Nano molar level chromogenic and fluorogenic sensing of heavy metal ions using multi-responsive novel Schiff base as a dual mode chemosensor

Abstract

A Schiff base centered chemosensor namely (2E,NE)-N-(2,4-dihydroxybenzylidene)-2-(quinolin-2-ylmethylene)hydrazinecarbothioamide i.e.SB has been synthesized and further utilized for the selective and sensitive detection of four metal ions through dual channels based on absorption and fluorescence properties. The distinctive recognition of Ni2+, Cu2+, Co2+, and Cd2+ in an aqueous solution was achieved by observation with the naked eye rapid color change, analyzing the absorption and fluorescence spectrum of chemosensor SB in the absence and presence of specific metal ion at respective concentration. The synthesized chemosensor SB offered extremely lower detection limit of nano molar level employing either the absorption or fluorescence method. The strong binding constant values such as 9.02 × 105 M−1 for Ni2+, 11.21 × 105 M−1 for Cu2+, 7.52 × 105 M−1 for Co2+ and 6.73 × 105 M−1 for Cd2+ indicate strong complexation between chemosensor SB and respective metal ion. The mechanism for change in the absorption and fluorescence properties of SB involves the interaction of respective metal ion with sulfur and oxygen functionality in SB molecule followed by the formation of ground state non-fluorescent complex via static quenching process. The TD-DFT studies for the theoretical calculations are in support of experimental interpretations. Pleasingly, the present approach allows quantitative determination of said metal ions in water samples at nano molar level.