Role of electrostatic interactions in the toxicity of titanium dioxide nanoparticles toward Escherichia coli

The increasing production and use of titanium dioxide nanoparticles (NP-TiO2) has led to concerns about their possible impact on the environment. Bacteria play crucial roles in ecosystem processes and may be subject to the toxicity of these nanoparticles. In this study, we showed that at low ionic strength, the cell viability of Escherichia coli was more severely affected at pH 5.5 than at pH 7.0 and pH 9.5. At pH 5.5, nanoparticles (positively charged) strongly interacted with the bacterial cells (negatively charged) and accumulated on their surfaces. This phenomenon was observed in a much lower degree at pH 7.0 (NP-TiO2 neutrally charged and cells negatively charged) and pH 9.5 (both NP-TiO2 and cells negatively charged). It was also shown that the addition of electrolytes (NaCl, CaCl2, Na2SO4) resulted in a gradual reduction of the NP-TiO2 toxicity at pH 5.5 and an increase in this toxicity at pH 9.5, which was closely related to the reduction of the NP-TiO2 and bacterial cell electrostatic charges.