Relationship between Nanostructure and Electrochemical/Biosensing Properties of MnO2 Nanomaterials for H2O2/Choline

Manganese dioxides of different crystalline structures with different dimensionality (amorphous MnO2 nanoparticles, alpha-MnO2 nanoparticles, and beta-MnO2 nanowires) were synthesized, characterized, and evaluated for their electrocatalytic activity to H2O2. Via a direct and facile electrochemical deposition method, MnO2 nanomaterials were codeposited onto glassy carbon (GC) electrodes with chitosan hydrogel. The catalytic oxidation current of amorphous MnO2 nanoparticles per unit mass to H2O2 is much larger than that of alpha-MnO2 nanoparticles and beta-MnO2 nanowires; however, the catalytic oxidation current of amorphous MnO2 per unit surface area to H2O2 is the same as that of alpha-MnO2 nanoparticles and much less than that of one-dimensional beta-MnO2 nanowires. The bicatalytic activity toward H2O2 of the electrodes modified with three different nanornaterials increases in the following order: amorphous MnO2 > alpha-MnO2 > beta-MnO2. Further codeposition of chitosan hydrogel, choline oxidase (ChOx), and different MnO2 nanornaterials onto GC electrodes was applied to form choline biosensors. The biosensors modified with crystalline MnO2 respond to choline far more quickly than that modified with amorphous MnO2 in amperometric measurements. For the same concentration of choline, the response time is 8 s, 25 s, and 5 min for biosensors modified with beta-MnO2, alpha-MnO2, and amorphous MnO2, respectively. The reasons for these phenomena were discussed in detail from the differences of the specific surface areas, the amounts of entrapped MnO2 on electrodes, crystalline structures, and dimensionality. The biosensors based on alpha-MnO2 nanoparticles and beta-MnO2 nanowires were applied on amperometric detections of choline chloride with the linear ranges of 2.0 x 10(-6)-5.8 x 10(-4) M and 1.0 x 10(-6)-7.9 x 10(-4) M with the detection limits of 1.0 and 0.3 mu M, respectively.