The ELISA results confirmed specificity of the functionalized gold nanoparticles to the anti-HA and anti-FLAG antibodies (Fig. 1 ). Evidence of cross reactivity between the gold nanoparticles presenting both peptides and both antibodies corroborates with QCM data. HA-FLAG-Au and FLAG-Au were detected with the anti-FLAG antibodies, while HA peptides and HA-Au were not. Correspondingly, HA peptides, HA-Au, and HA-FLAG-Au were recognized by anti-HA, while FLAG-Au was not. The LOD was calculated to be 0.146 OD using the average absorbance of wells incorporating buffer only in the initial step, which served as a negative control. The gold particles displaying only tiopronin (Au-tiopronin) appeared to nonspecifically bind with anti-HA. Nonspecific binding could be reduced in ELISAs with the use of higher concentrations of BSA or with the use of more stringent rinsing protocols, such as the use of Tween 20, a nonionic surfactant, between steps, which might also remove bound particles and lower the sensitivity of these assays [47] . MPCs functionalized without the PEG linker between the epitope and the cysteine linkage showed no binding with their respective antibodies in either ELISA or QCM experiments, but once the PEG link was incorporated into the mimic's a b designs, the antibodies were able to bind with the epitopefunctionalized nanoparticles. Calculations using Spartan computational software show the spatial projection of tiopronin molecules to be about 7 Å from the gold surface, and the peptides, due to the PEG addition, extend 20 Å, from the gold surface [48] . The addition of the six-mer PEG link between the epitope and the terminal cysteine enhanced the probability for cluster-epitope interaction with antibody by distancing the epitope from the gold surface. This enhanced binding was demonstrated with both ELISA and QCM studies.