Thermo-Plasmonics

Thermo-plasmonics is an emerging field in photonics which aims at harnessing the kinetic energy of light to generate heat at the nanoscale. Localized surface plasmon resonance (LSPR) supported by metallic nanostructures greatly enhance the interactions of light with the structure. By engineering the size, morphology and composition of metallic nanostructures, the absorption of light can be triggered and maximized, resulting in a totally controllable nano-source of heat.

To this end, the design and development of platforms consisting of metal nanostructures characterized by an high thermal efficiency (light-heat conversion factor) are analyzed by starting from colloidal solutions of spherical gold nanoparticles then passing to more complex shapes such as nano-rods, nano-stars, nano-shells and so on.

Applications of these complex nano-sources of heat can be found in various contexts including localized cancer therapy, drug delivery, nano-surgeries and theranostics.

Photo-thermal study of layers of gold nanoparticles

Experimental characterization and theoretical modeling of the macroscopic plasmonic heat production that takes place in a single layer of small gold nanoparticles (GNPs), uniformly distributed on a glass substrate, covered with different host media and acted on by a resonant radiation. Due to the macroscopic dimension of the spot size, the used laser irradiates a huge number of nanoparticles, inducing a broad thermo-plasmonic effect that modifies the thermal conductivity of the entire system. A theoretical approach has been extended to the macroscale, including an analysis of the effects predicted for different NP densities and laser spot size values, as well as for different values of the laser intensity. Theoretically predicted temperature variations are in excellent agreement with experimental results.

Thermoplasmonic Effects in Gain-Assisted Nanoparticle Solutions

The thermo-plasmonic effects of metallic nano-objects can be controlled by considering gain-assisted solutions of metallic nanoparticles. By addition of an organic dye to the solutions, whose emission band overlaps to the LSPR the photothermal efficiency can be enhanced if the solutions are excited far from the resonance.

This happens in the case of smaller Au nanoparticles (radius < 20 nm), due to a strong coupling effect between the two subunits, which causes an increase of the extinction cross section of the whole gain-assisted system. On the other hand, for larger nanoparticles (radius > 40 nm) , an opposite behavior is found: a loss compensation mechanism, based on a resonant energy transfer process from gain units to plasmonic nanoparticles, limits the increase of the absorption cross section with a consequent lowering of the photothermal efficiency. These studies represent a useful results for biomedical applications as well as for integrated plasmonic devices based on loss compensation effects, where the impact of undesirable thermal effects cannot be ignored.

Plasmon-mediated cancer phototherapy

A nanoplatform for simultaneous cellular imaging, photodynamic and photo-thermal therapies has been designed and realized by embedding a purposely synthesized highly luminescent water soluble iridium(III) compound into gold core–silica shell nanoparticles. These multifunctionalities arise mainly from the photo-physical properties of the cyclometalated complex: (i) the heavy atom promotes, through excited triplet state formation, energy transfer processes towards molecular oxygen, with the generation of ¹O₂(photodynamic effect); (ii) the overlap of the iridium(III) complex emission band with the plasmonic resonance of gold nanostructures allows excitation energy transfer towards the metallic core (photo-thermal effect); (iii) the remarkable iridium(III) complex luminescence feature, which is preserved despite energy transfer processes, makes the whole system an efficient luminescent bio-probe (imaging).