Thiol Functionalized Silica Nanoparticles: Solution for Toxic Heavy Metal Cadmium Cd2+ Stress

Abstract

The escalating contamination of agricultural soil and plants with cadmium poses significant challenges to plant health, ecosystem sustainability, and human health through the food chain. This study explores thiol-functionalized silica nanoparticles (SiO2-SH NPs) as a novel approach to address cadmium-induced stress in spinach, enhancing the antioxidant defense system and photosynthetic pigment. SiO2-SH NPs were synthesized using the two-step sol-gel process with a size range of ±43 nm and are tested in a pot experiment with nine treatments (T1-T9), combining Cd stress levels (0,10 and 15 mg/kg) and SiO2-SH NP treatments (0, 50 and 100 mg/kg) in a complete randomized design. Cd stress reduced spinach plant and root weight and Spad index, indicating growth and photosynthetic impairment. Conversely, SiO2-SH NPs significantly increased including root weight supporting development under Cd stress. Cd stress significantly (p<0.001) decreased chlorophyll levels and photosynthetic capacities, while SiO2-SH NP treatments significantly restored chl a, chl b, total chlorophyll, and carotenoids along with improved photosynthetic performance by significantly increasing phi2 and reducing phiNPQ and NPQt levels. Under Cd stress, H2O2, MDA, antioxidant defenses (SOD, POD, CAT, APX, AsA and GSH) were significantly (p<0.001) activated while SiO2-SH NPs treatments further significantly (p<0.001) enhance antioxidant activity, reducing H2O2 cand MDA content and mitigating oxidative damage. SiO2-SH NPs treatments notably reduced Cd uptake, especially at the highest dose (S100 mg/kg), potentially immobilizing Cd in soil and limiting uptake in roots and shoots. Transfer index (TI), Translocation factor (TF), and Cd uptake exhibited similar significant decreases with SiO2-SH NPs treatment. These SiO2-SH nanoparticles showcased their potential to mitigate Cd stress by chelation leading to active adsorption and Cd immobilization to reduce bioavailability. This results in enhanced growth, improved antioxidants, regulation of photosynthetic pigments and capacities. SiO2-SH NPs hold promise in sustainable agriculture and environmental remediation through their dual role in enhancing plant resilience and reducing cadmium toxicity. Thus, by decreasing and immobilizing the mobility of Cd and formation of complexes SiO2-SH NPs are proved to be efficient in alleviating toxic heavy metal Cd2+ stress.