Magnetic permeability measurements in bioanalysis and biosensors

A new transducer concept in bioanalysis and biosensors, utilizing measurements of magnetic permeability, is reported. A model system based on dextran ferrofluid, concanavalin A immobilized to a carrier (Sepharose), and glucose was used to demonstrate the feasibility of this approach. Direct ferromagnetic detection of the dextran ferrofluid was achieved by using a measuring coil (transducer) in a Maxwell bridge. A sensitivity of 21 μV/(μg Fe/mL) and a rsd value of 3.8% were obtained (n = 5). It was also demonstrated that a small, non-ferromagnetic metabolite (glucose) could be detected using a competitive approach. With an increasing concentration of glucose (20-40 mM), we observed a decrease in the response (0.59-0.11 mV). Reference measurements performed on Sepharose without the biorecognition element, concanavalin A, showed no significant response (0.01 mV). Some potential advantages and drawbacks of this novel type of magnetic transducer are discussed. The advantages include very low interference from the sample matrix, as the transducer is only sensitive to ferromagnetic substances, which rarely are present in samples. In addition, it is suggested that these transducers should be free from fouling. The new transducers are proposed to provide the basis for a new group of affinity biosensors suitable for in vivo and in vitro use.