Letter pubs.acs.org/NanoLett
Reconstitutable Nanoparticle Superlattices Boya Radha,†,‡,§ Andrew J. Senesi,⊥ Matthew N. O’Brien,†,‡ Mary X. Wang,‡,∥ Evelyn Auyeung,‡,§ Byeongdu Lee,*,⊥ and Chad A. Mirkin*,†,‡,§,∥ †
Department of Chemistry, ‡International Institute for Nanotechnology, §Department of Materials Science and Engineering, and Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ⊥ X-ray Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, United States ∥
S Supporting Information *
ABSTRACT: Colloidal self-assembly predominantly results in lattices that are either: (1) fixed in the solid state and not amenable to additional modification, or (2) in solution, capable of dynamic adjustment, but difficult to transition to other environments. Accordingly, approaches to both dynamically adjust the interparticle spacing of nanoparticle superlattices and reversibly transfer superlattices between solution-phase and solid state environments are limited. In this manuscript, we report the reversible contraction and expansion of nanoparticles within immobilized monolayers, surface-assembled superlattices, and free-standing single crystal superlattices through dehydration and subsequent rehydration. Interestingly, DNA contraction upon dehydration occurs in a highly uniform manner, which allows access to spacings as small as 4.6 nm and as much as a 63% contraction in the volume of the lattice. This enables one to deliberately control interparticle spacings over a 4−46 nm range and to preserve solution-phase lattice symmetry in the solid state. This approach could be of use in the study of distance-dependent properties of nanoparticle superlattices and for long-term superlattice preservation. KEYWORDS: DNA, nanoparticle, superlattice, rehydration, X-ray scattering, assembly
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Scheme 1. Reversible Rehydration and Dehydration of Nanoparticle Superlatticesa
anoparticle superlattices are of interest for optoelectronic, photonic, and plasmonic applications that depend upon the spacing, symmetry, and orientation of nanoparticles with respect to one another.1−4 Common approaches to making nanoparticle-based superlattices often rely upon the interaction of surface-bound ligands5−7 or crystallization through solvent evaporation.8,9 While lattices assembled through dryingmediated processes have been generated with a range of symmetries and different composition of building blocks,10−13 the structures remain static after assembly, fixed in the solid state. In contrast, DNA-mediated nanoparticle superlattice assembly,14−20 one of the most powerful and general approaches to ligand-mediated assembly, requires aqueousbased hybridization processes, and the resulting structures often must remain in solution to maintain order. To date, the only reported method to transition such systems to the solid state is by chemically growing a network of silica21 or some other material around the lattice16 in an irreversible encapsulation process. This observation prompted us to consider the possibility of reconstitutable lattices from DNA-assembled materials, such that structures can be assembled in solution, stored in the solid state, and reconstituted in solution by controlled solvation (Scheme 1). Herein, we explore this novel concept in the context of lattices grown off surfaces and solution-dispersed single crystalline superlattices. DNA has emerged as an ideal ligand to tune superlattice parameters over the 10−200 nm length scale with exquisite control over interparticle spacing, lattice symmetry, and © 2014 American Chemical Society
a
Within the superlattice, each nanoparticle (the core in each sphere) possesses a dense, radially-oriented arrangement of DNA ligands (the semi-transparent shell surrounding each core). The overlap of complementary DNA sequences on each particle (i.e., the sequence on the blue particle is complementary to that of the red particle) is responsible for superlattice assembly. The removal of water from assembled superlattices, i.e., dehydration, decreases the length of the DNA within the superlattice but does not change the symmetry of the lattice.
nanoparticle coordination due to the programmability of the DNA length, binding strength, and specificity. In this work, we study the reconstitution of: (1) monolayers of DNA-functionalized nanoparticles attached to gold films through DNA hybridization, (2) body-centered cubic (BCC) superlattices of DNA-functionalized nanoparticles grown on a surface, and (3) Received: February 5, 2014 Revised: February 20, 2014 Published: March 18, 2014 2162
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Figure 1. DNA reconstitution in the context of prism monolayers. (A) DNA hybridization scheme used in the assembly of prism monolayers. DNA design is composed of three unique regions, as described in the main text: a thiolated anchor (blue), a linker (red), and a duplexer (green). (B) GISAXS and (C) incident angle resolved (IAR)-GISAXS analysis of the prism monolayers in the hydrated, dehydrated, and rehydrated states. The dotted vertical lines in (B) represent the position of linecuts, which were used for constructing 2D-maps of IAR-GISAXS. (D) SEM image of the prism monolayers assembled by DNA. (E) The surface-to-surface distance between the prism and the surface, i.e., gap length (obtained by fitting the GISAXS data), increases linearly with number of base pairs.
behavior offers a simple model to investigate ligand behavior based upon the spacing between the prism and the substrate. In a typical experiment, Au prisms and Au-coated silicon substrates were each functionalized with thiolated DNA strands composed of three key regions: a thiol group to anchor the DNA to the surface of the prism, a single-stranded A10 spacer region, and an 18 base recognition sequence. Surface-bound oligonucleotides were then hybridized to a “linker” strand, composed of a complementary 18 base sequence, a duplexed “spacer” region of variable length to control the overall DNA length, and a short 5 base “sticky end” used to hybridize the prism to the substrate (Figure 1A). Assembly was performed by immersing the DNA-coated substrate into the prism solution23 at 35−40 °C. The resultant monolayers were characterized by GISAXS,24 as it provides an accurate way of measuring the distance between the nanoparticle and the substrate, as well as the in-plane arrangement of the nanoparticles over large areas in both the hydrated and the dehydrated states.
free-standing, single crystalline BCC superlattices formed from two complementary nanoparticle−oligonucleotide conjugates. In all three systems, we probe the state of DNA duplexes in the hydrated, dehydrated, and rehydrated states by synchrotron grazing incidence small-angle X-ray scattering (GISAXS) and circular dichroism (CD) spectroscopy as a function of solvation. The dehydrated states are also characterized by scanning electron microscopy (SEM). To probe the effect of dehydration on the individual nanoparticle building block decoupled from the lattice, we initially probed nanoparticle monolayers connected to gold films through complementary DNA interactions. We first considered DNA-modified triangular nanoprisms (hereafter, referred to as prisms for simplicity; edge length, 60 ± 10 nm; thickness, 7.0 ± 0.5 nm).16,22 The atomically flat triangular facets of prisms template a planar arrangement of DNA ligands, which results in a consistent orientation and spacing of nanoparticles with respect to the substrate. This assembly 2163
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Figure 2. Reconstitution of surface-assembled spherical nanoparticle superlattices. Panels for (A) monolayer and (B) ten-layer BCC superlattices are arranged from left to right as: Scheme of the nanoparticle assembly method, where each set of horizontally arranged spheres (red or blue) represents one layer; GISAXS scattering patterns in the hydrated, dehydrated, and rehydrated states; and SEM images in the dehydrated state, with a Fourier transform inset. In the GISAXS patterns, the scattering vector from the reflected beam, qr,z is plotted against parallel scattering vector, i.e., qxy. Additional GISAXS data are presented in Figures S9−S12.
The 2D-GISAXS patterns (Figure 1B) of as-assembled prism monolayers show interference peaks in the off-specular condition (2θf ≠ 0), known as Kiessig fringes, originating from constructive and destructive interference of X-rays scattered from two interfaces, that is, the prism monolayer and the substrate. The period of the Kiessig fringes is dependent on the distance between the prism and the substrate interfaces25,26 and thus provides a sensitive measure of the effective DNA length from nanoparticle surface to substrate surface, described hereafter as “gap length”. Upon drying, the number of fringes decreases, indicating a significantly reduced gap length due to DNA dehydration.27,28 Further, the near disappearance of in-plane peaks reveals an increase in disorder upon drying, likely due to a greater variation in gap length (Figure 1B). After rehydrating overnight, the Kiessig fringes return with the same periodicity as observed in the original hydrated state, and after 30 min of annealing at 35 °C (the deposition temperature) the in-plane peaks also reappear. Thus the process of dehydration does not impact the ability to fully recover the initial gap length and ordering of the prism monolayer upon rehydration. To compare gap lengths among different samples, incident angle resolved-GISAXS (IAR-GISAXS) was employed to resolve mixed scattering signals from various sources, indepth details of which can be found elsewhere.25 Briefly, a series of GISAXS patterns were measured at a range of incident angles (from 0.005° to 0.4°, step size: 0.005°), and 2D maps were generated from vertical linecuts through the Kiessig fringe (Figure 1B,C). Quantitative gap lengths were extracted by comparing the periodicity of the Kessig fringes to data simulated using the distorted-wave Born approximation (DWBA) at multiple incident angles24,29 (Figures 1E and S1 of the Supporting Information). Three different DNA lengths were examined, corresponding to 41, 81, and 121 duplexed DNA bases. Notably, the gap lengths for the original hydrated and rehydrated states (Table S3) are consistent with previously observed interparticle spacings of DNA-prism lamellae employing DNA sequences of similar length.16
In the dehydrated state, the gap length decreases to less than 25% of its original value. This indicates one of the following scenarios is occurring: (1) the DNA dehybridizes and undergoes coiling to present a steric barrier between the substrate and nanoparticle, (2) the DNA remains hybridized but lies nearly parallel to the substrate, perhaps with bends at the “flexor” positions (Figure 1A), or (3) DNA remains hybridized but undergoes compression. In order to probe the hybridization state of the DNA before and after rehydration, desorption analysis was performed while monitoring at 260 nm (Figure S3). The temperature at which prism dehybridization occurs qualitatively correlates with the number and strength of the DNA interconnects. Both hydrated and rehydrated samples show the same sharp desorption transitions for all three DNA lengths (Figure S3 and Table S4). This result, in addition to the ability to fully recover the initial DNA length after dehydration suggests that the DNA likely remains hybridized throughout our investigation. Furthermore, the small gap lengths (4.6, 6.6, and 9.9 nm for the three DNA lengths described above) observed in the dried state cannot be explained by models for coiled conformations such as the freely jointed chain30 or worm like chain models31 since the radius of gyration of coils is proportional to √N, where N is the number of duplexed sequences. The gap lengths in this work (Figure 1E) instead linearly increase with N. To investigate the effect of nanoparticle curvature on the dehydration and rehydration processes, we assembled monolayers of DNA-modified spheres (diameter, 9.7 ± 0.4 nm; Figure S5). These structures template a radial orientation of DNA ligands, compared to the planar arrangement templated by prisms, and therefore reduce the number and stability of DNA interconnects that can form with the substrate.6 Importantly, upon dehydration and subsequent rehydration, the Kiessig fringes and the in-plane scattering peaks return to the same initial position, although with an increased peak width (Figure 2A; Figures S6−S8). This suggests that there is minimal effect on the rehydration process associated with the nanoparticle shape, suggesting that these results may be generalizable to nanoparticles of different shapes. 2164
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Figure 3. (A) Horizontal linecuts of the 2D GISAXS pattern for BCC superlattices before and after consecutive dehydration−rehydration cycles. The inset shows that the interparticle gap length, a, is recovered upon rehydration up to 3 cycles of reconstitution. (B) CD spectra of BCC superlattices assembled on a quartz substrate, revealing a transition from A to B form DNA in the dehydrated and hydrated states, respectively.
The monolayer of spherical particles can also be used to form extended three-dimensional superlattices via a stepwise assembly process.23 This process can be used to investigate how dehydration and rehydration are impacted by nanoparticle coordination at each step in the formation of a lattice. When a second layer is introduced, both in-plane and out-of-plane DNA connections are formed between particles in the first and second layers. After drying, the second layer collapses into the first layer and forms a 2D hexagonal monolayer structure (Figure S9). Interestingly, the gap length of the nanoparticles in the dehydrated state was 5.5 nm, as measured by GISAXS, which is significantly larger than that observed for a monolayer (1.9 nm; Figures S2 and S5). Upon rehydration, the initial structure was fully recovered. Three layer samples, which form a full unit cell of the BCC crystal (Figure S10), also recover their structure and lattice parameter after dehydration and subsequent rehydration. However, unlike the two-layer sample, the three-layer sample did not collapse into a monolayer upon dehydration, likely because the network of DNA-connected particles prevents the structure from completely collapsing. This “network effect” becomes more obvious with increasing particle layers. A 10 layer superlattice was assembled on a substrate using the same stepwise deposition process and confirmed to possess a (100)-oriented polycrystalline BCC structure with a lattice parameter of 44.8 nm by GISAXS (Figure 2B). This lattice parameter corresponds to a surface-to-surface gap length of 29.8 nm. After drying, the 2D-GISAXS scattering pattern suggests a disordered body-centered tetragonal structure, with a = b = 34.1 nm and c = 28.7 nm (Figure S13), indicating a greater vertical collapse than in the plane of the substrate. This collapse corresponds to an overall superlattice contraction of 63% by volume. A closer examination of the morphology of the dehydrated BCC sample with SEM shows ordered domains of varied lattice projections, with many regions retaining obvious (100)-oriented BCC structure (Figures 2B, S11). The emerging picture that develops of dehydration suggests a relatively isotropic compression of the lattice, with some distortion due to the substrate, while retaining a high degree of order. Importantly, we determine that 10 layer samples can be rehydrated to fully recover the initial lattice parameters and that
this process of dehydration and rehydration can be repeated in a fully reversible fashion for at least three consecutive cycles (Figure 3A), with minimal change in the lattice parameters. While superlattice disorder is introduced during dehydration and rehydration cycles (note the increase in GISAXS peak width), the ordering may be fully recovered after brief annealing at 40 °C (several degrees below the melting temperature) (Figure S14). To better understand the state of the DNA during this transition from hydrated to dehydrated forms, we analyzed 10layer samples with circular dichroism (CD). In the hydrated state, the DNA within the superlattice shows a positive Cotton effect at 274 nm and a negative Cotton effect at 240 nm (Figure 3B), indicative of a native B-form of DNA.32−34 This conclusion is corroborated by previous theoretical simulations of hydrated DNA-nanoparticle superlattices.35 After drying, the ellipticity (θ) increased, the positive Cotton effect shifted to a longer wavelength (280 nm), and the negative peak at 210 nm increased in relative intensity, suggesting a higher energy Aform DNA.36−38 Geometrically, A-DNA is more compact with a wider diameter and shorter rise per turn than B-DNA27 and is favored as a stable conformation under low hydration conditions.34,36,39 The predicted length of the A-DNA is ∼65% of B-DNA and corresponds well with the dried in-plane distance of the BCC lattice. Previous studies on DNA thin films (without any nanoparticles) have also reported reversible B to A transitions upon dehydration and subsequent rehydration in controlled humidity environments;40 however this is the first time this has been shown in the context of a nanoparticle assembly. Solution-dispersed, BCC superlattices composed of spherical nanoparticles (Figure S15) were also examined to probe the importance of the surface in the dehydration process. Lattices were generated via a recently reported slow-cooling process, which results in single crystal superlattices with a well-defined rhombic dodecahedron crystal habit.41 This controlled habit enables us to investigate preservation of the macroscopic lattice morphology, in addition to local order. To probe changes in the lattice, transmission small-angle X-ray scattering (SAXS) was employed. SAXS analysis revealed a change in the surface-tosurface distance between nanoparticles from 21.7 nm in the 2165
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that DNA compression within free-standing superlattices is isotropic upon dehydration and thus that the anisotropy previously observed for multilayer thin film samples was likely induced by the surface. This represents a powerful method to reduce the interparticle spacing, while maintaining crystalline structure. In summary, we have shown the reversible contraction and expansion of DNA-assembled nanoparticles in both monolayer and superlattice forms through simple dehydration and rehydration. In this process, the DNA remains hybridized and changes from B to A form, with DNA orientation and position preserved through attachment to the nanoparticle surface. This work provides a route to transfer superlattices to the solid state without irreversibly fixing the structure and offers an attractive method to decrease interparticle spacing. Such changes, along with the ability to dynamically recover the initial parameters, could be of significant interest in the study of distancedependent properties, such as plasmonic coupling. Furthermore, this approach may also provide a means to preserve nanoparticle superlattices for long-term use.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS
This material is based upon work supported by the AFOSR under Award Nos. FA9550-09-1-0294, FA9550-11-1-0275, and FA9550-12-1-0280. This work was supported by the National Science Foundation’s MRSEC program (DMR-1121262) at the Materials Research Center of Northwestern University. This material is based upon work supported as part of the Nonequilibrium Energy Research Center (NERC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0000989. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center, and the Nanoscale Science and Engineering Center (EEC-0118025/003), both programs of the National Science Foundation; the State of Illinois; and Northwestern University. B.R. acknowledges support through an Indo-US postdoctoral fellowship. M.N.O. gratefully acknowledges support through a NSF Graduate Research Fellowship. M.X.W. acknowledges support through an NSF Graduate Research Fellowship and a Northwestern University Ryan Fellowship.
Figure 4. SEM images of single crystalline BCC superlattices after (A) silica embedding and (B) dehydration. Respective magnified images are shown in the right side panels.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 2166
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