Isomerization and redox tuning in ‘Maya yellow’ hybrids from flavonoid dyes plus palygorskite and kaolinite clays

Highlights

• Organic–inorganic hybrid materials from kaolinite and palygorskite clays and flavonoid natural dyes are prepared.

• The composition of the organic fraction is studied by solid state electrochemistry and mass spectroscopy.

• Thermal treatment above 100 °C determines significant isomerization and oxidation reactions.

• Dye attachment to the clays would define a peculiar ‘Maya chemistry’.

Abstract

The composition of the organic fraction of organic–inorganic hybrid materials prepared upon attachment of different natural, ‘historical’ flavonoid yellow dyes (zacatlaxcalli, fustic, marigold and cosmos) to palygorskite and kaolinite clays is described. Upon thermal treatment between 100 and 180 °C, significant isomerization and oxidation reactions occur thus resulting in the formation of polyfunctional materials potentially usable for therapeutic, catalytic and art purposes. The dye attachment to the clays would define a ‘Maya chemistry’ whose complexity could explain the versatile use of such materials in the pre-Columbian cultures.

Introduction

The Maya blue, a pigment widely used in wall paintings, pottery and sculptures by the Mayas and other ancient Mesoamerican peoples, has received considerable attention, since its discovery by Merwin [1], because of its intriguing durability and peculiar hue variability [2]. From the contemporary view, the Maya blue can be considered as a particular example of organic–inorganic hybrid materials [3], resulting from the association between indigotin or indigo (3H-indol-3-one-2-(1,3-dihydro-3-oxo-2H-indol-2-ylidene)-1,2-dihydro), a blue dye extracted from leaves of añil or xiuquitlitl (Indigofera suffruticosa) in Mesoamerica and a phyllosilicate clay, palygorskite, of ideal formula Si₈(Mg₂Al₂)O₂₀(OH)₂(H₂O)₄·4H₂O.

Apart from the still remaining controversy on the nature of the clay-dye association and the location of dye molecules in the clay framework [4], [5], [6], [7], [8], [9], [10], [11], the Maya blue has challenged to scientists due to the high symbolic value in the Maya culture contrasting with the absence of historical sources describing its preparation procedure. The studies of Gettens [12], Van Olphen [13], Shepard [14] and Kleber et al. [15] prompted the aforementioned view of the pigment as an organic–inorganic hybrid material. In parallel, Arnold discovered several locations in Yucatan peninsula where palygorskite has been found [16], [17] and, recently, has reported the first direct evidence of pre-Columbian sources of palygorskite for Maya blue [18].

The most extended view on the Maya blue, expanded to similar hybrid materials prepared from the dye and other clays [19], [20], [21] consider them as constituted by the original dye molecules distributed externally and/or inside pores and channels of the clay, with no other accompanying organic components [4], [5], [6], [7], [8], [9], [10], [11]. Based on solid state electrochemistry and spectral techniques, we proposed that, contrary to this dominating view, dehydroindigo, the oxidized form of indigo, accompanied the parent pigment in the clay and contributed significantly to the pigment hue and its variability [22], [23]. In this view, these hybrid materials can be described as a polyfunctional nanostructured systems where different dyes can be attached to the inorganic support in different sites [24], [25] the dye attachment to the clay determining the appearance of isomerization and redox tuning reactions [26], [27].

In this context, combination of electrochemical and spectroscopic data permitted to propose that the Maya blue pigment experienced local variations and evolved in time following a ramified scheme [23], [28] thus in agreement with the idea, expressed by Magaloni [29], that different technical schools existed. Additionally, recent archaeological discoveries suggested the existence of other associations between clays and indigoid dyes such as dehydroindigo and indirubin in the area Maya [30], [31], [32] so that leucoindigo [33] or isatin [30] could be present in several Mayan yellow pigments while dehydroindigo would be the majority component in remaining of a greenish plaster found in the site of La Blanca (Guatemala) [31], [32].

These results suggest a series of questions: is it possible to prepare other dye plus clay hybrid materials with stability comparable to that of the Maya blue?, or is Maya blue a unique material with regard to its significant redox tuning? Our more recent data on hybrids prepared from lapachol (2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone), a yellow dye extracted from guachupin plant (Diphysa robinoides, fam. Fabaceae) and porous phyllosilicates (palygorskite, sepiolite), reveals that isomerization, cyclization and redox tuning occurs accompanying the thermally assisted association of the dye to the clay, thus suggesting that there was a ‘Maya chemistry’ no limited to Maya blue [26], [27], [34].

In this report, we present a study on several dye-clay hybrid materials which could be used as yellow pigments in pre-Columbian civilizations [35]. Several yellow dyes were reported to be used on some Colonial Mexican documents [36]. Interestingly, the presence of yellow dyes associated to clays in the Codex Colombinus was reported by Torres et al. [37] and, more recently, in the Cospi Codex by Miliani et al. [38], thus suggesting the possibility of other than Maya blue dye-clay associations. Four species of dyes used in Mexico in pre-Columbian times [35], [38] have been selected for attaching to kaolinite (KA), a layered aluminosilicate (Al₂Si₂O₅(OH)₄), where only ‘external’ dye adsorption would be accessible and palygorskite (PL), which enables external attachment and inside channels or pores. Table 1 summarizes the chosen dyes with their nahuatl, Spanish and English names, botanic species of plants from which the dyes are extracted, and their main flavonoid chromophore molecules. The use of yellow dyes in molecular structure of the latter is illustrated in Fig. 1. Dyes will be labeled as zacatlaxcalli (ZAC), fustic (FUS), marigold (MAR) and cosmos (XOC). Analysis of the extracts from different dye plus clay complexes by high performance liquid chromatography with diode array detection (LC-DAD), and gas chromatography/mass spectrometry (GS–MS), combined with electron microscopy, visible and infrared spectroscopies reveals that, upon moderate thermal treatment, there is a significant modification in the composition of the guest dye. The chromatographic and spectroscopy study is combined with solid state electrochemistry of the hybrid materials in order to directly explore the possibility of dye redox tuning. Here, the voltammetry of microparticles (VMP), a methodology developed by Scholz et al. [39], [40] which provides analytical information on sparingly soluble solids attached to inert electrodes in contact with suitable electrolytes, has been applied. This technique is of application in a variety of fields [41] including archaeometry, conservation of heritage [42] and is complemented with scanning electrochemical microscopy (SECM) measurements. SECM is a technique used for characterizing surface properties providing an electrochemical topography image of surfaces, at the nanoscopic scale [43].