## 1 Introduction
Figure 1: The functional scheme of a typical sensor system. Figure 2: Molecular imprinting process (adapted from [14]). In the classical imprinting process, a complex is formed between the template and functional monomer(s), by covalent, semi-covalent or non-covalent bonds, the non-covalent approach being preferred because of the simplicity in complex forming and the ease of the template extraction. In this case, polymerization takes place in the presence of an initiator and a porogen solvent, the latter having the role to create pores in the polymer matrix that facilitate the access of target molecules near the imprinted sites, during the rebinding assays [15]. A large amount of crosslinker is needed, as well, in order to stabilize the structure. Finally, the template is extracted from the crosslinked polymer and, thus, the imprinted cavities are created. This approach is also called the bulk method [16] because a solid block is first obtained, which is later on crushed for obtaining irregular-shaped particles. This method has little productivity because many of the recognition sites are destroyed during the crushing. This is the main reason why other methods were developed for enhancing the specificity and selectivity of MIPs, such as suspension [17], emulsion [18] or precipitation polymerization [19]. Molecular imprinting (MI) was applied initially for organic small molecule templates and ions [20], due to the molecular size, complexity, conformational flexibility and diffusion difficulties of large molecules [8]. Nevertheless, in the last 15 years, large molecules, such as proteins, were also successfully imprinted. For instance, by using the so-called epitope approach [21], only a characteristic part of the biomacromolecule is imprinted, and the following rebinding process is based on recognizing this part alone. Another clever approach developed from necessity allows the preparing of MIPs for labile, dangerous or very expensive targets, by performing the imprinting using a \"dummy\" template, meaning a safer or more available compound with a similar structure to that of the target analyte [22]. MIPs can also be used to design enzymatic and immunoaffinity sensors. In agreement with the classification of biosensors, MIP-based devices can work according to two different detection schemes: (1) affinity sensors (\"plastic antibodies\") and (2) catalytic sensors (\"plasticenzymes\") [2; 23]. In this respect, MIP layers are usually deposited directly on the transducer surface of a sensor chip to produce the recognition unit [24; 25]. As a result of this rather simple procedure, many researchers have found various methodologies for producing MIP layers as receptors for producing sensing devices, such as spin coating of a precursor solution to form a thin film, followed by insitu chemical polymerization [26]; electropolymerization of a pre-assembled complex of an electroactive functional monomer with a template [27; 28]; dropcasting of a precursor polymer from solution [25; 29; 30]; dripping a composite solution containing a conducting material (e.g., graphene), MIP particles, and a binder (e.g., PVC) [31]; and, self-assembly of monolayers [32]. Thereby, the target of this review is to present both modern and dedicated methods for the preparation of MIP-sensitive layers, which are presented in detail in the next sections. In all cases, the authors describe the synthesis procedure and properties of prepared MIPs, giving quantifiable results relative to the control samples, non-imprinted polymers (NIP).