English

• 1. Introduction

  According to the World Health Organization, over 2.1 billion people globally lack access to clean, safe, and hygienic water [1]. Meeting the demand for clean water for human metabolism, daily life, and hygiene remains a significant challenge [2,3]. The proliferation of pathogenic microorganisms, including bacteria and viruses, has exacerbated this issue by facilitating the development of drug-resistant strains, highlighting the urgent need for integrated management strategies to control harmful microorganisms [4].

  Current disinfection methods for the aquatic environment include physical methods such as slow and fast sand filtration [5], heating and boiling [6], diffusion and reverse osmosis [7], as well as chemical methods such as chlorination and hydration [8]. Early studies have shown that slow and rapid sand filtration are simple to operate, reduce turbidity and pathogen counts, but have limited antimicrobial capacity, are generally effective at sterilising water, and tend to clog reactors [5]. Heating and boiling, although effective and widely used, are 2 methods that require significant additional energy supply, such as chemical fuel combustion or electrical heating input, and are not suitable for large-scale water disinfection [9]. Large-scale disinfection of water by chemical methods such as chlorination and ozone can strongly kill disease-causing microorganisms. However, strong oxidants are produced during the disinfection process and the disinfectants further react with natural organic matter (NOM) to produce foul odours and harmful disinfection by-products (DBPs) [10]. While these approaches offer broad-spectrum antimicrobial effects, they are often limited by various drawbacks, particularly when treating water contaminated with organic matter and pathogenic microorganisms [11]. Photothermal water disinfection emerges as a promising alternative, utilizing highly light-absorbing materials to generate heat and reactive oxygen species (ROS) through solar energy. This process enables effective heating or boiling of water across a range of light wavelengths. Under strong light irradiation, photothermal materials use solar energy to generate localized high temperatures or various reactive oxygen. The transfer of heat inactivates bacteria without producing DBPs, to achieve efficient antibacterial disinfection [12]. Consequently, solar energy decontamination using sunlight is more sustainable and economically viable than conventional methods.

  Photothermal water disinfection antimicrobials hold substantial potential for research, development, and practical application in water treatment. In recent years, a variety of photothermal composites have been used to exert antimicrobial effects through synergistic mechanisms, and different types of composites with different antimicrobial mechanisms have been developed for applications in water disinfection. For example, a spectrally tailored aerogel, D-HNb₃O₈/PAM, selectively exploits the entire solar spectrum to achieve synergistic conversion of integrated photocatalysis and photothermal activation [13,14]. In addition, photothermal materials at this stage face many challenges in achieving the goals of high light absorption, high photothermal conversion efficiency and low heat loss, which should be improved and enhanced in terms of functionality and adaptability in the future. In addressing the challenges, further development of antimicrobial water disinfection is promoted by reducing the use of toxic reagents and heavy metal ions.

  With continuous advances in water treatment technology and materials design research, many advanced photothermal materials are being developed to promote environmental protection and sustainable development [15,16]. The core of this overview focuses on the antimicrobial mechanisms and material types of photothermal composites. In addition, the categories of applications of photothermal materials involved in water disinfection, the challenges faced at this stage and future research directions are discussed to provide theoretical basis and guidance for further design of efficient antimicrobial disinfection photothermal materials (Fig. 1) [17–25].

  Figure 1

  

  Figure 1.  Recent advances in photothermal water disinfection. Reprinted with permission [17]. Copyright 2019, American Chemical Society. Reprinted with permission [18]. Copyright 2023, American Chemical Society. Reprinted with permission [19]. Copyright 2021, The Royal Society of Chemistry. Reprinted with permission [20]. Copyright 2020, American Chemical Society. Reprinted with permission [21]. Copyright 2018, The Royal Society of Chemistry. Reprinted with permission [22]. Copyright 2020, The Royal Society of Chemistry. Reprinted with permission [23]. Copyright 2019, Elsevier. Reprinted with permission [24]. Copyright 2021, American Chemical Society. Reprinted with permission [25]. Copyright 2022, The Royal Society of Chemistry.

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  2. Mechanisms of photothermal water disinfection

  Given the importance of photothermal materials in water disinfection, understanding the underlying conversion processes is crucial. Photothermal conversion processes typically involve localized heating of protons, nonradiative relaxation of electron-hole pairs, and thermal vibrations of molecules [18]. Unlike other energy conversion processes, photothermal conversion lacks the separation or release of electrons, the production of additional photons, or the process of elevation of the electronic energy state. Instead, the absorbed solar energy is dissipated as thermal or kinetic energy. Therefore, the application of photothermal materials in the water environment can elevate the water temperature serving as an effective disinfectant and antimicrobial agent [26]. The efficiency of this conversion process is pivotal, as it determines the effectiveness of the disinfection method. The photothermal conversion efficiency is shown in the following equation (Eq. 1) [27]:

  $ \eta=\frac{h S \Delta T_{\max }-Q_\mathrm{s}}{I\left(I-10^{-A}\right)} $

  (1)

  where h is heat-transfer coefficient, S is surface area exposed to light, ΔT_max is temperature difference of the solution, Q_s is the intensity of the incident light, I is the laser power, A is the absorbance of the solution at NIR irradiation.

  Introducing the parameter τ to calculate hS using the following equation (Eq. 2):

  $ \tau=\frac{m C_{\mathrm{p}}}{h S} $

  (2)

  where m is the mass, C_p is heat capacity of the solvent.

  The equation for establishing a connection between T and τ is (Eq. 3):

  $ T=-\tau \ln (\theta)=\tau \ln T_{\max }-T_{\mathrm{amb}} $

  (3)

  where T_amb is the room temperature, T_max is the maximum temperature during irradiation.

  Applying this formulation to practical scenarios enables the evaluation and optimization of the performance of photothermal materials in aqueous environments. The efficiency of photothermal conversion has been quantified, highlighting the potential of these materials in water disinfection. To elucidate the comprehensive impact of photothermal materials on microbial inactivation, a detailed examination of the physical, chemical, and synergistic mechanisms is warranted.

  2.1 Physical mechanism

  The physical mechanism of photothermal antimicrobial materials is primarily driven by photothermal conversion. These materials absorb solar energy and directly converts it into heat, resulting in a rapid increase in water temperature. This temperature elevation not only disrupts the integrity of bacterial cell membranes but also effectively achieves antibacterial disinfection. The elevated water temperature further inhibits bacterial activity by altering the function of proteins, nucleic acids, cell membranes, and cell walls, thereby complementing the direct thermal impact.

  The photothermal mechanism is divided into three processes: photoelectron excitation, relaxation and thermal diffusion. The plasma decay mechanism exemplifies the photothermal effect, which is attributed to lattice vibrations within the nanostructure of plasma materials, and they are excited to higher energy levels at the Fermi energy level to form a thermal electron cloud [28,29]. In contrast, semiconductor nanomaterials produce electron-hole pairs above the band gap. For carbon-based nanomaterials, loose electrons tend to form electron leaps (Fig. 2a) [26]. Where the relaxation process means that the excited electrons will eventually return to a lower state and energy will be transferred from the electrons to the lattice phonons, resulting in a higher local temperature. The absorbed light is rapidly converted into heat through a series of non-radiative processes and then transferred to the surroundings. Localized surface plasmon resonance (LSPR) within nanostructured materials is a key enhancer of the photothermal effect. It facilitates the generation of heat through the amplification of electron motion and increased collisions with lattice atoms (Fig. 2b) [30]. The LSPR effect is characterized by its strong and broad absorption spectrum, which is essential for the efficient conversion of light to heat. The presence of oxygen vacancies on the material surface further augments this absorption, with these surface defects significantly influencing the material's light absorption properties (Fig. 2c) [31,32].

  Figure 2

  

  Figure 2.  Generalization of the photothermal water disinfection mechanism. (a) Photothermal effects of different mechanisms with corresponding light absorption ranges. Reprinted with permission [26]. Copyright 2019, The Royal Society of Chemistry. (b) Schematic diagram of the photothermal conversion process. Reprinted with permission [30]. Copyright 2017, Elsevier. (c) Schematic diagram of the photothermal conversion mechanism. Reprinted with permission [32]. Copyright 2021, Wiley. (d) Synergistic killing of bacteria by ROS and photothermal effect under 660 nm light irradiation. Reprinted with permission [41]. Copyright 2021, Elsevier. (e) Conceptual diagram of farm to tap water treatment plant water treatment. Photosensitizers and dispersants are extracted from the plant by boiling or other simple methods and added to the water, which is placed in sunlight for disinfection. Reprinted with permission [36]. Copyright 2020, American Chemical Society.

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  2.2 Chemical mechanism

  The chemical mechanism of photothermal antimicrobial materials is predominantly mediated by the generation of ROS, which are instrumental in the disruption of cellular integrity. Castro-Alférez et al. [33] have shown that endogenous chromophores catalyze intracellular ROS production during solar water disinfection (SODIS), highlighting the significance of ROS in bacterial inhibition. The electronic reduction of oxygen leads to the formation of various ROS, including ¹O₂, ^•O₂⁻, ^•OH, and H₂O₂, with the following schematic and chemical formulas (Eqs. 4–9) [34]:

  $ \text { Material }+ \text { sunlight } \rightarrow \mathrm{h}^{+}+\mathrm{e}^{-} $

  (4)

  $ \mathrm{O}_2+\mathrm{e}^{-} \rightarrow{ }^{\cdot} \mathrm{O}_2^{-} $

  (5)

  $ ^ \cdot {\rm{O}}_2^ - + {{\rm{h}}^ + }{ \to ^1}{{\rm{O}}_2} $

  (6)

  $ ^ \cdot \mathrm{O}_2^{-}+\mathrm{e}^{-}+2 \mathrm{H}^{+} \rightarrow \mathrm{H}_2 \mathrm{O}_2 $

  (7)

  $\mathrm{H}_2 \mathrm{O}_2+h v \rightarrow 2 ^\cdot \mathrm{OH} $

  (8)

  $ { }^1 \mathrm{O}_2 /^ \cdot \mathrm{O}_2^{-} / ^ \cdot \mathrm{OH} / \mathrm{H}_2 \mathrm{O}_2+\text { bacteria } \rightarrow \text { cellular debris } $

  (9)

  Among these ROS, singlet oxygen (¹O₂) is particularly effective due to its low oxidation potential and longer lifetime, making it well-suited for disinfecting complex aqueous environments. While the hydroxyl radical (^•OH) is known for its high reactivity, the superoxide anion (^•O₂⁻), with its longer half-life, is more readily accumulated and exhibits significant antimicrobial activity. This highlights the importance of developing robust methods for ROS detection in wastewater treatment [35,36].

  Furthermore, photothermal disinfection affects cellular gene expression, as evidenced by Hong et al. [37], who found that it results in higher ROS yields compared to other treatments. This leads to significant gene up-regulation post-heating and photothermal treatment, contrasting with the down-regulation under solar radiation. The photothermal process may disrupt the expression of genes involved in ROS metabolism, such as catalase, causing an accumulation of ROS that damages the cell membrane and RNA, ultimately leading to cell death and effective disinfection.

  2.3 Synergistic mechanism

  Photothermal materials exert their antimicrobial effects through synergistic mechanisms that surpass individual approaches. The photothermal-photocatalytic synergy, which combines the effects of heat and reactive oxygen species (ROS), is particularly effective, offering a promising solution without adverse side effects [38,39]. Within the light absorption range and intensity, through multiple scattering and reflection, the material increases carrier photogeneration and enhances photothermal conversion through enhanced light trapping. On the one hand, the photothermal effect occurs to increase the carrier mobility, and on the other hand, the photothermal effect of the material contributes to the heating of the composite and increases the temperature during the photocatalytic process. Luo et al. [40] found that direct and indirect leaps coexist in r-CuS/g-C₃N₄ under LUV–vis illumination. The direct jump is almost independent of temperature, while the indirect jump is much smaller than the direct jump, and the photothermal effect of CuS may contribute slightly to promoting the photoexcitation efficiency, thus bringing a slight enhancement to the photocatalytic reaction. This method capitalizes on solar energy, minimizing energy consumption and reducing reliance on chemical reagents (Fig. 2d) [41].

  Photocatalysts, such as ZnO, TiO₂, MoO₃, CdS, and g-C₃N₄, have been developed for their near-infrared absorption and antimicrobial properties. Their combination with photothermal materials enhances the overall synergistic effect. Zhang et al. [42] demonstrated that Ni/reduced graphene oxide (Ni/rGO) nanomaterials, under 808 nm near-infrared irradiation, achieved a 35.78% photothermal conversion efficiency and over 99% inhibition rates against Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis).

  Photosensitizers also play a crucial role, generating ROS under light to facilitate disinfection, with natural photosensitizers being a safe and environmentally friendly option. Ryberg et al. [36] showed that plant-derived photosensitizers and dispersants can significantly enhance the inactivation of the MS2 phage when exposed to sunlight, offering a viable water disinfection method for low-income countries (Fig. 2e).

  3. Types of photothermal materials

  The evolution in photothermal materials for water disinfection reflects a transition from simple monomaterials to complex composites, all aimed at enhancing light absorption and photothermal efficiency. This discussion focuses on the properties and applications of these materials, from monomaterials such as gold (Au), silver (Ag), and carbon-based materials, to composites that integrate various components to boost their antimicrobial effectiveness. The analysis highlights the current effectiveness of these materials and foreshadows their potential in advancing environmental sanitation technologies.

  3.1 Monomaterials

  Monomer materials such as Au, Ag, Cu, Fe₃O₄, MoS₂, carbon nanotubes (CNTs), and red phosphorus (RP) have been extensively studied and reported for their use in photothermal antimicrobial water disinfection applications. For example, Au is recognized for its exceptional structural controllability, chemical stability, high photothermal conversion efficiency and LSPR effect, making it a high-performance photothermal material. The photothermal antimicrobial activity of a single material is closely related to its morphology, structure, particle size and environment. However, ongoing research has led to development of modified monomer materials, which often results in composites more excellent in photothermal conversion performance and antimicrobial performance. These composites not only improve photothermal properties but also expand the range of potential applications, thereby broadening the utility of photothermal materials in water disinfection.

  3.2 Composites

  The evolution to composite photothermal materials represents a leap forward in antimicrobial water disinfection, capitalizing on the inherent advantages of monomaterials while introducing synergistic enhancements. These composites harness the light absorption capabilities of metallic nanoparticles through surface plasmon resonance, particularly in the visible and near-infrared regions, and the size-dependent quantum confinement effects of semiconductor nanocrystals. The integration of these components results in an expanded light absorption spectrum and improved photothermal response [43–46]. Furthermore, the interfaces within composites, such as those at metal-semiconductor junctions, create conditions that enhance charge transfer and absorption efficiency [47,48]. This leads to a more effective conversion of light energy into heat via non-radiative processes, highlighting the aptness of composites for high-efficiency photothermal applications. The transition from monomaterials to composites indicates a significant advancement, promising an expanded repertoire of applications for photothermal materials in the realm of environmental sanitation and water disinfection.

  3.2.1   Metal-based materials

  Metal matrix composites, prevalent in the realm of photothermal disinfection, have been ingeniously tailored to harness the synergies of photothermal effects for potent antimicrobial action. A case in point is the Au@Bi₂WO₆ composite material, engineered by Li et al. [49]. This material, a strategic assembly of gold nanoparticles and Bi₂WO₆, exhibits a marked enhancement in photothermal properties and antimicrobial efficacy. Upon exposure to sunlight for 15 min, it achieves remarkable antimicrobial rates against E. coli and Staphylococcus aureus (S. aureus), reaching 99.93% and 99.87%, respectively. The synergistic effect of high temperature and high free radical yield disrupts cell membranes faster and eradicates bacteria effectively. Li et al. [22] developed a Bi@Bi₂Se₃ nanoparticle (NP) with small size and high stability to enhance the photothermal performance of Bi interleaved with Bi₂Se₃ energy levels. By utilizing the properties of Bi and the high stability of Bi₂Se₃, improvement in the photothermal conversion efficiency was achieved (Figs. 3a–c).

  Figure 3

  

  Figure 3.  (a) Schematic synthesis and mechanism of Bi@Bi₂Se₃ NPs with enhanced photothermal properties. (b) TEM image of Bi@Bi₂Se₃ NPs. (c) Temperature rises curves of Bi, Bi@Bi₂Se₃ and Bi@Bi₂Se₃ NPs under 808 nm laser irradiation. (a-c) Reproduced with permission [22]. Copyright 2020, The Royal Society of Chemistry. (d) Mechanism of inactivation of E. coli by Ag/MnO₂ PHMs under solar irradiation. (e) SEM image of 0.3% Ag/MnO₂ PHMs. (f) TEM image of 0.3% Ag/MnO₂ PHMs. (d-f) Reproduced with permission [23]. Copyright 2019, Elsevier. (g) SEM image of Fe₃O₄@PB NPs. (h) Schematic of Fe₃O₄@PB NPs for recyclable photothermal water sterilization under primary solar irradiation. (i) Photographs and (j) relative viability of total colonies in contaminated tap water treated with different concentrations of Fe₃O₄@PB dispersions and irradiated by the sun for 15 min. (g-j) Reproduced with permission [50]. Copyright 2019, Elsevier.

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  3.2.2   Metal oxide-based materials

  Researchers, addressing the challenges of metal photothermal materials, have pivoted towards metal compound-based photothermal materials like Fe₃O₄, MnO₂, and CuO, which offer high chemical stability, simplified processing, and cost-effectiveness, along with potent antimicrobial properties. Xia et al. [23] synthesized Ag/MnO₂ PHMs with enhanced antimicrobial effects due to the LSPR effect of Ag NPs, demonstrating the superiority over unmodified MnO₂ PHMs under sunlight. Furthermore, copper oxide nanomaterials have good stability and excellent antimicrobial activity (Figs. 3d–f). Further, Jiang et al. [50] successfully developed Fe₃O₄@PB NPs composites with strong magnetic properties, achieving 100% inhibition of E. coli and S. aureus under solar irradiation, highlighting their potential in antimicrobial applications (Figs. 3g–j). Additionally, Fang et al. [51] reported a CuO/ZnO composite with a 30% ZnO composition that reached a photothermal conversion efficiency of 97.35% under optimal conditions.

  3.2.3   Carbon-based material

  Carbon-based photothermal materials, recognized for their broad-spectrum absorption and economic viability, have emerged as a promising alternative to metal-based counterparts. They are categorized into zero-dimensional (e.g., carbon quantum dots and carbon black), one-dimensional (e.g., carbon nanotubes and fibers), and two-dimensional materials (e.g., graphene and MXene). Presently, these materials are widely used.

  Loeb et al. [52] reported that carbon black nanoparticles (CB NPs) outperformed gold nanoparticles (Au NPs) in photothermal conversion efficiency under solar irradiation, despite an initial lower antimicrobial effect. The antimicrobial efficacy of Au/CB composites surpassed that of individual components after extended irradiation periods, highlighting the potential of composites in photothermal disinfection. Cao et al. [53] developed MXene/PVA/HA composite aerogels that demonstrated remarkable antimicrobial activity and a photothermal conversion efficiency of 61% under solar irradiation (Figs. 4a–d). Zhang et al. [54] utilized chemical vapor deposition (CVD) to fabricate Ti-RP/GO membranes, which, under sunlight irradiation, significantly enhanced ROS generation and photothermal conversion efficiency, achieving nearly complete antimicrobial activity against S. aureus and E. coli within 20 min. Further functionalization of MXene materials through surface modification or hybridization with other materials have shown great deal to improve the photothermal conversion efficiency, chemical stability, and antimicrobial and antioxidant activities of MXene (Figs. 4e–j).

  Figure 4

  

  Figure 4.  (a) Schematic of the synthesis of MXene/PVA/HA aerogel. SEM images of the top surface of the MXene composite aerogel at (b) low and (c) high magnification. (d1) Schematic showing the accumulation of E. coli on the surface of the FTCS-MXene /PVA/HA aerogel after exposure to E. coli contaminated medium for 30 min. FTCS-MXene /PVA/HA aerogel (d4) under open sunlight and (d7) Schematic representation of the antifouling properties after polarization with a water thickness of 5 mm. E. coli fluorescence images of FTCS-MXene / PVA/HA aerogels (d2 and d3) in the pristine state, (d5 and d6) under open-air solar irradiation for 10 min, and (d8 and d9) after potential polarization for 30 min. (a-d) Reprinted with permission [53]. Copyright 2023, The Royal Society of Chemistry. (e) SEM images of Ti-RP/GO. (f) Raman spectra of Ti-RP, Ti-GO, and Ti-RP/GO samples. (g) Schematic diagram of the RP/GO membrane synthesis process. (h) Fluorescence measurement of S. aureus biofilm after LED light irradiation. Green color represents live bacteria and red color represents dead bacteria. (i) Schematic diagram of the antibacterial test for Ti-RP/GO light-driven inactivation. (j) Schematic of Ti-RP/GO biofilm dispersion. (e-j) Reproduced with permission [54]. Copyright 2020, Elsevier.

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  3.2.4   Phosphorus-based materials

  Phosphorus-based materials, while less explored than other photothermal materials, are gaining recognition for their semiconductor properties and light-absorbing capabilities, which are instrumental in antimicrobial applications. Red phosphorus (RP) stands out for its cost-effectiveness and practicality, making it a promising candidate for large-scale applications [35,36]. In a significant advancement, black phosphorus (BP), another member of the phosphorus family, has demonstrated even more pronounced photothermal properties. Zhang et al. [55] reported the enhancement of BP nanosheets through the integration with quaternary ammonium chitosan (QCS) via electrostatic adsorption, yielding a BP-QCS composite. This composite exhibited an impressive photothermal conversion efficiency of 55.7% when subjected to near-infrared irradiation at 808 nm and 1 W/cm². As an antimicrobial agent under these irradiation conditions for a duration of 10 min, the BP-QCS composite realized over 95% bacterial inactivation against S. aureus and E. coli. These findings not only highlight the effectiveness of phosphorus-based materials in photothermal disinfection but also illustrate the progression from RP to BP, showcasing the breadth of potential within this class of materials.

  The realm of photothermal antimicrobials is burgeoning, with materials like gold nanoparticles and metal oxides showcasing efficacy in thermally neutralizing microbes. Composites and phosphorus-based semiconductors further augment these capabilities, offering economic and environmental benefits. Anticipated advancements in nanotechnology and eco-friendly material integrations signal a promising trajectory for enhanced, sustainable disinfection solutions with broad implications for global health and ecology.

  4. Types of applications and design of photothermal materials

  Advancing the frontier of photothermal applications, nanostructured materials have made significant inroads in water disinfection. Acting as proficient photo-absorbers, these materials convert solar energy into heat through non-radiative photothermal conversion, proving indispensable in solar-driven water purification systems [56,57]. The localized heat generated is effective in inactivating microorganisms, making these systems particularly effective in sun-rich areas where surfaces or floating structures coated with photothermal materials can significantly raise water temperature to achieve disinfection.

  Portable photothermal water purifiers, designed as compact solar-powered devices, provide practical solutions for rural communities and outdoor users, ensuring a clean water supply during peak demand periods. Moreover, the application of photothermal antimicrobials in municipal and industrial wastewater treatment facilities promises to enhance disinfection efficiency and reduce reliance on chemical disinfectants [58]. In emergencies, such as natural disasters, these devices offer a critical, rapid means to prevent waterborne infections, highlighting their vital role in safeguarding public health. Furthermore, expanding their utility, photothermal antimicrobials are also integral to solar sterilization reactors, which are categorized based on operational mode-continuous flow or batch-and structural design, including fluidized bed, fixed bed, and near-radiation reactors. This diversity underscores the adaptable nature of photothermal technology across a spectrum of sterilization applications.

  4.1 Continuous flow reactor

  In the domain of continuous flow reactors, two principal types are recognized: fixed-bed and radiation reactors. These reactors maintain a relentless flow of water, ensuring a persistent photothermal disinfection process. Fixed-bed reactors utilize an immobilized array of photothermal agents, while radiation reactors are designed to optimize solar irradiation, both facilitating a continuous and efficient water treatment regimen.

  4.1.1   Internal water disinfection fixed-bed reactor

  Internal water disinfection fixed-bed reactor facilitates water disinfection by enabling continuous contact between the solution and photothermal antimicrobial materials. Loeb et al. [59] engineered a continuous flow system incorporating a photothermal composite membrane of Au NR-CTABs within the flow channel, enhancing microorganism-material interactions and antimicrobial potency. This reactor effectively treated 8 liters of drinking water within a 45 cm × 45 cm chamber, achieving temperatures sufficient to surpass the thermal limits of microbes like E. coli and MS2 (Figs. 5a, b, d). However, scalability in low-light scenarios presents a challenge for this reactor design, necessitating the use of optical concentrators such as Fresnel lenses to intensify light exposure in bright conditions.

  Figure 5

  

  Figure 5.  (a) Schematic of a fixed-bed reactor for a continuous flow reactor. (b) Schematic of the design of an experimental-scale fixed-bed reactor. (d) Photograph of a 3D printed fixed-bed reactor in front of a photothermal membrane module showing the planar illuminated area above the water flow path. (a, b, d) Reproduced with permission [59]. Copyright 2019, American Chemical Society. (c) Schematic of a radiative reactor for a continuous flow reactor. (e) Schematic of a typical tube-fin solar absorber. (f, g) Schematic of the experimental setup and experimental site for a radiation reactor with continuous flow reactor. (c, e-g) Reproduced with permission [61]. Copyright 2020, Elsevier. (h) Schematic of a fixed-bed reactor for a batch reactor. (i) Schematic of a fluidized bed reactor for a batch reactor. Reproduced with permission [69]. Copyright 2021, The Royal Society of Chemistry. (j) Schematic of a near-field radiation reactor for a batch reactor. Reproduced with permission [70]. Copyright 2018, Nature.

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  4.1.2   Radiation sterilization reactor

  Radiation sterilization reactors represent an alternative class of photothermal systems, relying on prolonged sunlight exposure to catalyze the production of reactive oxygen species, which degrade microbial proteins. These reactors leverage large-area solar collectors to maximize light absorption and achieve the desired antimicrobial effect [60]. Zhao et al. [61] introduced an innovative radiation reactor design utilizing a conventional tube and fin absorber. At temperatures exceeding 100 ℃, radiative heat transfer predominates, rendering this design independent of extensive solar collectors and tracking systems. This reactor is capable of producing steam at temperatures above 120 ℃, which significantly enhances sterilization efficacy. The radiation reactor's benefits include reduced reliance on light intensity and robust performance across varying weather conditions, underscoring its practical applicability and potential for widespread use (Figs. 5c, 5e-g).

  4.2 Batch reactor

  Batch reactors offer a discrete approach to photothermal antimicrobial treatments, intermittently processing water in distinct cycles [62]. Distinguished by their operational modes, they include fluidized bed reactors, which suspend particles for enhanced light interaction; fixed bed reactors, providing a packed environment for continuous water exposure; and near-field radiation reactors, optimizing proximity to light sources for intensified photothermal effects [63]. This classification reflects a strategic adaptation to various treatment needs within batch processing systems.

  4.2.1   External water disinfection fixed-bed reactor

  External water disinfection fixed-bed reactor integrating photothermal materials at the solution interface utilize the principle of steam sterilisation, which solves the nanoparticle recycling problem associated with fluidised-bed reactors but limits heat transfer to the evaporating surface [64,65]. These reactors provide the benefit of localized thermal effects in photothermal antimicrobial applications without leaving particulate residues (Fig. 5h). However, the face the challenge of clogging due to direct contact between the solar absorber and the solution. The interaction leads to increased ion concentration and subsequent crystal formation, which can obstruct the porous material's internal channels, hindering water flow and reducing solar absorption efficiency [66].

  4.2.2   Fluidized-bed reactor

  The fluidized-bed reactor employs the dispersion of nanoparticles within the solution to achieve photothermal bacterial sterilization [67]. Within this system, a solar absorber generates localized nanoscale heat, which, while not reaching boiling points, can induce liquid evaporation [68]. The subsequent condensation of saturated vapors releases latent heat, effectively targeting bacteria for sterilization. However, challenges such as the separation of residual metal nanoparticles like Au NPs post-sterilization and the high cost of precious metal-based materials persist. Future advancements necessitate the development of efficient nanoparticle separation and recycling methods, alongside the exploration of more sustainable and economical photothermal materials (Fig. 5i) [69].

  4.2.3   Near-field radiation reactor

  The near-field radiation reactor represents a non-contact approach to sterilization, utilizing thermal infrared radiation to isolate photothermal materials from the solution [70–72]. This design effectively prevents contamination of the light absorber and circumvents issues related to concentrated salts, bacteria, and other impurities (Fig. 5j) [70]. Nevertheless, the development of photothermal materials for complex aqueous environments, such as industrial wastewater, faces significant challenges. Future research should concentrate on enhancing pollutant removal, water purification and recycling, and energy conversion efficiency. Advancements in science and technology are expected to broaden the application of photothermal antimicrobials, extending their utility to a range of environmental contexts beyond water purification.

  5. Future research directions

  In the future, photothermal materials still face many challenges to achieve the goals of high light absorption, high photothermal conversion efficiency and low heat loss. Much of the previously reported work has focused on the study of the properties of photothermal materials, and there is still a need for continuous improvement in terms of application functionalization and environmental adaptability. Promoting the real application of solar water sterilization has benefits not only for clean water production but also for other environments.

  5.1 Removal of wastewater pollutants

  Industrial wastewater contains a wide range of complex pollutants, the discharge of which in large quantities seriously affects environmental protection and public health. These pollutants include organic reagents and metal ions that are highly mobile, toxic and non-biodegradable [73,74]. The mechanism of pollutant removal by photothermal effect is expected to combine photocatalytic and thermocatalytic processes [75]. Zhang et al. [76] designed a synergistic photocatalytic-photothermal system based on two-dimensional Ti₃C₂T_x MXene (TCM) membranes, which was capable of recovering Ag⁺ ions and removing rhodamine B (RhB) from wastewater. Through the surface plasma effect, Ag⁺ could be fully converted to Ag NPs on the TCM membrane, which enhanced the photothermal conversion efficiency (~81%) and further promoted the oxidation of RhB. Meanwhile, the photocatalytic oxidation of RhB also promoted the reduction of Ag⁺. These studies provide potential research directions and development pathways for the removal of pollutants in wastewater.

  5.2 Degradation of organic pollutants in wastewater

  In addition to the above trends, organic pollutants in wastewater can also be treated using energy conversion for wastewater purification. Jiao et al. [77] designed a synergistic polypyrrole/rGO/cobalt phosphate/gold foam nickel (PGCN) composite for the photocatalytic degradation of industrial wastewater containing organic pollutants and salt ions. Both polypyrrole and rGO have high photothermal conversion efficiencies, and PGCN exhibits excellent catalytic degradation performance with the addition of cobalt phosphate catalyst. Meanwhile, the nickel foam as the skeleton can effectively conduct the heat to the wastewater and the catalyst, and the conversion of organic pollutants was successfully realized.

  5.3 Solar cavitation reactors

  Photo-thermal sterilization combined with bubble technologies such as bubble nucleation and cavitation may offer additional opportunities for improved water treatment. Bubbles are stable spherical gas packets in liquids with interfacial properties that can improve the efficiency of biological wastewater treatment while also inactivating pathogens and mitigating biological contamination.

  Photoexcitation of photothermal nanostructures leads to intense nonequilibrium heating of the material lattice and subsequently the water shell near the surface of the nanostructures and explosive boiling. This explosive boiling is accompanied by drastic changes in bubble size and internal pressure, leading to intense and massive stresses in bubble nucleation, which contribute to the destruction of microbial cells. In addition, the cavitation effect is a phase transition phenomenon in fluids at low local pressures [78]. This rapid transition from liquid to gas is accompanied by the formation of large imploding bubbles in the fluid [79,80]. The collapse of the bubbles produces localized temperatures above 4000 ℃ and high-pressure points of 500 atm [81]. This cavitation energy (mechanical, thermal and chemical effects, responsible for water treatment) will be released into the surrounding liquid, inactivating microorganisms. At the same time, the thermal effect caused by cavitation reduces the heating requirements of the photothermal process and improves the heat utilization [79]. In water treatment, the effectiveness of the technology was demonstrated by the use of a hydrodynamic cavitation reactor that reduced viral activity by 75% [82]. Therefore, combining photothermal conversion with bubble technology for synergistic sterilization is a promising development trend.

  6. Conclusion

  This review offers an exhaustive review of current research, elucidating the antimicrobial mechanisms underpinning photothermal water disinfection and the diverse photothermal materials in use. The emergence of photothermal composites, endowed with superior conversion efficiency and robust antimicrobial potency, marks a significant advancement. Furthermore, the types and design of photothermal materials for applications are explored, and possibilities for future developments in wastewater pollutant removal, degradation of organic pollutants in wastewater, and solar cavitation reactors are suggested.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Ruiting Ni: Writing – review & editing, Writing – original draft, Investigation, Conceptualization. Kwame Nana Opoku: Writing – review & editing. Xingrong Li: Formal analysis. Yarao Gao: Formal analysis. Yanyun Wang: Writing – review & editing. Fu Yang: Writing – review & editing, Project administration, Conceptualization.

  Acknowledgments

  The research presented in this paper received financial support from National Natural Science Foundation of China (No. 21908085), Natural Science Foundation of Jiangsu Province (No. BK20241950), China Postdoctoral Science Foundation (No. 2023M731422), Open Project of State Key Laboratory of Materials Chemical Engineering (No. KL-NICE-23B03), Hubei Key Laboratory of Processing and Application of Catalytic Materials (No. 202441204) and the Science and Technology Plan School-Enterprise Cooperation Industry-University-Research Forward-looking Project of Zhangjiagang (No. ZKYY2341).

  
  
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  Figure 1  Recent advances in photothermal water disinfection. Reprinted with permission [17]. Copyright 2019, American Chemical Society. Reprinted with permission [18]. Copyright 2023, American Chemical Society. Reprinted with permission [19]. Copyright 2021, The Royal Society of Chemistry. Reprinted with permission [20]. Copyright 2020, American Chemical Society. Reprinted with permission [21]. Copyright 2018, The Royal Society of Chemistry. Reprinted with permission [22]. Copyright 2020, The Royal Society of Chemistry. Reprinted with permission [23]. Copyright 2019, Elsevier. Reprinted with permission [24]. Copyright 2021, American Chemical Society. Reprinted with permission [25]. Copyright 2022, The Royal Society of Chemistry.

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  Figure 2  Generalization of the photothermal water disinfection mechanism. (a) Photothermal effects of different mechanisms with corresponding light absorption ranges. Reprinted with permission [26]. Copyright 2019, The Royal Society of Chemistry. (b) Schematic diagram of the photothermal conversion process. Reprinted with permission [30]. Copyright 2017, Elsevier. (c) Schematic diagram of the photothermal conversion mechanism. Reprinted with permission [32]. Copyright 2021, Wiley. (d) Synergistic killing of bacteria by ROS and photothermal effect under 660 nm light irradiation. Reprinted with permission [41]. Copyright 2021, Elsevier. (e) Conceptual diagram of farm to tap water treatment plant water treatment. Photosensitizers and dispersants are extracted from the plant by boiling or other simple methods and added to the water, which is placed in sunlight for disinfection. Reprinted with permission [36]. Copyright 2020, American Chemical Society.

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  Figure 3  (a) Schematic synthesis and mechanism of Bi@Bi₂Se₃ NPs with enhanced photothermal properties. (b) TEM image of Bi@Bi₂Se₃ NPs. (c) Temperature rises curves of Bi, Bi@Bi₂Se₃ and Bi@Bi₂Se₃ NPs under 808 nm laser irradiation. (a-c) Reproduced with permission [22]. Copyright 2020, The Royal Society of Chemistry. (d) Mechanism of inactivation of E. coli by Ag/MnO₂ PHMs under solar irradiation. (e) SEM image of 0.3% Ag/MnO₂ PHMs. (f) TEM image of 0.3% Ag/MnO₂ PHMs. (d-f) Reproduced with permission [23]. Copyright 2019, Elsevier. (g) SEM image of Fe₃O₄@PB NPs. (h) Schematic of Fe₃O₄@PB NPs for recyclable photothermal water sterilization under primary solar irradiation. (i) Photographs and (j) relative viability of total colonies in contaminated tap water treated with different concentrations of Fe₃O₄@PB dispersions and irradiated by the sun for 15 min. (g-j) Reproduced with permission [50]. Copyright 2019, Elsevier.

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  Figure 4  (a) Schematic of the synthesis of MXene/PVA/HA aerogel. SEM images of the top surface of the MXene composite aerogel at (b) low and (c) high magnification. (d1) Schematic showing the accumulation of E. coli on the surface of the FTCS-MXene /PVA/HA aerogel after exposure to E. coli contaminated medium for 30 min. FTCS-MXene /PVA/HA aerogel (d4) under open sunlight and (d7) Schematic representation of the antifouling properties after polarization with a water thickness of 5 mm. E. coli fluorescence images of FTCS-MXene / PVA/HA aerogels (d2 and d3) in the pristine state, (d5 and d6) under open-air solar irradiation for 10 min, and (d8 and d9) after potential polarization for 30 min. (a-d) Reprinted with permission [53]. Copyright 2023, The Royal Society of Chemistry. (e) SEM images of Ti-RP/GO. (f) Raman spectra of Ti-RP, Ti-GO, and Ti-RP/GO samples. (g) Schematic diagram of the RP/GO membrane synthesis process. (h) Fluorescence measurement of S. aureus biofilm after LED light irradiation. Green color represents live bacteria and red color represents dead bacteria. (i) Schematic diagram of the antibacterial test for Ti-RP/GO light-driven inactivation. (j) Schematic of Ti-RP/GO biofilm dispersion. (e-j) Reproduced with permission [54]. Copyright 2020, Elsevier.

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  Figure 5  (a) Schematic of a fixed-bed reactor for a continuous flow reactor. (b) Schematic of the design of an experimental-scale fixed-bed reactor. (d) Photograph of a 3D printed fixed-bed reactor in front of a photothermal membrane module showing the planar illuminated area above the water flow path. (a, b, d) Reproduced with permission [59]. Copyright 2019, American Chemical Society. (c) Schematic of a radiative reactor for a continuous flow reactor. (e) Schematic of a typical tube-fin solar absorber. (f, g) Schematic of the experimental setup and experimental site for a radiation reactor with continuous flow reactor. (c, e-g) Reproduced with permission [61]. Copyright 2020, Elsevier. (h) Schematic of a fixed-bed reactor for a batch reactor. (i) Schematic of a fluidized bed reactor for a batch reactor. Reproduced with permission [69]. Copyright 2021, The Royal Society of Chemistry. (j) Schematic of a near-field radiation reactor for a batch reactor. Reproduced with permission [70]. Copyright 2018, Nature.

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