# Metal Nanozymes: New Horizons in Cellular Homeostasis Regulation
## 9 Nanozymes Combating Pathogens
aureus_-induced subcutaneous abscess in mice enhanced wound healing and proved bacteriostatic [257]. The cerium oxide nanoparticle (nanoceria) was shown to possess high peroxidase activity [257, 258]. Provided that high peroxidase activity is more conducive to promoting the production of ROS, the CeO\({}_{2}\)-H\({}_{2}\)O\({}_{2}\) system might be regarded generally a good solution for bacterial disinfection. However, as shown and discussed by [PERSON] et al. [259], CeO\({}_{2}\) nanoparticles show multiple enzyme-like activities depending on their shapes and sizes [26, 260]. Beside the high peroxidase activity, CeO\({}_{2}\) also have high SOD and antioxidant activities and depending on the conditions could as well be used as a ROS scavengers. In the system investigated by [PERSON] et al. [259], the peroxidase activity of CeO\({}_{2}\) did not contribute to the antibacterial effect; instead, the ROS scavenging capacity of CeO\({}_{2}\) could protect the bacteria from being killed by H\({}_{2}\)O\({}_{2}\). Recently, the highly photoluminescent glucosamine-derived nitrogen and zinc-doped carbon dots (N, Zn-CDs) were developed as a fluorescence sensor for the detection of Fe\({}^{3+}\) ions. At the same time, the bactericidal properties of the synthesized nanodots against gram-negative (_Escherichia_) and gram-positive (_S. aureus_) pathogens were investigated. It turned out that these nanodots showed good bactericidal activity against both pathogens under light conditions, and _E. coli_, also in the dark. According to the authors, the small size of the nanodots facilitated their entry into the bacteria, causing the enzymatic imbalance enhanced by the light energy and, as a result, the accumulation of ROS due to stress. The mechanism of action in dark conditions was explained by the release of Zn\({}^{2+}\) ions and their interaction with the cytoplasmic content of the bacterial cell, followed by the formation of ROS [261]. Nanozymes can be effective agents enhancing infection-fighting during wound healing. A group of researchers from the Xiamen University [262] constructed a nanoplatform based on Pd@Pt nanoparticles decorated with a synthetic sonosensitizer, meso-tetra(4-carboxyphenyl)porphine (T790). The nanoplatform was used for catalysis-enhanced SDT to treat myositis infected by methicillin-resistant _S. aureus_. Due to the covalent bond, the sonosensitizer showed good stability and did not release from nanoparticles. Pd@Pt-T790 accumulated effectively in a deeply embedded bacterial infection by using the above-discussed EPR effect, which was a good prerequisite for SDT. T790 anchored on Pd@Pt NPs could significantly block the CAT-like activity of NPs, whereas, upon US irradiation, the nanozyme activity was effectively recovered to catalyze the decomposition of endogenous H\({}_{2}\)O\({}_{2}\) into O\({}_{2}\). At the same time, the nanosystem was shown as an attractive medium for fluorescence imaging, PA, and CT in mice. MRI and PA imaging of oxyhemoglobin saturation were used to monitor the treatment process in real-time, revealing a complete eradication of bacterial myositis in mice [262]. It should also be noted that an important issue when designing nanozyme systems is the carrier which can be e.g., a hydrogel composition [263]. An interesting approach was presented by [PERSON], [PERSON] et al. [264]. When neutrophils encounter an invasive pathogenic microorganism in the human immune system, they typically form branch-like pseudopodia to surround and trap the bacteria for phagocytosis [265]. In the mentioned work [264], a construct was used containing iron-based NM-88 MOF as active centers, as a highly efficient peroxidase mimic, and hierarchical nanocavities produced by the growth of covalent organic structures (COF) as binding pockets to create an adapted microenvironment of pores around active sites to enrich and activate substrate molecules, to perform enhanced bacterial inhibition.