Electrochemical Interrogation and Sensor Applications of Nanostructured Polypyrroles

The polymerization of pyrrole in β-naphthalene sulfonic acid (NSA) gave nanotubules, nanomicelles or nanosheets of polypyrrole (PPy) depending on the amount of NSA in the polymer and the temperature of the reaction. Scanning electron microscopy (SEM) measurements showed that the diameters of the nanostructured polypyrrole-β-naphthalene sulfonic acid (PPyNSA) composites were 150–3000 nm for the tubules, 100–150 nm for the micelles and 20 nm for the sheets. A red shift in the UV-vis absorption spectra of PPy was observed for PPyNSA which indicates the involvement of bulky β-naphthalene sulfonate ion in the polymerization process. The UV-vis also showed the existence of polaron and bi-polaron in the polymer which may be responsible for the improved solubility of PPyNSA compared to PPy. All the characteristic IR bands of polypyrrole were observed in the FTIR spectra of PPyNSA, with slight variation in the absolute values. However, the absence of NH stretching at 3400 cm−1 and 1450 cm−1 usually associated with neutral polypyrrole confirms that the polymer is not in the aromatic state but in the excited polaron and bipolaron defect state. Electrochemical analysis of PPyNSA reveals two redox couples: a/a′ – partly oxidized polypyrrole-naphthalene sulfonate radical cation/neutral polypyrrole naphthalene sulfonate; b/b′ – fully oxidized naphthalene sulfonate radical cation/partly reduced polypyrrole-naphthalene sulfonate radical anion. The corresponding formal potentials measured at 5 mV/s, E°′(5 mV/s), are 181 mV and 291 mV, respectively. Amperometric phenol sensor constructed with PPyNSA on a glassy carbon electrode (GCE) gave a sensitivity of 3.1 mA M−1 and a dynamic linear range of 0.65–139.5 μM. The data for the determination of phenol on the GCE/PPyNSA electrode was consistent with the electrocatalytic Michaelis-Menten model, giving an apparent Michaelis-Menten constant (KM′) value of 160 μM.