The problem? It is hard to capture and scan a single quantum dot over another nanoscale object.
The UMD team's solution lies in a microfluidic device that manipulates and positions quantum dots using precision flow control. A computer algorithm analyzes the dots dispersed inside, selecting one to be the reporting probe. As the microfluidic device creates a fluid flow, the targeted dot begins to move. An image-guided feedback process continually tracks the dot's location and adjusts the flow accordingly. For example, if the dot is observed to be to the northwest of its desired location, a southeast flow is created to move it into place.
This technique gives researchers the ability to manipulate a single dot precisely, guiding it quickly to desired locations, and holding it in each position with nanometer accuracy so it can be used to scan objects. The dot's response to each scanned object is measured, providing information about the object's electromagnetic fields with nanoscale resolution. Since nothing mechanical touches the quantum dot or affects its interaction with the objects it scans, the images produced are distortion-free, clean and sharp.
A Superior, Less Expensive Technique"In other particle manipulation techniques—for example laser tweezers—the force applied to a particle scales with its volume," explains Clark School of Engineering Prof. Benjamin Shapiro (Fischell Department of Bioengineering and the Institute for Systems Research), one of the paper's co-authors. "But the viscous forces that the fluid flow applies scale with the diameter of the particle. At the nanoscale, fluid flow has a greater effect on the particle than competing techniques, allowing us to move, guide and immobilize the quantum dot more easily and accurately." In addition to its technical superiority, the new nanoscale manipulation system is far less expensive than near-field scanning optical microscopy, which requires equipment that costs hundreds of thousands of dollars.