2. Photothermal Therapy (PTT) and Photodynamic Therapy (PDT)
2.1. Photothermal Therapy (PTT)
PTT is a therapeutic method in which light energy is converted into heat energy after PTAs are irradiated by external light sources, such as NIR
[20]. This converted heat energy can effectively kill bacteria through a variety of thermal effects, such as cell membrane rupture, cell fluid evaporation, protein/enzyme degeneration, and cell hollowing. Previously, anti-cancer applications were the most used fields for PTT
[21]. However, researchers have also utilized PTT in various ways for antibacterial activity and wound healing
[13,20][13][20]. PTT is a non-invasive technique with low side effects and high specificity. Owing to these beneficial attributes, PTT has emerged as a promising strategy for combatting MDR pathogenic infections.
2.1.1. Mechanism of PTT
The working mechanism of PTT involves the conversion of light energy into heat energy for use in the surrounding environment by photothermal materials. Owing to the distinct photophysical characteristics of various materials, it is evident that photothermal conversion mechanisms are different for different materials. In this section, the photothermal conversion mechanisms of all the photothermal materials are broadly described.
PTTs using noble metal (for example, Ag and Au)-based nanomaterials are well-known
[22], and their excellent photothermal properties can be attributed to the localized surface plasmon resonance (LSPR) effect of the nanomaterials
[23]. Free electrons from the noble metallic nanoparticle surface are excited after the nanoparticles absorb the energy of photons at the appropriate wavelengths. The conduction band electrons then start to vibrate collectively at the same frequency. The LSPR effect is the term applied to this phenomenon
[24]. LSPR can decay both radiatively and non-radiatively. The plasmonic enhancement of the electric field in the near-field regime is mainly governed by the radiative decay process, whereas the formation of hot electrons is directed by the non-radiative decay process via intra- or inter-band transitions. As light absorption (for example, NIR light) can be increased by modifying a particle size or structure, the LSPR effect is significantly correlated with several characteristics, including particle morphology, size, and composition
[25]. The plasma coupling effect is another approach for enhancing the LSPR effect. Although noble metal (Ag and Au)-based nanomaterials are mostly used for the plasmonic effect, some materials, such as Al, Cu, Co, Ni, and CuS, have also been investigated for the same purpose
[23].
Another photothermal conversion mechanism comprises the generation and relaxation of electron (e
−)–hole (h
+) pairs; these often occur in semiconductors
[26]. In this process, the semiconductor absorbs photons to produce active e
−–h
+ pairs after being irradiated with light energy (the energy should be equal to or greater than the band gap energy of the semiconductor). After light irradiation, electrons are generated in the conduction band (CB), followed by electronic vacancies or holes in the valence band (VB). At this point, either the radiative (photons) or non-radiative (phonons) process is used for the subsequent relaxation from the higher excited states to the lower-energy states. This non-radiative process releases heat, resulting in a thermal (vibrational) energy increment of the lattice, which can be measured as an increase in their temperature.
Lattice vibrations are another photothermal conversion mechanism by which carbon and polymer-based materials exhibit excellent photothermal properties
[26]. In this mechanism, less tightly held electrons in ᴨ bonds from the ᴨ orbital can be easily excited to the ᴨ* orbital with lower energy input. Notably, the light-irradiated excitation of electrons (ᴨ→ᴨ*) induces a strong absorption in the NIR region. The excited electrons released the absorbed energy as heat during their return to the ground state, resulting in an increase in the temperature.
2.1.2. Advantages of PTT
PTT has emerged as a potential solution for treating MDR pathogens and a viable alternative to antibiotics
[13,20,27][13][20][27] owing to the following advantages: (1) broad-spectrum antibacterial effects for both Gram-negative (G−) bacteria and Gram-positive (G+) bacteria irrespective of the membrane structure, owing to the penetration ability of the light sources (for example NIR) utilized in PTT; (2) a good tissue penetration ability without causing tissue damage based on the commonly utilized NIR light source (700–1400 nm), which enhances the chances for successful bacterial treatments (even in deep tissues); (3) localized hyperthermia for the antibacterial activity to reduce the risk of damaging normal cells; (4) facilitation of antibacterial agent penetration inside the biofilm via hyperthermia; and (5) a non-invasive and non-contact mechanism which minimizes the opportunity for bacteria to obtain resistance against the therapy. From the above features, it is easy to understand why PTT-mediated therapy for combatting MDR pathogens is gathering significant attention.
2.1.3. Limitations of PTT
Despite the considerable interest in PTT, significant obstacles remain before PTT can be fully employed for practical applications. First, PTT usually requires high temperatures (≥60 °C) to kill bacteria
[20]. Such prolonged hyperthermia kills bacterial cells and thermally harms the normal tissues around bacterial infection sites
[16]. Therefore, a more strategic design is necessary to optimize the conditions, for example, with shorter treatment times at lower temperatures (~50 °C). Additionally, normal tissues may be critically affected by the necessity for a high excitation light power and high PTA dosage; these aspects require quick attention to ensure safer and more efficient PTT.
2.2. Photodynamic Therapy (PDT)
PDT combines photoresponsive materials such as photocatalysts and photosensitizers (PSs) and a light source to kill bacterial cells
[17]. Currently, PDT has mainly been applied to cancers as a clinical therapeutic approach to treating non-invasive tumors. It is regarded as a major step in anti-cancer applications, including surgery, chemotherapy, and radiotherapy
[28]. Additionally, PDT has been applied to other hard-to-treat diseases such as rheumatoid arthritis, actinic keratosis, and bacterial infections. Researchers have used PDT for antibacterial activity owing to its low toxicity, the negligible chance of drug resistance with mild adverse reactions, and excellent antibacterial potential.
2.2.1. Mechanism of PDT
The primary mechanism of PDT relies on the generation of ROS, including hydroxyl radicals (·OH), singlet oxygens (
1O
2), or superoxide radicals (·O
2−) after photoresponsive materials are exposed to laser irradiation
[29]. It is widely acknowledged that “toxic” cellular waste (i.e., ROS) can permanently damage macromolecules such as nucleic acids, lipids, and proteins after entering bacterial cells. The ROS generated by the PDT can directly or indirectly interfere with the physiological activities of the cells, ultimately leading to cell death
[30]. In a different procedure, ROS can attach to bacterial cell walls and membranes, leading to cell death.
The ROS generation mechanism for PSs differs from that for photocatalysts
[34][31]. There are two main molecular-level mechanisms in the photosensitization route
[35][32] for controlling PDT-mediated antibacterial activity. One of the mechanisms normally occurs in bacterial cell membranes. In this mechanism, molecules from the PS temporarily migrate from the ground state to the singlet state (
1PS) and subsequently reach the time-extended triplet state (
3PS) by inter-system scurrying. In this scenario, the
3PS produces ROS, such as ·OH and ·O
2−, through electron transfer after reacting with biomolecules in the surrounding environment. Subsequently, the ROS disrupt the bacterial cell membranes and increases their ion permeability for killing bacteria. In the second mechanism, the
3PS can directly react with oxygen molecules to undergo an energy transfer for the formation of
1O
2 with a very short lifetime (and only able to react within a micrometer range of its generation site). This singlet oxygen can cause oxidative damage to intracellular bacterial compounds and eventually kill them. The PS molecules return to their ground states for the next cycle of reactions after the reactions are complete.
SPR also has the potential to enhance PDT-mediated antibacterial activity
[36][33]. Photoinduced noble metals (Ag and Au) and metallic compounds (CuS) can produce SPR, which activates electrons that can be transferred to the CBs of the photocatalysts. As a second mechanism, the irradiation near the plasmon resonance frequency of noble metals can substantially enhance the local electric field, accelerating the separation of e
−-h
+ pairs. In this regard, PSs and noble metals can be applied together for better antibacterial activity
[37][34].
Up-conversion nanoparticles (UCNPs) represent another approach to resolving the poor penetration ability of PDT. UCNPs can convert the excitation of long-wavelength light into short-wavelength light for PDT based on visible light-responsive photocatalysts
[38][35].
2.2.2. Advantages of PDT
Researchers are eager to seek alternatives to antibiotics owing to the ever-increasing number of MDR pathogens. In this regard, PDT has emerged as a viable option for treating MDR pathogens, owing to the following advantages: (1) broad-spectrum activity with ROS production, which disrupts many metabolic pathways of bacteria and their cellular structures rather than focusing on a single process or structure; (2) light-irradiation-induced antibacterial activity with photoresponsive ROS-generating materials; correspondingly, the chances of bacteria becoming resistant to the photoresponsive materials are very minimal; (3) preferential control of the binding to bacteria at the infected site of the body, light time, and location
[39][36]; (4) low cytotoxicity to normal cells when treating bacterial infections in living organisms; (5) combinations with other therapies, such as radiotherapy, chemotherapy, and PTT; and (6) remote chances of any self-adaptability transferring to the next generation of bacterial cells in a very short time. Hence, the chances of bacteria becoming resistant are greatly reduced.
2.2.3. Limitations of PDT
Despite the above-mentioned advantages of PDT, certain limitations must be addressed before it can be fully utilized to treat bacterial infections in living organisms. ROS-mediated PDT depends on photoresponsive materials and a light source for the irradiation of biological tissues. However, owing to the thickness of human tissue, the poor penetration ability or shallow depth of short-wavelength light or both has hindered the application of PDT to human tissues for antibacterial infections
[40][37]. A longer-wavelength light source could provide greater tissue penetration. However, the ROS production for PDT is directly correlated with the energy of the light; therefore, light sources with longer wavelengths and low energies limit the production of ROS. Thus, UV light with a short wavelength and high energy can produce more ROS and is highly effective in killing microorganisms
[41][38]. However, UV irradiation can also damage normal cells and tissues. Hence, the selection of an appropriate light source must be addressed to improve PDT applications.
Additionally, the development of PDT for clinical use is constrained by the limited release distance and lifespan of the ROS, as well as the low stability, toxicity, and ineffective bacterial targeting of certain photoresponsive materials
[17]. Notably, PDT is less effective for G− bacteria than for G+ bacteria owing to the different membrane structures and limited penetration ability of ROS; this also requires further research. Therefore, the limitations of PDT need to be fully addressed before it can be further applied in practical applications.