Selective binding of sulphated glycosaminoglycans induces self-assembly of naphthalene diimide into fluorescent nanofibers.

The self-assembly of π-conjugated chromophores directed by biomolecular recognition offers a smart strategy to create bio-synthetic hybrid nanomaterials with emergent properties. Here, we report a novel amphiphilic, cationic naphthalene diimide (NDI) derivative that undergoes supramolecular polymerization upon interaction with anionic glycosaminoglycans (GAGs). Binding of GAGs like heparin to NDIs leads to their supramolecular polymerization in aqueous media. Interestingly, such binding events result in highly emissive fluorescent nanofibers due to the formation of a static excimer. Spectroscopic and microscopic investigations reveal that polyanionic heparin helps to bring the cationic NDIs into close proximity to promote π-π stacking and that amphiphilic self-assembly is essential for excimer formation. The heparin binding-induced excimer formation exhibits a clear emission color change from blue to bright green. Furthermore, the NDI selectively binds to sulphated GAGs such as heparin and chondroitin sulphate, but not to carboxylated hyaluronic acid, resulting in a differential fluorescence response. Thus, this study presents a heparin binding-induced supramolecular polymerization of a novel NDI derivative, providing a design strategy for controlling supramolecular order and for creating functional fluorescent nanomaterials for future biosensing and bioimaging applications.