Liquid Interface

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Assembly of Nanoscale Objects at the Liquid/Liquid Interface Samuel G. Booth, and Robert A.W. Dryfe J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Sep 2015 Downloaded from http://pubs.acs.org on October 4, 2015

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The Journal of Physical Chemistry

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, UK

Abstract The liquid-liquid interface provides a molecularly sharp, defect free 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 synthesise 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.

Keywords: nanoparticles, self-assembly, ITIES, graphene, Janus particles, SERS.

1. Introduction The interface between oil based and aqueous solutions, often termed the liquid/liquid interface, or 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 1 ACS Paragon Plus Environment


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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 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 a 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 far from comprehensive. 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 both a polar and non-polar 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

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(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 Equations 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:

(1)

(2)

/ = 2 1 + / = 2 1 −

/ = −/ 2 1 −

(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) 3 ACS Paragon Plus Environment


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the repulsive interaction between the particle and the organic phase Es/o and (3) the reduction in surface energy caused by the removal of aqueous phase/organic phase interaction Ew/o; r indicates the radius of the particle and z the vertical distance between the centre of the particle and the position of the interface as shown in Figure 1.

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 centre of the particle).

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 (Equation 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°. 4 ACS Paragon Plus Environment

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cos / =

/ − / /

(4)

Alongside the surface energy dependence on the contact angle formed at the three phase junction, there is also a strong particle size effect (Equation 5).12 The equation shows the case for preferential wetting by the aqueous phase, the sign within the brackets would be positive for the case of wetting by the organic phase. Equation 5 takes the same form as Equation 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 Equation 3.

= / (1 − cos / )

(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 cm 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 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.155 ACS Paragon Plus Environment


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However for the production of assemblies which are stable over a long time period, the surface

composition of the nanoparticles and inter-particle interactions must be carefully considered.

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, the same reduction would be seen if the contact angle were increased, indicating an increase in wetting by the organic phase.

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 long range 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 behaviour of these particles fits in well with the classic Derjaguin-Landau-Verwey6 ACS Paragon Plus Environment

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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 For a system under applied potential the lateral force acting on a particle when at the liquid/liquid interface can be defined as in Equation 6.25

=

1 3# # 1 + 2 ! 4 %& %' ! (

(6)

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



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attractive and the second is repulsive then the two particles can remain in a stable conformation at the liquid/liquid interface.

Figure 3. Scatter plot showing the positions of polymethyl methacrylate (PMMA) crystals held in a water droplet (24 µm radius). This conformation remained stable for more than 30 minutes. The inset shows a fluorescent image of the particle positions and the scatter plot indicates how the particles moved over a 5 minute time period (sampled every 30 seconds). The system was stable for more than half an hour. Reprinted with permission from reference 18 (Nikolaides, M. G.; Bausch, A. R.; Hsu, M. F.; Dinsmore, A. D.; Brenner, M. P.; Weitz, D. A.; Gay, C. ElectricField-Induced Capillary Attraction Between Like-Charged Particles at Liquid Interfaces Nature 2002, 420, 299-301). Copyright 2002 Macmillan Publishers Ltd.

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 adjacent organic phase as well.26 This work focused on the formation of Pickering emulsions27,

28

–

emulsions stabilised by the presence of solid particles. Here ion partitioning from the non-polar solvent into the aqueous phase was believed to be an important factor dictating the structure produced. The ion partitioning causes a build-up of positive charge on the aqueous side of the 8 ACS Paragon Plus Environment

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interface which attracts a monolayer of particles, despite the fact that they are almost completely non-wetting 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 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 9 ACS Paragon Plus Environment


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permission from reference 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.

2. Assembly at the liquid/liquid interface There are a number of different groups of particles which may be assembled at liquid/liquid interfaces. 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 non-spherical 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 stabilising ligands. The different nanoparticle structures are categorised 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 templated nanostructures, in these systems the assembly varies dramatically with shape as well as charge.


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Figure 5. Schematic showing some of the particles assembled at liquid/liquid interfaces in terms of particle dimension.

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 Whilst 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 Stabilisation and Exchange 11 ACS Paragon Plus Environment


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For metallic nanoparticles the surface stabilising 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-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 interface.38 Thiol capped gold nanoparticles offer very good stability in a bulk organic phase.39

Figure 6. Schematic showing the attachment of Au(I)-S “staples” to a central Au0 core. Gold(I) atoms are coloured in yellow and the smaller red atoms show the sulfur groups present in the structure.

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 12 ACS Paragon Plus Environment

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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 build-up of charge at the interface which inhibits further nanoparticle attachment.

Figure 7. 60 nm Au nanoparticles adsorbed at a heptane and 1,2-dichloroethane/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 reference 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.

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



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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 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 liquid like 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 system41-43 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, 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 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


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favour 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 inter-particle 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



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2.1.3

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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 towards the interface. The effect of potential is also strong enough to overcome the surface energy barrier enabling desorption of attached nanoparticles (< 20 nm) from the interface back into the bulk phase.16, 58 This is dependent on the size and charge on the particles, as large particles require a greater driving force to overcome a higher stabilisation energy (Figure 2).10, 12, 14

For micron size particles, which assemble too strongly to desorb under experimentally-

accessible potentials, an external potential bias can provide lateral control over the assembly. This 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 1 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 (CNT) have perhaps received the greatest attention. The interfacial assembly properties of CNTs are governed by their aspect ratio, quantity of single or multi-wall 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


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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 ligands - attached 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 are 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 in to 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 2 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



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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 vapour 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 achieved using exfoliated few layer graphene and CVD grown graphene.66, 69 In common with CNT dispersions, exfoliated graphene can be assembled at the liquid/liquid interface by sonication. Graphene oxide (GO) assembly at the liquid/liquid interface has also been achieved. Due to the variation in surface charge/extent of oxidation between GO samples based on their preparation method, the contact angle θ may vary quite dramatically. Therefore some samples are seen to adsorb spontaneously74 whilst others require a chemical modification step.75 The addition of ethanol can provide efficient GO assembly, as discussed above with spherical nanoparticles, this is through the reduction in surface charge on the GO.76

2.4 Janus particles Named after the Roman god with two faces (Figure 8a), Janus particles contain two segments with different surface chemistries or charges.77 Classical Janus particles are described by two faces of equal area on the particle surface although control of the preparation of hemispheres with exactly equal area can be very difficult on the nanoscale (Figure 8b). Particles where the


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two faces are not equal in size can also exhibit useful properties so the term has relaxed somewhat to incorporate a wider range of particle morphologies. Hydrophobic/hydrophilic Janus particles have the potential to offer a greater reduction in surface energy than homogenous particles through interfacial assembly. The case of Janus particles at the liquid/liquid interface has been examined by theory and in practice.78, 79

Figure 8. a) Coin depicting the Roman god Janus b) a schematic of a classical Janus nanoparticle showing two hemispheres which display different chemistries. c) Schematic of a non-classical Janus particle present at the liquid/liquid interface. The image of the two headed coin was reprinted from reference [80] (Larson, S. M. The Janus Project: The Remaking of Nuclear Medicine and Radiology J. Nucl. Med. 2011, 52, 3S-9S). Copyright 2011 the Society of Nuclear Medicine and Molecular Imaging, Inc.

From a theoretical standpoint, Janus particles should form a very stable interfacial assembly Binks and Fletcher developed a model which suggests that the strength of the adsorption can be increased by a factor approaching three, compared with a homogenous particle.78 This description of Janus particles at a liquid/liquid interface allows for particles where the polar and



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apolar regions of the particle are not equal in area. In this description the angle α describes the immersion depth of the particles at the liquid/liquid interface relative to the division between the polar and apolar regions of the particle, Figure 8c. The angle β describes the amphiphilic nature of the particle where a value of 90° corresponds to classical Janus particle (50:50 mixture), and 0° or 180° would correspond to homogeneous particles with completely polar or apolar surface groups, respectively. The reduction in surface energy on adsorption of a Janus particle can be described by the two equations given below (7 and 8).78, 81 Equation 7 describes the situation where θ ≤ α, the example depicted in Figure 8c. Equation 8 describes the reduction in interfacial energy when θ ≥ α. As before r corresponds to the radius of the particle and γ the surface tension – the subscripts are given as: p – polar surface of the particle, a – apolar surface of the particle, w – water phase and o – the organic phase.

) = 2 *+⁄ (1 + cos -) + .⁄ (cos − cos -) + .⁄ (1

(7)

1 − cos ) − ⁄ /01 2 2 ) = 2 *.⁄ (1 + cos ) + +⁄ (cos - − cos ) + .⁄ (1

(8)

1 − cos -) − ⁄ /01 - 2 2

By comparing dodecanethiol and octadecanethiol coated nanoparticles bound to iron oxide to form Janus particles, Glaser et al. were able to show that a more hydrophobic ligand further increased the reduction in interfacial tension.79 The orientation at the interface can be complicated if the preferential wetting of the solid surface is stronger than the repulsion between 20 ACS Paragon Plus Environment

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the two liquid phases. In this case non-equilibrium orientations could be favourable which could lead to attractive or repulsive capillary forces between particles at the interface based on their relative orientation.82 In practice the formation of uniform Janus particles with a 50:50 mixture of the hydrophilic and hydrophobic components on the micro or nanoscale scale by chemical synthesis is difficult, although treatment of particles from either the aqueous or the organic side following

assembly

at

the

liquid/liquid

interface

is

one

way

to

develop

these

hydrophilic/hydrophobic structures.83 Slight fluctuations in the position of the division between polar and apolar regions have been shown to increase capillary attractions between particles as the preferential wetting of the surface leads to greater distortions in the shape of the interface.84 So far there have been no methods developed to directly “image” the particles in situ, as with homogeneous particles, but the interfacial (surface) tension measurements clearly demonstrate the trend expected by theory. The measurement of liquid/liquid interfacial tension in the presence of Janus particles has shown that further variation in adsorption characteristics is based on the shape of the nanoparticles. Observations at a toluene/water interface have shown that cylindrical particles appear to offer a greater reduction in surface tension than either spheres or disks.85, 86 The greater reduction in surface tension is due to the ease of particle rearrangement involving individual particles which assemble “incorrectly”, e.g. when the hydrophobic side aligns with the aqueous phase and the hydrophilic side with the organic due to the slightly random nature of adsorption. For disk shaped particles the nanoparticles must overcome a large desorption energy in order to correct the conformational errors at the interface.14 If this energy barrier cannot be overcome then the unstable particle formation will reduce the beneficial s/w and s/o interactions, increasing the surface energy. In the case of cylinders, where the division between the apolar and polar regions is parallel to the longitudinal axis, misalignment may be readily overcome by



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rotation around the longitudinal plane of the particles. This has also been shown by molecular dynamics simulations which showed a single minimum for spherical or rod shaped Janus particles but a thermodynamic minimum (preferentially wetted faces aligned correctly) and a kinetic minimum (faces out of alignment) for disc shaped particles.87 The alignment of the discs were shown to be strongly size and aspect ratio dependent. Janus cylinders where the polar and apolar regions are at each end of the cylinder – in a configuration which resembles a bar magnet, have either one or two possible conformations at the liquid/liquid interface which is dependent on the aspect ratio of the cylinder.88 SEM images of the liquid/liquid interface following gelation showed that short rods are able to align perpendicular to the interface with each phase preferentially wetted by the aqueous or organic phase. However, for longer Janus cylinders tilted conformations are also possible as the reduction in contact between the two liquid phases is in competition with the preferential wetting and the higher number of possible conformations would be more entropically favourable. These conformations could possibly be controlled by the application of potential in the same way that spherical particles can be manipulated when present at the liquid/liquid interface.89 Liquid/liquid interfaces can also be used to direct the formation of Janus particles, such that hydrophobic and hydrophilic domains are formed in situ. In this system nanoparticles with hydrophilic and hydrophobic surface chemistries are able to combine at the liquid/liquid interface forming Janus structures.90, 91 Some reports have also shown that the stability provided by an aqueous/wax interface enables surface modification to produce Janus particles (Figure 9). Particles are initially assembled at the molten wax/air interface in an emulsion, this is then cooled fixing the particles at the interface and enabling selective modification of the exposed external surface structure.92, 93 Hong et al. used large spherical silica particles (800 nm and 1.5


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µm in diameter) which provided a very strong attachment at the interface.92 More recently it has been shown that smaller particles (100 nm in diameter) can also be formed in this manner.93 Cetyltrimethylammonium bromide (CTAB) was used as the surface stabiliser in this instance. By modifying the CTAB to silicon ratio the authors could manipulate the surface charge on the nanoparticles. This resulted in a controllable variation in the contact angle at the interface and hence control over the proportion of the nanoparticle surface area which could be template by gold nanoparticles (10-20 nm diameter), dispersed in the aqueous phase, after the wax had solidified. This was achieved by utilising the affinity shown between amide functionalised silica surfaces and gold nanoparticles. Silica nanoparticles have also been templated by gold at the liquid/liquid interface in a microfluidic system. Here assembly at the interface and the gold nanoparticle coating process is very rapid. Dwell times up to 30 minutes were shown to be effective, beyond this point the gold nanoparticles were shown to begin coating the whole surface of the silica particle rather than a single hemisphere.83

Figure 9. Schematic showing the steps involved to assemble and then functionalize nanoparticles at a water/wax interface by utilizing the solidification of the wax phase to immobilise the particles allowing functionalization of a single hemisphere to form Janus particles.



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2.5 Decorated nanostructures and composites As indicated with some of the examples of Janus particle synthesis, one of the most promising properties for interfacial assemblies is that they may be patterned in situ through chemical manipulation or electrodeposition in order to create composite structures which can be carefully tailored to improve the desired property of the material. The in situ deposition of metals at the liquid/liquid interface has been thoroughly researched producing some size control over the type of deposit formed. We have recently discussed this subject elsewhere.6, 94 Here we focus instead on the co-deposition of composite structures and possible structure formation on the surface of preformed particles, following their assembly at the liquid/liquid interface. The co-deposition of different metal species works well for species with similar reactivity.95 When different metal salts are dissolved together, alloys may form at the interface.96, 97 Pt and Sn were found to produce rod shaped composite structures98 whilst other metals produce roughly spherical particles which aggregate into 2D sheets at the interface. These materials can produce alloys which match the initial ratio of metallic salts in solution as with Ag-Au and Au-Cu; although for Ag-Au-Cu the presence of silver was shown to preferentially aid the deposition of copper instead of gold therefore altering the composition of the three in the film.97 3D MOF (metal organic framework) crystals may also be grown at the liquid/liquid interface.99 In order to form core-shell nanoparticles at the liquid/liquid interface, underpotential deposition has been utilised to form a monolayer of copper on the surface of gold nanoparticles following assembly. It was shown by electron energy loss spectroscopy (EELS), performed with a scanning tunnelling electron microscope, that the copper formed a smooth shell around the nanoparticle (Figure 10) indicating either that the particles can freely rotate at the interface to enable metal


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deposition on all sides or that the particles are almost completely immersed in the aqueous phase.100

Figure 10. High angle annular dark field images (a and b) and corresponding EELS mapping above the copper edge (c) above the gold edge (d) and overlaid maps (e) collected on a STEM microscope. Reprinted with permission from reference 100 (Gründer, Y.; Ramasse, Q. M.; Dryfe, R. A. W. A Facile Electrochemical Route to the Preparation of Uniform and Monoatomic Copper Shells for Gold Nanoparticles Phys. Chem. Chem. Phys. 2015, 17, 5565-5568). Copyright 2015 PCCP Owner Societies.

Once present at the interface it is possible to “decorate” graphene with metal nanoparticles, which opens up a possible route to device fabrication at the liquid/liquid interface. Templated films have been achieved through in situ metallic deposition onto nanorods, graphene oxide (GO) and reduced-graphene oxide (rGO).66, 69, 101 Assembled films may be templated, either by spontaneous chemical reactions, or through controlled electrodeposition.69, 96, 98, 101 The chemical reduction to form nanoparticles on the surface of GO and rGO can be conducted in unison with 25 ACS Paragon Plus Environment


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the stirring or sonication procedure required to assemble the structure at the liquid/liquid interface or, alternatively, as a two-step process whereby the rGO film is formed at the interface prior to metal reduction.101, 102 The one step process was shown to produce a higher surface coverage.102 rGO may be formed from GO by the same reducing reagent used to generate the metal nanoparticles in solution.96,

98

Self-assembly of carbon nanocomposites has been

demonstrated by combining few layer graphene with CNTs and C60 and metal nanoparticle/CNT hybrid films have also been formed.67,

103, 104

It was noted that the introduction of carbon

nanotubes caused the aqueous phase nanoparticles to very rapidly assemble at the interface. The driving force behind this rapid assembly process is, as yet, not well understood.103 The structure of these films has been shown to depend on the concentration of nanoparticles and also the ratio of nanoparticles to CNTs.105 Polyaniline has been used to coat carbon nanotubes at the liquid/liquid interface, enabling the addition of platinum nanoparticles as a different route to the formation of 2D nanotube/metal heterostructures.106 In the case of the assembled CVD-grown graphene it was shown that the deposition could be controlled in order to deposit a metal from either the aqueous or organic phase.69 This leads to the possible formation asymmetrical decoration onto graphene, using different metals present in the solutions on either side of the graphene sheet (Figure 11).107 Deposition of nanoparticles can be driven spontaneously or electrochemically, by varying the strength of the reducing reagent.


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Figure 11. Top – schematic of the two sided decoration gold, in a single phase solution, and subsequently palladium at the liquid/liquid interface. SEM (A), SEM with energy dispersive Xray spectroscopy (EDAX) (B), the EDAX elemental composition from B (C), and AFM of the graphene sheet following templating with nanoparticles (D) Reprinted in part with permission from reference 107 (Toth, P. S.; Velicky, M.; Ramasse, Q. M.; Kepaptsoglou, D. M.; Dryfe, R. A. W. Symmetric and Asymmetric Decoration of Graphene: Bimetal-Graphene Sandwiches Adv. Func. Mater. 2015, 25, 2899-2909). Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Monolayer assembly at the liquid/liquid interface has also recently been applied to fabricate a 3D structure with a discrete number of layers. Individually assembling monolayers of functionalized polystyrene were transferred to a solid substrate to form a 3D structure with the desired number of layers. Once the template was formed the reactant solution could be injected into the void space in the ordered nanostructure, in this case forming a hollow nanosphere film of tantalum nitride.108



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3. Experimental techniques As the study of particle assembly/deposition at the liquid/liquid interface expands, a number of techniques have been developed or adapted which are able to examine the particle adsorption process, or alternatively use interfacial assemblies to examine solution based species. However, the relationship between interfacial adsorption equilibria and particle structure, and the extent of ordering of interfacial structures, is still not well understood. As with the majority of research on nanoparticulate materials, a large amount of characterisation is conducted using either electron or probe microscopy techniques. These methods have been developed to the point where they can produce remarkable resolution providing very useful insights into the types of structures being examined. There is always however, a limit to the level of information that can be garnered about interfacial assembly as these techniques cannot generally be applied to the liquid-liquid interface in situ, and it is difficult to determine what type of structural alteration may have occurred on transferring the sample from the interface. One new possibility is the field of liquid-phase TEM, which is developing quickly and could in future lead to real time imaging of structures at the liquid/liquid interface. Some preliminary work has been conducted on the development of an in situ imaging technique using environmental TEM.109 Also, non-linear optical techniques such as sum frequency generation may provide some useful information about liquid/liquid assembly: this technique was the focus of a recent feature article.110

3.1 Surface Tension The classic experimental technique, which still offers useful insights into liquid/liquid assembly, as mentioned briefly before, is the measurement of variation in interfacial tension. This technique is sensitive to the adsorption of particles at the liquid/liquid interface and therefore


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verifies the predictions of size and contact angle dependency and may indicate the stability of the assembly by examining the magnitude of the surface tension change or the time required to reach a minimum.32,

38

Some information about the packing at the interface can also be found by

examining the concentration dependence of the surface tension to indicate the amount of particles required to form a monolayer.38 This has been particularly useful when comparing the assembly of Janus particles with pure particles or examining shape/size dependent stability.90, 111 The measurement of surface tension is appealing when studying liquid/liquid assembly as it is simple to measure and correlates well with the predictions based on interfacial stabilisation energy. The difficulties with the technique arise from the type of method used. Pendant drop measurements are often used to measure the surface tension, however the arrangement of particles in this system may not represent precisely the arrangement for particles when they are assembled at a flat interface. The technique also provides averaged information about the macroscopic interface, rather than on individual particles and their orientations: as such this technique is often used in conjunction with others.

3.2 X-ray scattering In contrast to tension based methods, X-ray scattering offers a route to determine the shape of structures formed at a surface. There are a number of different X-ray scattering configurations which provide different structural information about the sample. In the area of liquid/liquid Xray scattering the majority of studies have used X-ray reflectivity. This is a surface sensitive technique which detects the intensity of reflected X-rays in the specular direction and can provide information about the surface based on deviations in the expected intensity of the reflected beam. In the case of liquid/liquid systems this provides information which is complimentary to interfacial tension measurements as reflectance necessarily requires a flat 29 ACS Paragon Plus Environment


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interface whereas interfacial tension measurements are often based on a droplet of one phase suspended from a pipette in the bulk of the other.89, 112 Small angle X-ray scattering (SAXS) has also provided some useful information: the technique examines elastically scattered X-rays at small (0.1-10°) angles producing structural information indicating the shape of particles that are 10s of nm in size. These techniques have proven very effective in homogenous solutions providing clear structural information.113 X-ray reflectivity has proven to be an extremely useful tool for elucidating the molecular-level structure of “bare” liquid/liquid interfaces114 (yielding an interfacial thickness of 0.33 nm ± 0.025 nm for water/hexane) and also for examining the assembly of surfactant molecules115 and lipids.116 A review of this area has been written by Schlossman.117 The use of a novel diffractometer set-up to enable movement of both the source and detector relative to the liquid/liquid interface has enabled Magnussen and co-workers to follow the deposition and growth of PbFBr electrodeposited at a mercury/water quasiliquid/liquid interface.118,

119

For this model system, where mercury and lead were chosen

because of their high scattering intensity, the deposition can be followed very well providing information about sub-nanometer structures at the interface. The technique becomes more difficult to interpret however at an organic/water interface or with the deposition of a lighter metal. X-ray scattering, in this case SAXS, has been used in conjunction with Monte Carlo simulations to demonstrate the behaviour of gold nanoparticles at the interface under compression.120 When compressed there is a large increase in surface energy and the particles buckle from a monolayer state forming 3D conformations at the interface. The monolayer is able to reform if the interfacial area is subsequently increased again with only minor hysteresis seen due to the rate of


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compression being too rapid to allow the particles to reach a true equilibrium in energy at each point in the compression.

Figure 12. (a) The X-ray reflectivity (R) for gold nanoparticles at the water/1,2-dichloroethane interface normalised to the Fresnel reflectivity (RF) of an ideal interface as a function of wave vector transfer normal to the interface (Qz). Sample 1 and 2 were both equilibrated at an applied potential of 30 mV before the potential was varied in either a negative direction (1) or a positive direction (2).

(b) Variation in electron density perpendicular to the liquid/liquid interface

determined from the fits in (a). The schematic at the bottom of the figure indicates the position of the particles relative to the interface indicated by the data. Reprinted in part with permission from reference 89 (Bera, M. K.; Chan, H.; Moyano, D. F.; Yu, H.; Tatur, S.; Amoanu, D.; Bu, W.; Rotello, V. M.; Meron, M.; Kral, P. et al. Interfacial Localization and Voltage-Tunable Arrays of Charged Nanoparticles Nano Lett. 2014, 14, 6816-6822). Copyright 2014 American Chemical Society.



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Recently X-ray reflectometry (Figure 12) and grazing-incidence small-angle X-ray scattering (GISAXS) have been used in conjunction with DFT calculations to provide some remarkable observations about the potential dependent assembly of gold nanoparticles at the liquid/liquid interface.89 GISAXS provides the structural information of SAXS along with information about the

geometrical

arrangement

at

the

interface.

Gold

nanoparticles

coated

with

tetramethylammonium capped ligands were able to adsorb negative ions from the organic phase leading to an assembly on the organic side of the liquid/liquid interface by the nanoparticles, which were initially stable in the bulk aqueous phase. Application of an external potential bias was able to influence the particle spacing at the interface by changing the concentration of background electrolyte close to the interface. This caused the inter-particle spacing to expand and contract, although the particles remained at the interface under the potentials applied.

3.3 Surface enhanced Raman Scattering (SERS) Raman spectroscopy is attracting more interest as a potential structural probe of the liquid/liquid interface.121, 122 The technique is very sensitive to chemical changes but can be less effective with concentrations, due to the relative quantity of Rayleigh scattered light compared to Raman scattering. However in the case of Surface Enhanced Raman Scattering (SERS) very low concentrations can be detected. When the conditions are controlled carefully, SERS has proven sensitive enough to provide single molecule detection.123 As described above liquid/liquid assembly procedures offer a facile way to produce nanostructures which are more ordered than aggregates, offering improved enhancements. This is applicable both in the situation where a liquid/liquid interface has been used as a template for the assembly of a SERS active substrate, which is then analysed ex situ, and more recently


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for in situ measurements. For ex situ measurements the liquid/liquid interface directs the assembly process and the substrate may then be collected on a solid membrane in order to detect the target analyte. This method has been shown to be effective for metal nanoparticles,45, 56, 124, 125

metal nanocomposites,126-128 and metal nanoparticle assemblies on reduced graphene oxide

films.101

Figure 13. Schematic depicting recent detection methods used to enable in situ SERS from nanoparticles sequestered at the liquid/liquid interface. a) The interface has been carefully formed by design and chemical treatment of the glassware to produce a flat surface enabling total internal reflection.129, 130 b) Droplets of aqueous solution form an emulsion in the organic, the volume is then reduced to bring the particles closer together and the assembly is transferred to a cover slip.4 The Raman signal is then collected through the cover slip. c) Horizontal imaging of the interface. The z-height of the cell can then be altered to examine changes between the interface and bulk.17, 131

Analyte detection is also possible when the SERS substrate remains at the liquid/liquid interface (Figure 13).132 Surface coverage at a liquid/liquid interface has been shown to have a strong effect on the plasmon resonance for metallic nanoparticles whereby a reduction in inter-particle spacing causes a red shift in the plasmon peak position.133,

134

For direct detection at the

liquid/liquid interface, the interfacial shape can make detection more difficult or result in a higher limit of detection. Modifications to the interfacial structure can help avoid this issue of the



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meniscus shape. Through the use of a cell with a large interfacial area and pre-treatment with silane to make the glass hydrophobic it is possible to form a flat enough interface to perform total internal reflection SERS (TIR-SERS).129, 130 In this system the surface protecting ligands (oleate) on the nanoparticles were also the target analyte providing information about the nanoparticle assembly. Comparisons between spectra of the bulk aqueous solution and those of the liquid/liquid interface showed that the ligands could undergo a rearrangement on the nanoparticle surface. This resulted in the ligands in the aqueous phase arranging with the charged head group pointing into the solution and on the organic side the head group is involved in bonding with the nanoparticle surface while the non-charged alkyl tail group points into the solution (Figure 14).129 When thiophene was used as the organic phase solution it was observed that the organic solvent was able to replacing some of the citrate particles adsorbed on the surface of the nanoparticles held at the interface resulting in the formation of a Janus particle structure.45 These observations suggest that the assembly at the liquid/liquid interface may have a substantial influence on the ligand structure on the surface, indicating that some alteration of the surface chemistry may occur, driven by the reduction in surface energy.


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Figure 14. Schematic showing the arrangement of oleate ions on the surface of silver colloids at the liquid/liquid interface. The orientation is based on the observation of the SERS spectra produced when the nanoparticles were present in a bulk aqueous phase or at the liquid/liquid interface. Reprinted with permission from reference 129 (Yamamoto, S.; Fujiwara, K.; Watarai, H. Surface-Enhanced Raman Scattering from Oleate-Stabilized Silver Colloids at a Liquid/Liquid Interface Anal. Sci. 2004, 20, 1347-1352). Copyright 2004 The Japan Society for Analytical Chemistry.

Very low detection limits have now been reached from a close packed monolayer structure identifying non-resonant molecules at concentrations as low as 8.2 pmol for an aqueous analyte and 323 pm for an organic analyte and even lower limits were achieved for fluorescent dye molecules.4, 135 MeLLFs, discussed earlier, have been shown to produce a stable SERS response over a long time period (weeks).136, 137 SERS has been applied to the electrified liquid/liquid interface to follow the potential dependent liquid/liquid assembly of silver nanoparticles.17 When 35 ACS Paragon Plus Environment


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the nanoparticles were used as the SERS substrate for detection of an analyte present in the organic phase the movement of the particles could be detected by the variation seen in the SERS enhancement. Nanorods have also been applied as the SERS active substrate, either through assembly and transfer to a glass slide, or via an in situ experiment. Nanorods may offer a higher SERS enhancement than nanoparticles because they contain (110) faces as well as (100) and (111) seen on nanoparticles. This high energy surface offers superior adsorption of analyte improving the chemical component of the enhancement factor (Equation 9).138, 139

=

34564 ⁄789 36.:.; ⁄7

Liquid Interface

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