Integration of polymeric membrane/dielectric sphere assemblies in microfluidic chips for enhanced-contrast imaging with low-magnification systems

Abstract

Abstract. Current microscopy systems for the imaging of microorganisms are expensive because of their optimized design toward resolution maximization and aberration correction. In situations where such an optimization is not needed, for instance to merely detect the presence of pathogens in liquids for on-site analyses, a potential approach is to use highly refractive spheres in combination with low-magnification objectives to increase the resolution and the sensitivity of the optical sensing system in a cost-effective fashion. Indeed, for point-of-need assays, integration of optical elements on a microfluidic device can bring several advantages, such as test parallelization/automation and low-volume consumption. We report a study on BaTiO3 spheres that are partially embedded in thin polymeric membranes of mismatched refractive index. We computed the transformation that the polymeric membrane/dielectric sphere assembly (PMDSA) mediates on the light originating from the sample toward the optical detector and shows its enhanced-detection potential for a low-magnification objective. We then propose a method to easily fabricate chips with custom designs and precise location of such dielectric spheres relative to the microfluidic structures for enhanced imaging of microorganisms. We applied this concept to the detection of living fluorescent bacteria, either flowing in aqueous medium or immobilized in hydrodynamic traps. We quantified the contrast gain provided by the PMDSA for short exposure when used with a low-magnification objective. By comparing with a high-magnification objective, we also show how longer-term imaging can be still reliably performed with a more cost-effective system. Since the present PMDSA concept combines the optical enhancement of low-magnification systems with the flexibility of microfluidic handling, it can be highly suitable for portable and cost-effective systems for on-site analysis, from flow cytometry to longer-term antibiotic testing.