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Angewandte Chemie International Edition, Volume 51, Issue 42, pages 10652–10655, October 15, 2012.
Dr. Marina M. Lezhnina, Tobias Grewe,Dr. Hardo Stoehr, Prof. Dr. Ulrich Kynast
Institute for Optical Technologies, Munster University of Applied Sciences, Stegerwaldstrasse 39, 48565 Steinfurt (Germany)
Presently on leave from Mari Technical State University Yoshkar-Ola, Institute of Physics, Lenin-pl. 3, 424000 Yoshkar-Ola (Russia)
Abstract
The neutral organic dye indigo forms an inorganic–organic hybrid material with nanoclays (see picture; blue circles on disks symbolizing indigo, spheres indicating liberated cations) and can thus be transferred into aqueous solution. Solids recovered from these solutions resemble the ancient Maya Blue pigment. The method can also be applied to other hydrophobic species and may open the gate for novel solution chemistry, including photonic and catalytic applications.
Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Additional Information:
Nanosized clayparticles may be utilized for the mobilization of non-ionic and non-polar molecules into aqueous solution. Among the first examples is the virtually insoluble dye Indigo, which develops a striking solubility in the presence of the nanoclays. The insolubility of numerous organic dyes in aqueous media or even less polar media is often a nuisance to experimental chemists and in chemical technology.
In investigations on nanoclay surfaces interacting with non-polar organic and metallo-organic compounds, we noted strongly clay-adhering species, which remained adsorbed even after extraction with organic solvents. The fact that the nanoclays used (e.g. materials derived from the clay mineral hectorite, commercially available as “Laponite”, trademark of Rockwood Additives Inc.) are completely dispersible in water, prompted us to use them as shuttles for the solubilization of natively insoluble organic dyes into the aqueous phase. The solubility of the nanoclays as such arises from their nanoscaled dimensions, an anisotropic platelet-like morphology (diameter approximately 25 nm at 1 nm thickness), and high surface charge. As a prominent example for an insoluble dye, which is at the same time of appreciable industrial interest, we chose the famous Indigo (“Blue Jeans”). Much to our delight, we found that Indigo readily yields intensely blue colored, transparent aqueous solutions in the presence of the nanoclay, in which up to 400 molecules of Indigo per clay platelet were mobilized. While concentrations beyond 10-2 Mol/L with respect to Indigo are possible, dispersions of 100 molecules per clay platelet, i.e. about 3·10-3 Mol/L, were tested to be stable over weeks. The mechanism of adhesion can most likely be attributed to surface hydrogen bonds in case of the Indigo, which has no overall net dipole, but a local polarity due to its keto- and N-H functions. However, other dyes, e.g. of the perylene family, bare of polar substituents, could also be solubilized, thus raising fundamental questions as to the detailed microscopic nature of the adhesion. Interestingly, the spectra of solids recovered from the solutions are in close resemblance of the ancient Maya Blue, a surprisingly weatherfast pigment. Chemically, Maya Blue is an inorganic-organic hybrid material, composed of palygorskite, a zeolite-like porous host, and Indigo, for which the bonding between host and guest is still under dispute too. The “Laponite Blue” effect and its underlying solubilization concept can be
extended into other areas of optical functionalization as well, luminescent dyes for example, thus paving the road towards a novel class of bio-assays. Beyond optical functionalization, the general strategy has the potentials to impact other fields also, e.g. water borne coating technologies, or omogeneous (photo-) catalysis to name just two.
Contacts:
Dr. M. M. Lezhnina, T. Grewe, Dr. H. Stoehr, Prof. Dr. U. Kynast
Institute for Optical Technologies, Muenster University of Applied Sciences
Stegerwaldstraße 39, 48565 Steinfurt, Germany
E-mail: [email protected]
