Time-dependent luminescence loss for individual upconversion nanoparticles upon dilution in aqueous solution

Abstract

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.

Long‐Term Colloidal and Chemical Stability in Aqueous Media of NaYF4‐Type Upconversion Nanoparticles Modified by Ligand‐Exchange

“…UCNPs with a NaYF 4 host lattice have been associated for a long time with excellent chemical stability and show a very low solubility product of 1.6 × 10 −26 m 6 . Nevertheless, a small amount of the particles gets dissolved in water according to the equilibrium reported by the group of Mely and displayed in the following equation: …”

“…Due to the potential toxic effect of added fluoride ions and the limitation of this in the aspect of bioanalytical applications, it was also studied if the addition of the ligand itself or the addition of Na + would have a positive effect. However, for the dissolution of the nanoparticles not only the pathway in Equation but also a second one, described by the group of Mely, would be possible …”

“…problem at all. For single particle imaging as shown by the group of Mely [47] UCNPs based on NaYF 4 host materials stay stable when 1 mmol L −1 fluoride was added to the solution. In order to study the presence of different ions for the stability of PAH and CIT modified UCNPs dialysis not only against water but also against 1 × 10 −3 m KF solution, 1 × 10 −3 m NaNO 3 , a solution with 100 µg mL −1 of the ligand (e.g., PAH or Na 3 CIT), 1 × 10 −3 m Y(NO 3 ) 3 , and 1 × 10 −3 m of KF and Y(NO 3 ) 3 has been performed.…”

Critical Considerations on the Clinical Translation of Upconversion Nanoparticles (UCNPs): Recommendations from the European Upconversion Network (COST Action CM1403)

“…Good and promising examples are block copolymers and phosphonates with multiple phosphonic groups . Inorganic shells can be an alternative solution to prevent dissolution of UCNPs . A silica coating may be used for this purpose, but it cannot completely inhibit the UCNPs dissolution due to the relative porosity of the silica shell .…”

Recent progress in the development of upconversion nanomaterials in bioimaging and disease treatment

“…It was difficult to find a dual functional material that can serve as an imaging probe and a drug carrier simultaneously. As an effective drug delivery carrier, it should satisfy the following criteria: (1) effective drug delivery, (2) good biocompatibility, and (3) good stability of in vivo circulation [ 5 – 7 ]. Recently, the rapid development of nanomedicine has opened up a new way to overcome these shortcomings.…”