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Nanoparticles as photocatalysts: understanding their interaction with light

Final Report Summary - NANOPHOCAT (Nanoparticles as photocatalysts: understanding their interaction with light)

NANOPHOCAT is based on the design of light-driven strategies using nanomaterials to perform photocatalytic processes such as oxidations, among which are the photosensitized oxidation by singlet oxygen, and coupling reactions. Gold nanoparticles (AuNPs) and upconversion nanoparticles (UCNPs) were chosen as the nanomaterials whose surface would be properly modified to exploit their interaction with light in photocatalytic processes.
Regarding UCNPS, the focus was on NaYF4 (Yb3+, Er3+), because of their comparatively higher emissive properties than other UCNPs. It was relevant to study new strategies to functionalize them, to enhance their emissive properties, and to determine the reproducibility of their size/geometry for different batches.
Texture and phase recognition analysis (TPRA) based on electron nano-diffraction technique is used to characterize the geometry of up-conversion nanocrystals (UCNCs) synthetized by the common thermal-decomposition protocol in presence of a stoichiometric amount of NH4F.
Texture and phase recognition analysis based on the electron nano-diffraction technique were used to characterize the geometry of UCNCs synthetized by a common thermal-decomposition protocol. These studies confirmed that despite the presence of apparently different shapes (hexagons, rods, and cubes) in the transmission electron microscopy images, all the nanocrystals were really similar. This is of relevance since many biological features of nanostructures, such as cellular internalization and cytotoxicity, are governed by their geometry. In addition, reproducibility in biological experiments is paramount.
UCNPs were capped with a thin polymer shell by multidentate thiolate-grafting of P(MEO2MA-co-SEMA) copolymers. The UCNP@P(MEO2MA-co-SEMA) nanohybrids exhibited a tenfold enhanced emission as compared with that of their hydrophobic precursor in dichloromethane and even in water (twofold). Moreover, their thermo-responsiveness was modulated by the pH. The formation of stable water-dispersible UCNPs with enhanced emission, together with their amphiphilic and temperature-responsive polymer coating, is promising for building multifunctional nanostructures for intracellular imaging, therapy and drug delivery. Furthermore, water-dispersible upconverting NaYF4 (Yb3+, Er3+) nanoparticles were decorated with different dyes to build nanohybrids able to produce singlet oxygen under near-infrared excitation. These nanohybrids proved efficient for inducing cancer cell death.
The switchability of colloidal oleylamine capped-AuNPs in the presence of an acetamidine–based surfactant was explored. AuNPs underwent reversible water/organic phase exchange by gas bubbling (either carbon dioxide or nitrogen, respectively), thus corroborating our hypothesis in the research proposal. This strategy will be extremely useful to recover the AuNPs after a (photo)catalytic reaction. The study has also been extended to quantum dots.
Ultraclean water-dispersible AuNPs have been synthesized by using a new protocol based on laser ablation. This is crucial since ligands tend to poison the (photo)catalyst and slow down or inhibit the reaction. These ultraclean AuNPs were capped with cucurbituril[7] (CB) in the absence of metallic cations and organic ligands. The nanohybrids encapsulated dissolved oxygen and showed an enhanced catalytic activity for the electrochemical reduction of dissolved O2 due to a cooperative effect between their components by fixing oxygen to the nanoparticle surface and increasing the local concentration of oxygen. Remarkably, sodium and ammonium cations stopped oxygen entering the CB cavity of NP@CB. The importance of removing ligands and polymers used for the preparation of metallic nanoparticles in catalysis has been highlighted when supporting the nanoparticles in solids but it has never been proved before for nanohybrids in solution. Therefore, we emphasize the importance of preparing AuNPs free of organic ligands and metal cations in order to design more efficient catalytic nanohybrids in solution.
The ultraclean AuNPs proved effective as colorimetric sensors of a tumor biomarker. A simple, fast, and highly selective and sensitive colorimetric assay of spermine in human urine was developed. It was able to detect nanomolar levels (healthy donors and cancer patients). This assay is based on the absence of a competitive organic capping on the AuNPs together with the high affinity of the amine groups of the analyte for the nanoparticle surface.
Finally, the capacity of AuNPs to act as photocatalysts has been tested in Prof. Scaiano´s group (University of Ottawa), thus consolidating a lasting co-operation with Canada (where the researcher was working before joining the Photochemistry Reactivity Group (Prof. Perez-Prieto´s group). Fifteen international peer-reviewed publications have been published during the period of the Marie Curie Reintegration Grant.
Dr. González-Béjar has established collaborations with groups at the Host Institution (2 groups), Universitat de Girona, Instituto de Ciencia y Tecnología de Polímeros, Universidad de Santiago de Chile and Stephenson Institute for Renewable Energy (The University of Liverpool). Dr. González-Béjar has been awarded with a Ramon y Cajal contract to establish her career at the Host Institution (The Institute of Molecular Science, ICMOL/ Department of Organic Chemistry; University of Valencia).
Dr. González-Béjar has also been an invited speaker at international conferences and Universities: 1ST ICMS: International Conference on Materials Science for Nanotechnolgy, Catalysis and Biomedicine (Chile-2011); Pontificia Universidad Católica de Chile (2011, 2014); Stephenson Institute for Renewable Energy, The University of Liverpool (2013); Universidad de Santiago de Chile (2014) and Instituto de Investigaciones en Fisico-Química de Córdoba (Universidad Nacional de Córdoba) (Argentina-2014). The level of independence the researcher has reached is in accordance with the possibilities that the Host Institution offers to researchers at this stage. Thus, she was the principal investigator in a project funded by University of Valencia (UV-INV-PRECOMP12-80490).