Figure 1.

Experimental setup for magnetic tweezers force spectroscopy with evanescent illumination. The sample is illuminated by a 532 nm laser beam through a high numerical aperture lens (NA=1.45). The lateral position dx of the optical fiber from the laser source is adjusted with a motorized actuator and changes the position of the focused spot in the back-focal plane of the objective. The control of the laser spot position allows for adjustment of the laser angle θ incident to the glass/water interface. A thin layer (~12.5 nm) of gold (yellow) is present at the interface for reaction with thiolated DNA. The evanescent field propagates from the glass/water interface to illuminate the tethered magnetic-fluorescent probes (inset). Force F is applied to the probes by an electromagnet placed above the fluid cell. The magnetic field B is modulated by a current i through the coil of the electromagnet. Fluorescent emission from the probes I is filtered from the incident laser radiation by a dichroic mirror and a high pass filter and detected by a digital camera connected to a computer.