Motoyuki Iijima

Surface modification and characterization for dispersion stability of inorganic nanometer-scaled particles in liquid media

Abstract Inorganic nanoparticles are indispensable for science and technology as materials, pigments and cosmetics products. Improving the dispersion stability of nanoparticles in various liquids is essential for those applications. In this review, we discuss why it is difficult to control the stability of nanoparticles in liquids. We also overview the role of surface interaction between nanoparticles in their dispersion and characterization, e.g. by colloid probe atomic force microscopy (CP-AFM). Two types of surface modification concepts, post-synthesis and in situ modification, were investigated in many previous studies. Here, we focus on post-synthesis modification using adsorption of various kinds of polymer dispersants and surfactants on the particle surface, as well as surface chemical reactions of silane coupling agents. We discuss CP-AFM as a technique to analyze the surface interaction between nanoparticles and the effect of surface modification on the nanoparticle dispersion in liquids.

Anionic Surfactant with Hydrophobic and Hydrophilic Chains for Nanoparticle Dispersion and Shape Memory Polymer Nanocomposites

An anionic surfactant comprising a hydrophilic poly(ethylene glycol) (PEG) chain, hydrophobic alkyl chain, and polymerizable vinyl group was synthesized as a capping agent of nanoparticles. TiO(2) nanoparticles modified by this surfactant were completely dispersible in various organic solvents with a wide range of polarities, such as nitriles, alcohols, ketones, and acetates. Furthermore, these particles were found to be dispersible in various polymers with different properties, such as thermosetting epoxy resins and radical polymerized poly(methylmethacrylate) (PMMA). A polymer composite of surface-modified TiO(2) nanoparticles in epoxy resins prepared by using the developed surfactant also possessed temperature-induced shape memory properties.

Tuning the stability of TiO2 nanoparticles in various solvents by mixed silane alkoxides.

The surface of TiO(2) nanoparticles which were well dispersed into acidic aqueous solution was successfully modified by silane alkoxides without strong aggregate formations. By adding decyltrimethoxysilane (DTMS) as silane alkoxides into the TiO(2) aqueous solution which were carefully diluted with methanol, DTMS slowly attached onto the TiO(2) surface without rapid hydrolysis and condensation reaction among DTMS. Because of the hydrophobicity of DTMS, the dispersed TiO(2) nanoparticles slowly formed flocks as DTMS reacted on TiO(2). These flocks were able to be completely redispersed into nonpolar solvents even after they were collected by centrifugation and drying under vacuum as dry powder. Furthermore the surface of TiO(2) nanoparticles have been successfully tuned by combining silane alkoxides which contains hydrophobic and hydrophilic groups such as DTMS and 3-aminopropyltrimethoxysilane (APTMS), respectively, toward their complete redispersion into various solvents. While TiO(2) nanoparticles modified by DTMS were redispersible into toluene, those modified by mixed alkoxides of 50 mol % DTMS and 50 mol % APTMS were redispersible into a mixed solution of toluene and methanol. Further when they were modified by mixed alkoxides of 25 mol % DTMS and 75 mol % APTMS, they were redispersible into polar solvents such as methanol with a little addition of acids.

Direct measurement of interactions between stimulation-responsive drug delivery vehicles and artificial mucin layers by colloid probe atomic force microscopy.

A novel thermo- and pH-sensitive nanogel particle, which is a core-shell structured particle with a poly(N-isopropylacrylamide) (p(NIPAAm)) hydrogel core and a poly(ethylene glycol) monomethacrylate grafted poly(methacrylic acid) (p(MMA-g-EG)) shell, is of interest as a vehicle for the controlled release of peptide drugs. The interactions between such nanogel particles and artificial mucin layers during both approach and separation were successfully measured by using colloid probe atomic force microscopy (AFM) under various compression forces, scan velocities, and pH values. While the magnitudes of the compression forces and scan velocities did not affect the interactions during the approach process, the adhesive force during the separation process increased with these parameters. The pH values significantly influenced the interactions between the nanogel particles and a mucin layer. A large steric repulsive force and a long-range adhesive force were measured at neutral pH due to the swollen p(MMA-g-EG) shell. On the other hand, at low pH values, the steric repulsive force disappeared and a short-range adhesive force was detected, which resulted from the collapse of the shell layer. The nanogel particles possessed a pH response that was sufficient to protect the incorporated peptide drug under the harsh acidic conditions in the stomach and to effectively adhere to the mucin layer of the small intestine, where the pH is neutral. The relationships among the nanogel particle-mucin layer interactions, pH conditions, scan velocities, and compression forces were systemically investigated and discussed.

Layer-by-layer surface modification of functional nanoparticles for dispersion in organic solvents.

In order to prepare SiO(2) nanoparticles that are dispersible in various organic solvents, an anionic surfactant 1, which branches into a hydrophobic chain and a hydrophilic chain, was adsorbed on to SiO(2) nanoparticles through a layer-by-layer surface modification route using polyethyleneimine (PEI). First, the relationship among the additive content of PEI, adsorbed content of PEI, and the redispersion stability of the SiO(2) nanoparticles in water was investigated. While almost the entire PEI was adsorbed when the additive PEI content was lower than 67 mg/g of SiO(2), the adsorbed content of PEI became saturated when the additive content was increased above 90 mg/g of SiO(2). SiO(2) nanoparticles that were saturated with PEI could be redispersed into water at sizes close to their primary particle size without the large-scale formation of aggregates. Next, the anionic surfactant 1 was adsorbed on the SiO(2) nanoparticles by using a SiO(2) aqueous suspension saturated with adsorbed PEI. It was found that the adsorbed content of 1 increased almost linearly as the additive content was increased when the additive condition was below 1400 mg/g of SiO(2). Furthermore, SiO(2) nanoparticles adsorbed with 80 mg/g of SiO(2) of PEI and 810 mg/g of SiO(2) of 1 could be dispersed into various organic solvents with different polarities. This layer-by-layer modification technique can also be applied to Ag nanoparticles in order to prepare Ag nanoparticles that can be dispersed in various organic solvents.

Carbon Nanotube/Nanofibers and Graphite Hybrids for Li-Ion Battery Application

To improve the electrical conductivity of negative electrodes of lithium ion batteries, we applied a direct CVD synthesis of carbon nanomaterials on the surface of graphite particles. To prepare a catalyst, two alternative approaches were utilized: colloidal nanoparticles (NPs) and metal (Ni and Co) nitrate salt precursors deposited on the graphite surface. Both colloidal and precursor systems allowed us to produce carbon nanofibers (CNFs) on the graphite surface with high coverage under the optimum CVD conditions. Electrical measurements revealed that the resistivity of the actual electrodes fabricated from CNFs coated graphite particles was about 40% lower compared to the original pristine graphite electrodes.

Electrostatic Deposition of Aerosol Particles Generated from an Aqueous Nanopowder Suspension on a Chemically Treated Substrate

The state of electrostatically deposited aerosol particles from a suspension that contains TiO2 particles on the surface of a solid substrate using electrospray was demonstrated. The particles were initially electrosterically stable in 7.5 wt % aqueous solution with a mean particle size of 50 nm. During deposition, the particles were pumped with different flow rates between 0.6 and 1.2 mL/h through a stainless steel capillary tube of 0.1 mm inner diameter. The particles were emitted at the tip of the capillary tube as an electrified liquid cone before forming into a highly charged droplet. For comparison, two types of substrate surfaces with and without chemical treatment were prepared. Atomic force microscopy (AFM) scanning and contact angle measurements showed that surface treatment increased the substrate roughness and created a hydrophilic surface. Raman analysis also showed the existence of an oxide layer and a P–O network on the treated substrate. Field emission scanning electron microscopy FE-SEM image analysis showed that more TiO2 particles were deposited on the treated substrate than on the untreated substrate.

Rapid magnetic catch-and-release purification by hydrophobic interactions.

A reversible, conventional, and rapid purification method of hydrophobically tagged products using hydrophobic magnetic nanoparticles was developed. The reversible purification system entails simply controlling the polarity of solvents. First, for the catching procedure, poor solvents were added into a well-dispersed system of magnetic nanoparticles and tagged products. Once the poor solvents were added to the system, the products were recrystallized among the nanoparticles and the aggregation of magnetic nanoparticles occurred due to hydrophobic interactions. These aggregates with the products contained within them were able to be collected rapidly by magnets. Then, the releasing procedure can be easily performed by redispersing the collected aggregates into good solvents. The availability of this purification protocol was confirmed by using a hydrophobically tagged fluorescent model product. Furthermore, this rapid purification method was successfully applied to a peptide elongation reaction system which enabled the synthesis of peptides such as Leu-Enkephalin in high purity, in high yield, and in a short time.

Microstructure of iron particles reduced from silica-coated hematite in hydrogen

Poly- and nearly monocrystalline hematite particles having diameters of around 2 and 0.1 μm, respectively, were prepared by the gel-sol method and coated with a uniform silica layer by the sol-gel method. The core in the silica shell was reduced to iron without agglomerate formation between the particles by using a hydrogen stream. The microstructure and morphology of these cores and the silica layers were examined by high-resolution transmission electron microscopy, and electron and X-ray diffraction analysis. In hematite particles, around 2 μm in diameter, the reduced products were mostly α-Fe, but partially magnetite. In hematite particles, around 0.1 μm in diameter, only α-Fe was observed. Most of the raw hematite and iron particles produced were monocrystalline, and part of core grew hexagonal prism-shaped monocrystalline particles. In the case of the growth of a crystal to a hexagonal prism, a nanometer-scaled space at the interface between the iron crystal core and the silica layer was discovered.

Characterization of Surface Interaction between Chitosan-modified Liposomes and Mucin Layer by Using CNT Probe AFM Method

　In order to characterize the adhesion and deformation behavior between chitosan-modified liposomes and the mucin layer of the small intestine, mucin was coated on hydrophobic surface-modified carbon nanotube (CNT) probe of an atomic force microscope. The interaction between this mucin layer and the liposomes with or without chitosan modification in phosphoric acid buffer solution was determined by atomic force microscopy. The pH of the buffer solution was controlled at 2.8 and 7.0. The chitosan modification increased the attractive force between the liposomes and mucin layer during the separation process under both pH conditions. This result corresponded with that from a previous study about the liposome adhesion behavior on the surface of the small intestine of rats. By using the mucin-coated CNT probe, the long range and different types of attractive forces between the chitosan-modified liposomes and mucin layer was observed. Furthermore, the small-scaled deformation behavior change on the liposomal surfaces due to chitosan modification was also observed by the CNT probe. The detail deformation and adhesion behavior of the liposomes with or without chitosan modification was detected.