Disposable thick-film liposome immunosensor for theophylline using immunochromatographic matrix

A novel electrochemical immunosensor based on liposome signal amplification and immunochromatography has been developed for the determination of theoghylline, a therapeutic drug, as a model analyte. The immunosensor is composed of two major parts; a disposable screen-printed electrode and a nitrocellulose strip. The nitrocellulose strip contains a sample loading zone, an immobilized antibody competition zone, and a signal generation zone containing ferrocyanide-loaded liposomes in an ascending sequence. Upon loading a liquid sample to the immunosensor, capillary action causes the theophylline and theophylline-melittin conjugate to migrate through an anti-theophylline antibody zone, where competition occurs. The unbound theophylline-melittin conjugates further migrate into the signal generation zone, where liposomes are disrupted to release the electroactive ferrocyanides which are amperometrically detected by the screen-printed electrode. The current produced is directly proportional to the amount of free theophylline in the sample. The lowest detection limit obtained with this immunosensor is 5 μg/ml