Photofunctional materials and energy conversion systems involving photochemical reactions are indispensable to support the comfortable life of human beings. Localized surface plasmons (LSPs) induced by metal nanoparticles have attracted much attention as light-harvesting antenna because LSPs confine the incident light spatially and temporarily. Recently, the authors studied various properties of LSPs of precisely designed and fabricated metal nanostructures. In this review, we introduce the unique photochemical reactions promoted by LSP. In particular, we explain the plasmon-induced light-to-chemical energy conversion reaction upon irradiation by visible light.
Gold nanoparticles have large absorption cross sections due to their localized surface plasmon (LSP). Thanks to this property, the gold nanoparticles have been used as color materials for a long time. In recent years, there is a growing interest in heating accompanying the LSP light absorption. Since heating by plasmon is spatially limited, it is possible to cause a chemical reaction by generating a locally high temperature while keeping the whole system at ambient temperature. In addition, since the heating region is narrow, thermal inertia is small, and rapid temperature switching is possible. From these characteristics, heating by LSP is now applied to photothermal cancer treatment, catalytic reaction, and hydrothermal synthesis. In this review, we describe the photothermal conversion characteristics of LSP and its application to chemical reactions.
Metallic gold and silver nanoparticles (NPs) with diameters of several tens of nanometers exhibit brilliant colors, reflecting their size and shapes. The origin of coloring is resonance between collective oscillation of metal conduction electrons and light, and is called plasmon resonance. Due to plasmon resonance, such NP surfaces are covered with strong localized light. Electromagnetic interaction between the localized light and molecules on the NPs greatly enhances optical molecular responses including absorption, scattering, etc. The spectroscopy utilizing the enhancement is called plasmon-enhanced spectroscopy. In this review, we explain the enhancement mechanism using surface-enhanced Raman scattering (SERS), which is one type of plasmon-enhanced spectroscopy. Then we report SERS observation of molecular structural changes under the single level. We also report the advantages of plasmonic nanowires instead of NPs for plasmon-enhanced spectroscopy. Finally, we introduce interesting phenomena that occur beyond the limit of the enhancement mechanism.
Metal nanostructures can harvest light energy in nanospaces on the surfaces based on localized surface plasmon resonance. Appropriate design can realize various photoenergy conversion systems with higher efficiencies. We fabricated various photo-active electrodes consisting of metal nanoparticles, intended for applications in photoelectric conversion systems. Structural characteristics and photoelectrical properties of photo-active electrodes were analyzed in order to control the plasmon absorption band and the near-field effects for highly effective utilization of the light-harvesting effects for photoelectric conversion systems.
With the ballooning aged population, the rapid rise of health care costs and the shifting impact of non-communicable diseases in global health, the demand for developing biosensors which can detect target molecule markers rapidly and accurately at a reasonable price still remains. Especially, the study of a plasmonic biosensor is highly promising because of its label-free nature and simple detection of the optical change based on the binding affinity between the recognition molecule and the target. With this, the use of conventional and pricy labeling such as fluorescent dye or enzyme can be avoided. In addition, to keep a biosensor system reusable, the sensing substrate is typically made to be disposable. Thus, the reduction of the substrate's fabrication process is critical in advancing a biosensor’s development. Taking this into consideration, we studied the development of a plasmonic biosensor chip using nanoimprinting technology. In addition, we also introduce an LSPR imaging assay using hyper spectrum imaging system necessary for multiple analysis.