E. coli was cultured in LB media with 0, 1, 10, 100, or 1000 μg/mL of MSNs. Sampling was carried out at 2 h intervals during the course of 12 h of fermentation. Cell growth was determined by measuring the optical density at 600 nm (Lambda-25 spectrophotometer, Perkin-Elmer, USA) in six parallel measurements for each time-point.