Christian Friedmann

Bio-orthogonal “Double-Click” Chemistry Based on Multifunctional Coatings

As a consequence of recent progress in biotechnology, regenerative medicine, and developments concerning medical implants, an increasing need for precise and flexible conjugation methods has emerged. For the immobilization of biomolecules, chemical reactions with high specificity towards the molecule of interest, mild reaction conditions compatible with physiological milieu, and rapid as well as quantitative conversion are essential. If defined immobilization of two or more biomolecules on the same surface in controlled ratios is required, the individual reactions not only need to be orthogonal with respect to ongoing biological events, but also with respect to each other. This prerequisite puts major constraints on the type of chemical reactions that can be exploited for bio-orthogonal immobilization. The development of bio-orthogonal reaction schemes has been heavily influenced by the concept of “click” chemistry, which was first introduced by Sharpless and co-workers in 2001. As the archetypal example of click chemistry, the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes (CuAAC) has since been widely used as a surfacemodification strategy. CuAAC is a highly efficient reaction under mild conditions, with complete regioselectivity for the 1,4-triazole product. Triazoles are stable linkers that are resistant to hydrolysis, oxidation, or reduction. Initial work was conducted on model surfaces, such as gold and silicon, but was recently extended to a wide range of different substrates. Chemical vapor deposition (CVD) polymerization is a versatile coating process that effectively decouples the surface chemistry from the bulk composition. The CVD polymerization of functionalized [2.2]paracyclophanes can result in functionalized poly(p-xylylene) coatings with a wide range of different groups including active esters, aldehydes, ketones, or anhydrides. These coatings have been used for the immobilization of proteins, peptides, DNA, and cells. CVD coatings can be conformally deposited on a broad range of materials with different geometry and are useful for applications including functional electrically conductive polymer films, polymer gradients, protein-resistant surfaces, solventless adhesive bonding, 3D photoresists, and polymer/carbon nanotube composites. The CVD process has been successfully applied for the deposition of alkyne-functionalized polymers on a range of different substrates and can even support microand nanopatterning by CuAAC. 9] Despite the success of CuAAC, the requirement of a potentially cytotoxic copper catalyst may limit its biomedical applications. To develop Cu-free click chemistry, alkynes were activated by applying ring-strain, incorporating an electron-withdrawing group, or both. Bertozzi and co-workers conducted extensive studies on the synthesis of cyclooctyne derivatives for copper-free azide– alkyne cycloadditions. They successfully improved the cyclooctyne reactivity by introducing electron-withdrawing fluorine atoms and used the copper-free click reactions for selective modifications of biomolecules and living cells. Boons and co-workers achieved a similar rate enhancement by fusing two aryl rings to the cyclooctyne scaffold. The strain-promoted cycloaddition of functionalized cyclooctynes to azides is an efficient reaction, but their challenging synthesis has prevented them from being more widely investigated and applied to bioimmobilization. Herein, we report a synthetically straightforward approach towards reactive coatings for copper-free 1,3dipolar cycloadditions and demonstrate a bio-orthogonal reaction scheme based on two sequential click reactions. Our approach is based on CVD polymerization of appropriately functionalized [2.2]paracylophanes. The development of a CVD coating that presents alkyne groups capable of copperfree click chemistry poses a number of challenges: 1) The reactive groups must readily react with azide groups at room temperature in benign solvents (e.g., water); 2) The functional groups have to be compatible with the processing conditions during CVD polymerization without decomposition or side reactions; 3) The precursors should be accessible by straightforward synthesis. Herein, we chose to synthesize [2.2]paracyclophane-4-methyl propiolate, which provides an electron-deficient alkynyl group for the spontaneous reaction with azide groups even in the absence of a catalyst. Neighboring electron-withdrawing groups are known to increase the reactivity of alkyne groups. Functional moieties such as sulfonyl and carbonyl groups were investigated in different studies. Applications of electrondeficient alkyne moieties include DNA modification, gold nanoparticle functionalization, or hydrogel crosslinking. As shown in Scheme 1, [2.2]paracyclophane-4-methyl propiolate was sublimed at 100 8C and 0.3 mbar and then subjected to thermal pyrolysis at 510 8C in vacuum to generate [*] X. Deng, Dr. C. Friedmann, Prof. Dr. J. Lahann Departments of Chemical Engineering, Materials Science and Engineering, Macromolecular Science and Engineering and Biomedical Engineering, University of Michigan Ann Arbor, 48109 (USA) and Institute of Functional Interfaces, Karlsruhe Institute of Technology Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen (Germany) Fax: (+ 1)734-764-7453 E-mail: lahann@umich.edu

Improved synthesis of enantiopure 4-hydroxy[2.2]paracyclophane.

4-Hydroxy[2.2]paracyclophane is readily prepared via an improved synthetic protocol from unsubstituted [2.2]paracyclophane. The key step is a Dakin oxidation of 4-formyl[2.2]paracyclophane. This allows a rapid access to large quantities of the product and an easy synthesis of the enantiopure form.

Free-standing nanomembranes based on selective CVD deposition of functional poly-p-xylylenes

The precise engineering of ultrathin nanofilms with variable functionality remains an unmet challenge in nanotechnology. We report a strategy for generating free-standing nanomembranes based on the selective chemical vapor deposition polymerization of functional [2.2]paracyclophanes on micropatterned self-assembled monolayers of alkanethiolates on gold. This fabrication strategy can yield microstructured nanofilms that are between 2 and 5 nm thick. Subsequent release from the substrate results in free-standing nanoscale membranes with controlled pore size and geometry. The process allows for modification of important functional parameters, such as ultrasmall membrane thickness, membrane pore geometry, and chemical functionality.

Backbone-Degradable Polymers Prepared by Chemical Vapor Deposition

Polymers prepared by chemical vapor deposition (CVD) polymerization have found broad acceptance in research and industrial applications. However, their intrinsic lack of degradability has limited wider applicability in many areas, such as biomedical devices or regenerative medicine. Herein, we demonstrate, for the first time, a backbone-degradable polymer directly synthesized via CVD. The CVD co-polymerization of [2.2]para-cyclophanes with cyclic ketene acetals, specifically 5,6-benzo-2-methylene-1,3-dioxepane (BMDO), results in well-defined, hydrolytically degradable polymers, as confirmed by FTIR spectroscopy and ellipsometry. The degradation kinetics are dependent on the ratio of ketene acetals to [2.2]para-cyclophanes as well as the hydrophobicity of the films. These coatings address an unmet need in the biomedical polymer field, as they provide access to a wide range of reactive polymer coatings that combine interfacial multifunctionality with degradability.

Ultrasensitive in Situ Fluorescence Analysis using Modulated Fluorescence Interference Contrast at Nanostructured Polymer Surfaces

The precise modulation of fluorescence interference contrast is achieved by introducing a nanoscopically engineered spacer layer prepared by chemical vapor deposition (CVD) of functional polymers. These novel imaging substrates are chemically identical throughout their entire detection area, yet present patterns of nanoscale thickness. A protein binding cascade is studied in real time and in the presence of high background noise.

Polymeric Whispering Gallery Mode Resonators for Biosensing Applications

We report on polymeric high-Q microresonators and a method for spatially selective functionalization. Furthermore we present coupled resonators exhibiting a higher bulk refractive index sensitivity than single resonators making them promising candidates for high-sensitivity sensing.