Quantum Dots to Detect Disease
Several nanomaterials, including nanowires, quantum dots, and iron oxide, are being investigated to create nano-sensors designed to detect disease. Functionalized quantum dots have been made to fluoresce when they encounter certain disease indicators, making them a promising material for these diagnostic nanosensors.
Quantum dots are semiconductor nanoparticles that glow a particular color when you expose them to light. The resulting color depends upon the size of the nanoparticle. When quantum dots are illuminated by UV light, some of the electrons receive enough energy to break free from the atoms.
This allows them to move around the nanoparticle in a conductance band in which electrons are free to move through a material and conduct electricity. When these electrons drop back into the outer orbit around an atom (called the valance band), they emit light. The color of that light depends on the energy difference between the conductance band and the valance band.
The gap between the valance band and the conductance band, which is present for all semiconductor materials, causes quantum dots to fluoresce. The smaller the nanoparticle, the higher the energy difference between the valance band and conductance band, which results in a deep blue color. For a larger nanoparticle, the energy difference is lower, which results in a reddish glow.
Many semiconductor substances can be used as quantum dots, such as silicon, cadmium selenide, cadmium sulfide, or indium arsenide. Nanoparticles of these or any other semiconductor substance have the properties of a quantum dot.
You can improve the fluorescence of quantum dots by coating them with another semiconductor material. This coating prevents the surface of the quantum dots from being oxidized, which degrades their capability to fluoresce.
For example, researchers have found that if they treat a quantum dot made of cadmium selenide (CdSe) with a coating containing zinc sulfide (ZnS), the glow of the quantum dots increases. However, because you also want the quantum dot to mix well with water (because blood is mostly water), a coating of a hydrophilic polymer is added on top of the ZnS.
The antibodies that researchers use to attach the quantum dot to diseased cells, such as those in a cancer tumor, are attached to this polymer layer.
The fact that quantum dots glow different colors depending on their size is convenient for diagnosing a blood sample to determine if disease indicators are present. Each size of quantum dot is attached to different antibodies.
When you place these quantum dots in a blood sample containing a molecule that indicates a particular disease, the quantum dots with the corresponding antibody attach to the protein or virus and form clusters. When you illuminate the solution with ultraviolet light, those clusters glow with the color of that size quantum dot, revealing which virus or other disease indicator is contained in the sample.