(391c) Continuous Sizing and Characterization of Suspension-Based Nanomaterials
Nanoparticle characterization is an area of intense interest owing to its increasing importance in areas such as photovoltaics, catalysis, and sensors. Most of these applications involve processing nanoparticles in suspension, and demand precise control of properties such as chemistry, size, morphology and/or crystalline structure to achieve desired functionality. Unfortunately, characterization methods have generally failed to keep up with this rapid pace of discovery, introducing a significant bottleneck between scientific discovery and commercialization of nanomaterial-based products. Here we introduce an approach that overcomes limitations of conventional analysis methods, enabling continuous online quantification and characterization of nanoparticle composition, size, and morphology, independent of agglomeration state. Our microfluidic approach enables continuous analysis by exploiting localized fluorescent complexation of nanoparticles in suspension triggered by a sharp chemical discontinuity, yielding a signature easily detectable over a broad concentration range. We evaluated our approach by co-injecting fluorescein dye into a microchannel in parallel with suspensions of ZnO and TiO2 (anatase and rutile) nanoparticles. Interfacial fluorescence enhancement is observed in ZnO and anatase TiO2, whereas quenching is observed in rutile TiO2. In the case of ZnO, a pronounced concentration dependence can be clearly distinguished over 4 orders of magnitude in nanoparticle concentration. Interfacial fluorescence is also sensitively dependent on particle size regardless of the material’s agglomeration state. Our approach is also highly sensitive to morphology, as demonstrated in the case of TiO2 where fluorescence enhancement observed in the anatase form dramatically transforms to quenching in rutile samples. We also formulated a reaction-diffusion model applied it to verify experimental findings and capture the fundamental underlying transport and adsorption phenomena. The computational model enables the fluorescence signature to be correlated with particle size, and we have further refined the model to enable characterization of mixtures containing different particle species (e.g., anatase and rutile TiO2). In addition to continuous environmental monitoring, our analysis format enables online characterization of solution-based nanomaterials routinely employed as additives and coatings owing to its inherent ability to function in a portable and automated manner.