Exosomes which are 30 – 150 nm in diameter are extracellular vesicles secreted from most cell types [1–5]. Many biomolecules such as proteins, lipids and various types of nucleic acids are found in exosomes. Ordinarily, exosomes are circulated in biological fluids such as saliva, blood, and urine. Many studies recommended that these exosomes play a significant role in cell-to-cell interaction and some pathological processes such as cancers , pregnancy complications [7, 8] and neurodegenerative diseases . As such, during pregnancy, exosomes are one of signal pathways to communicate between the mother and the fetus . During gestation, the human placenta releases exosome into the maternal circulation in the first trimester. Oxygen tension and glucose concentration including placental mass regulated the release of placental exosomes [11, 12]. There has been reported that the expected mothers, who have a higher concentration of placental-derived exosomes, face the risk of pregnancy complications such as gestational diabetes  and preeclampsia  than that of normal pregnancy. Therefore, detecting unusual level of placental-derived exosomes early as first trimester of pregnancy can help expectant mothers at risk to be treated properly before the complication develops.
Thus, the accurate purification and detection of exosomes from bodily fluids is still challenging . Conventional methods such as gradient density centrifugation, ultrafiltration or immunological separation have been reported [14, 15]. However, these centrifugal isolations are time-consuming, and require expensive instrument and facilities, which may be inaccessible in poor-resource settings. Moreover, the commercial exosome isolation kit is also available. These kits shorten the process by precipitating exosome with polyethylene glycol or related chemicals, but the product may be contaminated with chemicals from the kits which possibly impact the downstream analysis.  Alternatively, flow cytometry [17, 18], nanoparticle tracking analysis (NTA) [19, 20], electrochemical-based method [21, 22], Enzyme-linked immunosorbent assay (ELISA) [23, 24] are successfully demonstrated to quantify and detect exosome. ELISA is known as a conventional method to detect and analyse various biomolecules, based on immunological affinity between antibodies and protein on exosome membranes. However, the standard protocol is time consuming with laborious sample loading, washing, and incubation step.
Paper-based platform enables a number of microfluidic sample preparation tasks such as preconcentration and heating. [25, 26] Paper-based ELISA is a promising platform for diagnostic application due to the low cost, friendly use and small sample needed. This platform was successfully demonstrated for detecting antibodies and exosomes [27–30]. In limited-resource environments, colorimetric assay is useful and easily observed by naked eyes, however minor colour change may be difficult to differentiate due to the broad range of colour spectrum.  Fluorescent-linked immunosorbent assay (FLISA) promotes narrow detection range due to specific excitation and emission wavelength. Therefore, the present study developed FLISA paper-based assay for exosome detection. The exosome target was captured with exosome-specific antibody and formed immunological complex on the paper. The fluorescent readout relies on detecting emission photon excited by a specific wavelength, which results from immunological affinity between exosomes and fluorophore-conjugated antibody.