Alex G. Shard

Plasma deposited metal Schiff-base compounds as antimicrobials

This paper details the synthesis, characterisation including crystal structure, and testing for antimicrobial efficacy of two compounds: a zinc centred bis(N-allylsalicylideneiminato)-zinc (ZSB) and its known copper analogue, bis(N-allylsalicylideneiminato)-copper (CSB). Differences in antimicrobial efficacy of the two compounds were observed, suggesting possible mechanisms for antimicrobial activity. The ZSB system was plasma deposited under various pulse conditions onto non-woven fabric, and the antimicrobial efficacy of the resultant film measured for Staphylococcus aureus and Pseudomonas aeruginosa. These results suggest the potential utility of this compound as an effective antimicrobial thin film, and confirm the critical role the ligands play in effecting antimicrobial activity.

Analysis of protein coatings on gold nanoparticles by XPS and liquid-based particle sizing techniques

The precise use of nanoparticles in technological applications requires control over their surface properties. This implies the ability to quantitatively describe, for example, molecular coatings in terms of their thickness, areal mass, or number of molecules. Here, the authors describe two different approaches to the measurement of these parameters by using gold nanoparticles ranging in diameter from 10 to 80u2009nm and coated with three different proteins: immunoglobulin G, bovine serum albumin, and a peptide. One approach utilizes ultraviolet-visible spectroscopy, dynamic light scattering, and differential centrifugal sedimentation to measure the protein shell refractive indices and thicknesses, from which the number of molecules in the protein shell can be derived. The other approach employs x-ray photoelectron spectroscopy to measure the thickness of the dry molecular coatings and also to derive the number of molecules in the protein shell. The authors demonstrate that the two approaches, although very different, produce consistent measurement results. This finding is important to extend the quantitative analysis of nanoparticle molecular coatings to a wide range of materials.

Chemical and biological characterisation of a sensor surface for bioprocess monitoring.

This paper describes the step-wise fabrication and characterisation of a multi-layer dual polarization interferometry (DPI) based biosensor utilising Protein G (ProG) as the bio-recognition layer for the detection of a fragment antibody (Fab). The biosensor is capable of monitoring the concentration of Fab product within the extracellular medium of a fed-batch fermentation after leakage from Escherichia coli (E.coli). The activity, stability and functionality of each sensor layer were analysed in situ using DPI, whilst the chemical identity and homogeneity of the chemical layers were assessed ex situ using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Two different biotin linkers were found to produce hugely differing surfaces after the capture of NeutrAvidin™ (NA) and biotinylated Protein G (b-ProG). The hydrophilic (PEG)(4)-biotin linker resulted in a surface where the b-ProG layer was deposited and organised above the NA layer producing an active and stable surface, whilst the hydrophobic LC-biotin linker generated a surface where the b-ProG layer was buried within the NA layer leading to variable surfaces and poor binding of the Fab target. The biosensor has a detection limit of 1.7 μg/ml with a dynamic range covering two orders of magnitude. The sensor can detect the onset of Fab leakage as early as 2h following product induction, with high signal-to-noise ratios and little interference from extracellular components. Leakage of Fab followed a biphasic profile, switching to a more rapid rate 20 h after induction, indicating accelerated product loss and the need for cultivation harvest.

Surface modification of PDMS via self-organization of vinyl-terminated small molecules

Polydimethylsiloxane (PDMS) elastomers are widely used in soft lithography, microfluidics and biomedical applications as they combine a range of desirable chemical and physical properties. We studied the surface of PDMS modified using surface enrichment with long-chain alkenes. These alkenes were immobilized during crosslinking of the PDMS pre-polymer mixture cast against a template of matching surface energy. A range of different functional groups was introduced, including perfluorinated and oligoethylene glycol groups, alkyl chains and initiators for controlled radical polymerization. A detailed analysis of the composition of the modified PDMS surfaces was carried out using contact angle measurements, AFM, SIMS and XPS. The results demonstrate that the PDMS was enriched with small molecules near the surface and show that the functional molecules follow the template surface energy on patterned surfaces with edge resolutions equal to the template. By introducing hydrophilic alkenes receding contact angles on PDMS can be lowered below 5°, and we investigated how these surfaces rearrange in air due to the chain mobility of the PDMS backbone and PDMS short-chain fragments.

Quantification of variable functional-group densities of mixed-silane monolayers on surfaces via a dual-mode fluorescence and XPS label.

The preparation of aminated monolayers with a controlled density of functional groups on silica surfaces through a simple vapor deposition process employing different ratios of two suitable monoalkoxysilanes, (3-aminopropyl)diisopropylethoxysilane (APDIPES) and (3-cyanopropyl)dimethylmethoxysilane (CPDMMS), and advances in the reliable quantification of such tailored surfaces are presented here. The one-step codeposition process was carried out with binary silane mixtures, rendering possible the control over a wide range of densities in a single step. In particular, APDIPES constitutes the functional silane and CPDMMS the inert component. The procedure requires only small amounts of silanes, several ratios can be produced in a single batch, the deposition can be carried out within a few hours and a dry atmosphere can easily be employed, limiting self-condensation of the silanes. Characterization of the ratio of silanes actually bound to the surface can then be performed in a facile manner through contact angle measurements using the Cassie equation. The reliable estimation of the number of surface functional groups was approached with a dual-mode BODIPY-type fluorescence label, which allows quantification by fluorescence and XPS on one and the same sample. We found that fluorescence and XPS signals correlate over at least 1 order of magnitude, allowing for a direct linking of quantitative fluorescence analysis to XPS quantification. Employment of synchrotron-based methods (XPS; reference-free total reflection X-ray fluorescence, TXRF) made the traceable quantification of surface functional groups possible, providing an absolute reference for quantitative fluorescence measurements through a traceable measurement chain.

Depth resolution, angle dependence, and the sputtering yield of Irganox 1010 by coronene primary ions.

A study is reported of the depth resolution and angle dependence of sputtering yields using the reference organic material, Irganox 1010, for a new coronene(+) depth profiling ion source at 8 and 16 keV beam energies. This source provides excellent depth profiles as shown by 8.5 nm marker layers of Irganox 3114. Damage occurs but may be ignored for angles of incidence above 70° from the surface normal, as shown by X-ray photoelectron spectroscopy (XPS) of the C 1s peak structure. Above 70°, XPS profiles of excellent depth resolution are obtained. The depth resolution, after removal of the thickness of the delta layers, shows a basic contribution of 5.7 nm together with a contribution of 0.043 times the depth sputtered. This is lower than generally reported for cluster sources. The coronene(+) source is thus found to be a useful and practical source for depth profiling organic materials. The angle dependencies of both the undamaged and damaged materials are described by a simple equation. The sputtering yields for the undamaged material are described by a universal equation and are consistent with those obtained for C60(+) sputtering. Comparison with the sputtering yields using an argon gas cluster ion source shows great similarities, but the yields for both the coronene(+) and C60(+) primary ion sources are slightly lower.

Exposure of mass-selected bimetallic Pt–Ti nanoalloys to oxygen explored using scanning transmission electron microscopy and density functional theory

The response of nanoparticles to exposure to ambient conditions and especially oxidation is fundamental to the application of nanotechnology. Bimetallic platinum–titanium nanoparticles of selected mass, 30 kDa and 90 kDa, were produced using a magnetron sputtering gas condensation cluster source and deposited onto amorphous carbon TEM grids. The nanoparticles were analysed with a Cs-corrected Scanning Transmission Electron Microscope (STEM) in High Angle Annular Dark Field (HAADF) mode. It was observed that prior to full Ti oxidation, Pt atoms were dispersed within a Ti shell. However, after full oxidation by prolonged exposure to ambient conditions prior to STEM, the smaller size 30 kDa particles form a single Pt core and the larger size 90 kDa particles exhibit a multi-core structure. Electron beam annealing induced a single core morphology in the larger particles. First principles density functional theory (DFT) calculations were employed to calculate the lowest energy structure of the Pt–Ti nanoparticles with and without the presence of oxygen. It was demonstrated that, as the concentration of oxygen increases, the lowest energy structure changes from dispersed Pt to multiple Pt cores and finally a single Pt core, which is in good agreement with the experimental observations.

Measuring the size and density of nanoparticles by centrifugal sedimentation and flotation

The successful translation of nanoparticle-based systems into commercial products depends upon the ability to reliably measure important physical and chemical properties of these particles. The density of nanoparticles is one such property, because it provides important information about the composition of the material. In this work, an analytical centrifugation approach based on line-start centrifugal sedimentation and flotation measurements is described. The two independent measurements permit both the size and the density of these nanoparticles to be determined with excellent precision. A set of monodisperse polystyrene nanoparticles of different sizes is used to demonstrate this method. The density and size measurements are validated by comparison to accurate Small Angle X-ray Scattering (SAXS) analysis for particles within the size range of SAXS, i.e. less than ∼300 nm in diameter. Both sedimentation and flotation measurements produce consistent high resolution size distributions of the particles and the measured size and density values are identical, within experimental uncertainty, to the SAXS results. This approach has the potential to provide useful characterisation of a range of particles of interest, for example, for medical application, such as liposomes and polymeric drug carriers.