[Preparation of silver nanoparticles in Brønsted acidic ionic liquid and its optical properties]

The use of room-temperature ionic liquids (RTILs) as green media for preparation of nanomaterials is very attractive. Herein, anisotropic silver nanoparticles were synthesized via chemical reduction silver nitrate with sodium hypophosphite, employing brønsted acidic functionalized ionic liquid 1-butyl-3-methyl-imidazolium dihydrogen phosphate ([bmim] H2PO4), which acted both as reaction medium and as a capping/shape-directing agent. The structure, morphology and optical absorption properties of anisotropic silver nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-Visible (UV-Vis) absorption spectra. The XRD patterns revealed that the crystal was cubic structure for anisotropic silver nanoparticles. The SEM image indicated that the anisotropic silver nanoparticles were block-like with average lateral dimensions of about 30 nm, and they were self-assembled into two-dimensional disordered multilayer on the silicon wafer, and the particles appeared to be closely packed. The UV-Vis absorption spectrum exhibited a strong absorption peak around 438 nm and a weak peak at 353 nm, which came from dipole and quadrupole oscillation resonance, indicating the formation of non-spherical silver nanoparticles. The surface-enhanced Raman scattering (SERS) active of the anisotropic silver nanoparticles were investigated using trans-1,2-bis(4-pyridyl)-ethylene (BPE) as probe molecule. The results indicate that self-assembled silver film of anisotropic silver nanoparticles on the silicon wafer is excellent substrate for SERS measurements, demonstrating significant enhancement, trace detection capability, and good stability. The minimum analytic concentration was 10(-9) mol x L(-1) for trans-1,2-bis (4-pyridyl)-ethylene (BPE) molecules. It was also found that silver film stored for 90 days in air shows no significant degradation in its sensitivity.