Fig. 2: Sensing of a biotin–streptavidin complex using the DigitISA platform.

a Monovalent streptavidin is added to a biotinylated and fluorophore-conjugated DNA strand. Binding of the streptavidin species reduces the electrophoretic mobility of biotinylated DNA probe. b Electropherogram as obtained by scanning the confocal volume across the cross-section of the channel in a stepwise manner for the biotin–streptavidin sample mixture (red line; average of N = 3 repeats, the shaded bands correspond to the standard deviation) and for the control sample (blue line; average of N = 3 repeats, the shaded bands correspond to the standard deviation) at the mid-height of the channel, demonstrating the presence and separation of both streptavidin-bound and nonbound biotinylated DNA molecules in the sample. The region shaded in grey was used to extract the number of streptavidin–biotin complexes that passed the device in a given time, and ultimately, its concentration (see main text). c Exemplary photon-count time traces for the control sample (left panel, blue) and the sample mixture (right panel, red) at the position where the concentration of the complex molecules was the highest as indicated with colored triangles in panel b. The number of molecules was estimated using a burst-search algorithm as detailed in the Methods section (Data analysis). Time traces in the upper panels are zoom-in views of the blue or red shaded areas in the lower panels, with dots indicating detected single-molecule events. The bin time was 1 ms in all traces. Source data are provided as a Source Data file.