Rheology and microstructure of thermoresponsive composite gels of hematite pseudocubes and Pluronic F127.

Stimuli-responsive materials or smart materials are designed materials whose properties can be changed significantly by applying external stimuli, such as stress, electric or magnetic fields, light, temperature, and pH. We report the linear and nonlinear rheological properties of thermoresponsive composite gels based on submicron-sized hematite pseudocube-shaped particles and a triblock copolymer Pluronic F127 (PF127). These novel composites form hard gels at an elevated temperature of 37 °C. For certain concentrations (<20 w/v. %) of hematite pseudocubes in 17.5 w/v. % of PF127, the gel strength is enhanced and the brittleness of the gels decreases. Higher concentrations (>20 w/v. %) of hematite pseudocubes in PF127 result in weaker and fragile gels. We develop an extensive rheological fingerprint using linear and nonlinear rheological studies. Adsorption of PF127 copolymer molecules on the hematite cube surfaces would further assist the formation of particle clusters along with magnetic interactions to be held effectively in the PF127 micellar network at elevated temperatures. The microscopic structure of these composite gels is visualized through a confocal microscope. Our experiments show that addition of hematite cubes up to 20 w/v. % does not change the rapid thermal gelation of PF127 solutions; hence, the hematite-PF127 composite, which transforms into a hard gel near human body temperature of 37 °C, could be suitable for use in smart drug delivery systems.

Ubiquity of complex coacervation of DNA and proteins in aqueous solution.

We report complex coacervation between a primarily hydrophobic protein, elastin, and a strong polyanion DNA (2 kbp) in aqueous and salty solutions at room temperature, 25 °C. The associative interaction at fixed elastin and varying DNA concentration, thereby maintaining a mixing ratio of r = [DNA] : [elastin] = 0.0027 to 0.093, was probed. What distinguishes this study from protein-DNA coacervation reported earlier is that the protein used here was mostly a hydrophobic polyampholyte with low linear charge density, and its complementary polyelectrolyte, DNA, concentration was chosen to be extremely small (1-35 ppm). The interaction profile was found to be strongly hierarchical in the mixing ratio, defined by three distinct regions: (i) Region I (r < 0.02) was defined as the onset of primary binding leading to condensation of DNA; (ii) Region II (0.02 < r < 0.08) indicated secondary binding which led to the formation of fully charge neutralized complexes signaling the onset of coacervation; and (iii) Region III (0.08 < r < 0.12) revealed growth of insoluble complexes of large size facilitating liquid-solid phase separation. The degree of complex coacervation was suppressed in the presence of a monovalent salt implying that screened Coulomb interactions governed the binding. Small angle neutron scattering data attributed an amorphous structure to the coacervates. The elastin-DNA system belongs to a rare class of interacting biopolymers where very weak electrostatic interactions may drive coacervation, thereby implying that coacervation between DNA and proteins may be ubiquitous.

Boron-doped carbon quantum dots: a 'turn-off' fluorescent probe for dopamine detection.

Boron-doped carbon quantum dots (size 2.3 nm) were fabricated by a modified hydrothermal carbonization one-pot synthesis protocol using 4-hydroxy phenylboronic acid as the common precursor that provided seed for the formation of carbon quantum dots as well as the dopant. These quantum dots exhibited excellent properties, namely good aqueous dispersion, strong fluorescence emission, good environmental stability, high selectivity and sensitivity towards the neurochemical dopamine even in the absence of any linker, functionalizing agents or enzyme. It is shown that this material can be used as a 'turn-off' fluorescent probe for the detection of even low concentrations of dopamine with a limit of detection (3σ/S) of about 6 µM. The simplicity of the synthesis protocol and the ease of dopamine detection define the novelty of this approach.

Boron Doped Carbon Quantum Dots: Turn-Off Fluorescent Probe for Dopamine Detection.

In this report, Boron-doped carbon quantum dots (BCQD, size = 2.3 nm) were fabricated by modified hydrothermal carbonization method by one-pot synthesis using phenyl boronic acid as the common precursor that provided seed for the formation of carbon quantum dots as well as the dopant. These quantum dots exhibited excellent properties including aqueous dispersibility, strong fluorescence emission, good environmental stability, highselectivity and sensitivity towards the neurochemical, dopamine even in the absence of any linker or functionalizing agents. It is shown that this material can be used as a "turn off" fluorescent probe for the detection of even low concentration of dopamine with a minimum detection limit of ~6 µM. The simplicity of synthesis protocol and the easiness of dopamine detection define the novelty of this approach.

Mixing ratio dependent complex coacervation versus bicontinuous gelation of pectin with in situ formed zein nanoparticles.

We report on the competitive phenomenon of complex coacervation versus bicontinuous gelation between pectin (P, a polyanionic carbohydrate, [P] = 0.01-2% (w/v)) and zein nanoparticles (Z, a hydrophobic protein and a weak polyampholyte, [Z] = 0.1 and 0.5% (w/v), in an ethanolic solution of effective concentration 4 and 27% (v/v)), which was studied below (pH ≈ 4), and above (pH ≈ 7.4) the pI (≈ 6.2) of zein at room temperature, 25 °C. The uniqueness of this study arises from the interaction protocol used, where the pectin used was in the extended polyelectrolyte (persistence length ≈ 10 nm) conformation while zein was used as a charged globular nanoparticle (size ≈ 80-120 nm) that was formed in situ. Their mixing ratio, r = [P] : [Z] (w/w), was varied from 0.02 to 4.0 (for [Z] = 0.5% (w/v)), and from 0.1 to 7.5 (for [Z] = 0.1% (w/v)) in the ionic strength range 10-4 to 10-2 M NaCl. Zeta potential data revealed that at pH ≈ 4, the complementary binding condition, r = 1 : 1 (equivalent to 1 : 5 molecule/nanoparticle) demarcated the coacervate from the gel region. The measured rigidity (G0, low frequency storage modulus) of these materials revealed the following: for r < 1, (low pectin content samples, coacervate region) the material had lower values of Gcoac0, whereas for r > 1, an excess of pectin facilitated gelation with Ggel0 ≫ Gcoac0. Above pI, surface patch binding caused associative interactions and complex coacervation though both biopolymers had similar net charge. The network density was used as a descriptor to distinguish between the coacervate and gel samples. Their microstructures were probed by small angle neutron scattering (SANS), and viscoelastic properties by rheology. Simple modeling shows that formation of the interpolymer complex was favored in higher protein containing samples. Mixing ratio dependent selective coacervation (a kinetic process) and bicontinuous gelation (a thermodynamic process) are rarely seen to coexist in biopolymer interactions.

Solvent hydrophobicity induced complex coacervation of dsDNA and in situ formed zein nanoparticles.

Zein, a predominantly hydrophobic protein, was sustained as a stable dispersion in ethanol-water (80 : 20, % (v/v)) binary solvent at room temperature (25 °C). Addition of aqueous dsDNA solution (1% (w/v)) to the above dispersion prepared with the protein concentration of Czein = 0.01-0.5% (w/v) caused a concomitant change in ethanol content from 14-35% (v/v), which in turn generated zein nanoparticles in situ of size 80-120 nm increasing with water content. The subsequent associative interaction between DNA (polyanion; 2000 bps) and the positively charged zein nanoparticles, (at pH = 4) was driven by Coulombic forces, and by the solvent hydrophobicity due to the ethanol content of the binary solvent. Experimentally, two interesting regions of interaction were observed from turbidity, zeta potential, particle sizing, and viscosity data: (i) for Czein < 0.2% (w/v), zein nanoparticles of size 80 nm bind to dsDNA (primary complex) causing its condensation (apparent hydrodynamic size decreased from ≈2100 to 560 nm), and (ii) for 0.2% < Czein < 0.5% (w/v) larger nanoparticles (>80 nm) were selectively bound to primary complexes to form partially charge neutralized interpolymer soluble complexes (secondary complexes), followed by complex coacervation. During this process, there was depletion of water in the vicinity of the nucleic acid, which was replaced by hydration provided by the ethanol-water binary solvent. Equilibrium coacervate samples were probed for their microstructure by small angle neutron scattering, and for their viscoelastic properties by rheology. The interplay of solvent hydrophobicity, electrostatic interaction, and zein nanoparticle size dependent charge neutralization had a commensurate effect on this hitherto unexplored coacervation phenomenon.

Hydrophobic hydration and anomalous diffusion of elastin in an ethanolic solution.

Elastin is an important structural protein that confers elasticity to tissues. It is widely used in the biosynthesis of human elastic tissues and exhibits interesting properties. This study reports an insight into the unusual dispersion and anomalous diffusion of elastin in an ethanolic solution. Due to its complex hydrophobic structure, its dispersibility was found to be sensitive towards the hydrophobicity of the solvent. Electrophoresis measurements (zeta-potential data) revealed that its net polarity changed from an anionic to a cationic state with the decreasing solvent hydrophobicity (ethanol content in the solvent). An interesting transition temperature of ∼297 K was observed above which the hydrophobic interactions among the protein molecules became dominant. Double-layer repulsion between protein molecules competes with attractive hydrophobic interactions and causes molecular self-organization. A DLVO-based theoretical model showed that hydrophobic interactions were facilitated by a binary solvent (ethanol-water), and the repulsive double layer screening provided sufficient energy to overcome the interactions between hydrophobic domains in the protein molecule and allow the self-assembly to occur.

Folic acid supramolecular ionogels.

Herein, we report on folic acid (FA, a low molecular weight gelator) thermoreversible supramolecular organo (in 1 : 1 (v/v) water-DMSO binary solvent), and ionogels made in 1-ethyl-3-methyl imidazolium chloride, [C2mim][Cl], and 1-octyl-3-methyl imidazolium chloride, [C8mim][Cl], solutions with 0.1 ≤ [IL] ≤ 5% (w/v). The self-assembled fibrils of folic acid were largely formed due to secondary forces, such as π-π stacking, H-bonding, and hydrophobic interactions above a gelator concentration of 0.2% (w/v) at room temperature, 20 °C. Fragile gels (FGs) having a low frequency storage modulus G0 ≈ 4-6 Pa were formed when 0.2 ≤ [FA] ≤ 0.5% (w/v) in the binary solvent, and at higher gelator concentration (0.5 ≤ [FA] ≤ 2.5% (w/v)) formation of strong gels (SGs) with a G0 value of 140 Pa-5 kPa was noticed. In the IL environment, for a given gelator concentration of [FA] = 1% (w/v), SG formation (with G0 ≈ 2-5 kPa) was noticed when 0.1 ≤ [IL] ≤ 0.5% (w/v), whereas very strong gels (VSGs) with remarkably high gel strengths were formed with G0 ≈ 11-15 kPa. Gelation temperature Tgel could be varied from 45 to 75 °C by varying the FA concentration in the binary solvent, whereas the ionogels exhibited an almost 10 °C rise in gelation temperature. The information obtained from the relative network density νr (ratio of network density in iono- to organo gel), differential free-energy and enthalpy of gelation, ΔGW-IL and ΔHW-IL (difference in free-energy and enthalpy of organo to ionogel), implied that [C2mim][Cl] ionogels had enhanced homogeneity, and higher crosslink density and gel strength. A 3-D plot of Tgel and G0versus gelator concentration clearly defines a phase diagram that describes the contour of the gelation domains of this biologically important gelator.

DNA ionogel: Structure and self-assembly.

DNA dissolved in ionic liquid (IL) solution (1-ethyl-3-methylimidazolium chloride, [C2mim][Cl]) showed a transition to the gel phase ([DNA] ≥ 1% (w/v)). The gelation time was 400 s for the 1% [IL] sample which reduced to 260 s for 5% [IL] concentration. Gelation times, obtained from the viscosity and ergodicity breaking from the dynamic structure factor data, were remarkably identical to each other. Correspondingly, the gelation temperature which was ∼60 °C increased to 67 °C with [IL] content. The small angle neutron scattering (SANS) structure factor profile revealed the presence of the following three distinct length scales: (a) mesh size, ξ ≈ 3 ± 0.5 nm for ionogels, and ≈0.73 ± 0.06 nm, for sol; (b) cross-sectional radius of DNA strand, Rc ≈ 1.6 ± 0.1 nm; and (c) the characteristic inter-cluster distance ≈33 ± 5 nm. Physical conformation of the DNA-IL complexes remained close to the Gaussian coil definition. It was observed that without IL, in the sol phase, the system was completely ergodic and did not gel, while on addition of IL a sudden transition to the non-ergodic (arrested) gel phase occurred. This was due to the formation of an amorphous network of DNA-IL complexes preceding gelation. In summary, it is shown that the DNA ionogels can be prepared with a tunable gel strength (27-70 Pa) and gelation temperature (60-67 °C). Further, the relaxation dynamics was found to be hierarchical in IL content of the gel, revealing considerable self-organization.

Interaction of Globular Plasma Proteins with Water-Soluble CdSe Quantum Dots.

The interactions between water-soluble semiconductor quantum dots [hydrophilic 3-mercaptopropionic acid (MPA)-coated CdSe] and three globular plasma proteins, namely, bovine serum albumin (BSA), ß-lactoglobulin (ß-Lg) and human serum albumin (HSA), are investigated. Acidic residues of protein molecules form electrostatic interactions with these quantum dots (QDs). To determine the stoichiometry of proteins bound to QDs, we used dynamic light scattering (DLS) and zeta potential techniques. Fluorescence resonance energy transfer (FRET) experiments revealed energy transfer from tryptophan residues in the proteins to the QD particles. Quenching of the intrinsic fluorescence of protein molecules was noticed during this binding process (hierarchy HSA<ß-Lg

Biocompatible capped iron oxide nanoparticles for Vibrio cholerae detection.

We report the studies relating to fabrication of an efficient immunosensor for Vibrio cholerae detection. Magnetite (iron oxide (Fe(3)O(4))) nanoparticles (NPs) have been synthesized by the co-precipitation method and capped by citric acid (CA). These NPs were electrophoretically deposited onto indium-tin-oxide (ITO)-coated glass substrate and used for immobilization of monoclonal antibodies against Vibrio cholerae (Ab) and bovine serum albumin (BSA) for Vibrio cholerae detection using an electrochemical technique. The structural and morphological studies of Fe(3)O(4) and CA-Fe(3)O(4)/ITO were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, and dynamic light scattering (DLS) techniques. The average crystalline size of Fe(3)O(4), CA-Fe(3)O(4) nanoparticles obtained were about 29 ± 1 nm and 37 ± 1 nm, respectively. The hydrodynamic radius of the nanoparticles was found to be 77.35 nm (Fe(3)O(4)) and 189.51 nm (CA-Fe(3)O(4)) by DLS measurement. The results of electrochemical response studies of the fabricated BSA/Ab/CA-Fe(2)O(3)/ITO immunosensor exhibits a good detection range of 12.5-500 ng mL(-1) with a low detection limit of 0.32 ng mL(-1), sensitivity 0.03 Ω/ng ml(-1) cm(-2), and reproducibility more than 11 times.

Ergodic-to-nonergodic phase inversion and reentrant ergodicity transition in DNA-nanoclay dispersions.

We have observed DNA concentration and hydration dependent inversion from ergodic to non-ergodic phase followed by reentry into the ergodic phase in DNA-nanoclay (laponite) dispersions at room temperature (25 °C), using results obtained from dynamic light scattering (DLS) and rheology data. The interaction between the DNA strand and the anisotropically charged discotic platelets of laponite (L) was found to be strongly hierarchical in DNA concentration. For a fixed laponite concentration (CL = 1% (w/v)) and varying DNA concentration (CDNA) from 0.3-2.3% (w/v), we observed three distinct phase regions characterized by the following: region (i): CDNA < 1.0% (w/v), ergodic region with weak DNA-L attractive interaction, region (ii): 1.0% < CDNA < 1.6% (w/v), non-ergodic regime having strong DNA-L associative interaction and region (iii): CDNA > 1.6% (w/v), showing phase reentry into the ergodic regime due to repulsion between DNA strands. Hydration study in these three regions revealed that a loss in the abundance of amorphous water, signified by Raman frequency 3460 cm(-1), caused the ergodic to nonergodic phase transition. In summary, it is shown that maximum stability and interaction between DNA and nanoclay platelets occurred at an intermediate concentration of DNA where the hydration was at its minimum. The present system is qualitatively different from the hard-sphere/polymer systems for which reentrant phase transition has been reported in the literature. However, some similarity between the two classes of systems is not ruled out.

Effect of solvent hydrophobicity on gelation kinetics and phase diagram of gelatin ionogels.

We present a systematic investigation of the effect of solvent hydrophobicity (alkyl chain length) on the gelation kinetics and the phase states of the polypeptide gelatin in imidazolium based ionic liquid (IL) solutions. We have observed that IL concentration and hydrophobicity had dramatic influences on the thermal and viscoelastic properties of gelatin ionogels. Gelation concentration cg was observed to increase from 1.75 to 2.75% (w/v) while the gelation temperature Tg was found to decrease from 32 to 26 °C with increase in 1-octyl-3-methyl imidazolium chloride [C8mim][Cl] (most hydrophobic) concentration as compared to the case of the least hydrophobic IL 1-ethyl-3-methyl imidazolium chloride [C2mim][Cl], where the corresponding changes were marginal. Gradual softening of the gel with increase in hydrophobicity and concentration of IL was clearly noticed. The viscosity of the gelling sol diverged as ηr ∼ ε(1)(-k) and storage modulus of gel grew as G0 ∼ ε(1)(t) where ε1 = |1 - c/cg| with the exponents having values k = 1.2-1.8 ± 0.08 and t = 1.2-1.6 ± 0.08, close to but not exactly the same as predicted by the percolation model: k = 0.7-1.3 and t = 1.9. Thus, the gelation kinetics involved in the growth of interconnected networks could be conceived to follow an anomalous percolation model. The temporal growth of self-assembled structures followed a power law dependence given by: ηr ∼ ε(2)(-α) and Rh ∼ ε(2)(-ß) where ε(2) = t > tg (α = 1-2.9 ± 0.08 and ß = 1-2.7 ± 0.08). The low frequency storage modulus G0, gelation temperature Tg, gelation concentration cg and gelation time tg adequately defined the sol-gel phase diagram. Results clearly revealed that by adjusting the hydrophobic chain length and concentration of IL it was possible to customize both thermal and mechanical properties of these ionogels to match specific application requirements.

Effect of persistence length on binding of DNA to polyions and overcharging of their intermolecular complexes in aqueous and in 1-methyl-3-octyl imidazolium chloride ionic liquid solutions.

The effect of persistence length on the intermolecular binding of DNA (200 bp, persistence length l(p) = 50 nm, polyanion) with three proteins, gelatin B (GB) (l(p) = 2 nm, polyampholyte chain), bovine serum albumin (BSA) (l(p) = 7 nm, polyampholyte colloid), gelatin A (GA) (l(p) = 10 nm, polyampholyte chain), and a polysaccharide chitosan (l(p) = 17 nm, polycation), was investigated in aqueous and in 1-methyl-3-octyl imidazolium chloride ionic liquid ([C8mim][Cl]) solutions. In DNA-GB and DNA-BSA solutions complexation primarily arises from surface patch binding whereas DNA-chitosan and DNA-GA binding was predominantly governed by electrostatic forces. These occurred at well defined pH values: (i) at pHc associative interactions ensued and soluble complexes were formed, (ii) at pHΦ soluble complexes coalesced to give rise to liquid-liquid phase separation (coacervation) and (iii) at pH(prep) formation of large insoluble complexes drove the solution towards liquid-solid phase separation. A universal phase diagram encapsulating the aforesaid interactions can be made using the persistence length of polyion as an independent variable. DNA formed overcharged intermolecular complexes with all these polyions when the polyion concentration was more than the concentration required to produce charge neutralized complexes (disproportionate binding). In IL solutions maximum binding occurred when 0.075 < [IL] < 0.10% (w/v) and the effect of overcharging was substantially screened. The extent of overcharge was a monotonous increasing function of the polyion persistence length. Results clearly revealed that DNA-polyion binding was hierarchical in polyion concentration and persistence length. Overcharging of the DNA-polyion complex was found to be ubiquitous for the polyions used in the present study.

Universal correlation between solvent polarity, fluorescence lifetime and macroscopic viscosity of alcohol solutions.

In this report, we show interesting correlation between solvent polarity of alcohol solutions and fluorescence lifetime (τ(av)), estimated from time-resolved fluorescence spectroscopy (TRFS), and macroscopic viscosity (η). Non-functionalized carbon nanoparticles (NCNP) were successfully used as flurophores in these measurements. The solvent polarity, described through polarizability (Δf) of dielectric continuum theory, could universally describe both τ(av)/τ(max) and η/η(max) through the relation, [Formula: see text] with X = τ(av)/τ(max) or η/η(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.

Universal sol state behavior and gelation kinetics in mixed clay dispersions.

Sol and gel state behavior, in aqueous salt free dispersions, of clays Laponite (L) and Na montmorillonite (MMT) was studied at various mixing ratios (L:MMT = r = 1:0.5, 1:1, and 1:2). In the sol state, the zeta potential and gelation concentration of L-MMT obeyed the universal relation, X(L-MMT) = (rX(L) + X(MMT))/(1 + r), where X is zeta potential or gelation concentration (c(g)), implying that these properties are linear combinations of the same of their individual components. The low frequency storage modulus (G(0)'), relative viscosity (η(r)), and apparent cluster size (R) could be universally described by the power-law, G(0)' ∼ ((c/c(g)) - 1)(t) (c > c(g)), and η(r), R ∼ (1 - (c/c(g)))(-k,ν) (c < c(g)), with t = 1.5, k = 1.1, and υ = 0.8 close to the gelation concentration, for r = 1:1 cogel, consistent with the percolation model description of gelation. Interestingly, the hyperscaling relation δ = t/(k + t) yielded δ = 0.56 not too different from the predicted value ∼0.7, while the experimental value of δ obtained from G''(ω) ∼ ω(δ) close to c ≈ c(g) yielded δ = 1.5, which was at variance with the hyperscaling result. The experimental data, on hand, mostly supported percolation type gelation mechanism. As the cogels were slowly heated, at a characteristic temperature, T(g), a sharp increase in G' value was noticed, implying a transition to gel hardening (a new phase state). The temperature-dependent behavior followed the power-law description, G' ∼ (T(g) - T)(-γ) (T < T(g)), with γ = 0.40 ± 0.05 invariant of composition of the cogel, whereas for MMT and Laponite, γ = 0.25 and 0.55, respectively. It has been shown that the cogel has significantly enhanced mechanical (G(0) increased by 10 times for r = 1:1 cogel) and thermal properties (T(g) increased by 13 °C for 1:1 cogel) that can be exploited to design customized soft materials.

Internal structures of agar-gelatin co-hydrogels by light scattering, small-angle neutron scattering and rheology.

Internal structures of agar-gelatin co-hydrogels were investigated as a function of their volumetric mixing ratio, [Formula: see text] , 1.0 and 2.0 using dynamic light scattering (DLS), small-angle neutron scattering (SANS) and rheology. The degree of non-ergodicity ( X = 0.2 ± 0.02) , which was extracted as a heterodyne contribution from the measured dynamic structure factor data remained less than that of homogeneous solutions where ergodicity is expected (X = 10. The static structure factor, I(q) , results obtained from SANS were interpreted in the Guinier regime (low-q , which implied the existence of ≈ 250 nm long rod-like structures (double-helix bundles), and the power law (intermediate-q regions) yielded I (q) ~ q(−α) with α = 2.3 , 1.8 and 1.6 for r = 0.5 , 1.0 and 2.0. This is indicative of the presence of Gaussian chains at low r , while at r = 2 there was a propensity of rod-shaped structures. The gel strength and transition temperatures measured from frequency sweep and temperature ramp studies were suggestive of the presence of a stronger association between the two biopolymer networks at higher r . The results indicate that the internal structures of agar-gelatin co-hydrogels were highly dependent on the volumetric mixing ratio.

Landau theory description of observed isotropic to anisotropic phase transition in mixed clay gels.

A characteristic new cooperative dehydration transition, in 1:1 Laponite-MMT cogel, was observed at T(c) ≈ 60 °C, a temperature at which the storage modulus (G(')) and depolarization ratio (D(p)) showed sharp increase, and the isotropic cogel turned into an anisotropic one. The dehydration dynamics could be described through power-law relations: G(') ∼ (T(c)-T)(-γ) and D(p) ∼ (T(c)-T)(-ß) with γ ≈ ß = 0.40 ± 0.05. The x-ray diffraction data revealed that the crystallite size decreased from 17 nm (at 20 °C) to 10 nm (at 80 °C) implying loss of free and inter-planar water. When this cogel was spontaneously cooled below T(c), it exhibited much larger storage modulii values which implied the existence of several metastable states in this system. This phase transition could be modeled through Landau theory, where the depolarization ratio was used as experimental order parameter (ψ). This parameter was found to scale with temperature, as ψ ∼ (T(c)-T)(-α), with power-law exponent α = 0.40 ± 0.05; interestingly, we found α ≈ ß ≈ γ.

Interaction of soot derived multi-carbon nanoparticles with lung surfactants and their possible internalization inside alveolar cavity.

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 approximately 150 nm, zeta potential approximately -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.

Multifunctional, fluorescent DNA-derived carbon dots for biomedical applications: bioimaging, luminescent DNA hydrogels, and dopamine detection.

Here, we describe the synthesis of 2-3 nm, hydrophilic, blue fluorescence-emitting carbon dots (C-Dots, made using a DNA precursor) by the hydrothermal route from the gelling concentration of 2% (w/v) DNA. These dots exhibited highly efficient internalization in pathogenic fungal cells, negligible cytotoxicity, good PL stability, and high biocompatibility, thus demonstrating their potential as nanotrackers in microbial studies. Bioimaging was performed using Candida albicans as the representative for microbial pathogens. The novelty of these dots is that they formed fluorescent nanocomposite hydrogels with the same DNA much below the gelation concentration (1% w/v) and the tunable gels possessed strength between 20 and 80 Pa with the corresponding gelation temperature Tgel between 40 to 50 °C. The network density and gelation free energy data supported the superior crosslinking ability of these dots. The as-prepared hydrogels can replace the existing toxic quantum dot-based hydrogels for drug delivery. We also demonstrated the use of a DNA hydrogel-fabricated working electrode (DNA-C-Dot/ITO electrode) for the biosensing of dopamine. Our electrochemical biosensor had a detection limit of 5 × 10-3 mM for dopamine. These multifunctional, fluorescent C-Dots and hydrogel after suitable conjugation or loading with molecules and drugs hold promising potential for further exploitation in bioimaging, targeted drug delivery, wound healing, and biosensing applications.