A strategy for inhibitors screening of xanthine oxidase based on colorimetric sensor combined with affinity chromatography technology

The discovery of enzyme inhibitors from natural products is a crucial aspect in the development of therapeutic drugs. However, the complexity of natural products presents a challenge in developing simple and efficient methods for inhibitor screening. Herein, we have developed an integrated analytical model for screening xanthine oxidase (XOD) inhibitors that combines simplicity, accuracy, and efficiency. This model utilizes a colorimetric sensor and affinity chromatography technology with immobilized XOD. The colorimetric sensor procedure can quickly identify whether there are active components in complex samples. Subsequently, the active components in the samples identified by the colorimetric sensor procedure were further captured, separated, and identified through affinity chromatography. The integrated analytical model can significantly enhance the efficiency and accuracy of inhibitor screening. The proposed method was applied to screen for an activity inhibitor of XOD in five natural medicines. As a result, a potential active ingredient for XOD, polydatin, was successfully identified from Polygoni Cuspidati Rhizoma et Radix. This work is anticipated to offer new insights for the screening of enzyme inhibitors from natural medicines.

Advances in the application of logic gates in nanozymes

Nanozymes are a class of nanomaterials with biocatalytic function and enzyme-like activity, whose advantages include high stability, low cost, and mass production. They can catalyze the substrates of natural enzymes based on specific nanostructures and serve as substitutes for natural enzymes. Their applied research involves a wide range of fields such as biomedicine, environmental governance, agriculture, and food. Molecular logic gates are a new cross-disciplinary discipline, which can simulate the function of silicon circuits on a molecular scale, perform single or multiple input logic operations, and generate logic outputs. A molecular logic gate is a binary operation that converts an input signal into an output signal according to the rules of Boolean logic, generating two signals, a high level, and a low level. The high and low levels represent the "true" and "false" values of the logic gates, and their outputs correspond to "l" and "0" of the molecular logic gates, respectively. The combination of nanozymes and logic gates is a novel and attractive research direction, and the cross-application of the two brings new opportunities and ideas for various fields, such as the construction of efficient biocomputers, intelligent drug delivery systems, and the precise diagnosis of diseases. This review describes the application of logic gates based on nanozymes, which is expected to provide a certain theoretical foundation for researchers' subsequent studies.

Ratio-fluorescent and naked-eye visualized dual-channel sensing strategy for Cu2+ and alkaline phosphatase activity assay

The levels of Cu2+ and alkaline phosphatase (ALP) are the important indicators of the developed stage of the relative diseases. Herein, a binary ratio-fluorescent and smartphone-assisted visual strategy basing on 4'-aminomethyl-4, 5', 8-trimethylpsoralen (AMT) and the oxidation of o-phenylenediamine was developed. Under the action of Cu2+, the fluorescent molecule, 3-diaminophenazine (DAP) formed which can act as a fluorescent acceptor of the ratio-fluorescent sensor. The emission spectrum of AMT overlapped with the excitation spectrum of DAP and, thus, it can act as the fluorescent donor of the ratio-fluorescent sensor. With the increasing concentration of Cu2+ and ALP, the fluorescent intensity of AMT decreased and the fluorescent intensity of DAP increased. The dual-emission reverse change ratio-fluorescent sensor realized the sensitive detection Cu2+ and ALP with the detection limits of 2 nM and 0.03 U/mL, respectively. In addition, the acceptable recoveries were obtained when the Cu2+ and ALP in spiked samples were detected. Furthermore, the relative activity of ALP was assessed by increasing the concentrations of the inhibitor Na3VO4 and IC50 of 25 μM was obtained. Importantly, the target concentration-dependent color change of DAP allowed us to utilize R/B ratio values to design the smartphone-assisting visual detection model of Cu2+ and ALP activity with the detection limits of 0.1 μM and 0.18 U/mL. This simple, flexible, dual-mode sensor strategy has a potential for disease diagnosis and drug screening.

Machine-Learning-Assisted Nanozyme Design: Lessons from Materials and Engineered Enzymes

Nanozymes are nanomaterials that exhibit enzyme-like biomimicry. In combination with intrinsic characteristics of nanomaterials, nanozymes have broad applicability in materials science, chemical engineering, bioengineering, biochemistry, and disease theranostics. Recently, the heterogeneity of published results has highlighted the complexity and diversity of nanozymes in terms of consistency of catalytic capacity. Machine learning (ML) shows promising potential for discovering new materials, yet it remains challenging for the design of new nanozymes based on ML approaches. Alternatively, ML is employed to promote optimization of intelligent design and application of catalytic materials and engineered enzymes. Incorporation of the successful ML algorithms used in the intelligent design of catalytic materials and engineered enzymes can concomitantly facilitate the guided development of next-generation nanozymes with desirable properties. Here, recent progress in ML, its utilization in the design of catalytic materials and enzymes, and how emergent ML applications serve as promising strategies to circumvent challenges associated with time-expensive and laborious testing in nanozyme research and development are summarized. The potential applications of successful examples of ML-aided catalytic materials and engineered enzymes in nanozyme design are also highlighted, with special focus on the unified aims in enhancing design and recapitulation of substrate selectivity and catalytic activity.

Ultrasensitive detection of glucose oxidase and alkaline phosphatase in milk based on valence regulated upconversion nanoprobes

Fresh milk should undergo sterilization before consumption to eliminate bacteria that can cause foodborne illnesses. Additional antimicrobial measures are beneficial to extend its shelf life. The nanoprobe developed herein can not only inspect the activity of alkaline phosphatase (ALP) for evaluating the degree of pasteurization, but also detect the activity of glucose oxidase (GOD), which is added as a chemical preservative. The facile preparation of the nanoprobe involved introducing gallic acid-Fe complex (GA-Fe) into lanthanide doped upconversion nanomaterials (UCNPs). Based on the alteration of iron's valence state in the complex through a straightforward redox reaction, both enzyme activities could be determined through colorimetric and luminometric dual-signal readouts. With detection limits of 1.669 × 10-5 for GOD and 9.81 × 10-6 U/mL for ALP respectively, this nanoprobe shows merits of easy operation and high sensitivity. Successful application in milk samples demonstrates its potential as an innovative and cost-effective approach to food safety inspection.

Specializing Carbon Nanozyme Active Sites for Sensitive Alkaline Phosphatase Activity Metal-Free Detection

As biological enzymes regulate metabolic processes, alkaline phosphatase (ALP) is a critical diagnostic indicator associated with many diseases. To accurately measure the enzyme activity, nanozymactic materials can offer sensitive strategies for ALP detection. However, nanozymes often lack specific target binding sites, and the presence of common co-components, e. g., metal ions, may cause false-positive or false-negative results in enzyme activity determination. Herein, we developed a colorimetric assay for ALP detection using metal-free nanozymatic carbon dots (CDs). The ALP hydrolysis of pyrophosphate ions (PPi) to phosphate ions (Pi) induces a "turn-on" response based on the nanozyme activity. This PPi-induced inhibition mechanism is extensively studied via the Michaelis-Menten model, revealing that PPi acts as a noncompetitive inhibitor for CDs at a binding site distinct from the common nanozyme active site. With superior responses to ALP substrates, a highly sensitive and selective method is established for sensing ALP activity with a linear range of 0.010-0.200 U/L and a detection limit of 0.009 U/L. This finding explores the recognition and binding behavior of nanozymes, allowing for precise and reliable measurements even in complex samples, and represents a significant breakthrough for nanozyme-based assays in biological analysis.

A ratiometric fluorescent probe for simultaneous detection of L-ascorbic acid and alkaline phosphatase activity based on red carbon dots/polydopamine nanocomposite

Herein, efficient red carbon dots (R-CDs) were synthesized by one-step hydrothermal treatment of N-(4-amino phenyl) acetamide and (2,3-difluoro phenyl) boronic acid. The optimal emission peak of R-CDs was at 602 nm (under 520 nm excitation) and the absolute fluorescence quantum yield of R-CDs was 12.9%. Polydopamine, which was formed by the self-polymerization and cyclization of dopamine in alkaline condition, emitted characteristic fluorescence with peak position of 517 nm (under 420 nm excitation) and affected the fluorescence intensity of R-CDs through inner filter effect. L-Ascorbic acid (AA), which was the hydrolysis product of L-ascorbic acid-2-phosphate trisodium salt under the catalytic reaction of alkaline phosphatase (ALP), effectively prevented the polymerization of dopamine. Combined with the ALP-mediated AA production and the AA-mediated polydopamine generation, the ratiometric fluorescence signal of polydopamine with R-CDs was correlated closely with the concentration of both AA and ALP. Under optimal conditions, the detection limits of AA and ALP were 0.28 μM during linear range of 0.5-30 μM and 0.044 U/L with linear range of 0.05-8 U/L, respectively. This ratiometric fluorescence detection platform can efficiently shield the background interference of sophisticated samples by introducing a self-calibration as reference signal in a multi-excitation mode, which can detect AA and ALP in human serum samples with satisfactory results. Such R-CDs/polydopamine nanocomposite provides a steadfast quantitative information and makes R-CDs be excellent candidate for biosensors via combining target recognition strategy.

Exceptional peroxidase-like activity of an iron and copper based organic framework nanosheet for consecutive colorimetric biosensing of glucose and kanamycin in real food samples

Two-dimensional metal-organic framework nanosheets are attractive as peroxidase mimicking nanocatalysts due to their rich chemical functional groups, large surface area, high porosity, and accessible active sites. In this study, we synthesized FeCu bifunctional 2D MOF nanosheets using a solvothermal method. Fe and Cu ions were added as metal precursors, while organic amine and acid served as the organic ligands to construct the FeCu-MOF nanosheets. These nanosheets demonstrated robust peroxidase-like catalytic activities and were employed to develop a visual detection system for multiple targets, such as glucose and kanamycin. In the detection mechanism, glucose was oxidized into gluconic acid by glucose oxidase (GOx), leading to the generation of H2O2. When H2O2 is present, the FeCu-MOF NSs demonstrate high intrinsic peroxidase-like activity, which might catalytically oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into a blue-coloured oxTMB product with a strong UV absorption at 654 nm. Subsequently, kanamycin was added to the above sensing system. The kanamycin strongly interacted with the FeCu-MOF NSs through H-bonding and blocked electron transfer, resulting in a colour change of the solution from blue to colourless with a weak UV absorption at 654 nm. Under the optimal conditions, the proposed colorimetric sensor exhibits an excellent linear response to glucose and kanamycin over the 0.25-5 μM and 0.02-0.1 μM ranges, respectively. The proposed colorimetric assay detection limits for glucose and kanamycin were found to be as low as 0.1 μM and 8 nM, respectively, and such a sensor shows excellent selectivity and sensitivity against different potential interferents. Thus, our proposed colorimetric assay was satisfactory when applied to glucose and kanamycin detection in agricultural and livestock husbandry samples.

Pd nanoparticles decorated ultrathin 2D metal-organic framework nanosheets with enhanced peroxidase-mimic activity and colorimetric assay of glucose

In addition to size, shape and morphology, enzyme-mimetic property could be efficiently regulated by controlling composition, forming complexes or hybrids, and surface modification. Herein, Pd nanoparticles with an average diameter of 2.52 nm were decorated on ultrathin 2D copper(ii)-porphyrin derived metal-organic framework (MOF) nanosheets by a simple reduction method for catalytic activity regulation. In comparison with other nanozymes, the as-synthesized Pd modified 2D MOF hybrid nanosheets (Pd@Cu-TCPP(Fe)) presented excellent peroxidase-mimic activity, exhibiting an even superior catalytic ability towards H2O2 with a Michaelis-Menten constant as low as 2.33 mM. Based on a cascade reaction between glucose oxidase and Pd@Cu-TCPP(Fe), a colorimetric method for the detection of glucose was established and validated with a wide linear range (0.2-8.0 mM), good recovery (89.5-94.2%) and nice reproducibility (3.65%). All these features guaranteed its excellent ability for glucose determination in human cerebrospinal fluids. This study could offer a valuable reference for constructing novel optical biosensors.

Stimuli-responsive nanozymes for biomedical applications

With the rapid development of nanotechnology, nanozymes are regarded as excellent substitutes for natural enzymes due to their high activity, convenient preparation, low cost, robust stability and other unique properties of nanomaterials. In biomedical applications, the always-on activity of nanozymes is undesirable as it poses a potential threat to normal tissues. Stimuli-responsive nanozymes were designed to manipulate the activities of nanozymes. This review introduces two types of stimuli-responsive nanozymes. One is smart responsive nanozymes with stimuli-switchable activities, further divided into those with on/off switchable activity and one/another switchable activity. Another is nanozymes exhibiting responsive release from specific carriers. Additionally, the biomedical applications of stimuli-responsive nanozymes in cancer therapy, antibacterial therapy, biosensing and anti-inflammatory therapy are briefly reviewed. Finally, we address the challenges and prospects in this field.

Nanozyme Rich in Oxygen Vacancies Derived from Mn-Based Metal-Organic Gel for the Determination of Alkaline Phosphatase

Vacancy engineering as an effective strategy has been widely employed to regulate the enzyme-mimic activity of nanomaterials by adjusting the surface, electronic structure, and creating more active sites. Herein, we purposed a facile and simple method to acquire transition metal manganese oxide rich in oxygen vacancies (OVs-Mn2O3-400) by pyrolyzing the precursor of the Mn(II)-based metal-organic gel directly. The as-prepared OVs-Mn2O3-400 exhibited superior oxidase-like activity as oxygen vacancies participated in the generation of O2•-. Besides, steady state kinetic constant (Km) and catalytic kinetic constant (Ea) suggested that OVs-Mn2O3-400 had a stronger affinity toward 3,3',5,5'-tetramethylbenzidine and possessed prominent catalytic performance. By taking 2-phospho-l-ascorbic acid as the substrate, which can be converted into reducing substance ascorbic acid in the presence of alkaline phosphatase (ALP), OVs-Mn2O3-400 can be applied as an efficient nanozyme for ALP colorimetric analysis without the help of destructive H2O2. The colorimetric sensor established by OVs-Mn2O3-400 for ALP detection showed a good linearity from 0.1 to 12 U/L and a lower limit of detection of 0.054 U/L. Our work paves the way for designing enhanced enzyme-like activity nanozymes, which is of significance in biosensing.

Air-Derived Inhibitor of Nanozymes

Nanozymes are functional nanomaterials with enzyme-mimicking activities, which have found wide applications in various fields. Investigation on nanozyme inhibitors not only helps to apply nanozymes in a controlled manner but also deepens our insight into the catalysis mechanism. Herein, we report an inorganic ion inhibitor, HCO₃^-, which can significantly inhibit the alkaline phosphatase-mimicking activities of Ce₆ cluster-based metal-organic framework (Ce-MOF) nanozymes. The inhibition of adsorption of the negatively charged fluorescence sodium on Ce₆ clusters in Ce-MOF nanoparticles (NPs) by HCO₃^- proves that HCO₃^- ions occupy and deactivate Ce₆ clusters (i.e., catalytic active sites), leading to the activity inhibition of Ce-MOF nanozymes. Tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) buffer is widely employed as the alkaline reaction medium. HCO₃^- ions can be formed in Tris-HCl buffer through adsorption of CO₂ in the air during storage in a sealed tube, which significantly inhibits the activity of Ce-MOF nanozymes. To our knowledge, this study is the first to demonstrate an air-derived inhibitor of nanozymes.

Pd(II)-based coordination polymer nanosheets for ratiometric colorimetric and photothermal dual-mode assay of serum alkaline phosphatase

Fabrication of a multi-signal readout assay with high sensitivity and selectivity is highly desirable for clinical and biochemical analysis, but remains a challenge due to laborious procedures, large-scale instruments, and inadequate accuracy. Herein, a straightforward, rapid, and portable detection platform based on palladium(II) methylene blue (MB) coordination polymer nanosheets (PdMBCP NSs) was unveiled for the ratiometric dual-mode detection of alkaline phosphatase (ALP) with temperature and colorimetric signal readout properties. The sensing mechanism is the ALP-catalyzed generation of ascorbic acid for competitive binding and etching PdMBCP NSs to release free MB in a quantitive means for detection. Specifically, ALP addition led to the decrease of temperature signal readout from the decomposed PdMBCP NSs under 808 nm laser excitation, and simultaneous increase of the temperature from the generated MB with a 660 nm laser, together with the corresponding absorbance changes at both wavelengths. Notably, this ratiometric nanosensor exhibited a detection limit of 0.013 U/L (colorimetric) and 0.095 U/L (photothermal) within 10 min, respectively. The reliability and satisfactory sensing performance of the developed method were further confirmed by clinic serum samples. Therefore, this study provides a new insight for the development of dual-signal sensing platforms for convenient, universal, and accurate detection of ALP.

Photo-responsive oxidase-like nanozyme based on a vanadium-docked porphyrinic covalent organic framework for colorimetric L-Arginine sensing

This study reports the development of a vanadium-docked porphyrinic covalent organic framework as a novel class of highly polar photoactive materials. Thanks to its extended π-electron conjugation and high chemical stabilities, this framework can serve as an oxidase-Like photo-nanozyme for photocatalytic oxidation of o-phenylenediamine (o-PDA) and a colorimetric substrate for the production of the yellow-colored oxidized o-PDA (o-PDAox). The physicochemical properties of the as-prepared photo-nanozyme were characterized by several analytical techniques. Its enhanced light harvesting and charge separation and transfer were also verified by electrochemical and spectroscopic analysis. This photo-nonenzymatic colorimetric assay was applied for the sensitive L-Arginine (L-Arg) detection as a typical amino acid in the linear range of 8.1 nM-330 μM with a limit of detection (LOD) of 3.5 nM. The findings of this research confirmed the safety and feasibility of the proposed photo-nonenzymatic colorimetric sensing strategy for the detection of L-Arg and other similar biomolecules in food samples. Kinetic investigation revealed that the photo-responsive oxidase mimic exhibits satisfactory K_m (0.47 mM) and V_max (42.0 μM/s) values. This work broadened our insight into the development of modified porphyrinic-COF-based visible light-responsive oxidase-like photo-nanozyme for environmentally friendly colorimetric biosensing.

Detection of Periodontal Disease Marker with Geometrical Transformation of Ag Nanoplates

Alkaline phosphatase (ALP) and interleukin-1beta (IL-1β) are crucial salivary biomarkers for the diagnosis of periodontal disease that harms the periodontal tissue along with tooth loss. However, there has been no way of sensitive and portable detection of both biomarkers in saliva with multivariate signal readout. In this work, we design the multicolorimetric ALP and IL-1β sensing platform based on geometrical transformation of silver nanoplate transducer. By utilizing enzymatic activity of ALP that dephosphorylates p-aminophenol phosphate (p-APP) to p-aminophenol (p-AP), localized surface plasmon resonance properties of silver nanoplate vary with ALP and show a distinct color change from blue to yellow based on a controlled seed transformation from triangular to hexagonal, rounded pentagonal, and spherical shape. The multicolor sensor shows an ALP detection range of 0-25 U/L with a limit of detection (LOD) of 0.0011 U/L, which is the lowest range of LOD demonstrated to date for state-of-the-art ALP sensor. Furthermore, we integrate the sensor with the conventional ELISA to detect IL-1β for multicolor signaling and it exhibits a linear detection range of 0-250 pg/mL and an LOD of 0.066 pg/mL, which is 2 orders of magnitude lower than the monochromic conventional ELISA (LOD of 3.8 pg/mL). The ALP multicolor sensor shows high selectivity with a recovery of 100.9% in real human saliva proving its reliability and suitability for the readily accessible periodontal diagnosis with multivariate signal readout.

A novel metal-organic framework of Co-hemin for portable and visual colorimetric detection of 2,4-dichlorophenoxyacetic acid

On-site quantitative analysis of 2,4-dichlorophenoxyacetic acid (2,4-D) is of significant importance for addressing increasing concerns about public health and environmental quality. Here, a novel metal-organic framework (MOF) of Co-hemin is synthesized and first used for on-site colorimetric monitoring of 2,4-D. 2,4-D as an inhibitor of alkaline phosphatase could specifically suppress the production of ascorbic acid, which restrained in situ etching of Co-hemin and further triggered the colorimetric response. In the colorimetric assay, Co-hemin displayed good oxidase-like activity without addition of H₂O₂, which could avoid the shortcomings of H₂O₂ such as toxicity and instability. The Co-hemin biosensor exhibited a relatively low detection limit of 33 ng mL^-1 for 2,4-D by the UV method. Moreover, a smartphone based RGB analysis system for the sensitive detection of 2,4-D was developed, and exhibited a good linear relationship between the RGB model parameter and the concentration of 2,4-D. The operability and accuracy of the Co-hemin biosensor were confirmed by the quantitative determination of 2,4-D in real samples, such as serum and tap water. Also, the Co-hemin based colorimetric biosensor showed good selectivity and specificity. Moreover, the developed assays displayed good application in constructing complex logic gates. This work not only provided a portable and visual platform for on-site monitoring of 2,4-D, but also expanded application prospects in the field of complex biological analysis.

Phosphatase-like activity of single-atom CeNC nanozyme for rapid detection of Al3

Single-atom nanozymes are a class of nanozymes with attractive enzyme-like activities. They usually mimic oxidoreductases and lack other types of enzyme-like activities. Hence, we verified a single-atom CeNC (SACeNC) nanozyme with an excellent phosphatase-like (PPA-like) activity, which could catalyze the dephosphorylation of inorganic phosphates. Meanwhile, we found that Al³⁺ could specifically combine with the O atom in its structure to form an Al-O bond, which could inhibit its PPA-like activity. Based on this principle, we have constructed a fast, portable, and efficient fluorescent liquid phase sensor to detect Al³⁺. The detection time was only 4 min, and the limit of detection (LOD) was 22.89 ng/mL in the linear range of 5-25 μg/mL. This study not only verified that single-atom nanozymes mimic phosphatase activities, but also applied its unique enzyme-like to the field of food safety rapid detection.

A ratiometric fluorescence method based on nitrogen-doped carbon quantum dots for the determination of the activity of alkaline phosphatase

Herein, a simple and sensitive ratiometric fluorescence sensing platform to detect alkaline phosphatase (ALP) activity is developed on the basis of yellow fluorescent nitrogen-doped carbon quantum dots (YNCDs). The hydrolysis of ascorbic acid 2-phosphate (AAP) into ascorbic acid (AA) is catalyzed by ALP. Then, AA will react with o-phenylenediamine (OPD) to form 3-(1,2-dihydroxyethyl)furo[3,4b]-quinoxaline (QXD) which is a blue fluorescent quinoxaline derivative with emission at 435 nm in the presence of Cu²⁺. YNCDs have yellow fluorescence emission at 555 nm, and can maintain stable in QXD reaction system. Therefore, by utilizing the fluorescence of YNCDs at 555 nm as reference signal and the fluorescence of QXD at 435 nm as report signal, we can detect the ALP activity by monitoring the fluorescence ratio (F₄₃₅/F₅₅₅). The linear range is 0.5-5 U/L, and the limit of detection is 0.14 U/L. An application of this method for the analysis of ALP in human serum has given satisfactory results. A ratiometric fluorescent nanoprobe for ascorbic acid and alkaline phosphatase detection with excellent biocompatible and high sensitivity was successfully constructed based on YNCDs and QXD.

Metal-organic frameworks for pharmaceutical and biomedical applications

Metal-organic framework (MOF) materials provide unprecedented opportunities for evaluating valuable compounds for various medical applications. MOFs merged with biomolecules, used as novel biomaterials, have become particularly useful in biological environments. Bio-MOFs can be promising materials in the global to avoid utilization above toxicological substances. Bio-MOFs with crystallin and porosity nature offer flexible structure via bio-linker and metal node variation, which improves their wide applicability in medical science.

Detection of Pyrophosphate and Alkaline Phosphatase Activity Based on PolyT Single Stranded DNA - Copper Nanoclusters

The determination of pyrophosphate and alkaline phosphatase activity plays a significant role in medical diagnosis. In this work, a label-free "ON-OFF-ON" fluorescence strategy is developed for the analysis of pyrophosphate and alkaline phosphatase activity. Using PolyT single strand DNA as templates to synthesize fluorescent copper nanoparticles, the coordination effect of pyrophosphoric acid on Cu²⁺ inhibited the generation of fluorescence. Afterwards, the addition of alkaline phosphatase into hydrolyze pyrophosphoric acid resulted in the release of Cu^2+, whereby the fluorescence intensity could be recovered. Thereupon enhanced-sensitivity for alkaline phosphatase was obtained (0.1 mU/L), much better than previously reported methods. Meanwhile, it could be performed directly in homogeneous solution, which was very close to the actual activity level of alkaline phosphatase under physiological conditions. Likewise, satisfactory results were also obtained in specificity assessment, which demonstrated its potential application in clinical diagnosis. Notably, a new, sensitive, low-cost, short-time, and high-sensitivity platform for alkaline phosphatase detection was constructed, and the design of biosensor using DNA-templated Copper nanoclusters (CuNCs) was instructed in this study.

2D material-based peroxidase-mimicking nanozymes: catalytic mechanisms and bioapplications

The boom in nanotechnology brings new insights into the development of artificial enzymes (nanozymes) with ease of modification, lower manufacturing cost, and higher catalytic stability than natural enzymes. Among various nanomaterials, two-dimensional (2D) nanomaterials exhibit promising enzyme-like properties for a plethora of bioapplications owing to their unique physicochemical characteristics of tuneable composition, ultrathin thickness, and huge specific surface area. Herein, we review the recent advances in several 2D material-based nanozymes, such as carbonaceous nanosheets, metal-organic frameworks (MOFs), transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and transition metal oxides (TMOs), clarify the mechanisms of peroxidase (POD)-mimicking catalytic behaviors, and overview the potential bioapplications of 2D nanozymes.

Flow-homogeneous electrochemical sensing system based on 2D metal-organic framework nanozyme for successive microRNA assay

Considering DNA-based homogeneous electrochemical assay allows identification of targets to be carried out in a homogeneous solution, it would be of significance to develop the successive homogeneous assay system in dynamic solution for rapid disease diagnosis and high-throughput bioanalysis. In homogeneous assay, the work electrodes generally have capability of DNA capture but lack signal amplification, restricting its sensitivity. Here, a flow-homogeneous sensing system was proposed to realize the successive assay of microRNA, a model biomarker. Ultrathin 2D metal-organic framework (MOF) nanozymes with thickness of about 1 nm were facilely prepared by ultrasonic approach. Due to the excellent enzyme-like activity and adsorption capacity towards single-strand DNA (ssDNA), MOF nanozymes adsorbed on electrode simultaneously played two roles of ssDNA collector and signal-amplifier. To adapt the recoverable electrode to on-line monitoring, duplex-specific nuclease-assisted circle reaction was conducted to produce the turn-on amplified signal. Flow injection device was employed to realize the recycling of electrodes and the successive microRNA assay. The assay strategy showed low limit of detection (0.12 pM, S/N = 3) for microRNA, excellent renewability and acceptable reliability for real sample assay. The established system exerts the advantages of DNA-based homogeneous electrochemical sensing strategy. This work would not only expand homogeneous electrochemical assay to successive bioassay, but also provide the possibility for practical application of homogeneous sensing strategy.

An electrochemiluminescence dual “turn-on” strategy for alkaline phosphatase detection using a dual quenching Ru(bpy)32+ encapsulated zeolite imidazole metal organic framework

An electrochemiluminescence (ECL) dual “turn-on” strategy is first designed to detect alkaline phosphatase (ALP) using a dual quenching Ru(bpy)32+ encapsulated zeolite imidazole metal organic framework (Ru(bpy)32+@ZIF-90).

Midas Touch: Engineering Activity of Metal-Organic Frameworks via Coordination for Biosensing

The ever-increasing attention on the highly sensitive biosensors pushes people to explore functional nanomaterials for signal amplification. To endow inert metal-organic frameworks (MOFs) with enzyme mimicking activity, a simple strategy of introducing Cu²⁺ via coordination with 2,2'-bipyridine ligands of Zr-MOF, just like "Midas touch," is proposed. More details on the coordination environment of Cu active sites in Zr-MOF-Cu are disclosed via electron paramagnetic resonance and synchrotron-radiation-based X-ray absorption fine structure analyses. The as-prepared Zr-MOF-Cu exhibits unparalleled catalytic ability, which can catalyze ascorbic acid (AA) to dehydroascorbic acid and further stimulate the reaction with o-phenylenediamine to produce fluorescent signal probes with 8-fold signal amplification. On the basis of catalyzing the dephosphorylation process of l-ascorbic acid-2-phosphate to yield AA via alkaline phosphatase (ALP) and AA-dependent signal responses, a universal fluorescent system has been successfully constructed for quantitative measurement of the activity of ALP and the ALP-related enzyme-linked immunosorbent assay with carcinoembryonic antigen as a model. Moreover, the stable loading of Cu active sites endows the sensing platform with anti-inference capacity and enables its reuse without loss of catalytic activity after 6 months.

Multicolor diagnosis of salivary alkaline phosphatase triggered by silver-coated gold nanobipyramids

Alkaline phosphatase (ALP) is one of the most versatile biomarkers for early detection of several diseases, such as oral carcinomas and periodontitis; therefore, great efforts have been dedicated for developing an ALP biosensor. Multicolor detection of ALP in saliva is ideal for a point-of-care diagnosis; however, this approach is very challenging since spectral responses over wavelengths of several tens of nanometers have thus far remained difficult to achieve. In this work, a colorimetric biosensor for ALP assay has been developed based on ALP affinity to dephosphorylate glucose phosphate into glucose, which has the affinity to deposit Ag nanoshells onto Au nanobipyramids with a multicolor response. This approach provides a blue shift of localized surface plasmon resonance (LSPR) as large as 190 nm corresponding to distinctive color changes, from yellowish brown to red based on the thickness of the formed Ag shell around the Au nanobipyramids. The change in the LSPR has been conducted for highly sensitive quantitative bioassay of ALP with a detectable multicolor change with linear dynamic range of 0.1-20 U/L and low limit of detection (LOD) of 0.085 U/L. Furthermore, the developed multicolor ALP biosensor exhibits high selectivity with high recovery of 98.6% demonstrating  its reliability and suitability for a point-of-care diagnosis.

Polyvinylpyrrolidone-assisted synthesis of highly water-stable cadmium-based metal-organic framework nanosheets for the detection of metronidazole

Recently, much effort has been dedicated to ultra-thin two-dimensional metal–organic framework (2D MOF) nanosheets due to their outstanding properties, such as ultra-thin morphology, large specific surface area, abundant modifiable active sites, etc. However, the preparation of high-quality 2D MOF nanosheets in good yields still remains a huge challenge. Herein, we report 2D cadmium-based metal–organic framework (Cd-MOF) nanosheets prepared in a one-pot polyvinylpyrrolidone (PVP)-assisted synthesis method with high yield. The Cd-MOF nanosheets were characterized with good stability and dispersion in aqueous systems, and were highly selective and sensitive to the antibiotic metronidazole (MNZ) with low limit of detection (LOD: 0.10 μM), thus providing a new and promising fluorescent sensor for rapid detection of MNZ in aqueous solution.

Except PVP was added for Cd-MOF nanosheets, the preparation process of bulk and Cd-MOF nanosheets was similar.

Copper (II) Ion-Modified Gold Nanoclusters as Peroxidase Mimetics for the Colorimetric Detection of Pyrophosphate

Copper (II) ions have been shown to greatly improve the chemical stability and peroxidase-like activity of gold nanoclusters (AuNCs). Since the affinity between Cu²⁺ and pyrophosphate (PPi) is higher than that between Cu²⁺ and AuNCs, the catalytic activity of AuNCs-Cu²⁺ decreases with the introduction of PPi. Based on this principle, a new colorimetric detection method of PPi with high sensitivity and selectivity was developed by using AuNCs-Cu²⁺ as a probe. Under optimized conditions, the detection limit of PPi was 0.49 nM with a linear range of 0.51 to 30,000 nM. The sensitivity of the method was three orders of magnitude higher than that of a fluorescence method using AuNCs-Cu²⁺ as the probe. Finally, the AuNCs-Cu²⁺ system was successfully applied to directly determine the concentration of PPi in human urine samples.

Urate oxidase loaded in PCN-222(Fe) with peroxidase-like activity for colorimetric detection of uric acid

In the past two decades, the number of reports on the construction of uric acid (UA) sensors has increased dramatically, as it is a vital factor in the diagnosis of physiological functions and diseases. Among these sensors, cascade colorimetric detection based on peroxidase mimics has received great attention owing to the advantages of easier operation and more intuitive results. Herein, we report a simple UA detection method via the integration of urate oxidase (UOx) and PCN-222(Fe) with peroxidase-like activity, in which UOx is immobilized in PCN-222(Fe) by physical adsorption. UOx could catalyze the UA oxidation to produce H₂O₂, and then PCN-222(Fe) catalyzed the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to generate a blue product with a maximum absorbance at 652 nm. Under optimal conditions, the increasing absorbance at 652 nm was proportional to the UA concentration. The linear range for UA detection was 10-800 μM with a limit of detection of 3.5 μM, and the cascade system has been successfully utilised for the detection of UA in human serum samples to demonstrate the accuracy of the proposed method. In conclusion, the cascade catalytic system based on the immobilization of UOx in PCN-222(Fe) has the potential to be used as a rapid and sensitive sensor for UA detection.

Two-Dimensional Material-Based Colorimetric Biosensors: A Review

Two-dimensional (2D) materials such as graphene, graphene oxide, transition metal oxide, MXene and others have shown high potential for the design and fabrication of various sensors and biosensors due to their 2D layered structure and unique properties. Compared to traditional fluorescent, electrochemical, and electrical biosensors, colorimetric biosensors exhibit several advantages including naked-eye determination, low cost, quick response, and easy fabrication. In this review, we present recent advances in the design, fabrication, and applications of 2D material-based high-performance colorimetric biosensors. Potential colorimetric sensing mechanisms and optimal material selection as well as sensor fabrication are introduced in brief. In addition, colorimetric biosensors based on different 2D materials such as graphene, transition metal dichalcogenide/oxide, MXenes, metal-organic frameworks, and metal nanoplates for the sensitive detection of DNA, proteins, viruses, small molecules, metallic ions, and others are presented and discussed in detail. This work will be helpful for readers to understand the knowledge of 2D material modification, nanozymes, and the synthesis of hybrid materials; meanwhile, it could be valuable to promote the design, fabrication, and applications of 2D material-based sensors and biosensors in quick bioanalysis and disease diagnostics.

Metal-Organic Frameworks Enhance Biomimetic Cascade Catalysis for Biosensing

Multiple enzymes-induced biological cascade catalysis guides efficient and selective substrate transformations in vivo. The biomimetic cascade systems, as ingenious strategies for signal transduction and amplification, have a wide range of applications in biosensing. However, the fragile nature of enzymes greatly limits their wide applications. In this regard, metal-organic frameworks (MOFs) with porous structures, unique nano/microenvironments, and good biocompatibility have been skillfully used as carriers to immobilize enzymes for shielding them against hash surroundings and improving the catalytic efficiency. For another, nanomaterials with enzyme-like properties and brilliant stabilities (nanozymes), have been widely applied to ameliorate the low stability of the enzymes. Inheriting the abovementioned merits of MOFs, the performances of MOFs-immboilized nanozymes could be significantly enhanced. Furthermore, in addition to carriers, some MOFs can also serve as nanozymes, expanding their applications in cascade systems. Herein, recent advances in the fabrication of efficient MOFs-involving enzymes/nanozymes cascade systems and biosensing applications are highlighted. Integrating diversified signal output modes, including colorimetry, electrochemistry, fluorescence, chemiluminescence, and surface-enhanced Raman scattering, sensitive detection of various targets (including biological molecules, environmental pollutants, enzyme activities, and so on) are realized. Finally, challenges and opportunities about further constructions and applications of MOFs-involving cascade reaction systems are briefly put forward.

2D bimetallic Ni/Fe MOF nanosheet composites as a peroxidase-like nanozyme for colorimetric assay of multiple targets

In this contribution, 2D Ni/Fe MOF nanosheets were synthesized by a simple two-step ultrasound strategy at room temperature, i.e. the 2D Ni-MOF with a lamellar structure was first synthesized by the top-down ultrasonic assisted stripping route, followed by introducing Fe3+ ions as a metal node and terephthalic acid as an organic ligand to form 2D Ni/Fe MOF nanosheets that exhibited weak oxidase-like and strong peroxidase-like properties. Relative to that of the single metal Ni-MOF and Fe-MOF, the peroxidase-mimicking capability of the 2D Ni/Fe MOF nanosheets increased by over 14-fold and 3-fold, respectively. Reactive oxygen trials indicated that the 2D Ni/Fe MOF nanosheets can efficiently catalyze the decomposition of H2O2 to generate the ˙OH and O2˙- radicals, which can oxidize TMB to oxTMB from colorless to blue. The kinetic trial demonstrated the high affinity of the 2D Ni/Fe MOF nanosheet to H2O2 with a Km of 0.037 mM, which was 100 times lower than that of HRP. These impressive characteristics are likely related to the good dispersion of the in situ formed Fe MOF in the 2D Ni-MOF nanosheet structure with coordinatively unsaturated metal sites. This allows the 2D Ni/Fe MOF nanosheets to expose more active metal sites and to enhance the intrinsic catalytic activity of each site due to the synergistic interaction between the two metals. Interestingly, glutathione can obviously restrict the peroxidase-like activity of the 2D Ni/Fe MOF nanosheet, while the inhibited TMB oxidation can be restored upon further introducing Hg2+ ions due to the high and specific affinity of Hg2+ to thiol groups in glutathione. Based on the above facts, the 2D Ni/Fe MOF nanozyme was used to construct a nanoplatform to determine multiple targets, i.e. H2O2, glutathione and Hg2+. The 2D Ni/Fe MOF nanozyme-based colorimetric assay exhibits a linear response to H2O2, glutathione and Hg2+ ions over the 0.01-100 μM, 0.02-100 μM, and 100 nM to 200 μM ranges, respectively. The limits of detection (3σ) for the determination of H2O2, glutathione and Hg2+ are 10 nM, 10 nM, and 100 nM, respectively. This method was used to determinate the content of Hg2+ ions in real water samples.

Ionic liquid-assisted chemiluminescent immunoassay of prostate specific antigen using nanoceria as an alkaline phosphatase-like nanozyme label

An alkaline phosphatase-like nanozyme was applied for an immunoassay for the first time. By using nanoceria as the alkaline phosphatase-like catalytic label and CDP-star as the substrate, the chemiluminescent detection of prostate specific antigen was demonstrated. More importantly, the addition of ionic liquid can significantly increase the sensitivity of the immunoassay. With the aid of ionic liquid, an order of magnitude improvement in the sensitivity was achieved with a detection limit of 53 fg mL^-1.

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.

Two-dimensional iron MOF nanosheet as a highly efficient nanozyme for glucose biosensing

A 2D Fe-BTC nanosheet was preparedviaa cation exchange route. Its peroxidase-like activity is 2.2 times that of 3D MIL-100(Fe) due to highly accessible surface active sites. This is helpful for substrate contact with the catalyst during the catalytic reaction.