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Microtubules are hollow protein cylinders of 25 nm diameter which are implicated in cytokinetics and proliferation in all eukaryotic cells. Here we demonstrate in vivo how multiwalled carbon nanotubes (MWCNTs) interact with microtubules in human cancer cells (HeLa) blocking mitosis and leading to cell death by apoptosis. Our data suggest that, inside the cells, MWCNTs display microtubule biomimetic properties, assisting and enhancing noncentrosomal microtubule polymerization and stabilization. These features might be useful for developing a revolutionary generation of chemotherapeutic agents based on nanomaterials.

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Excitation into the Yb 3+ 2 F 5/2 excited states leads to visible-by-eye green luminescence spanning the spectral region from 490 to 790 nm, with a quadratic power dependence. Optical absorption, luminescence, and excitation spectroscopy as well as pulsed measurements on single-crystal SrCl 2 :Yb(1%), Tb(1%) are used to determine the upconversion (UC) properties of this system. The upconverted luminescence is easily detectable by eye from RT to 100 K, at which point the intensity drops significantly and a change in colour from green to blue is observed. Pulsed measurements coupled with excitation spectroscopy lead to the unambiguous assignment of a phononassisted cooperative sensitization mechanism as the dominant UC process for T > 50 K with VIS NIR photon ratios on the order of 10 −2 % for a laser power of 56 W mm −2. Below 50 K, the dominant UC emission becomes the wellknown Yb-Yb cooperative luminescence around 500 nm, with a consequential reduction of Tb 3+ emission by more than three orders of magnitude from RT to 10 K.

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The design of upconversion phosphors with higher quantum yield requires a deeper understanding of the detailed energy transfer and upconversion processes between active ions inside the material. Rate equations can model those processes by describing the populations of the energy levels of the ions as a function of time. However, this model presents some drawbacks: energy migration is assumed to be innitely fast, it does not determine the detailed interaction mechanism (multipolar or exchange), and it only provides the macroscopic averaged parameters of interaction. Hence, a rate equation model with the same parameters cannot correctly predict the time evolution of upconverted emission and power dependence under a wide range of concentrations of active ions. We present a model that combines information about the host material lattice, the concentration of active ions, and a microscopic rate equation system. The extent of energy migration is correctly taken into account because the energy transfer processes are described on the level of the individual ions. This model predicts the decay curves, concentration, and excitation power dependences of the emission. This detailed information can be used to predict the optimal concentration that results in the maximum upconverted emission.