Toxicity of Silver–Chitosan Nanocomposites to Aquatic Microcrustaceans Daphnia magna and Thamnocephalus platyurus and Naturally Luminescent Bacteria Vibrio fischeri

Abstract

All novel materials should be analyzed for their potential environmental hazard. In this study, the toxicity of different silver–chitosan nanocomposites—potential candidates for wound dressings or antimicrobial surface coatings—was evaluated using environmentally relevant aquatic microcrustaceans Daphnia magna and Thamnocephalus platyurus and naturally luminescent bacteria Vibrio fischeri. Three silver-chitosan nanocomposites (nAgCSs) with different weight ratios of Ag to CS were studied. Citrate-coated silver nanoparticles (nAg-Cit), AgNO3 (ionic control) and low molecular weight chitosan (LMW CS) were evaluated in parallel. The primary size of nAgCSs was ~50 nm. The average hydrodynamic sizes in deionized water were ≤100 nm, and the zeta potential values were positive (16–26 mV). The nAgCSs proved very toxic to aquatic crustaceans: the 48-h EC50 value for D. magna was 0.065–0.232 mg/L, and the 24-h LC50 value for T. platyurus was 0.25–1.04 mg/L. The toxic effect correlated with the shedding of Ag ions (about 1%) from nAgCSs. Upon exposure of V. fischeri to nAgCSs for 30 min, bacterial luminescence was inhibited by 50% at 13–33 mg/L. However, the inhibitory effect (minimum bactericidal concentration, MBC) on bacterial growth upon 1 h exposure was observed at higher concentrations of nAgCSs, 40–65 mg/L. LMW CS inhibited bacterial luminescence upon 30-min exposure at 5.6 mg/L, but bacterial growth was inhibited at a much higher concentration (1 h MBC > 100 mg/L). The multi-trophic test battery, where D. magna was the most sensitive test organism, ranked the silver-chitosan nanocomposites from ‘extremely toxic’ [L(E)C50 ≤ 0.1 mg/L] to ‘very toxic’ [L(E)C50 > 0.1–1 mg/L]. Chitosan was toxic (EC(L)50) to crustaceans at ~12 mg/L, and ranked accordingly as ‘harmful’ [L(E)C50 > 10–100 mg/L]. Thus, silver-chitosan nanocomposites may pose a hazard to aquatic organisms and must be handled accordingly.