Project ID Assignment: Project 17: Effect of Wettability on the Transport and Fate of Metal Oxide
Nanoparticles
P. Somasundaran, X. Fang, S. Murthy Khandrika, I. Chernyshova, Partha Patra
Abstract:
To find out how aggregation properties of metal oxide nanoparticles (NPs) affect their fate and transport
in the environment and living organisms, we addressed the fate of ~30-nm TiO2, ZnO2 and CeO2 in the
N. Europea bacterial culture (as a model system demonstrating the route from formulation to
organelle/membrane surface). Results suggest that bacterial cell membranes were more densely
populated with the TiO2 NPs as compared to ZnO and CeO2. Furthermore, the population of TiO2 was
more prominent on the membranes compared to that in the interior of the cell, i.e., cytoplasm. CeO2
nanoparticles, unlike TiO2 and ZnO2, were observed in the form of aggregates at the bacterial and ‘cellwall’-media interface. TiO2 NPs are less aggregated due to higher surface energy compared to CeO2
and thus gets entrapped in cell wall whereas CeO2 aggregated much before to their contact with the
bacterial cell wall and probably does not penetrate the cell wall. Thus, even though the particles were of
comparable sizes their fates were different, which can be attributed to their different trends to aggregate.
Considering that the interaction of NPs with proteins and lipids is also of importance for their fate
and transport in biotic media, we studied the interactions of NPs with bovine serum albumin (BSA). We
found that in general for unbuffered solutions of 2.5mg/l BSA, pH of the ZnO TiO 2 and CeO2 range from
8.5-9.0, 6.9-7.5 and 6.0-6.75. The pH values of ZnO and TiO 2 suspensions decrease with time, while in
CeO2 particles an increase is observed. pH of ZnO suspension is less sensitive to particle concentration
as compared to TiO2 and CeO2 suspensions. ZnO and TiO2 particle suspensions also show similar
behavior in conductivity by showing incremental trend with time. CeO2 particle suspensions, however,
shows a decremental trend with time in conductivity. Addition of more particles enhances the conductivity
in all types of suspensions. Both the TiO2and CeO2 particles have incremental zeta potentials with time
and particle concentrations. The zeta potential for ZnO particles remains on the similar levels with time
and particle concentrations. Affinity of the NPs to adsorb on BSA varies in the order of ZnO> CeO 2> TiO2.
Higher affinity of ZnO towards BSA correlates with its high surface energy. Similarly in case of CeO 2 and
TiO2 their lower adsorption towards BSA could be reasoned with their lower surface energies. These
results show our hypothesis holds good for interaction of NPs with proteins. Thus particles with lower
surface energies tend to stay in bulk solution and pose the possibility of getting transported to longer
distances. In another study, mutual effects of interactions of NPs with common soil components are also
carried out.
We have also studied the interaction of ZnO, CeO 2 and TiO2 with natural minerals, kaolinite and
talc under various minerals and particle concentrations. pH, conductivity, turbidity/sedimentation of the
suspensions and the charge/size of the particles in suspension were measured as function of time. A
parameter called total surface energy loading density (SELD) was defined to assess the interaction of the
particles with the minerals. It was observed that sedimentation rates vary inversely with SELD. Presence
of kaolinite and talc slow down the sedimentation rate allowing particles to stay in suspension for longer
times. Since particles remained in suspension for longer times, it is higher probable for them to interact
with aquatic organisms and other environmental components. Zeta potentials of ZnO 30nm suspensions
with and without kaolinite were investigated. Zeta potential vs. ZnO amount in suspension was also
studied. Suspension pH increased with increasing amounts of ZnO from 8.1 to 9.3 and zeta potential
dropped from 12 mV to about zero mV. Suspension pHs of kaolinite (0.01g) with different amounts of
ZnO (0.01, 0.02 and 0.05g) ranged from 8.4 to 9.4 and corresponding zeta potentials became less
negative and ranged from -23 mV to about zero mV. This could be due to particle dissolution, dissolved
species and physicochemical properties of particles and surrounding medium. Isoelectric point (IEP) of
ZnO is at pH: 9.5 from literature and zeta potential of ZnO with and without kaolinite is approaching zero
mV at about pH: 9.5. This indicates that system behavior is dominated by ZnO and its species perhaps
by coating kaolinite particles.
We specified the adsorption forms of laurate on hematite. It was found that protonation of the outersphere complexes does not influence the conformational order of the surfactant tails. One monolayer,
which is filled through the growth of domains and is reached at the micellization/precipitation edge of
laurate, makes the particles superhydrophobic. These results contradict previous models of the fatty acid
adsorption and suggest new interpretation of literature data. The molecular-level understanding of the
adsorption of fatty acids on hematite presents the basis for the current study of the effects of NP size and
morphology on their aggregation in the presence of fatty acids.
Employing a combination of in situ FTIR and ex situ X-ray photoelectron spectroscopy (XPS) and using
the Mn(II) oxygenation on hematite (α-Fe2O3) and anatase (TiO2) NPs as a model catalytic reaction, we
discovered that the catalytic and sorption performance of the semiconducting NPs in the dark can be
manipulated by depositing them on different supports or mixing them with other NPs. We introduce the
electrochemical concept of the catalytic redox activity to explain the findings and to predict the effects of
(co)aggregation and deposition on the catalytic and corrosion properties of ferric (hydr)oxides. These
results provide a new framework for modeling the fate and transport of semiconducting metal oxide NPs
in the environment and living organisms. Using the same experimental approach, we found that oxidative
catalytic performance of hematite NPs degrades with decreasing their size. This unusual trend was
rationalized within the electrochemical paradigm.