Antimicrobial Efficacy of Silver Nanoparticles: Synthesis Routes, Structure–Activity Relationships and Recent Advances in Plant-Mediated Green Synthesis

Silver nanoparticles (Ag-NPs), typically sized between 1 and 100 nm, hold considerable promise in biomedicine. Owing to their broad biological activity—most notably their antibacterial action against drug-resistant bacteria—a growing number of research efforts are now directed toward the most environmentally friendly routes for Ag-NP synthesis. Ag-NPs can be prepared through chemical, physical or biological approaches. The chemical route is inexpensive and gives a high yield, but relies on reducing agents and stabilisers that carry a risk of toxicity. Physical methods avoid these chemicals—using a radiation beam as the reducing agent instead—but are energy-intensive and require specialised equipment. These drawbacks have progressively shifted research toward green biosynthesis. In this review, we summarise recent progress in the synthesis of Ag-NPs by physical, chemical and biological approaches and discuss their antimicrobial properties, with particular attention paid to the green synthesis of Ag-NPs from plant-derived (including plant-waste) extracts. Unlike earlier overviews, we compare the three families of methods against the same criteria (control of size and shape, scalability, environmental burden and suitability for biomedical use), relate particle properties to antimicrobial performance, and treat toxicity and bacterial silver resistance in dedicated sections. We further highlight developments published between 2023 and 2026 and outline the main challenges—toxicity, standardisation and scale-up—that will determine whether Ag-NPs can be translated into clinically and industrially viable antimicrobial agents.