A nicotinamide-anthracene conjugate based fluorescent probe 1 exhibits highly selective fluorescence enhancement at 535 nm only with sulfate in HEPES buffer-DMSO (9 : 1) at pH 7.4. The other anions viz.F À , Cl À , Br À , I À , AcO À , HSO 4 À , H 2 PO 4 À , CN À , OH À , NO 3 À , ClO 4 À , SCN À , even if present in excess, do not interfere in the estimation of sulfate. The limit of detection for sulfate ions by probe 1 is 0.5 mM. Probe 1 can also be used to determine the sulfate ions in tap water using EDTA as a metal ion masking agent.Titration experiments and Job's plot reveal the formation of a 1 : 1 stoichiometric complex between the probe 1 and sulfate ions and DFT studies show the electron transfer from SO 4 2À to anthracene moiety responsible for the observed optical changes.The sulfate ion plays a signicant role in the environment 1 and is crucial for many biochemical processes, such as in biosynthesis 2 and sulfate binding proteins. 3 In nature, Gram-negative bacterium Salmonella typhimurium possesses sulfate binding proteins (SBP) in the periplasmic space between the inner cytoplasmic membrane and the external outer membrane which are responsible for tight binding of the sulfate that has passively diffused into the periplasm across the external outer membrane. This protein selectively binds sulfate with an association constant ca. 10 6 M À1 in water and its affinity is independent of pH over the range pH 5 to pH 8. Sulfate is also responsible for the permanent hardness of water; 4 hampers the vitrication process 5 of nuclear waste sites and thus is a known inorganic pollutant in the environment. As a result, the design of suitable hosts for the selective recognition of sulfate remains a challenge in the area of anion binding chemistry. In nature, a seven-coordinate sulfate structure was observed in a sulfate binding protein, where three of four oxygens were linked with two hydrogen bonds. 6 Many synthetic receptors bearing amide, 7 pyrrole, 8 urea, 9 thiourea 10 as motifs for the selective binding of sulfate have been reported. The tripodal molecular probes with three urea/thiourea units appended with different substituents on urea/thiourea nitrogen as strong H-bond donor sites encapsulate sulfate in the presence of hydrophobic tetrabutylammonium as counter cation. 9a,b,10b More recently, Jiang, Lin et al. have shown that squaramide-based tripodal receptor selectively encapsulates sulfate over other anions. 11Such molecular probes have also been used for the quantitative determination of sulfate in solution phase but are in general applicable in aprotic solvents like CH 3 CN, CHCl 3 and DMSO or solvent systems with <5% water. 12 To the best of our knowledge, there are only couple of examples, 13,14 where sulfate has been recognized in methanol-H 2 O (1 : 1) using uores-cence as a tool 13 or a bis-hydroxyl Schiff base molecule has been used to develop ion-selective electrode 14 for the estimation of sulfate within pH range 3-9. Therefore, the design of optical probes for the determination/recognitio...
