Chemically Selective Force Mapping of Electrochemically Generated Two-Component ω-Substituted Alkanethiol Monolayer Gradients by Pulsed-Force-Mode Atomic Force Microscopy

Pulsed-force-mode atomic force microscopy (PFM-AFM) was used to map the adhesion force between an AFM tip and samples prepared by microcontact printing (μCP) and electrochemically generated ω-substituted alkanethiol monolayer gradients. AFM tips chemically derivatized to terminate with either or -COOH end functional groups were used to measure the adhesion force difference between and COOH-terminated regions on a μCP sample in both air and deionized Images acquired in air with a -terminated tip yielded mean adhesion force differences between and COOH-terminated sample regions, of 7.5 ± 4.8 nN for tips prepared from alkanethiols on Au and 11.1 ± 3.0 nN for tips prepared from alkylsiloxanes on Si. The variability in was significantly reduced in deionized PFM-AFM was also able to distinguish and COOH-terminated regions in an electrochemically generated gradient in air and deionized The contrast of the image and adhesion force differences measured on electrochemically generated gradients in were comparable to those measured on μCP samples. PFM-AFM images of electrochemically generated gradients indicated a transition region ∼75 mV wide in rough correspondence to the 60 mV width of the Fermi-Dirac distribution at 300 K.