Disposable liposome immunosensor for theophylline combining an immunochromatographic membrane and a thick-®lm electrode

A novel electrochemical immunosensor based on the principles of liposome signal ampli®cation and immunochromatography has been developed for the determination of theophylline. The immunosensor is composed of two major parts including a screen-printed electrode and an immunochromatographic nitrocellulose membrane strip. On the membrane, anti-theophylline antibody is immobilized in an antibody competition zone and hexacyanoferrate(II)-loaded liposomes are immobilized in a signal generation zone. When a theophylline sample solution is applied to the immunosensor pre-loaded with theophylline± melittin conjugate in sample loading zone, the theophylline and theophylline±melittin conjugate migrate through the antitheophylline antibody zone, where competitive binding occurs. Unbound theophylline±melittin conjugate further migrates into the signal generation zone, where it disrupts the liposomes to release the electroactive hexacyanoferrate(II) which is then detected amperometrically. The current produced is directly proportional to the concentration of hexacyanoferrate(II) which, in turn, can be related to the concentration of free analyte in the sample. The detection limit of 5 mg mlÿ1 enables the present immunosensor to be used to monitor theophylline over the clinically relevant ranges (10±20 mg mlÿ1) with a one-step assay within 20 min. # 1999 Published by Elsevier Science B.V.