Advances in metal–organic framework-based nanozymes and their applications

Portable Visual Photoelectrochemical Biosensor Based on a MgTi2O5/CdSe Heterojunction and Reversible Electrochromic Supercapacitor for Dual-Modal Cry1Ab Protein Detection

Reversible electrochromic supercapacitors (ESCs) have attracted considerable interest as visual display screens. The use of ESCs in combination with a photoelectrochemical (PEC) biosensor promises to improve the detection efficiency. Herein, a visual PEC biosensor is developed by introducing a circuit module between a PEC-sensing platform (PSP) and a reversible ESC for Cry1Ab protein detection. In PSP, a type II MgTi2O5/CdSe heterojunction effectively drives charge separation by their cross-matched band gap structures, generating an amplified photocurrent. Next, the circuit module is designed to connect the PSP and ESC, realizing the signal conversion from photocurrent to voltage. ESC, as a visual display screen, produces reversible color changes with different voltages. As the concentration of Cry1Ab increases, the photocurrent decreases due to the specific binding between the aptamer and Cry1Ab in PSP, while the color of the reversible ESC changes from green to blue. To improve the integrity of the device, a portable PEC biosensor is further constructed via three-dimensional printing for dual-modal Cry1Ab protein detection, thus collecting both PEC and visual signals. The linear ranges are 0.3-3000 ng mL-1 for PEC mode and 1-1000 ng mL-1 for visual mode. This work presents a portable, efficient, sensitive, and visualized detection system, providing an important reference for practical visualization applications.

Metal Site and Size-Controlled BTC-Based MOF as Cysteine Oxidase Mimic for Self-Cascade Detection of Cysteine and Hg2

Nanozyme-mediated strategy for sensing has been widely applied nowadays, in which the construction of a nanozyme cascade platform is an effective and challenging method to simulate the complexity and multifunctionality of natural systems. Herein, a simple and convenient self-cascade sensing platform was developed for the fluorescent detection of cysteine and Hg2+ by a BTC-based MOF through screening the metal sites and crystal sizes. By the introduction of polyvinylpyrrolidone, the as-prepared Cu-BTC possessed a metal center of Cu2+ and smaller size, which exhibited both cysteine oxidase- and peroxidase-like activities. The dual enzymic characters of Cu-BTC made a self-cascade reaction occur during which cysteine was first oxidized to cystine and generated H2O2 in the presence of O2, then H2O2 was decomposed into ·OH, and finally the ·OH triggered the turn-on fluorescence of Cu-BTC. Based on the self-cascade reactions and high affinity of Hg2+ and -SH within cysteine, a fluorescent method was developed to detect cysteine and Hg2+ with a range of 0-160/0-15 μM and a limit of detection of 0.04/0.09 μM, respectively. This work reveals the important role of the Cu2+ center for mimicking cysteine oxidase and gives a feasible strategy for constructing simple self-cascade reactions.

Exploring the potential of iron-based metal-organic frameworks as peroxidase nanozymes for glucose detection with various secondary building units

Finding materials in biosensing that balance enzyme-like reactivity, stability, and affordability is essential for the future. Because of their unique peroxidase properties, including variable pore size, surface area, and Lewis acid active sites, iron-based metal-organic frameworks (MOFs) have evolved as viable possibilities. In this study, we constructed a Fe-MOF and tested its peroxidase-like activity and responsiveness toward H2O2 colorimetric techniques. Using encapsulation, we incorporated glucose oxidase into the ZIF-90 PVP MOF and conducted a sequential reaction with the Fe-MOF to detect glucose. The results showed better peroxidase catalytic activity of the MIL-88B(Fe) (1,4-NDC) MOF and similar secondary building unit (SBU) Fe-MOFs were studied in other peroxidase nanozyme studies. When combined with an enzyme-encapsulating ZIF-90 PVP MOF, they could be sequentially employed for glucose detection purposes. This study highlights the potential of nanozymes as an alternative to natural enzymes, with promising applications in biosensing and beyond.

Two-Site Enhanced Porphyrinic Metal-Organic Framework Nanozymes and Nano-/Bioenzyme Confined Catalysis for Colorimetric/Chemiluminescent Dual-Mode Visual Biosensing

The rational design of efficient nanozymes and the immobilization of enzymes are of great significance for the construction of high-performance biosensors based on nano-/bioenzyme catalytic systems. Herein, a novel V-TCPP(Fe) metal-organic framework nanozyme with a two-dimensional nanosheet morphology is rationally designed by using V2CTx MXene as a metal source and iron tetrakis(4-carboxyphenyl)porphine (FeTCPP) ligand as an organic linker. It exhibits enhanced peroxidase- and catalase-like activities and luminol-H2O2 chemiluminescent (CL) behavior. Based on the experimental and theoretical results, these excellent enzyme-like activities are derived from the two-site synergistic effect between V nodes and FeTCPP ligands in V-TCPP(Fe). Furthermore, a confined catalytic system is developed by zeolitic imidazole framework (ZIF) coencapsulation of the V-TCPP(Fe) nanozyme and bioenzyme. Using the acetylcholinesterase (AChE) as a model, our constructed V-TCPP(Fe)/AChE@ZIF confined catalytic system was successfully used for the colorimetric/CL dual-mode visual biosensing of organophosphorus pesticides. This work is expected to provide new insights into the design of efficient nanozymes and confined catalytic systems, encouraging applications in catalysis and biosensing.

Secondary metal doped cuprous-cyanoimidazole frameworks for triple-mode detection of dopamine

BACKGROUNDS

Metal-organic framework-based nanozymes enable several opportunities for designing novel analysis methods for the detection of pesticides, heavy metal ions, and biomolecules; however, practical applications are still limited by a complicated synthesis route, lower catalytic activity, and single detection mode. Dopamine (DA) is a crucial catecholamine substance in the human body that acts as a neurotransmitter regulating a variety of physiological functions of the central nervous system. Therefore, it is highly significant to explore simple nanozymes synthesis methods for constructing a multiple analysis system to detection DA.

RESULTS

Herein, we elaborately selected cobalt ions as the secondary metal doping in cuprous-cyanoimidazole frameworks (CuCo-CIFs) with a mass-production strategy. CuCo-CIFs possess intrinsic peroxidase-like activity that can convert hydrogen peroxide into various reactive oxygen species (i.e., 1O2, OH·, O2·-) and thereby oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) and DA to blue oxTMB and orange polydopamine (PDA), respectively. The absorption of the detection system increases at 460 nm while decreases at 652 nm as the concentration of DA increases under near-neutral pH (6.1), resulting in a color transition from blue to orange. Consequently, an unprecedented triple-mode analysis system of DA monitored by naked eyes, ratiometric-absorption, and scanometric was constructed. The limit of detection for the ratiometric-absorption and scanometric mode can reach 20 nM and 28 nM, respectively. CuCo-CIFs were successfully used for the rapid and accurate detection of DA in practical samples.

SIGNIFICANCE

As a simple, low-cost, multi-mode colorimetric platform, this kind of nanozyme detection with peroxidase-like activity exhibits significant potential for the detection of DA. Our work not only expands the applications of MOFs in analytical fields but also addresses the general challenges faced by nanozyme-based colorimetric detection systems of DA. This work provides valuable insights for the rational application of nanozyme and the design of new analysis systems.

Reactive oxygen nanobiocatalysts: activity-mechanism disclosures, catalytic center evolutions, and changing states

Benefiting from low costs, structural diversities, tunable catalytic activities, feasible modifications, and high stability compared to the natural enzymes, reactive oxygen nanobiocatalysts (RONBCs) have become dominant materials in catalyzing and mediating reactive oxygen species (ROS) for diverse biomedical and biological applications. Decoding the catalytic mechanism and structure-reactivity relationship of RONBCs is critical to guide their future developments. Here, this timely review comprehensively summarizes the recent breakthroughs and future trends in creating and decoding RONBCs. First, the fundamental classification, activity, detection method, and reaction mechanism for biocatalytic ROS generation and elimination have been systematically disclosed. Then, the merits, modulation strategies, structure evolutions, and state-of-art characterisation techniques for designing RONBCs have been briefly outlined. Thereafter, we thoroughly discuss different RONBCs based on the reported major material species, including metal compounds, carbon nanostructures, and organic networks. In particular, we offer particular insights into the coordination microenvironments, bond interactions, reaction pathways, and performance comparisons to disclose the structure-reactivity relationships and mechanisms. In the end, the future challenge and perspectives for RONBCs are also carefully summarised. We envision that this review will provide a comprehensive understanding and guidance for designing ROS-catalytic materials and stimulate the wide utilisation of RONBCs in diverse biomedical and biological applications.

Impedimetric aptasensor based on MOF based composite for measuring of carcinoembryonic antigen as a tumor biomarker

In this research, gold nanoparticle (GNPs)-modified metal-organic framework/reduced graphene oxide (MOF(801)/rGO) hybrid was employed to design a new aptasensor for carcinoembryonic antigen (CEA) quantification in biological sample. The sensing ability of the electrode for CEA biomarker was examined with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry procedures. Besides, CEA was electrochemically quantified by the EIS method. With respect to the high surface-to-volume ratio of MOF(801) and the good electron transfer ability of rGO, the proposed sensor displayed notable sensitivity and reliability in the CEA analysis. The derived electrode showed an appreciable detection limit of 0.8 pg L-1 using EIS protocol. In addition, the present aptasensor revealed diverse advantages including anti-interference property, wide linear range (0.0025-0.25 ng L-1), convenience and high efficiency toward CEA quantification. More importantly, the performance of the suggested assay remains unchanged in analysis of CEA in body fluids. The established assay demonstrates that the suggested biosensor is a promising device in clinical diagnosis.

Rapid Trace Detection of Sulfite Residue in White Wine Using a Multichannel Colorimetric Nanozyme Sensor

As a commonly used food additive, sulfite (SO32-) is popular with food manufacturers due to the functions of bleaching, sterilizing, and oxidation resistance. However, excess sulfites can pose a threat to human health. Therefore, it is particularly important to achieve rapid and sensitive detection of SO32-. Herein, a colorimetric sensor was invented for visual, meticulous, and rapid detection of SO32- based on MIL-53(Fe/Mn). Bimetallic nanozyme MIL-53(Fe/Mn) was prepared by a one-pot hydrothermal reaction. The prepared MIL-53(Fe/Mn) can effectively catalyze the oxidation of colorless TMB to a blue oxidation product (oxTMB). The introduction of SO32- causes significant discoloration of the reaction system, gradually transitioning from a visible blue color to colorless. Hence, a sensitive colorimetric sensor for SO32- detection was developed based on the decolorization degree of the detection system. Further, the discoloration was ascribed to the inactivation of nanozyme and the strong reducing ability of SO32-. Under the optimal experimental conditions, there was a good linear relationship between the absorbance at 652 nm and SO32- concentration in the linear range of 0.5-6 μg mL-1 with a limit of detection (LOD) of 0.05 μg mL-1. The developed method was successfully applied to the detection of actual samples of white wine with good accuracy and recovery. Compared to traditional methods, this colorimetric sensor produces similar detection results but significantly reduces the detection time. Compared to traditional methods, this colorimetric sensor can not only reduce the detection costs effectively but also help the food industry maintain quality standards. Strong anti-interference capability, simple operation, and low detection limits ensure the excellent performance of the colorimetric sensor in detecting SO32- in white wine. The combination of a smartphone and a colorimetric analysis application has also greatly facilitated the semi-quantitative, visual on-site detection of SO32-, which has opened up an application prospect of an MIL-53(Fe/Mn)-based detection platform. Our work has indicated a new direction for the detection of SO32- and provided important assurance for food safety.

Hydrolase mimic via second coordination sphere engineering in metal-organic frameworks for environmental remediation

Enzymes achieve high catalytic activity with their elaborate arrangements of amino acid residues in confined optimized spaces. Nevertheless, when exposed to complicated environmental implementation scenarios, including high acidity, organic solvent and high ionic strength, enzymes exhibit low operational stability and poor activity. Here, we report a metal-organic frameworks (MOFs)-based artificial enzyme system via second coordination sphere engineering to achieve high hydrolytic activity under mild conditions. Experiments and theoretical calculations reveal that amide cleavage catalyzed by MOFs follows two distinct catalytic mechanisms, Lewis acid- and hydrogen bonding-mediated hydrolytic processes. The hydrogen bond formed in the secondary coordination sphere exhibits 11-fold higher hydrolytic activity than the Lewis acidic zinc ions. The MOFs exhibit satisfactory degradation performance of toxins and high stability under extreme working conditions, including complicated fermentation broth and high ethanol environments, and display broad substrate specificity. These findings hold great promise for designing artificial enzymes for environmental remediation.

Metal-organic framework (MOF) hybridized gold nanoparticles as a bifunctional nanozyme for glucose sensing

Inspired by natural enzymes that possess multiple catalytic activities, here we develop a bifunctional metal-organic frame-work (MOF) for biosensing applications. Ultrasmall gold nano-particles (AuNPs) are grown in the internal cavities of an iron (Fe) porphyrin-based MOF to produce a hybridized nanozyme, AuNPs@PCN-224(Fe), in which AuNPs and PCN-224(Fe) exhibit the catalytic activity of glucose oxidase (GOx) and horseradish peroxidase (HRP), respectively. We established that the bifunctional nanozyme was capable of a cascade reaction to generate hydrogen peroxide in the presence of d-glucose and oxygen in situ, and subsequently activate a colorimetric or chemiluminescent substrate through HRP-mimicking catalytic activity. The nanozyme was selective over a range of other saccharides, and 93% of the catalytic activity was retained after being recycled five times.

A PdPt nanoparticle-decorated thiol-functionalized MOF with high peroxidase-like activity for colorimetric sensing of D-glucose and chlorophenol isomers

The peroxidase (POD)-like catalytic activity of various nanozymes was extensively applied in many significant fields. In this study, a thiol-functionalized MOF-loaded PdPt nanocomposite (UiO-66-(SH)₂@PdPt) was fabricated, which possesses superior and selective POD-like activity with strong affinity towards H₂O₂ and 3,3',5,5'-tetramethylbenzidine under mild conditions. The POD-like property of UiO-66-(SH)₂@PdPt was used to sensitively detect the concentration of D-glucose under near-neutral (pH = 6.5) conditions. The detection limit of D-glucose was as low as 2.7 μM, and the linear range of D-glucose was 5-700 μM. In addition, UiO-66-(SH)₂@PdPt could accelerate the oxidative coupling chromogenic reaction of chlorophenol (CP) and 4-aminoantipyrine (4-AAP) in the presence of H₂O₂. Based on this phenomenon, a simple and visualized sensing array for the identification of chlorophenol contaminant isomers was further constructed to finally achieve the effective differentiation of three monochlorophenol isomers and six dichlorophenol isomers. Furthermore, a colorimetric detection method for 2-chlorophenol and 2,4-dichlorophenol was established. This work provides an effective means to improve the catalytic activity and selectivity of nanozymes by introducing an ideal carrier, which will be of significant value for the design of efficient nanozymes.

Multimetallic nanoparticles decorated metal-organic framework for boosting peroxidase-like catalytic activity and its application in point-of-care testing

Metal-organic frameworks (MOFs) are a sort of promising peroxidase-like nanozyme but face the challenge that the inorganic nodes in most of the MOF structures are generally blocked by the organic linkers. Further enhancement or activation of their peroxidase-like activity plays an important role in developing MOF-based nanozymes. Herein, a multimetallic nanoparticle (NP) decorated-MOF, Cu/Au/Pt NP decorated-Cu-TCPP(Fe) nanozyme (CuAuPt/Cu-TCPP(Fe)) was synthesized in situ and served as a peroxidase-like nanozyme. The peroxidase-like activity of this stable CuAuPt/Cu-TCPP(Fe) nanozyme was enhanced due to the decreased potential barriers for *OH generation in the catalytic process. Owing to the remarkable peroxidase-like activity, a CuAuPt/Cu-TCPP(Fe)-based colorimetric assay was established for the sensitive determination of H₂O₂ and glucose with the limit of detection (LOD) of 9.3 µM and 4.0 µM, respectively. In addition, a visual point-of-care testing (POCT) device was developed by integrating the CuAuPt/Cu-TCPP(Fe)-based test strips with a smartphone and was employed for a portable test of 20 clinical serum glucose samples. The results determined by this method agree well with the values deduced by clinical automatic biochemical analysis. This work not only represents an inspiration for the usage of MNP/MOF composite as a novel nanozyme for POCT diagnosis, but also provides a deeper insight and understanding into the enhanced enzyme-mimic effect of MNP-hybrid MOF composites, which in turn will guide the engineering of MOF-based functional nanomaterials. Graphical Abstract.

Highly Active CoRh Graphitic Nanozyme for Colorimetric Sensing in Real Samples

Rh-based nanozymes show high catalytic efficiency, specific surface area, good stability, and unique physicochemical properties, while magnetic nanozymes facilitate the magnetic separation of detection samples under an external magnetic field for improved sensitivity. However, magnetic Rh nanozymes, especially those with excellent stability, have not been reported. Herein, we apply the chemical vapor deposition (CVD) method to prepare a CoRh graphitic nanozyme (termed as CoRh@G nanozyme), which structurally consists of CoRh nanoalloy encapsulated by a few layers of graphene for sensitive colorimetric sensing applications. The proposed CoRh@G nanozyme has superior peroxidase (POD)-like activity, and it shows higher affinity of the CoRh@G nanozyme than horseradish peroxidase (HRP) toward 3,3',5,5'-tetramethylbenzydine (TMB) oxidation. In addition, the CoRh@G nanozyme shows high durability and superior recyclability owing to its protective graphitic shell. The outstanding merits of the CoRh@G nanozyme allow its use for quantitative colorimetric detection of dopamine (DA) and ascorbic acid (AA), showing high sensitivity and good selectivity. Moreover, it shows satisfactory performance for AA detection in commercial beverages and energy drinks. The proposed CoRh@G nanozyme-based colorimetric sensing platform shows great promise in point-of-care (POC) visual monitoring.

DNA controllable peroxidase-like activity of Ti3C2 nanosheets for colorimetric detection of microcystin-LR

The peroxidase-like activity of Ti₃C₂ nanosheets (Ti₃C₂ NSs) was evaluated by catalytic oxidation of colorless o-phenylenediamine (OPD) into orange-yellow 2,3-diaminophenazine (DAP) with the aid of H₂O₂. The catalytic behavior followed the typical Michaelis-Menten kinetics. Systematic studies about the catalytic activity of Ti₃C₂ NSs including cytochrome C (Cyt C) electron transfer experiments, radical capture experiments, and fluorescence analysis were conducted, revealing that the catalytic mechanism of Ti₃C₂ NSs was attributed to nanozyme-accelerated electron transfer between substrates and nanozyme-promoted generation of active species (superoxide anion free radical (·O₂^-) and holes (h⁺)). Single-stranded DNA (ssDNA) inhibited the peroxidase-like activity of Ti₃C₂ NSs, and the reduced catalytic activity was ascribed to DNA-hindered substrate accessibility to nanozyme surface. Based on the DNA controllable peroxidase-mimicking activity of Ti₃C₂ NSs, taking microcystin-LR (MC-LR) aptamer as an example, a label-free colorimetric aptasensor was proposed for the sensitive detection of MC-LR. The colorimetric aptasensor showed a wide linear range (0.01-60 ng mL^-1), low limit of detection (6.5 pg mL^-1), and high selectivity. The practicality of the colorimetric aptasensor was demonstrated by detecting different levels of MC-LR in spiked real water samples; satisfactory recoveries (97.2-102.1%) and low relative standard deviations (1.16-3.72%) were obtained.

Rapid and ultrasensitive activity detection of α-amylase based on γ-cyclodextrin crosslinked metal-organic framework nanozyme

α-Amylase plays a significant part in fermentation and the food industry, as this enzyme effectively regulates the content of different sugars in brewing systems and affects the yield and quality of alcoholic beverages. Nevertheless, current strategies suffer from unsatisfactory sensitivity and are time-consuming or are indirect methods which demand the assistance of tool enzymes or inhibitors. Therefore, they are unsuitable for the low bioactivity and non-invasive detection of α-amylase in fermentation samples. Rapid, sensitive, facile, and direct detection method of this protein remains challenging in actual applications. In this work, a nanozyme-based α-amylase assay was constructed. The colorimetric assay used the interaction between α-amylase and γ-cyclodextrin (γ-CD) which crosslinks MOF-919-NH₂. The determination mechanism bases on the hydrolysis of γ-CD by α-amylase, resulting in increased peroxidase-like bioactivity of the released MOF nanozyme. The detection limit was 0.12 U L^-1 with a wide linear range (0-200 U L^-1) and excellent selectivity. Additionally, the proposed detection method was successfully utilized in distilled yeasts to verify analytical capability in fermentation samples. The exploration of this nanozyme-based assay not only provides a convenient and effective strategy for enzyme activity determination in food industry, but also has promotion significance in clinical diagnosis and pharmaceutical production.

From liver fibrosis to hepatocarcinogenesis: Role of excessive liver H2O2 and targeting nanotherapeutics

Liver fibrosis and hepatocellular carcinoma (HCC) have been worldwide threats nowadays. Liver fibrosis is reversible in early stages but will develop precancerosis of HCC in cirrhotic stage. In pathological liver, excessive H₂O₂ is generated and accumulated, which impacts the functionality of hepatocytes, Kupffer cells (KCs) and hepatic stellate cells (HSCs), leading to genesis of fibrosis and HCC. H₂O₂ accumulation is associated with overproduction of superoxide anion (O₂^•−) and abolished antioxidant enzyme systems. Plenty of therapeutics focused on H₂O₂ have shown satisfactory effects against liver fibrosis or HCC in different ways. This review summarized the reasons of liver H₂O₂ accumulation, and the role of H₂O₂ in genesis of liver fibrosis and HCC. Additionally, nanotherapeutics targeting H₂O₂ were summarized for further consideration of antifibrotic or antitumor therapy.

•

Liver fibrosis and HCC are closely related because ROS induced liver damage and inflammation, especially over-cumulated H₂O₂.

•

Excess H₂O₂ diffusion in pathological liver was due to increased metabolic rate and diminished cellular antioxidant systems.

•

Freely diffused H₂O₂ damaged liver-specific cells, thereby leading to fibrogenesis and hepatocarcinogenesis.

•

Nanotherapeutics targeting H₂O₂ are summarized for treatment of liver fibrosis and HCC, and also challenges are proposed.

Intelligent Pd1.7Bi@CeO2 Nanosystem with Dual-Enzyme-Mimetic Activities for Cancer Hypoxia Relief and Synergistic Photothermal/Photodynamic/Chemodynamic Therapy

Reactive oxygen species-mediated therapeutic strategies, including chemodynamic therapy (CDT) and photodynamic therapy (PDT), have exhibited translational promise for effective cancer management. However, monotherapy often ends up with the incomplete elimination of the entire tumor due to inherent limitations. Herein, we report a core-shell-structured Pd_1.7Bi@CeO₂-ICG (PBCI) nanoplatform constructed by a facile and effective strategy for synergistic CDT, PDT, and photothermal therapy. In the system, both Pd_1.7Bi and CeO₂ constituents exhibit peroxidase- and catalase-like characteristics, which not only generate cytotoxic hydroxyl radicals (^•OH) for CDT but also produce O₂ in situ and relieve tumor hypoxia for enhanced PDT. Furthermore, upon 808 nm laser irradiation, Pd_1.7Bi@CeO₂ and indocyanine green (ICG) coordinately prompt favorable photothermia, resulting in thermodynamically amplified catalytic activities. Meanwhile, PBCI is a contrast agent for near-infrared fluorescence imaging to determine the optimal laser therapeutic window in vivo. Consequently, effective tumor elimination was realized through the above-combined functions. The as-synthesized unitary PBCI theranostic nanoplatform represents a potential one-size-fits-all approach in multimodal synergistic therapy of hypoxic tumors.

Amino-functionalized Fe(III)-Based MOF for the high-efficiency extraction and ultrasensitive colorimetric detection of tetracycline

In order to achieve the simultaneous extraction and detection of tetracycline (TC) in milk, the amino-functionalized Fe-based metal-organic frameworks (NH₂-MIL-88B) was synthesized via a solvothermal method with Fe³⁺ and 2-aminoterephthalic acid (NH₂-BDC) as precursor. Thanks to the unique structure of NH₂-MIL-88B, it could be used to highly effective extract of TC in milk. More interestedly, the introduced -NH₂ could react with -OH from TC by a hydrogen-bonding interaction to cause the electronic interactions that enhances the peroxidase-like activity of NH₂-MIL-88B, which result in the enhancement of Fenton reaction by the transfer of the electron between TC and NH₂-MIL-88B. Under the optimal testing conditions, the linear absorbance response is well correlated with the TC concentration range of 50-1000 nM, which can reach a low LOD of 46.75 nM. Besides, the sensor exhibits excellent selectivity to TC, and the proposed strategy can also be applied to milk with good recovery (83.33-107.00%). Finally, the NH₂-MIL-88B and cellulose acetate (CA) are combined to form nanozyme hybrid membranes through the non-solvent induced phase separation method, which can be used to prepare point-of-care testing (POCT) for rapid and in-situ detection of TC.

Dual-mode strategy for 2,6-dipicolinic acid detection based on the fluorescence property and peroxidase-like activity inhibition of Fe-MIL-88NH2

This work designed a fluorometric/colorimetric dual-mode sensor for detecting 2,6-dipicolinic acid (DPA) based on the blue emission property and peroxidase-like activity of Fe-MIL-88NH₂. The fluorescence of Fe-MIL-88NH₂ was obviously turned off by Cu²⁺, but DPA was able to bring it back because it has a strong chelate bond with Cu²⁺. Fe-MIL-88NH₂ also displayed high peroxidase-like activity, which accelerated the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to the blue oxidation product (oxTMB) when H₂O₂ was present. When DPA was added, it efficiently inhibited the peroxidase-like activity of Fe-MIL-88NH₂, causing less oxTMB and less absorbance at 652 nm. The fluorescence recovery of Fe-MIL-88NH₂ and the change in absorbance at 652 nm were used as analytical signals for dual-mode detection of DPA. The linear responses in the range of 10-60 μM and 60-160 μM were achieved for the fluorometric mode, and the limit of detection (LOD) was 1.46 μM. The respective values of linear range and LOD for the colorimetric mode were 5-25 μM and 3.00 μM, respectively. In summary, the dual-mode testing strategy successfully detected DPA in aqueous environmental samples, suggesting great potential in disease prevention and environmental analysis.

Application of Nanozymes in Environmental Monitoring, Management, and Protection

Nanozymes are nanomaterials with enzyme-like activity, possessing the unique properties of nanomaterials and natural enzyme-like catalytic functions. Nanozymes are catalytically active, stable, tunable, recyclable, and versatile. Therefore, increasing attention has been paid in the fields of environmental science and life sciences. In this review, we focused on the most recent applications of nanozymes for environmental monitoring, environmental management, and environmental protection. We firstly introduce the tuning catalytic activity of nanozymes according to some crucial factors such as size and shape, composition and doping, and surface coating. Then, the application of nanozymes in environmental fields are introduced in detail. Nanozymes can not only be used to detect inorganic ions, molecules, organics, and foodborne pathogenic bacteria but are also involved in the degradation of phenolic compounds, dyes, and antibiotics. The capability of nanozymes was also reported for assisting air purification, constructing biofuel cells, and application in marine antibacterial fouling removal. Finally, the current challenges and future trends of nanozymes toward environmental fields are proposed and discussed.

Bioinspired Coassembly of Copper Ions and Nicotinamide Adenine Dinucleotides for Single-Site Nanozyme with Dual Catalytic Functions

Nanozymes can imitate the catalytic properties of natural enzymes while overcoming the limitations of natural enzymes such as high cost, poor robustness, and difficulty in recycling. However, rational design and facile preparation of nanozymes are still in demand. Inspired by the chemical structure of laccase, we report an amorphous metal-organic coordination nanocomposite named CuNAD, which is composed of copper ions and nicotinamide adenine dinucleotide (NAD⁺) via a simple coordinating coassembly process. As a single-site nanozyme, CuNAD exhibits excellent robustness under extreme conditions, significantly stronger catalytic activity for phenolic compounds, and 4.02-fold higher sensitivity for epinephrine detection than laccase. Furthermore, by breaking through the functional constraints of laccase, CuNAD is also able to activate H₂O₂ at neutral pH, benefiting a one-step chromogenic detection platform for cholesterol. This facile approach demonstrates the potential to develop single-site nanozymes by biomimicking natural enzymes and may boost more insights into the structure-function relationship of nanozymes.

A versatile platform for colorimetric, fluorescence and photothermal multi-mode glyphosate sensing by carbon dots anchoring ferrocene metal-organic framework nanosheet

Concerns regarding pesticide residues have driven attempts to exploit accurate, prompt and straightforward approaches for food safety pre-warning. Herein, a nanozyme-mediated versatile platform with multiplex signal response (colorimetric, fluorescence and temperature) was proposed for visual, sensitive and portable detection of glyphosate (GLP). The platform was constructed based on a N-CDs/FMOF-Zr nanosensor that prepared by in situ anchoring nitrogen-doped carbon dots onto zirconium-based ferrocene metal-organic framework nanosheets. The N-CDs/FMOF-Zr possessed excellent peroxidase (POD)-like activity and thus could oxide colorless 3, 3', 5, 5'-tetramethylbenzidine (TMB) into a blue oxidized TMB (oxTMB) in presence of H₂O₂. Intriguingly, owing to the blocking effect triggered by multiple interaction between GLP and N-CDs/FMOF-Zr, its POD-like activity of the latter was remarkably suppressed, which can modulate the transformation of TMB into oxTMB, generating tri-signal responses of fluorescence enhancement, absorbance and temperature decrease. More significantly, the temperature mode can be facilely realized by a portable home-made mini-photothermal device and handheld thermometers. The proposed multimodal sensing was capable of providing sensitive results by fluorescence mode and simultaneously realized visual/portable testing by colorimetric and photothermal channels. Consequently, it exhibited more adaptability for practical applications, which can satisfy different testing requirements according to sensitivity and available instruments/meters, presenting a new horizon for exploiting multifunctional sensors.

MIL-101(CuFe) Nanozymes with Excellent Peroxidase-like Activity for Simple, Accurate, and Visual Naked-Eye Detection of SARS-CoV-2

The COVID-19 pandemic has spread to every corner of the world and seriously affected our health and daily activities in the past three years; thereby, it is still urgent to develop various simple, quick, and accurate methods for early detection of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission. Nanozymes, a kind of nanomaterial with intrinsic enzyme-mimicking activity, have emerged as a suitable alternative for both therapy and diagnosis of SARS-CoV-2. Here, ultrasensitive and ultrafast MIL-101(CuFe)-CD147 biosensors are established for the detection of SARS-CoV-2 by a simple colorimetric method. A MIL-101(CuFe) metal-organic framework has excellent peroxidase-like activity due to the synergistic effect of Fe and Cu atoms. In addition, the MIL-101(CuFe)-CD147 biosensor shows great potential to detect the various variants of SARS-CoV-2 due to the universal receptor of CD147. The enzyme-based biosensor for the detection of SARS-CoV-2 achieves a very low limit of detection (about 3 PFU/mL) within 30 min. Therefore, the present method provides a new generation of an alternative approach for highly sensitive and visual diagnosis of COVID-19.

Specific Nanodrug for Diabetic Chronic Wounds Based on Antioxidase-Mimicking MOF-818 Nanozymes

Chronic wound is a common complication for diabetic patients, which entails substantial inconvenience, persistent pain, and significant economic burden to patients. However, current clinical treatments for diabetic chronic wounds remain unsatisfactory. A prolonged but ineffective inflammation phase in chronic wounds is the primary difference between diabetic chronic wounds and normal wounds. Herein, we present an effective antioxidative system (MOF/Gel) for chronic wound healing of diabetic rats through integrating a metal organic framework (MOF) nanozyme with antioxidant enzyme-like activity with a hydrogel (Gel). MOF/Gel can continuously scavenge reactive oxygen species to modulate the oxidative stress microenvironment in diabetic chronic wounds, which leads to a natural transition from the inflammation phase to the proliferation phase. Impressively, the efficacy of one-time-applied MOF/Gel was comparable to that of the human epidermal growth factor Gel, a widely used clinical drug for various wound treatments. Such an effective, safe, and convenient MOF/Gel system can meet complex clinical demands.

The Mn-modified porphyrin metal-organic framework with enhanced oxidase-like activity for sensitively colorimetric detection of glutathione

The selective detection of glutathione (GSH) has been used as important colorimetric probe for human health. Herein, we used a facile method to synthesize manganese ions modified porphyrin metal-organic framework (PCN-224-Mn) with a size of 125.7 ± 14.2 nm and zeta potential of -3.9 ± 0.5 mV. We showed that PCN-224-Mn catalyzed oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the absence of H₂O₂, resulting in a blue-colored oxidized TMB (oxTMB) that exhibits oxidase-like activity. Furthermore, a simple colorimetric detection method for GSH was developed based on the oxidase-like activity of PCN-224-Mn. This method shows wide linear detection range of 0.5-60 μM for GSH with a much lower detection limit of 0.233 μM. Finally, the recovery of colorimetric sensor of PCN-224-Mn suggests its great potential as a biosensor. As the catalytically active site, the manganese porphyrin unit plays a major role in the oxidase-like property and detection ability of PCN-224-Mn. Our data suggest that GSH detection method using PCN-224-Mn has great potential in multiple applications in the future.

A Dual-Mode NADH Biosensor Based on Gold Nanostars Decorated CoFe2 Metal-Organic Frameworks to Reveal Dynamics of Cell Metabolism

Nicotinamide adenine dinucleotide (NADH) is central to metabolism and implicated in various diseases. Herein, nanohybrids of gold nanostars and metal-organic frameworks are devised and demonstrated as a dual-mode NADH sensor, for which colorimetric detection is enabled by its peroxidase-like nanozyme property and Raman detection is realized by its surface-enhanced Raman scattering property with the detection limit as low as 28 pM. More importantly, this probe enables real-time SERS monitoring in living cells, providing a unique tool to investigate dynamic cellular processes involving NADH. Our experiments reveal that metabolism dynamics is accelerated by glucose and is much higher in cancerous cells. The SERS results can also be verified by the colorimetric detection. This sensor provides a new potential to detect biomarkers and their dynamics in situ.

Design and Application of Electrochemical Sensors with Metal-Organic Frameworks as the Electrode Materials or Signal Tags

Metal-organic frameworks (MOFs) with fascinating chemical and physical properties have attracted immense interest from researchers regarding the construction of electrochemical sensors. In this work, we review the most recent advancements of MOF-based electrochemical sensors for the detection of electroactive small molecules and biological macromolecules (e.g., DNA, proteins, and enzymes). The types and functions of MOF-based nanomaterials in terms of the design of electrochemical sensors are also discussed. Furthermore, the limitations and challenges of MOF-based electrochemical sensing devices are explored. This work should be invaluable for the development of MOF-based advanced sensing platforms.

Protein-Mimicking Nanoparticles in Biosystems

Proteins are essential elements for almost all life activities. The emergence of nanotechnology offers innovative strategies to create a diversity of nanoparticles (NPs) with intrinsic capacities of mimicking the functions of proteins. These artificial mimics are produced in a cost-efficient and controllable manner, with their protein-mimicking performances comparable or superior to those of natural proteins. Moreover, they can be endowed with additional functionalities that are absent in natural proteins, such as cargo loading, active targeting, membrane penetrating, and multistimuli responding. Therefore, protein-mimicking NPs have been utilized more and more often in biosystems for a wide range of applications including detection, imaging, diagnosis, and therapy. To highlight recent progress in this broad field, herein, representative protein-mimicking NPs that fall into one of the four distinct categories are summarized: mimics of enzymes (nanozymes), mimics of fluorescent proteins, NPs with high affinity binding to specific proteins or DNA sequences, and mimics of protein scaffolds. This review covers their subclassifications, characteristic features, functioning mechanisms, as well as the extensive exploitation of their great potential for biological and biomedical purposes. Finally, the challenges and prospects in future development of protein-mimicking NPs are discussed.

Fabrication of multifunctional metal-organic frameworks nanoparticles via layer-by-layer self-assembly to efficiently discover PSD95-nNOS uncouplers for stroke treatment

Background

Disruption of the postsynaptic density protein-95 (PSD95)—neuronal nitric oxide synthase (nNOS) coupling is an effective way to treat ischemic stroke, however, it still faces some challenges, especially lack of satisfactory PSD95-nNOS uncouplers and the efficient high throughput screening model to discover them.

Results

Herein, the multifunctional metal–organic framework (MMOF) nanoparticles as a new screening system were innovatively fabricated via layer-by-layer self-assembly in which His-tagged nNOS was selectively immobilized on the surface of magnetic MOF, and then PSD95 with green fluorescent protein (GFP-PSD95) was specifically bound on it. It was found that MMOF nanoparticles not only exhibited the superior performances including the high loading efficiency, reusability, and anti-interference ability, but also possessed the good fluorescent sensitivity to detect the coupled GFP-PSD95. After MMOF nanoparticles interacted with the uncouplers, they would be rapidly separated from uncoupled GFP-PSD95 by magnet, and the fluorescent intensities could be determined to assay the uncoupling efficiency at high throughput level.

Conclusions

In conclusion, MMOF nanoparticles were successfully fabricated and applied to screen the natural actives as potential PSD95-nNOS uncouplers. Taken together, our newly developed method provided a new material as a platform for efficiently discovering PSD95-nNOS uncouplers for stoke treatment.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01583-7.

Semiconductor quantum dots for photodynamic therapy: Recent advances

Photodynamic therapy is a promising cancer treatment that induces apoptosis as a result of the interactions between light and a photosensitizing drug. Lately, the emergence of biocompatible nanoparticles has revolutionized the prospects of photodynamic therapy (PDT) in clinical trials. Consequently, a lot of research is now being focused on developing non-toxic, biocompatible nanoparticle-based photosensitizers for effective cancer treatments using PDT. In this regard, semiconducting quantum dots have shown encouraging results. Quantum dots are artificial semiconducting nanocrystals with distinct chemical and physical properties. Their optical properties can be fine-tuned by varying their size, which usually ranges from 1 to 10 nm. They present many advantages over conventional photosensitizers, mainly their emission properties can be manipulated within the near IR region as opposed to the visible region by the former. Consequently, low intensity light can be used to penetrate deeper tissues owing to low scattering in the near IR region. Recently, successful reports on imaging and PDT of cancer using carbon (carbon, graphene based) and metallic (Cd based) based quantum dots are promising. This review aims to summarize the development and the status quo of quantum dots for cancer treatment.

Confining enzymes in porous organic frameworks: from synthetic strategy and characterization to healthcare applications

Enzymes are a class of natural catalysts with high efficiency, specificity, and selectivity unmatched by their synthetic counterparts and dictate a myriad of reactions that constitute various cascades in living cells. The development of suitable supports is significant for the immobilization of structurally flexible enzymes, enabling biomimetic transformation in the extracellular environment. Accordingly, porous organic frameworks, including metal organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs), have emerged as ideal supports for the immobilization of enzymes because of their structural features including ultrahigh surface area, tailorable porosity, and versatile framework compositions. Specially, organic framework-encased enzymes have shown significant enhancement in stability and reusability, and their tailorable pore opening provides a gatekeeper-like effect for guest sieving, which is beneficial for mimicking intracellular biocatalysis processes. This immobilization technique brings new insight into the development of next-generation enzyme materials and shows huge potential in healthcare applications, such as biomarker diagnosis, biostorage, and cancer and antibacterial therapies. In this review, we describe the state-of-the-art strategies for the structural immobilization of enzymes using the well-explored MOFs and burgeoning COFs and HOFs as scaffolds, with special emphasis on how these porous framework-confined technologies can provide a favorable microenvironment for mimicking natural biocatalysis. Subsequently, advanced characterization techniques for enzyme conformation, the effect of the confined microenvironment on the activity of enzymes, and the emerging healthcare applications will be surveyed.

How to Define a Nanozyme

Over the past 15 years, many articles have considered "nanozymes" as ferromagnetic nanoparticles having an "intrinsic peroxidase-like activity" in the presence of hydrogen peroxide. However, the definition and the catalytic activity of these nanozymes have been questioned. The present Perspective reports the main criteria that are essential to classify a nanoparticle as a nanozyme. It is important to consider that not all nanoparticles able to generate hydroxyl radicals in the presence of hydrogen peroxide without catalytic activity can be registered as nanozymes.

AIEgen-Based Bionic Nanozymes for the Interventional Photodynamic Therapy-Based Treatment of Orthotopic Colon Cancer

Relative to traditional photosensitizer (PS) agents, those that exhibit aggregation-induced emission (AIE) properties offer key advantages in the context of photodynamic therapy (PDT). At present, PDT efficacy is markedly constrained by the hypoxic microenvironment within tumors and the limited depth to which lasers can penetrate in a therapeutic context. Herein, we developed platelet-mimicking MnO2 nanozyme/AIEgen composites (PMD) for use in the interventional PDT treatment of hypoxic tumors. The resultant biomimetic nanoparticles (NPs) exhibited excellent stability and were able to efficiently target tumors. Moreover, they were able to generate O2 within the tumor microenvironment owing to their catalase-like activity. Notably, through an interventional approach in which an optical fiber was introduced into the abdominal cavity of mice harboring orthotopic colon tumors, good PDT efficacy was achieved. We thus propose that a novel strategy consisting of a combination of an AIEgen-based bionic nanozyme and a biomimetic cell membrane coating represents an ideal therapeutic platform for targeted antitumor PDT. This study is the first to have combined interventional therapy and AIEgen-based PDT, thereby overcoming the limited light penetration that typically constrains the therapeutic efficacy of this technique, highlighting a promising new AIEgen-based PDT treatment strategy.

Modulating the Biomimetic and Fluorescence Quenching Activities of Metal-Organic Framework/Platinum Nanoparticle Composites and Their Applications in Molecular Biosensing

Nanoscale metal-organic frameworks (nMOFs) have gained considerable attention with significant potential applications. Although great efforts have been devoted to designing and fabricating nanoscaffold structures, approaches of deliberately regulating the intrinsic functionality of nMOFs have been poorly explored. Herein, we report a simple and novel strategy to regulate the catalytic and fluorescence quenching behaviors of nMOFs through coordination-driven self-assembly. As a proof-of-concept, we synthesized a synergistic and stable MOF-metal nanocomposite by loading platinum nanoparticles (PtNPs) on a commonly used Fe-MOF, i.e., MIL-88B-NH₂/Pt, as a MOF composite model for exploration. On one hand, the complexation with ATP effectively broke the pH limitation of the peroxidase-mimicking MIL-88B-NH₂/Pt nanozyme, bringing a 10-fold increased catalytic activity under alkaline condition. Based on the distinct catalytic enhancement between ATP and other nucleotides, real-time monitoring of apyrase activity as well as colorimetric detection of alkaline phosphatase (ALP) was performed. On the other hand, interactions of MIL-88B-NH₂/Pt with fluorescent DNA were tolerant of different nucleic acids and, more importantly, were further manipulated by inorganic molecules. As a result, H₂O₂ could only trigger the release of a G-rich sequence, while phosphates could readily induce desorption of various DNA molecules with varying lengths, sequences, and fluorescent dyes. Accordingly, fluorescent DNA and MIL-88B-NH₂/Pt as functional probe-quencher pairs were proposed, allowing the establishment of a fluorescence bioassay for ALP and PPase detection and Boolean logic calculations. This work offers a means to tune the intrinsic activities of nMOFs by surface engineering, benefiting design of functional nanomaterials and development of advanced biosensing systems.

Facile preparation of nanocellulose/Zn-MOF-based catalytic filter for water purification by oxidation process

Sulfate radical (SO₄^•-)-based advanced oxidation processes (SR-AOPs) have recently attracted much attention due to their potential in degrading organic pollutants. Metal-organic frameworks (MOFs) have been reported as effective materials to generate SO₄^•-. However, it is challenging to separate and recover the dispersed MOF particles from the reaction solution when MOFs are used alone. We used cellulose nanofibers (CNFs) as a porous filter template to immobilize Zn-based MOF, zeolitic imidazolate framework-8 (ZIF-8), and obtained a catalytic composite membrane having peroxymonosulfate (PMS) activating function to produce SO₄^•-. The CNF was effective in holding ZIF-8 nanoparticle and making a durable porous filter. The activated PMS-produced ^•OH and SO₄^•- radicals from ZIF-8 play an important role in the catalytic reaction. More than 90% of methylene blue and rhodamine B was degraded by ZIF-8/CNFs composite membrane in the PMS environment within 60 min. The ZIF-8/CNFs catalytic filters can be used several times without performance reduction for organic dye degradation. The results show that ZIF-8/CNFs catalytic membrane can be separated from organic pollution system quickly and used for the efficient separation and recovery of MOF particle-based catalytic materials. Therefore, this study provides a new perspective for fabricating the MOFs particles-immobilized catalytic filter by biomass nanocellulose-based materials for water purification. This method can be used for facile fabrication of the cellulose-based porous functional filter and open diverse applications.

Synthesis of a cerium-based nanomaterial with superior oxidase-like activity for colorimetric determination of glutathione in food samples

Enzyme-like nanomaterials have received significant attention for their high stability and low cost. However, most nanomaterials require complicated synthesis processes, limiting the range of their potential applications. In this study, a novel cerium-based nanomaterial was fabricated in a facile manner from a mixture of dipicolinic acid (DPA), guanosine 5'-monophosphate (GMP), and cerium acetate under ambient conditions. The obtained nanomaterial, designated as DPA-Ce-GMP, exhibited superior oxidase-like activity owing to the mixed valence (Ce³⁺/Ce⁴⁺) of cerium ions. DPA-Ce-GMP efficiently catalyzed the oxidation of 3,3,5,5-tetramethylbenzidine (TMB), achieving a color reaction without requiring hydrogen peroxide. Thus, DPA-Ce-GMP was incorporated into a simple, rapid, and sensitive colorimetric sensor for glutathione (GSH) detection. Within this sensor, TMB oxidation is inhibited by the reducibility of GSH. The sensor exhibits a linear response over two concentration ranges (0.05-10 and 10-40 μM), and its detection limit is 17.1 nM (3σ/slope). The proposed sensor was successfully applied to GSH quantification in food samples. The developed sensor provides an efficient biomimic oxidase for GSH detection in real samples. Facile approach to prepare cerium-based nanomaterial with superior oxidase-like activity for colorimetric detection of glutathione in food samples.

Regulation Mechanism of ssDNA Aptamer in Nanozymes and Application of Nanozyme-Based Aptasensors in Food Safety

Food safety issues are a worldwide concern. Pathogens, toxins, pesticides, veterinary drugs, heavy metals, and illegal additives are frequently reported to contaminate food and pose a serious threat to human health. Conventional detection methods have difficulties fulfilling the requirements for food development in a modern society. Therefore, novel rapid detection methods are urgently needed for on-site and rapid screening of massive food samples. Due to the extraordinary properties of nanozymes and aptamers, biosensors composed of both of them provide considerable advantages in analytical performances, including sensitivity, specificity, repeatability, and accuracy. They are considered a promising complementary detection method on top of conventional ones for the rapid and accurate detection of food contaminants. In recent years, we have witnessed a flourishing of analytical strategies based on aptamers and nanozymes for the detection of food contaminants, especially novel detection models based on the regulation by single-stranded DNA (ssDNA) of nanozyme activity. However, the applications of nanozyme-based aptasensors in food safety are seldom reviewed. Thus, this paper aims to provide a comprehensive review on nanozyme-based aptasensors in food safety, which are arranged according to the different interaction modes of ssDNA and nanozymes: aptasensors based on nanozyme activity either inhibited or enhanced by ssDNA, nanozymes as signal tags, and other methods. Before introducing the nanozyme-based aptasensors, the regulation by ssDNA of nanozyme activity via diverse factors is discussed systematically for precisely tailoring nanozyme activity in biosensors. Furthermore, current challenges are emphasized, and future perspectives are discussed.