Mass Transfer from Ion-Sensing Component-Loaded Nanoemulsions into Ion-Selective Membranes: An Electrochemical Quartz Crystal Microbalance and Thin-Film Coulometry Study.

Recent work has shown that ion-selective components may be transferred from nanoemulsions (NEs) to endow polymeric membranes with ion-selective sensing properties. This approach has also been used for nanopipette electrodes to achieve single-entity electrochemistry, thereby sensing the ion-selective response of single adhered nanospheres. To this date, however, the mechanism and rate of component transfer remain unclear. We study here the transfer of lipophilic ionic compounds from nanoemulsions into thin plasticized poly(vinyl chloride) (PVC-DOS) films by chronoamperometry and quartz crystal microbalance. Thin-film cyclic coulovoltammetry measurements serve to quantify the uptake of lipophilic species into blank PVC-DOS membranes. Electrochemical quartz crystal microbalance data indicate that the transfer of the emulsion components is insignificant, ruling out simple coalescence with the membrane film. Ionophores and ion-exchangers are shown to transfer into the membrane at rates that correlate with their lipophilicity if mass transport is not rate-limiting, which is the case with more lipophilic compounds (calcium and sodium ionophores). On the other hand, with less lipophilic compounds (valinomycin and cation-exchanger salts), transfer rates are limited by mass transport. This is confirmed with rotating disk electrode experiments in which a linear relationship between the diffusion layer thickness and current is observed. The data suggests that once the nanoemulsion container approaches the membrane surface, transfer of components occur by a three-phase partition mechanism where the aqueous phase serves as a kinetic barrier. The results help better understand and quantify the interaction between nanoemulsions and ion-selective membranes and predict membrane doping rates for a range of components.

Surfactants for Optode Emulsion Stabilization without Sacrificing Selectivity or Binding Constants.

We compare here the effect of surfactants on ion-selective membranes measured via voltammetry and optode emulsions measured optically. Cyclic voltammetry on a thin-film ion-selective membrane is shown to be a useful screening technique for the estimation of effective complex formation constants and selectivity coefficients for different surfactants with various cations. This technique is particularly useful for its ability to identify separate ion-transfer events (free, surfactant complexed, ionophore complexed) for a specific membrane. However, we also caution against the over-reliance on this technique as changes in membrane characteristics are observed following surfactant partitioning. Of the surfactants explored here, a zwitterionic sulfobetaine-based surfactant was found to stabilize sensors without reducing effective binding constants and selectivity, with greatly superior characteristics to other commonly utilized surfactants. Those include Brij-35, F-127, and Triton X-100, all of which showed significant binding to so-called free ions in the membrane, resulting in peak potential shifts of 199 ± 10, 180 ± 24, 278 ± 11 mV, respectively, for potassium following the subtraction of transducing layer effects. This peak shift translated to a much larger undesired free ion response in optode emulsions. The selectivity in emulsion-based systems was also shown to decrease in the presence of nonionic surfactants compared to that containing the zwitterion.

Synergy on Surfaces: Anti-Biofouling Interfaces Using Surface-Attached Antimicrobial Peptides PGLa and Magainin-2.

The synergistic effect of antimicrobial compounds is an important phenomenon that can increase the potency of treatment and might be useful against the formation of biofilms on surfaces. A strong inhibition of microbial viability on surfaces can potentially delay the development of biofilms on treated surfaces, thereby enhancing the performance of water-purification technologies and medical devices, for example, to prevent hospital-acquired infections. However, the synergistic effects of surface-immobilized antimicrobial peptides (AMPs) have not yet been reported. Here, we demonstrate the synergistic antimicrobial effects of the AMPs PGLa and magainin-2 on modified reverse-osmosis (RO) membranes. These AMPs are known to act synergistically in the free state, but their antimicrobial synergistic effects have not yet been reported in a surface-immobilized state. The AMPs were functionalized with alkyne linkers and covalently attached to RO membranes modified with azides, using a click chemistry reaction. The resulting RO membranes showed reduced contact angles, indicating increased wettability. X-ray photoelectron spectroscopy confirmed the presence of the two peptides on the membranes via changes in the amounts of carbon, oxygen, and sulfur, which led to an increased S/C ratio, probably because of the sulfur present in the methionine residue of the peptides. The synergistic activity was measured with the free peptides in solution and covalently bound on RO membrane surfaces by observing increased leakage of 5(6)-carboxyfluorescein from large unilamellar vesicles. The synergistic antimicrobial activity against Pseudomonas aeruginosa was observed using surface-activity assays, where the AMP-modified RO membranes showed an effective inhibition of P. aeruginosa biofilm growth, as compared with unmodified membranes. An enhanced activity of antimicrobials on surfaces might lead to potent antimicrobial surfaces, which could result in more fouling-resistant water-treatment membranes.

Ink-Jet Printing-Assisted Modification on Polyethersulfone Membranes Using a UV-Reactive Antimicrobial Peptide for Fouling-Resistant Surfaces.

Antimicrobial peptides (AMPs) are promising candidates for surface coatings to control biofilm growth on water treatment membranes because of their broad activity and the low tendency of bacteria to develop resistance to AMPs. However, general and convenient surface modification methods are limited, and a deeper understanding of the antimicrobial mechanism of action is needed for surface-attached AMPs. Here, we show a method for covalently attaching AMPs on porous ultrafiltration membranes using ink-jet printing and provide insight into the mode of action for the covalently tethered peptide RWRWRWA-(Bpa) (Bpa, 4-benzophenylalanine) against Pseudomonas aeruginosa. AMP-coated ultrafiltration membranes showed surface antibacterial activity and reduced biofilm growth. Fluorescence microscopy analysis revealed that the modified surfaces could cause cell membrane disruption, which was seen by live uptake of propidium iodide stain, and scanning electron microscopy images showed compromised cell membranes of attached bacteria. This study indicated that the mode of action of covalently tethered AMPs was similar to that of freely soluble AMPs. The deeper understanding of the mode of action of AMPs covalently attached to surfaces could lead to a more rational approach for designing surfaces with antibacterial activity.