1. What are the key interactions that mediate the population and crystallization at the interface of charged BNSLs formed by grafting AuNPs with PEG?
The key interactions that mediate the population and crystallization at the interface of charged BNSLs formed by grafting AuNPs with PEG are hydrophobic and Van der Waals interactions among PEG chains, and Coulombic interactions due to the charged terminals. These interactions play a crucial role in the formation of pseudo-stoichiometries and novel symmetries present in the charged BNSLs, which are distinct from those observed from neutral PEG systems. The manipulation of the pH and core size of the AuNPs allows for control over the total charge of the constituents, leading to the formation of various superstructures with different charge ratios and lattice structures.
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2. How does bulk mixing ratio affect superstructures at pH 3?
At pH 3, the bulk mixing ratio influences the formation of superstructures. Figure 2 shows GISAXS profiles of A:B and A:B' systems at different molar mixing ratios. A 1:1 mixing ratio of A' : B constituents leads to a 2x2 square lattice. Increasing the molar ratio results in a transition from a square lattice to a hexagonal lattice, which remains robust as the ratio increases to 4:1. Similarly, a 1:2 mixture of A:B' constituents forms a 2x2 square lattice. The A:B' system transitions from a 3x3 hexagonal lattice to a 2x2 square lattice with a 1:1 ratio. The surface charge density created by the COOH group is larger than the NH2 terminal group, and the total charge scales with the surface area of the core. Varying the pH can manipulate the net surface charge, affecting the crystallization of the system with fixed molar mixing ratios.
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3. What effect does pH have on the lattice constant of A ' : B system?
Lowering the pH in the A ' : B system reduces the lattice constant by approximately 20%. This is due to the balance of net charge on A ' and B particles caused by the addition of HCl. The decrease in pH increases the solubility of B particles, leading to a pseudo-stoichiometric A ' 2 B hexagonal unit cell. The lattice constant reduction is evident in the diffraction patterns shown in Figure 3 (a) and (b).
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4. How does pH affect the charge of PEG-grafted AuNPs?
In this study, the total surface charge of PEG-grafted AuNPs is controlled by adding carboxylic or amine terminal groups and varying the pH. The carboxylic acid group (COOH) is expected to be negatively charged, while the amine group (NH2) is expected to be positively charged. Under neutral pH conditions, COOH dissociates into COO-, resulting in fully dispersed AuNPs. NH2 is partially charged and migrates to the vapor/liquid interface. Lowering the pH reduces the charge in COOH and increases the charge in NH2. Zeta potential measurements confirm these findings, providing a quick path for predicting possible structures of BNSLs. Surface-sensitive X-ray diffraction techniques (GISAXS, XR) are used to test hypotheses and determine actual structures of AuNPs under various conditions. The study's main results are summarized in a pH-mixing-ratios phase diagram, showing the achievement of a checkerboard square lattice by lowering pH and manipulating molar mixing ratios. Further lowering pH or increasing molar ratio transforms the system into a hexagonal structure with pseudostoichiometry A'B, A'2B, and AB'2. This tunability enables tailoring material properties by manipulating molar ratios. In extreme low pH, one constituent forms a single-particle hexagonal structure similar to neutral PEG-grafted AuNPs at the vapor/liquid interface.
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