To F-SIMS/XPS chemical depth profiling of synthetic polymer hydrogels

Taylor, Michael (2017) To F-SIMS/XPS chemical depth profiling of synthetic polymer hydrogels. PhD thesis, University of Nottingham.

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Over the last decade the beneficial properties of hydrogels as artificial cell culture supports have been extensively investigated. Certain synthetic hydrogels have been proposed to be similar in composition and structure to the native extracellular matrix of the stem cell niche, their in vivo cell habitat, which is a powerful component in controlling stem cell fate. The stem cell differentiation pathway taken is influenced by many factors. When culturing cells within or upon hydrogels the choice can be strongly dependent on the underlying 3D hydrogel chemistry which strongly influences hydrogel-cell interactions.

The interrelationship between hydrogel chemistry and that of biomolecules in controlling cellular response ideally requires analysis methods to characterise the chemistry without labels and normally in 3D. Time-of-flight secondary ion mass spectrometry (ToF SIMS) has the potential to be utilised for through thickness characterisation of hydrogels. The frozen-hydrated sample format is well suited to minimise changes associated with dehydration or the complication of chemical ‘fixation’. There is however significant challenges associated with this sample format. Frost formation occurs on cold samples in the ambient atmosphere affecting the quality of chemical information acquired from depth profiling frozen hydrogel samples.

We develop a simple method to remove this frost by blowing with gas prior to entry into the instrument which is shown to produce remarkably good profiles on a poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel film where a model protein, lysozyme, is incorporated to demonstrated how biomolecule distribution within hydrogels can be determined. A comparison of lysozyme incorporation is made between the situation where the protein is present in the polymer dip coating solution and lysozyme is a component of the incubation medium. It is shown that protonated water clusters H(H2O)n+ where n=5-11 that are indicative of ice are detected through the entire thickness of the pHEMA and the lysozyme distribution through the pHEMA hydrogel films can be determined using the intensity of characteristic fragment secondary ions.

Quantitative TOF-SIMS analysis is highly desirable in biomaterial analysis as the amount and type of molecule in the material analysed may be determined.

This has significant interest in hydrogel chemical analysis as cellular development on or within hydrogels may be highly influenced by the concentration and type of soluble molecule. Unfortunately, the matrix effect in SIMS changes the measured signal intensity of the analyte, preventing accurate quantitation.

For this reason, we apply X-Ray Photoelectron Spectroscopy (XPS) on the equivalent samples as the ToF-SIMS in an attempt to correlate molecular ion yields to exact elemental concentrations. Similarly to ToF-SIMS the frozen-hydrated format in XPS is however still relatively unexplored. We apply the developed preparatory procedure in 3D XPS analysis of pHEMA/lysozyme hydrogel films in a hydrated state. We show that this format is suitable for alternative high vacuum analysis techniques. Furthermore, we show that lysozyme ingression and concentration can be determined through XPS.

This work describes the first example of the characterisation of a hydrogel by ToF-SIMS and XPS in a frozen hydrated format, characterising hydrogels in a format most reflecting its native hydrated state at culture conditions.

The described procedure allows for the mapping of biomolecules in a label free manner in 3D, furthermore allowing quantitative determination of biomolecule concentrations in hydrogels.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Alexander, Morgan
Zelzer, Mischa
Buttery, Lee
Keywords: Hydrogel, ToF-SIMS, XPS, Frozen-Hydrated
Subjects: Q Science > QD Chemistry > QD450 Physical and theoretical chemistry
R Medicine > R Medicine (General) > R855 Medical technology. Biomedical engineering. Electronics
Faculties/Schools: UK Campuses > Faculty of Science > School of Pharmacy
Item ID: 38755
Depositing User: Taylor, Michael
Date Deposited: 17 Jul 2017 04:40
Last Modified: 13 Oct 2017 17:19

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