5'-Hydroxymethyl fluorescein: A colorimetric chemosensor for naked-eye sensing of cyanide ion in a biological fluid.

A two-step synthetic method to prepare a highly sensitive and selective chemosensor 5'-hydroxymethyl fluorescein (5'-HMF) is described herein. This sensor was explored as a colorimetric sensor for naked-eye detection of cyanide ion in the biological fluid as well as in organic and aqueous media. The addition of cyanide ion to 5'-HMF resulted in a rapid change in color in aqueous medium from light green to dark fluorescent green, and in acetonitrile from light pink to purple. A significant bathochromic shift in the absorption spectra enables cyanide ion to be detected by naked eyes in water and acetonitrile without any interference of the competing anions such as, AcO-, F- and SCN- in aqueous solution. Using the 1HNMR titration experiments and Job's plot from absorbance spectroscopy, the interaction of CN- ion with 5'-HMF has been investigated and binding stoichiometry was found to be 1:2 (5'-HMF to CN-). The limit of detection (LOD) of the sensor for CN- was 3.68 µM in water with a linearity (R2 = 0.9923) in the range of 0.50 to 30.0 µM concentration assuming 1:2 (5'-HMF to CN-) binding stoichiometry. In addition, the sensor 5'-HMF sensed the CN- ion in human saliva with the LOD as 7.0 µM in aq. medium.

Mechanochemical synthesis of fluorescein-based receptor for CN- ion detection in aqueous solution and cigarette smoke residue.

A facile green method for the mechanochemical synthesis of Schiff base phenylhydrazono-N-methylene fluorescein (PHMF) with 95% yields has been established. The synthesized receptor assists in the naked-eye detection of CN- ions in organic and aqueous media, and F- ions in acetonitrile over a series of anions with a color transfer from colorless to pink. A redshift of 160 nm of PHMF-CN- complex in the absorbance spectrum and a turn-on response in the fluorescence spectrum were observed, respectively, at λmax 345 to 515 and 519 nm. A strong interaction of PHMF with CN- and F- ions forming a 1:3 binding stoichiometry has been noted in this study. In an aqueous medium for CN- ion, the lower limit of detection (LOD) is defined as 9.204 nM, which is quite better in terms of sensitivity. In addition, PHMF's paper-strip sensor for rapid real-time CN- ion sensing was found to be sufficiently sensitive to successfully detect CN- ion in water and a solid state, resulting in a portable device for detecting cyanide ions. In acetonitrile, the receptor's ability to detect CN- ion in cigarette smoke residue was also satisfactorily achieved. Graphical Abstract.

Mechanochemical Synthesis of a Fluorescein-Based Sensor for the Selective Detection and Removal of Hg2+ Ions in Industrial Effluents.

Environmentally benign mechanochemistry-assisted high-yielding synthesis of fluorescein-phenylalaninol (FPA) conjugates as a Schiff base receptor is reported herein. This newly synthesized fluorescent probe is found to be most exciting and efficient because of its simultaneous detection and removal of mercury ions (Hg2+) in aqueous medium and industrial effluents through precipitate formation. The receptor successfully worked as a chemosensor in selectively sensing the Hg2+ ion through the rapid transition from yellow to pink in the colorimetric as well as quenching of fluorescence intensity in the fluorometric assay. The removal of mercury ions was confirmed by the inductively coupled plasma analysis of the supernatant. The lower detection limit of Hg2+ ions for the receptor FPA is 1.65 and 0.34 µM as determined through absorption and fluorescence spectroscopic methods, respectively. The high removal efficiency (∼98%) of the mercury ions is promising and could be achieved via the formation of the complex in a 1:1 stoichiometric ratio of receptor to Hg2+ ions. Furthermore, this probe may be a practical alternative for use in a paper-based portable device for achieving on-site detection of mercury ions in solid, solution, and vapor phases.