An ultrasensitive and highly selective fluorescent and colorimetric chemosensor for citrate ions based on rhodamine B and its application as the first molecular security keypad lock based on phosphomolybdic acid and citrate inputs

Rhodamine B (Rh_B) has been developed as novel and efficient colorimetric and fluorometric chemosensor for citrate ions (Cit³⁻) in an absolutely aqueous media. The UV–vis absorption and fluorescent emission titrations experiments have been employed to study the sensing process. Rh_B could act as an efficient "ON–OFF" fluorescent chemosensor for phosphomolybdic acid (PMA) based on an electron transfer (ET) process. Also (Rh_B⁺)₃.PMA³⁻ could operate as an "OFF–ON" fluorescent chemosensor for citrate ions based on a ligand substitution process. The chemosensor Rh_B shows excellent fluorescence sensitivity and selectivity toward citrate in aqueous media, and displays ON–OFF–ON type fluorescence change with alternately adding PMA and citrate to the media along with reversible association–dissociation of the complex. The (Rh_B⁺)₃.PMA³⁻ can be applied to the quantification of citrate with a linear ranges covering from 0.053 to 0.83 and 0.08 to 1.6 µM by detection limits of 6.0 and 9.1 nM for fluorescence and colorimetric methods respectively. The keypad lock operation is particularly important, as the output of the system depends not only on the proper combination but also on the order of input signals, creating the correct password that can be used to "open" this molecular keypad lock through strong fluorescence emission at 575 nm. As a whole, its various logic gate properties may improve its impact for the development of new-generation "intelligence" digital devices. The ionic PMA and Cit³⁻ inputs to (Rh_B⁺)₃.PMA³⁻ have been mimicked as a superimposed electronic molecular keypad lock. Also indicates that Rh_B is suitable for the detection of Cit³⁻ ions in the biological environment. - Highlights: • Our probe is commercially available with good photostability and high quantum yield. • Both color and fluorescence change with long emission wavelength in aqueous media. • Characteristics of an ON–OFF–ON fluorescence switch. • The responses of the chemosensor toward citrate are very fast. • Mimic the function of a security keypad lock on sequential addition of PMA and Cit³⁻