Encapsulation in Silica Nanoparticles Increases the Phytotoxicity of Essential Oil from Thymus vulgaris in a Weed Species

Weed control poses a significant challenge to agriculture, warranting the development of effective but environmentally safe herbicides. Encapsulation of plant essential oils (EOs) with herbicidal properties in nanoscale polymers can offer high loading capacity as well as controlled and tunable agrochemical delivery. This study investigated the use of encapsulated thyme EO against redroot pigweed (Amaranthus retroflexus L.), a difficult-to-control weed resistant to multiple herbicides. Three volumes of thyme EO (500, 750, and 1000 μL) were encapsulated in a silica nanoparticle (SiNP) suspension to achieve 250 μL/mL (hereinafter “500”), 375 μL/mL (hereinafter “750”), and 500 μL/mL (hereinafter “1000”) EO concentrations. The efficacies of these preparations were compared to that of pristine EO. The loading efficiencies were 26, 42, and 64% for the “500”, “750”, and “1000” EO preparations, respectively. Transmission electron microscopy (TEM) revealed spherical and regular SiNPs with a size range of 220–300 nm. Fourier transform infrared (FT-IR) spectroscopy confirmed EO loading by the presence of characteristic peaks of isoprenoids and isomeric compounds. Herbicidal bioassays with pristine thyme EO in postemergence treatments on A. retroflexus seedlings exhibited a significant (p ≤ 0.05) concentration-dependent herbicidal activity, reducing shoot biomass by 85% at the highest tested concentration (“1000”), compared to the control (Tween 20). Encapsulation with SiNPs further enhanced the herbicidal efficacy compared to the control, reaching 96% at the highest concentration. Compared to the pristine EO, EO-SiNPs induced significant ROS production at the highest concentration, leading to cell membrane damage and an imbalanced antioxidant system, as demonstrated by the increased shoot malondialdehyde content (40%) and activities of the antioxidant enzymes ascorbate peroxidase (APX) (65%), catalase (CAT) (52%), and superoxide dismutase (SOD) (36%). These results suggest significant potential for developing an effective nanobioherbicide using thyme EO encapsulated in SiNPs.