Two-dimensional assembly of gold nanoparticles grafted with charged-end-group polymers

Ionic-Like Superlattices by Charged Nanoparticles

“…This trend of lowering the charge on the COOH group of A ′ particles, continues by lowering the pH and finally leading to a simple hexagonal structure consisting of the A ′ particles as shown in the diffraction pattern and depicted in Figure 3(d). 22 We rationalize that the surface predominance of particles A ′ can be attributed to the enriched charge density of particle B, which potentially augments the solubility of particles B, thereby encouraging their bulk presence. To summarize, the effect of lowering the pH, and the progression of structural transitions are illustrated in Figure 3(e) conceptually.…”

“…The relative charge on each particle is tuned by the manipulation of the pH of the suspension. We note that the net charge of a nanoparticle is influenced substantially by the charge of the terminal group, 22 and the residual charge inherent to the AuNPs. 23,24 Three key interactions mediate the population and crystallization at the inter-face: the hydrophobic and Van der Waals interactions among PEG chains and the Coulombic interactions due to the charged terminals.…”

“…A central conclusion from the present and previous sections is that the surface charge density created by the COOH group is larger than the NH 2 terminal group, consistent with ζ potential values and published results. 22 Also, the total charge on each particle scales with the surface area of the core. These differences can be taken advantage of by manipulating the pH in the suspension to vary the net surface charge.…”

“…14 To rationalize this, we note that previous results indicate that NH 2 (B particles) terminated particles spontaneously populate the surface due to a lower positive surface charge, while the COOH (A ′ particles) terminated particles carry a higher total negative charge density. 22 This results in a repulsion between the A ′ particles, leading to the increased unit cell size, as depicted schematically by a halo of the A ′ particles with an enlarged effective cross-section. To counteract this effect, we add HCl to the system, which balances the net charge on A ′ and B particles, and as a result, the lattice constant is reduced by ∼ 20% to a ≃ 43 nm as deduced from the diffraction patterns in Figure 3 (a) and (b).…”

Tuning Self-Assembly of Gold Nanoparticles Grafted with Charged Polymers by Varying Temperature and Electrolytes

“…18 Here, we note that, although K 2 CO 3 increases the pH and is supposed to make the −COOH terminal more charged (−COO − ) and the whole grafted AuNPs more soluble, we believe that in this case, the cation K + binds back to the COO − making it −COOK, which behaves similarly to neutral methyl (−CH 3 )-terminated PEG-grafted AuNPs. 16,18 With pK a values of 5 for −COOH and 10 for −NH 2 , the actual charge of each AuNP is strongly dependent on the solution conditions. At neutral pH, increasing ionic strength by adding K 2 CO 3 screens the end charge while making the PEG backbone insoluble, leading to the same phases reported for methyl-terminated PEG, thus making the charged end groups ineffective.…”