# Carbon Nanodots as Peroxidase Nanozymes for Biosensing
## 1 Introduction  
'Artificial enzymes', a term invented by [PERSON] [1], is an stimulating branch of biomimetics, which relies on the use of alternative molecules or materials to either reproduce the elusive structure of enzyme active sites or mimic the functions of the natural enzymes [2; 3], 'Nanozymes'--nanomeralists with enzyme-like characteristics--represent an emerging research area in the field of artificial enzymes and have generated enormous scientific excitement because of their superiority in terms of elegant response to external stimuli, self-assembly capability, large surface area, ease of desirable yet diversified structural modulations or bioconjugation, and most importantly, size-dependent catalytic activities [4; 5]. Not surprisingly, therefore, the past few years have witnessed the exploration of a variety of nanomaterials to mimic the structures and functions of natural enzymes, including, but not limited to, peroxidase (POD), catalase, oxidase, superoxide dismutase, phosphatase, and nuclease for diverse applications [5; 6; 7; 8; 9]. Carbon, is the 15 th most abundant natural element found in the Earth's crust. Carbon is abundant in the Sun, stars, comets, and in the atmosphere of most planets. This abundance, combined with the exceptional but unfamiliar polymer-forming ability of organic compounds at Earth's temperatures makes carbon the chemical basis of all known life. The carbon allotropes such as amorphous carbon or coal, graphite and diamond have found many usages over the centuries. From providing energy, to being the most expensive crystal (a sensation among female fans), to support a central need in education, giving us pencils, a tool for many; carbon has done it all. Recently, with the rapid rise of classical nanofabrication techniques, the carbonaceous nanomaterials such as carbon nanotubes (CNTs)and graphene are (and will continue to be) increasingly used in the imminent carbon age for a diverse range of catalytic applications, including enzyme mimics [10; 11; 12; 13; 14; 15; 16; 17]. Since their initial discovery in 2004 [18], zero-dimensional carbon nanodots (C-dots) or carbon quantum dots (CQDs) have gradually become a rising star as a new member in the family of carbonaceous nanomaterials and the scope of their utility has increasingly matured in the last few years for variety of applications such as biosensing, bioimaging, and drug delivery and therapy [19; 20; 21; 22; 23]. Owing to their strong luminescence, in contrast to macroscopic carbon, C-dots are also referred to as nanolights or fluorescent carbon. In general, C-dots are quasi-spherical carbon nanoparticles (C-NPs) that comprise amorphous to nanocrystalline cores with a diameter of less than 10 nm. A representative structure of C-dots is depicted in Figure 1.  
It has been demonstrated that the inner part of C-dots is primarily composed of sp\({}^{2}\)-hybridized carbon atoms while sp\({}^{3}\)-hybridized carbon atoms are present in the outer part, as inferred from nuclear magnetic resonance (NMR) measurements [24]. Similar to its predecessors, the semiconductor quantum dots (QDs), C-dots also exhibit an excellent electrochemiluminescence emission property. Nevertheless, the exceptional physicochemical properties such as an excellent water dispersibility (thanks to the abundant hydrophilic groups on their surface), robust chemical inertness, low toxicity, good biocompatibility, high resistance to photobleaching, ease of synthesis, excitation-dependent multicolor emission, and desirable surface engineering with all facets of C-dots make their profile a collection of very desirable and remarkable traits, which enable them as the nanomaterials-of-choice for enzymatic applications, in particular, a POD mimics. PODs (EC 1.11.1.7) or a group of oxidoreductases are a family of isoenzymes that play a critical role in different metabolic activities in living organisms.