Effect of metal ions on the thermo-optic properties of Rhodamine 6G-gold nanoparticle hybrids

The dye-plasmonic nanoparticle (NP) mixtures were reported to be an efficient probe for heavy metal detection in aqueous solution, particularly metals like Hg²⁺ ions. However, little is known about the variation in such dye-NP mixtures' thermal properties upon the addition of heavy metals. In this work, using the laser-based dual-beam thermal lens technique, the variation in thermal diffusivity value of Rhodamine6G (Rh6G)-gold nanoparticle (AuNP) mixture upon the addition of various concentrations of Hg²⁺ ions are investigated. The thermal property measurements are corroborated with optical absorption studies. The analysis reveals that the absorption peak wavelength and the shape of the spectrum are very sensitive to the addition of the Hg²⁺ ions. The measured thermal diffusivity values found to increase with the Hg²⁺ ion concentrations range from 0.1 nM to 10 µM and beyond which, the Hg²⁺ ions cause little variation in the thermal diffusivity value. The variation in the thermal diffusivity value originates from amalgam formation due to heavy metal-NP interaction. The investigation of dye molecules' emission spectral studies using a laser-based fluorescence setup reveal that the fluorescence behavior concurs with the data obtained from thermal lens studies. Additionally, the studies using the monovalent and divalent metal ions to dye-NP mixture elucidate that divalent metals are more efficient in tailoring the thermal and optical properties. The present study elucidates that optical sensor development based on the emission or absorption properties of the dye-plasmonic NP mixture for heavy metals have a concomitant variation in thermal properties.