Dehydroindigo: A New Piece into the Maya Blue ... - ACS Publications

J. Phys. Chem. B 2006, 110, 6027-6039

6027

Dehydroindigo: A New Piece into the Maya Blue Puzzle from the Voltammetry of Microparticles Approach Antonio Dome´ nech,*,† Marıá Teresa Dome´ nech-Carbo´ ,‡ and Marıá Luisa Va´ zquez de Agredos Pascual‡,§ Departament de Quı´mica Analı´tica, UniVersitat de Vale` ncia, Dr. Moliner, 50, 46100 Burjassot (Vale` ncia) Spain, Institut de Restauracio´ del Patrimoni/Departament de ConserVacio´ i Restauracio´ de Bens Culturals, UniVersitat Polite´ cnica de Vale` ncia, Camı´ de Vera 14, 46022, Vale` ncia, Spain, and Departamento de Historia del Arte, UniVersidad de Valencia, Passeig al Mar s/n, Vale` ncia ReceiVed: December 14, 2005; In Final Form: January 25, 2006

Combining a novel technique, the voltammetry of microparticles, with spectrometric, nuclear magnetic resonance, electron microscopy, and atomic force microscopy data, Maya Blue is detected in wall paintings of the Substructures A-3, A-5, and A-6, dated in the Early Classical period (440-450 a.c.), and the Substructure II-C, dated in the Late Preclassical period (150 b.C.), in the archaeological site of Calakmul (Campeche, Mexico), thus providing evidence on the use of the pigment 750 years prior to the date currently accepted. Electrochemical measurements, supported by spectrometric data, indicate that the presence of palygorskiteattached dehydroindigo, the oxidized form of indigo, contributes to the greenish color of Maya Blue. Enthalpy and entropy of attachment of such compounds to palygorskite are calculated from the temperature dependence of electrochemical data. Both attachment processes are endothermic, becoming thermodynamically spontaneous at moderate temperatures. Accordingly, ancient Mayas may modulate the hue of Maya Blue from turquoise to greenish blue by controlling the temperature during the crushing process.

Introduction Maya Blue is a famous indigo-based pigment produced by the ancient Mayas that has a peculiar color and significant stability that has been of interest over the years. Maya Blue can show different hues, ranging from a bright turquoise to a dark greenish blue. The pigment is surprisingly stable, being unaffected by the attack of acids, alkalis, oxidants, reducing agents, and organic solvents. Maya Blue has attracted attention recently1-12 because of its character of inorganic-organic material, so that the elucidation of its structure is not only a challenge for chemists and conservators but also a possible source of information in the design of new synthetic routes for advanced hybrid materials.1 Currently, there exists a general agreement in which the association of indigo to a local clay, palygorskite (known as attapulgite for historicians and restorers), determines the stability of the pigment, which can be described as a nanostructured material.1 The use of Maya Blue in Central America, mostly in Mexico, was until now documented from the eighth to the sixteenth centuries,13 being extended even to recent times.14 Although the procedure of preparation of Maya Blue is unknown, it is believed that the Mayas prepared the pigment by crushing indigo (obtained from several plants, mainly Indigofera suffruticosa Miller) and palygorskite with a moderate thermal treatment.15,16 However, the nature of the indigopalygorskite association was in the past controversial.17-23 In 1962, Gettens17 hypothesized that the pigment was purely composed by blue palygorskite mineral. Shepard18 first intro* Corresponding author. E-mail: [email protected] † Universitat de Vale ` ncia. ‡ Universitat Polite ´ cnica de Vale`ncia. § Universidad de Valencia.

duced the idea of Maya Blue being an unusual pigment consisting of a dye attached to certain clays in Yucatan. In 1966, van Olphen19 prepared a complex analogous to Maya Blue by heating indigo and palygorskite, which complex had a Fourier transform infrared (FTIR) spectra and powder X-ray diffractogram (XRD) that were roughly equivalent to those recorded for authentic Maya Blue samples and suggested that the indigo molecules are adsorbed only on the external surfaces of the clay mineral. Kleber et al.20 suggested that indigo molecules are intercalated inside the microporous tunnels of palygorskite. Arnold21 later identified palygorskite (or attapulgite) in Yucatan clays and commented on its importance as an ingredient in preCulombian (and present day) ceramicware. One village in the northern peninsula, Sacalum (a Spanish corruption of Sac lu’um), has apparently served as a major source of this clay for over 800 years. In the 1980s, Littmann22 thought that the pigment was a naturally blue montmorillonite but later23 acknowledged the validity of the palygorskite/indigo complex model. The main inorganic component of Maya Blue, palygorskite, is a fibrous phyllosilicate of ideal composition (Mg,Al)4Si8(O,OH,H2O)24‚nH2O, consisting of continuous, two-dimensional tetrahedral and octahedral sheets. Palygorskite can be described as a mixture of two different polytypes: one monoclinic and one orthorhombic.24,25 Both polytytpes show discontinuous 2:1 tetrahedral/octahedral layer framework with one 2:1 unit joined to the next by inversion of the SiO4 tetrahedra along Si-O-Si bonds. The tetrahedral and octahedral mesh gives rise to a series of rectangular tunnels of dimensions 6.4 × 3.7 Å. Sepiolite, having a similar structure, possess 10.6 × 3.7 Å tunnels. Such clays are therefore crossed by zeolite-like channels and permeated by weakly bound, nonstructural (zeolitic) water, as described by Giustetto and Chiari.26 Magnesium and aluminum

10.1021/jp057301l CCC: $33.50 © 2006 American Chemical Society Published on Web 03/07/2006

6028 J. Phys. Chem. B, Vol. 110, No. 12, 2006 cations complete their coordination with tightly bound water molecules (structural water). Crystal structure refinement of Maya Blue pigment prepared from deuterated indigo, using neutron powder diffraction, is also provided by Giustetto et al.27 Indigo is a blue dye widely used in several civilizations as a pigmenting agent. Natural indigo is formed by indigotin (3Hindol-3-one, 2-(1,3-dihydro-3-oxo-2H-indol-2-ylidene)-1,2dihydro), a quasi-planar molecule of approximate dimensions 4.8 × 12 Å having a slightly elongated central CdC bond and two elongated CdO bonds. Hydrogen bonds between adjacent molecules cause solid-state aggregation in which one indigo molecule is linked to four others.28 Indigotin is accompanied in plant extracts by indirubin (or indigo brown) (2H-indol-2one, 3-(1,3-dihydro-3-oxo-2H-indol-2-ylidene)-1,3-dihydro), isoindigotin (2H-indol-2-one, 3-(1,2-dihydro-2-oxo-3H-indol-3ylidene)-1,3-dihydro) and different precursor compounds. The nature of the indigo-palygorskite interaction and the reasons for its color and durability still remain open to discussion. Jose´-Yacamań et al.29 discovered that most palygroskite particles exhibit a superstructure along the a-axis about 14 Å, which roughly corresponds to three times the original lattice constant a and might well be explained by the presence of indigo in the channels. Such authors confirmed the presence of iron and iron oxide nanoparticles in Maya Blue, suggesting that these may at least partly account for the color via Mie dispersion.29 Additional results have been provided by Polette et al.12 that have reported the presence of iron nanoparticles outside the lattice of the crystallites of palygorskite as well as inside the channels. Iron oxide and an amorphous phase of FeO(OH) have been also found in Maya Blue samples, suggesting that they contribute to the optical properties of the pigment or in the characteristic brilliant color. In contrast, del Rio et al.3 measured at the ESRF ID26 beamline the Fe K-edge XANES spectra of the blue pigment in Maya Blue samples and synthetic ones, concluding that the iron found in Maya Blue pigment is related to the Fe exchanged in the palygorskite clay. These authors did not find iron in metallic form or goethite in archaeological Maya Blue. Hubbard et al.10 proposed a model in which indigo molecules are rigidly fixed to the clay mineral surface through hydrogen bonds of their carbonyl and amino groups with edge silanol units, and these molecules act to block nanotunnel entrances. Alternatively, Chiari et al.8 have proposed that indigo can fit into the channels of palygorskite, forming hydrogen bonds between the CdO group of the dye and structural water of the clay. Molecular dynamics simulations of the indigo-containing palygorskite predict that, after several tens of picoseconds, the molecules of indigo must set into stable sites along the channels of the clay and no further diffusion occurs. Host-guest electrostatic interactions and hydrogen bonds between the indigo carbonyl group and structural water may be accompanied, following Fois et al.,9 by a direct close interaction between such carbonyl groups and the clay octahedral ions at the edge of the channels. Giustetto et al.4 studied the indigo-palygorskite interactions by combining computational and spectroscopic techniques, concluding that formation of hydrogen bonds between structural water and the CdO and N-H groups of indigo occur. In the current report, a novel technique, the voltammetry of microparticles, has been used for studying Maya Blue samples. As described by Scholz et al.30,31 the voltammetry of microparticles is based on the record of the electrochemical response of small amounts of a sparingly soluble solid attached to the surface of an inert electrode in contact with a suitable electrolyte.

Domeńech et al. In this work, we combine that methodology with that of the electrochemistry of electroactive species encapsulated within microporous and mesoporous materials, recently reviewed by Bessel and Rolison.32 Electrochemical data are combined with the microscopic examination of samples by transmission electron microscopy (TEM), atomic force microscopy (AFM), FTIRATR, UV-vis, and solid state 13C NMR spectroscopies. Such techniques have been applied to the study of a series of 26 samples from Yucatań sites Chacmultuń, D’zula, Ek Balam, Acanceh, Kuluba´, Mulchic, and Campeche sites Dzibilnocac and El Tabasquenõ, all of the Late Classical period, and in the Yucatań sites of Chicheń Itza´ (Terminal Classical period) and Mayapań (Postclassical period). Four additional samples from the Substructures A-3, A-5, and A-6 of the site of Calakmul (Campeche), dated at 400-450 a.c., corresponding to the Early Classical period and the Substructure II-C of the same archaeological site, dated in the Late Preclassical period (150 b.c.) have been studied. These last samples are of particular interest because, until now, the use of Maya Blue in Yucatań wall paintings was documented in the Late Classical period only (600-800 a.c.). Experimental Section Sampling and Reference Materials. Micro- (ca. 1 µg) and eventually nanosamples (ca. 20-50 ng) of Maya Blue wall paintings were taken in the listed archaeological sites with the help of a microscalpel during routine examination and restoration. A light microscope Leica DMR (×25 - ×400) was used for selecting the samples to be analyzed and for morphological examination of them. Palygorskite was collected from the Sacalum classical deposits in Yucatan. Synthetic indigo (Fluka) and synthetic “Maya Blue” (Kremer), consisting of indigo adsorbed onto silica, were used as reference materials. Instrumentation and Procedures. Pictorial samples were examined with a JEOL JSM 6300 scanning electron microscope operating with a Link-Oxford-Isis X-ray microanalysis system. The analytical conditions were: accelerating voltage 20 kV, beam current 2 × 10-9 A, and working distance 15 mm. Samples were carbon coated to eliminate charging effects. Semiquantitative microanalysis was carried out using the ZAF method for correcting interelemental effects. The counting time was 100 s for major and minor elements. Microparticle voltammetry experiments were performed using paraffin-impregnated graphite electrodes immersed into acetic acid/sodium acetate buffer (total acetate concentrated 0.50 M, pH 4.85). Measurements were performed in a thermostated three-electrode cell under argon atmosphere using a AgCl (3 M NaCl)/Ag reference electrode and a platinum-wire auxiliary electrode. Cyclic and square wave voltammograms (CVs and SQWVs, respectively) were obtained with a CH I420 apparatus. Experiments at variable temperature were performed with the help of a conventional bath in the range between 0 and 75 °C. For modified electrode preparation, the samples were powdered in an agate mortar and pestle and extended forming a spot of finely distributed material. The lower end of the graphite electrode was pressed over that spot of sample to obtain a sample-modified surface. Visible spectra were obtained with a Minolta CM-503i spectrophotometer using a Xe-arc lamp and a Si photodiode detector. The instrument was calibrated with a standard white (coordinates Y ) 95.8; x ) 0.3167; y ) 0.3344). A transmission electron microscope, Philips CM10 with Keen view camera, soft imaging system was used operating at 100

Dehydroindigo

J. Phys. Chem. B, Vol. 110, No. 12, 2006 6029

kV. Maya blue pigment from Dzibilnocac site and palygorskite from Sakalum site were prepared by grinding a few micrograms of the samples in an agate mortar and then dispersing them by the help of an ultrasonic bath in dichloroethane. A drop of the dispersions was poured on TEM grids pretreated with a polymer film layer with holes in order to improve the images obtained. A Multimode AFM (Digital Instruments VEECO Methodology Group) with a NanoScope IIIa controller and equipped with a J-type scanner (maximum scan size of 150 × 150 × 6 um) was used. The topography of the samples was studied in contact mode. An oxide-sharpened silicon nitride probe Olympus, VEECO Methodology Group, model NP-S has been used with a V-shaped cantilever configuration. The spring constant is 0.06 N/m and the tip radius of curvature is 5-40 nm. For electrochemical measurements, the AFM was coupled to a Digital Instruments universal biopotentiostat (VEECO Methodology Group). All measurements were performed at room temperature in solutions previously deaerated with argon during 15 min. FTIR-ATR spectra of samples and reference materials were obtained with a Vertex 70 Fourier transform infrared spectrometer with a FR-DTGS (fast recovery deuterated triglycine sulfate) temperature-stabilized coated detector. Number of co-added scans: 32; resolution: 4 cm-1. The 1H to 13C cross-polarization spectrum was acquired on a Bruker AV400 spectrometer with a 7 mm Bruker BL-7 probe at a sample spinning rate of 5 kHz by using a 90° pulse for 1H of 5 microseconds, a contact time of 5 ms, and a recycle delay of 3 s. Results and Discussion Microscopic Examination of Samples. A typical TEM micrograph of pristine palygorskite is shown in Figure 1a. The image consists of aggregates of elongated crystals 0.5-1 µm sized having fiber structures with thickness from 30 to 60 nm. In contrast, TEM images of Maya Blue samples show narrow fibers with a corrugated structure as can be seen in Figure 1b. Textural differences between palygorskite crystals from pristine clay and Maya Blue samples were also found upon examination by AFM, as shown in Figure 2. Again, palygorskite crystals exhibit a “clean” fiber structure, as illustrated in Figure 2a. In contrast, Maya Blue samples present elongated crystals divided into almost square sections of 125 ( 5 nm size, as depicted in Figure 2b. The observed feature can tentatively be attributed to the presence of domains of regular distribution of indigo on the surface of palygorskite rods or to extensive dislocations of the palygorskite associated with the distribution of indigo molecules. It should be noted that the observed alteration of the topography of the surface of the palygorskite fibers can significantly influence the optical properties of the material because, as studied by Jose´-Yacamań et al.,12,29 radiation dispersion is conditioned by surface roughness. Similar TEM and AFM images were obtained for Maya Blue samples from all the studied archaeological sites. In particular, typical TEM and AFM Maya Blue images were obtained for microsamples from the Substructures A-3, A-5, A-6, and c-II of the site of Calakmul (Campeche), denoting te presence of Maya Blue in the Early Classical period and the Late Preclassical period. Such data indicate that Maya Blue was prepared and used at least 750 years before the date currently accepted. Solid-State Electrochemistry. The solid-state electrochemistry of synthetic indigo microparticles in contact with aqueous acetate buffer is illustrated in Figure 3a. CVs of indigo show two well-defined couples at equilibrium potentials of +475 (I) and -280 mV (II) vs AgCl/Ag, calculated as the half sum of

Figure 1. Micrographs obtained by TEM of (a) pristine palygorskite sample from Sacalum (12000x magnification); (b) Maya Blue sample (18000× magnification) Dzibilnocac-2 (Yucatań, Late Clasical period).

the anodic and cathodic peak potentials. Such equilibrium potentials are independent of the potential scan rate, denoting a reversible behavior. Accordingly, and in agreement with recent literature data,33,34 processes I and II can be ascribed, respectively, to the oxidation of indigo, H2IN, to dehydroindigo, IN, and the reduction of indigo to leucoindigo, H4IN. Such electrochemical processes can be represented as (see also Scheme 1): Process I:

H2IN (s) f IN (s) + 2H+ (aq) + 2e-

(1)

H2IN (s) + 2H+ (aq) + 2e- f H4IN (s)

(2)

Process II:

where (s) denotes solid phases. Notice that charge conservation is ensured in both cases by the issue/ingress of two protons and two electrons from/to the solid. The voltammetric pattern changes significantly in contact with alkaline media (0.10 M NaOH), where leucoindigo exhibits a significant solubility. As is depicted in Figure 3b, a reduction peak at -850 mV (III) appears clearly separated from its anodic couterpart at -590 mV. This response can be interpreted, following Bond et al.,33 in terms of the occurrence of a reductive dissolution process yielding leucoindigo in solution phase. The

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Domeńech et al.

Figure 2. AFM images of (a) pristine palygorskite from Sacalum, and (b) Maya Blue sample Dzibilnocac-2 (Yucatań, Late Classical Maya period). Pristine palygorskite crystals exhibit a smooth surface in contrast with the corrugated surface observed for Maya Blue samples.

electrochemical process III can be represented as

H2IN (s) + 2H+ (aq) + 2e- f H4IN (aq)

(3)

Voltammetry of Maya Blue samples was similar to that of indigo as can be seen in Figure 4 where square wave voltammograms initiated at -750 mV in the positive direction for (a) indigo, and samples from (b) Calakmul, Substructure II-C, (c) Dzibilnocac-2, and (d) Mayapań-2 are shown. Voltammetry of microparticles confirms the presence of indigo in all previously listed samples. In particular, samples from the substructures A-3, A-5, and A-6 (Early Classical period) and II-C (Late Preclassical period) of Calakmul exhibited the characteristic voltammetric profile of Maya Blue. In contact with 0.10 M NaOH, the voltammetric peaks almost entirely disappear. Now, as can be seen in Figure 5, a welldefined response was obtained. Upon convolution, overlapping peaks I, II, and III can be distinguished. SQWVs of Maya Blue samples exhibit some significant differences with those of indigo microparticles: (i) small, but consistent differences in peak potentials; (ii) peak broadening; (iii) remarkably different peak current ratio for processes I and II. Peak potential data are shown in Table 1. The values of the quotient between the peak current of processes I and II, ip(I)/ip(II) in SQWVs are summarized in Table 1. Values of ip(I)/ip(II) are clearly larger than those of indigo microparticles, suggesting that a different proportion of the different forms of indigo exists. Application of methods for

Figure 3. CVs of indigo microparticles attached to PIGEs immersed into (a) 0.50 M acetate buffer; (b) 0.10 M NaOH. Potential scan rate 50 mV/s.

SCHEME 1: Structural Formulas for Indigo, Dehydroindigo, and Leucoindigo

evaluating the proportion of depolarizers in different oxidation states33,34 provides that a significant amount of dehydroindigo, the oxidized form of indigo, is present in Maya Blue samples. The difference between the above peak current ratio for indigo and those recorded in Maya Blue samples can be clearly seen in Figure 6, where the variation with the square wave amplitude, ESW, of the peak current ip(I)/ip(II) ratio for indigo microparticles (squares), indigo attached to silica (triangles), and Maya Blue sample Acanceh-2 (rhomboids). Here, values of ip(I)/ip(II) for

Dehydroindigo


Figure 5. Voltammetry of Maya Blue sample from Mayapań (Mayapań-2) immersed into 0.10 M NaOH. (a) CV, potential scan rate 50 mV/s; (b) SQWV, potential step increment 4 mV; square wave amplitude 25 mV; frequency 5 Hz. Semi-derivative convolution of both voltammograms was performed for increasing peak resolution. Figure 4. SQWVs of (a) indigo microparticles, and samples from (b) Calakmul, Substructure II-C, Late Preclassical period; (c) Dzibilnocac2, Late Classical period; (d) Mayapań-2, Early Postclassical period. Electrolyte 0.50 M acetate buffer, pH 4.85. Potential scan initiated at -750 mV in the positive direction. Potential step increment 4 mV; square wave amplitude 25 mV; frequency 5 Hz.

solution phase as a result of ion exchange between the electrolyte and the porous solid. In both cases, charge conservation requires the issue/entrance of electrolyte countercations to the aluminosilicate pore/channel system. Accordingly, redox processes I and II observed in Maya Blue samples can be represented as

{H2IN + palygorskite}(s) f indigo microparticles differ significantly from those recorded for indigo attached to inorganic solids. It can be observed in Figure 6, that the peak current ratio remains essentially ESWindependent for indigo microparticles and indigo attached to silica. In contrast, peak current ratio values recorded for Maya Blue samples are notably enhanced at ESW values below 75 mV whereas they become almost identical to those obtained for indigo adsorbed on silica at ESW values larger than 75 mV. As depicted in Figure 7, the peak current ratio of Maya Blue samples is clearly larger than that measured for indigo microparticles at all square wave frequencies. These features can be regarded within the context of the electrochemistry of redox-active species attached to micro- and mesoporous aluminosilicate-type solids. Following Bessel and Rolison,37 the observed electrochemical response can be attributed to electron-transfer processes involving electroactive molecules located in the more external region (or boundary) of the solid particles and/or electroactive molecules passing to the

{IN + palygorskite}(s) + 2H+ (aq) + 2e- (4) and

{H2IN + palygorskite}(s) + 2H+ (aq) + 2e- f {H4IN + palygorskite}(s) (5) respectively. In alkaline media the reductive dissolution process III can be formulated as

{H2IN + palygorskite}(s) + 2H+ (aq) + 2e- f H4IN (aq) + {palygorskite}(s) (6) Following the scheme previously developed for different electroactive species associated with zeolites38-40 and mesoporous aluminosilicates,40,41 at relatively long-time experiments


Domeńech et al.

TABLE 1: Electrochemical Data for Indigo Microparticles and Maya Blue Samples Studied in This Worka Ep(I) (mV)

Ep(II) (mV)

ip(I)/ip(II)

sample

Ep(I) (mV)

Ep(II) (mV)

ip(I)/ip(II)

Indigo

sample

+475

-280

0.52

Chacmultuń-2 D′zula-2 D’zula-9 Ek Balam-2 Kuluba´-9 Acanceh-2 Acanceh-2A Mayapań-2 Mayapań-9A Mulchic-2 Mulchic-9A Mulchic-9B Chicheń Itza´-2 Chicheń Itza´-2A

+480 +450 +475 +480 +475 +460 +455 +460 +450 +465 +470 +470 +440 +465

-275 -270 -280 -265 -265 -285 -275 -275 -260 -270 -265 -265 -275 -275

1.08 1.83 2.00 0.84 0.86 1.69 1.62 0.77 1.82 0.93 0.90 0.99 1.06 1.00

Chicheń Itza´-2B Chicheń Itza´-2D Chicheń Itza´-9B Chicheń Itza´-9C Chicheń Itza´-9E Dzibilnocac-9 Dzibilnocac-9A El Tabasquenõ-2 El Tabasquenõ-2A El Tabasquenõ-9 El Tabasquenõ-9A El Tabasquenõ-9B Calakmul-A-3-2 Calakmul-A-5-9 Calakmul-A-6-2 Calakmul II-C

+475 +450 +465 +450 +450 +465 +445 +460 +460 +475 +485 +480 +465 +455 +455 +500

-280 -280 -280 -270 -275 -255 -275 -280 -265 -265 -265 -270 -280 -280 -260 -235

1.67 1.50 0.85 0.93 1.15 0.86 1.15 1.41 1.81 1.10 0.90 1.11 1.00 1.39 1.18 1.02

a Peak potentials recorded in SQWVs of sample-modified PIGEs immersed into 0.50 M acetate buffer, pH 4.85. Potential step increment 4 mV; square wave amplitude 25 mV; frequency 5 Hz.

Figure 7. Variation with the square wave frequency of the peak current ip(I)/ip(II) ratio for indigo microparticles (squares) and Maya Blue sample Mulchic-2 (rhomboids). From SQWVs in 0.50 M acetate buffer, pH 4.85. Potential step increment 4 mV; square wave amplitude 25 mV.

Figure 6. Variation with the square wave amplitude of the peak current ip(I)/ip(II) ratio for indigo microparticles (squares), indigo attached to silica (triangles), and Maya Blue sample Acanceh-2 (rhomboids). From SQWVs in 0.50 M acetate buffer, pH 4.85. Potential step increment 4 mV; frequency 5 Hz.

(roughly, low frequencies and/or low square wave amplitudes), the electrochemical response reflects the behavior of molecules strongly attached to the aluminosilicate matrix, while at relatively short-time experiments (high frequencies and/or large square wave amplitudes), the response of weakly associated molecules prevails. Then, data in Figures 6 and 7 can be rationalized on assuming that indigo externally adsorbed or weakly associated to the inorganic solid is responsible for the response observed for silica-attached indigo and Maya Blue samples at high f or ESW.

The significantly different voltammetric response obtained for Maya Blue at low frequencies and square wave amplitudes denotes that strongly palygorskite-associated indigo exists in those samples. Both the oxidation and reduction of indigo microparticles can be described on the basis of the model developed by Lovric, Scholz, Oldham et al.42-45 for describing the solid-state voltammetry of immobilized microcrystals. Following this scheme, the redox reaction is initiated at the three-phase electrode/electrolyte/ microparticle junction and expands via proton transfer across the electrolyte/particle interface, and electron transfer across the electrode/particle interface. This formulation can be applied for describing the electrochemistry of electroactive species attached to micro- and mesoporous inorganic solids.46 Chronoamperometric experiments were performed in order to correlate electrochemical data for Maya Blue and theory for the voltammetry of immobilized microcrystals. As described

Dehydroindigo


for indigo microparticles,47 equations developed by Schro¨der et al.45 can be adapted for describing the chronoamperometric current for a deposit of N cuboid microparticles of size a. At short times, the current i must satisfy the relationship

[(

De1/2 + DH1/2

i ) nFNc 4a g

1/2 1/2

2π t

)

+ (DeDH)1/2 - 4DH(2Det)1/2

]

(7)

where De, DH represent the coefficients of diffusion of electrons and protons in the solid (cm2/s), n is the number of electrons involved in the redox reaction, c represents the concentration (mol/cm3) of the electroactive centers in the solid, and g represents an effective length that corresponds to a mean size of the three-phase junction boxes in which the crystal is divided for applying simulation procedures. If the reaction rate is controlled by electron diffusion, the chronoamperometric current at long times approaches45-47

i)

2nNFa2cDe δ

∞

exp ∑ j)1

( ) π2Det 4a2

(8)

where δ denotes the thickness of the electroactive layer adjacent to the particle/electrode interface. If proton diffusion is ratedetermining, the long-time chronoamperometric response is given by45-47 ∞

i ) 3.3nNcFaDH

[ ]

exp ∑ j)1

2π2DHt 4δ2

(9)

The major drawback for testing such equations is the large uncertainty in estimating the number of cuboids. This can be estimated from the mass of the deposit, the density of the material, and the size of the cuboids. It should be noted, however, that (i) it is uneasy to control the exact amount of material mechanically transferred to the electrode surface; (ii) even transferring weighted amounts of the solid, the variable aggregation of the particles makes uncertain the amount of particles displaying an effective contact with the electrode. In our calculations it is assumed that the deposit of Maya Blue (typically 1.5 mg) consisted of a set of cuboid microcrystals of 1-2 µm size with a density of 2.3 g/cm3 and a concentration of indigo of 2% (w/w), equivalent to 3.3 × 10-5 mol/cm3. As can be seen in Figure 8, corresponding to a Maya Blue sample from Acanceh, experimental chronoamperometric data at short times agree with the predictions from eq 4. In this representation, the product it1/2 increases initially while time increases until a maximum value is reached at a certain transition time, t*. From which the product it1/2 decreases monotonically. This profile is characteristic of the electrochemical response of insertion solids.45 Interestingly, the current decay recorded at times longer than t* for Maya Blue samples was more smooth than that recorded for indigo microparticles.47 This is in agreement with the porous structure of Maya Blue, a material where charge transfer via proton and electron hopping between immobile indigo molecules is presumably allowed to a significant extent. In contrast, in indigo microparticles proton hopping is considerably hindered, so that only a narrow region in the external layer of the crystals is electroactive. As a result, proton transfer is rapidly exhausted and the current experiences a fast decay in chronoamperometric experiments.47

Figure 8. CA data for Maya Blue sample Acanceh-2 attached to PIGE in contact with 0.50 M acetate buffer at pH 4.85. Applied potential +550 mV.

Inserting the above-mentioned parameter values, the values of the mean diffusion coeffcients for protons and electrons and the thickness of the corresponding electroactive layer in Maya Blue were calculated. De values were of (2 ( 1) × 10-9 cm2/s. The values of De calculated for Maya Blue are lower than those obtained for indigo oxidation47 (3 × 10-7 cm2/s) and those reported for redox polymers, typically between 2 × 10-8 cm2/s and 2 × 10-9 cm2/s,48,49 but larger than those for electroactive transition metal complexes encapsulated into zeolites (2 × 10-11 cm2/s).38 The calculated values of DH for Maya Blue samples were of (2 ( 1) × 10-8 cm2/s, clearly larger than those estimated for proton transfer in indigo microparticles47 (3 × 10-10 cm2/s), as well as the diffusion coefficient of K+ into copper(II) hexacyanoferrate(II) (1.49 × 10-9 cm2/s),50 Li+ and Et4N+ cations diffusing in zeolites (1 × 10-9 cm2/s),38 and ferricinium ions in plasma-polymerized vinylferrocene (5 × 10-13 cm2/s).48 Characterization of Dehydroindigo. Electrochemical data indicate that palygorskite-attached dehydroindigo is present in a minor but significant proportion in Maya Blue. To confirm the presence of dehydroindigo, different spectral techniques were used. In Figure 9 it is shown the FTIR-ATR spectra of (a) indigo, (b) palygorskite from Sacalum, and Maya Blue samples from (c) Mulchic, and (d) the II-C Substructure of Calakmul. Indigo exhibits a characteristic band at 1629 cm-1, corresponding to the carbonyl frequency, while palygorskite shows a band at 1656 cm-1, corresponding to OH groups. Maya Blue samples exhibit a well-defined indigo band at 1629 cm-1, accompanied by a well-defined band at 1736 cm-1. According to the theoretical and experimental study of Klessinger and Luettke,51 this last band is characteristic of the carbonyl frequency of dehydroindigo. Such bands are accompanied by the band at 1656 cm-1, characteristic of OH groups of the palygorskite matrix, and a prominent band at 1408 cm-1. This last band must be ascribed to the stretching vibration of carbonate group. The presence of carbonate group, absent in the spectra of pristine palygorskite, can be associated with indigo because, during its process of preparation, lime was added to the aqueous suspension of Indigofera sp. leaves. Partial elimination of interfering carbonate by treatment of the sample with HCl leads to the record of well-defined indigo and dehydroindigo bands, as can be seen in Figure 9c.


Domeńech et al.

Figure 10. VIS spectra of (a) indigo; (b) dehydroindigo; (c) Maya Blue sample from A-3 Substructure of Calakmul, Early Classical period. The scale of this last spectrum is vertically shifted for clarity.

Figure 9. FTIR-ATR spectra of (a) indigo; (b) palygorskite from Sacalum; (c) sample Mulchic-2, Late Classical period; (d) Maya Blue sample from the II-C Substructure of Calakmul, Late Preclassical period. Sample (c) was previously treated with 1 M HCl and dried for eliminating carbonate interference (see text).

As can be seen in Figure 10, the main absorption band of indigo in the VIS region is located in the red-orange zone (λmax 6060 Å), so that it is perceived as the complementary blue hue. Absorption of dehydroindigo occurs mainly in the blue-violet region (λmax 4425 Å), as described by Klessinger and Luttke both theoretically and experimentally.51 Accordingly, the resulting hue will be orange-yellow. Spectra of Maya Blue samples show bands of indigo (λmax 6060 Å) and dehydroindigo (δmax 4425 Å) so that one can expect that the resulting hue will be blue-greenish, the color depending on the relative amounts of indigo and dehydroindigo. Additionally, 13C nuclear magnetic resonance data (13C NMR) of Maya Blue samples provide a multiple peak profile in the region between 100 and 200 ppm. Obtained spectra were compared with those reported by Hubbard et al.10 for indigo and crushed sepiolite-indigo, interpreted by such authors in terms of the presence of two species of indigo, one being unaffected pure indigo, and the other representing an altered state of the indigo molecules. 13C RMN data for Maya Blue samples exhibited a similar but more complicated pattern, the most

remarkable difference being the presence of an additional peak at 174 ppm. This peak is attributable to the carbon atoms of the central C-C bond of dehydroindigo and appears far from the signal at 127 ppm corresponding to the carbon atoms of the central CdC bond of that molecule, again in agreement with the idea that dehydroindigo accompanies indigo within the palygorskite framework. Distribution of Indigo Forms in the Palygorskite Matrix. Incorporation of indigo and dehydroindigo within the channels of palygorskite leads presumably to a nonuniform distribution of both indigo forms in the inorganic host. Qualitatively, two basic distribution models can be considered. (i) Taking into account the flexible character of the dehydroindigo molecule, where a single C-C bond replaces the rigid CdC bond of indigo, one can expect that this molecule can penetrate more easily than rigid indigo molecules into the palygorskite channels; accordingly, the inner part of the palygorskite crystals could be enriched in dehydroindigo with respect to their external region. However, (ii) as long as dehydroindigo is probably produced as a result of the oxidation of indigo in the palygroskite channel system, the major abundance of dehydroindigo could be located in the external region of the palygorskite particles. Voltammetric data can be used for testing information on the distribution of dehydroindigo in the palygorskite matrix on the basis of reported criteria for quantifying the relative amount of the oxidized and reduced forms of a depolarizer coexisting simultaneously.35,36 The relevant point to emphasize is that for reversible electron-transfer processes, the voltammetric profile is sensitive to the relative abundance of the oxidized and reduced forms, potential scan rate, and starting and switching potentials.

Dehydroindigo

Figure 11. Theoretical CVs for a reversible couple; potential scan initiated at the formal electrode potential in the negative direction for different percentages of the reduced form: A, 100%; B, 80%; C, 60%. Potential scan rate: (a) 10 mV/s; (b) 100 mV/s.

For our purposes, a case of particular interest is when the CV is initiated just at the formal electrode potential of the couple; i.e., at a potential intermediate between those of the cathodic and anodic peak. The following considerations will be made for a potential scan initiated at that equilibrium potential and scanning the potential in the negative direction. In that situation the first scan exhibits a profile sharply dependent on the composition of the system and the potential scan rate.36 If only the reduced form of a reversible couple is present, an anodic current initially flows. If the potential is scanned toward less positive values, the current will decrease because the applied potential separates from the potentials at which the reagent is oxidized. If the experiment is repeated but only the oxidized form of the pair is present, an initial cathodic current must flow. On prolonging the scan toward less positive potentials, there is a fast current decay, it reaches the region in which the oxidation is diffusion-controlled, and then a Cottrell-type current decay will be observed. In the intermediate case, when both the oxidized and reduced forms of the depolarizer are present, the current will depend on the composition of the system. Symmetrical considerations apply if the potential is scanned in the positive direction. In view of the close vicinity between the CV profiles of insertion solids and species in solution phase,30,31 working curves for a reversible electron transfer process were used. Figure 11 shows theoretical CVs (only the initial branch of the CV is represented) for mixtures of both forms containing 100% (A), 80% (B), and 60% (C) of the reduced form at (a) 10 mV/s and (b) 100 mV/s. A normalized current scale will be used where the actual current/current at time equal-to-zero ratio (i/io) is plotted as a function of the difference between the formal potential of the couple and the applied potential (Eo′ - E).

J. Phys. Chem. B, Vol. 110, No. 12, 2006 6035 At low scan rates, the current is initially anodic but its magnitude decreases monotonically along the potential scan. At relatively high potential scan rates, the initial anodic current increases at the beginning of the scan until it reaches a maximum (negative) value and then decreases monotonically. Experimental data for the Maya Blue sample from ChicheńItza´ are compared with those recorded for indigo microparticles in Figure 12. In both cases the background current recorded for blank experiments performed at a palygorskite-modified and unmodified PIGE, respectively, was subtracted. For indigo microparticles attached to PIGEs (Figure 12a, 12b), nominally composed of 100% indigo, an initial anodic current was obtained, followed by a fast decrease attributable, as already reported, to the exhaustion of the electroactive layer of indigo crystals because of the nonfacile proton hopping between the immobile indigo molecules in the solid.47 In contrast, Maya Blue samples exhibit a response close to that represented in Figure 11. As previously noted, charge transfer via proton and electron hopping is allowed in Maya Blue by the porous structure of the material. At low potential scan rates (Figure 12c), a monotonically rising curve was obtained fitting well with theoretical predictions for a system containing 80% indigo. At relatively high potential scan rates (Figure 12d), the initial current is cathodic, denoting the presence of a high proportion of dehydroindigo. Upon comparing theoretical curves and experimental CVs, the proportion of dehydroindigo is above 40%. This difference between the percentage of indigo estimated at “low” and “high” scan rates can be interpreted on the basis of general ideas on the voltammetry of insertions solids.42-45 Accordingly, “high” scan rate voltammograms result from the electrochemical response of the more external layer of the solid particles, whereas “low” potential scan rate CVs are representative of the composition of a more extensive region within the solid particle. At the expense of more detailed modeling, the foregoing set of data permits to draw a new picture of indigo attachment to palygroskite in Maya Blue. Now, rigid indigo molecules should be predominantly fixed to the channel entrances, presumably forming hydrogen bonds through their carbonyl and amino groups with edge silanol units of palygroskite, while dehydroindigo, a more flexible molecule, should be able to fit into the channels via hydrogen bond formation through its CdO groups. Consistent with that scheme, Witke et al.11 recently indicated that the Raman spectra of Maya Blue samples suggest the loss of the planarity of the indigo molecule as a result of a strong indigo-clay interaction. Thermochemical Parameters for Indigo Association to Palygorskite. Consistent with the foregoing set of considerations on the electrochemistry of Maya Blue, temperature dependence of peak potentials for oxidation process I was remarkably different for indigo microparticles and Maya Blue samples, as can be seen in Figure 13. For the indigo microparticles in contact with acetate buffer, formal electrode potentials, E°′, measured as the peak potential I in SQWVs, was negatively shifted on increasing temperature, T, with linear dependences of E°′ on T. In contrast, for Maya Blue samples the oxidation peak was positively shifted on increasing temperature. The dependence of peak potentials for the processes II and III, recorded, respectively, for modified electrodes immersed into acetate buffer and NaOH 0.10 M, also produced a different variation of E°′ on T for indigo microparticles and Maya Blue samples, as can be seen in Figure 14. The changes of standard-state free energy, enthalpy, and entropy for the attachment of the parent indigo and its reduced


Domeńech et al.

Figure 12. CVs of (a,b) indigo microparticles, and (c,d) Maya Blue sample Chicheń-Itza´-2, attached to PIGEs in contact with 0.50 M acetate buffer after subtraction of voltammograms at (a,b) unmodified PIGE, (c,d) palygorskite-modified PIGE. Potential scan initiated at (a,b) +475; (c,d) +450 mV, and continued in the negative direction. Potential scan rate: (a,c) 10 mV/s; (b,d) 100 mV/s.

Figure 13. Temperature dependence of formal electrode potentials for the oxidation of indigo microparticles (solid squares) and Maya Blue from Calakmul, Substructure C-3 (squares). From peak potentials in SQWVs in 0.50 M acetate buffer, pH 4.85. Potential step increment 4 mV; square wave amplitude 25 mV; frequency 5 Hz.

and oxidized forms to palygorskite (indigo ) IN, H2IN, and H4IN),

indigo (s) + palygorskite (s) f {indigo + palygorskite}(s) (10) ∆G°att, ∆H°att, ∆S°att, respectively, can be calculated, using the

Figure 14. Temperature dependence of formal electrode potentials for processes II and III for indigo microparticles (solid squares and triangles) and Maya Blue from Calakmul, Substructure C-3 (squares and triangles). From peak potentials in SQWVs in 0.50 M acetate buffer, pH 4.85 (A) and 0.10 M NaOH (B).

Nernst equation, from the temperature dependence of peak potentials as described for zeolite-associated probes from the slope and y-intercept of the E°′(vs NHE) vs T linear plots,38,39 namely, E°′(I) ) (614 ( 5) - (0.546 ( 0.015)T (N ) 7, r )

Dehydroindigo

J. Phys. Chem. B, Vol. 110, No. 12, 2006 6037 Using electrochemical data, thermochemical parameters for the attachment of indigo, dehdroindigo, and leucoindigo to palygorskite were obtained. ∆H°att were +101 ( 8, +65 ( 5, and +118 ( 15 kJ/mol for indigo, dehydroindigo, and leucoindigo, respectively. Values of ∆S°att were +0.32 ( 0.02, +0.46 ( 0.02, and +0.25 ( 0.02 kJ/mol K, respectively. Such data indicate that the attachment of all three forms to palygorskite is an endothermic process accompanied by an increase in enthropy. The values of ∆H°att are in the order leucoindigo > indigo > dehydroindigo, whereas the values of ∆S°att increase in the order leucoindigo < indigo < dehydroindigo. Accordingly, the attachment of dehydroindigo to palygorskite is more favored thermodinamically than that of indigo and leucoindigo. Such data indicate that the attachment of dehydroindigo and indigo become thermodynamically spontaneous at moderate temperatures. This result is consistent with reported procedures for Maya Blue preparation, consisting of crushing palygorskite with indigo and heating moderately. In agreement with the prior series of considerations, thermoelectrochemical data support the presence of dehydroindigo in Maya Blue. Formal electrode potentials yield that formation of palygorskite-associated dehydroindigo from palygorskiteassociated indigo, formally represented as

SCHEME 2: Thermochemical Diagram for Redox Processes Involving Indigo and Maya Blue Forms

{indigo - palygorskite}(s) + 1/2O2 f {dehydroindigo - palygorskite}(s) + H2O (14)

0.998); E°′(I,MB) ) (430 ( 3) + (0.164 ( 0.011)T (N ) 7, r ) 0.98), E°′(II) ) (-24 ( 9) - (0.93 ( 0.03)T (N ) 7, r ) 0.997), E°′(II,MB) ) (-590 ( 6) - (0.23 ( 0.02)T (N ) 7, r ) 0.96), E°′(III) ) (-376 ( 9) - (0.26 ( 0.04)T (N ) 7, r ) 0.98), E°′(III,MB) ) (-1115 ( 15) + (1.43 ( 0.04)T (N ) 7, r ) 0.998). For that calculation, one can use the thermochemical parameters for the solid-state oxidations of indigo (eq 1, ∆G°(I)) and Maya Blue (eq 4, ∆G°(I,MB), the corresponding solid-state reduction processes described by eqs 2 and 5 (∆G°(II) and ∆G°(II,MB), respectively), and the reductive dissolution processes III represented by means of eqs 3 and 6 (∆G°(III), ∆G°(III,MB), respectively). Such electrochemical processes can be related with the attachment reactions for indigo, dehydroindigo, and leucoindigo to palygorskite, generically represented by eq 7, as depicted in the thermochemical cycle drawn in Scheme 2. Combining such thermochemical parameters with those determined for the solid-state reduction and the reductive dissolution of indigo and Maya Blue, described by eqs 2, 3, 5, and 6, one can arrive at

∆G°att(H2IN) ) ∆G°(III) - ∆G°(III,MB)

(11)

enabling the calculation of the free energy of attachment of indigo to palygorskite. The free energy of attachment of dehydroindigo and leucoindigo to palygorskite can be obtained from the equations

∆G°att(IN) ) ∆G°(I) + ∆G°att(H2IN) - ∆G°(I,MB) (12) and

∆G°att(H4IN) ) ∆G°(II,MB) - ∆G°(II) + ∆G°att(H2IN) (13)

should be a spontanous process for which the standard-state free energy change, ∆G°, is -50.4 kJ/mol. Such data indicate that the attachment of dehydroindigo and indigo become thermodynamically spontaneous at moderate temperatures. This result is consistent with reported procedures for Maya Blue preparation, consisting of crushing palygorskite with indigo and heating moderately. As reported by Chiari et al.8 and Hubbard et al.,10 zeolitic water must be removed from pristine, hydrated palygorskite, for preparing Maya Blue. It should be noted that thermochemical data for attachment processes described by eq 10 formally correspond to the association of indigo to dehydrated palygorskite. As previously noted, this is calculated from electrochemical processes described by eqs 5 and 6. This last process, however, implies the release of indigo from Maya Blue in contact with an aqueous electrolyte. As a result, the resulting indigo-free palygorskite can eventually be partly rehydrated. Consistently, the value of ∆H°att obtained here for indigo is intermediate with those calculated by Chiari et al.8 using molecular modeling, for nonhydrated (-37 kJ/mol) and hydrated palygorskite (+150 kJ/ mol). Additionally, it should be noted that, although there is no direct procedure to evaluate the hydration degree of Maya Blue on the basis of the above thermochemical data, formation of dehydroindigo via indigo chemical oxidation as described by eq 14 involves the formation of water presumably remaining in the palygorskite system. This means that the formation of dehydroindigo should imply a certain rehydration of the clay matrix in agreement with ∆H°att values calculated from electrochemical data. Thermochemical data indicate that the attachment of dehydroindigo to palygorskite is thermochemically favored with respect to that of indigo, both processes being differently favored on increasing temperature. Thus, neglecting diffusion/kinetic effects, the indigo/dehydroindigo relationship can in principle be controlled by varying the temperature during the crushing process. Accordingly, it appears that a single crushing process

6038 J. Phys. Chem. B, Vol. 110, No. 12, 2006 with variable temperature allowed ancient Mayas to prepare pigments with hues ranging from blue to greenish. Reported data suggest that the hue of Maya Blue can result from a combination of several factors, namely, bathochromic shift of the indigo spectrum due to palygorskite-indigo interaction, Rayleigh/Mie dispersion in iron and iron oxide nanoparticles, superposition of the spectra of indigo and dehydroindigo (with the corresponding spectral shifts associated to palygorskite attachment), and eventually optical effects associated with the roughness of palygorskite/Maya Blue fibers. Final Considerations Application of the voltammetry of microparticles approach to Maya Blue samples in contact with aqueous electrolytes provides well-defined electrochemical responses that can be described in terms of the oxidation and reduction of palygorskite-associated indigo to palygorskite-associated dehydroindigo and palygorskite-associated leucoindigo, respectively. Voltammetric data fit well with current models on the electrochemistry of ion insertion solids and indicate that a significant amount of dehydroindigo accompanies indigo in Maya Blue samples. The presence of dehydroindigo is confirmed by spectroscopic and 13C nuclear magnetic resonance data. In addition to spectral shifts due to the interaction of indigo with the palygorskite matrix, dispersion effects associated with fiber roughness and iron oxide nanoparticles, the presence of dehydroindigo can contribute to explain satisfactorily the greenishblue hue of Maya Blue, as a result of the superposition of the absorption bands of indigo at 6060 Å and dehydroindigo at 4250 Å. Thermochemical data concerning the attachment of the different forms of indigo to palygorskite have been calculated from the temperature variation of formal electrode potentials. The attachment of all three forms, dehydroindigo, indigo, and leucoindigo, to the palygorskite framework is an endothermic process accompanied by an increase in entropy. As a result, attachment of indigo and dehydroindigo becomes spontaneous at moderate temperatures. Formation of palygorskite-associated dehydroindigo from the aerobic oxidation of palygorskiteassociated indigo is a spontaneous process. All these data suggest that the ancient Mayas were able to prepare Maya Blue by crushing indigo and palygorskite at moderate temperatures, thus modulating the color of the resulting pigmenting system by varying the amount of indigo and the temperature of operation, thus anticipating modern synthesis of hybrid organic-inorganic materials. In this sense, Mayas pioneered not only modern synthetic routes for preparing hybrid organic-inorganic materials but also thermal control of chemical reactions. Acknowledgment. Financial support is gratefully acknowledged from the Generalitat Valenciana GV04B/197 and GV04B/ 441 I+D+I Projects and the MEC projects CTQ2004-06754C03-01 and 02, which are also supported with FEDER founds. The authors would like to thank to Prof. Ramoń Carrasco Vargas, Director of the Calakmul Archaeological Project, Dr. Marıá Jose´ Sabater and Dr. Jose´ Alejandro Vidal Moya of the ITQ of the Polytechnical University of Valencia, Dr. Jose´ Luis Moya Lo´pez, technician responsible for the scanning electrochemical atomic force microscope, Mr. Manuel Planes Insausti, technician responsible for the Microscopy Service of the Polytechnical University of Valencia for their technical support.

Domeńech et al. Note Added after ASAP Publication. This manuscript was originally published on the Web March 7, 2006. The manuscript was reposted March 14, 2006 with a revised value in the third paragraph of the section titled “Characterization of Dehydroindigo”. References and Notes (1) Romero, P.; Sańchez, C. New J. Chem. 2005, 29, 57-58. (2) Chianelli, R. R.; Perez de la Rosa, M.; Meitzner, G.; Siadati, M.; Berhault, G.; Mehta, A.; Pople, J.; Fuentes, S.; Alonzo-Nun˜ez, G.; Polette, L. A. J. Synchroton Radiat. 2005, 12, 129-134. (3) del Rio, M. S.; Sodo, A.; Eeckhout, S. G.; Neisius, T.; Martinetto, P.; Dooryhee, E.; Reyes-Valerio, C. Nucl. Instrum. Methods Phys. Res. B 2005, 238, 50-54. (4) Giustetto, R.; Llabres i Xamena, F. X.; Ricchiardi, G.; Bordiga, S.; Damin, A.; Gobetto, R.; Chierotti, M. R. J. Phys. Chem. B 2005, 109, 19360-19368. (5) Vandenabeele, P.; Bode, S.; Alonso, A.; Moens, L. Spectrochim. Acta A 2005, 61A, 2349-2356. (6) del Rıó, M. S.; Martinetto, P.; Somogyi, A.; Reyes-Valerio, C.; Dooryheé, E.; Peltier, N.; Alianelli, L.; Moignard, B.; Pichon, L.; Calligaro, T.; Dran, J.-C. Spectrochim. Acta B 2004, 59, 1619-1625. (7) Reinen, D.; Ko¨hl, P.; Mu¨ller, C. Z. Anorg. Allg. Chem. 2004, 630, 97-103. (8) Chiari, G.; Giustetto, R.; Ricchiardi, G. Eur. J. Mineral. 2003, 15, 21-33. (9) Fois, E.; Gamba, A.; Tilocca, A. Microporous Mesoporous Mater. 2003, 57, 263-272. (10) Hubbard, B.; Kuang, W.; Moser, A.; Facey, G. A.; Detellier, C. Clays Minerals, 2003, 51, 318-326. (11) Witke, K.; Brzezinka, K.-W.; Lamprecht, I. J. Mol. Struct. 2003, 661-662, 235-238. (12) Polette, L. A.; Meitzner, G. Jose´-Yacamań, M.; Chianelli, R. R. Microchem. J. 2002, 71, 167-174. (13) Magaloni, K. D. Materiales y Te´ cnicas de la Pintura Maya. Facultad de Filosofıá y Letras, Universidad Nacional Autońoma de Me´xico, 1996. (14) Tagle, A.; Paschinger, H.; Richard, H.; Infante, G. Stud. ConserVat. 1990, 35, 156-159. (15) Reyes-Valerio, C. De Bonampak al Templo Mayor: el azul Maya en Mesoame´ rica; Siglo XXI: Madrid, 1993. (16) Torres, L. M. Materials Research Society Symposium Proceedings, Vol. 123, 1988. (17) Gettens, R. J. Am. Antiq. 1962, 27, 557-564. (18) Sheppard, A. O. Am. Antiq. 1962, 27, 565-566. (19) Van Olphen, H. Science 1967, 154, 645-646. (20) Kleber, R.; Masschelein-Kleiner, L.; Tissen, J. Stud. ConserVat. 1967, 12, 41-55. (21) Arnold, D. Am. Antiq. 1971, 36, 20. (22) Littmann, E. R. Am. Antiq. 1980, 45, 87-100. (23) Littmann, E. R. Am. Antiq. 1982, 47, 404-408. (24) Chisholm, J. E. Can. Mineral. 1990, 28, 329-339. (25) Chisholm, J. E. Can. Mineral. 1992, 30, 61-73. (26) Giustetto, R.; Chiari, G. Eur. J. Mineral. 2004, 16, 521-532. (27) Giustetto, R.; Levy, D.; Chiari, G. Eur. J. Mineral., in press. (28) Gordon, P. F.; Gregory, P. Indigoid Dyes in Organic Chemistry in Colour; Springer-Verlag: Berlin, 1983; pp 208-211. (29) Jose´-Yacamań, M.; Rendoń, L.; Arenas, J.; Serra Puche, M. C. Science 1996, 273, 223-225. (30) Scholz, F.; Meyer, B. Electroanalytical Chemistry, A Series of AdVances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1998; Vol. 20, pp 1-87. (31) The peak profile of the background current, obtained at PIGE electrodes modified with pristine palygorskite can be associated with redox processes involving quinone-type functonalities promoted by the scratching of the graphite surface. See Barisci, J. N.; Wallace, G. G.; Baughman, R. H. Electrochim. Acta 2000, 46, 509-517. (32) Rolison, D. R.; Bessel, D. A. Acc. Chem. Res. 2000, 33, 737-744. (33) (a) Bond, A. M.; Marken, F.; Hill, E.; Compton, R. G.; Hu¨gel, H. J. Chem. Soc., Perkin Trans. 1997, 2 1735-1742. (34) Grygar, T.; Kuckova, S.; Hradil, D.; Hradilova, D. J. Solid State Electrochem. 2003, 7, 706-713. (35) Scholz, F.; Hermes, M. Electrochem. Commun. 1999, 1, 345-348. (36) Domeńech, A.; Sańchez, S.; Domeńech, M. T.; Gimeno, J. V.; Bosch, F.; Yusa´, D. J.; Saurı´, M. Electroanalysis 2002, 14, 685-696. (37) Bessel, D. A.; Rolison, D. R. J. Phys. Chem. B 1997, 101, 11481157. (38) Domeńech, A.; Formentıń, P.; Garcıá, H.; Sabater, M. J. J. Phys. Chem. B 2002, 106, 574-582. (39) Domeńech, A.; Garcıá, H.; Alvaro, M.; Carbonell, E. J. Phys. Chem. B 2003, 107, 3040-3050.

Dehydroindigo (40) Domeńech, A.; Alvaro, M.; Ferrer, B.; Garcıá, H. J. Phys. Chem. B 2003, 107, 12781-12788. (41) Domeńech, A.; Garcıá, H.; Casades, I.; Espla´, M. J. Phys. Chem. B 2004, 108, 20064-20075. (42) Lovric, M.; Scholz, F. J. Solid State Electrochem. 1997, 1, 108113. (43) Lovric, M.; Scholz, F. J. Solid State Electrochem. 1999, 3, 172175. (44) Oldham, K. B. J. Solid State Electrochem. 1998, 2, 367-377. (45) Schro¨der, U.; Oldham, K. B.; Myland, J. C.; Mahon, P. J.; Scholz, F. J. Solid State Electrochem. 2000, 4, 314-324.

J. Phys. Chem. B, Vol. 110, No. 12, 2006 6039 (46) Domeńech, A. J. Phys. Chem. B 2004, 108, 20471-20478. (47) Domeńech, A.; Domeńech, M. T. J. Solid State Electrochem., in press. (48) Daumm, P.; Lenhard, J. R.; Rolison, D.; Murray, R. W. J. Am. Chem. Soc. 1980, 102, 4649. (49) Andrieux, C. P.; Saveánt, J.-M. J. Phys. Chem. 1988, 92, 67616767. (50) Kahlert, H.; Retter, U.; Lohs, H.; Siegler, K.; Scholz, F. J. Phys. Chem. 1998, 102, 8757. (51) Klessinger, M.; Lu¨ttke, W. Tetrahedron Lett. 1963, 19 (suppl. 2), 315-335.

Dehydroindigo: A New Piece into the Maya Blue ... - ACS Publications

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