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Posted: Mar 21, 2007
Quantum dots are ready for real world applications
(Nanowerk Spotlight) Nanocrystals, also called quantum dots (QD), are artificial nanostructures that can possess many varied properties, depending on their material and shape. For instance, due to their particular electronic properties they can be used as active materials in single-electron transistors. Because certain biological molecules are capable of molecular recognition and self-assembly, nanocrystals could also become an important building block for self-assembled functional nanodevices. The atom-like energy states of QDs furthermore contribute to special optical properties, such as a particle-size dependent wavelength of fluorescence; an effect which is used in fabricating optical probes for biological and medical imaging. So far, the use in bioanalytics and biolabeling has found the widest range of applications for colloidal QDs. Though the first generation of quantum dots already pointed out their potential, it took a lot of effort to improve basic properties, in particular colloidal stability in salt-containing solution. Initially, quantum dots have been used in very artificial environments, and these particles would have simply precipitated in 'real' samples, such as blood. These problems have been solved and QDs are ready for their first real applications.
"Due to their properties, nanocrystals are particularly interesting with regard to the construction of smaller and faster devices or multifunctional materials on the nanometer scale" Dr. Wolfgang Parak explains to Nanowerk. "Molecular recognition is a “key/lock” principle realized on a molecular scale: Receptor molecules (the lock) recognize certain ligand molecules (the “key”) with very high selectivity. Thus, only the appropriate ligand will bind to its receptor. Several important classes of receptor–ligand pairs exist, such as oligonucleotides and their complementary counterpart, antibodies and antigens, and the biotin/ avidin system."
"For this purpose, each building block has to be functionalized with ligand molecules" Parak says. "The building block–ligand conjugates will now bind to positions where corresponding receptor molecules are present. In this way the following three types of applications are possible: 1) The assembly of receptor–ligand-mediated groupings of building blocks to form new multifunctional building blocks, 2) the arrangement of ligand-modified building blocks on a surface that is patterned with receptor molecules, and 3) the labeling of specific bioreceptors with ligand-modified building blocks."
It is this third area where QD-based applications have progressed furthest so far.
In a recent free access review paper in The Journal of Materials Chemistry ("Bioanalytics and biolabeling with semiconductor nanoparticles (quantum dots)"), Parak, who heads the Nanomaterials research group in the Physics Department at The Ludwig-Maximilians University in Munich, Germany, together with collaborators from Spain and Taiwan, summarized the recent applications of quantum dots in bioanalytics and biolabeling.
a) Analyte detection by quenching of the quantum dot fluorescence (red) upon binding of the analyte (black) to the quantum dot surface. b) By binding an appropriate organic fluorophore (green) as acceptor to the surface of the donor quantum dot fluorescence resonance energy transfer (FRET) occurs. FRET is stopped upon displacement of the acceptor dye from the quantum dots surface by the analyte. c) Specific cellular receptors (black) can be labeled with quantum dots that have been modified with appropriate ligand molecules. d) If a cell (grey) within a cell colony is labeled with quantum dots this cells passes the quantum dots to all its daughter cells and the fate of this cell can be observed. (Reprinted with permission from the Royal Society of Chemistry)
When colloidal QDs, which basically are fluorescent dye particles, are photo-excited, electron–hole pairs are generated and upon their recombination fluorescence light is emitted. Due to their small size quantum effects play an important role, which finally results in size dependent wavelengths of fluorescence. The smaller the particles, the more blue-shifted their fluorescence. In this way all colors in the visible and infrared can be obtained by synthesizing nanoparticles of different size.
In contrast to organic fluorophores, colloidal QDs are based on inorganic semiconductor nanoparticles and have several advantages: Their emission spectra are quite narrow and symmetric, and do not show any red-tail. In this way many different colors can be excited with just one wavelength of excitation and can be spectrally well resolved. Their fluorescent lifetime is higher (still measured in nanoseconds, though); and their photobleaching is reduced. While QDs most likely will not replace organic fluorophores as such, they are expected to be the dominant fluorescence dyes in certain types of applications. In particular, they could have a considerable impact in single molecule tracing studies, in FRET based immunoassays, and in tracking the fate of cells in tissues.
QDs can be used in different ways to detect analytes.
As active sensor elements the fluorescence properties of the QDs are changed upon reaction with the analyte. While these applications are fairly simple, they appear to be restricted to sense just a few reactive small molecules or ions that are able to interact directly with the QDs' surface. Moreover, the reactions are usually not highly specific, and they also depend strongly on the surface properties of the QDs (including their coating).
"We think that the use of QDs as active element in sensors in industrial applications will be limited" predicts Parak. "Much more promising is the use of QDs as passive labels in sensor applications, in particular in FRET based assays".
In passive label probes, selective receptor molecules such as antibodies have been conjugated to the surface of QDs. In a first step, capture antibodies are immobilized on a substrate to which the analyte is added. In a second step QD-labeled antibodies are used to visualize and quantify the bound analyte. This technique allows for the design of simple multiplexed immunoassays.
Parak thinks that QDs will have a huge potential for cellular labeling. Here, individual molecules can be fluorescence-labeled with QDs to trace the movement of individual membrane proteins. Also, whole cellular structures, such as DNA, can can be QD-labeled and used as a fluorescent in situ marker.
"Nowadays the synthesis of quantum dots in organic solvents is well established, so that control of the size, shape, and even composition is possible" says Parak. "However, some problems still have to be overcome. One of them is the potential cytotoxicity, especially of the cadmium-containing materials"
While coverage of potentially toxic particles with additional shells can seal the Cd-containing core, Parak suggests that in the future other QDs that do not contain cadmium and therefore are more biocompatible will be made available, such as for example doped zinc selenide particles.
Another problem that is still not completely understood is the process of blinking, i.e. when luminescence switches on and off. Blinking limits quantitative single QD based sensor applications.
"There is still plenty of room for further development in all these directions" concludes Parak.