Mechanism of Photocurrent Generation from Bacteriorhodopsin on Gold Electrodes

Ferritin Single-Electron Transistor

We report on the fabrication of a single-electron transistor based on ferritin using wide self-aligned nanogap devices. A local gate below the gap area enables three-terminal electrical measurements, showing the Coulomb blockade in good agreement with the single-electron tunneling theory. Comparison with this theory allows extraction of the tunnel resistances, capacitances, and gate coupling. Additionally, the data suggest the presence of two separate islands coupled in series or in parallel: information that was not possible to distinguish by using only two-terminal measurements. To interpret the charge transport features, we propose a scenario based on the established configuration structures of ferritin involving either iron sites in the organic shell or two dissimilar clusters within the core.

Biomaterial-Based Biomimetic Visual Sensors: Inkjet Patterning of Bacteriorhodopsin

Biomimetic visual sensors utilizing bacteriorhodopsin (bR) were fabricated by using an inkjet method. The inkjet printer facilitated the jetting of the bR suspension, allowing for the deposition of bR films. The resulting inkjet-printed bR film exhibited time-differential photocurrent response characteristics similar to those of a dip-coated bR film. By adjusting the number of printed bR film layers, the intensity of the photocurrent could be easily controlled. Moreover, the inkjet printing technique enabled unconstrained patterning, facilitating the design of various visual information processing functions, such as visual filters. In this study, we successfully fabricated two visual filters, namely, a two-dimensional Difference of Gaussian (DOG) filter and a Gabor filter. The printed DOG filter demonstrated edge detection capabilities corresponding to contour recognition in visual receptive fields. On the other hand, the printed Gabor filter proved effective in detecting objects of specific sizes as well as their motion and orientation. The integration of bR and the inkjet method holds significant potential for the widespread implementation of highly functional biomaterial-based visual sensors. These sensors have the capability to provide real-time visual information while operating in an energy-efficient manner.

Formation of Supported Thylakoid Membrane Bioanodes for Effective Electron Transfer and Stable Photocurrent

The light-dependent reactions of photosynthesis use light energy to generate photoelectrons traveling through the thylakoid membranes (TMs). Extracting the photoelectrons from the TMs to form bioanodes can have various applications. Most studies focus on modifying the electrode materials to increase the collected photocurrent. Seldom studies have investigated how the orientation of the TMs influences photocurrent collection. In addition, the formation of reactive oxygen species (ROS) during photosynthesis is a challenge for stable photocurrent generation. Here, we enhanced the photoelectron transfer from the TMs to electrodes by depositing expanded thylakoids as planar supported membranes onto an electrode. The high contact area between the external electrodes and TMs per unit mass of thylakoid allows the thylakoid to more effectively transfer electrons to the electrodes, thereby reducing the free electrons available for the ROS generation. We expanded the naturally stacked thylakoids into liposomes through osmotic pressure and dropcasted them onto an Au electrode. The electrochemical impedance measurement showed that the supported membrane bioanode formed by the expanded liposomes had a lower photoelectron transfer resistance. Additionally, we observed that the expanded TM bioanode provided a higher photocurrent and was more durable to air/water interfacial tension. These results suggest that the effective contact between the expanded TM and electrodes can lead to more efficient electron transfer and increase the system robustness. The photo fuel cell (PFC) made by the expanded TM bioanode had a higher open-circuit voltage than the one made by the stacked TM bioanode. Interestingly, we found that PFCs made of high-load TM bioanodes had fast photocurrent decay under continuous operation at high cell voltages. The poor contact of large numbers of TMs with the electrodes at the high-load TM bioanodes could cause more ROS accumulation and therefore decreased the operational stability, supporting the importance of effective contact between TMs and the electrodes.

Electrosprayed Thylakoid-Alginate Film on a Micro-Pillar Electrode for Scalable Photosynthetic Energy Harvesting

Direct harvesting of electricity from photosynthesis is highly desired as an eco-friendly and sustainable energy harvesting technology. Photosynthetic apparatuses isolated from plants, such as thylakoid membranes (TMs), are deposited on an electrode by which photosynthetic electrons (PEs) are collected from water splitting. To enhance PE collection efficiency, it is critical to increase the electrochemical interfaces between TMs and the electrode. Considering the size of TMs to be around a few hundred nanometer, we hypothesize that an array of micropillar-shaped (MP) electrode can maximize the TM/electrode interface area. Thus, we developed MP electrodes with different heights and investigated the electrospraying of TM-alginate mixtures to fill the gaps between MPs uniformly and conformally. The uniformity of the TM-alginate film and the interaction between the TM and the MP electrode were evaluated to understand how the MP heights and film quality influenced the magnitude of the PE currents. PE currents increased up to 2.4 times for an MP electrode with an A/R of 1.8 compared to a flat electrode, indicating increased direct contact interface between TMs and the electrode. Furthermore, to demonstrate the scalability of this approach, an array of replicated SU-8 MP electrodes was prepared and PE currents of up to 3.2 μA were monitored without a mediator under 68 mW/cm². Finally, the PE current harvesting was sustained for 14 days without decay, demonstrating the long-term stability of the TM-alginate biophotoanodes.

The Unlimited Potential of Microbial Rhodopsins as Optical Tools

Microbial rhodopsins, a photoactive membrane protein family, serve as fundamental tools for optogenetics, an innovative technology for controlling biological activities with light. Microbial rhodopsins are widely distributed in nature and have a wide variety of biological functions. Regardless of the many different known types of microbial rhodopsins, only a few of them have been used in optogenetics to control neural activity to understand neural networks. The efforts of our group have been aimed at identifying and characterizing novel rhodopsins from nature and also at engineering novel variant rhodopsins by rational design. On the basis of the molecular and functional characteristics of those novel rhodopsins, we have proposed new rhodopsin-based optogenetics tools to control not only neural activities but also "non-neural" activities. In this Perspective, we introduce the achievements and summarize future challenges in creating optogenetics tools using rhodopsins. The implementation of optogenetics deep inside an in vivo brain is the well-known challenge for existing rhodopsins. As a perspective to address this challenge, we introduce innovative optical illumination techniques using wavefront shaping that can reinforce the low light sensitivity of the rhodopsins and realize deep-brain optogenetics. The applications of our optogenetics tools could be extended to manipulate non-neural biological activities such as gene expression, apoptosis, energy production, and muscle contraction. We also discuss the potentially unlimited biotechnological applications of microbial rhodopsins in the future such as in photovoltaic devices and in drug delivery systems. We believe that advances in the field will greatly expand the potential uses of microbial rhodopsins as optical tools.

Biomimetic Membranes with Transmembrane Proteins: State-of-the-Art in Transmembrane Protein Applications

In biological cells, membrane proteins are the most crucial component for the maintenance of cell physiology and processes, including ion transportation, cell signaling, cell adhesion, and recognition of signal molecules. Therefore, researchers have proposed a number of membrane platforms to mimic the biological cell environment for transmembrane protein incorporation. The performance and selectivity of these transmembrane proteins based biomimetic platforms are far superior to those of traditional material platforms, but their lack of stability and scalability rule out their commercial presence. This review highlights the development of transmembrane protein-based biomimetic platforms for four major applications, which are biosensors, molecular interaction studies, energy harvesting, and water purification. We summarize the fundamental principles and recent progress in transmembrane protein biomimetic platforms for each application, discuss their limitations, and present future outlooks for industrial implementation.

Biomimetic Membranes as a Technology Platform: Challenges and Opportunities

Biomimetic membranes are attracting increased attention due to the huge potential of using biological functional components and processes as an inspirational basis for technology development. Indeed, this has led to several new membrane designs and applications. However, there are still a number of issues which need attention. Here, I will discuss three examples of biomimetic membrane developments within the areas of water treatment, energy conversion, and biomedicine with a focus on challenges and applicability. While the water treatment area has witnessed some progress in developing biomimetic membranes of which some are now commercially available, other areas are still far from being translated into technology. For energy conversion, there has been much focus on using bacteriorhodopsin proteins, but energy densities have so far not reached sufficient levels to be competitive with state-of-the-art photovoltaic cells. For biomedical (e.g., drug delivery) applications the research focus has been on the mechanism of action, and much less on the delivery 'per se'. Thus, in order for these areas to move forward, we need to address some hard questions: is bacteriorhodopsin really the optimal light harvester to be used in energy conversion? And how do we ensure that biomedical nano-carriers covered with biomimetic membrane material ever reach their target cells/tissue in sufficient quantities? In addition to these area-specific questions the general issue of production cost and scalability must also be treated in order to ensure efficient translation of biomimetic membrane concepts into reality.

Protein bioelectronics: a review of what we do and do not know

We review the status of protein-based molecular electronics. First, we define and discuss fundamental concepts of electron transfer and transport in and across proteins and proposed mechanisms for these processes. We then describe the immobilization of proteins to solid-state surfaces in both nanoscale and macroscopic approaches, and highlight how different methodologies can alter protein electronic properties. Because immobilizing proteins while retaining biological activity is crucial to the successful development of bioelectronic devices, we discuss this process at length. We briefly discuss computational predictions and their connection to experimental results. We then summarize how the biological activity of immobilized proteins is beneficial for bioelectronic devices, and how conductance measurements can shed light on protein properties. Finally, we consider how the research to date could influence the development of future bioelectronic devices.

Photoinduced electron transfer pathways in hydrogen-evolving reduced graphene oxide-boosted hybrid nano-bio catalyst

Photocatalytic production of clean hydrogen fuels using water and sunlight has attracted remarkable attention due to the increasing global energy demand. Natural and synthetic dyes can be utilized to sensitize semiconductors for solar energy transformation using visible light. In this study, reduced graphene oxide (rGO) and a membrane protein bacteriorhodopsin (bR) were employed as building modules to harness visible light by a Pt/TiO2 nanocatalyst. Introduction of the rGO boosts the nano-bio catalyst performance that results in hydrogen production rates of approximately 11.24 mmol of H2 (μmol protein)(-1) h(-1). Photoelectrochemical measurements show a 9-fold increase in photocurrent density when TiO2 electrodes were modified with rGO and bR. Electron paramagnetic resonance and transient absorption spectroscopy demonstrate an interfacial charge transfer from the photoexcited rGO to the semiconductor under visible light.

Bacteriorhodopsin—Novel biomolecule for nano devices

The aim of this article is to provide insight on the use of a biological molecule--bacteriorhodopsin (bR) having all the basic properties necessary for the assembly of nanoscale electronic devices. Recent developments made during last decade supported by key references are reviewed in this contribution. Major emphasis on bR-based observations conducted in our laboratory has been elaborated. Important issues concerning structure, widely accepted photocycle of bR has been summarized. The possibility of nano-devices emanating from this biomolecule is briefly presented.

Approaches for biological and biomimetic energy conversion

This article highlights areas of research at the interface of nanotechnology, the physical sciences, and biology that are related to energy conversion: specifically, those related to photovoltaic applications. Although much ongoing work is seeking to understand basic processes of photosynthesis and chemical conversion, such as light harvesting, electron transfer, and ion transport, application of this knowledge to the development of fully synthetic and/or hybrid devices is still in its infancy. To develop systems that produce energy in an efficient manner, it is important both to understand the biological mechanisms of energy flow for optimization of primary structure and to appreciate the roles of architecture and assembly. Whether devices are completely synthetic and mimic biological processes or devices use natural biomolecules, much of the research for future power systems will happen at the intersection of disciplines.

Energy transfer between chlorophyll derivatives in silica mesostructured films and photocurrent generation

Layered silica/surfactant mesostructured thin films containing chlorophyllous pigments [C13(2)-demethoxycarbonyl-pheophytin b (pyroPheo b) or zinc C13(2)-demethoxycarbonyl-chlorophyll b (Zn-pyroChl b)] have been prepared on an indium tin oxide (ITO) electrode grafted with a chlorophyll derivative possessing a triethoxysilyl group (copper C13(2)-demethoxycarbonyl-chlorophyllide a 3-triethoxysilyl propylamide, Cu-APTES-Chl a) to achieve effective light harvesting and successive photocurrent generation by the mesostructured films. The incorporation of pyroPheo b and Zn-pyroChl b in the mesostructured film resulted in 1.2- and 1.6-fold increases of the photocurrent density, respectively, as compared to the case of an antenna pigment-free film also grafted to a surface-modified ITO electrode. The difference action spectra, between the electrodes with and without the antenna pigments, coincided well with the absorption spectra of the immobilized pigments. Because direct electron injection from the antenna pigments in the mesostructured films to the ITO electrode was scarcely observed, the energy transfer from the antenna pigments to Cu-APTES-Chl a plays an important role for the increase in photocurrent density. The usefulness of the mesostructured films as model systems is discussed in relation to the photosynthetic primary processes of higher plants.

Opto-electrical processes in a conducting polymer–bacteriorhodopsin system

In this report, we highlight the opto-electrical processes at a conducting polymer-bacteriorhodopsin (bR) interface in presence of a voltage bias. Oriented bR on a conducting polymer substrate forms a unique hybrid system where the oxidation state of the polymer controls the optically activated proton gradient in the bR side. The internal conversion of the intermediate deprotonated M-state and the proton transfer/transport of bR at the interface can be controlled by the electrostatic environment and leads to interesting device features in this process.

In situ characterization of functional purple membrane monolayers at the air-water interface

The purple membrane (PM) of Halobacterium salinarum contains a single type of protein, bacterio-rhodopsin (bR), which is a member of the seven alpha-helices transmembrane protein family. This protein is a photoactive proton pump, translocating one proton from the cytoplasmic to the extracellular side of the PM per photon absorbed. bR is found in trimers in PM, where they are assembled in a two-dimensional hexagonal lattice. We show herein that stable and functional films can be built in monolayers at the air-water interface by spreading aqueous suspensions of purified and native PM patches. In situ spectroscopic measurements at the air-water interface indicate that bR remains photoactive in this environment. Physical parameters of these PM films, such as protein molecular area, irreversible in-plane aggregation, z-axis orientation, film thickness, and surface roughness, were determined from surface pressure and surface potential-area isotherms, fluorescence spectroscopy, and X-ray reflectivity at the air-water interface. We find that PM do form organized monolayers of membranes, with an optimal packing density at a surface pressure of approximately 20 mN/m, although no preferential vectorial alignment, with respect to the plane normal to the membrane, can be detected from fluorescence quenching experiments.

Facile synthesis of chlorophyll analog possessing a disulfide bond and formation of self-assembled monolayer on gold surface

A chlorophyll analog forming self-assembled monolayers (SAMs) on a gold surface was synthesized for the first time. 13(2)-(Demethoxycarbonyl)pheophorbide-a, which was converted from naturally occurring chlorophyll-a, was condensed with 2-hydroxyethyl disulfide to give a chlorin dyad linked by a disulfide bond. The chlorin analog was spontaneously immobilized on a gold substrate by soaking in an acetone solution of the dyad for 24 h. The resulting gold plate exhibited a visible absorption spectrum with about 420- and 675-nm maxima as the Soret and Qy peaks, respectively, indicating that chlorin pi-conjugates were modified on the gold substrate through Au-S bonding. Both visible absorption and fluorescence emission bands of the chlorin chromophores on the gold substrate were red-shifted compared with those of the synthesized chlorin dyad in a homogeneous acetone solution. The measured absorbance at the Soret maximum suggests that the chlorin chromophores on the gold plate were densely packed on a gold surface to form a SAM. Cathodic photocurrents were generated from SAMs of the chlorins on a gold substrate with irradiation of visible-lights above 400 nm. Photoinduced electron transfer from chlorins on the gold substrate to oxygen molecules in an electrolyte solution were attributed to the cathodic photocurrent generation.

Proton transfer reactions in the F86D and F86E mutants of pharaonis phoborhodopsin (sensory rhodopsin II)

pharaonis phoborhodopsin (ppR, also called pharaonis sensory rhodopsin II, psRII), a negative phototaxis receptor of Natronobacterium pharaonis, can use light to pump a proton in the absence of its transducer protein. However, the pump activity is much lower than that of the light-driven proton-pump bacteriorhodopsin (BR). ppR's pump activity is known to be increased in a mutant protein, in which Phe86 is replaced with Asp (F86D). Phe86 is the amino acid residue corresponding to Asp96 in BR, and we expect that Asp86 plays an important role in the proton transfer at the highly hydrophobic cytoplasmic domain of the F86D mutant ppR. In this article, we studied protein structural changes and proton transfer reactions during the photocycles of the F86D and F86E mutants in ppR by means of Fourier transform infrared (FTIR) spectroscopy and photoelectrochemical measurements using a tin oxide (SnO2) electrode. FTIR spectra of the unphotolyzed state and the K and M intermediates are very similar among F86D, F86E, and the wild type. Asp86 or Glu86 is protonated in F86D or F86E, respectively, and the pK(a) > 9. During the photocycle, the pK(a) is lowered and deprotonation of Asp86 or Glu86 is observed. Detection of both deprotonation of Asp86 or Glu86 and concomitant reprotonation of the 13-cis chromophore implies the presence of a proton channel between position 86 and the Schiff base. However, the photoelectrochemical measurements revealed proton release presumably from Asp86 or Glu86 to the cytoplasmic aqueous phase in the M state. This indicates that the ppR mutants do not have the BR-like mechanism that conducts a proton uniquely from Asp86 or Glu86 (Asp96 in BR) to the Schiff base, which is possible in BR by stepwise protein structural changes at the cytoplasmic side. In ppR, there is a single open structure at the cytoplasmic side (the M-like structure), which is shown by the lack of the N-like protein structure even in F86D and F86E at alkaline pH. Therefore, it is likely that a proton can be conducted in either direction, the Schiff base or the bulk, in the open M-like structure of F86D and F86E.

Photoinduced electron transfer at solid surfaces: the TiO2–SiO2system

Small particles of TiO2 have been synthesized on porous silica. XRD and spectral measurements place the sizes of the particles at about 50 A. With these small particles the band gap of the semiconductor material is markedly blue shifted with respect to bulk TiO2. This enables the direct excitation of co-adsorbed arenes on the TiO2-SiO2 material. Direct excitation of TiO2 leads to Ti3+, by photoinduced extraction from the SiO2. Direct excitation of pyrene on the material leads to a shortened fluorescence lifetime and lower quantum yield. Meanwhile, the yield of pyrene cation radical increases compared to that on pure SiO2. The production of Ti3+ on the pyrene samples, by direct excitation of the TiO2, leads to a shortened pyrene fluorescence lifetime and decreased quantum yield, the quantum yield decreasing to zero at high enough Ti3+. Removal of Ti3+ by O2 causes the original properties of the pyrene fluorescence to return.

Effect of metal ion exchange on the photocurrent response from bacteriorhodopsin on tin oxide electrodes

The transient photocurrent response from bacteriorhodopsin (bR) on tin oxide electrodes was strongly influenced by metal ions bound to bR molecules. The photocurrent polarity reversal pH, which corresponded to the pH value for the reversal of the proton release/uptake sequence in the bR photocycle, of cation-substituted purple membrane (PM) was shifted to lower pH with the increase in the cation affinities to carboxyl groups and a close correlation was noted between the two values. This suggests that the metal ion present in the extracellular region of a bR molecule modulates the pK(a) of proton release groups of bR by stabilizing the ionized state of the proton-releasing glutamic acids. The behavior of photocurrents at light-off in alkaline media, reflecting the proton uptake by bR, was unchanged by binding monovalent (Na(+) and K(+)) or divalent cations (Mg(2+) and Ca(2+)), but was drastically changed by binding La(3+) ions. This can be explained by invoking a substantial slowing of the proton uptake process in the presence of La(3+).