Advanced Wearable Sensing Technologies for Sustainable Precision Agriculture – a Review on Chemical Sensors

Crop production is impacted by increased plant diseases and shifting environmental circumstances. Monitoring plant health is necessary to raise crop quality and productivity to meet population growth demands. Nanotechnology‐based sensor platforms provide real‐time plant monitoring capabilities, going beyond the constraints of conventional sensor technologies. Wearables are an evolving area of health monitoring and have been modified for agricultural purposes. Wearable sensors are placed on various plant organs in the agricultural industry to check the crops’ health continuously. The varieties of wearable sensor materials and their fabrications, followed by their sensing mechanisms, are highlighted in this review. Furthermore, monitoring plant micro‐environmental factors, including salinity, hazardous gases, and pesticides, are discussed. This text covers various internal plant growth factors monitoring, such as sap flow, transpiration, and signal monitoring. The challenges of wearable sensors in agriculture are mentioned toward the end.


Introduction
Agriculture continues to be a vital industry that supports the nation's economy and, more critically, provides food for the population.The quality and quantity of the agriculture sector are barely affected by rapid industrialization. [1]The percentage of cropland declined to 15% (0.21 ha) per capita as the world's population expanded, according to the Food and Agriculture Organisation of the United Nations Statistical Report 2020. [2]It resulted in 690 million people going without food, and 25.9% worldwide DOI: 10.1002/adsr.202300107 reported food insecurity (Figure 1). [2]As a result, food demand is predicted to increase to 98% by 2050. [3]Climatic issues such as drought and flooding negatively influence production and plant growth.In addition, diseases and pests have a yearly impact of 17-30% on substantial crop production. [4]Therefore, it is essential to reduce the food demand and ensure the growth of the agricultural industry.
Microorganisms cause most plant diseases.Furthermore, every living species uses a signal transduction system to correlate its activities and responses to its environment.Using a signal transduction mechanism, plants communicate locally and long-distance with each other. [5]They have a distinct defense mechanism that does not include immune cells like animals but rather complicated signaling networks.Environmental stress and pathogenic exposure cause abiotic and biotic stresses in crops, resulting in the production of phytohormones such as auxin, salicylic acid (SA), ethylene (ET), jasmonic acid (JA), abscisic acid (ABA), reactive oxygen species (ROS), nitric oxide (NO), and H 2 O 2 . [6]When a crop is stressed, the transmission of signaling molecules increases, which is the plant's defense response to the stress.During drought stress, the plant controls water content by stomata (pores in the leaf) closure, which helps to reduce transpiration.The accumulation of ABA in roots during this process is a symptom of drought stress. [7]The ABA is carried to leaf cells via the transpiration stream, which causes the synthesis of H 2 O 2 , which in turn causes NO production.NO initiates the defense mechanism by generating ET, which triggers the stomatal closure. [8]As a result, each signaling molecule is critical in regulating growth and the plant's defense mechanism.The detection of these signaling species can aid in the monitoring of plant health.Crop output losses can be avoided if plant diseases are detected early and addressed quickly. [9]s a result, various new techniques for monitoring plant health have emerged, including infrared thermography, proximal optical sensors, satellite imaging techniques, spectroscopy, and machine vision systems.However, the majority of the techniques rely heavily on optical functioning processes.Correspondingly, these systems have drawbacks; most of them require laboratories that are unsuitable for continuous plant monitoring in real time.The imaging techniques are not specific, resulting in low accuracy and sensitivity.Polymerase chain reaction (PCR), cell culture, enzyme-linked immunosorbent assay, and immunofluorescence are traditional laboratory techniques that require equipment, adequate reaction time, and a trained workforce, which is difficult for practical applications. [10]To address these issues, chemical sensors that come under the nanosensors category are used as alternatives for continuous monitoring.Furthermore, sensors contribute to sustainable agriculture by regularly monitoring plant water levels, soil conditions, and plant health, which helps apply water, pesticides, and fertilizers precisely. [11]earable electronics is an emerging area that has shown promising results in various applications lately.Wearable electronic skin research is enthralling in human health monitoring and agriculture.The development of Human wearable chemical sensors is well-known in healthcare regarding disease diagnostics and monitoring applications.On the other hand, the development of plant-wearable chemical sensors for agriculture applications is still in its infancy.Figure 2 depicts how plant wearables can be used in various agricultural applications.Wearable sensors for plant health monitoring can be placed in multiple locations, including stems, leaves, and roots, to monitor the plants' biological processes effectively.
Meanwhile, a wearable sensor can be positioned close to plant parts due to its flexibility.It also helps in continuous monitoring, which aids in the early detection of plant stress and the prevention of plant productivity loss.This understanding makes flexible, lightweight wearable sensors critical for monitoring plant health. [12]The published reviews in plant sensors highlighted all types of wearables.For example, Saoni Banerji et al. [13] discussed the plant signaling process and wearables utilizing physical, chemical, and imaging technologies.This review mainly focused on the chemical wearable sensors for plant health monitoring and their sensing mechanism.Additionally, it details the basic structure of nanosensors and the design and fabrication of wearables.

Nanosensor: Basic Structure and working principle
Wearable chemical sensors lie under the category of nanosensors.To get a better understanding of the sensor structure, we discussed the sensor's parts in brief.A nanosensor is a device that incorporates nanomaterials and is used in a variety of applications, including health care, [14] agriculture, [15] environmental monitoring, [16] food science, [17] biotechnology, and chemical industries.Nanosensors possess three main components: a recognition element, a transducer, and electronics. [18]Figure 3 depicts a schematic illustration of the nanosensor's essential components.

Recognition Element
The crucial part of the sensor is the recognition or sensing element, which recognizes the target molecule by interaction or reaction.The recognition elements should be specific to the target molecule with high binding affinity. [19]Furthermore, the recognition element will differ depending on the sensor type.For example, the recognition element in biosensors works on a biochemical principle in which enzymes, aptamers, and antibodies are used.Molecularly imprinted polymers and nanomaterials are used as recognition elements in physical and chemical sensors.The advancement of a recognition element, namely its integration of biomolecules with nanostructure, significantly enhances sensor sensitivity and selectivity. [19,20]

Transducer
The sensor transducer helps transform one form of energy into another.It converts the biological or chemical signal from the recognition element into a measurable signal, and this energy conversion is called signalization.The majority of the transducer produces an electrical or optical signal, and the produced signal is directly or inversely proportional to the concentration of the targeted molecule. [21]Based on the transduction mechanism, sensors are divided into several types, such as electrochemical sensors, optical sensors, piezoelectric sensors, calorimetric sensors, chemiresistive sensors, and quartz microbalance sensors. [22]

Electronics
The electrical portion of the sensor contains complicated circuits that process the signal produced by transduction.It also amplifies the signal and converts it from analog to digital.The processed signal is then quantized and displayed in a legible, user-friendly format. [23]

Wearable Sensors
The real-time monitoring of plant health is a challenging task.Integration of stretchable and flexible electronics into nanosensors has made the development of plant wearable sensors most feasible.The wearable sensor offers comfortable contact and longterm monitoring, proliferating wearables research in agriculture.

Basic Structure and Materials
Substrate material, sensing element, and electrode are the significant components of wearable sensors, along with electrical components for signal harvesting and wireless transmission. [24,25]In the sensor, the sensing element reacts with the stimuli (an analyte) and produces the signal.The signal processing unit processes the signal, converts it into readable data, and sends it to a user-friendly display.28]

Substrate Material
Selecting substrate material with essential qualities such as high flexibility, transparency, thinness, stretchability, corrosion resistance, and insulation is critical in wearable sensors. [27]Also, the substrate is used to support other components of the sensor, especially the sensing element.Several materials were widely used as flexible substrates: polymers, organic, and natural. [29]olymers play a leading role in flexible electronics due to the wide range of fabrication techniques.Polyimide (PI) is widely used because of its higher thermal stability.However, due to its yellow color, PI is limited to applications requiring optical transparency. [30]Other polymers, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), showed higher transparency but suffered from lower thermal stability and poor stretchability. [31][34] PDMS is still an excellent choice for flexible substrates due to its extraordinary thermal and chemical resistance.Polyurethane (PU) and polyethylene glycol (PEG) films were also often used as substrates. [35]In the case of a wearable sensor, natural materials were used because of their excellent biocompatibility, biodegradability, and sustainability, which are highly required properties for wearable sensors.Chitosan, wool, cotton, silk, and cellulose are examples of naturally derived substrate materials. [26]ntegrating naturally derived materials with inorganic conductive materials is emerging research in wearable sensors.Rosace et al. [36] constructed the multiwall carbon nanotube (MWCNT) coated cotton fabric wearable sensor for monitoring humidity and temperature.Additionally, introducing appropriate shapes and microstructure in the substrate helps to modify the properties such as elasticity, surface roughness, and adhesion, boosting the sensor performance. [25]These are the strategies that significantly improve the performance of the devices.

Sensing Elements
The sensing material is the wearable sensor's active layer, containing the recognition material, which is sensitive to the analyte.The interaction between the recognition material and the analyte changes the signal output.Furthermore, the sensor's fast response & recovery time and the complete recovery without carry-over are critical requirements for continuous real-time monitoring applications.In addition to the sensing performance, the sensing element must have adequate conductivity, which facilitates sensitivity.Carbon-based materials, [37] metal nanoparticles, semiconductors, conducting polymers, and ionic conductors were used as sensing elements.
The sensing element is made up of single or hybrid materials.Physical sensor requires a single material with a suitable electrical and mechanical response. [38]In contrast, hybrid or composite materials are used for the chemical or biochemical sensor.Here, the fabrication strategy varied depending on the nature of the material.The nanomaterials were patterned on the substrate by different techniques, such as screen printing and inkjet printing, followed by biological materials like enzymes and antibodies anchored in biochemical sensors.The sensor's sensitivity and specificity are enhanced by utilizing the multiple sensing elements.Li et al. [39] fabricated reduced graphene oxide (rGO)-based wearable sensor arrays functionalized with various ligands for monitoring plant-emitted volatile organic compounds (VOC) profile.Lan et al. [40] constructed the gold-coated fiber-based stretchable sensor for H 2 O 2 detection.The fiber consists of MXene and CNT combined with polyurethane constructed by the wet spinning technique.

Electrode
The electrode is a conducting part of the sensor and a base for the sensing element.It has input and output terminals.The characteristics of the electrode determine the sensor's sensitivity and stability.It requires higher robustness and high conductivity under complex mechanical deformation.In wearable sensors, highconducting metal films such as gold, silver, and graphite integrated electrodes were employed. [26]The ideal electrode would have reduced strain fatigue, be more flexible, and be biocompatible.The stretchability of the material is critical for device manufacturing.Nanowires and conducting polymers are combined with elastic elastomeric polymers to meet the need. [26]

Electronics
The electronics consist of a signal processing unit and an amplifier, which amplifies the signal from the sensing element and produces an output.In wearable sensors, the wireless communication module plays an important role.It is made up of a remotecontrol system and a wireless transceiver circuit.2.4 GHz wireless LAN, Wi-Fi, and Bluetooth are commonly used in wireless communication.Recently, researchers have reported low-energy wireless communication for wearables. [61]he electronic part possesses a power source for the sensors, and the majority of the sensors run on rechargeable Li-ion batteries.Wearable sensors require a long-lasting, maintenance-free power supply. [42]Few wearable sensors use nanogenerators that convert mechanical energy into electrical energy and harvest energy from wind and raindrops.Lan et al. [62] fabricated a selfpowered nanogenerator for plant wearable sensors.The gener-ators use raindrops and wind to generate electricity through triboelectrification and strong electrostatic induction.

Applications of Wearable Chemical Sensors in Agriculture
Traditional sensors retain limitations for real-time monitoring applications.Flexible sensors show potential for real-time plant health monitoring on account of their flexibility, thus facilitating comfortable contact on the irregular surface of the plants.In this section, we focus on the chemical sensors in agriculture, like plant physiological parameters such as moisture and transpiration.Also, discuss the sensors that support the microenvironment monitoring, such as temperature, light intensity, UV exposure, and relative humidity.The sensors related to monitoring the VOCs (signaling molecules), gases, and pesticides on and near the plant's environment are also discussed.The wearable chemical sensors in agriculture are tabulated in Table 2.

Plant Physiological and Microenvironment Monitoring
In plant physiology, transpiration is a necessary process that helps to maintain homeostasis.The process of water molecules passing through the leaf's stomata is known as transpiration.The hydraulic pathway involves transporting water through the stem up taken by roots and transpiration through the stomata. [80]95 to 97% of the water uptake is evaporated by stomatal transpiration, and only 3-5% is retained in the plant, leading to dehydration risk.Continuous measuring of transpiration helps to maintain the plant's water balance, control the soil moisture, and quantify irrigation level to obtain the maximum growth potential. [81]In addition, monitoring sap flow is also helpful in analyzing plant transpiration, water consumption, plant health, drought stress, and nutrient distribution. [82]The flexible and wearable sensor has the potential for real-time sap flow monitoring.Chai et al. [63] proposed a flexible, ultrathin, and lightweight wearable electronic sensor for continuous monitoring of sap flow in plants.The sensor monitors the sap flow using spatial anisotropic temperature distribution.The sap flow sensor consists of two temperature sensors and a positive temperature coefficient (PTC) thermistor between them.The sap flow creates an anisotropic temperature distribution between the temperature sensors, leading to the temperature rise in the PTC thermistor that helps to analyze the sap flow rate.In the fabrication, the copper (Cu) film was deposited over spin-coated PI film, and the serpentine patterns were created using a laser cutting process on a Cu film.Again, the PI was coated on the top of the Cu serpentine.The PI layer was removed in appropriate places using laser cutting to create the contact for the connection pad.The temperature sensors and thermistor were connected using solar pasting.Further, the whole sensor was laminated within the PDMS layer to make the sensor waterproof.The thermistor was connected to the wireless transmitter electronics for wireless monitoring (Figure 4a).Finally, the arrow-shaped sensor was attached to the watermelon plant's stem with the support of water-soluble tape (Figure 4a) for continuously monitoring the flow rate.
Nowadays, Laser-induced graphene (LIG) has emerged as a promising material in flexible electronics owing to its solventfree fabrication technique.Lan et al. [64] fabricated LIG interdigital 16 days in the field [ 63] 2.

Temperature, humidity, and plant growth
Change in resistance Ti/Au PI film and PDMS Plant leaves Temperature, Humidity -3 h, Plant growth-2 days 52 [ 67] 6.

Monitoring of ammonia
Change in resistance PANI/Ti 3 C 2 Tx PI agriculture yard 6 days [ 69] 8. NO 2 monitoring Change in resistance Au NPs/rGO electrode printed on Leaf Plant leaf - [ 70] 9.

Methyl parathion pesticide detection
Change in Current PDMS/OPH PI/PDMS spinach and apple surface On site detection [ 72] 11.

Carbendazim and diquat
Change in Current PLA/SPE PLA mat Apple and cabbage surface On site detection [ 73] 12.

Trifluralin detection Change in Current
Ag nano ink Gloves tomato, and mulberry leaves surface On site detection [ 74] 13.

Salicylic acid and pH Change in Current Cu MOF and PANI Resin
The cabbage plant's stem and root 12 hours [ 75] 14.

Salicylic acid
Change in current SA sensitive MIP PDMS Tobacco plant leaf 6days [ 79] electrode (IDE)-based sensor to monitor plant transpiration via leaf.In the process of transpiration, water vapor was released through the stomata, which increased the humidity on the surface of the leaves.Monitoring the leaf surface's humidity level helps monitor the plants' transpiration.The sensor was fabricated by drop-casting graphene oxide on LIG-IDE and dried at room temperature.For real-time monitoring, the sensor was placed at the backside of the Epipremnum aureum leaf, as shown in Figure 4b inset.The results for real-time monitoring of transpiration reported for 11 days are shown in Figure 4b.
The recorded capacitance value decreased when the plant's humidity decreased, denoting that the plant was under water stress (Figure 4b).The flexible GO-based LIG-IDE sensor is promising for new-generation wearables and mass production.
The sensor array for multifunctional sensing has recently been an emerging idea in various fields.Monitoring the plant's growth and microclimate, such as temperature, relative humidity, drought, and light intensity, is essential in agriculture to address the plant's stress.Using bio-impedance spectroscopy, Kim et al. [83] demonstrated drought and photo-damage monitoring  [63] Copyright 2021, Wiley.b) real-time monitoring of drought stress (the inset shows the photograph of humidity sensor, Reproduced with permission. [64]Copyright 2020, Elsevier c) photograph of polymer printed plant, d) frequency-dependent impedance response for dehydration, and e) photo-damage, Reproduced with permission. [83]Copyright 2019, Science.
using vapor-printed polymer leaf electrodes.P-doped poly(3,4propylenedioxythiophene) (PProDOT-Cl) polymer was used as the sensing element, and it was directly deposited on the leaves by vapor printing method with 3.18 mm inter-spacing electrode shown in Figure 4c.The sensor was connected to an impedance analyzer through the probe station.For drought monitoring, the polymer sensor printed pothos leaves were kept in the vacuum oven, and the results were recorded every 10 min.The sensor's impedance increased during the plant dehydration, as depicted in Figure 4d.The sensor printed leaves exposed to Ultra Violet A (UVA) radiation for measuring photodamage.The recorded results reveal that UVA-irradiated leaves show increased impedance compared to the fresh leaves (Figure 4e).The bio-impedance spectroscopic results clearly distinguished the drought and UVA-induced stress.Lu et al. [65] constructed a wearable multimodal sensor array for plant health monitoring.The sensor array consists of two humidity sensors for monitoring environmental and leaves humidity, an optical sensor to monitor the light intensity, and a temperature sensor to monitor the ambient temperature.The photograph of the sensor is shown in Figure 5a.
The multimodal sensor array was constructed on the PI film.The LIG-IDE was formed by laser scanning on the PI film for the humidity sensor, and ZnIn2S4 (ZIS) nanosheets were dropcasted.The humidity sensor monitors the transpiration of leaves as well as the environmental humidity.The plastic foam was attached as the spacing layer between the sensor and the leaf in the transpiration monitoring sensor.The temperature sensor was fabricated by drop-casting CNT/SnO 2 on the silver screen printed electrode.The resistance of the temperature sensor decreased while increasing the environmental temperature.The sensor array was positioned on the lower epidermis of Pachira macrocarpa leaves (Figure 5b) and connected to a data logger paired with the laptop.The sensor monitors the dehydration over 15 days.Thus, the multimodal sensor array has the potential for real-time plant health and microenvironment monitoring.Zhao et al. [66] built a skin-like stretchable sensor for long-term continuous microclimate and leaf physiology monitoring without creating any physical strain on the leaves.The multifunctional sensor consists of three chemical sensors for hydration, temperature, and light illumination and a strain sensor for growth monitoring, as shown in Figure 5c.In the sensor fabrication process, the PI was spincoated on the Cu foil and placed on the PDMS-coated glass substrate.The PI acts as an insulating layer between the sensing element and the Cu layer for hydration monitoring.Following this, a PI insulating layer was formed.Ti/Cu layer formed on top of that as a temperature sensing element.The strain sensor sensing element was fabricated with two perpendicular strain gauges mentioned as S1 and S2, made up of SWCNT.After coating the PI layer, the silicon-based phototransistor was attached to monitor the environmental light intensity.Finally, the sensor film was moved to the silicone elastomer membrane.The sensor array was connected to the wireless sensing circuit consisting of a printed circuit board.The hydration difference of the plant was measured by the impedance variation of the sensor (Figure 5d).Copyright 2020, American Chemical Society, c) a schematic of a leaf sensor, The sensor response for d) hydration, e) temperature, f) strain, and g) light intensity, Reproduced with permission. [66]Copyright 2019, American Chemical Society.
As shown in Figure 5f, the sensor's resistance increases along with the strain, which helps to monitor the plant's growth, and the transistor current was increased while increasing the light intensity (Figure 5g).
Nassar and their group fabricated another example of a wearable microclimate monitoring sensor. [67]The butterfly-shaped sensor array comprises temperature, relative humidity, and strain sensor modules.The microenvironment of the plant was monitored using temperature and humidity sensors, whereas the plant growth was monitored via strain sensors.The butterfly sensor array was constructed on the spin-coated PI.The Ti/Au (10 nm/180 nm) was sputtered on the PI film and transferred onto the PDMS layer through the peel-off method.Ti/Au acted as the temperature-sensing element, and the temperature was monitored by measuring the resistance of the electrode.Further, Ti/Au was deposited as a capacitive integrated structure as the electrode, and PI acted as the sensing element for the humidity sensor.The dielectric permittivity of the sensor increases while increasing the humidity.The sensor array was connected to low-power Bluetooth systems powered by a 3.7 V rechargeable Li-ion battery for continuous wireless data collection.The photograph of the sensor and its components is shown in Figure 6a.The comparative performance of the sensor with the commercial sensor is shown in Figure 6b,c.The sensor results are promising for the real-time monitoring of the plant environment.

Gas and Pesticide Monitoring
To improve crop yields, chemicals such as fertilizers, pesticides, insecticides, and fungicides are applied to the plants.Extensive utilization of these chemicals pollutes the soil.These chemicals persist for a longer time in the soil and severely strike the defense mechanism of plants.Also, the chemical produces volatile components and gases such as NH 4 , CO 2 , and O 3 in the environment.A higher concentration of such gases is highly toxic and severely affects the plant's growth by creating stress, leading to plant cell damage.Therefore, continuously monitoring toxic gas and pesticides is most important for sustainable agriculture.Even though ozone is beneficial in blocking UV radiation from entering the earth, its gaseous form in the environment impacts human and plant health.Agriculture disturbs the plant's metabolism by increasing chlorophyll content and respiration, decreasing photosynthesis, and damaging the leaves.Ramya et al. [84] studied the impact of elevated ozone stress in rice cultivars, which causes leaf bronzing.Kim et al. [68] fabricated the polymer tattoo sensor to monitor the ozone stress by monitoring the sensors' impedance changes.The PEDOT-Cl conducting polymer sensor was fabricated directly on the grapes' leaf surface by the oxidative chemical vapor deposition method.The sensor could selectively monitor the ozone stress and produce an impedimetric signal.The impedance was decreased upon ozone exposure.Ammonia is also toxic to the environment and impacts human and plant  and c) humidity response.Reproduced with permission. [67]Copyright 2018, Nature.
health.It causes visible foliar injury to the plant.In agricultural land, ammonia is mainly produced by fertilizer.Therefore, monitoring ammonia is vital.Li et al. [69] fabricated a chemiresistive flexible sensor for monitoring ammonia in the plant environment.PANI/Ti 3 C 2 T x composite film acts as a sensitive element coated on polyimide substrate by in situ self-assembly method.The sensor monitored ammonia volatilization for 6 days after fertilization, and the sensor resistance was increased in the ammonia atmosphere.The sensor was selective toward ammonia and showed excellent sensing properties even at higher relative humidity (80%) and low temperatures (10-40°C).These results are promising for the real-time monitoring of ammonia from agricultural products in the plant environment.Nitrogen oxides (NO x ) are critical environmental pollutants released from fertil-izer, and it is essential to monitor the level of NO x .Oikawa et al. studied the ecological influence of the NO x emitted from agricultural soil, and NO x contributes to ozone production.The emission of NO x varied nonlinearly with temperature, moisture, and fertilizer.Li et al. [70] fabricated a wearable NO 2 sensor array with metallic single-walled carbon nanotubes (m-SWCNT).For the NO x sensor, the flexible m-SWCNT integrated conducting electrodes were fabricated by spray coating.The Ag NPs decorated rGO were drop-casted onto the m-SWCNT electrode and acted as the sensing layer.The sensor could detect NOx as low as 0.2 ppm and showed excellent stability up to 3000 cycles.The sensor was printed on a leaf (Figure 7a), and the dynamic response of a leafprinted sample is shown in Figure 7b, and the sensor resistance increases on exposure to NO x .
Figure 7. a) The photograph of the leaf sensor and b) the dynamic response of NO 2 for leaf printed, Reproduced with permission. [70]Copyright 2020, Elsevier.
Zhao et al. [72] developed a 3D porous LIG-based enzymelinked electrochemical wearable biosensor to detect the methyl parathion pesticide.The commercial PI film was carved using the computer-controlled laser induction system to fabricate the flexible porous LIG three-electrode system and then covered with PDMS.The electrode was further modified with the organophosphorus hydrolase (OPH) enzyme to detect methyl parathion specifically.The sensor was connected to a hand-held electrochemical workstation, and the readout could be monitored on the smartphone wirelessly.The test was demonstrated on spinach and apple (Figure 8a,c) by SWV, and the results are shown in Figure 8b,d respectively.The sensor current increased while increasing the concentration of pesticide.Paschoalin et al. [73] fabricated a wearable and flexible electrochemical sensor using a carbon screen printed electrode (SPE) on polylactic acid (PLA) to detect carbendazim and diquat pesticides in agriculture and fruit samples.Adapting the blow spinning technique, the PLA mats were fabricated, and using carbon ink, a three-electrode system was printed on the PLA mat.Further, Ag/AgCl ink was used to draw the reference electrode.A customized connector was used to connect the flexible sensor and electrochemical unit.This flexible sensor was integrated into an apple and cabbage and used for on-site detection of pesticides.Adopting DPV and SWV electrochemical techniques, the sensor current increased on the exposure to pesticides.Pereira et al. [71] developed gloveembedded wearable and flexible non-enzymatic electrochemical sensors for detecting four classes of pesticides (carbendazim, diuron, paraquat, and fenitrothion).In the glove-embedded sensor, the sensor was printed on the three fingers of the glove.Each finger was modified with a carbon screen printed three electrode IDE such as working electrode (WE), reference electrode (RE), and auxiliary electrode (AE).The reference electrode was printed using conductive silver ink.The WE acted as the sensing element, and the WE of the index finger was modified with a carbon spherical shell (IF/CSS) to detect carbendazim.The middle finger was modified with printex carbon nanoballs (MF/PCNB) to detect diuron, and the ring finger (RF) was electrochemically pre-treated, which is selective towards paraquat and fenitrothion.The glove-printed electrodes were connected to a potentiostat using a flexible cable.The DPV technique analyzed the sensors, and the sensor current increased on the exposure of the selective pesticides.A glove-embedded sensor detects four pesticides by directly touching the cabbage and apple and dipping them into the juice.Farshchi et al. [74] constructed the glove-based electrochemical sensor for detecting the trifluralin by touching the surface of tomato and mulberry leaves.The sensor was designed with a three-electrode system on the glove finger using Ag nano ink, and the sensor was connected to a palm sense device for monitoring.Electrochemical signal changes induced by the direct touch of a gloved fingertip on the leaves and tomato were measured by DPV and SWV techniques.

Monitoring Signaling Molecules
Pathogens or environmental factors cause plant stress, resulting in minor changes in the plant.Monitoring these changes can thus aid in disease diagnosis.The stress plants release some phytohormones as signaling molecules, such as VOCs.As a result, monitoring VOCs is critical for identifying the infection.Abiotic and biotic stress are the two types of plant stress.This section describes the sensing signaling molecules affected by abiotic and biotic stressors.

Abiotic Stress
Abiotic stress is caused by inappropriate or unfavorable environmental circumstances such as drought, high temperature, salinity, and humidity.These circumstances interfere with the plant's normal metabolisms, and extreme stress results in cell wall breakdown, membrane leakage, and altered plant physiology. [6]Stress activates the plant's defense mechanisms and causes the release of various signaling molecules.The abiotic stress promotes the production of ROS, H 2 O 2 , NO, SA, and ABA as signaling molecules.Abiotic stress accounts for 51-82% of crop output losses worldwide. [85]The detection of these signaling molecules aids in the monitoring of abiotic stress.Yao et al. [86] fabricated ROS-detection using AuPt nanoparticles decorated flexible MoS 2 paper-based sensor.For the fabrication of flexible MoS 2 paper, the exfoliated suspension of MoS 2 nanosheets was vacuum-filtered over a nitrocellulose membrane and transferred to the Au-coated flexible PI electrode.Then, the flexible MoS 2 paper was dipped into ethanol to remove the nitrocellulose membrane.Finally, the flexible MoS 2 paper-based sensor was decorated with AuPt NPs by immersing MoS 2 paper into the Au and Pt precursors.Even though the flexible paper was not integrated with plants, the ROS in the plant extract was quantified using a standard electrochemical approach.Methyl jasmonate (MeJa) is one of the phytohormones for abiotic and biotic stress.Using FeS 2 -modified cellulose paper, Sha et al. [87] suggested a wireless smartphone-based chemiresistive sensor for MeJa detection.The resistance variation of the sensor was directly measured using a smartphone.
As mentioned above, SA is a phenolic phytohormone that regulates plant growth, particularly under drought stress.Drought stress induces growth reduction, affects photosynthesis, and reduces cell elongation and division.Li et al. [88] developed a disposable LIPG-based wireless mini-sensor for detecting SA in watermelon extract.The portable potentiostat of the sensor was connected to the computer via Bluetooth.Hossain et al. [75] constructed a wearable microneedle-based electrochemical sensor integrated with a pH sensor to monitor SA and pH levels in the plant stem.The sensor comprises two 3D-printed WEs, a shared CE, and a RE.Pyramid-shaped microneedle was patterned using biocompatible resin on each electrode.Followed by, WE and CE were painted with graphene ink and RE-painted with Ag/AgCl paste.The working electrode of the SA and pH sensor was modified with Cu-MOF and PANI, respectively.The fabricated microneedle sensor was attached to the stem of the cabbage plant, and the sensor was integrated with a hand-held potentiostat for real-time monitoring.The concentration of SA increases in the stress-induced plant while the sensor's current increases gradually.
Excessive salts are also considered an indication of drought stress.Janni et al. [76] constructed an organic electrochemical transistor named a bioristor to monitor drought stress.The bioristor consists of a textile fiber functionalized with poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) for continuous monitoring of the changes of xylem sap's composition, such as sodium (Na + ), potassium (K + ), calcium (Ca 2+ ), and magnesium (Mg 2+ ) salts upon drought.The sensor was directly inserted into the stem of the tomato plant for real-time monitoring and continuously monitored for 23 days.The device could measure plants' drought stress within 30 h of water deprivation.The response of the bioristor fell during the drought stress.
Chemiresistive sensor plays a vital role in monitoring signaling molecules.Zheng et al. [77] fabricated a wearable chemiresistive sensor to detect -pinene vapor from Platycladus orientalis (P.orientalis) plants.-pinene is a VOC signal for drought stress and mechanical damage.In the fabrication, Au IDE was printed on the PET substrate and dipped into a polyethylene-covinyl acetate (PEVA)/MWCNT mixture (sensing element).The PEVA/MWCNT modified Au IDE sensor was fixed onto the P.orientalis trunk, as shown in Figure 9, to measure the mechanical damage and water stress.The sensor resistance increased when the plant was exposed to bark damage and water stress.The results are presented in Figure 9a,b.In addition, the IDE sensor was integrated into NFC (Near Field Communication) tags for wireless plant stress monitoring.

Biotic Stress
Pests and pathogens such as bacteria, viruses, oomycetes, and fungi are primarily responsible for reducing crop production.The stress caused by these living organisms is known as biotic stress.Biotic stress alters the physiological response of plants, triggering an immunological response.To inhibit pathogen growth, it produces phytohormones. [6,89]Traditional pathogen detection methods are direct diagnosis procedures after visible physiological signatures; the severity of damages is considerable at that time.It results in the overuse of pesticides and hazardous chemicals, which disrupts agricultural sustainability and drastically reduces plant defense potential.Continuous monitoring of these signaling molecules aids in the early detection of infection, which reduces plant loss and production. [90]It is primarily helpful in predicting the proper pesticide and dosage and protecting the soil and food from toxicity.
ROS plays a vital role in the stress defense mechanism.ROS's defense mechanism pathway is explained as follows: The pathogens naturally have pathogen-associated molecular patterns (PAMPs).The surface of the plant cell contains specific recognition receptors that detect the PAMPs and initiate the plant's primary immune mechanism, known as PAMPs trigger immunity (PTI); the process produces ROS. [91]The ROS molecules catalyze the antimicrobial activity, causing the strengthening of the cell wall and acting as the secondary messenger in programmed cell death. [92]Monitoring the ROS helps to analyze the plant stress.In addition to ROS, ABA, SA, NO, and a few VOCs are produced by biotic stress.Usually, plants emit VOCs, and the emission concentration is much greater in infected plants. [93]The VOC profile differs according to the infected pathogens, as noted in Table 3. [94] For example, the higher concentration of C6-aldehydes ((E)−2-hexanal, (E)−2-hexenal, and (Z)−3-hexenal), alcohol, terpineol, and terpenes were observed from the infected tomato stems or leaves, helps to differentiate the healthy and unhealthy plants. [95]Mainly, Phytophthora infestans infected tomato leaves emitted 4 to 6 folds higher (E)−2hexenal than healthier plants. [96]iarpinijnun et al. proposed a sensor array with 12 different commercial sensors for monitoring Aspergillus fungal infection in jasmine brown rice by monitoring VOC emission. [97]However,  [77] Copyright 2023, Elsevier.
wearable sensors are more likely suitable for continuous field monitoring of signaling molecules.Thus, recently, the plant wearable chemiresistive sensor was proposed for the real-time monitoring of the VOC profile caused by Phytophthora infestans that produce late blight in tomatoes. [39]The sensor array consists of rGO sheet-coated silver nanowire electrode.The electrode was modified with Au nanoparticles attached to chemical ligands containing various recognition groups.The sensor array was placed on the tomato leaves for real-time monitoring.The sensor quantifies 13 VOC profiles at the sub-ppm level and also mechanical injury of the plants by monitoring the VOC profile.The sensor diagnosed the disease as early as 4 days post-inoculation, and the mechanical damage was analyzed within 1 h.In a different work, Ibrahim et al. [78] fabricated a wearable plant sensor for monitoring methanol emission in the plant head space.Methanol is one of the signaling molecules involved in plant defense mechanisms against insects.The three-finger carbon electrodes were printed on the PET substrate in the sensor fabrication.Ag/AgCl pasted carbon acted as the reference electrode, the bare carbon electrode acted as the counter electrode.The middle electrode, modified with conducting polymer (poly (2-amino-1,3,4-thiadiazole)) decorated with Pt NPs, served as the sensing element.The sensor was capped with a hydrophobic gas filter membrane to reduce the humidity influences and  ) the sensor connected with a potentiostat and wireless readout, Reproduced with permission. [79]Copyright 2022, Elsevier.
was fitted on maize plant leaves' surface.The sensor potentially the different concentrations of methanol in the plants.Bukhamsin et al. [79] constructed a paper-based wearable electrochemical biosensor for monitoring the fungal infection in tobacco.In the post-inoculation period, the concentration of SA increases while the sensor current increases.The IDE of the electrochemical sensor was modified with a microneedle, which is the size of the leaf's cuticle, fabricated by the micro molding process.The working electrode was the sensing element decorated with SA-sensitive magnetic molecular imprinted polymer (MIP).The sensor was placed on the leaf (Figure 10a) and connected to the portable potentiostat, which monitors the plant's SA level in 5 min post-inoculation period.The sensor was wirelessly connected to the smartphone, and results were read out continuously (Figure .10b).The monitoring VOCs have constraints because the VOC profile will differ according to various environmental conditions such as relative humidity, temperature, salinity, and drought. [98]

Conclusion
Meeting the expanding population's food requirements is a complex issue.Improving yield through sustainable agriculture protects natural resources and soil fertility.Sensors play an essential role in sustainable agriculture.Continuous monitoring of plant health and the microenvironment improves agricultural output and pesticide control.Wearable sensors are emerging tools in sensor technology, due to their exceptional flexibility and field adaptability.This report focused on wearable chemical sensors in agricultural applications such as monitoring plant physiology, microenvironment, and plant stress induced by abiotic and biotic variables.However, most agricultural research in wearable technology is currently at the laboratory level.Wearable technology requires further development to meet market demand.Significant issues in agricultural wearables are sensor fabrication, selectivity, power supply, and material toxicology.According to the aforementioned difficulties, the following tasks must be prioritized for agricultural wearables in the future: (i) Developing stretchable electronics is essential for improving the utility of wearable sensor fabrication.For field monitoring, the sensor must resist adverse circumstances such as high temperatures, rain, dampness, and wind, and (ii) the sensor's selectivity is still complicated.The sensor's cross-sensitivity investigations should be more focused.
Signaling molecule database studies on phytohormone monitoring are limited.Therefore, generating a VOC profile from such little information is difficult.As a result, proper research in the particular sector enhances the data, including various environmental circumstances, which aid in improving the VOC profile database and causes, and (iii) the sensor power consumption should be sustainable.The energy must be generated by solar or wind power, and (iv) the sensing material must be biocompatible.

Figure 1 .
Figure 1.Food insecurity level by region.

Figure 2 .
Figure 2. Applications of wearable sensors in agriculture.

Figure 4 .
Figure 4. a) Photograph of the sap flow sensor connected to a smartphone, Reproduced with permission.[63]Copyright 2021, Wiley.b) real-time monitoring of drought stress (the inset shows the photograph of humidity sensor, Reproduced with permission.[64]Copyright 2020, Elsevier c) photograph of polymer printed plant, d) frequency-dependent impedance response for dehydration, and e) photo-damage, Reproduced with permission.[83]Copyright 2019, Science.

Figure 5 .
Figure 5. a) and b) photos of the leaf sensor, Reproduced with permission[65] Copyright 2020, American Chemical Society, c) a schematic of a leaf sensor, The sensor response for d) hydration, e) temperature, f) strain, and g) light intensity, Reproduced with permission.[66]Copyright 2019, American Chemical Society.

Figure 6 .
Figure 6.a) Photographs of butterfly sensor array placed on the plant leaf, the fabricated sensor results in comparison with the commercial sensor toward b) temperature response and c) humidity response.Reproduced with permission.[67]Copyright 2018, Nature.

Figure 8 .
Figure 8. Photos of the sensor attached on a) spinach and c) apple.The SWV response for methyl parathion on b) spinach and d) apple, Reproduced with permission.[72]Copyright 2020, Elsevier.

Figure 9 .
Figure 9. Results for real-time monitoring of a) mechanical damage and b) drought stress in (P.orientalis) plants, Reproduced with permission.[77]Copyright 2023, Elsevier.

Figure 10 .
Figure10.The digital photograph of a) the sensor integrated with the leaf and b) the sensor connected with a potentiostat and wireless readout, Reproduced with permission.[79]Copyright 2022, Elsevier.

Table 1 .
Components of the flexible sensors.