Quite recently, my former group lead by Prof. Lene Oddeshede in Niels Bohr Institute, University of Copenhagen published two findings with regards to optical trapping, which definitely will pave the road for its applications in bio-related scientific research.

I don’t know how many people in Nature network knows about optical trapping (OT). Simply allows me to say a few words about OT prior to my brief introduction of the work described in the two recent papers. OT is invented by Ashkin and now becoming the-state-of-the-art tool in investigation of biological problems, because it renders the researchers with high spatial resolution (nm) and well-calibrated force (picoNewton). It is generated by tightly focusing a laser beam to an oil immersion objective with high numeric aperture ( NA). Since light carries momentum, once the laser photons hit on the transparent microspheres, where will be momentum change of the light (i.e. due to reflection, or more precisely scattering, and refraction because the differences of the refractive index of the sphere from water environment) and this give rises to the forces. For intuitive purposes, we categorize the force to two main components, namely scattering force and gradient force. The scattering force pushes the sphere away from the focus of the trap, while the gradient force acts on the direction towards the highest light intensity, which is of course the focus. And the gradient force is main player which keeps the sphere in trap, and has to balance against the scattering component.

Normally, for small displacements the sphere in trap, the trapping potential is safely assumed to be a harmonic one. And the trapping force is approximated as a hookean spring with a linear dependence on the displacements from the equilibrium position. It is known that the lateral trapping force is much larger than the axial component, and this is because the larger gradient force in the lateral than the axial direction. And thus trapped sphere (normally a polystyrene bead is used) tends to escape the trap from the axial direction before the full lateral trapping force can be utilized.

As is known that spherical aberration (due to the mismatch of the refractive index of the immersion oil with the water environment) plays a dominant role in diminishing the trapping power other than the focusing ability of the objective, previously Lene together with Nader Reihani who was a postdoc in our group but now works as a professor in the Institute for Advanced Studies in Basic Sciences in Iran found that by correcting the spherical aberration larger forces than in ordinary case that one can obtain from a single optical tweezers (Optics Letters, vol.32, p.1998-2000). By doing this they use optical tweezers to pull stiffer tethers, or even to unfold some covalent binding in proteins or such they wanted to know what extreme can optical tweezers go to. By driving a piezoelectric stage on which the sample is held at certain velocity, the bead in trap is dragged to an extreme position before it escapes, Andrew C. Richardson (a PhD student in our group, who has done most of the experimental work) together with Nader and Lene found an increase of trapping stiffness beyond normal linear trapping regime. And the trapping potential is found to be well approximated by two harmonic functions depending on the displacement regime, instead of a harmonic potential which has been long assumed for OT. This work is published on Optics Express, vol. 16, p.15709-15717 .

If this is not so much interesting to biologists for its application in their research, another recently published paper might attract their eyes. Normally, the bead size for OT experiment is in micrometer range, because they are somehow relatively big in comparison with the biomolecules that are under study such that perturbation to a large extent is expected. Now, good news is that single nano-sized colloidal quantum dots which can emit light of different colours depending upon their sizes can be used for OT experiment. Not only smaller perturbation for the biological system is anticipated, one can also observe the fluorescence signals simultaneously, which no doubt will bring new excitement for biological research. This work is mainly done by Liselotte Jauffred (a PhD student) and Andrew C. Richardson, and published on Nano Letters, vol.8, p.3376-3380.