Glutamate is an important excitatory neurotransmitter, while GABA is an inhibitory neurotransmitter. However, direct and accurate visualization of these important signaling agents by a chemical sensor is still very challenging. Here, a novel coumarin-based fluorescent sensor for the selective labeling and imaging of amino acids in neurons has been developed. This sensor system provides two binding sites for amino acids: an aldehyde group for the amine binding and a modified fluorobenzene for an unusual nucleophilic aromatic substitution (S