Susumu Inasawa

Optical recording media using laser-induced size reduction of Au nanoparticles

Silica-gel films containing Au nanoparticles were fabricated by a photoreduction method, which functioned as a medium for high-density optical recording. On irradiation of 532 nm laser light, which corresponds to the surface plasmon resonance absorption of Au particles, the absorption spectrum of the film showed a blueshift. Using this property, a hologram was recorded on the film. Transmitting electron microscopy supported that the change in absorption spectrum was due to size reduction of embedded particles caused by laser irradiation. For recording the hologram, a threshold of laser fluence existed which corresponded to the heat of vaporization of Au particles.

Real time observation and kinetic modeling of the cellular uptake and removal of silicon quantum dots.

The time courses of uptake and removal of silicon quantum dots (Si-QDs) by human umbilical endothelial cells (HUVECs) were observed via confocal laser scanning microscope. Si-QDs were internalized via endocytosis and transported to late endosomes/lysosomes. The number of internalized Si-QDs increased with time and gradually reached a plateau value. When Si-QD-internalized HUVECs were subsequently washed and exposed to fresh culture medium, HUVECs removed internalized Si-QDs via exocytosis. The number of internalized Si-QDs decreased with time and gradually reached a plateau value. Not all internalized Si-QDs were removed from the cell interior but large numbers of internalized Si-QDs remained accumulated inside cells. A kinetic model based on the mass balance of Si-QDs and receptors in a cell was proposed to describe the cellular uptake and removal of Si-QDs. Model calculation fitted well with experimental results. Using this model, the dissociation constant between receptors and Si-QDs in the endosome, K(d,in), was found to be a determinant factor for Si-QD accumulation in cells after the removal process.

Stable and color-tunable fluorescence from silicon nanoparticles formed by single-step plasma assisted decomposition of SiBr4

We describe a facile method to synthesize silicon nanoparticles through plasma-assisted decomposition of silicon tetrabromide, a novel precursor for gas phase synthesis. Raman spectroscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy analyses demonstrate that the obtained silicon nanocrystals were covered with a silicon oxide layer. The nanoparticles can be easily dispersed in ethanol and the dispersion exhibited stable optical properties for over half a year, showing strong photoluminescence (PL) under ultraviolet irradiation with quantum yields of up to 24%. The PL properties of the nanoparticles were tolerant to hydrofluoric acid (HF) etching. The PL intensity of the nanoparticles was slightly enhanced after removal of the surface oxide layer via HF etching. Replacement of the surface oxide layer with a hydrogen-terminated surface after etching would be one of the possible reasons for this PL enhancement. In addition, we were able to tune the color of PL of the nanoparticles from dark-blue to greenish-yellow by changing the total pressure of the plasma chamber during synthesis. These results indicated that our method could provide us with a simple and practical method to produce silicon nanoparticles.

Selective labeling of the endoplasmic reticulum in live cells with silicon quantum dots

A simple and novel approach was developed to obtain water-dispersible silicon quantum dots (Si-QDs) of low toxicity that were able to selectively label the endoplasmic reticulum (ER) in live cells. A block copolymer (Pluronic F127) was used to coat the surface of Si-QDs. Si-QDs form aggregates with diameters of 20-40 nm and show an outstanding optical stability upon UV irradiation. Our F127-treated Si-QDs would be a powerful tool for long-term real-time observation of the ER in live cells.

Size Controlled Formation of Gold Nanoparticles Using Photochemical Grwoth and Photothermal Size Reduction by 308 nm Laser Pulses

We synthesized gold nanoparticles (mean diameter 8.3 nm, standard deviation 2.7 nm) from tetrachloroaurate complex (AuCl4-) solution by 308 nm laser irradiation without the use of any stabilizers. The combination of photochemical particle growth with photothermal particle size reduction by 308 nm laser irradiation resulted in a narrow size distribution. Photothermal size reduction controlled the maximum diameter of gold nanoparticles which existed in the system, and photochemical growth controlled their size distribution. Using this technique, we were able to control the diameter and the size distribution of gold naoparticles. We propose a simple model for estimating the maximum diameter of gold nanoparticles formed in the system by the irradiation of nanosecond laser pulses. The maximum diameter of particles is determined by the competition between heating of a particle by absorbed photon energy and heat dissipation from the particle surface to the surroundings.

Size- and surface chemistry-dependent intracellular localization of luminescent silicon quantum dot aggregates

Aggregates of luminescent silicon quantum dots (Si-QDs) selectively label certain organelles, such as lysosomes, endoplasmic reticulum, cytosol and nuclei, depending upon the size of the aggregates and their surface properties. We used Si-QDs in an aggregated form and the size of aggregates was controlled from ca. 30 to 270 nm diameter. In fixed human umbilical vein endothelial cells (HUVECs), allylamine-terminated Si-QDs selectively labeled cell nuclei, while Si-QDs, treated by the amphiphilic block copolymer Pluronic® F127, uniformly labeled cytosol. On the other hand, in live HUVECs, allylamine-modified Si-QDs selectively labeled lysosomes, whereas F127-treated Si-QDs showed size-dependent intracellular localization: F127-treated Si-QD aggregates with a small diameter of ca. 30 nm selectively labeled the endoplasmic reticulum and those with a large diameter of ca. 270 nm labeled lysosomes. Our results indicate that specific organelle imaging can be achieved by controlling the surface properties and size of Si-QD aggregates, without using conventional antigen–antibody reactions. Physicochemical interactions between silicon nanomaterials and cells play a critical role in the observed intracellular localization. The possible mechanism and cytotoxicity of the silicon nanomaterials are discussed.

Self-organized pattern formation of cracks perpendicular to the drying direction of a colloidal suspension

A new cracking mode in silica colloidal films formed via quasi-one-dimensional directional drying is presented. Unlike typical cracking parallel to the drying direction, unique periodic cracks perpendicular to the drying direction are formed when a silica colloidal suspension dries in a thin glass cell. The mechanism of formation of the perpendicular cracks is discussed and a mathematical model based on the mass balance of water, which describes the experimental data well, is proposed. Our model reveals that the drying rate of water predominantly determines crack spacing and it is inversely related. Analogy of the inverse relation to the scaling law for the self-organized formation of columnar jointing, observed in desiccated corn starch and cooled lava, is also discussed.

Formation of Optically Anisotropic Films from Spherical Colloidal Particles

Colloidal silica films, formed by the drop evaporation method, showed birefringent spherulite optical properties. They displayed a Maltese cross pattern under crossed polarizers, and interference colors, such as blue and orange-red, under crossed polarizers with a compensator. The difference in refractive index was estimated to be 9x10(-4) from the interference colors. Scanning electron microscopy (SEM) results revealed anisotropic structures in the colloidal films. Particles formed radially ordered hexagonal arrays. The drop evaporation method used in this report, which dries from the edge to the center, resulted in a radially ordered colloidal film. When a colloidal silica film was prepared using a unidirectional drying method, particles were packed in an ordered structure corresponding to the drying direction and the resulting film showed different birefringent optical properties. Our results show that a variety of birefringent films can be obtained from spherical colloidal dispersions through control of the drying method.

Formation of Well-Aligned Thin Films of Rod-Like Nanoparticles via Solvent Evaporation: A Simulation Study

Thin film formation from a suspension of rod-like nanoparticles via solvent evaporation was studied by simulation. Two types of initial nanoparticle configurations were examined. From a suspension of randomly oriented rod-like nanoparticles, a thin film with various domains and a rough surface was obtained after solvent evaporation. On the other hand, when particles were strongly charged, they spontaneously aligned in suspension, resulting in a well-aligned thin film with a smooth surface after complete drying. In addition to the initial configuration of nanoparticles in a suspension, the vertical capillary force plays an important role in the formation of these thin films.

Optical anisotropy in packed isotropic spherical particles: indication of nanometer scale anisotropy in packing structure

We investigated the origin of birefringence in colloidal films of spherical silica particles. Although each particle is optically isotropic in shape, colloidal films formed by drop drying demonstrated birefringence. While periodic particle structures were observed in silica colloidal films, no regular pattern was found in blended films of silica and latex particles. However, since both films showed birefringence, regular film structure patterns were not required to exhibit birefringence. Instead, we propose that nanometer-scale film structure anisotropy causes birefringence. Due to capillary flow from the center to the edge of a cast suspension, particles are more tightly packed in the radial direction. Directional packing results in nanometer-scale anisotropy. The difference in the interparticle distance between radial and circumferential axes was estimated to be 10 nm at most. Nanometer-scale anisotropy in colloidal films and the subsequent optical properties are discussed.