Single Quantum Dot (QD) Imaging of Fluid Flow Near Surfaces
We have introduced the use of quantum dot (QD) nanoparticles for near-surface velocimetry. These particles are an order of magnitude smaller than what has been used to date in nano-PIV measurements.
Nanocrystal QDs are semiconductor nanoparticles that are chemically synthesized with precisely controlled sizes in the range 1-10 nm. Among the most developed QDs for fluorescence imaging, and the type we use in our work, are the core-shell dots composed of an optically-active nanocrystal core of CdSe surrounded by a protective shell of ZnS. The surface of QD is covered with a ligand shell that can be functionalized for broad chemical flexibility. Several properties of QDs make them extremely attractive for fluid flow studies. By modifying the functional groups in the ligand shell, QDs may be dispersed in specific chemical environments such as polar and non-polar liquids. Since QDs can be solubilized within the fluid, they are expected to behave more like molecules and some of the near-wall issues of “solid” particles should not manifest themselves to QD tracers (e.g. particle-fluid and particle-wall interactions). The emission wavelength of a QD depends on its size, and can be tuned across the entire visible spectrum by varying the diameter of the particle. The excitation band is very broad, and the emission is independent of the excitation wavelength. Thus a size series of QDs with different emissions can be excited with the same light source.
In our work to date, we have used evanescent wave illumination to image within a region of order 100 nm of a surface the motion of water-soluble (CdSe)ZnS QDs with a core size of 6 nm in order to determine the two in-plane components of the velocity field.
This work is supported by the CRC Program of the National Science Foundation.
Further details can be found in the journal article “Single Quantum Dot (QD) Imaging of Fluid Flow Near Surfaces“ by Pouya et al. (2005). It is also available from this link.
Image of individual quantum dots within the evanescent field (field of view 86µmx50µm)
Superposition of four consecutive frames showing the movement of two individual QDs (field of view 37µmx37µm)