Quantum Dots

Fluorescent semiconductor nanoparticles have gained tremendous interest in research over the past decades. They show significant advantages compared to conventional fluorescent dyes.

Fluorescent Quantum Dots in the visible range These nanocrystals come always along with an inorganic core whose size and composition determines their optical properties. As this core is stable against environmental conditions like temperature, irradiation or air it features robust fluorescence properties. Stabilizing organic molecules around this inorganic core allows for solubility in organic solvents, like hexanes, toluene or chloroform. Exchanging these organic ligands on the outer sphere through water soluble molecules the dispersibility in aqueous media can be achieved without losing the original properties of the Quantum Dot. The absorption of light used for excitation in the case of nanoparticles is generally larger compared to fluorescent dyes too. Thus, the detection of these particles is possible at very low concentrations, even a single particle could be investigated by spectroscopic methods.

Semiconductor nanoparticles
Semiconductor nanoparticles are probably the most investigated system among fluorescent particles. The band-gap of these systems and with that the emission wavelength can be influenced by changing the size of the particle. Therefore, these systems are very interesting. The smaller a Quantum Dot is the bigger the band-gap and, respectively, the more short-wavelength the emitted light gets. This effect is called size quantization.
Through concerted decision on material composition and size the whole spectrum of visible light up to the IR region can be covered. Our CANdot® Series A and C are designed using this concept. The Series A covers the visible region of the spectra with emission wavelengths from 500 to 650 nm and the Series C has emission maxima in the IR region greater than 1000 nm.

Since the absorption increases from the emission maximum going to shorter wavelengths it is basically possible to excite the nanoparticles with every wavelength below the emission. It is thus not so that compared to organic fluorescent dyes the excitation wavelength has to be reset for each dye. With a single excitation wavelength it is therefore possible to excite a whole set of different nanoparticles at once and also to detect their emission.

In semiconductor nanocrystals an electron hole pair is generated after excitation, so after the absorption of light. Until recombination, which means the emission of light, this electron hole pair – called exciton – is nearly free to move within the core. During this time, usually about 10 to 20 ns, the charge carriers can be bound elsewhere and the emission intensity decrease. To avoid this effect the bare semiconductor nanoparticles (cores) were surrounded by passivating inorganic shells. These shells increase the stability of the particle and also the quantum yield of the particle system, e.g. in our Series A core/shell (CdSe/CdS) and core/shell/shell (CdSe/ZnSe/ZnS) systems.