Positive and Negative Lattice Shielding Effects Co-existing in Gd (III) Ion Doped Bifunctional Upconversion Nanoprobes

Abstract

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.

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