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.

Herein, aqueous ammonium sensing characteristics of polyvinylpyrrolidone (PVP) capped silver nanoparticles (Ag-NPs) coated optical fiber-based sensors are presented. The PVP-capped Ag-NPs were prepared using cold and modified polyol synthesis methods. Aqueous ammonium detection was carried out by the surface plasmon resonance (SPR) effect observed in the Ag-NPs coated optical fiber system. The effect of cold and modified polyol synthesis methods on optical sensing performance was studied. The optical fiber cladding was modified with PVP-capped Ag-NPs according to the standard protocol for sensing investigation. The probe sensing response was analyzed for varying concentrations of ammonium ions on red, green, and blue LEDs. The sensor characteristics, viz., sensing response, repeatability, calibration curve, and ambient light effect, were investigated for PVP capped Ag-NPs coated optical fiber-based sensor. The PVP capped Ag-NPs synthesized via the polyol synthesis method showed a detection limit of 48.9 mM, 1.33 mV/M sensitivity, and an excellent linear relationship (R² = 0.9992) between voltage and ammonium ion concentration in the range of 0.054–13.4 M concentration. On the other hand, PVP capped Ag-NPs synthesized using the cold synthesis method showed a detection limit of 159.4 mM, a sensitivity of 0.06 mV/M, and a poor linear relationship (R² = 0.4588) between voltage and ammonium ion concentration in the range of 0.054–13.4 M concentration. The results indicate that the PVP-capped Ag-NPs synthesized using the polyol synthesis method exhibit enhanced ammonium ion sensing compared to the cold synthesis method.

Industrialization has led to an increasing need for specific and selective gas sensors in the past few decades. Environmental monitoring of certain volatile compounds such as ammonia is necessary. Advancements in the food storage sector have created the need for cheap and effective amine chemosensors. Classical chemosensors still face several issues, such as a lack of selectivity and low sensitivity toward ammonia and amines. Sensitivity is defined as the relative change in response expressed in percentage. In this work, we have resolved a few issues associated with the ammonia and amine sensors, such as low selectivity, long-term instability, and unreliability under higher temperatures using plasma-polymerized thiophene (PPTh) reduced graphene oxide (rGO) composite films. PPTh films were prepared using RF plasma polymerization with optimized deposition parameters. Several samples were evaluated for their sensing response to understand the optimal PPTh and rGO ratio in the PPTh-rGO composite. These composite PPTh-rGO films have shown 4 times higher sensitivity for ammonia/amines than individual PPTh and rGO films. Ammonia, methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA) were primary analytes and tested for sensing response of the PPTh-rGO composite. The sensitivity measured ranges from 1328 for trimethylamine to 2354 for methylamine at 1000 ppm. The order of sensitivity was found to be MA > Ammonia > DMA > TMA. Polymer swelling, reduced charge carriers, and disruption of conductive pathways can explain possible sensing mechanisms. PPTh-rGO composite films have shown selectivity as high as 110 for ammonia/amine over other commonly used volatile organic compounds. The sensing response of these films is stable for any temperature fluctuations from 30 °C to 150 °C. Additionally, films showed stable sensitivity for over 4 months. Thus, composite films of PPTh-rGO can be effectively used to develop highly selective and stable gas sensors for the environmental monitoring of ammonia/amines.

In this paper, we synthesized WO₃ thick films on a rough and smooth glass substrate by hydrothermal method and then heated to a temperature of 400-degree Celcius. Characteristic techniques such as XRD and SEM analysis were sequenced to determine the crystallite size and grain composition of the finished samples, respectively. We have discussed the results of the Rietveld refinement made using MAUD to determine useful information regarding the atomic sites, mesh parameters, and micro-stresses in the sample. Subsequently, FTIR analysis has been performed to note the critical bond vibrations associated with the material. AFM studies have also been included to determine the pore sizes and understand the surface-level differences between WO₃ films on rough and smooth substrates. The room temperature gas sensing mechanism was then discussed in the presence of humidity with methanol, ethanol, and benzene along with most of the targeted gases with different selective parameters at atmospheric pressure. We have tried to develop a theory incorporating the anomalous observation for the methanol gas sensing experiment and explained the future scope of this work.