The photothermal effect, a process that integrates principles from optics, thermodynamics, and quantum mechanics, has emerged as an important research area with broad applications, including photothermal therapy, imaging, biosensing, catalysis, energy conversion, and nanomanipulation. Metallic nanoparticles, with their high surface area and localized surface plasmon resonance (LSPR), significantly enhance photothermal conversion efficiency, offering potential in cancer treatment, high-resolution bioimaging, localized chemical reactions, and precise micro/nanofabrication and manipulation.

While the basic concept and theory of thermoplasmonics have been well established decades ago (Fig. 1), accurately determining nanoscale temperatures remains a challenge, despite several developed strategies (Fig. 2). Applications of photothermal effects using plasmonic nanoparticles have advanced significantly, especially in areas like high-resolution bioimaging, cancer treatment, energy harvesting, seawater desalination, and precise nanomanipulation and fabrication. Significant advancements have been made with the development of plasmonic nanoparticles that operate in the long-wavelength near-infrared (NIR) region, especially NIR-II (1000‒1700 nm), which allows for selective cancer cell destruction in the brain while minimizing trauma from procedures like craniotomy. Hybrid plasmonic nanoparticles with high photothermal efficiency and drug-loading capability are also increasingly attractive for photothermal applications (Fig. 3). In photothermal imaging, the development of photothermal microscopy [Fig. 4(a)] has advanced to achieve photothermal circular dichroism (PT CD) imaging, offering a simple method for chiral discrimination of nanoscaled chiral objects [Fig. 4(b)]. In biosensing, the photothermal effect enhances the speed of nuclei acid detection for viruses like the coronavirus, significantly reducing false-negative rates [Fig. 4(d)]. In photothermal catalysis, research has focused on fuel generation through methanol and CO₂ hydrogenation, facilitated by plasmonic nanoparticles [Figs. 5(a), (b)]. In addition, seawater desalination using plasmonic nanoparticles and an anodic aluminum oxide (AAO) template has proven to be a more efficient method [Fig. 5(c), (d)]. Photothermoelectric conversion efficiency is further improved by decorating carbon nanotubes with gold nanoparticles (AuNPs) [Figs. 5(e), (f)]. In photothermal-assisted nanomanipulation, new manipulation principles based on photothermal gradients have extended beyond photothermophoresis to include thermoelectrophoresis [Fig. 6(a)]. Fast-acting actuation using phase change materials has also emerged [Fig. 6(b)]. Direct particle manipulation on solid substrates is achieved through either interfacial surfactants [Fig. 6(c)] or photoacoustic surface waves [Figs. 6(d)‒(f)]. Other light-triggered propulsion systems based on nanoparticle jetting mechanisms can also mobilize nano-objects on solid surfaces [Fig. 6(g)]. In photothermal-assisted nanofabrication, the synergy between optical forces and photothermal effects leads to controlled patterning of colloidal particles [Fig. 7(a)] and laser-directed etching in both polymer and glass substrates [Fig. 7(b)]. In addition, the photothermal effect enables controlled nanoscale material growth around plasmonic nanoparticles, facilitating encapsulation with inorganic and polymer materials [Figs. 7(c), (d)].

The high photothermal conversion efficiency of metallic materials presents numerous opportunities across various fields. However, challenges remain in improving the efficiency and accuracy of the photothermal effect, necessitating the discovery of new materials with enhanced structural designs and more accurate control at targeted locations. The development of more stable and accurate nanothermometers is also crucial. Furthermore, scalable and cost-effective fabrication of photothermal materials is essential for advancing industrial applications. We believe in significant breakthroughs and progress in this field over the next decade, leading to a series of new applications.