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(notpH(3)) [notpH(6)=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(notpH(3)) affects its formal potential value (E(0)') considerably. Plotting E(0)' 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(notpH(3)) which shows an "on/off" switching at pH ∼ 4.0.