This yielded an equilibrium association constant for the binding between the HA protein and anti-HA of 7.14± 0.26×10 7 . This K a is in the range expected for antibodyantigen interactions, from 10 6 to 10 10 M −1 [57] . In fitting the binding of the nanoparticles to this Langmuir isotherm calibration (at the experimentally used nanoparticle concentration of 500 μg/mL), the sensor response to HA-FLAG-Au would have the same binding as 0.92±0.01 μM HA protein, and HA-Au would generate the same sensor response as 1.50±0.02 μM HA protein. The large response seen is at the maximum of the Langmuir isotherm. This demonstrates that even at lower concentrations, these functionalized nanoparticles can be used as a positive control. The functionalized MPC-antibody binding is not inhibited by the presence of an additional non-interacting epitope (either FLAG or HA) on the polyfunctionalized nanoparticle; therefore, multiple binding interactions can be explored simultaneously.