Hiroyasu Nishi

Potential-Scanning Localized Surface Plasmon Resonance Sensor.

Localized surface plasmon resonance (LSPR) sensors based on plasmonic nanoparticles attract much attention recently. Here we propose a new class of LSPR sensor, that is, a potential-scanning LSPR sensor, in which electron density of the plasmonic nanoparticles is controlled by potential scanning. The sensor exhibits a resonance peak during the potential scan, which negatively shifts with increasing local refractive index. Therefore, the present sensor can be applied to affinity biosensors and chemical sensors based on potential scan instead of wavelength scan. The potential-scanning LSPR sensors do not require space and a mechanical device for wavelength scanning, so the sensors are advantageous for miniaturization and cost reduction, in comparison with the conventional LSPR sensors. We explain the principle and theoretical sensitivities of the potential-scanning LSPR sensors, and refractometry is demonstrated using a sensor with an ITO electrode loaded with gold nanospheres (13 or 40 nm diameter) or nanorods. The smaller and larger nanospheres are suitable for sensing with a wider dynamic range and with a higher sensitivity, respectively. The use of nanorods further improves the sensitivity and figure of merit.

CuS nanoplates for LSPR sensing in the second biological optical window

Anisotropic gold nanoparticles are chemically stable and serve as localized surface plasmon resonance (LSPR) sensors operatable in the first biological optical window (650−950 nm). However, alternative materials are awaited because they are expensive and somewhat complicated to prepare. Here we employ CuS (covellite) nanoplates, which consist of earth-abundant elements and exhibit LSPR in the near-infrared region, as materials for LSPR sensors. The CuS nanoplates respond to refractive index changes of the surrounding medium in the second biological optical window (1000−1350 nm). The refractive index sensitivity (160−600 nm RIU−1) and the operation wavelength (1100−1250 nm) of the CuS nanoplates can be controlled by simply changing the composition of reaction suspension for nanoplate synthesis.

Oxidation Ability of Plasmon-Induced Charge Separation Evaluated on the Basis of Surface Hydroxylation of Gold Nanoparticles.

The oxidation ability of plasmonic photocatalysts, which has its origins in plasmon-induced charge separation and has not yet been studied quantitatively and systematically, is important for designing practical photocatalytic systems. Oxidation ability was investigated on the basis of surface hydroxylation of Au nanoparticles on TiO2 at various irradiation wavelengths and electrolyte pH values. The reaction proceeds only when the sum of the flat band potential of TiO2 and the irradiated photon energy is close to, or more positive than, the theoretical potential for the reaction.

Controlling Shape Anisotropy of ZnS–AgInS2 Solid Solution Nanoparticles for Improving Photocatalytic Activity

Independently controlling the shape anisotropy and chemical composition of multinary semiconductor particles is important for preparing highly efficient photocatalysts. In this study, we prepared ZnS-AgInS2 solid solution ((AgIn)xZn2(1-x)S2, ZAIS) nanoparticles with well-controlled anisotropic shapes, rod and rice shapes, by reacting corresponding metal acetates with a mixture of sulfur compounds with different reactivities, elemental sulfur, and 1,3-dibutylthiourea, via a two-step heating-up process. The chemical composition predominantly determined the energy gap of ZAIS particles: the fraction of Zn2+ in rod-shaped particles was tuned by the ratio of metal precursors used in the nanocrystal formation, while postpreparative Zn2+ doping was necessary to increase the Zn2+ fraction in the rice-shaped particles. The photocatalytic H2 evolution rate with irradiation to ZAIS particles dispersed in an aqueous solution was significantly dependent on the chemical composition in the case of using photocatalyst particles with a constant morphology. In contrast, photocatalytic activity at the optimum ZAIS composition, x of 0.35-0.45, increased with particle morphology in the order of rice (size: ca. 9 × ca. 16 nm) < sphere (diameter: ca. 5.5 nm) < rod (size: 4.6 × 27 nm). The highest apparent quantum yield for photocatalytic H2 evolution was 5.9% for rod-shaped ZAIS particles, being about two times larger than that obtained with spherical particles.

Plasmonic Enhancement of a Photocycloreversion Reaction of a Diarylethene Derivative Using Individually Dispersed Silver Nanoparticles

The fabrication of silver nanoparticles covered with polymers with a well-defined core-shell structure and the quantitative evaluation of the plasmonic enhancement effect on a photochemical reaction in the vicinity of these silver nanoparticles individually dispersed in a medium are described. The photocycloreversion reaction of a diarylethene polymer in the vicinity of silver nanoparticles was enhanced by 2-6 times relative to the reaction without the nanoparticles. The promotion of the photocycloreversion reaction is due to enhancement of the electromagnetic field near the surface of the silver core.

Potential-Scanning Localized Plasmon Sensing with Single and Coupled Gold Nanorods

Single plasmonic nanoparticles can potentially serve as optical sensors for detecting local refractive index changes. However, simultaneous monitoring of the scattering spectra from multiple nanoparticles is not practical. Here we perform potential-scanning localized surface plasmon resonance (LSPR) sensing. Gold nanorods are immobilized on an ITO electrode. Instead of collecting the full spectrum, as is done in conventional LSPR sensing, the electrode potential is scanned while the rod spectra are monitored at a single wavelength. We demonstrate that refractive index changes can be determined from single wavelength experiments and we further find that gold nanorod (NR) dimers exhibit higher refractive index sensitivities than single NRs in both potential-scanning and conventional wavelength-scanning based LSPR sensing.

Well-controlled synthesis of wurtzite-type Cu2ZnSnS4 nanoparticles using multiple sulfur sources via a two-step heating process

Wurtzite-type copper zinc tin sulfide (Cu2ZnSnS4) nanoparticles were prepared in dodecanethiol (DDT) solutions by reacting corresponding metal acetates with a mixture of sulfur compounds with different reactivities, elemental sulfur (S) and dibutylthiourea (DBTU), via a two-step heat treatment. Initial heating at 200 °C enabled the nucleation of metal sulfide nanoparticles composed of two crystal phases, and subsequent heat treatment at 240 °C resulted in the formation of Cu2ZnSnS4 nanoparticles with a wurtzite-type crystal structure. The resulting particles had rod shapes with a stoichiometric composition, the size and shape of which could be controlled by changing both the reaction time and the molar ratio of S and DBTU used in a sulfur precursor. In contrast, spherical Cu2ZnSnS4 nanoparticles with a kesterite-type crystal structure were produced by reaction with pure S as a precursor. The electronic energy levels of the conduction band and valence band edges were determined for wurtzite-type Cu2ZnSnS4 nanoparticles by photoelectrochemical measurement, the individual levels being comparable to those of kesterite-type ones. The light–electricity conversion efficiency varied remarkably depending on the kind of crystal structure: wurtzite-type Cu2ZnSnS4 particles exhibited an efficiency superior to that of the particles with a kesterite-type crystal structure.

Plasmon-induced charge separation at two-dimensional gold semishell arrays on SiO2@TiO2 colloidal crystals

Photoelectrodes based on plasmonic Au semishell (or halfshell) arrays are developed. A colloidal crystal consisting of SiO2@TiO2 core-shell particles is prepared on a TiO2-coated transparent electrode. A Au semishell (or halfshell) array is deposited by sputtering or evaporation on the colloidal crystal. An electrode with the semishell (or halfshell) array exhibits negative photopotential shifts and anodic photocurrents under visible light at 500-800 nm wavelengths in an aqueous electrolyte containing an electron donor. In particular, hydroquinone and ethanol are good electron donors. The photocurrents can be explained in terms of plasmon-induced charge separation at the Au-TiO2 interface.

Adipose Tissue-Derived Stem Cell Imaging Using Cadmium-Free Quantum Dots

Quantum dots (QDs) have received much attention for biomolecule and cell imaging applications because of their superior optical properties such as high quantum efficiency, size-tunable emission, and resistance to photobleaching process. However, QDs that are commercially available contain cadmium (Cd), a highly toxic element. Thus, the development of Cd-free and less toxic QDs is strongly desired. In this study, we developed Cd-free QDs (ZnS-coated ZnS-AgInS2 solid solution nanoparticles with a sulfo group: ZnS-ZAIS-SO3H) and investigated the ability of this material to label stem cells. ZnS-ZAIS-SO3H could be transduced into mouse adipose tissue-derived stem cells (mASCs) using octaarginine peptides (R8), known as cell-penetrating peptides. The optimal ratio of ZnS-ZAIS-SO3H:R8 was found to be 1:100 for labeling mASCs. More than 80% of mASCs labeled with 500 nM ZnS-ZAIS-SO3H were found to be alive, and the proliferation rates of labeled mASCs were maintained at the same rate as that of nonlabeled mASCs. In addition, no abnormalities in the morphology of mASCs labeled with ZnS-ZAIS-SO3H could be observed. These data suggest that ZnS-ZAIS-SO3H may be effective for the labeling of mASCs.

Labeling and in vivo visualization of transplanted adipose tissue-derived stem cells with safe cadmium-free aqueous ZnS coating of ZnS-AgInS 2 nanoparticles

The facile synthesis of ZnS-AgInS2 (ZAIS) as cadmium-free QDs and their application, mainly in solar cells, has been reported by our groups. In the present study, we investigated the safety and the usefulness for labeling and in vivo imaging of a newly synthesized aqueous ZnS-coated ZAIS (ZnS-ZAIS) carboxylated nanoparticles (ZZC) to stem cells. ZZC shows the strong fluorescence in aqueous solutions such as PBS and cell culture medium, and a complex of ZZC and octa-arginine (R8) peptides (R8-ZZC) can achieve the highly efficient labeling of adipose tissue-derived stem cells (ASCs). The cytotoxicity of R8-ZZC to ASCs was found to be extremely low in comparison to that of CdSe-based QDs, and R8-ZZC was confirmed to have no influence on the proliferation rate or the differentiation ability of ASCs. Moreover, R8-ZZC was not found to induce the production of major inflammatory cytokines (TNF-α, IFN-γ, IL-12p70, IL-6 and MCP-1) in ASCs. Transplanted R8-ZZC-labeled ASCs could be quantitatively detected in the lungs and liver mainly using an in vivo imaging system. In addition, high-speed multiphoton confocal laser microscopy revealed the presence of aggregates of transplanted ASCs at many sites in the lungs, whereas individual ASCs were found to have accumulated in the liver.