Iron oxide nanozyme catalyzed synthesis of fluorescent polydopamine for light-up Zn(2+) detection

Redox modulation of polydopamine surface chemistry: a facile strategy to enhance the intrinsic fluorescence of polydopamine nanoparticles for sensitive and selective detection of Fe3

In recent years, polydopamine (PDA) nanoparticles have attracted considerable attention in different research fields because of their many fascinating physicochemical properties. However, as an analogue of naturally occurring melanin, PDA nanoparticles (PDANPs) typically exhibit weak fluorescence properties. Herein, we report a facile one-pot method for synthesizing bright blue luminescent PDANPs through the redox modulation of PDA surface chemistry. The composition and morphology of the resultant NPs were systematically characterized by transmission electron microscopy and several spectroscopy methods, which verified the successful fabrication of PDANPs. More importantly, comparative chemical analysis of dopamine polymerization revealed the significant impacts of synthesis conditions and PDA surface chemistry on the luminescence properties of PDANPs. Remarkably, in addition to their excellent water-solubility, salt-tolerance and high photostability under extreme pH conditions, the as-prepared PDANPs possess the highest quantum yield (5.1%) among all the reported intrinsic fluorescent PDANPs. Moreover, based on the coordination interaction between phenolic hydroxyl groups of PDANPs and ferric ions (Fe3+), the synthesized PDANPs were successfully utilized as a turn-off sensing platform for sensitive and selective detection of Fe3+ without using any additional targeting molecules. Upon increasing the Fe3+ concentration in the range from 0.5 to 20 μM, the fluorescence intensity of PDANPs decreased linearly. The detection limit of Fe3+ was 0.15 μM. Finally, this fluorescent sensor was successfully used to determine Fe3+ in natural water samples, showing good prospects for practical applications and may pave the way for the development of new rational methodologies for further enhancing the intrinsic fluorescence of PDA and fabricating other novel fluorescent organic nanoparticles.

A Simple and Effective Colorimetric Assay for Glucose Based on MnO₂ Nanosheets

Simple and effective methods for the detection of the level of blood glucose are closely linked to the monitoring of people's health. In the study, MnO₂ nanosheets with absorption range of 300 nm~500 nm and obvious yellow color were easily prepared and applied to detect glucose through their absorbance and color. The proposed method is based on the fact that a specific concentration of glucose can be quantitatively transformed into hydrogen peroxide (H₂O₂) under the catalytic effect of glucose oxidase. Based on the redox reaction of MnO₂ with H₂O₂, yellow MnO₂ can be converted into colorless Mn²⁺ to monitor the concentration of glucose. Under optimal conditions, a simple and effective visual assay for the sensitive and reliable detection of glucose was developed. The linear range was estimated to the range from 0 μM to 100 μM, with a detection limit of 12.8 μM. Furthermore, the proposed colorimetric assay based on MnO₂ nanosheets can effectively detect blood glucose of clinical serum samples with accuracy and convenience.

Multifunctional nanozymes: enzyme-like catalytic activity combined with magnetism and surface plasmon resonance

Over decades, as alternatives to natural enzymes, highly-stable and low-cost artificial enzymes have been widely explored for various applications. In the field of artificial enzymes, functional nanomaterials with enzyme-like characteristics, termed as nanozymes, are currently attracting immense attention. Significant progress has been made in nanozyme research due to the exquisite control and impressive development of nanomaterials. Since nanozymes are endowed with unique properties from nanomaterials, an interesting investigation is multifunctionality, which opens up new potential applications for biomedical sensing and sustainable chemistry due to the combination of two or more distinct functions of high-performance nanozymes. To highlight the progress, in this review, we discuss two representative types of multifunctional nanozymes, including iron oxide nanomaterials with magnetic properties and metal nanomaterials with surface plasmon resonance. The applications are also covered to show the great promise of such multifunctional nanozymes. Future challenges and prospects are discussed at the end of this review.

Polydopamine nanodots are viable probes for fluorometric determination of the activity of alkaline phosphatase via the in situ regulation of a redox reaction triggered by the enzyme

The authors describe an environmentally friendly and fast (~14 min) method for the synthesis of homogeneously distributed fluorescent polydopamine nanodots (PDA-NDs) using KMnO₄ as the oxidant. Alkaline phosphatase (ALP) catalyzes the hydrolysis of ascorbic acid 2-phosphate to release free ascorbic acid which undergoes an in-situ redox reaction with KMnO₄. Depending on the activity of ALP, more or less KMnO₄ is consumed, and this affects the formation of the PDA-NDs. Based on this finding, a sensitive method was worked out to quantify the activity of ALP via real-time formation of fluorescent PDA-NDs. The fluorometric signal (best measured at excitation/emission peaks of 390/500 nm) is linear in the 1 to 50 mU·mL^-1 ALP activity range, and the limit of the detection is as low as 0.94 mU·mL^-1 (based on 3 σ/m). The method was successfully applied to the determination of ALP activity in spiked human serum and in MCF-7 cell lysates. It was also applied in a method to screen for inhibitors of ALP. Graphical abstract Schematic of a fluorometric method for the determination of alkaline phosphatase (ALP) activity. The method is based on the in-situ regulation of the formation of fluorescent polydopamine nanodots (PDA-NDs) through the competition between the KMnO₄-induced polymerization of dopamine and ALP-directed ascorbic acid 2-phosphate (Asc-2P) hydrolysis. AA: Ascorbic acid.

Highly active fluorogenic oxidase-mimicking NiO nanozymes

NiO nanoparticles can quickly catalyze oxidation of Amplex red to produce fluorescent products for intracellular imaging, much more efficiently than other types of tested nanozymes.

An iron hydroxyl phosphate microoctahedron catalyst as an efficient peroxidase mimic for sensitive and colorimetric quantification of H2O2 and glucose

An iron hydroxyl phosphate that possesses high peroxidase activity is synthesized via a one-step hydrothermal method and used for colorimetric glucose detection in human serum.

Iron Oxide Nanozyme: A Multifunctional Enzyme Mimetic for Biomedical Applications

Iron oxide nanoparticles have been widely used in many important fields due to their excellent nanoscale physical properties, such as magnetism/superparamagnetism. They are usually assumed to be biologically inert in biomedical applications. However, iron oxide nanoparticles were recently found to also possess intrinsic enzyme-like activities, and are now regarded as novel enzyme mimetics. A special term, "Nanozyme", has thus been coined to highlight the intrinsic enzymatic properties of such nanomaterials. Since then, iron oxide nanoparticles have been used as nanozymes to facilitate biomedical applications. In this review, we will introduce the enzymatic features of iron oxide nanozyme (IONzyme), and summarize its novel applications in biomedicine.

Boosting the Peroxidase-Like Activity of Nanostructured Nickel by Inducing Its 3+ Oxidation State in LaNiO3 Perovskite and Its Application for Biomedical Assays

Catalytic nanomaterials with intrinsic enzyme-like activities, called nanozymes, have recently attracted significant research interest due to their unique advantages relative to natural enzymes and conventional artificial enzymes. Among the nanozymes developed, particular interests have been devoted to nanozymes with peroxidase mimicking activities because of their promising applications in biosensing, bioimaging, biomedicine, etc. Till now, lots of functional nanomaterials have been used to mimic peroxidase. However, few studies have focused on the Ni-based nanomaterials for peroxidase mimics. In this work, we obtained the porous LaNiO₃ nanocubes with high peroxidase-like activity by inducing its 3+ oxidation state in LaNiO₃ perovskite and optimizing the morphology of LaNiO₃ perovskite. The peroxidase mimicking activity of the porous LaNiO₃ nanocubes with Ni³⁺ was about 58~fold and 22~fold higher than that of NiO with Ni²⁺ and Ni nanoparticles with Ni⁰. More, the porous LaNiO₃ nanocubes exhibited about 2-fold higher activity when compared with LaNiO₃ nanoparticles. Based on the superior peroxidase-like activity of porous LaNiO₃ nanocubes, facile colorimetric assays for H₂O₂, glucose, and sarcosine detection were developed. Our present work not only demonstrates a useful strategy for modulating nanozymes' activities but also provides promising bioassays for clinical diagnostics.

Hierarchical NiCo₂O₄ Hollow Sphere as a Peroxidase Mimetic for Colorimetric Detection of H₂O₂ and Glucose

In this work, the hierarchical NiCo₂O₄ hollow sphere synthesized via a “coordinating etching and precipitating” process was demonstrated to exhibit intrinsic peroxidase-like activity. The peroxidase-like activity of NiCo₂O₄, NiO, and Co₃O₄ hollow spheres were comparatively studied by the catalytic oxidation reaction of 3,3,5,5-tetramethylbenzidine (TMB) in presence of H₂O₂, and a superior peroxidase-like activity of NiCo₂O₄ was confirmed by stronger absorbance at 652 nm. Furthermore, the proposed sensing platform showed commendable response to H₂O₂ with a linear range from 10 μM to 400 μM, and a detection limit of 0.21 μM. Cooperated with GOx, the developed novel colorimetric and visual glucose-sensing platform exhibited high selectivity, favorable reproducibility, satisfactory applicability, wide linear range (from 0.1 mM to 4.5 mM), and a low detection limit of 5.31 μM. In addition, the concentration-dependent color change would offer a better and handier way for detection of H₂O₂ and glucose by naked eye.

Multicopper Laccase Mimicking Nanozymes with Nucleotides as Ligands

Using nanomaterials to achieve functional enzyme mimics (nanozymes) is attractive for both applied and fundamental research. Laccases are multicopper oxidases highly important for biotechnology and environmental remediation. In this work, we report an exceptionally simple yet functional laccase mimic based on guanosine monophosphate (GMP) coordinated copper. It forms an amorphous metal-organic framework (MOF) material. The ratio of copper and GMP is 3:4 as determined by isothermal titration calorimetry. It has excellent laccase-like activity and converts a diverse range of phenol containing substrates such as hydroquinone, naphthol, catechol and epinephrine. Comparative work shows that the activity is originated from guanosine coordination instead of phosphate binding in GMP. Cu²⁺ is required and cannot be substituted by other metal ions. At the same mass concentration, the Cu/GMP nanozyme has a higher V_max and similar K_m compared to the protein laccase. To achieve the same catalytic efficiency, the cost of the Gu/GMP is ∼2400-fold lower than that of laccase. The Cu/GMP is much more stable at extreme pH, high salt, high temperature and for long-term storage. This is one of the first laccase-mimicking nanozymes, which will find important applications in analytical chemistry, environmental protection, and biotechnology.