Seed geometry and hydrogen bonding dependent plasmonic tuning of silver nanocrystals in a citrate–hydrazine matrix and SERS spectroscopic detection of chromium†
By employing a ‘mild stabilizer–mild reductant’ system, a seed-mediated synthesis of tunable anisotropic and plasmonic silver nanocrystals (NCs) has been developed. Differently shaped seed NCs were synthesized using citrate and hydrazine hydrate as stabilizer and reductant respectively, and the same reagent system was retained for the subsequent overgrowth process as well. Spherical seed NCs always resulted in red-shifted plasmon peaks of the overgrown nanocrystals. However, we found that anisotropic seed NCs may cause either a red-shift or a blue-shift of the plasmon peak depending on the geometry of the seed NCs. Plate-like seed NCs result in red-shifted surface plasmon bands as a consequence of structural transformation from pentagonal to hexagonal plate-like structures. Second derivative FTIR and Raman spectroscopy revealed that the anisotropic overgrowth of the seed NCs is directed by the stabilizer–reductant i.e., citrate–hydrazine, hydrogen bonding network. As the concentration of hydrazine increases, the H-bonded network is strengthened and the plasmon peak shows a gradual red-shift (500 nm → 615 nm). In contrast, a pyramid-like seed causes a blue shift of the plasmon peak (790 nm → 775 nm), attributed to the loss of structural anisotropy of the pyramid like nanostructures. Based on the interaction of these NCs with inorganic oxoanions, a rapid and sensitive SERS-detection method for Cr(III) and Cr(VI) species has been developed with a limit of detection of 30 and 40 ppb, respectively. The simplicity is underscored as the detection method works under non-resonant conditions and in the solution phase without any “SERS tags”.