Composites of small Ag clusters confined in the channels of well-ordered mesoporous anatase TiO2 and their excellent solar-light-driven photocatalytic performance

Small Ag clusters confined in the channels of ordered mesoporous anatase TiO2 have been fabricated via a vacuum-assisted wet-impregnation method, utilizing well-ordered mesoporous anatase TiO2 with high thermal stability as the host. The composites have been characterized in detail by X-ray diffraction, X-ray photoelectron spectroscopy X-ray absorption fine structure （XAFS） spectroscopy, N2 adsorption, UV-visible diffuse reflectance spectroscopy and transmission electron microscopy. The results indicate that small Ag clusters are formed and uniformly confined in the channels of mesoporous TiO2 with an obvious confinement effect. The presence of strong AgO interactions involving the Ag clusters in intimate contact with the pore walls of mesoporous TiO2 is confirmed by XAFS analysis, and favors the separation of photogenerated electron-hole pairs, as shown by steady-state surface photovoltage spectroscopy and transient-state surface photovoltage measurements. The ordered mesoporous Ag/TiO2 composites exhibit excellent solar-light-driven photocatalytic performance for the degradation of phenol. This is attributed to the synergistic effects between the small Ag clusters acting as traps to effectively capture the photogenerated electrons, and the surface plasmon resonance of the Ag clusters promoting the absorption of visible light. This study clearly demonstrates the high-efficiency utilization of noble metals in the fabrication of high-performance solar-light-driven photocatalysts.

Simultaneous optical and electrochemical recording of single nanoparticle electrochemistry

Single nanoparticle collisions have become popular for studying the electro- chemical activity of single nanoparticles by determining the transient current during stochastic collisions with the electrode surface. However, if only the electrochemical current is measured, it remains challenging to identify and characterize the individual particle that is responsible for a specific current peak in a collision event; this hampers the understanding of the structure-activity relationship. Herein, we report simultaneous optical and electrochemical recording of a single nanoparticle collision; the electrochemical signal corresponds with the activity of a single nanoparticle, and the optical signal reveals the size and location of the same nanoparticle. Consequently, the structure （optical signal）- activity （electrochemical signal） relationship can be elucidated at the single nanoparticle level; this has implications for various applications including batteries, electrocatalysts, and electrochemical sensors. In addition, our previous studies have suggested an optical-to-electrochemical conversion model to independently calculate the electron transfer rate of single nanopartides from the optical signal. The simultaneous optical and electrochemical recording achieved in the present work enables direct and quantitative validation of the optical-to-electrochemical conversion model.

Uniform and reproducible plasmon-enhanced fluorescence substrate based on PMMA-coated, large-area Au@Ag nanorod arrays

Here we describe a plasmon-enhanced fluorescence substrate based on poly（methyl methacrylate） （PMMA）-coated, large-area Au@Ag nanorod arrays. The use of a PMMA medium enables precise control of the competition between enhancing and quenching processes as a function of the distance between Au@Ag nanorods and dye molecules. At the optimal PMMA layer thickness of 56 nm （for which the distance between nanopartides and dye molecules is 16 nm）, a maximum enhancement of fluorescence of up to N 27 times is measured. The competition mechanism between enhancing and quenching processes depends on the thickness of the PMMA layer, which has been confirmed by consistent experimental and theoretical modeling results. Notab136 the micropatterned metal-enhanced fluorescence （MEF） substrate exhibits high uniformity and reproducibility. The simple spin-coating process described herein provides an attractive, scalable, and low-cost strategy to produce uniform and reproducible large-area MEF substrates that can potentially be used in many fields, such as biochips, diagnostics, and photonics.

Spermine induced reversible collapse of deoxyribonucleic acid-bridged nanoparticle-based assemblies

DNA-linked 2D and 3D nano-assemblies find use in a diverse set of applications, ranging from DNA-origami in drug delivery and medical imaging, to DNA-linked nanoparticle structures for use in plasmonics and （bio）sensing. However, once these structures have been fully assembled, few options are available to modulate structure geometry. Here, we investigated the use of the polycation spermine to induce DNA collapse in small oligonucleotide-linked （54 bp） gold nanoparticle structures by monitoring shifts in the localized surface plasmon resonance （LSPR） peak and by comparing the data with finite-difference time-domain （FDTD） simulations. Our data shows that low concentrations of spermine can be applied to induce large changes in DNA conformation, leading to a significant reduction in interparticle distance （from - 25 to - 3 nm） and enhanced plasmonic coupling. The DNA collapse is near-instantaneous and reversible, and its application at low and high DNA densities is demonstrated with surface plasmon resonance imaging （SPRi）, showing the potential of spermine to dynamically modulate distances and geometry in DNA-based nano-assemblies.

Asymmetric growth of Au-core/Ag-shell nanorods with a strong octupolar plasmon resonance and an efficient second-harmonic generation

Colloidal Au-core/Ag-shell nanorods with an asymmetric transverse cross- section and a strong octupolar plasmon resonance are synthesized by the controlled growth of Ag shells on one side of the Au cores. A largely enhanced second harmonic generation （SHG） from these asymmetric core-shell nanorods is demonstrated for the first time by tuning the dipolar and the octupolar plasmon modes to make them resonant with the fundamental and harmonic frequencies, respectively. The SHG intensity of the Au-Ag nanorods with dual-frequency resonances is enhanced by 21 times compared to that of the bare Au nanorods. The co-existence of the dipolar, quadrupolar, and octupolar radiations in the SHG is revealed. Additionally, the cellular uptake of the Au-Ag nanorods is monitored and the evolution of the membrane bleb is successfully observed by the SHG imaging. Our observations provide a strategy for enhancing the SHG of the colloidal metal nanoparticles and can have applications in chemical process monitoring and biological sensing.

Improved plasmon-assisted photoelectric conversion efficiency across entire ultraviolet-visible region based on antenna-on zinc oxide/silver three-dimensional nanostructured films

ZnO has received tremendous attention for applications in photoelectrochemical water splitting, photocatalysis, and photovoltaic devices. However, the photoelectric conversion efficiency of ZnO is limited by the rapid recombination of photoexcited electron-hole pairs and the wide band gap, which allows only a small fraction of the solar spectrum to be absorbed. Recently, substantial research efforts have aimed to increase the photoelectric conversion efficiency across the entire ultraviolet-visible （UV-vis） spectrum by coupling semiconductors such as ZnO with noble metal nanoparticles （NPs）. In this stud~ we compare the performance of a pure ZnO film and ZnO/Ag nanostructured films as photoelectrodes. We show that under broad-spectrum UV-vis illumination, the photocurrent generated in the ZnO/Ag three-dimensional （3D） nanostructured films increases 3.75 times relative to the photocurrent generated in the pure ZnO films. We attribute the high photocurrent to the electric-field enhancement associated with the localized surface plasmon resonance of the Ag NPs, which are present at a high density in the 3D nanostructured films, and to the creation of photoexcited hot electrons in Ag that are transferred to ZnO, promoting electron-hole pair separation. We propose a mechanism to explain the observed enhancement of the photoelectric conversion efficiency.