Overcoming Diffusion-Limited Trapping in Nanoaperture Tweezers Using Opto-Thermal-Induced Flow

Nanoaperture-based plasmonic tweezers have shown tremendous potential in trapping, sensing, and spectroscopic analysis of nano-objects with single-molecule sensitivity. However, the trapping process is often diffusion-limited and therefore suffers from low-throughput. Here, we present bubble- and convection-assisted trapping techniques, which use opto-thermally generated Marangoni and Rayleigh-Bénard convection flow to rapidly deliver particles from large distances to the nanoaperture instead of relying on normal diffusion, enabling a reduction of 1-2 orders of magnitude in particle-trapping time (i.e., time before a particle is trapped). At a concentration of 2 × 10⁷ particles/mL, average particle-trapping times in bubble- and convection-assisted trapping were 7 and 18 s, respectively, compared with more than 300 s in the diffusion-limited trapping. Trapping of a single particle at an ultralow concentration of 2 × 10⁶ particles/mL was achieved within 2-3 min, which would otherwise take several hours in the diffusion-limited trapping. With their quick delivery and local concentrating of analytes at the functional surfaces, our convection- and bubble-assisted trapping could lead to enhanced sensitivity and throughput of nanoaperture-based plasmonic sensors.