A new method is described for surface functionalization of silicon nanocrystals. Organic monolayers were self-assembled via gas-phase adsorption of amines, alkenes, alkynes, and aldehydes onto the surfaces of aerosolized crystalline silicon nanoparticles of 12.2 nm diameter in an atmospheric pressure tube reactor. Assembly took place within 4 s at temperatures between 200 and 500 C. The extent of adsorption was measured by using tandem differential mobility analysis (T-DMA), an on-line diagnostic method for measuring changes in particle size. Functionalized particles were further characterized by high-resolution transmission electron microscopy and diffuse reflectance Fourier transform infrared spectroscopy. The apparatus described in this work can be used for continuous mass production of functionalized silicon nanoparticles. Moreover, the overall strategy of using T-DMA for monitoring monolayer uptake could be generally applied to study surface processing of other aerosolized nanoparticle systems.