Stepwise dansyl grafting on the kaolinite interlayer surface

doi:10.1016/j.jcis.2012.02.033

Journal of Colloid and Interface Science

Volume 375, Issue 1, 1 June 2012, Pages 112-117

Dedicated to the memory of Klaus Josef Mueller.

https://doi.org/10.1016/j.jcis.2012.02.033 Get rights and content

Abstract

Here we describe the step-wise grafting of the fluorophore dansyl chloride on the interlayer aluminol groups of kaolinite. The modified clay was characterized by powder RD, TGA, FT-IR and ²⁷Al, ¹⁹Si, ¹³C MAS-NMR, which confirmed the achievement of the clay functionalization. The photophysical properties of the resulting nanohybrid material were evaluated by photoluminescence excitation and emission measurements.

Graphical abstract

Highlights

► Dansyl residue was grafted to the internal kaolinite surface by a step-wise process. ► Emission luminescence properties were measured in the solid state. ► Multinuclear MAS-NMR characterization of all intermediates was carried out.

Introduction

In recent years, many functional materials have been obtained from the covalent grafting of organic fragments on inorganic surfaces, mainly for analytical applications [1], [2], [3]. Natural and synthetic layered inorganic structures represent a very promising platform for the construction of hybrid organic–inorganic materials. On the other hand, the preparation of hybrid systems by interlayer covalent grafting offers the possibility to obtain materials where the host imposes a specific and organized orientation to the active molecules (guests) [2], [3].

Within layered inorganic materials of natural origin, kaolinite that is widely abundant in nature is particularly suitable for covalent grafting because it is characterized by a much higher density of hydroxyl groups compared to the other clay minerals. Nevertheless the interlayer chemistry of this 1:1 dioctahedral layered mineral whose layers are formed of silicon tetrahedral sheets linked to aluminum octahedral sheets is much less developed that of the smectites. The strong van der Waals and hydrogen bonds between the stacked layers [4] make more difficult the access to the aluminol groups (Al–OH) of the interlayer space, suitable for grafting reactions. The preparation of kaolinite inorganic–organic hybrid materials requires the preliminary expansion of the space between two consecutive layers by the intercalation of small polar molecules [5], followed by their displacement by functional organic molecules suitable for grafting onto the interlayer aluminol surfaces of the clay. The last decade has seen the report of a few examples of organic moieties grafted onto the interlayer aluminol surfaces of kaolinite [4], [6], [7], [8], [9], [10], [11], [12], [13], [14]. However, to the best of our knowledge, any communication concerning the covalent incorporation of fluorescent organic probes has appeared up to date in the scientific literature.

Covalent immobilization of fluorescent dyes on nanosized inorganic materials is widely considered a very promising approach for opening up new perspectives in the field of chemosensors and fluorescent labels, which are of particular interest because of their high sensitivity and selectivity [15], [16], [17], [18], [19], [20]. Among the different fluorescent dyes, dansyl chloride, (5-dimethylaminonaphthalene-sulfonyl chloride), in virtue of its strong sensibility to the polarity of the medium, is one of the most studied, since it allows modulation of the materials behavior by changing the environmental conditions [21], [22].

Here we describe the step-wise grafting of the dansyl group in the interlayer spacing of kaolin, the characterization of the products by XRD, TGA, DRIFT-IR, ²⁷Al, ¹⁹Si, ¹³C MAS-NMR and the evaluation of the fluorescence properties of the resulting hybrid material.

Section snippets

Materials

Kaolin (Sigma–Aldrich) was used without further purification. Dichloromethane (Sigma–Aldrich) and toluene (Sigma–Aldrich) were dried using standard procedures. Dimethysulfoxide (DMSO, Riedel), 1,4-dioxane (Sigma–Aldrich), tris(hydroxymethyl) aminomethane (THAM, Sigma–Aldrich), dansyl chloride 5-(dimethylamino)naphthalene-1-sulfonyl chloride, 98% (Acros Organics), tripropylamine (Sigma–Aldrich) and pyridine (Sigma–Aldrich) were used as supplied without further purification.

Sample preparation

The complete step-wise

X-ray powder diffraction

The X-ray diffraction patterns of the samples K, K-DMSO, K-THAM and K-Dansyl are shown in Fig. 1.

The d₀₀₁ peak characteristic of kaolinite, corresponding to an interlayer distance of 0.72 nm, is fully replaced, after intercalation with DMSO, by a new one, corresponding to a basal spacing of 1.12 nm, in agreement with literature data [24], [25]. Such enlargement of the basal distance that is less than the dimensions of the DMSO molecule is attributed to the keying of one of the DMSO methyl groups

Conclusions

A new kaolinite-based hybrid material, containing chromophore molecules covalently bound to the aluminol groups of the clay interlayer space, has been successfully prepared by a step-wise procedure. The presence of organic moieties (THAM and dansyl chloride) in the interlayer space of the clay was proved by powder XRD, TGA and FT-IR spectroscopy. Moreover, multinuclear (¹³C, ²⁷Al, ²⁹Si) MAS-NMR measurements confirmed the structure of all the synthetic intermediates and of the final grafted

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^¹: Deceased author.

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Stepwise dansyl grafting on the kaolinite interlayer surface

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials

Sample preparation

X-ray powder diffraction

Conclusions

Appl. Clay Sci.

Langmuir

J. Am. Chem. Soc.

Chem. Mater.

J. Phys. Chem. Solids

Chem. Mater.

Chem. Rec.

The Chemistry of Clay–Organic Reactions

Chem. Mater.

Chem. Mater.

Langmuir

J. Mater. Chem.

J. Mater. Chem.

Clay Miner.

Clay Miner.

Chem. Commun.