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Magnetic particles have become popular in recent years for immunoassays due to their high surface-to-volume ratio and the ease of their manipulation. However, such assays also require multiple reaction and washing steps that are both time-consuming and manually laborious. Here, we describe a setup and methodology for performing rapid immunoassays on magnetic particles in continuous flow via their deflection through multiple laminar flow streams of reagents and washing solutions. In particular, we focus on the use of the microfluidic platform for a C-reactive protein (CRP) sandwich immunoassay in less than 60 s.

We demonstrate the application of a multilaminar flow platform, in which functionalized magnetic particles are deflected through alternating laminar flow streams of reagents and washing solutions via an external magnet, for the rapid detection of the inflammatory biomarker, C-reactive protein (CRP). The two-step sandwich immunoassay was accomplished in less than 60 s, a vast improvement on the 80-300 min time frame required for enzyme-linked immunosorbent assays (ELISA) and the 50 min necessary for off-chip magnetic particle-based assays. The combination of continuous flow and a stationary magnet enables a degree of autonomy in the system, while a detection limit of 0.87 μg mL(-1) makes it suitable for the determination of CRP concentrations in clinical diagnostics. Its applicability was further proven by assaying real human serum samples and comparing those results to values obtained using standard ELISA tests.

The need for fast, specific and portable diagnostic systems for clinical assays has, in recent years, led to an explosion of research into microfluidic chip-based immunoassays towards rapid point-of-care analysis. Such devices exploit small dimensions, superior fluidic control and low reagent volumes to allow a number of clinically important procedures to be achieved with improvements on conventional methods, many of which rely on the surface-based binding of antigens to antibodies. Here, we discuss recent developments and innovations in the area of on-chip surface-based immunoassays and provide an outlook on the potential of such platforms for future diagnostics.

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 degree and 45 degree, 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 microm 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.