Quantum-Dots-Doped ORMOSIL Nanoparticles as Optical Probes for Total Internal Reflection Fluorescence Imaging of Cancer Cells

We report the novel use of organically modified silica (ORMOSIL) nanoparticles to encapsulate hydrophobic quantum dots (QDs), thus making them hydrophilic and yet maintaining stable optical properties. QDs are resistant to photobleaching, and the cationic charge on the surfaces of the ORMOSIL nanoparticles can facilitate them being uptaken at the cell substrate regions. An objective-lens-based total internal reflection fluorescence (TIRF) microscope is used to observe the staining and dynamics of HeLa cancer cells, which have been targeted by the synthesized QD-doped ORMOSIL nanoparticles. Since the same microscope objective is used for launching the incident light beam and the collection lens, the observation and manipulation of live samples under a TIRF microscope is therefore much easier than that in the case of using the conventional prism-based TIRF setup. The present nanoparticle-assisted approach has made TIRF imaging a much more powerful tool for real-time monitoring of intracellular biological metabolic activities.

I. Introduction

Total internal reflection fluorescence (TIRF) microscopy, which utilizes a generated evanescent wave as the excitation source, provides a good way to selectively excite fluorophores in an aqueous or cellular environment very near to an interface (within about 100 nm) [1]. Since an evanescent field cannot excite fluorescence at positions far from the interface region, the resulting images have very low background (noise) fluorescence (virtually no out-of-focus fluorescence) and minimal exposure of cells to light in the region away from the interface. The unique features of TIRF can give many potential applications, such as sectioning visualization of cell membrane [2], observing single-molecule fluorescence very close to the surface [3], micromorphological structures and dynamics of live cells [4], measurements of the kinetic rates of binding of extracellular and intracellular proteins to cell surface receptors, and artificial membranes [5].