Design Traits for Diblock Copolymer Coating Properties on NaGdF₄ Upconversion Nanoparticles

Inorganic upconversion nanoparticles used in biological and medical applications are typically coated with materials to make these particles biocompatible, colloidally stable in aqueous media, and bioactive. However, the design of the molecules used in this coating must be carefully tuned to achieve these desired properties, and the molecular-scale design rules required for this are not known. In this work, a molecular dynamics simulation strategy is introduced to predict the structures and properties of a family of diblock copolymers that are used to coat the surface of upconversion nanoparticles in liquid water. These polymers comprise blocks of poly(oligo (ethylene glycol) methyl ether acrylate) and monoacryloxy ethyl phosphate, where the length of the former block is varied to have 6, 13, 35, and 55 units. The optimal polymer size for a range of properties is identified by modeling their interactions with the aqueous NaGdF4 interface. The simulations suggest that interparticle aggregation is likely for the smaller polymers and that intrachain folding effects in the larger polymers strongly influence the polymer/inorganic interaction. This in turn affects the structure of the polymer/solvent interface and consequently governs the efficiency of bioconjugation for these coated nanoparticles. The outward presentation of carboxylate groups to the solvent is crucial to the antibody binding efficiency, and this was predicted to be significantly reduced for larger chains, consistent with recent experimental data. Informed by this, mathematical models are formulated to predict the relation between polymer sizes and three key properties: surface charge, maximum loading, and maximum thickness of the coatings. This molecular simulation strategy is generally applicable to determine the optimal properties of polymer coatings prior to experiment.