Masaki Ujihara

Sensitizing of pyrene fluorescence by β-cyclodextrin-modified TiO2 nanoparticles.

TiO(2) nanoparticles were synthesized by hydrolysis of tetraisopropyl orthotitanate in an aqueous solution of cyclodextrin. The β-cyclodextrin-modified spherical TiO(2) nanoparticles were water-dispersible and had an average particle diameter of 4.4 ± 1 nm. Pyrene fluorescence was enhanced by increasing the concentration of β-cyclodextrin-modified TiO(2) nanoparticle and the sensitization effect was triply stronger than the case of the β-cyclodextrin only. The increase in a concentration of host (β-cyclodextrin) changes its microenvironment for guest (pyrene), that is, the interaction of pyrene with apolar cavity of β-cyclodextrin increases, resulting in enhancement of fluorescence. The sensitization behavior of pyrene fluorescence in the presence of TiO(2) nanoparticles occurs from the increase in the extinction coefficient of pyrene, demonstrating the charge transfer between pyrene and metal oxide nanoparticle.

A solution-based nano-plasmonic sensing technique by using gold nanorods

We have successfully demonstrated a novel sensing technique for monitoring the variation of solution concentrations and measuring the effective dielectric constant in a medium by means of an ultra-small and label-free nanosensor, the mechanism of which is based on the localized surface plasmon resonance (LSPR) of gold nanorods. The nanorods are fabricated in a narrow size distribution, which is characterized by transmission electron microscopy and optical absorption spectroscopy. In addition, we employ a simple analytical calculation to examine the LSPR band of the absorption spectrum, which provides excellent consistency with aspect ratio. The plasmonic sensing is performed by detecting the diffusion process and saturation concentration of hexadecyltrimethylammonium bromide in water, and tracing the effective dielectric constants of the medium simultaneously. This promising sensing and analytical technique can be easily used for investigating the nano-scale variations of mixing or reaction process in a micro/nanofluidic channel or the biological interaction in the cytoplasm of the cell.

Massive-exfoliation of magnetic graphene from acceptor-type GIC by long-chain alkyl amine

Graphene can be prepared from a graphite intercalation compound (GIC) with acceptor-type intercalator, FeCl3. When the FeCl3–GIC is treated with primary amines at 90 °C for 6 h, the GIC expands to a few layers. The expansion is further facilitated, as the alkyl chain of primary amines becomes longer, while tertiary amines cannot penetrate inside the GIC because of their structural steric hindrance. The primary amine adsorbed in the GIC is oriented to form a bilayer by an indirect reactions among FeCl3–graphene–amine, and this process plays an important role in the expansion of the GIC, in contrast to the reaction of primary amines with donor-type GICs. Then the expanded-GIC is sonicated to exfoliate the graphene sheets. The obtained material exhibited a superparamagnetic property due to the remaining iron compounds. This approach using FeCl3–GIC and primary amine is preferable for the mass production of graphene because of the mild reaction conditions and the short treatment time for exfoliation from the chemically stable FeCl3–GIC. Moreover, the magnetization of graphene nano-composites could be useful for magnetic-recovery processes, electromagnetic heating, and the other applications.

Size-dependent effect of cystine/citric acid-capped confeito-like gold nanoparticles on cellular uptake and photothermal cancer therapy

Physiochemical changes, including size, are known to affect gold nanoparticle cellular internalization and treatment efficacy. Here, we report the effect of four sizes of cystine/citric acid-coated confeito-like gold nanoparticles (confeito-AuNPs) (30, 60, 80 and 100nm) on cellular uptake, intracellular localization and photothermal anticancer treatment efficiency in MDA-MB231 breast cancer cells. Cellular uptake is size dependent with the smallest size of confeito-AuNPs (30nm) having the highest cellular internalization via clathrin- and caveolae-mediated endocytosis. However, the other three sizes (60, 80 and 100nm) utilize clathrin-mediated endocytosis for cellular uptake. The intracellular localization of confeito-AuNPs is related to their endocytosis mechanism, where all sizes of confeito-AuNPs were localized highly in the lysosome and mitochondria, while confeito-AuNPs (30nm) gave the highest localization in the endoplasmic reticulum. Similarly, a size-dependent trend was also observed in in vitro photothermal treatment experiments, with the smallest confeito-AuNPs (30nm) giving the highest cell killing rate, whereas the largest size of confeito-AuNPs (100nm) displayed the lowest photothermal efficacy. Its desirable physicochemical characteristics, biocompatible nature and better photothermal efficacy will form the basis for further development of multifunctional confeito-AuNP-based nanotherapeutic applications.

A combination of hard and soft templating for the fabrication of silica hollow microcoils with nanostructured walls

Hollow silica microcoils have been prepared by using functionalized carbon microcoils as hard templates and surfactant or amphiphilic dye aggregates as soft templates. The obtained materials have been characterized by electron and optical microscopy, nitrogen sorption and small angle X-ray scattering. The obtained hollow microcoils resemble the original hard templates in shape and size. Moreover, they have mesoporous walls (pore size ≈ 3 nm) with some domains where pores are ordered in a hexagonal array, originated from surfactant micelles. The obtained silica microcoils also show preferential adsorption of cationic fluorescent dyes. A mechanism for the formation of silica microcoils is proposed.

Solution-Phase Synthesis of Branched Metallic Nanoparticles for Plasmonic Applications

This review discusses wet processes to synthesize metallic nanoparticles with many surface projections. Such projections can be formed by the aggregation of seed nanoparticles or by anisotropic crystal growth from specific facets on a base nanoparticle. The aggregation process can be controlled by protecting agents, which also play a key role in determining the morphology of the projections and the size of the nanoparticles. The reducing agents used for this purpose are mostly moderate and therefore allow seed aggregation before crystal growth. Some reducing agents act catalytically on specific crystal facets to promote anisotropic crystal growth. Branched nanostructures with high symmetry can be prepared from monocrystalline nanoparticles by site-selective growth and etching. The optical and plasmonic properties of the nanoparticles thus obtained can be used for various applications in surface-enhanced spectroscopy and in plasmon photocatalysts.

Confeito-like Au/TiO 2 nanocomposite: synthesis and plasmon-induced photocatalysis

Nanocomposites of confeito-like Au nanoparticles (CAuNPs) and TiO2 were synthesized under different irradiation conditions (darkness, UV light, and visible light) and time spans by the reaction of a Ti-citrate-peroxo complex with CAuNPs. The TiO2 synthesized under irradiation formed mesoporous films with embedded CAuNPs. The photocatalytic activity of the CAuNP/TiO2 nanocomposites was measured by the degradation of methylene blue (MB) under different irradiation conditions (darkness, UV light, and visible light). The results demonstrated that the bare CAuNPs decomposed MB under visible light and that this activity was enhanced by hybridization with TiO2. The activity of the CAuNPs was associated with the plasmon-induced effect, which the TiO2 enhanced by suppressing electron-hole recombination via acceptance of the hot electrons from the CAuNPs. This synergistic effect of the CAuNP/TiO2 nanocomposite varied with the amount of TiO2, and a thick layer of TiO2 decreased the activity as the surface of the CAuNPs was covered by TiO2. This behavior indicates that to design effective plasmonic devices and catalysts, an optimum balance between the amounts of CAuNPs and TiO2 must be achieved.

Chapter 4 – Surface-Enhanced Spectroscopy for Surface Characterization

Metal nanostructures are used to enhance optical responses by means of surface-enhanced spectroscopies such as Raman scattering, infrared absorption, and fluorescence spectroscopies. Metal nanostructures have been designed to improve efficiency in surface enhancement, and preparation methods of specimen materials are also important depending on the purpose of measurements. These methods can achieve the sensitivity of single-molecule level today, and provide unique insights on molecular behaviors and especially on surface properties in many research fields. In this chapter, mechanisms of enhancements in these spectroscopies, physicochemical behaviors of metals and surrounding materials, preparation methods of nanostructures, and techniques in measurements are reviewed.