Tuning the Interaction of Nanoparticles from Repulsive to Attractive by

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Tuning the Interaction of Nanoparticles From Repulsive to Attractive by Pressure Martin A. Schroer, Florian Schulz, Felix Lehmkühler, Johannes Möller, Andrew James Smith, Holger Lange, Tobias Vossmeyer, and Gerhard Grübel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06847 • Publication Date (Web): 15 Aug 2016 Downloaded from http://pubs.acs.org on August 20, 2016

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The Journal of Physical Chemistry

Tuning the Interaction of Nanoparticles From Repulsive to Attractive by Pressure Martin A. Schroer,∗,†,‡,# Florian Schulz,¶,§ Felix Lehmkühler,†,‡ Johannes Möller,k Andrew J. Smith,⊥ Holger Lange,¶,§ Tobias Vossmeyer,¶ and Gerhard Grübel†,‡ †Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany ‡The Hamburg Centre for Ultrafast Imaging (CUI), Luruper Chausee 149, 22761 Hamburg, Germany ¶Institute of Phyical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany §The Hamburg Centre for Ultrafast Imaging (CUI), Luruper Chaussee 149, 22761 Hamburg, Germany kESRF - The European Synchrotron, CS 40220, 38043 Grenoble Cedex 9, France ⊥Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Didcot, UK # present address: European Molecular Biology Laboratory (EMBL) Hamburg c/o DESY, Notkestr. 85, 22607 Hamburg, Germany E-mail: [email protected] Phone: +49-40-89902650

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Abstract We report a high pressure small angle x-ray scattering (SAXS) study on concentrated aqueous suspensions of gold nanoparticles coated with a poly(ethylene glycol) (PEG)-based ligand shell. Analyzing the experimental SAXS data with a liquid-state theory approach, we find a pressure-induced transition of the interparticle interactions from repulsive to attractive in the pressure range from 1 bar to 4000 bar that also depends on the particle concentration. This transition is related to structural changes of the ligand shell which is compressed with increasing pressure. Our study highlights the important, yet unexplored role of pressure for the interaction of nanoparticle systems.

INTRODUCTION Structure and phase transitions in liquids are of general interest both for fundamental research and technical applications. Similar phenomena are also present in nanoparticle suspensions. In contrast to molecular liquids, the interaction potentials between nanoparticles can be modified at will, allowing detailed studies of their influence on the resulting structures. This can be achieved either by chemical or by physical means. While modification of the particles by chemical methods allows tailoring of the properties such as surface charge density, hydrophobicity, or selectivity, 1–3 the interaction potential between particles can also be changed externally by the addition of salt, 4,5 or cosolvents, 6 or by changing temperature, 7 or application of external forces. 8–10 Interparticle interactions determine the stability of nanoparticle suspensions, nanoparticle assembly and their reversible or irreversible aggregation, which is especially relevant for high particle concentrations. To control interparticle interactions and chemical and colloidal stability it is a common approach to coat the nanoparticles with tailored polymer ligands, by that obtaining a hard core – soft shell particle. Thus, the understanding of such systems is of high interest. Many theoretical and experimental approaches have been conducted, motivated by the relevance for applications but also by fundamental interest. 11–17 Here, gold nanoparticles (AuNP) are particularly interesting as 2


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these exhibit low dispersity, and the possibility to synthesize and purify poly(ethylene glycol) (PEG)-coated AuNP with high chemical and colloidal stability in a controlled and reproducible manner. 18–20 AuNP with a polymer coating of poly(ethylene glycol)-based ligands are well established and can be considered as reference system, especially in the context of nanomedicine. 11 Besides their technological and biomedical relevance, suspensions of AuNPs are also useful and interesting model systems to study nanoparticle interactions, and how these can be influenced by pressure. As such studies demand stable, concentrated nanoparticle suspension, PEG-based ligands are perfectly suited for this purpose. Only recently pressure has been employed as an important parameter to modify the interparticle interactions of nanoparticle and colloidal-like systems, such as colloidal crystallites and protein suspensions. 21–24 It has been demonstrated to affect the interaction potential of proteins, 22 even allowing the control of liquid-liquid phase separation in protein solutions. 23 Although pressure is a fundamental thermodynamic parameter affecting the system’s volumetric properties, corresponding studies using high pressures are still very limited due to experimental challenges. The structure and phase transitions of concentrated particle suspensions can be studied in detail using scattering techniques. In particular, these methods can be employed to determine the underlying interactions. 22,23,25–27 For high pressure measurements on nanoparticle systems, small angle x-ray scattering (SAXS) is a powerful technique as it allows the interrogation of structures up to several hundred nanometers in size and the use of hard x-rays enables high pressure studies due to their penetration power through the windows of pressure cells. The high scattering contrast of AuNP qualifies them as a model system for SAXS studies at high concentrations and high pressures. Here, we report a high pressure SAXS study on concentrated aqueous suspensions of AuNPs that are coated with a PEG-based ligand, α-methoxypoly(ethylene glycol)-ω-(11mercaptoundecanoate) (PEGMUA). This ligand provides high chemical and colloidal stabilization 20 and is therefore perfectly suited for studies with high concentrations as needed for the determination of the interparticle interactions. We find a reversible pressure-induced

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transition of the interparticle interactions from purely repulsive to attractive, which takes place in the pressure range from 1 bar to 4000 bar. This transition, which also depends on the particle concentration, can be related directly to structural changes of the ligand shell that is compressed with increasing pressure.

METHODS The synthesis of AuNP with very low dispersity of a sufficiently large quantity and their effective stabilization with PEGMUA, a tailored PEG-ligand, were described previously. 18–20 The PEGMUA ligand used in this study had a molecular weight of about 5000 g/mol (5 kDa) corresponding to a Flory radius in the range 5-6 nm, 28 and a molecule length of roughly 45 nm in an all-trans conformation. The hypothetical thickness of the polymer brush lies between these values. 28 The ability to synthesize large amounts of homogeneous AuNP with very high colloidal stability allowed an increase of the concentrations of the colloidal solutions by a factor of about 1000, to achieve volume fractions in the low percent range without affecting the colloidal integrity of the samples. High pressure SAXS measurements were performed at beamline I22, Diamond Light Source, Didcot, U.K., using a high pressure sample cell. 29 An x-ray energy of E = 18 keV (wavelength λ = 0.68 ˚ A) was utilized to penetrate the two sealing diamond windows of the pressure cell. Concentrated suspensions of AuNPs with PEGMUA coatings were filled into polycarbonate capillaries, sealed, and used for SAXS measurements. Two-dimensional scattering patterns were recorded with a Pilatus3-2M detector at a sample-detector distance of 6.704 m using a 160 × 300 (v × h) µm2 x-ray beam. The exposure time per SAXS pattern was 1 s. No radiation damage has been observed for any conditions reported herein. A pressure range of p = 1 – 4000 bar was covered at a fixed temperature T = 21 ◦ C. The pressure value was determined by full bridge strain gauge sensors. 29 The AuNPs with PEGMUA coating were suspended in tris(hydroxymethyl)aminomethane (Tris, 25 mM) solution to

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keep the pH value at high pressures constant at pH 7.4. 30 The scattering patterns were azimuthally integrated and corrected for the background signals from the buffer and the pressure cell, which were measured separately at each pressure. The total SAXS signal is given as I(Q) ∝ P (Q) · S(Q) wherein Q = 4π/λ sin(Θ/2) denotes the wave vector transfer and Θ the scattering angle. A diluted particle concentration (c = 20 nM, Φ < 0.05 vol%) was measured to determine the particle form factor P (Q) for all pressures. For higher concentrations (c = 3 – 6 µM, Φ = 6 – 12 vol%) interparticle correlations give rise to a structure factor contribution S(Q). Studying this structure factor under varying conditions allows determination of their effect on the interparticle interaction potential. 22,23 In the case of concentrated AuNPs covered with PEGMUA the experimental data can be well described using the Baxter sticky hard sphere (SHS) potential, which consists of an attractive interaction between the particles, additional to a hard core repulsion. It reads 31,32

    ∞    

, for r < σ

VSHS (r) = ln 12 τ ∆ σ+∆  kB T      0

, for σ ≤ r ≤ σ + ∆ .

(1)

, for σ + ∆ < r

Herein, r denotes the interparticle distance, T the absolute temperature, kB is Boltzmann’s constant, and σ the particle diameter. The inverse stickiness parameter τ −1 characterizes the attraction of the interaction potential. In the limit of ∆ → 0, the Baxter model has an analytical solution for the structure factor S(Q) in Percus-Yevick approximation. 32 The stickiness parameter is related to the reduced second virial coeffiecent, which characterizes the interaction via 23,33

b2 = 1 −

5

1 . 4τ


(2)


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Herein, b2 is normalized to the contribution of a pure hard sphere potential. For b2 > 0 the interaction is mainly repulsive, whereas for b2 < 0 it is attractive. The experimental structure factors are determined by dividing the SAXS curves from the concentrated suspensions by the diluted ones for all pressures. The so-determined structure factor data are refined by employing the sticky hard sphere structure factor in the PercusYevick approximation using the effective particle radius RSHS , the volume fraction φ, and the stickiness parameter τ as independent fitting parameters.

RESULTS AND DISCUSSION a)

b) p p

P(Q)

Figure 1: a) SAXS intensity of a suspension of AuNPs covered with PEGMUA as a function of pressure p (particle concentration c = 5.7 µM). The SAXS curve for a highly diluted sample reflecting the particle form factor P (Q) is shown for comparison (black diamonds). b) Corresponding structure factor S(Q) as a function of pressure. Curves were shifted for clarity. Solid lines are fits to the data within the sticky hard sphere model. The SAXS intensity I(Q) of a concentrated suspension of AuNPs covered with 5 kDa PEGMUA as a function of hydrostatic pressure is shown in Fig. 1a) (circles) and the corresponding structure factors S(Q) in Fig. 1b). The pronounced oscillations of I(Q) for Q > 0.7 nm−1 stem from the form factor and are not changing with increasing pressure. Refinement of the scattering signal from a diluted suspension (diamonds) with the form factor of polydispere spheres yields a particle radius of Rcore = (6.15 ± 0.05) nm and a size polydispersity

∆Rcore Rcore

= 7 %. This value is in agreement with that obtained by transmission 6


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electron microscopy for the gold core of the AuNPs (TEM, RTEM = 5.98 nm,

∆RTEM RTEM

= 5 %,

e Fig. 2a)). Due to the low scattering contrast of the PEGMUA shell in water (∆ρ ≤ 40 nm 3) −

e compared to that of the gold core (∆ρ ≈ 4300 nm 3 ), the dominant scattering contribution −

arises from the latter one, i.e. the PEGMUA shell is hardly visible for the diluted suspension, and therefore the form factor only refers to the gold core. For Q < 0.7 nm−1 , the SAXS curves of the concentrated suspensions exhibit maxima due to interparticle correlations that shift to larger Q and whose amplitude increases as p is enhanced. In addition, the forward scattering intensity I(Q → 0) becomes stronger, indicating the transition from repulsive to attractive interaction. These pressure-induced changes are more directly visible from the structure factor obtained by dividing the intensity by the form factor (Fig. 1b)). a)

b)

1.12 Q max

1.1

S(Q max)

1.08

Qmax [nm -1]

1.06 1.04

S(Qmax)

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1.02

0.24 0.2 0.16 0.12

0

500 1000 1500 2000 2500 3000 3500 4000

p [bar]

c)

d)

Figure 2: a) TEM image of the AuNPs. b) Position, Qmax , and amplitude, S(Qmax ), of the first maximum of the structure factor S(Q) as a function of pressure. c) Particle radius obtained from the refinement using the sticky hard sphere potential, RSHS , radius of the Au core, Rcore , and the determined shell thickness, Rshell , as a function of pressure. d) Volume fraction φ as a function of pressure. For all subfigures, closed, black symbols refer to a particle concentration of c = 3.0 µM and open, red symbols to c = 5.7 µM. The position, Qmax , and amplitude, S(Qmax ), of the first structure factor maximum with

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increasing pressure for two different particle concentrations are shown in more detail in Fig. 2b). For both concentrations (c = 3.0 µM and 5.7 µM), pressure leads to a continuous shift of Qmax to larger Q-values indicating a pressure-induced reduction of the average interparticle distance dmax =

2π Qmax

from dmax = (42 ± 3) nm to dmax = (27 ± 1) nm. This effect is

independent of the sample concentration. The amplitude S(Qmax ) exhibits a more complex pressure-dependence. For c = 3.0 µM, pressures up to 750 bar lead to a decrease of the amplitude. Increasing pressures above this results in a continuous enhancement. A similar behavior is also present for the higher particle concentration, although both the amplitude and the transition pressure are shifted to higher values. In order to quantify the observed pressure-induced changes of S(Q), the experimental data were refined using the structure factor model of sticky hard sphere systems with the effective radius RSHS , the volume fraction Φ, and the stickiness parameter τ as the fitting parameters (solid lines in Fig. 1, see methods for details). For pressures up to 1000 bar, the system behaves similar to hard spheres interacting via short range repulsion. When increasing pressure, the system changes becoming attractive. The effective radii RSHS obtained by the fits as a function of pressure are shown in Fig. 2c) (circles). A value of RSHS = (20 ± 1) nm at 1 bar is determined which is larger than that of the gold core and does not depend on the particle concentrations. Thus, in case of the concentrated suspensions, the obtained radius refers to a full effective particle radius, i.e. Au core and PEGMUA shell. Although the shell thickness cannot be determined from form factors directly, using concentrated particle suspensions allows it to obtain information on its size Rshell = RSHS − Rcore . The thickness of the PEGMUA shell agrees well with theory 28 and complementary dynamic light scattering experiments yielding the hydrodynamic radii of both pure AuNPs, i.e. citrate stabilized, and coated particles (data not shown), indicating a stretched conformation of the polymer chains due to a high grafting density. For both concentrations studied the particles are in close contact as the interparticle distance is close to the particle diameter. Increasing pressure leads to a decrease of RSHS . Since the Au core of the NPs does not

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change with pressure (form factor measurement, solid line), the change of the particle size is due to a compression of the thickness of the PEGMUA shell (diamonds in Fig. 2c)). The pressure-induced decrease of the shell thickness thus leads to a decrease of the interparticle distance dmax as discussed above. Whereas there is no indication of a crowding induced compression of the ligand shell with increasing particle number for the concentrations studied, pressure effectively reduces the shell thickness. As the PEGMUA ligand at 1 bar is strongly hydrated and thus extended, increasing pressure is likely to induce a collapse of the ligand chain and thus leads to a partial deswelling. Pressure-induced deswelling has been reported also for microgel particles of poly-(N -isopropylacrylamide) 34,35 and was attributed to a decreasing solubility of the polymer with increasing pressure. 34 The volume fraction Φ, referring to the total particle size RSHS , is determined by the refinement of the structure factor data and depicted in Fig. 2d). For particle concentrations of c = 3.0 µM and 5.7 µM, volume fractions of Φ = 0.05 and 0.09 are obtained for 1 bar, respectively, from the fitting. An increase of pressure up to 1500 bar leads, basically, to a decrease of Φ for both concentrations. For the lower concentration sample a steeper decrease is present for larger pressures. For c = 5.7 µM an increase of pressure beyond 1500 bar does not reduce Φ anymore and the volume fraction stays approximately constant. It has to be noted that within the experimental error, the decrease of Φ for c = 3.0 µM can be fully explained by the compression of the PEGMUA shell resulting in a smaller effective size and thus volume fraction occupied by the particles. For the higher particle concentration, there are differences in the pressure range of 1000 - 2000 bar that might point to a different compressibility of the particle suspension. Besides the pure particle volume reduction with pressure, interparticle interactions can also affect the volume reduction. The pressure-dependent stickiness parameter τ as determined by fitting S(Q) with the sticky hard sphere model for both concentrations is shown in Fig. 3a). With increasing pressure, τ decreases indicating a transition from repulsive to attractive. This can be seen

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a)

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b)

Figure 3: a) Stickiness parameter τ and b) reduced second virial coeffiecent b2 as a function of pressure. The dashed line refers to b2 = 0 at which repulsive and attractive interaction compensate each other. more clearly by looking at the corresponding reduced second virial coefficient b2 (Fig. 3b)), which is known to be sensitive to the type and strength of interactions. 23,33,36,37 For a particle concentration of c = 3.0 µM, pressure leads to a continuous change from repulsive to attractive where the transition is reached at p ≈ 2000 bar. At the higher particle concentration, c = 5.7 µM, there is also a rapid decrease of b2 for pressures smaller than 1500 bar. Further increase in pressure, between 1500 bar and 3500 bar, does not change the interaction significantly. At the highest pressures, the attractive interaction becomes dominant. This reveals that pressure allows continuous manipulation of the pair interaction between the PEGMUA-coated AuNPs from repulsive to attractive. This effect seems to be concentration dependent, in particular at higher pressures above 1500 bar. For concentrated suspensions, it becomes more complex, most probably due to additional volume effects as is suggested also by the pressure dependence of the volume fraction Φ. The application of pressure leads both to a decrease of the PEGMUA shell size, and an increase of the particle attraction. Hence, it is likely that there is a correlation between both findings. Without a stabilizing ligand shell, AuNPs aggregate due to attractive vander-Waals interactions. 2,16,17 In order to stabilize the colloidal suspension against this, the particles are covered with PEGMUA to avoid aggregation by counteraction van-der-Waals attraction with steric repulsion. Compressing the PEGMUA shell by pressure and decreasing 10


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its thickness understandably seems to reduce the steric repulsion. Thus, the underlying attractive van-der-Waals interaction will become more dominant. This increase of attraction due to the reduced repulsion of the ligand shell is then reflected by a decrease of the stickiness parameter and the second virial coefficient. Interestingly, it is only a slight decrease of the shell thickness of 4 nm that already leads to a drastic change in the pair interaction. Notably, after pressure release, no aggregates were detected in the samples by dynamic light scattering (data not shown) on the same samples the SAXS measurements have been performed. This indicates that there was no severe radiation damage and that the pressure-induced changes are reversible. Such changes, from repulsive to attractive interactions, as observed here for pressure, are typically achieved in colloidal suspensions by addition of cosolutes. In the case of aqueous suspensions of charge stabilized NPs, addition of salt leads to a screening of the particle charges by the corresponding ions and thus reduces the repulsive Coulomb interaction. 4,5 Increasing the ion concentration thus allows enhancement of this screening effect. The addition of non-adsorbing polymers can also induce a change of the interaction. This so-called depletion interaction depends solely on the polymers’ radius of gyration and concentration. 6 As shown here, pressure can similarly induce a transition of particle interactions from repulsive to attractive. However, in contrast to additives, pressure allows reversible switching between the states.

CONCLUSIONS Summarizing, the presented study shows that pressure allows to influence the interaction potential between PEGMUA-coated gold nanoparticles. Using concentrated, highly stable AuNP suspensions, we were able to determine the particle size, the ligand shell thickness, and the second virial coefficient by SAXS. We found an increase of the interparticle attraction with increasing pressure that correlates to a decrease of the shell thickness. For a pressure

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range from 1 bar to 4000 bar, the effective interaction changes from repulsive to attractive. This effect is fully reversible when releasing the pressure. These findings reveal that pressure is an important parameter for the interaction potential of colloidal suspensions, allowing a change in the sense of the interaction, as demonstrated here for coated AuNPs. Future experimental studies might address and compare the pressure-response of thinner PEG-based ligand shell thicknesses. Additional theory and simulation studies, moreover, might allow for a molecular description of our findings. More generally, the application of pressure gives the possibility of inducing structural transitions in soft matter systems such as aggregation, cluster formation, or phase separation phenomena, without the addition of additional components. This enables in-situ measurements on the exact same sample, crucial for many fundamental studies. Moreover, as pressure only affects the volumetric properties of the system, phase transitions might be induced as well, leading to even richer phase diagrams.

Acknowledgement This work has been supported by the excellence cluster “The Hamburg Centre for Ultrafast Imaging, Structure, Dynamics, and Control of Matter at the Atomic Scale” of the DFG. We acknowledge Diamond Light Source for time on I22 under proposal SM11192.

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