Embedding lanthanide-functionalized polymers into hollow mesoporous silica spheres: a ship-in-a-bottle approach to luminescent hybrid materials

Abstract

Photoluminescent hybrid materials were designed and synthesized according to a ship-in-a-bottle approach, comprising hollow mesoporous silica spheres (HMSS) and lanthanide coordination compounds supported by poly(4-vinylpyridine) and 1,10-phenanthroline.

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Silica-based organic–inorganic hybrid materials have attracted considerable attention during the past decades due to synergistic combinations of the inherent properties of the organic and inorganic components.^1–3 Such tailor-made hybrid materials have revealed promising applications as for example in catalysis,⁴ optical sensing,⁵ and controlled drug release.⁶ Well-established synthesis strategies include impregnation,⁷ doping,⁸ grafting,⁹ various sol–gel approaches such as co-condensation^10,11 or direct condensation of functionalized bridged organosilanes,^4,12 as well as the assembly of sub-building blocks via organic linkers.^13,14 Alternatively, the ship-in-a-bottle strategy has been explored to fabricate functional hybrid materials by encapsulation of functional building blocks inside porous matrices, such as zeolites,^15–17 porous polymers,¹⁸ metal–organic frameworks,¹⁹ mesoporous silica particles,²⁰ and macroporous alumosilicates.^21,22 Generally, ship-in-a-bottle syntheses are based on porous host materials with larger cavities (e.g., zeolites with a cage-like pore configuration) and small building blocks, which can enter the porous matrix via diffusion and convert into larger molecular products. The “ships” formed via self-assembly (metal complex formation) or polymerization remain physically entrapped within the large cavities since their molecular size is too big to escape the porous matrix.^23,24

Lanthanide complexes display excellent luminescent properties because of their large Stokes shift, narrow-band emission and high color purity.^25–27 However, the low mechanical strength and poor thermal stabilities of lanthanide complexes often limit their practical applications.²⁸ To overcome these shortcomings, lanthanide complexes were incorporated into solid matrices, such as sol–gel-derived porous materials, via doping or post-treatment chemical immobilization, affording lanthanide-containing hybrid materials of enhanced mechanic and thermal stability.^8,29–33 Up to now, investigations of lanthanide-based inorganic–organic hybrid silica materials primarily focused on grafting or immobilization of small molecular lanthanide complexes.⁸ Polymer-bound lanthanide coordination compounds entrapped into hollow mesoporous silica spheres (HMSS) are still rare to date. Herein we present a feasible method to design and synthesize HMSS-entrapped functionalized polymers (poly(4-vinylpyridine) = P4VP) by the ship-in-a-bottle strategy. Linking 1,10-phenanthroline(Phen)-lanthanide(III) chelates to the confined polymer via the pyridine donor groups generates efficient luminescent hybrid silica materials (Scheme 1). The luminescent properties can be altered by simply choosing different lanthanide ions.

Scheme 1 Ship-in-a-bottle strategy for the preparation of Ln(III) complex-functionalized polymers entrapped into hollow mesoporous silica spheres. Herein Phen = 1,10-phenanthroline, Ln(III) = Tb(III), Eu(III). The small grey dots present monomer 4-vinylpyridine (4VP), while the grey, green and red larger dots present poly(4-vinylpyridine) (P4VP), Phen-Tb-P4VP, and Phen-Eu-P4VP, respectively.

The HMSS host with an average diameter of 510 nm (Fig. 1a and Fig. S1a†) and an average pore size of 2.3 nm was prepared according to a slightly modified spontaneous self-transformation approach.³⁴ The calcined host material was subsequently loaded with commercially available 4-vinylpyridine (4VP) via impregnation in ethanolic solution. A ship-in-a-bottle synthesis was achieved by radical polymerization of the 4VP monomer in the presence of initiator azobisisobutyronitrile (AIBN), generating P4VP@HMSS. Owing to the small molecular size of 4VP (ø ca. 0.65 nm) compared to the mesopores of HMSS, it can readily access the void space of the host material. P4VP attached to the external surface or pore entrances of HMSS can be removed easily through ethanol washings, while HMSS-entrapped P4VP cannot be washed away due to its relatively large molecular size compared to the pore sizes. UV-visible spectroscopic monitoring of the supernatants was applied to ensure that all externally bound polymers were removed completely (Fig. S2†). Crucially, the polymer P4VP features hard nitrogen donor atoms suitable for rare-earth metal coordination (salt or complex) to target luminescent HMSS materials. Previously, block copolymers such as poly(ethylene glycol)-block-poly(4-vinylpyridine) and polystyrene-block-poly(4-vinylpyridine), proved to be useful components for the design of luminescent lanthanide(III) ions-containing polymers and corresponding hybrid materials, such as silica-based hybrid materials, micelles and self-organized films.^35–38

Fig. 1 TEM images of (a) HMSS, (b) P4VP@HMSS, (c) Phen-Tb-P4VP@HMSS, and (d) Phen-Eu-P4VP@HMSS.

Compared to parent HMSS (Fig. 2b), the IR spectrum of P4VP@HMSS (Fig. 2c), revealed the appearance of an adsorption band at 1597 cm⁻¹ attributable to the stretching vibration characteristic of the pyridine ring, hence clearly confirming that the polymer has been successfully encapsulated into the hollow silica matrix. The appearances of asymmetric v_as(C–H) and symmetric v_s(C–H) stretching vibrations at 2934 and 2981 cm⁻¹ further corroborate the formation of HMSS-entrapped P4VP.^35,36,39 This scenario is also supported by the ¹³C solid-state NMR spectrum of P4VP@HMSS (Fig. S7†), which shows characteristic signals assignable to carbon atoms of the pyridine ring, vinylic moieties (–CH₂–CH–), as well as those of Me₂C(CN) groups or non-decomposed AIBN.⁴⁰ C/N elemental analyses of P4VP@HMSS indicated that the polymer loading amounts to about 16%. Overall, the polymer loading can be adjusted by varying the concentration of monomer in ethanol (see ESI†).

Fig. 2 FTIR spectra of (a) 1,10-phenanthroline (orange), (b) HMSS (black), (c) P4VP@HMSS (blue), (d) Phen-Tb-P4VP@HMSS (green), and (e) Phen-Eu-P4VP@HMSS (red).

As shown in Fig. 3, the nitrogen adsorption–desorption isotherm of P4VP@HMSS is of type-IV without hysteresis loop. The pore parameters suggest that marked changes of the specific BET surface area did not occur compared to that of parent HMSS (Table S1†). But it is worthy to note that the unit cell parameter (3.9 vs. 4.3 nm) and calculated pore wall thickness (1.6 vs. 2.4 nm) increased, as well as the mesopore diameter (2.3 vs. 1.9 nm) and the pore volume (0.72 vs. 0.50 cm³ g⁻¹) considerably decreased, in accordance with the incorporation of polymer P4VP into the void space and intrapores of HMSS. Although FTIR and NMR spectroscopy, nitrogen physisorption and elemental analysis support the entrapment of P4VP into the HMSS matrix, it is difficult to be observed in the representative TEM image of P4VP@HMSS shown in Fig. 1b due to the low mass density of the polymer.⁴¹

Fig. 3 Nitrogen adsorption–desorption isotherms and the BJH pore size distribution curves (inset) of parent HMSS (black), P4VP@HMSS (blue), Phen-Tb-P4VP@HMSS (green) and Phen-Eu-P4VP@HMSS (red).

In order to obtain luminescent hybrid materials, europium(III) and terbium(III) cations were introduced into P4VP@HMSS, using impregnation with aqueous solutions of commercially available hydrated chlorides. By exploiting the coordination ability of the nitrogen donor atoms of P4VP and addition of the potentially chelating ligand 1,10-phenanthroline (Phen) this should result in the final product Phen-Ln-P4VP@HMSS (Ln = Eu or Tb). The Phen ligand is known as an efficient antenna ligand which strongly absorbs UV light which can be transferred and acts as excitation energy to Tb(III) or Eu(III) ions in order to enhance the emission intensity.^25,42 Non polymer-coordinated “free” lanthanide ions and Phen can be washed away completely with ethanol until no luminescence is monitored in the filtrate. The metal contents of the hybrid materials were analyzed as 1.6 (Tb) and 1.1 wt% (Eu) via ICP-MS.

Fig. 2d and e show the IR spectra of Phen-Ln-P4VP@HMSS (Ln = Eu or Tb). Compared to that of P4VP@HMSS, on the one hand, the characteristic peak of the pyridine is blue-shifted from 1597 cm⁻¹ to 1607 cm⁻¹, indicating the coordination of lanthanide ions to the nitrogen donor atoms in P4VP.³⁷ On the other hand, preservation of the characteristic peak (at 1558 cm⁻¹) of the Phen ligand coordinated to lanthanide(III) ions^35,36 in Phen-Ln-P4VP@HMSS (Ln = Eu or Tb) further demonstrated the existence of a stable Phen-Ln(III) complex. These findings confirmed that both Phen and P4VP nitrogen donor groups engage in the coordination with the lanthanide(III) ions in hybrid luminescent materials Phen-Ln-P4VP@HMSS (Ln = Eu or Tb).

For materials Phen-Ln-P4VP@HMSS (Ln = Eu or Tb), typical IV isotherms are retained with a specific BET surface area of 894 m² g⁻¹ and pore volume of 0.44 cm³ g⁻¹ for the Tb-hybridized material (Eu(III) hybrid: 883 m² g⁻¹, 0.47 cm³ g⁻¹) as shown in Fig. 3 and Table S1.† Compared to P4VP@HMSS, the decrease of BET surface area and pore volume, and retention of pore size (1.9 nm) confirmed clearly that the Phen-chelated lanthanide complexes have been successfully incorporated into the P4VP@HMSS matrix. Although TEM images (Fig. 1c and d) do not show any obvious difference in size and structure before and after the coordination of the lanthanide complexes, the emergence of lanthanide ion signals in the energy-dispersive X-ray (EDX, Fig. S5†) spectra of Phen-Ln-P4VP@HMSS (Ln = Tb, Eu) further corroborated the presence of lanthanide(III) ions and the successful coordination of Phen-chelated lanthanide ions to the entrapped P4VP.

The photoluminescent properties of the obtained hybrid mesoporous silica spheres Phen-Ln-P4VP@HMSS (Ln = Eu, Tb) were examined using fine powders dispersed in absolute ethanol. The europium-based material showed red emission at ambient temperature under an UV lamp, and its excitation spectrum was obtained by monitoring the emission wavelength of the Eu(III) ion at 615 nm (Fig. S6a†). Upon excitation at the strong excitation wavelength of 303 nm, the obtained emission spectrum (Fig. 4a) displayed the characteristic peaks of Eu(III) ions in the range of 500–710 nm, which could be assigned to ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) intra-4f⁶ transitions.^33,43 Among these peaks, the most intense emission at 615 nm is attributed to the ⁵D₀ → ⁷F₂ transition induced by an electric dipole. The intensity ratio of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) is usually used for assessing the symmetry of the Eu(III) cation. For Phen-Eu-P4VP@HMSS this value is ca. 4.38, indicating an (expected) low symmetry of the Eu(III) ion in the europium polymer complex of this hybrid material.⁴⁴ Similarly, the terbium-based hybrid HMSS exhibited green emission at ambient temperature under excitation with UV light. The excitation spectrum was measured by using the emission wavelength at 545 nm (Fig. S6b†). The characteristic peaks of the emission spectrum of Tb(III) ions (Fig. 4b) appeared in the range of 450–650 nm, which are assigned to ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions at 490, 545, 586 and 621 nm, respectively.⁴⁵ The most intense peak at 545 nm ascribed to the ⁵D₄ → ⁷F₅ transition indicated the green-color emission. For both of the lanthanide hybrid materials, their emission spectra displayed the bands characteristic of Eu(III) or Tb(III) ions and the absence of emission bands originating from organic ligands, indicative of ligand-mediated luminescence.

Fig. 4 Emission spectra of (a) Phen-Eu-P4VP@HMSS (red) and (b) Phen-Tb-P4VP@HMSS (green).

Finally, the thermal stability of the obtained hybrid materials Phen-Ln-P4VP@HMSS (Ln = Eu or Tb) was investigated by TGA/DTA measurements. As viewed in Fig. 5, three stages of weight-loss could be observed by increasing the temperature from 30 to 800 °C. The ca. 3.8% weight loss below 100 °C is ascribed to physically absorbed water. The following weight loss of ca. 10% centered at 300 °C is probably due to the first thermal displacement/decomposition of organic components/ligands in the hybrid materials. The exothermic mass loss of about 19% at 430 °C can be attributed to further decomposition of organic components and the europium complex. Similar TGA/DTA curves were observed for the terbium-containing hybrid material (see Fig. S8†). From the TGA/DTA curves it can be concluded that the encapsulation of polymer lanthanide complexes into HMSS can improve their thermal stability compared to the bare P4VP europium complexes whose thermal decomposition usually occurs around 300 °C.^46,47 The enhancement of thermal stability was in accordance with previously reported silica hybrid materials.⁴⁸

Fig. 5 Thermogravimetric analysis (TGA, solid line) and differential thermal analysis (DTA, dashed line) curves of Phen-Eu-P4VP@HMSS.

A ship-in-a-bottle approach using hollow mesoporous silica spheres (HMSS) as host (“bottle”) and 4-vinylpyridine (4VP), Ln(III) chlorides, and 1,10-phenanthroline (Phen) as “ship” components gives facile access to luminescent hybrid materials. Confined polymer P4VP constitutes the core of this system, affording efficient bonding of the Ln(III)-Phen fragments to the pyridine functional groups. Through adequate choice of functional polymers, metal complexes, and porous matrices, this strategy might provide access to a library of luminescent materials. Such tailor-made luminescent materials might open new avenues in biomedical imaging and for the detection of controlled drug release.

Acknowledgements

Ning Yuan acknowledges financial support from the China Scholarship Council (CSC). We are also grateful to the University of Tübingen for funding within the program EXPAND as well as Dr Markus Ströbele and Elke Nadler for TGA and SEM measurements, respectively.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, SEM images, UV spectra, small angle PXRD patterns, structural parameters, nitrogen physisorption isotherms, EDX spectra, excitation spectra, ¹³C MAS NMR spectra, elemental analysis, TGA/DTA data. See DOI: 10.1039/c5ra11612k

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