The fluorescence images were recorded using an Andor Zyla sCMOS camera from Andor Oxford Instruments using the Andor Solis software. Videos were sampled at 2000 frames per second with an image dimension of 45 × 200 pixels. The analogue-to-digital-units (ADU) fluorescence intensity was converted to the number of photons by calibrating the camera detector gain from a series of gradient images (Figure ).