Peptide self-assembly is a facile route to the development of bioorganic hybrid materials that have sophisticated nanostructures towards diverse applications. Here, we report the synthesis of self-assembled peptide (Fmoc-diphenylalanine, Fmoc-FF)/graphitic carbon nitride (g-C3N4) hydrogels for light harvesting and biomimetic photosynthesis through non-covalent interactions between aromatic rings in Fmoc-FF nanofibers and tris-s-triazine in g-C3N4 nanosheets. According to our analysis, the photocurrent density of the Fmoc-FF/g-C3N4 hydrogel was 1.8 times higher (0.82 μA cm-1) than that of the pristine g-C3N4. This is attributed to effective exfoliation of g-C3N4 nanosheets in the Fmoc-FF/g-C3N4 network, facilitating photo-induced electron transfers. The Fmoc-FF/g-C3N4 hydrogel reduced NAD+ to enzymatically active NADH under light illumination at a high rate of 0.130 mole g-1 h-1 and drove light-responsive redox biocatalysis. Moreover, the Fmoc-FF/g-C3N4 scaffold could well-encapsulate key photosynthetic components, such as electron mediators, cofactors, and enzymes, without noticeable leakage, while retaining their functions within the hydrogel. The prominent activity of the Fmoc-FF/g-C3N4 hydrogel for biomimetic photosynthesis resulted from the easy transfer of photo-excited electrons from electron donors to NAD+ via g-C3N4 and electron mediators as well as the hybridization of key photosynthetic components in a confined space of the nanofiber network.
Abnormal aggregation of β-amyloid (Aβ) peptides is a major hallmark of Alzheimer’s disease (AD). In spite of numerous attempts to prevent the β-amyloidosis, no effective drugs for treating AD have been developed to date. Among many candidate chemicals, methylene blue (MB) has proved its therapeutic potential for AD in a number of in vitro and in vivo studies; but the result of recent clinical trials performed with MB and its derivative was negative. Here, with the aid of multiple photochemical analyses, we first report that photoexcited MB molecules can block Aβ42 aggregation in vitro. Furthermore, our in vivo study using Drosophila AD model demonstrates that photoexcited MB is highly effective in suppressing synaptic toxicity, resulting in a reduced damage to the neuromuscular junction (NMJ), an enhanced locomotion, and decreased vacuole in the brain. The hindrance effect is attributed to Aβ42 oxidation by singlet oxygen (1O2) generated from photoexcited MB. Finally, we show that photoexcited MB possess a capability to disaggregate the pre-existing Aβ42 aggregates and reduce Aβ-induced cytotoxicity. Our work suggests that light illumination can provide an opportunity to boost the efficacies of MB toward photodynamic therapy of AD in future.
The self-assembly of amyloidogenic peptides into β-sheet-rich aggregates is a general feature of many neurodegenerative diseases, including Alzheimers disease, which signifies the need for the effective attenuation of amyloid aggregation toward alleviating amyloid-associated neurotoxicity. In the present study, we report that photoluminescent carbon nanodots (CDs) can effectively suppress Alzheimers β-amyloid (Aβ) self-assembly and function as a β-sheet breaker disintegrating preformed Aβ aggregates. We synthesized CDs using ammonium citrate through one-pot hydrothermal treatment and passivated their surface with branched polyethylenimine (bPEI). The bPEI-coated CDs (bPEI@CDs) exhibited hydrophilic and cationic surface characteristics, which interacted with the negatively charged residues of Aβ peptides, suppressing the aggregation of Aβ peptides. Under light illumination, bPEI@CDs displayed a more pronounced effect on Aβ aggregation and on the dissociation of β-sheet-rich assemblies through the generation of reactive oxygen species from photoactivated bPEI@CDs. We verified the light-triggered attenuation effect of Aβ aggregation using a series of experiments, including photochemical and microscopic analysis. Furthermore, our cell viability test confirmed the ability of photoactivated bPEI@CDs for the suppression of Aβ-mediated cytotoxicity, indicating bPEI@CDs’ potency as an effective anti-Aβ neurotoxin agent.
Enoate reductases from the family of Old Yellow Enzymes (OYEs) can catalyze stereoselective trans-hydrogenation of activated C=C bonds. Despite their potential, however, their application is limited by the necessity for continuous supply of redox equivalents such as nicotinamide cofactors [NAD(P)H]. Here, we report visible light-driven activation of OYEs through NAD(P)H-free, direct transfer of photoexcited electrons from xanthene dyes to the prosthetic flavin moiety. Our spectroscopic and electrochemical analyses verified spontaneous association of rose bengal and its derivatives with OYEs. Illumination of a white light-emitting-diode triggered photoreduction of OYEs by xanthene dyes, which facilitated the enantioselective reduction of C=C bonds in the absence of NADH. The photoenzymatic conversion of 2-methylcyclohexenone resulted in enantiopure (ee >99%) (R)-2-methylcyclohenanone with conversion yields as high as 80-90%. The turnover frequency was significantly affected by the substitution of halogen atoms in xanthene dyes. The NADH-free, xanthene-sensitized photobiocatalytic platform was successfully applied to different homologues of OYEs from Thermus scotoductus and Bacillus subtilis. This work demonstrates a simple and versatile way of activating OYEs by direct coupling of OYE-catalysis with molecular photocatalysis.
Biomineralization is a biogenic process that produces elaborate inorganic and organic hybrid materials in nature. Inspired by the natural process, this study explores novel mineralization approach to create nanostructured CaCO3 films composed of amorphous CaCO3 hemispheres using catechol-rich polydopamine (PDA) as a biomimetic mediator. We successfully transformed thus-synthesized biomimetic CaCO3 to nanostructured films of metal oxide minerals, such as FeOOH, CoCO3, NiCO3, and MnOOH, via a simple procedure. CaCO3-templated metal oxide minerals functioned as an efficient electrocatalyst; CaCO3-templated CoPi (nanoCoPi) film exhibited high stability as a water oxidation electrocatalyst with a current density of 1.5 mA cm-2. The nanostructure of nanoCoPi consisting of individual nanoparticles (~70 nm) and numerous internal pores (BET surface area: 3.17 m2g-1) facilitated an additional charge transfer pathway from the electrode to individual active sites of catalysts. This work demonstrates a plausible strategy for facile and green synthesis of nanostructured electrocatalysts through biomimetic CaCO3 mineralization.
Organic redox compounds represent an emerging class of cathode materials in rechargeable batteries for low-cost and sustainable energy storage. However, the low operating voltage (< 3 V) and necessity of using lithium-containing anodes have significantly limited their practical applicability to battery systems. Here, we introduce a new class of p-type organic redox centers based on N, N’-substituted phenazine (NSPZ) to build ready-to-charge organic batteries. In the absence of lithium-containing anodes, NSPZ cathodes facilitate reversible two-electron transfer at 3.7 and 3.1 V accompanying anion association, which results in a specific energy of 622 Wh kg-1 in dual-ion batteries.
Natural photosynthesis is an effective route for clean and sustainable conversion of CO2 to high-energy chemicals. Inspired by the natural scheme, we designed a tandem-photoelectrochemical (PEC)-cell-integrated-with-enzyme-cascade (TPIEC) system, which transfers photogenerated electrons to a multi-enzyme cascade for biocatalyzed reduction of CO2 to methanol. We applied a hematite photoanode and a bismuth ferrite photocathode to fabricate the iron oxide-based tandem PEC cell for visible light-assisted regeneration of nicotinamide cofactor (NADH). The cell utilized water as an electron donor and spontaneously regenerated NADH. To complete the TPIEC system, a superior three-dehydrogenase cascade system was employed in the cathodic part of the PEC cell. Using applied bias, the TPIEC system achieved high methanol conversion output, providing a PEC platform for highly selective synthesis of hydrocarbon fuel using readily-available solar energy and water.
Peroxygenases are very promising catalysts for oxyfunctionalization reactions. Their practical applicability, however, is hampered by their sensitivity against the oxidant (H2O2), therefore necessitating in situ generation of H2O2. Here, we report a photoelectrochemical approach to provide peroxygenases with suitable amounts of H2O2 while reducing the electrochemical overpotential needed for the reduction of molecular oxygen to H2O2. When tethered on single-walled carbon nanotubes (SWNT) under illumination, flavins allowed for a marked anodic shift of the oxygen reduction potential in comparison to pristine-SWNT and/or non-illuminated electrodes. This flavin-SWNT-based photoelectrochemical platform enabled peroxygenases-catalyzed, selective hydroxylation reactions.
We report a photoelectrocatalytic way for suppressing beta-amyloid (Abeta) self-assembly using a visible light-active, hematite-based photoelectrode platform. Upon illumination of a light-emitting diode with anodic bias, we found that hematite photoanodes generate reactive radical species such as superoxide ions and hydroxyl radical via the photoelectrocatalytic process. According to our analyses, the hole-derived radicals, in particular the hydroxyl radical, played a significant role of oxidizing Abeta peptides, which effectively blocked further fibrillation. The efficacy of photoelectrocatalytic inhibition on Abeta aggregation was enhanced by introducing cobalt phosphate (Co-Pi) as a co-catalyst on the hematite photoanode, which facilitated the separation of electron-hole pairs. We verified that both bare and Co-Pi@hematite photoanodes are biocompatible and effective in reducing Abeta aggregation-induced cytotoxicity.
The abnormal aggregation of extracellular beta-amyloid (Abeta) peptides is a major pathological event of Alzheimers disease. Recently, photodynamic suppression of the assembly of Abeta peptides into beta-sheet-rich aggregates and the resulting neurotoxicity were suggested; however, its application has been limited by the low tissue penetration-depth of UV or visible light. Herein, we report rose bengal (RB)-loaded upconverting nanocomposites as a NIR-responsive inhibitor of Abeta aggregation. Rattle-structured, organosilica shell (ROS) deposited on NaYF4:Yb,Er nanocrystals (UCNPs) was adopted as an efficient photosensitizer carrier with high loading capacity and disaggregation effect of RB. We demonstrated that the UCNP@ROS exhibited high energy transfer efficiency to the loaded RB under the irradiation of 980 nm NIR light and generated singlet oxygen efficiently inhibiting Abeta self-assembly. Furthermore, RB-loaded UCNP@ROS is not only biocompatible, but also effective in suppressing Abeta-induced cytotoxicity under NIR light, suggesting its potential towards photodynamic treatment of of Alzheimers disease in future.
In natural photosynthesis, solar energy is converted to chemical energy through a cascaded, photoinduced charge transfer chain that consists of primary and secondary acceptor quinones (i.e., QA and QB), which leads to exceptionally high quantum yield near unity. Inspired by the unique multistep charge transfer architecture in nature, we have synthesized catecholamine-functionalized, reduced graphene oxide (RGO) film as a redox mediator that can mimic quinone acceptors in the photosystem II. We utilized polynorepinephrine (PNE) as a redox-shuttling chemical, as well as to coat graphene oxide (GO) and to reduce GO to RGO. The two-electrons-and-two-protons-involving charge transfer characteristic of quinone ligands in PNE acted as an electron acceptor that facilitated charge transfer in photocatalytic water oxidation. Furthermore, PNE-coated RGO film promoted fast charge separation in [Ru(bpy)3]2+ and over two-fold increased the activity of cobalt phosphate on photocatalytic water oxidation. The results suggest that our bio-inspired strategy for the construction of forward charge transfer pathway can provide more opportunities to realize efficient artificial photosynthesis.
Green conversion of carbon dioxide to fuels has attracted high interest recently due to the global issues of environmental sustainability and renewable energy sources. In this study, we present photoelectrochemical (PEC) regeneration of nicotinamide cofactors (NADH) coupled with enzymatic synthesis of formate from CO2 towads mimicking natural photosynthesis. The water oxidation-driven PEC platform exhibited high yield and rate of NADH regeneration compared to many other homogeneous, photochemical systems. We successfully coupled solar-assisted NADH reduction with enzymatic CO2 reduction to formate under continuous CO2 injection.
Redox enzymes are industrially important for catalyzing highly complex reactions because of their excellent regio- and stereo-selectivity; however, broad application of redox enzymes has been often limited by the requirement of stoichiometric supply of cofactors such as β-nicotinamide adenine dinucleotide (NADH). Here, we report light-driven cofactor regeneration coupled with water oxidation by employing a photoelectrochemical cell platform consisted of a FeOOH/Fe2O3 photoanode and a black silicon photocathode. The FeOOH layer deposited on Fe2O3 surface decreased reaction barriers for water oxidation. The black silicon photocathode exhibited high photocurrent response and superior capacity to drive cofactor reduction. The cofactor regeneration yield in the photoelectrochemical cell was almost two-fold higher than that obtained in homogenous system, which demonstrates that photoelectrochemical cell is a promising platform for redox biocatalytic reactions using water as an electron donor.
We report on a silicon-based photoelectrochemical cell that integrates a formate dehydrogenase from Thiobacillus sp. (TsFDH) to convert CO2 to formate using water as an electron donor under visible light irradiation and an applied bias. Our results revealed that sequential transfer of electrons, extracted via a water oxidation reaction at a npp+ triple-junction silicon on ITO (3-jn-Si/ITO/CoPi) photoanode, to a a hydrogen-terminated silicon nanowire (H-SiNW) photocathode, and further to TsFDH, leads to effective formate production with a faradaic efficiency of 16.18% under the applied bias of 1.8 V, while no formate was synthesized directly at the H-SiNW photocathode alone. The formate yield increased significantly through the integrated PEC system, which continuously regenerated NADH for TsFDH-catalyzed CO2 reduction. Moreover, we demonstrated that our silicon-based biocatalytic system could be operated under natural sunlight using a solar tracking module, which is a highly desirable result for the practical utility of the PEC as a sustainable solar energy harvesting system. The current study suggests that the deliberate integration of biocatalysis to a PEC platform can provide an opportunity to synthesize valuable chemicals with the use of earth-abundant materials and sustainable resources. With our biocatalysis-integrated PEC platform, further engineering of enzymes and photoelectrode materials would provide more opportunity to improve efficiency of the system.
In nature, quinone plays a vital role in numerous electrochemical reactions for energy transduction and storage; such processes include respiration and photosynthesis. For example, fast proton-coupled electron transfer between primary and secondary quinones in green plants triggers the rapid charge separation of chlorophyll molecules, achieving unparalleled photosynthesis with near-unity quantum yield. In addition, quinone-rich polymers such as eumelanin and polydopamine show unique optical and electrical properties (e.g., strong broadband absorbance or a switching response to external stimuli), mostly arising from their chemically disordered structures. Understanding the unique features of quinone and its derivatives can provide solutions to the construction of bio-inspired systems for energy harvesting and conversion. This paper reviews recent advances in the design of quinone-functionalized hybrid materials based on quinones redox, electrical, optical, and metal chelating/reducing properties to determine these materials applications in energy-harvesting and -storage systems, such as artificial photosynthetic platforms, rechargeable batteries, pseudocapacitors, phototransistors, plasmonic light harvesting platforms, and dye-sensitized solar cells.
FROM WASTE TO VALUABLES: Human urine is studied as a potential source of energy for light-driven redox biocatalytic reactions. The urea-rich human urine functions as an efficient chemical fuel in a photoelectrochemical cell regenerating nicotinamide cofactor (NADH), an essential hydride mediator that is required for numerous redox biocatalytic reactions. We demonstrate the utility of human urine as a chemical fuel for driving redox biocatalysis in a photoelectrochemical cell. Ni(OH)2-modified alpha-Fe2O3 is selected as a photoanode for the oxidation of urea in human urine and black silicon (bSi) is used as a photocathode material for NADH regeneration. The electrons extracted from human urine are used for the regeneration of NADH. The catalytic reactions at both the photoanode and the photocathode were significantly enhanced by light energy that lowered the overpotential and generated high currents in the full cell system.
Graphitic carbon nitride (g-C3N4) is a metal-free material of only carbon and nitrogen-based structure called tri-s-triazine, which is non-toxic, abundant, and cost-effective. While there are only a few studies about biomedical applications of g-C3N4, good biocompatibility of g-C3N4 has been confirmed recently through its use in cancer diagnosis, drug delivery, and cell imaging. Here we report that g-C3N4 has a suppressive ability toward Alzheimers beta-amyloid (Abeta) aggregation under light illumination. Under commercial white light emitting diode light, photo-induced electrons of g-C3N4 with a 2.6 eV bandgap generated reactive oxygen species (ROS), such as superoxide anion and singlet oxygen; then, the ROS blocked further Abeta aggregation as a way of photo-oxidation, impacting the conformational structure of Abeta. Through metal doping into a g-C3N4 framework, we further demonstrated that Fe-doped g-C3N4 showed enhanced optical properties and stronger inhibition on Abeta aggregation than bare g-C3N4. Both g-C3N4 and Fe-doped g-C3N4 had negligible cytotoxicity and exhibited significant reduction in Abeta-induced cell death.
Harnessing solar energy has recently attracted much attention due to the increased importance of environmental and energy issues. In particular, the photolysis of water using photocatalysts, so called artificial photosynthesis, has been receiving great attention in terms of the direct and efficient solar energy conversion system to produce O2 and H2 as chemical fuels. The effectiveness of water splitting using photocatalysts is determined by the utilization of visible light of the solar spectrum, capacity of the harvested light to generate charge carriers, and the extent of charge separation and transfer. Thus, the selection of semiconducting photocatalyst materials with proper band position, bandgap energy, and long-lived stability is critical for the viable water splitting system. Herein we report on the synthesis of highly porous, 1-D tungsten-doped BiVO4 nanofibers (W:BiVO4 NFs). To facilitate photocatalysis, we introduced nickel nanoparticles (NiOx NPs) as co-catalysts on the surface of the W:BiVO4 NFs. The outstanding water oxidation performance of the NiOx NPs-functionalized W:BiVO4 NFs were obtained through (i) the control of polymer/precursor to achieve porous W:BiVO4 NFs (for higly increased surface area), (ii) the control of tungsten-doping level (for fast charge transfer), and (iii) the optimization of the loading amounts of NiOx NPs (for efficient charge pathway suppression of charge recombination).
In the past 50 years, cytochrome P450 monooxygenases (P450s) have been given significant attention for the synthesis of natural products (e.g., vitamins, steroids, lipids) and pharmaceuticals. Despite their potential, however, costly nicotinamide cofactors such as NAD(P)H are required as reducing equivalents; thus, in situ regeneration of NAD(P)H is essential to sustaining P450-catalyzed reactions. Furthermore, poor stability of P450s has been considered as a hurdle, hampering industrial implementations of P450-catalyzed reactions. Herein we describe the development of an economic and robust process of P450-catalyzed reactions by the combination of P450 immobilization and solar-induced NADPH regeneration. The P450 monooxygenase could be efficiently immobilized on a P(3HB) biopolymer, which enabled simple purification from the E. coli host. We clearly demonstrated that the P450-P(3HB) complex exhibited much higher enzymatic yield and stability than free P450 did against changes of pH, temperature, and concentrations of urea and ions. Using the robust P450-P(3HB) complex and solar-tracking module, we successfully conducted P450-catalyzed artificial photosynthesis under the irradiation of natural sunlight in a preparative scale (500 mL) for multiple days. To the best of our knowledge, this is the largest reactor volume in P450-catalyzed reactions reported so far. We believe that our robust platform using simple immobilization and abundant solar energy promises a significant breakthrough for the broad applications of cytochrome P450 monooxygenases.
Cellulose, a main component of green plants, is the most abundant organic chemical on Earth, produced 1011 tons per year in the biosphere. The polysaccharide consists of D-glucose units linked by beta-1,4-glycosidic bonds and has been widely utilized in diverse engineering fields because of its biocompatibility, abundance, and high chemical stability. In this work, we have demonstrated the utility of carboxymethyl cellulose (CMC) fibers as a sacrificial template to produce binary and tertiary metal oxides fibers. The electrostatic interaction between metal ions and the carboxyl groups in CMC fibers induced a hierarchical structure of metal oxides. The morphologies of synthesized metal oxides (e.g., CeO2, ZnO, and CaMn2O4) could be controlled according to synthetic conditions, such as metal precursor concentration, calcination temperature, and the amount of CMC fibers. Thus-synthesized CMC-templated metal oxide fibers exhibited enhanced performances for photocatalytic, photochemical, and electrocatalytic reactions. The CeO2 fibers showed much higher photocatalytic activity than CeO2 nanoparticles due to superior ability to generate reactive oxygen species which can degrade organic pollutants. We also demonstrated that hierarchical ZnO fibers hybridized with g-C3N4 could provide directional charge transfer pathway and showed their utility for biocatalyzed artificial photosynthesis through visible light-driven chemical NADH regeneration coupled with redox enzymatic reaction. The electrochemical properties of CaMn2O4 fibers enabled bi-functional reactions of oxygen reduction and evolution reactions. We expect that the economical and environmentally friend approach could be extended to green synthesis of hierarchically structured materials of other metal oxides.
The abnormal assembly of beta-amyloid (Abeta) peptides into neurotoxic, beta-sheet rich amyloid aggregates is a major pathological hallmark of Alzheimers disease (AD). Photodynamic therapy (PDT) is a promising strategy for treating various diseases due to its temporal and spatial controllability and reduced side effects. However, PDT for neurodegenerative diseases has not been explored yet. Here, we show that light-induced photosensitizing molecules can regulate Abeta amyloidogenesis. Our multiple photochemical analyses using circular dichroism, atomic force microscopy, dot blot, and native gel electrophoresis verified that photo-activated meso-tetra(4-sulfonatophenyl) porphyrin (TPPS) successfully inhibits Abeta aggregation in vitro. Furthermore, we demonstrate that Abeta toxicity was relieved in the photoexicited-TPPS-treated Drosophila AD model. TPPS suppresses neural cell death, synaptic toxicity, and behavioral defects in the Drosophila AD model under blue light illumination. Behavioral phenotypes, including larval locomotion defect and short lifespan caused by Abeta overexpression, were also rescued by blue light-excited TPPS.
The practical limits of coinage metal-based plasmonic materials demand sustainable, abundant alternatives with a wide plasmonic range of the solar energy spectrum. Aluminum (Al) is an emerging alternative, but its instability in aqueous environments critically limits its applicability to various light-harvesting systems. Here, we present a novel design strategy to achieve a robust platform for plasmon-enhanced light-harvesting using Al nanostructures. The incorporation of mussel-inspired polydopamine nano-layers in the Al nanoarrays allows for the reliable use of Al plasmonic resonances in a highly corrosive photocatalytic redox solution, and provides nanoscale arrangement of organic photosensitizers on Al surfaces. Resulting Al-photosensitizer core-shell assemblies exhibit plasmon-enhanced light absorption, which enables a 300% increase in photo-to-chemical conversion. Our strategy opens a path to realizing the stable and advanced use of aluminum for plasmonic light-harvesting.
Photoelectrochemical (PEC) detection is an attractive biosensing strategy because it inherits the benefits of electrochemical sensors, such as low cost, simple instrumentation and high sensitivity. Furthermore, PEC sensing can reduce undesired background noise and enhance sensitivity by using two separate forms of signals: light (for excitation) and electricity (for detection). Hematite is a promising photoanode material because of its strong absorption of visible light (Eg ~2.1 eV), high stability, low price, and environmentally benign characteristics. Here, we report the first hematite-based PEC biosensor platform to detect NADH under visible light. To enhance the electrical signal of photoanodes, we employed mussel-inspired polydopamine which immobilize redox mediators on hematite. The enzymatic PEC biosensor enabled the detection of glucose, ethanol, and lactate, and even showed successful detection of glucose in human plasma suggesting the practical usefulness of our platform.
Peptide self-assembly is an attractive route to the synthesis of intricate organic nanostructures that possess remarkable structural variety and biocompatibility. Recent studies on peptide-based, self-assembled materials have been expanding beyond the construction of high-order architectures; they are now reporting new functional materials that have applications in the emerging fields, such as artificial photosynthesis and rechargeable batteries. Nevertheless, there have been rather scarce reviews particularly concentrating on such versatile, emerging applications. Herein we selectively review recent advances in the synthesis of self-assembled peptide nanomaterials (e.g., cross beta-sheet-based amyloid nanostructures, peptide amphiphiles, etc.) and describe their new applications in diverse, interdisciplinary fields ranging from optics, energy storage/conversion to healthcare. We highlight the applications of peptide-based self-assembled materials in unconventional fields, such as photoluminescent peptide nanostructures, artificial photosynthetic peptide nanomaterials, and lithium-ion battery components. We also discuss relation of such functional materials to the rapidly progressing biomedical applications of peptide self-assembly, which include biosensors/chips and regenerative medicine. The combination of strategies shown in respective applications would further promote the discovery of novel, functional small materials.
Titanium dioxide has long been pursued as a promising material for many photocatalytic applications because of its chemical activity and stability as well as low cost. On the other hand, graphene oxide (GO) can serve as a scaffold for functional hybrid materials by interacting with various organic and inorganic chemicals. Herein, we synthesized graphene oxide-wrapped anatase TiO2 nanoparticles (GO-TiO2 NPs) as a self-adhesive photocatalyst for UV-activated colorimetric oxygen indicators. Our multiple analyses with zeta potential, UV-Vis spectrophotometry, and cyclic voltammetry revealed that methylene blue (MB), a widely used redox dye for colorimetric oxygen indication, strongly adsorbs onto GO-TiO2 NPs by both electrostatic and pi-pi stacking interactions. We successfully fabricated UV-activated visual oxygen indicator films using MB, GO-TiO2 NPs, glycerol, and hydroxyethyl cellulose (HEC) as a redox dye, a UV-absorbing self-adhesive photocatalyst, a sacrificial electron donor, and an encapsulation polymer, respectively. The chemical attraction between GO and MB significantly reduced dye leakage problem, a major drawback of conventional oxygen indicators; MB leaching from GO-TiO2-based film was 4.8 times lower than that from TiO2-based film. This novel MB/GO-TiO2/glycerol/HEC film was photobleached by UV irradiation within 6 min and regained its blue color in the air within 20 min, demonstrating its useful functionality as a UV-activated colorimetric oxygen indicator.
Cytochromes P450 (P450 or CYP) belong to a superfamily of multifunctional monooxygenases that contain heme molecules (i.e., Fe-porphyrin) as a prosthetic group. They can catalyze various oxidative metabolic reactions of endogenous and exogenous compounds in living organisms. Their catalytic diversity and vast substrate range with regio- and stereo-specificity have high potential in applications to drug metabolism as well as in the fine chemical synthesis of steroids, lipids, vitamins, and natural products. Here, we have designed a novel visible light-driven platform for cofactor-free, whole-cell P450 photo-biocatalysis using eosin Y (EY) as a photosensitizer. EY can easily enter into the cytoplasm of Escherichia coli and bind specifically to the heme domain of P450. The catalytic turnover of P450 was mediated through the direct transfer of photo-induced electrons from the photosensitized EY to the P450 heme domain under visible light illumination. The photoactivation of the P450 catalytic cycle in the absence of cofactors and redox partners is successfully conducted using many bacterial P450s (variants of P450 BM3) and human P450s (CYPs 1A1, 1A2, 1B1, 2A6, 2E1, and 3A4) for the bioconversion of different substrates, including marketed drugs (simvastatin, lovastatin, and omeprazole) and a steroid (17beta-estradiol), to demonstrate general applicability of the light-driven, cofactor-free system.
The abnormal aggregation of beta-amyloid (Abeta) peptides in the brain is a major pathological hallmark of Alzheimers disease (AD). The suppression (or alteration) of Abeta aggregation is considered to be an attractive therapeutic intervention for treating AD. We report on visible light-induced inhibition of Abeta aggregation by xanthene dyes, which are widely used as biomolecule tracers and imaging markers for live cells. Among many xanthene dyes, rose bengal (RB) under green LED illumination exhibited a much stronger inhibition effect upon photo-excitation on Abeta aggregation than RB under dark conditions. We found that RB possesses high binding affinity to Abeta; it exhibits a remarkable red shift and a strong enhancement of fluorescence emission in the presence of Abeta. Photo-excited RB interfered with an early step in the pathway of Abeta self-assembly and suppressed the conformational transition of Abeta monomers into beta-sheet-rich structures. Photo-excited RB is not only effective in the inhibition of Abeta aggregation, but also in the reduction of Abeta-induced cytotoxicity.
Cytochrome P450 monooxygenases that catalyze a remarkable variety of oxidative transformation are of exceptional interest for the synthesis of fine chemicals. However, due to their instability and the requirment of expensive cofactors, P450s have not been used extensively for industry yet. Here, we developed new platform of P450-catalyzed reaction toward preparative scale process by the immobilization of P450s on polyhydroxybutyrate granules. Using the fusion with phasin, P450s could be efficiently immobilized on P(3HB) granules in the cytoplasm of Escherichia coli, and the complex was simply purified by centrifugation after cell disruption. Under various harsh environmental conditions (pH, temperature, urea, and ionic strength), the immobilized P450s exhibited much higher stability and activity compared to those of non-immobilized P450s.
The use of biologically occurring redox centers holds a great potential in designing sustainable energy storage systems. Yet, to become practically feasible, it is critical to explore optimization strategies of biological redox compounds, along with in-depth studies regarding their underlying energy storage mechanisms. Here, we report a molecular simplification strategy to tailor the redox unit of pteridine derivatives, which are essential components of ubiquitous electron transfer proteins in nature. We first apply pteridine systems of alloxazinic structure in lithium/sodium rechargeable batteries, and unveil their reversible tautomerism during energy storage. Through the molecular tailoring, the pteridine electrodes can show outstanding performance, delivering 533 Wh kg-1 within 1 hour and 348 Wh kg-1 within 1 minute, as well as high cyclability retaining 96% of the initial capacity after 500 cycles at 10 A g-1. Our strategy combined with experimental and theoretical studies suggests guidance for the rational design of organic redox centers.
In this report, photoelectroenzymatic synthesis of chemical compounds employing platinum nanoparticle-decorated silicon nanowire (Pt-SiNW) is presented. Pt-SiNW was proved to be an efficient material for photoelectrochemical cofactor regeneration because silicon nanowire absorbs a wide range of solar spectrum and platinum nanoparticle serves as an excellent catalyst for electron and proton transfer. By integrating the platform with redox enzymatic reaction, visible light-driven electroenzymatic synthesis of L-glutamate was achieved. Compared to electrochemical and photochemical methods, this approach is free from side reactions caused by sacrificial electron donor and has advantages of applying low potential to realize energy-efficient and sustainable synthesis of chemicals by photoelectroenzymatic system.
Efficient harvesting of unlimited solar energy and its conversion into valuable chemicals is one of the ultimate goals of scientists. With the ever-increasing concerns about sustainable growth and environmental issues, numerous efforts have been made to develop artificial photosynthetic process for the production of fuels and fine chemicals mimicking natural photosynthesis. Despite the research progresses made over the decades, the technology is still in its infancy because of the difficulties in kinetic coupling of whole photocatalytic cycles. Here, we report a new type of artificial photosynthesis system that can avoid such problems by integrally coupling biocatalytic redox reactions with photocatalytic water-splitting. We found that photocatalytic water-splitting reaction can be efficiently coupled with biocatalytic redox reactions by using tetra-cobalt polyoxometalate and Rh-based organometallic compound as hole and electron scavengers, respectively, for photoexcited Ru(bpy)32+ dye. Based on these results, we could successfully photosynthesize a model chiral compound (L-glutamate) using a model redox enzyme (glutamate dehydrogenase) upon in-situ photo-regeneration of cofactors.
We report on the capability of polydopamine (PDA), a mimic of mussel adhesion proteins, as an electron gate as well as a versatile adhesive for mimicking natural photosynthesis. This work demonstrates that PDA accelerates the rate of photoinduced electron transfer from light-harvesting molecules through two-electron and two-proton redox-coupling mechanism. The introduction of PDA as a charge separator significantly increased the efficiency of photochemical water oxidation. Furthermore, simple incorporation of PDA ad-layer on the surface of conducting materials (such as carbon nanotubes) facilitated fast charge separation and oxygen evolution through the synergistic effect of PDA-mediated, proton-coupled electron transfer and substrate materials high conductivity. Our work shows that PDA is an excellent electron acceptor as well as a versatile adhesive; thus, it opens a new electron gate for harvesting photoinduced electrons and designing artificial photosynthetic systems.
Solar energy has attracted much attention because of the huge amount of energy continuously transferred from the sun to the Earth. While numerous photosensitizing systems had been studied over the decades for light harvesting, most photosensitizers possess a large bandgap (> 1.7 eV) requiring ultraviolet and visible light for their activation. Considering that over 46% of solar energy is in the near-infrared (NIR) range, almost half of overall solar spectrum cannot be utilized to activate those photosensitizers. Herein, we first report on NIR light-driven biocatalytic artificial photosynthesis using upconversion nanoparticles. Upconversion refers to nonlinear optical processes that occur through anti-Stokes emission, in which an emitted photon has more energy than the absorbed photon by sequential absorption of photons. For NIR light-driven photoenzymatic synthesis, we synthesized silica-coated upconversion nanoparticles, such as Si-NaYF4:Yb,Er and Si-NaYF4:Yb,Tm, for efficient photon-conversion through Forster resonance energy transfer (FRET) with rose bengal (RB), a photosensitizer. We observed NIR-induced electron transfer using linear sweep voltammetric analysis, which indicated photoexcited electrons of RB/Si-NaYF4:Yb,Er were transferred to NAD+ through a Rh-based electron mediator. RB/Si-NaYF4:Yb,Er nanoparticles, which exhibited higher FRET efficiency due to more spectral overlap than RB/Si-NaYF4:Yb, resulted in much better performance for photoenzymatic conversion. Our work shows that upconversion nanoparticles with anti-Stokes emission are promising light harvesters for versatile usage of NIR light in solar-to-chemical conversion processes.
We present a simple and versatile approach for the construction of plasmonic metal/ photosensitizer core-shell nanohybrids for efficient light harvesting by adopting multi-purpose polydopamine (PDA) nanolayers inspired by mussel adhesion. In our plasmonic core-shell assembly, PDA coating plays multiple roles: (1) a reducing agent for the synthesis of metal nanoparticles, (2) a scaffold for the encapsulation of photosensitizing dye molecules, and (3) an adhesive layer between the nanohybrid and the substrate. In contrast to nanolithography processes, the entire synthetic procedure can be handled in an aqueous solution under mild conditions and requires no intricate equipment, which confers advantages in large-scale production. Also, by virtue of the remarkable adhesive versatility of PDA coating, this approach can be applied to the development of elaborate core-shell nanostructures regardless of material type and morphology of substrates. We found that the resulting plasmonic nanohybrids exhibit strongly enhanced photocatalytic activity during visible light-induced artificial photosynthesis as a result of amplified light absorption by molecular photosensitizers through LSPR from the plasmonic metal nanoparticles. We expect that a diverse range of metal core (e.g., gold and silver) and dye molecule combinations are possible through the use of our strategy to facilitate the synthesis of assorted sets of nanohybrids with desired optical properties, allowing design flexibility in solar energy conversion applications.
Bio-inspired organic electrodes that imitate natural energy metabolisms, such as respiration and photosynthesis, can facilitate the design of sustainable batteries. For example, the electro-active carbonyl compounds mimicking biological quinone cofactors that can be obtained from biomass through eco-friendly processes are intriguing candidates for such electrode materials. Also, flavin-based electrodes that function through the imitation of the cellular energy transduction mechanism are promising candidates. The practical use of organic-based electrodes, however, suffers from sluggish kinetics and poor capacity retention, which originate from low electronic conductivity and dissolution of electroactive compounds into electrolytes. Here, we report a novel and facile design strategy for organic electrodes to achieve high energy and power densities combined with excellent cyclic stability. Non-covalent nanohybridization of electroactive aromatic molecules with single-walled carbon nanotubes (SWNTs) by exploiting pi-pi interactions leads to a rearrangement of electroactive molecules from bulk crystalline particles into molecular layers on conductive scaffolds. The nanohybrid electrode in the form of a flexible, free-standing paper (free of binder/additive and current collector) results in ultrafast kinetics delivering 510 Wh/kg within 30 minutes (204 mAh/g ~ 98% of theoretical capacity) and 272 Wh/kg of energy even within 46 seconds. Moreover, the stable anchorage of electroactive molecules on SWNTs enables above 99% capacity retention upon 100 cycles, which was hardly achieved for organic electrodes. Our approach can be extended to other aromatic organic electrode systems, bringing organic redox chemicals a step closer to practical cathodes in rechargeable batteries.
Solar-driven water oxidation is an essential way to provide electrons for artificial photosynthesis. IrO2 colloids have been used as one of highly-efficient water oxidation catalysts due to their distinguished catalytic activity for water oxidation. However, IrO2 nanoparticles (NPs), which posses a hydrous nature, often suffer from corrosive surface degradation and enter into unstable Ir oxidation states, thereby limiting their cycling characteristics. In this work, we propose a new water oxidation catalyst for enhanced oxygen evolution and long-term recyclability through the functionalization of highly crystalline IrO2 NPs on semiconducting TiO2 nanofibers (NFs). The effects of IrO2 NPs immobilized on TiO2 NFs were investigated in terms of decoration position (inner and outer layers of NFs), crystallite size (10 nm and 30 nm), and loading amount (0 - 5.17 wt %). IrO2 (10 nm)-decorated TiO2 NFs exhibited a high turnover number (TON: 322) and superior recyclability for repeated water oxidation (90% O2 evolving capability after 10 cycles). X-ray photoelectron spectroscopy analysis verified that TiO2 NFs anchored to discrete IrO2 NPs can maintain the oxidation state of IrO2 by self-reduction of TiO2 scaffold. Our synthetic strategy offer a promising route for fabricating efficient and robust catalyst via immobilization of crystalline water oxidation catalysts on semiconducting metal-oxide scaffold.
We introduce shell cross-linked nanocapsules as an efficient tumor-targeted systemic delivery nanocarrier for highly luminescent, heavy-metal-free Cu0.3InS2/ZnS (CIS/ZnS) core-shell quantum dots (QDs). The CIS/ZnS QDs are synthesized by using a hot injection method with copper iodide, indium acetate, zinc stearate, and dodecanethiol. A mixture of the prepared QDs and amine-reactive six-armed poly(ethylene glycol) (PEG) in dichloromethane was emulsified into an aqueous solution containing human serum albumin (HSA). The resulting shell cross-linked nanocapsules show excellent dispersion stability in a serum-containing medium and high luminescence comparable to QDs in a non-polar organic solvent. Folic acid is introduced as a tumor-targeting ligand. In vivo tumor targeted delivery is demonstrated by measuring the fluorescence intensity of several major organs and tumor tissue after an intravenous tail vein injection of the nanocapsules into nude mice. The cytotoxicity of the QD-loaded HSA-PEG nanocapsules is also examined in several 32 types of cells. Our results show that the cellular uptake of the QDs is critical for cytotoxicity. Moreover, a significantly lower level of cell death is observed in the CIS/ZnS QDs compared to nanocapsules loaded with cadmium-based QDs. This study suggests that the systemic tumor targeting of heavy metal-free QDs using shell cross-linked HSA-PEG hybrid nanocapsules is a promising route for in vivo tumor diagnosis with reduced non-specific toxicity
Natural photosynthesis, a solar-to-chemical energy conversion process, occurs through a series of photo-induced electron transfer reactions in nanoscale architectures that contain light-harvesting complexes, protein-metal clusters, and many redox biocatalysts. Artificial photosynthesis in nanobiocatalytic assemblies aims to reconstruct man-made photosensitizers, electron mediators, electron donors, and redox enzymes for solar synthesis of valuable chemicals through visible light-driven cofactor regeneration. The key requirement in the design of biocatalyzed artificial photosynthetic process is an efficient and forward electron transfer between each photosynthetic component. This review introduces recent research outcomes in the development of nanobiocatalytic assemblies that can mimic natural photosystems I and II, respectively. Current issues in biocatalytic artificial photosynthesis and future perspectives are discussed.
The self-assembly of peptide-based building blocks is an attractive route for fabricating functional materials due to their unique features, such as functional flexibility and molecular recognition as well as environmental compatibility. Here, we report on the development of artificial light-harvesting hydrogel generated by the self-assembly of Fmoc-FF peptides and metalloporphyrins. We utilized the self-assembled peptide nanostructure of Fmoc-FF as a template to assemble metalloporphyrins into efficient light- harvesting antenna. The metalloporphyrins were placed in close enough proximity to each other to enable excited energy transfer, increasing the photosensitization efficiency, as observed in natural light-harvesting complexes in green plants. The metalloporphyrins in the light-harvesting hydrogel increased the efficiency of photocatalytic water oxidation by iridium oxide nanoparticles up to about 3.7 times compared to their physical mixture. The peptide-based platform could further extend possible sets of functional molecules simply by adding or modifying amino acids in the motif peptides. Scientific insights into the effects of nanoscale-assembled structures of photosensitizers on excited energy transfer can broaden the potential application of biomimetic approaches for light-driven energy systems and photosensitive sensor devices.
Artificial photosynthesis is an attractive way to utilize solar energy through inspiration from natural photosynthesis in green plants. Water-splitting is critically required to establish an artificial photosynthetic system that consists of sequential charge-obtaining and transferring reactions. The oxidation of water is a limiting step to achieving water-splitting because of its multi-hole-related characteristics. A key to the development of effective water oxidation catalysts is the optimized control of material structure and composition through a facile synthetic method. This work synthesized polycrystalline RuO2/Co3O4 core/shell nanofibers by electrospinning and evaluated their photocatalytic water oxidation performance using a Ru(bpy)32+/persulfate system under visible light illumination. Our results show that RuO2/Co3O4 nanofibers exhibit significantly enhanced efficiency of photocatalytic water oxidation with a higher number of turnover frequency than those of pristine Co3O4 nanoparticles, Co3O4 nanofibers, and RuO2 nanofibers, respectively. The unique core-shell structure of RuO2/Co3O4 nanofibers comprising the inner region of highly conductive RuO2 and the outer region of catalytic Co3O4 provided a fast and effective transport highway for holes to O2-evolving sites. This work highlights the potential of tailored 1D binary composite nanofibers for the development of efficient oxygen-evolving catalysts and offers a new viewpoint for exploring multi-component catalysts through electrospinning.
Cellular metabolism comprises energy transduction machineries that operate by a series of redox-active components to store energies from nutrients, which are transduced into high-energy intermediates for cellular works such as chemical synthesis, transport, and movement. Biological energy transduction mechanism hints at the construction of a man-made energy storage system. Herein, we present a bio-inspired strategy to design high-performance energy devices based on the analogy between energy storage phenomena of mitochondria and lithium rechargeable batteries. Flavins, a key redox element in respiration and photosynthesis, facilitate either one- or two-electron-transfer redox processes accompanying proton transfer at nitrogen atoms of diazabutadiene motif during cellular metabolism. We have successfully demonstrated flavins as a molecularly tunable cathode material that exhibits reversible reactivity with two lithium ions and electrons per formula unit. Analysis of both the ex situ characterizations and density-functional theory (DFT)-based calculations revealed that the redox reaction occurs via two successive single-electron transfer steps, which is analogous to the proton-coupled electron transfer mechanism of flavoenzymes. Tailored flavin analogues obtained via chemical substitution on the isoalloxazine ring showed fine tunability of electrochemical properties, exhibiting a gravimetric capacity of 174 mAh/g and an average redox potential of 2.65 V, and its expected energy density is comparable to that of LiFePO4.
The control of cell-material interaction is a key issue in the design of suitable scaffold materials for tissue engineering because the physicochemical properties (e.g., surface chemistry, topography) of substrate materials significantly influence cell behaviors. We studied the effect of mussel-inspired polydopamine (PDA) functionalization of the substrate surface in combination with topographical cues on the behavior of skeletal myoblasts. The formation of the PDA ad-layer on the scaffold surface was analyzed using multiple tools including atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. When myoblasts were grown on planar glass substrates, the PDA ad-layer well-supported the adhesion and proliferation of myoblasts, and enhanced the differentiation of myoblasts into multinucleate myotubes. We further developed well-aligned nanofibrous scaffolds to resemble the highly ordered architectures of skeletal muscle tissues, followed by PDA-based surface functionalization. On PDA-modified nanofibers, myogenic protein expression and the fusion of myoblasts were increased significantly compared with those on unmodified nanofibers. The multinucleate myotubes on the aligned nanofibers were oriented in a direction parallel to the nanofibers. Our results suggest that the combination of mussel-inspired surface functionalization and uniaxial topography is a useful strategy for scaffold design in skeletal tissue engineering.
Solar energy utilization is accomplished in green plants through a cascade of photo-induced electron transfer, which remains a target model for realizing artificial photosynthesis. In this article, we introduce the concept of about how to design biocatalyzed artificial photosynthesis through coupling redox biocatalysis and photocatalysis to mimic natural photosynthesis. Key design principles for reaction components, such as electron donors, photosensitizers, and electron mediators, are described for artificial photosynthesis involving biocatalytic assemblies. Recent research outcomes that serve as a proof of the concept are summarized and current issues are discussed to provide a future perspective.
We report the synthesis of a 3D-structured graphene/Rh-complex hydrogel that works as a robust catalyst for electroenzymatic reactions. Pyridine nucleotide cofactors [NAD(P)H] are critically required as a reducing power for many reactions catalyzed by redox enzymes. Thus, in-situ regeneration of reduced cofactors is essential to ensuring redox enzymes continue their turnover. We successfully designed the graphene/Rh-complex hydrogel by immobilizing Rh complex, an organometallic mediator, in the network of graphene hydrogel having large surface area and high conductivity. The pi-electron system in the aromatic heterocyclic region of Rh complex played a critical role in the immobilization and stabilization of Rh complex in the graphene hydrogel for electrochemical NADH regeneration. The catalytic activity of graphene/Rh-complex that has phenanthroline as a ligand remained almost the same through repeated tests. When a-ketoglutarate was electroenzymatically converted to L-glutamate in the presence of graphene/Rh-complex hydrogel, L-glutamate yield increased more than 10 times than that of free Rh complex. This work demonstrates that graphene hydrogel can boost industrially important reactions catalyzed by redox enzymes.
Graphene-based nanomaterials have received much attention in biomedical applications for drug/gene delivery, cancer therapy, imaging, and tissue engineering. Despite the capacity of 2D carbon materials as a nontoxic and implantable platform, their effect on myogenic differentiation has been rarely studied. We investigated the myotube formation on graphene-based nanomaterials, particularly graphene oxide (GO) and reduced graphene oxide (rGO). GO sheets were immobilized on amine-modified glass to prepare GO-modified glass, which was further reduced by hydrazine treatment for the synthesis of rGO-modified substrate. We studied the behavior, including adhesion, proliferation, and differentiation, of mouse myoblast C2C12 on unmodified, GO-, and rGO-modified glass substrates. According to our analyses of myogenic protein expression, multi-nucleated myotube formation, and expression of differentiation-specific genes (MyoD, Myogenin, Troponin T, and MHC), myogenic differentiation was remarkably enhanced on GO, which resulted from serum protein adsorption and nanotopographical cues. Our results demonstrate the ability of GO to stimulate myogenic differentiation, showing a potential for skeletal tissue engineering applications.
Ceria attracted much attention due to its unique redox properties and high reactivity, which has been widely applied for solid oxide fuel cells, catalysis, and sensors. However, the use of ceria as a photocatalyst is limited due to its large optical bandgap (3.19 eV). In this study, we successfully synthesized ceria sheets that exhibited distinct polycrystalline sheet-like structure with grains, the size of which varied with calcination temperatures. The grain size of ceria sheets influenced the concentration of cerium ions on their surface, thus affecting their bandgap. The nano-grained ceria sheets exhibited a red-shift in the UV-visible absorption spectrum and a much narrower bandgap (2.71-2.83 eV). Visible light-responsive photocatalytic activity was observed with nano-grained ceria sheets at a rate constant that was much higher than that of ceria nanoparticles.
Carbon-based nanomaterials such as graphene sheets and carbon nanotubes possess unique mechanical, electrical, and optical properties that present new opportunities for tissue engineering, a key field for the development of biological alternatives that repair or replace whole or a portion of tissue. Carbon nanomaterials can also provide a similar micro-environment as like a biological extracellular matrix in terms of chemical composition and physical structure, making them a potential candidate for the development of artificial scaffolds. In this review, we summarize recent research advances in the effects of carbon nanomaterial-based substrates on cellular behaviors, including cell adhesion, proliferation, and differentiation into osteo- or neural- lineages. The development of 3D scaffolds based on carbon nanomaterials (or their composites with polymers and inorganic components) is introduced, and the potential of these constructs in tissue engineering, including toxicity issues, is discussed. Future perspectives and emerging challenges are also highlighted.
Cytochrome P450 monooxygenases are multi-functional biocatalyst with potential applications in chemoenzymatic synthesis of complex chemicals as well as in studies of metabolism and xenobiotics. Widespread application of cytochrome P450s, however, is encumbered by the critical need for redox equivalents in their catalytic function. To overcome this limitation, we studied visible light-driven regeneration of NADPH for P450-catalyzed O-dealkylation reaction; we used eosin Y as a photosensitizing dye, triethanolamine as an electron donor, and Cp*Rh(bpy)H2O as an electron mediator. We analyzed catalytic activity of cell-free synthesized P450 BM3 monooxygenase variant (Y51F/F87A, BM3m2) in the presence of key components for NADPH photoregeneration. The P450-catalyzed O-dealkylation reaction sustainably maintained its turnover with the continuous supply of photoregenerated NADPH. Visible light- driven, non-enzymatic NADPH regeneration provides a new route for efficient, sustainable utilization of P450 monooxygenases.
Biomineralization, the natural pathway of assembling biogenic inorganic compounds, inspires us to exploit unique, effective strategies to fabricate functional materials with intricate structures. In this article, we review the recent advances in bio-inspired synthesis of minerals, mainly those of calcium-based minerals, and its applications to the design of functional materials for energy, environment, and biomedical fields. Biomimetic mineralization is extending its application range to unconventional area such as the design of component materials for lithium-ion batteries and elaborately structured composite materials utilizing carbon dioxide gas. Materials with highly enhanced mechanical properties are synthesized through emulating the nacre structure. Studies of bioactive minerals-carbon hybrid materials show an expansion of potential applications to fields ranging from interdisciplinary science to practical engineering such as the fabrication of reinforced bone-implantable materials.
Silicon nanowires have been widely used in many nanoscale devices, including solar cells, photoelectro- chemical cells, transistors, and battery electrodes. Herein, we report a new possible application of hydrogen- terminated silicon nanowires (H-SiNWs) as a rechargeable template for hydride transfer in redox biocatalysis. Redox enzymes can catalyze various types of complex organic synthesis under mild conditions but often require a stoichiometric amount of expensive nicotinamide cofactors (NADH) for their catalytic activities. We found that H-SiNWs transfer hydride efficiently to regenerate NADH from NAD+ via an Rh-based electron mediator. During the regeneration of NADH, the Si-Hx bonds on H-SiNWs were oxidized to form Si-OH and Si-O-Si bonds on the nanowire surface and evolve hydrogen. The oxidized H-SiNWs were readily recharged by treatment in a diluted HF solution for the repeated generation of NADH and continuous enzymatic reactions for the synthesis of D-lactate from pyruvate catalyzed by lactate dehydrogenase.
This study successfully demonstrates that hydrogen-terminated silicon nanowires (H-SiNWs) are an ideal artificial photosynthetic material, which possesses suitable photocatalytic properties to regenerate reducing power (i.e., NADH) and synthesize chemicals by photoenzymatic reaction. H-SiNWs, fabricated by a metal-assisted chemical etching process, possessed an enlarged band gap from the effect of quantum confinement and enabled a cascading electron transfer from electron donor to NAD via an Rh-based electron mediator. Approximately 80% of NADH was photo-regenerated from NAD by H-SiNWs within 2 hrs of light irradiation (wavelength > 420 nm), which was successfully coupled with the photoenzymatic synthesis of L-glutamate. Our work suggests that H-SiNWs are an ideal artificial photosynthetic material, which possesses suitable photocatalytic properties to regenerate NADH and synthesize chemicals by photoenzymatic reaction.
CuO possesses high theoretical capacity and safety with low cost and limited environmental toxicity, but a large volumetric change of CuO electrodes during the insertion and extraction of lithium ions can destroy its crystal structure and cause capacity decay in a short time. According to the present work, graphene-wrapped CuO hybrid material can highly enhance the stability and recyclability of CuO anode for lithium ion batteries. We successfully synthesized nanostructured graphene/CuO by converting a carbon dioxide-mineralized graphene oxide/calcium carbonate precursor to Cu-based minerals. Graphene/CuO exhibited nanoribbon-like CuO aggregates well-hybridized with graphene nanosheets. The excellent electrochemical performance of graphene/CuO is attributed to the synergic effect of CuO wrapped by highly conductive graphene sheets and graphene itself capable of Li-ion storage. Furthermore, flexible graphene sheets hybridized with CuO were beneficial for reducing the strain caused by volume changes during the charge/discharge process to show good cyclic performance. The synthesis of graphene/CuO and its application to lithium ion battery electrodes suggest a new possibility for hybridizing graphene and metal oxide nanoparticles using the inspiration of natural mineralization.
A graphene oxide (GO)-based immunosensor is developed for the detection of interleukin-5 (IL-5), a key cytokine associated with asthma pathology and eosinophilia. The immunosensing platform utilizes innate fluorescence of GO, not demanding biomolecules labeled with fluorescent dyes. For the construction of GO-based immunosensors, anti-IL-5 antibodies were immobilized on GO surface, then IL-5 and horseradish peroxidase (HRP)-linked antibody conjugates were consequently introduced to form a sandwich immune-complex on GO, which was investigated by using multiple analytical tools such as UV/Vis absorption, fluorescence, Raman spectroscopies, and atomic force microscopy. We found that HRP-catalyzed polymerization of 3,3-diaminobenzidine directly quenched the fluorescence of GO. The degree of GO fluorescence quenching was closely correlated to the concentration of IL-5 with a detection limit of approximately 4 pg/ml. The GO-based immunoassay system exhibited high specificity for IL-5 among other cytokines and was not affected by non-specific proteins in human serum.
The interactions between cells and materials play critical roles in the success of new scaffolds for tissue engineering, since chemical and physical properties of biomaterials regulate cell adhesion, proliferation, migration, and differentiation. We have developed nanofibrous substrates that possess both topographical cues and electroactivity. The nanofiber scaffolds were fabricated through the electrospinning of polycaprolactone (PCL, a biodegradable polymer) and polyaniline (PANi, a conducting polymer) blends. We investigated the ways in which those properties influenced myoblast behaviors. Neither nanofiber alignment nor PANi concentration influenced cell growth and proliferation, but cell morphology changed significantly from multipolar to bipolar with the anisotropy of nanofibers. According to our analyses of myosin heavy chain expression, multinucleate myotube formation, and the expression of differentiation-specific genes (myogenin, troponin T, MHC), the differentiation of myoblasts on PCL/PANi nanofibers was strongly dependent on both nanofiber alignment and PANi concentration. Our results suggest that topographical cues and the electroactivity of nanofibers synergistically stimulate muscle cell differentiation to make PCL/PANi nanofibers a suitable scaffold material for skeletal tissue engineering.
For the past decades, biomaterials have been extensively studied mostly for medical applications, such as new pharmaceuticals, tissue engineering, and artificial organs, due to their excellent biocompatibilities. Nowadays, biomaterials further expand their boundaries to various functionalities in sensor, display, and energy devices. With the move towards the use of greener materials to power vehicles, environmentally-benign synthesis of energy materials is becoming an important aspect. Here, energy storage capability of Cu-based biomineral, copper oxychloride, from the jaws of Glycera dibranchiate, a marine bloodworm, is demonstrated. Copper oxychloride electrode delivered approximately 500 mAh/g with a reasonably good cycling through the conversion reaction. This study demonstrates that inorganic biominerals, which are in-vivo synthesizable, can be utilized as energy storage materials, and furthermore, suggests the applicability of sustainable production of energy devices from bio-factory. While we have examined the Cu-based biomineral in this study, there are various natural biominerals containing other transition metal ions, such as Fe and Mn, which can serve as excellent redox elements. Therefore, significant unexplored opportunities exist in natural biominerals with different electrochemical properties.
We describe on the successful coupling of photochemical NADH regeneration with redox enzymatic synthesis by using proflavine as a light-harvesting molecule. Proflavine, a promising photosensitizer, exhibited a high capacity to drive the reduction of NAD into NADH in the presence of a Rh-based electron mediator, and the photoregenerated NADH was enzymatically active to be oxidized by NADH-dependent L-glutamate dehydrogenase for the synthesis of L-glutamate. Both the wavelength and intensity of incident light were found to significantly affect the efficiency of photochemical NADH regeneration. In contrast to proflavine, flavin derivatives, such as FAD, FMN, lumichrome, and riboflavin, accelerated solely the rate of NADH oxidation, not that of NAD reduction. Our results indicate that proflavine has the potential to become an efficient light harvesting component in biocatalytic photosynthesis driven by solar energy.
We first report on chemiluminescence resonance energy transfer (CRET) between graphene nanosheets and chemiluminescent donors. In contrast to fluorescence resonance energy transfer, CRET occurs via non-radiative dipole-dipole transfer of energy from a chemiluminescent donor to a suitable acceptor molecule without an external excitation source. We designed a graphene-based CRET platform for homogenous immunoassay of C-reactive protein, a key marker for human inflammation and cardiovascular diseases, using a luminol/hydrogen peroxide chemiluminescence (CL) reaction catalysed by horseradish peroxidase. According to our results, anti-CRP antibody conjugated to graphene nanosheets enabled the capture of CRP at the concentration above 1.6 ng/mL. In the CRET platform, graphene played a key role as an energy acceptor, which was more efficiently than graphene oxide, while luminol served as a donor to graphene, triggering the CRET phenomenon between luminol and graphene. The graphene-based CRET platform was successfully applied to the detection of CRP in human serum samples in the range observed during acute inflammatory stress.
Enzymes have long been successfully employed as biocatalysts in organic synthesis because they possess high specificity and catalytic activity even under mild conditions. However, the applications of redox enzymes were limited, mainly because of their strict requirement of reduced nicotinamide coenzyme (i.e., NADH). For the first time, employment of NAD analogs has overcome the limitations of NAD through photochemical regeneration. We investigated four different NAD analogs (i.e., APAD, PAAD, TNAD, and NAAD) that possess substituted functional groups in their pyridine part and exhibit different spectral and redox properties from NAD. We found that APAD and PAAD were photochemically reduced more efficiently than NAD, while their reduced products showed coenzyme activity comparable to natural NAD. In contrast, TNAD formed a complex with photosensitizer, and NAAD possessed more negative reduction peak potential and a negative charge, making both TNAD and NAAD poorer than NAD in photoregeneration. The higher reduction efficiency of APAD significantly enhanced the yield of redox reaction coupled with in situ visible light-driven coenzyme regeneration. Our work shows that NAD analogs can be excellent coenzymes to be regenerated via photochemical regeneration method and to be applied to redox enzymatic reactions.
Titanium dioxide, an oxide semiconductor, is regarded as a suitable material for various photocatalytic applications because of its strong oxidizing power, high chemical inertness, low cost, and long-term stability. However, a large band gap (3.2 eV) of anatase titanium dioxide restricts its use only to the narrow light-response range of ultraviolet (only 3~5% of total sunlight). We report on the synthesis of novel graphene-wrapped anatase titanium dioxide nanoparticles (NPs) that highly enhance the photocatalytic activity of titanium dioxide under visible light irradiation. We have prepared graphene-anatase titanium dioxide hybrid NPs by wrapping amorphous titanium dioxide NPs with graphene oxide (GO), followed by a one-step GO reduction and titanium dioxidecrystallization via hydrothermal treatment. Graphene-titanium dioxide NPs exhibited a red shift of the band edge and a significant reduction of the band gap (2.80 eV). We found that graphene-titanium dioxide NPs possess excellent photocatalytic property under visible light for the degradation of methylene blue with a rate constant of 0.0341/min, which was much higher than those of other titanium dioxide- based photocatalytic materials. The strategy presented in this study will enable a ready integration of functional semiconductor NPs and graphene nanosheets for the synthesis of highly photoactive graphene-based metal oxide hybrid materials.
Self-assembled light-harvesting peptide nanotubes are synthesized for artificial photosynthesis. Light-harvesting by natural photosynthesis occurs by means of two large protein complexes called photosystem I and II, which are composed of light-harvesting antenna (i.e., chlorophyll a and b) and catalytic metal clusters embedded within proteins. We have succeeded in the development of light-harvesting peptide nanotubes that integrate photosynthetic units, thus mimicking natural photosynthesis. Light-harvesting peptide nanotubes were synthesized by the self-assembly of diphenylalanine (Phe-Phe, FF) and porphyrin. We found that the J-aggregation of porphyrin occurs during the self-assembly of the FF nanotubes via electrostatic attraction and hydrogen bonding. The light-harvesting peptide nanotubes were suitable for mimicking photosynthesis because of their structure and electrochemical properties similar to natural photosystem. We demonstrated that the integrated photocatalytic system is effective for visible light-driven NADH regeneration coupled with redox enzymatic synthesis of fine chemicals such as L-glutamate.