<p>We have developed a technique to control the structure of plasmonic nanoparticle–photofunctional material composites at the nanoscale by using analytical chemical methods. We have also developed a highly sensitive sensing system that maximizes the photo-harvesting effect of plasmonic nanoparticles, and a novel photo-detecting system that utilizes a charge separation system at the interface between plasmonic nanoparticles and p-type semiconductors. The research results have been successfully applied to the photocatalyst for hydrogen evolution and the elucidation of the reaction mechanism through analysis and measurement. These research results propose a new method to effectively utilize weak light energy and to dramatically improve environmental and spectroscopic measurement techniques using light. This review outlines our efforts about these topics, primarily based on our recent achievement.</p> Graphical Abstract <p></p>