Direct electrochemistry of hemoglobin adsorbed on self-assembled monolayers with different head groups or chain length

The electrochemical behaviors of hemoglobin (Hb) were studied by adsorbing Hb on the gold electrodes modified with self-assembled monolayers (SAMs) of different terminal groups and alkyl chain length. Specifically, through adsorbing the Hb molecules onto the SAMs of 3-mercaptopropanoic acid (MPA), 1-propanethiol (PT) and cysteamine hydrochloride (cys), the influences of the terminal groups of alkanethiols on the electron transfer of Hb were examined. A quasi-reversible redox process was observed when Hb was adsorbed on the MPA-modified electrode. However, an irreversible reduction process and no redox response were shown when Hb molecules were adsorbed on the SAMs of PT and cys-modified electrodes, respectively. The dependence of the direct electrochemical response of Hb on the alkyl chain length of alkanethiols (n) was further investigated on the SAMs of HS(CH2)(n)COOH (n = 1, 2, 5 and 10). When 11 was 1, Hb showed a weak current response. When n were 2 and 5, quasi-reversible redox processes with nearly similar electron transfer rate constants (k(s)) of 0.49 and 0.47 s(-1) were obtained, respectively. Increasing the number of methylene groups within carboxyl alkanethiol to 10 resulted in a significant decrease of the electron transfer rate and current response of Hb. The observations indicated that the SAM of MPA can provide suitable properties to keep Hb in a favorable adsorption state for direct electron transfer (DET). Furthermore, the direct electrochemistry of the Hb adsorbed on the SAM of MPA was studied in detail. (c) 2009 Elsevier B.V. All rights reserved.