Ammonia Sensing Behavior of Green- Synthesized ZnO Nanoparticles Using Optical Density as Sensing Parameter

Detection of ammonia in liquid medium at lower concentration levels [50 parts per million (ppm)] is a crucial need for environmental health. In this work, we reported a novel approach that used zinc oxide (ZnO) nanoparticles as the sensing substrate coupled with an optical interrogation method. Instead of using the conventional method to synthesize ZnO nanoparticles, we utilized a green synthesis (environmentally friendly) method that used commonly available lemon and orange peel extracts, where these extracts acted as the reducing and stabilizing agents. ZnO samples synthesized using lemon and orange peel extracts and calcined at 600 °C for 1 h, i.e., ZnO-L3 and ZnO-O3, respectively, that exhibited higher optical density, were selected for sensing liquid ammonia. We conducted parallel investigations to characterize the salient properties of these sensing substrates. Briefly, the optical band gaps of ZnO-L3 and ZnO-O3 were estimated to be 3.15 and 3.05 eV, respectively. Le-Bail refinement of the X-ray diffraction (XRD) patterns of ZnO-L3 and ZnO-O3 revealed that the cell parameters agreed well with prior reported data. The average particle sizes of ZnO-L3 and ZnO-O3 obtained from transmission electron microscope (TEM) images were 52 and 48 nm and were close to the crystallite size estimated from the Williamson–Hall equation. Subsequently, we tested these samples for sensing liquid ammonia of different concentrations (0–500 ppm) using optical density as the independent sensing parameter. The coefficients of determination (
${R} ^{{2}}$
) of the prediction models for liquid NH3 sensing were higher than 0.98 for both the ZnO-L3 and ZnO-O3, indicating the proof-of-the-concept validation of this approach for real-world applications.