Development of a bioreactor with an integrated non-dispersive infrared CO₂ sensor for rapid and sensitive detection of Cr(VI) toxicity in water

Whole-cell bioreactors equipped with external physico-chemical sensors have gained attention for real-time toxicity monitoring. However, deploying these systems in practice is challenging due to potential interference from unknown wastewater constituents with liquid-contacted sensors. In this study, a novel approach using a bioreactor integrated with a non-dispersive infrared CO₂ sensor for both toxicity detection and real-time monitoring of microbial growth phases was successfully demonstrated. Online detection of microbial-generated CO₂ in gas-phase which is considered as a non-invasive method could significantly improve real-time monitoring of microbial growth phases while addressing some of the aforementioned limitations of conventional whole-cell toxicity biosensors. Pseudomonas koreensis ICTTOX1, which was isolated from wastewater and identified using MALDI-TOF mass spectrometry and 16S rRNA sequencing, was employed as the microbial source for Cr(VI) toxicity testing. Using the cells selected in the early-log phase, where its protective mechanisms have not yet fully activated, and conducting parallel toxic and blank runs contribute to the promising limit of detection, sensitivity, and reproducibility of the method. By achieving an IC₅₀ of 0.16 mg/L for Cr(VI), this method demonstrates significantly higher sensitivity compared to other reported toxicity assays. Moreover, the sensitivity of the early-log cells was maintained for 24 h of storage at 10°C. A statistically significant difference between the blank sample and 0.01 mg/L Cr(VI) solution confirms the ability of the method to detect Cr(VI) toxicity at the limited value of surface water quality in Vietnam. The largest coefficient of variation of inhibition was found to be 12 %, aligning with the recommended value for the validation of bioanalytical methods according to USFDA. These findings support the development of an eco-friendly and sensitive method for rapid detection of Cr(VI) toxicity, enabling early warning of pollution events to enhance environmental safety. However, the study is limited by the absence of chromium speciation analysis during microbial metabolism, which should be explored in future research.