Selective and swift-responsive “off-on” rhodamine B based chemosensors: Recognition of multi-metal ions, on-site sensing of Fe(III) in water samples and bioimaging in aqueous media

• Distinct “off–on” rhodamine B based chemosensors for sensing and bioimaging. • High selectivity towards Fe +3 and naked eye detection in < 1 min. • Enhanced fluorescence rationalized by DFT and TD-DFT methods for Fe +3 sensing. • 2a , 2b possess potential application in real water samples and in-vitro imaging. Distinct “off–on” rhodamine B based chemosensors 3',6'-bis(diethylamino)-2-(2-(diethylamino)ethyl) spiro[isoindoline-1,9′-xanthen]-3-one ( 2a ); 3′,6′-bis(diethylamino)-2-(2-(dimethylamino)ethyl) spiro[isoindole-1,9′-xanthen]- 3-one ( 2b ) are synthesized in a facile single-step reaction and characterized by X-ray crystallography, NMR, Mass, and FT-IR techniques. 2a , 2b ; selectively detected Fe +3 , Sn +2 , and Cr +3 , in aqueous media over 17 other environmentally relevant cations. Furthermore, 2a , 2b showed high selectivity towards Fe +3 ion with ∼ 62 fold ( 2a ), ∼21 fold ( 2b ) enhanced emission intensity. A prominent pink color, in < 1 min (∼40 s) was detected under naked eye on sensing Fe +3 . Enhanced emission was rationalized by natural transition orbital analysis using DFT and TD-DFT methods. Detection limit and binding constant of Fe +3 for 2a and 2b are estimated as 51.25 ppb (73 nM), 40.3 ppb (59.7 nM), and 0.80 × 10 3 M −1 , 4.59 × 10 3 M −1 , each respectively, indicating their efficiency towards Fe +3 sensing. A test strip method was developed for the on-site qualitative detection of target metal ions, and practical applicability was demonstrated by determining the concentration of Fe +3 ion in real water samples. Probes 2a , 2b further tested in human gastric adenocarcinoma (AGS) cell-lines via in-vitro imaging. The increased emission intensity in 2a , 2b on binding to Fe +3 in AGS cells indicate their potentiality as small-molecule based biological fluorescent probes.