The current study reports the facile design of quantum dot (QD)-conjugated lipids and their application to high-speed tracking experiments on cell surfaces. CdSe/ZnS core/shell QDs with two types of hydrophilic coatings, 2-(2-aminoethoxy)ethanol (AEE-coating) and a 60:40 molar mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy (polyethyleneglycol-2000] (LIPO-coating), are conjugated to sulfhydryl lipids via maleimide reactive groups on the QD surface. Prior to lipid conjugation, the colloidal stability of both types of coated QDs in aqueous solution is confirmed using fluorescence correlation spectroscopy. A sensitive assay based on single lipid tracking experiments on a planar solid-supported phospholipid bilayer is presented that establishes conditions of monovalent conjugation of QDs to lipids. The QD lipids are then employed as single molecule tracking probes in plasma membranes of several cell types. Initial tracking experiments at a frame rate of 30 fps corroborate that QD-lipids diffuse like dye-labeled lipids in the plasma membrane of COS-7, HEK-293, 3T3, and NRK cells, thus confirming monovalent labeling. Finally, QD-lipids are applied for the first time to high-speed single molecule imaging by tracking their lateral mobility in the plasma membrane of NRK fibroblasts with up to 1000 fps. Our high-speed tracking data, which are in excellent agreement to previous tracking experiments with larger 40nm Au labels, not only push the time resolution in long-time, continuous fluorescence-based single molecule tracking, but also show that highly photostable, photoluminescent nanoprobes of 10nm size can be employed (AEE-coated QDs). These probes are also attractive because, unlike Au nanoparticles, they facilitate complex multicolor experiments.

The sonochemically driven synthesis of CdSe quantum dots (QDs) and their subsequent sonochemical ZnS shelling is described. By adapting the use of Cd(OAc) 2 , TOPO, and hexadecylamine to an ultrasounddriven synthesis and by applying a subsequent sonochemical ZnS shelling procedure to CdSe QD cores using Zn-ethylxanthate, highly luminescent QDs with quantum yields of 50% to 60%, narrow emission spectra (fwhm ∼25 nm), and size distributions of ∼10% were obtained. Furthermore, QD synthesis via ultrasound has several attractive features, such as a straightforward process control in the absence of any outside heat source and the ability to achieve nucleation and growth at bulk temperatures notably lower than those required for thermal techniques. The approach presented herein is accessible to laboratories with limited synthetic expertise to create CdSe/ZnS QDs.

Amphiphilic lipopolymers are known to form 2D thermoreversible gels at the air-water interface. Recently, we have reported surface rheology and film balance experiments on poly(ethylene glycol) (PEG) lipopolymers of different molecular weights, which indicated that a sufficient cross-sectional area mismatch between polymer and lipid moieties is necessary to form stable 2D gels (J. Coffman and C. Naumann, Macromolecules 2002, 35, 1835). In the current studies, we have investigated the influence of the hydrophobic anchor on the gelation properties by surface rheology and film balance technique. Experiments on PEG lipopolymers carrying saturated and partially unsaturated alkyl chains and on poly[(2-n-nonyl-2-oxazoline)x-b-(2-methylor 2-ethyl-2-oxazoline)y] (NxMy or NxEy) diblock copolymers of different block length show that the gel formation is not merely the result of the area mismatch between hydrophilic and hydrophobic moieties of the amphiphile (cone shape), but that a sufficient strength of van der Waals interaction within the hydrophobic moiety is necessary for the 2D gel to form, thus verifying earlier predictions that an alkyl chain condensation is a necessary precursor for the gelation process to occur. We also present neutron reflectometry data on PEG lipopolymers above and below the alkyl chain condensation and gelation transitions, which, in agreement to previous neutron and X-ray scattering experiments, reveal that both transitions occur after surface micelles of lipopolymers are formed at the air-water interface. On the basis of these findings, we assume that the gelation process of lipopolymers at the air-water interface is caused by a surface micellization of lipopolymers, which can be seen as the 2D analogue to the 3D gel formation observed for polymeric colloids with grafted polymer chains, such as copolymers and star polymers.