Raman characterization of alkanethiols and the application of alkanethiols to electroreflectance spectroscopy

A series of novel Raman bands has been observed employing surface-enhanced Raman scattering (SERS) on self-assembled monolayers (SAMs) of alkanethiols on Au and Ag surfaces. The novel Raman bands display a resonance effect, observed with excitation in the 630 to 710 nm range. The bands appear superimposed on the normal SERS spectrum and can be very strong. At room temperature, the presence of the novel bands displays a chain length dependence, occurring in SAMs with n ≥ 8 methylene groups. At liquid nitrogen temperatures (--196°C), alkanethiol monolayers with n ≥ 5 methylene groups display the bands, indicating a dependence on monolayer crystallinity. Isotopic labeling studies show no novel Raman band changes, indicating the bands are not of vibrational origin. Electrode potential studies revealed that charge transfer effects were not sources of the bands. A model utilizing image potential states is proposed as the source of the novel bands, indicating surface-enhanced electronic Raman scattering;SAMs of carboxylic acid terminated alkanethiols on evaporated Au electrodes were utilized as model membranes to investigate electron transfer (ET) rates of immobilized cytochrome c (cyt. c). UV-Vis electroreflectance spectroscopy was used to determine the ET rates. The effects of alkanethiol chain lengths, electrolyte ionic strength, pH and solvent viscosity were studied. For thiol chain lengths with n ≥ 9, the ET rate was dominated by electron tunneling through the chain, according to Marcus theory. At lower ionic strengths 40 mM) the ET rate appears nearly constant while at higher ionic strengths a decrease in ET rate in noted. It is believed at higher ionic strengths the electrostatic attraction between the carboxylic acid terminal group and the cyt. c is shielded, increasing the distance between the two and slowing the ET rate. The ET rate pH dependence is attributed to changes in the cyt. c orientation on the SAM. Solution viscosity effects the ET rates through protein reorientation. At higher viscosities (eta ≥ 5.3 cP) the immobilized cyt. c is in a configuration yielding slower ET rates. A gating mechanism is proposed to account for the ET reaction mechanism between cyt. c and the gold electrodes