Influence of the type and concentration of oxidant on the photoacoustic response of polypyrrole nanoparticles for potential bioimaging applications

The photoacoustic response of polypyrrole particles was studied in relation to the type and amount of oxidant used for their synthesis by photoacoustic spectroscopy. At the same time, a detailed study of the molecular structure of these particles was performed using infrared and Raman spectroscopy. It was found that the particles prepared using superstoichiometric amounts of the oxidizer, nPy:nOxi = 1:3, in the case of FeCl3 and stochiometric nPy:nOxi = 1:1, in the case of (NH4)2S2O8 - showed the best photoacoustic output. At ratios nPy:nOxi oxidants equal to or less than 1:2 in the case of FeCl3, oligomers and polypyrrole with a longer kinetic chain length might be formed, whereas the higher oxidant content no longer affects the quality of the product. In the case of oxidation by (NH4)2S2O8, amounts of oxidant higher than 1:2 led to the formation of over-oxidized products. Although nanoparticles prepared at the stoichiometric ratios of both oxidizing agents were equivalent in terms of the molecular structure determined by the IR and Raman spectra, the particles prepared by (NH4)2S2O8 show a slightly stronger PA signal.