We report the use of Tat peptide-conjugated quantum dots (Tat-QDs) to examine the complex behavior of nanoparticle probes in live cells, a topic that is of considerable current interest in developing advanced nanoparticle agents for molecular and cellular imaging. Dynamic confocal imaging studies indicate that the peptide-conjugated QDs are internalized by macropinocytosis, a fluid-phase endocytosis process triggered by Tat-QD binding to negatively charged cell membranes. The internalized Tat-QDs are tethered to the inner vesicle surfaces and are trapped in cytoplasmic organelles. The QD loaded vesicles are found to be actively transported by molecular machines (such as dyneins) along microtubule tracks. The destination of this active transport is an asymmetric perinuclear region (outside the cell nucleus) known as the microtubule organizing center (MTOC). We also find that Tat-QDs strongly bind to cellular membrane structures such as filopodia and that large QD-containing vesicles are released from the tips of filopodia by vesicle shedding. These results provide new insights into the mechanisms of Tat peptide-mediated delivery as well as toward the design of functionalized nanoparticles for molecular imaging and targeted therapy.

A number of procedures are currently available to encapsulate and solubilize hydrophobic semiconductor Quantum Dots (QDs) for biological applications. Most of these procedures are based on the use of small-molecule coordinating ligands, amphiphilic polymers, or amphiphilic lipids. However, it is still not clear how these different surface coating molecules affect the optical, colloidal, and chemical properties of the solubilized QDs. Here we report a systematic study to examine the effects of surface coating chemistry on the hydrodynamic size, fluorescence quantum yield, photostability, chemical stability, and biocompatibility of water-soluble QDs. The results indicate that quantum dots with the smallest hydrodynamic sizes are best prepared by direct ligand exchange with hydrophilic molecules, but the resulting particles are less stable than those encapsulated in amphiphilic polymers. For stability against chemical oxidation, QDs should be protected with a hydrophobic bilayer. For high stability under acidic conditions, the best QDs are prepared by using hyperbranched polyethylenimine. For stability in high salt buffers, it is preferable to have uncharged, sterically-stabilized QDs, like those coated with polyethylene glycol (PEG). These insights are expected to benefit the development of quantum dots and related nanoparticle probes for molecular and cellular imaging applications.

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Controlled transport of multiple individual nanostructures is crucial for nanoassembly and nanodelivery but is challenging because of small particle size. Although atomic force microscopy and optical and magnetic tweezers can control single particles, it is extremely difficult to scale these technologies for multiple structures. Here, we demonstrate a "nano-conveyer-belt" technology that permits simultaneous transport and tracking of multiple individual nanospecies in a selected direction. The technology consists of two components: nanocontainers, which encapsulate the nanomaterials transported, and nanoconveyer arrays, which use magnetic force to manipulate individual or aggregate nanocontainers. This technology is extremely versatile. For example, nanocontainers encapsulate quantum dots or rods and superparamagnetic iron oxide nanoparticles in <100 nm nanocontainers, the smallest magnetic composites to have been simultaneously moved and optically tracked. Similarly, the nanoconveyers consist of patterned microdisks or zigzag nanowires, whose dimensions can be controlled through micropatterning. The nanoconveyer belt technology could impact multiple fields, including nanoassembly, biomechanics, nanomedicine, and nanofluidics.

Semiconductor quantum dots are luminescent nanoparticles that are under intensive development for use as a new class of optical imaging contrast agents. Their novel properties such as optical tunability, improved photostability, and multicolor light emission have opened new opportunities for imaging living cells and in vivo animal models at unprecedented sensitivity and spatial resolution. Combined with biomolecular engineering strategies for tailoring the particle surfaces at the molecular level, bio-conjugated quantum dot probes are well suited for imaging single-molecule dynamics in living cells, for monitoring protein-protein interactions within specific intracellular locations, and for detecting diseased sites and organs in deep tissue. In this article, we describe the engineering principles for preparing high-quality quantum dots and for conjugating the dots to biomolecular ligands. We also discuss recent advances in using quantum dots for in vivo molecular and cellular imaging.

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Because of their extraordinary brightness and photostability, quantum dots (QDs) have tremendous potential for long-term, particle tracking in heterogeneous systems (e.g., living cells, microfluidic flow). However, one of their major limitations is blinking, an intermittent loss of fluorescence, characteristic of individual and small clusters of QDs, that interrupts particle tracking. Recently, several research groups have reported "nonblinking QDs". However, blinking is the primary method used to confirm nanoparticle aggregation status in situ, and single or small clusters of nanoparticles with continuous fluorescence emission are difficult to discern from large aggregates. Here, we describe a new class of quantum dot-based composite nanoparticles that solve these two seemingly irreconcilable problems by exhibiting near-continuous, alternating-color fluorescence, which permits aggregation status discrimination by observable color changes even during motion across the focal plane. These materials will greatly enhance particle tracking in cell biology, biophysics, and fluid mechanics.

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