Ultrasonic enhancement of coated particle agglutination immunoassays: influence of particle density and compressibility.

The detection rate and sensitivity (analyte concentration limit) of coated particle agglutination immunoassays are increased in ultrasonic standing waves. The influence of particle volume, density and compressibility, properties that modify the ultrasonic radiation, and interaction forces the particles experience, on assay sensitivity with latex and silica particles in the range 0.25-1.0 microm is examined here. Streptavidin-coated 0.3-microm silica particles and 0.25-microm and 1.0-microm latex particles were examined for agglutination with biotinylated bovine serum albumin (bBSA) following exposure on axis in a 4.6-MHz radial standing wave. The lowest detection limit, 2 ng/mL bBSA, was achieved with the 0.3-microm silica. The detection limit decreased with increasing latex particle size. The limit of an ultrasound-enhanced agglutination immunoassay of rabbit antimouse immunoglobulin was 6-fold better with 1.0-microm coated silica than with equal-sized latex particles. Calculations show that the particle density-dependent ultrasonic interaction force dominates the particle compressibility force for the present case. The density-dependent force on silica, but not on latex particles, is shown to be comparable in magnitude to both the long-range van der Waal's attractive force and the electrostatic repulsion between the particles. This density-dependent force may explain the improved enhancement of analyte detection by coated silica compared with latex particles.