Direct Detection by Atomic Force Microscopy of Single Bond Forces Associated with the Rupture of Discrete Charge-Transfer Complexes

Abstract

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.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…The work of Skulason and Frisbie [388] deals with the adhesion between gold and S-acetate-, Oacetate-, SH-and OH-terminated SAMs in different solvents, and the accent is put on the detection of the discrete rupture forces or force quanta (see also [389]). The authors found a force quantum of 0.1 nN in histograms of the adhesion force and assigned it to the rupture of individual chemical bonds.…”

“…Several experiments also have been performed with DNA strands [1078,[1143][1144][1145][1146], DNA-ligands [1147][1148][1149][1150], RNA [1151], RNA-ligands [1152], peptides [1153], and other complementary molecules [389,1154,1155]. In particular, Schumakovitch et al [1156] have studied the temperature dependence of unbinding forces between complementary DNA strands.…”

Effect of Intramonolayer Hydrogen Bonding of Carboxyl Groups in Self-assembled Monolayers on a Single Force with Phenylurea on an AFM Probe Tip

“…20,23,[28][29][30][31][32][33][34][35]37,38 We have evaluated the single force for specific hydrogen bonding between the phenylurea and carboxyl groups by a statistical method using autocorrelation analysis of the values of the adhesion force. In this analysis, an autocorrelation function is computed for the histogram data of the observed pull-off forces.…”

“…The values for the single force determined here are in a similar order of magnitude to the single rupture forces of the bond in various molecule pairs, 12,24 including the complementary DNA bases, 23 which were determined by AFM using chemically modified tips. According to the equation E = Fsd, 29,34 assuming that the distance (d) is approximately 0.1 nm, the product of Fs (the single force) and d affords an approximate bond energy (E) of 3 -4 kJ mol -1 , which is in a reasonable order of magnitude for the energy of general hydrogen bondings.…”

“…[8][9][10][11][12][13][14][15][16][17][18][19]36 Furthermore, the individual force for a variety of intermolecular forces at a single-molecule level can also be evaluated by statistical analyses of the forces obtained by repetitive adhesion force measurements. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]37,38 In our previous studies, we measured specific intermolecular forces attributable to the characteristic molecular functionalities of the functional molecules, such as crown ether 36,37 and spirobenzopyran 38 derivatives, attached covalently on the probe tips. These functionalized AFM probe tips, which possess a powerful capability of detecting supramolecular interactions, can be potentially utilized for developing a novel analytical tool for surface analysis based on force sensing by AFM.…”