Selective, direct detection of acetylcholine in PBS solution, with self-assembled fluorescent nano-particles: experiment and modelling.
Résumé
We report synthesis, characterization and mol. modeling of a new fluorescent cyclotriveratrylene probe for acetylcholine in aq. media, with emission around 430 nm thanks to extended conjugation. The probe discriminates acetylcholine from choline, with resp. binding consts. 540 and 240 M-1 in PBS buffered saline soln., an order of magnitude improvement over the previous best performance. Dynamic light scattering and transmission electron microscopy show the new probe self-assembles in ca. 5 nm diam. particles in PBS medium. Mol. modeling suggests that the high fluorescence quantum yield of the probe, 20% in aq. media, is due to features of the mol. arrangement in the nano-particles, contributing both to exposure of the complexation site and to shielding of the fluorescent π system from quenching by water. Titrn. data for other quaternary ammoniums and modeling indicate that recognition of acetylcholine vs. choline depends on specific electrostatic interactions, and to a lesser extent on exclusion of water by hydrophobic-hydrophilic segregation. Probe-substrate interactions enhance the fluorescence of the probe by shielding against water and by flattening the π system.