A Reaction-based Ratiometric Fluorescent Probe with Large STOKES Shift for Cu2+ in Neat Aqueous Solution.

A novel reaction-based ratiometric fluorescent probe 1 for Cu2+ using picolinate as the reaction site and hemicyanine as the fluorophore was developed. 1 displayed maximum absorption peak at 355 nm and fluorescence emission peak at 500 nm, with large Stokes shift of 145 nm. Upon reaction with Cu2+, the maximum absorption and fluorescence emission peaks red-shifted to 390 nm and 570 nm respectively, owing to Cu2+-induced hydrolysis of the picolinate moiety in 1. Meanwhile, the solution of 1 turned from green to orange under a 365 nm UV lamp. 1 not only could detect Cu2+ ratiometrically by the ratios of both absorbance (A390 nm/A355 nm) and fluorescence intensity (F570 nm/F500 nm), but also displayed large Stokes shift, fast response, high sensitivity and excellent selectivity over other metal ions in neat aqueous solution.

Anthracene-based dual channel donor-acceptor triazine-containing covalent organic frameworks for superior photoelectrochemical sensing.

Covalent organic frameworks (COFs) exhibit excellent photoelectrically active structures and serve as channels for photon capture and charge carrier transport. However, their relatively high charge-carrier recombination rates and lack of specific recognition sites limit their application in photoelectrochemical sensing. This paper reports a functionalized donor-acceptor (D-A) COF comprising electron-rich polycyclic aromatic moieties and electron-deficient triazines (Tz) incorporating boronic acid through ligand exchange. The number of aromatic rings in the polycyclic aromatic moiety is crucial for establishing an efficient D-A system within COF. In the absence of an external electron donor, the anthracene-based COF exhibited a five-fold enhancement in photocurrent compared to the naphthalene-based COF. The resulting anthracene-based D-A COF exhibited enhanced orbital overlap and electron push-pull interactions, facilitating more effective charge separation. Furthermore, introducing boronic acid enabled the selective enrichment of low-concentration external electron donors, such as dopamine, in the inner Helmholtz plane. This ingenious approach establishes a unique dual-channel D-A system that allows direct measurement of dopamine in serum. Under optimized conditions, the test platform achieves good correspondence for dopamine at 1 to 100 nM and 0.5 to 100 µM with a detecting limit of 0.36 nM (3σ/S, n = 11). This strategy introduces a novel dimension to photoelectrochemical sensing, focusing on the effect of spatial separation between the external electron donor and the photoelectrode interface that intricately shapes the behavior and enhances the performance of the photoelectric system.

Synergistic co-catalytic nanozyme system for highly efficient one-pot colorimetric sensing at neutral pH: Combining molybdenum trioxide and Fe(III)-Modified covalent triazine framework.

This study investigates the co-catalytic capabilities of MoO3 nanosheets in enhancing the enzyme-like catalytic activity of a two-dimensional ultrathin Fe(III)-modified covalent triazine framework (Fe-CTF) under neutral pH conditions. The unique physicochemical surface properties and two-dimensional structures of Fe-CTF enable the direct immobilization of native enzymes (glucose oxidase (GOD) and xanthine oxidase (XOD)) through adsorption, eliminating the need for chemical processes. Efficient immobilization of the native enzymes within the Fe-CTF/GOD(XOD) hybrid is achieved through multipoint attachment involving various interactions. The Fe-CTF/MoO3 co-catalytic system exhibits enzyme-mimicking activity at neutral pH and, when combined with the high catalytic activity of the immobilized native enzymes, enables the development of a colorimetric method for glucose detection. This method demonstrates excellent facilitation, rapidity, sensitivity, and selectivity, with a linear detection range of 50-1000 µM and a limit of detection of 8.8 µM for glucose. Furthermore, a straightforward one-pot colorimetric method is established for screening XOD inhibitors. The inhibitory potential of a crude extract derived from Chinese water chestnut peel on XOD activity is evaluated using this method. The findings of this study pave the way for the utilization of nanozyme/native enzyme hybrids in pH-neutral conditions for one-pot colorimetric sensing. This work contributes to the advancement of enzyme-based sensing technologies and holds promise for various applications in biosensing and biomedical research.

Cu-MOF derived CuO@g-C3N4 nanozyme for cascade catalytic colorimetric sensing.

The use of peroxidase mimics has great potential for various real applications due to their strong catalytic activity. Herein, a facile strategy was proposed to directly prepare CuO@g-C3N4 by Cu-MOF derivatization and demonstrated its efficacy in constructing a multiple enzymatic cascade system by loading protein enzymes onto it. The resulting CuO@g-C3N4 possessed high peroxidase-like activity, with a Michaelis constant (Km) of 0.25 and 0.16 mM for H2O2 and 3,3',5,5'-tetramethylbenzidine (TMB), respectively. Additionally, the high surface area of CuO@g-C3N4 facilitated the loading of protein enzymes and maintained their activity over an extended period, expanding the potential applications of CuO@g-C3N4. To test its feasibility, CuO@g-C3N4/protein oxidase complex was prepared and used to sense the ripeness and freshness of fruits and meat, respectively. The mechanism relied on the fact that the ripeness of fruits increased and freshness of food decreased with the release of marked targets, such as glucose and xanthine, which could produce H2O2 when digested by the corresponding oxidase. The peroxidase mimics of CuO@g-C3N4 could then sensitively colorimetric detect H2O2 in present of TMB. The obtained CuO@g-C3N4/oxidase complex exhibited an excellent linear response to glucose or xanthine in the range of 1.0-120 µmol/L or 8.0-350 µmol/L, respectively. Furthermore, accurate quantification of glucose and xanthine in real samples is achieved with spiked recoveries ranging from 80.2% to 120.0% and from 94.2% to 112.0%, respectively. Overall, this work demonstrates the potential of CuO@g-C3N4 in various practical applications, such as food freshness detection.

Ligand-variable metal clusters charge transfer in Ce-Por-MOF/AgNWs and their application in photoelectrochemical sensing of ronidazole.

A photoelectrochemical sensing platform based on ligand-variable metal clusters charge transfer was established for the quantitative assay of ronidazole (RNZ) using Ce-porphyrin-metal-organic frameworks/silver nanowires (Ce-Por-MOFs/AgNWs). Rod-like Ce-Por-MOFs and well-dispersed sub-50 nm AgNWs were prepared using a hydrothermal method and polyol strategy, and then through simple drop coating to yield Ce-Por-MOFs/AgNWs nanocomposites. We investigated the intrinsic semiconducting properties of the composites. More importantly, it was found that the variable-valence metal node can provide electronic defect states similar to those caused by multi-metal doping, synergizing with the surface plasmon effect of AgNWs, which significantly improved the photoelectric conversion efficiency, thereby resulting in excellent optoelectronic properties. In combination with molecular imprinting, a competitive type trace photoelectrochemical sensor for RNZ was constructed using Fe2+ as the electron donor and probe. Under optimal conditions, the sensor response is proportional to the logarithm of RNZ concentration in the range 0.1-104 nM with a detected limit of 0.038 nM. The recoveries ranged from 87.2 to 116% with relative standard deviations (RSDs) < 6.5% (n = 3) in milk sample. This work reveals the charge-transfer process of variable-valence metal nodes in MOFs during photoelectrochemical processes, which will provide new insights for the sensing application of variable-valence metal MOFs.

Dextran-coated Gd-based ultrasmall nanoparticles as phosphatase-like nanozyme to increase ethanol yield via reduction of yeast intracellular ATP level.

Nanozymes-functional materials that possess intrinsic enzyme-like characteristics-have gained tremendous attention in recent years owing to their unique advantages; however, further research is required to understand their scope in biological applications. In this study, dextran-coated nanogadolinia (DCNG) was synthesised, and its phosphatase mimetic activity was demonstrated. Specifically, the dephosphorylation of adenosine triphosphate (ATP), an important biomolecule, by DCNG was investigated. The results showed that DCNG could selectively catalyse the hydrolysis of the terminal high-energy phosphate bonds of ATP under physiological conditions. Furthermore, the biocompatible DCNG, with remarkable phosphatase mimicking activity, decreased the intracellular ATP content by dephosphorylation and increased ethanol yield during glucose fermentation by S. cerevisiae. These results indicate potential alternatives for improving ethanol yields and exploring novel biological applications of nanozymes.

Green photoreduction synthesis of dispersible gold nanoparticles and their direct in situ assembling in multidimensional substrates for SERS detection.

Gold nanoparticles (AuNPs) and their composites have been applied in surface-enhanced Raman scattering (SERS) detection methods, owing to their stable and excellent surface plasmon resonance. Unfortunately, methods for synthesizing AuNPs often require harsh conditions and complicated external steps. Additionally, removing residual surfactants or unreacted reductants is critical for improving the sensitivity of SERS detection, especially when employing AuNPs-assembled multidimensional substrates. In this study, we propose a simple and green method for AuNPs synthesis via photoreduction, which does not require external surfactant additives or stabilizers. All the processes were completed within 20 min. Along this way, only methanol was employed as the electron acceptor. Based on this photoreduction synthesis strategy, AuNPs can be directly and circularly assembled in situ in multidimensional substrates for SERS detection. The removal of residual methanol was easy because of its low boiling point. This strategy was employed for the preparation of three different dimensional SERS substrates: filter paper@AuNPs, g-C3N4@AuNPs, and MIL-101(Cr)@AuNPs. The limit of detection of filter paper@AuNPs for thiabendazole SERS detection was 1.0 × 10-7 mol/L, while the limits of detection of g-C3N4@AuNPs and MIL-101(Cr)@AuNPs for malachite green SERS detection were both 5.0 × 10-11 mol/L. This strategy presents potential in AuNP doping materials and SERS detection.

Ultrasmall phosphatase-mimicking nanoceria with slight self-colour for nonredox nanozyme-based colorimetric sensing.

Nanozyme-based colorimetric sensing has attracted significant interest in recent years, and a number of redox-type nanozyme-based colorimetric sensors based on peroxidase and oxidase mimics have been reported. However, conventional redox-type nanozyme-based colorimetric sensing is affected by interference from the endogenous reductants present in actual samples. Herein, we describe the development of a homogeneous nonredox-type nanozyme-based colorimetric sensor that exploits the intrinsic phosphatase-like activity of CeO2. Specifically, colorimetric detection platforms for fluoride ions, zearalenone, and hydrogen peroxide were constructed based on activity inhibition, aptamer-assisted gate control, and plasmonic nanoparticle growth, respectively. Thus, this study reports a versatile route for constructing nonredox nanozyme-based colorimetric sensors that are not affected by interference from endogenous reductants and are thus highly promising for use in food safety and bioassay applications.

Colorimetric detection of creatinine based on specifically modulating the peroxidase-mimicking activity of Cu-Fenton system.

Creatinine (CR) has always been considered as a prime important indicator for evaluating of renal dysfunction. Nevertheless, the literature still lacks in methods that fulfill the requirements for detecting CR effectively. Therefore, the development of a visual sensing method for the detection of CR specifically and sensitively is definitely desirable. To provide a solution, a colorimetric sensing platform for monitoring CR was constructed based on the regulation effect of CR on the peroxidase-mimicking activity of MoO3-Cu2+ system. Here, the prepared MoO3 itself does not have the catalytic activity of peroxidase mimics, but it can be used as a co-catalyst. In the presence of CR, the Cu2+ Fenton-like reaction can be promoted through co-catalysis of MoO3, so as to facilitate the production of more reactive oxygen species (ROS) and accelerate the oxidation process of 3,3',5,5'-tetramethylbenzidine (TMB). As a result, the regulation of MoO3-Cu2+ peroxidase-like activity can be achieved by adding different concentrations of CR. The higher the concentrations of CR, the stronger the catalytic activity for MoO3-Cu2+/CR. Experimental results showed that the colorimetric sensing strategy proposed in this paper can be successfully applied to the analysis of CR in human serum samples.

Enhancing the peroxidase-like activity of MIL-88B by ligand exchange with polydopamine.

Metal-organic frameworks (MOFs) as nanozymes have been widely used in biosensing. However, MOFs have inherent defects of easy agglomeration, leading to the stacking of active surfaces. In addition, the low conductivity of MOFs is not conducive to the electron migration in the Fenton-like reaction, which leads to a further decrease in catalytic activity and severely restricts their application. In response to the above problems, it makes sense to develop a method to improve both the dispersion and conductivity of MOFs. Here, a simple ligand exchange method with polydopamine (PDA) was used to synthesize MOF PDA-MIL-88B. PDA-MIL-88B shows stronger peroxidase-like activity than MIL-88B. One reason is the good dispersibility of PDA-MIL-88B, which is conducive to exposing the active surface. In addition, the reduced electrochemical impedance of PDA-MIL-88B increases its electrical conductivity, which is favorable for electron migration in the Fenton-like reaction. As a result, PDA-MIL-88B can better catalyze 3,3',5,5'-tetramethylbenzidine to achieve redoximorphic color changes. PDA-MIL-88B can be used to detect glucose in human serum with good sensitivity and selectivity. This work can provide a strategy for MOFs to enhance the enzyme-like activity.

Biomimetic Synthesis of Ultrafine Mixed-Valence Metal-Organic Framework Nanowires and Their Application in Electrochemiluminescence Sensing.

Metal-organic frameworks (MOFs) prepared via typical procedures tend to exhibit issues like poor water stability and poor conductivity, which hinder their application in electrochemical sensing. Herein, we report a strategy for the preparation of mixed-valence ultrafine one-dimensional Ce-MOF nanowires based on a micelle-assisted biomimetic route and subsequent investigation into their growth mechanism. The prepared mixed-valence Ce-MOF nanowires exhibited a typical size of ∼50 nm and were found to present good water stability and high conductivity. On this basis, we examined the introduction of these nanowires into the luminol hydrogen peroxide luminescence system and proposed a novel dual-route self-circulating electrochemiluminescence (ECL) catalytic amplification mechanism. Finally, in combination with molecular imprinting, a MOF-based ECL sensor was developed for the detection of trace amounts of imidacloprid in plant-derived foods. This sensor exhibited a linearity of 2-120 nM and a detection limit of 0.34 nM. Thus, we proposed not only a novel route to MOF downsizing but also a facile and robust methodology for the design of a MOF-based molecular imprinting ECL sensor.

Porous Oxyhydroxide Derived from Metal-Organic Frameworks as Efficient Triphosphatase-like Nanozyme for Chromium(III) Ion Colorimetric Sensing.

The dephosphorylation that involves the removal of a phosphate group from a substrate molecule plays a significant role in living organisms. An enzyme mimic (nanozyme) with phosphatase-like catalytic activity has recently received attention in terms of its capacity for dephosphorylation. In this study, three types of highly porous oxyhydroxide with remarkable triphosphatase-like catalytic activities, ZrOOH, GdOOH, and HfOOH, have been prepared through the transformation of metal-organic frameworks (MOFs) using a simple alkaline hydrolysis method. The triphosphatase mimetic activities of ZrOOH, GdOOH, and HfOOH were then thoroughly investigated and verified. In particular, an isotopic tracing experiment revealed that abundant surface hydroxyls could serve as nucleophilic agents to directly attack the electropositive phosphorus atom, causing the cleavage of the terminal phosphoester bonds of phosphoester substrate molecules. The kinetic analysis provided calculated values of Km of 105.7, 90.5, and 46.1 µM, while the Vmax values were 3.57, 4.76, and 2.74 × 10-8 M s-1 and Ea values were estimated to be 47.52, 41.15, and 52.79 kJ/mol for ZrOOH, GdOOH, and HfOOH, respectively. The chromium(III) ions acting as "poisoning" inhibitors efficiently downregulated the triphosphatase mimetic activity of GdOOH. On the basis of this effect, a colorimetric chromium(III) ion-sensing system was explored, which provided a relevant linear response range for the detection of chromium(III) ions of 5.0-200 µM and a low detection limit of 0.84 µM. This work not only shows the great potential of ZrOOH, GdOOH, and HfOOH as triphosphatase nanozymes but also deepens our understanding of the role of surface hydroxyls on phosphatase-mimicking nanozyme catalytic dephosphorization, which could be used in the rational design of phosphatase-mimicking nanozymes. Furthermore, the developed colorimetric sensing system could be applied to chromium(III) ion detection in biological systems.

Synthesis of Zr-coordinated amide porphyrin-based two-dimensional covalent organic framework at liquid-liquid interface for electrochemical sensing of tetracycline.

Highly-conductive two-dimensional covalent organic framework (COF) displays prominent applications in various fields of science and technology. This paper reports the design and liquid-liquid interface synthesis of a novel Zr-coordinated amide porphyrin-based 2D COF (Zr-amide-Por-based 2D COF). The COF adopts a graphene-like multilayer structure with the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) band gap of 1.6 eV. The ordered multilayer structure of the amide COF was confirmed through a series of characterization techniques, including scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. In particular, the inherent-ordered structure of Zr-amide-Por-based 2D COF with Zr as the catalytically active center confers several distinct advantages to the material, such as high conductivity and high electrocatalysis performance. A molecularly imprinted tetracycline electrochemiluminescence sensor was constructed based on the Zr-amide-Por-based 2D COF, and gate control effect was used as a signal-generation mechanism. Under optimal conditions, the sensor showed a good linear relationship with tetracycline in the concentration range of 5-60 pM, with a detection limit of 2.3 pM. Because the sensor is rapid, cost-efficient, highly sensitive, and specific, it can be considered as a viable platform for veterinary drug residue monitoring.

A Cu(II)-anchored unzipped covalent triazine framework with peroxidase-mimicking properties for molecular imprinting-based electrochemiluminescent detection of sulfaquinoxaline.

The authors describe a method of electrochemiluminescent quantitation of the antibiotic sulfaquinoxaline (SQX). It relies on the use of a molecularly imprinted polymer and a Cu(II)-anchored unzipped covalent triazine framework (UnZ-CCTF) with excellent dispersibility, electrical conductivity, and peroxidaze-like activity. The framework was prepared by unzipping a covalent triazine framework under retention of basic triazine units. It was morphologically and structurally characterized by a range of instrumental techniques. The excellent peroxidase-mimicking effect of UnZ-CCTF on the electrochemiluminescence of the luminol/H2O2 system was exploited to design an ultrasensitive SQX assay with a 1.0-20 pM detection range and a detection limit of 0.76 pM (at 3δ/m). The technique was used for SQX quantitation in spiked milk samples, achieving recoveries of 94.0-104.8%. Graphical abstract Scheme of the sulfaquinoxaline molecularly imprinted electrochemiluminescence sensor based on Cu-anchored unzipped covalent triazine frameworks.

Water dispersed two-dimensional ultrathin Fe(iii)-modified covalent triazine framework nanosheets: peroxidase like activity and colorimetric biosensing applications.

Peroxidase mimics have attracted increasing attention as emerging artificial enzymes due to their promising applications in many fields, including bionanotechnology, sustainable chemistry, and environmental remediation. Although many types of peroxidase mimics have been successfully exploited in the past decade, the development of an innovative peroxidase mimic that does not contain noble metals, but exhibits remarkable peroxidase-like activity and low cytotoxicity still remains a major challenge in this field. Herein, we present a bulk covalent triazine framework cleavage and metal atom anchoring strategy for the synthesis of iron-modified two-dimensional covalent triazine frameworks (2D Fe-CTFs) that demonstrate excellent peroxidase-like activity. Furthermore, three kinds of colorimetric sensing platforms for sarcosine, ochratoxin A, and fluoride ions were constructed based on the intrinsic peroxidase-like activity, salt-induced coagulation, and coordination competition of the 2D Fe-CTF, respectively. This work may provide a new synthetic method for peroxidase mimics that can be used in various colorimetric sensors.

Bioinspired Synthesis of Cu2+ -Modified Covalent Triazine Framework: A New Highly Efficient and Promising Peroxidase Mimic.

Artificial enzymes is an emerging field of research owing to the remarkable advantages of enzyme mimics over their natural counterpart, including tunable catalytic efficiencies, lower cost, ease of preparation, and excellent tolerance to variations of the reaction system. Herein, we report an efficient peroxidase mimic based on a copper-modified covalent triazine framework (CCTF). Owing to its unique specific surface area, atomically dispersed active Cu sites, efficient electron transfer, and enhanced photo-assisted enzyme-like activity, the CCTF showed enhanced peroxidase-like enzyme activity. Therefore, copper modification represents an effective route to tailor the peroxidase-like activity of the covalent triazine frameworks. Furthermore, the mechanism of the enhanced peroxidase-like activity and stability of the CCTF were investigated. As a proof of concept, the CCTF was used for the colorimetric detection of H2 O2 and decomposition of organic pollutants. This work provides a new strategy for the design of enzyme mimics with a broad range of potential applications.

Prussian blue nanoparticles encapsulated inside a metal-organic framework via in situ growth as promising peroxidase mimetics for enzyme inhibitor screening.

Artificial enzyme mimics are of current research interest owing to their remarkable advantages over natural enzymes. Herein, as a novel peroxidase mimic material, MIL-101(Cr)@PB was fabricated by encapsulating Prussian blue (PB) nanoparticles into the host matrix of MIL-101(Cr) via a facile and mild in situ growth synthetic strategy. The crystallographic characteristics, morphology, and porosity of the as-synthesized MIL-101(Cr)@PB composites were carefully studied using XRD, SEM, TEM, TGA, and BET. The results show that the synthesized MIL-101(Cr)@PB possesses a reproducible and impressive intrinsic peroxidase-like activity even under extreme conditions. Exploiting this, a colorimetric platform for screening xanthine oxidase inhibitors was constructed. We hope that this work will elucidate the applications of metal-organic frameworks as carriers for enzyme mimics and enable a wider application in drug screening.

Synthesis of a mixed valence state Ce-MOF as an oxidase mimetic for the colorimetric detection of biothiols.

We demonstrate a facile and rapid in situ partial oxidation synthetic strategy for the fabrication of a mixed valence state Ce-MOF (MVCM) which exhibits intrinsic oxidase-like activity. Furthermore, on the basis of the excellent catalytic activity of the MCVM, a colorimetric approach for the high-throughput detection of biothiols in serum samples was established.

Preparation of polyhedral oligomeric silsesquioxane based hybrid monoliths by thiol-ene click chemistry for capillary liquid chromatography.

A facile organic-silica hybrid monolith was prepared by a thiol-ene click reaction of polyhedral oligomeric silsesquioxane methacryl substituted (POSS-MA) with 1,4-bis(mercaptoacetoxy) butane (BMAB) using toluene and dodecanol as a porogenic system. The effects of the ratio of POSS-MA-BMAB and porogenic solvents and click reaction temperature on the morphology, permeability and column performance of the resulting POSS-BMAB hybrid monoliths were studied in detail. A uniform monolithic network with a high porosity was obtained. The POSS-BMAB hybrid monolith exhibited good permeability and high thermal and mechanical stability. A series of test compounds, including alkylbenzenes, polycyclic aromatic hydrocarbon, phenols and, anilines were used to evaluate the retention behaviors of the POSS-BMAB hybrid monolith in capillary liquid chromatography. The prepared POSS-BMAB hybrid monolith exhibited typical reversed-phase chromatographic behavior toward neutral solutes. The minimum plate height of this hybrid monolith was determined as 12.6 and 13.7 µm for thiourea and benzene, respectively. These results demonstrate that thiol-ene click chemistry can provide a facile and robust approach for the preparation of a POSS-based hybrid monolith.