Fluorescence Imaging of the Oxidative Desorption of a BODIPY-Alkyl-Thiol Monolayer Coated Au Bead

Abstract

The reductive and oxidative desorption of a BODIPY labeled alkylthiol self-assembled monolayer (SAM) on Au was studied using electrochemical methods coupled with fluorescence microscopy and image analysis procedures to monitor the removal of the adsorbed layer. Two SAMs were formed using two lengths of the alkyl chain (C10 and C16). The BODIPY fluorescent moiety used is known to form dimers which through donor-acceptor energy transfer results in red-shifted fluorescence. Fluorescence from the monomer and dimer were used to study the nature of the desorbed molecules during cyclic step changes in potential. The reductive desorption was observed to occur over a small potential window (0.15 V) signified by an increase in capacitance and in fluorescence. Oxidative readsorption was also observed through a decrease in capacitance and a lack of total removal of the fluorescent layer. Removal by oxidative desorption occurred at positive potentials over a broad potential range near the oxidation of the bare Au. The resulting fluorescence showed that the desorbed molecules remained near the electrode surface and were not dispersed over the 20 s waiting time. The rate of change of the fluorescence for oxidative desorption was much slower than the reductive desorption. Comparing monomer and dimer fluorescence intensities indicated that the dimer was formed on the Au surface and desorbed as a dimer, rather than forming from desorbed monomers near the electrode surface. The dimer fluorescence can only be observed through energy transfer from the excited monomer suggesting that the monomers and dimers must be in close proximity in aggregates near the electrode. The fluorescence yield for longer alkyl chain was always lower presumably due to its decreased solubility in the interfacial region resulting in a more efficient fluorescence quenching. The oxidative desorption process results in a significantly etched or roughened electrode surface suggesting the coupling of thiol oxidative removal and Au oxide formation which results in the removal of Au from the electrode.

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

“…Because the thiolated molecules diffuse away in the bulk of the solution, only a small part can be readsorbed as the potential returns to less negative values, and thus the capacitance remains relatively high. [21,32] In the fluorescence versus potential curve, the fluorescence intensity also remains approximately constant from + 0.10 to À0.50 V. It increases abruptly around À0.50 V and immediately decreases. Around À0.90 V, a second but smaller increase in the fluorescence intensity is observed, and then the fluorescence intensity decreases again.…”

“…[21,22] This technique provides spatial information, at the typical resolution of optical microscopy ( % 0.5 mm), on the homogeneous or heterogeneous nature of SAMs, and it is thus highly complementary to surface-averaged electrochemical data. In addition, it takes advantage of the fact that metal substrates can alter the fluorescence of dyes located in their vicinity to convey information regarding the electrode j electrolyte interface.…”

Frequency response analysis of potential-modulated orientation changes of a DNA self assembled layer using spatially resolved fluorescence measurements.

“…The modified surface was characterized using in-situ fluorescence microscopy as described previously [44,36,35,34,57]. Briefly, a custom spectroelectrochemical cell with a 250 m glass optical window as the bottom is used with an inverted microscope (Olympus IX-70) configured as shown in Figure 1.…”

“…Their objective is to analyze the potential dependent changes in the adsorbed layer rather than measurement of a rate of change of the interfacial characteristics. We have also used this approach in the analysis of potential induced changes in an adsorbed organic layer on gold electrodes [33][34][35][36] which will be further demonstrated in this contribution.…”

“…Labeling the distal end of the DNA-alkylthiol with a fluorescent moiety allows an estimation of the movement or re-orientation of the DNA-SAM molecules since the measured fluorescence intensity is dictated by the efficiency of metal-mediated fluorescence quenching which strongly depends on the separation of the fluorophore from the gold surface [40][41][42][43]. This distance dependent quenching characteristic has been used to examine the quality of the electrode surface modification and the influence of electrical potential (or surface charge or energy) on the adsorbed layer [44,36,35]. These "electrically switchable" fluorophore-labeled DNA layers were further characterized using ac methods with the measured response being the change in fluorescence intensity driven by changes in the electrode potential or charge density at the gold surface [37,39,45,46].…”