Mechanistic Insights Into Noble Metal Nanoparticle Antimicrobial Activity through Correlative SEM-SEI-XPS Analysis

This article presents a correlative analytical strategy for studying nanoparticle-microbe interactions. Understanding the intracellular transformations of metallic nanoparticles (MNPs) provides critical insights into their antimicrobial mechanisms and informs the design of safe, durable antimicrobial materials. Conventional analytical techniques, however, are limited in their ability to simultaneously capture nanoscale morphological changes and the chemical-state evolution of nanoparticles within microbial cells. To address this, a strategy integrating high-resolution scanning electron microscopy (HR-SEM) with sputtering-enabled intracellular x-ray photoelectron spectroscopy (SEI-XPS) was developed, enabling depth-resolved, chemically specific analysis of nanoparticle-microbe interactions. Using AgNPs and PtNPs as model systems, their fate was investigated in one bacterium (Arthrobacter sp.) and two phylogenetically distinct fungi (Alternaria obovoidea and Cladosporium sp.). SEM revealed rapid membrane disruption in bacteria and slower, species-dependent structural impairment in fungi. SEI-XPS demonstrated biologically induced oxidation of AgNPs, with increasing Ag(I) and higher-order oxidized species correlating with exposure time and microbial damage, while PtNPs remained predominantly metallic, confirming an ion-release-independent antimicrobial mechanism. Depth profiling enabled direct chemical-state mapping of intracellular nanoparticle residues, revealing persistent metallic cores for both AgNPs and PtNPs. This approach provides mechanistic insight inaccessible through conventional assays and establishes SEI-XPS as powerful, laboratory-based method for probing nanoparticle transformations in living microbial systems. The methodology also offers a framework for rational design of next-generation antimicrobial coatings, including those intended for cultural heritage conservation.