Improving the MR Imaging Sensitivity of Upconversion Nanoparticles by an Internal and External Incorporation of the Gd3+ Strategy for in Vivo Tumor-Targeted Imaging

Gd3+-ion-doped upconversion nanoparticles (UCNPs), integrating the advantages of upconversion luminescence and magnetic resonance imaging (MRI) modalities, are capturing increasing attention because they are promising to improve the accuracy of diagnosis. The embedded Gd3+ ions in UCNPs, however, have an indistinct MRI enhancement owing to the inefficient exchange of magnetic fields with the surrounding water protons. In this study, a novel approach is developed to improve the MR imaging sensitivity of Gd3+-ion-doped UCNPs. Bovine serum albumin (BSA) bundled with DTPA-Gd3+ (DTPA(Gd)) is synthesized both as the MR imaging sensitivity synergist and phase-transfer ligand for the surface engineering of UCNPs. The external Gd3+ ion attachment strategy is found to significant improve the MR imaging sensitivity of Gd3+-ion-doped UCNPs. The relaxivity analysis shows that UCNPs@BSA center dot DTPA(Gd) exhibit higher relaxivity values than do LICNPs@BSA without DTPA(Gd) moieties. Another relaxivity study discloses a striking message that the relaxivity value does not always reflect the realistic MRI enhancement capability. The high concentration of Gd3+-ion-containing UCNPs with further surface-engineered BSA center dot DTPA(Gd) (denoted as UCNPs(-H)@BSA center dot DTPA(Gd)) exhibits a more pronounced MRI enhancement capability compared to the other two counterparts [UCNPs(-N)@BSA center dot DTPA(Gd) and UCNPs(-L)@BSA center dot DTPA(Gd) ((-N) and (-L) are denoted as zero and low concentrations of Gd3+ ion doping, respectively)], even though it holds the lowest r(1) of 1.56 s(-1) per mmol L-1 of Gd3+. The physicochemical properties of UCNPs are essentially maintained after BSA center dot DTPA(Gd) surface decoration with good colloidal stability, in addition to improving the MR imaging sensitivity. In vivo T-1-weighted MRI shows potent tumor-enhanced MRI with UCNPs(-H)@BSA center dot DTPA(Gd). An in vivo biodistribution study indicates that it is gradually excreted from the body via hepatobiliary and renal processing with no obvious toxicity. It could therefore be concluded, with improved MR imaging sensitivity by an internal and external incorporation of Gd3+ strategy, that UCNPs(-H)@BSA center dot DTPA(Gd) presents great potential as an alternative in tumor-targeted MR imaging.