  Plasmon-polaritons are among the most promising candidates for next
generation optical sensors due to their ability to support extremely confined
electromagnetic fields and empower strong coupling of light and matter. Here we
propose quantum plasmonic immunoassay sensing as an innovative scheme, which
embeds immunoassay sensing with recently demonstrated room temperature strong
coupling in nanoplasmonic cavities. In our protocol, the
antibody-antigen-antibody complex is chemically linked with a quantum emitter
label. Placing the quantum-emitter enhanced antibody-antigen-antibody complexes
inside or close to a nanoplasmonic (hemisphere dimer) cavity facilitates strong
coupling between the plasmon-polaritons and the emitter label resulting in
signature Rabi splitting. Through rigorous statistical analysis of multiple
analytes randomly distributed on the substrate in extensive realistic
computational experiments, we demonstrate a drastic enhancement of the
sensitivity up to nearly 1500% compared to conventional shifting-type plasmonic
sensors. Most importantly and in stark contrast to classical sensing, we
achieve in the strong-coupling (quantum) sensing regime an enhanced sensitivity
that is no longer dependent on the concentration of antibody-antigen-antibody
complexes -- down to the single-analyte limit. The quantum plasmonic
immunoassay scheme thus not only leads to the development of plasmonic
bio-sensing for single molecules but also opens up new pathways towards
room-temperature quantum sensing enabled by biomolecular inspired protocols
linked with quantum nanoplasmonics.
