A pH responsive electrochemical switch sensor based on Fe(notpH3) [notpH6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)]

The electrochemistry of a macrocyclic metal complex Fe(notpH3) [notpH6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)] reveals that the protonation/deprotonation of the non-coordinated P–OH groups in Fe(notpH3) affects its formal potential value (E0′) considerably. Plotting E0′ as function of solution pH gives a straight line with a slope of −585 mV pH−1 in the pH range of 3.4–4.0, which is about ten times larger than the theoretical value of −58 mV pH−1 for a reversible proton-coupled single-electron transfer at 20 °C. A sensitive pH responsive electrochemical switch sensor is thus developed based on Fe(notpH3) which shows an “on/off” switching at pH ∼ 4.0.