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Conjugating Quantum Dots to surface-functionalized magnetic microspheres

Researchers at the Shanghai Key Laboratory of Tuberculosis have improved upon a tool for detecting mycobacterium tuberculosis (MTB) in human samples of deep-lung mucus. The tool combines magnetic, fluorescent, and immunologic sorting techniques to increase test sensitivity and portability. Tuberculosis-specific antibodies and proteins are conjugated to the surfaces of quantum dots (QDs) and magnetic microspheres (MMSs). In this functionalized state the quantum dots and microspheres can freely bind to the mycobacterium. At optimal concentrations, along with sufficient incubation time, the QDs and MMSs each serve as a slice of bread in a QD-MTP-MMS sandwich. The sample is magnetically sorted to collect the MMSs before quantifying the amount of QD fluorescence with a spectrofluorometer.

The presence of a strong fluorescent signal in a sample after magnetic separation indicates the presence of bacteria in a ternary complex with QDs and MMSs. The authors of this paper optimized the specific antibodies and proteins used, the concentrations, and the incubation times needed to form clear limits of detection. As a result, the authors created an accurate tuberculosis test where all six samples taken from infected patients tested positive, and the two control samples tested negative.

More about quantum dots

First discovered in 1981 by Alexey Ekimov, quantum dots are nanocrystals made of semiconducting materials. These nanocrystals are sharply fluorescent at wavelengths determined by their size. Semiconducting materials are defined by the presence of electron-hole pairs known as excitons. The distance between the electron and the hole is called a band gap. The band gap represents the potential energy that is released as light when an excited electron relaxes back into a hole. Imagine water inside of a bottle where the surface of the water is where the electrons float, while the bottom of the bottle is where the holes line up. The bulk of the water represents the free space of the band gap. If you squeeze the sides of the water bottle the surface of the water rises and the distance between the electrons and the holes increases, which in turn increases the energy potential between them.

This is essentially what is happening in quantum dots; the “bottle” is squeezed by decreasing the size of the crystal enough that the potential energy of the band gap between electrons and holes increases. This is called quantum confinement, and is highly tunable. The amount of potential energy released when an electron drops down to a lower energy hole determines the color of light emitted. These colors are bright and distinguishable from biological autofluorescence because of their narrow wavelengths.

A potentially useful feature is that many different colors of QDs can be emitted at once from one incident wavelength of light. The only requirement is that the excitation wavelength is shorter than all of the emission wavelengths of the different QDs in a sample. This useful effect has already been capitalized upon to develop immunohistochemical QD stains currently on the market. Quantum dots have a bright future in biotechnology.

For more information about the development of this tuberculosis test, see the paper entitled “Detection of Mycobacterium tuberculosis based on H37Rv binding peptides using surface functionalized magnetic microspheres coupled with quantum dots—a nano detection method for Mycobacterium tuberculosis.” It was published December 17, 2014 in the International Journal of Nanomedicine.

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Lluis M. Martínez | SEPMAG Chief Scientific Officer

Founder of SEPMAG, Lluis holds a PhD in Magnetic Materials by the UAB. He has conducted research at German and Spanish academic institutions. Having worked in companies in Ireland, USA and Spain, he has more than 20 years of experience applying magnetic materials and sensors to industrial products and processes. He has filed several international patents on the field and co-authored more than 20 scientific papers, most of them on the subject of magnetic particle movement.

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