Nidhi Joshi

Influence of Structure, Charge, and Concentration on the Pectin-Calcium-Surfactant Complexes.

Polymer-surfactant complex formation of pectin with different types of surfactants, cationic (cetyltrimethylammonium bromide, CTAB and dodecyl trimethylammonium bromide, DTAB), anionic (sodium dodecyl sulfate, SDS), and neutral (Triton X-100, TX-100), was investigated at room temperature in the presence and absence of cross-linker calcium chloride using light scattering, zeta potential, rheology, and UV-vis spectroscopic measurements where the surfactant concentration was maintained below their critical micellar concentration (CMC). Results indicated that the interaction of cationic surfactant with pectin in the presence and absence of calcium chloride was much stronger compared to anionic and neutral surfactants. The neutral surfactant showed identifiable interaction despite the absence of any charged headgroup, while anionic surfactant showed feeble or very weak interaction with the polymer. The pectin-CTAB or DTAB complex formation was attributed to associative electrostatic and hydrophobic interactions. On comparison between the cationic surfactants, it was found that CTAB interacts strongly with pectin because of its long hydrocarbon chain. The morphology of complexes formed exhibited random coil structures while at higher concentration of surfactant, rod-like or extended random coil structures were noticed. Thus, functional characteristics of the complex could be tuned by varying the type of surfactant (charge and structure) and its concentration. The differential network rigidity (pectin-CTAB versus pectin-DTAB gels) obtained from rheology measurements showed that addition of a very small amount of surfactant (concentration ≪ CMC) was required for enhancing network strength, while the presence of a large amount of surfactant resulted in the formation of fragile gels. No gel formation occurred when the surfactant concentration was close to their CMC values. Considering the importance of pectin in food and pharmaceutical industry, this study is relevant.

Coexistence of iso-nonergodic laponite gel and glass in 1-methyl-3-octylimidazolium chloride ionic liquid solutions.

We report unique colloidal gel-glass coexistence in aqueous laponite dispersion (2% w/v) in the presence of 1-methyl-3-octylimidazolium chloride ionic liquid (IL, [C8mim][Cl], concentration 0.01 to 0.05% w/v), where both of the phases had identical nonergodicity and were dynamically interactive. With aging, the nascent heterogeneous dispersion exhibited spontaneous two-phase separation, and the time-dependent relative viscosity followed: η(r) = |ε|(-k) where ε = (t - t(g))/t(g) and t(g) is the time required for the system to get arrested, with k decreasing from 3.13 to 2.54 as the IL concentration was increased from 0 to 0.03% (w/v), implying slowing down of the arrest kinetics. This time was measured from viscosity and rheology studies, revealing the formation of IL-mediated finite size colloidal networks on a time scale of ~4 × 10(3) s, whereas the dispersion developed a large viscosity one decade in time later (~4 × 10(4) s). Homogeneous transparent upper phase was an entropic glass and exhibited substantial storage modulus gain (300-3000 Pa) with an increase in IL concentration (0 to 0.05% (w/v)). The translucent lower gel phase had a much higher storage modulus. Dynamic light scattering measured bimodal relaxation time of concentration fluctuations. The degree of nonergodicity in the two phases was approximately the same, implying laponite-IL cluster exchange across the interface (identical slow-mode diffusivity). In summary, IL-induced first-order phase separation in laponite dispersion produced a homogeneous colloidal gel coexisting with a glass not commonly observed in soft matter systems. This implied that the two phases were dynamically coupled on long time scales, whereas their short-time behavior was distinctively different.

Evolution of Self-organization and gelation in Laponite Ionogels

We report formation of colloidal gel network in aqueous laponite dispersions containing ionic liquid (IL). With the passage of time, these flowing dispersions reorganized and attained a solid-like rigid structure. Structural reorganization of clay platelets in the IL environment caused reduction in the inter platelet repulsion due the IL-double layer screening and facilitated formation of stronger ionogels for samples with [IL] 0.03% (w/v)). The structure factor comprised two relaxation modes diffusion fast mode followed by an anomalous slow mode. The slow mode was frozen at all IL concentrations whereas the fast mode contained two diffusion coefficients both strongly dependent on IL concentration. In summary, laponite gels could be systematically customized in IL solutions to generate a range of soft materials with properties solely dependent on IL concentration which may not exceed 0.05% (w/v).

Effect of hydrogen ion implantation on cholesterol sensing using enzyme-free LAPONITE®-montmorillonite electrodes

Ion irradiation experiments were performed on LAPONITE®-montmorillonite/indium tin oxide (L-MMT/ITO) films using a 20 keV H2+ ion beam with variable fluence ranging from 1012 to 1016 ions per cm2, and the electrochemical profiling of these irradiated electrodes was done using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used for pre and post-irradiation morphology study and binding analysis. FTIR spectroscopy indicated the implantation of low-energy H2+ ions resulting in the formation of new bonds. The enhanced cholesterol sensitivity of irradiated films up to a fluence of 1013 ions per cm2 was observed due to morphological changes taking place in L-MMT films. Close to 20% enhancement in cholesterol sensitivity was noticed, when the ion fluence was ≈1013 ions per cm2. The sensitivity for cholesterol detection of the L-MMT electrode formed through H2+ ion implantation clearly exhibited a strong dependence on the fluence of the ion beam. The radiation-induced enhanced sensitivity can be proposed as a platform for development of a more effective enzyme-free strip sensor.

Comparative evaluation of enzyme-free nanoclay-ionic liquid based electrodes for detection of bioanalytes

Enzyme-free electrodes were fabricated using nanoclay LAPONITE® and montmorillonite (MMT) and imidazolium based ionic liquids (ILs), 1-ethyl-3-methyl imidazolium chloride ([C2mim][Cl]) and 1-methyl-3-octyl imidazolium chloride ([C8mim][Cl]). Here, we have presented a set of quantitative results which conclude that these enzyme-free electrodes can be used successfully for the development of strip-sensors for detection of bioanalytes. Introduction of ILs into nanoclay dispersions resulted in the enhancement of stability of the thin film electrodes formed of the NC-IL materials. The fabricated electrodes were used for sensing various bioanalytes, such as ascorbic acid (AA), oxalic acid (OA), urea (U), citric acid (CA), glucose (Glu) and cholesterol (Chox). The influence of different alkyl chain lengths of ionic liquids, and the aspect ratio of nanoclay platelets were studied with respect to their electrochemical response to different analytes, which covered seven distinct matrices coated on the electrode surface. Further, the effect of oxygen ion beam irradiation on the electrochemical profiles of these electrodes was explored. The irradiation leads to reduction of the electrochemical properties by blocking certain active charge transfer sites. The electrochemical characterization of these electrodes was done using cyclic voltammetry (CV), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).