Correlating the photoluminescence (PL) properties of nanomaterials like semiconductor nanocrystals (QDs) and lanthanide-based upconversion nanocrystals (UCNPs) at the ensemble and single particle level is increasingly relevant for applications of these nanomaterials in the life sciences like bioimaging studies or their use as reporters in microfluidic assays. Aiming to derive particle architectures well suited for spectroscopic and microscopic applications, we compared the spectroscopic properties of different QDs like II/VI QDs and cadmium-free AIS/ZnS QDs as well as UCNPs of different chemical composition for particle ensembles and single particles. This included the PL spectra, fluorescence quantum yields (ΦF), brightness values, blinking behavior, and PL decay kinetics of these nanomaterials. Special emphasis was focused on measurements of the fluorescence quantum yield (ΦF) and brightness, that determine the signal size and provide a measure for the quality of nanocrystals like QDs and UCNPs which are prone to surface quenching and stabilized with coordinatively bound surface ligands. [1-3] In this respect, the potential of fluorescence correlation spectroscopy (FCS) for relative ΦF measurements of nanoparticles at ultralow concentration was explored exemplarily for ligand-stabilized CdTe nanocrystals in water. 
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