This Small Business Innovation Research (SBIR) Phase I project will demonstrate proof of concept and validate the feasibility of translating a molecular receptor for nitrate anion into a highly-selective and sensitive soil probe. Ultimately, these sensors will fulfill the need for real-time monitoring of fertilizer application in environmentally sustainable precision agriculture. Both the ion-selective electrode and chemically modified field effect transistor interfaces currently used for nitrate monitoring are capable of measurements only in aqueous media. These sensors rely solely upon non-specific interactions for their selectivity due to a general lack of nitrate selective receptor components. This limitation preempts their use in soil media where highly competitive interferents diminish response. The first innovation proposed herein is the development of a sensor incorporating a rationally designed and intrinsically selective host molecule, which will provide the affinity for nitrate needed to enable monitoring in soils on a molecular level. This technology will then enable a second innovation: a field-embeddable soil sensor network that wirelessly reports fertilizer levels during application in real-time. These innovations will enable molecularly selective sensing in soil, and will pave the way for the development of future molecular sensors for monitoring difficult-to-target anionic and neutral substrates in complex media.

The broader impact/commercial potential of this project is the simple need for feeding the world sustainably. Increasing food production capacity by two-fold in the next 30 years, while concurrently decreasing the environmental impact of nonpoint-source pollution has been identified as one of the grand challenges facing the sciences. Nitrate-based fertilizer accounts for almost 60% of the 21M tons of fertilizer applied annually and almost 30% of this is wasted due to seepage, runoff and volatilization. Conserving even 20% of the 2.5M tons of domestic fertilizer that ultimately contribute to nonpoint-source pollution would save growers an average of $45/acre annually, giving rise to an annual market in the U.S. worth approximately $2.1B. Additionally, real-time monitoring of soil macronutrients will enhance understanding of soil chemistry by providing snapshots of the in situ behavior and fate of these chemicals. On a global scale the development of a low-cost and universal probe for soil quality would offer developing areas a novel method for optimizing yields and enabling self-sufficiency in food production. Additionally, these sensors will address the environmental dilemma of groundwater contamination with a foundational solution: limiting the wasteful over-application of fertilizers in food, flower and grain production.

Project Report

") support enabled SupraSensor Technologies, LLC (SST) to fabricate, test and validate a new sensor for in-soil agricultural fertilizer monitoring using a nitrate-selective small molecule. Managing the nitrogen cycle and ensuring access to clean water have been identified as "Grand Challenges" facing humanity by the National Academy of Engineering. These challenges are intimately related in agriculture, where nitrogen fertilizer application generates a host of nonpoint source pollution problems, most notably ground and surface water fouling by nitrate and generation of the greenhouse gas nitrous oxide. The world’s growing population and challenge to feed it continues to drive the need for maximizing the crop yield per acre farmed; however, the limited toolbox for monitoring the introduction of anthropogenic nitrogen sources has led to a systemic over-application of fertilizers in order to ensure consistent yields. Unlike other agricultural nutrients, nitrate is highly mobile in irrigated fields. Current fertilizer monitoring technology takes a post-application, "top-down" approach to ensure enough fertilizer is present to grow healthy plants. The innovation central to SST’s work under NSF Phase I/IB support is distinct from these technologies by a "ground-up" approach of directly measuring the actual fertilizer molecules present throughout the rooting region in real-time. Proper placement of these sensors in and below the root zone will stem over-application and warn of fertilizer below the reach of plant roots and before it enters the water table. These data are acquired not by an indirect metric based on the effect of the fertilizer on plants (e.g., leaf-measurement or surface water methodologies), but rather by molecule-molecule interactions in the active areas of the field soil. SST’s in-situ sensor measures the nutrient molecules in real-time rather than solely monitoring the results post-application. The development of a low-cost sensor for fertilizer monitoring that is robust and reliable enough for continuous deployment in soil year-round will enable precision control of fertilizer application in the current market, and would create new markets in developing countries as a readily available alternative to more expensive and less available laboratory testing. A direct molecule-molecule "lock and key" mechanism that recognizes nitrate by size and shape provides much greater selectivity and increased sensitivity for nitrate ions in field-moist soil. The small molecule receptor central to this molecular interaction was first developed with NSF support in academic laboratories at the University of Oregon (CHE-0718242 and DGE-059503). This approach incorporates the sensor interface developed during Phase I/IB support and soil moisture monitoring for in-situ molecular recognition of nitrate at specific depths across soil types. The sensor’s output in loam, silt and clay soils was validated by common colorimetric methods in lab, and by EPA standard cadmium reduction methods by a third-party analytical laboratory. Additionally, NSF SBIR Phase I support enabled SST to secure external funding for further development of the power and signal processing electronics, which triggered the Phase IB funding for development and validation of a novel low-cost reference electrode for use with the sensor developed during Phase I. The intellectual merit of the Phase I/IB supported development is the development of an in-soil sensor capable of measuring and reporting available nitrate levels in-situ by use of a rationally designed molecular receptor for nitrate. This achievement avoids the aqueous measurement requirement of ion-selective electrodes and standard CHEMFET technologies in current practice. Additionally, overcoming the interference of chloride and phosphate that is common in field soils allowed SST to make in-soil measurements that were reliable and repeatable down to single digit parts-per-million (ppm) nitrate levels even in the presence of thousands of ppm chloride. The organic anion and herbicide glyphosate had absolutely no effect on the sensor’s output. The broader impacts and commercial potential are enormous. Simply put, the market pull for fertilizer management tools stems from the amount of nitrate fertilizer applied annually (~59% of all applied fertilizer, 12.29M tons), and the estimated waste rate of >30%. This lost input over 382M acres of US farmland and the cost of nitrate remediation to recover the potability of well-water in agricultural communities exceeds $30B annually.

National Science Foundation (NSF)
Division of Industrial Innovation and Partnerships (IIP)
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Muralidharan S. Nair
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Suprasensor Technologies, LLC
United States
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