Visible-light photocatalytic regeneration of NADH using P-doped TiO2 nanoparticles

Redox Biocatalysis: Quantitative Comparisons of Nicotinamide Cofactor Regeneration Methods

Enzymatic processes, particularly those capable of performing redox reactions, have recently been of growing research interest. Substrate specificity, optimal activity at mild temperatures, high selectivity, and yield are among the desirable characteristics of these oxidoreductase catalyzed reactions. Nicotinamide adenine dinucleotide (phosphate) or NAD(P)H-dependent oxidoreductases have been extensively studied for their potential applications like biosynthesis of chiral organic compounds, construction of biosensors, and pollutant degradation. One of the main challenges associated with making these processes commercially viable is the regeneration of the expensive cofactors required by the enzymes. Numerous efforts have pursued enzymatic regeneration of NAD(P)H by coupling a substrate reduction with a complementary enzyme catalyzed oxidation of a co-substrate. While offering excellent selectivity and high total turnover numbers, such processes involve complicated downstream product separation of a primary product from the coproducts and impurities. Alternative methods comprising chemical, electrochemical, and photochemical regeneration have been developed with the goal of enhanced efficiency and operational simplicity compared to enzymatic regeneration. Despite the goal, however, the literature rarely offers a meaningful comparison of the total turnover numbers for various regeneration methodologies. This comprehensive Review systematically discusses various methods of NAD(P)H cofactor regeneration and quantitatively compares performance across the numerous methods. Further, fundamental barriers to enhanced cofactor regeneration in the various methods are identified, and future opportunities are highlighted for improving the efficiency and sustainability of commercially viable oxidoreductase processes for practical implementation.

Improving the Enzymatic Cascade of Reactions for the Reduction of CO2 to CH3OH in Water: From Enzymes Immobilization Strategies to Cofactor Regeneration and Cofactor Suppression

The need to decrease the concentration of CO2 in the atmosphere has led to the search for strategies to reuse such molecule as a building block for chemicals and materials or a source of carbon for fuels. The enzymatic cascade of reactions that produce the reduction of CO2 to methanol seems to be a very attractive way of reusing CO2; however, it is still far away from a potential industrial application. In this review, a summary was made of all the advances that have been made in research on such a process, particularly on two salient points: enzyme immobilization and cofactor regeneration. A brief overview of the process is initially given, with a focus on the enzymes and the cofactor, followed by a discussion of all the advances that have been made in research, on the two salient points reported above. In particular, the enzymatic regeneration of NADH is compared to the chemical, electrochemical, and photochemical conversion of NAD+ into NADH. The enzymatic regeneration, while being the most used, has several drawbacks in the cost and life of enzymes that suggest attempting alternative solutions. The reduction in the amount of NADH used (by converting CO2 electrochemically into formate) or even the substitution of NADH with less expensive mimetic molecules is discussed in the text. Such an approach is part of the attempt made to take stock of the situation and identify the points on which work still needs to be conducted to reach an exploitation level of the entire process.

Efficient Nicotinamide Adenine Dinucleotide Regeneration with a Rhodium-Carbene Catalyst and Isolation of a Hydride Intermediate

Regeneration of nicotinamide adenine dinucleotide (NADH) has been the primary interest in the field of enzymatic transformation, especially associating oxidoreductases given the stoichiometric consumption. The synthesized carbene-ligated rhodium complex [(η⁵-Cp*)Rh(MDI)Cl]⁺ [Cp* = pentamethylcyclopentadienyl; MDI = 1,1'-methylenebis(3,3'-dimethylimidazolium)] acts as an exceptional catalyst in the reduction of NAD⁺ to NADH with a turnover frequency of 1730 h^-1, which is over twice that of the higher catalytic activity of the commercially available catalyst [Cp*Rh(bpy)Cl]⁺ (bpy = 2,2'-bipyridine). Offsetting the contentious atmosphere currently taking place over the specific intermediate of the NADH regeneration, this study presents pivotal evidence of a metal hydride intermediate with a bis(carbene) ligand: a stable form of the rhodium hydride intermediate, [(η⁵-Cp*)Rh(MDI)H]⁺, was isolated and fully characterized. This enables thorough insight into the possible mechanism and exact intermediate structure in the NAD⁺ reduction process.

Developing and Regenerating Cofactors for Sustainable Enzymatic CO2 Conversion

Enzymatic CO2 conversion offers a promising strategy for alleviating global warming and promoting renewable energy exploitation, while the high cost of cofactors is a bottleneck for large-scale applications. To address the challenge, cofactor regeneration is usually coupled with the enzymatic reaction. Meanwhile, artificial cofactors have been developed to further improve conversion efficiency and decrease cost. In this review, the methods, such as enzymatic, chemical, electrochemical, and photochemical catalysis, developed for cofactor regeneration, together with those developed artificial cofactors, were summarized and compared to offer a solution for large-scale enzymatic CO2 conversion in a sustainable way.

Preparation of phosphorus-doped tungsten trioxide nanomaterials and their photocatalytic performances

In this study, a highly efficient and stable phosphorus-doped Tungsten trioxide (P-WO₃) photocatalyst was successfully synthesized using a combination of hydrothermal and post-calcination method. The microstructures, morphologies and optical properties of the obtained WO₃ and P-WO₃ samples were characterized. The results showed that P was uniformly doped into the WO₃ lattice in a pentavalent-oxidation state (P⁵⁺). The charge carrier traps were also formed, which could accept the photoelectrons. Furthermore, the band gap energy was reduced from 2.4 to 2.33 ev. The photocatalytic performance of the obtained P-WO₃ samples with different P concentrations were then tested by photocatalytic degradation of methyl blue (MB). It was found that the 6%-P-WO₃ sample exhibited the highest photocatalytic activity, with 96% of MB being able to be degraded within 120 min, which was more than four times higher than that of the pure WO₃. The practicality of the prepared P-WO₃ was also evaluated using samples from two domestic wastewater treatment plants. The P-WO₃ had a high photodegradation performance in treating low concentration of organic matters from real wastewater. The photocatalysis of P-WO₃ could be mainly initiated by the production of hydroxyl radical (·OH) and photogenerated hole (h⁺).

Covalent Triazine Framework as an Efficient Photocatalyst for Regeneration of NAD(P)H and Selective Oxidation of Organic Sulfide

Covalent triazine frameworks (CTFs), belonging to the super-family of covalent organic frameworks, have attracted significant attention as a new type of photosensitizer due to the superb light harvesting ability and efficient charge transfer originating from the large surface area. However, the wide optical band gap in CTFs, which is larger than 3.0 eV, hinders the efficient light harvesting in the visible range. To overcome this limitation, we developed the new type CTFs photocatalyst based on the donor-acceptor conjugation scheme by using melamine (M) and 2,6-diaminoanthraquinone (AQ) as monomeric units. The melamine-2,6-diaminoanthraquinone based covalent triazine frameworks (M-AQ-CTFs) photocatalyst shows the excellent light harvesting capacity with high molar extinction coefficient, and the suitable optical band gap involving the internal charge transfer character. Combination of M-AQ-CTFs and artificial photosynthetic system including the organometallic rhodium complex, acting as an electron mediator, exhibited the excellent photocatalytic efficiency for the regeneration of the nicotinamide cofactors such as NAD(P)H. In addition, this photocatalyst showed the high photocatalytic efficiency for the metal-free aerobic oxidation of sulfide. This study demonstrates the high potential of CTFs photocatalyst with the donor-acceptor conjugated scheme can be actively used for the artificial photosynthesis.

Photocatalyst-enzyme hybrid systems for light-driven biotransformation

Enzymes catalyse target reactions under mild conditions with high efficiency, as well as excellent regional-, stereo-, and enantiomeric selectivity. Photocatalysis utilises sustainable and environment-friendly light power to realise efficient chemical conversion. By combining the interdisciplinary advantages of photo- and enzymatic catalysis, the photocatalyst-enzyme hybrid systems have proceeded various light-driven biotransformation with high efficiency under environmentally benign conditions, thus, attracting unparalleled focus during the last decades. It has also been regarded as a promising pathway towards green chemistry utilising ubiquitous solar energy. This systematic review gives insight into this research field by classifying the existing photocatalyst-enzyme hybrid systems into three sections based on different hybridizing modes between photo- and enzymatic catalysis. Furthermore, existing challenges and proposed strategies are discussed within this context. The first system summarised is the cofactor-mediated hybrid system, in which natural/artificial cofactors act as reducing equivalents that connect photocatalysts with enzymes for light-driven enzymatic biotransformation. Second, the direct contact-based photocatalyst-enzyme hybrid systems are described, including two different kinds of electron exchange sites on the enzyme molecules. Third, some cases where photocatalysts and enzymes are integrated into a reaction cascade with specific intermediates will be discussed in the following chapter. Finally, we provide perspective concerning the future of this field.

Enhanced visible light photoelectrocatalytic degradation of tetracycline hydrochloride by I and P co-doped TiO2 photoelectrode

Elimination of antibiotics such as tetracycline hydrochloride (TC) from wastewater is of great significance, but still faces challenges. Herein, for the first time, I and P co-doped TiO₂ catalysts were prepared via a hydrolysis method. We also reported a simple method to prepare I and P co-doped TiO₂ photoelectrodes, which exhibited preeminent photoelectrocatalytic (PEC) performance for the decomposition of TC. The synergistic effect of I and P co-doping could significantly improve the charge separation rate and enhance the light absorption capacity of TiO₂, leading to an enhancement of PEC activity. The main factors affecting the PEC performance were investigated, and the highest degradation rate constant (4.20 × 10^-2 min^-1) was achieved when the doping content of P was 4 at% (ITP-4 photoelectrode) at pH 11.02 under visible light. The Langmuir-Hinshelwood kinetic model and active species trapping experiments were selected to investigate the degradation mechanism of TC. The results suggest that the hydroxyl radicals and photogenerated holes were the main active species that were responsible for the decomposition of TC. Moreover, the degradation pathways of TC based on the intermediates also demonstrated that the hydroxyl radicals and holes showed a principal role in degrading TC.

Recent Progress and Perspectives on Electrochemical Regeneration of Reduced Nicotinamide Adenine Dinucleotide (NADH)

NAD is a cofactor that maintains cellular redox homeostasis and has immense industrial and biological significance. It acts as an enzymatic mediator in several biocatalytic electrochemical reactions and undergoes oxidation/reduction to form NAD⁺ or NADH, respectively. The NAD redox couple (NAD⁺ /NADH) mostly exists in enzyme-assisted metabolic reactions as a coenzyme during which electrons and protons are transferred. NADH shuttles these charges between the enzyme and the substrate. In order to understand such complex metabolic reactions, it is vital to study the bio-electrochemistry of NADH. In addition, the regeneration of NADH in industries has attracted significant attention due to its vast usage and high cost. To make biocatalysis economically viable, primary methods of NADH regeneration including enzymatic, chemical, photochemical and electrochemical methods are widely used. This review is mainly focused on the electrochemical reduction of NAD⁺ to NADH with specific details on the mechanism and kinetics of the reaction. It provides emphasis on the different routes (direct and mediated) to electrochemically regenerate NADH from NAD⁺ highlighting the NAD dimer formation. Also, it describes the electrocatalysts developed until now and the scope for development in this area of research.

Review of First-Principles Studies of TiO2: Nanocluster, Bulk, and Material Interface

TiO2 has extensive applications in the fields of renewable energy and environmental protections such as being used as photocatalysts or electron transport layers in solar cells. To achieve highly efficient photocatalytic and photovoltaic applications, ongoing efforts are being devoted to developing novel TiO2-based material structures or compositions, in which a first-principles computational approach is playing an increasing role. In this review article, we discuss recent computational and theoretical studies of structural, energetic, electronic, and optical properties of TiO2-based nanocluster, bulk, and material interface for photocatalytic and photovoltaic applications. We conclude the review with a discussion of future research directions in the field.

2D black phosphorus and tungsten trioxide heterojunction for enhancing photocatalytic performance in visible light

A novel, efficient and stable 2D black phosphorus and tungsten trioxide heterojunction (WO3-BPNs) was successfully synthesized using a combined hydrothermal, liquid phase exfoliating and co-precipitation method. The as-obtained WO3-BPNs composite was characterized by using XRD, SEM, XPS, UV-vis, etc. The results showed that the bandgap energy of the WO3-BPNs50 sample was 2.2 eV, which was lower than that of pure WO3. BPNs in the WO3-BPNs heterojunction as a co-catalyst effectively enhanced photo-generated electron-hole pairs separation. The synthesized WO3-BPNs sample significantly improved the photocatalytic performance in degrading rhodamine B (RhB) and metoprolol (MET) compared to pure WO3 and BPNs under visible-light. The maximum RhB and MET removal efficiencies were 92% and 87%, respectively, in the WO3-BPNs50 (added 50 mL BPNs dispersion) sample within 120 minutes. The relevant photocatalysis mechanisms were discussed. In addition, the intermediate products in the MET photodegradation process were investigated by LC-MS technology, and the degradation pathway of MET was proposed.

Metal Oxide Mediated Extracellular NADPH Regeneration Improves Ethanol Production by Engineered Synechocystis sp. PCC 6803

The ethanol synthesis pathway engineered Synechocystis sp. PCC 6803 (hereafter Synechocystis) was used to investigate the influence of metal oxide mediated extracellular NADPH regeneration on ethanol synthesis. The in-vitro studies proved that the metal oxides have the potential to generate the NADPH in the presence of electron donor, the usual components of photoautotrophic growth conditions. When the NADPH regeneration was applied in Synechocystsis, the strain showed improved growth and ethanol production. This improved ethanol synthesis is attributed to the increased availability of NADPH to the ethanol synthesis pathway and redirection of closely related carbon metabolism into the ethanol synthesis. Under optimized light intensity and NADP addition, the maximum ethanol production of 5,100 mg/L was observed in MgO mediated extracellular NADPH regeneration after 25 days of cultivation, which is 2-fold higher than the control. This study indicates the feasibility of metal oxide mediated extracellular NADPH regeneration of Synechocystis to increase the production of ethanol.

Recent Advances and Applications of Semiconductor Photocatalytic Technology

Along with the development of industry and the improvement of people’s living standards, peoples’ demand on resources has greatly increased, causing energy crises and environmental pollution. In recent years, photocatalytic technology has shown great potential as a low-cost, environmentally-friendly, and sustainable technology, and it has become a hot research topic. However, current photocatalytic technology cannot meet industrial requirements. The biggest challenge in the industrialization of photocatalyst technology is the development of an ideal photocatalyst, which should possess four features, including a high photocatalytic efficiency, a large specific surface area, a full utilization of sunlight, and recyclability. In this review, starting from the photocatalytic reaction mechanism and the preparation of the photocatalyst, we review the classification of current photocatalysts and the methods for improving photocatalytic performance; we also further discuss the potential industrial usage of photocatalytic technology. This review also aims to provide basic and comprehensive information on the industrialization of photocatalysis technology.

Boosting the Oxygen Evolution Reaction Activity of NiFe2O4 Nanosheets by Phosphate Ion Functionalization

Here, we demonstrate an effective strategy to constitutionally increase the conductivity and electrocatalytic property of NiFe₂O₄ by phosphate ion functionalization. The phosphate-ion-modified NiFe₂O₄ (P-NiFe₂O₄) nanosheets are readily grown on a carbon cloth by a simple hydrothermal method and followed by a phosphating process. The introduction of phosphate ions on the NiFe₂O₄ surface is highly beneficial for increasing the charge transport rate and electrocatalytic active sites. As a result, the as-prepared P-NiFe₂O₄ nanosheets show outstanding electrocatalytic activity toward oxygen evolution reaction (OER), with a low overpotential (231 mV at 10 mA/cm²) and Tafel slope (49 mV/dec). Furthermore, the P-NiFe₂O₄ electrode has a remarkable stability with no activity fading after 50 h. In addition, the as-fabricated water electrocatalysts exhibit excellent flexibility at the foldable state. These features make the phosphate-ion-functionalized NiFe₂O₄ electrodes open a new way to develop OER electrocatalysts with high electrochemical property.

Phosphorous-doped TiO2 nanoparticles: synthesis, characterization, and visible photocatalytic evaluation on sulfamethazine degradation

Mesoporous phosphorous-doped TiO₂ (TP) with different wt% of P (0.5, 1.0, and 1.5) was synthetized by microwave-assisted sol-gel method. The obtained materials were characterized by XRD with cell parameters refinement approach, Raman, BET-specific surface area analysis, SEM, ICP-OES, UV-Vis with diffuse reflectance, photoluminescence, FTIR, and XPS. The photocatalytic activity under visible light was evaluated on the degradation of sulfamethazine (SMTZ) at pH 8. The characterization of the phosphorous materials (TP) showed that incorporation of P in the lattice of TiO₂ stabilizes the anatase crystalline phase, even increasing the annealing temperature. The mesoporous P-doped materials showed higher surface area and lower average crystallite size, band gap, and particle size; besides, more intense bands attributed to O-H bond were observed by FTIR analysis compared with bare TiO₂. The P was substitutionally incorporated in the TiO₂ lattice network as P⁵⁺ replacing Ti⁴⁺ to form Ti-O-P bonds and additionally present as PO₄^3- on the TiO₂ surface. All these characteristics explain the observed superior photocatalytic activity on degradation (100%) and mineralization (32%) of SMTZ under visible radiation by TP catalysts, especially for P-doped TiO₂ 1.0 wt% calcined at 450 °C (TP1.0-450). Ammonium, nitrate, and sulfate ions released during the photocatalytic degradation were quantified by ion chromatography; the nitrogen and sulfur mass balance evidenced the partial mineralization of this recalcitrant molecule.

Phosphor and nitrogen co-doped rutile TiO2 covered on TiN for oxygen reduction reaction in acidic media

Phosphor and nitrogen atoms were co-doped into rutile TiO₂ phase on TiN to produce new active sites for oxygen reduction reaction.

A thiophene-modified doubleshell hollow g-C3N4 nanosphere boosts NADH regeneration via synergistic enhancement of charge excitation and separation

ATCN-DSCN enabled boosted NADH photo-regeneration and FDH-assisted CO₂ reduction.

Sodium hypophosphite induced a simultaneous P doping and hollowing process of TiO2 spherical nanostructures with enhanced photocatalytic activity

Sodium hypophosphite induced a simultaneous P doping and hollowing process of TiO₂ spherical nanostructures and produced P-doped hollow spherical nanostructures via a simple one-step hydrothermal method.

Tetra(4-carboxyphenyl)porphyrin for efficient cofactor regeneration under visible light and its immobilization

TCPP was successfully used for visible light-driven NADH regeneration with a high yield of 81.5% and its immobilization was attempted.

Regulation of the Electroanalytical Performance of Ultrathin Titanium Dioxide Nanosheets toward Lead Ions by Non-Metal Doping

Three non-metallic elements, sulfur, fluorine, and iodine, were used to dope the ultrathin two-dimensional TiO₂ nanosheets, which would regulate their electroanalytical properties toward heavy metal ions. Among these doped materials, fluorine-doped TiO₂ nanosheets shows the highest electrochemical sensitivity and a superior detection limit toward Pb(II) when the doping concentration is 10%. When compared with the bare TiO₂ nanosheets, the sensitivity increased by 102%, and the detection limit decreased by 36.4%. Through combining further electrochemical experiments and density-functional theory calculations, the enhanced electrochemical performance stemming from element doping was then investigated in detail. The theoretical calculation demonstrated that fluorine doping could greatly increase the adsorption energy of Pb(II) on the TiO₂ nanosheets and enhance their loading capacity. Both cyclic voltammetric and electrical impedance spectroscopy analysis indicated the enhanced electron transfer rate on the electrode modified by fluorine-doped TiO₂ nanosheets. Further measurement on the desorption performance showed the better stripping response of Pb(II) on the electrode with TiO₂ nanosheets after fluorine doping, which suggests that fluorine doping is beneficial for Pb(II) diffuse onto the electrode surface for the reduction and stripping reaction. Therefore, the element doping of two-dimensional TiO₂ nanosheets provides a facile method to extend the electronic materials toward detection of heavy metal ions in the environment.

Artificial Photosynthesis: Hybrid Systems

Oxidoreductases are promising catalysts for organic synthesis. To sustain their catalytic cycles they require efficient supply with redox equivalents. Today classical biomimetic approaches utilizing natural electron supply chains prevail but artificial regeneration approaches bear the promise of simpler and more robust reaction schemes. Utilizing visible light can accelerate such artificial electron transport chains and even enable thermodynamically unfeasible reactions such as the use of water as reductant.This contribution critically summarizes the current state of the art in photoredoxbiocatalysis (i.e. light-driven biocatalytic oxidation and reduction reactions).

Product Selectivity in Homogeneous Artificial Photosynthesis Using [(bpy)Rh(Cp*)X]n+-Based Catalysts

Due to the limited amount of fossil energy carriers, the storage of solar energy in chemical bonds using artificial photosynthesis has been under intensive investigation within the last decades. As the understanding of the underlying working principle of these complex systems continuously grows, more focus will be placed on a catalyst design for highly selective product formation. Recent reports have shown that multifunctional photocatalysts can operate with high chemoselectivity, forming different catalysis products under appropriate reaction conditions. Within this context [(bpy)Rh(Cp*)X]n+-based catalysts are highly relevant examples for a detailed understanding of product selectivity in artificial photosynthesis since the identification of a number of possible reaction intermediates has already been achieved.

Unique band structure enhanced visible light photocatalytic activity of phosphorus-doped BiOI hierarchical microspheres

In this article, phosphorus (P)-doped BiOI hierarchical microspheres were prepared via a facile hydrolytic method at room temperature.

Recent advances in non-metal modification of graphitic carbon nitride for photocatalysis: a historic review

This review provides a comprehensive survey and critical comments on the development of photocatalysts with a focus on the metal-free materials.

An integrated photocatalytic/enzymatic system for the reduction of CO2 to methanol in bioglycerol-water

A hybrid enzymatic/photocatalytic approach for the conversion of CO2 into methanol is described. For the approach discussed here, the production of one mol of CH3OH from CO2 requires three enzymes and the consumption of three mol of NADH. Regeneration of the cofactor NADH from NAD(+) was achieved by using visible-light-active, heterogeneous, TiO2-based photocatalysts. The efficiency of the regeneration process is enhanced by using a Rh(III)-complex for facilitating the electron and hydride transfer from the H-donor (water or a water-glycerol solution) to NAD(+). This resulted in the production of 100 to 1000 mol of CH3OH from one mol of NADH, providing the possibility for practical application.

Rhodium-coordinated poly(arylene-ethynylene)-alt-poly(arylene-vinylene) copolymer acting as photocatalyst for visible-light-powered NAD⁺/NADH reduction

A 2,2'-bipyridyl-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene) polymer, acting as a light-harvesting ligand system, was synthesized and coupled to an organometallic rhodium complex designed for photocatalytic NAD(+)/NADH reduction. The material, which absorbs over a wide spectral range, was characterized by using various analytical techniques, confirming its chemical structure and properties. The dielectric function of the material was determined from spectroscopic ellipsometry measurements. Photocatalytic reduction of nucleotide redox cofactors under visible light irradiation (390-650 nm) was performed and is discussed in detail. The new metal-containing polymer can be used to cover large surface areas (e.g. glass beads) and, due to this immobilization step, can be easily separated from the reaction solution after photolysis. Because of its high stability, the polymer-based catalyst system can be repeatedly used under different reaction conditions for (photo)chemical reduction of NAD(+). With this concept, enzymatic, photo-biocatalytic systems for solar energy conversion can be facilitated, and the precious metal catalyst can be recycled.