Catalysis

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Catalysis

A range of industrial catalysts in pellet form

An air filter that uses a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air.

Catalysis (/kəˈtæləsɪs/) is the increase in rate of a chemical reaction due to an added substance known as a catalyst (/ˈkætəlɪst/). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst.

The rate increase occurs because the catalyst allows the reaction to occur by an alternative mechanism which may be much faster than the noncatalyzed mechanism. However the noncatalyzed mechanism does remain possible, so that the total rate (catalyzed plus noncatalyzed) can only increase in the presence of the catalyst and never decrease.

Catalysis may be classified as either homogeneous, whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant, or heterogeneous, whose components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category.

Catalysis is ubiquitous in chemical industry of all kinds. Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.

The term "catalyst" is derived from Greek καταλύειν, kataluein, meaning "loosen" or "untie". The concept of catalysis was invented by chemist Elizabeth Fulhame, based on her novel work in oxidation-reduction experiments.

General principles

Example

An illustrative example is the effect of catalysts to speed the decomposition of hydrogen peroxide into water and oxygen:

2 H₂O₂ → 2 H₂O + O₂

This reaction proceeds because the reaction products are more stable than the starting compound, but this decomposition is so slow that hydrogen peroxide solutions are commercially available. In the presence of a catalyst such as manganese dioxide this reaction proceeds much more rapidly. This effect is readily seen by the effervescence of oxygen. The catalyst is not consumed in the reaction, and may be recovered unchanged and re-used indefinitely. Accordingly, manganese dioxide is said to catalyze this reaction. In living organisms, this reaction is catalyzed by enzymes (proteins that serve as catalysts) such as catalase.

Another example is the effect of catalysts on air pollution and reducing the amount of carbon monoxide. Development of active and selective catalysts for the conversion of carbon monoxide into desirable products is one of the most important roles of catalysts. Using catalysts for hydrogenation of carbon monoxide helps to remove this toxic gas and also attain useful materials.

Units

The SI derived unit for measuring the catalytic activity of a catalyst is the katal, which is quantified in moles per second. The productivity of a catalyst can be described by the turnover number (TON) and the catalytic activity by the turn over frequency (TOF), which is the TON per time unit. The biochemical equivalent is the enzyme unit. For more information on the efficiency of enzymatic catalysis, see the article on enzymes.

Catalytic reaction mechanisms

Main article: catalytic cycle

In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction mechanism (reaction pathway) having a lower activation energy than the noncatalyzed mechanism. In catalyzed mechanisms, the catalyst is regenerated.

As a simple example occurring in the gas phase, the reaction 2 SO₂ + O₂ → 2 SO₃ can be catalyzed by adding nitric oxide. The reaction occurs in two steps:

2 NO + O₂ → 2 NO₂ (rate-determining)

NO₂ + SO₂ → NO + SO₃ (fast)

The NO catalyst is regenerated. The overall rate is the rate of the slow step

v=2k₁[NO]²[O₂].

An example of heterogeneous catalysis is the reaction of oxygen and hydrogen on the surface of titanium dioxide (TiO₂, or titania) to produce water. Scanning tunneling microscopy showed that the molecules undergo adsorption and dissociation. The dissociated, surface-bound O and H atoms diffuse together. The intermediate reaction states are: HO₂, H₂O₂, then H₃O₂ and the reaction product (water molecule dimers), after which the water molecule desorbs from the catalyst surface.

Reaction energetics

Generic potential energy diagram showing the effect of a catalyst in a hypothetical exothermic chemical reaction X + Y to give Z. The presence of the catalyst opens a different reaction pathway (shown in red) with lower activation energy. The final result and the overall thermodynamics are the same.

Catalysts enable pathways that differ from the uncatalyzed reactions. These pathways have lower activation energy. Consequently, more molecular collisions have the energy needed to reach the transition state. Hence, catalysts can enable reactions that would otherwise be blocked or slowed by a kinetic barrier. The catalyst may increase the reaction rate or selectivity, or enable the reaction at lower temperatures. This effect can be illustrated with an energy profile diagram.

In the catalyzed elementary reaction, catalysts do not change the extent of a reaction: they have no effect on the chemical equilibrium of a reaction. The ratio of the forward and the reverse reaction rates is unaffected (see also thermodynamics). The second law of thermodynamics describes why a catalyst does not change the chemical equilibrium of a reaction. Suppose there was such a catalyst that shifted an equilibrium. Introducing the catalyst to the system would result in a reaction to move to the new equilibrium, producing energy. Production of energy is a necessary result since reactions are spontaneous only if Gibbs free energy is produced, and if there is no energy barrier, there is no need for a catalyst. Then, removing the catalyst would also result in a reaction, producing energy; i.e. the addition and its reverse process, removal, would both produce energy. Thus, a catalyst that could change the equilibrium would be a perpetual motion machine, a contradiction to the laws of thermodynamics. Thus, catalysts do not alter the equilibrium constant. (A catalyst can however change the equilibrium concentrations by reacting in a subsequent step. It is then consumed as the reaction proceeds, and thus it is also a reactant. Illustrative is the base-catalyzed hydrolysis of esters, where the produced carboxylic acid immediately reacts with the base catalyst and thus the reaction equilibrium is shifted towards hydrolysis.)

The catalyst stabilizes the transition state more than it stabilizes the starting material. It decreases the kinetic barrier by decreasing the difference in energy between starting material and the transition state. It does not change the energy difference between starting materials and products (thermodynamic barrier), or the available energy (this is provided by the environment as heat or light).

Related concepts

Some so-called catalysts are really precatalysts, which convert to catalysts in the reaction. For example, Wilkinson's catalyst RhCl(PPh₃)₃ loses one triphenylphosphine ligand before entering the true catalytic cycle. Precatalysts are easier to store but are easily activated in situ. Because of this preactivation step, many catalytic reactions involve an induction period.

In cooperative catalysis, chemical species that improve catalytic activity are called cocatalysts or promoters.

In tandem catalysis two or more different catalysts are coupled in a one-pot reaction.

In autocatalysis, the catalyst is a product of the overall reaction, in contrast to all other types of catalysis considered in this article. The simplest example of autocatalysis is a reaction of type A + B → 2 B, in one or in several steps. The overall reaction is just A → B, so that B is a product. But since B is also a reactant, it may be present in the rate equation and affect the reaction rate. As the reaction proceeds, the concentration of B increases and can accelerate the reaction as a catalyst. In effect, the reaction accelerates itself or is autocatalyzed. An example is the hydrolysis of an ester such as aspirin to a carboxylic acid and an alcohol. In the absence of added acid catalysts, the carboxylic acid product catalyzes the hydrolysis.

Switchable catalysis refers to a type of catalysis where the catalyst can be toggled between different ground states possessing distinct reactivity, typically by applying an external stimulus. This ability to reversibly switch the catalyst allows for spatiotemporal control over catalytic activity and selectivity. The external stimuli used to switch the catalyst can include changes in temperature, pH, light, electric fields, or the addition of chemical agents.

A true catalyst can work in tandem with a sacrificial catalyst. The true catalyst is consumed in the elementary reaction and turned into a deactivated form. The sacrificial catalyst regenerates the true catalyst for another cycle. The sacrificial catalyst is consumed in the reaction, and as such, it is not really a catalyst, but a reagent. For example, osmium tetroxide (OsO₄) is a good reagent for dihydroxylation, but it is highly toxic and expensive. In Upjohn dihydroxylation, the sacrificial catalyst N-methylmorpholine N-oxide (NMMO) regenerates OsO₄, and only catalytic quantities of OsO₄ are needed.

Classification

Catalysis may be classified as either homogeneous or heterogeneous. A homogeneous catalysis is one whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant's molecules. A heterogeneous catalysis is one where the reaction components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category. Similar mechanistic principles apply to heterogeneous, homogeneous, and biocatalysis.

Heterogeneous catalysis

Main article: Heterogeneous catalysis

The microporous molecular structure of the zeolite ZSM-5 is exploited in catalysts used in refineries

Zeolites are extruded as pellets for easy handling in catalytic reactors.

Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture. Important heterogeneous catalysts include zeolites, alumina, higher-order oxides, graphitic carbon, transition metal oxides, metals such as Raney nickel for hydrogenation, and vanadium(V) oxide for oxidation of sulfur dioxide into sulfur trioxide by the contact process.

Diverse mechanisms for reactions on surfaces are known, depending on how the adsorption takes place (Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen). The total surface area of a solid has an important effect on the reaction rate. The smaller the catalyst particle size, the larger the surface area for a given mass of particles.

A heterogeneous catalyst has active sites, which are the atoms or crystal faces where the substrate actually binds. Active sites are atoms but are often described as a facet (edge, surface, step, etc.) of a solid. Most of the volume but also most of the surface of a heterogeneous catalyst may be catalytically inactive. Finding out the nature of the active site is technically challenging.

For example, the catalyst for the Haber process for the synthesis of ammonia from nitrogen and hydrogen is often described as iron. But detailed studies and many optimizations have led to catalysts that are mixtures of iron-potassium-calcium-aluminum-oxide. The reacting gases adsorb onto active sites on the iron particles. Once physically adsorbed, the reagents partially or wholly dissociate and form new bonds. In this way the particularly strong triple bond in nitrogen is broken, which would be extremely uncommon in the gas phase due to its high activation energy. Thus, the activation energy of the overall reaction is lowered, and the rate of reaction increases. Another place where a heterogeneous catalyst is applied is in the oxidation of sulfur dioxide on vanadium(V) oxide for the production of sulfuric acid. Many heterogeneous catalysts are in fact nanomaterials.

Heterogeneous catalysts are typically "supported", which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes its cost. Supports prevent or minimize agglomeration and sintering of small catalyst particles, exposing more surface area, thus catalysts have a higher specific activity (per gram) on support. Sometimes the support is merely a surface on which the catalyst is spread to increase the surface area. More often, the support and the catalyst interact, affecting the catalytic reaction. Supports can also be used in nanoparticle synthesis by providing sites for individual molecules of catalyst to chemically bind. Supports are porous materials with a high surface area, most commonly alumina, zeolites, or various kinds of activated carbon. Specialized supports include silicon dioxide, titanium dioxide, calcium carbonate, and barium sulfate.

Electrocatalysts

Main article: Electrocatalyst

In the context of electrochemistry, specifically in fuel cell engineering, various metal-containing catalysts are used to enhance the rates of the half reactions that comprise the fuel cell. One common type of fuel cell electrocatalyst is based upon nanoparticles of platinum that are supported on slightly larger carbon particles. When in contact with one of the electrodes in a fuel cell, this platinum increases the rate of oxygen reduction either to water or to hydroxide or hydrogen peroxide.

Homogeneous catalysis

Main article: Homogeneous catalysis

Homogeneous catalysts function in the same phase as the reactants. Typically homogeneous catalysts are dissolved in a solvent with the substrates. One example of homogeneous catalysis involves the influence of H⁺ on the esterification of carboxylic acids, such as the formation of methyl acetate from acetic acid and methanol. High-volume processes requiring a homogeneous catalyst include hydroformylation, hydrosilylation, hydrocyanation. For inorganic chemists, homogeneous catalysis is often synonymous with organometallic catalysts. Many homogeneous catalysts are however not organometallic, illustrated by the use of cobalt salts that catalyze the oxidation of p-xylene to terephthalic acid.

Organocatalysis

Main article: Organocatalysis

Whereas transition metals sometimes attract most of the attention in the study of catalysis, small organic molecules without metals can also exhibit catalytic properties, as is apparent from the fact that many enzymes lack transition metals. Typically, organic catalysts require a higher loading (amount of catalyst per unit amount of reactant, expressed in mol% amount of substance) than transition metal(-ion)-based catalysts, but these catalysts are usually commercially available in bulk, helping to lower costs. In the early 2000s, these organocatalysts were considered "new generation" and are competitive to traditional metal(-ion)-containing catalysts.

Organocatalysts are supposed to operate akin to metal-free enzymes utilizing, e.g., noncovalent interactions such as hydrogen bonding. The discipline organocatalysis is divided into the application of covalent (e.g., proline, DMAP) and noncovalent (e.g., thiourea organocatalysis) organocatalysts referring to the preferred catalyst-substrate binding and interaction, respectively. The Nobel Prize in Chemistry 2021 was awarded jointly to Benjamin List and David W.C. MacMillan "for the development of asymmetric organocatalysis."

Photocatalysts

Main article: Photocatalysis

Photocatalysis is the phenomenon where the catalyst can receive light to generate an excited state that effect redox reactions. Singlet oxygen is usually produced by photocatalysis. Photocatalysts are components of dye-sensitized solar cells.

Enzymes and biocatalysts

Main article: Enzyme catalysis

In biology, enzymes are protein-based catalysts in metabolism and catabolism. Most biocatalysts are enzymes, but other nonprotein-based classes of biomolecules also exhibit catalytic properties including ribozymes, and synthetic deoxyribozymes.

Biocatalysts can be thought of as an intermediate between homogeneous and heterogeneous catalysts, although strictly speaking soluble enzymes are homogeneous catalysts and membrane-bound enzymes are heterogeneous. Several factors affect the activity of enzymes (and other catalysts) including temperature, pH, the concentration of enzymes, substrate, and products. A particularly important reagent in enzymatic reactions is water, which is the product of many bond-forming reactions and a reactant in many bond-breaking processes.

In biocatalysis, enzymes are employed to prepare many commodity chemicals including high-fructose corn syrup and acrylamide.

Some monoclonal antibodies whose binding target is a stable molecule that resembles the transition state of a chemical reaction can function as weak catalysts for that chemical reaction by lowering its activation energy. Such catalytic antibodies are sometimes called "abzymes".

Significance

Left: Partially caramelized cube sugar, Right: burning cube sugar with ash as catalyst

Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture. In 2005, catalytic processes generated about $900 billion in products worldwide. Catalysis is so pervasive that subareas are not readily classified. Some areas of particular concentration are surveyed below.

Energy processing

Petroleum refining makes intensive use of catalysis for alkylation, catalytic cracking (breaking long-chain hydrocarbons into smaller pieces), naphtha reforming and steam reforming (conversion of hydrocarbons into synthesis gas). Even the exhaust from the burning of fossil fuels is treated via catalysis: Catalytic converters, typically composed of platinum and rhodium, break down some of the more harmful byproducts of automobile exhaust.

2 CO + 2 NO → 2 CO₂ + N₂

With regard to synthetic fuels, an old but still important process is the Fischer–Tropsch synthesis of hydrocarbons from synthesis gas, which itself is processed via water-gas shift reactions, catalyzed by iron. The Sabatier reaction produces methane from carbon dioxide and hydrogen. Biodiesel and related biofuels require processing via both inorganic and biocatalysts.

Fuel cells rely on catalysts for both the anodic and cathodic reactions.

Catalytic heaters generate flameless heat from a supply of combustible fuel.

Bulk chemicals

Typical vanadium pentoxide catalyst used in sulfuric acid production for an intermediate reaction to convert sulfur dioxide to sulfur trioxide.

Some of the largest-scale chemicals are produced via catalytic oxidation, often using oxygen. Examples include nitric acid (from ammonia), sulfuric acid (from sulfur dioxide to sulfur trioxide by the contact process), terephthalic acid from p-xylene, acrylic acid from propylene or propane and acrylonitrile from propane and ammonia.

The production of ammonia is one of the largest-scale and most energy-intensive processes. In the Haber process nitrogen is combined with hydrogen over an iron oxide catalyst. Methanol is prepared from carbon monoxide or carbon dioxide but using copper-zinc catalysts.

Bulk polymers derived from ethylene and propylene are often prepared using Ziegler–Natta catalyst. Polyesters, polyamides, and isocyanates are derived via acid–base catalysis.

Most carbonylation processes require metal catalysts, examples include the Monsanto acetic acid process and hydroformylation.

Fine chemicals

Many fine chemicals are prepared via catalysis; methods include those of heavy industry as well as more specialized processes that would be prohibitively expensive on a large scale. Examples include the Heck reaction, and Friedel–Crafts reactions. Because most bioactive compounds are chiral, many pharmaceuticals are produced by enantioselective catalysis (catalytic asymmetric synthesis). (R)-1,2-Propandiol, the precursor to the antibacterial levofloxacin, can be synthesized efficiently from hydroxyacetone by using catalysts based on BINAP-ruthenium complexes, in Noyori asymmetric hydrogenation:

Food processing

One of the most obvious applications of catalysis is the hydrogenation (reaction with hydrogen gas) of fats using nickel catalyst to produce margarine. Many other foodstuffs are prepared via biocatalysis (see below).

Environment

Catalysis affects the environment by increasing the efficiency of industrial processes, but catalysis also plays a direct role in the environment. A notable example is the catalytic role of chlorine free radicals in the breakdown of ozone. These radicals are formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs).

Cl^· + O₃ → ClO^· + O₂

ClO^· + O^· → Cl^· + O₂

History

The term "catalyst", broadly defined as anything that increases the rate of a process, is derived from Greek καταλύειν, meaning "to annul", or "to untie", or "to pick up". The concept of catalysis was invented by chemist Elizabeth Fulhame and described in a 1794 book, based on her novel work in oxidation–reduction reactions. The first chemical reaction in organic chemistry that knowingly used a catalyst was studied in 1811 by Gottlieb Kirchhoff, who discovered the acid-catalyzed conversion of starch to glucose. The term catalysis was later used by Jöns Jakob Berzelius in 1835 to describe reactions that are accelerated by substances that remain unchanged after the reaction. Fulhame, who predated Berzelius, did work with water as opposed to metals in her reduction experiments. Other 18th century chemists who worked in catalysis were Eilhard Mitscherlich who referred to it as contact processes, and Johann Wolfgang Döbereiner who spoke of contact action. He developed Döbereiner's lamp, a lighter based on hydrogen and a platinum sponge, which became a commercial success in the 1820s that lives on today. Humphry Davy discovered the use of platinum in catalysis. In the 1880s, Wilhelm Ostwald at Leipzig University started a systematic investigation into reactions that were catalyzed by the presence of acids and bases, and found that chemical reactions occur at finite rates and that these rates can be used to determine the strengths of acids and bases. For this work, Ostwald was awarded the 1909 Nobel Prize in Chemistry. Vladimir Ipatieff performed some of the earliest industrial scale reactions, including the discovery and commercialization of oligomerization and the development of catalysts for hydrogenation.

Inhibitors, poisons, and promoters

An added substance that lowers the rate is called a reaction inhibitor if reversible and catalyst poisons if irreversible. Promoters are substances that increase the catalytic activity, even though they are not catalysts by themselves.

Inhibitors are sometimes referred to as "negative catalysts" since they decrease the reaction rate. However the term inhibitor is preferred since they do not work by introducing a reaction path with higher activation energy; this would not lower the rate since the reaction would continue to occur by the noncatalyzed path. Instead, they act either by deactivating catalysts or by removing reaction intermediates such as free radicals. In heterogeneous catalysis, coking inhibits the catalyst, which becomes covered by polymeric side products.

The inhibitor may modify selectivity in addition to rate. For instance, in the hydrogenation of alkynes to alkenes, a palladium (Pd) catalyst partly "poisoned" with lead(II) acetate (Pb(CH₃CO₂)₂) can be used (Lindlar catalyst). Without the deactivation of the catalyst, the alkene produced would be further hydrogenated to alkane.

The inhibitor can produce this effect by, e.g., selectively poisoning only certain types of active sites. Another mechanism is the modification of surface geometry. For instance, in hydrogenation operations, large planes of metal surface function as sites of hydrogenolysis catalysis while sites catalyzing hydrogenation of unsaturates are smaller. Thus, a poison that covers the surface randomly will tend to lower the number of uncontaminated large planes but leave proportionally smaller sites free, thus changing the hydrogenation vs. hydrogenolysis selectivity. Many other mechanisms are also possible.

Promoters can cover up the surface to prevent the production of a mat of coke, or even actively remove such material (e.g., rhenium on platinum in platforming). They can aid the dispersion of the catalytic material or bind to reagents.

Prebiotic catalysis in the origin of life

Life is based on an interplay between information processing and catalytic activity carried out by biological polymers. A possible evolutionary pathway for the emergence of catalytic functions in prebiotic information coding polymers was proposed. It has also been proposed that life emerged as an RNA-protein system in which the two components cross catalyzed the formation of each other.

Bioorthogonal chemistry

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Bioorthogonal_chemistry

The term bioorthogonal chemistry refers to any chemical reaction that can occur inside of living systems without interfering with native biochemical processes. The term was coined by Carolyn R. Bertozzi in 2003. Since its introduction, the concept of the bioorthogonal reaction has enabled the study of biomolecules such as glycans, proteins, and lipids in real time in living systems without cellular toxicity. A number of chemical ligation strategies have been developed that fulfill the requirements of bioorthogonality, including the 1,3-dipolar cycloaddition between azides and cyclooctynes (also termed copper-free click chemistry), between nitrones and cyclooctynes, oxime/hydrazone formation from aldehydes and ketones, the tetrazine ligation, the isocyanide-based click reaction, and most recently, the quadricyclane ligation.

Shown here is a bioorthogonal ligation between biomolecule X and reactive partner Y. To be considered bioorthogonal, these reactive partners cannot perturb other chemical functionality naturally found within the cell.

The use of bioorthogonal chemistry typically proceeds in two steps. First, a cellular substrate is modified with a bioorthogonal functional group (chemical reporter) and introduced to the cell; substrates include metabolites, enzyme inhibitors, etc. The chemical reporter must not alter the structure of the substrate dramatically to avoid affecting its bioactivity. Secondly, a probe containing the complementary functional group is introduced to react and label the substrate.

Although effective bioorthogonal reactions such as copper-free click chemistry have been developed, development of new reactions continues to generate orthogonal methods for labeling to allow multiple methods of labeling to be used in the same biosystems. Carolyn R. Bertozzi was awarded the Nobel Prize in Chemistry in 2022 for her development of click chemistry and bioorthogonal chemistry.

Etymology

The word bioorthogonal comes from Greek bio- "living" and orthogōnios "right-angled". Thus literally a reaction that goes perpendicular to a living system, thus not disturbing it.

Requirements for bioorthogonality

To be considered bioorthogonal, a reaction must fulfill a number of requirements:

• Selectivity: The reaction must be selective between endogenous functional groups to avoid side reactions with biological compounds
• Biological inertness: Reactive partners and resulting linkage should not possess any mode of reactivity capable of disrupting the native chemical functionality of the organism under study.
• Chemical inertness: The covalent link should be strong and inert to biological reactions.
• Kinetics: The reaction must be rapid so that covalent ligation is achieved prior to probe metabolism and clearance. The reaction must be fast, on the time scale of cellular processes (minutes) to prevent competition in reactions which may diminish the small signals of less abundant species. Rapid reactions also offer a fast response, necessary in order to accurately track dynamic processes.
• Reaction biocompatibility: Reactions have to be non-toxic and must function in biological conditions taking into account pH, aqueous environments, and temperature. Pharmacokinetics are a growing concern as bioorthogonal chemistry expands to live animal models.
• Accessible engineering: The chemical reporter must be capable of incorporation into biomolecules via some form of metabolic or protein engineering. Optimally, one of the functional groups is also very small so that it does not disturb native behavior.

Staudinger ligation

The Staudinger ligation is a reaction developed by the Bertozzi group in 2000 that is based on the classic Staudinger reaction of azides with triarylphosphines. It launched the field of bioorthogonal chemistry as the first reaction with completely abiotic functional groups although it is no longer as widely used. The Staudinger ligation has been used in both live cells and live mice.

Bioorthogonality

The azide can act as a soft electrophile that prefers soft nucleophiles such as phosphines. This is in contrast to most biological nucleophiles which are typically hard nucleophiles. The reaction proceeds selectively under water-tolerant conditions to produce a stable product.

Phosphines are completely absent from living systems and do not reduce disulfide bonds despite mild reduction potential. Azides had been shown to be biocompatible in FDA-approved drugs such as azidothymidine and through other uses as cross linkers. Additionally, their small size allows them to be easily incorporated into biomolecules through cellular metabolic pathways.

Mechanism

Classic Staudinger reaction

The nucleophilic phosphine attacks the azide at the electrophilic terminal nitrogen. Through a four-membered transition state, N₂ is lost to form an aza-ylide. The unstable ylide is hydrolyzed to form phosphine oxide and a primary amine. However, this reaction is not immediately bioorthogonal because hydrolysis breaks the covalent bond in the aza-ylide.

Staudinger ligation

The reaction was modified to include an ester group ortho to the phosphorus atom on one of the aryl rings to direct the aza-ylide through a new path of reactivity in order to outcompete immediate hydrolysis by positioning the ester to increase local concentration. The initial nucleophilic attack on the azide is the rate-limiting step. The ylide reacts with the electrophilic ester trap through intramolecular cyclization to form a five-membered ring. This ring undergoes hydrolysis to form a stable amide bond.

Limitations

The phosphine reagents slowly undergo air oxidation in living systems. Additionally, it is likely that they are metabolized in vitro by cytochrome P450 enzymes.

The kinetics of the reactions are slow with second order rate constants around 0.0020 M⁻¹•s⁻¹. Attempts to increase nucleophilic attack rates by adding electron-donating groups to the phosphines improved kinetics, but also increased the rate of air oxidation.

The poor kinetics require that high concentrations of the phosphine be used which leads to problems with high background signal in imaging applications. Attempts have been made to combat the problem of high background through the development of a fluorogenic phosphine reagents based on fluorescein and luciferin, but the intrinsic kinetics remain a limitation.

Copper-free click chemistry

Main article: Copper-free click chemistry

Copper-free click chemistry is a bioorthogonal reaction first developed by Carolyn Bertozzi as an activated variant of an azide alkyne Huisgen cycloaddition, based on the work by Karl Barry Sharpless et al. Unlike CuAAC, Cu-free click chemistry has been modified to be bioorthogonal by eliminating a cytotoxic copper catalyst, allowing reaction to proceed quickly and without live cell toxicity. Instead of copper, the reaction is a strain-promoted alkyne-azide cycloaddition (SPAAC). It was developed as a faster alternative to the Staudinger ligation, with the first generations reacting over sixty times faster. The bioorthogonality of the reaction has allowed the Cu-free click reaction to be applied within cultured cells, live zebrafish, and mice.

Copper toxicity

The classic copper-catalyzed azide-alkyne cycloaddition has been an extremely fast and effective click reaction for bioconjugation, but it is not suitable for use in live cells due to the toxicity of Cu(I) ions. Toxicity is due to oxidative damage from reactive oxygen species formed by the copper catalysts. Copper complexes have also been found to induce changes in cellular metabolism and are taken up by cells.

There has been some development of ligands to prevent biomolecule damage and facilitate removal in in vitro applications. However, it has been found that different ligand environments of complexes can still affect metabolism and uptake, introducing an unwelcome perturbation in cellular function.

Bioorthogonality

The azide group is particularly bioorthogonal because it is extremely small (favorable for cell permeability and avoids perturbations), metabolically stable, and does not naturally exist in cells and thus has no competing biological side reactions. Although azides are not the most reactive 1,3-dipole available for reaction, they are preferred for their relative lack of side reactions and stability in typical synthetic conditions. The alkyne is not as small, but it still has the stability and orthogonality necessary for in vivo labeling. Cyclooctynes are traditionally the most common cycloalkyne for labeling studies, as they are the smallest stable alkyne ring.

Mechanism

The reaction proceeds as a standard 1,3-dipolar cycloaddition, a type of asynchronous, concerted pericyclic shift. The ambivalent nature of the 1,3-dipole should make the identification of an electrophilic or nucleophilic center on the azide impossible such that the direction of the cyclic electron flow is meaningless. [p] However, computation has shown that the electron distribution amongst nitrogens causes the innermost nitrogen atom to bear the greatest negative charge.

Regioselectivity

Although the reaction produces a regioisomeric mixture of triazoles, the lack of regioselectivity in the reaction is not a major concern for most current applications. More regiospecific and less bioorthogonal requirements are best served by copper-catalyzed Huisgen cycloaddition, especially given the synthetic difficulty (compared to the addition of a terminal alkyne) of synthesizing a strained cyclooctyne.

Development of cyclooctynes

Cyclooctyne	Second order rate constant (M−1s−1)
OCT	0.0024
ALO	0.0013
MOFO	0.0043
DIFO	0.076
DIBO	0.057
BARAC	0.96
DIBAC (ADIBO)	0.31
DIMAC	0.0030

OCT was the first cyclooctyne developed for Cu-free click chemistry. While linear alkynes are unreactive at physiological temperatures, OCT was able readily react with azides in biological conditions while showing no toxicity. However, it was poorly water-soluble, and the kinetics were barely improved over the Staudinger ligation. ALO (aryl-less octyne) was developed to improve water solubility, but it still had poor kinetics.

Monofluorinated (MOFO) and difluorinated (DIFO) cyclooctynes were created to increase the rate through the addition of electron-withdrawing fluorine substituents at the propargylic position. Fluorine is a good electron-withdrawing group in terms of synthetic accessibility and biological inertness. In particular, it cannot form an electrophilic Michael acceptor that may side-react with biological nucleophiles. DIBO (dibenzocyclooctyne) was developed as a fusion to two aryl rings, resulting in very high strain and a decrease in distortion energies. It was proposed that biaryl substitution increases ring strain and provides conjugation with the alkyne to improve reactivity. Although calculations have predicted that mono-aryl substitution would provide an optimal balance between steric clash (with azide molecule) and strain, monoarylated products have been shown to be unstable.

BARAC (biarylazacyclooctynone) followed with the addition of an amide bond which adds an sp²-like center to increase rate by distortion. Amide resonance contributes additional strain without creating additional unsaturation which would lead to an unstable molecule. Additionally, the addition of a heteroatom into the cyclooctyne ring improves both solubility and pharmacokinetics of the molecule. BARAC has sufficient rate (and sensitivity) to the extent that washing away excess probe is unnecessary to reduce background. This makes it extremely useful in situations where washing is impossible as in real-time imaging or whole animal imaging. Although BARAC is extremely useful, its low stability requires that it must be stored at 0 °C, protected from light and oxygen.

The synthesis was designed by the Bertozzi group as a modular route to facilitate future modifications in SAR analysis. The first step is Fischer indole synthesis. The product is alkylated with allyl bromide as a handle for future probe attachment; TMS is then added. Oxidation opens the central rings to form a cyclic amide. The ketone is treated as an enolate to add a triflate group. Reaction of the terminal alkene generates a linker for conjugation to a molecule. The final reaction with CsF introduces the strained alkyne at the last step.

Further adjustments variations on BARAC to produce DIBAC/ADIBO were performed to add distal ring strain and reduce sterics around the alkyne to further increase reactivity. Keto-DIBO, in which the hydroxyl group has been converted to a ketone, has a three-fold increase in rate due to a change in ring conformation. Attempts to make a difluorobenzocyclooctyne (DIFBO) were unsuccessful due to the instability.

Problems with DIFO with in vivo mouse studies illustrate the difficulty of producing bioorthogonal reactions. Although DIFO was extremely reactive in the labeling of cells, it performed poorly in mouse studies due to binding with serum albumin. Hydrophobicity of the cyclooctyne promotes sequestration by membranes and serum proteins, reducing bioavailable concentrations. In response, DIMAC (dimethoxyazacyclooctyne) was developed to increase water solubility, polarity, and pharmacokinetics, although efforts in bioorthogonal labeling of mouse models is still in development.

Reactivity

Computational efforts have been vital in explaining the thermodynamics and kinetics of these cycloaddition reactions which has played a vital role in continuing to improve the reaction. There are two methods for activating alkynes without sacrificing stability: decrease transition state energy or decrease reactant stability.

The red arrow shows the direction of energy change. Black arrows show the difference in activation energy before and after the effects.

Decreasing reactant stability: Houk has proposed that differences in the energy (E_d ^‡) required to distort the azide and alkyne into the transition state geometries control the barrier heights for the reaction. The activation energy (E ^‡) is the sum of destabilizing distortions and stabilizing interactions (E_i ^‡). The most significant distortion is in the azide functional group with lesser contribution of alkyne distortion. However, it is only the cyclooctyne that can be easily modified for higher reactivity. Calculated barriers of reaction for phenyl azide and acetylene (16.2 kcal/mol) versus cyclooctyne (8.0 kcal/mol) results in a predicted rate increase of 10⁶. The cyclooctyne requires less distortion energy (1.4 kcal/mol versus 4.6 kcal/mol) resulting in a lower activation energy despite smaller interaction energy.

Relationship between activation energy, distortion energy, and interaction energy

Decreasing transition state energy: Electron withdrawing groups such as fluorine increase rate by decreasing LUMO energy and the HOMO-LUMO gap. This leads to a greater charge transfer from the azide to the fluorinated cyclooctyne in the transition state, increasing interaction energy (lower negative value) and overall activation energy. The lowering of the LUMO is the result of hyperconjugation between alkyne π donor orbitals and CF σ* acceptors. These interactions provide stabilization primarily in the transition state as a result of increased donor/acceptor abilities of the bonds as they distort. NBO calculations have shown that transition state distortion increases the interaction energy by 2.8 kcal/mol.

The hyperconjugation between out-of-plane π bonds is greater because the in-plane π bonds are poorly aligned. However, transition state bending allows the in-plane π bonds to have a more antiperiplanar arrangement that facilitates interaction. Additional hyperconjugative interaction energy stabilization is achieved through an increase in the electronic population of the σ* due to the forming CN bond. Negative hyperconjugation with the σ* CF bonds enhances this stabilizing interaction.

Regioselectivity

Although regioselectivity is not a great issue in the current imaging applications of copper-free click chemistry, it is an issue that prevents future applications in fields such as drug design or peptidomimetics.

Currently most cyclooctynes react to form regioisomeric mixtures. [m] Computation analysis has found that while gas phase regioselectivity is calculated to favor 1,5 addition over 1,4 addition by up to 2.9 kcal/mol in activation energy, solvation corrections result in the same energy barriers for both regioisomers. While the 1,4 isomer in the cycloaddition of DIFO is disfavored by its larger dipole moment, solvation stabilizes it more strongly than the 1,5 isomer, eroding regioselectivity.

Symmetrical cyclooctynes such as BCN (bicyclo[6.1.0]nonyne) form a single regioisomer upon cycloaddition and may serve to address this problem in the future.

Applications

The most widespread application of copper-free click chemistry is in biological imaging in live cells or animals using an azide-tagged biomolecule and a cyclooctyne bearing an imaging agent.

Fluorescent keto and oxime variants of DIBO are used in fluoro-switch click reactions in which the fluorescence of the cyclooctyne is quenched by the triazole that forms in the reaction. On the other hand, coumarin-conjugated cyclooctynes such as coumBARAC have been developed such that the alkyne suppresses fluorescence while triazole formation increases the fluorescence quantum yield by ten-fold.

coumBARAC fluorescence increases with reaction

Spatial and temporal control of substrate labeling has been investigated using photoactivatable cyclooctynes. This allows equilibration of the alkyne prior to reaction in order to reduce artifacts as a result of concentration gradients. Masked cyclooctynes are unable to react with azides in the dark but become reactive alkynes upon irradiation with light.

Copper-free click chemistry is being explored for use in synthesizing PET imaging agents which must be made quickly with high purity and yield in order to minimize isotopic decay before the compounds can be administered. Both the high rate constants and the bioorthogonality of SPAAC are amenable to PET chemistry.

Other bioorthogonal reactions

Nitrone dipole cycloaddition

Copper-free click chemistry has been adapted to use nitrones as the 1,3-dipole rather than azides and has been used in the modification of peptides.

This cycloaddition between a nitrone and a cyclooctyne forms N-alkylated isoxazolines. The reaction rate is enhanced by water and is extremely fast with second order rate constants ranging from 12 to 32 M⁻¹•s⁻¹, depending on the substitution of the nitrone. Although the reaction is extremely fast, it faces problems in incorporating the nitrone into biomolecules through metabolic labeling. Labeling has only been achieved through post-translational peptide modification.

Norbornene cycloaddition

1,3 dipolar cycloadditions have been developed as a bioorthogonal reaction using a nitrile oxide as a 1,3-dipole and a norbornene as a dipolarophile. Its primary use has been in labeling DNA and RNA in automated oligonucleotide synthesizers, and polymer crosslinking in the presence of living cells.

Norbornenes were selected as dipolarophiles due to their balance between strain-promoted reactivity and stability. The drawbacks of this reaction include the cross-reactivity of the nitrile oxide due to strong electrophilicity and slow reaction kinetics.

Oxanorbornadiene cycloaddition

The oxanorbornadiene cycloaddition is a 1,3-dipolar cycloaddition followed by a retro-Diels Alder reaction to generate a triazole-linked conjugate with the elimination of a furan molecule. Preliminary work has established its usefulness in peptide labeling experiments, and it has also been used in the generation of SPECT imaging compounds. More recently, the use of an oxanorbornadiene was described in a catalyst-free room temperature "iClick" reaction, in which a model amino acid is linked to the metal moiety, in a novel approach to bioorthogonal reactions.

Ring strain and electron deficiency in the oxanorbornadiene increase reactivity towards the cycloaddition rate-limiting step. The retro-Diels Alder reaction occurs quickly afterwards to form the stable 1,2,3 triazole. Problems include poor tolerance for substituents which may change electronics of the oxanorbornadiene and low rates (second order rate constants on the order of 10⁻⁴).

Tetrazine ligation

The tetrazine ligation is the reaction of a trans-cyclooctene and an s-tetrazine in an inverse-demand Diels Alder reaction followed by a retro-Diels Alder reaction to eliminate nitrogen gas. The reaction is extremely rapid with a second order rate constant of 2000 M⁻¹–s⁻¹ (in 9:1 methanol/water) allowing modifications of biomolecules at extremely low concentrations.

Based on computational work by Bach, the strain energy for Z-cyclooctenes is 7.0 kcal/mol compared to 12.4 kcal/mol for cyclooctane due to a loss of two transannular interactions. E-cyclooctene has a highly twisted double bond resulting in a strain energy of 17.9 kcal/mol. As such, the highly strained trans-cyclooctene is used as a reactive dienophile. The diene is a 3,6-diaryl-s-tetrazine which has been substituted in order to resist immediate reaction with water. The reaction proceeds through an initial cycloaddition followed by a reverse Diels Alder to eliminate N₂ and prevent reversibility of the reaction.

Not only is the reaction tolerant of water, but it has been found that the rate increases in aqueous media. Reactions have also been performed using norbornenes as dienophiles at second order rates on the order of 1 M⁻¹•s⁻¹ in aqueous media. The reaction has been applied in labeling live cells and polymer coupling.

[4+1] Cycloaddition

This isocyanide click reaction is a [4+1] cycloaddition followed by a retro-Diels Alder elimination of N₂.

The reaction proceeds with an initial [4+1] cycloaddition followed by a reversion to eliminate a thermodynamic sink and prevent reversibility. This product is stable if a tertiary amine or isocyanopropanoate is used. If a secondary or primary isocyanide is used, the produce will form an imine which is quickly hydrolyzed.

Isocyanide is a favored chemical reporter due to its small size, stability, non-toxicity, and absence in mammalian systems. However, the reaction is slow, with second order rate constants on the order of 10⁻² M⁻¹•s⁻¹.

Tetrazole photoclick chemistry

Photoclick chemistry utilizes a photoinduced cycloelimination to release N₂. This generates a short-lived 1,3 nitrile imine intermediate via the loss of nitrogen gas, which undergoes a 1,3-dipolar cycloaddition with an alkene to generate pyrazoline cycloadducts.

Photoinduction takes place with a brief exposure to light (wavelength is tetrazole-dependent) to minimize photodamage to cells. The reaction is enhanced in aqueous conditions and generates a single regioisomer.

The transient nitrile imine is highly reactive for 1,3-dipolar cycloaddition due to a bent structure which reduces distortion energy. Substitution with electron-donating groups on phenyl rings increases the HOMO energy, when placed on the 1,3 nitrile imine and increases the rate of reaction.

Advantages of this approach include the ability to spatially or temporally control reaction and the ability to incorporate both alkenes and tetrazoles into biomolecules using simple biological methods such as genetic encoding. Additionally, the tetrazole can be designed to be fluorogenic in order to monitor progress of the reaction.

Quadricyclane ligation

The quadricyclane ligation utilizes a highly strained quadricyclane to undergo [2+2+2] cycloaddition with π systems.

Quadricyclane is abiotic, unreactive with biomolecules (due to complete saturation), relatively small, and highly strained (~80 kcal/mol). However, it is highly stable at room temperature and in aqueous conditions at physiological pH. It is selectively able to react with electron-poor π systems but not simple alkenes, alkynes, or cyclooctynes.

Bis(dithiobenzil)nickel(II) was chosen as a reaction partner out of a candidate screen based on reactivity. To prevent light-induced reversion to norbornadiene, diethyldithiocarbamate is added to chelate the nickel in the product.

These reactions are enhanced by aqueous conditions with a second order rate constant of 0.25 M⁻¹•s⁻¹. Of particular interest is that it has been proven to be bioorthogonal to both oxime formation and copper-free click chemistry.

Uses

Bioorthogonal chemistry is an attractive tool for pretargeting experiments in nuclear imaging and radiotherapy.

Artificial enzyme

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Artificial_enzyme 

Schematic drawing of artificial phosphorylase

An artificial enzyme is a synthetic organic molecule or ion that recreates one or more functions of an enzyme. It seeks to deliver catalysis at rates and selectivity observed in naturally occurring enzymes.

History

Enzyme catalysis of chemical reactions occur with high selectivity and rate. The substrate is activated in a small part of the enzyme's macromolecule called the active site. There, the binding of a substrate close to functional groups in the enzyme causes catalysis by so-called proximity effects. It is possible to create similar catalysts from small molecules by combining substrate-binding with catalytic functional groups. Classically, artificial enzymes bind substrates using receptors such as cyclodextrin, crown ethers, and calixarene.

Artificial enzymes based on amino acids or peptides have expanded the field of artificial enzymes or enzyme mimics. For instance, scaffolded histidine residues mimic certain metalloproteins and enzymes such as hemocyanin, tyrosinase, and catechol oxidase.

Artificial enzymes have been designed from scratch via a computational strategy using Rosetta. A December 2014 publication reported active enzymes made from molecules that do not occur in nature. In 2016, a book chapter entitled "Artificial Enzymes: The Next Wave" was published.

Nanozymes

Nanozymes are nanomaterials with enzyme-like characteristics. They have been explored for applications such as biosensing, bioimaging, tumor diagnosis and therapy, and anti-biofouling.

1990s

In 1996 and 1997, Dugan et al. discovered superoxide dismutase (SOD)-mimicking activities of fullerene derivatives.

2000s

The term "nanozyme" was coined in 2004 by Flavio Manea, Florence Bodar Houillon, Lucia Pasquato, and Paolo Scrimin. A 2005 review article attributed this term to "analogy with the activity of catalytic polymers (synzymes)", based on the "outstanding catalytic efficiency of some of the functional nanoparticles synthesized". In 2006, nanoceria (CeO₂ nanoparticles) was reported to prevent retinal degeneration induced by intracellular peroxides (toxic reactive oxygen intermediates) in rat. This was seen as indicating a possible route to a treatment for certain causes of blindness. In 2007 intrinsic peroxidase-like activity of ferromagnetic nanoparticles was reported by Yan Xiyun and coworkers as suggesting a wide range of applications in, for example, medicine and environmental chemistry, and the authors designed an immunoassay based on this property. Hui Wei and Erkang Wang then (2008) used this property of easily prepared magnetic nanoparticles to demonstrate analytical applications to bioactive molecules, describing a colorimetric assay for hydrogen peroxide (H
₂O
₂) and a sensitive and selective platform for glucose detection.

2010s

As of 2016, many review articles have appeared. A book-length treatment appeared in 2015, described as providing "a broad portrait of nanozymes in the context of artificial enzyme research", and a 2016 Chinese book on enzyme engineering included a chapter on nanozymes.

Colorimetric applications of peroxidase mimesis in different preparations were reported in 2010 and 2011, detecting, respectively, glucose (via carboxyl-modified graphene oxide) and single-nucleotide polymorphisms (in a label-free method relying on hemin−graphene hybrid nanosheets), with advantages in both cost and convenience. A use of colour to visualise tumour tissues was reported in 2012, using the peroxidase mimesis of magnetic nanoparticles coated with a protein that recognises cancer cells and binds to them.

Also in 2012, nanowires of vanadium pentoxide (vanadia, V₂O₅) were shown to suppress marine biofouling by mimicry of vanadium haloperoxidase, with anticipated ecological benefits. A study at a different centre two years later reported V₂O₅ showing mimicry of glutathione peroxidase in vitro in mammalian cells, suggesting future therapeutic application. The same year, a carboxylated fullerene dubbed C3 was reported to be neuroprotective in a primate model of Parkinson's disease.

In 2015, a supramolecular nanodevice was proposed for bioorthogonal regulation of a transitional metal nanozyme, based on encapsulating the nanozyme in a monolayer of hydrophilic gold nanoparticles, alternately isolating it from the cytoplasm or allowing access according to a gatekeeping receptor molecule controlled by competing guest species; the device, aimed at imaging and therapeutic applications, is of biomimetic size and was successful within the living cell, controlling pro-fluorophore and prodrug activation. An easy means of producing Cu(OH)
₂ supercages was reported, along with a demonstration of their intrinsic peroxidase mimicry. A scaffolded "INAzyme" ("integrated nanozyme") arrangement was described, locating hemin (a peroxidase mimic) with glucose oxidase (GOx) in sub-micron proximity, providing a fast and efficient enzyme cascade reported as monitoring cerebral brain-cell glucose dynamically in vivo. A method of ionising hydrophobe-stabilised colloid nanoparticles was described, with confirmation of their enzyme mimicry in aqueous dispersion. De novo designed metallopeptides with self-assembling properties carry out the oxidation reaction of dimethoxyphenol.

Field trials in West Africa were announced of a magnetic nanoparticle–amplified rapid low-cost strip test for Ebola virus. H
₂O
₂ was reported as displacing label DNA, adsorbed to nanoceria, into solution, where it fluoresces, providing a highly sensitive glucose test. Oxidase-like nanoceria was used for developing self-regulated bioassays. Multi-enzyme mimicking Prussian blue was developed for therapeutics. A review on metal organic framework (MOF)-based enzyme mimics was published. Histidine was used to modulate iron oxide nanoparticles' peroxidase-mimicking activities. Gold nanoparticles' peroxidase-mimicking activities were modulated via a supramolecular strategy for cascade reactions. A molecular imprinting strategy was developed to improve the selectivity of Fe₃O₄ nanozymes with peroxidase-like activity. A new strategy was developed to enhance the peroxidase-mimicking activity of gold nanoparticles by using hot electrons. Researchers designed gold nanoparticle–based integrative nanozymes with both surface-enhanced Raman scattering and peroxidase-mimicking activities for measuring glucose and lactate in living tissues. Cytochrome c oxidase mimicking activity of Cu₂O nanoparticles was modulated by receiving electrons from cytochrome c. Fe₃O₄ nanoparticles were combined with glucose oxidase for tumor therapeutics. Manganese dioxide nanozymes were used as cytoprotective shells. An Mn₃O₄ nanozyme for Parkinson's disease (cellular model) was reported. Heparin elimination in live rats was monitored with two-dimensional MOF-based peroxidase mimics and AG73 peptide. Glucose oxidase and iron oxide nanozymes were encapsulated within multi-compartmental hydrogels for incompatible tandem reactions. A cascade nanozyme biosensor was developed for detection of viable Enterobacter sakazakii. An integrated nanozyme of GOx@ZIF-8(NiPd) was developed for tandem catalysis. Charge-switchable nanozymes were developed. Site-selective RNA splicing nanozyme was developed. A nanozymes special issue in Progress in Biochemistry and Biophysics was published. Mn₃O₄ nanozymes with the ability to scavenge reactive oxygen species were developed and showed in vivo anti-inflammatory activity. A proposal entitled "A Step into the Future – Applications of Nanoparticle Enzyme Mimics" was presented. Facet-dependent oxidase and peroxidase-like activities of palladium nanoparticles were reported. Au@Pt multibranched nanostructures as bifunctional nanozymes were developed. Ferritin-coated carbon nanozymes were developed for tumor catalytic therapy. CuO nanozymes were developed to kill bacteria in a light-controlled manner. Enzymatic activity of oxygenated CNT was studied. Nanozymes were used to catalyze the oxidation of L-tyrosine and L-phenylalanine to dopachrome. Nanozymes were presented as an emerging alternative to natural enzyme for biosensing and immunoassays. A standardized assay was proposed for peroxidase-like nanozymes. Semiconductor quantum dots were utilized as nucleases for site-selective photoinduced cleavage of DNA. Two-dimensional MOF nanozyme-based sensor arrays were constructed for detecting phosphates and probing their enzymatic hydrolysis. Nitrogen-doped carbon nanomaterials as specific peroxidase mimics were reported. Nanozyme sensor arrays were developed to detect analytes from small molecules to proteins and cells. A copper oxide nanozyme for Parkinson's disease was reported. Exosome-like nanozyme vesicles for tumor imaging were developed. A comprehensive review on nanozymes was published by Chemical Society Reviews. A progress report on nanozymes was published. eg occupancy as an effective descriptor was developed for the catalytic activity of perovskite oxide–based peroxidase mimics. A Chemical Reviews paper on nanozymes was published. A single-atom strategy was used to develop nanozymes. A nanozyme for metal-free bioinspired cascade photocatalysis was reported. Chemical Society Reviews published a tutorial review on nanozymes. Cascade nanozyme reactions to fix CO₂ were reported. Peroxidase-like gold nanoclusters were used to monitor renal clearance. A copper–carbon hybrid nanozyme was developed for antibacterial therapy. A ferritin nanozyme was developed to treat cerebral malaria. Accounts of Chemical Research reviewed nanozymes. A new strategy called strain effect was developed to modulate metal nanozyme activity. Prussian blue nanozymes were used to detect hydrogen sulfide in the brains of living rats. Photolyase-like CeO₂ was reported. An editorial on nanozymes titled "Can Nanozymes Have an Impact on Sensing?" was published.

2020s

A single-atom nanozyme was developed for sepsis management. Self-assembled single-atom nanozyme was developed for photodynamic therapy of tumors. An ultrasound-switchable nanozyme against multidrug-resistant bacterial infection was reported. A nanozyme-based H₂O₂ homeostasis disruptor for chemodynamic tumor therapy was reported. An iridium oxide nanozyme for cascade reaction was developed for tumor therapy. A book entitled Nanozymology was published. A free radical–scavenging nanosponge was engineered for ischemic stroke. A minireview was published on gold-conjugate-based nanozymes. SnSe nanosheets as dehydrogenase mimics were developed. A carbon dot–based topoisomerase I mimic was reported to cleave DNA. Nanozyme sensor arrays were developed to detect pesticides. Bioorthogonal nanozymes were used to treat bacterial biofilms. A rhodium nanozyme was developed for treat colon disease. A Fe-N-C nanozyme was developed to study drug–drug interactions. A polymeric nanozyme was developed for second near-infrared photothermal cancer ferrotherapy. A Cu5.4O nanozyme was reported for anti-inflammation therapy. A CeO₂@ZIF-8 nanozyme was developed to treat reperfusion-induced injury in ischemic stroke. Peroxidase-like activity of Fe₃O₄ was explored to study the electrocatalytic kinetics at the single-molecule/single-particle level. A Cu-TA nanozyme was fabricated to scavenge reactive oxygen species from cigarette smoke. A metalloenzyme-like copper nanocluster was reported to have anticancer and imaging activities simultaneously. An integrated nanozyme was developed for anti-inflammation therapy. Enhanced enzyme-like catalytic activity was reported under non-equilibrium conditions for gold nanozymes. A density functional theory method was proposed to predict the activities of peroxidase-like nanozymes. A hydrolytic nanozyme was developed to construct an immunosensor. An orally administered nanozyme was developed for inflammatory bowel disease therapy. A ligand-dependent activity engineering strategy was reported to develop a glutathione peroxidase–mimicking MIL-47(V) metal–organic framework nanozyme for therapy. A single-site nanozyme was developed for tumor therapy. A SOD-like nanozyme was developed to regulate the mitochondria and neural cell function. A Pd12 coordination cage as a photoregulated oxidase-like nanozyme was developed. An NADPH oxidase-like nanozyme was developed. A catalase-like nanozyme was developed for tumor therapy. A defect-rich adhesive molybdenum disulfide/reduced graphene oxide nanozyme was developed for anti-bacterial activity. A MOF@COF nanozyme was developed for anti-bacterial activity. Plasmonic nanozymes were reported. Tumor microenvironment–responsive nanozyme was developed for tumor therapy. A protein-engineering-inspired method was developed to design highly active nanozymes. An editorial on nanozymes definition was published. A nanozyme therapy for hyperuricemia and ischemic stroke was developed. Chemistry World published a perspective on artificial enzymes and nanozymes. A review on single-atom catalysts, including single-atom nanozymes, was published. Peroxidase-like mixed-FeCo-oxide-based surface-textured nanostructures (MTex) were used for biofilm eradication. A nanozyme with better kinetics than natural peroxidase was developed. A self-protecting nanozyme was developed for Alzheimer's disease. CuSe nanozymes was developed to treat Parkinson's disease. A nanocluster-based nanozyme was developed. Glucose oxidase–like gold nanoparticles combined with cyclodextran were used for chiral catalysis. An artificial binuclear copper monooxygenase in a MOF was developed. A review on highly efficient design of nanozymes was published. Ni–Pt peroxidase mimics were developed for bioanalysis. A POM-based nanozyme was reported to protect cells from reactive oxygen species. A gating strategy was used to prepare selective nanozymes. A manganese single-atom nanozyme was developed for tumor therapy. A pH-responsive oxidase-like graphitic nanozyme was developed for selective killing of Helicobacter pylori. An engineered FeN₃P-centred single-atom nanozyme was developed. Peroxidase- and catalase-like activities of gold nanozymes were modulated. Graphdiyne–cerium oxide nanozymes were developed for radiotherapy of esophageal cancer. Defect engineering was used to develop nanozyme for tumor therapy. A book entitled Nanozymes for Environmental Engineering was published. A palladium single-atom nanozyme was developed for tumor therapy. A horseradish peroxidase–like nanozyme was developed for tumor therapy. The mechanism of a GOx-like nanozyme was reported. A review on nanozymes was published. A mechanism study on nanonuclease-like nanozyme was reported. A perspective on nanozyme definition was published. Aptananozymes were developed. Ceria nanozyme loaded microneedles helped hair regrowth. A catalase-like platinum nanozyme was used for small extracellular vesicles analysis. A book on Nanozymes: Advances and Applications was published by CRC Press. A review on nanozyme catalytic turnover was published. A nanozyme was developed for ratiometric molecular imaging. A Fe₃O₄/Ag/Bi₂MoO₆ photoactivatable nanozyme was developed for cancer therapy. Co/C as an NADH oxidase mimic was reported. An iron oxide nanozyme was used to target biofilms causing tooth decay. A new strategy for high-performance nanozymes was developed. A high-throughput computational screening strategy was developed to discover SOD-like nanozymes. A review paper entitled "Nanozyme-Enabled Analytical Chemistry" was published in Analytical Chemistry. A nanozyme-based therapy for gout was reported. A data-informed strategy for discovery of nanozymes was reported. Prussian blue nanozyme was used to alleviates neurodegeneration. A dual element single-atom nanozyme was developed. A valence-engineered method was developed to design antioxidant banozyme for biomedical applications. Combined with small interfering RNA, ceria nanozyme was used for synergistic treatment of neurodegenerative diseases. A universal assay for catalase-like nanozymes was reported. A nanozyme catalyzed CRISPR assay was developed. A nanozyme-based tumor-specific photo-enhanced catalytic therapy was developed. Single-atom nanozymes for brain trauma therapy were reported. An edge engineering strategy was developed to fabriacte single atom nanozymes. A single atom nanozyme was developed to modulate tumor microenvironment for therapy. A new mechanism for peroxidase-like Fe3O4 was proposed. A plant virus cleaving nanozyme was reported. Nanozymes is selected as one of the IUPAC Top Ten Emerging Technologies in Chemistry 2022. A book entitled "Nanozymes: Design, Synthesis, and Applications" was published by ACS. Nanozymes were used to remove and degrade microplastics. A cold-adapted nanozyme was reported. A MOF-818 nanozyme with antioxidase-mimicking activities was used to treat diabetic chronic wounds. Cu single-atom nanozymes were developed for catalytic tumor-specific therapy. Machine learning was employed to search for nanozymes. Enzyme-like meso-bacroporous carbon sphere was developed. A combination of DNAzyme and nanozyme was reported. A peroxidase-like photoexcited Ru single-atom nanozyme was reported. A probiotic nanozyme hydrogel for Candida vaginitis therapy was developed. A method to determine the maximum velocity of a peroxidase-like nanozyme was proposed. Antisenescence nanozymes for atherosclerosis therapy were reported. A book entitled 'Biomedical Nanozymes: From Diagnostics to Therapeutics' was published by Springer. 2023 Dalton Division Horizon Prize was awarded to High-Performance Nanozyme Designer. Nanozyme-cosmetic contact lenses were developed. Biogenic ferritins act as natural nanozymes were reported. An integrated computational and experimental framework for inverse screening of nanozymes was developed. A diatomic iron nanozyme was reported. Mechanism of carbon dot-based SOD-like nanozyme was studied. A hybrid ceria nanozyme was developed for arthritis therapy. A chiral nanozyme was reported for Parkinson's disease. A dimensionality-engineered single-atom nanozyme was reported. A liposome-base nanozyme was developed to treat infected diabetic wounds. A single-site iron nanozyme was developed for alcohol detoxification. A Pt nanozyme was developed to treat gouty arthritis. Two nature reviews on nanozymes were published, focusing on nanohealthcare and in vivo applications. Combination of nanozyme and probiotics for IBD therapy. An artificial metabzyme for tumour-cell-specific metabolic therapy was reported. Inhalable nanozyme for viral pneumonia therapy. A strategy to modulate the microenvironmental pHs of nanozymes was developed and the modulated nanozymes were used for analysis including chiral analysis. Certain nanozymes have the potential for treating ischemic stroke and traumatic brain injury due to their ability to mitigate the harmful effects of excessive free radical production, oxidative brain damage, inflammation, and blood-brain barrier disruption.