Combining Fluorescent Imaging and Magnetic Force in Single Molecule Studies

Studying single nanoparticles and molecules has led to a number of important discoveries in biology and nanoscience. In particular, single quantum dots (semiconductor nanocrystals) have been successfully used to image and track biological processes at single molecule level in vitro and in vivo. On the other hand, magnetic tweezers have been used to apply force on micron-sized magnetic beads. Combining the fluorescence of quantum dots and the force of magnetic tweezers, we have achieved the first concurrent magnetic manipulation and fluorescent tracking of single nanoparticles (smaller than 100 nm).

Quantum dots and superparamagnetic iron oxide nanoparticles were first loaded into nanocontainers with diameters adjustable from 10 to 100 nm, forming nanocomposites with both fluorescence and superparamagnetism properties. Magnetic tweezers were then used to apply force on the composite nanoparticles, while the movement of these nanoparticles was tracked by a fluorescent microscope. It was found that the composite nanoparticles had fluorescent intermittency (i.e. blinking), indicating that they were single nanoparticles (rather than large aggregates of nanoparticles). The ability of concurrent magnetic manipulation and fluorescent tracking of single sub-100 nm nanoparticles could have broad impact in many areas, including biomechanics and nanofabrication.