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.

Highlights

  • Dielectric Spheres in Optical Imaging and Integration of Optical Elements on Microfluidic ChipsIn the past decade, dielectric microspheres have been widely studied because of their properties of improving the performance of optical systems in a cost-effective fashion: the effective field of view and magnification created by the μSs were characterized,[1,2] and resolution analyses were performed in the case of μSs immersed in liquid media[3] or elastomers.[4]

  • Based on our previous resolution analysis on dielectric spheres immersed in homogenous media,[3] we deduced the optimal configuration for a highly refractive sphere embedded in a PDMS membrane (PDMS, refractive index nPDMS 1⁄4 1.43) [Fig. 1(a)]: it consists of a sphere only partially embedded in PDMS, where the index

  • This effect can be used to generate resolution and contrast gains for low-magnification objectives: in Fig. 1(b), we show the presence of an enhanced-detection zone (EDZ) for a 10 × ∕0.25 NA objective, which approaches the collection angle of a 100 × ∕0.75 NA objective with the same immersion medium

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Summary

Introduction

Dielectric microspheres (μSs) have been widely studied because of their properties of improving the performance of optical systems in a cost-effective fashion: the effective field of view and magnification created by the μSs were characterized,[1,2] and resolution analyses were performed in the case of μSs immersed in liquid media[3] or elastomers.[4] Such μSs were used for various applications, such as microtopography measurements,[5,6] biological structure imaging,[7,8,9,10] single-molecule detection,[11] single-nanoparticle detection,[12] and fluorescence correlation spectroscopy.[13,14] Previous examples of technical implementation employed scanning probes,[6,15] arrays,[16] or thin polymeric films[17] to perform the imaging. Placing μSs in precise locations with high yield can prove

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