Optical chromatography is a powerful optofluidic technique enabling label-free fractionation of microscopic bioparticles from heterogenous mixtures. However, sophisticated instrumentation requirements for precise alignment of optical scattering and fluidic drag forces is a fundamental shortcoming of this technique. Engineers at University of California, Santa Cruz introduce a subwavelength thick (<200 nm) Optofluidic PlasmonIC (OPtIC) microlens that effortlessly achieves objective-free focusing and self-alignment of opposing optical scattering and fluidic drag forces for selective separation of exosome size bioparticles. The optofluidic microlens provides a self-collimating mechanism for particle trajectories with a spatial dispersion that is inherently minimized by the optical gradient and radial fluidic drag forces working together to align the particles along the optical axis. The developers demonstrate that this facile platform facilitates complete separation of small size bioparticles (i.e., exosomes) from a heterogenous mixture through negative depletion and provides a robust selective separation capability for same size nanoparticles based on their differences in chemical composition. Unlike existing optical chromatography techniques that require complicated instrumentation (lasers, objectives and precise alignment stages), this OPtIC microlenses with a foot-print of 4 μm × 4 μm open up the possibility of multiplexed and high-throughput sorting of nanoparticles on a chip using low-cost broadband light sources.
(a) A 4 μm × 4 μm OPtIC microlens consisting of a patch array of small circular nanoholes and an enlarged center aperture for nanofluidic integration. (b) Fluidic flow profile through the OPtIC microlens with a fluidic flow rate of 1.3 μm/s at the focal point. (c) Separation of two different size particle is illustrated at the focal point region. Fluidic drag (Fd), optical scattering (Fs) and thermo-plasmonic convection (Ftp) forces are inherently aligned against each other along the optical axis by the OPtIC microlens.