# Electrochemical Detection of Penicillin G Using Molecularly Imprinted Conductive Co-Polymer Sensor
## 1 Introduction
These tests are in the form of small narrow sheets impregnated with various compounds which show the presence or absence of molecules of interest in a medium when the strips are dipped into it. This method is one of the most used for the detection of PenG in milk. In this method, the first antibody, labeled with a tracer (usually a dye) and complementary to the antigen to be detected, is impregnated on the surface of the strip but remains mobile. Two others are immobilized on the surface of the strip at two specific locations corresponding to the test and control windows (Figure 1). During the analysis, the strips, often made of nitrocellulose, are dipped into the solution to be analyzed. The solution flows through the strip by capillary action and first passes through the area containing the labeled antibodies. If the antigen is present in the solution, the immune complex is formed. The solution then continues its journey to the two test windows. The test window interacts with the antigens bound to the labeled antibodies whereas the control window binds the labeled antibodies alone and not the immune complex. By comparing their coloration, it is possible to determine if the antigen to be detected is present or not in the solution being tested. Depending on the presence or absence of antigens, the test area will be colored or not, while the control area will always be colored [16]. Figure 1: Diagram of an antibody-antigen test strip with its different constituents (sandwich method). It must be noted that it is also possible to determine the antigen coloration using spectrophotometer analysis. This makes it possible not only to determine whether or not the molecule of interest is present, but also to know in which quantity. However, this kind of device is way more cumbersome and expensive than the test strips cost. A more recent and promising development in antibiotic sensors are the molecularly imprinted polymers (MIP) sensors. These are synthetic materials resulting from biomimicry of antibody/antigen interactions (key/lock system). The properties obtained are comparable to those of biological receptors in terms of sensitivity, but especially in terms of selectivity [17]. They present the advantage of being able to perform measurements in milk without the interference of the milk matrix [12]. This kind of materials can be coupled to chemical sensors through their combination with conducting polymers (CPs). Indeed, conducting polymers can be doped or undoped by a molecule and, hence, see their physical properties modified. It is more interesting to choose conducting polymers because of all the quantifiable properties linked to the conductivity variation. However, lots of conducting polymers are not stable under ambient conditions. Among the stable CPs, polyanilines and polypyrrole are the two most interesting. Since the properties of polyaniline also vary with pH, polypyrrole is preferred for more stable measurements [18]. MIPs are obtained by polymerization of monomers around a template molecule. After the elimination of this molecule, the polymer obtained contains cavities, and imprints, with specific recognition sites (see Figure 2) [19]. These cavities are specific to the template used while doing the polymerization. This template will be the analyte to be detected and significantly increase the selectivity. This recognition technique is strongly inspired by the key-lock interaction process allowing the antibody to recognize antigens by binding with antigen epitopes. Molecularly imprinted polymers have therefore been nicknamed \"antibody mimics\". It has been shown that they can be substituted for biological receptors in certain formats of immunoassays and biosensors. In the case of conductive polymers, they are called molecularly imprinted conductive polymers (MICPs). For each MIP synthesis, a similar polymer without imprint will be synthesized. This is called a non-imprinted polymer (NIP or NICP in the case of a non-imprinted conductive polymer).