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In Situ Electron Microscopy Imaging andQuantitative Structural Modulation ofNanoparticle SuperlatticesJuyeong Kim,, Matthew R. Jones, Zihao Ou, and Qian Chen*,,,

Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, and Department of Chemistry,University of Illinois, Urbana, Illinois 61801, United StatesDepartment of Chemistry, University of California, Berkeley, California 94720, United States

*S Supporting Information

ABSTRACT: We use liquid-phase transmission electron micros-copy (LP-TEM) to characterize the structure and dynamics of asolution-phase superlattice assembled from gold nanoprisms atthe single particle level. The lamellar structure of the superlattice,determined by a balance of interprism interactions, is maintainedand resolved under low-dose imaging conditions typicallyreserved for biomolecular imaging. In this dose range, wecapture dynamic structural changes in the superlattice in realtime, where contraction and smaller steady-state lattice constantsare observed at higher electron dose rates. Quantitative analysisof the contraction mechanism based on a combination of directLP-TEM imaging, ensemble small-angle X-ray scattering, and theoretical modeling allows us to elucidate: (1) thesuperlattice contraction in LP-TEM results from the screening of electrostatic repulsion due to as much as a 6-fold increasein the effective ionic strength in the solution upon electron beam illumination; and (2) the lattice constant serves as ameans to understand the mechanism of the in situ interaction modulation and precisely calibrate electron dose rates withthe effective ionic strength of the system. These results demonstrate that low-dose LP-TEM is a powerful tool for obtainingstructural and kinetic properties of nanoassemblies in liquid conditions that closely resemble real experiments. Weanticipate that this technique will be especially advantageous for those structures with heterogeneity or disorder thatcannot be easily probed by ensemble methods and will provide important insight that will aid in the rational design ofsophisticated reconfigurable nanomaterials.

KEYWORDS: nanoassemblies, reorganization kinetics, liquid-phase TEM, gold nanoprism superlattice, interaction modeling, SAXS

Self-assembly of nanoparticles into complex architecturespromises access to collective photonic,13 plasmonic,46electronic,79 catalytic,10,11 and magnetic12 functional-ities that are tunable and may even be adaptable in response tochanges in external conditions.1316 An understanding ofinternanoparticle interactions and structuring kinetics in liquidsis essential for predictive nanoparticle assembly, which remainsa challenge.17,18 Classical colloidal theories developed forinteractions between micron-sized particles do not simplyrescale at nanometer dimensions because of multiscalecollective effects and the nonadditivity of nanoscale inter-actions.1821 Ideally one would address this by analyzingdirectly the dynamic motion and assembly of individualnanoparticles. However, experimental studies of this sort arecomplicated by the need to acquire images with the appropriatetime and spatial resolution while nanoparticles interact freely inliquids to form into superstructures.

Most in situ observations of nanoparticle assembly in solutionuse small-angle X-ray scattering (SAXS), which probesassembled structures in reciprocal space.1315,22,23 SAXS hasminimal disturbance to the native sample solution duringmeasurement and high resolution in detecting structural order.It is limited, however, by its ensemble nature, which offers littleinsight into the real-time motions and interactions of singlenanoparticles and complicates interpretation of structuralheterogeneity/disorder.On the other hand, liquid-phase transmission electron

microscopy (LP-TEM) readily achieves single-nanoparticleimaging in liquids2432 and allows experimental interpretationof nanoparticle interactions and assembly dynamics.3338 Forexample, LP-TEM has been used to observe nanoparticle

Received: August 4, 2016Accepted: October 10, 2016

Artic

lewww.acsnano.org

XXXX American Chemical Society A DOI: 10.1021/acsnano.6b05270ACS Nano XXXX, XXX, XXXXXX

www.acsnano.orghttp://dx.doi.org/10.1021/acsnano.6b05270

monolayer formation during solvent drying33 and the assemblyof gold spheres,3436 gold rods,37 and CdSe/CdS octapods38

into linear chains. Based on a statistical analysis of singlenanoparticle motion trajectories, our earlier work reported aquantitative map of nanoparticle interactions inside LP-TEM.37

The limitation of LP-TEM is the effect of the imaging electronbeam, which has been shown to greatly alter the liquidcomponents through radiolysis reactions.2729 For instance, theradiolysis of water,39 the widely used solvent for metallicnanoparticle and biomolecule-directed assemblies, generatesionic and reactive species (OH, H+, hydrated e, OH, etc.) as afunction of electron beam dose rates (number of electronsimposed onto the sample per unit area per unit time). Thesereactions frequently perturb the solution considerably,convoluting the interpretation of the data and making theresults challenging to be used to guide nanoparticle assembly instandard experimental conditions outside TEM. It is thuscritical to experimentally quantify how local changes in theliquid environment within the illuminated area consequentlyinfluence nanoparticle interactions and assembly dynamics.Here we use low-dose LP-TEM to characterize one-

dimensional (1D) nanoprism superlattices. We resolve thereal-space structure of a hydrated crystalline nanoparticlesuperlattice and its structural change in response to localchanges in the liquid. The quantitative correlation among LP-TEM images, theoretical modeling, and ensemble measure-ments of lattice constants by SAXS enables us to understandthe mechanism of electron beam-mediated nanoparticleinteractions and to reproduce identical structural reorganizationof the assembly outside TEM. We observe that the dose ratestypically used in cryogenic EM imaging of biomolecules (110e/(2s)),40 which are lower than the dose rates commonlyused in earlier LP-TEM studies (Figure S1 and Table S1), arerequired to prevent the superlattices from collapsingcompletely. Within the optimized dose rate range (16 e/(2s)), the contraction of the lattice constant, i.e., the center-to-center d spacing between prisms (Figure 1A), isquantitatively modulated by electron beam dose rates. We seethe in situ and outside TEM correlation we establish here asa critical step to generalize single-nanoparticle insights onnanoparticle assembly learned from LP-TEM, at a spatiotem-poral resolution not accessible by other means, to directpredictable assembly in real experimental conditions.

RESULTS AND DISCUSSIONThe superlattices used in this work are defined by evenlyspaced gold triangular nanoprisms aligned face-to-face, here-tofore referred to as meta-rods (Figure 1A). While the meta-rods represent a rich and important class of nanoparticlesuperlattices, with collective properties that are sensitive to thespatial arrangement of component nanoparticles,1,4,5,79,11,12

they have previously been characterized at the ensemble level.41

The meta-rods are prepared by functionalizing gold prisms(89.3 9.1 nm in edge length and 7.5 0.3 nm in thickness)with a dense monolayer of alkyl-thiol ligands, rationallydesigned to induce robust solution-phase assembly behavior(see Materials and Methods in the Supporting Information,Figure S2). Specifically, carboxyl-terminated thiols areexchanged with the native cetyltrimethylammonium bromide(CTAB) ligands on the prism surface.4244 The strong goldthiol bond allows for the system to be prepared in such a wayso as to have a low concentration of free ligands. Thisminimizes the contribution of depletion forces41 and facilitates

theoretical modeling of interprism interactions for mechanisticunderstanding. An aqueous pH = 8 phosphate buffer solution(PBS) is used as the liquid medium to ensure completedeprotonation of the ligands into negatively charged COOgroups (pKa = 3.53.7, Figure S3A), which excludesinterparticle hydrogen-bonding effects (COOH OHOC).43 The high buffer concentration (0.15 M) maintainsa constant pH and keeps the surface charge density of prismsconstant even with potential radiolysis-generated H+ or OH

species (see the evaluation of beam-induced pH effects inFigure S3B). Under these conditions, prisms assemble face-to-face through a balance of electrostatic repulsion and van derWaals attraction.Figure 1A shows a typical LP-TEM image of 1D meta-rods

lying flat on the SiNx window composing the liquid cell samplechamber, showing visible interstices between nearest-neighborprisms which indicate preservation of the solvent/ligand layersbetween the particles. The LP-TEM image was taken at a doserate of 10 e/(2s) and an accelerating voltage of 200 kV. Thereal-space TEM image allows us to measure the d spacingdirectly from the intensity profile across parallel neighboringprisms (Figure 1A); the d spacing is consistent betweendifferent pairs of neighboring prisms (12.2 1.0 nm). MovieS1 shows that at the stable state the intensity profile fluctuatesand still generates a consistent d spacing over time. Such direct

Figure 1. Characterizations of meta-rods. (A) A schematic (left, notdrawn to the scale) of meta-rods in 0.15 M pH = 8 PBS buffersolution labeling the d spacing and the meta-rod length h, a LP-TEM image showing a meta-rod lying flat on the SiNx liquidwindow (middle), and the intensity profile of the TEM imageregion boxed by red dotted lines showing the intensity averagedhorizontally as a function of the vertical axis due to alternatingprisms (dark in TEM) and interstices (bright in TEM), from whichwe measure the d spacing in LP-TEM (ri