|Aggregation behavior of nanomaterials in water : roles of material intrinsic property and engineered DNA surface coating
|Jiang, Yi (CEE)
Li, Xiang-dong (CEE)
|Nanostructured materials -- Environmental aspects
Hong Kong Polytechnic University -- Dissertations
|Department of Civil and Environmental Engineering
|x, 188 pages : color illustrations
|Engineered nanomaterials (ENMs) are manufactured materials with at least one nanoscale dimension (ca. 1-100 nm). Their high surface area-to-volume ratio often leads to high surface energy and colloidal instability (i.e., aggregation) compared to their bulk counterparts. Aggregation of ENMs affects not only their functionality, but also their toxicity, bioavailability, fate and transport in the environment. Understanding the aggregation behavior of ENMs in water remains challenging because of their complex physicochemical properties and dynamic transformations in aquatic environments. Discrepancies on the effect of certain physicochemical properties (e.g., size, zeta potential, surface functionality) of ENMs on their aggregation behavior are often observed among studies, and there is also a critical knowledge gap in understanding the role of (transformed) engineered single-stranded DNA (ssDNA) coating in the aggregation of coated nanomaterials, despite their rapidly emerging applications (e.g., sensors, hydraulic tracers).
Therefore, this thesis aims to (1) resolve the seemingly contradictory effects of specific physicochemical properties on nanomaterial aggregation and examine the corresponding predictions by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, based on a meta-analysis of previous individual studies; and (2) understand the transformation(s) of the engineered ssDNA coating in typical aquatic environments at a molecular-level and reveal the subsequent impact on the aggregation behavior of coated nanomaterials.
Through a meta-analytic approach, this thesis reveals that, for graphene oxide and carbon nanotube, a threshold hydrodynamic size of ca. 200 nm exists, below which their critical coagulation concentration (CCC) increases with decreasing material size. This finding is consistent with the early prediction of the DLVO theory, but the observed threshold size is smaller (i.e., 200 nm vs. 2 µm). Beyond the threshold size, the effect of size is minimum on CCC, and material zeta potential can be mathematically correlated to CCC as predicted by the DLVO theory. However, this correlation is only observed for nanomaterials with moderate/low surface charge but not high surface charge, which is likely due to the underestimation of surface charge by zeta potential for the highly charged nanomaterials. A positive correlation between CCC and material surface oxidation degree is also observed.
Through experimental work combined with molecular dynamics simulations, this thesis reveals that transformations of the engineered ssDNA coating significantly affect the aggregation of coated nanomaterials. The ssDNA coating shows distinct features compared to conventional polymer coatings when interacting with surrounding counterions and natural organic matter (NOM). The strong interaction of cations with guanine bases affects the ion distribution along the DNA strand and the guanine-rich DNA strand leads to a thicker DNA coating on the coated nanomaterials, thereby substantially mitigating their aggregation. Furthermore, the ssDNA coating preferentially interacts with NOM of high aromaticity via strong π-π interaction. Divalent cations facilitate a complete shielding of the ssDNA coating by highly aromatic NOM (i.e., Aldrich humic acid), whereas NOM with low aromaticity (e.g., Suwannee River humic acid) only partially shields the ssDNA coating. The extent of ssDNA shielding by NOM has a major impact not only on the aggregation behavior of coated nanomaterials, but also on the performance of engineered DNA-based applications.
Altogether, this thesis resolves the inconsistency among previous observations of certain physicochemical properties on ENM aggregation and provides new knowledge regarding the unconventional ssDNA coating, thereby considerably improving our understanding of the aggregation behavior of ENMs in water. Knowledge obtained from this thesis will facilitate the rational design of ENMs and engineered DNA for their sustainable applications.
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