Precise Sorting of Gold Nanoparticles in a Flowing System

Precise sorting of gold nanoparticles is important, but it still remains a big challenge. Traditional methods such as centrifugation can separate nanoparticles with a high throughput but at the cost of low precision. Optical tweezers enable the precise manipulation of a single nanoparticle in steady liquid environments. However, this method may become problematic when dealing with a considerable amount of nanoparticles in a flowing system due to the difficulties in balancing the additional Stokes forces by the fast velocity of streams and in controlling all dispersed nanoparticles with disorderly positions. Here, we exploit optical and hydrodynamic forces to sort gold nanoparticles in the flowing system, obtaining simultaneously high precision and considerable throughput. This is accomplished by utilizing opposite impinging streams to generate a stagnation point, near which the flow velocity becomes very small to reduce the Stokes force and to prolong the optical acting time. Nanoparticles of different sizes, confined in a narrow region by the hydrodynamic focusing, can then be separated by a laser beam of moderate power. Experimental demonstrations have been presented by sorting gold nanoparticles with diameters of 50 nm from those of 100 nm, and 100 nm from 200 nm. The sorting fidelities is ≥92% for the 50/100 nm combination and ≥86% for the 100/200 nm set, with a sorting throughput of 300 particles/min. Sorting of gold nanoparticles with smaller heterogeneity (50 and 70 nm) has also been realized with a lower throughput of <100 particles/min. Our method can also be extended to separate nanoparticles of different shapes and compositions, which shows its great promise in the fields of plasmonics and nanophotonics.