**4.1 Organic dye molecules**

Methylene blue (MB) is a kind of derivative of phenothiazine and widely used a redox indicator and electron transfer medium in electrochemical analysis. For a typical DNA detection using MB labels, the distance between the MB and the electrode surface is adjusted by the change of conformation of the DNA probes labeled with MB, so that the peak current or the change in impedance can indicate the presence of the target DNA and quantitative concentration. This method is simple and versatile, however easy to be influenced by the solution environment. Despite MB, other organic dyes, such as gentian violet, ethyl green, Hoechst 33258 are also utilized in the electrochemical works. They are not as popular as MB, but show good performance in some biosensors.
