Yolk–Shell‐Structured Aluminum Phenylphosphonate Microspheres with Anionic Core and Cationic Shell

Spherical materials with yolk‐shell structure have great potential for a wide range of applications. The main advantage of the yolk‐shell geometry is the possibility of introducing different chemical or physical properties within a single particle. Here, a one‐step hydrothermal synthesis route for fabricating amphoteric yolk‐shell structured aluminum phenylphosphonate microspheres using urea as the precipitant is proposed. The resulting microspheres display 3D sphere‐in‐sphere architecture with anionic core and cationic shell. The controllable synthesis of aluminum phosphates with various morphologies is also demonstrated. The anionic core and cationic shell of the aluminum phenylphosphonate microspheres provide docking sites for selective adsorption of both cationic methylene blue and anionic binuclear cobalt phthalocyanine ammonium sulphonate. These new adsorbents can be used for simultaneous capture of both cations and anions from a solution, which make them very attractive for various applications such as environmental remediation of contaminated water.

Adsorption of salmon sperm DNA. ys-AlPhPO was dried at 80 °C for 24 h in a vacuum oven prior to adsorption of salmon sperm DNA. Adsorption isotherms were obtained by preparing a series of salmon sperm DNA solutions with concentration ranging from 2 to 440 gmL -1 in pure water. In each adsorption experiment, 5 mg of the ys-AlPhPO sample was added to 10 mL of salmon sperm DNA solutions with different concentrations, and the resulting mixture was continuously shaken in a shaking bath at room temperature for 24 h. The supernatant was separated from the solid material by repetitive centrifugation and the salmon sperm DNA content in the supernatant was measured using UV absorption at 260 nm.
The kinetics of salmon sperm DNA adsorption was investigated by suspending 5 mg of the sample in salmon sperm DNA solution (200 gmL -1 , 10 mL). The mixture was shaken in a shaking water bath at room temperature for different intervals and measured periodically. For each measurement, the upper solution was separated from the solid material by repetitive centrifugation and the salmon sperm DNA content of upper limpid solution was measured using UV absorption at 260 nm.
The amount of salmon sperm DNA adsorbed on ys-AlPhPO was calculated as the difference in the concentration of salmon sperm DNA before and after adsorption according to the following equation: M = [C i V-C e V]/m, M (gmg -1 ) is the amount of salmon sperm DNA adsorbed per microgram of the material studied; C i (gmL -1 ) is the initial concentration of salmon sperm DNA; C e (gmL -1 ) is the equilibrium concentration of salmon sperm DNA in the upper solution after adsorption; V (mL) is the volume of salmon sperm DNA solution; m (mg) is the weight of the adsorbent material.
Adsorption models. Two adsorption models, Langmuir and Freundlich, were applied to select the most appropriate adsorption isotherm for the system studied. The Langmuir model is applicable for an ideal monolayer adsorption on a homogeneous surface. Namely, adsorption takes place on energetically homogeneous sites present on the adsorbent surface; all adsorption sites are identical and energetically equivalent; each site can accommodate only one molecule or atom; and the lateral interactions between adsorbed molecules are neglected. The adsorption capacity can be expressed by the following equation, where q e is the solid-phase concentration of adsorbate at the equilibrium (gmg -1 ), C e is the equilibrium adsorbate concentration in the aqueous phase (gmL -1 ), K L (mLmg -1 ) is Langmuir constant and q m (gmg -1 ) is the monolayer adsorption capacity.
A linear expression of the Langmuir equation can be represented by The Freundlich model can be applied for adsorption on heterogeneous surfaces and is expressed by the following equation: where q e is the solid-phase concentration of adsorbate at the equilibrium (gmg -1 ), C e is the equilibrium adsorbate concentration in the liquid phase (gmL -1 ), K F is the Freundlich parameter [mL 1/n g 1-1/n mg -1 ], and 1/n is the heterogeneity factor. A linear form of the Freundlich expression can be obtained by taking Adsorption kinetics models. The most common kinetics models, pseudo-first-order and pseudo-secondorder reaction rate equations, were used to investigate the adsorption kinetics experimental data. The pseudo-first-order equation is given as follows: where q t is the amount adsorbed at a given time (g/mg); q e is the corresponding equilibrium adsorption (g/mg); k 1 is the rate constant (1/min) and t is the time (min). The plot of ln(q e  q t ) against t should give a straight line with slopek 1 and intercept lnq e .
The pseudo-second-order equation is expressed as follows: where q t is the amount adsorbed at a given time (g/mg); q e is the is the corresponding equilibrium adsorption (g/mg); k 2 is the rate constant (mgg -1 min -1 ) and t is the time (min). The k 2 and the equilibrium adsorption (q e ) can be obtained experimentally from the slope and intercept of the plot of t/q t versus t.
I. Langmuir, J. Am. Chem. Soc. 1918, 40, 1362. H. M. F. Freundilich, Z. Phys. Chem. 1906 Enrichment Tests. Enrichment tests towards peptide and protein digests were performed using low concentrated solutions prepared by a step-wise dilution method. The microspheres were dispersed in water at a concentration of 10 mgmL -1 and 10 L of the slurry was directly added to the prepared solutions. Then the supernatant was removed by centrifugation after 10 min enrichment and the microspheres were collected. For matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS), the microspheres were incubated with 1 L of matrix solution (10 (2) (2) (3) (4) (6) (5) e m L m e e 1 1 C q K q q C   mg/mL CHCA in TFA/ACN/water, 0.1%/49.9%/50%, v/v/v) and analyzed on the plain steel MALDI plate. The MS spectra were collected accumulating 500 laser shots at 10 different spots on Bruker Autoflex II Smartbeam system and no smooth spectra were used.  Table S3. Adsorption kinetics constants for salmon sperm DNA on ys-AlPhPO.
Pseudo-first-order model Pseudo-second-order model  The resonance signal at 128 ppm can be assigned to the quaternary carbon atom, in the ipso position, which is linked with phosphonate group. The resonance peaks at 130.9 ppm and 135.0 ppm can be attributed to carbon atoms in the meta and para positions, respectively. In comparison to the 13 C CP/MAS NMR spectrum of phenylphosphonic acid reported in the previous reference, in which there are four resonance signals located at 128.0, 129.7, 132.0, and 135.0 ppm corresponding to four different carbon atoms (ipso, ortho, meta, and para positions) in the benzene ring, Fig. S3 is consistent with the coordination of aluminum with phenylphosphonate in the sample resulting in the increase of steric hindrance of the ortho carbon atoms and the partial loss of conjugation between phosphonyl and phenyl. The resonance signal at 162.4 ppm is due to the carbon atom of carbonyl group in urea. Besides, the resonance peak observed at 31.3 ppm can be assigned to the carbon atom of methyl group in acetone, while the resonance signal at 207.3 ppm corresponding to the carbon atom of carbonyl group in acetone is overlaid by the rotation side band. The strong and wide bands located in the range of 1000 cm -1 1170 cm -1 are due to the AlPO stretching vibration, indicating that the materials studied were constructed by coordination of aluminum and phenylphosphonate species. The presence of characteristic bands at 1487 cm -1 , 1597 cm -1 and 3000 cm -1 3100 cm -1 indicates that the samples contain benzene rings. The existence of sharp bands at 695, 725 and 750 cm -1 further illuminates that the benzene groups in the samples are monosubstituted. The strong and sharp band at 1438 cm -1 corresponding to the CP stretching vibration is largely enhanced because of the direct connection of phosphorus atom with benzene ring. The strong band at 1630 cm -1 and broad band at 3445 cm -1 are associated with the OH bending and stretching vibration of the adsorbed water, respectively. The disappearance of the band at 1222 cm -1 indicates the absence of a PO bond in the framework of ys-AlPhPO and hs-AlPhPO microspheres.  A B Figure S8. The linear plots representing data for salmon sperm DNA on ys-AlPhPO obtained by using (a) pseudofirst-order and (b) pseudo-second-order adsorption kinetics models.