The current status of the use of nanoparticles for photothermal treatments is reviewed in detail. The different families of heating nanoparticles are described paying special attention to the physical mechanisms at the root of the light-to-heat conversion processes. The heating efficiencies and spectral working ranges are listed and compared. The most important results obtained in both in vivo and in vitro nanoparticle assisted photothermal treatments are summarized. The advantages and disadvantages of the different heating nanoparticles are discussed.

Rare-earth-doped luminescent nanothermometers are not reliable as their emission spectra can be affected by numerous environmental and experimental factors.

Quantum dot based thermometry, in combination with double beam confocal microscopy, was used to investigate the absorption/heating efficiency of gold nanoparticles with different morphologies (nanorods, nanocages, nanoshells, and nanostars), all of them with an intense localized surface plasmon resonance within the first biological window, at around 808 nm. The heating efficiency was found to be strongly dependent on the geometry of the nanostructure, with the largest values found for gold nanorods and long-edge gold nanostars, both of them with heating efficiencies close to 100%. Gold nanorods and nanocages were found to have the largest absorption cross section per unit mass among all the studied geometries, emerging as optimum photothermal agents with minimum metal loading for biosystems.

The importance of high-resolution intracellular thermal sensing and imaging in the field of modern biomedicine has boosted the development of novel nanosized fluorescent systems (fluorescent nanothermometers) as the next generation of probes for intracellular thermal sensing and imaging. This thermal mapping requires fluorescent nanothermometers with good biocompatibility and high thermal sensitivity in order to obtain submicrometric and subdegree spatial and thermal resolutions, respectively. This review describes the different nanosized systems used up to now for intracellular thermal sensing and imaging. We also include the later advances in molecular systems based on fluorescent proteins for thermal mapping. A critical overview of the state of the art and the future perspective is also included.

Precise knowledge and control over the orientation of individual upconverting particles is extremely important for full exploiting their capabilities as multifunctional bioprobes for interdisciplinary applications. In this work, we report on how time-resolved, single particle polarized spectroscopy can be used to determine the orientation dynamics of a single upconverting particle when entering into an optical trap. Experimental results have unequivocally evidenced the existence of a unique stable configuration. Numerical simulations and simple numerical calculations have demonstrated that the dipole magnetic interactions between the upconverting particle and trapping radiation are the main mechanisms responsible of the optical torques that drive the upconverting particle to its stable orientation. Finally, how a proper analysis of the rotation dynamics of a single upconverting particle within an optical trap can provide valuable information about the properties of the medium in which it is suspended is demonstrated. A proof of concept is given in which the laser driven intracellular rotation of upconverting particles is used to successfully determine the intracellular dynamic viscosity by a passive and an active method.

Non-contact thermometry is essential in biomedical studies requiring thermal sensing and imaging with high thermal and spatial resolutions. In this work, we report the potential use of Er:Yb:NaYF4 and Er:Yb:NaY2F5O up-conversion nanoparticles as thermal sensors by means of lifetime based luminescent thermometry. We demonstrate how Er:Yb:NaY2F5O nanocrystals present a higher thermal sensitivity than the Er:Yb:NaYF4 ones and that their lifetime thermal coefficient is comparable to those corresponding to other nano-sized luminescent systems already used for high resolution lifetime fluorescence thermal sensing. We evaluate the potential use of Er:Yb:NaY2F5O nanoparticles as lifetime based thermal probes by providing the first experimental evidence on sub-tissue lifetime fluorescence thermal sensing by using up-conversion nanoparticles in an ex vivo experiment.

Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in: Yb 3+ excitation provides thermal sensing during optical manipulation. Thus, three-dimensional particle scanning allows for the measurement of thermal gradients in the surroundings of individual cancer cells subjected to a plasmonic-mediated photothermal therapy. It is found that such thermal gradients extends for distances larger than 10 microns, avoiding real single cell photothermal treatments under in vitro conditions. This work introduces to the scientific community a novel and simple approach for high resolution thermal sensing at the cellular level that could constitute a powerful tool for a better understanding of cell dynamics during thermal treatments.

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