Cubic versus hexagonal – effect of host crystallinity on the T1 shortening behaviour of NaGdF4 nanoparticles

Abstract

Sodium gadolinium fluoride (NaGdF₄) nanoparticles are promising candidates as T₁ shortening magnetic resonance imaging (MRI) contrast agents due to the paramagnetic properties of the Gd³⁺ ion. Effects of size and surface modification of these nanoparticles on proton relaxation times have been widely studied. However, to date, there has been no report on how T₁ relaxivity (r₁) is affected by the different polymorphs in which NaGdF₄ crystallizes: cubic (α) and hexagonal (β). Here, a microwave-assisted thermal decomposition method was developed that grants selective access to NaGdF₄ nanoparticles of either phase in the same size range, allowing the influence of host crystallinity on r₁ to be investigated. It was found that at 3 T cubic NaGdF₄ nanoparticles exhibit larger r₁ values than their hexagonal analogues. This result was interpreted based on Solomon–Bloembergen–Morgan theory, suggesting that the inner sphere contribution to r₁ is more pronounced for cubic NaGdF₄ nanoparticles as compared to their hexagonal counterparts. This holds true irrespective of the chosen surface modification, i.e. small citrate groups or longer chain poly(acrylic acid). Key aspects were found to be a polymorph-induced larger hydrodynamic diameter and the higher magnetization possessed by cubic nanoparticles.