Enzyme-linked immunosorbent assay Initial tests to determine whether the functionalized nanoparticles were suitable as both a viral simulant and a synthetic calibrant were performed using ELISAs. Positive activity was established by adhering gold nanoparticles (250-500 ng/per well) with a variety of functionalities (Au-Tiop, HA-Au, FLAG-Au, and HA-FLAG-Au) to a 96-well Immulon 2HB plate. Buffer and free peptides (25 μg/well) were plated and tested in a similar fashion. Blocking was achieved with BSA (1 mg/mL), followed by exposure to either anti-FLAG or anti-HA primary antibodies (at recommended dilutions, monoclonal mouse). Horseradish peroxidase linked antimouse antibodies (1:5000 dilution) were then added, followed by exposure to tetramethylbenzidine (TMB, Sigma) substrate solution. The reaction was halted with 2 M H 2 SO 4 , and the plates were read at 450 nm (BioTek Synergy HT plate reader).