Monitoring scanning electrochemical microscopy approach curves with mid-infrared spectroscopy: toward a novel current-independent positioning mode.

Single-bounce attenuated total reflection infrared spectroscopy in the 3-20 mum range (mid-infrared, MIR) has been combined with scanning electrochemical microscopy (SECM) for in situ spectroscopic detection of electrochemically induced localized surface modifications using an ultramicroelectrode (UME). In this study, a novel current-independent approach for positioning the UME in aqueous electrolyte solution is presented using either changes of infrared (IR) absorption intensity associated with borosilicate glass (BSG), which is used as shielding material of the UME wire, or by monitoring IR changes of the water spectrum within the penetration depth of the evanescent field due to displacement of water molecules in the volume between the sample surface and the UME within the evanescent field. The experimental results show that the UME penetrates into the exponentially decaying evanescent field in close vicinity (a few micrometer) to the attenuated total reflection (ATR) crystal surface. Hence, the resulting intensity changes of the IR absorption spectra for borosilicate glass (increase) and for water (decrease) can be used to determine the position of the UME relative to the ATR crystal surface independent of the current measured at the UME.