Letter pubs.acs.org/Langmuir
Photosystem I on Graphene as a Highly Transparent, Photoactive Electrode Darlene Gunther,† Gabriel LeBlanc,‡ Dhiraj Prasai,† Jamie R. Zhang,§ David E. Cliffel,‡ Kirill I. Bolotin,∥ and G. Kane Jennings*,§ †
Interdisciplinary Materials Science Graduate Program, ‡Department of Chemistry, §Department of Chemical and Biomolecular Engineering, and ∥Department of Physics, Vanderbilt University, Nashville, Tennessee 37235, United States S Supporting Information *
ABSTRACT: We report the fabrication of a hybrid light-harvesting electrode consisting of photosystem I (PSI) proteins extracted from spinach and adsorbed as a monolayer onto electrically contacted, large-area graphene. The transparency of graphene supports the choice of an opaque mediator at elevated concentrations. For example, we report a photocurrent of 550 nA/cm2 from a monolayer of PSI on graphene in the presence of 20 mM methylene blue, which yields an opaque blue solution. The PSI-modified graphene electrode has a total thickness of less than 10 nm and demonstrates photoactivity that is an order of magnitude larger than that for unmodified graphene, establishing the feasibility of conjoining these nanomaterials as potential constructs in next-generation photovoltaic devices.
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INTRODUCTION
Here, we demonstrate the ability to adsorb PSI onto graphene to achieve a photoactive electrode that is less than 10 nm thick. Since pioneering work reporting the extraction of graphene from bulk graphite via micromechanical exfoliation,8 improvements in synthesis and characterization methods have provided the opportunity to exploit this novel material as a transparent, highly conductive electrode.9 Graphene sheets are 2D layers of sp2-hybridized carbon atoms that have interesting and unique properties such as high carrier mobility,10 high elasticity and breaking strength,11 record thermal conductivity (∼3000 W/ m·K in plane),12 and high transparency over the visible spectrum (97.7%).13 These extraordinary electronic, mechanical, and optical properties of graphene are inviting for applications from solar cells to transistors.14 By combining the unique photodiode properties of PSI with the exceptional optical and electronic properties of graphene, we herein establish the foundation for the integration of these ultrathin materials into a new ultrathin photoactive electrode.
Photosynthesis is the process by which plants, algae, and cyanobacteria convert solar energy into stored energy in the form of reduced carbon. This intricate process requires the use of protein complexes that perform various tasks such as light absorption, water splitting, charge separation, electron transport, and carbon fixation.1 Photosystem I (PSI) is one of these protein complexes located in the thylakoid membrane of most photosynthetic organisms. PSI derived from spinach is a 500 kDa complex composed of 17 proteins.2 Coordinated with these proteins are ∼180 antennae chlorophyll molecules that capture photons of the appropriate energy, transferring the energy to a pair of special chlorophyll molecules known as the P700 reaction center.3 When an electron within P700 acquires sufficient energy, rapid charge separation occurs and the electron is transported to a terminal iron−sulfur complex known as FB.3 This charge separation, with quantum efficiency near unity3 coupled with the nanoscale size of PSI, its vast abundance, and renewability, has inspired its use in biohybrid material systems.4 PSI extracted from green plants and cyanobacteria has been assembled onto a variety of substrates, including gold, glass, ITO, alumina, TiO2, ZnO, and Si.5−7 © 2013 American Chemical Society
Received: December 18, 2012 Revised: March 4, 2013 Published: March 18, 2013 4177
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Letter
MATERIALS AND METHODS
Graphene films were grown by chemical vapor deposition on 25-μmthick Cu foils using the recipe developed by Ruoff’s group15 and transferred onto designated substrates (Si/SiO2, Au, and glass). Each sample was cut into 1 cm × 1 cm squares with electrical contact made with Cu tape. Raman spectra were taken to verify the graphene film (Supporting Information, Figure S1). The ratio of G:2D peaks was 0.53 with a small D peak at 1343 cm−1 that is attributed to minor defects in pristine graphene.16 PSI complexes were extracted from commercial baby spinach as described previously17,18 and characterized using UV−vis absorbance spectroscopy according to Baba et al.,19 resulting in a chlorophyll concentration of 0.6 mg/mL and a PSI concentration of 3.2 μM. The extracted PSI was directly deposited as monolayer films onto graphene-coated silicon, gold, and glass substrates by the vacuumassisted, drop-casting method20 developed within our group.
Figure 1. Cyclic voltammograms of the SiO2 substrate (blue), unmodified graphene electrode (black), and PSI-modified graphene electrode (red) taken in the dark using ruthenium hexamine trichloride (2 mM) in an aqueous phosphate buffer (5 mM) solution with potassium chloride (0.1 M). The potential of the working electrode was scanned from −0.35 to 0.05 V vs Ag/AgCl at a rate of 0.1 V/s.
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RESULTS AND DISCUSSION Spectra were collected to analyze the surface composition of PSI monolayer films prepared on graphene. Polarized modulation infrared reflectance-absorption spectroscopy (PMIRRAS) revealed amide I and amide II peaks at ∼1662 and ∼1540 cm−1, respectively, indicative of the protein complex21,22 (Figure S2a). Additionally, UV−vis spectra revealed two distinct peaks at ∼680 and ∼440 nm, consistent with the chlorophylls of PSI present on the surface of the graphene electrode (Figure S2b). Together, the PM-IRRAS and the UV− vis spectra indicate that PSI is adsorbed onto the graphene surface. Ellipsometric thicknesses of the PSI films were measured to assess the coverage of protein deposited on the graphene substrate. The average thickness measurement for PSI films was 58 Å with a standard deviation of ±15 Å using a Cauchy model. On the basis of the reported size of PSI23 and the considerable free volume contained in a complete monolayer of PSI in which the proteins are oriented with their electron-transfer vectors normal to the substrate, we estimate that a complete monolayer would yield an ellipsometric thickness of ∼80 Å, which agrees well with the greatest thicknesses reported for PSI monolayers.21 Therefore, a measurement of 58 Å represents approximately 70% surface coverage of PSI on the graphene surface. To investigate the effective coverage of PSI on the electrode further, electrochemical experiments were performed in the dark to assess the ability of the PSI film to block the access of redox-active species to the underlying graphene. Figure 1 shows cyclic voltammograms (CV) recorded in the absence of light, where the applied potential to the working electrode was cycled from −0.35 to 0.05 V versus Ag/AgCl in the presence of ruthenium hexamine trichloride (RuHex) as the current was measured continuously. The underlying substrate (SiO2) acts as an insulator so that no current is observed at any potential. The unmodified-graphene substrate reveals an increasing current as the voltage sweep becomes more negative, which is attributed to the reduction of the [Ru(NH3)6]3+ species. As the voltage sweeps more positive, an oxidation peak appears at −0.1 V, which is the expected formal potential (versus Ag/AgCl) of the [Ru(NH3)6]2+ species,24 demonstrating that the graphene film is conductive and an appropriate electrode for this process. After the deposition of a PSI monolayer, the observed reductive and oxidative currents decrease as PSI blocks mediator access to a majority of the electrode surface. Assuming that PSI completely blocks the electrode area that it occupies from mediator interaction, a PSI surface coverage of ∼70% was
estimated from the decrease in the integrated charge for the reduction peak after the adsorption of PSI, consistent with that gleaned from ellipsometric measurements. Photochronoamperometric analyses of PSI-modified graphene electrodes were performed to determine their photocurrent activity and to compare the results with unmodified graphene electrodes. Figure 2a shows the photoresponse of the PSI-modified graphene electrode (red) in comparison to an unmodified graphene electrode (black) measured at opencircuit potential. Upon illumination with white light at an intensity of 82 mW/cm2, an electron is excited from the P700 to the FB site of PSI. The graphene electrode draws the electron from the FB− site of PSI, and the [Ru(NH3)6]2+ mediator reduces the resulting P700+ site, resulting in a negative photocurrent or an anodic response. Unmodified graphene is also capable of light-assisted mediator oxidation in the absence of PSI; however, the observed photocurrent is 8-fold lower than that of the PSI-modified electrode. Changing the potential of the electrode can change the direction of electron flow (Figure 2b). When a −100 mV overpotential is applied to the electrode, the ability of the electrode to give up electrons increases so that the photoreduction of P700+ and the oxidized mediator is observed as a positive or cathodic photocurrent. The “sawtooth” shape of this curve is consistent with the diffusional response of the mediator to the additional reducing ability of the PSI/graphene electrode in the light. Additionally, the photocurrent observed is significantly higher than when the system is at open-circuit potential or at a positive overpotential. The fact that photocurrent switches from anodic to cathodic suggests that PSI is deposited onto graphene in different orientations, which produce electron-transfer vectors leading both to and from the graphene surface, yielding a photocurrent that reflects a net average of the overall processes occurring at the electrode surface held at a constant potential. Importantly, these results show that the applied potential can be used to direct the flow of current between graphene and the PSI film. One benefit of using graphene as a transparent, conducting electrode is the flexibility it allows with mediator choice and concentration. For example, by mounting graphene on glass, we can irradiate the cell through the transparent graphene electrode rather than through the solution, enabling the use of opaque mediators at higher concentrations to boost photocurrents.24 Figure 3 shows a photochronoamperometry 4178
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Figure 2. Photochronoamperometric analyses of PSI-modified graphene electrodes. All measurements were made using ruthenium hexamine (0.2 mM), potassium chloride (0.1 M), and phosphate buffer (5 mM) as the mediator solution. White light at an intensity of 82 mW/cm2 was irradiated onto the surfaces for 30 s. (a) Comparison between an unmodified graphene electrode (black) and a PSI-modified graphene electrode (red). Both samples were measured while holding the working electrode at the open circuit potential. The light was on from 30 s to 60 s. (b) Comparison of a PSI-modified graphene electrode held at 0 mV or open-circuit potential (black), a positive 100 mV overpotential (red), or a negative 100 mV overpotential (blue). The light was on from 20 s to 50 s.
Figure 3. (a) Photochronoamperometric analysis of a PSI-modified graphene electrode on glass (red) as compared to that of an unmodified graphene electrode (black) irradiated through the electrode and compared to graphene electrodes irradiated through the mediator solution (blue). All measurements were made using a 20 mM MB aqueous mediator with 0.1 M KCl. Irradiation using a 633 nm filter resulting in a 98 mW/cm2 intensity was supplied at 20 s and removed at 40 s for each measurement. (b) Effect of MB mediator concentration on the measured photocurrent density, measured 20 s after turning on the light. Error bars reflect the standard deviation from six independently prepared samples. If no error bar is present, then the size of the symbol reflects the error.
Similar to the previous electrochemical measurements in transparent RuHex, the graphene was able to reduce the mediator in the absence of PSI. The PSI-modified graphene electrode shows a 10-fold enhancement over the bare graphene electrode at 20 s after irradiation in this case, as compared to the RuHex experiments exhibiting an 8-fold enhancement. The effect of mediator concentration on current density is shown in Figure 3b. The current density for these PSI-modified graphene electrodes increased by a factor of 20 as the MB mediator concentration increased from 0.2 to 20 mM. Here, the integration of PSI monolayer films with graphene as an atomically thin, transparent electrode enables the achievement of these high photocurrents, whereas traditional opaque electrodes would not allow testing under these conditions. This unique system, combined with the reported long-term stability25 and enhanced performance of thicker PSI films,5 offers exciting opportunities to amplify biohybrid cell performance.
experiment where we have used methylene blue (MB) as an opaque, redox-active mediator and red light with an intensity of 98 mW/cm2. MB has not been used with more traditional electrodes because it absorbs in the red regime (650−750 nm) where it deprives PSI of incident photons that would be absorbed by the chlorophyll network of PSI (Figure S2b). Figure 3 shows the results of measured photocurrent taken through the opaque mediator (blue), compared to the measured photocurrent taken through the graphene substrate for both PSI-modified (red) and unmodified graphene (black). The absence of photocurrent confirms the absorption of light by the mediator when the light initially passes through the solution on its path to the PSI-modified or unmodified graphene substrate. In contrast, the photocurrent measured when the light passes through the transparent graphene electrode first (∼550 nA/cm2) confirms that photons are absorbed by PSI, initiating the measured photoresponse, which is highly complex and diffusional as a function of the time dependence of the local concentration of MB at the electrode. 4179
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(10) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Ultrahigh electron mobility in suspended graphene. Solid State Commun. 2008, 146, 351−355. (11) Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385−388. (12) Seol, J. H.; Jo, I.; Moore, A. L.; Lindsay, L.; Aitken, Z. H.; Pettes, M. T.; Li, X.; Yao, Z.; Huang, R.; Broido, D.; Mingo, N.; Ruoff, R. S.; Shi, L. Two-dimensional phonon transport in supported graphene. Science 2010, 328, 213−216. (13) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine structure constant defines visual transparency of graphene. Science 2008, 320, 1308. (14) Celebrating graphene. Nat. Photonics 2010, 4, 731. (15) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 2009, 324, 1312−1314. (16) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman spectrum of graphene and graphene layers. Phys. Rev. Lett. 2006, 97, 187401. (17) Kincaid, H. A.; Niedringhaus, T.; Ciobanu, M.; Cliffel, D. E.; Jennings, G. K. Entrapment of photosystem I within self-assembled films. Langmuir 2006, 22, 8114−8120. (18) Reeves, S. G.; Hall, D. O. [8] Higher Plant Chloroplasts and Grana: General Preparative Procedures (Excluding High Carbon Dioxide Fixation Ability Chloroplasts). In Photosynthesis and Nitrogen Fixation: Part C; San Pietro, A., Ed.; Methods in Enzymology; Academic Press: New York, 1980; Vol. 69, pp 85−94. (19) Baba, K.; Itoh, S.; Hastings, G.; Hoshina, S. Photoinhibition of photosystem I electron transfer activity in isolated photosystem I preparations with different chlorophyll contents. Photosynth. Res. 1996, 47, 121−130. (20) Faulkner, C. J.; Lees, S.; Ciesielski, P. N.; Cliffel, D. E.; Jennings, G. K. Rapid assembly of photosystem I monolayers on gold electrodes. Langmuir 2008, 24, 8409−8412. (21) Ko, B. S.; Babcock, B.; Jennings, G. K.; Tilden, S. G.; Peterson, R. R.; Cliffel, D. Effect of surface composition on the adsorption of photosystem I onto alkanethiolate self-assembled monolayers on gold. Langmuir 2004, 20, 4033−4038. (22) Yada, L. D. S. Organic Spectroscopy; Kluwer Academic Publishers: New Delhi, 2005; p 324. (23) Amunts, A.; Toporik, H.; Borovikova, A.; Nelson, N. Structure determination and improved model of plant photosystem I. J. Biol. Chem. 2010, 285, 3478−3486. (24) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001. (25) Ciesielski, P. N.; Hijazi, F. M.; Scott, A. M.; Faulkner, C. J.; Beard, L.; Emmett, K.; Rosenthal, S. J.; Cliffel, D.; Kane Jennings, G. Photosystem I-based biohybrid photoelectrochemical cells. Bioresour. Technol. 2010, 101, 3047−3053.
CONCLUSIONS We have demonstrated the adsorption of PSI monolayers directly onto graphene electrodes for the conversion of light to electrical current. This system provides a photoelectrochemical response to irradiation as measured in an electrochemical cell that can be enhanced with the use of an overpotential. The transparent nature of graphene enables the use of a highly effective and opaque mediator (MB) to amplify photocurrents greatly. This research represents the first report of integrating PSI with atomically thin graphene to create a photoactive biohybrid electrode that is thinner than 10 nm. This work establishes a foundation for exploiting the unique properties of these nanomaterials in future biohybrid devices.
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ASSOCIATED CONTENT
S Supporting Information *
Information regarding the synthesis of graphene. Raman spectra to confirm single-layer graphene. Ellipsometry modeling details. PM-IRRAS and UV−vis spectra to confirm the presence of PSI. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +1 615 322 2707. Fax: +1 615 343 7951. E-mail: kane.g.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Science Foundation (DMR 0907619, REU - DMR 1005023) and the NSF EPSCoR (EPS 1004083). In addition, this research was supported in part by the Scialog program from the Research Corporation for Science Advancement.
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REFERENCES
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