Exosomes are endosome-derived membrane vesicles carrying proteins and nucleic acids that are involved in cellular functions such as intercellular communication, protein and RNA secretion, and antigen presentation, and therefore serve as potential biomarkers for many diseases including cancer. Since exosomes are hard to be enriched or purified from biofluids, quantification of exosomes is tedious and inaccurate. Here, researchers from the National Center for Nanoscience and Technology, China present a real-time, label-free, and quantitative method to detect and characterize tumor-derived exosomes without enrichment or purification. Utilizing surface plasmon resonance imaging (SPRi) in combination with antibody microarrays specific to the extracellular domains of exosome membrane proteins, exosomes in tumor cell culture medium can be quantitatively detected. The researchers found a positive correlation between metastasis and exosome secretion. This method provides an easy, efficient and novel way to detect exosome secretion and thus an avenue towards the diagnosis and prognosis prediction of cancer.