FRET-Enhanced Optical Imaging of Nanoparticle Collision on a Gold Ultramicroelectrode

While optical imaging can help better study nanoparticle collision electrochemistry, it remains challenging to establish an optical signal that can directly reflect the redox state of individual particles during their transient collision process. Herein, we report a method based on the use of Förster resonance energy transfer (FRET) for enhancing the optical contrast of nanoparticle collision events on a gold ultramicroelectrode (UME). We first showed that methylene blue (MB), a redox-active fluorophore, exhibits fluorescence turn-off upon reduction on the electrode. To further amplify the optical signal, MB was used as a FRET acceptor and paired with Rhodamine 101 (R101) as the donor, such that MB quenches R101 emission via energy transfer, while reduced MB would disrupt this process, leading to the recovery of R101's fluorescence emission. Using polystyrene nanoparticles (PSNPs) labeled with both R101 and MB, we demonstrated fluorescence imaging of nanoparticle collision events on an Au UME, where individual particles exhibited fluorescence turn-on of R101 emission upon collision owing to the electrochemical reduction of surface-bound MB. Compared with direct imaging in the MB channel, the FRET readout provided substantially higher optical contrast and enabled clearer visualization of single-particle electrochemical processes. This work demonstrates a useful fluorescence transduction strategy for improving optical tracking in nanoparticle collision electrochemistry.