drugs that are also toxic to normal cells, which could lead to severe side effects in the case of long-term use. Radiotherapy lacks the precision in targeting cancer cells and, thus, can also harm boundary normal cells. Surgery may not always remove the entire cancer and may have the possibility of mistakenly injuring surrounding healthy tissues. To minimize the side effects and enhance therapeutic effectiveness, the development of novel methods is highly needed. Nanomedicine may be the new promising approach to cancer theranosis (a combined word of diagnosis and therapy). During the last decade, nanomedicine has been the main research focus in cancer theranosis. Researchers have explored various types of nanomaterials for cancer treatments, including gold nanoparticles [5; 6; 7; 8], quantum dots (QDs) [9; 10; 11; 12], upconversion nanoparticles [13; 14; 15; 16], and organic dye-loaded nanoparticles [17; 18; 19; 20] for imaging-guided cancer therapy. However, these materials were often hindered from applications owing to poor water solubility, high toxicity, high costs, and poor photostability. Since the discovery of carbon dots (CDs), they have garnered significant attention for their potential use in cancer theranosis [21; 22]. CDs were first identified accidentally by [PERSON] et al. in 2004 in purifying single-walled carbon nanotubes (SWCNTs) [23]. After this, CDs have been extensively studied with a particular focus on developing various synthesis methods, understanding their diverse physicochemical properties, and exploring their applications in various fields [24; 25; 26; 27; 28; 29]. Based on research outcomes up to now, CDs are known for their simple preparation methods, rich hydrophilic surface functional groups, easy surface modifications, high water solubility, stable optical properties, various therapeutic properties (Figure 2), and low toxicity because they are mainly made of nontoxic carbon (C) with additional nontoxic elements such as hydrogen (H) and oxygen (O) [30]. CDs are composed of sp\({}^{2}\) and sp\({}^{3}\) carbons and can be made to have hydrophilic functional groups on their surfaces, such as carboxyl (-COOH), amino (-NH\({}_{2}\)), and hydroxyl (-OH) groups [31; 32; 33]. These rich hydrophilic  
Figure 1: Cancer incidence and mortality worldwide in both sexes and all ages for the year 2022. Graph from Global Cancer Observatory (GLOBOCAN) 2022, International Agency for Research on Cancer (IARC), World Health Organization [2]. functional groups of CDs make easy surface modifications to have high loading capacity of various cargoes such as drugs [34; 35; 36; 37; 38], photosensitizers [39; 40; 41; 42; 43], nucleic acids [44; 45; 46], and targeting moieties (i.e., transferrin, folic acid, and hyaluronic acid) [47; 48; 49; 50; 51; 52]. Compared with toxic metal-based QDs [53], CDs are much safer and eco-friendly for biomedical applications as they are free from heavy metals and toxic elements. One of the most notable attributes of CDs is their excellent optical properties, covering absorption and emission from ultraviolet (UV) to near-infrared (NIR) [54], useful for cancer theranosis. These various valuable physicochemical, toxicological, and theranostic properties of CDs make them very promising materials for use in cancer theranosis. The multifunctionality of CDs is useful for cancer theranosis, including fluorescent imaging, drug delivery, photodynamic therapy (PDT), photothermal therapy (PTT), gene therapy, sonodynamic therapy (SDT), and gas therapy, offering new methods for cancer theranosis (Figure 2) [55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65]. Furthermore, the combination of several therapeutic modalities can lead to synergistic effects, further enhancing the therapeutic efficacy. Despite many review papers on CDs so far [66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81], there is still a need for an overview focusing on surface functionalization and applications of CDs to cancer theranosis.