Modified Metal-Organic Framework (MOF-818) inspired by natural enzymes for intelligent detection of total antioxidants and bisphenol A in infant beverages.

BACKGROUND: Accurate and quick judgement of the food quality can protect the legitimate rights of consumers. Currently, nanozymes are widely employed in the rapid detection of food due to their stability and economy. The contents of bisphenol A and antioxidant can be used to measure the quality of beverages. However, due to the complexity of the actual samples, it is still challenging to achieve the sensitive detection of both at the same time. The development of nanozyme with high enzyme activity is essential for sensitive detection of targets in complex foods. RESULTS: In this work, a novel nanomaterial (ZrTGA) was synthesized based on thioglycolic acid-modified Metal-Organic Framework (MOF-818). The interaction between Cu-S bonds and increase in the proportion of Cu1+ resulted in ZrTGA exhibiting higher peroxidase-like and polyphenol oxidase-like activities. These enzyme activities were 317 % and 200 % of the original values, respectively. With high enzyme activity can sensitively detect two important indicators of bisphenol A and antioxidants in beverages. The increased enzyme activity of ZrTGA enabled the content of both substances to be detected by smartphone extraction of RGB. Finally, through the output of the ''0″ and ''1″ signals of the logic gates, it is possible to quickly determine the level of the two substances and thus directly assess the quality of the beverages. SIGNIFICANCE: The modification of nanozyme enables the detection of substances at low concentrations based on enhancing dual-enzyme activity. The combination of mobile phone photography and logic gate technology enables the continuous detection of two important indicators in beverages, overcoming the limitations of traditional large-scale instruments. It also provides an alternative strategy for food quality detection.

Fluorescent sensor based on bismuth metal-organic frameworks (Bi-MOFs) mimic enzyme for H2O2 detection in real samples and distinction of phenylenediamine isomers.

Although peroxidase-like nano-enzymes have been widely utilized in biosensors, nano-enzyme based biosensors are seldom used for both quantitative analysis of H2O2 and differentiation of isomers of organic compounds simultaneously. In this study, a dual-functional mimetic enzyme-based fluorescent sensor was constructed using metal-organic frameworks (Bi-MOFs) with exceptional oxidase activity and fluorescence properties. This mimetic enzyme sensor facilitated quantitative analysis of H2O2 and accurate discrimination of phenylenediamine isomers. The sensor exhibited a wide linear range (0.5-400 µM) and low detection limit (0.16 µM) for the detection of H2O2. Moreover, the sensor can also be used for the discrimination of phenylenediamine isomers, in which the presence of o-phenylenediamine (OPD) leads to the appearance of a new fluorescence emission peak at 555 nm, while the presence of p-phenylenediamine (PPD) significantly quenched its fluorescence due to the internal filtration effect. The proposed strategy exhibited a commendable capability in distinguishing phenylenediamine isomers, thereby paving the way for novel applications of MOFs in the field of environmental science.

A label-free biosensor for selective detection of Gram-negative bacteria based on the oxidase-like activity of cupric oxide nanoparticles.

A label-free biosensor based on cupric oxide (CuO) nanoparticles was constructed for the selective detection of Gram-negative bacteria. CuO possesses oxidase-like activity and can catalyze the oxidation of o-phenylenediamine (OPD) to produce oxidized OPD, which has a fluorescence emission at 573 nm under excitation at 423 nm. The mechanism study suggests that the oxygen vacancies of CuO can activate the dissolved oxygen to form superoxide anions, which in turn oxidize OPD. Gram-negative bacteria can reduce part of Cu(II) in CuO to Cu(I) based on their copper homeostasis system, thus inhibiting the oxidation of OPD and decreasing the fluorescence intensity of the catalytic system. This principle was utilized to construct a biosensor to realize the selective detection of Gram-negative bacteria successfully. The biosensor exhibited a good linear correlation toward the logarithm concentration of three Gram-negative bacteria with R2 ≥ 0.985. It was applied to detect three Gram-negative bacteria in eggshell, Chinese cabbage, and the Pearl River water samples, with recoveries ranging from 92.4 to 107%. Moreover, a smartphone-based portable device was designed and fabricated to realize the on-site detection of bacteria. The results of the portable device were comparable to those of fluorescence spectrophotometry, suggesting that the portable device has tremendous potential in the on-site detection of bacteria.

Enhancing enzymatic activity of Mn@Co3O4 nanosheets as mimetic nanozyme for colorimetric assay of ascorbic acid.

In nanozyme-based assays, increasing enzymatic activity is very desirable for enhancing sensitivity and lowering the detection limit. In this study, novel Mn doped cobalt oxide nanosheets (Mn@Co3O4 NSs) were synthesized by hydrothermal process. The obtained Mn@Co3O4 possessed enhanced dual-enzyme mimetic, oxidase and peroxidase, and can catalytically oxidize of 3, 3', 5, 5'-tetramethylbenzidine (TMB), to a blue product of oxidized TMB. The enzyme kinetics were well-described mathematically using a common Michaelis-Menten and Lineweaver Burk model. The enzyme kinetics constant (Km) was found to be 0.15 mM, which is relatively low comparing with pure Co3O4 nanosheets (0.35 mM) and natural enzyme HRP (0.434 mM). Therefore, the efficient colorimetric method was achieved for determination of H2O2 and ascorbic acid. The limit of detection (LOD) of H2O2 was 8.0 µM and the linear range was 20-200 µM based on direct turn on of the peroxidase-like activity of Mn@Co3O4. While, for ascorbic acid detection based on turn-off approach, the linearity range for the ascorbic acid was 1-8 µM with LOD of 0.4 µM. Moreover, the colorimetric system exhibited good stability and selectivity toward the detection of ascorbic acid effectively in real samples (vitamin C tablets) with satisfactorily accuracy and precision.

Cell membrane-coated nanoparticles as peroxidase mimetics for cancer cell targeted detection and therapy.

The development of novel and efficient recognition molecules that can be easily modified by nanomaterials to achieve ultra-sensitive and specific cancer cell analysis is of great significance for its early diagnosis and timely prognosis. Herin, a new nanostructured hybrid based on cell membrane-coated Au cores- ultrathin Pt skins composite nanoparticles (Au@Pt@CM NPs) were developed for in vitro detection and treatment of cancer cells. In this strategy, the Au@Pt NPs acted as the signal transducer, and the cell membrane were used as the cancer-cell recognition tool. The synthesized Au@Pt@CM NPs could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of the hydrogen peroxide and were demonstrated to have excellent peroxidase-like activity. Coated with the source cancer cell membrane, the nanoparticles achieved highly specific self-recognition to the source cell. Therefore, the colorimetric method based on Au@Pt@CM NPs could detect the cancer cells in the linear range from 50 to 100000 cells/mL with a limit of detection of 5 cells/mL, which is much lower than other colorimetric detection methods. Afterwards, the nanoparticles as a mimetic enzyme were used for therapeutics of cancer cells through the ROS-mediated oxidative damage. Due to the change of the redox state in the cells by the Au@Pt@CM NPs, the hybrid can achieve the growth inhibitory effect and the selective killing effect on cancer cells. It can be expected that this novel hybrid membrane coating method will bring new insight into developing targeted nanomaterials for tumor treatment and detection.

Newly Constructed NiCo2O4 Derived from ZIF-67 with Dual Mimic Enzyme Properties for Colorimetric Detection of Biomolecules and Metal Ions.

Integration of novel bio-/nanostructures as effective sensing platforms is still of great significance for robust and rapid analysis. Herein, a novel metal-organic framework-derived NiCo2O4 was synthesized via a feasible templating method. Significantly, redox couples of both Ni3+/Ni2+ and Co3+/Co2+ provided richer oxidation-reduction reactions, thereby leading to an enhanced catalytic activity. Furthermore, NiCo2O4 as an enzyme mimic with peroxidase-like activity and oxidase-like activity could oxidize colorless thylbenzidine (TMB) to blue oxTMB in the absence of H2O2. Thus, a sensitive chromogenic sensing platform for detecting Fe2+, thiourea, cysteine (Cys), and epigallocatechin-3-gallate (EGCG) was proposed. The colorimetric detection methods exhibited great features of low limit of detection (LOD) and broad linear range. Owing to the complexation reaction, the chromogenic sensing system of TMB + NiCo2O4 + Cys achieved effective detection of Cu2+ and Mn2+ with the LODs of 0.0022 and 0.0181 mM, respectively. Developed detection methods with wide linear ranges of 0.008-0.1 mM for Cu2+ and 0.08-1 mM for Mn2+ had excellent practical potential. Similarly, the reaction system of TMB + NiCo2O4 + EGCG could achieve the colorimetric detection of Cu2+ and Fe3+. The great chromogenic sensing performance for detecting Cu2+ and Fe3+ with a broad linear range and a low LOD could be also realized.

A Simple Colorimetric Assay for Sensitive Cu2+ Detection Based on the Glutathione-Mediated Etching of MnO2 Nanosheets.

In this paper, we developed a quick, economical and sensitive colorimetric strategy for copper ions (Cu2+) quantification via the redox response of MnO2 nanosheets with glutathione (GSH). This reaction consumed MnO2 nanosheets, which acted as a catalyst for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to a blue product (oxTMB). In the presence of Cu2+, the GSH was catalyzed to GSSG (oxidized glutathione), and the solution changed from colorless to deep blue. Under the optimum conditions, the absorption signal of the oxidized product (oxTMB) became proportional to Cu2+ concentration in the range from 10 to 300 nM with a detection limit of 6.9 nM. This detection system showed high specificity for Cu2+. Moreover, the system has been efficaciously implemented for Cu2+ detection in actual tap water samples. The layered-nanostructures of MnO2 nanosheets make it possess high chemical and thermal stability. TMB can be quickly oxidized within 10 min by the catalyzing of MnO2 nanosheets with high oxidase-like activity. There is no need of expensive reagents, additional H2O2 and complicated modification processes during the colorimetric assay. Therefore, the strategy primarily based on MnO2 nanosheets is promising for real-time, rapid and highly sensitive detection of Cu2+ under practical conditions.

Catalase active metal-organic framework synthesized by ligand regulation for the dual detection of glucose and cysteine.

Mimic enzymes greatly improve the inherent insufficiencies of natural enzymes. Therefore, mimic enzyme sensors attract increasing research interest. Metal-organic framework (MOF) is emerging in the field of mimic enzyme catalysis due to its remarkable structural properties. In this paper, a colorimetric method is designed for rapid and sensitive detection of glucose and cysteine levels. The MOF Eu-pydc (pydc-2,5-pyridinedicarboxylic acid) is synthesized by a new strategy which is regulated by ligands at room temperature and found to have peroxidase activity. Then, the MOF is used as a mimic enzyme to catalyze chromogenic substrate (3,3',5,5'-tetramethylbenzidine, TMB) for colorimetric sensing of glucose. The developed method can accurately detect glucose in the range of 10 µM-1 mM (R2 = 0.9958) with a relatively low detection limit about 6.9 µM. Moreover, a cysteine sensor with a detection limit of 0.28 µM is also established based on the disappearance of the color of oxTMB. Additionally, the proposed glucose sensor exhibits excellent selectivity and is successfully applied to blood glucose detection. At the same time, the detection of cysteine is also highly sensitive. In short, the dual sensor is fast, low cost, and convenient, and has great application potential in the diagnosis of disease.

Colorimetric zearalenone assay based on the use of an aptamer and of gold nanoparticles with peroxidase-like activity.

An aptamer based colorimetric assay is described for the determination of zearalenone (ZEN). It is based on the inhibition of the peroxidase-mimicking activity of gold nanoparticles (AuNPs) by the ZEN aptamer. However, in the presence of ZEN, the aptamer is bound by ZEN and can no longer inhibit the peroxidase-mimicking activity of AuNPs. The color change of solution is related to ZEN concentration and observed with bare eyes. Under optimal conditions, the absorbance (at 630 nm) increases linearly in the ZEN concentration range of 10-250 ng·mL-1, and the limit of detection is 10 ng·mL-1. The specificity of the assay was verified by studying the effect of potential interferents. The recoveries from ZEN spiked corn and corn oil range from 92 to 110%, and the relative standard deviations are between 2.4 and 6.4%. The results are in good agreement with those obtained by an ELISA. Graphical abstract Schematic presentation of colorimetric assay for rapid and sensitive determination of zearalenone (ZEN) based on the inhibition of ZEN aptamer on the the peroxidase-like activity of gold nanoparticle (AuNPs).

Copper (II)-ploy-L-histidine functionalized multi walled carbon nanotubes as efficient mimetic enzyme for sensitive electrochemical detection of salvianic acid A.

In this work, carboxylated multi walled carbon nanotubes (CMWCNTs) were firstly prepared and functionalized with poly-L-histidine (PLH), which were then chelated with copper (II) ions to from the nanocomposites of Cu(II)-PLH-CMWCNTs. The nanocomposites could be exploited as an efficient mimic enzyme for sensitive electrochemical detection of salvianic acid A (SAA). Cu(II)-PLH-CMWCNTs owned good charge transfer property and excellent synergetic catalytic effect between the overoxidized imidazole groups and the copper redox-active units. Therefore, highly sensitive electrochemical response to SAA was achieved under optimum experimental conditions. A good linear relationship between differential pulse voltammetry (DPV) peak current and the SAA concentration was established in the range of 0.4-1000 µM. A low detection limit of 0.037 µM and a sensitivity of 0.27 µA µM-1 cm-2 were achieved. The developed biosensor also had advantages of good repeatability, stability and high selectivity, thus, it was successfully applied to the determination of SAA in real samples with satisfactory results, which may have great potential for further exploitation of electroanalysis applications.

A novel electrogenerated chemiluminescence biosensor for histone acetyltransferases activity analysis and inhibition based on mimetic superoxide dismutase of tannic acid assembled nanoprobes.

A simple and sensitive turn-off electrogenerated chemiluminescence (ECL) biosensor was designed for the analysis of histone acetyltransferases (HATs) activity and inhibitor evaluation based on the anti-hydrolysis ability of acetylated peptide and mimetic superoxide dismutase (SOD) features of tannic acid (TA) assembled gold nanoparticles (AuNPs) nanoprobes. In this strategy, after the acetylated reaction in the presence of HATs, the acetylated peptide on electrode was resistance to the hydrolysis of trypsin, and can absorb AuNPs@TA-Fe probe onto the electrode by the hydrophobic interaction and hydrogen bonding interaction. Thus, the ECL signal of the modified electrode in luminol solution decreased significantly owing to the mimetic SOD features of the TA assembled nanoprobe that can eliminate the reactive oxygen species. The ECL intensity changes can be utilized for sensitive HATs activities detection and inhibitor screening. The detection limit of the as-prepared ECL biosensor was 0.074 nM (S/N = 3). Moreover, the as designed ECL biosensor was also applied in MCF-7 cell lysates for HATs activity analysis and drug inhibition, which is feasible to HATs activity analysis and inhibitor screening, and presents highly promise in HAT-related clinical diagnostics.

Determination of Glucose Based on Carbon Quantum Dots as Peroxidase Mimetic Enzyme / 分析化学

Water soluble carbon quantum dots ( CQDs) were prepared by using soot as carbon source. The obtained CQDs showed an excellent intrinsic peroxidase-like activity, which could catalyze the oxidization of 3,3′,5,5′-tetramethylbenzidine (TMB) by H2O2and thus resulted in color change. Glucose could react with dissolved oxygen to produce H2O2in the presence of glucose oxidase ( GOx) . A colorimetric method using CQDs as peroxidase mimetic enzyme was developed for glucose determination. When TMB was acted as a substrate, the effect of a series of conditions, such as temperature and pH on the catalytic activity of the obtained CQDs, was systematically studied. Under optimal conditions, e. g. pH 3. 5 and temperature 35℃, 0. 5 mmol/L TMB and 1 μg/mL CQDs, the absorbance at 652 nm showed linear response with glucose concentrations ranging from 0. 025 mmol/L to 0. 40 mmol/L, with detection limit of 5. 10 μmol/L (3σ/k). The proposed method exhibited excellent selectivity and the common substances did not interfere with detection of glucose. This method was successfully applied to detect glucose in real samples with recoveries of 95. 0%-105. 1% .

Progress of Mimetic Enzymes and Their Applications in Chemical Sensors.

The need to develop innovative and reformative approaches to synthesize chemical sensors has increased in recent years because of demands for selectivity, stability, and reproducibility. Mimetic enzymes provide an efficient and convenient method for chemical sensors. This review summarizes the application of mimetic enzymes in chemical sensors. Mimetic enzymes can be classified into five categories: hydrolases, oxidoreductases, transferases, isomerases, and induced enzymes. Potential and recent applications of mimetic enzymes in chemical sensors are reviewed in detail, and the outlook of profound development has been illustrated.

Dendritic DNA-porphyrin as mimetic enzyme for amplified fluorescent detection of DNA.

In this work, a novel dendritic DNA-porphyrin superstructure was designed as mimetic enzyme for the amplified fluorescent detection of DNA. The dendritic DNA superstructure was in situ assembled with three auxiliary DNAs via hybridization chain reaction. With groove interaction between iron porphyrin (FeTMPyP) and double-stranded DNA, the dendritic DNA superstructure is capable to gather abundant FeTMPyP molecules to form dendritic DNA-FeTMPyP mimetic enzyme. Using tyramine as a substrate, the dendritic DNA-FeTMPyP demonstrated excellent peroxidase-like catalytic oxidation of tyramine into fluorescent dityramine in the presence of H2O2. Based on an amplified fluorescence signal, a signal on strategy is proposed for DNA detection with high sensitivity, good specificity and practicability. The assembly of porphyrin with dendritic DNA not only provided the new avenue to construct mimetic enzyme but also established label-free sensing platform for a wide range of analytes.