Luminescence is ubiquitously used in numerous scientific areas, as it providesa high degree of versatility and sensitivity. Its versatile aspect is twofold, sincethe nature of luminescence makes it well suited to be incorporated as a readoutin various setups, and the rich palette of lumiphores, with unique photophysi-cal and photochemical properties, supports numerous applications. The use ofluminescence has proven especially useful in biomedical areas, where lumines-cent labels aid in, for instance, localizing subcellular structures or canceroustissue. These luminescence measurements are typically conducted entirely inthe visible region with excitation and emission wavelengths between 400 nmand 700 nm. This spectral region is, however, associated with issues of aut-ofluorescence, scattering of light, and poor tissue transparency. As a remedyfor these concerns, there has been a growing interest in extending to the nearinfrared (NIR; broadly defined as 700–1700 nm), as moving to longer wave-lengths diminishes the issues of light-tissue interactions.In this thesis, I explored the use of two particular lumiphore classes, namelyDNA-stabilized silver nanoclusters (DNA-AgNCs) and upconversion nanopar-ticles (UCNPs), for novel applications in the NIR I region (700–1000 nm).While the applications range from new imaging techniques to sensing in com-plex biological media, the unifying concept has been the NIR I region, as ex-citation and/or luminescence wavelengths have been kept within this spectralwindow. For instance, we were able to image a DNA-AgNC with an emissionmaximum around 960 nm at the single molecule level; bordering the NIR IIregion (1000–1700 nm), this proved to be the most NIR emissive DNA-AgNCdetected at the single molecule level to date. Since some DNA-AgNCs exhibitdual-emission with a short- (ns) and long-lived (μs) component, we developeda technique that could capture the dynamics on both timescales in a singlemeasurement. By introducing a secondary delayed NIR laser to this experi-ment, we were able to deepen our understanding of the relatively unexploredoptically activated delayed fluorescence phenomenon.UCNPs, on the other hand, were used for the development of a plasma compat-ible immunoassay based on upconversion cross-correlation spectroscopy. Themany long-lived energy levels of lanthanide ions in UCNPs lead to anti-Stokesemission, circumventing autofluorescence from biological samples, and supportexcited state absorption when co-illuminated with a secondary light source atwavelengths specific to particular lanthanide ions. We used this specificity fordemonstrating a new multiplexing method able to resolve spectrally and spa-tially overlapping UCNPs by encoding the signal of interest with a frequency.