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 wa ves. The influence of particle volume, density and compressibility, propert ies that modify the ultrasonic radiation, and interaction forces the partic les experience, on assay sensitivity with latex and silica particles in the range 0.25-1.0 mu m is examined here. Streptavidin-coated 0.3-mu m silica particles and 0.25-mu m and 1.0-mu m latex particles were examined for aggl utination 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-mu m silica. The detection limit decrea sed with increasing latex particle size. The limit of an ultrasound-enhance d agglutination immunoassay of rabbit antimouse immunoglobulin was 6-fold b etter with 1.0-mu m coated silica than with equal-sized latex particles. Ca lculations show that the particle density-dependent ultrasonic interaction force dominates the particle compressibility force for the present case. Th e density-dependent force on silica, but not on latex particles, is shown t o be comparable in magnitude to both the long-range van der Waal's attracti ve force and the electrostatic repulsion between the particles. This densit y-dependent force may explain the improved enhancement of analyte detection by coated silica compared with latex particles. (C) 1999 World Federation for Ultrasound in Medicine & Biology.