Size variant spherical gold nanoparticles for in situ detection of trace contaminants by surface enhanced Raman spectroscopy

Spherical gold nanoparticles (AuNPs) of different sizes have been synthesized for the in situ detection of trace contaminants by Surface Enhanced Raman Spectroscopy (SERS) with Rhodamine 6G (R6G) as a probe molecule. TEM analyses revealed that most of the AuNPs are uniform in size and spherical in shape with an average dia. of ~ 12 ± 1 nm and ~ 25 ± 3 nm for solution pH 10.0 (GNP-10) and pH 6.0 (GNP-6), respectively. HR TEM lattice fringe analyses exhibit a consistent expansion of d-spacing for smaller GNP-10 compared to those of larger GNP-6 nanoparticles elucidated from both Au(111) and Au(200) planes. The SERS enhancement factor was calculated to be ~ 20-fold larger for GNP-6 compared to that of GNP-10 while ~ 5 μL of AuNPs solution added in situ atop the 0.2 × 10⁻³ M R6G droplet (size ~ 4 mm) for Raman spectra recorded for ~ 5 s. Following this liquid phase SERS protocol, the detection limit as low as 10⁻¹¹ M of R6G was successfully demonstrated.