Modification of the Electrochemical Properties of Nile Blue through Covalent Attachment to Gold As Revealed by Electrochemistry and SERS

Conventional electrochemistry and surface-enhanced Raman scattering (SERS) spectroelectrochemistry are used to probe the redox reaction of Nile Blue immobilized on gold electrodes. Covalent attachment of Nile Blue (NB) to gold through carbodiimide cross-linking shows the appearance of a second, new redox reaction, unobserved by solution phase or physisorbed NB. Each redox reaction is characterized by differential pulse voltammetry to reveal two individual one-proton, one-electron electrochemical reactions. SERS spectroelectrochemistry along with electrochemical characterization of structurally similar Cresyl Violet are used to assign the electrochemical (de)­protonation of the terminal amine and phenoxazine nitrogen to the lower and higher energy redox reactions, respectively. Analysis of covalently bound NB via azide–alkyne click chemistry supports the hypothesis that the electron-withdrawing carbonyl formed during the carbodiimide cross-linking induces a change in the electronic structure of NB, causing a shift in the terminal amine redox reaction to lower energy.