Synthesis of Gold Nanorods under the Influence of NIR Light

Abstract

Gold nanorods (GNRs) have attracted considerable attention due to their tunable optical properties and strong localized surface plasmon resonance (LSPR), making them relevant for applications in sensing, imaging, and photothermal therapy. Unlike spherical gold nanoparticles, GNRs have both transverse and longitudinal plasmon modes, where the longitudinal resonance strongly depends on particle aspect ratio. Precise control over nanorod growth is therefore essential for tailoring the optical response. This thesis investigates how near-infrared (NIR) laser irradiation influences the seedmediated synthesis of gold nanorods. Laser-irradiated samples were compared with non-irradiated controls prepared under identical conditions. Additional temperaturecontrolled experiments were conducted to distinguish the thermal effects from irradiationrelated effects. Characterization was performed using UV-Vis spectroscopy, scanning electron microscopy (SEM), and dynamic light scattering (DLS). UV-Vis analysis focused on longitudinal plasmon peak position, absorbance intensity, full width at half maximum (FWHM), rod index, and aspect ratio estimations. These observations were supported by SEM images analysed with ImageJ, which were used to evaluate morphology and aspect ratio distributions, while DLS measurements provided information on hydrodynamic diameter and colloidal stability. The results show that laser irradiation affects both the synthesis process and the optical properties of the resulting nanorods. Laser-irradiated samples generally exhibited blue-shifted longitudinal plasmon peaks and reduced FWHM values compared to control samples, indicating lower aspect ratios and narrower particle distributions. This is supported by the SEM analysis, although aggregation and sampling limitations introduced uncertainty in the quantitative measurements. Lastly, DLS measurements were strongly affected by clustering and were therefore mainly used for qualitative evaluation. In conclusion, NIR laser irradiation can influence GNRs growth during the synthesis and modify both morphology and optical properties.