Rational design and synthesis of covalent organic polymers with hollow structure and excellent antibacterial efficacy

Abstract

A covalent organic polymer with hollow structure (COP-H) has been synthesized via Sonogashira coupling from the precursors containing positive charge. The formation of COP-H was confirmed by FTIR, solid-state NMR, SEM and TEM. COP-H showed excellent anti-microbial activity because of the cationic charge on the surface of COP-H.

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Covalent organic polymers (COPs) have received considerable attention not only for their porosity and large surface area, easy designing and functionalization, but also for their strong covalent bonds and high stability compared to metal-organic frameworks (MOFs) counterparts, which made COPs appealing in catalysis, gas storage and separation, chemical sensors, light emitting materials and bioscience.¹ Recently, some progress has been made in studying the micro-morphology of the hollow structures, which can viewed as capsules or carriers for drug delivery or nanoreactors, and hollow spheres also showed better efficiencies in catalytic performance than non hollow ones.² In general, porous polymers with hollow structures, such as SiO₂, TiO₂, some polymers like polystyrene and poly(lactic-co-glycolic acid), were prepared by layer-by-layer method or templated synthesis.³ For example, Li and his co-workers reported the hollow micro-porous organic capsule templated by SiO₂,⁴ and Kim et al. also prepared the hollow micro-porous organic network with Sonogashira coupling reaction using SiO₂ as template.⁵ There are also some reports on different templates like ZnO, poly(methyl methacrylate) (PMMA) and even MOFs.⁶ However, the removal of the templates is quite essential in these systems. The waste of templates, removal agents and energy consumption prevents this method to be universal and appropriate for green chemistry. Therefore, it is highly desirable to develop a simple method without the use of any templates and post-modification techniques for making COPs with hollow structure.

Very recently, Zhao et al. reported the single-step preparation of 2D polymers and their evolution into hollow sphere, which was driven by the reduction of the surface energy.⁷ However, the unstable connection of borate, especially in aqueous solution, limited the application of this hollow polymeric sphere. Inspired by the formation of polymeric vesicles through self-assembling of amphiphilic copolymers in aqueous solution,⁸ we hypothesized that the organic polymers with hollow structure could be prepared via similar rational design. Herein, we choose fluorene decorated with positive charge (Fluorene-C) and 1,3,5-triethynylbenze (TEB) as building blocks to synthesize COP with hollow structure (COP-H) in the absence of templates. We used the non-charged fluorene (Fluorene-N) with TEB to synthesize COP with non-specific morphology as a control experiment. The anti-microbial activity of COP-H was studied towards gram-negative bacteria.

Scheme 1 shows the design and preparation of the COP-H particles. The particles were obtained through Sonogashira coupling reaction between Fluorene-C and TEB, after being washed with DMF, water and ethanol for several times, then dried under vacuum at 70 °C (details can be seen in ESI† for the preparation). To confirm our strategy in designing the COP-H, control experiment with non-charged Fluorene-N and TEB was carried out under the same conditions, which formed the covalent organic polymer, denoted as COP-C. The structure of the COP-H and COP-C were verified by Fourier transform infrared spectroscopy (FTIR) (Fig. S1, ESI†) and solid-state nuclear magnetic resonance (NMR) (Fig. 1). In Fig. S1,† the peaks centered at 2200 cm⁻¹ are ascribed to the C–C triple bonds, and the peaks at 2929 cm⁻¹ are due to the C–H stretching vibration of the alkyl chain of the fluorene precursors for both the COP-H and COP-C. While the peak centered at 3400 cm⁻¹ only appeared in COP-H, which is for the vibration of the positive charged parts. In Fig. 1a and b, for the solid-state NMR ¹³C spectra, the peak at 92.0 ppm is attributed to the C–C triple bonds, whereas the peak at 65.7 ppm is attributed to cationic of COP-H, which is not shown in COP-C, and other peaks can fit well with the structures of COP materials.^6c,9 These results confirmed the formation of COP-H and COP-C. Thermal gravimetric analysis (TGA) was carried out and is shown in Fig. S2 (ESI†). Note that both COP-H and COP-C are stable over 200 °C, and each of the COPs has a weight loss of 100% at nearly 600 °C. Fluorescent spectra were recorded to study the emission behaviors of the COPs as synthesized, and the results are shown in Fig. S3.† The COPs show different fluorescence spectra for the charged and non-charged alkyl chains. The COP-H has an emission at 525 nm while COP-C shows no fluorescence. This is because the bromide atoms on alkyl chains of the COP-C quenched the fluorescence.

Scheme 1 Preparation of COP-H and COP-C.

Fig. 1 Solid-state NMR ¹³C spectra of COP-H (a) and COP-C (b).

The morphology of COP-H was examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). As shown in Fig. 2a and b, the particles with spherical structure can be seen clearly, and hollow structures were found in Fig. 2b. SEM images in Fig. 2c also show the sphere particles, and the hollow structure was further confirmed when the particle was in a broken state (Fig. 2d). The average size of the particles was about 120 nm from TEM pictures, which was further confirmed by the result of DLS, as presented in Fig. S4 (ESI†). The sizes of the particles range from 50 nm to 200 nm, which is due to the growing layers with the reaction going on. Generally, COP materials prepared under such conditions have 2D or 3D non-specific morphology,⁹ which was also supported by control experiment, as presented by SEM in Fig. S5a and b.† The possible mechanism for this hollow structure is that a 2D material formed during the reaction, while the electrostatic repulsion between the positive charges drove the 2D COP into curling state, and then lead the formation of hollow structure.⁷ Moreover, the positive charged part of the polymer is hydrophilic while the other parts are hydrophobic, with stirring during reaction, and the phase separation drove the planar polymer into a hollow sphere, similar to the hydrophobic force driving the self-assembly of vesicles and micelles.^8,10 The positive charge was verified by zeta potential value of +19.15 mV. PXRD result is presented in Fig. S6 (ESI†) and shows the non-crystallization state of the COP-H, which is reasonable for the formation of covalent bonds that are not dynamic and reversible.

Fig. 2 TEM (a) and (b) and SEM (c) and (d) of COP-H. The scale bars are 500 nm for (a) and (b), 2 μm for (c) and 500 nm for (d).

Many antibacterial materials based on heavy metal ions like Ag⁺ and Cu²⁺ are limited in some aspects because of the high toxicity of these metal ions.¹¹ Looking for antibacterial materials without metal is interesting. Recently, Wang et al. reported photocatalytic disinfection by graphitic carbon nitride polymers under visible light.¹² Yang et al. reported a metal-free triblock polymer with positive charge, achieving the selective lysis of microbial membranes.¹³ To the best of our knowledge, there is no report on the COP as antibacterial material. Herein, the antibacterial properties were studied using COP-H. We hypothesized the particles could work as a nano-scalpel to gash the bacterial, and finally destroy the germs.^13,14 With this idea in mind, we carried out the antibacterial experiment towards the gram-negative bacterial Escherichia coli (E. coli). After COP-H with various concentrations were incubated with the E. coli for 0 h, 4 h and 16 h, optical density (OD) value was tested to evaluate the efficiency of anti-bacterial, and the result is shown in Fig. 3. The result shows that COP-H has inhibited the growth of E. coli efficiently compared to the control group with 0 mg ml⁻¹ COP-H material. With the increase of COP-H concentration, the inhibiting efficiency increases, due to the total amount of positive charge increase. Note that the material has a certain antibacterial activity at a quite low concentration. The minimal inhibitory concentration (MIC) is defined as the lowest polymer concentration to inhibit visible bacterial growth,¹⁵ and as reported by Lin et al., the concentration of the demarcation sample (whose solution remains clear among the series) was determined as a MIC of 0.25 mg ml⁻¹ for COP-H,¹⁶ which is comparable to excellent cationic micelle materials.^14a,14b We also considered the control experiment with COP-C for the antibacterial tests, and COP-C showed no antimicrobial activity even at a concentration of 10 mg ml⁻¹, as can seen from the results in Fig. S7.†

Fig. 3 OD values at 600 nm for the E. coli incubated with COP-H.

In summary, we have synthesized a covalent organic polymer (COP-H) with hollow structure by introducing the positive charge on the monomers. The formation mechanism of hollow structure was revealed by using non-charged monomer control experiment. COP-H indicated efficient anti-bacterial properties towards inhibiting the growth of gram negative bacteria. We hope our strategy could provide a new way to synthesize hollow structure materials and highlight the potential application of COP materials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Project no. 91227118 and 21104075).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06118g

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