A Liposomal Platform for Sensing of Extracellular Analytes Near Cells

Cell-permeable fluorescent chemosensors (calcein, monochlorobimane, and a recently reported spiropyran-based sensor SP2) have been incorporated into yeast total lipid extract-based liposomes to suppress inherent cell permeability to allow the detection of extracellular Ca²⁺, GSH, and Zn²⁺, respectively. The repurposed sensors have enhanced aqueous solubility and the ability to quantitatively measure biologically relevant concentrations of Ca²⁺ (0.25 mM⁻1 mM), Zn²⁺ (6.25 µM⁻50 µM), and GSH (0.25 mM⁻1 mM) by fluorescence in aqueous media. In addition, the liposomal sensors are nontoxic to HEK293 cells and have the ability to detect exogenously added Zn²⁺ (1 mM), Ca²⁺ (1 mM), or GSH (1 mM) near cells without internalisation. This new sensing platform provides a means to repurpose a range of intracellular fluorescent sensors to specifically detect extracellular analytes, while also improving biocompatibility for overall enhanced use in a wide range of biomedical applications.