From Mayan pigments to innovative nanocomposite materials and beyond
Roberto Giustetto
Dept. of Earth Sciences, University of Turin, via Valperga Caluso 35, 10125 Torino; NIS - Nanostructured Interfaces and Surfaces Centre, via Quarello 15/A, 10135 Torino; INFN - National Institute of Nuclear Physics, via Giuria 5, 10125 Torino (Italy)
Abstract
The ancestor of modern nanocomposite materials was synthesized in Central America, probably more than 1500 years ago. Maya Blue was (and is!) a famous pigment used by the ancient Mayas for mural paintings and decorations. When re-discovered in modern times (circa 1930), it soon captured the interest of the Scientific Community due to its astounding stability. In fact, it can resist the attack of acids, alkalis and solvents, without losing its structural features nor showing any colour fading. The secret about the Maya Blue ‘invulnerability’ lasted until the rise of the new millennium, when it was acknowledged [1] that the pigment forms through a heating-induced encapsulation and bonding of an organic dye – indigo into the tunnels permeating the structure of microporous clay minerals – such as palygorskite and sepiolite. The incorporation and shielding of the guest dye offered by the host framework de facto stabilizes the indigo molecule (usually frail and decomposable), conferring to the resulting hybrid composite its renowned stability.
These achievements issued a new challenge: if Mayas made it Blue, why couldn’t someone make it some different colour? After several attempts, a stable, ‘Mayan-inspired’ red nanocomposite was obtained by grinding and heating palygorskite with methyl red, a well-known azo dye used as a pH indicator [2]. As well as its renowned blue predecessor, the palygorskite@methyl red hybrid composite maintains its purplish-red hue in spite of severe acid or alkali attacks, showing no appreciable change after drastic pH fluctuations. Such an evidence suggested the existence of supramolecular host/guest interactions between the clay and the dye, responsible for the composite stabilization. The accommodation of methyl red inside the palygorskite framework and the nature of their mutual interactions were investigated by means of a multi-analytical approach, including molecular mechanics simulations and experimental methods such as UV-visible, FT-IR, SER and FT-Raman spectroscopies, synchrotron XRPD and TGA coupled with GC-MS. Such a protocol proved that the palygorskite@methyl red composite forms when different forms of the dye (in neutral, zwitterionic and/or protonated forms) spread inside the clay tunnels while heating during synthesis. Incorporation within the host channels favours partial transformation of the guest dye from the neutral to the protonated form. The outstanding stability of the hybrid complex is guaranteed by the existence of several kinds of host/guest interactions, formed between the dye COOH group and the palygorskite framework, whose evolution and/or reversibility vary as a function of the temperature reached during synthesis. The dye encapsulation in the host framework prevents the structural folding typical of palygorskite and modifies its dehydration mechanism with temperature rise. Besides, once sheltered inside the host pores, the thermal stability of methyl red is dramatically enhanced with respect to the isolated dye. The unambiguous existence, in the clay tunnels, of different topological methyl red isomers causes the palygorskite@methyl red complex to be suitably considered a poly-functional organic/inorganic hybrid composite – a definition that fits also for its famed blue-analogue, formed by indigo fixation in the same host [3]. These peculiar physico-chemical properties cause the studied nanocomposite to be a valid, ecological surrogate to other synthetic pigments, used nowadays in the Materials Science and Cultural Heritage fields. Future perspectives aim at synthesizing analogous hybrid composites with different hues as well as testing the suitability of the palygorskite framework as a carrier for other kinds of organic guests (e.g., active principles of drugs).
[1] R. Giustetto, F. Llabres I Xamena, G. Ricchiardi, S. Bordiga, A. Damin, M.R. Chierotti, R. Gobetto, J. Phys. Chem. B 2005, 109 (41), pp. 19360-19368.
[2] G. Batchelder & V.W. Meloche (1932). Contrib. from Chem. Lab. of the Univ. of Wisconsin, 1932, pp.1319-1323. [3] A. Doménech, F.M. Valle-Algarra, M.T. Doménech-Carbò, M.E. Domine, L. Osete-Cortina, J.V.
