Force Titrations and Ionization State Sensitive Imaging of Functional Groups in Aqueous Solutions by Chemical Force Microscopy

Abstract

Chemical force microscopy (CFM) was used to probe interactions between ionizable and neutral functional groups in aqueous solutions. Force microscope probe tips and sample substrates have been covalently modified with self-assembled monolayers (SAMs) terminating in distinct functional groups. SAMs were prepared by treating Au-coated or uncoated tips and substrates with functionalized thiols or silanes, respectively. A force microscope has been used to characterize adhesive and frictional interactions between probe tips and substrates modified with SAMs that terminate in -NH 2 , -COOH, -OH, and -CH 3 functional groups as a function of solution pH and ionic strength. In general, adhesion and friction forces were observed to depend on the chemical identity of the tip and sample surface and also were found to be highly sensitive to the changes in the ionization state of the terminal functionalities induced by varying the solution pH. Adhesion measurements made as a function of pH (force titrations) on amine-terminated surfaces exhibit a sharp decrease in the adhesion force at pH values below 4.5. The pK a estimated from this drop in adhesion, which is due to the protonation of amine groups on the sample and tip, 3.9, is similar to the value determined by conventional contact angle wetting studies of these same surfaces. The large decrease in the pK of the amine group relative to homogeneous solution was attributed to the relatively hydrophobic environment of the amine group in these SAMs. Adhesion measurements made as a function of pH on -COOH-terminated surfaces exhibited a large drop in adhesion force for pH values greater than 5. The pK a estimated from these data, 5.5, is similar to the free aqueous solution value. In addition, the adhesion force between nonionizable -OH and -CH 3 groups was found to be independent of solution pH. The measured adhesive forces were interpreted using a contact mechanics model that incorporated the effects of the double layer free energy. Analyses of repulsive electrostatic forces and adhesion data recorded as a function of ionic strength were used to determine properties of the double layer. The pH dependence of the friction force between tips and samples modified with SAMs terminating in -COOH, -OH, and -CH 3 groups was measured as a function of applied load. For a given pH, these data exhibit a linear dependence on load with the slope corresponding to the coefficient of friction. The coefficient of friction for -OH and -CH 3 groups was independent of pH, while the friction coefficient for -COOH-terminated surfaces drops significantly at a pH corresponding to the pK a determined by adhesion measurements. The pHdependent changes in friction forces for ionizable groups were exploited to map spatially changes in ionization state on surfaces terminating in -COOH and -OH functional groups.

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

“…The density of the thiols or disulfides on the gold surface is well determined, and as a result forces between defined chemical groups can be measured [120]. Various rest groups have been used such as carboxyl (-COOH) [121], hydroxyl (-OH) [122], methyl (-CH 3 ), acetate (-OCOCH 3 ) [123,124], amide [125], and amino [126,127]. One problem of chemically modified tips is that they might be destroyed by the interaction with another surface.…”

“…glass, silica, and silicon nitride [15,16,187,[427][428][429][430][431][432][433][434]; gold [327,429,435,436]; copper and nickel [437]; zinc and lead sulphide [171,172,438]; titanium oxide [169,339,340]; zirconia [165,166,410]; iron oxide [439]; tungsten [175]; cobalt [175]; and alumina [16,[439][440][441][442]. Different polymers [177,189,[443][444][445][446][447], Langmuir-Blodgett and other thin organic layers [126,329,411,448,449] have been analyzed. In recent years special interest in biologically relevant surfaces materials evolved.…”

“…A way of measuring the force between two surfaces with a well-defined density of functional groups is to coat the tip with thiols or disulfides. Titration experiments have been done with a number of differently coated surfaces [121,122,[124][125][126][127].…”

Ion exclusion by sub-2-nm carbon nanotube pores

“…It is also likely that the rejection mechanism involves charge repulsion due to the interaction of the ions with the ionized carboxylic groups at the CNT mouth (24,25). To test the importance of the electrostatic interaction, we measure the exclusion characteristics of the CNT membrane at two different solution pH values, one above the pK a of the COOH group on the surface (pK a ϭ 5.5) (33) [and, also, on a CNT tip, pK a ϭ 4.5 (34,35)] and one below it. For these experiments we choose to use a 0.5 mM pyrenetetrasulfonic acid tetrasodium salt solution (Na 4 PTS) because the large PTS 4Ϫ ion remains ionized over a wide range of solution pH values (36).…”

pH-control of the protein resistance of thin hydrogel gradient films

“…30 In addition, the lack of pH-dependence in the interactions of hydroxylated surfaces (and in particular the difference between hydroxylated and carboxylated surfaces in this regard) has been demonstrated by others. 31 Force measurements on the hydrogel gradients were made in buffers, and the forces involved in the interaction were normalized with the probe radius and plotted against the separation.…”