Hydration Layer-Mediated Pairwise Interaction of Nanoparticles

Dec 28, 2015 - See Wee Chee , Zhaslan Baraissov , N. Duane Loh , Paul T. Matsudaira , and Utkur Mirsaidov .... Utkur Mirsaidov , Utkarsh Anand , Guanh...
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Letter pubs.acs.org/NanoLett

Hydration Layer-Mediated Pairwise Interaction of Nanoparticles Utkarsh Anand,†,‡,§,∥ Jingyu Lu,†,‡,§,∥ Duane Loh,†,‡ Zainul Aabdin,†,‡,§,∥ and Utkur Mirsaidov*,†,‡,§,∥ †

Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore, 117551 Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore, 117543 § Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546 ∥ NanoCore, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 ‡

S Supporting Information *

ABSTRACT: When any two surfaces in a solution come within a distance the size of a few solvent molecules, they experience a solvation force or a hydration force when the solvent is water. Although the range and magnitude of hydration forces are easy to characterize, the effects of these forces on the transient steps of interaction dynamics between nanoscale bodies in solution are poorly understood. Here, using in situ transmission electron microscopy, we show that when two gold nanoparticles in water approach each other at a distance within two water molecules (∼5 Å), which is the combined thickness of the hydration shell of each nanoparticle, they form a sterically stabilized transient nanoparticle dimer. The interacting surfaces of the nanoparticles come in contact and undergo coalescence only after these surfaces are fully dehydrated. Our observations of transient steps in nanoparticle interactions, which reveal the formation of hydration layer mediated metastable nanoparticle pairs in solution, have significant implications for many natural and industrial processes. KEYWORDS: Hydration force, steric force, intermolecular forces, DLVO, nanoparticle, in situ TEM

H

nanoparticle attachment17−20 and coalescence21 during the growth and the assembly of nanostructures. In these studies, tracking the movement of interacting nanoparticles, separated by a few nanometers, showed that dipolar interactions assist in pairwise approach and alignment of nanoparticles. This method provides an opportunity to directly probe the effect of shortrange solvation forces on interaction dynamics between nanoparticles in solution. Here, using dynamic in situ TEM imaging,22−24 we visualized and quantified the pairwise interaction between gold nanoparticles in water, showing that their approach toward each other is stalled by the primary hydration shell of each gold nanoparticle, which is one water molecule thick. By contrasting the pairwise interaction between pure gold nanoparticles with the pairwise interaction of surfactant-coated gold nanoparticles in water, we found that the interaction range is set by the size of surface-bound molecules. Our analysis reveals that these nanoparticles jump to contact only when the primary hydration shells with the one-molecule-thick water layer of both interacting surfaces are expelled. Our experimental setup consisted of a thin (∼20−30 nm) layer of a liquid specimen25 with nanoparticles sandwiched

ydration forces between nanoscale objects in solution play an important role in many areas of science and engineering. They are responsible for colloidal stability,1 crystallization,2 the interactions between biological molecules,3 and lubrication.4,5 The repulsive hydration force between two surfaces in water is believed to arise due to a layer of surfacebound water molecules and steric effects of other water molecules trapped between these interacting surfaces6,7 and thus regulates the interaction between nanoparticles in solution. From macro- and microscopic studies, it is known that these short-range repulsive forces may prevent surfaces from approaching closer than a distance of several water molecules.6 The exact origin of these forces is still not fully understood, and it cannot be explained by classical continuum models that account only for a repulsive diffuse double-layer force and the attractive van der Waals (vdW) force in solution.7,8 At short distances between two macroscopic hydrophilic surfaces, these forces may dominate over other intermolecular forces and give rise to oscillatory9,10 and monotonically repulsive forces.11−16 However, it remains unclear how repulsive hydration forces affect the interaction dynamics at the nanoscale. For example, the effects of hydration forces and hydration layers on nanoparticle attachment remain experimentally unresolved. Recent in situ transmission electron microscopy (TEM) observations of nanoparticle interactions in liquids revealed that electrostatic forces between nanoparticles play a crucial role in © 2015 American Chemical Society

Received: November 25, 2015 Revised: December 23, 2015 Published: December 28, 2015 786

DOI: 10.1021/acs.nanolett.5b04808 Nano Lett. 2016, 16, 786−790

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Figure 1. Pairwise interaction of gold nanoparticles in aqueous solution. Typical time-series TEM images of two gold nanoparticles undergoing coalescence (A) in pure water and (B) in the presence of 1 mM CTAB. (C,D) The corresponding schematic of these nanoparticle interactions in liquid (blue spheres, water molecules; orange sphere, gold nanoparticle; green molecules, CTAB) that arise due to overlapping surface-bound molecules. (E,F) Magnified TEM image of the interface between two nanoparticles 0.1 s prior to their contact. (G) The spacing between the surfaces of these coalescing gold nanoparticles as a function of time in pure water (black curve) and in the presence of 1 mM CTAB (red curve).

and 7);26 however, charging during imaging may enhance the movement of the nanoparticles.27 The time-resolved TEM images in Figure 1A show a pair of gold nanoparticles interacting in water and compare this interaction with the pairwise interaction of surfactant-coated gold nanoparticles shown in Figure 1B. Tracking the spacing between nanoparticle surfaces as they interact reveals that the pairwise distance for these two cases changes differently: the uncoated pair and the surfactant-coated pair of gold nanoparticles undergo a rapid approach when the corresponding pairwise distances reach ∼5 and ∼20 Å, respectively (Figures 1E−G), followed by a sudden jump to contact, which results in pairwise attachment (Figure 1A,B). The difference in the pairwise distances prior to the jump in these two cases suggests that this jump that leads to pairwise attachment occurs immediately after surface-bound molecules (water with a molecular diameter of σwater ∼ 3 Å and surfactant with a linear length of lCTAB ∼ 15 Å) are detached from the surface as these particles interact (Figure 1C,D). We analyzed the pairwise attachment between 76 uncoated and surfactant-coated nanoparticle pairs in water (Figure 2). Although the individual trajectories and jump-to-contact distances may vary (Figure 2A,B), the average pairwise separation for uncoated and coated nanoparticles exhibits a jump-to-contact distance that is consistent with the diameters of two water molecules and the length of two overlapping surfactant molecules, respectively (Figure 2A). The spread in the distribution of the jump-to-contact distances may be

between two electron transparent membranes of a liquid cell,22,23 which protects liquid samples from the high vacuum of a TEM column. The translational and rotational diffusion of nanoparticles within these thin liquid films near liquid−solid interfaces is strongly suppressed25−28 (Supporting Information Figure S2), which allows for the real-time imaging and quantification of the interaction force fields20 of an otherwise rapid phenomenon. We used two kinds of aqueous precursor solutions: 1 mM HAuCl4 in pure water (final pH is 2.5), and the same solution in the presence of 1 mM cetrimonium bromide (CTAB) surfactant, which were left at room temperature for 48 h prior to imaging to allow for the formation of naked and surfactant-coated gold nanocrystals, respectively. These freshly formed clean and hydrophilic gold nanoparticles in pure water are expected to have a hydration shell around them. In contrast, when CTAB, which has a strong affinity to gold, is added it forms a capping layer surrounding the gold nanoparticles. We then loaded approximately 400 nL of this solution into liquid cells in which top and bottom membranes were separated by an 80 nm spacer. Each of these liquid cells was then sealed with a gasket and inserted into the TEM using a custom specimen holder. In situ imaging was performed using a JEOL 2010FEG TEM operated at 200 kV with electron doses ranging from 4000 to 12 000 e/(Å2·s) and at a rate of 10 frames per second. The effects of the beam on the movement of nanoparticles in liquid cells due to heating or momentum transfer from energetic electrons is negligible under these imaging conditions (Supporting Information Sections 6 787

DOI: 10.1021/acs.nanolett.5b04808 Nano Lett. 2016, 16, 786−790

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Figure 2. Statistics of 76 coalescence events. (A) Spacing between 39 pairs of bare (gray) and 37 pairs of surfactant-coated (orange) gold nanoparticles during the last 1 s prior to contact at t − t0 = 0. Solid thick lines represent average spacing for uncoated (black) and surfactant-coated (red) nanoparticles in water. (B) Distribution of interparticle spacing, and (C) effective radius, R eff = A proj/π , of all coalescing nanoparticles in pure water (gray open circles) and in the presence of CTAB coating (orange open circles) measured prior to coalescence at t − t0 = −0.1 s. Here, Aproj is the projected nanoparticle area from the TEM image. The time-averaged effective radii for uncoated and coated nanoparticles in water are 2 and 4.5 nm, respectively.

Figure 3. Combined distribution of all pairwise separation distances for (A) 39 gold nanoparticle pairs and (B) 37 surfactant-coated gold nanoparticle pairs. The dark blue curve is a fit to the Boltzmann distribution (see text for details). The inset in (A) is a zoomed out (0 to 8 nm) distribution of interparticle spacings. Pairwise interaction energies for (C) gold (black open circles) and (D) surfactant-coated gold (red open circle) nanoparticle pairs as a function of pairwise separation obtained from the distributions in (A,B), respectively, using eq 1. Here, green curves represent the pairwise interaction forces obtained from the fits to interaction energies (blue curves).

⎛ −U (D) ⎞ P(D) ∼ exp⎜ ⎟ ⎝ kBT ⎠

attributed to roughness, irregular faceting, and high curvature of the interacting nanoparticle surfaces, which may also be responsible for smoothing out oscillatory features of the hydration force associated with multiple water layering that can usually be observed between smooth macroscopic surfaces.9 Moreover, jump-to-contact distances do not show a dependence on nanoparticle sizes, as shown in Figure 2C (see also Supporting Information Section 9). This again indicates that the jump-to-contact distance is primarily associated with the removal of the hydration layer or the surfactant layer of nanoparticles even in the presence of other intermolecular forces whose net strength depends on nanoparticle geometry.29 Prior to jumping to contact, at short distances of a few nanometers or less nanoparticles experience a pairwise repulsive steric force due to the overlap repulsion of surfacebound water or surfactant molecules.6 The strength and the range of these steric short-range pairwise interaction forces, F(D) = −

dU (D) , dD

(1)

where we chose to represent the interaction potential through a fit function of U(D) = W exp(−D/δ) − (AR/12D). Here, the first term is an approximation of a monotonically repulsive force with the decay length scale of δ for a hydration force12,14,30 or steric force for a surface coated with a polymer chain.29,31,32 The second term is an approximation of the vdW interaction between two spheres with the radius of R, where A is the Hamaker constant.29 The time-averaged radii of nanoparticles in water and in the presence of surfactant are ∼2 and ∼4.5 nm, respectively (Figure 2C). By fitting (eq 1) to a combined pairwise separation for all uncoated nanoparticle pairs, we find that the decay length of δwater = 1.4 Å for a hydration interaction lies within the reported range of 0.6−10 Å.13−16 Similarly, in the presence of surfactant, we obtained δCTAB = 5.6 Å,33 which is also consistent with the linear length of the polymer chain LCTAB ≈ πδCTAB ≈ 18 Å (for details, see Supporting Information Section 10).29 Notably, for nanoparticle pairs in water the interaction minimum occurs at a pairwise separation of D ∼ 5 Å (Figure 3C), approximately at

between any two

gold nanoparticles can be extracted from their interparticle spacing distributions (Figure 3) using the Boltzmann distribution: 788

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of Au−water bond at a separation of ∼2.5 Å (Supporting Information Section 10). It must also be noted that hydration forces “kick in” only when the nanoparticles are separated by a few water molecules. Beyond this distance, long-range double layer repulsion due to the surface charge of the nanoparticles and vdW attraction govern the pairwise interaction. The secondary peak at D ∼ 4 nm in the distribution of pairwise separations, shown in the inset to Figure 3A, is due to a local minimum in the energy curve, where an attractive vdW and repulsive double-layer forces are balanced, as described by classical DLVO theory29 (Supporting Information Section 11 and Figures S7−9). This potential well in most cases is sufficient to keep charged nanoparticles dispersed and to prevent their aggregation and coalescence. In fact, we found that during our observation time ∼75% of nanoparticle pairs in water never contacted each other and always remained more than 14 Å apart (Supporting Information Figure S8). Only for the nanoparticle pairs that overcame this long-range repulsion due to thermal fluctuations, the transition from secondary minima down to a ∼ 5 Åseparation occurs, which is eventually followed by pairwise attachment (Figure 4). In summary, our study reveals that free nanoparticles in water can form water-bonded transient pairs stabilized by steric hydration and attractive van der Waals forces. The formation of transient pairs associated with the hydration layers, has important implications for the kinetics that governs the shortrange pairwise interactions between nanoparticles or molecules in solution. The formation of transient pairs with hydration shells in direct contact may allow sufficient time for nanoscale bodies to reorient and explore optimal configurations prior to attachment, and it may play a crucial role in processes such as the oriented attachment of crystals36 or site-specific binding between biomolecules.37,38 We believe that future studies similar to ours will play an important role in exploring the details of interaction pathways of other intermolecular forces that are essential in many areas of science and engineering.39,40

the thickness of two water molecules,6 indicating that the repulsive hydration force balances the attractive vdW force. We propose that the peak in the pairwise distribution and subsequent local energy minima at D0 ∼ 5 Å (Figure 3A,C) occurs when the one-water-molecule-thick hydration shells of each nanoparticle in a pair come into direct contact. At this point, any further pairwise approach stalls for a period of time, as shown by individual traces of pairwise distances in Figure 4.

Figure 4. Pairwise separation between interacting gold nanoparticles in water. The plots of the pairwise separations between nanoparticles show the transition from a secondary minimum at a D ∼ 4 nm down to a D ∼ 0.5 nm separation set by the combined thickness of the hydration layer of each nanoparticle, at which point these nanoparticles form a sterically stabilized transient pair. Once the surfacebound water molecules (blue spheres) between nanoparticles are drained, the nanoparticle surfaces come into contact at t = t0 and are followed by coalescence, as schematically illustrated in the top panel. The inset shows the distribution of dwell times at D < 0.8 nm. The dashed line shows the spacing, which corresponds to two diameters of water molecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04808. Materials and methods, discussion of experimental parameters, and detailed analysis of relevant intermolecular forces between nanoparticles in solution. PDF) Attachment of gold nanoparticles in water. See corresponding Figure S5A. (AVI) Attachment of gold nanoparticles in aqueous 1 mM CTAB. See corresponding Figure S5B. (AVI) Long-range interaction between gold nanoparticles in water. See corresponding Figure S7. (AVI)

The contact of hydration shells of nanoparticles leads to transiently stable nanoparticle pairs with pairwise separation of D0 ∼ 5 Å. For these metastable transient nanoparticle pairs, the stiffness of the hydration layer mediated bond can be approximated as k ≈ (d2U/dD2)D0 ≈ 100 mN/m (see Supporting Information Section 13 for details). The jump-tocontact between nanoparticle pairs occurs only when the interacting portion of the nanoparticle surfaces is fully dehydrated, and the pair forms a single larger nanoparticle. It must be noted that the water molecules in a hydration layer move on the gold surface34 and even exchange with bulk water molecules.35 Therefore, when two surfaces are pressed against each other their hydration layers are drained as a result of an outward driven movement of these surface-bound water molecules. Consequently, hydration forces vanish and attractive vdW forces dominate, driving the attachment of nanoparticles in the pair. Interestingly, we do not observe a peak in the distribution of the pairwise separations (Figure 2A) at distances less than 5 Å, which would correspond to a single water layer trapped between the nanoparticle surfaces. The absence of a metastable water monolayer between the particles is consistent with the strength of the vdW attraction exceeding the strength

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

U.A., J.L., and D.L. contributed equally. Notes

The authors declare no competing financial interest. 789

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(35) Limmer, D. T.; Willard, A. P.; Madden, P. A.; Chandler, D. J. Phys. Chem. C 2015, 119, 24016−24024. (36) De Yoreo, J. J.; Gilbert, P. U.; Sommerdijk, N. A.; Penn, R. L.; Whitelam, S.; Joester, D.; Zhang, H.; Rimer, J. D.; Navrotsky, A.; Banfield, J. F. Science 2015, 349, 6760. (37) Ball, P. Chem. Rev. 2008, 108, 74−108. (38) Nel, A. E.; Madler, L.; Velegol, D.; Xia, T.; Hoek, E. M. V.; Somasundaran, P.; Klaessig, F.; Castranova, V.; Thompson, M. Nat. Mater. 2009, 8, 543−557. (39) Min, Y.; Akbulut, M.; Kristiansen, K.; Golan, Y.; Israelachvili, J. Nat. Mater. 2008, 7, 527−538. (40) Silvera Batista, C. A.; Larson, R. G.; Kotov, N. A. Science 2015, 350, 1242477.

ACKNOWLEDGMENTS This work was supported by the Young Investigator Award (NUSYIA-FY14-P17) from the National University of Singapore and the Singapore National Research Foundation’s Competitive Research Program funding (NRF-CRP9-2011-04).

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