We demonstrate a novel epitaxial layer-by-layer growth on upconverting NaYF(4) nanocrystals (NCs) utilizing Ostwald ripening dynamics tunable both in thickness and composition. Injection of small sacrificial NCs (SNCs) as shell precursors into larger core NCs results in the rapid dissolution of the SNCs and their deposition onto the larger core NCs to yield core-shell structured NCs. Exploiting this NC size dependent dissolution/growth, the shell thickness can be controlled either by manipulating the number of SNCs injected or by successive injection of SNCs. In either of these approaches, the NCs self-focus from an initial bimodal distribution to a unimodal distribution (σ <5%) of core-shell NCs. The successive injection approach facilitates layer-by-layer epitaxial growth without the need for tedious multiple reactions for generating tunable shell thickness, and does not require any control over the injection rate of the SNCs, as is the case for shell growth by precursor injection.

Electrospinning of chitosan solutions with poly(ethylene oxide) (PEO) in an aqueous solution of 2 wt% acetic acid was studied. The properties of the chitosan/PEO solutions, including conductivity, surface tension and viscosity, were measured. Morphology of the electrospun chitosan/PEO was observed by using scanning electron micrographs. Results showed that the ultrafine fibers could be generated after addition of PEO in 2:1 or 1:1 mass ratios of chitosan to PEO from 4-6 wt% chitosan/PEO solutions at 15 kV voltage, 20 cm capillary-collector distance and flow rate 0.1 ml/h. During electrospinning of the chitosan/PEO solutions, ultrafine fibers with diameters from 80 nm to 180 nm were obtained, while microfibers with visually thicker diameters could be formed as well. Results of X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and differential scanning calorimeter exhibited the larger electrospun microfibers were almost entirely made from PEO, while the electrospun ultrafine fibers mainly contained chitosan.

Ultrafine fibers of bisphenol-A polysulfone (PSF) were prepared by electrospinning of PSF solutions in mixtures of N,N-dimethylacetamide (DMAC) and acetone at high voltages. The morphology of the electrospun PSF fibers was investigated by scanning electron microscopy. Results showed that the concentration of polymer solutions and the acetone amount in the mixed solvents influenced the morphology and the diameter of the electrospun fibers. The processing parameters, including the applied voltage, the flow rate, and the distance between capillary and collection screen, were also important for control of the morphology of electrospun PSF fibers. It was suggested that uniform ultrafine PSF fibers with diameter of 300-400 nm could be obtained by electrospinning of a 20 % (wt/v) PSF/DMAC/acetone (DMAC:acetone = 9:1) solution at 10-20 kV voltages when the flow rate was 0.66 ml h −1 and capillary-screen distance was 10 cm.

Cation exchange in lanthanide fluoride nanoparticles is reported. Typically, dispersible LnF(3) nanoparticles were exposed to another lanthanide ion that was roughly 5 times the amount of Ln(3+) in the nanoparticles. Results show that cation exchange of GdF(3) nanoparticles with La(3+) was almost complete in 1 min, and it also happens reversibly although the degree of exchange is not as much as the forward reaction. However, cation exchange with lanthanide ions close to each other, such as GdF(3) with Eu(3+) and NdF(3) with La(3+), did not end up with nearly full exchange, but with a significant amount of the two lanthanides. A relatively small driving force for the cation exchange is suggested by the experimental results, which is also confirmed by calculations based on a thermodynamic cycle. This unprecedented finding in the field of lanthanide-based nanoparticles raises the question whether reported core-shell structures were indeed made and, at the same time, it opens up new pathways to make nanomaterials that cannot be made directly.

Cation exchange was performed on up-conversion NaYF4:Yb,Tm nanoparticles, resulting in NaYF4:Yb,Tm-NaGdF4 core–shell nanoparticles as indicated by electron energy-loss spectroscopy 2D mapping. Results show that core–shell nanoparticles with a thin, tunable, and uniform shell of subnanometer thickness can be made via this cation exchange process. The resulting NaYF4:Yb,Tm-NaGdF4 core–shell nanoparticles have an enhanced up-conversion intensity relative to the initial core nanoparticles. As potential magnetic resonance imaging (MRI) contrast agents, they were tested for their proton relaxivities. The r1 relaxivity per Gd3+ ion of the nanoparticles with a thin NaGdF4 shell (ca. 0.6 nm thick) measured at 9.4 T was found to be 2.33 mM–1·s–1. This r1 relaxivity is among the highest in all the reported NaYF4–NaGdF4 core–shell nanoparticles. The r1 relaxivity per nanoparticle is 1.56 × 104 mM–1·s–1, which is over 4000 times higher than commercial Gd3+-complexes. The very high proton relaxivity per nanoparticle is critical for targeted MRI as such nanoparticles provide strong contrast even in low concentrations. The presented cation exchange method is a promising way to manufacture core–shell nanoparticles with up-conversion NaYF4:Yb,Tm core and paramagnetic NaGdF4 shell for bimodal imaging, i.e. MR and optical imaging.

In this feature article we will critically discuss the synthesis and characterisation aspects of Ln(3+)-doped nanoparticles (NPs) that show upconversion, upon 980 nm excitation. Upconversion is a non-linear process that converts two or more low-energy photons, often near-infrared photons, into one of higher energy, e.g. blue and 800 nm from Tm(3+) and green and red from Er(3+) or Ho(3+). Nearly all researchers use the absorption of 980 nm light by Yb(3+) as the sensitiser for the co-doped emissive Ln(3+) ions. The focus will be on LnF(3) and MLnF(4) (M = alkali metal) as the host matrix, because most progress has been made with these. In particular we will argue that a detailed understanding of how the dopant ions and the host Ln(3+) ions are distributed (in the core) and how (doped) shell growth occurs is not well understood. Moreover, their use as optical and magnetic resonance imaging contrast agents will be discussed. We will argue that deep-tissue imaging beyond 600 μm with retention of optical resolution, i.e. to see fine structure such as blood capillaries in brain tissues, has not yet been achieved. Three key parameters have been identified as impediments: (i) the low absorption efficiency of the Yb(3+) sensitiser, (ii) the low quantum yield of upconversion, and (iii) the long-lived excited states. On the other hand, there are very encouraging results that suggest that these nanoparticles could be developed into very potent magnetic resonance imaging (MRI) contrast agents.

A series of lanthanide fluoride nanoparticles were prepared with a simple colloidal approach at 75 °C. All the nanoparticles are highly water-dispersible with sizes in the range of 3−10 nm. The light lanthanide fluoride salts, LaF3, CeF3, and NdF3, have the same trigonal crystal structure as the corresponding bulk materials. However, for the fluoride salts of the heavy Dy, Ho, Er, and Yb, nonstoichiometric cubic structures of Na x Ln y F z composition were identified. Particularly, the middle GdF3 and EuF3 nanoparticles have both trigonal and orthorhombic crystal phases instead of a single orthorhombic phase as for the corresponding bulk materials, which is attributed to kinetically formed products. Study of La3+-doped GdF3 nanoparticles showed that 15% La3+ doping is sufficient for GdF3 nanoparticles to crystallize completely in the trigonal phase, i.e., the same as LaF3, which is dramatically different than 50% La3+ doping for the bulk. Finally, on the basis of the thermodynamic cycle for the preparation of the doped materials, calculations carried out explain well the published experimental results for the bulk. However, this approach failed to explain the results of the nanoparticles. Evidence is provided that the formation of kinetic products is responsible.

Lanthanide-based nanoparticles have many optical applications such as biolabels, lasers, optical amplifiers, and opticaldisplay phosphors. 1À11 These applications are generally accomplished by doping emissive Ln 3+ ions into an optically inactive matrix, such as fluoride, 12 phosphate, 13 oxide, 14 or vanadate, 15 so that maximum optical efficiency can be achieved. 16 Recently, Gd 3+ -based nanoparticles have been reported as magnetic resonance imaging (MRI) contrast agents because one nanoparticle easily contains thousands of Gd 3+ ions, which gives rise to very high proton relaxivities as compared to Gd 3+ complexes. 6,17À22 In these nanoparticles, emissive Ln 3+ ions were also doped as an optical probe, for instance, for multimodal imaging. 6,19À21 Very little attention is being paid to the dopant ion distribution, probably because the trivalent lanthanide ions have similar chemical properties and these dopant ions in lanthanide-based nanoparticles are generally assumed to be statistically distributed in the nanoparticle. However, evidence that confirms this is generally not provided. Recently, the work conducted by our group has shown that due to cation exchange, some nanoparticles prepared in aqueous media, using a procedure intended for an alloy structure do not have a true alloy structure; that is, the dopant ions are not statistically distributed in the nanoparticle. 23 This raises the obvious question whether this is also the case for ' AUTHOR INFORMATION