Self-Reporting Molecularly Imprinted Polymer with Covalently Immobilized Ferrocene Redox Probe for Selective Electrochemical Sensing of P-Synephrine

Molecularly imprinted polymers (MIPs) belong to the illustrious examples of bio-mimicking recognizing materials.1 They have found numerous applications in the fabrication of selective chemosensors.2 Their analytical parameters, such as sensitivity, selectivity, and detectability, are almost as high as those of biosensors. Additionally, MIP based chemosensors are superior to biosensors concerning their ease of fabrication, durability, and tolerance to harsh conditions, including elevated or decreased temperature, high ionic strength, extreme pH values, the presence of heavy metal ions and organic solvents in the samples. Conductive MIPs have recently become more frequently applied. That is mainly due to the easy control of MIPs deposition as thin films by electropolymerization.3 For the electrochemical determination of non-electroactive analytes, some external redox probe is usually added to the test solution. It is assumed that target analyte molecules' binding into molecular cavities causes MIP film swelling or shrinking. According to the so-called "gate effect" mechanism, this polymer "breathing" causes changes in the redox probe permeability through an MIP film, thus changing faradaic current corresponding to the redox probe's reduction or oxidation in cyclic voltammetry (CV) and differential pulse voltammetry (DPV) determinations.4-5 This mechanism is operative for nonconductive MIP films. Another mechanism may be considered for surface imprinted macromolecular compounds, e.g., proteins. A drop in the faradaic current of the redox probe accompanying protein adsorption originates from physical blocking of the electrode surface by their bulky nonconductive molecules.6 But both of these mechanisms seem to be invalid in case of electrochemical sensors based on conductive MIP films. In our previous studies, we demonstrated that a drop in the DPV current, caused by the appearance in a solution of an analyte, at conductive MIP film-coated electrodes might originate not from hindering the diffusion of the redox probe through the film but from changes in electrochemical properties of the film itself 7. Suppose the redox probe diffusion through the MIP film is not a decisive parameter for the faradaic current involving. Then, in the, e.g., DPV, determinations of electroinactive analytes at conductive MIP film-coated electrodes, this diffusion may be eliminated. For that the redox probe could be immobilized inside the MIP film matrix. Herein, we propose to deposit a self-reporting MIP film and apply it for fabrication of the selective electrochemical sensor determining the target analyte in the redox probe free test solutions. For that purpose, a ferrocene redox probe was covalently immobilized in a bis-bithiophene polymer molecularly imprinted with the p-synephrine template. Simultaneously, this polymer was deposited on the Pt electrode as a thin film. After the template extraction from the film, the analyte was determined with differential pulse voltammetry (DPV) in a redox probe free solution. That was possible because the internal ferrocene redox probe generated the DPV analytical signal. The thickness and morphology of the film were crucial for the sensor's performance. The mechanism of this redox self-reporting MIP film-based chemosensor was examined with electrochemical methods, simultaneous piezomicrogravimetry and electrochemistry at an electrochemical quartz crystal microbalance, and surface plasmon resonance spectroscopy. The devised chemosensor was applied for selective p-synephrine determination in a concentration range of 2.0 to 75 nM. 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