In this framework, several interesting methods for small-scale thermometry, based on complex materials that are sensitive to changes in temperature, have been recently developed, It is worth mentioning here the set of luminescent thermometers based on organic dyes, quantum dots, Ln3+ and complex hybrid matrices with emitting centers, along with the set of non-luminescent thermometers such as scanning thermal microscopy, nanolithography thermometry, carbon nanotubes, and biomaterials thermometry. Liquid crystal thermography has also attracted great attention, due to its capability to quickly reconstruct temperature fields in surface and volumes. Nonetheless, all the methods developed so far were not able to provide a temperature micro-sensor suitable for aqueous media while avoiding strong perturbations of the surrounding environment.
In their article recently published in Optics Express, Petriashvili et al. have shown an innovative, easy-to-use, and really promising strategy to accurately measure temperature at the microscale in aqueous environment. This smart method will likely have a huge impact on nano-biotechnology. Notably, the authors exploited thermochromic cholesteric liquid crystals (CLCs) that are already known to shift the bandgap position when subjected to temperature changes, due to supramolecular helical structures. Their valuable contribution was to develop an emulsion in which a cholesteric liquid crystal forms a microdroplet suspension in an aqueous environment. The micron-sized droplets composed of CLCs act as microthermometers, providing temperature information about the local environment. The authors used the method to study the photothermal behavior of plasmonic noble metal nanoparticles, obtaining in this way a proof of its practical applicability. Silver nanoparticles were optically excited with a laser beam in the regime of plasmon resonance, and the propagation of the heat to the surrounding medium was monitored thanks to the spectral shift of the selective reflection peak of CLC, obtaining a straightforward micro-scale map of the temperature inside the sample.
This technique has the potential to have a profound impact in several areas of science, in which a precise mapping of the temperature is required. Moreover, we envision that further scaling-down of thermographic material micro-emulsion to nano-emulsion could pave the way to temperature nanosensors of great interest to nanoscience (e.g., nanophotonics, nanochemistry) that for example could be able to penetrate inside cells to monitor the temperature of single cells or even of the intra-cellular local environment, without perturbing cellular functions.
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