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 (muCP) and electrochemically generated omega-substituted alkanethiol monolayer gradients. AFM tips chemically derivatized to terminate with either -CH3 or -COOH end functional groups were used to measure the adhesion force difference between CH3- and COOH-terminated regions on a mCP sample in both air and deionized H2O. Images acquired in air with a CH3-terminated tip yielded mean adhesion force differences between CH3- and COOH-terminated sample regions, \DeltaF\, 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 \DeltaF\ was significantly reduced in deionized H2O. PFM-AFM was also able to distinguish CH3- and COOH-terminated regions in an electrochemically generated gradient in air and deionized H2O. The contrast of the image and adhesion force differences measured on electrochemically generated gradients in H2O were comparable to those measured on mCP samples. PFM-AFM images of electrochemically generated gradients indicated a transition region similar to75 mV wide in rough correspondence to the 60 mV width of the Fermi-Dirac distribution at 300 K. (C) 2002 The Electrochemical Society.