Effect of Mediator Spacing on Electrochemical and Enzymatic Response of Ferrocene Redox Polymers

Abstract

The effect of mediator distance from the polymer backbone on the redox behavior, electron transport, electrochemical stability, and electrical communication with redox enzymes was studied with novel linear poly(ethylenimine) redox polymers. To measure these effects, we synthesized two new ferrocene redox polymers having a three-carbon (Fc-C 3 -LPEI) and a six-carbon (Fc-C 6 -LPEI) spacer to complement our previous synthesized polymer (Fc-C 1 -LPEI) having a 1-carbon spacing. Increasing the spacer length to either three or six carbons resulted in a single peak redox behavior over the entire pH tested (3-11), which is in contrast with the multiwave redox behavior observed with a 1-carbon spacing under neutral or basic pH solutions. In addition increasing the spacer also resulted in a six-fold increase in the electrochemical stability of crosslinked redox polymer films. In contrast with previous reports of electron transport increasing with spacer length, we observed no correlation between mediator spacing and electron transport or electrical communication with the enzyme glucose oxidase. Surprisingly, the redox polymer (Fc-C 3 -LPEI) with the lowest electron transport produced the highest enzymatic response (>1 mA/cm 2 ), suggesting that other factors (e.g., polymer-enzyme complexation) were important. These results demonstrate how small changes in the redox polymer structure can affect its properties.

Electrochemical Characterization of Layer-By-Layer Assembled Ferrocene-Modified Linear Poly(ethylenimine)/Enzyme Bioanodes for Glucose Sensor and Biofuel Cell Applications

“…In contrast, LBL assembled (Fc-C 3 -LPEI/p-GOX) 16 biosensors are capable of generating as much, and even more, current than solution cast films. 6 When using the short wash method, the films actually surpass the current output of the previously reported glucose biosensors using the same materials. This is important for two reasons: processing time and amount of material.…”

“…6 They reported that decreasing the mediator spacing between ferrocene and LPEI from six to three methylene groups increased the sensitivity, maximum enzyme response, and stability of the films. On the contrary, Mao et al reported a significant increase in electron diffusion when osmium redox centers are extended further away from the polymer backbone.…”

“…which is consistent with previous results from solution cast films. 6 Decreasing the wash time also led to an increase in sensitivity for both (Fc-C 6 -LPEI/p-GOX) n and (Fc-C 3 -LPEI/p-GOX) n films. We had previously reported that the sensitivity of (Fc-C 6 -LPEI/p-GOX) n films assembled using the long wash 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 21 procedure compared favorably with LBL films fabricated using osmium-based redox polymers, and were significantly higher than LBL films constructed using other ferrocene-based redox polymers.…”

“…The CV also showed a characteristic decrease in the half-wave potential (E 1/2 ) with the decrease in tether length that corresponds to previous studies. 6 The E 1/2 values for Fc-C 6 -LPEI and Fc-C 3 -LPEI using the long wash were 0.33 V and 0.27 V, respectively. Significant changes in E 1/2 with the number of additional bilayers were not observed, suggesting that the ferrocene moieties are, on average, in the same electronic environment.…”

How the reduction of O2 on enzymes and/or redox mediators affects the calibration curve of “wired” glucose oxidase and glucose dehydrogenase biosensors

“…This is likely the case for a given hydrogel composition hydrated in a specific environment. Indeed, it has been noticed that the degree of reticulation [22,25], the nature of the enzyme and its weight percent in the hydrogel [26], the nature of the polymer [27,28], the oxidation state of its complexes [29] and the nature of the buffer (temperature, pH, ionic strength) [16,22,24,30,31] can affect the degree of hydration of the hydrogel and therefore its thickness. Since our electrodes were modified with the same hydrogel composition and were evaluated under the same experimental conditions, in the following, we assume that the hydrogel thickness increased with .…”