Application of SiO2 nanoparticles to address CdS NPs contamination in spinach

Rising cadmium (Cd) contamination poses significant threats to crop productivity, quality, and human health. To address this, nano-enabled techniques have recently gained attention for their potential to enhance crop yields and remediate contamination due to heavy metals. This study explores the efficacy of silicon dioxide nanoparticles (SiO2 NPs) in mitigating the effects of cadmium sulfide (CdS) NPs in spinach. Field experiments were conducted growing spinach plants subjected to cultivation with 1 mg/L CdS NPs contamination, with foliar application of SiO2 NPs at concentrations of 1, 20, and 100 mg/L. The phenotypic, biochemical, and metabolic responses of the plants to stress conditions were examined following exposure to CdS and SiO2 for four weeks. The results showed that SiO2 NPs increased the fresh and dry weights of both roots and shoots. Furthermore, CdS NPs exposure reduced chlorophyll content by 66.76 %, whereas SiO₂ NPs co-exposure increased chlorophyll levels by up to 42 % compared to the CdS NPs and control groups. However, elevated malondialdehyde (MDA) levels were observed in leaves for the CdS-only group and roots for all treatments indicating oxidative stress was most pronounced for the CdS case. Results demonstrated that SiO₂ application significantly reduced Cd accumulation in spinach by up to 34.92 %. Also, enhanced mineral accumulations were recorded in both roots and shoots, whereas decreased levels were found in the co-exposure groups, except for Zn. The exposure to SiO2 resulted in upregulation of metabolites including galactonic acid, d-aspartic acid and others, and UDP-d-galactose was downregulated in the group exposed only to CdS NPs. The upregulation of these metabolites by SiO2 NPs demonstrates their mitigating effect against CdS NPs induced stress. This work enhances understanding of phenotypic and metabolic alterations induced in spinach by CdS and SiO2 NPs, and independently and through their co-exposure. Overall, our findings indicate that Cd contamination can be reduced in spinach using SiO2 NPs when applied at low levels, and the mechanisms are discussed.