Piling up excited states to reach upconversion (UC) is severely restricted by vibrational quenching mechanisms, especially when one looks at discrete molecular entities in solution. By carefully controlling the supramolecular assembly processes resulting from the strong electrostatic interactions between negatively charged Yb complexes and Tb 3+ cations in aqueous solutions, we engineered the formation of heteropolynuclear complexes of [(YbL) 2 Tb x ] compositions (x = 1 and 2). These edifices display a phenomenon of cooperative photosensitization UC with green emission of the Tb cations upon NIR excitation at 980 nm in the Yb absorption band. The photophysical properties of the complexes were carefully investigated by steady-state and time-resolved luminescence experiments in D 2 O, allowing to quantify the impact of the composition and pD of the solution on the emission intensity, as well as clarifying the exact cooperative photosensitization upconversion mechanism. Using optimized conditions, the energy transfer UC process could be observed for the first time in non-deuterated water with discrete molecular compounds.
Single-particle luminescence microscopy is a powerful method to extract information on biological systems that is not accessible by ensemble-level methods. Upconversion nanoparticles (UCNPs) are a particularly promising luminophore for single-particle microscopy as they provide stable, non-blinking luminescence and allow the avoidance of biological autofluorescence by their anti-Stokes emission. Recently, ensemble measurements of diluted aqueous dispersions of UCNPs have shown the instability of luminescence over time due to particle dissolution-related effects. This can be especially detrimental for single-particle experiments. However, this effect has never been estimated at the individual particle level. Here, the luminescence response of individual UCNPs under aqueous conditions is investigated by quantitative wide-field microscopy. The particles exhibit a rapid luminescence loss, accompanied by large changes in spectral response, leading to a considerable heterogeneity in their luminescence and band intensity ratio. Moreover, the dissolution-caused intensity loss is not correlated with the initial particle intensity or band ratio, which makes it virtually unpredictable. These effects and the subsequent development of their heterogeneity can be largely slowed down by adding millimolar concentrations of sodium fluoride in buffer. As a consequence, the presented data indicate that microscopy experiments employing UCNPs in an aqueous environment should be performed under conditions that carefully prevent these effects.
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