More Symmetrical “Hot Spots” Ensure Stronger Plasmon-Enhanced Fluorescence: From Au Nanorods to Nanostars

Plasmon-enhanced fluorescence (PEF) is considered to be a powerful signal amplification technology to overcome intrinsic shortcomings of photobleaching and brightness of the traditional fluorescent dyes. Nevertheless, exploitation of PEF-based probes for bioimaging application is still at a very early stage. In this work, a simple but powerful gold nanostar (Au NST)@SiO2-based PEF probe with 20 symmetric "hot spots" was developed for highly sensitive "lighting up" in situ imaging of intracellular microRNAs (miRNAs). By regulating the thickness of the silica shell, the distance between Au NSTs and fluorescent dyes was controlled, and the optimum fluorescence enhancement (21-fold) was obtained with the silica shell thickness of approximately 22 nm. Thanks to the 20 more powerful "hot spots" that can produce stronger localized electric fields, the Au NST-based PEF probe exhibits stronger PEF effects than the traditional plasmonic nanostructures such as gold nanorods (Au NRs), gold nanobipyramids (Au NBPs), and triangular gold nanoprisms (Au NPRs), resulting in high sensitivity and improved detection limit (LOD) of 0.21 pM for miRNA-21 analysis. Moreover, not only cancer cells (MCF-7 and Hela) and normal cells (L02) with distinct miRNA-21 expression levels can be discriminated but also tumor cells in co-cultured mixtures can be recognized, indicating its promising potential in clinical diagnosis.