Fig. 5. (a) Upper panels: SECM imaging of PYO (∼2.7 μM) release from a P. aeruginosa aggregate in a microtrap. The SECM tip was biased at 0 V vs. Ag/AgCl to oxidize PYO. Lower panels: PYO was used as a proxy for P. aeruginosa QS-controlled communication between physically separated populations using two mutant strains. Aggregates of ≥5000 Δphz cells induced PYO production in the neighbouring ΔrhlI aggregate. (b) Electrochemical imaging of 2 day anaerobically grown ΔphzH biofilms after 4.25 h of oxygenation based on SWV. Phenazine concentration was calculated from the peak currents. Bottom left: exemplary SWV with peak indication. Bottom right: exemplary cross-section from the electrochemical image. (a) Adapted with permission from ref. 65. Copyright 2014 National Academy of Sciences. (b) Adapted from ref. 67. Copyright 2016 Springer Nature.