A NIR phosphorescent osmium(ii) complex as a lysosome tracking reagent and photodynamic therapeutic agent

Article abstract of DOI:10.1039/c7cc07776a

A novel near infrared (NIR) phosphorescent osmium complex (Os1) was developed for lysosome tracking and photodynamic therapy. Owing to its NIR photophysical properties, cellular imaging ability and phototoxicity, it has advantages over its ruthenium analogue (Ru1).

Full text of DOI:10.1039/c7cc07776a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Received 00th January
NIR phosphorescent osmium(II) complex as lysosome tracking
reagent and photodynamic therapeutics†
20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Pingyu Zhang*a, Yi Wanga, Kangqiang Qiua,b, Zhiqian Zhaoa, Rentao Hua, Chuanxin Hea, Qianling
Zhang*a
and
Hui
Chao*a,b
A novel near infrared (NIR) phosphorescent osmium complex samples, and can also improve tissue depth penetration.7 Potential
(Os1) was developed for lysosome tracking and photodynamic uses span from optoelectronic/solar energy materials, to research
therapy. Its NIR photophysical property, cellular imaging and imaging probes and, to fluorescence guided precision surgery, such
phototoxicity have advantages over its ruthenium analogue as photodynamic therapy.8 However, most of the available NIR
(Ru1).
fluorescent probes are mainly limited to organic dyes,9
semiconductor quantum dots (QDs)10, metalꢀorganic framework
(MOF)
nanoꢀparticles8d
and
lanthanide
upconversion
Metal coordination complexes have attracted a great interest in the
fields of chemical sensors and bioimaging because they possess
tunable and intense emission, long emission lifetimes, large Stokes
shifts, and high photostability.1 In this regard, Ru(II) and Ir(III)
complexes are particularly promising because of their excellent
photophysical and electrochemical properties, which can also be
systematically modulated to a significant extent by changing the
ligands, as well as due to their robust structure.2 These metal
complexes have emerged as promising alternative choices to organic
dyes as diagnostic probes and tracking agents for diseases.2e
Whilst osmium complexes featuring labile ligands have been the
focus of numerous important studies as antiꢀcancer agents,3 few
osmium polypyridyl complex was examined to date in the context of
their capacity as cellular imaging probes.4 Although the emission
quantum yields and lifetimes of Os(II) polypyridyl complexes tend
to be lower, they share many of the photophysical advantages of
their Ru(II) analogues. Firstly, Os(II) polypyridyl complexes
typically exhibit emission maxima in the NIR spectral region,
strongly coincident with the biological optical window.5 Secondly,
because of their increased crystal field splitting, the osmium d–d
states cannot be accessed by thermal crossover from its excited
triplet state, as is the case for ruthenium.6
nanophosphors,11 these compounds have several significant
limitations such as poor photostability, larger size and small Stokes
shift, etc.12
Compared with these NIR fluorescent probes, Os(II) complexes
offer advantages of long MLCT absorption, high photostability and
excellent NIR emission.13 Recently, McFarland and coꢀworkers
reported
[Os(biq)2(phen)](PF6)2 (TLD1822, biq = 2,2′ꢀbiquinoline, phen =
1,10ꢀphenanthroline), [Os(biq)2(IP)](PF6)2 (TLD1829, IP
imidazo[4,5ꢀf][1,10]phenathroline) and [Os(biq)2(dppn)](PF6)2
three
osmiumꢀbased
photosensitizers,
=
(TLD1824, dppn = benzo[i]dipyrido[3,2ꢀ′:2′,3′ꢀc]phenazine). These
photosensitizers were panchromatic, activatable from 200 to 900 nm
and had strong resistance to photobleaching. In vitro studies showed
photodynamic therapy efficacy with both red and NIR light in
normoxic and hypoxic conditions, which translated to good in vivo
efficacy of TLD1829 in a subcutaneous murine colon cancer model.5
Herein, we designed a pair of Ru(II) and Os(II) analogues (Fig.1)
and studied their photophysics, cellular imaging and photodynamic
therapy. The results suggested that Os1 exhibited NIR
phosphorescence emission (700ꢀ850 nm) and interestingly
accumulated in lysosome, allowing for NIR imaging. However, Ru1
showed red luminescence (550ꢀ700 nm) in mitochondria.
Furthermore, Os1 showed obvious photocytotoxicity to the cancer
cells after NIR light irradiation. This novel Os(II) complex is the
first report for NIR lysosome tracking and NIR photodynamic
therapy.
Nearꢀinfrared (NIR) fluorophores with emission wavelengths in
the region of 650–900 nm can markedly decrease light scattering and
background absorption, minimize photodamage to biological
The syntheses of Ru1 and Os1 were described in the experimental
section and Fig. S1 in the supporting information. They were
characterized by the MS, NMR and elemental analysis (Figs. S2ꢀS8).
The wavelength of the MLCT absorption of Ru1 was 459 nm and its
red phosphorescence of Ru1 emitted from 550 to 700 nm in the PBS
†Electronic Supplementary Information (ESI) available: Experimental details, MS and
NMR spectra and figures referenced throughout the text.
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intensities of MTG and LTG decreased obviously after irradiation of
488 nm light for 30 min. The phosphorescence intensities of Ru1
and
Fig. 1. The chemical structures of Ru1 and Os1.
Fig. 3. Confocal images of the A549 cells colabeled with Ru1 or
Os1 (20 ꢀM, 4 h) in the RPMIꢀ1640 medium (with 1% DMSO) and
(a, c) MitoTracker Green (MTG, 100 nM, 30 min), and (b, d)
LysoTracker Green (LTG, 100 nM, 30 min). Ru1: λex = 458 nm,
λem = 600±30 nm; Os1: λex = 561 nm, λem = 720±30 nm; MTG/LTG:
λex = 488 nm, λem = 520 ± 30 nm; Hoechst 33258: λex = 405 nm, λem
= 450 ± 30 nm; Scale bar: 20 ꢀm.
Fig. 2. (a) UVꢀvis and (b) emission spectra of Ru1 and Os1 (20 ꢀM)
in the PBS solution (with 1% DMSO); the wavelengths of excitation
of Ru1 and Os1 were 465 nm and 633 nm, respectively.
solution, whereas Os1 showed the MLCT absorption at 488 nm and
683 nm, and exhibited NIR emission between 700ꢀ850 nm (Fig. 2).
This is probably due to strong spinꢀorbit coupling induced by heavy
Os1 remained mostly unchanged after photobleaching in aqueous
solution and living A549 cancer cells. In addition, we observed that
Os1 still showed strong phosphorescence in lysosome after 12 h and
48 h incubation (Fig. S14). These results indicated that Ru1 and Os1
exhibited outstanding photostabilities and were suitable for longꢀ
time tracking of lysosomes in the living cells.
3
Os(II), then the spinꢀforbidden MLCT absorption bands were seen
in the NIR region.13b In contrast to its Ru(II) analogue and most of
Ir(III) complexes,14 Os1 exhibited longer wavelength of emission,
which is usefully situated inside the biological window and out of
the range of cellular autofluorescence. In addition, both Ru1 and
Os1 were highly stable in the cell culture medium (RPMIꢀ1640) for
72 h (Fig. S9).
It was interesting to find out that Ru1 and Os1 targeted different
cellular organelles. Ru1 and Os1 have the same ligands and charge.
The difference between them lies in the metal center. We firstly
thought that this is due to their abilities in responding to biological
pH. Thus, the emission intensities of Ru1 and Os1 were measured at
various pH values ranging from 4.11 to 10.48 in the PBS solution.
The phosphorescence intensity exhibited no obvious change in these
pH values for both Ru1 and Os1 (Fig. S15). Then we speculated that
the hydrophilicity influenced by metal center was responsible for the
different targets of Ru1 and Os1. To determine the hydrophilicities
of Ru1 and Os1, the nꢀoctanol/water partition coefficients (log P)
were measured. The results showed that Os1 was more lipophilic,
with a log P of ꢀ1.24±0.11. However, Ru1 was more hydrophilic,
with a log P of ꢀ2.23±0.28. Previous report suggested that difference
in log P might make a difference in the cellular targets in the living
cells.14
Next we measured whether O2 affects the phosphorescence
intensities of Ru1 and Os1. The result showed that Ru1 and Os1 had
much stronger phosphorescence under N2 than that in the air (Fig.
S16), and higher Φem under N2 environment (0.047 for Ru1 and
0.012 for Os1, Table 1). The emission lifetimes of Ru1 and Os1
were longer under N2 than (89 ns for Ru1, 72 ns for Os1) that in the
air (43 ns for Ru1, 24 ns for Os1) (Table 1 and Fig. S17). As
Due to their strong phosphorescence properties, the cellular
distribution in A549 cancer cells was investigated by confocal laser
scanning microscopy. As shown in Fig. 3, the colocalization
experiment of Ru1 with MitoTracker Green (MTG) in the A549
cells demonstrated high overlap, with a Pearson’s colocalization
coefficient of 82.3% (Fig. 3a). Meanwhile, little colocalization for
Ru1 and LysoTracker Green (LTG) or Hoechst 33258 was
observed (Fig. 3b and Fig. S10). However, Os1 was found to have
high overlap with lysosomes, with a Pearson’s colocalization
coefficient of 87.6%, but few in either nucleus or mitochondria (Fig.
3c,d and Fig. S10). Furthermore, for Ru1, the inductively coupled
plasma mass spectrometry (ICPꢀMS) results revealed that the level
of cellular Ru in the mitochondria (80.3%) was much higher than
that in the cell membrane, lysosomes and nucleus. For Os1, we
found that most of cellular Os were located in the lysosomes and
very few Os was in the nucleus, membrane or mitochondria
(Fig. S11).
To explore the application of Ru1 and Os1 as cellular dyes, the
photostabilities of the complexes were examined in comparison to
MTG and LTG. Figs. S12, S13 showed that the fluorescence
2 | J. Name., 2012, 00, 1-3
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expected, the presence of oxygen had a great influence on the
phosphorescence and lifetimes of Ru1 and Os1.
Table 1. Photophysical data for Ru1 and Os1.
b
Φem
Lifetimes τ[ns]c
Φ(1O2)d
Complex
λ
abs[nm](ε[Mꢀ1cmꢀ1×104])a
λ
em[nm]a
Air
N2
Air
43
N2
89
72
465 nm
633 nm
0.001
0.05
Ru1
459 (0.67)
619
736
0.016
0.009
0.047
0.012
0.10
Os1
488 (0.50); 683 (0.14)
24
0.003
a
Absorption and emission spectra recorded in H2O (with 1% DMSO) at 310 K; b Φem, luminescence quantum yield in H2O (with 1%
DMSO); c τ, lifetime, determined in H2O (with 1% DMSO). d Φ(1O2), quantum yields for O2 determined under 465 nm and 633 nm light
1
irradiation, respectively. Ru(bpy)3]2+ was used as a standard photosensitizer (Φem = 0.028, Φ(1O2) = 0.22 in water15)
1O2 generation by Ru1 and Os1 upon 465 nm (blue) and 633 nm irradiation or Os1 upon 633 nm light irradiation. However, the cells
(red) light irradiation was detected by electron paramagnetic treated with Ru1 upon 633 nm irradiation or Os1 upon 465 nm
resonance (EPR) spectroscopy using 2,2,6,6ꢀtetramethylpiperidine irradiation showed weak fluorescence (Fig. S21).
(TEMP) as the spinꢀtrap. As illustrated in Fig. S18, the characteristic
tripletꢀofꢀtriplets
(3450ꢀ3510
G)
for
the
2,2,6,6ꢀ
tetramethylpiperidineꢀ1ꢀoxyl radical was observed in the Ru1
sample after 465 nm light irradiation and Os1 sample after 633 nm
light irradiation. No 1O2 signal was observed either in the Ru1
sample upon 633 nm light irradiation or in the Os1 sample upon 465
nm light irradiation. Φ(1O2) of Ru1 and Os1 upon irradiation were
determined (the quenching of the absorbance of ρꢀnitrosodimethyl
aniline (RNO) by a transꢀannular peroxide adduct formed by 1O2 and
an imidazole derivative16) as 0.10 (Ru1, 465 nm light) and 0.05
(Os1, 633 nm light), respectively (Fig. S19 and Table 1). Φ(1O2) of
Ru1 upon 633 nm light irradiation and Os1 upon 465 nm light
irradiation were much lower.
The phototoxicities of Ru1 and Os1 towards various kinds of cells
were further studied (Fig. 4 and Table S1). After treatment with
different concentrations of Ru1 and Os1 for 4 h, and incubated
without nonꢀcomplexes medium for another 44 h. The result showed
that Ru1 and Os1 were no cytotoxicity (IC50 = 425 and 406 ꢀM,
respectively) in the dark. The cancer cells exhibited no loss of
viability after being irradiated with these lights in the absence of the
Ru/Os complexes (control + irradiation, Fig. S20). However, Ru1
was highly toxic to A549 cancer cells (IC50 = 12.3 ꢀM) after 465 nm
light irradiation (4.8 mW/cm2, 1 h) but not 633 nm light irradiation
(11.1 mW/cm2, 3 h, IC50 = 442 ꢀM). Interestingly, Os1 showed great
phototoxicity after 633 nm light irradiation (IC50 = 31.7 ꢀM) and its
phototoxicity index upon 633 nm (PI633 = IC50 (dark) / IC50 (633nm
light)) was 12.8 (Table S1). Ru1 and Os1 also showed toxicity
towards HeLa and HepꢀG2 upon irradiation but almost nonꢀtoxicity
Fig. 4. Growth curves for the A549 cells treated with (a) Ru1 (b)
Os1 for 4 h in the dark or followed by irradiation with 465 nm (4.8
mW/cm2, 1 h) or 633 nm light (11.1 mW/cm2, 3 h). The cells
transferred to fresh medium, then incubated for a further 44 h.
Conclusions
We present the first application of an Os(II) polypyridyl
complex for lysosome NIR imaging in the living cells. In
contrast to its Ru(II) analogue and most of iridium complexes,
Os1 exhibited NIR emission centred around 736 nm, which is
usefully situated inside the biological window and out of the
range of cellular autofluorescence. Interestingly, we observed
that Os1 accumulated in lysosome with NIR emission, whereas
Ru1 located in mitochondria with red luminescence.
Furthermore, Os1 exhibited more significant phototoxicity
towards the cancer cells than Ru1 upon 633 nm light irradiation.
Overall, this study shows that Os(II) polypyridyl complex may
offer an useful addition to the growing repertoire of NIR
imaging and NIR photodynamic therapy.
We appreciate the financial support of the National Natural
Science Foundation of China (NSFC, 21701113) and the
Science and Technology Foundation of Shenzhen
(JCYJ20170302144346218) for PZ, the NSFC (21471164 and
21525105) and the 973 program (2015CB856301) for HC, the
NSFC (21374064, 21471101 and 21671137) and the
Foundation for Emerging Industries of Strategic Importance of
Shenzhen (JCY20150324140036843) for QZ.
to MRCꢀ5 normal cells both in the dark and upon irradiation (IC50
>
100 ꢀM, Table S1). And the phototoxicity of Os1 was more
excellent than the reported Os(II) complexes (TLD1822, TLD1824
and TLD1829)5a. Under the same experimental conditions, 5ꢀALA
(5ꢀaminoꢀlevulinic acid) and cisplatin displayed almost no
phototoxicity.
1
To demonstrate that Ru1 and Os1 can produce cellular O2 after
light irradiation, A549 lung cancer cells were incubated with Ru1
and Os1 and the fluorescence probe 2,7ꢀdichlorodihydroꢀfluorescein
diacetate (DCFHꢀDA). The cells treated with DCFHꢀDA and the
complexes in the dark showed no enhancement of fluorescence (Fig.
S21). In contrast, a significant increase in fluorescence from DCFHꢀ
DA was observed in the cells treated with Ru1 upon 465 nm light
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In comparison to ruthenium(II) complex, osmium(II) complex has great advantages of NIR
phosphorescence imaging and NIR photodynamic therapy.