What Are Photothermal Effects?

Photothermal effects are the thermal consequences that arise when a material absorbs light and converts the absorbed optical energy into heat rather than re-emitting it as fluorescence or scattering. The process is distinct from photochemical effects, in which absorbed photons drive chemical reactions, and from photoacoustic effects, in which rapid thermal expansion generates acoustic waves. In photothermal phenomena, the primary output is a sustained or pulsed increase in local temperature at the absorption site.

The physical basis of photothermal conversion spans several mechanisms depending on the class of material. In bulk absorbers, nonradiative recombination of photoexcited carriers and phonon emission dissipate energy as lattice heat. In nanoscale structures, the dominant pathway depends on the electronic band structure and surface properties of the material. Because the conversion can be confined to volumes well below the optical diffraction limit when nanoparticle absorbers are used, photothermal effects have drawn strong interest in targeted biomedical therapy, solar energy collection, and microscale thermal actuation.

Light-to-Heat Conversion Mechanisms

Three principal mechanisms govern how different classes of materials convert light to heat. In noble-metal nanostructures such as gold nanorods and silver nanospheres, incident photons at frequencies matching the localized surface plasmon resonance (LSPR) drive coherent oscillations of conduction electrons. These plasmons decay through electron-electron scattering on a timescale of tens to hundreds of femtoseconds, producing hot electrons that thermalize with the metal lattice through electron-phonon coupling within one to a few picoseconds. The resulting heat dissipates to the surrounding medium on a nanosecond timescale. By tuning nanoparticle size and shape, the LSPR peak can be shifted across the visible and near-infrared spectrum to match a desired excitation wavelength.

In semiconductors and carbon-based materials, photoexcited electron-hole pairs recombine nonradiatively through Shockley-Read-Hall and Auger processes, releasing energy through phonon emission. Graphene and carbon nanotubes absorb broadly from the ultraviolet through the mid-infrared due to their linear or quasi-linear band structures, producing efficient heat generation across a wide range of illumination wavelengths. Organic conjugated polymers absorb through pi-electron excitation, with excess energy released as heat via internal conversion. A detailed review of these mechanisms and their material-class dependence is provided in a 2023 Chemical Reviews survey of photothermal nanomaterials and in a PMC-hosted review of photothermal nanomaterials from ACS.

Nanomaterial Photothermal Agents

The field of photothermal agent design has expanded well beyond gold nanostructures to include a broad palette of materials engineered for specific applications. Metal oxide nanoparticles such as iron oxide and copper sulfide provide magnetic or multimodal optical properties alongside photothermal activity. Two-dimensional materials including MXenes (transition metal carbides and nitrides) combine broadband absorption with high electrical conductivity, making them candidates for both thermal and electronic integration. Organic nanoagents based on porphyrin derivatives and croconaine dyes offer biodegradability advantages for in vivo use.

A critical design parameter is the photothermal conversion efficiency (PCE), defined as the ratio of heat generated to total optical power absorbed. Gold nanorods with optimized aspect ratios achieve PCEs above 90%, while some copper sulfide-based systems and organic agents also approach or exceed this value. Equally important for biomedical applications is absorption in the first or second near-infrared biological windows (700 to 950 nm and 1000 to 1350 nm), where tissue scattering and water absorption are minimized, allowing optical energy to penetrate centimeters into tissue. The clinical potential of these systems is examined in a PMC study of photothermal nanomaterials for biomedical therapy.

Applications

Photothermal effects have applications in a wide range of disciplines, including:

• Photothermal therapy for cancer treatment, using localized heating to destroy tumor cells
• Solar water evaporation and desalination driven by sunlight-absorbing floating films
• Photothermal catalysis, in which local heating lowers reaction activation energy
• Light-driven soft actuators and microrobotic systems
• Antibacterial therapy using hyperthermia to inhibit multidrug-resistant bacteria