Intracellular viscosity is a crucial parameter that indicates the functioning of cells. In this work, we demonstrate the utility of TPE-Cy, a cell-permeable dye with aggregation-induced emission (AIE) property, in mapping the viscosity inside live cells. Owing to the AIE characteristics, both the fluorescence intensity and lifetime of this dye are increased along with an increase in viscosity. Fluorescence lifetime imaging of live cells stained with TPE-Cy reveals that the lifetime in lipid droplets is much shorter than that from the general cytoplasmic region. The loose packing of the lipids in a lipid droplet results in low viscosity and thus shorter lifetime of TPE-Cy in this region. It demonstrates that the AIE dye could provide good resolution in intracellular viscosity sensing. This is also the first work in which AIE molecules are applied in fluorescence lifetime imaging and intracellular viscosity sensing.

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Activation of the thermogenic brown and beige fat is an effective means to increasing whole-body energy expenditure. In this work, a unique label-free method was developed to quantitatively assess the metabolism and thermogenesis of mouse adipose tissues in vivo. Specifically, an optical redox ratio (ORR) based on the endogenous fluorescence of mitochondrial metabolic coenzymes (nicotinamide adenine dinucleotide and flavin adenine dinucleotide) was used to measure the metabolic state of adipocytes. Our findings demonstrate that the ORR provides a label-free and real-time biomarker to determine the thermogenic response of brown, beige and white adipose tissues to a variety of physiological stimulations. In addition, the redox ratio also can be used to evaluate the degree of browning in the white fat of cold-acclimated mice. This technique is important to understand the recruitment and activation of thermogenic adipocytes in mammals and thus can help to develop therapeutic strategies against obesity.