Radiation pressure control in photonic force microscopy and application to biophysical membrane binding studies

Optical traps have been widely used to trap and manipulate microscopic particles for about two decades. A recent application of optical trapping is the photonic force microscope (PFM). In a PFM, an optically weakly trapped particle acts as probe which, driven by Brownian motion, scans its local environment. The thermal position fluctuations of the trapped bead are tracked interferometrically in three dimensions with a precision of a few nanometers and a time sampling up to one MHz. By varying the axial radiation pressure of the focused beam, we control both the probe’s trapping position relative to the focus and the size and linearity of the interferometric detection region. The PFM is used to study the binding of particles to the membrane of living macrophage cells. Macrophages are specialized cells that ingest particles such as bacteria or synthetic objects like latex beads. An optically trapped bead is moved to the membrane of a macrophage cell. The thermal position fluctuations of the bead change in close proximity to the membrane. The following process of membrane binding is recorded over a brought dynamic range.