Single-type reporter multiplexing with A single droplet through bead-based digital microfluidics.

With the limited sample volume, the droplet-based microfluidic becomes attractive in biomedical diagnosis, especially for measuring multiple analytes. Usually, for multiplexing by parallel processing, a larger sample volume is required. In our previous study, simultaneously detecting two analytes from a single droplet was first achieved by measuring different fluorescence wavelengths for different analytes. However, the number of detectable analytes could be limited by the spectral resolution of fluorescence. Here a different approach is proposed for multiplexing by sharing a single droplet in multiple sub-assays. Therefore, only a single-type reporter, i.e., the fluorescence with the same wavelength, is needed for detection of different analytes from a single sample droplet, called single-type reporter multiplexing (STRM). The standard curves of two analytes, human IL-1β and human TNF-α, are demonstrated. The required sample volume for one measurement is only 520 nL; the total duration of the on-chip process is less than 50 min. The limits of detection (LOD) of human IL-1β and human TNF-α are about 1.14 and 0.97 pg/mL, respectively. It is shown that the proposed bead-based digital microfluidic immunoassay can achieve multiple analytes detection with low LOD from a single sample droplet using the single-type reporter, which has never been achieved before.