Organic-inorganic hybrid materials have applied in catalysis and sensing. We proposed that dehydroindigo (oxidized form of indigo), is a component of Maya Blue, and contributes to the peculiar greenish hue of the pigment,¹ but the presence of this compound has become controversial.² Synthetic specimens prepared upon indigo attachment to laminar clays and channeled clays exhibited characteristic blue-greenish hue after application of thermal treatments. Analysis of extracts provides evidence of dehydroindigo and isatin presence by LC-DAD, UPLC-MS and Py-GC-MS.³ For specimens prepared from laminar clays, the dye is almost quantitatively extracted from the clay support in both thermally treated and unheated specimens, with the idea that dye molecules are ‘externally’ bound to the clay crystals. However, samples prepared from porous/channeled supports retained significant amount of the dye after equivalent extraction experiments. The reported data indicate that indigo oxidations are ubiquitous reactions occurring, upon heating, in all tested clay specimens, the formation of dehydroindigo being particularly favored by thermal treatments removing zeolitic water in the channeled clays.

Aknowledgements. This work has been performed by members of the microcluster Grupo análisis científico de bienes culturales y patrimoniales y estudios de ciencia de la conservación (Ref. 1362) belonging to the Valencia International Campus of Excellence. Financial support is gratefully acknowledged from the Spanish “I+D+I MICINN” projects CTQ2011-28079-CO3-01, 02 and 03 supported by ERDEF. Also by project P1.1B2012-36 of UJI.

References

1.Doménech-Carbó, A. et al. J. Phys. Chem. B 2006, 110, 6027.

2.Doménech-Carbó, A. et al. J. Phys. Chem. C 2009, 113, 12118.