Chapter 1 Molecular imprinting

1.1 Introduction

The molecular recognition is a strategy adopted by living systems for promoting interactions at the molecular level and supporting their physiological functions. Simply, it is based on the ability exhibited by a molecule (host) to selectively recognize another molecule (guest) and interact with it by means of weak chemical bonds. These kinds of specific interactions, which play an essential role in biology and molecular engineering, have been attracted more and more the attention of scientists in developing selective tools for obtaining products with high-purity degree to meet the needs of society. A contribution in pursuing this objective was coming from the introduction of biological molecular recognition elements (i.e., affinity ligands and antibodies) in different materials for producing advanced molecularly recognition devices [1, 2, 3, 4]. However, the production of these biological receptor-based systems is expensive, tedious, and time-consuming. In addition, they present low density of recognition sites, limited application, and low stability. From this standpoint, for producing synthetic more stable and robust recognition systems that imitate the natural ones, the scientific community did a hard work. These efforts led to the development of the imprinting technology, which is an approach allowing to synthesize polymeric materials (polymers and membranes) awarded with specific and selective recognition properties.

Up to now, different imprinted polymers and membranes with high specificity versus compounds of particular interest in many fields, such as chemical trade diagnostics, foods, pharmaceuticals, and sensing platforms, have been successfully developed [5, 6]. The target compounds range from ions and small molecules to macromolecules and microorganisms such as viruses, bacteria, cells, and plant tissues. Different routes produce imprinted materials exploiting the formation of either covalent or noncovalent bonding between the analyte and the newborn-imprinted matrix. This chapter discusses the concept of molecular recognition by introducing the readers to the fascinating world of the molecular imprinting technology (MIT) in the perspective of miming biological systems. In addition, it presents an overview of the interactions characterizing imprinted polymers as well as about the synthesis and characterization of imprinted polymers, whose birth preceded that of the imprinted membranes (IMs). A look to their potential applications is also given.

1.2 The concept of molecular recognition

Molecular recognition is a key process of biological systems comprising specific interactions at the molecular level, allowing the formation of supramolecular structures. These kinds of interactions permit the processes that support life and evolution. The basis of the molecular recognition phenomenon comes from the strategic ability of biological molecules to distinguish other compounds (on the basis of their chemical complementarity), interact with them, and regulate the functions of their cells consequently. Noncovalent bonds (coordination forces, electrostatic forces, hydrogen bonds, van der Waals interactions, hydrophobic forces, π–π interactions, etc.) govern these specific interactions. Some examples of molecular recognition are the interaction between DNA and proteins, RNA and ribosomes, enzyme and substrate, and antigen and antibody [1, 2, 3].

Hermann Emil Fisher, who received the Nobel Prize in Chemistry, pioneered the concept of molecular recognition at the end of the nineteenth century (in 1894). The scientist hypothesized the “lock-and-key” model for explaining the method used by an enzyme (lock) for the recognition of its substrate (key). This theory postulated that only in the case of an exact geometric complementarity between the enzyme and the substrate, the functional groups of the latter could perfectly interact with the active site of the enzyme [1, 2, 3, 7].

However, this model did not explain all the aspects of the enzyme catalysis, for example, owing to the fact that certain enzymes are highly specific near their substrate while others might house some structurally different substrates. Over 60 years ago, Daniel Koshland proposed the induced fit recognition mechanism, which is supposed to show certain flexibility of the interacting molecules during the binding process. In particular, the binding of the substrate leads to a change in the three-dimensional relationship of the active site of the enzyme. The induced fit model is also stated as the “hand in glove model.” This definition reflects the fact that during the binding process, the enzyme and its substrate mutually adjust to each other, similar to that which occurs when a hand tucks into a glove [8, 9, 10].

Today, the molecular recognition is regarded as an efficient method for identifying and separating specific compounds of multicomponent mixtures. In fact, studies on the recognition process of living systems, as well those concerning the molecular and supramolecular interactions, have stimulated the scientists to build up synthetic strategies for producing biomimetic materials exhibiting high recognition specificity.

It is possible to distinguish two different kinds of molecular recognition: static molecular recognition and dynamic molecular recognition. Figure 1.1 reports a schematic representation of the two different strategies.

Fig. 1.1: Static (a) and dynamic (b) molecular recognition.

The static molecular recognition, based on the lock-and-key model, entails the binding of a single molecule (guest) to a specific receptor (host). The dynamic molecular recognition occurs when at least two guest molecules are involved in the process. In this case, the binding of the first guest molecule with the first receptor site determines conformational changes in the host molecule, thus promoting the interaction of its second receptor site with the second guest molecule. The dynamic molecular recognition reflects the induced fit model [1, 10, 11, 12, 13].

One of the first approaches used for the production of materials having specific recognition ability toward a given molecule was the synthesis of three-dimensional “host” structures and natural macromolecular recognition systems, such as crown ethers and cyclodextrins, respectively.

Today, one of the most usefully employed methods is the imprinting technology, which produces imprinted materials for applications in different fields, like sensors, selective extraction, drug delivery and enantiomeric separation, enzyme activity like catalysis, and antibody mimic [1, 5, 6, 11, 12, 13, 14, 15, 16, 17, 18, 19].

1.3 Imprinting technology

The development of man-made materials that are able to recognize selectively molecules or ions of particular interest, by mimicking the molecular recognition mechanism typical of living systems, is an object pursued by the entire scientific community. As already mentioned, an advanced strategy suitable for producing materials having these characteristics is the imprinting technology. It is an approach that permits to create artificial specific recognition sites in polymeric matrices for producing imprinted materials (polymers or membranes) that are able to establish specific interactions with target compounds via either covalent or noncovalent binding. The production strategy of entails the employment of different routes for creating the molecular (or ionic) memory of the target compound (atom, ion, molecule, complex, microorganisms, etc.) into these artifical smart materials during their synthesis. The compound of interest is called template (or target/print molecule as well as target/print ion) and the material endowed with its specific memory named “imprinted polymer” or “imprinted membrane,” depending on its format. Imprinted materials possess high sensitivity, stability, and specificity, which permit them to work in water or organic phase and under a wide range of pH, ionic strength, and temperature. In addition, they are prepared in a short time. They can also be simply regenerated and stored at room temperature, without losing their efficiency [14, 15, 16, 17, 18, 19]. Today, different imprinted materials are prepared using a wide range of compounds and they have a great potential in sensing and separating efficiently target ions or molecules of specific interest from a mixture containing similar compounds, like structural homologues, or other ingredients. Furthermore, they are good candidates for designing new biomimetic smart catalytic materials and drug delivery devices. Owing to their features, imprinted materials led to overcome the problem of high product purification/separation costs of traditional separation techniques (such as solvent extraction and chromatographic separation). Finally, they are promising tools for reducing waste streams and consequently the environmental impact with benefit for human health [6, 18, 19, 20, 21, 22,...