Polyelectrolyte microcapsules with urease and paramagnetic particles as a basis for a potentiometric biosensor for determining urea

A biosensor is proposed based on a pH-sensitive field-effect transistor for detecting urea. The bioreceptors are polyelectrolyte microcapsules (size of 3–4 μm) with encapsulated paramagnetic particles of Fe3O4 and enzyme urease, capable of biotransforming urea. The bioreceptor was formed on the surface of a transistor gate using a constant magnetic field; the sedimentation of microcapsules in the gate zone proceeded because of the presence of paramagnetic particles in the bioreceptor. The microcapsules were prepared separately and stored at 4°C. This method of biosensor formation took several seconds and did not require additional chemical reagents for treating the electrode surface before and after the measurement. The lower limit of the detection of urea was 0.03 mM in the range of 0.03–100 mM. The biosensor is characterized by high sensitivity (~3.58 pH/mM) and time of signal formation of about 30–150 s, depending on the concentration of urea. The biosensor was tested on milk samples.