Researchers are faced with a paradox in quantum dot-based particle/molecule tracking: on one hand, blinking is a problem for quantum dot-based tracking as it breaks up tracking trajectories; on the other hand, blinking is also a very useful indicator as it offers the best - and often only practical - in situ indication of aggregation status. Thus, when a researcher conducts a tracking experiment, s/he is annoyed by blinking but at the same time would be equally disappointed if blinking was not present to confirm single, or near single, particle status. Previous strategies to manipulate blinking dynamics involve changing the structure (surface or core chemistry) of a quantum dot. Scientists have now taken a fundamentally different approach: quantum dots employed remain unchanged.
Quantum dots have been receiving extensive attention from researchers because they can be widely used for basic physics study, quantum computing, biological imaging, nanoelectronics, and photonics applications. Current major fabrication methods for semiconductor quantum dots all have certain drawbacks. Comparing all these methods, the electrical depletion method has many advantages, such as electrical tunability by gate contacts, smooth confinement boundaries, good control and uniformity if the top gate patterns are uniform enough. Researchers have now, for the first time, applied the electrical depletion method to quantum wells and generate a large area of uniform quantum dots using a uniform metallic nanoholes array on top of quantum wells. This design for forming quantum dots has inherited all the advantages of the electrical depletion method. Furthermore, it can produce millions of uniform quantum dots easily, precisely, and controllably, which will help realize the wide applications of quantum dots in many areas.
While the dye sensitized photovoltaic cell is a fairly mature design, researchers are still trying to improve its efficiency with various techniques, including structuring nanoporous electrodes to provide higher surface area and better charge transport, replacement of the liquid electrolyte by a solid one in order to prevent the electrolyte evaporation, and ways to widen the narrow absorption spectra of molecular dyes. In a standard DSSC, an organic molecule adsorbed on the surface of a porous electrode absorbs light and then initiates the charge separation process eventually leading to generation of photocurrent. One major difficulty in such cells is that very few dyes can absorb a broad spectral range, essentially covering the solar spectrum. In contrast, broad spectral coverage is an inherent property of semiconductor nanocrystals. The latter, however, turn out to do a rather lousy job in separating the charges. Researchers in Israel have now presented a new configuration for quantum dot sensitized DSSCs via a FRET process.
Oncogenes are genes that are associated with the development of cancer - when mutated or expressed at high levels, they can help turn a normal cell into a tumor cell. Promising new chemotherapeutic strategies have therefore focused on suppressing oncogenes. One such approach is based on RNA interference (RNAi), a technique wherein small double-stranded RNA molecules can sequence-specifically inhibit the expression of targeted oncogenes. The idea here is that with the help of small interfering RNA (siRNA), key oncogenes that modulate signaling pathways and thereby regulate the behavior of malignant tumor cells can be manipulated. To harness the full potential of this approach, the prime requirements are to deliver the siRNA molecules with high selectivity and efficiency into tumor cells and to monitor both siRNA delivery and the resulting knockdown effects at the single-cell level.
The usefulness of quantum dots comes from their peak emission frequency's extreme sensitivity to both the dot's size and composition. QDs have been touted as possible replacements for organic dyes in the imaging of biological systems, due to their excellent fluorescent properties, good chemical stability, broad excitation ranges and high photobleaching thresholds. In order for quantum dots to be useful as as nanoemitters for biological imaging, they need to be linked with a specific sensor molecule that exclusively targets a biomolecule of interest. These conjugations are usually made using small linker molecules although this often demands multistep procedures and may suffer from QD colloidal instabilities during the coupling reactions. Demonstrating hyaluronic acid-QD conjugates, researchers in Korea have now solved this problem by simple electrostatic conjugation which offers stable and size-tunable conjugates.
In a recent Nanowerk Spotlight we reported on a single molecule approach to directly visualize and map protein binding sites on DNA using fluorescent quantum dots. One of the challenges the researchers in this work had was to measure distances between probes bound to combed DNA with nanometer resolution. Whereas very short distance (below 10 nm) can be assessed by FRET measurements and distances above the Rayleigh criterion can be measured, say, with a standard microscopy picture and a ruler, distances in between need to be addressed differently. This is were a novel approach by scientists at UCLA fills the gap, and, as they claim, better than other techniques do.
Proteins that bind to specific sites of DNA are essential to all biological functions of DNA. These DNA-binding proteins include transcription factors which modulate the process of transcription, various polymerases, nucleases which cleave DNA molecules, and histones which are involved in chromosome packaging in the cell nucleus. Developing methods to precisely determine the locations and occupancy of DNA-binding proteins is instrumental to scientists' understanding of cellular processes like gene expression and regulation. Motivated by the desire to overcome some of the inherent limitations of existing biochemical techniques for mapping protein binding sites on DNA, scientists at UCLA have now demonstrated the viability of a single molecule approach to directly visualize and map protein binding sites on DNA using fluorescent quantum dots, allowing multicolor, nanometer-resolution localization.
The properties of a quantum dot are not only determined by its size but also by its shape, composition, and structure, for instance if it's solid or hollow. A reliable manufacturing technology that makes use of nanocrystals' properties - for a wide-ranging number of applications in such areas as catalysis, electronics, photonics, information storage, imaging, medicine, or sensing - needs to be capable of churning out large quantities of nanocrystals where each batch is produced according to the exactly same parameters. In a recent review article, Sara Skrabalak from Indiana University and Younan Xia from Washington University in St. Louis, describe recent advances in seeded growth as the ultimate approach to producing metal nanocrystals with precisely controlled sizes, shapes, and compositions - the necessary first step toward their use and assembly for large-scale applications.