Electrostatically Driven Multielectron Transfer for the Photocatalytic Regeneration of Nicotinamide Cofactor

Bio-Inspired Photosynthesis Platform for Enhanced NADH Conversion and L-Glutamate Synthesis

Inspired by the layered structure, light absorption, and charge carrier pathway of chloroplast thylakoids in natural photosynthesis, we propose a novel artificial photosynthesis platform, which is composed of layered structured vaterite as the scaffold with gold nanoparticles (AuNPs), photosensitizer eosin Y (EY), and redox enzyme L-glutamate dehydrogenase (GDH) as the functional components. The EY exhibited significantly enhanced light absorption and charge carrier generation due to the localized surface plasmon resonance (LSPR) around the AuNPs and light refraction within the layers. This artificial photosynthesis platform can regenerate reduced nicotinamide adenine dinucleotide (NADH) under visible light and promote the rapid conversion of α-ketoglutarate to L-glutamate (0.453 Mm/h). The excellent biocompatibility of layered vaterite significantly enhances the resistance of GDH to harsh conditions, including high pH (pH = 10) and elevated temperatures (37-57 °C).

Atomically Precise Regulation of the N-Heterocyclic Microenvironment in Triazine Covalent Organic Frameworks for Coenzyme Photocatalytic Regeneration

Artificial photosynthesis represents a sustainable strategy for accessing high-value chemicals; however, the conversion efficiency is significantly limited by its difficulty in the cycle of coenzymes such as NADH. In this study, we report a series of isostructural triazine covalent organic frameworks (COFs) and explore their N-substituted microenvironment-dependent photocatalytic activity for NADH regeneration. We discovered that the rational alteration of N-heterocyclic species, which are linked to the triazine center through an imine linkage, can significantly regulate both the electron band structure and planarity of a COF layer. This results in different separation efficiencies of the photoinduced electron-hole pairs and electron transfer behavior within and between individual layers. The optimal COF catalyst herein achieves an NADH regeneration capacity of 89% within 20 min, outperforming most of the reported nanomaterial photocatalysts. Based on this, an artificial photosynthesis system is constructed for the green synthesis of a high-value compound, L-glutamate, and its conversion efficiency significantly surpasses the enzymatic approach without the NADH photocatalytic cycle. This work offers new insights into the coenzyme regeneration by means of regulating the distal heterocyclic microenvironment of a COF skeleton, holding great potential for the green photosynthesis of important chemicals.

Metal-semiconductor heterojunction accelerates the plasmonically powered photoregeneration of biological cofactors

Photocatalysis with plasmonic nanoparticles (NPs) is emerging as an attractive strategy to make and break chemical bonds. However, the fast relaxation dynamics of the photoexcited charge carriers in plasmonic NPs often result in poor yields. The separation and extraction of photoexcited hot-charge carriers should be faster than the thermalization process to overcome the limitation of poor yield. This demands the integration of rationally chosen materials to construct hybrid plasmonic photocatalysts. In this work, the enhanced photocatalytic activity of gold nanoparticle-titanium dioxide metal-semiconductor heterostructure (Au-TiO2) is used for the efficient regeneration of nicotinamide (NADH) cofactors. The modification of plasmonic AuNPs with n-type TiO2 semiconductor enhanced the charge separation process, because of the Schottky barrier formed at the Au-TiO2 heterojunction. This led to a 12-fold increment in the photocatalytic activity of plasmonic AuNP in regenerating NADH cofactor. Detailed mechanistic studies revealed that Au-TiO2 hybrid photocatalyst followed a less-explored light-independent pathway, in comparison to the conventional light-dependent path followed by sole AuNP photocatalyst. NADH regeneration yield reached ~70% in the light-independent pathway, under optimized conditions. Thus, our study emphasizes the rational choice of components in hybrid nanostructures in dictating the photocatalytic activity and the underlying reaction mechanism in plasmon-powered chemical transformations.

Hybrid CdSe/ZnS Quantum Dot-Gold Nanoparticle Composites Assembled by Click Chemistry: Toward Affordable and Efficient Redox Photocatalysts Working with Visible Light

A new modular, easy-to-synthesize photocatalyst was prepared by assembling colloidal CdSe/ZnS quantum dots (QD) and gold nanoparticles (AuNP) via their ligands thanks to copper-catalyzed azide to alkyne cycloaddition (CuAAC) click chemistry. The resulting composite (QD-AuNP) photocatalyst was tested with a benchmark photoredox system previously reported by our group, for which QD alone acted as a photocatalyst but with a modest quantum yield (QY = 0.06%) and turnover number (TON = 350 in 3 h) due to poor charge separation. After optimization, the QD-AuNP composites exhibited much improved photocatalytic performances: up to five times higher TON (2600 in 3 h) and up to 24 times faster reaction in the first 10 min of visible irradiation. Such an improvement is attributed to an efficient electron transfer from QD to AuNP in the photoexcited QD-AuNP composites, which ensures a much better charge separation than that in QD alone. This was confirmed by studying both (i) the quenching of the QD photoluminescence during the synthesis of the QD-AuNP composites and (ii) the blue shift of the AuNP plasmon absorption band due to the accumulation of up to 7400 electrons per AuNP in QD-AuNP composites under visible light irradiation in the presence of electron donors.

Plasmon enabled Claisen rearrangement with sunlight

Plasmonic-heat generated from the solar irradiation of gold nanoparticles is used as the thermal energy source for the Claisen rearrangement of allyl phenyl ether to 2-allylphenol, which is conventionally performed with electrical heating at 250 °C. The use of a closed reactor enables the physical separation of the reactants from the source of plasmonic-heat, thereby preventing the interference of the hot-charge carriers in the plasmon-driven Claisen rearrangement. In this way, the sole effect of plasmonic-heat in driving a high temperature organic transformation is demonstrated. Our study reveals the prospects of plasmonic nanostructures in conducting energy intensive chemical synthesis in a sustainable fashion.

Ligand-mediated electron transport channels enhance photocatalytic activity of plasmonic nanoparticles

Photoexcitation of noble metal nanoparticles creates surface plasmons which further decay to form energetic charge carriers. These charge carriers can initiate and/or accelerate various chemical processes at nanoparticle surfaces, although the efficiency of such processes remains low as a large fraction of these carriers recombine before they can reach the reaction sites. Thus efficient utilization of these charge carriers requires designing nanostructures that promote the separation of charges and their transport toward the reaction sites. Here we demonstrate that covalently bound surface-coating ligands with suitable orbital alignment can provide electron transport channels boosting hot electron extraction from a gold nanostructure leading to a huge enhancement in the rate of hydrogen evolution reaction (HER) under NIR excitation. A (p)Br-Ph-SH substituted gold nanoprism (AuTP) substrate produced ∼4500 fold more hydrogen compared to a pristine AuTP substrate under 808 nm excitation. Further experimental and theoretical studies on a series of substituted benzene-thiol bound AuTP substrates showed that the extent of the ligand-mediated HER enhancement depends not only on the polarity of the ligand but on the interfacial orbitals interactions.

Nanoparticle Self-Assembly: From Design Principles to Complex Matter to Functional Materials

The creation of matter with varying degrees of complexities and desired functions is one of the ultimate targets of self-assembly. The ability to regulate the complex interactions between the individual components is essential in achieving this target. In this direction, the initial success of controlling the pathways and final thermodynamic states of a self-assembly process is promising. Despite the progress made in the field, there has been a growing interest in pushing the limits of self-assembly processes. The main inception of this interest is that the intended self-assembled state, with varying complexities, may not be "at equilibrium (or at global minimum)", rendering free energy minimization unsuitable to form the desired product. Thus, we believe that a thorough understanding of the design principles as well as the ability to predict the outcome of a self-assembly process is essential to form a collection of the next generation of complex matter. The present review highlights the potent role of finely tuned interparticle interactions in nanomaterials to achieve the preferred self-assembled structures with the desired properties. We believe that bringing the design and prediction to nanoparticle self-assembly processes will have a similar effect as retrosynthesis had on the logic of chemical synthesis. Along with the guiding principles, the review gives a summary of the different types of products created from nanoparticle assemblies and the functional properties emerging from them. Finally, we highlight the reasonable expectations from the field and the challenges lying ahead in the creation of complex and evolvable matter.

On-nanoparticle monolayers as a solute-specific, solvent-like phase

In addition to modifying surface properties, self-assembled monolayers, SAMs, on nanoparticles can selectively incorporate small molecules from the surrounding solution. This selectivity has been used in the design of substrate-specific catalytic systems but its degree has not been quantified. This work uses catalytic centers embedded in on-nanoparticle hydrophobic SAMs to monitor and quantify the partitioning of molecules between the bulk solvent and these monolayers. A combination of experiments and theory allows us to relate the logarithm of the incorporation-into-SAM constant to the "bulk" log P values, characterizing the incoming substrates. These results are in line with classic, semi-empirical linear free energy relationships between partitioning solvent systems; in this way, they substantiate the view of nanoscopic on-particle SAMs acting akin to a bulk solvent phase.

Strong synergy between gold nanoparticles and cobalt porphyrin induces highly efficient photocatalytic hydrogen evolution

The reaction efficiency of reactants near plasmonic nanostructures can be enhanced significantly because of plasmonic effects. Herein, we propose that the catalytic activity of molecular catalysts near plasmonic nanostructures may also be enhanced dramatically. Based on this proposal, we develop a highly efficient and stable photocatalytic system for the hydrogen evolution reaction (HER) by compositing a molecular catalyst of cobalt porphyrin together with plasmonic gold nanoparticles, around which plasmonic effects of localized electromagnetic field, local heating, and enhanced hot carrier excitation exist. After optimization, the HER rate and turn-over frequency (TOF) reach 3.21 mol g-1 h-1 and 4650 h-1, respectively. In addition, the catalytic system remains stable after 45-hour catalytic cycles, and the system is catalytically stable after being illuminated for two weeks. The enhanced reaction efficiency is attributed to the excitation of localized surface plasmon resonance, particularly plasmon-generated hot carriers. These findings may pave a new and convenient way for developing plasmon-based photocatalysts with high efficiency and stability.

Cr3+-ZnGa2O4@Pt for Light-Triggered Dark Catalytic Regeneration of Nicotinamide Coenzymes without Other Electron Mediators

Photocatalysts for regeneration of reduced nicotinamide adenine dinucleotide (NADH) usually work with continuous lighting and electron mediators, which causes impracticability under dark conditions, risk of NADH reoxidation, and complex separation. To solve these problems, we present a new catalyst of tiny Pt nanoparticles photodeposited on chromium-doped zinc gallate (CZGO@Pt). Upon being light-triggered, the photogenerated electrons are stored in the traps of CZGO and then gradually released and transferred by Pt to directly reduce NAD⁺ after stoppage of illumination. Three lighting modes are compared to demonstrate the feasibility and advantage of this light-triggered dark catalysis. Within 4 h of reaction, the in-the-dark NADH yield reaches 75.0% under prelighting CZGO@5%Pt and it reaches 80.0% under prelighting CZGO@5%Pt and triethanolamine (TEOA). However, the NADH yield is only 53.5% under continuous lighting of CZGO@5%Pt, TEOA, and NAD⁺. Consequently, the light-triggered dark catalytic regeneration of NADH not only saves energy and operates easily but also significantly elevates the NADH yield. It thus would secure wide interests and applications in places where no light or only intermittent light is available.

Proving Cooperativity of a Catalytic Reaction by Means of Nanoscale Geometry: The Case of Click Reaction

Establishing whether a reaction is catalyzed by a single-metal catalytic center or cooperatively by a fleeting complex encompassing two such centers may be an arduous pursuit requiring detailed kinetic, isotopic, and other types of studies─as illustrated, for instance, by over a decade-long work on single-copper versus di-copper mechanisms of the popular "click" reaction. This paper describes a method to interrogate such cooperative mechanisms by a nanoparticle-based platform in which the probabilities of catalytic units being proximal can be varied systematically and, more importantly, independently of their volume concentration. The method relies on geometrical considerations rather than a detailed knowledge of kinetic equations, yet the scaling trends it yield can distinguish between cooperative and non-cooperative mechanisms.

Metal-Polymer Heterojunction in Colloidal-Phase Plasmonic Catalysis

Plasmonic catalysis in the colloidal phase requires robust surface ligands that prevent particles from aggregation in adverse chemical environments and allow carrier flow from reagents to nanoparticles. This work describes the use of a water-soluble conjugated polymer comprising a thiophene moiety as a surface ligand for gold nanoparticles to create a hybrid system that, under the action of visible light, drives the conversion of the biorelevant NAD⁺ to its highly energetic reduced form NADH. A combination of advanced microscopy techniques and numerical simulations revealed that the robust metal-polymer heterojunction, rich in sulfonate functional groups, directs the interaction of electron-donor molecules with the plasmonic photocatalyst. The tight binding of polymer to the gold surface precludes the need for conventional transition-metal surface cocatalysts, which were previously shown to be essential for photocatalytic NAD⁺ reduction but are known to hinder the optical properties of plasmonic nanocrystals. Moreover, computational studies indicated that the coating polymer fosters a closer interaction between the sacrificial electron-donor triethanolamine and the nanoparticles, thus enhancing the reactivity.

Gram-Selective Antibacterial Activity of Mixed-Charge 2D-MoS2

Development of nanomaterial-based antibiotics can be the most potent alternative due to the increasing resistance against conventional antibiotics. But one of the important parameters in development of antibacterial agent is...

Nanocatalysis under Nanoconfinement: A Metal-Free Hybrid Coacervate Nanodroplet as a Catalytic Nanoreactor for Efficient Redox and Photocatalytic Reactions

Nature utilizes cellular and subcellular compartmentalization to efficiently drive various complex enzymatic transformations via spatiotemporal control. In this context, designing of artificial nanoreactors for efficient catalytic transformations finds tremendous importance in recent times. One key challenge remains the design of multiple catalytic centers within the confined space of a nanoreactor without unwanted agglomeration and accessibility barrier for reactants. Herein, we report a unique blend of nanoscience and chemical catalysis using a metal-free hybrid synthetic protocell as a catalytic nanoreactor for redox and photocatalytic transformations, which are otherwise incompatible in bulk aqueous medium. Hybrid coacervate nanodroplets (NDs) fabricated from 2.5 nm-sized carbon dots (CDs) and poly(diallyldimethyl)ammonium chloride have been utilized toward reductive hydrogenation of nitroarenes in the presence of sodium borohydride (NaBH₄). It has been found that the reduction mechanism follows the classical Langmuir-Hinshelwood (LH) model at the surface of embedded CDs inside the NDs via the generation of reactive surface hydroxyl groups. These NDs show excellent recyclability without any compromise on reaction kinetics and conversion yield. Importantly, spatiotemporal control over the hydrogenation reaction has been achieved using two mixed populations of coacervates. Moreover, efficient visible light-induced photoredox conversion of ferricyanide to ferrocyanide and artificial peroxidase-like activity have also been demonstrated inside these catalytic NDs. Our findings indicate that the individual polymer-bound CD inside the NDs acts as the catalytic center for both the redox and photocatalytic reactions. The present study highlights the unprecedented catalytic activity of the metal-free CD-based coacervate NDs and paves the way for next-generation catalytic nanoreactors for a wide range of chemical and enzymatic transformations.

Coupling plasmonic catalysis and nanocrystal growth through cyclic regeneration of NADH

In a typical colloidal synthesis, the molecules of the reducing agent are irreversibly oxidized during nanocrystal growth. Such a scenario is of questionable sustainability when confronted with naturally occurring processes in which reducing agent molecules are cyclically regenerated. Here we show that cofactor molecules once consumed in the nucleation and growth of metallic nanocrystals can be photoregenerated using metallic nanocrystals as photocatalysts and reused in the subsequent nucleation process. Cyclic regeneration of cofactor molecules opens up the possibilities for the sustainable synthesis of inorganic nanoparticles.

Zirconia supported gold-palladium nanocatalyst for NAD(P)H regeneration via two-step mechanism

The regeneration cycle of expensive cofactor, NAD(P)H, is of paramount importance for the bio-catalyzed redox reactions. Here a ZrO₂supported bimetallic nanocatalyst of gold-palladium (Au-Pd/ZrO₂) was prepared to catalyze the regeneration of NAD(P)H without using electron mediators and extra energy input. Over 98% of regeneration efficiency can be achieved catlyzed by Au-Pd/ZrO₂using TEOA as the electron donor. Mechanism study showed that the regeneration of NAD(P)H took place through a two-step process: Au-Pd/ZrO₂nanocatalyst first catalyzed the oxidation of triethanolamine (TEOA) to glycolaldehyde (GA), then the generated GA induced the non-catalytic reducing of NAD(P)⁺to NAD(P)H under an alkaline environment maintained by TEOA. This two-step mechanism enables the decoupling of the regeneration of NAD(P)H in space and time into a catalytic oxidation and non-catalytic reducing cascade process which has been further verified using a variety of electron donors. The application significance of this procedure is further demonstrated both by the favorable stability of Au-Pd/ZrO₂nanocatalyst in 5 successive cycles preserving over 90% of its original activity, and by the excellent performance of the regenerated NADH as the cofactor in the catalytic hydrogenation of acetaldehyde using an ethanol dehydrogenase.

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.

Graphene oxide-grafted plasmonic Au@Ag nanoalloys with improved synergistic effects for promoting hot carrier-driven photocatalysis under visible light irradiation

Plasmonic metallic nanostructure with unique hot carrier-driven photocatalysis has recently emerged as a promising photocatalyst. Herein, we show that the plasmonic photocatalysis can be significantly promoted by supporting bimetallic Au@Ag nanoalloys (NAs) on graphene oxide (GO). The obtained Au₃@Ag₁/GO (molar ratio of Au to Ag∼3:1) with improved synergistic effects provides a remarkable higher visible-light (>400 nm) photocatalytic activity for a complete degradation (99.36%) of tetracycline hydrochloride (TCH) molecules within 70 min, while about 61.74% or 62.38% via monometallic Au/GO or Ag/GO. The optimum photocatalytic performance is attributed to the production of high yield hot carriers on NAs with enhanced localized surface plasmon resonance property and the pronounced photoinduced electron-transfer ability of modified GO support by overgrowth of NAs. These findings enable the optimal Au₃@Ag₁/GO to become an appealing high-performance photocatalyst for promoting diverse photochemical reactions.

Intensifying Electron Utilization by Surface-Anchored Rh Complex for Enhanced Nicotinamide Cofactor Regeneration and Photoenzymatic CO2 Reduction

Solar-driven photocatalytic regeneration of cofactors, including reduced nicotinamide adenine dinucleotide (NADH), reduced nicotinamide adenine dinucleotide phosphate (NADPH), and reduced flavin adenine dinucleotide (FADH₂), could ensure the sustainable energy supply of enzymatic reactions catalyzed by oxidoreductases for the efficient synthesis of chemicals. However, the elevation of cofactor regeneration efficiency is severely hindered by the inefficient utilization of electrons transferred on the surface of photocatalysts. Inspired by the phenomenon of ferredoxin-NADP⁺ reductase (FNR) anchoring on thylakoid membrane, herein, a homogeneous catalyst of rhodium (Rh) complex, [Cp∗Rh(bpy)H₂O]²⁺, was anchored on polymeric carbon nitride (PCN) mediated by a tannic acid/polyethyleneimine (TA/PEI) adhesive layer, acquiring PCN@TA/PEI-Rh core@shell photocatalyst. Illuminated by visible light, electrons were excited from the PCN core, then transferred through the TA/PEI shell, and finally captured by the surface-anchored Rh for instant utilization during the regeneration of NADH. The TA/PEI-Rh shell could facilitate the electron transfer from the PCN core and, more importantly, achieved ~1.3-fold elevation of electron utilization efficiency compared with PCN. Accordingly, the PCN@TA/PEI-Rh afforded the NADH regeneration efficiency of 37.8% after 20 min reaction under LED light (405 nm) illumination, over 1.5 times higher than PCN with free Rh. Coupling of the NADH regeneration system with formate dehydrogenase achieved continuous production of formate from carbon dioxide (CO₂). Our study may provide a generic and effective strategy to elevate the catalytic efficiency of a photocatalyst through intensifying the electron utilization.

Early instability of MIL-125-NH2 in aqueous solution and mediation of the visible photogeneration of an NADH cofactor

MIL-125-NH₂ hydrolysis can be minimized by regulating the pH of the medium, thus defining a stability window where it is possible to use it as a photocatalyst for visible light-driven production of NADH.