Responsive and Nonequilibrium Nanomaterials - The Journal of

Perspective pubs.acs.org/JPCL

Responsive and Nonequilibrium Nanomaterials Scott C. Warren,† Ozge Guney-Altay,‡ and Bartosz A. Grzybowski*,†,‡ †

Department of Chemistry and ‡Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ABSTRACT: Nanoscience has been promoted as a major technological revolution, and yet its influence outside of the laboratory has been relatively small. From our survey of recent progress, we conclude that as nanoscience fragments into subdisciplines and researchers become ever more specialized, there is increasingly little advancement toward the emergence of research themes that may unite and elevate nanoscience toward having an impact of the magnitude achieved by the steam engine, electricity, medicine, and the Internet. We suggest that one avenue for nanoscience to break this impasse is to venture beyond static structures into domain of dynamic nanomaterials that organize and/or function when displaced from thermodynamic equilibrium. We highlight recent work from our laboratory in this emerging area and also suggest some possible future applications for responsive and nonequilibrium nanosystems/materials.

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dynamic nanomaterials of the future should be displaced from equilibrium and should be able to reside in multiple metastable states between which they could be interconverted.25−27 The rationale for this vision is that it is only outside of thermodynamic equilibrium28 that life-like properties such as multifunctionality, adaptability, reconfigurability, taxis,29 internal feedback, or self-replication,30,31 can be realized.32−36 In other words, creating any (nano)materials that are “intelligent” and adaptive requires venturing beyond the confines of equilibrium/static structures.

ver the past 2 decades, the focus of nanoscience has been gradually shifting from the synthesis of individual nanocomponents to the synthesis/assembly of larger nanostructures and materials. At first glance, this approach has been quite successful as it generated an impressive variety of structures  from molecule-like nanoclusters,1−3 to 2D nanoparticle (NP) arrays4−6 and 3D crystals,7,8 to DNA origami,9,10 to mesoporous materials,11−13 to name just a few. Although these materials are being used to address important challenges in catalysis,14 energy conversion,15,16 information storage and processing,17 sensing,18−20 diagnostics,21−23 and therapeutics,24 upon closer inspection, the progress appears less spectacular in the sense that these applications existed prior to the advent of nanoscience. It is hard to escape a conclusion that the field of nanomaterials is conceptually saturating, that much of the work done to control nanoscale structure does not address fundamental or societal challenges, and that as nanomaterials research fragments into subdisciplines, the improvements become more incremental. While this assessment might appear provocative or even unfair, it is perhaps instructive to remind ourselves what nanoscience was originally expected to become. In its early days, it was heralded (and generously funded!) as a technological revolution (after steam, electricity, computing, and medicine) that would create materials directly impacting and improving the human condition. We think it is safe to say that although nanomaterials have led to the improvement of preexisting devices, they have not yet revolutionized our lives in the ways the steam engine, the light bulb, antibiotics, or the Internet did. If nanomaterials are to deliver on their much-publicized promise, something conceptually new has to happen. The thesis of this Perspective is that this “something” is the construction of responsive and nonequilibrium nanomaterials that could utilize externally delivered energy to change their internal structures and overall functions on demand. Unlike most traditional materials, which are locked into global or local thermodynamic minima, © 2012 American Chemical Society

It is hard to escape a conclusion that the field of nanomaterials is conceptually saturating, that much of the work done to control nanoscale structure does not address fundamental or societal challenges, and that as nanomaterials research fragments into subdisciplines, the improvements become more incremental. In this quest, the unique advantage of nanomaterials is that their components can combine multiple properties without the need for advanced chemical synthesis. Nowadays, it is straightforward to make nanoobjects from various materials,6,37−47 in various sizes44,48,49 and shapes,38,40,50 and with different types of stabilizing ligands.51,52 Moreover, in the context of dynamical materials, one can build in various forms of responsiveness Received: May 8, 2012 Accepted: July 12, 2012 Published: July 12, 2012 2103

dx.doi.org/10.1021/jz300584c | J. Phys. Chem. Lett. 2012, 3, 2103−2111

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not anywhere close to making walking57 or swimming58,59 nanorobots,60−62 our laboratory has recently demonstrated several types of nanomaterials where the “dynamic” nature translates into switchable/responsive function with potential applications in catalysis63 and electronics.55 The ultimate goal of the present Perspective is not so much to showcase these specific proof-ofconcept experiments but to use them to illustrate the opportunities that lie ahead; in this spirit, we allow ourselves to supplement the examples of what has already been demonstrated with the visions of what we think could and should be done. From Responsive NPs to Metastable Crystals and Dynamic Catalysts. Conceptually, the most intuitive approach to dynamic nanomaterials is to make them from responsive nanocomponents. In this spirit, we have been developing a class of metallic NPs functionalized with ligands terminated in moieties that change their properties upon changes in the environment or upon external stimulus.55,64 In arguably the simplest possible scenario, the NPs are covered with self-assembled monolayers (SAMs)51,52 of ligands terminated in phenol groups that change from neutral to negatively charged when the pH of the surrounding solution is lowered below the SAM’s pKa, which can be regulated by particle size.65,66 One implementation of this form of switchability is to couple the NPs to so-called chemical oscillators67 (Figure 1a), which are systems of reactions in which

Creating any (nano)materials that are “intelligent” and adaptive requires venturing beyond the confines of equilibrium/static structures. ranging from optical addressability (e.g., due to the plasmonic excitation of metallic nanostructures53), through the modulation of surface properties (e.g., by coating with switchable ligands54), to the on-demand control of the nanomaterial’s electrical properties55 (e.g., by linking nearby NPs with conjugated, photoactive ligands56). Importantly, these properties can often be added without mutual interference; for instance, the properties of a magnetic NP core remain largely unchanged when the particle is coated with a monolayer of photoswitchable ligands that permit the surface properties to be changed from hydrophobic to hydrophilic. In contrast, synthesizing a purely organic magnetic molecule that, on demand, becomes water-soluble/insoluble is a much harder proposition and would require precise control of both electronic and steric effects responsible for the two properties. These general considerations are but a starting point for the development of dynamic nanomaterials. While we are definitely

Figure 1. Dynamic, oscillatory dispersion and aggregation in NP assemblies. (a) A chemical oscillator cycles the pH between 6.8 and 9.3, which drives the dispersion and aggregation of pH-sensitive NPs. The oscillator runs as long as a sufite/bisulfite buffer and “fuel” (formaldehyde/ gluconolactone) are added. The NPs are capped by 2-fluoro-para-mercaptophenol, which has a pKa of 8.3. (b) Aggregation leads to plasmonic coupling between NPs, inducing a red shift and overall increase in optical extinction of the gold NPs. Measurements of particle size by light scattering provide evidence for reversible, pH-oscillator-driven dispersion and aggregation. (c) An alternate vision for oscillatory switching is based on mixtures of two types of NPs (silver, shown in gray, and gold) that compete for adsorption at binding sites on a polymer. The gold NPs are covered with ligands incorporating DNP (1,5-dioxynaphthalene) that can form pseudorotaxanes with CBPQT4+ (cyclobis(para-quat-p-phenylene) on the polymers. Redox-sensitive TTF (tetrathiafulvalene) ligands on silver NPs allow the relative binding strength of gold and silver NPs to be reversed. The addition of a reducing or an oxidizing agent leads to the alternating dispersion and aggregation of gold and silver NPs. UV−vis absorption spectroscopy was used to monitor the plasmon resonances for silver (gray circles) and gold NPs in solution. Adapted from refs 67 and 71. 2104



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of the ease of synthesis but also because their cis−trans isomerization creates/annihilates dipole moments that can translate into dipole−dipole interactions72 between the nanoobjects that these molecules coat. This property underlies the design of dynamic NP crystals13,25 illustrated in Figure 2. Here, the NPs are covered with a mixed monolayer incorporating ligands bearing azobenzene units. In the absence of UV irradiation, the azobenzenes are in the trans state and have no appreciable dipole moment; as a result, these NPs are stable in a nonpolar solution. Upon UV irradiation, however, the azobenzenes isomerize to the cis form characterized by a dipole moment of ∼5 D; the attractions between these dipoles then cause the NPs to assemble. Importantly, by adjusting the fraction of azobenzene ligands on the NPs, the magnitudes of the light-induced dipole−dipole interactions are commensurate with the thermal energy 3/2kT but are strong enough to cause particle aggregation. Under these circumstances, the NPs form assemblies that persist only when UV irradiation continues; in the absence of irradiation, the azobenzene units reisomerize to the trans form, and the thermal noise causes the NP aggregates to fall part. This is perhaps the first demonstration of metastable/ nonequilibrium crystals that persist only as long as energy (here, light) is externally delivered to them. Because our focus is on applications of dynamic nanomaterials, the key question is whether these types of transient assemblies have any potential practical uses. In one applicationoriented project that we have been pursuing, the light-switchable particles are embedded in a thin sheet of an organogel (Figure 3a).73 When localized light is applied, it causes NP aggregation and a color change (due to a shift in the surface plasmon resonance) in a specific location; in this way, using a light pen or shadow mask, images can be written into the material with a resolution down to a few micrometers and with the ability to achieve multiple colors depending on the light intensity and the degree of self-assembly (Figure 3b,c). The key dynamic feature of such materials is, however, that once written, the images gradually self-erase as the nanoassemblies fall apart. An obvious use of this sort of material is in secure communications where sensitive documents would self-erase after a predetermined period of time. Interestingly, a similar concept is being developed by Xerox, but it allows for only a single self-erasure time determined by the photoisomerization of individual photochromic molecules.

complex, nonlinear kinetics gives rise to periodic changes in the solution’s pH.68−70 When the NPs’ pKa's falls within the pH range of the oscillations, the NPs periodically repel (when negatively charged) and attract (when neutral and interacting by van der Waals forces). This behavior translates into rhythmic dispersion and assembly of the NPs, which, in turn, manifests itself in periodic shifts of the NPs’ surface plasmon resonance and of the solution’s color (Figure 1b). When the same types of phenomena are implemented in a gel matrix, the oscillations start at a given location and propagate through the material as chemical waves. While this system is clearly a toy model, perhaps most relevant to the study of nonlinear chemical phenomena and chemical systems, its value lies in demonstrating that switchability on the level of nanocomponents can be coupled to the environment and give rise to dynamic/nonequilibrium phenomena at the macroscopic level.

Switchability on the level of nanocomponents can be coupled to the environment and give rise to dynamic/nonequilibrium phenomena at the macroscopic level. Beyond this demonstrative value, however, the above system suffers from a rather serious and hard to overcome limitation, namely, the relative slowness of switching that is limited by the delivery (diffusion) of the acid/base needed to protonate/ deprotonate the carboxylic acids on the NPs. This limitation is, in fact, common to other systems using molecules switching upon chemical stimuli, even the chemically elaborate pseudorotaxanebased NP constructs that can be, for example, captured and released on-demand from polymeric “sponges”71 (Figure 1c). Although the individual rotaxanes can switch on the order of milliseconds,54 the switching at the level of the entire material is very slow, on the order of minutes to hours.71 A potentially much more rapid and also more versatile mode of switchability is to use light-sensitive nanocomponents. Light signals can be delivered instantaneously and the molecular switches can be made responsive to a desired wavelength. In the context of our discussion, azobenzene units are advantageous not only because

Figure 2. Dynamic, metastable crystals made of Au NPs covered with SAMs incorporating alkane thiols terminated in azobenzene units. In the absence of UV irradiation, the azobenzenes are in the trans form (see Figure 4a), and the NPs are dispersed, as evidenced by the solution’s red color due to Au NP surface plasmon resonance at ∼520 nm. Upon UV irradiation, the azobenzenes isomerize to the cis form (see also Figure 4a), and the light-induced dipoles mediate formation of NP crystals such as the dodecahedral crystal shown in the right-most SEM image (scale bar = 100 nm; the inset zooms on the individual NPs on the crystal’s faces). The crystals slowly disassemble in the absence of UV light or, more rapidly, upon irradiation with visible light. Adapted from ref 25. 2105



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Figure 3. Reversible image storage by NP assemblies. (a) Scheme of the writing into self-erasable NP films using a light pen. (b) Multicolor images written into AuNP films. In the “flowers” picture, the purple regions were irradiated for shorter times than the purple−bluish ones. In the Union Jack, the whitish−blue regions were irradiated longest, so that all NPs in these regions aggregated, shifting the plasmonic resonance of the material into the infrared. (c) A multicolor image written in two steps into a AgNP film. The entire film was first exposed to UV light, causing a color change from yellow to pale red. The film was then bent, and the pattern of squares was “written in” (transition from pale red to purple). Finally, the film was flattened, and an image of azobenzene was created by exposure to visible light, which caused disassembly of the NP aggregates in the irradiated region and return to the original light-orange hue. (d) Two images of self-erasing films. The upper image was created in a film in which the surface coverage of the azobenzene ligands on the NPs was lower than that in the lower image; consequently, the upper image erases faster than the lower one. All scale bars are 1 cm. Adapted from ref 73.

Figure 4. Photoswitchable catalysis. (a) Gold NPs functionalized with a mixed SAM of an azobenzene-terminated alkane thiol and an alkylamine allows NPs to be repeatedly dispersed and aggregated via the cis−trans isomerization of azobenzene. (b) Photograph of dispersed (i) and aggregated (ii) NPs. The yield of a gold-catalyzed hydrosilylation reaction was monitored as the catalyst was irradiated with either UV light (beginning at times labeled in blue) or visible (beginning at times labeled in red). (c) Scheme for a proposed catalytic cycle in which the aggregation state of two types of NPs can be independently addressed by the use of different light-responsive ligands. Au NPs bear the Grubbs catalyst for alkene metathesis, while the silver NPs bear imidazole for ester hydrolysis. For either NP, light-driven aggregation leads to a decrease in surface area that inhibits catalysis. Such switchable catalysts could enable the development of one-pot multistep reactions. Adapted from ref 63. 2106



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Figure 5. Dynamic, reconfigurable electronic components. (a) A film composed of gold NPs with N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride (HS-C11H25−NMe3+Cl−) ligands was deposited between two electrodes. Application of a potential across the film produced a concentration gradient of chloride ions. (b) After migration of chloride ions toward the positive electrode, the resulting internal polarization (Ei) opposes the applied field (Eapp). This leads to an overall decrease in current and the emergence of a low conductance state. If the applied potential is switched, Eapp and Ei are in the same direction, and a high conductance state is achieved. (c) Six wedge-shaped electrodes were deposited onto a NP film. A positive potential was applied to electrodes 2 and 3 with respect to electrodes 5 and 6, polarizing the film (top left). The red lines show qualitative ion distributions, and the red dots show the mobile ion distribution, with low chloride ion concentration near electrodes 5 and 6. Polarization leads to the development of an internal electric field, a component of which lies along vectors E15 and E13 (top right). Subsequently, electrode 1 was biased positive with respect to electrodes 3 and 5 (bottom left). The applied field coincided with the internal field between electrodes 1 and 3, thereby increasing current; meanwhile, the current was lowered between electrodes 1 and 5 because the applied and internal fields opposed each other. (d) The current between two electrodes in the absence of polarization (dashed line) is smaller than the current when enhanced by polarization (1−3) and larger than the current when opposed by polarization (1−5). Measurements of the current between 1 and 3 and 1−5 were inverted every 10 s. Adapted from ref 55.

In contrast, the times of self-erasure in our nanostructured materials can be varied flexibly from minutes to days (Figure 3d) by adjusting the numbers of switchable ligands on the NPs (more ligands translate into stronger dipole−dipole forces and longer disassembly times).73 Another use of light-switchable NPs is in light-controlled catalysis.63 In a system illustrated in Figure 4a, Au NPs are covered with a mixed SAM of a switchable azobenzene-terminated alkane thiol and weaker binding dodecyl amine (DDA) ligands (DDA is used because if the SAMs were all made of thiolate ligands, the particles would be fully passivated and catalytically inactive). In the absence of irradiation (Figure 4a, left), the NPs do not attract each other, are dispersed in the solvent, and can catalyze a hydrosilylation reaction. When the UV light is turned on (Figure 4a, right), however, the NPs aggregate; the catalytic surface area of the NPs decreases, and the catalyzed reaction effectively comes to a halt. This light off−light on cycle can be

repeated multiple times (Figure 4b), translating into on-demand photocatalysis. Although interesting, this system is only a precursor toward more elaborate constructs that we would like to ultimately achieve. One example is illustrated in Figure 4c in which two types of NPs catalyzing two different transformations are present in solution and are covered by photoswitchable ligands responsive to light of different wavelengths (λ1 and λ3). The idea here is that by selectively assembling only one type of these particles, it would be possible to translate a sequence of irradiation (i.e., first λ1 and then λ3 versus first λ3 and then λ1) into a different desired sequence of chemical reactions. In this way, one could “program” multistep chemical synthesis by the sequences of light pulses. Responsive Nanomaterials Based on Dynamic Internal Gradients. In the examples above, the switchability was achieved by toggling the responsive nanocomponents between disassembled and assembled states. Another and, in some applications, more desirable form of switchability would be to reconfigure a solid-state 2107



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Figure 6. Possible future directions in responsive and nonequilibrium materials. For further details, see refs 79−84 and references therein.

“nanoionic”74−78 NPs covered by SAMs of ligands terminated in charged groups (e.g., −N(CH3)3+ or −COO−). These particles are cast from aqueous solution as thin films and thoroughly dried, and metal electrodes are evaporated onto the film in a desired configuration by use of a shadow mask. In the absence of any external bias, there is nothing in the material itself that suggests its switchability; things change, however, when bias is

nanostructured material without necessarily disassembling/ assembling its constituent pieces. To do this, some parts of the material, those responsible for the overall structure, should be immobile, while others should be able to switch or move. This heuristic underlies operation of nanostructured materials that we recently developed and demonstrated as reconfigurable electronic elements.55 Here, the building blocks are metallic 2108



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relatively complex functions by using relatively simple nanochemistry. Naturally, it could be argued that none of the systems that we have described outclass technologies that already exist (e.g., conventional silicon-based electronics) or that alternative technological solutions can be designed (e.g., series of separate reactors in lieu of mixtures of photoswitchable nanocatalysts in Figure 4c). However, this argument may miss the point. It is more worthwhile to ask whether dynamic nanomaterials may provide a range of functions that are not obtained in static, equilibrium materials (e.g., reconfigurable, adaptable, and selfrepairing), thereby enabling the emergence of entirely new applications. For example, in the context of electronic applications, if we had a material that could become a diode, thermistor, transistor, or memory device depending on the external inputs, why make all of these components separately?

applied. Under these circumstances, the small counterions surrounding the large (and jammed/immobile) charged NPs can move to create a gradient of ions within the material. Importantly, these gradients are long-ranged (for details, see ref 55) and can give rise to internal electric fields that can modulate the externally applied field. This property underlies a range of dynamic behaviors and on-demand reconfigurability of the materials’ electrical properties. Let us first consider a simple example in which the material’s conductance is monitored in the same direction as the applied bias. Because there are no Faradaic reactions at the electrodes, the current flow in this material is due to the electrons tunneling or hopping between the metallic cores of nearby NPs. When an external field, Eapp, is applied, it not only drives the current flow but also sets up a counterion gradient and a gradient-derived internal field, Ei, that is in the direction “opposing” Eapp (Figure 5a) The typical current−voltage characteristics of the material reveal an interesting property, namely, that for a certain Eapp, the internal field, Ei, becomes strong enough to create a no-field situation within the material (Figure 5b, top). In other words, the material changes/reconfigures from an electronic conductor to an insulator. This behavior is dynamic in the sense that the internal gradients persist only as long as the applied field is on; in its absence, the counterion gradients equilibrate, and the material reverts to its conductive state. Moreover, if the applied potential is inverted following ion polarization, the internal field Ei assists current flow, resulting in a highly conductive state (Figure 5b, bottom). The range of accessible dynamic behaviors becomes much wider if the ionic gradients are not set up along the direction of electron flow. This is illustrated in Figure 5c, where the counterions are first redistributed along the horizontal direction (between electrodes (2,3) and (5,6)) by applying a polarizing voltage. Because the region near electrode 1 is roughly halfway between polarizing electrodes 6 and 2, the concentration of counterions (represented by red dots) is similar along the (1−5) and (1−3) directions. The situation is very different near electrodes 5 and 3; near 5, the couterions are depleted, but near 3, they are accumulated. Consequently, the concentration of couterions increases from 5 toward 1 and from 1 toward 3. These gradients then translate into internal fields E15 and E13. When a potential difference is subsequently applied along these directions, E15 opposes the externally applied field E along (1−5), whereas E13 enhances the applied field along (1−3). Overall, the current is steered toward electrode 3 (that is, the currents are such that I15 ≪ I13; see Figure 5d). The explanation is similar when the direction of the polarizing field is reversed (from (2,3) toward (5,6)). In this case, I15 ≫ I13. Overall the redistribution of counterions within the material allows the current to be “steered” along different directions. The relative currents in directions 1−3 and 1−5 are compared in the plot of Figure 5d. When the dynamic phenomena similar to the ones described above are extended to materials containing both positively and negatively charged NPs, the counterion gradients are not necessarily monotonic, and the materials can behave akin to p−n junctions (i.e., diodes55) and, as we will describe in detail in a series of upcoming papers, can serve as a basis for building sophisticated electronic elements and circuits that are made entirely of nanoionic metallic NPs that are cast from solution and maintain their function when bent (e.g., on a flexible substrate). Importantly, the operation of these devices rests on the creation of dynamic configurations of counterions. The forward-looking conclusion from these examples is that it is indeed possible to achieve internal reconfigurability and

It is more worthwhile to ask whether dynamic nanomaterials may provide a range of functions that are not obtained in static, equilibrium materials (e.g., reconfigurable, adaptable, and selfrepairing), thereby enabling the emergence of entirely new applications. We close with some suggestions (summarized in Figure 6) of the potential applications for responsive and nonequilibrium nanomaterials. While the selection in this figure is certainly subjective, we hope it will provide a stimulus for rapid development of this exciting emerging field at the intersection of materials science, chemistry, and physics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Scott C. Warren completed his Ph.D. with Frank DiSalvo and Uli Wiesner at Cornell, designing fuel cell electrodes by self-assembly. He developed record-setting water splitting electrodes with Michael Grätzel at EPFL and directed NanoPEC, a European consortium on water splitting. With Bartosz Grzybowski, he studies electronic circuits, solar cells, and self-assembly. http://s-warren.com Ozge Guney-Altay received her Ph.D. from Texas A&M, studying controlled drug delivery for cancer and hormone therapy. She developed nanoparticle-based sensors using molecular imprinting and solidstate photopolymerization and controlled delivery devices for orthodontic applications at VCU. She is currently the Director of Operations of NERC at Northwestern University. http://nercenergy.com/ Bartosz A. Grzybowski, K. Burgess Professor of Physical Chemistry and Chemical Systems Engineering at Northwestern, pioneered nonequilibrium self-assembly processes in chemical systems and complex organic−chemical networks. Grzybowski is an author of over 180 articles on topics that include reaction−diffusion, contact electrification, light-controlled materials, nanoscience, plasmonics, energy storage, systems’ chemistry, and cell biology. http://dysa. northwestern.edu/ 2109



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ACKNOWLEDGMENTS This work was supported by the Non-Equilibrium Energy Research Center (NERC), which is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0000989.

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