## 2 Materials and Methods  
Chemicals and materials used for nanoparticle synthesis: analytical grade iron (II) sulfate heptahydrate and alconox were purchased from Sigma-Aldrich. All aqueous solutions were prepared with double-distilled deionized (DI) H\({}_{2}\)O. The glassware used in all experiments was washed with alconox and rinsed thrice with DI H\({}_{2}\)O. To prepare the aqueous avocado seed husk extract, the air-dried avocado seeds were collected from the waste. Seeds were separated from the husks and the seed husks were washed with deionized water and dried in air for 24 h. The dried seed husks were then crushed to powder using a blender; 10 g of the powder was added to 200 mL of deionized water and heated at 70 \({}^{\circ}\)C for 1 h and allowed to cool at room temperature to obtain the extract. The synthesis of hydronium jarosite nanoparticles was completed by following a nanochemistry approach and the method of [PERSON] et al., 2021 [13]. Next, 100 mL solution of FeSO\({}_{4}\cdot\)7H\({}_{2}\)O was added to 100 mL of the extract and stirred for 3 h at 70 \({}^{\circ}\)C as the black precipitate formed. The aqueous mixtures containing avocado seed husk extract and nanoparticles were left to cool. For downstream analysis, the nanoparticles were separated from the solution and isolated using centrifugation at 4000 rpm for 20 min. The pellet obtained after centrifugation was washed with DI H\({}_{2}\)O and dried in a Memmert hot air oven at 50 \({}^{\circ}\)C for 2 h.  
Characterization techniques: Morphological and elemental analysis was characterized using a Tescan MIRA3 SEM, whilst elemental composition was determined with EDX performed using a Nova NanoSEM at 20 kV. A Balzer's evaporation coater was used to carbon coat SEM samples. HRTEM images were captured using a Tecnai F20 FEG-TEM in bright field mode at 200 KeV, ImageJ was used to evaluate particle size distribution. Phase identification was done using XRD (Bruker Advanced AXS D8 diffractometer with monochromated Cu K\(\alpha\) radiation [\(\lambda\) (wavelength) = 1.54060 A] operating in the Bragg-Brentano geometry and a step size of 0.2\({}^{\circ}\) with 0.2 s per step for angles between 5\({}^{\circ}\) and 85\({}^{\circ}\) was used for phase identification and sample crystallography. To characterize chemical bonds and functional groups, Fourier transform infrared spectroscopy (FTIR) using potassium bromide pellet technology and a PerkinElmer 100 Spectrometer in the wave number 400-4000 cm\({}^{-1}\) range was used. Cytotoxicity: The murine macrophage cells (Raw 264.7) was purchased from the American Type Culture Collection (ATCC TIB-71, Manassas, VA, USA). Standard tissue culture conditions were used to maintain the cells in a complete medium. Cells were sub-cultured approximately every 3-4 days at 37 \({}^{\circ}\)C in a humidified 5% CO\({}_{2}\) environment, using DMEM (Lonza, Cape Town, South Africa), 10% heat-inactivated fetal bovine serum (FBS), glutaMAX, antibiotic/antimycotic (Sigma-Aldrich, St. Louis, MO, USA), and gentamicin were added as supplements (Sigma-Aldrich, St. Louis, MO, USA). To evaluate the cytotoxicity of hydronium jarosite (MTT) (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolzolium bromide assays were employed as previously described with minor modifications. MTT assay is based on the ability of nicotinamide adenine dinucleotide phosphate (NADPH)-dependent cellular oxidoreductase enzymes to reduce the tetrazolium dye MTT to its insoluble formazan (purple color). Briefly, cells were seeded at 5 \(\times\) 10\({}^{4}\) cells per well in a 96-well plate and incubated as earlier described using the cell culture standard procedure. After treatment with the NPs for 24 h, 48 h, and 72 h, the test medium was removed, and cells were incubated with 100 \(\upmu\)L of MTT solution (0.5 mg/mL MTT diluted in the media without FBS) for 1 h at 37 \({}^{\circ}\)C, 5% CO\({}_{2}\). Subsequently, the MTT solution was removed and 100 \(\upmu\)L of DMSO was added to each well.