Crystallization of nanoparticles induced by precipitation of trace polymeric additives

Orthogonal to guided growth of nanoparticle (NP) crystals using DNA or supramolecules, a trace amount of polymeric impurities (<0.1 wt.%) leads to reproducible, rapid growth of 3D NP crystals in solution and on patterned substrates with high yield. When polymers preferentially precipitate on the NP surfaces, small NP clusters form and serve as nuclei for NP crystal growth in dilute solutions. This precipitation-induced NP crystallization process is applicable for a range of polymers, and the resultant 3-D NP crystals are tunable by varying polymeric additives loading, solvent evaporation rate, and NP size. The present study elucidates how to balance cohesive energy density and NP diffusivity to simultaneously favor nuclei formation energetically and kinetic growth in dilute solutions to rapidly crystalize NPs over multiple length scales. Furthermore, the amount of impurities needed to grow NP crystals (<0.1%) reminds us the importance of fine details to interpret experimental observations in nanoscience.


Synthesis of Au NPs(1):
In a typical synthesis of ~ 4 nm Au NPs, an orange precursor solution of tetralin (10 mL), OAm (10 mL), and HAuCl4•3H2O (100 mg) was prepared at room temperature (r.t., ~20°C) and degassed three times. The solution was then kept in an ice bath and N2 was purged through. A reducing solution containing 0.5 mmol of TBAB (43 mg), tetralin (1 mL), and OAm (1 mL) was mixed under sonication and quickly injected into the precursor solution. The reduction was instantaneously initiated and the solution changed to a deep purple color within 5 s. The mixture was allowed to react in the ice bath for 1 hour before acetone (60 mL) was added to precipitate the Au NPs. Au NPs were collected by centrifugation (7,200 × g, 10 min), washed with methanol twice, and re-dispersed in toluene. The resulting NPs have the oleylamine as the stabilizing ligand and are denoted as OAm-Au NPs. The average NP core size is 3.8 nm

Synthesis of thiol end-functionalized polystyrene:
(Radical Addition Fragmentation Transfer) RAFT polymerization was employed to synthesize polystyrene (PS). In a typical synthesis of PS, targeting Mn = 3k and conversion of 60%, styrene monomer, chain transfer agent and AIBN were mixed at a molar ratio of 237.5 : 5 : 1 and degassed with three freeze-pump-thaw cycles. The mixture solution was then put into a pre-heated 75°C oven until the aliquot became viscous. 2 mL hydrazine mono-hydrate was added to the reaction solution and dissolved using 10 mL THF under vigorous stirring. The thiol-terminated PS was precipitated with 30 mL methanol and was collected by centrifugation (7,200 × g, 5 min). The purification procedure was repeated three times to remove the unreacted monomers and initiators.
The final product was dried in vacuum oven at r.t. over-night. Styrene monomer was purified by distillation to remove the inhibitor prior to the synthesis. AIBN was recrystallized before use.

PS characterization:
1 H nuclear magnetic resonance (NMR) spectra were obtained with a Bruker Advance 400 spectrometer (400 MHz) using a 5-mm Z-gradient broadband observe probe. PS molecular weight was determined using gel permeation chromatography with an Agilent 1260 Infinity series instrument equipped with two Agilent PolyPore columns (300 mm × 7.5 mm), calibrated using PS standards. The runs were performed with tetrahydrofuran as mobile phase at 1 mL min -1 .

Thiol Functionalization of OAm-Au NPs(2):
A toluene solution of OAm-Au NPs with a NP concentration of 5 mg mL -1 was mixed with a 40 mg mL -1 PS-SH solution under sonication. The mixture solution was stirred vigorously for 3 days. The PS-modified Au NPs were collected by methanol precipitation and centrifugation at 7,200 × g. for 3 min. The supernatant was removed and the PGNPs were redispersed in toluene.
The above solvent-nonsolvent cleaning procedure was repeated at least 6 times to remove excess unbound PS.  Ligand grafting density ( ) was calculated (8)   NPs in (c) and (d).

Characterization of polymer additives
Methods:

PGNP assembly with different polyolefin additives:
In order to understand how chemical composition, molecular weight and crystallinity influence  Fig. 8). The molecular weight of the precipitants also affected the formation of PGNP assemblies. As shown in Supplementary  Fig. 9). Polyolefins with high crystallinity are not preferred since they are more likely to precipitate by themselves rather than interacting with the PGNPs. Thus, PP/PE as an amorphous mixture of PP and PE with low Mn is the most effective precipitant to induce crystallization of PGNPs.
Polyethylene CH2 protons are highlighted. The red lines represent the Gaussian fit to the size histograms.