Molecular imprinting is a new, innovative technique which makes it possible to develop synthetic receptors for target molecules with a relatively low molecular weight. This is done by the formation of polymer-based microparticles containing nanocavities, which are complementary to the target molecules with respect to size, shape, and chemical functionalities. Ideally, the MIPs can come close to the specificity and selectivity of enzymes and antibodies. Special advantages of MIPs are their comparatively low production cost and robust behavior in a wide range of environmental conditions such as temperature, pH, ionic strength, and presence of heavy metals. Moreover, in case of non-covalent molecular imprinting, the MIPs have the potential to be regenerated and to perform under subsequent recognition-extraction cycles.
We have synthesized MIPs-type receptors for several types of target molecules, including nicotine, histamine, malachite green, and serotonin. These molecules have special relevance in medical diagnostics or in food-safety analysis, or even in both domains together (histamine and other biogenic amines). The detection of the target molecules is either performed by electrochemical impedance spectroscopy (EIS) or by quartz crystal microbalances (QCM). The typical detection limits are in the nanomolar range in case of EIS and the QCM-based sensor operates in the micromolare concentration regime. The selective response of these sensors was proven by reference experiments with competitor molecules, e.g. cotinine instead of nicotine and histidine instead of histamine. The results are negative except for the upper micromolar range, where competitors start getting bound to the sensor surfaces by non-specific adsorption.
Also, we have modeled the pH-dependent response of the MIP-based sensors with a combinatorial approach, taking into account the pH-related protonation states of the target molecules and the imprinted molecular cavities. To illustrate the feasibility of MIP based impedimetric sensing in the case of food samples, we have recently measured dose-response curves of histamine-spiked tuna brine, where concentrations in the ppb range could be clearly detected.