What Happens to the Thiolates Created by Reductively Desorbing SAMs? An in Situ Study Using Fluorescence Microscopy and Electrochemistry

Abstract

In situ examination of the reductive desorption process for Au microelectrodes modified with a thiol self-assembled monolayer (SAM) using fluorescence microscopy enabled the study of the fate of the desorbed thiolate species. The Bodipy labeled alkyl-thiol SAM, when adsorbed, is not fluorescent due to quenching by the Au surface. Once reductively desorbed, the thiolate molecules fluoresce and their direction and speed are monitored. At moderately negative reduction potentials, the thiolate species hemispherically diffuse away from the microelectrode. Also observed is the influence of a closely positioned counter electrode on the direction of the desorbed thiolate movement. As the potential becomes more negative, the molecules move in an upward direction, with a speed that depends on the amount of dissolved H(2) produced by water reduction. Shown is that this motion is controlled, in large part, by the change in the electrolyte density near the electrode due to dissolved H(2). These results should help in explaining the extent of readsorption at oxidative potentials observed in cyclic voltammetry (CV) reductive desorption measurements, as well as improving the general understanding of the SAM removal process by reductive desorption. The electrogenerated H(2) was also shown to be able to reductively remove the thiol SAM from the Pt/Ir particles that decorate the microelectrode glass sheath.

Highly Stable Perylenediimide‐Based Self‐Assembled Monolayers Studied with Spectroelectrochemistry

“…We chose to add alkanethiols (non-fluorescent molecules) to the solution near the Au-PDI substrate in order to inducet he desorption of 4 through exchange reactions. [1,[31][32][33] Very quickly, we could again observe the characteristic intense fluorescenceo ft he PDI unit ( Figure 6). [8] Accordingly,t he molecular luminescence of 4 is quenched by the metallics ubstrate, validating the classical energy-transfer theory.…”

“…A possible way to prove the fluorescent quenching in the SAM involves desorbing the immobilized molecules. We chose to add alkanethiols (non‐fluorescent molecules) to the solution near the Au–PDI substrate in order to induce the desorption of 4 through exchange reactions . Very quickly, we could again observe the characteristic intense fluorescence of the PDI unit (Figure ) …”

In Situ Fluorescence Microscopy Study of the Interfacial Inhomogeneity of DNA Mixed Self‐Assembled Monolayers at Gold Electrodes

“…In contrast to the case of the potential-dependent orientation of DNA, the fluorescence changes are not reversible, as the integrity of the monolayer is destroyed and the molecules diffuse away. This leads to a decrease in the fluorescence intensity, which is dependent on the diffusion rate, [36][37][38] as manifested by the broad tail in the fluorescence bands of Figure 1 b.…”

Selective Electrochemical Bleaching of the Outer Leaflet of Fluorescently Labeled Giant Liposomes

“…This also indicates that excited NBD-fluorophores were not quenched near the thin metal surface( 20 nm thickness) as often observed at the surfaceo ft hick metallic materials. [14,15,43,44] On the other hand, polarization of the working electrode at À0.9 Va lso triggers water and oxygen reduction (see the cyclic voltammogramo f NBD in the Supporting Information). Reduction of NBD could possibly be achieved according to aH 2 -mediatedp rocess, and/ or OH À that is known to switch off the NBD fluorescence through ap Hj ump.…”