This study investigates the physical and chemical properties of silver nanoparticles (SNPs) that contribute to their antimicrobial activities. The researchers synthesized spherical SNPs with an average diameter of approximately 9 nm using borohydride reduction of Ag+ ions. They found that partially oxidized SNPs exhibit antibacterial activities, while zero-valent SNPs do not. The levels of chemisorbed Ag+ on the particle surface, as revealed by changes in surface plasmon resonance (SPR) absorption during oxidation and reduction, correlate well with the observed antibacterial activities. Silver nanoparticles are tolerated by bacteria resistant to Ag+, and their antibacterial activities are size-dependent, with smaller particles showing higher activities. SNPs aggregate in media with high electrolyte content, leading to a loss of antibacterial activity, but complexation with albumin can stabilize the nanoparticles and retain their activity. The results suggest that the antibacterial activities of SNPs are dependent on chemisorbed Ag+, which forms readily due to the nanoparticles' sensitivity to oxygen, and on optimally displayed oxidized surfaces in well-dispersed suspensions.This study investigates the physical and chemical properties of silver nanoparticles (SNPs) that contribute to their antimicrobial activities. The researchers synthesized spherical SNPs with an average diameter of approximately 9 nm using borohydride reduction of Ag+ ions. They found that partially oxidized SNPs exhibit antibacterial activities, while zero-valent SNPs do not. The levels of chemisorbed Ag+ on the particle surface, as revealed by changes in surface plasmon resonance (SPR) absorption during oxidation and reduction, correlate well with the observed antibacterial activities. Silver nanoparticles are tolerated by bacteria resistant to Ag+, and their antibacterial activities are size-dependent, with smaller particles showing higher activities. SNPs aggregate in media with high electrolyte content, leading to a loss of antibacterial activity, but complexation with albumin can stabilize the nanoparticles and retain their activity. The results suggest that the antibacterial activities of SNPs are dependent on chemisorbed Ag+, which forms readily due to the nanoparticles' sensitivity to oxygen, and on optimally displayed oxidized surfaces in well-dispersed suspensions.