In situ O-PTIR spectroscopy study on single-bacterium response to plasma

As a complex multi-component system, plasma exerts multifaceted and intricate effects on biological organisms. In situ detection of functional group changes at the single-bacterium level holds great promise for revealing the molecular-level mechanisms of plasma action. In this work, based on a custom-designed calcium fluoride-based plasma discharge setup, in situ single-bacterium detection during plasma treatment was achieved using optical photothermal infrared (O-PTIR) spectroscopy. Based on quantitative analysis of gaseous products, two distinct discharge modes (O3 mode and NOx mode) of surface dielectric barrier discharge plasma were identified. The O-PTIR results reveal that plasma exposure induces both carbonylation and nitration in individual bacterial cells, as evidenced by significantly enhanced infrared absorption bands at 1730 cm−1 (C = O) and 1365 cm−1 (N–O). Compared with the O3 mode, the NOx mode generates a substantially stronger N–O band signal, indicating more pronounced nitration, which correlates with its higher bacterial inactivation efficacy (96.96% vs 88.26%). Time-resolved in situ O-PTIR spectroscopy further uncovered a combined inactivation mechanism involving chemical modification (carbonylation and nitration) and physical etching. This work establishes O-PTIR spectroscopy as a powerful single-cell-level tool for deciphering plasma–bacteria interactions.