Imprinting modes

As one of the modes of template-monomer interaction, covalent imprinting (Cl), or otherwise referred to as the pre-organized approach [6], refers to procedures in which one or some monomeric species are joined through reversible covalent bounds to create the so-called template-monomers before the main polymerization or copolymerization process starts, through adding the required reagents (the crosslinker) and inducing the reaction conditions. 

The different covalent interactions used in the Cl of MIPs have been classified in many references [30-35]. In the case of the MIP produced through this strategy, the removal of the template commonly requires chemical treatment of the produced MIP. Covalent imprinting has been further categorized into different branches [29]. 

Condensation of readily reversible covalent bonds, which is the classical method of covalent imprinting, is one of these methods, where the template monomers are 

Some of the disadvantages of the covalent imprinting mode are the fact that it is applicable for a narrow group of monomers and templates like alcohols, amines, aldehydes, ketones or carboxylic acids [50], and also that the separation of the template from the MIP involves rather harsh procedures. This is also indicative that the rebinding of the MIP with the target species will be slow since this necessitates the reformation of covalent bonds [50,51]. 

An approach taken to overcome these deficiencies is the use of the so-called sacrificial spacers [29]. In this method the intermediate species used during the pre-polymerization of the MIP to link the monomers and the template is chosen so that it is eliminated—or in other words sacrificed—during the removal of the template. This way not only is the template attached to the monomer, the chances of steric influences of the residual moe-ities during the future rebindings with the target species are also avoided. Instances of the common sacrificial intermediate species are spacers with carbonyl groups [59-70] or the less frequent instances of salicylate (2-hydroxybenzoate) [60,71], dimethyl silyl group of silyl ethers [72] and silyl esters [73]. 

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Keywords Molecularly imprinted polymer, biomimetic molecules, chemical sensor, drug delivery, pharmaceutical separation, imprinting mode, polymerization, smart polymers... 

The properties of the solvent can also impact the interactions among the monomers and the template complexes formed in the pre-polymerization stage in the noncovalent imprinting mode, which will be described later in the text. So the choice of the imprinting approach is highly dependent on the nature and physical properties of the solvents [17,19,20]. 

Figure 6.10. The operation mode of microgravimetric detection of S-propanolol using S-imprinted polymer.

Fig. 8 (a) Schematic representation of an ordered assembly of cyclodextrin molecules with each one binding a part of the template, (b) Binding modes of (a) cholesterol, (b) 4-hydroxyazobenzene and (c) 4-phenylphenol to the guest-binding sites in the cholesterol-imprinted beta-cyclodextrin polymer. Reprinted with permission from [89]. Copyright 1999 American Chemical Society... 

A molecularly imprinted polypyrrole film coating a quartz resonator of a QCM transducer was used for determination of sodium dodecyl sulphate (SDS) [147], Preparation of this film involved galvanostatic polymerization of pyrrole, in the presence of SDS, on the platinum-film-sputtered electrode of a quartz resonator. Typically, a 1-mA current was passed for 1 min through the solution, which was 0.1 mM in pyrrole, 1 mM in SDS and 0.1 M in the TRIS buffer (pH = 9.0). A carbon rod and the Pt-film electrode was used as the cathode and anode, respectively. The SDS template was then removed by rinsing the MlP-film coated Pt electrode with water. The chemosensor response was measured in a differential flow mode, at a flow rate of 1.2 mL min-1, with the TRIS buffer (pH = 9.0) as the reference solution. This response was affected by electropolymerization parameters, such as solution pH, electropolymerization time and monomer concentration. Apparently, electropolymerization of pyrrole at pH = 9.0 resulted in an MIP film featuring high sensitivity of 283.78 Hz per log(conc.) and a very wide linear concentration range of 10 pM to 0.1 mM SDS. 

Matsushita T, Nishikawa T, Yamashita H et al (2008) Development of a new single-mode waveguide surface plasmon resonance sensor using a polymer imprint process for high throughput fabrication and improved design flexibility. Sensors Actuators B Chem 129 881-887... 

Conventionally, MlPs are obtained by bulk co-polymerization from a mixture consisting of a functional monomer, cross-linker, chiral template, and a porogenic solvent mixture. Nowadays, imprinting via non-covalent template binding is preferred over the covalent mode and involves three major steps (see Fig. 9.9). (i) Functional monomers (e.g. methacrylic acid, MAA) and a cross-linker (e.g. ethyleneglycol dimethacrylate, EDMA) assemble around the enantiomeric print molecule, e.g. (S)-phenylalanine anilide (1), driven by non-covalent intermolecular interactions, e.g. ionic interactions, hydrogen bonding, dipole-dipole interaction. Tr-rt-interaction. (ii) By thermally or photochemi-... 

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