Fluorescence from Alexa 488 Fluorophore Immobilized on a Modified Gold Electrode

The fluorescence intensity was measured from Alexa 488 fluorophore immobilized on a modified polycrystalline gold electrode. In the range of potentials −0.5 to +0.2 V vs Ag(Ag/Cl), the fluorescence signal was small and of the order expected from known fluorescence quenching by metal surfaces. Immobilization of Alexa 488 with a larger separation between the fluorophore and the electrode surface resulted in an increased fluorescence signal in the potential range mentioned above. For potentials more negative than −0.5 V and more positive than +0.2 V, the fluorescence signal increased exponentially. The increase was attributed to a potential driven increase in the separation between the fluorophore and the electrode surface brought about by reorientation and dechemisorption of the fluorophore. For potentials more positive than −0.7 V vs Ag(Ag/Cl) the change in fluorescence intensity was quasi-reversible, for potentials more negative than −0.8 V the change was irreversible, and the fluorescence vanished on returning to more positive potentials. The dependence of the fluorescence intensity on potential was interpreted in terms of sequential occurrence of reorientation( E > −0.7 V) and dechemisorption ( E < −0.8 V). The fluorescence signal yields significant additional information about the behavior of molecules at electrode surfaces.