Magnetically localized and wash-free fluorescence immunoassay (MLFIA): proof of concept and clinical applications.

Immunoassays are used for many applications in various markets, from clinical diagnostics to the food industry, generally relying on gold-standard ELISAs that are sensitive, robust, and cheap but also time-consuming and labour intensive. As an alternative, we propose here the magnetically localized and wash-free fluorescence immunoassay (MLFIA): a no-wash assay to directly measure a biomolecule concentration, without mixing nor washing steps. To do so, a fluorescence no-wash measurement is performed to generate a detectable signal. It consists of a differential measurement between the fluorescence of fluorophores bound to magnetic nanoparticles specifically captured by micro-magnets against the residual background fluorescence of unbound fluorophores. Targeted biomolecules (antibodies or antigens) are locally concentrated on micro-magnet lines, with the number of captured biomolecules quantitatively measured without any washing step. The performance of the MLFIA platform is assessed and its use is demonstrated with several biological models as well as clinical blood samples for HIV, HCV and HBV detection, with benchmarking to standard analyzers of healthcare laboratories. Thus, we demonstrated for the first time the versatility of the innovative MLFIA platform. We highlighted promising performances with the successful quantitative detection of various targets (antigens and antibodies), in different biological samples (serum and plasma), for different clinical tests (HCV, HBV, HIV).

Magnetophoretic induced convective capture of highly diffusive superparamagnetic nanoparticles.

Micro-magnets producing magnetic field gradients as high as 106 T m-1 have been used to efficiently trap nanoparticles with a magnetic core of just 12 nm in diameter. Particle capture efficiency increases with increasing particle concentration. Comparison of measured capture kinetics with numerical modelling reveals that a threshold concentration exists below which capture is diffusion-driven and above which it is convectively-driven. This comparison also shows that two-way fluid-particle coupling is responsible for the formation of convective cells, the size of which is governed by the height of the droplet. Our results indicate that for a suspension with a nanoparticle concentration suitable for bioassays (around 0.25 mg ml-1), all particles can be captured in less than 10 minutes. Since nanoparticles have a significantly higher surface-to-volume ratio than the more widely used microparticles, their efficient capture should contribute to the development of next generation digital microfluidic lab-on-chip immunoassays.