Research Article www.acsami.org
Thermoresponsive Oligo(ethylene glycol)-Based Polymer Brushes on Polymer Monoliths for All-Aqueous Chromatography Nan Li,†,‡ Li Qi,*,† Ying Shen,†,‡ Yaping Li,†,‡ and Yi Chen† †
Beijing National Laboratory of Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Graduate School, University of the Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Porous polymer monoliths onto which were grafted a thermoresponsive copolymer, poly(2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA)-cooligo(ethylene glycol) methacrylate (OEGMA)), were synthesized by the two-step atom transfer radical polymerization (ATRP) method. The copolymer-grafted monoliths were characterized by elemental analysis, scanning electron microscopy, and mercury intrusion porosimetry. They were further used as the thermoresponsive stationary phase for all-aqueous high-performance liquid chromatography (HPLC). The chromatograms of three steroids demonstrated that the chain length of the grafted copolymer, which was regulated by varying the grafting time, could affect the separation by providing different amounts of hydrophobic interaction sites with analytes. Additionally, the elution profiles of steroids on the stationary phase could also be tuned by the comonomer composition. The results showed that the porous polymer monoliths enabled separation of the test mixture in pure aqueous mobile phase under isocratic conditions. Furthermore, the proposed method provides a simple and promising tool in the design and construction of responsive surfaces for chromatography applications. KEYWORDS: poly(2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate) brushes, two-step atom transfer radical polymerization, thermoresponsive monolith, separation, hydrophobic interaction
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INTRODUCTION Responsive surfaces, usually prepared by modification of responsive polymer brushes on solid substrates, are of current interest in a number of research areas, including biosensing, cell culture, and drug delivery.1−4 One of the most interesting applications is responsive stationary phases.5,6 Especially, thermoresponsive stationary phases, emerging as a new class of chromatography supports for separation of bioanalytes, have been drawing a considerable amount of attention.7−10 The main advantage of thermoresponsive chromatography is that the separation can be achieved by only changing the column temperature without using an organic solvent as the mobile phase. Thus, it contributes to maintaining the biological activity of analytes and reducing the environmental burden. To date, most thermoresponsive stationary phases for all-aqueous chromatography have been produced from poly(N-isopropylacrylamide) (PNIPAM),8,11 which exhibits a lower critical solution temperature (LCST) of 32 °C in water.12 Thus, the hydrophobic property of the PNIPAM-grafted surface could be easily altered when the temperature is changed across the LCST. Recently, polymers with oligo(ethylene glycol) side chains have been attracting a lot of attention as a new family of thermoresponsive polymers.13 Among them, copolymers of 2(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and oligo(ethylene glycol) methacrylate (OEGMA) were reported to exhibit LCST values that can be tuned in the range of 26−90 °C by varying the co-monomer composition.14 In addition, © 2013 American Chemical Society
their LCST values were found to be less affected by factors such as ionic strength, concentration of the copolymer in water, and chain length.15 Therefore, the P(MEO2MA-co-OEGMA) copolymer appears as a reliable choice instead of conventional PNIPAM for chromatography applications. However, oligo(ethylene glycol)-based thermoresponsive stationary phases have been constructed only on silica monoliths.16 Porous polymer monoliths were developed in the early 1990s and have been occupying an important position in separation science because of the simple fabrication, improved mass transfer properties, and good tolerance to extreme pH.17−19 However, research on thermoresponsive chromatography based on polymer monoliths has remained scarce. Thus, introducing novel thermoresponsive polymer brushes onto polymer monolith surfaces, combining the advantages of two kinds of materials in chromatography applications, is necessary and desirable. Among the several techniques to prepare polymer monoliths, atom transfer radical polymerization (ATRP), which is the most versatile method of living free-radical polymerization,20,21 offers great advantages such as allowing reaction under mild conditions and control of the grafting polymer length.22−25 Therefore, the two-step ATRP method could Received: August 21, 2013 Accepted: November 20, 2013 Published: November 20, 2013 12441
dx.doi.org/10.1021/am403510g | ACS Appl. Mater. Interfaces 2013, 5, 12441−12448
ACS Applied Materials & Interfaces
Research Article
waves. Next, hydrazine (37 μL, 0.6 mmol) was added to the above solution, and the resulting mixture was sonicated for 10 s. The color of the mixture was found to turn reseda at the same time. Subsequently, the reaction was carried out at 35 °C by pumping the polymerization solution through the polymer monolith at a flow rate of 0.05 mL/min. After 4 h of grafting polymerization, the monolith was washed with 100 mL of water to wash out all of the soluble residues remaining in the monolithic column. Characterization of the Prepared Monoliths. The prepared porous monoliths (both nongrafted and grafted) were removed from the columns and subjected to elemental analysis using a Flash EA 1112 elemental analyzer. The amount of the bromic group, which served as the surface initiator for grafting polymerization, was calculated using the equation
provide a simple and effective approach for preparing and grafting monoliths to generate responsive stationary phases. In this study, P(MEO2MA-co-OEGMA)-grafted porous polymer monoliths were prepared via the two-step ATRP method and applied as the novel stationary phase for thermoresponsive chromatography. P(MEO2MA-co-OEGMA) brushes with different chain lengths for various interaction sites were obtained by changing the grafting time. Moreover, the comonomer composition was altered to modulate the hydrophobicity of P(MEO2MA-co-OEGMA). Characterizations of the responsive copolymer-grafted polymer monoliths were wellperformed. Furthermore, the separation abilities of the prepared columns were evaluated using three steroids in high-performance liquid chromatography (HPLC).
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ATRP initiator =
EXPERIMENTAL SECTION
%Br × 106 %Br
%Br
calcd
[1 −
%Br calcd
]MS
where %Br is the percent bromine as determined by elemental analysis, %Brcalcd is the calculated weight percent of bromine in the initiator unit, M is the formula weight of the initiator unit (g/mol), and S is the specific area of the prepared monolith. The amount of grafted copolymer on the polymer monolith surface was calculated using the equation
Materials. Ethylene glycol dimethacrylate (EDMA) was freshly distilled under vacuum prior to use. Cuprous bromide (CuBr) was washed with acetic acid and methanol and vacuum-dried before use. 2(Dimethylamino)ethyl methacrylate (DMAEMA), ethyl 2-bromopropionate (EBP), 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA), cupric bromide (CuBr2), MEO2MA, OEGMA (Mn = 475 g/mol), hydrocortisone, testosterone, medroxyprogesterone acetate, and other chemicals were commercially available and used directly without purification. Milli-Q water prepared by an ultrapure water purification system (Millipore, Billerica, MA) was used in this study. Preparation of Porous Monoliths. The porous polymer monoliths were synthesized by ATRP. The detailed preparation process was similar to that reported by our lab26 and is described as follows: A mixture of EDMA (0.5 mL, 0.26 mmol), EBP (1.9 μL, 0.015 mmol), cuprous bromide (6.5 mg, 0.045 mmol), methanol (0.5 mL), and hexane (0.5 mL) was placed in a dry sample vial, homogenized by ultrasonic waves, and then deoxygenized by purging with Ar for 10 min. Subsequently, PMDETA (9.2 μL, 0.045 mmol) was quickly added to the mixture, and the solution was injected into a 50 mm × 4.6 mm I.D. column. With both ends sealed, the column was placed at room temperature to react for 12 h and then was connected to the HPLC system. The residual compounds that may remain in the polymer monolith were washed out by pumping 100 mL of methanol and 50 mL of water successively through the column at a flow rate of 0.20 mL/min. Grafting of Porous Monoliths. The grafting of P(MEO2MA-coOEGMA) on prepared monoliths via surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) was performed as displayed in Scheme 1. The detailed polymerization procedure was as follows: CuBr2 (34 mg, 0.15 mmol), PMDETA (32 μL, 0.15 mmol), MEO2MA (0.76 mL, 4.11 mmol), and OEGMA (0.14 mL, 0.32 mmol) were dissolved in 20 mL of water. The mixture formed a uniform blue solution under ultrasonic
grafted copolymer =
%Cp %Ccalcd p [1
−
%Cp %Ccalcd p
−
%Ci %Ccalcd i
]S
where %C is the percent carbon increase over that of the original monolith as determined by elemental analysis and %Ccalcd is the calculated weight percent of carbon in the initiator or copolymer monomer. The subscript i denotes the initiator (%Ci equals zero because the original monolith itself served as the macroinitiator) and the subscript p denotes the copolymer. The morphologies of the prepared monoliths were characterized using scanning electron microscopy (SEM) on a model S-4300 scanning electron microscope (Hitachi, Japan). Mercury intrusion porosimetry was performed to determine the pore size distribution of the prepared monoliths on an Autopore III 9220 mercury intrusion porosimeter (Micromeritics, USA). Temperature-Modulated Elution of Steroids. The P(MEO2MA-co-OEGMA)-grafted polymer monoliths were connected to an HPLC system (LC-20A, Shimadzu, Japan) with a UV−vis detector (SPD-20A) for chromatographic evaluation. Hydrocortisone, testosterone, and medroxyprogesterone acetate were selected as model analytes at concentrations of 0.02, 0.42, and 0.75 mg/mL, respectively. Table 1 presents the molecular weight and logP values of the steroids.
Table 1. Properties of Model Steroids
Scheme 1. Route for Preparing the P(MEO2MA-coOEGMA)-Grafted Polymer Monoliths by the Two-Step ATRP Method
a
analyte
mol wt (g/mol)
logPa
hydrocortisone testosterone medroxyprogesterone acetate
360.49 288.42 384.51
1.96 2.44 3.31
Partition coefficient in the n-octanol/water system.
Milli-Q water was pumped through the monolith column as the mobile phase, and the elution behavior of the analytes was recorded with a flow rate of 1.0 mL/min at different temperatures. The detection wavelength was set at 254 nm. Van’t Hoff plots were built to investigate the retention behavior of the analytes on the thermoresponsive monolithic columns. The value of the retention factor k′ was calculated as
k′ = (t R − t0)/t0 where tR is the retention time of the model steroid at a specific temperature and t0 is the retention time of potassium nitrate. 12442
dx.doi.org/10.1021/am403510g | ACS Appl. Mater. Interfaces 2013, 5, 12441−12448
ACS Applied Materials & Interfaces
Research Article
Table 2. Characterization of P(MEO2MA-co-OEGMA)-Grafted Monoliths elemental composition (%)b monolitha
grafting time (h)
C0-0 C3-15 C4-15 C8-15 C4-10 C4-20
0 3 4 8 4 4
C 58.40 59.70 59.90 63.46 59.80 59.90
± ± ± ± ± ±
Br 0.17 0.09 0.01 0.01 0.12 0.03
amount of initiator (μmol/m2)c
1.90 ± 0.15
