Solubility and transport of cationic and anionic patterned nanoparticles

We analyze bulk diffusion and transport through hydrophobic nanochannels of nanoparticles (NPs) with different hydrophobic-hydrophilic patterns achieved by coating a fraction of the NP sites with positive or negative charges via explicit solvent molecular dynamics simulations. Ten different charge pattern types including Janus charged-hydrophobic NPs are studied. The cationic NPs are more affected by the patterns and have higher diffusion constants and fluxes than their anionic NPs counterparts. The NP-water interaction dependence on surface pattern and field strength explains these observations. The NP-water Coulomb interaction of anionic NPs in the bulk, which are much stronger than the hydrophobic NP-water interactions, are stronger for NPs with higher localized charge, and stronger than in the cationic NPs counterparts. The diffusion and transport of anionic NPs such as proteins and protein charge ladders with the same total charge but different surface charge patterns are slowest for the highest localized charge pattern, which also adsorb strongest onto surfaces. Our model demonstrates the separation (by reverse osmosis, capillary electrophoresis, or chromatography) of cationic NPs, including proteins with equal net charge but different surface charge distributions.

Figure 1

(a) All NP types have $\pm 6e$ total charge, and 60 atoms (right to left, top to bottom), with charge per coated atom in parentheses: $1+(q=+0.1e)$, $1-(q=-0.1e)$, $2+(q=+0.2e)$, $3+(q=+0.12e)$, $4+(q=+0.2e)$, and $5+(q=+0.4e)$. The anionic types $2-(q=-0.2e)$, $3-(q=-0.12e)$, $4-(q=-0.2e)$, and $5-(q=-0.4e)$; the corresponding negative NP types (not shown) have the same charge arrangement with $-$ sign. Green and red represent positive and negative charges, respectively, while blue denotes neutral. (b) A snapshot of the MD simulation system showing the hydrophobic nanometer water channel, of length $L=2.564$ nm and diameter $D=1.616$ nm and the two membrane sheets (turquoise green) in a periodic box with water molecules, wherein five NPs of radius $R=0.333$ nm (bright green) are driven through it by an external electric field $\mathbf{E}$.

Figure 2

(a) The mean squared displacement of NPs for type 1+ and type $1-$ as a function of time under $E=0$. The resulting diffusion constants are $1.527\times {10}^{-5}{\mathrm{cm}}^2/\mathrm{s}$ for type 1+ and $0.732\times {10}^{-5}{\mathrm{cm}}^2/\mathrm{s}$ for type $1-$, respectively. (b) Nanoparticle-water interactions for type 1+ and 5+ (left axis) and type $1-$ (right axis) as a function of the electric field. There is one nanoparticle and 1728 water molecules in each simulation, and an additional 25-ns MD run for each nanoparticle type as a function of the electric field.

Figure 3

Average flux of NPs (a) for types $1\pm$ and (b) for types $3\pm$, $4\pm$, and $5\pm$ as a function of the electric field $\mathbf{E}$. Note that error bars in (a) are shown for two data points and most of them are smaller than the symbols; the right axis (indicated in the plot by $☆$ symbols) is the ratio flux (1+)/flux($1-$).