Engineering pH-Responsive trans-Ferulic Acid/κ-Carrageenan Beads for On-Demand Micronutrient Delivery in Plants

Abstract Micronutrient deficiencies in soils are a critical challenge in agriculture, particularly in acidic soil environments where nutrient availability is strongly limited by fixation, leaching, and altered metal speciation. These constraints contribute to inefficient nutrient uptake and reduced crop yields. Conventional micronutrient supplementation methods, such as bulk fertilizer application and foliar sprays, are often inefficient, environmentally harmful, and unsustainable, underscoring the need for smarter delivery systems tailored to soil pH conditions. In this study, we developed biodegradable, pH-responsive microbeads fabricated from κ-carrageenan (κ-CG), sulphated polysaccharide derrived from algal biomass and trans-ferulic acid (TFA), a naturally occurring phenolic antioxidant with radical-scavenging activity. The κ-carrageenan/TFA-based microbeads, designated as κ-TF-B, were synthesized through an eco-friendly process and optimized for size, morphology, and stability. Characterization confirmed the successful incorporation of the constituent functional groups, while swelling, degradation, and release studies demonstrated pH-responsive delivery of essential micronutrients under acidic conditions (pH 4.0), mimicking acidic soil environments. The optimized κ-TF-B microbeads exhibited enhanced swelling at acidic pH and sustained micronutrient release throughout the study period. In addition, the microbeads showed strong antioxidant activity, with a radical-scavenging efficiency of 93.4%, attributed to the presence of TFA. For plant applications, κ-TF-All, defined as the κ-TF-B microbead formulation loaded with all four essential micronutrients (Mn2+, Zn2+, Cu2+, and Fe2+), was evaluated in soil and plant growth assays. κ-TF-All improved micronutrient availability and promoted plant growth, resulting in larger leaf area and a 3.3-fold increase in chlorophyll content in Arabidopsis thaliana compared with the negative control, without observable phytotoxic effects. Collectively, these findings demonstrate the potential of κ-TF-All microbeads as an eco-friendly and pH-responsive platform for sustainable micronutrient delivery and improved plant growth under micronutrient-limited conditions.