Hyperthermal Intact Molecular Ions Play Key Role in Retention of ATRP Surface Initiation Capability of Plasma Polymer Films from Ethyl α-Bromoisobutyrate

TrapREMI: A reaction microscope inside an electrostatic ion beam trap

A new experimental setup has been developed to investigate the reactions of molecular ions and charged clusters with a variety of projectile beams. An Electrostatic Ion Beam Trap (EIBT) stores fast ions at keV energies in an oscillatory motion. By crossing it with a projectile beam, e.g., an IR laser, molecular reactions can be induced. We implemented a Reaction Microscope (REMI) in the field-free region of the EIBT to perform coincidence spectroscopy on the resulting reaction products. In contrast to prior experiments, this unique combination of techniques allows us to measure the 3D momentum-vectors of ions, electrons, and neutrals as reaction products in coincidence. At the same time, the EIBT allows for advanced target preparation techniques, e.g., relaxation of hot molecules during storage times of up to seconds, autoresonance cooling, and recycling of target species, which are difficult to prepare. Otherwise, the TrapREMI setup can be connected to a variety of projectile sources, e.g., atomic gas jets, large-scale radiation facilities, and ultrashort laser pulses, which enable even time-resolved studies. Here, we describe the setup and a first photodissociation experiment on H₂ ⁺, which demonstrates the ion-neutral coincidence detection in the TrapREMI.

Functional nanothin films plasma-deposited from 2-isopropenyl-2-oxazoline for biosensor applications

Plasma polymers derived from oxazoline precursors present a range of versatile properties that is fueling their use as biomaterials. However, coatings deposited from commonly used methyl and ethyl oxazoline precursors can be sensitive to the plasma deposition conditions. In this work, we used various spectroscopic methods (ellipsometry, x-ray photoelectron spectroscopy, and time of flight secondary ion mass spectrometry) and cell viability assays to evaluate the transferability of deposition conditions from the original plasma reactor developed by Griesser to a new wider, reactor designed for upscaled biosensors applications. The physicochemical properties, reactivity, and biocompatibility of films deposited from 2-isopropenyl-2-oxazoline were investigated. Thanks to the availability of an unsaturated pendant group, the coatings obtained from this oxazoline precursor are more stable and reproducible over a range of deposition conditions while retaining reactivity toward ligands and biomolecules. This study identified films deposited at 20 W and 0.012 mbar working pressure as being the best suited for biosensor applications.

To be a radical or not to be one? The fate of the stable nitroxide radical TEMPO [(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl] undergoing plasma polymerization into thin-film coatings

The stable nitroxide radical TEMPO [(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl] has a multitude of applications in fields ranging from energy storage to biomedical applications and many more. However, to date, the processes of incorporating nitroxide radicals into thin-film coatings are laborious and not cost-effective, which hinders their wider use in many applications. In contrast, the authors have recently demonstrated the facile method of plasma polymerization of TEMPO into thin-film coatings that retain the stable nitroxide radicals. In this work, we are using three types of mass spectroscopic methods (plasma-mass spectrometry, time of flight secondary ion mass spectrometry, and high-performance liquid chromatography-mass spectrometry) and electron spin resonance to track the fate of the TEMPO molecule from monomer flask through the plasma and inside the resulting coatings. The results of this study demonstrate that TEMPO is a versatile monomer that can be used across different plasma reactors and reliably retain the stable nitroxide radical in the resulting thin-film coatings if certain process conditions are observed, namely, higher process pressures and lower powers.

The Physics of Plasma Ion Chemistry: A Case Study of Plasma Polymerization of Ethyl Acetate

Deposition chemistry from plasma is highly dependent on both the chemistry of the ions arriving at surfaces and the ion energy. Typically, when measuring the energy distribution of ions arriving at surfaces from plasma, it is assumed that the distributions are the same for all ionic species. Using ethyl acetate as a representative organic precursor molecule, we have measured the ion chemistry and ion energy as a function of pressure and power. We show that at low pressure (<2 Pa) this assumption is valid; however, at elevated pressures ion-molecule collisions close to the deposition surface affect both the energy and chemistry of these ions. Smaller ions are formed close to the surface and have lower energy than larger ionic species which are formed in the bulk of the plasma. The changes in plasma chemistry therefore are closely linked to the physics of the plasma-surface interface.

Highly-reactive haloester surface initiators for ARGET ATRP readily prepared by radio frequency glow discharge plasma

New surface initiators for ARGET ATRP (activators regenerated by electron transfer atomic transfer radical polymerization) have been prepared by the plasma deposition of haloester monomers. Specifically, methyl 3-bromopropionate (M3BP), methyl 2-chloropropionate, and ethyl 2-fluoropropionate (E2FP) were plasma deposited onto glass discs using RF glow discharge plasma. This technique creates surface coatings that are resistant to delamination and rich in halogen species making them good candidates for surface initiators for ARGET ATRP. Of all the plasma polymerized surface coatings, M3BP showed the highest halogen content and was able to grow 2-hydroxyethyl methacrylate (HEMA) polymer brushes on its surface via ARGET ATRP in as little as 15 min as confirmed by XPS. Surprisingly, E2FP, a fluoroester, was also able to grow HEMA polymer brushes despite fluorine being a poor leaving group for ARGET ATRP. The versatility of RF glow discharge plasma offers a clear advantage over other techniques previously used to immobilize ARGET ATRP surface initiators.

Effective Synthesis of Patterned Polymer Brushes with Tailored Multiple Graft Densities

This paper reports an effective method to prepare patterned polymer brushes on surfaces with tailored graft densities. High-density (concentrated), moderate-density (semidiluted), and low-density (diluted) polymer brushes were fabricated in patterned manners, offering defined three-dimensional patterned structures. This method uses a middle/near-UV (≥250 nm) lamp and needs only a short time (≤10 min) to fabricate prepatterns of the initiator, in sharp contrast to the previous high-energy lithography and time-consuming processes. The obtained patterned brush served as a molecular (protein) repellent/adsorptive interface based on a graft-density-dependent size-exclusion effect. This method is facile and accessible to wide ranges of tunable density and pattern shapes, which are attractive for extensive use.

Substrate-Independent Micropatterning of Polymer Brushes Based on Photolytic Deactivation of Chemical Vapor Deposition Based Surface-Initiated Atom-Transfer Radical Polymerization Initiator Films

Precise microscale arrangement of biomolecules and cells is essential for tissue engineering, microarray development, diagnostic sensors, and fundamental research in the biosciences. Biofunctional polymer brushes have attracted broad interest in these applications. However, patterning approaches to creating microstructured biointerfaces based on polymer brushes often involve tedious, expensive, and complicated procedures that are specifically designed for model substrates. We report a substrate-independent, facile, and scalable technique with which to prepare micropatterned biofunctional brushes with the ability to generate binary chemical patterns. Employing chemical vapor deposition (CVD) polymerization, a functionalized polymer coating decorated with 2-bromoisobutyryl groups that act as atom-transfer radical polymerization (ATRP) initiators was prepared and subsequently modified using UV light. The exposure of 2-bromoisobutyryl groups to UV light with wavelengths between 187 and 254 nm resulted in selective debromination, effectively eliminating the initiation of ATRP. In addition, when coatings incorporating both 2-bromoisobutyryl and primary amine groups were irradiated with UV light, the amines retained their functionality after UV treatment and could be conjugated to activated esters, facilitating binary chemical patterns. In contrast, polymer brushes were selectively grown from areas protected from UV treatment, as confirmed by atomic force microscopy, time-of-flight secondary ion mass spectrometry, and imaging ellipsometry. Furthermore, spatial control over biomolecular adhesion was achieved in three ways: (1) patterned nonfouling brushes resulted in nonspecific protein adsorption to areas not covered with polymer brushes; (2) patterned brushes decorated with active binding sides gave rise to specific protein immobilization on areas presenting polymer brushes; (3) and primary amines were co-patterned along with clickable polymer brushes bearing pendant alkyne groups, leading to bifunctional reactivity. Because this novel technique is independent of the original substrate's physicochemical properties, it can be extended to technologically relevant substrates such as polystyrene, polydimethylsiloxane, polyvinyl chloride, and steel. With further work, the photolytic deactivation of CVD-based initiator coatings promises to advance the utility of patterned biofunctional polymer brushes across a spectrum of biomedical applications.

Structure and Stability of C:H:O Plasma Polymer Films Co-Polymerized Using Dimethyl Carbonate

C:H:O plasma polymer films (PPFs) were deposited by means of plasma-enhanced chemical vapour deposition using the non-toxic, biodegradable organic compound dimethyl carbonate (DMC) at various plasma powers and pressures in order to control the degradation properties related to the carbonate ester group. Coating properties using pure DMC monomer vapours were compared to co-polymerized films from gaseous mixtures of DMC with either ethylene (C2H4) or carbon dioxide (CO2) affecting deposition rate and chemical composition. C:H:O film properties were found to depend primarily on the amount of oxygen in the plasma. To investigate the PPF stability during aging, changes in the composition and properties were studied during their storage both in air and in distilled water over extended periods up to 5 months. It was shown that aging of the films is mostly due to oxidation of the plasma polymer matrix yielding slow degradation and decomposition. The aging processes and their rate are dependent on the intrinsic amount of oxygen in the as-prepared C:H:O films which in turn depends on the experimental conditions and the working gas mixture. Adjustable film properties were mainly attained using a pure DMC plasma considering both gas phase and surface processes. It is thus possible to prepare C:H:O PPFs with controllable degradability both in air and in water.

Promiscuous hydrogen in polymerising plasmas

Historically, there have been two opposing views regarding deposition mechanisms in plasma polymerisation, radical growth and direct ion deposition, with neither being able to fully explain the chemistry of the resultant coating. Deposition rate and film chemistry are dependent on the chemistry of the plasma phase and thus the activation mechanisms of species in the plasma are critical to understanding the relative contributions of various chemical and physical routes to plasma polymer formation. In this study, we investigate the roles that hydrogen plays in activating and deactivating reactive plasma species. Ethyl trimethylacetate (ETMA) is used as a representative organic precursor, and additional hydrogen is added to the plasma in the form of water and deuterium oxide. Optical emission spectroscopy confirms that atomic hydrogen is abundant in the plasma. Comparison of the plasma phase mass spectra of ETMA/H₂O and ETMA/D₂O reveals that (1) proton transfer from hydronium is a common route to charging precursors in plasma, and (2) hydrogen abstraction (activation) and recombination (deactivation) processes are much more dynamic in the plasma than previously thought. Consideration of the roles of hydrogen in plasma chemistry may then provide a more comprehensive view of deposition processes and bridge the divide between the two disparate schools of thought.

Synthesis of highly functionalised plasma polymer films from protonated precursor ions via the plasma α–γ transition

Functional group retention in plasma polymers is maximised by tuning the pressure/power to the α to γ transition.