Development of excitation power-responsive anti-stokes emission wavelength switching and their energy saving induced by localized surface plasmon resonance.

We designed an external stimulus-responsive anti-Stokes emission switching using dual-annihilator-based triplet-triplet annihilation upconversion systems. This system, which was constructed by incorporating a palladium porphyrin derivative as a sensitizer and 9,10-diphenylanthracene (DPA) and 9,10-bis(triisopropylsilyl)ethynylanthracene (TIPS) as annihilators into polymer thin films, produced TIPS- and DPA-based anti-Stokes emission under low and high excitation powers, respectively. The mechanism involves the following: under low excitation power, triplet energy transfer from triplet-excited PdOEP to DPA is induced, followed by relay to TIPS. This results in the generation of triplet-excited TIPS, and the subsequent triplet-triplet annihilation between them produces TIPS-based anti-Stokes emission. Conversely, under high excitation power, the high-density triplet-excited DPA, generated through triplet energy transfer from PdOEP, undergoes triplet-triplet annihilation among themselves, resulting in the generation of DPA-based anti-Stokes emission. Additionally, we achieved energy savings by reducing the required excitation power for switching through the utilization of plasmonic metal nanoparticles. The strong local electromagnetic fields associated with the localized surface plasmon resonance of metal nanoparticles enhance the photoexcitation efficiency of PdOEP, subsequently increasing the density of triplet-excited DPA. As a result, anti-Stokes emission switching becomes feasible at lower excitation powers.

Optimizing the Distance between Upconversion Thin Films and Silver Nanoprisms for the Design of a High-Performance Plasmonic Triplet-Triplet Annihilation Upconversion System.

While the distance dependence of metal-enhanced fluorescence has been extensively studied for composite systems comprising fluorophores and metal nanoparticles, the corresponding distance dependence of triplet-triplet annihilation upconversion (TTA-UC) systems remains unexplored. Herein, we investigated the influence of the spatial distance between Ag nanoprisms (AgPRs) and TTA-UC thin films consisting of a palladium octaethylporphyrin (PdOEP) sensitizer and a 9,10-diphenylanthracene (DPA) emitter, aiming at enhancing the upconverted (UC) emission as efficiently as possible. Results indicated that the optimal distance for the examined system was significantly longer (12.6 nm) than those of typical metal-enhanced fluorescence systems (about 2 nm). We demonstrated that the UC emission enhancement factor can be expressed as a product including factors of the PdOEP photoexcitation rate, triplet-triplet energy transfer (TTET) efficiency from PdOEP to DPA, triplet excited DPA lifetime, and fluorescence efficiency of singlet excited DPA. We discovered that the AgPRs play a beneficial role in enhancing the PdOEP photoexcitation, whereas they exert detrimental effects on the other three factors. Among these three factors, quenching contributions by the decrease of the triplet excited DPA lifetime and DPA fluorescence efficiency were significant, making these the primary and secondary factors, respectively, for the UC emission quenching, particularly at short distances. These results demonstrate that the characteristic distance dependence of the UC emission enhancement is determined by the competing effects of beneficial PdOEP photoexcitation enhancement and the detrimental localized surface plasmon (and/or AgPR)-induced nonradiative decay of the triplet- and singlet excited DPA molecules. The findings offer valuable guidelines for the design of high-performance plasmonic TTA-UC systems.

Photothermal therapeutic ability of copper open-shell nanostructures that are effective in the second biological transparency window based on symmetry breaking-induced plasmonic properties.

In this study, a photothermal therapy agent that works efficiently in the second biological transparency window was developed based on the localized surface plasmon (LSP) resonance of symmetry-broken open-shell nanostructures of low-cost Cu (CuOSNs). The strong LSP resonance and superior photothermal conversion ability in the second biological transparency window were achieved by generating the dipolar bonding mode due to the plasmon hybridization between the nanoshell dipole and the nanohole dipole at the opening edge in CuOSNs derived from the symmetry breaking of a Cu nanoshell. Oxidative dissolution of CuOSNs in water was significantly suppressed by successive coating with the self-assembled monolayer of 16-mercaptohexadecanoic acid and a thin silica layer. Furthermore, the stability in phosphate buffered saline, which models the biological environment, was attained by further coating the nanoparticles with polyethylene glycol. It was demonstrated from in vitro cell tests using HeLa cells that the cytotoxicity of CuOSNs was effectively suppressed by the surface protection. The viability of HeLa cells incubated with CuOSNs was decreased under the irradiation of low intensity 1060 nm laser with increasing number of CuOSNs. These results demonstrate that low-cost symmetry-broken Cu-based nanostructures can act as an excellent photothermal therapy agent in the second biological transparency window.

Plasmonic Metal Nanostructures Meet Triplet-Triplet Annihilation-Based Photon Upconversion Systems: Performance Improvements and Application Trends.

Improving the performance of upconversion systems based on triplet-triplet annihilation (TTA-UC) can have far-reaching implications for various fields, including solar devices, nano-bioimaging, and nanotherapy. This review focuses on the use of localized surface plasmon (LSP) resonance of metal nanostructures to enhance the performance of TTA-UC systems and explores their potential applications. After introducing the basic driving mechanism of TTA-UC and typical sensitizers used in these systems, we discuss recent studies that have utilized new sensitizers with distinct characteristics. Furthermore, we confirm that the enhancement in upconverted emission can be explained, at least in part, by the mechanism of "metal-enhanced fluorescence", which is attributed to LSP resonance-induced fluorescence enhancement. Next, we describe selected experiments that demonstrate the enhancement in upconverted emission in plasmonic TTA-UC systems, as well as the emerging trends in their application. We present specific examples of studies in which the enhancement in upconverted emission has significantly improved the performance of photocatalysts under both sunlight and indoor lighting. Additionally, we discuss the potential for future developments in plasmonic TTA-UC systems.

Development of Plasmonic Cu2O/Cu Composite Arrays as Visible- and Near-Infrared-Light-Driven Plasmonic Photocatalysts.

We describe efficient visible- and near-infrared (vis/NIR) light-driven photocatalytic properties of hybrids of Cu2O and plasmonic Cu arrays. The Cu2O/Cu arrays were prepared simply by allowing a Cu half-shell array to stand in an oxygen atmosphere for 3 h, which was prepared by depositing Cu on two-dimensional colloidal crystals with a diameter of 543 or 224 nm. The localized surface plasmon resonances (LSPRs) of the arrays were strongly excited at 866 and 626 nm, respectively, at which the imaginary part of the dielectric function of Cu is small. The rate of photodegradation of methyl orange was 27 and 84 times faster, respectively, than that with a Cu2O/nonplasmonic Cu plate. The photocatalytic activity was demonstrated to be dominated by Cu LSPR excitation. These results showed that the inexpensive Cu2O/Cu arrays can be excellent vis/NIR-light-driven photocatalysts based on the efficient excitation of Cu LSPR.