Nina C. Shapley, Chemical Engineering, Columbia University, 500 W. 120th Street, MC 4721, New York, NY 10027, Mona Utne Larsen, Department of Chemical Engineering, Columbia University, 500 W. 120th Street, MC 4721, New York, NY 10027, Matthew Seward, Division of Engineering, Brown University, Bio-Med Building Box G, 69 Brown Street, Providence, RI 02912, and Anubhav Tripathi, Engineering, Brown University, Box D, Providence, RI 02912.
This study reveals an exciting phenomenon of stimulated bacteria clustering. Rapid aggregation and microbial arrest are shown to occur in Escherichia coli solutions of neutral pH when chitosan nanoparticles with positive zeta potential are added. Since chitosan nanoparticles can easily be dispersed in aqueous buffers, the rapid clustering phenomenon requires only miniscule nanoparticle concentrations and will be critical in developing new methods for extricating bacterial pathogens. This work establishes the dominant role of electrostatic attraction in bacteria-nanoparticle interactions by varying the nanoparticle zeta potential from highly positive to strongly negative values, and by exploring concentration effects. For strongly negative nanoparticles, no clusters form, while aggregates are small and loose at intermediate conditions. In addition, optical density measurements indicate that over 90% of the suspended bacteria flocculate within seconds of being mixed with chitosan nanoparticles of a highly positive surface charge. Finally, the nanoparticles are significantly more efficient as a clustering agent compared to an equal mass of molecular chitosan in solution, since the bacteria-nanoparticle clusters formed are substantially larger. The bacteria-nanoparticle aggregation effect demonstrated here promises a rapid separation method for pathogen elimination.