Ratiometric Fluorescence Imaging of Intracellular MicroRNA with NIR-Assisted Signal Amplification by a Ru-SiO 2 @Polydopamine Nanoplatform.

Accurate and sensitive fluorescence imaging of intracellular miRNA is essential for understanding the mechanism underlying some physiological and pathological events, as well as the prevention and diagnosis of diseases. Herein, a highly sensitive ratiometric fluorescent nanoprobe for intracellular miRNA imaging was fabricated by integrating a Ru-SiO ₂ @polydopamine (Ru-SiO ₂ @PDA) nanoplatform with a near-infrared light (NIR)-assisted DNA strand displacement signal amplification strategy. The Ru-SiO ₂ @PDA spheres have excellent biosafety, high photothermal effect, and unique photophysical properties that can both emit a stable red fluorescence and well quench the fluorophores getting closer to them. So, when the fuel DNA and carboxyfluorescein (FAM)-labeled signal DNA are co-assembled on their outer surfaces, the FAM's green fluorescence is quenched, and a low ratiometric signal is obtained. However, in the presence of miRNA, the target displaces the signal DNA from the capture DNA, releasing the signal DNA far away from the Ru-SiO ₂ @PDA. Then, the green fluorescence recovers and leads to an enhanced I _green / I _red value. Under NIR light irradiation, the Ru-SiO ₂ @PDA increases the local temperature around the probe and triggers the release of fuel DNA, which thus recycles the target miRNA and effectively amplifies the ratiometric signal. Using A549 cells as a model, the nanoprobe realizes the highly sensitive ratiometric fluorescence imaging of miRNA let-7a, as well as its in vivo up- and down-regulation expressions. It provides a facile tool for highly sensitive and accurate intracellular miRNA detection through one-step incubation and may pave a new avenue for single-cell analysis.