Design and synthesis of two novel functional metal–organic microcapsules; an investigation into ligand expansion effects on the metal–organic microcapsules' properties

Abstract

In this study, two novel metal–organic capsules were synthesized based on coordination-directed organization of two new extended tetrazolate ligands (1,4-bis((1H-tetrazol-5-yl)methyl)benzene (btmbenzene), 4,4′-bis((1H-tetrazol-5-yl)methyl)biphenyl (btmbiphenyl)) and Zn²⁺ ions. Furthermore, the function of these capsules in the inclusion of inorganic nanoparticles (Fe₃O₄), dyes (Rhodamine B) and drugs (5-fluorouracil) was investigated. Moreover, the effects of ligand expansion on the size, morphology, loading capacity and other properties of these capsules were studied. In addition, other morphologies of these coordination polymers were fabricated under different conditions . Finally, the obtained microcapsules were utilized for the synthesis of crystalline ZnO hollow spheres via a calcination method.

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Introduction

Infinite coordination polymers (ICPs), consisting of polymerized metal–ligand networks which can be synthesized by self-assembly of metal nodes and a polydentate organic ligand, have recently been regarded as a captivating innovative class of inorganic–organic materials.^1–3 Metal–organic capsules with the capability of adaptive entrapment of various functional species are new ICP materials which have appealed to researchers in recent years. Recently, various guest materials such as drugs, dyes, small nanoparticles and nucleotides, have been included into self-supported ICP coordination networks.^4,5,7–11 In 2009, Kimizuka et al. reported the preparation of supramolecular networks constructed from lanthanide ions and nucleotides which exhibited adaptive encapsulation features. Moreover, the ability of these capsules to act as contrast enhancing agents for magnetic resonance imaging (MRI) were proven.⁵ However, the main drawback of lanthanide-containing compounds is their toxicity toward living systems.⁶ Concurrently, inorganic–organic empty spheres consisting of Zn(II) metal ions and 1,4-bis(imidazol-1-ylmethyl) benzene (bix) were synthesized based on the self-assembly technology reported by Maspoch et al.⁴ The significance of these capsules was in their magnitude and scope in dimensions, from about 100 nm to over 1 μm. These capsules are bigger than metal–organic polyhedra that commonly have sizes of a few nanometers, with a narrow ability for inclusion of small numbers of guest molecules.^7,8 Subsequently, in 2010, discus-like nanoscale coordination polymers comprising a π-conjugated dicarboxylate ligand and lanthanide ions were fabricated by Uvdal et al. which demonstrated guest inclusion properties. Interestingly, the loading quantity of guest molecules into these ICPs can be regulated by altering the synthesis temperatures.⁹ Next, Mao and coworkers prepared ICP nanoparticles from the b-nicotinamide adenine dinucleotide and terbium ions with encapsulation abilities.¹⁰ In 2014, our group revealed the synthesis of ICP nanocapsules assembled from metal nodes (Zn²⁺) and ditopic organic ligands (btb: 1,3-bis(tetrazol-5-ylmethyl)benzene) Zn(btb) with a spherical morphology. Accordingly, some core–shell particles could also be synthesized via calcination of the tetrazole-based ICP capsules.¹¹ Recently reported metal–organic capsules with the ability to confine inorganic particles were synthesized by Zhang et al. which were prepared via photodecomposition of ferrocenedicarboxylic acid in methanol.¹² A dominant challenge in the fabrication of functional metal–organic capsules is to change the chemical components and increase host–guest interactions without transforming the fundamental morphology. Expansion is a common aspect of metal–organic framework (MOF) fabrication as well as directional-bonding synthesis routes for supramolecular coordination compounds.¹³ Generally in this method, ligands with additional phenyl or ethynyl groups are employed that can increase the length between Lewis basic centers of a polydentate linker.¹³ Addition of phenyl or ethynyl spacers leads to enlargement of the dimensions of MOF cavities and the internal pores of supramolecular coordination compounds without much influence on the synthesis route. Applying expanded linkers with a specific secondary building unit (SBU) results in fabrication of a series of analogous MOFs which have the same network topology.¹³ The effect of adding a phenyl group on the size and other properties of metal–organic capsules has not been investigated yet.

We suppose that addition of a phenyl group would result in larger dimensions of the metal–organic spheres and that the prepared capsules could entrap larger amounts of the guest molecules.¹³ Moreover, an additional phenyl ring in the linker leads to an increase in the electron density of the structure and favorable π–π interactions. Hence, such capsules could have better interactions with guest molecules.¹⁴ Therefore, we aimed to design two novel metal–organic ICP capsules based on two new tetrazolate ligands of different lengths to investigate the expansion effects on these materials (Scheme 1). Moreover, other morphologies of these coordination polymers were synthesized under different conditions. We hope that this work can open up a new route for the synthesis of various tetrazole-based metal–organic microcapsules with novel extended linkers of the same type, while maintaining the original morphology.

Scheme 1 N-Donor-based building blocks used in this project.

Results and discussion

All ligand syntheses were carried out under an argon atmosphere by reacting the corresponding 2,2′-(1,4-phenylene)diacetonitrile and 2,2′-(biphenyl-4,4′-diyl)diacetonitrile with sodium azide and triethylamine hydrochloride in accordance with literature procedures (ESI†).¹⁵ To prepare btmbenze ligand, 2,2′-(1,4-phenylene)diacetonitrile, sodium azide and triethylamine hydrochloride were refluxed in dry toluene for 72 h under an inert argon atmosphere. The work-up was performed according to detailed information available in the ESI.† The prepared product was characterized by ¹H NMR and ¹³C NMR spectroscopy (Fig. S2 and S3 respectively†) and elemental analysis. All analyses confirmed the formation of the btmbenzene ligand. In an experiment, a methanolic solution of Zn(NO₃)₂·6H₂O was added to a DMF solution of tetrazolate ligand (btmbenzene, Scheme 1) under stirring and the homogenized mixture poured into a Teflon-lined stainless steel autoclave. Subsequently, it was heated at 120 °C for 12 h and then cooled to room temperature for 3 h, which led to preparation of a white powder denoted Zn(btmbenzene)-1. To purify the product, the mixture was washed with DMF several times and centrifuged. Field-emission scanning electron microscopy analysis of Zn(btmbenzene)-1 showed that the product consists of spheres with an average diameter of 5 μm that have smooth surfaces. Fig. 1a and b shows SEM images of bare Zn(btmbenzene)-1 microspheres. Fig. 1c shows the corresponding high resolution transition electron microscopy (HRTEM) image of Zn(btmbenzene)-1 which confirms that these microcapsules are hollow. FT-IR spectra of the compounds displayed two strong peaks at 1431 and 1657 cm⁻¹ which are attributed to deprotonation of the tetrazole rings and coordination of the ditopic ligands to Zn²⁺ cations (Fig. S7†). Thermogravimetric analysis (TGA) of the ICPs was performed in the range of 25–800 °C under a flow of nitrogen gas at a constant rate of 10 °C min⁻¹ to investigate the weight loss as a function of temperature and to evaluate the thermal stability.

Fig. 1 (a) Low-magnification SEM image of the Zn(btmbenzene)-1 spheres, (b) high-magnification SEM image and (c) high-resolution TEM image of an individual Zn(btmbenzene)-1 sphere.

This analysis demonstrated that a major loss of weight occurred between 350 °C and 420 °C (Fig. S11†). X-ray diffraction (XRD) analysis of the material was carried out to study its structure. According to the XRD analysis of the microcapsules, the structure is amorphous without any crystalline area (Fig. S9†). Elemental analysis data for the ICPs confirmed the formula [Zn(btmbenzene)]_n for Zn(btmbenzene)-1. Luminescence is a well-known feature of inorganic–organic coordination polymers which is due to the presence of aromatic or conjugated linkers or lanthanide ions in their structures.¹⁶ The photoluminescence characteristics of the ICPs were studied by photoluminescence spectroscopy. Excitation of the Zn(btmbenzene)-1 microcapsules at 350 nm led to emission with a peak maximum at 450 nm (Fig. 2b). Fluorescence microscopy images of Zn(btmbenzene)-1 showed an intense blue emission for these microcapsules (Fig. 2a). The inclusion capability of these self-assembled hollow microspheres was investigated by loading of Rhodamine B fluorescent dye. Rhodamine B is regularly utilized as a tracer dye, a hydrophilic model drug and widely in biotechnology applications.^17–19 Therefore, a certain amount of the dye was added to the reaction mixture to be entrapped inside the polymeric sphere through an in situ encapsulation process.

Fig. 2 (a) Fluorescence microscopy image of the Zn(btmbenzene)-1 microspheres deposited onto glass (λ_ex = 359–371 nm, λ_em > 440 nm) and (b) PL spectrum collected at λ_exc = 350 nm.

Fluorescence optical microscopy and fluorescence spectroscopy analyses were carried out to study the dye inclusion ability of these hollow microspheres. As can be seen in Fig. 3, red emission is associated with excitation at 540 nm which indicates the inclusion of dye molecules into Zn(btmbenzene)-1, denoted RhB@Zn(btmbenzene)-1. The quantity of RhB loaded into Zn(btmbenzene)-1 was also evaluated by a UV-vis spectrophotometer. RhB@Zn(btmbenzene)-1 was dispersed in deionized water at pH 5 and stirred for 72 h. Next, the mixture was centrifuged and the absorbency of the supernatant was determined at 554 nm utilizing a UV-vis spectrophotometer. According to the UV-vis analysis, the loading capacity of Rhodamine B into Zn(btmbenzene)-1 was 20 μmol g⁻¹ (Table S2 and Fig. S19†). Fig. S20† shows FT-IR spectra of Zn(btmbenzene)-1 and RhB@Zn(btmbenzene)-1, which are the same. Moreover, a FESEM image of RhB@Zn(btmbenzene)-1 confirms that the product maintains its spherical morphology after the in situ encapsulation process (Fig. S21†). Therefore, the guest molecules did not interfere with the overall synthesis of the host polymer during in situ encapsulation. Nowadays a common approach for smart drug delivery to tumor cells is applying microcapsules containing human therapeutic drugs.^20–22 5-Fluorouracil (5-FU) is an anticancer drug that is broadly applied in clinical chemotherapy for the treatment of different tumors such as breast adenocarcinoma and colon cancer.^23–25

Fig. 3 (a) Fluorescence optical microscopy image of RhB@Zn(btmbenzene)-1 (λ_exc = 540–552 nm, λ_em > 590 nm) and (b) PL spectrum collected at λ_exc = 350 nm.

To exhibit the diversity in ability of these hollow spheres for encapsulation of molecules, 5-FU was chosen as a guest. Following a similar experimental procedure, Zn(btmbenzene)-1 microspheres that could entrap 5-FU were synthesized (ESI†). 5-FU@Zn(btmbenzene)-1 was dispersed in deionized water at pH 5 and stirred for 72 h.

UV-vis analysis was performed at 260 nm on the supernatant solution to estimate the quantity of 5-FU loaded into the 5-FU@Zn(btmbenzene)-1 microspheres. The quantity of 5-FU encapsulated in the microspheres was found to be 158 μmol g⁻¹ (Table S1 and Fig. S18†). Moreover, the influence of different synthesis strategies and solvents on the morphology of this infinite coordination polymer was studied (Scheme 2). The conventional method reported for the preparation of metal–organic capsules is fast precipitation, referring to the addition of a poor solvent to the reaction mixture which leads to precipitation of the polymerized hollow metal–organic capsules.⁴ Therefore, a solution of Zn(NO₃)₂·6H₂O in methanol was added to the tetrazolate ligand btmbenzene in DMF under vigorous stirring. Fig. 4a displays a SEM image of the product obtained from the reaction denoted Zn(btmbenzene)-2 with an average size of 100 nm. A FT-IR spectrum of the sample showed two strong peaks at 1374 and 1667 cm⁻¹, similar to Zn(btmbenzene)-1, related to tetrazole coordination to Zn²⁺ ions (Fig. S7†).^26,27 XRD analysis showed an amorphous phase for Zn(btmbenzene)-2 (Fig. S9†). Thermogravimetric analysis of the sample showed a 20% weight loss related to DMF and the main weight loss region is between 320 and 420 °C (Fig. S12†). Fe₃O₄ nanoparticles were selected as a model to investigate the inclusion affinity of infinite coordination polymers for the insoluble guest. Fe₃O₄ nanoparticles were dispersed in a DMF solution of btmbenzene ligand and a methanolic solution of Zn(NO₃)₂·6H₂O was added to the mixture gradually (ESI†). Fig. 5 shows the corresponding FESEM and TEM images of the product. HRTEM images obviously show Fe₃O₄ nanoparticles entrapped inside the polymeric spheres. The magnetic properties of the obtained Fe₃O₄@Zn(btmbenzene)-2 capsules were examined utilizing a vibrating sample magnetometer. According to the magnetization curve of the magnetic polymeric nanocapsules, the saturation magnetization for Fe₃O₄@Zn(btmbenzene)-2 is 10 emu g⁻¹ (Fig. 6).

Scheme 2 Synthesis of btmbenzene-based micro- and nano-structures: (a) under MeOH/DMF/solvothermal conditions, (b) using a MeOH/DMF/fast precipitation method, (c) using H₂O/DMF/reflux and (d) under H₂O/DMF/solvothermal conditions.

Fig. 4 SEM images of (a) Zn(btmbenzene)-2 (scale bar = 1 μm), (b) Zn(btmbenzene)-3 (scale bar = 1 μm), (c) Zn(btmbenzene)-4 (scale bar = 30 μm) and (d) Zn(btmbenzene)-2 after transformation to cubic structures under H₂O/DMF/solvothermal conditions (scale bar = 5 μm).

Fig. 5 (a and b) SEM images of Fe₃O₄@Zn(btmbenzene)-2 (the scale bar in a and b = 2 and 1 μm, respectively) and (c and d) HRTEM images of Fe₃O₄@Zn(btmbenzene)-2 spheres (the scale bar in c and d = 200 and 100 nm, respectively).

Fig. 6 Magnetic hysteresis loop of Fe₃O₄@Zn(btmbenzene)-2 measured at room temperature.

In practice, an aqueous solution of zinc nitrate was added to a btmbenzene ligand solution in DMF and refluxed for 72 h. Next, the product was separated by centrifugation and characterized by FESEM. The FESEM image of this compound, denoted Zn(btmbenzene)-3, shows that the morphology of this coordination polymer is rod-like in structure (Fig. 4b). XRD analysis of the Zn(btmbenzene)-3 rods demonstrated some sharp peaks, indicating a semi-crystalline nature (Fig. S9†). Moreover, flower-like structures, denoted Zn(btmbenzene)-4, could be synthesized by reaction of a aqueous solution of zinc nitrate and btmbenzene ligand in DMF at 140 °C in a Teflon-lined stainless steel autoclave (Fig. 4c). The structure of the material was studied by XRD analysis, and exhibited a crystalline character (Fig. S9†). Elemental analyses were performed for these four morphologies, which all displayed the same percentages of carbon, nitrogen and hydrogen atoms and confirmed the formula of [Zn(btmbenzene)]_n for all the infinite coordination polymers. As can be seen in Fig. S7,† FT-IR spectra of these three morphologies resemble each other with two distinctive strong peaks at about 1400 and 1600 cm⁻¹, indicating the coordination of tetrazole groups to Zn²⁺ ions. This evidence confirm that all five morphologies of [Zn(btmbenzene)]_n have the same constituents. In addition, transformation of these structures upon changes to their exterior environments was studied. Zn(btmbenzene)-2 nanocapsules were separated by centrifugation from the reaction mixture, washed with DMF and dried, then put into a Teflon-lined stainless steel autoclave containing DMF and methanol and heated at 120 °C for 24 h. Remarkably, an important morphological transformation occurred and small nanocapsules of Zn(btmbenzene)-2 turned to spherical microcapsules of Zn(btmbenzene)-1 (Fig. S15†). In an experiment, nanocapsules of Zn(btmbenzene)-2 were heated at 140 °C for 72 h in a Teflon-lined stainless steel autoclave containing DMF and H₂O. FESEM images of the product showed that these nanocapsules changed to cubic-like structures (Fig. 4d and S17†).

Zinc oxide nanostructures have been considered as significant key materials due to their potential in a wide range of applications such as photocatalysis, gas sensing, as antimicrobial materials, and in electronic and optoelectronic devices. Various micro- and nanostructures of zinc oxide have been synthesized as exemplified by rods, belts, combs, saws, spirals, rings and hollow spheres.^28–35 Hollow zinc oxide spheres have been considered as an applicable desired morphology of this material due to their special advantages such as good surface area and low density.^36–43 We decided to use these hollow metal–organic microspheres to prepare hollow zinc oxide microspheres through calcination. Therefore, hollow Zn(btmbenzene)-1 spheres were calcined at 450 and 550 °C for 3 h in air to find the best temperature to prepare ZnO while retaining the spherical morphology. FESEM images of the products show that the spheres keep their spherical morphology after calcination at 450 °C but that the morphology changed under calcination at 550 °C (Fig. 11). The sample produced at 450 °C was characterized by XRD analysis. The obtained XRD pattern can be indexed to the wurtzite structure of zinc oxide (Fig. S14†). It is noteworthy that no other diffraction peaks were observed, confirming that the spherical ICPs entirely changed into the ZnO phase. The diversity and development of supramolecular coordination compounds are obviously indebted to the use of larger extended decorated bridging ligands.¹³ We chose a longer linker of the same type (btmbiphenyl) to synthesize an extended network with the morphology of the spherical capsule (Scheme 1). To prepare the btmbiphenyl ligand, 4,4′-bis(bromomethyl)biphenyl was used as a precursor. The alkyl halide underwent nucleophilic aliphatic substitution with sodium cyanide in distilled H₂O under reflux conditions for 24 h. The obtained dinitrile product was transformed into the tetrazolate ligand through reaction with sodium azide and triethylamine hydrochloride under reflux in dry toluene for 72 h and an inert argon atmosphere.

The work-up procedure can be found in the ESI†. Elemental analysis, ¹H NMR (Fig. S5†) and ¹³C NMR (Fig. S6†) spectroscopy confirmed the synthesis of the btmbiphenyl ligand. Following a similar procedure, Zn(btmbiphenyl)-1 microcapsules were prepared by solvothermal reaction of the btmbiphenyl ligand in DMF and a methanolic solution of Zn(NO₃)₂·6H₂O salt at 120 °C. The morphology of the as-synthesized product was studied by FESEM analysis. In accordance with the FESEM images (Fig. 7), the particles can be seen to have a spherical morphology with an average size of 20 μm. As anticipated, by employing the extended linker, the size of the prepared spheres increased obviously. Moreover, unlike the Zn(btmbenzene)-1 microcapsules, the surface of these expanded ICPs is completely unsmooth. The structure of the Zn(btmbiphenyl)-1 compound was analyzed by HRTEM as well. Fig. 7 shows the TEM images of the Zn(btmbiphenyl)-1, which validate the formation of hollow microcapsules. The infrared spectrum of the sample resembles that of Zn(btmbenzene)-1, displaying peaks at 1403 and 1604 cm⁻¹ which prove the polymerization of Zn²⁺ ions and tetrazole groups (Fig. S8†). A thermogravimetric study of Zn(btmbiphenyl)-1 showed that the major weight decline domain was between 310 °C and 430 °C, relating to the destruction of the ICPs (Fig. S13†). A powder X-ray diffraction experiment was carried out to investigate the crystallinity of the sample. Surprisingly, the related XRD pattern displayed sharp peaks indicating the crystalline structure of these capsules (Fig. S10†). The elemental composition of these hollow capsules was examined by elemental analysis. According to the elemental analysis, a formula of [Zn(btmbiphenyl)]_n was suggested for Zn(btmbiphenyl)-1. The emission properties of Zn(btmbiphenyl)-1 were studied using photoluminescence spectroscopy. When the microcapsules were excited at 420 nm, the obtained emission spectrum exhibited a peak with λ_max at 660 nm (Fig. 8). The PL spectrum of Zn(btmbiphenyl)-1 showed an obvious red-shift in comparison with the Zn(btmbenzene)-1. Increasing the π-conjugation in the ligand led to a decrease in the HOMO–LUMO gap which meant that ligand to metal charge transfer became ineffective and consequently ligand-based emission occurred and a red-shift was observed in the PL spectrum. To investigate the potential of Zn(btmbiphenyl)-1 for the loading of 5-FU in a similar procedure, it was encapsulated into the microcapsules (ESI†). The amount of 5-FU loaded into the microcapsules was determined by utilizing a UV-vis spectrophotometer (Table S1 and Fig. S18†). The outcomes revealed that the 5-FU loading content was 188 μmol g⁻¹ which, interestingly, is more than Zn(btmbenzene)-1. Moreover, encapsulation of Rhodamine B as a guest was also performed and analyzed through an identical method (Table S2 and Fig. S19†). According to the UV-vis spectrophotometry analysis, the Rhodamine B loading capacity of RhB@Zn(btmbiphenyl)-1 was 25 μmol g⁻¹. Fluorescence microscopy images of RhB@Zn(btmbiphenyl)-1 showed intense green emission related to the encapsulated fluorescent dye (Fig. 9). Fig. S22† displays FT-IR spectra of Zn(btmbiphenyl)-1 and RhB@Zn(btmbiphenyl)-1 which have the same pattern. In addition, an FESEM image of RhB@Zn(btmbiphenyl)-1 proves that the sample keeps its spherical morphology after the in situ encapsulation process (Fig. S23†). Therefore, our expectations were fulfilled and the Zn(btmbiphenyl)-1 microcapsules showed a higher loading capacity for both 5-FU and Rhodamine B in comparison to Zn(btmbenzene)-1, which can be attributed to stronger host–guest interactions due to the additional aromatic ring of the btmbiphenyl organic linker (Scheme 1) and the larger dimensions of the capsules. Afterward we tried to perform encapsulation of Fe₃O₄ magnetic nanoparticles inside the polymeric spheres through an in situ process by a solvothermal method, but FESEM analysis of the samples did not show any special structure.

Fig. 7 (a) A low-magnification SEM image and (b) a high-magnification SEM image of the Zn(btmbiphenyl)-1 hollow spheres. (c and d) High-resolution TEM images of an individual Zn(btmbiphenyl)-1 sphere.

Fig. 8 PL spectrum of the Zn(btmbiphenyl)-1 hollow spheres collected at λ_exc = 420 nm.

Fig. 9 Fluorescence optical microscopy images of RhB@Zn(btmbiphenyl)-1 (a and b) λ_exc = 359–371 nm and λ_em > 397 nm.

Furthermore, the influence of the operating conditions and solvents on the morphology of these ICPs was also inspected. To study the effects on the morphology of the conventional method, a methanolic solution of Zn(NO₃)₂·6H₂O was added to the btmbiphenyl ligand in DMF dropwise at RT under stirring. The separated product was denoted Zn(btmbiphenyl)-2 and analyzed by FESEM. A FESEM image of the Zn(btmbiphenyl)-2 demonstrated that flower-like structures were synthesized (Fig. 10a). XRD analysis of the sample showed some sharp peaks indicating the semi-crystallinity of the compound (Fig. S10†). In next step, an aqueous solution of Zn(NO₃)₂·6H₂O was reacted with a btmbiphenyl solution in DMF under solvothermal conditions at 140 °C. The morphology of the product denoted Zn(btmbiphenyl)-3 was studied by FESEM analysis. As can be seen in Fig. 10b and Scheme 3c flower-like structures were obtained that have much thicker branches in comparison to Zn(btmbiphenyl)-2. The powder was characterized by XRD analysis and it indicated that the compound is crystalline (Fig. S10†). FT-IR spectra of Zn(btmbiphenyl)-n (n = 2, 3) appeared similar to the Zn(btmbiphenyl)-1 infrared spectrum (Fig. S8†). According to the elemental analyses of Zn(btmbiphenyl)-n (n = 2, 3), these morphologies have the same formula expressed as [Zn(btmbiphenyl)]_n, indicating that these three morphologies have similar compositions. Morphological transformation of Zn(btmbiphenyl)-2 under solvothermal conditions was also examined. Thereupon, Zn(btmbiphenyl)-2 powder was heated at 120 °C under solvothermal conditions in DMF and methanol for 12 h (ESI†). Surprisingly, the flower-like constructions changed into spherical structures (Fig. S16†). Subsequently, the Zn(btmbiphenyl)-1 microcapsules were used to synthesize zinc oxide hollow spheres via calcination. Hence, the Zn(btmbiphenyl)-1 microcapsules were heated at 450 and 550 °C for 3 h in air. According to the FESEM images of the samples shown in Fig. 11, both of the resultant products preserved their spherical morphology. Pursuant to the perforated sphere shown in Fig. 11, the emptiness of these structures even after calcination was also proved. The white powder prepared at 450 °C was examined by XRD analysis. The XRD pattern indicated that ZnO with a wurtzite structure and a hexagonal unit cell was prepared (Fig. S14†). Moreover, the determined lattice constants were a = 3.2 and c = 5.2 Å. It is worth noting that the morphology of the Zn(btmbenzene)-1 microcapsules and the previously reported Zn(btb)¹¹ changed after calcination at 550 and 450 °C respectively, unlike the Zn(btmbiphenyl)-1 microcapsules which could keep the desired hollow spherical morphology under both conditions. Therefore, using the Zn(btmbiphenyl)-1 microspheres also seems a better choice for obtaining hollow zinc oxide structures, achieving greater thermal stability for their morphologies.

Fig. 10 SEM images of (a) Zn(btmbiphenyl)-2 (scale bar = 2 μm) and (b) Zn(btmbiphenyl)-3 (scale bar = 1 μm).

Scheme 3 Synthesis of btmbiphenyl-based nano- and micro-structures: (a) under MeOH/DMF/solvothermal conditions, (b) using the MeOH/DMF/fast precipitation method and (c) under H₂O/DMF/solvothermal conditions.

Fig. 11 FESEM images of ZnO architectures obtained from calcination of the prepared ICPs after calcination of (a) Zn(btmbenzene)-1 (scale bar = 1 μm), (b) Zn(btmbiphenyl)-1 (scale bar = 20 μm) in air at 550 °C for 3 h, (c) Zn(btmbenzene)-1 (scale bar = 1.5 μm) and (d) Zn(btmbiphenyl)-1 (scale bar = 5 μm), in air at 450 °C for 3 h.

Conclusions

In brief, we report the successful synthesis of two novel self-assembled hollow spherical metal–organic microcapsules based on two new extended tetrazolate ligands of the same type and Zn²⁺ ions by a solvothermal method. Moreover, by changing the reaction conditions and solvents, different morphologies of these ICPs were prepared, which included flower-like structures, cubes and rods. In addition, entrapment of functional materials, such as an anticancer drug, a fluorescent dye and inorganic nanoparticles within these coordination networks was examined through an in situ process as a potential application. The loading capacities of these carriers for the examined anticancer drug and fluorescent dye were determined and compared. Furthermore, these spherical polymeric microcapsules were applied as precursors for the fabrication via calcination of hollow crystalline ZnO spheres, which are considered a preferred morphology of zinc oxide due to their potential applications as drug delivery carriers and in catalysis.

Acknowledgements

We appreciatively thank Tarbiat Modares University for support of this investigation.

References

1. (a) M. Oh and C. A. Mirkin, Nature, 2005, 438, 651; (b) M. Oh and C. A. Mirkin, Angew. Chem., Int. Ed., 2006, 45, 5492.
2. Y.-M. Jeon, J. Heo and C. A. Mirkin, J. Am. Chem. Soc., 2007, 129, 7480.
3. (a) H. Maeda, M. Hasegawa, T. Hashimoto, T. Kakimoto, S. Nishio and T. Nakanishi, J. Am. Chem. Soc., 2006, 128, 10024; (b) X. Sun, S. Dong and E. Wang, J. Am. Chem. Soc., 2005, 127, 13102; (c) H. Wei, B. Li, Y. Du, S. Dong and E. Wang, Chem. Mater., 2007, 19, 2987; (d) S. Jung and M. Oh, Angew. Chem., Int. Ed., 2008, 47, 2049; (e) K. Wang, X. Ma, D. Shao, Z. Geng, Z. Zhang and Z. Wang, Cryst. Growth Des., 2012, 12, 3786; (f) L. Zhang, X. Gao, L. Yang, P. Yu and L. Mao, ACS Appl. Mater. Interfaces, 2013, 5, 8120; (g) S. Zhong, H. Jing, Y. Li, S. Yin, C. Zeng and L. Wang, Inorg. Chem., 2014, 53, 8278.
4. I. Imaz, J. Hernando, D. Ruiz-Molina and D. Maspoch, Angew. Chem., Int. Ed., 2009, 48, 2325.
5. R. Nishiyabu, N. Hashimoto, T. Cho, K. Watanabe, T. Yasunaga, A. Endo, K. Kaneko, T. Niidome, M. Murata, C. Adachi, Y. Katayama, M. Hashizume and N. Kimizuka, J. Am. Chem. Soc., 2009, 131, 2151.
6. V. Gonzalez, D. A. L. Vignati, C. Leyval and L. Giamberini, Environ. Int., 2014, 71, 148.
7. N. R. Champness, Angew. Chem., Int. Ed., 2009, 48, 2274.
8. (a) K. Suzuki, J. Lida, S. Sato, M. Kawano and M. Fujita, Angew. Chem., 2008, 120, 5864 (Angew. Chem., Int. Ed., 2008, 47, 5780); (b) S. M. Biros, R. M. Yeh and K. N. Raymond, Angew. Chem., 2008, 120, 6151 (Angew. Chem., Int. Ed., 2008, 47, 6062); (c) D. J. Tranchemontagne, Z. Ni, M. O'Keeffe and O. M. Yaghi, Angew. Chem., 2008, 120, 5214 (Angew. Chem., Int. Ed., 2008, 47, 5136).
9. X. Zhang, M. A. Ballem, M. Ahrén, A. Suska, P. Bergman and K. Uvdal, J. Am. Chem. Soc., 2010, 132, 10391.
10. P. Huang, J. Mao, L. Yang, P. Yu and L. Mao, Chem.–Eur. J., 2011, 17, 11390.
11. Z. Sharifzadeh, S. Abedi and A. Morsali, J. Mater. Chem. A, 2014, 2, 4803.
12. B. Zhang, B. Liu, G. Chen and D. Tang, Biosens. Bioelectron., 2015, 64, 6.
13. (a) M. Eddaoudi, et al., Science, 2002, 295, 469O. M. Yaghi, M. O'Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Nature, 2003, 423, 705; (b) H. Furukawa, Y. B. Go, N. Ko, Y. K. Park, F. J. Uribe-Romo, J. Kim, M. O'Keeffe and O. M. Yaghi, Inorg. Chem., 2011, 50, 9147; (c) H. C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673; (d) A. Schneemann, V. Bon, I. Schwedler, I. Senkovska, S. Kaskel and R. A. Fischer, Chem. Soc. Rev., 2014, 43, 6062.
14. G. Ferey, Chem. Soc. Rev., 2008, 37, 191.
15. K. Sumida, M. L. Foo, S. Horike and J. R. Long, Eur. J. Inorg. Chem., 2010, 24, 3739.
16. Y. Cui, Y. Yue, G. Qian and B. Chen, Chem. Rev., 2012, 112, 1126.
17. G. Reina, S. Orlanducci, C. Cairone, E. Tamburri, S. Lenti, I. Cianchetta, M. Rossi and M. L. Terranova, J. Nanosci. Nanotechnol., 2015, 15, 1022.
18. K. Gulati, S. Maher, S. Chandrasekaran, D. M. Findlay and D. Losic, J. Mater. Chem. B, 2016, 4, 371.
19. D. Zheng, J. Li, C. Li, Z. Xu, S. X. Cheng and X. Z. Zhang, J. Mater. Chem. B, 2015, 3, 3483.
20. A. P. Esser-Kahn, N. R. Sottos, S. R. White and J. S. Moore, J. Am. Chem. Soc., 2010, 132, 10266.
21. M. Ambrosi, E. Fratini, P. Baglioni, C. Vannucci, A. Bartolini, A. Pintens and J. Smets, J. Phys. Chem. C, 2016, 120, 13514.
22. W. Cui, J. Li and G. Decher, Adv. Mater., 2016, 28, 1302.
23. H. Zhao, H. Feng, D. Liu, J. Liu, N. Ji, F. Chen, X. Luo, Y. Zhou, H. Dan, X. Zeng, J. Li, C. Sun, J. Meng, X. Ju, M. Zhou, H. Yang, L. Li, X. Liang, L. Chu, L. Jiang, Y. He and Q. Chen, ACS Nano, 2015, 9, 9638.
24. S. Yu, X. Gao, H. Baigude, X. Hai, R. Zhang, X. Gao, B. Shen, Z. Li, Z. Tan and H. Su, ACS Appl. Mater. Interfaces, 2015, 7, 5089.
25. E. A. K. Nivethaa, S. Dhanavel, V. Narayanan, C. A. Vasu and A. Stephen, RSC Adv., 2015, 5, 1024.
26. M. Dinca, A. F. Yu and J. R. Long, J. Am. Chem. Soc., 2006, 128, 17153.
27. S. Jeong, X. Song, S. Jeong, M. Oh, X. Liu, D. Kim, D. Moon and M. S. Lah, Inorg. Chem., 2011, 50, 12133.
28. T. Guo, M. S. Yao, Y. H. Lin and C. W. Nan, CrystEngComm, 2015, 17, 3551.
29. J. Wang, X. Li, Y. Xia, S. Komarneni, H. Chen, J. Xu, L. Xiang and D. Xie, ACS Appl. Mater. Interfaces, 2016, 8, 8600.
30. S. Park, S. An, H. Ko, C. Jin and C. Lee, ACS Appl. Mater. Interfaces, 2012, 4, 3650.
31. R. Kumar, S. Anandan, K. Hembram and T. N. Rao, ACS Appl. Mater. Interfaces, 2014, 6, 13138.
32. C. J. Tainter and G. C. Schatz, J. Phys. Chem. C, 2016, 120, 2950.
33. S. G. Kumar and K. S. R. K. Rao, RSC Adv., 2015, 5, 3306.
34. L. Wang and M. Muhammed, J. Mater. Chem., 1999, 9, 2871.
35. L.-Y. Yang, S.-Y. Dong, J.-H. Sun, J.-L. Feng, Q.-H. Wu and S.-P. Sun, J. Hazard. Mater., 2010, 179, 438.
36. J. Yu and X. Yu, Environ. Sci. Technol., 2008, 42, 4902.
37. G. Patrinoiu, M. Tudose, J. M. Calderon-Moreno, R. Birjega, P. Budrugeac, R. Ene and O. Carp, J. Solid State Chem., 2012, 186, 17.
38. K. An and T. Hyeon, Nano Today, 2009, 4, 359.
39. (a) D. M. Vriezema, M. C. Aragonès, J. A. A. W. Elemans, J. J. L. M. Comelissen, A. E. Rowan and R. J. M. Nolte, Chem. Rev., 2005, 105, 1445; (b) Y. Zhao and L. Jiang, Adv. Mater., 2009, 21, 3621.
40. (a) F. Caruso, R. A. Caruso and H. M. Ohwald, Science, 1998, 282, 1111; (b) B. D. G. Geest, N. N. Sanders, G. B. Sukhorukov, J. Demeester and S. C. D. Smedt, Chem. Soc. Rev., 2007, 36, 636.
41. F. Caruso, Chem.–Eur. J., 2000, 6, 413.
42. X. W. Lou, L. A. Archer and Z. C. Yang, Adv. Mater., 2008, 20, 3987.
43. L. L. Wang, Z. Lou, T. Fei and T. Zhang, J. Mater. Chem., 2012, 22, 4767.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23075j

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