Martin Caldarola
Leiden University
Martin Caldarola is a postdoctoral researched at the Single-Molecule Optics group at Leiden University, led by Prof. Michel Orrit. He holds a PhD in Physics from the University of Buenos Aires and his expertise is in the field of nanophotonics. His research interest are focused on enhanced light-matter interactions, including fluorescence enhancement, nonlinear effects in the nanoscale and the interaction between single molecules and photonic nanostructures.
Introduction
Electrochemical (EC) reactions are of crucial importance in diverse fields of nanoscience. Here we achieved electrochemical measurements with fluorescent readout of the redox-sensitive dye, Methylene Blue (MB) at single-molecule (SM) level. Figure 1 shows the two-electron reduction reaction for MB, which is fluorescent in its oxidized state and non-fluorescent in the reduced state. Thus, by detecting the emitted fluorescent photons we can optically read the oxidation state of MB molecules.
Methods
In order to control the oxidation state of the molecules accurately and measure the optical response simultaneously we combined a fluorescence confocal microscope with a potentiostat. We work with immobilized MB molecules on a glass surface and we used two configurations, shown in figure 2: an ensemble of MB molecules (see Figure 2a), where no enhancement is needed, and single-molecule scheme, where we used gold nanorods to enhance fluorescence (see figure 2b). The surface chemistry was similar in both cases, with the addition of the gold nanorods and a lower MB superficial concentration to ensure that there is only one enhanced molecule at the tip of the nanorod.
Results and discussion
Firstly, we measured the fluorescence intensity from an ensemble of MB molecules at different applied electrochemical potentials to show our capability of reading the redox state optically, as evidenced in the curve of figure 3a. From these data we obtained a mid-point potential of 51 mV, shown with the dashed vertical line.
Secondly we studied the redox-induced blinking of single MB molecules. We measured the same molecule at different EC potentials (see figure 3b) and found that the blinking off-times increase when decreasing the EC potential (and vice versa for the on-times). We then plotted and fitted the ratio of on and off mean times at different potentials to extract the mid-point potential for every measured SM using the Nernst equation (figure 4a), obtaining the histogram of mid-point potentials shown in figure 4b.
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.
