Abstract
The application of on-chip optical trapping and Raman spectroscopy using a dual-waveguide trap has so far been limited to relatively big synthetic and biological particles (e.g., polystyrene beads and blood cells). Here, from simulations, we present the capabilities of dual-waveguide traps built from composite SiO2-Si3N4 waveguides for optical trapping of extracellular vesicles (EVs). EVs, tiny cell-derived particles of size in the range 30-1000 nm, strongly attract attention as potential biomarkers for cancer. EVs are hard to trap, because of their smallness and low index contract w.r.t. water. This poses a challenge for on-chip trapping. From finite-difference time-domain simulations we obtain the narrow beam emitted from the waveguide facet into water, for λ = 785 nm. For a pair of such beams, in a counter-propagating geometry and for facet separations of 5, 10 and 15 µm, we derive the inter-facet optical field, which has a characteristic interference pattern with hot spots for trapping, and calculate the optical force exerted on EVs of size in the range 50-1000 nm, as a function of EV position. We use two refractive index models for the EV optical properties. Integration of the force curves leads to the trapping potentials, which are well-shaped in the transverse and oscillatory in the longitudinal direction. By applying Ashkin's criterion, the conditions for stable trapping are established, the central result of this work. Very small EVs can be stably trapped with the traps by applying a power also suitable for Raman spectroscopy, down to a smallest EV diameter of 115 nm. We thus argue that this dual-waveguide trap is a promising lab-on-a-chip device with clinical relevance for diagnosis of cancer.
Highlights
A class of lab-on-a-chip devices for optical analysis of particles is based on using integrated photonic waveguides for manipulating and spectroscopic fingerprinting the particles, with the goal of their identification
From simulations, we present the capabilities of dualwaveguide traps built from composite SiO2-Si3N4 waveguides for optical trapping of extracellular vesicles (EVs)
For a pair of such beams, in a counter-propagating geometry and for facet separations of 5, 10 and 15 μm, we derive the inter-facet optical field, which has a characteristic interference pattern with hot spots for trapping, and calculate the optical force exerted on EVs of size in the range 50−1000 nm, as a function of EV position
Summary
A class of lab-on-a-chip devices for optical analysis of particles is based on using integrated photonic waveguides for manipulating and spectroscopic fingerprinting the particles, with the goal of their identification. Prominent examples of on-chip versions are dual-fibre or dual-waveguide traps, often combined with microfluidics for particle supply These traps are based on two opposing fibres or integrated photonics waveguides, from which counter-propagating beams emanate that define the concentrated optical field for trapping and Raman spectroscopy. These in turn, via Ashkin’s criterion [19], lead to the conditions for stable EV trapping, which constitute the central results of this work It follows that very small EVs can be trapped with the dual-waveguide traps, highlighting the clinical relevance of this lab-on-achip as a basic device for high throughput platforms. 2. Device geometry and optical models for EVs The building block of the dual-waveguide trap is a waveguide with a box-shaped composite Si3N4−SiO2 structure. The core-shell model will be more appropriate for small vesicles since for these the shell contribution to the total optical force is expected to be rather high
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