Supramolecular assembly of a pyrrole-based squaraine into multifunctional nanoparticles for chemical sensing and photochemical applications

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Abstract
Herein, we report the synthesis of a pyrrole-based symmetric squaraine (SQL) and its supramolecular organization into nanoparticles (SQLNPs) in aqueous media. The aggregation process, driven by non-covalent interactions, gives rise to J- and H-type assemblies and produces pronounced changes in the optical properties of the squaraine. The resulting nanoparticles display selective colorimetric responses toward Zn²⁺, Hg²⁺ and Pd²⁺ and toward low-molecular-weight thiol-containing compounds, with particularly pronounced sensitivity toward glutathione. The photochemical properties of SQL and SQLNPs were also investigated using complementary reactive-oxygen-species assays. Under the experimental conditions employed, blue-light irradiation produced a detectable radical-type ROS response for the aggregated nanoparticles, whereas no measurable response was observed under orange or red-light irradiation. The absence of detectable singlet-oxygen generation together with the response to radical-sensitive assays is consistent with a predominantly Type I radical-mediated photochemical pathway. Computational analysis further indicates that supramolecular aggregation modifies the electronic structure of SQL, particularly for the H-type aggregate. Finally, SQLNPs display blue-light-dependent phototoxicity toward A549 cells. These results demonstrate how supramolecular organization of a multifunctional squaraine can modulate its optical, sensing and photochemical properties and provide a platform for further development of squaraine-based functional nanomaterials.