Conjugated polymers feature with excellent optical properties for bioimaging. The limited tissue penetration depth of optical imaging modalities constrains the applications of conjugated polymers for in vivo imaging. Since conjugated polymers can be paramagnetic, developing conjugated polymer based contrast agents for magnetic resonance imaging (MRI) can overcome the tissue penetration problem in optical imaging. This is because magnetic resonance imaging feature with unlimited tissue penetration and non-ionization irradiation. However, tuning paramagnetic conjugated polymers into MRI agents requires further investigation. Recently, Shao’s group reports on T2-MRI active agents based on conjugated polymers (polypyrrole) by regulating the interactions between paramagnetic polarons and surrounding molecules.

    The oxidized forms of conjugated polymers are paramagnetic polarons (cation) and diamagnetic bipolarons (dication). The strong “spin-spin” interactions between polarons hinder the interactions between polarons and water protons. Thus, polarons cannot interact with water protons to generate MRI contrasts. Based on above speculations, Shao’s group develops a “partially complexed” method using catechol derivatives to release polarons from the strong “spin-spin” interactions. Meanwhile, the population of bipolarons is elevated. Biopolarons can enhance the interactions between polarons and water protons by electrostatistic repulsion interactions. As-prepared polypyrrole nanoparticles can be used for in vivo magnetic resonance imaging. Besides, this “partially complexed” method works for preparing T2-MRI active agents based polypyrrole, polyaniline and polythiophene derivatives.

    This research titled “Non-metallic T2-MRI agents based on conjugated polymers” is published in the journal of Nature Communications with Dr. Lin as the first author, Prof. Shao and Dr. Yang as corresponding authors. See please: https://www.nature.com/articles/s41467-022-29569-x.

Fig 1 Schematic illustration of the “partially complexed” method for activating conjugated polymer nanoparticles in T2 MRI.