Plasmon-Induced Photocatalysis Based on Pt–Au Coupling with Enhanced Oxidation Abilities

Pt has been used as a cocatalyst for semiconductor photocatalysis because of its high catalytic activity. Although Pt catalysts are highly active for both oxidation and reduction reactions, photocatalysts have exploited the activity of Pt almost exclusively for reduction reactions. In the case of Au nanoparticles combined with a semiconductor, oxidation reactions take place at the Au surface due to plasmon-induced charge separation (PICS). If Au nanoparticles are replaced with Pt nanoparticles, oxidation reactions may occur at the Pt surface. However, Pt is much less plasmonic in comparison with Au, in the visible wavelength range. In order to make Pt nanoparticles resonant with visible light, here we coupled small Pt nanospheres (PtNSs) with large Au nanocubes (AuNCs) electromagnetically on TiO2, so that a coupling resonance mode arose at ∼600 nm. The bimetallic coupling allowed the PtNS and the AuNC to serve as a charge separation and catalytic reaction unit and a light-harvesting antenna unit, respectively. Light collected by the AuNC is transferred to the PtNS, where hot electron–hole pairs are generated. The electrons are injected into TiO2, and the holes drive the oxidation reactions at the Pt surface. We performed oxidation of water at the PtNSs. As a result of coupling, the external quantum efficiency of PICS was enhanced by a factor of 28 because of the amplified interparticle electric field and intensified light absorption in the PtNS

Photoinduced Chirality Switching of Metal-Inorganic Plasmonic Nanostructures

Chiral plasmonic nanodevices whose handedness can be switched reversibly between right and left by external stimulation have attracted much attention. However, they require delicate DNA nanostructures and/or continuous external stimulation. In this study, those issues are addressed by using metal-inorganic nanostructures and photoinduced reversible redox reactions at the nanostructures, namely, site-selective oxidation due to plasmon-induced charge separation under circularly polarized visible light (CPL) and reduction by UV-induced TiO2 photocatalysis. We irradiate gold nanorods (AuNRs) supported on TiO2 with right- or left-CPL to generate electric fields with chiral distribution around each AuNR and to deposit PbO2 at the sites where the electric fields are localized, for fixing the chirality to the AuNR. The nanostructures thus prepared exhibit circular dichroism (CD) based on longitudinal and transverse plasmon modes of the AuNRs. Their chirality given by right-CPL (or left-CPL) is locked until PbO2 is rereduced under UV light. After unlocking by UV, the chirality can be switched by left-CPL (or right-CPL) irradiation, resulting in reversed CD signals and locking the switch again. The handedness of the chiral plasmonic nanodevice can be switched reversibly and repeatedly

Rotationally displaced electric field intensity distribution around square nanoantennas induced by circularly polarized light

An optical field around regular polygon metal nanostructures excited by circularly polarized light can exhibit rotationally displaced intensity distributions. Although this phenomenon has been recognized, its underlying mechanisms has not been sufficiently explained. Herein, finite-difference time-domain simulations and model analyses reveal that the rotationally displaced optical intensity distribution can be generated when each of the linear polarization components that constitute circular polarization excites a superposition of multiple modes. The proposed model reasonably explains the rotationally displaced patterns for a square nanoantenna and other regular-polygon nanoantennas

Magneto-Plasmonic Response Enhancement of Au@Fe₂O₃ Nanocomposites Fabricated by Plasmon-Induced Charge Separation

Here, we developed magneto-plasmonic Au@Fe2O3 core–shell nanocomposites by plasmonic photocatalysis, and we observed enhanced magnetic circular dichroism (MCD). Plasmonic Au nanoparticles were adsorbed onto TiO2 and exposed to visible light in a solution containing Fe2+, and FeOOH was deposited photocatalytically on the Au nanoparticles due to plasmon-induced charge separation. MCD and extinction of the Au nanoparticles at ∼640 nm were enhanced, and their enhancement factors were almost the same, because both of them were due to the increased local refractive index around the Au core. After the nonmagnetic FeOOH shells on the Au cores were converted to magnetic α-Fe2O3 by aerobic annealing, MCD was enhanced further, and its enhancement factor was higher than that of the extinction. This surplus enhancement of MCD is explained in terms of local enhancement of an external magnetic field applied during the MCD measurements by the magnetic α-Fe2O3 shell

Real-Time Dynamic Adsorption Processes of Cytochrome <i>c</i> on an Electrode Observed through Electrochemical High-Speed Atomic Force Microscopy

<div><p>An understanding of dynamic processes of proteins on the electrode surface could enhance the efficiency of bioelectronics development and therefore it is crucial to gain information regarding both physical adsorption of proteins onto the electrode and its electrochemical property in real-time. We combined high-speed atomic force microscopy (HS-AFM) with electrochemical device for simultaneous observation of the surface topography and electron transfer of redox proteins on an electrode. Direct electron transfer of cytochrome <i>c</i> (cyt <i>c</i>) adsorbed on a self-assembled monolayers (SAMs) formed electrode is very attractive subject in bioelectrochemistry. This paper reports a real-time visualization of cyt <i>c</i> adsorption processes on an 11-mercaptoundecanoic acid-modified Au electrode together with simultaneous electrochemical measurements. Adsorbing cyt <i>c</i> molecules were observed on a subsecond time resolution simultaneously with increasing redox currents from cyt <i>c</i> using EC-HS-AFM. The root mean square roughness (<i>R</i>_RMS) from the AFM images and the number of the electrochemically active cyt <i>c</i> molecules adsorbed onto the electrode (<i>Γ</i>) simultaneously increased in positive cooperativity. Cyt <i>c</i> molecules were fully adsorbed on the electrode in the AFM images when the peak currents were steady. This use of electrochemical HS-AFM significantly facilitates understanding of dynamic behavior of biomolecules on the electrode interface and contributes to the further development of bioelectronics.</p></div

Real-time cyt <i>c</i> desorption processes from the MUA-modified gold electrode.

<p>(A) Continuous AFM images of desorbing cyt <i>c</i> molecules. Frame rate, 2 frames/s; image area, 150 × 150 nm². (B) Time evolution of <i>R</i>_RMS values at the higher ionic strengths. (C) Schematic of desorbing cyt <i>c</i> molecules from the MUA-modified electrode. </p

Time-course analysis of <i>R</i>_RMS and <i>Γ</i> values.

<p>(A) Time evolution of <i>R</i>_RMS values from the HS-AFM images (open circle) and the level of electrochemically active cyt <i>c</i> (<i>Γ</i>) from the cyclic voltammograms (open square). (B) Schematic of the adsorbing cyt <i>c</i> molecules on the MUA-modified electrode at each time point.</p

AFM images show (A) the MUA-modified gold surface and (B) cyt <i>c</i> adsorbed on the MUA SAM at 450 sec.

<p>Continuous AFM images of adsorbing cyt <i>c</i> molecules with real-time labels. Frame rate, 2 frames/s; image area, 150 × 150 nm². The cross sections of the images in (A) and (B), along the short white line, are shown in the lower right. </p