# Metal Nanozymes: New Horizons in Cellular Homeostasis Regulation
## 6 Metal Nanozymes as Cellular Probes for Biosensing
The on/off colorimetric sensors have been also designed, for example, based on the reversible surface passivation of noble metal nanozymes with single-stranded DNA [142] (ssDNA) or an aptamer [143]. Self-regulated colorimetric sensors use the nanozyme activity of nanoceria to detect acetylcholinesterase-nesterase, nerve agents, drugs, and bioactive ions, by in situ modulating the oxidase-like activity of nanoceria through proton-producing (or proton-consuming) enzymatic reactions [144]. Several other sensitive colorimetric sensors based on functional nanozymes have been reported, among others for the detection of cocaine [145], immunomod chromatographic analysis of the _Ebola virus_[146], and _Escherichia coli_[147]. In addition, a sensor array based on combined analysis of analytes' influence on Pt Ru and IrNPs' peroxidase-like activities has been designed to discriminate between biothis, proteins, and cancer cells [148]. In terms of sensitivity, one of the most spectacular improvements brought by nanozymes concerns their application in the popular enzyme-linked immunosorbent assay (ELISA). ELISA is the most widely used technique for the detection and quantification of peptides and antigens. The enzyme-conjugated primary antibody (direct ELISA) or the secondary antibody (indirect or sandwich ELISA) in the ELISA test specifically recognizes the antigen. The most commonly used enzyme reporter in the ELISA test is HRP. It catalyzes the oxidation of TMB in the presence of H\({}_{2}\)O\({}_{2}\) to produce a colorimetric signal, and the intensity of the signal is proportional to the concentration of the recognized antigen. However, HRP has a lot of limitations that are typical of proteins. It is not resistant to many preservatives such as sodium azide. It is also subject to proteolytic degradation. Its enzymatic activity is limited to a narrow range of pH and temperature [10]. In addition, the sensitivity of a conventional  
Figure 5: (a) Nanozyme as peroxidase mimic for colorimetric sensing of H\({}_{2}\)O\({}_{2}\), and glucose when combined with glucose oxidase. (b) The sensing format in (a) could be extended to other targets (substrate 1 here) when combined with a suitable oxidase. (c) The target of interest as substrate 0 could be determined if it could be converted into an oxidase substrate. Numerous transduction signals can be adopted for sensing (such as colorimetric, fluorometric, chemiluminescent, and surface enhanced Raman spectroscopy (SERS) signals when the corresponding substrates are used; and electrochemical signals when a nanozyme is immobilized on an electrode). Adapted with permission from [17]. Copyright (2013) Royal Society of Chemistry. ELISA is limited [149]. To overcome these limitations, numerous nanozymes have been developed, considered direct substitutes for HRP [150]. For instance, [PERSON] and [PERSON] [151], to avoid the occurrence of false positives and false negatives caused by conventional ELISA, have recently developed a nanozyme-linked immunosorbent assay based on metal-organic frameworks (MOFs) for sensitive detection of aflatoxin B1. As a result, the limit of detection (LOD) of the MoFLISA method was 0.009 ng/mL with a linear working range from 0.01 to 20 ng/mL, thus having a 20-fold improved LOD value compared with the conventional test. In a fluorescent sensor, nanozyme usually converts a non-fluorescent substrate into a fluorescent-active substrate by catalyzing a hydrolysis or oxidation reaction [152; 153; 154], including application in fluorescent ELISA tests [155; 156]. For example, [PERSON] et al. demonstrated that the ability of poly (acrylic acid)-coated nanoceria to mimic oxidase depends on the pH of the solution [156]. In the pH range 4\(-\)7, HRP/H\({}_{2}\)O\({}_{2}\) oxidized ampliflu to nonfluorescent resazurin. By contrast, at pH 6-8 nanoceria oxidized ampliflu to the intermediate oxidation fluorescent product resourfin, while at or below pH 5.0, nanoceria also yielded the terminal oxidized nonfluorescent product resazurin.