A Silver Colloid Produced by Reduction with Hydrazine as Support for Highly Sensitive Surface-Enhanced Raman Spectroscopy

Reduction of aqueous silver nitrate by hydrazine dihydrochloride in weakly alkaline solution results in a polydisperse colloid that is stable for many months without addition of any stabilizing compounds. The average size of the predominantly spherical particles depends on the initial concentration of silver ions, ranging between 40 and 70 nm in diameter. The colloidal solutions exhibit a characteristic absorption in the blue region of the visible spectrum and are not turbid below a formal silver concentration of 4.5 × 10-4 M. With colloids prepared from 1.5 × 10-4 M silver(I), the SERS spectra of dyes such as nile blue A could be recorded from a solution with concentrations as low as 10-10 M, whereas no SERS signal was observed for dye concentrations higher than 10-4 M. The maximum signal intensity was obtained at a concentration of about 10-7 M. With colloids prepared from ≥3 × 10-4 M silver(I), no SERS signal was obtained from highly diluted solutions, but the concentration limit for the maximum signal intensity of around 10-7 M became even sharper. The thus prepared silver colloids can therefore be recommended for qualitative detection of certain organic compounds in the parts per billion range as well as for a semiquantitative determination in the parts per million range.