Perspectives of upconverting luminescent nanoparticles for (bio)-analytical applications

The thesis describes the synthesis, characterization, surface modification, and (bio)-analytical applications of upconverting luminescent nanoparticles (UCLNPs). In Chapter 1 an overview of nanomaterials used for (bio)-analytical sensing and imaging is provided with special emphasis on luminescent nanomaterials. UCLNPs as one class of luminescent nanomaterials are introduced. The aims of this work such as synthesis of small UCLNPs with a narrow size-distribution, their surface engineering, and a study on their luminescence properties are addressed in Chapter 2. Chapter 3 deals with surface-functionalized multicolor UCLNPs suitable for protein conjugation. The preparation and characterization of monodisperse silica-coated UCLNPs (average diameter of 38 nm) modified with poly(ethylene glycol) spacers carrying N-hydroxysuccinimde groups is presented. It is demonstrated that such UCLNPs can be employed as luminescent labels due to their strong binding to proteins. A hybrid material consisting of streptavidinylated magnetic beads labeled with amino-reactive UCLNPs is prepared which can be separated from a colloidal dispersion by applying an external magnetic force. These magnetic beads/UCLNPs conjugates display visible upconversion luminescence upon 980 nm continuous wave laser excitation. The synthesis of amphiphilic polymer-coated core-shell UCLNPs is reported in Chapter 4. Using such a core-shell architecture, the upconversion luminescence intensity at 475 nm is increased by a factor of ~ 60. It is demonstrated that the upconversion emission of core-shell UCLNPs based on β-NaYF4(Yb3+/Tm3+)@NaYF4 spectrally matches the absorption of the coenzyme FAD and the enzyme cosubstrate NADH. This spectral match is exploited to fluorescently monitor the formation of NADH and the consumption of FAD during enzymatic reactions using 980 nm photoexcitation. A sensing scheme based on an inner filter effect employing UCLNPs as a kind of nanolamps is developed which allows for the quantification of ethanol and glucose levels. In Chapter 5 an optimized synthesis protocol for the large scale production of oleate-capped UCLNPs based on β-NaYF4(Yb3+/Er3+) is described. Such UCLNPs are characterized by a high crystallinity, an extremely narrow size distribution, and a comparatively high quantum yield. The impact of nine different surface modifications, which allow for a phase transfer of initially hydrophobic UCLNPs into water, on their upconversion luminescence properties is described. Limitations and challenges of UCLNPs are addressed in Chapter 6. Their future directions and perspectives are highlighted