D G | Georgia Southern University (original) (raw)

Papers by D G

The polymerization of N-vinylcarbazole was conducted in bulk in presence of ZnO without any exter... more The polymerization of N-vinylcarbazole was conducted in bulk in presence of ZnO without any external initiator and a nanocomposite of poly(N-vinylcarbazole)-ZnO (PNVC-ZnO) was isolated from the system. The polymerization of N-vinylcarbazole by ZnO impregnated with acetylene black (AB) and Fe 3+ was also conducted to isolate the respective AB and Fe 3+ loaded PNVC-ZnO composites, PNVC-ZnO(AB) and PNVC-ZnO(Fe 3+ ). The formation of the poly(N-vinylcarbazole) in these systems was confirmed by FTIR, UV-vis and emission spectroscopic analysis. TGA, DSC and SEM characteristics of these composites were evaluated in details. XRD analysis revealed no crystanillity in poly(N-vinylcarbazole) moiety. PNVC-ZnO was nonconducting but PNVC-ZnO(AB) and PNVC-ZnO(Fe 3+ ) systems exhibited conductivities in the range 0.12 and 10 −3 S/cm respectively. A carbocationic propagation pathway was suggested to explain the initiation of N-vinylcarbazole by Zn ++ moiety in ZnO. Kinetic studies revealed that the polymerization is first order with respect to ZnO and the monomer concentration respectively.

Alkyltrimethylammonium bromide Surfactant Micelle Chain length Micelle-water interface Rotational... more Alkyltrimethylammonium bromide Surfactant Micelle Chain length Micelle-water interface Rotational relaxation a b s t r a c t Photophysics and rotational relaxation dynamics of a b-carboline analog, 3-acetyl-4-oxo-6,7-dihydro-12H-indolo-[2,3-a] quinolizine (AODIQ) have been investigated in cationic alkyltrimethylammonium bromide (nTAB) micelles using steady-state and time-resolved fluorometric techniques. The study reveals modification of its photophysics by the conjugate effect of polarity and rigidity of the micellar environments with varying alkyl chain lengths of the surfactants. Furthermore, it suggests that the fluorophore resides at the micelle-water interfacial domain. Contrary to the single exponential nature of the fluorescence anisotropy decay of AODIQ in aqueous medium, the decay is found to be biexponential in all the micellar environments studied. The enhancements in the steady-state anisotropy and rotational relaxation time in the micellar media compared to that in pure aqueous solution reflect that the fluorophore resides in a motionally restricted environment introduced by the cationic micelles. The rotational correlation time increases marginally with an increase in the surfactant chain length. The rotational relaxation of AODIQ in the micellar environments has been discussed in the light of the two-step and wobbling in a cone model. The model helps in evaluating different rotational parameters and in ascertaining the location of the fluorophore in the micellar media. This technique provides valuable information regarding the rotational relaxations of the fluorophore within an organized assembly. When the lifetime measurements and orientational relaxation measurements are combined, significant inferences can be made regarding the partitioning of the probe in different regions of the micelles.

Effect of variation of length of nonionic surfactants in terms of the headgroup as well as the ta... more Effect of variation of length of nonionic surfactants in terms of the headgroup as well as the tail part on the photophysical and rotational dynamical properties of a -carboline analogue, 3-acetyl-4-oxo-6,7-dihydro-12H-indolo-[2,3-a]quinolizine (AODIQ) has been investigated. Steady-state and time-resolved fluorescence and fluorescence anisotropy have been exploited for the purpose. The experiments revealed modification of the photophysics of AODIQ by the conjugate effect of polarity and rigidity of the micellar environments with varying poly(ethylene oxide) chain length in the case of Triton X series and the alkyl chain length in the case of Tween series surfactants. Fluorometric studies suggest that the fluorophore resides at the micelle-water interface in all these systems. The enhancements in the steady-state anisotropy in all the micellar media compared to those in pure aqueous solution reflect that the fluorophore is located in motionally restricted regions introduced by the nonionic micelles. Contrary to the single exponential nature of the fluorescence anisotropy decay of AODIQ in aqueous medium, they were found to be biexponential in the micellar environments. The rotational relaxation of AODIQ in the micellar environments has been discussed in light of the two-step and wobbling in a cone model. The model helps to evaluate different rotational parameters and to ascertain the location of the fluorophore in the micellar media. The significant feature is that the motional restriction decreases with an increase in the poly(ethylene oxide) chain length while it increases with an increase in the alkyl chain length. The difference in the extent of water penetration due to variation in the thickness of the palisade layer and therefore a variation in the micellar polarity with a variation of the length of poly(ethylene oxide) and alkyl chain has been argued to be responsible. * Corresponding

Steady state and time resolved fluorometric and circular dichroism (CD) techniques have been expl... more Steady state and time resolved fluorometric and circular dichroism (CD) techniques have been exploited to explore the binding interaction of a ketocyanine dye, namely, 2-[3-(N-methyl-N-phenylamino)-2-propenylidene] indanone (MPAPI) with transport proteins, bovine serum albumin (BSA) and human serum albumin (HSA). The emission spectrum of buffered solution of the dye is found to be perturbed remarkably upon binding with the proteins. An explicit study with respect to modification of fluorescence and fluorescence anisotropy upon binding, effect of denaturant, fluorescence lifetime and CD measurements reveal that the dye binds with both BSA and HSA; the binding being stronger with the latter. Denaturation and CD studies reveal that stability of the proteins increases upon binding with the dye. The probable binding sites of the dye in the proteinous environments have been assessed from fluorescence resonance energy transfer (FRET) study. The probe is argued to be located in the inter domain cleft region of HSA (near Trp-214). From the similarity in the fluorescence behavior of the dye in BSA and HSA it is inferred that in BSA environment the probe is located near Trp-212 rather than Trp-132.

Electronic absorption and fluorescence spectroscopic techniques reveal that the pK b for neutral ... more Electronic absorption and fluorescence spectroscopic techniques reveal that the pK b for neutral form (N) of 3-aminoquinoline (3AQ) is 5-5.7, in the ground as well as the first electronically excited state. However, the case is quite different for the second pK b , involving the monocation and the dication. The emissive state is found to be monocationic, even in highly acidic conditions and in nafion membrane, where the ground state of the molecule is predominantly dicationic. This indicates that the dication loses a proton in its electronically excited state, possibly due to the delocalization of the lone pair on the amino nitrogen atom over the ring, due to an intramolecular charge transfer. The pK b values have been computed using density functional theory (DFT) for the equilibria of neutral monocation and monocation dication respectively and found to be reasonably in good agreement with the experimental observations.

We present here, a detailed photophysical and rotational relaxation dynamical study of three stru... more We present here, a detailed photophysical and rotational relaxation dynamical study of three structurally analogous cationic dyes, namely, phenosafranin ͑PSF͒, safranin-T ͑ST͒, and safranin-O ͑SO͒, carried out in well characterized, monodispersed biomimicking anionic reverse micellar nanocavities composed of sodium bis͑2-ethylhexyl͒sulfosuccinate ͑AOT͒/heptane with increasing water contents. The dyes belong to the phenazinium family and they differ in terms of methyl substitution on the planar phenazinium skeleton. The objective of the present study is to investigate the modification in the photophysical and dynamical behavior of the dyes with the change in the size of the water pool of the reverse micelle and thereby to explore the role of methyl substitution. Steady state and time resolved emission and anisotropy studies have been exploited for the purpose. The dyes are found to exhibit a marked decrease in the fluorescence anisotropy with increasing water/surfactant mole ratio ͑w͒, i.e., the water pool size in the reverse micellar core, implying that overall motional restriction experienced by the molecules are decreased with increasing hydration. Some of the depth dependent fluorescence parameters such as fluorescence maximum, fluorescence anisotropy ͑r͒ have been monitored for exploring the microenvironment around the probes in the reverse micelles. Fluorescence studies suggest that at low w values, the probes do not penetrate into the reverse micellar core; rather it binds at the interfacial region. Estimates of the micropolarity at the binding sites of the probe molecule have been determined as a function of w. Finally, dynamic studies reveal that both the lifetime and rotational relaxation time decrease with an increase in w for all the three probes, the extent of the decrease being more for PSF than ST and SO. This indicates a stronger binding of the reverse micelle with ST and SO compared to that with PSF which is rationalized in terms of an increase in the hydrophobicity of the former two dyes because of the methyl substitution on the phenazinium moiety.

Excited-state proton transfer (ESPT) and fluorescence resonance energy transfer (FRET) have been ... more Excited-state proton transfer (ESPT) and fluorescence resonance energy transfer (FRET) have been linearly coupled leading to an efficient pH-sensitive energy transfer from 2-naphthylamine (2NA) to a potentially bioactive cationic phenazinium dye, phenosafranin (PSF). The prototropic product produced exclusively from the photoexcited 2NA in the presence of added alkali serves as the donor for the energy transfer process. The energy transfer process is turned on at pH g 12, whereas the process is turned off at a pH lower than that. Within the range of pH 12 to 13, the energy transfer efficiency (E) has been shown to follow a linear relation with the solution pH establishing the governing role of pH of the solution on the energy transfer process. The energy transfer follows a long-range dipole-dipole interaction mechanism. The critical energy transfer distance (R 0 ) and the distance between the acceptor and the donor (r) have been determined for the ESPT-promoted FRET process at an optimum pH of 13. The present study involving the coupled processes is simple but has its implication due to its potential to be exploited for designing a pH-sensitive molecular switch.

Interaction of a cationic phenazinium dye, phenosafranin (PSF), with the anionic liposomal vesicl... more Interaction of a cationic phenazinium dye, phenosafranin (PSF), with the anionic liposomal vesicle/bilayer of dimyristoyl-l-␣-phosphatidylglycerol (DMPG) has been demonstrated using steady state and time resolved fluorescence and fluorescence anisotropy techniques. The charge transfer emission spectrum of PSF shows a dramatic modification in terms of fluorescence yield together with an appreciable hypsochromic shift in the lipid environment. The blue shift indicates a lowering in polarity inside the vesicle as compared to that in bulk water. The fluorescence and fluorescence quenching studies and micropolarity determination reveal that the cationic fluorophore has a profound binding interaction with the anionic DMPG membrane. Anisotropy study indicates the imposition of a motional restriction on the probe inside the bilayer. The electrostatic interaction between the cationic dye and the anionic lipid membrane has been argued to be the reason behind all these observations. The results could be useful in analyzing membrane organization and heterogeneity in natural membranes exploiting PSF or alike compounds as fluorescent probes.

Tungsten oxide and tungsten blues (WB) prepared by reducing the former with stannous chloride wer... more Tungsten oxide and tungsten blues (WB) prepared by reducing the former with stannous chloride were capable of initiating the bulk polymerization of Nvinylcarbazole (NVC) at the melting temperature of the latter (65 C). The polymerizations were initiated by a g 2 -type of complex formation involving p-clouds of vinyl group and the available d-orbitals of W 6þ ions. A composite of PNVC with WB was isolated and characterized. Formation of PNVC and its incorporation in the composite was confirmed by FTIR, NMR, and UV absorption spectroscopic studies. Scanning electron micrographic analyses of PNVC-WB composite revealed the presence of close packed aggregates comprising particles of average sizes between 100-200nm range. TGA revealed the thermogravimmetric stability trend as WB > PNVC-WB > PNVC. Dc conductivities of PNVC-WB nanocomposite were found to depend on the WB : PNVC weight ratio in the composite and varied from 3.5 Â 10 À4 (weight ratio 26) to 3 Â 10 À2 S/cm (weight ratio 100) at room temperature. The threshold WB : PNVC weight ratio for the manifestation of conductivity in the composite was around 25 under the experimental condition of this study.

Interaction of phenosafranin (PSF), a biologically potent cationic dye molecule, has been studied... more Interaction of phenosafranin (PSF), a biologically potent cationic dye molecule, has been studied with zwitterionic and anionic lipid membranes of dimyristoyl-L-R-phosphatidylcholine (DMPC) and dimyristoyl-L-R-phosphatidylglycerol (DMPG), respectively. The effect of cyclic oligosaccharide, -cyclodextrin ( -CD), on the stability of these probe-bound lipid bilayers has also been investigated exploiting steady state and time-resolved fluorescence, steady state fluorescence anisotropy, and dynamic light scattering techniques. An interpretation of membrane destabilization upon interaction of cyclodextrin with the lipids was drawn exploiting PSF as an extrinsic fluorescent probe. The fluorophore showed discernible interactions with DMPC and DMPG vesicles. Experimental results reveal that the extent of interaction of PSF with DMPG is greater compared to that with DMPC. Addition of -CD into the PSF-bound lipids showed a differential effect for the two lipids of varying surface charge characteristics. In the case of DMPC, addition of -CD resulted in a preferential interaction of the probe with CD. However, addition of -CD to PSF-bound DMPG resulted in the selective interaction of DMPG with the added CD leading to the release of the probe into the bulk aqueous medium.

Dielectric constant and dielectric loss parameters of poly(N-vinylcarbazole) homopolymer and seve... more Dielectric constant and dielectric loss parameters of poly(N-vinylcarbazole) homopolymer and several nanocomposites of poly(N-vinylcarbazole) with ZnO were studied as a function of frequency. In the low frequency range (0-20 kHz) the dielectric constant values of the base polymer varied from ∼30 to ∼2, and the same for the composite varied from 8500 to 2000 (4.54), 5000 to 1000 (2.63) and 2000 to 500 (1.17), the figures in parenthesis denoting the ratio of ZnO:PNVC in the nanocomposites. Likewise, dielectric loss parameters were found to be (7-10 × 10 −3 ) for the homopolymer and 4.0, 2.5 and 1.25 for the three PNVC-ZnO composites respectively. Notably, a mechanical mixture of ZnO and PNVC (1.17) exhibited much lower dielectric constant (400-25) and loss parameters (0.14-0.065). These features imply polarization was differently affected depending on factors such as grain size and grain-boundary interfaces being formed in these systems. Tan ı-temperature variation for the composites revealed the occurrence of a maximum between 60 and 70 • C. These features signified dipole group loss in the composite. Dielectric constant of a conducting nanocomposite of poly(N-vinylcarbazole) with acetylene black revealed very low negative value tending to zero at high frequency.

The interaction of b-cyclodextrin with the non-ionic micelle-forming surfactant Triton X-165 (TX-... more The interaction of b-cyclodextrin with the non-ionic micelle-forming surfactant Triton X-165 (TX-165) has been studied using steady state fluorescence and fluorescence anisotropy techniques. Both extrinsic and intrinsic fluorescence have been exploited for the purpose. Phenosafranin (PSF), a cationic phenazinium dye, has been used as the extrinsic probe while fluorescence of TX-165 has served as the intrinsic one. PSF shows discernible interactions with both TX-165 and b-CD. The experimental results reveal that the extent of interaction of PSF with TX-165 is greater than with b-CD. However, addition of b-CD to a micellar solution of TX-165 containing PSF leads to a disruption of the micelles whereby the fluorophore is released from the micellar environment to the bulk aqueous phase. It has been substantiated that an inclusion complex is formed between the non-ionic surfactant and the cyclodextrin. A 1:1 stoichiometry of the TX-165-b-CD inclusion complex has been proposed. Such a complexation between TX-165 and b-CD results in an inhibition in the micellization process of TX-165 leading to an enhancement in the apparent CMC value. The inferences are drawn from a series of experiments, viz., binding studies, determination of micropolarity, heavy-ion quenching studies and steady state fluorescence anisotropy experiments monitoring both extrinsic and intrinsic fluorescences.

Intramolecular charge transfer (ICT) promoted fluorescence transfer has been investigated in two ... more Intramolecular charge transfer (ICT) promoted fluorescence transfer has been investigated in two polar solvents, acetonitrile and water. The ICT species of 4-N,N-dimethylaminobenzonitrile (DMABN) produced exclusively in the photoexcited state serves as the donor while phenosafranin (PSF), a cationic phenazinium dye, acts as the acceptor. A transfer of the fluorescence occurs from the ICT state of DMABN to PSF in both the media. Time-resolved fluorescence decay analysis in combination with the steady state fluorometric observations conclusively rule out the involvement of the fluorescence resonance energy transfer (FRET) process and establish the fluorescence transfer as a consequence of re-absorption of the fluorescence of the donor by the acceptor. Thus, apart from citing an example of a coupled interaction of ICT and fluorescence transfer, importance of the present work lies in providing a demonstration in differentiating between the Förster resonance energy transfer and the trivial energy transfer through re-absorption.

Contrary to the expected quenching, unprecedented and remarkable enhancement is observed in the f... more Contrary to the expected quenching, unprecedented and remarkable enhancement is observed in the fluorescence of an anionic fluorophore, 8-anilino-1-naphthalene sulfonate, with the addition of bromide ion in cationic cetyltrimethylammonium bromide micellar medium. Electrostatic pushing effect of the halide ion on the anionic fluorophore that forces the probe to penetrate further into the micellar interior has been assigned to be responsible for the novel observation. Experiments with other probes and surfactants propose that the electrostatic pushing effect is rather a general phenomenon. While applying to the ionic drugs in real biosystems, potential application of the unorthodox effect remains in enhancing the solubilization of the drugs into the active target region leading to a radical enhancement in the drug efficacy.

The citrate reduction method of synthesis of gold nanoparticles (AuNPs) as introduced by Frens ha... more The citrate reduction method of synthesis of gold nanoparticles (AuNPs) as introduced by Frens has been standardized to enable one to prepare AuNPs of desired dimension by controlling the composition of the reactants. The standardization has been made through characterization of the nanoparticles by UV-vis spectroscopy and from the transmission electron microscopic (TEM) measurements. Linearity of the plot of the plasmon absorption maximum ( max of the synthesized AuNPs against their diameter as measured from TEM, as well as the plot of max with the fractional concentration of citrate in the reaction mixture provides a convenient and easy route to dictate the size of the synthesized AuNPs from a control on the composition of the reactants. The standardization reveals that a calculated composition of citrate (in terms of fractional concentration) in the reaction mixture produces AuNPs of a desired dimension within the range of 15-60 nm. The diameter of the synthesized gold nanoparticles can be confirmed simply from the UV-vis spectrophotometric technique. This essentially makes the use of costly TEM unnecessary, at least for the primary purposes.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.

Binding interaction of 3-hydroxyflavone (3HF), a bioactive flavonoid, with calf-thymus DNA (ctDNA... more Binding interaction of 3-hydroxyflavone (3HF), a bioactive flavonoid, with calf-thymus DNA (ctDNA) has been explored exploiting various experimental techniques. The dual fluorescence of 3HF resulting from the excited state intramolecular proton transfer (ESIPT) is modified remarkably upon binding with the biomacromolecule. The determined binding constant, fluorescence quenching experiment, circular dichroism (CD) study, comparative binding study with the known intercalative binder ethidium bromide and thermometric experiment relating to the helix melting of ctDNA confirm the groove binding of 3HF with the DNA. This is in contrast to two other members of the flavonoid group, namely, fisetin and quercetin, where the bindings are established to be intercalative. The structural difference of 3HF from the other two probes with respect to the absence/presence of the additional hydroxyl groups is ascribed to be responsible for the difference in the mode of binding.

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Hyper-efficient quenching of fluorescence of non-conjugated polymer poly(N-vinylcarbazole) by sil... more Hyper-efficient quenching of fluorescence of non-conjugated polymer poly(N-vinylcarbazole) by silver nanoparticles is demonstrated. The quenching efficiency increases with an increase in the length of the polymer chain containing the pendant chromophore units. The Stern-Volmer constants (10 9 -10 10 mol À1 dm 3 ) are found to be orders of magnitude higher than the normal photochemical quenching processes. Although nanoparticle induced hyper-quenching is reported for conjugated polymers, similar observation in non-conjugated polymers is unprecedented. A versatile mechanism is proposed that rationalizes the hyper-quenching of both conjugated and non-conjugated polymers.

Photophysics of the nonconjugated vinyl polymer poly(N-vinylcarbazole) (PNVC) has been explored i... more Photophysics of the nonconjugated vinyl polymer poly(N-vinylcarbazole) (PNVC) has been explored in the presence of coumarin 153 (C153) exploiting steady state and time-resolved fluorometric techniques. Dual emission from the two distinct excimers of PNVC adds importance to the study and makes it interesting. The study substantiates the occurrence of Forster resonance energy transfer (FRET) from PNVC to C153. The differential involvement of the two excimers in the energy transfer process has been established. Considering the fact that FRET is a long distance dipole induced phenomenon, this differential effect has been rationalized from a difference in the dipole moments of the two excimers. Determination of the quenching constants reveals an order of magnitude more quenching of the high energy excimer than the low energy one in the presence of the quencher C153.

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