A study by Luo et al. (2021) reported the photosulfitochemistry mechanism of MB dye as demonstrated in Figure 2. The mechanism outlines a sequence of chemical reactions involved in the wastewater treatment process using MB as a photosensitizer involving five processes namely;

(i) Photo-excitation of MB⁺: where the ground state MB⁺ becomes excited to the single state ¹MB⁺* after absorbing visible light. This process is followed by the generation of triplet state ³MB⁺* from ¹MB⁺* after going through fast intersystem crossing (ISC). For the sake of simplicity in this process, the ISC reaction is not included here, and MB⁺* stands for the excited MB⁺'s triplet state.

As a result, the use of MB⁺ and its recovery in this process is advantageous due to its faster redox turnover compared to transition metals as catalysts for sulfite activation. This efficiency facilitates significant As(III) oxidation and requires lower MB⁺ concentrations (Luo et al., 2021). Intrinsically, it is worth noting that the overall success of this wastewater treatment process depends on factors such as reaction kinetics, efficiency of radical generation, and the ability to recycle thiazine dyes i.e., MB effectively. Additionally, the specifics of the ISC reaction and any possible side reactions not mentioned may influence the overall performance of the system. Notably, MB has been widely employed in the photooxidation of both synthetic and natural compounds (Tardivo et al., 2005; Luo et al., 2021; Yhon et al., 2023). As shown on Scheme 1, two main types of photochemical process can be observed, which can be explained as follows:

Type I, is where reducing agents gives an electron to the MB triplet, generating the semi-reduced radical MB, Reaction (3), in Scheme 1, and Type II, is where the triplet energy is transferred to oxygen, forming singlet oxygen (¹O₂, Reaction (2)) (Scheme 1). The ground state MB molecules can function as reducing agents by themselves at high dye concentrations (D − D* mechanism) (Junqueira et al., 2002; Severino et al., 2003). As such, the schematic representation demonstrates the effectiveness of using thiazines such as MB as photosensitizers.

Scheme 1 also shows the photochemical pathways for MB in which MB⁺, ¹MB⁺*, ³MB⁺* represent the ground state, singlet, and triplet excited states of MB, respectively, which was reported by Tardivo et al. (2005) (Tardivo et al., 2005). In here, MB⋅ and MB⋅²⁺ represent the semi-reduced and semi-oxidized radicals of methylene blue, respectively; Ω ₁ denotes light absorption; Φ _f, Φ _nr, Φ _T are fluorescence, non-radiative, and triplet quantum yield.

The light that is typically applied in the visible and near-infrared wavelengths must stimulate photosensitizers in order to start the photodynamic process. The initial state of the photosensitizer is a singlet state in which paired electrons in frontier molecular orbitals have opposing spins (Zhao et al., 2021; Li and Pu, 2019). Upon exposure to light of the appropriately designated wavelength, the photosensitizers absorb the light, resulting in the excitation of a single electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), while the spin remains unchanged during the process (Rossi, 2023; Kenry and Liu, 2022). When the photosensitizer is exposed to light with the right wavelength, it absorbs the light and one electron is excited from HOMO to the LUMO, with the spin staying unaltered throughout. This excited state, which lasts for only a few nanoseconds at most, is referred to as the excited singlet state (Rossi, 2023; Kenry and Liu, 2022; Huang et al., 2014). The photosensitization mechanism, as illustrated in Figure 3, involves electrons from the dyes HOMO excited to their LUMO and then injected to the CB of TiO₂. This process occurs once the dyes absorb photons in the visible light energy range of the solar spectrum (Li et al., 2020).

The occurrence of this excited singlet state is pivotal in initiating subsequent photochemical reactions and is fundamental to the overall functionality of photosensitizers in diverse applications such as photodynamic therapy, solar energy conversion, and photocatalysis (Zhao et al., 2021; Li and Pu, 2019; Xu et al., 2022; Monro et al., 2019). In this context, the singlet-excited state of photosensitizers can manifest in different pathways. It may either emit light in the form of fluorescence or undergo radiationless relaxation, converting the excitation energy into heat internally (Ortiz-Rodríguez et al., 2021). Alternatively, the excited electron has the option to reverse its spin, a process that, for most organic compounds, is relatively slow and requires a singlet state with an extended lifetime. The majority of organic compounds exhibit a prolonged lifetime for the singlet state due to the sluggish nature of the spin reversal process (Ortiz-Rodríguez et al., 2021; Stoll and Schweiger, 2006; Li et al., 2022). It is noteworthy that the excited electron, having a parallel spin relative to its unexcited, paired electron, transitions into the triplet state, which boasts a considerably longer lifetime. However, the return of the excited electron to the ground state in the triplet state is constrained by the Pauli exclusion principle, as this process is considered “spin forbidden” (Stoll and Schweiger, 2006; Li et al., 2022; Minaev et al., 2022). Consequently, the excited electron in the triplet state cannot simply revert to the ground level, leading to a substantially prolonged lifetime for the triplet state (Minaev et al., 2022). These intricate dynamics play a pivotal role in determining the fate of the excited state and influence the subsequent photochemical and photophysical processes in which the photosensitizer is involved. The triplet state is much longer lived because the excited electron possesses a parallel spin to its paired, unexcited electron. In the triplet state, the excited electron cannot easily fall back to ground level (a “spin forbidden” process such as this would violate the Pauli Exclusion Principle) (Ortiz-Rodríguez et al., 2021; Lynch et al., 2019).

The PDT exhibits its distinctive oxidative damage through the capability of photosensitizers to interact with oxygen during their prolonged triplet state. Exposure to light triggers electron-hole pairs in TiO₂, generating redox reactions that produce ROS, such as superoxide ions and hydroxyl radicals, which can damage organic pollutants in wastewater (Gilson et al., 2017). The extended triplet state of photosensitizers in PDT and the resulting oxidative damage pathways are analogous to the generation of ROS during the photocatalytic activity of TiO₂ in wastewater treatment (Tanielian et al., 2003). The corresponding pathways are illustrated in Figure 4, delineating the ensuing photochemistry and potential mechanisms of damage to various targets. In this process, the photosensitizer, residing in its triplet state with an extended lifetime, engages in interactions with molecular oxygen. This interaction initiates a cascade of events leading to the generation of ROS, such as singlet oxygen.

The ROS, particularly singlet oxygen, is highly reactive and can induce oxidative damage to biomolecules and cellular structures. These damaging effects are the basis for the therapeutic efficacy of PDT, as the targeted generation of ROS selectively damages cancer cells or other pathogenic targets (Das and Roychoudhury, 2014; Zapata et al., 2022; Abdal Dayem et al., 2017). The pathways depicted in Figure 4 provide a visual representation of the intricate photochemical processes and the subsequent mechanisms by which PDT induces oxidative damage to its intended targets (Das and Roychoudhury, 2014). Understanding these pathways is crucial for optimizing PDT protocols and expanding its applications in medical treatments and other fields. In the context of wastewater treatment, the oxidative damage caused by the extended triplet state of photosensitizers aligns with the oxidative degradation of pollutants facilitated by TiO₂ photocatalysis (Lima and Reis, 2023; Zhao et al., 2021). The pathways illustrated in Figure 4, along with the resulting photochemistry and potential mechanisms of damage to various targets, could be conceptually linked to the intricate processes involved in the photocatalytic degradation of contaminants in water using TiO₂ (Lima and Reis, 2023; Monro et al., 2019). Understanding these mechanisms aids in optimizing TiO₂-based photocatalysis for effective wastewater treatment and pollutant removal.

The elucidation of photocatalytic mechanisms involving TiO₂-based dye photosensitizers is crucial for a comprehensive understanding of how these systems work and for optimizing their performance in various applications (Schneider et al., 2014; Sakar et al., 2019; Chen et al., 2020; Serpone, 2018). Researchers have employed a combination of experimental techniques and theoretical modelling to uncover these mechanisms. Figure 5 shows the mechanism of the dye sensitization process with the dye adsorbed onto the catalyst surface and dye in the bulk.

Thiazine dyes play a critical role in extending the light absorption range of TiO₂ (Rochkind et al., 2015; Bindhu et al., 2022). When exposed to light, thiazine dyes can absorb photons in the visible region, promoting electron excitation from their HOMO to their LUMO ground state to higher energy levels (Luo et al., 2021; Rochkind et al., 2015; Senthilkumar et al., 2010; Vara and Ortiz, 2016). Upon excitation, the thiazine dye molecules transfer electrons to the conduction band of TiO₂. This injection of photoexcited electrons into TiO₂ is facilitated by the difference in energy levels of the dye LUMO and the bandgap of TiO₂, particularly its CB position, and this is a key step in the photocatalytic process (Khan et al., 2022; Rochkind et al., 2015; Senthilkumar et al., 2010; Parrino et al., 2022; Ronca et al., 2012; Mezyen et al., 2023). The injected electrons can then participate in redox reactions on the surface of TiO₂ with adsorbed species or in the reduction of oxygen to form superoxide radicals. Simultaneously, when electrons are injected into the TiO₂ conduction band, holes (positive charge carriers) are created in the valence band of TiO₂ (Ronca et al., 2012; Elg et al., 2021). These holes can participate in oxidative reactions, such as the oxidation of water or organic pollutants, which is crucial for overall photocatalytic activity.

The efficiency of a TiO₂-based dye sensitizer system depends on the extent of charge separation and minimization of electron-hole recombination. Photogenerated electrons in the conduction band and holes in the valence band should be efficiently separated to avoid recombination, which would lead to a decrease in photocatalytic activity (Elmorsy et al., 2023; Zani et al., 2021). Photogenerated electrons and holes can interact with adsorbed species, leading to the formation of intermediate radicals. These radicals can participate in the degradation of organic pollutants or in other chemical reactions. For example, superoxide radicals formed from oxygen reduction can contribute to the degradation of organic compounds. Oxygen and water are often present in photocatalytic reactions. Oxygen can accept electrons from the conduction band, forming superoxide or other oxygen radicals. Water can serve as an electron donor to reduce holes in the valence band (Chakhtouna et al., 2021; Goodarzi et al., 2023; Humayun et al., 2022; Li et al., 2013). These reactions are crucial for pollutant degradation and hydrogen production.

The interaction of thiazine dye-sensitized TiO₂ with target molecules or pollutants on the surface is essential for the photocatalytic process (Zani et al., 2021; Senthilkumar et al., 2010; Barakat et al., 2022). Surface adsorption, electron transfer, and radical reactions play a significant role in the degradation or transformation of target compounds. Researchers often study the quantum yields of photocatalytic reactions to understand the efficiency of the process. Additionally, reaction kinetics are examined to determine rate constants, reaction pathways, and the role of intermediates (Senthilkumar et al., 2010; Araujo et al., 2022; Liu et al., 2014). To elucidate these mechanisms, researchers use techniques like transient absorption spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, surface science experiments, and computational modelling, including density functional theory (DFT) calculations (Qin et al., 2023). These methods provide valuable insights into the dynamic processes occurring during photocatalysis, helping to optimize TiO₂-based thiazine dye systems for various applications, including water treatment, pollutant degradation, and hydrogen production (Luo et al., 2021; Senthilkumar et al., 2010; Acar et al., 2015; Yang and Wang, 2018). Figure 6 demonstrates the application of EPR spectroscopy in TiO₂and Nb₂O₅ photocatalysis.

As shown in Figure 6, the application of EPR spectroscopy in the observation of paramagnetic centres formed upon irradiation of TiO₂ and niobium oxide (Nb₂O₅) photocatalysts was reported by Al-Madanat et al. (2021). They showed that electron paramagnetic resonance spectroscopy is a powerful technique that can be used to monitor the photoinduced phenomena occurring in semiconductors like TiO₂ and Nb₂O₅ photocatalysts. As such, the EPR spectroscopy provides precise insights into the dynamic and reactivity of the photocatalyst under different experimental conditions (Al-Madanat et al., 2021). It can characterize paramagnetic centers formed upon irradiation of these photocatalysts. The application of EPR in the observation of paramagnetic centers formed upon irradiation of TiO₂ and Nb₂O₅ photocatalysts has been studied. The EPR spectroscopy differentiates itself from other techniques by providing exceptional sensitivity and specificity towards species with unpaired electrons, including organic and inorganic radicals, crystal defects, dopant atoms, and both free and trapped charge carriers (Al-Madanat et al., 2021; Hurum et al., 2003). The results obtained from EPR spectroscopy can help in understanding the electron-hole recombination, the nature of trapped charge carriers, and the reactions involved in the photocatalytic process.

Electron and energy transfer processes are fundamental mechanisms that play a critical role in a wide range of scientific and technological applications, including photophysics, photochemistry, and photobiology (Parrino et al., 2022). Understanding these processes is essential for optimizing various technologies and advancing our knowledge of chemical and physical systems. Electron transfer refers to the movement of an electron from one molecular entity (donor) to another (acceptor). This process can be categorized into two types such as chemical electron transfer, where the transfer of electrons is accompanied by changes in the chemical structure of the donor and acceptor (Beranek, 2011; Bayard et al., 2021). This process is often observed in redox reactions, where the donor loses an electron (oxidation), and the acceptor gains an electron (reduction). Meanwhile, photoinduced electron transfer (PET) refers to a special case of electron transfer that occurs upon absorption of light by a photosensitizer (Beranek, 2011; Speirs et al., 2023). In this process, the excited photosensitizer (electron donor) transfers an electron to an acceptor molecule. The PET is essential in various photophysical and photochemical reactions, such as fluorescence quenching and charge separation in photovoltaic devices (Speirs et al., 2023; Buglak et al., 2022; El-Khouly et al., 2004). The photosensitization mechanism and the role of electron energy transfer in photosensitizers, including the generation of ROS and the potential for tuning ROS generation through anionization are shown in Figure 7.

As shown in Figure 7, the photosensitizers undergo excitation upon absorbing a photon, transitioning from the singlet ground state to a higher-energy state. In this regard, the energy is transferred from one molecular entity to another without an actual transfer of electrons. Hence, in photosensitizers, energy transfer occurs through the process of intersystem crossing (Yu et al., 2022). There are two main types of energy transfer, which are Radiative Energy Transfer (Forster Resonance Energy Transfer, (FRET)) and Non-Radiative Energy Transfer (Dexter Energy Transfer (DET)) (Calçada et al., 2019; Enhanced Photodynamic et al., 2022; van der Meer, 2013; Tavakkoli Yaraki et al., 2022). The FRET is an Radiative Energy Transfer process that occurs between two molecules (usually chromophores) nearby. When the energy levels of the donor and acceptor molecules are appropriately matched, energy can be transferred from the excited donor to the acceptor (Calçada et al., 2019; van der Meer, 2013). FRET is widely used in applications like fluorescence microscopy, molecular biology, and energy transfer-based sensors. While the dexter energy transfer is a non-radiative process, in which energy is transferred between molecules through electron-electron interactions without the emission of a photon (Chou and Dennis, 2015), (Sekar and Periasamy, 2003). As a result, this process is more distance-dependent than FRET and typically requires closer proximity between donor and acceptor molecules.

The rate and efficiency of both electron and energy transfer processes depends on various factors, such as the degree to which the electronic states of the donor and acceptor molecules overlap the distance between donor and acceptor, the solvent polarity, temperature, and surrounding conditions. The quantum yield represents the probability of a transfer event occurring and is a measure of the efficiency of the process (Sekar and Periasamy, 2003), (DeVine et al., 2015; Kubas, 2021; Roduner and Krüger, 2022). Electron and energy transfer processes are essential in many applications, such as light-harvesting systems in organic photovoltaic cells and solar panels, photoluminescence and fluorescent labelling techniques, sensors and probes for detecting molecular interactions and electron transport in biological systems, including respiration and photosynthesis (DeVine et al., 2015; Kaushal et al., 2023; Wang and Lu, 2022; Chen et al., 2015). Therefore, understanding and controlling electron and energy transfer processes have led to numerous technological advancements and continue to be areas of active research in chemistry, physics, and biology. These processes are crucial for the development of sustainable energy technologies, improved sensors and imaging techniques, and a deeper understanding of biological and chemical systems.

Thiazine dyes are a class of synthetic organic dyes that contain a thiazine ring structure as shown in Figure 1. These dyes are commonly used in various industrial applications, including textile dyeing, paper colouring, and as biological stains. The toxicity of thiazine dyes in water can vary depending on the specific dye and its chemical composition (Handbook of Green and Sustainable Nanotechnology, 2023; Rani and Shanker, 2023; Abilaji et al., 2023). In general, some thiazine dyes may pose environmental and health risks due to their potential toxicity. Many synthetic dyes, including MB dyes, are known to be persistent in the environment and may be resistant to biodegradation. Consequently, they can accumulate in water bodies, leading to potential adverse effects on aquatic ecosystems (Selvaraj et al., 2021; Zosenko et al., 2022). Scheme 2 outlines general considerations regarding the potential toxic effects of thiazine dyes.

To assess the toxicity of a specific thiazine dye, it is necessary to consider factors such as its chemical structure, concentration in water, and the sensitivity of the organisms present in the aquatic environment. Toxicity studies and risk assessments are typically conducted to evaluate the potential harm of specific chemicals, including dyes, to aquatic organisms and human health (Oladoye et al., 2022; Khan et al., 2023). It is important to note that regulations and guidelines exist to limit the release of certain dyes into water bodies to mitigate potential environmental and health risk.

Thiazine dyes, particularly exemplified by methylene blue, are recognized as versatile organic dyes with well-established applications in various scientific and technological domains. Their significance extends across chemical, biological, and medical applications, making them indispensable in these fields. In the realm of wastewater treatment, thiazine dyes such as methylene blue play a crucial role in addressing environmental concerns (Paul and Suresh Kumar, 2013). Notably, thiazine dyes exhibit unique properties, including solva-tochromism and metachrosis, making them highly sensitive to changes in their surroundings. Additionally, their high singlet oxygen quantum yields contribute to their efficacy in specific applications. In the context of wastewater treatment, these dyes can be strategically modified to create derivatives with enhanced functionality, allowing for the introduction of new substituents at the nitrogen or CH groups (Montalti et al., 2006; Talman and Atun, 2006). As such, thiazine dyes, like many other synthetic dyes, have both advantages and disadvantages in various applications, including their use in water-based systems. Scheme 3 outlines of some potential advantages and disadvantages of thiazine dyes in water.

The majority of dye producers and consumers, especially those in the textile industry, discharge large amounts of wastewater that contains dye in amounts between 0.001 and 0.7% w/v (see Figure 8) (Langhals and Zollinger, 2004). A review of papers on textile industry wastewater treatment for the removal of different dyes revealed that advanced oxidation processes, such as photocatalysis and Fenton reactions, showed the highest colour removal efficiency (GilPavas et al., 2018; Rashid et al., 2021). However, the research did not clearly demonstrate the impact of green technologies on energy consumption, carbon footprint, and waste generation. New technologies need to be developed and evaluated in a sustainable context with real wastewater. The denim textile industry generates wastewater with persistent pollutants, which can be toxic and carcinogenic. Wastewater treatment is important to reduce risks to aquatic life and public health (GilPavas et al., 2018; Rashid et al., 2021; Kishor et al., 2021).

As shown in Figure 8, the textile dyeing and finishing processes involve the extensive use of dyes, and the wastewater generated during these processes often contains significant amounts of dye residues. The most widely used thiazine dyes i.e., MB is applied in dye silk, cotton, and wood. The wastewater containing dyes is characterized by elevated alkalinity, low biodegradability, total dissolved solids (TDS), chemical oxygen demand (COD), and biochemical oxygen demand (BOD) (Srinivasan and Viraraghavan, 2010). Since dye is colourful, it is simple to detect its presence in water bodies, and it easily blocks sunlight and eventually the water bodies ability to reoxygenate thereby interfering with aquatic life's biological processes. Industrial dye effluent can be colored-free using a variety of techniques. However, due to the wide variety of dyes on the market and the presence of additional chemicals in industrial effluent, many treatment techniques may not be effective when used alone and may need to be combined with other techniques to remove most of the colouring. There are three types of dye wastewater treatment methods viz., chemical, biological, and physical treatments (Crini and Lichtfouse, 2019; Hung et al., 2017; Hung et al., 2020). When not properly treated, this wastewater can be discharged into rivers and other water bodies, causing environmental pollution thiazine dye derivatives, when employed as photosensitizers in PDT processes, offer several advantages. These include high selectivity, resulting in reduced toxicity to healthy tissue, improved cosmetic outcomes, dual functionality during application, a lower risk of infection, absence of organ damage, diminished likelihood of inducing resistance, and minimal side effects such as infertility. Moreover, thiazine photosensitizers in PDT hold promise for combination therapies due to their unique method of action, which makes them resistant to many common chemotherapeutic resistance mechanisms (Callaghan and Senge, 2018; Senge and Radomski, 2013; Spring et al., 2015). Despite these strengths, PDT faces challenges in becoming the primary treatment choice for certain tumours, primarily due to limitations in tissue penetration, light dosage management, and photosensitivity. Nevertheless, PDT remains a clinically useful therapy, and there is a pressing need for innovative approaches to enhance existing therapies. This involves accelerating the development of the third or fourth generation of thiazine photosensitizers to overcome current limitations and potentially revolutionize treatment modalities in relevant fields. Efforts should focus on refining existing methodologies, exploring novel techniques, and fostering collaboration between disciplines to drive transformative advancements in the application of thiazine dyes, such as methylene blue, in wastewater treatment and related areas (Spring et al., 2015).

Thiazine dyes, such as phenothiazine dyes, have limitations that restrict their further optimization for high-efficiency DSSCs (Buene and Almenningen, 2023). However, recent studies have shown the importance of these dyes in achieving higher photovoltaic conversion efficiency (Han et al., 2023). For example, N-phenylphenothiazine dyes with different π-bridges have demonstrated improved J_SC and VOC, leading to higher overall efficiency in DSSCs (Hirakawa and Mori, 2021). Additionally, the inclusion of auxiliary acceptors, such as benzothiadiazole, in phenothiazine-based dyes has been shown to broaden their spectral response range and improve device performance (Li et al., 2021). These findings suggest that thiazine dyes can still play a role in sustainable environmental solutions, particularly in the development of highly efficient DSSCs (Wang et al., 2022). However, it is important to consider the potential secondary pollution caused by these dyes and explore alternative options for sustainable environmental solutions. Thiazine dyes have garnered considerable attention for their unique properties, making them promising candidates in various aspects of solar energy conversion. From enhancing light absorption to serving as sensitizers in DSSCs, thiazine dyes play a crucial role in advancing the efficiency and versatility of solar cell technologies (Buene and Almenningen, 2023; Han et al., 2023; Li et al., 2021). Table 3 provides an overview of key applications, showcasing the potential of thiazine dyes across different solar cell architectures and technologies.

As shown on the table, different types of dye sensitizers used in various photovoltaic solar cell devices underscores the diverse strategies employed to harness solar energy for electricity generation. Organic dyes, inorganic dyes, and perovskite materials represent distinct categories of sensitizers, each with unique advantages and challenges (Li et al., 2022; Rani and Shanker, 2023; Takahashi et al., 2023). Organic dyes, characterized by their tunable chemical structures, offer versatility and ease of modification. They have been extensively used in DSSCs, demonstrating good light absorption and electron injection capabilities. However, challenges related to stability and limited absorption in the red and near-infrared regions have prompted exploration into alternative sensitizers (Liu et al., 2023; Li et al., 2021; Lee et al., 2017).

Inorganic dyes, particularly metal complexes like ruthenium-based compounds, exhibit remarkable photostability and extended absorption spectra (Roy et al., 2023). Their utilization in DSSCs has shown significant efficiency improvements, yet concerns regarding the scarcity and cost of some metal components stimulate ongoing research for more sustainable alternatives. Perovskite materials have emerged as a groundbreaking class of sensitizers, demonstrating rapid advancements in photovoltaic technology (Takahashi et al., 2023). Perovskite solar cells exhibit high efficiency, low-cost fabrication, and a broad absorption spectrum. However, issues related to stability, toxicity, and scalability remain focal points of research efforts.

The TiO₂ structure, morphology, and particle size are known to have a significant impact on its optical properties and, consequently, its photoactivity. There are several methods for achieving titanium dioxide photosensitizers, including surface modification using the right species (Moma and Baloyi, 2018). A visible-light-induced electron or hole injection into the conduction or valence band, respectively, is necessary for the photosensitization process (Hamza et al., 2023; Moma and Baloyi, 2018; Dlamini et al., 2021). The photosensitizer moiety's (surface complex) electrical interaction with the TiO₂ particle determines the process's efficiency. Quantum yields of photoinduced electron transfer processes are also impacted by surface modification of TiO₂ (Hamza et al., 2023; Elmorsy et al., 2023; Monika et al., 2023). Specifically, TiO₂ photosensitization attracts a lot of interest because this material is used as a photoactive material in photovoltaics, optoelectronics, and photocatalysis (Hagfeldt and Grätzel, 2000; Moser et al., 1998; Szaciłowski et al., 2005).

At the surface of TiO₂, several Ti(IV) complexes are generated in situ. There may be correlations between the application of TiO₂ phonium and the structures of Ti(IV) complexes, particularly those involving oxygen-ligands (Hug and Bahnemann, 2006). In addition, there are bidentate bridging structures made up of chelating ligand binding two nearby Ti(IV) centres, which is the bidentate chelating structure with the ligand occupying two coordination sites. One oxo ligand must be substituted for other titanium centers coupled with five ligands to create a bidentate chelating complex, although monodentate or bridging complexes can be formed by any of the five surfaces Ti(IV) sites. Meanwhile, the monodentate complexes with coordination between aliphatic alcohols and the surface of titanium dioxide is extensively researched, primarily due to its significant impact on photocatalytic reactions that take place when these molecules are present. The hydroxyl group on the surface of TiO₂ facilitates the physical (molecular) and chemical adsorption of simple aliphatic alcohols, including methanol, ethanol, and 1- and 2-propanol (Kim et al., 1988; Lusvardi et al., 1995). On the other hand, both phenol and 4-chlorophenol bind to the TiO₂ surface with a moderate affinity (Lusvardi et al., 1995; Al-ekabi et al., 1989). The complexes that are produced exhibit poor stability constants. The incapacity of phenol and 4-chlorophenol to form ring-structured surface complexes could be the cause of the observed weak chemisorption.

As summarised in Table 1, bidentate complexes are formed only when the ligand has two donor group photosensitizers or one group with two donor atoms. Physisorption and the monodentate complexation mode are not excluded by the potential for bidentate structures to be formed. Polynuclear complexes results through anchoring of the group photosensitizers, in which different organometallic complexes of transition metals may be attached to the titanium surface centers. In this regards, the carboxyl group photosensitizers can act as anchors leading to the formation of a polynuclear complex with bridging ligands that can coordinate with Ti(IV) and other metal ions. There are many methods used to prepare TiO₂-based material to be used as sensitizers reported in the literature coated on fluorine-doped tin oxide (FTO) glass substrate as thin films. Vinaayak et al. (2022), used a doctor-blade approach to coat TiO₂ nanoparticles on an FTO substrate as a photoanode. Meanwhile, Dhanasekaran and Marimuthu (2023), reported the synthesis of TiO₂ paste from TiO₂ nanopowders, actylacetone, and nitric acid. It was Alaya et al. (2023), who prepared TiO₂ thin films on FTO glass substrates using spray pyrolysis deposition and annealed them at different temperatures for at least 3 hours per thin film. The preparation of these TiO₂ materials was followed by treatment with organic materials such as thiazine dyes, or plant extracts to generate TiO₂-based sensitizers.

Thiazine dyes known as phenothiazines have two benzene rings condensed to the thiazine ring in addition to a central thiazine core. These dyes have lately been explored as photosensitizers due to their biological, photochemical, and photophysical properties (Padnya et al., 2023) with an emphasis on the inactivation of bacterial, viral, and parasite strains (Vara and Ortiz, 2016; Almeida et al., 2020; Shen et al., 2020). Methylene blue and toluidine blue O are two of the most well-known phenothiazines. Methylene blue has been used in clinical settings to treat bacterial infections, including dental conditions, and has shown to be safe, effective, and aesthetically pleasing (Shen et al., 2020; Theodoro et al., 2021). Moreover, methylene blue derivatives, or 3,7-disubstituted dyes of the phenothiazine nucleus, have been produced and functionalized at these sites with a variety of amines for further research into photobiological and optoelectronic uses (Lusvardi et al., 1995). Some compounds derived from phenothiazines are still developed within the purview of their application in cancer PDT, albeit being less prevalent than their antibacterial action. One of the most appealing instances happens when the conjugation of methylene blue with camptothecin via an activatable linker containing a disulfide bond that is prone to breakage by GHS (Yang et al., 2022). When administered in vivo, no physiological toxicity was observed, indicating safety and biocompatibility.

Thiazine-based photosensitizers hold significant promise for innovative and effective wastewater treatment strategies. These photosensitizers, derived from thiazine dyes like methylene blue, exhibit unique characteristics that can be harnessed for environmental remediation purposes (Vara and Ortiz, 2016; Bindhu et al., 2022). As such, thiazine dyes are well known for their versatility and well-established applications in various scientific fields. When applied as photosensitizers in wastewater treatment processes, these compounds can demonstrate exceptional photodynamic properties (Bindhu et al., 2022). The key attributes that make thiazine and methylene blue-based photosensitizers advantageous in this context include high sensitivity to environmental changes, which thiazine dyes, with their solva-tochromism and metachrosis properties, become highly responsive to alterations in the surrounding environment. This sensitivity is crucial for detecting and targeting contaminants in wastewater (Luo et al., 2021; Acar et al., 2015; Bindhu et al., 2022). The high singlet oxygen quantum yields of methylene blue, contribute to their efficacy in inducing photodynamic reactions. This property is particularly beneficial for the degradation of organic pollutants in wastewater.

In this regard, the thiazine-based photosensitizers can also be tailored to selectively target specific contaminants in wastewater (Thandu et al., 2015). This selectivity minimizes the impact on non-target organisms and reduces the overall environmental toxicity of the treatment process. As such, the use of thiazine and methylene blue-based photosensitizers ensures low toxicity in water, which is a critical factor for sustainable and environmentally friendly wastewater treatment practices (Kenry and Liu, 2022; Thandu et al., 2015). They also offer the potential for combination therapies, allowing for synergistic approaches to enhance wastewater treatment effectiveness. This can involve coupling photodynamic processes with other treatment methods to address a broader range of contaminants. The unique method of action of thiazine photosensitizers makes them less prone to common chemotherapeutic resistance mechanisms (Kenry and Liu, 2022; Barakat et al., 2022; Thandu et al., 2015). This characteristic enhances their reliability and effectiveness in the treatment of wastewater. While thiazine-based photosensitizers present these advantages, it is essential to acknowledge the existing challenges in wastewater treatment, such as the need for optimized dosages, penetration depth, and potential photosensitivity issues (Barakat et al., 2022; Anjum et al., 2019). Ongoing research and development efforts should focus on refining the application methodologies, exploring novel delivery systems, and addressing these challenges to maximize the potential of thiazine-based photosensitizers in wastewater treatment technologies. Through harnessing the unique properties of these compounds, we can advance towards more sustainable and efficient approaches for treating wastewater and safeguarding our environment (Barakat et al., 2022). This led to the conclusion thiazine dye-sensitized TiO₂ could overcome the limits of both camptothecin and methylene blue in terms of their uniqueness.