Plasmofluidic Microlenses for Label-Free Optical Sorting of Exosomes

Abstract

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. Here, we 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. Our 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. We 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), our 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.

Plasmonic Nanopores: Optofluidic Separation of Nano-Bioparticles via Negative Depletion

“…The total thickness of the microlens is h = t Au + t Ti + t Si3N4 = 205 nm, whereas the lateral dimension of the finite size NHA is 4 μm  4 μm. Recent research findings suggest that (quasi)periodic arrays of nanoplasmonic apertures behave as microconvex lenses focusing broadband incoherent light beams to spot sizes comparable to wavelength of light [11][12][13]. Such tight light focusing capability can be harnessed to realize sufficiently strong optical scattering forces suitable for OC using collimated broadband light sources [14,15].…”

“…The high intensity region around the nanopore opening may lead to increased optical scattering force F s , causing undesired rejection of smaller/lowerrefractive index particles that managed to pass the focal region and carried towards the central nanopore. However, in addition to tailoring the central nanopore dimension, one can also take advantage of Stokes flow [1,9,11], where the fluidic drag forces scale with the relative velocity of nano-bioparticles (u) with respect to the flow rate (v) of medium (i.e., F d ∝|u-v|). As fluidic velocity v is approximately three orders of magnitude higher close to the central nanopore opening with respect to that of the focal point (Figure 1b), fluidic drag forces F d are significantly larger too.…”

“…OPtIC nanopore device enables both size-and refractive-index based separation of nano-bioparticles by utilizing a delicate balance of counteracting forces, i.e., F s , F d , F tp and W (gravitational force). Since these forces act along the optical axis within the focal point region, selective particle elution can be readily achieved by adjusting the net force F net = F s + F d + F tp + W [11]. We calculated F net for varying spherical bioparticles with a mass density of 1.05 g/cm 3 and refractive index of 1.55.…”

“…In a recent publication, we introduced a novel plasmonic nanopore device that eliminates the shortcomings of the conventional OC technique [11]. Here, we review this hybrid Optofluidic PlasmonIC (OPtIC) device merging light focusing and fluidic flow through a tiny (4 μm  4 μm footprint) plasmonic microlens housing an integrated nanopore channel.…”

“…(b) Temperature distribution and heat-induced fluidic flow pattern on a perpendicular plane crossing the optical axis. The arrows represent the velocity vector v. copyright 2020 nature publishing group adapted with permission[11].…”