Surface modified Mg-doped ZnO QDs for biological imaging

Nanocrystals of ZnO are currently attracting great interest as potential labels for biological applications, such as theranostic devices, due to their luminescent properties and low toxicity in vivo. It has been reported that doping with Mg2+ ions could enhance the luminescence of ZnO quantum dots (QDs). In the present study Mg-doped ZnO QDs were synthesized by a hydrolysis and condensation reaction. Surface modification of the QDs was performed using oleic acid (OA) to hinder their aggregation and to provide them colloidal stability in non-polar environments. Mg2+ ions could be incorporated into the ZnO wurtzite lattice owing to the very close values of the Mg2+ and Zn2+ ion radii. However, the dopant ions strongly influenced the growth and final size of ZnO nanocrystals, as evidenced by time-resolved synchrotron SAXS -measurements. The presence of Mg prevented the aggregation of the primary nanoparticles. Doping with Mg2+ ions widened the band gap of ZnO QDs and enhanced their visible luminescence. With increasing proportion of Mg2+ ions, both the absorption and emission spectra experienced a blue shift. The luminescence went through a maximum for a 20 mol% nominal concentration of Mg2+ ions in the reaction medium. The quantum yield (QY) of 20 mol% Mg-doped ZnO colloidal suspension (64%) was about 6 times higher than that of the ZnO suspension (10%). Mg-doped ZnO QDs capped by OA formed stable colloidal dispersions in chloroforme, with strong visible fluorescence (QY = 38%), promising for biological imaging.