Volume 25 (2009) Issue 2 Pages 285-291
Realisation of a device intended for the manipulation and detection of bead-tagged DNA and other bio-molecules is presented. Acoustic radiation forces are used to manipulate polystyrene micro-beads into an optical evanescent field generated by a laser pumped ion-exchanged waveguide. The evanescent field only excites fluorophores brought within ∼100 nm of the waveguide, allowing the system to differentiate between targets bound to the beads and those unbound and still held in suspension. The radiation forces are generated in a standing-wave chamber that supports multiple acoustic modes, permitting particles to be both attracted to the waveguide surface and also repelled. To provide further control over particle position, a novel method of switching rapidly between different acoustic modes is demonstrated, through which particles are manipulated into an arbitrary position within the chamber. A novel type of assay is presented: a mixture of streptavidin coated and control beads are driven towards a biotin functionalised surface, then a repulsive force is applied, making it possible to determine which beads became bound to the surface. It is shown that the quarter-wave mode can enhance bead to surface interaction, overcoming potential barriers caused by surface charges. It is demonstrated that by measuring the time of flight of a microsphere across the device the bead size can be determined, providing a means of multiplexing the detection, potentially detecting a range of different target molecules, or varying bead mass.