What are quantum dots?

Quantum dots are semiconductor crystals built on the size of only several nanometers. They are actually a collection of atoms arranged in usually a circular or elliptical manner that confines electrons and holes within a central area. The electrons and holes are confined in way that they behave as though they existed in zero dimensions, and the quantum dot behaves as if it were a singular atom and is sometimes called an "artificial atom". Quantum dots possess rather remarkable physical properties (especially their optical ones) that make them useful in numerous fields: electronics, quantum computing, solar cells, and of course medical imaging.

How they work

Quantum dots can be used to enhance a process known as fluoroscopy. Fluoroscopy produces real-time images of internal structures of the body by using a constant input of x rays. It is often used for procedures when constant feedback is required. To image anything from a section of tissue down to a few molecules of drug agents, these things must first be somehow tagged or stained.

Different quantum dots respond to different electromagnetic radiation (light, x-rays) wavelengths depending on their size. When quantum dots are excite by such radiation stimuli, they exhibit luminescence that is several times stronger than that of conventional dyes. To image internal body parts such as cells, drug particles, or larger tissues, quantum dots must be attached to these things. To do this, researchers attach an antibody to the quantum dot that bonds to some protein on the target structure or attach it using a peptide bond. Once this is done, infrared light or other radiation can be shone onto the site. The quantum dots absorb and re-emit light that can penetrate flesh and (though invisible to the eye) image onto a screen. [19]

Fabrication

The new technique developed by a team led by T.J. Mountziaris, Ph.D., professor of chemical and biological engineering in the University of Buffalo School of Engineering and Applied Sciences, enables precise control of particle size by using a microemulsion template formed by through self-assembly. The process involves the direct mixing of a nonpolar substance (heptane), a polar substance (formamide) and a surfactant to form a uniform dispersion of heptane droplets in formamide, stabilized by the surfactant.

Using the technique, the UB researchers were able to synthesize zinc selenide quantum dots. When excited by ultraviolet light, the quantum dots emit a particular fluorescent color and brightness, depending on the dot’s size. The problem for scientists has been devising simple techniques to control the size of quantum dots, which would give them the ability to control a quantum dot’s color properties. Such control is a critical factor in the quantum dot’s functionality. [18]
 

Applications

Cancer: Detection and Monitoring

When doctors are diagnosing a tumor, they want to know if it has spread to other regions of the body. To do this, they focus on the lymph nodes, an interconnected network of bean-shaped structures that filter the fluid around our cells and fight against disease. Doctors must surgically remove the specific lymph node connected to the tumor, but lymph nodes are often bunched together, making the task complicated. After the operation, a pathologist examines the lymph node carefully for evidence that the cancer cells have started to spread.

Research teams from the Beth Israel Deaconess Medical Center and the Harvard Medical School have attached quantum dots to the lymph nodes and used a combination imaging technique to see the tissues surrounding the lymph node and the fluorescing dots with the node. The imaging involves an infrared camera juxtaposed with a visible light camera, which makes the quantum dots stand out tremendously. In a trial surgery on a pig, this technique showed the surgeon exactly where to cut and which lymph node to remove for examination to see if a cancer had spread. [22]

Benefits and Molecular Tracking

New nanoscale materials have increased intensity of fluorescent light emission when illuminated with excitation radiation, have longer lifetimes for fluorescing, and provide a much broader spectrum of excited colors than that obtainable with conventional materials. Quantum dots allow for efficient multicolor imaging of biological samples and should be especially useful for fluorescence imaging in living tissues. Researchers at UCLA have and observed with high-sensitivity imaging cameras a single protein tagged with a fluorescent quantum dot inside a living cell. The imaging was produced in three dimensions and within a few nanometers of accuracy. [22]

In biological samples it is often difficult to track the movements of individual molecules. For medical purposes, a technique to do this would allow for the imaging of how a drug particle spreads throughout the body or to track actual cells. Quantum dots aid in this pursuit because they fluoresce brighter and last longer with the body. Experiments have shown that with appropriate surface coatings the quantum dots may be made stable for internal imaging for periods up to four months. Cells tagged with quantum dots may permit antigen-tracking for great lengths of time. [21]

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Copyright © 2005 Nanogroup Beta: Jason Feng, Maryam Liaqat, Eric Shubo Ma | Physics 87N: Prof. Hari Manoharan
Last modified: 12/09/05