NIR-to-Vis Handheld Platforms for Detecting miRNA Level and Mutation Based on Sub-10 nm Sulfide Nanodots and HCR Amplification

Sub-10 nm monodisperse alkaline-earth sulfide nanodots (ASNDs) with bright near-infrared (NIR)-excitation fluorescence and adjustable emission wavelength were prepared by a thermal decomposition method for the first time. The ASNDs exhibited high NIR-to-vis conversion efficiency and served as multicolor fluorescent labels in the proposed miR-224 assay. Targeted detection of the miR-224 level and single-nucleotide variation in miR-224 was carried out on a smartphone-based platform using a hybridization chain reaction (HCR) amplification strategy. In the presence of miR-224, the ASND-labeled HCR probes self-assembled on the surface of the diagnosis kits, generating strong fluorescent signals linearly proportional to miR-224 contents in the range of 10–2000 fM. Significantly, mutations in miR-224 led to the variation in the fluorescence intensity ratio in RGB channels. Simultaneously, evident changes of fluorescent brightness and color were easily visualized by the naked eye, which enabled on-site discrimination of miR-224 with different mutant loci. This work provides a novel preparation approach for ultrasmall NIR excitation sulfide nanodots and reveals the potential of the as-synthesized ASNDs in point-of-care (POC) nucleic acid testing. Further, it may provide a handheld platform for miRNA single-nucleotide polymorphism analysis.