REPOSITORY > RESULTS

Abstract

An everyday awareness of the presence of harmful chemicals around us has led to the development of a way to report overexposure through the reactions of these compounds with specifically prepared materials - sensory elements. In this doctoral dissertation we focused on the electrochemical synthesis (electropolymerization) of conductive polymer polyaniline (PANI) and on its ability to detect ammonia (NH3). The main objectives included the electrochemical synthesis of PANI, the study of its electronic and electrochemical properties and its use for detecting NH3. The latter was detected in the gaseous and aqueous states, which divided this work into three chapters presented in scientific articles.
NH3 is a well-known compound, indispensable in industry, part of the natural nitrogen cycle and a common decomposition product of nitrogen compounds. Due to its extremely irritating and harmful effects when exposed to excessive concentrations and its presence in exhaled air as a consequence of organ failure, the detection of NH3 gas is of great scientific interest. PANI, the most commonly studied and widely used conductive polymer, is known for its specific interaction with NH3, which is why it is also a commonly used material for constructing NH3 sensors.
In our study, PANI was prepared electrochemically from 0.1M aniline (ANI) in 1.0 M HCI solution by cyclic voltammetry (CV) from −0.3 to 1.0 V vs Ag reference electrode at 50 mV s−1. The resulting polymer was directly deposited on a working electrode of commercial screen-printed electrodes (SPEs). Since PANI’s electrochemical, optical and electronic properties are interconnected, their correlation during the PANI formation was observed with the spectro-electrochemical technique. The experiment was performed in-situ in the specifically designed electrochemical cell for SPEs with space for the optical probe. The CV result shows the ANI oxidation at a high positive potential (0.9 V vs Ag), the successful deposition of PANI during each cycle, observed in a current increase, and characteristic electrochemical transformations of already-deposited PANI (from leucoemeraldine, emeraldine, to pernigraniline). While the spectroscopic results showed characteristic absorbance peaks such as for the formation of dimers, the formation of charge carriers during the PANI polymerization, and the structural changes in PANI (i.e., quinoid ring formation). The PANI electropolymerization process carried out by CV under certain conditions produces a compact deposition of PANI directly on the SPE in a conductive protonated emeraldine form suitable for further use in sensorics.
Such an electrochemically synthesized PANI layer, compactly attached to the SPE (PANI-Au-SPE), was the starting material for producing NH3 gas-sensor platforms. The PANI-deposit was characterized with scanning electron microscopy (SEM), profilometry, and Fourier-transformation infrared spectroscopy (FTIR) to determine the morphology, surface roughness and to confirm the form of emeraldine salt. The electrochemical preparation requires using a three-electrode system, while for subsequent sensor measurements, only two electrodes connected via the PANI were needed. The SPE was converted into a system suitable for electrical measurements by creating a new contact with Au sputtering, connecting the PANI to the counter and reference electrodes. The electrodes prepared in this way were exposed to the humidity and NH3 in the gas chamber, and the material's response was monitored by observing changes in the PANIs’ resistance. The simultaneous use of a commercial NH3 analyser confirms the immediate response and possibility for realistic measurements. Considering the system's simplicity from the point of view of preparation and the PANI itself (simple acid dopant, without the addition of nanomaterials), detections of low NH3 concentrations (detection limit 23 ppb) were achieved, giving the developed system an applied value.
As a highly water-soluble gas, NH3 is also present in water sources and biological fluids in the body, where it can indicate diseases and organ failure. The last part of the thesis focuses on the electrochemical detection of liquid NH3 in a neutral medium. In the previous study, electrochemically prepared PANI (PANIel) in the form of emeraldine salt demonstrated an excellent affinity for NH3 gas. Therefore, the same material was also tested against aqueous NH3. Due to the aim of the biomedical application, the phosphate buffered saline (PBS, pH = 7) was used as a working electrolyte. Awareness of PANI's electronic behaviour was important, as its conductivity depends on the electrochemical conditions and environmental pH. The PANIel electrochemical behaviour was therefore observed in acidic (0.1 M H2SO4) and neutral (PBS) media to determine the most appropriate conditions for further use in sensorics. NH3 detection was performed using chronoamperometry (CA) at a constant applied potential of 0.2 V vs Ag by injecting a 1μL sample aliquot into a 50 μL drop of PBS, placed directly on the SPE. The current response is explained by the mechanism involving PANIel deprotonation, PANIel re-oxidation due to the previous reduction caused by NH4+ oxidation. To improve the PANIel responses, Au NPs of different sizes were added, where 20-nm Au NPs (PANIel-Au20) showed the highest contribution to the PANIel’s electronic properties. Thus, the detection limit was lowered from 24.64 μM NH3 to 1.44 μM, i.e., 17 times, and the quantification limit form 51 μM NH3 to 2.55 μM, i.e., 20 times lower. The capability for real-sample measurement was studied by observing the PANIel-Au20 response to NH3 in artificial saliva of different pH values, where it exhibits a recovery rate of 90–99.5 %. Thus, the PANIel-Au20 demonstrated suitability for measuring more complex samples containing a variety of ions and consequently proved the high affinity of PANI for NH3.
Overall, the PANI prepared by electropolymerization on SPE demonstrated the applicability for NH3 gas and aqueous sensing, demonstrating the versatility of the studied material.