Ajayan Vinu

Challenges and breakthroughs in recent research on self-assembly

Abstract The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.

Recent advances in functionalization of mesoporous silica

Mesoporous silica with regular geometries have been recently paid much attention owing to their scientific importance and great potentials in practical applications such as catalysis, adsorption, separation, sensing, medical usage, ecology, and nanotechnology. Especially, applications often require immobilization of the related functional groups in the mesopores. In order to achieve desire applications, modification of these mesoporous silica are indispensable. In this review, recent progresses of functionalization of mesoporous silica are comprehensively summarized. In the first parts, advances in three major methods, grafting (post-synthetic modification), co-condensation (direct synthesis), and techniques related with periodic mesoporous organosilicates, are explained. In the latter parts, new concepts for functionalization of mesoporous silica including functional template method and lizard template method are introduced. Most of the examples described here have been published in a new millennium.

Gold Nanoparticles Embedded in a Mesoporous Carbon Nitride Stabilizer for Highly Efficient Three-Component Coupling Reaction†

Support with triple function: Au nanoparticles with sizes of less than 7 nm were fabricated in the channels of a mesoporous carbon nitride (MCN) support, which acts as a stabilizing, size-controlling, and reducing agent (see picture; Au NPs in green). The embedded, well-dispersed Au nanopartides are a highly active, selective, and recyclable catalyst for the three-component coupling reaction of benzaldehyde, piperidine, and phenylacetylene for the synthesis of propargylamine.

Layer‐by‐Layer Films of Graphene and Ionic Liquids for Highly Selective Gas Sensing

The best of both worlds: Graphene/ionic liquid (G–IL) layered films were obtained by direct reduction of graphene oxide in the presence of ionic liquids, followed by reassembly through electrostatic layer-by-layer (LbL) adsorption (see picture). The layer spacing of the graphene sheets is regularly expanded upon insertion of ionic liquid molecules (green discs). Selective sensing of aromatic compounds (red spheres) by using the G–IL LbL films was also achieved.

Highly Ordered Mesoporous Carbon Nitride Nanoparticles with High Nitrogen Content: A Metal‐Free Basic Catalyst

Nitrogen-rich carbon nitride is obtained as ultrasmall, discrete, mesoporous nanoparticles (see picture) by reaction of ethylenediamine with CCl in the nanochannels of a mesoporous silica template. Its high nitrogen content, large surface area, and large pore volume make it a highly active catalyst for transesterification of β-keto esters of aryl, aliphatic, and cyclic primary alcohols.

New families of mesoporous materials

Abstract Mesoporous materials have been paid much attention in both scientific researches and practical applications. In this review, we focus on recent developments on preparation and functionalization of new families of mesoporous materials, especially non-siliceous mesoporous materials invented in our research group. Replica synthesis is known as the method to synthesize mesoporous materials composed of various elements using originally prepared mesoporous replica. This strategy has been applied for the syntheses of novel mesoporous materials such as carbon nanocage and mesoporous carbon nitride. Carbon nanocage has a cage-type structure with huge surface area and pore volume, which exhibits superior capabilities for biomolecular adsorption. Mesoporous carbon nitride was synthesized, for first time, by using mixed material source of carbon and nitrogen simultaneously. As a totally new strategy for synthesis of mesoporous materials, the elemental substitution method has been recently proposed by us. Direct substitution of component elements in original mesoporous materials, with maintaining structural regularity, provided novel mesoporous materials. According to this synthetic strategy, mesoporous boron nitride and mesoporous boron carbon nitride have been successfully prepared, for first time. In addition to these material inventions, hybridization of high functional materials, such as biomaterials, to mesoporous structure has been also developed. Especially, immobilization of proteins in mesopores was systematically researched, and preparation of peptidehybridized mesoporous silica was demonstrated. These new families of mesoporous materials introduced in this review would have high potentials in future practical applications in wide ranges from electronics and photonics to environmental and medical uses.

Layer-by-Layer Films of Dual-Pore Carbon Capsules with Designable Selectivity of Gas Adsorption

Stable, homogeneous ultrathin films of uniformly dimensioned dual-pore carbon capsules with mesoporous walls and macroscopic empty cores were fabricated using layer-by-layer methods based on electrostatic interaction between a polyelectrolyte and a surfactant coating of the carbon capsules. The resulting dual-porous carbon capsule films were investigated as a sensor substrate for vapors of different organic solvents. The carbon capsule films have much higher adsorption capacities than conventional electrolyte films and even than noncapsular mesoporous carbon films. The dual-pore carbon capsules have greater affinities for aromatic volatiles over their aliphatic counterparts, probably due to stronger pi-pi interactions. Additionally, the adsorption selectivity can be designed. Impregnation of additional recognition components into the carbon capsules permits further control over adsorption selectivity between aromatic and nonaromatic substances and between acids and bases in the prevailing atmosphere. Therefore, it is anticipated that the dual-pore carbon capsule films developed in this work will find application in sensing and separation applications because of their designable selectivity.

A Layered Mesoporous Carbon Sensor Based on Nanopore‐Filling Cooperative Adsorption in the Liquid Phase

Tea or coffee? The layer-by-layer assembly of mesoporous carbon onto a quartz crystal microbalance results in a structure (see picture) that can be used for highly selective in situ measurement of nonionic aqueous guests, such as tea components.

Stimuli-free auto-modulated material release from mesoporous nanocompartment films

Mesoporous nanocompartment films composed of silica particles and hollow silica capsules were prepared by the layer-by-layer (LbL) technique. The resulting mesoporous nanocompartment films possess special molecular encapsulation and release capabilities so that stimuli-free auto-modulated stepwise release of water or drug molecules was achieved through the mesopore channels of robust silica capsule containers embedded in the film. Stepwise release of water was reproducibly observed that originates in the non-equilibrated rates between evaporation of water from the mesopore channels to the exterior and the capillary penetration of water from container interior to the mesopore channels. It was generalized to evaporation of other substances, fragrances, limonene. Application was also tested in the controlled release of the sunscreen UV-absorber (UV-S1) for circumvention of its rapid dissolution in water and prolongation of its prophylactic effect toward harmful ultraviolet radiation. UV-S1 was successfully entrapped within the mesoporous nanocompartment films and was released in a prolonged stepwise mode. The nanocompartment films developed in this research are promising materials for drug delivery since they allow gradual release of therapeutic agents with likely related improvements in their efficacy.

Highly Crystalline and Conductive Nitrogen‐Doped Mesoporous Carbon with Graphitic Walls and Its Electrochemical Performance

We present a rational and simple methodology to fabricate highly conductive nitrogen-doped ordered mesoporous carbon with a graphitic wall structure by the simple adjustment of the carbonization temperature of mesoporous carbon nitride without the addition of any external nitrogen sources. By simply controlling the heat-treatment temperature, the structural order and intrinsic properties such as surface area, conductivity, and pore volume, and the nitrogen content of ordered graphitic mesoporous carbon can be controlled. Among the materials studied, the sample heat-treated at 1000  °C shows the highest conductivity, which is 32 times higher than that for the samples treated at 800  °C and retains the well-ordered mesoporous structure of the parent mesoporous carbon nitride and a reasonable amount of nitrogen in the graphitic framework. Since these materials exhibit high conductivity with the nitrogen atoms in the graphitic framework, we further demonstrate their use as a support for nanoparticle fabrication without the addition of any external stabilizing or size-controlling agent, as well as the anode electrode catalysts. Highly dispersed platinum nanoparticles with a size similar to that of the pore diameter of the support can be fabricated since the nitrogen atoms and the well-ordered porous structure in the mesoporous graphitic carbon framework act as a stabilizing and size-controlling agent, respectively. Furthermore the Pt-loaded, nitrogen-doped mesoporous graphitic carbon sample with a high conductivity shows much higher anodic electrocatalytic activity than the other materials used in the study.