Chirantan Kar

NIR- and FRET-based sensing of Cu2+ and S2- in physiological conditions and in live cells.

We have synthesized a new indole functionalized rhodamine derivative L(1) which specifically binds to Cu(2+) in the presence of large excess of other competing ions with visually observable changes in their electronic and fluorescence spectral behavior. These spectral changes are significant enough in the NIR and visible region of the spectrum and thus enable naked eye detection. The receptor, L(1), could be employed as a resonance energy transfer (RET) based sensor for detection of Cu(2+) based on the process involving the donor indole and the acceptor Cu(2+) bound xanthene fragment. Studies reveal that L(1)-Cu complex is selectively and fully reversible in presence of sulfide anions. Further, fluorescence microscopic studies confirmed that the reagent L(1) could also be used as an imaging probe for detection of uptake of these ions in HeLa cells.

Zn2+ and Pyrophosphate Sensing: Selective Detection in Physiological Conditions and Application in DNA-Based Estimation of Bacterial Cell Numbers

A diformyl-quinoline based receptor (L1) exhibits selective colorimetric and fluorometric sensing of Zn(2+) in aqueous medium at pH 7.4 based on the intraligand charge transfer (ICT) process. The in situ formed phenoxo-bridged complex, L1·2Zn can selectively and specifically sense PPi among all the other biologically important anions including ATP through reversible binding. The detection limit for Zn(2+) and PPi were found to be approximately 56 and 2 ppb, respectively. The unique selectivity of the PPi by the L1-Zn ensemble could be used as an analytical tool to probe PPi generation in a prototype polymerase chain reaction (PCR) setup and track DNA amplification with higher sensitivity as compared to conventional agarose gel electrophoresis. Interestingly, the principle of PPi estimation in PCR rendered rapid estimation of bacterial cell numbers with a limit of detection of 10 CFU of Escherichia coli MTCC 433 in as early as 10 PCR cycles. The proposed method of PPi sensing offers interesting application potential in PCR-based rapid diagnostics for pathogenic agents and microbiological quality control.

Selective sensing and efficient separation of Hg2+ from aqueous medium with a pyrene based amphiphilic ligand

We have synthesized a new fluorogenic compound L, which can selectively bind and sense Hg2+ in aqueous medium over a broad pH range. It exhibits excellent selectivity for Hg2+ over a large number of competitive cations (Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Pb2+, Ca2+, Mg2+, Na+, K+). The sensing ability of the ligand is studied by fluorescence and UV-vis spectroscopy. Hg2+ present even in a nanomolar range can be detected. The effectiveness of L for detecting Hg2+ inside live human cancer cells (HeLa) is also examined. The hydrophobic part of L is efficiently employed for the quantitative extraction of Hg2+ from an aqueous medium into the organic layer. The extraction ability of L is also estimated by NMR, fluorescence and atomic absorption spectroscopy showing that approximately 99% of the Hg2+ ions are extracted. These results imply that the compound has potential application for sensing and removal of Hg2+ ions from waste water in a large cross-section of mercury threatened zones around the world.

A simple and efficient fluorophoric probe for dual sensing of Fe3+ and F−: application to bioimaging in native cellular iron pools and live cells

A new quinoline functionalized fluorophoric Schiff base L1 was synthesized and its colorimetric and fluorescence responses toward various metal ions in mixed aqueous media were explored. The ligand exhibited high selectivity towards Fe3+ in the presence of a large excess of other competing ions with certain observable optical and fluorescence changes. These spectral changes are significant enough in the visible region of the spectrum and thus enable naked eye detection. The efficiency of L1 in detecting Fe3+ ions was also checked in the presence of relevant complex biomacromolecules viz. methaemoglobin, fetal bovine serum and human serum albumin. L1 was also found to be sensitive enough for visual detection of Fe3+ ions in native iron pools of banana pith. Studies revealed that L1–Fe complex formation is fully reversible in the presence of the fluoride anion with very high selectivity. Furthermore, fluorescence microscopic studies demonstrated that compound L1 could also be used as an imaging probe for detection of uptake of these ions in model human cells. This selective sensing behaviour of L1 towards Fe3+ was explained via the CHEF process where theoretical calculations also supported the premise.

Synthesis, crystal structure and bio-macromolecular interaction studies of pyridine-based thiosemicarbazone and its Ni(II) and Cu(II) complexes

A new pyridine-based heterocyclic thiosemicarbazone ligand and its Ni(II) and Cu(II) complexes have been synthesized and characterized by structural, analytical and spectral methods. The mono-deprotonated anionic form of the ligand coordinates via NNS donor atoms to yield an octahedral Ni(II) complex and distorted square planar Cu(II) complex. UV-visible and fluorescence-based spectroscopic techniques revealed that both metal complexes interact with double stranded DNA via intercalation. A comparative assessment indicated that the Ni(II) complex displayed superior DNA binding. The interaction of these compounds with bovine serum albumin (BSA) suggested that the ligand and its Cu(II) complex quenched the intrinsic fluorescence of BSA in a static quenching process, whereas for the Ni(II) complex, fluorescence quenching of BSA was a combination of both static and collision/dynamic quenching processes. The quenching of the fluorescence of BSA is owing to energy transfer from the tryptophan residues of BSA to the compounds bound to BSA. Cytotoxicity tests based on the standard MTT assay revealed that the Cu(II) complex displayed prominent anti-proliferative activity against HeLa cells.

A novel amphiphilic thiosemicarbazone derivative for binding and selective sensing of human serum albumin

The interaction of ligands and drug molecules with protein is of major interest in drug pharmacokinetics and pharmacodynamics. In this study, we synthesized a novel thiosemicarbazone-based amphiphilic molecule for selective binding and detection of human serum albumin (HSA) with significant increase in fluorescence intensity. The compound 5-(octyloxy) naphthalene substituted salicylaldehyde thiosemicarbazone was designed to interact with site I of HSA. The weak fluorescence of the probes in aqueous solution showed a dramatic increase in fluorescence intensity upon binding with HSA, while the responses to various other proteins and enzymes were negligible under similar experimental conditions. Changes in fluorescence intensity and formation of a new emission maximum of the compound in the presence of HSA as well as an increase in steady-state anisotropy values reflected well the nature of binding and location of the probe inside the protein environment.

Colorimetric and Fluorometric Discrimination of Geometrical Isomers (Maleic Acid vs Fumaric Acid) with Real-Time Detection of Maleic Acid in Solution and Food Additives

Heterobis imine Schiff base probe L is able to discriminate geometrical isomers (maleic acid vs fumaric acid) through sharp colorimetric as well as fluorogenic responses even conspicuous with the naked eye. Colorimetric as well as fluorogenic sensing of maleic acid among various carboxylic acids was also demonstrated in ethanol-buffer medium. Sensing behavior of L was corroborated by (1)H NMR spectra, mass spectrometry, and theoretical calculations. Subsequently sensing behavior of L was used to probe maleic acid in starch rich food samples.

Synthetic amphiphiles as therapeutic antibacterials: lessons on bactericidal efficacy and cytotoxicity and potential application as an adjuvant in antimicrobial chemotherapy

In this paper, we present a critical assessment of the therapeutic potential of low molecular weight pyridine-based synthetic amphiphiles based on structure-guided bactericidal activity and a rational evaluation of their cytotoxic potential. Fluorescence-based structure-function studies revealed that the amphiphiles were membrane-acting and displayed a hierarchical pattern of bactericidal activity, which could be correlated with their charge density and hydrophobicity. The membrane-targeting activity of the most potent cationic amphiphile (compound 6) was vindicated as it induced extensive membrane-disruption and dissipation of the transmembrane potential (ΔΨ) in pathogenic bacteria. At concentrations equivalent to the minimum inhibitory concentration (MIC) against the Gram-positive pathogen S. aureus MTCC 96, none of the amphiphiles exerted any cytotoxic effect on model human cell lines (HeLa, MCF-7 and HT-29). However, at elevated concentrations, a distinct gradation in the cytotoxic effect was manifested, which is probably accounted by the charge density and conformational flexibility of the amphiphiles. A viable therapeutic application of compound 6 is demonstrated in combinatorial assays, wherein the proclivity of the amphiphile to disrupt bacterial membranes at very low concentration is exploited to enhance the uptake and bactericidal efficacy of erythromycin against Gram-negative pathogenic bacteria.

A near-infrared emissive Al3+ sensing platform for specific detection in solution, cells and probing DNase activity

A new tricarbocyanine-based chemosensor exhibited a dramatic Al(3+)-specific fluorescence turn-on response in the near-infrared (NIR) region. The receptor was found to be highly selective towards Al(3+) over other metal ions in physiological condition. The sensor was non-toxic and could thus be employed as an imaging probe for detecting intracellular Al(3+) in live cells. Interestingly, upon interaction with DNA in solution, the L-Al(3+) ensemble rendered tracking of DNase activity in solution through a systematic reduction in the fluorescence emission intensity.

Fluorescent naphthalene-based benzene tripod for selective recognition of fluoride in physiological condition

AbstractAluminium complex of a naphthalene-based benzene tripod ligand system has been reported for the selective recognition of fluoride in aqueous medium in physiological condition. The ligand can selectively recognize Al3+ through enhancement in the fluorescence intensity and this in situ formed aluminium complex recognizes fluoride through quenching of fluorescence. The receptor system detects fluoride in nanomolar range. The sensing property was extended for practical utility to sense fluoride in tap water, pond water and river water. Graphical AbstractA highly fluorescent tripodal aluminum complex was used for the sensing of fluoride in physiological condition and in waste water.