Exploiting Hydrophobic Interactions at the Nanoscale - The Journal of

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Exploiting Hydrophobic Interactions at the Nanoscale Marek Grzelczak*,†,‡ and Luis M. Liz-Marzán*,†,‡ †

Bionanoplasmonics Laboratory, CIC biomaGUNE, Paseo de Miramón 182, 20009 Donostia-San Sebastián, Spain Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain

‡

ABSTRACT: Hydrophobic effects are ubiquitous and manifest themselves in everyday processes such as solubilizing oil, precipitating molecules, and formation of particles or foam. Although this phenomenon is often intuitively recognized, it is not straightforward to predict it and, in particular, to control it experimentally. Hydrophobic effects are however progressively gaining recognition as an important tool providing control at the nanoscale, which may ultimately lead to the design of responsive metamaterials with unprecedented functionalities under nonequilibrium conditions.

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ydrophobic interactions can be intuitively understood as a chemical incompatibility between solute and solvent molecules. These interactions explain the simple aggregation process of hydrophobic molecules or particles in a polar solvent, which are often observable by the naked eye. The reason that scientists developed the term “hydrophobic interactions” was the need of understating the organization of biological molecules into complex architectures including the formation of vesicles and membranes or the folding of globular proteins.1 The organization of living matter via hydrophobic interactionsas well as synthetic nanomaterialsis usually explained in terms of thermodynamics. As indicated by Chandler,2 the solvation free energy (the reversible work needed for solvent molecules to solvate the solute) is the main driving force for the self-assembly of nanoparticles because the solvation free energy is lower for clustered nanoparticles than for fully dispersed ones. When the aggregate gets larger, the ratio between volume and surface area increases, suggesting that at the nanoscale regime the free energy is proportional to the hydrophobic surface area.3 In addition, the driving force becomes stronger at higher temperature, which explains the entropic origin behind hydrophobic interactions as postulated by Kauzmann in the late 1950s.4 In the context of nanoparticles self-assembly, hydrophobic interactions originate at the molecular level. Thus, occurrence of the assembly process requires stronger attraction between solvent molecules than between solvent and surface ligands. Detailed measurements of hydrophobic interactions between hydrophobic surfaces across water are possible by using the surface force apparatus.5 Israelachvili and co-workers have proposed a general interaction potential to account for hydrophobic interactions, showing that these forces operate in the range of 1−2 nm and decay exponentially when increasing the distance.6 The same mathematical model has been applied to a colloidal system, in which polystyrene-stabilized nanoparticles in THF aggregate upon addition of water.7 Whereas the interplay © XXXX American Chemical Society

Hydrophobic interactions are underexploited because scientists prefer to see interparticle interactions from the point of view of nanoparticles properties rather than those of the dispersing medium. between attractive van der Waals and repulsive polymer brush (steric) interactions was unable to correctly model the aggregation process, the hydrophobic component revealed attractive interactions for different solvent compositions. This simple model shows that by changing the interfacial energy hydrophobic interactions can overcome strong steric repulsions between linear polymers grafted on the particles surface. Although large progress has been made in understanding the role of hydrophobic interactions in molecular and colloidal events, the hydrophobic effect is not fully understood. One of the reasons is that many experimental and theoretical discussions regard water as a solvent, which forms extended hydrogen bonding and supports proton transport, thus behaving rather as the solvent. Additionally, we lack comparison with the behavior of solvents different to water because hydrophobic interactions are biologically relevant, and in biological systems, water is not just a solvent but also an active constituent.8 Despite of these challenges, hydrophobic interactions are very useful in materials science, especially at the nanoscale regime, where intermolecular Received: May 17, 2014 Accepted: June 25, 2014

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composition (Figure 1). Thus, we can picture now two possible scenarios: (1) exploit switchable ligand chemistry through reversible hydrophilic/hydrophobic transitions in the ligand under external stimuli and (2) design a switchable solvent that, under an external stimulus, undergoes a reversible hydrophilic/ hydrophobic transition, while the ligand chemistry remains unchanged (by hydrophilic and hydrophobic we mean that the solvent has high or low dielectric constant, respectively). The term switchable refers to modulation of the physical properties of either the ligand or the solvent through stimuli-induced hydrophobic/hydrophilic transitions. Although sometimes the field of nanoparticles research seems to focus exclusively on understanding the physical properties of inorganic nanocrystals as a function of size, shape, and composition, the surface chemistry of the nanocrystals plays an essential role and can be considered as one the most intensively studied parameters toward further handling and application. The reason is that the relatively young field of nanoparticles research benefits from the enormous legacy of organic, polymer, and supramolecular chemistry. A clear example is the self-assembly of nanoparticles through surface ligands, which requires knowledge on the physical chemistry of intermolecular interactions.14,15 Still, directing nanoparticles into a desired organization can be seen as the easy part of the process. Disassembly is often much more challenging because of the effect of short-range attractive (van der Waals) interactions, which lock the assemblies into a thermodynamic minimum. Therefore, to ensure the reversibility of self-assembly one may use either small nanoparticles, where the attractive forces are rather small, or large enough ligands that will ensure sufficient steric repulsion and interparticle separation in the assembled state, thus preventing thermodynamic trapping. Using larger molecules, however, is often less attractive because wide gaps between particles may prevent the observation of the collective properties within the ensemble. This is of particular relevance for plasmonic nanoparticles, where plasmon coupling at small interparticle gaps leads to local electric field enhancement,16 which in turn is highly desired toward surface enhanced spectroscopies,17 energy transfer between nanoparticles,18 or even photocatalysis.19 Therefore, to fully control reversible selfassembly with predictable physical properties, size-dependent attractive forces, and switchable molecular shells need to be balanced. Switchable Ligands. Reversible self-assembly requires ligands that are capable of switching their conformation, and this switch can be used to modify solute−solvent interactions. As mentioned above, hydrophobic interactions are entropy-driven and depend on temperature, which can be a suitable stimulus

forces provide a unique tool to develop materials that are unavailable in nature. In this Perspective, we aim to illustrate the role of hydrophobic interactions in dynamic events that occur at the nanoscale. We propose a general strategy for the successful execution of reversible nanoparticles self-assembly, where switchable hydrophobic interactions can lead to metastable systems. If properly applied, hydrophobic interactions not only can dynamically change nanoscale arrangements but also can provide a switchable tool for on demand phase transitions that can be applied for example in reversible (bio)catalysis or can induce spatiotemporal cargo release from plasmonic containers. We show that external stimuli, such as light, temperature, or even greenhouse gases, can modulate hydrophobic interactions, which are expected to lead to the development of stimuliresponsive actuators. After three decades of development of synthetic protocols for nanoparticles synthesis, the scientific community is actively exploiting the possibility of controlling nanoparticle self-assembly to devise novel nanostructured materials.9 Experimental techniques and theoretical self-assembly models originate from more traditional fields such as colloid chemistry. In fact, considerable effort has been devoted toward systematically understanding the forces that operate at the nanoscale and drive nanoparticles self-organization.10,11 Hydrophobic interactions have, however, remained in the shadow of the usual interactions, and they are often believed to play a role of rather qualitative auxiliary interactions in the theoretical description of experimental achievements. The reason why we often avoid exploiting hydrophobic interactions is that scientists prefer to see interparticle interactions from the points of view of particle properties rather than those of the liquid medium. We thus look after highly specific interactions (e.g., hydrogen bonding), which are omnipresent in biological systems and rather easy to assess by experimental means. Nonspecific interactions are of course difficult to control, thus leading to disordered gel-like forms, especially if they depend on solute−solvent interactions. Aside from this drawback, hydrophobic interactions exhibit important advantages over other interactions, such as the possibility of reversible solvation of the solute by solvent molecules, thus providing an important tool to develop stimuliresponsive nanomaterials. In fact, molecular self-assembly is probably the most advanced field of nanofabrication where hydrophobic interactions are controlled with great precision. Recent reviews on molecular self-assembly confirm the broad scope of this field.12,13 To exploit self-assembly of nanoparticles via hydrophobic interactions, one needs to precisely devise the system, taking into account both surface chemistry and solvent

Figure 1. Strategies for exploiting hydrophobic interactions between nanosized objects. Switchable ligands: under an external stimulus, surface ligands undergo a structural transition that affects solvent−ligand interactions, which in turn invoke attractive hydrophobic forces. Removing the stimulus or applying a different one brings the particles back to the initial disperse phase. Switchable solvent: An external stimulus induces changes in solvent chemistry, affecting solvent−ligand interactions and thereby facilitating particles assembly. 2456



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become a promising analytical tool to monitor biological events. Light is an excellent alternative trigger to modulate hydrophobic interactions between nanoparticles,22,23 mainly because it is minimally invasive and can be readily tuned through wavelength, intensity, illuminated area, and so forth. The use of light, however, requires specific functional groups to produce switchable hydrophilic/hydrophobic transitions. Zhao and co-workers used gold nanoparticles stabilized by copolymers containing azobenzene functional groups in trans conformation, which ensured colloidal stability of the nanoparticles in water (Figure 2b).24 Upon UV irradiation, the cis conformation of azobenzene renders the grafted copolymer insoluble, leading to aggregation of the nanoparticles. The initial dispersion was recovered by visible light irradiation, recovering the trans conformation. Switchable Surfactants. In the above examples, hydrophobic interactions were induced between molecules that were covalently attached to the nanoparticles surface. Although this strategy is appealing, it often requires complex ligand exchange protocols that require large amounts of ligand molecules. In this context, progressive attention has been given to switchable surfactants that can drive nanoparticles organization via hydrophobic interactions. In surfactants, both hydrophobic and hydrophilic moieties coexist within single molecules, and therefore, they have the ability to self-organize at interfaces by decreasing the interfacial energy. Surfactants with a switching ability exhibit properties that make possible the design of smart materials with on demand dynamically changing properties and structure in response to environmental changes. Thus, this class of

Hydrophobic interactions exhibit important advantages over other interactions, such as the possibility of reversible solvation of the solute by solvent molecules. toward reversible assembly. Thermoresponsive polymers are very interesting surface ligands that undergo a hydrophilic to hydrophobic phase transition at a certain temperature. The most usual thermoresponsive polymer is probably poly(isopropylacrylaminde) (pNIPAM). Its solubility in water at low temperature is driven by enthalpy and governed by hydrogen bonding between water and the polymer. With increasing temperature, H-bonding between solvent and polymer breaks, thereby increasing entropy and expelling water into the bulk solution, which results in further attraction between polymer molecules. Hamner et al.20 have recently proposed an elegant procedure for reversible aggregation of gold nanoparticles stabilized with a thiolated thermosensitive copolymer containing pNIPAM blocks (Figure 2a). The nanoparticles maintain colloidal stability below 50 °C but form extended aggregates above this temperature. Small angle X-ray scattering at different temperatures confirmed the temperature-dependent interparticle distances, corresponding to the extended and collapsed configurations of the copolymer shell. Temperature-tunable interactions between plasmonic nanoparticles are very interesting for the development of nanothermometers,21 which have

Figure 2. Switchable ligands and surfactants for hydrophobic interactions. (a) An example of temperature-controlled self-assembly of gold nanoparticles through the hydrophilic-to-hydrophobic transition of a polymer shell. (b) Conformational changes on the surface chemistry facilitate nanoparticles aggregation. (c) Light-responsive phase transfer of gold nanoparticles between water and toluene phases mediated by an azofunctionalized surfactant. (d) Reversible dispersion of carbon nanotubes using an amidine-functional surfactant and CO2 gas as the trigger. Reprinted from refs 20, 24, 27, and 33. 2457



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molecules has gained substantial attention not only on the level of fundamental science but also toward a number of technological processes. The switching ability of surfactants has been reported through optical, thermal, pH, or CO2 triggers, which induce changes in macroscopic properties such as viscosity or solubility.25,26 For example, a switchable surfactant has been recently used to reversibly transfer gold nanoparticles between organic and aqueous phases.27 In the proof of concept experiments, α-cyclodextrin-stabilized gold nanoparticles were dispersed in an aqueous phase while a photoswitchable surfactant containing trans-azobenzene moieties was solubilized in toluene (Figure 2c). The nanoparticles were converted from hydrophilic to hydrophobic via host−guest interactions between α-cyclodextrin (ligand) and trans-azobenzene (surfactant), thereby undergoing spontaneous transfer into the organic phase. Upon UV light irradiation, the trans-azo group transformed into cis-form, dissociating from the gold nanoparticles surface and allowing their transfer back to the aqueous phase. Thus, light-modulated hydrophobic interactions open new possibilities to shuttle cargo between phases.28 Switchable transitions of nanoparticles between water and organic phases can also be triggered by dissolved gas, in particular by carbon dioxide, which is an alternative stimulus capable of modulating the solubility of surfactants.29,30 For example, the mixture of CO2 and water (H2CO3) can protonate amidine functional groups to form imidinium ions. The protonation process is reversible through bubbling of an inert gas such as argon. Surfactants containing amidine moieties in the absence of the CO2 are soluble in organic solvents, whereas upon treatment with CO2, they became soluble in water. This class of molecules has been recently used to induce nanoparticles phase transfer31 based on the interdigitation of a switchable surfactant between nonpolar ligands on the nanoparticles. Upon CO2 bubbling, amidine-containing surfactants gain a positive charge, and thereby the nanoparticles can be transferred into water, whereas subsequent bubbling with nitrogen reverses the transfer into the organic phase. Reversible CO2 capture and subsequent changes of physical properties constitute a promising strategy to resolve environmental issues such as CO2 fixation or even as a resource for fuel production.32 In the context of self-assembly processes driven by tunable hydrophobic interactions, one may explore CO2-triggered surfactant switching within a single phase solution, that is, without phase transfer. Although the self-assembly of nanoobjects induced by CO2 still needs further development, recent examples regarding reversible aggregation of carbon nanotubes are inspiring toward greenhouse gas-controlled self-organization. Ding et al. used a CO2-responsive surfactant that contained hydrophobic pyrene and switchable amidine functional groups.33 Although pyrene strongly attaches to the carbon nanotubes’ hydrophobic surface, the amidine moieties become protonated by capturing CO2 and provide the nanotubes with colloidal stability in water. Bubbling the solution with inert gas leads to deprotonation of the amidinium group, which in turn induces carbon nanotubes aggregation (Figure 2d). Reversible Solvent Composition. Another interesting strategy to tune hydrophobic interactions is based on compositional changes in the environment (Figure 1). A visual example of such a strategy is the aggregation of hydrophobic, polystyrene-capped gold nanoparticles upon gradual changes in solvent composition, for example, from pure THF to THF/water mixtures. The particles gradually aggregate and eventually precipitate after several hours, which can be monitored by simple color changes.7

This visible manifestation of the hydrophobic interactions is due to the decreased solubility of polystyrene molecules with increasing water content. Therefore, this strategy for directing nanoparticle self-assembly exploits changes in the chemical composition of the solvent under the external stimulus. In this context, linear copolymers become a very attractive class of molecules because they can organize themselves into a wide variety of structures depending on solvent composition, in analogy to low molecular weight surfactants. The first experimental evidence of copolymer self-assembly was reported back in 1995,34 showing that a copolymer containing hydrophobic polystyrene and hydrophilic poly(acrylic acid) blocks (PS-b-PAA) formed micelles, vesicles, or planar architectures upon addition of water to a solution of the copolymer in an organic solvent (DMF, THF, dioxane). Interestingly, the structural diversity of the assemblies depends on the kinetics of water addition.35 Thus, upon fast water addition the copolymer molecules reorganize into less thermodynamically stable micelles. On the other hand, slow water addition (∼mL/h) allows for the dynamic rearrangement of the different blocks in the copolymer, leading to thermodynamically stable vesicles. The slow dynamics of the copolymers, as compared with conventional surfactants, is due to a larger molecular weight (5 kDa) and allows for kinetic freezing of the system while maintaining it out of thermodynamic equilibrium. Such properties are particularly beneficial for the spatial organization of additives (e.g., nanoparticles), and indeed, linear copolymers have been widely exploited as templates in the self-assembly of nanoparticles.36−40 For example, Eisenberg and co-workers have shown that gold nanoparticles could be selectively distributed inside the membrane of copolymer vesicles.41 The particles were coated with diblock copolymers with a structure similar to that of the vesicle components, thereby allowing the particles to be preferentially localized in the central region of the vesicle membrane. On the other hand, nanoparticles can also achieve a spatial organization in the form of 3D clusters inside the copolymer micelles. This process is particularly interesting for bottom-up nanofabrication because it offers control over the number of nanoparticles per cluster by simply tuning the aggregation kinetics before the polymeric surfactant (copolymer) encapsulates the aggregating nanoparticles.7,42 Suitable combinations of nanoparticle shape and spatial ligand distribution (patchiness) may lead the system to behave as an amphiphilic copolymer upon changing solvent composition. Kumacheva and co-workers have developed patchy anisotropic nanoparticles consisting of gold nanorods coated with hydrophobic polystyrene at their tips while keeping the lateral facets hydrophilic.43 In this manner, nanoparticles that were initially stable in DMF formed chains upon addition of water through strong hydrophobic interactions between the polymer molecules located on the tips. Interestingly, mechanistic studies showed that the number of nanoparticles in the chains increased linearly with time, resembling conventional step-growth polymerization of the molecular system.44 Thus, the self-assembly of nanoparticles via hydrophilic interactions has strong foundations in polymer science, and this is why this class of self-assembled systemsmostly comprising gold nanoparticleshas been termed “plasmonic polymers”18 or even “plasmonic copolymers”.45 The examples above show that indeed hydrophobic interactions constitute a pivotal point that bridges polymer chemistry and colloid chemistry. The sensitivity toward solvent composition and the reversible nature of hydrophobic interactions additionally allow for dynamic restructuring of the assemblies, thereby generating 2458



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Figure 3. Structural transformations at the nanoscale through solvent-mediated hydrophobic interactions. (a) Side-to-side dimers of gold nanodumbbells transform into a cross-like conformation inside polymeric micelles (b) Coiling of thin gold nanowires through encapsulation in polymeric micelles and subsequent increase in water content. (c) Division of plasmonic micelles with increasing amount of dioxane. (d) Structural transition from globular to chain-like assemblies through increasing the amount of water. Reprinted from refs 46, 47, 48, (Copyright 2013, Royal Society of Chemistry) and 49.

micelles so that with increasing dioxane content, the interfacial energy decreases and changes the interfacial area by decreasing micelle size. In another example of structural transition driven by hydrophobic interactions, Kumacheva and co-workers used polystyrene-stabilized spherical nanoparticles to demonstrate structural transitions between linear and globular assemblies in DMF/water mixtures49 (Figure 3d). The transitions were governed by the subtle competition between attractive hydrophobic and repulsive electrostatic forces. At low water content (5 wt %), hydrophobic interactions dictate the formation of stable globular systems that lead to decreased surface energy. At higher water content (15 wt %), however, the electrostatic repulsions balance the attractive hydrophobic forces, thereby yielding chainlike assemblies. In the presence of salt, the transition from globules to chain-like assemblies does not take place because of the suppression of electric double layer interactions, clearly suggesting that electrostatic repulsions can finely balance hydrophobic interactions.

different types of nanostructures from the same type of nanoparticles. Such properties thus offer an exciting strategy toward the design of responsive nanomaterials. Our group has recently shown that side-to-side dimers of polystyrenefunctionalized gold nanodumbbells encapsulated inside polymeric micelles of PS-b-PAA switched into cross-like clusters upon increasing solvent polarity (addition of water).46 The mechanical stress invoked by hydrophobic interactions eventually overcomes steric hindrance, leading to a structural transformation of the assemblies that is unique for dumbbelllike building blocks (Figure 3a). Chen and co-workers also studied the effect of hydrophobic forces on mechanical stress at the nanoscale.47 Ultrathin gold nanowires were encapsulated in PS-b-PAA copolymer in a DMF/water mixture. By inducing hydrophobic interactions in the presence of water, the initially elongated copolymer domains contracted into spheres, forcing the embedded nanowires to coil into rings (Figure 3b). Interestingly, the nanowires spontaneously sprang back to the initial elongated form upon removal of the polymer shell, showing that the coiled nanowires stored mechanical energy in the form of elastic potential energy. Solvent-induced division of polymeric micelles is another example of structural responsiveness of the assemblies. We recently proposed a new type of binary plasmonic clusters encapsulated inside copolymer micelles.48 Upon addition of dioxane (i.e., decreasing solvent dielectric constant) to the mixture containing the assemblies and excess copolymer, the spherical clusters underwent division to produce smaller units with evenly distributed nanoparticle building blocks (Figure 3c). This effect is ruled by changes of the interfacial energy of the

The sensitivity toward solvent composition and the reversible nature of hydrophobic interactions additionally allow for dynamic restructuring of the assemblies. Switchable Solvents. Beyond conceptual novelty, the above examples suffer from the need for direct intervention of the 2459



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drug delivery (Figure 4a).53 Such plasmonic vesicles consist of amphiphilic nanoparticles stabilized with hydrophobic and hydrophilic polymer brushes. Although the hydrophobic component is embedded in the vesicle membrane, the hydrophilic brushes extend into the aqueous environment to stabilize the structure. Drug molecules can thus be released by disassembly of the vesicles in the acidic endocytic organelles, showing that this multifunctional drug carrier not only allows for efficient cargo release but can also generate independent optical and spectroscopic feedback. The same group expanded their strategy to photoresponsive plasmonic vesicles that allowed active delivery of anticancer payloads onto cancer cells.54 These vesicles must contain a photoresponsive hydrophobic component, such as poly(2-nitrobenzyl acrylate) (PNBA), which upon photoexposure converts into the hydrophilic poly(acrylic acid), leading to vesicle disintegration. Nie and co-workers went one step further and applied a multifunctional theranostic platform based on photosensitizer-loaded plasmonic vesicles for in vivo cancer imaging and treatment.55 The vesicular architecture ensures not only strong plasmonic coupling between nanoparticles in the membranes but also the capability of encapsulating photosensitizer cargo molecules, therefore enabling trimodal imaging (fluorescent, thermal, and photoacoustic) and bimodal therapy (photothermal and photodynamic). This strategy for theranostic applications has been recently expanded by the same group to use biodegradable plasmonic vesicles composed of poly(ethylene

experimentalist on solvent composition. Self-assembly takes place only upon addition of a nonsolvent, which has at least an important drawback, namely the unavoidable dilution of the mixture. Therefore, we face the challenge to find solvents that can undergo hydrophilic/hydrophobic transitions under external stimuli. An interesting class of solvents are so-called “switchable hydrophilicity solvents”, which are typically liquids containing amidines, tertiary amines, or ionic liquids and which undergo hydrophobic/hydrophilic transitions in the presence of CO2. In the context of self-assembly, switchable solvents are very promising because they exhibit significant polarity changes upon application of external stimuli.29,50 For example, the mixture of alkaneamidine and water (1:1 v/v) in the presence of CO2 becomes a single phase with high polarity. In the absence of CO2, however, amidine has lower polarity and expels water, consequently leading to phase separation. Thus, carefully designed experiments with adequate amidine/water volume fractions may lead to controlled self-assembly of nanoparticles using CO2 as a switchable trigger. Applications. Responsive Assemblies for Bioapplications. Drug delivery nanosystems that can release a cargo in response to an external trigger are currently under intense investigation in pharmaceutical chemistry and nanomedicine.51,52 Selfassembled nanoparticles constitute a promising theranostic tool for drug delivery, imaging, and therapy. Duan’s group reported the use of plasmonic vesicles for cancer-targeted

Figure 4. Application of reversible hydrophobic interactions. (a) Plasmonic vesicles release drug molecules under external stimuli inside cancer cells. (b) Multifunctional plasmonic vesicles for in vivo imaging and therapy. (c) Photoswitchable nanoparticles stabilize a Pickering emulsion as a container for biphasic biochemical reactions. (d) Mechanical locking and unlocking of DNA origami by hydrophobic interactions. Reprinted from refs 54 (Copyright 2013, Royal Society of Chemistry), 56 (Copyright 2013, Wiley-VCH), 58, and 60 (Copyright 2014, Wiley-VCH). 2460



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glycol)-b-poly(α-caprolactone).56 After completing their therapeutic function, the biodegradable vesicles disintegrated into smaller discrete units, which undergo rapid clearance (Figure 4b). Hydrophobic interactions for switchable catalysis. In (bio)catalysis, spatially increased concentration of the substrate (compartmentalization) is crucial for enhanced performance.57 In fact, a large body of work has been reported on the fabrication of nanoreactors comprising permeable membranes or biphasic emulsions. In the latter case, the switchable hydrophobic interactions can provide additional functionalities, especially when dealing with catalyst recycling. For example, photoswitchable Pickering emulsions have been recently developed by using spiropyran-stabilized upconverting nanophosphors as colloidal emulsifiers.58 Upon absorption of NIR light, the upconverting nanophosphors emited photons in the UV region that induced isomerization of spiropyran from hydrophilic to hydrophobic structures, thereby driving emulsion inversion (Figure 4c). Interestingly, by loading bacteria in the aqueous phase, the authors could perform water−toluene biphasic enantioselective biocatalysis across the nanoparticle “membrane”. Owing to the inversion ability of the emulsion, a multiple recovery process could be realized including the reaction products, biocatalyst, and colloid emulsifiers. Hydrophobic Actuators. Biomacromolecules such as DNA and RNA have been demonstrated as an attractive class of molecules for the design and experimental realization of artificial molecular actuators that can change their geometry and mechanical properties and can even capture or release nanoobjects.59 Recently, Simmel and co-workers introduced a “hydrophobic switching” mechanism for reversible rearrangement of DNA− cholesterol conjugates in aqueous solution (Figure 4d).60 Hydrophobic interactions are responsible for folding cholesterol-modified flat DNA origami into sandwich-like bilayer structures, which actually hide the cholesterol modifications in their interior. The DNA bilayer structures could be unfolded back upon surfactant addition. Interestingly, the opening of the bilayer was also achieved in the presence of molecular nanocontainers. Hydrophobic interactions between cholesterol from DNA origami and vesicular lipid membranes were strong enough to overcome similar hydrophobic forces between cholesterol molecules within closed DNA origami. Therefore, it becomes progressively obvious that hydrophobic interactions expand the repertoire of switching mechanisms for reconfigurable nucleic-acid-based nanostructures and may make it possible to access structural features that are usually found in proteins. We close this article with a clear take-home message: hydrophobic effects indeed constitute a very attractive chemical tool, with particular interest in the area of colloidal nanofabrication. By collecting the few examples above, we hope to stimulate further the rapid development of this exciting field, in which solvent−solute interactions operating at the length scale below 2 nm can drive bottom-up fabrication of macroscopic systems using nanoscale objects as building blocks. In addition, the sensitivity of hydrophobic interactions toward external stimuli is a fundamental parameter for the fabrication of smart functional materials that will find applications in nanomedicine or catalysis. Of course, the fast development of this field leads to facing important challenges at both the experimental and theoretical levels. First of all, evaluation of the forces for a given system (e.g., solvent, ligand, core material) is of major importance and would facilitate the exploitation of hydrophobic interactions in colloidal nanofabrication. Stronger efforts

toward switchable systems, in which hydrophobic interactions may play an important role, will provide elegant combinations of nanoparticles self-organization and spatiotemporal functionality. Finally, theoretical modeling of hydrophobic interactions will help us to understand existing systems and to predict the formation of novel structures from specific components and with relevant functionalities.

Stronger efforts toward switchable systems will provide elegant combinations of nanoparticles self-organization and spatiotemporal functionality.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. http://www. bionanoplasmonics.com *E-mail: [email protected]. http://www. bionanoplasmonics.com. Notes

The authors declare no competing financial interest. Biographies Marek Grzelczak received an M.Sc. degree from Adam Mickiewicz University (2004) and Ph.D. from University of Vigo (2008). After postdoctoral stays at Universty of Trieste and Max Planck Institute of Colloids and Interfaces, he is Ikerbasque Research Fellow at CIC biomaGUNE. His research focuses on the synthesis and self-assembly of multifunctional nanostructures. Luis M. Liz-Marzán holds a Ph.D. from University of Santiago de Compostela and was postdoc at Utrecht University and visiting professor at various universities and research centers. After being Full Professor at the University of Vigo, he is currently Ikerbasque Research Professor and Scientific Director of CIC biomaGUNE.

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ACKNOWLEDGMENTS This work has been supported by the European Research Council (ERC Advanced Grant #267867 Plasmaquo). REFERENCES

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