Hydroxyl-triggered fluorescence for location of inorganic materials in polymer-matrix composites

We present a locating technique for inorganic materials in polymer-matrix composites through a post-labeling approach based on specific covalent binding.


Introduction
Incorporation of inorganic materials (e.g., ions, molecules and supramolecules) into organic matrices has been regarded as an effective approach to advance functional composite materials. 1-3 Therefore, the recognition and location of the incorporated materials within the matrices are essential not only for subsequent research but also for understanding the critical processing conditions necessary for quality control. [4][5][6][7] Several techniques have been developed for the spatial location of incorporated materials, such as electron paramagnetic resonance, crystallographic and spectroscopic methods. [8][9][10][11][12] Recently, uorescence imaging techniques have become attractive alternatives for spatial visualization of individual incorporated materials in organic matrices. However, the established imaging techniques are mostly based on pre-modication of the inorganic materials with luminescent molecules before they are incorporated into the organic matrices. 13,14 Actually, from an industry point of view, it is desirable to evaluate the inherent structural and functional behaviors of the incorporated materials in organic matrices without any addition of extraneous substances. Therefore, it is a topic of interest and signicance to develop a simple location technique for inorganic materials in composites through a postlabeling process, instead of pre-modication.
Molecular targeted tracing techniques have been widely applied in biological elds, including tumor diagnostics, biomolecular detection and drug delivery. 15,16 Such methods are mainly based on in situ visualization of the cell/biomolecules in the organism via labeling with luminescent molecules, keeping the inherent properties of the targeted molecules. [17][18][19] These superiorities inspired us to introduce this type of technique into materials science, avoiding the changes to the inherent properties of the incorporated materials that occur in techniques with a pre-labeling aspect.
It has been reported that boronic acid-based covalent receptors possess a high targeting specicity towards hydroxyl groups on guests. 20,21 Much effort has been made to sense carbohydrates by boronic acid in combination with uorescence, colorimetry, and surface plasmon resonance. [22][23][24] There is apparently no good reason to disregard the applications of hydroxyl-triggered luminescence techniques to recognize hydroxyl groups in materials science. This situation may be ascribed to the aggregation-caused quenching in solid-state material. Encouragingly, Sun and his co-workers recently employed an AIE-active tetraphenylethene-cored diboronic acid (TPEDB) for sensing glucose when the uorogen was oligomerized with glucose through B-O bonds. 25 Therefore, it is reasonable to anticipate the application possibilities of hydroxyl-triggered luminescence techniques in materials science using boronic acid-based covalent receptors. Two-dimensional layered materials (e.g., montmorillonite (MMT) and layered double hydroxides (LDHs)) are the most widely chosen additives to improve the properties of organicinorganic composites. 26,27 The abundant hydroxyl groups on the surfaces of two-dimensional layered materials have been used to construct a hydrogen-bond network between the hydroxide layers and the organic guests, 28,29 facilitating the fabrication of various functional organic-inorganic composites. 30,31 In this contribution, we observed that strong emissions are generated via specic B-O bonds between the hydroxyl groups of layered materials and AIE-active boronic acid. Furthermore, the visualization of incorporated MMT/LDHs in polymer-matrix composites has been implemented by simply dipping the composite lm in AIE-active boronic acid solution without the aid of any pre-modication before the lm formation (Scheme 1). The generality and simplicity of a post-labeling process are the highlights of our proposed approach: (1) the used AIE-active boronic acid reagent is commercially available; and (2) technicians could potentially adopt this approach without substantial training in chemistry. Therefore, the proposed strategy has great potential in the in situ screening of advanced properties for organic-inorganic composites.

Results and discussion
To explore the possibility of hydroxyl-triggered luminescence between AIE-active boronic acid and layered materials with hydroxyl groups, MgAl-LDHs were synthesized via a hydrothermal method (Fig. S1 †). As a typical AIE compound with two boronic acid groups attached to the TPE core, TPEDB can be ionized and is molecularly soluble in alkaline medium. 32 Fig. S2 † shows that TPEDB can emit weak blue light at 415 nm with the excitation at 330 nm. However, the emissions of TPEDB were signicantly intensied with the addition of LDHs (Fig. 1A). The strongest cyan emission for the TPEDB-LDH composites, which was $25-fold higher than that of pristine TPEDB, was achieved when the concentration of LDHs reached 1.8 mM. The greatly promoted uorescence could also be visualized by photographs under UV irradiation (Fig. 1B). Moreover, the quantum yields of TPEDB-based samples have also been investigated, with the lowest value of 2.05% obtained for pristine TPEDB and the highest yield of 17.46% for TPEDB-LDHs (1.8 mM). These results indicated that the uorescence intensity of TPEDB can be signicantly enhanced in the presence of LDHs.
A series of control experiments were performed to explore the specic B-O bonds between the hydroxyl groups of LDHs and the boronic acid of TPEDB. The precursors for the preparation of LDHs were studied, and the results showed that these precursors had no effect on the uorescence emission intensities of TPEDB (Fig. S3 †). On the other hand, the TPE core in the absence of boronic acid groups was also investigated; however, the emission spectra of the TPE remained the same aer the addition of the LDHs (Fig. 2A). These results demonstrated that the LDH-enhanced TPEDB luminescence was not due to the adsorption of phenyl groups onto the surface of the LDHs. Moreover, the electropositive polymer poly(diallyldimethylammonium chloride) (PDDA) was added into the TPEDB solution. Interestingly, no obvious change in the uorescence spectra was observed (Fig. 2B), indicating that the electrostatic interaction was not responsible for the TPEDB-LDH interactions. In conclusion, the above results implied that the binding affinity between the hydroxyl groups of LDHs and the boronic acid of TPEDB might exist.
In order to verify the binding affinity property, FT-IR measurements were conducted. Fig. S4A † shows the IR transmittance spectrum of LDHs, where the broad band at 3100-3600 cm À1 represents the hydroxyl groups and the peaks around 1350-1380 cm À1 stand for nitrate/carbonate in the interlayer of Scheme 1 Schematic representation of hydroxyl-triggered fluorescence targeting techniques for layered materials in polymer-matrix composites. the LDHs. 33,34 Unfortunately, the region of interlaminar nitrate/ carbonate is coincident with the stretching of B-O bonds, which is in the range of 1310-1430 cm À1 . 35,36 As an alternative, another layered material with abundant hydroxyl groups, MMT, was also studied. As expected, obvious bands in the range of 1310-1430 cm À1 appeared for MMT aer the addition of TPEDB (Fig. S4B †), indicative of B-O bond formation. 38,39 We can draw the conclusion that the specic B-O bonds were formed between the boronic acid of TPEDB and hydroxyl groups of layered materials, which contributes to the restricted rotation of the phenyl units. Therefore, the non-radiation process of TPEDB was inhibited, resulting in the improved luminescence activity of TPEDB as illustrated in Fig. 1. [37][38][39] It has been reported that carbohydrates with different structures (e.g., number, relative distance and arrangement of hydroxyl groups) exhibit different binding affinities towards boronic acid. [21][22][23] In this study, different LDHs and exfoliated LDH nanosheets (ELDH) with tunable hydroxyl groups were prepared. As seen in Fig. 2C, the ELDH could slightly increase the uorescence intensity of TPEDB in comparison with the LDHs illustrated in Fig. 1. This phenomenon may be due to the hydroxyl sites occupied by formamide in order to maintain the monolayer state of the exfoliated LDH nanosheets. 40 Furthermore, different metal ionic radii can change the lattice parameters of LDHs. Accordingly, the distance between adjacent hydroxyl groups is different. [41][42][43][44] Herein, Gd-doped MgAl-LDHs and MgGa-LDHs were investigated (Fig. S5 †). 44 The results indicated that MgAl-LDH, with the closer hydroxyl arrangement, exhibited the larger uorescence enhancement for TPEDB (Fig. 2C); however, the Gd-doped MgAl-LDHs and MgGa-LDHs, with remote hydroxyl distances, only displayed slight uorescence enhancements (Fig. 2D). Therefore, the uorescence intensities of TPEDB could be used for distinguishing the hydroxyl arrangement of different LDHs.
The precise and selective recognition of hydroxyl groups on the surfaces of LDHs by virtue of TPEDB emissions inspired us to develop a general method for targeted tracing of hydroxylcontaining layered materials in organic-inorganic composites by the confocal uorescence microscopy (CFM) technique. To achieve this goal, we rst investigated the binding affinity of TPEDB towards the LDH powder. The results showed that the non-emissive LDHs exhibited a strong cyan luminescence at 470 nm ( Fig. 3A and B) with the addition of a certain amount of TPEDB (50 mL, 100 mM), indicative of the efficient binding between TPEDB and LDHs.
Next, LDHs were blended in polyethylene (PE) at a ratio of 5% and a stochastic area of the PE-5% LDH lm was selected and then dipped into a 100 mM TPEDB solution for 10 min. As shown in Fig. S6, † great numbers of uorescent cyan dots were captured to map the location of the LDHs in the PE matrix. However, no uorescent dots appeared in the pure PE lm in the absence of LDHs (Fig. S7 †). Moreover, for spatial distribution characterization, three-dimensional (3D) imaging involving image collecting techniques at different depths along the Z-axis were adopted. Fig. 3C shows that the LDHs incorporated in the PE matrix can be clearly observed through uorescence labeling by TPEDB in 3D (600 Â 600 mm 2 ). In comparison with the non-selective physical absorption process, these CFM results reect the stable and specic location of LDHs in polymer-matrix composites by the proposed hydroxyltriggered uorescence method.
To verify the accuracy of targeting hydroxyl groups on the surface of LDHs in polymer-matrix composites, two-color co-  staining experiments were performed. In this work, the LDHs were pre-stained with red emission quantum dots (QDs) through electrostatic interaction to prepare the molded PE-5% (QD@LDH) lm. The obtained PE-5% (QD@LDH) lm was then dipped into the TPEDB solution to mark the LDHs via the B-O bonds. As depicted in Fig. 4A, the positions of the LDHs are represented as red dots of QDs in the PE matrix through a double-channel detection of CFM. On the other hand, the PE-5% (QD@LDH) lm aer post-staining by TPEDB showed cyan colored dots, as displayed in Fig. 4B. Interestingly, the merged picture of channel A and channel B reected an almost perfect match between the QD pre-stained dots and TPEDB postbonded particles (Fig. 4C). A 3D representation of this costaining approach was also produced. It was observed that the coincident mapping of LDHs was acquired in composite lms, even in different focal planes along the Z-axis (Fig. 4D and E). These results demonstrate that the proposed visualization strategy is a newly non-invasive platform to precisely target and trace the incorporated LDHs in polymer-matrix composites.
We further investigated the universality of this proposed method for mapping the spatial distribution of inorganic layered materials with hydroxyl groups in polymer matrices. The other typical layered material, MMT, [45][46][47] was investigated to verify the specic binding between hydroxyl groups and boronic acid. The strong emission of MMT appeared in the presence of TPEDB as a result of the formation of B-O bonds (Fig. S8 †). In addition, the incorporated MMT (5%) in PE can be distinctly screened through the precise match between the hydroxyl groups and boronic acid (Fig. S9 †). To further assess this strategy, another polymer, polypropylene (PP), was also examined as a matrix for the LDH/MMT. Excitingly, the binding selectivity and molecular recognition were reproduced for the PP-5% LDH and PP-5% MMT lms (Fig. S10-S12 †), and the spatial distributions of the LDHs and MMT in PP were similar to those observed for PE. Therefore, our experimental data demonstrated the possibility of the in situ recognition and visualization of inorganic layered materials in polymer matrices through specic B-O bonds by a rapid and simple approach.

Conclusions
In conclusion, we have demonstrated the presence of a strong solid-state emission of layered materials due to the formation of specic B-O bonds between hydroxyl groups on the surfaces of layered materials and AIE-active emissive boronic acid. This hydroxyl uorescence location technique enabled us to establish a powerful imaging platform to precisely target and trace the incorporated inorganic materials in polymer-matrix composites. This post-labelling approach exhibited superiority in evaluating the inherent structural and functional behaviors of the inorganic-organic composites in comparison with the traditional pre-modication procedures. More importantly, such a unique uorescent probe can be easily achieved by simply dipping polymer-matrix composite lms in AIE-active boronic acid solution combined with high-resolution uorescent imaging. Our strategy opens up new possibilities for optimization of critical processing conditions, while retaining the initial structural nature of polymer-matrix composites. This facile method can also be applied for the in situ targeted tracing of other inorganic materials by changing the binding affinity between the inorganic materials and AIE molecules.

Conflicts of interest
There are no conicts to declare.