Maya blue (MB) simulants were prepared from mixtures of indigo and palygorskite (ind–paly) and investigated by resonance Raman and UV–vis absorption spectroscopies together with thermogravimetric analysis, aiming to enlarge the understanding of the dye–clay interaction, the relationship between color and chemical stability, and the alleged formation of dehydroindigo (DHI); for comparative purposes, other simulants were prepared using sepiolite, laponite, and montmorillonite. The results obtained here suggest that the greenish hue that develops when the ind–paly is heated seems to be linked with a decrease in the indigo molecular symmetry, which causes an increase in the oscillator strength of an absorption band at 500 nm (forbidden under the C 2h symmetry) and a bathochromic shift and narrowing of the intense electronic transition at 657 nm (indigo). The DHI characteristic features are not observed in the resonance Raman spectrum (457.9 nm) of the ind–paly system, contrary to what happens with sepiolite, Laponite and montmorillonite that, however, do not present the chemical stability observed for MB. Resonance Raman, UV–vis absorption spectroscopy, and thermal analysis provided firm evidences that in ind–paly, the dye is inside the micropores, interacting through hydrogen bonding with water molecules coordinated with the metal ions of the clay framework.
Maya Blue is a puzzling pigment found in objects produced by the ancient Maya civilization. It is a combination of indigo and palygorskite, and it is well-known for the high chemical and photochemical stability of the dye promoted by the clay confined environment. This pigment has survived over 1500 years, and it was first thought to be purely inorganic. The reasons for such stability have been investigated over the past years, and it may involve hydrogen bonds, complexation, and oxidation to dehydroindigo. However, these theories are not completely understood, and more evidence about indigo/palygorskite interactions must be obtained. In this study indigo and a Maya Blue simulant pigment were, for the first time, studied by transient absorption (TA) and time-resolved infrared (TRIR) spectroscopy. From such analysis it was possible to investigate the electronic excited states of indigo and the photochemistry behavior of the dye when interacting with the palygorskite. Concerning the TRIR measurements, the shifts of the C O and N−H vibrations indicate that hydrogen bonds are formed involving the dye and the coordinated water molecules present in the clay. Furthermore, a red shift is observed in the absorption of electronic ground state (50 nm) and also in the electronic excited state (27 nm) of Maya Blue simulant, suggesting that the excited states are stabilized by the clay. Indigo in DMSO solution presents a lifetime of ca. 120 ps while in the clay it becomes much shorter, ca. 3 ps. The shorter lifetime and also the red shift observed in the TA results suggest a stabilization of the first electronic excited state, which promotes a more efficient energy relaxation through conical intersection and, as a consequence, a faster excited state decay. Such factors can be important for the Maya Blue photostability, as they are also believed to be responsible for the high photostability of DNA and melanin.
In this work fibers from pre-Columbian Peruvian textiles and synthetic analogs of the natural dyes previously found in Andean historical textiles (indigo, carmine, indigo carmine, purpurin, alizarin and luteolin) were analyzed using a Raman microscope which was also used as a microspectrofluorimeter. SEM-EDS and FTIR were used as complimentary techniques. The efficiency of HNO 3 etched copper surface as SERS substrates was investigated aiming at its application in archaeometry; in this particular case, the capability of such SERS active substrate to cope with luminescence presented by the dyed textile was evaluated. The archaeological fibers were identified as dyed wool by FTIR, FT-Raman and SEM; the red dye (carmine) was identified by resonance Raman and SERS, however, the blue dyed fiber presented a strong fluorescent background in the visible and, in the NIR, the FT-Raman spectrum was not conclusive, therefore the identification was performed using SERS through the dye in-situ reduction on a Cu etched disk. Specifically in the case of synthetic dyes, although all of them had already their SERS spectra reported in the literature, it is here shown that the copper etched surfaces provide SERS and SERRS information that are in full agreement with previously reported data, with the advantage of a much simpler and faster preparation.
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