Gold nano-particles were coated with the spike protein (S protein) of SARS-CoV-2 and exposed to increasingly acidic conditions . Their responses were investigated by monitoring the surface plasmon resonance (SPR) band shift . As the external pH was gradually changed from neutral pH to pH â¼2 the peak of the SPR band showed a significant red-shift, with a sigmoidal feature implying the formation of the gold-protein aggregates . The coating of S protein changed the surface property of the gold enough to extract the coverage fraction of protein over nano particles, & #920;, which did not exhibit clear nano-size dependence . The geometrical simulation to explain & #920; showed the average axial length to be a = 7 . 25 nm and b =8.00 nm when the S-protein was hypothesized as a prolate shape with spiking-out orientation . As the pH value externally hopped between pHâ¼3 and pHâ¼10, a behavior of reversible protein folding was observed for particles with diameters> 30 nm . It was concluded that S protein adsorption conformation was impacted by the size (diameter, d) of a core nano-gold, where head-to-head dimerized S protein was estimated for d & #8804; 80 nm and a parallel in opposite directions formation for d = 100 nm.