Bioconjugate nanocapsules were fabricated by using polystyrene sulfonate (PSS) to encapsulate gold nanoparticles (AuNPs) bearing adsorbed horseradish peroxidase (HRP). The average size of nanocapsule was in a range 150–400 nm. The efficiency of the capsules to enhance signals in an immunoassay was demonstrated by using an enzyme linked immunosorbent assay (ELISA) to detect the food-borne pathogen – Listeria monocytogenes. The antibody adsorbed onto the PSS shell of the nanocapsules provided the recognition molecule. For a given quantity of antibody, the bioconjugate nanocapsules showed 30 times greater sensitivity and a shorter assay time (5 min) when compared to conventional ELISA using an HRP labelled antibody. This proof-of-concept encapsulation of HRP through PSS nanocapsules may pave the way for alternative signal enhancement strategies where sensitivity is a priority.