Single quantum dot (QD) imaging of fluid flow near surfaces

We introduce the use of quantum dot (QD) nanoparticles for near-surface velocimetry and provide preliminary data to demonstrate its feasibility. Evanescent wave illumination is used to image the motion of water-soluble (CdSe)ZnS QDs with a core size of 6 nm within a region of order 100 nm of a surface . Results are presented for the two in-plane components of the velocity field. With the recent surge of interest in microfluidics and nanofluidics research, various fundamental and practical issues necessitate the interrogation of flow and transport near the surfaces of channels of increasingly smaller size. The most common method for measuring velocities in microchannels is micro-particle image velocimetry, lPIV (Santiago et al. 1998; Meinhart et al. 1999). In lPIV, fluorescent particles with diameters typically in the range 200–300 nm are used to obtain velocity data with an out-of-plane spatial resolution typically of O(1 lm). Near-wall measurements with this method have so far been obtained no closer than 450 nm from the surface. Recently, nano-PIV (nPIV) has been introduced as a technique specifically designed for near-surface interrogation of flow (Zettner and Yoda 2003; Jin et al. 2004; Sadr et al. 2004). This method relies on an evanescent wave illumination generated by the total internal reflection (TIR) of light at the interface between two materials of different refractive indices (Axelrod et al. 1984), e.g., a glass–liquid interface. With this method, the motion of particles within a region of order 100 nm of the surface is measured. In nPIV studies, to date, the size of the seed particles are in the 100–300 nm range; i.e., they are of the same order of, or larger than, the illuminated region. It would be desirable to use much smaller seed particles to reduce some of the complications that can be caused by larger particles such as the modification of evanescent wave field, and particle–fluid interactions that compromise the accuracy of fluid velocity measurement and the ability of particles to move close to the surface. In this paper we introduce 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 nPIV. Nanocrystal QDs are semiconductor nanoparticles that are chemically synthesized with precisely controlled sizes in the range of 1–10 nm (Murray et al. 1993; Dabboussi et al. 1997; Bruchez et al. 1998; Mattoussi et al. 2000; Murray et al. 2000). Among the most developed QDs for fluorescence imaging, and the type which has been used in this paper are the coreshell 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 nonpolar 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. S. Pouya Æ M. Koochesfahani (&) Department of Mechanical Engineering, Michigan State University, East Lansing, MI 48824, USA E-mail: [email protected] P. Snee Æ M. Bawendi Æ D. Nocera Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Experiments in Fluids (2005) 39: 784–786 DOI 10.1007/s00348-005-0004-x