Non-Destructive Sub-Surface Chemical Characterization of Air-Exposed Plasma Polymers by Energy-Resolved XPS Pierre-Luc Girard-Lauriault, Iris Retzko, Sufal Swaraj, Nobuyuki Matsubayashi, Thomas Gross, Renate Mix, Wolfgang E. S. Unger* Introduction Plasma polymers can be deposited on a wide range of substrates in order to introduce a desired surface chemistry, which can be tailored for a wide range of applications. [1–4] For many of these, only the chemistry of the outermost surface layer is relevant; a good example are biomedical applications, where the cells ‘‘sense’’ and ‘‘recognise’’ only the outermost molecular layer. The surface chemistry of a material can often differ significantly from that of the bulk material; however, proposed deposition-kinetic models [5] predict that this effect should be small for the case of plasma polymers. Unfortunately, there presently exist very few published data to support this view. Many works dealing with plasma-polymerised and plasma-modified surfaces have favoured the use of surface- specific techniques such as X-ray photoelectron spectro- scopy (XPS), time of flight secondary ion mass spectrometry (ToF-SIMS), Fourier transform infrared spectroscopy (FTIR), near-edge X-Ray absorption fine structure spectroscopy (NEXAFS), and contact angle goniometry. While described as ‘‘surface specific’’, each technique senses the surface-near region to within a certain depth, which is often much more than the relevant outermost layer of the material; its nature can very rarely be determined thoroughly, thus often rendering conclusions on the specific influence of a surface chemistry ambiguous. XPS is one of the most frequently Full Paper P.-L. Girard-Lauriault, I. Retzko, S. Swaraj, T. Gross, R. Mix, W. E. S. Unger BAM -Bundesanstalt fu ¨r Materialforschung und –pru ¨fung, D-12203 Berlin, Germany Fax: þ48 30 8104 1827; E-mail: wolfgang.unger@bam.de N. Matsubayashi National Institute of Advanced Industrial and Science Technology (AIST), Tsukuba, Ibaraki 305, Japan Plasma polymers containing oxygen- and nitrogen-based surface chemistries were prepared from allyl alcohol, allylamine and acrylic acid. Established selective chemical derivatisation procedures with trifluoroacetic anhydride (TFAA) and 4-trifluoromethyl benzaldehyde (TFBA) were used to determine alcohols and amines, respect- ively. Samples were analysed by energy-resolved XPS (ERXPS), enabling XPS depth resolution. We demon- strated that slightly aged plasma polymers prepared from allyl alcohol and allylamine present a homo- geneous sub-surface chemistry within the range of the XPS information depth (10 nm); this was clearly not the case for their acrylic acid counterparts. Further- more, it is shown that standard derivatisation reac- tions result in homogeneous changes of the chemistry. 474 Plasma Process. Polym. 2010, 7, 474–481 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/ppap.200900130