Mechanistic Insight into the Effect of Polymer and NOM Coatings on Adhesion and Interactions between Nanoparticles and Bacteria

Engineered nanomaterials may be released to the environment and adversely affect the microbial community. Three generalized modes of interaction between NPs and bacteria that lead to observed toxicity are commonly described: physical contact between nanoparticles and cells (Type I), production of reactive oxygen species (ROS) (Type II), and release of toxic metal ions (Type III). Previous studies demonstrated that polymeric coatings and natural organic matter (NOM) may reduce antibacterial activities of nanoparticles. Thus, application of coatings is a mean to mitigate nanoparticle toxicity. However, coatings may have different effects depending on the mode of interaction between the NP and bacteria and the toxicity mechanism. The reasons for the different effects of coatings observed on nanoparticles having these different modes are still unclear.

The primary objectives of this thesis are (i) to assess the effect that organic macromolecular coatings such as synthetic polymers and natural organic matter have on nanoparticle-microorganism interactions with type I, II, and III mode of action, and (ii) to determine the reasons for these effects.

NZVI (type I), nano-TiO2 (type II), and AgNPs (type III) were studied as representative nanoparticles. Poly(styrene sulfonate) (PSS), polyaspartate (PAP), humic acid, and carboxy methyl cellulose (sodium salt) (CMC) were used to coat nanoparticles. E. coli was exposed to bare and coated nanoparticles, and the bacterial concentration was determined at specific times using a plate counting technique. By observing different effects for the same coating on each type of nanoparticle, conclusions are made regarding the exact nature by which the adsorbed coatings (or in solution) are affecting the observed growth inhibition of E. coli (toxicity).

For NZVI (type I), exposure to 100 mg/L of bare NZVI with 28% Fe⁰ content resulted in a 2.2-log inhibition after 10 minutes and a 5.2-log growth inhibition after 60 minutes. Adsorbed poly(styrene sulfonate) (PSS), poly(aspartate) (PAP), or NOM on NZVI with the same Fe0 content significantly decreased its toxicity, causing less than 0.2-log inhibition after 60 minutes. TEM images and heteroaggregation studies indicate that bare NZVI adheres significantly to cells, and that the adsorbed polyelectrolyte or NOM prevents adhesion. This is the proposed mechanism for decreasing NZVI toxicity.

For nano-TiO2 (type II), exposure to 100 mg/L bare and PAP coated nano-TiO2 with UV irradiation resulted in 5-log growth inhibition after 3 hours. Thus, PVP, a coating that prevents direct contact between E. coli and the TiO2 was not sufficient to decrease the toxicity of TiO2 as it did for NZVI. However, adsorbed NOM significantly decreased nano-TiO2 toxicity, causing less than 0.3-log inhibition with UV irradiation for 3 hours. It was demonstrated that NOM scavenged ROS produced by the TiO2 and thereby decreased it toxicity. The other coating evaluated (PAP) could not scavenge ROS produced by TiO2. This study suggests that NOM, or other coatings capable of scavenging ROS may be used to mitigate the toxic effects of nano-TiO2.

For AgNPs, exposure of cells to 1 mg/L bare AgNP resulted in 6-log inhibition after 3 hours. Dissolved coal-derived humic acid at 100 mg/L decreased AgNP toxicity, resulting in less than 1-log inhibition over the same period. However, dissolved polyvinylpyrrolidone (PVP) and carboxy methyl cellulose (CMC) had no impact on the toxicity of AgNP. Results show that NOM scavenges Ag⁺ ion released by AgNP more than CMC and PVP. A correlation between sulfur content and NOM effects on AgNP toxicity is observed. Humic acid containing higher sulfur content resulted in less toxicity. This suggests that Ag⁺ scavenging by NOM, and the resulting decrease in toxicity, may be correlated with its ability to complex with NOM through thiol type interactions with Ag ion. These findings suggest that the potential for detrimental impacts of nanoparticles that interact with bacteria through the release of toxic metal ions can be decreased by NOM or other polymeric coatings that scavenge toxic metal ions.

Our results show that the presence of natural organic matter, ubiquitous in terrestrial systems, greatly reduces the toxicity of the nanoparticles, regardless of the mode of toxicity. We demonstrate that the decrease of toxicity is closely tied to the chemistry of both the coating and nanoparticle.