Two-Component Fluorescent-Semiconducting Hydrogel from Naphthalene Diimide-Appended Peptide with Long-Chain Amines: Variation in Thermal and Mechanical Strengths of Gels

Abstract

Two-component fluorescent hydrogels have been discovered containing mixtures of naphthalenediimide (NDI)-conjugated peptide-functionalized bola-amphiphile and primary amines with long alkyl chains at physiological pH 7.46. The aggregation induced enhanced emission (AIEE) associated with NDI-appended peptide in aqueous medium is rare, as water is known to be a good quencher of fluorescence. In this study, NDI-containing gelator peptide forms a highly fluorescent aggregate in aqueous medium. Absorption and emission spectroscopic techniques reveal the formation of J-aggregates among the chromophoric moieties in their aggregated state in aqueous medium. However, this NDI-containing peptide doesn’t form any gel in aqueous medium. In presence of primary amines with long alkyl chains in buffer solution, it forms two-component fluorescent hydrogels exhibiting a bright yellow fluorescence under the UV-lamp (365 nm). Probably, the acid-amine interaction between the amines and the bola-amphiphile triggers the gel formation, as it is evident from Fourier transformed infrared (FTIR) data indicating the presence of carboxylate group (-COO-) and ammonium species (NH3+) in the co-assembled two-component gel system. Low and wide angle powder X-ray diffraction (PXRD), small angle X-ray scattering (SAXS) further supports the fact that the co-assembled state in the gel form is produced by the supramolecular interaction between the NDI-based bola-amphiphile and the long chain amines. Field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM) images reveal that the π-conjugated co-assembled hydrogels exhibit nano-fibrillar network morphologies. Interestingly, the co-assembled hydrogels exhibit an enhanced fluorescence emission, excited state lifetime and quantum yield compared to those of the NDI-containing amphiphile alone in its self-assembled state in aqueous medium. Moreover, thermal stability and mechanical strength of these gels have been successfully tuned by varying the alkyl chain length of the corresponding amine. Moreover, these NDI-peptide conjugated soft-materials exhibit semiconducting behavior in their respective co-assembled states. This holds future promise to use these peptide appended NDI-based co-assembled soft materials for the application in opto-electronic and other devices.