Development of a gas sensing nanofibrous membrane for asthma detection

Abstract

In this study, a membrane consisting of zinc oxide nanofibers was formed through electrospinning process in order to be used in gas sensing tests for determining the nitrogen oxides (NOx) gases which exist in the exhaled breath of asthma patients more than those of healthy people. Various attempts had been made to get the best result concerning the ease and continuity of the process at production step. Polyvinyl alcohol (PVA) and Zinc acetate dihydrate (Zn(OAc)2·2H2O) were used as precursor substances for membrane formation. Zinc oxide (ZnO) nanofibers-based membranes were obtained after calcination process. In order to determine the best polymer concentration for electrospinning solutions; PVA solutions at concentrations ranging from 7% to 20% (w/v) were prepared, firstly. According to the rheological study results, 15% PVA (w/v) was found the most proper concentration among them in terms of the level of the polymer chain entanglement that provided sufficient viscosity. Four different mass ratio of Zn(OAc)2·2H2O with respect to the polymer concentration; 1:0.5, 1:1, 1:1.5 and 1:2 were introduced to the solutions for finding the most proper electrospinnable solution for ZnO formation. Some physicochemical and rheological analyzes such as pH, conductivity, viscosity and surface tension measurements were carried out in order to evaluate the electrospinnability of each solution and also to suggest the possible reactions of intermediate products formed in aqueous medium. The conductivity, surface tension and viscosity values of the solutions increased with an increase in Zn(OAc)2·2H2O concentration; however, a decrease in pH was observed. This was probably due to the consumption of the hydroxide ions in PVA / Zn(OAc)2·2H2O solutions, in order to form an aqueous intermediate product, zinc hydroxide Zn(OH)2. Moreover, thermogravimetric analysis (TGA) was carried out for defining suitable thermal parameters for calcination process. The scanning electron microscopy (SEM) analysis was employed to the membranes prepared both before and after calcination processes. As far as the all analyzes and measurements were concerned, it was estimated that the formation of ZnO overlapped with the late decomposition of PVA during the thermal process, around 600°C. The formation of ZnO proceeded through the transformation of Zn(OH)2 by loss of water molecule. Among the prepared samples, 15% PVA solution with equivalent amount of Zn(OAc)2·2H2O showed the best performance considering the fineness of nanofibers and the amount of products or reactants produced or used; however, the continuity and stability at the region of taylor cone and jet were not found satisfactory enough. That is the reason why, various attempts were applied to all samples in order to get more optimized electrospinning process. In the last part of the study, the solutions were re-prepared and investigated in acidic and basic medium, in water:ethanol binary solvent system and in the presence of surfactant. After conducting all necessary analyses, it was concluded that adding 1% (w/v) non-ionic surfactant to the solution of 15% PVA: 15% Zn(OAc)2·2H2O provided more stable and continuous electrospinnability predominantly thanks to the decrease in surface tension of the solution.