Biogenic Nickel Ferrite Nanoparticles Synthesized From Agro-Waste: Green Synthesis, Physicochemical Characterization, and Potential for Magnetic Hyperthermia Applications.

INTRODUCTION: Nickel ferrite (NiFe2O4) nanoparticles are promising magnetic hyperthermia agents, but conventional synthesis relies on toxic reagents. Plant-extract-mediated biogenic synthesis offers an eco-friendly alternative; however, the influence of specific agro-waste phytochemical matrices and precursor chemistry on nanoparticle structure and heating efficiency remains poorly characterized. METHODS: Aqueous extracts from pomegranate peel (Punica granatum L) and raspberry leaves/shoots (Rubus idaeus L) were characterized for total phenolic content (TPC), total flavonoid content (TFC), and DPPH radical scavenging activity. Cherry stalk extract showed insufficient reactivity and was excluded. Two precursor systems - nitrate (Fe(NO3)3/Ni(NO3)2) and acetate-sulfate (Ni(CH3COO)2/FeSO4) - were combined with each extract, yielding four nanoparticle batches (M1, M2, G1, G2) after calcination at 500°C for 4 h. Nanoparticles were characterized by XRD, XRF, FTIR, TEM, DLS, and zeta potential. Magnetic heating performance was assessed under AMF (532.4 kHz, 3.1 kA·m-1); SAR and ILP were calculated with ferrite-mass correction based on XRF data. RESULTS: Pomegranate extract showed the highest phytochemical activity (DPPH: 1283.9 μM TE·g-1). XRD confirmed the spinel NiFe2O4 structure in all batches, with crystallite sizes of 15.0-21.8 nm and lattice parameters of 8.35-8.38 Å. The acetate-sulfate batches (M2, G2) contained substantially higher Fe/Ni fractions (M2: Fe 49.0%, Ni 11.5%) than the nitrate batches (M1, G1: Fe ≤2.9%, Ni ≤1.7%). DLS indicated hydrodynamic diameters of 200-400 nm; zeta potential values (-14.0 to +7.8 mV) indicated limited colloidal stability. Under AMF, M2 and G2 reached maximum temperatures of 46.0°C and 43.4°C (baseline 31°C), with SAR of 15.7 and 15.0 W·g-1 and ILP of 3.08 and 2.93 nH·m2·kg-1, respectively. Sample M1 showed negligible heating (SAR 0.93 W·g-1; ΔT =1.1°C). DISCUSSION: Precursor chemistry critically determines ferrite yield and heating efficiency, with the acetate-sulfate system consistently outperforming the nitrate system. The obtained ILP values are comparable to literature reports for NiFe2O4 under similar AMF conditions. However, the observed aggregation and low zeta potential limit biomedical applicability. As no cytotoxicity or in vitro/in vivo data were obtained, these findings represent proof-of-concept for the thermal conversion potential of biogenically synthesized NiFe2O4, pending biological validation.