## 5 aPDT Performance in Clinical Studies
### Quantum Dots in Antimicrobial Photodynamic Therapy  
Quantum dots are nanocrystals created from semiconductors and are readily synthesized by processes of self-organization of particles, such as sol-gel [80; 81]. Their nano dimension of up to 10 nm confer to them unique optical features and influence their fluorescence wavelength. Moreover, these particles have a high photostability [82]. The fluorescence of quantum dots is greatly attributed to their smaller size more so than the exciton Bohr radius. The exciton Bohr radius is the distance between the electron-hole pair. When the particle's radius is smaller than the electron-hole pair, quantum confinement occurs [83]. The random switching of bright and dark, a usual behavior of quantum dots, was observed by the fluorescence microscopy due to illumination-induced charging (on off). Simultaneously, a non-radiative Auger recombination predominates, and the re-neutralization period (off, on) occurs while radiative recombination prevails [84; 85]. This switching effect is observed as a photoemission blinking, which was also indicated when zinc oxide quantum dots were incorporated into experimental adhesives for therapeutic purposes [80].  
Quantum dots of tantalum oxide [86], zinc oxide [87], and titanium dioxide [88] (oxides that may show bioactivity) were already synthesized and incorporated into dental materials to provide antibacterial activity. CdSe/ZnS core-shell quantum dots were also incorporated in a resin to tailor dental composites' fluorescence [89]. In recent years, quantum dots have emerged as novel PS in photodynamic therapies [82] due to the aforementioned optical properties. An interesting approach is based on the quantum dots' ability to carry antibiotics, proteins, drugs to combat tumors, and other biomolecules beyond the fluorescence property [82].  
Despite the increased bioengineering application, these particles are poorly investigated in photodynamic therapy for disinfection purposes. In 2019, graphene quantum dots doped with curcumin, a natural PS extracted from turmeric roots, were tested against periodontal pathogens mixed biofilms [90]. The particles were irradiated (435 nm for 1 min), and the quantity of ROS formed, as well as the biofilm-formation ability and the changes in gene expressions implicated in the biofilm formation, were evaluated. As an outcome, the photoexcited particles showed antimicrobial activity against planktonic and biofilm forms and regulated the gene expression, evidencing a great alternative PS against perio-pathogens [90].  
Moreover, photoactivable polymers with cadmium quantum dots conjugated with crystal violet were tested against important clinical multidrug-resistant bacteria: methicillin-resistant _S. aureus_ and a carbapenemase-producing strain of _Escherichia coli_[91]. Crystal violet was added in this study to improve ROS generation. The analyzed photoluminescence lifetime and the ROS generation showed a chemical interaction between the crystal violet and the quantum dots. Furthermore, the particles evidenced great antimicrobial activity against these strains [91]. So far, none of the quantum dots strategies have been investigated in the endodontics field for proof of concept. However, quantum dots seem to be a promising approach to treat biofilms along the root canal due to their ability to be functionalized, conjugate with other drugs, penetrate through narrow canals, emit electromagnetic energy, and be identified via microscopic images.