## 2 Molecular Imprinting by Surface Polymerization
In recent decades, an extensive interest in the application of controlled radical polymerization methods (CRPs) for the imprinting of biomacromolecules has been observed [48]. Generally, MIPs are prepared by a conventional free radical polymerization mechanism, mainly due the fact that it is not disturbed by a large range of functional groups of the monomers and of the template, but also due to mild reaction conditions that can be employed. However, free radical polymerization has a major drawback due to the fact that chain propagation and termination reactions are hard to control, which makes the synthesis of surface imprinted polymer films with constant and targeted thickness difficult. Furthermore, free radical polymerization normally yields crosslinked polymer networks with heterogeneous structures, which might be responsible for the increased heterogeneity of binding sites, and thus, for the decreased affinity and selectivity [49]. On the other hand, the negligible chain termination in CRPs and their thermodynamically-controlled processes allow for more constant rates of the polymer chain growth, leading to more homogeneous polymer networks with narrower distributions of the chain length. This is the reason why several CRPs were developed so far for surface polymerization, in which case the most preferred methods refer to iniferter-induced radical polymerization [50; 51], atom transfer radical polymerization (ATRP) [38; 52] and reversible addition-fragmentation chain transfer (RAFT) polymerization [38; 53]. Some of the successful procedures are provided next. A selective surface plasmon resonance (SPR) sensor, based on surface polymerization, for the detection of Ochratoxin A (OTA) contamination in dried fig was developed by [PERSON] et al. [54]. OTA is one of the most common mycotoxins that contaminate a wide range of agricultural products, which is why its assessment is very important for the monitoring of the food quality. The MIP layer was produced onto the SPR sensor chip by light-initiated polymerization of \(N\)-methacryloyl-L-phenylalanine (MAPA) and 2-hydroxyethyl methacrylate (HEMA) using OTA as a template. In a first step, the gold surface of the chip sensor was modified with allyl groups, by allyl mercaptan. This pre-treatment was performed because thiol-end will bind to the gold SPR chip, and the other end, allyl group, will interact with the polymer, insuring the good adherence of the MIP film on the chip. For the preparation of the MIP film, MAPA and OTA were mixed to obtain a pre-complex with the molar ratio of 1:3 for OTA:MAPA. The pre-polymerization complex was then mixed with HEMA, a crosslinker and a radical initiator in methanol. This reaction mix was dripped onto the SPR gold chip surface, distributed uniformly using a spin coater and, finally, polymerized by UV-light to produce the MIP film. The MIP-SPR chip was able to detect the OTA with high specificity (around 4.24 higher than that of the NIP), while the detection limit was close to 1 ng/mL and the response time was about 8 min [54]. Another interesting study on surface imprinting via the grafting (grafting from) method is provided by [PERSON] [55], which described the obtaining of a sensor for testosterone starting from porous silica by covalently binding azo-initiators and then photografting. First, glycidoxy groups were immobilized on the surface of silica particles by the reaction of silanol groups with 3-glycidoxy-propyltrimethoxysilane (GPS), after which the glycidoxy groups were modified with an azo-initiator (4,4\({}^{\prime}\)-Azobis(4-cyanopentanoic acid)) to yield azo group-introduced silica particles. The following polymer grafting was carried out by photopolymerization of MAA as the functional monomer, ethylene glycol dimethacrylate (EGDMA) as the crosslinker, and testosterone as the template, on the surface of the azo-modified particle, which served as the initiator. The prepared particles with specific recognition ability for testosterone (imprinting factor of 1.52) were used for liquid chromatography [55].