Techniques for precisely arranging nanoscopic objects are essential to fully realizing their potential in emerging technologies. Controlled placement of quantum dots (QDs) on a surface could be key to engineering photonic or plasmonic structures on the nanoscale. C. Ropp and a multidisciplinary team from the University of Maryland have recently developed a microfluidic technique for moving quantum dots with nanometer precision before individually immobilizing them within a polymer shell.
Published in the November 10, 2010 issue of Nano Letters (DOI: 10.1021/nl1029557; p. 4673), their work makes use of the electroosmotic control of CdSe/ZnS QDs in microfluidic channels. These particles are suspended in a unique water-based photoresist, which fills two channels formed between molded poly(dimethylsiloxane) and a glass slide. The X-shaped channels connect two pairs of electrodes which allow two-dimensional control of the electric field in the 100 μm square intersecting region. The flow direction of the fluid in this region, along with the suspended QDs can be precisely controlled using the electroosmotic effect of the applied field (see figure).
A microfluidic device for precisely positioning quantum dots on a surface using electroosmotic flow control (EOFC). A UV laser is used to polymerize the fluid and trap individual quantum dots in place. Reproduced with permission from Nano Lett. 10 (11) (2010) DOI: 10.1021/nl1029557; p. 4673. © 2010 American Chemical Society.
The real strength of this technique, however, is a continuous feedback loop between the electrodes controlling the movement and a camera imaging the position of the QD, which emit light when illuminated with a green laser. Using a precise, sub-pixel, imaging algorithm, a specific QD can be manipulated into position with more and more slight adjustments to the flow, while other QDs drift in a divergent fashion. When the QD has reached the desired location, the spot is automatically irradiated with a UV laser, which causes cross-linking of the photoresist around the QD and effectively encapsulates it. The nature of the photoresist fluid, comprised of a photoinitiator, a water-soluble acrylic monomer, and a viscosity modifier, is crucial to the process. Using high concentrations of the monomer causes segregation of the QDs near the glass surface, providing an element of control over the third dimension and enhancing the actuation effect.
Immobilizing the QDs allows successive objects to be moved without disturbing them, and introduces the possibility of surface patterning. The team demonstrates this, as well as the ability to spectrally select for QDs, by organizing a 3 × 3 array of alternating color QDs, 5 μm apart. The technique could easily be extended to positioning any imageable nanoparticle on water-compatible surfaces, and should prove to be a powerful tool to the nanotechnologist.
