Several observations can be made by inspection of the QCM responses (Fig. 2 , recorded data and schematic of adsorption) during sensor formation. Both antibodies are found to bind well to protein G. Observed spikes during assembly occur when reagent flowing through the peristaltic pump is momentarily interrupted for reactant exchange. A time delay (1-2 min) between spike and sensor response was due to the requisite transport time for the analyte to move through the peristaltic pump to the QCM sensor. binding of the nanoparticles and the HA protein to the HAantibody (Fig. 3a) and to the FLAG-antibody (Fig. 3b) . Detection of the HA protein confirms the ability of this assay to detect HA specific to the H5 HA subtype, and thus the use of this HA-functionalized nanoparticle as a viral simulant. This change in mass is based on the relationship that both frequency and resistance changes have on mass load. Originally, the Sauerbrey equation was applied in air or in a vacuum, and the resulting equation was only valid for thin solid layers deposited on the resonator [49]. Since then, extensive work has been done to establish the use of the QCM to probe interactions in a liquid environment, involving suitable oscillator circuits, fluid modeling in viscous and lossy fluids, as well as determination of the relationship between motional resistance and mass load [12, 45, 50] . The sensing layers utilized in this study should yield at most a layer 30-nm thick, using liberal estimates, where the Sauerbrey equation can be applicable to thin films less than 250 nm [51] [52] [53] [54] . Previous work, which uses a sucrose calibration, modifies the Sauerbrey equation to account for the changes that do occur in part from motional resistance, and therefore allows the ideology behind the Sauerbrey equation to apply in environments where energy is dissipated in the non-rigid liquid environment [30] . The nanoparticles increase the resistance of the crystals, and thus the rigidity, as well as decreased the frequency, resulting in a detectable increase of mass adsorbed on the surface of the crystal. Binding to the HA-antibody (positive Δm) occurs if the gold nanoparticle is functionalized with either HA or both HA and FLAG epitopes, but does not occur if the cluster is only FLAG functionalized (Fig. 3a) .