Signal amplification strategy for electrochemical immunosensing based on a molybdophosphate induced enhanced redox current on the surface of hydroxyapatite nanoparticles

The authors describe a strategy for improving the sensitivity of electrochemical immunoassays that is based on the use of hydroxyapatite nanoparticles (HAP-NPs). Signal generation is based on the reaction of phosphate groups present in HAP-NPs with molybdate to form a redox-active molybdophosphate precipitate on the surface of the electrode that facilitates the generation of an electrochemical current. The cancer biomarker α-fetoprotein (AFP) was chosen as a model antigen (analyte). Antibodies against AFP (Ab 2 ) were immobilized on the HAP-NPs to obtain the signal probe. The reaction of HAP-NPs with molybdate and the performance of the immunoelectrode were characterized in detail. Based on this signal amplification approach, an immunoassay was worked out that has a linear range that extends from 0.1 pg·mL −1 to 1 ng·mL −1 and a detection limit as low as 50 fg·mL −1 . The assay was successfully applied to the determination of AFP in serum samples. In our perception, this new signal amplification scheme can be adapted to numerous other immunoassays. Schematic of the electrochemical assay for the cancer biomarker α-fetoprotein based on hydroxyapatite (HAP) nanoparticles as signal probe. The reaction of HAP nanoparticles with molybdate leads to the formation of a redox-active molybdophosphate precipitate on the electrode surface and generates an electrochemical current.