Continuous Nanoparticle Sizing and Characterization via Microfluidics

Abstract

High-throughput manufacturing of nanomaterial-based products demands robust online characterization and quality control tools capable of continuously probing the in suspension state. But existing analytical techniques are challenging to deploy in production settings because they are primarily geared toward small-batch ex-situ operation in research laboratory environments. Here we introduce an approach that overcomes these limitations by exploiting surface complexation interactions that emerge when a micron-scale chemical discontinuity is established between suspended nanoparticles and a molecular tracer. The resulting fluorescence signature is easily detectable and embeds surprisingly rich information about composition, quantity, size, and morphology of nanoparticles in suspension independent of their agglomeration state. We show how this method can be straightforwardly applied to enable continuous sizing of commercial ZnO nanoparticles, and to instantaneously quantify the anatase and rutile composition of multi-component TiO2 nanoparticle mixtures pertinent to photo catalysis and solar energy conversion. A transport model of the interfacial complexation process is formulated to qualitatively confirm the experimental discovery and to provide understanding of the transport and binding processes.

Practical utility is demonstrated by combining our detection method with a cyclone sampler to enable continuous monitoring of airborne nanoparticles. Our method uniquely combines ultra-high flow rate sampling (up to thousands of liters per minute) with sensitive detection based on localized fluorescent complexation, permitting rapid quantitative measurement of airborne nanoparticle concentration. By coupling these components, we show initial results demonstrating detection of airborne ultrafine Al2O3 nanoparticles at environmental concentrations below 200 μg m^−3 in air sampled at 200 L min^−1. This capability suggests potential for online monitoring, making it possible to establish dynamic exposure profiles not readily obtainable using current-generation personal sampling instruments. The underlying fluorescent complexation interactions are inherently size and composition dependent, offering potential to straightforwardly obtain continuous detailed characterization.

The increasing commercial prevalence of nanoparticle-based materials also introduces a new demand for robust online characterization tools amenable toward online monitoring in manufacturing settings. We address this need by showing how electrical conductivity measurements can be exploited to instantaneously obtain size and species information in oxide nanoparticle suspensions. This approach is readily implemented in an easy to build platform that can be employed either online to provide real-time feedback during continuous synthesis and processing, or offline for evaluation of test samples obtained from larger batches. Our implementation enables accurate results to be obtained using inexpensive digital multimeters, suggesting broad potential for on-site deployment in industrial settings.