Counting and sizing of particles and particle agglomerates in a microfluidic device using laser light scattering: application to a particle-enhanced immunoassay

A microfluidic device for counting and sizing particles and particle agglomerates based on laser light scattering is demonstrated. The particles were confined hydrodynamically and passed through a focused laser beam. Scattering at two different angles, 15° and 45°, was detected. At an acquisition rate of 10 kHz, a throughput of 150 particles s⁻¹ was achieved. Scattering intensity was found to depend on particle volume for 2 to 9 µm diameter particles. Size discrimination of particles with a diameter ratio of 1 : 2 was accomplished. In addition, the scattering signals of particle agglomerates formed in a particle-enhanced immunoassay for C-reactive protein (CRP) were measured. Scattering intensity was found to be dependent on the CRP concentration, 100 ng CRP per mL could be detected. The particle counting method presented is generic and can be employed in a wide variety of assays as well as for cell counting and particle counting.