Local atomic structure of lanthanide complexes in cubic ordered mesoporous silica

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Abstract

The aim of this work was to analyze the local atomic environment around lanthanide atoms anchored inside silica matrices by means of X-ray absorption spectroscopy (XAS). Europium and cerium complexes were incorporated in cubic FDU-1 and SBA-16 ordered mesoporous silica (OMS) by a post-synthesis method. Small angle X-ray scattering (SAXS) showed the presence of an ordered arrangement of pores for all investigated samples. A much higher Eu incorporation for the dibenzoylmethane complex is attained compared to picrate complex with a weight percent similar to the nominal concentration of Eu, as determined by Rutherford back-scattering spectrometry (RBS). The XAS results showed that the local atomic arrangement around Eu is composed by a first neighbor sphere similar to the complex and different second neighbors compared to pristine material, indicating an interaction of the complexes with the silica walls, in such an extent that it does not interfere on the complexes luminescent properties. The first and second spheres around the incorporated Ce are analogous to the pure cerium picrate complex, showing that the complex is anchored on the porous surface. The solvent does not interfere on the local atomic structure, but on the incorporation capacity.

Highlights

Lanthanide complexes are successfully incorporated in cubic mesoporous silica. ► Eu incorporation is more efficient for DbmTppo complex compared to PicLeu. ► The dielectric constant of the solvent influences on the incorporation capacity.

Introduction

Lanthanide (Ln) materials, due to their Lewis acidity and spectroscopic, magnetic and redox properties, have extensive use as, for example, catalysts in the treatment of car gaseous emissions and cracking of oil; as luminescent material in the manufacture of lasers, in cathodic ray tubes for television, “matches” in the manufacture of fluorescent light bulbs, etc. and contrast agents [1].

The use of lanthanide compounds to check the properties and functions of biochemical systems, and also to determine biologically active substances, has been increased [1], [2], [3], [4], [5]. The lanthanides are used mainly as spectroscopic sounding leads in the study of bio-molecules functions [3], [4], [5]. For example, they can be used as biological tracers to follow the path of drugs in men and animals [3], [6]; as labels in immunology essays (fluoro-immunoassays) [6], [7], [8] and also, as agent of contrast in non-invasive diagnosis of pathologies in tissues by Nuclear Magnetic Resonance (NMR) images [1], [3], [4], [5].

As we can verify the lanthanides have innumerable applications, but the majority of them demands sufficiently stable compounds, as it is in the case of contrast agents. However, many of these applications are limited, because many Ln compounds are not much stable. The incorporation of them in mesoporous silica, silica gel or glasses can improve their stability [9], [10].

The highly ordered mesoporous silica (OMS) have been deserved much scientific attention because they have great potentialities for various applications, including host for compounds. These materials are hydro-thermal and thermally very stable and possess large pore diameter (2 nm < D < 30 nm), that can be tailored for a diversity of nano-technological applications, such as sensors, electrodes, separation processes, selective adsorption, and mainly in the area of catalysis. Mesoporous silica is prepared by means of different methods, which depend on the structure template, producing a variety of pore sizes; moreover, the surface can be modified by a number of silylants agents and by incorporation of metals inside the structure [11], [12], [13].

Studies show that the incorporation of lanthanide ions in silica, beyond improving their stability, makes them to generate high luminescence, by protecting them from non-irradiative processes [14], [15], [16], [17]. For example, the emission of lanthanide ions in water solution is highly decreased due to the loses of vibration energy of the excited Ln (III) state to water molecules, but for compounds incorporated in silica this effect does not occur [17]. Thus, it is of great importance to modify to local environment of Ln (III) ions in order to improve their absorption characteristics and to reduce non-irradiative mechanisms of the excited state [15], [16], [17], [18].

In our previous work we report on the post-synthesis incorporation of Eu luminescent complexes on only one type of cubic OMS, FDU-1, and we analyzed the incorporation capacity, thermal stability and luminescence efficiency of these new materials [14]. In this work we demonstrated the incorporation of the Eu complexes inside the silica matrix by analyzing the SAXS results, since the decrease of the peak intensities is an indication for complexes inclusion within the silica mesopores. The present work aims to complete the characterization of lanthanide complexes (Eu(dbm)3⋅TPPO, where dbm = dibenzoylmethane and TPPO = triphenylphosphine oxide and Ln(pic)3⋅2Leu⋅5H2O, where Ln = Ce and Eu, pic = picrate and Leu = L-leucine) embedded in three cubic OMS by looking at the local atomic structure around the lanthanides. X-ray absorption spectroscopy (XAS) at Ce and Eu L3 edges, is a powerful experimental tool to analyze the insertion of the lanthanide compounds in silica and its atomic coordination [19], [20], [21], [22]. Therefore, in this research we investigated the local atomic structure of lanthanide compounds, more specifically, Ce and Eu, incorporated in cubic mesoporous silica (FDU-1 and SBA-16) by means of XAS in order to check the interaction of these luminescent complexes with the silica host.

Section snippets

Synthesis

The triblock copolymers used in this work as templates for syntheses of OMS (FDU-1 and SBA-16) are (i) B50-6600, poly(ethylene oxide)–poly(butylene oxide)–poly(ethylene oxide), EO39BO47EO39 from Dow Chemicals, (ii) Vorasurf 504, EO38BO46EO38 from Dow Chemicals and (iii) Pluronic F-127, EO106BO70EO106 from Basf.

The synthesis of FDU-1 was carried out using the same synthesis gel composition (expressed as molar ratio) as reported by Yu et al. [23], [24], [25]: 1 TEOS:0.00735 triblock copolymer:6

Results and discussion

The experimental data allow analyzing how the complex loading and local atomic structure depend on four different synthesis parameters: (i) cubic OMS structure, (ii) complex composition, (iii) solvent type and (iv) Ln atom.

Therefore, the results presented below will compare these four variable parameters on the material properties.

Conclusions

In this work we demonstrated that ordered mesoporous silicas with cubic pore symmetry, independently on porous network arrangement, are suitable for incorporation of europium and cerium complexes, keeping their chemical integrity and luminescent properties, maintaining the Ln local atomic structure. A much higher Eu incorporation is achieved for the dibenzoylmethane complex compared to picrate and, with a weight percent similar to the nominal concentration of Eu set at the synthesis process.

Acknowledgments

The authors acknowledge the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the financial support. The Laboratório Nacional de Luz Síncrotron (LNLS), Brazil, is acknowledged for the use of D04B-XAFS1 beamline (project number 5261/06), with assistance of Dr. Gustavo de Medeiros Azevedo and Dr. Ana Paula Rodrigues. LNLS is also acknowledged for the use of D11A-SAXS1 beamline (project number 4868/05),

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