Frances Neville

Fabrication and activity of silicate nanoparticles and nanosilicate-entrapped enzymes using polyethyleneimine as a biomimetic polymer

In nature, some peptides induce precipitation of silicic acid into silica nanoparticles such as is found in marine algae called diatoms. However, polybasic polymers can act as peptide mimics; one such polymer, polyethyleneimine (PEI), has the advantage that it is stable at room temperature and is inexpensive, in comparison with synthetic peptides. We describe the fabrication and characterization of biosilicate nanoparticles formed by mimicking the peptides using PEI. Brownian motion nanoparticle tracking analysis and field emission gun scanning electron microscopy have been used for the first time to characterize nanoparticles made with tetramethyl orthosilicate (TMOS) and PEI to investigate the fundamental factors that affect particle properties. These factors include the effect of phosphate concentration, PEI molecular weight, TMOS concentration, and species of alkoxy-silane used. The properties of the particles are compared with other particles made with polymers that induce silication. Our results show that using PEI gives differences in particle size compared with previous work using other polymers that induce silication. The entrapment of enzymes during the silication process, rationale for using nonphosphate and phosphate buffers during enzyme entrapment, and the analysis of enzyme activity are also presented. Because enzymes can be entrapped during fabrication, it means that there are many future possibilities for the use of silicate nanoparticles containing enzymes, such as biosensors and biocatalytic reactors.

Novel one-pot synthesis and characterization of bioactive thiol-silicate nanoparticles for biocatalytic and biosensor applications

A novel one-pot neutral synthesis using bioinspired polymers to fabricate thiol-nanoparticles is presented. The thiol-particles may be directly tethered to metal surfaces such as gold, allowing the production of self-assembled nanostructured biocatalytic or biosensor surfaces. This one-pot method has also been used to entrap enzymes within the thiol-nanoparticles; it is apparent that once enzyme entrapment is carried out a bimodal distribution of particles is formed, with particles of one mode being very similar in size to thiol-nanoparticles without enzyme entrapped, and particles of the other mode being much larger in size. To this end, efforts have been made to separate the two modes of particles for the sample containing enzyme and it has been observed that the larger mode thiol-nanoparticles do indeed contain significant amounts of enzyme in comparison to the smaller mode ones. As the enzyme-containing thiol-nanoparticles can now be isolated, this means that there are many future possibilities for the use of thiol-particles containing enzyme, as they may be used in a wide range of processes and devices which require catalytic functionalized surfaces, such as biosensors and biocatalytic reactors.

A comparative study on the interactions of SMAP-29 with lipid monolayers

This work investigates the discrimination of lipid monolayers by the ovine antimicrobial peptide SMAP-29 and compares it to that of the human LL-37 peptide. Fluid phospholipid monolayers were formed in a Langmuir trough and subsequently studied with the X-ray scattering techniques of X-ray reflectivity and grazing incidence X-ray diffraction. Any changes in the phospholipid structure after injection of peptide under the monolayer were considered to be due to interactions between the peptides and lipids. The data show that SMAP-29 discriminates against negatively charged phospholipids in a similar way to LL-37. However, it is even more interesting to note that despite a higher concentration of SMAP-29 near the monolayer, ensured by its greater charge as compared to LL-37, the amount of SMAP-29 needed to observe monolayer disruption was around three and a half times the number of molecules of LL-37 used to see similar changes with the same system. This result suggests that the structure, amino acid sequence or size of the peptide may well be as important as electrical charge and therefore gives many implications for the further study of antimicrobial peptides with regards to novel drug design and development.

The interaction of antimicrobial peptide LL-37 with artificial biomembranes: epifluorescence and impedance spectroscopy approach

Membrane interactions of the human antimicrobial peptide LL-37 have been studied by a variety of techniques including insertion assay, epifluorescence microscopy and impedance spectroscopy. This study makes use of lipid monolayers at the air–aqueous interface to mimic bacterial or eukaryotic membranes. It was found that LL-37 readily inserts into phosphatidylglycerol (PG) and lipid A monolayers, significantly disrupting their structure. In contrast, the structure of phosphatidylcholine (PC) monolayers remains virtually unaffected by LL-37, which is evident both from epifluorescence and electrochemical measurements. Impedance spectroscopy showed that the LL-37 rich PC monolayer remains an ideal capacitor while LL-37 enriched lipid A capacitance decreases significantly, suggesting an increase in layer thickness from peptide–lipid binding.

Real-time monitoring of in situ polyethyleneimine-silica particle formation.

Silica particles are traditionally made via the hydrolysis and condensation of tetraalkoxysilanes with the use of methanol and ammonia as a basic catalyst. More recently, bioinspired polyamines have been used in place of ammonia. Particle formation via the use of tetraalkoxysilanes typically occurs extremely quickly with cloudy precipitates forming immediately, making it practically impossible to characterize the reaction in real time. Our study uses trimethoxymethylsilane (TMOMS) and the polyamine polyethyleneimine (PEI) to form PEI-silica particles via a reaction that takes place over several minutes, allowing us to study the reaction in real time. The acidic hydrolysis of TMOMS and basic polymerization condensation of TMOMS via PEI to form solid PEI-silica particles were observed in situ over time using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and dynamic light scattering (DLS) for the first time. The ATR-FTIR data suggest that dimer formation occurs during acidic hydrolysis followed by PEI-catalyzed condensation to form silsesquioxane structures. The results for the particles formed in situ were then compared with those for particle samples that had been washed to remove excess reactants. The ATR-FTIR results were corroborated via scanning electron microscopy and DLS, which suggest that the growth of PEI-silica particles occurs by aggregation of smaller particles to larger ones, because the data show the presence of small particles and much larger particles at the same time throughout the whole particle growth process.

Influence of Shell Thickness on the Colloidal Stability of Magnetic Core-Shell Particle Suspensions

We present a Discrete Element study of the behavior of magnetic core-shell particles in which the properties of the core and the shell are explicitly defined. Particle cores were considered to be made of pure iron and thus possessed ferromagnetic properties, while particle shells were considered to be made of silica. Core sizes ranged between 0.5 and 4.0 μm with the actual particle size of the core-shell particles in the range between 0.6 and 21 μm. The magnetic cores were considered to have a magnetization of one tenth of the saturation magnetization of iron. This study aimed to understand how the thickness of the shell hinders the formation of particle chains. Chain formation was studied with different shell thicknesses and particle sizes in the presence and absence of an electrical double layer force in order to investigate the effect of surface charge density on the magnetic core-shell particle interactions. For core sizes of 0.5 and 4.0 μm the relative shell thicknesses needed to hinder the aggregation process were approximately 0.4 and 0.6 respectively, indicating that larger core sizes are detrimental to be used in applications in which no flocculation is needed. In addition, the presence of an electrical double layer, for values of surface charge density of less than 20 mC/m2, could stop the contact between particles without hindering their vertical alignment. Only when the shell thickness was considerably larger, was the electrical double layer able to contribute to the full disruption of the magnetic flocculation process.