Upconversion
luminescence where visible light can be obtained through low energy near-infrared
radiation is an interesting research topic especially within the scope of biomedical
applications using optical detection. The benefits of using upconverting nanoparticles
are low background due to low autofluorescence and narrow bandwidth of
lanthanide luminescence. However, their use is still being hampered by the challenges
arising from the water based biological matrices. The research for creating hydrophilic
nanoparticle surfaces for aqueous environment and simultaneously preventing the
quenching of the upconversion luminescence is still ongoing.

The
aim of the study conducted in this thesis was to produce uniform and highly luminescent
upconverting NaYF4:Yb3+,Er3+ nanoparticles suitable for biomedical applications
accounting for the challenges of water based matrices. To produce such nanoparticles
a synthesis route was modified and selected parameters affecting the nanoparticle
structure, size and uniformity were studied and their upconversion luminescence
behaviors were measured.

To
make the upconverting nanoparticles biocompatible, a layer-by-layer approach
was chosen for the surface modification method as it has not been studied with upconversion
nanoparticles in detail. Two types of bilayer structures in the surface modifications
were used, one combining negative polyelectrolytes and positive metal ions and
one where both components were polyelectrolytes that could be crosslinked to
produce a more rigid bilayer structure. The layer deposition conditions
affecting the bilayer structure formation such as polyelectrolyte length,
polyelectrolyte concentration, and ionic concentration were studied. The
formation of the coating by the layer-by-layer method was confirmed for both
bilayer structures and their effect on the upconversion luminescence was
studied. It was observed that with the selected coatings and the number of
bilayers the obtained upconversion luminescence could be enhanced. The
enhancement could be maintained even with five bilayers of coating using
additional fluoride during the layer formation.

In
addition, studies to observe the possible disintegration of the coated
upconversion nanomaterials were conducted by both optical and solution concentration
methods. It was found that in polyelectrolyte/metal ion coated nanoparticles
prepared with selected conditions the emission was maintained in pure water for
24 hours. Crosslinking of the two polyelectrolyte coatings was successful in
effectively hindering the fluoride loss from the core nanomaterial. This study
demonstrates the usefulness of a widely modifiable layer-by-layer method in the
surface modification of upconverting nanoparticles. It offers possibilities to
create surface structures where the luminescence and the core particle can be
shielded from the deleterious environmental effects while rendering the
nanoparticles with functionality for further biomodifications.