Lok Kumar Shrestha

Amphiphile nanoarchitectonics: from basic physical chemistry to advanced applications

Amphiphiles, either synthetic or natural, are structurally simple molecules with the unprecedented capacity to self-assemble into complex, hierarchical geometries in nanospace. Effective self-assembly processes of amphiphiles are often used to mimic biological systems, such as assembly of lipids and proteins, which has paved a way for bottom-up nanotechnology with bio-like advanced functions. Recent developments in nanostructure formation combine simple processes of assembly with the more advanced concept of nanoarchitectonics. In this perspective, we summarize research on self-assembly of amphiphilic molecules such as lipids, surfactants or block copolymers that are a focus of interest for many colloid, polymer, and materials scientists and which have become increasingly important in emerging nanotechnology and practical applications, latter of which are often accomplished by amphiphile-like polymers. Because the fundamental science of amphiphiles was initially developed for their solution assembly then transferred to assemblies on surfaces as a development of nanotechnological techniques, this perspective attempts to mirror this development by introducing solution systems and progressing to interfacial systems, which are roughly categorized as (i) basic properties of amphiphiles, (ii) self-assembly of amphiphiles in bulk phases, (iii) assembly on static surfaces, (iv) assembly at dynamic interfaces, and (v) advanced topics from simulation to application. This progression also represents the evolution of amphiphile science and technology from simple assemblies to advanced assemblies to nanoarchitectonics.

Fullerene Nanoarchitectonics: From Zero to Higher Dimensions

The strategic design of nanostructured materials, the properties of which could be controlled across different length scales and which, at the same time, could be used as building blocks for the construction of devices and functional systems into new technological platforms that are based on sustainable processes, is an important issue in bottom-up nanotechnology.Such strategic design has enabled the fabrication of materials by using convergent bottom-up and top-down strategies. Recent developments in the assembly of functional fullerene (C60) molecules, either in bulk or at interfaces, have allowed the production of shape-controlled nano-to-microsized objects that possess excellent optoelectronic properties, thus enabling the fabrication of optoelectronic devices. Because fullerene molecules can be regarded as an ideal zero-dimensional (0D) building units with attractive functions, the construction of higher-dimensional objects, that is, 1D, 2D, and 3D nanomaterials may realize important aspects of nanoarchitectonics. This Focus Review summarizes the recent developments in the production of nanostructured fullerenes and techniques for the elaboration of fullerene nanomaterials into hierarchic structures.

Fullerene crystals with bimodal pore architectures consisting of macropores and mesopores.

A new class of fullerene (C(60)) crystals with bimodal pore architectures consisting of macropores and mesopores was synthesized by using a liquid-liquid interfacial precipitation (LLIP) method involving an interface between isopropyl alcohol (IPA) and a saturated solution of C(60) in a mixture of benzene and carbon tetrachloride (CCl(4)). By varying the mixing fraction of CCl(4) in benzene, the porosity and electrochemically active surface area can be flexibly controlled.

Dimensionally integrated nanoarchitectonics for a novel composite from 0D, 1D, and 2D nanomaterials: RGO/CNT/CeO2 ternary nanocomposites with electrochemical performance

We report a one-step conversion of a dimensionally mixed ternary nanocomposite from zero-dimensional (0D) cerium oxide (CeO2), one-dimensional (1D) carbon nanotubes (CNTs), and two-dimensional (2D) reduced graphene oxide (RGO) nanomaterials by the chemical precipitation method. The RGO/CNT/CeO2 ternary nanocomposite showed excellent electrochemical performance (electrical double layer capacitor properties) in an aqueous electrolyte followed by long term cyclic stability and high energy density compared to its binary counterparts.

Aligned 1-D nanorods of a π-gelator exhibit molecular orientation and excitation energy transport different from entangled fiber networks.

Linear π-gelators self-assemble into entangled fibers in which the molecules are arranged perpendicular to the fiber long axis. However, orientation of gelator molecules in a direction parallel to the long axes of the one-dimensional (1-D) structures remains challenging. Herein we demonstrate that, at the air-water interface, an oligo(p-phenylenevinylene)-derived π-gelator forms aligned nanorods of 340 ± 120 nm length and 34 ± 5 nm width, in which the gelator molecules are reoriented parallel to the long axis of the rods. The orientation change of the molecules results in distinct excited-state properties upon local photoexcitation, as evidenced by near-field scanning optical microscopy. A detailed understanding of the mechanism by which excitation energy migrates through these 1-D molecular assemblies might help in the design of supramolecular structures with improved charge-transport properties.

Nanoporous Carbon Tubes from Fullerene Crystals as the π‐Electron Carbon Source

Here we report the thermal conversion of one-dimensional (1D) fullerene (C60) single-crystal nanorods and nanotubes to nanoporous carbon materials with retention of the initial 1D morphology. The 1D C60 crystals are heated directly at very high temperature (up to 2000 °C) in vacuum, yielding a new family of nanoporous carbons having π-electron conjugation within the sp(2)-carbon robust frameworks. These new nanoporous carbon materials show excellent electrochemical capacitance and superior sensing properties for aromatic compounds compared to commercial activated carbons.

Tunable, functional carbon spheres derived from rapid synthesis of resorcinol-formaldehyde resins.

In this article, the rapid synthesis of colloidal, spherical polymer resins via enhanced copolymerization and polycondensation of resorcinol with formaldehyde is presented. The ultrasound-mediated technique assembles perfectly spherical resins in less than 5 min due to generated active species and free radicals produced in an aqueous ammonia-ethanol-water solvent. In this report, numerous controlled experiments account for and support the important role of high intensity ultrasounds in the rapid cluster formation, condensation, and gelation process of resorcinol with formaldehyde in the presence of ammonia catalyst. After a controlled heat treatment process, amorphous carbon spheres are obtained from these spherical polymer resins. The effect of temperature (up to 1100 °C) on the structural evolution of these carbon spheres is meticulously studied which is lacking in the previous literature. The resorcinol-formaldehyde resins carbonized at 600 and 900 °C demonstrate BET surface areas of 592.4 m(2)/g and 952.5 m(2)/g with specific capacitances of 17.5, and 33.5 F/g (scan rate of 5 mV/s), respectively.

Hierarchically Structured Fullerene C70 Cube for Sensing Volatile Aromatic Solvent Vapors

We report the preparation of hierarchically structured fullerene C70 cubes (HFC) composed of mesoporous C70 nanorods with crystalline pore walls. Highly crystalline cubic shape C70 crystals (FC) were grown at a liquid-liquid interface formed between tert-butyl alcohol and C70 solution in mesitylene. HFCs were then prepared by washing with isopropanol of the FC at 25 °C. The growth directions and diameters of C70 nanorods could be controlled by varying washing conditions. HFCs perform as an excellent sensing system for vapor-phase aromatic solvents due to their easy diffusion through the mesoporous architecture and strong π-π interactions with the sp(2) carbon-rich pore walls. Moreover, HFCs offer an enhanced electrochemically active surface area resulting in an energy storage capacity 1 order of magnitude greater than pristine C70 and fullerene C70 cubes not containing mesoporous nanorods.

Wormlike micelles in mixed amino acid-based anionic/nonionic surfactant systems.

We present the formation of viscoelastic wormlike micelles in mixed amino acid-based anionic and nonionic surfactants in aqueous systems in the absence of salt. N-Dodecylglutamic acid (designated as LAD) has a higher Krafft temperature; however, on neutralization with alkaline amino acid l-lysine, it forms micelles and the solution behaves like a Newtonian fluid at 25 degrees C. Addition of tri(oxyethylene) monododecyl ether (C(12)EO(3)) and tri(oxyethylene) monotetradecyl ether (C(14)EO(3)) to the dilute aqueous solution of the LAD-lysine induces one-dimensional micellar growth. With increasing C(12)EO(3) or C(14)EO(3) concentration, the solution viscosity increases gradually, but after a certain concentration, the elongated micelles entangle forming a rigid network of wormlike micelles and the solution viscosity increases tremendously. Thus formed wormlike micelles show a viscoelastic character and follow the Maxwell model. Tri(oxyethylene) monohexadecyl ether (C(16)EO(3)), on the other hand, could not form wormlike micelles, although the solution viscosity increases too. The micelles become elongated; however, they do not appear to form a rigid network of wormlike micelles in the case of C(16)EO(3). Rheological measurements have shown that zero shear viscosity (eta(0)) increases with the C(12)EO(3) concentration gradually at first and then sharply, and finally decreases before phase separation. However, no such maximum in the eta(0) plot is observed with the C(14)EO(3). The eta(0) increases monotonously with the C(14)EO(3) concentration till phase separation. In studies of the effect of temperature on the wormlike micellar behavior it has been found that the eta(0) decays exponentially with temperature, following an Arrehenius behavior and at sufficiently higher temperatures the solutions follow a Newtonian behavior. The flow activation energy calculated from the slope of log eta(0) versus 1/T plot is very close to the value reported for typical wormlike micelles. Finally, we also present the effect of neutralization degree of lysine on the rheology and phase behavior. The formation of wormlike micelles is confirmed by the Maxwell model fit to the experimental rheological data and by Cole-Cole plots.

Vortex-Aligned Fullerene Nanowhiskers as a Scaffold for Orienting Cell Growth

A versatile method for the rapid fabrication of aligned fullerene C60 nanowhiskers (C60NWs) at the air-water interface is presented. This method is based on the vortex motion of a subphase (water), which directs floating C60NWs to align on the water surface according to the direction of rotational flow. Aligned C60NWs could be transferred onto many different flat substrates, and, in this case, aligned C60NWs on glass substrates were employed as a scaffold for cell culture. Bone forming human osteoblast MG63 cells adhered well to the C60NWs, and their growth was found to be oriented with the axis of the aligned C60NWs. Cells grown on aligned C60NWs were more highly oriented with the axis of alignment than when grown on randomly oriented nanowhiskers. A study of cell proliferation on the C60NWs revealed their low toxicity, indicating their potential for use in biomedical applications.