Towards nanovesicle-based disease diagnostics: a rapid single-step exosome assay within one hour through in situ immunomagnetic extraction and nanophotonic label-free detection.

Exosomes have been considered as high-quality biomarkers for disease diagnosis, as they are secreted by cells into extracellular environments as nanovesicles with rich and unique molecular information, and can be isolated and enriched from clinical samples. However, most existing exosome assays, to date, require time-consuming isolation and purification procedures; the detection specificity and sensitivity are also in need of improvement for the realization of exosome-based disease diagnostics. This paper reports a unique exosome assay technology that enables completing both magnetic nanoparticle (MNP)-based exosome extraction and high-sensitivity photonic crystal (PC)-based label-free exosome detection in a single miniature vessel within one hour, while providing an improved sensitivity and selectivity. High specificity of the assay to membrane antigens is realized by functionalizing both the MNPs and the PC with specific antibodies. A low limit of detection on the order of 107 exosome particles per milliliter (volume) is achieved because the conjugated MNP–exosome nanocomplexes offer a larger index change on the PC surface, compared to the exosomes alone without using MNPs. Briefly, the single-step exosome assay involves (i) forming specific MNP–exosome nanocomplexes to enrich exosomes from complex samples directly on the PC surface at the bottom of the vessel, with a >500 enrichment factor, and (ii) subsequently, performing in situ quantification of the nanocomplexes using the PC biosensor. The present exosome assay method is validated in analyzing multiple membrane proteins of exosomes derived from murine macrophage cells with high selectivity and sensitivity, while requiring only about one hour. This assay technology will provide great potential for exosome-based disease diagnostics.