# Emerging Biomedical Applications of Carbon Dot and Polymer Composite Materials
## 2 General Aspects of Carbon Dot/Polymer Composites
### Carbon Dots and Polymer Hybrid Composite Materials  
#### 2.4.1 Features of Carbon Dot/Polymer Composites  
CDs and polymer composite materials demonstrate a polymer-carbon hybrid structure rather than a central carbon body structure, contributing to the predominant polymeric surface properties, which is different from the traditional carbonized CDs. Therefore, composite hybrid materials possess superior optical properties compared to conventional carbonized CDs and bare polymers [17; 46]. In general, polymers can act as stabilizing agents for high-volume-to-surface nanoparticles such as CDs. Polymer structures tend to diffuse through the nanoparticle surface to compensate for the starving nature of the nanoparticle surface energy. For these CDs, identical phenomena have been encountered in the polymer chains, and desirable synergistic features were presented. The newly formed CD-based hybrid features have higher optical properties, retaining all other polymer features intact. A remarkable advantage of polymer materials is their reliability in the era of functional product development. The durability of polymer materials is sometimes influenced by improper stress depletion, lack of uniformity in reinforcement, and thermal stability. Under these conditions, CDs play a crucial role in remediating inherent demerits. Physical interactions, H-bonding, polar-polar anchoring, polymeric entanglement, and surface passivation allow CDs to achieve better physisorption-aided anchoring of polymer chains onto the surfaces of CDs [13; 47]. The polymeric hybridization of CDs mainly includes the following three features: (1) abundant functional groups and attachment of polymer chains, (2) predefined polydispersity in structures and properties influenced by the polymeric conjugation process, and (3) superior optical properties generated by the process of surface passivation. CDs with tunable optical properties have been extensively developed. For example, nitrogen-doped CDs emit bright blue fluorescence when subjected to UV excitation [48]. In addition, CDs showed red fluorescence when rhodamine B was added as a donor molecule, resulting in these unique optical tuning properties to the transfer of nano-radioactive energy from excited donor to acceptor [48]. Furthermore, oxygen-containing CDs obtained from lemon juice via the hydrothermal method showed bright blue-green emissions when excited with UV [49]. CDs (1.8 nm) synthetized from carbon tetrachloride by hydride-reducing agents showed a wide absorption band at 270 nm. By increasing the mean diameter of CDs, a distinct shift was found in wavelength absorption to 304 and 306 nm [29]. Carbon dot/polymer composites are distinguished from a molecular state, where the surface state is the synergistic hybridization of a polymeric backbone. The energy gap of the surface state, where electrons and holes (e/h) pair and recombine to emit fluorescence, corresponds to the extent of the \(\pi\)-electron and surface chemistry. New sub-levels can be created by surface passivation or other effective modification methods, thus roughly controlling the photoluminescence (PL). Moreover, several studies have demonstrated that most of the fluorescence of the CD/polymer composite originates from the surface state and highlights its potential to change the photoluminescence (PL) properties [50; 51; 52]. Bare CDs usually exhibit light absorption in the UV-vis region, which has an extremely strong UV absorption. In the CD/polymer composite structure, light absorption results from multiple hybrid structures in the polymer/carbon nanoparticles. The \(\pi\)-\(\pi^{\star}\) transition is typically attributed to the aromatic carbon structure, and the n-\(\pi^{\star}\) transition is assigned to the functional chemical groups with lone pairs of electrons, including the amino-based chromospheres. Molecular state-connected structures possessing a band gap similar to that of organic fluorescent molecules may also generate specific absorption peaks.