Dual-fluorescent starch biopolymer films containing 5-(4-nitrophenyl)-1,3,4-thiadiazol-2-amine powder as a functional nanofiller

Physical and photophysical properties of starch-based biopolymer films containing 5-(4-nitrophenyl)-1,3,4-thiadiazol-2-amine (NTA) powder as a nanofiller were examined using atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), stationary UV-Vis and fluorescence spectroscopy as well as resonance light scattering (RLS) and time-resolved measurements, and where possible, analyzed with reference to pristine NTA solutions. AFM studies revealed that the addition of NTA into the starch biopolymer did not significantly affect surface roughness, with all examined films displaying similar Sq values ranging from 70.7 nm to 79.7 nm. Similarly, Young's modulus measurements showed no significant changes after incorporating the 1,3,4-thiadiazole. Adhesion force and water contact angle assessments demonstrated that the films maintained high hydrophilicity (water wetting) across all examined films. Color analysis corroborated the anticipated trend, showing that increasing additive content resulted in decreased lightness and increased yellowness. Interestingly, however, while in polar isopropanol solvent at low concentration, NTA shows a typical single-band emission, centered at 410 nm and a slight enhancement of the band on the long-wavelength side around 530 nm, its incorporation into the biopolymer matrices results in the appearance of dual fluorescence signal with maxima at 430 and 530 nm. Concentration-dependence emission experiments, demonstrating that with even a slight increase of the amount of NTA in solution, an additional, weak long-wavelength emission band emerged within the spectral range corresponding to the intensive band in the biopolymer film, along with results of the performed quantum-chemical studies, including both the monomeric and aggregated (dimer and trimer) models, conclusively unveil that the dual fluorescence observed in starch/NTA films is due to molecular aggregation effects resulting in aggregation-induced emission. This study underscores the potential of NTA as an additive in biobased polymer films, furnishing them with new photophysical features without substantially altering their surface properties and thus enabling their extended applications.