Leiden University, Netherlands
Martin Caldarola is a Postdoctoral Researcher at the Single-Molecule Optics group, Leiden University, Netherlands. He holds a PhD in Physics from the University of Buenos Aires and his expertise is in the field of nanophotonics. His research interest is focused on light-matter interactions, specifically the interaction between single-molecules and nanostructures, including fluorescence enhancement and nonlinear effects in the nanoscale.
Electrochemical (EC) reactions are of crucial importance in diverse fields of nanoscience. Methylene Blue (MB) is well-known redox indicator that can be used as nanoscale probe of the electrochemical environment. With this aim we studied the fluorescence redox-induced blinking of single MB molecules: this molecule is fluorescent in the oxidized state and non-fluorescent in the reduced state. Due to the low fluorescence yield of this molecule Single Molecule (SM) optical sensitivity was achieved employing the high fluorescence enhancement factors provided by individual gold nanorods for weak red fluorophores. Our experimental approach combined single-molecule sensitive confocal setup with an electrochemical cell which allowed the fluorescence readout for an externally-controlled oxidation state of the MB molecule. We work with immobilized MB molecules on a glass surface and we observe the molecules that are enhanced near the tip of the nanorod. We could study the same MB molecule at different electrochemical potentials and we observed that the fluorescence blinking responds to the EC potential change. We performed this type of measurement for several single molecules and from the Nernst Equation we extracted the mid-point potential value, the potential where the molecule spends half of the time in the reduced (dark) state and half of the time in the oxidized state (fluorescent). We found a small dispersion for this value of about 21 mV in a mean value of 78 mV. In conclusion, we accessed the electrochemical properties of methylene blue at single-molecule level using an optical method based on fluorescence enhancement by individual gold nanorods.