Sensitive Imaging of Electroactive Species in Plasmonic Electrochemical Microscopy Enabled by Nanoconfinement

Spatially resolved sensing is a burgeoning area of electrochemistry that, in contrast to traditional electrochemical techniques, allows for the analysis of heterogeneous systems such as neurotransmitter release from cells. Of these techniques, optical microscopy methods are valued for real-time high throughput sensing. However, improving the sensitivity of many optical techniques remains a challenge. Here, we modify the gold (Au) electrode of the standard plasmonic electrochemical microscopy (PEM) setup with a mesoporous silica film (MSF) to achieve sensitive imaging of the electroactive species. Sensitivity enhancement occurs via species nanoconfinement from the attraction of ions to the negatively charged silica films, thereby increasing the local concentration change and magnifying the PEM signal. The performance of Au-MSF electrodes in the PEM setup was investigated using 1,1'-ferrocenedimethanol, whose oxidized form carries a positive charge. Results revealed enhancement of the sensing signal, with up to 37-fold improvement in the detection limit and up to 23 times improvement in the sensitivity. Importantly, Au-MSF electrodes allowed for the quantification of detected concentrations, in contrast to Au electrodes, for which R² values were unacceptably low. Furthermore, Au-MSF electrodes also showed increased sensitivity for dopamine detection compared to Au electrodes and were able to visualize localized dopamine release, showing this setup's great promise for biological applications, such as real-time imaging of the neurotransmitter release.