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Interface Coassembly and Polymerization on Magnetic Colloids: Toward Core–Shell Functional Mesoporous Polymer Microspheres and Their Carbon Derivatives

Abstract Core–shell structured magnetic mesoporous polymer or carbon‐based microspheres not only possess the combined merits of magnetic particles and stable mesoporous shell but also provide various organic functional groups for further modification and immobilization of active sites, thus opening up more possibility for various applications. Herein, a bottom‐up soft‐templating strategy is developed to controllably synthesize core–shell magnetic mesoporous polydopamine microspheres (MMP) and their derivative magnetic mesoporous carbon (MMC) microspheres via an amphiphilic block copolymer‐directed interface assembly and polymerization (denoted as abc‐DIAP) approach. The obtained uniform MMP microspheres have a well‐defined structure consisting of magnetic core, silica middle layer and mesoporous PDA shell, uniform mesopores of 11.9 nm, high specific surface areas (235.6 m2 g−1) and rich functional groups. They show fast magnetic separation speed and superior performance in selective adsorption of Cyt.C from complex biosample solutions. Moreover, they can be in situ converted into core–shell magnetic mesoporous carbon (MMC) for efficient in‐pore immobilization of ultrafine Au nanoparticles for high‐efficiency catalytic epoxidation of styrene with high conversion (88.6%) and selectivity (90.1%) toward styrene oxide.

wt%) were purchased from Shanghai Chemical Company. Deionized water was used for all experiments. All chemicals were used as received without any further purification.

Synthesis of Fe3O4@nSiO2 Microspheres
Fe3O4 particles with a mean diameter of ~100 nm were synthesized as we reported previously. [1] An aqueous dispersion of Fe3O4 particles (2 mL, 40 g/mL) was added to a three-neck round bottom flask with ethanol (105 mL), H2O (35 mL) and concentrated ammonia solution (2.0 mL) under the mechanical stirring (200 rpm) at 30 o C. After stirring for 30 min, 3.0 ml of TEOS (2.79 g) was added dropwise and the reaction was allowed to proceed for 8 h with continuous mechanical stirring. After that, a magnet was used to separate and collect the core-shell Fe3O4@nSiO2 microspheres, followed by washing three times with ethanol and water, respectively.

Synthesis of Core-Shell Magnetic Mesoporous Polydopamine Microspheres with vertically aligned cylindrical mesopores (MMP-V)
In a typical synthesis, 100 mg of F127 and 0.1 mL TMB were gradually dissolved in ethanol (4.7 mL) and deionized water (5 mL) by ultrasonication. The resultant transparent solution gradually turned into pale blue due to the micellization of F127 block copolymers [2] . Then, 0.3 ml of ethanol solution containing 20 mg of Fe3O4@nSiO2 microspheres and 120 mg of DA were added with stirring (280 rpm) at 25 ºC. After continuous stirring for 1h, 0.06 mL NH3·H2O (28 wt%) was dropwise added, and the reaction was allowed to proceed for 2 h under continuous mechanical stirring. The as-made samples (Fe3O4@nSiO2@PDA/F127 microspheres) were collected by magnetic separation, followed by washing with deionized water and ethanol three times.

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After collected, the as-made samples were redispersed in 80 mL acetone of and refluxed at 80 °C overnight to remove F127 templates. The extraction was repeated 6 times and then thoroughly washed with ethanol. After vacuum drying, core-shell magnetic mesoporous polydopamine microspheres (Fe3O4@nSiO2@mPDA) with vertically aligned cylindrical mesopores were obtained.
For the synthesis of core-shell magnetic mesoporous polydopamine nanochains with vertically aligned cylindrical mesopores, an applied magnetic field was introduced into the interface coating of silica as we reported previously. [3] Then, a layer of PDA/F127 was coassembled and deposited onto the Fe3O4@nSiO2 nanochain with the same method as prepared according to previously reported procedures. [4] The synthesis procedure is similar to that for MMP-V spheres, except that the reaction solution was ethanol/H2O/THF (The volume ratio was 1:2:1) instead of ethanol/H2O (The volume ratio was 1:1), and meanwhile the amount of template was 20 mg of PEO108-b-PS210 instead.
Firstly, standard curves were fitted according to the characteristic UV-vis absorption peak intensity of various concentrations (5-100 mg/L) of protein buffer solution at a wavelength of 409 nm and 285 nm for Cyt.C and at for BSA, respectively (shown as below).

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The characteristic UV-vis spectrum (a, c) and the corresponding standard curves (b, d) of Cyt.C (a, b) and BSA (c, d) in a buffer (pH 9.5).
The saturated adsorption capacity of Cyt.c by MMP-V was studied through monitoring the adsorption capacity in various initial concentrations (5-150 mg/L) of Cyt.C solution in NaOH/NaHCO 3 buffer (pH 9.5). 0.5 mg of MMP-V was added into the Cyt.C/NaOH/ NaHCO 3 buffer (pH 9.5) solution and the mixture was then incubated in a shaker at room temperature for above 6 h. Then MMP-V was separated from the mixture using an external magnet and washed three times with buffer. UV-vis spectroscopy measurement was employed to determine the amount of retained Cyt.C in the magnetic supernatant and washing buffer by monitoring the characteristic peak intensity of 409 nm. The amount of adsorbed proteins was calculated by subtracting the free proteins in the magnetic supernatant and washing buffer after a specific time from the initial amount of proteins.
For size-selective protein adsorption measurement, typically, 0.5 mg of MMP-V was transferred to the protein NaOH/NaHCO 3 buffer (pH 9.5) solution containing 5 mL of Cyt.C or 5 mL of BSA with the same concentration (100 mg/L). The mixture was then incubated in a shaker at room temperature for above 6 h. Then MMP-V was separated from the mixture using an external magnet and washed three times with buffer. UV-vis spectroscopy measurement was employed to determine the retained protein of the magnetic supernatant and washing buffer by monitoring the characteristic peak intensity of 409 nm and 285 nm for Cyt.C and at for BSA, respectively. The amount of adsorbed proteins was calculated by subtracting the free proteins in the magnetic supernatant and washing buffer after a specific time from the initial amount of proteins. Besides the alkaline buffer solution, the sizeselective protein adsorption was also measured in NaH 2 PO 4 /HCl buffer solution (pH 3.0) with the same method.

Synthesis of Au-loaded MMC-V microspheres (Au@MMC-V)
Au nanoparticles were loaded into MMC-V microspheres via an in situ reduction method. In a typical synthesis, 20 mg of MMC-V microspheres were dispersed in 10 mL ethanol solution by ultrasonication treatment. Then, 0.8 mL of HAuCl4 (0.2 mg/mL) was added, followed by 0.2 mL of ice-cold, freshly prepared 0.1M of NaBH4 solution. The products were centrifuged after reaction for 2 h, washed with deionized water and ethanol three times, respectively.
After drying in a vacuum at 60 ºC, the obtained sample was denoted as Au@MMC-V microspheres.
Catalytic epoxidation of styrene 20 mg of catalyst (Au@MMC-V) was added to a mixture of styrene (2.6 mL, 20 mmol), and acetonitrile (10 mL). The dispersion was bubbled with high purity Ar for 60 min at room temperature. After adding t-butyl hydroperoxide (10.0 g, 76 mmol, 70 wt% in water), the reaction vessel was immersed in an oil bath and heated at 80 ºC. During the reaction process, a minor amount of reaction solution (20 mL) was withdrawn at different time intervals for gas chromatography-mass spectrometer (GC-MS) measurements. After reaction for 40 h, the reaction system was cooled down, and the catalyst was recycled using a magnet, washed 3 times with acetonitrile, and vacuum dried at 40 ºC for reuse.        runs.