Debes Ray

Synthesis and characterization of high concentration block copolymer-mediated gold nanoparticles.

The formation of high concentration gold nanoparticles at room temperature is reported in block copolymer-mediated synthesis where the nanoparticles have been synthesized from hydrogen tetrachloroaureate(III) hydrate (HAuCl(4)·3H(2)O) using block copolymer P85 (EO(26)PO(39)EO(26)) in aqueous solution. The formation of gold nanoparticles in these systems has been characterized using UV-visible spectroscopy and small-angle neutron scattering (SANS). We show that the presence of additional reductant (trisodium citrate) can enhance nanoparticle concentration by manyfold, which does not work in the absence of either of these (additional reductant and block copolymer). The stability of gold nanoparticles with increasing concentration has also been examined.

Investigations on microstructural changes in pH responsive mixed micelles of Triton X-100 and bile salt

Aqueous solution behaviour of a nonionic surfactant Triton X-100 is investigated in the presence of two bile salts namely sodium deoxycholate (NaDC) and sodium cholate (NaC) at different pH, temperatures and in the presence of sodium chloride and the resultant structural changes to accordingly formed mixed micelles were analyzed by using cloud point (CP), viscosity and scattering techniques. Both the bile salts increased the CP and showed a corresponding decrease in viscosity and apparent hydrodynamic diameter (Dh), which can further be subsided with the progressive addition of sodium chloride and an increase in temperature. Interestingly, in the presence of bile salt below pH ∼5, CP decreased with corresponding increase in viscosity, while a reversed trend was observed above pH∼8. Small angle neutron scattering data reveal that nearly spherical mixed micelles were formed in the presence of bile salt which grow and transform to prolate ellipsoidal ones at pH∼3. These morphological changes are facilitated by the protonation of carboxylic acid group of bile salt and deeper penetration of bile acid molecules into TX-100 micelles at lower pH. Proposed molecular interactions are extremely informative to understand more about these biologically important compounds playing a crucial role in digestion processes.

Interaction of imidazolium based ionic liquids with Triton X-100 micelles: investigating the role of the counter ion and chain length

The micellar characteristics of an aqueous solution of the polyoxyethylene based alkyl aryl type non-ionic surfactant Triton X-100 (TX-100) in the presence of several 1-alkyl-3-methylimidazolium (Cnmim+) based non-amphiphilic, amphiphilic and biamphiphilic ionic liquids (ILs) varying in the alkyl chain length of the cation (C4–C10) and different anions viz. chloride [Cl−], tetrafluoroborate [BF4−], trifluoromethanesulfonate [CF3SO3−], hexaflurophosphate [PF6−] and octylsuphate [C8SO4−] have been systematically investigated by cloud point (CP), viscosity, dynamic light scattering (DLS), nuclear magnetic resonance (NMR), fluorescence and small-angle neutron scattering (SANS) measurements. These ILs interact with TX-100 micelles on account of their hydrophobicity; ILs with a hydrophilic counter ion (Cl−) favour demicellization, increase CP, decrease the solution viscosity and apparent hydrodynamic diameter (Dh) of micelle, whereas ILs with a hydrophobic counter ion (PF6−) increase the solution viscosity and exhibit micellar growth. SANS data on the size/shape of TX-100 micelles in the presence of different ILs are reported to provide information on microstructural changes in the micelles. The location of ILs, which has a dominating effect on the structure of the micelles, was evaluated using fluorescence and 1H NMR studies, and these results are discussed in terms of the insertion of the ILs in TX-100 micelles. IL molecules with long alkyl chains penetrate deeper in the TX-100 micelles. The present study provides useful information on tuning the solution characteristics of TX-100 in the presence of ILs that may have industrial applications.

1-Hexanol triggered structural characterization of the worm-like micelle to vesicle transitions in cetyltrimethylammonium tosylate solutions

The cationic surfactant cetyltrimethylammonium tosylate (CTAT) forms highly viscous/viscoelastic solutions and worm-like micelles at relatively low concentrations. The effect of 1-alkanols with short to long alkyl chains viz. ethanol, 1-butanol, 1-hexanol and 1-octanol on CTAT micelles was examined. In particular, a detailed study on the effect of 1-hexanol was carried out by viscosity, cryogenic transmission electron microscopy (cryo-TEM), nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS) to observe microstructural changes in CTAT micelles. 1-Hexanol displays a distinct interaction with CTAT micelles strongly dependent on CTAT concentration. Up to a certain critical CTAT concentration, 1-hexanol molecules penetrate into the micelles and show growth. Characterization by direct cryo-TEM imaging implies that upon progressive addition of 1-hexanol, worm-like CTAT micelles first grow and finally transform into vesicles. The course of vesicle formation was followed by SANS measurements. The site of 1-hexanol in the micelles close to the palisade layer was evaluated using 2D NMR. This study devotes a fundamental knowledge for controlling the shape and size of worm-like micelles that find many industrial applications particularly in personal- and home-care products.

Mechanically robust dual responsive water dispersible-graphene based conductive elastomeric hydrogel for tunable pulsatile drug release

Nanohybrid hydrogels based on pristine graphene with enhanced toughness and dual responsive drug delivery feature is opening a new era for smart materials. Here pristine graphene hydrogels are synthesized by in situ free radical polymerization where graphene platelets are the nanobuiliding blocks to withstand external stress and shows reversible ductility. Such uniqueness is a mere reflection of rubber-like elasticity on the hydrogels. These nanobuilding blocks serve also the extensive physisorption which enhances the physical crosslinking inside the gel matrix. Besides the pH-responsive drug release features, these hydrogels are also implemented as a pulsatile drug delivery device. The electric responsive drug release behaviours are noticed and hypothesized by the formation of conducting network in the polyelectrolytic hydrogel matrix. The hydrogels are also tested as good biocompatibility and feasible cell-attachment during live-dead cell adhesion study. The drug release characteristics can also be tuned by adjusting the conducting filler loading into the gel matrix. As of our knowledge, this type of hydrogels with rubber-like consistency, high mechanical property, tunable and dual responsive drug delivery feature and very good human cell compatible is the first to report.

Supramolecularly Engineered Amphiphilic Macromolecules: Molecular Interaction Overrules Packing Parameters

We report molecular interaction-driven self-assembly of supramolecularly engineered amphiphilic macromolecules (SEAM) containing a single supramolecular structure-directing unit (SSDU) consisting of an H-bonding group connected to a naphthalene diimide chromophore. Two such SEAMs, P1-50 and P2-50, having the identical chemical structure and hydrophobic/hydrophilic balance, exhibit distinct self-assembled structures (polymersome and cylindrical micelle, respectively) due to a difference in the H-bonding group (hydrazide or amide, respectively) of the single SSDU. When mixed together, P1-50 and P2-50 adopted self-sorted assembly. For either series of polymers, variation in the hydrophobic/hydrophilic balance does not alter the morphology reconfirming that self-assembly is primarily driven by directional molecular interaction which is capable of overruling the existing norms in packing parameter-dependent morphology control in an immiscibility-driven block copolymer assembly.

Microstructure of copolymeric micelles modulated by ionic liquids: investigating the role of the anion and cation

Here we report comprehensive analysis of the influence of ionic liquids (ILs) on the physicochemical properties of an ethylene oxide–propylene oxide (EO–PO) star shaped block copolymer, Tetronic®1304 (total mol. wt = 10 500 and %PEO = 40%) hereafter referred to as T1304, by employing cloud point (CP), viscosity, dynamic light scattering (DLS), small-angle neutron scattering (SANS), high sensitivity differential scanning calorimetry (HSDSC) and fluorescence measurements and 1H NMR spectroscopy. 1-Alkyl-3-methylimidazolium based ILs varying in alkyl chain length (C4–C10) and anion, namely chloride, tetrafluoroborate, trifluoromethanesulfonate and hexafluorophosphate, were used. At lower concentrations, the ILs with different anions showed identical effects on the T1304 micelles but a significant change was noticed at higher concentrations. ILs with longer alkyl chains formed smaller mixed micelles while those with shorter chains remained in the bulk. Moreover, an increase in the alkyl chain length significantly increased the CP while the apparent hydrodynamic diameter (Dh) of the micelles decreased. In line with this, the variation in alkyl chain length has no significant effect on the CMT up to C8mimCl but decreases efficiently for C10mimCl. On the other hand, the variation in counter-ions seldom influenced the CP/Dh/CMT of the T1304 solutions at lower concentrations of the IL, but showed sufficient change at higher concentrations. The presence of NaCl suppressed the effect of the IL on the T1304 micelles. The microstructural changes as observed for the T1304 micelles in the presence of different ILs are explained using SANS data. Fluorescence studies with coumarin-481 as a probe showed shifts in the emission spectra and enhancement in the fluorescence decay, suggesting incorporation of the ILs in the micelles. The penetration of ILs with longer alkyl chains in the T1304 micelles was further confirmed using 1H NMR spectroscopy. The present study provides valuable information on tuning the solution behaviour of T1304 with ILs that may prove to be beneficial for different industrial applications.

Bile salt assisted morphological changes of cationic gemini surfactant (12-4-12) micelles

Microstructural evolution of a cationic gemini surfactant butanediyl-1,4-bis(dodecyldimethylammonium bromide) (hereafter referred to as 12-4-12) micelles in the presence of two bile salts viz. sodium deoxycholate (NaDC) and sodium cholate (NaC) was investigated using surface tension, viscosity, nuclear magnetic resonance (NMR), and small angle neutron scattering (SANS) measurements. A negative value of interaction parameter (β) evaluated from surface tension measurements is a signature of strong synergistic interaction between oppositely charged surfactants. Other micellar parameters have also been calculated at different mole fractions. Both the bile salts induced a shape transitions in 12-4-12 micelles on account of their hydrophobicity. Viscosity measurements disclose that loading of bile salts induces morphological changes in gemini micelles; NaDC is more efficient in altering the aggregation behaviour compared to NaC and presents a pronounced increase in viscosity and micellar growth which is suppressed at elevated temperatures. A remarkable growth observed in micelles in the presence of NaDC at low pH has been ascribed to the solubilization of bile acids formed in acidic medium. The size and shape of gemini mixed micelles obtained from SANS measurements are explicated on the basis of the hydrophobicity of bile salts. The location of bile salts in the micelle was determined from NOESY. The present study characterizes gemini–bile salt mixed systems which significantly enriches our knowledge and such a structural transition provides an opportunity to use these bioamphiphiles (bile salt containing mixed micelles) as delivery vehicles and in some pharmaceutical formulations.

Step-addition Method for Enhancing the Yield of Gold Nanoparticles in Block Copolymer Solution

Triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) has been used to synthesize gold nanoparticles from hydrogen tetrachloroaureate (III) hydrate (HAuCl4·3H2O) salt in aqueous solution at room temperature. Measurements were performed using the triblock copolymer Pluronic P85 (EO26PO39EO26) at a fixed concentration (1 wt%) mixed with varying HAuCl4·3H2O concentration in the range of 0.001 to 0.1 wt%. The surface plasmon resonance (SPR) band in UV-visible absorption spectra confirmed the formation of the gold nanoparticles. The maximum yield of the nanoparticles was found at 0.005 wt% of the salt solution. Small-angle neutron scattering (SANS) does not show any significant change in the scattering profile in these suspensions of the nanoparticles. A similar behavior was also observed in dynamic light scattering (DLS) experiments where autocorrelation function was found to be independent of the salt concentration. This can be understood since a high-block copolymer-to-gold ion ratio (r ∼ 22) is required in the reduction reaction to produce gold particles. As a result, a very small fraction of the block copolymers were associated with the gold nanoparticles, and hence lead to a very low yield. Both SANS and DLS basically see the micelles of most of these block copolymers, which are not associated with nanoparticles. Based on this explanation, a step-addition method was used to enhance the yield of gold nanoparticles by manifold, where the gold salt is added in small steps to maintain higher value of r (>22), and therefore continuous formation of nanoparticles.

Influence of hydrotropic coions on the shape transitions of sodium dioctylsulfosuccinate aggregates in an aqueous medium

The effect of hydrotropic organic coions on the physicochemical behavior of ionic surfactants has not yet been understood clearly, although as counterions they are known to have a remarkable effect on the microstructures of ionic surfactants. In this paper therefore the aggregation behavior of sodium dioctylsulfosuccinate (AOT) has been studied in aqueous media in the presence of sodium benzoate (NaB)/sodium salicylate (NaSa)/sodium meta-hydroxybenzoate (Na-mHB)/sodium para-hydroxybenzoate (Na-pHB). Surface tension and small angle neutron scattering (SANS) techniques are mainly used. AOT is known to have a special counterion binding behavior (SCB) with respect to sodium counterion, viz., at about 0.015 mol kg−1 of added NaCl the counterion binding constant of AOT increases suddenly by two fold which is attributed to the shape change of the AOT micelle. The SCB of AOT is found to exist in the presence of benzoate, m-hydroxybenzoate and p-hydroxybenzoate coions, but not in the presence of o-hydroxybenzoate (salicylate) coions. The SANS data, on the other hand, showed that shape transition of the AOT aggregate takes place in the presence of all the four hydrotropic coions; the concentration of NaSa required to induce the shape transition was however higher. A sharp increase in the value of the hydrodynamic diameter determined by the dynamic light scattering (DLS) experiments also indicates the shape transition of AOT aggregates in the presence of the salts. Chemical shift values of benzoate and hydroxybenzoates confirmed the binding of these coions to an AOT micelle. The binding of the salicylate coion is shown to be stronger due to its low hydration energy and high pKh (Kh is the hydrolysis constant) values.