Amorphous mixed-valent Mn-containing nanozyme with cocklebur-like morphology for specific colorimetric detection of cancer cells via Velcro effects

Designing nanozymes with excellent catalytic activity through valence state engineering and defect engineering is a widely applicable strategy. However, their development is hindered by the complexity of the design strategies. In this work, we employed a simple calcination method to regulate the valence of manganese and crystalline states in manganese oxide nanozymes. The oxidase-like activity of the nanozymes was found to benefit from a mixed valence state dominated by Mn (III). And the amorphous structure with more active defect sites significantly enhanced the catalytic efficiency. Moreover, we demonstrated that amorphous mixed-valent Mn-containing (amvMn) nanozymes with unique cocklebur-like biomimetic morphology achieved specific binding to cancer cells through the Velcro effects. Subsequently, the nanozymes mediated TMB coloration through their oxidase-like activity, enabling the colorimetric detection of cancer cells. This work not only provides guidance for optimizing nanozyme performance, but also inspire the development of equipment-free visual detection methods for cancer cells.

Research on engineered electrocatalysts for efficient water splitting: a comprehensive review

Water electrolysis plays an interesting role toward hydrogen generation for overcoming global environmental crisis and solving the energy storage problem. However, there is still a deficiency of efficient electrocatalysts to overcome sluggish kinetics for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Great efforts have been employed to produce potential catalysts with low overpotential, rapid kinetics, and excellent stability for HER and OER. At present, hydrogen economy is driven by electrocatalysts with excellent characteristics; thus, systematic design strategy has become the driving force to exploit earth-abundant transition metal-based electrocatalysts toward H₂ economy. In this review, the recent progress on newer materials including metals, alloys, and transition metal oxides (manganese oxides, cobalt oxides, nickel oxides, PBA-derived metal oxides, and metal complexes) as photocatalysts/electrocatalysts has been overviewed together with some methodologies for efficient water splitting. Metal-organic framework (MOF)-based electrocatalysts have been highly exploited owing to their interesting functionalities. The photovoltaic-electrocatalytic (PV-EC) process focused on harvesting high solar-to-hydrogen efficiency (STH) among various solar energy conversion as well as storage systems. Electrocatalysts/photocatalysts with high efficiency have become an urgent need for overall water splitting. Also, cutting-edge achievements in the fabrication of electrocatalysts along with theoretical consideration have been discussed.

Investigation of electrocatalytic activity of a new mononuclear Mn(II) complex for water oxidation in alkaline media

Water splitting is a promising way to alleviate the energy crisis. In nature, water oxidation is done by a tetranuclear manganese cluster in photosystem II. Therefore, the study of water oxidation by Mn complexes is attractive in water splitting systems. In this report, a new mononuclear Mn(II) complex, MnL₂ (HL = (E)-3-hydroxy-N'-(pyridin-2-ylmethylene)-2-naphthohydrazide) was prepared and characterized by spectroscopic techniques and single-crystal X-ray diffraction. Crystallographic analysis indicated that the geometry around the Mn(II) ion is distorted octahedral. The MnN₄O₂ coordination moiety is achieved by bounding of oxygen and two nitrogen donor atoms of two hydrazone ligands. The synthesized complex was also investigated for electrochemical water oxidation using electrochemical techniques, scanning electron microscopy, energy dispersive spectrometry, and PXRD analysis. Linear sweep voltammetry experiment showed that the modified carbon paste electrode by the complex displays high activity for water oxidation reaction with an overpotential of 565 mV at a current density of 10 mA cm^-2 and Tafel slope of 105 mV dec^-1 in an alkaline solution. It was found that the complex structure finally changes during the reaction and converts to Mn oxide nanoparticles which act as active catalytic species and oxidize the water.

Promoting Proton Transfer and Stabilizing Intermediates in Catalytic Water Oxidation via Hydrophobic Outer Sphere Interactions

The outer coordination sphere of metalloenzyme often plays an important role in its high catalytic activity, however, this principle is rarely considered in the design of man‐made molecular catalysts. Herein, four Ru‐bda (bda=2,2′‐bipyridine‐6,6′‐dicarboxylate) based molecular water oxidation catalysts with well‐defined outer spheres are designed and synthesized. Experimental and theoretical studies showed that the hydrophobic environment around the Ru center could lead to thermodynamic stabilization of the high‐valent intermediates and kinetic acceleration of the proton transfer process during catalytic water oxidation. By this outer sphere stabilization, a 6‐fold rate increase for water oxidation catalysis has been achieved.

Systematic Influence of Electronic Modification of Ligands on the Catalytic Rate of Water Oxidation by a Single-Site Ru-Based Catalyst

Catalytic water oxidation is an important process for the development of clean energy solutions and energy storage. Despite the significant number of reports on active catalysts, systematic control of the catalytic activity remains elusive. In this study, descriptors are explored that can be correlated with catalytic activity. [Ru(tpy)(pic)₂ (H₂ O)](NO₃ )₂ and [Ru(EtO-tpy)(pic)₂ (H₂ O)](NO₃ )₂ (where tpy=2,2' : 6',2"-terpyridine, EtO-tpy=4'-(ethoxy)-2,2':6',2"-terpyridine, pic=4-picoline) are synthesized and characterized by NMR, UV/Vis, EPR, resonance Raman, and X-ray absorption spectroscopy, and electrochemical analysis. Addition of the ethoxy group increases the catalytic activity in chemically driven and photocatalytic water oxidation. Thus, the effect of the electron-donating group known for the [Ru(tpy)(bpy)(H₂ O)]²⁺ family is transferable to architectures with a tpy ligand trans to the Ru-oxo unit. Under catalytic conditions, [Ru(EtO-tpy)(pic)₂ (H₂ O)](NO₃ )₂ displays new spectroscopic signals tentatively assigned to a peroxo intermediate. Reaction pathways were analyzed by using DFT calculations. [Ru(EtO-tpy)(pic)₂ (H₂ O)](NO₃ )₂ is found to be one of the most active catalysts functioning by a water nucleophilic attack mechanism.

Jahn-Teller Effects in a Vanadate-Stabilized Manganese-Oxo Cubane Water Oxidation Catalyst

Heuristic rules that allow identifying the preferred mixed‐valence isomers and Jahn‐Teller axis arrangements in the water oxidation catalyst [(Mn₄O₄)(V₄O₁₃)(OAc)₃]ⁿ⁻ and its activated form [(Mn₄O₄)(V₄O₁₃)(OAc)₂(H₂O)(OH)]ⁿ⁻ are derived. These rules are based on computing all combinatorially possible mixed‐valence isomers and Jahn‐Teller axis arrangements of the Mn^III atoms, and associate energetic costs with some structural features, like crossings of multiple Jahn‐Teller axes, the location of these axes, or the involved ligands. It is found that the different oxidation states localize on different Mn centers, giving rise to clear Jahn‐Teller distortions, unlike in previous crystallographic findings where an apparent valence delocalization was found. The low barriers that connect different Jahn‐Teller axis arrangements suggest that the system quickly interconverts between them, leading to the observation of averaged bond lengths in the crystal structure. We conclude that the combination of cubane‐vanadate bonds that are chemically inert, cubane‐acetate/water bonds that can be activated through a Jahn‐Teller axis, and low activation barriers for intramolecular rearrangement of the Jahn‐Teller axes plays an essential role in the reactivity of this and probably related compounds.

Spectral Signatures of Oxidation States in a Manganese-Oxo Cubane Water Oxidation Catalyst

We report IR and UV/Vis spectroscopic signatures that allow discriminating between the oxidation states of the manganese‐based water oxidation catalyst [(Mn₄O₄)(V₄O₁₃)(OAc)₃]³⁻. Simulated IR spectra show that V=O stretching vibrations in the 900–1000 cm⁻¹ region shift consistently by about 20 cm⁻¹ per oxidation equivalent. Multiple bands in the 1450–1550 cm⁻¹ region also change systematically upon oxidation/reduction. The computed UV/Vis spectra predict that the spectral range above 350 nm is characteristic of the managanese‐oxo cubane oxidation state, whereas transitions at higher energy are due to the vanadate ligand. The presence of absorption signals above 680 nm is indicative of the presence of Mn^III atoms. Spectroelectrochemical measurements of the oxidation from [Mn2III Mn2IV ] to [Mn4IV ] showed that the change in oxidation state can indeed be tracked by both IR and UV/Vis spectroscopy.

Evaluation of Manganese Cubanoid Clusters for Water Oxidation Catalysis: From Well-Defined Molecular Coordination Complexes to Catalytically Active Amorphous Films

With a view to developing multimetallic molecular catalysts that mimic the oxygen-evolving catalyst (OEC) in Nature's photosystem II, the synthesis of various dicubanoid manganese clusters is described and their catalytic activity investigated for water oxidation in basic, aqueous solution. Pyridinemethanol-based ligands are known to support polynuclear and cubanoid structures in manganese coordination chemistry. The chelators 2,6-pyridinedimethanol (H₂ L¹ ) and 6-methyl-2-pyridinemethanol (HL² ) were chosen to yield polynuclear manganese complexes; namely, the tetranuclear defective dicubanes [Mn^II ₂ Mn^III ₂ (HL¹ )₄ (OAc)₄ (OMe)₂ ] and [Mn^II ₂ Mn^III ₂ (HL¹ )₆ (OAc)₂ ] (OAc)₂ ⋅2 H₂ O, as well as the octanuclear-dicubanoid [Mn^II ₆ Mn^III ₂ (L² )₄ (O)₂ (OAc)₁₀ (HOMe/OH₂ )₂ ]⋅3MeOH⋅MeCN. In freshly prepared solutions, polynuclear species were detected by electrospray ionization mass spectrometry, whereas X-band electron paramagnetic resonance studies in dilute, liquid solution suggested the presence of divalent mononuclear Mn species with g values of 2. However, the magnetochemical investigation of the complexes' solutions by the Evans technique confirmed a haphazard combination of manganese coordination complexes, from mononuclear to polynuclear species. Subsequently, the newly synthesized and characterized manganese molecular complexes were employed as precursors to prepare electrode-deposited films in a buffer-free solution to evaluate and compare their stability and catalytic activity for water oxidation electrocatalysis.

Polymeric Design for Electron Transfer in Photoinduced Hydrogen Generation through a Coil-Globule Transition

To realize a renewable energy society, a polymeric system for photoinduced hydrogen generation utilizing a copolymer containing an electron acceptor was designed. In this system, the redox changes of viologen introduced into poly(N-isopropylacrylamide) cause cyclic conformational changes owing to the shifting of the phase transition temperature (PTT). The polymeric coil-globule transitions with hydrophilic/hydrophobic changes accelerate the electron transfer for hydrogen generation. In particular, hydrogen generation using visible-light energy with high efficiency is achieved around the PTT. In contrast to conventional solution systems, our polymeric system enables efficient hydrogen generation in a close molecular arrangement without the aggregation of catalytic nanoparticles. The utilization of conformational changes will provide a new strategy for synthesizing artificial photosynthetic hydrogels that split water to generate both hydrogen and oxygen.

Recent Advances in the Development of Molecular Catalyst-Based Anodes for Water Oxidation toward Artificial Photosynthesis

Catalytic water oxidation represents a bottleneck for developing artificial photosynthetic systems that store solar energy as renewable fuels. A variety of molecular water oxidation catalysts (WOCs) have been reported over the last two decades. In view of their applications in artificial photosynthesis devices, it is essential to immobilize molecular catalysts onto the surfaces of conducting/semiconducting supports for fabricating efficient and stable water oxidation anodes/photoanodes. Molecular WOC-based anodes are essential for developing photovoltaic artificial photosynthesis devices and, moreover, the performance of molecular WOC on anodes will provide important insight into designing extended molecular WOC-based photoanodes for photoelectrochemical (PEC) water oxidation. This Review concerns recent progress in the development of molecular WOC-based anodes over the last two decades and looks at the prospects for using such anodes in artificial photosynthesis.

Electrochemically Deposited Nickel Oxide from Molecular Complexes for Efficient Water Oxidation Catalysis

A facile method for the electrodeposition of amorphous nickel oxyhydroxide is described and discussed in which well-defined nickel complexes with pyridinedimethanol ligands are employed as single-source molecular precursors. No buffering agent is required to assist the anodic deposition process. The deposited nickel oxyhydroxide shows high robustness and efficiency for electrocatalytic water oxidation.

Visible-Light-Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster

Photosynthetic water oxidation in plants occurs at an inorganic calcium manganese oxo cluster, which is known as the oxygen evolving complex (OEC), in photosystem II. Herein, we report a synthetic OEC model based on a molecular manganese vanadium oxide cluster, [Mn4 V4 O17 (OAc)3 ](3-) . The compound is based on a [Mn4 O4 ](6+) cubane core, which catalyzes the homogeneous, visible-light-driven oxidation of water to molecular oxygen and is stabilized by a tripodal [V4 O13 ](6-) polyoxovanadate and three acetate ligands. When combined with the photosensitizer [Ru(bpy)3 ](2+) and the oxidant persulfate, visible-light-driven water oxidation with turnover numbers of approximately 1150 and turnover frequencies of about 1.75 s(-1) is observed. Electrochemical, mass-spectrometric, and spectroscopic studies provide insight into the cluster stability and reactivity. This compound could serve as a model for the molecular structure and reactivity of the OEC and for heterogeneous metal oxide water-oxidation catalysts.

Catalytic Activity and Impedance Behavior of Screen-Printed Nickel Oxide as Efficient Water Oxidation Catalysts

We report that films screen printed from nickel oxide (NiO) nanoparticles and microballs are efficient electrocatalysts for water oxidation under near-neutral and alkaline conditions. Investigations of the composition and structure of the screen-printed films by X-ray diffraction, X-ray absorption spectroscopy, and scanning electron microscopy confirmed that the material was present as the cubic NiO phase. Comparison of the catalytic activity of the microball films to that of films fabricated by using NiO nanoparticles, under similar experimental conditions, revealed that the microball films outperform nanoparticle films of similar thickness owing to a more porous structure and higher surface area. A thinner, less-resistive NiO nanoparticle film, however, was found to have higher activity per Ni atom. Anodization in borate buffer significantly improved the activity of all three films. X-ray photoelectron spectroscopy showed that during anodization, a mixed nickel oxyhydroxide phase formed on the surface of all films, which could account for the improved activity. Impedance spectroscopy revealed that surface traps contribute significantly to the resistance of the NiO films. On anodization, the trap state resistance of all films was reduced, which led to significant improvements in activity. In 1.00 m NaOH, both the microball and nanoparticle films exhibit high long-term stability and produce a stable current density of approximately 30 mA cm(-2) at 600 mV overpotential.

Molecular Cobalt Clusters as Precursors of Distinct Active Species in Electrochemical, Photochemical, and Photoelectrochemical Water Oxidation Reactions in Phosphate Electrolytes

Three cobalt model molecular compounds, Co-cubane ([Co4 (µ3 -O)4 (µ-OAc)4 py4 ]), Co-trimer ([Co3 (μ3 -O)(µ-OAc)6 py3 ]PF6 ), and Co-dimer ([Co2 (μ-OH)2 (µ-OAc)(OAc)2 py4 ]PF6 ), are investigated as water oxidation reaction (WOR) catalysts, using electrochemical, photochemical, and photoelectrochemical methodologies in phosphate electrolyte. The actual species contributing to the catalytic activity observed in the WOR are derived from the transformation of these cobalt compounds. The catalytic activity observed is highly dependent on the initial compound structure and on the particular WOR methodology used. Co-cubane shows no activity in the electrochemical WOR and negligible activity in the photochemical WOR, but is active in the photoelectrochemical WOR, in which it behaves as a precursor to catalytically active species. Co-dimer also shows no activity in the electrochemical WOR, but behaves as a precursor to catalytically active species in both the photochemical and photoelectrochemical WOR experiments. Co-trimer behaves as a precursor to catalytically active species in all three of the WOR methodologies.

Molecular Mixed-Metal Manganese Oxido Cubanes as Precursors to Heterogeneous Oxygen Evolution Catalysts

Well-defined mixed-metal [CoMn3 O4 ] and [NiMn3 O4 ] cubane complexes were synthesized and used as precursors for heterogeneous oxygen evolution reaction (OER) electrocatalysts. The discrete clusters were dropcasted onto glassy carbon (GC) and indium tin oxide (ITO) electrodes, and the OER activities of the resulting films were evaluated. The catalytic surfaces were analyzed by various techniques to gain insight into the structure-function relationships of the electrocatalysts' heterometallic composition. Depending on preparation conditions, the Co-Mn oxide was found to change metal composition during catalysis, while the Ni-Mn oxides maintained the NiMn3 ratio. XAS studies provided structural insights indicating that the electrocatalysts are different from the molecular precursors, but that the original NiMn3 O4 cubane-like geometry was maintained in the absence of thermal treatment (2-Ni). In contrast, the thermally generated 3-Ni develops an oxide-like extended structure. Both 2-Ni and 3-Ni undergo structural changes upon electrolysis, but they do not convert into the same material. The observed structural motifs in these heterogeneous electrocatalysts are reminiscent of the biological oxygen-evolving complex in Photosystem II, including the MMn3 O4 cubane moiety. The reported studies demonstrate the use of discrete heterometallic oxide clusters as precursors for heterogeneous water oxidation catalysts of novel composition and the distinct behavior of two sets of mixed metal oxides.

The role of nano-sized manganese oxides in the oxygen-evolution reactions by manganese complexes: towards a complete picture

Eighteen Mn complexes with N-donor and carboxylate ligands have been synthesized and characterized. Three Mn complexes among them are new and are reported for the first time. The reactions of oxygen evolution in the presence of oxone (2KHSO5·KHSO4·K2SO4) and cerium(iv) ammonium nitrate catalyzed by these complexes are studied and characterized by UV-visible spectroscopy, X-ray diffraction spectrometry, dynamic light scattering, Fourier transform infrared spectroscopy, electron paramagnetic resonance spectroscopy, transmission electron microscopy, scanning electron microscopy, membrane-inlet mass spectrometry and electrochemistry. Some of these complexes evolve oxygen in the presence of oxone as a primary oxidant. CO2 and MnO4(-) are other products of these reactions. Based on spectroscopic studies, the true catalysts for oxygen evolution in these reactions are different. We proposed that for the oxygen evolution reactions in the presence of oxone, the true catalysts are both high valent Mn complexes and Mn oxides, but for the reactions in the presence of cerium(iv) ammonium nitrate, the active catalyst is most probably a Mn oxide.

Enhancing catalytic activity by narrowing local energy gaps--X-ray studies of a manganese water oxidation catalyst

Changes in the local electronic structure of the Mn 3d orbitals of a Mn catalyst derived from a dinuclear Mn(III) complex during the water oxidation cycle were investigated ex situ by X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) analyses. Detailed information about the Mn 3d orbitals, especially the local HOMO-LUMO gap on Mn sites revealed by RIXS analyses, indicated that the enhancement in catalytic activity (water oxidation) originated from the narrowing of the local HOMO-LUMO gap when electrical voltage and visible light illumination were applied simultaneously to the Mn catalytic system.

Nanostructured manganese oxides as highly active water oxidation catalysts: a boost from manganese precursor chemistry

We present a facile synthesis of bioinspired manganese oxides for chemical and photocatalytic water oxidation, starting from a reliable and versatile manganese(II) oxalate single-source precursor (SSP) accessible through an inverse micellar molecular approach. Strikingly, thermal decomposition of the latter precursor in various environments (air, nitrogen, and vacuum) led to the three different mineral phases of bixbyite (Mn2 O3 ), hausmannite (Mn3 O4 ), and manganosite (MnO). Initial chemical water oxidation experiments using ceric ammonium nitrate (CAN) gave the maximum catalytic activity for Mn2 O3 and MnO whereas Mn3 O4 had a limited activity. The substantial increase in the catalytic activity of MnO in chemical water oxidation was demonstrated by the fact that a phase transformation occurs at the surface from nanocrystalline MnO into an amorphous MnOx (1<x<2) upon treatment with CAN, which acted as an oxidizing agent. Photocatalytic water oxidation in the presence of [Ru(bpy)3 ](2+) (bpy=2,2'-bipyridine) as a sensitizer and peroxodisulfate as an electron acceptor was carried out for all three manganese oxides including the newly formed amorphous MnOx . Both Mn2 O3 and the amorphous MnOx exhibit tremendous enhancement in oxygen evolution during photocatalysis and are much higher in comparison to so far known bioinspired manganese oxides and calcium-manganese oxides. Also, for the first time, a new approach for the representation of activities of water oxidation catalysts has been proposed by determining the amount of accessible manganese centers.

Energy from Photosystem II: Manganese Water Oxidation Catalysts

Photosystem II is a blueprint for the design of water oxidation catalysts for incorporation into photoelectrochemical devices capable of efficient solar hydrogen production. In this chapter, we review ongoing efforts to develop manganese water oxidation catalysts. These catalytic systems embody one or more of the key features observed in the PSII water oxidizing complex – the concentration of high energy oxidation states of multiple manganese centres, the ability to facilitate di-oxygen bridge formation, a dynamic supporting environment that prevents dissociation of the complex, assists in electron and proton removal, and aids coupling to a photoactive charge separation centre – with the most successful examples incorporating most or all of these key features. Promising advances have been made towards achieving solar water oxidation, ranging from the direct coupling of Mn complexes to Ru dyes or TiO2 to demonstrate successful oxidation of Mn centers, to achieving direct light driven water oxidation by coupling a Nafion supported Mn catalysts to a Ru-dye sensitized TiO2 electrode, which should stimulate further interesting developments.

Solar Energy Utilisation

The relevant properties of, as well as subsequent factors influencing, the net solar irradiance incident at the Earth's surface are examined. The methods currently employed (and those under near term development) able to convert solar energy into other usable energy forms, such as thermal, electrical and chemical energy, are surveyed. The efficiencies and likely cost structures of these conversion processes are compared. The direct conversion of solar energy to heat is identified to be the most efficient process (&gt;70%) and employs well-established technologies which are generally cost competitive with other heat sources when sited in favourable locations (at latitudes below 45°). Additionally, the solar powered generation of electricity is shown to be of comparable cost and overall efficiency (∼20–30% of incident flux on the area utilised), whether based on the latest implementations of ‘conventional’ silicon Generation I photovoltaic technologies or high temperature concentrating solar heat capture systems when coupled to thermal engines driving electric generators. Exotic, but more efficient (&gt;30%), non silicon multi-junction (Generation II) photovoltaic technologies are further from practical application and are not cost effective unless combined with solar concentration systems. Generation III (for example organic photovoltaic) technologies hold promise for low cost/unit area but are currently less efficient (&lt;5%) than semi-conductor photovoltaics. Bio-solar approaches, employing natural photosynthetic organisms, allowing the conversion of solar energy to fuels, particularly hydrogen, have the lowest overall efficiencies of those technologies examined (∼1–2% maximum). The use of biomimetic catalysts to enhance hydrogen production using solar generated electricity is considered.