Hironobu Tahara

The development of anisotropic gold nanostructures supporting localized surface plasmon (LSP) resonances in the near-infrared (NIR) biological window is of great interest for diagnostic and therapeutic nanotechnologies. Here, we report gold open-shell nanoparticles (AuOSNs), a symmetry-broken nanoshell architecture exhibiting strong NIR surface-enhanced Raman scattering (SERS) activity. AuOSNs were fabricated via a surfactant-free strategy combining bottom-up silica sphere assembly with a simple top-down gold deposition process, without using highly cytotoxic surfactants such as cetyltrimethylammonium bromide (CTAB). Boundary element method (BEM) simulations revealed that the asymmetric open-shell geometry induces NIR LSP resonances with pronounced electromagnetic field localization near the opening edges, depending on excitation configuration. Consistent with these predictions, extinction spectra of AuOSNs dispersed in water showed an LSP resonance peak at ~793 nm, close to the 785 nm excitation wavelength for SERS. In aqueous dispersion, AuOSNs modified with 4-mercaptobenzoic acid (4-MBA) exhibited strong SERS activity with enhancement factors of ~10⁶. Furthermore, polyethylene glycol (PEG)-modified MBA/AuOSNs showed negligible cytotoxicity in vitro. SERS imaging confirmed that PEG/MBA/AuOSNs enable visualization of HeLa cells via characteristic MBA SERS signals. These results demonstrate that surfactant-free AuOSNs provide a biocompatible platform for NIR-excited SERS sensing and cellular imaging, highlighting their potential in plasmonic bioimaging applications.

We present the synthesis and characterization of a viologen-based molten poly(ionic liquid), VPIL(TFSI), and its application to electrochromic (EC) devices. VPIL(TFSI) was obtained as a highly viscous liquid with a glass transition temperature of −23 °C, enabling its use in a molten state without additional solvent. Electrochemical analysis by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of neat VPIL(TFSI) revealed a unique conduction mechanism: while ionic conductivity is dominated by the counter-anion (TFSI) migration, charge transport during redox cycling involves electron hopping between viologen units. Diffusion coefficient analysis indicated that electron hopping is slower than counter-anion migration, suggesting that the reorientation of viologen moieties, rather than ion migration, determines the transport kinetics. An EC device was fabricated using an equimolar mixture of VPIL(TFSI) and a ferrocene-based RAIL as cathodic and anodic components, respectively, without any supporting electrolyte. The device exhibited distinct coloration with strong absorption bands at 530 and 890 nm, attributed to π-dimerization of reduced viologen species, along with high contrast and coloration efficiency comparable to theoretical values. These findings demonstrate the potential of molten poly(ionic liquids) as promising redox-active media for solvent-free and durable electrochromic devices.

Densely‐packed thin films of plasmonic nanoparticles enhance Raman scattering. The effect of size uniformity of plasmonic nanoparticles in such thin films contributes to the development of highly sensitive Raman scattering spectroscopy for practical use. In this research, densely‐packed thin films containing a mixture of gold nanoparticles with two different sizes are fabricated and evaluated about their surface‐enhanced Raman scattering properties. The enhancement efficiency of gold nanoparticle films with different sizes is lower than that of single‐sized gold nanoparticle films. This tendency suggests a relationship between Raman scattering enhancement efficiency and the electric field generated by localized surface plasmon resonance among gold nanoparticles of different sizes.

Redox-active ionic liquids (RAILs) exhibit high redox density and non-volatility. A low-melting RAIL, [C4VC7][TFSI]2, was developed using asymmetric design and anion selection. Chargetransfer interactions were introduced by mixing with a carbazole-based RAIL, tuning viscosity and redox behavior. A semi-empirical method based on the Nernst-Planck equation was applied to evaluate the apparent diffusion coefficient without supporting electrolytes. An all-in-one electrochromic device using viologen and ferrocene-based RAILs showed clear color change, cycling stability, and oxygen resistance. These results demonstrate the promise of RAILs as functional electrolytes for electrochemical applications.

Electrochromic (EC) devices have been gaining attention for display, electronic paper, and smart windows because of the low-power consumption, low-voltage drive, and memory effect of the colored state holding sharp contrast. They could be useful for the intended usage, which organic light emitting diode and liquid crystal display do not cover. The minimum components of EC devices are (i) redox-active EC material for a transparent electrode of interest, (ii) supporting electrolyte, (iii) medium (solvent), and (iv) charge-compensating (charge-balancing) redox material for another electrode. The medium (solvent) is required for electrolytes, which provide electric conductivity to the device. However, it is difficult to solve the problems of solvent evaporation and leakage of the electrolyte solution from the device. Recently, ionic liquids (ILs) are the focus of extensive investigations for electrochemical devices due to their non-volatility, moderate viscosity, and chemical stability. ILs can play roles of both solvent and supporting electrolytes. We have been focused on ILs as a designable liquid material with the above useful properties and developed electrochromic redox-active ionic liquids (RAILs). In RAILs, either or both constituent ions are redox-active. RAILs are a new class of ILs, which are non-volatile, electro-conductive, and viscous liquid materials. In addition, RAILs consist of concentrated redox-active ions, resulting in large redox density. In the case of RAILs with electrochromic ions, which are electrochromic RAILs (EC-RAILs), the concentrated electrochromic species give a large light absorption resulting in a very thin device thickness. The non-volatility and chemical stability provide the long-term durability of the EC devices. EC-RAILs play the roles of (i)-(iv) required in EC devices. In this study, we developed EC-RAILs, the coloration process by the voltage control, and their application to the EC devices without using solvents and supporting electrolytes. Because of the concentrated redox species and absence of the supporting electrolyte, the resistance of the EC cells could be dominant for the coloration kinetics. In this study, we have analyzed the electrode kinetics of the EC device composed of pure EC-RAILs.

Electrochromism is a phenomenon in which reversible color change occurs by redox reaction and devices with such characteristics are called electrochromic (EC) devices. The EC devices are often composed of EC materials, compensators for the reaction charge, electrolytes, solvents, and electrodes. Such EC devices, however, contain potential issues such as solvent volatilization and damage caused by physical cracks. By the way, ionic liquids (IL) are non-volatile, flame retardant, chemically stable, and possess high ionic conductivity. In addition, ILs play double roles as electrolytes and solvents. Therefore, fabricating all components of the EC device using IL leads to not only clearing up the issues at once but also the development of the eco-friendly one. In this work, we have fabricated two types of EC devices. In this work, we have fabricated two types of EC devices. One is an all-in-one type that consists of only ferrocene (Fc)-viologen (V)-linked IL.¹ The other is a composite type that is composed of an Fc IL and V IL without any additives: solvent, supporting, and electrolyte.² As a result, both EC devices showed good EC performance. The efficiency of the former and later devices were 70 and 91.4 cm² C^-1 at 1.0 V, respectively; the EC process was stable even after over 10 000 potential cycles for the former device and 13 000 ones for the latter device. In this presentation, we also describe the synthesis of ILs and their electrochemistry in detail.

The α-C(sp ³ )–H difluoroalkylation of tetrahydroisoquinolines and isochromans via electrochemical oxidation was developed using organozinc reagents to achieve a broad substrate scope under mild reaction conditions.