1-Naphthyl phosphate as an enzymatic substrate for enzyme-linked immunomagnetic electrochemistry

We demonstrate substitution of the custom-synthesized alkaline phosphatase (AP) substrate, p-aminophenyl phosphate (pAPP), with the commercially available l-naphthyl phosphate (1-NP) as applied in the enzyme-linked immunomagnetic electrochemical (ELIME) detection of the pathogenic bacterium, Escherichia coli O157:H7. ELIME entails 'sandwiching' bacterial analyte between antibody-coated magnetic beads and an AP-conjugated antibody. The beads (with or without bound bacteria) were localized onto the surface of magnetized graphite ink electrodes in a multi-well plate format. Enzyme substrate (pAPP or 1-NP) was added and conversion to an electroactive product was measured using Osteryoung square wave voltammetry. Using this technique, quantitative detection of E. coli O157:H7 bacterial cells was achieved with a minimum detectable level of less than or equal to 4.7 x 10(3) cells ml(-1) in buffer or porcine carcass wash water within ca. 80 min. (C) 1999 Elsevier Science S.A. All rights reserved.