Measuring Nanoscale Heating using Silicon Photonic Thermometry

Abstract

Genetically encoded ion channels that respond to magnetic fields -‘Magnetogenetics’ — enable wireless stimulation of specific neurons deep in the brain and provide a powerful tool for studying neural correlates of behavior in freely moving animals. Recently, several magnetogenetic proteins were constructed using heat-sensitive channels such as TRPV1/4 attached to biogenic iron storage protein, ferritin. Magnetic fields have been proposed to activate these channels via a thermal mediated pathway. Ferritin can be heated in an alternating magnetic field through relaxation losses or in a low frequency field via the magnetocaloric effect. Classical heat transfer laws predict that because nanoparticles dissipate heat quickly and efficiently to the surroundings, the small amount of heat produced in ferritin would not be sufficient to gate channels. However, experimental evidence has emerged recently that suggest that temperatures of magnetically heated iron oxide nanoparticles are many degrees higher than that of the bulk, and that the heat dissipates much slower than expected. The actual dissipation rates have been shown to depend on whether the nanoparticles are in a suspension or on the surface of a cell membrane. Here, we propose a novel method for measuring temperature in the vicinity of magnetic nanoparticles and their heat dissipation rates using silicon photonic thermosensors. This method relies on temperature dependent optical properties of silicon. A change in temperature changes the resonant wavelength condition of a silicon micro-resonator. Temperature near the surface of nanoparticles attached to these resonators can then be measured based on this optical readout. This approach allows us to measure the heat dissipation rates of nanoparticles present on a surface. Silicon resonators have been shown to have temperature sensitivities of few tens of mK and response times of 6 μs, making them suitable candidates for precise measurement of nanoscale heating.