Biological applications of lanthanide doped nanomarkers

Abstract

Fluorescence is one of the most commonly used methods of biodetection, mainly due to the high sensitivity, non-invasiveness, simplicity, and also due to the availability of the whole range of powerful light sources, a wide range of photodetectors, and numerous and sensitive measuring methods. From the point of view of biodetection and bioimaging, the important characteristics of such fluorophores are large Stokes shift, narrow absorption/emission lines as well as stable and efficient luminescence. Traditional organic dyes applied in biology reveal very fast photobleaching and limited opportunities for simultaneous detection of many biomolecules, what stimulate development of new fluorescent markers. Fast and intensive development of nanotechnology and chemical engineering observed in recent years, aims at designing the nanophosphors or luminescent nanoplatforms, that demonstrate desirable properties and devised functionality. However, new phosphors are not included yet in broad practical applications, mainly because of the need to adapt the measuring apparatus so as to fully exploit their potential. From among the fluorescent nano-particles, silica dye doped nanoparticles, quantum dots, nanocolloidal metallic nanoparticles, and lanthanide doped nanoluminophores show the largest application potential. This article discusses the unique physico-chemical properties of lanthanide doped nanoparticles, which beside very long luminescence lifetimes and narrow emission bands, enable to obtain a visible emission under the near infrared photoexcitation (called anti-Stokes emission), offering improved sensitivity, stability, repeatability and accuracy of the fluorescent biodetection and bioimaging methods. In this review, physico-chemical properties of lanthanide doped nanoluminophores and many examples of their biological applications have been discussed. The first chapter presents spectral characteristics of rare-earth ions with particular regard to the mechanism of energy transfer and up-conversion, which is a fundamental difference and the decisive advantage compared with other known fluorescent markers. The luminescent properties of lantanides are demonstrated based on the most commonly used nanomaterials singly doped with Eu^3+ and Tb^3+ and the codoped matrices, like Yb^3+-Tm^3+, Yb^3+-Er^3+or Yb^3+-Ho^3+ co-doped phosphores. The features of these materials are best suited from the point of view of biodetection and bioimaging. The next chapter gives an overview of the applications of lanthanide doped nanoluminophores in biological sciences. Different types of hetero-/homo-genous tests and luminesce based sensors for pH, CO_2, the level of glucose, and other analytes are presented. Then, basic aspects of bioimaging, photodynamic and thermo-therapy, nanotermometry as well as nano-bio-technology platforms have been summarized. In conclusion suggestions of new research directions and new biological applications of lanthanide doped nanoparticles have been presented