Analysis of Iron Complexes of Tannic Acid and Other Related Polyphenols as Revealed by Spectroscopic Techniques: Implications in the Identification and Characterization of Iron Gall Inks in Historical Manuscripts

Abstract

In this work, a structural analysis of the polyphenol complexes with iron and copper at several conditions is reported. The investigated polyphenols were tannic acid, gallic acid, pyrogallol, and syringic acid, being components and molecular models of the gallnuts usually employed in the past in fabrication of iron gall inks (IGIs). Commercial tannic acid extracted from gallnuts, which is a complex mixture of different gallotannins and simpler galloylglucoses, was also employed in this analysis. This analysis comprised the use of Raman, Fourier-transform infrared (FTIR), UV–vis absorption, and fluorescence spectroscopy. The complexation of iron with these molecules leads to a strong change in color due to the deep restructuring of the polyphenol that can be clearly seen by Raman and FTIR spectra. Three main Raman bands appeared at 1450–1490 cm –1 (ν 1 ), 1320–1345 cm –1 (ν 2 ), and 400–650 cm –1 (ν 3 ), which are characteristic of the metal complexes. The structural changes of the polyphenol complexes with iron were also investigated at different pHs and different polyphenol/iron stoichiometries. Other effects of the interaction of polyphenols with iron are the pH decrease of the mixture upon metal complexation and fluorescence quenching induced by the interaction of iron. This quenching is important since it facilitates the Raman inspection of manuscripts since polyphenols show a strong fluorescence emission that overlaps the Raman spectrum. Furthermore, DFT calculations were performed for the first time on the gallic acid complex with iron in order to elaborate a detailed assignment of the vibrational modes of polyphenols and their metal complexes, something that was missed in previous applications of Raman to IGIs.

Super-Stretchable Hydrogel Films with High Fracture Energy Enabled by Coordination Nanoparticles as Multifunctional Wound Dressings

“…The similar sizes of FTCNs-2 and FTCNs-3 are likely due to the presence of more Fe 3+ , which may also increase the particle size by extensive cross-linking, consistent with TGA data showing the increase of iron content from FTCNs-1 to FTCNs-3 (6.47, 7.31, and 9.16%, respectively) (Figure D). According to previous work, , the peaks around 550 cm –1 from Raman spectra indicate the coordination bonding between Fe 3+ and TA (Figure E). Other peaks around 1457 and 1519 cm –1 are attributed to the stretching mode of gallic benzene ring, and those around 850 cm –1 are assigned to the −CH deformation (Figure E).…”

Solid-State Encapsulation of Urea via Mechanochemistry-Driven Engineering of Metal–Phenolic Networks

“…Thus, the electron density of the conjugate system is reduced, which increases the bond order of the C�C bonds in the aromatic ring, and leads to a more intense υ 2 band. 51 Our findings reveal that the binding strength of the Ti IV −TA complexes enhances, thereby improving the stability of the urea−MPN matrix. SEM analyses of TA, ground urea, and urea−MPN matrix indicated that both TA and urea crystals underwent shape restructuring after coordination with Ti IV metal ions during the MPN assembly (Figure 1f−h).…”

Characterization of materials in the late 15th century Ottoman period Animal Encyclopedia and detection of deterioration products by HPLC-DAD

“…various spectroscopic techniques such as Raman, Fourier-transformed infrared (FTIR), UV-Vis absorption, and uorescence spectroscopy [17]. Haskovic and Ibragic used XRF and FTIR techniques for the detection of inks and pigments in manuscripts [15].…”

“…Later, black inks obtained from ivory and bone were used. In addition to these inks used, iron-gall inks used by the Late Roman Empire in the west until the 19th century and even the 20th century were the primary choices for the creation of many manuscripts [14,[16][17][18][19][20]. It is known that the expression gallotannate, which indicates iron-gall ink, was used in Pliny's book in the 1 s t century AD [14,16,21].…”

“…Iron-gall inks are composed of four main components: tannin, vitriol (iron or copper sulfate), water, and gum arabic [11,17,18,22,26,39]. Of these materials, vitriol is referred to as copperas, sal martis, vitriolum commune, vitriolum romanorum and vitriolum cyprinum in some recipe books.…”