Skulason Hjalti, Frisbie C Daniel
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455, USA.
J Am Chem Soc. 2002 Dec 18;124(50):15125-33. doi: 10.1021/ja028089r.
Atomic force microscopy (AFM) was used to measure the chemical binding force of discrete electron donor-acceptor complexes formed at the interface between proximal self-assembled monolayers (SAMs). Derivatives of the well-known electron donor N,N,N',N'-tetramethylphenylenediamine (TMPD) and the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) were immobilized on Au-coated AFM tips and substrates by formation of SAMs of N,N,N'-trimethyl-N'-(10-thiodecyl)-1,4-phenylenediamine (I) and bis(10-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl) disulfide (II), respectively. Pull-off forces between modified tips and substrates were measured under CHCl(3) solvent. The mean pull-off forces associated with TMPD/TCNQ microcontacts were more than an order of magnitude larger than the pull-off forces for TMPD/TMPD and TCNQ/TCNQ microcontacts, consistent with the presence of specific charge-transfer interactions between proximal TMPD donors and TCNQ acceptors. Furthermore, histograms of pull-off forces for TMPD/TCNQ contacts displayed 70 +/- 15 pN periodicity, assigned to the rupture of individual TMPD-TCNQ donor-acceptor (charge-transfer) complexes. Both the mean pull-off force and the 70 pN force quantum compare favorably with a contact mechanics model that incorporates the effects of discrete chemical bonds, solvent surface tensions, and random contact area variations in consecutive pull-offs. From the 70 pN force quantum, we estimate the single bond energy to be approximately 4-5 kJ/mol, in reasonable agreement with thermodynamic data. These experiments establish that binding forces due to discrete chemical bonds can be detected directly in AFM pull-off measurements employing SAM modified probes and substrates. Because SAMs can be prepared with a wide range of exposed functional groups, pull-off measurements between SAM-coated tips and substrates may provide a general strategy for directly measuring binding forces associated with a variety of simple, discrete chemical bonds, e.g., single hydrogen bonds.
原子力显微镜（AFM）用于测量在近端自组装单分子层（SAMs）界面形成的离散电子供体 - 受体复合物的化学结合力。通过形成N，N，N'-三甲基 - N' -（10 - 硫代癸基）-1,4 - 苯二胺（I）和双（10 -（2 -（（2,5 - 环己二烯 - 1,4 - 二亚基）二丙二腈）癸基）二硫化物（II）的SAMs，将著名的电子供体N，N，N'，N'-四甲基苯二胺（TMPD）和电子受体7,7,8,8 - 四氰基对苯二醌二甲烷（TCNQ）的衍生物分别固定在涂金的AFM针尖和基底上。在CHCl₃溶剂中测量修饰针尖与基底之间的拉脱力。与TMPD/TCNQ微接触相关的平均拉脱力比TMPD/TMPD和TCNQ/TCNQ微接触的拉脱力大一个数量级以上，这与近端TMPD供体和TCNQ受体之间存在特定的电荷转移相互作用一致。此外，TMPD/TCNQ接触的拉脱力直方图显示出70±15 pN的周期性，这归因于单个TMPD - TCNQ供体 - 受体（电荷转移）复合物的破裂。平均拉脱力和70 pN的力量子与一个考虑了离散化学键、溶剂表面张力和连续拉脱中随机接触面积变化影响的接触力学模型相当吻合。根据70 pN的力量子，我们估计单键能量约为4 - 5 kJ/mol，与热力学数据合理一致。这些实验表明，在使用SAM修饰探针和基底的AFM拉脱测量中，可以直接检测到由离散化学键引起的结合力。由于可以制备具有广泛暴露官能团的SAMs，SAM涂层针尖与基底之间的拉脱测量可能提供一种直接测量与各种简单离散化学键（例如单氢键）相关的结合力的通用策略。
