Direct electrochemistry of cytochrome c on a phosphonic acid terminated self-assembled monolayers

The direct electrochemistry of cytochrome c (cyt c) on a gold electrode modified with 3-mercaptopropylphosphonic acid [HS–(CH2)3–PO3H2, MPPA] self-assembled monolayers (SAMs) was for the first time investigated. Electrochemical measurements and surface-enhanced infrared absorption spectroscopic reveal that the adsorption kinetics of cyt c on the MPPA-SAMs is very fast (saturation adsorption is completed within 5 s) and the immobilized cyt c molecules retain their native secondary protein structure. The nature of interaction between cyt c and –PO3H2 groups is mainly the electrostatic interaction. The direct electrochemistry of the immobilized cyt c on the –PO3H2 terminated SAMs with short chain is nearly reversible. Its formal potential (E0′ = 18 ± 3 mV vs. SCE) is very close to that of cyt c in an aqueous solution (E0′ = 18–22 mV vs. SCE). In addition, the electron transfer rate of cyt c immobilized on –PO3H2 terminated SAMs is relatively slow as compared to –SO3H and –COOH terminated SAMs, indicating excess negative charge density on the SAMs surface will decrease the electron transfer rate of cyt c.