Individual Ultrabright Fluorescent Nanoparticles as Nanothermometers

Temperature measurement at the nanoscale is important in various scientific and engineering studies, from assessing temperature distribution in the cellular medium to conducting thermal imaging of integrated circuit devices. A serious challenge in nanoscale temperature measurement lies in achieving high spatial resolution, which can be overcome by utilizing individual nanoparticles.<br/>For this purpose, we selected fluorescent silica nanoparticles with mesoporous structure, which have encapsulated fluorescent dyes. As has been shown, such particles demonstrate ultrahigh fluorescent brightness, which is sufficient to image single nanoparticles. To be used as thermometers, two different types of dye molecules, reference molecules, and temperature-sensitive ones, were used. The measured sensitivity, reversibility, and reproducibility of measurements of temperature using these individual nanoparticles have shown the superiority of the reported particles compared to the previously reported ones. For example, the obtained individual nanothermometers exhibited a temperature fluctuation of ~0.5 K at the measurement time of 0.69 msec and excitation power density of (temperature resolution of 0.22 K.Hz^-1/2), showing a relative sensitivity of 8% within the physiologically relevant range of 25-50^oC. This sensitivity is ~4 times beter and the temperature resolution (at an excitation power of 0.26 <i>μ</i>W) is ~2 times better than the previously reported best temperature measuring fluorescent particles. <br/>These superior properties are presumably the result of the fluorescence resonance energy transfer (FRET) between the encapsulated dye molecules, which act as donors and acceptors. The enhanced brightness of these nanosensors offers numerous benefits, including the improved signal-to-noise ratio and the ability to locate the particles with high spatial precision. This work was supported by NSF grants CBET 1911253 and 2110757 (IS).