Kaushik Pal

pH dependent supramolecular recognition of dapoxyl sodium sulfonate with 2-hydroxypropyl β-cyclodextrin: an application towards food-additive formulation

pH dependent host–guest complexation of dapoxyl sodium sulfonate (DSS), an intramolecular charge transfer dye, with 2-hydroxypropyl beta-cyclodextrin (HP-β-CD) has been investigated. Complexation of DSS with HP-β-CD has been studied at four different pH values using steady-state and time-resolved spectroscopy. Cyclodextrin encapsulation alters the acid–base properties of DSS and results in host-induced deprotonation. A large fluorescence enhancement of DSS was observed upon HP-β-CD binding at different pH values and it enabled us to evaluate the pH dependent binding affinity of DSS with HP-β-CD. The binding affinity of non-protonated DSS was much higher when compared to its protonated state. Subsequently, the encapsulation induced fluorescence enhancement at ca. pH 7.0 has been implemented for developing a fluorescence displacement method for assaying the binding affinity of food-additives namely monosodium glutamate, trans-ferulic acid, p-coumaric acid, gallic acid and its methyl, ethyl and propyl ester derivatives with HP-β-CD. Other than monosodium glutamate, all other tested food-additives show preferential binding to the hydrophobic cavity of HP-β-CD. Therefore, this quick method of assaying the binding affinity of food additives with a water-soluble macrocyclic host molecule can offer better food processing ability, controlled release, and stability.

Probing microenvironment of micelle and albumin using diethyl 6-(dimethylamino)naphthalene-2,3-dicarboxylate: An electroneutral solvatochromic fluorescent probe.

Synthesis, single-crystal X-ray characterization, and spectroscopic investigation of small, non-charged diethyl 6-(dimethylamino)naphthalene-2,3-dicarboxylate (DMNDC) by UV-Visible, steady-state, and time-resolved fluorescence reveal a series of interesting photophysical properties originating from the intrinsic intramolecular charge transfer (ICT) state, leading to diverse applications. Stokes shift, lifetime, and emission maxima of DMNDC show a very good correlation with ET(30) solvent polarity scale for a series of different polarity solvents, confirms that it has very good environment sensitivity. Furthermore, this dye has been found to be an exceptionally suitable probe for determining Critical Micelle Concentration (CMC) and probing self-organization processes of five different type of surfactant with structural diversity. A 20-60nm blue shift in emission maxima accompanied by a large fluorescence lifetime enhancement (ca. 23ns) was observed upon relocation of DMNDC into a hydrophobic microenvironment. Along with this, the small size, electroneutrality, pH stability, and excellent solvatochromic fluorescent properties are employed for deciphering the number of hydrophobic binding pockets with strong affinity and their local microenvironment present in Bovine Serum Albumin (BSA).

Investigation of the effect of cucurbit[7]uril complexation on the photophysical and acid–base properties of the antimalarial drug quinine

Host-guest complexation of mono and dicationic quinine with cucurbit[7]uril (CB7), a water-soluble macrocyclic host molecule, has been investigated. Jobs plot, time-resolved anisotropy as well as concentration dependent NMR titration confirm the binding of two CB7 macrocycles with one quinine molecule. The binding affinity of dicationic quinine with CB7 is one order of magnitude higher than the binding constant of mono-cationic quinine. Such preferential binding results in one unit pKa shift in the ground-state of the quinoline ring. However, using fluorescence spectroscopy we have obtained two acid-dissociation constants, one for quinoline ring nitrogen and the other for the nitrogen of the quinuclidine moiety. In the excited state, CB7 complexation causes one unit pKa shift for the quinoline ring and 1.9 unit shift for the quinuclidine moiety. Interestingly, a large enhancement of fluorescence lifetime and anisotropy of quinine at pH 2.7 and pH 9.0 upon CB7 complexation was observed due to the restriction of conformational flexibility. Moreover, at pH 3.0, a large fluorescence enhancement of quinine due to CB7 complexation was observed and it was quite significant as compared to that of quinine in 0.1 (M) HCl without CB7. We believe that this study of quinine complexation with CB7 will reduce phototoxicity, increase bioavailability and offer an alternative standard for quantum yield measurements in an amiable condition.

Rationally Designed Solvatochromic Fluorescent Indoline Derivatives for Probing Mitochondrial Environment

A new class of solvatochromic, robust, and multifunctional fluorescent probes derived from indoline is presented. Specificity of mitochondria targeting was achieved and utilized for probing polarity under normal and apoptotic conditions. A large Stokes shift, high quantum yield, thermal, photochemical, and pH stability, tolerance to buffer compositions, and a bioconjugation tool-kit make it a promising candidate for live-cell fluorescence imaging.

Polarization induced dynamic photoluminescence in carbon quantum dot-based ionic fluid

We have recently reported the synthesis of water-dispersible, polymer-passivated and redox-active carbon quantum dots (CQDs). The CQDs were converted into a solvent-less conductive fluid through a step-wise surface modification technique. The material has a core–corona–canopy structure with CQD as the core, passivating-polymer as the corona and polyetheramine (Jeffamine®) as the canopy. These materials are unique in characteristics and are designated as nano-ionic materials (NIMs). Structure and properties of CQD-NIMs were determined by dynamic light scattering, thermogravimetry, differential scanning calorimetry, photoluminescence (PL) and cyclic voltammetry (CV). Dynamic changes in extrinsic PL maxima (λem) of the CQDs were observed during and after CV. Such fluctuations in λem helped to understand the sequential ordering and disordering of the Jeffamine® canopy on the CQD surface during polarization during CV. This phenomenon enables us to understand molecular canopy dynamics in NIMs and further showcases redox-active CQDs as a sustainable material for future electrochemical applications.

Differentiation of Folate-Receptor-Positive and -Negative Cells Using a Substrate-Mimicking Fluorescent Probe

Diagnosis and therapy exploiting overexpressed receptors on the cell surface is one important strategy in medicine. Determination of the over expression level of a particular receptor is prerequisite for it to be of clinical use. Differentiation between FR-positive (FR=folate receptor) and -negative cells via fluorescence microscopy using a substrate mimetic fluorophore is presented in this work. The strategy adopted here is not the classical FA-conjugated (FA=folic acid) fluorescent probe but a small and environment-sensitive pterin-based (pterin is part of folate, i.e., vitamin B9) fluorescent probe. Electronically diverse pterin-based fluorescent probes have been designed and synthesized to understand the effect of the binding environment on the receptor-substrate interactions. By utilizing steady-state UV/Vis and fluorescence along with time-resolved fluorescence spectroscopy, the effects on the electronic and acid-base properties of the substrate were investigated. Evidently, one synthesized probe showed FA-mimicking behavior with strong binding interaction with FR.

Synthesis of Two-Photon Active Tricomponent Fluorescent Probe for Distinguishment of Biotin Receptor Positive and Negative Cells and Imaging 3D-Spheroid

A fluorescence microscopy-based distinguishment between biotin receptor (BiR) positive and negative cell lines via receptor-mediated endocytosis has been demonstrated. A water-soluble, three-component, two-photon (2P) active solvatofluorochromic probe has been designed and synthesized. The applicability of the probe for 2P microscopy and 3D-spheroid was also assessed.

Deciphering micro-polarity inside the endoplasmic reticulum using a two-photon active solvatofluorochromic probe

A new class of two-photon active and solvatofluorochromic dyes for the determination of ER polarity is reported. The fluorescent colour spans almost the entire visible spectrum. One of the derivatives is rationally designed for specific ER targeting. Finally, the fluorescence spectral scanning technique has been utilised to determine the micro-polarity inside the ER which is found to be much lower than that of water.