Development of high-sensitivity immunoassays is a continuous interest in medical diagnostics, especially in the case of such diseases where higher sensitivity analyte measurements improve the prognosis of treatment. One such analyte is cardiac troponin I, used for detection of cardiac events. One of the key factors determining immunoassay sensitivity is the reporter, which labels the analytes in the assays and produces the measurable signal. Upconverting nanoparticles (UCNPs) are promising candidates for reporters in sensitive immunoassays. Their unique ability to convert near-infrared light into higher energy visible light by stacking photons, producing emission exhibiting anti-Stokes shift. As no other natural materials are capable of the process, measurement of UCNPs can be designed to completely dismiss any background signal originating from autofluorescence. However, reaching the maximal sensitivity they theoretically enable has been hindered by their tendency to non-specifically bind to solid surfaces in assays and to each other, forming nanoparticle clusters of varying sizes. The extent of these tendencies has been linked to the surface chemistry of the UCNPs.

The aim of this thesis was to study the surface chemistry of UCNPs and apply them as reporters in different immunoassay technologies for detection of cTnI. During the research, surface coating of UCNPs with poly(acrylic acid) was studied and successfully improved, leading to reduced non-specific binding and cluster formation tendency. The performance of the UCNPs with the novel surface was compared to other surface chemistry approaches in microtiter plate assays utilizing either analog or digital readout method, and a lateral flow assay.

Another aim was to develop the used assay technologies utilizing upconversion to reach extreme sensitivities. Reagents and conditions in analog microtiter plate assay for cTnI were thoroughly investigated to fine-tune the performance, and the limit of detection (LoD) reached an unprecedented low value of 0.13 ng/L for an analog assay. In addition, a mechanical actuator for automation of a lateral flow assay for cTnI was fabricated via 3D-printing, and when combined with the improved UCNPs, an LoD of 1.5 ng/L was reached, bringing high-sensitivity pointof- care detection of cTnI a step closer to reality.