Eu3+-induced aggregates of diblock copolymers and their photoluminescent property
Highlights
► A general protocol to prepare self-assembled polymeric aggregates was proposed. ► Multiple morphologies could be observed by adjusting the parameter of reactants. ► The internal mechanism and photoluminescent property were discussed in detail.
Introduction
There has been an arising interest in the research of lanthanide complexes due to their remarkable properties on electricity, optics and catalytic, etc. [1], [2], [3], [4], [5], [6]. All of these distinct features result from the fact that the 4f electronic states of these lanthanide ions are shielded by the outer 5s and 5p electronic orbits, while the parity electronic transitions between the 4f electronic states are strictly forbidden, either [2]. The most studied lanthanide elements are europium (Eu) [7], [8], [9] and terbium (Tb) [10], [11], whose photoluminescent (PL) property is one of the most important applications [3], [12], [13], [14], [15].
However, the PL intensity of lanthanide compounds alone is relatively weak, and these compounds are hard to be fabricated, either. A common solution to this problem is incorporating them into organic or inorganic ligands, forming organic–inorganic hybrid compounds [3], [16], [17]. The PL intensity could be strengthened through the antenna effect of ligands [14], [18], [19]. The most widely used ligands include small molecules like β-diketones and other conjugated compounds [8], [13], [20], [21], [22]. However, more and more researchers now have turned to using polymers as high molecular ligands due to their better processing ability, lighter weight, and lower cost [9], [23], [24], [25], [26]. There are mainly two ways to prepare this kind of complexes [1]: the first one is simply doping the lanthanide complexes into the polymer matrices, making them blend uniformly; and the second one is to make the lanthanide ions coordinate with certain groups along the polymer chains. That is to say, the former way is a physical approach while the latter one is chemical. Although the doping method is relatively simpler, its limitation lays in the bad compatibility between the lanthanide complexes and the polymer matrices, resulting in the phase separation phenomenon which degrades the performance of the resultant materials. Taking advantage of coordination bonds formed by the chemical approach, the PL intensity is enhanced greatly [27]. In fact, in early 1963, Wolff and Pressley studied the fluorescence properties of Eu3+-containing poly (methyl methacrylate) (PMMA) [28]. Since then, a large amount of literatures on lanthanide-polymer complexes with photoluminescent character came into being [5].
At the same time, the development of polymerization techniques has provided a variety of polymer ligands for complexation, especially the rise of living free radical polymerization represented by atom transfer radical polymerization (ATRP) [29] and reversible addition-fragmentation chain transfer (RAFT) polymerization [30], which could offer polymers with low polydispersity and various chain structures like blocks, brushes, and stars [31].
The overall preparation procedure of the lanthanide-polymer complexes by the chemical approach is similar to that of small molecule-lanthanide complexes. Generally speaking, the complex products will precipitate from the solution and form solid-like powders or crystals, which may not be suitable for further processing like spin-coating and casting. Jin et al. introduced a novel method called ion-induced micellation of double hydrophilic block copolymers (DHBCs) to prevent the problem mentioned above [32]. However, their method was restricted to the solvents system of alcohols with hydroxyl groups, which may induce the fluorescence quenching. The simple self-assembled micelles of diblock copolymer were firstly reported by Eisenberg in 1995 [33], and it has been developed rapidly for its potential usages like drug delivery [34], [35], [36]. As for the amphiphilic diblock copolymers, there are mainly three ways to prepare micelles: (1) dissolve the polymers directly in the “selective solvent”, which is soluble to one segment of the polymer while insoluble to the other segment; (2) dissolve the polymer in a “non-selective solvent”, which is soluble to both segments of copolymer, and then add selective solvent slowly; (3) External stimulus like chemical reactions of side groups along polymer chains and tuning the pH values or temperatures of the system [34], [37], [38], [39]. The solution containing polymer micelles is still kept homogeneous, which is an excellent method to prevent precipitation.
Herein, we present the synthesis and characterization of self-assembled polymeric aggregates using the coordination-induction technique. Firstly, amphiphilic diblock copolymer, PS-b-PAA, was synthesized as the polymer ligand by RAFT polymerization. Then, europium chloride (EuCl3) was added into the polymer solution in N,N-dimethyl formamide (DMF). By coordination of the Eu3+ ions with carboxyl groups along the polymer chains, the resultant Eu3+-PAA forms cross-linked networks, which act as the core, while the PS segments still keep their solubility to DMF by fully stretching into the solvent phase, and then, the self-assembled aggregates form in the solution. The overall procedure is illustrated in Fig. 1. A rich variety of morphological structures of these aggregates could be observed by adjusting the molar ratio of acrylic acid groups (AA) in PAA segment to Eu3+ ions or the concentration of overall reactants. Meanwhile, the underlying formation mechanism was discussed in detail. Moreover, the PL property of these aggregates was another focus of this article.
Section snippets
Materials
All the reagents used for the synthesis of RAFT agent were bought from Sigma–Aldrich and were used as received. The monomers, styrene, and acrylic acid (AA) were distilled under reduced pressure before use; 2,2-azobisisobutyronitrile (AIBN) was purified by recrystallization from ethanol; the europium oxide (99.99%) was purchased from Darui Company (Shanghai, China) and used directly. 1,10-phenanthroline and all the solvents, including dioxane, methanol, petroleum ether and N,N-dimethyl
The structure of coordinated polymeric aggregates
The UV/vis absorptions of PS-b-PAA and Eu3+ ion coordinated block copolymer (Eu3+-BCP) are presented in Fig. 3a. The block copolymer shows strong absorption ranging from 260 nm to 350 nm with its peak at 269 nm mainly due to the π–π* electronic transition of phenyl groups in PS segment and carboxyl groups in PAA segment. However, the absorption platform of BCP between 280 nm and 350 nm disappears in the spectrum of Eu3+-BCP, whose peak red-shift to 271 nm from BCP’s 269 nm at the same time. It could
Conclusions
In summary, we have prepared a novel series of polymeric aggregates by a coordination-induction self-assembly method. The amphiphilic diblock copolymer, PS-b-PAA synthesized by RAFT polymerization, was used as the polymer ligand and Eu3+ ions acted as the cross-linkers. Experiments indicate that these aggregates present various morphological structures such as ordinary micelles, rod-wrapped micelles, sun-shaped micelles, vesicles and LCMs at different Eu3+ concentrations and different molar
Acknowledgments
This study was supported by: (1) Natural Scientific Foundation of China (Grant #51273096); (2) International Collaborative Program of Qingdao Science & Technology Bureau, Grant #10-1-4-97-hz; (3) Shandong Province Project: Tackle Key Problem in Key Technology, #2010GGX10327; (4) Program of Qingdao Science & Technology (2012–2014).
References (52)
- et al.
Opt. Mater.
(2004) - et al.
J. Colloid Interface Sci.
(2003) - et al.
J. Colloid Interface Sci.
(2009) - et al.
Polymer
(2008) - et al.
Eur. Polym. J.
(2009) - et al.
Prog. Polym. Sci.
(2007) - et al.
Prog. Polym. Sci.
(2012) Prog. Polym. Sci.
(2003)- et al.
J. Colloid Interface Sci.
(2011) - et al.
J. Colloid Interface Sci.
(2012)
Prog. Polym. Sci.
J. Colloid Interface Sci.
Chem. Rev.
Chem. Rev.
Chem. Soc. Rev.
ACS Nano
Chem. Rev.
Chem. Soc. Rev.
J. Am. Chem. Soc.
J. Phys. Chem. C
J. Am. Chem. Soc.
J. Phys. Chem. B
J. Phys. Chem. C
J. Am. Chem. Soc.
Chem. Mater.
Anal. Chem.
Cited by (25)
Highly sensitive color fine-tuning of diblock copolymeric nano-aggregates with tri-metallic cations, Eu(III), Tb(III), and Zn(II), for flexible photoluminescence films (FPFs)
2021, Journal of Materials Science and TechnologyCitation Excerpt :While, their aggregates may affect the distribution uniform in the matrix [27,28]. Our group have synthesized Eu3+-induced aggregates with rich morphological structures [13]. Following this origin, we fabricate a series of nano aggregates by introducing three kinds of metal ions into solutions of amphiphilic diblock copolymers of PS-b-PAA.
Advances in non-covalent crosslinked polymer micelles for biomedical applications
2021, Materials Science and Engineering CSelective Sensing of Cu<sup>2+</sup> and Fe<sup>3+</sup> Ions with Vis-Excitation using Fluorescent Eu<sup>3+</sup>-Induced Aggregates of Polysaccharides (EIAP) in Mammalian Cells and Aqueous Systems
2020, Journal of Hazardous MaterialsCitation Excerpt :Fluorescent lanthanide complexes offer improved photo-stability, sharp emission spectra and high quantum yield. Through the design of lanthanide (Ln) complexes, the most promising fluorescent properties can be obtained, such as large Stokes shifts, long fluorescence lifetimes, intense emissions and insensitivity to quenching by oxygen (Liu et al., 2016b; Xu et al., 2013; Wang et al., 2017; Resende Filho et al., 2017). Consequently, due to their low cost, colour purity and high quantum efficiencies (Zhang et al., 2018; Shavaleev et al., 2015; G.Y. et al., 2013), these Ln-complexes are favorable candidates for applications in environmental monitoring, human health, food safety, bio-imaging and fluorescence sensing and detection (Zhang et al., 2013; Mikhalyova et al., 2015; Bouzigues et al., 2011).
Structure and solvation thermodynamics of asymmetric poly (acrylic acid)-b-polystyrene polyelectrolyte block copolymer micelle in water: Effect of charge density and chemical composition
2018, PolymerCitation Excerpt :For XPS = 0.2 and 0.7, it is observed that micelle aggregation number shows some fluctuation and these do not match with the value estimated using the scaling model. For XPS = 0.7, two smaller micelles (denoted as M1 and M2) were formed after 95 ns with aggregation numbers 9 (for M1) and 6 (for M2) which remained separated after 120 ns At XPS = 0.2, the formation of a micelle was observed in a system containing 5 copolymer chains after 53 ns, instead of 9 chains (value estimated from scaling relation) even after 80 ns It is well known that micellar aggregation numbers (m) have statistical distribution and m has deviation range as 5–14% as observed from experimental findings [28–32]. Hence the deviation in m, which we have observed in the systems from our simulations is not unusual.
Red light emitting nano-PVP fibers that hybrid with Ag@SiO<inf>2</inf>@Eu(tta)<inf>3</inf>phen-NPs by electrostatic spinning method
2018, Optical MaterialsCitation Excerpt :Rare-earth complexes noted for their photoluminescence properties have greatly drawn scientists' attention in a variety of promising applications, such as laser materials [1], magnetic materials [2] and phosphors [3]. One of the attractive features is emission bands arising from f-f transitions [4], along with extremely narrow bandwidth, which leads to a high color purity of the emitted light [5,6]. The intramolecular energy transfer is assigned to the well-known “antenna effect” [7,8].