Recent development of nanoplasmonics has stimulated chemists to utilize plasmonic nanomaterials for efficient and distinctive photochemical applications, and physicists to boldly go inside the "wet" ...chemistry world. The discovery of plasmon-induced charge separation (PICS) has even accelerated these trends. On the other hand, some confusion is found in discussions about PICS. In this perspective, we focus on differences between PICS and some other phenomena such as co-catalysis effect and plasmonic nanoantenna effect. In addition, materials and nanostructures suitable for PICS are shown, and characteristics and features unique to PICS are documented. Although it is well known that PICS has been applied to photovoltaics and photocatalysis, here light is shed on other applications that take better advantage of PICS, such as chemical sensing and biosensing, various photochromisms, photoswitchable functionalities and nanoscale photofabrication.
Electrons transfer from plasmonic nanoparticles to semiconductors by exploiting the energy of light, and this effect is applied to photovoltaics, photocatalysis, sensing, photochromisms, photoswitchable functionalities and nanofabrications.
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.
External quantum efficiency of plasmon-induced charge separation (PICS) at a plasmonic nanoparticle–semiconductor interface is given as the product of the plasmonic light absorption efficiency and ...the internal quantum efficiency (IQE) of charge separation. As the size of the plasmonic nanoparticle increases, the absorption efficiency is improved, but the IQE is often deteriorated. In order to address the limitation caused by those opposite dependencies, we developed a novel PICS system in which a large Au nanocube (AuNC) as a light-harvesting antenna unit is electromagnetically coupled with a small Au nanosphere (AuNS) as a charge separation unit on a TiO2 semiconductor substrate. The AuNS–AuNC structure exhibited PICS efficiency enhancement of a factor of two or higher in the 650–800 nm wavelength range in comparison with the system without AuNC antennae. Simulation of electric field distributions shows that photons are captured by the AuNC antenna, transferred to the AuNS unit via the asymmetric plasmon coupling, and relaxed to an electron–hole pair, which are separated at the AuNS–TiO2 interface, resulting in the improved PICS efficiency.
To clarify effects of plasmonic electric field intensity and distributions on the internal quantum efficiency (IQE) of plasmon-induced charge separation (PICS), we took advantage of interparticle ...plasmon coupling of Au nanoparticles (AuNPs) on TiO2. For the isolated AuNPs without coupling, the electric field localized at the Au–TiO2 interface contributes to the IQE value. As the plasmon coupling is formed, the plasmonic absorption peak of the AuNPs is red-shifted and the IQE value is increased. The electric field intensity at the AuNP surface is remarkably increased, whereas that at the interface is hardly changed, by the plasmon coupling. Therefore, it is concluded that the electric field at the AuNP surface is enhanced by the plasmon coupling, resulting in the improved IQE of PICS.
Phosphorus usage is irreplaceable in agriculture; however, its excessive use leads to wastage of invaluable resources and significant soil surplus. Agronomic soil phosphorus surplus in Asian regions ...has a much higher level than the global average. And with rapid urbanization and population growth in the recent decades, Asian countries have seen a rise in environmental pollution levels also. This study assessed the detailed phosphorus budget in the Yamato River catchment, an urbanized coastal catchment in Asia, from 1940s to 2010s using Soil and Water Assessment Tool, comprehensively analyzed the effect of anthropogenic factors on long-term phosphorus loading and agronomic soil phosphorus balance. The results showed the peak period of total phosphorus loading and agronomic soil phosphorus surplus occurred in 1970s, at 895 tons/year and 36.6 kg/ha, respectively. The major reasons for increased phosphorus loading and soil surplus during 1940–1970 were rapid population growth and increased fertilizer usage, respectively. Since the 1980s, the construction of wastewater treatment system and reduction in agricultural land contributed to environmental improvement. These anthropogenic factors had a much stronger impact on phosphorus budget than climate change in the study catchment. Soil phosphorus balance is affected by a combination of factors, such as soil properties, fertilizer usage and applied schedule, precipitation event, and crop types. And soil phosphorus surplus may be severely overestimated if the non-point source loss due to precipitation factor is not fully considered.
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•Excessive phosphorus fertilizer increases soil phosphorus surplus and pollution.•Phosphorus loading and budget were estimated for 80 years of Yamato River catchment.•Agronomic soil budget assessment using SWAT is better compared to MFA-based methods.•Total phosphorus loading and agronomic soil phosphorus surplus peaked in the 1970s.
Although there are many mechanistic studies for plasmon-induced charge separation (PICS), most of them have been devoted to energetic electron injection (or hot electron injection) from plasmonic ...metal nanoparticles into electron transport materials (ETMs) including n-type semiconductors. Here we also studied energetic hole injection (or hot hole injection) into five different organic hole transport materials (HTMs) with different ionization energies, by using solid-state photovoltaic cells fabricated by introducing gold nanoparticles (AuNPs) in between TiO2, which is an ETM, and a HTM. As a result, photocurrents based on PICS are obtained even if the height of the energy barrier at the Au–HTM interface is 0.9 eV. Therefore, the present PICS processes involve both energetic electron injection from AuNPs to TiO2 and hole injection from AuNPs to the HTMs. In this case, simultaneous electron–hole injection, in which one electron–hole pair gives simultaneous electron injection into TiO2 and hole injection into a HTM, is often postulated. However, there is another possibility of stepwise electron–hole injection, in which one electron–hole pair leads to electron injection into TiO2 and another pair gives hole injection into a HTM. The stepwise injection process is accompanied by recombination of the residual hole of the former pair and the electron of the latter. The present cells generated the photocurrents even if the incident photon energy was lower than the sum of the TiO2–Au and Au–HTM barrier energies (≤2.1 eV). In addition, experimentally obtained photocurrent action spectra of the cells can be reproduced on the basis of plasmonic light absorption and theoretically analyzed carrier injection efficiencies, when the stepwise injection is assumed. It was therefore concluded that the stepwise carrier injection chiefly contributes to the present PICS processes.
Acetyl xylan esterase (AXE) catalyzes the hydrolysis of the acetyl bonds present in plant cell wall polysaccharides. Here, we determined the crystal structure of AXE from
(
AXEA), providing the ...three-dimensional structure of an enzyme in the Esterase_phb family.
AXEA shares its core α/β-hydrolase fold structure with esterases in other families, but it has an extended central β-sheet at both its ends and an extra loop. Structural comparison with a ferulic acid esterase (FAE) from
indicated that
AXEA has a conserved catalytic machinery: a catalytic triad (Ser119, His259, and Asp202) and an oxyanion hole (Cys40 and Ser120). Near the catalytic triad of
lAXEA, two aromatic residues (Tyr39 and Trp160) form small pockets at both sides. Homology models of fungal FAEs in the same Esterase_phb family have wide pockets at the corresponding sites because they have residues with smaller side chains (Pro, Ser, and Gly). Mutants with site-directed mutations at Tyr39 showed a substrate specificity similar to that of the wild-type enzyme, whereas those with mutations at Trp160 acquired an expanded substrate specificity. Interestingly, the Trp160 mutants acquired weak but significant type B-like FAE activity. Moreover, the engineered enzymes exhibited ferulic acid-releasing activity from wheat arabinoxylan.
Hemicelluloses in the plant cell wall are often decorated by acetyl and ferulic acid groups. Therefore, complete and efficient degradation of plant polysaccharides requires the enzymes for cleaving the side chains of the polymer. Since the Esterase_phb family contains a wide array of fungal FAEs and AXEs from fungi and bacteria, our study will provide a structural basis for the molecular mechanism of these industrially relevant enzymes in biopolymer degradation. The structure of the Esterase_phb family also provides information for bacterial polyhydroxyalkanoate depolymerases that are involved in biodegradation of thermoplastic polymers.
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