Thermal properties of lipid bilayers determined using upconversion nanothermometry

Luminescent nanomaterials have shown promise for thermal sensing in bio-applications, yet little is known of the role of organic coatings such as supported lipid bilayers on the thermal conductivity between the nanomaterial and its environment. Additionally, since the supported lipid bilayer mimics the cell membrane, its thermal properties are fundamentally important to understand the spatial variations of temperature and heat transfer across membranes. Herein we describe a new approach that enables direct measurement of these thermal properties using a LiYF4: Er3+ / Yb3+ upconverting nanoparticle encapsulated within a conformal supported lipid bilayer and dispersed in water as a temperature probe yielding the temperature gradient across the bilayer. The thermal conductivity of lipid bilayer was measured as a function of the temperature, being 0.20±0.02 W·m‒1·K‒1 at 300 K. For the uncapped nanoparticles dispersed in water, the temperature dependence of the thermal conductivity was also measured in the 300‒314 K range as [0.63‒0.69]±0.11 W·m‒1·K‒1. Using a lumped elements model, we calculate the directional heat transfer at each of the system interfaces, namely nanoparticle-bilayer and bilayer-nanofluid, opening a new avenue to understand the membrane biophysical properties as well as the thermal properties of organic and polymer coatings.