Three-dimensional manipulation and control of single and multiple nanoparticles is key for a variety of applications, including single molecule spectroscopy, colloidal dynamics, tailored particle assembly, protein isolation, high resolution surface studies, controlled investigation of biological processes and surface enhanced spectroscopy [1]. The potential applications of nanoparticle manipulation may be further extended if the nanoparticles in question also act as fluorescent probes, so that their luminescence properties may be used to provide physical and/or chemical cues. Such single particle manipulation may be used for physical/chemical imaging of a variety of systems with reduced sizes (ranging from photonic devices to single cells).

UCNPs have been successfully used for high resolution intracellular dynamical studies, bioimaging and accurate intracellular thermal measurements [2]. However, to the best of our knowledge, optical trapping of single or multiple UCNPs has not yet been demonstrated. Such a step would facilitate exciting new applications. As an example, intracellular fluorescence sensing and imaging is presently achieved by intracellular incorporation of UCNPs, so that UCNPs are distributed over the entire cell volume. The trapping of one single nanoparticle would allow for the mapping of the cell with only one nanoparticle, thus minimizing the perturbations to the system.

We report on the first experimental observation of stable optical trapping of dielectric NaYF4:Er3+,Yb3+ upconverting fluorescent nanoparticles (~26 nm in diameter) using a continuous wave 980 nm single-beam laser. The laser serves both to optically trap and to excite visible luminescence from the nanoparticles. Sequential loading of individual nanoparticles into the trap is observed from the analysis of the emitted luminescence. We demonstrate that the trapping strength and the number of individual nanoparticles trapped is dictated by both the laser power and nanoparticle density. The possible contribution of thermal effects has been investigated by performing trapping experiments in heavy water in addition to distilled water. For the case of heavy water, thermal gradients are negligible and optical forces dominate the trap loading behaviour. The results provide a promising path towards real three dimensional manipulation of single NaYF4:Er3+,Yb3+ nanoparticles for precise fluorescence sensing in biophotonics experiments.

References

[1] D. J. Stevenson, F. Gunn-Moore, K. Dholakia, J Biomed Opt 2010, 15 (4), 041503

[2] Y. Zhang, W.  Li, Optical Society of America: 2012; p AS3E.1