17712 Phys. Chem. Chem. Phys., 2011, 13, 17712–17721 This journal is c the Owner Societies 2011 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 17712–17721 A modular molecular photovoltaic system based on phospholipid/ alkanethiol hybrid bilayers: photocurrent generation and modulationw Hong Xie, Kai Jiang and Wei Zhan* Received 25th May 2011, Accepted 22nd August 2011 DOI: 10.1039/c1cp21701a Monolayer quantities of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), incorporated with either fullerenes or ruthenium tris(bipyridyl) (Ru(bpy) 3 2+ ) complexes, were formed on ferrocene-terminated C11-alkanethiol self-assembled monolayers (SAMs) through lipid fusion. Thus formed hybrid structures are characterized by quartz crystal microbalance, UV-vis spectroscopy, cyclic voltammetry and impedance analysis. In comparison to lipid monolayers deposited on C12-alkanethiol SAMs, photocurrent generation from these ferrocene-based structures is significantly modulated, displaying attenuated anodic photocurrents and enhanced cathodic photocurrents. While a similar trend was observed for the two photoagents studied, the degree of such modulations was always found to be greater in fullerene-incorporated bilayers. These findings are evaluated in the context of the film structure, energetics of the involved photo(electrochemical) species and cross-membrane electron-transfer processes. 1. Introduction A multicomponent photocurrent generation system, based on photoactive agents (either fullerene or ruthenium tris(bipyridyl) (bipyridyl = 2,2 0 -bipyridine) species) and ferrocene co-assembled in a hybrid lipid bilayer, is described here. In both cases, it is found that the presence of a ferrocene layer in between the photoactive layer and the electrode can significantly modulate the observed photocurrents, leading to an enhanced cathodic current and an attenuated anodic current. This work presents an alternative strategy of organizing multiple photo-/redox-active agents on electrodes for photocurrent generation and may also provide a modular model system for studying photoinduced electron transfer across lipid membranes. There has been a lasting research interest 1–3 in mimicking natural photosynthesis using synthetic approaches and for over three decades, lipid-bilayer based structures 4,5 have been employed in building such biomimetic systems. Because lipid bilayers typically are only 3–5 nm thick, electron tunneling and photoinduced electron transfer across the lipid membrane can occur under favorable conditions. Calvin and his coworkers 6 were among the first to recognize the utility of lipid bilayers in building artificial photosynthetic systems. By using liposomes, for example, they showed that efficient photosensitized electron transfer across a lipid bilayer could be established between membrane-bound ruthenium tris(bipyridyl) complexes and aqueous viologen and EDTA. 7,8 Using surfactant vesicles and black lipid membranes (BLMs), Fendler and coworkers 9,10 produced a series of biomimetic photosynthetic systems in the 1980–90s. Specifically, photovoltages could be reliably generated across BLMs 11 decorated with photoactive CdS particles. In still another approach, Gust, Moore and Moore 12,13 demonstrated that a quinone–porphyrin–carotene triad conjugate could be reconstituted in liposomes and upon light excitation, this synthe- tic complex could drive the production of proton potential 14 as well as ATP synthesis 15 at the liposome hosts. These results, together with numerous others, 4,5,16 clearly point to the useful- ness and versatility of liposomes and BLMs in building artificial photosynthetic systems. Continuing research in membrane biophysics and bio- technology has brought forth new lipid-bilayer structures in recent years. For example, the work of McConnell, 17,18 Boxer, 19,20 Sackmann 21,22 and others has established that well- defined lipid bilayers can be formed on hydrophilic substrates such as glass and oxidized silicon. Structurally, these bilayers are symmetrical, with controllable fluidity, and stable over a long period of time (i.e., hours or longer) when immersed in aqueous media, which warrants their use as a model of cellular membranes. More recently, it has also been shown that a monolayer of lipids can be deposited on a pre-formed self-assembled monolayer, thus producing a hybrid bilayer structure 23–25 on a solid support. Several types of hybrid bilayers have been successfully constructed on alkane/lipid SAMs on gold 26,27 as well as oxide substrates. 28,29 Importantly, since these bilayers are formed in two separate steps, it becomes possible to control the chemical makeup of each leaflet of the bilayer. Department of Chemistry and Biochemistry, Auburn University, Auburn, AL 36849, USA. E-mail: [email protected]; Tel: +1 (334) 844-6984 w Electronic supplementary information (ESI) available. See DOI: 10.1039/c1cp21701a PCCP Dynamic Article Links www.rsc.org/pccp PAPER
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17712 Phys. Chem. Chem. Phys., 2011, 13, 17712–17721 This journal is c the Owner Societies 2011
A modular molecular photovoltaic system based on phospholipid/
alkanethiol hybrid bilayers: photocurrent generation and modulationw
Hong Xie, Kai Jiang and Wei Zhan*
Received 25th May 2011, Accepted 22nd August 2011
DOI: 10.1039/c1cp21701a
Monolayer quantities of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), incorporated
with either fullerenes or ruthenium tris(bipyridyl) (Ru(bpy)32+) complexes, were formed on
ferrocene-terminated C11-alkanethiol self-assembled monolayers (SAMs) through lipid fusion.
Thus formed hybrid structures are characterized by quartz crystal microbalance, UV-vis
spectroscopy, cyclic voltammetry and impedance analysis. In comparison to lipid monolayers
deposited on C12-alkanethiol SAMs, photocurrent generation from these ferrocene-based
structures is significantly modulated, displaying attenuated anodic photocurrents and enhanced
cathodic photocurrents. While a similar trend was observed for the two photoagents studied,
the degree of such modulations was always found to be greater in fullerene-incorporated bilayers.
These findings are evaluated in the context of the film structure, energetics of the involved
photo(electrochemical) species and cross-membrane electron-transfer processes.
1. Introduction
A multicomponent photocurrent generation system, based on
photoactive agents (either fullerene or ruthenium tris(bipyridyl)
(bipyridyl = 2,20-bipyridine) species) and ferrocene co-assembled
in a hybrid lipid bilayer, is described here. In both cases, it is
found that the presence of a ferrocene layer in between the
photoactive layer and the electrode can significantly modulate
the observed photocurrents, leading to an enhanced cathodic
current and an attenuated anodic current. This work presents an
alternative strategy of organizing multiple photo-/redox-active
agents on electrodes for photocurrent generation and may also
provide a modular model system for studying photoinduced
electron transfer across lipid membranes.
There has been a lasting research interest1–3 in mimicking
natural photosynthesis using synthetic approaches and for
over three decades, lipid-bilayer based structures4,5 have been
employed in building such biomimetic systems. Because lipid
bilayers typically are only 3–5 nm thick, electron tunneling and
photoinduced electron transfer across the lipid membrane can
occur under favorable conditions. Calvin and his coworkers6
were among the first to recognize the utility of lipid bilayers in
building artificial photosynthetic systems. By using liposomes,
for example, they showed that efficient photosensitized electron
transfer across a lipid bilayer could be established between
membrane-bound ruthenium tris(bipyridyl) complexes and
aqueous viologen and EDTA.7,8 Using surfactant vesicles and
black lipid membranes (BLMs), Fendler and coworkers9,10
produced a series of biomimetic photosynthetic systems in the
1980–90s. Specifically, photovoltages could be reliably generated
across BLMs11 decorated with photoactive CdS particles. In still
another approach, Gust, Moore and Moore12,13 demonstrated
that a quinone–porphyrin–carotene triad conjugate could be
reconstituted in liposomes and upon light excitation, this synthe-
tic complex could drive the production of proton potential14 as
well as ATP synthesis15 at the liposome hosts. These results,
together with numerous others,4,5,16 clearly point to the useful-
ness and versatility of liposomes and BLMs in building artificial
photosynthetic systems.
Continuing research in membrane biophysics and bio-
technology has brought forth new lipid-bilayer structures
in recent years. For example, the work of McConnell,17,18
Boxer,19,20 Sackmann21,22 and others has established that well-
defined lipid bilayers can be formed on hydrophilic substrates
such as glass and oxidized silicon. Structurally, these bilayers
are symmetrical, with controllable fluidity, and stable over
a long period of time (i.e., hours or longer) when immersed
in aqueous media, which warrants their use as a model of
cellular membranes. More recently, it has also been shown
that a monolayer of lipids can be deposited on a pre-formed
self-assembled monolayer, thus producing a hybrid bilayer
structure23–25 on a solid support. Several types of hybrid bilayers
have been successfully constructed on alkane/lipid SAMs on
gold26,27 as well as oxide substrates.28,29 Importantly, since these
bilayers are formed in two separate steps, it becomes possible to
control the chemical makeup of each leaflet of the bilayer.
Department of Chemistry and Biochemistry, Auburn University,Auburn, AL 36849, USA. E-mail: [email protected];Tel: +1 (334) 844-6984w Electronic supplementary information (ESI) available. See DOI:10.1039/c1cp21701a
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Fig. 10 Fluorescence emission spectra of 2% Ru(bpy)32+–DOPE
assembled in either the POPC/Fc-C11SH SAM (spectra 1 and 3) or
the POPC/C12SH SAM (spectra 2 and 4) bilayers. In all samples, the
oxygen was removed from the solution using an enzymatic cocktail
solution containing glucose oxidase, catalase and glucose (see
the Experimental section). Of these, samples used to acquire spectra
2 and 4 in addition contained 50 mM ascorbate. Excitation was made
at 470 � 20 nm.
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 17712–17721 17721
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