Optical trapping of coated microspheres

Opt Express. 2008 Sep 1;16(18):13831-44. doi: 10.1364/oe.16.013831.


In an optical trap, micron-sized dielectric particles are held by a tightly focused laser beam. The optical force on the particle is composed of an attractive gradient force and a destabilizing scattering force. We hypothesized that using anti-reflection-coated microspheres would reduce scattering and lead to stronger trapping. We found that homogeneous silica and polystyrene microspheres had a sharp maximum trap stiffness at a diameter of around 800 nm--the trapping laser wavelength in water--and that a silica coating on a polystyrene microsphere was a substantial improvement for larger diameters. In addition, we noticed that homogeneous spheres of a correct size demonstrated anti-reflective properties. Our results quantitatively agreed with Mie scattering calculations and serve as a proof of principle. We used a DNA stretching experiment to confirm the large linear range in detection and force of the coated microspheres and performed a high-force motor protein assay. These measurements show that the surfaces of the coated microspheres are compatible with biophysical assays.

MeSH terms

  • Coated Materials, Biocompatible / chemistry*
  • Coated Materials, Biocompatible / radiation effects
  • DNA / chemistry*
  • DNA / radiation effects
  • DNA / ultrastructure*
  • Microspheres
  • Optical Tweezers*
  • Silicon Dioxide / chemistry*
  • Silicon Dioxide / radiation effects


  • Coated Materials, Biocompatible
  • Silicon Dioxide
  • DNA