Fluorescent homopolypeptide toroids†
Toroids are important ring-like nanostructures in living systems; intrinsically luminogenic toroids are promising in bioimaging but it remains challenging to synthesize such nanoparticles. Herein, we report a fluorescent toroid that is self-assembled from the homopolypeptide poly(γ-benzyl-L-glutamate) (TPE-PBLG28, where TPE refers to the end-group tetraphenylethene and the subscript refers to the degree of polymerization). We found that both water content and temperature are crucial for the formation of toroids during self-assembly. The morphological evolution pathway goes from twisted nanoparticles to curved nanoparticles, and eventually, to toroids; this was analyzed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), etc. This new mechanism of end-to-end closure of twisted nanoparticles that leads to toroids offers new insight for preparing ring-like nanostructures, as it is different from the traditional ‘loop-and-wind’ route of high molecular weight PBLG. The fluorescent TPE end-group confers the final low cytotoxic toroids suitable for cell imaging. Overall, this work provides insight into a new mechanism for the formation of fluorescent polypeptide toroids.