Regulation of a cerium(iv)-driven O2 evolution reaction using composites of liposome and lipophilic ruthenium complexes

Article abstract of DOI:10.1039/c5dt00793c

A composite containing a liposome and a lipophilic ruthenium complex was synthesized to regulate an O₂ evolution reaction using cerium(iv) ammonium nitrate as an oxidizing reagent. We found that the surrounding environment of the reaction centre is an important factor for controlling the O₂ evolution catalytic reaction. We successfully regulated the reaction activity using the linker length of the lipophilic ligand and using the head groups of the phospholipid component.

Full text of DOI:10.1039/c5dt00793c

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Regulation of a cerium(IV)-driven O2 evolution
reaction using composites of liposome and
lipophilic ruthenium complexes†
Tomomi Koshiyama,*a Nao Kanda,a Koki Iwata,a Masayuki Honjo,a Sana Asada,a
Tatsuru Hatae,a Yasuhiro Tsuji,a Masaki Yoshida,b Masaya Okamura,b Reiko Kuga,b
Shigeyuki Masaokab and Masaaki Ohba*a
Cite this: DOI: 10.1039/c5dt00793c
Received 24th February 2015,
Accepted 29th April 2015
DOI: 10.1039/c5dt00793c
www.rsc.org/dalton
A composite containing a liposome and a lipophilic ruthenium (nm–μm scale) and their surface charge from the various types
complex was synthesized to regulate an O2 evolution reaction of phospholipids that can be employed, and because of their
using cerium(IV) ammonium nitrate as an oxidizing reagent. We high stability in environmentally friendly aqueous solutions.
found that the surrounding environment of the reaction centre is Hence, catalytic systems based on liposomes, used in organic
an important factor for controlling the O2 evolution catalytic reac- conversion,3,4 electron transfer,5 photocatalytic water oxi-
tion. We successfully regulated the reaction activity using the dation,6 and hydrogen production,7,8 have been reported. Pre-
linker length of the lipophilic ligand and using the head groups of vious researchers have succeeded in utilizing lipid membranes
the phospholipid component.
as platforms for reactions, whereas regulation of the reactions
has been addressed using functional molecules themselves,
and not by the environment on the membrane surface.
Sophisticated design of molecular catalysts for medical and
industrial applications has been a long-standing goal in the
field of catalytic chemistry. While molecular catalysts are
designable for high activity and selectivity by adjusting the
substituents of their ligands, such designs often require com-
plicated organic synthesis. Furthermore, the low solubility of
molecular catalysts interferes with their applications to “green
chemistry”. One of the promising methods to overcome these
limitations involves deposition of molecular catalysts in a con-
fined environment, such as a nanoreactor, constructed from
synthetic and biological molecules.1
A spherical vesicle, such as a liposome, consisting of a
phospholipid bilayer has four types of regions: a hydrophilic
inner water phase, a hydrophobic lipid bilayer, and inner and
outer surfaces. A hierarchical incorporation of different func-
tional molecules can occur in these vesicles. Such spherical
composites that integrate functional molecules are attractive
for fabricating catalysts, sensors, drug delivery systems, and
nanomaterials.1,2 In addition, liposomes have advantages as
“reaction spaces” because of the ease of controlling their size
Here, we report on the preparation of new liposome compo-
sites with metal complexes and on a strategy for regulating
their catalytic reactions (Fig. 1a). For the functionalization of
the liposome surface, we designed new lipophilic metal com-
plexes and succeeded in fixing them on liposomes. In this
work, O2 evolution from water was selected because of its
potential application in artificial solar energy conversion and
storage.9–11
To fix the oxygen-evolving mononuclear ruthenium
complex [Ru(terpy)(bpy)(OH2)]2+ (Ru core unit = cRu) on the
liposome surface,12,13 two lipophilic Ru complexes having
cholesterol derivatives were designed: [Ru(terpy)(L1)(sol)]2+
(1, L1 = cholesterol 2,2′-bipyridine-5-ylmethylsuccinate) and
[Ru(terpy)(L2)(sol)]2+ (2, L2 = cholesterol 2,2′-bipyridine-5-
carboxylate) (Fig. 1b). The high affinity of the cholesterol moiety
for phospholipids allowed 1 and 2 to become fixed tightly onto
the lipid bilayer.14 Different linker lengths between bpy and
the cholesterol moieties were designed to adjust the position
of the Ru cores on the liposome surface and to influence the
accessibility of oxidizing reagents. Compounds 1 and 2 were
synthesized as described in the ESI.† The UV/Vis spectra of 1
and 2 in acetonitrile were identical to that of the precursor cRu
(Fig. S1†).12 The absorption band observed around 460 nm cor-
responded to the metal-to-ligand charge transfer (MLCT)
band. Cyclic voltammograms of 1 and 2 in acetonitrile showed
redox couples from Ru(II)/Ru(III) at E1/2 = 0.91 and 0.94 V vs. Fc/
Fc+, respectively (Fig. S2†). In addition, cyclic voltammograms
of 1 and 2 in an aqueous 0.5 M H2SO4 solution showed that
aDepartment of Chemistry, Faculty of Science, Kyushu University, 6-10-1 Hakozaki,
Higashi-ku, Fukuoka, Japan. E-mail: ohba@chem.kyushu-univ.jp,
koshi@chem.kyushu-univ.jp; Fax: +81-92-642-2570; Tel: +81-92-642-2570
bInstitute for Molecular Science, Higashiyama 5-1, Myodaiji, Okazaki 444-8787,
Japan
†Electronic supplementary information (ESI) available: Details of synthesis of
lipophilic Ru complexes, physical measurements and additional experimental
data. CCDC 1050758. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5dt00793c
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Fig. 3 The crystal structure of [Ru(terpy)(L3)(CH3CN)](PF6)2.
5-carboxylate). The crystal structure shows acetonitrile occupy-
ing a reaction site on the Ru core and the relative disposition
of the coordinated acetonitrile and the pyridyl ring bearing the
cholesterol substituent being in a trans configuration (Fig. 3).
This implies that the reaction sites of 1 and 2 were fixed in the
lipid bilayers, facing the outer water phase of the liposomes,
and oxidants and water molecules could easily access the
Ru cores.
The type of phospholipid used is important in the design
of the reaction space surrounding the Ru cores on the lipo-
some surface. Three phospholipids were used in this work: the
negatively charged lipids 1,2-distearoyl-sn-glycero-3-phosphate
(DSPA) and 1,2-distearoyl-sn-glycero-3-phospho-(1′-rac-glycerol)
(DSPG), and the zwitterionic lipid 1,2-distearoyl-sn-glycero-3-
phosphocholine (DSPC) (Fig. 1c). The size and charge of the
phospholipid head group was expected to influence the
packing and surface charge of the composites, thereby regulat-
ing the accessibility of oxidants to the Ru cores. To stabilize
the liposomes under the reaction conditions used (high ionic
strength and acidic solutions), phospholipids with a stearoyl
group were chosen. Yasuda et al. reported that liposomes con-
taining phospholipids having saturated and relatively long
fatty acid chains are stable in acidic solutions.15 In addition,
3.6 mol% of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-
Fig. 1 (a) A schematic representation of the composite of a liposome
and a lipophilic Ru complex. The molecular structures of (b) 1 and 2 and
(c) the phospholipids used.
electrocatalytic oxygen evolution from water occurred at poten-
tials of ca. 1.3 V vs. Ag/AgCl (Fig. 2a). These results show that
the properties of the prepared lipophilic Ru complexes were
the same as that of cRu.
N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG2000
) was
added to each composite to reduce any aggregation and fusion
of the liposomes.16
We succeeded in crystallizing an analogue of 2, [Ru(terpy)-
(L3)(CH3CN)](PF6)2 (L3 = cholesterol 5′-methyl-2,2′-bipyridine-
Liposome composites were prepared using a lipid thin film
hydration method with a 9 mol% chloroform solution of 1 or 2
followed by sonication (Table 1). Formation of the compo-
site was confirmed directly using confocal laser scanning
microscopy (CLSM), in which a giant unilamellar vesicle was
used for visualization (Fig. S3†), and the contents of 1 and 2
were determined using inductively coupled plasma atomic
emission spectroscopy. The UV-Vis spectra of the samples are
shown in Fig. 2b. 1·PC and 1·PG exhibited an MLCT absorp-
tion maximum at 457 nm, which is almost the same as that of
1 in acetonitrile. However, the MLCT absorption maximum in
1·PA and that in 2·PG were red-shifted to 495 nm and 472 nm
when compared with that of 1 (at 456 nm) and 2 (at 465 nm)
in acetonitrile, respectively. The Ru cores of 1·PA and 2·PG
could be influenced by the phosphate groups of DSPA and by
those of DSPG, because the MLCT band of cRu in 50 mM
Na2HPO4 showed a shift similar to those of 1·PA and 2·PG
(Fig. S4†).
Fig. 2 (a) Cyclic voltammograms of 1,
2 and a blank solution in
aqueous 0.5 M H2SO4 under a N2 atmosphere. (b) UV-visible spectra of
1·PA (green), 1·PC (blue), 1·PG (red), and 2·PG (black) in H2O (Ru concen-
tration = 6 μM).
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Table 1 Zaverage diameter and zeta potential of the liposome composites
Lipophilic
Zave. diameter
(nm)
Zeta potential
(mV)
Turnover number
after 12 h
Composite
Ru complexa
Phospholipidb
1·PA
1·PC
1·PG
2·PG
PA
PC
PG
1
1
1
2
—
—
—
DSPA
DSPC
DSPG
DSPG
DSPA
DSPC
DSPG
65.1
108.1
41.6
56.7
74.4
63.3
50.7
−56.5
30.5
133
118
145
53
—
—
−49.9
−50.8
−78.5
−36.3
−70.1
N.D.
a The concentration of the lipophilic Ru complexes was 9 mol%. Liposomes without lipophilic Ru complexes contained 9 mol% of cholesterol
instead of 1 and 2. b All composites contained 3.6 mol% DSPE-PEG2000
.
The average particle size of the composites was measured
using dynamic light scattering (Table 1). The Zave. diameters of
1·PA, 1·PG, and 2·PG were in the range 40–65 nm, and were
almost the same as that of the liposomes without lipophilic Ru
complexes, except for 1·PC. The average particle size of 1·PC
(108 nm) was about twice as large as that of PC. Because larger
liposomes commonly have a smaller degree of curvature and
have a more closely packed lipid bilayer,17 the Ru complex
moieties of 1 in 1·PC could interact electrostatically with DSPC
head groups. Furthermore, the fixation of the lipophilic Ru
complexes on the liposome surface was confirmed from the
zeta potential. The zeta potentials of all the liposome compo-
sites were more positive than those of the non-complexed lipo-
somes (Table 1). This observation is attributed to the positive
charge of the Ru core of 1 and of 2. The zeta potentials of the
composites depended on the type of phospholipid head group.
1·PC containing zwitterionic DSPC was determined to have a
positive surface charge (30.5 mV), although other composites
had a negative surface charge. This result suggests the lipo-
some surface as the location of the Ru cores, and that the
charges surrounding the Ru cores were controlled by the types
of phospholipid head groups.
The evolution of O2 was examined at 20 °C under an Ar
atmosphere in the presence of 195 mM (NH4)2[Ce(NO3)6] as an
oxidant and 0.08 mM of the liposome composites (Fig. 4). The
stability of the composites under the reaction conditions was
confirmed using CLSM. The reaction between 1·PG and 40
equiv. of (NH4)2[Ce(NO3)6] was monitored spectrophotometri-
cally (Fig. S5†). The spectrum of the mixture after 12 h showed
the bleaching of the MLCT band, corresponding to the oxi-
dation of the RuII species by Ce4+.12 After addition of an excess
of ascorbic acid to the reaction mixture, the MLCT band of 1
was recovered.12 In addition, the reaction mixture evolved a
negligible amount of carbon dioxide during the reaction,
which was confirmed by gas chromatography. These results
provided good corroboration for the robustness of 1·PG during
the O2 evolution reaction. 1·PG exhibited the highest catalytic
activity, which was of the same order as that of cRu.12 The
total amount of O2 evolved by 1·PG was 23.2 μmol, which is 2.7
times greater than that evolved by 2·PG. 1·PG and 2·PG have
comparable surface environments regarding Zave. diameter and
Fig. 4 The evolution of oxygen from an aqueous 195 mM (NH4)2-
[Ce(NO3)6] solution in the presence of 1·PA (green), 1·PC (blue), 1·PG (red),
2·PG (black), and PG (orange).
zeta potential; the difference in catalytic reactivity can thus be
explained by the location of the Ru cores on the liposome
surface, which was adjusted by the linker lengths of L1 and L2.
The distance between the Ru and the ester oxygen atom O2 in
the crystal structure of [Ru(terpy)(L3)(CH3CN)](PF6)2 (analogue
of 2) is ca. 5.4 Å, which is less than the thickness of the PG
head group (ca. 9 Å) (Fig. 3).18 Thus, the Ru core of 2·PG would
be embedded in the head group region, resulting in a reduced
accessibility of Ce4+ ions. In addition, the O2 evolution behav-
iour of these composites changed depending on the types of
phospholipids. The activities of 1·PG and 1·PA, whose surfaces
are net negatively charged, were significantly higher than that
of 1·PC, whose surface is net positively charged. In the case of
1·PA, the O2 evolution began without an induction period. To
help understand the initial process of the reaction, the reac-
tion between the composites and 40 equiv. of (NH4)2[Ce(NO3)6]
was monitored by UV-visible spectroscopy (Fig. S6†). Soon
after mixing 1·PA with Ce4+, the spectrum showed rapid
bleaching of the MLCT band. In contrast, the MLCT bands of
1·PC and 1·PG gradually decayed. This means that the rela-
tively small and negatively charged head group of PA is
effective in promoting the accessibility of Ce4+ ions to the Ru
core. This result suggests that (1) the location of the Ru cores
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is on the liposome surface, and (2) the charge of the phospho-
lipid head group is an important consideration in the develop-
ment of catalytic systems based on liposome space.
Notes and references
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Conclusions
We fabricated O2 evolution catalysts using liposomes and lipo-
philic ruthenium complexes. We found that the surrounding
environment of the ruthenium reaction centre is an important
factor controlling the O2 evolution reaction. In particular, the
position of the Ru cores on the liposome surface influences
the accessibility of oxidizing reagents, and the reaction activity
can be regulated by the linker length of the lipophilic ligand
and by the phospholipid head group. This is a new method for
regulating molecular catalyst systems. Compared with the reac-
tivity of the photocatalytic water oxidation system using Ru
complexes with vesicles reported by Hansen et al.,6 our system
showed lower catalytic reactivity than our system. Because the
types of catalysts and reactions are completely different in
these two systems, it is not easy to determine the reason for
the different reactivity. One possible reason is that the lipophi-
lic complexes were installed on the outer and inner surfaces in
the current method. Because oxidants could not pass through
the lipid bilayer, the complexes fixed on the inner surface do
not participate in the reaction. The catalytic activity of the
composites would be improved by establishing a method that
selectively fixes the metal complexes on the outer surface.
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Acknowledgements
This work was supported by a Grant-in-Aid for Challenging
Exploratory Research (no. 26620049) from MEXT, Japan. T. K.
thanks the MEXT for a Grant-in-Aid for Young Scientists (B)
(no. 30467279), Kyushu University Interdisciplinary Program
in Education and Projects in Research Development. This
work was partly supported by Nanotechnology Platform
Program (Molecule and Material Synthesis) of MEXT, Japan,
and the Center of Advanced Instrumental Analysis, Kyushu
University.
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