Research ArticleAPPLIED PHYSICS

Nanometer-precision linear sorting with synchronized optofluidic dual barriers

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Science Advances  05 Jan 2018:
Vol. 4, no. 1, eaao0773
DOI: 10.1126/sciadv.aao0773
  • Fig. 1 Damping diagram unfolds the complete map of trapping and sorting particles in optofluidic systems: Classic trapping, microscale sorting, and nanoscale sorting.

    (A) Oscillation diagram with stiffness of optical tweezers (k) and viscosity of the medium (η). Functions of optical manipulation from left to right are loosely overdamped system for nanoscale sorting (k ~ 10−10 to 10−8 N/m), optical chromatography for microscale sorting (k ~ 10−8 to 10−6 N/m), and optical trapping (k ~ 10−6 to 10−4 N/m). The optical trapping can be divided into liquid and gas trapping based on different viscosities. Parameters of m and R in the equation (the dividing line of underdamped and overdamped systems) are the mass and radius of particle, respectively. (B to F) Illustration of optical sorting of gold nanoparticles with radii of 30, 35, and 40 nm using technologies in the diagram. (B) Optical trapping is shown to be unable to sort nanoparticles because of the overlap of potential wells of different nanoparticles. (C) Calculation of trapping positions of 30- to 50-nm gold nanoparticles in the optical chromatography. Individual gold nanoparticles with different radii can be trapped in different positions using the Gaussian beam with a numerical aperture (NA) of 0.12. However, the theoretical minimum separation distance between them is only 0.9 μm. When the gold nanoparticles are trapped in the beam, the smaller nanoparticle absorbs light and greatly influences the trapping positions of larger gold nanoparticles. Therefore, 35-, 40-, 45-, and 50-nm gold nanoparticles will eventually aggregate. Detailed proof has been provided in the Supplementary Materials. (D) A loosely overdamped system is an ideal platform for nanoparticle sorting, in which 30-, 35-, and 40-nm gold nanoparticles can be separated with a large distance (>15 μm) away from each other. (E) Nanoparticles oscillate in microscale without escaping the system with stiffness of 10−10 to 10−8 N/m under the control of synchronized optical and fluidic barriers. (F) The experimentally observed sorting distance increases linearly with the radius, implying that each nanoparticle has a specific trapping position in the system. The separation sensitivity (position/radius) is shown on the right y axis.

  • Fig. 2 Microscopic oscillation via synchronized dual optofluidic barriers.

    (A) Physical model of the microscopic oscillation of a single nanoparticle. The movement of the nanoparticle is confined by the optical and fluidic barriers. (B) The size of the focal point is inversely proportional to the NA in a conventional beam, whereas the quasi-Bessel beam has a tight focus with a radius of 0.25 μm and a low NA of 0.04 at the same time. Trapping distance (C) and oscillation amplitude (D) of the 40-nm gold nanoparticle can be tuned by synchronizing optical and fluidic barriers.

  • Fig. 3 Nanometer-precision sorting of nanoparticles and microorganisms in the loosely overdamped system.

    (A) Light intensity profile of the quasi-Bessel beam. (B) Self-healing property of the quasi-Bessel beam. The quasi-Bessel beam reconfigures itself after propagating through an absorbing particle (0.5 μm in radius) that is completely blocking the main lobe of the beam (radius ~ 0.25 μm). (C) Trapped nanoparticles with different diameters in the microchannel. The laser power is 400 mW, and the flow rate is 300 μm/s. Scale bar, 20 μm. (D) Separation of polystyrene nanoparticles with radii of 100 and 150 nm. Two clusters of 100- and 150-nm polystyrene nanoparticles were trapped in isolated locations. (E) Separation of E. coli and Cryptosporidium in the microchannel.

  • Fig. 4 Characterization of the microscopic oscillations of nanoparticles in the loosely overdamped system.

    (A) Potential energies of 30-, 40-, and 50-nm gold nanoparticles trapped in the loosely overdamped system. The potential wells have widths of 14, 16.8, and 25.6 μm for 30-, 40-, and 50-nm gold nanoparticles when the energy is 10kBT, respectively. Inset shows the potential energy of a 40-nm gold nanoparticle trapped in a single-beam optical tweezer, with an NA of 1.4 and laser power of 30 mW. The width for 10kBT is only 40 nm in this single-beam trapping. (B) Square root of MSD of gold nanoparticles in their trapped positions. The red circles indicate the situations when the individual systems approach 99% of the steady states. (C) Experimentally measured displacements of 100-nm polystyrene nanoparticles and 40-nm gold nanoparticles during oscillation. (D) Experimental observation of local microscopic oscillation of the 100-nm polystyrene nanoparticle in the microchannel. Scale bar, 10 μm.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/1/eaao0773/DC1

    Supplementary Materials and Methods

    fig. S1. Sorting of gold nanoparticles using conventional optical chromatography.

    fig. S2. Simulation of temperature on gold nanoparticles in the flow stream.

    fig. S3. Tuning of trapping stiffness by coordinating laser power and flow velocity.

    fig. S4. Determination of the refractive indices by fitting the spectra.

    fig. S5. Micrograph of the optofluidic chip.

    fig. S6. Calculation of optical extinction forces in water.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials and Methods
    • fig. S1. Sorting of gold nanoparticles using conventional optical chromatography.
    • fig. S2. Simulation of temperature on gold nanoparticles in the flow stream.
    • fig. S3. Tuning of trapping stiffness by coordinating laser power and flow velocity.
    • fig. S4. Determination of the refractive indices by fitting the spectra.
    • fig. S5. Micrograph of the optofluidic chip.
    • fig. S6. Calculation of optical extinction forces in water.

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