Co-precipitation of oppositely charged nanoparticles: the case of mixed ligand nanoparticles

Abstract

Colloid stability is of high importance in a multitude of fields ranging from food science to biotechnology. There is strong interest in studying the stability of small particles (of a size of a few nanometres) with complex surface structures, that make them resemble the complexity of proteins and other natural biomolecules, in the presence of oppositely charged nanoparticles. While for nanoparticles with homogeneously charged surfaces an abrupt precipitation has been observed at the neutrality of charges, data are missing about the stability of nanoparticles when they have more complex surface structures, like the presence of hydrophobic patches. To study the role of these hydrophobic patches in the stability of nanoparticles a series of negatively charged nanoparticles has been synthesized with different ratios of hydrophobic content and with control on the structural distribution of the hydrophobic moiety, and then titrated with positively charged nanoparticles. For nanoparticles with patchy nanodomains, the influence of hydrophobic content was observed together with the influence of the size of the nanoparticles. By contrast, for nanoparticles with a uniform distribution of hydrophobic ligands, size changes and hydrophobic content did not play any role in co-precipitation behaviour. A comparison of these two sets of nanoparticles suggests that nanodomains present at the surfaces of nanoparticles are playing an important role in stability against co-precipitation.

Anionic nanoparticle-lipid membrane interactions: the protonation of anionic ligands at the membrane surface reduces membrane disruption

“…The shaded area shows a conservative estimate of the indeterminacy with which the pKa of the ligand in water is known: the free ligand has a pKa of 5 (bottom edge of the shaded area for z = 3 nm from the membrane center), while cooperative effects on the NP surface have been predicted to shift the pK a of similar ligands up to 7.6 34 (upper edge of the shaded area). Using 6.3 as reference pK a value in water 35 , the light-blue profile is obtained and the error bars deriving from our metadynamics calculations lay within the shaded area. The shaded grey areas indicate the distribution of lipid heads and glycerol groups.…”

“…We set up surface have been predicted to shift the pKa of similar ligands up to 7.6 34 (upper edge of the shaded area). Using 6.3 as reference pKa value in water 35 , the light-blue profile is obtained and the error bars deriving from our metadynamics calculations lay within the shaded area. The shaded grey areas indicate the distribution of lipid heads and glycerol groups.…”

“…When adsorbed on the surface of a NP, though, their pKa changes and shifts to larger values. Moglianetti et al 35 have measured an average pKa of 6.3 for MUA ligands adsorbed on the surface of 4-5 nm Au NPs, suggesting that about one tenth of the MUA ligands are indeed protonated at physiological pH. The interaction with the membrane can induce changes of the protonation state, as well 36,37 .…”

“…The offset between the two free energy profiles (right vertical segment of the cycle in Fig. 3) is provided by the pKa of the ligand in the water phase, which we assume to be 6.3 as measured by Moglianetti et al 35 .…”

Monovalent ion-mediated charge–charge interactions drive aggregation of surface-functionalized gold nanoparticles

“…In fact, because of their nanometer scales, NPs are characterized by an extremely high surface-to-volume ratio and, therefore, NPs in solution tend to aggregate to minimize the surface energy, resulting in the rapid settling of the suspension. 1 Numerous previous studies have focused on the role of charges, 20 solvent ion concentration, 21 solubility, and wettability, 22 and it has been demonstrated that the charge and morphology of the ligand shell play a fundamental role on determining NPs' properties. 23 In particular, the role of ions in promoting NP aggregation has been thoroughly investigated 24,25 in large part due to the well-known ability of multivalent cations to bridge charged ligands and to promote NP aggregation.…”