We present a systematic study on the effect of surface ligands on the luminescence properties and colloidal stability of β-NaYF4:Yb(3+),Er(3+) upconversion nanoparticles (UCNPs), comparing nine different surface coatings to render these UCNPs water-dispersible and bioconjugatable. A prerequisite for this study was a large-scale synthetic method that yields ∼2 g per batch of monodisperse oleate-capped UCNPs providing identical core particles. These ∼23 nm sized UCNPs display an upconversion quantum yield of ∼0.35% when dispersed in cyclohexane and excited with a power density of 150 W cm(-2), underlining their high quality. A comparison of the colloidal stability and luminescence properties of these UCNPs, subsequently surface modified with ligand exchange or encapsulation protocols, revealed that the ratio of the green (545 nm) and red (658 nm) emission bands determined at a constant excitation power density clearly depends on the surface chemistry. Modifications relying on the deposition of additional (amphiphilic) layer coatings, where the initial oleate coating is retained, show reduced non-radiative quenching by water as compared to UCNPs that are rendered water-dispersible via ligand exchange. Moreover, we could demonstrate that the brightness of the upconversion luminescence of the UCNPs is strongly affected by the type of surface modification, i.e., ligand exchange or encapsulation, yet hardly by the chemical nature of the ligand.

CONSPECTUS: Photon upconversion nanoparticles (UCNPs) have emerged as a promising new class of nanomaterials due to their ability to convert near-IR light into visible luminescence. Unfortunately, most efficient methods for preparing UCNPs yield hydrophobic materials, but water-dispersibility is needed in the major fields of applications of UCNPs, that is, in bioimaging, labeling, and bioassays. Numerous methods therefore have been reported in the past years to convert the hydrophobic surface of UCNPs to a more hydrophilic one so to render them dispersible in aqueous systems. We present a classification respective for these strategies and assess the main methods. These include (A) chemical modification of the hydrophobic (typically oleate) ligand on the surface, (B) addition of an extra layer, (C) addition of a thin shell on top of the UCNP, and (D) complete replacement of the original ligand by another one. Chemical modification (A) involves oxidation of the oleate or oleylamine and leads to particles with terminal oxygen functions. This method is less often used because solutions of the resulting UCNPs in water have limited colloidal stability, protocols are time-consuming and often give low yields, and only a limited number of functional groups can be introduced. Methods B and C involve coating of UCNPs with amphiphiles or with shells made from silica oxide, titanium oxide, or metallic gold or silver. These methods are quite versatile in terms of further modifications, for example, by further cross-linking or by applying thiol-gold chemistry. Growing an extra shell is, however, often accompanied by a higher polydispersity. Method D can be divided into subgroups based on either (i) the direct (single-step) replacement of the native ligand by a new ligand or (ii) two-step protocols using nitrosyltetrafluoroborate (NOBF4) or strong acids as reagents to produce ligand-free UCNPs prior to the attachment of a new ligand. These methods are simple and versatile, and the distance between the new ligand and the luminescent particle can be well controlled. However, the particles often have limited stability in buffer systems. The methods described also are of wider interest because they are likely to be applicable to other kinds of nanomaterials. We additionally address the need for (a) a better control of particle size and homogeneity during synthesis, (b) more reproducible methods for surface loading and modification, (c) synthetic methods giving higher yields of UCNPs, (d) materials displaying higher quantum yields in water solution without the need for tedious surface modifications, (e) improved methods for workup (including the suppression of aggregation), (f) new methods for surface characterization, and (g) more affordable reagents for use in surface modification. It is noted that most synthetic research in the area is of the trial-and-error kind, presumably due to the lack of understanding of the mechanisms causing current limitations. Finally, all particles are discussed in terms of their biocompatibility (as fa...

Upconversion nanoparticles (UCNPs) are attractive candidates for energy transfer-based analytical applications. In contrast to classical donor-acceptor pairs, these particles contain many emitting lanthanide ions together with numerous acceptor dye molecules at different distances to each other, strongly depending on the particle diameter. UCNPs with precisely controlled sizes between 10 and 43 nm were prepared and functionalized with rose bengal and sulforhodamine B by a ligand-exchange procedure. Time-resolved studies of the upconversion luminescence of the UCNP donor revealed a considerable shortening of the donor lifetime as a clear hint for Förster resonance energy transfer (FRET). FRET was most pronounced for 21 nm-sized UCNPs, yielding a FRET efficiency of 60%. At larger surface-to-volume ratios, the FRET efficiency decreased by an increasing competition of nonradiative surface deactivation. Such dye-UCNP architectures can also provide an elegant way to shift the UCNP emission color, since the fluorescence intensity of the organic dyes excited by FRET was comparable to that of the upconversion emission of smaller particles.

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.