Heterogeneous Rate Constant for Amorphous Silica Nanoparticle Adsorption on Phospholipid Monolayers.

The interaction of amorphous silica nanoparticles with phospholipid monolayers and bilayers has received a great deal of interest in recent years and is of importance for assessing potential cellular toxicity of such species, whether natural or synthesized for the purpose of nanomedical drug delivery and other applications. This present communication studies the rate of silica nanoparticle adsorption on to phospholipid monolayers in order to extract a heterogeneous rate constant from the data. This rate constant relates to the initial rate of growth of an adsorbed layer of nanoparticles as SiO2 on a unit area of the monolayer surface from unit concentration in dispersion. Experiments were carried out using the system of dioleoyl phosphatidylcholine (DOPC) monolayers deposited on Pt/Hg electrodes in a flow cell. Additional studies were carried out on the interaction of soluble silica with these layers. Results show that the rate constant is effectively constant with respect to silica nanoparticle size. This is interpreted as indicating that the interaction of hydrated SiO2 molecular species with phospholipid polar groups is the molecular initiating event (MIE) defined as the initial interaction of the silica particle surface with the phospholipid layer surface promoting the adsorption of silica nanoparticles on DOPC. The conclusion is consistent with the observed significant interaction of soluble SiO2 with the DOPC layer and the established properties of the silica-water interface.

Significance of particle size and charge capacity in TiO2 nanoparticle-lipid interactions.

HYPOTHESIS: The activity of submicron sized titanium oxide (TiO2) particles towards biomembrane models is coupled to their charge carrying capacity and their primary particle size. EXPERIMENTS: Electrochemical methods using a phospholipid layer on mercury (Hg) membrane model have been used to determine the phospholipid monolayer activity of TiO2 as an indicator of biomembrane activity. The particles were characterised for size, by dynamic light scattering (DLS) and scanning electron microscopy (SEM), and for charge, by acid-base titration. FINDINGS: TiO2 nanoparticles aggregate in 0.1moldm(-3) solutions of KCl. The charge capacity of TiO2 nanoparticles depends on their primary particle size and is unaffected by aggregation. TiO2 particles of ∼40nm primary particle size interact significantly with phospholipid layers. Aggregation of these particles initially has a small effect on this interaction but long term aggregation influences the interaction whereby the aggregates penetrate the lipid layer rather than adsorbing on the surface. Fulvic acid does not inhibit the ∼40nm particle/phospholipid interaction. P25 TiO2 particles of larger particle size interact less strongly with phospholipid layers and the interaction is alleviated following particle aggregation. The semiconductor properties of TiO2 are evident in voltammograms showing electron transfer to TiO2 adsorbed on uncoated Hg.

ZnO nanoparticle interactions with phospholipid monolayers.

Aqueous ZnO nanoparticle dispersions interaction with a dioleoyl phosphatidylcholine (DOPC) monolayer is reported in this paper. ZnO-DOPC interactions were investigated using rapid cyclic voltammetry (RCV) by focusing on the effect of the interactions on the characteristics of the capacitance current peaks representing two potential induced phase transitions. Results showed: - (1) The order of interaction of common commercially sourced nanoparticles with DOPC coated Hg electrodes was NanoTek>NanoShield>metals basis. This extent of interaction was inversely related to the ZnO particle size where the metals basis nanoparticles were strongly aggregated. The contribution of the non-ionic dispersant added by manufacturer to the NanoTek and NanoShield interaction was uncertain. (2) Freshly prepared aqueous Nanosun ZnO nanoparticle (~25 nm) dispersions interacted with and penetrated DOPC coated Hg electrodes. Aggregation of the nanoparticles, coating of the ZnO with phosphate and coating of the ZnO with fulvic acid minimised ZnO-DOPC interaction. (3) In-house synthesised ZnO nanoparticles of lower primary particle size (~6 nm) than Nanosun ZnO nanoparticles interacted strongly with DOPC coated Hg electrodes with no evidence of penetration of the nanoparticle in the DOPC monolayer. Even after considerable aggregation of the particle to between 1 and 10 µm, a strong interaction of the in-house synthesised ZnO with DOPC was observed.

Chemical modification and immobilisation of laccase from Trametes hirsuta and from Myceliophthora thermophila.

Laccase from two different source organisms, Myceliophthora thermophila and Trametes hirsuta, were subjected to chemical modification in solution by (i) two bifunctional reagents, ethylene-glycol-N-hydroxy succinimide (EGNHS) and glutaraldehyde and (ii) by the monofunctional citraconic anhydride. The untreated and chemically modified forms of both enzymes were then immobilised onto three different types of mesoporous silicate (MPS) particle (MCM, CNS and SBA-15). Thermal stabilities of native, modified-soluble and immobilised laccases were then evaluated. Although the two laccases have similar lysine contents, those of M. thermophila are clearly more amenable to chemical modification. Treatment of the M. thermophila enzyme with EGNHS led to a 8.7-fold increase in thermal stability over the free soluble enzyme while glutaraldehyde gave a 5.7-fold increase. Increased activity of M. thermophila laccase occurred only with citraconic anhydride modification (a 3-fold increase), while the glutaraldehyde modification marginally increased the activity of the T. hirsuta enzyme (by 1.2-fold). Upon immobilisation onto MPS, the greatest increase in stability was for the glutaraldehyde-treated M. thermophila preparation on SBA-15 (24-fold over the soluble enzyme). Chemical modification of laccase from T. hirsuta with both glutaraldehyde and EGNHS gave only a 2-fold increase in stability, increasing >4-fold upon immobilisation onto SBA-15 and MCM-41/98.