Volume 1 photovoltaic solar energy 1 31 – artificial leaves towards bio inspired solar energy converters

Volume 1 photovoltaic solar energy 1 31 – artificial leaves towards bio inspired solar energy converters Volume 1 photovoltaic solar energy 1 31 – artificial leaves towards bio inspired solar energy converters Volume 1 photovoltaic solar energy 1 31 – artificial leaves towards bio inspired solar energy converters Volume 1 photovoltaic solar energy 1 31 – artificial leaves towards bio inspired solar energy converters Volume 1 photovoltaic solar energy 1 31 – artificial leaves towards bio inspired solar energy converters Volume 1 photovoltaic solar energy 1 31 – artificial leaves towards bio inspired solar energy converters Volume 1 photovoltaic solar energy 1 31 – artificial leaves towards bio inspired solar energy converters

A Pandit and RN Frese, VU University Amsterdam, Amsterdam, The Netherlands

© 2012 Elsevier Ltd All rights reserved

1.31.1 The Design of Natural Photosynthesis

1.31.1.1 Photosynthetic Light-Harvesting Antennae

1.31.1.3.4 Light harvesting and charge transport in purple bacteria

1.31.2 Design Principles of Natural Photosynthesis

1.31.2.1 Photon Absorption, Excitation Energy Transfer, and Electron Transfer

1.31.2.1.1 Photon absorption

1.31.2.1.2 Excitation energy transfer

1.31.2.1.3 Electron transfer

1.31.2.2 Photochemical Thermodynamics of Energy Storage

1.31.3 The Design of an Artificial Leaf

1.31.3.1 Interfacing Proteins onto Solid-State Surfaces

1.31.3.2 Protein Maquettes for Artificial Photosynthesis

1.31.3.3 Bio-Inspired Self-assembled Artificial Antennae

1.31.3.3.1 Chlorosome-based, light-harvesting antennae

storing or consuming energy; other forms being via fat and

used as reducing power

macrocycle made from four

1.31.1 The Design of Natural Photosynthesis

Photosynthetic organisms are ubiquitous on the surface of the Earth and, in fact, responsible for the development and sustenance of all life on the planet They all use the same basic pattern whereby light energy from the sun is initially absorbed and concentrated by

an antenna system, then transferred to a reaction center (RC) where charge separation takes place, followed by reactions that convert the captured light energy into a chemical form Photosynthesis can be divided into oxygenic photosynthesis, carried out by

ADP ATP

ATPase bc1

RC

Cytochrome c2 H+

LH1 are LHCs that absorb light and transfer excited-state energy among intra- and intercomplex bound pigments The terminal acceptor is the RC, which is completely encircled by the LH1 complex There the excited-state energy induced ETs among cofactors until two electrons reside on the terminal acceptor, which is a quinone molecule This molecule becomes double protonated upon which it exits the complex into the lipid phase of the membrane, diffusing toward a proton-translocating pump, the bc1 complex A small protein cytochrome c 2 shuttles between the bc1 complex and the RC to re-reduce the primary electron donor, thus closing the circuit When protons are actively transported back from the periplasm to the cytoplasm through the ATP synthase complex, the energy is chemically stored as ATP Reproduced from Figure 1 in Hu XC, Damjanovic A, Ritz T, and Schulten K (1998) Architecture and mechanism of the light-harvesting apparatus of purple bacteria Proceedings of the National Academy of Sciences of the United States of America 95:

5935–5941 [1], with permission of National Academy of Sciences USA

cyanobacteria, algae, and plants that produce oxygen, and nonoxygenic photosynthesis, carried out by purple, green sulfur, and

Figure 1 for a schematic representation of nonoxygenic photosynthesis The factory where the photosynthetic process takes place is the cell, or a specialized compartment within the cell Each reaction is carried out by specific proteins or complexes of proteins that often bind functional molecules for light absorption, redox chemistry, and charge transfer Proteins are nanometric-sized biopo­lymers of specific shape and function, and are dynamically organized within, or associated with, a lipid membrane Membranes allow the compartmentalization of the various reactions and are especially important for ATP formation, which is driven by the electrochemical gradient formed by the protons that are pumped across the membrane and enable the synthesis of ATP during controlled back transfer through the ATP synthase enzyme

1.31.1.1 Photosynthetic Light-Harvesting Antennae

Natural light-harvesting antenna complexes (LHCs) are complexes of proteins that are organized to collect and deliver light excited-state energy to the RC where charge separation takes place [2] They permit an organism to increase greatly the absorption cross section for light without having to build an entire RC and associated electron-transfer (ET) system for each pigment, which would be very costly in terms of biosynthesis and cellular resources The intensity of sunlight is dilute so that any given pigment molecule absorbs at most a few photons per second By incorporating many pigments into a single antenna unit and creating supramolecular assemblies of antenna units, large photosynthetic membrane surfaces are covered, ensuring that photons striking any spot on the surface will be absorbed The antenna units are interconnected to carry light energy through exciton migration over

Photosynthetic organisms are equipped with a light-harvesting antenna containing the pigments (bacterio) chlorophylls ((B)Chl), carotenoids (Cars), or phycobilins While they share their function of funneling the excitation energy into the RC, there is a large variety in the antenna structures and macro-organization of different species

Most organisms have antenna systems that are embedded in the photosynthetic membrane, but green sulfur bacteria and cyanobacteria contain aggregated antenna structures that are associated with the membrane Green sulfur bacteria contain antennae built from large 3D tubular aggregates of self-assembled (B)Chls with small amounts of Cars and quinones, contained in specialized

to operate at extremely low light intensities and therefore is extremely large compared to the antennae of other species This is the only species in which the antenna structures are self-assembled from pigment molecules without the aid of a protein environment Cyanobacteria use rod-like antenna structures called phycobilisomes [5], containing protein assemblies in which the proteins have prosthetic groups of linear pyrroles, the phycobilin pigments The phycobilisomes are associated with the photosynthetic membranes and transfer their excitation energy to two photosystems that are embedded in the membrane: photosystem 1 (PS1) and photosystem

2 (PS2) that contain the RCs This is the only species in which its antenna pigments are covalently bound to proteins

energy transfer via excitonic interactions The combination of Cars in van der Waals contact with the (B)Chls allows efficient and rapid energy transfer from the Cars (absorbing in the blue-green region of the solar spectrum) to the (B)Chls that absorb in the red

(a) (b)

(c)

Reaction center

TRENDS in Plant Science

complex 1–RC (LH1–RC) complexes of purple nonsulfur bacteria, (b) chlorosome antenna of green sulfur bacteria, (c) phycobilisome antenna of cyanobacteria, and (d) light-harvesting complex II (LHCII) antenna attached to photosystem 2 in plants Reproduced from Figure 2 in Mullineaux CW

or infrared region, covering a larger part of the solar spectrum The Cars also protect the organism from photo-oxidative damage by

pigment interactions within the antennae broaden the lowest excited-state absorption spectrum (increasing the cross section for

pigment with the lowest excited-state energy The arrangements of the antenna complexes are such that light energy can migrate among the complexes via Förster energy transfer and that the antennae surround the RC so that light energy migration is funneled into the RCs from different sites

1.31.1.2 Photosynthetic RCs

RCs are complexes of proteins with embedded cofactors that act simultaneously as chromophores, redox groups, and ET factors; see Reference 6 and Figure 3 for a detailed description of the structural and functional characteristics of RCs In RCs, the excited-state energy generated by photon absorption is utilized for the release of electrons after which energy back transfer has become impossible ETs are directional among the cofactors and are accompanied by a loss of free energy that ensures a fast-forward and slow-backward transfer rate The primary electron donor and the terminal acceptor are separated in space close to either side of a membrane where they are coupled to other proteins or molecules via redox chemistry; at that point, the energy is stored [8]

As such, RCs can carry out the primary photochemical reactions without the need of LHCs discussed in the previous section While Nature has developed a large variety of LHCs among the species [9], RCs have remained surprisingly homologous throughout billions of years of evolution This may be a reflection of the much more stringent design principles for ET compared to energy transfer, for instance, the distance dependency of the rates All RCs contain dimers of proteins that covalently link the cofactors Attached to the protein dimers are other proteins enabling redox chemistry via again other proteins or redox molecules Differentiations between the species regarding the RCs are the type of chromophores used, the wavelengths of absorption, the redox potential between first electron donor and final acceptor, and the redox reactions leading to re-reduction of the primary donor and the re-oxidation of the terminal acceptor [10]

In Figure 4, the redox potentials of the components active in ETs within the four types of RCs are depicted Two types are found

in anoxygenic photosynthesis, which are the evolutionary precursors of the other two oxygenic types The latter are part of the photosystem 1 and 2 supercomplexes (PS1 and PS2) that are coupled systems allowing the formation of NADPH When the need for NADPH is low, PS1 and PS2 can become uncoupled and ATP is produced by cyclic electron transport, similar to the process in purple bacterial RCs

As can be seen in Figure 4, there is a common theme among all types of RCs regarding the redox potential generated after the initial electron donors (P870, P680, P700, or P840) have been excited (indicated by P*) Light energy is used to promote the primary electron donors to a redox state that is more negative than the subsequent electron acceptors, thus promoting electro­chemically downhill ETs A most striking difference between the primary donor of photosystem 2 and that of the other RCs is the extremely large redox potential of P680, +1.2 V versus standard hydrogen electrode; it can oxidize water into protons and oxygen [12]

Q side

H-polypeptide

QA quinone

Fe

QB quinone

white, green, and yellow tubes, respectively The approximate position of the membrane is shown as a gray box, with the primary donor side (P side) and quinone side (Q side) of the RC labeled The embedded cofactors are shown as sticks, with the Car (bottom left) shown with teal carbons (b) Enlarged view of the BChl, BPhe, and quinone cofactors, color coded as in (a) with oxygens in red, nitrogens in blue, and Fe and Mg atoms in brown or magenta spheres Membrane-spanning ET starts from the primary donor pair of BChls (PA/PB – pink carbons) and proceeds via the BA BChl (green carbons), HA

BPhe (yellow carbons), and QA ubiquinone (cyan carbons) Electrons are passed on to the dissociable QB ubiquinone (cyan carbons), which can exchange with exogenous ubiquinone The BB BChl and HB BPhe do not participate in ET For clarity, the large hydrocarbon side chains of the BChl, BPhe, and quinone cofactors are not shown (c) Structure of the RC–LH1 complex from Rhodopseudomonas palustris [7]; Protein DataBank (PDB) entry 1PYH – resolution 0.48 nm The central RC, colored as for (a) and viewed from the same direction, is surrounded by concentric cylinders of multiple copies of α (cyan ribbons) and β (magenta ribbons) polypeptides, sandwiching a ring of BChls (colored alternately red and orange) (d) Absorbance spectra of the Rhodobacter sphaeroides RC and the Rhodopseudomonas acidophila RC–LH1, showing band attributions Note the real contribution of the RC pigments

to the absorbance of RC–LH1 complexes can be viewed in the RC–LH1 spectrum at 800 and 750 nm (Ba/Bb and Ha/Hb pigments, respectively) Structure and pigment organization in the photosynthetic RC of purple bacteria Courtesy of Prof Michael Jones, Sheffield University

In fact, only P680 possesses a redox potential higher than the constituent Chl molecules [13] The high redox potential of P680 is most

shielding [14] As will be discussed in Section 1.31.4, the utilization of photon energy to split water into protons and oxygen has yet to

be accomplished by a man-made catalyst In fact, since protons can be redirected to form molecular hydrogen or other energy-rich compounds, PS2 mimics can be regarded as the Holy Grail in artificial photosynthesis

1.31.1.3 Supramolecular Organization

1.31.1.3.1 Supercomplexes

As discussed in the previous sections, photosynthesis combines complexes of proteins for light-absorbing and ETs in combination

unit very well capable of performing primary photochemistry But because of the dim- or low-light conditions within their

−1.5

P700*

P680

cofactors The initial electron donors are indicated by their wavelength of absorption P870, P680, P700, and P840 Reprinted from Figure 3 in Hillier W and Babcock GT (2001) Photosynthetic reaction centers Plant Physiology 125: 33–37 [11], with permission of American Society of Plant Physiologists

[16] are large photosystems consisting of RCs linked to a variable set of LHCs In purple bacteria, the core complex is an RC surrounded by an LHC consisting of a ring of proteins [7] This core complex can form dimers with another, similar, core complex, which in turn may be surrounded by other smaller ring-like LHCs [17]

1.31.1.3.2 Supramolecular complexes

organization of the photosynthetic components within the membranes In plants, PS1 and PS2 are well separated in different

the supramolecular organization has been found to be dependent on the light conditions [22] Most strikingly, the photosynthetic membranes are heavily packed with RCs and LHCs, much like a 2D crystal, which places much strain on diffusive processes [23] While distinct supramolecular organizations exist, there is no underlying structure holding the components in place Therefore, a thermodynamic model has been derived showing that entropy can be a driving force for domain formation of RCs and LHCs (see Reference 24 and Figure 5) Due to heavy packing, the clustering of larger photosystems, separated from the smaller LHCs, enhances the volume that can be occupied by the LHCs such that the total entropy is actually maximized This effect has been shown for colloidal particles before and may be generalized for the nanometric-sized protein complexes as well The more fluid domain of smaller LHCs may also enable diffusive pathways necessary for self-repair and adaptability The model predicts how different supramolecular organizations may be inferred by specific tuning the size and shape of the components Due to packing, also the ultrastructure of the entire photosynthetic membrane can be predicted from the same self-organizing principles [24]

1.31.1.3.3 Membranes

The membrane in which the photosynthetic complexes primarily reside is a two-dimensional (2D) structure and can be regarded as

a monolayer film of energy-transducing nanometric components Photosynthetic species enhance their photosynthetic volume by folding the membranes in three dimensions (see Figure 6) Plant membranes containing PS2 are layered close to each other [20]; purple bacterial membranes may be folded similarly or form small spheres, budded off from a linear membrane [25] Whether or not there exists functionality from interacting complexes embedded in different membranes has to be established

1.31.1.3.4 Light harvesting and charge transport in purple bacteria

To illustrate how the different components work together as a solar energy converter, we describe here how natural photosynthesis proceeds in one of the most simple and well-characterized photosynthetic systems: the photosynthetic machinery of purple nonsulfur bacteria

100 nm

50 nm

Photon

Energy transfer

consists of many light-absorbing components, also named pigments that also transfer the energy among them The target is an RC where this energy triggers a series of ET events starting off at a special site, the primary donor (b) Image of protein complexes embedded within a bacterial photosynthetic membrane obtained by atomic force microscopy by Bahatyrova and co-workers (Enschede, the Netherlands) In this particular membrane, RC–LH1 complexes form dimeric supercomplexes, which in turn are arranged in rows Adapted from Figure 1 in Bahatyrova S, Frese RN, Siebert CA, et al (2004) The native architecture of a photosynthetic membrane Nature 430: 1058–1062 [17], with permission of Nature Publishing Group (c) Result of coarse-grained modeling of the photosynthetic membrane based on colloidal theory by Frese et al [24] A 2D lattice, occupied equally with dimeric

RC–LH1 and RC–LH2 complexes, is equilibrated by a Monte Carlo method Energy minimization is based only on differences in size and shape of the two components Besides the two different domains as observed in the AFM images, the model also predicts the correct shape of the entire membrane Various subsequent realizations show the green domain to be highly fluid, while the red domain remains rigid Adapted from Figure 6d in Frese RN, Pamies JC, Olsen JD, et al (2008) Protein shape and crowding drive domain formation and curvature in biological membranes Biophysical Journal 94: 640–647 [24], with permission of Biophysical Society, USA

picture on the right panel PS1 is indicated in blue, ATP synthase in red, and b6f complexes in orange Adapted from Figures 7 and 9 in Dekker JP and Boekema EJ (2005) Supramolecular organization of thylakoid membrane proteins in green plants Biochimica et Biophysica Acta—Bioenergetics 1706:

12–39 [20], with permission of Elsevier Science B.V

The process starts with capture of light in the peripheral light-harvesting antennae, called LH2 The LH2 are ring-shaped protein oligomers that contain a ring of eight or nine BChl a monomers, called B800, and a second ring of eight or nine BChl a dimers, called B850 [26, 27] (see Figure 7) The pigment names are based on their lowest excited-state absorption maximum In addition, the LHs contain Cars that are located in van der Waals contact with the BChls, allowing efficient and rapid energy transfer [28] Excitations of the B800 BChls are localized on a single chromophore and move by Förster-type electronic energy transfer (EET) to

are elliptical-shaped oligomers and contain Cars and a ring of 15 or 16 BChl a dimers, called B875 The B875 BChls are also strongly coupled and their arrangement is homologous to the B850 BChls in LH2 [7, 32]

bacterial RCs organize the pigments into two parallel ET pathways, termed the A side and the B side However, the RC only utilizes

absorbs a photon, it ejects an electron, which is transferred via another molecule of BChl to a bacteriopheophytin (BPheo) This

8700.5

and the RC are from the crystal structures, and LH1 is simulated by analogy to LH2 Adapted from Figure 5 in Hu XC, Damjanovic A, Ritz T, and Schulten K

States of America 95: 5935–5941 [1], with permission of National Academy of Sciences (b) Dynamics of energy-transfer processes between bacteriochlorophylls B850 and B800 in LH2, B875 in LH1, and the special pair P870 in the RC Adapted from Figure 1 in Yang M, Agarwal R, and Fleming

[29], with permission of Elsevier Science B.V (c) AFM image of arrangement of LH2–RC (highlighted in green) and LH1–RC (red) complexes in the bacterial membrane The bright spots are the LH1–RC complex and the inset in the second panel is a model using the known crystal structures to reproduce the image Adapted from Figure 1 in Bahatyrova S, Frese RN, Siebert CA, et al (2004) The native architecture of a photosynthetic membrane Nature 430: 1058–1062 [17], with permission of Nature Publishing Group (d) Energies and timescales of ET within the RC Adapted from Figure 6.3 in Blankenship RE (2002) Molecular Mechanisms of Photosynthesis Oxford, UK: Blackwell Science Ltd [2], with permission

membrane (see also Figure 1) The generated electrochemical gradient is used to drive synthesis of ATP by the membrane-bound

1.31.2 Design Principles of Natural Photosynthesis

synthetic units can be described by a defined set of length and energy scales that may be used as guidelines for the design of a so-called artificial leaf An excellent overview of these engineering guidelines is presented in the work of Noy and Dutton [33, 34]

1.31.2.1 Photon Absorption, Excitation Energy Transfer, and Electron Transfer

1.31.2.1.1 Photon absorption

s−1

1.31.2.1.2 Excitation energy transfer

Coulomb interactions in a Förster mechanism according to

6

r0

r

length scale of exciton migration in the antenna is large, the exciton lifetimes should be long enough to allow for photons striking

Once excitons are trapped in the RC, the process of charge separation should take place faster compared to back transfer to the

the RC Furthermore, surrounding of the RCs by assemblies of antenna pigments gives a spatial arrangement in which energy

1.31.2.1.3 Electron transfer

The third length scale is connected to the ET processes In the RC, electron transport via multiple steps is necessary because transfer across the membrane in a single step would be too slow to compete with decay of the excited state to the ground state and dissipation of the energy into useless heat If the electron and the hole can be further separated before charge recombination occurs, the electronic coupling and therefore the rate constant for recombination is drastically reduced The use of multiple cofactors

and the electronic interaction between the donor and the acceptor are described well by theory Equation [3] shows how the ET rate

2π

V2 1 – ð Δ G0 þ λ Þ 2 = 4λkT

the nuclear reorganization (structural change) within the donor, the acceptor, and the solvent required for the reaction to occur One

of the key features of eqn [3] is that it predicts that the rate of an ET reaction will slow down when the free energy of reaction

by application of the dielectric continuum model of a solvent

1.31.2.2 Photochemical Thermodynamics of Energy Storage

The photosynthetic apparatus is connected to the steady-state network of catalytic conversion reactions in the organism that is continuously dissipating energy If there is a shortage at one spot, it can be smoothly compensated from other dynamic reservoirs in the network

The storage and downstream utilization can be described as a single-step conversion of solar energy into Gibbs free energy In its simplest form, the photosynthetic solar cell is a heat engine that produces charge separation (see Figure 8) Inside the engine, a molecular absorber is excited that produces charge separation There are three basic conversion processes in the primary mechanism

of the photosynthetic solar cell [38]:

1 Excitation of a molecular absorber (Chl) with rate g

2 Energy conversion by charge separation with rate I into an electron and hole in dynamic equilibrium with the absorber

emitting heat flow Iqout is excited due to the heat flow Iqin from the solar reservoir at temperature T = 5800 K with a rate g to an excited state separated by an energy hν from the ground state The excited Chl state either decays with loss rate 1/τ or produces an electron–hole pair with net charge separation rate I and free energy Δμ = μe − μh Adapted from Figure 5 in Markvart T and Landsberg PT (2002) Thermodynamics and reciprocity of solar energy conversion Physica E 14: 71–77 [37], with permission of Elsevier Science B.V

The importance of a tight connection between energy conversion and energy storage was noted at an early stage [39] First, excitation

of a molecular Chl absorber in exchange with the field of solar irradiation leads to a difference in chemical potential:

½4

½Chl

originating from the different temperatures of the incoming radiation and of the heat reservoir

A photovoltaic solar cell produces electricity, while a photosynthetic RC produces a photochemical steady state with a voltage over the membrane and charge separation in dynamic equilibrium with the absorber When the light is switched on and the absorber is coupled to a storage reservoir, a steady state is produced, where the formation of the excited state is balanced by its decay

p0 ′q0 ′ represents the difference in chemical potential or free energy produced by light-induced charge separation and contains an

ATP, an intermediate energy carrier When a storage reservoir is connected to the absorber, energy flows can be forwarded from the absorber into the reserve and backwarded from the reserve back into the absorber, where it can be emitted In this way, assimilation has to compete against depletion of the reserve by backward reaction coupled with radiative loss in the absorber The entire system has to operate linearly and close to the thermodynamic limit to minimize losses The net production rate of charge separation and energy extraction can be described by the solar cell equation:

Δμ

BT

The energy storage from an elementary steady-state process can be generalized according to the equation

favor the forward reaction over the backward process, the energy of the combined products is lower than that for the excited state by

Equation [7] can be generalized to storage processes further downstream and longer timescales, where the entire system is in

reactions and represents the work that can be obtained, for example, from sugar or biomass

When the solar radiation is interrupted, the unavoidable back reaction causes gradual depletion of the accumulated energy For instance, a plant in the dark produces a small flux of photons (luminescence) from its reserve that was stored during illumination

 

periods This leads to a generalized expression for the thermodynamic limit of energy storage in a steady-state reserve at all time and

t

with t being the storage time

The highest yields and efficiencies are attainable when energy is used as soon as it is assimilated, avoiding intermediate storage, for instance, by generating and using electricity, by running nanoscale catalytic converters directly from the photoconverter and storing the energy in a redox couple, by coupling a chemical cycle directly to a photoconverter like in the chloroplast, by connecting the chloroplast directly to respiration like in the plant, or by using light-driven cell factories for production of food and chemical feedstock

1.31.3 The Design of an Artificial Leaf

What is an artificial leaf? The answer depends on the focus point that one has when talking about a leaf Is a leaf a high surface area for the capture of photons? Or is it a natural design of a light energy to fuel converter? Or perhaps, is it an example of high­density-packed nanomachines that capture and utilize light energy?

self-assembly, self-repair, and adaptability Furthermore, a leaf constitutes different, well-defined nanometric systems, each of them precisely tuned to carry out a specific task and to interrelate with each other As such, the most prominent feature of how a leaf utilizes light energy is the supramolecular functioning of the several building blocks

The artificial leaf represents a different approach toward (photosynthesis) solar energy conversion compared to bio-based fuel production where natural or genetically modified organisms are used in an agricultural fashion An artificial leaf is a device where biological material may well be entirely absent At the same time, it has to feature some of the crucial aspects of natural photosynthesis Minimally, this is the self-assembly of building blocks into a functional supramolecular network that operates

on fine-tuned length and timescales constrained by physical rules

The hypothetical device constitutes three aspects of photosynthesis: (1) light capture, (2) charge separation and ET, and (3) catalysis The first two cover also the main aspects of photovoltaics In photosynthesis though, ETs can trigger the splitting of water into oxygen and protons, the cycling of electrons, or the generation of proton-motive force Artificial photosynthesis research targets all these aspects of natural photosynthesis and can be separated into two main approaches: bio-based systems and bio-inspired synthetic systems The first approach utilizes integrated biological components, isolated via biochemistry techniques from living organisms The latter approach mimics the biological systems by means of synthetic chemistry or surface physics For both lines of research, there is a necessity to interconnect compounds or systems with conducting material in order to extract electrons and re-reduce the primary electron donor In other words, the photovoltaic aspect of the artificial leaf constitutes the primary mode of operation and has to be solved for any artificial photosynthetic design

1.31.3.1 Interfacing Proteins onto Solid-State Surfaces

Biology may serve as a template for a bottom-up approach in nanotechnology [41] Ever since the discovery of the protein, it has been recognized that biological function is carried out starting from the nanoscale to higher length scales At the nanoscale, matter has several properties that are different from larger scales, notably large reactive surfaces and a molecular thermodynamic regime In biology, these properties lead to dynamic properties of the systems, allowing rapid response to changing impulses from the outside world Measuring

or utilizing environmental impulses that interact with biology in a technological fashion may utilize biological components [42] Such devices have been constructed, most famously the electrochemical blood glucose test strips for diabetes [43] Here, for an artificial leaf harboring features as in a biological cell but which is not a (synthetic) biological cell itself, adaptability implies nanometric components that are embedded within a flexible matrix To date, proteins or protein complexes are the only compounds known that may provide this adaptability outside a cell And for solar energy uses, photosynthetic proteins are the first to be utilized

Early research on interfacing photosynthetic proteins and conducting surfaces concentrated on the purple bacterial RC since it has been the best characterized photosynthetic complex [44] The RC encompasses the key features sought after in an artificial leaf: a self-assembling and high-pigment-density nanometric structure, with a near-unity quantum yield of directional energy and electron-transferring capability Likewise, LHCs enhance the absorption cross section considerably with only minimal energy losses [9] Ongoing photosynthesis research elucidates to more and more detail how pigments are organized by the proteins to accomplish these feats [45, 46] Fine-tuning includes the mutual orientation and distances of cofactors within degrees and angstroms relative to each other, leading to very precise energy-transfer and ET reactions While the research on protein functionality transferred into a device may have started (and perhaps still is) a proof-of-principle type of investigation, it will be hard to find any other matrix that can arrange light-active molecules with such precision [34] In this section, we will present an overview of the investigations on the interfacing of protein complexes and conducting material