A sensitive fluorescent assay for thiamine based on metal-organic frameworks with intrinsic peroxidase-like activity

Biodegradable bio-film based on Cordyceps militaris and metal-organic frameworks for fruit preservation

In this study, Cordyceps militaris matrix was employed for the first time to fabricate a biodegradable food packaging. Carmine and Ag@CuBTC were introduced to cross-link with mycelium and were uniformly dispersed within the matrix to enhance the water resistance, antimicrobial, and antioxidant properties of the bio-films. The bio-film displayed high biodegradability, with nearly 100 % degradation achieved after three weeks. The bio-film exhibited exceptional resistance to oxidation (49.30 % DPPH and 93.94 % ABTS•+), as well as effective inhibitory capabilities against E. coli and S. aureus, respectively. The composite film maintained a high CO2/O2 selective permeability, which was advantageous for mitigating fruit metabolism and extending shelf life. Simultaneously, food preservation experiments confirmed that these bio-films can decelerate the spoilage of fruits and effectively prolong the shelf-life of food. The experimental findings indicated that the prepared Bio-R-Ag@Cu film held promise as an environmentally friendly biodegradable material for food packaging.

Recent Progress and Prospect of Metal-Organic Framework-Based Nanozymes in Biomedical Application

A nanozyme is a nanoscale material having enzyme-like properties. It exhibits several superior properties, including low preparation cost, robust catalytic activity, and long-term storage at ambient temperatures. Moreover, high stability enables repetitive use in multiple catalytic reactions. Hence, it is considered a potential replacement for natural enzymes. Enormous research interest in nanozymes in the past two decades has made it imperative to look for better enzyme-mimicking materials for biomedical applications. Given this, research on metal-organic frameworks (MOFs) as a potential nanozyme material has gained momentum. MOFs are advanced hybrid materials made of inorganic metal ions and organic ligands. Their distinct composition, adaptable pore size, structural diversity, and ease in the tunability of physicochemical properties enable MOFs to mimic enzyme-like activities and act as promising nanozyme candidates. This review aims to discuss recent advances in the development of MOF-based nanozymes (MOF-NZs) and highlight their applications in the field of biomedicine. Firstly, different enzyme-mimetic activities exhibited by MOFs are discussed, and insights are given into various strategies to achieve them. Modification and functionalization strategies are deliberated to obtain MOF-NZs with enhanced catalytic activity. Subsequently, applications of MOF-NZs in the biosensing and therapeutics domain are discussed. Finally, the review is concluded by giving insights into the challenges encountered with MOF-NZs and possible directions to overcome them in the future. With this review, we aim to encourage consolidated efforts across enzyme engineering, nanotechnology, materials science, and biomedicine disciplines to inspire exciting innovations in this emerging yet promising field.

Fe-Ni dual-single atoms nanozyme with high peroxidase-like activity for sensitive colorimetric and fluorometric dual-mode detection of cholesterol

Cholesterol is widely existed in nerve myelin sheath and various membrane structures, whose abnormal level would deteriorate human cells or even cause diseases. Herein, Fe-Ni dual-single-atom nanozyme was efficiently incorporated into N-doped carbon nanosheets (FeNi DSAs/N-CSs) by a simple calcination method. Its nanozyme activity and catalytic mechanism were investigated in details. The FeNi DSAs/N-CSs nanozyme showed superior peroxidase-like property, which was applied for the dual-mode determination of hydrogen peroxide (H2O2) and cholesterol. The colorimetric/fluorometric assays of H2O2 displayed the linear ranges of 1-50 mM and 5-40 mM with low limits of detection of 0.45 mM and 3.33 mM, respectively. In parallel, there exhibited the linear ranges of 0.5-5.0 mM and 0.25-5.0 mM for the colorimetric/fluorometric analysis of cholesterol, coupled with the limits of detection down to 0.19 mM and 0.044 mM, respectively. This work provided a rapid, cost-effectiveness and simple colorimetric/fluorometric method for sensitive dual-mode detection of cholesterol in human serum samples.

Preparation of novel HKUST-1-glucose oxidase composites and their application in biosensing

Copper-based metal-organic frameworks (MOF) and multi-walled carbon nanotubes (HKUST-1-MWCNTs) composite were synthesized by one-step hydrothermal method, and PDA-enzyme-HKUST-1-MWCNTs composite was prepared by one-pot method for the construction of glucose biosensors, which realized the sensitive amperometric detection of glucose at 0.7 V (vs. SCE). The sensitivity of the sensor for glucose detection was 178 μA mM^-1cm^-2 in the wide linear range of 0.005 ~ 7.05 mM, the detection limit was 0.12 μM and the corresponding RSD was 3.8%. Its high performance is mainly benefitted from the high porosity and large specific surface area of HKUST-1, the good conductivity of MWCNTs, and the excellent adhesion and dispersion of PDA. The strategy of combining PDA and MWCNTs to improve the dispersion and conductivity of MOF is expected to achieve a wider application of MOF-based materials in the electrochemical biosensing field.

Aptamer Trimode Biosensor for Trace Glyphosate Based on FeMOF Catalytic Oxidation of Tetramethylbenzidine

The stable and highly catalytic Fe metal-organic framework (FeMOF) nanosol was prepared and characterized by electron microscopy, and energy and molecular spectral analysis. It was found that FeMOF strongly catalyzed the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ to produce TMBox, which had a fluorescence (FL) peak at 410 nm. When silver nanoparticles were added, it exhibited strong resonance Rayleigh scattering (RRS) activity and surface-enhanced Raman scattering (SERS) effect. This new FeMOF nanocatalytic trimode indicator reaction was combined with the glyphosate aptamer reaction to establish a new SERS/RRS/FL trimode biosensor for glyphosate. The sensor can be used for the analysis of environmental wastewater, and a new method for detecting glyphosate content in wastewater is proposed. The linear range of the sensor is 0.1-14 nmol/L, the detection limit is 0.05 nmol/L, the recovery is 92.1-97.5%, and the relative standard deviation is 3.6-8.7%.

New Insight into Assembled Fe3O4@PEI@Ag Structure as Acceptable Agent with Enzymatic and Photothermal Properties

Metal-based enzyme mimics are considered to be acceptable agents in terms of their biomedical and biological properties; among them, iron oxides (Fe₃O₄) are treated as basement in fabricating heterogeneous composites through variable valency integrations. In this work, we have established a facile approach for constructing Fe₃O₄@Ag composite through assembling Fe₃O₄ and Ag together via polyethyleneimine ethylenediamine (PEI) linkages. The obtained Fe₃O₄@PEI@Ag structure conveys several hundred nanometers (~150 nm). The absorption peak at 652 nm is utilized for confirming the peroxidase-like activity of Fe₃O₄@PEI@Ag structure by catalyzing 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂. The Michaelis–Menten parameters (K_m) of 1.192 mM and 0.302 mM show the higher catalytic activity and strong affinity toward H₂O₂ and TMB, respectively. The maximum velocity (V_max) value of 1.299 × 10⁻⁷ M∙s⁻¹ and 1.163 × 10⁻⁷ M∙s⁻¹ confirm the efficiency of Fe₃O₄@PEI@Ag structure. The biocompatibility illustrates almost 100% cell viability. Being treated as one simple colorimetric sensor, it shows relative selectivity and sensitivity toward the detection of glucose based on glucose oxidase. By using indocyanine green (ICG) molecule as an additional factor, a remarkable temperature elevation is observed in Fe₃O₄@PEI@Ag@ICG with increments of 21.6 °C, and the absorption peak is nearby 870 nm. This implies that the multifunctional Fe₃O₄@PEI@Ag structure could be an alternative substrate for formatting acceptable agents in biomedicine and biotechnology with enzymatic and photothermal properties.

Bifunctional Nitrogen and Fluorine Co-Doped Carbon Dots for Selective Detection of Copper and Sulfide Ions in Real Water Samples

Copper ions (Cu²⁺) and sulfur ions (S²⁻) are important elements widely used in industry. However, these ions have the risk of polluting the water environment. Therefore, rapid and quantitative detection methods for Cu²⁺ and S²⁻ are urgently required. Using 2,4-difluorobenzoic acid and L-lysine as precursors, nitrogen and fluorine co-doped dots (N, F-CDs) were synthesized in this study via a hydrothermal method. The aqueous N, F-CDs showed excellent stability, exhibited satisfactory selectivity and excellent anti-interference ability for Cu²⁺ detection. The N, F-CDs, based on the redox reactions for selective and quantitative detection of Cu²⁺, showed a wide linear range (0–200 μM) with a detection limit (215 nM). By forming the N, F-CDs@Cu²⁺ sensing platform and based on the high affinity of S²⁻ to Cu²⁺, the N, F-CDs@Cu²⁺ can specifically detect S²⁻ over a linear range of 0–200 μM with a detection limit of 347 nM. In addition, these fluorescent probes achieved good results when used for Cu²⁺ and S²⁻ detection in environmental water samples, implying the good potential for applications.

Sensitive and highly selective colorimetric biosensing of vitamin-C and vitamin-B1 by flavoring agent-based silver nanoparticles

A sensitive scheme was established for the detection of vitamin C (Ascorbic acid) and vitamin B1 (Thiamin HCl) using Maltol capped AgNPs (McAgNPs) as colorimetric sensor. The designed scheme showed an instant alteration in color from yellow to orange and green for vitamin-C and vitamin B1 sequentially. The probe was sensitive in a concentration range of (0-1 µM) with limit of detection 0.064 and 0.038 µM for vitamin C and vitamin B1 sequentially. The interaction mechanism between vitamin C and vitamin B1 and McAgNPs was evaluated by visible spectroscopy, FTIR, and AFM. Vitamin C attaches on the surface of nanoparticles by C=O group, while OH, C-S-C, and NH₂ groups are involved in the binding of vitamin B1 with McAgNPs. The Vit-C/Vit-B1-McAgNPs complexes were stable over a wide range of pHs. The size of McAgNPs increased after the interaction of vitamin C/vitamin B1 from 30-40 nm to 500 and 400 nm sequentially. The scheme was successfully applied for the detection of vitamin C and vitamin B1 in urine, plasma, water, and commercial pharmaceutical tablets with good recoveries. The scheme was ascertained to be more sensitive than many other formerly described schemes.

RuO2/rGO heterostructures as mimic peroxidases for colorimetric detection of glucose

The successful synthesis of ruthenium oxide/reduced graphene oxide (RuO₂/rGO) heterostructures by one-pot hydrothermal method using graphene oxides and RuCl₃ as precursors is reported. The heterostructures had high peroxidase-like (POD-like) activities, which catalyzes the oxidation of classical peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to create a blue colored reaction product. The catalytic activity was significantly enhanced by the synergistic effect between RuO₂ nanoparticles and rGO. RuO₂/rGO had a low K_m of 0.068 mM and a high v_max of 1.228 × 10^-7 M·s^-1 towards TMB in the TMB-H₂O₂ catalytic oxidation system. In addition, the POD-like activity originating from the electron transfer mechanism was confirmed by cytochrome C (Cyt C) oxidation experiment. A colorimetric method based on RuO₂/rGO heterostructures was developed with good sensitivity and selectivity for glucose detection with a limit of detection of 3.34 μM and a linear range of 0-1500 μM. The RuO₂/rGO heterostructures have potential applications in the biomedical areas, such as biosensor and diagnostics.

Catalase-integrated metal-organic framework with synergetic catalytic activity for colorimetric sensing

As a platform for enzyme immobilization, metal-organic frameworks (MOFs) can protect enzyme activity from the interference of external adverse environment. Although these strategies have been proven to produce good results, little consideration has been given to the functional similarity of MOFs to the encapsulated enzyme. Here, catalase (CAT) was encapsulated in Fe-BTC with peroxidase-like activity to obtain a stable composite (CAT@Fe-BTC) with synergistic catalytic activity. Depending on the superior selectivity and high catalytic activity of CAT@Fe-BTC, colorimetric sensing for the detection of hydrogen peroxide and phenol was developed. This work demonstrates that the integration of functional MOFs with natural enzyme can be well applied to the construction of efficient catalysts.

Glutamate Oxidase-Integrated Biomimetic Metal-Organic Framework Hybrids as Cascade Nanozymes for Ultrasensitive Glutamate Detection

The hybrid coupling of biocatalysts and chemical catalysts plays a vital role in the fields of catalysis, sensing, and medical treatment due to the integrated advantages in the high activity of natural enzymes and the excellent stability of nanozymes. Herein, a new nanozyme/natural enzyme hybrid biosensor was established for ultrasensitive glutamate detection. The MIL-88B(Fe)-NH₂ material with remarkable peroxidase mimic activity and stability was used as a nanozyme and carrier for immobilizing glutamate oxidase (GLOX) through Schiff base reaction to construct a chem-enzyme cascade detector (MIL-88B(Fe)-NH₂@GLOX). The resultant MIL-88B(Fe)-NH₂@GLOX exhibited a wide linear range (1-100 μM), with a low detection limit of 2.5 μM for glutamate detection. Furthermore, the MIL-88B(Fe)-NH₂@GLOX displayed excellent reusability and storage stability. After repeated seven cycles, MIL-88B(Fe)-NH₂-GLOX (GLOX was adsorbed on MIL-88B(Fe)-NH₂) lost most of its activity, whereas MIL-88B(Fe)-NH₂@GLOX still retained 69% of its initial activity. Meanwhile, MIL-88B(Fe)-NH₂@GLOX maintained 60% of its initial activity after storage for 90 days, while free GLOX only retained 30% of its initial activity. This strategy of integrating MOF mimics and natural enzymes for cascade catalysis makes it possible to design an efficient and stable chemo-enzyme composite catalysts, which are promising for applications in biosensing and biomimetic catalysis.

Explaining chemical clues of metal organic framework-nanozyme nano-/micro-motors in targeted treatment of cancers: benchmarks and challenges

Nowadays, nano-/micro-motors are considered as powerful tools in different areas ranging from cleaning all types of contaminants, to development of Targeted drug delivery systems and diagnostic activities. Therefore, the development and application of nano-/micro-motors based on metal–organic frameworks with nanozyme activity (abbreviated as: MOF-NZs) in biomedical activities have received much interest recently. Therefore, after investigating the catalytic properties and applications of MOF-NZs in the treatment of cancer, this study intends to point out their key role in the production of biocompatible nano-/micro-motors. Since reducing the toxicity of MOF-NZ nano-/micro-motors can pave the way for medical activities, this article examines the methods of making biocompatible nanomotors to address the benefits and drawbacks of the required propellants. In the following, an analysis of the amplified directional motion of MOF-NZ nano-/micro-motors under physiological conditions is presented, which can improve the motor behaviors in the propulsion function, conductivity, targeting, drug release, and possible elimination. Meanwhile, by explaining the use of MOF-NZ nano-/micro-motors in the treatment of cancer through the possible synergy of nanomotors with different therapies, it was revealed that MOF-NZ nano-/micro-motors can be effective in the treatment of cancer. Ultimately, by analyzing the potential challenges of MOF-NZ nano-/micro-motors in the treatment of cancers, we hope to encourage researchers to develop MOF-NZs-based nanomotors, in addition to opening up new ideas to address ongoing problems.

Application of Peroxidase-Mimic Mn2BPMP Boosted by ADP to Enzyme Cascade Assay for Glucose and Cholesterol

The Mn2BPMP complex has an intrinsic peroxidase-like activity in the pH range of 5 to 8, especially a maximum activity at pH 7, while most peroxidase mimics operate at an acidic pH (mainly pH 4). Its peroxidase-like activity is high among small-molecule-based peroxidase mimics with a high reproducibility. In addition, we recently revealed that adenosine mono/diphosphate (AMP and ADP) significantly boosted the peroxidase-like activity of Mn2BPMP. These advantages imply that Mn2BPMP is suitable for biosensing as a substitute for horseradish peroxidase (HRP). Herein, we established a colorimetric one-pot assay system using the enzyme cascade reaction between analyte oxidase and ADP-boosted Mn2BPMP. The simple addition of ADP to the Mn2BPMP-based assay system caused a greater increase in absorbance for the same concentration of H2O2, which resulted in a higher sensitivity. It was applied to one-pot detection of glucose and cholesterol at 25 °C and pH 7.0 for a few minutes.

Biocatalytic Metal-Organic Framework: Promising Materials for Biosensing

The high-efficient and specific catalysis of enzyme allow it to recognize a myriad of substrate that impels the biosensing. Nevertheless, the fragility of natural enzymes severely restricts their practical applications. Metal-organic frameworks (MOFs) with porous network and attractive functions have been intelligently employed as supports to encase enzymes for protecting them against hash environments. More importantly, the customizable construction and composition affords the intrinsic enzyme-like activity of some MOFs (known as nanozymes), which provides an alternative guideline to construct robust enzymes mimics. Herein, this review will introduce the concept of these biocatalytic MOFs, with the special emphasis on how the biocatalytic processes operated in these MOFs materials can reverse the plight of native enzymes-based biosensing. In addition, the present challenges and future outlooks in this research field are briefly put forward.

Highly sensitive colorimetric detection and effective adsorption of phosphate based on MOF-808(Zr/Ce)

MOF-808(Zr/Ce) has been successfully used for the sensitive and rapid detection of phosphate and phosphate removal by effective adsorption.

Metal Organic Frameworks as Biosensing Materials for COVID-19

The novel coronavirus disease (COVID-19) pandemic outbreak is the most startling public health crises with attendant global socio-economic burden ever experienced in the twenty-first century. The level of devastation by this outbreak is such that highly impacted countries will take years to recover. Studies have shown that timely detection based on accelerated sample testing and accurate diagnosis are crucial steps to reducing or preventing the spread of any pandemic outbreak. In this opinionated review, the impacts of metal organic frameworks (MOFs) as a biosensor in a pandemic outbreak is investigated with reference to COVID-19. Biosensing technologies have been proven to be very effective in clinical analyses, especially in assessment of severe infectious diseases. Polymerase chain reactions (PCR, RT-PCR, CRISPR) - based test methods predominantly used for SARS-COV-2 diagnoses have serious limitations and the health scientists and researchers are urged to come up with a more robust and versatile system for solving diagnostic problem associated with the current and future pandemic outbreaks. MOFs, an emerging crystalline material with unique characteristics will serve as promising biosensing materials in a pandemic outbreak such as the one we are in. We hereby highlight the characteristics of MOFs and their sensing applications, potentials as biosensors in a pandemic outbreak and draw the attention of researchers to a new vista of research that needs immediate action.

Enhanced oxidase-mimicking activity of Ce4+ by complexing with nucleotides and its tunable activity for colorimetric detection of Fe2

Complexing with adenosine-5'-monophosphate (AMP) was proven to be a facile way to enhance the oxidase-mimicking activity of Ce⁴⁺, and enabled nanoenzyme recovery and reuse. Additionally, the oxidase-mimicking activity of AMP-Ce⁴⁺ infinite coordination polymers (ICPs) could be specifically inhibited by Fe²⁺. Based on this finding, we developed a simple and highly selective colorimetric assay to detect Fe²⁺.

Light-responsive Au nanoclusters with oxidase-like activity for fluorescent detection of total antioxidant capacity

A fluorescent assay for total antioxidant capacity (TAC) detection based on the light-responsive oxidase-like activity of bovine serum albumin-stabilized gold nanoclusters (BSA-AuNCs) has been developed. Thiamine (TH) as the peroxidase substrate usually works at alkaline conditions and thus limits its practical applications. Here, by utilization the light-responsive oxidase-like activity of BSA-AuNCs, TH is oxidized to fluorescent thiochrome under neutral condition in two minutes due to the single oxygen generated by BSA-AuNCs upon light irradiation. After the introduction of antioxidants into the BSA-AuNCs-TH system, the formation of thiochrome is inhibited resulting in the fluorescence decrease. On the basis of the above facts, BSA-AuNCs-TH-based assay has been fabricated and applied successfully to detect antioxidants and the TAC of vitamin C tablets as well as some commercial fruit juice with satisfied results. This work may provide novel insights into developing light-responsive nanozymes-based fluorescent assays.

Tailoring metal-organic frameworks-based nanozymes for bacterial theranostics

Nanozymes are next-generation artificial enzymes having distinguished features such as cost-effective, enhanced surface area, and high stability. However, limited selectivity and moderate activity of nanozymes in the biochemical environment hindered their usage and encouraged researchers to seek alternative catalytic materials. Recently, metal-organic frameworks (MOFs) characterized by distinct crystalline porous structures with large surface area, tunable pores, and uniformly dispersed active sites emerged, that filled the gap between natural enzymes and nanozymes. Moreover, by selecting suitable metal ions and organic linkers, MOFs can be designed for effective bacterial theranostics. In this review, we briefly presented the design and fabrication of MOFs. Then, we demonstrated the applications of MOFs in bacterial theranostics and their safety considerations. Finally, we proposed the major obstacles and opportunities for further development in research on the interface of nanozymes and MOFs. We expect that MOFs based nanozymes with unique physicochemical and intrinsic enzyme-mimicking properties will gain broad interest in both fundamental research and biomedical applications.

Biomimetic two-dimensional nanozymes: synthesis, hybridization, functional tailoring, and biosensor applications

Biological enzymes play important roles in mediating the biological reactions in vitro and in vivo due to their high catalytic activity, strong bioactivity, and high specificity; however, they have also some disadvantages such as high cost, low environmental stability, weak reusability, and difficult production. To overcome these shortcomings, functional nanomaterials including metallic nanoparticles, single atoms, metal oxides, alloys, and others have been utilized as nanozymes to mimic the properties and functions of natural enzymes. Due to the development of the synthesis and applications of two-dimensional (2D) materials, 2D nanomaterials have shown high potential to be used as novel nanozymes in biosensing, bioimaging, therapy, logic gates, and environmental remediation due to their unique physical, chemical, biological, and electronic properties. In this work, we summarize recent advances in the preparation and functionalization, as well as biosensor and immunoassay applications of various 2D material-based nanozymes. To achieve this aim, first we demonstrate the preparation strategies of 2D nanozymes such as chemical reduction, templated synthesis, chemical exfoliation, calcination, electrochemical deposition, hydrothermal synthesis, and many others. Meanwhile, the structure and properties of the 2D nanozymes prepared by conjugating 2D materials with nanoparticles, metal oxides, biomolecules, polymers, ions, and 2D heteromaterials are introduced and discussed in detail. Then, the applications of the prepared 2D nanozymes in colorimetric, electrochemical, fluorescent, and electrochemiluminescent sensors are demonstrated.

Facile Colorimetric Nanozyme Sheet for the Rapid Detection of Glyphosate in Agricultural Products Based on Inhibiting Peroxidase-Like Catalytic Activity of Porous Co3O4 Nanoplates

The rapid and onsite detection of glyphosate herbicides in agricultural products is still a challenge. Herein, a novel colorimetric nanozyme sheet for the rapid detection of glyphosate has been successfully prepared through the physical adsorption of porous Co3O4 nanoplates on a polyester fiber membrane. Glyphosate can specifically inhibit the peroxidase-mimicking catalytic activity of porous Co3O4 nanoplates, thereby the visual detection of glyphosate can be realized by distinguishing the change in the color intensity of the established nanozyme sheet. The prepared nanozyme sheet has good sensitivity and selectivity, with a detection limit of 0.175 mg·kg-1 for glyphosate detection just by the naked eyes. It can effectively detect glyphosate within 10 min, and the color spots can maintain more than 20 min. The nanozyme sheet is not easily affected by the external environment in detection and storage. The merits of the nanozyme sheet facilitate its practical application in the large-scale preliminary screening of glyphosate residues in agricultural products.

Determination of butyrylcholinesterase activity based on thiamine luminescence modulated by MnO2 nanosheets

In this paper, a novel strategy for biosensing butyrylcholinesterase (BChE) activity is developed based on manganese dioxide (MnO₂) nanosheets to modulate the photoluminescence of thiamine (TH). The oxidase-like activity of MnO₂ nanosheets enables them to catalyze the oxidation of non-fluorescent substrate TH to generate strong fluorescent thiochrome (TC). When the target BChE is introduced to form thiocholine in the presence of S-butyrylthiocholine iodide (BTCh), MnO₂ nanosheets are reduced by thiocholine to Mn²⁺, resulting in the loss of their oxidase-like activity and the reduction of TC fluorescence. Based on this, a BChE activity fluorescence biosensor is constructed utilizing the luminescence behavior variation of TH and the oxidase-like activity of MnO₂ nanosheets. The fluorescence biosensor shows a sensitive response to BChE, and the detection limit reaches 0.036 U L^-1. In addition, the feasibility of the biosensor in real samples analysis is studied with satisfactory results.

Sensitive Fluorescence Assay for the Detection of Alkaline Phosphatase Based on a Cu2+-Thiamine System

The authors describe a novel, facile, and sensitive fluorometric strategy based on a Cu²⁺-thiamine (Cu²⁺-TH) system for the detection of alkaline phosphatase (ALP) activity and inhibition. The principle of the method is as follows. Under a basic conditions, TH, which does not exhibit a fluorescence signal, is oxidized into fluorescent thiochrome (TC) by Cu²⁺. Ascorbic acid 2-phosphate (AAP), which is the enzyme substrate, is hydrolyzed to produce ascorbic acid (AA) by ALP. The newly formed AA then reduces Cu²⁺ to Cu⁺, which prevents the oxidation of TH by Cu²⁺; as a result, the fluorescent signal becomes weaker. On the contrary, in the absence of ALP, AAP cannot reduce Cu²⁺; additions of Cu²⁺ and TH result in a dramatic increase of the fluorescent signal. The sensing strategy displays brilliant sensitivity with a detection limit of 0.08 U/L, and the detection is linear in the concentration range of 0.1 to 100 U/L. This approach was successfully applied to ALP activity in human serum samples, indicating that it is reliable and may be applied to the clinical diagnosis of ALP-related diseases.

Metal-Organic-Framework-Engineered Enzyme-Mimetic Catalysts

Nanomaterial-based enzyme-mimetic catalysts (Enz-Cats) have received considerable attention because of their optimized and enhanced catalytic performances and selectivities in diverse physiological environments compared with natural enzymes. Recently, owing to their molecular/atomic-level catalytic centers, high porosity, large surface area, high loading capacity, and homogeneous structure, metal-organic frameworks (MOFs) have emerged as one of the most promising materials in engineering Enz-Cats. Here, the recent advances in the design of MOF-engineered Enz-Cats, including their preparation methods, composite constructions, structural characterizations, and biomedical applications, are highlighted and commented upon. In particular, the performance, selectivities, essential mechanisms, and potential structure-property relations of these MOF-engineered Enz-Cats in accelerating catalytic reactions are discussed. Some potential biomedical applications of these MOF-engineered Enz-Cats are also breifly proposed. These applications include, for example, tumor therapies, bacterial disinfection, tissue regeneration, and biosensors. Finally, the future opportunities and challenges in emerging research frontiers are thoroughly discussed. Thereby, potential pathways and perspectives for designing future state-of-the-art Enz-Cats in biomedical sciences are offered.

Fluorometric Detection of Thiamine Based on Hemoglobin-Cu3(PO4)2 Nanoflowers (NFs) with Peroxidase Mimetic Activity

Component analysis plays an important role in food production, pharmaceutics and agriculture. Nanozymes have attracted wide attention in analytical applications for their enzyme-like properties. In this work, a fluorometric method is described for the determination of thiamine (TH) (vitamin B1) based on hemoglobin-Cu3(PO4)2 nanoflowers (Hb-Cu3(PO4)2 NFs) with peroxidase-like properties. The Hb-Cu3(PO4)2 NFs catalyzed the decomposition of H2O2 into ·OH radicals in an alkaline solution that could efficiently react with nonfluorescent thiamine to fluoresce thiochrome. The fluorescence of thiochrome was further enhanced with a nonionic surfactant, Tween 80. Under optimal reaction conditions, the linear range for thiamine was from 5 × 10-8 to 5 × 10-5 mol/L. The correlation coefficient for the calibration curve and the limit of detection (LOD) were 0.9972 and 4.8 × 10-8 mol/L, respectively. The other vitamins did not bring about any obvious changes in fluorescence. The developed method based on hybrid nanoflowers is specific, pragmatically simple and sensitive, and has potential for application in thiamine detection.

ZIF-67 as a Template Generating and Tuning "Raisin Pudding"-Type Nanozymes with Multiple Enzyme-like Activities: Toward Online Electrochemical Detection of 3,4-Dihydroxyphenylacetic Acid in Living Brains

Due to its unique structure and high porosity, metal-organic frameworks (MOFs) can act not only as nanozyme materials but also as carriers to encapsulate natural enzymes and thus have received extensive attention in recent years. However, a few research studies have been conducted to investigate MOF as a template to generate and tune nanozymes in the structure and performance. In this work, the "raisin pudding"-type ZIF-67/Cu_0.76Co_2.24O₄ nanospheres (ZIF-67/Cu_0.76Co_2.24O₄ NSs) were obtained by rationally regulating the weight ratio of ZIF-67 and Cu(NO₃)₂ in the synthesis process. Here, ZIF-67 not only acts as a template but also provides a cobalt source for the synthesis of cobalt copper oxide on the surface of ZIF-67/Cu_0.76Co_2.24O₄ NSs with multiple enzyme-like activities. The ZIF-67/Cu_0.76Co_2.24O₄ NSs can mimic four kinds of enzymes with peroxidase-like, glutathione peroxidase-like, superoxide dismutase-like, and laccase-like activities. Based on its laccase-like activity, an online electrochemical system for continuous monitoring of 3,4-dihydroxyphenylacetic acid with good linearity in the range of 0.5-20 μM and a detection limit of 0.15 μM was established. Furthermore, the alteration of DOPAC in the brain microdialysate before and after ischemia of the rats' brain was also successfully recorded. This work not only raises a new idea for the synthesis of nanozyme materials with multiple enzyme activities but also provides a new solution for the detection of neurotransmitters in living brains.

Hemin-Bridged MOF Interface with Double Amplification of G-Quadruplex Payload and DNAzyme Catalysis: Ultrasensitive Lasting Chemiluminescence MicroRNA Imaging

Here, we report a double-amplified sensing platform for ultrasensitive chemiluminescence (CL) miRNA detection in real patients' blood in which a hemin-bridged metal-organic framework (MOF) is employed as a functional interface to boost the payload and catalysis of G-quadruplex (G4) DNAzymes. Hemin is here used as the organic ligand for the MOF synthesis, which endows the MOF with an intrinsic peroxidase-like catalytic activity. Most importantly, the MOF surface provides a large amount of binding sites for polymeric G4 DNAzymes that are produced by miRNA-triggered rolling circle amplification reactions, and meanwhile, the interfaced G4 DNAzymes on MOFs (G4/MOFzymes) display an about 100-fold higher catalytic activity than those in solution. By using the G4/MOFzyme catalysts in the luminol/H₂O₂ CL system, the amplification detection of two acute myocardial infarction (AMI)-related miRNAs (low to 1 fM seen with naked eyes) is achieved in human serum with a smartphone as a portable imaging detector, which provides a facile methodology for point-of-care (POC) diagnosis of AMI. Compared with previous smartphone-based counterparts not requiring sophisticated equipment, this new facile methodology shows both 6 orders of magnitude higher sensitivity and an ∼50-fold longer duration for CL miRNA imaging. These unique features allow our developed G4/MOFzymes to be further employed as a novel luminescent ink for printing commonly used patterns.

Nanomagnet-Silica Nanoparticles Decorated with Au@Pd for Enhanced Peroxidase-Like Activity and Colorimetric Glucose Sensing

Nanomagnet-silica shell (Fe₃O₄@SiO₂) decorated with Au@Pd nanoparticles (NPs) were synthesized successfully. The characterization of Fe₃O₄@SiO₂-NH₂-Au@PdNPs was achieved using several spectroscopic and microscopic techniques. The quantitative surface analysis was confirmed using X-ray photoelectron spectroscopy. The Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs exhibited excellent peroxidase-like activity by effectively catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂. The absorption peaks at 370 and 652 nm confirmed the peroxidase-like activity of the Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs. The Michaelis-Menten constant (K_m) of 0.350 and 0.090 mM showed strong affinity toward H₂O₂ and TMB at Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs. The mechanism of the peroxidase-like activity was found to proceed via an electron transfer process. A simple colorimetric sensor based on glucose oxidase and Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs showed excellent selectivity and sensitivity towards the detection of glucose. The fabricated glucose biosensor exhibited a wide linear response toward glucose from 0.010 to 60.0 μM with an limit of detection of 60.0 nM and limit of quantification of 200 nM. The colorimetric biosensor based on Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs as a peroxidase mimic was also successfully applied for the determination of glucose concentrations in serum samples. The synthesized Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs nanozymes exhibited excellent potential as an alternative to horseradish peroxidase for low-cost glucose monitoring.

Metal–organic framework based nanozymes: promising materials for biochemical analysis

Metal–organic frameworks with enzyme-like catalytic features (MOF nanozymes) exhibit great promise in detecting various analytes with amplified signal outputs.

Glutenin-directed gold nanoclusters employed for assaying vitamin B1

Herein, glutenin-directed gold nanoclusters (AuNCs@Glu) with the red-fluorescence have been originally synthesized. Moreover, AuNCs@Glu could be employed to assay vitamin B₁.

Progress and Trend on the Regulation Methods for Nanozyme Activity and Its Application

Natural enzymes, such as biocatalysts, are widely used in biosensors, medicine and health, the environmental field, and other fields. However, it is easy for natural enzymes to lose catalytic activity due to their intrinsic shortcomings including a high purification cost, insufficient stability, and difficulties of recycling, which limit their practical applications. The unexpected discovery of the Fe3O4 nanozyme in 2007 has given rise to tremendous efforts for developing natural enzyme substitutes. Nanozymes, which are nanomaterials with enzyme-mimetic catalytic activity, can serve as ideal candidates for artificial mimic enzymes. Nanozymes possess superiorities due to their low cost, high stability, and easy preparation. Although great progress has been made in the development of nanozymes, the catalytic efficiency of existing nanozymes is relatively low compared with natural enzymes. It is still a challenging task to develop nanozymes with a precise regulation of catalytic activity. This review summarizes the classification and various strategies for modulating the activity as well as research progress in the different application fields of nanozymes. Typical examples of the recent research process of nanozymes will be presented and critically discussed.

Peroxidase-like Activity of Metal-Organic Framework [Cu(PDA)(DMF)] and Its Application for Colorimetric Detection of Dopamine

A metal-organic framework (MOF) [Cu(PDA)(DMF)] was synthesized under mild mixed solvothermal conditions. It is constructed by 1,10-phenanthroline-2,9-dicarboxylic acid (H₂PDA) and Cu²⁺ ions. The complex exhibits high peroxidase-like activity and can catalytically oxidize the colorless substrate 3,3',5,5'-tetramethylbenzidine to a blue product in the presence of H₂O₂. However, the peroxidase-like activity of [Cu(PDA)(DMF)] can be potently inhibited in the presence of dopamine. Based on this phenomenon, the colorimetric detection of dopamine was demonstrated with good selectivity and high sensitivity. [Cu(PDA)(DMF)] showed good stability and robust catalytic activity, which has been employed in the detection of dopamine in human urine and pharmaceutical samples.

Alkaline Hydrolysis Behavior of Metal-Organic Frameworks NH2-MIL-53(Al) Employed for Sensitive Immunoassay via Releasing Fluorescent Molecules

Nanosized metal-organic frameworks (MOFs) NH₂-MIL-53(Al) were synthesized from 2-aminoterephthalic acid (NH₂·H₂BDC) and AlCl₃ by a facile hydrothermal method. The synthesized MOFs displayed good stability and a uniform particle size in a netural medium and were hydrolyzed in alkaline medium to release a large amount of fluorescent ligand NH₂·H₂BDC. Therefore, they can act as large-capability nanovehicles to load signal molecules for investigating various biorecognition events. In this work, based on the alkaline hydrolysis behavior of MOFs NH₂-MIL-53(Al), a sensitive immunoassay method was developed for the detection of aflatoxin B₁ (AFB₁) by employing them as fluorescent signal probes. With a competitive immunoassay mode on microplate, AFB₁ can be detected within a linear range of 0.05-25 ng mL^-1. The method was successfully employed to detect AFB₁ spiked in Job tears, Polygala tenuifolia and with acceptable recovery values of 83.00-114.00%. The detection results for moldy Fructus xanthii displayed an acceptable agreement with those from the high-performance liquid chromatography method, with relative errors of -14.21 to 3.49%. With the merits of high sensitivity, facile manipulation, and ideal reliability, the approach can also be extended to other areas such as aptasensor and receptor-binding assay.

Development and validation of a dried blood spot test for thiamine deficiency among infants by HPLC-fluorimetry

Thiamine deficiency, if detected early in infancy, can be treated with thiamine supplementation and can prevent seizures, other disabilities and death. The dried blood spot (DBS) sampling technique is an attractive sample collection technique for infants. The present study reports the development and validation of a highly sensitive and precise method for quantification of thiamine diphosphate from DBS. The method utilizes full-spot analysis of a volumetrically deposited 40 μl DBS. The analyte was extracted from the DBS using 50% methanol and then derivatized using potassium ferricyanide to thiochrome. Separation was achieved with the help of an Inertsil ODS C₁₈ column (5.0 μm, 250 × 4.6 mm) using 150 mm phosphate buffer pH 7-acetonitrile (90:10, % v/v) as the mobile phase. The use of a fluorimetric detector gave a good response to the thiochrome derivative offering good sensitivity for the method. The excitation and emission wavelengths were 367 and 435 nm, respectively. The limit of detection and lower limit of quantification were 5 and 10 ng/ml, respectively. Linearity was demonstrated from 10 to 1000 ng/ml, and precision (CV) was <12.08%, at all tested quality control levels. The method accuracy was 89.34-118.89% with recoveries >80%. Bland-Altman analysis of DBS sampling vs. whole blood demonstrated a mean bias of only 1.16 ng/ml, with a majority of the 60 investigated patient samples lying within 7.2% of the corresponding concentration measured in blood, thereby meeting the clinical desirable biological specification criterion and showing that the two methods are comparable.

Discriminative Detection of Glutathione in Cell Lysates Based on Oxidase-Like Activity of Magnetic Nanoporous Graphene

As the most abundant intracellular biothiol, glutathione (GSH) plays a central role in many cellular functions and has been proved to be associated with numerous clinical diseases. Nevertheless, it is still a challenge to detect GSH over other mercaptoamino acids owing to their similar structures and activities. In this paper, magnetic nanoporous graphene (MNPG) nanocomposites were prepared for the first time through partial combustion of graphene oxide (GO) and ferric chloride. Due to the combination of porous graphene and magnetic nanoparticles, the MNPG nanocomposites exhibited large specific surface area, fast mass, and electron transport kinetics, resulting in remarkable oxidase mimic activity and easy separation. On the basis of the inhibition effect of GSH on the MNPG-catalyzed oxidation of thiamine, a novel and simple method for fluorescence determination of GSH was established. The sensor displayed a good linear response in the range of 0.2-20 μM toward GSH with a limit of detection of 0.05 μM. High sensitivity and selectivity facilitated its practical application for discriminative detection of GSH levels in PC12 cell lysates. The presented assay will be a simple and powerful tool to monitor intracellular GSH levels for biomedical diagnosis. Furthermore, the MNPG nanocomposites will provide insights to construct nanoporous graphene-based hybrids and push forward the advancement of porous graphene for wide applications.

Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications

Because of the high catalytic activities and substrate specificity, natural enzymes have been widely used in industrial, medical, and biological fields, etc. Although promising, they often suffer from intrinsic shortcomings such as high cost, low operational stability, and difficulties of recycling. To overcome these shortcomings, researchers have been devoted to the exploration of artificial enzyme mimics for a long time. Since the discovery of ferromagnetic nanoparticles with intrinsic horseradish peroxidase-like activity in 2007, a large amount of studies on nanozymes have been constantly emerging in the next decade. Nanozymes are one kind of nanomaterials with enzymatic catalytic properties. Compared with natural enzymes, nanozymes have the advantages such as low cost, high stability and durability, which have been widely used in industrial, medical, and biological fields. A thorough understanding of the possible catalytic mechanisms will contribute to the development of novel and high-efficient nanozymes, and the rational regulations of the activities of nanozymes are of great significance. In this review, we systematically introduce the classification, catalytic mechanism, activity regulation as well as recent research progress of nanozymes in the field of biosensing, environmental protection, and disease treatments, etc. in the past years. We also propose the current challenges of nanozymes as well as their future research focus. We anticipate this review may be of significance for the field to understand the properties of nanozymes and the development of novel nanomaterials with enzyme mimicking activities.

Fluorometric determination of the activity of inorganic pyrophosphatase and its inhibitors by exploiting the peroxidase mimicking properties of a two-dimensional metal organic framework

A copper(II)-based two-dimensional metal-organic framework with nanosheet structure (CuBDC NS) that possesses peroxidase (POx) mimicking activity was prepared. In the presence of hydrogen peroxide, the system catalyses the oxidation of terephthalic acid to a blue-fluorescent product (excitation = 315 nm; emission = 425 nm). Pyrophosphate has a very strong affinity for Cu²⁺ ion and blocks the POx-mimicking activity of the CuBDC NS. If, however, inorganic pyrophosphatase is present, the POx mimicking activity is gradually restored because pyrophosphate is hydrolyzed. The findings were used to design a method for the determination of the activity of inorganic pyrophosphatase by fluorometry. Fluorescence increases linearly in the 1-50 mU·mL^-1 inorganic pyrophosphatase activity range. The limit of detection is 0.6 mU·mL^-1 (S/N = 3). Graphical abstract A copper(II)-based two-dimensional metal-organic framework (CuBDC NS) is described that possesses POx-mimicking activity. Inorganic pyrophosphate (PPi) was hydrolyzed to phosphate in the presence of inorganic pyrophosphatase (PPase). Hence, it cannot coordinate with Cu²⁺ in CuBDC NS, its structure was well-conserved to catalyses the oxidation of terephthalic acid (H₂BDC) to produce a blue fluorescent product (oxBDC) in the presence of hydrogen peroxide (H₂O₂).

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes (II)

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.

Colorimetric determination of mercury(II) via the inhibition by ssDNA of the oxidase-like activity of a mixed valence state cerium-based metal-organic framework

This work demonstrates the inhibition effects of single-stranded (ssDNA) on the oxidase-like activity of a mixed-valence state cerium-based metal-organic framework, denoted as MVC-MOF. The MVC-MOF was synthesized by partial oxidation of cerium(III) which leads to the presence of both Ce(III) and Ce(IV) ions. The latter endows the MVC-MOF with a typical oxidase-like activity. However, on addition of ssDNA, the catalytic activity of the MVC-MOF is inhibited because it binds the MVC-MOF and thereby shield its active sites. This prevents the access of substrates. The inhibition by ssDNA depends on its length but not its sequence. By contrast, negligible changes in the oxidase-mimicking activity are observed if double-stranded DNA (dsDNA) is added. By employing a thymine-rich ssDNA (T-ssDNA) as a model DNA, a colorimetric assay was developed for the determination of Hg(II). This ion binds to T-ssDNA and causes the formation of T-dsDNA. Hence, the oxidase-mimicking activity is compromised. By using the oxidase substrate 3,3',5,5'-tetramethylbenzidine that gives a colored product in the presence of oxygen, the assay has a linear response that covers 0.05 to 6 μM Hg(II) with a detection limit of 10.5 nM, and exhibits high selectivity over other metal ions. The assay was successfully applied to the determination of Hg(II) in environmental water samples. Graphical abstract Schematic of the inhibition effect of ssDNA on the oxidase-like activity of MVC-MOF that converts colorless TMB to oxTMB with blue color in the presence of oxygen, and its application in the construction of a colorimetric assay for Hg(II) determination.