Embedding Mn²⁺ in polymer coating on rod-like cellulose nanocrystal to integrate MRI and photothermal function

Grafting Mn ions on 1D particles can produce high-performance MRI agents. However, the motion of Mn ions may cause Mn aggregation and then attenuate imaging contrast. Here, we proposed an MRI-promotion strategy based on embedding Mn²⁺ into crosslinked polydopamine (PDA) coating on rod-like cellulose nanocrystals. The results indicate that the slow relaxation of PDA molecular chains restricts the motion of embedded Mn²⁺. When the embedded Mn²⁺ content raises up to 2.25 wt%, the particles display a longitudinal relaxivity of 38.08 mM^-1·s^-1. Such a relaxivity is over 6.6 times of the sample with adsorbed 0.65 wt% Mn²⁺. The aspect ratio of nanoparticles even increases from 14.2 to 20.7 as embedding Mn²⁺ concentration increases. Then, the photothermal conversion efficiency of nanoparticles increases from 17.7 % to 44.4 %. This high photothermal conversion efficiency contributes this kind of MRI agent with embedded Mn²⁺ to the application potential of integrating tumor diagnosis and treatment.