Chinese Chemical Letters  2019, Vol. 30 Issue (2): 465-469   PDF    
A novel composite stationary phase composed of polystyrene/divinybenzene beads and quaternized nanodiamond for anion exchange chromatography
Peng Yaoa, Zhongping Huanga,*, Qiulian Zhua, Zuoyi Zhub, Lili Wanga,*, Yan Zhuc     
a College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China;
b Institute of Quality and Standard for Agricultural Products, Zhejiang Academy of Agricultural Science, Hangzhou 310021, China;
c Department of Chemistry, Zhejiang University, Hangzhou 310028, China
Abstract: An approach for preparation of a novel composite anion exchanger composed of polystyrene/ divinylbenzene (PS/DVB) beads and quaternized nanodiamods (QND) were proposed. Oxidized nanodiamonds (OND) were quaternized by the condensation polymerization between methylamine (MA) and 1, 4-butanediol diglycidyl ether (BDDE), which were characterized by Fourier transform infrared (FTIR) spectra, X-ray phtoelectron spectroscopy (XPS), thermogravimetric analysis (TGA). QND with layers of cationic polyelectrolyte was attached onto the surface of sulfonated PS/DVB beads electrostatically. Subsequently, hyperbranched reaction of QND agglomerated on the PS/DVB bead surface was performed by the alternate reaction between MA and BDDE to increase the exchange capacity. The composite anion exchanger showed good stability in organic solvent and a wide pH range. The surface of these microspheres was characterized by scanning electron microscopy. In addition, ion exchange selectivity and separation efficiency of the anion exchangers were assessed using the mixtures of anions (F-, Cl-, NO2-, Br-, NO3-, HPO42- and SO42-) with carbonate/bicarbonate as eluent, and the anion exchanger with high exchange capacity could be used to analyze chloride in aqueous solution with high concentration of fluoride. This work explores the potential of nanodiamods as an agglomerated material for ion chromatography stationary phases for the separation of inorganic anions.
Keywords: Quaternized nanodiamonds     Ion chromatography     Polystyrene/divinylbenzene     Hyperbranched     Anion exchangers    

The application of ion chromatography is becoming more and more extensive in many fields such as electronics [1], scientific research [2], pharmacy and so on. Today, many new technologies introduced in ion chromatography are anticipated to improve the rapidity of analysis, sensitivity and resolution of separations [3-5], which not only depend on the species of stationary phase substrate, but also the size and surface chemistry of packing particles. Therefore, the development of new separation media is still the major trend in ion chromatography.

The majority of stationary phase used in ion chromatography, as is well-known, are based on silica or organic polymers. However, the limited pH range from 2 to 8 suitable for its exploitation [4] prevents silica from being used for ion chromatography, because the strongly acidic or basic eluents are commonly used in ion chromatography. Naturally, organic polymers become the preferred substrates for ion exchangers for they can tolerate a wide pH range. Organic polymers after functional modification, such as chemical derivatization, polymer-grafting or hyperbranching, surface coated with bulky hydrophobic ions or charged latex, have been exploited as the commercial anion exchangers. The most widely used polymers are styrene or ethylvinylbenzene copolymers cross-linked with divinylbenzene [6].

The preparation of latex-based agglomerated anion exchangers was proposed by Small et al. in 1975 for the first time [7]. Latexbased anion exchangers are consist of a surface-sulfonated polymeric substrate and fully aminated latex beads with diameter around 100 nm [8]. Nowadays high percentage of commercially available anion exchangers are latex-agglomerated ones due to the significant advantages of the approach to anion exchanger design including compatibility with mobile phase pH 0–14, excellent long-term chemical stability and especially the ability to make large batches of anion exchanger independent of substrate synthesis. Traditionally, the ion-exchange latex is the fine polymer particles with low cross-linking degree, such as polystyrene, polyacrylate [9] or poly vinylbenzylchloride/divinylbenzene [8, 10]. However, low crosslinking degree of the latex lead to their limited mechanical strength. Therefore, the development of ion-exchange latex with good mechanical stability at high pressure is necessary for the improvement of the rapidity of analysis, sensitivity and resolution of separations in ion chromatography.

Several carbon nanomaterials such as carbon nanotubes, graphene and nanodiamond, have been utilized in the chromatographic separation materials. In our previous work, multiwalled carbon nanotubes had been employed as an agglomerated material for ion chromatography stationary phases for separating inorganic anions [11, 12]. However, joints between polystyrene/ divinylbenzene (PS/DVB) beads were formed due to the length of multi-walled carbon nanotubes. The agglomeration of PS/DVB beads would decrease the capacity and separation efficiency of anion exchangers. Therefore, nanodiamond as the three-dimensional nanomaterial was selected in this study to avoid the joint problem. Nanodiamond has many beneficial properties which have been used in different areas of nanotechnology. The mechanical stability at high pressure, excellent stability over the whole pH range, thermal stability, absence of shrinking or swelling in different water solutions and organic solvents [13, 14], the possibility of surface modification with different functional groups and relative adsorptive inertness of the matrix make nanodiamond a prospective material for use as a stationary phase in different separation modes of liquid chromatography. Microdisperse sintered nanodiamond was used as weak cation-exchangers in ion chromatography directly by Nesterenko et al. [15]. Chromatographic of high pressure high temperature synthetic diamond in ion-exchange chromatography was investigated by Peristyy et al. [16]. Pellicular particles with spherical carbon cores and porous nanodiamond/polymer shells were prepared by Wiest et al. [17] for reversed-phase high performance liquid chromatography. Diamond as stationary phase of solid-phase extraction (SPE) was proposed by Wiest et al. [18]. Qiu et al. [19] prepared an novel nanodiamond/silica stationary phase for hydrophilic interaction chromatography. There were also other applications of nanodiamond in electrophoresis [20], gas chromatography [21] and mass spectrometry [22]. To the best of our knowledge, there are few literatures about the applications of the composite stationary phase composed of polymers and nanodiamond in anion exchange chromatography.

The main aim of this work was to prepare a composite stationary phase composed of polystyrene/divinybenzene beads and quaternized nanodiamond for anion exchange chromatography. Quaternized nanodiamods (QND) with cationic polyelectrolytes obtained by the condensation polymerization of amine and diepoxide, were attached onto the surface of sulfonated PS/DVB beads through electrostatic interaction. The reliability of the prepared anion exchangers in practical applications was demonstrated by the separation of common inorganic anions.

Equipments and reagents can be found in Supporting information. PS/DVB substrate beads were synthesized according to the literature that we reported before [23]. Sulfonated PS/DVB was synthesized as following procedure. 3 g PS/DVB beads were dispersed in 20 mL glacial acetic acid and stirred for 10 min at 30 ℃ in a 250-mL three-neck flask. Subsequently 5 mL dichloromethane was added and stirred for another 30 min at 30 ℃. Finally, PS/DVB beads were sulfonated with 95% sulfuric acid for 3 min, then quenching with 1.0 mol/L iced sulfuric acid. The obtained suspension was filtered and washed repeatedly until the filtrate was neutral. PS/DVB beads with negative charge were obtained.

Pristine nanodiamonds (PND) were oxidized by concentrated sulfuric and nitric acid. A 3:2 mixture of concentrated sulfuric and nitric acid was added into a 250-mL three-neck flask which containing 1 g PND. The mixture was diluted with cold deionized water after stirred for 12 h at 80 ℃. Oxidized nanodiamonds (OND) were collected by a centrifuge at 8000 rpm and washed with deionized water repeatedly until the deionized water was neutral. The final solid was dried under vacuum for 24 h at 60 ℃.

OND were quaternized with methylamine (MA) and 1, 4- butanediol diglycidyl ether (BDDE) as shown in Fig. 1. 40 mL aqueous mixture of MA (2.8%, v/v) and BDDE (7.2%, v/v) was added to 1 g OND, then the suspension was heated to 65 ℃ for 1 h, later filtered and rinsed with deionized water. QND with one layer of cationic polyelectrolyte (1QND) were obtained. The above QND were sequentially reacted with MA and BDDE to produce more quaternary ammonium groups. First, 1QND were dispersed into 100 mL deionized water followed by adding 20 mL MA (4%, v/v), then the mixture was heated to 65 ℃ for 30 min. After that, the product was filtered and rinsed with deionized water. Second, the residue of the previous step was reacted with 20 mL BDDE (10%, v/v) under the same conditions as above. The above procedures (step a and b shown in Fig. 1) were named a cyclic reaction. After repeating the cyclic reaction one time, one more layer of cationic polyelectrolyte was bonded onto the surface of nanodiamond. Finally, QND with 5 layers of cationic polyelectrolyte (5QND) were obtained. QND with different layers of cationic polyelectrolyte was dispersed in deionized water before the agglomeration reaction with sulfonated PS/DVB beads.

Fig. 1. Synthesis route of the titled stationary phase. (a) First reaction cycle with primary amine on the surface of resin; (b) First reaction cycle with diepoxide monomer on the surface of resin.

Latex-agglomerated/hyperbranched anion exchangers (LAHAE) were prepared as following procedure. Sulfonated PS/DVB beads were added into the 5QND dispersed aqueous solution, and then stirred for 12 h at room temperature. After filtered, PS/DVB beads coated with 5QND were obtained. Then, these particles were hyperbranched by MA and BDDE. The approach was similar to the synthesis of QND as shown in Figs. 1a and b. Finally, particles with 2 and 4 layers of cationic polyelectrolyte (LAH-AE2 and LAH-AE4) were obtained respectively, after repeating the cyclic reaction.

Agglomerated/hyperbranched anion exchangers (AH-AE) were prepared as following procedure. 40 mL BDDE (10%, v/v) was added into a 500-mL three-neck flask which contained 100 mL deionized water. After the temperature increased to 60 ℃, 40 mL MA (4%, v/v) was dropped into the above solution, and then stirred for 30 min. Finally, cationic polyelectrolyte aqueous solution was obtained after cooling to room temperature. After that, 3 g sulfonated PS/ DVB beads were added into the above 100 mL cationic polyelectrolyte aqueous solution and then stirred for 24 h at 30 ℃. The suspension was filtered and washed repeatedly; continuously, these particles were hyperbranched with the method above. Finally, AH-AE2 was obtained through 2 cyclic reactions.

All the prepared anion exchangers were slurry packed into the stainless steel tube (150 mm × 4.6 mm), through pressing with deionized water as packing solvent under a pressure of 20 MPa. The volume of packing solvent passing through the column should be 300 mL at least. All of the columns were washed with sodium carbonate/bicarbonate solution before connected to ion chromatographic instrument.

The chemical stability of LAH-AE2 and AH-AE2 were investigated at the conditions of pH 1-13 and organic solvent. With the eluting time of 6 h, an eluent of 10 mmol/L Na2CO3 and 8 mmol/L NaOH (pH 13) was used for the alkaline test, while 30 mmol/L methanesulfonic acid (pH 1) was employed for the acidic test.100% methanol was utilized for the organic solvent test and eluted for 10 h. After the stability test, the column was washed with deionized water before the subsequent column efficiency evaluation. The mechanical stability of LAH-AE2 and AH-AE2 were tested by Waters 1525 HPLC system. The variation of pressure was recorded through increasing the flow rate of eluent. Capacities of anion exchangers were determined according to the following steps: Columns were flushed with 1 mol/L sodium chloride aqueous solution for 1 h at 1 mL/min and then flashed with deionized water for 1 h at 1 mL/min, subsequently flushed with 10 mmol/L Na2CO3. Chloride in the effluent was quantified by ion chromatography using a Dionex Ionpac AS23 column.

Carboxyl groups were introduced onto the surface of nanodiamonds after the acid treatment of concentrated sulfuric and nitric acid. Subsequently, quaternary ammonium groups were bonded on the surface of nanodiamonds, based on condensation polymerization of carboxyl, BDDE and MA. To prove the presence of polyelectrolytes attached on the nanodiamonds, Fourier transform infrared (FTIR) spectra was performed to characterize the functional groups on the surface of nanodiamonds. The FTIR spectra of pristine, oxidized and quaternized nanodiamonds were shown in Fig. 2. It was found that oxygen-containing groups existed on the surface of pristine nanodianmonds due to the process of synthesis. As shown in Fig. 2 curve a, the broad absorption band at 3340 cm-1 and the peak at 1635 cm-1 are assigned to the stretching and deformation vibrations of hydroxyl group. The absorption peak at 1771 cm-1 is assigned to the mode of C=O stretching vibration in carbonyl group. The results are similar as those reported previously in the references [13, 24]. The peak at 1400 cm-1 is assigned to the in-plane bending vibration of C-H group and it disappeared after oxidized by concentrated sulfuric and nitric acid in Fig. 2 curve b. New absorption peaks of acidified nanodiamonds were observed in Fig. 2 curve b after the oxidation treatment. The peaks at 1200 cm-1 and 1053 cm-1 are attributed to the epoxy group stretching vibrations in cyclic ether or cyclic ester chains [25]. The peak at 874 cm-1 is assigned to the symmetrical stretching vibration of epoxy group [26], and the peak at 593 cm-1 may be assigned to the bending vibration of C-CO-C. The results demonstrated that epoxy groups were also formed on the surface of nanodianmonds due the strong oxidation of concentrated sulfuric and nitric acid. However, the above four peaks disappeared after quaternized modification (Fig. 2 curve c), probably the epoxy groups were reacted with MA. Differing from those of acidified nanodiamonds, distinct signals from polyelectrolytes can be observed on curve c. The C—H stretching vibrations of the grafted polyelectrolytes can be observed at 2910 cm-1 and 2860 cm-1. The small peak present at 1460 cm-1 could be assigned to the bending vibrations of CH, CH2 and CH3. The significant peak at 1113 cm-1 is due to the stretching vibration of the C—N group of polyelectrolytes.

Fig. 2. FTIR spectra of (a) PND, (b) OND, (c) 5QND.

XPS analysis was performed to investigate the elemental composition of nanodiamonds surface. The peaks of O 1s (530.3 eV), N 1s (399.1 eV) and C 1s (284.4 eV) were observed in Fig. S1 (Supporting information). By the comparison of PND and OND, the content of oxygen on nanodiamond's surface after oxidization increased from 10.16% to 27.12% while the content of carbon decreased from 88.21% to 68.45%. Compared with PND, the content of nitrogen on QND's surface increased from 1.68% to 2.03% while the content of carbon decreased from 88.21% to 84.36%.

To further prove the presence of functional groups bonded on the nanodiamonds, TGA were performed to characterize the thermal stability. As shown in Fig. S2 (Supporting information), the first weight-loss stage for PND and OND occurs below ~150 ℃, which is assigned to the desorption of water. Although they were dried at the same temperature, moisture content of OND was significantly higher than that of PND. The temperatures of functional groups decomposed completely were at 230 ℃ and 300 ℃ respectively for PND and OND. In addition, the weight loss of OND at the second stage was also larger than that of PND. After quaternization treatment, the first weight-loss stage for QND occurs at 250 ℃ and it has finished completely at 400 ℃, which is differ from PND and OND. Therefore, the results show that new functional groups on the surface of nanodiamonds were formed after oxidation treatment.

The evaluation of chromatographic characteristics for the home-made columns was performed using a test mixture of inorganic anions composed of fluoride, chloride, nitrite, bromide, nitrate, sulfate and hydrophosphate. In our experiment, QND with five layers of cationic polyelectrolyte was chose due to it showed good dispersion in water, enough positive charge and especially no severe self-agglomeration. Overreaction would lead to nanodiamonds agglomerating together and result in an uneven coating, because the two epoxide groups at the chain ends of BDDE could react with the secondary amines from different nanodiamond. The hyperbranched reaction was further performed on the surface of these particles to increase the exchange capacity. The surfaces of the anion exchangers could be observed in Fig. 3. Chromatogram of inorganic anions separated on LAH-AE2 was shown in Fig. 4a and the exchange capacity of the column was 0.181 mmol/column. In order to explore the effect of QND, AH-AE2 was prepared. The exchange capacity of the column was 0.154 mmol/column. Chromatogram of inorganic anions separated on AH-AE2 was shown in Fig. 4b.

Fig. 3. The SEM images of (a) AH-AE2 (5000 times) and (b) LAH-AE2 (5000 times).

Fig. 4. Separation of seven common inorganic anions with LAH-AE2 and AH-AE2. (a) LAH-AE2, peaks: fluoride (2 mg/L), chloride (5 mg/L), nitrite (10 mg/L), bromide (20 mg/L), nitrate (20 mg/L), hydrophosphate (30 mg/L) and sulfate (10 mg/L). (b) AH-AE2, peaks: fluoride (6 mg/L), chloride (15 mg/L), nitrite (30 mg/L), bromide (60 mg/L), nitrate (60 mg/L), hydrophosphate (150 mg/L) and sulfate (30 mg/L). Column was used stainless steel tube (150 mm × 4.6 mm), flow rate 1 mL/min, injection volumn 25 μL, suppressor current 25 mA, eluent was composed of 2 mmol/L Na2CO3 and 2 mmol/L NaHCO3.

To assess the mechanical stability of LAH-AE2 and AH-AE2, the materials were slurry packed with water into 4.6 mm × 150 mm steel columns. These columns were then subjected to increasing pressures up to 4000 psi with a constant flow rate pump with water as eluent. The variation of pressure was manually recorded at each flow rate. The results were illustrated in Fig. S3 (Supporting information). As the experiment showed, the pressure-flow rate curve of AH-AE2 almost kept a linear relationship under increasing column pressures up to 3100 psi. However, microsphere fracturing or compression deformation happened when the flow rate at 5 mL/min, which lead to the column pressure increasing continuously. It was demonstrated that this destruction was irreversible. Conversely, the column pressure of LAH-AE2 microspheres remained linear below 4000 psi in this experiment, indicating that the composite stationary phase could tolerate higher pressure.

Chemical stability test was performed under the extreme pH conditions. The result was shown in Fig. S4 (Supporting information), which indicated a general trend in decreasing retention time of anions after initial test of alkaline or acidic eluent. It might be caused by residual function group sloughed off the surface of PS/ DVB. However, there was no marked change in separation effect. The test result demonstrated that both LAH-AE2 and AH-AE2 could tolerate the conditions of pH 1-13.

The stability test of compatibility with organic solvent was performed using 100% methanol as mobile phase. The comparison of LAH-AE2 and AH-AE2 was given in Table S1 (Supporting information). Obviously, the compatibility with organic solvent of LAH-AE2 was better than that of AH-AE2. Not only the column efficiency of AH-AE2 decreased, but also the peaks broadened visibly. What is worse, the column pressure of AH-AE2 increased from 898 psi to 1213 psi after the test accomplished, which made us believe that the functional layer of AH-AE2 shed in methanol to some extent. On the contrary, LAH-AE2 was more stable in methanol due to the functional layer bonded to nanodiamond directly which was hardly dissolved in organic solvent.

LAH-AE with different exchange capacities could be prepared through controlling the time of cyclic reactions. The retention of anions was enhanced with the increase of exchange capacity, which was demonstrated by the separation of inorganic anions on LAH-AE4 (0.412 mmol/column). As shown in Fig. S5a (Supporting information), 7 anions were separated in 60 min under the condition of 8 mmol/L Na2CO3 and 8 mmol/L NaHCO3. LAH-AE4 was chosen to analyze chloride in aqueous solution with high concentration of fluoride due to the long time interval between them, which could be covered by the tailed peak of fluoride if used the common column. The result was shown in Fig. S5b (Supporting information).

In summary, composite anion exchangers composed of PS/DVB beads and QND with different ion exchange capacities were prepared. 5QND were attached evenly onto the surface of sulfonated PS/DVB beads with water as dispersant. A defined time of bonded layers with desirable positive charge could be grafted onto QND at the surface of PS/DVB substrate beads, to increase their exchange capacity. LAH-AE2 and AH-AE2 could be used to successfully separate seven common inorganic anions. Furthermore, both of them could tolerate extreme conditions of pH 1-13 and showed good linearity under the normal flow rate. Compared to AH-AE2, LAH-AE2 was more stable in organic solvent and could tolerate higher pressure. This work explores the potential of nanodiamod as an agglomerated material for ion chromatography stationary phases for the separation of inorganic anions.


This research was financially supported by the National Natural Science Foundation of China (No. 51503182), Analysis and Measurement Foundation of Zhejiang Province (No. 2017C37064), Zhejiang Provincial Natural Science Foundation (No. LQ15C200006) and Zhejiang University of Technology Natural Science Foundation (No. 2014XY002).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at

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