Investigation of carboxylic-functionalized and n-alkanethiol self-assembled monolayers on gold and their application as pH-sensitive probes using scanning electrochemical microscopy

We investigated the insulating properties of n-alkanethiol self-assembled monolayers (SAMs) of varying chain lengths [CH3(CH2)nSH; n = 7, 9, 11, 15] on polycrystalline gold electrodes using scanning electrochemical microscopy (SECM) and cyclic voltammetry. On the basis of SECM approach curves we examined the local ET through monolayers with increasing chain length in different redox mediators. We were able to distinguish the monolayers because of their different insulating properties and in addition, the status of SAM formation after immersion times of 2 h and 24 h, respectively, could be observed. Cyclic voltammetric measurements confirmed the SECM results and were in good agreement with other experimental data in the literature. High-resolution SECM images of hexadecanethiol SAM micropatterns down to 4 μm in diameter formed by microcontact printing (μCP) were obtained in the feedback mode. Furthermore, we studied the ET and the pH-dependent behavior of mercaptoundecanoic acid monolayers on gold at varying pH and in different redox mediator solutions to test their application as pH-sensors. An additional influence on the ET could be established based on Coulomb/ionic interactions between the charged monolayer and the redox mediator at changing pH. Therefore, we present a new approach for designing pH-sensitive SECM probes using 11-mercaptoundecanoic acid-coated 10 μm-diameter gold ultramicroelectrodes (HOOC-C11SH/Au UMEs) in aqueous solutions containing hexacyanoferrate. Voltammetric measurements at HOOC-C11SH/Au UMEs at different pH values enabled us to estimate the degree of dissociation of the carboxylic-terminated monolayers.