Modulation of inner filter effect between persistent luminescent particles and 2, 3-diaminophenazine for ratiometric fluorescent assay of ascorbic acid and ascorbate oxidase activity

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

Physiological and biochemical roles of ascorbic acid on mitigation of abiotic stresses in plants

Under conditions of abiotic stress several physiological and biochemical processes in plants can be modified. The production of reactive oxygen species (ROS) is toxic at high concentrations and promotes RNA, DNA and plant cell membrane degradation. Plants have enzymatic and non-enzymatic adaptation mechanisms to act against ROS detoxification. Ascorbic acid (AsA) is the non-enzymatic compound essential for several biological functions, which acts in the elimination and balance of ROS production and with the potential to promote several physiological functions in plants, such as the photosynthetic process. For plant development, AsA plays an important role in cell division, osmotic adjustment, hormone biosynthesis, and as an enzymatic cofactor. In this review, the redox reactions, biosynthetic pathways, and the physiological and biochemical functions of AsA against abiotic stress in plants are discussed. The concentration of AsA in plants can vary between species and depend on the biosynthetic pathways d-mannose/l-galactose, d-galacturonate, euglenids, and d-glucuronate. Although the endogenous levels of AsA in plants are used in large amounts in cell metabolism, the exogenous application of AsA further increases these endogenous levels to promote the antioxidant system and ameliorate the effects produced by abiotic stress. Foliar application of AsA promotes antioxidant metabolism in plants subjected to climate change conditions, also allowing the production of foods with higher nutritional quality and food safety, given the fact that AsA is biologically essential in the human diet.

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.

Fluorescent molecular probes for imaging and detection of oxidases and peroxidases in biological samples

Oxidases and peroxidases are two subclasses of oxidoreductases. The abnormal expression of oxidases (such as tyrosinase, cytochrome P450 oxidases, and monoamine oxidases) and peroxidases (such as glutathione peroxidase, myeloperoxidase, and eosinophil peroxidase) is relative with some diseases. Therefore, the analysis of oxidases and peroxidases is great important for disease diagnosis and treatment. Fluorescent probes present simple protocol, high sensitivity and good stability in sensing field. Molecule fluorescent probes are constructed with chemical groups that tunes their fluorescence emission in response to binding events, chemical reactions, and the surrounding environment. A fluorescent probe is an efficient tool for visualizing the activity of enzymes in living organisms on the basis of its high specificity, sensitivity, and noninvasiveness characteristics. In this review, we focus on the sensing of oxidases and peroxidases by molecule fluorescent probes, and hope to bring new insight to wide researchers about oxidases and peroxidases in biological samples.

Nanoporous ZIF-8 Microparticles as Acetylcholinesterase and Alkaline Phosphatase Mimics for the Selective and Sensitive Detection of Ascorbic Acid Oxidase and Copper Ions

In this study, the alkaline phosphatase (ALP)-like activity of zeolitic-imidazolate framework-8 (ZIF-8) is reported for the first time. Then, colorimetric sensors for the ascorbic acid oxidase (AAO) and copper ion (Cu²⁺) detection were developed based on the acetylcholinesterase (AChE)- and ALP-like activities of ZIF-8. The ZIF-8 has good mimetic enzyme activity and exhibits high affinity to the substrates. Its AChE- and ALP-like activities also have good reusability and storage stability. Good linear dependences are obtained in the range of 1.3-250.0 μM (AChE-like activity-based) and 4.5-454.5 μM (ALP-like activity based) for Cu²⁺ detection. Furthermore, good linear dependence is also obtained based on the ALP-like activity of ZIF-8 for AAO detection in the range of 2.3-454.5 U/L. Their limits of detection (LODs) are calculated to be 0.7 µM, 2.8 µM, and 1.8 U/L, respectively. Finally, the sample spiked recoveries of Cu²⁺ in tap water, Cu²⁺, and AAO in human serum and rabbit plasma were measured, and the results are in the range of 80.0-119.3%. In short, the preparation of ZIF-8 is simple, environmentally friendly, and harmless, and can realize highly selective detection of AAO and Cu²⁺ in an efficient and fast process.