Metal-Enhanced Fluorescence via Spacer-Free and Electrogenerated Nanomaterials at Oil-Fouled Electrodes

Fluorescence-based imaging and assays are essential in biomedical diagnostics, environmental monitoring, and materials science. The capabilities of these techniques are further expanded via metal-enhanced fluorescence (MEF), which exploits plasmonic interactions to amplify emission signals and reduce photobleaching. However, the broad implementation of MEF is hindered by the need of a fine-tuned spacer around the metal nanoparticles to ensure optimal metal-to-fluorophore separation. Here, we demonstrate spacer-free MEF through nanomaterials that are electro-generated in a reactor consisting of an aqueous tetrachloroauric acid-fluorescein solution in contact with an ITO-glass working electrode that is strategically fouled with insulating oil droplets. Spectroscopic data indicate that the 1:4 complexation of Au(III) with zwitterionic fluorescein is critical to achieve the nanoparticle morphology that leads to optimal MEF. Microscopy data reveal that the application of an appropriate reduction potential to the reactor results in current-heterogeneity-induced convection toward the insulator-electrode-electrolyte interface (triple-point), thereby generating arrays of suitably spaced nanoparticle-fluorophore complexes and, consequently, a characteristic MEF "ring". More importantly, we report a maximum bulk fluorescence enhancement of 115%, which we attribute to potential-dependent nanoparticle growth and hyperbranching. This study lays the groundwork for spacer-free MEF and it advances the understanding of hydrophobic effects.