Salt-Induced, Continuous Deposition of Supramolecular Iron(III)-Tannic Acid Complex

One-step assembly of iron(III)-tannic acid (Fe³⁺-TA) complex forms nanothin (∼10 nm) films on various substrates within minutes. In this deposition scheme, however, the film does not grow continuously over time even though Fe³⁺-TA complex is still abundant in the coating solution. In this paper, we report that the salt addition dramatically changes the one-off coating characteristic to continuous one, and each salt has its optimum concentration (C_MFT) that produces maximum film thickness. For detailed investigation of the salt effects, we employed various salts, including LiCl, NaCl, KCl, CaCl₂, SrCl₂, BaCl₂, NaBr, and NaNO₃, and found that only cations played an important role in the continuous deposition of the Fe³⁺-TA complex, with smaller C_MFT values for the cations of higher valency and larger size. On the basis of the results, we suggested that the positively charged cations screened the negative surface charges of Fe³⁺-TA complex particles, leading to coagulation and continuous deposition, further supported by the ζ-potential measurement and time-resolved dynamic light-scattering analysis.