Takashi Tachikawa

Evidence for Crystal-Face-Dependent TiO2 Photocatalysis from Single-Molecule Imaging and Kinetic Analysis

According to the concept of active sites, the activity of heterogeneous catalysts correlates with the number of available catalytic sites and the binding affinity of the substrates. Herein, we report a single-molecule, single-particle fluorescence approach to elucidate the inherent photocatalytic activity of exposed surfaces of anatase TiO(2), a promising photocatalyst, using redox-responsive fluorogenic dyes. A single-molecule imaging and kinetic analysis of the fluorescence from the products shows that reaction sites for the effective reduction of the probe molecules are preferentially located on the {101} facets of the crystal rather than the {001} facets with a higher surface energy. This surprising discrepancy can be explained in terms of face-specific electron-trapping probability. In situ observation of the catalytic events occurring at the solid/solution interfaces reveals the hidden role of the crystal facets in chemical reactions and their impact on the efficiency and selectivity of heterogeneous (photo)catalysts.

Au/TiO2 superstructure-based plasmonic photocatalysts exhibiting efficient charge separation and unprecedented activity.

Plasmonic photocatalysts were successfully synthesized by the modification of TiO2 mesocrystals with Au nanoparticles (NPs) by a simple impregnation method. The Au NP sensitizers show a strong photoelectrochemical response in the visible-light region (400-800 nm) due to their surface plasmon resonance (SPR). The diffuse reflectance spectroscopy measurements on a wide range of time scales (from picoseconds to minutes) demonstrate that a substantial part of electrons, injected from the Au NPs to the TiO2 mesocrystals through the SPR excitation, directionally migrate from the basal surfaces to the edges of the plate-like mesocrystals through the TiO2 nanocrystal networks and are temporally stored there for further reactions. This anisotropic electron flow significantly retarded the charge recombination of these electrons with the holes in the Au NPs, thereby improving the visible-light-photocatalytic activity (for organic-pollutant degradation) by more than an order of magnitude, as compared to that of conventional Au/TiO2 NP systems.

Single-Particle Study of Pt-Modified Au Nanorods for Plasmon-Enhanced Hydrogen Generation in Visible to Near-Infrared Region

Pt-modified Au nanorods (NRs) synthesized by anisotropic overgrowth were used for producing H2 under visible and near-infrared light irradiation. The Pt-tipped sample exhibited much higher activity compared with fully covered samples. Using single-particle spectroscopies combined with transmission electron microscopy, we observed obvious quenching phenomena for photoluminescence and light scattering from individual Pt-tipped NRs. The analysis of energy relaxation of plasmon-generated hot electrons indicates the electron transfer from the excited Au to Pt.

Design of a Highly Sensitive Fluorescent Probe for Interfacial Electron Transfer on a TiO2 Surface

Electron transfer (ET) is a key process in various chemical and biochemical reactions. For example, interfacial ET in nanoscale TiO2-based systems governs their photocatalytic performance, and thus a proper understanding of the ET mechanism can provide valuable information for designing highly efficient photocatalysts. Fluorescence at the singlemolecule or single-particle level has recently evolved as an important tool for studying catalytic reactions on solid surfaces, because of its high sensitivity, simplicity of data collection, and high spatial resolution in microscopic imaging techniques. For instance, several organic dye probes have been successfully employed to detect the generated reactive oxygen species and identify the active sites on individual TiO2 nanoparticles by utilizing single-molecule fluorescence spectroscopy. Nevertheless, there are very limited single-molecule studies on dyes that respond to the reduction reactions involved in ET; hence, there is a tremendous need to develop new suitable fluorescent probes for exploring ET processes in heterogeneous catalysis. We have now designed and synthesized a redox-responsive boron dipyrromethane fluorescent probe, namely, 3,4dinitrophenyl-BODIPY (DN-BODIPY, Figure 1A) on the basis of a photoinduced intramolecular ET mechanism. Both ensemble-averaged and single-molecule fluorescence experiments demonstrated that DN-BODIPY can act as a highly sensitive nanosensor to monitor photoinduced ET process on the TiO2 surface. The dye DN-BODIPY is composed of a fluorescent chromophore (BODIPY core) and a reactive site (dinitrophenyl group). The BODIPYs are of interest as fluorophores due to their attractive properties, such as a high extinction coefficient e, high fluorescence quantum yield Ffl , and good chemical and photostability, which facilitate their use in chemical and biosensor applications. On the other hand, reduction of aromatic nitro compounds to the corresponding hydroxylamines or amines is one of the most important transformations in synthetic organic chemistry and biochemistry, and thus has been used as a model system to investigate (photo)catalytic reduction reactions with semiconductor and metal nanoparticles. However, in order to develop a fluorogenic probe for monitoring ET process that exploits the reduction of a nitro-substituted benzene moiety, a major drawback must be overcome: nitrobenzene and its reduction products (i.e., phenylhydroxylamine or aniline) are believed to be strong quenchers of fluorescence dyes. The nitro group greatly lowers the LUMO energy level of the benzene moiety at the meso position of the BODIPY core because of its strongly electron withdrawing nature, and thus significant quenching of BODIPY fluorescence occurs by intramolecular ET from the excited fluorophore to the nitro-substituted benzene moiety (donor-excited ET). Oppositely, because the electron-donating hydroxylamino and amino groups greatly Figure 1. A) Photocatalytic generation of fluorescent HN-BODIPY from nonfluorescent DN-BODIPY. B) Normalized UV/Vis absorption (fine lines) and fluorescence (bold lines, excitation at 470 nm) spectra of DN-BODIPY (solid lines), HN-BODIPY (dashed lines), and ANBODIPY (dotted lines). The inset shows the magnified absorption peaks. C) Cyclic voltammograms of 1.0 mm DN-BODIPY, BODIPY, and o-dinitrobenzene (DNB) in Ar-saturated anhydrous solutions of electrolytes in acetonitrile.

Plasmon-Enhanced Formic Acid Dehydrogenation Using Anisotropic Pd–Au Nanorods Studied at the Single-Particle Level

Plasmonic bimetal nanostructures can be used to drive the conventional catalytic reactions efficiently at low temperature with the utilization of solar energy. This work developed Pd-modified Au nanorods, which work as the light absorber and the catalytically active site simultaneously, and exhibit efficient plasmon-enhanced catalytic formic acid dehydrogenation even when below room temperature (5 °C). Plasmon-induced interface interaction and photoreaction dynamics of individual nanorods were investigated by single-particle photoluminescence measurement, and a complete quenching phenomenon at the LSPR region was observed for the first time. More importantly, the spatial distribution of the SPR-induced enhancement, analyzed by the finite difference time domain (FDTD) simulation, shows that only tip-coated Pd can be affected for the occurrence of plasmon resonance energy transfer. This finding provides a route to decrease the amount of Pd species by the selective deposition only at the field-enhanced sites.

Far-red fluorescence probe for monitoring singlet oxygen during photodynamic therapy.

Singlet oxygen ((1)O2), molecular oxygen in the lowest excited state, has a critical role in the cell-killing mechanism of photodynamic therapy (PDT). Although (1)O2 phosphorescence measurement has been mainly used to monitor (1)O2 formation during PDT, its intensity is far insufficient to obtain two-dimensional images of intracellular (1)O2 with the subcellular spatial resolution using the currently available near-IR detector. Here, we propose a new far-red fluorescence probe of (1)O2, namely, Si-DMA, composed of silicon-containing rhodamine and anthracene moieties as a chromophore and a (1)O2 reactive site, respectively. In the presence of (1)O2, fluorescence of Si-DMA increases 17 times due to endoperoxide formation at the anthracene moiety. With the advantage of negligible self-oxidation by photoirradiation (ΦΔ < 0.02) and selective mitochondrial localization, Si-DMA is particularly suitable for imaging (1)O2 during PDT. Among three different intracellular photosensitizers (Sens), Si-DMA could selectively detect the (1)O2 that is generated by 5-aminolevulinic acid-derived protoporphyrin IX, colocalized with Si-DMA in mitochondria. On the other hand, mitochondria-targeted KillerRed and lysosomal porphyrins could not induce fluorescence change of Si-DMA. This surprising selectivity of Si-DMA response depending on the Sens localization and photosensitization mechanism is caused by a limited intracellular (1)O2 diffusion distance (∼300 nm) and negligible generation of (1)O2 by type-I Sens, respectively. For the first time, we successfully visualized (1)O2 generated during PDT with a spatial resolution of a single mitochondrial tubule.

pH-Induced Intramolecular Folding Dynamics of i-Motif DNA

Using the combination of fluorescence resonance energy transfer (FRET) and fluorescence correlation spectroscopy (FCS) technique, we investigate the mechanism and dynamics of the pH-induced conformational change of i-motif DNA in the bulk phases and at the single-molecule level. Despite numerous studies on i-motif that is formed from cytosine (C)-rich strand at slightly acidic pH, its detailed conformational dynamics have been rarely reported. Using the FRET technique to provide valuable information on the structure of biomolecules such as a protein and DNA, we clearly show that the partially folded species as well as the single-stranded structure coexist at neutral pH, supporting that the partially folded species may exist substantially in vivo and play an important role in a process of gene expression. By measuring the FCS curves of i-motif, we observed the gradual decrease of the diffusion coefficient of i-motif with increasing pH. The quantitative analysis of FCS curves supports that the gradual decrease of diffusion coefficient (D) associated with the conformational change of i-motif is not only due to the change in the intermolecular interaction between i-motif and solvent accompanied by the increase of pH but also due to the change of the shape of DNA. Furthermore, FCS analysis showed that the intrachain contact formation and dissociation for i-motif are 5-10 times faster than that for the open form. The fast dynamics of i-motif with a compact tetraplex is due to the intrinsic conformational changes at the fluorescent site including the motion of alkyl chain connecting the dye to DNA, whereas the slow intrachain contact formation observed from the open form is due to the DNA motion corresponding to an early stage interaction in the folding process of the unstructured open form.

Superstructure of TiO2 Crystalline Nanoparticles Yields Effective Conduction Pathways for Photogenerated Charges.

Materials with intricate nanostructures display fascinating properties, which have inspired extensive research on the synthesis of materials with controlled structures. In this study, we investigated the properties of superstructures of TiO2 to understand the inter-relationship between structural ordering and photocatalytic performance. The nanoplate anatase TiO2 mesocrystals were chosen as the typical investigation objects, which were newly synthesized by a topotactic structural transformation. The TiO2 mesocrystals displayed the superstructure of crystallographically ordered alignment of anatase TiO2 nanocrystals with high surface area and large high-energy surface {001} planes exposed. The photoconductive atomic force microscopy and time-resolved diffuse reflectance spectroscopy were utilized to determine the charge transport properties of TiO2 mesocrystals, and their features were highlighted by a comparison with reference TiO2 samples, for example, anatase TiO2 nanocrystals with similar surface area and single crystal structure. Consequently, it was found for the first time that such a superstructure of TiO2 could largely enhance charge separation and had remarkably long-lived charges, thereby exhibiting greatly increased photoconductivity and photocatalytic activity.

Selective Oxidative Degradation of Organic Pollutants by Singlet Oxygen-Mediated Photosensitization: Tin Porphyrin versus C60 Aminofullerene Systems

This study evaluates the potential application of tin porphyrin- and C(60) aminofullerene-derivatized silica (SnP/silica and aminoC(60)/silica) as (1)O(2) generating systems for photochemical degradation of organic pollutants. Photosensitized (1)O(2) production with SnP/silica, which was faster than with aminoC(60)/silica, effectively oxidized a variety of pharmaceuticals. Significant degradation of pharmaceuticals in the presence of the 400-nm UV cutoff filter corroborated visible light activation of both photosensitizers. Whereas the efficacy of aminoC(60)/silica for (1)O(2) production drastically decreased under irradiation with λ > 550 nm, Q-band absorption caused negligible loss of the photosensitizing activity of SnP/silica in the long wavelength region. Faster destruction of phenolates by SnP/silica and aminoC(60)/silica under alkaline pH conditions further implicated (1)O(2) involvement in the oxidative degradation. Direct charge transfer mediated by SnP, which was inferred from nanosecond laser flash photolysis, induced significant degradation of neutral phenols under high power light irradiation. Self-sensitized destruction caused gradual activity loss of SnP/silica in reuse tests unlike aminoC(60)/silica. The kinetic comparison of SnP/silica and TiO(2) photocatalyst in real wastewater effluents showed that photosensitized singlet oxygenation of pharmaceuticals was still efficiently achieved in the presence of background organic matters, while significant interference was observed for photocatalyzed oxidation involving non-selective OH radical.

Tin-porphyrin sensitized TiO2 for the production of H2 under visible light

Hydrogen was successfully produced under visible light irradiation in a tin porphyrin (SnP)-sensitized TiO2 system in the wide pH range (pH 3–11) although SnP hardly adsorbs on TiO2. The number of H2 produced in the SnP/TiO2 system after 9 h irradiation corresponds to the turnover number of 410. The apparent photonic efficiency for H2 evolution was estimated to be 35% with the monochromatic radiation of 550 ± 10 nm. The photochemical production of hydrogen is mediated through the formation of the π-radical anion (SnP˙−) that subsequently transfers electron to TiO2. The photogenerated SnP˙− was monitored by transient absorption spectroscopy and its lifetime is long enough to survive the slow diffusion from the solution bulk to the TiO2 surface, which makes the adsorption of SnP on TiO2 not required for hydrogen production. This is clearly contrasted with the common ruthenium complex-sensitized TiO2 system where the adsorption of the sensitizer complex is essentially required and the hydrogen production is limited to the acidic condition where the adsorption of the sensitizers on TiO2 is allowed. The photocatalytic activity of SnP was mainly attributed to the Q-band (500–650 nm), not to the Soret band (420–430 nm) of which absorption intensity is much higher.