Nanoparticles for Biological Imaging
6 Feb, 2006 09:09 am
Luminescent nanoparticles give to biology attractive tools for the detection since nanoparticles used as a platform or as a container for fluorophores are able to enhance the signal detection and/or for long term study because of the high photostability of some nanoparticles (quantum dots).
To overcome these drawbacks, nanoparticles seems to be well-suited. Two main classes of nanoparticles for biological imaging can be distinguished. The first class is composed of nanoparticles which are rendered luminescent by their functionalization with organic fluorophores whereas the second one is constituted by intrinsically luminescent nanoparticles. Despite the aforementioned drawbacks of organic fluoropores, their incorporation onto or within nanoparticles is very interesting for lowering the detection threshold. In this case, nanoparticles behave as plateforms or containers (especially for vesicles) which are able to host a large amount of organic fluorophores immobilized on the particles surface or entrapped within the particles. The observation of biological substrates labelled by such particles are easier because there are detected by at least ten light emitting molecules instead of a few units when dyes are directly attached to the target.
Besides the signal amplification, the incorporation of the organic fluorophores in the nanoparticles leads to an improvement of their photostability. However it is not sufficient for a long term study. Such studies are very crucial for a better understanding of cell machinery. Up to now, the behaviour of cells has been monitored by the observation of several cells population at different time: their history was tailed on the basis of snapshots. Obviously, the reliability of this kind of scenario is scientifically questionable. The continuous observation of a cell colony would provide more reliable data. The recent development on the synthesis of semi-conducting nanoparticles (quantum dots QD) such as cadmium selenide core embedded in zinc sulphide layer (CdSe@ZnS) opened the door to the long term study of living cells. First the very small size (a few nanometers) facilitates the labelling of biological substrates. Moreover these luminescent particles are characterized by a high photostability and by a size-dependent wavelength. By increasing the diameter (1-5 nm), the wavelength of the light emitted by CdSe@ZnS nanoparticles shifts from blue to red. Wathever the color (i.e. the size of the QD) the emission band is narrow whereas the absorption band is very broad. It means that a single light source allows the excitation of a multicoloured mixture of QD whose each color can be obviously distinguished. Such particles are therefore very attractive for multicolour imaging and long term study. However they suffer from intermittent emission (the signal vanishes suddenly and reappears) : this feature can be detrimental for the observation. Moreover the toxicity of Cd2+ which can be released from the core of the QD must not be neglected. Recently a new class of highly photostable luminescent nanoparticles with narrow emission band is emerging. They are composed of lanthanide ions like Eu3+, Tb3+… Contrary to the QD, the emitted light (without intermittency) does not depend on the size (whose control at the atomic level is not trivial) but only on the nature of the rare earth ions.
In conclusion, luminescent nanoparticles give to biology attractive tools for the detection since nanoparticles used as a platform or as a container for fluorophores are able to enhance the signal detection and/or for long term study because of the high photostability of some nanoparticles (QD). Thanks to the nanoparticles the era of the photo-story of cell life is over and the quest to the detection of single molecule is not a pipe dream anymore.
