Investigating the Competitive Adsorption of Polymer-Bisphosphonate Ligands on Lanthanide Nanoparticles

We describe the binding of five "stealth" polymers to the surface of NaYF₄:Yb,Er NPs with diameters of 20 and 24 nm. All polymers had a neridronate (Ner) end group that provided a bisphosphonate to promote strong binding to the lanthanide NP (LnNP) surface. The polymers included four poly(ethylene glycol) (PEG) samples with M_n = 2000 and 5000 and a zwitterionic poly(sulfobetaine methacrylate) (PSBMA, DP = 35). At surface saturation, the polymers had densities ranging from 2.3 to 3.8 nm²/polymer, and all provided excellent colloidal stability in PBS buffer. Ligand exchange experiments for samples aged 24 h after treatment with the first polymer ligand showed that PEG-Ner was more effective at displacing PSBMA-Ner than the reverse reaction. One surprising result is that PEG-Ner polymers with a distal azide group bound to the nanoparticles (NPs) at higher surface density than the corresponding methoxy-PEG polymers. Another set of surprising results is that when methoxy-PEG-Ner samples competed with PSBMA-Ner, more PSBMA than PEG became bound to the surface. However, when the azide-PEG-Ner samples competed with PSBMA-Ner, more PEG than PSBMA became bound to the surface.