Corrigendum: Monitoring the orientation of rare-earth-doped nanorods for flow shear tomography.

This corrects the article DOI: 10.1038/nnano.2017.111.

Monitoring the orientation of rare-earth-doped nanorods for flow shear tomography.

Rare-earth phosphors exhibit unique luminescence polarization features originating from the anisotropic symmetry of the emitter ion's chemical environment. However, to take advantage of this peculiar property, it is necessary to control and measure the ensemble orientation of the host particles with a high degree of precision. Here, we show a methodology to obtain the photoluminescence polarization of Eu-doped LaPO4 nanorods assembled in an electrically modulated liquid-crystalline phase. We measure Eu3+ emission spectra for the three main optical configurations (σ, π and α, depending on the direction of observation and the polarization axes) and use them as a reference for the nanorod orientation analysis. Based on the fact that flowing nanorods tend to orient along the shear strain profile, we use this orientation analysis to measure the local shear rate in a flowing liquid. The potential of this approach is then demonstrated through tomographic imaging of the shear rate distribution in a microfluidic system.

Measuring Fast and Slow Enzyme Kinetics in Stationary Droplets.

We present a new microfluidic platform for the study of enzymtatic reactions using static droplets on demand. This allows us to monitor both fast and slow reactions with the same device and minute amounts of reagents. The droplets are produced and displaced using confinement gradients, which allows the experiments to be performed without having any mean flow of the external phase. Our device is used to produce six different pairs of drops, which are placed side by side in the same microfluidic chamber. A laser pulse is then used to trigger the fusion of each pair, thus initiating a chemcial reaction. Imaging is used to monitor the time evolution of enzymatic reactions. In the case of slow reactions, the reagents are completely mixed before any reaction is detected. This allows us to use standard Michaelis-Menten theory to analyze the time evolution. In the case of fast reactions, the time evolution takes place through a reaction-diffusion process, for which we develop a model that incorporates enzymatic reactions in the reaction terms. The theoretical predictions from this model are then compared to experiments in order to provide measurements of the chemical kinetics. The approach of producing droplets through confinement gradients and analyzing reactions within stationary drops provides an ultralow consumption platform. The physical principles are simple and robust, which suggests that the platform can be automated to reach large throughput analyses of enzymes.

Parallel measurements of reaction kinetics using ultralow-volumes.

We present a new platform for the production and manipulation of microfluidic droplets in view of measuring the evolution of a chemical reaction. Contrary to existing approaches, our device uses gradients of confinement to produce a single drop on demand and guide it to a pre-determined location. In this way, two nanoliter drops containing different reagents can be placed in contact and merged together, in order to trigger a chemical reaction. The reaction rate is extracted from an analysis of the observed reaction-diffusion front. We show that the results obtained using this platform are in excellent agreement with stopped-flow measurements, while decreasing the sample consumption 5000 fold. We also show how the device operation can be parallelized in order to react an initial sample with a range of compounds or concentrations, on a single integrated chip. This integrated chip thus further reduces sample consumption while reducing the time required for the experimental runs from hours to minutes.

Combining rails and anchors with laser forcing for selective manipulation within 2D droplet arrays.

We demonstrate the combination of a rails and anchors microfluidic system with laser forcing to enable the creation of highly controllable 2D droplet arrays. Water droplets residing in an oil phase can be pinned to anchor holes made in the base of a microfluidic channel, enabling the creation of arrays by the appropriate patterning of such holes. The introduction of laser forcing, via laser induced thermocapillary forces to anchored droplets, enables the selective extraction of particular droplets from an array. We also demonstrate that such anchor arrays can be filled with multiple, in our case two, droplets each and that if such droplets have different chemical contents, the application of a laser at their interface triggers their merging and a chemical reaction to take place. Finally by adding guiding rails within the microfluidic structure we can selectively fill large scale arrays with monodisperse droplets with significant control over their contents. In this way we make a droplet array filled with 96 droplets containing different concentrations of fluorescent microparticles.