## 1 Introduction  
Carbon dots (CDs) were serendipitously discovered in 2004 by professor [PERSON] during the purification by electrophoresis of single wall carbon nanotubes [1]. CDs are nanospheres with a diameter usually below 10 nm. They are made of carbon, with small amounts of other heteroatoms such as H, N and O appearing on the surface. Structurally, they have a graphitic core with sp2 hybridization coating and a crust of amorphous carbon [2]. This nanomaterial presents very interesting properties such as high water solubility, low toxicity, characteristic luminescent properties, high biocompatibility, high chemical stability and easy functionalization. Those properties also arise from the nano-size and surface functional groups, and can therefore be modified or even 'tuned' according to the synthesis pathway, reaction condition (temperature, time) or the reagents used, which is mainly the carbon source. Thus, CDs constitute a powerful tool in many scientific fields such as controlled drug delivery [3], clinic diagnose [4, 5], catalysis [6], bioimaging [7] and energy conversion [8], among others. Moreover, CDs can be easily synthesized following the 12 principles of 'green chemistry', as proposed by [PERSON] and [PERSON] in 1998 [9], which seek the reduction in use and generation of dangerous substances. The concept of 'green-chemistry' evolved and originated from the term'sustainable chemistry', which was defined by the Organization for Economic Co-operation and Development, OECD, as \"_a scientific concept that seeks to improve the efficiency with which natural resources are used to meet human needs for chemical products and services. Sustainable chemistry encompasses the design, manufacture and use of efficient, effective, safe, and more environmentally benign chemical products and processes\" [10]. The synthesis of sustainable carbon dots usually follows a bottom-up approach, with methods such as microwave-assisted pyrolysis, ultrasonication or hydrothermal/solvothermal decomposition. Hydrothermal decomposition is one of the most used and studied methodologies. Carbon dots obtained by this pathway form through a four-stage process: dehydration, polymerization, carbonization and passivation [11]. In the first stage, heating breaks the hydrogen bonds between the molecules, which convert into graphitic core during polymerization and carbonization. These cores grow by diffusion of other molecules to the surface. Furthermore, the cores passivate themselves and, therefore, no strong acids or passivation processes are required afterwards [12]. On the other hand, biocompatible antioxidant materials that could have pharmacologic applications are of paramount importance, as oxidative stress and reactive species of oxygen (ROS) are known agents of pathologies such as inflammatory processes, cancer or even aging [13; 14]. The use of natural products such as tomato juice in order to obtain water-soluble biocompatible anti-oxidant nanomaterials is, therefore, consistent with the sustainable chemistry philosophy. Tomato is the second most raised vegetable in the world, just after potato. It is an extraordinary source of antioxidants, including carotenoids (\(\alpha\)-carotene, \(\beta\)-carotene, lycopene and lutein) and phenolic compounds (phenolic acids, flavonoids and tannins), although the most important one is lycopene, since it presents the most antioxidant power, a higher ability to quench atomic oxygen and constitutes between 80% and 90% of total carotenoids in ripe tomatoes [15]. Thus, the synthesis of CDs from tomato (TCDs) juice is expected to show a high antioxidant capability. Likewise, taking into account the biocompatible origin of the carbon source, TCDs are expected to maintain their high biocompatibility.