Surface Engineering of Lanthanide-Doped Nanoparticles for Assays, Sensing and Theranostic Applications

This thesis presents the design, synthesis, surface modification and characterization of lanthanide doped nanoparticles for bioanalytical applications in assays, sensing and theranostic approaches. In Chapter 1 nanoparticles are classified regarding their properties. Lanthanide doped nanoparticles are introduced as an interesting alternative to commercial dyes. The mechanism of downconversion and upconversion are explained in detail. Several strategies are introduced to enhance the luminescence properties of upconverting nanoparticles in aqueous solutions. In Chapter 2 the aim of the work is described as the identification and preparation of lanthanide doped nanoparticles for (bio)analytical applications considering their unique optical properties. For sensing, assays or theranostics different properties of the nanomaterial are required, additionally intelligent surface engineering may be necessary. GdVO4 nanoparticles doped with europium are studied as a potential luminescent probe for hydrogen peroxide in Chapter 3. The sensitivity to H2O2 is depended on the europium concentration inside the nanocrystals. Higher europium concentrations (>10%) lead to lower luminescent quantum yields, but higher sensitivities. GdVO4:Eu (50%) are identified as the efficient luminescent probe for H2O2 with a dynamic range from 5 to 250 µM and with a limit of detection of 1.6 µM. Selectivity tests show no significant changes in luminescence for physiological relevant ions (Ca2+, Mg2+, Zn2+). The ability to monitor enzyme-based reaction is proven by glucose oxidase catalyzed detection of glucose. The applicability of the fluorescent nanoparticles is illustrated by the determination of glucose in fetal bovine serum by the standard addition method. Here, good accordance with a commercial sensor can be demonstrated. In Chapter 4 functionalization aspects of water dispersible upconversion nanoparticles are discussed. After a short introduction the three main synthesis strategies for upconverting nanoparticles are introduced: thermolysis, Ostwald ripening and hydrothermal strategy. The necessity of surface modifications is explained and ligand exchange method, coating with an amphiphilic polymer and formation of a silica shell are presented as the most interesting techniques to achieve water dispersibility. The reliability of the surface modifications is evaluated according the functionalization of the upconverting nanoparticles with proteins, nucleic acids or dyes. A guideline for the choice of surface engineering for specific bioanalytical applications is established. For sensor applications of the upconverting nanoparticles via energy transfer processes to organic dyes short distances between emitters and absorbers are favored, which can be accomplished by the ligand exchanges method or by mesoporous silica shells. Coatings with an amphiphilic polymer and with silica are suitable for applications of UCNPs in electrophoresis and cellular imaging. Both surface modification techniques are stable against the detachment of the surface layer. The suitability of the ligand exchange method for cellular application strongly depends on the binding sites to the nanoparticle of the inserted ligand. Here, polymers like polyacrylic acid with many coordinating groups have better stability compared to monodentate ligands. Chapter 5 describes the application of upconverting nanoparticles in photodynamic therapy. In the first step small and bright core-shell nanoparticles NaYF4:Yb,Er,Gd@NaYF4 are synthesized by thermal decomposition combined with Ostwald ripening method in high boiling solvents. The ligand oleate is replaced by L-lysine to achieve water dispersibility and to bind the photosensitizer Rose Bengal close to the surface of the nanoparticles. Intensity based measurements indicate an energy transfer from the green emission of the Er3+ doped UCNPs, confirmed by lifetime measurements, where a FRET efficiency of 25% is calculated. The production of singlet oxygen after NIR excitation is proven with the fluorescent probe 9,10-anthracenediyl-bis(methylene)dimalonic acid. Cellular uptake in human breast cancer cells (SK-BR 3) is confirmed by confocal and two photon microscopy. MTT assays of SK-BR 3 cells show already significant cell death at particle concentrations of 10 µg·mL-1 after NIR excitation. After cell death a staining of the cell nuclei with the functionalized nanoparticles is observed in confocal microscopy allowing live/dead imaging without further treatment. In Chapter 6 UCNPs are entrapped in electrospun 1 D nanofibers to enable ratiometric sensing in microfluidic channel systems. The nanoparticles are distributed homogeneously inside the nanofibers without forming aggregates. The mass concentration of the nanolamps inside the nanofibers is up to 254 ± 9 mg·mL-1 showing a significant increase (up to 50 times) compared to common mass concentration achieved in aqueous solutions. The optical properties of the green (541 nm) and red emission bands (656 nm) of Yb,Er doped UCNPs maintain unchanged after NIR excitation and visible light can be directly generated in the nanofibers. The fibermats are transferred to a microfluidic chip system by hot bonding. The luminescence signal of nanolamps inside the microfluidic channels is not affected by ionic strength or different pH values (2-14), however, shows remarkable and reproducible changes for molecules without the ability to quench the upconversion luminescence, e.g. deuterium oxide (D2O). Ratiometric sensing is tested with the dye Rose Bengal, which absorbs the green light while the red emission keeps unaffected. The dynamic range is from 1 to 1000 µM with a limit of detection of 0.1 µM. Chapter 7 provides a critical discussion about the advantages and requirements of lanthanide doped nanoparticles for (bio)analytic application. Future perspectives and challenges of the different nanomaterials in assays, sensing and theranostic applications are validated and different approaches are addressed