Self assembly of iron protoporphyrin and its binding with carbon monoxide on dithiol modified gold electrode as probed by in situ ATR-SEIRAS

Real time surface enhanced infrared absorption spectroscopy in ATR configuration (ATR-SEIRAS) has been applied to monitor the self assembly process of iron(III) protoporphyrin IX (FeIIIPP or hemin) onto an alkanedithiol modified Au film (HSRS-Au) electrode through the thioether linkage. This chemical grafting assembly is a two-stage process with a rapid increase of surface coverage in the initial 1000 s followed by a gradual saturation for the HSRS-modified Au electrode in a 50 μM FeIIIPP-contained phosphate buffer solution (PBS) (pH 7.4). The cyclic voltammogram of thus obtained electrode shows clearly a pair of quasi-reversible FeIIIPP/FeIIPP redox peaks in PBS of pH 7.4 with the formal potential ca. −0.36 V (SCE), and the surface coverage of the attached FePP is estimated to be ca. 2.55 × 10−11 mol/cm2 from the charge contained in the redox peak. In situ ATR-SEIRAS is also extended to study the potential-dependent coordination of CO to the as-grafted FeIIPP monolayer. The mid-point transition potential for de-ligating CO off the (CO)FeIIPP is centered at ca. −0.21 V and 0.01 V (SCE) in PBS solutions of pH 7.4 and pH 3.8, respectively. No spectral feature indicated the formation of the bicarbonyl adduct.

► Assembly process of hemin (or FePP) on alkanedithiol modified Au film electrode monitored by real time ATR-SEIRAS. ► Quasi-reversible redox behavior seen for well-defined surface FeIIIPP/FeIIPP. ► CO coordination to and decoordination off surface FePP investigated by electrochemical ATR-SEIRAS.