Acid synthesis of luminescent amine-functionalized or erbium-doped silica spheres for biological applications

In this work we discuss and investigate the morphological and optical properties of luminescent silica spheres which can have interesting applications in bioimaging and biosensing. The spheres are synthesized following an acid route by the hydrolysis and condensation of tetraethylortosilicate (TEOS) and can be functionalized by incorporation of aminopropyl-triethoxysilane (APTES) during the synthesis, inducing a significant luminescence that can be attributed to a recombination mechanism from localized organic defects related to -NH2 groups. It is shown that the acid synthesis route produces very regular spherical particles, but their diameter vary in the range of 200-4,000 nm. The luminescence properties have been investigated and optimized by variation of the annealing temperature for the functionalized spheres, obtaining the most efficient PL emission after a thermal treatment of 1 h at 600 C in air. Moreover, the possibility to introduce rare earths like erbium in the spheres was also studied and the corresponding Er-3 luminescence emission at 1.53 mu m is reported in terms of intensity and lifetime, pointing out that erbium can be easily and efficiently incorporated during the acid synthesis giving high PL intensity with a good lifetime of 3.9 ms.