Since the colorimetric assay is performed in a pH 4 environment, where the surface charge of the OA nanozyme can shift, surface charge changes were observed depending on the incubation time in different pH environments. The surface charge of the OA nanozyme shifted to a positively charged one in the acid range (e.g., pH 3/4) after up to 1 hour of incubation (Figure ). However, there are transition periods required to alter their surface charge completely. The transition periods were identified through zeta potential measurements, as up to 3 minutes after pH environment alternation due to buffering activity. It was found that a significant portion of the negative charge of the surface in OA nanozyme was observed within a short time ~ 1 minute) even in the pH 4 range, which demonstrates that the charge interaction-dependent binding systems remain active as all assays and measurements can be conducted within 30 seconds (Figure ). Histamine detection was conducted using an ABTS colorimetric assay with OA nanozyme-ABTS-H2O2 substrates similar to the previous assay. The oxidation of the ABTS molecule was performed by varying the concentrations of histamine range between 10pgmL -1 to 100 μgmL -1 (Figure ).