Molecularly Imprinted\nPolymers Electrochemical Sensing:\nThe Effect of Inhomogeneous Binding Sites on the Measurements. A Comparison\nbetween Imprinted Polyaniline versus nanoMIP-Doped Polyaniline Electrodes\nfor the EIS Detection of 17β-Estradiol

Molecularly imprinted polymers (MIPs) are synthetic receptors\nmade\nby template-assisted synthesis. MIPs might be ideal receptors for\nsensing devices, given the possibility to custom-design selectivity\nand affinity toward a targeted analyte and their robustness and ability\nto withstand harsh conditions. However, the synthesis of MIP is an\ninherently random process that produces a statistical distribution\nof binding sites, characterized by a variety of affinities. This is\nverified both for bulk MIP materials and for MIP’s thin layers.\nIn the present work, we aimed at assessing the effects of inhomogeneous\nversus homogeneous imprinted binding sites on electrochemical sensing\nmeasurements, and the possible implications on the sensor’s\nperformance. In the example of an Electrochemical Impedance Spectroscopy\n(EIS) sensor for the 17β-estradiol (E2) hormone, the scenario\nof inhomogeneous binding sites was studied by modifying electrodes\nwith an E2-MIP polyaniline (PANI) thin layer, called the “Imprinted\nPANI layer”. In contrast, the condition of discrete and uniform\nbinding sites was epitomized by electrodes modified with a thin PANI\nlayer purposedly doped with E2-MIP nanoparticles (nanoMIPs), which\nwere referred to as “nanoMIP-doped PANI”. The behaviors\nof the two EIS sensors were compared. Interestingly, the sensitivity\nof the nanoMIP-doped PANI was almost twice with respect to that of\nthe imprinted PANI layer, strongly suggesting that the homogeneity\nof the binding sites has a fundamental role in the sensor’s\ndevelopment. The nanoMIP-doped PANI sensor, which showed a response\nfor E2 in the range 36.7 pM–36.7 nM and had a limit of detection\nof 2.86 pg/mL, was used to determine E2 in wastewater.