Chang, M. et al. Opt. Express 25, 32189–32197 (2017).

Single-molecule force spectroscopy is often combined with optical imaging approaches. Total internal reflection fluorescence (TIRF) microscopy and confocal microscopy provide high signal-to-noise ratios. However, imaging beyond a single focal plane is difficult, as TIRF microscopy is limited in imaging depth, while in confocal microscopy, movement of the sample stage or of the objective interferes with force measurements. Chang et al. introduce an electrically tunable lens (ETL) into the imaging arm of their optical tweezer setup. The focal length of the ETL could be changed at millisecond temporal resolution while extending the depth range to tens of micrometers. The researchers used their system to visualize the axial motion of fluorescently labeled epidermal growth factor receptors that were optically trapped.