Final report for GS20-220
Growing concerns among small farmers about the overuse of water and soil health underscore the benefits of sustainable and affordable monitoring systems in crop production. Parameters such as moisture and soil nutrients that affect crop production could be optimized in real-time using a sensor such as by controlling irrigation water and nitrogen supply. Cellulose has an inherent moisture absorption property and the adsorb moisture on the cellulose fibers was converted into an electrical signal by printing graphite electrodes on the cellulose surface using a pencil. The electrical signal intensity can be recorded with a microelectronic device. Nitrate concentration in moist soil can also be measured using an electrochemical technique with a nitrate ion-selective electrode (ISE). Therefore, we propose to develop a cost-effective cellulose-based biosensor derived from cotton residues to monitor the relative humidity and soil nutrients. The specific objectives were 1) to extract pure cellulose from left-over cotton linter and to fabricate a cellulose-based sensor; 2) to characterize the sensing performance of the biosensor to monitor soil condition such as moisture, and 3) to disseminate the results through scientific conferences and meetings. The successful results of the proposed project would be integrated with low-cost microcontroller devices, allowing sensing data available for farming practices that could improve the profitability of crop production.
The proposed project has three objectives:
- To extract nanocellulose from left-over cotton linter and to fabricate cellulose-based biosensor
- To characterize sensing properties of the cellulose-based biosensor
- To disseminate the results through presentation to the target audiences
The following sections delineate the scientific approaches that will be used to resolve the problems regarding the successful completion of the proposed project under each objective.
Objective 1. To extract pure cellulose from left-over cotton linter and to fabricate a cellulose-based sensor
a. Extraction of cellulose from cotton linter. The leftover cotton linter will be collected from the cotton fields, dried in an oven, and ground. The powder will be treated in the mixture of 1 N NaOH and 3% hydrogen peroxide solution at a pH of 11.05 under continuous stirring for 2 h at 50 oC. This step dissolves the hemicellulose and lignin from the cotton linter. After chemical treatment, cellulose will be rinsed with hot water until neutral pH and then washed with acetone to remove the impurities by soaking for 10 min. The suspension will be mechanically treated in a low-frequency (20 kHz) ultrasonic probe for 30 min to obtain a homogeneous suspension.
b. Cellulose sheet preparation. Homogeneous cellulose suspension obtained from objective 'a' will be vacuum filtered through Whatman filter paper to remove excess water. The samples will be sandwiched between filter paper discs and oven-dried at ~105 ºC for 1 h, and will be hot-pressed. The resultant smooth cellulose sheet (~0.25 mm to 0.30 mm thickness) will be used for sensors.
c. Sensor fabrication using a pencil draw electrode. The cellulose sheet will be cut around 2 cm x 1 cm dimension to form a single electrode. A line will be drawn by using a regular graphite pencil available in a bookstore by hand. The drawn line will be an electrode to transfer current. The consistency of the line will be checked by measuring its resistance with a simple multimeter. The line will be insulated using sellotape, making sure that the top end remains open for connection with the instrument via alligator clips.
Objective 2. To characterize the sensing performance of the biosensors to monitor moisture
Moisture sensor characterization setup and electrical measurement. The homemade characterization setup will consist of a polytetrafluoroethylene chamber. The synthetic air (nitrogen and oxygen) supply will be controlled by the gas mass flow controller. The nitrogen flow will humidify by bubbling through deionized water inside the chamber. Two conductive electrodes will be connected to the equipment (Keithley 2400), which is available for the resistance measurement from the extraction of electrical signal. The resistance change of the sensing materials will be recorded in the presence or absence of moisture under continuously blowing synthetic air. The moisture will be introduced into the sensing chamber with an alternative cycle of 10 min on/off at room temperature by changing the concentration of the moisture.
Objective 3. To disseminate the results through presentations for target audiences
a. Results dissemination. The obtained results from the proposed research will be demonstrated at the conferences to various target audiences such as researchers, students, and farmers.
Cellulose extraction and sheet/film preparation from cotton linter
Cotton linter was collected from the George Washington Carver Agricultural Experiment Station at Tuskegee University. Cotton fiber was collected from the cotton linter cleaned with deionized water. Then cotton fiber was pretreated with hydrogen peroxide (AHP) solution to remove hemicellulose and lignin. The undissolved cellulose pulp was collected and resuspended into a solution (1% H2O2 solution, 30% glycerol, and TEMPO) and irradiated using an ultrasonic probe. After irradiation, cellulose pulp is transformed into a gel-like transparent slurry. The slurry was filtrated and applied in a hot press, and film was obtained. Prepared cellulose film was used as a moisture sensor device.
Hydrogen peroxide in AHP solution promotes delignification and dissolves hemicellulose by its oxidative action, resulting in almost pure cellulose fibers . The alkaline conditions break the intermolecular ester bond between lignin and carbohydrates; thus, the compact structure relaxes and dissolves lignin [1,2]. Ultrasonic irradiation dissociates H2O2 into many reactive species radicals such as HO. and .O2- and generates H+, derived radicals that can depolymerize lignin and H+ break down hemicellulose backbone . Cellulose surface structure can be modified using TEMPO oxidation, this process converted C6-OH groups of cellulose to sodium C6-carboxylate group through C6-aldehyde . Surface-modified cellulose is used in print electronics . The presence of glycerol may increase -OH groups, enabling the adsorption of moisture or water-soluble gases, thus improving the conductivity. The Fourier Transform Infrared (FTIR) analysis was conducted to analyze the chemical property of cellulose in each successive step during the cellulose fiber extraction and the film production process. The FTIR analysis verified that cellulose with TEMPO-oxidation results in changes in chemical structure with glycerol, and the presence of hydroxyl groups. The dominant peaks were observed at around 3500 and 3000 cm-1 due to OH-stretching and CH- stretching, more intense and prominent peaks might be due to glycerol, which provides more -OH group in the films . The difference is the carboxyl group's appearance (C=O) stretching band at around 1645, 1602, and 1410 cm-1 in TEMPO-oxidized films, indicating that hydroxyl groups at the C6 position of cellulose molecules are converted to sodium carboxylate (-COONa).
Characteristics of cellulose sensor and its moisture sensing property
The initial resistance of the prepared cellulose sensor was ~2.0 M Ohm. The detection of moisture sensors is based on the resistivity change of the materials. The resistance of the sensor decreased upon exposure to the moisture with relative humidity (RH) at the different levels (25%, 50%, and 75%), while the resistance recovered to its initial value when moisture is replaced by nitrogen gas. The response of the sensor was observed regardless of the humidity, however, the response and recovery data of the sensor at 50% HR was better than data at both 30% and 75% RH. The change in resistance was proportional to the moisture concentration, with the lowest limit of single-digit detection of ppm at ambient humidity (50%). Nanocellulose and its derivatives have been used as a matrix for humidity sensing applications due to their high surface-to-volume ratio and many accessible hydroxyls (-OH) groups on their surface, which provides an inherent hydrophilic property. For instance, it is reported that fabricated self-standing wide-range humidity sensors (20 to 90%) from bagasse-derived CNFs composites . Another study reported the ammonia gas sensor based on cellulose acetate that detects 1 ppm of ammonia . It can be found that the as-prepared sensor exhibits moisture-sensing capability. However, sensing performance of the sensors could be different under humidity conditions, it may be because hydroxyl groups in the cellulose chains take a longer time to adsorb water molecules to reach saturation . This study is in the early stage and further research is recommended for the reproducibility of the data.
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Educational & Outreach Activities
Shahi N, Lee E, Min B, and Kim D. 2021. Nanocellulose derived from agricultural byproducts and its utilization for sensing materials. ACS National Meeting, Atlanta, GA, August 22-26.
Key highlights of project outcomes are as follows:
- Applied methods were feasible for the extraction and characterization of cellulose from cotton residues.
- Cellulose derived from cotton residues could be used as a sensing device for gas and moisture.
- Research findings were disseminated to various audiences such as scientific communities, students, and producers.
Through collaboration with Dr. Kim's research team at Auburn University and research teams at the College of Agriculture, Environment, and Nutrition Sciences at Tuskegee University, the cellulose-based sensor derived from cotton residues was evaluated for gases and moisture sensing abilities. Research outputs from the proposed proposal were presented at an internationally recognized academic conferences and meetings which include the American Chemical Society (ACS). In addition, the project activity and its potential application were also disseminated to visitors and intern students in the PI's research laboratory at Tuskegee University as well as the summer high school students through AgriTREK/SciTREK and AgDiscovery Summer Program at Tuskegee University. The source material (cellulose) used for sensor and research design is economical, affordable, and accessible. It is thought that it might be scaled up as a part of sustainable farm management for small-scale farmers, however, further research is necessary to test and apply sensing devices in the field.
During the tenure of this project, my knowledge was widened in the impact of effective utilization of resources and emerging technologies for sustainable agriculture.