Preparation and Characterization of Metal Oxide Sensors and Their Application for the Detection of Organophosphate Insecticides
Authors: Bravo Hualpa Fabiola , Trevejo-Pinedo Jorge , Sun Kou María del Rosario , Cárcamo Henry 
Published: May 2020

This study focuses on the synthesis and characterization of metal oxides doped with transition metals with the purpose of enhancing their sensitivity to detect organophosphate insecticides, Chlorpyrifos and Malathion, on air. The moderate toxicity of these organophosphorus pesticides and their extensive use in small-scale agriculture reveals the urgency of continuous air quality monitoring in the work environments of communities with restricted access to sophisticated techniques for detecting these contaminants, such as high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS).
The selected metal oxides are Zr doped zinc oxide (ZnO) and tin oxide (SnO2) doped with both Pt and Zr. These semiconductors (ZnO and SnO2) have the advantage of showing a sensing response at low working temperatures, in the range of 200°C and 250°C. Additionally, both have great thermal and chemical stability. The synthesis of ZnO is performed through the application of a hydrothermal synthesis methodology in an autoclave; while ZnO doping is conducted by adding the dopant precursor (ZrOCl2) to the reaction mixture. In the case of SnO2, commercial tin oxide is used for doping with Zr and Pt. The doping process was carried out in two separate processes: 1) mechanical mixing and sintering of ZrO2 and SnO2, and 2) chemical reduction of H2Cl6Pt with SnSO4 on the surface of SnO2. The characterization of these materials is carried out by employing X-ray diffraction (DRX), Fourier-transformed infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), scanning electron microscopy / energy dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM) and sorption of N2 analysis. In addition, the quantification of pesticide concentration in commercial samples, that are used in the sensing essays, is performed with HPLC.
The Pt doped SnO2 is prepared with a content of Pt between 0.05 and 0.2 wt. %, while the concentration of Zr is in the range of 0.3 and 0.7 wt. %. In the case of ZnO, the concentration of Zr is between 0.9 and 2.0 wt. %. The characterization by DRX reveals a substitutional doping for both materials. Additionally, the dopants produce an increase of the lattice strain and a small contraction of the unit cell in both metal oxides. The SEM analysis of Zr doped ZnO allows to identify changes in their morphology, which confirms an increase in particle size at Zr concentration of 2.0 wt. % in comparison to 1.2 wt. %.
The sensing tests of the doped metal oxides are evaluated in an electronic nose. This equipment allows to test four sensors each time and the vapors of the pesticides are produced through a bubbling process of a liquid suspension of one organophosphate insecticide in water. The prepared doped oxides present an enhanced sensibility for the detection of organophosphates on air. Response signals are obtained with greater stability for long sensing times with zinc oxide sensors doped with Zr at 2.0 wt. % in comparison to sensors of Zr doped ZnO with lower Zr content.
A study of the optimum temperature conditions (between 210°C and 220°C) and concentration of the dopant in zinc oxide and tin oxide is carried out. The temperature has a positive effect on the sensitivity of all the sensors tested. At the test temperature of 220°C, the sensitivity of the best sensors (AT-Zr-2.0-ZnO and Pt-0.13-ZrO2-0.15-SnO2) for Malathion can be maximized. In the case of Chlorpyrifos, better results are obtained for the Pt-0.13-ZrO2-0.15-SnO2 sensor, whose signal is favored at 210°C. In addition, the sensitivity of Pt-0.13-ZrO2-0.15-SnO2 with respect to AT-Zr-2.0-ZnO at 220°C is higher for Chlorpyrifos, but lower for Malathion.
The statistical treatment method of principal component analysis (PCA) allows the evaluation of the signals of samples with different concentrations of pesticides. The best PCA, obtained with the data of the measurements using the most sensitive sensors, show a total explained variance greater than 90% and a better differentiation between air samples contaminated with pesticides and air samples without contamination.

Figure 1