Nanostructure of Poly(<i>N</i>‑isopropylacrylamide) Brush at the Air/Water Interface and Its Responsivity to Temperature and Salt

Nanostructure and transition of the poly­(<i>N</i>-isopropylacrylamide (PNIPAm) brush at the air/water interface were investigated by π-<i>A</i> isotherm and X-ray reflectivity, and an interesting behavior was observed with the change in temperature and salt. The polymer monolayer of poly­(<i>n</i>-butyl acrylate)­(P<i>n</i>BA)-<i>b</i>-PNIPAm on the water surface showed a transition between carpet-only/carpet+brush structures as a function of brush density, which was controlled by compression/expansion, as was the case for ionic brush systems. The brush stretching factor was about 50%, which was slightly less than that for a strongly ionic brush. The number of water molecules inside the brush layer was estimated to be 11–13 per repeating unit of PNIPAm chain. This value is very close to the number of hydrated water molecules reported, which means that all the water molecules inside the brush layer were hydrated water. With elevating temperature, the PNIPAm brush shrank, and the number of water molecules in the brush layer was reduced to 3. These observations certainly indicated a dehydration process. Interestingly, a part of the PNIAPm chain formed a “hydrophobic PNIPAm layer” on the carpet layer under the P<i>n</i>BA hydrophobic layer. A similar transition was observed also by the addition of salt to the water subphase. Although the formation of “hydrophobic PNIPAm layer” was not observed in this case, shrinking of the brush was observed with increasing salt concentration, and finally it became a carpet-only structure, which contained no water molecules. This salt effect was found to be ion specific, and its effectiveness was in the order of F^– > Cl^– > Br^–, which is in agreement with the Hofmeister series

Layer-by-Layer Epitaxial Growth of Scalable WSe₂ on Sapphire by Molecular Beam Epitaxy

Molecular beam epitaxy (MBE) provides a simple but powerful way to synthesize large-area high-quality thin films and heterostructures of a wide variety of materials including accomplished group III–V and II–VI semiconductors as well as newly developing oxides and chalcogenides, leading to major discoveries in condensed-matter physics. For two-dimensional (2D) materials, however, main fabrication routes have been mechanical exfoliation and chemical vapor deposition by making good use of weak van der Waals bonding nature between neighboring layers, and MBE growth of 2D materials, in particular on insulating substrates for transport measurements, has been limited despite its fundamental importance for future advanced research. Here, we report layer-by-layer epitaxial growth of scalable transition-metal dichalocogenide (TMDC) thin films on insulating substrates by MBE and demonstrate ambipolar transistor operation. The proposed growth protocol is broadly applicable to other TMDCs, providing a key milestone toward fabrication of van der Waals heterostructures with various 2D materials for novel properties and functionalities

Structural Analysis of Unimer Nanoparticles Composed of Hydrophobized Poly(amino acid)s

Amphiphilic random copolymers of poly­(γ-glutamic acid)-<i>graft</i>-l-phenylalanine (γ-PGA-Phe), with various lengths of γ-PGA main chains (molecular weight of 70, 140, and 220 kDa) plus hydrophobic moieties of Phe groups (grafting degree 12–60%), self-assembled in aqueous media to form nanoparticles (NPs). The aggregation number (<i>N</i>_agg) could be adjusted according to their molecular structures as well as the preparative methods/conditions. The γ-PGA-Phe NPs were further characterized by means of dynamic and static light scattering, small-angle neutron scattering, as well as steady-state fluorescence measurements/quenching techniques. The single chain state had hydrophobic domains, and formed spherical structures, herein called unimer NPs, and were obtained using molecular weights of γ-PGA higher than 140 kDa conjugated with Phe at 27–42% due to the balance of hydrophobicity/hydrophilicity along the single polymer chain. The number of hydrophobic domains in one NP (<i>N</i>_domain), estimated by means of fluorescence quenching techniques and the rigidity of the inner particles detected by dipyrene fluorescence demonstrated that the <i>N</i>_domain and the rigidity were affected by the particle size and preparative methods. In addition, the effect of pH on the stability of the unimer NPs indicated a reduction of the <i>N</i>_domain upon the pH, supporting a loose packing due to hydrophobic association under alkaline conditions

pH-Sensitive Adsorption Behavior of Polymer Particles at the Air–Water Interface

pH-Sensitive adsorption of polymer particles bearing poly­[2-(dimethylamino)­ethyl methacrylate] hairs at the air–water interface was investigated using a surface tensiometer, a Langmuir–Blodgett trough, and an X-ray reflectometer. We clarified that the particles are adsorbed at the interface at basic pH; by contrast, at acidic pH, only a small number of particles are adsorbed, whereas the majority are dispersed in the water phase. X-ray reflectometry analysis revealed that a particle monolayer was formed at the air–water interface, which was packed densely under increasing surface pressure, as determined by the electron density profile change. The contact angles of the particles at the air–water interface were estimated to be 29° and 34° at pH 3 and 10, respectively, by direct visualization of the air–water interface position of the particles using a polycyanoacrylate trapping method

Near-Infrared Dye-Conjugated Amphiphilic Hyaluronic Acid Derivatives as a Dual Contrast Agent for In Vivo Optical and Photoacoustic Tumor Imaging

Amphiphilic hyaluronic acid (HA) derivatives bearing hydrophobic indocyanine green dye derivatives and hydrophilic poly­(ethylene glycol) were synthesized through the use of condensation and copper-catalyzed click cyclization reactions. The amphiphilic HA derivatives dissolved in water and formed self-assemblies in which the near-infrared dyes were tightly packed and arranged to form dimers or H-aggregates. By irradiating an aqueous solution of HA derivatives with near-infrared light, photoacoustic signals were detected along with fluorescence emission. Self-assemblies consisting of HA derivatives could smoothly accumulate in tumor tissues by passive tumor targeting. By utilizing HA derivatives as a contrast agent, tumor sites were clearly visualized by optical imaging as well as by photoacoustic tomography

Polymeric Self-Assemblies with Boron-Containing Near-Infrared Dye Dimers for Photoacoustic Imaging Probes

Polymers containing pyrrolopyrrole aza-BODIPY (PPAB) and thiophene-bridged BODIPY dimers (TBD) having poly­(ethylene glycol) (PEG) or PEGylated hyaluronic acid (HA) were prepared by facile conjugation approaches. Self-assemblies consisting of TBD-conjugated polymers more efficiently generated photoacoustic (PA) signals than PPAB–PEG conjugate upon irradiation with near-infrared pulsed laser light. Among dye-conjugated polymers examined, TBD-HA-PEG conjugates efficiently generated photoacoustic signals, 1.49–1.83 times stronger than that of commercially available indocyanine green (ICG). We found that the following two factors are essential to enhance PA signals from self-assemblies: (1) the formation of strongly interacting TBD aggregates and (2) enhancement of the elastic modulus of self-assemblies by conjugating TBDs with HA. TBD-conjugated HA derivatives circulated in blood vessels for a longer time (15.6 ± 4.9% injected dose (ID) in blood 24 h after injection) and more specifically accumulated in tumor tissues (17.8 ± 3.5% ID/g in tumor 24 h after injection) than ICG-conjugated HA derivatives, visualizing a tumor site more clearly. The cell uptake experiment of dye–HA conjugates indicates that ICG-conjugated polymers internalized into cells or merged with cell walls to emit strong fluorescence, while TBD-conjugated polymers were not internalized into cells. Because the disassembly of the TBD-conjugated HA derivatives is suppressed, aggregated TBDs emit weak fluorescence but efficiently generate strong PA signals in tumor tissues

Near-Infrared Dye-Conjugated Amphiphilic Hyaluronic Acid Derivatives as a Dual Contrast Agent for In Vivo Optical and Photoacoustic Tumor Imaging

Amphiphilic hyaluronic acid (HA) derivatives bearing hydrophobic indocyanine green dye derivatives and hydrophilic poly­(ethylene glycol) were synthesized through the use of condensation and copper-catalyzed click cyclization reactions. The amphiphilic HA derivatives dissolved in water and formed self-assemblies in which the near-infrared dyes were tightly packed and arranged to form dimers or H-aggregates. By irradiating an aqueous solution of HA derivatives with near-infrared light, photoacoustic signals were detected along with fluorescence emission. Self-assemblies consisting of HA derivatives could smoothly accumulate in tumor tissues by passive tumor targeting. By utilizing HA derivatives as a contrast agent, tumor sites were clearly visualized by optical imaging as well as by photoacoustic tomography