Chemical Force Titration of Plasma Polymer-Modified PDMS Substrates by Using Plasma Polymer-Modified AFM Tips

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• 作者：Alexandre Geissler ; Marie-France Vallat ; Loï ; c Vidal ; Jean-Claude Voegel ; Joseph Hemmerl&eacute ; Pierre Schaaf ; Vincent Roucoules
• 刊名：Langmuir
• 出版年：2008
• 出版时间：May 6, 2008
• 年：2008
• 卷：24
• 期：9
• 页码：4874 - 4880
• 全文大小：274K
• 年卷期：v.24,no.9(May 6, 2008)
• ISSN：1520-5827

文摘

Plasma polymerization has gained increasing attention in surface functionalization. We use here chemical forcetitration to characterize PDMS (polydimethylsiloxane) substrates modified by maleic anhydride-pulsed plasmapolymerization. The coating is hydrolyzed to promote the formation of dicarboxylic acid groups. To enhance thevariation of the adhesion forces as a function of pH, we use AFM tips modified in the same way as the substrates.The pH-dependent adhesion measurements are performed at different KCl concentrations. The dicarboxylic natureof the maleic acid groups clearly emerges from the force titration curves. The surface pK_a values (pK_a1 = 3.5 ± 0.5and pK_a2 = 9.5 ± 0.5) of the dicarboxylic acids are evaluated from low electrolyte concentration solutions. The valuesare shifted toward higher pK_a values when compared to maleic acid in solution. The first pK_a appears in the titrationforce curve for low salt concentration as a peak. This peak changes to a sigmoidal shape at higher salt concentrations.The appearance of a peak is attributed to the formation of strong hydrogen bonds between the tip and the substrateas reported in the literature. The effect of the ionic strength on the force curves is explained by the condensation ofcounterions on the carboxylate groups. At high pH, the adhesion force almost vanishes. On the approach, at high pH,one first observes repulsion between the tip and the substrate, which varies exponentially with the tip/substratedistance. The decay length of this repulsion force is in good agreement with theoretical predictions of the Debye length,attesting to the electrostatic nature of the interactions. We also find that the replacement of monovalent cation K⁺by the divalent cation Ca²⁺ leads to significant changes in the force titration curve at high pH where the dicarboxylicgroups are fully ionized. We observe that the adhesion force no longer vanishes at high pH but even slightly increaseswith pH, an effect that is explained by Ca²⁺ ions bridging between two carboxylate groups.