We have removed these two obstacles by designing a PS-I biophotovoltaic whose IV characteristics can be easily studied under regular sunlight and its design and fabrication are amenable to low-cost, iterative optimization. To avoid denaturation, we treated PS-I with designer peptide surfactants1. To improve photovoltaic performance we increased the light absorption cross-section without changing the footprint by departing from the traditional flat electrode geometry in favor of mesoscopic, high-surface area semiconducting electrodes (TiO 2 nanocrystals and ZnO nanowires). Finally, we showed how high affinity peptide motifs10 bioengineered to promote selective adsorption to specific substrates can enhance photovoltaic performance. These materials, geometries and design resulted in simple, robust biophotovoltaic devices of unprecedented performance.

Photochemically active, trimeric PS-I was isolated and characterized from the thylakoids of the thermophilic cyanobacteria Thermosynechococcus elongatus as described in detail in Fig. 2 of Iwuchukwu et al.11. This PS-I was stabilized for several months in solution12 and in dry form by designer peptide surfactants1 ( Fig. 2 ). To build devices, PS-I molecules were air-dried on nanostructured semiconducting substrates and the circuits were completed by liquid electrolyte and platinized glass as is common for conventional DSCs ( Fig. 1 b ). We used nanocrystalline TiO 2 and ZnO nanowires to provide a large effective surface area (A eff ) for PS-I adsorption and light harvesting ( Fig. 3 ) and without any further optimization, these devices achieved electrical power outputs P out of up to 81 µW/cm2 and area-normalized short-circuit current densities I sc Norm of up to 362 µA/cm2 ( Fig. 4 ). These parameters are to be compared to the over 10,000 times lower I sc Norm (∼30 nA/cm2) reported previously with monochromatic illumination tuned to the ∼800 nm absorption peak of a monolayer of PS-I on a thin film of gold (actual power efficiency not reported)7and to the up to 120 µA/cm2 I sc Norm observed when isolated RCs were chemically bound to a series of evaporated metallic and semiconducting thin films and illuminated with 10 W/cm2 laser light (one-hundred times the power density of AM1.5 sunlight) all concentrated into the 808 nm absorption peak of RCs5. While there have been various estimated and theoretical maximum efficiencies (e.g. an upper possible limit of 20% efficiency for RC-biophotovoltaics5), there have been no reports of conversion efficiency η for PS-I (where , P in the total power of the incident light and P out the total resulting electrical power). Our photocurrent measurements were carried out under AM1.5 standard simulated sunlight with precisely controlled active surface areas (0.159 cm2) and continuously-calibrated, spectral-mismatch corrected sunlamps, as is the standard in the conventional photovoltaic industry13. Under these conditions, that closely emulate outdoor sunlight, the total external efficiency of conversion of incident sunlight to useable electricity was η ∼ 0.08%. This must not to be confused with the sometimes very high quantum or internal efficiencies routinely reported for organic optoelectronics.

Figure 2 (a) To promote attachment and orientation of the entire PS-I complex to ZnO nanowires, we fused the ZnO-binding peptide tag RSNTRMTARQHRSANHKSTQRARS10 (expressed in E.coli) to the N-terminus of the PsaE subunit. Upon exchanging native PsaE in favor of PsaE-ZnO and self-assembly, the modified PS-I preferentially binds to ZnO nanowires by the electron acceptor side, minimizing distance between electron acceptor and electrode and maximizing electron transfer. (b) The marked increase in the rate of methyl viologen (MV)-mediated oxygen reduction by PS-I in the presence of the designer surfactant peptide A 6 K, indicates that A 6 K maintains the ability of PS-I to catalyze photochemical charge separation and MV-mediated O 2 uptake relative to the control treatment with either the non-ionic detergent Triton X-100 (middle) or DDM present in the isolation buffer (left). Increased O 2 uptake activity cannot be due to free chlorophyll-mediated O 2 consumption (via3chl) since treating cyanobacterial PS-I with strong detergents (1% SDS) leads to only minimal loss of chlorophyll from PS-I22. All activity tests are normalized per mol of PS-I. (c) Low-temperature fluorescence of PS-I self-assembled in the presence of excess PsaE-ZnO (red) peaks at the same two wavelengths as unaltered PS-I extract (blue) indicating that bulk chlorophyll organization is preserved12 and the stabilizing interaction with A 6 K is likely similar in both cases. Full size image

Figure 3 SEM of nanostructured TiO 2 and ZnO photoanodes and schematic of an ideal ultra-low cost biophotovoltaic arrangement. (a) 3.8 µm-thick, 60 nm-pore TiO 2 nanocrystalline photoanode of roughness factor ρ TiO2 ∼200 (i.e. surface area increases by roughly fifty times per µm of film thickness) fabricated as described previously13. (b) 3 µm tall, ZnO nanowires grown on Zn-nanoparticle-seeded ITO-glass as described elsewhere15, ρ ZnO ∼30. Round graphic at top left of inserts represents a PS-I trimer drawn to scale. (c), (d), (e) ideal arrangement of PS-I and designer surfactant peptide stabilizers on ZnO nanowires that could be grown at room temperature on a variety of flexible and inexpensive substrates16. Full size image

Figure 4 Photocurrent measurements of PS-I biophotovoltaic devices under AM1.5 simulated insolation at 298°K. Illuminated surface 0.159 cm2 (a) 40 µl of PS-I (0.2 mg/mL) stabilized by 1∶1 0.1%w/v designer surfactant peptide A 6 K (resulting in a total of 8 µg of protein) dried on a 3.8 µm thick layer of 60 nm-pore TiO 2 produces an IV curve typical of a DSC. Fill factor (ff) ranged from 64% to 71% (b) Eliminating ultraviolet (UV) wavelengths below 350 nm resulted in a ∼20% reduction in the normalized short circuit current (J sc Norm) and a ∼10% reduction in the open circuit voltage (V oc ) indicating that 80% of the total electrical power generated is due to PS-I (the rest due to UV photovoltaic response of TiO 2 ). These photocurrents cannot be attributed to sensitization of TiO 2 by leached chlorophyll derivatives1,17. A blank control containing A 6 K generated no power when exposed to UV-less sunlight of any intensity, neither did controls built with PS-I denatured by boiling for 10 minutes, nor devices built with PS-I not treated with A 6 K (data not shown). Total incident-light to electrical external power conversion efficiency η was 0.08% with UV, 0.07% without. (c) Linearity test of PS-I photocurrent at intensities from 0.01x to 1.0x AM1.5 shows behavior typical of a DSC. (d) IV of PS-I self-assembled in the presence of an overabundance of PsaE-ZnO electron-accepting subunit yields a total power conversion efficiency, η = 0.03%. (e) Control: IV of PS-I self-assembled with an overabundance of non-ZnO specific histidine-tag containing PsaE subunit yields lower V oc , J sc Norm and η = 0.00% as expected, suggesting that the PsaE-ZnO tag either enhanced binding of PS-I to the ZnO nanowires or favored the optimal orientation, or both. Z813Co(II)/Co(III) electrolyte14 and platinized glass were used to complete all devices. Full size image

PS-I Biophotovoltaic Solar Cell

In photosynthetic organisms, PS-I catalyses light-driven electron transfer from reduced plastocyanin located in the lumen, to ferredoxin in the stroma providing a path across the membrane consisting of a chain of cofactors ( Fig. 1a ). Light absorption results in excitation of the primary electron donor (P700), transfer to primary and secondary electron acceptors and finally crossing of the membrane. In our biophotovoltaic solar cell, the role of plastocyanin was played by the Co(II)/Co(III) ion-containing electrolyte Z81314 and ferredoxin was replaced by either nanocrystalline TiO 2 ( Fig. 1 b left) or ZnO-nanowires ( Fig. 1 b right) to provide large-surface area electron acceptors.

When using TiO 2 , we chose the pore size of the nanocrystalline film to be double the diameter of our PS-I particles to ensure a high probability of physisorption. When using ZnO nanowires, we substituted (by a self-assembly exchange reaction) the naturally-occurring electron acceptor PsaE subunit with one that contained an amino acid sequence with high affinity for ZnO: RSNTRMTARQHRSANHKSTQRARS10 (PsaE-ZnO) thus promoting adhesion and minimizing the distance that electrons must travel to the anode ( Fig. 2 a ).

Stabilization of Native and Bioengineered PS-I

Stabilization of dry PS-I extract on glass and on the transparent conductor Indium-Tin-Oxide (ITO) for at least three weeks has been described elsewhere1. Here, we mixed PS-I at 0.4 mg/mL in a 1∶1 ratio with 0.1% (w/v) of the 2.4 nm long cationic peptide surfactant Ac-AAAAAAK-NH 2 (A 6 K) consisting of six alanines and a lysine at the amidated C-terminus and observed enhanced stability ( Fig. 2 b ). The bioengineered, self-assembled PS-I containing PsaE-ZnO exhibited low-temperature fluorescence peaks identical to unmanipulated PS-I extract undergoing identical treatment ( Fig. 2 c ), indicating that subunit substitution did not adversely affect structure and we expect the photochemical-activity enhancing effect of A 6 K to be similar in both cases.

Nanocrystalline TiO 2 and ZnO Nanowires as Large Surface Area Photoanodes

I sc is directly proportional to electrical power output and is controlled by light absorption. To increase the useful light incidence angle and optical cross-section of our biophotovoltaics, we used two types of rough, large surface area semiconductors as photoanodes. This made our devices able to absorb light from nearly a 2π solid angle and provided an increased effective area for PS-I SAM adsorption: A eff ∼200 times that of the flat footprint for TiO 2 nanocrystals (Fig 3 a) and ∼30 times with ZnO nanowires (Fig 3 b). In addition to providing an inexpensive alternative to TiO 2 , the charge carrier mobility of ZnO nanowires is one-hundred times faster than TiO 2 15 and large-scale, ambient temperature solution-growth of ZnO nanowires is simple, requiring fewer steps, less energy and is easily adaptable to flexible conducting substrates16. However, ZnO DSC photoanodes have so far always underperformed15 when compared with identically sensitized TiO 2 . This is due to their lower roughness factor ρ, poor dye loading and the shunting of the photocurrent by the corrosion of ZnO by common DSC dyes and electrolytes. The IV behavior of our biophotovoltaics indicated that tagging PS-I with an amino acid sequence that binds to ZnO promoted orientation and/or binding to ZnO nanowires with η = 0.03%, for PsaE-ZnO, while η = 0.00% for the histidine-tagged control (Fig. 4 d and e respectively). The I sc achieved with ZnO (Fig. 4 d) is roughly a factor of ten lower than that with TiO 2 , consistent with the ratio of the two A eff (ρ ZnO ∼30, ρ TiO2 ∼200) suggesting that A eff is the primary control of I sc .

Photocurrent Measurements Under Realistically Simulated Solar Illumination