Synthesis of Uniform NaLnF₄ (Ln: Sm to Ho) Nanoparticles for Mass Cytometry

Over the past decade, there have been extensive developments in the field of lanthanide-based nanoparticles (NPs). Most studies have focused on the application of upconverting NaYF₄-based NPs for deep tissue imaging and paramagnetic NaGdF₄ NPs for MRI. Current applications for the remaining members of the lanthanide series are rather limited. Recently, a novel bioanalytical technique known as mass cytometry (MC) has been developed which can benefit from the entire lanthanide series of NPs. MC is a high-throughput multiparametric cell-by-cell analysis technique based on atomic mass spectrometry that uses antibodies labeled with metal isotopes for biomarker detection. NaLnF₄ NPs offer the promise of high sensitivity coupled with multiparameter detection, provided that NPs can be synthesized with a narrow size distribution. Here we describe the synthesis of six members of this NP family (NaSmF₄, NaEuF₄, NaGdF₄, NaTbF₄, NaDyF₄, NaHoF₄) with the appropriate size (5–30 nm) and size distribution (CV < 5%) for MC. We employed the coprecipitation method developed by Li and Zhang [<i>Nanotechnology</i> <b>2008</b>, <i>19</i>, 345606], and for each member of this series, we examined the heating rate, final reaction temperature, and composition of the reaction mixture in an attempt to optimize the synthesis. For each of the six NaLnF₄, in the range of the target sizes, we were able to identify “sweet spots” in the reaction conditions to obtain NPs with a narrow size distribution. In addition, we investigated the oleate surface coverage of the NPs and the effect of long-term storage (2 years) on the colloidal stability of the NPs. Finally, NaTbF₄ NPs were rendered hydrophilic via lipid encapsulation and tested for nonspecific binding with KG1a and Ramos cells by mass cytometry

Influence of Lu³⁺ Doping on the Crystal Structure of Uniform Small (5 and 13 nm) NaLnF₄ Upconverting Nanocrystals

While there have been many advances in techniques to synthesize uniform lanthanide-doped upconversion nanoparticles (UCNPs), it is still a challenge to synthesize small (ca. 5 nm) hexagonal phase UCNPs that are also bright. The most common method to obtain strongly emissive UCNPs is to synthesize core–shell structures with a passivating shell coating the luminescent core. This approach normally results in larger NPs (>20 nm) and requires two-step procedures. Here, we report a one-pot synthesis of 4 nm NaLuF₄:Gd­(37%),Yb­(16%),Er­(2%) UCNPs, whose colloidal solutions show upconversion luminescence (UCL) visible to the eye. We initially hypothesized that the origin of UCL from such small UCNPs was due to a Gd-rich hexagonal upconverting core containing Yb and Er with a Lu-rich passivating shell. This idea is based on the different nucleation rates of the NaLnF₄ NPs. Interestingly, the 4 nm NaLuF₄-based UCNPs are in the cubic phase, and subsequently undergo a phase transformation with prolonged heating to form larger (12–14 nm) uniform hexagonal phase UCNPs. We also found that if the molar ratio of Lu:Gd in the reaction mixture was decreased from 45:37 to 20:62, the resulting UCNPs still initially nucleated in the cubic phase. Additional studies in which we varied other reaction parameters (temperature, ratios of Na⁺/Ln³⁺ and F^–/Ln³⁺, and solvent composition) also resulted in initial nucleation in the cubic phase. In contrast, both the NaGdF₄:Yb,Er and NaYF₄:Gd,Yb,Er UCNPs nucleated in the hexagonal phase. Our results suggest that the presence of Lu in the reaction mixture influences the nucleation of NaLnF₄ NPs. Lanthanide compositions that would normally nucleate in the hexagonal phase appear to nucleate in the cubic phase when Lu is present

Quantification of Surface Ligands on NaYF₄ Nanoparticles by Three Independent Analytical Techniques

There have been important advances in characterizing the surface coverage of ligands on colloidal inorganic nanoparticles (NPs), but our knowledge of ligand coverage on lanthanide NPs is much more limited. The as-synthesized NPs are often coated with hydrophobic ligands that need to be replaced with hydrophilic ligands such as poly­(ethylene glycol) (PEG) for biomedical applications. The two challenges in terms of characterizing ligand coverage on NPs are first to show that different analytical methods give consistent results and second to show how the sample preparation protocol affects ligand density. Here, we report a quantitative study of the native oleate content of as-synthesized NaYF₄ and NaTbF₄ NPs, as well as the surface coverage after ligand exchange with three methoxyPEG-monophosphates with <i>M</i>_n = 750, 2000, and 5000 Da. For NaYF₄, we obtained consistent results for both oleates and PEGs by three independent methods (TGA, ¹H NMR, and ICP-AES). The oleate coverage was very sensitive to the sample isolation/purification protocol, with a high surface coverage (5.5 to 8 nm^–2) for ethanol/hexane sedimentation/redispersion but only 2 nm^–2 if THF was used in place of hexanes. The surface coverages PEG750 (∼1.1 nm^–2), PEG2000 (∼1.7 nm^–2), and PEG5000 (∼0.2 nm^–2) suggest that corona repulsion limits the number of PEG5000 molecules that can graft to the surface. For NaTbF₄ NPs, we compared the surface coverage of PEG2000-monophosphate with a PEG2000-tetraphosphonate ligand shown to provide enhanced colloidal stability in PBS buffer. We found the surprising result that the footprints of these ligands were comparable, suggesting that there was insufficient room for all four phosphonate groups of the tetradentate ligand to bind simultaneously to the NP surface