Effects of electrolyte and pH on the behavior of cross-linked films of ferrocene-modified poly(ethylenimine)

Ferrocene redox polymers based on the coupling of ferrocenecarboxaldehyde to both linear and branched poly(ethylenimine) (PE1) have been prepared to investigate the effects of pH, electrolyte, and cross-linking on electron charge transport and film swelling. The redox behavior of both ferrocene-modified linear PEI and ferrocene-modified branched PEI was investigated by cyclic voltammetry, while electron diffusion coefficients reported for PEI-based redox polymers were determined by electrochemical impedance spectroscopy. In phosphate solutions at pH > 7, cross-linked films of both redox polymers exhibited multiple redox wave behavior and were unstable. In contrast, in non-phosphate solutions, cross-linked films exhibited stable electrochemical behavior and fast electron transfer in solutions with pH < 11. Gel swelling experiments suggested that the multiple wave behavior and instability exhibited in either phosphate solutions or at high pH in non-phosphate solutions were related to a combination of film collapse and electrolyte binding within the hydrogel. The electron diffusion coefficients for these polymers are on the order of 10(-8) (mol cm(-1) s(-1/2)), which are approximately 40 times greater than other ferrocene-modified polymers. Incorporation of the enzyme, glucose oxidase, into these films demonstrated that these redox polymers were able to electrically communicate with the enzyme's flavin adenine dinucleotide (FAD) redox centers. Glucose sensors based on these films exhibited enzyme saturation current densities that ranged from 240 to 48 mu A/cm(2) in response to glucose, which were dependent upon the supporting electrolyte and pH. The sensitivity of these sensors at 5 mM glucose ranged from 10 to 48 mu A center dot cm(-2)center dot mM(-1).