Lanthanide based upconversion nanoparticles (UCNP), such as NaYF4: Yb3+/Er3+, are inorganic nanocrystals that exhibit upconversion luminescence; a phenomenon that involves emission of higher energy photons upon the excitation of the nanomaterial with lower energy photons. As a result, these particles show distinct non linear optical properties that make them suitable candidates to be used in applications such as bioimaging, drug release and drug delivery, photovoltaics and biosensing. However, the use of these nanoparticles is still limited due to numerous challenges. Amongst those are the small absorption cross sections and narrow absorption bands of UCNP along with the relatively low photoluminescence quantum yields. Additionally, the cytotoxicity of these particles is still a matter of investigation with very few research outcomes assessing the biocompatibility of UCNP despite their enormous potential in biological and life science applications. This thesis focusses on new means that can help to tackle these challenges. One strategy to deal with the first challenge and boost UCNP brightness is sensitization using near infrared (NIR) absorbing and emitting dyes. This strategy was used and is presented in the first project discussed in this thesis. A custom made NIR dye (1859 SL) ,synthesized by our collaboration partners, was coupled with NaYF4: 20%Yb3+, 2%Er3+ in a micellar encapsulation approach using two different surfactants, namely Pluronic F 127 and Tween 80. The relatively broad absorption band and the strong absorption cross section of the dye made it an ideal light harvester and energy donor for UCNP. The optimum ratio of dye to UCNP and the best value of excitation power density for the measurements were investigated by measuring the luminescence intensity for systems with different particle to dye ratios, and at variable excitation power densities. In the second project, dissolution of UCNP in aqueous media was investigated as a possible source for toxicity of UCNP due to the release of fluoride and lanthanide ions. Investigation of the ability of surface passivation and silica coating to inhibit ions release from the UCNP surfaces was studied. UCNP dissolution was quantified electrochemically using a fluoride ion selective electrode (ISE) and by inductively coupled plasma optical emission spectrometry (ICP OES) that provided the amount of released fluoride and lanthanide ions respectively. In addition, dissolution was monitored fluorometrically. The chemical composition of the aqueous environment on UCNP dissolution has a critical influence. For example, the formation of a layer of adsorbed molecules (organic compounds constituting DMEM, such as aminoacids) on the UCNP surface was observed for particles aged in a cell medium (DMEM). This layer protected the UCNP from dissolution and enhanced their fluorescence. X ray photoelectron spectroscopy (XPS) and mass spectrometry (MS) were used to investigate the chemical nature of this layer. This outcome offers a very practical and biocompatible mean for inhibiting ions release from UCNP, which is a cause of cytotoxicity of the particles. In a third project carried out in collaboration with the group of Prof. Christina Graf (FUB), a new, simple approach for growing a silica shell with an adjustable thickness between 5 and 250 nm onto oleate coated UCNP was investigated. Two different synthesis methods were combined to achieve that: oil in water microemulsion synthesis, and Stöber like synthesis. Firstly, this method included the growth of silica on the oleate coated particles in a multi step reverse microemulsion reaction performed consecutively without the need for intermediate isolation or purification steps. Then the particles were isolated and further grown in one step up to a diameter of more than 500 nm in a modified Stöber process. The importance of this procedure is due to its ability to grow thick silica shells (of thicknesses larger than 50 nm) onto the hydrophobic UCNP, which could not have been achieved using either of the two synthesis methods separately. The methods used to confirm the silica shell thickness and quality of the UCNP@SiO2 systems included dynamic light scattering (DLS), zeta potential measurements and scanning electron microscopy (SEM).
