Artificial Oxidase Research Articles

An 1,4,7,10-tetraazacyclododecane-nickel complex functionalized polymolybdate acting as artificial oxidase in removing methylene blue

The reactive oxygen species (ROS), serving as a crucial biochemical reaction medium, is intricately involved in a myriad of physiological and pathological processes. The proper use of ROS can solve many practical problems, for example killing harmful microorganisms or cells, removing pollutants in water, etc. Therefore, the design and synthesis of artificial oxidase with catalytic activity in producing ROS become a hot research spot from bio-inorganic chemistry to catalytic industry. Herein, a cyclen-nickel complex functionalized polymolybdate, [Ni(cyclen)(H2O)2]2[(AsO)6(NiMo6O24)]·3H2O (abbreviated as NiAM, cyclen = 1,4,7,10-tetrazacyclododecane), which can acting as a catalyzed oxidase to generating ROS, has been structurally characterized by means of single-crystal X-ray diffraction (SXRD), elemental analyses, IR and UV–vis spectra, powder X-ray diffraction (PXRD), and bond valence sums (Σs) calculation. The NiAM's principal element, as revealed by SXRD, indicates that it is an ionic complex that composed by two [Ni(cyclen)(H2O)2]2+and one modified Anderson-type cluster [(AsO)6(NiMo6O24)]4–. Interestingly, NiAM possesses effective ability in catalyzing the ROS generation in aqueous solution. As a result, it can remove methylene blue (MB) dyes from the stable solution system.

The nature of local oxygen radical boosts electrocatalytic ethanol to selectively generate CO2

Developing a high-activity and antitoxic electrocatalyst is still a demanding task. Enhancing the enrichment of oxygen species on catalysts is beneficial for thorough oxidation of ethanol to generate CO2, but the role of oxygen radicals in the process of ethanol oxidation is still ambiguous. Herein, an artificial oxidase that can catalyze oxygen to generate reactive oxygen species (ROS) in-situ has been applied in EOR for the first time and the roles of •OH, •O2−, and 1O2 in complete oxidation of ethanol were investigated. The mass activity of EOR is 18.2 A mgPt−1 in 1 mol L−1 KOH and the CO2 selectivity is 98.7%. The research showed that Sn element could optimize coordination mode on catalyst surface, which enhanced oxidase activity of the catalyst. Explored the intermediates of the reaction and evaluated the performance of the catalyst using in-situ infrared testing technology. Theoretical calculations indicate that C–C bond breakage of *CH3CO to generate *CH3 and *CO is potential determination steps in the C1 pathway. When singlet oxygen is present on the PtSn IM/C surface, the dissociation energy of C–C bond is –0.51 eV, which is lower than the 1.07 eV of hydroxyl radicals and –0.47 eV of superoxide anions.

Facile Strategy to Output Fluorescein from Nucleic Acid Interactions.

Biomolecular operations, which involve the conversion of molecular signals or interactions into specific functional outputs, are fundamental to the field of biology and serve as the important foundation for the design of diagnostic and therapeutic systems. To maximize their functionalities and broaden their applicability, it is crucial to develop novel outputs and facile chemical transformation methods. With this aim, in this study, we present a straightforward method for converting nucleic acid signals into fluorescein outputs that exhibit a wide range of functionalities. This operation is designed through a DNA-templated reaction based on riboflavin-photocatalyzed oxidation of dihydrofluorescein, which is readily prepared by simple NaBH4 reduction of the fluorescein with no complicated chemical caging steps. The templated photooxidation exhibits high efficiency (kapp = 2.7 × 10-3/s), generating a clear fluorescein output signal distinguishable from a low background, originating from the high stability of the synthesized dihydrofluorescein. This facile and efficient operation allows the nucleic acid-initiated activation of various fluorescein functions, such as fluorescence and artificial oxidase activity, which are applied in the design of novel bioanalytical systems, including fluorescent and colorimetric DNA sensors. The operation presented herein would expand the scope of biomolecular circuit systems for diagnostic and therapeutic applications.

A nanoscale ferromagnetic nickel-supercluster functionalized giant heterometallic arsenototungstate with ability in catalyzing reactive oxygen species generation

In this work, a high-nuclear functionalized giant heterometallic arsenototungstate with artificial oxidase property, [Na(H2O)5]2[Ni(H2O)(en)2]2[Ni(H2O)3(en)][H4Ni6 (OH)2(en)2.5(B-α-AsVW9O34)]2[W4O16][Ni4(H2O)2(en)2]2[Ni(H2O)(en)][(α-AsVW6O26)]2·3H2O (abbreviated as NiAW, en = ethylenediamine) has been designed and synthesized successfully. NiAW was structurally characterized by X-ray photoelectron spectrum (XPS), infrared (IR) spectrum, ultraviolet-visible (UV-vis) spectrum, single-crystal X-ray diffraction (SXRD) and bond valence sums (Σs) calculation. The SXRD results indicated that the skeleton of NiAW is constructed by two sandwich-type [(B-α-AsVW9O34)H4Ni6(OH)2(en)2.5(α-AsVW6O26)]4– sub-unit linking by a [W4O16]8– building block, in which a novel tetra-Ni cluster [Ni4(H2O)2(en)2O10]12– graft to the hexa-vacant Keggin [α-AsW6O26]11– fragment. Finally, with the embellishment of several [Ni(en)2(H2O)]2+, [Ni(en)(H2O)]2+, and [Na(H2O)5]+ cations, NiAW forms its complete structure. Notably, the Ni-cluster in NiAW exhibited the ferromagnetic coupling interactions. Furthermore, it is also demonstrated that NiAW is capable of catalyzing the generation of ROS efficiently by the synergistic effects of POM fragments and nickel complexes.

A Cyclen-Functionalized Cobalt-Substituted Sandwich-Type Tungstoarsenate with Versatility in Removal of Methylene Blue and Anti-ROS-Sensitive Tumor Cells.

Oxidative degradation by using reactive oxygen species (ROS) is an effective method to treat pollutants. The synthesis of artificial oxidase for the degradation of dyes is a hot spot in molecular science. In this study, a nanoscale sandwich-type polyoxometalate (POM) on the basis of a tetra-nuclear cobalt cluster and trivacant B-α-Keggin-type tungstoarsenate {[Co(C8H20N4)]4}{Co4(H2O)2[HAsW9O34]2}∙4H2O (abbreviated as CAW, C8H20N4 = cyclen) has been synthesized and structurally examined by infrared (IR) spectrum, ultraviolet–visible (UV–Vis) spectrum, X-ray photoelectron spectrum (XPS), single-crystal X-ray diffraction (SXRD), and bond valence sum (Σs) calculation. According to the structural analysis, the principal element of the CAW is derived from modifying sandwich-type polyanion {Co4(H2O)2 [HAsW9O34]2}8– with four [Co(Cyclen)]2+, in which 1,4,7,10-tetraazacyclododecane (cyclen) is firstly applied to modify POM. It is also demonstrated that CAW is capable of efficiently catalyzing the production of ROS by the synergistic effects of POM fragments and Co–cyclen complexes. Moreover, CAW can interfere with the morphology and proliferation of sensitive cells by producing ROS and exhibits ability in specifically eliminating methylene blue (MB) dyes from the solution system by both adsorption and catalytic oxidation.

Bifunctional Mn-Doped N-Rich Carbon Dots with Tunable Photoluminescence and Oxidase-Mimetic Activity Enabling Bimodal Ratiometric Colorimetric/Fluorometric Detection of Nitrite.

Multimodal detection is a promising paradigm because of its advantages of expanding usage scenarios and improving reliability. However, it is very challenging to design reasonable strategies to achieve the multimodal sensing of targets. Herein, we developed an unprecedented bimodal ratiometric colorimetric/fluorometric method by exploring a novel bifunctional artificial oxidase mimic, Mn-doped N-rich carbon dots (Mn-CDs), to achieve the high-performance determination of nitrite in complicated matrices. The Mn-CDs exhibited both tunable photoluminescence and high oxidase-like activity, effectively catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to generate blue TMB+. When nitrite was introduced, the TMB+ species generated would specifically react with nitrite to produce diazotized TMB+, resulting in a color change from blue to green and finally to yellow. Simultaneously, the fluorescence of Mn-CDs was quenched by the diazotized TMB+ product via the inner filter effect. Hence, the existence of nitrite could lead to the simultaneous variations of visual color and photoluminescence, providing the principal basis for the bimodal ratiometric colorimetric/fluorometric quantification of the target. With the method, excellent sensitivity, selectivity, reliability, and practicability for nitrite detection were verified. Our work proposes a new bimodal strategy for nitrite measurement using bifunctional CDs-based enzyme mimics, which will inspire future effort on the exploration of promising multifunctional nanozymes and their advanced applications in biochemical sensing.

A Novel Tri-Coordination Zinc Complex Functionalized Silicotungstate with ROS Catalytic Ability and Anti-Tumor Cells Activity

Reactive oxygen species (ROS) can be used as an effective method to treat tumors. Artificial oxidase has received increasing attention as a catalyst for ROS generation in fields ranging from bioinorganic chemistry to pharmaceutical chemistry. In this study, an artificial oxidase based on a binuclear zinc complex and Keggin-type silicotungstate [Zn2(4,4′-bpy)(Phen)2][SiW12O40] (ZSW) (4,4′-bpy = 4,4′-bipyridine; Phen = 1,10-phenanthroline) was synthesized and structurally featured in terms of its X-ray photoelectron spectrum (XPS), bond valence sum (Σs) calculation, IR spectra, and single-crystal X-ray diffraction (SXRD). ZSW is an ionic compound in which the cation is a binuclear zinc complex [Zn2(4,4′-bpy)(Phen)2]4+ and the anion is a α-Keggin-type silicotungstate [SiW12O40]4– cluster. Notably, the Zn ions in the [Zn2(4,4′-bpy)(Phen)2] exist in tri-coordination, which was first obtained in polyoxometalate (POM) chemistry. It was also demonstrated that ZSW is capable of efficiently catalyzing the production of ROS, which, according to the computational calculations, may be due to the synergistic action of zinc complexes and POM building blocks. Furthermore, ZSW exhibited inhibition ability toward ROS-sensitive tumor cells, such as PC12 cells.

Nanostructured MnO2 nanosheets grown on nickel foam: an efficient and readily recyclable 3D artificial oxidase for the colorimetric detection of ascorbic acid

.An efficient and readily recyclable 3D artificial oxidase (NF@MnO2) for the colorimetric detection of ascorbic acid was well constructed with nickel foam as the substrate.

Target-triggered inhibiting oxidase-mimicking activity of platinum nanoparticles for ultrasensitive colorimetric detection of silver ion

The development of efficient methods for the detection of hazardous and toxic elements is extremely important for environmental security and public health. In this work, we developed a facile colorimetric assaying system for Ag+ detection in aqueous solution. Chitosan-stabilized platinum nanoparticles (Ch-PtNPs) were synthesized and severed as an artificial oxidase to catalyze the oxidation of the substrate 3,3′,5,5′-tetramethylbenzidine (TMB) and generate color signal. In the presence of Ag+, due to the strong metallophilic interactions between Ag+ and Pt2+ on the surface of Ch-PtNPs, Ag+ can weaken the affinity to the substrates and inactivate the catalytic activity of Ch-PtNPs, leading to decreased absorbance signal to varying degrees depending on Ag+ amount. Combing the specific binding between Ch-PtNPs and Ag+ with signal amplification procedure based on the Ch-PtNPs-catalyzed TMB oxidation, a sensitive, selective, simple, cost-effective, and rapid detection method for Ag+ can be realized. Ag+ ions in tap and lake waters have been successfully detected. We ensured that the proposed method can be a potential alternative for Ag+ determination in environmental samples.

Metal organic framework MIL-53(Fe) as an efficient artificial oxidase for colorimetric detection of cellular biothiols

Biothiols play critical roles in many biological processes and their aberrant is related to a variety of syndromes. A simple and reliable colorimetric method is developed in this work for biothiols detection based on an oxidase mimic, a metal organic framework (MOF) MIL-53(Fe), and a peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB). In this design, MIL-53(Fe) is utilized to catalyze the conversion of TMB to a blue colored 3,3′,5,5′-tetramethylbenzidine diimine, which can be read on a spectrophotometer at 652 nm. The oxidation-induced blue color generation can be efficiently inhibited by biothiols, thus a colorimetric analytical method is proposed for biothiols detection based on the above system. Under optimal conditions, a linear relationship in a range from 1 to 100 μM and a limit of detection (LOD) at 120 nM are achieved with Cys as a model target. The developed platform is further applied to evaluate cellular biothiols in normal (RWPE-1) and cancer (LNCap) cell lines, revealing that the overall biothiols level in LNCap is much higher than that in RWPE-1. This work renders a powerful tool for identifying cancer cells in a simple manner for biomedical diagnosis associated with biothiols.

Expansion of Redox Chemistry in Designer Metalloenzymes.

Many artificial enzymes that catalyze redox reactions have important energy, environmental, and medical applications. Native metalloenzymes use a set of redox-active amino acids and cofactors as redox centers, with a potential range between -700 and +800 mV versus standard hydrogen electrode (SHE, all reduction potentials are versus SHE). The redox potentials and the orientation of redox centers in native metalloproteins are optimal for their redox chemistry. However, the limited number and potential range of native redox centers challenge the design and optimization of novel redox chemistry in metalloenzymes. Artificial metalloenzymes use non-native redox centers and could go far beyond the natural range of redox potentials for novel redox chemistry. In addition to designing protein monomers, strategies for increasing the electron transfer rate in self-assembled protein complexes and protein-electrode or -nanomaterial interfaces will be discussed. Redox reactions in proteins occur on redox active amino acid residues (Tyr, Trp, Met, Cys, etc.) and cofactors (iron sulfur clusters, flavin, heme, etc.). The redox potential of these redox centers cover a ∼1.5 V range and is optimized for their specific functions. Despite recent progress, tuning the redox potential for amino acid residues or cofactors remains challenging. Many redox-active unnatural amino acids (UAAs) can be incorporated into protein via genetic codon expansion. Their redox potentials extend the range of physiologically relevant potentials. Indeed, installing new redox cofactors with fined-tuned redox potentials is essential for designing novel redox enzymes. By combining UAA and redox cofactor incorporation, we harnessed light energy to reduce CO2 in a fluorescent protein, mimicking photosynthetic apparatus in nature. Manipulating the position and reduction potential of redox centers inside proteins is important for optimizing the electron transfer rate and the activity of artificial enzymes. Learning from the native electron transfer complex, protein-protein interactions can be enhanced by increasing the electrostatic interaction between proteins. An artificial oxidase showed close to native enzyme activity with optimized interaction with electron transfer partner and increased electron transfer efficiency. In addition to the de novo design of protein-protein interaction, protein self-assembly methods using scaffolds, such as proliferating cell nuclear antigen, to efficiently anchor enzymes and their redox partners. The self-assembly process enhances electron transfer efficiency and enzyme activity by bringing redox centers into close proximity of each other. In addition to protein self-assembly, protein-electrode or protein-nanomaterial self-assembly can also promote efficient electron transfer from inorganic materials to enzyme active sites. Such hybrid systems combine the efficiency of enzyme reactions and the robustness of electrodes or nanomaterials, often with advantageous catalytic activities. By combining these strategies, we can not only mimic some of nature's most fascinating reactions, such as photosynthesis and aerobic respiration, but also transcend nature toward environmental, energy, and health applications.

Facile approach to prepare HSA-templated MnO2 nanosheets as oxidase mimic for colorimetric detection of glutathione

In this work, a simple, rapid, and highly sensitive colorimetric assay for the determination of glutathione (GSH) was developed. It employs human serum albumin (HSA)-templated MnO2 nanosheets as an artificial oxidase. HSA-templated MnO2 nanosheets can oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to a blue oxTMB product with a significant increase in absorbance at 652 nm in the absence of H2O2. When GSH is introduced, the MnO2 nanosheets are reduced to Mn2+ ions, thereby inhibiting the formation of oxTMB. Based on these findings, a simple colorimetric assay was developed for the detection GSH in the range of 10 nM to 5 μM with a low detection limit of 5.6 nM. Importantly, the proposed method was successfully used for quantitative determination of GSH in biological fluids, such as human serum samples.

Colorimetric immunoassay for rapid detection of Vibrio parahaemolyticus

Vibrio parahaemolyticus (V. parahaemolyticus) is one of the most common food-borne pathogens. The authors describe a rapid colorimetric assay for V. parahaemolyticus that is based on a combination of a magnetic bead-based sandwich immunoassay and signal amplification via an enzmye mimic. MnO2 nanoparticles are used as an artificial oxidase that oxidizes 3,3′,5,5′-tetramethylbenzidine in the presence of oxygen to form a blue (and readily visible) product with an absorption maximum at 652 nm. By combining the superior capture efficiency of magnetic beads with the high catalytic activity of the enzmye mimic, this method can detect V. parahaemolyticus concentration in the range between 10 to 105 cfu·mL−1 without pre-enrichment, and the limit of detection is as low as 10 cfu·mL−1. Recoveries ranging from 87.5% to 106.0% are found when analyzing spiked oyster samples. The assay is rapid, sensitive, and specific and specific. In our perception, it shows promise in rapid instrumental and on-site visual detection of V. parahaemolyticus.

Rapid and sensitive colorimetric detection of ascorbic acid in food based on the intrinsic oxidase-like activity of MnO2 nanosheets.

In this paper, we report a colorimetric sensor for the rapid, selective detection of ascorbic acid (AA) in aqueous solutions. Single-layered MnO2 nanosheets were established as an artificial oxidase; consequently colorless 3,3´,5,5´-tetramethylbenzidine (TMB) was oxidized to a blue product (oxTMB), with increase in absorbance at 650 nm. The absorbance of the reaction system decreased after introduction AA, which reduced MnO2 into Mn2+ . Under optimum conditions, a detection limit of 62.81 nM for AA in aqueous solutions could be achieved. The linear response range for AA was 0.25-30 μM with a correlation coefficient of 0.996. Importantly, the MnO2 nanosheet-TMB chromogenic reaction exhibited great selectivity as there was no interference from other metal ions, amino acids and small biological molecules. The proposed colorimetric sensing of AA could be applied for fruit, juice and pharmaceutical samples. Moreover, the proposed sensor showed satisfying performance, including low cost, easy preparation, rapid detection, and good biocompatibility.

Colorimetric sensing of silver ions based on glutathione-mediated MnO2 nanosheets

We described a rapid and highly sensitive colorimetric method for the detection and quantification of silver ions employing glutathione (GSH)-mediated MnO2 nanosheets as an artificial oxidase. In this assay,the absorption intensity of the chromogenic reaction system enabled the quantification of silver ions. Under the optimum conditions, the linear response ranged for Ag+ was from 10nM to 800nM with a correlation coefficient of 0.996. Remarkably, limit of detection in aqueous solutions was 4.23nM, which was well below United States Environmental Protection Agency (USEPA) permissible limit in drinking water (460nM). This sensing assay had highly specificity because no significant interference occurred with 10-fold concentration of other metal ions. Moreover, the approach also could be employed to the Ag+ detection in real samples. And the recovery of Ag+ spiked in tap water ranged from 99.62% to 108.01%. Importantly, the oxidation reaction of TMB to oxTMB by MnO2 nanosheets completed instantaneously even by naked-eye readout. Therefore, the assay based on GSH-mediated MnO2 nanosheets is simple, rapid, highly sensitive and selective for the quantification of Ag+ in water and practical samples.

MnO2 nanosheets as an artificial enzyme to mimic oxidase for rapid and sensitive detection of glutathione

Nanozymes are increasingly used as components in assays and diagnostics. Here, we describe a rapid and highly sensitive colorimetric assay for the detection and quantification of glutathione (GSH) employing MnO2 nanosheets as an artificial oxidase. In the assay pale yellow 3,3´,5,5´-tetramethylbenzidine (TMB) is oxidized to a blue product (oxTMB) under catalyzing of MnO2 nanosheets with a significant change in absorption at 650nm. GSH selectively inhibits this reaction with a detection limit of 300nM. The high specificity of inhibition by GSH allows this system to be used to determine the GSH concentrations in human serum samples. The MnO2 nanosheet-based assay is simple, rapid, sensitive and selective for the quantification of GSH and surpasses detection methods based on other MnO2 nanomaterials.

Catalysis without a headache: Modification of ibuprofen for the design of artificial metalloenzyme for sulfide oxidation

A new artificial oxidase has been developed for selective transformation of thioanisole. The catalytic activity of an iron inorganic complex, FeLibu, embedded in a transport protein NikA has been investigated in aqueous media. High efficiency (up to 1367t), frequency 459TONmin−1 and selectivity (up to 69%) make this easy to use catalytic system an asset for a sustainable chemistry.

Cyclodextrin-based artificial oxidases with high rate accelerations and selectivity

Three cyclodextrin derivatives with one to four 2-O-formylmethyl groups attached to the secondary rim were prepared and investigated as catalysts for the oxidation of aminophenols in buffered dilute hydrogen peroxide. The derivatives were found to be Michaelis–Menten catalysts and to give rate accelerations of up to 20,000 for the oxidation of 2-aminophenol to 2-amino-phenoxazin-3-one, and 12,000 for the oxidation of 2-amino-p-cresol to 2-nitro-p-cresol. While a range of differently substituted substrates was oxidized the success of the reaction was highly dependent on the substituent pattern. The ability of one of the new artificial enzymes to oxidize selectively one aminophenol from a mixture of two was investigated giving substrate selectivities of up to 16:1.

Substrate structure governs maximum rate of catalysis exerted by cyclodextrin oxidase chemzymes

Selectively modified α- and β-cyclodextrin ketones or aldehydes act as artificial oxidases on a variety of small lipophilic substrates. The structure of the substrate is a highly important factor governing how effectively the oxidation reaction can be catalyzed. Amino acid-type substrates were not prone to catalysis, which yields new information about the limits of CD catalysis. Aniline showed some non-quantifiable catalysis, but for quinones and benzyl alcohols no net catalysis was detected. For aminophenol oxidation, o-aminophenols are far better substrates than p-aminophenols. The CD-catalyzed reaction follows Michaelis–Menten kinetics, involves CD cavity binding of the substrate and substrate recognition, and thus encompasses many of the hallmarks of natural enzymatic catalysis. Strong binding of the cooxidant H2O2 to the CD catalytic carbonyl group is a prerequisite for the subsequent oxidation of the substrate and in accordance with this, the binding of H2O2 to β-CD dialdehyde was shown to be strong (K d = 1.4 mM). β-CD 6A,6D-diketone which binds H2O2 weaker than an aldehyde was accordingly a less efficient oxidase. The wide range of substrates applicable to CD chemzyme catalysis brings about optimism for future scopes of synthetic biology.

Simple cyclodextrin aldehydes as excellent artificial oxidases

Cyclodextrin based oxidases, with a ketone as functional group are well known as good artificial enzyme mimics (Fenger et al. Org Biomol Chem 7:933–943; Marinescu and Bols Angew Chem Int Ed 45:4590–4593; Bjerre et al. Eur J Org Chem 704–710; Marinescu et al. J Am Chem Soc 127:17578–17579). We here report a series of modified cyclodextrins, having aldehydes as functional groups. The aldehyde based artificial enzymes have, in most cases, better catalysis than the ketones, because of their powerful covalent binding of hydrogen peroxide. Among the modified cyclodextrins studied are mono and di aldehydes on the 6 positions, with or without methylated hydroxyl groups. The aldehyde functionality was also introduced close to the secondary side, by attaching ethoxy-2-al or propoxy-3-al to the 2 position. The modified cyclodextrins showed excellent enzymatic activity towards oxidation of different aminophenols, and 4-methoxy benzyl alcohol with hydrogen peroxide as a stoichiometric oxidant. Rate enhancements up to 4,600 were achieved for oxidation of 4-methoxy benzyl alcohol, where as oxidation of amines gave rate enhancements up to 3,400. The artificial oxidases catalyses oxidations under enzymatic conditions (water, pH 7, 25 °C), following Michaelis–Menten kinetics. To confirm the enzyme activity, inhibition studies with sodium naphthalene-2-sulfonate were carried out. These studies showed competitive inhibition of the enzymes, verifying the cyclodextrins enzyme like character.