Article pubs.acs.org/IECR
Preparation and Selectivity Evaluation of Glutathione Molecularly Imprinted Polymers from Aqueous Media Xiaojiao Liu,†,‡ Zhenbin Chen,*,†,‡ Jiapeng Long,†,‡ Hui Yu,†,‡ Xueyan Du,*,†,‡ Yaming Zhao,§ and Jingbo Liu∥ †
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, Gansu, China ‡ School of Material Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, Gansu, China § Lanzhou Institute of Industrial, Lanzhou, 730050, Gansu, China ∥ The Department of Chemistry, Texas A&M University-Kingsville, MSC 161, 700 University Blvd, Kingsville, Texas 78363, United States ABSTRACT: A novel polymer, by molecularly imprinting glutathione (GSH), capable of being directly utilized in the aqueous phase, was developed for uptaking GSH. To significantly improve cross-linking density of this molecular imprinted polymer (MIP), the pseudoternary phase diagram of N,N′-methylene-biacrylamide, methanol, and water was investigated to increase solubility of NMBA. Subsequently, key fabrication variables, including a functional monomer, cross-linker, and initiator quantity, polymerization temperature, and time were optimized. Under optimum conditions, the maximum adsorption capacity of GSH approached 27 mg g−1, and the separation degree was as high as 2.84. The Fourier transform infrared spectrometer (FTIR), scanning electron microscope (SEM), and Brunauer−Emmett−Teller (BET) analysis results from GSH-MIP and the control indicated that the high adsorption capacity and selectivity of GSH originated from the arrangement of functional groups and cavities. Additionally, this MIP demonstrated appealing long-time stability and reusability. The adsorption kinetics could be well described by a pseudo-first-order model, while the thermodynamics of the adsorption process could be described by neither classic Langmuir nor Freundlich model. The adsorption and separation study was applied in urine, and the adsorption rate was 90.30%, the desorption rate was 75.32%, and the purity was 58.74%.
1. INTRODUCTION GSH is a type of water-soluble, high value-added compound. Due to its various physiological functions, such as detoxification, radioprotection, antiallergic, liver protection, and antioxidant defense, it has been utilized extensively in the pharmacy, food, and cosmetic industries. Chemical synthesis and extraction from animals, plants, and microorganisms are two alternative routes to obtain GSH. Compared with the former, the latter are more appealing due to its environmentally friendliness and abundant natural origin. GSH was prepared by yeast fermentation and chemical synthesis.2 Unfortunately, the separation and enrichment, currently, by a subsequently stepped procedure of extraction, salting out, ion exchange (or membrane separation), concentration, and crystallization,2,3 is a disadvantage of environmental pollution and escalating costs.4,5 Therefore, a more efficient and low-cost method for separation and enrichment is necessary. Molecular imprinting technology provides an attractive method for selective-adsorbents preparation due to its high selectivity, durability, and structural predictability. Therefore, MIPs are widely applied in chiral resolution,6 chromatographic technology,7 and biosensors.8 Nevertheless, relatively few groups investigated its potential of separating and purifying high value-added compounds.9−11 Although varieties of GSHMIP synthesis have been reported,12−15 the requirement of a complex preparation procedure,12 specialized instruments,13 and an aprotic environment14,15 still remains. © 2014 American Chemical Society
This contribution presented a simple and convenient method for GSH-MIP preparation with high adsorption selectivity and capacity. The copolymerization and cross-linking of NVP and AM were achieved by VC, with H2O2 as an initiator and NMBA as a cross-linker, in a mixture of methanol and water. An interference compound, L-homocystiene (L-cy), smaller than GSH, with a similar chemical structure was introduced to improve the adsorption selectivity. The adsorption and separation study was applied in urine, and the adsorption rate was 90.30%, the desorption rate was 75.32%, and the purity was 58.74%.
2. MATERIAL AND METHODS 2.1. Materials. Acrylamide (AM, AR, Kermel Chemical Reagent Company, Tianjin, China) was recrystallized in acetone after being received. N-vinypyrrolidone (NVP, CP, Boai Nky Pharmaceuticals Co., Ltd. Henan, China) was purified by reduced pressure distillation (bp 77−78 °C, 2.0 mmHg; during this process, paradioxybenzene, sulfur deposition, and copper powder were used). N,N′-methylenebiacrylamide (NMBA, AR, Zhongtai Chemical Reagent Company, Shanghai, China) was recrystallized in 66% methanol at 50 °C Received: Revised: Accepted: Published: 16082
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Scheme 1. Schematic Representation for the Synthesis of GSH-MIP
in a vacuum for 24 h, then ground and screened with 50 and 150 mesh screens, and the samples between the two sieves were selected. Nonmolecularly imprinted polymer (NMIP) was also prepared as a control under the same conditions. Schematics of the synthesis procedure are illustrated in Scheme 1. 2.4. Characterizations of MIP and NMIP. The structure and morphology of MIP and NMIP were characterized by FTIR (Nicolet NEXUS 670, American Nicolet Corporation, Madison, Wisconsin, USA) and SEM (JSM-5600LV SEM, JEOL, Tokyo, Japan). The average pore size, specific surface area, and pore volume of GSH-MIP and NMIP were characterized by nitrogen gas adsorption measurements using BET analysis (ASAP 2020, Micromeritics Co., Norcross, Georgia, USA). The parameters were analyzed by N 2 adsorption, at 75.9 K with each pressure step carried for 10 s to reach equilibrium. 2.5. Determination of Adsorption Selectivity of MIP. A total of 0.2000 g of MIP was added into a 50 mL conical flask loaded with 25.00 mL of mixed solution of GSH and L-cy (the concentration of each component was 0.1000 g L−1) for the adsorption experiment. MIP was removed after adsorbing for 6 h at 10 °C, and the peak areas of raffinate were measured. The adsorption capacities of MIP to GSH and L-cy were calculated according to following eq 1:
and dried at a low temperature. L-Ascorbic acid (VC, AR, Tianjin Deen Chemical Reagent Co., Ltd. Tianjin, China), hydrogen peroxide (H2O2, 30%, AR, Tianjin Hengxing Chemical Reagent Co., Ltd. Tianjin, China), methanol (AR, Yantai Shuangshuang Chemical Reagent Co., Ltd. Yantai, Shandong, China), acrylic acid (AA, AR, Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China), hydroxyethyl methacrylate (HEMA, AR, Tianjing Chemical Reagent Research Institute, Tianjin, China), glutathione (GSH, AR, Shanxi Sciphar Biotechnology Co., Ltd. Shanxi, China), Lhomocysteine (L-cy, AR, Shanghai Zhongqin Chemical Co., Ltd. Shanghai, China), phosphoric acid (AR, Tianjin Fuyu Fine Chemical Co., Ltd. Tianjin, China), potassium dihydrogen phosphate (CP, Tianjin Beichen District Reagent Factory Founder, Tianjin, China), sodium 1-heptanesulfonate, and methanol (CP, Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China) were used as received. 2.2. HPLC Conditions. All samples were analyzed by HPLC on a Hitachi L-2400 series HPLC system (Hitachi Ltd., Tokyo, Japan) with a C18 column according to reported literature (AT. LICHROM, 150 mm × 4.6 mm, 5 μm, analysis technology Co., LTD, China).16 2.3. Synthesis of GSH-MIP. 2.3.1. Pseudoternary Phase Diagram. Eight mixture solutions of methanol and deionized water with different volume ratios (0:1, 1:4, 3:7, 2:3, 1:1, 7:3, 4:1, and 1:0) were prepared; NMBA was added until the mixture become turbid, followed by filtration, evaporation, and completely drying in a vacuum oven at 60 °C. The solubility was calculated by weight. The pseudoternary phase diagram was plotted accordingly. 2.3.2. Synthesis of MIP. GSH-MIPs were synthesized by solution polymerization. A total of 5.00 mL of GSH solution (200.00 mmol L−1), 10.00 mL of NVP (600.00 mmol L−1), and 15.00 mL of AM (600.00 mmol L−1) solution were added into a three-necked flask with magnetic stirring for 10 min; then 35.00 mL of NMBA (648.50 mmol L−1) methanol/water solution (7:3, V/V) was added followed by nitrogen purging for 15 min. A total of 0.20 mL of H2O2 (705.88 mmol L−1) and 0.30 mL of VC (136.20 mmol L−1) aqueous solutions were introduced to initiate polymerization until high viscosity and then immersed in a 35 °C water bath for 30 h. The polymer was purified by Soxhlet extraction with mixture of methanol/ acetic acid (9:1, V/V). The extracted MIPs were dried at 60 °C
Q GSH =
(C0(GSH) − C(GSH))V W
Q L ‐ cy =
(C0(L‐cy) − C(L‐cy))V W
(1)
where QGSH and QL‑cy are equilibrium adsorption capacities of GSH and L-cy (mg g−1), C0(GSH) and C0(L-cy) are the concentrations of GSH and L-cy in the initial mixture, respectively, C(GSH) and C(L-cy) are the concentrations of GSH and L-cy in raffinate (g L−1) after adsorption equilibrium, V is the volume of the initial solution (mL), and W is the mass of MIP (g). The total adsorption capacity (Qt, mg g−1) of MIP was calculated according to eq 2: Q t = Q GSH + Q L ‐ cy 16083
(2)
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Figure 1. (a) Pseudoternary phase diagrams of methanol, water, and NMBA. (b) Solubility curve of pseudoternary phase diagram.
was identical as a thermodynamics measurement. The desorption was performed in methanol/acetic acid (9:1, V/V) for 6 h. After that, the MIP was completely dried under a vacuum for next cyclic measurement.
The separation degree (D) of MIP to GSH and L-cy could be calculated by eq 3: D=
Q GSH Q L ‐ cy
(3)
3. RESULTS 3.1. The Pseudoternary Phase Diagram of NMBACH3OH-H2O. To develop suitable recognition sites and keep the specific adsorption performance for the template molecule, MIP is typically highly cross-linked (cross-linking degree up to 70−90%),17 meaning sufficient concentration of the template molecule and cross-linker is a prerequisite to ensure high adsorption capacity and selectivity. In this work, a water-soluble cross-linker, NMBA, is selected in MIP synthesis because GSH always existed in the aqueous phase due to its strong polarity. However, the solubility of NMBA in water is still insufficient to achieve the minimum cross-linking demand of MIP; therefore, methanol/water mixed solvent was adopted to improve the solubility of NMBA. Figure 1 showed that the solubility of NMBA appeared at a maximum at a Vmethanol/VH2O ratio of 7:3. Acylamino-terminal groups in the NMBA structure induce hydrophilicity and therefore can be dissolved in water. However, the hydrophobic ethenyl-terminal group and methylene group abate the solubility of NMBA in water. After methanol was introduced, the solubility of NMBA in the water/methanol mixture increased due to the improved compatibility of ethenyl and methylene in methanol. However, as methanol exceeds optimal composition, the solubility of NMBA oppositely decreased, which can be ascribed to polarity incompatibility of the methanol hydroxyl group and acylaminoterminal groups in NMBA. 3.2. Structure Characterization of MIP. FTIR spectra of MIP and NMIP are shown in Figure 2. Characteristic
2.6. Measurement of Adsorption Kinetics. Fourteen MIP samples 0.2500 g in weight were added into 14 100 mL conical flasks containing 50.00 mL of 0.5000 g L−1 GSH aqueous solution; then those conical flasks were immersed into a water bath at 10 °C simultaneously and was allowed to adsorb for 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 7, 8, 9, and 10 h, respectively. The adsorption capacity of MIP was calculated according to eq 4 based on the concentration of raffinates:
Q=
(C 0 − C ) V W
(4) −1
Q is the adsorption capacity of MIP (mg g ), C0 is the concentration of GSH in the initial solution (g L−1), and C is the concentration (g L−1) of GSH in the raffinate. 2.7. Adsorption Thermodynamics. The thermodynamics were measured by MIP adsorption (0.1000 g) of GSH solutions (50.00 mL) ranging from 0.0300 to 4.0000 g L−1 at 10 °C for 6 h and calculated by eq 5 according to the peak areas of raffinates:
Q=
(C0 − Ce) V W
(5)
where Ce was the equilibrium concentration of GSH (g L−1). 2.8. The Performance of GSH-MIP. 2.8.1. The Long-Time Stability of GSH-MIP. A 0.1000 g sample preserved for three years was used to verify the long-time stability of GSH-MIP in the same aforementioned procedure, and the long-time stability of MIP was assessed by eq 6 and 7: RL(Q ) =
RL(D) =
Q3 Q0
(6)
D3 D0
(7)
where RL(Q) and RL(D) are the relative adsorption capacity and separation degree of MIP. Q3 and Q0 and D3 and D0 denoted the adsorption capacity and separation degree of samples. 2.8.2. The Reusability of GSH-MIP. The reusability of GSHMIP was evaluated by measuring the adsorption capacities of eight desorption−adsorption cycles. The adsorption procedure
Figure 2. FTIR spectra of MIP and NMIP. 16084
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Figure 3. SEM of MIP and NMIP with magnification factors of 20 000 and 50 000.
absorption peaks (υN−H = 3420 cm−1; υ−CH3/−CH2 = 2940 cm−1; υCO = 1650 cm−1; υOC−NH2 = 1380 cm−1), confirmed synthesis of NVP−AM copolymer. The subtle shift of NMIP characteristic peaks (3420 cm−1 to 3430 cm−1, 1650 cm−1 to 1660 cm−1, respectively) indicated that alteration of functional groups arrangement resulted from self-assembly among GSH, NVP, and AM. Particularly, a small shift particularly of oxygencontaining group adsorption peaks suggested the assembly was much likely achieved by electrostatic interaction and possible hydrogen bonding. To further verify the difference between MIP and NMIP, SEM and BET analyses were also adopted to characterize surface morphology and performance parameters, and the results are shown in Figure 3, Figure 4, and Table 1. SEM results (Figure 3a was NMIP and b was MIP) showed a rougher surface. SEM results in Figure 3 qualitatively demonstrate a rough surface morphology with high porosity for both NMIP and MIP samples, but MIP showed a higher pore size. The molecular sized cavity in MIP induced by an imprinting template cannot be discerned under SEM magnification. The quantitative comparison of NMIP and MIP in the BET results shows a much higher porous volume and surface area over the entire pore size range, and a characteristic pore size (about 12 nm) both in NMIP and MIP. This might be attributed to weakened compatibility between the polymer and solvent resulting from stronger interactions between functional groups of monomers and GSH, giving rise to a larger extent of phase separation. Additionally, a much higher porous volume at pore size 2−5 nm, similar to the size of the GSH molecule, merely occurred in MIP, which might be the leftover footprint of the removed GSH template. 3.3. Optimization of the Key Fabrication Variables. 3.3.1. Influence of nAM/nNVP on Adsorption Performance of MIP. Figure 5a displayed that QGSH, QL‑cy, Qt, and D of the GSH-MIP increased with nAM/nNVP and reached a maximum at nAM/nNVP = 2.25. Adsorption sites in MIP embrace complete imprinting cavities (CIC), defected imprinting cavities, and free
Figure 4. BJH adsorption dV/dD pore volume (a) and BJH desorption dV/dD pore volume (b).
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Table 1. Performance Parameters of MIP and NMIP
increased initially with nH2O2/nNVP and then decreased, while QL‑cy increased slowly. The value of nH2O2/nNVP corresponded to the maximum. QGSH, Qt, and D were 0.0235, 0.0235, and 0.0176, respectively. When the amount of nH2O2 is much lower than that of NVP, slow polymerization and low cross-linking lead to the generation of defective imprinting cavities and a soluble polymer in network.20 Thus, the quantities of FAL increased, accompanied by the increment of pore collapse during elution and drying processes, resulting in low QGSH, QL‑cy, and Qt. With the excessive amount of nH2O2, imploded polymerization leads to dissolution of soluble polymer in MIP. Consequently, FAL and the pore collapse increased dramatically so that low QGSH and Qt were expected. The continuous increasing of QL‑cy beyond optimal was likely ascribed to disintegration of the complex and collapse of pores caused by intense heat and high cross-linking in imploded polymerization. The trend of D can also be explained by quantity variation of CIC with an increasing n/n ratio. 3.3.5. Influence of nVC/nH2O2 on Adsorption Performance of MIP. The influence of nVC/nH2O2 on QGSH, QL‑cy, Qt, and D is shown in Figure 5e. Obviously, QGSH, Qt, and D demonstrated a similar trend to that described above. The maximum QGSH, Qt, and D occurred at ratios of 3.0, 3.0, and 2.0, respectively. At a low ratio regime, oxidized GSH by H2O2 resulted in complex dissociation, FAL elevation, and phase separation difficulty, QGSH, QL‑cy, and Qt, correspondingly. Nevertheless, QGSH, Qt, and QL‑cy were compromised when stepped into a high ratio regime, which was caused by instability of the complex due to the competitive assembly between VC and GSH. 3.3.6. Influence of Polymerization Time on Adsorption Performance of MIP. Figure 5f showed the effect of polymerization time (t) on QGSH, QL‑cy, Qt, and D of MIP, suggesting the optimal t was 30 h. The cross-linking of MIP was achieved in vinyl monomers via chain polymerization and amide groups via step polymerization. When polymerization is shorter than 30 h, chain polymerization dominated, while when longer than 30 h, step polymerization overwhelmed.18 During the initial stage, a large quantity of soluble components were present due to the incompleteness of chain polymerization, resulting in a FAL increment, which leads to pore collapse and phase separation difficulty; the values of QGSH and Qt were relatively low. When polymerization lasted longer than 30 h, stepwise polymerization might have overwhelmed other mechanisms, which was unfavorable for ideal imprinting cavities;21 thus QGSH and Qt decreased correspondingly. 3.3.7. Influence of Polymerization Temperature on Adsorption Performance of MIP. Figure 5g showed the influence of polymerization temperature (T) on QGSH, QL‑cy, Qt, and D. Maximum QGSH, Qt, and D occurred at 40, 40, and 35 °C, respectively. At low T, inadequate initiation led to incomplete polymerization and cross-linking that a significant amount of soluble components and FAL preserved in MIP.21 At high temperatures, implosion of polymerization gave rise to a damaged complex structure and collapsed cavities,22 both of which resulted in lower QGSH, QL‑cy, Qt, and D. 3.4. The Performance of GSH-MIP. 3.4.1. The Long-Time Stability of GSH-MIP. The adsorption performance of longpreserved and optimally prepared samples was investigated and compared with that of a freshly prepared one. The results showed that RL(Q) and RL(D) reached 90% and 95%, respectively, suggesting appealing long-time stability. For
performance parameters material name
pore volume (cm3 g−1)
pore size (nm)
surface area (m2 g−1)
MIP NMIP
0.312740 0.046575
12.42644 11.13657
96.6201 16.3574
acting loci (FAL).18 The adsorption and selectivity of CIC is strongest while FAL is weakest. When nAM/nNVP is lower than 2.25, QGSH, QL‑cy, and Qt are relatively low, because the selfassembly between GSH and the functional monomer was incomplete due to the imbalance of hard−soft ratios in the complex, resulting in dissociation of the labile complex and numerous FAL in the network.18 During solvent evaporation, strong interactions between FAL and the porogen induced pores collapse, and phase separation is difficult in MIP.19 As AM increased, CIC content increased monotonically resulting from the complete assembly between GSH and functional monomers and reduction of collapsed pores. When the AM amount exceeded the optimum point, the hard−soft ratios became imbalanced once more, causing the dissolution of the complex and the collapse of pores. On the other hand, D reaches a maximum as well when nAM/nNVP approaches 2.25 due to the highest CIC and lowest FAL content at this point. 3.3.2. Influence of nGSH/nNVP on Adsorption Performance of MIP. The influence of nGSH/nNVP on QGSH, QL‑cy, Qt, and D is shown in Figure 5b. It was found that QGSH, Qt, and D increased with nGSH/nNVP and reached a maximum when nGSH/ nNVP was 0.5, while QL‑cy increases slowly. For QGSH, QL‑cy, and Qt, when nGSH/nNVP was relatively low, excessive NVP and AM not engaged in the assembly process would form FAL, leading to the difficulty of phase separation and collapse of pores. With the rising of nGSH/nNVP, the increasing completeness of assembly among GSH and monomers facilitates phase separation and impedes pore collapse. Thus, QGSH, QL‑cy, and Qt increased accordingly. With the exorbitant increase of nGSH/ nNVP, the complex failed to form the stable structure caused by the insufficiency of NVP and AM, and numerous pores collapsed during drying.19 So QGSH, and Qt decreased relatively. However, as QGSH and Qt increased to the maximum, QL‑cy continued increasing, which possibly due to the FAL generated by the disintegrated complex in the polymerization, and then preserved in highly viscous and cross-linked network. The trend of D can also be explained by the similar mechanism described above. 3.3.3. Influence of nNMBA/nNVP on Adsorption Performance of MIP. Figure 5c displayed the influence of nNMBA/nNVP on QGSH, QL‑cy, Qt, and D. It could be observed that QGSH, QL‑cy, Qt, and D all increased with nNMBA/nNVP and maximize at nNMBA/ nNVP = 2.5.When nNMBA/nNVP was low, MIP contained a large quantity of soluble polymer and fragile cavities due to low cross-linking of GSH-MIP. During the elution step, the soluble polymer was removed and cavities deformed, which elevated the level of FAL, the deformation, and collapse of imprinting cavities.18 However, when NMBA exceeded the optimal quantity, severe cross-linking of polymers led to a large extent of deformation and even collapse of imprinting cavities,19 which unfavorably decreased QGSH, QL‑cy, Qt, and D. 3.3.4. Influence of nH2O2/nNVP on Adsorption Performance of MIP. The influence of nH2O2/nNVP on QGSH, QL‑cy, Qt, and D is shown in Figure 5d. It was found that QGSH, Qt, and D 16086
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Figure 5. Optimization of the preparation conditions (a, the influence of nAM/nNVP on Q and D; b, the influence of nGSH/nNVP on Q and D; c, the influence of nNMBA/nNVP on Q and D; d, the influence of nH2O2/nNVP on Q and D; e, the influence of nVC/nH2O2 on Q and D; f, the influence of polymerization time (t) on Q and D; g, the influence of polymerization temperature (T) on Q and D).
3.4.2. The Reusability of GSH-MIP. The reusability of the GSH-MIP study shown in Figure 6 indicates that Q plateaus at about 70% after each adsorption−desorption cycle, though values of which were 30% lower than the first cycle, yet still
those samples, chain motion and conformation were severely arrested due to high cross-linking and the absence of a plasticized solvent. Consequently, the adsorption capacity and selectivity were preserved. 16087
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more resistant layer, lowering the adsorption rate in the late stage. Thermodynamically, the desorption rate elevated when adsorption quantities built up.23 Therefore, the plateau Q is an equilibrated adsorption rather than a saturated process. In order to further analyze the adsorption characteristics of GSH-MIP, the pseudo-first-order and pseudo-second-order kinetics models24 were adopted to fit the experimental data. Pseudo-first-order kinetics model: ln(Q e − Q t *) = ln Q e − K1t *
(6a)
Pseudo-second-order kinetics model: t* 1 1 = + t* 2 Qt Qe K 2Q e (7a) * where Qe and Qt* were the adsorption capacity at equilibrium and at any time, respectively. K1 and K2 were the rate constants of the pseudo-first-order and pseudo-second-order models, respectively. The regression of the experimental data with the above equations was performed, and the results are shown in Table 2. It could be found that the pseudo-first-order model was more suitable to describe the adsorption kinetics than the pseudosecond-order model.25 Second, the rate-limiting step was determined by the regression of experimental data with the liquid film diffusion model and particle diffusion model.26 Equation of the liquid film diffusion model:
Figure 6. Reusability of GSH-MIP.
suggesting fairly significant reusability. This discrepancy between the first cycle and the subsequent ones was possibly due to the incomplete desorption of GSH after the first cycle as well as the collapse and deformation of pores and cavities. To identify the predominant origin of Q reduction, the desorption ratio of the first cycle was investigated, showing that only 75% of it was desorbed, corroborating partial desorption was the primary contribution. High cross-linking hindered chain motion and kinetically trapped adsorbed GSH molecules. Additionally, the relatively low thermodynamic dissociation constant of adsorbed GSH in CIC also contributed to lower Q in the second cycle. 4.1. Adsorption Kinetics of GSH-MIP. Figure 7 described the relationship between adsorption capacity at equilibrium
−ln(1 − Q t /Q e) = K3t * Equation of the particle diffusion model:
(8)
1 − 3(1 − Q t /Q e)2/3 + 2(1 − Q t /Q e) = K4t (9) * * where K3 and K4 were respectively the rate constants of the liquid film diffusion model and the particle diffusion model, respectively. The simulated results were also shown in Table 2. It could be found that a better fitted liquid film diffusion model indicated that the adsorption mechanism of GSH-MIP was liquid film diffusion-controlled. Since the adsorption occurred only in the solution immersed region in MIP, the whole process was essentially controlled by liquid diffusion. 4.2. Study on Adsorption Thermodynamics. The adsorption thermodynamics of GSH-MIP are shown in Figure 8a. It could be found that Qe increased quickly with C until C reached 2.80 g L−1. Adsorption was initially achieved at CIC, followed by defect imprinting and FAL, eventually. At a low GSH concentration, almost all GSH molecules were undertaken by CIC due to the higher affinity of CIC to GSH; thus Qe increased fast. As the concentration increased, adsorption sites gradually shifted to defect imprinting cavities and FAL. The
Figure 7. Relationship between equilibrium adsorption capacity (Qe) and adsorption time (t*).
(Qe) and adsorption time (t*). It could be seen that Qe increased quickly and leveled with the extension of t* until 360 min. The fast adsorption rate at the initial stage originated from reduction of the surface energy by adsorbing free GSH and fast diffusion of GSH in a shallow layer. With the elongation of t*, the increasing occupation in the shallow layer forced upcoming GSH molecules to penetrate into a deeper,
Table 2. Regression Results of the Kinetics Experimental Data for GSH model
K
Qe(mgg−1)
R2
pseudo-first model
ln(Q e − Q t ∗) = ln Q e − K1t
0.6104
27
0.9123
pseudo-second model
t* 1 1 = + t Qt Qe K 2Q e2 * − ln(1 − Q t /Q e) = K3t *
0.0012
27
0.6835
0.6104
27
0.9123
1 − 3(1 − Q t /Q e)2/3 + 2(1‐Q t /Q e) = K4t * *
0.1563
27
0.8599
liquid diffusion model particle diffusion model
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Figure 8. (a) The adsorption thermodynamics curve of GSH-MIP. (b) Scatchard graph of GSH-MIP.
Table 3. Regression Results of Langmuir Model and Freundlich Model K
model Langmuir model Freundlich model
0.0006
C 1 C = + Qe Q t KL Qt * * ⎛1⎞ ln Q e = ln KF + ⎜ ⎟ ln C ⎝n⎠
C
=
ln Q e = ln KF +
C Qe C
0.9905
⎛1⎞ ⎜ ⎟ln C ⎝n⎠
(13)
Langmuir equation: C 1 C = + Qe Q t KL Qt (14) * * where KF was a constant indicative of the relative adsorptive capacity of the adsorbent, the constant 1/n indicated the intensity of the adsorption, and KL was the dissociation constant. As shown in Table 3, the simulated results showed that the Freundlich model was better to describe the adsorption of GSH-MIP than the Langmuir model. The Langmuir model applies to the uniformed monolayer, while the Freundlich model deals with ununiformed multilayer adsorption, which favorably described the multilayer FAL and defected imprinting cavity adsorption. However, it should be pointed out that the Freundlich model could not agree with the experimental data completely due to the existence of monolayer CIC adsorption.
(10)
where C (g L−1) was the equilibrium concentration of GSH, Qe (mg g−1) represented the equilibrium adsorption amount, Qmax (mg g−1) stood for the maximum amount of adsorption, and Kd (g L−1) was the equilibrium dissociation constant of the binding sites. Figure 8b showed the Scatchard graph of GSH-MIP when the concentration of GSH was higher than 0.96 g L−1. It showed that the Scatchard plot could be divided into two distinct sections; the two linear equations could be expressed as follows: Qe
1.0431
Finally, the Freundlich and Langmuir models26 were adopted to explore the adsorption behavior of the GSH-MIP. Freundlich equation:
Q max − Q e Kd
R2 0.9896
0.0174
desorption enhanced due to the weak affinity of MIP to GSH as well as the increasing adsorption amounts;27 the derivative of Qe versus concentration decreased until it reached equilibrium. Scatchard analysis was employed to further analyze the binding isotherms. The Scatchard equation28 could be expressed as follows: Qe
n
= −0.3413Q e + 12.692, R2 = 0.9991
(11)
= −0.7059Q e + 16.33, R2 = 0.9879
(12)
5. CONCLUSIONS By optimizing polymerization conditions (such as the molar ratio of template, functional monomer, cross-linker and initiators, the polymerization time and temperature), GSHMIP with specific adsorption selectivity and high GSH uptake was successfully synthesized in methanol and water media. The optimum condition was obtained as molar ratios of nGSH/nNVP = 0.5, nAM/nNVP = 2.25, nNMBA/nNVP = 2.5, nH2O2/nNVP = 0.01765, and nVC/n H2O2 = 2; t = 30 h; and T = 35 °C, and the sample prepared under optimal conditions showed an adsorption capacity of 27 mg g−1, and a separation degree of 2.84 when L-cy was used as interference. FTIR, SEM, and BET characterization revealed the structure of GSH-MIP with numerous recognition cavities. The performance study showed
Kd and Qmax values calculated from the slopes and intercepts of the two linear portions were 2.93 g L−1 and 37.18 mg g−1 for higher affinity sites and 1.42 g L−1 and 23.19 mg g−1 for lower affinity sites, respectively. Two imprinted binding sites generated during GSH-MIP synthesis, i.e., CIC and FAL. Due to the dissimilarity of the adsorption ability of two sites, almost all adsorption was undertaken by CIC at a low concentration of GSH, and a high Ka was obtained. As GSH concentration increased, adsorption shifted to FAL gradually. Consequently, two distinct segments were displayed on the Scatchard plot. The lower R-square between Qe/C and Qe during FAL adsorption suggested a transition stage of GSH adsorption by defected imprinting cavities. 16089
dx.doi.org/10.1021/ie501126e | Ind. Eng. Chem. Res. 2014, 53, 16082−16090
Industrial & Engineering Chemistry Research
Article
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that GSH-MIP possessed high long-time stability and reusability. The pseudo-first-order model was more suitable to describe the adsorption kinetics, and the adsorption was a liquid film diffusion-controlled step. Scatchard analysis revealed that the heterogeneous binding sites were formed in the polymers. The Langmuir and Freundlich models could not describe the adsorption behavior of GSH-MIP completely. The adsorption and separation study was applied in urine, and the adsorption rate was 90.30%, the desorption rate was 75.32%, and the purity was 58.74%.
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86 931 297 3563. Fax: +86 931 297 3661. E-mail:
[email protected]. *Tel.: +86 931 297 3563. Fax: +86 931 297 3661. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Doctorial Program Funds of the Lanzhou University of Technology (Grant No. SB01200806), the Graduate Tutor Program Funds of Gansu Provincial Department of Education (Grant No. 1001ZSB002), and the Natural Science Foundation of China (Grant No. SB01200806).
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