Meike Koenig

Combined QCM-D/GE as a tool to characterize stimuli-responsive swelling of and protein adsorption on polymer brushes grafted onto 3D-nanostructures

A combined setup of quartz crystal microbalance and generalized ellipsometry can be used to comprehensively investigate complex functional coatings comprising stimuli-responsive polymer brushes and 3D nanostructures in a dynamic, noninvasive in situ measurement. While the quartz crystal microbalance detects the overall change in areal mass, for instance, during a swelling or adsorption process, the generalized ellipsometry data can be evaluated in terms of a layered model to distinguish between processes occurring within the intercolumnar space or on top of the anisotropic nanocolumns. Silicon films with anisotropic nanocolumnar morphology were prepared by the glancing angle deposition technique and further functionalized by grafting of poly-(acrylic acid) or poly-(N- isopropylacrylamide) chains. Investigations of the thermoresponsive swelling of the poly-(N-isopropylacrylamide) brush on the Si nanocolumns proved the successful preparation of a stimuli-responsive coating. Furthermore, the potential of these novel coatings in the field of biotechnology was explored by investigation of the adsorption of the model protein bovine serum albumin. Adsorption, retention, and desorption triggered by a change in the pH value is observed using poly-(acrylic acid) functionalized nanostructures, although generalized ellipsometry data revealed that this process occurs only on top of the nanostructures. Poly-(N-isopropylacrylamide) is found to render the nanostructures non-fouling properties.

Adsorption of enzymes to stimuli-responsive polymer brushes: Influence of brush conformation on adsorbed amount and biocatalytic activity.

Polyelectrolyte brushes can be utilized to immobilize enzymes on macroscopic surfaces. This report investigates the influence of the pH value of the surrounding medium on the amount and the activity of enzymes adsorbed to poly(2-vinylpyridine) and poly(acrylic acid) brushes, as well as the creation of thermoresponsive biocatalytically active coatings via the adsorption of enzymes onto a mixed brush consisting of a polyelectrolyte and temperature-sensitive poly(N-isopropylacryl amide). Spectroscopic ellipsometry and attenuated total reflection-Fourier transform infrared spectroscopy are used to monitor the adsorption process. Additionally, infrared spectra are evaluated in terms of the secondary structure of the enzymes. Glucose oxidase is used as a model enzyme, where the enzymatic activity is measured after different adsorption conditions. Poly(acrylic acid) brushes generally adsorb larger amounts of enzyme, while less glucose oxidase is found on poly(2-vinylpyridine), which however exhibits higher specific activity. This difference in activity could be attributed to a difference in secondary structure of the adsorbed enzyme. For glucose oxidase adsorbed to mixed brushes, switching of enzymatic activity between an active state at 20°C and a less active state at 40°C as compared to the free enzyme in solution is observed. However, this switching is strongly depending on pH in mixed brushes of poly(acrylic acid) and poly(N-isopropylacryl amide) due to interactions between the polymers.

Nanocomposite coatings with stimuli-responsive catalytic activity

Stimuli-responsive catalytic coatings are fabricated by in situ-synthesis of metallic nanoparticles in binary PNIPAM–P2VP (poly(N-isopropyl acrylamide)–poly(2-vinyl pyridine)) brushes. The amount of immobilized nanoparticles is found to be controllable by the polymer ratio, since solely P2VP interacts with the nanoparticles. The in situ-synthesis of nanoparticles is studied by attenuated total reflection Fourier transform infrared spectroscopy and atomic force microscopy, whereas the swelling behavior of the polymer films is investigated by spectroscopic ellipsometry and grazing incidence small angle X-ray scattering. Temperature dependent catalytic activity is probed by use of the reduction of nitrophenol to aminophenol as a model reaction. Control of the catalytic activity by the temperature induced deswelling of PNIPAM is observed.

Bioinstructive Coatings for Hematopoietic Stem Cell Expansion Based on Chemical Vapor Deposition Copolymerization

We report the chemical vapor deposition (CVD) of dual-functional polymer films for the specific and orthogonal immobilization of two biomolecules (notch ligand delta-like 1 (DLL1) and an RGD-peptide) that govern the fate of hematopoietic stem and progenitor cells. The composition of the CVD polymer and thus the biomolecule ratio can be tailored to investigate and optimize the influence of the relative surface concentrations of biomolecules on stem cell behavior. Prior to cell experiments, all surfaces were characterized by infrared reflection adsorption spectroscopy, time-of-flight secondary ion mass spectrometry, and X-ray photoelectron spectroscopy to confirm the presence of both biomolecules. In a proof-of-principle stem cell culture study, we show that all polymer surfaces are cytocompatible and that the proliferation of the hematopoietic stem and progenitor cells is predominantly influenced by the surface concentration of immobilized DLL1.

Salt Sensitivity of the Thermoresponsive Behavior of PNIPAAm Brushes

We report investigations on the salt sensitivity of the thermoresponsive behavior of PNIPAAm brushes applying the quartz crystal microbalance coupled with spectroscopic ellipsometry technique. This approach enables a detailed study of the optical and mechanical behavior of the polymer coatings. Additional conclusions can be drawn from the difference between both techniques due to a difference in the contrast mechanism of both methods. A linear shift of the phase-transition temperature to lower temperatures with the addition of sodium chloride was found, similar to the behavior of free polymer chains in solution. The thermal hysteresis was found to be decreased by the addition of sodium chloride to the solution, hinting to the interaction of the ions with the amide groups of the polymer, whereby the formation of hydrogen bonds is hindered. The results of this study are of relevance to the application of PNIPAAm brushes in biological fluids and demonstrate the additional potential of the ion sensitivity besides the better known thermosensitivity.

Vapor-based polymers: From films to nanostructures

While the traditional and popular realm of polymer synthesis is the liquid phase, the use of vapor-based techniques to deposit polymers has been met with increasing interest over the past decades. The perhaps most relevant example, the deposition of poly(p-xylylenes) via the Gorham process, has been of industrial use in the fabrication of isolating or protective coatings in electronics and biomaterials for many years [1,2]. More recently, vapor deposition polymerization has been extended to a broad variety of reactive polymers [3], additionally using techniques such as plasma-, initiated-, or oxidative chemical vapor deposition polymerization [4,5]. The reason for the ongoing interest in this research field is that, analogue to the deposition of inorganic coatings by chemical vapor deposition, the deposition of polymer coatings from the vapor phase has many advantages over traditional, wet chemistry methods. These advantages result in a variety of benefits for vapor-based polymer coatings and their broad utility.

Nanotopographical control of surfaces using chemical vapor deposition processes

In recent years much work has been conducted in order to create patterned and structured polymer coatings using vapor deposition techniques – not only via post-deposition treatment, but also directly during the deposition process. Two-dimensional and three-dimensional structures can be achieved via various vapor deposition strategies, for instance, using masks, exploiting surface properties that lead to spatially selective deposition, via the use of additional porogens or by employing oblique angle polymerization deposition. Here, we provide a concise review of these studies.