Study of the size and separation of pinholes in the self-assembled thiol-porphyrin monolayers on gold electrodes

Abstract

Self-assembled monolayers (SAMs) of thiol-porphyrins with different alkyl chain length (abbreviated as H 2TPPO(CH 2) n SH, n = 3, 6, 9 and 12) on gold electrode have been studied by electrochemical impedance spectroscopy in electrolytes containing a one-electron redox couple. From the size and separation of pinholes on SAMs, both estimated the values of charge-transfer resistance and pore size model, we get the surface coverage ( θ). The results demonstrate that the surface coverage of saturated porphyrin modified gold electrode is close to unity (> 99%). But on the SAMs, a large number of pinholes still exist and acting as the microelectrodes, with average pinhole radius of 4–6 μm and the separations are ten times greater than the pinhole size, of 40–70 μm. Moreover, long chain length thiol-porphyrin SAM shows high surface coverage, larger pinhole size and excellent blocking of electron-transfer reactions between the redox mediator and the gold substrate. We demonstrated the large pinhole size is due to the aggregation of mass individual “micro-holes”.