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Sensors & Transducers Journal, Vol. 113, Issue 2, February 2010, pp. 1-17

improvements required in sensor designing is low cost, small size, lower weight, greater sensitivity and better specificity all of which may be achieved through nanotechnology.

  • 3.

    Types of Biosensors

    • 3.1.

      Quantum Dots Biosensors

Quantum dots (QD) appears to be the most promising functional and reproducible nanostructures available in the nanotechnology research circle. Quantum dots are very small and the smallest objects that can be synthesized on the nanoscale. Its structure is like a small dot which suggests its name. Different kinds of quantum dots can be synthesized in the laboratory through different techniques and the shapes normally appear to be pyramids, cylinders, lens shapes, and spheres. It confines electrons in three dimensions. The total diameter of a quantum dot varies between 3-60 nm depending on its application. QD exhibits quantum confinement properties in all three dimensions, i.e., the electrons are not allowed to move freely around in any direction. As its behavior is similar to atoms, it is also called an ‘artificial atom’. This has a lot of important consequences for researchers. First of all, they exhibit quantized energy levels like an atom. For a particular incident radiation, for instance, a quantum dot will only emit certain specific spectra of light. The quantum theory also predicts that with the decreasing diameters of quantum dots, there will be a corresponding increase in energy of emitted light. From the solution of Schrödinger equation for an electron confined in an 1-dimentional box of length L, the energy difference between two successive levels En and En+1 can be given as, E = (2n+1) {h2/8mL2}. This equation shows that if the length of the box decreases the energy difference between the levels increases and for L   (E will be 0, i.e., the electrons are delocalized and there is no quantization). This particular emission property of QDs has huge applications in diagnostics. Quantum dots are already in use as markers that are inserted into patients’ body. These markers can be seen under medical scanners helping detection of biological processes as they occur.

Quantum dots can be fabricated with either top-down or bottom-up techniques. Top down techniques are very effective for generating a uniform distribution of diameters. This is important if it is desirable to create a large array of dots that will emit the same wavelength of light. The top down approaches like lithography are diffraction limited and cannot create dense networks of quantum dots. This approach inherently implies material damage and many quantum dots produced with these techniques have defects that reduce their effectiveness.

The commonly used methods for producing quantum dots are bottom-up approaches. This can be done either with chemical vapor deposition or molecular beam epitaxy on a highly mismatched substrate. By layering a desired material that does not fit properly with the lattice of the substrate, high strain occurs at the interface and that layer will start nucleating into small quantum dots. Bottom-up approaches are acceptable ways to create quantum dots in dense arrays that will self-assemble in an orderly manner. However, the uniformity of their size distribution is not as precise as that produced through top-down approach mainly because it's impossible to control their formation as strictly.

From early eighties, quantum dots are being deployed in nano-scale computing applications, where light is used to process information. However, this technology is now being used in medicine. The QD crystals are one ten-millionth of an inch in size and can be dissolved in water, which when illuminated, act as molecule-sized LEDs, and can be used as probes to track antibodies, viruses, proteins, or DNA within the human body. Biomolecules labeled with luminescent colloidal semiconductor quantum dots (QDs) have various applications to fluoro-immunoassays and biological imaging. Because of their small size, quantum dots can be used to visualize, measure, and track individual molecular events using fluorescence techniques, as they have the ability to visualize and track dynamic molecular

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