Highly Stable Nanoparticle Supercrystals Formed by Aldol Reaction in Conjunction with Slow Solvent Evaporation

The nanoparticle supercrystals (NPSCs) have been of great interests for their collective emergent properties. While various NPSCs have been successfully fabricated using intermolecular forces, the limited structural stability of NPSCs due to the weak nature of the intermolecular forces still remains a major hurdle for practical applications. Herein, we report a new method to fabricate highly stable three-dimensional NPSCs by using aldol reaction, a model covalent bond forming reaction, in conjunction with slow solvent evaporation. Gold nanoparticles functionalized with thiol poly-ethylene glycol formyl are linked to each other by carbon-carbon covalent bonds formed by aldol reaction as the particle dispersion in aqueous NaOH solution is slowly evaporated, resulting in highly faceted three-dimensional NPSCs. As-synthesized NPSCs show excellent structural stability in solvents of different polartities as well as the dried condition and at temperature up to 160 °C, which is far superior to NPSCs stabilized by intermolecular forces such as hydrogen bonding and van der Waals interactions. The new covalent bonding appraoch opens up new opportunities in the synthesis of NPSCs and their applications.


Introduction
Nanoparticle supercrystals have been of great interest for their new emergent properties including plasmonic 1-3 , optical 4-6 , electrical 7,8 , and magnetic 9,10 properties which cannot be achieved by individual nanoparticles. The emergent properties of nanoparticle supercrystals originate from collective interaction between nanoparticles and are determined by the crystal symmetry 11,12 and lattice parameters 13 as well as composition 14 . Various intermolecular interactions such as hydrogen bonding 15,16 and van der Waals interaction 17,18 have been successfully utilized to synthesize and stabilize NPSCs of different symmetries and lattice parameters. However, the limited structural stability of NPSCs fabricated by the relatively weak intermolecular interaction has been a major hurdle for realizing the potential applications. For example, the NPSCs formed by the DNA-mediated method (stabilized by hydrogen bonds) is stable only in saline solution and below DNA melting temperature 19 and the NPSCs formed by a slow solvent evaporation method (stabilized by van der Waals interaction) are stable only in dried condition 20 .
The intramolecular interactions are much stronger than the intermolecular interactions. Therefore, if intramolecular interactions are used to form and stabilize the NPSC, the structural stability of NPSC would be significantly enhanced. Covalent bonding interactions are one of the most representative intramolecular interactions and a broad spectrum of covalent bonding interactions are available and have been extensively utilized to form small molecules and covalent organic framework 21 . Therefore, the covalent bonds can be an excellent candidate for forming new NPSCs with significantly enhanced structural stability. In addition to the wellestablished covalent bonds chemistry and its versatility, the relatively much cheaper price of molecules for covalent bond forming reaction than the DNA counterpart would be a great advantage for practical applications. In the past few years, the molecular cross-linking has been used to increase the stability of pre-formed NPSCs by thermal treatment 22 or ultraviolet irradiation 23,24 . However, the utilization of covalent bonding interaction as a main driving force in the formation of NPSCs has not been reported yet.
Here, we report a new method to fabricate highly stable NPSCs by using covalent bonding interaction in conjunction with the slow solvent evaporation process. Gold nanoparticles (Au NPs) functionalized with thiol poly-ethylene glycol formyl (HS-PEG-CHO) are dispersed in aqueous NaOH solution and slowly evaporated on a substrate, resulting in NPSCs stabilized with covalent bonds, as illustrated in Figure 1. In this method, the aldol reaction is used as a model covalent bond forming reaction in which formyl groups (-CHO) in the presence of a base react each other to form β-hydroxy carbonyl (called aldol group) compounds. The NPSCs fabricated with this method are highly stable in solvents of different polarity ranging from water to toluene and at temperature up to 160 °C. As-synthesized NPSCs show superior solvent and 4 thermal stability than NPSCs fabricated by previous methods and provide new opportunities for a wide range of potential applications.  Figure S5). The Au-CHO particles are dispersed in water at 136 mg ml -1 . 10 μL of the dispersion is mixed with 5 μL of 0.1 M NaOH (aq.), and then the mixture is allowed to evaporate on a silicon wafer placed inside a petri dish (with 2 ml of water added) for 5 days at room temperature. All the procedures described above are performed for both spherical and octahedral Au NPs, respectively.

Characterization of Au NPSCs: Structures and Covalent Bonding
To investigate the morphology of NPSCs, the scanning electron microscopy (SEM) measurements of the aggregates on silicon wafers after 5 days of evaporation were performed.
The SEM images show that highly faceted micrometer-sized three-dimensional NPSCs are formed for both spherical and octahedral Au NPs (Figure 2a and 2c). The NPSCs formed with spherical particles also show truncated octahedral shape and the NPSCs formed with octahedral 6 particles show rhombic dodecahedral shape. It should be noted that both of the NSPC shapes are Wulff polyhedral which are obtained when the surface energy is minimized 25 . To understand the effects of aldol reaction on the formation of NPSCs, the aggregates formed by slow evaporation without aldol reaction are prepared and compared. For this purpose, the Au NPs functionalized with HS-PEG-CHO are slowly evaporated at the same condition as described above without adding NaOH (5 μL of water is used instead of 5 μL of 0.1 M NaOH (aq.)). The SEM measurements show that spherical and octahedral Au NPs form two-dimensional NPSCs (with one or a few layers of hexagonally packed particles), which is typical for NPSCs formed by slow solvent evaporation (Figure 2b and 2d) 26 . This is in stark contrast with the morphology of NPSCs formed in the presence of NaOH. The differences clearly indicate that the aldol reaction plays a key role in the formation of highly faceted three-dimensional NPSCs.
To confirm the formation of covalent bonds in the NPSCs prepared with addition of NaOH, To investigate the structure of NPSCs, small-angle x-ray scattering (SAXS) measurements are performed for the Au NPSCs formed by aldol reaction in conjunction with slow solvent evaporation. The SAXS intensity of NPSCs made of spherical Au NPs shows distinct peaks, which can be indexed with the face-centered cubic (fcc) symmetry with the lattice parameters of 39.7 nm. The presence of high order peaks indicates that NPSCs are highly ordered. The fcc symmetry has the highest packing density of 74% for spherical particles and the highest coordination number of 12. Therefore, for the spherical particles which bind each other through an aldol reaction, the fcc symmetry provides the minimum system energy both entropically and enthalpically 28 . The distance between the nearest neighboring spherical Au NPs estimated from the lattice parameter is 28.1 nm. Considering that the diameter of spherical Au NPs is 26.3 nm, the gap between neighboring particles filled with covalently connecting molecules is 1.8 nm.
The domain size estimated from the (111) peak using the Scherrer's equation is ca. 1.5 μm, which is consistent with the size of NPSCs estimated from SEM images. This suggests that the NPSCs are mostly single crystal.

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The SAXS intensity of NPSCs made of octahedral Au NPs shows distinct high order peaks up to more than 10 th order, indicating that highly ordered NPSCs are formed. All the peaks can be indexed by the body-centered cubic (bcc) symmetry with a lattice parameter of 31.5 nm. This is consistent with previous Monte Carlo simulation, free energy calculation, and experimental studies which show that the bcc phase is most stable for octahedral particles [29][30][31] .
Using the measured lattice parameter and the size of octahedral particle, the particle packing density, and the orientation of octahedral particles relative to the axis of bcc unit cell are estimated. The particle packing density is ca. 70.5 % and the line connecting two opposite vertices of the octahedral particle is tilted ca. 12.1° from the c-axis of a unit cell. In this estimation, the molecular shell of 0.9 nm thickness on octahedral Au NPs is included in the volume of octahedral particles. The domain size estimated from the (110) peak using the Scherrer's equation is ca. 1.4 μm. This is consistent with the SEM measurements which predominantly show NPSCs of 1-2 μm size. This also suggests that the NPSCs are mostly single crystal.
The formation of three-dimensional NPSCs by aldol reaction in conjunction with slow solvent evaporation can be attributed to the nucleation in the solution bulk and the reversibility of aldol reaction. Considering that the slow solvent evaporation typically induces a thin film of NPSCs by nucleation and growth at the air-liquid interface, 26 the formation of threedimensional NPSCs suggests that nucleation and growth may have occured mainly in the solution bulk by aldol reaction between functionalized Au NPs. To form highly ordered NPSCs, particles should be allowed to re-adjust their positions or orientations after clustering to minimize the free energy. In this method, this is provided by the reversibility of aldol reaction, i.e. the retro-aldol reaction in the presence of NaOH temporally decomposes the aldol group back to the initial formyl groups 32 .

Structural Stability of Au NPSCs Formed by Aldol Reaction
The carbon-carbon covalent bonds formed by the aldol reaction is highly stable in a wide range of solvents. Therefore, the Au NPSCs fabricated by aldol reaction in conjunction with

Synthesis of Au nanoparticle supercrystals (NPSCs) by aldol reaction in conjunction with
slow solvent evaporation. A mixture of 10 μL of 136 mg ml -1 Au-CHO aqueous solution and 5 μL of 0.1M NaOH aqueous solution was dropped on a silicon wafer placed inside a petri dish (with 2 ml of water added) and is allowed to evaporate for 5 days at room temperature. This was performed for spherical and octahedral Au NPs, respectively.

Small-Angle X-Ray Scattering Measurements.
Small-angle X-ray scattering (SAXS) measurements were performed at the beamline 4C of the Pohang Accelerator Laboratory (PAL), Republic of Korea. X-rays with a wavelength (λ) of 0.1217 nm and a wavelength spread (Δλ/λ) of 2 × 10 −4 delivered by a Si(111) double crystal monochromator were used. A 2D CCD camera (SX165, Mar USA, Inc. CCD 165) was used to collect scattered X-rays. The sample-to-detector distance of 2 m was used to cover the q range of 0.1 nm −1 < q < 2 nm −1 , where q = (4π/λ) sin(θ/2) is the magnitude of the scattering vector and θ is the scattering angle. The q values were calibrated using silver behenate (AgO2C(CH2)20CH3). The temperature was controlled by using a water circulation bath (Lauda, Germany). All the samples were equilibrated for at least 30 minutes at each temperature before measurement.