Multipotent Polymer Coatings Based on Chemical Vapor Deposition Copolymerization

The controlled and stable immobilization of one or multiple types of (bio)molecules to a surface has been identified as one of the critical challenges in several emerging research fields, such as the regulation of cell shape, the development of advanced biological assays and scaffolds for regenerative medicine, or the fabrication of increasingly complex micrototal analytical systems (lTAS). This is partly motivated by the need for defined surface architectures to simultaneously present multiple biological entities in controlled ratios. While a range of methods have been developed for the immobilization of a single type of biomolecule to artificial substrates, fewer concepts are available for the precise immobilization of multiple biomolecules in a controllable fashion. Recently, we established a suite of surface modification techniques based on chemical vapor deposition (CVD) polymerization of substituted [2.2]paracyclophanes. This novel coating technology resulted in a diverse class of functionalized poly p-xylylenes containing a wide variety of functional groups such as amines, esters, aldehydes, and alcohols. The resulting polymers provide a flexible solution to surface-engineering challenges, as they decouple surface design from bulk properties. In essence, CVD technology is a one-step coating procedure that generates functionalized surfaces without the need for further post-treatments once the films are deposited. In a significant extension of our previous work, we now report on the use of CVD copolymerization to fabricate multipotent and modular coatings. The term “multipotent coating” in this context, refers to a reactive coating that is compatible with the simultaneous presentation of multiple biomolecules in controllable ratios. Prior to CVD copolymerization, 4-trifluoroacetyl [2.2]paracyclophane (1) and 4-aminomethyl-[2.2]paracyclophane (2) were synthesized from commercially available [2.2]paracyclophane following established synthetic routes. CVD copolymerization of 1 and 2 was then conducted and resulted in a vacuum-deposited film of copolymer 3 on the substrate (Scheme 1). For CVD copolymerization, mixtures of carefully purified dimers 1 and 2 were initially sublimated under a reduced pressure of 56 Pa at 90–100 °C. The sublimation temperatures of 1 and 2 were sufficiently similar, and so to ensure that the compounds were exposed to comparable sublimation conditions they were placed near each other within the CVD system. Sublimated 1 and 2 were then transferred to the pyrolysis zone, which was heated to 670 °C to ensure cleavage of the aliphatic C–C bonds, resulting in the corresponding quinodimethanes (monomers). In the last step, monomers were adsorbed on the substrate at approximately 10 °C and spontaneously polymerized. CVD copolymerization of 1 and 2 resulted in transparent and topologically uniform polymer films (Figure S2, Supporting Information) with thicknesses between 50 and 200 nm. The film thickness is mainly determined by the total amount of [2.2]paracyclophanes used for polymerization. For instance, the thickness of a film produced by the deposition of 20 mg of equimolar amounts of 1 and 2 was determined by means of imaging ellipsometry to be 115 ± 15 nm. Moreover, the multifunctional coatings showed excellent chemical stability in a dry-air environment. No significant change in composition or chemical behavior was found for samples stored in a dry-air atmosphere for several weeks compared to freshly prepared samples. All copolymers as well as the two homopolymers 4 and 5 remained intact after rinsing with standard solvents such as water, chloroform, acetone, and ethanol. The elemental composition of the copolymer poly[4-aminomethyl-p-xylylene-co-4-trifluoroacetyl-p-xylylene-co-p-xylylene] (3) was studied by X-ray photoelectron spectroscopy (XPS). XPS is capable of detecting atomic composition within a depth of about 10 nm. Copolymer 3 was compared to the individual polymers, poly[4-trifluoroacetyl-p-xylylene-co-pxylylene] (4) and poly[4-aminomethyl-p-xylylene-co-p-xylylene] (5, see Table 1). Characteristic chemical elements detected for the individual polymers 4 and 5, such as oxygen (polymer 4), fluorine (polymer 4), and nitrogen (polymer 5), were simultaneously detected in the copolymer, indicating the presence of both functional groups on the surface. Because nitrogen is only present in the aminomethyl group of polymer 5, while fluorine is present in only the ketone function of polymer 4, the ratio of the elemental composition of nitrogen and fluorine is a good indicator of the chemical quality of the copolymer films. Using XPS, we found a N/F ratio of 0.332, which is in good acC O M M U N IC A IO N S