Three-Mediator Enhanced Collisions on an Ultramicroelectrode for Selective Identification of Single Saccharomyces cerevisiae

Several studies deal with the use of redox mediators for probing the electrochemical activity of a bacterium.

“In order to develop single-impact electrochemistry toward the identification of bacterial cells, several studies deal with the use of redox mediators for probing the electrochemical activity of the bacterium [32][33][34].”

Menadione, a hydrophobic mediator, can selectively penetrate the S. cerevisiae membrane, access its intracellular redox center, and react with K3Fe(CN)6 in bulk solution.

“The hydrophobic mediator (menadione) can selectively penetrate through the S. cerevisiae membrane and get access to its intracellular redox center, then can further react with K 3 Fe(CN) 6 in the bulk solution (Fig. 3b, top left) [34].”

Chen et al. reported a three-mediator system (K3Fe(CN)6, K4Fe(CN)6, and menadione) that achieves redox activity analysis and selective identification of single Saccharomyces cerevisiae without antibodies.

“In a recent study, Chen et al. have reported a three-mediator system (K 3 Fe(CN) 6 , K 4 Fe(CN) 6 , and menadione) for achieving redox activity analysis and selective identification of single Saccharomyces cerevisiae (S. cerevisiae, a yeast) without the use of antibodies (Fig. 3b, top) [34].”

Single collisions of microorganisms onto the UME surface involve a blocking effect on potassium ferrocyanide diffusion near the UME surface.

“Thus, single collisions of microorganisms onto the UME surface involve a blocking effect on the potassium ferrocyanide diffusion near the UME surface (as discussed in the first section of this review) [34].”

Redox reactions between S. cerevisiae cells and mediators reduce K3Fe(CN)6 to K4Fe(CN)6 and increase the local concentration of K4Fe(CN)6.

“However, the redox reactions between the S. cerevisiae cells and mediators also reduce K 3 Fe(CN) 6 to K 4 Fe(CN) 6 and increase the local concentration of K 4 Fe(CN) 6 at the same time.”

Because the current signal generated by S. cerevisiae is opposite to those from E. coli and S. aureus in the three-mediator system, S. cerevisiae can be selectively detected and distinguished in a mixed microbial suspension.

“Due to the opposition between the current signal generated by S. cerevisiae and the ones from E. coli and S. aureus in the three-mediator system, S. cerevisiae can be selectively detected and distinguished in the mixed microbial suspension [34].”

When the three-mediator system is extended to E. coli and S. aureus, different electrochemical collision signals are observed.

“In contrast, different electrochemical collision signals have been observed when the three-mediator system experiments are extended to E. coli (Fig. 3b, bottom) and Staphylococcus aureus (S. aureus, Gram-positive) bacteria (not shown here) [34].”

The three-mediator system work provides a proof of concept for selective detection of single S. cerevisiae cells from a mixed microbial consortium based on the electrochemical collision technique.

“Hence, this works provides a proof of concept for the selective detection of single S. cerevisiae cells from a mixed microbial consortium, based on the electrochemical collision technique [34].”

In the three-mediator system, i-t curves for S. cerevisiae show positive current step transients combined with a steady-state current increase, corresponding to impacts and adsorption of S. cerevisiae cells onto the UME surface.

“Hence, the corresponding i-t curves for this microorganism in the threemediator system show positive current step transients, combined with a steady state current increase (Fig. 3b, top right), corresponding to the impacts and adsorption of S. cerevisiae cells onto the UME surface [34].”

In the three-mediator system, an initial current arises from continuous oxidation of K4Fe(CN)6 to K3Fe(CN)6 at a carbon fiber UME (7 μm diameter) polarized at +0.7 V vs Ag/AgCl, corresponding to the diffusion-limited steady state.

“In the three-mediator system, there is an initial current due to the continuous oxidation of K 4 Fe(CN) 6 to K 3 Fe(CN) 6 at the carbon fiber UME (diameter: 7 µm) polarized at +0.7 V vs. Ag/AgCl (corresponding to the diffusion-limited steady state) [34].”

In the three-mediator system, E. coli and S. aureus only generate downward current steps because blocking of mediator diffusion suppresses their redox activities, so only blocking collisions are observed in their i-t curves.

“Indeed, both E. coli and S. aureus only generate downward current steps because the blocking effect of mediator diffusion suppresses their redox activities (illustrated in Fig. 3b, bottom left for E. coli), thus only blocking collisions are observed in the i-t curves (shown in Fig. 3b, bottom right for E. coli) for these two types of bacteria in the three-mediator system [34].”