Avijit Kumar Das

Dual channel selective fluorescence detection of Al(III) and PPi in aqueous media with an ‘off–on–off’ switch which mimics molecular logic gates (INHIBIT and EXOR gates)

In this study, we have synthesized a simple Schiff base type isophthaloyl salicylaldehyde hydrazone (ISH) moiety which selectively detects Al(III) and PPi with a fluorescence enhancement at two different wavelengths in aqueous solution. The sensing phenomenon is also reversible and thus the sensor beautifully mimics logic gates (INHIBIT and EXOR gates).

Nanomolar detection of hypochlorite by a rhodamine-based chiral hydrazide in absolute aqueous media: application in tap water analysis with live-cell imaging.

By employing the oxidation property of hypochlorite (OCl(-)), a novel rhodamine-based hydrazide of the chiral acid ((S)-(-)-2-pyrrolidone-5-carboxylic acid) (RHHP) was designed and synthesized for detection of OCl(-) absolutely in aqueous medium at nanomolar level. The structure of the chiral sensor was also proved by the X-ray crystallography. The bioactivity and the application of the probe for detection of OCl(-) in natural water system have been demonstrated. A plausible mechanism for oxidation of the sensor followed by hydrolysis is also proposed. The sensibility of the receptor toward OCl(-) was studied in absolute aqueous media, and the detection limit of hypochlorite-mediated oxidation to the receptor in nanomolar level makes this platform (RHHP) an ultrasensitive and unique system for OCl(-) oxidation.

A naphthalimide–quinoline based probe for selective, fluorescence ratiometric sensing of trivalent ions

A new naphthalimide–quinoline based probe (NAQ) is designed and synthesized and its structure is confirmed through single crystal analysis. It detects the trivalent ions (Fe3+ or Al3+ or Cr3+) selectively among other alkali and transition metal ions studied. NAQ shows a distinct ratiometric fluorescence behavior upon addition of trivalent metal ions in CH3CN–HEPES buffer solution (40/60, v/v, pH = 7.4). This fluorogenic sensing of NAQ to M3+ (M3+ = Fe3+ or Al3+ or Cr3+) can be observed by the naked eye, when illuminated under the UV light.

A highly selective and sensitive probe for colorimetric and fluorogenic detection of Cd2+ in aqueous media

A new rhodamine-quinoline based dyad has been synthesized. It shows a highly selective response to Cd(2+) in the presence of other competing metal ions in aqueous media (pH = 7.1). The detection limit of the sensor is in the 10(-7) M level.

ESIPT based Hg2+ and fluoride chemosensor for sensitive and selective ‘turn on’ red signal and cell imaging

An excited state intramolecular proton transfer (ESIPT) enabled fluorescent sensor phenanthroline diimino phenol (PDP) for Hg2+ has been designed and synthesized. PDP acts as a dual sensor and selectively detects only Hg2+ in mixed aqueous medium and fluoride in acetonitrile medium over other competing metal ions and anions. The binding of PDP with Hg2+ is supported by DFT. The ESIPT phenomenon in PDP is favored in the presence of Hg2+, which is rarely reported, along with an intense red fluorescence suppressing other competing metal ions. Among different anionic analytes, only fluoride shows a visually detectable exciting color change from pale yellow to pink with almost similar emission characteristics. PDP also demonstrates its importance in the fluorescent imaging of Hg2+ ions in human cancer cells.

Single sensor for multiple analytes: fluorogenic detection of Al3+ in aqueous media and AcO− in organic media

A simple novel receptor, designed to have a combination of both oxygen and nitrogen-binding sites for metal ion and hydrogen bond donor sites for anion, was synthesised. The receptor has been explored for the selective detection of cation Al3+ over the other interfering metal ions and anion AcO− against a range of physiologically relevant anions in the fluorescence spectroscopy. The receptor shows a different response to aluminium and acetate in the emission spectra. The binding isotherm and detection limit demonstrate that the receptor is an excellent fluorometric probe for Al3+ and AcO− . Our receptor HNQ represents a ‘Single sensor for multiple analytes’. A new array for the ‘naked-eye’ detection of cation (Al3+, blue colour) and anion (AcO− , green colour) in the emission spectra. The dual sensing to Al3+ and AcO− is rare.

A FRET-based rhodamine–benzimidazole conjugate as a Cu2+-selective colorimetric and ratiometric fluorescence probe that functions as a cytoplasm marker

On the basis of fluorescence resonance energy transfer (FRET) from benzimidazole to a rhodamine moiety, a rhodamine–benzimidazole conjugate (RBC) ratiometric fluorescent probe has been designed and synthesized. The RBC selectively binds to Cu2+, showing visually observable changes in absorption and emission behavior, and demonstrates an effective intracellular Cu2+ imaging ability, allowing it to function as a cytoplasm marker.

Selective colorimetric and ratiometric probe for Ni(II) in quinoxaline matrix with the single crystal X-ray structure

A quinoxaline based colorimetric nickel sensor, HQAP [2-(quinoxalin-2-ylmethyleneamine)phenol] with high selectivity and sensitivity toward Ni2+ ions is shown to have potential for practical use. The absorption maximum of HQAP shows a large ratiometric shift from 306 to 570 nm (ΔI = 264 nm) in the presence of Ni2+ ions, and the color changes from colorless to deep violet only upon addition of Ni2+ which is very easily observed by the naked eye (the detection limit of Ni2+ is as low as 4.16 μM in solution). The predicted binding mode (2 : 1) from spectral analysis and Jobs plot was confirmed by the single crystal X-ray structure of the complex.

Neighbouring group participation of thiol through aldehyde group assisted thiolysis of active ether: ratiometric and vapor phase fast detection of hydrogen sulfide in mixed aqueous media

The “Addition–NGP (neighbouring group participation)–Elimination” technique has been used for the ratiometric sensing of H2S/HS−. We report herein the sensing of H2S by the nucleophilic addition of H2S/HS− to a –CHO group of R1 adjacent to naphthyl hydroxyl followed by NGP of thiol through a probable six-membered transition state that undergoes the cleavage of the dinitrophenyl ether moiety. In contrast, R2 undergoes a probable addition and elimination mechanism forming a Meisenheimer complex, which also cleaves the ether moiety. However, herein, the reaction is very slow compared to R1, which works through NGP. The probe can be highly promising for selectivity and sensitivity, to detect H2S/HS− with rapid response (<3 min) and it opens a new way for the thiolysis of the ortho-hydroxy aldehyde platform by cleavage of an active ether linkage. Test strips based on R1 were fabricated, which could act as a convenient and efficient H2S/HS− test kit to detect H2S/HS− in an aqueous solution and in a solid phase for practical application.

Relay recognition of F− and a nerve-agent mimic diethyl cyano-phosphonate in mixed aqueous media: discrimination of diethyl cyanophosphonate and diethyl chlorophosphate by cyclization induced fluorescence enhancement

Anion to nerve agent simulant detection by relay recognition has been designed and realized for the first time with sequence specificity (F− → DCNP) via a fluorescence “off–on–on” mechanism. The discrimination of DCNP and DCP via a CIFE (cyclization induced fluorescence enhancement) mechanism has also been demonstrated here. Test strips based on the sensors with F− and DCNP are fabricated, which can act as convenient and efficient nerve agent and F− test kits. The origin of the sequence specificity of different fluorescence recognition was revealed through single X-ray crystal and NMR analysis.