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

Abstract

IntroductionDuring the recent decade, atomic force microscopy (AFM) has attracted much attention as a promising analytical tool for precisely measuring intermolecular forces.1-5 AFM can be used to detect the force between its probe tip and substrate surfaces, even at the pN level. It is thus of great importance that there is almost no limitation of the environment and conditions where the AFM force measurements are carried out. Thus, if the AFM probe tip and the substrate are modified chemically with selfassembled monolayers (SAMs) 6,7 terminating with target functional groups, the intermolecular force in their molecular pairs can be evaluated by force-curve measurements in appropriate solvents by AFM. [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.In such AFM force measurements, SAMs are often used for conveniently established covalent bonding of the target molecules onto the surfaces of the tip and substrate. Then, the intramonolayer interaction between the components in the SAMs can be expected to affect the force observed by the AFM technique. In other words, the AFM technique could detect such effects as the intramonolayer interaction on the interaction between the tip and the substrate. Intramonolayer hydrogen bonding is crucial in the SAMs of thiols or silanes bearing a terminal functional group, which can form hydrogen bondings between themselves. For example, terminal carboxyl groups in the SAMs of ω-carboxyalkanethiol can easily form hydrogen bonding within the monolayers. 39-42The COOH-terminated SAMs, such as thiol and silane, have been analyzed by FT-IR spectroscopy. 44 Especially, in the case of COOH-terminated SAMs, the effect of the intramonolayer hydrogen bonding between the neighboring carboxyl groups in the SAMs had been considered with respect to their surface property, such as wettability 45 and surface acidity (pKa) of the carboxyl groups. [41][42][43] In the present paper, we report an AFM force spectroscopic study concerning the effect of intramonolayer hydrogen bonding in COOH-terminated SAMs of 6-mercaptohexanoic acid (MHA) on the adhesion force observed between a SAM of MHA and a phenylurea derivative attached to the substrate and probe tip, respectively.Urea derivatives have a powerful ability to form hydrogen bondings, sin...

Characterization of Bromide Ions in Charge-Stacked Zwitterionic Micellar Systems

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Analytical and Separation Chemistry by Taking Advantage of Organic Photochromism Combined with Macrocyclic Chemistry

“…54 We had successfully measured the complexation force of 18-crown-6 with ammonium ion by AFM using probe tips modified chemically with an alkanethiol derivative having a terminal group of 18-crown-6, and had demonstrated the applicability of AFM to the study of cation binding processes on a molecular scale in synthetic host-guest chemistry, which exhibits weaker interactions than the biological and coordination-chemical systems. [55][56][57] Also, the single complexation force of 18-crown-6 with ammonium ion was evaluated by a histogram treatment by an autocorrelation method. Very recently, we succeeded in the two-dimensional mapping of metal ions on a substrate by using cantilevers modified chemically by a crown ether derivative.…”