1. Nanobiosensors in Diagnostics and Precision Agriculture
The unprecedented increase in the use of agrochemicals and fertilizers has led to an accumulation of nutrients and toxins in ground and surface waters. These toxic concentrations are responsible for higher costs of water purification, reduced fisheries, and decreased recreational activities 
. Conventional agricultural practices are deteriorating soil quality and are responsible for the eutrophication of water bodies. In addition, bad farming practices damage the ecosystems of beneficial insects and other wild organisms and, therefore, must be replaced by precision agricultural methods.
Precision agriculture includes wireless field networking and nanosensors for observing and controlling farming practices. It manages site-specific crops and pre- and post-harvesting aspects 
. Under precision agriculture, exploring the fascinating properties of functional materials from which nanobiosensors are built could help accurately analyze soil humidity, water, nutrients, and phytopathogens 
). Biosensors are now available for detecting odors in food spoilage, and such sensors 
are called “electronic noses”, followed by the development of other sensor types. The electronic nose uses an array of gas sensors to identify various kinds of odors. The gas sensors are composed of NPs like ZnO nanowires 
and nanorods 
, which could detect impurities in vapor mixtures 
. Such sensors work on the principle of change in their resistance with the passage of different gases resulting in variation in the generated electrical signals, which are used as a fingerprint for gas detection. A typical biosensor consists of four units: (1) a sensor, (2) a signal conditioning block, (3) a microprocessor chip, and (4) a radio module for wireless communications between the sensor and the monitoring station 
Figure 1. Functional representation of nanosensors in precision agriculture.
Recent nanotechnological leaps have enabled us to study biochemical interactions in plant cells and tissues due to various pathogens. The method uses a probe inserted in the xylem vessel at the root base. The probe measures xylem pressure, radial electrical gradients, and ionic activity 
. Such tools help better understand pathogenicity mechanisms to improve crop disease treatment strategies 
. However, the previous approach relied on the destructive sampling of pathogenic bacteria colonizing the xylem, which failed to provide helpful information about colorization patterns, biofilm development, movement, and re-colonization of bacterial pathogens in new tissues. However, implementing microfabricated xylem vessels containing nano-sized features lets us understand the features that were impossible with conventional methods 
1.1. Monitoring of Soil Quality Parameters
Biomonitoring is a technique used to collect and analyze organisms, tissues, or fluids to determine their exposure to natural and synthetic chemicals. The information gleaned from these observations is valuable, as it provides insight into the number of chemicals that have entered the organism and led to corresponding changes. Biomonitoring is also an effective method for estimating the total dose absorbed by the organism, which can provide indirect access to monitor target site concentrations. The advancement of sensor technology has improved its sensitivity and reduced its size compared to conventional biosensors. Such biosensors are used to monitor fertilizers, herbicides, pesticides, insecticides, pathogens, soil moisture, and pH 
. An ideal nanobiosensor should be stable over long storage periods and possess a lower reaction time. In addition, it should be small, biocompatible, non-toxic, non-antigenic, inexpensive, portable, accurate, and capable of producing repeatable findings 
. Nanobiosensors are ultrasensitive devices and can detect viruses at ultra-low concentrations as they operate at the atomic scale with the highest efficiency and accuracy.
1.2. Monitoring Soil Pesticides/Herbicides
The insects are cosmopolitan in distribution and hold the highest population among pests. They infest all plants and products by injuring their parts or attack storage products to incur heavy crop losses. The regular use of pesticides in fields to combat pests can lead to the development of resistance among pest groups 
. Additionally, pesticide chemicals degrade in the environment over time, which reduces their effectiveness for agricultural use. NM use in pesticide formulations could aid in reducing usage and attaining agricultural sustainability. NM includes C nanotubes, quantum dots, gold NPs, carbon black, and nanocomposites. Many nanostructured biosensors have been developed for pesticide detection in water and food 
. Based on consumption rates, toxicological information, and environmental residual levels, the U.S. Environmental Protection Agency (EPA) proposed a limit of 0.9 mg/L glyphosate in drinking water and an acceptable daily intake of 0.3 mg/kg/day 
1.3. Monitoring Soil Nutrients
Nanosensors are being developed as a promising real-time technology for monitoring soil nutrients. These sensors detect and quantify nutrients such as nitrogen, phosphorus, and potassium in soil samples. They use nanomaterials, such as carbon nanotubes, graphene, and nanoclays, to detect and bind with specific nutrients in the soil 
. Nanosensors can provide farmers with accurate and timely information about soil nutrient levels, which can help them make more informed decisions about fertilizer application and crop management. This technology can assist in decreasing fertilizer waste, boost fertilizer efficiency, and lessen the potentially negative environmental implications of typical fertilizer application methods. One example of a nanosensor for soil nutrient monitoring is a graphene-based sensor that can detect nitrogen levels in soil 
. The sensor is designed to be integrated into a wireless sensor network that can provide real-time data on soil nutrient levels to farmers.
1.4. Monitoring Soil Humidity
To ensure successful crop production, it is necessary to regularly analyze soil texture and moisture content. Relative humidity measurements determine the amount of water vapor in a gas mixture at a specified temperature. Standard-level deviations in soil moisture can significantly impact agricultural yields since these parameters vary spatially and temporally. Although conventional methods are available for estimating soil moisture levels, their accuracy is often low. Such methods require frequent calibration, reducing their stability and making them less preferable for use in agricultural settings.
Humidity-based nanosensors are increasingly replacing conventional methods for measuring soil moisture 
. These sensors utilize electrical transduction with a hygroscopic probe, which changes its dielectric properties upon water absorption. Nanosensors fabricated from polymers, ceramics, and composites provide several benefits, such as increased stability, prolonged chemical and thermal durability, and enhanced environmental adaptability 
. The widespread use of nanosensors in agriculture could significantly improve the precision of soil temperature and moisture measurements. Many of these devices are equipped with wireless communications systems that are economical, user-friendly, and can provide real-time data. Examples of nanosensors commonly used for soil measurements include carbon nanotube and graphene-based nanosensors 
. For instance, a graphene oxide-based sensor is a type of humidity-based nanosensor that can detect changes in humidity levels from 0.1% to 90% 
1.5. Monitoring Plant Disease and Stress
Plant stress and nutrient deficiency are detected by monitoring plant physiology through imaging, spectroscopy, and fluorescence 
. The described remote sensing methods provide vital information about leaf area, chlorophyll content, stomatal conductance 
, transpiration rate 
, water potential 
, and leaf temperature 
. However, the methods are not helpful for the early diagnosis of plant stress and nutrient deficiency and are not economical for installation in individual plants 
. NPs-based sensors are now being utilized to monitor plant disease and stress by providing an early detection system for plants. These systems measure the volatile organic compounds (VOCs) released by plants during biotic and abiotic stress or disease conditions. Nanoparticle based sensors can detect these VOCs by analyzing their physical and chemical properties, allowing for the identification of the specific stress or disease affecting the plant.
One example of a nanoparticle-based system for plant disease detection is a gold nanoparticle-based sensor that can detect the presence of bacterial pathogens in plants 
. The sensor works by detecting the VOCs released by the bacteria, allowing for early detection of the disease before visible symptoms appear. Similarly, NPs have been used to monitor abiotic stress in plants, such as drought stress, by detecting changes in VOC emissions. Carbon nanotubes have been utilized in a sensor that can detect changes in VOCs associated with drought stress in plants 
. The sensor can detect VOCs with high sensitivity and specificity, allowing for early detection of drought stress in plants.
1.6. Monitoring Irrigation
Due to the uncertainties posed by climate change, land water availability has reduced globally, and droughts and erratic monsoon patterns are becoming more frequent 
. The current decade is facing a challenge in getting clean and needed water for human use, industrial purposes, and agriculture. The escalating use of agrochemicals in agriculture has exacerbated groundwater pollution. Our water resources are getting contaminated with microbial pathogens, salts, metals, agrochemicals, pharmaceutical compounds, personal care products, and radioactive elements 
. A specific type of contaminant in water bodies is primarily due to anthropogenic activities like oil and gas production, mining, or natural processes like leaching 
, which require thorough treatment procedures for water recycling. Water treatment requires novel and sustainable technologies for recycling purposes.
Precision and site-specific irrigation management have emerged as potential solutions to enhance crop productivity under adverse climate change conditions 
. The concept has appeared as a possible solution for improved crop productivity under adverse climate change. The method uses advanced technologies such as GPS, GIS, and automated machine guidance to apply water judiciously. This approach can be complemented with low-flying drones or sensitive satellites with high-resolution imaging capabilities to determine the water content of soil or plants and induce precise irrigation at the site of need. As a result, water consumption for irrigation can be reduced. However, several bottlenecks, such as cloud interference and high data processing requirements, still need to be addressed. Integration of crop simulation models with remote sensing technology enhances the efficacy of agricultural management and decision-making processes. The application of nanotechnology to microirrigation can enhance water quality and filtering techniques. Nanoparticle-based biosensors can detect and measure water-based contaminants in real-time and remove them using nanofiltration membranes 
. Nanoparticle-based membranes can also desalinate water, reducing the likelihood of clogging on the filters and membranes.
1.7. Summary of Biosensors in Precision Agriculture
The section discussed the role of nanobiosensors in diagnostics and precision agriculture. These sensors monitor soil parameters such as quality, pesticide/herbicide levels, nutrient content, and humidity. They also play a crucial role in monitoring plant disease and stress and managing irrigation. The summary of biosensors’ application in precision agriculture is also mentioned, highlighting their importance in achieving more efficient and sustainable farming practices (Table 1).
Table 1. Nanobiosensors in diagnostics and precision agriculture.
|Type of Biosensors
|Environmental biosensors, chemiresistor sensors
||monitoring of soil quality parameters
||polymers, metal oxides
|Pesticide biosensors, electrochemical biosensors
||monitoring soil pesticides/herbicides
||enzymes, conducting polymers
|Nutrient biosensors, potentiometric biosensors
||monitoring soil nutrients
||ion-selective electrodes, polymers
|Moisture sensors, capacitive humidity sensors
||monitoring soil humidity
|Plant disease biosensors, fluorescence-based biosensors
||monitoring plant disease and stress
||quantum dots, fluorescent proteins
|Irrigation biosensors, soil moisture sensors
||ceramics, metal oxides