Synthesis and Characterization of Polyvinylpyrrolidone Coated

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The synthesis and characterization of PVP coated cerium oxide nanoparticles. Ruth Corrin Merrifield, Zhiwei Wang, Richard Edward Palmer, and Jamie R Lead Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es402541z • Publication Date (Web): 17 Sep 2013 Downloaded from http://pubs.acs.org on September 22, 2013

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Environmental Science & Technology

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The synthesis and characterization of PVP coated

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cerium oxide nanoparticles.

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Ruth C. Merrifield1, Zhiwei Wang2, Richard E. Palmer2, Jamie R. Lead1*

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5 6

1

School of Geography, Earth and Environmental science, University of Birmingham, Birmingham, UK

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2

School of Physics and Astronomy, University of Birmingham, Birmingham, UK

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* Corresponding author. ADDRESS: Dr J Lead, ENHS University of south Carolina,

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Columbia, SC 29208. EMAIL: [email protected]. TEL: +01 803 777 0091.

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1 ACS Paragon Plus Environment


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Abstract

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There is a pressing need for the development of standard and reference nanomaterials for

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environmental

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polyvinylpyrrolidone (PVP)-coated ceria nanoparticles (NPs) were produced. Four differently

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sized monodispersed samples were produced by using different PVP chain lengths. The

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chemical and physical properties of these NPs were characterized as prepared and in different

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ecotoxicology exposure media. Dynamic light scattering analysis showed that the samples

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were monodispersed, with an unchanged size when suspended in the different media over a

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72 hour period. Electron microscopy confirmed this and revealed that the larger (ca 20 nm)

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particles were aggregates composed of the smaller individual particles (4-5 nm). Electron

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energy loss spectroscopy (EELS) showed that the smallest and largest samples were

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composed almost entirely of cerium(III) oxide, with only small amounts of cerium(IV)

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present in the largest sample. Dissolved cerium concentrations in media were low and

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constant, showing that the NPs did not dissolve over time. The simple synthesis of the these

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NPs and their physical and chemical stability in different environmental conditions make

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them potentially suitable for use as reference materials for (eco)toxicology and surface water

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environmental studies.

nanoscience

and

nanotoxicology.

To

that

29 30

TOC art


aim,

suspensions

of

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31 32

Key words

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Cerium oxide, nanoparticle, PVP, toxicology, nanomaterial,

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35

Introduction

36

Nanoparticles (NPs) are defined as materials where at least one dimension is between 1 and

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100 nm and are of great interest to a variety of different sciences and industries due to their

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unique physiochemical properties.1 Advances in nanoparticle production have meant that

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their properties can be potentially tuned to perform a specific task and they are now widely

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used in industry and in commercial products. Their extensive use means that discharge to the

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environment is increasing,2 although the behavior and impacts of these particles at

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environmentally or toxicologically relevant conditions has been largely overlooked.3 In

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addition, there is an absence of well controlled and suitable test, or reference materials, for

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(eco)toxicological and environmental studies. Production of such materials and knowledge of

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their behavior under relevant conditions is an important scientific advance and will help to

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underpin a safe and sustainable nanotechnology industry.4

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Cerium oxide NPs are now being used in a variety of different industrial applications, the

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main ones being; glass polishing,5,6 chemical mechanical polishing (CMP) in the

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microelectronics industry7,8 and as a catalyst in diesel to improve fuel burn efficiency.9

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These applications use very different properties that the NPs possess but which the bulk

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material cannot appropriately provide. Such as the increased specific surface area of the NPs

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and the ability of cerium oxide to cycle between (III and IV) oxidation states with very little

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required energy10 (a feature which other rare earth metals do not possess), making Ceria of

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interest due to its catalytic, redox potential and oxygen storage capabilities.

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Although larger ceria NPs are usually in the Ce(IV) oxidation state, small ceria NPs are

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generally found in the Ce(III) oxidation state and will reduce quickly,11 with particles of a

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few nm being entirely Ce(III) and particles larger than 20 nm in the Ce(IV) state.

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Nanoparticles that are released in to the environment may therefore exist as either Ce(III) or


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Ce(IV). However, despite their benefits, using such materials inevitably produces airborne

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and waterborne cerium and ceria which might become widely dispersed in the environment.

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Most NPs used in commercial applications are mass produced, uncapped, have large size

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distributions,12,13 and are likely to aggregate or dissolve when exposed to natural water

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systems releasing ions, NPs and small aggregates, all of which are likely to stay in the water

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column, and large aggregates, which will sediment at the bottom, to be exposed to the

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environment.14,15 Although these bare ceria nanoparticles are widely used there is a growing

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need for smaller more well defined particles with very specific property needs. These are

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usually produced with a polymer coating and are already being used in the CMP industry,16

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and biomedical applications.17 Despite the growing number of applications for both ceria and

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polymer coated ceria NPs there are only a few studies on their biological effects, interaction

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with natural environments or fate in water systems. Finally, most available studies examine

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uncoated ceria NPs.18

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The current literature on biological responses to ceria NPs is very unclear with the NPs

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being pro-oxidant, anti-oxidant or neither.18,19 Ceria NPs have generally been shown to be

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largely low hazard,20,21 although high toxicity has been shown elsewhere,22 with toxicity

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measurements ranging from 0.012 mg/l to nontoxic. The origin of such differences and of

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toxicity mechanisms are not entirely clear.21 It has been hypothesized that the cycling

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between oxidation states could account for the large discrepancy in the toxicological data,

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and in some cases cause oxidative stress through the generation of reactive oxygen species

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(ROS).

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In most toxicological studies the effect of the media on size, aggregation and surface

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oxidation state of the particles is not characterized. A substantial current limitation to

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development of further knowledge is the lack of well controlled NPs which are fully

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characterized for use as reference materials in toxicological, biological and environmental



84 85

studies and many NPs used in these studies are poorly constrained and poorly characterized 23,24

meaning that conclusions about property-activity relationships are difficult to make.

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Thus reference materials, which have suitable purity and stability and are referenced for

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specific properties such as size, monodispersity25 and chemical state, are needed.

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Two of the challenges faced when trying to characterize particles in environmental or

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toxicology studies are observing the particles at low concentrations and accurately measuring

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dose. Low concentrations of particles are very difficult to find and analyze, extracting enough

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particles from suspension to analyze using traditional techniques (XRD and XPS) is

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challenging and will miss out on subtle changes in state (such as surface oxidation state or

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adsorption) due to their all-encompassing nature. Here we show that it is possible to measure

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the oxidation state and chemical composition of NPs using the STEM-EELS combination

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down to an Å resolution, thus providing vital information about the surface chemistry that

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might have previously been overlooked. The second problem of accurately measuring particle

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number concentration is thus far unanswered in the community (and is out of the scope of this

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work). Current techniques are of limited use as they either require a large particle size or high

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concentration, a more accurate measurement method for small or dilute NP suspensions is

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required.

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There are many different techniques employed to produce cerium oxide NPs, the most

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common of which is the hydrothermal method.26,27 In this method a solution containing a

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cerium salt, a reducing reagent and/or a capping agent are mixed in an appropriate manner

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and heated. The method is simple, cheap and does not use toxic solvents which might

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interfere with biological systems. In addition to shape, size can be controlled by using

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correct reaction conditions.

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This paper reports the synthesis of ceria NPs which simulate both monodispersed NPs and

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monodispersed aggregates of NPs which are stable in selected toxicologically relevant media.


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The suspensions have tightly controlled physicochemical parameters which have been

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thoroughly characterized (size, aggregation, shape, dissolution, oxidation state, purity,

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stability) and will potentially be usable as (certified) reference materials.28,29

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Methodology

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Four NP suspensions were synthesized and characterized for use (eco)toxicology studies

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including algae, daphnia and human cell lines. This paper underpins these studies which are

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either in progress or manuscripts in preparation.

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A synthesis method was adapted from previous work30 and used here. Approximately 130

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mg Ce(III)NO3 (Sigma Aldrich plc) was dissolved in a 5 mM solution of PVP, with different

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molar masses (10k, 40k, 160k and 360k) and the mixture heated for 3 hours at 105 0C. After

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heating, the reaction was quenched by dipping the flask into cold water. When the reaction

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mixture was cooled, the excess PVP was removed by adding acetone to the solution in a 4:1

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(acetone:particle suspension) and then centrifuging at 4000 rpm for 10 minutes. The yellow

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pellet was retained and the excess liquid was discarded. The pellet was re-suspended in ultra

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high purity water (UHP, resistivity 18.2 MΩ-cm, total organic carbon < 10 ppb). This

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procedure was repeated to ensure all excess PVP was removed. Finally, the NPs were re-

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dispersed in UHP water and stored at room temperature in the dark.

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The stability of the NPa suspension in media was assessed by diluting 5 ml of stock

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solution into 50 ml of either solutions of 0-100 mM NaCl or OECD algal or daphnia media.

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The composition of algae and daphnia media can be found in S1.

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Ultrafiltration using a 10 ml capacity stirred ultrafiltration cell and a 1 kDa regenerated

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cellulose membrane (Millipore Corporation). Filtration was carried out at a pressure of

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100psi (nitrogen gas, BOC) using 4 ml of solution added to the cell with 4 ml of ultra high

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purity water (UHP, resistivity 18.2 MΩ cm-1, total organic carbon < 10 ppb) and filtered

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slowly to reduce the volume in the cell down to 4 ml. This process was carried out three

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times to ensure the removal of any cerium ions in the suspensions. ICP-MS measurements

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were performed on the using an Agilent 4500ce and standards with mass of 140 and 142 for

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cerium. Both retentate and permeate were measured to quantify the total and dissolved


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concentrations. Cerium ion standards were used to correct for membrane losses of dissolved

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cerium, which were measured to be 17.1 %.

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Samples for electron microscopy (EM) were produced by diluting the stock suspensions 10

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fold to have a cerium concentration of approximately 100 ppm. Copper grids, coated in a thin

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film of holey carbon (Agar Scientific), were suspended in the NP suspensions overnight to

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allow NPs to adhere to the surface. The grids were then removed from the suspensions and

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gently dipped several times into UHP water and the excess water removed carefully with

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tissue. The washing procedure is necessary to detach any loose material from the grid and

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remove any excess salts. The adsorption method relies on the diffusion of the NPs onto the

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grid, this allows the observation of particles that remain within the water column and are thus

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expected to travel further and interact with waterborne species such as daphnia or algae.

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However in most cases the suspensions are sufficiently monodisperse and the particles

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sufficiently small to ensure representative images of the suspension. However for less

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monodisperse samples a method such as ultracentrifugation would give a more representative

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representation. The grids were allowed to air dry and kept covered to prevent contamination.

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Transmission EM (TEM) images were obtained on an FEI Tecnai F20 microscope, using an

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accelerating voltage of 200 keV and an extraction voltage of 4400 eV, with other instrument

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parameters chosen to achieve the highest possible resolution images while minimizing

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sample damage.31

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Scanning transmission EM (STEM) images and electron energy loss spectroscopy (EELS)

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were obtained on a Jeol 2100F-Cs coupled with a spherical aberration probe corrector and an

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Enfina EELS. The STEM probe size is ~1 angstrom. The spatial resolution of the EELS is

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comparable to that of the STEM probe size and the energy resolution ~1eV. Pixel sizes of

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0.065 – 1 nm were used in recording one-dimensional EELS spectrum images, dependent on

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the particle size. Cerium oxidation state is sensitive to the electron beam with prolonged



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beam time. To minimise this damage the total time over a single point was kept to 3 seconds,

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as performed previously.31 The acquisition software was programmed to take three 1 second

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spectra at each point and average them.

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The EELS spectra of cerium is characterized by two sharp peaks that rise near the energy of

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the white lines and is due to the transition of a core electron to an unbound state, 3d3/2→4f5/2

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and the 3d5/2→4f7/2 for the two different peaks. These are denoted as M4 and M5 levels

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respectively (discussion can be found in the supporting information, S7). There are many

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different methods employed to analyze the EELS spectra to obtain the Ce(III):Ce(IV)

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ratio32,33 but here we chose to use the second derivative method as it is less susceptible to

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alterations in the sample thickness.34 Either the integrated intensity of each peak between the

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points of inflection or the height of the each peak is measured to find the ratios. The raw

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EELS signal is background corrected, and band pass filtered (using the low band pass filter

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function) and the second derivative calculated using the digital micrograph software. The

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M5/M4 ratio is then determined using the intensities of the peaks in the second derivative.

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The relative peak positions, intensities and shapes of the M4 and M5 peaks present in the

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EELS spectra are different for the Ce(III) spectra and the Ce(IV) spectra (Figure S7). In

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comparison to the Ce(III) signal the peaks in the Ce(IV) spectra are shifted to a slightly

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higher energy and also have a shoulder on the M4 peak.35 The ratio of the peak intensities

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also alters. The ratios of these peaks (M5/M4) can be very different in the literature and have

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been quoted to be anywhere between 0.79-0.91 and 1.11-1.31 for Ce(IV) and Ce(III)

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respectively,32,36 showing the importance of calibrating to a known standard. In this work, an

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M5/M4 ratio of 0.83(0.1) and 1.2 (0.15) was obtained for the Ce(IV) and Ce(III) standards,

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respectively (details in the SI)

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The size distribution and form factor ((4π*area)/(perimeter2)) of the NP suspensions were

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measured using Digital Micrograph (DM)software. The edges of the particles were defined


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by using a watershed algorithm in DM where the background is removed by excluding pixels

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of a threshold value, leaving just the particles contributing to the image. The diameters, areas,

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circularity, roundness and major and minor Feret values were then automatically calculated.

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Particles at the edges of the image and particles that were less than four pixels were excluded

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(to reduce noise from the sample). At least 100 particles were analyzed to get an appropriate

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distribution.37 AFM images (S3) were analyzed in the same way but using the XEI software.

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The size distributions seen for AFM and TEM were collected over 3 batches for each sample

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to show that the average size of the nanoparticles for each sample remained similar between

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batches (A DLS comparison of multiple batches for each sample can also be found in S4

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along with ICP-MS results measuring the ionic Ce concentrations of each batch).

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For atomic force microscopy (AFM) imaging, samples were collected on freshly cleaved

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mica, after placing the mica a vial of the diluted (100 ppm) suspension. The mica was left

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overnight, removed and washed and dried in a similar way to the TEM grid.

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The AFM images were collected on a Park Systems XE100 microscope, recorded using

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XEP software and analyzed with XEI software. The AFM tip was rastered in the non-contact

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mode to ensure that very little force was exerted on the sample. Non-contact mode silicon

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AFM Cantilevers were used (PPP-NCHR, spring constant 42 N m-1 from LOT-Oriel). X-Y

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scan sizes were 5 x 5 µm with resolution of 250 pixels per line were used. The scanning rates

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were optimized to acquire a stable and clear image without damaging the tip or detaching

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particles from the surface during scanning, usually 0.3 to 1 Hz. The maximum heights of the

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individual particles were taken as the diameter as the x-y distances are overestimated due to

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the convolution of the tip radius.

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DLS measurements were performed on a Malvern Instruments Nanosizer. Ten consecutive

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measurements were collected and averaged to calculate a Z average size. The results were

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taken at 20 0C with samples equilibrated for 2 minutes before measurements were started.



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The Stokes-Einstein algorithm was used to calculate the hydrodynamic diameter of the

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particles.

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Field-flow fractionation (FFF) separations and sizing were carried out on a Postnova,

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asymmetrical field-flow fractionation from. The accumulation wall was a 1 KDa regenerated

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cellulose membrane. The eluent was 0.01 M NaCl (pH 7.5). The channel flow was 1 ml/min

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and cross flow 2 ml/min, to ensure a good separation between the void peak and particle

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elution time. The injection volume was 0.5 ml (particle concentration of 100 ppb) which was

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injected into the channel after 6 minutes. The channel volume was calculated using 20, 30

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and 60 nm polystyrene bead standards (Duke Scientific Corp.). All particles were detected

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with a UV detector at 254 nm. Diffusion coefficients were calculated using FiFFF theory and

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converted to size using the Stokes-Einstein relationship. At least 3 replicates were collected

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and averaged to obtain the size.

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Results and discussion

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Synthesis and Characterization

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A simple bench top method of cerium NP production was adapted30 to produce four

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samples of NPsa-d of four nominally different sizes by using different molar masses of PVP

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(10K, 40K, 160K and 360K for samples NPa-d respectively). All four NP suspensions (a-d)

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had a pH of 5.5 ± 0.5, were yellow in color immediately after preparation and were found to

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be stable over a period of several months when kept in water in the dark at low temperature.

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The distinctive yellow color is indicative of cerium being in the Ce3+ oxidation state.38,39

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The PVP is thought to attach, via oxygen containing moieties, to Ce(III) ions in

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solution.40 This attachment forms a nucleation site from which the NPs grow. The PVP

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chains then wraps around the NP via an attachment between the carbonyl groups of the PVP

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and the hydroxyls on the surface of the NPs40 thus strongly stabilizing the cerium oxide NPs.

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When the PVP chain length is increased more NPs are able to nucleate along a single PVP

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chain causing small clusters of NPs, as seen in samples NPc and NPd. Steric stabilization

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mechanisms ensure colloidal stability of the NPs.41

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After synthesis, a multi method approach was adopted to measure the properties of the

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four NP suspensions.13,42 Figure 1a-d show high resolution TEM images of the four different

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particle suspensions NPa-d respectively. It can be seen that samples in figure 1a and 1b are

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single nanocrystals, while 1c and 1d are agglomerates of particles. Lattice fringes are clearly

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visible in all images showing that the individual particles are crystalline and not amorphous.

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The atomic resolution on the STEM analysis for sample NPa also showed that out of 113 NPs

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counted (in 18 images) all had a crystalline structure (Supporting information S2. Lower

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resolution images of NPa and NPd are also shown here).

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Figure 2a-d shows the size distributions of the particles taken from the TEM images

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in Figure 1. A Gaussian curve was fitted to the data to find the mean and standard deviation.



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The mean (standard deviations in parentheses) core sizes are 4(0.6), 8.5(2), 10.7(4) and

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15.2(6) nm for samples a-d, respectively. However it should be noted that there are a few

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outlying points in sample NPd with a second smaller peak appearing around 40 nm and a few

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particles in between so the Gaussian fit only represents the bulk of the particles and not the

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outliers. The form factor (f) of the particles in suspensions NPa and NPd were calculated as

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being 0.82 (0.11) and 0.42 (0.14) respectively, with the higher value reflective of the near

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spherical nature of the individual particle and the latter value indicative of non-sphericity of

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the aggregates formed (see STEM data, later in this section). The TEM images show the

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crystalline structure of the NPs but does not show the passivating PVP layer,43 due to its low

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electron density. Aggregate formation of sample NPd is most likely due to the longer PVP

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chains having many nucleation sites causing small particles to form along the chain.

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Table 1 shows a comparison between four sizing techniques for samples NPa-d. (The

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size data for the AFM, DLS and FFF can be found in the supporting information S3 and S4).

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The data suggests good agreement in the case of NPsa-c, for all methods but discrepancies

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between the microscopy results and the DLS results for NPd where the DLS sizes are larger.

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The difference is expected and due to the different parameters measured (core and

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hydrodynamic size), along with DLS based bias induced by the presence of the larger

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particles in the suspension (seen in small numbers with the TEM), and the low form factor of

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the particles in NPd leading to the DLS being a poor tool to measure the size of the

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particles.43

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For suspension NPa, there is good agreement between data from all methods.

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However, the small differences between the AFM and FFF data are instructive. The

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differences are due to the nature of the techniques used and the parameters measured. By

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comparing the difference between the AFM/TEM and the FFF size we can estimate the PVP


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thickness for the NPa suspension is 2.2 nm, which is in agreement with similar work carried

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out to find the length of 10K PVP coated NPs.43

277 278

Lattice structure at a sub-nanometre scale

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HR-STEM imaging alongside EELS is a powerful combined tool to analyze both the

280

detailed physical structure of NPs but also their chemical composition and the oxidation state

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of individual atoms within the NP.

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Figure 4a-c are aberration corrected STEM images of three different particles from

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suspension NPa showing three orientations: (111), (110) and (211), respectively. Lattice

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fringes and atomic structure can be seen in almost all of the HR-TEM and STEM images

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acquired on all of the NP suspensions. Most of the particles in samples a and b show

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symmetry associated with the 2 fold (110) crystal axis, and are surrounded by 4 (111)

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surfaces and 2 (100) surfaces suggesting a truncated octahedral structure, as found

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elsewhere.13,44 Images and further discussion are in the supporting information, S5. The NP

289

projections are along the (110) projection plane exposing the (111) lattice fringes. Ceria NPs

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preferentially form polyhedral fluorite structures in a face centred cubic (FCC)-like structure

291

bound by the low energy (100), (110) and (111) facets.45,46 Theoretical calculations have

292

shown that despite the initial shape of a crystal they will preferentially re-crystallize to form

293

truncated octahedral structures with large exposed (111) (oxygen terminated) planes

294

truncated by smaller (100) (cerium terminated) facets as seen in NPa-d. There is an absence of

295

(110) (oxygen and cerium terminated) facets despite the fact that the (110) plane is

296

energetically more stable than the (100) plane.30 A discussion on the lattice parameters can

297

be found in S5.

298

It has been shown that although in dry conditions the (111) surface is the most energetically

299

stable, in an aqueous solution, the cerium will hydroxylate, causing the (111) to become the



300

least stable surface in water and the (100) surface becomes energetically favourable.47 As the

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amount of water attached to the surface of the NP increases so does the size of the (100)

302

planes. However, in this case the PVP coating will preferentially bind to the oxygen rich

303

(111) facet through its own hydroxyl bonds.48,49 Due to these two processes occurring

304

simultaneously during the synthesis of these particles they form slightly non-spherical shapes,

305

as discussed earlier, showing more step defects and the presence of higher energy surfaces.

306

The higher energy surfaces found here are absent in most published literature50-52 due to

307

drying and calcination at high temperatures to form powders causing their structure to reform

308

into the (111) lowest energy surface; this distinction could affect toxicological response to

309

differently treated NPs.

310 311

Oxidation state at a sub-nanometre scale

312

Cerium oxide can take two main oxidation states of Ce(III) and Ce(IV) oxide. It is known

313

that bulk ceria is predominantly in the Ce(IV) state, while small NPs take on the Ce(III) state

314

to release lattice strain.36,53 The oxidation states of cerium can be identified by their energy

315

loss signal in EELS, where white lines relating to the M-edge region of the spectra are

316

observed. These hold information on the initial 4f occupancy of the atoms. The M4 and M5

317

peaks relate to the 3d3/2→4f5/2 and the 3d5/2→4f7/2 transitions, respectively, which are

318

associated with the spin-orbit splitting. The ratio of these peaks can be used to determine the

319

valence occupancy of the cerium ions and thus the oxidation state.11,49

320

Figures 5a and 5c are STEM images for NPa and NPd, respectively while 5b and 5d are

321

graphs corresponding to the line profiles (blue) and M5/M4 ratio (red) across the particles

322

(marked as white lines in figures 5a and 5d) . The pixel sizes used in recording the STEM

323

images and EELS spectra are 0.068 nm and 0.068 nm for figure 5(a-b) and 0.159 nm and

324

0.41 nm for figure 5(c-d). It can be seen that in figure 5a the cerium content of the particle is


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atomically mapped and that each atomic column is in the Ce(III) oxidation state. The feature

326

around 1.8 nm into the particle shows that the probe measuring the EELS signal at that point

327

has gone straight between two atomic columns in the particle thus showing no energy loss of

328

those electrons. Figure 5c shows two particles and is of lower absolute resolution given the

329

larger NP size; damage to the NP, sample drift and acquisition time were all substantially

330

reduced, while still maintaining a very high spatial resolution. It is clear from the STEM

331

image that the particle is formed of many smaller crystals, each of approximately the same

332

size as the nanocrystals in NPa, and are similar to the TEM images in figure 1 albeit at higher

333

resolution. The STEM data shows non-sphericity and some polydispersity which, along with

334

the permeability of the larger polymer shell explains the discrepancy between microscopy

335

and the DLS data for this sample. Although the particles are still primarily Ce(III), there are

336

small patches in both of the particles present in Figure 5c, at 20-23 nm and 46-50 nm in the

337

graphical representation, where the M5/M4 ratio changes to 1 - 1.05 corresponding to

338

patches of increased intensity (thickness) in the STEM profile, showing that some Ce(IV)

339

oxide is present at the centre of these NPs. These patches within the NPd sample are ca 60%

340

Ce(III) and 40% Ce(IV), equivalent to literature data on solid particles of ca 9-15 nm.11,33

341 342

Behavior of particles in toxicology exposure media.

343

To examine the effect of ionic strength to the NPs, varying amounts of NaCl

344

(concentrations from 0 to 100 mM) were added to the NPa suspensions. This range of

345

concentrations covers the ionic strength of daphnia media, algal media and EPA synthetic

346

soft water (the ionic strength and composition of which can be found in table S1 supporting

347

information). The z average ranged from 6.8 (in 5.0 mM solution) to 7.9 (in the 0.1 mM

348

solution) with a mean of 7.5 with no trend observed with ionic strength14,54 and no significant

349

difference in mean or standard deviation values (graphical representation and a table of



350

results can be found in S6). The mean PDI was 0.27 with a standard deviation of 0.04 and

351

spanned a range of 0.24 to 0.32 with only one measurement over 0.3. The PVP sterically

352

stabilizes the NPs and is largely unaffected by salts in the external medium explaining the

353

colloidal stability of the NPs.55,56

354

Figure 3 a-d show the DLS size of the particles after 72 hours in ultra high purity

355

(UHP) water, OECD algal toxicity media or OECD daphnia toxicity test media; time periods

356

were chosen so that they are equal to or greater than the exposure period of algal toxicity tests

357

and the acute daphnia toxicity test.57,58 The pH of the particles diluted into the media was

358

taken and found to be 7.17 (0.13) and 7.20 (0.87) for the daphnia and algae media

359

suspensions respectively. There is an increase in size between all stock suspensions and that

360

of the particles in media (table S4), which can be explained by the higher viscosity of the

361

stock suspensions compared with the media dilutions (1.1 mPa.s and 7.8 mPa.s for NPa

362

diluted into media and NPa stock suspension respectively) resulting in larger sizes in the

363

media due to the viscosity-size relationship in the Stokes-Einstein relationship ௞்

݀ ሺ݄ሻ ൌ ଷగ௡஽

364

Equation 1

365

where d(h) hydrodynamic diameter, k Boltzmann constant, T temp, D translational

366

diffusion coefficient and n is viscosity). However the z average size, corresponding PDI and

367

peak intensity can be found in S4, and are similar when diluted into both media and water.

368

Thus the only effect on DLS results is due to media viscosity and there are no observable

369

changes in NP properties.

370

The dissolution of the NPs in algal media was measured at four points over a 72 hour

371

period using ultra-filtration. The cerium ions, measured by ICP-MS, were not significantly

372

different over a three day exposure period (0.84 ± 0.72 ppb, n=12); the total Ce concentration

373

in the media was 4.0 ± 0.5 ppm, showing that the particles are highly insoluble in relevant

374

media and do not appreciably change over time in the relevant media. Use of the NPs in 18 ACS Paragon Plus Environment

Page 18 of 35

Page 19 of 35


375

toxicology tests will therefore reflect the particle behavior and not the dissolved metal

376

behavior. Taken together the lack of aggregation and dissolution suggest that these materials

377

will make excellent model compounds for (eco)toxicology and surface water environmental

378

and transportation studies.

379



380

Acknowledgments

381

The

UK

Natural Environment and

the

Facility

Research Council for

Environmental

Page 20 of 35

(NE/H008764/1; Nanoscience

NE/I0083/1;

382

NE/G010641/1

Analysis

and

383

Characterisation, FENAC) along with the Center for Environmental Nanoscience and Risk,

384

University of South Carolina is acknowledged for financial support. Steve Baker is

385

acknowledged for measurements by ICP-MS.

386 387

Supporting Information Available

388

Composition of algae and daphnia media, AFM and DLS results, cerium oxide lattice

389

information and TEM micrographs of NPs from suspension NPa and the STEM images and

390

EELS analysis for the standards used to calibrate the results. This information is available

391

free of charge via the Internet at http://pubs.acs.org/.

392


Page 21 of 35

393


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394

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568

569

570 571


57.

OECD OECD guidelines for testing of chemicals, No 202, Daphnia sp. acute

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572 573 574



575

Graphics

576 577

Figure 1: TEM images of samples NPa,-d (a, b, c and d, respectively). The white scale

578

bar in figure 1a is 10nm and corresponds to all images. The inserts highlight the lattice

579

fringes for these samples; the corresponding black scale bar is 5nm.

580 581


Page 30 of 35

Page 31 of 35


582

583 584

Figure 2: Size distributions of NPa-d (a,b,c and d respectively –from TEM images (grey, bin

585

size of 1nm), with corresponding Gaussian fit (black line).

586



587

588 589

Figure 3: a-d show the size measurement, by DLS, of the four NP samples NPa-d respectively

590

after 72 hours exposure to water (black), algae media (grey) and daphnia media (light grey).

591

The graphs have all been normalized to 1 for comparison and thus is an arbitrary unit of

592

intensity).

593 594


Page 32 of 35

Page 33 of 35


595

596 597

Figure 4: STEM images of different particles looking down different atomic plan directions.

598



599

600 601 602

Figure 5: a and c are STEM images of particles from suspensions NPa and NPd respectively.

603

The accompanying graphs (b and d) show the intensity profile (blue) of the STEM image

604

corresponding to the left hand axis, and the M5/M4 ratio (red) of different points along the

605

NPs corresponding to the axis on the right. The green and maroon lines correspond to the

606

M5/M4 ratios of the Ce(IV) and Ce(III) reference materials respectively (data in S7).

607


Page 34 of 35

Page 35 of 35


608

Suspension PVP chain length DLS Mean Size(PDI) AFM Mean Size(SD) TEM Mean Size(SD) FFF Mean Size(SD)

NPa

NPb

NPc

NPd

10K

40K

160K

360K

5.5(0.4)

10.7(0.2)

13.9(0.2)

37.4(0.5)

5.0(1.0)

4.8(1.2)

10.0 (1.8)

17.0 (3.0)

4.0(0.6)

8.5(2.0)

10.7(4.0)

15.2(6.0)

6.7(0.2)

609

610

Table 1. This table compares the different sizes obtained for suspensions NPa-d using different

611

sizing methods.

612


Synthesis and Characterization of Polyvinylpyrrolidone Coated

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