| Summary: | Worldwide, more than a million people die from tuberculosis (TB) every year. Although the disease is curable, treatment is complicated by multi-drug resistant and extensively drug-resistant TB strains. To detect TB and differentiate between its strains, a sensitive and specific point-of-care device is required. Previous studies show that carminic acid (CA), an anthraquinone derivative, is suitable as an electrochemical molecular beacon due to the ability to switch on and off its electrochemical activity on its dimerization.
Characterization of the electrochemical activity of CA at low concentrations (1 μM to 1 mM) over a range of pH values was performed using methods such as cyclic voltammetry, square wave voltammetry and Koutecky-Levich analysis on a rotating disk electrode. CA species of different protonation, which are predominant at pH 1.1, pH 4.1, pH 6.6 and pH 10.5, were examined in more detail. All measurements were carried out on a glassy carbon electrode in phosphate buffer solution electrolyte.
It was found that CA undergoes a diffusion limited two proton two electron redox reaction with an overall peak potential shift of 61 mV per pH unit. Electrochemical measurements of the fully protonated CA resulted in additional current peaks that were assigned to an adsorption process of a CA reduction product. Generally, CA has faster electron transfer kinetics in more acidic environment and no electrochemical activity was observed for the fully deprotonated CA species at pH 10.5. While SWV could be used for quantitative analysis of CA for the concentrations up to 1 mM, its redox current signal was determined not to be concentration dependent at high measurement frequencies. These frequencies can also be adjusted to be more sensitive towards either the redox peak potentials with sharper peaks at low frequencies or the electron transfer kinetics based on kinetic dependent peak currents at high frequencies. The limit of detection for CA at pH 7.0 was found to be as low as 10 nM when measured using 200 Hz SWV. === Science, Faculty of === Chemistry, Department of === Graduate
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