Size Characterization of Ultrasmall Silver Nanoparticles Using MALDI-TOF Mass Spectrometry

Experimental

Synthesis of Silver Nanoparticles. 0.17 g of silver nitrate (Strem Chemical) was added to 0.5 mL of oleylamine (Acros Organics) and 4.5 mL of oleic acid (Sigma-Aldrich Inc.). The mixture was degassed at 70 ℃ for 90 min under vacuum and the reaction vessel was purged with argon. For the synthesis of 1.8 nm-sized nanoparticles, the solution was heated to 180 ℃ at a heating rate of 10 ℃/min and maintained for 2 min. For 3.7 nm-sized nanoparticles, the heating rate was set to 1 ℃/min and the reaction was stopped when the temperature just reached 180 ℃. After the reaction, the reaction vessel was cooled to room temperature and washed with 10 mL of toluene and 50 mL ethanol. The nanoparticles were dispersed in chloroform.

Characterization. MALDI-TOF MS was performed on a Voyager-DETM STR Biospectrometry Workstation (Applied Biosystems). Nanoparticles dispersed in chloroform were mixed with 9-nitroanthracene dissolved in chloroform (Sigma-Aldrich Inc.) in a weight ratio of 1:1 and spotted onto a target plate. The mass spectra were obtained with the 40-50% of the laser’s full power. All of the mass spectra were smoothed with a simple average of 100 data points. TEM images were taken with a JEOL-2010 electron microscope and TGA data were collected with a Q-5000 IR (TA Instrument).