Synthesis of non-stoichiometric FeS₂ nanoparticles and their applications in antibacterial phototherapy

Antibiotic resistance in bacteria has become a critical global issue, prompting increased interest in the development of non-antibiotic antibacterial strategies. Recent studies have emphasized the importance of multifunctional antibacterial therapies particularly those integrating photothermal, photodynamic, gas therapy, and chemotherapy approaches due to their enhanced antibacterial efficacy. Among these, metal chalcogenide nanoparticles have gained prominence owing to their combined photothermal and photodynamic effects and their broad-spectrum antibacterial activity. This thesis investigates the synthesis of FeS₂ nanoparticles via the hot-injection method and examines their antibacterial properties. FeS₂ nanoparticles were selected due to their non-toxic, environmentally friendly, hydrophilic nature, and dual photothermal and photodynamic activities. Initially, the synthesis parameters were optimized, followed by the production of FeS₂ nanoparticles with varying stoichiometries. The produced nanoparticles were thoroughly characterized in terms of their crystal structure, morphology, and optical properties. Non-stoichiometric FeS₂ nanoparticles were synthesized by altering the molar ratios of precursors (iron and sulfur sources) during the reaction. X-ray photoelectron spectroscopy (XPS) analysis revealed Fe/S ratios of 1.012:2, 0.960:2, 0.905:2, and 0.900:2 for samples prepared with precursor molar ratios of 1:6, 1:8, 1:10, and 1:12, respectively. Photothermal tests conducted under 808 nm near-infrared (NIR) irradiation showed temperature increases of 48°C, 49°C, 51°C, and 53°C for the samples with Fe/S ratios of 1.012:2, 0.960:2, 0.905:2, and 0.900:2, respectively. An increase in crystal defect density was observed to enhance photothermal performance but reduce photodynamic efficacy. For the sample with an Fe/S ratio of 0.900:2, the photothermal conversion efficiency was calculated to be 44.3% in pure water and 52.6% in phosphate-buffered saline (PBS). In vitro antibacterial activity tests revealed that non-stoichiometric FeS₂ nanoparticles exhibited strong antibacterial activity against Escherichia coli (98.00% inactivation at 100 μg/mL) and Staphylococcus aureus (99.89% inactivation at 125 μg/mL). This potent activity is attributed to the synergistic effects of reactive oxygen species (ROS) generation and hyperthermia. These findings indicate that non-stoichiometric FeS₂ nanoparticles hold significant potential as antibacterial agents, particularly for treating bacterial infections under NIR light irradiation.