Charge transfer interactions exist in extracellular polymeric substances: Comparison with natural organic matter

Extracellular polymeric substances (EPS) and natural organic matter (NOM) are widely present in the environment. While the molecular basis of NOM's optical properties and reactivity after treatment with sodium borohydride (NaBH₄) has been successfully explained by the charge transfer (CT) model, the corresponding structure basis and properties of EPS remain poorly understood. In this work, we investigated the reactivity and optical properties of EPS after NaBH₄ treatment, comparing them to the corresponding changes in NOM. After reduction, EPS exhibited optical properties and a reactivity with Au³⁺ similar to NOM, manifesting an irreversible loss of visible absorption (≥70%) associated with blue-shifted fluorescence emission (8-11 nm) and a lower rate of gold nanoparticles formation (decreasing by ≥ 32%), which can be readily explained by the CT model as well. Furthermore, the absorbance and fluorescence spectra of EPS were solvent polarity dependent, contrary to the superposition model. These findings contribute to an original understanding of the reactivity and optical properties of EPS and facilitate further cross-disciplinary studies.