A microfluidic microparticle-labeled impedance sensor array for enhancing immunoassay sensitivity.

An impedimetric biosensor is used to measure electrical impedance changes in the presence of biomolecules from sinusoidal input voltages. In this paper, we present a new portable impedance-based biosensor platform to improve the sensitivity of immunoassays with microparticles as a label. Using a 2 × 4 interdigitated electrode array with a 10/10 µm electrode/gap and a miniaturized impedance analyzer, we performed immunoassays with microparticles by integrating a microfluidic channel to evaluate signal enhancement. First, to understand the material dependency of microparticles on the sensor array, magnetic, silica, and polystyrene microparticles were tested. Among these microparticles, magnetic microparticles presented a high signal enhancement with relevant stability from the sensor array. With the magnetic microparticles, we demonstrate a series of immunoassays to detect human tumor necrosis factor (TNF-α) and compare the level of signal enhancement by measuring the limit of detection (LOD). With the microparticles, we achieved over ten times improvement of LOD from sandwich immunoassays. By incorporating with sample preparation and flow manipulation systems, this impedance sensor array can be utilized for digital diagnostics for a real sample-in answer-out system.

A capillary flow-driven microfluidic system for microparticle-labeled immunoassays.

A simple, reliable, and self-powered capillary flow-driven microfluidic platform is developed for conducting microparticle-labeled immunoassays. To obtain the washing forces and binding kinetics appropriate for microparticle-labeled immunoassays, both microchannel networks and sample access holes are designed and characterized to confirm the fluidic routes. To demonstrate two different types of immunoassays, serial and parallel capillary-driven microfluidic platforms were developed for mouse immunoglobulin G (IgG) and cardiac troponin I (cTnI) using detection antibody-conjugated microparticles, respectively. With the serial capillary-driven microfluidic platform, we successfully demonstrated IgG quantification using direct immunoassay and achieved a limit of detection (LOD) of 30 pM by using pre-immobilized mouse IgG. In the parallel capillary-driven microfluidic platform, a sandwich immunoassay for detecting cTnI was demonstrated and a clinically relevant LOD as low as 4.2 pM was achieved with minimal human intervention. In both assays, the association rate constants (Ka) were measured to estimate the overall assay time. According to these estimations, microparticle-labeled immunoassays could be conducted in a few minutes using the proposed capillary-driven microfluidic devices. By coupling with various magnetic sensors, these simple immunoassay platforms enable us to achieve a true sample-in-answer-out device that can screen for a variety of targets without relying on external power sources for fluidic manipulation.