Bridging the Gap

Assay sensitivity is governed by the assay’s signal-to-noise ratio. High assay sensitivity requires both high signal and low background noise. Significant efforts have been made to develop analytical technologies that achieve large signals with only small quantities of biomarker, such as surface-enhanced Raman scattering (SERS). However, comparatively little attention has been dedicated to reducing background noise, an equally important component. Low background noise depends on effective bioseparations that capture target analytes with high specificity; i.e., without nonspecific binding of other molecules. For example, magnetic microbeads are widely used in bioseparations to immobilize affinity reagents because they respond sharply to a modest magnetic field. However, a limiting factor is their large size—magnetic beads must be >1 µm to be separated from fluid samples by a magnetic field. Beads of this size exhibit a small surface-to-volume ratio resulting in low antibody loading, limited analyte capture, and low signal. Compensating by using more microbeads leads to greater nonspecific binding and higher noise. The >1 µm magnetic particles also diffuse poorly, leading to long incubation times for antigen binding.

To improve the sensitivity and speed of the many assays that use magnetic beads, we have developed stimuli-responsive (“smart”) polymer-coated magnetic nanoparticles that exhibit the rapid diffusion and high surface area of small nanoparticles during binding, and the high magnetophoretic mobility of larger particles during separation, by undergoing switchable aggregation[1, 2]. We have developed magnetic nanoparticles that are decorated with temperature-responsive polymers such as pNIPAAm, that can be used as soluble reagents to capture diagnostic targets in a microfluidic device at a controlled time point and channel position[2]. We have also applied pH-responsive magnetic nanoparticles to bioseparations under continuous flow processing conditions[1].

1. Lai JJ, Nelson KE, Nash MA, Hoffman AS, Yager P, Stayton PS. Dynamic bioprocessing and microfluidic transport control with smart magnetic nanoparticles in laminar-flow devices. Lab on a Chip. 2009;9(14):1997-2002.

2. Lai JJ, Hoffman JM, Ebara M, Hoffman AS, Estournès C, Wattiaux A, Stayton PS. Dual magnetic-/temperature-responsive nanoparticles for microfluidic separations and assays. Langmuir. 2007;23(13):7385-91.