9 research outputs found
Highly Enhanced Affinity of Multidentate versus Bidentate Zwitterionic Ligands for Long-Term Quantum Dot Bioimaging
High colloidal stability in aqueous conditions is a prerequisite
for fluorescent nanocrystals, otherwise known as âquantum dotsâ,
intended to be used in any long-term bioimaging experiment. This essential
property implies a strong affinity between the nanoparticles themselves
and the ligands they are coated with. To further improve the properties
of the bidentate monozwitterionic ligand previously developed in our
team, we synthesized a multidentate polyzwitterionic ligand, issued
from the copolymerization of a bidentate monomer and a monozwitterionic
one. The nanocrystals passivated by this polymeric ligand showed an
exceptional colloidal stability, regardless of the medium conditions
(pH, salinity, dilution, and biological environment), and we demonstrated
the affinity of the polymer exceeded by 3 orders of magnitude that
of the bidentate ligand (desorption rates assessed by a competition
experiment). The synthesis of the multidentate polyzwitterionic ligand
proved also to be easily tunable and allowed facile functionalization
of the corresponding quantum dots, which led to successful specific
biomolecules targeting
SulfobetaineâVinylimidazole Block Copolymers: A Robust Quantum Dot Surface Chemistry Expanding Bioimagingâs 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, QDsâ potential as a bioimaging platform relies upon a precise control over the surface chemistry modifications of these nano-objects. Here, a zwitterionâvinylimidazole 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
Real-Time in Situ Probing of High-Temperature Quantum Dots Solution Synthesis
Understanding the formation mechanism
of colloidal nanocrystals is of paramount importance in order to design
new nanostructures and synthesize them in a predictive fashion. However,
reliable data on the pathways leading from molecular precursors to
nanocrystals are not available yet. We used synchrotron-based time-resolved <i>in situ</i> small and wide-angle X-ray scattering to experimentally
monitor the formation of CdSe quantum dots synthesized in solution
through the heating up of precursors in octadecene at 240 °C.
Our experiment yields a complete movie of the structure of the solution
from the self-assembly of the precursors to the formation of the quantum
dots. We show that the initial cadmium precursor lamellar structure
melts into small micelles at 100 °C and that the first CdSe nuclei
appear at 218.7 °C. The size distributions and concentration
in nanocrystals are measured in a quantitative fashion as a function
of time. We show that a short nucleation burst lasting 30 s is followed
by a slow decrease of nanoparticle concentration. The rate-limiting
process of the quantum dot formation is found to be the thermal activation
of selenium
Electrolyte-Gated Field Effect Transistor to Probe the Surface Defects and Morphology in Films of Thick CdSe Colloidal Nanoplatelets
The optical and optoelectronic properties of colloidal quantum dots strongly depend on the passivation of their surface. Surface states are however difficult to quantify using optical spectroscopy and techniques based on back gated field effect transistors are limited in the range of carrier density that can be probed, usually significantly below one charge carrier per particle. Here we show that electrolyte gating can be used to quantitatively analyze the increase of defects in a population of nanoparticles with increasing surface irregularities. We illustrate this method using CdSe nanoplatelets that are grown in their thickness using low temperature layer-by-layer method. Spectroscopic analysis of the samples confirm that the nanoplatelet thickness is controlled, on average, with atomic precision, but structural analysis with transmission electron microscopy shows that the number of surface defects increases with the nanoplatelet thickness. The amount of charge defects is probed quantitatively using electrolyte-gated field effect transistor (EFET). We observe that the threshold voltage of the EFET increases with the NPL thickness, in agreement with the structural analysis. All samples displayed n-type conduction with strong current modulation (subthreshold swing slope of 100 mV/decade and on/off ratio close to 10<sup>7</sup>). We also point out that an efficient electrolyte gating of the film requires a fine control of the nanoparticle film morphology
Coupled HgSe Colloidal Quantum Wells through a Tunable Barrier: A Strategy To Uncouple Optical and Transport Band Gap
Among
semiconductor nanocrystals (NCs), 2D nanoplatelets (NPLs)
are a special class of nanomaterials with well controlled optical
features. So far, most of the efforts have been focused on wide band
gap materials such as cadmium chalcogenide semiconductors. However,
optical absorption can be pushed toward the infrared (IR) range using
narrow band gap materials such as mercury chalcogenides. Here we demonstrate
the feasibility of a core/shell structure made of a CdSe core with
two HgSe external wells. We demonstrate that the optical spectrum
of the heterostructure is set by the HgSe wells and this, despite
the quasi type II band alignment, makes the band edge energy independent
of the inner core thickness. On the other hand, these core/shell NPLs
behave, from a transport point of view, as a wide band gap material.
We demonstrate that the introduction of a wide band gap CdSe core
makes the material less conductive and with a larger photoresponse.
Hence, the heterostructure presents an effective electric band gap
wider than the optical band gap. This strategy will be of utmost interest
to design infrared effective colloidal materials for which the reduction
of the carrier density and the associated dark current is a critical
property
Engineering Bicolor Emission in 2D Core/Crown CdSe/CdSe<sub>1â<i>x</i></sub>Te<sub><i>x</i></sub> Nanoplatelet Heterostructures Using Band-Offset Tuning
Colloidal
2D nanoplatelets (NPLs) are a class of nanoparticles
that offer the possibility of forming two types of heterostructures,
by growing either in the confined direction or perpendicular to the
confined direction, called core/crown NPLs. Here, we demonstrate that
bicolor emission can be obtained from 2D NPLs with a core/crown geometry.
To date, for CdSe/CdTe NPLs with type-II band alignment, only charge
transfer emission has been observed due to the very fast (1â<i>x</i>Te<sub><i>x</i></sub> alloys crowned with the right composition and lateral extension
enables the observation of bicolor emission at the single-particle
level. One source of emission originates from recombination at the
core/crown interface (<i>X</i><sub>int</sub>), and the other
emission source originates from direct recombination in the crown
(<i>X</i><sub>crown</sub>). This crown emission, which is
nonvisible in pure CdSe/CdTe core crown NPLs, results from the large
binding energy compared to the reduced conduction band offset existing
in the alloy with intermediate (60%) Te content. These observations
are only made possible by the 2D geometry of the NPLs
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 QDâprotein
A immobilization platform displays remarkable antibody functionality
retention after binding, usually a compromised property in antibody
conjugation to surfaces. Typical QDâprotein Aâantibody
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 ÎŒM
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 QDâprotein A nanoconjugates have thus a
latent potential to become an off-the-shelf tool destined to unresolved
biological questions
Clickable-Zwitterionic Copolymer Capped-Quantum Dots for in Vivo Fluorescence Tumor Imaging
In
the last decades, fluorescent quantum dots (QDs) have appeared as
high-performance biological fluorescent nanoprobes and have been explored
for a variety of biomedical optical imaging applications. However,
many central challenges still exist concerning the control of the
surface chemistry to ensure high biocompatibility, low toxicity, antifouling,
and specific active targeting properties. Regarding in vivo applications,
circulation time and clearance of the nanoprobe are also key parameters
to control the design and characterization of new optical imaging
agents. Herein, the complete design and characterization of a peptide-near-infrared-QD-based
nanoprobe for biomedical optical imaging is presented from the synthesis
of the QDs and the zwitterionic-azide copolymer ligand, enabling a
bio-orthogonal coupling, till the final in vivo test through all the
characterization steps. The developed nanoprobes show high fluorescence
emission, controlled grafting rate, low toxicity, in vitro active
specific targeting, and in vivo long circulating blood time. This
is, to our knowledge, the first report characterizing the in vivo
circulation kinetics and tumor accumulation of targeted zwitterionic
QDs
Influence of Luminescence Quantum Yield, Surface Coating, and Functionalization of Quantum Dots on the Sensitivity of Time-Resolved FRET Bioassays
In
clinical diagnostics, homogeneous time-resolved (TR) FRET immunoassays
are used for fast and highly sensitive detection of biomarkers in
serum samples. The most common immunoassay format is based on europium
chelate or cryptate donors and allophycocyanin acceptors. Replacing
europium donors with terbium complexes and the acceptors with QDs
offers large photophysical advantages for multiplexed diagnostics,
because the Tb-complex can be used as FRET donor for QD acceptors
of different colors. Water-soluble and biocompatible QDs are commercially
available or can be synthesized in the laboratory using many available
recipes from the literature. Apart from the semiconductor material
composition, an important aspect of choosing the right QD for TR-FRET
assays is the thickness of the QD coating, which will influence the
photophysical properties and long-term stability as well as the donorâacceptor
distance and FRET efficiency. Here we present a detailed time-resolved
spectroscopic study of three different QDs with an emission maximum
around 605 nm for their application as FRET acceptors (using a common
Tb donor) in TR-bioassays: (i) Invitrogen/Life Technologies Qdot605,
(ii) eBioscience eFluorNC605 and iii) ter-polymer stabilized CdSe/CdS/ZnS
QDs synthesized in our laboratories. All FRET systems are very stable
and possess large FoÌrster distances (7.4â9.1 nm), high
FRET efficiencies (0.63â0.80) and low detection limits (0.06â2.0
pM) within the FRET-bioassays. Shapes, sizes and the biotin/QD ratio
of the biocompatible QDs could be determined directly in the solution
phase bioassays at subnanomolar concentrations. Both commercial amphiphilic
polymer/lipid encapsulated QDs and self-made ligand-exchanged QDs
provide extremely low detection limits for highly sensitive TR-FRET
bioassays