Characterizing nanoparticle size and composition using microfluidic Raman diffusion-ordered spectroscopy

We show that microfluidic Raman Diffusion-Ordered SpectroscopY (Raman-DOSY) can be used to simultaneously characterize the size and the chemical composition of nanoparticles in solution. In these experiments, we inject a solution of nanoparticles into one half of a microfluidic channel, and solvent into the other. After the injection, the nanoparticles diffuse from the solution-filled part into the solvent-filled part of the channel, at a rate determined by their diffusion coefficient. By analyzing the time-dependent Raman spectrum in the initially solvent-filled part of the channel, we determine the size (from the diffusion coefficient) and characterize the chemical composition (from the corresponding Raman spectra) of the nanoparticles. Within about 1 hour we obtain both the size and chemical-structure information of polystyrene beads with diameters of 20, 50, and 100 nm. We further demonstrate that this method can distinguish nanoparticles of varying sizes in mixed samples. These results show that microfluidic Raman-DOSY is a promising method for combined size- and composition analysis of nanoparticles.