Quantum dots have proven themselves as powerful fluorescent probes, especially for longer term, multiplexed, and quantitative imaging and detection. Newly engineered quantum dots with integrated targeting, imaging and therapeutic functionalities have become excellent material to study drug delivery in cells and small animals. This fluorescent 'prototype' will provide important information in the rational design of biocompatible drug carriers and will serve as a superior alternative to magnetic and radioactive imaging contrast agents in preclinical drugs, validation and delivery research.
Quantum dots which range from about 2 to 10 nanometers across have distinct advantages over conventional fluorescent dyes. By simply varying the crystal size, scientists can produce dots that emit light in a wide range of wavelengths, or colours, that are less prone to overlap than those of organic dyes. And whereas each organic dye must be excited with a specific wavelength of light, a single light source can excite quantum dots of many colours, so scientist's can use the dots to label and detect multiple targets simultaneously. Because of increased multiplexing potential of quantum dots for detecting cancer markers. By filling polymer beads with multiple colours and intensity of dots in various combinations, the researchers created "quantum beads" with distinct optical signatures analogous to merchandise barcodes. When linked to different antibodies, peptides, or oligonucleotide probes, the bar-coded beads should enable sensitive, high throughput detection of tens of thousands of different proteins or gene sequences in clinical specimens or other samples. Quantum beads may provide a faster, more flexible, and cheaper alternative to other technologies used for analyses.
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Key words: - Quantum dots, Diagnosis, Quantitative imaging, Dyes and Analytics.
A quantum dot can be made from a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. The confinement can be due to electrostatic potentials (generated by external electrodes, doping, strain, impurities), the presence of an interface between different semiconductor materials (e.g. in core-shell nanocrystal systems), the presence of the semiconductor surface (e.g. semiconductor nanocrystal), or a combination of these. A quantum dot has a discrete quantized energy spectrum. The corresponding wave functions are spatially localized within the quantum dot, but extend over many periods of the crystal lattice. A quantum dot contains a small finite number (of the order of 1-100) of conduction band electrons, valence band holes, or excitons, i.e., a finite number of elementary electric charges.
Small quantum dots, such as colloidal semiconductor nanocrystals, can be as small as 2 to 10 nanometers, corresponding to 10 to 50 atoms in diameter and a total of 100 to 100,000 atoms within the quantum dot volume. Self-assembled quantum dots are typically between 10 and 50Â nm in size. Quantum dots defined by lithographically patterned gate electrodes, or by etching on two-dimensional electron gases in semiconductor heterostructures can have lateral dimensions exceeding 100Â nm. At 10Â nm in diameter, nearly 3 million quantum dots could be lined up end to end and fit within the width of a human thumb.
Quantum dots can be contrasted to other semiconductor nanostructures:
1) QuantumHYPERLINK "http://en.wikipedia.org/wiki/Quantum_wire" wires, which confine the motion of electrons or holes in two spatial directions and allow free propagation in the third.
2) QuantumHYPERLINK "http://en.wikipedia.org/wiki/Quantum_well" wells, which confine the motion of electrons or holes in one direction and allow free propagation in two directions.
Quantum dots containing electrons can also be compared to atoms: both have a discrete energy spectrum and bind a small number of electrons. In contrast to atoms, the confinement potential in quantum dots does not necessarily show spherical symmetry. In addition, the confined electrons do not move in free space, but in the semiconductor host crystal. The quantum dot host material, in particular its band structure, does therefore play an important role for all quantum dot properties. Typical energy scales, for example, are of the order of ten electron volts in atoms, but only 1 millielectron volt in quantum dots. Quantum dots with a nearly spherical symmetry or flat quantum dots with nearly cylindrical symmetry can show shell filling according to the equivalent of HundHYPERLINK "http://en.wikipedia.org/wiki/List_of_Hund's_rules"'HYPERLINK "http://en.wikipedia.org/wiki/List_of_Hund's_rules"s rules for atoms. Such dots are sometimes called "artificial atoms". In contrast to atoms, the energy spectrum of a quantum dot can be engineered by controlling the geometrical size, shape, and the strength of the confinement potential. Also in contrast to atoms it is relatively easy to connect quantum dots by tunnel barriers to conducting leads, which allows the application of the techniques of tunneling spectroscopy for their investigation.
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Like in atoms, the energy levels of small quantum dots can be probed by optical spectroscopy techniques. In quantum dots that confine electrons and holes, the interband absorption edge is blue shifted due to the confinement compared to the bulk material of the host semiconductor material. As a consequence, quantum dots of the same material, but with different sizes, can emit light of different colors.
Quantum dots are particularly significant for optical applications due to their theoretically high quantum yield. In electronic applications they have been proven to operate like a single-electron transistor and show the Coulomb blockade effect. Quantum dots have also been suggested as implementations of qubits for quantum information processing.
One of the optical features of small excitonic quantum dots immediately noticeable to the unaided eye is coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, more significant in terms of coloration is the size. The larger the dot, the redder (the more towards the red end of the spectrum) the fluorescence. The smaller the dot, the bluer (the more towards the blue end) it is. The coloration is directly related to the energy levels of the quantum dot. Quantitatively speaking, the band gapHYPERLINK "http://en.wikipedia.org/wiki/Bandgap_energy" energy that determines the energy (and hence color) of the fluoresced light is inversely proportional to the square of the size of the quantum dot. Larger quantum dots have more energy levels which are more closely spaced. This allows the quantum dot to absorb photons containing less energy, i.e. those closer to the red end of the spectrum. Recent articles in nanotechnology and other journals have begun to suggest that the shape of the quantum dot may well also be a factor in the colorization, but as yet not enough information has become available. Furthermore it was shown recently,that the lifetime of fluorescence is determined by the size. Larger dots have more closely spaced energy levels in which the electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longer and thus these large dots shown a larger lifetime.
The ability to tune the size of quantum dots is advantageous for many applications. For instance, larger quantum dots have spectra shifted towards the red compared to smaller dots, and exhibit less pronounced quantum properties. Conversely the smaller particles allow one to take advantage of quantum properties.
It was in early 1980s that the first quantum dots were successfully made in a research laboratory. About five years earlier Leo Ascii et al. had demonstrated one dimensional confinement in quantum well structures. In less than a decade Aleksey Ekimov et al. obtained evidence for three dimensional confinement in quantum dots. The trick was to grow a semiconductor crystal of atomic dimensions. The construction of Q-dots took two separate paths. In one method the dot is constructed using lithography techniques of microchip manufacturing; creating one Q-dot at a time. In the second method chemical processes are used to grow Q-dots in bulk.
In large numbers, quantum dots may be synthesized by means of a colloidal synthesis. Colloidal synthesis is by far the cheapest and has the advantage of being able to occur at benchtop conditions. It is acknowledged to be the least toxic of all the different forms of synthesis.
Highly ordered arrays of quantum dots may also be self assembled by electrochemical techniques. A template is created by causing an ionic reaction at an electrolyte-metal interface which results in the spontaneous assembly of nanostructures, including quantum dots, on the metal which is then used as a mask for mesa-etching these nanostructures on a chosen substrate.
Another method is pyrolytic synthesis, which produces large numbers of quantum dots that self-assemble into preferential crystal sizes.
1:The first advantage of quantum dots is their tunable bandgap. It means that the wavelength at which they will absorb or emit radiation can be adjusted at will: the larger the size, the longer the wavelength of light absorbed and emitted
. Â The greater the bandgap of a solar cell semiconductor, the more energetic the photons absorbed, and the greater the output voltage. On the other hand, a lower bandgap results in the capture of more photons including those in the red end of the solar spectrum, resulting in a higher output of current but at a lower output voltage. Thus, there is an optimum bandgap that corresponds to the highest possible solar-electric energy conversion, and this can also be achieved by using a mixture of quantum dots of different sizes for harvesting the maximum proportion of the incident light.
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3:Another advantage of quantum dots is that in contrast to traditional semiconductor materials that are crystalline or rigid, quantum dots can be molded into a variety of different form, in sheets or three-dimensional arrays. They can easily be combined with organic polymers, dyes, or made into porous films ("Organic solar power", this series). In the colloidal form suspended in solution, they can be processed to create junctions on inexpensive substrates such as plastics, glass or metal sheets.
When quantum dots are formed into an ordered three-dimensional array, there will be strong electronic coupling between them so that excitons will have a longer life, facilitating the collection and transport of 'hot carriers' to generate electricity at high voltage. In addition, such an array makes it possible to generate multiple excitons from the absorption of a single photon (see later).
Quantum dots are offering the possibilities for improving the efficiency of solar cells in at least two respects, by extending the band gap of solar cells for harvesting more of the light in the solar spectrum, and by generating more charges from a single photon.
How Quantum Dots Work
A Special Class of Semiconductors
Quantum dots, also known as nanocrystals, are a special class of materials known as semiconductors, which are crystals composed of periodic groups of II-VI, III-V, or IV-VI materials. Semiconductors are a cornerstone of the modern electronics industry and make possible applications such as the Light Emitting Diode and personal computer. Semiconductors derive their great importance from the fact that their electrical conductivity can be greatly altered via an external stimulus (voltage, photon flux, etc), making semiconductors critical parts of many different kinds of electrical circuits and optical applications. Quantum dots are unique class of semiconductor because they are so small, ranging from 2-10 nanometers (10-50 atoms) in diameter. At these small sizes materials behave differently, giving quantum dots unprecedented tunability and enabling never before seen applications to science and technology.
The usefulness of quantum dots comes from their peak emission frequency's extreme sensitivity to both the dot's size and composition, which can be controlled using Evident Technologies' proprietary engineering techniques. This remarkable sensitivity is quantum mechanical in nature, and is explained as follows.
Artificial atoms or quantum dots (QDs) constructed from semiconductors are expected to provide the basis for future generations of device technologies such as threshold-less lasers and ultra-dense memories. The quantum dots can be induced by interface fluctuations (top of the figure) in a quantum well, self-assembled with the driving force being lattice mismatch (bottom) or formed with lithographic techniques. There are many material systems being explored in this advanced and rapidly changing field, but much of the physics is common to all of them. In particular, issues such as size, position, composition and strain fluctuations, interface roughness and the physics of energy relaxation are often cited as limiting the application of low-dimensional structures. To go forward these issues must be systematically addressed.
However, to do this requires not only great control and knowledge of the growth process at the atomic level, but also advanced measurement techniques and directed theoretical modeling. In the Electronic Materials Branch and the Microwave Technology Branch there have been significant advances in MBE growth and understanding of new, ultra small, high quality, semiconductor structures. For example, techniques have been developed to grow, measure and calculate the properties of quantum wells, quantum wires and quantum dots in a variety of semiconductor materials. Aperture nanoscopic spectroscopes invented in the Electronic Materials Branch and fabricated by the Nanoelectronic Processing Facility can now be performed on individual ultra small semiconductor structures. In addition, structural imaging at the atomic level with proximal probes within the growth environment of the Epicenter is now routine, and self-organized ten nanometer quantum dots have been grown by MBE. Coupled with these experimental advances, the analytical machinery for modeling electronic, optical and vibrational properties of nanoscopic semiconductor structures has been developed.
In the past two years we have made a number of unexpected discoveries arising from the breakthrough of single quantum dot spectroscopy based on ultra high resolution techniques. Examples include the observation of excited states of the artificial atoms (E1-E4), the discovery of fine structure splitting (see inset in figure), strong hyperfine interactions and the observation of random telegraph signals in the spectral intensities. Similar to the scientific developments that occurred during the early days of atomic spectroscopy, these observations are forcing the development of new theoretical concepts. If we can further develop this initial conceptual understanding, we will have methods to determine directly the magnitude, size, shape, and orientation of the quantum potentials that are determining the optical, electronic and vibrational properties of QDs with extraordinary selectivity and sensitivity. Such a complete nanoscopic picture will put us in position to attack a number of important fundamental problems that have already been identified, and no doubt some that have not yet been recognized. One example is the relaxation bottleneck problem that is predicted to be caused by a size-induced decoupling of the vibrational and electronic motions. Relaxation bottlenecks are often cited as a fundamental problem that will limit the application of quantum dots in such devices as lasers, yet there is not a good understanding of such bottlenecks in real systems.
In this project, we are exploring properties of nanostructure semiconductor materials down to the atomic level using the advanced state of the art growth, measurement and theoretical techniques. The structural, electronic, vibrational and optical properties of nanostructure materials, self-assembled in the MBE growth process, are measured in situ in the Epicenter using RHEED and scanning probe microscopy (see AFM image of self-assembled quantum dots of GaSb on GaAs), and ex situ using high spatial and spectral resolution spectroscopies such as optical-near-field PL, Raman and NMR. Particular emphasis is placed on defining novel electronic structures that may have potential device applications, exploring their properties, and laying the scientific groundwork necessary for recognizing and assessing future opportunities and limitations.
Quantum dot-biotin conjugates:
Biotinylated, highly luminescent CdSe-ZnS quantum dot (QD) conjugates were prepared and used in immunofiltration assays. Water-soluble quantum dot surfaces having a homogeneous negative charge density at basic pH were initially coated with a two-domain recombinant maltose-binding protein appended with a positively charged leucine zipper. Biotin fictionalization of these electro statically stabilized QD-protein complexes was then carried out using amine-reactive NHS biotin. These protein-coated biotin-functionalized quantum dot conjugates were incorporated into flow immunofiltration/displacement assays employing Affi-gel agarose resin for antibody immobilization, analyte capture, and immune complex formation with a second biotinylated antibody. A key component of the assay was the use of tetranitromethane-modified NeutrAvidin, used to link the biotinylated QDs to the immune complexes and facilitate their release at basic pH for subsequent quantification. This assay methodology was used to detect as little as 10 ng/mL staphylococcal enterotoxin type-B.
Quantum Dot Applications:
Quantum Dots - A Versatile Research Tool in Life Science
EviTag Quantum Dot Labels and EviFluor Quantum Dot Conjugates are ideally suited for life science research applications and can be used to displace many of the applications currently accomplished via organic dyes and fluorophores.
Evident Technologies has the highest quality lipid coated quantum dots EviTags available for life science research applications. We also offer quantum dots conjugated to secondary antibodies and proteins EviFluors.
At Evident, we've developed protocols for the most common life science research applications for use with our quantum dots, including:
Advantages of Quantum Dot Cell Staining
EviTag Quantum Dot Labels and EviFluor quantum dot conjugates have been successfully introduced into live, fixed and permeabilized cells. They have been used to preferentially stain the cytoskeleton, centromeres and kinetocores, and other organelles as well as track the evolution of growth of frog and nematode embryos. Due to the unprecedented photostability and color multiplexing properties, continuous observations using fluorescence and confocal microscopes of multiple cellular components and cellular metabolism/transport can be made over a period of hours to days.
Quantum Dot Live Cell Uptake
The use of quantum dots as intracellular probes has great potential. Their high resistance to photobleaching allows them to remain bright for much longer than traditional organic fluorophores.
In vivo Imaging:
Non-Cadmium Quantum Dots for In Vivo Imaging
Quantum Dot Labels have been introduced into large multicellular animals (mice) to preferentially stain the vascular and lymphatic systems, tumors etc. The photo stability and ability to engineer quantum dot surfaces for injection into live animals opens the door to many in vivo studies and diagnostic applications. The quantum dots proved to be nontoxic and easily cleared by the kidneys and liver of the test organism.
Micro arrays & DNA/RNA Assays:
Quantum Dots in Cancer Protein Micro arrays with EviTags
Evident Technologies teamed with Protea Biosciences, Inc. (Morgantown, WV) to bridge the nano- and bio-technology worlds. Protea's mission is to create a leading Proteomics business by developing and commercializing West Virginia University - based proteomics technology and applying it to the discovery of new, disease-specific protein targets for use in the development of new pharmaceuticals and biomarkers for the improved management of cancer and other human disease.
Quantum Dot Immunoassays
quantum dots conjugated to primary antibodies have been used to preferentially stain the surface of pathogenic E. Coli vs. nonpathogenic E. Coli and observed through microscopes and other instruments. Accurate, multiplexed assays to determine the presence of dangerous pathogens can be performed. Inexpensive detection platforms can be designed with far fewer laser and optical components because of the unique absorption and emission characteristics of quantum dots.
Quantum Dots as FRET Donors
EviTags quantum dots are very efficient FRET donors with organic fluorophores, due to the large overlap between the quantum dot emission wavelength and the absorption spectra of the dyes. Because the emission characteristics of the EviTags can be continuously tuned, it is possible to create a FRET donor for any number of organic dyes that emit between approximately 510nm and 640nm.
Advantages of Quantum Dot Flow Cytometry
Quantum Dot Conjugates are ideally suited for flow cytometry applications. The number of emission colors made possible by quantum dots, as well as the possibility of eliminating multiple lasers sources of conventional flow cytometers and replacing with simplified LEDs and optics, greatly increase throughput and will reduce the costs of next generation flow cytometer systems.
Quantum dots, which range from about 2 to 10 nanometers across (roughly equivalent to a medium-sized protein), have distinct advantages over conventional fluorescent dyes. By simply varying the crystal size, scientists can produce dots that emit light in a wide range of wavelengths, or colors, that are less prone to overlap than those of organic dyes. And whereas each organic dye must be excited with a specific wavelength of light, a single light source can excite quantum dots of many colors, so scientists can use the dots to label and detect multiple targets simultaneously. In addition to this "multiplexing" capability, quantum dots are much brighter than organic dyes and retain their glow much longer.
There are many potential cancer-related applications, said Min Song, Ph.D., a program director at the National Cancer Institute. They include sensitive in vitro diagnostic tests; high-throughput, multiplex analysis of gene and protein expression in clinical specimens; and long-term observation of biomolecules during malignant transformation of cells in culture.
Until about 5 years ago, quantum dots were not suitable for biological applications. The dots, as synthesized, have a water-repelling outer layer that makes them insoluble in watery biological milieus. A key advance came in September 1998, when two research groups reported in Science that they had not only prepared water-soluble quantum dots but also had devised ways to conjugate, or chemically join, the dots to biological molecules such as antibodies so they could be used to label specific biological targets. To make the dots water soluble and permit conjugation to biomolecules, researchers either replace the dots' hydrophobic outer layer with a different substance or coat the dots with an additional layer of material.
But many technical limitations remained. The first generation of water-soluble quantum dots fluoresced weakly in biological environments and tended to clump together and stick to things other than their intended targets. Some were toxic to living cells
These breast cancer cells are labeled with a quantum dot conjugate. The breast cancer marker Her2 was detected on the cells with a combination of the drug Herceptin, biotinylated goat anti-human immunoglobulin G, and quantum dots conjugated to streptavidin.
Quantum dots detect viral infections:
In what may be one of the first medical uses of nanotechnology, a chemist and a doctor who specializes in infectious childhood diseases have joined forces to create an early detection method for a respiratory virus that is the most common cause of hospitalization among children under five.
Respiratory syncytial virus (RSV) sends about 120,000 children to the hospital in the United States each year. Although it is only life-threatening in one case out of every 100, it infects virtually all children by the time they are five. Few children in the U.S. die from RSV, but it also attacks the elderly, causing some 17,000 to 18,000 deaths annually. Individuals with impaired immune systems are another highly susceptible group. Vanderbilt researchers report that not only can a quantum dot system detect the presence of RSV particles in a matter of hours, rather than the two to five days required by current tests, but it is also more sensitive, allowing it to detect the virus earlier in the course of an infection
Current methods of detecting the virus can take from two to six days, postponing effective treatment. The new, high-tech method uses multi-colored, microscopic fluorescent beads, called quantum dots, which bind to molecular structures that are unique to the virus's coat and the cells that it infects. In a paper appearing in the June issue of the journal Nanoletters, the Vanderbilt researchers report that not only can a quantum dot system detect the presence of particles of the respiratory syncytial virus (RSV) in a matter of hours, rather than the two to five days required by current tests, but it is also more sensitive, allowing it to detect the virus earlier in the course of an infection.
"The problem with current detection technologies is that they take too long," says Professor of Pediatrics James E. Crowe, Jr. who collaborated with Associate Professor of Chemistry David W. Wright in the development. "When a patient with a respiratory illness comes in to the doctor, emergency room or clinic, some times their symptoms are caused by bacteria and some times they are caused by viruses. There are specific medicines to treat some viral infections and there are definitely antibiotics to treat bacteria. Yet current detection tests take up to five days to tell you if a virus is present and another day or so to tell you which virus it is."
Crowe lists three potential benefits for such an early detection system. It can:
-- Increase the proper use of antiviral medicines. Although such medicines have been developed for some respiratory viruses, they are not used often as therapy because they are only effective if given early in the course of infection. By the time current tests identify the virus, it is generally too late for them to work.
-- Reduce the inappropriate use of antibiotics: Currently, doctors often prescribe antibiotics for respiratory illnesses. However, antibiotics combat respiratory illness caused by bacteria and are ineffective on viral infections. An early virus detection method would reduce the frequency with which doctors prescribe antibiotics for viral infections inappropriately, thereby reducing unnecessary antibiotic side-effects and cutting down on the development of antibiotic-resistance in bacteria.
-- Allow hospital personnel to isolate RSV patients: RSV is extremely infectious so early detection would allow hospital personnel to keep the RSV patients separate from other patients who are especially susceptible to infection, such as those undergoing bone-marrow transplants.
Quantum dots are available in a dozen different colors, and antibodies specific to the other four respiratory viruses have been identified and can be used as linker molecules. Such a test would be able to diagnose more than 90 percent of all the cases of viral respiratory infection, he says.
The existence of such a test could encourage the development of improved therapies for respiratory viruses, Crowe says. Without a good diagnostic for a specific viral infection, drug companies don't have much motivation to develop effective treatments because doctors are unlikely to prescribe them very often.
Currently, there are three diagnostic tests available for identifying respiratory viruses like RSV. The "gold standard" involves incubating an infected sample in a tissue culture for five days and then using a fluorescent dye to test for the presence of the virus. The main problem with this technique is that the virus is multiplying in the patient at the same time as it is growing in the culture.
This has caused many hospitals to switch to a technique called real time PCR, which is extremely sensitive but still takes 36 to 48 hours because of the need for a highly trained molecular biologist to conduct the test in a reference laboratory. There is also a third method, called the antigen test, which only takes 30 minutes. However, it is not sensitive enough to detect the presence of the virus at the early stages of an infection.
Quantum dot DNA test:
Indiana University researchers have shown how to identify tens of thousands of genes all at once by using tiny semiconductor crystals that dazzle in ultraviolet light.
The technique works like a bar code with each color and intensity combination corresponding to an individual gene. The researchers predict that up to 40,000 genes or proteins could be studied in as little as 10 minutes.
Competing technologies include the lab-on-a-chip, or biochip, in which miniature DNA-decoding troughs are etched onto flat surfaces. These devices can take as long as 24 hours to identify a group of genes.
Researchers have tried for years to use tiny crystals, called quantum dots, as glowing labels for genes, proteins, and other biological molecules. Quantum dots promise faster, more flexible, less costly tests for on-the-spot biological analysis or patient diagnosis. But they have been difficult to collect and manipulate with enough precision to be useful.
Quantum dots display a rainbow of colors. Each dot is made from semiconductor crystals of cadmium selenide encased in a zinc sulfide shell as small as 1 nanometer in diameter (one millionth of a millimeter). In ultraviolet light, each dot radiates a brilliant color.
To capture the quantum dots in specific quantities and in a wide range of colors and various intensities. Using six colors, each with 10 intensity levels, it would be possible to code for 1 million genes. But the group said that for accurate detection without any spectral overlap, a reasonable range would be 10,000 to 40,000 different codes.
To capture the quantum dots, they made porous microbeads of polystyrenes (which is used to make Styrofoam brand plastic foam) and seeded these with the zinc sulfide-capped cadmium selenide nanocrystals. They made both the beads and the quantum dots water repellent. This encouraged the quantum dots to move into the pores.
To demonstrate the use of these quantum dots in DNA analysis, the researchers prepared microbeads of three colors, or spectral wavelengths, and attached them to strips of genetic material. Each color corresponded with a specific DNA sequence. These were used as probes to seek out complementary pieces of genetic material in a DNA mixture.
Among the advantages of the quantum dot system is its flexibility. If you want to add a new gene code to the test, you mix a new batch of beads. This takes about half an hour. Adding a new gene to a DNA chip means going back to the manufacturing plant to design and fabricate a new chip.
Nanoparticles which are 1o,ooo times smaller than human hair have unique quantum properties that is change in color according to minute difference in size. biological q dots are collection of nanoparticlas embedded in tiny beads made up of polymer materials.
Because q dots have a cadmium core scientist have concerned about their potential toxicity if infused into the blood stream of patient. Using them in laboratories to detect biomarker in the cell and tissue outside the human body eliminate this concern.
Q dots also have advantage over traditional dyes and stains often used in imaging. they are more brightly fluorescent, they resist photo bleaching and they can emit a broad rang of colors simultaneously.
The properties make biconjugate q dots more useful in laboratories in diagnosis of diseases. They are particularly useful in detecting biomarkers of cancer, RNA, proteins, carbohydrets etc.
The semiconductor material Gallium-Arsenide (GaAs) is a compound semiconductor made of gallium (Ga) and arsenic (As). It has a band-gap of 1.43 eV, can be combined with impurities to create number of variations in its semiconductor properties, and is very efficient in absorption/emission of light. It is therefore very much favored in optoelectronic applications (this is in contrast with silicone based semiconductors - with a band gap of 1.1 eV - that are commonly used in electronic applications). Addition of impurities such as aluminum (Al) and phosphorous (P) to GaAs allows for creation of regions with higher or lower electron or hole densities. Fabrication of light emitting semiconductors uses lithography to create layers of these materials in a design that converts electric energy supplied to its electrodes into light. In the case of semiconductor lasers, such as the ones used in CD players, presence of impurities in the semiconductor creates energy levels between the semiconductors energy bands. These energy levels then play a similar role in making lasing transitions possible.
A typical laser diode has dimensions of few to one hundred micrometers (see pages on Quantum Application). Therefore optical lithography can be successfully used to create such structures. In the case of Q-dot diode lasers that require engineering at the level of afew nanometers, optical lithography cannot provide necessary resolution.
One of the methods of obtaining higher resolution is to employ shorter wavelengths by using X-ray or particle beams (electrons or ions, for example). In one method of Q-dots fabrication a layer of only a few nm in thickness (of say GaAs), is deposited on a bulk semiconductor material (AlGaAs). This layer is covered with another bulk layer of the semiconductor (say AlGaAs ) thus creating a Q-well. Then through a series of steps involving electron beam etching and chemical removal using solvents a Q-dot is created on a tower of bulk semiconductor. By using a mask this process can be repeated over and over to fabricate an array of Q-dot. An alternative to particle beam removal is to deposit gates and electrodes, in nm scale of course, to use electric fields to confine electrons in the Q-well to nm by nm areas and thus create regions of Q-dots on the Q-well layer. One advantage of this technique is that by controlling the gate potentials one could affect the Q-dot's size.
Arrays of Q-dot lasers are far more efficient than standard heterojunction diode lasers. But the primary advantage of Q-well and Q-dot lasers is that by adjusting dimensions the output wavelength can be controlled in a range that is not possible using bulk materials only.
Fluorescence Tagging Applications:
By far the most common application of Q-dots is in fluorescence tagging, where they replace molecular dyes. These range from biological to environmental applications. In these applications when the tagging substance is irradiated with light it absorbs the light and then re-emits it at a different wavelength. The emission spectrum then is the tell-tale sign for the presence of the tagging substance. For example, injecting a tagging substance into a particular biological cell makes it possible to identify that cell amid other cells from its fluorescence.
There are several reasons that make Q-dots more advantageous to molecular competition. The first of these is that Q-dots can absorb a wide band of light for their excitation, but they emit in a very narrow band. In contrast, most molecular dyes can absorb only a very narrow band of wavelength, so most of the illuminating light is not used. Also, these molecules emit is a much wider band of wavelengths. As a result, to distinguish separate features one needs to use very different molecular dyes, each with its own required excitation wavelengths. In the case of Q-dots their emission wavelength depends on their size. So, Q-dots made of the same semiconductor material, but of different sizes all can be excited by the same light source, but then they emit distinctly different wavelengths.
Another feature of Q-dots that makes them a good candidate for tagging purposes is that their tagging property is controlable - with proper chemistry these objects can be attached to "molecules with a purpose". This is in contrast to traditionally used molecular tags have well defined binding characteristics. As a result a particular molecular tag may or may not bind with a given molecule or surface. Since Q-dots have a surface that could bind with a variety of molecules, they could be prepared (funtionalized) so as to attach to well defined targets, even at the molecular level.
A third feature of Q-dots that makes them desirable for tagging is due to their nonlinear optical behavior. Through freqency-doubling nonlinear optical materials can absorb two photons of longer wavelength and create a single photon of lower wavelength. But most optical materials have a small non-linearity. Q-dots are made with large nonlinear properties that have allowed researchers to employ them for deep noninvasive imaging. This is done by focusing long wavelength laser light that is not absorbed by tissue beneath the skin of a rat injected with Q-dots in the tail blood vessels. As the dots reach the focal point of the illuminating laser they frequency-double the laser radiation (absorb two of the long wavelength photons simultaneously) and emit light well into the visible. This "fluorescence" allows the imaging of the blood vessels/tumors without opening the tissue.
One of the drawbacks of Q-dots for medical applications is that their safe use in biological environments is not well understood. Many of the substances, such as arsenic, used in manufacturing semiconductors are toxic. Even though the Q-dots can be made with a "protective coating", the long term integrity of the coating and its overall effect on biological environments it is not well understood at this time. However, the high sensitivity of Q-dots as fluorescent tags often require such small quantities for use in most applications that the issue of safety may not be a major concern.
Manufacturing of Q-dots for fluorescence is very different than their production for optoelectronics applications. Q-dots used for tagging are prepared from nucleation in oxides or by thermodynamically controlled precipitation in a vapor phase. These dots are manufactured in a colloidal suspension that could then be made into other forms, such as thin films or coatings. One common nanocrystal Q-dot is made of a core of cadmium sellenide (CdSe), few nanometers in diameter, followed by a shell of ZnS. Because the core surface is not perfectly uniform, dangling bond at this surface tend to quench optical transitions. The shell's "protective" layer therefore enhances the fluorescence by more than an order of magnitude. In addition, this coating happens to also increase the broad-band absorption characteristic of the Q-dot.
Quantum Dot Products:
Core & core-shell quantum dots. EviDots are available as core quantum dots in their fundamental state, or enhanced with our proprietary coating technologies as core-shell semiconductor nanocrystal quantum dots. EviDots are available in wavelengths ranging from 490nm - 2100nm.
Quantum dot composites. EviComposites use the properties of Evident's proprietary EviDot quantum dots as well as common insulating polymer matrix materials.
Water soluble quantum dots. EviTags are conjugation-ready with a bio-active surface. Carboxyl or amine functionalized dots are available in wavelengths ranging from 490nm - 680nm.
Water soluble quantum dots conjugated to antibodies and proteins. EviFluors are ready-to-use high quality, activated quantum dots coupled to secondary antibodies and proteins. Goat anti-Mouse, Goat anti-Rabbit, Goat anti-Rat, Streptavidin, and Biotin conjugated quantum dots are available in wavelengths ranging from 520nm - 680nm.