Enantioselective screen-printed amperometric biosensor for the determination of d-amino acids

d-amino acids are generally considered to be important markers of bacterial contamination of food products. A screen-printed amperometric biosensor for the detection of d-amino acids has been constructed by the immobilization of d-amino acid oxidase on a graphite working electrode of a screen-printed strip modified with Prussian Blue and Nafion layers. Enzyme immobilization was then carried out by cross-linking of a mixture of the enzyme and bovine serum albumin with glutaraldehyde. As a result of the mediator addition and because of the multi-layer construction of the biosensor, including a polymer layer to avoid the interferences, the limit of the detection of the developed biosensor was two orders of magnitude improved in comparison to other screen-printed biosensors, as far as the determination of amino acids is concerned. Additional modification of the graphite electrode with carbon nanotubes led to a significant enhancement of the signal magnitude. A fast linear response of the developed biosensor was subsequently observed in static measurements for d-alanine in the concentration range from 5 to 200 μM. Excellent enantioselectivity towards d-amino acids was discovered. During the experiment, d-amino acids were detected in fruit juices and some milk samples. The complex matrix of natural milk samples had no influence on the response of the biosensor. The results were in good agreement with those obtained by capillary electrophoresis measurements.