New hydrophilic 2D graphene oxide (GO) nanosheets with various oxygen functional groups were employed to maintain high sensitivity in highly unfavorable environments (extremely high humidity, strong acidic or basic). Novel one-headed polymer optical fiber sensor arrays using hydrophilic GO and hydrophobic reduced graphene oxide (rGO) were carefully designed, leading to the selective sensing of volatile organic gases for the first time. The two physically different surfaces of GO and rGO could provide the sensing ability to distinguish between tetrahydrofuran (THF) and dichloromethane (MC), respectively, which is the most challenging issue in the area of gas sensors. The eco-friendly physical properties of GO allowed for faster sensing and higher sensitivity when compared to previous results for rGO even under extreme environments of over 90% humidity, making it the best choice for an environmentally friendly gas sensor.

In the present situation, plants have to meet the food supply demand for a large and increasing population. In order to get high yield, it is essential for the plantations to be nourished with soil containing an appropriate amount of nutrients like Nitrogen (N), Phosphorus (P) and Potassium (K). Various methods such as, physical (optical) and chemical (electrochemistry) have been adopted to analyze the soil nutrients. This paper reviewed optical methods of soil nutrient detection suitable for building a portable sensor because it can sense nutrients in dry soil samples directly without the need for complicated sample pretreatments. We concentrate and elaborate on optical methods of experimentation. Starting from laboratory testing standards followed in India we move on to off the lab crude methods like soil testing kits and colorimetric approaches. Further, we review the effective and utilitarian spectroscopic approaches and also the technologically advanced and latest methods like imaging systems, microfluidic, and micro-electromechanical system (MEMs) based sensors. However, optical methods are affected by environmental factors that affect the accuracy of sensor results. This paper then discusses boons and curses of optical methods of soil nutrient sensing. It also explains briefly the working of each method and mentions the most recent advancements made in the given testing method. We hope that this paper can serve as a guide for the experimenters and give a direction for carrying out further work required in developing a portable and efficient soil NPK detection sensor.