Figure 2. Optimization studies of COLOR.

(a) Study assessing the incubation period needed for functionalization of ACE2 onto the surface of the cotton swabs upon activation of the aldehyde groups with potassium periodate. The optimal incubation period to ensure ACE2 functionalization was determined to be 7 hours (b) Kinetic study of the interaction between the cotton swab-immobilized ACE2 and SP, which revealed an optimal incubation time of 1 min. (c) Study of incubation time between the cotton swab biosensor and AuNano-cys-ACE2, yielding an optimal time of 3 min for best colorimetric results. All results were optimized by evaluating the sensitivity parameters obtained from analytical curves using 1×10⁻¹¹ g mL⁻¹ to 1×10⁻⁸ g mL⁻¹ of SP for 3 different biosensors, color shaded bands represent the uncertainty regions (error bars).