We introduce a novel electrochemical protein quantitation based on the shotgun biotin tagging of proteins prior to their interfacial immunocapture and polymeric enzyme tagging. The highly amplified faradaic signals generated from a novel ferrocene-tyramine adduct enable fg mL-1 (attomolar) levels of detection and span cross a 5 orders of magnitude dynamic range. This work supports ultrasensitive protein marker detection in a single antibody immunoassay format.