Transition metal-based nanozymes: Classification, catalytic mechanisms and emerging biomedical applications

Transition metal-based nanozymes, which exhibit intrinsic enzyme-like capacities, have several advantages over natural enzymes, including excellent stability, abundant metal sources, controllable activity, and inexpensive preparation processes. Nanozymes are characterized by interesting physicochemical properties, including photoluminescence, superparamagnetism, and special optical properties. Recently, various transition metal-based nanozyme platforms have been developed to target single or multiple substrates. The catalytic properties of nanozymes can be regulated by microenvironmental factors such as pH, oxygenation level, and concentration of H2O2, and also by a magnetic field, light, ultrasound, and heat. Thus, nanozyme signals can be maximized and tailored for disease diagnosis and treatment. Prompted by these inherent advantages, new approaches for diagnosis, treatment, and theranostics are emerging and gaining momentum. In this review, we summarize the preparation, catalytic mechanisms, and properties of transition metal-based nanozymes and highlight their emerging biomedical applications, including disease diagnosis, cancer therapy, imaging, and antibacterial infections. We anticipate that this review will be significant for improving our understanding of the capacities of metal-based nanozymes and motivating broader applications in several biomedical fields.