. Lett., 2008, 10, 29; (b)
T. Bura, P. Retailleau and R. Ziessel, Angew. Chem., Int. Ed., 2010,
49, 6659.
8 (a) S. Atilgan, Z. Ekmekci, A. L. Dogan, D. Guc and E. U. Akkaya,
Chem. Commun., 2006, 4398; (b) S. Erbas, A. Gorgulu,
M. Kocakusakogullari and E. U. Akkaya, Chem. Commun., 2009,
4956; (c) H. He, P.-C. Lo, S.-L. Yeung, W.-P. Fong and
D. K. P. Ng, Chem. Commun., 2011, 47, 4748.
least 12 h (Fig. S5, ESI†), suggesting the absence of significant
cytotoxicity.
In conclusion, we have demonstrated a new strategy of the indirect
S0 / S2 excitation for enhancing the brightness of the near-infrared
distyryl Bodipy derivative (B1). For the application of the strategy,
B2 was designed and synthesized by incorporating a Hg2+ receptor to
the B1 platform for fluorescence turn-on detection of Hg2+ based on
the PET mechanism. The results obtained disclosed that by the
indirect S0 / S2 excitation, B2 could detect Hg2+ with high sensi-
tivity. In addition, B2 was also applied to biological imaging of Hg2+
inside HeLa cells.
9 S. Erten-Ela, M. D. Yilmaz, B. Icli, Y. Dede, S. Icli and E. U. Akkaya,
Org. Lett., 2008, 10, 3299.
ꢀ
10 (a) J. M. Serin, D. W. Brousmiche and J. M. J. Frechet, J. Am. Chem.
Soc., 2002, 124, 11848; (b) D. W. Brousmiche, J. M. Serin,
We acknowledge the Natural Science Foundation of China
(NSFC nos 21172137 and 21072121) for support of this work.
ꢀ
J. M. J. Frechet, G. S. He, T.-C. Lin, S. J. Chung and P. N. Prasad,
J. Am. Chem. Soc., 2003, 125, 1448; (c) M. D. Yilmaz,
O. A. Bozdemir and E. U. Akkaya, Org. Lett., 2006, 8, 2871; (d)
O. A. Bozdemir, M. D. Yilmaz, O. Buyukcakir, A. Siemiarczuk,
M. Tutas and E. U. Akkaya, New J. Chem., 2010, 34, 151.
11 J. Fan, K. Guo, X. Peng, J. Du, J. Wang, S. Sun and H. Li, Sens.
Actuators, B, 2009, 142, 191.
12 (a) J. M. L. Pecourt, J. Peon and B. Kohler, J. Am. Chem. Soc., 2001,
123, 10370; (b) H. Kang, B. Jung and S. K. Kim, J. Chem. Phys., 2003,
118, 6717.
Notes and references
1 (a) J. Fabian, Chem. Rev., 1992, 92, 1197; (b) L. D. Lavis and
R. T. Raines, ACS Chem. Biol., 2008, 3, 142; (c) J.-A. Richard,
M. Massonneau, P.-Y. Renard and A. Romieu, Org. Lett., 2008,
10, 4175; (d) T. Geiger, H. Benmansour, B. Fan, R. Hany and
€
F. Nuesch, Macromol. Rapid Commun., 2008, 29, 651; (e)
U. Mayerhoffer, K. Deing, K. Grub, H. Braunschweig,
€
ꢀ
13 P. Didier, G. Ulrich, Y. Mely and R. Ziessel, Org. Biomol. Chem.,
€
K. Meerholz and F. Wurthner, Angew. Chem., Int. Ed., 2009, 48,
8776; (f) P.-A. Bouit, D. Rauh, S. Neugebauer, J. L. Delgado, E. Di
Piazza, S. Rigaut, O. Maury, C. Andraud, V. Dyakonov and
N. Martin, Org. Lett., 2009, 11, 4806; (g) T. Weil, T. Vosch,
2009, 7, 3639.
14 M. Albota, D. Beljonne, J.-L. Bredas, J. E. Ehrlich, J.-Y. Fu,
ꢀ
A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder,
€
€
J. Hofkens, K. Peneva and K. Mullen, Angew. Chem., Int. Ed.,
D. McCord-Maughon, J. W. Perry, H. Rockel, M. Rumi,
€
€
2010, 49, 9068; (h) U. Mayerhoffer, B. Fimmel and F. Wurthner,
Angew. Chem., Int. Ed., 2011, 50, 1; (i) N. Karton-Lifshin, E. Segal,
L. Omer, M. Portnoy, R. Satchi-Fainaro and D. Shabat, J. Am.
Chem. Soc., 2011, 133, 10960; (j) M. Fu, Y. Xiao, X. Qian, D. Zhao
and Y. Xu, Chem. Commun., 2008, 1780; (k) Y. Koide, Y. Urano,
K. Hanaoka, T. Terai and T. Nagano, ACS Chem. Biol., 2011, 6, 600.
2 (a) G. I. Jones, in Dye Laser Principles, ed. F. J. Duarte and L. W.
Hillman, Academic Press, New York, 1990, pp. 287–343; (b)
B. N. G. Giepmans, S. R. Adams, M. H. Ellisman and R. Y. Tsien,
Science, 2006, 312, 217; (c) N. Johnsson and K. Johnsson, ACS
Chem. Biol., 2007, 2, 31; (d) Y. Koide, Y. Urano, K. Hanaoka,
T. Terai and T. Nagano, J. Am. Chem. Soc., 2011, 133, 5680; (e)
T. Egawa, K. Hanaoka, Y. Koide, S. Ujita, N. Takahashi,
Y. Ikegaya, N. Matsuki, T. Terai, T. Ueno, T. Komatsu and
T. Nagano, J. Am. Chem. Soc., 2011, 133, 14157; (f) L. Yuan,
W. Lin, Y. Yang and H. Chen, J. Am. Chem. Soc., 2012, 134, 1200.
3 X. He, K. Wang and Z. Cheng, Nanomed. Nanobiotech., 2010, 2, 349.
4 (a) A. Mishra, R. K. Behera, P. K. Behera, B. K. Mishra and
G. B. Behera, Chem. Rev., 2000, 100, 1973; (b) E. Sasaki,
H. Kojima, H. Nishimatsu, Y. Urano, K. Kikuchi, Y. Hirata and
T. Nagano, J. Am. Chem. Soc., 2005, 127, 3684; (c) X. Peng,
F. Song, E. Lu, Y. Wang, W. Zhou, J. Fan and Y. Gao, J. Am.
Chem. Soc., 2005, 127, 4170; (d) N. Karton-Lifshin, E. Segal,
L. Omer, M. Portnoy, R. Satchi-Fainaro and D. Shabat, J. Am.
Chem. Soc., 2011, 133, 10960; (e) T. Myochin, K. Kiyose,
K. Hanaoka, H. Kojima, T. Terai and T. Nagano, J. Am. Chem.
Soc., 2011, 133, 3401; (f) H. S. Choi, K. Nasr, S. Alyabyev,
G. Subramaniam, W. W. Webb, X.-L. Wu and C. Xu, Science,
1998, 281, 1653.
15 (a) S. L. Niu, C. Massif, G. Ulrich, R. Ziessel, P.-Y. Renard and
A. Romieu, Org. Biomol. Chem., 2011, 9, 66; (b) R. Ziessel,
G. Ulrich, J. H. Olivier, T. Bura and A. Sutter, Chem. Commun.,
2010, 46, 7978.
16 D. W. Cho, M. Fujitsuka, J. H. Ryu, M. H. Lee, H. K. Kim,
T. Majima and C. Im, Chem. Commun., 2012, 48, 3424.
17 S. Kim, T. Y. Ohulchanskyy, A. Baev and P. N. Prasad, J. Mater.
Chem., 2009, 19, 3181.
18 D. W. Cho, M. Fujitsuka, J. H. Ryu, M. H. Lee, H. K. Kim,
T. Majima and C. Im, Chem. Commun., 2012, 48, 3424.
19 (a) H. H. Harris, I. J. Pickering and G. N. George, Science, 2003, 301,
1203; (b) P. B. Tchounwou, W. K. Ayensu, N. Ninashvili and
D. Sutton, Environ. Toxicol., 2003, 18, 149.
20 (a) E. M. Nolan and S. Lippard, Chem. Rev., 2008, 108, 3443; (b)
X. Zhang, Y. Xiao and X. Qian, Angew. Chem., Int. Ed., 2008, 47,
8025; (c) S. Atilgan, T. Ozdemir and E. U. Akkaya, Org. Lett.,
2010, 12, 4792; (d) X. Chen, S.-W. Nam, M. J. Jou, Y. Kim,
S.-J. Kim, S. Park and J. Yoon, Org. Lett., 2008, 10, 5235; (e)
H. N. Kim, S.-W. Nam, K. M. K. Swamy, Y. Jin, X. Chen,
Y. Kim, S.-J. Kim, S. Park and J. Yoon, Analyst, 2011, 136, 1339;
(f) Q. Zou, L. Zou and H. Tian, J. Mater. Chem., 2011, 21, 14441;
(g) H. Yu, Y. Xiao, H. Guo and X. Qian, Chem.–Eur. J., 2011, 17,
3179; (h) B. Leng, J. Jiang and H. Tian, AIChE J., 2010, 56, 2957;
(i) W. Lin, X. Cao, Y. Ding, L. Yuan and L. Long, Chem.
Commun., 2010, 46, 3529.
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