Tapas K. Paira

Ascorbate-assisted growth of hierarchical ZnO nanostructures: sphere, spindle, and flower and their catalytic properties.

A simple solution-based method to prepare mainly flowerlike zinc oxide (ZnO) nanostructures using the ascorbate ion as a shape-directing/capping agent at relatively low temperature (ca. 30 and 60 degrees C) was described. However, we observed that different shapes of hierarchical ZnO nanostructures such as flowerlike, spindlelike, and spherical could be obtained with an increase in the synthesis temperature from 60 to 90 degrees C. The effects of other organic capping agents on the shape of hierarchical ZnO nanostructures were also studied. FTIR, FESEM, and XRD characterization were performed on the formed ZnO nanostructures to understand the role of ascorbate in the growth of flowerlike morphology. The nucleation and growth process can regulate by changing the metal precursor and ascorbate ion concentrations. We were able to identify intermediate nanostructures such as spherical/quasi-spherical and spindle that are very much on the pathway of formation of large, flowerlike ZnO nanostructures. Electron microscopy results indicated that these spherical/quasi-spherical ZnO nanoparticles might aggregate through oriented attachment to produce spindlelike and flowerlike nanostructures. On the basis of these results, a possible growth mechanism for the formation of flowerlike ZnO nanostructures was described. The optical properties of these differently shaped ZnO nanostructures were also described. The catalytic activities of the as-synthesized spherical and flowerlike ZnO nanostructures were tested in the Friedel-Crafts acylation reaction of anthracene with benzoyl chloride. The catalysis results indicated that the catalytic activity of flowerlike ZnO nanostructures is slightly higher than the spherical counterpart.

Reversible self-assembly of carboxylated peptide-functionalized gold nanoparticles driven by metal-ion coordination.

Carboxylated peptide-functionalized gold nanoparticles (peptide-GNPs) self-assemble into two- and three-dimensional nanostructures in the presence of various heavy metal ions (i.e. Pb(2+), Cd(2+), Cu(2+), and Zn(2+)) in aqueous solution. The assembly process is monitored by following the changes in the surface plasmon resonance (SPR) band of gold nanoparticles in a UV/Vis spectrophotometer, which shows the development of a new SPR band in the higher-wavelength region. The extent of assembly is dependent on the amount of metal ions present in the medium and also the time of assembly. TEM analysis clearly shows formation of two- and three-dimensional nanostructures. The assembly process is completely reversible by addition of alkaline ethylenediaminetetraacetic acid (EDTA) solution. The driving force for the assembly of peptide-GNPs is mainly metal ion/carboxylate coordination. The color and spectral changes due to this assembly can be used for detection of these heavy-metal ions in solution.

Synthesis and Self-Aggregation of Poly(2-ethyl-2-oxazoline)-Based Photocleavable Block Copolymer: Micelle, Compound Micelle, Reverse Micelle, and Dye Encapsulation/Release

We report on the synthesis of photocleavable poly(2-ethyl-2-oxazoline)-block-poly(2-nitrobenzyl acrylate) (PEtOx-b-PNBA) block copolymers (BCPs) with varying compositions via combination of microwave-assisted cationic ring-opening polymerization (CROP) and atom transfer radical polymerization (ATRP) using α-bromoisobutyryl bromide as an orthogonal initiator. The amphiphilic nature of this BCP causes them to self-assemble into primary micelles in THF/H2O, which further undergo secondary aggregation into nanostructured compound micelles as established through DLS, FESEM, and TEM. Upon UV irradiation (λ = 350 nm), the photocleavage of the PNBA block of the PEtOx-b-PNBA BCP takes place, and that leads to the formation of the doubly hydrophilic poly(2-ethyl-2-oxazoline)-b-poly(acrylic acid) (PEtOx-b-PAA) BCP causing the rupture of compound micelles as confirmed by spectroscopic and microscopic techniques. Encapsulation of a model hydrophobic guest molecule, nile red (NR), into the photocleavable BCP micellar core in aqueous solution and its UV-induced release is also investigated by fluorescence emission measurements. PEtOx-b-PNBA BCP amphiphiles are also shown to self-assemble into spherical nanostructures (∼90 nm) in dichloromethane as established by DLS and TEM analysis. These are referred to as reverse micelles and are able to encapsulate anionic hydrophilic dye, Eosin B, and facilitate its solubilization in organic media.

Surface confined atom transfer radical polymerization: access to custom library of polymer-based hybrid materials for speciality applications

A significant advance has been achieved in the techniques for growing polymers from a surface via atom transfer radical polymerization (ATRP). In this review, we highlight recent advances and developments in the field of surface-confined atom transfer radical polymerization (SC-ATRP) with a brief overview of the fundamentals of ATRP. Some exciting current and forthcoming applications of the materials made by SC-ATRP are presented. We conclude with an outline of the future directions in the applications of SC-ATRP towards the design of advanced functional materials with designed performances for many high-value future applications.

Synthesis, characterization, and solution behavior of well-defined double hydrophilic linear amphiphilic poly ( N -isopropylacrylamide)- b -poly ( ε -caprolactone)- b -poly ( N -isopropylacrylamide) triblock copolymers

Well-defined linear dihydrophilic amphiphilic ABA-type triblock copolymers of ε-caprolactone (CL) and N-isopropylacrylamide (NIPAAm) have successfully been synthesized with a high yield by combining the ring opening polymerization (ROP) and xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization methods. The resulted block copolymer shows the formation of micelles in water as supported by light scattering. The critical micelle concentration (cmc) value of the micelle increases with the increase in the chain length of the poly (N-isopropylacrylamide) (PNIPAAm) block. Cloud point of the block copolymers decreases with the decrease in the PNIPAAm chain length. The TGA analysis shows a one-step degradation and a lower thermal stability of the triblock copolymer than the PNIPAAm. The DSC analysis of the triblock copolymer shows the lowering of glass transition temperature (Tg), and melting temperature (Tm) peaks possibly due to the partial miscibility of the poly (ε-caprolactone) (PCL) block with the amorphous PNIPAAm block through the interaction of ester groups of PCL with the amide groups of PNIPAAm. The XRD pattern of the triblock copolymer shows a broad peak due to the suppression of the crystallization of PCL block owing to the mixing of PNIPAAm block with the PCL block.

Amino-acid-based zwitterionic polymer and its Cu(II)-induced aggregation into nanostructures: a template for CuS and CuO nanoparticles.

A convenient and water-based approach is described for the synthesis of an l-lysine-based zwitterionic polymer, poly(ε-l-lysinyl acrylamide) (PLAM), without using protecting group chemistry, chromatographic purifications, and organic solvents as the reaction media. PLAM contains both amine and carboxylic acid groups in each repeating unit, which can either be protonated or deprotonated just by altering the pH of the solution to obtain overall positive or negative charge. PLAM is tested for its applicability as a zwitterionic polymeric buffer in water. Cu(II) ion-induced aggregation of PLAM as a function of solution pH is studied. Spherical nanogel aggregates are formed at pH 9.5 due to aggregation of PLAM through its complexation with Cu(II) ion. Spherical aggregates appear to dissociate via breaking of the complexation at a pH < 5.5 resulting in molecular dissolution of PLAM. This aggregation process is pH reversible. The Cu(II)-PLAM aggregates are used as a template for fabrication of CuO and CuS nanoparticles.

Gelation of amino acid-based amphiphiles in water-based mixed solvent systems: reusable catalytic templates for nanostructured silica and silica–zirconia photocatalyst

We describe a gelation study of a series of different fatty acid–amino acid conjugated amphiphiles in various mixed solvent systems with water. Field emission scanning electron microscopy (FESEM), polarized optical microscopy (POM) and X-ray diffraction (XRD) study reveals that the self-organization of the amphiphile molecules in the mixed solvent leads to the formation of crystalline fibers, which form the opaque white gel. This amino acid amphiphile-based gel acts as a catalyst as well as a template for the hydrolysis/condensation of tetraethoxysilane (TEOS) to silica and forms a composite gel. The methanol extraction of the opaque white as-prepared composite gel results in the formation of a transparent nanostructured silica gel. The recovered amphiphilic gelator, after a methanol wash, can be reused for preparing a gel, which can subsequently be used as a catalyst to prepare a nanostructured silica gel again. The FESEM study confirms that the formed nanostructured silica gel is made up of spherical silica nanoparticles. We also extend this gel-based catalysis strategy to prepare silica–zirconia mixed oxide nanostructures. TEM examination reveals the formation of spherical silica–zirconia nanoparticle of high surface area as confirmed through BET surface area measurement. Finally, the photocatalytic activity of silica–zirconia mixed oxide is investigated towards methylene blue degradation. The mixed oxide shows higher photocatalytic efficiency than neat zirconia nanostructures.

Synthesis and self-assembly properties of well-defined four-arm star poly( ε -caprolactone)- b -poly( N -vinylpyrrolidone) amphiphilic block copolymers

Abstract Well-defined amphiphilic four-arm star diblock copolymers of poly(ε-caprolactone) (PCL) and poly(N-vinylpyrrolidone) (PNVP) have successfully been synthesized by combining the ring-opening polymerization (ROP) of ε-caprolactone (CL) and xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization of N-vinylpyrrolidone (NVP). The resulting block copolymer shows the formation of spherical micelles in water as revealed by transmission electron microscopy (TEM) and supported by light-scattering study. The critical micellar concentration (cmc) value of the micelle increases with the increase in the PNVP block length. Hydrodynamic diameter distribution of the micelles decreases with the increase in the PNVP block length. The effective hydrodynamic ratio (Rh) remains almost constant over the angles of scattering measurements above the corresponding cmc value. The usefulness of the synthesized star amphiphilic block copolymers was checked by the successful synthesis of silver nanoparticles.Graphical abstract Well-defined four-arm star poly(ε-caprolactone)-b-poly(N-vinylpyrrolidone) amphiphilic block copolymers are prepared by the combination of ring opening polymerization and xanthate-mediated reversible addition-fragmentation chain transfer polymerization and their self-assembly properties are studied using 1H NMR, fluorescence spectroscopy, dynamic light scattering, and TEM.

Ultrasound-Induced In Situ Formation of Coordination Organogels from Isobutyric Acids and Zinc Oxide Nanoparticles

The discovery of ultrasound-induced in-situ formation of coordination organogels using various isobutyric acids (such as isobutyric acid or 2-methylisobutyric acid or 2-bromoisobutyric acid) and zinc oxide nanoparticles was described. FTIR and XRD results suggest that ultrasound irradiation triggers the quick dissolution of zinc oxide nanoparticles by isobutyric acids, resulting in the in-situ formation of zinc isobutyrate complexes that undergoes fast sonocrystallization into gel fibers. FESEM results clearly demonstrate the formation of well-defined networks of fibers with several micrometers in length, but the average diameter of the fiber ranges from 30 to 65 nm, depending upon the nature of the isobutyric acids used. A combination of single-crystal structure analysis and powder XRD result was used to envisage the molecular packing present in the gel state. This is probably a very rare case of ultrasound-induced organogelation where metal oxide NPs are used as the precursor.

Free radical polymerization of alkyl methacrylates with N,N-dimethylanilinium p-toluenesulfonate at above ambient temperature: a quasi-living system

N,N-Dimethylanilinium p-toluenesulfonate (PTSA-DMA) has been successfully used as a versatile initiator for the quasi-living free radical polymerization of several alkyl methacrylate monomers such as methyl, ethyl, n-butyl, tert-butyl, and benzyl methacrylates (MMA, EMA, n-BuMA, t-BuMA and BzMA respectively) at 60 °C in dry THF. The initiator, PTSA-DMA was prepared by a simple complexation reaction of readily available p-toluenesulfonic acid (PTSA) and N,N-dimethylaniline (DMA). The yield of this polymerization system was moderate to good (60–75%). The produced poly(alkyl methacrylates) had narrow polydispersities (PDIs) (Mw/Mn = 1.16–1.45). Although, this polymerization follows first order kinetics but the obtained molecular weight remains almost unchanged with conversion. This polymerization proceeds through radical mechanism as confirmed by electron paramagnetic resonance spectroscopy. The ‘quasi-living’ nature of this polymerization was verified from the chain extension experiment as well as the successful synthesis of a block copolymer, poly(methyl methacrylate)-b-poly(methyl methacrylate-co-benzyl methacrylate), by the sequential addition of the respective monomers. The obtained block copolymer was characterized by 1H NMR, differential scanning calorimetric and atomic force microscopic studies.