Synthesis of water soluble PEG-functionalized iridium complex via click chemistry and application for cellular bioimaging

Article abstract of DOI:10.1016/j.inoche.2010.07.042

A water soluble iridium (III) complex was prepared via click chemistry. It shows the bright red phosphorescence centered at 625 nm with a quantum yield of ～ 1.4% in the phosphate buffered saline (PBS) solution. Furthermore, it has low cytotoxicity, good m

Full text of DOI:10.1016/j.inoche.2010.07.042

Inorganic Chemistry Communications 13 (2010) 1387–1390
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis of water soluble PEG-functionalized iridium complex via click chemistry
and application for cellular bioimaging
Hong Yang a, Liutao Li a, Liqiang Wan b, Zhiguo Zhou c, Shiping Yang a,
⁎
a
Department of Chemistry, Shanghai Normal University, Shanghai 200234, PR China
b
School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China
c
Department of Chemistry, Fudan University, Shanghai 200234, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
water soluble iridium (III) complex was prepared via click chemistry. It shows the bright red
Received 29 June 2010
Accepted 30 July 2010
Available online 6 August 2010
phosphorescence centered at 625 nm with a quantum yield of ~1.4% in the phosphate buffered saline
(PBS) solution. Furthermore, it has low cytotoxicity, good membrane permeability and exclusive staining in
cytoplasm, which can be an excellent promising candidate for the design of new generation luminescent
bioprobes.
Keywords:
PEG
© 2010 Elsevier B.V. All rights reserved.
Iridium complex
Click chemistry
Cellular bioimaging
Fluorescence imaging is a powerfull technique which provides
subcellular level information that cannot be resolved by ultrasound or
magnetic resonance imaging [1–4]. Up to now, most of the fluorescent
probes are organic fluorescent dyes, but there are some limitations
because of their small stoke shift, easy photobleaching and interfering
by autofluorescence [5]. Compared with organic fluorophores,
phosphorescent complexes based on metal-to-ligand-charge-transfer
mechanism (MLCT) have become the better candidate for bioimaging
due to their large stoke shift, long life time and good stability [6–11].
Among the various phosphorescent complexes with MLCT, iridium
complexes have been investigated in detail due to their versatile
photophysical and photochemical properties [12,13]. Therefore, they
have been applied in many fields such as organic light-emitting
diodes, [14–16] solar energy conversion devices [17] and phospho-
rescent chemosensors [18–26]. For the application in luminescent
imaging, Lo et al. reported the first example of luminescent
cyclometalated iridium (III) complexes as labeling reagents for
biological substrates [27]. Up to date, more detailed investigations
in this field have been reported [28–32]. For example, Li et al.
demonstrated two cationic iridium (III) complexes as phosphorescent
dyes with good stability for imaging in living cells [33]. In order to
improve the solubility of iridium (III) complexes in water, Bian et al.
presented zwitterionic iridium complexes for cell imaging instead of
cationic complexes [34].
Generally, luminescent probes should be water-dispersible, has
low cytotoxicity and biocompatible. One of the most common
methods is to combine a water soluble polymer with luminescent
probes. Poly(ethylene glycol) (PEG) is a biocompatible polymer and
has been widely regarded as a typical non-toxic, non-immunogenic,
protein-resistant polymer [35]. Although π-conjugated polymer with
iridium complexes has been widely applied in the field of organic
light-emitting diodes, to the best of our knowledge, few reports of the
synthesis and application of phosphorescent polymers containing
iridium (III) complexes for cellular bioimaging have been published
[36,37]. Click chemistry is a chemical philosophy introduced by K.
Barry Sharpless in 2001 and describes chemistry tailored to generate
substances quickly and reliably by joining small units together [38]. In
this paper, we present a strategy to prepare water soluble iridium (III)
complexes containing PEG polymer as phosphorescent dyes for
exclusive staining in the cytoplasm of living cells by click chemistry.
The synthetic route toward the water soluble iridium complex is
outlined in Scheme 1. Click chemistry was used to attach the PEG
chain to iridium (III) complexes. As shown in the IR spectrum of PEG-
N3 (Fig. 1), the strong absorption characteristic of the azide groups at
2102 cm−1 was observed, demonstrating that PEG-N3 was obtained
quite efficiently. After click reaction, the IR spectrum of complex 1
showed a strong and sharp C–O stretch ether linkage at 1107 cm−1
,
which was due to the presence of the polyether backbone of PEG,
while other important peaks were at 1587 cm−1 (weak, νN = N),
which confirmed the success of conjugation [39,40]. It is important
that the strong absorption characteristic of the azide groups at
⁎
Corresponding author. Tel.: +86 21 64322343; fax: +86 21 64322511.
E-mail address: shipingy@shnu.edu.cn (S. Yang).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.07.042

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H. Yang et al. / Inorganic Chemistry Communications 13 (2010) 1387–1390
Scheme 1. Synthesis of water soluble PEG-functionalized iridium complex 1.
excited by 370 nm, the broad and structureless emission bands
centered at 592 nm, while the emission peak shifted to 625 nm in the
PBS solution. The phosphorescence quantum efficiencies in the PBS
solution is 1.4% [41].
Generally, the cytotoxicity of complex 1 is critical to the
biomedical application. The cytotoxicity of complex 1 was evaluated
using an MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide] assay with the KB cell lines. The concentration-dependent
effect of compound 1 on the cell viability at 12 and 24 h was
determined as presented in Fig. 3. It is clear that the high cell viability
can still be attained even at a concentration of 100 μM. This indicates
that complex 1 can be considered to have low cytotoxicity and may be
applied in the field of biology and medicine.
Due to the low energy MLCT absorption and emission properties in
the visible region, it is convenient for bioimaging in living cells. After
incubation of the KB cells with complex 1 at 37 °C under a 5% CO2
atmosphere for 10 min, the efficient interiorization of 1 was observed
by the laser-scanning confocal microscopy with an excitation
wavelength at 488 nm as shown in Fig. 4a. The overlay of confocal
luminescence and brightfield images confirmed that complex 1 was
mainly located in the cytoplasm of living cells.
Fig. 1. FTIR spectra of PEG-N3 and complex 1 at room temperature.
In conclusion, combined with click chemistry, a water soluble
iridium (III) complex has been obtained with the incorporation of PEG
polymer chain onto the neutral ligand. Its photophysical property in
different solvent was investigated in detail. More importantly, it
2102 cm−1 disappeared, and the typical weak absorption of 2110 cm−1
suggested the presence of –C≡CH group in complex 1. Combined IR and
GPC data showed that one of the two alkynyl groups of complex 2 is
conjugated by the click reaction.
The photophysical properties of 1 in CHCl3, EtOH, THF and PBS
were investigated. The data are summarized in Table 1. Fig. 2 shows
the UV–vis absorption spectra recorded in different solvents. Complex
1 displays intense absorption bands below 370 nm in different
solvents, which are assigned to spin-allowed intraligand LC (π→π*)
transitions. The absorption bands in the lower energy region, starting
around 370 nm and extending to around 600 nm, are observed. It is
resulted from the weak and broad metal-to-ligand-charge-transfer
(MLCT) transition of the iridium complexes. From the room
temperature photoluminescence spectra in CHCl3, EtOH and THF
Table 1
Photophysical data of compound 1 in CHCl3, EtOH, THF and PBS at room temperature.
Solvent
Absorption (λ, nm)
Emission
(λmax, nm)
φa (λex, nm)
CHCl3
EtOH
THF
272, 507
271, 449
270, 526
270, 410
592
592
592
625
0.072 (365)
0.033 (295)
0.030 (325)
0.014 (310)
PBS
aThe quantum efficiency was measured with [Ru(bpy)3]Cl2 as an external standard
(φ=2.8%).

H. Yang et al. / Inorganic Chemistry Communications 13 (2010) 1387–1390
1389
Fig. 2. UV–vis absorption spectra (Left) and normalized emission spectra (right) of 1 in different solvents at room temperature.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China, (20971086, 20801015, 50703007), the
Shanghai Municipal Education Commission (09YZ159, 10ZZ84), the
Shanghai Science and Technology Development Fund (09QA1404300)
and the Excellent Youth Teachers of Shanghai University (RE902).
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