The length of Ag nanorods is uniquely controlled by irradiation with visible light using anatase TiO 2 -containing tubular SiO 2 mesoporous templates. Spherical Ag nanoparticles and Ag nanorods of controlled length were fabricated by irradiation with visible light of selected wavelengths.Noble metal nanoparticles are attracting a great deal of attention, in particular because of their surface plasmon resonance (SPR)-related optoelectronic properties. 1-5 Nanoparticles (NPs) with one-dimensional (1-D) structures, such as rods and wires, possess interesting properties. For example, noble metal nanorods (NRs) exhibit anisotropic SPR, where the wavelength may be controlled by changing the length of the NRs. 6-8 Among a variety of noble metal NPs, Ag NPs show SPR with the strongest field enhancement factor, which is favorable for their application in surface enhanced Raman scattering (SERS), surface enhanced fluorescence (SEF), and molecular labeling (ML). 9-15 However, the preparation of Ag NRs of controlled size, which should be relatively small (5-10 nm in diameter, and 10-100 nm in length) to effectively exhibit SPR, has seldom been reported, in contrast to the large quantity of research on the preparation and application of Au NRs of controlled length. [16][17][18] This is because of the stronger oxidation and ionization tendencies of Ag compared with Au and some other noble metals, which make it more difficult to work with. 19 The preparation of Ag NRs using a SiO 2 template with tubular mesopores has been reported. [20][21][22] In this procedure, Ag was chemically or thermally deposited in the tubular mesopores, so continuous loading of Ag led to the formation of Ag NRs in the mesopores. However, the length of the Ag NRs was not controlled, which caused wide SPR bands. This means that the obtained samples do not possess wavelength selectivity, which is very important to obtain a high quantum efficiency in SERS, SEF, and ML applications. On the other hand, it was recently reported that Ag NPs deposited on TiO 2 crystals exhibited multicolor photochromism. 23-26 Irradiation with monochromatic light caused the size distribution of Ag NPs to change, which led to color changes. The change in size distribution was attributed to the partial dissociation of Ag NPs caused by electron transfer from Ag NPs exhibiting SPR to TiO 2 crystals. These results suggest the possibility of fabricating monodisperse Ag NPs by irradiation with light of a specific wavelength, which would induce SPR only on Ag NPs of unintended size and shape, thus removing undesired NPs from the sample.In this work, the work of ref. 24 and 25 was extended. Namely, the length of Ag NRs was controlled by light irradiation. Ag NRs were chemically deposited in the tubular mesopores of SiO 2 templates that contained TiO 2 nanocrystals. During or after the deposition of Ag NRs, the samples were exposed to light with a wavelength that induced SPR on Ag NRs of certain length. This caused lengthselective dissociation of Ag NRs, which allowed precise control of ...