Sub-micron particle behaviour and capture at an immuno-sensor surface in an ultrasonic standing wave

The capture of 200 nm biotinylated latex beads from suspensions of concentration 107 to 2.5 × 108 particle/ml on an immuno-coated surface of the acoustic reflector in an ultrasound standing wave (USW) resonator has been studied while the acoustic pathlength was less than one half wavelength (λ/2). The particles were delivered to the reflector's surface by acoustically induced flow. The capture dependencies on suspension concentration, duration of experiments and acoustic pressure have been established at 1.09, 1.46 and 1.75 MHz. Five-fold capture increase has been obtained at 1.75 MHz in comparison to the control (no ultrasound) situation. The contrasting behaviours of 1, 0.5 and 0.2 μm fluorescent latex beads in a λ/4 USW resonator at 1.46 MHz have been characterized. The particle movements were observed with an epi-fluorescent microscope and the velocities of the particles were measured by particle image velocimetry (PIV). The experiments showed that whereas the trajectories of 1 μm particles were mainly affected by the direct radiation force, 0.5 μm particles were influenced both by the radiation force and acoustic streaming. The 0.2 μm latex beads followed acoustic streaming in the chamber and were not detectably affected by the radiation force. The streaming-associated behaviour of the 200 nm particles has implications for enhanced immunocapture of viruses and macromolecules (both of which are also too small to experience significant acoustic radiation force).