This study investigated the possible natural formation of silver nanoparticles (AgNPs) in Ag⁺–fulvic acid (FA) solutions under various environmentally relevant conditions (temperature, pH, and UV light). Increase in temperature (24–90 °C) and pH (6.1–9.0) of Ag⁺–Suwannee River fulvic acid (SRFA) solutions accelerated the appearance of the characteristic surface plasmon resonance (SPR) of AgNPs. The rate of AgNP formation via reduction of Ag⁺ in the presence of different FAs (SRFA, Pahokee Peat fulvic acid, PPFA, Nordic lake fulvic acid, NLFA) and Suwannee River humic acid (SRHA) followed the order NLFA > SRHA > PPFA > SRFA. This order was found to be related to the free radical content of the acids, which was consistent with the proposed mechanism. The same order of AgNP growth was seen upon UV light illumination of Ag⁺–FA and Ag⁺–HA mixtures in moderately hard reconstituted water (MHRW). Stability studies of AgNPs, formed from the interactions of Ag⁺–SRFA, over a period of several months showed that these AgNPs were highly stable with SPR peak reductions of only ∼15%. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements revealed bimodal particle size distributions of aged AgNPs. The stable AgNPs formed through the reduction of Ag⁺ by fulvic and humic acid fractions of natural organic matter in the environment may be transported over significant distances and might also influence the overall bioavailability and ecotoxicity of AgNPs.

We report a new procedure, exploiting “green” chemistry, resulting in biocompatible magnetically-driven nanocomposites with high antibacterial and antifungal activities against ten tested bacterial strains and four Candida species. The nanocomposites consist of silver nanoparticles (NPs), biogenic magnetite NPs isolated from magnetotactic bacteria, and an environmentally friendly polymer – chitosan. These hybrids were prepared using an acidified chitosan suspension to cover biogenic magnetite NPs by a cross-linking method, followed by employing the NH₂ groups of chitosan for the reduction of silver salt in an alkaline medium. Thus, in the procedure, chitosan acts as (i) a biocompatible matrix surrounding the magnetite NPs, (ii) a reducing agent for the silver ions, and (iii) a linker between magnetic and silver NPs. The size of the resulting silver NPs and the total amount of silver involved in the nanocomposites can be simply controlled by the initial concentration of the silver salt used in the reaction mixture. The as-prepared nanocomposites reveal increased bactericidal and antifungal activity when compared to previously reported magnetic silver NPs systems which were not prepared by green synthetic routes. The use of biogenic magnetite with an uniform shape and size, the absence of any other reducing agent during synthesis, the simple control of silver NPs size and loading, the biocompatible character of chitosan matrix, and a high antimicrobial effect predetermine the developed nanocomposites for targeted applications in biomedicine.