On-site determination of water toxicity based on freeze-dried electrochemically active bacteria

Our previous studies have reported water toxicity determination with a fresh electrochemically active bacteria (EAB) suspension as the sensing element, which exhibits high sensitivity and has great prospects in providing early warning about water pollution. However, because the preparation of fresh EAB suspensions is time-consuming, these studies are not suitable for the on-site determination of water toxicity. To solve this problem, this study investigated the rapid preparation of an EAB suspension by the rehydration of freeze-dried EABs and established a novel method for the on-site determination of water toxicity based on the freeze-dried EAB model strain Shewanella oneidensis MR-1. The results demonstrate that the optimal cryoprotectant for S. oneidensis MR-1 freeze drying is 7.5 % (w/v) skimmed milk powder. Compared with fresh S. oneidensis MR-1, freeze-dried S. oneidensis MR-1 exhibits similar extracellular electron transfer (EET) performance (74.7 % ± 0.3 %) and slightly lower sensitivity for water toxicity determination (65.8 % ± 2.2 %) with the optimal cryoprotectant. On-site determination of water toxicity was realized by using freeze-dried S. oneidensis MR-1, and the detection limits of five common toxic pollutants (Cd²⁺, Pb²⁺, Cu²⁺, phenol and dichlorophenol) reached 0.5 mg/L. Water toxicity determination is capable of resisting common interferences, e.g., glucose, lactate, nitrate and nitrite, and shows high accuracy in practical applications.