SIMS2015 Session LS-ThP: Life Science and Biotechnology Poster Session

Characterization of protein adsorption from an aqueous solution onto a surface is an important element of research in surface science, from both fundamental and practical perspectives. The adsorption of human serum albumin (HSA) and lactoferrin (LF) onto Fe(II) and Cu(II) terpyridine-based complexes self-assembled on a gold surface has been investigated. This contribution deals with an integrated characterization approach aimed to obtain detailed insights into the nature of the surface-induced conformational changes in co-adsorbed proteins. This approach involves time-of-flight secondary ion mass spectrometry measurements coupled with multivariate data treatment and parallel experiments using quartz crystal microbalance with dissipation monitoring (QCM-D). In particular, Principal Component Analysis (PCA) was used to identify similarities and differences in TOF-SIMS spectra of protein-based films and to classify the spectra into groups. PCA results shown that single-protein films can certainly be discriminated. Films prepared by mixed-component solutions revealed peculiar positions in the scores plot. The ToF-SIMS spectra of proteins adsorbed onto Fe(II) complex were clustered close to the HSA single-component layer. Similarly, films prepared on Cu(II) have surface mass spectra similar to that of the FN single-component film. On the other hand, QCM-D studies show that HSA exhibits a strong affinity with the Cu(II)-containing self assembled monolayer, whereas LF interacts much strongly with Fe(II)-based SAMs rather than with HSA. Consequently, we conclude that the PCA results on TOFSIMS data can be interpreted in terms of competitive multilayer adsorption. In other words, when allowing a mixed-component (HSA/FN) protein to interact with the metal complex monolayer, HSA competitively interacts with Cu(II)-based complexes and adsorbs on them. This stage is followed by the subsequent adsorption of FN on top of the HSA layer. By contrast, the low interaction of HSA with Fe(II) complex favours the formation of a first layer of LF, followed by the adsorption of HSA on top of it.

Storage of protein modified surfaces in air can become necessary for several applications, e.g. during the implantation of biomedical devices or for sensor applications. Protein coatings are very sensitive to dehydration and can undergo significant and irreversible alterations of their conformations if exposed to air. Xia et al. previously investigated the stabilizing effects of trehalose on protein self-assembled monolayers (SAMs) (N. Xia, C.J. May, S.L. McArthur, D.G. Castner, Langmuir 2002, 18, 4090) and could retain the protein activity for 2h.

With the use of compatible solutes from extremophilic bacteria, ectoine and hydroxyectoine, we were able to extend the lifetime of dried protein SAMs up to 24h. Horseradish peroxidase immobilized on compact TiO₂ was used as a model system. Structural differences between compatible solute stabilized and unstabilized protein films were analyzed with static time-of-flight secondary ion mass spectrometry (ToF-SIMS). The biological activity difference observed in a colorimetric activity assay was correlated to changes in protein conformation by application of principle component analysis (PCA) to the static ToF-SIMS spectra.

Furthermore, the effect of nanostructured TiO₂ in the form of TiO₂-nanotubes on the protein SAM lifetime was investigated without and with the addition of compatible solutes. In the nanotubes the protein coatings could be kept active for several days, as a complete dehydration seems to be prevented by the nanostructure.

Thin film coatings are widely used in medical devices in order to improve the biological response to the device without compromising its mechanical performance. These coatings are frequently organic in nature and are applied to a wide range of substrate materials. The challenge of ensuring a stable linkage between the coating and the underlying substrate is common to all of these systems. Defects at the interface between the coating and the substrate can result in failure of the medical device with potentially serious consequences. The study of buried interfaces in organic systems, like those common in medical devices, has in the past been a nearly intractable problem because sputter depth profiling with monatomic ions destroys the relevant chemical information. Recent advances in Gas Cluster Ion Beam technology have opened up exciting possibilities to better understand these buried interfaces. In this work, we have studied adhesion between an bacterial-biofilm resistant polymer coating and an oxygen plasma-treated polymer surface using Argon Cluster 3D-imaging Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Raman Microscopy. Analysis has been performed in both dry and hydrated state. The analysis provided several analytical challenges. Because the overlayer was not of uniform thickness, a depth scale correction was needed to reduce misleading artefacts at the interface. Analysis of the hydrated catheters required cryogenic analysis conditions. From the ToF-SIMS data we have been able to observe the presence of particles, cracks and water, and to monitor hydrophobic recovery at the interface between the coating and the catheter. Raman analysis has provided complementary information on the Van-Der-Waal interactions at the interface. The results have been compared to mechanical adhesion tests and help to provide a better understanding of the processes that influence adhesion between the coating and the catheter.