Joshua B. Edel

Pillar-induced droplet merging in microfluidic circuits

A novel method is presented for controllably merging aqueous microdroplets within segmented flow microfluidic devices. Our approach involves exploiting the difference in hydrodynamic resistance of the continuous phase and the surface tension of the discrete phase through the use of passive structures contained within a microfluidic channel. Rows of pillars separated by distances smaller than the representative droplet dimension are installed within the fluidic network and define passive merging elements or chambers. Initial experiments demonstrate that such a merging element can controllably adjust the distance between adjacent droplets. In a typical scenario, a droplet will enter the chamber, slow down and stop. It will wait and then merge with the succeeding droplets until the surface tension is overwhelmed by the hydraulic pressure. We show that such a merging process is independent of the inter-droplet separation but rather dependent on the droplet size. Moreover, the number of droplets that can be merged at any time is also dependent on the mass flow rate and volume ratio between the droplets and the merging chamber. Finally, we note that the merging of droplet interfaces occurs within both compressing and the decompressing regimes.

Quantitative detection of protein expression in single cells using droplet microfluidics

We demonstrate that single cells can be controllably compartmentalized within aqueous microdroplets; using such an approach we perform high-throughput screening by detecting the expression of a fluorescent protein in individual cells with simultaneous measurement of droplet size and cell occupancy.

Self-assembled nanoparticle arrays for multiphase trace analyte detection

Nanoplasmonic structures designed for trace analyte detection using surface-enhanced Raman spectroscopy typically require sophisticated nanofabrication techniques. An alternative to fabricating such substrates is to rely on self-assembly of nanoparticles into close-packed arrays at liquid/liquid or liquid/air interfaces. The density of the arrays can be controlled by modifying the nanoparticle functionality, pH of the solution and salt concentration. Importantly, these arrays are robust, self-healing, reproducible and extremely easy to handle. Here, we report on the use of such platforms formed by Au nanoparticles for the detection of multi-analytes from the aqueous, organic or air phases. The interfacial area of the Au array in our system is ≈25 mm(2) and can be made smaller, making this platform ideal for small-volume samples, low concentrations and trace analytes. Importantly, the ease of assembly and rapid detection make this platform ideal for in-the-field sample testing of toxins, explosives, narcotics or other hazardous chemicals.

DNA tunneling detector embedded in a nanopore.

We report on the fabrication and characterization of a DNA nanopore detector with integrated tunneling electrodes. Functional tunneling devices were identified by tunneling spectroscopy in different solvents and then used in proof-of-principle experiments demonstrating, for the first time, concurrent tunneling detection and ionic current detection of DNA molecules in a nanopore platform. This is an important step toward ultrafast DNA sequencing by tunneling.

Development of quantitative cell-based enzyme assays in microdroplets.

We describe the development of an enzyme assay inside picoliter microdroplets. The enzyme alkaline phosphatase is expressed in Escherichia coli cells and presented in the periplasm. Droplets act as discrete reactors which retain and localize any reaction product. The catalytic turnover of the substrate is measured in individual droplets by monitoring the fluorescence at several time points within the device and exhibits kinetic behavior similar to that observed in bulk solution. Studies on wild type and a mutant enzyme successfully demonstrated the feasibility of using microfluidic droplets to provide time-resolved kinetic measurements.

Micro- and nanofluidic systems for high-throughput biological screening.

High-throughput screening (HTS) is a method of scientific experimentation widely used in drug discovery and relevant to the fields of biology. The development of micro- and nanofluidic systems for use in the biological sciences has been driven by a range of fundamental attributes that accompany miniaturization and massively parallel experimentation. We review recent advances in both arraying strategies based on nano/microfluidics and novel nano/microfluidic devices with high analytical throughput rates.

Microfluidic routes to the controlled production of nanoparticlesElectronic supplementary information ESI available: image of the central portion of the micromixer chip. See http://www.rsc.org/suppdata/cc/b2/b202998g/

A microfluidic procedure for the controlled production of cadmium sulfide nanoparticles is described.

A microdroplet dilutor for high-throughput screening

Pipetting and dilution are universal processes used in chemical and biological laboratories to assay and experiment. In microfluidics such operations are equally in demand, but difficult to implement. Recently, droplet-based microfluidics has emerged as an exciting new platform for high-throughput experimentation. However, it is challenging to vary the concentration of droplets rapidly and controllably. To this end, we developed a dilution module for high-throughput screening using droplet-based microfluidics. Briefly, a nanolitre-sized sample droplet of defined concentration is trapped within a microfluidic chamber. Through a process of droplet merging, mixing and re-splitting, this droplet is combined with a series of smaller buffer droplets to generate a sequence of output droplets that define a digital concentration gradient. Importantly, the formed droplets can be merged with other reagent droplets to enable rapid chemical and biological screens. As a proof of concept, we used the dilutor to perform a high-throughput homogeneous DNA-binding assay using only nanolitres of sample.

Monitoring of real-time streptavidin-biotin binding kinetics using droplet microfluidics.

Rapid kinetic measurements are important in understanding chemical interactions especially for biological molecules. Herein, we present a droplet-based microfluidic platform to study streptavidin-biotin binding kinetics with millisecond time resolution. With integration of a confocal fluorescence detection system, individual droplets can be monitored and characterized online to extract kinetic information. Using this approach, binding kinetics between streptavidin and biotin were observed via fluorescence resonance energy transfer. The binding rate constant of streptavidin and biotin was found to be in a range of 3.0 x 10 (6)-4.5 x 10 (7) M (-1) s (-1).

Ultrafast Surface Enhanced Resonance Raman Scattering Detection in Droplet-Based Microfluidic Systems

The development of ultrafast Raman-based detection is one of the most interesting challenges underpinning the application of droplet-based microfluidics. Herein, we describe the use of surface-enhanced resonance Raman spectroscopy (SERRS) with submillisecond time resolution as a powerful detection tool in microdroplet reactors. Individual droplets containing silver nanoparticle aggregates functionalized with Raman reporters are interrogated and characterized by full spectra acquisitions with high spatial resolution in real time. Whereas previous works coupling SERRS with droplet-based microfluidics acquire a single spectrum over single or multiple droplets, we build upon these results by increasing our temporal resolution by 2 orders of magnitude. This allows us to interrogate multiple points within one individual droplet. The SERRS signals emitted from the aggregates are utilized to access the influence of flow rate on droplet size and throughput. Accordingly, our approach allows for high-throughput analysis that facilitates the study of other biological assays or molecular interactions.