Exploring the PhotophysicalProperties of Self-AssembledPhenylalanine: Aggregation-Induced Emission, Fluorescent HydrogelFormation and Selective Interaction with Hemin

The aggregation of proteins and amino acids has garnered significant attention due to its involvement in various health disorders. In this study, we investigated the self-assembly of unmodified l-phenylalanine (Phe) monomers into unique fluorescence-enhanced assemblies and provided a detailed understanding of the time-dependent structural transformations of Phe assemblies, primarily using fluorescence spectroscopy and field emission scanning electron microscopy (FE-SEM). Phe, typically a weak fluorophore, demonstrates a remarkable enhancement in fluorescence intensity upon self-assembly, with a shift in emission maximum from 290 nm (λ_ex = 240 nm) for the monomer to 490 nm (λ_ex = 410 nm) for the assembly. This is accompanied by a nearly 10-fold increase in quantum yield (ϕ = 0.207) compared to Phe monomers (ϕ = 0.022), which can be attributed to aggregation-induced emission (AIE), offering valuable insights into the previously unexplored optical behavior of Phe. FE-SEM revealed the formation of well-defined stacked sheet-like structures, distinct from the typical fibrillar assemblies, highlighting a unique self-assembly process driven by π–π stacking and hydrogen bonding interactions. This structural transformation underscores the intrinsic ability of Phe to organize into ordered architectures with remarkable optical properties. The fluorescence intensity of these Phe-assemblies can be selectively quenched by hemin, enabling their potential use as a biosensor for sensitive detection of heme-proteins with high precision. The interaction mechanism, involving both static and dynamic quenching, was further elucidated through Stern–Volmer analysis, revealing a detection limit as low as 50 pM for hemin. Additionally, the Phe-assemblies form a fluorescent hydrogel with hygroscopic properties, opening avenues for moisture absorption, environmental remediation and biomedical applications. The Phe-assemblies’ remarkable selectivity and sensitivity for heme and heme-containing proteins, coupled with their potential for various biomedical and environmental applications, make them a promising material for fluorescence-based sensing and gel technologies.