Nonstatistical Dopant Distribution of Ln³⁺-Doped NaGdF₄ Nanoparticles

Abstract

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

Quenching Pathways in NaYF4:Er3+,Yb3+ Upconversion Nanocrystals

“…For now we assume that Γ rad and Γ Q are the only decay pathways for the luminescent centers. Assuming that the dopant centers are homogeneously distributed within the NC (which may in fact be not exactly true), 40 we obtain the expected decay dynamics of an ensemble of doped NCs by integrating Γ Q over the NC volume:where a and V are the radius and volume of the luminescent core of the NC, respectively,is the monoexponential radiative decay component (eq 3), andis the multiexponential solvent-quenching component to the decay (eq 2). …”

Ligand-dependent luminescence of ultra-small Eu3+-doped NaYF4 nanoparticles

“…In the simplest case, the electric field may result from uncoordinated Eu 3+ and Y 3+ ions present on the NPs surface. We also need to take into account that the distribution of dopant ions is not statistical within the NP volume (Dong et al 2011). For this reason, we have decided to assume equal surface charge density ( Q surface ) for all types of investigated NPs, and thus roughly even electric field strength F z , at a fixed distance from the NPs surface, for all three types of investigated NPs.…”

Probing the nature of upconversion nanocrystals: instrumentation matters

“…The high sensitivity (down to ppm) of this technique, combined with the ability to measure the surface structure, makes this technique ideal for probing subtle variations on the surface of UC nanocrystals. 119,166,167 They found that the distribution of Y 3+ , Tb 3+ or Nd 3+ in the NaGdF 4 nanoparticles is inhomogeneous. 38,165 Depth-dependent composition measurement by XPS provides an alternative method to study the dopant distribution in UC nanocrystals.…”

“…64,124,171 For example, the replacement of Y 3+ in NaYF 4 by a trace amount of Gd 3+ can be readily confirmed by XPS measurement but not other techniques, including XRD and EDS. 165 They further extended this approach to characterizing core-shell UC nanocrystals by analyzing the relative signal intensity of lanthanides as a function of the photoelectron kinetic energy. By using synchrotron radiation, van Veggel and co-workers reported precise control over the penetration depth of X-rays in NaGdF 4 nanoparticles.…”