# Metal Nanozymes: New Horizons in Cellular Homeostasis Regulation
## 6 Metal Nanozymes as Cellular Probes for Biosensing  
Over the past decade, nanozymes have proved to be excellent providers of high-performance and ultra-sensitive biosensors, including colorimetric, fluorometric, chemi-luminescent, surface-enhanced Raman scattering, and electrochemical assays [139]. Due to their oxidase-like activity, metallic, metaloxide, and metal-organic nanoparticles are used for colorimetric and fluorimetric sensing of H\({}_{2}\)O\({}_{2}\), O\({}_{2}\) and H\({}_{2}\)S, glucose, ascorbic acid, cysteine, GSH, and other bio-thiols, cholesterol, uric acid, dopamine, epinephrine, phenol, Hg\({}^{2+}\), cancer cells, thrombin, ALP, HIV DNA, sulfadimethoroxine, xanthine, bacteria, heavy metal ions, and many others (a detailed review on analytical applications of MOF-based nanozymes can be found in [79]; Detailed reviews on metal nanozyme biosensors can be found in [19, 140]). Colorimetric detection provides a quick response (color change), often allowing the naked eye to evaluate the sample parameters and subsequent quantification in UV-VIS. The advantage of naked-eye detection is that it can be used as a first-pass screening test for rapid disease diagnosis. This feature of colorimetric sensors makes them suitable for \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Nanozyme** & **Ref.** &
\begin{tabular}{c} **Target** \\ **Enzyme** \\ \end{tabular} & **Enzyme-Nanozyme Interaction Type** & **Application** \\ \hline AuNPs & [132] & ChT & electrostatic interactions & Protein interaction \\ ZnONPs & [135] & Beta-Gal & Hydrogen bonding, Van der Vaals, molecular shape & Ovarian cancer, cell aging \\
2 DMoS2 & [138] & \(\beta\)-Gal, ChT & electrostatic interactions & diagnosis, treatment \\
2 DMoS2 & [136] & \(\beta\)-lactamase & electrostatic interactions, steric hindrance & overcoming penicillin resistance \\ \hline \hline \end{tabular} Abbreviations: AuNPs, gold nanoparticles; ZnONPs, ZnO nanoparticles; ChT, R-chymortrypsin \(\beta\)-Gal, beta-galactosidase.  
\end{table} 
Table 2: Nanozymes inhibit natural enzymes by various interaction modes. developing fast and inexpensive screening tools in medicine (i.e., detection of disease-specific molecules, proteins, and cells), biotechnology, and environmental sciences. Nanozymes can oxidize a variety of chromogenic substrates, such as (e.g., TMB), 2,2\({}^{\prime}\)-azino-bis(3-ethylbenzthiazol-6-sulfonic acid) (ABTS), 3,3\({}^{\prime}\)-diaminobenzidine (DAB), and \(o\)-phenylenediamine dihydrochloride (OPD) in the presence of hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) and produce a distinguishable color. The analytical determination of hydrogen peroxide is of considerable importance for medical diagnosis because H\({}_{2}\)O\({}_{2}\) is formed as an intermediate product in a large number of important detection processes (Equations (3) and (4), Figure 5). Nanozymes can easily replace peroxidase in conventional assays, often extending the field of application of tests by, e.g., a larger pH range. This makes them interesting for the investigation of combined multienzyme assays with oxidases that require a narrower pH range for their optimum activity, as shown for the glucose/glucose oxidase catalytic test [141]. \[\text{Substrate + substrate oxidase + O}_{2}\rightarrow\text{oxidation product +H}_{2}\text{O}_{2}, \tag{3}\]  
H\({}_{2}\)O\({}_{2}\) + Peroxidase nanozyme mimic + Oxygen acceptor (Colorless) \(\rightarrow\) H\({}_{2}\)O + Oxidized acceptor (Colored), (4)  
Figure 5: _Cont._  
Sometimes a nanozyme may fulfill a tandem role, as obtained by [PERSON] et al., of \"non-naked\" AuNP coated with protein (bovine serum albumin). Although the \"naked\" Au NPs have peroxidase-like activity and GOx-like activity, the optimal pH ranges of the two activities are different. Fortunately, synthesized \"non-naked\" Au NPs exhibit dual enzymatic activity at the same pH, benefiting the establishment of one-pot enzyme-free glucose detection.