Quantum Dots | Quantum Dots
Key Words: photobleaching, confocal, time-lapse imaging, Fluorescence, fluorophore, resolution, live cell imaging, immunofluorescence, environmental control, signal-to-noise, 3-D imaging
Definition:Quantum dots are nanometre-scale semiconductors used as fluorescent probes in microscopy (light and electron microscopy)
Quantum dots are nanocrystals composed of inorganic semiconductor materials, such as CdSE, InGaP, carbon and silicon, in which electron-hole pairs can be created and confined. When the quantum dots are exposed to light, electron-hole pairs are excited and fluoresce. The emission wavelength of the fluorescence is related to the size of the quantum dot - the larger the quantum dot the longer the emission wavelength (for the same excitation wavelength). During manufacture, the size of quantum dots can be determined with nanometre precision to create a series of quantum dots with distinct emission characteristics. A CdSe quantum dot of 3 nm, for example, emits at 520 nm (green) while one of 5.5 nm emits at 630 nm (red). Quantum dots are usually coated in zinc sulphite to improve fluorescence intensity (by further confining the electron-hole pairs) and then covered in a water-soluble, biocompatible shell to enable uptake and retention by cells.
Quantum-dots provide greater sensitivity in live cell imaging compared with conventional organic fluorophores: imaging can be 1,000 times brighter. Quantum dots also have the advantage that they do not photobleach on exposure to light. Additionally, quantum dots of different sizes can be excited by a single wavelength light source, allowing simultaneous detection and multicolour imaging.
Quantum dots emitting fluorescence at different wavelengths can be targeted to selected cellular components using antibodies. They provide a powerful means of visualising multiple cellular components over time - for example in tracking cells and visualising structures deep in tissues. In combination with confocal imaging, quantum dots enable high resolution, 3-D imaging inside living tissues. The exceptionally bright fluorescence of quantum dots enables single quantum dots to be observed and monitored over time.
As with other fluorescent techniques, quantum dot fluorescence can be observed with widefield and confocal fluorescence microscopes, either in the upright or inverted configuration. Confocal imaging is ideally suited to quantum dots imaging, enabling high resolution 3-D imaging inside tissues and cells. AOM / AOTF technologies can help to target excitation power to defined areas of interest.
Ti series inverted microscopes with environmental controls and confocal C1, C1si or LiveScan Sweptfield systems. The C1si spectral imaging system enables separation of overlapping spectra. Nikon's Ti series inverted microscopes include noise-terminator technology to improve fluorescence signal-to-noise ratios. A number of specialised fluorescence objectives are available (Plan Fluor, Super Fluor, Plan Apochromat, Plan Apochromat VC).