Detection abilities on tested subjects of sensors should be closely connected to the sensing unit numbers. Herein, two anion sensors ICZ-o-1S and ICZ-o-2S were synthesized by using indolo (2,3-a) carbazoles as fluorescent chromophore and salicylaldehyde as recognition site. Though UV-Vis and fluorescent ways, it demonstrated that F- can induce the sensor solutions becoming colored from colorless to yellow green, and can endow them with bright green turn-on fluorescence, proving their sensitive and selective sensing on F- . Accordingly, the F ion sensing studies including anti-interference abilities against to other anions on fluorescence response, stoichiometric ratios of sensor-F- in 1 : 1 and 1 : 2, -OH deprotonation sensing mechanism confirmed by 1H NMR titration and theoretical calculation were fully covered. Most importantly, fluoride ion detection limits achieved by ICZ-o-1S and ICZ-o-2S were 1.8 x 10-7 M and 6.0 x 10-8 M, respectively, the latter with two sensing units exhibited 3 times lower detection limit outcompeted to the former with only one sensing unit, rendering the sensor design strategy of improving detecting ability by increasing sensing unit number was rational. The practical application of F- detection in water-containing environment calibrated from the standard curve between the fluorescence intensity of sensor-F- system and the changing F- concentration was conducted. In addition, the accuracy of the sensor on detecting F- was evaluated by the spiked recovery experiment, therefore, the fast and convenient F- concentration detection based on the fluorescence color RGB values of the tested sensor-sample mixture was investigated. Consequently, the results obtained by these two sensors should deliver effective supports on designing high-performance sensors featuring naked-eye and fluorescence turn-on anion sensing by altering the response unit numbers.
