Redox switching of carbohydrate binding to ferrocene boronic acid

The redox properties of ferrocene boronic acid in aqueous buffers from pH 4 to pH 12 have been investigated using cyclic voltammetry. It is shown that the proton transfer equilibrium is under thermodynamic control over the range of scan rates employed (20-5000 mV/s), leading to pK(a) values of 10.8 and 5.8 for the ferrocene and ferrocenium forms, respectively. In the presence of sorbitol, fructose, or glucose, the voltammetric behaviour of ferrocene boronic acid at pH 7 is consistent with reversible formation of a carbohydrate - boronic acid complex. However, in all cases, the complexation-dissociation is under kinetic control. Analysis of the voltammetric response as a function of scan rate and carbohydrate concentration, together with an independent spectroscopic determination of the binding constant to the ferrocene form, allows all of the kinetic and thermodynamic constants for each system to be determined. In general, it is found that the binding constants ferrocenium form are about two orders of magnitude greater than those for the ferrocene form. It is possible that this redox dependent switching of the carbohydrate binding can be exploited in amperometric or potentiometric sensing devices.