Citrate–hydrazine hydrogen-bonding driven single-step synthesis of tunable near-IR plasmonic, anisotropic silver nanocrystals: implications for SERS spectroscopy of inorganic oxoanions

A simplified, single-step aqueous synthesis route to tunable anisotropic silver nanocrystals (NCs) has been developed by tailoring the hydrogen-bonding interactions between a mild stabilizer, sodium citrate, and a mild reductant, hydrazine hydrate. The structure directing ability of the H-bonding interaction was harnessed by keeping a stoichiometric excess of hydrazine under ambient conditions (pH 7, 25 °C). Decreasing the synthesis temperature to 5 °C imparts rigidity to the citrate-hydrazine H-bonding network, and the plasmon peak moves from 500 to 550 nm (using 40 mM hydrazine). On lowering the pH from 7 to 5, the H-bonding is further strengthened due to partial protonation of citrate and the plasmon peak is tuned to 790 nm. Further, we found that, at 5 °C and pH 5, there also exists a sub-stoichiometric regime in which maximum tunability of the plasmon peak (790→1010 nm) is achieved with 1 mM hydrazine. HR-TEM reveals that the near-IR plasmonic NCs are nanopyramids having a pentagonal base with edge length varying from 15 nm to 30 nm. Through second derivative FTIR analysis, a correlation between hydrogen-bonded molecular vibrations and the plasmon tunability has been established. The anisotropic NCs exhibit significant Raman enhancement on the citrate molecules. Further, a solution-phase, non-resonant SERS spectroscopic detection method for an inorganic contaminant of ground water, arsenite, has also been developed.