Layer-by-layer assembly of ferrocene-modified linear polyethylenimine redox polymer films

Herein, both electrostatic and covalent layer-by-layer assembly were used for the construction of multicomposite thin films using a ferrocene-modified linear poly(ethylenimine) redox polymer (Fc-C6-LPEI) as the cationic polyelectrolye, and poly(acrylic acid) (PAA), poly(glutamic acid) (PGA), or glucose oxidase (GOX) as the negative polyelectrolyte. The assembly of the multilayer films was characterized by cyclic voltammetry (CV), UV/Vis spectroscopy, and ellipsometry with the enzymatic response of the films containing GOX being characterized via constant potential amperometry. CV measurements suggested that the successful buildup of multilayer films was dependent upon the nature of the anionic polyelectrolyte used. Electrostatic assembly of films composed of Fc-C6-LPEI and either PAA or PGA produced large oxidation peak current densities of 630 and 670 μA cm(-2), respectively, during cyclic voltammetry. Increased measured absorbance by UV/Vis spectroscopy and increased measured film thicknesses (400-600 nm) by ellipsometry provided additional evidence of successful film formation. In contrast, the films incorporating GOX that were electrostatically assembled surprisingly produced significantly lower electrochemical responses (12 μA cm(-2)), low absorbance values, and reduced film thicknesses (~15 nm), and glucose electro-oxidation current densities less than 1 μA cm(-2), which all suggested unstable or minimal film formation. Subsequently, we developed a covalent layer-by-layer approach to fabricate films of Fc-C6-LPEI/GOX by covalently linking the amine groups of Fc-C6-LPEI to the aldehyde groups of periodate-oxidized glucose oxidase. Covalent assembly of the Fc-C6-LPEI/GOX films produced oxidation peak current densities during cyclic voltammetry of 40 μA cm(-2) and glucose electro-oxidation current densities of 220 μA cm(-2). These films also showed an increase in their thicknesses (~140 nm) relative to the electrostatic GOX films. For the films containing either PAA or PGA, the pH of the polymer solutions used for construction was found to have a significant effect on the response of the multilayer films, and the electrochemical response of the Fc-C6-LPEI/PAA, Fc-C6-LPEI/PGA, or covalently assembled Fc-C6-LPEI/GOX films could be tuned by varying the number of bilayers (n=1-16) in the film. These results are important because this is the first report of the use of the novel Fc-C6-LPEI redox polymer in the successful development of multicomposite layer-by-layer films. The electrochemical response achieved with the covalently assembled Fc-C6-LPEI/GOX films demonstrates that this redox polymer and layer-by-layer assembly technique can be used for possible biosensor and biofuel applications, and the success of multiple anionic polyelectrolytes could lead to additional applications with other enzyme systems.