Enhanced deposition of Fe(III)-tannic acid complex nanofilm by Fe(III)-embedded dextran nanocoating

Nanofilm deposition with Fe(III)-tannic acid (TA) complex serves as a surface nanoengineering technique for a wide variety of substrate materials. However, it has been still elusive to tune the fundamental nanofilm properties, such as thickness and roughness, for a single Fe(III)-TA layer. Here we demonstrate Fe(III)-embedded dextran nanocoating serving as a boosting layer for enhanced deposition of Fe(III)-TA complex nanofilm with higher roughness. The formation of Fe(III)-embedded dextran layer with 0.37 mM FeCl₃·6H₂O shows maximum three times deposition enhancement of Fe(III)-TA layer in film thickness (ca. 9 nm), compared with the conventional deposition. pH stability tests show that the multilayer structures formed with Fe(III)-TA layers and Fe(III)-embedded dextran layers are highly stable at pH 7.4 but gradually and rapidly degrade at pH 10 and pH 2, respectively. Further surface characterizations disclose that solution pH, correlated with the concentration of Fe(III) source (FeCl₃·6H₂O), is the major factor governing the deposition enhancement effect: more than 1.85 mM FeCl₃·6H₂O makes significantly acidic chemical environments hampering the further deposition of Fe(III)-TA nanofilms. The experimental findings herein provide clues for deeper understanding of metal-polyphenol complex nanofilm deposition as well as contribute to the future development of biocompatible nanocoating utilizing promising combinations of polysaccharides and polyphenols.