Force titration of amino group-terminated self-assembledmonolayers using chemical force microscopy

Chemical force microscopy (CFM) was used to measure the adhesion force between an Au-coated Si₃N₄ tip functionalized with OH groups and a silicon substrate functionalized with NH₂ groups. The curve of the adhesion force as a function of the solution pH value (force titration curve) was obtained. The pK_1/2 of surface NH₂ groups estimated from the force titration curve is 7:4, about 3 pK units lower than the pK value of NH₂ functionality in aqueous solution, and in nice agreement with the value of pK_1/2 D 7:6 determined by conventional contact angle titration. The surface pK_1/2 value that we obtained from the force titration or contact angle titration is the apparent surface pK value, which is expressed as the corresponding bulk solution pH value at which the functional groups at the interface are half-ionized.