Conductive polymers have been extensively researched for the creation of polymer electronics, such as thin film transistors, photovoltaics and organic light emitting diodes. In these devices, the processability and mechanical properties of conductive polymers are improved in comparison to traditional inorganic materials. Though less explored, these same properties encourage the use of conductive polymers in biomedical applications. In particular, conductive polymers may be chemically and physically tailored to be similar to established biomaterials, with the added feature of possessing unique electrical properties. The incorporation of these electrical properties in biomaterials presents opportunities for systems integration in biomaterials.

In this work, the conductive polymer polyaniline (PANI) complexed with poly(2-acrylamido-2-methylpropane sulfonic acid) (PAAMPSA), referred to as PANI-PAAMPSA, is used for biomolecular sensing. The development of PANI-PAAMPSA as an important material for polymer electronics was recently noted, and the inherent doping system of the material may be useful as an integrated biomolecular sensor. PANI-PAAMPSA is generated by a template synthesis process, where the PAAMPSA is dissolved in water with aniline monomer, allowing the aniline monomers to complex with the sulfonic acid groups of the PAAMPSA. Oxidative polymerization is initiated to form the PANI chain, which is doped by the dissociated acid functional groups of the PAAMPSA. The resulting material is highly conductive, and the processability of the conductive polymer is significantly enhanced. The premise of this research is that the introduction of biomolecules with ionic functional groups will disturb the complexation between the PANI and the PAAMPSA, leading to a decrease in dopant charge and therefore a decrease in the polymer conductivity.

Initial testing with the synthesized PANI-PAAMPSA material demonstrated its potential for use as a biomolecular sensor. To explore this effect further, it was desired to have a polymer film adhered to a device substrate, with electrodes for probing the film conductivity, which could be repeatedly exposed to solutions with various biomolecules, rinsed with water. To ensure a fast response time, a film of nanosized thickness was also desired. To accomplish these objectives, a self-assembled monolayer technique was used to adsorb the PANI-PAAMPSA to the device substrate. Silane treated device substrates were immersed in the PANI-PAAMPSA reaction solution, and the polymers adsorbed to the surface as the chains polymerized. This synthesis method has been previously described as an in situ polymerization method, however our methods differ in that we are using the template synthesis process to generate a film with PAAMPSA incorporated as a stable large molecule dopant, which does not easily rinse out with water or with a pH or ionic change in the environmental conditions, enabling the film to be used as a functional biomolecular sensor.

The produced films were characterized by profilometry, x-ray photoelectron spectroscopy (XPS), UV-VIS spectroscopy, and the electrical conductivity was measured using a 4 point probe technique. The films were characterized in their as-synthesized form, and upon exposure to a range of pH-controlled buffers. The films demonstrated a 3 order of magnitude change in conductivity when the pH was increased to 7.4, and the response time was less than 5 minutes. Additionally, upon rinsing with deionized water, the films recovered their conductivity.

The fabricated devices are highly sensitive and show promise as a platform for a biomolecular sensing device. The material's electrical properties, coupled with mechanical and physical properties ascribed to the PAAMPSA, enable the introduction of electronic properties into customizable biomaterials with integrated sensing properties.

This work was supported by the NSF-IGERT program grant DGE-0333080.