Feature Article pubs.acs.org/JPCC
Assembly of Nanoscale Objects at the Liquid/Liquid Interface Samuel G. Booth and Robert A. W. Dryfe* School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K. ABSTRACT: The liquid−liquid interface provides a molecularly sharp, defectfree focal plane for the assembly of solid materials. In this article we discuss the various materials which have been successfully assembled at the liquid/liquid interface such as metallic nanoparticles, Janus particles, and carbon nanomaterials. Strategies to induce particle assembly include manipulation of surface chemistry, surface charge, and potential control. Liquid/liquid assembly can be exploited to synthesize materials in situ and template preformed structures. We go on to discuss the difficulties encountered when attempting to fully understand the structure of assemblies present at the liquid/liquid interface and the development of experimental techniques to elucidate information about the structure, stability, chemical composition, and reactivity of interfacial assemblies.
1. INTRODUCTION The interface between oil-based and aqueous solutions, often termed the liquid/liquid interfaceor in the presence of electrolytes in both phases, the interface between two immiscible electrolyte solutions (ITIES)has been studied intensely in recent years as it offers a simple, easy to manipulate system for the assembly of nanostructures.1,2 The liquid/liquid interface is a highly reproducible, defect-free surface for the assembly of a large variety of solid particles. Generally driven by a lowering of surface tension between the fluid phases, discussed below, solid particles readily form stable structures at the liquid/liquid interface. These solid structures have revealed a wide variety of applications particularly in catalysis and sensing.3−7 Around a decade ago, one of the present authors wrote an article, which attempted to summarize current and likely future directions for particle deposition and assembly at the liquid/liquid interface.8 In the interim, activity in this area has increased considerably with a vast range of materials, both in pure form and as more complex composites, being adsorbed to and/or formed at the liquid/liquid interface. There has been some progress in understanding the different types of forces driving interfacial particle assembly and ordering, although the picture is still incomplete. A more serious challenge, identified in the 2006 article, is still the relative scarcity of structural probes, able to reveal particle/array geometry in situ. The various ways in which particles may be assembled at the interface will be examined as a main theme of this text, which in some ways can be viewed as an “update” of the earlier summary, although given the amount of activity in this area, coverage of recent activity is necessarily selective. At the time of writing there are a number of questions which remain including a thorough understanding of the assembly process, the adsorption equilibrium between material initially present in one of the bulk phases and that which assembles at the interface, and the perennial lack of detail about how particles arrange at the liquid/liquid interface or how their surface chemistry is affected by being in simultaneous contact with © 2015 American Chemical Society
both a polar and nonpolar solvent. This review will aim to examine the types of structures whose assembly at liquid/liquid interfaces has been reported recently, including: carbon nanomaterials (nanotubes and graphene), metallic nanomaterials (nanoparticles and nanorods), and colloidal polymer materials. The variation in assembly methods through chemical or electrochemical (potential) control has spawned a number of different assemblies, with the goal of fine-tuning of the resultant assemblies still on the horizon. Finally we will discuss the techniques available to examine particles and their ensembles at liquid/liquid interfaces, how they are developing, and how they may be combined in order to garner different structural and chemical information. 1.1. Free Energy Model for Particle Assembly. The most common description of the assembly process for dispersed nanoparticles at a liquid/liquid interface is driven by the drop in the total free energy of the system brought about by the reduction in the organic/water surface area. As the two solutions are immiscible, the interaction is purely repulsive. For a solid phase without any surface charge the sum of the surface tensions between the solid and the organic (γs/o) and the solid and the aqueous phase (γs/w) is lower than the surface tension between the two liquid phases (γw/o). This reduction in surface tension has been described by Pieranski in eqs 1−3.9 In this work the interface in question was the liquid/air interface although the arguments can be applied to any fluid/fluid boundary ⎛ ⎛ z ⎞⎞ Es/w = γw 2πr 2⎜1 + ⎜ ⎟⎟ ⎝ r ⎠⎠ ⎝
(1)
⎛ ⎛ z ⎞⎞ Es/o = γo2πr 2⎜1 − ⎜ ⎟⎟ ⎝ r ⎠⎠ ⎝
(2)
Received: August 9, 2015 Revised: September 16, 2015 Published: September 23, 2015 23295
DOI: 10.1021/acs.jpcc.5b07733 J. Phys. Chem. C 2015, 119, 23295−23309
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The Journal of Physical Chemistry C ⎛ ⎛ z ⎞2 ⎞ Ew/o = −γw/o2πr 2⎜1 − ⎜ ⎟ ⎟ ⎝r ⎠ ⎠ ⎝
(3)
In the above equations E denotes the contribution to the surface energy of the liquid/liquid interface from (1) the repulsive interaction between the particle and the aqueous phase Es/w, (2) the repulsive interaction between the particle and the organic phase Es/o, and (3) the reduction in surface energy caused by the removal of the aqueous phase/organic phase interaction Ew/o; r indicates the radius of the particle and z the vertical distance between the center of the particle and the position of the interface as shown in Figure 1.
Figure 2. Plot showing the calculated variation in surface energy with increasing particle radius for the attachment at a toluene/water interface, γo/w = 36.0 mN m−1.10 The values were calculated for particles with contact angles varying from 90° to 60°. These values are calculated for an increase in wetting by the aqueous phase, and the same reduction would be seen if the contact angle were increased, indicating an increase in wetting by the organic phase.
Figure 1. Schematic of the interaction between a single particle and the liquid/liquid interface indicating the values of r (radius of the particle) and z (vertical distance between the interface and the center of the particle).
nanoparticles (1−5 nm) thermal fluctuations can result in removal of particles from the liquid/liquid interface. Although this makes for a less stable film it does enable the possible reordering of the particles meaning that defects in the film formed may be readily corrected, a phenomenon that has been exploited to demonstrate control over reversible particle assembly.15−17 However, for the production of assemblies which are stable over a long time period, the surface composition of the nanoparticles and interparticle interactions must be carefully considered. 1.2. Long-Range Ordering in Particle Assemblies. The observations made about surface tension work well for the adsorption of a single particle at the liquid/liquid interface although this does not completely explain the interaction(s) between adjacent particles assembled at the interface. Some very interesting assemblies have been reported, showing longrange order between particles producing an almost crystalline conformation of particles fixed at the interface (Figure 3).18 This ordering appears to arise from interactions between the particles and the interface itself. When in a bulk solution the interaction between particles can be described simply by the repulsion or attraction between surface charges. The behavior of these particles fits in well with the classic Derjaguin− Landau−Verwey−Overbeek (DLVO) theory. In contrast, when sequestered at a liquid/liquid interface particles have been shown to exhibit repulsive forces over a much larger range.9,19,20 For some systems only repulsive dipole interactions have been observed, irrespective of particle concentration at the interface.21 However, for some carefully controlled systems the ordered positions of the particles observed at the liquid/liquid interface clearly point to a long-range attractive interaction.18,22 An individual particle adsorbed at the liquid/liquid interface induces a force normal to the interface which causes a distortion in the shape of the interface. This distortion results in an attractive capillary wave action between particles at the interface. What is less clear is the origin of the force normal to the liquid/liquid interface which induces the capillary waves with the suggestion that it is electrostatically induced being the subject of debate.23,24
A simpler description of the position of the particle at the liquid/liquid interface, which is now used more readily due to its ease of measurement, is the interfacial contact angle. For a real particle the contact angle at the liquid/liquid interface reflects the preferential wetting by either the organic or the aqueous phase solution (eq 4).10 The dynamics of particle wettability have been examined with a suggestion that in some cases it can take a very long period of time (months) to reach the equilibrium position.11 A liquid/liquid system reaches a maximum in particle stability when θ = 90° (z = 0), described by the familiar Young−Dupré equation below; however, the preferential assembly at the interface relative to the bulk phase drops off rapidly when the contact angle deviates from 90°. γs/w − γs/o cos θw/o = γw/o (4) Alongside the surface energy dependence on the contact angle formed at the three-phase junction, there is also a strong particle size effect (eq 5).12 The equation shows the case for preferential wetting by the aqueous phase, and the sign within the brackets would be positive for the case of wetting by the organic phase. Equation 5 takes the same form as eq 3 with the position of the particle described in terms of the particle contact angle. In this instance E refers to the reduction in surface energy so there is no negative term as in eq 3. E = πr 2γw/o(1 − cos θw/o)2
(5)
For micron-scale particles the free energy reduction is on the order of 107 kBT, where kB denotes the Boltzmann constant and T temperature, resulting in irreversible self-assembly which produces very stable structures. Macroscale structures (1−10 mm) have been assembled at the liquid/liquid interface with good control over the shape of the structure.13 These polymeric solids could further assemble at the interface through tiling to produce assemblies on the centimeter scale with even greater stability at the liquid/liquid interface. With nanoparticles the reduction in surface energy is approximately equal to 101−104 kBT (Figure 2).10,12,14 This means that for very small 23296
DOI: 10.1021/acs.jpcc.5b07733 J. Phys. Chem. C 2015, 119, 23295−23309
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adjacent organic phase as well.26 This work focused on the formation of Pickering emulsions27,28emulsions stabilized by the presence of solid particles. Here ion partitioning from the nonpolar solvent into the aqueous phase was believed to be an important factor dictating the structure produced. The ion partitioning causes a buildup of positive charge on the aqueous side of the interface which attracts a monolayer of particles, despite the fact that they are almost completely nonwetting with reported contact angles ≥165° and often 180°. Beyond the monolayer the particles were excluded from a zone of liquid, beyond which a Wigner crystal conformation was seen in the organic phase. This conformation is brought about when the potential energy barrier is greater than the kinetic energy of the particles leading to a stable ordered pattern of particles due to their mutual electrostatic repulsion. Because of the low polarity of the phase this effect occurs over a long range (Figure 4). These ordered structures are less apparent in most systems as smaller particles produce a lower distortion of the interface, and the presence of electrolyte in the solutions reduces the range of the interaction, as the charge effects due to the presence of the particles are screened by the ions in solution. However, particles have been shown to exhibit a long-range repulsion even at high concentrations of electrolyte in the aqueous phase.19,29 This is attributed to residual charges in water droplets trapped on the hemisphere of the particles immersed in the organic phase due to surface roughness. This only applies to organic phases with low relative permittivity and does not hold for systems with a more polar organic phase or in the presence of an organic electrolyte.
Figure 3. Scatter plot showing the positions of poly(methyl methacrylate) (PMMA) crystals held between a water droplet (24 um radius) and a bulk oil phase. This conformation remained stable for more than 30 min. The inset shows a fluorescent image of the particle positions, and the scatter plot indicates how the particles moved over a 5 min time period (sampled every 30 s). The system was stable for more than half an hour. Reprinted with permission from ref 18 (Nikolaides, M. G.; Bausch, A. R.; Hsu, M. F.; Dinsmore, A. D.; Brenner, M. P.; Weitz, D. A.; Gay, C. Electric-Field-Induced Capillary Attraction Between Like-Charged Particles at Liquid Interfaces. Nature 2002, 420, 299−301). Copyright 2002 Macmillan Publishers Ltd.
For a system under applied potential the lateral force acting on a particle when at the liquid/liquid interface can be defined as in eq 6.25 3pp ww 1 i j 1 i j Fl = + 2πγ R 4πε0εL R4 (6)
2. ASSEMBLY AT THE LIQUID/LIQUID INTERFACE A variety of different types of particles may be assembled at the liquid/liquid interface. The simplest are spherical particles: the dimensionality of these particles may be approximated as 0D, or 3D, depending on their size relative to the length scale of the interface. Rod-shaped 1D and “sheet”- or “platelet”-shaped 2D structures have also been formed, many of which can undergo further assembly to produce 3D structures. With nonspherical particles there are additional complications to the assembly brought about by the particles’ orientation with respect to the interface, as well as differing in the type of distortion exerted on the interface itself.30 The properties of particle assemblies are dependent on their shape and size as well as surface charge effects often induced by the stabilizing ligands. In this review, the different nanoparticle structures are categorized by the dimensionality of the structures they form at the interface, as described in Figure 5, and will be introduced separately as different assembly techniques are prevalent for each category. Finally in this section we examine particles which exhibit anisotropic charge distributions such as Janus particles or
This equation is defined in terms of particles i and j where w is a vertical force acting on the particle and p is the induced dipole. R describes the separation of the two particles; γ is the interfacial tension; ε0 is the permittivity of free space; and εL is the permittivity of the lower liquid phase. In this equation the first term describes the capillary force induced by the vertical force of the particle on the interface, and the second term is the contribution from the induced dipole between particles i and j. As can be seen the first term is active over a longer range than the second given their 1/R and 1/R4 dependence, respectively. If the first term in the equation is attractive and the second is repulsive then the two particles can remain in a stable conformation at the liquid/liquid interface. The ordered conformation is not limited to interfacial assembly. It has also been seen that the liquid/liquid interface may induce order in the particles present in the “bulk” of an
Figure 4. Series of cross-sectional images through an organic droplet (cyclohexyl bromide and cis-decalin) in water. The spherical particles are PMMA, coated with a fluorescent dye (rhodamine isothiocyanate), which could be imaged by confocal laser microscopy. Reprinted with permission from ref 26 (Leunissen, M. E.; van Blaaderen, A.; Hollingsworth, A. D.; Sullivan, M. T.; Chaikin, P. M. Electrostatics at the Oil−Water Interface, Stability, and Order in Emulsions and Colloids. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 2585−2590). Copyright 2007 National Academy of Sciences, USA. 23297
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interface.38 Thiol-capped gold nanoparticles offer very good stability in a bulk organic phase.39 In general, the arrangement of ligands on the surface of the nanoparticles produces an even surface charge which enables manipulation of the nanoparticles through simple variations in the solution conditions such as pH, electrolyte concentration, or interfacial potential. Manipulation of pH and electrolyte concentration has been shown to enable reversible interfacial assembly of gold nanoparticles.15 The addition of the salt caused a screening of the surface charge on the nanoparticles reducing the barrier to adsorption. The adsorption barrier is a characteristic of the particles themselves, whereby nanoparticle assembly results in a buildup of charge at the interface which inhibits further nanoparticle attachment. In contrast to this adsorption/desorption method based on the manipulation of salt concentration and pH, it has been shown that in carefully controlled systems it is possible to form a nanoparticle assembly at the liquid/liquid interface which takes on the appearance of the bulk metal producing a highly reflective layer, with the appearance of a solid but still behaving as a suspension (Figure 7). Unlike the majority of nanoparticle
Figure 5. Schematic showing some of the particles assembled at liquid/liquid interfaces in terms of particle dimension.
templated nanostructures, and in these systems the assembly varies dramatically with shape as well as charge. 2.1. Assembly of Spherical Particles at the Liquid/ Liquid Interface. When examining the assembly of solids at a liquid/liquid interface spherical nanoparticles appear to facilitate the highest level of control over the assembly. Spherical particles are often able to form monolayer or stacked 3D structures depending on concentration. As mentioned in the Introduction, size plays an important role in the stability of a nanoparticle assembly at liquid/liquid interfaces. The size of spherical structures that have been assembled ranges from particles of a few nanometers, such as metallic nanoparticles or semiconductor “quantum dots”, up to micron-scale PMMA particles.18,31 Increasing particle size offers a dramatic increase in stability for particles, scaling with r2 as shown previously.9 The stability of nanoparticle assemblies has also been shown to depend on the concentration of electrolyte and the type of solvent.15,32 While larger particles offer more stable assemblies, smaller metallic nanoparticles can offer greater control over the assembly due to a higher surface charge density which enables greater manipulation of electrostatics. Smaller nanoparticles also offer the possibility of reversible assembly as there are a number of simple methods to overcome an adsorption energy of ≤100 kBT as detailed below. 2.1.1. Ligand Stabilization and Exchange. For metallic nanoparticles the surface stabilizing ligands are generally assumed to form a single self-assembled monolayer on the particle surface, although the bonding appears to be weak enough to enable rearrangement and ligand exchange.33,34 Thiol-protected gold nanoparticles contain a strong chemical bond between the sulfur and gold(I) present on the surface of the nanoparticles in a “staple” motif (Figure 6).35,36 Both [RS-
Figure 7. 60 nm Au nanoparticles adsorbed at a heptane and 1,2dichloroethane/water interface. The optical image shows the appearance for a Au concentration equivalent to a surface coverage of 1.6 monolayers. (b) The image under laser light irradiation (532 nm) shows the highly reflective metallic nature of the adsorbed nanoparticles. Reprinted with permission from ref 40 (Fang, P. P.; Chen, S.; Deng, H. Q.; Scanlon, M. D.; Gumy, F.; Lee, H. J.; Momotenko, D.; Amstutz, V.; Cortes-Salazar, F.; Pereira, C. M. et al. Conductive Gold Nanoparticle Mirrors at Liquid/Liquid Interfaces. ACS Nano 2013, 7, 9241−9248). Copyright 2013 American Chemical Society.
assemblies at the liquid/liquid interface, the formation of 3D multilayer interfacial structures is required to produce high levels of reflectivity. These films are often termed metal liquidlike films (MeLLF). Their formation has been demonstrated by careful control of the reduction of aqueous silver ions in the presence of a surfactant, in a two-phase system,41−43 and by ligand exchange of preformed gold nanoparticles.40,44,45 To assemble preformed particles it has been shown that sonication can be required to avoid the formation of black aggregate structures, which are an alternative to the highly reflective surfaces.44 The mechanism for assembly is unclear in this instance because the particles are initially stable in bulk aqueous solution and only undergo aggregation in the presence of the organic phase. By comparison, the work of Turek et al.15 mentioned previously did not form a highly reflective surface but appeared to be far more robust during the assembly due to the exchange of the citrate ligands for a more stable
Figure 6. Schematic showing the attachment of Au(I)-S “staples” to a central Au0 core. Gold(I) atoms are colored in yellow, and the smaller red atoms show the sulfur groups present in the structure.
Au-SR] and [RS-Au-S(R)-Au-SR] units have been found on the surface of the metallic core. As a result of these structures the interaction with thiol-protecting groups is thought to be more stable than that with citrate ligands, although exchange of ligands between particles is still possible.37 With organic surface groups such as thiols, the reduction in surface energy on assembly at the liquid/liquid interface varies strongly with alkyl chain length as longer chains result in an increase in hydrophobicity, lowering the stability of the particles at the 23298
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approach has been demonstrated using a mixture of two particles with different surface charges assembled at the interface: different cluster conformations were found, based on the relative sizes of the particles.25 2.2. Assembly of One-Dimensional Materials at the Liquid/Liquid Interface. 1D rod and cylinder-shaped particles have also been assembled at the liquid/liquid interface. Of these, carbon nanotubes (CNTs) have perhaps received the greatest attention. The interfacial assembly properties of CNTs are governed by their aspect ratio, quantity of single or multiwall structures, tendency to form bundles and the surface charge (itself caused by impurities, which can often be difficult to quantify). Nanorod assemblies formed of CNTs or CdSe rods exhibit the greatest reduction in surface energy when their longitudinal axis is parallel to the interface.59−61 An exception occurs for nanorods with strongly hydrophilic or hydrophobic ligands, which may instead adopt a perpendicular orientation with respect to the interface, with the majority of the nanorod in the phase it is preferentially wetted by and just the tip which contains a lower density of ligandsattached at the interface.62 This possibility of material or surface chemistry induced preferential orientation means that 1D nanorods, randomly oriented in bulk solution, can be rapidly arranged into an ordered structure at the interface.63 For CNTs, when a mixture of single tubes and bundles is present, the bundles are likely to preferentially assemble at the interface as their larger surface area can cause a larger reduction in surface energy. To form a dispersion of nanotubes it is necessary to sonicate the nanotubes to produce a homogeneous suspension. Once suspended, the second liquid phase may be brought into contact. The addition of ethanol and further sonication has been applied to assemble single and multiwall carbon nanotubes at the liquid/liquid interface.60,64 Assembly has also been demonstrated in the presence of electrolyte.65−67 The tendency of nanotubes to form bundles, and the presence of illdefined surface contaminants, can make the assembly of nanotubes at the interface more difficult than the corresponding process with nanoparticles. 2.3. Assembly of Two-Dimensional Structures at the Liquid/Liquid Interface. Unlike spheres, which can undergo rotation at the interface, and 1D materials, which can exhibit compositional-dependent orientation, 2D structures should only be able to assemble horizontally (i.e., with the long axis parallel to the interface) at the liquid/liquid interface due to the large difference in liquid/liquid interfacial area reduction. There are a number of deposition products such as aggregated nanoparticles which can be considered to form 2D structures at the liquid/liquid interface; however, there are fewer examples of individual 2D materials assembled at the liquid/liquid interface, although there have been some reports on the assembly of graphene and, more recently, tungsten diselenide at the liquid/ liquid interface.68−70 In typical applications, graphene must be held on some form of support structure. Graphene grown by chemical vapor deposition (CVD) is grown on a metal support, usually Ni or Cu.71,72 When transferred to an alternative solid support, the graphene structure may be distorted because of differences between materials with respect to the adhesion of the graphene to the surface.73 The alternative offered by “floating” at the liquid/liquid interface allows the graphene sheet to be supported without being in direct contact with another solid structure; i.e., effectively a “soft contact” is made to the graphene. Assembly at the liquid/liquid interface has been
mercaptododecanoic acid. The ligand exchange technique relies on the use of ligands with a surface group which is not too hydrophilic, enabling interfacial assemblies to form with a contact angle θ close to 90°.46,47 However, by manipulating the hydrophilic or hydrophobic nature of ligands it is possible to form controlled multilayer structures: for instance silica nanoparticles have been assembled at the interface and then bound to hydrophilic nanoparticles in the aqueous phase and hydrophobic nanoparticles in the organic phase forming a three-tiered structure.48 Ligand exchange may also drive particle desorption: CdSe nanoparticles have been successfully desorbed from the liquid/liquid interface by light irradiated in the presence of a strongly hydrophilic dye molecule. In this case the light irradiation caused the dye to bind to the nanoparticles, shifting the interfacial contact angle far enough away from 90° to favor desorption into the bulk phase.49 CdSe nanoparticles assembled at the liquid/liquid interface have been shown to induce electron transfer reactions across the interface under illumination, as have palladium, TiO2, and dye-sensitized TiO2 nanoparticles.50−53 2.1.2. Addition of a Mutually Miscible Solvent. Particles may also be assembled through the addition of a solvent which is miscible with both the aqueous and the organic phase such as acetone or ethanol, or simply heating.44,54−56 Ethanol was shown to reduce the surface charge on citrate-coated nanoparticles through exchange with chemisorbed citrate molecules on the gold surface. These particles do not undergo aggregation in the bulk phase, as seen when the electric double layer is manipulated, but instead adsorb more readily at the interface as there is less electrostatic interparticle repulsion and a contact angle (θ) close to 90°.54 When controlling nanoparticle assemblies it is desirable to avoid the formation of aggregated structures as these can have adverse effects on the nanoparticles either by loss of control over assembly, shifting the position of the surface plasmon, or in the case of MeLLF films, the loss of reflectivity. Reflective MeLLF films can also be assembled by the careful injection of nanoparticles suspended in a methanol solution close to the aqueous/organic interface.57 Gold has proven to yield a more stable MeLLF film than silver due to the inert nature of the surface, whereas silver nanoparticles may contain a more active oxide layer. Once formed the MeLLF film is very stable to disturbance of the interface and is able to distort and reform without losing its optical properties, demonstrating the repairable nature of structures at a liquid/liquid interface.44 2.1.3. Control over Interfacial Potential. Control of the potential on the interface offers another route to particle assembly at the ITIES. Here the aqueous phase is biased with respect to the organic phase. In this case the liquid/liquid interface functions in a manner analogous to the solid electrode/solution interface. In the presence of an aqueous dispersion of negatively charged nanoparticles application of a negative potential difference (aqueous phase with respect to the organic phase) thus drives the charged nanoparticles toward the interface. The effect of potential is also strong enough to overcome the surface energy barrier enabling desorption of attached nanoparticles (
