Nanohybrids of edible dyes intercalated in ZnAl layered double hydroxides

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Abstract

Edible dyes such as Allura® Red AC (C18H14N2O8S22−), Sunset Yellow FCF (C16H10N2O7S22−), and Brilliant Blue FCF (C37H34N2O9S32−) were successfully intercalated into the gallery spaces of ZnAl layered double hydroxides (LDHs). Those organic–inorganic nanohybrids were synthesized by co-precipitating the aqueous solution of zinc nitrate and aluminum nitrate simultaneously in the presence of anionic dyes. From the X-ray diffraction and infrared spectroscopic analyses, we found that the dye molecules could be intercalated into hydroxide layers without any changes in their chemical and structural properties. The basal spacings were determined to be 24.0, 20.3, and 24.7 Å for Allura® Red AC, Sunset Yellow FCF, and Brilliant Blue FCF intercalated LDHs, respectively. According to the solid UV–vis spectroscopic analyses, the absorbance intensity of dye-intercalated LDHs was clearly enhanced with the band narrowing compared to those of the corresponding dye salts due to the charge transfer between the host lattice and spatially confined dye molecules.

Introduction

Increasing attention has been paid to the layered double hydroxides (LDHs), anionic clays or hydrotalcite-like compounds [1], owing to their lamellar-type crystal structure and unique anion exchange capability [1], [2]. The LDHs are widely applied to the fields like catalysis, electrochemistry, separation technology, and drug delivery [3], [4], [5], [6].

The LDHs consist of positively charged metal hydroxide sheets with anions located between the layers to compensate the positive layer charges. The compositions are generally represented as [M1-x2+Mx3+(OH)2][An-]x/n·mH2O, where M2+ and M3+ are divalent and trivalent cations, respectively, x is the ratio M3+/(M2++M3+), and Ax/nn- is an anion with a charge of n (such as NO3, CO32−, Cl or SO42−). M2+and M3+ species represent Zn2+, Ni2+, Mg2+ or Cu2+, and Al3+, Cr3+, Fe3+, or Ga3+, respectively. Various kinds of organic and inorganic anions have been immobilized into such layers by the ion-exchange or coprecipitation method [1], [2].

Among various organic substances used as the interlayer guest species, a dye molecule is one of the interesting materials because its host–guest interaction may provide unique structural features and physicochemical properties [7]. Especially, edible dyes are widely used as color additives for foods, drugs, and cosmetics due to their high biocompatibility. To be useful, the edible dyes are often incorporated with polymers, which, unfortunately, did not exhibit high stability due to the incorporated processing parameters, such as heat, light, etc. [8], [9]. Thus, it would be one of the critical factors to enhance the thermal stability of the dyes.

In this study, we employed various anionic edible dyes, such as Allura® red AC (AR), sunset yellow FCF (SY), and brilliant blue FCF (BB), and intercalated them directly into our ZnAl–LDH nanoparticles by coprecipitation. It should be noted that the dyes used in this study, although they are synthetic, are certified as edible dyes meeting strict government specifications. The enhanced physicochemical properties of the dyes after intercalation are presented in detail.

Section snippets

Experimental section

A ZnAl–LDH containing edible dyes was prepared by a conventional coprecipitation method. The Zn/Al molar ratio was adjusted to 3. The edible dyes used in this study were AR (C18H14N2O8S22−: FD&C red no.40), SY (C16H10N2O7S22−: FD&C yellow no.6), and BB (C37H34N2O9S32−: FD&C blue no.1). A mixed aqueous solution containing Zn2+ (0.075 mol, from Zn(NO3)2·6H2O) ions, Al3+ (0.025 mol, from Al(NO3)3·9H2O) ones, and AR, SY, or BB (0.05 mol) was titrated dropwise with a NaOH (0.5 M) solution with vigorous

Results and discussion

The powder XRD patterns and the suggesting hybrid structures for the LDHs intercalated with Allura® red AC (AR–LDH), sunset yellow FCF (SY-LDH), and brilliant blue FCF (BB–LDH) are shown in Fig. 1. The observed basal d003 spacings were 24.0, 20.3, and 24.7 Å for AR–LDH, SY–LDH, and BB–LDH, respectively. Considering the thickness of 4.8 Å for the brucite layer, the gallery heights could also be estimated to be 19.2, 15.5, and 19.9 Å for AR–LDH, SY–LDH, and BB–LDH, respectively. Each value suggested

Conclusion

In the present study, we demonstrated that dye–LDH hybrids could be prepared by the direct coprecipitation method. The edible dyes, while intercalated into LDH layers, exhibited significant enhancement in color development and thermal stability compared to their salt forms. Such organic–inorganic-polymer hybrid systems can be very useful for various industrial applications due to their advanced and synergistic functions.

Acknowledgments

This work was financially supported by the SRC/ERC program of MOST/KOSEF through the Center for Intelligent Nano-Bio Materials (Grant: R11-2005-008-00000-0) at Ewha Womans University (Grant: R11-2005-008-01001-0).

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