Nano-velocimetry using Quantum Dots

Our research focuses on the application of semiconductor nanocrystals or quantum dots (QDs) in optical diagnostics for micro and nanoscale fluid, thermal and mass transport. Advanced statistical tracking algorithms are used to track the dynamics of QDs near fluid/solid interfaces to estimate fluid velocity profiles. Additional research projects include microscale laser induced fluorescence (LIF) thermometry and microscale aerosol dynamics using QDs.

Quantum dots (QDs) are self-assembled semiconductor nanocrystals with diameters ranging from 2 to 20 nm. Unlike popular fluorescent dyes typically used in microfluidic and biofluidic applications, QDs are resistant to photo-bleaching, have emission wavelengths tunable with the particle diameter and can be coated with various ligands for either organic or inorganic solvents. Additionally, QDs exhibit fluorescence intermittency (blinking), where they cycle through bright and dark states under continuous illumination. Smaller tracer particles, like QDs, are desirable in micro and nanofluidics to probe fluid motion closer to solid boundaries. However, traditional particle tracking algorithms such as Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) have difficulty tracking smaller tracers due to their low intensity, blinking and high levels of thermal motion (Brownian motion). We have developed a statistical PTV (SPTV) algorithm to track such small tracer particles based on the statistical nature of these phenomena.

QDs are being used as tracer particles in a variety of organic and inorganic solvents including long-chain polymers (silicon oil), water and glycerol. By measuring tracer displacements, we can simultaneously determine diffusive properties, measure velocity profiles or velocity vector fields, and meausre temperature fields (using two-color measurement techniques). Standard flood illumination is used to measure displacements in bulk fluid and also, Total Internal Reflection Fluorescence Microscopy (TIRFM) allows illumination within less than 200 nm of fluid just above the solid surface for near-wall velocimetry.

 

Single quantum dot diffusing in water, imaged using TIRFM