## 5 Molecular Imprinting by Sol-Gel Derived Techniques
Due to the good optical and semi-conductive properties, sol-gel thin films can be used for the development of optical sensors as well as for electrochemical sensors. Thereby, in this section of the review, sensors based on sol-gel MIP films are presented (according to the two procedures presented in Figure 6). Thus, in the first part of the section, MIP sensor recognition elements based on sol-gel are presented, while the second part presents some examples of hybrid methods based on the combined sol-gel/electropolymerization method. Table 4 presents the use of sol-gel technique for MIP-based sensors and their performances. [PERSON] and his team [146] proposed the preparation of an MIP film using sol-gel method, in order to develop an electrochemical sensor for theophylline (TP) detection. The film was deposited on a GCE surface, which was modified prior to MIP deposition. The functionalization was made using a functional monomer: CoG-TEICPS (Crocein orange G-triethoxy(3-isocyanatoppropyl)siliane) and TEOS (tetraethyl orthosilicate) in acidic conditions, after which the MIP film was deposited by spin coating using the same recipe but with the addition of the template, TP. The electrochemical behavior of the films was studied using two methods: CV and DPASV (differential pulse anodic stripping voltammetry), which revealed that the sensor was selective and attained a low limit of detection of 1.4 \(\times\) 10\({}^{-9}\) M [146]. Figure 6: Sol–gel and combined sol–gel/electropolymerization methods for the preparation of MIP layers. [PERSON] et al. [147] developed a sensor based on an MIP membrane for electrochemical detection of melamine. The gold electrode was first modified with a mercaptopropy-ltrimethoxysilane (MPTMOS) layer. Subsequently, the MIP layer was obtained in acidic conditions using TEOS, phenyltrimethoxysilane (PTMOS) and methyltrimethoxysilane (MTMOS), after immersing the modified electrode in a solution of melamine. The final sensors were characterized by CV and SWV, resulting in a LOD of 0.4 \(\times\) 10\({}^{-9}\) M [147]. A selective SPR sensor based on an MIP film was developed by [PERSON] and co-workers [155], for detecting amoxicillin. The sol-gel precursor solution for MIP films consisted of methacrylamide (MAAM, functional monomer), TEOS, vinyltrimethoxysilane (VTES, coupling agent) and the template (amoxicillin), which was further used to coat uniformly the gold surface of an SPR sensor. The obtained sensors, tested using SPR and CV, recorded an LOD of 73 pM and a good selectivity for amoxicillin [148]. Another example of sensor based on MIP film is presented by [PERSON] et al. [149]. In this case, a LSPR (localized surface plasmon resonance) sensor for cis-jsamone detection was developed using a sol-gel technique. The synthesis of the films involved the preparation of a solution containing TBOT (Tetrabutoxy titanium), the monomer (PTMOS, PTEOS), initiated with TiCl\({}_{4}\). Prior to MIP film deposition by spin coating, the glass substrate was modified with gold. For such LSPR sensors, the registered LOD was 3.5 ppm for cis-jsamone detection in vapor state [149]. In 2016, [PERSON] and co-workers [150] proposed an MIP-based SPR sensor for trinitrotoluene (TNT) detection. The sol-gel films were prepared by dissolving the silanes (APTES, TEOS and MPTMOS) and template (TNT) in ethanol and gelation took place in basic conditions. The films were deposited by spin coating onto the SPR substrate and the final sensor was analyzed by SPR in the 4.9 \(\pm\) 2.8 ppb concentration range. In this case, a limit of detection for TNT of 0.26 ppb was estimated [150]. [PERSON] and her team [151] studied a sensor for the detection of chlorogenic acid in food samples. In a first step, the GCE electrode surface was modified with a layer consisting of MWCNTs and VTES. Following this procedure, an MIP sol-gel layer was deposited on