Correlation of the Structural Decomposition and Performance of Pyridinethiolate Surface Modifiers at Gold Electrodes for the Facilitation of Cytochrome c Heterogeneous Electron-Transfer Reactions

Abstract

This paper describes the results of an electrochemical and spectroscopic (infrared reflection and X-ray photoelectron spectroscopies) investigation of the modified gold electrode surfaces prepared from dilute ethanolic solutions of 4-mercaptopyridine (PySH) and 4,4‘-dipyridyl disulfide (PySS). Both precursors have been used extensively as facilitators for the electron transfer of redox proteins like cytochrome c (cyt c). During the course of an investigation of the interfacial architectures formed from the two different precursors, a previously unreported structural instability in the adlayers was discovered. This instability manifests itself as a decrease in the ability of the modified surfaces to facilitate the electron transfer of cyt c that correlates with an increase of the immersion time in the precursor solutions. Results are presented that delineate the decrease in facilitator performance and probe the structural changes resulting in the decrease in performance. Together, the electrochemical and surface spectroscopic findings reveal that the modified surfaces spontaneously decompose to yield an adlayer composed largely of adsorbed atomic and oligomeric sulfur, an adlayer that we found to be ineffective in the facilitation of the electron transfer reaction of cyt c. The implications of these findings on the use of this type of modifier to studies of electron transfer reactions of redox proteins and to issues of the general structural stability of organosulfur-based monolayers are briefly discussed.

5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol on gold: SAM Formation and electroactivity

“…The E rd value (-0.55V) is almost equal to that reported for the SAM formed with pyS on gold, E rd = -0.56V. 16,18 For the pyS SAM, surface-enhanced Raman scattering (SERS) data indicated a sigma ( ) interaction with the gold surface atoms. 19 With this consideration in mind, the Hpyt adsorption on gold must involve the same type of interaction, i.e.…”

“…According to literature, [14][15][16] the reductive potential sweep in alkaline electrolyte (0.5 mol L -1 KOH) of gold surfaces modified Vol. 19, No.…”

“…This value is smaller than those observed for the desorption of monolayers formed with n-alkanethiolates, 17 ca. 75 C cm -2 , or 4-mercaptopyridine (pyS), 16 ca. 50 C cm -2 , on gold.…”

“…This result reinforces those obtained from the reductive desorption measurements which suggest that the Hpyt SAM is less packed and ordered than SAMs formed with alkanethiolates on gold. 17 Even comparing the Hpyt values to those obtained for the very similar pyS molecule, 16 the Hpyt SAM is still less packed. This observation may be assigned to weak lateral forces between the Hpyt adjacent molecules as suggested by the STM results.…”

Self-Assembly Process of Flatly Adsorbed Porphyrin Self-Assembled Monolayer on a Au(111) Surface Studied by ex situ Scanning Tunneling Microscopy and Electrochemical Reductive Desorption Measurements

“…It is well known that alkylthiol SAMs are electrochemically desorbed from a gold surface by the following reduction process in an alkaline aqueous solution: [56][57][58][59][60][61][62][63][64][65] Au Figure 2 shows LSVs of the Au(111) electrodes modified with the TMMPP SAMs, which were prepared for various dipping periods, measured in 0.1 M KOH at a scan rate of 20 mV s ¹1 when the potential was negatively scanned from 0 to ¹1.4 V. Between ¹0.7 and ¹1.1 V, three cathodic peaks, which seemed to be due to the reductive desorption of the SAM, were observed. The two more negative peaks observed around ¹0.9 and ¹1.0 V were reported only in the case of the mercaptopyridine and mercaptopyrrole SAMs.…”