Aza-CyBz is a selective NaCl-responsive fluorescent probe. It visualizes plant salt stress by forming ordered aggregates with NaCl, leading to reduced near-infrared fluorescence signals. Aza-CyBz utilizes the electrostatic interactions between cationic cyanine dye and NaCl to form J-aggregates, resulting in fluorescence quenching, with a detection limit for NaCl as low as 170 μM.