Tunable Molecular Logic Gates Designed for Imaging Released Neurotransmitters

Tunable dual‐analyte fluorescent molecular logic gates (ExoSensors) were designed for the purpose of imaging select vesicular primary‐amine neurotransmitters that are released from secretory vesicles upon exocytosis. ExoSensors are based on the coumarin‐3‐aldehyde scaffold and rely on both neurotransmitter binding and the change in environmental pH associated with exocytosis to afford a unique turn‐on fluorescence output. A pH‐functionality was directly integrated into the fluorophore π‐system of the scaffold, thereby allowing for an enhanced fluorescence output upon the release of labeled neurotransmitters. By altering the pH‐sensitive unit with various electron‐donating and ‐withdrawing sulfonamide substituents, we identified a correlation between the pK_a of the pH‐sensitive group and the fluorescence output from the activated fluorophore. In doing so, we achieved a twelvefold fluorescence enhancement upon evaluating the ExoSensors under conditions that mimic exocytosis. ExoSensors are aptly suited to serve as molecular imaging tools that allow for the direct visualization of only the neurotransmitters that are released from secretory vesicles upon exocytosis.