In order to understand the contribution of each metal cation to the bactericidal efficacy, we conducted a spike addition-like approach to produce a series of designed compositions as two-metal (Co and Mn), three-metal (Co/Mn/Cu), four-metal (Co/Mn/Cu/Zn), and five-metal (Co/Mn/Cu/Zn/Ag) cases. Yet the hand sprayer-based approach is not reliable enough, since a significant loading uncertainty may be caused from user to user. Alternatively, we adopted an "immersion" route as a much more reliable and repeatable version of preparation to generate samples with well-controlled compositions.