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Fabrication and characterization of thin film molecularly imprinted polymers

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Sara E Campbell
Abstract:
Molecularly imprinted polymers are artificial networks designed to have many selective binding sites that are suitable for various applications. In order to develop more of these systems, the fabrication and characterization of imprinted polymers has become a significant area of study in recent years. Though there are several ways to fabricate imprinted polymers, the focus of this study was on a modified phase inversion procedure. A polymer and target of interest were dissolved together in a casting solvent, and then thin film imprinted polymer systems were created via spin coating. Main characterization methods included infrared spectroscopy, atomic force microscopy, and nanoindentation. Before nanoindentation could be used to study the desired imprinted polymer systems, a protocol for testing was determined. Two different series of templates were examined in this study of imprinted polymers. The first series of templates investigated were the carbohydrates fructose, glucose, and maltose. Imprinted polymer films were made using poly(4-vinylphenol) as the polymer host. The surfaces of these carbohydrate imprinted films were examined, and the successful removal of the template was observed. Some advances were made toward reinserting the carbohydrate template molecules, though the system could be optimized further. Quantification of reinsertion was possible through a chemical assay. Additionally, the solvent N,N-dimethylformamide was used for the fabrication of these imprinted polymers and was found to be an extremely good porogen. The second series of templates investigated were aromatic compounds, primarily methyl 4-nitrobenzoate. Imprinted polymer films were made using both Saran® F-310 and poly(4-vinylphenol) as the polymer hosts. Removal of the template was successful, but some difficulties were found in the reinsertion process. Rather than focus on the reinsertion of methyl 4-nitrobenzoate, a study was made of the surface morphologies of aromatic templates with differing functionalities. Computational studies were also performed with both template systems to study the polymer/template interactions. Hydrogen bonding interactions were found for both template systems.

Table of Contents

Abstract….….….……………………………………………………………………… ii

Acknowledgements….….…………………………………………………………….. iv

Table of Contents……….….………………………………………………………..... vi

List of Tables………………………………………………………………………….. viii

List of Figures….….………………………………………………………………….. ix

Chapter One: Introduction to Molecularly Imprinted Polymers Introduction…………………………………………………………………….. 1 Fabrication……………………………………………………………………... 4 Characterization………………………………………………………………... 7

Chapter Two: Nanoindentation of Thin Film Molecularly Imprinted Polymers Introduction…………………………………………………………………….. 11 Experimental Details…………………………………………………………… 16 Results …………………………………………………………………………. 18 Conclusions & Future Work…………………………………………………… 30

Chapter Three: Carbohydrate Templated Molecularly Imprinted Polymers Introduction…………………………………………………………………….. 32 Experimental Details Chemicals………………………………………………………………. 35 Instrumentation………………………………………………………… 36 Sample Fabrication…………………………………………………….. 37 Removal/Reinsertion Protocol…………………………………………. 39 Results Poly(4-vinylphenol) and Fructose, Ethanol/Water Solvent……………. 40 Poly(4-vinylphenol) and Fructose, DMF Solvent……………………… 58 Poly(4-vinylphenol) and Glucose, Ethanol/Water Solvent…………….. 84 Poly(4-vinylphenol) and Glucose, DMF Solvent……………………… 90 Poly(4-vinylphenol) and Maltose, DMF Solvent……………………….101 Conclusions & Future Work…………………………………………………… 110

Chapter Four: Aromatic Templated Molecularly Imprinted Polymers Introduction…………………………………………………………………….. 112 Experimental Details…………………………………………………………… 116 Chemicals………………………………………………………………. 116 Instrumentation………………………………………………………… 117 Sample Fabrication…………………………………………………….. 117 Removal/Reinsertion Protocol…………………………………………. 118

vii

Results Saran ® F-310 and Methyl 4-nitrobenzoate…………………………….. 119 PVP and Methyl 4-nitrobenzoate……………………………………… 139 Surface Studies of Other Template Molecules………………………… 161 Conclusions & Future Work…………………………………………………… 196

Chapter Five: Computation Studies of Molecularly Imprinted Polymers Introduction…………………………………………………………………….. 199 Computational Details…………………………………………………………. 203 Results PVP and Fructose………………………………………………………. 204 PVP and Methyl 4-nitrobenzoate............................................................ 218 Conclusion & Future Work.................................................................................. 234

Appendix One: Load Functions Used for Nanoindentation........................................... 236

Appendix Two: Geometric Coordinates for Geometry Optimization Calculations....... 242

Appendix Three: Geometric Coordinates for Energy Calculations............................... 283

References....................................................................................................................... 292

viii

List of Tables

Chapter Two Table 2.1: Specifications of the transducer……………………………………………. 16 Table 2.2: Summary of modulus and hardness data for fructose templated films…….. 22 Table 2.3: Summary of modulus and hardness data for load functions with and without holding segment....……………………………………………………………... 30

Chapter Three Table 3.1: Summary of modulus and hardness data for glucose templated films…….. 99

Chapter Four Table 4.1: Summary of film thicknesses for Saran ® F-310 polymer………………….. 120

Chapter Five Table 5.1: Total energy for optimized geometries of vinyl phenol monomer, dimer, trimer, and fructose.............................................................................................. 206 Table 5.2: Total energy for optimized geometries of various combinations of vinyl phenol and fructose.............................................................................................. 207 Table 5.3: Total energy for optimized geometries of various combinations of vinyl phenol and methyl 4-nitrobenzoate..................................................................... 220 Table 5.4: Total energy for fixed geometries of various combinations of vinyl phenol and methyl 4-nitrobenzoate........................................................................................ 230

ix List of Figures

Chapter One Figure 1.1: Sample imprinting process………………………………………………... 4 Figure 1.2: Sample phase inversion process…...…………………………………….... 5

Chapter Two Figure 2.1: Sample force/depth curve............................................................................. 13 Figure 2.2: Sample multi-load function.......................................................................... 14 Figure 2.3: Sample force/depth curve for Saran ® F-310 control films........................... 19 Figure 2.4: Chart of hardness values for Saran ® F-310 films......................................... 20 Figure 2.5: Representation of different testing locations................................................ 22 Figure 2.6: Image of PVP control film after nanoindentation........................................ 23 Figure 2.7: Charts of time- and load- dependent data for PVP control film...................24 Figure 2.8: Representation of film thickness.................................................................. 25 Figure 2.9: Images of methyl 4-nitrobenzoate templated PVP after nanoindentation... 27 Figure 2.10: Chart of hardness for methyl 4-nitrobenzoate templated PVP.................. 28 Figure 2.11: Comparison of hardness values for load functions with and without holding segment................................................................................................................ 29

Chapter Three Figure 3.1: Structure for simple carbohydrates.............................................................. 33 Figure 3.2: Structures for polymers with hydrogen bonding functionality.................... 35 Figure 3.3: IR spectra for control and 10% fructose templated PVP............................. 42 Figure 3.4: AFM micrographs for control and 10% fructose templated PVP from 90/10 ethanol/water casting solvent............................................................................... 43 Figure 3.5: AFM micrographs for control and 10% fructose templated PVP from 80/20 ethanol/water casting solvent............................................................................... 45 Figure 3.6: AFM micrographs for control PVP films from both ethanol/water casting solvents................................................................................................................ 45 Figure 3.7: AFM micrographs for mixing time study for 10% fructose templated PVP ............................................................................................................................. 48 Figure 3.8: AFM micrographs for sonication time study for 10% fructose templated PVP...................................................................................................................... 50 Figure 3.9: AFM micrographs for PVP templated with varying concentrations of fructose ............................................................................................................................. 51 Figure 3.10: IR spectra for heated 10% fructose templated PVP................................... 54 Figure 3.11: AFM micrographs for heated 10% fructose templated PVP...................... 55 Figure 3.12: IR spectra for 10% fructose templated PVP reinserted via water.............. 56 Figure 3.13: AFM micrographs for 10% fructose templated PVP with template removed and reinserted via water....................................................................................... 57 Figure 3.14: IR spectra for varying casting solvents...................................................... 59 Figure 3.15: IR spectra for control and 10% fructose templated PVP on KCl substrate ..............................................................................................................................59 Figure 3.16: AFM micrographs comparing control and 10% fructose templated PVP cast from ethanol/water and DMF.............................................................................. 61

x Figure 3.17: IR spectra for varying concentrations of fructose in templated films........ 63 Figure 3.18: AFM micrographs for varying concentrations of fructose in templated films..................................................................................................................... 64 Figure 3.19: AFM micrographs for varying casting speeds for 10% fructose templated films..................................................................................................................... 66 Figure 3.20: Charts comparing film thickness and weights for various casting speeds ............................................................................................................................. 67 Figure 3.21: IR spectra for 10% fructose templated PVP as-produced and with template removed................................................................................................................69 Figure 3.22: IR spectra comparing removal times for 10% fructose templated films.... 69 Figure 3.23: IR spectra for 10% fructose templated PVP reinserted via water.............. 70 Figure 3.24: IR spectra comparing reinsertion times for 10% fructose templated PVP ..............................................................................................................................71 Figure 3.25: IR spectra comparing reinsertion times for control PVP............................72 Figure 3.26: Set up for suspension of sample film during reinsertion............................ 73 Figure 3.27: IR spectra comparing reinsertion via diffusion or moving reinsertion solution................................................................................................................. 74 Figure 3.28: IR spectra comparing reinsertion via water or 4% acetic acid reinsertion solution................................................................................................................. 75 Figure 3.29: IR spectra for reinsertion via 4% acetic acid reinsertion solution.............. 75 Figure 3.30: IR spectra for control PVP immersed in 4% acetic acid reinsertion solution ............................................................................................................................. 76 Figure 3.31: IR spectra comparing reinsertion times in a 4% acetic acid reinsertion solution................................................................................................................. 77 Figure 3.32: IR spectra comparing as-produced 10% fructose templated PVP with rinsed samples................................................................................................................. 79 Figure 3.33: AFM micrographs comparing as-produced 10% fructose templated PVP with rinsed samples.............................................................................................. 79 Figure 3.34: Calibration curve for fructose assay........................................................... 81 Figure 3.35: Two calibration curves for fructose assay.................................................. 81 Figure 3.36: Amounts of fructose in samples with varying concentration..................... 83 Figure 3.37: IR spectra for 10% glucose templated and control PVP............................ 86 Figure 3.38: IR spectra comparing glucose and fructose templated PVP, ethanol/water solvent ................................................................................................................. 86 Figure 3.39: AFM micrographs for 10% glucose templated PVP, ethanol/water solvent ............................................................................................................................. 87 Figure 3.40: IR spectra for 10% glucose templated PVP reinserted via water............... 89 Figure 3.41: IR spectra comparing glucose and fructose templated PVP, DMF solvent ............................................................................................................................. 91 Figure 3.42: AFM micrographs for 10% glucose templated PVP, DMF solvent........... 92 Figure 3.43: IR spectra showing removal of glucose template.......................................93 Figure 3.44: AFM micrographs for 10% glucose templated PVP with template removed ............................................................................................................................. 93 Figure 3.45: IR spectra for 10% glucose templated PVP reinserted via water............... 96 Figure 3.46: IR spectra for reinsertion into control PVP................................................ 96 Figure 3.47: IR spectra for reinsertion via 4% DMF solution........................................ 97

xi Figure 3.48: IR spectra for reinsertion via 4% acetic acid..............................................98 Figure 3.49: IR spectra comparing fructose, glucose, and maltose templated PVP....... 102 Figure 3.50: AFM micrographs for 10% maltose templated PVP.................................. 103 Figure 3.51: AFM micrographs for 10% maltose templated PVP made at various casting speeds................................................................................................................... 104 Figure 3.52: IR spectra for 10% maltose templated PVP with template removed......... 106 Figure 3.53: AFM micrograph comparing 10% maltose templated PVP with removed sample.................................................................................................................. 106 Figure 3.54: IR spectra for 10% maltose templated PVP reinserted via water.............. 107 Figure 3.55: AFM micrographs of reinserted 10% maltose templated PVP.................. 108 Figure 3.56: IR spectra for 10% fructose templated PVP reinserted with maltose via water..................................................................................................................... 109

Chapter Four Figure 4.1: Structures for trinitrotoluene and methyl 4-nitrobenzoate........................... 123 Figure 4.2: Components of Saran ® F-310 polymer........................................................ 124 Figure 4.3: Structures for some polymers with hydrogen bonding or aromatic functionality......................................................................................................... 130 Figure 4.4: Chart of Saran ® F-310 film thicknesses based on weight percent............... 130 Figure 4.5: Chart of Saran ® F-310 film thickness based on casting speeds................... 132 Figure 4.6: AFM micrographs for 10% methyl 4-nitrobenzoate templated Saran ® F-310 at different casting speeds.................................................................................... 123 Figure 4.7: AFM micrographs for control and 10% methyl 4-nitrobenzoate templated Saran ® F-310........................................................................................................ 124 Figure 4.8: AFM micrographs of 10% methyl 4-nitrobenzoate templated Saran ® F-310 ............................................................................................................................. 126 Figure 4.9: IR spectra for 10% methyl 4-nitrobenzoate templated Saran ® F-310.......... 128 Figure 4.10: Literature IR spectrum for methyl 4-nitrobenzoate................................... 128 Figure 4.11: AFM micrograph for Saran ® F-310 control film soaked in methanol....... 130 Figure 4.12: IR spectra for control Saran ® F-310 as-produced and soaked in methanol ............................................................................................................................. 130 Figure 4.13: IR spectra for 10% methyl 4-nitrobenzoate templated Saran ® F-310 as produced and with template removed.................................................................. 131 Figure 4.14: AFM micrographs for 10% methyl 4-nitrobenzoate templated Saran ® F-310 after soaking in methanol..................................................................................... 132 Figure 4.15: IR spectra for 10% methyl 4-nitrobenzoate templated Saran ® F-310 reinserted via chloroform..................................................................................... 133 Figure 4.16: IR spectra for 10% methyl 4-nitrobenzoate templated Saran ® F-310 reinserted via methanol/acetone/toluene mixture................................................ 135 Figure 4.17: IR spectra for 10% methyl 4-nitrobenzoate templated Saran ® F-310 reinserted via methanol/acetonitrile mixture....................................................... 135 Figure 4.18: IR spectra for two reinsertion cycles using methanol/acetonitrile mixture ............................................................................................................................. 137 Figure 4.19: AFM micrograph for 10% methyl 4-nitrobenzoate templated Saran ® F-310 reinserted via methanol/acetonitrile.....................................................................137 Figure 4.20: AFM micrographs for 10% methyl 4-nitrobenzoate templated PVP......... 139

xii Figure 4.21: AFM micrographs for multiple 10% methyl 4-nitrobenzoate templated PVP films..................................................................................................................... 140 Figure 4.22: AFM micrographs for multiple control PVP films.................................... 141 Figure 4.23: AFM micrographs for control films made from two different PVPs......... 142 Figure 4.24: AFM micrographs for control PVP cast from MEK.................................. 143 Figure 4.25: IR spectra for control and methyl 4-nitrobenzoate templated PVP cast from MEK..................................................................................................................... 144 Figure 4.26: AFM micrographs for 10% methyl 4-nitrobenzoate templated PVP cast from MEK............................................................................................................ 145 Figure 4.27: AFM micrographs for 5% methyl 4-nitrobenzoate templated PVP........... 146 Figure 4.28: IR spectra for 10% methyl 4-nitrobenzoate templated PVP with template removed................................................................................................................148 Figure 4.29: IR spectra for 10% methyl 4-nitrobenzoate templated PVP reinserted via acetonitrile............................................................................................................150 Figure 4.30: IR spectra for 10% methyl 4-nitrobenzoate templated PVP reinserted via toluene ................................................................................................................. 150 Figure 4.31: IR spectra for 3 reinsertions via toluene with chloroform rinse................. 152 Figure 4.32: IR spectra comparing 10% methyl 4-nitrobenzoate templated PVP cast from different solvents......................................................................................... 155 Figure 4.33: AFM micrographs for control and 10% methyl 4-nitrobenzoate templated PVP cast from DMF............................................................................................ 156 Figure 4.34: AFM micrographs for 10% methyl 4-nitrobenzoate templated PVP......... 156 Figure 4.35: IR spectra for 10% methyl 4-nitrobenzoate templated PVP with template removed................................................................................................................159 Figure 4.36: AFM micrographs for 10% methyl 4-nitrobenzoate templated PVP with template removed................................................................................................. 159 Figure 4.37: IR spectra for 10% methyl 4-nitrobenzoate templated PVP rinsed with water..................................................................................................................... 160 Figure 4.38: Template molecules for surface morphology study................................... 162 Figure 4.39: IR spectra for 10% methyl 2-chloro-4-nitrobenzoate templated PVP....... 164 Figure 4.40: Literature IR spectrum for methyl 2-chloro-4-nitrobenzoate.................... 164 Figure 4.41: AFM micrographs for 10% methyl 2-chloro-4-nitrobenzoate templated PVP ............................................................................................................................. 166 Figure 4.42: AFM micrographs comparing methyl 4-nitrobenzoate and methyl 2-chloro- 4-nitrobenzoate templated PVP........................................................................... 166 Figure 4.43: IR spectra comparing methyl 4-nitrobenzoate and methyl 2-chloro-4- nitrobenzoate templated PVP............................................................................... 167 Figure 4.44: AFM micrographs for 5% methyl 5-chloro-2-nitrobenzoate templated PVP ............................................................................................................................. 168 Figure 4.45: IR spectra for 10% methyl 3-nitrobenzoate templated PVP...................... 170 Figure 4.46: Literature IR spectrum for methyl 3-nitrobenzoate................................... 170 Figure 4.47: AFM micrographs for 10% methyl 3-nitrobenzoate templated PVP......... 171 Figure 4.48: AFM micrograph for 10% methyl 3-nitrobenzoate templated PVP with template removed................................................................................................. 173 Figure 4.49: IR spectra for 10% 4-nitrophenol templated PVP...................................... 175 Figure 4.50: Literature IR spectrum for 4-nitrophenol................................................... 175

xiii

Figure 4.51: AFM micrographs for 10% 4-nitrophenol templated PVP........................ 176 Figure 4.52: AFM micrograph for 10% 4-nitrophenol templated PVP with template removed................................................................................................................177 Figure 4.53: IR spectra for 10% benzoic acid templated PVP....................................... 179 Figure 4.54: Literature IR spectrum for benzoic acid..................................................... 179 Figure 4.55: AFM micrographs for 10% benzoic acid templated PVP.......................... 180 Figure 4.56: IR spectra for 10% biphenyl templated PVP............................................. 182 Figure 4.57: Literature IR spectrum for biphenyl........................................................... 182 Figure 4.58: AFM micrographs for 10% biphenyl templated PVP................................ 183 Figure 4.59: AFM micrograph for 10% biphenyl templated PVP with template removed ..............................................................................................................................184 Figure 4.60: IR spectrum for 10% methyl caproate templated PVP.............................. 186 Figure 4.61: Literature spectrum for methyl caproate.................................................... 186 Figure 4.62: AFM micrographs for 10% methyl caproate templated PVP.................... 187 Figure 4.63: IR spectra for 10% adipic acid templated PVP.......................................... 190 Figure 4.64: Literature IR spectrum for adipic acid....................................................... 190 Figure 4.65: AFM micrographs for 10% adipic acid templated PVP............................. 191 Figure 4.66: AFM micrograph for 10% adipic acid templated PVP with template removed................................................................................................................192 Figure 4.67: High magnification AFM micrographs comparing aromatic templated PVP films..................................................................................................................... 194 Figure 4.68: High magnification AFM micrographs for 10% 4-nitrophenol templated PVP...................................................................................................................... 195 Figure 4.69: High magnification AFM micrographs comparing non-aromatic templated PVP films............................................................................................................. 196

Chapter Five Figure 5.1: Optimized geometries for vinyl phenol monomer and dimer...................... 205 Figure 5.2: Optimized geometry for fructose................................................................. 205 Figure 5.3: Optimized geometries for fructose/vinyl phenol monomer complexes....... 208 Figure 5.4: Optimized geometries for fructose/two vinyl phenol monomer complexes ............................................................................................................................. 210 Figure 5.5: Optimized geometries for fructose/vinyl phenol dimer complexes............. 213 Figure 5.6: Optimized geometry for fructose/three vinyl phenol monomer complex.... 215 Figure 5.7: Optimized geometry for fructose/vinyl phenol trimer complex...................216 Figure 5.8: Optimized geometry for fructose/two vinyl phenol dimer complex............ 216 Figure 5.9: Optimized geometry for methyl 4-nitrobenzoate......................................... 219 Figure 5.10: Optimized geometries for two methyl 4-nitrobenzoate complexes........... 222 Figure 5.11: Optimized geometries for methyl 4-nitrobenzoate/vinyl phenol monomer complexes............................................................................................................ 223 Figure 5.12: Optimized geometry for methyl 4-nitrobenzoate/two vinyl phenol monomer complex................................................................................................................ 224 Figure 5.13: Optimized geometries for methyl 4-nitrobenzoate/vinyl phenol dimer complexes............................................................................................................ 226 Figure 5.14: Optimized geometry for methyl 4-nitrobenzoate/two vinyl phenol dimer complex................................................................................................................ 228

xiv Figure 5.15: Geometries for two methyl 4-nitrobenzoate molecules in the same orientation............................................................................................................ 231 Figure 5.16: Geometries for two methyl 4-nitrobenzoate molecules in the opposite orientation............................................................................................................ 233

Appendix Figure A.1: Load function used for air indentation and calibration of the instrument... 239 Figure A.2: Load function with three cycles at the same maximum load...................... 240 Figure A.3: Load function with four cycles at the same maximum load........................241 Figure A.4: Load function with four cycles with increasing maximum load................. 242 Figure A.5: Load function with nineteen cycles with increasing maximum load.......... 243 Figure A.6: Load function with nineteen cycles with increasing maximum load and holding segments................................................................................................. 244

1

Chapter One: Introduction to Molecularly Imprinted Polymers

Introduction In recent times, molecularly imprinted polymers have attracted considerable interest. These polymers are designed and fabricated in such a way that they possess recognition sites selective to a specific target, leading to many applications as biological mimics, in separation science, transport of specific substances, catalysis, directed synthesis, or even for use as sensor components. 1-5 Imprinted polymer systems exhibit a variety of properties that make them such a popular area of study, including a high degree of selectivity, inexpensive fabrication, resistance to high pressure and temperature, tolerance to different solvents, and an abundance of binding sites. 6-9

The first step toward the use of molecularly imprinted polymers is to design the system. The functional monomer/s must be chosen so that it/they interact/s with the target or template molecule to form a network in solution. 3 The monomers are then polymerized, effectively locking this network into place and creating the recognition sites for the target. Once the polymer network is locked in place, the target can be removed and reintroduced any number of times. There are several methods of interaction between the monomer/polymer and template that are possible to create the polymer/template network. The most obvious method is to covalently link the monomers with the template molecules before polymerization, which gives very specific binding/rebinding. Other options for polymer/template interaction and networking include non-covalent bonds, such as hydrogen bonding, π – π interaction, ionic attraction, and even van der Waals interactions. 2, 4, 10, 11

2

The first type of interaction, covalent bonding, seems like it would be the best option. Once the monomer/template complex has formed and the monomer has been polymerized, the templated sites are very specific to the template molecule because of the covalent interaction. The hard part of the process is the removal and reinsertion of the template molecule. The covalent bond needs to be cleaved and then re-formed over and over in order for the imprinted polymer to work in its intended fashion. This can lead to harsh solvent conditions such as strong acids or bases in order to cleave covalent bonds. A study by Whitcombe et al investigated a polymer imprinted with cholesterol. This was accomplished by creating an ester linkage through the hydroxyl groups on the monomer and the template. 12 In this case, hydrogen bonding between cholesterol and the MIP was shown to be selective enough for rebinding. Non-covalent interactions are the most commonly exploited interactions in the literature, though they have their own difficulties. They are less specific, since the bonds holding together the network of polymer and template are weaker. Other molecules can sometimes be more easily substituted for the template during the rebinding process. This does, however, mean that it is easier to remove and reinsert the template molecule under much less stringent conditions. The different types of interactions (hydrogen bonding, π – π interaction, ionic bonding, and van der Waals interaction) must be selected carefully when choosing the monomers and the templates to give the best overall opportunity for network formation. Solvent effects on these interactions must also be considered, particularly if using a polar solvent. Though it may seem that there are too many factors to make non-covalently bound MIPs specific, it is encouraging to remember that enzymes, antibodies, and other biological receptors rely on non-covalent interactions to

3

perform their duties. 4 An example of hydrogen bonding as the main networking interaction is found in the study of dimethoate by Yongqin Lv et al. Dimethoate has a carbonyl as well as two phosphoro-ester groups available to complement the functional monomer. Several functional monomers were screened, but methyl methacrylate, which has an ester functional group available for hydrogen bonding, was determined to make the best imprinted polymer. 13 The principle of π – π interaction was demonstrated by Chen et al. who imprinted a co-polymer of 4-vinylpyridine and ethyleneglycol dimethacrylate with the herbicide 2,4-dichlorophenoxyacetid acid (2,4-D). A according to molecular modeling calculations, interaction of the pyridine group in the polymer and the aromatic ring in the template contributed to the binding energy of the imprinted polymer. 10 In a different study involving 2,4-D, ionic interactions and a hydrophobic effect played a large role in the templating process. 14

One unique approach is the “semi-covalent” imprinting technique. The polymers are made with a covalent bond between the target and the monomer, but only non- covalent interactions are involved when the template rebinds. 1 Caro et al used this approach when investigating the extraction of 4-nitrophenol from water. They compared non-covalently 4-nitrophenol imprinted polymers to semi-covalently 4-nitrophenol imprinted polymers and determined that the non-covalent MIPs were more selective, but the semi-covalent polymer showed slightly higher recoveries of the target molecule. 1

Fabrication The standard method for creating imprinted polymers can be seen in Figure 1.1. Monomers and template molecules are dissolved together and allowed to mix to form the

4

monomer/template complex. After a length of time, the monomers are polymerized to form the imprinted polymer. There are several ways to do this, but typically, monomers are polymerized in solution and the imprinted polymer is precipitated either during this polymerization process or after. Depending upon the desired application, the polymer can then be ground or milled until particles of a certain size are reached, normally in the micrometer range, in order to give good surface area. 2, 15, 16 Surface area is very important to imprinted polymers, because it gives more room for the template to diffuse into and out of the porous network. Another option for creating more surface area is to create thin- film surfaces, which are ideal for use in sensor components. Typically this is only possible when the monomer is UV curable, and can be polymerized in situ. 17 Otherwise, the milled particles can be used to make a film. These particle films are less stable than those created in situ because the particles have some mobility. Another way of fabricating MIPs is by using a method called phase inversion, shown in Figure 1.2. A polymer is used rather than a monomer, cutting out the costly and time consuming polymerization step. The polymer and the template are dissolved in a common solvent and allowed time to mix and/or network based on non-covalent interactions. 4 Once the network has had sufficient time to form, a second solvent, typically water or a combination of water and the original solvent, is added to the selection self-assembly polymerization extraction Figure 1.1: Sample imprinting process

5

mixture. This second solvent, also called a “non”solvent has poor solvation properties for the polymer/template complex, and so the MIP is precipitated out of the solvent mixture. 9, 18, 19 As with the polymerization method, this commonly leaves precipitated polymer in a bulk form, though thin membranes can be made if the original casting solution is spread thin before the addition of the poor solvent. 4

A third fabrication method makes use of parts of the phase inversion method but produces the imprinted polymers as thin films rather than in bulk form. Only one solvent is used, and it is chosen to dissolve both the polymer and the template. The chosen solvent must also be volatile. The solution, after allowing time for polymer/template mixing/networking, is deposited onto a substrate and then subjected to spinning at very high speeds, called spin casting, in order to evaporate the solvent. 2 The spinning of the sample serves two main purposes. Firstly, it thins the material to a uniform thickness. Then, after the material is evenly dispersed on the substrate, the spinning causes the solvent to evaporate. Once the solvent has completely evaporated, the imprinted polymer

6

is dry and in the form of a thin film upon the chosen substrate. 20 The thin film has a large amount of surface area, which is desirable in imprinted polymers. This third method employs the pre-polymerized polymers used in phase inversion, cutting out the costly and time consuming polymerization step, while also yielding the MIP as a thin film, typically not possible with the phase inversion method. If there is concern about residual solvent, the films can be heated (below the T g of the polymer) to drive off any solvent molecules remaining. Many times, the target of interest is a dangerous or toxic molecule, or very expensive. In those cases, it is important to go through a proof-of-concept period, using a similarly structured molecule to help investigate the ideal parameters for MIP fabrication before the true template of interest is investigated. By using a similar molecule, the system created will be close to what is needed in the end-stage product. In certain cases, a model or “dummy” molecule (similar in structure but without the toxicity) can be used for the templating stages, but the actual target molecule will bind just as easily to the polymer. Because of this dummy effect, it is necessary to test the imprinted polymer against a range of similar molecules to check that the binding or rebinding is selective to the target molecule. For example, Zhang et al studied the binding of phenoxyacetic herbicides on a vinyl pyridine based polymer that had been templated with phenoxyacetic acid as the dummy template. 21

Characterization Once the films have been made, they need to be characterized, and removal/reinsertion of the template molecule needs to be studied. The methods of

7

characterization are as varied as the polymers. When the standard fabrication method involving monomers is used, the polymers themselves must be studied in order to determine molecular weights and polydispersity index. With phase inversion or spin coating, these polymer properties are available from the manufacturer, and the focus can remain on characterizing the imprinted polymer. Removal and reinsertion can be tested in a variety of ways, including fluorescence, IR spectroscopy, or even by chemical assay. For thin film imprinted polymers, an important property to measure is the thickness of the film. This can be easily accomplished using a step-profilometer. The profilometer measures the height difference between the film and the substrate. Though this is a fairly accurate method, there can be variations in the films, so several should be tested for average values. Another important characterization tool is infrared (IR) spectroscopy, a useful way to study thin films. Spectra of control films (i.e., films cast with just polymer and no template) are compared to spectra of templated films. If peaks are seen the in templated films that are not present in the control film, they are due to the presence of the template. 22 Typically, these peaks can be explained by the structure of the template and specific bends or stretches associated only with the template. Comparison of the templated polymer film with a literature spectrum for just the template usually shows an alignment of these peaks. Upon removal of the template, these characteristic IR peaks will disappear from the spectrum, and upon reinsertion, the peaks will reappear. 22 This disappearance and reappearance of template characteristic peaks shows that this is a good qualitative test for removal and reinsertion. The method is not purely qualitative; it can be quantitative to a certain extent, with different concentrations of template displaying

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Abstract: Molecularly imprinted polymers are artificial networks designed to have many selective binding sites that are suitable for various applications. In order to develop more of these systems, the fabrication and characterization of imprinted polymers has become a significant area of study in recent years. Though there are several ways to fabricate imprinted polymers, the focus of this study was on a modified phase inversion procedure. A polymer and target of interest were dissolved together in a casting solvent, and then thin film imprinted polymer systems were created via spin coating. Main characterization methods included infrared spectroscopy, atomic force microscopy, and nanoindentation. Before nanoindentation could be used to study the desired imprinted polymer systems, a protocol for testing was determined. Two different series of templates were examined in this study of imprinted polymers. The first series of templates investigated were the carbohydrates fructose, glucose, and maltose. Imprinted polymer films were made using poly(4-vinylphenol) as the polymer host. The surfaces of these carbohydrate imprinted films were examined, and the successful removal of the template was observed. Some advances were made toward reinserting the carbohydrate template molecules, though the system could be optimized further. Quantification of reinsertion was possible through a chemical assay. Additionally, the solvent N,N-dimethylformamide was used for the fabrication of these imprinted polymers and was found to be an extremely good porogen. The second series of templates investigated were aromatic compounds, primarily methyl 4-nitrobenzoate. Imprinted polymer films were made using both Saran® F-310 and poly(4-vinylphenol) as the polymer hosts. Removal of the template was successful, but some difficulties were found in the reinsertion process. Rather than focus on the reinsertion of methyl 4-nitrobenzoate, a study was made of the surface morphologies of aromatic templates with differing functionalities. Computational studies were also performed with both template systems to study the polymer/template interactions. Hydrogen bonding interactions were found for both template systems.