Controlled Synthesis of Gold Nanoparticles on Fluorescent Nanodiamond via Electron-Beam-Induced Reduction Method for Dual-Modal Optical and Electron Bioimaging

Hybrid nanoparticles are emergent nanomaterials that combine particles with different characteristic properties to enhance their original functions or modulate their original physical or chemical properties for application in catalysis, sensing, and imaging. Fluorescent nanodiamonds (fNDs) have recently become more attractive for bioimaging because of their characteristic physicochemical properties and biocompatibility. Their wide applicability in bioimaging has been utilized in the single-particle tracking of biomolecules, local environmental sensors in cells, and stem cell tracking in tissues. However, the use of fNDs as multiscale spatial mapping probes for multiple biomolecules and cells in optical and electron microscopy techniques has been limited because of their broad fluorescence spectrum and composition of mainly light elements (C, O, H, N, etc.). On the other hand, metal nanoparticles (metal NPs) with unique photonic properties have been employed as functional labeling probes in bioimaging. Therefore, an efficient synthesis strategy to produce fND/metal NP nanocomposites with regulated shapes is required to develop molecular and cellular bioimaging probes with simultaneous use in multiple imaging techniques. Here, we report the synthesis of dual-modal hybrid gold NP–fND (Au-ND) nanoparticles with a mean diameter of less than 20 nm using an electron-beam-induced reduction method. The resultant Au-NDs exhibited stable Au-NP-induced plasmonic modulation of fluorescence lifetimes in cellular environments, which is useful for fluorescence lifetime imaging microscopy to detect multiple molecules or cells. Furthermore, Au NPs modified on fNDs function as surrogate markers with sub-10 nm spatial resolution for electron microscopy in mammalian cells. Our findings indicate that the electron-beam reduction method will enable us to make simplified formations of metal NPs with characteristic plasmonic structures on fNDs for multimodal bioimaging probes.