Martínez-Julca, Milton A.

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    Effect of the incorporation of defects in ZnO nanocrystals on the generation of reactive oxygen species for applications in photodynamic therapy
    (2015) Martínez-Julca, Milton A.; Perales-Pérez, Oscar J.; College of Arts and Sciences - Sciences; Fernández, Félix; Castellanos, Dorial; Department of Physics; Colon Reyes, Omar
    Photodynamic therapy (PDT) is an alternative to traditional cancer treatments. This approach involves the use of photosensitizer (PS) agents and their interaction with light. As a consequence, cytotoxic reactive oxygen species (ROS) are generated that will kill cancer cells. On the other hand, ZnO is a biocompatible and nontoxic material with the capability to generate ROS, specifically singlet oxygen (SO), which makes this material a promising candidate for PDT. Unfortunately up to now, the photo-excitation of ZnO-based requires the use of ultraviolet (UV) light, which limits their biomedical applications. Doping of ZnO is expected to induce defects in the host oxide structure and favor the formation of trap states that should affect the electronic transitions related to the generation of SO. Accordingly, the present work reports the effect of the type and level of dopant element (Lithium, Li, and Titanium, Ti species) on the ZnO structure and its capability to generate SO. Pure and doped ZnO nanoparticles were synthesized under size-controlled conditions using a modified version of the polyol method. These modified version was conducive to the synthesis of highly monodisperse nanoparticles with a particle size in the 87-140 nm range. Photoluminescence (PL) spectroscopy measurements of pure and doped ZnO nanoparticles, excited by UV light (345 nm), revealed the characteristics emission peaks for ZnO as well as other peaks associated to defect states in the band gap. The observed increase of the emission intensity of the emission peak corresponding to defect states, relative to the intensity of the main emission peak, was attributed to the promotion of trap states associated to interstitial Zn or to the substitution of Zn ions by dopant species. Use of a sensor green kit evidenced the enhancing effect of the dopant type and concentration on the capability of the ZnO-based nanoparticles to generate SO species under UV illumination. This dopant-dependence of SO generation was attributed to the enhancement of the concentration of trap states in the host ZnO, a fact that was supported by PL measurements. The two-photon excitation of ZnO-based nanoparticles would enable the use of visible or NIR light to excite this material, opening actual possibilities to expand their use to the biomedical field. As an attempt to verify the capability of ZnO-based nanoparticles to be used in 2-photon PDT, a preliminary work based on the two-photon fluorescence microscopy (TPFM) technique, using near-infrared light of 690nm, was carried out. The observed PL transitions will favor triplet states formation necessary to yield cytotoxic reactive oxygen species. Accordingly, the fact that ZnO-based nanoparticles can be excited by the 2-photon approach using a 690nm light would enable this nanomaterial to become cytotoxic to cancer cells via photo-induced ROS generation.