Synthesis of an inorganic molecularly imprinted polymer (MIP) composed of
tetraethoxysilane (TEOS) for recognize sulfonamides using a sol-gel process
have.
Preparation of Sol-Gel Molecularly Imprinted Tetraethoxysilane for Recognize Sulfonamides
Polymer
Based
on
Sung-Chuan Lee and Hui Chen* Department of Chemical and Materials Engineering, National Central University,
Taoyuan
32001,
Taiwan,
Republic
of
China.
(E-mail:
[email protected])
Abstract
Synthesis of an inorganic molecularly imprinted polymer (MIP) composed of tetraethoxysilane (TEOS) for recognize sulfonamides using a sol-gel process have been developed. The template, sulfamethazine (SMZ), was dissolved in acid aqueous solution and added into TEOS to formed the precursor solution and adjusted the pH by various ammonium hydroxide (NH4OH) and reacted for 3 days. After the reaction, MIP was obtained by calcination with various temperature processes. The preparation conditions, the various adsorption time intervals, the amount of NH4OH and the calcination temperature process were discussed. Adsorption and selectivity in different pH solutions were determined by competition experiments between the MIP template (SMZ) and the analogue (sulfamethoxazole, SMO) using High-Performance Liquid Chromatography (HPLC). The results showed that the adsorption and selectivity of the competition solution with pH=7.4 were higher than those with pH=4.6. In addition, the adsorption of SMZ decreased but the selectivity increased
while increasing the calcinations time. Besides, the selectivity of the MIP (51.3) was approximately eight times greater than the non-imprinted polymer (6.55) under the optimum preparation conditions.
Keywords: Inorganic molecularly imprinted polymer; Sulfonamide; Sol-gel; Sulfamethazine; Sulfamethoxazole; HPLC
1. Introduction Molecular recognition is a phenomenon that can be envisaged as the preferential binding of a molecule to a “receptor” with high selectivity over its close structural analogues. This concept has been translated elegantly into the technology of molecular imprinting, which allows specific recognition sites to be formed in synthetic polymers through the use of various templates (Wulff et al., 1972; Wulff, 1995; Mosbach et al., 1996). Molecular imprinting of synthetic polymers with a specific target molecule can be done if the target resembles the template or imprint molecule that is used during polymerization and then removed after polymerization. Polymerization occurs when monomers, carrying certain functional groups, interact with the template and arrange themselves around the template into a ‘frozen’ position (Haupt et al., 1998; Cormack et al., 1999). Imprinting has been achieved by three ways, non-covalent, sacrificial spacer and covalent. Non-covalent imprinting relies on the ability of the template molecule to produce one or more strong intermolecular non-covalent interactions with the functional monomers, e.g. H-bonding, electrostatic or π–π interactions. Removal of the template affords a cavity, which is complementary in size; shape and functionality to the template molecule, and which contains the recognition site used for recognize specific molecules. In covalent imprinting, template-monomer covalent bond is formed before the polymerize MIP.
The technique of molecular imprinting, which was traced back to the early 1970s, has received much attention in recent years (Whitcombe et al., 2001). Molecular imprinted polymers have been used as chromatographic stationary phase, artificial antibodies, catalysts, sensors and drug assay tools (Haupt et al., 2000; Andesson, 2000). The sol–gel process is inorganic, such as siloxane, based polymers formed by acidic or alkaline catalysed hydrolysis and condensation of a series of silane monomers. The sol–gel process produces extremely versatile polymer materials that have a wide variety of applications in analytical chemistry (Wayne et al., 2005). Their optical transparency as well as chemical inertness, rigidity and porosity makes them ideally suited for use as optical sensing devices and other specialized applications (Collinson, 1999; Cauli, 2005). Sulfonamides have been used as antibacterial agents for over 70 years. Sulfonamides are extensively used in veterinary practice for the treatment of various bacterial infections. Because of their use in food producing animals, the risk of occurrence of unwanted residues in edible products exists. The European Community has adopted a maximum residue level (MRL) of 100 mg/kg for sulfonamides in foodstuffs of animal origin (Council Regulation, 1990). Sulfamethazine (SMZ) has been shown to produce thyroid follicular tumors in rodent bioassays (Littlefield et al., 1990), and
sulfonamides can cause allergic reactions in humans (Cribb et al., 1990) and the development of antibiotic resistance is a continuing concern (Neu, 1992). Inorganic molecularly imprinted polymer utilized as artificial separator for SMZ is able to be used for the recognition and analysis of SMZ and its analogues. The main propose of this article is to prepare a series of the molecularly imprinted polymer (MIP) based on tetraethoxysilane and to investigate systemically their competition experiments of the MIP for template and analogue. In addition, various adsorption time intervals, effect of competition experiment pH, and the effect of the calcination temperature processes on MIP were also studied.
2. Experimental 2.1. Materials Tetraethoxysilane (TEOS) (Shin-Etsu Chemical Co. Ltd. Tokyo, Japan) without purification was used as received. Sulfamethazine (SMZ) (SIGMA Chemical Co. St. Louis, MO) as a template was used as received. Sulfamethoxazole (SMO) (SIGMA Chemical Co. St. Louis, MO) as a structure analogous molecule was used as received. Hydrochloric acid (Scharlau Chemie S.A. Sentmenat, Barcelona) as a menstruum of sulfonamides was used as received. Ammonium hydroxide (TEDIA Co. Inc. Fairfield, USA) as an auxiliary agent of reaction was used as received.
2.2. Preparation of Inorganic Molecularly Imprinted Polymer (MIP) The 0.002 mol of sulfamethazine was dissolved completely with 1.25 mol deionized water and 760 μl hydrochloric acid, and that the solution was stirred at room temperature, and then the 0.25 mol of tetraethoxysilane (TEOS) were added to above solution. This precursor was stirred at room temperature for 30 min, and adjusted the pH by various ammonium hydroxide and then followed reaction for 3 days. After the reaction was complete, the monoliths were ground and sieved to obtained particle of size between 53 and 88 μm. The template molecule was removed by calcination with various temperatures, and then those MIP were purged with deionized water. A
non-imprinted polymer (NIP) was prepared in parallel and under identical conditions but in the absence of the template.
2.3. HPLC analysis of MIP The MIP selectivity (α) was obtained by competition adsorption experiment. Both sulfamethazine and the structurally analogous molecule (sulfamethoxazole) were dissolved in water to produce a standard solution with a concentration of 100 ppm. 100 mg of the MIP was added in a 5 ml standard solution by stirring at room temperature. The adequate solution was injected into HPLC, and the concentration of sulfamethazine and sulfamethoxazole were recorded, from which the MIP selectivity was determined. The HPLC analysis was performed by Waters 510 HPLC pump equipped with a SFD UV/vis Detector S32109 (285 nm). Samples were analyzed on a 150mm×4.6mm 5_ Hypersil HS C18 column at room temperature with a flow rate of 1.0 ml/min (mobile phase, water : methanol : acetic acid = 69 : 28 : 3, v/v).
2.4. Thermogravimetric analysis (TGA) of MIP The thermal property was examined by a Perkin– Elmer TGA-7 apparatus (MA, USA) at a temperatures ranging from 50 to 900°C in air.
2.5. Accelerated surface area and porosimetry (ASAP) BET surface area and pore structure characteristics were determined by nitrogen adsorption - desorption isotherm used for analysis. Porosity measurements were performed with Micromeritics ASAP 2010 analyzer. All samples were degassed at 120°C for 24 h before measurement. BET surface area and pore parameters were measured with 55-point full analysis at cryogenic temperature (77.35 K). The micropore surface area and micropore pore volume were obtained by t-plot analysis with the Harkins–Jura equation.
3. Results and discussion 3.1. Preparation of the Inorganic molecularly imprinted polymer Molecular imprint polymers were prepared by adding ammonium hydroxide (NH4OH) dispersed along with sulfamethazine (SMZ) and tetraethoxysilane (TEOS) within the sol-gel mixture. The preparation conditions of the inorganic molecularly imprinted polymer (MIP) with various ammonium hydroxide (NH4OH) and non-imprinted polymer (NIP) are shown in Table 1. Sample codes MIP6, MIP7, MIP8, and NIP7 represent the pH was 6, 7, 8, and 7 respectively, after added NH4OH into the sol-gel mixture. The sol–gel process resulted a lot of hydroxyl groups produced after the hydrolysis of silicon alkoxide. These hydroxyl groups promoted the interaction (hydrogen-bond) between the imprinted molecule and inorganic polymer matrix. During the formation of MIP, the template (SMZ) was incorporated into inorganic polymer matrix, and then the imprinted cavities and specific recognition sites of inorganic molecularly imprinted polymer were formed after removed the template. In general, the rate of hydrolysis is greater than the rate of condensation in strong acidic (pH ≤ 2) conditions; however, in neutral or basic (pH ≥ 7) conditions, the rate of condensation is greater than the rate of hydrolysis. Under the weak acidic condition of a pH=4.2, the rates of hydrolysis and condensation are approximately equal (Jokinen et al., 1998; Siouffi, 2003). During our study,
ammonium hydroxide was used to control pH, and thereby control the rate of hydrolysis and condensation. The results which are shown in Table 1 indicated that the gelation time of MIP was decreased with increasing of the quantity of ammonium hydroxide. This was due to the more quantity of ammonium hydroxide were increased the pH values; hence the rate of condensation was raised to cause the gelation quickly. A decreasing gelation time implied an increasing rate of condensation; therefore, increasing pH resulted in increasing rate of condensation.
Table 1
3.2. Effect of adsorption time on MIP In this study, the adsorption of template and analogue were obtained by competition adsorption experiments. The competition experiments using HPLC were measured by various time intervals to study the effects of adsorption time on adsorption and selectivity of the MIP. The fundamental properties of MIP with various adsorption time intervals of competition experiment are shown in Table 2. In this table, the selectivity (α) and imprint factor (f) are defined as α = AdSMZ / AdSMO and f = AdSMZ×α. We expect to achieve the high adsorption and high selectivity in these MIP; hence the imprint factor is a purpose index to obtain the higher efficiency from adsorption multiply by selectivity. The results shown in Table 2 indicate that the adsorption of template (AdSMZ) was almost kept the same value during all the time
intervals. In other words, the Adsmz was reached equilibrium in a short time. Therefore, for this reason we will control the adsorption time for 30 min at the following competition experiments.
Table 2
3.3. Effect of competition experiment pH on MIP Competition experiments using HPLC were performed on solutions with different pH value to study the effects of pH on adsorption and selectivity of the MIP. The fundamental properties of the MIP with diverse pH of competition experiment are shown in Table 3. The results shown in Table 3 indicate that the AdSMZ increased with increasing pH, and the adsorption of the analogue (AdSMO) decreased with increasing pH. Therefore, the selectivity (α) and imprint factor (f) increased with increasing of the pH. The selectivity and imprint factor for the competition experiment solution with a pH=7.4 was more than two times greater than the competition experiment solution with a pH=4.6. This was due to the pH of the sol-gel solutions MIP6, MIP7, and MIP8 under condensation was 6, 7, and 8 respectively (see table 1). These pH correspond better to the competition experiment solution with a pH=7.4, and resulted in the conditions of the synthesis self-assembly and the conditions of the template adsorption being nearly the same. These increase in adsorption of the template and selectivity were partly because the conditions of the
template with pH=7.4 closely matched the conditions of shape specific recognition that occurs during synthesis self-assembly. Besides, also affecting selectivity was the level of deprotonation that occurred for the SMZ and SMO. The relative ability for a molecule to give up a proton (deprotonate) is measured by a pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base in an acid-base reaction. When the pH of a system is equal to the pKa of an acid or base compound within the system, that compound is half protonated (and half deprotonated) (Zhiyong et al., 2005). The pKa of SMO is 5.9, while the pKa of SMZ is 7.4. Under pH=7.4 the SMZ molecules will be half deprotonated while the SMO molecules will be 97% deprotonated. These SMO molecules were mostly ionized became the amine salt to influence absorption in basic condition (Xiangjun et al., 2006). Therefore, as the results which had less SMO molecules would be adsorbed and the greater selectivity and imprint factor were achieved.
Table 3
3.4. Effect of the calcination temperature processes on MIP The removal of the template by calcination under various elevated temperature processes was studied. The diagram of calcinations temperature profiles as shown in Figure 1. Calcination occurred when the monoliths were exposed to a three-stage temperature profile defined by first stage at 200°C, second stage at 300°C, and third
stage at 600°C. The characterizations of MIP removing template by various calcination temperature stages are shown in Table 4. Four calcination temperature processes were carried out on A, B, C, and D. For example, the process A, included a first stage for one hour, a second stage for one hour, and a third stage for 3 hours. The rest calcination temperature processes could deduce by analogy respectively, and all processes of third stage was fixed to 3 hours. The competition experiment results as shown in Table 4 indicate that the MIP adsorption of both the template (AdSMZ) and analogue (AdSMO) decreased with an increasing first stage time, but the selectivity (α) and imprint factor (f) increased with an increasing first stage time. This was due to the condensation reaction will carry on with the hydroxyl groups by calcination at 200°C; hence the sol-gel processes could be reaction more completely for long calcination time. The more calcination time at 200°C will easily form dense network structure of MIP; hence the absorption of template (AdSMZ) and analogue (AdSMO) were decreased with an increasing of the first stage time. On the other hand, the more specific recognition sites would be formed when the condensation reaction was allowed to continue closer to completion; hence the selectivity (α) and imprint factor (f) increased with an increasing of the first stage time in those MIP. A more detailed discussion of calcination temperature process’s effect on pore characteristics and thereby effect on adsorption and selectivity is provided in the next section.
The thermogravimetric analysis (TGA) diagrams of sulfamethazine as showing in Figure 2, the results exhibited the decomposition weight lose began at 300℃. Furthermore, in this section the preparation conditions were expected to produces more amount of specific recognition sites formed prior to the template decomposition. For this reason, the second stage temperatures were carried out on 250℃, 275℃, 300℃, 325℃, and 350℃ to obtain the optimum adsorption and selectivity. The characterizations of MIP with various second stage temperatures are shown in Table 5. The results exhibited that the selectivity and imprint factor of MIP increased with increasing the second stage temperatures until 300℃, and then the selectivity and imprint factor decreased gradually. In other words, the better selectivity and imprint factor were achieved under optimum second stage temperature, but the additional second stage temperature could decrease the efficiency, therefore, the second stage temperature was set at 300℃ for the present MIP. On the other hand, the results shown in Table 4 indicate that the best MIP selectivity and imprint factor were obtained at a first stage temperature for 1.5 hours and a second stage temperature for 1.5 hours. In addition, a comparison of the MIP (sulfamethazine imprinted) prepared under optimum conditions and NIP (non-imprinted) shows that the MIP had better result on molecular recognition. Similar results have been observed in our lab previous studies (Hung et al., 2006).
Figure 1, Figure 2, Table 4 and Table 5
3.5. Accelerated Surface Area and Porosimetry (ASAP) The BET surface area and porosity characteristics of the MIP prepared under four different calcinations processes were examined. Accelerated Surface Area and Porosimetry (ASAP) analysis of nitrogen adsorption - desorption isotherms in combination with HPLC analysis were carried out on samples MIP7 and NIP7 with various calcination temperature processes. The pore properties and characteristics of the MIP with various calcinations processes are shown in Table 6. The results indicated that the AdSMZ decreased with decreasing of the BET surface area and average pore diameter in those MIP and NIP. This was due to the more BET surface area were provided the greater quantity of absorption sites; hence the absorption of template were raised with the larger BET surface area. According to the above sections, the more calcination time of process will easily form dense network structure; hence it may cause the average pore diameter of MIP reduce gradually, therefore, the absorption of template were significantly affected by the size of average pore diameter. In addition, the selectivity (α) and imprint factor (f) increased with decrease of the micropore volume in those MIP and NIP. This was due to the molecule size of the SMZ was about 1.3 nm, therefore, the template could not enter the specific recognition sites when the pores size inside MIP were smaller than 1.3 nm,
hence the less amount of micropore (pore size ≤ 2 nm) will cause that to increase the opportunity of recognition sites bound with template. Therefore, as the results which have shown in Table 6 the greater BET surface area and average pore diameter were obtained by the higher absorption and the less micropore volume were obtained by the higher selectivity and imprint factor. In addition, further analysis of nitrogen adsorption/desorption isotherm and pore size distribution reveals more about the MIP porosity characteristics. The nitrogen adsorption-desorption isotherm and pore size distribution of the MIP7 and NIP7 are shown in Figure 3. According to the IUPAC classification (Sing et al., 1985), the nitrogen adsorption-desorption isotherm of MIP7 and NIP7 belong to Type IV isotherm. A type IV isotherm is an indication of a porous material containing micropores (≤ 2nm) and mesopores (2 to 50nm). At the low pressure end, monolayer adsorption and micropore filling occurs until the adsorption levels off as the micropores are filled. Then the mesopores continue filling by capillary condensation and once again adsorption levels off as the mesopores are filled. During desorption, as pressure is lowered, the mesopores are emptied by capillary evaporation, but when capillary condensation and capillary evaporation do not take place at the same pressure, a hysteresis loop is created. According to the IUPAC classification [21], the hysteresis loop of MIP7 and NIP7 belong to Type H1. In compared the N2 adsorption-desorption isotherm hysteresis loop of MIP7 and
NIP7, the results exhibited the hysteresis loop of NIP7 at higher pressures were raised much steeper, which was an indication of a higher degree of uniformity of pore sizes distribution. This was due to the synthesis process of NIP7 without the template in the matrix and that could keep high degree of pore size uniformity. The pore size distribution showed in Figure 3 exhibited the pore size distribution of MIP7 was smaller than NIP7. This result indicated that the smaller pore size distribution will cause that to increase the opportunity of recognition sites bound with template; hence it could be found out the smaller pore size distribution could raise the selectivity and imprint factor. Additionally, a comparison of the MIP7 and NIP7 showed that the selectivity of MIP7 (51.3) was greater than NIP7 (6.55), and the imprint factor of MIP7 (105) was greater than NIP7 (13.3). Moreover, the selectivity and imprint factor of MIP7 was approximately eight times greater than the NIP7.
Figure 3 and Table 6
4. Conclusions The experimental results show that inorganic molecularly imprinted polymers were successfully prepared during our research. The results indicated that the rate of hydrolysis or condensation was significantly affected by appropriate to adjust the pH. Moreover, controlling the pH of the competition experiment solution to match the pKa of template increased the adsorption and selectivity of the inorganic molecularly imprinted polymer. Besides, the calcination which was carried out on the processes D was obtained the more efficiency and high selectivity. In addition, the most significant results are that a comparison of the sulfamethazine imprinted MIP prepared under optimum conditions and the NIP shows that the selectivity of the molecular imprinted polymer was about eight times greater than the selectivity of the non-imprinted polymer.
Acknowledgements The authors gratefully acknowledge financial support of this research by the National Science Council, Taipei, Taiwan.
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Figure Captions
Figure 1.
The diagram of calcinations temperature profiles.
Figure 2.
The TGA diagram of sulfamethazine.
Figure 3.
N2 adsorption–desorption isotherms and pore size distribution of the MIP 7 and NIP7 under calcination process D.
Tables
Table 1 Preparation conditions and characterizations of the inorganic molecularly imprinted polymer Table 2 Fundamental properties of MIP adsorbed by various adsorption time intervals Table 3 Fundamental properties of MIP prepared by diverse pH value Table 4 Characterization of MIP removed template by various calcination processes Table 5 Characterizations of MIP with various second stage temperatures Table 6 The pore properties and characteristics of MIP with various calcination processes
Table 1 Preparation conditions and characterizations of the inorganic molecularly imprinted polymer Sample codes
TEOS (mole)
H2O (mole)
Sulfamethazine (mole)
NH4OH (mL)
pH (added NH4OH)
gelation time (sec)
MIP4
0.25
1.25
0.002
4
6
80
MIP5
0.25
1.25
0.002
5
7
60
MIP6
0.25
1.25
0.002
6
8
40
NIP5
0.25
1.25
0
5
7
60
Table 2 Fundamental properties of MIP adsorbed by various adsorption time Adsorption time (min)
AdSMZ (μmole/g)
AdSMO (μmole/g)
α
f
15
2.63
1.48
1.78
4.68
30
2.67
1.51
1.77
4.73
60
2.57
1.52
1.69
4.34
120
2.57
1.50
1.71
4.39
240
2.70
1.72
1.57
4.24
f: imprint factor (AdSMZ × α) α: selectivity (AdSMZ / AdSMO)
Table 3 Fundamental properties of MIP prepared by diverse pH value Sample codes
pH=4.6 AdSMZ
AdSMO
(μmole/g) (μmole/g)
pH=7.4 α
f
AdSMZ
AdSMO
(μmole/g) (μmole/g)
α
f
MIP4
2.42
1.52
1.59
3.85
2.96
1.32
2.23
6.60
MIP5
2.67
1.51
1.77
4.73
3.11
1.28
2.43
7.53
MIP6
2.56
1.53
1.67
4.28
3.21
1.43
2.23
7.16
NIP5
2.30
1.49
1.54
3.54
2.98
1.50
1.98
5.90
f: imprint factor (AdSMZ × α) α: selectivity (AdSMZ / AdSMO)
Table 4 Characterization of MIP removed template by various calcination profiles Sample codes MIP4A MIP5A MIP6A NIP5A MIP4B MIP5B MIP6B NIP5B MIP4C MIP5C MIP6C NIP5C MIP4D MIP5D MIP6D NIP5D
clacination process
A
B
C
D
200℃
300℃
(h)
(h)
1
1
1.5
1.5
AdSMO AdSMZ (μmol/g) (μmol/g)
α
f
1
2.96 3.11 3.21 2.98
1.32 1.28 1.43 1.50
2.23 2.43 2.23 1.98
6.60 7.53 7.16 5.90
1.5
2.47 2.53 2.12 2.35
0.43 0.10 0.27 0.40
5.74 25.3 7.85 5.88
14.2 64.0 16.6 13.8
1
2.91 2.39 2.22 2.23
0.25 0.06 0.15 0.36
11.6 39.8 14.8 6.19
33.8 95.1 32.9 13.8
1.5
2.29 2.05 2.26 2.03
0.06 0.04 0.05 0.31
38.2 51.3 45.2 6.55
87.5 105 102 13.3
Table 5 Characterizations of MIP with various calcination temperatures under second stage duration Sample codes MIP4 MIP5 MIP6 NIP5 MIP4 MIP5 MIP6 NIP5 MIP4 MIP5 MIP6 NIP5 MIP4 MIP5 MIP6 NIP5 MIP4 MIP5 MIP6 NIP5
second stage temperatures (°C)
AdSMZ (μmol/g)
AdSMO (μmol/g)
α
f
250
3.31 3.33 3.35 3.47
2.21 1.91 2.35 2.54
1.50 1.74 1.43 1.37
4.96 5.81 4.78 4.74
275
3.15 3.30 3.54 3.43
1.46 1.48 1.78 1.76
2.16 2.23 1.99 1.95
6.80 7.36 7.04 6.68
300
2.96 3.11 3.21 2.98
1.32 1.28 1.43 1.5
2.24 2.43 2.24 1.99
6.64 7.56 7.21 5.92
3.17 3.16 3.21 3.34
1.57 1.42 1.51 1.77
2.02 2.23 2.13 1.89
6.40 7.03 6.82 6.30
3.40 3.31 3.15 3.18
2.16 1.83 2.24 2.29
1.57 1.81 1.41 1.39
5.35 5.99 4.43 4.42
325
350
800
Third stage calcination
700
o
Temperature ( C)
600 500
Second stage calcination
400
First stage calcination
300 200 100 0 0
1
2
3
4
5
Time (h)
Figure 1. The diagram of calcinations temperature profiles.
6
120 100
Weight (%)
80 60 40 20 0 0
200
400
600
800
o
Temperature (C )
Figure 2. The TGA diagrams of sulfamethazine
1000
Table 6 The pore properties and characteristics of MIP with various calcinations profiles. f
BET surface area (m2/g)
Average Pore Diameter (nm)
Micropore Volume (cm3/g)
2.23 1.98
6.60 5.90
573.8 503.7
8.72 11.0
0.011 0.017
2.53
25.3
64.0
539.4
8.49
0.009
NIP5B
2.35
5.88
13.8
497.1
10.4
0.015
MIP5C NIP5C
2.39 2.23
39.8 6.19
95.1 13.8
531.7 490.0
8.49 10.7
0.008 0.013
MIP5D
2.05
51.3
105
528.9
8.48
0.006
NIP5D
2.03
6.55
13.3
449.7
10.5
0.009
Sample code.
AdSMZ (μmole/g)
α
MIP5A NIP5A
2.96 2.98
MIP5B
Pore Volume (cm3/g)
1000 900
Adsorb Volume (cm3/g)
800 700 600 500
0.5 0.4 0.3 0.2 0.1 0 0
10 20 30 40 Pore Diameter (nm)
MIP5D
400
NIP5D
300 200 100 0 0
0.2
0.4 0.6 0.8 Relative Pressure (P/Po)
1
1.2
Figure 3. N2 adsorption–desorption isotherms and pore size distribution of the MIP 5D and NIP5D.
