Fluorescent Heterodoped Nanotetrapods as Synergistically Enhancing Positive and Negative Magnetic Resonance Imaging Contrast Agents

In this work, we report Mn-Fe heterodoped ZnSe tetrapod nanocrystals (NCs) synthesized to synergistically enhance contrast in both T₁- and T₂-weighted magnetic resonance imaging (MRI). The proposed NCs were prepared using a customized heteroarchitecture such that the manganese (Mn) is confined in the core and iron (Fe) in the branches of the tetrapods. The elemental composition and profile of these NCs were studied using X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma mass spectroscopy. Photoluminescence quantum yield of these heterodoped NCs in water is ∼30%. Magnetic measurements reveal the simultaneous presence of superparamagnetic and paramagnetic behavior in these NCs because of the coexistence of Mn²⁺ and Fe²⁺ dopants. Their potential as simultaneous positive and negative MRI contrast agents was demonstrated by relaxivity measurements and in vivo MRI. From the in vivo studies, we also found that these NCs (with a hydrodynamic diameter of 20 nm) are excreted from the body within 24 h after the injection. Therefore, these heterodoped tetrapods NCs, while being fluorescent and safe, hold great future as a synergistically enhancing dual-modal MRI contrast agent.