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Published in final edited form as: Acc Chem Res. 2012 Mar 26;45(11):1916–1926. doi: 10.1021/ar200305f

Dynamic Nanoparticles Assemblies

LIBING WANG 1,#, LIGUANG XU 1,#, HUA KUANG 1,#, CHUANLAI XU 1,*, NICHOLAS A KOTOV 2,*
PMCID: PMC3479329  NIHMSID: NIHMS366702  PMID: 22449243

CONSPECTUS

Importance

Although nanoparticle (NP) assemblies are at the beginning of their development, their unique geometrical shapes and media-responsive optical, electronic and magnetic properties have attracted significant interest. Nanoscale assembly bridges multiple sizes of materials: individual nanoparticles, discrete molecule-like or virus-like nanoscale agglomerates, microscale devices, and macroscale materials. The capacity to self-assemble can greatly facilitate the integration of nanotechnology with other technologies and, in particular, with microscale fabrication. In this Account, we describe developments in the emerging field of dynamic NP assemblies, which are spontaneously formed superstructures containing more than two inorganic nanoscale particles that display ability to change their geometrical, physical, chemical, and other attributes. In many ways, dynamic assemblies can represent a bottleneck in the ‘bottom-up’ fabrication of NP-based devices because they can produce a much greater variety of assemblies, but they also provide a convenient tool for variation of geometries and dimensions of nanoparticle assemblies.

Classification

Superstructures of NPs (and those held together by similar intrinsic forces) are classified into two groups: Class 1 where media and external fields can alter shape, conformation, and order of stable superstructures with a nearly constant number same. The future development of successful dynamic assemblies requires understanding the equilibrium in dynamic NP systems. The dynamic nature of Class 1 assemblies is associated with the equilibrium between different conformations of a superstructure and is comparable to the isomerization in classical chemistry. Class 2 assemblies involve the formation and/or breakage of linkages between the NPs, which is analogous to the classical chemical equilibrium for the formation of a molecule from atoms. Finer classification of NP assemblies in accord with established conventions in the field may include different size dimensionalities: discrete assemblies (artificial molecules), one-dimensional (spaced chains) and two-dimensional (sheets) and three-dimensional (superlattices, twisted structures) assemblies. Notably, these dimensional attributes must be regarded as primarily topological in nature because all of these superstructures can acquire complex three-dimensional shapes.

Preparation

We discuss three primary strategies used to prepare NP superstructures: (1) anisotropy-based assemblies utilizing either intrinsic force field anisotropy around NPs or external anisotropy associated with templates and/or applied fields; (2) assembly methods utilizing uniform NPs with isotropic interactions; and (3) methods based on mutual recognition of biomolecules, such as DNA and antigen-antibody interactions.

Applications

We consider optical, electronic, and magnetic properties of dynamic superstructures, focusing primarily on multiparticle effects in NP superstructures as represented by surface plasmon resonance, NP-NP charge transport, and multibody magnetization. Unique properties of NP superstructures are being applied to biosensing, drug delivery, and nanoelectronics. For both Class 1 and Class 2 dynamic assemblies, biosensing is the most dominant and well-developed area of dynamic nanostructures being successfully transitioned into practice. We can foresee the rapid development of dynamic NP assemblies toward applications in harvesting of dissipated energy, photonics, and electronics. The final part of the review is devoted to the fundamental questions facing dynamic assemblies of NPs in the future.

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Introduction

A wide variety of individual nanoparticles (NPs) was synthesized by different methods of nanoscale synthesis. Although the synthetic challenges to make intricate nanoscale shapes still persist, many basic shapes of common materials used in nanotechnology became routine. Instead challenges related to making the complex structures using NPs as building blocks emerged. Nanoparticle superstructures offer even greater variety of nano/microscale systems than individual NPs and enable investigations of collective behavior/properties. The research on nanoparticles assemblies is constantly increasing (Figure 1) and there are many reasons to believe that it will continue with increasing rate.

FIGURE 1.

FIGURE 1

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Research papers on NP assemblies (Source: Web of Science).

One of the most intriguing parts of the research continuum on NP assemblies is the dynamic superstructures. Dynamic NP assemblies can be defined as spontaneously formed superstructures containing more than two inorganic nanoscale particles that display ability to change their geometrical, physical, chemical, and other attributes. The dynamic NP assemblies are scientifically attractive because they

  • - open the pathway to understanding collective interactions in NP assemblies;

  • - increase diversity of NP assemblies;

  • - enable functional tuning/optimization of supestructures;

  • - facilitate integration with microscale technologies;

  • - mimic characteristic processes in live organisms.

In respect to practical applications dynamic assemblies are potentially suitable for

  • - a variety of sensor devices;

  • - stimuli-responsive optoelectronic materials;

  • - drug delivery vehicles;

  • - energy harvesting.

Other applications based on stimuli- and media-dependent restructuring, aka “smart” nanomaterials, should also be considered. In this review, we summarize recent progress in dynamic NPs assemblies. Properties and emerging applications of dynamic NPs assemblies are also briefly discussed.

Classification

Considering current and constantly emerging new examples of dynamic processes (Figure 1), we can tentatively identify two classes of dynamic NP assemblies.

Class 1 encompasses superstructures that change their shape, conformation, size, topology etc. The total number of the NPs does not change appreciably.

Class 2 encompasses superstructures in which the total number of NPs in superstructures greatly changes while the organizational motif remains the same.

Within each class one can also distinguish superstructures that are different in their spatial and topological characteristics: discrete assemblies,1,2 extended one-dimensional (1D),3,4 two-dimensional (2D),5 and three-dimensional (3D) assemblies.6,7 The dimensionality classification follows the accepted terminology in the current literature and refers primarily to topology of NP “connections” within the assemblies rather than on their actual geometry. For instance, NP chains are typically referred to as 1D assemblies and correspond to the case when most NPs have two neighbors. It is implicitly understood, however, that in solutions and on surfaces they can acquire complex 3D and 2D conformations. Similarly, the number of neighbors in flexible NP sheets is limited to, for instance, six particles localized within the equatorial plane but the sheets can acquire many 3D shapes.

Understanding equilibrium and kinetic effects8 is exceptionally important for all dynamic and more so than for static NP systems. Note there are also dynamic superstructures responsive to external stimuli in a non-equilibrium fashion, which eventually result in static assemblies. Interparticle fusion to form the mesocrystals7 and conformational transition of flat ribbons into twisted ribbons upon illumination6 exemplify such systems.

The differences between Class 1 and 2 dynamic superstructures are fundamental. Class 1 assemblies are associated with equilibrium between different conformations of a superstructure. The transitions between them can be compared to isomerization in classical chemistry. Class 2 assemblies involve formation/breakage of linkages between NPs. It can be compared to equilibrium in synthetic reactions.

Synthesis

Three primary strategies were used to synthesize static and dynamic NP superstructures. Historically the first was based on anisotropy of NP interactions with each other (intrinsic), templates or external fields (extrinsic)9-13. The second strategy is based on drying highly uniform NPs with isotropic interactions14-16. Such assemblies were made from semiconductor14, metal15,17, and magnetic16 NPs. Interestingly, they can also be made from non-spherical NPs, for instance from gold nanorods18 and mixtures of NPs with different shapes19. The third and probably the most wide-spread strategy is the method utilizing biomolecules, when NPs are assembled following mutual recognition of biomolecules using DNA20,21 or antigen-antibody pairs22. DNA is arguably the most attractive platform for dynamic assembly due to the base-pairing regularity and relatively simple synthetic strategies. They are also attractive to many applications especially sensing23. Both DNA and proteins give both Class 1 and Class 2 assemblies as well as different degree of reversibility in the superstructures8.

Discrete Assemblies refer to nanoscale superstructures made from a finite and typically small number of NPs. Most of them take advantage of biomacromolecular linkages24. They allow for reconfigurable topology and conformational changes of superstructures. The assembly of NP dimers via hybridization of partially complementary single-stranded DNA (ssDNA) is the simplest case of such dynamic superstructures (Figure 2). The distance between adjacent NPs in them can also be modified switching between distinct conformational states.2

FIGURE 2.

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DNA-based discrete assemblies. (A) NP dimers B) Reconfigurable satellite assemblies with hydrodynamic diameters in hairpin and extended states. (C) NP triangles and dimers from 15 and 5 nm NPs; Ref 2, 25 and 26.

Discrete dynamic Class 1 superstructures can also be built using hairpin DNA sequences2,25 (Figure 2B). The interparticle distance increased upon addition of complementary ssDNA. Precise control over the spatial assembly of homogeneous or heterogeneous NPs has been achieved using cyclic ssDNA as templates26 to produce triangles and rectangles from individual NPs. Upon addition of a different specific ssDNA, these assemblies could be disassembled and reconfigured (Figure 2C)2.

The next step in discrete DNA-based assemblies is exploration of increasing complexity of the superstructures. The first example of an assembly with NPs of different geometries is the assemblies from NPs and nanorods (NRs). They are made with 85+ % yield and regiospecific selectivity (Figure 3).27 Incorporation of NRs stimulates strong Raman scattering and can be a convenient spectroscopic tool in addition to UV-vis spectroscopy to observe the dynamic behavior.

FIGURE 3.

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TEM images of DNA-based regioselective discrete assemblies (A) End, (B) Side and (C) Satellite assemblies with (D) yield analysis; Ref 27.

Self-organizing supraparticles also represent Class 1 discrete NP system (Figure 4).28 They can be made from a large number of materials. Their existence and the fact that they can be exceptionally uniform are the manifestations of the dynamic assembly-disassembly process. They are formed in a self-limiting growth process that was governed by the equilibrium of repulsive-attractive interactions that makes possible fabrication of complex assemblies with great ease. Ultimately, they display similarities in size, core-shell structure, and self-organization behavior with viruses.

FIGURE 4.

FIGURE 4

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TEM (A-D) and SEM (E-H) of CdSe supraparticles of different sizes. (I-L) Intermediate stages of formation and corresponding. (M,O) computer simulations and (N,P) TEM image of core-shell supraparticles from CdSe and Au NPs; Ref 28.

1D Assemblies

1D dynamic NPs chains can be constructed using all three assembly strategies. A discernible similarity between the 1D Class 2 self-assembly of Au NRs, and polymerization was pointed out by Kumacheva.29 Linear, branched, and cyclic ensembles of NRs were obtained in a DMF/water mixture (Figure 5). To reduce the surface energy of the system in the poor solvent, the hydrophilic NRs tethered with hydrophobic PS molecules organized into chains.30 NRs were assembled into rings, nanochains, bundles, nanospheres, and bundled nanochains, merely by changing the water content in different organic solvents.

FIGURE 5.

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(A) Self-assembly of polymer-modified NRs. (B) Absorption spectra, (C) TEM images (D) and SERS spectra as they assemble; Ref 30, 50.

Antibody–antigen interactions can also be used to assemble Au NRs into 1D superstructures.31 Extended Class 2 chains from end-modified NRs were observed in presence of an antigen (Figure 6b, d). Chains from ZnO pyramids assembled by excluded volume interactions present a system with rich dynamic behavior and transitions of the assembly motif between head-to-tail, parallel, and antiparallel patterns.32

FIGURE 6.

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Schematics of (A) side-by-side and (B) end-to-end NR assemblies for toxin detection, (C,D) representative TEM images. (E,F) Hydrodynamic diameter of assemblies at different concentrations of microcystin-LR; Ref 31.

A distinct example of Class 1 dynamic 1D NP assembly is the hybrid Au NP+CdTe NW superstructures assembled via bifunctional PEGs acting as molecular springs. The dynamic behavior manifests in them as solvent- and temperature response of PEG chains altering the distance between the NPs (Figure 7). The conformation of the PEG and NP-NW distance is altered with increase of the temperature.33

FIGURE 7.

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(A) Schematics and (B) TEM image of dynamic assemblies with molecular springs; Ref 65.

2D Assemblies

Fluid–fluid interfaces offer many opportunities to make dynamic 2D assemblies where the NPs are in constant movement at the interface and bulk phase(s).9-13 Fluid–fluid interfaces provide the opportunity to rapidly reach equilibrium, which can also manifest as NP size selectivity34.

Spontaneous assembly of 2D sheets as static systems was achieved for CdTe NPs5. The structure of NP sheets was compared with the structure of proteins and other biomacromolecules resembled the assembly of S-proteins35 or chaperonines. Adjustment of the strength of the attractive interactions can lead to true Class 1 or Class 2 dynamic systems.36

3D Assemblies

3D dynamic assemblies can be formed using biological linkages19, 21, 37, 38, isotropic19, 39 and anisotropic NP interactions7. Their formation can be induced by pH40, redox reaction, polarity of solvent30, or illumination with light6, As such, microscale helical twisted ribbons are formed in Class 1 irreversible dynamic process from straight ribbons6. The change of conformation occurs in ribbons due to photooxidation-induced mechanical strain in the assembled structures. Uniquely-shaped assemblies -- ellipsoidal clouds, dog-bone agglomerates and ribbon bunches - undergo dynamic transitions in the same system (Figure 8).

FIGURE 8.

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SEM images of intermediate NP superstructures and twisted ribbons after (A) 12, (B) 24, (C) 48, and (D) 72 hours of assembly. (E and F) twisted ribbons prepared at different light intensities: (E) 61 and (F) 21 mW; Ref 6.

Polymeric hydrogels are typical representatives of “smart” dynamic materials. Due to intrinsic anisotropy of NPs manifesting in formation of chains, it is also possible to fabricate gels from NPs41 but their dynamic still to be investigated well. Along with irreversible transitions described by Eycmuller,42 the reversible sol-gel transitions of fluorescent hydrogels from CdTe NPs induced by sonication (Figure 9) accompanied by reversible emission-color switching are also possible.43

FIGURE 9.

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(A) Photographs and (B) schematic of CdTe sols and gels before and after sonication with corresponding (C) TEM image of CdTe gels; Ref 43.

3D character of the NP assemblies can also originate from the special shape of individual NPs that can acquire extreme anisotropy and anisometry, however, the assemblies from them remains challenging due to “jammed” states. It was possible however for octapod-shaped NPs (Figure 10) precisely due to unfreezing these states into dynamic equilibrium.7

FIGURE 10.

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3D TEM of individual octapod NPs (A) and a octapod NPs chain (B). (C) SEM of octapod sheets; Ref 7.

Properties

Well-characterized dynamic NP systems are still not very common. However, the optical, electrical and magnetic properties of both static and dynamic assemblies often overlap. Therefore discussing properties we used examples of both dynamic and a limited number of static assemblies to illustrate promising research directions.

Optical Properties

The energy of the surface plasmon resonance (SPR) bands is dependent on the number of NPs27, their shape, orientation, and distance44. All these effects have been extensively investigated by Mirkin45, EI-Sayed46, Alivisatos44, Liz-Marzan21,38, Gang2, Mulvaney47, Kumacheva30, Xia48, and many others. Great examples can be found for chains made of gold NPs exhibiting strong red-shift when assembled. For NRs the shift can be vastly different for the longitudinal plasmon band and transverse plasmon depending on the assembly fashion (Figure 6).31

SERS is of particular interest for NP assemblies with SPR. High SERS intensity originates from the dramatic amplification of the electromagnetic fields in the gaps between the plasmonic particles.18, 48, 49 The geometry of NP assembles is therefore of great importance. Kumacheva reported that SERS intensity varied nonlinearly with the aggregation number50. The field intensity was the highest for NR assembles containing three NRs.

Chirality is a geometrical and optical property that describes an object whose mirror image is not non-superimposable with the original. This property represents one of the most interesting research areas for dynamic MP assemblies adue to high sensitivity to small geometrical changes. Alivisatos demonstrated dynamic Class 1 NP assemblies in a chiral tetrahedral configuration in 2009.51 Kotov and Xu made dynamic NPs assemblies with strong chirality (Figure 11)1 associated with formation of trimer and tetramer assemblies in 2009. Liz-Marzan observed chiral effects in helical NR assemblies in 2011.52 Circular dichroism (CD) band representing the superstructures appeared in the 400-700 nm region coinciding with SPR. While the origin of these CD bands is still being investigated, the strong chirality of plasmonic superstructures could be a pathway to negative refractive index materials (NIMs). Class 1 assemblies of plasmonic and semiconductor particles can also help realization of NIMs serving as energy “pump” for NIMs in the visible range.53,54

FIGURE 11.

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TEM images of early PCR products and schematics of their chiral isomers; Ref 1.

Electronic Properties

The grand challenge of nanoscale electronics is to self-organize of NPs, NRs, NW, etc in a specific configuration. Care needs to be taken to attain high crystallinity of the superstructures and avoid gaps because electron transport can be strongly frustrated by the grain boundaries55 and gaps made by structure-determining ligands.56 Considerable attention in NP assemblies for nanoelectronics was focused on ordered 2D metallic NPs structures exemplified by single electron transistors. 2D assemblies of 5.4 nm dodecanethiol-protected Au NPs assembled at an air-water interface57 revealed Coulomb blockade effect at 4.2K. Dynamic nature of these assemblies resulted into large noise.

The assemblies from anisotropic NPs with minimal surface coating are advantageous because they allow for more facile charge transfer from one NP to another. Spontaneous recrystallization of NP chains into monocrystalline NWs observed for CdTe3 is also very encouraging for solar cell. Assembly of NP superlattices and subsequent removal of the insulating coatings around NPs demonstrated by Talapin and Shevchenko presents an alternative pathway58 and unusual electronic properties.59

Magnetic Properties

The best examples of magnetic studies in Class 2 superstructure can be probably found in the works by Pileni. The reported differences in the magnetic properties of single Co NPs and NP chains.60 Magnetic measurements showed increases in the remnant magnetization and coercivity for the 1D Co chains, due to the orientation of the NPs’ magnetic moments in the direction of the magnetic field. External magnetic field organizing cobalt NPs in dynamic architectures provides a convenient tool for tuning the collective magnetic response, which is interesting for magnetic storage devices.

Applications

Sensors

Over the past decade, the application of dynamic NP assemblies as sensors has been widely investigated61,62. Most of these studies involve Class 2 NP superstructures with strong SPR. They utilize the shift of SPR band and the color change of the NP dispersion from red to blue or vice versa in presence of analytes associated with change the total number of NP in the assembly. The works of Mirkin demonstrated their exceptional sensitivity for biomedical purposes, with detection limit of 10fmol of oligonucleotide.45 El-Sayed used oral cancer cell as the template to assemble NRs conjugated to anti-epidermal growth factor receptor antibody for Raman signal enhancement.46

The challenges for detection of environmental pollutants and toxins could be as hard as those for cancer. Rapid assessment of minute amounts of toxins is much needed31 for environmental sensors.62,63 For example, Au NR ensembles with side-by-side and end-to-end attachment motifs have been prepared as for detection of mircrocystin-LR, a widespread toxin and resulting in liver failure or cancer. The method based on Class 2 superstructures from NRs had a detection limit of 5 pg/mL making it more sensitive than enzyme linked immunosorbent assay (ELISA). The detection time was 15-20 min which is 36-48-fold faster than ELISA. The use of magnetic NP with antigens allows one to improve the detection limit to 0.6 ppb64.

The photoluminescence of Class 1 superstructures made from semiconductor NWs with metal NPs connected by molecular springs with antibodies transduced protein concentration into the change of emission wavelength.65 The advantage the wavelength read-out is the elimination of internal standards.

Class 1 superstructures could be used as intracellular probes. Incubation of the label-free NP-NR assemblies with HeLa cells indicated sufficient Raman intensity enhancement to detect lipids of mitochondria and provided the first proof-of-concept data for the possibility of real-time probing of local organelle environment in live cells.27

Drug Delivery

Site-specific delivery and controlled release of drugs to physiological targets remain major challenges for therapeutics of cancer and other diseases. Since dynamic NP superstructures are sensitive to external conditions, they are attractive for targeted drug delivery and mitigation of chemotherapy side effects. Nanostructures can also incorporate multiple therapeutic and targeting agents. Potential applications of dynamic superstructures in drug delivery66,67 can be exemplified by DNA nanotubes incorporating capsules/compartments along its length (Figure 12).66 They opened when specific DNA strands were added releasing the cargo.

FIGURE 12.

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DNA nanotubes (A) without and (B) with encapsulated NPs; Ref 66.

Nanoelectronics

Self-assembly processes could potentially be a cost-efficient manufacturing approach for nanoelectronics. Self-organization of NPs at solid interfaces was successfully used in light-emitting devices68. Stochastic agglomerates of colloidal particles emerged as an alternative to single-crystalline PV devices for dye- and NP-sensitized solar cells.69 Greater conversion efficiency values are expected for NP PV elements after optimization of both the absorption of incident photons and carrier collection pathways which is likely to require non-planar assembly geometries.70

Particular attention must be given to NP assemblies for photovoltaics (PV). Self-assembly approaches were not articulated well for NP-based PV, however, a number of devices were demonstrated using self-assembly of conducting polymers. PV devices require materials with complex nanoscale architectures that facilitate charge transport, maximize light absorption, and minimizing charge carrier entrapment. Potentially NP superstructures for PV and other areas of nanoelectronics can replicate organization of photosynthetic centers in plants and bacteria.

Conclusions

Further work should also address the following questions: 1) How should building blocks and ligands be chosen (or designed) for the formation of dynamically self-assembled superstructures? 2) How can dynamic assembly processes be monitored in real time? 3) How do laws of thermodynamics apply to these assemblies? 4) How close the dynamic superstructures emulate the functions of living cells? There is also a high probability that the dynamic processes in NP superstructures will display unpredictable behavior due to collective interactions between NPs and entropy-enthalpy compensation effects. All these questions have both fundamental and practical importance and can sustain the development of this field for decades to come.

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (21071066, 20835006, 91027038, 21101079, 21175034), the Key Programs from MOST (2010AA06Z302, 2010DFB3047, 2008BAK41B03, 2009BAK61B04, 2010GB2C100167), and grants from Natural Science Foundation of Jiangsu Province, MOF and MOE (BK2010001, BK2010141, JUSRP10921, JUSRP11019). This work is based upon work partially supported by the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number #DE-SC0000957 (NAK). We acknowledge support from NSF under grant ECS-0601345; EFRI-BSBA 0938019; CBET 0933384; CBET 0932823; and CBET 1036672. (NAK) The work is also partially supported by AFOSR MURI 444286-P061716 and NIH 1R21CA121841-01A2.

BIOGRAPHICAL INFORMATION

Libing Wang received his PhD degrees from Jiangnan University. He is a full professor with research focused on the analytical aspects of nanoparticle assemblies.

Liguang Xu is currently a PhD candidate under the guidance of Prof. Chuanlai Xu. His research is focused on the assembly of nanoparticles.

Hua Kuang received her PhD degree in China Agricultural University in 2009. Her research area is nanoparticle assemblies for biodetection.

Chuanlai Xu received his BS, MS and PhD degrees from Jiangnan University. His research focuses on NP-based biosensor and optical properties of nanoassemblies.

Nicholas A. Kotov obtained his PhD in Chemistry from Moscow State University in 1990. He is currently a Professor at University of Michigan. His research interests in the field include synthesis of new nanocolloids, their organization in functional assemblies, layer-by-layer assembly, ultrastrong materials, biosensing, photovoltaics, interfacing nanomaterials with living cells, cancer treatment, and diagnostics with nanoparticle superstructures.

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