Enhanced image contrast in biological second harmonic imaging microscopy (SHIM) has previously been reported via quantitative assessments of forward- to epi-generated signal intensity ratio and by polarization analysis. Here we demonstrate a new form of contrast: the material-specific, wavelength-dependence of epi-generated second harmonic generation (SHG) excitation efficiency, and discriminate collagen and myosin by ratiometric epi-generated SHG images at 920 nm and 860 nm. Collagen shows increased SHG intensity at 920 nm, while little difference is detected between the two for myosin; allowing SHIM to characterize different SHG-generating components within a complex biological sample. We propose that momentum-space mapping of the second-order non-linear structure factor is the source of this contrast and develop a model for the forward and epi-generated SHG wavelength-dependence. Our model demonstrates that even very small changes in the assumed material fibrillar structure can produce large changes in the wavelength-dependency of epi-generated SHG. However, in the case of forward SHG, although the same changes impact upon absolute intensity at a given wavelength, they have very little effect on wavelength-dependency beyond the expected monotonic fall. We also propose that this difference between forward and epi-generated SHG provides an explanation for many of the wavelength-dependency discrepancies in the published literature.

Plasma polymers are often used in applications requiring aqueous immersion; therefore, it is important to understand how this exposure affects the physical and chemical properties of the films. Three different plasma polymer films were deposited at different distances from the electrode, and the film properties were characterized using contact angle, ellipsometry, and x-ray photoelectron spectroscopy. The film behaviors in aqueous solutions were studied via quartz crystal microbalance with dissipation (QCM-D). Exposure to buffer solutions produced significant swelling of the plasma polymerized acrylic acid films, with swelling increasing with distance from the powered electrode, results that could be correlated with changes in film chemistry. Plasma polymerized octadiene and allylamine exhibited little swelling. These films exhibited changes in thickness and contact angle with respect to distance from the electrode, but this had little influence on their behavior in aqueous solution. By combining QCM-D with the more traditional surface chemical analysis techniques, the authors have been able to explore both swelling behavior and the effect that sample position and thus deposition parameters have on film properties and aqueous behavior. This approach gives the authors the basis to define deposition parameters to assist the engineering of thin films for applications such as biosensing and tissue engineering applications where specific chemistries and film behaviors are desired.

The effect of ionic strength and pH on the structure of hydrated plasma polymerized films of allylamine (ppAAm) and acrylic acid (ppAAc) has been analyzed in situ using quartz crystal microbalance with dissipation techniques, electrochemical impedance spectroscopy, ellipsomtery, and X-ray photoelectron spectroscopy. Both materials showed a salt concentration and pH dependent uptake and release of water and ions. Depending on the type of monomer used, the effects showed reversible or non-reversible behavior. The investigation of the electrochemical properties of the film further revealed a non-homogeneous structure, especially in the case of ppAAc films, with regions of higher and lower cross-linking density. The use of complimentary techniques to characterize the films in situ allowed for a deeper understanding of processes happening inside the plasma polymerized films, which can help to optimize film preparation conditions for selected applications.

Model lipid vesicle and supported lipid bilayer (SLB) systems are used in a variety of applications including biosensing, cell membrane mimics, and drug delivery. Exposure of a surface to a vesicle solution provides a straightforward method for creating such systems via vesicle adsorption and collapse. However, this process is complex and the relationship between the surface physicochemical properties and vesicle collapse is poorly understood. Plasma polymers are thin conformal films that can be applied to a variety of materials to modify surface properties. This paper uses quartz crystal microbalance with dissipation and fluorescence recovery after photobleaching (FRAP) to explore lipid vesicle interactions with plasma polymerized acrylic acid (ppAAc), allylamine (ppAAm), and ppAAc/ppAAm micropatterns. Vesicle interactions were dependent on plasma polymer chemistry and pH of the buffer solution. Vesicles readily and stably adsorbed to ppAAm over a wide pH range. ppAAc demonstrated limited interactions at pH 7 and vesicle adsorption at pH 4. Vesicle collapse and SLB formation could be induced using a pH change. FRAP was used to explore the fluidity of the lipid structures on both the patterned and unpatterned plasma polymer films. On ppAAm/ppAAc micropatterns, pH transitions combined with the presence of chemically distinct regions on the same substrate enabled immobile lipid islands on ppAAc to be surrounded by fluid lipid regions on ppAAm. This work demonstrates that plasma polymer films could enable spatially controlled vesicle adsorption and SLB formation on a wide variety of different substrates.