## 3 Antimicrobial Photodynamic Therapy (aPDT)  
Antimicrobial Photodynamic Therapy (aPDT) is an adjuvant technique used to improve root canal disinfection without inducing bacterial resistance [29]. In this technique, a light at a specific wavelength is used to activate PSs, generating reactive oxygen species (ROS) [30]. The aPDT mechanisms are mainly divided into Type I and Type II, as shown in Figure 2.  
**Type I**: The light-excited PS interacts with the surrounding molecules by an electron or a hydrogen atom exchange. This process triggers specific environmental changes that lead to hydroxyl radicals, one of ROS's responsive forms [31]. After light activation of the PS, a series of released oxygen ions and free radicals kill the targeted cells [32; 33]. One of the significant advantages of aPDT is the reliance on the enhanced penetration and accessibility of the reactive oxygen species [34; 35]. **Type II**: Similarly, the PS is activated by light and reacts with the ground state molecular oxygen, generating excited singlet-state oxygen (\({}^{1}\)O\({}_{2}\)) that directly targets biofilm-triggered diseases [36]. Both types of reactions could happen simultaneously. The ratio between these mechanisms depends on the type of PS used and the PS molecules' microenvironment [31; 37]. The photosensizers that were studied for the elimination of microorganisms belong to different classes of compounds: phenothiazzinium derivates, such as toluidine blue and methylene blue; porphyrin [36] as described in Table 1; phthalocyanine derivatives (disulphonated aluminum phthalocyanine and cationic Zn(II)-phthalocyanine) [37]; halogenated xanthenes derivates, such as Rose Bengal; triarymethane dyes, such as Malachite green, acidines; some conjugates of chlorins; and perylenequinones, such as hypericin [37]. However, the most used class for aPDT in endodontics is phenothiazzinium derivates, such as toluidine blue and methylene blue. Figure 2: A light source with a specific wavelength can activate a photosensitizer to an excited singlet state and then to a triplet singlet state in antimicrobial photodynamic therapy. The triplet singlet state can induce antimicrobial killing via the generation of the reactive oxygen species (Type I), singlet oxygen (Type II), or both. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Chemical Class** & **Photosensitizer** & **Spectra Absorption** & **Basic Chemical Structure** \\ \hline Zinc phthalocyanine & 600–700 nm & & \\ \hline Aluminum disulphonated phthalocyanine (AlPcS2) & 675 nm & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: General features of main photosensizers used in aPDT against oral bacteria. Most of them possess intense absorption bands in the red end of the visible spectrum (\(\Psi>600\) nm). The absorption in red provides a relatively high penetration power in most human tissues and is not substantially absorbed by endogenous cell/tissue constituents, thus minimizing the risk of undesired side effects [38]. Differences in antibacterial action concerning the phenothiazzinium photosensizers are known for toluidine blue O (TBO) and MBO. Against _E. coli_, TBO is known to be membrane active since it interacts more easily with the bacterial membrane than the methylene blue because this dye presents great solubility in the hydrophobic region of the membrane. This dye's efficient antibacterial photoactivity causes increased permeability, whereas MB causes strand breaks in the organism's nucleic acid. These two compounds present physico-chemical structures and properties that are similar, but the photodynamic efficiency varies between different microorganisms. In the early 2000 s, aPDT has become more exciting and promising in the dental field as a non-to-minimally invasive approach [38]. The reasonable easy access to the oral cavity by the light source/PS solution and the effective killing of putative pathogens in vitro have granted clinical investigations in many dental specialties, such as periodontitis, cardiology, and endodontics [39] over the last twenty years.