Hironobu Tahara

Metal-enhanced fluorescence platforms based on plasmonic ordered copper arrays: wavelength dependence of quenching and enhancement effects.

Ordered arrays of copper nanostructures were fabricated and modified with porphyrin molecules in order to evaluate fluorescence enhancement due to the localized surface plasmon resonance. The nanostructures were prepared by thermally depositing copper on the upper hemispheres of two-dimensional silica colloidal crystals. The wavelength at which the surface plasmon resonance of the nanostructures was generated was tuned to a longer wavelength than the interband transition region of copper (>590 nm) by controlling the diameter of the underlying silica particles. Immobilization of porphyrin monolayers onto the nanostructures was achieved via self-assembly of 16-mercaptohexadecanoic acid, which also suppressed the oxidation of the copper surface. The maximum fluorescence enhancement of porphyrin by a factor of 89.2 was achieved as compared with that on a planar Cu plate (CuP) due to the generation of the surface plasmon resonance. Furthermore, it was found that while the fluorescence from the porphyrin was quenched within the interband transition region, it was efficiently enhanced at longer wavelengths. It was demonstrated that the enhancement induced by the proximity of the fluorophore to the nanostructures was enough to overcome the highly efficient quenching effects of the metal. From these results, it is speculated that the surface plasmon resonance of copper has tremendous potential for practical use as high functional plasmonic sensor and devices.

Magnetic Field Effects on Photoelectrochemical Reactions of Porphyrin?Viologen Linked Compounds in an Ionic Liquid

Magnetic field effects (MFEs) on photoelectrochemical reactions of three porphyrin–viologen linked compounds with various methylene groups [ZnP(n)V (n=4,6,8)] were examined in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) as an ionic liquid using a two-electrode cell. Stable anodic photocurrents are produced by irradiating ZnP(n)V (n=4,6,8) in [BMIM][BF4] with visible light, and the MFEs on photocurrents were clearly observed in ZnP(n)V (n=4,6,8). The MFEs on photocurrents increase with magnetic field for lower magnetic fields (B ≤200 mT) and are constant for higher magnetic fields (B > 200 mT). The magnitude of the MFEs in ZnP(n)V (n=6,8) are larger than that in ZnP(4)V. The MFEs can be explained by radical pair mechanism. The magnitude of the MFEs is larger than those in electrodes modified with ZnP(n)V (n=4,6,8) as Langmuir–Blodgett films. The results are most likely attributable to the properties of [BMIM][BF4] and the mechanism of photoelectrochemical reaction.

Dependence on Electrode Potential of the Magnetic Field Effect on Photoelectrochemical Reactions of Electrodes Modified with Porphyrin–Viologen Linked Compounds

Modified indium tin oxide (ITO) electrodes are prepared by depositing monolayers containing porphyrin–viologen linked compounds with various length spacer chains [ZnP(n)V (n = 4,6,8)] and arachidic acid (1:10) using the Langmuir–Blodgett method. The electrochemical properties of the ZnP(n)V (n = 4,6,8)-modified electrodes indicate that the linked compounds [ZnP(n)V (n = 4,6,8)] are immobilized on the ITO electrode surface so with the molecules oriented to put the viologen moiety nearest to the electrode. A stable anodic photocurrent is produced by irradiating the modified electrodes with visible light, and the photocurrent is clearly enhanced by the application of a magnetic field. The influence of the electrode potential upon the magnetic field effects on the photocurrent in the ZnP(8)V-modified electrode is also examined. With increasing electrode potential, the photocurrent increases but the magnetic field effects weaken. The influence of electrode potential on the magnetic field effects is explained by the radical pair mechanism at the ITO electrodes.

Magnetic Field Effects on Photoelectrochemical Reactions of a Porphyrin-Viologen Linked Compound in an Ionic Liquid

Magnetic field effects (MFEs) on photoelectrochemical reaction of a porphyrin-viologen linked compound with six methylene group (ZnP(n)V(n = 6)) were examined in an ionic liquid ([BMIM][BF4 ]) using two-electrode cell. A stable anodic photocurrent is produced by irradiating ZnP(6)V in [BMIM][BF4 ] with visible light, and the photocurrent clearly increased in the presence of a magnetic field. The MFEs are explained by radical pair mechanism. The magnitude of the MFEs is larger than those in electrodes modified with ZnP(n)V(n = 4,6,8) as Langmuir-Blodgett films. The results are most likely ascribed to the properties of [BMIM][BF 4 ].

Photocurrent enhancement of porphyrin molecules over a wide-wavelength region based on combined use of silver nanoprisms with different aspect ratios

We succeeded in controlling the wavelength range in which the photocurrent of porphyrin is enhanced by tuning as well as expanding the wavelength ranges in which the localized surface plasmon resonance (LSPR) occurs. We fabricated photoelectric conversion systems consisting of 5,10,15,20-tetrakis(p-carboxyphenyl)porphyrin (TCPP) and silver nanoprisms with small (SAgPRs) and large (LAgPRs) aspect ratios as plasmonic nano-antennae. Their photocurrents were much larger than those from TCPP-modified Ag planar electrodes at the specific wavelengths corresponding to their LSPR bands (SAgPRs: 460–610 nm; LAgPRs: 610–690 nm). The maximum enhancement factors (EFs) for the SAgPRs and the LAgPRs were 37 and 35, respectively. In order to enhance the photocurrents, we expanded the LSPR bands by the combined use of SAgPRs and LAgPRs. The system consisting of the mixture (MAgPRs) showed enhancement of the photocurrent over the entire Q-band region (480–690 nm). Finally, the total EFs of the photocurrents were evaluated by irradiation with AM1.5G sunlight through a long-pass filter of 480 nm and the results revealed that the EFs were in the order of MAgPRs > SAgPRs > LAgPRs. Furthermore, the system showed stability without loss of the enhancement property for at least 10 min under the solar irradiation.

Refractive index susceptibility of palladium nanoplates with plasmonic resonance in the visible region

We synthesized anisotropic Pd nanoplates for use as a novel refractive index (RI) sensing material in the visible region. The nanoplates showed an extinction peak that was attributed to the excitation of the localized surface plasmon resonance at ~620 nm in the visible region. It was found that the peak was red-shifted with increasing the RI of the surrounding medium. The susceptibility was calculated to be 250 nm per RI unit, comparable to some anisotropic Au nanoparticles that are excellent RI sensing materials.

Efficient Photocurrent Enhancement from Porphyrin Molecules on Plasmonic Copper Arrays: Beneficial Utilization of Copper Nanoanntenae on Plasmonic Photoelectric Conversion Systems.

We demonstrated the usefulness of Cu light-harvesting plasmonic nanoantennae for the development of inexpensive and efficient artificial organic photoelectric conversion systems. The systems consisted of the stacked structures of layers of porphyrin as a dye molecule, oxidation-suppressing layers, and plasmonic Cu arrayed electrodes. To accurately evaluate the effect of Cu nanoantenna on the porphyrin photocurrent, the production of Cu2O by the spontaneous oxidation of the electrode surfaces, which can act as a photoexcited species under visible light irradiation, was effectively suppressed by inserting the ultrathin linking layers consisting of 16-mercaptohexadecanoic acid, titanium oxide, and poly(vinyl alcohol) between the electrode surface and porphyrin molecules. The reflection spectra in an aqueous environment of the arrayed electrodes, which were prepared by thermally depositing Cu on two-dimensional colloidal crystals of silica with diameters of 160, 260, and 330 nm, showed clear reflection dips at 596, 703, and 762 nm, respectively, which are attributed to the excitation of localized surface plasmon resonance (LSPR). While the first dip lies within the wavelengths where the imaginary part of the Cu dielectric function is moderately large, the latter two dips lie within a region of a quite small imaginary part. Consequently, the LSPR excited at the red region provided a particularly large enhancement of porphyrin photocurrent at the Q-band (ca. 59-fold), compared to that on a Cu planar electrode. These results strongly suggest that the plasmonic Cu nanoantennae contribute to the substantial improvement of photoelectric conversion efficiency at the wavelengths, where the imaginary part of the dielectric function is small.

Driving Quick and Large Amplitude Contraction of Viologen-Incorporated Poly-l-Lysine-Based Hydrogel by Reduction

A glutaraldehyde-cross-linked poly-l-lysine-based hydrogel with pendant viologens was synthesized with various [viologen unit]/[Lys unit] ratios. The hydrogel with the ratio of 25% was extensively characterized. Characterization of the hydrogel revealed that (i) water content reaches 95.8%, (ii) unbound viologen unit is absent, and (iii) lyophilized gel, showing porous structure, is rewettable. The hydrogel in contact with a gold, glassy carbon, pyrolytic graphite, or ITO electrode was electroactive, showing voltammetric responses involving diffusion process. Occurrence of electron transfer between viologen sites was verified by ESR measurements. The electroreflectance spectrum demonstrated significant dimerization of one-electron-reduced forms, viologen radical cations. Most remarkably, reduction of the hydrogel by dithionite showed a 93% volume decrease, being near to the water content, at 100 s and finally 97% contraction at 380 s. The initial rate of contraction was 0.02 s-1, and the swelling weight ratio was over 17. The contraction rate was much faster than that observed when adding of redox-inactive salts in the outer medium. Recovery by reoxidation was relatively sluggish even when O2-saturated water was used. The quick and large-amplitude contraction with removal of water from the gel body should be the consequence of permeability of reductant, interchain π-stacking of one-electron-reduced forms (viologen monoradical monocation), desolvation of viologen upon reduction, electron hopping between viologen units and its acceleration with contraction, and effective osmotic pressure difference.