We have studied the growth behaviour of sub-10 nm NaYF₄ upconversion nanocrystals of the hexagonal β-phase and the cubic α-phase. Ostwald-ripening of such particles in oleic acid/octadecene solvent results in broadening of the particle size distribution if the colloid contains particles of one crystal phase only. Narrow size distributions are formed only if β-phase particles grow in the presence of an excess of α-phase particles. Such binary mixtures of α-phase and β-phase particles form intrinsically when colloids of α-phase particles are heated for a sufficiently long time, because seeds of the β-phase nucleate in the solution after some time at high temperatures. Since the number of seeds determines the final size of the β-phase product, control of the nucleation is crucial for controlling the final particle size. We show that the number of β-phase seeds strongly depends on the composition of the α-phase known to form solid solutions Na₁-xYF₄-x in the range from x = 0 to x = 4/9. Sodium-deficient α-phase particles form a negligible number of β-phase seeds whereas α-phase particles with high sodium content yield a very large number of seeds. By taking advantage of this dependence and modifying the synthesis of the α-phase particles accordingly, small phase-pure β-NaYF₄:Yb,Er particles with a size smaller than 6 nm can be prepared in oleic acid/octadecene just as well as much larger particles.

The origin of the narrow particle size distributions obtained in the oleic acid-based synthesis of hexagonal phase β-NaREF(4) nanocrystals (RE = Sm, Eu, Gd, Tb) has been investigated. Compared to the standard synthesis, the growth conditions were simplified by using small purified particles of either α-NaREF(4) (cubic phase) or β-NaREF(4) (hexagonal phase) as single-source precursors, thereby avoiding the complications arising from the simultaneous presence of molecular educts and intermediately formed small particles. The study shows that α-phase as well as β-phase particles grow by Ostwald-ripening but narrow particle size distributions of the β-NaREF(4) product particles are only obtained when α-phase precursor particles are employed. Since the small particles are also formed as intermediate products in the standard synthesis of β-NaSmF(4), β-NaEuF(4), β-NaGdF(4) and β-NaTbF(4) particles, their crystal phase is an important parameter to obtain a narrow size distribution in these systems.

Watering of Phalaenopsis and Arabidopsis plants with an aqueous colloidal solution of NaYF 4 :Yb,Er nanoparticles leads to uptake and transport of nanoparticles into the plants within a few days. Characteristic upconversion emission of the particles can be excited in the shoot and the leaves of the plants. Uptake of particles by the root was studied by confocal laser-scanning microscopy using excitation in the near-infrared. We demonstrated that nanoparticles are able to enter the stele of the plant root and thus the long distance transport system despite their relatively large size in comparison to the size of the structures responsible for the transport.

Doped nanocrystals of NaYF(4) and NaGdF(4) are currently studied as upconversion luminescence markers and magnetic resonance imaging contrast agents. An EPR investigation on the growth mechanism of NaYF(4):Gd and NaGdF(4) nanocrystals showed that these nanomaterials grow in the standard oleic acid-based reaction medium by a dissolution/recrystallization mechanism and not by the aggregation or oriented attachment of smaller particles.

We have investigated the kinetics of the uptake and the translocation of nanoparticles of different size in plants.