Bipolar Electrochemiluminescence at Single Gold Microbeads Trapped by Micropipettes

Electrochemiluminescence (ECL) reactions can take place at single entities, allowing for the studying of reaction mechanisms at the micro/nanoscale level. However, the ECL generation usually relies on the substrate electrode, where single entities of interest are positioned, leading to the overlap of ECL-emitting regions. Here, we report the fabrication of an open bipolar electrode (BPE) based on a single gold-coated microbead (GMB), thus investigating ECL generation merely from a single bead via a wireless mode. In this configuration, a single GMB was tightly positioned near the tip of a glass micropipet electrode (MPE) while maintaining the flow of electrolyte solution through the orifice of the micropipette. Thanks to the spatial confinement of the micropipette, ECL generation from tris-(2,2'-bipyridyl)-ruthenium-(II) and tri-n-propylamine can be initiated at a driving potential as low as 4 V, which is significantly lower than the theoretical value. Furthermore, the variation of ECL patterns with an external potential was resolved. In the narrow potential range from 4 to 5 V, the ECL-emitting region spatially extends from the anodic pole to the equator of the GMB, while it sharply shrinks at 5.5 V, most likely due to the competition reaction of water oxidation. The contraction of the ECL-emitting region is attributed to the suppression of ECL reactions by a decrease in local pH. This work offers a promising method of modulating the thickness of the ECL-emitting layer at a wireless BPE and provides informative insight into ECL generation in a confined volume.