Drop-casted Photosystem I/cytochrome c multilayer films for biohybrid solar energy conversion.

One of the main barriers to making efficient Photosystem I-based biohybrid solar cells is the need for an electrochemical pathway to facilitate electron transfer between the P700 reaction center of Photosystem I and an electrode. To this end, nature provides inspiration in the form of cytochrome c6, a natural electron donor to the P700 site. Its natural ability to access the P700 binding pocket and reduce the reaction center can be mimicked by employing cytochrome c, which has a similar protein structure and redox chemistry while also being compatible with a variety of electrode surfaces. Previous research has incorporated cytochrome c to improve the photocurrent generation of Photosystem I using time consuming and/or specialized electrode preparation. While those methods lead to high protein areal density, in this work we use the quick and facile vacuum-assisted drop-casting technique to construct a Photosystem I/cytochrome c photoactive composite film with micron-scale thickness. We demonstrate that this simple fabrication technique can result in high cytochrome c loading and improvement in cathodic photocurrent over a drop-casted Photosystem I film without cytochrome c. In addition, we analyze the behavior of the cytochrome c/Photosystem I system at varying applied potentials to show that the improvement in performance can be attributed to enhancement of the electron transfer rate to P700 sites and therefore the PSI turnover rate within the composite film.

Surface-Initiated, Catechol-Containing Polymer Films for Effective Chelation of Aluminum Ions.

We present a new route for obtaining surface-tethered polymer films containing pendant catechol functional groups via surface-initiated activators regenerated by electron-transfer atom-transfer radical polymerization (SI-ARGET ATRP) of glycidyl methacrylate (GMA) and post-polymerization modification of the resulting poly(glycidyl methacrylate) (pGMA) films with dopamine. This method enables a high degree of functionalization of pGMA films with catechol groups at a controlled level, depending on the duration of the post-polymerization modification reaction. The dopamine-pGMA films readily absorbs Al3+ and Zn2+ ions, as verified by quartz crystal microbalance with dissipation (QCM-D) under continuous flow conditions, and demonstrates a four-fold molar selectivity to Al3+ over Zn2+. The ions desorb from the films upon rinsing with pure deionized (DI) water, which regenerates the catechol sites in the dopamine-pGMA film. Subsequent exposure to metal ions after rinsing steps yields reproducible levels of loading.

Layer-by-Layer Assembly of Photosystem I and PEDOT:PSS Biohybrid Films for Photocurrent Generation.

The design of electrode interfaces to achieve efficient electron transfer to biomolecules is important in many bioelectrochemical processes. Within the field of biohybrid solar energy conversion, constructing multilayered Photosystem I (PSI) protein films that maintain good electronic connection to an underlying electrode has been a major challenge. Previous shortcomings include low loadings, long deposition times, and poor connection between PSI and conducting polymers within composite films. Here, we show that PSI protein complexes can be deposited into monolayers within a 30 min timespan by leveraging the electrostatic interactions between the protein complex and the poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) polymer. Further, we follow a layer-by-layer (LBL) deposition procedure to produce up to 9-layer pairs of PSI and PEDOT:PSS with highly reproducible layer thicknesses as well as distinct changes in surface composition. When tested in an electrochemical cell employing ubiquinone-0 as a mediator, the photocurrent performance of the LBL films increased linearly by 83 ± 6 nA/cm2 per PSI layer up to 6-layer pairs. The 6-layer pair samples yielded a photocurrent of 414 ± 13 nA/cm2, after which the achieved photocurrent diminished with additional layer pairs. The turnover number (TN) of the PSI-PEDOT:PSS LBL assemblies also greatly exceeds that of drop-casted PSI multilayer films, highlighting that the rate of electron collection is improved through the systematic deposition of the protein complexes and conducting polymer. The capability to deposit high loadings of PSI between PEDOT:PSS layers, while retaining connection to the underlying electrode, shows the value in using LBL assembly to produce PSI and PEDOT:PSS bioelectrodes for photoelectrochemical energy harvesting applications.

pH-Responsive Copolymer Films Prepared by Surface-Initiated Polymerization and Simple Modification.

We report the preparation of pH-responsive, ester/carboxylic acid random copolymer films via simple modification of poly(norbornene diacyl chloride) (pNBDAC), prepared via surface-initiated ring-opening metathesis polymerization, with mixtures of water and ethanol to form carboxylic acid and ethyl ester side groups. The pNBDAC film serves as a compositionally versatile platform to controllably obtain copolymers with multiple functionalities. In modifying the pNBDAC to form the copolymer film, ethanol exhibits a significantly higher reactivity with acyl chloride groups within the film than does water. The magnitude and range of the pH-responsive performance are highly dependent on the carboxylic acid content in the copolymer films, which demonstrates the effect of film hydrophilicity on the pH-responsive switching of ionic barrier properties. The resistance of the film against ion transfer can be decreased by a factor of 104 through pH change, demonstrating pH-induced switching from hydrophobic and insulating to swollen and ion-permeable films. The interactions of the copolymer films with water at different pH values were also explored. When the copolymer contains 34% carboxylic acids, a 4× greater film thickness is obtained in high pH solution than in low pH solution due to ionically driven water swelling. The reversibility of the pH-responsive performance of these copolymer films is high based on measurements using quartz crystal microbalance with dissipation (QCM-D).

Polyviologen as Electron Transport Material in Photosystem I-Based Biophotovoltaic Cells.

The photosynthetic protein complex, photosystem I (PSI), can be photoexcited with a quantum efficiency approaching unity and can be integrated into solar energy conversion devices as the photoactive electrode. The incorporation of PSI into conducting polymer frameworks allows for improved conductivity and orientational control in the photoactive layer. Polyviologens are a unique class of organic polycationic polymers that can rapidly accept electrons from a primary donor such as photoexcited PSI and subsequently can donate them to a secondary acceptor. Monomeric viologens, such as methyl viologen, have been widely used as diffusible mediators in wet PSI-based photoelectrochemical cells on the basis of their suitable redox potentials for accepting electrons. Polyviologens possess similar electronic properties to their monomers with the added advantage that they can shuttle electrons in the solid state. Depositing polyviologen directly onto a film of PSI protein results in significant photocurrent enhancement, which confirms its role as an electron-transport material. The polymer film not only improves the photocurrent by aiding the electron transfer but also helps preserve the protein film underneath. The composite polymer-PSI assembly enhances the charge-shuttling processes from individual protein molecules within the PSI multilayer, greatly reducing charge-transfer resistances. The resulting PSI-based solid-state platform demonstrates a much higher photocurrent than the corresponding photoelectrochemical cell built using a similar architecture.

Surface-Initiated Ring-Opening Metathesis Polymerization of Dicyclopentadiene from the Vapor Phase.

Surface-attached polydicyclopentadiene (pDCPD) films were prepared on gold and silicon substrates via surface-initiated ring-opening metathesis polymerization (SI-ROMP) of dicyclopentadiene (DCPD). The films were grown utilizing monomer in both the vapor phase and the solution phase with the former process exhibiting rapid kinetics, producing ∼400-nm-thick pDCPD films in less than 1 min of polymerization. No significant differences in thickness were observed for films grown from monomer in the vapor phase with the different isomers (exo and endo) of DCPD. Decane was used as an inert additive to control the concentration of DCPD monomer in the vapor phase enabling the preparation of pDCPD films with thickness ranging from tens of nanometers to hundreds of nanometers. The thickness of pDCPD films polymerized using monomer in the vapor phase was enhanced by the presence of a rinse solvent on the surface of the ROMP-active gold substrates. The choice of ROMP catalyst was found to be an important consideration when SI-ROMP was conducted on different substrates. Electrochemical impedance spectroscopy was used to reveal that the films provide effective barriers to the diffusion of aqueous ions in excess of 1 × 106 Ω·cm2. The mechanical properties of the surface-tethered pDCPD films were quantified with AFM PeakForce quantitative nanomechanical mapping (QNM) with a measured reduced Young's modulus (Er) of 15 GPa. The measured Er was greater than that of a non-cross-linked surface-tethered polymer, pNB, indicating that the pDCPD films are stiffer.

Electrical Control of near-Field Energy Transfer between Quantum Dots and Two-Dimensional Semiconductors.

We investigate near-field energy transfer between chemically synthesized quantum dots (QDs) and two-dimensional semiconductors. We fabricate devices in which electrostatically gated semiconducting monolayer molybdenum disulfide (MoS2) is placed atop a homogeneous self-assembled layer of core-shell CdSSe QDs. We demonstrate efficient nonradiative Förster resonant energy transfer (FRET) from QDs into MoS2 and prove that modest gate-induced variation in the excitonic absorption of MoS2 leads to large (∼500%) changes in the FRET rate. This in turn allows for up to ∼75% electrical modulation of QD photoluminescence intensity. The hybrid QD/MoS2 devices operate within a small voltage range, allow for continuous modification of the QD photoluminescence intensity, and can be used for selective tuning of QDs emitting in the visible-IR range.

Construction of a Semiconductor-Biological Interface for Solar Energy Conversion: p-Doped Silicon/Photosystem I/Zinc Oxide.

The interface between photoactive biological materials with two distinct semiconducting electrodes is challenging both to develop and analyze. Building off of our previous work using films of photosystem I (PSI) on p-doped silicon, we have deposited a crystalline zinc oxide (ZnO) anode using confined-plume chemical deposition (CPCD). We demonstrate the ability of CPCD to deposit crystalline ZnO without damage to the PSI biomaterial. Using electrochemical techniques, we were able to probe this complex semiconductor-biological interface. Finally, as a proof of concept, a solid-state photovoltaic device consisting of p-doped silicon, PSI, ZnO, and ITO was constructed and evaluated.

Photosystem I protein films at electrode surfaces for solar energy conversion.

Over the course of a few billion years, nature has developed extraordinary nanomaterials for the efficient conversion of solar energy into chemical energy. One of these materials, photosystem I (PSI), functions as a photodiode capable of generating a charge separation with nearly perfect quantum efficiency. Because of the favorable properties and natural abundance of PSI, researchers around the world have begun to study how this protein complex can be integrated into modern solar energy conversion devices. This feature article describes some of the recent materials and methods that have led to dramatic improvements (over several orders of magnitude) in the photocurrents and photovoltages of biohybrid electrodes based on PSI, with an emphasis on the research activities in our laboratory.

Photoactive films of photosystem I on transparent reduced graphene oxide electrodes.

Photosystem I (PSI) is a photoactive electron-transport protein found in plants that participates in the process of photosynthesis. Because of PSI's abundance in nature and its efficiency with charge transfer and separation, there is a great interest in applying the protein in photoactive electrodes. Here, we developed a completely organic, transparent, conductive electrode using reduced graphene oxide (RGO) on which a multilayer of PSI could be deposited. The resulting photoactive electrode demonstrated current densities comparable to that of a gold electrode modified with a multilayer film of PSI and significantly higher than that of a graphene electrode modified with a monolayer film of PSI. The relatively large photocurrents produced by integrating PSI with RGO and using an opaque, organic mediator can be applied to the facile production of more economic solar energy conversion devices.

Reproducing superhydrophobic leaves as coatings by micromolding surface-initiated polymerization.

Micromolding surface-initiated polymerization enables the fabrication of polymer coatings that reproduce the microscale surface topography of superhydrophobic leaves onto solid supports. Here, the surfaces of superhydrophobic leaves from Trifolium repens and Aristolochia esperanzae are molded and reproduced as the topography of a partially fluorinated polymer coating through the surface-initiated ring-opening metathesis polymerization of 5-(perfluorooctyl)norbornene (NBF8). The polymer coatings have thicknesses exceeding 100 µm, which can be tailored by the amount of monomer added to the mold. These coatings are robustly bound to the substrate, contain compositions not found in nature, and achieve superhydrophobicity that is comparable to the actual leaf.

Interfacing poly(p-anisidine) with photosystem I for the fabrication of photoactive composite films.

Photosystem I (PSI) is an intrinsically photoactive multi-subunit protein that is found in higher order photosynthetic organisms. PSI is a promising candidate for renewable biohybrid energy applications due to its abundance in nature and its high quantum yield. To utilize PSI's light-responsive properties and to overcome its innate electrically insulating nature, the protein can be paired with a biologically compatible conducting polymer that carries charge at appropriate energy levels, allowing excited PSI electrons to travel within a composite network upon light excitation. Here, a substituted aniline, 4-methoxy-aniline (para-anisidine), is chemically oxidized to synthesize poly(p-anisidine) (PPA) and is interfaced with PSI for the fabrication of PSI-PPA composite films by drop casting. The resulting PPA polymer is characterized in terms of its structure, composition, thermal decomposition, spectroscopic response, morphology, and conductivity. Combining PPA with PSI yields composite films that exhibit photocurrent densities on the order of several µA cm-2 when tested with appropriate mediators in a 3-electrode setup. The composite films also display increased photocurrent output when compared to single-component films of the protein or PPA alone to reveal a synergistic combination of the film components. Tuning film thickness and PSI loading within the PSI-PPA films yields optimal photocurrents for the described system, with ∼2 wt% PSI and intermediate film thicknesses generating the highest photocurrents. More broadly, dilute PSI concentrations show significant importance in achieving high photocurrents in PSI-polymer films.

Simultaneous Spin Coating and Ring-Opening Metathesis Polymerization for the Rapid Synthesis of Polymer Films.

We report a highly controlled technique for the synthesis of polymer films atop a substrate by combining spin coating with ring-opening metathesis polymerization (ROMP), herein termed spin coating ROMP (scROMP). The scROMP approach combines polymer synthesis and deposition into one process, fabricating films of up to 36 cm2 in under 3 min with orders-of-magnitude reduction in solvent usage. This method can convert numerous norbornene-type molecules into homopolymers and random copolymers as uniform films on both porous and nonporous substrates. Film thickness can be varied from a few hundred nanometers to a few tens of micrometers based on spin speed and monomer concentration. The resulting polymers possess high MW (>100 kDa) and low polydispersity (PDI) (<1.2) values that are similar to ROMP polymers made in solution. We also devise a model to investigate the balance between convective monomer spin-off and polymer growth from the surface, which allows the determination of critical kinetic parameters for scROMP. Finally, translation of scROMP to porous supports enables the synthesis of thin film composite membranes that demonstrate the ability to dehydrate ethanol by pervaporation.

Amplification of surface-initiated ring-opening metathesis polymerization of 5-(perfluoro-n-alkyl)norbornenes by macroinitiation.

This article reports the enhanced rate of the surface-initiated polymerization (SIP) of 5-(perfluoro-n-alkyl)norbornenes (NBFn) by combining two SIP techniques, namely surface-initiated atom-transfer polymerization (SI-ATRP) to grow a macroinitiator and surface-initiated ring-opening metathesis polymerization (SI-ROMP) to produce the final coating. This polymerization approach promotes the rapid growth of dense partially fluorinated coatings that are highly hydrophobic and oleophobic and yield thicknesses from 4-12 µm. Specifically, the growth rate and the limiting thickness of pNBFn with different side chain lengths (n = 4, 6, 8, and 10) at various monomer concentrations and temperatures are evaluated through two approaches: growing the polymer from an initiator-terminated monolayer (control) or from a modified poly(2-hydroxyethyl methacrylate) (PHEMA) macroinitiator. X-ray photoelectron spectroscopy (XPS) analysis shows that 38% of the hydroxyl termini in the macroinitiator react with a norbornenyl diacid chloride (NBDAC) molecule, and 7% of such anchored norbornenyl groups react with a catalyst molecule. The kinetic data have been modeled to determine the propagation velocity and the termination rate constant. The PHEMA macroinitiator provides thicker films and faster growth as compared to the monolayer, achieving a 12 µm thick coating of pNBF8 in 15 min. Increasing the monomer side chain length, n, from 4 to 10 improves the growth rate and the limiting polymer thickness. Performing the polymerization process at higher temperature increases the growth rate and the limiting thickness as evidenced by an increase in the film growth rate constant. Arrhenius plots show that the reactions involved in the macroinitiation process exhibit lower activation energies than those formed from a monolayer. Electrochemical impedance spectroscopy reveals that the films exhibit resistance against ion transport in excess of 1 × 10(10) Ω·cm(2).

Photosystem I on graphene as a highly transparent, photoactive electrode.

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/cm(2) 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.

Photoactive and conductive biohybrid films by polymerization of pyrrole through voids in photosystem I multilayer films.

The combination of conducting polymers with electro- and photoactive proteins into thin films holds promise for advanced energy conversion materials and devices. The emerging field of protein electronics requires conductive soft materials in a composite with electrically insulating proteins. The electropolymerization of pyrrole through voids in a drop-casted photosystem I (PSI) multilayer film enables the straightforward fabrication of photoactive and conductive biohybrid films. The rate of polypyrrole (PPy) growth is reduced by the presence of the PSI film but is insensitive to its thickness, suggesting that rapid diffusion of pyrrole through the voids within the PSI film enables initiation at vacant areas on the gold surface. The base thickness of the composite tends to increase with time, as PPy chains propagate through and beyond the PSI film, coalescing to exhibit a tubule-like morphology as observed by scanning electron microscopy. Increasing amounts of PPy greatly increase the capacitance of the composite films in a manner almost identical to that of pure PPy films grown from unmodified gold, consistent with a high polymer/aqueous interfacial area and a conductive composite film. While PPy is not photoactive here, all composite films, including those with large amounts of PPy, exhibit photocurrents when irradiated by white light in the presence of redox mediator species. Optimization of the Py electropolymerization time is necessary, as increasing amounts of PPy lead to decreased photocurrent density due to a combination of light absorbance by the polymer and reduced accessibility of redox species to active PSI sites.

Photosystem I in Langmuir-Blodgett and Langmuir-Schaefer monolayers.

Photosystem I (PSI) is a membrane protein complex that generates photoinduced electrons and transfers them across the thylakoid membrane during photosynthesis. The PSI complex, separated from spinach leaves, was spread onto the air-water interface as a monolayer and transferred onto a gold electrode surface that was precoated with a self-assembled monolayer (SAM). The electrochemical properties of the transferred PSI monolayer, including cyclic voltammetry and photoinduced chronoamperometry, were measured. The results showed that PSI retained its bioactivity after the manipulation. Its capability of converting photoenergy into electrical potential was demonstrated by its reducing an electron acceptor, dichloroindophenol (DCIP), and by oxidizing an electron donor, sodium ascorbate (ASC). We have shown that the protein has two possible orientations at the water interface. The orientation distribution was determined by comparing the controlled reductive and oxidative photocurrents generated from Langmuir-Blodgett and Langmuir-Schaefer monolayers.

Photoreduction of catalytic platinum particles using immobilized multilayers of Photosystem I.

Using the abundance of available electrons generated by immobilized multilayers of the photoactive protein complex Photosystem I (PSI), we have photoreduced platinum particles that are catalytically active for the H(2)/H(+) redox couple. The resulting platinized PSI films were optimized using electrochemical measurements and then characterized using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and scanning electrochemical microscopy (SECM). These results demonstrate a novel method for generating immobilized platinum catalysts that are readily available on the surface of a photoactive PSI multilayer.

Frictional properties of mixed fluorocarbon/hydrocarbon silane monolayers: a simulation study.

Because of small surface area to volume ratios nanoscale devices can exhibit dominant surface forces that can quickly degrade unlubricated contacting surfaces. While fluorinated materials have been widely used as lubricants, because of their low critical surface tension and high thermal and mechanical stability, fluorinated monolayer coatings, which are suitable for lubricating nanoscale devices, are less effective as lubricants. Although fluorinated monolayers are more stable than their hydrocarbon counterparts against elevated temperature and humidity, they are known to exhibit higher frictional forces. To overcome this issue, here we study mixed monolayers composed of both hydrocarbon and fluorocarbon chains. Hydrocarbon-based monolayers have been widely studied and shown to improve frictional properties and device life. To investigate the frictional behavior of mixed fluorocarbon/hydrocarbon monolayers, molecular dynamics simulations of pure hydrogenated and fluorinated chains and mixed fluorinated/hydrogenated chains on silica surfaces have been performed. The adhesion and friction between the nanoconfined monolayers as a function of normal load, chain length, and chemical composition of the monolayer coating have been investigated, and mixed fluorocarbon/hydrocarbon monolayers found to outperform both pure fluorocarbon and pure hydrocarbon monolayers. Surface coverage was found to have a significant effect on the performance of all systems studied with higher surface coverage resulting in lower frictional forces. The simulations also show that when the hydrocarbon chains in the monolayer are longer than the fluorocarbon chains, a liquidlike layer is formed by the longer hydrocarbon chains that protrudes above the shorter fluorocarbon chains and aids in friction reduction. A frictional load dependence is also seen in these mixed monolayer systems because of repulsive interactions between the fluorocarbon base layer and the hydrocarbon liquidlike layer. A chain length difference of eight carbons between the base layer and the liquidlike layer was found to provide the lowest friction, while both a larger (because of increased entanglement) and a smaller (insufficient atoms between the contacting base layers to form a liquidlike layer) chain length difference increased friction.

Examining the frictional forces between mixed hydrophobic-hydrophilic alkylsilane monolayers.

Monolayers presenting methyl-terminated (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces on silica have been studied by molecular dynamics simulation and the effects of hydrogen bonding, chain length, and chain mixing on the frictional properties determined. The hydroxyl-terminated monolayers were found to show large adhesion zones as a result of strong interfacial interlayer hydrogen bonds; the interfacial sliding forces observed in the hydroxyl-terminated monolayers being one order of magnitude higher than the interfacial forces for the hydrophobic surfaces at the characteristic point of zero-load. Mixed hydroxyl- and methyl-terminated monolayers of equal length were found to exhibit intermediate shear stress values between those observed for pure monolayers, with the magnitude of the shear stress depending on the surface content of the hydroxyl-terminated chains. For mixed monolayers of unequal chain lengths, at high loads a maximum in the magnitude of the shear stress as a function of the length of the methyl-terminated chain was observed due to the creation of a buffer zone between the hydroxyl-terminated chains that produces strong hydrogen-bonding interactions. The effect of a constant normal load or constant separation simulation ensemble on the results has also been studied and in general found to have minimal influence on the observed behavior, although some differences are observed for the shear stress at intermediate normal loads due to the formation of stronger hydrogen bond networks at constant load compared to constant separation.