Bichitra Nandi Ganguly

Characterization of ZnO nanoparticles grown in presence of Folic acid template

BackgroundZnO nanoparticles (grown in the template of folic acid) are biologically useful, luminescent material. It can be used for multifunctional purposes, e.g., as biosensor, bioimaging, targeted drug delivery and as growth promoting medicine.MethodsSol–gel chemical method was used to develop the uniform ZnO nanoparticles, in a folic acid template at room temperature and pH ~ 7.5. Agglomeration of the particles was prevented due to surface charge density of folic acid in the medium. ZnO nanoparticle was further characterized by different physical methods.ResultsNanocrystalline, wurtzite ZnO particles thus prepared show interesting structural as well as band gap properties due to capping with folic acid.ConclusionsA rapid, easy and chemical preparative method for the growth of ZnO nanoparticles with important surface physical properties is discussed. Emphatically, after capping with folic acid, its photoluminescence properties are in the visible region. Therefore, the same can be used for monitoring local environmental properties of biosystems.

A critical assessment of the bubble model for positronium annihilation in liquids

The bubble model conventionally used to fit the observed characteristics of the pick-off component of ortho-positronium decay in liquids is subjected, in the present study, to a critical assessment. It is demonstrated that in its usual form (namely that of a bubble with a sharp boundary) the model is untenable, when confronted conjointly with experimental data on the lifetime and angular correlation of the decay gammas. A modified version of the model that is relatively free from such shortcomings is presented.

A modified bubble model for positronium atoms in liquids

Positronium self-trapping in liquids is frequently described through the bubble model in order to account for the observed decay characteristics, namely, the enhanced pick-off lifetime and the details of the angular correlation of the decay gammas. This model is improved upon in the present paper by modifying the idealised picture of a sharp bubble radius and thereby a formalism is developed which is not only more realistic but also provides elegant analytic solutions.

Structural aspects of synthetic zeolite: A comparative assay through positron annihilation and gas adsorption methods

Structural arrangement in alumino-silicate compounds (synthetic zeolite species), which encompasses large cavities and channels, have been discussed on the basis of their molecular organization and is further illustrated with the help of micrographs as perceived through transmission electron microscopic studies. The crystallinity aspects and structural defects were investigated through line broadening in x-ray diffraction and the grain sizes through the Debye-Scherrer method. Further, assays of the void size (micro- as well as mesopores) have been performed through positronium annihilation lifetime spectroscopy (PALS) and Brunauer-Emmett-Teller adsorption-desorption isotherm and compared. Despite of some of the limitations of the latter, the final results are in close agreement. The utility of PALS as a microprobe is illustrated through our models. Finally, it has been justified that PALS not only probes the subnanometer micropore sizes efficiently but it also senses the mesopores.

Modification of the bubble model for positronium atoms in high surface tension liquids

The bubble model is widely used for the analysis of the decay characteristics of the positronium atom in liquids. However, according to some authors this description is inappropriate in the case of polar liquids with high surface tension. It has been advocated that for such media, rather than employing the value of the bulk surface tension (which appears as a parameter in the model), one should incorporate the notion of a transition layer between the liquid phase and the cavity that encloses the positronium. Accordingly, the usual bubble model with sharp boundaries is modified in the present work through the introduction of a diffusivity in the bubble surface, thus developing the proper setting (without involving any extra free parameters) such that liquids with high surface tension may also be meaningfully discussed in this context and confronted with experimental data.

A molecular basis of the bubble model of positronium annihilation in liquids

The bubble model of positronium annihilation in liquids universally used for the description of positron annihilation in liquids involves macroscopic notions, based on a continuum description of the liquid (such as surface tension extended to nano-particle dimensions), which is somewhat unsatisfactory. An elementary molecular level description is presented. The consequent clarification of underlying concepts (such as the location of the surface of tension) is injected into the bubble model to obtain a modified version. It is shown that there is a considerable difference between the effective surface tension in such microbubbles and the corresponding bulk values. The concept of a work function for the positronium in the liquid is introduced on the basis of this model from which some conclusions are drawn regarding the quasi-positronium, the precursor of the positronium in the bubble, as being a rather extended delocalized entity.

Dependence of positronium formation on critical micelle concentration: A phenomenological approach

Employing and enlarging the scope of the diffusion-recombination model, proposed by Byakov and Stepanov (1993), for positronium formation in aqueous solutions of surfactants, the general trend of the dependence of positronium formation on Critical Micelle Concentration (CMC) has been successfully explained.

Positron Annihilation Study of ZnO Nanoparticles Grown Under Folic Acid Template

Positron lifetime spectroscopy (PAL), Doppler broadening (DB) as well as coincidence Doppler broadening (CDB) spectroscopy of a new variety of Folic acid (FA) capped zinc oxide nano-particle samples has been performed at room temperature. The results show interesting patterns of observation, hither-to unobserved in ZnO wurtzite crystalline samples, such as predominance of positronium formation, as reflected in the (PAL) analysis, phase transition in the nano crystalline samples at ~0.8-1.0 % FA concentration as depicted from DB results. Also, the chemical environment of the samples has been analysed from the ratio curves of CDB studies. Beside these, other independent results from X-ray diffraction (XRD) data and Debye- Scherrer method, Transmission electron microscopic (TEM) observations and as well as Fourier transformed infrared spectroscopic (FT-IR) analysis are reported for comparison.

Enigmatic features of positronium acceptor reactions in associative solvents

Abstract Positronium (Ps) reactions with the diamagnetic acceptor molecule, namely, nitrobenzene, have been critically studied by us at various temperatures in water and water/methanol mixture (1:1 v/v), with the special emphasis on the Arrhenius plot of the overall reaction rate constant. While the Ps–Ac reaction rate constant in water takes over an anomalous course at higher temperatures (departing from normal Arrhenius paradigm), the reaction in water/methanol mixture, however, shows an entirely different systematics. The temperature dependences of surface tensions and viscosities of these solvents when taken in conjunction with the other physical parameters, enable us to explain and compare the plausible mechanism in the respective solvents.

Temperature, pressure and solvent dependence of positronium acceptor reactions

Abstract Positronium (Ps) reaction rates ( κ ) with weak Acceptors (Ac) leading to the formation of Ps–Ac complexes show several intriguing features: non-monotonic temperature dependence of κ (departing from the usual Arrhenius paradigm), considerable variability of κ with respect to different solvents, and anomalies in response to external pressure at ambient temperature (large changes of κ in some media and hardly any in others). We explain all these phenomena, introducing the novel concept of a critical surface tension, which unifies observations in diverse non-polar solvents at different temperatures and pressures.