Controlled Self-Assembly of a Close-Packed Gold Octahedra Array for SERS Sensing Exosomal MicroRNAs.

MicroRNAs (miRNAs) in exosomes can be transferred from parental cells to recipient cells by trafficking exosomes, and they are effective in regulating the gene expression of the recipient cells. Therefore, exosomal miRNAs play a vital role in cancer biology and could be potential biomarkers for cancer diagnosis and therapeutic responses. However, accurate detection of exosomal miRNAs is still challenging due to the low abundance of any given miRNA in exosomes. Herein, a surface-enhanced Raman scattering (SERS)-based sensor was developed for the quantitative determination of let-7a miRNAs in MCF-7 cell-derived exosomes (MCF-7 exosomes) using a close-packed and ordered Au octahedral array as a sensing platform. Au octahedra in the array uniformly stand on their triangular face. This kind of orientation produces "hot surfaces" rather than "hot spots" and greatly improves the detection sensitivity and uniformity. Let-7a detection with single-base specificity was thus achieved from the SERS intensity change induced by the structural switch of the probing DNA from a hairpin to a duplex in the presence of the target. The sensor showed a broad linear range (10 aM to 10 nM) and a low detection limit (5.3 aM) without using any signal amplification strategy. Moreover, this sensor could accurately detect target let-7a in MCF-7 exosomes and further value the impact of drug treatment on exosomal let-7a expression, indicating promising applications of the developed sensor for cancer diagnostics and therapy.