Positive and negative lattice shielding effects Co-existing in Gd (III) ion doped bifunctional upconversion nanoprobes

Gadolinium (Gd) doped upconversion nanoparticles (UCNPs) have been well documented as T₁-MR and fluorescent imaging agents. However, the performance of Gd³⁺ ions located differently in the crystal lattice still remains debatable. Here, a well-designed model was built based on a seed-mediated growth technique to systematically probe the longitudinal relaxivity of Gd³⁺ ions within the crystal lattice and at the surface of UCNPs. We found, for the first time, a nearly 100% loss of relaxivity of Gd³⁺ ions buried deeply within crystal lattices (> 4 nm), which we named a "negative lattice shielding effect" (n-LSE) as compared to the "positive lattice shielding effect" (p-LSE) for the enhanced upconversion fluorescent intensity. As-observed n-LSE was further found to be shell thickness dependent. By suppressing the n-LSE as far as possible, we optimized the UCNPs' structure design and achieved the highest r₁ value (6.18 mM^-1s^-1 per Gd³⁺ ion) among previously reported counterparts. The potential bimodal imaging application both in vitro and in vivo of as-designed nano-probes was also demonstrated. This study clears the debate over the role of bulk and surface Gd³⁺ ions in MRI contrast imaging and paves the way for modulation of other Gd-doped nanostructures for highly efficient T₁-MR and upconversion fluorescent bimodal imaging. Positive and negative lattice shielding effects were found to co-exist in lanthanide-ion-doped upconversion fluorescent nanoparticles (UCNPs), and were used to advance the T₁-MR contrast imaging and fluorescent imaging capabilities of the water soluble bifunctional nanocrystals. This resulted in highly enhanced magnetic and optical imaging modalities in comparison with previous literature reports.