Synthesis, characterization, and evaluation of ferrocene-theophylline conjugates for use in electrochemical enzyme immunoassay

A series of 8-(ferrocenylalkyl)theophylline conjugates were synthesized for evaluation in a homogeneous, competitive electrochemical immunoassay for theophylline with amperometric detection of the ferrocene label at +320 mV. The electrical signal was amplified via redox cycling with the glucose oxidase/ glucose system. The resulting catalytic current was strongly inhibited upon binding of the conjugates to anti-theophylline antibodies such that a large excess of theophylline was required to achieve complete reversal leading to an assay with poor sensitivity in the clinical range. A study of the nonspecific interaction of the antibodies with various ferrocene derivatives indicated that this was reduced when a charged functional group was present on the metallocene ring. Consequently, a conjugate was synthesized with a quaternary ammonium group which when incorporated into the assay resulted in improved sensitivity.