Preparation of a Ruthenium-Complex-Functionalized Two-Photon-Excited Red Fluorescence Silicon Nanoparticle Composite for Targeted Fluorescence Imaging and Photodynamic Therapy in Vitro

Silicon nanoparticles (SiNPs), especially those emitting red fluorescence, have been widely applied in the field of bioimaging. However, harsh synthetic conditions and strong biological autofluorescence caused by short wavelength excitation restrict the further development of SiNPs in the field of biological applications. Here, we report a method for synthesizing a ruthenium-complex-functionalized two-photon-excited red fluorescence silicon nanoparticle composite (SiNPs–Ru) based on fluorescence resonance energy transfer under mild experimental conditions. In the prepared SiNPs–Ru composite, silicon nanoparticles synthesized by atmospheric pressure microwave-assisted synthesis served as a fluorescence energy donor, which had two-photon fluorescence properties, and tris(4,4′-dicarboxylic acid-2,2-bipyridyl)ruthenium(II) dichloride (LRu) acted as a fluorescence energy acceptor, which could emit red fluorescence as well as had the ability to produce singlet-oxygen for photodynamic therapy. Therefore, the synthesized SiNPs–Ru could emit red fluorescence by two-photon excitation based on fluorescence resonance energy transfer, which could effectively avoid the interference of biological autofluorescence. Fluorescence imaging tests in zebrafish and nude mice indicated that the as-prepared SiNPs–Ru could act as a new kind of fluorescence probe for fluorescence imaging in vivo. By coupling folic acid (FA) to SiNPs–Ru, the prepared composite (FA–SiNPs–Ru) could not only serve as a targeted two-photon fluorescence imaging probe but also kill cancer cells via photodynamic therapy in vitro.