CAREER: Introducing Dynamic Sulfur Chemistry into Hydrogels to Promote Water Retention and Healthy Microbe Growth in Soil

世界人口正在迅速增长，这就要求大幅度扩大粮食生产。再加上城市化、气候变化和水资源竞争加剧，这对农业构成了极大的挑战。这些变化本身很难管理，更不用说可持续地实现，对经济、公共卫生和周围生态系统的影响有限。农业是美国用水量最大的行业。近年来，美国西部大部分地区经历了极端干旱，我们必须更有效地利用现有的水资源。虽然已经确定硫对植物有很多好处，但直到最近，由于燃烧化石燃料，大量的硫沉积到土壤中。随着社会逐渐远离石油和空气质量标准的提高，土壤中的硫将继续减少，增加了向作物添加含硫肥料的压力。尽管硫对农作物的成功生产是必要的，但它也带来了污染风险。由于常用肥料被作物吸收的比例不到50%，含硫肥料也不太可能被完全吸收，并可能渗入地表水，对环境产生负面影响。这项工作旨在创造吸收性的硫基材料，以帮助保持水分，并为土壤提供包括硫在内的关键营养物质的可控输送。更有效地向植物输送养分可以提高作物产量，同时限制对周围生态系统的影响。将这些材料暴露于土壤中常见的细菌中，将有助于确定这些材料对土壤中微生物的影响。此外，本研究为本科生提供毕业后加入STEM劳动力所需的培训和指导。它还为化学系的所有研究生建立了一个专注于科学交流的暑期研究经验（SRE）。SRE将提高学生有效地向其他科学家学习和分享数据的能力，并更好地与公众分享他们的结果。随着时间的推移，SRE将包括该地区服务不足和低收入的高中学生，在他们与STEM高等教育之间建立联系。硫作为农业杀虫剂已经使用了几个世纪。然而，它作为肥料的应用直到最近才被考虑。几十年来，燃烧化石燃料使硫沉积到环境中。随着社会逐渐远离石油，空气质量不断改善，土壤中的硫将减少，增加了向作物添加含硫肥料的压力。在过去的十年中，反硫化（IV）使硫基材料得以扩展。IV利用S8中的热不稳定键作为引发剂、溶剂和单体，得到硫含量在20-90%之间的多硫化物。然而，S8的疏水性使得极性单体的聚合具有挑战性，产生很少的材料可以与水相互作用。大蒜精油（GEO）富含二烯丙基二硫化物，提供了一种将硫与极性单体结合的策略。通过将动态硫化学引入亲水凝胶平台，这项工作旨在开发一类新的硫基聚合物，并深入了解硫环、GEO和3D聚合物结构对材料性能的作用。这些材料将暴露于已知与硫进行氧化还原化学的微生物中，以确定它们是否可以利用这些凝胶中的硫。进一步的分析将把我们对聚合物结构和硫的氧化态的理解与细菌的反应联系起来，并为细菌对多硫化物的修饰提供见解。将对土壤细菌进行额外的测试，以开始了解硫雾如何影响土壤微生物群。进一步的分析将确定微生物是否能够接触到聚合物中的硫，并将其转化为硫酸盐等植物可获得的形式。这些分析将有助于开发能够向作物提供有控制的水、养分和有益细菌的材料。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARY The world’s population is growing rapidly, requiring the dramatic expansion of food production. Combined with increased urbanization, climate change, and competition for water, this poses an extreme challenge for the agricultural industry. These changes alone are difficult to manage, much less achieve sustainably, with limited impacts on economics, public health, and surrounding ecosystems. Agriculture is the top consumer of water in the United States. With much of the western U.S. experiencing extreme drought in recent years, the water we do have must be used more efficiently. Although it is established that sulfur provides many benefits to plants, until recently, there has been ample sulfur deposition into soils from burning fossil fuels. As society moves away from petroleum and air quality standards improve, sulfur deposited in soil will continue to decrease, increasing the pressure to add sulfur-based fertilizers to crops. Despite the necessity of sulfur for successful crop production, it also poses a pollution risk. Since commonly used fertilizers have less than 50% uptake by crops, sulfur-based fertilizers are also unlikely to be fully assimilated and could leach into surface waters, negatively impacting the environment. This work aims to create absorbent, sulfur-based materials to aid in water retention and provide controlled delivery of key nutrients, including sulfur, to the soil. More effective nutrient delivery to plants could enhance crop production while limiting the impact on surrounding ecosystems. Exposing these materials to common bacteria found in soil will help determine the impacts of these materials on microbes in the soil. Additionally, this research provides undergraduates with the training and mentoring necessary to join the STEM workforce after graduation. It also establishes a summer research experience (SRE) focused on scientific communication for all research students in the Chemistry Department. The SRE will enhance students’ ability to effectively learn from and share data with other scientists as well as better share their results with the general population. Over time, the SRE will include underserved and low-income high school students in the region creating connections between them and higher education in STEM.TECHNICAL SUMMARYSulfur has been used for centuries as a pesticide in agriculture. However, its application as a fertilizer has not been considered until recently. Burning fossil fuels has deposited sulfur into the environment over many decades. As society moves away from petroleum and air quality continues to improve, sulfur deposited into the soil will decrease, increasing the pressure to add sulfur-based fertilizers to crops. Inverse vulcanization (IV) has enabled the expansion of sulfur-based materials over the past decade. IV uses the heat labile bonds in S8 to act as the initiator, solvent, and monomer, leading to polysulfides with sulfur-contents ranging from 20-90%. However, the hydrophobicity of S8 makes polymerizing polar monomers challenging, yielding very few materials that can interact with water. Garlic essential oil (GEO), rich in diallyl disulfides, offers a strategy to combine sulfur with polar monomers. By introducing dynamic sulfur-chemistry into a hydrophilic gel platform, this work aims to develop a new class of sulfur-based polymers and provide insight into the role of sulfur-loops, GEO, and the 3D polymer structure on the material properties. These materials will be exposed to microbes, known to perform redox chemistry with sulfur, to determine if they can utilize the sulfur within these gels. Further analysis will connect our understanding of the polymer structure and oxidation state of sulfur to the bacterial response and provide insight into bacterial modification of the polysulfides. Additional testing with soil bacteria will be performed to begin to understand how sulfogels impact the soil microbiome. Further analyses will determine if microbes are able to access the sulfur within the polymer and convert it to a form accessible to plants such as sulfates. These analyses would aid in the development of materials that can provide the controlled delivery of water, nutrients, and beneficial bacteria to crops.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.