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Abstract

Osmotic pressures play essential roles in biological processes and numerous technological applications. However, the measurement of osmotic pressures in-situ with spatiotemporal resolution has not been achieved so far. Here, we introduce a novel kind of osmotic pressure sensors based on liposomes (average hydrodynamic diameter ≈ 1 μm) loaded with highly water-soluble fluorescent dyes exhibiting resonance energy transfer (FRET). The liposomes are prepared with a simple extrusion method and experience volume changes in response to osmotic pressure due to water outflux through their semipermeable membrane. The FRET efficiency depends on the effective concentration of the entrapped dyes and thus provides a direct measure of the osmotic pressure surrounding each liposome at a given point in space and time. The sensors show quick response and high sensitivity to osmotic pressures in the biologically relevant range of 0 - 0.3 MPa in aqueous solutions of salt, small organic molecules, and macromolecules. With the help of FRET microscopy, we demonstrate the feasibility of spatiotemporal osmotic pressure imaging, which can be a promising new tool to investigate phenomena involving osmotic pressures and their dynamics in biology and technology.