Green synthesis of gold and silver nanoparticles from Harpagophytum procumbens (Devil’s Claw) extract and their biomedical applications.
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Date
2025
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University of the Western Cape
Abstract
Nanotechnology refers to the research of materials with dimensions of one billionth of a meter (1-100 nm). Metallic nanoparticles (MNPs), such as gold (Au) and silver (Ag), are undoubtedly some of the most popular nanoparticles (NPs) owing to their physicochemical properties. Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) have antibacterial and anticancer properties which allows them to be used in various biomedical applications. Green synthesis
of MNPs is a safe, cost-effective, and sustainable method that mitigates against highly toxic chemicals and the formation of dangerous byproducts seen in most physical and chemical methods. Plant extracts contain phytochemicals, which act as reducing and stabilising agents in the synthesis of NPs. Harpagophytum procumbens subsp. procumbens (Burch.) DC. Ex Meisn, commonly known as Devil's Claw, is a weedy perennial tuberous plant native to the Kalahari region in southern Africa. Its secondary bioactive compounds have been identified, including amino acids, carbohydrates, iridoids, flavonoids, and phytosterols. The therapeutic effects of Harpagophytum procumbens (Hp) are attributed to iridoid glycoside, harpagoside, compound found in high concentrations in Harpagophytum species that is responsible for its anti-inflammatory activity. In addition, research has reported the analgesic, antioxidant, anti- microbial, anti-cancer and anti-viral effects of Hp. In the current study, Harpagophytum procumbens aqueous extracts (HpAE) was used for the green synthesis of AuNPs and AgNPs. Various parameters such as pH, temperature, extract concentration, metal precursor concentration, ratio and synthesis reaction time were investigated to determine the optimal conditions for HpAE-MNP synthesis. Due to the biocompatibility and well-defined surface chemistry of AuNPs, ciprofloxacin (CIP) was
functionalized onto the surface of HpAE-AuNPs for use in drug delivery and as a therapeutic agent. The green synthesized HpAE-MNPs further characterized using UV-vis spectroscopy (UV-vis), Dynamic Light Scattering (DLS), Zeta (ζ) potential, Fourier-transform Infrared (FTIR), Induced Coupled Plasma-Optical Emission Spectrometry (ICP-OES), and High transmission electron microscopy (HR-TEM). The stability of HpAE-MNPs were evaluated in various biological media, including deionized water (ddH2O), Müeller-Hinton broth (MHB), Phosphate-buffered saline (PBS), complete and incomplete Dulbecco's Modified Eagle's Medium (DMEM). Agar well diffusion and microdilution assays were used to investigate the antibacterial activities of HpAE and HpAE-MNPs against several bacterial strains associated with antimicrobial resistance (AMR) (Escherichia coli, Staphylococcus aureus, Methicillin-resistant Staphylococcus aureus, Klebsiellae pneumoniae, Acinetobacter baumanni, Pseudomonas aeruginosa and Escherichia cloacae). In addition, the antioxidant potential of HpAE and HpAE-MNPs were evaluated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH),
2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and Ferric-Reducing Antioxidant Power (FRAP) assays. The HpAE-AuNPs had a SPR peak at 540 nm, a hydrodynamic diameter of 148.47 ± 29.4 nm, an average core size of 96.2 ± 38.2 nm (prisms), 40.02 ± 8.2 nm (pentagons), 33.46 ± 9.5 nm (hexagons) and 43.55 ± 12.4 nm (irregular spheres). Upon functionalization, HpAE-AuNPCIPs displayed a reduced hydrodynamic diameter of 134.85 ± 27.29 nm and an increased average core size of 104.27 ± 21.18 nm (prisms), 99.63 ± 17.68 nm (pentagons), 85.67 ± 10.27 nm (hexagons) and 74.35 ± 21.45 nm (irregular spheres). On the other hand, HpAE-AgNPs had a SPR peak at 404 nm, a hydrodynamic diameter of 45.71 ± 4.84 nm, and an average core size of 27.55 ± 7.99 nm. The HpAE-AuNP had a polydispersity index (PDI) of 0.5 ± 0.03 and a ζ potential of -16.97 ± 0.49 mV, while HpAE-AuNPCIPs had a PDI of 0.32 ± 0.12 and a ζ potential of -33.39 ± 2.84 mV. The HpAE-AgNPs had a PDI of 0.24 ± 0.07 and a ζ potential of -23.5 ± 3.5 mV. Both HpAE-AuNPs and HpAE-AuNPCIPs were larger and anisotropic in comparison to smaller spherical HPAE-AgNPs. HpAE-MNPs were determined to have high phenolic and flavonoid content, suggesting the involvement of phytochemicals in the reduction and capping of metal ions during the synthesis process. HpAE-AgNPs exhibited dose-
dependent and enhanced free radical scavenging activity, while HpAE-AuNPs displayed limited antioxidant activity. The HpAE-AgNPs exhibited broad-spectrum antibacterial effects against both Gram-positive and Gram-negative antimicrobial resistant bacteria, with higher susceptibility observed against Gram-negative bacteria. HpAE-AuNPs alone were non-toxic to the tested bacteria, however, enhanced antibacterial activity against Gram-negative bacteria
were only observed after functionalization of HpAE-AuNPs with CIP. Therefore, the biosynthesized HpAE-AgNPs and HpAE-AuNPCIPs provide potential alternatives for treating ESKAPE infections and subsequently AMR, while HpAE-AgNPs show promise as antioxidants to manage oxidative stress-related conditions.
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Keywords
Gold nanoparticles (AuNPs), Silver nanoparticles (AgNPs), Devil’s claw, Nanotechnology, Green synthesis