Baio J E, Weidner T, Brison J, Graham D J, Gamble Lara J, Castner David G
National ESCA and Surface Analysis Center for Biomedical Problems, University of Washington, Departments of Bioengineering and Chemical Engineering, Box 351750, Seattle, WA 98195.
J Electron Spectros Relat Phenomena. 2009 May 1;172(1-3):2-8. doi: 10.1016/j.elspec.2009.02.008.
Self-assembled monolayers (SAMs) on gold prepared from amine-terminated alkanethiols have long been employed as model positively charged surfaces. Yet in previous studies significant amounts of unexpected oxygen containing species are always detected in amine terminated SAMs. Thus, the goal of this investigation was to determine the source of these oxygen species and minimize their presence in the SAM. The surface composition, structure, and order of amine-terminated SAMs on Au were characterized by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), sum frequency generation (SFG) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. XPS determined compositions of amine-terminated SAMs in the current study exhibited oxygen concentrations of 2.4 ± 0.4 atomic %, a substantially lower amount of oxygen than reported in previously published studies. High-resolution XPS results from the S(2p), C(1s) and N(1s) regions did not detect any oxidized species. Angle-resolved XPS indicated that the small amount of oxygen detected was located at or near the amine head group. Small amounts of oxidized nitrogen, carbon and sulfur secondary ions, as well as ions attributed to water, were detected in the ToF-SIMS data due to the higher sensitivity of ToF-SIMS. The lack of N-O, S-O, and C-O stretches in the SFG spectra are consistent with the XPS and ToF-SIMS results and together show that oxidation of the amine-terminated thiols alone can only account for, at most, a small fraction of the oxygen detected by XPS. Both the SFG and angle-dependent NEXAFS indicated the presence of gauche defects in the amine SAMs. However, the SFG spectral features near 2865 cm(-1), assigned to the stretch of the methylene group next to the terminal amine unit, demonstrate the SAM is reasonably ordered. The SFG results also show another broad feature near 3200 cm(-1) related to hydrogen-bonded water. From this multi-technique investigation it is clear that the majority of the oxygen detected within these amine-terminated SAMs arises from the presence of oxygen containing adsorbates such as tightly bound water.
由胺基封端的烷硫醇制备的金表面自组装单分子层(SAMs)长期以来一直被用作带正电荷的模型表面。然而,在以往的研究中,总是在胺基封端的SAMs中检测到大量意想不到的含氧物种。因此,本研究的目的是确定这些氧物种的来源,并尽量减少它们在SAM中的存在。通过X射线光电子能谱(XPS)、飞行时间二次离子质谱(ToF-SIMS)、和频产生光谱(SFG)和近边X射线吸收精细结构(NEXAFS)光谱对金表面胺基封端的SAMs的表面组成、结构和有序性进行了表征。XPS测定的本研究中胺基封端的SAMs的组成显示氧浓度为2.4±0.4原子%,比以前发表的研究报告的氧量低得多。来自S(2p)、C(1s)和N(1s)区域的高分辨率XPS结果未检测到任何氧化物种。角分辨XPS表明检测到的少量氧位于胺头基处或其附近。由于ToF-SIMS的更高灵敏度,在ToF-SIMS数据中检测到少量氧化的氮、碳和硫二次离子,以及归因于水的离子。SFG光谱中缺乏N-O、S-O和C-O伸缩振动与XPS和ToF-SIMS结果一致,共同表明仅胺基封端的硫醇的氧化最多只能解释XPS检测到的一小部分氧。SFG和角度相关的NEXAFS均表明胺基SAMs中存在gauche缺陷。然而,在2865 cm(-1)附近的SFG光谱特征,归因于末端胺单元旁边的亚甲基的伸缩振动,表明SAM是合理有序的。SFG结果还显示在3200 cm(-1)附近有另一个与氢键水相关的宽峰。通过这项多技术研究清楚地表明,在这些胺基封端的SAMs中检测到的大部分氧来自于含氧吸附物的存在,如紧密结合的水。