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Yuan and colleagues were motivated in their search for a new approach by the difficulties they encountered in their work studying the morphology of synaptic structures. In considering how to move forward, "We hypothesized that the key breakthrough would be to discover a medium possessing both a high refractive index and high hydrophilicity -- something neither traditional organic solvents nor aqueous solutions can simultaneously provide," says Yuan.
Yuan's team saw potential in ionic liquids, non-conventional solvents with properties that can be fine-tuned by manipulating their ionic composition. However, compared to conventional reagents, "ionic liquids are far more complex, and their interactions with tissues and fluorescent molecules were virtually uncharted territory," notes Yuan. Careful screening and optimization led them to formulate APIL, an ionic liquid with the desired characteristics. Importantly, APIL exhibits vitrification into a 'glass-like' state on cooling and boosts the fluorescent signal of proteins commonly used to label biological substrates.