Non-Invasive Study of Bacteria Mobility by Lithium Niobate Crystal-Based Light-Induced Photovoltaic Tweezers.

We report the results of an experimental study of the movement and trapping of Gram-negative Escherichia coli (E. coli) bacteria in broth suspensions, under photovoltaic fields generated by an optical Bessel beam illumination of the surface of a lithium niobate crystal (photovoltaic tweezers). The study was performed using a phase-sensitive transmission microscope. Experiments showed that the direct electrical interactions are suppressed due to the screening of the negative charge of bacteria by Na⁺ counterions present in broth media because of the dissociation of NaCl in the water. The immobilization and trapping of ~95% of bacteria in the area of the recorded hologram (~2 mm²) require ~40 min. The velocity map of bacteria movement in the photovoltaic lattice is constructed. The formation and destruction of bacterial chains are also observed and discussed. The proposed method is a non-invasive microscopy technique with inserted chip-scale photovoltaic tweezers.