Dr. Pradip K. Gangwar

PbS crystals of cubic, octahedral, and
dendritic shapes are synthesized in an aqueous solution
that contains supramolecular complexes of b-cyclodextrin
(CD) and hexadecyltrimethylammonium bromide
(CTAB). The morphology of the PbS crystals depends
on the concentration of CD or CTAB in the reaction
solution; for example, the branched dendritic structures
evolve with an appropriate molar ratio of CD/CTAB
supramolecular complexes and reaction time. Regardless
of the CD/CTAB molar ratios, octahedral PbS
crystals are observed at all compositions of CD/CTAB
for the reaction times of 1–5 h, while self-assembled
branched/dendritic structures are obtained only for CD/
CTAB molar ratios of 0.5, 1, and 2 after a prolonged
reaction, e.g., for 24–48 h. Systematic investigation
reveals that both reaction time and CD/CTAB molar
ratio are responsible for self-assembled branched/dendritic
structures of octahedral crystals.

We report solvent dependent spectroscopic study of unique non-functionalized fluorescent carbon nanoparticles (NCNPs) dispersed in 15 organic solvents: aromatic (three), hydrogen bonded (five) and aprotic (seven). Absorption spectra were found to be independent of the solvent nature, with absorption bands located around 430, 405 and 385 nm whereas photoluminescence (PL) spectra exhibited considerable solvent dependence with PL emission peaks lying in the region 405 to 500 nm. Emission life time measured by time resolved fluorescence spectroscopy revealed that in aromatic solvents the average lifetime (τav), did not change significantly with solvent polarity, which was, typically 4–5 ns under the excitation of 405 nm. In hydrogen bonded solvents, τav was observed to decrease with solvent polarity, but in case of aprotic solvents, τav was observed to increase with solvent polarity for a particular excitation. Emission data in hand revealed possible quantum confinement of these nanoparticles inside the cavity of rings of THF, p-xylene, benzene and toluene molecules.► Carbon nanoparticles dispersed in common organic solvents without surface functionalization. ► Fluorescence was observed from all organic solvent dispersions. ► Solvent dependent UV–vis, steady-state, and time-resolved fluorescence reported. ► Steady-state fluorescence and lifetime measurements showed strong polarity and solvent structure dependence. ► Hydrodynamic radius determined was found to be independent of solvent polarity.

Candle soot deposited on copper plate was collected, and dispersed in various organic solvents,and in water. These non-functionalized samples were probed with an array of experimental techniques.Results of energy-dispersive X-ray analysis confirmed the absence of metallic elements and X-ray diffraction (XRD) study confirmed the presence of amorphous as well as graphitized carbon in these nanostructures with minimum grain size &2 nm. TEM data revealed the presence of 30 nm diameter spherical carbon nanoparticles and dynamic light scattering determined the average hydrodynamic diameter &120 nm in water, implying the packing of these nanoparticles into clusters. Raman spectroscopy showed characteristic peaks located at 1324 and 1591 cm-1 corresponding to the D (diamond) and G (graphite) phase of carbon with the characteristic ratio ID/IG & 1.77, yielding 2.4 nm grain size consistent with XRD data. The electrophoresis measurements yielded mean zeta potential values & - 22 mV in water. The UV–Vis absorption and photoluminiscence (PL) spectra were found to be independent of the solvent nature and polarity, with absorption bands located around 430, 405, 385, and 335 nm, and PL emission peaks lying in the region 390 to 465 nm. Average emission lifetime measured by time resolved fluorescence spectroscopy was observed to decrease with increase in solvent polarity for a particular excitation, and with increasing excitation wavelength in all solvents. It is shown that these nanoparticles have the potential to be used as green fluorescence probes.

In this report, we show interesting correlation between solvent polarity of alcohol solutions and fluorescence lifetime (av), estimated from time-resolved fluorescence spectroscopy (TRFS), and macroscopic viscosity ().  Non-functionalized carbon nanoparticles (NCNP) were successfully used as flurophores in these measurements. The solvent polarity, described through polarizability (f) of dielectric continuum theory, could universally describe both av/max and /max through the relation,  with X = av/max or /max, (subscript OH represents corresponding values for alcohols) for alcohol solutions of methanol, ethanol and 1-propanol at room temperature. We show that fluorescence lifetime and solvent viscosity are universal functions of solvent polarity for alcohol solutions.

We report fluorescence behavior in non-functionalized carbon nanoparticles (NCNP) prepared from lamp soot and their application in imaging of normal and cancer cells. Structural characterization of these particles by Raman spectroscopy showed characteristic peaks located at 1350 and 1590 cm−1 corresponding to the diamond-like (D) and graphite-like (G) bands of the carbon allotropes respectively with the characteristic ratio ID/IG = 2.24. X-ray diffraction study confirmed the presence of amorphous as well as graphitized carbon in these nanostructures with minimum grain size ≈2 nm. A typical luminescence lifetime measured by time resolved fluorescence spectroscopy was obtained 3.54 ns. The photoluminescence behavior of these particles was excitation dependent and gave off blue, green and red fluorescence under UV, blue and green excitation, respectively. Cellular uptake of these NCNP yielded excellent results for cell imaging of human embryonic kidney, lung carcinoma and breast adenocarcinoma cells. Cell imaging was further correlated with cytotoxicity in the above mentioned cell lines and also in leukemia cell lines. Dose dependant cytotoxicity was observed after 24 h up to 48 h of incubation of nanoparticles. Fluorescence microscopy of nanoparticle-cell interaction clearly indicated aggregation of the particles.

We present a systematic investigation of hydration and gelation of the polypeptide gelatin in water–glycerol mixed solvent (glycerol solutions). Raman spectroscopy results indicated enhancement in water structure in glycerol solutions and the depletion of glycerol density close to hydration sheath of the protein molecule. Gelation concentration (cg) was observed to decrease from 1.92 to 1.15% (w/v) while the gelation temperature (Tg) was observed to increase from 31.4 to 40.7 °C with increase in glycerol concentration. Data on hand established the formation of organogels having interconnected networks, and the universal gelation mechanism could be described through an anomalous percolation model. The viscosity of sol diverged as η  (1 – cg/c)−k as cg was approached from below (c < cg), while the elastic storage modulus grew as G′  (c/cg – 1)t (for c > cg). It is important to note that values determined for critical exponents k and t were universal; that is, they did not depend on the microscopic details. The measured values were k = 0.38 ± 0.10 and t = 0.92 ± 0.17 whereas the percolation model predicts k = 0.7–1.3 and t = 1.9. Isothermal frequency sweep studies showed power-law dependence of gel storage modulus (G′) and loss modulus (G′′) on oscillation frequency ω given as G′(ω)  ωn′ and G′′(ω)  ωn′′, and consistent with percolation model prediction it was found that n′ ≈ n′′ ≈ δ ≈ 0.73 close to gelation concentration. We propose a unique 3D phase diagram for the gelatin organogels. Circular dichroism data revealed that the gelatin molecules retained their biological activity in these solvents. Thus, it is shown that the thermomechanical properties of these organogels could be systematically tuned and customized as per application requirement.

A systematic investigation of interaction of multi-carbon nanoparticles, obtained from soot, with dipalmitoyl phosphatidylcholine (DPPC), a clinical pulmonary phospholipid surfactant, sold under trade name “Survanta”, was undertaken to establish a model for internalization of these nanoparticles inside alveolar cavity. In vitro experiments were carried out to establish the phospholipid assisted dispersion mechanism of carbon nanoclusters (size 150 nm, zeta potential  -15 mV) in water. Results obtained from an array of experimental methods, like dynamic laser light scattering, electrophoresis, UV-absorption spectroscopy, surface tension studies and transmission electron microscopy, revealed that the carbon nanoparticles interacted with DPPC predominantly via hydrophobic interactions. Selective surface adsorption of DPPC molecules on nanoparticle surface was found to be strongly dependent on the concentration of the phospholipid. DPPC, a gemini surfactant, formed a rigid monolayer around the carbon nanocluster even at nanomolar concentration and provided excellent stability to the dispersion. Based on the experimental data it is proposed that the free-energy gain involved in the hydrophobic interactions will facilitate the internalization of these nanoparticles on the inner wall of the alveolar cavity.

Molecular structure of surfactants governs the dispersion stability of, poorly soluble and mainly hydrophobic, carbon nanoparticles in solvents. A systematic study was carried out to establish the surfactant assisted dispersion mechanism of carbon nanoclusters (size ~150nm, zeta potential ~-15mV), obtained from soot, in water. An array of surfactants, SDS, CTAB, TX-100, sodium cholate (bile salt) and dipalmitoyl phosphatidylcholine (DPPC, a clinical pulmonary surfactant preparation called Survanta), were used in a wide range of concentration (0.01CMC to 2CMC) to probe the dispersion mechanism. Results revealed that the adsorption of surfactant molecules on the nanoparticle surface was interplay of ionic, hydrophobic and @p-@p stacking forces. The CTAB molecules (cationic) formed a bilayer on the carbon nanoclusters providing robust dispersion stability whereas SDS molecules (anionic) were poorly adsorbed through hydrophobic interactions. TX-100 molecules (neutral) stabilized the dispersion via hydrophobic and @p-@p stacking interactions. Sodium cholate, adsorbed on nanoclusters mostly through hydrophobic interaction and generated large asymmetric complexes. DPPC, a gemini surfactant, formed a rigid monolayer around the carbon nanocluster even at nanomolar concentration and provided excellent stability to the dispersion. Binding constant for adsorption onto a hydrophobic surface or being part of membrane/micelles was found to be energetically most favorable for TX-100 followed by CTAB, NaC and SDS in that order. A comparison with carbon nanotubes data indicates that surfactant assisted dispersion stability is sensitive to the size and morphology of the carbonaceous nanostructure.

Polydisperse preparations of multicarbon nanoparticles were observed to undergo anomalous time-dependent self-association leading to the formation of nanoclusters in carbon tetrachloride (nonpolar and hydrophobic solvent, high solubility), water (polar and nonhydrophobic solvent, low solubility), and ethanol-water binary solvents (polarity and hydrophobicity dependent on ethanol concentration, marginal solubility) at room temperature (20 °C). The association kinetics in CCl4, water, and binary solvents could be described through the Smoluchowski aggregation model where the average cluster diameter dav was found to scale with time, t as dav ∼ t
, where
) 0.58 ( 0.07, 0.45 ( 0.05, 0.57 ( 0.08, and 0.65 ( 0.09 for CCl4, water, and 45% (v/v) and 70% (v/v) ethanol binary solvents, respectively. The corresponding mass fractal dimensions, df of these clusters were df ) 1/
) 1.72 for CCl4, 2.22-2.95 for water (depending on salt concentration), and
1.75 and 1.54 for 45% (v/v) and 70% (v/v) ethanol binary solvents, respectively. The intensity of scattered light, Is(t) did not exhibit such universal behavior: for the CCl4 sample, Is(t) ∼ tR with R ) 0.65, whereas, for all other solvents, a linear time-dependent increase was observed mimicking a Derjaguin-Landau-Verwey-
Overbeek (DLVO) type growth mechanism. The
values obtained for clusters dispersed in CCl4 and the binary solvents implied cluster-cluster aggregation while the same in water behaved like diffusion limited aggregation (DLA) clusters. The physical morphology of these clusters was observed through high-resolution transmission electron microscopy (HRTEM) studies. UV spectroscopy data showed characteristic solute-solvent
interactions. The electrophoretic mobility studies in the binary solvents indicated interesting polyampholytetype behavior of these clusters. The results taken together indicate the specific role played by hydrophobicity
in solubilization of carbonaceous nanoparticles.