Ultrathin Amorphous Alumina Nanoparticles with Quantum-Confined Oxygen-Vacancy-Induced Blue Photoluminescence as Fluorescent Biological Labels

Ultrathin alumina nanoparticles (NPs) with an average size of less than 4 nm are produced from porous anodic alumina membranes. The alumina NPs in a suspension produce strong blue tunable photoluminescence (PL) with a high quantum efficiency of ∼15% and Stokes shift as large as 1.0 eV. An obvious blue-shift and diminished line width are observed after storing the suspension in air. The tunable blue PL which is closely related to the oxygen vacancy (OV) defect centers at different depths beneath the surface depends on the NP size. The experimental observations are corroborated by theoretical derivation demonstrating that the electron wave functions of the OV-induced defect levels are extended in space, and quantum confinement takes place when the alumina NP is smaller than the spread of the wave functions. It is thus possible to control the PL behavior by changing the NP size and OV depth distribution and the alumina NPs are experimentally demonstrated to be robust and nontoxic biological probes.