POMOF/SWNT Nanocomposites with Prominent Peroxidase-Mimicking Activity for l-Cysteine "On-Off Switch" Colorimetric Biosensing

Recent research progress of selenotungstate-based biomolecular sensing materials

Polyoxometalates (POMs) have drawn significant attention on account of their structural designability, compositional diversity and great potential applications. As an indispensable branch of POMs, selenotungstates (SeTs) have been synthesized extensively. Some SeTs have been applied as sensing materials for detecting biomarkers (e.g., metabolites, hormones, cancer markers). To gain a comprehensive understanding of advancements in SeT-based sensing materials, we present an overview that encapsulates the sensing performances and mechanisms of SeT-based biosensors. SeT-based biosensors are categorized into electrochemical catalytic biosensors, electrochemical affinity biosensors, "turn-off" fluorescence biosensors and "turn-on" fluorescence biosensors. We anticipate the expansive potential of SeT-based biosensors in wearable and implantable sensing technologies, which promises to catalyze significant breakthroughs in SeT-based biosensors.

Nitrogen doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets for enhanced peroxidase activity

Heteroatom doping and phase engineering are effective ways to promote the catalytic activity of nanoenzymes. Nitrogen-doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets N-1 T/2H-MoS2/CuS are prepared by a simple hydrothermal approach using polyoxometalate (POM)-based metal-organic frameworks (MOFs) (NENU-5) as a precursor and urea as nitrogen doping reagent. The XPS spectroscopy (XPS) and Raman spectrum of N-1 T/2H-MoS2/CuS prove the successful N-doping. NENU-5 was used as the template to prepare 1 T/2H-MoS2/CuS with high content of 1 T phase by optimizing the reaction time. The use of urea as nitrogen dopant added to 1 T/2H-MoS2/CuS, resulted in N-1 T/2H-MoS2/CuS with an increase in the content of the 1 T phase from 80 % to 84 % and higher number of defects. N-1 T/2H-MoS2/CuS shows higher peroxidase activity than 1 T/2H-MoS2/CuS and a catalytic efficiency (Kcat/Km) for H2O2 twice as high as that of 1 T/2H-MoS2/CuS. The enhanced catalytic activity has probably been attributed to several reasons: (i) the insertion of urea during the hydrothermal process in the S-Mo-S layer of MoS2, causing an increase in the interlayer spacing and in 1 T phase content, (ii) the replacement of S atoms in MoS2 by N atoms from the urea decomposition, resulting in more defects and more active sites. As far as we know, N-1 T/2H-MoS2/CuS nanosheets have the lowest detection limit (0.16 µm) for the colorimetric detection of hydroquinone among molybdenum disulfide-based catalysts. This study affords a new approach for the fabrication of high-performance nanoenzyme catalysts.

Co-based Nanozymatic Profiling: Advances Spanning Chemistry, Biomedical, and Environmental Sciences

Nanozymes, next-generation enzyme-mimicking nanomaterials, have entered an era of rational design; among them, Co-based nanozymes have emerged as captivating players over times. Co-based nanozymes have been developed and have garnered significant attention over the past five years. Their extraordinary properties, including regulatable enzymatic activity, stability, and multifunctionality stemming from magnetic properties, photothermal conversion effects, cavitation effects, and relaxation efficiency, have made Co-based nanozymes a rising star. This review presents the first comprehensive profiling of the Co-based nanozymes in the chemistry, biology, and environmental sciences. The review begins by scrutinizing the various synthetic methods employed for Co-based nanozyme fabrication, such as template and sol-gel methods, highlighting their distinctive merits from a chemical standpoint. Furthermore, a detailed exploration of their wide-ranging applications in biosensing and biomedical therapeutics, as well as their contributions to environmental monitoring and remediation is provided. Notably, drawing inspiration from state-of-the-art techniques such as omics, a comprehensive analysis of Co-based nanozymes is undertaken, employing analogous statistical methodologies to provide valuable guidance. To conclude, a comprehensive outlook on the challenges and prospects for Co-based nanozymes is presented, spanning from microscopic physicochemical mechanisms to macroscopic clinical translational applications.

Polyoxometalate-Guanosine Monophosphate Hydrogels with Haloperoxidase-like Activity for Antibacterial Performance

Haloperoxidases represent an important class of enzymes that nature adopts as a defense mechanism to combat the colonial buildup of microorganisms on surfaces, commonly known as biofouling. Subsequently, there has been tremendous focus on the development of artificial haloperoxidase mimics that can catalyze the oxidation of X- (halide ion) in the presence of H2O2 to form HOX. The natural intermediate HOX disrupts the bacterial quorum sensing, thus preventing biofilm formation. Herein, we report a simple method for the formation of supramolecular hydrogels through the self-assembly of Keggin-structured polyoxometalates, phosphotungstic acid, and silicotungstic acid with the small biomolecule guanosine monophosphate (GMP) in an aqueous medium. The polyoxometalate-GMP hydrogels that contained highly entangled nanofibers were mechanically robust and showed thixotropic properties. The gelation of the polyoxometalates with GMP not only rendered manifold enhancement in biocompatibility but also the fibril network in the hydrogel provided high water wettability and the polyoxometalates acted as an efficient haloperoxidase mimic to trigger oxidative iodination, as demonstrated by a haloperoxidase assay. The antifouling activity of the phosphotungstic acid-GMP hydrogel was demonstrated against both Gram-positive and Gram-negative bacteria, which showed enhanced antibacterial performance of the hydrogel as compared to the polyoxometalate alone. We envision that the polyoxometalate-GMP hydrogels may facilitate mechanically robust coatings in a simple pathway that can be useful for antifouling applications.

Polyoxometalates as Potential Artificial Enzymes toward Biological Applications

Artificial enzymes, as alternatives to natural enzymes, have attracted enormous attention in the fields of catalysis, biosensing, diagnostics, and therapeutics because of their high stability and low cost. Polyoxometalates (POMs), a class of inorganic metal oxides, have recently shown great potential in mimicking enzyme activity due to their well-defined structure, tunable composition, high catalytic efficiency, and easy storage properties. This review focuses on the recent advances in POM-based artificial enzymes. Different types of POMs and their derivatives-based mimetic enzyme functions are covered, as well as the corresponding catalytic mechanisms (where available). An overview of the broad applications of representative POM-based artificial enzymes from biosensing to theragnostic is provided. Insight into the current challenges and the future directions for POMs-based artificial enzymes is discussed.

High Peroxidase-Mimicking Metal-Organic Frameworks Decorated with Platinum Nanozymes for the Colorimetric Detection of Acetylcholine Chloride and Organophosphorus Pesticides via Enzyme Cascade Reaction

The sensitive detection of acetylcholinesterase (AChE) and organophosphorus pesticides (OPs) is very important for the protection of human health. Herein, a hybrid material, Pt NPs/Fe-MOF, consisting of a metal-organic framework (MIL-88B-NH2, Fe-MOF) decorated with platinum nanoparticles (Pt NPs), was prepared first and exhibited remarkably improved and excellent peroxidase-mimicking activity compared to the Fe-MOF material resulting from the synergistic catalysis effect between Fe-MOF and Pt NPs, which can effectively catalyze 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to generate a blue product (oxidized TMB, oxTMB). Interestingly, in the presence of AChE and acetylcholinesterase, the peroxidase-mimicking activity from Pt NPs/Fe-MOF was inhibited obviously, and thus, a colorimetric sensing platform for AChE can be constructed; more importantly, after the addition of OPs, this nanozyme activity can be recovered, inducing the further successful construction of a sensitive colorimetric sensing platform for OPs. The related sensing mechanism and condition optimization were studied, and the as-prepared Pt NPs/Fe-MOF nanozyme-based colorimetric method for AChE and OP detection displayed superior analytical performances with wide linearities and low detection limits. Furthermore, the designed method offers satisfactory real application ability. We expect the as-proposed Pt NPs/Fe-MOF nanozyme-based colorimetric sensing platform for AChE and OPs via the enzyme cascade reaction to show great potential application.

Graphdiyne Oxide Quantum Dots: The Enhancement of Peroxidase-like Activity and Their Applications in Sensing H2O2 and Cysteine

As one of the typical carbon nanomaterials, graphdiyne (GDY) with unique chemical, physical, and electronic properties has a great potential in various fields. Although it is an important member of carbon nanozymes, the research on its intrinsic enzyme mimetic properties and applications is still limited. Herein, graphdiyne oxide quantum dots (GDYO QDs) have been synthesized through oxidative cleavage, which exhibit enhanced peroxidase-like activity with lower K_m and higher V_max than those of most carbon-based nanozymes. The catalytic mechanism is explored, showing that the enhanced catalytic performance is attributed to the good conjugated structure, large number of oxygen-containing groups, and small-sized nanosheets with few layers. As a kind of peroxidase mimetic, the GDY-based nanozyme has excellent potential in sensing H₂O₂ and biological antioxidants through the colorimetric assay, with a linear range from 5 to 500 μM and detection limit of 1.5 μM for H₂O₂ and a linear range from 0 to 90 μM and detection limit of 0.48 μM for l-cysteine. Our work will be beneficial to develop high-performance artificial enzymes and to understand their mechanism for better applications.

Recent Advances in Polyoxometalates with Enzyme-like Characteristics for Analytical Applications

Artificial enzymes based on inorganic solids with both enzyme-mimetic activities and the special material features has been a promising candidate to overcome many deleterious effects of native enzymes in analytical applications. Polyoxometalates (POMs) are an importance class of molecular metal-oxygen anionic clusters. Their outstanding physicochemical properties, versatility and potential applications in energy conversion, magnetism, catalysis, molecular electronics and biomedicine have long been studied. However, the analytical applications of them is limited. Recently, the intrinsic enzymatic activities of POMs have also been found and become an area of growing interest. In this review, along with other reports, we aimed to classify the enzymatic activity of POMs, summarize the construction of POMs-based enzymes, and survey their recent advances in analytical fields. Finally, the current challenges and trends of the polyoxometalates with enzymatic activity in future chemo-/bio-sensing applications are briefly discussed.

Multifunctional carbon nanomaterials for diagnostic applications in infectious diseases and tumors

Infectious diseases (such as Corona Virus Disease 2019) and tumors pose a tremendous challenge to global public health. Early diagnosis of infectious diseases and tumors can lead to effective control and early intervention of the patient's condition. Over the past few decades, carbon nanomaterials (CNs) have attracted widespread attention in different scientific disciplines. In the field of biomedicine, carbon nanotubes, graphene, carbon quantum dots and fullerenes have the ability of improving the accuracy of the diagnosis by the improvement of the diagnostic approaches. Therefore, this review highlights their applications in the diagnosis of infectious diseases and tumors over the past five years. Recent advances in the field of biosensing, bioimaging, and nucleic acid amplification by such CNs are introduced and discussed, emphasizing the importance of their unique properties in infectious disease and tumor diagnosis and the challenges and opportunities that exist for future clinical applications. Although the application of CNs in the diagnosis of several diseases is still at a beginning stage, biosensors, bioimaging technologies and nucleic acid amplification technologies built on CNs represent a new generation of promising diagnostic tools that further support their potential application in infectious disease and tumor diagnosis.

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Carbon nanomaterials (CNs) are systematically introduced in this review.

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This review studies the application of CNs in infectious diseases and tumors diagnosis.

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CNs act as potent nanostructures for biosensing, bioimaging, & nucleic acid amplification.

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New CN-based detection methods were introduced to detect SARS-CoV-2.

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The challenges and prospects of CN-based diagnostic assays are also discussed.

Co-N/C-900 metal-organic framework-derived nanozyme as a H2O2-free oxidase mimic for the colorimetric sensing of L-cysteine

Nanozymes have attracted considerable attention as a new type of promising artificial enzyme in recent years. Here, an oxidase-like cobalt-nitrogen-carbon (Co-N/C-900) nanozyme with well-regulated metal atom spatial distribution has been derived from Co-Zn bimetal zeolitic imidazolate framework precursors and used to develop a facile colorimetric sensing method for L-cysteine. With the aid of Co-N/C-900, the colorless 3,3',5,5'-tetramethylbenzidine (TMB) was oxidized to blue oxidized TMB in the absence of H₂O₂. However, the oxidation was inhibited after the addition of L-cysteine, and the blue color faded to colorless. Thus, Co-N/C-900 exhibited quite good oxidase-like activity with high catalytic efficiency. Therefore, a facile and efficient colorimetric method to sensitively determine L-cysteine with a low detection limit of 33 nM (S/N = 3) has been developed. Furthermore, favorable selectivity and anti-interference ability towards the determination of L-cysteine based on this approach have also been achieved. It is believed that this colorimetric method for the detection of L-cysteine based on Co-N/C-900 will show potential applications in bioscience and bioengineering.

The BODIPY-based chemosensor for the fluorometric determination of organochlorine pesticide dicofol

Dicofol is an organochlorine pesticide, which is widely used in fruits, tea and other crops, and is moderately toxic to humans. Therefore, the monitoring of organochlorine pesticide-dicofol is critical for food safety. In this work, a fluorometric chemosensor based on mercaptoethanol and boron dipyrromethene (BODIPY) was first constructed to detect the dicofol. The chemosensor displayed turn-off fluorescence behavior upon dicofol with a detection limit of 200 ppb. The nucleophilicity of the glutathione and other biological thiols was studied to evaluate the reactivity of thiols with dicofol. In practical applications, an obvious color difference was observed on a paper based microfluidic device modified by phenyltriethoxysilane (PTES). We designed an integrated device for pretreatment and paper-based detection, and successfully used for the detection of dicofol in tea. The applicability was demonstrated by detection of dicofol in real tea samples with good recovery ranging from 86% to 109%. The apparatus was convenient and could be used for on-site evaluation of dicofol.

Carbon Nanomaterials (CNMs) and Enzymes: From Nanozymes to CNM-Enzyme Conjugates and Biodegradation

Carbon nanomaterials (CNMs) and enzymes differ significantly in terms of their physico-chemical properties—their handling and characterization require very different specialized skills. Therefore, their combination is not trivial. Numerous studies exist at the interface between these two components—especially in the area of sensing—but also involving biofuel cells, biocatalysis, and even biomedical applications including innovative therapeutic approaches and theranostics. Finally, enzymes that are capable of biodegrading CNMs have been identified, and they may play an important role in controlling the environmental fate of these structures after their use. CNMs’ widespread use has created more and more opportunities for their entry into the environment, and thus it becomes increasingly important to understand how to biodegrade them. In this concise review, we will cover the progress made in the last five years on this exciting topic, focusing on the applications, and concluding with future perspectives on research combining carbon nanomaterials and enzymes.

Pd-Fe3O4 Janus nanozyme with rational design for ultrasensitive colorimetric detection of biothiols

Although nanozyme-based colorimetric assays have been broadly used for biosensing, some limitations such as low catalytic activity of nanozyme, poor sensitivity to analytes and lack of understanding the structure-activity relationship remain unsolved. In this work, we developed an ultrasensitive colorimetric method for biothiols detection based on density functional theory-assisted design of janus Pd-Fe₃O₄ nanozyme. The Pd-Fe₃O₄ dumbbell-like nanoparticles (DBNPs) prepared by seed-mediated approach shows a uniform heterodimeric nanostructure. Ultrasensitive biothiols detection is achieved from two aspects. On one hand, due to the synergistic effect between Pd and Fe₃O₄ in the dumbbell structure, Pd-Fe₃O₄ DBNPs show enhanced peroxidase-mimic activity compared to the individual components. On the other hand, when the target biothiols molecule is present, its inhibition effect on the janus Pd-Fe₃O₄ nanozyme is also significantly enhanced. The above results are confirmed both in experiment and theoretical calculation. Based on the rational design, a simple, highly selective and urtrasensitive colorimetric and quantitative assay for biothiols is developed. The limit of detection (LOD) can reach as low as 3.1 nM in aqueous solution. This assay is also successfully applied to the detection of biothiols in real urine samples. Moreover, the Pd-Fe₃O₄ nanozyme is used to discriminate biothiols levels in normal and cancer cells with high sensitivity at the cell density of 15,000/mL, which demonstrates its great potential in biological and clinical analysis. This work not only shows the great promise of janus bimetallic nanozymes' excellent functionalities but also provides rational guidelines to design high-performance nanozymes for biosensing and biomedical applications.

Co-N-C single-atom nanozymes with oxidase-like activity for highly sensitive detection of biothiols

Biothiol detection is of great importance for clinical disease diagnosis. Previous nanozyme-based colorimetric sensors for biothiol detection showed unsatisfactory catalytic activity, which led to a high detection limit. Therefore, developing new nanozymes with the high catalytic activity for biothiol detection is extremely necessary. Recently, single-atom nanozymes (SAzymes) have attracted much attention in biosensing due to their 100% atom utilization and excellent catalytic activity. Most previous works focus on the peroxidase-like activity of Fe-based SAzymes by using unstable and destructive H₂O₂ as the oxidant. It is essential to develop new SAzymes with high oxidase-like activity for biosensing to break through the limitation. Herein, Co-N-C SAzymes with high oxidase-like activity are explored. Furthermore, Co-N-C SAzymes are used as a biosensor for colorimetric detection of biothiols (GSH/Cys) based on the inhibition of thiols toward the oxidase-like activity of Co-N-C SAzymes, which showed high sensitivity with a low detection limit of 0.07 µM for GSH and 0.06 µM for Cys. Besides, the method showed good reproducibility and high selectivity against other amino acids. This work offers new insights using Co-N-C SAzymes in the biosensing field.

Carbon Nanotube Based Metal-Organic Framework Hybrids From Fundamentals Toward Applications

Metal-organic frameworks (MOFs) materials constructed by the coordination chemistry of metal ions and organic ligands are important members of the crystalline materials family. Owing to their exceptional properties, for example, high porosity, tunable pore size, and large surface area, MOFs have been applied in several fields such as gas or liquid adsorbents, sensors, batteries, and supercapacitors. However, poor conductivity and low stability hamper their potential applications in several attractive fields such as energy and gas storage. The integration of MOFs with carbon nanotubes (CNTs), a well-established carbon allotrope that exhibits high conductivity and stability, has been proposed as an efficient strategy to overcome such limitations. By combining the advantages of MOFs and CNTs, a wide variety of composites can be prepared with properties superior to their parent materials. This review provides a comprehensive summary of the preparation of CNT@MOF composites and focuses on their recent applications in several important fields, such as water purification, gas storage and separation, sensing, electrocatalysis, and energy storage (supercapacitors and batteries). Future challenges and prospects for CNT@MOF composites are also discussed.

Colorimetric determination of cysteine based on Au@Pt nanoparticles as oxidase mimetics with enhanced selectivity

A H₂O₂-free colorimetric protocol based on urchin-like Au@Pt nanoparticles (Au@Pt NPs) has been developed for the sensitive and selective determination of cysteine (Cys). We verified the intrinsic oxidase-like activity of the Au@Pt NPs. They can act as artificial mimic oxidases to catalyse the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) with the assistance of dissolved oxygen, avoiding the use of H₂O₂ in the colorimetric determination of Cys. In addition, the discrimination of Cys from the other two biothiol analogues, homocysteine and glutathione, can be easily realized through a simple ageing process. HNO₃ is added to this colorimetric system to terminate the reaction by oxidizing ox-TMB (oxidized form of TMB) to diphenoquinone (DPQ), thus generating a characteristic absorption peak of DPQ at 450 nm. By recording the absorbance at 450 nm, interference from the aggregated Au@Pt NPs (absorption peak at 670 nm) when 650 nm (the characteristic absorption peak of ox-TMB) is used as the absorption wavelength can be eliminated. We investigated this H₂O₂-free colorimetric protocol and obtained high sensitivity, with a detection limit of 1.5 nM and relatively high selectivity. The analytical performance for real samples was further explored. The Au@Pt NP-based H₂O₂-free colorimetric protocol is of great significance for the sensitive and selective determination of Cys in practical samples in different scenarios.

A single nucleotide polymorphism electrochemical sensor based on DNA-functionalized Cd-MOFs-74 as cascade signal amplification probes

An ultrasensitive electrochemical sensor has been constructed for the detection of single nucleotide polymorphisms (SNPs) based on DNA-functionalized Cd-MOFs-74 as cascade signal amplification probe under enzyme-free conditions. Interestingly, the introduction of an auxiliary probe did not disturb the detection of SNP targets, but could bind more Cd-MOFs-74 signal elements to enhance the different pulse voltammetry electrochemical signal 2~3 times as compared to sensing system without auxiliary probe, which obviously improves the sensitivity of the proposed sensor. Experimental results taking p53 tumor suppressor gene as SNP model demonstrated that the proposed method can be employed to sensitively and selectively detect target p53 gene fragment with a linear response ranging from 0.01 to 30 pmol/L (detection limit of 6.3 fmol/L) under enzyme-free conditions. Utilizing this strategy, the ultrasensitive SNP electrochemical sensor is a promising tool for the determination  of SNPs in biomedicine. Graphical Abstract.

Synthesis of POMOFs with 8-fold helix and its composite with carboxyl functionalized SWCNTs for the voltammetric determination of dopamine

Although many satisfactory studies have been developed for biomolecule detection, the complexity of biofluids still poses a major challenge to improve the performance of nanomaterials as electrochemical sensors. Herein, unprecedented polyoxometalate-based metal-organic frameworks (POMOFs) with 8-fold meso-helical feature, [Ag₅(trz)₄]₂[PMo₁₂O₄₀] (PAZ), were synthesized and explored as electrochemical sensors to detect dopamine (DA). To improve the conductivity of PAZ and the binding ability with single-walled carbon nanotubes (SWCNTs), the nanocomposite of carboxyl functionalized SWCNTs (SWCNTs-COOH) with nano-PAZ (NPAZ), NPAZ@SWCNTs-COOH, was fabricated, and transmission electron microscopy (TEM) shows that NPAZ can interact stably and uniformly with SWCNTs-COOH, owing to more defect sites on the surface of SWCNTs-COOH. The electrochemical result of NPAZ@SWCNTs-COOH/GCE towards detecting DA shows that the linear range was from 0.05 to 100 μM with a detection limit (LOD) of 8.6 nM (S/N = 3). A new electrochemical biosensing platform by combining 8-fold helical POMOFs with SWCNTs-COOH was developed for enhancing detection of dopamine for the first time, exhibiting the lowest detection limit to date.

Hollow POM@MOF-derived Porous NiMo6 @Co3 O4 for Biothiol Colorimetric Detection

Developing highly active and sensitive peroxidase mimics for _L -cysteine (_L -Cys) colorimetric detection is very important for biotechnology and medical diagnosis. Herein, polyoxometalate-doped porous Co₃ O₄ composite (NiMo₆ @Co₃ O₄ ) was designed and prepared for the first time. Compared with pure and commercial Co₃ O₄ , NiMo₆ @Co₃ O₄ (n) composites exhibit the enhanced peroxidase-mimicking activities and stabilities due to the strong synergistic effect between porous Co₃ O₄ and multi-electron NiMo₆ clusters. Moreover, the peroxidase-mimicking activities of NiMo₆ @Co₃ O₄ (n) composites are heavily dependent on the doping mass of NiMo₆ , and the optimized NiMo₆ @Co₃ O₄ (2) exhibits the superlative peroxidase-mimicking activity. More importantly, a sensitive _L -Cys colorimetric detection is developed with the sensitivity of 0.023 μM^-1 and the detection limit at least 0.018 μM in the linear range of 1-20 μM, which is by far the best enzyme-mimetic performances, to the best our knowledge.

Anderson polyoxometalates with intrinsic oxidase-mimic activity for "turn on" fluorescence sensing of dopamine

Anderson-type polyoxometalate containing Fe³⁺ and Mo⁶⁺, (NH₄)₃[H₆Fe(III)Mo₆O₂₄] (FeMo₆), was found to work as an oxidase-mimicking nanoenzyme for the first time, exhibiting the ability of catalytic oxidation of o-phenylenediamine (OPD), 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTs), and 3,3',5,5'-tetramethylbenzidine (TMB), which features easy synthesis, low cost, simple operation, and low consumption. Attributed to the nature of FeMo₆ and Fenton-like effect, a novel sensor based on two consecutive "turn on" fluorescence was developed for detecting dopamine (DA) by employing the FeMo₆-OPD system, and the linear range was from 1 to 100 μM with the detection limit 0.0227 μM (3σ/s). Moreover, to increase oxidase-mimic activity of FeMo₆, reduced graphene oxide (rGO) loading FeMo₆ composites (FeMo₆@rGO (n), n = 5%, 10%, 15%) was fabricated, and results show that oxidase-like activities of FeMo₆@rGO (n) are dependent on the mass ratio of FeMo₆/rGO, and FeMo₆@rGO (10%) exhibits the highest oxidase-mimic activity and the fastest respond time (4 min) among all reported oxidase mimic of DA to date. Graphical abstract Anderson-type Mo-POMs FeMo₆ was found to work as an oxidase-mimicking nanoenzyme for the first time and was used to detect DA for two consecutive "turn on" fluorescence sensor modes.

Highly Efficient Synthesis of p-Benzoquinones Catalyzed by Robust Two-Dimensional POM-Based Coordination Polymers

Selective oxidation of alkyl-substituted phenols offers efficient access to p-benzoquinones (BQs) that serve as key components for synthesizing biologically active compounds, but rational manufacture of efficient recyclable catalysts for such a reaction remains a severe challenge. Herein, two crystalline 2D polyoxometalate-based coordination polymers (POMCPs), formulated as H₃[Cu^I₃(L)₃]₂[PM₁₂O₄₀]·xH₂O (M = Mo, x = 4 for 1; M = W, x = 6 for 2; and HL = 4-(1H-tetazol-5-yl)pyridine), are prepared by a mineralizer-assisted one-step synthesis strategy and explored as heterogeneous catalysts for p-BQs synthesis. Both compounds have been characterized through elemental analysis, EDS analysis, infrared spectroscopy, UV-vis diffuse reflectance spectrum, EPR, XPS, BET, single-crystal, and powder X-ray diffraction. Single-crystal X-ray diffraction analysis indicates that both 1 and 2 exhibit an interesting 2D sheet structure composed of 2-connected Keggin type anions [PM₁₂O₄₀]^3- and hexa-nuclear {Cu^I₆(HL)₆} cluster-based metal-organic chains via Cu···O interactions. When used as catalysts, POMCPs 1 and 2 have excellent catalytic activities in the selective oxidation of substituted phenols to p-BQs with H₂O₂. Notedly, in the model reaction from 2,3,6-trimethylphenol (TMP) to the vitamin E key intermediate trimethyl-p-benzoquinone (TMBQ), the catalytic activities expressed by turnover frequency (TOF) of 1 and 2 can reach an unprecedented 2400 and 2000 h^-1, respectively, at close to 100% TMBQ yield. The truly heterogeneous nature, stability, and structural integrity of both catalysts were ascertained by FTIR, PXRD techniques, and the following cycles. Mechanism studies reveal that both catalysts can involve a dual reaction pathway through a heterolytic oxygen atom transfer mechanism and homolytic radical mechanism. Moreover, the 2D POMCPs with highly accessible bilateral active sites and efficient mass transfer efficiency possess superior catalytic performance to their analogous 3D species.

Encapsulation of Ultrafine Metal-Organic Framework Nanoparticles within Multichamber Carbon Spheres by a Two-Step Double-Solvent Strategy for High-Performance Catalysts

Carbon-encapsulated metal-organic framework (MOF) composite is one kind of emerging new catalyst with high efficiency and has gained much attention. However, for this kind of composite catalyst, the key to improving its catalytic activity and durability is to realize the effective dispersion of MOF nanoparticles (NPs) and enhance the interaction between MOF NPs and the carbon matrix, which remain a significant challenge. Herein, ultrafine MOF NPs within multichamber carbon spheres (MOF@MCCS), for the first time, have been rationally synthesized by a two-step double-solvent strategy for high-performance catalysts. The precise loading of guest MOFs can be achieved by adjusting the multichamber structure and calcination extent of the multichamber polymer (MCP), and the particle size of MOFs can be as low as 13.2 nm. Due to the formation of abundant carbon defects in the pyrolysis process of MCPs, the special structure and synergistic effect make the material exhibit higher catalytic activity and durability. More importantly, this method is universal and can be extended to different MOF systems. The two-step double-solvent strategy not only prepares a unique structure of MOF@MCCS-type host-guest-encapsulated catalysts but also provides a new idea for the design of high-efficiency catalysts with better performance and higher durability.

Construction of the POMOF@Polypyrrole Composite with Enhanced Ion Diffusion and Capacitive Contribution for High-Performance Lithium-Ion Batteries

Polyoxometalate (POM) as an "electronic sponge" can store a great number of electrons; however, shortcomings of poor conductivity and solubility in electrolytes cause a significant decrease in specific capacity and poor rate capability. To address the aforementioned disadvantages, a dual strategy was proposed, including coating the conductive polypyrrole (PPy) and utilizing nitrogenous ligands (1,10-phenanthroline monohydrate = 1,10-phen) for metal-organic frameworks (MOFs) to fabricate a [Cu(1,10-phen)(H₂O)₂]₂[Mo₆O₂₀]@PPy (Cu-POMOF@PPy) composite, effectively confining the POM in MOFs to avoid dissolution of POM in the electrolyte and improve electrochemical stability. Simultaneously, the PPy shell could improve the conductivity, contribute extra capacity, and alleviate volume variation of Cu-POMOF during cycling. Therefore, the final Cu-POMOF@PPy composite provides an excellent specific capacity of around 769 mA h g^-1 at 0.1 A g^-1 after 160 cycles and good rate performance, associated with great cycling stability (319 mA h g^-1 at 2 A g^-1 after 500 cycles). Moreover, the electrochemical reaction mechanism of Cu-POMOF@PPy was investigated by ex situ XPS measurements, indicating that storage of electrons results from the reduction/oxidation of Mo atoms (Mo⁶⁺ ↔ Mo⁴⁺) and Cu atoms (Cu²⁺ ↔ Cu⁰). As a consequence, this work not only proposes a novel method for preparing POM-based lithium-ion batteries but also expands the variety of anode materials.

Fe–N/C single-atom nanozyme-based colorimetric sensor array for discriminating multiple biological antioxidants

Fe–C/N single-atom nanozyme with oxidase-like activity was applied to constructed a triple-channel colorimetric sensor array for discriminating l-Cys, GSH, UA, AA and MT.

Metal–organic frameworks as photoluminescent biosensing platforms: mechanisms and applications

Recent progress of MOF-based photoluminescent platforms: a comprehensive overview of their applications in biosensing and underlying mechanisms.

Polyelectrolyte-functionalized reduced graphene oxide wrapped helical POMOF nanocomposites for bioenzyme-free colorimetric biosensing

For the sake of effective colorimetric sensing-pattern, a sensitive colorimetric sensor was conceived based on polyoxometalates based metal-organic frameworks (POMOFs) and polydiallyldimethylammonium chloride functionalized reduced graphene oxide (PDDA-rGO) for the first time, in which PDDA as a "glue" molecule turns rGO nanosheets into general platforms for bonding POMOFs nanoparticles. Herein, a new POMOF compound with fascinating helices-on-helices feature, [Ni₄(Trz)₆(H₂O)₂][SiW₁₂O₄₀]^.4H₂O (Trz = 1,2,4-triazole) (abbreviated as Ni₄SiW₁₂), was synthesized and characterized, then PDDA-rGO sheet as dispersive and conductive material was successfully introduced to Ni₄SiW₁₂ fabricating new PDDA-rGO/Ni₄SiW₁₂-n nanocomposites, (abbreviated as PMPG-n). The resulting PMPG-n nanocomposites as peroxidase mimetic show excellent catalytic activities under extreme condition (pH value 2.5), attributed to the nature and synergies from POMs, MOFs and PDDA-rGOs. Note that the peroxidase-like activity of PMPG-1 (the mass ratio of Ni₄SiW₁₂ to PDDA-rGO is 1:1) exhibits higher sensitivity (1-60 μM), faster response (10 min) and the lowest limit of detection (2.07 μM) among all reported materials to citric acid (CA) to date. This work opens up new application prospects in colorimetric sensing system for food quality control and safety, biotechnology and clinical diagnosis.

Engineering Inorganic Nanoflares with Elaborate Enzymatic Specificity and Efficiency for Versatile Biofilm Eradication

Nanozyme has emerged as a versatile nanocatalyst yet is constrained with limited catalytic efficiency and specificity for various biomedical applications. Herein, by elaborately integrating the recognition/transduction carbon dots (CDs) with platinum nanoparticles (PtNPs), an exquisite CDs@PtNPs (CPP) nanoflare is engineered as an efficient and substrate-specific peroxidase-mimicking nanozyme for high-performance biosensing and antibacterial applications. The intelligent CPP-catalyzed hydrogen peroxide (H₂ O₂ )-generated reactive oxygen species realize the sensitive diagnosis-guided enhanced disinfection of pathogens. Significantly, the CPP nanozyme shows the prominent biofilm eradication and wound healing in vivo by virtue of endogenous H₂ O₂ in acidic infection tissues, which can substantially preclude the annoying antibiotics resistance. A fundamental understanding on the present CPP nanoflare would not only facilitate the advancement of various prospective biocatalysts, but also establish a multifunctional means for versatile biosensing and smart diagnostic applications.

Assembly of 12-Tungstovanadate-Templated Nanocage and Nanocomposites with Single-Walled Carbon Nanotubes as Anodes in Lithium-Ion Batteries

A new 12-tungstovanadate-templated 3D nanocage framework, Ag₁₀(μ₄-ttz)₄(H₂O)₄(VW₁₂O₄₀) (VW₁₂@MOCF), was designed based on a "molecular library", hydrothermally synthesized, structurally characterized, and explored as anode material for lithium-ion batteries (LIBs). Combination of the structural superiority of VW₁₂@MOCF with the good electrical conductivity of the single-walled carbon nanotubes (SWNTs) renders the VW₁₂@MOCF/SWNT-2 nanocomposite reasonable electrochemical performance and stability as anode materials of LIBs. The successful cooperative fabrication of nanocages and polyoxometalate (POMs) must initiate extensive research interests.

Reprogrammable fluorescence logic sensing for biomolecules via RNA-like coenzyme A-based coordination polymer

In this study, coenzyme A (CoA)-based coordination polymers (CPs) have been generated in situ by exploiting the reaction of thiols with metal ion (Au(III) or Ag(I)), which are dependent on both thiol-metal and aurophilic metal∙metal interaction. It is interesting to note that CPs-related biosensing capabilities toward some biomolecules including ascorbic acid (AA), cysteine (Cys) and glutathione (GSH) are also investigated via SYBR Green II (SGII)-derived fluorescence switchable mechanisms. The synthesized CPs display especial RNA-like structure and are capable of initiating the fluorescence of SGII. Conversely, AA, Cys or GSH can give rise to the structural destruction of RNA-like CPs, thus inhibiting the fluorescence signal, and quantitative detection of these biomolecules are achieved favorably with a detection limit of 7.2, 0.55 and 0.48 nM, respectively. Meanwhile, the fascinating fluorescence on-off property and simple synthetic process are employed to build a series of basic logic gates (YES, NOT, OR, AND, INHIBIT and NOR) and multiple configurable logic gates (OR-AND and OR-OR-INHIBIT) along with different logic inputs. In view of these, developing CoA-based CPs as a new material to execute logic operations provides a valuable platform to establish the next generation of advanced molecular devices for clinic diagnostic and biomedical research.

A polypyrrole-coated eightfold-helical Wells-Dawson POM-based Cu-FKZ framework for enhanced colorimetric sensing

To explore a novel colorimetric biosensor with high sensibility and selectivity, a new Wells-Dawson-type polyoxometalate (POM)-based metal-organic framework (MOF) with an eightfold helix, [Cu₉(FKZ)₁₂(H₂O)₈][H₃P₂W₁₈O₆₂]₂·4H₂O (CuFKZP₂W₁₈) (HFKZ = 1-(2,4-difluorophenyl)-1,1-bis[(1H-1,2,4-triazol-1-yl)methyl] ethanol), was successfully synthesized; then, polypyrrole (PPy) was introduced to fabricate CuFKZP₂W₁₈/PPy(n) nanocomposites (n = 7%, 15%, 30%) via a facile in situ oxidation polymerization process. All the results indicate that CuFKZP₂W₁₈/PPy(15%) as a colorimetric biosensor exhibits lower limits of detection (0.07 μM towards H₂O₂ and 0.627 μM towards ascorbic acid), smaller K_m values (0.106 mM for H₂O₂ and 0.042 mM for o-phenylenediamine) and higher sensitivity (0.0227 1 μM^-1 towards H₂O₂ and 0.0025 1 μM^-1 to ascorbic acid) than most reported enzyme mimetics to the best of our knowledge.

Cobalt-based metal organic frameworks: a highly active oxidase-mimicking nanozyme for fluorescence “turn-on” assays of biothiol

With Co-MOFs as an oxidase-mimicking nanozyme, the AR oxidized product, non-fluorescent resazurin could be reduced to fluorescent resorufin by l-cysteine, which is specifically applied for fluorescence “turn-on” detection of l-cysteine.

High peroxidase-mimicking activity of gold@platinum bimetallic nanoparticle-supported molybdenum disulfide nanohybrids for the selective colorimetric analysis of cysteine

A sensitive and selective colorimetric sensor was designed for cysteine detection based on a high activity peroxidase-mimicking gold@platinum bimetallic nanoparticle-supported molybdenum disulfide nanohybrid.

Exploratory studies of a multidimensionally talented simple MnII-based porous network: selective “turn-on” recognition @ cysteine over homocysteine with an indication of cystinuria and renal dysfunction

Selective and real field detection of biothiols (Cys and Hcy) from aqueous and extra bio-matrices by a simple Mn^II-MOF.

Ni–Mo modified metal–organic frameworks for high-performance supercapacitance and enzymeless H2O2 detection

The growth process for A(B)-Ni_xMo_y-MOFs@AAC hybrids.

Multiunit Catalysts with Synergistic Reactivity: Three-Dimensional Polyoxometalate-Based Coordination Polymers for Highly Efficient Synthesis of Functionalized p-Benzoquinones

The rational design of highly efficient catalysts for the synthesis of functionalized p-benzoquinones (p-BQs) is of great significance for the manufacture of bioactive compounds. Herein, two 3D crystalline polyoxometalate-based coordination polymers (POMCPs) are used as heterogeneous catalysts for the synthesis of p-BQs, which are H[Cu^II(ttb)(H₂O)₃]₂[Cu^II(ttb)Cl]₂[PW₁₂O₄₀]·4H₂O (1) (Httb = 1-(tetrazol-5-yl)-4-(triazol-1-yl)benzene) and [ClCu₆^I(trz₎₄][ClCu₅^I(trz)₄]₂[Cu^II(H₂O)][PW₁₂O₄₀] (2) (trz = 1,2,4-triazole). Both compounds were characterized by elemental analysis, IR, XPS, solid diffuse reflective spectroscopy, TG analysis, and single-crystal X-ray diffraction. In 1, Keggin anions [PW₁₂O₄₀]^3- locate in 1D square channels constructed from wave-like Cu-ttb layers to form a 3D POMCP by coordinating to Cu ions, and in 2, [PW₁₂O₄₀]^3- anions situate in eight-membered Cu-trz channels via Cu···O interactions to yield a 3D POMCP structure. The catalytic activities of 1 and 2 have been evaluated in the selective oxidation of alkylphenols/alkoxybenzenes/methylnaphthalene, especially in the oxidation reaction of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate), with H₂O₂ as oxidant. By using catalysts 1 and 2 under optimal conditions, the yield of TMBQ can reach 99% and 96% within 10-20 min, respectively. Both catalysts demonstrated high turnover frequencies (300 h^-1 for 1 and 600 h^-1 for 2) and the truly heterogeneous nature. 1 and 2 catalyzed the synthesis of p-BQs on the basis of effective cooperative catalytic activities by POMs and metal nodes.

Heightened Integration of POM-based Metal-Organic Frameworks with Functionalized Single-Walled Carbon Nanotubes for Superior Energy Storage

To increase the conductivity of polyoxometalate-based metal-organic frameworks (POMOFs) and promote their applications in the field of energy storage, herein, a simple approach was employed to improve their overall electrochemical performances by introducing a functionalized single-walled carbon nanotubes (SWNT-COOH). A new POMOF compound, [Cu₁₈ (trz)₁₂ Cl₃ (H₂ O)₂ ][PW₁₂ O₄₀ ] (CuPW), was successfully synthesized, then the size-matched functionalized SWNT-COOH was introduced to fabricate CuPW/SWNT-COOH composite (PMNT-COOH) by employing a simple sonication-driven periodic functionalization strategy. When the PMNT-COOH nanocomposite was used as the anode material for Lithium-ion batteries (LIBs), PMNT-COOH(3) (CuPWNC:SWNT-COOH=3:1) showed superior behavior of energy storage, a high reversible capacity of 885 mA h g^-1 up to a cycle life of 170 cycles. The electrochemical results indicate that the uniform packing of SWNT-COOH provided a favored contact between the electrolyte and the electrode, resulting in enhanced specific capacity during lithium insertion/extraction process. This fabrication of PMNT-COOH nanocomposite opens new avenues for the design and synthesis of new generation electrode materials for LIBs.

Enzyme-Like Metal-Organic Frameworks in Polymeric Membranes for Efficient Removal of Aflatoxin B1

Biodegradation is a mild and efficient way to protect humans and animals from mycotoxins. However, microbes and enzymes are susceptible to environmental change, lack of stability, and reusability. In this work, three peroxidase-like metal-organic frameworks (MOFs), as artificial substitutes of natural peroxidase, are used for aflatoxin B₁ (AFB₁) removal, demonstrating the strong removal ability for AFB₁ and anti-interference ability toward other substances. There are distinct adsorption and catalytic properties among these MOFs that are mainly because of the differences in structure and Fe ion active sites. Then, we immobilized these MOFs into ultrafiltration membranes to form a multifunctional membrane (i.e., filtration, adsorption, and catalysis) for AFB₁ removal with good reusability that can be operated in simultaneous adsorption/catalysis or adsorption followed by catalysis/regeneration modes. Physicochemical analysis and animal experiments showed that the degradation products are probably several low-carbon substances whose toxic groups are cleaved.