With the recent advancement in artificial intelligence and machine learning (AI/ML), the electronic nose (e-Nose) technology has improved significantly in the last decade. However, relying on the signals produced by an array of gas and volatile organic compound (VOC) sensors used as the hardware of an e-Nose, the overall performance of the system is limited by the sensor types and sensing mechanisms. The majority of the commercially available gas/VOC sensors are using metal-oxide (MOX) thin films. Although MOX sensors are relatively stable, they are extremely power-hungry. Therefore, other materials have been investigated for gas and VOC sensing. In this work, we have focused on the application of organometallic compounds as a low-cost and low-power alternative to the MOX sensors. Copper Phthalocyanine (CuPc) has been tested in both forms of a chemiresistor and an electrochemical cell. In the electrochemical design, the material presented a unique selectivity to formic acid due to a protonation reaction. However, removing contaminants from the electrolyte of an electrochemical cell is challenging and limits the repeatability of the sensor response. In contrast, in a thin-film design (chemiresistor), CuPc responds to various VOCs including ethanol, isoproapanol, and acetone, but due to the lack of the protonation mechanism, the selectivity response was not observed. Our studies on ZnPc and CoPc, also, are promising for designing an array of metal phthalocyanines (MPcs) as the hardware of an e-Nose to address the shortcomings in the MOX technology.
Metalated phthalocyanines (MPcs) are a special class of organic semiconductors having unique chemical structures and properties. The material’s conjugated 18-π electron structure enables a strong fusion with carbon nano tubes (CNTs) which have a highly delocalized π electron structure. This strong binding between MPc and CNT facilitates the study of the material in an electrochemical cell. Here in this account, we report a novel method of identifying the selectivity and sensitivity trend of several MPcs with different metal cores through an electrochemical approach. Sensor electrode was prepared by drop casting the MPc solution on paper strips coated with CNT ink, and then tested in electrochemical cell in a 2-electrode fashion. The nature of the redox reactions between MPc and the VOC analytes (i.e., acetone, ethanol, isopropanol, and methanol) were studied through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometry. Four different metalated phthalocyanines (CoPc, CuPc, MgPc, and ZnPc) are studied here. Experimental results suggest large shift in conductivity in the CoPc sensor upon exposure to ethanol, MgPc sensor upon exposure to isopropanol, and ZnPc sensor upon exposure to acetone.
With the net zero carbon emissions target by 2050, in the agricultural sector, it is essential to employ technologies to reduce the consumption of energy and resources while enhancing the yield of crops. Learning about how measurable signals can indicate the growth status of various plants will be beneficial for designing plant health monitoring systems (PHMSs) that can be used around the globe for the efficient growth of plants. In this work, we have designed and employed an array of gas sensors, acting as an electronic nose, to monitor the health status of lettuce being grown in a chamber by measuring the emission and consumption of various gases and volatile organic compounds (VOCs). While emission of ethylene is a strong indicator, we have found that accurate concentration measurements of CO2 and alcohols can also be used to assess the health status of the plant at its different stages of growth, particularly at the seedling and vegetative stages. ~20% change in the alcohol concentration and more than 2 folds increase in the equivalent CO2 level was observed when brown leaves started growing before the plant died. The results of the studies can help to design a simple PHMS that can help grow vegetables at a high yield with minimum supervision
In the area of environmental monitoring, Volatile Organic Compounds (VOCs) have become a major concern. Specifically, in indoor environments, VOCs have been found to be linked with various health conditions-ranging from benign to lethal. Early detection of VOC gases can play a crucial role in ensuring a safe environment. In this work, we have looked into two different geometrical arrangements of methylammonium lead iodide (MAPbI3) perovskite to study how their selectivity and sensitivity towards an array of VOC gases under different illumination conditions may be affected. For the purpose of a comparative study, we tested the sensitivity of the device using two different photo-resistor designs, namely, (a) capillary-filled microchannels, where microchannels were created on indium tin oxide (ITO) coated plastic substrate, and leveraging the capillary motion force the microchannels were filled with the perovskite precursor solution, and (b) thin-film approach, where perovskite layers were spin coated on substrates with two conductive pads across. Samples of both designs were exposed to four analytes: acetone, ethanol, isopropanol, and methanol. In our previous study on the capillary design, we reported a decrease in photocurrent by about 22% upon exposure to methanol under illuminated conditions. The goal of this comparative study is to identify the viability of the photo-resistor design for fabricating a low-cost and fast-response gas sensor for the purpose of environmental monitoring.
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