Advances in nanotechnology have resulted in a library of functional nanomaterials with well-defined size, shape, composition, and surface functionalities that enable the conversion of low-density light energy to thermal energy, known as photothermal materials. They have attracted extensive research interest among scientists for their great promise in many practical applications ranging from biomedical to environmental fields. In biomedical applications, the photothermal effect is typically stimulated by near infrared (NIR) light, which enables high penetration depth and minimal invasiveness to biological tissues. The emerging methods in the diagnosis and treatment of cancer that rely on photothermal effects show benefits of high selectivity and precision as well as low invasiveness to normal cells which reduce the significant side effects of the conventional treatment methods. Upon NIR laser irradiation, the induced photothermal effect elevates the temperature within the tumor and causes irreversible destruction to cancer cell membranes, protein denaturation, and followed by cell death, while sparing the healthy cells.^1,2  Similarly, nanomaterials with NIR-responsive photothermal effects can have good capacity for fighting bacterial infections using the produced heat to inhibit the growth of bacteria as well as prevent bacterial biofilm formation.

Photothermally active materials have also emerged as a frontier area of research for their potential application in solar-driven vapor generation in aqueous systems.^3–6  In this regard, photothermal conversion is the simplest way to utilize NIR light in the solar light spectrum for practical transformation of solar energy into thermal energy. Accordingly, the NIR radiation in the range of 780–2500 nm constitutes nearly half of the solar energy. The solar-driven water evaporation (e.g. seawater desalination and wastewater purification) represents one of the most promising green and sustainable solutions through combining two of the most abundant resources on Earth (i.e. solar energy and water) for low-energy fresh water production. This technology is based on the harvesting of solar energy by a photothermal material and converting it to thermal energy for heating up liquid water to generate steam.^5,7 

Material and structural design are the most important criteria for photothermal applications. Over recent decades, a great number of NIR-responsive photothermal materials and structural engineering strategies have been developed to suit different applications. In general, photothermal materials can be categorized into four functional categories, such as plasmonic metals, semiconductors, carbon-, and polymer-based materials. The photothermally active plasmonic metals are mainly comprised of nanostructures of noble metals (e.g. Au, Ag, Pd) while semiconductors are composed of transition metal oxides (e.g. WO₃, Fe₃O₄), transition metal chalcogenides (e.g. CuS, Cu₂Se), and transition metal dichalcogenides (e.g. WS₂, MoS₂). The diversified carbon-based nanomaterials are based on 1D, 2D, and 3D architectures (e.g. carbon nanotubes, graphene) and polymer-based materials including conjugated polymers (e.g. polyaniline, polypyrrole) as well as crystalline porous organic polymers (e.g. covalent organic framework). In this book chapter, we will first (1) introduce the fundamentals of various photothermal conversion mechanisms and (2) review the recent development of unique nanomaterials and related nanostructures that exhibit outstanding photothermal performance, as well as their (3) potential applications in photothermal therapy (PTT), photothermal sterilization and solar-driven water evaporation. Finally, we (4) present a summary and the perspectives of photothermal materials (see Figure 1.1).

Photothermal effect refers to the temperature increase of a material due to the absorption of light. The distinct photothermal effect induced by nanostructured photothermal materials can reduce the defined region of heat modulation to the nanoscale. Considering the different light–matter interactions in various materials that relate to their inherent electronic or bandgap structures, photothermal conversion mechanisms can be categorized into three main groups: (1) plasmonic localized heating of metals, (2) electron–hole generation and relaxation of semiconductors, and (3) HOMO–LUMO excitation and lattice vibration of molecules.

Metallic nanomaterials (e.g. gold and silver) have gained great scientific and technological interest for their key ability to interact with light in the visible to NIR region. When interacting with light at the appropriate wavelengths, free electrons on the metallic nanoparticle surface are excited and conduction-band electrons collectively oscillate at the same frequency. This phenomenon is identified as localized surface plasmon resonance (LSPR).^10,11  The LSPR is able to decay through two competitive pathways namely the radiative and nonradiative decay processes. The radiative decay process provides the main role in the plasmonic enhancement of the electric field in the near-field regime, while the nonradiative decay process via intraband or interband transitions is responsible for forming energetic or hot electrons, leading to particle heating. A typical example of the decay dynamics of photoexcited gold nanoparticles is described in three phenomena¹²  (see Figure 1.2a): firstly, relaxation from a non-Fermi to Fermi electron distribution via ‘electron–electron scattering’ (<100 fs), secondly, cooling of hot-electron gas through ‘electron–phonon scattering’(1–10 ps), and lastly, heat dissipation from gold nanoparticles to the surrounding environment via ‘phonon–phonon scattering’ (∼100 ps).^10,13,14  The LSPR effect is strongly dependent on many factors, including the metallic particle morphology, size, composition, interparticle distance, dielectric constant or medium around a particle.^15,16  Up to now, noble metals such as gold and silver nanostructures have been the most frequently used plasmonic metals, while several other non-noble metals that show metal-like optical properties in specific wavelength ranges include transition metals (e.g. Al, Cu, Co, Ni),^17–20  transition metal oxides (e.g. WO_3−x and MoO_3−x),^21–23  and transition metal chalcogenides (e.g. Cu_2−xE, E = S, Se).^24,25 

The generation and relaxation of electron–hole pairs usually occur in semiconductors (see Figure 1.2b).²⁶  When irradiated by an incident light with an energy equal to or greater than the band gap, the semiconductor absorbs photons to produce active electron–hole pairs. The act of photoexcitation generates electrons in the conduction band and leaves electronic vacancies or holes in the valence band. The subsequent relaxation from the higher excited states to the lower energy states can occur either via radiatively in the form of photons, or non-radiatively in the form of phonons. The latter causes a heat liberation when carriers distribute part of their energy to the crystal lattice. As a result, the thermal (vibrational) energy of the lattice rises, which is measured as an increase in its temperature. A temperature distribution is thus established, depending on the optical absorption and nonradiative bulk/surface recombination. This establishment of a temperature distribution in the material charge by carrier diffusion and recombination is referred to as the photothermal effect.²⁷ 

Carbon- and polymer-based materials have been investigated as photothermal materials due to their strong light-absorbing and photon-to-heat conversion abilities through lattice vibrations (see Figure 1.2c).²⁶  In these materials, the less tightly held electrons in π bonds can be easily excited from the π orbital to π* orbital with a lower energy input. Notably, the conjugation (π–π or p–π) and hyperconjugation effects (σ–π) facilitate the excitation of electrons by light irradiation (π →π*)and induce a strong absorption in the NIR region, where the excited electrons return to the ground state by releasing the absorbed energy into heat.^28–30  Accordingly, the excited electron is promoted from the ground state (highest occupied molecular orbital, HOMO) to a higher energy orbital (lowest unoccupied molecular orbital, LUMO) upon illumination with light energy that matches a possible electronic transition within the molecule. Then, the relaxation from the higher excited states to the lower energy states can occur by means of electron–phonon coupling. Therefore, the energy gained is transferred from the excited electrons to vibrational modes within the atomic lattices, resulting in an increase in the temperature of the material.³¹ 

Plasmonic metals, like silver and gold, exhibit photothermal effects arising from their LSPR. For instance, triangular silver nanoplates have been widely explored for their photothermal effects under NIR laser irradiation.^32–35  As for gold, a typical absorption band of gold nanospheres is presented in the 500 to 550 nm region due to their LSPR.³⁶  When the particle size increases, there is some LSPR red shift due to electromagnetic retardation in larger particles. With continuous improvement and development in the synthetic methodologies, various gold nanomaterials including rods, shells, prisms, plates, and cages/boxes, hollow, and branched nanostructures have been synthesized successfully.^37–41  By changing their sizes or shapes, the plasmon resonance peak can be tuned to the NIR spectral region to achieve maximum thermal conversion for photothermal application.^42–44  Gold nanorods are among the most studied metal nanostructures as inorganic photothermal materials, owing to their anisotropic shape, tunable aspect ratio, and strong NIR absorption of the longitudinal plasmon band with high thermal conversion efficiency.^45–49  The absorption range of gold nanorods can be tuned through altering the aspect ratio, so the heating efficiency can be optimized by using an absorption maxima ∼800 nm. Our group synthesized multiple-branched gold nanostructures that exhibited a distinct near- and mid-IR LSPR via a template-directed approach.^50,51  The synergistic coupling between the branches enables the multiple-branched nanocrosses to efficiently absorb IR light (see Figure 1.3a–f). As compared to gold nanospheres and gold nanorods, the absorption cross-section of gold nanocrosses of 100 nm in the longitudinal and traverse directions was calculated to be 7.5 × 10⁻¹⁵ m² by using discrete dipole approximation simulations,⁵²  which is higher than the gold nanospheres of 40 nm (2.93 × 10⁻¹⁵ m²)^53,54  and 150 nm (5.73 × 10⁻¹⁵ m²)⁵⁵  in size as well as gold nanorods of 45 nm × 20 nm in dimension (1.83 × 10⁻¹⁵ m²).⁵⁵  Thus, the gold nanocrosses can effectively improve the light absorption with a great extension to the longer wavelengths relative to the gold nanospheres or nanorods at shorter wavelengths.

Despite all the advantages associated with the gold nanostructures, they often suffer from poor photothermal stability and hence easy loss of shape and NIR SPR properties upon strong NIR laser irradiation (see Figure 1.3g).^56–58  As such, palladium nanostructures have been developed to overcome the photothermal instability drawback because of its significantly higher melting point. Huang et al. prepared ultrathin hexagonal palladium nanosheets that exhibited unusual optical features with tunable (826–1068 nm) and intense LSPR absorption (molar extinction coefficient of 4.1 × 10⁹ M⁻¹ cm⁻¹) in the NIR region, which is comparable with the most studied gold nanorods (5.5 × 10⁹ M⁻¹ cm⁻¹).^59,60  Most importantly, these palladium nanosheets do not undergo a shape transformation into spherical particles under high intensity NIR laser irradiation. Further study on the palladium nanosheets demonstrated a high photothermal conversion efficiency of 52.0% at 808 nm.⁶¹  In another study, palladium nanocorolla composed of unidirectionally aligned, well-spaced, and connected ultrathin palladium nanosheets was synthesized through an etching growth strategy (see Figure 1.3h).⁶²  Upon 808 nm laser irradiation, the photothermal effect of palladium nanocorolla induced by the NIR SPR absorption gave rise to a temperature increase from 26.6 to 50.4 °C. Likewise, Xiao et al. reported palladium nanoparticles with a porous architecture (see Figure 1.3i), exhibiting high NIR absorption (6.3 × 10⁷ M⁻¹ cm⁻¹) nearly two times that of solid palladium nanocubes at the same mass concentration.⁶³ 

Several narrow bandgap semiconductors, such as hydrogenated black TiO₂,⁶⁴  Ti₂O₃ nanoparticles,⁶⁵  oxygen-deficient MoO₃ nanostructures,²³  and magnetic microspheres (e.g. Fe₃O₄, MnFe₂O₄, ZnFe₂O₄, and CoFe₂O₄),⁶⁶  have been studied as photothermal materials in recent years. Zhu et al. constructed black TiO₂ nanocages with enhanced absorption due to the light trapping effect.⁶⁴  The black TiO₂ efficiently absorbed the solar irradiation and the well-crystallized interconnected nanograins structure accelerated the heat transfer in the system, thus achieving a light-to-thermal conversion efficiency of 70.9%. Wang et al. reported Ti₂O₃ nanoparticles with outstanding absorption capability across the full solar spectrum.⁶⁵  It was suggested that the small bandgap (≈0.1 eV)and nano-sized features of the Ti₂O₃ nanoparticles deliver a remarkable solar-to-thermal conversion efficiency of ∼92%.

Unlike conventional wide-bandgap TiO₂ which only absorbs UV light, Ti₂O₃, which can be considered as oxygen-deficient phases of TiO₂, is able to absorb the full solar spectrum. Likewise, there are other oxygen-deficient metal oxides such as WO_3−x and MoO_3−x, which exhibit strong photoabsorption properties in the broad wavelength range up to the NIR region.^67,68  Among the active photothermal metal oxides, WO_3−x exhibits strong LSPR due to oxygen vacancies contributing free electrons. In contrast to WO₃ which is yellow in color, WO_3−x are generally blue in color, with many forms of oxygen-deficient stoichiometries such as WO_2.9, WO_2.83, and WO_2.72.^21,68  Apart from the intrinsic absorption of WO₃ at 480 nm (i.e. indirect band gap 2.6 eV), the oxygen-deficient WO_3−x exhibits a broad absorption peak in the 480–1800 nm region which is ascribed to the new discrete energy bands below the conduction band, generated by the oxygen vacancies and the collective oscillations of surface-free conduction band electrons (700–1800 nm) that induce the SPR.⁶⁹  Among the transition metal oxides, iron oxide nanoparticles possess dual functionalities of NIR absorption and magnetism. In comparison with individual magnetic Fe₃O₄ nanoparticles, studies have shown that clustered Fe₃O₄ nanoparticles can induce a higher temperature increase because of their strong absorption in the NIR region.⁷⁰  Likewise, the formation of self-assembled Fe₃O₄ architectures is capable of improving the photothermal performance, exhibiting rapid temperature increments in larger Fe₃O₄ superstructures than the smaller ones due to the enhanced molar extinction coefficient in the NIR region.⁷¹ 

Transition metal chalcogenides are another important group of inorganic photothermal materials that are receiving intense research due to their strong absorption in the NIR region, good photostability, and other benefits such as low cytotoxicity, low cost, and abundance. Particularly, a well-known p-type semiconductor material, copper sulfide (CuS), has been found to show intrinsic NIR region absorption originating from d–d energy band transitions of Cu²⁺ ions rather than SPR. A notable example of CuS photothermal materials is the synthesis of 3D flower-like CuS superstructures which exhibited high NIR photothermal conversion efficiency.⁷²  The CuS superstructures can rapidly convert 980 nm laser energy into thermal energy, leading to an increase in the temperature of CuS superstructure aqueous dispersion as a function of irradiation time and concentration. It was revealed that the faceted end planes of the crystalline CuS superstructures function as the laser-cavity mirrors for the 980 nm laser, giving rise to the enhancement of reflection, absorption, and photothermal conversion. Our group has synthesized the multiply-voided Cu₁₂Sb₄S₁₃ tetrahedron architecture with significant absorption in the NIR region. Upon radiation from an 808 nm laser, the temperature of the aqueous dispersions of tetrahedrite nanostructures elevates with increasing concentrations, showing an increment of 18.2 °C at a concentration of 0.4 mg mL⁻¹ compared to 2.4 °C in pure water (see Figure 1.4a–c).⁷³ 

The phenomenon of intra-band transition appears to be a unique characteristic of CuS nanostructures of 1 : 1 stoichiometry, as nonstoichiometric copper chalcogenides (e.g. Cu_2−xS/Cu_2−xSe) have been found to show stable LSPR behavior similar to metals.^74,75  Upon irradiation by NIR light at 800 nm, the Cu_2−xSe nanocrystals generate an intense NIR absorbance peak relating to the plasmon resonance. The significant photothermal heating effect resulted in a photothermal transduction efficiency of 22%.²⁵  Copper sulfides such as Cu₉S₅ and Cu_7.2S₄ nanocrystals (see Figure 1.4d–e) demonstrated photothermal conversion efficiencies up to 25.7% and 56.7% respectively, which are both higher than that of gold nanorods of 24.6% under 980 nm light irradiation.^76,77  The outstanding photothermal conversion efficiencies can be explained by strong absorption in the NIR region due to the LSPR arising from p-type carriers in vacancy-doped nanocrystals.⁷⁸  In the case of Cu_7.2S₄ nanocrystals, the plasmon resonance peak is centered at 968 nm which is close to the wavelength of the NIR laser of 980 nm, leading to higher NIR absorption than that of Cu₉S₅ nanocrystals. When Cu_1.94S nanocrystals were self-assembled into ordered plant-like structures, the NIR photothermal conversion efficiency of the assembled architectures was enhanced by about 50% compared with individual Cu_1.94S nanocrystals, upon irradiation with a 1064 nm laser.⁷⁹  In addition, 2D transition metal dichalcogenides such as MoS₂ and WS₂ also show strong absorbance in the NIR region. In particular, MoS₂ nanosheets presented a higher mass extinction coefficient at 808 nm than that of graphene.^80,81 

Several classes of carbon-based materials including carbon nanotubes (CNTs), graphene, graphene oxide/reduced graphene oxide, carbon black, graphite, and carbon composites have been nominated as potential photothermal materials owing to their excellent light absorption over a wide range of wavelengths and superior light-to-heat conversion efficiency.^82–90  Moreover, they are relatively low cost and abundant compared to metallic materials. Diversified carbon-based nanomaterials based on 1D, 2D, and 3D architectures have been designed and synthesized for photothermal applications.^7,91  Unlike conventional amorphous carbon, the conjugation and hyperconjugation in the molecular structure of CNTs and graphene permit strong absorption in the NIR region and increasing the π-conjugation gives rise to a red-shift of the absorption light spectrum.

CNTs are made of sp² carbons and regarded as typical 1D nanomaterials. They can be categorized into either single-walled (SWCNTs) or multi-walled (MWCNTs). The structure of SWCNTs is a rolled-up tubular shell of a graphene sheet, while MWCNTs consist of a stack of graphene sheets rolled up into concentric cylinders.^92,93  Each type has different properties and specific photothermal effects. As CNTs are potentially toxic in their bare state and hydrophobic in nature, one effective strategy is to chemically modify or functionalize the CNTs to render solubility in aqueous medium, particularly for biomedical applications. Du et al. introduced amino groups onto the surface of SWCNTs by HNO₃ oxidation and amidation treatment to enhance dispersibility of SWCNTs.⁹⁴  This significantly improved the solar-thermal conversion under sunlight irradiation experiments. In many other cases, CNTs are noncovalently functionalized by simple adsorption of different kinds of molecules including polymers, biomolecules such as saccharides/polysaccharides, proteins, enzymes, and DNA. Wang et al. modified the SWCNTs using PEG-grafted amphiphilic polymer, leading to stable dispersion in various physiological media.⁹⁵  The PEG-coated SWCNTs exhibited strong absorbance in the NIR region and were able to warm up rapidly under 808 nm NIR laser irradiation, resulting in significant photothermal effects.

Graphene is a 2D material composed primarily of sp² hybridized carbon. Due to the hydrophobic feature of graphene, additional hydrophilic treatments are often required. Reports have shown that functionalization of graphene through nitrogen doping⁹⁶  and hydrophilic groups (e.g. hydroxyl and carboxyl)⁹⁷  can enhance the photothermal performance. On the other hand, graphene oxide is a chemically modified graphene with additional reactive oxygen functional groups, such as hydroxyl, carboxyl, and epoxy groups. It is hydrophilic due to the oxygen-containing functionalities and therefore it can be easily dispersed in aqueous media. It can also be converted to reduced graphene oxide at high yields. Robinson et al. developed biocompatible reduced graphene oxide sheets of ∼20 nm through noncovalent functionalization with amphiphilic PEGylated polymer chains and yielded a 6-fold increase in NIR absorbance than with graphene oxide.⁹⁸  Besides PEG coating, the photothermal absorbing ability of reduced graphene oxide has been studied by using different functional groups.⁹⁹  Arginine modification of reduced graphene oxide increases the stability in aqueous solutions and shows a higher absorption cross-section of 3.2 times that of graphene oxide at 808 nm.⁸⁸  When graphene sheets were assembled into a vertically aligned graphene sheet membrane, the 3D graphene-based materials exhibited outstanding light absorption in the full solar spectrum range (250–2500 nm) for excellent photothermal transduction, featuring high efficiency of 86.5% under one sun illumination.¹⁰⁰ 

Apart from inorganic nanomaterials, organic materials particularly the polymeric ones have been reported for their outstanding light-harvesting capability and photothermal conversion efficiency with excellent biocompatibility.¹⁰¹  Conjugated polymers are macromolecules characterized with π-conjugated backbones, which have emerged as new generation NIR-absorbing photothermal materials.^102–105  The successful examples of biocompatible conjugated polymer-based photothermal materials mainly include polyaniline,^106,107  poly(3,4ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS),^108,109  polypyrrole,^110,111  polydopamine,^112,113  and donor–acceptor (D–A) structured polymer molecules.^103,114 

Covalent organic frameworks (COFs) are newly emerged crystalline porous organic polymers with extended structures, in which their backbones are constructed entirely from light elements (B, C, N, O, Si), and connected via covalent bonds into 2D or 3D structures.¹¹⁵  The key features of COFs include low density, high surface area, high thermal and mechanical stability, highly ordered π–π stacking structure, easy functional modification, amenable topologies, and are structurally pre-designable, etc.^9,116,117  In general, COFs were formed by reversible condensation reactions between various organic building blocks. They were first demonstrated by Yaghi and co-workers in 2005 using condensation reactions of phenyl diboronic acid and hexahydroxytriphenylene.¹¹⁸  The versatility enables COFs to be modified with many kinds of functional groups for different applications. Accordingly, COFs were widely employed as a perfect support for integration or encapsulation of functional nanoparticles in a controllable and predictable way. Tan et al. developed COF–Fe₃O₄ core–shell microspheres through an amorphous-to-crystalline transformation process, whereby Fe₃O₄ nanoclusters were encapsulated by an imine-linked COF network.¹¹⁹  Due to the well-organized stacking in shell, the resultant COF–Fe₃O₄ microspheres exhibited NIR absorbance and obtained a high molar extinction coefficient of 4.2 × 10¹⁰ m⁻¹ cm⁻¹ at 785 nm. As such, the photothermal conversion efficiency of COF–Fe₃O₄ microspheres was determined to be 21.5%, which is 2–3 times higher than that of Fe₃O₄ nanoclusters alone. Hu et al. synthesized COF–CuSe nanocomposites with NIR photothermal conversion ability via a solution-phase synthesis approach under ambient conditions.¹²⁰  Upon 808 nm laser irradiation, the corresponding COF–CuSe nanocomposites displayed a photothermal effect with a photothermal conversion efficiency of 26.34% (see Figure 1.5).

Organic–inorganic hybrid crystalline porous materials such as metal–organic frameworks (MOFs), are similar to COFs, but instead of being composed of light elements, the structure of MOFs is composed of metal ions or clusters cross-linked by organic linkers. The photothermal effect has been demonstrated in most representative subfamilies of MOFs, including HKUST-1, UiO-66, ZIF-8, CPO-27, Fe–MIL-101–NH₂, and IRMOF-3, with significant heating of up to 167 °C (for CPO-27–Ni) achieved within 5 min of UV–vis irradiation. This effect was attributed to the d–d transitions of the metal ion centers, as observed in their broad absorption bands within the irradiation range (300–650 nm); while MOFs without absorption bands in this range showed only a small temperature increment even after irradiation for 30 min.¹²¹  The photothermal effect in MOFs has been used for solvent removal and chemical activation, with localized heating resulting in a more rapid activation than heating with an external source, as well as chemical modification in solid-state reactions.^121,122  They have also been combined with polymers such as polyaniline for photothermal therapy.¹²³ 

Cancer refers to a large group of diseases characterized by the development of abnormal cells that spread uncontrollably. It is a global public health crisis due to its high incidence and mortality rates.¹²⁴  To date, there is no comprehensive approach for cancer treatment; common options are primarily focused on chemotherapy, radiotherapy, immunotherapy, and surgery, while surgery in many cases is not able to remove cancerous tissue fully. Despite these approaches offering considerable therapeutic efficacy, they are limited by their risk to normal cells and tissues as well as their potential to destroy the immune system. For this reason, it is highly desirable to develop an effective cancer therapy to overcome the limitations of conventional therapies, particularly focusing on minimal invasiveness and reducing undesirable side effects while enhancing efficacy. Toward this goal, photothermal therapy (PTT) is emerging as a promising alternative therapeutic approach for cancer treatment, involving the application of benign light wavelengths (λ = 700–1100 nm) in conjunction with photothermal agents that transform light energy to localized heat to ablate cancer cells. The ideal photothermal agents should possess several distinct features, including (1) biocompatibility and low toxicity, (2) strong absorption in the NIR region, (3) large extinction coefficient and high photothermal conversion efficiency, and (4) size between 30–200 nm to prolong circulation and enhance tumor accumulation.^1,125 

In the pursuit of increasing therapeutic efficiency, continuous efforts have been devoted to enhancing the intrinsic photothermal performance of photothermal agents. Accordingly, the photothermal performance is related to the nanoscale properties of materials in terms of composition, size, structure, morphology, and surface functionalities. At present, the successful examples of photothermal agents with a relatively high tissue transparency in the NIR window are mainly focused on noble metal nanostructures, tungsten-, copper-, and iron-based semiconductors, carbon-based materials, as well as polymer-based materials. Among these, gold-based nanostructures were the earliest nanomaterials used in cancer photothermal therapy research and have remained one of the most widely studied inorganic photothermal agents today.¹²⁶  Examples of gold-based structures for photothermal study include nanoparticles,^127,128  nanorods,^129–131  nanoshells,^132–134  nanocages,^135,136  nanostars,^137–139  nanoflowers,⁴⁰  and nanocrosses,^50,52  which are capable of inducing localized hyperthermia effects. Our group demonstrated that branched gold nanostructures such as nanocrosses and nanoflowers could act as efficient absorbers for NIR-assisted photothermal destruction of living cells. The percentage of cell death is dependent on the laser intensity as well as the exposure time. Upon 900 nm laser irradiation (4.2 W cm⁻²), human lung cancer cells (A549) associated with gold nanocrosses were killed rapidly within 30 s, indicating the hyperthermia effect induced by gold nanocrosses.⁵⁰  This observation is also demonstrated in gold nanoflowers for both in vitro and in vivo photothermal therapy under 808 nm laser irradiation. In vitro photothermal heating of gold nanoflowers in the presence of MCF-7 cancer cells caused the destruction of the cancer cells after irradiation for 30 s. Meanwhile, the gold nanoflower-mediated photothermal ablation of MCF-7 tumors in mice led to effective ablation of the tumors using an NIR laser, suggesting an excellent in vivo photothermal therapeutic efficacy of gold nanoflowers (see Figure 1.6).⁴⁰  Apart from gold nanostructures, other inorganic nanostructured materials such as palladium (e.g. nanosheets,⁶¹  nanocorolla,⁶²  and porous/hollow nanoparticles^63,140 ) have shown great potential in NIR photothermal cancer therapy. Zhang and co-workers demonstrated in vitro photothermal heating of liver cancer cells in the presence of palladium nanocorolla, causing almost 100% cell death upon 808 nm NIR irradiation (1.4 W cm⁻², 2 min).⁶² 

Semiconductors, like transition metal oxides (WO_3−x, MoO_3−x)^21–23,68  and transition metal chalcogenides (Cu_2−xE, E = S, Se)^24,25  have emerged as a new class of plasmonic materials owing to crystal lattice oxygen vacancies that contribute free electrons. With an LSPR signal at around 900 nm,¹⁴¹  WO_3−x is an excellent candidate for biomedical applications.^142,143  Particularly, nanostructured tungsten oxides (nanoparticles,¹⁴⁴  nanorods,^145,146  nanowires,¹⁴⁷  nanosheets²² ) have been developed as photothermal agents for in vitro and in vivo cancer therapy. When oxygen-deficient tungsten oxide nanosheets were subjected to 808 nm NIR irradiation (2.5 W cm⁻²), the temperature of the tungsten oxide dispersion increased rapidly to 64.2 °C at a dose of 100 μg mL⁻¹.²²  This photothermal effect is responsible for >90% of MCF-7 cell death upon 2 min irradiation and an efficient tumor inhibition rate of 96.8%. Likewise, nanostructured copper sulfide (nanoparticles, nanoplates,¹⁴⁸  hollow structures^149,150 ) are promising new materials for photothermal cancer therapy.^151–153  It has been reported that the PEGylated copper sulfide nanoparticles of pure covellite phase possess strong LSPR absorption in the NIR, yielding an outstanding photothermal heat conversion efficiency of 71.4%.¹⁵⁴  Li et al. presented a nuclear-targeted PTT strategy based on RGD and TAT peptide-modified copper sulfide nanoparticles, to effectively destroy residual cancer cells and prevent local cancer recurrence.¹⁵⁵  Upon 980 nm NIR laser irradiation, the copper sulfide nanoparticles rapidly elevated the temperature of the nucleus, causing DNA damage and protein denaturation, thereby leading to an exhaustive apoptosis of the cancer cells. The therapeutic effect of the designed nanoparticles was demonstrated by the cell activity experiments, which showed 84% mortality arising from targeted nanoparticles compared to that of non-targeted nanoparticles which showed 42% mortality. Moreover, in vivo experiments showed that the xenografted tumor was fully eliminated after 14 days with only a single treatment and no recurrence of the cancer (see Figure 1.7).

Carbon-based materials (e.g. CNTs,^156–161  graphene, and its derivatives^86,162–165 ) have been extensively applied as photothermal agents in biomedical applications. They are often assembled with other functional molecules or nanostructures to afford hybrid functional materials in order to provide better properties than their individual counterparts. Due to their large surface area to volume ratio and distinct surface properties, graphene can act as a platform in PTT by incorporating drugs or varieties of other nanomaterials via covalent and non-covalent conjugation means. For instance, through integrating reduced graphene oxide and gold superparticles via an emulsion-based self-assembly method, Lin et al. were able to develop a theranostic nanoplatform for PTT of cancer based on the reduced graphene oxide coated gold superparticles.¹⁶³  This combination improves the photothermal conversion properties due to the plasmonic coupling of adjacent gold nanoparticles and the interaction of gold superparticles with reduced graphene oxide, leading to efficient photothermal ablation of U87MG tumors in vivo (see Figure 1.8).

All the above-described inorganic nanomaterials have shown favorable absorbance features, high photothermal conversion efficiencies, and good photo-stabilities. Unfortunately, their potential long-term safety concerns due to poor biodegradability may restrict their further application in clinical translation. On the other hand, organic photothermal agents exhibit excellent biodegradability and biocompatibility as an alternative approach for PTT in cancer treatment. Nevertheless, it must be considered that organic nanomaterials have their own limits associated with poor photostability, low absorptivity, or limited photothermal conversion efficiency. In many cases, combining PTT with other therapies such as chemotherapy and immunotherapy can further improve on the suppression of tumor growth and metastasis.^166,167  For example, PEG-modified PEDOT:PSS was reported as a drug carrier to load doxorubicin and SN38 for combined photothermal- and chemotherapy, to realize a synergistic effect in cancer cell killing.¹⁰⁹  A study by Yu et al. revealed that both NIR dye and chemotherapeutic agent were covalently conjugated for combating doxorubicin resistance in breast cancer.¹⁶⁸  In this study, a hybrid micelle with a pH and NIR light dual-responsive property was developed based on enzyme-responsive doxorubicin polymeric prodrugs and cypate-linked polymers. Upon NIR laser irradiation, the hybrid micelles induced a hyperthermia effect and improved tumor penetration and cytoplasm release of doxorubicin, thus significantly improving the therapeutic efficacy for treatment of doxorubicin-resistant MCF-7/ADR breast cancer.

The increasing development of antibiotic resistance in bacteria has become a major global health threat. In particular, the eradication of antibiotic-resistant bacteria (i.e. multidrug-resistant bacteria) and their biofilms is very challenging with conventional antibiotics. It is therefore important to develop a non-antibiotic strategy with high antimicrobial efficacy against multidrug-resistant bacteria. Among which, nanomaterials with photothermal effect under irradiation of NIR have a good capacity for fighting bacterial infections, making use of the produced heat to inhibit antibiotic-resistant bacterial growth as well as bacterial biofilm formation.¹⁶⁹  Our group has developed multi-branched gold nanocrosses with strong NIR absorption to destroy antibiotic resistant bacteria, P. aeruginosa and its biofilms, with an 800 nm laser (3.0 W cm⁻², 5 min).⁵²  The NIR-assisted photothermal effect was clearly shown through the effective inhibition of the growth of P. aeruginosa by means of conjugated gold nanocrosses with pathogen-specific antibodies (see Figure 1.9a).

Graphene derivatives such as graphene oxide, reduced graphene oxide, and other chemically modified graphene have been widely investigated in antibacterial applications.¹⁶⁹  In various graphene-based photothermal anti-infection agents, they are mostly integrated with other functional molecules or nanostructures to achieve more specific and effective photothermal sterilization. Hui et al. reported a polyelectrolyte-stabilized reduced graphene oxide antibacterial surface that is able to kill >90% airborne bacteria including antibiotic-tolerant persisters, on contact upon minutes of solar irradiation.¹⁷⁰  Jia et al. employed a photothermal antibiotic agent based on chitosan and Fe₃O₄ functionalized graphene oxide for capturing and inhibiting the growth of both Staphylococcus aureus and Escherichia coli, as well as destroying bacterial biofilms with NIR irradiation.¹⁷¹  In the presence of nanocomposites and NIR radiation, the cell membranes of E. coli were broken down and lost their integrity while some intracellular components of S. aureus clearly leaked out, indicating effective sterilization through photothermal lysis (see Figure 1.9b).

Solar-driven evaporation is considered as a promising and sustainable approach for clean water production which is important to alleviate global water scarcity issues. In recent years, various types of NIR absorbing materials, including metallic nanostructures,^172–175  semiconductors,^176–178  and carbon-based materials^179–181  have been designed to optimize the solar spectrum absorption and achieve efficient light-to-vapor conversion. In particular, various gold solar absorbers including nanoparticles,¹⁸²  nanoshells,^172,183  and nanorods,¹⁸⁴  have been widely used in steam and clean water generation. Furthermore, gold nanostructures can be hybridized with other materials to improve photothermal efficiency or to form multifunctional composites.¹⁸⁵  Deng and co-workers introduced a surface evaporation approach via localized plasmonic heating by a self-assembled gold nanoparticle film¹⁸⁶  and an airlaid-paper-based gold nanoparticle film¹⁸⁷  at the air–water interface. When most of the thermal energy is confined and utilized directly at the evaporative surface, this localized heating system reduces heat loss and improves heat-to-evaporation conversion efficiency. With increased surface roughness and good thermal insulating properties, the paper substrate yielded high absorption of incident light and reduced heat loss, thus achieving a higher evaporation efficiency (77.8%) than freestanding plasmonic films (47.8%).

Successful examples of semiconductor-based solar adsorption materials with significant water evaporation efficiency include WO_2.72 nanoparticles,¹⁸⁸  magnetic microspheres (Fe₃O₄ and MFe₂O₄, M = Mn, Zn, Co),⁶⁶  and Cu₇S₄ nanocrystals.¹⁸⁹  As for carbon-based materials, they generally appear in black and are suitable for efficient broadband absorption.¹⁹⁰  With excellent light-to-heat conversion properties, carbon-based absorbers can induce rapid water evaporation upon illumination under different environmental conditions, therefore they are widely used in a solar-driven steam generation system.^3,83,191  A comparative experiment was conducted by Ni et al. to study three different carbon-based nanofluids (graphite carbon black, carbon black, and graphene) in a solar-driven steam generation system.⁸³  The findings revealed that graphite carbon black and graphene nanofluids significantly outperformed the carbon black by 7% under 1.5 h of illumination. Wang et al. developed a solar thermal evaporation system based on reduced graphene oxide modified with magnetic Fe₃O₄ nanoparticles, aiming for desalination of seawater.¹⁹²  With high solar light absorption of over 95%, the evaporation efficiency yielded 70% in a 3.5% NaCl solution with dispersed nanocomposite, under solar illumination of 1 kW m⁻² (see Figure 1.10). Moreover, this system offers the benefit of recyclability because magnetic nanoparticles can be easily separated from seawater.

This book chapter provides a succinct review of the different classes of photothermal nanomaterials, such as plasmonic metal nanostructures, semiconductors, and carbon-, and polymer-based materials, which have been broadly explored as highly promising candidates for a variety of photothermal-related applications. The different photothermal materials along with their photothermal conversion mechanisms such as plasmonic localized heating of metals, electron–hole generation and relaxation of semiconductors as well as HOMO–LUMO excitation and lattice vibration of molecules are summarized. To realize a highly efficient photothermal material, the light absorbing capability is an essential factor. For the plasmonic light absorbing materials, the light absorption can be greatly improved through controlling the size, shape, and composition which in turn tune the LSPR peaks to the NIR region. On the other hand, enhancement of light absorption in a semiconductor can be obtained through element doping or introducing oxygen vacancies in the lattice. Also, carbon-based and organic polymeric materials are often assembled with other functional molecules or nanostructures to afford hybrid functional materials that can demonstrate better properties compared to their individual counterparts.

Apart from improving the light absorbing capability, the photothermal conversion performance of photothermal materials is discussed with respect to their application in biomedical and environmental fields. Due to the great advantages of PTT and the high demand for targeted cancer therapy, widespread studies on engineering photothermal materials have been reported and have demonstrated promising results in cancer treatment. When combined with NIR light, these inorganic and organic photothermal materials are capable of generating sufficient heat to raise the local temperature and thus result in tumor cell death. Among which, gold nanostructures have received great attention due to their strong optical absorption properties, outstanding photothermal conversion efficiencies, and good photo-stabilities. Moreover, they have achieved encouraging therapeutic efficacies in many in vivo animal studies, which make them promising photothermal materials in cancer treatment. Despite all the glaring prospects, application of photothermal materials is still associated with many challenges that may restrict their further application in clinical translation. For instance, thermal stability is a highly critical parameter particularly for gold nanostructures. When the heating rate far exceeds the cooling rate, the accumulated heat in the lattice will result in structural changes in terms of the shape or integrity of nanoparticles. Another concern of using photothermal materials in PTT is the possible long-term cytotoxicity of accumulated and aggregated nanomaterials, therefore, knowledge about their potential toxicity and health impact is essential before these nanomaterials can be used in real clinical settings.

Undoubtedly, photothermal material-based solar-driven water evaporation represents a green, efficient, low-cost, and environmentally benign technology for clean water generation. Material and structural design with favorable light absorption over the full solar spectrum and excellent photothermal conversion efficiencies are the prerequisites for a high-performance solar evaporation device. Although high conversion efficiencies of greater than 90% have been achieved in recently studied photothermal materials, more efforts are required to develop robust photothermal materials enabling good thermal/chemical stability and recyclability. Additionally, to facilitate practical applications of solar-driven water evaporation in different water quality (e.g. seawater and industrial wastewater), the properties of the photothermal materials, including photothermal stability, corrosion resistance, anti-biofouling ability, as well as cost effectiveness, long-term stability, and durability should be taken into consideration. We anticipate that, in the coming years, more photothermal material and structural engineering strategies will be developed to effectively enhance the light absorption and light-to-heat conversion or even emerging new properties for a wider range of applications.