NIR Imaging of the Integrin-Rich Head and Neck Squamous Cell Carcinoma Using Ternary Copper Indium Selenide/Zinc Sulfide-Based Quantum Dots.

The efficient intraoperative identification of cancers requires the development of the bright, minimally-toxic, tumor-specific near-infrared (NIR) probes as contrast agents. Luminescent semiconductor quantum dots (QDs) offer several unique advantages for in vivo cellular imaging by providing bright and photostable fluorescent probes. Here, we present the synthesis of ZnCuInSe/ZnS core/shell QDs emitting in NIR (~750 nm) conjugated to NAVPNLRGDLQVLAQKVART (A20FMDV2) peptide for targeting αvß6 integrin-rich head and neck squamous cell carcinoma (HNSCC). Integrin αvß6 is usually not detectable in nonpathological tissues, but is highly upregulated in HNSCC. QD-A20 showed αvß6 integrin-specific binding in two-dimension (2D) monolayer and three-dimension (3D) spheroid in vitro HNSCC models. QD-A20 exhibit limited penetration (ca. 50 µm) in stroma-rich 3D spheroids. Finally, we demonstrated the potential of these QDs by time-gated fluorescence imaging of stroma-rich 3D spheroids placed onto mm-thick tissue slices to mimic imaging conditions in tissues. Overall, QD-A20 could be considered as highly promising nanoprobes for NIR bioimaging and imaging-guided surgery.

Zwitterionic polymer ligands: an ideal surface coating to totally suppress protein-nanoparticle corona formation?

In the last few years, zwitterionic polymers have been developed as antifouling surface coatings. However, their ability to completely suppress protein adsorption at the surface of nanoparticles in complex biological media remains undemonstrated. Here we investigate the formation of hard (irreversible) and soft (reversible) protein corona around model nanoparticles (NPs) coated with sulfobetaine (SB), phosphorylcholine (PC) and carboxybetaine (CB) polymer ligands in model albumin solutions and in whole serum. We show for the first time a complete absence of protein corona around SB-coated NPs, while PC- and CB-coated NPs undergo reversible adsorption or partial aggregation. These dramatic differences cannot be described by naïve hard/soft acid/base electrostatic interactions. Single NP tracking in the cytoplasm of live cells corroborate these in vitro observations. Finally, while modification of SB polymers with additional charged groups lead to consequent protein adsorption, addition of small neutral targeting moieties preserves antifouling and enable efficient intracellular targeting.

In Vivo Imaging of Single Tumor Cells in Fast-Flowing Bloodstream Using Near-Infrared Quantum Dots and Time-Gated Imaging.

Whereas in vivo fluorescence imaging of cells immobilized within tissues provides a valuable tool to a broad range of biological studies, it still lacks the sensitivity required to visualize isolated cells circulating fast in the bloodstream due, in particular, to the autofluorescence from endogenous fluorophores. Time-gated imaging of near-infrared emitting ZnCuInSe/ZnS quantum dots (QDs) with fluorescence lifetimes in the range of 150-300 ns enables the efficient rejection of fast autofluorescence photons and the selection of QD fluorescence photons, thus significantly increasing sensitivity. We labeled model erythrocytes as well as lymphoma cells using these QDs coated with a stable zwitterionic polymer surface chemistry. After reinjection in the bloodstream, we were able to image and count individual QD-labeled cells circulating at mm·s-1 velocities in blood vessels.

Oriented Bioconjugation of Unmodified Antibodies to Quantum Dots Capped with Copolymeric Ligands as Versatile Cellular Imaging Tools.

Distinctive optical properties of inorganic quantum dot (QD) nanoparticles promise highly valuable probes for fluorescence-based detection methods, particularly for in vivo diagnostics, cell phenotyping via multiple markers or single molecule tracking. However, despite high hopes, this promise has not been fully realized yet, mainly due to difficulties at producing stable, nontoxic QD bioconjugates of negligible nonspecific binding. Here, a universal platform for antibody binding to QDs is presented that builds upon the controlled functionalization of CdSe/CdS/ZnS nanoparticles capped with a multidentate dithiol/zwitterion copolymer ligand. In a change-of-paradigm approach, thiol groups are concomitantly used as anchoring and bioconjugation units to covalently bind up to 10 protein A molecules per QD while preserving their long-term colloidal stability. Protein A conjugated to QDs then enables the oriented, stoichiometrically controlled immobilization of whole, unmodified antibodies by simple incubation. This QD-protein A immobilization platform displays remarkable antibody functionality retention after binding, usually a compromised property in antibody conjugation to surfaces. Typical QD-protein A-antibody assemblies contain about three fully functional antibodies. Validation experiments show that these nanobioconjugates overcome current limitations since they retain their colloidal stability and antibody functionality over 6 months, exhibit low nonspecific interactions with live cells and have very low toxicity: after 48 h incubation with 1 µM QD bioconjugates, HeLa cells retain more than 80% of their cellular metabolism. Finally, these QD nanobioconjugates possess a high specificity for extra- and intracellular targets in live and fixed cells. The dithiol/zwitterion QD-protein A nanoconjugates have thus a latent potential to become an off-the-shelf tool destined to unresolved biological questions.

Sulfobetaine-Vinylimidazole Block Copolymers: A Robust Quantum Dot Surface Chemistry Expanding Bioimaging's Horizons.

Long-term inspection of biological phenomena requires probes of elevated intra- and extracellular stability and target biospecificity. The high fluorescence and photostability of quantum dot (QD) nanoparticles contributed to foster their promise as bioimaging tools that could overcome limitations associated with traditional fluorophores. However, QDs' potential as a bioimaging platform relies upon a precise control over the surface chemistry modifications of these nano-objects. Here, a zwitterion-vinylimidazole block copolymer ligand was synthesized, which regroups all anchoring groups in one compact terminal block, while the rest of the chain is endowed with antifouling and bioconjugation moieties. By further application of an oriented bioconjugation approach with whole IgG antibodies, QD nanobioconjugates were obtained that display outstanding intra- and extracellular stability as well as biorecognition capacity. Imaging the internalization and intracellular dynamics of a transmembrane cell receptor, the CB1 brain cannabinoid receptor, both in HEK293 cells and in neurons, illustrates the breadth of potential applications of these nanoprobes.

Comparing intracellular stability and targeting of sulfobetaine quantum dots with other surface chemistries in live cells.

The in vivo labeling of intracellular components with quantum dots (QDs) is very limited because of QD aggregation in the cell cytoplasm and/or QD confinement into lysosomal compartments. In order to improve intracellular targeting with QDs, various surface chemistries and delivery methods have been explored, but they have not yet been compared systematically with respect to the QD intracellular stability. In this work, the intracellular aggregation kinetics of QDs for three different surface chemistries based on ligand exchange or encapsulation with amphiphilic polymers are compared. For each surface chemistry, three delivery methods for bringing the nanoparticles into the cells are compared: electroporation, microinjection, and pinocytosis. It is concluded that the QD intracellular aggregation behavior is strongly dependent on the surface chemistry. QDs coated with dihydrolipoic acid-sulfobetaine (DHLA-SB) ligands diffuse freely in cells for longer periods of time than for QDs in the other chemistries tested, and they can access all cytoplasmic compartments. Even when conjugated to streptavidin, these DHLA-SB QDs remain freely diffusing inside the cytoplasm and unaggregated, and they are able to reach a biotinylated target inside HeLa cells. Such labeling was more efficient when compared to commercial streptavidin-conjugated QDs, which may be due to the smaller size of DHLA-SB QDs and/or to their superior intracellular stability.

Small and stable sulfobetaine zwitterionic quantum dots for functional live-cell imaging.

We have developed a novel surface coating for semiconductor quantum dots (QDs) based on a heterobifunctional ligand that overcomes most of the previous limits of these fluorescent probes in bioimaging applications. Here we show that QDs capped with bidentate zwitterionic dihydrolipoic acid-sulfobetaine (DHLA-SB) ligands are a favorable alternative to polyethylene glycol-coated nanoparticles since they combine small sizes, low nonspecific adsorption, preserved optical properties, and excellent stability over time and a wide range of pH and salinity. Additionally, these QDs can easily be functionalized with biomolecules such as streptavidin (SA) and biotin. We applied streptavidin-functionalized DHLA-SB QDs to track the intracellular recycling of cannabinoid receptor 1 (CB1R) in live cells. These QDs selectively recognized the pool of receptors at the cell surface via SA-biotin interactions with negligible nonspecific adsorption. The QDs retained their optical properties, allowing the internalization of CB1R into endosomes to be followed. Moreover, the cellular activity was apparently unaffected by the probe.