Polyelectrolyte-Coated Cerium Oxide Nanoparticles: Insights into

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Polyelectrolyte-Coated Cerium Oxide Nanoparticles: Insights into Adsorption Process Zlatko Brkljaca, Nikolina Lesic, Katarina Bertovic, Goran Draži#, Klemen Bohinc, and Davor Kovacevic J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07115 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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The Journal of Physical Chemistry

Polyelectrolyte-coated Cerium Oxide Nanoparticles: Insights into Adsorption Process Zlatko Brkljača,1 Nikolina Lešić,1 Katarina Bertović,1 Goran Dražić,2 Klemen Bohinc3 and Davor Kovačević1,* 1

Department of Chemistry, Faculty of Science, University of Zagreb, 10000 Zagreb, Croatia

2

Department for Materials Chemistry, National Institute of Chemistry, SI-1000 Ljubljana,

Slovenia 3

Faculty of Health Sciences, University of Ljubljana, SI-1000 Ljubljana, Slovenia

ACS Paragon Plus Environment

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ABSTRACT: We investigated ceria nanoparticle/strongly charged polyelectrolyte solution interface on the example of adsorption of a strong polyelectrolyte poly(sodium 4styrenesulfonate), PSS, on cerium oxide nanoparticles in order to quantitatively characterize the above-mentioned interface. The obtained results could enable a better prediction of nanoparticles behavior in polyelectrolyte solution which could lead to improved applications of ceria nanoparticles. We initially both synthetized and characterized ceria nanoparticles. For the characterization X-ray powder diffraction (XRD), bright-field scanning transmission electron microscopy (BF-STEM), dynamic light scattering (DLS) and electrophoretic mobility measurements were used. Results revealed that nanoparticles are spherical with primary ceria nanoparticles diameter (nanocrystallite size) of approximately 3.5 nm. Results of high-resolution transmission electron microscopy (HRTEM), DLS and electrophoretic mobility measurements after adsorption of PSS showed that polyelectrolyte-coated nanoparticles were obtained. The comparison of average hydrodynamic diameters of uncoated and PSS-coated ceria nanoparticles leads to the assumed thickness of the polyelectrolyte layer of δ ≈ 9 nm. Finally, for the quantitative interpretation of polyelectrolyte adsorption on ceria nanoparticles the application of the modified Ohshima model was examined. In that respect, the adsorption parameters such as charge density of the polyelectrolyte layer electrophoretic softness and adsorption density were determined. It was thus shown that Ohshima model could be applied for studying ceria nanoparticle/strongly charged polyelectrolyte solution interface and that the obtained results could serve as a basis for further tailored applications of polyelectrolytes as stabilizing agents for nanoparticles.


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INTRODUCTION Interest for investigation of cerium oxide (ceria) nanoparticles (shorten CNPs) has been increasing recently due to their specific properties1 and due to the importance of their potential applications in various areas, e.g. catalysis2, biomedicine3, sensors4 and solid oxide fuel cells5. Cerium oxide’s catalytic ability, which is largely affected by redox behavior and is related to cerium’s redox state6,7, is gaining significant attention in the field of environment and energy related applications. The CNPs are often largely composed of Ce3+ (up to 60% in some cases) as well as Ce4+. The larger content of Ce4+, the stronger the catalase mimetic activity becomes.8 Moreover, the catalytic activity of nanoparticles could be enhanced by reducing their size.9 Understanding surface chemistry of ceria nanoparticles plays a very important role in predicting their catalytic activity. Wöll and coworkers10 showed recently, using carbon monoxide as a probe molecule, that the rod-shaped CNPs actually restructure and expose {111} nanofacets. Recent studies1,11-15 have also shown that the biomedical aspects of ceria nanoparticles deserve further attention. For example, CNPs were shown to have specific regenerative properties due to their low reduction potential and due to the coexistence of both Ce3+/Ce4+ on their surface. Since the local microenvironment, as well as the applied synthesis method, influence the Ce3+/Ce4+ ratio, they both play a very important role in the process of obtaining CNPs with specific biological activity and toxicity.1 One of the main driving forces for applications of CNPs in nanomedicine is their antioxidant activity. In that respect, Das et al.11 showed that the antioxidant behavior and biocompatibility of CNPs could be used to prevent diseases of the central nervous system. It was also shown12 that it is possible to accelerate cutaneous wound healing using chitosan-coated cerium oxide nanocubes. Regarding the field of environmental biomedical applications, it should be stressed that CNPs showed stronger inhibitory effects on microbes production of biogas than


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titania, gold or silver nanoparticles.13 Additionally, polymer-coated ceria nanoparticles showed also significant antibacterial activity.14 The activity of CNPs against Escherichia coli was studied by Peletier et al.15, whereas Fang and coworkers16 reported the antibacterial activity against Nitrosomonas europaea. One of the main advantages of possible CNPs biomedical applications is their biocompatibility and relatively low health risks.17,18 It was shown18 that ceria nanoparticles caused, even at the highest dose, very little inflammatory response in human aortic endothelial cells making them rather benign in comparison with, for example, Y2O3 and ZnO nanoparticles. On the other hand, caution is advised since it was recently shown that cerium dioxide nanoparticles affect in vitro fertilization in mice.19 Even very low concentration of CNPs could induce DNA damage (however, no loss of vitality was reported) in human spermatozoa20. Pulido-Reyes and coworkers21 found that the possible toxicity of cerium oxide nanoparticles depends on the percentage of Ce3+ at the surface of nanoparticles. A very important issue related to the application of CNPs in biological environment is a requirement for a stable dispersion of nanoparticles, especially at physiological pH. It is known that the stability of nanoparticles, in addition to pH, depends on nanoparticle concentration and ionic strength of the solution.22 A number of different pathways used for stabilization of nanoparticles can be found in the literature.23-29 For example, it was shown23 recently that lipidbased nanovesicles (liposomes) could stabilize suspensions of metal oxide nanoparticles. Moreover, aminopolycarboxylic acids could also be used for that purpose which was shown24 on the example of ceria nanoparticles. In this respect, one of the most promising tools for stabilization of CNPs is the adsorption of polyelectrolytes.25 Polyelectrolytes are chain molecules consisting of linked charged or chargeable groups. The interactions of polyelectrolytes with nanoparticles have


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been analyzed in detail by Skirtach and coworkers26,27 at the example of gold nanoparticles. They revealed that polyelectrolytes can be efficiently used for controlling the aggregation of gold nanoparticles. It was also shown by Tian et al28 that polyelectrolyte-stabilized Pt nanoparticles can be applied as new electrocatalysts for low temperature fuel cells. Additionally, the stability of CNPs induced by adsorption of weak polyelectrolyte polyacrylic acid (PAA) was systematically studied by Seal and coworkers29. They showed that PAA coated ceria nanoparticles successfully preserved the stability and surface chemistry of CNPs. As a special and very interesting case of nanoparticle-polyelectrolyte interactions, the build-up of polyelectrolyte multilayers on nanoparticles should also be mentioned here.30,31 Such a layer-by-layer technique opened a new area for functionalization and stabilization of nanoparticles. As an example, stimuli-responsive polyelectrolyte multilayer capsules and nanoshells could be used as drug delivery carriers.32 Motivated by the aforementioned results, we decided to explore the interactions between ceria nanoparticles and a strong anionic polyelectrolyte poly(sodium 4-styrenesulfonate), PSS.33-35 The main reason for our selection of PSS was its ability to retain its charge in a broad pH range. Also PSS is a frequently investigated polyelectrolyte, so the comparison of the obtained results with the literature data can be readily performed. Therefore, we synthetized ceria nanoparticles and characterized them systematically before and after adsorption of PSS. For that purpose we used dynamic light scattering (DLS) and electrophoretic mobility measurements, as well as transmission electron microscopy (TEM) and X-ray diffraction (XRD) experiments. Since PSS is a strong polyanion, it adsorbs readily on positively charged ceria nanoparticles in pH region below the isoelectric point. For the interpretation of obtained results we used electrophoretic soft particles model, namely a modified


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Ohshima model,29,36,37 which is based on the measurements of mobility of nanoparticles covered with polyelectrolyte layer.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ceria Nanoparticles. Cerium oxide nanoparticles (CNPs) were synthetized by hydrolysis of cerium(III) nitrate hexahydrate (Ce(NO3)3 × 6H2O; obtained by Fluka) at room temperature according to the procedure described in the literature.38,39 Sodium hydroxide aqueous solution (V = 125 mL, c = 1.0 mol dm−3) was added to cerium(III) nitrate aqueous solution (V = 5 L, γ = 2.5 g dm−3) under constant stirring. The obtained suspension was stirred continuously for 24 h at room temperature which led to the change in the color of suspension from violet to yellow. The pH of suspension was monitored and it was determined to be pH  11. The precipitate was purified by intensive washing with deionized water until the conductivity of the supernatant reached the conductivity of deionized water. Dry ceria powder was stored in a glass bottle at room temperature. 2.2. PSS Coating on CNPs. Strongly charged polyelectrolyte poly(sodium 4-styrenesulfonate), PSS (Mw ≈ 70000 g mol−1, f = 0.83, Sigma-Aldrich) was used as received. PSS solution was prepared by dissolving the appropriate amount of PSS in water. Suspension of ceria nanoparticles was prepared in HCl solution and NaCl was added. HCl and NaCl concentrations were chosen depending on the required final concentration. The acidic pH range was chosen in order to prevent the aggregation of particles40 and to assure for ceria nanoparticles to be positively charged. Being a strong polyanion, PSS is negatively charged throughout the entire pH range. Finally, PSS solution (initial monomer PSS concentration was c = 0.1 mol dm–3) was added to CNP suspension (final monomer PSS concentration was c = 5∙10–3 mol dm–3) with the aim of obtaining final CNPs


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mass concentration of γ = 3 g dm−3 (for DLS measurements) and γ = 1 g dm−3 (for electrophoretic mobility measurements). The system was mixed for one hour. 2.3. Characterization of Uncoated and Coated Ceria Nanoparticles. Uncoated and polyelectrolyte-coated ceria nanoparticles were characterized by various experimental methods. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS) and electrophoretic light scattering techniques were used. X-ray powder diffraction experiments were performed on Philips PW 1850 diffractometer in the Bragg-Brentano geometry employing the Cu Kα radiation (λ = 1.542 Å), operated at 40 kV and 30 mA. The patterns were collected in the angle region between 20 and 65 (2 θ). A JEOL ARM-200 CF probe Cs-corrected Scanning Transmission Electron Microscope, operated at 80 kV and equipped with Dual EELS system (QuantumGIF, Gatan, USA) was used for the study of size, shape, crystallinity and Ce valence state. High-resolution transmission electron microscopy, bright-field scanning transmission electron microscopy and high-angle annular dark-field (HAADF-STEM) imaging techniques were used in the study. The effective hydrodynamic diameter, DH, of uncoated and polyelectrolyte-coated ceria nanoparticles was determined from dynamic light scattering experiments. For that purpose, a Brookhaven 90Plus Particle Size Analyzer (Brookhaven Instruments Corporation) was used. We employed quasi-elastically light scattering method (QELS) using 15 mW solid state laser. The detector was in all cases placed at 90° with respect to the incident beam. The diffusion coefficients of particles were calculated from the autocorrelation function, and the diameters were obtained from the Einstein-Stokes equation.


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Electrophoretic mobilities of the nanoparticles were determined by means of ZetaPlus instrument (Brookhaven Instruments Corporation) at 25 °C from the measured Doppler shift in angular frequency and the applied electric field.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Physico-Chemical Characterization of Ceria Nanoparticles. Ceria nanoparticles used in this study were synthetized using the method presented in the Experimental section. The specific surface area was determined by the multiple Brunauer, Emmett and Teller (BET) method (Micromeritics, Gemini) using liquid nitrogen and it was found to be 133 m2 g−1. Prior to the determination of the surface area, the sample was outgassed at 100 C for one hour. The synthetized CNPs were additionally characterized in details by means of various aforementioned experimental methods such as X-ray powder diffraction, dynamic light scattering, bright-field scanning transmission electron microscopy and electrophoretic light scattering techniques to determine their size (crystallite size and hydrodynamic diameter), shape and electrophoretic mobility. 3.1.1. X-ray Powder Diffraction. The X-ray diffractogram of ceria sample is presented in Figure 1.


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Figure 1. XRD diffraction pattern of ceria nanoparticles. The observed diffraction maxima can be indexed to the cubic fluorite structure, confirming that the prepared sample was predominantly cerium(IV) oxide. The diffraction peaks are broad due to the small particle size. The diameter of primary ceria nanoparticles (nanocrystallite size) was estimated using Scherrer equation41,42 to be Dp ≈ 3.5 nm (k = 0.95, θ = 28.25 ̊, β1/2 = 0.048 rad). 3.1.2. Bright-field Scanning Transmission Electron Microscopy. BF-STEM micrographs of the cerium oxide samples (Figure 2) revealed that the nanoparticles were of a fairly uniform size. The distribution of estimated diameters was found to be ≈ 3.5 nm. Moreover, from the Figure 2 it could be confirmed that the synthetized ceria nanoparticles are roughly spherical.

Figure 2. BF-STEM micrographs of the synthetized ceria nanoparticles. Left: scale bar = 5 nm, right: scale bar = 2 nm. From the spectra obtained by electron energy loss spectroscopy (EELS), presented in Figure 3, the presence of Ce4+ was confirmed based on the shape, position and intensity ratio of M5 and M4 white lines43. Pre-edge line at O K edge and post-edge lines (Y and Y’ line) at M 4,5 edge are characteristic for Ce4+.44 All those extra lines are absent in the case of pure Ce3+.45 However, certain


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amount of Ce3+ could be detected at the surface of the samples (lower two spectra in Figure 3). The pre-edge line at O K edge is very faint, energies of M5 and M4 edges are lower, post-edge lines (Y, Y’) are relatively lower and the ratio of M5/M4 edge integrals (obtained with second derivative method) increased.

Figure 3. EELS spectra collected at the center and surface part of the nanoparticle indicating that beside Ce4+ small amount of Ce3+ is also present on the surface of the particle (normally observed on CeO2 nanoparticles inside the vacuum under electron irradiation).


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3.1.3. Dynamic Light Scattering. The hydrodynamic diameter of ceria nanoparticles was estimated using DLS method with the measurements being conducted at pH = 2. The hydrodynamic diameter as a function of time is shown in Figure 4.

Figure 4. Time variation of hydrodynamic diameter, DH, of synthetized CeO2 nanoparticles dispersed in NaCl solution. Clustering method is used to divide the particles in two groups according to their hydrodynamic diameter: larger nanoparticles (full symbols) and smaller nanoparticles (open symbols). Ic = 1∙10−2 mol dm−3, pH = 2, γ = 3 g dm−3,  = 25 °C. Inset: histogram of the presented data (red rhombi) overlapped with double Gaussian function (blue line).

It can be observed from Figure 4 that the synthetized ceria nanoparticles are not homogeneous in size, whereby they range from ≈ 5 nm to ≈ 25 nm in hydrodynamic diameter, seemingly showing two distinct distributions with regard to their size. We quantified these differences by employing clustering method (K-means clustering algorithm)46 which divided the particles into two groups.


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The average hydrodynamic diameters of these two groups of nanoparticles were DHL(CNP) = (23.82 ± 1.65) nm (for the group of larger (DHL) nanoparticles) and DHS(CNP) = (10.49 ± 4.32) nm (for the group of smaller (DHS) nanoparticles), respectively. It is worth noting that, upon clustering, the coefficient of variation of the more dispersed group (group of smaller nanoparticles, empty blue circles, Figure 4) is cv(DHS) = 7%. On the other hand, prior to the clustering (i.e. taking the entire set of measured values into account) coefficient of variation was evaluated to be cv(DH) = 37%, implying significantly larger dispersion of the measured values. This strongly suggests that the particles, with regard to their hydrodynamic diameter, belong to two distinct groups. The observed variation of the hydrodynamic diameter values (especially in the case of the group of smaller nanoparticles) could be due to the limitations of the accuracy of the measurement method since the results are close to the minimum of the particle size range measurable with the available instrument. 3.1.4. Electrophoretic Mobility Measurements. One of the prerequisites for studying the adsorption behavior of polyelectrolytes on metal oxide aqueous interface is to determine the isoelectric point. Therefore, in our study the isoelectric point of pure ceria nanoparticles was obtained from electrophoretic mobility measurements at two ionic strength values (Ic = 110−2 mol dm−3 and Ic = 110−3 mol dm−3). The isoelectric point is determined to be pHiep = 6.7 ± 0.3 at Ic = 110−3 mol dm−3 and pHiep = 6.8 ± 0.3 at Ic = 110−2 mol dm−3 (Figure 5) which is in accordance with the literature values for ceria nanoparticles.47 No significant influence of ionic strength on the isoelectric point was observed in the examined ionic strength range.


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Figure 5. The pH dependence of electrophoretic mobility of ceria nanoparticles in the presence of NaCl at Ic = 110−2 mol dm−3 and Ic = 110−3 mol dm−3;  = 1 g L−1,  = 25 °C. 3.2. Physico-chemical Characterization of Ceria Nanoparticles Coated with PSS. As stated in the introduction, the main aim of the study presented here was to explore the interactions between CNPs and polyelectrolytes in more details. Therefore, the CNPs were coated with PSS using the procedure defined in Section 2.2. As a first test a visual examination (sedimentation test) of both PSS-coated and uncoated samples was performed at various pH values. Sedimentation of uncoated samples which was observed at pH = 5 (Figure 6) proved that stability is increased by coating CNPs with PSS, but also served as an indicator that PSS coating actually occurs.


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Figure 6. Digital images of suspensions of uncoated and PSS-coated ceria nanoparticles. From left to right suspensions of: ceria nanoparticles (pH = 2), ceria nanoparticles (pH = 5), PSS-coated ceria nanoparticles (pH = 2) and PSS-coated ceria nanoparticles (pH = 5).

3.2.1. High-resolution Transmission Electron Microscopy. HRTEM micrographs of the ceria samples coated with PSS (Figure 7) revealed that the polyelectrolyte chains are adsorbed on the surface of ceria nanoparticles.


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Figure 7. HRTEM micrograph of the coated ceria nanoparticles (with enlarged detail). Red arrow indicates the polyelectrolyte layer. Left: scale bar = 10 nm, right: scale bar = 5 nm.

3.2.2. Dynamic Light Scattering. The same technique as in the case of uncoated ceria nanoparticles was used to determine the hydrodynamic diameter of PSS-coated ceria nanoparticles, with the measurements again performed at pH = 2 as shown in Figure 8.

Figure 8. Time variation of hydrodynamic diameter, DH, of PSS-coated CeO2 nanoparticles dispersed in NaCl solution. Clustering method is used to divide the particles in two groups according to their hydrodynamic diameter: larger nanoparticles (full symbols) and smaller nanoparticles (open symbols). Ic = 110−2 mol dm−3, pH = 2, γ = 3 g dm−3,  = 25 °C. Inset: histogram of the presented data (red rhombi) overlapped with double Gaussian function (green line).


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Due to the fact that the ceria nanoparticles are positively charged at pH = 2 (see Figure 5), and PSS being negatively charged in the entire pH range, the coating of ceria nanoparticles can occur. In this respect, following the trend observed in the uncoated case, the DLS measurements also show two distributions of nanoparticles. Therefore we again applied clustering algorithm to characterize the data and separate the particles in groups according to their size. The results revealed that, after the adsorption of polyelectrolyte, the average hydrodynamic diameters of the particles belonging to the groups of larger and smaller nanoparticle are DHL(CNP-PSS) = (32.88 ± 4.69) nm and DHS(CNP-PSS) = (10.26 ± 3.52) nm, respectively. By comparing the average hydrodynamic diameters of uncoated and PSS-coated ceria nanoparticles, one can observe that there is no significant difference in the size of smaller particles (compare DHS(CNP) and DHS(CNPPSS)) before and after polyelectrolyte adsorption. This, together with TEM measurements, implies that the effect of the PSS coating on small particles is too small to be detected with DLS method. However, if one compares groups of nanoparticles with larger hydrodynamic diameter (compare DHL(CNP) and DHL(CNP-PSS)), one finds that the difference in their diameter is δ ≈ 9 nm. This suggests that polyelectrolyte coating indeed took place and that the subsequent analysis can be performed on the data gathered on the larger group of particles, both in uncoated and coated case (Figure 4 and Figure 8, respectively). In this respect, the aforementioned size difference (δ) can be interpreted as the thickness of the polyelectrolyte layer. 3.2.3. Electrophoretic Mobility Measurements. Additionally to the estimation of the isoelectric point, electrophoretic mobility measurements could also be applied for confirming the adsorption of polyelectrolytes (in this case PSS) on metal oxide (nano)particles, as presented in Figure 9.


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Figure 9. The pH dependence of electrophoretic mobility of ceria nanoparticles coated with PSS in the presence of NaCl, γ = 1 g dm−3, Ic = 110−2 mol dm−3, c(PSS) = 510−3 mol dm−3,  = 25 °C. The experimental results obtained using multiple techniques confirm that adsorption of PSS actually takes place at examined experimental conditions. These results present the experimental basis for the interpretation of data in terms of the modified Ohshima model. 3.3. Interpretation of Experimental Data using the Modified Ohshima Model. In addition to characterizing uncoated and PSS-coated ceria nanoparticles, the aim of this study was to obtain the adsorption parameters for the studied system. Using the Smoluchowski theory one can in principle derive zeta potential, , from the electrophoretic mobility, 𝜇, with the implicit assumption that the particles are perfectly rigid. However, in the regime of soft particles, it is necessary to apply corrections, i.e. models which account for porous nature of such particles. In that respect, we decided to employ Ohshima model29,36,37, where electrophoretic mobility, 𝜇, shows dependence on several parameters such as charge density of the polyelectrolyte layer, ZeN, ionic strength of the solution, Ic, and the value 1/λ known as the electrophoretic softness37,48,49. κm is the effective


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Debye-Hückel parameter of the surface polyelectrolyte layer. The expression derived by Ohshima36 and used in this work is given by

𝜓0 𝜓𝐷𝑂𝑁 𝜀0 𝜀𝑟 𝜅𝑚 + 𝜆 𝑏 𝑍𝑒𝑁 𝜇= 𝑓( )+ 2 , 1 1 𝜂 𝑎 𝜂𝜆 𝜅𝑚 + 𝜆

(1)

where εr represents the relative permittivity, ε0 is the permittivity of the free space, e is the elementary charge and η is the viscosity of the solvent. The number density and valence of charge that is present on polyelectrolyte-coated layer are denoted by N and Z, respectively. Two potentials in the above expression, namely surface potential at the boundary between the surface layer and the surrounding electrolyte solution (potential of the polyelectrolyte layer) and the Donnan potential, are represented by 𝜓0 and 𝜓𝐷𝑂𝑁 , respectively. The diameter of polyelectrolyte-coated nanoparticles is represented by b, and the diameter of the bare nanoparticle is denoted by a. In order to apply the above model it is necessary to experimentally determine the dependence of mobility of (in this case) polyelectrolyte-coated nanoparticles on ionic strength of solution. Detailed procedure and the obtained parameters are presented in the following section.


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3.3.1. Parameters for Soft Particle Electrophoretic Mobility Modeling. To calculate the correction 𝑏

factor in Eq. 1 (i.e. 𝑓 (𝑎)), we used the hydrodynamic diameter of the uncoated and coated ceria nanoparticles, both obtained using DLS. More precisely, average hydrodynamic diameters of the larger group of uncoated (DHL(CNP) = 23.82 nm, see section 3.1.3.) and coated (DHL(CNP-PSS) = 32.88 nm, see section 3.2.2.) ceria nanoparticles denote parameters a and b, respectively. On the

Figure 10. Mobility of polyelectrolyte-coated cerium oxide nanoparticles with changing ionic strength of prepared suspension (red circles). Blue line represents the best fit of the modified Ohshima model. other hand, as the expressions of zeta and Donnan potential, as well as the effective Debye-Hückel parameter, depend on two of the parameters that are also incorporated in Eq. 1, namely the product of the number density and valence of charge present on polyelectrolyte (ZeN) and electrophoretic softness (1/λ), one can expand the above equation so that it explicitly depends on these two parameters. These parameters represent the two unknowns and can be readily determined


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experimentally by the procedure mentioned in ref 29. We obtained the parameters by fitting the experimentally measured electrophoretic mobility as a function of ionic strength (see Figure 10) of the solution according to Eq. 1. The resulting fit is also presented in Figure 10. We can observe that the result of the fit nicely follows the experimentally measured values, enabling us to determine two aforementioned parameters of the system. Thereby we found the electrophoretic softness to be 1/λ = 3.03 nm, while the charge density of the polyelectrolyte amounts to ZeN = −0.032 mol dm−3. The value we obtained for the softness parameter for CNP-PSS system lies in a reasonable range when compared with the value determined by Lowry and coworkers50 for PSScoated reactive nanoiron particles (1/λ = 4.2 nm). 3.3.2. Adsorption Density. Electrophoretic softness, 1/λ, and charge density of polyelectrolyte layer, ZeN, represent parameters via which it is possible to calculate the adsorption density of, in this case, PSS on ceria nanoparticles.29 In this respect, the charge density is the consequence of the ionized sulfonate groups present in every monomer, increasing proportionally with the PSS adsorption. More precisely, each monomer contributes one unit charge stemming from –SO3 group. Adsorption density of polymer on CNPs surface was derived using

𝛤=𝑁

(𝐷𝐶𝑁𝑃 + 2𝛿)3 𝑀, (𝐷𝐶𝑁𝑃 )2

(2)

where 𝛤 represents the adsorption density of PSS onto ceria nanoparticles (i.e. mass of the adsorbed polyelectrolyte per surface area, given in mg m‒2). Moreover, molar mass of monomer unit is denoted as M (M = 206.191 g mol−1), DCNP is the diameter of nanoparticles estimated via XRD (nanocrystallite size) and confirmed via BF-STEM (≈ 3.5 nm), while δ represents the


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thickness of the polyelectrolyte layer found to be 9 nm (see section 3.2.2). We thereby find that, under the measured conditions, the adsorption density of PSS on CNPs amounts to Γ = 5.4 ± 2.7 mg m−2. It is interesting to compare the results obtained here for PSS-coated ceria nanoparticles with earlier investigations of PAA adsorption of CNPs. Interpolation of the data obtained by Saraf et al.29 to the values of ionic strength used in this study gives the same order of magnitude for , showing that the results are comparable. Moreover, herein obtained electrophoretic softness value roughly corresponds to the value obtained for the lowest molecular weight case from the ref 29. Seemingly, there is no significant difference in adsorption between weakly and strongly charged polyelectrolytes, at very least in the regime of low polyelectrolyte concentration. Since the experiments with ceria nanoparticles in biological environment require a stable dispersion, the results obtained in our study could be used to predict the behavior of nanoparticles in polyelectrolyte (i.e. PSS) solution which could enable the preparation of suspension having sufficient stability, in turn leading to even better future applications of ceria nanoparticles in the field of biomedicine. Moreover, the obtained results could also present the basis for the further investigations and possible applications of ceria nanoparticles as the templates for the formation of polyelectrolyte multilayers.

4. CONCLUSIONS The significant changes of physical properties induced by adsorption of polyelectrolytes on metal oxide nanoparticles presented in this paper confirm the importance of studying this phenomenon. Synthetized ceria nanoparticles are spherical, with the diameter (nanocrystallite size) being ≈ 3.5 nm. It was shown that, at the examined conditions, isoelectric point (determined to be pHiep = 6.75


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± 0.35) does not depend on the ionic strength of solution significantly. According to the hydrodynamic diameters CNPs could be divided into two groups, with the average of the larger and smaller nanoparticles being DHL(CNP) = (23.82 ± 1.65) nm and DHS(CNP) = (10.49 ± 4.32) nm, respectively. We clearly demonstrated (from HRTEM, DLS and electrophoretic mobility measurements) that adsorption of PSS on ceria nanoparticles takes place in the examined experimental conditions. After the adsorption of PSS, the average hydrodynamic diameter of the larger and smaller nanoparticles is DHL(CNP-PSS) = (32.88 ± 4.69) nm and DHS(CNP-PSS) = (10.26 ± 3.52) nm, respectively. By comparing the average hydrodynamic diameters of uncoated and PSS-coated ceria nanoparticles, in the case of nanoparticles with larger hydrodynamic diameter, one finds that the difference in their diameter is δ ≈ 9 nm. The electrophoretic mobility measurements of PSScoated nanoparticles as a function of pH also show that the suspension is stable up to pH = 9. We applied the modified Ohshima model for determination of adsorption parameters. The obtained parameters confirmed the applicability of that model for studying the electrical interfacial layer between ceria nanoparticles and aqueous polyelectrolyte solution. Additionally, we find that the calculated parameters are comparable with the results from the previous studies.29 In summary, it should be again stressed that the obtained results provide systematic characterization of cerium oxide/polyelectrolyte aqueous solution interface which facilitates the understanding of the fundamental principles governing the adsorption process. Moreover, it was shown that addition of polyelectrolytes (that act as the stabilizers) to the suspension of nanoparticles just after their synthesis increases the stability of the system. However, that also serves as an indicator that PSS coating actually occurs. Ultimate goal of these studies would be the preparation of polyelectrolyte-coated nanoparticles with finely tunable properties, tailored to


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specifications of each possible application. This is particularly important due to the potential use of such coated nanoparticles in various branches of industry and biomedicine. Our study further gives an insight into the prediction of the behavior of nanoparticles in a strong polyelectrolyte solution. This in turn could enable the preparation of even more stable suspensions of nanoparticles, thus leading to improved applications of ceria nanoparticles.

ACKNOWLEDGMENT The Croatian Science Foundation under the project IP-2014-09-6972 supported this research. Results presented in this work were partially obtained in the framework of CEEPUS networks Training and research in environmental chemistry and toxicology and Colloids and nanomaterials in education and research. The authors are thankful to Marijana Đaković for XRD experiments and Branka Njegić Džakula for BET analysis. Corresponding Author *E-mail: [email protected]

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