Unlocking the synthesis of ultra-small silver nanoparticles via a reactive formamide-driven nanoemulsion within Ac-Dex nanoparticles

The development of innovative nanomaterials for antimicrobial applications has become increasingly important in the context of rising multidrug-resistant (MDR) infections. Silver nanoparticles (AgNPs) are potent antimicrobial agents due to their unique physicochemical properties and broad-spectrum bactericidal activity. However, the synthesis of ultra-small and monodisperse AgNPs with controlled size, shape, and stability remains a significant challenge. In this study, we present a novel reactive nanoemulsion approach that leverages the dual functionality of formamide as both an emulsifying solvent and a reducing agent, allowing rapid in situ reduction of Ag + to Ag 0 and the formation of AgNPs ranging from 2.5 to 8.0 nm (mean 4.8 ± 1.0 nm) with low polydispersity. The Ac-Dex polymer network ensured particle stability and enabled co-encapsulation of doxycycline (DOX), producing a dual antimicrobial formulation (Ac-Dex-Ag-DOX NPs). These nanoparticles showed minimal cytotoxicity against MCF-7 cells and exhibited strong antibacterial activity against E. coli , with a minimal inhibitory concentration of 3.0 μg mL −1 (calculated based on Ag concentration). The resultant hybrid material constitutes a hierarchical nano-in-nano architecture, a class of hybrid nanomaterials in which a primary functional nanoparticle (ultra-small AgNPs) is nested within a secondary nanoscale carrier (Ac-Dex NPs). This organization confers multilevel stabilization, precise size control, and dual antimicrobial functionality. The unprecedented combination of reactive nanoemulsion synthesis and polymer-mediated stabilization holds great potential for developing next-generation antimicrobial nanomaterials. • Ultra-small AgNPs synthesized via a reactive formamide-driven nanoemulsion. • Ac-Dex nanoparticles act as confined nanoreactors and stabilizing carriers. • AgNPs and doxycycline co-loaded for dual antimicrobial functionality. • Nano-in-nano architecture enables size control and colloidal stability. • Antimicrobial activity demonstrated against Gram-negative E. coli .