Particles that can be trapped in optical tweezers range in size from tens of nanometres to tens of micrometres.
Notably, this size range includes large single molecules. We show experimentally, in agreement with theoretical
expectations, that optical tweezers can be used to manipulate single molecules of polyethylene oxide suspended in
water. The trapped molecules accumulate without aggregating, so the optical trap offers a method of controlling
the concentration of macromolecules in solution.
Potential applications are the micromanipulation of nanoparticles, nanoassembly, microchemistry, and the
study of biological macromolecules.
The ability to exert optical torques to rotationally manipulate microparticles has developed from an interesting curiosity to seeing deployment in practical applications. Is the next step to genuine optically-driven micromachines feasible or possible? We review the progress made towards this goal, and future prospects.
Recently we have shown that protein crystals could be grown while they were three-dimensionally trapped by optical tweezers. This permitted studies of modifications of single crystals while gradually changing the conditions in the growing solution. Furthermore it allowed the crystals to grow far away from container walls favoring high quality crystal growth. Many protein crystals themselves consist of fairly large molecules, with sizes up to tens of nanometers. Here we present experiments studying the effect of optical trapping potentials on large molecules, with the aim to explore ways to further enhance crystal growth. For this purpose we extended our tweezers setup with a specially developed detection system allowing us to monitor changes in the molecule concentration of a solution. Using polyethylene oxide (PEO) molecule solutions we were able to demonstrate that the trapping potential of an optical trap is sufficient to collect large single molecules. Our results show that the optical trap induces an increase in the molecule concentration in the focal region. As expected only molecules above a certain molecular weight could be manipulated, and the concentration in the focal region depended on the power of the trapping laser. The ability to locally increase the concentration of molecules may be useful in assisting nucleation of crystals.
We report here on the use of optical tweezers in the growth and manipulation of protein and inorganic crystals. Sodium chloride and hen egg-white lysozyme crystals were grown in a batch process, and then seeds from the solution were introduced into the optical tweezers. The regular and controllable shape and the known optical birefringence in these structures allowed a detailed study of the orientation effects in the beam due to both polarization and gradient forces. Additionally, we determined that the laser tweezers could be used to suspend a crystal for three-dimensional growth under varying conditions. Studies included increasing the protein concentration, thermal cycling, and a diffusion-induced increase in precipitant concentration. Preliminary studies on the use of the tweezers to create a localized seed for growth from polyethylene oxide solutions are also reported.
Several methods to rotate and align microscopic particles controllably have been developed. Control of the orientation of a trapped particle allows full three dimensional manipulation, whereas rotating particles are tools for the development of optically-driven micromachines. It has been shown that the orientation of an object in the laser trap depends on its birefringence as well as on its shape. The effect of shape is often referred to as form-birefringence. We report on the trapping, rotation, and in-situ growth of birefringent tetragonal lysozyme crystals in optical tweezers operating at a wavelength of 1064 nm. Variation of the temperature, pH and lysozyme concentration of the solution during growth was used to alter the size, as well as the length to width ratio of the crystals, and hence their orientation in the tweezers. Thus this system serves as a model to study the relative importance of birefringence versus form-birefringence for particle orientation. Crystals with the optical axis skewed or perpendicular to the trapping-beam axis could be rotated by changing the orientation of linearly polarized light. We observed spontaneous spinning of some asymmetric crystals in the presence of linearly polarized light, due to radiation pressure effects. Addition of protein to the solution in the tweezers permitted real-time observation of crystal growth.