Constructing bifunctional metal–organic framework based nanozymes with fluorescence and oxidase activity for the dual-channel detection of butyrylcholinesterase

Ultra-trace Ag doped carbon quantum dots with peroxidase-like activity for the colorimetric detection of glucose

Carbon quantum dots (CQDs) have significant applications in nanozymes. However, previous studies have not elucidated the structure-activity relationship and enzyme mechanism. In this study, we employed a one-step microwave method to synthesize ultra-trace Ag-doped carbon quantum dots (Ag-CQDs). In the presence of hydrogen peroxide (H2O2), we used the oxidative coupling reaction of 3,3',5,5'-tetramethylbenzidine (TMB) to evaluate the intrinsic peroxidase-like activity, kinetics, and mechanism of Ag-CQDs. The trace amount of doped Ag (1.64 %) facilitated electron transfer from the CQDs interior to the surface. The electron transfer triggered the peroxide activity of CQDs, producing hydroxyl radical (·OH), which oxidized the colorless TMB to blue-colored TMB (oxTMB). By coupling with glucose oxidase (GOx), the Ag-CQDs/H2O2/TMB system has been used for colorimetric glucose determination. The system demonstrated a low detection limit (0.17 µM), wide linear range (0.5-5.5 µM), and satisfactory results when fruit juice was analyzed. This study reports a feasible method for the colorimetric detection of glucose by synthesizing ultra-trace Ag-doped carbon quantum dots with peroxidase-mimicking activity.

Construction of Mn-decorated zeolitic imidazolate framework-90 nanostructure as superior oxidase-like mimic for colorimetric detection of glucose and choline

A Mn decorated zeolitic imidazolate framework-90 (ZIF-90) nanozyme (Mn/ZIF-90) was constructed through an effective and rapid post-synthetic strategy for the first time. The Mn in Mn/ZIF-90 exists in mixed valence states, which is doped to the ZIF-90 through the formation of Mn-O bond. The Zn-N coordination structure of ZIF-90 may change the electronic arrangement of oxygen atoms in the free carbonyl groups (-CHO), allowing the coordination of Mn with O. The prepared Mn/ZIF-90 possesses outstanding oxidase-like activity and remarkable stability. Besides, the catalytic activity of Mn/ZIF-90 can be inhibited in the presence of H2O2. Therefore, using the Mn/ZIF-90-triggered chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) as an amplifier, a versatile enzyme cascade-based colorimetric method for the detection of glucose and choline with good sensitivity and selectivity was developed. The linear ranges for glucose and choline are 6.25-500 μM and 5-1000 μM, respectively. Furthermore, the developed method was applied in the detection of glucose and choline in rabbit plasma samples, and the recoveries are 89.5-107.3 % and 96.0-109.3 %, respectively. In short, the simple and efficient post-synthetic doping method may provide a new thought for the rational designs of enzyme mimics with improved catalytic performance. Moreover, the colorimetric method based on the excellent catalytic activity of Mn/ZIF-90 may be extended to detect other H2O2-generating or consuming molecules and evaluate the activity of bio-enzymes that can catalyze the generation of glucose or choline.

Dual-Emission Single Sensing Element-Assembled Fluorescent Sensor Arrays for the Rapid Discrimination of Multiple Surfactants in Environments

Surfactants are considered as typical emerging pollutants, their extensive use of in disinfectants has hugely threatened the ecosystem and human health, particularly during the pandemic of coronavirus disease-19 (COVID-19), whereas the rapid discrimination of multiple surfactants in environments is still a great challenge. Herein, we designed a fluorescent sensor array based on luminescent metal-organic frameworks (UiO-66-NH2@Au NCs) for the specific discrimination of six surfactants (AOS, SDS, SDSO, MES, SDBS, and Tween-20). Wherein, UiO-66-NH2@Au NCs were fabricated by integrating UiO-66-NH2 (2-aminoterephthalic acid-anchored-MOFs based on zirconium ions) with gold nanoclusters (Au NCs), which exhibited a dual-emission features, showing good luminescence. Interestingly, due to the interactions of surfactants and UiO-66-NH2@Au NCs, the surfactants can differentially regulate the fluorescence property of UiO-66-NH2@Au NCs, producing diverse fluorescent "fingerprints", which were further identified by pattern recognition methods. The proposed fluorescence sensor array achieved 100% accuracy in identifying various surfactants and multicomponent mixtures, with the detection limit in the range of 0.0032 to 0.0315 mM for six pollutants, which was successfully employed in the discrimination of surfactants in real environmental waters. More importantly, our findings provided a new avenue in rapid detection of surfactants, rendering a promising technique for environmental monitoring against trace multicontaminants.

Two-mode sensing strategies based on tunable cobalt metal organic framework active sites to detect Hg2

Heavy metal pollution poses a major threat to human health, and developing a user-deliverable heavy metal detection strategy remains a major challenge. In this work, two-mode Hg2+ sensing platforms based on the tunable cobalt metal-organic framework (Co-MOF) active site strategy are constructed, including a colorimetric, and an electrochemical assay using a personal glucose meter (PGM) as the terminal device. Specifically, thymine (T), a single, adaptable nucleotide, is chosen to replace typical T-rich DNA aptamers. The catalytic sites of Co-MOF are tuned competitively by the specific binding of T-Hg2+-T, and different signal output platforms are developed based on the different enzyme-like activities of Co-MOF. DFT calculations are utilized to analyze the interaction mechanism between T and Co-MOF with defect structure. Notably, the two-mode sensing platforms exhibit outstanding detection performance, with LOD values as low as 0.5 nM (colorimetric) and 3.69 nM (PGM), respectively, superior to recently reported nanozyme-based Hg2+ sensors. In real samples of tap water and lake water, this approach demonstrates an effective recovery rate and outstanding selectivity. Surprisingly, the method is potentially versatile and, by exchanging out T-Hg2+-T, can also detect Ag+. This simple, portable, and user-friendly Hg2+ detection approach shows plenty of promise for application in the future.

Integrating Intelligent Logic Gate Dual-Nanozyme Cascade Fluorescence Capillary Imprinted Sensors for Ultrasensitive Simultaneous Detection of 2,4-Dichlorophenoxyacetic Acid and 2,4-Dichlorophenol

Herein, a dual-nanozyme cascade catalysis triemission fluorescence capillary imprinted sensor integrated with intelligent logic gates was constructed for simultaneous detection of 2,4-dichlorophenoxyacetic acid (2,4-DA) and 2,4-dichlorophenol (2,4-DCP). The novel nanozyme fluorescence organic framework (Bi, Co-MOF) was grafted on the surface of Fe3O4 modified with histidine to form a nanozyme composite (FBM) with dual-enzyme activity, which was imprinted with 2,4-DA to prepare a fluorescence molecularly imprinted polymer (FBM@MIP). Carbon dots (CDs) coupling with FBM@MIP (FBM@MIP/CDs) was inhaled into a capillary to construct a dual-nanozyme capillary imprinted sensor directly. The FBM@MIP/CDs capillary sensor realized to detect 2,4-DA and 2,4-DCP simultaneously within a linear concentration range of 1.0 × 10-12-1.2 × 10-9 M and 1.0 × 10-12-4.8 × 10-9 M with the detection limit of 0.75 and 0.68 pM, respectively. Interestingly, a smartphone-assisted portable capillary fluorescence intelligent sensing platform was developed that can detect 2,4-DA and 2,4-DCP visually without tedious operations such as soaking and drying. Combined with a smartphone, the linear relationships between RGB ratios and concentrations of 2,4-DA and 2,4-DCP were established with the detection limit of 0.93 and 0.81 pM, respectively. The integrated logic gates provided a promising way for intelligent sensing of multiple targets simultaneously, which provided a new strategy for ultrasensitive simultaneous detection of multiple pollutants with a microvolume (18 μL/time) in complex environments.

Cu, I-doped carbon dots as simulated nanozymes for the colorimetric detection of morphine in biological samples

As newly developed synthetic enzymes with exceptional catalytic capabilities and outstanding stability, nanozymes have drawn considerable interest in the realm of sensing. Using a simple hydrothermal process, iodine and copper-doped carbon dots (Cu,I-CDs) with simulated enzymes were fabricated in the current investigation. Cu,I-CDs demonstrate peroxidase-mimicking function together with high catalytic effectiveness due to aforementioned features. This led to generation of a colorimetric sensor for quick and accurate quantitative assessment of morphine (MOR). The outcomes showed the method's usefulness for the colorimetric detection of MOR. The linear range for MOR detection is 0.25-25 μg/mL having a reduced detection limit of 64 ng/mL. This sensor's successful use in the analysis of MOR in biological material is more noteworthy.

Adsorption of methyl orange by porous membranes prepared from deep eutectic supramolecular polymer-modified chitosan

The fabrication of an adsorbent with excellent performance has been a focus of attention because of the toxicity, mutagenicity and carcinogenicity of methyl orange (MO)-containing wastewater discharged from the textile, tannery and pharmaceutical industries. In this study, chitosan (CS) membranes were modified with a deep eutectic supramolecular polymer (DESP), and adsorbent membranes with porous structures were prepared with polyethylene glycol (PEG). Microstructural characterization of the CS-DESP-PEG composite membranes with FT-IR, XRD and SEM showed that the membranes had amorphous crystalline structures and that hydrogen bonding interactions weakened the crystallinity and formed loose porous structures. Optimization of the chitosan to β-cyclodextrin ratio, pH, PEG proportion, MO concentration and adsorbent dose significantly improved the adsorption efficiencies of the membranes. The adsorption behaviours of the membranes were fit with pseudo-second-order adsorption kinetics and the Freundlich adsorption isotherm model. Regeneration experiments showed that the membranes were reusable multiple times and maintained good adsorption capacities.

Ultra-sensitive detection of 5-fluorouracil by flow injection chemiluminescence immunoassay based on Fenton-like effect of single atom Co nanozyme

Single atom nanozymes (SAzymes) are considered as the most hopeful candidates for replacing natural enzymes. In this work, a flow-injection chemiluminescent immunoassay (FI-CLIA) based on a Fenton-like activity single atom cobalt nanozyme (Co SAzyme) was developed for the rapid and sensitive detection of 5-fluorouracil (5-Fu) in serum for the first time. Co SAzyme was prepared by an in-situ etching method at room temperature using ZIF-8 metal-organic frameworks (ZIF-8 MOFs). With excellent chemical stability and ultra-high porosity of ZIF-8 MOFs as the core, Co SAzyme presents high Fenton-like activity which can catalyze the decomposition of H₂O₂ to produce large amounts of superoxide radical anions, thus effectively amplifying the chemiluminescence of the Luminol-H₂O₂ system. In addition, carboxyl-modified resin beads were used as the substrate to load more antigens due to its advantages of good biocompatibility and large specific surface area. Under optimal conditions, the detection range of 5-Fu was 0.001-1000 ng mL^-1 with a limit of detection of 0.29 pg mL^-1 (S/N = 3). Furthermore, the immunosensor was successfully applied for the detection of 5-Fu in human serum samples with satisfactory results, displaying the potential application of this strategy for bioanalysis and clinical diagnosis.

Rational design of three-in-one Pt/MnO2/GO hybrid nanozyme with enhanced catalytic performance for colorimetric detection of ascorbic acid and cysteine

In this work, a novel three-in-one Pt/MnO₂/GO hybrid nanozyme with wide pH and temperature working range was rational prepared using simple hydrothermal and reduction strategy. The prepared Pt/MnO₂/GO displayed enhanced catalytic activity than single active component due to the excellent conductivity of GO, the increased active sites, the increased electron transfer capacity, the synergistic effect between each component and the decreased binding energy for adsorbed intermediates. Combing chemical characterization and theoretical simulation calculations, the O₂ reduction process on the Pt/MnO₂/GO nanozymes and the generated reactive oxygen species in the nanozyme-TMB system were thoroughly illustrated. Based on the excellent catalytic activity of Pt/MnO₂/GO nanozymes, a colorimetric strategy was proposed to detect the ascorbic acid (AA) and cysteine (Cys), and the experimental results indicated that detection range of AA was 0.35-56 μM with a LOD of 0.075 μM and detection range of Cys was 0.5-32 μM with a LOD of 0.12 μM. Good recoveries were achieved in the human serum and fresh fruit juice detection procedures, demonstrating the potential applications of Pt/MnO₂/GO-based colorimetric strategy in complex biological and food samples.

Sensitive detection of butyrylcholinesterase activity based on a stimuli-responsive fluorescence reaction

A fluorogenic reaction between the chelate of Mn(II)-citric acid and terephthalic acid (PTA) was discovered, which was carried out through heating the aqueous mixture of Mn²⁺, citric acid and PTA. Detailed investigations indicated the reaction products were 2-hydroxyterephthalic acid (PTA-OH), which was attributed to the reaction between PTA and OH, formed by the triggering of Mn(II)-citric acid in the presence of dissolved O₂. PTA-OH showed a strong blue fluorescence, peaked at 420 nm, and the fluorescence intensity presented a sensitive response to pH of the reaction system. Based on these mechanisms, the fluorogenic reaction was used for the detection of butyrylcholinesterase activity, achieving a detection limit of 0.15 U/L. The detection strategy was successfully applied in human serum samples, and it was also extended for the detection of organophosphorus pesticides and radical scavengers. Such a facile fluorogenic reaction and its stimuli-responsive properties offered an effective tool for designing detection pathways in the fields of clinical diagnosis, environmental monitoring and bioimaging.

Enzyme-Regulated In Situ Formation of Copper Hexacyanoferrate Nanoparticles with Oxidase-Mimetic Behaviour for Colorimetric Detection of Ascorbate Oxidase

In this study, a copper hexacyanoferrate nanoparticle with excellent oxidase-mimetic behaviour has been synthesized through a simple precipitation method. The synthesized copper hexacyanoferrate nanoparticle has intrinsic oxidase-like activity, which can catalyze the chromogenic reaction of 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulphonate) through an O₂^•- reactive oxygen-species-participated process. On the other hand, K₃[Fe(CN)₆] can be reduced by ascorbic acid (AA) to produce K₄[Fe(CN)₆], thereby inhibiting the formation of the copper hexacyanoferrate nanoparticles. Furthermore, ascorbate oxidase (AAO) can catalyze the oxidation of AA to produce dehydroascorbic acid, which cannot reduce K₃[Fe(CN)₆]. Thus, a system for an AAO-regulated in situ formation of copper hexacyanoferrate nanoparticles was constructed by coupling a prepared copper hexacyanoferrate nanozyme with AA for the detection of AAO activity. This colorimetric sensing assay shows high sensitivity and selectivity for the detection of AAO activity (the limit of detection is 0.52 U/L) with a linear range of 1.1-35.7 U/L. Finally, the developed method was applied to detect the activity of AAO in normal human serum with a satisfactory sample spiked recovery (87.4-108.8%). In short, this study provides a good strategy for the construction of nanozyme-based multi-enzyme cascade-signal amplification assay.

A turn-on fluorescent probe based on ESIPT and AIEE mechanisms for the detection of butyrylcholinesterase activity in living cells and in non-alcoholic fatty liver of zebrafish

Butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) are two important cholinesterase enzymes in human metabolism which are closely related to various diseases of the liver. BChE and AChE are difficult to be distinguished due to their similarity in biochemical properties. Therefore, developing BChE-specific probes with high sensitivity and low background reading is desirable for the relevant biological applications. Herein, we reported the design and synthesis of a fluorescent probe HBT-BChE for biological detection and imaging of BChE. The probe is triggered by BChE-mediated hydrolysis, releasing a fluorophore that holds AIEE and ESIPT properties with large Stokes shift (>100 nm), rendering the probe features of low background interference and high sensitivity. The probe can also distinguish BChE from AChE with a low detection limit of 7.540 × 10^-4 U/mL. Further in vitro studies have shown the ability of HBT-BChE to detect intracellular BChE activity, as well as to evaluate the efficiency of the BChE inhibitor. More importantly, the in vivo studies of imaging the BChE activity level in liver tissues using zebrafish as the model animal demonstrated the potential of HBT-BChE as a powerful tool for non-alcoholic fatty liver disease.

Bioconjugation of nanozyme and natural enzyme for ultrasensitive detection of cholesterol

When nanozymes are used in biological analysis, higher activity can improve the detection sensitivity, and better selectivity can eliminate other interference. To improve the specificity and sensitivity, we fabricated an innovative bioconjugated nanozyme with natural enzyme (BNNZ), in which natural ChOx was immobilized onto histidine-modified Fe₃O₄ (His-Fe₃O₄) with hydrophilic poly(ethylene glycol) (PEG) as a linker. ChOx could specifically catalyze the oxidation of cholesterol to generate H₂O₂ molecule, and then the newly formed H₂O₂ oxidized the colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue ox-TMB by peroxidase-like His-Fe₃O₄. According to the above cascade reaction, the BNNZ-based colorimetric strategy was proposed for the detection of cholesterol. Wherein, natural enzymes specifically catalyzed substrates, which endowed BNNZ with excellent specificity for target molecules; meanwhile, the introduction of histidine on His-Fe₃O₄ effectively increased the peroxidase-like activity of BNNZ, which provided a guarantee for sensitivity. Furthermore, BNNZ after reaction could be rapidly separated by an external magnetic field without interfering with colorimetric quantitative detection. The proposed strategy exhibited excellent sensitivity with limit of detection of 0.446 μM and was successfully used for the detection of cholesterol in spiked human serum sample with recovery and relative standard deviation in the range of 97.9-103.5% and 2.5-4.0%, respectively. This work indicates that the bioconjugation of nanozyme and natural enzyme may be a universal strategy for synthesis of high-performance enzyme-nanozyme systems, and the new-type BNNZ will be widely used in biological detection and disease treatment.