Synthesis, characterization, and application of polyaniline supramolecular structures
Authors:   , Oliveira Kairon Márcio de 
Published: June 2024

Polyaniline (PANI) is a well-known semiconductor polymer that can change its conductive characteristics by doping and dedoping processes using acidic and basic solutions, respectively. These characteristics allow these polymers to be applied in electronic, optical, and electrochemical sensor equipment. An alternative to improving the PANI polymer's applicability and efficiency is the production of supramolecular structures such as particles, tubes, rods, plates, and core shells. The production of these structures occurs by adding amphiphilic molecules (stabilizers) such as surfactants and polymers during the aniline polymerization, generating different morphologies as the final product. In this work, our objective was to produce PANI particles using the polymer Poly(vinyl pyrrolidone) (PVP) as a stabilizer and analyze the characteristics of these particles in optical tweezer measurements compared to inorganic semiconductor particles. Using the Dynamic Light Scattering (DLS) technique, it was possible to note that the hydrodynamic radius of the particles varies with pH, showing that when the polymer has a charge, the tendency is for the particles to have a smaller size. The scanning electron microscopy technique allowed us to evaluate whether the particles produced are spherical and not amorphous. The results of Raman spectroscopy confirmed the presence of PANI and PVP in the particles. Finally, characterization using optical tweezers showed that the absorption in PANI particles changes with laser power. It is possible to note that the particles do not follow the linear optics for small sizes; however, the behavior of the results was similar to that of the theoretical model. We also aimed to produce supramolecular structures of PANI and the stabilizer Sodium Dodecyl Sulfate (SDS) in the shape of tubes, rods, and plates and evaluate the influence of the oxidizing agent (ammonium persulfate (APS)) and the synthesis temperature on the morphology of the structures. It was found that for the ratio of [APS]/[Ani] equal to 1, the formation of plates occurs, and with a decrease in this ratio, the formation of rods is favored. Structures such as rods have a 1:1 ratio of aniline and SDS molecules. For high concentrations of APS, this ratio is equal to 4:1, showing that the number of aniline and SDS molecules in the unit cell that form the structures plays a fundamental role. Results from Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) show that for small concentrations of APS, there is no formation of polymers and only the presence of oligomers, making the samples poorly conductive, for the ratio of [APS]/[Ani] equal to 1, the formation of conductive polymers and the presence of oligomers occurs. The X-ray Diffraction technique shows that SDS makes the samples more crystalline than pure PANI. Electrochemical analyses were carried out to characterize the structures for application in ascorbic acid sensors. The Cyclic Voltammetry results made it possible to observe the presence of PANI oxidation (reduction) peaks for all samples, showing the quality of the samples. Chronoamperometry measurements were carried out in 15 different ascorbic acid concentrations, revealing that samples with SDS have a current response twice as high as pure PANI, increasing the sensor's sensitivity. Tests with commercial orange juice showed that the sensors are selective, reusable, and accurate, with an expected value variation of less than 10%. Finally, the influence of adding a p-phenylenediamine (PDA) copolymer on the synthesis of aniline with and without SDS was evaluated to improve previously produced sensors. The replacement of hydrochloric acid as a dopant with sulfuric acid and camphor sulfonic acid was also evaluated. Raman and FTIR showed that the formation of hybrid structures between aniline and PDA occurs, proving the formation of copolymers. It can be noted that the addition of sulfuric and camphor sulfonic acid increased the crystallinity of the structures without SDS. When SDS is added, the crystallinity of the structures does not change. Cyclic voltammetry measurements showed that samples synthesized using sulfuric acid have the highest capacitance values, making them chosen for applications in ascorbic acid sensors. Chronoamperometry measurements were carried out in 15 different concentrations of ascorbic acid, revealing that samples with SDS have a current response slightly higher than PDA and aniline copolymer, increasing the sensitivity of the sensor. Tests with commercial orange juice showed that the sensors are selective, reusable, and accurate, with an expected value variation of less than 17%. Keywords: Polyaniline; Conductive Polymers; Sensors; Polymer-surfactant composites.