Manganese-Based Nanozymes

Introduction

Manganese-based nanozymes combine the physical and chemical properties of manganese-based nanomaterials with the catalytic activity of natural enzymes, and have the characteristics of easy preparation, low cost, environment friendly and excellent physicochemical properties. As Mn is one of the essential elements in the human body, it is a biocompatible element that plays a physiological role, which allows manganese-based nanozymes for significant applications in the biomedical field, especially in tumor therapy, biosensing and bacterial inhibition. It is worth noting that the conversion of multiple valence states enables Mn to exhibit exquisite catalytic activity, resulting in the exploitation of various manganese-based nanozymes.

Preparation

The common methods for the preparation of manganese-based nanozymes are the hydrothermal/solvothermal method, chemical co-precipitation method and sol-gel method.

• Hydrothermal/solvothermal. The hydrothermal synthesis is a method of synthesis using chemical reactions of substances in aqueous solutions under high temperature and high pressure conditions. Different from this, due to the different affinity of different substances to solvents, the solvothermal synthesis method dissolves the raw materials in a good solvent rather than an aqueous solution. Both hydrothermal synthesis method and solvothermal synthesis method can control the concentration, temperature, reaction time and other parameters of the reactants, thereby controlling the morphology and size of the product, and producing various manganese-based nanozymes.
• Chemical co-precipitation. The co-precipitation strategy is to prepare nanozymes by mixing different reactants in solution and controlling the conditions to make the solution precipitation at the nanoscale. Even if the same method is used, manganese-based nanozymes with different morphologies and properties can be obtained by controlling different reaction conditions such as concentration, temperature, pH, etc. Furthermore, strict specifications of stoichiometric ratios can produce high-purity manganese-based nanozymes.
• Sol-gel. The sol-gel process generally involves the preparation of a sol from a Mn-containing feedstock to further prepare a gel substance. Then, gel-like nanozymes can be obtained by processing the gel. The morphology and size of the resulting nanozymes can also be controlled by selecting appropriate complexing agents, chemical additives, temperature and concentration.

Applications

Manganese-based nanozymes have diverse applications in biomedicine. The major applications include tumor therapy, biosensing and antibacterial.

• Tumor therapy. Conventional tumor treatment strategies for manganese-based nanozymes are relief of hypoxia, magnetic resonance imaging (MRI), photo dynamic therapy (PDT), radiation therapy (RT), and starvation therapy. It is worth noting that reactive oxygen species (ROS) can induce tumor necrosis or apoptosis, which also gives manganese-based nanozymes more possibilities to achieve tumor therapy. They can effectively kill tumor cells by producing ROS.
• Biosensing. In the biosensing field, a lot of manganese-based nanozymes are used in colorimetric biosensors to detect bioactive molecules (H₂O₂, glucose, glutathion, cholesterol, etc.) or other small molecules (O₂, o-phenylenediamine, 3,3',5,5'-tetramethylbenzidine, etc.). They can use enzymatic activity to catalyze the substrate and detect the difference in color before and after the reaction so that a clear color change can be observed without visual aid.
• Antibacterial. Many nanozymes have been applied to treat bacterial infections by using enzymatic activity to produce ROS, and this method is also known as reactive oxygen therapy. Among the many compounds of Mn, MnO₂ is an ideal material for bacterial inhibition. For example, researchers prepared a manganese-based nanozyme OA-MnO₂. The OXD-like activity of OA-MnO₂ nanozyme catalyzes the conversion of oxygen to ROS when it encounters the site of bacterial infection, which effectively inhibits bacteria^[1].

Fig.1 OA-MnO₂ for antibacterial and its preparation process.

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