Stability, UV shielding properties, and light conversion behavior of Eu(BMDM)3@polysiloxane nanoparticles in water and polyurethane films

https://doi.org/10.1016/j.matchemphys.2012.09.070Get rights and content

Abstract

Bifunctional Eu(BMDM)3@polysiloxane nanoparticles were prepared through reprecipitation–encapsulation methods using 1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione (BMDM) ligand and octyltrimethoxysilane (OTS) precursor and embedded into waterborne polyurethane (PU) coatings to fabricate transparent optical composite films. The photostability and thermostability of the nanoparticles in water and their ability to block UV and convert light when embedded in PU films were investigated. In comparison with the control Eu(BMDM)3 nanoparticles, the Eu(BMDM)3@polysiloxane nanoparticles, especially those prepared at a Eu(BMDM)3/OTS mole ratio of 1:2, exhibited far superior stability under storage conditions, UV irradiation, and heating. They also showed excellent UV-shielding and highly efficient light conversion properties because of the protective polysiloxane.

Highlights

► Eu(BMDM)3@polysiloxane nanoparticles were successfully prepared. ► The nanoparticles show excellent stability to storage, UV light, and heat. ► PU film with 0.3 wt% nanoparticles exhibited excellent UV shielding performance. ► The nanoparticles can be used as bifunctional additives for agriculture film.

Introduction

In the past few decades, lanthanide-complex-based luminescent materials have attracted much attention due to their excellent performance in biological imaging, temperature sensors, luminescent solar concentrators, agriculture, and horticulture [1], [2], [3], [4], [5], [6], [7], [8], [9]. To date, many organic ligands, such as β-diketones, carboxylic acids, and heterocyclic derivatives have been used to assemble lanthanide ions into lanthanide complexes [9], [10], [11], [12]. β-diketones are the most frequently used ligands [9], [13]. The lanthanide-β-diketone complexes have great potential in applications involving lasers, NMR shift reagents, and organic light emitting diodes (OLEDs) and as catalysts in organic reactions [13], [14], [15], [16], [17]. Ever since the emission characteristics of lanthanide β-diketone complexes were first reported by Weissman, they have received a great deal of interest [13], [18], [19], [20], [21]. However, these studies have covered only the photoluminescent behavior, crystal structures, and photophysical processes. The stabilities of lanthanide β-diketone complexes under UV-irradiation, heat, and etc. as well as the UV-absorption behavior of lanthanide β-diketone complexes were seldom concerned.

The most prominent properties of lanthanide β-diketone complexes are their photoluminescent properties, which are the result of effective energy transfer from β-diketone ligands to lanthanide ions. This is called the antenna effect. Meanwhile, the strong UV absorption of β-diketone ligands ascribed to their π–π* transition can endow lanthanide β-diketone complexes with excellent UV shielding properties [19]. In this way, lanthanide β-diketone complexes show great potential as both UV shielding and light conversion agents for agricultural films, in which they would absorb harmful UV light from sunlight and convert it to blue or red light that is necessary for photosynthesis. However, unmodified lanthanide complexes are usually not suitable to practical application due to their poor thermostability and photostability [22]. To solve this problem, scientists have used microemulsion method and Stöber method to encapsulate the lanthanide complexes in a silica layer [23], [24], [25], [26]. Unfortunately, the as-obtained lanthanide complexes and silica core–shell nanoparticles cannot absorb UV as well as their corresponding lanthanide complexes and organic ligands [24]. It is highly desirable to develop novel methods to improve the stability of lanthanide complexes without loss of their strong UV absorption.

In this paper, 1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione (BMDM), one of the most commonly used and commercially available sunscreen agents, was employed as the organic ligand to fabricate the europium β-diketone complex (Eu(BMDM)3). BMDM was chosen for its high absorption coefficient in the UV range of 300–400 nm [27]. The Eu(BMDM)3 complex together with octyltrimethoxysilane (OTS) was further transformed into Eu(BMDM)3@polysiloxane composite nanoparticles by reprecipitation–encapsulation method [2], [28]. The as-obtained composite nanoparticles not only improved the photostability and thermostability of lanthanide complexes but also retained UV absorption of lanthanide complexes. When they were embedded into polyurethane (PU) dispersion, the PU film displayed excellent UV shielding properties and efficient light conversion.

Section snippets

Materials

Aqueous ammonia solution (NHH2O, 28 wt%), europium oxide (Eu2O3), concentrated hydrochloric acid, absolute ethanol, octyltrimethoxysilane (OTS) and Rhodamine B were purchased from Sinopharm Chemical Reagent Co. 1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione (BMDM, 98 wt%) was obtained from J&K Chemical Ltd. Waterborne PU dispersion (solid content 34%, NEOREZ R-972) was provided by DSM Co. Titania nanoparticles (P25) with the average size of 25 nm were purchased from Degussa Co.,

Preparation of Eu(BMDM)3@polysiloxane nanoparticles

Commercial sunscreen agent (BMDM) was used as the organic ligand for the fabrication of lanthanide complexes [Eu(BMDM)3]. BMDM serves as a highly efficient absorber of UV and as an antenna to transfer the absorbed energy into Eu3+ to facilitate the light emission of Eu3+. In order to improve photostability and thermostability, a protective polysiloxane layer derived from the hydrolysis and condensation of OTS was encapsulated the Eu(BMDM)3 complex via the reprecipitation–encapsulation method,

Conclusions

In this paper, dual functional Eu(BMDM)3@polysiloxane nanoparticles were prepared through reprecipitation–encapsulation using BMDM ligand and an OTS precursor. At an excitation wavelength of 365 nm, the Eu(BMDM)3@polysiloxane nanoparticles exhibit one strong emission at 613 nm and two weak emissions at 579 and 590 nm. These were assigned to the transitions 5D0-7F0, 5D0-7F1 and 5D0-7F2 of Eu3+. The stabilities of these excited emissions under storage, UV-irradiation, and heat conditions were

Acknowledgments

This work was supported by Nature Science Foundation of China (Grant No. 51073038), the innovative team of Ministry of Education of China (Grant No. IRT0911) and Key Project of Nature Science Foundation of China (Grant No. 51133001).

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