Toxicity of Silver–Chitosan Nanocomposites to Aquatic Species ^†

According to the World Health Organization, antimicrobial resistance (AMR) is one of the top ten global public health threats, justifying the need for new effective antimicrobials for biomedical applications. For the successful commercialization of any new material, data on its environmental safety are obligatory. Silver (Ag) compounds are widely used in wound dressings as well as disinfectants in healthcare due to their antibacterial activity and assumingly low risk of developing AMR.

In this study, the potential environmental hazard of silver–chitosan nanocomposites (nAgCSs) was evaluated. nAgCSs were synthesized by the reduction of the AgNO₃ with trisodium citrate and stabilized by coating with low molecular-weight chitosan (50–190 kDa). The amount of chitosan (CS) in the nanocomposites varied, resulting in three different types of nanocomposites with the weight ratio of Ag to CS 1:0.3 (nAgCS-0.3), 1:1 (nAgCS-1) and 1:3 (nAgCS-3). The toxicities of different Ag-CS nanocomposites and citrate-coated Ag nanoparticles were tested on the naturally luminescent bacterium Vibrio fischeri and microcrustaceans Daphnia magna and Thamnocephalus platyurus.

The primary size of nAgCSs was ~50 nm. In deionized water, the average hydrodynamic sizes were in the nanoscale (≤100 nm) and the surface charges were positive (16–26 mV). The toxicity of the studied Ag nanomaterials was evaluated using the bacterial kinetic bioluminescence inhibition and viability test [1,2] and acute immobilization/mortality tests with crustaceans [3,4]. The nAgCSs were about 10–500 fold more toxic to microcrustaceans D. magna (48-h EC₅₀ = 0.044–0.077 mg Ag/L) and T. platyurus (24-h EC₅₀ = 0.19–0.261 mg Ag/L) than to bacterium V. fischeri (30-min EC₅₀=3–26 mg Ag/L). Taking into the account the data obtained with this multi-trophic test battery the synthesized silver-chitosan nanocomposites could be classified as “extremely toxic” [L(E)C₅₀ ≤ 0.1 mg/L] [5]. Interestingly, the nanocomposites with the highest chitosan content (nAgCS-3) were the most toxic to bacteria V. fischeri but the least toxic to the crustaceans. The data obtained show that the chitosan–silver nanocomposites may pose a hazard to aquatic organisms and must be handled accordingly.