Electronically tuned sulfonamide-based probes with ultra-sensitivity for Ga³⁺ or Al³⁺ detection in aqueous solution

Three electronically tuned fluorescent probes (1-3) were synthesized by conjugating a fluorescent unit to N,N-bis-(hydroxyethyl)ethylenediamine. Probe 1 bearing an electron-deficient naphthalenedimide unit did not give a fluorescence response to the presence of various metal ions including monovalent metal ions (Na⁺, K⁺, and Ag⁺), divalent metal ions (Ca²⁺, Cd²⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺, Pb²⁺, and Zn²⁺) and trivalent metal ions (Al³⁺, Ga³⁺, Fe³⁺, and Cr³⁺) in an aqueous solution. By contrast, probes 2 and 3 possessing 1,8-naphthalimide and pyrene fluorophores, respectively, exhibited selective fluorescent "OFF-ON" behaviors as a result of Ga³⁺/Al³⁺ binding among the diverse metal ions, suggesting the importance of fluorophore electronic character with regard to metal ion sensing. The ethylenediamine analog of probe 3, corresponding to probe 4, was unable to yield a significant change in fluorescence intensity in the presence of any metal ions tested here, revealing the essential role of two hydroxyl groups for metal ion binding. A high association constant of K_a = 2.99 × 10⁵ M^-1 was obtained for probe 3 with Ga³⁺, with a limit of detection (LOD) of 10 nM. This LOD is the lowest value known for Ga³⁺ detection using chemical sensors. Along with an increase in aggregate sizes, PET suppression of probes upon metal ion binding was the primary contributor to the enhancement in fluorescence emission necessary for the sensitive detection of the target ions. The probe-metal ion complexes were fully characterized via TEM, FE-SEM, ¹H NMR, fluorescence spectroscopy techniques and DFT calculations.